progress in energy and combustion...

Progress in Energy and Combustion Science 67 (2018) 158�187

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Progress in Energy and Combustion Science

journal homepage: www.elsevier.com/locate/pecs

Underground coal gasification� Part I: Field demonstrations and process
performance
TaggedPGreg PerkinsTaggedPMartin Parry Technology, Brisbane, 4001 Queensland, Australia

TAGGEDPA R T I C L E I N F O

Article History:Received 27 February 2017Accepted 20 February 2018Available online xxx

E-mail address: [email protected]

http://dx.doi.org/10.1016/j.pecs.2018.02.0040360-1285/© 2018 Elsevier Ltd. All rights reserved.

TAGGEDPA B S T R A C T

Underground coal gasification can convert deep coal resources into synthesis gas for use in the productionof electricity, fuels and chemicals. This paper provides a review of the various methods of undertakingunderground coal gasification and observations from demonstrations of the process in the field. A generalrepresentation of the underground process is presented, along with an identification of the various zonesand associated governing phenomena. The main factors affecting the performance of underground coal gas-ification, such as coal rank, depth and thickness and oxidant composition and injection rate are examined indetail. A brief assessment of the economic and environmental considerations relevant to underground coalgasification projects is presented. Finally, guidelines for site and oxidant selection are provided based onthe learnings from prior demonstration projects.

© 2018 Elsevier Ltd. All rights reserved.

TaggedPKeywords:
Underground coal gasificationFluid flowChemical reactionField demonstrations
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1592. Underground coal gasification methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
2.1. Early efforts in former Soviet Union . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1602.2. Linked vertical wells method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1602.3. Steeply dipping coal seammethod. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1612.4. Controlled retracting injection point method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1612.5. Long tunnel method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

3. Summary of field demonstrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633.1. American era experience. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

3.1.1. Large block experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633.1.2. Partial Seam CRIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1633.1.3. Rocky Mountain I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164

3.2. Thulin, Belgium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1643.3. Chinchilla, Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1663.4. Bloodwood Creek, Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1673.5. Swan Hills, Canada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1683.6. “Barbara” andWieczorek, Poland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1693.7. General representation of the UCG process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

4. Factors affecting process performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1704.1. Effect of oxidant. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1714.2. Effect of coal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1714.3. Effect of coal seam depth. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1744.4. Effect of coal seam thickness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1744.5. Effect of process scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1744.6. Effect of gasifier design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1754.7. Effect of site conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177

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5. Economics of syngas production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1785.1. Impact of oxidant choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1785.2. Impact of coal areal energy density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1795.3. Impact of production well capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1795.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

6. Environmental considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1806.1. Surface impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1806.2. Groundwater pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1806.3. Groundwater depletion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1826.4. Greenhouse gas emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1836.5. Carbon capture and storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

7. Guidelines for site and oxidant selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1848. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

Fig. 1. Summary of the underground coal gasification sites, including prior test sites,pilot operations and areas thought to be prospective for CO2 sequestration (fromCouch [16]).

1. Introduction

TaggedPUnderground coal gasification (UCG) is the process of convertinghydrocarbon materials into synthesis gas in-situ. Underground coalgasification is sometimes referred to as in-situ coal gasification(ISCG). The process has been developed over more than a century,though only a few projects currently operate on a continuous basis.Various articles and studies indicate that UCG is technically feasibleand economically attractive as a method to utilise the energy ofdeep coal resources (e.g. [1�6]). Recent reviews on the fundamen-tals, applications and modelling have been reported by Bhutto et al.[7], Shafirovich et al. [8] and Khan et al. [9], respectively. Fig. 1 showsthe worldwide locations of UCG commercial and demonstrationprojects as well as areas that are thought to be prospective for car-bon sequestration.

TaggedPIn its simplest form UCG involves: i) drilling an injection well intothe coal seam and linking it with a production well, ii) igniting thecoal seam and iii) injecting an oxidant (e.g. air, oxygen/steam etc.)and iv) recovering syngas from the production well. In practice, theprocess involves performing coal gasification within an open systemlocated hundreds of metres below ground and controlling it thougha limited number of injection and production wells to convert andextract the hydrocarbons. Like surface coal gasification, under-ground coal gasification, requires high temperatures, managementof dangerous fluids such as pure oxygen and handling of hot rawproducts of syngas, tars and produced water. In addition, the processis undertaken in a geo-reactor, wherein the natural surroundingstrata are used to contain the process and provide adequate barriersto ensure isolation, in terms of chemical, thermal and mechanicalimpacts.

TaggedPThe treated product syngas from UCG can then be used in a vari-ety of processes such as:

TaggedP� combustion of syngas in a combined cycle gas turbine to pro-duce electricity (e.g. [10,11])

TaggedP� conversion of syngas into synthetic crude oil to produce naph-tha, diesel and kerosene via the Fischer�Tropsch process (e.g.[12])

TaggedP� conversion of syngas into methanol, which may be furtherrefined into dimethyl ether (a potential transport fuel), olefinsand acetic acid

TaggedP� conversion of syngas into hydrogen to produce ammonia andurea or for use in fuel cells (e.g. [13�15])

TaggedP� conversion of syngas into synthetic natural gas, via the metha-nation reaction

TaggedPFig. 2 shows a schematic of the important products that can bemade with UCG. Carbon dioxide may be separated from the syngasand used for enhanced oil recovery (EOR) or sequestered in geologi-cal formations in so-called carbon capture and storage (CCS) proj-ects.

TaggedPThe focus of this paper, which is Part I of a two-part series, is ondescribing the main methods of undertaking underground coal gasi-fication, summarising the results of field demonstrations, analysingthe factors which affect the process performance and reviewing theeconomic and environmental considerations of developing projects.In Part II of the series, a detailed examination of the fundamentalphenomena in various zones of the process is performed, includingat the cavity sidewall, in the permeable bed of ash and char, in thevoid spaces and in the near- and far-fields surrounding the activegasification zones.

2. Underground coal gasification methods

TaggedPThe main methods for underground coal gasification are:

TaggedP� chamber methodTaggedP� streammethodTaggedP� linked vertical wells methodTaggedP� controlled retracting injecting point methodTaggedP� steeply dipping beds method

TaggedPThe dependent variables in underground coal gasification areshown in Table 1. Like mining and oil and gas extraction, site selec-tion determines most of the important independent variables whichdetermine the technical and economic performance.

TaggedPA schematic of several of the methods is shown in Fig. 3. Severalother methods, such as the blind-borehole method, a small-boremethod and the long tunnel system of gasifying manually-minedchannels have also been trialled [17�20]. Table 2 provides a sum-mary of significant underground coal gasification projects world-wide since the 1930s.

List of Symbols

csyngas syngas cost ($/GJ)copex total operating cost ($/year)Ctotal total capital cost ($)Cwells capital cost of well pair ($)Erecovered energy recovered per well-pair (GJ)GCV gross calorific value (GJ/ton)H height of coal seam (m)L horizontal length of well (m)_msyngas mass flow of syngas (kg/s)n, m exponentNg number of gasifierspfuel price of fuel ($/GJ)Poperating operating pressure (Pa)R discount rate (%/year)R

0 effective discount rate (%/term)T project lifetime (year)T

0 gasifier lifetime (year)W width of cavity (m)a areal energy density (GJ/m2)e cold gas efficiency (%)h width factor ratio (�)z cavity shape factor (�)r density (ton/m3)

160 G. Perkins / Progress in Energy and Combustion Science 67 (2018) 158�187

2.1. Early efforts in former Soviet Union

TaggedPEfforts to develop UCG methods started in the 1930s in the for-mer Soviet Union. Initial experiments were conducted on a range ofcoals in three coal basins: the Moscow, Donetsk and Kuznetsk withsome of the early attempts aimed at gasifing pillars left behind from

Fig. 2. Processes and products of u

TaggedProom and pillar underground coal mining using the chamber method[35]. Fig. 4 shows the configuration of the first UCG experiment con-ducted at the Krutova mine in the Moscow basin in 1933 [36,37]. Apanel of 10£ 10 m coal was separated from the seam using brickwalls, with air injected on one side and produced gas extracted fromthe other side. In these early UCG experiments the flame front prop-agations were found to be difficult to control due to the heteroge-neous nature of the coal seams and were judged to beunsatisfactory.

TaggedPIn 1933 a group of engineers from the Donetsk Institute of CoalChemistry (DUKhI) suggested gasifying coal in a channel [35]. Themethod involved mining two shafts to the coal seam and mining adrift shaft between them. Oxidant would be injected via boreholesin one shaft and syngas extracted from boreholes in the second.They applied this technique, which became known as the streammethod, in a dipping coal seam in the Donetsk basin [35]. Fig. 5shows a schematic of the streammethod.

TaggedPIn the 1940s the Soviets began to experiment with using a pat-tern of boreholes drilled into the coal seams [21,35,38,39]. The earlyshaftless methods used rectangular and circular arrays of boreholesspaced very close together (on the order of several metres). The insitu combustion process was started by dropping hot charcoalsdown one set of wells, while using another set of wells to inject airand a third set of wells to recover the produced gases. These meth-ods were generally called “percolation”methods. Due to the low nat-ural permeability of the coal seams, the Soviets struggled to achievesatisfactory performance, and the costs were prohibitively high dueto the need for boreholes spaced extremely close together.

2.2. Linked vertical wells method

TaggedPThe linked vertical wells (LVW) technique evolved over a periodof several decades as the Soviets experimented with a number of so-

nderground coal gasification.

Fig. 3. Schematic of the several techniques used for conducting underground coalgasification: (a) linked vertical wells, (b) controlled retracting injection point (CRIP),and (c) steeply dipping coal seams.

Table 1Independent and dependent variables of interest for UCG.

Independent variables Dependent variables

Coal properties Product gasProximate analysis Composition, heating valueUltimate analysis Flowrate

Coal seam geology, hydrogeology Process efficiencyThickness, depth and dip ThermalPermeability to gas, liquid Chemical

Strata geology, hydrogeology Resource recoveryoperating conditions Interaction with environment

Injection composition Water consumptionInjection flowrate SubsidenceOperating pressure Dispersion of pollutantsWell layout


TaggedPcalled “linking” methods, to increase the permeability of the coalseams between boreholes, including reverse combustion, electricresistive heating, hydraulic fracturing and directional drilling[35,40,41]. Reverse combustion linking was found to be a reliablemethod, in relatively shallow and relatively low rank coals (lignite,sub-bituminous coal) and could be operated with well centresbetween 20 m and 50 m apart. Fig. 6 shows a typical example of thewell array and sequence of gasification for shaftless UCG methodsdeveloped for flat lying coal seams. This method was used at theShatskaya UCG station in the Moscow basin, producing several hun-dred million m3 of syngas each year from a 2 to 4 m thick lignite atshallow depth [22]. Reverse combustion linking remains a majorlimiting factor for the LVW concept as it requires relatively perme-able coal seams, short distances between wells and is slow to com-plete a link, with typical linking speeds of between 1 and 4 m/day[42].

2.3. Steeply dipping coal seam method

TaggedPMany coal seams dip at significant angles, and for these theSoviets developed an alternate technique. The scheme involvedusing the shaft mined to access one seam to provide access to anoverlying seam and the mining of a firing drift. Injection boreholes,typically inclined, were drilled from the surface and productionboreholes were drilled into the underlying seam to act as productionwells [38]. The gasification of a steeply dipping coal seam was per-formed at the Rawlins site in the United States of America[35,43,44]. Coal char is converted at the bottom of the gasifier andthe produced gases move upwards causing moisture and volatilematter in the coal to be released, which in turn, results in the coalfalling into the gasification region below as seen in Fig. 3. Gas qualityfrom steeply dipping coal seams is very good [24] � probablybecause of the high surface area for reaction � however, applicationof this technique is limited by the availability of steeply dipping coalseams in suitable locations. Since ash, slag and rock rubble has thepropensity to create a zone of low permeability above the injectionwell, multiple injection wells are needed to gasify a steeply dippingcoal seam [38].

2.4. Controlled retracting injection point method

TaggedPThe CRIP technique uses directional drilling to locate an injectionwell, low and horizontal in the seam, as seen in Fig. 7 [45]. By havinga retractable igniter/injector, the injection location can be moved bythe operators at the surface, allowing a staged access of the coalseam. In the first stage, the injection point is located close to the pro-duction well. In the second and subsequent stages the injectionpoint is retracted away from the production well to form a new gasi-fication cavity in fresh coal. The technique was first implemented atscale in the Partial Seam CRIP test at Centralia [46]. An advantage of

TaggedPthe CRIP technique is that injection location remained low in theseam, injection well damage from subsidence was minimized andlarge amounts of coal could be gasified with a single injection/pro-duction well pair. Various embodiments have been reported (e.g.[47]) and well configurations include a linear CRIP (L-CRIP), a knife-edge CRIP (K-CRIP) and a parallel CRIP (P-CRIP). The US R&D pro-gramme culminated with the Rocky Mountain I demonstration proj-ect in Hanna, Wyoming, in which 11,000 tons of coal were gasifiedin 100 days using a knife-edge CRIP gasifier [26].

2.5. Long tunnel method

TaggedPThe long tunnel method has been developed at the Chinese Uni-versity for Mining and Technology (CUMT) and is a mixture of a shaftand shaftless UCG method [29,49,50]. The process is called the “longtunnel, large section, two stage” UCG method by the Chinese [29]. Itinvolves boreholes, an airflow tunnel and a gasification tunnel asshown in Fig. 8. The size of each gasifier is approximately

Table 2Summary of significant underground coal gasifiers operated worldwide.

Country Site Startup year Coal type Technique Injected gas Seam depth (m) Seam thickness (m) Gas heating value (MJ/m3) Reference

Former Lisichansk 1935 Bit. SDB Air 30 1 3.8 [21]U.S.S.R. Podmoskovna 1947 L LVW Air 55 3 3.4 [21]

Yuzno-Abinskaja 1955 Bit. SDB Air » 100 3 4.1 [21]Shatskaya 1959 L LVW Air 50 2 3.2 [22]Angrenskaja 1961 L LVW Air 150 9 3.4 [23]

USA Hanna I 1973 HVC LVW Air 120 9 4.2 [24]Hanna II 1975 HVC LVW Air 85 9 5.3 [24]Hanna III 1977 HVC LVW Air 85 9 4.1 [24]Hoe Creek I 1976 HVC LVW Air 40 8 3.6 [24]Hoe Creek IIA 1977 HVC LVW Air 40 8 3.4 [24]Hoe Creek IIB 1977 HVC LVW O2/H2O 40 8 9.0 [24]Hoe Creek IIIA 1979 HVC LVW Air 40 8 3.9 [24]Hoe Creek IIIB 1979 HVC LVW O2/H2O 40 8 6.9 [24]Pricetown I 1979 Bit. LVW Air 270 2 6.1 [24]Rawlins IA 1979 SB SDB Air 105 18 5.6 [24]Rawlins IB 1979 SB SDB O2/H2O 105 18 8.1 [24]Centralia A 1984 SBC K-CRIP O2/H2O 75 6 9.7 [25]Centralia B 1984 SBC LVW O2/H2O 75 6 8.4 [25]Rocky Mountain IA 1987 SB K-CRIP O2/H2O 110 7 9.5 [26]Rocky Mountain IB 1987 SB LVW O2/H2O 110 7 8.8 [26]

UK Newman-Spinney P5 1949 SB BH Air 75 1 1.4 [17]Belgium Thulin 1986 A LVW Air 860 6 7.0 [27]Spain El Tremedal 1997 SB L-CRIP O2/H2O 580 2 10.9 [27]Poland Wieczorek 2014 SB SM Air, O2, CO2 464 5.5 3.4 [28]China Xinhe 1994 Bit. LT Air/Steam 80 3.5 11.8 [29]

Liuzhuang 1996 HVC LT Air/Steam 100 3 12.2 [29]Xinwen 2000 HVC LT Air/Steam 100 1.8 10.4 [29]Feichang 2001 Bit. LT Air 90 1.5 5.1 [29]Xiyang 2001 A LT Air/Steam 190 6 11.9 [29]

Australia Chinchilla G1 2000 SB LVW Air 132 10 4�5 [30]Chinchilla G3 2007 SB LVW Air 132 10 4�5 [31]Chinchilla G4 2009 SB P-CRIP Air 132 10 4�8 [31]Chinchilla G5 2011 SB L-CRIP Air & O2/H2O 132 5.5 4�11 [31]Bloodwood Ck, Panel 1 2009 SB P-CRIP Air & O2/H2O 200 9 5�12 [32]Bloodwood Ck, Panel 2 2011 SB P-CRIP Air 200 9 5�6 [33]

Canada Swan Hills 2011 HVB L-CRIP O2/Water 1400 4.5 16 [34]

Guide to symbols Coal type: L=lignite, SB=subbituminous, SBC=subbituminous C, Bit.=bituminous, HVB=high volatile bituminous B, A=Anthracite. Technique: LVW=linked verticalwells, CRIP=controlled retracting injecting point, L-CRIP=linear CRIP, P-CRIP=parallel CRIP,K-CRIP=Knife-edge CRIP, SDB=steeply dipping beds,BH=borehole method, LT=long tun-nel, SM=shaft method.


TaggedP200 m£ 200 m [51]. In the first stage of operation, air is injected intothe gasifier, to support combustion, heating the face of the coalseam to over 1000 °C. In the second stage, steam is injected andreacts with the coal at high temperature to form carbon monox-ide and hydrogen, with the H2 concentration reaching about50 vol% in the produced syngas [52]. The main objective of themethod is to access additional coal resources from existing mine

Fig. 4. First UCG experiments in the former Soviet Union using the chamber methodat the Krutova mine in 1933 (from Klimenko [37]).

TaggedPshafts and it has the advantage of producing a high quality syn-gas when using air as the oxidant [53]. Production capacities foreach gasifier are reported to be on the order of 1500 Nm3/h ofsyngas [29]. Since the 1990s over 15 long tunnel gasifiers havebeen constructed and operated [51].

Fig. 5. Schematic of the stream method of UCG as proposed by engineers from theDonetsk Institute of Coal Chemistry (from Olness and Gregg [35]).

Fig. 6. Linked vertical wells concept as used at the Shatskaya underground coal gasifi-cation station (from Olness [22]).


3. Summary of field demonstrations

TaggedPObservations of performance of UCG, excavations and post-burncoring of the cavities are important to develop fundamental insightsinto the gasification phenomena, to develop conceptual models ofthe process, and finally, to formulate mathematical models by whichthe process can be described quantitatively. In this section, pertinentobservations from projects conducted in the USA, Europe, Australiaand Canada are summarized.

3.1. American era experience

TaggedPThe excavations and post-burn coring activities of UCG projectsconducted as part of the American program in the 1980s still providethe most detailed and illustrative insights into how coal gasificationproceeds underground.

TaggedP3.1.1. Large block experimentsTaggedPA series of five experiments (LBK-1 to LBK-5) were conducted by

the Lawrence Livermore National Laboratory (LLNL) in 1981/82 inan exposed coal face in the WIDCO coal mine near Centralia,

Fig. 7. Controlled retracting injection point (CRIP) process for un

TaggedPWashington in the Tono basin [54]. The coal at this location was sub-bituminous with considerable ash in the form of stringers and eachexperiment lasted between 3 and 7 days and affected between 20and 50 m3 of coal. Steam/oxygen ratios of 1:1, 2:1 and 3:1 alongwith air were used as the oxidant. Table 3 provides a summary ofthe operating conditions and performance of the experiments.

TaggedPFig. 9 shows pictures of the excavated cavities from the tests.Inspections showed an ellipsoid profile in plan view with cavitycross-sections having an oval shape, with vertical to horizontalaspect ratios of between 1:1 and 2:1 [55]. The cavities were found tocontain a permeable pile of dried coal, char, ash and in most casesroof rock material. In no case was there a completely open channelthrough the coal seam. Grens and Thorsness report that in many cir-cumstances the cavity is filled with char rubble, fused and rubblizedroof rock, with a void space at the top and an ash layer on the cavityfloor [56].

TaggedP3.1.2. Partial Seam CRIPTaggedPFollowing on from the large block experiments, the LLNL devel-

oped a larger experiment at the WIDCO coal mine to field test thecontrolled retracting injection point concept, called the Partial SeamCRIP trial. The project utilized the high wall geometry of the Cen-tralia open pit mine to gasify coal over a thirty day period in 1983[58,59]. The project included a horizontal injection well, a slant pro-duction well (PRD-1) and a vertical production well (PRD-2). A30 day operations period resulted in 2000 tons of coal being con-verted into syngas using mostly an oxidant of oxygen and steam.Fig. 10 shows a plan view of the 14 slices through the affected coalthat were made during the post-burn excavation activities.

TaggedPThe volume of coal affected was calculated from excavations tobe 1150 m3, while material balance estimates were 1388 m3 with achar accumulation ratio of 0.3 [46]. Fig. 11 shows the schematic of atypical slice through the cavity in the vicinity of the injection pointlocation. It can be seen that the cavity grew upwards consumingcoal to the roof of the seam and then expanded into the overburden.Cavity shapes exhibited an hourglass shape with pinching aroundthe midpoint of the coal seam height. Ash accumulated around theinjection point, with unconverted char present at the lateral extrem-ities of the cavity. Spalled and collapsed overburden materials werepresent above the ash and char and were distinguished by variouscolours during the excavations. According to Cena et al., the ash andchar appeared highly permeable, while the rock rubble was less

derground coal gasification (adapted from Cena et al. [48]).

Fig. 8. Long tunnel method of gasifying coal seams (from Li [29]).


TaggedPpermeable, having large areas where the material had been fusedtogether at high temperatures [46].

TaggedPThe outflow channel consisted of packed char rubble with aver-age particle size of 1

2 cm or less. A V-shape channel formed fromshrinkage and slumping of the heated coal leading to the formationof a void space. The hot syngas dries coal to the sides and above thevoid space, while the sagging and slumping dried coal insulates thebottom [46]. As more coal is dried and falls down, the gas flowmigrates upwards leading to the creation of V-shaped channel.

TaggedP3.1.3. Rocky Mountain ITaggedPThe Rocky Mountain I project was conducted in south western

Wyoming near the town of Hanna and was designed to demonstratethe technologies of CRIP and ELW (Extended Linked Well) alongsideeach other in the same coal seam at a commercial production scale[48,60]. The coal was sub-bituminous, at a depth of 110 m with athickness of approximately 7.6 m.

TaggedPThe CRIP module ran from November 1987 to February 1988operating for 93 days and converting 11,023 tons of coal [48]. TheCRIP module included a horizontally drilled injection and productionwell and a vertical well used only for linking. The ELW moduleincluded a horizontally drilled production well and two verticalinjection wells.

TaggedPDue to the depth of the Rocky Mountain I project, the gasificationcavities could not be excavated. Instead an 18 well post-burn coring

Table 3Injected gas and product gas for the five large-block experim

Experiment LBK-1

Phase Pre-CRIPFlow ramp up FastSteam/oxygen ratio (mol/mol) 3.0Product gas heat of combustion (kJ/mol) 231Product gas composition (vol%)H2 42CO 25CO2 28CH4 3.5C2H4 0.4C2H6 0.1N2 0.4Coal consumed (m3) 19

TaggedPprogram was developed to delineate the areal extent of the cavities,identify the extent of roof collapse, obtain samples of cavity materi-als and to characterise the area near the ignition and CRIP locations[61]. Fig. 12 shows the interpretation of a section through the cavityin the vicinity of the initial ignition. An ash and slag layer is found onthe bottom of the cavities, with unconverted char thought to existon the lateral regions of the cavity. Unconverted coal remains inplace below the level of the wells and remains unaffected due to theinsulative properties of the ash and slag. In the bulk of the affectedzone, rubblized rock material is found to account for between about25 and 50% of the cavity height. Void spaces are found, but are rela-tively small and not uniform across the cavity cross section. Alteredover-burden roof material is found and penetrates up to 50% of theseam height into the overlying rock layer [61].

3.2. Thulin, Belgium

TaggedPIn the 1980s a first UCG experiment at great depth was carriedout within the framework of the Belgo�German project on under-ground coal gasification [62,63]. The coal at this location was a lowvolatile semi-anthracite, with a thickness of 4 � 6 m at a depth of860 m [64]. Two vertical wells (TH1 and TH2) were drilled into thecoal seam separated by 35 m. The plan was to link the wells byreverse combustion, however after two years of unsuccessful

ents (from Hill et al. [54]).

LBK-1 LBK-2 LBK-3 LBK-4 LBK-5

Post-CRIP � � � O2-onlyFast Slow Slow Fast Fast2.8 1.2 3.0 1.1 1.8242 251 251 242 241

43 38 43 43 4614 33 23 22 2034 22 27 28 286.0 4.0 5.3 5.0 4.80.4 0.3 0.2 0.2 0.20.6 0.3 0.4 0.4 0.30.5 1.1 0.6 0.6 1.17 7 32 24 6

Fig. 9. Excavation of cavity of the Large Block tests: (a) Slice 13 from LBK-1 and (b)Slice 3 from LBK-4 (from Thorsness and Cena [57]).

Fig. 11. Schematic of the excavated Slice VII positioned around the injection point ofthe first CRIP manoeuvre in the Partial Seam CRIP field demonstration (from Cenaet al. [46]).


TaggedPattempts it was decided to link the wells using short radius drilling[62].

TaggedPGasification operations started on 2nd October 1986 and endedon 25th May 1987. The process gradually transitioned to a filtrationgasification process under high pressure (20�30 MPa) through thecoal seam, leading to the generation of medium BTU gas with a highmethane content [62]. During the project’s duration three oxidantswere tested: air, oxygen enriched air and a mixture of oxygen andfoamy water. Flow modelling during the gasification operationsshowed that the flow behaviour was relatively dispersive [64].Fig. 13 shows an interpretation of the Thulin project during

Fig. 10. Plan view of the Partial Seam CRIP field trial showing slices which were sub-sequently excavated (from Cena et al. [46]).

TaggedPoperations, including indicative locations for the high temperaturezone and the pyrolysis zone.

TaggedPThree post-burn wells (PB1, PB2, PB3) were drilled to examinethe affected materials from the test. Analysis of the samples col-lected from the post burn drilling revealed that temperatures at least900 °C and possibly up to 1100 °C existed in the vicinity of PB2, whilelower temperatures not exceeding 450 °C were observed in well PB1[62]. Post-burn drilling showed affected and rubblized overburdenmaterial along with affected coal and samples of ash and slag. Minorvoid spaces were also identified, but due to the high pressure, thegasification process is thought to have operated mostly in the

Fig. 12. Rocky Mountain I typical post-burn coring analysis (from Lindblom [61]).

Fig. 13. Top view diagram of the underground coal gasification at Thulin: 1, high tem-perature zone for production; 2, pyrolysis zone; 3, gasification zone; 4, zone of retro-gression of the Boudouard and water-gas-shift reactions (from Dafaux et al.. [63]).


TaggedPfiltration / permeable bed gasification regime wherein gases diffusethrough the coal seam at low velocity over a relatively wide frontalarea [63].

3.3. Chinchilla, Australia

TaggedPThe Chinchilla site located approximately 350 km west of Bris-bane, Australia was the site of five demonstration gasifiers from1999 to 2013 conducted in a sub-bituminous coal seam up to 10 mthick. At its peak, the Chinchilla Demonstration Facility consisted ofUCG facilities, a 5 to 10 bpd GTL plant, a waste water treatmentplant, a tank farm, chemical laboratory and supporting infrastruc-ture, see Fig. 14 [31]. The coal seam targeted for UCG was the

Fig. 14. Plot plan of the Chinchilla Demonstration Facility showing the locations of Gas

TaggedPcombined Macalister A and B seams within the Juandah Coal Meas-ures, located circa 130 m below the surface and with an average of 6and 4 m in thickness, respectively. The overburden (Springbok For-mation Sandstones) comprises a sequence of medium grey inter-bedded carbonate cemented lithic-feldspathic siltstones and finesandstones with minor carbonaceous mudstone, mudstone and coalstringers. Between the surface and the bottom of the Macalister coalseams, there are no high yielding aquifers. The coal quality at thesite is sub-bituminous with a calorific value between 15 to 21 MJ/kg,with an average of 19 MJ/kg.

TaggedPTable 4 provides a summary of the five gasifiers operated at Chin-chilla. Gasifier 1 operated from 1999 to 2002 and has been describedin detail elsewhere [30,65�67]. Gasifier 2 and 3 used variants of thelinked vertical well concept with varying degrees of success [12].Gasifier 4 used a parallel CRIP design with uncased open hole hori-zontals. Around 14,759 tons of coal were converted to syngas overtwo years of operations, mostly using air as an oxidant [31]. Trials ofO2 enriched air (up to 40 mol%) and CO2/air gasification were alsoundertaken [68].

TaggedPGasifier 5 was commissioned in October 2011 and consisted of amodified Linear CRIP design with three vertical service wells, andwith directionally drilled injection and production wells, with themain well heads spaced 900 m from each other [31,69]. The injectionand production horizontals intersected the service wells as seen inFig. 15, forming a drilled link from the injection side to the produc-tion side. The production well was directionally drilled, mostly as aprecaution, so that coiled tubing tools could be run in the well toremove any particulate or tarry matter that might build up afterignition of the gasifier. The injection well used a proprietary 4 1/20 0

liner which had alternating sections of blank casing and casing sec-tions with laser-cut apertures encased in a HDPE sheath [70]. The

ifiers 1 to 5, water treatment plant, gas to liquids plant and other surface facilities.

Table 4Summary of UCG gasifiers operated at the Chinchilla Demonstration Facility.

Gasifier Operated Well Configuration Oxidants Coal Converted (tons)

1 1999�2001 LVW (reverse combustion) Air > 20,0002 2007 LVW (reverse combustion) Air < 2003 2008�2009 LVW (drilled link) Air 24724 2010�2012 Parallel CRIP Air, enriched air 14,7595 2011�2013 Linear CRIP Air, enriched air, O2/water > 19,000

Fig. 15. Schematic of the well layout used in Gasifier 5 at Chinchilla, Australia (from Perkins et al. [31]).

Fig. 16. Picture of the oxygen-water lance used in Gasifier 5 being tested at surface.


TaggedPproduction well liner was 70 0 and included laser-cut apertures alongthe horizontal section to increase the collection surface area for theproduced syngases. A total of 20 thermocouples were installed onthe liner of the injection well and 12 thermocouples were installedon the liner of the production well and provided real-time tempera-ture measurement over the life of wells’ operation. The length of thein-seam horizontal was limited to 360 m by the surface layout of thesite. The initial ignition of Gasifier 5 was performed using a down-hole electrical heater in Service Well 2. Subsequently, the location ofthe active gasification zone was moved using a natural gas/airburner deployed on coiled tubing or through operational changes tothe air injection rate. A water quench system was installed into Ser-vice Well 1 to control the downhole temperature at the heel of theproduction well to below 350 °C.

TaggedPGasifier 5 operated continuously from October 2011 till Novem-ber 2013, converting more than 19,000 tons of coal into syngas [31].The average coal conversion rate over 24 months was circa 27 tpd,while the peak conversion rate of circa 45 tpd was achieved usingoxygen blown gasification (see Table 10 for performance data). Overthe gasifier’s life the active gasification zone was retracted morethan 300 m from the ignition point close to Service Well 2, to» 30 m in front of the heel of the injection well.

TaggedPFor oxygen blown gasification, liquid oxygen was supplied bytruck and stored at site and vaporized at surface. Oxygen and waterinjection was achieved by using a concentric coiled tubing consistingof 3.50 0 outer diameter tubing, a 20 0 stainless steel coil to convey thepure oxygen and a thermocouple. The oxygen and water were mixeddownhole in a nozzle designed to handle temperatures of over1000 °C and sprayed into the active gasification zone. Fig. 16 showsa picture of the oxygen-water nozzle being tested at surface in thepresence of natural gas burners used to simulate the high tempera-tures present downhole near oxidant injection zone. The nozzle alsoincluded check-valves to prevent syngas from entering into thecoiled tubing.

TaggedPFor gasification operations on air, the oxidant was injected intothe main injection wellbore and the dry syngas heating value was 4to 5 MJ/Nm3 with an oxygen utilization of » 950 MJ/kmol. The

TaggedPoxygen utilization (MJ/kmol) is a key measure of performance forUCG:

O2 utilization ¼ Energy in syngas ðMJ=sÞInjected oxygen rate ðkmol=sÞ ð1Þ

TaggedPFor UCG it is a more reliable indicator than the cold gas efficiency,which relies on a calculation of the coal converted to syngas, whichcan not be measured directly. On oxygen blown gasification the syn-gas heating value was in the range 10 to 11 MJ/Nm3 and the oxygenutilization was around 1200 MJ/kmol.

3.4. Bloodwood Creek, Australia

TaggedPThe Bloodwood Creek site located near Kogan west of Brisbane,Australia was the site of two UCG gasifiers operated by CarbonEnergy [33,71]. The UCG projects were conducted in the Macalisterseam of the Walloon coal measures � the same one targeted at thenearby Chinchilla site. At the Bloodwood Creek location the coal wasdeeper at » 200 m below the surface with a thickness of up to 13 m[72].

Fig. 17. Aerial view of the Bloodwood Creek UCG site in Australia in 2010 showinglocation of Panel 1 (from Mallett [32]). Panel 2 was later drilled parallel to Panel 1 in2011.

Table 5The quantum of chemicals removed from Panel 2 during steam cleaning, chemi-cals remaining in cavity water and percentage removed during cleaning (fromMallett [72]).

Mechanism Total kg BTEX PAH Ammonia Phenolics

Vented with steam 3065.75 2221.48 165.36 542.4 118.32In cavity water 274.10 1.03 0.36 159.52 113.20Total in cavity at start 3339.85 2222.51 165.72 701.92 231.52Removed during

venting (%)91.79 99.95 99.78 77.27 51.11


TaggedPCarbon Energy used a P-CRIP gasifier design, which was ignitedvia a vertical well positioned at the apex of the two horizontals eachhaving 500 m of in-seam length and which were spaced 30 m apart[32,72]. Panel 1 was started up in October 2008 and was operatedmostly on air with a short period of steam/oxygen gasification. Panel2 operated from March 2011 to October 2012, converting more than10,000 tons of coal into syngas. Carbon Energy also built andcommissioned a 5 MWe gas engine power plant which used the syn-gas from UCG to generate up to 1.5 MWe of electricity and exportthis into the local grid. Fig. 17 shows an aerial view of the BloodwoodCreek site.

TaggedPFig. 18 shows the injection and production flow rates and thehigher heating value of the produced syngas from a period of opera-tion of Panel 2. While the injection flow is stable, it is observed thatperiodically the syngas flow rate and quality will drop steeply, dueto poor gasification efficiency (poor contact of reactants with coal,combustion of produced syngas, excess heat losses). This drop inperformance is used to indicate when its time to move the oxidantinjection zone to a new location. Fig. 18 shows that after a CRIPmanoeuvre, the syngas flowrate and quality improves back to itsbaseline value. Thus, a field of UCG gasifiers is able to provide a con-sistent production flow and quality of syngas to a downstream user.

TaggedPThe main environmental concern of UCG is contamination oflocal groundwaters by pyrolysis products formed during the gasifi-cation process. Carbon Energy has published several reports of theenvironmental performance of the Bloodwood Creek UCG projectsduring and after operations [72,73]. Table 5 shows estimates of the

Fig. 18. Gas quality and CRIP manoeuvres in Panel 2 UCG pilot project at BloodwoodCreek (from Haines and Mallett [33]).

TaggedPamounts of BTEX, PAHs, Ammonia and Phenols that existed in thecavity after gasification and the amounts removed via venting duringthe shutdown and decommissioning process. It is observed that over91% of the chemicals of interest were removed during venting. Fur-ther studies concluded that safe levels of benzene would exist within27 m of the cavity and that the best long term remediation method ismonitored natural attenuation [72].

3.5. Swan Hills, Canada

TaggedPBetween 2009 and 2011 Swan Hills Synfuels LP undertook a dem-onstration project of UCG in Alberta, Canada [34]. The target was theMannville formation with a coal seam located » 1450 m belowground level, a reservoir pressure exceeding 100 bar, a permeabilityless than 1 md, an average thickness of 4.5 m and a lower calorificvalue of » 29 MJ/kg. Fig. 19 shows a schematic of the well layoutand surface facilities of the Swan Hills UCG project.

TaggedPLiquid oxygen and nitrogen were vaporized and pumped througha coiled tubing unit into the injection well. The Swan Hills projectused the L-CRIP configuration with a directionally drilled injectionwell of 4 1/20 0 diameter and a horizontal section of » 1600 m. Acoiled tubing unit (see Fig. 20) using a concentric coil with 2 3/80 0

outer coil, an instrument string and a 1/20 0 ignition tubing was usedto deliver oxygen and ignition fluids into the injection well, withwater delivered via the annulus between the coiled tubing and cas-ing. The production well was drilled vertically and incorporated awater quench to control the bottom hole temperature.

TaggedPSuccessful ignition with a pyrophoric fluid and operations usingwater and oxygen injection at mass ratios of between 2:1 and 3:1were conducted at the site [34]. While the design coal conversionrate was 118 tpd, it is unlikely this was achieved, due to the presenceof a restricted flow path between the toe of the injection well andthe production well [34]. Swan Hills experimented with hydraulicpumping, water jet micro-drilling, forward combustion and direct

Fig. 19. Schematic of the Swan Hills UCG project (from Swan Hills Synfuels [34]).

Fig. 20. Injection wellpad of the Swan Hills UCG project (from Swan Hills Synfuels[34]).

Table 6Forecast syngas compositions from the Swan Hills UCG project.

Parameter CH4 CO2 CO H2 C2þ

Dry gas (N2 free) mol% 37 41 5 15 2


TaggedPintercept drilling to improve the connection between the injectionand production wells, with limited success [34]. Table 6 provides theforecast syngas compositions from the Swan Hills demonstration.Due to the high pressures, the syngas contains high proportions ofmethane and carbon dioxide. Between February and April 2011 thetemperature at the oxygen injection nozzle installed at the end ofthe coiled tubing ranged from 200 to 350 °C, which is comparable tothat measured during oxygen blown operation in Chinchilla, Gasifier5 [34]. The major advantages of undertaking UCG at depth, includingisolation from shallow water aquifers and the generation of highquality syngas were achieved.

TaggedPHowever, on October 10thD4X X2011, a complex sequence of eventsled to a blowout of the injection well and an explosion and fire inthe surface facilities [74]. The root cause analysis identified a longlist of factors that contributed to the incident. A major factor wasthat the coiled tubing was corroded by an oxygen and water mixtureat high pressure, which caused D5X Xsyngas and oxygen to mix, leading toan ignition event in the vertical section of the injection well whichcontinued until it reached the surface, resulting in the blowout andfire. The mixing and ignition of oxygen and syngas in the injectionwell and uncontrolled burn-back to surface should have been pre-vented. The controls which could have mitigated the ignition andburn-back include: better operating procedures, automatic oxygenshut-off and water quench on the injection well, use of check-valveson the oxygen supply coil and the installation of a fire-break mate-rial at the heel of the injection well (see detailed list prepared byAlberta Energy Regulator [74]). These control systems were imple-mented in the Gasifier 5 project at Chinchilla, and successfully pre-vented failure and uncontrolled burn-back of the oxygen filledcoiled tubing when operating on pure oxygen.

Table 7The average performance of Stage I operation of the “Barara”

Stage Average Flow Dry gas composition (mol%)

(Nm3/h) H2 CO CH4 C2H6

I 75.2 42.2 37.7 2.5 0.07

TaggedPWhile Swan Hills Synfuels applied to redrill the injection and pro-duction wells after the incident, these applications do not appear tohave been successful [75]. The Swan Hills project highlights the chal-lenges of conducting UCG at high pressures. The combination of hightemperatures, pure oxygen, water and corrosive and flammablegases poses challenges for materials selection and for the design ofsafety systems. Applying the lessons learnt from this project and theoperating experience from Gasifier 5 on oxygen/water mixtures willbe critical for future UCG projects at high pressures, where an oxy-gen/steammixture can not be used.

3.6. “Barbara” and Wieczorek, Poland

TaggedPThe Central Mining Institute in Poland has been activelyresearching and developing UCG methods for application in aban-doned and working underground coal mines as part of the HUGE(Hydrogen-orientated Underground Gasification for Europe) andHUGE 2 research projects (e.g. [76�79]). A 6 day UCG pilot test wasundertaken during August 2013 in the “Barbara” coal mine in South-ern Poland [79]. The target coal was located in coal seam 310 at adepth of 30 m and the gasifier was operated with oxygen blown gas-ification at a pressure of » 1 bar. A total of 5.3 tons of coal was con-verted with an overall cold gas efficiency of 70% [79]. The averagegas composition and gas heating value for the first 4 days of theexperiment are shown in Table 7.

TaggedPIn 2015, a larger and longer duration UCG pilot was undertaken atthe Wieczorek hard coal mine in Poland located in the central part ofthe Upper Silesian Coal Basin [28]. The target coal, was the no. 501coal seam, which has lower calorific value of 25.5 MJ/kg, a thicknessof 5.5 m and is located at a depth of 464 m below ground level at thegasification site. Injection and production piping of diameters200 mm and 300 mm, respectively, was run to and from the UCGgasifier via the existing mine shafts. The research program for thegasification test, included operation on air + oxygen (Phase I), air(Phase II), air + carbon dioxide (Phase III), air (Phase IV) and air +nitrogen (Phase V) [28]. The gas composition versus time for thesefive phases is shown in Fig. 21.

TaggedPDuring the 60 day operations period, a total of 230 tons of coalwas gasified yielding an average syngas heating value (LHV) of3.4 MJ/Nm3. The average coal consumption rate was » 4.3 tpd, theaverage gas production rate was 786 Nm3/h, and the maximum gasproduction rate reached 2400 Nm3/h [28].

TaggedPThe UCG pilot tests in coal mines show that the coal gasificationcan be undertaken in this manner, however the production ratesdemonstrated to date are moderate and the gas quality is only suit-able for combustion in furnaces. There is a lot of further workrequired to demonstrate a technically feasible operation at a com-mercially viable scale.

3.7. General representation of the UCG process

TaggedPThe excavations and post-burn drilling of both small scale andlarge scale and both low pressure and high pressure UCG projects,complemented by laboratory experiments enable the developmentof a general conceptual view of how coal is affected and convertedinto syngas during the in situ gasification process. Figs. 22�25

UCG pilot test (fromWiatowski [79]).

Gas heating value

H2S CO2 N2 O2 (LHV, MJ/Nm3)

0.27 15.5 1.3 0.38 10.3

Fig. 21. Gas compositions versus time for the Wieczorek UCG pilot project (fromMocek et al. [28]).


TaggedPprovide depictions of this general picture using the Linear CRIP con-figuration, from the scale of a coal block up to the scale of multiplegasifiers. The observations are consistent with descriptions of theprocess as reported in the former Soviet Union (e.g. [38,80]), thedemonstration projects in America and Europe (e.g. [27,62]) andrecent demonstrations in Australia, Canada and Poland. Fig. 22shows a general representation of the UCG process from a sideviewperspective. As the coal is converted, ash builds up above the hori-zontal well. Injected oxidant disperses through the bed, gasifyingthe char within the bed and generating high temperatures. Abovethe bed, combustion and gasification reactions occur in the voidspace, producing a hot radiant turbulent gas. Heat is transferred tothe walls of the cavity causing wall recession by chemical reactionand thermo-chemical failure (spalling) of coal and overburden rockas shown in the cross-sectional view provided in Fig. 23. Spalledchar and ash fall onto the top of the ash bed. As the hot gases movetowards the production well they cool down due to the endothermicgasification reactions, the heat required to dry and pyrolyze coal andfrom heat losses to the over- and under-burden formations. Towardsthe production well, pyrolyzed coal leads to a build up of char form-ing a permeable bed. In the coal seam surrounding the growing cav-ity, water and gases flow through the porous media of the coaldriven by the local pressure gradient. Typically the gasifier is

Fig. 22. General sideview representation of a UCG cavity, showing coal and

TaggedPoperated at a pressure lower than the surrounding pressure (hydro-static) and this results in water and gas to flow towards the produc-tion well.

TaggedPFig. 24 shows a schematic of the coal wall where it interfaceswith the hot cavity. From the void space, hot turbulent gas radiatesheat towards the surface of the wall, leading to the subsequent dry-ing, pyrolysis and gasification of the coal. Steam, volatiles and syngasfrom the gasification process migrate into the void space. Ash maybuild up on the surface as the carbon is consumed. Invariably, cracksdevelop in the weakened and pyrolyzed coal, leading to small piecesof coal breaking off from the wall and spalling into the cavity. Thelength scale over which the coal is converted is only a few tens ofcentimetres depending upon the conditions [81,82].

TaggedPIn a commercial project multiple UCG gasifiers will be operatedsimultaneously to produce the syngas required for the downstreamprocess. Fig. 25 shows a schematic of howmultiple Linear CRIP gasif-iers operating in parallel could be drilled and operated.

TaggedPFrom this general picture, we can identify several regions withinthe UCG process: i) the void space formed above the permeable bedof ash and char and below the roof, ii) the permeable bed of ash andchar formed due to spalling, iii) the sidewall and roof boundarieswhere coal is heated, pyrolyzes, reacts and spalls, iv) a near fieldregion around the gasification cavity where thermal, mechanicaland chemical changes occur due to the formation of the cavity itselfand v) the far field region which is mostly affected by the gasifier interms of changes in the pressure and concentration of componentsfrom their natural values. Table 8 provides a summary of these zonesand the relevant processes and physics that dominate in each ofthem.

4. Factors affecting process performance

TaggedPAttempts have been made to gasify a wide range of coal typesin different coal seam conditions using the techniques discussed inSection 2. Table 2 provides a summary of the underground coal gasif-iers that have been operated in the past. Table 9 shows a set of keyperformance indicators (KPIs) for UCG. Fundamentally, it is desirableto have low cost, high efficiency, high resource recovery and reliableoperations with low environmental impacts! In practice there aremany decisions and trade-offs to be made in order to achieve a

overburden, injection and production wells, ash, char and void zones.

Fig. 23. General cross-sectional representation of a UCG cavity, showing coal andoverburden, injection and production wells, ash, char and void zones and cavitygrowth zones: 1) sidewall coal wall zone, 2) roof coal wall zone and 3) roof overbur-den wall zone.


TaggedPviable process. While performance should be comprehensivelyjudged by considering efficiency, economic and environmentalaspects, this section looks at a more limited set of factors, includingthe effect of oxidant, coal properties, coal seam depth, coal seamthickness, process scale, gasifier design and site conditions.

4.1. Effect of oxidant

TaggedPThe injected oxidant is usually either air, enriched air, or a mix-ture of steam/oxygen or water/oxygen. The rate of gas productionand its composition varies widely depending upon the oxidant used,operating conditions such as gas injection rate and site conditions,including type of coal, and the behaviour of the surrounding strataand hydro-geological conditions. When air is used, the product gashas a lower heating value of 3�6 MJ/m3 (STP), and the nitrogen con-tent of the gas is 40�50 vol%. When a steam/oxygen mixture is usedas the oxidant, the product gas heating value can reach 11 MJ/m3 ormore. Table 10 shows the performance of Gasifier 5 at the Chinchillasite in Australia for both air blown and oxygen blown operations.Under air blown conditions the gas heating value was typicallyaround 5 MJ/Nm3 while under oxygen blown conditions (with waterinjection) the gas heating value was around 10 MJ/Nm3 [31]. It canbe seen that the oxygen utilization is improved under oxygen blownconditions, as nitrogen does not need to be heated up to process

Fig. 24. General representation of the sidewall coal zone of a UCG cavity, showing gas flowmass flux of syngas and volatile molecules from within the coal into the void space.

TaggedPconditions and does not extract sensible energy from the reactionzone. These results are typical of UCG projects conducted in sub-bituminous coal seams at relatively low pressure.

TaggedPIn the USA demonstration projects conducted in the 1980s,steam/oxygen ratios of 1:1, 2:1 and 3:1 were trialled. Table 11 showsthe effect of changes in the steam/oxygen ratio of the oxidant on theresulting product gas composition. Increases in the steam/oxygenratio lead to significant drops in the CO content of the syngas, andsmaller rises in the H2 and CO2 content [54]. Hill and Thorsness alsoreport that changes in the system were most pronounced at low oxi-dant injection rates [54]. In the Rocky Mountain I demonstration asteam/oxygen ratio of 2:1 was used. Subsequent analysis showedthat the optimum steam/oxygen ratio would have been 1.3:1. [83].

TaggedPIn the long tunnel method, a two-stage gasification technique isutilized. In the first stage air is used to heat up the coal face and thenin the second stage steam is injected to produce a high quality syn-gas. During the steam gasification stage, the gas quality is typically11�12 MJ/m3 and the gas composition is reported to be: H2

47�58 vol%, CO 8�14 vol%, CO2 19�23 vol%, CH4 9�12 vol% and N2

5�9 vol% [29]. The influence of the oxygen content in air on theproduct syngas quality from field tests of the long tunnel method isshown in Fig. 26. The use of high O2-enrichment levels in air, leadsto higher CO, H2 and CH4 content in the product gas and lower N2

content, causing the overall heating value of the syngas to increase.TaggedPThe gas from UCG typically has a methane content similar to

moving bed gasification due to the low exit temperatures which isinsufficient to D6X Xcompletely crack the volatiles from pyrolysis [84]. Inhigh pressure operations, the formation of methane via the reactionof H2 with char is favoured. Yields of hydrocarbon liquids (tars) fromUCG are typically lower than in moving bed gasifiers, because someof them are cracked in the high temperature zones and othersremain trapped in the underground cavity.

TaggedPThe concentration of CO in the UCG product gas is lower than inentrained flow systems due to the lower temperatures, and the con-centration of CO2 is correspondingly higher. However, in a welldesigned and operated underground gasifier, it is expected that thegreater production of methane can offset the lower CO content, thusmaking the overall product gas heating value comparable to thatobtainable from surface gasification [48].

4.2. Effect of coal properties

TaggedPCoal properties have a substantial effect on the operation andperformance of UCG. Historically, demonstrations have mostly beenconducted in low rank coals which are reactive and which spall

in the void, heat transfer into the coal through the ash, char and dry coal layers and the

Fig. 25. General representation of a module of four gasifiers operating simultaneously.


TaggedPupon heating, which help the conversion of coal into gas [85]. Someof the most successful UCG demonstrations such as Rocky MountainI, Bloodwood Creek Panel 2 and Chinchilla Gasifier 5 have beenundertaken in sub-bituminous coals with relatively high volatilefractions, moderate moisture contents and moderate ash contents[26,31,33].

TaggedPLi et al. have conducted a controlled set of experiments in largecoal blocks to establish the influence of coal rank on the likely per-formance of UCG [29]. Table 12 shows the produced gas composi-tions, syngas heating value and gas production rate from three typesof coal. It can be seen that sub-bituminous coals give high methanecontent, moderate to high heating value and a high production rateof gas per unit of coal. Lignites produce a syngas with lower heatingvalue and produce less gas per ton of coal. In the laboratory experi-ments, bituminous coals produce a syngas lower heating value simi-lar to that of sub-bituminous coals with slightly higher gasproduction per ton of coal.

TaggedPThe rank of the coal, its propensity to spall when heated and thebehaviour of mineral matter upon heating are key factors in deter-mining the performance of large scale UCG. Low rank coals generallyshrink and spall upon heating, enabling renewal of the sidewall and

Table 8Governing process and physics in the various zones of an underground coa

Process Sub-process Void Spa

Chemical Gasification major products xGasification minor productsChemical reaction x

Fluid Flow Turbulent convection xLaminar convectionDiffusion

Thermal ConductionConvection xRadiation x

Mechanical Thermo-mechanicalGeo-mechanical

Phases Gas xLiquidSolid

Characteristic Temperature High High

TaggedProof surfaces and the establishment of a permeable bed of coal andash through which the injected oxidant can readily flow and pene-trate to the surface in order to gasify the remaining fixed carbon[86]. The excavations of UCG projects summarized in Section 3 wereall performed with low rank coals. High rank coals are less reactiveand have a tendency to swell upon heating, potentially causing prob-lems with diffusion of volatiles from within the pore structure toreach the surface of the coal. In addition, the spalling behaviour ofhigh rank coals is not well understood.

TaggedPThe behaviour of the mineral matter during gasification is impor-tant. If an ash layer builds up on the coal, then this will significantlyimpede the diffusion of oxygen, steam and carbon dioxide to the car-bon, becoming a rate limiting step in the conversion of the coal intosyngas [82]. Similarly, if the mineral matter has a propensity to slag,it may also significantly impede the gasification process.

TaggedPField demonstrations of the UCG process in high rank coals havehighlighted a number of challenges. The Pricetown project was thefirst attempt to gasify bituminous Eastern coals of the United States[87]. A poor link resulting from reverse combustion linking andswelling of the high rank coal led to operational problems and only17 days of forward gasification [24,87,88].

l gasifier.

Zonece Pemeable Bed Sidewall & Roof Near Field Far Field

x xx x

x xx

x x xx x x xx x xx x x

xx

xx x x

x x xx x x xHigh to Low Low Ambient

Table 9Key performance indicators of underground coal gasification.

KPI Metric How to achieve

Reliable start-up andignition

% Start-up procedure andi D1X Xgnition system testedunder representativeconditions

Syngas availability % Continuous operationswith downstreamsyngas user

Coal resource recovery % Multi-gasifier fieldoperations

Oxygen and steamutilization

MJ/kmol-O2 Site selectionMJ/kmol-steam Oxidant selection

Gasifier designOperating procedures

Acceptable environ-mental impact

Surface subsidence Site selectionSub-surface impacts Gasifier designSyngas containment Operating proceduresGroundwater impacts Operational monitoring

Acceptable water useand management

Water consumption Site selectionand discharge Gasifier designquality Facilities design

Cost of syngas $/GJ Site selectionOxidant selectionGasifier designOperating procedures Fig. 26. Influence of O2 in injected air on product syngas quality (from Wang et al.

[53]).


TaggedPThe Thulin UCG project conducted in the mid-1980s envisagedcreating a permeable link between two vertical wells in a deepanthracite [62]. However, all attempts to link the wells failed andshort radius drilling was used to evenutally connect the injectionand production wells. The anthracite, being a high rank coal, was notvery reactive as evidenced from field data showing the presence ofoxygen in the product gases [63]. Thus, satisfactory gasification con-ditions could not be established and less than 300 tons of coal wereconverted to syngas.

Table 11Impact of change in steam/oxygen ratio of the oxidant on resulting syn-gas composition in the Large Block Experiment LBK-5 (from Hill andThorsness [54]).

Change in steam/oxygen ratio From 1:1 to 2:1 From 2:1 to 3:1

Change in gas compositionH2 +3% +2%CO ¡19% ¡11%CO2 +6% +3%

Table 10Performance of the Chinchilla, Gasifier 5 demonstration project for air and oxy-gen blown operations (from Perkins et al. [31]).

Parameter Unit Air blown Oxygen blown

Gas compositionH2 mol% 18�20 40�45CO mol% 8�10 5�10CO2 mol% 15�20 30�35CH4 mol% 5�10 10�13N2 mol% 40�45 0�3Dry syngas heating value (LHV) MJ/Nm3 5�6 10�11Oxygen utilization MJ/kmol 900�1000 1100�1300Operating pressure bara 7 7

TaggedPThe Pricetown and Thulin trials highlight that when applyingUCG in high rank coals, a good flow path between the injection andproduction wells must be established and the area available for reac-tion maximized. The low moisture and volatile content of anthraciteand bituminous coals, together with low permeability, mean thatthere are few reactants (steam, CO2, volatiles) for gasification thatcan be supplied from within the coal itself. Thus, oxygen blown gasi-fication together with the injection of steam or water should be usedunder these conditions to ensure high enough reaction temperaturesand a sufficient supply of reactants for the steam gasification reac-tion.

TaggedPHigh rank coals, generally have lower volatile contents than lowrank coals, and together with the low reactivity are therefore harderto ignite. High sulphur levels, high chlorine levels and other contam-inants have significant implications for the design of the surfacefacilities, raw gas treatment and syngas clean up. The presence ofCO2 and water at high temperature and pressure is potentially verycorrosive. The presence of minor species such as sulphur and chlo-rine can significantly exacerbate the situation. The selection of mate-rials for production tubings and surface piping are impacted by thesyngas composition, temperature and pressure. In general, high sul-phur, chlorine and other contaminants lead to higher costs for han-dling and treating the produced syngas.

Table 12Effect of coal properties on process performance as measured experimentally bygasification of large coal blocks (from Li et al. [29]).

Parameter Unit Lignite Soft coal Hard coalSub-bituminous Bituminous

Gas compositionH2 mol% 36�45 33�42 35�45CO mol% 20�30 25�35 25�35CO2 mol% 25�35 20�25 25�30CH4 mol% 1�5 4�10 2�8N2 mol% 1�3 2�3 1�3Dry syngas heatingvalue (LHV)

MJ/Nm3 8.5�9.5 9.6�11 9.5�10

Production rate m3/kg 1.2�1.4 1.8�2.3 1.9�2.5


4.3. Effect of coal seam depth

TaggedPThere are many site factors which influence the UCG process,and coal seam depth is a significant one. The operating pressureis determined by coal seam depth. As a rule of thumb, the oper-ating pressure increases by » 1 MPa (» 10 bar) for every 100 mof depth.

TaggedPFig. 27 shows prior UCG projects plotted against mean coal seamdepth and mean coal seam thickness. The cutoffs shown delineatingthin/thick and shallow/deep coals have arbitrarily been set at 5 mand 300 m, respectively. It can be seen that field demonstrations canbe grouped into two categories � those conducted at shallow depth(< 300 m) and those conducted in thin seams (< 5 m). For example,there are no trials which have been conducted in thick seams atappreciable depth. Qualitatively it can be expected that at shallowdepths there is likely to be increased interaction between the cavityand the surface and operating pressures will be low. As a rule ofthumb, UCG should be undertaken in coals deeper than 200 m toensure sufficient containment pressure from surrounding ground-waters [72]. As the depth increases, operating pressure increase andso do drilling costs.

TaggedPThe effect on performance with coal seam depth is shown inTable 13 using the gas composition and heating value resultsfrom four demonstration projects. As the pressure increases,there is a general trend towards more CO2 and CH4 in the syngasand lower amounts of CO and H2, which is consistent with ther-modynamic calculations. In general, the (nitrogen free) heatingvalue of the syngas increases with pressure. It is expected thatthe oxygen utilization should also increase in well designed andoperated projects due to higher cold gas efficiencies from lowergas migration, lower water influx and lower heat loss from lowpermeability deep coals.

Fig. 27. Summary of UCG projects in terms of mean seam thickness and mean seamdepth.

Table 13Dry syngas compositions from underground coal gasification demonstr

Site Depth Pressure (bara) Gas composition

CO CO2 H

Rocky Mountain I 110 8 12 37 4El Tremedal 550 50 14 4 2Thulin 860 186 1 46 5Swan Hills 1400 120 5 41 1

nr=not reported

4.4. Effect of coal seam thickness

TaggedPApplication of UCG in thick coal seams (> 5 m thick) has beenidentified as being attractive in many parts of the world [91]. Thickerseams offer a proportionately higher potential resource per unit ofprojected surface area, which should translate into lower costs perunit of coal gasified. On the other hand the gradient of hydrostaticpressure is proportional to seam thickness, so water influx may bemore difficult to control in thick seams which are also generallymore permeable � though this does not seem to have been borneout in field demonstrations to date [24].

TaggedPFig. 28 shows the effect of coal seam thickness on the heatingvalue of the syngas from air blown gasification as measured by theSoviets and reported by Gregg et al. [21]. The syngas heating value isvery low (< 4 MJ/Nm3) when the coal seam is less than 3 m thickand when the specific water inflow (ton-water/ton-coal-gasified) isgreater than 2 ton/ton. Thicker coal seams lead to less heat loss perunit volume of coal affected and hence higher syngas heating values.

4.5. Effect of process scale

TaggedPLike other gasification methods, the economics of UCG are a func-tion of the scale at which the process is undertaken. Here, the pro-cess scale, refers to the syngas production capacity per injection-production well pair. If the process scale is small, then many wellpairs will need to be drilled to deliver the desired syngas capacityfor the downstream process making the economics less attractive.On the other hand, process wells have standard sizes and drillinglarge diameter wells can be very expensive and may not be feasible.

TaggedPA succession of gasification tests at different process scales undersimilar process conditions were performed in the USA by the LLNL,starting with small scale laboratory experiments in drums, followedby the Large Block Tests, and then by the Partial Seam CRIP test andending in the Rocky Mountain I field demonstration. Key processcharacteristics are provided in Table 14. As can be seen the UCGdevelopment was conducted across significant scales: about fiveorders of magnitude in terms of coal affected and about three ordersof magnitude in terms of the rate of coal conversion. Across theseprocess scales, despite some differences in pressure and steam/oxy-gen ratios, performance was relatively stable. The main observationswith regards to the process scale are:

TaggedP� CO content decreased with scaleTaggedP� CH4 content increased with scaleTaggedP� H2 and CO2 content were reasonably stable with scaleTaggedP� Oxygen utilization increased with scale

TaggedPThe CO, CH4 and oxygen utilization observations are thought tobe related to the residence time and also to the relative increase insize of the pyrolysis zone in the outlet channel. Heat loss characteris-tics of the different tests would also have played a role. Greater resi-dence time, lower temperatures and heat loss, lead to reduced COcontent in the syngas. While a larger pyrolysis zone would lead tohigher CH4 content in the syngas and better oxygen utilization.

ation projects using steam and oxygen at different depths.

(vol%) Gas heating value

2 CH4 CH2þ N2 N2 free (LHV, MJ/m3) Refs.

0 9 nr 2 9.5 [89]7 14 nr < 10 10.9 [90]

24 nr nr 12.4 [63]5 37 2 < 2 16.3 [34]

Fig. 28. Effect of coal seam thickness on the syngas heating value for various specificwater inflow rates using air blown gasification (data from Gregg et al. [21]).


TaggedPThe fact that even relatively small scale experiments were able toprovide a meaningful representation of the UCG process is veryencouraging, as such experiments are significantly cheaper to con-duct than full scale UCG demonstrations. The results of laboratoryexperiments should be combined with mathematical model D7X Xing toenable greater scale up factors when commercializing the UCG pro-cess in future.

TaggedPFig. 29 shows the effect of coal gasification rate on the syngasheating value as measured by the Soviets and reported by Gregget al. [21]. Greater coal gasification rate leads to higher syngas heat-ing value, due to the lower specific heat loss and specific waterinflow. Generally, coal gasification rates between 100 and 200 tpdlook attractive for delivering a reasonable quality syngas (> 4 MJ/Nm3 air blown) with specific water inflows of between 0 to 1 ton/ton.If the specific water inflow is high and the coal gasification rate islow, then the cold gas efficiency suffers and the syngas quality ispoor.

Table 14Process scale up of the underground gasification of coal using the CRI

Parameters Laboratory Experiments

Year 1981Location Livermore, CAExperiment WID-1Coal Parameters

Depth (m) N/AThickness (m) N/AType sub-bit.

Process ParametersProduction Pressure (kPa) 100Oxidant steam/oxygen (mol:mol) 2:1Oxygen Injected (Mmol) 167x10�6

Oxygen utilization (MJ/kmol) 644Coal converted (m3) 0.014Duration (days) 0.2Coal conversion rate (m3/d) 0.07

Dry Gas Compositions (mol%)H2 36.7CO 23.8CO2 33.5CH4 2.7

Scale Up ParametersRelative coal affected 10�3

Relative coal conversion rate 10�2

References [92]

4.6. Effect of gasifier design

TaggedPFig. 30 shows schematics of the common UCG designs: LinkedVertical Wells, Linear-CRIP and Parallel-CRIP. The K-CRIP design isan approximation to the P-CRIP design, where straight injection andproduction wells intersect at a point. In the Partial Seam CRIP andthe Rocky Mountain I field demonstrations a K-CRIP design wasimplemented, largely due to limitations with directional drillingtechnologies at the time. Generally, in the K-CRIP and P-CRIPdesigns, a vertical well is also drilled which is used as the ignitionwell for the gasifier. The Gasifier 4 project at Chinchilla and Panel’s 1and 2 at the Bloodwood Creek project used a P-CRIP design[12,31,33]. In the L-CRIP design a horizontal injection well connectsto a vertical production well. The Swan Hills Synfuels project inAlberta, Canada used a L-CRIP design [34]. In the case of Gasifier 5 atChinchilla, a modified L-CRIP design was used with a directionallydrilled production well [31]. In future, the P-CRIP and L-CRIP designsare most likely to be used, along with variants involving the use ofmultiple injection and/or production wells.

TaggedPThe main characteristics of the L-CRIP and P-CRIP designs aresummarized in Table 15. While there are many issues, the main onesrelate to the performance of each design over a full life cycle fromstart up to shut down.

TaggedPThe L-CRIP design is economically attractive due to the require-ment for only a single horizontal well, however care needs to betaken in the design to avoid potential geomechnical impacts on theproduction well and to manage high temperatures that exist at startup. The design is suited to geological settings where the overburdenmaterial is strong and has low propensity to undergo bulk collapse.While the concept of using discrete ignitions was the basis of the L-CRIP method, recent operational experience with Gasifier 5 at Chin-chilla in Australia, shows that continuous retraction and gasificationcan be undertaken under air blown conditions and also appears fea-sible for oxygen blown gasification [31]. A remaining uncertainty ishow very long L-CRIP gasifiers (> 500 m) will perform in terms ofthe impacts of heat loss on gasification performance towards end oflife.

TaggedPThe P-CRIP design is attractive for geological settings which mayhave a high propensity to collapse, as the goafing behind the

P method.

Large Block Tests Partial Seam CRIP Rocky Mountain I

1981�1982 1983 1987�1988Centralia, WA Centralia, WA Hanna, WYLBK-1 CRIP CRIP

15�22 61 1106 6 7.6sub-bit. sub-bit. sub-bit.

105 425 8153:1 3:1 2:10.5 17.3 102887 794 » 120016 1400 80772.4 30 936.7 46.7 86.9

42.2 34.2 38.524.9 17.5 11.827.5 34.4 37.63.5 5.5 9.4

1 88 5041 7 13[54,55] [93] [26,48,85,94]

Fig. 29. Effect of air blown gasification rate on the syngas heating value for variouswater inflow rates (data from Gregg et al. [21]).

Fig. 30. Common gasifier designs: (a) Linked Vertical Wells, (b) L-CRIP and (c) P-CRIP(from Perkins [95]).


TaggedPgasification front will not catastrophically block the flow path of theoxidant and syngas. However, as the P-CRIP design requires the dril-ling of two horizontal wells it is generally more costly than the L-CRIP design. A major uncertainty of the P-CRIP is whether a perpen-dicular retraction of the gasification face can be made over a longdistance (> 500 m). In the case of the Bloodwood Creek Panel 2 proj-ect, the transition from the Knife-edge section to the parallel wellsection was not completed and the electroseismic data indicate thatthe gasification front was elongated towards the production well[33]. However, Haines and Mallett indicate this was because of lowoxidant injection rates required by regulators [33]. In principal, irre-spective of the flow rate, the temperatures in the combustion zone(near injection well) should be hotter than those in the reducingzone (near production well), implying that the front should movealong the injection well more quickly than along the productionwell, leading to a skewed gasification front and incomplete resourcerecovery between the two wells. In extreme situations, the cavitydevelopment of the P-CRIP could evolve into something similar to avery long L-CRIP. If the coal in the vicinity of the production well cannot be converted to syngas, then the addition of the horizontal pro-duction well may increase the cost of producing syngas. Anotherissue with the P-CRIP design is the propensity of tars to cool andrestrict or block the production well during start up. This issue wasobserved during the Gasifier 4 project at Chinchilla and was a con-tributing factor to the selection of the L-CRIP design in Gasifier 5.However similar issues were not reported at the Bloodwood Creekproject. If the target coal is high in volatiles and produces significanttar then the start up operations of a long cold horizontal productionwell may become very challenging. It is often hard to distinguishbetween design and operational effects when evaluating the perfor-mance of a particular design. For example, it is not known whether amore aggressive ramp up of oxidant flows in Gasifier 4 could haveavoided the condensation of tars and what role coal cuttings leftover from drilling operations played in the start up operations.

TaggedPThere is sufficient experience with both the L-CRIP and P-CRIPdesign to state that they appear feasible from both the technical andeconomic perspectives. Each design has its own characteristics andthe specific gasifier design should be chosen with respect to site con-ditions, in particular the propensity of the over-burden to spall andcollapse. Despite the very different nature of the gas flow paths inthe L-CRIP and P-CRIP design, the process performance of the twomethods appears quite similar. Table 16 shows a comparison of theRocky Mountain I K-CRIP performance with steam/oxygen with theChinchilla Gasifier 5 L-CRIP performance on water/oxygen. Gas com-positions, gas heating values and oxygen utilization efficiency are

TaggedPquite comparable. Table 17 shows a comparison of the BloodwoodCreek Panel 2 P-CRIP performance on air with the Chinchilla Gasifier5 L-CRIP performance on air (these two projects are operated in thesame coal). The gas compositions show an effect of the water gasshift, especially in the P-CRIP design and the gas heating values andoxygen utilization efficiency are quite comparable. It appears thatthe P-CRIP design led to a slightly higher heating value of syngas inthe Bloodwood Creek demonstration and this may have been due tothe longer pyrolysis zone in the P-CRIP design and/or the slightlyhigher pressure of the gasifier at the location of this particular site.

TaggedPIn the Rocky Mountain I demonstration a P-CRIP module and aELW module were operated in the same coal seam using the samefacilities. This side-by-side demonstration enables some insights

Fig. 31. Description of the cavity development of the Bloodwood Creek, Panel 2 proj-ect (from Haines and Mallett [33]).

Table 15Characteristics of the L-CRIP and P-CRIP gasifier design.

Design Characteristics

L-CRIP Single horizontal well; reduced drilling costPotential geomechanical impact on production wellHigher temperatures at production well at start-of-life; lower tem-peratures at end-of-life

Uncertainty of residence time and heat loss on process performanceover life

P-CRIP Two horizontal wells; higher drilling costLower geomechanical impact on production wellPotential for higher coal conversion per well-pairLower temperatures at production well at start-of-life; higher tem-peratures at end-of-life

Potential tar blockage of production well during start upUncertainty of the feasible length of the gasification front (lateral dis-tance between wells)

Table 16Comparison of performance of the Rocky Mountain I project and Chinchilla Gas-ifier 5 project under oxygen blown conditions.

Parameters Rocky Mountain I Chinchilla Gasifier 5

Gasifier design K-CRIP L-CRIPYear 1987�1988 2011�2013Coal ParametersDepth (m) 110 130Thickness (m) 7.6 5.5Type sub-bit. sub-bit.

Process ParametersProduction Pressure (kPa) 815 » 700Injected Oxidant (mol:mol) 2:1 steam/O2 2:1 water/O2

Oxygen utilization (MJ/kmol) » 1200 1167Coal converted (m3) 8077 19,500Duration (days) 93 730Coal conversion rate (m3/d) 86.9 26.7

Dry Gas Compositions (mol%)H2 38.5 44.5CO 11.8 10.1CO2 37.6 31.9CH4 9.4 10.6N2 2 2.7

Dry Gas CV (LHV, MJ/Nm3) 9.5 9.9References [26,48,85,94] [31]

Table 17Comparison of performance of the Bloodwood Creek, Panel 2 project and the ChinchillaGasifier 5 project under air blown gasification conditions.

Parameters Bloodwood Creek Panel 2 Chinchilla Gasifier 5

Gasifier design P-CRIP L-CRIPYear 2011�2013 2011�2013Coal Parameters

Depth (m) 200 130Thickness (m) 13 5.5Type sub-bit. sub-bit.

Process ParametersProduction Pressure (kPa) 1200 » 700Injected Oxidant (mol:mol) air airOxygen utilization (MJ/kmol) N/A 950Coal converted (m3) » 17,000 19,500Duration (days) 577 730Coal conversion rate (m3/d) 29.5 26.7

Dry Gas Compositions (mol%)H2 20.9 20CO 2.6 10CO2 21.6 15CH4 8.6 10N2 44.7 45

Dry Gas CV (LHV, MJ/Nm3) 5.7 5References [33] [31]


TaggedPinto the effect of the well configuration and gasification method onprocess performance. The CRIP process had better process efficiencyand better resource recovery than the ELWmodule [26,96].

TaggedPIn summary, it can be concluded that a CRIP design will havesuperior performance to a LVW or ELW design and that a P-CRIP andL-CRIP design will likely give similar process performance in thesame coal. The choice of CRIP design should therefore take intoaccount the details of the under- and over-burden properties andcoal behaviour upon heating and make an evaluation of the costs ofsyngas production in each case. Uncertainties in both designs relatemostly to the overall resource recovery and changes in performancewhich may occur from start up to shut down due to very long hori-zontal well lengths.

4.7. Effect of site conditions

TaggedPThere are many site factors which influence the process, includ-ing under- and over-burden properties and hydro-geological proper-ties of the formation, in addition to those already considered such asoxidant and coal properties.

TaggedPFig. 32 shows the effect of specific water inflow on the syngasheating value using air blown gasification. In general the higher thespecific water inflow then the lower the syngas heating value. Toachieve high gasification efficiency and a good quality syngas, thespecific water inflow should be between 0 and 1 ton/ton based onthis data. The specific water inflow will be a function of the perme-ability and saturation of the formations in the vicinity of the UCGproject, the operating pressure and the selected coal gasificationrate per injection-production well pair for the project.

Fig. 32. Effect of specific water inflow on the syngas heating value using air blowngasification (data from Gregg et al. [21]).

Table 18Selection of several more successful and less successful underground coal gasification demonstration projects.

Category Site Startupyear

Coaltype

Technique Injectedgas

Seamdepth (m)

Seamthickness (m)

Gas heatingvalue(MJ/m3)

Coalconverted(tons)

Reference

More Angrenskaja 1961 L LVW Air 150 9 3.4 > 1,000,000 [23]Successful Chinchilla Gasifier 5 2011 SB L-CRIP Air & O2/H2O 132 5.5 4�11 > 19,000 [31]

Rocky Mountain I 1987 SB K-CRIP O2/H2O 110 7 9.5 11,194 [26]Bloodwood Ck P2 2011 SB P-CRIP Air & O2/H2O 180 13 5�12 10,300 [33]

Less Hoe Creek IIIB 1979 HVC LVW O2/H2O 40 8 6.9 3950 [24,98]Successful Pricetown I 1979 Bit. LVW Air 270 2 6.1 350 [24,101]

Thulin 1986 A LVW Air 860 6 7.0 < 300 [62,64]El Tremedal 1997 SB L-CRIP O2/H2O 550 2 10.9 209 [27]Swan Hills 2011 HVB L-CRIP O2/H2O 1400 4.5 16.3 nr [34]

nr=not reported


TaggedPThe overall performance of the project generally depends simul-taneously on a number of factors related to the coal properties, siteconditions, gasifier design and operations. A common performancegoal is to successfully convert large quantities of coal into syngas.Therefore, for the purposes of reviewing the effect of site conditions,the quantity of coal converted is taken to be the primary indicator ofsuccess. On this measure, Table 18 shows some of the more success-ful and some of the less successful UCG demonstration projects thathave been conducted. Based on the historical data, the more success-ful projects have typically been performed in relatively shallow coals(< 300 m), which are moderately thick (6�10 m), have low rank(sub-bituminous coals or lignite), surrounded by shales, clays orsandstones and have utilized a CRIP gasifier design. The less success-ful projects have been performed in either very shallow or relativelydeep coals, which are reasonably thin (< 7 m) and which are ofhigher rank, such as bituminous coals and anthracites.

TaggedPExamination of each individual project can reveal the particularissues which led to the poor results and can now serve as lessons tobe heeded in future development. For example, in the Hoe Creek tri-als conducted in the USA, the shallow nature of the Felix coal seams(< 50 m deep, with Felix #1 and #2 separated by only 5 m) and thevery weak and wet sandy over-burden materials were the main sitefactors which lead to poor process control and eventually to signifi-cant subsidence and release of volatile hydrocarbons into the envi-ronment [97,98]. The removal of large amounts of coal in the HoeCreek II and III trials served to hydraulically connect three aquifers(two coal seams and a sand aquifer) via collapse of over-burden rub-ble [99]. The Hoe Creek trials highlight that UCG should be under-taken with sufficient competent over-burden materials and awayfrom good quality water aquifers. Care must also be taken, whenperforming UCG in locations with multiple coal seams, where thereis a chance that operations in the deepest coal, may interact with theoverlying coal seam.

TaggedPThe El Tremedal project was highly sophisticated using moderndirectional drilling and coiled tubing technologies to drill and con-trol the processes within the wells. Unfortunately, the site chosenhad a very sandy over-burden and overlying acquifer. Oxygen injec-tion volumes were relatively low at 100 to 400 Nm3/h [100]. Anexplosion occurred in the underground reactor, and within a shorttime, uncontrollable water influx made control of the gasificationprocess very difficult. A second attempt at gasification operationswas made, however it only lasted a few days [90].

TaggedPThe Swan Hills project used oil and gas technology and equip-ment and successfully produced syngas in a very deep coal(> 1400 m) for the first time [34]. However, the blow out of theinjection well highlights the challenges of undertaking UCG reliablyat high pressures.

TaggedPFrom a bird’s eye perspective, this summary indicates that futureUCG projects in high rank and/or relatively deep (> 500 m) coalswill need to pay particular attention to design and operationalissues, in order to mitigate the historical challenges of operating

TaggedPUCG under these conditions. From an environmental and economicperspective, development of UCG technology to operate successfullyin deeper coals is of particular importance.

5. Economics of syngas production

TaggedPThe economics of UCG has been reported on extensively over theyears, particularly for specific project conditions [5,102,103]. Themajor capital cost items associated with UCG are: 1) oxidant produc-tion facilities, 2) injection piping and surface facilities, 3) well dril-ling and completion, production piping and surface facilities.Recently, Perkins and Vairakannu have developed a simplified modelwhich estimates the oxidant and drilling costs [104]. The net presentcost of syngas ($/GJ) is given by Eq. (2):

csyngas ¼ 1Erecovered ¢Ng

Ctotal þcopexR

1� 1

1þ Rð ÞT !

þ Cwells

R0 1� 1

1þ R0ð ÞT 0�1

!" #:

ð2ÞTaggedPUsing the above relation, it is possible to derive that the cost

of syngas is a function of the following main variables as shown inEq. (3):-

csyngas / 1a LHhz ɛ

m_syngasɛ

� �n

þpfuel ¢ PmoperatingþCwells

� �; ð3Þ

where a is the areal energy density a ¼ GCVrH; GCV is the gross cal-orific value of the coal, r is the density, L is the length of the horizon-tal well, H is the thickness of the coal seam, W is the width of acavity, h is the ratio of width to height, z is the cavity shape factorand e is the cold gas efficiency, which is adversely affected by heatloss and excessive water influx. Therefore, the unit cost of syngas isreduced by: higher areal energy density, a; higher cold gas effi-ciency, e; higher shape factor, z ; higher width to height ratio, h;thicker coal, H and larger project scales, _msyngas. The cost of syngas isincreased by: higher fuel costs, pfuel and higher drilling costs, Cwells.

TaggedPThe cost of drilling wells may be estimated as a function of thedepth (pressure) and length of the horizontal well, i.e.: Cwells/f(Poper-ating, L). Typically, longer horizontal wells lead to lower costs per vol-ume of coal accessed, up to a point. Similarly, higher operatingpressures, Poperating, increase drilling costs but lead to improved coldgas efficiency. In the following sections, the economic impacts ofseveral design and operational choices are examined in detail. Basecase conditions and further details of the model are given in Perkinsand Vairakannu [104].

5.1. Impact of oxidant choice

TaggedPFig. 33 shows indicative syngas costs based on the oxidant anddrilling cost for air, steam/O2 and water/O2 blown gasification. Per-kins and Vairakannu have discussed the considerations for oxidantand gasifying medium selection in UCG [104]. When considering

Fig. 33. Effect of oxidant selection on the cost of syngas due to oxidant and drillingcosts: (a) Effect of production scale and (b) Effect of well pair drilling cost (data fromPerkins and Vairakannu [104]).

Fig. 34. Effect of areal energy density on the cost of syngas due to oxidant and drillingcosts for a range of drilling costs per well pair.


TaggedPproduction scale, Fig. 33(a) indicates that air yields the lowest syngascosts for demonstration and small scale projects, however above anenergy production rate of » 200 MWch (which corresponds toapproximately 50 MWe using open-cycle and 100 MWe using com-bined-cycle gas turbines) oxygen blown gasification with water and(potentially) steam become more attractive. Fig. 33(b) shows theimpact of the cost of drilling an injection-production well pair for anunderground coal gasifier on the costs at a scale of 300 MWch.

5.2. Impact of coal areal energy density

TaggedPFig. 34 shows the cost of oxidant and drilling costs as a functionof the coal’s areal energy density, a, for injection-production well

Table 19Areal energy density of selected UCG projects.

Project Coal Thickness, H Coal Density,(m) (kg/m3)

Bloodwood Ck, Australia 13 1350Chinchilla, Australia 9 1350Rocky Mountain I, USA 7.6 1363Angren, former U.S.S.R 10 1300Swan Hills, Canada 4.5 1425Liuzhang, China 3 1390Thulin, Belgium 2 1400Pricetown, USA 1.8 1450

TaggedPpair drilling costs of 1�5 $million USD. It can be seen that costsdecrease inversely with areal energy density. For coal seams witha� 100 GJ/m2 oxidant and drilling costs are very high. For coalseams with a� 200 GJ/m2 the oxidant and drilling costs are rela-tively low and do not reduce significantly with further increases inthe areal energy density [104]. Table 19 shows the calculated arealenergy density of the coal seams from a variety of UCG projects con-ducted in the past. Also note, that the more successful UCG projectsgenerally had high areal energy density.

5.3. Impact of production well capacity

TaggedPWhen considering an injection-production well pair, what is apreferred production capacity? The cold gas efficiency drops off atlow oxidant injection rates [54,55]. In addition, if the productioncapacity of each well-pair is low, then more well pairs must bedrilled to produce the syngas required by the downstream user.Therefore, it is generally desirable to have high syngas productioncapacity per well pair. However, there are practical limits to drillingwells suitable for handling high temperature syngas. Fig. 35 showsthe oxidant and drilling costs as a function of the syngas flow perproduction well for four different oxidants. It can be seen that highersyngas flow per well reduces the syngas cost, but if the syngas flowper production well is » 10,000 Nm3/h then any additional cost sav-ings are relatively small. Fortunately, at » 10,000 Nm3/h, productiontubing sizes of 40 0 to 70 0 are satisfactory, which is within the range oftypical oil and gas well sizes. The successful UCG demonstrationprojects at Chinchilla and Bloodwood Creek both produced up to5000 Nm3/h of syngas [31,33].

r Gross calorific value, GCV Areal Energy Density, a(MJ/kg) (GJ/m2)

19 33419 23120 20715 19529 18622 9225 7026 68

Fig. 35. Effect of syngas capacity per production well on the cost of syngas due to oxi-dant and drilling costs for four different oxidants.


5.4. Summary

TaggedPIn summary, this simple analysis of syngas production economicsfacilitates the development of two rules of thumb: 1) to select siteswith areal energy density of » 200 GJ/m2 or greater and 2) to sizeeach injection-production well pair to produce » 10,000 Nm3/h ormore of syngas.

TaggedPRecently, Pei et al. compared the costs of UCG combined cycle(UCGCC) power projects with conventional pulverized coal (PC)plants, natural gas combined cycle (NGCC) and integrated gasifica-tion combined cycle (IGCC) configurations [105,106]. UCGCC wasforecast to cost » 45 $/MWh and provide the lowest cost of powerwhen the natural gas (NG) price was less than » 4.50 $/GJ [105].When carbon capture and storage (CCS) was also included, NGCC-CCS had the lowest generation costs when the NG price was lessthan 4.50 $/GJ and UCG-CCS had the lowest generation costs whenNG was priced more than 4.50 $/GJ [105].

6. Environmental considerations

TaggedPIn UCG, the process considerations cannot be separated from theenvironmental considerations because they are intimately related.Unlike with conventional surface coal gasification facilities whereinthe reactor environment is totally engineered and controlled, theperformance of in situ gasification is significantly impacted by thegeologic and hydrogeologic setting in which it is operated [107]. As aresult the design and operation of any UCG operation must be basedon a fundamental knowledge of the gasification site. Fig. 36 shows aschematic of how a UCG operation could impact on the air, surfacesoils, surface waters, underground rocks and groundwaters. In thefollowing sections the potential surface impacts, subsurface impactsand greenhouse gas emissions are briefly reviewed.

6.1. Surface impacts

TaggedPUCG operations can in principle lead to the escape of productgases into the air and surface soils and the deformation and subsi-dence of the surface topography. The most likely pathway for prod-uct gases to reach the surface air or shallow soils is via the wells(injection, production, monitoring) drilled from the surface. There-fore, the design of the casing and cement of the wells is very impor-tant. Younger and Bhat discuss various well design issues [108,109].High temperature cements are available from major oil field servicecompanies (e.g. [110]). To reduce the risk of well integrity issues,design rules and best practices can be adapted from the oil and gas

TaggedPindustries, in particular using experience from thermal wells usedfor heavy oil recovery [111]. UCG production wells which can handletemperatures up to 400 °C have been operated for several yearswithout issue, when designed and constructed according to theseguidelines [31]. Operational procedures and safety systems mustalso be implemented to prevent well blowouts like the one experi-enced at Swan Hills [74].

TaggedPSubsidence of the surface ground due to UCG is another potentialimpact, and it depends heavily on the formation depth (D) and thick-ness (H), strength of formation materials and the width (W) of thezones of coal conversion [112,113]. The subsidence, d, from UCM iscorrelated as d/W ¢H ¢D�1 [114]. Thus, surface subsidence has beenobserved in many UCG projects operated at shallow depth and inmoderate to thick coal seams. For example, the Hoe Creek III demon-stration developed a substantial surface crater, because the chosensite was only 40 m deep and also had weak and wet over-burdenmaterial [97,98]. Derbin et al. reports on Soviet experience wheresubsidence of up to 2.2 m was measured in projects at depths of50 m or less; and subsidence of up to 1.2 mwas measured in projectsat depths with clay overburden at depths up to 120 m [112]. WhenUCG has been conducted in deeper coal seams with strong over-bur-den materials there has been negligible surface subsidence. Forexample, there have been no recorded subsidence issues at the dem-onstration projects conducted at Chinchilla and Bloodwood Creek inAustralia [31,65]. Similarly, when UCG has been conducted in deepercoal seams at El Tremedal, Thulin and Swan Hills there have been nosurface subsidence impacts [27,34,62,115]. The risk of surface subsi-dence can be mitigated through site investigation and selection andvia application of geomechanical models to appropriately design thegasifier field layout.

6.2. Groundwater pollution

TaggedPThe major concerns for pollution of the groundwater comes fromthe release of organics such as BTEX, phenols and polyaromatichydrocarbons (PAHs) during coal pyrolysis and gasification, therelease of heavy metals during gasification and from leaching ofinorganic materials from residual char and ash once gasification hasceased [116]. The main inorganics include a wide range of ionic spe-cies, such as sodium (Naþ), calcium (Ca2þ), sulphate (SO2�

4 ), bicarbon-ate (HCO�

3), chlorine (Cl�), ammonia (NH3), fluoride (F�) and bromide(Br�) [116]. Other substances of interest include metals, arsenic andboron.

TaggedPThe extent and concentration of groundwater pollution dependsprimarily on groundwater flow velocity, dispersion and the reactionand adsorption of various contaminants. Managing the risk ofgroundwater contamination in the vicinity of UCG needs to be amajor focus for site selection, design and operational activities.

TaggedPMany organic contaminants such as BTEX, phenols and PAHs aredense in comparison to syngas and condense preferentially into theliquid phase at lower temperatures, thus they do not travel far fromthe UCG cavity. In addition, they are not very soluble in water [109].According to field results reported by Mead et al., the phenols con-centration decreases rapidly with time and with distance from thegasification zone [117]. The adsorption of coal and surroundingstrata makes a substantial contribution to the decrease of contami-nant concentrations over time and with distance from the gasifica-tion zone. Table 20 shows the decrease of phenolic concentrationsmeasured as a function of distance and time in the vicinity of theHoe Creek I UCG demonstration project. The reduction of phenolconcentration with time has been attributed to the sorption of phe-nols by coal, as confirmed in laboratory experiments [117]. Similarresults have been reported for UCG projects conducted in the formerU.S.S.R [116].

TaggedPIn regards to organic contaminants, Table 21 shows the concen-trations of various substances at 6, 83 and 762 days after gasification

Fig. 36. Potential environmental impacts of UCG.

Table 21Comparison of the concentrations of various organic com-ponents found in the water near the How Creek I UCGproject in well EM-4 (located 44m or 146 ft from the gas-ifier) on days 6, 83 and 762 following gasification (fromCampbell et al. [118]).

Parameters Concentration (ppb)days after gasification

6 83 762

Volatile Componentsn-pentane 1.4 � �n-hexane 4.8 5.00 �Benzene 180 290 �Thiophene 0.7 0.87 �Toluene 0.8 tr 0.92n-octane 0.1 nr ndEthyl Benzene 0.4 0.25 0.16


TaggedPmeasured in a well 44 m from the UCG cavity. It can be seen, thatwithin 2.5 years after gasification, the concentrations of almost allorganics of interest were zero. Trace concentrations of toluene, ethylbenzene and xylene were present.

TaggedPSeveral studies have identified inorganic substances in ground-water due to the coal gasification process. In general it is expectedthat the concentration of inorganic contaminants increases as aresult of leaching of the ash and char materials left behind after gasi-fication. Table 22 shows results of water analyses before and afterthe Hoe Creek I experiment.

TaggedPDuring the Rocky Mountain I project, thorough investigation andanalysis of the baseline water conditions, the produced water qualityand the water quality in the coal seam after gasification was con-ducted. In addition, reports of the project from site characterisationthrough operations to shutdown are available. Detailed reports ofthe changes in groundwater quality have been published � seefor example Barbour et al. [60], Boysen et al. [96], Lindblom[61,119,120] and Covell et al. [121]. Table 23 shows a comparison ofthe baseline water analyses, the produced water analyses and thehighest reported concentrations in the coal seam wells for the CRIPmodule. It can be seen that the water produced from the process ishigh in phenols, organics, ammonia, total dissolved solids and somemetals. This produced water is collected from the production wellduring operations and is often considered representative of thatwhich could be in the cavity (if temperatures were low enough).However, within less than three years of the gasification operations,

Table 20Decrease of phenolic concentrationswith distance and time from an under-ground coal gasifier (from Liu et al.[116]).

Time Distance from cavity (m)

days 3 15 30

3 8 0.09 0.00883 0.6 0.03 0.004182 0.09 0.007 < 0.001280 0.04 0.003 < 0.001762 0.02 0.001 < 0.001

TaggedPmost of the baseline water quality conditions had been achieved inthe coal seam wells surrounding the project. As of June 1991 theonly baseline condition that was not met, was the elevated levels ofboron detected in both the ELW and CRIP modules of the project[119].

TaggedPGroundwater monitoring results from the first gasifier at Chin-chilla, Australia showed that the concentration levels of benzene,phenol and PAHs in the vicinity of the gasifier were » 10 mg/L,» 100 mg/L and » 10 mg/L, respectively [66]. Outside of the active

Xylenes 1.4 0.49 0.11n-nonane 4.1 nr ndn-decane 0.2 nr ndNapthalene tr 0.53 �Biphenyl nr 6.20 �

Semi-Volatile ComponentsPhenol 138 93 �Dimethyl phenol isomers 721 412 �Cresol isomers 607 388Ethyl phenol isomers 522 420 �n-hexanal tr tr �n-heptanal nr nr �n-octanal 3 nr �n-nonanal 34 66 �All Phenols 1988 1312 �

tr = trace. nr = not reported. nd = not detected.

Table 22Results of water analyses before and after the Hoe Creek I UCG project(from Campbell et al. [118]).

Species Units Baseline Inside cavity 30m from cavity280 days 280 days

Al3þ mg/l 0�10 4 0Br� mg/l 0�0.1 0.6 0.2Ca2þ mg/l 36 § 10 350 61Cl� mg/l 13 § 5 19 10CN� mg/l 0�0.001 0.19 0.01HCO�

3 mg/l 496 § 40 48 690Pb2þ mg/l 0�1 4 �Mg2þ mg/l 10 § 4 19 18NHþ

4 mg/l 0.55 § 0.05 10 0.69Phenols mg/l 1 § 1 12 1Naþ mg/l 214 § 15 280 310SO2�

4 mg/l 154 § 80 1500 320Zn2þ mg/l 224 § 200 8 4pH � 7.5 § 0.1 9.5 6.3


TaggedPgasification zone concentrations were less than 1 mg/L and withinthe range of the measured background values for the region [66].

TaggedPMallett has reported on the environmental performance of theUCG projects at the Bloodwood Creek site in Australia [72,73]. InPanel 2, within 48 h of the air injection being halted, the gasificationreactions had ceased and the gasifier pressure was reduced to pro-mote groundwater inflow, to produce steam and reduce the cavitytemperatures below 300 °C [72]. Groundwater inflow and steamproduction continued for nine months, during which time » 92% ofthe organic chemicals were entrained in the vent gases produced tosurface (see Table 5). Contaminant fate and transport modellingshowed that a safe concentration of benzene of 1 mg/L would beobtained within 27 m of the cavity; a safe concentration of phenol of85 mg/L would be obtained within 4.5 m of the cavity and that a safeconcentration of naphthalene of 2 mg/L would be obtained within0.5 m of the cavity [72]. Since in situ groundwater does not pose anysignificant health or environmental risk, and no use of the ground-water is envisaged over the period of declining concentrations, theremedial approach adopted is monitored natural attenuation; i.e.wait for groundwater conditions to recover [72].

Table 23Analyses of baseline, produced and post-gasification water quality from

Parameters Baseline conditions Produced water(1986�1987) mg/L (January 1988) m

ComponentsBenzene nr 60Toluene nr 59Ethylbenzene nr 18Xylenes nr 86Phenols < 0.1 2950Ammonia 2.4�7.8 12,860Sulfates 300�1400 130Cyanide < 0.02 0.5Boron 0�0.037 0.21Aluminium 0�0.253 1.7Cadnium < 0.01 0.03Chromium < 0.01 < 0.02Copper < 0.01 0.04Lead < 0.05 0.12Mercury < 0.00005 < 1Nickel < 0.01 < 0.02Iron 0.042�1.06 0.27Zinc < 0.015 0.05

PropertiesAlkalinity < 10�858 meqCaCO3 33,600pH 7.7�9.2 8.7Total dissolved solids 1400�2700 914Total organic carbon 11�43 4400

Reference [119] [60]

nr = not reported.

TaggedPThe Rocky Mountain I, Chinchilla and Bloodwood Creek UCGprojects show that groundwater pollution is not intrinsic to the UCGprocess and impacts can be managed with appropriate site selection,gasifier design and operations [122]. In these projects, the so-called“Clean Cavern Concept” was implemented, in which the gasfier isoperated such that:

TaggedP1. the operating pressure is kept below the surrounding formationpressure during and after operations to minimise escape ofpyrolysis products and ensure a positive flux of groundwatertowards the cavity

TaggedP2. once gasification is terminated, pyrolysis products are ventedfrom the cavity

TaggedP3. cavity temperatures are reduced via groundwater influx asquickly as possible once gasification is terminated, to minimisethe release of more pyrolysis products

TaggedPWhile Boysen et al. argue that injection of steam in the post-gasi-fication cavity is beneficial to strip residual hydrocarbons from thecavity [96], the results from Rocky Mountain I and Bloodwood Creekshow that most of the steam used to entrain the organic chemicals ismade from the evaporation of groundwater flowing into the hot cav-ities. In the Rocky Mountain I project, the cavity water was pumpedtwice and treated before being released into the environment at sur-face. The total water pumped and treated per ton of coal gasifiedwas circa 0.85 t/t.

TaggedPRecent European work on the potential of groundwater contami-nation from organic and inorganic pollutants generated in the pro-cess of coal gasification has been reported [77,123�126].

6.3. Groundwater depletion

TaggedPMost of the UCG demonstration projects that have been con-ducted show a drawdown of the formation pressure during opera-tions. This is a result of the fact, that the operating pressure of UCGis kept below that of the surrounding formation pressure, thus pro-viding a driving force for water influx. The coal gasification processconsumes water to produce CO and H2 via the steam gasification

the Rocky Mountain I project.

from CRIP module Highest concentrations in coal seam wellsg/L (December 1990) mg/L

< 5nrnrnr< 0.028.61310< 0.020.90.108< 0.01< 0.008< 0.006< 0.05< 0.0002< 0.021.36< 0.003

1040 meqCaCO3

7.7�8.9339051[119]


TaggedPreaction. The drawdown in pressure during operations implies thatthe UCG surface facilities will need to be designed for a range ofpressures. This is one reason why targeting deeper coal seams forUCG in the future should be attractive, as in these cases, the draw-down as a percentage of the baseline pressure will be much lower,making the design of surface facilities much easier.

TaggedPThe impacts of groundwater depletion may be mitigated byinjecting freshwater or re-injecting treated produced water backinto the formation surrounding the UCG gasifiers. The effectivenessof this approach will likely depend on a variety of factors, includingthe permeability of the coal and over-burden formations. There is noreported literature examining these issues using numerical models.

Fig. 38. Emissions of SOx and NOx from UCG for power generation as compared withconventional coal and natural gas technologies (data from Blinderman and Jones[65]).

6.4. Greenhouse gas emissions

TaggedPThe carbon emissions of energy technologies are of high socialimportance. It is generally accepted that global carbon emissionsmust fall if the world is to limit greenhouse gas warming to within1.5�2 °C of pre-industrial levels [127]. While coal is generally plenti-ful and relatively cheap, it is a high carbon fuel, which releases highlevels of CO2 when combusted per unit of energy produced.

TaggedPFig. 37 shows a comparison of the lifecycle grenhouse gas emis-sions from conventional power generation technologies and UCG,with and without CCS. The reported values of IGCC and UCG withCCS exclude the use of a water gas shift unit. Beath et al. calculatedthat with the inclusion of CCS and shift the greenhouse gas emis-sions would be 149 kgCO2eq/MWh for IGCC and 333 kgCO2eq/MWhfor UCG [91]. The emissions for the IGCC unit with shift are lowerthan for UCG due to the fact that almost all of the carbon in the syn-gas is in the form of CO, whereas in UCG the carbon present in CH4

cannot be shifted to H2 and CO2. A recent study by Hyder et al. com-pared the emissions for power generation from UCG using combinedcycle gas turbines (CCGT), integrated surface gasification with CCGT,pulverized coal combustion with supercritical steam and pulverizedcoal combustion with saturated steam [128]. This work estimatedgreenhouse gas emissions of UCG to power projects of 774 kgCO2eq/MWh. A recent study by Doucet et al. estimates that a UCG projectwith combined cycle gas turbines and CO2 capture from the syngas(without a water gas shift unit) would emit 389 kgCO2eq/MWhwhich is similar to both the emissions from a surface IGCC plantwith CO2 capture and the emissions from natual gas using combinedcycle gas turbines [11].

Fig. 37. Lifecycle greenhouse gas emissions of UCG for power generation as comparedwith conventional coal and natural gas technologies (data from Wibberley et al. [129],Beath et al. [91], Hyder et al. [128] and Doucet et al. [11]).

TaggedPIn the case of making liquid fuels, Perkins et al. estimate that thewell to wheels emissions from a coal to liquids project using UCGwith CO2 capture for use in enhanced oil recovery would be similarto existing emissions from crude oil based transportation fuels [12].

TaggedPThe emissions of SOx and NOx from a variety of coal and naturalgas power generation technologies are summarized in Fig. 38. TheUCG SOx emissions are much lower than pulverized coal combustiondue to the gas clean up unit (typically either Amine, Selexol� or Rec-tisol�). The NOx emissions are also very low when compared to bothpulverized coal and natural gas combustion.

6.5. Carbon capture and storage

TaggedPWhile CCS has been implemented on a large scale for several nat-ural gas production projects, its application to coal is much lesswidespread. The major challenges with coupling CCS with coal com-bustion and gasification are both technical and economic. From thetechnical perspective, finding suitable geologic storage sites withinthe vicinity of locations where coal is being used has been a chal-lenge. From the economic perspective, when using coal, there is amuch greater amount of CO2 per unit of energy produced. Thus sepa-ration and re-compression of CO2 from either coal derived syngas orcoal derived flue gas is energy intensive and expensive. Where proj-ects have been undertaken, the CO2 is mostly being used forenhanced oil recovery and therefore can be assigned a monetaryvalue in producing more oil. A number of coal projects with CCShave recently been constructed including Saskpower’s post-combus-tion capture Boundary Dam project in Saskatchewan, Canada [130];the Petra Nova post-combustion capture project in Texas, USA [131];and the Kemper County IGCC project with pre-combustion capturelocated in Mississippi, USA [132].

TaggedPAssessment of the potential to sequester CO2 inside UCG cavitieshas been reported by several authors (e.g. [3,109,133]). Estimatesshow that only a portion of the CO2 emitted from UCG could bestored in the cavities based on the volumes available [134]. Youngerhas proposed that the excess CO2 could be stored in the deformedstrata above the coal seam and has calculated that a height ofbetween 10 and 60 times the coal seam thickness would be suffi-cient to store all of the CO2, depending upon the porosity of the over-lying strata [109]. There are significant uncertainties regarding theability of the formations subjected to UCG to safely contain the CO2

[134,135]. The concept may be technically feasible, if the coal islocated deep enough and the formation materials are suitable.


7. Guidelines for site and oxidant selection

TaggedPIt is now well established that the use of the CRIP technique withdirectionally drilled wells provides compelling advantages overmethods like the linked vertical wells (LVW) or the extended linkwell (ELW). As such the main decisions that a UCG project designerwill need to make to achieve both economic and environmental KPIsare related to the selection of the site and selection of the oxidant tobe used in the project.

TaggedPTable 24 provides a summary of guidelines for the selection of asuitable UCG site, based on a combination of experience and priorwork [51,109,136�138]. It should be recognized that since experi-ence with UCG at large scales is still immature, the site selectionparameters can only be considered as guidelines, and can not be

Table 24Guidelines for site selection.

Category Parameter Guideline

General Resource size > 100 MT preferred

Location Not in remote/urban areas

Topography Flat to gentle undulation

Structural complexity Low

Coal seam Rank Sub-bituminous and low swelling p

Quality Low ash and low sulphur

Thickness > 6 m preferred

Depth > 200 m preferred

Permeability Low, < 100 mD preferredFaulting, partings LowGeometry Flat (< 20° dip)

Over- and under-burden Quality Shale, siltstone, mudstone, cementepreferred

Thickness High, > 40 m preferred

Permeability Low, < 20 mD preferred

Strength Greater than lithostatic

Hydrogeology Proximity to aquifers Prefer no acquifers within < 60 mWater quality Low quality

Hydraulic head Prefer > 20 bar

Table 25Guidelines for oxidant and gasifying medium selection (from Perkins and Vairakannu [104

Oxidant Syngas Use Coal Depth Coal Prop

Water/O2 All Shallow to Deep Prefer lowinflux. C

Steam/O2 All Shallow to Moderate Prefer lowinflux. C

CO2/O2 All (with CO2 removed) Shallow to Deep Prefer higwater in

N2/O2 (air and enriched air) Power generation Shallow Prefer coavalue.

TaggedPconsidered as criteria. In any particular project, there could to goodreasons to vary one or more parameters. In general a good UCG sitewill have several characteristics including: flat lying sub-bituminouscoal at a moderate or greater depth, with a thickness of more than6 m and with low structural complexity, surrounded by strong andlow permeability formations in a location with a high existinghydraulic pressure and no significant acquifers in the vicinity. Whenprojects are proposed with parameters substantially outside of theseproposed guidelines, additional risk management strategies shouldbe utilized to address the fact that there is a lot less experience withUCG in the proposed conditions.

TaggedPIn terms of oxidant selection, Table 25 provides guidelines basedon the coal properties, coal depth and desired end use for the syngas.In general air blown gasification may be considered for

Comments

Resource must be sufficient for proposed projectcapacity.

Prefer site to be accessible with existing infrastructurebut neither remote nor in an urban area.

Prefer to avoid steep topographies which will limit place-ment of wells and surface facilities.

Minimal faulting or fractures in both coal seam and sur-rounding formations.

referred Coals of all ranks can be gasified. Significant experiencewith low rank coals.

Ash content < 40%. High ash and sulphur impact eco-nomics of gas production and gas cleanup.

Thin seams have more heat loss and require more drillingto access the same volume of coal.

Require minimum head/pressure for process. Minimisesurface impacts.

Low permeability reduces water influx.Low faulting, parting, intrusions. Away frommajor faults.Low drilling complexity.

d sandstones These strata are typically strong and have lowpermeability.

Prefer thick over- and under-burden layers to help pro-cess containment

Low permeability for low water influx and gascontainment.

Prefer strong and stable to support cavity span, minimisesubsidence and prevent heave.

Reduce potential to connect to overlying acquifers.Reduce impact on environment and any existing uses ofthe water.

Higher pressure better for efficiency and downstreamprocessing.

]).

erties Comments

moisture content and wateran cope with high ash.

Flexible for all conditions, though leads tohigher oxygen requirement than Steam/O2. Only viable oxidant for deep coalseams.

moisture content and wateran cope with high ash.

Source of steam required. Preferably steamgenerated from waste-heat in down-stream plant to ensure high efficiency andavoid additional fuel gas and equipmentcosts.

h moisture content with someflux.

High cost for CO2 removal from syngas andrecompression into coal formation. Lim-ited practical experience to date.

ls with moderate to high calorific Air blown popular for demonstrations, butviable only in shallow formations and forpower generation applications. Enriched-air blown gasification more competitivefor power generation at large scale withoxygen blown gasification.


TaggedPdemonstration projects and when the coal is at shallow depth.Enriched-air blown gasification may be suitable for power genera-tion applications. For large scale projects and/or when coal seamsare deeper, oxygen blown gasification using water/O2 and steam/O2

should be considered [104]. There is very little experience with theuse of CO2/O2 oxidant mixtures in UCG projects.

8. Conclusions

TaggedPThis paper has provided a comprehensive review of undergroundcoal gasification, with a particular emphasis on describing UCGmethods and the results of previous field demonstrations. Under-ground coal gasification is not a new concept, however despitemany decades of development, the process has not been imple-mented commercially outside of the former U.S.S.R. The complexityof UCG results from the fact that there are strong interactionsbetween chemical, thermal and mechanical processes occurring dur-ing gasification. The UCG process is strongly impacted by the proper-ties of the coal, surrounding strata and the environment. The factorswhich affect the performance of UCG such as oxidant choice, coalproperties, coal seam depth and thickness, process scale, gasifierdesign and site conditions have been reviewed. Work in the past sev-eral decades has shown that application of the controlled retractinginjection point (CRIP) method has many advantages over earliermethods such as linked vertical wells and results in superior perfor-mance. A brief review of the economic factors and environmentalconsiderations for undertaking UCG projects has been performed.Guidelines for the selection of sites and the choice of oxidant infuture UCG projects have been provided.

Acknowledgement

TaggedPThe review comments provided by Dr. Cliff Mallett on an earlydraft of the manuscript are gratefully acknowledged.

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