encyclopedia of electrochemical power sources || fuels – hydrogen production | coal gasification

Coal Gasification

27

DAJ Rand, CSIRO Energy Technology, Clayton, VIC, AustraliaRM Dell, Formerly of Atomic Energy Research Establishment, Harwell, UK

& 2009 Elsevier B.V. All rights reserved.

The Role of Hydrogen as a Fuel

There is mounting concern over the sustainability ofglobal energy supplies. Among the key drivers are (1)climate change, ocean surface acidification, and airpollution; these imply the need to control and reduceanthropogenic emissions of greenhouse gases, especiallyemissions from transportation and thermal powerstations; (2) the diminishing reserves of oil and naturalgas; (3) the need for energy security adapted to eachcountry, such as decreasing the dependence on fossil-fuel imports from regions where there is political oreconomic instability; (4) the expected growth in worldpopulation with an ever-increasing expectation of animproved standard-of-living for all, especially in de-veloping and poor nations. The world population ispredicted to grow from 6.7 billion today to 9 billionby around 2050, before leveling off at 9–10 billion.Couple this to the energy aspirations of developingcountries and it is clear that the demand for energy,in all its forms, will grow inexorably. Where is thisenergy to come from, and what will be the impact onthe environment?

Hydrogen is being promoted worldwide as a panaceafor energy problems in that it may eventually replace, orat least greatly reduce, the reliance on fossil fuels,while being itself a clean-burning fuel that releases nogreenhouse gases into the atmosphere. Although themost abundant element in the universe – the stuff fromwhich stars are made – hydrogen does not occur freelyon earth, but is predominantly found in combinationwith oxygen as water, and with carbon as fossil fuels.Chemical, thermal or electrical energy has to be ex-pended to extract hydrogen from these sources.Hydrogen is therefore not a new form of primary en-ergy, but a vector (or carrier) for storing and trans-porting energy from any one of a myriad of sources towhere it may be utilized. In this respect, it is analogousto electricity, which is also a secondary form of energy.Hydrogen and electricity are complementary: electricityis used for a multitude of applications for whichhydrogen is not suitable, whereas hydrogen, unlikeelectricity, has the attributes of being both a fuel andan energy store. These two energy vectors are, inprinciple, inter-convertible; electricity may be used togenerate hydrogen by the electrolysis of water, whilehydrogen may be converted into electricity by means ofa fuel cell.

6

Specifically, hydrogen has the following key attributes.

• It can be derived from fossil and nonfossil sources(renewable or nuclear energy).

• It can serve as an alternative fuel for internal com-bustion engines.

• It is ideal for use in fuel cells to drive electric modesof transportation.

• It is oxidized cleanly to water with no emissions ofgreenhouse gases; when obtained from water usingrenewable energy, the fuel cycle is closed and nopollutants are released in the overall process.

The proposal to use hydrogen as a sustainable medium ofenergy has come to be known as the ‘Hydrogen Economy’;the overall scheme is illustrated conceptually in Figure 1.The upper part of the diagram is generally referred to asthe transitional phase, during which hydrogen is producedfrom fossil fuels. The lower part relates to the long-term,post fossil-fuel age when hydrogen will be manufacturedfrom renewable energy sources and used as a storagemedium and as a super-clean fuel, particularly for certaintypes of fuel cell that are seen as a key enabling tech-nology. Not unexpectedly, the building of a HydrogenEconomy presents great scientific and technologicalchallenges in production, delivery, storage, conversion, andend-use. In addition, there are many policy, regulatory,economic, financial, investment, environmental, and safetyquestions to be addressed.

The world production of hydrogen is around45–50 Mt per year. This is used principally in petroleumrefining, the synthesis of ammonia (for use in fertilizers)and methanol, vegetable oil hydrogenation, and the re-duction of metal oxides. Most of this hydrogen is derivedfrom natural gas by steam reforming; the remainder isobtained principally from oil and coal by partial oxi-dation processes. In all these processes, the carboncomponent of the fossil fuel ends up as carbon dioxide,which is a key greenhouse gas and has to be separatedfrom the hydrogen and stored indefinitely if it is not tocontribute to climate change. By comparison, the prep-aration of hydrogen by electrolysis of water at presentaccounts for only B3% of the world output. Thetransformation of natural gas and liquid hydrocarbonfeedstocks into hydrogen is a straightforward catalyticprocess, but the route from coal requires an initial step ofhigh-temperature gasification.

Coal is a more variable commodity than natural gas asregards its composition, structure, and properties. It is

Hydrogen distribution• pipeline• trucking• rail• marine shipping

Hydrogen storage• compressed gas• cryogenic liquid• metal hydrides• complex hydrides• chemical hydrides• chemical carriers• nano materials

Hydrogen end-use• fuel cells• engines• industrial processes• rocket fuel

Natural gas

Oil

Coal

Energyinput

Carbonsequestration

Steamreforming

Gasification

Nuclear fission

Electricityfor

electrolysis

ThermolysisThermochemical

Renewable energy

Geothermal

Solar−thermal

Hydro

Solar photovoltaic

Wind

GasificationPhoto-biochemical

Fermentation

Nuclear energy

Fossil fuels

Hydrogen productionPrimary energy Hydrogen energy

Thermochemical

Photo-electrochemical

Biomass

Figure 1 A sustainable Hydrogen Economy.

Fuels – Hydrogen Production | Coal Gasification 277

also more difficult to extract and handle. The argumentfor coal as a feedstock for future hydrogen productionlies in the prodigious quantities of hydrogen required fora full Hydrogen Economy – far more than may beavailable from natural gas – coupled with the widespreadoccurrence of coal measures and the very substantialreserves that are known to exist. Supplies of natural gasare better conserved for the manufacture of chemicals,and for space heating and cooking. Most countries withindigenous coal seams will wish to utilize them, not leastbecause of the security of energy supply that they pro-vide and the favorable impact on trade balances. Coal canbe exploited in an environmentally friendly fashion ei-ther (1) by combustion followed by capture of the carbondioxide from the exhaust gases and its indefinite storage(sequestration), or (2) through conversion into hydrogenthrough gasification – with subsequent sequestration ofthe accompanying carbon dioxide. The hydrogen canthen serve as a clean fuel in gas turbines, fuel cells, orinternal combustion engines.

The present use of hydrogen for electricity generationthrough fuel cells is negligible. There is, however, anenvironmental benefit to be gained that stems from therelative ease of pollution management at a central pro-duction facility rather than at dispersed sites. Moreover,in principle, emissions of carbon dioxide are more easily

captured and stored at a single plant than when fossilfuels are deployed in the field.

Coal Production and Reserves

It is generally accepted that most deposits of coal are ofplant origin and were laid down in geological time, es-pecially during the Carboniferous Period between 345and 280 million years ago. The first product of decay andconsolidation is peat, which has a relatively low carboncontent and a high moisture content. Under forces of heatand pressure, peat gradually converts first into bitumin-ous coal and, ultimately, to hard coal (anthracite).

Although coal has been widely used as a fuel forcenturies (notably, however, King Edward I of England in1306 banned the burning coal on the grounds of airpollution!), it reached preeminence in the eighteenthcentury with the invention of the steam engine in Britainand the subsequent launching of the Industrial Revo-lution. The characteristic physical and chemical prop-erties of coal have long been studied. The diversity of theoriginal plant materials that ultimately formed coal, thevariations in the depositional environment and the age ofthe coal since it was laid down (the ‘rank’ of the coal)have resulted in an exhaustive literature that classifies

278 Fuels – Hydrogen Production | Coal Gasification

coal by its appearance (macroscopic and microscopic), itschemical composition, the occluded mineral matter, andits physical properties. The various grades include, interms of increasing carbon content: lignite (or browncoal), soft bituminous coal, and anthracite. Each type ofcoal is suitable for different applications. The quality ofcoal is determined by its calorific value, its moisturelevel, its volatile hydrocarbon content, its ash contentand, given present concerns over the adverse effects ofemissions on the environment, its sulfur and mercurycontents. During coal combustion, sulfur impurities areconverted into sulfur dioxide, which in the atmosphereis oxidized to sulfur trioxide and then precipitates asacid rain.

As mentioned above, coal is by far the most abundantof the conventional fossil fuels, with proven economically-recoverable reserves close to 1012 t worldwide; about aquarter of these are in the USA, with large deposits foundin Russia and the other States of the former Soviet Union,the People’s Republic of China, India, and Australia. Thetotal reserves are equivalent to the energy in 3.5� 1012

barrels of oil, and represent about 150 years of coalproduction at present rates. Natural gas is also associatedwith some of the coal. According to the World Coal In-stitute, coal provided 26% of global primary energy needsin 2007 through the worldwide mining of 5543 Mt of hardcoal and 945 Mt of brown coal (lignite); 41% of the totalproduction was employed in electricity generation. TheEnergy Information Administration in the USA has pro-jected that by 2030 the consumption of coal will haverisen by a further 64%. This forecast, made in 2008, hasassumed the absence of national policies and/or bindinginternational agreements that would limit or reducegreenhouse gas emissions. The USA projection is based onthe widespread availability of coal (in many countries, it isthe only indigenous fossil fuel) and the relatively low costof the fuel. It should be reiterated that these are the majorfactors driving the commercial expansion of the coalmarket; other issues such as pollution, climate change andenergy security are more the concerns of governmentsand politicians, who must provide the legislative frame-work and financial disincentives (e.g., carbon taxes) toensure the emergence and growth of clean coaltechnologies.

Coal-Fired Electricity Plant

In the past, coal was employed as a fuel for both sta-tionary and traction steam engines, for ships, for centralheating boilers, and for the manufacture of coal gas, coaltar and coal-based chemicals. Most of these markets forcoal have declined or disappeared altogether due to theuptake of more convenient petroleum and natural gas.With the advent of refined petroleum, the internal

combustion engine replaced the steam engine. As dis-cussed above, electricity generation is now the mainmarket for coal throughout the world. Coal-fired elec-tricity drives the economies of China and India, and alsomakes a large contribution to those of key industrialcountries such as the USA and Germany. Despite all theassociated environmental concerns, there is no doubt thatcoal will continue to play a leading role in world energyfor the foreseeable future. Mankind will simply have tolearn to extract the energy from coal without releasing itscarbon content to the atmosphere as carbon dioxide.

If a full Hydrogen Economy were ultimately toemerge, enormous quantities of hydrogen would berequired. Ideally, this would be produced by the elec-trolysis of water using electricity generated fromlow-carbon renewable or nuclear sources. In the earlyyears, these energy forms would be inadequate to meetthe requirement and, as an interim measure, it would benecessary to convert fossil fuels into hydrogen, with se-questration of carbon dioxide. The direct route tohydrogen from coal is by gasification (as describedbelow), but the problems of filtering and purifying the gasto a standard appropriate for use in fuel cells present amajor challenge. An alternative is to use coal for theelectrolytic splitting of water. At first sight, it would seemcircuitous and nonsensical to use coal to generate elec-tricity, then feed the electricity to an electrolyzer toproduce hydrogen, and then to utilize the hydrogen in afuel cell to generate electricity. When, however, accountis taken of (1) the well-established technology of large-scale electricity generation in coal-fired plant, (2)subsequent distribution of the electricity through theexisting national grid, and (3) the high purity of hydrogenproduced by electrolysis, it is not yet clear whichroute would be the more practical and economic. Indeed,gasification and electrolysis are both technically fea-sible and the choice may vary from place to placeaccording to circumstances. For these reasons, a de-scription of electricity generation in coal-fired plant nowfollows.

Modern coal-fired power stations are often, althoughnot always, large in size, that is, 1–4 GW output. In aconventional pulverized fuel coal combustion (PFCC)plant, the fuel is first milled to a fine powder and thenblown into the combustion chamber of a boiler to raisesteam. The burner mixes the pulverized coal in the airsuspension with additional preheated combustion air andforces it out of a nozzle similar in action to fuel beingatomized by an injector in modern cars. Under operatingconditions, there is sufficient heat in the combustion zoneto ignite all the incoming fuel.

Burners can be designed to accept any type of coal;lignite (brown coal), sub-bituminous, or bituminous coalsare generally used, with hard coals (anthracite) reservedfor premium applications such as steel making. The


economics of generating electricity depend very much onthe cost of the coal that, in turn, depends on fuel quality,the thickness and the depth of the coal seams, and thedistance over which the coal should be transported.Open-cut mining is usually the cheapest. After removingthe topsoil, coal (lignite) is simply scooped up withdredgers and deposited on a conveyor belt for directtransfer to the power station.

In many countries, the coal reserves, though plentiful,are deep underground in narrow seams and thereforeexpensive to mine. This is the basic reason why some coalmines have been closed in favor of importing cheapercoal. The United Kingdom, for example, obtains much ofits coal from South Africa, Russia, and elsewhere.Moreover, coal from South Africa is low in sulfur contentand thus its use reduces atmospheric pollution.

A schematic layout of a typical 2-GW PFCC plant isgiven in Figure 2. The station has four parallel lines, eachof which is capable of generating 500 MW; the schematicshows just one of these. Coal is transported on a conveyorbelt (14) from a stockpile to a pulverizing mill (16) whereit is ground to a fine powder, picked up by a stream of hotair and blown into the boiler system to burn like a gas(each of the four lines has a boiler). The heat produced

3 4

2

1

5 56 9 10

14

11312

11

7

8

1. Cooling tower2. Cooling water pump3. Pylon (termination tower)4. Unit transformer5. Generator6. Low-pressure turbine7. Boiler feed pump

8. Conde9. Interm10. Stea11. High12. Deae13. Feed14. Coal

Figure 2 Schematic of a 500-MW line in modern, coal-fired, 2-GW

converts extremely pure boiler water into steam in thetubing that forms the boiler walls. The steam leaves theboiler at 841 K and B17 MPa pressure and passes throughthe high-pressure stage of a steam turbine (11) to turn theblades and shaft at 3000 rpm. The steam returns tothe boiler for reheating and then is directed back to theintermediate- and low-pressure stages of the turbine,items (9) and (6), respectively. The turbine shaft is linkedto a 500-MW generator (5). The resultant electricity is fedat 23.5 kV to a transformer (4) where it is raised to 400 kVfor transmission along the national grid (3).

The spent steam from the turbine goes to a condenser(8) where it is cooled (usually by river water) and the purecondensate is pumped back to the boiler for reuse. Thewarmed water from the condensers passes to large coolingtowers (1) where it is sprayed over packing in the base ofthe tower and cooled by evaporation in the natural up-draught of air. The tower is mounted on stilts to allow theair to enter at the base. Before the boiler exhaust gases aredischarged from the main chimney stack (27), 99% of thefine dust is removed by electrostatic precipitators (25).Much of the ash from the coal (18) is sold for use in theconstruction industry. The remainder may serve to filldisused gravel pits that are later restored to agricultural

15

17 19 21

22

27

23

2526

2420186

nsorediate-pressure turbinem governor-pressure turbinerator heater conveyor

Key15. Coal hopper16. Pulverized fuel mill17. Boiler drum18. Ash hopper19. Superheater20. Forced draught fan21. Reheater

22. Air intake23. Economizer24. Air preheater25. Precipitator26. Induced draught fan27. Chimney stack

power station.

Table 1 Advantages and disadvantages of electricity

generation by coal

Advantages Disadvantages

Large reserves of low-cost

coal available

Carbon-rich fuel leads to

extensive liberation of

carbon dioxideWell-established industry

High sulfur content of many

coals leads to pollution by

sulfur dioxide and acid rain

Indigenous fuel source for

many countries


use. The levels of sulfur dioxide (SO2) and nitrogen oxides(NOx) in the exhaust gases are closely monitored. A 2-GW power station produces sufficient electricity to supplythe needs of nearly two million people.

In the United Kingdom, for example, each powerstation is granted a licence for the amount of pollutantsthat may be discharged per year. In order to maximizethe quantity of electricity generated and stay within thelicence, there is an incentive for the station manager toburn low-sulfur coal. Some power stations have beenequipped with flue-gas desulfurization units where thesulfur dioxide is absorbed in limestone (CaCO3) to formcalcium sulfate (gypsum, CaSO4), which finds a market asplaster board. (Note that the amount of carbon dioxideproduced by the reaction of sulfur dioxide with lime-stone, namely, CaCO3þ SO2þ 1

2 O2 - CaSO4þCO2, isminimal compared with that released from the com-bustion of coal.) Although this permits the use of coalswith higher sulfur content, it is an expensive option interms of both capital and running costs. Flue-gas de-sulfurization units have been fitted to the largest coal-fired station in Europe, the 3.975-GW plant at Drax inNorth Yorkshire, UK. This station can handle 10� 106 tof coal per year.

Over 90% of the world’s coal-fired power stations areof the PFCC type, with the remainder being either cir-culating fluid-bed combustion or pressurized fluid-bedcombustion designs. The PFCC plants generally operatewith a thermal efficiency (ratio of electrical output toheat input) in the range of 35–39%. Modern plant usingsupercritical and ultra-supercritical steam may have ef-ficiencies of 45%, or even higher. A 1% increase in ef-ficiency reduces carbon dioxide emissions by around 2%,which is a significant gain. Some generating stations havebeen adapted to burn gas as well as coal when this iseconomically justified, that is, they are ‘dual fuel’ stations.

The advantages and disadvantages of using coal togenerate electricity are summarized in Table 1. The factthat coal has the highest carbon content of all the fossilfuels is a serious disadvantage, particularly given theconcern over climate change caused by greenhouse gases,especially carbon dioxide. What is to be done about it?Attempts are being made to develop practical technologyfor the capture of carbon dioxide, as well as sulfur di-oxide, from the exhaust stacks of power stations. This so-called ‘post-combustion capture’ (discussed later in moredetail) is not an easy task as there is no low-costequivalent to limestone to absorb carbon dioxide. Rather,attempts are focused on the use of liquid amines assorbents – a technology employed in the steam reformingof natural gas to manufacture hydrogen. The exhaustgases from power stations, however, are not so easy totreat, not least on account of their high volume andthroughput. The alternative approach, and a more radicalone, is to gasify the coal and remove the carbon, sulfur,

and other impurities before combustion; not surprisingly,this is termed ‘pre-combustion capture’ and is alsoexamined in more detail below.

Hydrogen-from-Coal Technologies

Coal is generally perceived as being a ‘dirty’ fuel – notonly in terms of its handling, but also on account of thevolatile matter liberated on burning. Among the productsof combustion are aliphatic and aromatic hydrocarbonsthat include carcinogenic polycyclic compounds, par-tially oxidized hydrocarbons, toxic gases such as sulfurdioxide and nitrogen oxides, tar, soot, and smoke. Afterthe dreadful urban smogs of the 1940s and early 1950s,notably in London but also in cities elsewhere, clean airlegislation was introduced into various countries. InBritain, for example, the burning of bituminous (soft) coalon open fires was banned and householders were requiredto switch to ‘smokeless fuel’, such as anthracite (hardcoal) of much lower volatile content. This legislation,together with the changeover to natural gas for spaceheating, completely transformed the urban environment.(Note, however, there followed a period when urban airquality again deteriorated as a result of exhaust pollutionfrom the growing number of vehicles, before the intro-duction of catalytic converters for the clean-up of vehicleexhaust.)

As mentioned above, 41% of all coal mined today isused in electricity generation. This includes both ligniteand bituminous coals. As higher-grade fuels such asnatural gas and petroleum become depleted, it is prob-able that the world will turn again to its huge reserves ofcoal as a source of energy. Fortunately, coal need not be adirty fuel and numerous clean technologies exist, or arebeing developed, to utilize coal on an industrial scale.These technologies fall broadly into two categories: coalliquefaction and coal gasification. The latter processyields hydrogen which, in principle, can be utilized ineither a gas turbine or a fuel cell.

Coal Liquefaction

Commercial plants for the manufacture of petrol from coalwere operated in Germany during World War II and, later,


in South Africa (the Sasol process). In the 1970s, there wasa plant operating at 5000 t coal per day in South Africa forthe domestic production of petrol. The conversion processof coal to the so-called ‘syncrude’ requires addition ofhydrogen to the coal so as to raise the hydrogen-to-carbonratio. There are three basic methods (shown schematicallyin Figure 3), each of which has been investigated on asignificant scale to yield, typically, three barrels of pet-roleum from 1 t of coal. These methods are as follows.

• Direct hydrogenation at high temperatures (723 K)and pressures (13–27 MPa) in the presence of acatalyst.

• Solvent refining, which involves dissolution of the coalunder pressure in a solvent (anthracene oil), followedby separation of the ash and catalytic hydrogenationand hydro-desulfurization of the solution.

• Conversion of coal into water gas (COþH2), followedby the production of liquid hydrocarbons and alcoholsby catalytic synthesis in the presence of addedhydrogen according to the general equations:

nCOþ ð2nþ 1ÞH2 -200 1C

nickel; cobalt or thorium catalystCnH2nþ2 þ nH2O ½I�

nCOþ 2nH2-CnH2nþ1OHþ ðn� 1ÞH2O ½II�

This catalytic process for making fuels from carbonmonoxide and hydrogen was invented in 1923 by Fischerand Tropsch. The resulting hydrocarbon mixture is sep-arated into a higher-boiling fraction for diesel engines anda lower-boiling fraction for petrol engines. The latterfraction contains a high proportion of straight-chainhydrocarbons (alkanes) and has to be reformed (‘cracked’)by catalytic action into branched-chain alkanes for use inmotor fuel. The process used at the Sasol plant in SouthAfrica is based on Fischer–Tropsch synthesis.

Coal liquefaction has tended to go into abeyance withthe discovery of large supplies of petroleum and naturalgas (the production of liquid fuels and chemicals fromnatural gas is economically more attractive than from

Coalsolution

PressureAnthracene oil

H2/catalystHydrogenationHydro-desulfurization

Coal Syncrude

Water gasCO + H2

Water-gas reaction Fischer−Tropschprocess

Steam H2/catalyst

H2/catalyst, 723 K, 13–27 MPa

Figure 3 Possible routes for the synthesis of petroleum from

coal.

coal). Nevertheless, when it becomes necessary to placegreater demands on coal reserves for primary energysupplies, the above three processes will be available tomanufacture gaseous and liquid fuels for use in internalcombustion engines. Coal liquefaction is relevant tohydrogen production only insofar as the internal com-bustion engine is a competitor to the hydrogen fuel cellfor transportation applications. The cost targets that thefuel cell will have to meet will be determined in part bythe cost of liquid fuels derived from coal.

Coal Gasification

Gasification is defined as the transformation of solids intocombustible gases in the presence of steam and (op-tionally) air, oxygen, or carbon dioxide; these oxidantscombust some of the fuel to provide the necessary heatfor the high-temperature (41000 K) endothermic re-action to produce hydrogen. Almost any fossil fuel can betreated in this way to produce hydrogen. The mostwidely-distributed fossil fuels are the various types(ranks) of coal, but other possibilities exist such as tarsands (now renamed ‘oil sands’), asphalts, heavy oils ex-tracted from shale, refinery residues, and petroleum coke.Biomass, which includes agricultural waste, forestrywaste, energy crops and municipal solid waste, may alsobe processed by gasification technology to producehydrogen and is discussed in a separate article of thisencyclopedia.

The gasification of coal has long been practiced. Whenheated in a restricted supply of air, coal or coke is con-verted to carbon monoxide that is heavily diluted bynitrogen. This is a low-grade fuel known as ‘producer gas’and has been employed in industry as a reducing at-mosphere. Because of the low calorific value of producergas, transport costs are an important factor and thus it ismainly manufactured close to where it is needed. DuringWorld War II, when petrol was in short supply, somebuses in the United Kingdom were converted to operateon producer gas. The bus towed a trailer equipped with asmall anthracite or coke oven and the gas was stored in aninflatable bag carried on top of the bus. Conventionalpetrol engines were employed, although their perform-ance was heavily degraded.

When heated coal or coke is reacted with steam alone,the ‘water-gas reaction’ occurs:

CþH2O-COþH2 ½III�

Water gas found widespread use before World War II inproducing hydrogen for the manufacture of ammoniathrough the Haber process. As mentioned earlier, mosthydrogen for this purpose is today obtained from syn-thesis gas (also known as ‘syngas’), which is made fromnatural gas by steam reforming; it is a cleaner andcheaper option. The water-gas reaction is highly


endothermic (heat absorbing) and thus soon ceases unlessheat is supplied. Conversely, the combustion of coal orcoke in air is highly exothermic (heat evolving). It istherefore usual to pair off the two reactions so as tobalance the heat evolved with that absorbed. The tworeactions may either be conducted consecutively, in shortbursts or, more usually, simultaneously by feeding amixture of air and steam to the heated bed. The resultinggas is a mixture of carbon monoxide, hydrogen, carbondioxide, and nitrogen. A gas of higher calorific value canbe obtained by using oxygen rather than air, but for manyapplications this is not affordable. From time to time,proposals have been advanced for the underground gas-ification of coal as a more efficient and safer alternative todeep mining, but such an approach runs into similarproblems, namely, air gives a gas which is too dilute to beuseful, while oxygen, prepared by cryogenic fractionationof liquid air, is too expensive to employ. Research is beingconducted on membrane processes for gas separation thatmay prove to be a considerably cheaper option for thepreparation of oxygen.

The product from the water-gas reaction may beupgraded in terms of hydrogen content by the ‘water-gasshift reaction’. The mixed gases are reacted with steamover a catalyst that converts carbon monoxide to carbondioxide and increases the amount of hydrogen, that is,

COþH2OðgasÞ-CO2 þH2 ½IV�

The carbon dioxide is then removed from the gas byscrubbing to leave a mixture of, predominantly, hydrogenand nitrogen; the latter enhances the usefulness of water

Coal GasificationCO/H2 W

r

Air separationunit

O2

Air

Gas cleaning

Figure 4 Schematic representation of an integrated coal gasificatio

gas in the synthesis of ammonia. The water-gas shiftreaction may be undertaken in two steps by which thecarbon monoxide content is first reduced to o2 vol% at673 K, and then to o0.2 vol% at 473 K. If ultra-purehydrogen is required for use in fuel cells, the remainingsmall quantity of carbon monoxide is selectively oxidizedto o0.002 vol% by admitting air at 373 K. The use ofexcess steam facilitates the water-gas shift reaction andenhances the yield of hydrogen, although at the expenseof a reduction in overall thermal efficiency. A conceptualflow sheet for the production of hydrogen by the gasifi-cation of coal is shown in Figure 4.

By adjusting the fuel, the gases employed (i.e., thesteam and air/oxygen mixture) and the operating con-ditions, it is possible to tailor-make syngas of a desiredcomposition. Until now, the gasification of coal has fo-cused predominantly on chemical synthesis (as discussedabove), but there is growing interest in its use for moreefficient and cleaner electricity generation (see below)and for hydrogen production for use in fuel cells.

Coal gasification plants are being constructed aroundthe world, particularly in China – primarily for themanufacture of chemicals and fertilizers, but also formore efficient generation of electricity. A world survey in2008 identified 140 operating plants with 420 gasifiersand a total capacity of 58 500 MWth. Of these, approxi-mately half were coal-fired and the remainder were op-erating with petroleum residues and other fuels. Thebulk of the syngas is employed in the manufacture ofchemicals and liquid fuels, with much less in electricityproduction and as gaseous fuels. Worldwide gasificationcapacity is projected to grow by 70% by 2015.

Combined-cyclepower

generation

CO2/H2 Gasseparation

ater-gasshift

eaction

CO2 tosequestration

H2

Electricity

Finalgas polishing

High-purityhydrogen

H2

n process.


Gasification Technology

The process engineering of coal gasification is quitecomplex and several large-scale processes have beendeveloped. There are three basic designs of coal gasifier,as follows.

• Entrained-flow.

• Moving-bed (sometimes also referred to as fixed bed!).

• Fluidized-bed.

These technologies are shown schematically in Figure 5and their main operating features are as follows.

Entrained-flow gasifier

Entrained flow is the most aggressive form of gasification,in which pulverized coal and oxidizing gas flow co-cur-rently. Optionally, the pulverized coal may be fed to thegasifier in the form of a slurry with water that thenprovides the source of the steam required for the re-action. Under operating conditions of high pressure(2–3 MPa) and high temperature (1273–1873 K), almostcomplete gasification is achieved with little formation oftars or char. Under the extremely turbulent flow, the coalparticles experience significant back-mixing, and resi-dence times are measured in seconds. There are at leastseven different proprietary technologies for this type ofgasifier. Some of these feed coal and air/steam mixturesfrom the reactor top (as shown in Figure 5) and othersfrom the bottom.

Entrained-flow gasification is specifically designed forlow-reactivity coals and high coal throughput. Single-pass carbon conversions are in the range of 95–99%. Toexperience smooth operation, with the removal of the ashas a molten slag, the temperature of the gasifier must lie

Moving

Coal slurry

Water

Oxygen

Slag Slag

Steam

Entrained-flow

Syngas

Oxygen

Steam

Coal, flu

Figure 5 Schematic representations of entrained-flow, moving-bed

above the coal-ash fusion temperature. Alternatively, it isnecessary to add fluxes to lower the melting temperatureof the mineral matter in the coal. A problem stemmingfrom the high operating temperature of these gasifiers isthat their refractory liners can be susceptible to some ofthe oxides present in the coal slag (silicon dioxide(SiO2), calcium oxide (CaO), magnetite (Fe2O3)), whichcan penetrate into the liner and eventually cause cracks.

Moving-bed gasifier

Moving-bed gasifiers (also known, perversely, as ‘fixed-bed gasifiers’) operate at about 3 MPa and closely re-semble a blast furnace. Crushed coal, from which fineshave been removed, and fluxes are placed on the top of adescending bed in a refractory-lined vessel. The mainrequirement of moving-bed gasifiers is good bed per-meability to avoid pressure drops and burning in thechannels. On moving downward, the coal is graduallyheated and contacted with steam and oxygen-enrichedair flowing upward counter-currently. Pyrolysis, chargasification, combustion and ash melting occur sequen-tially. The oldest and best-known gasifier of this type isthe Lurgi moving-bed gasifier, although other types havesince been developed. The temperature at the top of thebed, where the syngas off-take is located, is typically723 K, and at the bottom can approach 2273 K, as dic-tated by the composition of the gas employed. Mineralmatter in the coal melts and is tapped as an inert slag.The characteristics of the ash melt influence bed per-meability, and fluxes may have to be added to modifyaspects of the slag flow. The off-gas contains tars thatmust be condensed and recycled. The production of tarsmakes downstream gas-cleaning more complicated than

Fluidized-bed-bed

Ash

Syngas

Syngasxes

Coal

Air/oxygen Steam

Fluidizedbed

Bed offeed

Grate

and fluidized-bed gasifiers.


with other gasification processes. The residence time isbetween 30 min and 1 h, which places stringent re-strictions on the physical and chemical properties of thecoal. Long residence times mean that moving-bed gasi-fiers have a low throughput and hence have limited ap-plication in large-scale electricity plants.

Fluidized-bed gasifier

A fluidized-bed reactor is a vessel in which fine solids arekept in suspension by an upward-flowing gas such thatthe whole bed exhibits fluid-like behavior. Finely dividedcoal is injected into a bed of inert particles that is flu-idized by steam and air (or oxygen) at high pressure.Rising oxygen-enriched gas reacts with suspended coal ata temperature of 1223–1373 K and a pressure of 2–3 MPa.High levels of back-mixing give rise to a uniform tem-perature distribution within the gasifier. This type ofreacting system is characterized by high rates of heat andmass transfer (i.e., increased reaction rates) between thesolid and the gas. The unusual characteristic of fluidized-bed gasifiers is that the majority of the bed material is notcoal but accumulated mineral matter and sorbent (for in

situ desulfurization). Operating with a high inventory ofinert bed material has two advantages: (1) the coal ex-periences a high rate of heat transfer on entry; (2) thegasifier can operate at variable load so that the rate ofsyngas production can be varied at will within widelimits, that is, the gasifier has a high ‘turn-down’ flexi-bility. In order to avoid ash agglomeration in the fluidizedbed, it is necessary to use coals with a higher ash-fusiontemperature than the operating temperature of thegasifier. Impurities in the coal, such as iron pyritesand high sodium content (which gives rise to sodiumsilicates during gasification), can also lead to particleagglomeration.

A variant is to use limestone as the inert material ofthe bed. The sulfur in the coal is oxidized to sulfur di-oxide and this reacts with the limestone and furtheroxygen to form calcium sulfate. Between 90% and 95%of the sulfur dioxide is removed from the exhaust gases.As ash builds up in the bed and the limestone is depleted,both materials are tapped off and further limestoneadded. The limestone will also absorb any other acidgases released from the coal during gasification.

The comparatively low temperature of operation limitsthe use of fluidized-bed gasifiers to reactive and pre-dominantly low-rank coals such as lignite or brown coal.Most of the units require recycling of entrained fines toachieve 95–98% carbon conversion. To reduce the extentof fines recycling, it has been proposed that the gasifier belinked with a fluidized-bed combustor (i.e., an ‘air-blowngasification cycle’). In this process, the coal is first gasifiedto 70–80% carbon conversion. The unreacted char is thenfed to the combustor where generated heat is used forsteam production. The gasifier–combustor combination

would enable the use of low-reactivity coals in an inte-grated gasification combined-cycle (IGCC) electricityplant (see below). In general, though, the type of gasifiershould be matched to the properties of the coal available,especially with respect to its gasifying characteristics andmineral content (ash melting temperature, chemicalcomposition). The three major classes of gasifier in theirvarious modifications can, between them, cope with mosttypes of coal.

Gas Cleaning

Gas cleaning is an essential part of the overall gasificationtechnology, both to protect catalysts from poisoning andalso to meet the end specification for the syngas, de-pendant upon whether it is to be used in gas turbines, forchemicals manufacture, or for the pure hydrogen re-quired by low-temperature fuel cells. Compared with thesteam reforming of natural gas, gasification of coal yieldsa syngas that has: (1) higher levels of carbon monoxide,which have to be shifted to carbon dioxide and hydrogen;(2) generally higher levels of impurities, which have to becleaned from the gas before use. The main contaminantsin syngas produced from coal are particulates, sulfurdioxide, nitrogen oxides (NOx), the alkali metals sodiumand potassium, and mercury. Minor or trace amounts ofammonia, arsenic, beryllium, cadmium, chromium,hydrogen chloride, hydrogen fluoride, lead, nickel andselenium can also be present. All these species have to beremoved to acceptable limits by final gas polishing inorder to protect downstream process equipment (inparticular, the gas turbines) against fouling, erosion and/or corrosion, to prevent poisoning of the catalyst used forthe shift reaction, and to meet environmental regulations.For low-temperature fuel cells, it is necessary to polishthe hydrogen fuel especially well to remove the finaltraces of impurities and carbon monoxide that can poisonthe platinum electrocatalyst. At present, barrier ‘candle’filters are used to remove solid contaminants. The filtershave a porous tubular structure and can be classified intotwo broad designs: (1) ‘ceramic’ filters, which are madefrom materials such as alumina, silica, and zeolites; (2)‘metallic’ filters, which are made from nanofibers or wiresof alloys, for example, iron aluminide, Incoloy, Monel,Hastelloy, and are protected from corrosion by a ceramiccoating. A detailed description of such filters is given in aseparate article in this encyclopedia.

Conventionally, sulfur is removed from raw syngas bymeans of low-temperature solvent/adsorption processes,but these are energy-intensive and highly expensive formany applications. A range of high-efficiency sulfur-capture techniques is being developed and includeimproved physical and chemical sorbents, advancedcatalytic processes, and selective separation membranes.


To avoid degradation of the construction materialsused for the gas-treatment unit, and also because of thethermal instability of the solvents, it has been customaryto undertake cleaning after the gas has been cooled tobelow 373 K. This necessitates large heat-exchangers thatare costly, and also degrade the overall energy efficiencyof the plant. Syngas is required at much higher tem-peratures and pressures when destined for use in com-bined-cycle electricity plant. Hence, there are obviousoperational advantages to be gained from the develop-ment of ‘hot-gas cleaning’ systems. Indeed, such an ad-vance is considered essential if the comparativeeconomics of coal gasification plants are to become at-tractive. Hot-gas cleaning technologies are in the earlydemonstration stage with units being evaluated at tem-peratures above 873 K (which necessitates considerablecooling of the gas) and at pressures up to 2.5 MPa.Calcium-based sorbents (limestone and dolomite) andzinc titanate are the leading candidates for use in thedesulfurization of high-temperature fuel gas, but eventhese have serious limitations that will have to be over-come before they may be regarded as satisfactory.

Combined-Cycle Electricity Generation

Combined-cycle gas turbines are widely used for elec-tricity generation from natural gas. The gas is burnt in ahigh-temperature gas turbine that is coupled to a gen-erator. The exhaust gas from the turbine is used to raisesteam, which is then fed to a conventional steam turbineand a further generator.

A combined-cycle plant for generating electricityfrom coal is based on a similar methodology – the IGCCprocess. The gas leaving the gasifier is an impure mixtureof hydrogen and carbon monoxide. Hydrogen sulfide(formed from sulfur impurity in the coal) is removed byabsorption in a polyethylene-based solvent. Mineralmatter in the coal forms a slag at these high temperaturesand is discharged from the base of the gasifier. Thecleaned syngas replaces natural gas as fuel for a large gasturbine coupled to a generator. Again, the gas turbineexhaust passes through a heat-exchanger (the ‘heat-re-covery steam generator’) to raise high-pressure steam.This is mixed with the steam from the gas cooler andused to operate a conventional steam turbine that gen-erates more electricity. The IGCC process has potentialto increase thermal efficiencies to over 50% with cor-respondingly reduced emissions of carbon dioxide.

The scale of the IGCC operation is impressive. Techno-economic studies have indicated that a large gasifier willhave a coal feed in the range of 2000–3000 t per day andwill produce syngas at an energy efficiency of 75–80%. Oncombusting this syngas in an IGCC scheme, the net powergenerated will lie in the range of 270–420 MWe, with an

overall thermal efficiency (coal-to-electricity) of 38–45%,although eventual efficiencies of 50% are widely antici-pated. This performance is significantly better than that ofmost conventional coal-fired (PFCC) electricity plants. A400-MWe plant will emit around 6500 t of carbon dioxideper day.

A refinement of the process is to convert the carbonmonoxide in the syngas into carbon dioxide and thenseparate it from the hydrogen, for storage, before thehydrogen is fed to the gas turbine for combustion. This‘pre-combustion capture’ of carbon dioxide is an exampleof ‘clean coal’ technology. It is easier than ‘post-com-bustion capture’ (where the exhaust gases are heavilydiluted with nitrogen from the air) and hence the costs ofcarbon dioxide sequestration will be lower. The exit gasfrom the gasifier is reacted with additional steam over asuitable catalyst (shift reaction, eqn [IV]) to convert thegas into a mixture of hydrogen and carbon dioxide.The carbon dioxide may then be separated in a formsuitable for sequestration (e.g., in geological structures),while the hydrogen is used to fuel the gas turbine. Inprinciple, the hydrogen could be fed to large-scale sta-tionary fuel cell installations, or used to provision a fleetof electric vehicles powered by fuel cells. These possi-bilities, however, are well into the future. To date, nolarge-scale IGCC plant with carbon capture has beenbuilt, although small-scale trials are thought to be inprogress. The ultimate aim is ‘zero-emissions power fromcoal’. Such plants could also be used for the production ofsynthetic liquid fuels and chemicals, a so-called ‘poly-generation plant,’ as shown schematically in Figure 6.

Capture of Carbon Dioxide

The overall sequestration of carbon dioxide involves thefollowing four steps.

• Capture of the gas from the emission source.

• Dehydration and compression of the gas.

• Transport to the storage site.

• Injection into the storage geological reservoir.

The capture of carbon dioxide from exhaust gases istaken to be the most difficult part of the overall seques-tration activity, and also the most costly. There are threemain processes for capturing carbon dioxide from powerplants, and then compressing and drying it ready forunderground storage, namely:

• Post-combustion gas scrubbing.

• Precombustion decarbonization.

• Oxy-fuel combustion.

The options are shown schematically in Figure 7. Notethat oxy-fuel combustion is essentially a variant of post-combustion capture; oxygen rather than air is the oxidant

Gas turbine

Gas coolingand cleaning

Contaminantse.g., sulfur

Syngas CO/H2

Shiftreactor

CO2/H2separator

Fuel cells

Solids

Gaseousconstituents

Liquid fuels and chemicals

Gas turbine

Steam turbine

Gasifier

Oxygen

Glassy slag

Air Generator

Generator

Heat recoverysteam generator

Steam

Combined cycle

Fuel cells

CO2 to storage

Coal feedstock

H2

Solids

Gaseousconstituents

Fischer–Tropschsynthesis Liquid fuels

and chemicals

Figure 6 Concept of a poly-generation plant based on the integrated gasification combined-cycle (IGCC) process.

Coalgas

biomass

CO2compression

anddehydration

Process+

CO2 separation

GasificationReformer

+CO2 separation

Airseparation

Pre-combustion

Post-combustion

Oxy-fuelcombustion

CO2 separationPower

+Heat

Coalbiomass

Air

Air/O2steam

Coalgas

biomass

Coalgas

biomass

Air

Air

Air/O2

Gas, ammonia, steelRaw material

N2O2

CO2

CO2

N2

O2

CO2

N2

CO2

Gas, oil

Industrialprocesses

Power+

Heat

O2

Power+

Heat

Figure 7 Principal routes for managing carbon dioxide emissions from power stations.


and thereby the need to separate carbon dioxide fromnitrogen is eliminated. Carbon dioxide is also generatedfrom many large industrial operations (petroleum re-fineries, ammonia plants, etc.) and is sometimes separatedfor use in chemical processes or in the food industry.

Current processes for separating carbon dioxide fromhydrogen, such as pressure swing adsorption (PSA) andsolvent scrubbing, are mature but energy-intensive. Amajor thrust for reducing the cost and improving theperformance of hydrogen and carbon dioxide separation is


the development of membranes that are selective forhydrogen diffusion. Ideally, membranes capable of oper-ation at high temperatures would be employed to obviatethe need for cooling the gas, and thereby save energy andremove the capital cost of extra heat exchangers. Evenbetter would be to integrate the hydrogen separationmembrane into the water-gas shift reactor, as shownschematically in Figure 8. Membrane separators are dis-cussed elsewhere in this encyclopedia.

Post-Combustion Capture

In a conventional combustion process, such as a powerstation boiler, the carbon dioxide is contained in the exitflue gas. The concentration is generally quite low andranges from about 4 vol% for a combined-cycle systemoperating on natural gas to 12–14 vol% for a traditionalboiler fired by pulverized coal. The exhaust can be‘scrubbed’ with an amine solution, typically mono-etha-nolamine, which is then heated to release the absorbedcarbon dioxide. The low concentration of carbon dioxidein the exit gas means that a huge volume would have tobe handled, and this would entail the installation of largeand expensive equipment. Also, considerable energy isrequired to desorb carbon dioxide from the amine solu-tion and subsequently to re-pressurize it. The amine,which is a valuable chemical, can be recycled but soondegrades through the action of high temperature, oxygen,and impurities in the gas; it therefore has to be replaced.This is particularly true for exhaust gases from coal-firedstations that contain sulfur dioxide and other acid im-purities. Research is being undertaken to develop moreefficient and less expensive processes.

Given the drawbacks of the amine route, post-combustion capture has not been demonstrated to be cost-effective in most power stations. It has been estimated thatthe process might well double the capital cost of a com-bined-cycle operation running on natural gas, as well asreducing the overall plant efficiency and therefore in-creasing the fuel consumption and the running costs. In

H2

Membrane tube

Catalyst in annular space

Syngas

H2-richstream

CO2-richstream

Reactor shell

Sweepgas

Water-gas shift reactionCO + H2O → CO2 + H2

Permeation

Figure 8 Schematic representation of a packed-bed membrane

reactor.

principle, retro-fitting to existing utilities is possible butthere may be practical considerations such as the avail-ability of land. Post-combustion capture has, however,been employed in some chemical manufacturing plantsand is being thoroughly investigated by power companies.Many new coal-fired power stations are being built ‘cap-ture ready,’ which implies making provision for futurepossible adoption of carbon dioxide separation processes.

Oxy-Fuel Combustion

The concentration of carbon dioxide in flue gas can beincreased greatly by using oxygen instead of air forcombustion, either in a boiler or in a gas turbine. Theoxygen would be produced by cryogenic air separationwhich, although expensive, is already employed on alarge scale, for example in the steel and glass-makingindustries. When fuel is burnt in pure oxygen, the flametemperature is excessively high, so some exhaust gas isrecycled to the combustor in order to hold the flametemperature similar to that in a normal air-blown boiler.The advantage of oxygen-blown combustion is that theflue gas has a carbon dioxide concentration of B80 vol%compared with 4–14 vol% for air-blown combustion, andtherefore separation of the carbon dioxide is relativelysimple or even unnecessary. After combustion, the gasstream is first cooled and compressed to remove thewater vapor. It may be possible to omit some of the gas-cleaning equipment that is used in modern coal-burningpower stations (such as scrubbers for desulfurization offlue gas), and this would reduce the net cost of carbondioxide capture. Some sulfur compounds and other im-purities would then remain in the carbon dioxide fed tostorage, which may be acceptable. A possible downside ofomitting the desulfurization stage is that recycled, wetflue gas will contain sulfuric acid that is highly corrosiveto the plant.

The disadvantage of oxy-fuel combustion is that alarge quantity of expensive oxygen is required. Advancesin oxygen production processes, such as new and im-proved membranes that can operate at high temperaturescould improve overall plant efficiency and economics. Todate, oxy-fuel combustion aimed at power generation hasonly been demonstrated in test rigs. Nevertheless, thereis active interest in the technology within the electricityindustry because of its potential for retro-fitting to pul-verized coal plants that constitute the majority of theworld’s generating capacity. This would have the ad-vantage of improving the generating efficiency and soreducing the carbon emissions.

Chemical Looping Combustion

This process is a highly speculative alternative to oxy-fuel combustion that has been proposed to separatecarbon dioxide from nitrogen and excess air. A metal


oxide (e.g., copper, cobalt, nickel, iron or manganeseoxides) serves as an oxygen carrier that transfers oxygenfrom the air to the fuel. Two reactors in the form ofinterconnected fluidized beds are employed: (1) a fuelreactor, in which the metal oxide is reduced by reactionwith the fuel; (2) an air reactor, in which the reducedmetal oxide from the fuel reactor is re-oxidized with air.A schematic of the process is given in Figure 9.

The outlet stream from the fuel reactor consists ofcarbon dioxide and water, while that from the air reactorcontains only nitrogen and some unused oxygen. Theoverall chemical reaction is the same as for normalcombustion with the same amount of heat released, butwith the important difference that carbon dioxide is in-herently separated from nitrogen, so that no extra energyand costly external equipment are required for gas sep-aration. Although the technology is very much in itsinfancy, particularly with respect to chemical engineeringissues, chemical looping combustion offers the sameadvantages as oxy-fuel combustion, but with the addedprospects of higher thermal efficiency and no require-ment to extract oxygen from air.

Pre-combustion Capture

After gasifying the coal, the product syngas is firstcleaned and then subjected to the water-gas shift re-action, as described earlier. It is then a mixture ofhydrogen and carbon dioxide that has to be separated.This is generally accomplished by PSA, which involvesisolating the carbon dioxide and impurity gases by ad-sorbing them at high pressure (up to 4 MPa) on a suitableadsorbent (e.g., a molecular sieve or activated carbon) in apacked bed. Impurities are selectively adsorbed, whilepure hydrogen is withdrawn at high pressure. To allowcontinuous operation, multi-beds connected in parallelare often used. Once a bed becomes saturated with im-purities, the feed is switched automatically to anotherfresh bed to maintain a continuous flow of hydrogen.Reducing the pressure in a number of discrete steps,

Compression

Turbin

Metal oxC

Airreactor

Metal

Air

Fuel

Combustion Expan

Turbin

Compressor

Fuelreactor

Figure 9 Principle of chemical looping combustion.

which releases the adsorbed gases, regenerates spentabsorbent. Usually, for existing hydrogen plants, all thedesorbed carbon dioxide is vented to the atmosphere,though in principle it is possible to separate it out forstorage. Indeed, there are instances where it is pumpedunderground as a means of facilitating enhanced oil re-covery (see below).

Pressure swing adsorption yields high-purity hydro-gen (up to 99.999%) and recoveries of up to 90% areroutinely achieved in industrial plants. The process ishighly reliable, flexible, and easy to operate with modestenergy requirements. It does, however, suffer from lim-ited capacity and from the requirement to cool the gasesto low temperature to effect separation. The producthydrogen is therefore now at low pressure and lowtemperature, which is not ideal as feedstock for a gasturbine or a high-temperature fuel cell if either system isto operate at maximum efficiency. On the other hand, itwould be very suitable for a low-temperature fuel cell.Owing to the modular nature of the PSA process, scaling-up can be readily achieved with no loss of efficiency.Industrial units have production capacities that rangefrom 500 to 100 000 N-m3 h�1, and typically require upto 12 beds for maximum hydrogen recovery. A photo-graph of an industrial PSA unit is given in Figure 10.

There are numerous ways, other than PSA, of separ-ating hydrogen from carbon dioxide. These includetemperature swing adsorption, physical and chemicalabsorption processes, and cryogenic separation. In the last-mentioned process, carbon dioxide is separated fromhydrogen and other gases by cooling and condensation.The technology is used commercially but is energy-in-tensive and, when water is present, may experience op-erational problems through the formation of carbondioxide clathrates and ice that can result in seriousblockages of the system. Nevertheless, there is the ad-vantage that, after re-warming under pressure, carbondioxide is produced in a liquid state that can be easilytransported to a disposal site by a tanker. Selectivemembrane diffusion processes also offer a promising

eO2-depleted air

(~14% O2)

Compressionand storage

H2O

ideO2 + H2O

sion

Cooling andwater

condensatione

CO2

Figure 10 Industrial pressure swing adsorption unit.


approach to separating hot gases, as discussed in othercontributions to this volume.

Storage of Carbon Dioxide

Geological Storage

There are a number of possible options for the bulkstorage of carbon dioxide, among which deep geologicalstorage is favored now. The most suitable sites arefound in sedimentary basins, for example, depleted oil orgas reservoirs, saline formations (aquifers), and deepunmineable coal seams. Oil and gas reservoirs have animportant advantage in that they are capped by im-permeable rock and are known to have stored oil or gassafely for many millennia. An additional attraction lies inthe fact that oil reservoirs are normally abandoned asuneconomic when they contain substantial reserves ofpetroleum. By injecting high-pressure gas, it is possible todrive out more oil from the pores of the sandstone inwhich it is held. The process is known as ‘enhanced oilrecovery’ and the value of the oil recovered serves tooffset the cost of gas storage. This is a well-demonstratedtechnology that is in use in various places (e.g., USA,Canada). The same procedure is possible with gas res-ervoirs, although mixing of the natural gas with the in-jected carbon dioxide limits the scope for furthermethane recovery.

The second preferred option, after enhanced oil re-covery, is storage in fully depleted oil or gas reservoirswhere there is no prospect for further extraction of fuel.This has the merit of storage in secure locations whereleakage is extremely unlikely, although there is no bonusto be obtained.

Storage in saline aquifers is the third favored option.The possibility of leakage from these formations over thelong term is less certain because they may not have a capof impermeable rock to hold the gas in place and havenot been demonstrated over millennia as secure storagesites. On the contrary, the attraction of aquifers is that

they are far more numerous than oil and gas reservoirsand are widespread, both on land and under the con-tinental shelf. Access to the latter would be throughpipelines from the shore or from off-shore platforms.Obviously, the shorter the pipeline from the capture unitto the store, the less is the capital cost. Also, operationson land are cheaper to conduct than those at sea. Off-shore aquifer storage of carbon dioxide is practiced in theNorth Sea. The natural gas from the Norwegian Sleipnerfield contains too high a content of carbon dioxide(9 vol%) to be usable so this is separated out (to leave2.5 vol% in the fuel) and injected into an aquifer at adepth of 800–1000 m below the sea floor. This aquifer isestimated to have a massive storage capacity of 1–10 GtCO2; the rate of separation and injection is B1 Mt CO2

per annum. An on-shore project is being conducted by InSalah Gas at a field in the Algerian desert. Carbon di-oxide is separated from the natural gas and injectedunder pressure into a brine formation at two kilometerbelow the surface.

Unmineable coal seams are another potential form ofgeological storage. These are either located at too great adepth to be mined, or are too thin to be economicallyexploitable. Coal is usually associated with methane. Thegas adsorbs on the surface of the micropores in the coaland is confined by water that usually fills the fractures ofthe host material. Substantial quantities of methane canbe adsorbed and are well worth recovering. To desorb thegas, its partial pressure must be reduced to allow both gasand water to move through the coal-bed and up the wells.Methane is a by-product of the process by which plantmaterial is converted into coal and up to 25 m3 maybecome trapped per ton of coal at the prevailing pres-sures. A substantial amount of coal-bed methane is re-trieved in the USA, whereas in Australia, the vastQueensland coalfields are being actively explored andmapped with a view to gas recovery.

Carbon dioxide has great affinity for coal. In generalterms, coal can store at least twice the amount of carbondioxide as methane on a molecular basis. This ratio willvary from coal-to-coal, as well as with the pressure,temperature and physical conditions of storage. In volu-metric terms, the ratio of the adsorbed amount of carbondioxide to that of methane ranges from as low as unity formature coals such as anthracite, to 10 or more foryounger, immature coals such as lignite. It has thereforebeen suggested that unworkable coal seams might bepurged with pressurized carbon dioxide to provide notonly a means of driving out and harvesting methane, butalso an underground store for the waste gas. The carbondioxide will flow through the cleat system of the coal,diffuse into its matrix, and then be adsorbed on the innermicropore surfaces to unlock methane.

Enhanced coal-bed methane (ECBM) recovery isbeing undertaken in the San Juan Basin in north-western


New Mexico, USA. More than 0.1 Mt carbon dioxidehave been injected over a 3-year period. In the USAalone, it is claimed that ECBM operations could provide4.25 Tm3 of technically recoverable methane and afurther 0.8 Tm3 of ‘proven reserve’, with the storage ofcorrespondingly large quantities of carbon dioxide. Ac-cording to the International Energy Agency, the worldpotential for carbon dioxide storage through ECBM is100–150 Gt, of which 40 Gt are considered to beviable now. To put this capability into perspective, it isinteresting to compare the predictions with the worldenergy-related emissions of carbon dioxide which, in2005, amounted to 28.1 Gt carbon dioxide. All projectionsof methane recovery and the associated storage of carbondioxide must be treated with caution, however, sincethe science involved in the uptake of gases by coal isimperfectly understood and complications may arise.There are also safety and environmental issues to beevaluated.

Ocean Storage

A number of authorities favor deep ocean storage ofcarbon dioxide, in one form or another. The oceans arebelieved to contain some 50 times as much carbon as theatmosphere. Moreover, oceans are mainly responsible forthe natural sequestration of approximately half of thecarbon dioxide that is produced by mankind. Therefore,why not inject the other half for ultimate disposal?

Shallow injection of carbon dioxide into an ocean isnot an option as it would cause a regional lowering of thepH with almost certain disastrous ecological con-sequences. Even now, there is grave concern amongmarine biologists over the acidification of the oceans thathas already taken place.

Deep injection is a different matter. The density ofseawater is almost independent of depth (1.040–1.045 g cm�3), whereas that of liquid carbon dioxide in-creases with depth. Down to about 2500 m, liquid carbondioxide is less dense than seawater and tends to float up-ward. Deeper than 3000 m, the liquid is denser than sea-water and sinks to the ocean floor where it accumulates asa lake, over which a solid layer of crystalline hydratesforms as an ice-like combination of carbon dioxide andwater. Within its stability range (low temperature, highpressure), solid carbon dioxide-hydrate would inspiregreater confidence as a permanent store than dissolved orliquid carbon dioxide. Storage on the ocean floor requiresthe carbon dioxide to be in liquid form. It could be con-veyed to the storage site either by pressurized pipeline orcryogenically by ship. The objections to this form ofstorage revolve around the unknown ecological impact onmarine species that live at these great depths. Also, there isthe threat that ocean currents or volcanic activity mightlead to dispersal of the crystalline hydrate and progressive

absorption into the ocean water, again lowering the pHwith all the anticipated consequences.

Recently, an entirely new procedure for ocean dis-posal has been proposed by scientists working at Harvardand Columbia Universities. They suggest that the gasshould be pumped a few 100 m into the porous sedimentthat covers the deep ocean floor. Under the prevailingconditions, the carbon dioxide would be in liquid formand would soon convert into crystalline hydrate, whichwould then be locked into the structure of the sediment.It is surmised that the crystalline hydrate would dissolveonly very slowly into the ocean over a period of hundredsof years and that no ecological damage would result. Thisproposition is seen to have material advantages overother methods of sea disposal. The scientists furtherhighlight the fact that the number of ocean sites forcontainment in porous sediment is virtually unlimitedand, therefore, would allow for the storage of all of theworld’s captured carbon dioxide indefinitely, with little orno ecological ramifications. Liquid carbon dioxide wouldbe conveyed to the given location by ship or pipeline andthen injected into the sea floor by means of the standardtechnology used in oil extraction. At present, this conceptis little more than an interesting proposal; considerablework is required to evaluate the prospects, determine thecosts, and provide the necessary reassurance to marinebiologists. Meanwhile, it is probable that land-basedgeological storage will continue to be developed.

Institutional Issues

Although geological storage of carbon dioxide does ap-pear to be technically feasible, and indeed is beingpracticed in a few places, its extension to effect the ul-timate disposal of most of the global emissions from fossilfuels does raise numerous institutional and safety issues.In economic terms, storage space will have a financialvalue in an era of emission permits and/or carbon taxes.The worth of a particular facility will depend upon manyfactors, for example, its location, size, ease of access, in-tegrity. Owners of suitable underground sites, of what-ever nature, will be able to charge customers for theirservices. Already this is the case where storage is in op-eration. A free market is likely to develop, in which thedisposer of the carbon dioxide will negotiate a price withthe facility owner. This raises the question of who is thelegal owner of each underground site. Most countrieshave defined mineral rights, but these do not generallyextend to great depths where mining is impracticable. Foroil and gas exploitation within territorial waters, gov-ernments normally invite tenders and sell franchises tothe highest bidder. But who owns deep-lying salineaquifers that have no other obvious use? On-shore, arethese the property of the landowner? Off-shore, willgovernments put such storage sites out to tender as with


oil and gas franchises? Furthermore, who would approvedeep ocean storage in international waters? Satisfactoryanswers have yet to be found.

Further legal issues relate to the status of capturedcarbon dioxide. Is it a waste that, under internationaltreaty, cannot be disposed of at sea? Or is it a ‘resource’when used, for example, to enhance oil recovery? And dothe conventions relating to dumping at sea apply tostorage under the sea bed, particularly if the only accessis through a pipe from the land? Hence there is muchscope for international debate before an agreement canbe reached. In the meantime, some new power stationsare being built ‘capture ready’ rather than incur theconsiderable capital costs of building capture units beforethe ground rules relating to storage have been agreed. Ithas even been suggested that stored carbon dioxide mightbe seen as a long-term resource to increase the green-house effect during the next ice age!

These and many other institutional, legal, and safetymatters remain to be resolved before the large-scalestorage of carbon dioxide can be adopted, particularlywhen off-shore or in the deep ocean. For the present, itappears that geological storage in disused oil and gaswells, or saline aquifers, are the most likely possibilities,although storage in ocean bed sediments remains aninteresting possibility.

Future Prospects of Coal Gasification

With the huge global reserves of coal available as a pri-mary source of energy, nations are now seriously con-sidering the various options for clean coal technology andhydrogen as a fuel. It is recognized that the effective andeconomic separation of carbon dioxide, both from post-combustion flue gases and from pre-combustion syngas,are key steps in developing clean coal technology as aprelude to the Hydrogen Economy. The problem withcapture is that the bulk extraction of carbon dioxide fromother gases is costly. Consequently, more research anddevelopment is required on novel separation techniques,particularly on membrane technology as applied to theprocessing of hot, pressurized gases to produce purehydrogen. Membrane processes for the separation ofoxygen from air are also important in the further de-velopment of gasification procedures.

Commercial coal gasifiers are now well establishedand there are many different types in operation. Thedevelopment of the different designs is, in part, a re-flection on the variability of coal as a feedstock; it isnecessary to match the gasifier employed to the type ofcoal available. Key issues when specifying a gasifier are:the scale of operations; the desired flexibility of feed-stock; the composition of the product syngas; energyefficiency and environmental considerations; reliability

and maintainability; ash/slag removal arrangements (andpossible markets for this by-product); and, of course,capital and running costs. Each of these broad areasbreaks down into many detailed considerations that in-variably lead to the requirement for improved technol-ogy. For instance, technical factors include the following.

• Procedures adopted for preparation of the coal andthe impact of these on the life of the feed injectors andthe performance of the gasifier.

• Improved feeding systems for high-pressure gasifiers.

• Robust refractory liners in high-temperature slagginggasifiers and the need for new materials of longeroperating life and reduced cost.

• Better instrumentation and a requirement for on-linemonitoring of conditions in the gasifier.

• Advanced hot gas-cleaning and particulate filtrationprocedures.

These are mentioned merely as examples of the detailedconsiderations that have to be given to gasifier selectionand design.

There are other important issues (beyond the scope ofthis article) that will determine how rapidly coal gasifi-cation is expanded in the electricity supply industry.Chief among these is the fervor with which society ad-dresses the issue of climate change and the level of car-bon emission taxes imposed. Another important factor isthe long-term future of the electricity supply industry.Will the current practice of centralized generation andlong-distance transmission through the grid be con-tinued, or will there be a movement toward more local-ized generation and distribution? If hydrogen-based fuelcells are to make a significant impact, then the lattermodel becomes more likely. Also the growth of combinedheat and power schemes (co-generation) would makelocalized generation more probable. These are im-ponderables at present. What does seem likely is that, inthe medium term, all energy prices will continue to riseand consequently there will be a premium on processeswith high-energy efficiency and low cost.

Finally, there is the question of carbon dioxide se-questration, which must be addressed earnestly. Coal willbe deemed unacceptable as a fuel unless sequestration isincluded. At present though, pending advances in tech-nology, carbon dioxide capture will result in a seriousdegradation of energy efficiency and increased carbondioxide emissions, which are equally unacceptable. Thisis a dilemma that can only be solved by aggressive re-search and development on gas separation techniques.

Experience of geological disposal is limited to just adozen or so projects, in which gas is injected into oil orgas reservoirs or saline aquifers. Many of the activitiesare in their early stages and, although so far promising, itwill be some time before the results can be fully evalu-ated. While these and future demonstration projects are


on-going, there is much to be done in terms of geologicalmapping, matching large point sources of carbon dioxideto prospective storage sites, and establishing the under-lying institutional framework for the long-term moni-toring and control of the repositories.

Taken in the round, it is clear that society facesmassive issues in the large-scale implementation of cleancoal technology for the production of electricity and themanufacture of hydrogen to be used in fuel cells. Some ofthese can be solved by industry alone, but many arecontingent upon governments setting the appropriatelegislative and financial frameworks within which in-dustry must act. If climate change is to be contained then,according to the climate scientists, there is no time to belost.

Nomenclature

Abbreviations and Acronyms

ECBM en
hanced coal-bed methane
IGCC in
tegrated gasification combined
cycle

PFCC pu
lverized fuel coal combustion
PSA pr
essure swing adsorption
See also: Fuel Cells – Molten Carbonate Fuel Cells:

Overview; Fuel Cells – Overview: Introduction; Fuel

Cells – Phosphoric Acid Fuel Cells: Systems; Fuel

Cells – Proton-Exchange Membrane Fuel Cells: Cells;

Overview Performance and Operational Conditions; Fuel

Cells – Solid Oxide Fuel Cells: Systems; Fuels –

Hydrogen Production: Autothermal Reforming; Gas

Cleaning: Barrier Filters; Gas Cleaning: Membrane

Separators; Gas Cleaning: Pressure Swing Adsorption;

Natural Gas: Conventional Steam-Reforming.

Further Reading

Childress J and Childress R (2004) 2004 World gasification survey: Apreliminary evaluation. Proceedings of Gasification Technologies2004. Washington, DC, USA, 4–6 October.

Collot A-G (2003) Prospects for Hydrogen from Coal Report CCC/78.London: International Energy Agency Clean Coal Centre.

Dell RM and Rand DAJ (2004) Clean Energy. Cambridge: The RoyalSociety of Chemistry.

Freese B (2003) Coal: A Human History. Chatham: Mackays ofChatham plc.

Gasification Technologies Council (2008) Gasification: Refining CleanEnergy. Arlington, VA: Gasification Technologies Council.

International Energy Agency (2006) Key World Energy Statistics 2006.Paris: International Energy Agency.

National Research Council and National Academy of Engineering of theNational Academies (2004) The Hydrogen Economy: Opportunities,Costs, Barriers, and R&D needs. Washington, DC: The NationalAcademies Press.

Rand DAJ and Dell RM (2008) Hydrogen Energy: Challenges andProspects. Cambridge: The Royal Society of Chemistry.

US Department of Energy (2000) Clean Coal Technology; TopicalReport No 20: The Wabash River Coal Gasification RepoweringProject, An Update. Washington, DC: US Department of Energy.

US Department of Energy (2004) Basic Research Needs for theHydrogen Economy. Washington, DC: Office of Science, USDepartment of Energy.

World Coal Institute (2004) The Role of Coal as an Energy Source.London: World Coal Institute.

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