coal fired fossil fuels

8/13/2019 Coal Fired Fossil Fuels

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Control and Minimiof Coal-fired

W O R K I N G P A R T Y O N F O S S I L

I N T E R N A T I O N A L E N E R G Y A G


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CONTROL AND MINIMISATION OF COAL-FIRED POW

INTRODUCTION

Worldwide, coal-fired power generation presently accounts foelectricity production. In some countries, such as China and Imuch as 50%. While coal use in some of the more developed

or is in decline, significant increases in coal-fired generation cin many of the developing nations and large capacity increconsequence of the extensive investments being made in manybecause coal resources are far more abundant than other fossil fupower plants have a long working life, coal will remain an impfor many years.

Coals on-going role underlines the importance of improving

fired power plants and the minimisation of their environmeeconomic and environmental reasons. A number of pollutafired power plants require consideration, although effective already exist in some cases. For instance, systems exist to cont(SOX) and nitrogen oxides (NOX), or particulates. Power planof CO2. With few exceptions, however, CO2 control technologieAnd yet, incentives can be introduced for deploying technolo

eliminate, CO2 emissions. Key examples are mandatory standinternalising external costs, notably through taxes and tradaSuch measures must be carefully balanced to encourage investmhigh standards have to be met, and thus prevent resources other countries.

A range of technologies has thus been developed to minimise tof undesirable substances emitted from coal-fired power plants

see the Website of the IEA Clean Coal Centre1

). Collectively,referred to as Clean Coal Technologies (CCTs). A CCT is a teeconomically viable manner, reduces plant emissions to enablexceed any emissions standards in force. CCTs are becoming as they provide a means for coal-fired plant to meet the requiremstringent environmental legislation applied in many countrie


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4 - CONTROL AND MINIMISATION OF COAL-FIRED POWER PLANT EMISSIONS

Climate change is a problem of global proportions. A number of agases are largely responsible for driving this process forward, the mcontributor being carbon dioxide (CO2) produced by the burningThe latter provide a large proportion (>85%) of the worlds commneeds and will continue to do so for the foreseeable future (Figure

that substantial reductions in atmospheric CO2 levels can be mapresent century and beyond, technological solutions urgedevelopment and application in order to control the increasingCO2 being produced. The International Energy Agency

2(IEA) is p

role in addressing this problem. Recognising the potential of COstorage technologies, the IEAs Working Party on Fossil Fuels

3(W

its strategy for Zero Emissions Technologies (ZETs) in 2001. Withalmost all conventional pollutants produced by the burning of f

eliminated and CO2 is captured and stored, thus precluding its the atmosphere. The capture of CO2 from commercial and industrfollowed by its storage in geological formations, is viewed as strategy for achieving substantial reductions in emissions levelsdeployment of CO2 capture and storage technologies will dependthe widespread introduction of appropriate mandatory standards ofor pricing CO2 emissions.

World Primary Energy Demand in a Business-as-UsualFigure 1


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As the IEA has commented:

Numerous technology solutions offer substantial CO2-reincluding renewable energies, fossil-fuel use with CO2 cnuclear fission, fusion energy, hydrogen, biofuels, fuel

energy end use. No single technology can meet this cDifferent regions and countries will require differentechnologies to best serve their needs and best exploiresources. The energy systems of tomorrow will rely onadvanced, clean, efficient technologies for energy supp

Energy Technology: Facing the Climate Challenge - paper prepaof the IEA Governing Board at Ministerial Level, 28-29 A

While emissions need to be reduced, it is clear that fossil fuel rnot be the driver of emissions reductions in the foreseeable futhe case, in spite of anticipated cost increases, as the cheapest depleted and transport distances increase for obtaining new supdemand is rising, particularly in the developing world, where pgrowth are greater than in developed countries and where therural to urban areas is significantly higher. Developing and decan be expected to continue using their abundant coal reservesof action, CO2 levels can be expected to continue increasing (

Energy-Related CO2 Emissions by Regionin a Business-as-Usual Scenario

Figure 2


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Continued fossil-fuel use in a CO2 emissions-constrained world wefficient fossil-fuel combustion technology, CO2 capture and storagamong fossil fuels. In a future Hydrogen Economy, control of COas well, because hydrogen will be produced mainly from fossil fuels

MAIN TYPES OF COAL-FIREDPOWER GENERATING PLANT

A number of types of coal-fired power plant technology are currentlythe world. In terms of potential for further significant developmehowever, efforts are focused largely on pulverised fuel (PF) and integcombined cycle (IGCC) systems. In terms of both number of placapacity, PF plants dominate the world electricity generating markgrowing, number of IGCC plants are now operating either in demonin commercial use.

Layout of a PF Power PlantFigure 3


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advanced steam cycles that allow for greater plant efficienccharacterised by overall thermal efficiencies of some 36% (Lowe- basis), but in some developing nations this figure can be mhigher steam temperatures and pressures can attain up to aroudevelopments take place, efficiencies of 50-55% may ultimat

In integrated gasification combined cycle (IGCC) plants, coaand oxygen in a gasifier, generating a fuel gas that consists prhydrogen. This gas is cleaned using a number of available techgas turbine. The exhaust heat is used to drive a steam cycleelectricity. IGCC plants allow high efficiencies to be attainegrade coals. Several important demonstration/commercial projin Europe and the United States (Figure 4), and several otpreparation stage.

The coal-fired Demkolec IGCC Plant at Buggenum

f f b b

Figure 4


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EMISSIONS CONTROLTECHNOLOGIES APPLIED

A range of pollutants are generated from coal-fired and coal co-fired plants and some are more specific to a particular technology. Histohas focused mainly on controlling emissions of SO2, NOX and particusystems have been developed and applied for their control. Some coone type of pollutant, whereas others may integrate several control systefor the control of two or more pollutants (e.g. combinations ofparticulates). Here, a range of technologies are either being demonscommercially. Most on-going development work, for either one pollucontrol systems, is focused on increasing process efficiency and/or

and operating costs, often through the adoption of simplified properation. To date, work has concentrated largely on controlling more traditional pollutants. However, emphasis has increased more restrategies for a range of other pollutant species that are present in low

When the addition of emissions control systems to a plant is beingnumber of issues require consideration in order to determine the mvariant(s), and many will reflect the configuration and age of the Where, for instance, an ageing PF-fired power plant is involved, there aoptions that may be pursued. Clearly, if the plant currently has liteffective control systems, as may be the case in some of the develooption is to retrofit off-the-shelf equipment, available on a turn-keyequipment vendors. But, if the plant is nearing the end of its worklow overall efficiency, it may be more cost-effective to re-power it wefficient PF technology or a system based on fluidised bed comb

technology; this is a particularly attractive option for older plants anincreasing power demands. Such options can be effective for significplants efficiency, as well as reducing its environmental impact. Wheefficient power plant is involved, it may be more appropriate to upgrexisting emissions control systems, or to replace them with more eAgain the replacement of the existing combustion plant may also


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MAJOR POLLUTANTS AND ME

FOR THEIR CONTROL

Sulphur dioxide (SO2). This is a major precursor for acid rain fo

technical solutions have been developed and are widely appliedof control system are used, one working internally and the ocases, these systems remove SO2 from combustion gases exitingas desulpurisation (FGD) systems operate within existing duplant, and are capable of reducing SO2 emissions, typically byplant is involved, FGD systems based on scrubber technologiare more efficient and can achieve reductions of up to more thextracted SO

2

can be commercialised for use in producing gypsuch as Germany, all major power plants are equipped with somHowever, globally, application is patchy, especially in some of

Large FGD Unit Fitted to Power Plant in JapanFigure 5


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Nitrogen oxides (NOX). NOXs are also involved in the formation of aas contributing to the formation of urban smog. In industrialised from power plants are today captured on a broad scale. There are types of technique for controlling and minimising NOX formation.plant, NOX can be controlled through primary measures such as airand other combustion modifications. Special designs of low-NO

retrofitted, resulting in NOX reductions of up to some 60%. A tecreburning can also be applied, whereby natural or coal-derived gathe main combustion zone in such a way that NOX is broken downitrogen; reduction levels of up to 70% can be attained. There adownstream NOX control measures which rely on the injection of into the flue gases. These are termed selective catalytic reduction (SCnon-catalytic reduction (SNCR). Such techniques can reduce NOXto 90%, but they are more expensive than other control measures.

Particulates. Coal combustion inevitably produces small particles anmajor pollutants, numerous national and international limits are ithe levels emitted into the atmosphere. Several main types of techncontrol particulate emissions from coal-fired power plants and large indThese are described below.

Electrostatic precipitators (ESPs). These units rely on the trans

charge to particles suspended in a gas stream and their subsequeelectric field to a suitable collecting electrode. They are widely plants and are capable of achieving collection efficiencies of mor

Fabric filters. Here, particles carried in a gas stream are retainpasses through multiple filter bags manufactured from hsynthetic fibres, usually at temperatures of up to some 300

oC. Fa

found growing application for both utility and industrial uses.

Wet particles scrubbers. A large number of variants (foam, film, spare available, most based on the use of a liquid medium toparticulates. They are used widely for industrial coal-fired applicatibeen used in high-temperature and pressure applications, as in IGCfluidised bed combustion (PFBC) plant. In some cases, particulatcombined with the removal of other species such as SO2, HF and


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CONTROL OF OTHER POLLUTAN

Apart from emissions of SO2, NOX and particulates, there has over recent years in controlling and minimising emissions of a nuproduced from coal-fired power plant in much smaller amou

trace elements from such plant are generally low and are not ror requiring control through the adoption of appropriate conare being made, however, to control emissions of mercury, tmost concern.

Coal-fired power plants and waste incinerators are responsibleanthropogenic mercury emissions. Mercury is difficult to cother trace metals that tend to be present as particulates, it i

vapour (in either elemental or ionic form). As a low concentrathe range 5-20 g/m

3), much of it passes through particulate

fabric filters and ESPs. Although such conventional control sypercentage of mercury (ESPs can remove roughly 24%, fabriadditional control measures are required in order to achieve Other emission control systems may also be effective in capturipresent. For instance, while FGD scrubbers are used to contr

is also increasing interest in using these to remove simultaneometals, including mercury (average mercury removal is arounsystems used to minimise NOX emissions have also proved to mercury levels. In a few countries, legislation limiting mercadopted, and levels emitted from coal-fired power plant fall wNevertheless, future implementation of more stringent envmay require lower mercury levels and thus remedial action.

A number of mercury capture methods are presently being waste incinerators. But, although new control systems are cuspecifically for coal-fired power plant, these are not yet being For instance, through its Fossil Energy Program, the UnitedEnergy (US DoE) is funding investigations into a range of developing more effective options that will reduce emission


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devices, but PM2.5s are considerably more difficult to control. PMoften be a complex mixture of pollutants emitted directly from combwhereas, in other cases, gases like SO2, NOX and volatile organic cominteract with other compounds in the air to form fine particlescomposition is defined largely by the sampling methods adopted andcriteria. This is an area of on-going debate and development. PM2.5s ha

with a number of harmful effects on human health and on ecosycontributing to atmospheric haze and power plant plume opacity. Tphysical properties of PM2.5s can vary significantly between regionoften containing varying levels of sulphate (from SO2), organic and enitrate and crustal materials derived from soil dust. In the United Sestimated that annually, roughly 138 000 tonnes of PM2.5s are emfired plants, accounting for some 3% of the countrys total primary P

R&D efforts to provide greater understanding of the issues involvvarious projects are underway, notably: data collection to better establand levels of PM2.5s in air samples; data collection to shed light on forand composition of fine particulates from coal-fired systems and thequality; and development of improvements in PM2.5 control techn

PRODUCTS FROM COAL CO-FIRINWITH OTHER MATERIALS

Increasingly, coal is being co-fired with different materials such as bstreams. Pulverised fuel, fluidised bed combustion and gasification-bhave all been applied to co-firing a range of materials that inclu

miscanthus, sewage sludge, refuse-derived fuel (RDF), agricultural waand tyre-derived fuel. Several major programmes sponsored by the Euroand US DoE have investigated the co-combustion and co-gasificabiomass and wastes. Presently, a number of commercial-scale power plaup to a scale of 635MWe) are operational using a number of co-combuF i PF l i l d di d b f d


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plants. When the exhaust gases are cooled, dioxins can beappropriate, rapid cooling and further cleanup stages are intrprior to release into the atmosphere. Many questions associatedof dioxins are still issues for debate, and further investigation

CO2 CONTROLMany systems for controlling the pollutants noted above are alin different parts of the world. In parallel with the on-going drand lower costs for such control systems, however, efforts increasingly on the control and minimisation of CO2 emittedplants and other large industrial processes. Several possible ro

objective.

Improved plant efficiency. Increased plant efficiency means t(producing less CO2) for the same power output. This may beadvanced development of existing (PF) plant, for instance, by aconditions (Figure 6). At present, the average thermal efficiencountries is roughly 36% (Lower Heating Value). Howevedeveloping world, efficiencies are much lower. Clearly, a signamount of CO2 emitted could be achieved by bringing sucstandard. The latest developments in PF technology have puabove 40%, which means a drop in the level of CO2 emitted

The Avedore Power Plant in DenmarkFigure 6


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Alternatively, a different form of coal-fired technology may be adopcirculating fluidised bed combustion (CFBC) power plants are often clow grade coals much more efficiently than a corresponding PFmore advanced coal-based systems such as PFBC and IGCC combineand gas turbine-based cycles, resulting in higher efficiencies than power plant. The levels of CO2 emitted are correspondingly lower th

PF facilities.CO2 capture. The main approach to controlling CO2 emissions is to cacombustion flue gases. Some types of CO2 capture technologies (basedand physical absorption) are well established and have been in use foThe majority of chemical-based methods rely on scrubbing systems tsolutions to remove CO2 from exhaust gases. Amine scrubbers have alrto different types of coal-fired industrial process and power station. I

systems used are similar in concept and configuration and usually empamine, such as monoethanolamine (MEA) as the working solvent. Dparticular application and type of flue gas being treated, such systemto 98% of the CO2 present, and produce a CO2 stream of up to 99% pumany processes have relied on MEA. Recently, however, more advanbeen developed, for instance by Mitsubishi Heavy Industries (MHI), aapplied commercially. Such new amines are claimed to suffer less de

have lower consumption rates and energy requirements than conventsolvents; significant improvements in performance have been technological developments have been instrumental in both improvinand reducing operational costs.

A number of commercial-scale physical absorption-based technologigenerally applied to systems operating at higher pressures. These rsolvents that include methanol and propylene carbonate. For IGCC appl

based on the use of proprietary solvents such as Union Carbides Selexto be the most applicable. Such solvents are favoured where high concare present in the flue gas stream. They also impose low energy reqsystem. In the United States, Selexol-based systems have been demonstrCool Water IGCC plant and used commercially at the Destec-based PlIn general further development of physical solvent based systems would


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RD&D REQUIREMENTS

AND DEVELOPMENTS

Since coal-fired power plants are likely to remain dominant in tfurther technical development will be a crucial factor foperformance, energy security and environmental protection. Tby the adoption of increasingly stringent legislation, and thecompete in what is often a highly competitive market. Importataking place are described below.

For PF technology, a major effort is on-going to develop reliab

for increasing the steam conditions in place. The applicationconditions, focusing on boilers capable of operating at stea300 bar, at 600

oC/620

oC, is resulting in higher plant effici

well as reducing fuel costs and levels of CO2, SO2 and NOX eof improved construction materials is playing a major role idevelopments are also taking place with emissions control systeFor instance, SO2 control is being tackled by improving existdesulphurisation efficiency and plant reliability, while reducand costs) and the development of innovative technologies electron beam treatment and use of dry sorbents. NOX-related isthrough the continuing development of improved low NOXair and fuel staging techniques, plus the evolution of advWith SCR-based NOX control systems, efforts are being mcosts and improve their lifetime and performance. Improvemparticulate capture technologies include enhanced designs o

systems, as well as hot gas cleanup technologies, based on the uceramic and metallic materials. Novel control strategies fomercury are also being pursued.

There are a number of major coal-fired IGCC plants nodemonstration or pre-commercial mode. This technology curr


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With both PF and IGCC technologies, the increasing importance oapplying advanced emissions control systems has been recognized. controls are becoming an essential component in the increasingenvironmental control systems now coming into use. Such systems optimisation and integration of boiler operating conditions and emissthereby increasing plant efficiency and minimising environmental imp

approximately 35 PF power plants (representing roughly 20 000 MWnow been equipped with an artificial intelligence-based system knowNOX Control Intelligent System (GNOCIS). Application of GNOCreduced NOX levels and amounts of unburned carbon in flyash, as overall plant efficiency. Similar systems addressing other pollutantsof pollutants are under development.

CONCLUDING REMARKS

A number of important configurations of coal-fired power plant exown advantages, disadvantages and requirements for further developthis, there are also a number of systems and techniques for the congenerated from each, some being specific to a particular type of po

being equally applicable to several variants. Systems for the control ofsuch as SO2, NOX and particulates are available commercially andalthough, in all cases, efforts remain on-going to deliver further proceand reduce associated costs. Consideration is also being given increasinof various trace elements present. Technological progress is a withstanding the pressure of increasingly competitive energy maprecondition for meeting future obligations in response to environHowever, an additional requirement that now needs to be consminimising emissions of CO2 into the atmosphere. This can be aincreased process efficiency and/or the application of CO2 capture teconly limited application of the latter has occurred. There is thereforcapture technologies to be optimised for large-scale power plant apassociated costs to be reduced significantly and, ideally, for integration


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