coal gasification and its applications

A challenge for clean coal technology is to produce power andsequester carbon dioxide at a price that is competitive with alternative power sources.Integrated gasification combined cycle

COAL GASIFICATION AND ITS

APPLICATIONS

DAVID A BELL

BRIAN F TOWLER

MAOHONG FAN

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The product of gasification is syngas, which is primarily a mixture of carbonmonoxide and hydrogen Most syngas, however, is not currently made by gasification,but rather by the steam reforming of natural gas In this process, steam and natural gas arefed to catalyst-packed tubes, which are held inside a furnace to provide the endothermicheat of reaction.Figure 0.1also shows other gases, which can be blended with syngas forfurther processing One such gas under consideration is hydrogen, which can beproduced by electrolyzing water using off-peak power from a nuclear power plant In

a few cases, carbon dioxide from an external source may supplement the carbonmonoxide in syngas

Just as coal is not the only feedstock for gasification, gasification is not the only use ofcoal Most coal is burned to produce electric power Chapter 2 describes a few of thenon-gasification uses of coal

Gasification is described in Chapters 3, 4, and 5 Chapter 3 describes gasification as

a chemical reaction system Although this chapter may look complex, our knowledge ofthe chemistry of gasification is far from complete Chapter 4 covers several gasifier designs.These designs were selected because they are now in commercial use or development, orbecause they illustrate interesting concepts One gasification approach is sufficientlydifferent that it deserves its own chapter, underground coal gasification, covered inChapter 5 Instead of mining coal and transporting it to a gasifier, the coal is left in placeunderground, and the reactant gases are brought to the coal Deeply buried coal seams,which are uneconomic to mine, may be exploited by underground coal gasification

Syngas leaving the gasifier contains numerous impurities The inorganic fraction ofthe feedstock leaves as solid ash or molten slag Ash or slag removal is usually an integralpart of the gasifier design If the gasification occurs at relatively low temperatures, thentar will be produced Tar removal is also an integral part of gasifier design Higher-temperature gasifiers do not produce significant tar The syngas also contains sulfur in the

ix

form of H2S, with lesser quantities of COS Sulfur must be removed from syngas either

to prevent emission of SO2when syngas is burned, or to prevent catalyst poisoning indownstream reactors Sulfur removal is described in Chapter 6

Carbon dioxide removal can occur either as a part of impurity removal, or after watergas shift, as shown inFigure 0.1 The traditional carbon dioxide removal techniques areclosely related to sulfur removal, and are described in Chapter 6 The ability to removecarbon from syngas and sequester it in a geological formation is one of the majorattractions of coal gasification This allows coal to be used while minimizing greenhousegas emissions A major objection to this approach is that carbon capture and sequestrationare expensive This prompted a great deal of research into new carbon dioxide separationtechnologies, and which is described in Chapter 10

Syngas contains a number of minor impurities, and one of the more significant ismercury, a neurotoxin Removal of mercury is discussed in Chapter 9

For some applications, a nearly pure hydrogen stream is desired In others, such asmethanol synthesis, a specific ratio of carbon monoxide to hydrogen is required In either

coal petcoke biomass natural

gas

other gas

water gas shift

CO2removal

steam sequestration

Syngas processing

hydrogen, electric power

ammonia, nitrogen fertilizers

methanol, dimethyl ether, hydrocarbons

Products

substitute natural gas, Fischer-Tropsch hydrocarbons

Figure 0.1 Gasification and related technologies.

case, the gasifier usually produces a higher ratio of carbon monoxide to hydrogen thandesired This ratio needs to be shifted towards a greater hydrogen content The usual way

to do this is through the water gas shift reaction in which carbon monoxide reacts withsteam to form hydrogen and carbon dioxide, as described in Chapter 7 Hydrogen canthen be burned in a turbine to generate electric power, an application known as inte-grated gasification combined cycle This is a means of producing electric power fromcoal with minimal greenhouse gas emissions

Hydrogen is also a potential transportation fuel The usual approach is to produceelectric power from hydrogen in a fuel cell, and then use that power in an electric motor.One of the main technical obstacles is a practical means of storing hydrogen in a vehicle.Chapter 9 explores hydrogen storage for this application

Nearly all synthetic nitrogen chemicals start as ammonia, synthesized from hydrogenand nitrogen gas Nitrogen fertilizers are, by far, the largest volume synthetic nitrogenchemicals Chapter 11 describes ammonia synthesis and some of the more commonnitrogen fertilizer compounds

Methanol is a major commodity chemical made from syngas, as described inChapter 12 Methanol is an intermediate used to make a wide range of products One ofthese, dimethyl ether (DME), is especially interesting DME can be used as a fuel orconverted to hydrocarbons, including gasoline and olefins for polymer production

Chapter 13 describes the direct conversion of syngas to hydrocarbons, includingsubstitute natural gas (methane) and Fischer-Tropsch liquid, a synthetic crude oil TheFischer-Tropsch liquid is then refined to meet petroleum product specifications

Coal is an inexpensive feedstock, but gasification-based plants tend to have very highcapital construction costs In concept, one could build a single plant that wouldincorporate all of the elements shown inFigure 0.1, but such a complex plant would beextraordinarily expensive to build Instead, gasification-based plants have a more limitedset of features dictated by economics and the regulatory environment

There are two major trends that prompt current interest in coal gasification The first

is the widely held belief that conventional petroleum supplies are declining, whiledemand for transportation fuels continues to rise This has led to heightened interest inalternative energy supplies, including coal The second major trend is concern aboutglobal warming Gasification offers a relatively cost-effective means of using coal whileminimizing greenhouse gas emissions

CHAPTER 1

The Nature of Coal

Contents

THE GEOLOGIC ORIGIN OF COAL

Coal is fossilized peat A peat bog is a marsh with lush vegetation Plant matter dies andfalls into the water, where partial decomposition occurs Aerobic bacteria deplete thewater of oxygen, and bacterial metabolic products inhibit further decomposition byanaerobic bacteria Plant matter accumulates on the marsh bottom faster than itdecomposes, and, over a period of many years, a layer of peat forms The peat thatbecame today’s coal was laid down millions of years ago

Buried peat is converted to coal when high pressure and elevated temperature isapplied to the buried layer This process is known as coalification The physical andchemical structure of the coal changes over time As shown inFigure 1.1, the youngest(least converted) coal is known as lignite, which can be further converted to sub-bitu-minous coal, bituminous coal, and finally anthracite These coal types strongly influence theproperties and use of coal, and will be discussed further

Peat Lignite Sub-bituminous Bituminous Anthracite

Increasing age, conversion

Figure 1.1 Coalification.

1

Petrography is the visual inspection of a rock sample to determine the mineral types inthe sample When applied to coal, the different coal types are known as macerals.Table1.1lists coal macerals, and shows how they are derived from plant material.

COAL ANALYSIS AND CLASSIFICATION

Coal is used primarily as a fuel, so its most important property is its heat ofcombustion Gross calorific value, also known as higher heating value (HHV), is determined

by measuring the heat released when coal is burned in a constant-volume calorimeter,with an intitial oxygen pressure of 2 to 4 MPA, and when the combustion products arecooled to a final temperature between 20 and 35
C (ASTM D 5865-04) The testsmentioned in this book are primarily based on the American Society for Testing andMaterials (ASTM) specifications.1 Coal is a variable, widely distributed and widelyused material so a wide range of standard tests have been developed by a variety ofindividuals and organizations

Coal is a porous medium, and these pores, especially in low rank coals, can containsubstantial quantities of water even though the coal appears to be dry The water is eitheradsorbed onto hydrophilic surface sites or held in pores by capillary forces When thismoist coal is burned or gasified, a substantial fraction of the combustion heat is requiredTable 1.1 Coal macerals, based on ASTM D121-05 and ASTM D 2799-05a 1

Maceral

Vitrinite Vitrinite Woody tissue of plants (cellulose,

lignin)

Most common maceral

materials Cutinite Waxy coating (cuticle) of leaves, roots

and stems Resinite Plant resins Sporinite Spores and pollen grains Intertite Fusinite Some structures of plant cell wall still

visible

Derived from strongly altered and degraded peat

Inertodentrinite Fragments incorporated within other

macerals.

Macranite No plant cell wall structure, larger than

10mm Micranite No plant cell wall structure, less than 10

mm, and typically 1 to 5mm Funginite Fungi

Secretinite No obvious plant structure, sometimes

containing fractures, slits or notch.

Semifusinite Like fusinite, but with less distinct

evidence of cellular structure.

to vaporize water Since the final temperature in the gross calorific value test is 20 to

35
C, most of the water is condensed, thereby recovering the heat of vaporization.

Water in the HHV test is primarily a non-combustible diluent For example,

a Wyoming Powder River Basin coal typically has an HHV of 19.8 MJ/kg (8500Btu/lb) and a 28% moisture level One can then calculate an HHV value for the coal if

it is dried:

HHV ; dry ¼ 19:8 MJ=kg

1 0:28 ¼ 27:5

MJkg

LHV ; moist ¼ 19:8 MJkg coal 2:395kg water MJ 0:28kg waterkg coal

is burned Fixed carbon is the fraction of coal that is not moisture, volatiles, or ash.Fixed carbon, which is mostly carbon but can contain other elements represents thecombustible portion of the coal char that remains after the volatiles have beenremoved

Proximate analysis results are sometimes reported on a dry mineral matter-free basis.Mineral matter is calculated using the following equation:

The 1.08 factor presumes that minerals in the coal are hydrated This water of hydration

is lost when the coal is burned The 0.55 factor assumes that sulfur is present as pyrites,which in many areas are converted to the corresponding oxides during combustion.Ultimate analysis (ASTM D 3176) describes coal in terms of its elemental composi-tion For a dried coal, weight percentages of carbon, hydrogen, nitrogen, sulfur, and ashare measured The remainder of the coal sample is assumed to be oxygen

COAL RANK

In the coalification process, the coal rank increases from lignite to anthracite, as shown in

Figure 1.1 Coal rank is useful in the market, because it is a quick and convenient way todescribe coal without a detailed analysis sheet A more detailed description of coal rank isshown inTables 1.2 and 1.3

Bituminous and sub-bitumous coals are the primary commercial coals A relativelysmall amount of anthracite is available In the USA, anthracites are produced only in north-eastern Pennsylvania Lignites are abundant But the economics of hauling a low-gradefuel long distances are unfavorable; so most lignite is consumed close to where it is mined.Peat is also mined and generally used close to where it is mined Peat may be eitherconsidered old biomass or very young coal In nations that regulate greenhouse gasemissions, the difference between the two is more than mere semantics Carbon dioxideemissions from biomass combustion are not considered a contributor to global warming,because these emissions are offset by carbon dioxide uptake by growing biomass On theother hand, the same emissions from fossil fuels, are restricted Emissions from peatcombustion are a regulatory gray area

Some coal, particularly bituminous coal, has the tendency to cake With increasingtemperature, coal particles simultaneously pyrolize and partially melt, causing the coalparticles to stick to one another Some gasification reactors, especially moving bed andfluidized bed gasifiers, are limited to processing coal that does not cake

Table 1.2 Classification of anthracitic and bituminous coals by rank (ASTM D 388-05) 1

greater than Lessthan Greaterthan Equal orless than

ASH THERMAL PROPERTIES

The melting temperatures of coal ash impose temperature limits for coal gasification.Fluidized bed gasifiers and dry-bottom moving bed gasifiers, such as the Lurgi gasifier,require free-flowing ash The maximum operating temperature for these gasifiers is theinitial deformation temperature When the temperature rises above the initial defor-mation temperature the ash becomes sticky Fluidized bed gasifiers often run near theinitial deformation temperature to maximize carbon conversion

Entrained flow gasifiers and slagging moving bed gasifiers such as the BGL gasifierrequire a fluid slag, so they must operate at a sufficiently high temperature to comp-letely melt the ash Operation at significantly higher temperatures increases oxygenconsumption

Ash is a complex mixture of minerals, which will cause the coal ash to melt over

a temperature range rather than at a fixed temperature Temperatures in this range arespecified by ASTM D-1857-04 A coal ash cone, 19 mm high and with an equilateraltriangle base 6.4 mm on each side, is placed in an oven Temperatures are reported forreducing or oxidizing gas environments The initial deformation temperature (IDT) occurswhen rounding of the cone tip first occurs The softening temperature (ST) occurs whenthe cone has fused to produce a lump which has a height equal to its base The hemi-spherical temperature (HT) occurs when the lump height is half the length of its base Thefluid temperature occurs when the fused mass has spread out in a nearly flat layer with

a maximum height of 1.6 mm

A number of researchers have attempted to correlate ash thermal properties withash composition The most extensive effort was by Seggiani and Pannocchia,2 whocorrelated the behavior of 433 ash samples, based on nine elemental concentrations

Table 1.3 Classification of bituminous, sub-bituminous and lignite coals by rank (ASTM D 388-05) Note that high volatile A bituminous coal is the only rank that is listed in both Table 1.2 and Table 1.3.

High volatile B bituminous coal 13 000 14 000 30.232 32.557 High volatile C bituminous coal 11 500 13 000 26.743 30.232

Note that mineral elemental compositions are reported as if the mineral sample were

a blend of simple metal oxides For example, the fraction of aluminum in a sample istypically reported as the equivalent weight percent of Al2O3 Seggiani and Pannocchia’scorrelations are based on mole percents, rather than weight percents, on a normalized,

SO3-free basis

The correlation for initial deformation temperature is given as:

IDT;
C ¼ 2; 040 exp

0:1

2SiO2þ Fe2O3þ CaO þ MgO

2

þ 83:4P2O5

þ 2:12 Al2O3þ 39:3TiO2þ 0:335 ðFe2O3Þ2þ 0:118 ðAl2O3Þ2

þ 0:135 ðCaOÞ2 0:116 ðSiO2Þ ðFe2O3Þ þ 0:0768 ðSiO2Þ ðAl2O3Þ

þ 0:533 ðFe2O3Þ ðCaOÞ þ 2:42

SiO2

ST;
C ¼ 5; 360 exp

0:1

2SiO2þ Fe2O3þ CaO þ MgO

2

þ 91:3 P2O5

þ 0:282 ðFe2O3Þ2þ 0:178 ðCaOÞ2þ 0:939ðMgOÞ2þ 0:630 ðFe2O3Þ

 ðCaOÞ  1:03 ðFe2O3Þ ðMgOÞ þ 2:34

SiO2

The correlation for hemispherical temperature is given as:



þ 0:285

SiO2

Al2O3

2

þ 910exp

0:1

Fe

2O3þ CaO þ MgO þ K2O þ Na2OSiO2þ Al2O3þ TiO2þ P2O5  1

2

þ 41:9

Fe

2O3þ CaO þ MgO þ K2O þ Na2OSiO2þ Al2O3þ TiO2þ P2O5

2

þ 86:4

Fe

2O3þ CaO þ MgO þ K2O þ Na2OSiO2þ Al2O3þ TiO2þ P2O5  1

2

þ 216

2SiO2þ Fe2O3þ CaO þ MgO

2SiO2þ Fe2O3þ CaO þ MgO

2

þ 6:13Al2O3

þ 58:0TiO2 13:8MgO þ 0:259 ðFe2O3Þ2þ 0:278 ðAl2O3Þ2

þ 0:736 ðMgOÞ2þ 0:259 ðFe2O3Þ ðCaOÞ  0:730 ðFe2O3Þ ðMgOÞ

þ 2:03

SiO2

2

þ 231

2SiO2þ Fe2O3þ CaO þ MgO

2

 1; 340

Eqn 1.7

The temperature of critical viscosity, Tcv, is not part of the ASTM D1857 test but

it is important for slagging gasifiers because it marks the transition of slag from a

difficult-to-handle Bingham plastic, below Tcv, to a more easily handled Newtonianfluid, above Tcv The correlation for temperature of critical viscosity is given as:

Tcv;
C ¼ 935P2O5þ 4:11Al2O3þ 2; 580ðP2O5Þ2þ 0:254ðAl2O3Þ2

 0:139ðNa2OÞ2þ 0:108 ðSiO2Þ ðFe2O3Þ þ 0:0377 ðSiO2Þ ðAl2O3Þ

þ 14:0

SiO2

Al2O3



þ 3:05

SiO2



 113

Fe

2O3þ CaO þ MgO þ K2O þ Na2OSiO2þ Al2O3þ TiO2þ P2O5

2

 5:48 ðNa2OÞ

Fe

2O3þ CaO þ MgO þ K2O þ Na2OSiO2þ Al2O3þ TiO2þ P2O5

88oC.Table 1.4compares experimental results for four American coals from Baxter3tothe temperatures predicted by these correlations The predicted results are very close tothe experimental results for the lignite and the sub-bituminous coals The exception isthe predicted temperatures are substantially higher than the experimental values for thebituminous coals

Inorganic additives have been added to coal gasifiers to modify ash thermal erties For example; alkaline materials such as sodium, potassium and calciumcompounds tend to lower ash melting temperatures These can be added to an entrainedflow gasifier to lower slag viscosity Care must be taken with refractrory-lined gasifiers,because these compounds may attack the refractory The opposite approach was taken byvan Dyk and Waanders.4They sought to increase the ash fusion temperature (ISO 540and 1195E) to allow higher temperature operation in a Lurgi gasifier Tests with Al2O3,TiO2, and SiO2showed that Al2O3was most effective Addition of 6 weight % Al2O3boosted the ash fusion temperature of a mixture of South African coals from 1,340
C togreater than 1,600
C.

prop-Ash is typically land-filled If the landfill is unlined, then water percolating throughthe ash pile may affect surface and groundwater quality Many ashes are alkaline and there

is the possibility that toxic heavy metals in the ash may be leached by rainwater Slagginggasifiers produce glassy, non-leachable slag

Some coal ashes are pozzolanic, which means that they tend to set up like cementwhen mixed with water These ashes are often used as road base High calcium ash has ananalysis that is similar to commercial cement Pozzolanic ashes are less likely to poseleachate problems than unconsolidated ashes

COAL AS A POROUS MATERIAL

Coal is a porous material.5Pores are classified as macropores (greater than 50 nm), whichare measured using mercury porosimetry, mesopores (2.0 to 50 nm), measured by nitrogenadsorption at 77 K, and micropores (0.4 to 2.0 nm), measured by carbon dioxideadsorption at 298 K Micropores are due to the voids from imperfect packing of largeorganic molecules Coals typically have surface areas in the range of 100 to 400 m2/g,

Table 1.4 Coal ash thermal properties: comparison of experimental values to values predicted by the Seggiani and Pannocchia correlations.

Coal rank Beulah lignite Wyodak sub-bituminous Pittsburgh No 8bituminous Illinois No 6bituminous

Mineral Moleweight Weight% Mole% Weight% Mole% Weight% Mole% Weight% Mole%

Al 2 O 3 101.94 13.968 12.666 14.218 11.562 20.657 17.262 16.904 13.348 CaO 56.08 16.358 26.963 24.845 36.725 2.085 3.167 5.180 7.435

which is due almost entirely to micropores For comparison, a typical atomic diameter is0.25 nm, so only small molecules may penetrate micropores Olague and Smith6studiedgas diffusion in coal.

SPONTANEOUS COMBUSTION

Coal oxidizes when it is exposed to oxygen at ambient conditions Low grade coals areespecially prone to low temperature oxidation The effect of long-term air exposure oncoal quality is known as weathering Oxidation at low temperatures is exothermic,resulting in increased temperatures that accelerate the rate of coal oxidation Thissometimes leads to spontaneous combustion of coal

Itay et al.7studied low temperature oxidation of South African coal They found thatthe quantity of oxygen adsorbed was greater than the quantity of oxygen-containingproduct gasses (CO2, CO, H2O), so most of the adsorbed oxygen remains in the coal.Other investigors8,9found an increase in carboxylic acids in weathered coal Itay et al.found that oxygen uptake with repeated oxygen exposures declines This same effect isshown inFigure 1.210 Small particles tend to oxidize faster than large particles, but theparticle size effect is not large This suggests that the rate of oxidation is limited by oxygendiffusion in coal micropores or by surface reaction rates

As-mined low grade coals typically have high water content, and the water-filledpores tend to block low temperature oxidation As shown in Equation 1.1, the heatcontent of these coals can be greatly increased by drying Unfortunately, the dry coalscannot be safely stored or shipped under ambient conditions due to their tendency tospontaneously combust Exposure of dry coals to high humidity or liquid water

Figure 1.2 Low temperature oxidation of about 1.5 g dried, sub-bituminous, Adaville coal from Kemmerer, Wyoming in a microcalorimeter10 Note that the heat released diminishes with repeated oxygen exposures.

accelerates the rate of low-temperature oxidation, possibly because water adsorption

on coal is exothermic Processes have been developed11,12 to convert high moisture,low grade coals to low moisture fuels with reduced spontaneous combustiontendencies

RESERVES, RESOURCES, AND PRODUCTION

Throughout history coal has played a very small role in the world’s energy mix Locally

it was a curiosity because it was an interesting rock that could be made to burn.However, commencing in about the year 1500, it began to be mined for small scaleenergy use in England and Germany When the Industrial Revolution dawned inEngland in the eighteenth century coal became a significant energy source that fueledthe English factories that were the hallmark of the Industrial Revolution However, interms of total energy use biomass (particularly wood) remained the major source ofenergy for the world until about 1900, when coal overtook biomass as the chief worldenergy source In the United States, where coal was abundant and easier to mine, ithad become the chief energy source in about 1880 Throughout the first half of thetwentieth century coal was the major world energy source, until it was overtaken by oil

in about 1960 Even though coal production has continued to increase since then itremains in second place behind oil and just ahead of natural gas This is illustrated inFigure 1.3

In the near future it is likely to remain in the second position until oil productionpeaks and starts to decline It is conceivable that when this happens coal will again

Figure 1.3 World energy sources since 1800.

become the most consumed energy source in the world In terms of energy reservesthe world has much more coal than any other energy source Some might argue thatcoal production will be restricted because of the amount of CO2that it produces But

as we learn to capture and sequester the CO2economically, coal production will likelycontinue to increase

The entire quantities of coal present, regardless of the cost or practicality ofrecovery, are known as resources A 5 cm thick layer of low quality coal buried under

2000 m of overburden contributes to total resources, but it is unlikely that it will ever bemined A more practical measure of the quantities of coal available are known as reserves,which is the subset of resources that can be mined at current prices using currenttechnology There are an estimated13 275 billion tons of coal reserves in the UnitedStates, compared to approximately 4 trillion tons of coal resources Table 1.5 showsestimates of worldwide reserves, recent annual production and reserve/production (R/P) ratios, which is the number of years these reserves will last at current productionlevels.14

Table 1.5 Worldwide coal production and reserves 14

Region and selected

countries 2006 coal production,million short tons 2003 coal reserves,million short tons Reserves/production,years

It does not mean that the world will run out of coal in the next 148 years Forexample, Luppens, et al.15 studied coal reserves and resources in the Gillette CoalField, a 5,180 km2 portion of the 57,000 km2 Powder River Basin in northeasternWyoming and southeastern Montana Coal is abundant throughout the Powder RiverBasin, but mining is restricted to coal with the lowest mining cost Most mining inthe basin occurs within the Gillette Coal Field, where thick coal seams lie close to thesurface There are multiple coal beds and the thickest of these, the Anderson, has anaverage 15 m thickness The beds dip slightly from east to west, so coal is producedfrom 13 strip mines along a 78 km northesouth line along the eastern edge of thedistrict These 13 mines produced over 42% of the coal produced in the USA in

2007.14

Because of the abundance of this coal, and because of its low mining cost, the openmarket mine mouth price14 for this sub-bituminous coal in 2007 was $9.67/ton;compared to $47.63/ton for bituminous coal from West Virginia As shallow coal in theGillette Coal field is depleted, mining moves to the west, with gradually increasingoverburden and gradually increasing mining cost When the overburden becomes toothick for strip mining, underground mining may be used to further extract coal Luppens

et al estimated the volume of coal available in the Gillette Coal Field versus price, andthese data are shown inFigure 1.4 At the 2008 production rate of 464 million tons/year,the coal reserves in the Gillette Coal Field will last only 21 years if coal is priced at $10/ton If the price of coal rises to $60/ton, then the coal will last 176 years at the 2008production rate Of course, the Gillette Coal Field is only one mining district, amongstmany scattered throughout the world As mining costs in the Gillette Coal field rise,mining will shift to other portions of the Powder River Basin and to other coal prov-inces On a worldwide basis, coal will be available for a very long time, but coal prices areexpected to gradually increase due to increasing mining costs

Coal reserves, millions of tons

Figure 1.4 Coal reserves versus cost in the Gillette Coal Field 15

In-situ coal gasification is the process of partially burning a coal seam in place toproduce a synthesis gas This technology may greatly expand coal reserves, becausedeeply buried coal seams may be exploited at a reasonable cost In-situ coal gasificationhas its own set of issues, however, which will be discussed in Chapter 4.

We cannot continue to mine coal until we completely exhaust the resource Otherphenomena will curb mining before then One concept, popular in peak oil discussions,

is called Energy Returned on Energy Input (EROEI) This concept states that if theenergy consumed in producing energy is greater than energy produced, then energyproduction will halt regardless of price Coal production will probably never reach theEROEI limit Instead, the limit to coal mining will be set by the principle of economicsubstitution

Wooly mammoths were once a major human food source When mammoths werehunted to extinction during the last ice age, our ancestors did not starve Instead, theyfound something else to eat This is an early example of economic substitution.Coal initially became popular when the growing human population and increasedurbanization made wood scarce The technology of coal use developed to the pointthat coal became a major source of chemicals, fuel gas and transportation fuel Thesecoal uses fell out of favor when natural gas and crude oil became abundant andinexpensive in the mid twentieth century Current interest in coal technology is largelythe result of rising natural gas and crude oil prices and concern about future energysupplies

At the time this was written, almost all coal consumed in the industrialized nationswas burned to produce electric power In the near future, coal-burning power plants willprobably be required to capture and sequester carbon dioxide to reduce global warming.When this happens, coal may no longer be a low cost fuel for power generation Theestimated cost16,17of pulverized coal-fired power production with carbon capture andsequestration is about double the cost of pulverized power production without carboncapture and sequestration Residential power customers would see a 50% increase intheir power bills, assuming that distribution costs would change

At this price, non-fossil fuel sources of electric power, such as nuclear and windpower, are attractive Currently, solar power is too expensive, but advancing technologypromises to lower costs A challenge for clean coal technology is to produce power andsequester carbon dioxide at a price that is competitive with alternative power sources.Integrated gasification combined cycle (IGCC) coal plants with carbon capture andsequestration (CCS) have attracted a great deal of interest because the cost of electricpower from these plants is only 60% higher than conventional pulverized coal generatedelectricity.16The cost and complexity of replacing coal-fired power plants is enormous,

so coal-fired power plants will be a substantial source of electric power for many years.Coal has a bright future as a raw material for liquid fuels, fuel gas, and chemicals.Projected crude oil prices suggest that liquid fuels will be produced from coal at a lower

cost than the same fuels produced from crude oil Coal technology, and in particular, coalgasification, will have a large role in energy production for many years.

3 Baxter L Brigham Young University Coal database, www.et.byu.edu/%7Elarryb/CoalDatabase.htm

4 van Dyk JC, Waanders FB Manipulation of gasification coal feed in order to increase the ash fusion temperature of the coal enabling the gasifiers to operate at higher temperatures Fuel 2007;86:2728- 2735.

5 Gan H, Nandi SP, Walker L Nature of the porosity in American coals Fuel 1972;51:272-277.

6 Olague NE, Smith DM Diffusion of gases in American coals Fuel 1989;68:1381-1387.

7 Itay M, Hill CR, Glasser D A study of the low temperature oxidation of coal Fuel Proc Tech 1989;21:81-97.

8 Yun Y, Meuzelaar HLC Development of a reliable coal oxidation (weathering) indexdslurry pH and its applications Fuel Proc Tech 1991;27:179-202.

9 Hayashi J, Aizawa S, Kumagai H, et al Evaluation of a brown coal by means of oxidative degradation in aqueous phase Energy & Fuels 1999;13:69-76.

10 Balasubramani R Calorimetric Investigation of the kinetics of low-temperature oxidation of dry coal, M.S Thesis, University of Wyoming (2003).

11 Sethi VK, Dunlop DD A coal upgrading technology for sub-bituminous and lignite coals, < www westernresearch.org/uploadedFiles/Energy_and_Environmental_Technology/Coal/Upgrading_

(Including_Headwaters)/CoalUpgradingFMI.pdf >.

12 Evergreen Energy, K-Fuel and K-Direct, < www.evgenergy.com/fact_sheets/K-Fuel.pdf >.

13 U.S Energy Information Administration, “U.S coal reserves, 1997 Update,” DOE/EIA-0529(97), 1999.

14 U.S Energy Information Administration, < www.eia.doe.gov/fuelcoal.html >.

15 Luppens JA, Scott DC, Haacke JE, et al Assessment of coal geology, resources, and reserves in the Gillette Coalfield, Powder River Basin, Wyoming, U.S Geological Survey, Open-File Report 2008-

CHAPTER 2

Non-gasification Uses of Coal

Contents

HOME HEATING AND COOKING VS INDUSTRIAL USE

The simplest use of coal is to burn it for heat Coal was once used as a household heatingand cooking fuel in Western nations; but it was largely replaced by natural gas, propane,electricity and fuel oil Coal is still used for household heating and cooking in China,where it is a major source of air pollution

In Western nations, coal is used primarily as a fuel for large industrial boilers,especially for electric power generation Large users are able to get more completecombustion, which reduces odor and soot, and are able to install complex and expensiveair pollution equipment Since pollution control equipment strongly influences theconfiguration of a modern coal-burning plant, emissions from coal combustion will bedescribed next

COAL COMBUSTION POLLUTANTS

The US Environmental Protection Agency developed a list of priority pollutants, whichare common air pollutants that are primarily generated by combustion The following is

a partial list of these pollutants

SOxconsists primarily of SO2but may also contain small amounts of SO3 In theatmosphere, SO2 oxidizes to SO3 This combines with water to form sulfuric acid,

H2SO4, the primary acid component in acid rain Combustion of sulfur-containingfuels creates SOx, and coal typically has high sulfur levels compared to other fossilfuels

17

NOx consists of several nitrogen-oxygen compounds that contribute to chemical smog, ozone depletion and global warming There are two primary sources

photo-of NOx Fuel NOx forms when nitrogen-containing fuel is burned Not all nitrogen

in the fuel forms NOx Some of the fuel nitrogen may be converted to N2 Thermal

NOx is created by direct combination of N2 and O2 in a flame Thermal NOx isfavored by high flame temperatures and high oxygen concentrations At ambientconditions, NOxis not thermodynamically stable; but it is very difficult to decomposeonce formed

CO is formed when carbon-containing fuels are burned In a flame, carbon is burned

to form CO; which is then further oxidized, at a slower reaction rate, to CO2 In nearlyall combustion processes, some of the intermediate product, CO, escapes into the fluegas Carbon monoxide emissions are favored by low oxygen/fuel ratios

Particulates are divided into two categories, PM10, which consists of particles less than

10 microns in diameter; and PM2.5, a subset of PM10which consists of particles less than2.5 microns in diameter When inhaled, these particles, especially PM2.5, tend to remain

in the lungs This can lead to chronic health conditions such as black lung in coal miners,silicosis in people who have prolonged exposure to dust and smoker’s lungs Some dust isgenerated when coal is mined, crushed, and shipped When coal is used for homeheating and cooking, the flue gas can contain significant quantities of soot, which is a finecarbon-rich dust In industrial boilers, combustion is more complete and little soot isproduced Particulate emissions are primarily due to fly ash, which are the fine ashparticles entrained in the flue gas

Volatile organic compounds, VOCs, are nearly all organic compounds that have

a significant vapor pressure at ambient conditions In home heating and cookingapplications, VOCs in the flue gas cause disagreeable odors Since combustion is morecomplete in industrial boilers, little odor is produced VOCs are a major issue in organicchemical plants, including coal-to-chemical and coal-to-liquid fuels plants

Air toxics include a long list of specific toxic compounds Coal contains smallquantities of volatile heavy metals; which vaporize during combustion and may leavewith the flue gas Mercury1 has received the most attention Mercury is a neurotoxin,and tends to accumulate in aquatic systems Mercury bio-accumulates, meaning thatlarge fish that eat smaller mercury-containing fish do not excrete the mercury Conse-quently, mercury concentrations are highest at the top of the food chain, including largefish and the people who eat them The US Environmental Protection Agency issued thefirst Clean Air Mercury Rule in 2005

Greenhouse gasses include CO2, CH4, and NOx compounds Because of the largevolume emitted, CO2 has received the most attention Most members of the scientificcommunity believe that global warming is largely due to greenhouse gasses released byfossil fuel combustion Since coal has lower H/C ratios than other fossil fuels, coalcombustion releases more CO2per unit of energy than other fossil fuels

PULVERIZED COAL COMBUSTION

The most common type of coal-fired power plant is pulverized coal combustion (PCC),shown inFigure 2.1 A mixture of pulverized coal and air is blown into a low NOxburner This burner has an annular arrangement Coal and a portion of the air are fed tothe center tube The remainder of the air is fed through the space between the inside andoutside tubes The main portion of the flame has a low oxygen/fuel ratio and a relativelylow temperature, both of which inhibit formation of NOx The additional air oxidizes

CO to CO2 A low NOxburner reduces NOxemissions from about 11 kg to about 5.5

kg per ton of sub-bituminous coal.2

The walls of the furnace have a water wall construction, meaning that side-by-sidetubes are welded together to form a continuous wall Hot combustion gas first risesthrough the boiler section where pressurized water is boiled to make steam Next comesthe superheater section, where the steam temperature is raised above its boiling point.Then the economizer section preheats the boiler feed water Finally there is a rotatingplate exchanger Iron plates rotate into the path of the warm flue gas The warm platesthen rotate out of the flue gas path and into the air path, where the plates preheatcombustion air

Flue gas then enters the selective catalytic reactor (SCR) Ammonia is injected intothe SCR where it reacts with NOx(here shown as NO) to form N2and H2O Thiseliminates 75 to 85% of the NOx

Rotating plate exchanger

Boiler section

Superheater section

Economizer section

flue gas

air

Flue gas to SCR

Figure 2.1 Pulverized coal combustion plant.

6NOþ 4NH3/5N2þ 6H2O R-2.1The flue gas then enters the bag-house, which removes fly ash The flue gas is forced

to flow through a bag filter that captures the fly ash Some power plants use an trostatic precipitator in place of a bag-house In this process the flue gas flows betweentwo electrified parallel plates These plates attract the fly ash to the surface of the plateswhere it is held through the electro-static force The de-ashed flue gas flows on throughthe plates

elec-A flue gas desulfurization unit (FGD) uses a wet or dry limestone (CaCO3) stream toconvert SO2to gypsum (CaSO4$2H2O), which is land-filled

SO2ðgÞ þ CaCO3ðsÞ/CaSO3ðsÞ þ CO2ðgÞ R-2.22CaSO3ðsÞ þ O2ðgÞ þ 4H2OðlÞ/2CaSO4$2H2OðsÞ R-2.3

At the time this was written, there was no standard method for mercury control.Control techniques under consideration include flotation of the coal to remove mineralmatter and injection of activated carbon into the flue gas ahead of the bag-house.Mercury in the flue gas can be in either oxidized or un-oxidized form Halide salts havebeen used to converted un-oxidized mercury to mercuric halides, which are morereadily removed in the bag-house or FGD The Western Research Institute has alsopatented a process where mercury is removed by preheating the coal to a particulartemperature prior to combustion

SUPERCRITICAL PULVERIZED COAL COMBUSTION

In a simplistic thermodynamic analysis, a pulverized coal combustion plant may be viewed

as a classic heat engine, shown inFigure 2.2 Although a real PCC unit is much morecomplex than what is shown here, this simplistic picture can be used to illustrate a trend.The theoretical maximum efficiency,hmax, is given by the Carnot cycle:

Figure 2.2 Pulverized coal combustion plant as a classic heat engine.

In theory, one may increase the efficiency by increasing THor decreasing TC Mostpower plants use cooling water from an evaporative cooling tower Smaller numbers ofpower plants use cooling water on a once-through basis or use air cooling Thetemperature of the water or air used to cool the power plant effectively sets TC Thismeans that to increase efficiency TH must be increased.

The maximum steam temperature and pressure is set by the steam tube materials ofconstruction Metal strengths fall with increasing temperature, therefore these tubes mustresist the corrosive environment in the furnace Steam tube metallurgy is an activeresearch area The latest steam tubes allow operation above the critical pressure of water,

as shown in Table 2.1

The higher efficiency of the supercritical plant means that less coal is needed toproduce the same amount of power This also reduces the corresponding emissions This

is a small but significant effect

CARBON CAPTURE WITH PULVERIZED COAL COMBUSTION PLANTS

The clean coal concept generally refers to a power plant that burns coal, or a coal-derivedfuel such as the syngas produced by a coal gasifier It then separates the CO2 andsequesters it to prevent emission of CO2 to the atmosphere Sequestration can take

a variety of forms, but the most common approach is to compress CO2 and store itunderground

In a pulverized coal combustion plant, the following three steps are required:

1 Separate CO2from flue gas

2 Compress CO2, typically to about 15 MPa

3 Inject CO2into a porous geologic formation

To illustrate the difficulty of step 1, consider a perfect membrane illustrated inFigure 2.3.This hypothetical membrane has perfect selectivity for CO2and offers no resistance for

CO2transport

Since the membrane offers no resistance for CO2transport, the CO2partial pressure

is the same on both sides of the membrane:

Table 2.1 Steam conditions and efficiency for subcritical and supercritical pulverized coal

combustion 3

Subcritical pulverized coal combustion Supercritical pulverizedcoal combustion Water criticalpoint

If the flue gas is at standard atmospheric pressure, 101 kPa and the flue gas contains13% CO2; then CO2separation will not begin until the pressure on the CO2side dropsbelow 13 kPa Removal of 90% of the CO2requires a 1.3 kPa CO2pressure and 99%removal requires a 0.13 kPa CO2pressure Even a perfect membrane would require largeand expensive vacuum pumps to separate CO2from the flue gas Real membranes withless than perfect selectivities and significant transport resistance would be more costly.The usual approach to CO2removal is to use a liquid absorbent or a solid adsorbentthat has an affinity for CO2 This allows the CO2to be removed at atmospheric pressure.

A weak bond is formed between the CO2 and the liquid or solid This bond is thenbroken, usually by heating This will regenerate the absorbent or adsorbent and free the

CO2 A strong bond between CO2 and the liquid or solid leads to fast and nearlycomplete removal of CO2from flue gas This strong bond requires a large amount ofenergy to break; so selection of the absorbent or adsorbent is a compromise betweeneffective CO2removal and ease of regeneration

Aqueous amine solutions have long been used for removal of CO2 and other acidgasses, such as SO2and H2S, from gas streams One of the more common commercialamines is mono-ethanol-amine (MEA), shown inFigure 2.4

Figure 2.5shows a simplified process for the removal of CO2from power plant fluegas The warm flue gas is cooled by water evaporation in a direct contact cooler Anaqueous solution of MEA is then used to absorb CO2from the flue gas A water washsection above the MEA absorption section removes traces of MEA from the flue gas The

CO2-loaded MEA solution is then sent to a stripper, where CO2is boiled off the MEAsolution The regenerated MEA solution is cooled and sent to the absorber The CO2iscompressed and sequestered

Woods et al.3compared designs for a subcritical pulverized coal combustion powerplant with and without carbon capture and sequestration (CCS) For both cases, the feed

was an Illinois No 6 bituminous coal The CCS case used an MEA process similar to thatshown inFigure 2.5.Table 2.2shows a comparison of estimated costs and efficiencies forthese two plant designs.

The carbon capture and sequestration system requires considerable energy, especiallyfor the stripper reboiler heat and for the CO2compressors This lowers the net HHVefficiency from 36.8% to 24.9% This study assumed that new power plants would be

CO2 – loaded solution

cooler MEA

solution water

Absorber

reboiler

Stripper

water condenser

CO2-free flue gas

to atmosphere

CO2 to compression

Figure 2.5 Mono-ethanol-amine (MEA) based process for CO 2 removal from flue gas.

Table 2.2 A comparison of the costs and efficiencies for a subcritical pulverized coal combustion

power plant with and without carbon capture and sequestration (CCS).3

built, as opposed to modifying an existing power plant This increases the coal feed ratefor the CCS case was from 219 to 323 t/hr, a 47% increase, in order to maintain 550 MW

of net output power If a CCS system were added to an existing power plant, then the netpower output would be reduced to about 68% of its former level Widespread retrofitting

of existing power plants would require substantial construction of new power plants tomaintain power production levels

Adding a CCS system nearly doubles both the plant cost and the cost of electricity.Katzer et al.4reviewed the CCS literature and concluded that adding CCS to coal-firedpower production would about double the cost of electric power production.Distribution costs would not change, so CCS would increase residential power costs byabout 50%

The enormous investment cost and steep electric power cost predicted for PCC withCCS prompted intense research into alternative CO2absorbents and adsorbents If a newpower plant is to be built, then PCC with CCS is not the most economical approach toproducing electricity with low CO2 emissions The large number of existing powerplants, however, provides a powerful incentive for adapting PCC technology

Alternative clean coal power production technologies are also being investigated Forexample, much of the current interest in coal gasification is due to the predicted cost ofelectric power for coal-based integrated gasification combined cycle (IGCC) plant withCCS is substantially lower than PCC with CCS Woods et al.3studied IGCC with CCSusing three different gasifiers The concluded that electric power could be made for 10.3cents/kw-hr This is a substantial increase over PCC without CCS, but less than PCCwith CCS The high cost of clean coal technology has raised interest in powerproduction technologies that produce less CO2

OXY-COMBUSTION

In simplistic terms, removal of CO2 from flue gas may be regarded as a CO2/N2separation The need for this separation may be eliminated if the furnace is fed oxygeninstead of air This is the basic concept of oxy-combustion.Figure 2.6shows a simplifiedversion of an oxy-combustion plant designed by Haslbeck et al.5Oxygen is produced by

an air separation unit (ASU) Typically, this is a cryogenic air distillation process; butother air separation techniques, such as pressure swing adsorption, have also been used.The oxygen purity from the cryogenic distillation unit is 95% The impurities are argon,3.4% and nitrogen, 1.6% Flames fed nearly pure oxygen are much hotter than flames fedair The materials of construction in the furnace cannot withstand these highertemperatures Consequently, CO2-rich flue gas is recycled to the furnace to give anoxygen partial pressure that is comparable to air Flue gas leaving the flue gas desulfur-ization unit is nearly saturated with water So the flue gas is reheated slightly to avoidwater droplets in the recycled flue gas

A portion of the flue gas is withdrawn, compressed and then dried using

a temperature swing adsorption unit The flue gas is rich in CO2, but containssignificant quantities of other gasses This entire stream may be compressed andsequestered Alternatively, the gas may be purified by cooling the gas and then sepa-rating liquid CO2from a vent gas that contains most of the impurities.Table 2.3showsthe stream compositions when the purification process is used The combined weightpercent of the vent gas and the sequestered gas is less than 100% due to the smallquantity of water removed from the moist flue gas Note that the vent gas contains CO2and SO2 These emissions can be eliminated if the purification process is removed Theneed for CO2 purification depends on the gas specification limits required forsequestration

Separation Unit

Coal Combustion

Ash

Flue Gas Desulfurization

Limestone, Water

Gypsum Reheat

Fan

Flue Gas Recycle

Drier

Compressor

Cool Flash

Water Vent gas

Flue gas to sequestration Pump

Figure 2.6 Oxy-combustion power plant based on the design by Haslbeck et al 5

Fogash and White6studied a process that would further purify the sequestered CO2and reduce the release of pollutants in the vent gas They used multistage flue gascompression combined with interstage cooling and condensation of water Most of the

NOxin the flue gas consists of NO The conditions of the compression train favor theoxidation of NO to NO2 Given sufficient residence time, NO2reacts with SO2to form

SO3this in turn, reacts with water to form sulfuric acid (H2SO4) The NO2also reactswith water to form nitric acid (HNO3), and mercury reacts with nitric acid to formmercuric nitrate Consequently, the bulk of the SO2, NOxand mercury in the flue gasleaves with the condensed water

The values shown in Table 2.3 are data from Haslbeck et al for a supercriticalpulverized coal combustion unit fed Illinois No 6 bituminous coal Haslbeck et al.noted that the flue gas desulfurization unit could be eliminated and that SO2could beco-sequestered with CO2 This would substantially reduce capital and operating costs.They kept the flue gas desulfurization unit in their design because, without it, therecycled flue gas would increase the SO2concentration to 3.4 to 3.5 times as high asthe same unit without a flue gas recycle With a high sulfur coal like Illinois No 6, thiswould cause corrosion problems in the boiler Haslbeck et al suggested eliminatingthe flue gas desulfurization unit when a low sulfur coal, such as Powder River Basincoal, is used

Table 2.4, also using data from Haslbeck et al., compares the cost and efficiency of

a pulverized coal combustion plant with and without oxy-combustion The addition of

an oxy-combustion system substantially lowers the efficiency and increases the cost ofelectric power production Comparing the values inTable 2.4andTable 2.2, we see that,compared to carbon capture and sequestration using amine absorption, oxy-combustion

is a more efficient and less costly means of capturing and sequestering CO2 For

a greenfield plant, the cost of power from an oxy-combustion plant is comparable to anIGCC plant Oxy-combustion, unlike IGCC, can be retrofitted to an existing pulverizedcoal combustion plant

Table 2.3 Flue gas compositions when CO 2 is puri fied by liquefaction 5

Figure 2.3illustrates that part of the difficulty in separating CO2from flue gas is due to itslow partial pressure As will be shown later, one of the attractive features of IGCC is that

CO2is separated from a high pressure gas stream, with a higher CO2partial pressure

A similar approach is used by the Sargas process,7,8in which CO2is separated from fluegas from a pressurized combustion process

A Sargas demonstration plant was installed at the Va¨rtan combined heat and powerplant in Stockholm, Sweden This plant uses a pressurized fluidized bed combustor (ABBCarbon P200 PFBC cycle) As shown inFigure 2.7, air is fed to a compressor/turbine on

a common shaft Air is compressed to about 1.3 MPa, and fed to the pressurized fluidizedbed combustor Coal is fed to the combustor as a coal/water slurry Limestone fed to thecombustor reduces SO2 emissions The low bed combustion temperature, typically850e880C, reduces NOxemissions A hydro-cyclone is used to remove fly ash fromthe flue gas

The flue gas then is cooled in a heat exchanger, and fed to a Benfield process9e11toseparate CO2 This process is similar to the MEA process shown in Figure 2.5, withpotassium carbonate (K2CO3) used instead of MEA The Benfield process, as marketed

by UOP,11also uses a proprietary soluble catalyst and a corrosion inhibitor An advantage

of the Benfield process is that, unlike amines, the inorganic chemicals used in the process

do not degrade in the presence of oxygen Flue gas is initially contacted with water toremove residual dust, NO2, and partially remove SO2 The gas then contacts the K2CO3solution, where CO2absorbs and reacts to form potassium bicarbonate (KHCO3) Thebicarbonate solution is heated in a stripping column to decompose the bicarbonate,releasing CO2and regenerating K2CO3

The CO2-free flue gas is used to cool the flue gas fed to the Benfield process Thisalso reheats the flue gas, which is then fed to the turbine side of the compressor/turbineset Warm, low pressure flue gas leaving the turbine is used to preheat boiler feed waterbefore it is vented to the stack The flue gas turbine produces about 20% of the powergenerated by the plant The other 80% is generated by the steam turbine

Table 2.4 A comparison of the costs and efficiencies for a supercritical pulverized coal combustion

power plant with and without oxycombustion.5The oxy-combustion case does not use CO2puri fication.

Without oxy-combustion With oxy-combustion

The production of liquid fuels from coal is expected to be a major application of coalgasification The following is a brief description of coal-to-liquid processes that do notrely on gasification

ENCOAL

Coking is the oldest form of processing coal, other than simply burning it Coal is heated

in the absence of oxygen to produce solid coke, liquid coal tar, and a flammable gas Coaltar was a major source of liquid fuel and chemical feedstock until petroleum becameabundant in the 1950s and 1960s

The ENCOAL process12 is a mild coking process that was demonstrated in a 1000ton per day plant near Gillette, Wyoming, in the 1990s An updated version of thisprocess is marketed by ConvertCoal.13A goal of this process is to upgrade PRB sub-bituminous coal to a solid fuel product called Process-Derived Fuel (PDF); which has an

Pressurized fluidized bed combustion

Stack

Steam

Benfield process

CO2

Fly ash CO2 -free

flue gas

Boiler feed water

Coal/

limestone slurry

Flue gas

Generator Figure 2.7 The Sargas process combines a pressurized fluidized bed combustion process with a post- combustion CO2removal process, here shown as the Ben field process.