Energy & Fuels 1993, 7, 7-13
7
Greenhouse Gas Emissions, Abatement and Control: The Role of Coal I. M. Smith' and K. V. Thambimuthut IEA Coal Research, London, U.K. Received June 3,1992. Revised Manuscript Received August 25, 1992
The basis for quantifying the relative effect of greenhouse gas emissions from coal utilisation is discussed. Emission factors (g of C/MJ) need to include greenhouse gas emissions and energy losses throughout the fuel cycle in order to compare the fuels. Nevertheless, C02 is the most important greenhouse gas from coal. Emission factors have decreased due to improved efficiency of coal use. The scope for further improvements in efficiency of conventional and advanced power generation is assessed. Control of CO2 emissions is viewed as a less promising option owing to the high cost and energy penalty of most methods. It is concluded that there is no firm basis for evaluating the effect of reducing emissions on their global warming potential. However, it is desirable that available technologies be implemented to reduce emissions by improved efficiency. Introduction Greenhouse gases from coal include principally carbon dioxide $ 0 2 ) and methane (CHJ, which is released during deep hard coal production, as well as nitrous oxide (NzO) which is emitted in small quantities when coal is used. Ozone (03) is another greenhouse gas formed indirectly from complex reactions with several pollutants in the atmosphere. Nitric oxide and nitrogen dioxide (jointly referred to as NO,) are also released from coal combustion and, although not greenhouse gases, they play a role in ozone formation. The topic of NO, emissions from coal combustion was reviewed at IEA Coal Research.' Chlorofluorocarbons (CFC) are important greenhouse gases, but coal is not involved in these emissions. Policy choices over the next few years to reduce greenhouse gas emissions are likely to have an effect on the use of coal in many countries. It is therefore essential to examine and monitor the scientific and technical basis for reducing emissions of greenhouse gases from coal use. A major scientific assessment of the greenhouse gas issue is provided by the reports of the Intergovernmental Panel on Climate Change (IPCC), set up in 1988 by the World Meteorological Organization and the United Nations Environment Pr~gramme.~J The IPCC considered that the stabilization of atmospheric concentrations of greenhouse gases would require a reduction in emissions of C02 >60%, CHI 15-2075, N2O 70-8075, CFC-1170-75%, and CFC-12 75-8575. IEA Coal Research, which is one of the collaborative projects of member countries of the International Energy Agency (IEA), has been keeping a watching brief on this issue since 1977.4-9 This paper + Presently at Energy Research Laboratories, CANMET, Energy Mines and Resources, Ottawa, Canada. (1)Sloss, L. Nitrogen oxides from coal combustion; IEA Coal Research London, 1991; 107 pp. (2) Houghton,J. T.; Jenkins, G. J.; Ephraums, J. J. Climate change: theIPCCscienttfrc assessment;Cambridge University Press: Cambridge, UK, 1990; 414 pp. (3) Houahton, J. T.: Callander. B. A.: Varnev. S.K. Climate change 1992 the aumlementarv report to the IPCC scientific assessme&: University P&: Cambhdge; UK, 1992; p 212. (4) Smith, I. M. Carbon dioxide and the 'greenhouse' effect-an unresolved problem; ICTWER 01; IEA Coal Research London, 1978; 40 PP.
examines the basis of estimates of emissions and their contribution to the greenhouse effect and assesses the state-of-the art of abatement and control technologies as they relate to coal use. Greenhouse Gas Emissions The relative contributions (%) of trace gases to the greenhouse effect may be estimated from an increase in the concentration of each gas in the atmosphere. The effect of each gas is expressed in terms of the equivalent C02 concentration. The concept of relative global warming potentials takes account of the differing times that gases remain in the atmosphere, their greenhouse effect while in the atmosphere, and the time period over which climatic changes are of concern (the IPCCIOJ1 include a detailed discussion). There are difficulties in the determination of realistic global warming potentials, mainly due to indirect effects of emissions which need to be quantified in addition to direct effects of greenhouse gas emissions. The indirect effects of CH4 emissions (the production of C02, stratospheric water vapor, and tropospheric 0 3 from CH4 oxidation) are potentially significant but dWicult to quantify. This applies also to the indirect effects from other trace gases: CO, NO,, and non-methane hydrocarbons. The IPCC acknowledged that their quantification (5) Smith, I. M. Carbon dioxide-emissions and effects; ICTIS/TR 18; IEA Coal Research London, 1982; 132 pp.
(6) Smith, I. M. CO, and climatic change; IEACRIO7; London, UK, IEA Coal Research London, 1988,52 pp. (7) Vernon, J. L. Market mechanism for pollution control: impacts on the coal industry; IEACR/27; IEA Coal Research London, 1990; 38 PP. (8) Vernon, J. L. Carbon taxes; IEAPER/Ol; IEA Coal Research London, 1992; 15 pp. (9) Smith, I. M.; Thambimuthu, K. V. Greenhouse gases, abatement and control: the role of coal; IEACR/39; IEA Coal Research London, 1991; 88 pp. (10) Shine, K. P.; Derwent, R. 0.; Wuebbles, D. J.; Morcrette, J. J. Radiative forcing of climate. In Climate change: the IPCC scientific assessment; Houghton, J. T., Jenkins, G. J., Ephraums, J. J., Eds.; Cambridge University Press: Cambridge, UK, 1990, pp 41-68. (11)Isakeen, I. S.A.; Ramaewamy, V.; Rodhe, H.; Wigley, T. M. L. Radiative forcingof climate. In Climote change 1992: thesupplementary report to the IPCC Scientific assessment; Houghton, J. T., Callander, B. A,, Varney, S.K., Eds.; Cambridge University Press: Cambridge, UK, 1992; pp 47-87.
0887-0624/93/2507-07$04.00/0Q 1993 American Chemical Society
8 Energy & Fuels, Vol. 7,No. 1, 1993
Smith and Thambimuthu
T a b l e I. Global Warming Potentials (by Weight) Relative t o T h a t of COz over Different T i m e HorizonsLoJL (a) IPCC 1990 Assessment trace gas
estimated lifetime, years
20
integration time horizon, years 100
500
-120 10 150 60 130
1 63 270 4500 7100
1 21 290 3500 7300
1 9 190 1500 4500
carbon dioxide methane, including indirect nitrous oxide
CFC-11 CFC-12
(b) IPCC 1992 Assessment ~
trace gas carbon dioxide methane, direct only nitrous oxide
CFC-11 CFC-12
~~
estimated lifetime, years
-
120 10.5 132 55 116
20
integration time horizon, years 100
500
sign of "indirect" effect
1 35 260 4500 7100
1 11 270 3400 7100
1 4 170 1400 4100
none positive uncertain negative negative
of the indirect effects needs further revision and evaluation and does not include some important chemical reactions.1° Also, the indirect effects vary spatially as well as with time.12 Hence a single global warming potential is not applicable to emissions in different regions. The recent revision of the 1990 IPCC report" does not attempt to quantify indirect effects but indicates whether they are positive or negative (see Table I). Additionally, the indirect effect of NO, is uncertain. I t is noted in the revision that the concept of a global warming potential may prove inapplicable to CO, NO, and non-methane hydrocarbons. Further uncertainties arise from relating the effect of greenhouse gases to that of COZas the reference gas (see Table I). The concentration of most greenhouse gases in the atmosphere declines exponentially with time; that of COz declines nonexponentially, fast over the first 10-year period, more gradually over the next 100years, and rather slowly over the thousand-year time scale. This is because COz is taken up from the atmosphere on varying time scales into different carbon storage sinks, in vegetation, soils, and the deep oceans. The IPCC used a carbon cycle model involving only an oceanic sink to describe COn uptake. The approximate lifetime of COZin the atmosphere is then about 120 years within a possible range of 50-200 years.13J4 There are uncertainties in accounting for all the COZ emissions which do not remain in the atmosphere. The carbon cycle is not balanced, the sources exceeding the known sinks. Other carbon sinks in the terrestrial biosphere16 and transport of carbon to16 and within" the oceans may be removing the excess. Use of a carbon cycle model with only an ocean sink is likely to overestimate concentrations of C02 resulting in an underestimate of the global warming potentials of other (12) Victor, D. G. Calculating greenhouse budgets. Nature 1990,347, 6292. (13) Watson, R. T.; Rodhe, H.;Oeschger, H.; Siegenthaler, U. Greenhouse gases and aerosols. In Climate change: the IPCC scientific assessment; Houghton, J . T., Jenkins, G. J., Ephraums, J. J., Eds.; Cambridge University Press: Cambridge, UK, 1990; pp 1-40. (14) Watson, R. T.;Meira Filho, L. G.; Sanhueza, E.; Janetos, A. Greenhouse gases: sources and sinks. In Climate change 1992 the supplementary report t o the IPCC scientific assessment; Houghton, J. T., Callander, B. A. Varney, S. K., Eds.; Cambridge University Press: Cambridge, UK, 1992; pp 23-46. (15) Tans,P. P.; Fung, I. Y.; Takahashi, T.Observational constraints on the global atmospheric COz budget. Science 1990, 247, 1431-1438. (16) Sarmiento, J. L.; Sundquist, E. T. Revised budget for the oceanic uptake of anthropogenic carbon dioxide. Nature 1992,356,589-593. (17) Broecker,W. S.;Peng,T.-H. Interhemispheric transport of carbon dioxide by ocean circulation. Nature 1992, 356, 587-589.
greenhouse gases. Increased concentrations of COz in the atmosphere in future would also result in higher global warming potentials for other greenhouse gases than the values reported in Table I.ll Sulfur dioxide is only a weak greenhouse gas with a short lifetime of about 5 days.18 I t may, however, have a regional cooling effect through the formation of sulfate aerosols, which increase the reflectivity of the earth both directly and as a result of cloud formation.lS-2l The cooling effect of SO2 emissions would offset the greenhouse effect of other emissions. More than 90% of industrial SO2 is emitted in the Northern Hemisphere. This might explain differences in the temperature records between the Northern and Southern Hemispheres. The much shorter residence time of sulfate aerosols compared with most greenhouse gases means that this effect is concentrated closer to emission sources. By contrast, the effect of CO2 operates globally because it is long-lived and mixes throughout the atmosphere. A continued growth in SO2 emissions is not necessarily a benefit in counteracting the effect of C02 and would lead to a rapidly widening contrast between the net radiation balance in the Northern and Southern Hemispheres with possibly disruptive effects on the climatic system.20 Further study of this effect is required, especially regarding the role of clouds.
Contributions to the Greenhouse Effect Comparisons of fuels must include all the contributions to potential global warming. Other greenhouse gas emissions as well as COZ,their by products, and other emissions which contribute indirectly to the greenhouse effect should be considered, together with any additional net consumption of energy (and hence COz emissions) in the production and distribution of the fuels. All these indirect effects need to be quantified in terms of their COZequivalent over the whole fuel cycle. Average COZemission factors for bituminous coal, crude oil, and natural gas, based on the higher heating value (18) Wuebbles,D. J.; Edmonds, J. Aprimerongreenhousegcrses;DOE/ NBB0083 TR040; U S . Department of Energy: Washington, DC, 1988; 100 PP. (19) Charlson, R. J.; Schwartz, S. E.; Hales, J. M.; Cese, R. D.; Coakley, Jr., J. A.; Hansen, J. E.; Hofmann, D. J. Climate forcing by anthropogenic aerosols. Science 1992,255, 423-430. (20) Wigley, T. M. L. Could reducing fossil-fuelemissionscause global warming? Nature 1991,349, 503-506. (21)Kaufman, Y. J.; Fraser, R. S.; Mahoney, R. L. Foesil fuel and biomass burning effect on climate: heating or cooling? J.Climate 1991, 4, 518-588.
Greenhouse Gas Emissions
(HHV), are 24.1, 19.9, and 13.8 g of C/MJ.22*23Based on the lower heating value (LHV), the factors are 25.8 g of C/MJ for all coals, 20.0 g of C/MJ for crude oil, and 15.3 g of C/MJ for natural gas with a range of variation of &3%.24 Coals of high and low rank have emission factors at the higher end of the range; for example, anthracites average 26.0 (HHV) and 26.7 g of C/MJ (LHV); brown coals 25.2 (HHV) and 30.6 g of C/MJ (LHV as received) or 26.4 g of C/MJ (LHV dry).g The latter show greater variability due to differences in moisture and ash content. When allowance is made for the effect of CH4, in terms of its C02 equivalent, the emission factor for natural gas increases relative to that of coal and oil due to CH4 leakage from natural gas distribution systems. The concept of global warming potentials was used to compare emission factors for coal and natural gas.25 The two fuels were esimated to have the same emission factors at a CHI leakage rate of 11.5% over a 100-year time horizon. Over 20 years, the breakeven point would occur a t a CH4 leakage rate of about 5 % . Such CH4 leakage rates are not uncommon in some systems. However, it is unlikely that leakage from new gas supplies for large scale use would exceed 1% .26 Energy consumption or emission penalties are incurred when coal is upgraded. For example, a coke from a bituminous coal had an effective emission factor of 31.8 gof C/MJ (HHV)whencoalandcokeproductionoverheads were included.27 Synthetic fuels from coal generally have much higher C02 emission factors than the original coal-even where gasoline is produced at a high thermal efficiency of 65% or more, the C02 emission factor is 38 gC/MJeg However, this penalty may be offset somewhat if the subsequent use of the synthetic fuel is more efficient than the burning of coal. The contribution of global coal use to the enhanced greenhouse effect is evaluated from COZemissionsin 1990% for the low and high estimates of the contribution from deforestation and land use adopted by the IPCC (see Table 11). The calculation is made for both the effect over 1 yearlo and for the global warming potential over a time horizon of 100 years using the weighting factors in Table Ib. Taking a mean value for deforestation and land use, coal and oil use are each responsible for about 23 % of the enhanced greenhouse effect due to C02 emissions and natural gas for 10% based on a 100-year time horizon. There would be a small addition for CH4 from hard coal mining and gas pipeline leakage and also from emissions of N20, and possibly NO,, from coal use. These other contributions have not been quantified reliably and are not therefore included in Table 11. However, it may be (22) Marland, G. Carbon dioxide emission rates for conventional and synthetic fuels. Energy 1983,8 (12), 981-992. (23) Marland, G.; Rotty, R. M. Carbon dioxide emissions from fossil fuele: a procedure for estimation and results for 1950-1982. Tellus 1984,
36B, 232-261. (24) Greenhouse gas emissions: the energy dimension: OECD/IEA: Paris, 1991; 199 pp. (25) Mitchell, C.; Sweet, J. A study of leakage from the UK natural gas supply system with reference to global warming; Greenpeace UK: London, 1990,112 pp. (26) James, C. G. Natural gas and the greenhouse. Nature 1990,347, 720. (27) Thurlow, G. Emissions of greenhouse gases. In Proceedings of the conference on technological respomes to the greenhouse effect; Thurlow, C., Ed.: Rooster Books: London, 1990, pp 16-24. (28) Marland, G. Oak Ridge National Laboratory, Oak Ridge, TN, personal communication (May, 1992).
Energy & Fuels, Vol. 7, No. 1, 1993 9
Table 11. Contributions to the Greenhouse E f f e ~ t ~ ~ J * J ~ ~ ~ COZemisaions, 1990 greenhouse effect, % Gt of C/year
%
over 1 year
over 100yeare
coal oil gas fossil fuel
2.4 2.4 1.0
36-28 36-28 15-12
20-15 20-15 8-7
26-20 26-20 11-9
5.8
87-68
48-37
63-49
cement manufacture deforestation and land use total coz
0.2 0.6-2.6
3.0-2.3 9-30
1.7-1.3
5-17
2.2-1.7 6-22
6.6-8.6
100
55
72
other greenhouse gases: CHI, Mt/year NzO, Mt/year halocarbons
351 7.2
total
15
10
6 24
5 13
I00
100
estimated from emissions data29 that the direct CH4 contribution to the enhanced greenhouse effect from all energy sources is about 2.6% and that of NzO about 0.4%. Therefore, it may be concluded that coal use contributes in the order of 24% to the enhanced greenhouse effect. Worldwide, power generation accounts for about half the coal used.30 Hence the greenhouse effect from coal-fired power generation is about 12 % of the total due to human activities. Abatement This section will concentrate on technologies which decrease COZemissions per unit of useful energy in the large-scaleuse of coal for power generation. All estimates of the energy consumption and cycle efficiencies are based on the LHV of the fuel, unless mentioned otherwise in the text. The emissions in g of C/(kW h), are derived taking into consideration the thermal to electrical conversion (or cycle) efficiency of the process. When expressed in this manner, the emissions are independent of the LHV or HHV of the fuel. These emissions are also based on the direct conversion of carbon in the fuel and therefore do not include overheads due to mining, fuel processing or transportation. The efficiency of coal use in steam power plants has already improved greatly this century. For example, in Germany increasingoperating pressures and temperatures and the use of steam reheat in conventional power plants have resulted in a reduction in the heat rate from 35 MJ/ (kW h), in the early 1900s to about 9 MJ/(kW h), in the 1 9 8 0 ~ . ~Emissions l and waste heat losses are now only about 25% of what they were in the early steam-based power plants. Current and future C02 abatement measures for power generation are summarized in Table 111. For the utilization of coal in conventional power plants, atmospheric pulverized coal (PC) and fluidized bed combustion (AFBC) are by far the most widely used methods. For a reference 500-MWe bituminous coal-fired power plant with wet limestone flue gas desulfurization, the US DOE32cites a (29) Leggett, J.;Pepper, W.J.; Swart, R. J. Emissions scenarioefor the IPCC: an update. In Climate change 1992: the supplementary report to the IPCC scientific assessment; Houghton, J. T., Callander, B. A., Varney, S. K., Me.Cambridge , University Presa: Cambridge, UK,1992; pp 69-95. (30)Daniel, M. Coal demand in power stations. Draft Report, IEA Coal Research, London, May 1991; 71 pp. (31) Schilling,H. D. Technologytrends in large powerplants. Modern Power S y s t e m Suppl. 1990,21-26.
10 Energy & Fuels, Vol. 7, No. 1, 1993
Smith and Thambimuthu
Table 111. Summary of COZAbatement M e a s u r e s f o r P o w e r Generation. ~~
~
~
C?, emissions,
steam conditions
net power station efficiency (LHV), %
subcritical subcritical supercritical (25 MPa) supercritical (30 MPa) subcritical subcritical subcritical
35 37 45.3 47.5 36-37.5 35 27.6-28.6
subcritical
8.4
subcritical subcritical
43 29.9-38.7
212.7
18.9 (90)
PFBC PFBC-gasification cycle pressurized pulverized coal
subcritical supercritical (25 MPa) subcritical
42 48-49 48
226.7 195.6-199.5 194.2
13.5 23.9-25.4 26
PC-natural gas + FGD, deNO, PC-natural gas + FGD, deNO, MHD
subcritical supercritical (30 MPa) subcritical
40 49 40-45"
202.1 166.1 192.5-216.5
22.lb 36.6b 17-27
fuel cells
subcritical
45-58"
147.1-192.5
26-44
77-84
108.9-118.5
54-59
technology conventional PC + FGD AFBC PC + FGD PC + FGD PC-natural gas cofiring PC design, coal oil mixture PC-C02flue recycle
PC design, Hz combustion
combined cycles IGCC IGCC
cogeneration combined heat and power (CHP) subcritical
a
CO? reduction, g of C/(kW h), % 262.1 257 208.1 198.7 233.3-242.1 251.2
0
1.95 20 31.9 7.4-llb 4.2b (100P
(100)
commenta reference plant for COZ emissions in-bed desulfurization, deNO, 540-560 OC, 0.002 MPa condenser 580 'C, 0.002 MPa condenser 15% natural gas input 40% fuel oil input energy use for air separation 187-220 (kW h),/t of COz, additional energy use for COzgas disposal energy use for Hzseparation 76% of coal heating value, additional energy use for carbon disposal low-temperature gas cleaning CO shift with low-temperature as cleaning; CO2 reduction or gas disposal high-temperature gas cleaning high-temperature gas cleaning additional energy use for gas cleaning, increased CO2 emission from desulfurization 33 76 natural gas input 33% natural gas input additional COn emissions from desulfurization additional COZ emissions from desulfurization
f
+
range for PC FGD, PFBC, and PFBC-gasification cycle; COz reduction figure for the direct re lacement of PC plant without Pow-grade heat recovery; COz reduction of 34-39% when replacing existing and separate coal-fired power and district heating plants
HHV.b Partial C02 emission reductions from fuel substitution. Numbers in parentheses apply t o % CO:!reduction by disposal.
net power plant efficiency of 35%. For these reference conditions using a 3.5 w t % sulfur coal, plant COz emission factors of 255.9 and 6.2 g of C/(kW h)e (total 262.1 g of C/(kW h),) respectively are cited for fuel combustion and lime use. For much smaller capacity AFBC plants, net power plant efficiencies at 37 % are roughly 2 percentage points higher due to lower energy use by in-bed desulfurization technique^.^^ However, with a slightly higher emission of 9.9 g of C/(kW h), of COZ from the use of in-bed sorbents, the total plant emission factor of 257 g of C/(kW h), is a very marginal improvement relative to the reference PC-fired plant. This comparison ignores the effect of NO, control technologies on the efficiency of power generation. Electricity consumption for control of particulates and sulfur and nitrogen oxides from 15power plants in six countries using a variety of technologiesvaried from 0.9 to 3.8% of net electricity production.33 It is expected that the energy use penalties for NO, control will be higher for PC combustion than for AFBC. NO, emissions from AFBC boilers are generally lower due to reduced fuel combustion temperatures, but future strategies to combat the marginally higher N20 emissions could increase the energy penalties for this system. The above US power plant efficiencies may also be 1-3 percentage points lower than the average for conventional PC plants currently in use in the other OECD countries. (32) A Fossil energy perspective on global climate change; DOE/FE0164; US Department of Energy: Washington, DC, 1990,99 pp. (33) Hjalmarsson, A. K. Interactions in emissions control for coalfired plants; IEACR/47; IEA Coal Research: London, 1992; 81 pp.
Improvements in the efficiency of power generation in conventional steam plants are possible through the use of supercritical steam cycles. The efficiency of a state of the art pulverized coal-fired power plant of 350-MWe capacity due to be commissioned in 1992at the Esbjerg unit 3 power station in Denmark is likely to be as high as 45.3 % .34 This plant efficiency assumes the use of a flue gas desulfurization unit with supercritical steam at 25 MPa and 560 "C, once through or cold sea-water condenser cooling and double steam reheat. As shown in Table 111, there is an efficiency improvement of 10 percentage points and an overall 20% reduction in the plant COz emissions relative to the reference PC-fired plant (see above). The relative improvement in the plant efficiency by 29.4 %, { 100 X (45.3 - 35)/35)also reduces coal use, and hence the nitrogen and sulfur input to the boiler by a similar amount. It is apparent that these supercritical steam conditions represent an upper limit on the maximum allowable stresses of sophisticated and expensive steel alloys used to fabricate steam tubing for high temperature and pressure service.35 Material availability and cost limitations and the irreversible waste heat losses from low-grade heat rejection (other than cogeneration; see below) suggest that there (34) Kjaer, S. Coal dust-fired power station unit with advanced water/ steam process; E P 90-01E; Elsamprojekt A / S Frederica, Denmark, 1990; 29 PP. (35) Williams, R. H.; Larson, E. D. Expanding roles for gas turbines in power generation. InElectricity: efficient end me and new generation technologies and theirplanning implications;Johansson,T. B., B d u n d , B., Williams, R. H., Eds.; Lund University Press: Lund, Sweden, 1989; 960 pp.
-
Greenhouse Gas Emissions
are limited marginal benefits to be gained from additional refinements to conventional steam (Rankine cycle) based power plants. An upper limit of a 47.5% plant efficiency for a PC plant with 30 MPa and 580 "C, supercritical steam conditions, has been proposed.34 Efficiency limitations in conventional steam cycles may be overcome by the use of combined cycles. In their simplest configuration, combined cycles generate power from the expansion of a hot pressurized flue gas through a gas turbine, with heat recovery from the gas turbine exhaust driving a bottoming steam cycle. An improved power to heat recovery ratio arising from a higher inlet working fluid temperature of the gas turbine (Brayton) cycle is the principal factor contributing to higher overall cycle efficiencies. Table I11 includes data on COZabatement (relative to the reference PC plant) for a number of emerging coal-based combined cycle schemes which include integrated gasification combined cycles (IGCC), pressurized fluidized bed combustion (PFBC) and hybrid cycles,and pressurized pulverized coal or direct coal-fired turbine cycles (DCFT). Other options include the pressurized natural gas-atmospheric pulverized coal combined cycles described below. The net power plant efficiency of 43% for an IGCC plant shown in Table I11 is derived for the use of a hightemperature entrained bed gasifier with low-temperature cleaning of the fuel gas.36 With cooling of the fuel gas, a significant fraction of the sensible heat in the gas leaving the coal gasifier bypasses the gas turbine cycle. Although much of this sensible heat is recovered as lower grade process heat by the steam cycle, the power plant efficiency is reduced to something less than the maximum available combined cycle e f f i ~ i e n c y .The ~ ~ development and use of high-temperature gas cleaning schemes would increase the overall power plant efficiency by several percentage points.38 Another means of abatement is offered by IGCC power plants which use solvent absorption for HzS and COZr e m o ~ a l .Although ~ COz separation will be dealt with in the next section on control technologies, removal of COZprior to fuel gas combustion in the gas turbine is strictly an abatement measure which is examined here. Solvent absorption when used with water gas shift reactors (which convert CO to CO2 with hydrogen production) yields a hydrogen-rich fuel gas with practically zero COz emissions at the power plant stack. However, there is agreater energy use penalty for the water gas shift reaction and for COZ separation and disposal. A recent study39 by the Electric Power Research Institute (EPRI) has shown that 90% CO2 removal from the fuel gas reduces the power plant efficiency by 6.9percentage points and increases the bus bar cost of electricity by 70% in comparison to an IGCC process without CO2 separation. The same EPRI study which evaluated the cost of CO2 removal from a PC power plant (see below) found that an IGCC power plant with COz separation has a higher power generation efficiency (by 7.3 percentage points) and a 18.5% lower bus bar cost (36) Isles, J. High temperature turbine favours IGCC. Modern Power Systems 1990, 19-23. (37) Takematsu, T.; Maude, C. Coal gasification for IGCC power generation; IEACR/37; IEA Coal Research London, 1991; 90 pp. (38) Thambimuthu,K. V. Gas cleaning for adoanced coal-basedpower generation; IEA Coal Research, London. Report in preparation.
CO2 disposal
Energy & Fuels, Vol. 7, No. 1, 1993 I1
Steam
Steam
02
u-1
4
Fuelgas
"
Compressor
p a -:; ration
Gas turbine
'02
9 to gasifier
1 Dryer
Reject N2
Figure 1. Schematicof IGCC with C02/02 combustionand C02 disposal.4l
of electric it^.^^ A similar study in The Netherlands using a slightly different IGCC power plant configuration concludes that COZseparation reduces the cycle efficiency by 10 percentage points with a correspondingly higher cost of electricity relative to a comparable power plant without COZremoval.40 In addition to the shift conversion and COZseparation by solvent absorption examined in these alternative IGCC power plant configurations, a new approach has been proposed41 in which the energy use penalties for CO2 removal and disposal can be further reduced by operating the gas turbine in a COZ/OZcombustion mode. Figure 1 illustrates the concept as applied to an IGCC gas turbine cycle. In this instance, a pure COZexhaust gas stream is obtained by recyclingthe turbine exhaust gas to the engine combustor and by burning the fuel gas in an 02-enriched and nitrogen-deficient gas atmosphere. A moisture dried and partially recompressed purge stream of pure COZgas is continuously removed for ita disposal from the power plant. Although eliminating the use of the more energy intensive upstream shift conversion and COz separation stages used in fuel gas treatment, more moderate energy penalties will be incurred from the need to recycle COZ from the gas turbine exhaust, in removing residual moisture and from the need to compress the COZpurge stream for final disposal from the power plant. Pressurized fluidized bed combustion (PFBC) combined cycle power plants rely on the direct expansion of a hot pressurized flue gas through a gas turbine. In-bed heat extraction and waste heat recovery from the gas turbine exhaust provides higher quality steam conditions and higher steam cycle efficiencies relative to gasification processes. However, due to a lower gas turbine inlet temperature of 700-900 "C (cf. 1100-1300 "C inlet temperature for an IGCC process) the overall combined cycle efficiency is marginally lower at 42% (see Table I11 (39) Smelser, S. C.; Booras, G. S. An engineering and economic evaluation of COz removal from fossil fuel-fired power plants. In Proceedings of the EPRI 9th annual conference on gasification power plants,Palo Alto, CA, 17-190ct 1990;ElectricPowerResearchInstitute, Washington, DC, 1990,pp 856-2471. (40) "Carbon dioxide disposal from coal based combined cycle power stations in depleted gaa fields in the Netherlands";Report No. 91; Ministry of Volkshuisvesting, Ruimtelijke Ordening en Milieubeheer (Housing, Physical Planning and Environment): Leidschendam,The Netherlands, 1990; 42 pp. (41) Thambimuthu, K. V. Development of gas cleaning technologies for advanced coal-basedpower generation cycles. Paper presented at the IEA New Electricity 21 Conference, Tokyo, Japan, 12-14th May 1992.
12 Energy & Fuels, Vol. 7, No. 1, 1993
Smith and Thambimuthu
combined cycle gasification plants suggest the technology and ref 42). With COz emissions from limestone use for could potentially reduce COZemissions by up to 44 9%47 in-bed desulfurization, the total plant emission at 226.7 (see Table 111). g of C/(kW h), is incrementally higher than that for a Finally, in areas where there is a supporting infraatruccomparable IGCC process.9 Hybrid PFBC schemes which ture for the use of low-grade process steam or heat in rely on the partial gasification of coal in a topping cycle district heating schemes, it is possible to operate cogenin addition to the generation of a pressurized gas from residual char combustion have also been p r o p o ~ e d . ~ ~ ?eration ~ ~ or combined heat and power (CHP) plants. The use of CHP schemes with conventional steam cycles With burning of the fuel gas from coal gasification, higher removes low-grade heat of limited use in a power generation gas turbine inlet temperatures and combined cycle efficycle and the increased heat extraction results in fuel ciencies are possible. The high cycle efficiencies may also energy use efficiencies of 82% or more.g CHP schemes be supplemented by supercritical steam conditions. Escan also be operated with combined cycles, which generally timate@ suggest that net power plant efficiencies as high as 48-49 % are possible (see Table 1111, and these higher increase the power to heat output ratio of the plant. As efficiencies reduce the power plant COZemissions by 23shown in Table 111, CHP plants by conserving energy, can 25% compared to the reference PC power plant. reduce COZemissions by 54-59 % when replacing existing The use of natural gas as a substitute or blending fuel power plants without cogeneration. For an alternative in coal-fired power stations has gained widespread interest scenario involving the replacement of separate coal-fired as a method of meeting more stringent environmental power and district heating plants, CHP schemes reduce controls on acid gas emissions. With lower fuel emission the net COZemissions by approximately 34-39%. factors (see above), natural gas substitution also reduces COz emissions. Cofiring of natural gas in conventional Control Technologies PC plants at levels of up to 15 % of the coal thermal input In contrast to abatement measures, the control of COz have been found to reduce SO, emissions by a similar emissions by recovery and disposal is generally regarded amount (by fuel dilution) and NO, emissions by up to as a last resort owing to the high energy costs involved. A 50% in a gas reburning mode. With a marginal 1-1.5 study by the US Electric Power Research Institute39 percentage point improvement in the net plant efficiencies, concluded that 90% COZrecovery by MEA scrubbing, estimates suggest that the COz emissions are reduced by solvent regeneration, compression, and removal for dis7.4-11% .g Greater reductions in the COZemissions are posal in the ocean at a depth of 457 m would incur an possible with gas use in a combined cycle mode.46 In this energy penalty of 37Ck374 (kW h)e/t of COa. The busbar scheme, natural gas combustion under high excess air is cost of electricity would increase by 159-178% relative to used to drive a gas turbine cycle. Exhaust gas from the a coal-fired power plant without COPcontrol. A study in turbine is then fed to a conventional PC-fired boiler driving the nether land^^^ concludes that MEA scrubbing would a bottoming steam cycle. The refurbishment of existing decrease the efficiency of a PC plant by 11.3 percentage PC plants to this mode of operation can increase the net points (energy penalty of 222.5 (kW h)e/t of COz) and power plant efficiency by 4-5 percentage points, while increase the bus bar cost of electricity by 80%. These and new construction employing supercritical steam cycles other results lie within a range of 222-988 (kW h)Jt of could yield efficiencies as high as 49 9%.45 For a 33/67 % COZ for the energy penalty estimated by several authors natural gas/coal thermal input, power plant emissions of for control technologies using solvent absorption or 166.1-202.1 g of C/(kW h), for these combined cycles result cryogenic separation at 90% COZremoval or membrane in a 22-37 % reduction in COZemissions compared to the separation at 80% removal. The estimates of energy use reference PC plant (Table 111). are in the range of 21-95 % of the energy released from the More futuristic schemes such as coal-fired MHD (magbasic coal combustion process and are thus impractical netohydrodynamics) and molten carbonate (MCFC) and and c ~ s t l y .However, ~ the development of integrated solid oxide fuel cells (SOFC) offer the potential of achieving systems for combined COZ, SO,, and NO, removal offers even higher power generation efficiencies. A review of some scope for energy savings relative to the use of separate the efficiencyof MHD schemes46suggests that commercial systems for the removal of these gases. MHD generators could reduce COZemissions by up to Disposal options include the oceans or sites on land such 27% in the near term and even more in future with a as salt domes and depleted natural gas or oil fields. For greater efficiency of heat r e c ~ v e r y .The ~ development of disposal in the oceans, the transfer of COz as solid blocks molten carbonate and solid oxide fuel cell technology is of dry ice or hydrate was found to be less practical and currently in its infancy but conceptual designs with more mostly than gas or liquid injection owing to the large volumes involved. It is desirable to inject the COz into (42) Jansson, S. A. The status of pressurised fluidised bed combustion: an energy technology for the future. In Proceedings of the clean the ocean at depths >lo00 m and preferably at 3000-4000 coal conference,London U K , 15-16 Jun 1988; Edwards, M., Ed.;Friends m in order to minimize the transfer of COz from the ocean of the Earth: London, 1989 pp 121-132. (43) Dawes, S. G.;Arnold, M. St. J.; Cross, P. J. I.; Holmes, J. British to the atmosphere.49 More research is required on the Coal development of advanced power generation technologies. Paper long-term effects of macromixing in the oceans and on presented at the Institution of Mechanical Engineers Conference on Steam Plant for the 19%, London, UK, 4-6 Apr 1990; 9 pp. (44) Martin, H. Assessing options for burning coal in the 1990s and beyond. Generation Technol. 1989, 42-48. (45) Hebel, G.;Kotachenreuther, H. Wirkungsgradverbessernde Maasnahmen an bestehenden Kraftwerken (Measuresto improvethe efficiency of existing power plants). In VGB-Konferenr Kraftwerkstechnik 2 ~ R e s s o u r c e m c h o n u n und g COTMinderung ( VGB conference power plant engineering 2000-Conservation of resources and COn control), Essen, FRG, 21-22 Feb 1990; VGB-Kraftwerkstechnik GmbH: Essen, FRG, 1990; pp 200-204. (46) Morrison,G.F. Coal-fired MHD;IEA/CRO6; IEA Coal Research: London, 1988; 32 pp.
(47) Kinoehita, K.;McLarnon,F. R.; Cairns,E. Fuel c e l k a handbook; DOE/METF-88/6096; United StatesDepartment of Energy, Morgantown Energy Technology Center: Morgantown, WV, 1988; 156 pp. (48) Blok, K.; Hendriks, C. A.; Turkenburg, W. C. The role of carbon dioxide removal in the reduction of the greenhouse effect. InZEAIOECD experts' seminar on energy technologies for reducing emissions of greenhouse gases, Paris, France, 12-14 Apr 1989; OECD/IEA Paris, 1989; Vol. 1, pp 135-155. (49) Hoffert, M. I.; Wey, Y. C.; Callegari, A. J.; Broecker, W. S. Atmospheric response to deep-sea injections of fossil-fuel carbon dioxide. Climatic Change 1979,2 (l),53-68.
Greenhouse Gas Emissions
CO2 retention in the deep oceaneg On land there is potential for storage of COZin salt domes. For example, in the USA, the existing storage capacity of strategic crude oil reserves could accommodate about 15 Gt of C or approximately 3 % of the national, annual COz emissions from coal combustion. Global 1980natural gas production values suggested that the storage capacity of depleted gas reservoirs could increase at about 0.7 Gt of C/year with an overall capacity as large as that of past and present reserves of natural gas.50 Storage of COZin depleted oil fields can be combined with its use as a solvent for enhanced oil recovery by miscible flooding. In the USA it is estimated that this use could only take up 7.7 Mt of C/year of C02.51 However, there would be a substantial release or recycling of injected gas at active production wells. Alternative means of controlling COz emissions include its use or recycle by natural or industrial processes. On land, commercially managed forests in temperate ecosystems could sequester 2.9-5.4 Gt of C/year but securing the necessary land areas of 465-733 Mha52153would pose problems. Unused land in tropical ecosystems (865 Mha) that previously supported forests might however be used to accumulate up to 1.5 Gt of C/year over a 100-year period.54 Two tropical reafforestation projects by US and Dutch utilities are to offset the C02 emissions from a new 180-MW e coal-fired power plant in the USA and two 600-MW e coal-fired power plants in The Netherlands.55~56 The reuse of COZin fuel or chemical synthesis is economically viable only if the total energy used can be produced at a lower cost than the fuel or chemical value of the product. Here, biofuels, relying on solar radiation as a "free" energy source for direct removal of COZ from the atmosphere, offer a means to slow the growth of fossil fuel use while simultaneously reducing the net accumulation of COZin the atmosphere? The present consumption of COZfor process use and the synthesis of industrial (50) Baes, C. F.; Beall, S. E.; Lee, D. W. The collection, disposal, and storage of carbon dioxide. In Proceedings of the international conference: Interactions of energy and climate, Miinster, FRG, 3-6 Mar 1980; Bach, W., Pankrath, J., Williams, J., Eds.; D. Reidel: Dordrecht, Netherlands, 1980; pp 495-519. (51) Abel, A,; Holt, M. E.; Parker, L. B. "Controllingcarbon dioxide emissions: CRS report for Congress";CRS-89-157-ENR; Congressional Research Service, Library of Congress, Washington, DC, 1989; 34 pp. (52) Marland, G. The role of forests in addressingthe COz greenhouse. InProceedings ofthe conference onglobalclimate change linkages;White, J. C., Wagner, W., Beale, C. N., Eds.; Elsevier: New York, 1989; pp 199212. (53) Sedjo, R. A.; Solomon, A. M. Climate and forests. In Greenhouse warming: abatement and adaptation workshop, Washington,DC, 1415 Jun 1988;Resources for the Future: Washington, DC, 1989; pp 105119. (54) Houghton, R. A. The future role of tropical forests in affecting the carbon dioxide concentration of the atmosphere. AMBIO 1990, 19 (4); 204-209. (55) Flavin, C. Slowing global warming: a worldwide strategy; Worldwatch Institute: Washingon, DC, 1989 pp 94. (56) European Energy Report. Dutch consider rain forest planting to offer power station emissions. Eur. Energy Rep. 1990, No. 312, 4-5.
Energy & Fuels, Vol. 7, No. 1, 1993 13
chemicals suggests that this sector is unlikely to make a significant contribution to reducing the overall global emissions of COz.9 Conclusions
Currently, the best available means of evaluating policies to reduce greenhouse gas emissions uses the concept of the global warming potential of each greenhouse gas relative to that of COZ, integrated over different time horizons. This exercise, is, however, complicated by indirect effects from some, as yet, unquantifiable chemical reactions and by uncertainties regarding the effective residence time of COZin the atmosphere. It is clear that COZ is the most important greenhouse gas from coal. Consideration of CH4 as well as COZin terms of their global warming potential makes no difference to the relative ranking of coal > oil > natural gas over the long term. Coal use worldwide is responsible for in the order of 24 % of the enhanced greenhouse effect (integrated over 100years and excluding indirect effects), half of which is attributable to coal-fired power generation. There are many options for the abatement of emissions from coal use in power generation. Apart from reducing the overall demand for energy by conservation, there are technologieswhich will reduce emissionsfrom an improved efficiency of coal use. Existing technologies based on supercritical steam cycles can achieve C02 emission reductions of up to 20%. Over the next decade, technologies such as PFBC and gasification combined cycle offer improvements up to 25 or 37% with the use of pulverized coal-natural gas combined cycles. For the future, MHD and fuel cells should reduce COZemissions by 17-44%. All of these technologies could be improved further with cogeneration for COZemission reductions approaching 60%, close to that required to stabilize concentrations of COZin the atmosphere. The control of COZemissions by recovery and disposal appears to incur too high an energy penalty with uncertain environmental consequences for a practical solution. However, systems which offer the possibility of COZ separation in an IGCC process and the combined removal of COZ,SO,, and NO, from flue gases might have some role in future in conjunction with energy savings. Reafforestation and recycling of COZin fuels potentially have more scope for reducing the concentration of C02 in the atmosphere but probably not on the scale required to stabilize it. Considerable progress has been made to improve efficiency for economic reasons in the past. Now there is an added impetus to achieve more for both economic and environmental benefits. There is scope for reducing emissions from coal use by both abatement and control measures, but their success depends on how quickly they can be implemented.
