Quantifying and comparing fuel-cycle greenhouse-gas emissions

Quantifying and comparing fuelcycle greenhouse-gas emissions Coal, oil and natural gas consumption Deborah Wilson

Greenhouse forcings from natural gas, coal and oil consumption are compared. Carbon dioxide, methane and nitrous oxide emissions are addressed using a complete fuel-cycle approach. The near- and long-term CO2-equivalent forcings resulting from consuming one MJ of each fuel are presented. Compared on that basis, naturalgas exploitation is less offensive than either coal or oil exploitation in both time-frames. The greenhouse forcing caused by fossil-fuel based electricity~heat systems using a variety of technologies is compared on a per kWh e basis. Keywords: Fossil fuels; Global warming; Greenhouse gases

The quantity of fossil fuels, especially coal, available for combustion in the future is large (Table 1). The fraction of available fossil fuels that will be extracted, delivered and combusted in the future depends largely on political and industrial decisions. The emissions of greenhouse gases that result from future exploitation of fossil resources depend largely on the technological advances that are made in emissions reduction and avoidance measures, including energy conservation, and the extent to which these are employed. Fuel-switching from coal and oil to natural gas is often suggested as a partial solution to the problem of reducing the emissions of greenhouse gases, especially for the near-term. Such suggestions are usually based on the assumption that natural gas is a comparatively clean fuel from the greenhouse gas perspective: switching to natural gas is expected to greatly Deborah Wilson is a Visiting Research Scientist at the

Department of Environmental and Energy Systems Studies, Institute of Science and Technology, Lund University, Gerdagatan 13, S-223 62 Lund, Sweden. 550

reduce the emissions of carbon dioxide (C02) that are associated with burning coal and, to a lesser extent, oil. This assumption has recently been questioned by researchers who point out that it is based on estimations that address neither the emissions of greenhouse gases other than CO2 nor a complete fuel-cycle approach to accounting for the sources of emissions from fossil fuels. Both of these issues and their impacts on the relative greenhouse effects of the three fuels are addressed in this report) The most important ignored gases in the fossil-fuel cycles are methane (CH4: the primary constituent of natural gas) and nitrous oxide (N20). In addition, nitrogen oxide(s) (NOx) and carbon monoxide (CO), although not greenhouse gases themselves, are important precursors to chemical reactions in the atmosphere that, interacting with other gases, contribute to the production of ozone (03) in the troposphere (the lower 10 to 15 km of the atmosphere) and can affect the atmospheric residence times of both 03 and methane. Chlorofluorocarbons (CFCs) are also important greenhouse gases. They are released through the production and use of energy-consuming end-use devices such as heatpumps and refrigerators - the final stage of complete fuel cycles (Figure 1). Such emissions are not specific to any one of the fossil-fuel cycles and are, therefore, beyond the scope of this study. Complete fossil-fuel cycles begin with extraction from the earth and end with services (such as light, warmth, and material products) provided to consumers (Figure 1). There are many options for choosing a specific path toward providing most services. Each fuel-cycle path results in a different amount of greenhouse-gas emissions. Carbon dioxide, CH4, and N20 are emitted to the atmosphere at several stages of fossil-fuel cycles; during extraction and production, distribution, and combustion. During coal extraction, CH4 is unear0301-4215/90/060550-13 (~ 1990 Butterworth-Heinemann Ltd

Fuel-cycle greenhouse-gas emissions

condition is a quality of the specific stock of pipelines in place today, rather than inherent to pipeline gas distribution technology in general. Highpressure long-distance pipelines used to deliver natural gas from production sites to utilities and large industrial customers are continuous (without joints) and made of welded steel. These pipelines are essentially leak-free under normal use (some leakage from pipe-failure and maintenance does take place). Low-pressure networks used to disperse gas to smaller industries and residential and commercial customers, however, have been the source of significant gas leaks. This is due, in part, to the decentralized and complex nature of the networks that complicates pipe maintenance and tracking and repair of leaks. Most of these leaks, however, exist in old cast-iron jointed-piping networks and can be virtually eliminated using modern polyethylene piping. The bulk of C O 2 and N20 emissions in fossil-fuel cycles are released during fuel combustion. If combustion is complete, resultant CO2 emissions are independent of where in the fuel cycle the combustion takes place (at utilities or at industry or consumer end-use sites). Nitrous-oxide emissions, however, may be closely tied to the combustion technology used (see below). In addition to the direct sources described above, fuel used within the energy industry for driving fuel production, processing and distribution is an important indirect source of greenhouse-gas emissions. This includes fuel used for coal processing and oil refining (often coal and oil respectively), natural gas used for running pumps in gas-distribution networks, and a variety of fuels used for powerplant construction, transporting coal and oil, etc. Emissions from such indirect sources have been excluded from this analysis because they are not generalizable

Table 1. 1988 global fossil fuel reserves and production."

Fuel

Anthracite and bituminous coal Sub-bituminous coal and lignite Total coal Crude oil Natural gas

Proved reserves b (gigatonnes) (exajoules)

579 443 1 022 124 112c

Production (exajoules)

17 382 6 24 5 4

97

649 031 580 532

22 119 136 78

a Dividing reserves by production for a given year gives the number of years that the remaining reserves will last if production continues at the given level. These ratios for the 1988 figures are 202 years for coal; 41 years for crude oil; 58 years for natural gas. Adapted from BP, BP Statistical Review of World Energy, July 1989, London, UK, 1989. b BP defines proved reserves as follows: generally taken to be those quantities which geological and engineering information indicate with reasonable certainty can be recovered in the future from known reservoirs/deposits under existing economic and operating conditions c Units: 1012m3.

thed and released to the atmosphere. Methane is also released from oil fields when associated natural gas leaks out and is not recovered. Some of this gas is flared, but the remainder is vented to the atmosphere. Associated natural gas is also captured and pumped back into some oil wells in order to increase well pressure and facilitate the extraction of the remaining oil. Some natural-gas losses can be expected from this process, but quantitative data describing them are not available. Leaks also occur at the well-head at natural-gas production sites. The extent to which such leaks can be and are controlled, varies from site to site. Measurement data quantifying well-head leaks are not available. Leaks from natural-gas distribution pipelines appear to be significant on a global average. This

m

m

P Cool

R O D

R E F

Ci I T I I' O N

N I N G

Oil

Gas

m

T R A N S P Om R T A T I O

E N

m

C O M B U S T I O N --

-

Electricity

Heot

-

-

D -/ S U E S R E __V I D C E E V S I C E S

Figure 1. Complete fossil fuel cycles.

ENERGY POLICY July/August 1990

551

Fuel-cycle greenhouse-gas emissions

(are case/application specific), not quantifiable with information available, and/or variable (involve multiple fuel-mix options). Some of the excluded emissions may be significant. Further research is needed to identify, quantify, and analyse their greenhouse contributions. Direct emissions from coal mining and oil and gas production, natural gas distribution, and utility combustion of all three fuels are included in the calculations. Energy efficiency affects total emissions at virtually every stage in the fuel cycles described above. Optimizing the energy efficiency of any given path through the fuel cycles to the provision of services also results in minimizing emissions for that path. Energy-efficiency retrofits adopted at any point(s) in the paths to services allow for additional provision of services without subsequent emissions increases. Efficiency improvements reduce growth in emissions when they reduce growth in energy demand, and reduce total emissions when they reduce total demand.

Fuel-cycle sources of emissions Estimates of the emissions of each greenhouse gas in each stage of the fuel cycles are required for quantifying the CO2 equivalent emissions from fossil-fuel cycles. Estimated values describing average emissions from technologies in place today have been collected where available. For comparison, estimates of emissions from low-emission technologies in place today have also been collected. In modern large-scale combustion processes, effectively all of the carbon contained in the combusted fossil fuel (Table 2) ends up in the atmosphere as CO2. The carbon content of bituminous coal is used in the per MJ emissions estimates in order to compare the best case situation for coal with oil and natural gas. In 1988, roughly 30% of the coal consumed globally was lignite and brown (subbituminous) coal. 2 Brown coal and lignite contain 6% more carbon per MJ on average than bituminous coal does (Table 2). Table 2. Average fuel carbon contents. Fuel

gC/MJ

gCO~MJ

Brown coal and lignitea Bituminous coal" Oil Natural gas

25.2 23.7 19.9 13.5

92 87 73 49

a Average values. The range can be at least + 10 percent.

Source: adapted from G. J. MacDonald, ed, The Long-Term Impacts of Increasing Atmospheric Carbon Dioxide Levels, Ballinger Publishing Co, Cambridge MA, USA, 1982.

552

Limestone mixed with coal for sulphur removal adds a small increment of carbon to the emissions from fluidized-bed processes. The amount of extra carbon emitted is roughly equal, on a molar basis, to the amount of sulphur originally contained in the fuel. Hard coal has, on average, a sulphur content of roughly 0.3 g/MJ resulting in a carbon emission increment of 0.1 gC/MJ (0.3 gS/MJ x 12 gC/32 gS) for fluidized-bed combustion processes. 3 For all three fuels, source emissions of both CH 4 and N20 are very poorly known. Methane is known to be leaked in the production processes of all three fuels, although accurate measurements of the amounts leaked are not available. According to Okken and Kram, CH4 emissions from coal mining vary substantially depending on the shape and depth of the coal reserve; they cite a US survey reporting national average C H 4 emissions of 6.6 m3/tonne (0.2 gCHa/MJ coal). 4 The global average has been estimated by Crutzen as 18 to 19 m 3 CH4/tonne of coal mined 5 (0.5 gCH4/MJ coal), and by CEPCEO as 35 million tonnes/year (0.3 gCH4/MJ coal). 6 The CEPCEO estimate is used in the calculations. Okken and Kram also estimate average global natural gas emissions from oil production and consumption of 10 million tonnes/year with an uncertainty range of 5 to 30 million tonnes/year. These numbers are based on assumptions regarding the flaring of associated gas. The 10 million tonnes/year estimate was derived assuming 80% is flared and the remainder vented, while the 30 million tonnes/year maximum estimate was derived assuming that 70% is flared. The adopted 10 million tonnes gas/year figure converts to roughly 0.22 gCH4/MJ oil (assuming annual oil production of approximately 3 000 million tonnes/ year. 7 Estimates in the research literature indicate that venting and leakage from natural-gas production and distribution totals roughly 2-3% of production for Europe 8 and 3-4% on a global average. 9 Recent natural-gas industry publications dispute these figures, claiming that global gas-industry emissions total no more than l% of production. 1° The Alphatania Partnership further estimates that, for new supply systems, leakage rates of 0.05% of production should be achievable. 11 If this estimate - or even an estimate of 1% - i s accurate, high system-leakage rates may exist, especially in older gas-systems networks, but are not necessary given today's technological and economic situation. In order to ensure that significant gas leakage does not occur, all gas systems should be subjected to strict monitoring, ideally by non-industry institutions. The effect of the high global average natural-gas distribution leakage

ENERGY POLICY July/August 1990

Fuel-cycle greenhouse-gas emissions

assumption on the ranking of coal, oil and natural gas, is demonstrated here by applying a global leakage rate of 3.5% of production. The sensitivity of the fuel ranking to this assumption is discussed below. Accurate measured data are not currently available on NzO emissions in the fuel cycles of coal, oil, and natural gas. Data gathered to date using grabsample collection methods are erroneous because N20 production takes place in sample flasks between the time of collection and the time of analysis. 12 New tests indicate that N20 emissions from combustion technologies are much smaller than previously believed: 'N20 levels emitted from the 800 MW pulverized coal-fired boiler were closer to 3 ppm than 130 ppm'. 13 The direct forcing effect of N20 emissions at such low levels would not warrant inclusion in the quantitative comparison given below of the greenhouse forcing caused by fossil-fuel consumption. Indirect forcing from N20 formation in the atmosphere caused by SO2 and NOx emissions from fossil combustion may be important however, and should therefore be studied further.14 Prior to the grab-sample error discovery, N20 emissions were thought to be closely tied to the specific process of combustion used. 15 In general, N20 emissions were believed to be produced downstream of the combustion flame through lowtemperature reactions between fuel-nitrogen and N20 precursors. The number of precursor molecules available for post-flame N20 production was t h o u g h t to be related to c o m b u s t i o n flame temperature, 16 and nitrogen availability was believed dependent on the nitrogen content of the fuel used and the amount of nitrogen combusted in the flame. 17 Because of these relationships, clean coal technologies that reduce SO2 - such as circulating fluidized beds with substantially less-than-flame temperatures - were thought to produce higher N20 emissions than conventional combustion technologies. 18 A comprehensive new database of measured NzO emissions for a variety of specific combustion technologies is needed to determine whether or not any of the former theories on NzO production from combustion still hold, and to quantify the role of Table 3. Direct radiative forcings relative to CO2. per mole in atmosphere:

per kg in atmosphere:

CO2 cn 4 N20 03

CO2 CH 4 N20 03

1 25 250 2 000

ENERGY POLICY July/August 1990

1 70 250 1 800

combustion-related NzO emissions as anthropogenic sources of greenhouse gases.

Comparing fuel-cycle greenhouse-gas emissions In order to compare the greenhouse forcings of CO2, C H 4 and N20 emissions it is necessary to put them into common units. Because the relative abilities of these gases to absorb long-wave radiation in the atmosphere are well understood, they can be compared on a mole-to-mole (or kg-to-kg) basis in a straightforward way. The cumulative greenhouse forcings caused by emissions of these gases, however, are more difficult to compare because emissions occur in varying quantities (ie not on a one-to-one basis) and because of differences in the gases' atmospheric residence times. These differences cause the relative greenhouse impacts of emissions to vary over time. There are two basic rationales for choosing a timeframe under which to compare the climatic effects of greenhouse-gas emissions. One leads to comparisons over the near-term (for example, 20-50 years) and the other leads to long-term comparisons (100 years or more, depending on the atmospheric lifetime of the most persistent gas under consideration). The adopted timeframe can have a significant effect on conclusions resulting from such comparisons. Subsequently, the policy implications of studies adopting near-term v long-term perspectives may differ in important ways. Take, for example, the comparison of the direct-radiative effects (see below) of C H 4 and CO2 emissions. Methane in the atmosphere absorbs long-wave radiation more effectively than CO2 but remains in the atmosphere roughly one tenth as long. The relative importance from the greenhouse perspective of emissions of two gases depends, therefore, on the quantities of the gases released and on the timeframe under consideration.

The long-term view If the greenhouse forcing of fossil-fuel C O 2 and CH4 emissions are compared over a time period equal to or longer than the atmospheric lifetime of CO2 (about 100 years), the CO2 emitted is more important than the CH4, unless a very large amount of CH4 is released. Because of the timelag between the emissions of the gases and their total climatic impacts, the total effect of today's emissions will not be experienced until well into the long-term future. Once the emissions of both gases are released to the atmosphere, however, the climatic changes they will

553

Fuel-cycle greenhouse-gas emissions

eventually cause are inevitable: methods for reversing or avoiding the climatic impacts of gases emitted to the atmosphere are unknown to science today. The near-term view

From a policy perspective, the rate of climatic change can be considered more important than the total magnitude of climatic change over the longterm (over centuries). This is true for two reasons: man's and the environment's ability to adapt to climatic change is much greater if the change takes place slowly; and man's chances of devising methods of a b o r t i n g or r e v e r s i n g the p r o c e s s e s of anthropogenically-induced climatic change increase as the amount of time available for doing so increases. Buying time by reducing emissions of gases with large near-term (within decades) forcing potential may prove to be an important policy objective. Arguments for such policy objectives become stronger if environmental threshold effects are considered: many sub-systems of the earth's climate as we know it today may have threshold temperature levels above which they will change irreparably (sub-systems such as the Gulf Stream). 19 If such sub-system changes may seriously threaten the environment within the next 50 years, long-term climatic impacts are less relevant to current political debate than are near-term impacts. If the greenhouse forcings from fossil-fuel emissions are compared over the first few decades following their release, all of the direct radiation absorption resulting from the emitted methane will have taken place, while only a fraction of the radiation absorption from the CO2 (and N20) will have taken place. Thus smaller amounts of methane emissions are required to 'equal' the near-term greenhouse forcing of CO2 emissions than is the case for the long-term perspective. The uncertainties that remain in man's ability to predict accurately the impacts of the greenhouse effect both regionally and in different timeframes make it difficult to choose between solutions aimed at mitigating either the near- or long-term effects of emissions. If policy aims encompass both near- and long-term climate preservation, neither emissions time-perspective should be ignored. Avoiding anthropogenically-induced climatic changes from emissions in either timeframe requires action in the present.

Greenhouse properties of the gases Complex computer models, such as General Circulation Models (GCMs), are used to estimate the

554

effects of emissions on global climate. GCMs take into account the radiative activity (absorption of incoming solar and outgoing long-wave radiation) and chemical activity (interaction with other molecules) of greenhouse gases and their resulting feedback effects. Some of these relationships are not fully understood today. In this analysis, descriptions of the behaviour of atmospheric CO2, N20 and, especially, CH4 have been simplified, and much less complex calculations than those used in GCMs are used to describe the relative importance of fossil fuels as potential sources of greenhouse-forcing gases. These calculations take into account assumptions regarding the chemical compositions of the fuels, points and quantities of emissions in the fuel cycles, the atmospheric residence times of COa, CH4, N20 and 03, the relative direct-radiative effects of the same gases in the atmosphere (which are well known) and the indirect-radiative effects of CH4 (through less well-understood chemical activity in the atmosphere). Areas of uncertainty in the assumptions are identified. The effect of uncertain assumptions on the robustness of the results has been determined through sensitivity calculations. Carbon dioxide

Carbon dioxide is the most talked about and most written about of the greenhouse gases that are associated with fossil-fuel use. Tropospheric COa absorbs outgoing infrared radiation thus contributing to a warming of the air at the earth's surface. At the same time, CO2 acts to enhance cooling in the middle and upper stratosphere (the stratosphere is the section of the atmosphere between approximately 10 and 50 km above the earth's surface), thereby increasing the amount of solar radiation that enters the troposphere. The atmospheric residence time of CO2 reported in the literature ranges from 2-500 years. In general, references that report the residence time as two years refer to the average time required for CO2 molecules to be fixed by photosynthesis and re-released to the atmosphere, 2° while references reporting 500 years refer to the combined lifetime through the carbon cycle including the atmosphere, biosphere and upper ocean. 21 For incremental emissions of CO2 (emissions exceeding the natural-source rates for CO2 released from the biosphere) the atmospheric residence time is widely accepted to be roughly 100 years, a2 Methane

In the troposphere, CH4 is both chemically and radiatively active. Because of this, there is greater uncertainty in quantitative estimates of the 'green-

ENERGY POLICY July/August 1990

Fuel-cycle greenhouse-gas emissions

house effectiveness' of CH4 than there is for CO2 or N20. Radiatively, CH4 absorbs outgoing infrared radiation in the IR-window (8-13 ~tm),23 ie the wavelengths where infrared radiation escapes to the stratosphere virtually uninhibited by CO2 and water vapour. 24 Methane is 20-30 times as effective as CO2 at absorbing outgoing long-wave radiation on a per mole basis. 25 The atmospheric residence time of c n 4 is shorter than that of CO2, however; roughly nine years, z6 Because the spectral regions in which several greenhouse gases absorb radiation overlap, the radiative effects of the gases may not be entirely addable. 27 Methane and N20 have overlapping radiative absorptivity in the 6-8 ~tm range. Based on today's mixing ratios, the radiation absorptivities of these two gases are assumed to be roughly addable. Should the tropospheric mixing ratios approach a level high enough to saturate radiation absorptiveness in the 6-8 ~tm range, this assumption would no longer hold. The primary removal process for CH4 in the atmosphere is through oxidation. Approximately 85% of CH 4 input to the atmosphere oxidizes in the troposphere. The rest enters the stratosphere. 28 Methane oxidation begins with chemical reactions b e t w e e n CH4 and h y d r o x y l r a d i c a l ( O H ) molecules 29 and, through a series of reactions, results in the net production of CO2 and water and destruction of OH. 3° Carbon monoxide also reacts with and removes OH from the atmosphere. Thus increases in CO emissions can affect the quantity of OH in the atmosphere and thereby indirectly affect the atmospheric residence time and concentration of CH4 .31 Similarly, tropospheric OH is a sink for tropospheric ozone (03, another highly radiatively active greenhouse gas), so increases in CH4 emissions can (by removing OH) increase tropospheric ozone concentrations. However, because 03 reacts with H20 to produce OH, 32 thereby increasing the availability of OH as an 03 sink, the net effect of reactions with OH on tropospheric 03 is unclear. The oxidation reactions for CH4 are known to alter the tropospheric concentration of ozone. Methane is oxidized in two different series of chemical reactions, depending on the amount of nitric oxide (NO) that is present in the surrounding atmosphere. In NO-rich atmosphere, the net of these reactions leads to the production of 3.7 molecules of ozone for every CH4 molecule oxidized. In nitrogenpoor atmosphere, the net of these reactions destroys 1.7 molecules of ozone. 33 Following Crutzen's postulation, 34 the estimate that roughly half of the CH 4 oxidation occurs through each of the two

ENERGY POLICY July/August 1990

processes is used, leading to an average production of 1 molecule 03 per molecule CH4 oxidized in the troposphere. CO2 production through the oxidation of CH4 is also assumed to take place on a one-to-one basis: 1 molecule of CH4 oxidation produces 1 molecule of CO2. 35 In the stratosphere, increases in the concentrations of CH4 will enhance cooling through radiative losses to space, 36 but this effect is an order of magnitude less for a doubling of CH4 than for a doubling of CO2 .37 Stratospheric C H 4 also reacts with CI to form HCI molecules that act as a temporary reservoir for CI atoms (CI destroys stratospheric 03). 38 Stratospheric water vapour produced from CH4 oxidation acts as a greenhouse gas and may therefore enhance the indirect greenhouse effect of CH4 significantly.39 The indirect greenhouse effects of methane in the stratosphere are not included in the calculations.

Nitrous oxide Nitrous oxide is a radiatively active greenhouse gas. N20 is also chemically active in the atmosphere. Although much less NzO than COe is emitted from fossil-fuel use, even small emissions are important because N20 is a stronger long-wave radiation absorber and has a longer atmospheric lifetime than CO2. Nitrous oxide molecules absorb outgoing longwave radiation approximately 250 times as effectively on a per mole basis as do COz molecules4° and have an atmospheric residence time of approximately 150 years. 41 In the stratosphere, N20 becomes a major source of NOx .42 Further stratospheric chemical reactions orginating with NzO both cause and impede the destruction of 03. The chemical changes brought about by N20 in the stratosphere are so complicated, however, that it is unknown whether the net result on stratospheric 03 is positive or negative. 43 Like CH4, N 2 0 enhances cooling in the stratosphere, but with an effectiveness that is an order of magnitude smaller than that of CO2 (for a doubling of both gases). 44 Tropospheric ozone Ozone is not directly emitted through the use of fossil fuels. Ozone is produced, however, through photo-chemical interactions in the atmosphere involving gases emitted in fossil-fuel cycles (primarily CH4 and CO). Thus increases in atmospheric concentrations of these gases caused by fossil-fuel use indirectly affect the atmospheric concentration of ozone. Ramanathan et a145 estimate that the greenhouse effect caused by tropospheric 03 created through CH 4 oxidation (see above) can be compara-

555

Fuel-cycle greenhouse-gas emissions

ble to that of the CH4 increase. Although the atmospheric residence time of 03 is short (0.1-0.3 years), tropospheric ozone is a highly effective absorber of long-wave radiation. One mole of 03 in the troposphere absorbs outgoing radiation approximately 2 000 times as effectively as one mole of CO2 .46

As stated previously, on a global average the oxidation of one molecule of CH4 is assumed to lead to the production of one molecule of 03 and 85% of the CH4 emitted to the atmosphere oxidizes in the troposphere (where 03 contributes to the greenhouse effect). It should be noted, however, that the actual 03 production from C H 4 oxidation may vary significantly on a local/regional basis. The primary precursor for 03 production through CH4 oxidation is NO. Nitric oxide is present in high concentrations primarily over continents in the northern hemisphere because anthropogenic NOx emissions do not have time to mix evenly in the atmosphere due to NO's short atmospheric residence time.47 Because of this, 03 production is not expected to occur uniformly over the globe. 48 This hypothesis has been supported by measurements of tropospheric o z o n e . 49

Comparing greenhouse forcing of emissions In order to compare emissions of CO2, CH4 and N20, their impacts must be described in comparable units. At any given point in time, one unit of each of the gases has the capacity to absorb a different amount of long-wave radiation. In Table 3, the radiative forcings of CH4 and N20 and 03 are described relative to the forcing of CO2. The cumulative radiative forcing caused by emissions of these gases is calculated by integrating the product of their CO2-relative radiative forcing and the quantity of emitted gas present in the atmosphere over time. The near- v long-term impacts of emissions of these gases are demonstrated by solving this integral for different timeframe boundaries. The results of using boundaries of 0-20 years, 0-50 years and zero to infinity are given throughout the remainder of this report. These time-frame boundaries are used for illustrative purposes: timeframe boundaries for specific policy analyses will vary depending on the goals of the study and points of reference. A lower boundary might, for example, be based on the shortest time before commitment to irreversible impacts is expected. The gases are assumed to decay exponentially in the atmosphere. The total radiative forcing of CH4 includes the radiative forcing of 03 and CO2 produced in the CH4 oxidation process. In Table 4, the cumulative greenhouse forcing caused

556

Table 4. Greenhouse forcings of methane and nitrous oxide emissions relative to that of carbon dioxide. Per mole (kg) emitted to atmosphere: After 20 years After 50 years

CO2 1 CH4 28 N20 258

CO2 1 (78) CH 4 15 ( 2 5 8 ) N20 270

Lifetime

CO2 1 (41) CH4 5 ( 2 7 0 ) N20 375

(10) (375)

by one mole (or kg) of C H 4 and N 2 0 respectively are presented relative to the forcing caused by an equal amount of CO2. The relative forcings of the emissions vary significantly depending on the timeframe examined. Twenty years after a simultaneous emission of amounts of C H 4 and CO2, each mole (kg) of CH4 will have caused 28 (78) times more greenhouse forcing than each mole (kg) of CO2. In other words, seen in a 20-year perspective emitting one mole of CH4 has the equivalent greenhouse-forcing effect of emitting 28 moles of CO2. After 50 years, essentially all of the C H 4 will have decayed, but the CO2 will continue to absorb outgoing radiation, as will the CO2 created as a result of the CH4 oxidation process. By the time the emission of each of the gases has decayed completely, the mole of c n 4 will have caused only five times as much greenhouse forcing as the CO2. The relative importance of emitting one mole of C H 4 v one mole of CO2 varies, therefore, depending on whether a near-term or long-term view is taken. The relative forcings of atmospheric N20 and CO2 are easier to follow intuitively because the atmospheric residence time of N20 is longer than that of CO2 while its relative greenhouse forcing is greater. When fossil fuels are consumed, CO2, CH4 and N20 are emitted in different amounts. The greenhouse forcing caused by emissions from the fuel cycles of coal, oil and natural gas respectively are estimated by multiplying the amount of each gas emitted by relative forcing for each gas as given in Table 4. The resulting products describe the amount of greenhouse forcing that would take place if an equivalent amount of CO2 were emitted. For purposes of simplicity, these CO2 equivalents are treated as addable. The precise effect of this assumption on the accuracy of the total CO2 equivalents is not known, but is assumed to be negligible (see argument above). The total greenhouse forcing for each fuel cycle is equal to the sum of the CO2 equivalents resulting from emissions of the individual gases. The CO2-equivalent forcings resulting from the consumption of one MJ of each of the three fossil

ENERGY POLICY July/August 1990

Fuel-cycle greenhouse-gas emissions

fuels are presented in Table 5. These figures were calculated based on the assumptions regarding partial fuel-cycle emissions, radiative forcing and residence times given above. Uncertainties in Table 5 surround the estimates of methane leakage in all three fuel cycles. High globalaverage leakage assumptions for CH4 from naturalgas production and distribution (3.5% of production) and low global-average emissions assumptions for coal (bituminous coal fuel carbon content and industry estimation for global CH4 emissions) have been made. The purpose of making these assumptions is to demonstrate that the emission-range extremes in the literature do not affect the fossil-fuel rankings. The effects of changing the methane emissions from natural-gas production and distribution to the industry estimate (1% of production) are given in the table in parentheses. Low methane emissionrate assumptions (~< 1% of production) are appropriate for evaluating potential new natural-gas systems. Measured data are required to compare effectively existing natural-gas and all coal systems because emissions from both vary significantly from site to site. The long-term view

Despite the biased assumptions used in creating Table 5, the long-term impacts of emissions from the three fuels result in a clear greenhouse-perspective ranking from best to worst: natural gas, oil, coal. If the assumption for natural-gas leakage is reduced from 3.5 to 1% of production (the value claimed by the natural-gas industry), CH4 emissions from gas production and distribution become equal to those from oil production, and amount to only two-thirds of the global average CH4 leaks from coal production reported by the Association of the Coal Producers of the European Community. Under those assumptions, accounting for CH4 emissions in effect Table 5. Per MJ cumulative CO2-equivalent forcing. After 20 years

After 50 years

Lifetime

25 87 112

13 87 100

3 87 90

For oil: CH4 from production CO: from combustion Total:

17 73 90

9 73 82

2 73 75

For natural gas: CH 4 from prod./dist. COe from combustion Total:

50 (14) 49 99 (63)

26 (7) 49 75 (56)

6 (2) 49 55 (51)

For bituminous coal: CH 4 from mining CO2 from combustion Total:

ENERGY POLICY July/August 1990

worsens the long-term picture for coal and has essentially no impact on the comparison between natural gas and oil. Adding N20 emissions to the calculations would not cause a re-ranking. The near-term view

The relative importance of the per MJ CO2equivalent emissions from CH4 is greater for all three fuels in the near-term than in the long-term. From a 20-year perspective, the high CH4-1eakage rate (3.5% of production) for natural-gas production and distribution causes the forcing from CH4 and CO2 emissions to equal each other. Lowering this assumption to 1% of production, which is probably a more relevant estimate at least for new systems, causes the CO2 emissions to resume their place of dominance in the natural-gas fuel cycle. Although coal remains the worst of the three fuels even in the worst-case scenario for natural gas, the near-term perspective for this case indicates that oil and natural gas are essentially equivalent. That picture changes radically, however, under the low-CH4-emission natural-gas case.

Electricity and combined heat and power production Electricity and combined heat and power (CHP) plants have varying efficiencies, meaning that they burn different amounts of fuel per unit of electricity (or electricity and heat) produced. Efficiency improvements in electricity and CHP plants can, therefore, contribute to reducing the emissions of CO2, C H 4 , and N20. A comparison of today's average v today's 'best available' utility technologies demonstrates the greenhouse advantage (in grams of CO2equivalent forcing per kWh) of efficient production of electricity and heat. Best-case CH4 and CO2 assumptions (from Table 5) for each of the fuels are used in these comparisons. Because the N20 emissions may be tied to specific plant designs and operating conditions, the effect of NzO emissions on the relative ranking of different plant types within each fuel category is unknown. In Tables 6 and 7, rough averages for lower-heating-value (LHV) system efficiencies are used. 5° In applications where heat is demanded as well as power, CHP plants can be used to increase the efficiency of electricity production by exploiting energy that would otherwise be lost as waste heat. District-heating systems used to provide space heating in urban settings and industrial-process heat production are examples of such applications. The energy required by CHP plants for electricity pro-

557

Fuel-cycle greenhouse-gas emissions

Table 6. System-efficiency impacts on COz-equivalent forcing resulting from large-scale electricity production. Assumed efficiency C02-equivalent forcing Plant type

(per kWh~)

% (LHV)

After After 20 50 years years Lifetime Average systems in place today: Conventional coal-fired steam turbine Oil-fired steam turbine Combined-cycle gas turbine Single-cycle gas turbine a Best available technologies: Conventional coal-fired steam turbine Coal PFBC/IGCC b Oil-fired combined-cycle gas turbine Steam-injected gas turbine (STIG) Gas-fired combined-cycle gas turbine Advanced technologies:c Intercooled STIG (ISTIG) Re-heat (RH) ISTIG RH ISTIG with steam recovery

35 40 40 25

1 152 1 029 810 738 567 504 907 806

926 675 459 734

40 43

1 008 938

900 837

810 753

50

648

590

540

44

515

458

417

50

454

403

367

52 55

436 412

388 367

353 334

59

384

342

311

a Single-cycle gas turbines are usually used strictly for backup and peak power. They are sometimes preferred for these applications because of their low capital costs. b PFBC and IGCC systems are preferred over conventional coal-fired steam turbines because they allow for significant reductions in SO2 and NOx emissions. c Not commercially available.

duction can be considered equal to the energy input to the system minus the energy that would have been consumed for the heat production had it been produced separately. The amount of electricity that can be produced per unit of heat produced (E/H ratios) in CHP plants varies depending on plant type/dsign. In most applications heat demand is limited. The design capacity of CHP plants built to meet that demand therefore determines the maximum amount of electricity that can be provided through CHP applications in a given setting. Electricity demand that exceeds this maximum must be met with alternative systems (see Table 6). The CO2equivalent forcing resulting from electricity production in various types of CHP systems is presented in Table 7. For all cases, a boiler with an efficiency of 90% is assumed as the alternative method of heat production that the CHP plant replaces. The CO2-equivalent forcings in Tables 6 and 7 represent the results of summations of forcing caused by emissions released at various points within the fuel cycles (see Figure 1). Because the precombustion emissions for each of the fuels are held

558

constant, however, the effect of combustiontechnology efficiencies on the fuel-cycle emissions can be seen in the tables. The differences between the per kWh CO2-equivalent forcing values represent the effects of both fuel and technology choices for producing heat and electricity. For example, the long-term CO2-equivalent forcing from coal-based power production can be reduced by 19% by switching from an average conventional coal-fired steam turbine to the best known coal-fired PFBC or IGCC technology. Similarly, fuel-switching from a conventional coal-fired steam turbine to a state-of-the-art gas-fired combined-cycle gas turbine would result in long-term per kWh forcing reduction of 60%. The same relationships hold for near-term comparisons of the CO2-equivalent forcings in Tables 6 and 7. In settings where heat can be used to meet a fraction of overall energy demand, CHP plants can provide that heat and part of the electricity demand. The amount of electricity that can be generated per unit heat production (E/H ratio), however, varies significantly amongst the CHP technologies available today. The combination of technologies chosen to meet the total electricity demanded of a system has, therefore, a significant effect on the resultant CO2-equivalent forcing caused by the system as a whole. Examples of the forcing caused by different combinations of CHP and central-station power plants are demonstrated in Table 8. Equal units of electricity and heat demand are assumed (10 each) for all of Table 7. CO2-equivalent forcing resulting from electricity production in plants.

CHP plant type Average systems in place today: Conventional coalfired steam turbine Best available technologies: Coal-fired steam turbine PFBC (coal) Oil-fired steam turbine STIG a Gas-fired combinedcycle gas turbine

CO2-equivalent forcing (per kWhe) Assumed After After efficiency E/H 20 50 % (LHV) ratio years years Lifetime

80

0.50

616

550

495

85 89

0.60 0.65

519 460

463 411

417 370

85 82

0.60 0.90

417 304

380 270

347 246

85

1.00

282

250

228

a For STIG systems, the system efficiency and E/H ratio given assumes the plant is configured to optimize for combined heat and power production. In practice such systems are not applied in this way today but are configured instead for power production alone (see Table 6).

ENERGY POLICY July/August 1990

Fuel-cycle greenhouse-gas emissions Table 8. CO2-equivalent forcing resulting from different electricity generating systems that deliver 10 units heat and 10 units electricity. CO2-equivalent forcing System technology combinations CHP

After 20 Additional non-CHP years

10 average coal-fired 5 best coal-fired steam turbines steam turbines 10 coal PFBCs 4 coal PFBCs 10 oil-fired steam 4 oil-fired turbines combined-cycle gas turbines 10 gas-fired combined-cycle gas turbines

(per kWh.) After 50

years Lifetime

11 200 10 000 8 352 7 458

9 000 6 712

6 762

6 160

5 630

2 800

2 500

2 280

the system combinations to demonstrate the maximum reductions in CO2-equivalent forcing that can be achieved by meeting a given heating requirement with CHP. Among the system combinations above, the same amount of heat and electricity production result in a wide range of CO2-equivalent forcing. Compared to the first coal case, the best available coal case is 25% less offensive and the best available gas technologies would result in 75% less forcing. From a policy perspective it is important to note that the differences in CO2-equivalent forcing that result from technology choices of systems (as in Table 8) or individual technologies (as in Tables 6 and 7) are the same regardless of the timeframe examined. Percentage differences amongst the technologies are equal for the near- and long-term perspectives.

Conclusions The question put forth in this analysis was whether or not the assumption that natural gas is a cleaner fuel than coal and oil was accurate, from the greenhouse perspective. The greenhouse impacts of coal, oil and natural gas were compared in order to answer this question, using an evaluation methodology that included three requirements: • • •

inclusion of emissions from the complete fuelcycles for each of the fuels; inclusion of C H 4 and N20 as well as CO2 emissions; evaluation of both near- and long-term greenhouse-forcing effects.

Using these evaluation criteria and the data available for describing emissions and the relative greenhouse forcing they cause, the results of the comparisons of the three fuels are unambiguous. Natural-gas exploitation is less offensive from a greenhouse perspective than either coal or oil ex-

ENERGY POLICY July/August 1990

ploitation compared on a per MJ basis. Sensitivity analyses were used to assure that this conclusion holds throughout the uncertainty ranges for the data assumptions regarding emissions and atmospheric chemistry, and holds for both near- and long-term comparisons. Although the forcing effects of C H 4 and N20 are significant for all three fuels, the CO2 emissions provide the driving factor in the ranking of the three fuels in long-term comparisons. Because the forcing effects of CH4 emissions are concentrated in time compared to those of CO2, the impacts of CH4 emissions can be as important as those of CO2 emissions in near-term comparisons, especially if natural-gas leakage is large (on the order of 3-4% of production). Methane emissions from natural-gas use must exceed even such high-leakage rates, however, to cause a re-ranking of the three fuels. 51 Assuming that leakage-rate claims by the naturalgas industry are correct, C H 4 emissions from natural-gas production and distribution (even in complex residential distribution grids) can be kept very low given today's technology and economic setting. It is important, therefore, that policy measures be taken to ensure that existing natural-gas leaks are brought under control and that existing and future natural-gas networks be subjected to regular maintenance and leakage monitoring. Network monitoring should be required by all policies that encourage fuel-switching to natural gas. Energy efficiency is a driving factor in determining the greenhouse forcing that results from the use of large-scale energy-conversion technologies compared per kWhc produced. Conversion-efficiency advantages in natural-gas technologies available today widen the gap in greenhouse-forcing comparisons between gas and coal or oil exploitation. Fuelswitching from coal or oil to natural gas for largescale power and/or CHP production would, therefore, lead to a reduction of anthropogenic emissions of greenhouse gases. End-use efficiency improvements and emission-abatement technologies can also be used to reduce greenhouse-gas emissions. Further analysis is needed to determine the extent to which global annual emissions can be reduced or their growth restrained through implementation of one or any combination of these measures. This article was sponsored in part by the Stockholm Environment Institute (SEI). The views and opinions herein do not necessarily reflect those of the SEI. The author accepts full responsibility for the accuracy of the data disclosed. The author gratefully acknowledges the invaluable advice and comments received from Professor Dean Abrahamson (University of Minnesota), Lars Kristoferson (Stockholm Environment Institute), and Professors Thomas B. Johansson and Bo Wiman (both of Lund University).

559

Fuel-cycle greenhouse-gas emissions IThe use of biomass has also been promoted as a viable alternative energy source, both on the grounds that it is renewable and that it is cleaner from a greenhouse perspective. There are no CO2 emissions from renewable biomass consumption, for example, when consumption is balanced by increased biomass production. In addition, no methane emissions should occur from complete combustion of biomass resources. Although the study of biomass is beyond the scope of this study, similar research using a complete fuel-cycle approach is needed to quantify what greenhouse forcing, if any, would be caused by widespread biomass use as a primary energy source. 2personal communication, Andrew Sprague, British Petroleum, October 1989. 3Statens Vattenfallsverk, Kolets Haelso- och Miljoeeffekter, Slutrapport April 1983, Underlagsdel 1: Teknik foer Kolanvaendning, Projekt KHM, Statens Vattenfallsverk, Sweden, 1983. 4p.A. Okken and T. Kram, 'CH4/CO-emission from fossil fuels: global warming potential', paper prepared for IEA-ETSAP Workshop, Paris, France, June 1989. 5p.j. Crutzen, 'Role of tropics in atmospheric chemistry', in R.E. Dickinson, ed, The Geophysiology of Amazonia, John Wiley and Sons, New York, USA, 1987, pp 107-132. 6CEPCEO (Association of the Coal Producers of the European Community), European Coal and the Greenhouse Effect, CEPCEO Research Committee, June 1989. 7British Petroleum, BP Statistical Review of World Energy, July 1989, BP, London, UK, 1989. sW. Seiler, 'Contribution of biological processes to the global budget of CH4 in the atmosphere', in M.J. Klug and C.A. Reddy, eds, Current Perspectives in Microbial Ecology, American Society of Meteorology, Washington DC, USA, 1984, pp 468--477. 9n.-J. Bolle, W. Seiler and B. Bolin, 'Other greenhouse gases and aerosols', in B. Bolin, B.R. Doeoes, J. Jaeger and R.A. Warrick, eds, Scope 29." The Greenhouse Effect, Climate Change, and Ecosystems, John Wiley and Sons, Chichester, UK, 1986, pp 157-203. 1°Gas Matters, 'Heart of the matter: people who live in green houses should not throw stones', 30 June 1989, pp 15-18; Alphatania Partnership, The, Methane Leakage from Natural Gas Operations: Results of an Investigation Conducted by The Alphatania Group in July and August 1989 The Alphatania Partnership, 19 Barlby Road, London Wl0 6AN, UK, August 1989. 11Alphatania Partnership, 1989, op cit, Ref 10. 12L.j. Muzio, M.E. Teague, J.C. Kramlich, J.A. Cole, J.M. McCarthy and R.K. Lyon, 'Errors in grab sample measurements of N20 from combustion sources', Journal of the Air and Waste Management Association, Vol 39, No 3, March 1989, pp 287-293. 13Ibid. 141bid. 15W.M. Hao, S.C. Wofsy, and M.B. McElroy, 'Sources of atmospheric nitrous oxide from combustion', Journal of Geophysical Research, Vol 92, No D3, 20 March 1987, pp 3098-3104; EPA (Environmental Protection Agency, USA), EPA/NOAA/ NASA/USDA N20 Workshop, Vol 1: Measurement Studies and Combustion Sources, Boulder CO, September 1987, EPA-600/888--079, Research Triangle Park NC, USA, May 1988. 16EPA, 1988, op cit, Ref 15. 17Ibid. 181bid; K. Dahlberg, A. Lindskog and B. Steen, Emissions of N20, CO, CH4, COS, and CS2 from Stationary Combustion Sources, IVL Report B891, Swedish Environmental Research Institute, Stockholm, Sweden, May 1988. 19For a discussion of potential sudden or threshold climatic responses to global temperature change, including global oceancirculation patterns, see W.S. Broecker, 'Greenhouse surprises', testimony given in a joint hearing before the Subcommittees on Environmental Protection and Hazardous Wastes and Toxic Substances of the Committee on Environment and Public Works, US Senate, One-hundredth Congress, first session, 28 January 1987. Reprinted in D.E. Abrahamson, ed, The Challenge of Global Warming, Island Press, Washington DC, USA, 1989, pp 196-209.

560

2°Personal communication, Professor Dean Abrahamson, University of Minnesota, USA, 30 December 1989. ~ID.J. Weubbles, A Primer On Greenhouse Gases, US DOE/ NBB0083, March 1988. 22T.E. Graedel and P.J. Crutzen, 'The changing atmosphere', Scientific American, Vol 261, No 3, September 1989, pp 28-36. 230p cit, Ref 9. 24V. Ramanathan et al, 'Climate-chemical interactions and effects of changing atmospheric trace gases', Review of Geophysics, Vol 25, No 7, August 1987, pp 1441-1482; V. Ramanathan, 'The greenhouse theory of climatic change: a test by an inadvertent ~lobal experiment', Science, Vol 240, April 1988, pp 293--299. See D. Abrahamson, ed, 1989, op cit, Ref 19. Estimates of 20x are given in D.R. Blake and F.S. Rowland 'Continuing worldwide increase in tropospheric methane, 1978 to 1987', Science, Vo1239, March 1988, pp 1129-1131; 32x in H.J. Wagner, C02 Emissions and Ways of Restructuring Energy Supplies in the FRG in View of Goals Stipulated at the "'Toronto Conference", Kernforschungsanlage Juelich GmbH, Programmgruppe Systemforschung und Technoiogische Entwicklung, Postfach D-5170, Juelich, FRG, 1989; and 32x in H. Grassl, 'Minderun des Treibhauseffektes: auf dem Weg zu einer CO2-Konvention', Energiewirtschafiliche Tagesfragen, 39 Jg (1989), Heft 1/2, pp 48-50. 26 M.A. Khalil and R.A Rasmussen, Causes of increasing atmospheric methane: depletion of hydroxyl radicals and the rise of emissions', Atmospheric Environment, Vol 19, No 3, 1985, pp 397-407, estimate 7-8 years; G.I. Pearman and P.J. Fraser, 'Sources of increased methane', Nature, Vol 332, April 1988, pp 489-490, estimate 7-11 years; R.J. Cicerone and R.S. Oremland, 'Biogeochemical aspects of atmospheric methane', Global Biogeochemical Cycles, Vol 2, No 4, December 1988, pp 299-327, estimate 8.1-11.8 years. 27Ramanathan et al, op cit, Ref 24. 2SCicerone and Oremland, op cit, Ref 26. 29Seiler, op cit, Ref 8; Ramanathan et al, 'Trace gas trends and their potential role in climate change', Journal of Geophysical Research, Vol 90, June 1985, pp 5547-5566; M.A. Khalil and R.A. Rasmussen, 'Causes of increasing atmospheric methane: depletion of hydroxyl radicals and the rise of emissions', Atmospheric Environment, Vol 19, No 3, 1985, pp 397-407; P.J. Crutzen, 'Tropospheric ozone: an overview', in I.S.A. Isaksen, cd, Tropospheric Ozone: Regional and Global Scale Interactions NATO ASI Series, D. Reidel Publishing Co, Dordrecht, Holland, 1988, pp 3-32. 3°D.J. Weubbles, K.E. Grant, P.S. Connell and J.E. Penner, 'The role of atmospheric chemistry in climate change', Journal of the Air and Waste Management Association, Vol 39, No 1, January 1989, pp 22-28; Cicerone and Oremland, 1988, op cit, Ref 26. 31Ramanathan, 1988, op cit, Ref 24. 3ZWeubbles et al, 1989, op cit, Ref 30. 33Crutzen, 1988, op cit, Ref 29. 341b/d. 35Cicerone and Oremland, 1988, op cit, Ref 26. 36Ramanathan et al, 1987, op cit, Ref 24; Cicerone and Oremland, 1988, op cit, Ref 26. 37Ramanathan et al, 1987, op cit, Ref 24. 38Cicerone and Oremland, 1988, op cit, Ref 26. 39Weubbles et al, 1989, op cit, Ref 30. 4°Statens Naturvaardsverk, Olika Gasers bidrag till Vaexthuseffekten - en Jaemfoerelse, Naturvaardsverket report 3647, Stockholm, Sweden, September 1989. 4~Weubbles et al, 1989, op cit, Ref 30. 42M. Kavanaugh, 'Estimates of future COe, N20 and NOx emissions from energy combustion', Atmospheric Environment Vol 21, No 3, 1987, pp 463--468. 43Ramanathan et al, 1987, op cit, Ref 24. 441bid. 451bid.

~Op cit, Ref 40.

D.H. Ehhalt and J.W. Drummond, 'NOx sources and tropospheric distribution of NOx during Stratoz III', in I.S.A. Isaksen,

ENERGY POLICY July/August 1990

Fuel-cycle greenhouse-gas emissions ed, Tropospheric Ozone: Regional and Global Scale Interactions, NATO ASI Series, D. Reidel Publishing Co, Dordrecht, Holland, 1988, pp 217-237. 48Ramanathan et al, 1985, op cit, Ref 29; Crutzen, 1988, op cit, Ref 29; Weubbles et al, 1989, op cit, Ref 30. 49H.-J. Bolle et al, op cit, Ref 9; Ramanathan et al, 1985, op cit, Ref 29. 5°Personal communication, Per Svenningsson, Environmental and Energy Systems Studies, Lund University, October 1989, and Robert Williams, Center for Energy and Environmental Studies,

Princeton University, September 1989. 51For comparisons under a timescale of 20 years, natural-gas leakage must exceed 3.6% of production for gas to rank more offensive than oil and must exceed 5.9% of production to rank worse than coal. For long-term comparisons (over the atmospheric lifetimes of the emissions), the gas-leakage rates must become extreme before the ranking of the fuels changes. Natural-gas leakage rates must exceed 22% and 38% of production for gas to become worse than oil and coal respectively.

Appendix The following general conversion factors were used in the text: 1 Btu = 1055 Joules 1 MTOE = 45 x 1015 Joules l k W h = 3.6MJ Some of the C H 4 rates were reported in reference material as natural-gas leakage rather than C H 4 leakage. In such cases, the natural-gas leaked is assumed to contain (on average) 75% CH4 by weight. Table 1: The production and reserves data are reported in BP (1989) in 10 6 t for coal, in 10 9 t (reserves) and 10 6 t (production) for crude oil, and in 1012 m 3 (reserves) and 10 6 t of oilequivalent (production) for natural gas. Conversions to energy units have been made using the following global average conversion factors: Anthracite and bituminous coal: 1 MTOE = 1.5 x 106t Sub-bituminous coal and lignite: 1 MTOE = 3.0 × 106t Natural gas: 1 MTOE = 111.1 x 10 9 m 3 Table 2: Coal carbon contents are reported in MacDonald (1982) in gC per Btu (British thermal unit). One g of C emitted as COz is equivalent to 3.66 g CO2. Table 3: Forcings per mole in the atmosphere are converted to forcings per kg in the atmosphere using the atomic weights of the gases: C H 4 : 1 mole = 16.05 g N 2 0 : 1 mole = 44.02 g 03: 1 mole = 48 g CO2:1 mole = 44.01

relative to that of CO2 for each gas (gasi) is calculated by solving the equation: S~

GFi=

( r ~ X 17i

e-t/l:i))

(1 -

S~ (rfcoz x ~co2 (1

-

dt

e-t/~c°2))dt

where rf = relative forcing values given in Table 3 and x (the atmospheric residence time) for each gas is assumed to be: for for for for

CH4:9 years CO2:100 years N 2 0 : 1 5 0 years 03: 0.2 years.

For the lifetime case, t = infinity and the solution to the above equation becomes simply: rj~ X Ti /fco2

X 1~CO 2

For CH4, the indirect forcing caused by 03 and CO2 generated as a result of CH4 oxidation has been included. Each mole of CH4 that oxidizes is assumed to produce one mole of 03 and one mole of CO2. The resulting relative forcing of each has been calculated separately and added to the direct relative forcing caused by the CH4. Table 5: Emissions assumptions (per MJ of fuel consumed) given previously in the text have been multiplied by the forcings relative to COz given in Table 4 and subsequently tallied. For example, the per MJ CO2-equivalent forcing caused by bituminous coal 20 years after consumption has been calculated as follows:

Table 4: The greenhouse forcing (GFi)

ENERGY POLICY July/August 1990

C H 4 (mining) = 78 gCOzeq/gCH4 × 0.32 gCH4/MJ

= 25

COz (combustion) = 1 gCO2eq/gCOz x 87 gCO2/MJ = 87 NzO (combustion) = 258 gCO2eq/gN20 x 0.09 gN20/MJ = 23 Total = 135 gCO2eq/MJ Table a: For large-scale electricity production, the CO2-equivalent forcing caused per kWhe produced is a function of the conversion technology's system efficiency. Lower-heatingvalue system efficiencies are used in this table. The values in Table 6 are calculated by dividing the per MJ CO2-equivalent forcing for a given fuel (Table 5) by an assumed conversion-system efficiency. For a conventional coal-fired steam turbine, the forcing per kWh~ 20 years after generation is calculated as follows: (25 + 87) gCO2eq MJ

×

3.6 MJ

- -

kWh 0.35

1152 gCO2eq kWhe Table 7: Forcing resulting from electricity production carried out in combined heat and power plants is calculated similarly to that for centralstation power plants. The total system efficiency of the CHP plant is adjusted to give separate efficiencies for the heat and electricity production. The energy requirement for electricity production is taken as the total energy supplied minus that which would have been required to produce tthe heat independently. The energy requirement for independent heat production is calculated assuming a system effi-

561

Fuel-cycle greenhouse-gas emissions ciency of 90% in all cases. For a conventional coal-fired steam turbine with an E/H ratio of 1/2 and a total L H V system efficiency of 80%, the energy required (E) for electricity production is calculated as follows: E

(2 kWh + 1 kWh)

kWhe

0.8

562

A t 20 y: 5.5 MJ -

-

systems demonstrated produces 10 units of electricity and 10 units of heat. The E/H ratios for the CHP plant types (Table 7) determine the number of non-CHP plants (Table 6) required. For the first system combination, 10 CHP and 5 non-CHP plants are required, producing the following forcing value:

112 gCO2eq x

kWh

MJ

= 616 gCO2eq kWh

2 kWh _ _ _ 1.53 kWh 0.9 3.6 MJ × _ _ _ kWh

The E values are multiplied by the forcing/MJ values (excluding N20 ) in Table 5:

5.51MJ

Table 8: The C O 2 equivalent forcing caused by the systems in Table 8 are simple summations of the forcing caused by the CHP and non-CHP system components. Each of the four

At 20 y: 10 x 616 gCO2eq/kWh + 5 x 1008 gCOEeq/kWh = 11 200 gCOEeq/kWh

ENERGY POLICY July/August 1990