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Future CO2 emissions from combustion of natural and synthetic gases
Energy Vol. IO, No. 9, PP. 1043-1049. Printed m Great Britain.
0360-5442/85 $3.00 + .@I 0 1985 Pergamon Prw Ltd.
1985
FUTURE CO2 EMISSIONS FROM COMBUSTION NATURAL AND SYNTHETIC GASEST
OF
DAVID B. REISTER Institute for Energy Analysis, Oak Ridge Associated Universities, Post Office Box I 17, Oak Ridge, TN 37831-0117, U.S.A.
and JOHN A. LAURMANN Gas Research Institute, 8600 W. Bryn Mawr Avtinue, Chicago, IL 6063 I, U.S.A. (Received 12 October 1984)
Abstract-We assess the contribution of future gas combustion to the global emissions of carbon dioxide. Presently available natural gas resources are too small to make a significant contribution to the ultimate carbon emissions. However, if the production and combustion of synthetic gas becomes a major component of the global energy system, then carbon emissions from gas can become significant-in some cases, up to 50% of the total. Increased use of synthetic gas does not increase carbon emissions, unless the gas displaces electricity produced from nuclear power or solar energy.
ANALYSIS
Carbon dioxide (COz) in the atmosphere is a colorless gas that is transparent to shortwavelength radiation from the sun, but absorbs the long-wavelength radiation from the earth. As the amount of COz in the atmosphere increases, the average temperature of the earth will probably increase. If the amount of carbon (C) in the atmosphere doubles, climate models forecast that the average temperature of the earth’s surface will increase by 1 to 4”C, thus leading to significant modification of the climate. In 1980 there were about 720 petagrams (Pg) of C in the atmosphere, an increase of about 50 Pg (7%) from the 1959 value of 670 Pg. In the next two decades the C emissions will probably be less than 130 Pg.? Since about 50% of the C emissions remain in the atmosphere, with the remaining 50% being sequestered in the ocean and biosphere, the C in the atmosphere will probably increase by less than 65 Pg (9%) in the next two decades. Thus, the C in the atmosphere will increase gradually and will probably not double before 2050. Although the annual rate of C emissions after 2000 is quite uncertain, an estimate can be made of the ultimately recoverable carbon in the fossil-fuel reservoirs. The best estimate of Rotty and Marland is 4000 Pg with an upper limit of 17,000 Pg.2 Combustion of 4000 Pg of C could increase the C in the atmosphere by a factor of three, while combustion of 17,000 Pg could cause an increase of 12. Thus, the amount of C in the fossil-fuel reservoirs is large enough to cause significant climate changes if it were all released as CO2 to the atmosphere. For the best estimate of Ref. 2, coal provides 85% of the C, while natural gas provides only 3%. For the upper limit of Ref. 2, unconventional petroleum provides 58% of the C, coal provides 38%, and natural gas provides only 1%. The ultimately recoverable C provided by unconventional natural gas (gas from tight gas formations or from geopressured reservoirs) will probably be about the same as from conventional natural gas. Thus,
t This article is based on work performed under Gas Research Institute Contract No. 5082-253-0758. Oak Ridge Associated Universities also performs complementary work for the U.S. Department of Energy under Contract No. DE-ACOS-7600R00033. t In the recent National Research Council report on climate change,’ Nordhaus and Yohe estimated that there is a 95% chance that annual C emissions in 2000 will be less than 8 Pg. If the annual emissions increase linearly from 5 Pg in 1980 to 8 Pg in 2000, the cumulative emissions will be 130 Pg. 1043
1044
D. B. REISTERand J. A. LAURMANN Table 1. Average C emissions (in kg of C/GJ) for the secondary types of energy for the base case. Year
Liquids
Gases
Solids
Electricity
1975
19.7
13.8
23.9
53.0
2000
20.7
14.7
22.9
38.9
2025
25.5
16.9
20.5
23.9
2050
33.0
19.8
17.5
17.4
2075
40.0
23.8
16.5
8.9
2100
42.6
28.0
16.8
5.8
natural gas cannot be a significant source of atmospheric C. However, the production and combustion of synthetic gas from coal could be a significant source of C. The four types of secondary energy are liquids, gases, solids, and electricity. Although energy services may be provided by any of these forms of energy, they are not perfect substitutes. Typically, the substitution of one form of secondary energy for another requires a new piece of capital stock (motor, boiler, furnace, etc.). The average C emissions for the four secondary types of energy are not equal; currently, the C emissions to produce a joule of electricity are 3.8 times as large as the C emissions per joule of gas (see Table 1). Although, a joule of electricity may produce more energy service than a joule of gas, substitution of gas for electricity reduces C emissions today. What about future substitution? Electricity can be produced from solar energy or nuclear power without using coal and without C emissions. As synthetic gas replaces natural gas, C emissions per joule of gas will increase, while C emissions per joule of electricity could decrease (see Table 1). Thus future substitution of synthetic gas for electricity produced from nuclear power or solar energy could increase C emissions. The Gas Research Institute (GRI) asked the Institute for Energy Analysis (IEA) to assess the contribution of future gas combustion to the global emissions of C02. An assessment of CO* emissions requires a global context. Furthermore, an assessment
3000
I
r
I
I
I
I
1
2025
2050
2075
Year Fig. 1. World energy use projections.
1
I
2100
1045
CO2 emissions from natural and synthetic gases
60
50
High Carbon Case
World
1975
2000
2025
2050
2075
)O
Year Fig. 2. Carbon emissions, United States and the world.
requires a very long time frame (100 years or more). To forecast C emissions for the world, a computer model developed at IEA by Edmonds and Reilly has been used.394 To assess the contribution of gas, a base case and a high C emissions case were developed, and the impact of four options that increase the demand for gas were analyzed. Many plausible scenarios for global C emissions could be developed. In close consultation with the staff of GRI, IEA developed a base-case scenario that agreed with existing GRI forecasts of energy demand patterns in the United States through 2020. For the base case, IEA assumed that the world makes the transition from fossil fuel to nuclear power and solar energy by 2100; that is, from C-based energy to non-C energy. For the high C case the world continues to rely on fossil fuel for energy. World energy demand for the two cases is shownt in Fig. 1, and the C emissions for the two cases are displayed in Fig. 2. By 2100 the C emissions for the base case are quite low, while the C emissions for the high case are quite high. The detailed assumptions on population and GNP for the two cases are given in the report by IEA to GRI.’ For the base case the C emissions increase until 2050 and remain almost constant until 2100. The cumulative C emissions to 2100 are 1130 Pg; assuming that 50% of the t For the high-C case there is a large supply of low-cost fossil fuel. Compared to the base case, the costof energy is lower for the high-C case, and the energy demand is higher.
D. B. REISTER andJ. A. LAURMANN
1046
Table 2. Carbon emissions (Pg/yr) associated with the production and combustion of gas. Carbon from Gas as Percent of total Carbon Emissions
Carbon Emissions from Gas
a
Year United States
World
A.
World Base
Case
1975
0.51
0.27
11
2000
1.22
0.27
17
2025
1.99
0.29
22
2050
2.22
0.35
21
2075
2.37
0.41
22
2100
2.51
0.44
24
B. 1975
0.51
High-Carbon
United States
Case
0.27
11
2000
1.30
0.32
16
2025
2.72
0.37
ia
2050
4.46
0.62
16
2075
8.28
1.35
19
2100
12.25
2.19
20
I I
emissions remain in the atmosphere, the C in the atmosphere increases by about 80% by 2 100. For the high C case the annual C emissions continually increase, and by 2 100 the cumulative C emissions are 3190 Pg. If 50% of the emissions remain in the atmosphere, then the C in the atmosphere would increase by 220% by 2100. Thus, the climate impact of the high C case will be much larger than for the base case. In 1975, 11% of the worldwide C emissions resulted from natural gas combustion, and gas combustion in the United States contributed 6% of the worldwide C emissions (see Table 2). By 2100 for the base case the contribution of natural and synthetic gases increases to 24%, while the contribution by the United States to the world total decreases slightly to 4%. By 2100 for the high-C case, the gas contribution increases to 20%, while the contribution by the United States is 4%. The gas contribution is about the same share of the total emissions for the two cases. The share is determined by the relative cost of the four secondary types of energy. For the base case, fossil fuel is expensive; for the high-C case, fossil fuel is inexpensive. But the relative cost of each of the secondary types of energy is about the same for the two cases. For both cases the gas industry remains strong until 2100. For the base case the annual worldwide demand for gas increases from 35 exajoules (EJ) in 1975 to 133 EJ in 2050 before declining to 122 EJ in 2 100 (see Table 3). For the high-C case the demand for gas increases steadily from 35 EJ in 1975 to 375 EJ in 2100. By 2100 synthetic gas from biomass and coal provides 64% of the gas supply for the base case and 90% of the gas supply for the high-C case (see Table 3). Sensitivity analysis was performed to assess the contribution of gas combustion to the global emissions of COz. Four options to increase the demand for gas were analyzed: (1) reduction of the nonfuel costs for producing synthetic gas; (2) reduction of the amount of coal required to produce synthetic gas; (3) reduction of the cost of unconventional natural gas; (4) introduction of new technology that uses gas to provide energy services with the same end-use efficiency as electricity. For the base case each of the four options increases the demand for gas. The combined
1047
CO2 emissions from natural and synthetic gases Table 3. Gas supply (EJ/yr) for the world. Total
Synthetic
Natural Year
Base
A.
Case
1975
34.6
0.0
34.6
2000
82.2
3.6
85.9
2025
109.0
17.7
126.7
2050
96.8
36.0
132.8
2075
71.0
59.0
130.0
2100
43.7
78.7
122.3
B.
High-C
Case
1975
34.6
0.0
34.6
2000
91.7
2.0
93.7
2025
143.3
23.6
166.9
2050
140.4
74.5
214.9
2075
72.8
210.7
283.6
2100
36.2
339.2
375.4
effect of all options is to increase gas demand by more than a factor of two after 2050. Each of the options decreases the demand for electricity. Compared to the base case, their combined effect causes demand for electricity to decrease by more than a factor of two in 2050, but they have only a small impact on C emissions. Compared to the base case, the combined effect of all four options is a 10% reduction of global C emissions in 2000 and a 9% increase in 2100. Since the price of unconventional natural gas was already low for the high-C case, only three options can be analyzed [(l), (2), and (4)]. For the high-C case the combined impact of the three options increases gas demand by about a factor of two by 2 100 (see Fig. 3), decreases worldwide electricity demand by about 35% after 2025, and reduces the
0 1975
I 2000
I 2025
I 2050
I 2075
Year Fig. 3. Gas share of world secondary energy demand, high-carbon case.
2100
D. B. RENTERand J. A. LAURMANN
1048
Table 4. Average C emissions (kg of C/GJ) for the secondary types of energy for the high-C case. Liquids
Gases
Solids
19.70
13.80
23.90
54.51
20.71
14.24
23.32
47.82
27.71
16.86
23.62
55.64
36.21
21.35
23.73
65.39
42.12
30.07
23.79
68.42
45.24
33.60
23.81
68.09
Electricity
total global C emissions by about 10% after 2000. For both cases most of the reduction in C emissions is caused by option four, which improves the end-use efficiency of gas. The explanation for the paradox that the options reduce COz emissions initially for the base case and increase C emissions later is one of the more interesting results of this study. The increase in demand for gas results in a decrease in the demand for electricity. As shown in Table 1, combustion of a gigajoule of gas in 2000 releases 15 kg of C, while consumption of a gigajoule of electricity in 2000 releases 39 kg of C. By 2100 the situation has reversed: 28 kg for gas and 6 kg for electricity. The reason for the reversal is that, by 2 100, electricity is primarily produced by nuclear power and solar energy
I
2000
I
I
2050
2025
I
2075
Year Fig. 4. Contributionof gas use to the C emissions rate.
i
2100
CO1 emissions from natural and synthetic gases
I049
than by combustion of fossil fuel. In contrast, for the high-C case, consumption of electricity always releases more C than the combustion of gas (see Table 4) because fossil fuel is always the primary source of electricity. Thus, substitution of gas for electricity can reduce C emissions when the electricity is produced from fossil fuel. Carbon emissions associated with the production and combustion of gas were displayed in Table 2 for the base case and high-C case. The assumptions made for all four cases in the sensitivity analysis resulted in an increased demand for gas. The C emissions for the high gas-demand cases are displayed in Fig. 4. For the high gas-demand base case, C emissions from gas increase from 12 to 50% for the world and from 5 to 11% for the United States. For the high gas-demand high-C case, carbon emissions increase from 12% to 45% for the world and from 5% to 10% for the United States. Thus, the production and combustion of gas becomes the major source of C emissions for the high gasdemand cases. Successful research and development, with results similar to the four assumptions in the sensitivity analysis, will increase demand for gas and will help the gas industry, but such efforts will not have a major impact, positive or negative, on the total emissions of COz. More unconventional natural gas cannot significantly reduce C emissions because its resource base is much smaller than coal. Increased use of synthetic gas does not increase C emissions unless the gas displaces electricity produced from nuclear power or solar energy. If the gas industry is successful in increasing demand for gas, an unavoidable consequence is that synthetic gas could become the major source of COz emissions in the next century. rather
REFERENCES
1. National Research Council, Changing Climate. National Academy Press, Washington, DC. (1983).
2. R. M. Rotty and G. Marland, Constraints on fossil fuel use in Interactions of Energy and Climate, Reidel, Boston ( 1980). 3. J. Edmonds and J. Reilly, Energy Economics 5, 74 (1983). 4. J. Edmonds and J. Reilly, Energy 8, 419 (1983). 5. D. B. Reister, An Assessment of the Contribution of Gas to the Global Emissions of Carbon Dioxide. Gas Research Institute, Chicago, Ill. (1984).
0360-5442/85 $3.00 + .@I 0 1985 Pergamon Prw Ltd.
1985
FUTURE CO2 EMISSIONS FROM COMBUSTION NATURAL AND SYNTHETIC GASEST
OF
DAVID B. REISTER Institute for Energy Analysis, Oak Ridge Associated Universities, Post Office Box I 17, Oak Ridge, TN 37831-0117, U.S.A.
and JOHN A. LAURMANN Gas Research Institute, 8600 W. Bryn Mawr Avtinue, Chicago, IL 6063 I, U.S.A. (Received 12 October 1984)
Abstract-We assess the contribution of future gas combustion to the global emissions of carbon dioxide. Presently available natural gas resources are too small to make a significant contribution to the ultimate carbon emissions. However, if the production and combustion of synthetic gas becomes a major component of the global energy system, then carbon emissions from gas can become significant-in some cases, up to 50% of the total. Increased use of synthetic gas does not increase carbon emissions, unless the gas displaces electricity produced from nuclear power or solar energy.
ANALYSIS
Carbon dioxide (COz) in the atmosphere is a colorless gas that is transparent to shortwavelength radiation from the sun, but absorbs the long-wavelength radiation from the earth. As the amount of COz in the atmosphere increases, the average temperature of the earth will probably increase. If the amount of carbon (C) in the atmosphere doubles, climate models forecast that the average temperature of the earth’s surface will increase by 1 to 4”C, thus leading to significant modification of the climate. In 1980 there were about 720 petagrams (Pg) of C in the atmosphere, an increase of about 50 Pg (7%) from the 1959 value of 670 Pg. In the next two decades the C emissions will probably be less than 130 Pg.? Since about 50% of the C emissions remain in the atmosphere, with the remaining 50% being sequestered in the ocean and biosphere, the C in the atmosphere will probably increase by less than 65 Pg (9%) in the next two decades. Thus, the C in the atmosphere will increase gradually and will probably not double before 2050. Although the annual rate of C emissions after 2000 is quite uncertain, an estimate can be made of the ultimately recoverable carbon in the fossil-fuel reservoirs. The best estimate of Rotty and Marland is 4000 Pg with an upper limit of 17,000 Pg.2 Combustion of 4000 Pg of C could increase the C in the atmosphere by a factor of three, while combustion of 17,000 Pg could cause an increase of 12. Thus, the amount of C in the fossil-fuel reservoirs is large enough to cause significant climate changes if it were all released as CO2 to the atmosphere. For the best estimate of Ref. 2, coal provides 85% of the C, while natural gas provides only 3%. For the upper limit of Ref. 2, unconventional petroleum provides 58% of the C, coal provides 38%, and natural gas provides only 1%. The ultimately recoverable C provided by unconventional natural gas (gas from tight gas formations or from geopressured reservoirs) will probably be about the same as from conventional natural gas. Thus,
t This article is based on work performed under Gas Research Institute Contract No. 5082-253-0758. Oak Ridge Associated Universities also performs complementary work for the U.S. Department of Energy under Contract No. DE-ACOS-7600R00033. t In the recent National Research Council report on climate change,’ Nordhaus and Yohe estimated that there is a 95% chance that annual C emissions in 2000 will be less than 8 Pg. If the annual emissions increase linearly from 5 Pg in 1980 to 8 Pg in 2000, the cumulative emissions will be 130 Pg. 1043
1044
D. B. REISTERand J. A. LAURMANN Table 1. Average C emissions (in kg of C/GJ) for the secondary types of energy for the base case. Year
Liquids
Gases
Solids
Electricity
1975
19.7
13.8
23.9
53.0
2000
20.7
14.7
22.9
38.9
2025
25.5
16.9
20.5
23.9
2050
33.0
19.8
17.5
17.4
2075
40.0
23.8
16.5
8.9
2100
42.6
28.0
16.8
5.8
natural gas cannot be a significant source of atmospheric C. However, the production and combustion of synthetic gas from coal could be a significant source of C. The four types of secondary energy are liquids, gases, solids, and electricity. Although energy services may be provided by any of these forms of energy, they are not perfect substitutes. Typically, the substitution of one form of secondary energy for another requires a new piece of capital stock (motor, boiler, furnace, etc.). The average C emissions for the four secondary types of energy are not equal; currently, the C emissions to produce a joule of electricity are 3.8 times as large as the C emissions per joule of gas (see Table 1). Although, a joule of electricity may produce more energy service than a joule of gas, substitution of gas for electricity reduces C emissions today. What about future substitution? Electricity can be produced from solar energy or nuclear power without using coal and without C emissions. As synthetic gas replaces natural gas, C emissions per joule of gas will increase, while C emissions per joule of electricity could decrease (see Table 1). Thus future substitution of synthetic gas for electricity produced from nuclear power or solar energy could increase C emissions. The Gas Research Institute (GRI) asked the Institute for Energy Analysis (IEA) to assess the contribution of future gas combustion to the global emissions of C02. An assessment of CO* emissions requires a global context. Furthermore, an assessment
3000
I
r
I
I
I
I
1
2025
2050
2075
Year Fig. 1. World energy use projections.
1
I
2100
1045
CO2 emissions from natural and synthetic gases
60
50
High Carbon Case
World
1975
2000
2025
2050
2075
)O
Year Fig. 2. Carbon emissions, United States and the world.
requires a very long time frame (100 years or more). To forecast C emissions for the world, a computer model developed at IEA by Edmonds and Reilly has been used.394 To assess the contribution of gas, a base case and a high C emissions case were developed, and the impact of four options that increase the demand for gas were analyzed. Many plausible scenarios for global C emissions could be developed. In close consultation with the staff of GRI, IEA developed a base-case scenario that agreed with existing GRI forecasts of energy demand patterns in the United States through 2020. For the base case, IEA assumed that the world makes the transition from fossil fuel to nuclear power and solar energy by 2100; that is, from C-based energy to non-C energy. For the high C case the world continues to rely on fossil fuel for energy. World energy demand for the two cases is shownt in Fig. 1, and the C emissions for the two cases are displayed in Fig. 2. By 2100 the C emissions for the base case are quite low, while the C emissions for the high case are quite high. The detailed assumptions on population and GNP for the two cases are given in the report by IEA to GRI.’ For the base case the C emissions increase until 2050 and remain almost constant until 2100. The cumulative C emissions to 2100 are 1130 Pg; assuming that 50% of the t For the high-C case there is a large supply of low-cost fossil fuel. Compared to the base case, the costof energy is lower for the high-C case, and the energy demand is higher.
D. B. REISTER andJ. A. LAURMANN
1046
Table 2. Carbon emissions (Pg/yr) associated with the production and combustion of gas. Carbon from Gas as Percent of total Carbon Emissions
Carbon Emissions from Gas
a
Year United States
World
A.
World Base
Case
1975
0.51
0.27
11
2000
1.22
0.27
17
2025
1.99
0.29
22
2050
2.22
0.35
21
2075
2.37
0.41
22
2100
2.51
0.44
24
B. 1975
0.51
High-Carbon
United States
Case
0.27
11
2000
1.30
0.32
16
2025
2.72
0.37
ia
2050
4.46
0.62
16
2075
8.28
1.35
19
2100
12.25
2.19
20
I I
emissions remain in the atmosphere, the C in the atmosphere increases by about 80% by 2 100. For the high C case the annual C emissions continually increase, and by 2 100 the cumulative C emissions are 3190 Pg. If 50% of the emissions remain in the atmosphere, then the C in the atmosphere would increase by 220% by 2100. Thus, the climate impact of the high C case will be much larger than for the base case. In 1975, 11% of the worldwide C emissions resulted from natural gas combustion, and gas combustion in the United States contributed 6% of the worldwide C emissions (see Table 2). By 2100 for the base case the contribution of natural and synthetic gases increases to 24%, while the contribution by the United States to the world total decreases slightly to 4%. By 2100 for the high-C case, the gas contribution increases to 20%, while the contribution by the United States is 4%. The gas contribution is about the same share of the total emissions for the two cases. The share is determined by the relative cost of the four secondary types of energy. For the base case, fossil fuel is expensive; for the high-C case, fossil fuel is inexpensive. But the relative cost of each of the secondary types of energy is about the same for the two cases. For both cases the gas industry remains strong until 2100. For the base case the annual worldwide demand for gas increases from 35 exajoules (EJ) in 1975 to 133 EJ in 2050 before declining to 122 EJ in 2 100 (see Table 3). For the high-C case the demand for gas increases steadily from 35 EJ in 1975 to 375 EJ in 2100. By 2100 synthetic gas from biomass and coal provides 64% of the gas supply for the base case and 90% of the gas supply for the high-C case (see Table 3). Sensitivity analysis was performed to assess the contribution of gas combustion to the global emissions of COz. Four options to increase the demand for gas were analyzed: (1) reduction of the nonfuel costs for producing synthetic gas; (2) reduction of the amount of coal required to produce synthetic gas; (3) reduction of the cost of unconventional natural gas; (4) introduction of new technology that uses gas to provide energy services with the same end-use efficiency as electricity. For the base case each of the four options increases the demand for gas. The combined
1047
CO2 emissions from natural and synthetic gases Table 3. Gas supply (EJ/yr) for the world. Total
Synthetic
Natural Year
Base
A.
Case
1975
34.6
0.0
34.6
2000
82.2
3.6
85.9
2025
109.0
17.7
126.7
2050
96.8
36.0
132.8
2075
71.0
59.0
130.0
2100
43.7
78.7
122.3
B.
High-C
Case
1975
34.6
0.0
34.6
2000
91.7
2.0
93.7
2025
143.3
23.6
166.9
2050
140.4
74.5
214.9
2075
72.8
210.7
283.6
2100
36.2
339.2
375.4
effect of all options is to increase gas demand by more than a factor of two after 2050. Each of the options decreases the demand for electricity. Compared to the base case, their combined effect causes demand for electricity to decrease by more than a factor of two in 2050, but they have only a small impact on C emissions. Compared to the base case, the combined effect of all four options is a 10% reduction of global C emissions in 2000 and a 9% increase in 2100. Since the price of unconventional natural gas was already low for the high-C case, only three options can be analyzed [(l), (2), and (4)]. For the high-C case the combined impact of the three options increases gas demand by about a factor of two by 2 100 (see Fig. 3), decreases worldwide electricity demand by about 35% after 2025, and reduces the
0 1975
I 2000
I 2025
I 2050
I 2075
Year Fig. 3. Gas share of world secondary energy demand, high-carbon case.
2100
D. B. RENTERand J. A. LAURMANN
1048
Table 4. Average C emissions (kg of C/GJ) for the secondary types of energy for the high-C case. Liquids
Gases
Solids
19.70
13.80
23.90
54.51
20.71
14.24
23.32
47.82
27.71
16.86
23.62
55.64
36.21
21.35
23.73
65.39
42.12
30.07
23.79
68.42
45.24
33.60
23.81
68.09
Electricity
total global C emissions by about 10% after 2000. For both cases most of the reduction in C emissions is caused by option four, which improves the end-use efficiency of gas. The explanation for the paradox that the options reduce COz emissions initially for the base case and increase C emissions later is one of the more interesting results of this study. The increase in demand for gas results in a decrease in the demand for electricity. As shown in Table 1, combustion of a gigajoule of gas in 2000 releases 15 kg of C, while consumption of a gigajoule of electricity in 2000 releases 39 kg of C. By 2100 the situation has reversed: 28 kg for gas and 6 kg for electricity. The reason for the reversal is that, by 2 100, electricity is primarily produced by nuclear power and solar energy
I
2000
I
I
2050
2025
I
2075
Year Fig. 4. Contributionof gas use to the C emissions rate.
i
2100
CO1 emissions from natural and synthetic gases
I049
than by combustion of fossil fuel. In contrast, for the high-C case, consumption of electricity always releases more C than the combustion of gas (see Table 4) because fossil fuel is always the primary source of electricity. Thus, substitution of gas for electricity can reduce C emissions when the electricity is produced from fossil fuel. Carbon emissions associated with the production and combustion of gas were displayed in Table 2 for the base case and high-C case. The assumptions made for all four cases in the sensitivity analysis resulted in an increased demand for gas. The C emissions for the high gas-demand cases are displayed in Fig. 4. For the high gas-demand base case, C emissions from gas increase from 12 to 50% for the world and from 5 to 11% for the United States. For the high gas-demand high-C case, carbon emissions increase from 12% to 45% for the world and from 5% to 10% for the United States. Thus, the production and combustion of gas becomes the major source of C emissions for the high gasdemand cases. Successful research and development, with results similar to the four assumptions in the sensitivity analysis, will increase demand for gas and will help the gas industry, but such efforts will not have a major impact, positive or negative, on the total emissions of COz. More unconventional natural gas cannot significantly reduce C emissions because its resource base is much smaller than coal. Increased use of synthetic gas does not increase C emissions unless the gas displaces electricity produced from nuclear power or solar energy. If the gas industry is successful in increasing demand for gas, an unavoidable consequence is that synthetic gas could become the major source of COz emissions in the next century. rather
REFERENCES
1. National Research Council, Changing Climate. National Academy Press, Washington, DC. (1983).
2. R. M. Rotty and G. Marland, Constraints on fossil fuel use in Interactions of Energy and Climate, Reidel, Boston ( 1980). 3. J. Edmonds and J. Reilly, Energy Economics 5, 74 (1983). 4. J. Edmonds and J. Reilly, Energy 8, 419 (1983). 5. D. B. Reister, An Assessment of the Contribution of Gas to the Global Emissions of Carbon Dioxide. Gas Research Institute, Chicago, Ill. (1984).