- Email: [email protected]
The Role of Catalysis in Achieving a Sustainable Society
K.J. Smith, E.C. Sanford (Editors), Progress in Catalysis 0 1992 Elscvier Scicncc Publishers B.V. All rights reserved.
I79
The Role of Catalysis in Achieving a Sustainable Society Eric L. Tollefson Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, Alberta, Canada T2N 1N4 Abstract This paper describes changes occurring in the environment which could radically alter the nature of society in the decades ahead unless corrective action is taken to create sustainable conditions. The upward trend in carbon emissions together with the effects of other greenhouse gases on the average global temperature are considered and suggestions made as to how a sustainable state might be achieved by replacing non-renewable energy sources with renewable ones such as trees, biomass and hydrogen as well as hydro, solar, tidal and wind. INTRODUCTION The Brundtland Report "Our Common Future" [l] states "Humanity has the ability to make development sustainable - to ensure that it meets the needs of the present Rithout compromising the ability of future generations to meet their own needs". To make "development sustainable" certain limits would have to be imposed, for example, so that the biosphere would be capable of absorbing the effects of human activities. McLaren [2] has noted that over the past 150 years the world population has grown from 1 to 5 billion with a current doubling time of 30 to 40 years, that a comparable increase in the usage of fossil fuels is leading to global pollution, to changes in climate and sea level and that destruction of the habitat of life is accelerating causing extinction of many species. The destruction of the forests, soil erosion, overuse of ground water and the production of wastes have also been accelerated. In McLaren's opinion one cannot talk about "stabilization" or "equity" or use the term "sustainable development" while these influences are disrupting our home planet and "most of them are growing exponentially or greater". Of all these destabilizing factors in terms of environmental security, the buildup of carbon dioxide in the atmosphere is the greatest cause for concern. In State of the World 1988, Brown and Flavin [3] provide data which show that carbon emissions from combustion of fossil fuels have risen from 1.7 billion tons in 1950 to 5.5 billion tons in 1988 while the world population has grown from 2.5 billion in 1950 to 5.1 billion in 1988. The Brundtland Report [l] predicts a world population of 8.2 billion by 2025. This would imply that carbon emissions from the combustion of fossil fuels will reach 7.5 billion tons per year by that time unless action is taken to alter energy consumption patterns. The purpose of this paper is to discuss the changes occurring in the
I80
environment and to consider what action should be taken to assure the future sustainability of society and of the earth's environment. Greenhouse Gases The so-called "greenhouse gases" include carbon dioxide (CO ) the chlorofluorocarbons (CFC's), methane (CH4) and nitrous oxide (N20). %hew gases, distributed throughout the atmosphere, trap the infra red radiation leaving the earth's surface and cause the temperature to rise. Carbon dioxide contributes 55% of the radiative heating while the other three contribute 24, 15 and 6%, respectively [4].Carbon dioxide is responsible for most of this heating effect, and creates the greatest concern although the CFC's and methane are also causing problems. It has been estimated that the globally averaged surface temperature of the earth will increase by 1.5 to 4.0°C if the carbon dioxide concentration in the atmosphere is allowed to double from its pre-industrial value of 280 ppm to 560 pprn [l]. Since 1950 the average carbon dioxide concentration in the atmosphere has increased from 310 ppm to 350 ppm in 1990, in parallel with the increased emissions of carbon (as carbon dioxide) indicated earlier. Stephen Schneider [5] has published a correlation between the temperature change from the present and the carbon dioxide concentrations in the atmosphere over the past 160,000years which shows remarkable correspondence indicating that carbon dioxide in the atmosphere has related closely with the warming of the atmosphere. There are those who do not agree with this correlation in to-day's world arguing that there are many factors such as dust in the atmosphere which could prevent the temperature of the atmosphere from rising even though the carbon dioxide concentration has steadily risen. However, many scientists are convinced that global warming is beginning to happen and that there is no time to lose in reducing carbon dioxide emissions. What can be done? Where does "catalysis" enter the picture? RENEWABLE AND NON-RENEWABLE ENERGY SOURCES "Renewable energy" sources include hydropower, wind power, solar power and biomass. Biomass is included because during photosynthesis it sequesters as much carbon dioxide as it releases at the end of its life cycle. None of these sources therefore add to the carbon dioxide content of the atmosphere. "Non-renewable energy" sources include coal, oil, gas and nuclear energy. (Nuclear energy does not contribute emissions of carbon dioxide except during fuel processing.) The carbon in the other "fossil-fuel" sources has been stored for millions 01 years and adds to the load of carbon dioxide in the atmosphere when they are burned. Every effort should therefore be made to reduce the combustion of these non-renewable energy sources to reduce the "greenhouse effect". Flavin and Lenssen [6]suggest that the global emissions of carbon dioxide should be reduced from the present 6 billion tons per year from fossil fuels to about 2 billion tons per year to reduce melting of the ice caps, to slow the greenhouse effect, and to prevent inundation of coastal areas. Achieving such a goal would create a very different world from the one in which we live to-day. One such scenario would have the consumption of oil halved by the year 2030, coal
consumption would be reduced to one-tenth of its present rate, natural gas would be the same as at present while renewable energy sources would increase by a factor of four. According to this scenario, nuclear energy would be phased out for environmental reasons. The energy from the fuel consumed in 2030 would be the same as in 1990 but the world population would have increased by 58%. This would be possible only if large improvements in the efficiencies with which energy is used are achieved. Whether such a large increase in the use of biomass could be achieved is questionable but it indicates the direction towards a sustainable society by controlling C02 emissions. VARIOUS STRATEGIES FOR REDUCING EMISSIONS OF CARBON DIOXIDE Using data from Flavin and Lenssen [6] it can be calculated that the energy available per ton of car on emissions from natural gas, coal and oil is 65.2, 34.7 and 48.2 gigajoules (10 Joules), respectively. Energy available per ton of carbon emissions from natural gas is almost double that from coal and approximately 35 percent greater than that from oil. These figures indicate the importance in planning of using fuels with high H/C atomic ratios such as in methane, CH4 with a value of 4.0 rather than oils with ratios of 1.4 to 1.9 or coals with ratios of 0.5 to 1.l. Hydrogen's role in the catalytic hydrogenation of low H/C ratio hydrocarbonsto high H/C hydrocarbons thus creating fuels of higher energy and lower CO, production on combustion is of major importance in the program to reduce C02 emissions. Several proposals for reducing global warming have emphasized the need to plant more trees, the purpose being to increase the extent of carbon dioxide absorption through photosynthesis until the trees mature. This "carbon sequestration" strategy is not permanent but does buy time while alternative renewable energy sources are being developed. little attention has been paid to the use of biomass as a substitute for fossil-fuel energy. The value of this approach in reducing the C02 in the atmosphere depends on the fuel displaced and the efficiency with which the energy might be produced in each case. Hal et al. [7] compared combustion of coal with that of biomass each being converted with equal efficiency to produce energy. If coal containing 1.0 GJ of energy were combusted about 0.025tonnes of carbon as C02 would be released. Likewise if biomass containing 1.OGJ of energy and 50% carbon were burned 0.025 tonnes of carbon as C02 would be produced. The carbon in the latter case, however, was sequestered from the atmosphere during growth of the biomass. Therefore substituting the biomass for the coal is equivalent to carbon sequestration in its effect on atmospheric COP Biomass can be grown indefinitely on land to replace fossil fuel. One scenario (71 suggests that a reduction in C02 emissions to one half of the 1985 level by 2050 by reversingdeforestation and emphasizing the efficient use of energy could be achieved by displacing 5.4 billion tonnes per year of carbon from fossil fuels by an equivalent productionof energy using biomass. Coal, would be replaced by biomass, one third coming from agricultural and industrial biomass residues, the remaining two-thirds coming from biomass plantations. These plantations would involve some 600 million hectares of land and would have an average productivity
8
I82
of 12 dry tonnes per hectare per year. Estimates made of possible tropical reforestation lands indicate that there are 800 million hectares potentially available as well as some 1500 million hectares of tropical grasslands half of which are burned each year. It will be necessary to give high priority to research and development needed for the sustainable production and conversion of biomass to energy if this relatively new approach to energy production is to be fully developed. It offers a possible solution to the C02 emissions problem. Before leaving reforestation,forestation and biomass production as solutions to the carbon emissions problem, it should be pointed out that the growth of trees and biomass will play roles in solving major problems relating to deforestation, desertification, soil erosion and the supply of fuel wood. Deforestation is proceeding at an alarming rate some 11 million hectares of tropical forest disappearing per year and 31 million hectares being damaged by air pollution and acid rain [3].Six million hectares of new desert are formed annually by land mismanagement. An estimated 26 billion tons of topsoil are lost each year in excess of new soil being formed. Approximately 1.2 billion people mainly in the Third World meet their needs for firewood by cutting wood faster than nature replaces it, thus exceeding the sustainable yields of the forests in the areas. Well planned forestation and biomass plantations could do much to alleviate these problems.
REDUCING CO2 EMISSIONS FROM THE USE OF NON-RENEWABLE FUELS While major steps need to be taken to control C02 emissions from use of renewable sources of energy, for several decades society will demand that nonrenewable fuels such as gasoline and diesel fuel be used for transportation. Every effort therefore should be made to use as little as possible of these fuels and with the highest efficiency so that the CO, emissions are minimized. Hall et al. [7] claim that methanol derived from biomass by thermochemical processes and ethanol produced by enzymatic hydrolysis of lignocellulosic feed materials could be competitive with gasoline by the year 2000. Adding these alcohols to gasoline would reduce the non-renewable fraction of gasoline and thereby decrease this type of CO emission. With the ban on the use of?ead tetraethyl in gasoline in December 1990, as well as a limitation on the percentage of benzene, a known carcinogen, allowed in gasoline, there has been a scramble to find replacements to provide performance in new automobile engines. The Environmental Protection Agency (EPA) in the United States [S] has negotiated regulations under the 1990 amendments to the Clean Air Act which require certain utban areas having excessive ozone and carbon monoxide levels to start using oxygenated and reformulated gasolines in the winter of 1992-93. There are 41 cities with serious carbon monoxide problems which will require oxygenated gasolines while nine with ozone problems will require reformulated gasolines. Other areas not meeting the ozone air quality guidelines may use the reformulated gasolines. The oxygenated fuels are produced by adding methanol, ethanol or methyl tertiary butyl ether (MTBE) at the refineries. Oxygenated fuels can be burned under leaner operating conditions and reduce the emissions of carbon monoxide in cold
183
weather. The regulationswill require an average of 2.7% by weight of oxygen. The EPA claims that this will reduce the carbon dioxide emissions by 17%. This implies a significant improvement in efficiency of utilization of the fuels. Regulations for reformulated gasoline are more involved. Under them refiners will provide new gasolines by January 1995 to meet the ozone guidelines in 87 cities. Approximately 55% of the gasoline used in the US will have to meet the requirements. These include an average oxygen content of 2.1% by weight, no more than 1.3% of benzene by volume and an average Reed vapor pressure of 7.4 psi. In addition, standards will be set on sulfur, olefins and the boiling point range. To provide the necessary oxygenated blending components, MTBE plants having a total capacity of 55,100 barrels per day will be added to the existing capacity of 100,000 bld which is used for improving octane ratings of gasolines [9]. MTBE has become the fastest growing petrochemical largely due to its ability to supply the oxygenatedfuel component requiredfor reformulatedgasolines. By 1995 when year-around ozone requirements of amendments to the Clean Air Act go into effect, the demand could be as high as 388,000 bld in the winter months. This would require about a dozen plants of 12,000 bld capacity being built to provide the needs for MTBE at various refineries around the United States. MTBE is made by reaction of methyl alcohol with isobutene in the presence of an acid catalyst. Its production requires two basic raw materials, methane and n-butane or i-butane. The methane is reformed with steam over a nickel catalyst to produce hydrogen and carbon monoxide. This "synthesis gas" then is reacted over a copper catalyst at 250°C and 100 atmospheresto yield methanol. n-Butane and i-butane are produced from fluid catalytic cracking. The n-butane fraction is isomerized using an AIC13-HCI catalyst to yield isobutane which is then dehydrogenated catalytically to isobutene. This is reacted with the methyl alcohol to produce MTBE thus, CH3OH
+
CH2 = C
H+
- ( C H 3 ) 2 + CH3O
-C-
(CH3)3
(1)
This program should create significant improvements in the atmosphere with respect to ozone and CO concentrations in certain areas. THE ROLE OF HYDROGENIN ACHIEVING ENVIRONMENTAL SUSTAINABILITY Hydrogen is a clean fuel which can be burned without adding carbon dioxide to the atmosphere. It can be produced by (a) electrolysis of water, (b) steam reforming of methane, (c) partial oxidation of hydrocarbons and (d) dehydrogenation of hydrocarbons such as ethane during ethylene production. Hydrogen can be used to provide energy for fuel cells which offer significantly higher efficiencies e.g. 6268%, compared with 30 to 50% for various fossil-fueled power plants, in the conversion of energy to electric power [lo]. Significant progress is being made in the development of solar cells which can geneiate power and can be used to electrolyze water producing hydrogen and oxygen. However, the costs of manufacturing solar cells are still too high to be
184
competitive except in restricted applications. If the carbon tax being considered in Europe were applied on oil, this difference would shrink making this source of hydrogen more attractive [ l l ] . Over the past 20 years the cost of photovoltaic electricity has fallen from $30 per kilowatt-hour to 30 cents per kWh. It is predicted that by the end of this decade the cost will be 10 cents per kWh and that by 2030, photovoltaics will be supplying a large share of the electricity required for approximately 4 cents per kWh [6]. According to H.A. Aulich [12] mass-produced solar cells of single-crystal silicon are currently achieving efficiencies of 14 to 16% while experimental high performance solar cells of single crystal silicon have achieved 24% compared with the theoretical maximum of 28%. Development of these renewable energy sources will do much in the future to reduce carbon emissions from power generation. Approximately 6000 water pumps driven by photovoltaic power supplies have been installed around the world. For small installations in remote areas their operation is more economical than the use of diesel power. Such installations based on photovoltaic power are notable because they have no C02 emissions [13]. HYDROGEN PRODUCTION AND USE IN CANADA. On a per capita basis, Canada produces more hydrogen than any other country in the world [14]. In 1989 production was two million tonnes, most of it being produced by the steam-methane reforming reaction. This corresponds to the production of 10 million tonnes per year of carbon dioxide which is dischargedto the atmosphere using present technology. Canadian consumption of hydrogen includes, 23% going into petroleum refining, 21% into methanol manufacture and 10% each in synthetic crude production. While hydrogen production is largely based on non-renewable fuels, research is proceeding in central Canada on the electrolysis of water as a source. Other sustainable forms of energy such as hydro, tidal, wind, and solar along with nuclear power are being considered to drive electrolytic production of hydrogen. Hydrogen is viewed as a clean fuel for fuel cells to drive locomotives, buses and other vehicles. It could also be used for heating homes with the aid of catalytic converters
.
Cost of Hydrogen Production Bailey and Logan [15] presented a paper to the Canadian Heavy Oil Association in May 1991 in Calgary, Alberta in which they compared the costs of producing hydrogen by (a) catalytic steam-methane reforming (b) partial oxidation of hydrocarbons and (c) electrolysis of water. The capital costs for plants to produce 132 million standard cubic feet per day of hydrogen for a 60,000 Wd bitumen upgrader are given in Table 1. With methane as feed and fuel at $1.!XI CDN per thousand standard cubic feet (MSCF), steam-methane reforming is the most attractive process providing hydrogen at $1.24 per MSCF. In this operation, carbon dioxide is produced by the reforming reaction together with the hydrogen product. CHq + 2H20 = C02 + 4H2 (2)
185
Half of the hydrogen is derived from the methane, the other half coming from the steam. The partial oxidation of hydrocarbons of low value residues has some economic advantages. The hydrocarbon feed and approximately the same mass of water are reactedwith oxygen producing hydrogen and carbon dioxide. However, power costs are high due to the need to operate the oxygen plant. Plant capital costs at $355 million are high making the total cost of hydrogen by this process equal to $2.00/MSCF. The carbon dioxide from this process is higher than by steam-methane reforming due to the lower H/Cratio of the feedstock. Table 1 Comparison of hydrogen costs by three different methods of productiona Method
Capital costsb (Millions $) (CANADIAN)
Hydrogen cost (Dollars per/MSCF)
Carbon dioxide produced (Vd)
-Steam Methane Reformingd -Partial Oxidation of Hydrocarbons -Electrolysis of Water
152 355
1.24 2.00
3534 5313
250
3.71'
(a) From data presented by R.T. Bailey and A. Logan at a meeting of the Canadian Heavy 3il Association in Calgary, Alberta, Canada, May 13, 1991 [15]. Cost comparisons were made for production of 132 million standard cubic feet of hydrogen per day to supply a 60,000 barrel per day bitumen upgrading plant. (b) Capital costs are for the steam-methane reformer, the hydrocarbon oxidation unit and the electrolysis unit. (c) Capital cost of the power plant is included in the cost of electricity at $0.03 kW/h assuming it to be produced from hydropower. (d) Cost of methane assumed was $1 .5O/MSCF.
Environmentallyspeaking, for steam-methane reformingand partial oxidation of hydrocarbons to be more acceptable there is a need to find a "sink" other than the atmosphere in which to deposit the carbon dioxide produced. Recently tests have been done in western Canada [15] in which CO has been charged into an oil field and enhanced oil production followed. A s t d y has shown that some 15 million tonnes per year of C02 could be utilized in this manner. Estimates are being made of the costs of collecting C02 emissions and pumping them into depleted reservoirs for permanent storage. In view of the concern over the rising concentration of C02 in the atmosphere, it would seem appropriate for the governments to encourage industry to reinject CO produced from steam-methane reforming into reservoirs even though this may no? be profitable. It should be noted that the combination of steam-methane reforming or partial oxidation Jf hydrocarbons with injection of the C02 product into permanent storage provides a means of producing hydrogen from hydrocarbons without the emission of C02 to the atmosphere. In view of the vast supplies of methane in natural gas, coal gas and methane hydrates, the use of this method should be encouraged for the reduction of C02 emissions.
186
A recent paper by C. Marchetti of the International Institute of Applied Systems Analysis in Austria [16] suggests that a joint venture between Western Europe and the Soviet Union be set up to steam reform the gas flowing by pipeline to Europe using a high temperature nuclear reactor to supply the necessary heat. The C02 from the reforming reaction would be permanently stored in reservoirs or illion cubic meters of used for enhanced recovery of oil. It was proposed that 8O natural g s p r year be reformed producing 200 billion m y of hydrogen and 50 billion m' y-' of C02. The design of the plant necessary to do such a large catalytic reforming operation would be a challenge to engineers and scientists.
-9
CONCLUSIONS Global warming due to greenhouse gases must be reduced by shifting to renewable energy sources such as biomass, hydrogen, solar, tidal and wind power, and by permanently storing some carbon dioxide in reservoirs. Use of hydrogen to upgrade f9ssil fuels reduces COPemissions. Oxygenatedfuels such as methanol, ethanol and MTBE in reformulated gasolines may reduce C02 emissions and problems due to ozone and CO in the atmosphere. REFERENCES 1. Brundtland, G.H. "Our Common Future" Report by a World Commission on Environmentand Development,Oxford University Press, March 20, (1987), 400. 2. Digby McLaren "Are Global Changes and SustainableDevelopment in Conflict?" Presented to the 40th Pugwash conference on Science and World Affairs at Egham, U.K. September 15-20, 1990. Reprinted in Pugwash Papers, 3(2) (1990) 4. 3. Brown, L.R. and C. Flavin, "The Earth's Vital Signs" in State of the World 1988, A Worldwatch Institute Report on Progress Towards a Sustainable Society, W.W. Norton and Company, New York (1988) 1-21. 4. O'Sullivan, D.A., C. and E. News, October 29 (1990) 26. 5. Schneider, S. H., Scientific American, Special Issue, September (1989) 70. 6. Flavin, C. and N. Lenssen, State of the World 1991, A Worldwatch Institute Report on Progress Towards a Sustainable Society, p. 21-38. 7. Hall, D.O., H.E. Mynick, and R.H. Williams, Commentary, Nature, Vol. 353, September 5 (1991) 11. 8. Hansen, D., C. and E. News, August 26 (1991) 4. 9. Ainsworth, S.J., C. and E. News, June 10 (1991) 13. 10. Riedle, K., Siemens Review, (1991) 15. 11. Government Concentrates, C. and E. News, October 29 (1991) 17. 12. Aulick, H.A., Siemens Review, (1991) 20. 13. Thiessen, T., GEOS, 19 (4) (1990) 1. 14. "Hydrogen: a Unique Industrial Opportunity for Canada". The Hydrogen Industry Council, Offices in Montreal and Calgary, Canada. 15. Bailey, R.J. and A. Logan, Canadian Heavy Oil. Quarterly Meeting, Calgary, Canada, May 13 (1991). 16. Marchetti, C., Int. J. Hydrogen Energy 44 (8)(1989) 493.
I79
The Role of Catalysis in Achieving a Sustainable Society Eric L. Tollefson Department of Chemical and Petroleum Engineering, The University of Calgary, Calgary, Alberta, Canada T2N 1N4 Abstract This paper describes changes occurring in the environment which could radically alter the nature of society in the decades ahead unless corrective action is taken to create sustainable conditions. The upward trend in carbon emissions together with the effects of other greenhouse gases on the average global temperature are considered and suggestions made as to how a sustainable state might be achieved by replacing non-renewable energy sources with renewable ones such as trees, biomass and hydrogen as well as hydro, solar, tidal and wind. INTRODUCTION The Brundtland Report "Our Common Future" [l] states "Humanity has the ability to make development sustainable - to ensure that it meets the needs of the present Rithout compromising the ability of future generations to meet their own needs". To make "development sustainable" certain limits would have to be imposed, for example, so that the biosphere would be capable of absorbing the effects of human activities. McLaren [2] has noted that over the past 150 years the world population has grown from 1 to 5 billion with a current doubling time of 30 to 40 years, that a comparable increase in the usage of fossil fuels is leading to global pollution, to changes in climate and sea level and that destruction of the habitat of life is accelerating causing extinction of many species. The destruction of the forests, soil erosion, overuse of ground water and the production of wastes have also been accelerated. In McLaren's opinion one cannot talk about "stabilization" or "equity" or use the term "sustainable development" while these influences are disrupting our home planet and "most of them are growing exponentially or greater". Of all these destabilizing factors in terms of environmental security, the buildup of carbon dioxide in the atmosphere is the greatest cause for concern. In State of the World 1988, Brown and Flavin [3] provide data which show that carbon emissions from combustion of fossil fuels have risen from 1.7 billion tons in 1950 to 5.5 billion tons in 1988 while the world population has grown from 2.5 billion in 1950 to 5.1 billion in 1988. The Brundtland Report [l] predicts a world population of 8.2 billion by 2025. This would imply that carbon emissions from the combustion of fossil fuels will reach 7.5 billion tons per year by that time unless action is taken to alter energy consumption patterns. The purpose of this paper is to discuss the changes occurring in the
I80
environment and to consider what action should be taken to assure the future sustainability of society and of the earth's environment. Greenhouse Gases The so-called "greenhouse gases" include carbon dioxide (CO ) the chlorofluorocarbons (CFC's), methane (CH4) and nitrous oxide (N20). %hew gases, distributed throughout the atmosphere, trap the infra red radiation leaving the earth's surface and cause the temperature to rise. Carbon dioxide contributes 55% of the radiative heating while the other three contribute 24, 15 and 6%, respectively [4].Carbon dioxide is responsible for most of this heating effect, and creates the greatest concern although the CFC's and methane are also causing problems. It has been estimated that the globally averaged surface temperature of the earth will increase by 1.5 to 4.0°C if the carbon dioxide concentration in the atmosphere is allowed to double from its pre-industrial value of 280 ppm to 560 pprn [l]. Since 1950 the average carbon dioxide concentration in the atmosphere has increased from 310 ppm to 350 ppm in 1990, in parallel with the increased emissions of carbon (as carbon dioxide) indicated earlier. Stephen Schneider [5] has published a correlation between the temperature change from the present and the carbon dioxide concentrations in the atmosphere over the past 160,000years which shows remarkable correspondence indicating that carbon dioxide in the atmosphere has related closely with the warming of the atmosphere. There are those who do not agree with this correlation in to-day's world arguing that there are many factors such as dust in the atmosphere which could prevent the temperature of the atmosphere from rising even though the carbon dioxide concentration has steadily risen. However, many scientists are convinced that global warming is beginning to happen and that there is no time to lose in reducing carbon dioxide emissions. What can be done? Where does "catalysis" enter the picture? RENEWABLE AND NON-RENEWABLE ENERGY SOURCES "Renewable energy" sources include hydropower, wind power, solar power and biomass. Biomass is included because during photosynthesis it sequesters as much carbon dioxide as it releases at the end of its life cycle. None of these sources therefore add to the carbon dioxide content of the atmosphere. "Non-renewable energy" sources include coal, oil, gas and nuclear energy. (Nuclear energy does not contribute emissions of carbon dioxide except during fuel processing.) The carbon in the other "fossil-fuel" sources has been stored for millions 01 years and adds to the load of carbon dioxide in the atmosphere when they are burned. Every effort should therefore be made to reduce the combustion of these non-renewable energy sources to reduce the "greenhouse effect". Flavin and Lenssen [6]suggest that the global emissions of carbon dioxide should be reduced from the present 6 billion tons per year from fossil fuels to about 2 billion tons per year to reduce melting of the ice caps, to slow the greenhouse effect, and to prevent inundation of coastal areas. Achieving such a goal would create a very different world from the one in which we live to-day. One such scenario would have the consumption of oil halved by the year 2030, coal
consumption would be reduced to one-tenth of its present rate, natural gas would be the same as at present while renewable energy sources would increase by a factor of four. According to this scenario, nuclear energy would be phased out for environmental reasons. The energy from the fuel consumed in 2030 would be the same as in 1990 but the world population would have increased by 58%. This would be possible only if large improvements in the efficiencies with which energy is used are achieved. Whether such a large increase in the use of biomass could be achieved is questionable but it indicates the direction towards a sustainable society by controlling C02 emissions. VARIOUS STRATEGIES FOR REDUCING EMISSIONS OF CARBON DIOXIDE Using data from Flavin and Lenssen [6] it can be calculated that the energy available per ton of car on emissions from natural gas, coal and oil is 65.2, 34.7 and 48.2 gigajoules (10 Joules), respectively. Energy available per ton of carbon emissions from natural gas is almost double that from coal and approximately 35 percent greater than that from oil. These figures indicate the importance in planning of using fuels with high H/C atomic ratios such as in methane, CH4 with a value of 4.0 rather than oils with ratios of 1.4 to 1.9 or coals with ratios of 0.5 to 1.l. Hydrogen's role in the catalytic hydrogenation of low H/C ratio hydrocarbonsto high H/C hydrocarbons thus creating fuels of higher energy and lower CO, production on combustion is of major importance in the program to reduce C02 emissions. Several proposals for reducing global warming have emphasized the need to plant more trees, the purpose being to increase the extent of carbon dioxide absorption through photosynthesis until the trees mature. This "carbon sequestration" strategy is not permanent but does buy time while alternative renewable energy sources are being developed. little attention has been paid to the use of biomass as a substitute for fossil-fuel energy. The value of this approach in reducing the C02 in the atmosphere depends on the fuel displaced and the efficiency with which the energy might be produced in each case. Hal et al. [7] compared combustion of coal with that of biomass each being converted with equal efficiency to produce energy. If coal containing 1.0 GJ of energy were combusted about 0.025tonnes of carbon as C02 would be released. Likewise if biomass containing 1.OGJ of energy and 50% carbon were burned 0.025 tonnes of carbon as C02 would be produced. The carbon in the latter case, however, was sequestered from the atmosphere during growth of the biomass. Therefore substituting the biomass for the coal is equivalent to carbon sequestration in its effect on atmospheric COP Biomass can be grown indefinitely on land to replace fossil fuel. One scenario (71 suggests that a reduction in C02 emissions to one half of the 1985 level by 2050 by reversingdeforestation and emphasizing the efficient use of energy could be achieved by displacing 5.4 billion tonnes per year of carbon from fossil fuels by an equivalent productionof energy using biomass. Coal, would be replaced by biomass, one third coming from agricultural and industrial biomass residues, the remaining two-thirds coming from biomass plantations. These plantations would involve some 600 million hectares of land and would have an average productivity
8
I82
of 12 dry tonnes per hectare per year. Estimates made of possible tropical reforestation lands indicate that there are 800 million hectares potentially available as well as some 1500 million hectares of tropical grasslands half of which are burned each year. It will be necessary to give high priority to research and development needed for the sustainable production and conversion of biomass to energy if this relatively new approach to energy production is to be fully developed. It offers a possible solution to the C02 emissions problem. Before leaving reforestation,forestation and biomass production as solutions to the carbon emissions problem, it should be pointed out that the growth of trees and biomass will play roles in solving major problems relating to deforestation, desertification, soil erosion and the supply of fuel wood. Deforestation is proceeding at an alarming rate some 11 million hectares of tropical forest disappearing per year and 31 million hectares being damaged by air pollution and acid rain [3].Six million hectares of new desert are formed annually by land mismanagement. An estimated 26 billion tons of topsoil are lost each year in excess of new soil being formed. Approximately 1.2 billion people mainly in the Third World meet their needs for firewood by cutting wood faster than nature replaces it, thus exceeding the sustainable yields of the forests in the areas. Well planned forestation and biomass plantations could do much to alleviate these problems.
REDUCING CO2 EMISSIONS FROM THE USE OF NON-RENEWABLE FUELS While major steps need to be taken to control C02 emissions from use of renewable sources of energy, for several decades society will demand that nonrenewable fuels such as gasoline and diesel fuel be used for transportation. Every effort therefore should be made to use as little as possible of these fuels and with the highest efficiency so that the CO, emissions are minimized. Hall et al. [7] claim that methanol derived from biomass by thermochemical processes and ethanol produced by enzymatic hydrolysis of lignocellulosic feed materials could be competitive with gasoline by the year 2000. Adding these alcohols to gasoline would reduce the non-renewable fraction of gasoline and thereby decrease this type of CO emission. With the ban on the use of?ead tetraethyl in gasoline in December 1990, as well as a limitation on the percentage of benzene, a known carcinogen, allowed in gasoline, there has been a scramble to find replacements to provide performance in new automobile engines. The Environmental Protection Agency (EPA) in the United States [S] has negotiated regulations under the 1990 amendments to the Clean Air Act which require certain utban areas having excessive ozone and carbon monoxide levels to start using oxygenated and reformulated gasolines in the winter of 1992-93. There are 41 cities with serious carbon monoxide problems which will require oxygenated gasolines while nine with ozone problems will require reformulated gasolines. Other areas not meeting the ozone air quality guidelines may use the reformulated gasolines. The oxygenated fuels are produced by adding methanol, ethanol or methyl tertiary butyl ether (MTBE) at the refineries. Oxygenated fuels can be burned under leaner operating conditions and reduce the emissions of carbon monoxide in cold
183
weather. The regulationswill require an average of 2.7% by weight of oxygen. The EPA claims that this will reduce the carbon dioxide emissions by 17%. This implies a significant improvement in efficiency of utilization of the fuels. Regulations for reformulated gasoline are more involved. Under them refiners will provide new gasolines by January 1995 to meet the ozone guidelines in 87 cities. Approximately 55% of the gasoline used in the US will have to meet the requirements. These include an average oxygen content of 2.1% by weight, no more than 1.3% of benzene by volume and an average Reed vapor pressure of 7.4 psi. In addition, standards will be set on sulfur, olefins and the boiling point range. To provide the necessary oxygenated blending components, MTBE plants having a total capacity of 55,100 barrels per day will be added to the existing capacity of 100,000 bld which is used for improving octane ratings of gasolines [9]. MTBE has become the fastest growing petrochemical largely due to its ability to supply the oxygenatedfuel component requiredfor reformulatedgasolines. By 1995 when year-around ozone requirements of amendments to the Clean Air Act go into effect, the demand could be as high as 388,000 bld in the winter months. This would require about a dozen plants of 12,000 bld capacity being built to provide the needs for MTBE at various refineries around the United States. MTBE is made by reaction of methyl alcohol with isobutene in the presence of an acid catalyst. Its production requires two basic raw materials, methane and n-butane or i-butane. The methane is reformed with steam over a nickel catalyst to produce hydrogen and carbon monoxide. This "synthesis gas" then is reacted over a copper catalyst at 250°C and 100 atmospheresto yield methanol. n-Butane and i-butane are produced from fluid catalytic cracking. The n-butane fraction is isomerized using an AIC13-HCI catalyst to yield isobutane which is then dehydrogenated catalytically to isobutene. This is reacted with the methyl alcohol to produce MTBE thus, CH3OH
+
CH2 = C
H+
- ( C H 3 ) 2 + CH3O
-C-
(CH3)3
(1)
This program should create significant improvements in the atmosphere with respect to ozone and CO concentrations in certain areas. THE ROLE OF HYDROGENIN ACHIEVING ENVIRONMENTAL SUSTAINABILITY Hydrogen is a clean fuel which can be burned without adding carbon dioxide to the atmosphere. It can be produced by (a) electrolysis of water, (b) steam reforming of methane, (c) partial oxidation of hydrocarbons and (d) dehydrogenation of hydrocarbons such as ethane during ethylene production. Hydrogen can be used to provide energy for fuel cells which offer significantly higher efficiencies e.g. 6268%, compared with 30 to 50% for various fossil-fueled power plants, in the conversion of energy to electric power [lo]. Significant progress is being made in the development of solar cells which can geneiate power and can be used to electrolyze water producing hydrogen and oxygen. However, the costs of manufacturing solar cells are still too high to be
184
competitive except in restricted applications. If the carbon tax being considered in Europe were applied on oil, this difference would shrink making this source of hydrogen more attractive [ l l ] . Over the past 20 years the cost of photovoltaic electricity has fallen from $30 per kilowatt-hour to 30 cents per kWh. It is predicted that by the end of this decade the cost will be 10 cents per kWh and that by 2030, photovoltaics will be supplying a large share of the electricity required for approximately 4 cents per kWh [6]. According to H.A. Aulich [12] mass-produced solar cells of single-crystal silicon are currently achieving efficiencies of 14 to 16% while experimental high performance solar cells of single crystal silicon have achieved 24% compared with the theoretical maximum of 28%. Development of these renewable energy sources will do much in the future to reduce carbon emissions from power generation. Approximately 6000 water pumps driven by photovoltaic power supplies have been installed around the world. For small installations in remote areas their operation is more economical than the use of diesel power. Such installations based on photovoltaic power are notable because they have no C02 emissions [13]. HYDROGEN PRODUCTION AND USE IN CANADA. On a per capita basis, Canada produces more hydrogen than any other country in the world [14]. In 1989 production was two million tonnes, most of it being produced by the steam-methane reforming reaction. This corresponds to the production of 10 million tonnes per year of carbon dioxide which is dischargedto the atmosphere using present technology. Canadian consumption of hydrogen includes, 23% going into petroleum refining, 21% into methanol manufacture and 10% each in synthetic crude production. While hydrogen production is largely based on non-renewable fuels, research is proceeding in central Canada on the electrolysis of water as a source. Other sustainable forms of energy such as hydro, tidal, wind, and solar along with nuclear power are being considered to drive electrolytic production of hydrogen. Hydrogen is viewed as a clean fuel for fuel cells to drive locomotives, buses and other vehicles. It could also be used for heating homes with the aid of catalytic converters
.
Cost of Hydrogen Production Bailey and Logan [15] presented a paper to the Canadian Heavy Oil Association in May 1991 in Calgary, Alberta in which they compared the costs of producing hydrogen by (a) catalytic steam-methane reforming (b) partial oxidation of hydrocarbons and (c) electrolysis of water. The capital costs for plants to produce 132 million standard cubic feet per day of hydrogen for a 60,000 Wd bitumen upgrader are given in Table 1. With methane as feed and fuel at $1.!XI CDN per thousand standard cubic feet (MSCF), steam-methane reforming is the most attractive process providing hydrogen at $1.24 per MSCF. In this operation, carbon dioxide is produced by the reforming reaction together with the hydrogen product. CHq + 2H20 = C02 + 4H2 (2)
185
Half of the hydrogen is derived from the methane, the other half coming from the steam. The partial oxidation of hydrocarbons of low value residues has some economic advantages. The hydrocarbon feed and approximately the same mass of water are reactedwith oxygen producing hydrogen and carbon dioxide. However, power costs are high due to the need to operate the oxygen plant. Plant capital costs at $355 million are high making the total cost of hydrogen by this process equal to $2.00/MSCF. The carbon dioxide from this process is higher than by steam-methane reforming due to the lower H/Cratio of the feedstock. Table 1 Comparison of hydrogen costs by three different methods of productiona Method
Capital costsb (Millions $) (CANADIAN)
Hydrogen cost (Dollars per/MSCF)
Carbon dioxide produced (Vd)
-Steam Methane Reformingd -Partial Oxidation of Hydrocarbons -Electrolysis of Water
152 355
1.24 2.00
3534 5313
250
3.71'
(a) From data presented by R.T. Bailey and A. Logan at a meeting of the Canadian Heavy 3il Association in Calgary, Alberta, Canada, May 13, 1991 [15]. Cost comparisons were made for production of 132 million standard cubic feet of hydrogen per day to supply a 60,000 barrel per day bitumen upgrading plant. (b) Capital costs are for the steam-methane reformer, the hydrocarbon oxidation unit and the electrolysis unit. (c) Capital cost of the power plant is included in the cost of electricity at $0.03 kW/h assuming it to be produced from hydropower. (d) Cost of methane assumed was $1 .5O/MSCF.
Environmentallyspeaking, for steam-methane reformingand partial oxidation of hydrocarbons to be more acceptable there is a need to find a "sink" other than the atmosphere in which to deposit the carbon dioxide produced. Recently tests have been done in western Canada [15] in which CO has been charged into an oil field and enhanced oil production followed. A s t d y has shown that some 15 million tonnes per year of C02 could be utilized in this manner. Estimates are being made of the costs of collecting C02 emissions and pumping them into depleted reservoirs for permanent storage. In view of the concern over the rising concentration of C02 in the atmosphere, it would seem appropriate for the governments to encourage industry to reinject CO produced from steam-methane reforming into reservoirs even though this may no? be profitable. It should be noted that the combination of steam-methane reforming or partial oxidation Jf hydrocarbons with injection of the C02 product into permanent storage provides a means of producing hydrogen from hydrocarbons without the emission of C02 to the atmosphere. In view of the vast supplies of methane in natural gas, coal gas and methane hydrates, the use of this method should be encouraged for the reduction of C02 emissions.
186
A recent paper by C. Marchetti of the International Institute of Applied Systems Analysis in Austria [16] suggests that a joint venture between Western Europe and the Soviet Union be set up to steam reform the gas flowing by pipeline to Europe using a high temperature nuclear reactor to supply the necessary heat. The C02 from the reforming reaction would be permanently stored in reservoirs or illion cubic meters of used for enhanced recovery of oil. It was proposed that 8O natural g s p r year be reformed producing 200 billion m y of hydrogen and 50 billion m' y-' of C02. The design of the plant necessary to do such a large catalytic reforming operation would be a challenge to engineers and scientists.
-9
CONCLUSIONS Global warming due to greenhouse gases must be reduced by shifting to renewable energy sources such as biomass, hydrogen, solar, tidal and wind power, and by permanently storing some carbon dioxide in reservoirs. Use of hydrogen to upgrade f9ssil fuels reduces COPemissions. Oxygenatedfuels such as methanol, ethanol and MTBE in reformulated gasolines may reduce C02 emissions and problems due to ozone and CO in the atmosphere. REFERENCES 1. Brundtland, G.H. "Our Common Future" Report by a World Commission on Environmentand Development,Oxford University Press, March 20, (1987), 400. 2. Digby McLaren "Are Global Changes and SustainableDevelopment in Conflict?" Presented to the 40th Pugwash conference on Science and World Affairs at Egham, U.K. September 15-20, 1990. Reprinted in Pugwash Papers, 3(2) (1990) 4. 3. Brown, L.R. and C. Flavin, "The Earth's Vital Signs" in State of the World 1988, A Worldwatch Institute Report on Progress Towards a Sustainable Society, W.W. Norton and Company, New York (1988) 1-21. 4. O'Sullivan, D.A., C. and E. News, October 29 (1990) 26. 5. Schneider, S. H., Scientific American, Special Issue, September (1989) 70. 6. Flavin, C. and N. Lenssen, State of the World 1991, A Worldwatch Institute Report on Progress Towards a Sustainable Society, p. 21-38. 7. Hall, D.O., H.E. Mynick, and R.H. Williams, Commentary, Nature, Vol. 353, September 5 (1991) 11. 8. Hansen, D., C. and E. News, August 26 (1991) 4. 9. Ainsworth, S.J., C. and E. News, June 10 (1991) 13. 10. Riedle, K., Siemens Review, (1991) 15. 11. Government Concentrates, C. and E. News, October 29 (1991) 17. 12. Aulick, H.A., Siemens Review, (1991) 20. 13. Thiessen, T., GEOS, 19 (4) (1990) 1. 14. "Hydrogen: a Unique Industrial Opportunity for Canada". The Hydrogen Industry Council, Offices in Montreal and Calgary, Canada. 15. Bailey, R.J. and A. Logan, Canadian Heavy Oil. Quarterly Meeting, Calgary, Canada, May 13 (1991). 16. Marchetti, C., Int. J. Hydrogen Energy 44 (8)(1989) 493.