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Thermoeconomics and CO2-emissions
Energy Vol. 15, No. 2: pp. 73-80, 1990 Printed in Great Brimn. AU rights reserved
03646442/90 $3.00 + 0.00 copyright 0 1990 Pergamoo Press plc
THERMOECONOMICS H.-M.
AND
COrEMISSIONSt
GROSCURTH and R. K~~MMEL
Physikalisches
Institut der Univenitiit D-8700 Wiirzburg, FRG
(Received
2 October
Wiirzburg
1989)
Abstract - Strategies of reducing carbon dioxide emissions (COZ) are evaluated by an optimization model for the Federal Republic of Germany (FRG) that was originally designed to optimize the use of primary energy and the costs of the energy system. The model is extended to included a third objective function which monitors the production of carbon dioxide in the system. By vector-optimization, we find that the application of energy-saving technologies, together with COz-removal techniques, may reduce the overall emissions of CO2 by more than 70%. This combination will increase the total costs of the energy system by nearly 100% and decrease energy consumption by 25%. In addition, we examine compromises between the three objectives. There are conflicting strategies of reducing COz-emissons.
1
INTRODUCTION
Carbon dioxide (CO*) and other anthropogenic trace gases are among the main causes of possible and likely man-made changes of climate. Since the emissions of COZ, which represent the largest single contribution to the greenhowe-efiecl, are mostly due to the combustion of fossil fuels, it is necessary to reconsider the way we use energy thoroughly.l-3 Commonly, the extraction of CO* from the flue gases is thought to be impossible or impracticable. With that premise, CO+missions can only be reduced by energy conservation, by shifting towards fuels that emit less carbon dioxide (e.g. natural gas or nuclear fuels), or by utilizing renewable energy sources like solar energy.4 There are, however, a few theoreticti models that have been used to investigate the removal of CO2 from exhaust gases of power plants. These will now be described. Steinberg proposed a chemical absorption and stripping process in which an alkanolamin (RNH2, where R is an organic group) binds the carbon dioxide in an absorber at a temperature of about 4O”C.5-7 The reaction can be reversed in a regenerator at temperatures around 150°C. This will, of course, cost a considerable amount of energy. However, low-pressure steam from the turbine and its condensation energy may be utilized in the regenerator, thereby keeping exergy losses small. After the removal, the isolated CO2 has to be liquefied. If one assumes that the entire energy necessary for removing 90% of the carbon dioxide comes from the power plant itself, the overall efficiency of electricity generation will be reduced from 38% to 29%.s These figures hold for hard-coal-fired power plants. If oil or gas are burnt, the efficiency decrease is smaller because of the larger hydrogen fraction in the fuel. SchiifYer and Kiimmel have shown in a model calculation that the carbon dioxide can also be frozen out under pressm.s-*O They find that the efficiency of a coal-fired power plant is reduced tThis work was supported,
in part, by the Deutsche
Forschungsgemeinschaft. 73
H.-M. GROSCURTH and R. K~MMEL
14
from 38% to 26% if 90% of the COz is removed. On the other hand, the electrical energy generated per kg COz discharged to the environment is increased from 1.2 kWk, to 7.2 kWh,. This figure is reduced to 5.0 kWh. if transportation and disposal of the liquefied carbon dioxide are taken into account. This value must be compared with the value of 2.15 kWh, per kg COz that can be reached if one uses natural gas instead of coal. With advanced technologies, such as gasification of coal and combined gas- and steam-turbine power plants, a part of the high pressure needed to freeze out the CO1 is generated during the energy-conversion process. In combination with the fact that these technologies
have higher energy efficiencies,
this fact may help to limit the decrease of the average
power plant efficiency. Another possibility
for removing carbon dioxide is to remove nitrogen from the air that is supplied to the combustion chamber. 11 The exhaust gas will then consist of 91% CO*, 7% HZO, and 2% Oz by volume, together with small quantities of SOz, NO,, etc. After further conditioning, the exhaust gas may be used for oil recovery by carbon dioxide flooding, a technique which is one of the fastest growing oil-production methods in use today.12 The largest possible disposal site is the deep sea, 5 where much more CO* is now dissolved than is produced by the combustion of fossil fuels. Other than in depleted oil fields, only COz, but no SOZ or NO,, should be deposited in the deep sea. A way of reusing the carbon which is bound in the CO, was proposed by Martin in 1980.13 He suggests to decompose the carbon dioxide in a solar-thermal process. The process will lead to the production of some sort of petroleum that may be used as fuel. While Martin utilizes solar energy in a chemical process, Brown has tried to set up a biological system in which microalgae convert the COz into fuels.14 It is self-evident that extensive research on possible environmental damages of all of these approaches is necessary before any one of them can be realized. In the following sections we will demonstrate the effects of a (theoretical) carbon dioxide removal technique on the primary energy demand and the costs of a national energy system with the help of an optimization model.
2 Without the possibility separate objective function
THE C02-FUNCTION
of removing COz from exhaust gases, it is not necessary to have a for COz-emissions in an energy optimization model since energy con-
servation and COz-reduction are parallel objectives. Considering COS-removal changes the picture substantially because now the reduction of carbon dioxide requires additional energy. Thus, the two objectives are now in conflict with each other. The linear vector-optimization model LEO was originally designed to minimize both the primary energy needed to satisfy aggregated process-heat and electricity-demand profiles and the cost of the energy system. It has been described in detail in Refs. 15 and 16. The model is now extended to include a third objective function, the emissions of CO*. The goal is to analyze the interactions between strategies that reduce COz-emissions by energy-conservation on the one hand and strategies which use COz-removal techniques on the other hand. In LEO, an amount of enthalpy H is rated by its quality Q = [l- (Tc/Z’)], which is a measure of the exergy contained in H. T is the temperature of H and To is usually given by the temperature of the environment. Using an amplified quality scale p = 10 Q, the demand and supply levels are characterized by integer qualities running from zero to ten. In the model, only a single primary energy carrier F with quality q = 10 is used. Energy that would be discharged to the environment after utilization for a well defined process, if no saving technologies are applied, is called secondary energy in this paper. We define as primary conversion efficiency r)(q, F) the fraction of one unit of demand enthalpy q generated from one unit of fuel F (direct heating). The secondary conversion efficiency q-(q, q’, d,,t) is the fraction of one unit of demand enthalpy q generated from one unit of secondary energy q’ via heat exchangers (ez). The parameter dPP, gives the distance of transportation between the location where secondary energy q’ is created and the location where heat of quality q is demanded. For heat pumps, the secondary conversion efficiency qJq,q’) is defined analogously, but it is assumed that no heat-transportation is necessary. Finally, qc(q, F, ds) gives the fraction of one unit of demand enthalpy q generated from one unit of fuel F via cogeneration, if
Thermoeconomics and CO,-emissions
75
the distance of transportation between the cogeneration plant and the location of demand q is dw. For the transportation of heat from heat exchangers and cogeneration units, extra electrical energy is necessary to pump the heat-carrying liquid through a pipeline, while heat pumps need electricity to pump secondary energy q’ to demand levels q > q’. Therefore, L(q, q’, dgpl), Uq, F, dq> and X,(q, q’) are defined to be the specific amounts of electricity necessary to use one unit of input energy via the respective technologies. The input energy is secondary energy for heat exchangers and heat pumps and primary energy in the case of cogeneration. In this study, electricity is chosen to be the only form of exergy input for saving technologies. It is, however, possible to chose other exergy forms, e.g. natural gas for driving heat pumps. For a detailed discussion of the q’s and A’s see Appendix A of Ref. 16. Energy demand and supply is described by n(q) = number of annually demanded enthalpy units of quality q (q = 1,. . . ,9); n(q’) = number of available secondary energy units of quality q’ (q’ = 0,. . . ) 9); n(l0) = demand of enthalpy units of electricity (q = 10); N(F) = total number of enthalpy units of primary energy F (fuel, q = 10); z;(q,q’)n(q’) = fraction of the total amount of available secondary energy q’ supplied to demand sector q via technology i (i = et, p); z,(q)n,(lO) = amount of electricity cogenerated with heat of quality q. The sum of zc(q) over all q is equal to 1, nc(lO) therefore gives the total amount of cogenerated electricity. The I,,, xp, and x,n, are the optimization variables. CO2 emissions are taken into account by W = total amount of annually emitted CO*; IV0 = total amount of annually emitted COz if no energy conservation technologies or COs removal techniques are applied. The specific CO* production factors are w(q, F) = amount of COs released by burning one unit of fuel F in order to supply heat to quality level q; w( 10, F) = amount of CO2 released by burning one unit of fuel F in all-electric power plants; w,(lO, F) = amount of CO* released by burning one unit of fuel F in a cogeneration plant. Calculations are done as if no CO2 were emitted by heat production in cogeneration plants, the whole CO1-output is charged to electricity production. This is just a formal procedure that does not aiiect the results. The total CO2 emissions of process-heat and electricity production IV,, without using energysaving technologies and without applying COr-removal techniques can be written as
(2.1) The first term on the rhs of Eq. (2.1) gi ves the CO2 produced at various demand levels q = 1,. . . ,9. The required amount of primary energy, n(q)/q(q, F), is multiplied by the specific COs-production factor w(q, F). Th e second term analogously indicates the COr-output of all-electric power plants. Application of any of the energy conservation technologies will reduce the emissions of COP due to the decreased input of primary energy. Since electricity is used to operate the technology, there will be an extra COz-output from the production of this electricity. The two effects are accounted for by defining a specific COs-reduction exchangers, we obtain
function
0; for each technology
i (i = ez, c,p).
For heat
The first term of Eq. (2.2) g ives the saved amount of CO2 per unit of secondary energy q’ supplied to demand level q via heat exchangers. The COz-savings result from the fact that for any demand unit satisfied by secondary energy no fossil fuel has to be burnt and thus, no COs is generated. The second term indicates the CO2 released from all-electric power plants during the production of electricity for pumping the heat-carrying liquid through pipelines. The specific COs-reduction function for cogeneration u= in Eq. (2.3) is made up of the CO1 saved by supplying cogenerated heat to a demand level q (1st term), the COz-emissions of the cogeneration plant (2nd term), the decrease of COz-output by the reduced eIectricity production in all-electric power plants (positive part of 3rd term) and, finally, the COz from producing the pumping-electricity in all-electric power plants (negative part of 3rd term), viz
w(q,F) vc(q,F, 4,) wc(lO,F) 410,F) v(q,F) vc(lO,F) - ~c(lO,F) + dlO,F)
%(Q)= -
1
_
Us F, do& ~c(lo,F)
1 ’
(2.3)
76
The specific COs-production Eq. (2.2) as
H.-M. G~oscumi and R. K~MEL
function for heat pumps given in Eq. (2.4) is defined similarly to (2.4)
Subtracting the COs-balances of the energy-conservation technologies determined by Eqs. (2.2) - (2.4) from Wc of Eq. (2.1), one finds that the total annual emissions W of CO? are given by a function which has exactly the same structure as the cost function C given by Eq. (3.5) of FLef. 16, i.e.
The Heavyside-function 4 (e(z) = 1 if z > 0 and zero otherwise) is used to restrict cogeneration to q < 6 and to make sure that not more exergy is supplied to drive a technology than may be saved by using it. This is achieved by the pi (i = ez,p,c) which are well defined functions of fixed technological parameters like conversion efficiencies etc. They become negative if the exergy balance does.r5Js The technology involved will then be excluded by B{p;}. In order to describe the total annual cost of the energy system, specific cost functions wi sre used in place of the COs-reduction functions oi. In Ref. 16, w&q, q’) was defined as a measure of the saved cost per unit of secondary energy q’ supplied to demand level q via heat exchangers, the investment cost of the heat exchangers and the heat-transportation facilities, and of the cost of the electricity needed to pump the heat-carrying liquid. The specific cost function for cogeneration w,(q) comprises the cost saved by supplying cogenerated heat to a demand level q, the cost of the cogeneration plant and its transportation facilities, the cost saved by the reduced electricity production in all-electric power plants, and the cost of the electricity for pumping the heat-carrying liquid. Finally, wP(q, q’) includes the cost saved by supplying secondary energy to demand level q, the cost of the heat pumps and the cost of producing the electricity that drives the heat pumps. In the same way in which the cost function C represents the financial burden on the users of the energy system, the COr-function W stands for the environmental burden. With the help of the functions pi, it is possible to regroup Eq. (2.1) of Ref. 16, which describes the total amount of primary energy N(F), into the same structure as Eq. 2.5. Similar to the COsreduction functions Ui or cost functions wi, the pi act as specific energy-saving functions. They stand for the amount of energy that is saved by applying technology i (i=es, c,p) and the energy that is necessary to operate the respective technology. For the specific COs-emissions associated with the single primary energy carrier F in the LEOmodel, we take the values given for hard coal by Fricke, Schii6ler and Kiimmel (see Tab.l).s If COs is frozen out from the flue gases or absorbed chemically, its emissions to the environment may be cut down by 9O%.s This is only possible for the centralized all-electric power plants and perhaps for cogeneration plants. There is no chance to reduce the COr-emissions from direct heating. Thus, we consider three scenarios: The standard scenario without COr-removal, a second scenario where only the all-electric power plants have reduced COr-emissions and a third one with COs-removal for all power plants including cogeneration plants. The removal of CO2 will lead to reduced efhciencies vi for power plants and cogeneration units. 81sThe parameter values used in the different scenarios are given in Tab. 1. Up to now, the cost of CO*-removal has not been evaluated in detail. Steinberg et al estimate that the realization of their chemical process could double the price of electricity for the consumer in the United States.5 In our study, we use investment and capital costs for COr-reduced all-electric power plants that are five times higher than for standard ones. This corresponds to approximately 125% higher prices of one unit of electrical energy at todays fuel prices. For cogeneration plants, the same additional costs as for all-electric power plants are assumed.
Thermocconomics and CO,-emissions Table
1. Energy scenarios
conversion
efficiencies
and C&
production
wo,F) wm,F) kg
2 3
3
(standard)
factors for different
(1 kg COz /kWh,k = 277.4 kt COz /PJ).
4% F) r1(10, F) %(lO,F) f-4!?,F) 1
n
0.85 0.85 0.85
0.40 0.30 0.30
0.25 0.25 0.15
0.31 0.31 0.31
CO2
/kWh
0.31 0.031 0.031
VECTOR-OPTIMIZATION OF ENERGY, C02-EMISSIONS
0.31 0.31 0.031
COSTS AND
LEO now has three objective functions, namely the amount of primary energy N(F) needed to satisfy the demand of process heat and electricity, the costs C of the energy system and the total carbon dioxide emissions W. Vector optimization is performed by minimizing the weighted sum of these three functions, which is given by Y = P * [~(q/Nl(~)l+
7 * [C/Co1 + Cl-
P - 7) * [W/WI
,
(3.1)
where p and 7 are weight factors. The subscript 0 indicates the magnitudes of N(F), C and W when no energy-saving technologies and COz-removal techniques are applied. Each combination of weight factors /3 and 7 will lead to the identification of a pareto optimal solution, which means that none of the objective functions can be reduced further without increasing one of the others. If the objectives of vector-optimization are not parallel but conflicting with each other, there will be many such pareto optimal solutions that may be viewed as compromises between the different objectives. By computing solutions for many sets of weight factors, it is possible to determine the trade-off between the objective functions, i.e. one quantifies the increase of one function if another one is reduced by one unit due to optimization. The results that will be described below should be viewed with caution for several reasons: (a) The COz-removal technologies considered have never been tested, they are just theoretical concepts. (b) The demand profiles used are not based on proper statistics but result from estimations (although the best available ones), (c) LEO is a static model. Thus, all results should not be treated as final solutions that can be realized, but rather as indicators of strategies for which further research seems worthwhile. Figure 1 shows some optimization results for the FRG’s industry-plus-households demand. The demand profile was obtained from the industrial demand profile used in Ref.16 by adding the roughly estimated low-temperature demand of households. This demand should be included since it can very well be satisfied via heat exchangers from higher quality levels and may thus increase the (relative) energy-saving potential. Figure 1 gives, among others, the solutions with least energy, least costs and least C02production for the scenarios l-3 which have been described in the previous section. They are obtained by optimizing a single-valued objective function, i.e. by assigning weight 1 to the energy, cost and COz-production term in Eq.(3.1), respectively. The least-cost result for the first scenario (no COz-removal, double-hatched bars) also represents the reference point, where each objective function has the value 100. Applying energy optimization leads to an energy-requirement of 58.8% of No(F), i.e. a saving potential of 41.2%. The CO2 emissions are also 58.8% (of Wo), since. they are a direct result of the energy consumption. For the second scenario (C02-reduced all-electric power plants, diagonally hatched bars), the least possible costs are higher than in the first scenario since power plants are now much more expensive. They are substituted completely by cogeneration plants, which were assumed to be more expensive than usual power plants, 16 but not as expensive as COz-reduced power plants. Applying energy optimization leads to the same kind of substitution, but this time caused by the increased advantage in energy efficiency for cogeneration units. In addition, the other energy-saving technologies, namely heat pumps and heat exchangers, are applied. This leads to the considerable cost increase that has been determined in Ref.16. Seeking for the least COz-production will reverse
78
H.-M.
GROSCURIH and R. KCJMMEL
CO2 -Production
Fig.1.
Optimization results for the industry-plus-households demand profile of the FRG at a fuel price of 4 ACU/GJ [k 249 (1986) per barrel oil]. All results have been normalized to the standard case, in which neither energy-conservation technologies nor COr-removal techniques are used. This case is represented by the value 100 on each axis. Also, all values for energy and cost have been rounded off to fit into equidistant bins that are necessary for the three-dimensional plot.
the substitution of power plants by cogeneration units. The carbon dioxide emissions are then reduced to 26.7% of the reference value Wo. Due to higher costs and lower energy efficiency of power plants this achievement leads to increases of both total energy and costs with respect to the least-energy case. Energy conservation techniques are still applied, therefore less energy is necessary than in the cost-optimal case. In the third scenario (COr-reduction for power plants and cogeneration units, vertically hatched bars), the substitution of power plants to reduce the costs is no longer possible since cogeneration units suffer the same increase in costs. The least costs, which are still 35% higher than for the standard case, are reached by not applying any of the energy-saving technologies. Due to the energy consuming COz-reduction techniques, an 5%-increase in primary energy demand is found, while the COz-production is reduced by 20%. The results for energy and for CO2 optimization are nearly identical, since power plants and cogeneration units suffer the same losses of efficiency in this scenario. Application of energy-saving technologies together with the CO*-reduction techniques leads to the same low COr-production as in scenario 2. The application of cogeneration plants reduces primary energy consumption and cost with respect to the second scenario. The reduction in primary energy cannot be seen in Fig. 1 due to the limited resolution. All the results discussed so far were obtained by optimizing a single-valued objective function. For the third scenario, some results of vector optimization, i.e. for a multi-valued objective function with different weight factors, are displayed in Fig. 1 (white bars). Each of these represents a paretooptimal compromise between the three objectives. Especially interesting is the fact that a 45% reduction in CO? can be reached with a 60% cost increase, while the full COz-reduction of 73% requires 90% higher costs.
Thermoeconomics
and CO,-emissions
79
If the investment costs of COz-reduced power pkants are only 100% higher than those of standard power plants (instead of the 400% assumed before), optimization leads to the same results as far as energy and COz-production are concerned. Naturally, the related costs are reduced significantly, e.g. the largest cost increase is reduced from a 100% to 65%. Since we are using fixed energy demand profiles and are concentrating on optimizing the efficient use of energy, it is necessary to pay some attention to energy conservation me&sums that are complementary to our approach. These are mainly techniques that will reduce the energy demand itself. The insulation of private houses and office building, for example, is such a technique. Optimistic studies claim that up to 90% of the energy demand can be saved for new houses and nearly 80% for old ones.17 Along this line, in an additional scenario, we assume that the energy demand of households is reduced to one third without creating additional costs by investing the amount of money in insulation that would have been spent on the heating system otherwise. The remaining third may then not be satisfied by heat-exchangers or cogeneration units. This assumption is reasonable since it would be too expensive to build a pipeline system for the largely reduced demand. The decrease in energy demand alone leads to a reduction of COz emissions by 25%. If now the power plants are COz-reduced, COz-emissions can be cut down by more than 40%. This is about the same value that was determined for the energy optimization in scenarios 1 and 2. The cost-increase with respect to the standard values, however, is only 40% instead of more than 50%, but this holds only if there are really no additional costs related with the insulation measures. Substituting again cogeneration plants for the COz-reduced power plants, the required costs are only 30% higher than in the standard scenario, while the COz-emissions are not increased significantly. On the other hand, seeking the COr-minimum for the reduced profile leads to larger absolute CO2 emissions than in the COz-minimum of the full demand profile. This results from the fact that the reduced demand profile is less suitable for the application of energy-saving technologies, especially since households cannot be served via heat-exchangers or cogeneration due to the assumption made above. While the demand for energy services is reduced by 25%, the (optimized) amount of primary energy required to satisfy this demand decreases only by 3%. This result shows that different strategies of reducing the anthropogenic CO? emissions may be conflicting with each other. By realizing one reduction path, the potential of another one may be deminished substantially. Therefore, increased attention should be paid to the interactions between different strategies.
4
CONCLUSIONS
We have demonstrated that the removal of CO2 from exhaust gases of power plants and cogeneration units in combination with energy optimization strategies may reduce the emissions of this infrared-active gas by more than 70%. Assuming that the investment costs for COz-reduced power plants will be five times higher than todays costs, we find that the full realization of the reduction potentials may nearly double the total cost of the energy system. Still significant reductions in COz-emissions (40%) may be obtained if a SO-SO% increase in cost is accepted. This can be done either by applying the COz-removal techniques or by optimizing the efficiency of the energy system. It should be stressed that the COr-removal techniques considered in this paper are based on theoretical concepts, which have not yet been tested even on a small scale. We think that further research and thorough discussion is necessary to answer the following questions: (a) The installation of COz-removal facilities in power plants will need an enormous effort not only for the extraction units but also for providing the capacities of COz-disposal including a COz-transportation network. Is this the most efficient way to use the available money? Isn’t it therefore more effective to focus on energy conservation, thus realizing one of the less expensive paretooptimal compromises, and spend the remaining money on the improvement of energy efficiency in developing countries? The answer to this question depends on whether techniques can be developed that allow the reuse of COz on a large scale rather than require the disposal of it. Therefore, prototypes of all removal processes should be built. This is also necessary in order to determine the real-life efficiencies of the removal facilities. (b) The removal of CO2 requires (primary) energy, which leads in return to increased emissions of other pollutants like SOz and NO,. Which is the strategy that utilizes
H.-M. GROSCURTH and R. Km
80
energy with the least environmental impact? (c) Since different strategies of energy conservation may be conflicting with each other, which combination will lead to the least CO,-emissions?
References 1.
“Schutz der ErdatmosplGre: Eine Intemationale Herausforderung”, Zwischenbericht der Enquetekommission des 11. Deutschen Bundestages, BOM (1988).
2.
Manifesto of Basel, Recommendations of European natural scientists to the European Ecumenic Assembly “Peace with Justice” in Base& Switzerland (May 1989); German text in Physikalische Bliitter 45,340 (1989).
3.
German Physical Society and German Meteorological Society, “The Threat of Man-Made Global Changes in Climate”, Bad Honnef (1989); German text in Physikafiscbe Bliitter 43, 347 (1987).
4.
H.J. Wagner and G. Kolb, “CO2 Emissions for Germany, Present and Projected Trend with a Technology Scenario for Their Reduction”, Contribution at the IEA/OECD Seminar on Energy Technologies for Reducing Emissions of Greenhouse Gases, Paris (April 1989).
5.
M. Steinberg, H.C. Cheng and F. Horn, “A Systems Study for the Removal, Recovery and Disposal of Carbon Dioxide from Fossile Fuel Power Plants in the US”, BNL35666 Informal Report, Brookhaven Natl. Lab., Upton, N.Y. (1984).
6.
M. Steinberg, “An Analysis of Concepts for Controlling Atmospheric Carbon Dioxide”, BNL33960, Brookhaven Natl. Lab., Upton, N.Y. (1983).
7.
M. Steinberg and S. Albanese, “Environmental Control Technolgy for Atmospheric Carbon Dioxide”, Final Report, DOE/EV- 0079, U.S. Department of Energy, Washington (1980).
8.
J. F%ke, U. Schii8ler and R. Kiimmel, Physik in unsemr i&it 20,168 (1989).
9.
U. Schil6ler and R. Kiimmel, “Carbon Dioxide Removal from Fossil Fuel Power PhIdB by Refrigeration Under Pressure” in Proceedings of the 24th Intersociety Energy Conversion Engineering Conference (IECEC ‘89) pp. 1789-1794, W.D. Jackson ed, Institute of Electrical and Electronics Engineers, New York (1989).
10.
U. Schii6ler and R. Kiimmel, “Energieaufwand fur das Ausfrieren von CO2 aus den Rauchgasen von Steinkohle-Kraftwerken”, unpublished discussion paper (1989).
11.
A.M. Wolsky and C. Brooks, “Recovering CO2 from Large Stationary Combustors” in Proceedings of the IEA/OECD Expert Seminar on Energy Technologies for Reducing Emissons of Greenhouse Gases, Paris (April 1989).
12.
J.J. Taber, “Need, Potential and Status of CO2 for Enhanced Oil Recovery” in Proceedings of a Workshop Held in Pacific Grove, California, Feb 11-13, 1985, Argonne National Laboratory Report ANL/CNSV-TM-166, Argonne, IL (1985).
13.
L.R. Martin, Solar Energy 24, 271 (1980).
14.
L.M. Brown, “Lipid Fuel From Microalgae, Air Pollution Abatement and the Greenhouse Effect”, talk presented at the 24th Intersociety Energy Conversion Engineering Conference (IECEC ‘89), Washington (1989).
15.
H.-M. Groscurth, R. Kiimmel and W. van Gool, Energy 14, 241 (1989).
16.
H.-M. Groscurth and R. Kiimmel, Energy 14, 685 (1989).
17.
A.B. Lovins, L.H. Lovins, F. Krause and W. Bach, Least-Cost Energy: Solving the CO2 Problem, Rocky Mountain Institute, Snowmass, CO (1989); reprinted from the Brick House edition (1981).
03646442/90 $3.00 + 0.00 copyright 0 1990 Pergamoo Press plc
THERMOECONOMICS H.-M.
AND
COrEMISSIONSt
GROSCURTH and R. K~~MMEL
Physikalisches
Institut der Univenitiit D-8700 Wiirzburg, FRG
(Received
2 October
Wiirzburg
1989)
Abstract - Strategies of reducing carbon dioxide emissions (COZ) are evaluated by an optimization model for the Federal Republic of Germany (FRG) that was originally designed to optimize the use of primary energy and the costs of the energy system. The model is extended to included a third objective function which monitors the production of carbon dioxide in the system. By vector-optimization, we find that the application of energy-saving technologies, together with COz-removal techniques, may reduce the overall emissions of CO2 by more than 70%. This combination will increase the total costs of the energy system by nearly 100% and decrease energy consumption by 25%. In addition, we examine compromises between the three objectives. There are conflicting strategies of reducing COz-emissons.
1
INTRODUCTION
Carbon dioxide (CO*) and other anthropogenic trace gases are among the main causes of possible and likely man-made changes of climate. Since the emissions of COZ, which represent the largest single contribution to the greenhowe-efiecl, are mostly due to the combustion of fossil fuels, it is necessary to reconsider the way we use energy thoroughly.l-3 Commonly, the extraction of CO* from the flue gases is thought to be impossible or impracticable. With that premise, CO+missions can only be reduced by energy conservation, by shifting towards fuels that emit less carbon dioxide (e.g. natural gas or nuclear fuels), or by utilizing renewable energy sources like solar energy.4 There are, however, a few theoreticti models that have been used to investigate the removal of CO2 from exhaust gases of power plants. These will now be described. Steinberg proposed a chemical absorption and stripping process in which an alkanolamin (RNH2, where R is an organic group) binds the carbon dioxide in an absorber at a temperature of about 4O”C.5-7 The reaction can be reversed in a regenerator at temperatures around 150°C. This will, of course, cost a considerable amount of energy. However, low-pressure steam from the turbine and its condensation energy may be utilized in the regenerator, thereby keeping exergy losses small. After the removal, the isolated CO2 has to be liquefied. If one assumes that the entire energy necessary for removing 90% of the carbon dioxide comes from the power plant itself, the overall efficiency of electricity generation will be reduced from 38% to 29%.s These figures hold for hard-coal-fired power plants. If oil or gas are burnt, the efficiency decrease is smaller because of the larger hydrogen fraction in the fuel. SchiifYer and Kiimmel have shown in a model calculation that the carbon dioxide can also be frozen out under pressm.s-*O They find that the efficiency of a coal-fired power plant is reduced tThis work was supported,
in part, by the Deutsche
Forschungsgemeinschaft. 73
H.-M. GROSCURTH and R. K~MMEL
14
from 38% to 26% if 90% of the COz is removed. On the other hand, the electrical energy generated per kg COz discharged to the environment is increased from 1.2 kWk, to 7.2 kWh,. This figure is reduced to 5.0 kWh. if transportation and disposal of the liquefied carbon dioxide are taken into account. This value must be compared with the value of 2.15 kWh, per kg COz that can be reached if one uses natural gas instead of coal. With advanced technologies, such as gasification of coal and combined gas- and steam-turbine power plants, a part of the high pressure needed to freeze out the CO1 is generated during the energy-conversion process. In combination with the fact that these technologies
have higher energy efficiencies,
this fact may help to limit the decrease of the average
power plant efficiency. Another possibility
for removing carbon dioxide is to remove nitrogen from the air that is supplied to the combustion chamber. 11 The exhaust gas will then consist of 91% CO*, 7% HZO, and 2% Oz by volume, together with small quantities of SOz, NO,, etc. After further conditioning, the exhaust gas may be used for oil recovery by carbon dioxide flooding, a technique which is one of the fastest growing oil-production methods in use today.12 The largest possible disposal site is the deep sea, 5 where much more CO* is now dissolved than is produced by the combustion of fossil fuels. Other than in depleted oil fields, only COz, but no SOZ or NO,, should be deposited in the deep sea. A way of reusing the carbon which is bound in the CO, was proposed by Martin in 1980.13 He suggests to decompose the carbon dioxide in a solar-thermal process. The process will lead to the production of some sort of petroleum that may be used as fuel. While Martin utilizes solar energy in a chemical process, Brown has tried to set up a biological system in which microalgae convert the COz into fuels.14 It is self-evident that extensive research on possible environmental damages of all of these approaches is necessary before any one of them can be realized. In the following sections we will demonstrate the effects of a (theoretical) carbon dioxide removal technique on the primary energy demand and the costs of a national energy system with the help of an optimization model.
2 Without the possibility separate objective function
THE C02-FUNCTION
of removing COz from exhaust gases, it is not necessary to have a for COz-emissions in an energy optimization model since energy con-
servation and COz-reduction are parallel objectives. Considering COS-removal changes the picture substantially because now the reduction of carbon dioxide requires additional energy. Thus, the two objectives are now in conflict with each other. The linear vector-optimization model LEO was originally designed to minimize both the primary energy needed to satisfy aggregated process-heat and electricity-demand profiles and the cost of the energy system. It has been described in detail in Refs. 15 and 16. The model is now extended to include a third objective function, the emissions of CO*. The goal is to analyze the interactions between strategies that reduce COz-emissions by energy-conservation on the one hand and strategies which use COz-removal techniques on the other hand. In LEO, an amount of enthalpy H is rated by its quality Q = [l- (Tc/Z’)], which is a measure of the exergy contained in H. T is the temperature of H and To is usually given by the temperature of the environment. Using an amplified quality scale p = 10 Q, the demand and supply levels are characterized by integer qualities running from zero to ten. In the model, only a single primary energy carrier F with quality q = 10 is used. Energy that would be discharged to the environment after utilization for a well defined process, if no saving technologies are applied, is called secondary energy in this paper. We define as primary conversion efficiency r)(q, F) the fraction of one unit of demand enthalpy q generated from one unit of fuel F (direct heating). The secondary conversion efficiency q-(q, q’, d,,t) is the fraction of one unit of demand enthalpy q generated from one unit of secondary energy q’ via heat exchangers (ez). The parameter dPP, gives the distance of transportation between the location where secondary energy q’ is created and the location where heat of quality q is demanded. For heat pumps, the secondary conversion efficiency qJq,q’) is defined analogously, but it is assumed that no heat-transportation is necessary. Finally, qc(q, F, ds) gives the fraction of one unit of demand enthalpy q generated from one unit of fuel F via cogeneration, if
Thermoeconomics and CO,-emissions
75
the distance of transportation between the cogeneration plant and the location of demand q is dw. For the transportation of heat from heat exchangers and cogeneration units, extra electrical energy is necessary to pump the heat-carrying liquid through a pipeline, while heat pumps need electricity to pump secondary energy q’ to demand levels q > q’. Therefore, L(q, q’, dgpl), Uq, F, dq> and X,(q, q’) are defined to be the specific amounts of electricity necessary to use one unit of input energy via the respective technologies. The input energy is secondary energy for heat exchangers and heat pumps and primary energy in the case of cogeneration. In this study, electricity is chosen to be the only form of exergy input for saving technologies. It is, however, possible to chose other exergy forms, e.g. natural gas for driving heat pumps. For a detailed discussion of the q’s and A’s see Appendix A of Ref. 16. Energy demand and supply is described by n(q) = number of annually demanded enthalpy units of quality q (q = 1,. . . ,9); n(q’) = number of available secondary energy units of quality q’ (q’ = 0,. . . ) 9); n(l0) = demand of enthalpy units of electricity (q = 10); N(F) = total number of enthalpy units of primary energy F (fuel, q = 10); z;(q,q’)n(q’) = fraction of the total amount of available secondary energy q’ supplied to demand sector q via technology i (i = et, p); z,(q)n,(lO) = amount of electricity cogenerated with heat of quality q. The sum of zc(q) over all q is equal to 1, nc(lO) therefore gives the total amount of cogenerated electricity. The I,,, xp, and x,n, are the optimization variables. CO2 emissions are taken into account by W = total amount of annually emitted CO*; IV0 = total amount of annually emitted COz if no energy conservation technologies or COs removal techniques are applied. The specific CO* production factors are w(q, F) = amount of COs released by burning one unit of fuel F in order to supply heat to quality level q; w( 10, F) = amount of CO2 released by burning one unit of fuel F in all-electric power plants; w,(lO, F) = amount of CO* released by burning one unit of fuel F in a cogeneration plant. Calculations are done as if no CO2 were emitted by heat production in cogeneration plants, the whole CO1-output is charged to electricity production. This is just a formal procedure that does not aiiect the results. The total CO2 emissions of process-heat and electricity production IV,, without using energysaving technologies and without applying COr-removal techniques can be written as
(2.1) The first term on the rhs of Eq. (2.1) gi ves the CO2 produced at various demand levels q = 1,. . . ,9. The required amount of primary energy, n(q)/q(q, F), is multiplied by the specific COs-production factor w(q, F). Th e second term analogously indicates the COr-output of all-electric power plants. Application of any of the energy conservation technologies will reduce the emissions of COP due to the decreased input of primary energy. Since electricity is used to operate the technology, there will be an extra COz-output from the production of this electricity. The two effects are accounted for by defining a specific COs-reduction exchangers, we obtain
function
0; for each technology
i (i = ez, c,p).
For heat
The first term of Eq. (2.2) g ives the saved amount of CO2 per unit of secondary energy q’ supplied to demand level q via heat exchangers. The COz-savings result from the fact that for any demand unit satisfied by secondary energy no fossil fuel has to be burnt and thus, no COs is generated. The second term indicates the CO2 released from all-electric power plants during the production of electricity for pumping the heat-carrying liquid through pipelines. The specific COs-reduction function for cogeneration u= in Eq. (2.3) is made up of the CO1 saved by supplying cogenerated heat to a demand level q (1st term), the COz-emissions of the cogeneration plant (2nd term), the decrease of COz-output by the reduced eIectricity production in all-electric power plants (positive part of 3rd term) and, finally, the COz from producing the pumping-electricity in all-electric power plants (negative part of 3rd term), viz
w(q,F) vc(q,F, 4,) wc(lO,F) 410,F) v(q,F) vc(lO,F) - ~c(lO,F) + dlO,F)
%(Q)= -
1
_
Us F, do& ~c(lo,F)
1 ’
(2.3)
76
The specific COs-production Eq. (2.2) as
H.-M. G~oscumi and R. K~MEL
function for heat pumps given in Eq. (2.4) is defined similarly to (2.4)
Subtracting the COs-balances of the energy-conservation technologies determined by Eqs. (2.2) - (2.4) from Wc of Eq. (2.1), one finds that the total annual emissions W of CO? are given by a function which has exactly the same structure as the cost function C given by Eq. (3.5) of FLef. 16, i.e.
The Heavyside-function 4 (e(z) = 1 if z > 0 and zero otherwise) is used to restrict cogeneration to q < 6 and to make sure that not more exergy is supplied to drive a technology than may be saved by using it. This is achieved by the pi (i = ez,p,c) which are well defined functions of fixed technological parameters like conversion efficiencies etc. They become negative if the exergy balance does.r5Js The technology involved will then be excluded by B{p;}. In order to describe the total annual cost of the energy system, specific cost functions wi sre used in place of the COs-reduction functions oi. In Ref. 16, w&q, q’) was defined as a measure of the saved cost per unit of secondary energy q’ supplied to demand level q via heat exchangers, the investment cost of the heat exchangers and the heat-transportation facilities, and of the cost of the electricity needed to pump the heat-carrying liquid. The specific cost function for cogeneration w,(q) comprises the cost saved by supplying cogenerated heat to a demand level q, the cost of the cogeneration plant and its transportation facilities, the cost saved by the reduced electricity production in all-electric power plants, and the cost of the electricity for pumping the heat-carrying liquid. Finally, wP(q, q’) includes the cost saved by supplying secondary energy to demand level q, the cost of the heat pumps and the cost of producing the electricity that drives the heat pumps. In the same way in which the cost function C represents the financial burden on the users of the energy system, the COr-function W stands for the environmental burden. With the help of the functions pi, it is possible to regroup Eq. (2.1) of Ref. 16, which describes the total amount of primary energy N(F), into the same structure as Eq. 2.5. Similar to the COsreduction functions Ui or cost functions wi, the pi act as specific energy-saving functions. They stand for the amount of energy that is saved by applying technology i (i=es, c,p) and the energy that is necessary to operate the respective technology. For the specific COs-emissions associated with the single primary energy carrier F in the LEOmodel, we take the values given for hard coal by Fricke, Schii6ler and Kiimmel (see Tab.l).s If COs is frozen out from the flue gases or absorbed chemically, its emissions to the environment may be cut down by 9O%.s This is only possible for the centralized all-electric power plants and perhaps for cogeneration plants. There is no chance to reduce the COr-emissions from direct heating. Thus, we consider three scenarios: The standard scenario without COr-removal, a second scenario where only the all-electric power plants have reduced COr-emissions and a third one with COs-removal for all power plants including cogeneration plants. The removal of CO2 will lead to reduced efhciencies vi for power plants and cogeneration units. 81sThe parameter values used in the different scenarios are given in Tab. 1. Up to now, the cost of CO*-removal has not been evaluated in detail. Steinberg et al estimate that the realization of their chemical process could double the price of electricity for the consumer in the United States.5 In our study, we use investment and capital costs for COr-reduced all-electric power plants that are five times higher than for standard ones. This corresponds to approximately 125% higher prices of one unit of electrical energy at todays fuel prices. For cogeneration plants, the same additional costs as for all-electric power plants are assumed.
Thermocconomics and CO,-emissions Table
1. Energy scenarios
conversion
efficiencies
and C&
production
wo,F) wm,F) kg
2 3
3
(standard)
factors for different
(1 kg COz /kWh,k = 277.4 kt COz /PJ).
4% F) r1(10, F) %(lO,F) f-4!?,F) 1
n
0.85 0.85 0.85
0.40 0.30 0.30
0.25 0.25 0.15
0.31 0.31 0.31
CO2
/kWh
0.31 0.031 0.031
VECTOR-OPTIMIZATION OF ENERGY, C02-EMISSIONS
0.31 0.31 0.031
COSTS AND
LEO now has three objective functions, namely the amount of primary energy N(F) needed to satisfy the demand of process heat and electricity, the costs C of the energy system and the total carbon dioxide emissions W. Vector optimization is performed by minimizing the weighted sum of these three functions, which is given by Y = P * [~(q/Nl(~)l+
7 * [C/Co1 + Cl-
P - 7) * [W/WI
,
(3.1)
where p and 7 are weight factors. The subscript 0 indicates the magnitudes of N(F), C and W when no energy-saving technologies and COz-removal techniques are applied. Each combination of weight factors /3 and 7 will lead to the identification of a pareto optimal solution, which means that none of the objective functions can be reduced further without increasing one of the others. If the objectives of vector-optimization are not parallel but conflicting with each other, there will be many such pareto optimal solutions that may be viewed as compromises between the different objectives. By computing solutions for many sets of weight factors, it is possible to determine the trade-off between the objective functions, i.e. one quantifies the increase of one function if another one is reduced by one unit due to optimization. The results that will be described below should be viewed with caution for several reasons: (a) The COz-removal technologies considered have never been tested, they are just theoretical concepts. (b) The demand profiles used are not based on proper statistics but result from estimations (although the best available ones), (c) LEO is a static model. Thus, all results should not be treated as final solutions that can be realized, but rather as indicators of strategies for which further research seems worthwhile. Figure 1 shows some optimization results for the FRG’s industry-plus-households demand. The demand profile was obtained from the industrial demand profile used in Ref.16 by adding the roughly estimated low-temperature demand of households. This demand should be included since it can very well be satisfied via heat exchangers from higher quality levels and may thus increase the (relative) energy-saving potential. Figure 1 gives, among others, the solutions with least energy, least costs and least C02production for the scenarios l-3 which have been described in the previous section. They are obtained by optimizing a single-valued objective function, i.e. by assigning weight 1 to the energy, cost and COz-production term in Eq.(3.1), respectively. The least-cost result for the first scenario (no COz-removal, double-hatched bars) also represents the reference point, where each objective function has the value 100. Applying energy optimization leads to an energy-requirement of 58.8% of No(F), i.e. a saving potential of 41.2%. The CO2 emissions are also 58.8% (of Wo), since. they are a direct result of the energy consumption. For the second scenario (C02-reduced all-electric power plants, diagonally hatched bars), the least possible costs are higher than in the first scenario since power plants are now much more expensive. They are substituted completely by cogeneration plants, which were assumed to be more expensive than usual power plants, 16 but not as expensive as COz-reduced power plants. Applying energy optimization leads to the same kind of substitution, but this time caused by the increased advantage in energy efficiency for cogeneration units. In addition, the other energy-saving technologies, namely heat pumps and heat exchangers, are applied. This leads to the considerable cost increase that has been determined in Ref.16. Seeking for the least COz-production will reverse
78
H.-M.
GROSCURIH and R. KCJMMEL
CO2 -Production
Fig.1.
Optimization results for the industry-plus-households demand profile of the FRG at a fuel price of 4 ACU/GJ [k 249 (1986) per barrel oil]. All results have been normalized to the standard case, in which neither energy-conservation technologies nor COr-removal techniques are used. This case is represented by the value 100 on each axis. Also, all values for energy and cost have been rounded off to fit into equidistant bins that are necessary for the three-dimensional plot.
the substitution of power plants by cogeneration units. The carbon dioxide emissions are then reduced to 26.7% of the reference value Wo. Due to higher costs and lower energy efficiency of power plants this achievement leads to increases of both total energy and costs with respect to the least-energy case. Energy conservation techniques are still applied, therefore less energy is necessary than in the cost-optimal case. In the third scenario (COr-reduction for power plants and cogeneration units, vertically hatched bars), the substitution of power plants to reduce the costs is no longer possible since cogeneration units suffer the same increase in costs. The least costs, which are still 35% higher than for the standard case, are reached by not applying any of the energy-saving technologies. Due to the energy consuming COz-reduction techniques, an 5%-increase in primary energy demand is found, while the COz-production is reduced by 20%. The results for energy and for CO2 optimization are nearly identical, since power plants and cogeneration units suffer the same losses of efficiency in this scenario. Application of energy-saving technologies together with the CO*-reduction techniques leads to the same low COr-production as in scenario 2. The application of cogeneration plants reduces primary energy consumption and cost with respect to the second scenario. The reduction in primary energy cannot be seen in Fig. 1 due to the limited resolution. All the results discussed so far were obtained by optimizing a single-valued objective function. For the third scenario, some results of vector optimization, i.e. for a multi-valued objective function with different weight factors, are displayed in Fig. 1 (white bars). Each of these represents a paretooptimal compromise between the three objectives. Especially interesting is the fact that a 45% reduction in CO? can be reached with a 60% cost increase, while the full COz-reduction of 73% requires 90% higher costs.
Thermoeconomics
and CO,-emissions
79
If the investment costs of COz-reduced power pkants are only 100% higher than those of standard power plants (instead of the 400% assumed before), optimization leads to the same results as far as energy and COz-production are concerned. Naturally, the related costs are reduced significantly, e.g. the largest cost increase is reduced from a 100% to 65%. Since we are using fixed energy demand profiles and are concentrating on optimizing the efficient use of energy, it is necessary to pay some attention to energy conservation me&sums that are complementary to our approach. These are mainly techniques that will reduce the energy demand itself. The insulation of private houses and office building, for example, is such a technique. Optimistic studies claim that up to 90% of the energy demand can be saved for new houses and nearly 80% for old ones.17 Along this line, in an additional scenario, we assume that the energy demand of households is reduced to one third without creating additional costs by investing the amount of money in insulation that would have been spent on the heating system otherwise. The remaining third may then not be satisfied by heat-exchangers or cogeneration units. This assumption is reasonable since it would be too expensive to build a pipeline system for the largely reduced demand. The decrease in energy demand alone leads to a reduction of COz emissions by 25%. If now the power plants are COz-reduced, COz-emissions can be cut down by more than 40%. This is about the same value that was determined for the energy optimization in scenarios 1 and 2. The cost-increase with respect to the standard values, however, is only 40% instead of more than 50%, but this holds only if there are really no additional costs related with the insulation measures. Substituting again cogeneration plants for the COz-reduced power plants, the required costs are only 30% higher than in the standard scenario, while the COz-emissions are not increased significantly. On the other hand, seeking the COr-minimum for the reduced profile leads to larger absolute CO2 emissions than in the COz-minimum of the full demand profile. This results from the fact that the reduced demand profile is less suitable for the application of energy-saving technologies, especially since households cannot be served via heat-exchangers or cogeneration due to the assumption made above. While the demand for energy services is reduced by 25%, the (optimized) amount of primary energy required to satisfy this demand decreases only by 3%. This result shows that different strategies of reducing the anthropogenic CO? emissions may be conflicting with each other. By realizing one reduction path, the potential of another one may be deminished substantially. Therefore, increased attention should be paid to the interactions between different strategies.
4
CONCLUSIONS
We have demonstrated that the removal of CO2 from exhaust gases of power plants and cogeneration units in combination with energy optimization strategies may reduce the emissions of this infrared-active gas by more than 70%. Assuming that the investment costs for COz-reduced power plants will be five times higher than todays costs, we find that the full realization of the reduction potentials may nearly double the total cost of the energy system. Still significant reductions in COz-emissions (40%) may be obtained if a SO-SO% increase in cost is accepted. This can be done either by applying the COz-removal techniques or by optimizing the efficiency of the energy system. It should be stressed that the COr-removal techniques considered in this paper are based on theoretical concepts, which have not yet been tested even on a small scale. We think that further research and thorough discussion is necessary to answer the following questions: (a) The installation of COz-removal facilities in power plants will need an enormous effort not only for the extraction units but also for providing the capacities of COz-disposal including a COz-transportation network. Is this the most efficient way to use the available money? Isn’t it therefore more effective to focus on energy conservation, thus realizing one of the less expensive paretooptimal compromises, and spend the remaining money on the improvement of energy efficiency in developing countries? The answer to this question depends on whether techniques can be developed that allow the reuse of COz on a large scale rather than require the disposal of it. Therefore, prototypes of all removal processes should be built. This is also necessary in order to determine the real-life efficiencies of the removal facilities. (b) The removal of CO2 requires (primary) energy, which leads in return to increased emissions of other pollutants like SOz and NO,. Which is the strategy that utilizes
H.-M. GROSCURTH and R. Km
80
energy with the least environmental impact? (c) Since different strategies of energy conservation may be conflicting with each other, which combination will lead to the least CO,-emissions?
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