774
Energy & Fuels 2003, 17, 774-778
Thermoneutral Coproduction of Calcium Oxide and Syngas by Combined Decomposition of Calcium Carbonate and Partial Oxidation/CO2-Reforming of Methane M. Halmann*,† and A. Steinfeld‡,§ Weizmann Institute of Science, Department of Environmental Sciences and Energy Research, Rehovot 76100, Israel, and Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland, and ETH-Swiss Federal Institute of Technology, Department of Mechanical and Process Engineering, CH-8092 Zurich, Switzerland Received October 1, 2002. Revised Manuscript Received January 14, 2003
The calcination of limestone to produce lime and cement is characterized by its high-energy consumption and concomitant CO2 emissions to the atmosphere. By combining the calcination reaction with the partial oxidation and CO2-reforming of methane, an overall thermo-neutral process can be designed, with high-quality syngas as a useful byproduct, and a substantial decrease in CO2 release. The thermochemical boundary conditions for such a combined process are established. The thermodynamics are also examined with coal instead of methane as the fuel and carbon source. Adding H2O in the reaction mixture results in the coproduction of CaO and syngas with a H2/CO molar ratio of 2, suitable for methanol synthesis. A preliminary economic analysis indicates favorable competitiveness, even before the application of credit for CO2 avoidance. The fuel saving of the proposed coproduction vs the separate production may amount up to 70%, and the avoidance of CO2 emissions may reach 74%.
Introduction The thermal decomposition of calcium carbonate to calcium oxide,
CaCO3 f CaO + CO2
∆H0298 ) 178 kJ mol-1 (1)
is the main endothermal step in the production of lime1a and cement.1b In 1998, the world production of hydrolic cement amounted to about 1.52 billion metric tons, 57% of it produced in the U. S.2a The CO2 emissions derived from this reaction are due both to the combustion of fossil fuels for supplying the required high-temperature process heat, and to the inherent CO2 release by the reaction itself. These together amount annually to about 0.7 billion tons CO2, and contribute approximately 5% to the global anthropogenic CO2 emissions.3 About 50% of these emissions are derived from the combustion of fuels for process heat. The calcination of CaCO3 is also an essential step in the recovery of the CaO required in the pulp and paper industry.4 * Corresponding author. E-mail:
[email protected]. Fax: +972-89-344-124. Phone: +972-89-471-834. † Weizmann Institute of Science. ‡ Paul Scherrer Institute. § ETH-Swiss Federal Institute of Technology. (1) Ullmann’s Encyclopedia of Industrial Chemistry, VCH Publishing: Weinheim, Germany. (a) Oates, T. Lime Limestone 1990, A15, 317-345. (b) Locher, F. W., Kropp, J. Cement Concr. 1986, A5, pp 489537. (2) (a) www.global-cement.dk/filos.htm. (b) www.qualical.com. (c) www.ppc.za. (d) Thoma, A. Ecological Consideration of the Solar Production of Lime; Semesterarbeit, ETH: Zurich, 2002. (In German.) (3) Steinfeld, A.; Thompson, G. Energy 1994, 19, 1077-1081. (4) Miner, R., Upton, B. Energy 2002, 27, 729-734.
Another industrial highly endothermal reaction is the production of syngas by either steam or CO2 reforming of natural gas,
CH4 + H2O f 3H2 + CO ∆H°298 ) 206 kJ mol-1 (2) CH4 + CO2 f 2H2 + 2CO ∆H0298 ) 247 kJ mol-1 (3) By combining the CO2-releasing decomposition of CaCO3 with the CO2-consuming reforming of CH4, it is possible to simultaneously coproduce CaO and syngas in a single reaction, represented by
CaCO3 + CH4 f CaO + 2CO + 2H2 ∆H0298 ) 426 kJ mol-1 (4) Such a combined process was experimentally demonstrated to proceed at above 1300 K.3 The use of concentrated solar energy as the source of high-temperature process heat can eliminate the part of CO2 emissions derived from the combustion of fossil fuels. Reaction 1 has been experimentally demonstrated in a solar furnace using cyclone and rotary kiln solar reactors.5 However, a drawback of using solar energy is its intermittent nature, requiring a hybrid solar-fossil fuel plant for continuous operation. (5) Steinfeld, A.; Imhof, A.; D. Mischler, D. J. Solar Energy Eng. 1992, 114, 171-174.
10.1021/ef020219u CCC: $25.00 © 2003 American Chemical Society Published on Web 03/13/2003
Coproduction of Calcium Oxide and Syngas
Energy & Fuels, Vol. 17, No. 3, 2003 775
An alternative path for syngas production from natural gas is to combine the endothermic steam or CO2 reforming of CH4 (eqs 2 and 3) with the exothermal partial oxidation of CH4,
CH4 + 1/2O2 f 2H2 + CO ∆H0298 ) -38 kJ mol-1 (5) thus achieving overall thermoneutral reactions, with significant fuel economy, and decreased CO2 emission.6-9 The same principle of combining endothermal and exothermal reactions has been applied recently for the thermoneutral coproduction of metals and syngas by reacting metal oxides with calculated amounts of CH4 and O2. The predicted reactions were experimentally confirmed for the reduction of ZnO to Zn, and of Fe2O3 to Fe using thermogravimetric and gas-chromatographic measurements.10 Such combined reactions are environmentally beneficial by decreasing both fuel consumption and the release of CO2. Furthermore, these kind of cleaner processes may be adopted more readily by industry because they are also economically advantageous.11 Since the global demand for cement is projected to increase by 60-105% over current levels by year 2020, there exists an urgent need to mitigate the expected much increased CO2 emissions.12 In the present work, the chemical thermodynamics boundary conditions are determined for the thermoneutral coproduction of CaO and syngas by the combined decomposition of CaCO3 and partial oxidation/CO2reforming of CH4 or carbon. Complete and rapid calcination of limestone is known to occur above 1173 K.1a In commercial lime kilns, the surface temperature of the limestone particles should not exceed 1373 to 1423 K, to prevent the sintering of the calcined product.2b Thus, the calculation for the combined process was performed for the temperature range of 1200-1400 K. Thermochemical Computations To derive the conditions for thermo-neutrality, the thermochemical equilibrium compositions for the reactions of CaCO3CH4 with O2 in the absence and presence of H2O at 1 bar and 1200 or 1400 K were computed, using existing programs.13,14 The enthalphies were calculated with the data given in the NIST Chemistry Webbook.15 The composition for zero enthal(6) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, O. D. F. Nature 1991, 352, 225-226. (7) Choudhary, V. R.; Rajput, A. M.; Prabhakar, B. Catal. Lett. 1995, 32, 391-396. (8) O’Connor, A. M.; Ross, J. R. H. Catal. Today 1998, 46, 203-210. (9) Inui T. Appl. Organometall. Chem. 2001, 15, 87-94. (10) Halmann, M.; Frei, A.; Steinfeld, A. Energy 2002, 27, 190691084. (11) Halmann, M. M.; Steinberg, M. Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology; Lewis Publishers: Boca Raton, 1999. (12) Mahasenan, N.; Smith, S.; Humphreys, K. The Cement Industry and Global Climate Change: Current and Potential Future Cement Industry CO2 Emissions. In Proceedings of the 6th International Conference on Greenhouse Gas Control Technologies, Kyoto, Japan, Oct. 1-4, 2002; paper J3-1. See also: www.wbcsdcement.org. (13) Thermochemical Software & Database Package F*A*C*T; Centre for Research in Computational Thermochemistry, Ecole Polytechnique de Montreal: Montreal, Canadal; www.crct.polymtl.ca. (14) Gordon, S.; J. B. McBride, J. B. NASA SP-273. NASA Lewis Research: Cleveland, OH, 1976. A PC version prepared by T. Kappauf, M. Pipho, and E. Whitby for E. A. Fletcher at the University of Minnesota was used in the present study.
Figure 1. Equilibrium composition vs temperature for CaCO3CH4-O2-H2O, initially with molar ratio 1:3:2.55:3.2 at 300 K and 1 bar. phy was derived by interpolation of the plot of ∆H vs CH4/O2 molar ratio. For reactions at 1200 K and 1400 K, thermoneutrality was reached at molar ratios of 0.79 or 0.86, respectively. The equilibrium composition in the temperature range 8001400 K for a reaction mixture of initially CaCO3-CH4-O2H2O (molar ratio 1:3:2.55:3.2) at 1 bar is shown in Figure 1. The conversion of CaCO3 to CaO is seen to appear between 1000 and 1100 K, while maximal H2 yield is predicted to occur between 900 and 1000 K. As an approximate estimate for the reaction of limestone with coal and oxygen, the thermochemical equilibrium composition of the systems CaCO3-C-O2 and CaCO3-C-O2-H2O was also calculated.
Results For the system CaCO3-CH4-O2 (molar ratio 1:3:2) at 1000 K and 1 bar, the equilibrium molar ratio of CaO: CaCO3 is still only 0.8:0.2, but at 1050 K the equilibrium is 100% toward formation of CaO. The calculated equilibrium compositions for thermoneutral reactions, which include complete conversion of CaCO3 to CaO, are presented in Table 1. In the absence of added H2O, the H2/CO molar ratios are rather low, in the range 1.21.4. To enhance the H2/CO ratio, water was included in the calculated reactant mixtures, thus promoting the water-gas shift reaction,
H2O + CO ) H2 + CO2
(6)
For the reactions described in runs 5-7 and 10 of Table 1, the resulting H2/CO molar ratios are close to 2.0, suitable for methanol synthesis. Obviously, this increases the amount of CO2 released, as can been seen in Table 1. At this temperature, there should not be any (15) National Institute of Standards and Technology. Standard Reference Data Program, Chemistry Webbook; http://webbook.nist.gov/ chemistry.
776
Energy & Fuels, Vol. 17, No. 3, 2003
Halmann and Steinfeld
Table 1. Thermochemical Equilibrium Composition of Products CaO, H2, CO, H2O, CO2, H2/CO Molar Ratios, and Exergy Efficiencies for Thermoneutral Reactions of CaCO3 (1 mol) with Various Initial Amounts of CH4, O2, and H2O at 1 Bar and 1200 or 1400 K run no.
temp K
CH4 initial mol
O2 initial mol
H2O initial mol
CaO final mol
H2 final mol
CO final mol
H2O final mol
CO2 final mol
H2/CO molar ratio
ηSyngas %
ηMeOH %
1 2 3 4 5 6 7 8 9 10
1200 1200 1200 1200 1200 1200 1200 1400 1400 1400
2.00 2.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00 3.00
1.80 2.00 2.37 2.50 2.50 2.55 2.55 2.57 2.97 3.10
3.50 2.00 2.50 3.00 3.20
1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00
2.39 2.70 4.21 4.47 4.55 4.56 4.59 3.74 3.89 3.84
2.01 1.30 3.05 2.53 2.45 2.34 2.31 3.12 2.17 1.96
1.61 4.81 1.79 3.54 3.95 4.44 4.61 2.26 7.11 8.66
0.99 1.70 0.95 1.47 1.55 1.66 1.69 0.88 1.83 2.04
1.19 2.07 1.38 1.77 1.96 1.95 1.99 1.20 1.79 1.96
63.4 57.9a 68.3 66.0 66.0a 65.1a 65.1a 64.7 57.4 55.0a
55.6b 49.6 59.7 57.5 57.5 56.8 56.8 56.6 50.2 48.0
5.00 6.50
a In these Runs, an in situ water-gas shift reaction provided directly a molar ratio H /CO ∼ 2. In all other runs, a subsequent separate 2 water-gas shift was assumed to reach that H2/CO ratio. b For the calculations of ηMeOH assumed 90% yield of conversion of syngas to methanol.
Table 2. Thermochemical Equilibrium Composition of Products CaO, H2, CO, H2O, CO2, H2/CO Molar Ratios, and Exergy Efficiencies for Thermoneutral Reactions of CaCO3 (1 mol) and Carbon with Various Initial Amounts of O2 and H2O at 1 Bar and 1200 K, Producing 1 Mol of CaOa run no.
carbon initial mol
O2 initial mol
H2O initial mol
H2 product mol
CO product mol
H2O product mol
CO2 product mol
H2/CO molar ratio
ηsyngas %
ηMeOH %
11 12
2.0 2.0
1.2 1.7
8.0
0.40
1.60 0.20
7.60
1.40 2.80
1.99
56.8b 25.8c
50.3 23.4
a For the thermochemical calculations carbon was taken as representative of coal. b In run 11, a subsequent separate water-gas shift was assumed. c In run 12, an in situ water-gas shift reaction provided directly a molar ratio H2/CO ∼ 2.
Ca(OH)2 present since it decomposes above 810 K to CaO and H2O.1a In the reaction of limestone with carbon, in the presence of water (CaCO3-C-O2-H2O in the molar ratio 1:2:1.7:8.0), a product H2/CO molar ratio of 1.99 is predicted, as required for methanol synthesis (see Table 2, run 12). However, in this case, the yield of conversion of carbon to syngas is quite low, as most of the carbon is oxidized to CO2. Instead, it seems much more efficient to carry out the water-gas shift conversion as a separate subsequent step, following a combined calcination and coal-O2 gasification (as shown in Table 2, run 11). This procedure is taken for the preliminary economic evaluation (see below). While these calculations show that the combined calcination of limestone with methane partial oxidation/ CO2-reforming is thermochemically favorable, the actual process could in principle be kinetically limited. Of the reaction steps involved, the calcination itself (equation 1) is of course well established. The calcination-CH4CO2 reforming indirectly involved in eq 4 had been confirmed experimentally above 1300 K, using concentrated sunlight as the heat source.3 Only the methane partial oxidation (eq 5) has not been directly confirmed under the conditions of the calcination. However, the actual mechanism is probably a primary step of total oxidation of part of the CH4 to CO2 and H2O, followed by H2O/CH4 and CH4/CO2 reforming. Such an “indirect” mechanism had been proposed for the combined partial oxidation and CO2 reforming of CH4 over supported Pt catalysts at temperatures of up to 1073 K.8 Preliminary Economic Evaluation A tentative economic evaluation of the coproduction of lime and methanol (via syngas) by the reaction CaCO3-CH4-O2-H2O is presented in Table 3. The proposed plant would have a designated capacity of 100
Table 3. Economic Assessment of CaO and Methanol Production from CaCO3 and NG design parameters design CaCO3 feed (ton/day) daily CaCO3 feed (kmol/day) annual CaCO3 feed (kmol/year) daily NG feed (kmol/day) daily NG feed (mmbtu/day) annual NG feed (mmbtu/year) daily O2 feed (kmol/day) annual O2 feed (ton/year) annual CaO production (kmol/year) annual CaO production (ton/year) annual methanol production (kmol/year)b annual methanol production (ton/year) capital cost (million U.S. $) equipment and facilityc interest during construction (10% of facility investment start-up expenses and working capital (10%) total annual cost (million U. S.$) capital cost (15% of total) operation and maintenance (2% of total) NG cost (U. S. $5.00/mmbtu)d O2 cost (U. S. $40/ton)e total annual sales (million U.S. $) CaO (U. S. $40/ton)f methanol (U. S. $229/ton)g total
100 1000 0.35 × 106 3000 2.54 × 103 0.89 × 106 2550 0.0287 × 106 0.35 × 106 0.020 × 106 0.81 × 106 0.026 × 106 2.0 0.2 0.2 2.4 0.36 0.05 4.45 1.15 6.01 0.80 5.95 6.75
a A designated mixture of CaCO -CH -O -H O (molar ratio 3 4 2 2 1:3:2.55:3.2) at 1200 K and 1 bar, calculated to produce in a thermoneutral reaction CaO-H2-CO-H2O-CO2 (molar ratio 1:4.59:2.31:4.61:1.69), followed by methanol synthesis. Ton ) metric ton. Operation: 350day/year. b Assume 100% conversion of syngas to methanol. c Including the lime kiln, PSA, methanol synthesis reactor, and other related equipment and facility. d January 2003. Source: International Herald Tribune. e See: Basye, L.; Swaminathan, S. Hydrogen Production Costs; 1997; Report by SENTECH, Inc., for DOE/GO/101-778. f December 2002. See: www.ppc.co.za. g January 2003. See: www.methanex.com/ methanol
tons of limestone per day, typical of small sized plants.2b It is assumed to operate 24h/day, 350d/year. Such a
Coproduction of Calcium Oxide and Syngas Table 4. Economic Assessment of CaO and Methanol Production from CaCO3 and Coal: The Designated Reaction Mixture Is CaCO3-C-O2 (Molar Ratio 1:2:1.2), Calculated to Produce in a Thermoneutral Reaction at 1200 K and 1 Bar CaO, CO, and CO2 (Molar Ratio 1:1.6:1.4)a design parameters design CaCO3 feed (ton/day) daily CaCO3 feed (kmol/day) annual CaCO3 feed (kmol/year) annual coal feed (kmol/year) annual coal feed (ton/year) annual O2 feed (kmol/year) annual O2 feed (ton/year) annual CaO production (kmol/year) annual CaO production (ton/year) annual methanol production (kmol/year)b annual methanol production (ton/year) capital cost (million U. S. $) equipment and facilityc interest during construction (10% of facility investment) start-up expenses and working capital total annual cost (million U. S. $) capital costs (15% of total) operation and maintenance (2% of total) coal cost (U. S. $40/ton)d O2 cost (U. S.$40/ton)e total annual sales (million U. S.$) CaO (U. S.$40/ton)f methanol (U. S.$229/ton)g total
100 1o00 0.35 × 106 0.70 × 106 0.0084 × 106 0.42 × 106 0.01344 × 106 0.35 × 106 0.020 × 106 0.187 × 106 0.00597 × 106 2.0 0.2 0.2 2.4 0.36 0.05 0.34 0.53 1.28 0.80 1.37 2.17
a After water-gas shift of 2/3 of CO to H , the syngas is con2 verted to methanol. b Assume 100% conversion of syngas to methc anol. Including the lime kiln, PSA, methanol synthesis reactor, and other related equipment and facility. d July-Dec 2001. See: www.eia.doe.gov/cneaf/coal/cia/html. e See: Basye, L.; Swaminathan, S. Hydrogen Production Costs; 1997; Report by SENTECH, Inc., for DOE/GO/101-778. f December 2002. See: www. ppc.co.za. g January 2003. See: www.methanex.com/methanol.
process appears profitable. The economics are even more favorable when using the less expensive coal as the reducing agent, as shown in Table 4. However, with coal, as noted above, the ratio of CO2 released to products is much higher. In these calculations, the cost of limestone has not been considered, as this strongly depends on locality. No credit has been assumed for the sale of the purified CO2 (reported price U. S. $12/ton17). Also, no carbon tax (U. S. $38/ton CO2) has been assumed as a credit for the avoided CO2 emissions. The main uncertainties in these evaluations are the capital costs, the cost of the oxygen (which would probably be produced on-site by an air separation plant), and the strongly variable prices of natural gas and methanol. Obviously, the syngas could be converted also to other products, such as hydrogen. If the hydrogen is destined to be used for ammonia synthesis, air instead of pure oxygen may be used as the oxidant in the initial reaction mixtures. In the economic evaluations of Tables 3 and 4, methanol was taken as the product from syngas because of the large variety of its uses, and because its market price is readily accessible.18 Major current markets for methanol are formaldehyde (35.5%, mainly for the production of polymers), various chemicals and (16) Leites, I. L.; Sama, D. A.; Lior, N. Energy 2003, 28, 55-97. (17) See: www.ieagreen.org.uk/util4.htm. (18) See: www.methanex.com/methanol.
Energy & Fuels, Vol. 17, No. 3, 2003 777
solvents (30.8%), methyl-tert-butylether (the fuel additive MTBE, 27.4%), and acetic acid (6.4%). All of these uses, except MTBE, are long-term sinks for carbon.11 The annual production of 0.026 × 106 tons of methanol, as described in Table 3, would supply 0.085% of the worldwide demand for methanol, which in 2003 amounts to 30.5 × 106 tons.19 Fuel Economy As described in Table 3, the annual thermoneutral coproduction of 0.020 × 106 ton CaO and of 2.44 × 106 kmol (54.7 × 106 m3) syngas (2H2 + CO) requires 0.9 × 106 mmbtu (0.94 × 106 GJ) of natural gas (NG). The conventional separate production of CaO in modern efficient lime kilns requires 3.5 × 103 kJ/kg.2b Thus, the production of 0.020 × 106 ton CaO requires 0.07 × 106 GJ. Also, the conventional steam-methane reforming of NG to produce 1 kg syngas requires 0.501 kg NG for the synthesis, and additionally 12.09 MJ fuel (any fuel) for process heat. Hence, 1 kmol of syngas production requires 1 kmol CH4, or 0.89 GJ for the synthesis (taking the HHV of CH4 as 891 kJ/mol), and 0.387 GJ for fuel. The total fuel consumption is thus 1.28 GJ/kmol syngas. For the production of 2.44 × 106 kmol syngas (assuming 1 kmol CH4 or 0.89 GJ for the synthesis + 0.387 GJ for process heat, and taking the HHV of CH4 as 890.8 kJ/mol), the fuel requirement for syngas production is therefore 3.12 × 106 GJ. The separate conventional production of CaO and syngas require 0.07 × 106 + 3.12 × 106 ) 3.19 × 106 GJ/year. The fuel saving of the proposed coproduction vs the separate production amounts thus to 100(3.19-0.94)/3.19 ) 70.5%. The calcination of CaCO3 combined with the partial oxidation of coal to CO, followed by partial water-gas shift of CO to syngas (2H2 + CO), according to Table 4, results in the annual production of 0.020 × 106 ton CaO and 0.37 × 106 kmol syngas, using 0.084 × 106 ton coal (∼0.70 × 106 kmol carbon). Taking the HHV of carbon (graphite) as 393.5 kJ/mol, or 0.3935 GJ/kmol, the use of the above amount of coal involves an energy input of 0.275 × 106 GJ. The separate production of 0.020 × 106 ton CaO requires 0.07 × 106 GJ (see above). The production of 0.37 × 106 kmol syngas by steam methane reforming requires 0.47 × 106 GJ. Thus, the total fuel use by separate CaO and syngas productions is 0.07 × 106 and 0.47 × 106 ) 0.54 × 106 GJ. In this case the fuel saving is only 100(0.54-0.275)/0.54 ) 49.1%. Environmental Assessment By the conventional annual world production of 1,520 × 106 tons of cement and lime, 700 × 106 tons of CO2 are emitted, or 0.46 tons CO2 per ton of cement and lime products. Also, the conventional annual production of 2 × 1011 m3 of syngas by steam-methane reforming results in the emission of 300 × 106 tons of CO2.3 In modern efficient lime kilns, with limestone preheaters, the production of 1 ton CaO requires about 200 kg coal, or a C/CaO molar ratio of 0.93.2c Thus, the molar ratio of the total CO2 released (including that derived from the coal) to the CaO produced is 1.93. With natural gas (19) See: www.methanol.org.
778
Energy & Fuels, Vol. 17, No. 3, 2003
as reducing agent, such lime kilns results in a molar ratio of total CO2 released to CaO produced of 1.38.2d For the proposed coproduction of lime from limestone and natural gas, the reaction according to run 7 (Table 1), as described in the economic analysis of Table 3, leads to the annual production of 0.02 × 106 tons CaO and 54.7 × 106 m3 syngas (2.44 × 106 kmol), as well as the release of 0.026 × 106 tons CO2. For comparison, by the conventional calcination of limestone, using natural gas as fuel, and separate production of syngas, the annual production of the same 0.02 × 106 tons CaO and 54.7 × 106 m3 syngas will release 0.021 × 106 and 0.079 × 106 tons CO2, respectively, or a total of 0.100 × 106 tons CO2. Therefore, the proposed coproduction of CaO and syngas would avoid 74% of the CO2 emission by conventional separate production. For the reaction according to run 12 (Table 2), by the economic model described in Table 4, using coal as the carbon and fuel source, the annual production of 0.020 × 106 tons CaO and 8.36 × 106 m3 syngas is predicted, with the release of 0.038 × 106 tons CO2. By conventional production of CaO with coal as fuel, and of syngas production by steam-methane reforming, the amounts of CO2 emitted will be 0.030 × 106 and 0.012 × 106 tons/ year, or a total of 0.042 × 106 tons/year. Hence, the proposed coproduction by this route would avoid 9.5% of the CO2 emission by conventional separate production. Thus, from the point of view of minimizing CO2 release during the coproduction of lime and methanol, the preferred reducing agent is natural gas. Exergy Efficiency Exergy, based on the second law of thermodynamics (also called thermodynamic availability), represents the theoretical optimum work that can be performed as a result of the change of the state of a system to that of an equilibrium state.16 The exergy efficiency ηexergy is here defined by the ratio of the theoretical maximum work output that can be extracted from the products (i.e., the ∆G of their complete combustion) to the heats of combustion (HHV) of the reactants, all calculated at 298 K. Values of ∆G for the full oxidation of H2(g) to
Halmann and Steinfeld
H2O(l), CO(g) to CO2(g), and CH3OH(l) to CO2(g) and 2H2O(l) are 237, 257, and 706 kJ mol-1, respectively. For CaO(s), ∆G is taken as the free energy change of its hydration to Ca(OH)2(s), i.e., 57 kJ mol-1. The HHV of CH4 and C(graphite) are 890.8 and 393.5 kJ mol-1, respectively. Assuming attainment of thermochemical equilibrium for the reaction at 1200K and 1 bar, as indicated in Table 1 (run 6),
CaCO3 + 3CH4 + 2.55O2 + 3H2O f CaO + 4.56H2 + 2.34CO + 4.44H2O + 1.66CO2 (7) the exergy efficiency for syngas production is ηsyngas ) 65.1%. For methanol production (assuming 90% chemical yield of conversion of syngas to methanol), ηMeOH ) 56.3%. Taking carbon instead of CH4 as indicated in Table 2 (run 12),
CaCO3 + 2C + 1.7O2 + 8H2O f CaO + 0.4H2 + 0.2CO + 7.6H2O + 2.8CO2 (8) the exergy efficiency for syngas production is only ηsyngas ) 25.8%, because most of the carbon is “wasted” by its oxidation to CO2. By carrying out the water-gas shift of CO to H2 as a subsequent separate step (as described in Table 2, run 11), much higher exergy efficiencies of the production of syngas and methanol are predicted, of 56.8 and 50.3%, respectively. Conclusions The advantages of the combined decomposition of CaCO3 and partial oxidation/CO2 reforming of CH4 are 3-fold: Production of both lime and syngas in a single reactor, fuel saving relative to separate production, and considerably decreased CO2 release to the atmosphere. With coal as the reducing agent, the economics are much improved, but a large fraction of the coal would be oxidized to CO2. The inclusion of H2O in the reactant mixtures enables the production of syngas with the H2/ CO molar ratio required for methanol synthesis. EF020219U
