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Chapter 7: Mineral carbonation and industrial uses of carbon dioxide 335 Box 7.3. Energy gain or penalty in using CO2 as a feedstock instead of carbon. CO2 can be used as a provider of carbon atoms for chemical synthesis, as an alternative to standard processes where the carbon atom source is fossil carbon, as coal or methane or other. This includes processes where the carbon atom in the CO2 molecule is either reduced by providing energy, for example methanol synthesis, or does not change its oxidation state and does not need energy, synthesis of polycarbonates for example. For the sake of simplicity let us consider a reaction from carbon to an organic final product A (containing n carbon atoms) that takes place in a chemical plant (standard process): nC → A (7) Let us also consider the alternative route whereby CO2 captured from the power plant where carbon has been burnt is used in the chemical plant where the synthesis of A is carried out. In this case the sequence of reactions would be: nC→nCO2→ A (8) The overall energy change upon transformation of C into A, ΔH, is the same in both cases. The difference between the two cases is that in case (8) this overall energy change is split into two parts – ΔH=ΔHcom+ΔHsyn – one for combustion in the power plant and the other for the synthesis of A from CO2 in the chemical plant (ΔHcom will be –400 which means 400 are made available by the combustion of carbon). If ΔH is negative, that means an overall exothermic reaction (1), then ΔHsyn will be either negative or even positive. If ΔH is positive, that means an overall endothermic reaction (7), then ΔHsyn will be even more positive. In both cases, exothermic or endothermic reaction, the chemical plant will lack 400 kJ/molC energy in case (2) with respect to case (1). This energy has already been exploited in the power plant and is no longer available in the chemical plant. It is worth noting that large-scale chemical plants (these are those of interest for the purpose of carbon dioxide emission mitigation) make the best possible use of their energy by applying so-called heat integration, for example by optimizing energy use through the whole plant and not just for individual processes. In case (1) chemical plants make good use of the 400 kJ/ molC that are made available by the reaction (7) in excess of the second step of reaction (8). Therefore, in terms of energy there is no benefit in choosing path (8) rather than path (7). In terms of efficiency of the whole chemical process there might be a potential improvement, but there might also be a potential disadvantage, since route (7) integrates the heat generation associated with the oxidation of carbon and the conversion to product A. These effects are of second order importance and have to be evaluated on a case-by-case basis. Nevertheless, the scale of the reduction in CO2 emissions would be rather small, since it would be even smaller than the scale of the production of the chemicals that might be impacted by the technology change, that is by the change from path (7) to path (8) (Audus and Oonk, 1997). and emissions. However, the analysis above demonstrates that, although the precise figures are difficult to estimate and even their sign is questionable, the contribution of these technologies to CO2 storage is negligible. Research is continuing on the use of CO2 in organic chemical polymer and plastics production, but the drivers are generally cost, elimination of hazardous chemical intermediates and the elimination of toxic wastes, rather than the storage of CO2. References Arakawa, H., 1998: Research and development on new synthetic routes for basic chemicals by catalytic hydrogenation of CO2. In Advances in Chemical Conversions for Mitigating Carbon Dioxide, Elsevier Science B.V., p 19-30. Aresta, M., I. Tommasi, 1997: Carbon dioxide utilization in the chemical industry. Energy Convers. Mgmt 38, S373-S378. Audus, H. and Oonk, H., 1997, An assessment procedure for chemical utilization schemes intended to reduce CO2 emission to atmosphere, Energy Conversion and Management, 38 (suppl, Proceedings of the Third International Conference on Carbon Dioxide Removal, 1996), S 409- S 414 Barnes, V. E., D. A. Shock, and W. A. Cunningham, 1950: Utilization of Texas Serpentine, No. 5020. Bureau of Economic Geology: The University of Texas. Bearat, H., M. J. McKelvy, A. V. G. Chizmeshya, R. Sharma, R. W. Carpenter, 2002: Magnesium Hydroxide Dehydroxylation/ Carbonation Reaction Processes: Implications for Carbon Dioxide Mineral Sequestration. Journal of the American Ceramic Society, 85 (4), 742-48. Benemann, J. R., 1997: CO2 Mitigation with Microalgae Systems. Energy Conversion and Management 38, Supplement 1, S475-S79. Blencoe, J.G., L.M. Anovitz, D.A. Palmer, J.S. Beard, 2003: Carbonation of metal silicates for long-term CO2 sequestration, U.S. patent application. Brownlow, A. H., 1979. Geochemistry. Englewood Cliffs, NJ: Prentice-Hall Butt, D.P., Lackner, K.S., Wendt., C.H., Conzone, S.D., Kung, H., Lu., Y.-C., Bremser, J.K., 1996. Kinetics of thermal dehydroxilation and carbonation of magnesium hydroxide. J. Am. Ceram. Soc.PDF Image | CARBON DIOXIDE CAPTURE AND STORAGE
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