Reciclado de CO2 mediante reformado de gas de coquería para la producción de metanol

Resumo

[EN] The steelmaking industry is the largest energy-consuming manufacturing sector. As a consequence of this, the CO2 emissions from this sector account for about 5-7 % of the total anthropogenic CO2 emissions. For this reason, increasing efforts are being made to find solutions that might help diminish these emissions and increase energy efficiency. A better management of the coke oven gas (COG) surplus is one of the proposed solutions. This study deals with the CO2 reforming of COG surplus. By means of this technology it is possible to obtain a synthesis gas with a composition suitable for use in the production of methanol. Thus, a highly valuable product, with many applications in different industries is obtained from two residual streams: the surplus of coke oven gas and CO2. Examined from a more global perspective this process constitutes a partial recycling of CO2, since part of the CO2 emitted when methanol is used is consumed in the production process. It has been established that, from the thermodynamic point of view, the most favourable operating conditions for carrying out the CO2 reforming of COG are temperatures higher than 800 ºC and the lowest possible pressures. In addition, the CH4/CO2 ratio must be as near to the stoichiometric ratio as possible. Otherwise, the process yield will be very low and/or the syngas thus obtained will not be suitable for methanol production. Since the CO2 reforming of methane is a heterogeneous catalytic reaction, it is necessary to use an appropriate catalyst. Several catalysts were tested, and the physical mixtures of activated carbon and a conventional Ni/Al2O3 catalyst were found to be the most promising. Such mixtures have a synergetic effect that leads to higher conversions of methane and carbon dioxide than those predicted by the law of mixtures. Moreover, the production of by-products, such us water, are lower than what is predicted by this law. It was found that the CO2 reforming of COG can take place via two different reaction mechanisms: on the one hand, the classical dry reforming, consisting of methane decomposition followed by gasification of the carbon deposits and on the other hand, due to the large amount of H2 present in the feed, the reverse Water Gas Shift followed by steam reforming. This latter reaction path appears to be the main mechanism, which would result in a lower deactivation rate than that of dry reforming. An assessment of the whole process, from the coke oven gas to the use of the methanol produced, has shown that this novel technology has certain advantages over conventional methanol production, the most important being lower CO2 emissions. Indeed, these emissions can be reduced by as much as 30%, depending on the location of the plant and the energy integration of the process. Moreover, COG-based production allows the maximum exploitation of the raw materials while purification costs are kept down to a minimum. From the energy point of view, COG-based production entails lower energy consumption than conventional production, whereas conventional production allows a higher energy recovery, which could eventually result in lower energy requirements provided that an adequate energetic integration strategy is adopted.