Improving Process Performances In Coal Gasification For Power And synfuel Production

摘要

This paper is aimed at developing process alternatives of conventional coal gasification. A number of possibilities are presented, simulated, and discussed in order to improve the process performances, to avoid the use of pure oxygen, and to reduce the overall CO_2 emissions. The different process configurations considered include both power production, by means of an integrated gasification combined cycle (IGCC) plant, and synfuel production, by means of Fischer-Tropsch (FT) synthesis. The basic idea is to thermally couple a gasifier, fed with coal and steam, and a combustor where coal is burnt with air, thus overcoming the need of expensive pure oxygen as a feedstock. As a result, no or little nitrogen is present in the syngas produced by the gasifier; the required heat is transferred by using an inert solid as the carrier, which is circulated between the two modules. First, a thermodynamic study of the dual-bed gasification is carried out. Then a dual-bed gasification process is simulated by Aspen Plus, and the efficiency and overall CO_2 emissions of the process are calculated and compared with a conventional gasification with oxygen. Eventually, the scheme with two reactors (gasifier-combustor) is coupled with an IGCC process. The simulation of this plant is compared with that of a conventional IGCC, where the gasifier is fed by high purity oxygen. According to the newly proposed configuration, the global plant efficiency increases by 27.9% and the CO2 emissions decrease by 21.8%, with respect to the performances of a conventional IGCC process. As a second possibility, the same gasifier-combustor scheme is coupled with a coal-to-liquid (CTL) process to convert the syngas into synthetic fuels by a FT reactor. It is shown that, if compared with a conventional CTL plant, the mass yield of liquid synthetic fuel is increased by 39.4%, the CO_2 emissions per unit of liquid fuel are decreased by 31.9% and energy efficiency increases by 71.1%.