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 CO2 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, to 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 PlusTM and the efficiency and overall CO2 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 configuration newly proposed 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 CO2 emissions per unit of liquid fuel are decreased by 31.9 % and energy efficiency increases of by 71.1 %.