Modelling and energy analysis of an integrated coal gasification and pyrolysis system for synthetic natural gas

The entrained-flow coal gasification is an important and attractive technology in large scale coal-based synthetic natural gas (SNG) process. In this study, an integrated entrained-flow coal gasification and pyrolysis system is proposed and investigated. The gasifier is divided into two stages in this system: one is mainly for the gasification of char and the other is mainly for the coal pyrolysis. The integrated system is studied by using a process simulation model and compared with a coal gasification system with the radiant and convective coolers. The effects of the operating conditions on the gasification performance are studied and the optimized operating conditions are obtained. It is found that the optimized steam coal ratio of the integrated system is about 250~300 kg(steam)/t (dry coal) while the gasification temperature is 1 400 ℃. The cold gas efficiency of 88.18% of the integrated system is higher than that of 84.14% of the gasification and radiant+conductive cooler system. The coal and O₂ consumptions are relative low in the integrated system. The performance of the integrated system is highly related to the yield of tar and CH₄ in the pyrolysis stage. The overall energy conversion efficiency of the integrated system (92.26%) is slightly lower than that of the gasification and radiant+conductive cooler system (93.39%). However, the exergy efficiency is enhanced by more than 2.2% in the integrated system. The integration of the gasification and pyrolysis can effectively recover and promote energy grade of the sensitive heat in the hot syngas.