Low temperature gasification of low-rank coal using a circulating fluidized bed gasifier

An integrated coal-gasification combined-cycle (IGCC) power plant, which represents the most environmentally friendly coal-fired power generation technology, has been developed and its efficiency is higher than that of conventional coal-fired power plants. More recently, an advanced-type integrated coal-gasification combined-cycle system (A-IGCC) was proposed[1,2]. The A-IGCC system can drastically improve the thermal efficiency; as a result, its overall efficiency is theoretically higher than that of a conventional IGCC system. In the A-IGCC system, coal is gasified at a relatively low temperature (<1173 K) using only the steam generated by the exhaust heat of the GT. The design of the reactor for low-temperature steam gasification of coal is essential for development of the A-IGCC system. In the present study, a new pyrolyzer-separated circulating fluidized-bed gasification reactor was developed. Coal is continuously pyrolyzed in the downer and the resultant char is sent to the bubbling bed gasifier for steam gasification while evacuation of the volatiles avoids the charvolatile interactions. Any ungasified char is transferred from the gasifier to the bubbling bed combustor, where it is either completely combusted or partially combusted and then recycled to the downer. Steam gasification was successfully performed in the absence of pyrolysis-derived volatiles, which strongly inhibit gasification. The recycling of the partially combusted char greatly increased its concentration in the downer thereby enhancing the reforming of the volatiles, in particular that of tar over that of char, and the resultant gas formation. The total yield of gases from the downer and bubbling bed gasifier was 45% higher than that obtained under direct feeding of the coal into the bubbling bed gasifier and full combustion of the char, i.e., with steam gasification of char in the presence of volatiles and subsequent reforming over char at a much lower concentration.