Promoting gas production by controlling the interaction of volatiles with char during coal gasification in a circulating fluidized bed gasification reactor

A novel circulating fluidized bed (CFB) gasifier, which consists of a downer and two bubbling fluidized beds, 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 char-volatile 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.