Flow structure simulation for a novel coal feeding system of a high-density downer reactor

Abstract

For steam gasification of coal, it is found that the produced tar, light hydrocarbon gases and inorganic gases at the initial pyrolysis stage severely hinder the gasification of the char. Therefore, the produced volatiles in pyrolysis stage should be removed out from the char surface in order to enhance the efficiency of char gasification. In our study, a high density triple-bed combined circulating fluidized bed (TCFB) system, which is composed of a downer pyrolyzer, a bubbling fluidized bed (BFB) gasifier, and a riser combustor, is proposed. The coal could be pyrolyzed rapidly in the downer at first, and then, the obtained gas and tar are separated from the char using a gas-solid separator. The charcoal enters the BFB to be gasified with the steam. The unreacted char flows into the riser to be combusted with air. The produced heat can be carried by inert solid medium such as sand, and circulated into the downer with a high mass flux and the BFB to provide the heat needed in the pyrolysis and gasification processes. In this system, coal should be introduced into the downer pyrolyzer and mixed with the heat carried particles homogeneously in order to use the heat effectively. The objective of this study is to develop an optimum coal feeding system for such a high-density downer pyrolyzer. A coal feeding system for an i.d.100mm downer was designed. The flow structure and the residence time distributions of two particles (coal and sand) were simulated using CFD method. The obtained results were analyzed and discussed in this report.