Numerical study on coal gasification in the downer reactor of a triple-bed combined circulating fluidized bed

Abstract

Owing to the advantages of cocurrent gas-solids flow without back mixing, short residence time, and uniform residence time distribution, the downer has emerged as an ideal reactor for gasification with high selectivity. In this study, the coal gasification in the down reactor of a triple-bed combined circulating fluidized bed was studied through computation fluid dynamics simulations using an Eulerian-Lagrangian method. The influences of nozzle arrangement for coal feeding, coal particle, and air/steam/coal feeding rates on the gasification were investigated. It was found that the tangential arrangement of feeding resulted in comparable H2 production with the normal arrangement, and higher CO production. When coal particle size was smaller than 500 μm, the particle size had little influence on the produced gas composition. In contrast, with increasing particle size beyond 500 μm, coal particles could not be gasified completely due to shorter residence time, leading to decreasing production of CO and H2. Steam gasification had a higher volume fraction of CO and H2, as well as higher char conversion ratio. With increasing coal feeding rates, the volume fraction of CO increased monotonically, while that of H2 increased first and then approached a constant value due to limited moisture availability in the coal samples. With increasing air feeding rates, more char and volatiles could be decomposed into light gases. As a result, the volume fraction of CO increased first and then started to decrease. These results had great significance in designing a better downer reactor with improved efficiency. © 2014 American Chemical Society.