Hydrodynamics, heat transfer and gasification characteristics of coal/biomass in a dual circulating fluidized bed reactor

Abstract

Due to the current high energy price, the gasification technology to convert solid fuels to the product gases ($H_2$ +CO) for the integrating gasification of combined cycle (IGCC), coal-to-liquid (CTL), chemical synthetic processes is under developing actively in many countries. Conventional fluidized bed gasifier by using air/steam as a gasifying agent produces the low calorific value gas (4-6 MJ/ $Nm^3$) due to nitrogen dilution. Whereas, a dual fluidized bed gasification technology enables to produce the medium calorific value gas (12-18 MJ/ $Nm^3$) by separating the combustion and gasification zones in which steam is used as a gasifying agent. In the present study, a rectangular dual circulating fluidized bed reactor (combustor: 0.04 m × 0.11 m × 4.5 m high; gasifier: 0.04 m × 0.285 m × 2.13 m high) was designed and constructed for gasification of coal, biomass, and their blends. Prior to gasification operation, the hydrodynamic properties such as solids circulation, bubble, and cluster in a cold-model dual circulating fluidized bed reactor (riser: 0.04 m × 0.11 m × 4.5 m high; bubbling fluidized bed: 0.04 m × 0.285 m × 2.13 m high) were determined to provide prerequisite knowledge for the optimum operation of the combustor and gasifier. The flow regime velocity, such as minimum fluidization velocity and transport velocity of bed material (silica sand particle) were determined. The riser gas velocity and recycle chamber aeration velocity were found to be decisive variables affecting solid circulation rate and a correlation is proposed to predict solid circulation rate. Hydrodynamic characteristics were identified by computational fluid dynamics (CFD) simulation where a two-dimensional (2D) multiphase Eulerian model incorporating with the kinetic theory of solid particles was used. Bubble properties (bubble chord length, rising velocity, frequency, fraction) were determined in the bubbling fluidized bed, whereas cluster properties were determined at the ris...