Operational investigation of a bubbling fluidized bed biomass gasification system

Abstract

The operational characteristics of a bubbling fluidized bed biomass gasifier (BFBG) are discussed. A gas cleaning and cooling (GCC) system that consists of cyclones and natural and forced convection air-to-gas heat exchangers is connected to the BFBG to facilitate removal of particulates, moisture and tar. The performance of the GCC with and without a second condenser and its effect is presented. The secondary condenser doubles the condensate removal rate. Temperatures across the components of the GCC remain constant after a period of time. The fluidization regime of the BFBG with varying air-flow rate was investigated by measuring the pressure drop (ΔP) with a manometer. A change in the slope of the ΔP curve indicated a change in fluidization regime from homogeneous fluidization to bubbling fluidization. The fluidization regime changes to bubbling fluidization once the flow rate exceeds 110 kg/hr regardless of the bed height, which causes intense solid particle mixing. The varying air-flow rate gives different equivalence ratio (ER) values. The upper limit of ER is constrained by the lower calorific value (LCV) of producer gas. The bed temperature and fluidization regime of the BFBG is determined by ER and has to be regulated so as to obtain maximum cold gas efficiency from the BFBG. Low ER corresponds to homogeneous fluidization and low bed temperature, while higher ER gives bubbling fluidization and higher bed temperatures. Further investigation on the relationship between these two parameters is still going on.