Combustion characteristics of ultrafine gasified semi‐char in circulating fluidized bed

Abstract

Coal fluidized bed gasification generates large amounts of semi‐char, which has a high carbon content. Effectively re‐burning semi‐char has become a promising way to improve overall conversion efficiency of coal gasification and use the residue carbon in the semi‐char. However, due to the ultrafine particle size (mean particle size ≈ tens of microns), nearly zero volatile content, and the graphitization structure of the semi‐char, the fluidization and combustion organization of semi‐char in a circulating fluidized bed (CFB) is difficult, leading to poor self‐sustained combustion stability and low combustion efficiency. The physicochemical properties of the semi‐char from an industrial CFB gasifier were studied. The combustion characteristics and activation energy of the semi‐char were tested using thermogravimetric analysis. The occurrence of sulphur and nitrogen in the semi‐char and fly ash after combustion were characterized with X‐ray photoelectron spectroscopy (XPS). A re‐burning experiment was carried out in a 5 t/day CFB pilot facility. The key is to build a balance between sufficient residence time to burnout and a proper fluidization velocity for bed material cycle. The effects of residence time, preheating temperature, and the ratio of preheating air to fuel on the combustion efficiency and pollutant emission was studied. The balance between the preheating time and residence time in the furnace was discussed. Results show that it can reach stable combustion in a specially designed CFB furnace without auxiliary fuel and the combustion efficiency can be higher than 98 %.