Experimental Study On The Characteristics Of Agglomeration And Defluidization Of Biomass Combustion In Fluidized Bed

In the process of biomass combustion in fluidized bed, the alkali component mainly including potassium-K in biomass will react with the silica in bed materials, leading to the formation of low-melting silicates, which is directly responsible for the bed agglomeration at high temperature. If this agglomeration process is not recognized, partial or total defluidization of the fluidized bed will occur. Study on the characteristics of agglomeration and defluidization of biomass combustion in fluidized bed is of great importance to the utilization of biomass fluidization.Taking the typical biomass of China as research object, the experiments were carried out in a lab-scale bubbling fluidized bed combustor (5 kW BFBC) and a 0.15 MWt circulating fluidized bed combustor(CFBC) in order to investigate the characteristics of agglomeration and combustion of the biomass. The FactSage6.1 was applied to analyze the characteristics of chemical reaction, element distribution and transfer of biomass combustion in fluidized bed. Further study on the characteristics of agglomeration and defluidization of biomass combustion in fluidized bed was conducted with the experimental and theoretical analysis.The experiment system of 5 kW bubbling fluidized bed combustor with the fuel of wheat straw was designed to study the defluidization characteristics of biomass combustion. The experimental results show that defluidization time decreases with the combustion temperature increasing, and when the gas velocity increases, the defluidization time increases obviously, but defluidization still occurs. The size of the sand particles has little influence on the defluidization time. The attachment of smelt produced by the reaction between biomass ash and quartz sand to the bed materials is the main reason for defluidization. However, the Umf of sand particles after the combustion experiment increases with the increase in temperature.Using the dolomite and the mixture of quartz sand-calcium oxide as bed materials respectively, the effect of the earth alkali metals(calcium and magnesium) on the agglomeration and defluidization was investigated. The results indicate that the Ca and Mg can significantly extend fluidization time, but couldn't avoid agglomeration. When the content of Si in bed materials decreases, and that of Ca and Mg increases, the agglomeration formed by the adherent of bed materials to the unburned fuel pellets is the main reason for defluidization. It is obvious that the K content in bed materials increases after the experiments, the melting silicate has high concentration of Si, K and Ca elements.Agglomeration of bed materials at the cyclone bottom and loop seal is mainly responsible for defluidization when the biomass is combusted in CFB combustor. The results present that there is high content of K, Na, Ca, Mg, Fe, Al and S in fly ash, and the circulating materials contain more of these elements than the bottom ash do. The K, S and Na in fly ash are water soluble, and these three kinds of water-soluble elements are aslo found to be highly contained in bed materials and circulating ash of wheat straw and king grass. The Cl- and SO42- are mainly in fly ash, and there is higher content of Cl- and SO42- in the circulating materials of wheat straw and king grass, compared with that in corn stalk and cotton stalk. The Cl- and SO42- in the circulating materials can promote agglomeration.