Fluidization characteristics of torrefied forest waste biomass and pulverized coal mixture particles with different mixing ratios

Fluidized bed co-gasification of biomass and coal is a new technology for efficient and clean coal utilization, as well as an innovative method for the large-scale efficient utilization of biomass energy. The effective fluidization of gasification feedstocks can promote the heat transfer, mass transfer between gas-solid phases and increase the chemical reaction rate. Using torrefied biomass to replace raw biomass can improve the fluidization characteristics of gasification feedstocks. The investigations on fluidization characteristics of binary particle mixtures (BPMs) of torrefied biomass and pulverized coal are of great importance to the design and stable operation of fluidized bed reactors. In this paper, the fluidization experiments for the BPMs of forest waste biomass and pulverized coal were carried out with a fluidization experimental setup. The fluidization curves for the BPMs were obtained, the characteristic velocities including initial fluidization velocity, minimum fluidization velocity and complete fluidization velocity, were determined, and then "composition-characteristic velocity" phase equilibrium diagram was drawn. The effect of torrefied biomass mass fraction (0, 20%, 40%, 60%, 80% and 100%) on the fluidization characteristics of the BPMs were investigated. An empirical formula for predicting minimum fluidization velocity of the BPMs was proposed. The results showed that the individual fluidization curves for torrefied biomass and pulverized coal, respectively, can be divided into four regions: I fixed bed region, II transition region, III initial fluidization region, and IV complete fluidization region. The sequence of standardized bed pressure drop (Rp) in region IV for four different raw materials was as follows: anthracite 1 (0.90) > anthracite 2 (0.86) > torrefied biomass 1 (0.84) > torrefied biomass 2 (0.53), indicating the decrease of fluidization quality. The fluidization curves for the BMPs were similar to those of single-component particle system, while the gas velocity ranges corresponding to region II and region III were narrower. As torrefied biomass mass fraction increased, the values of Rp in complete fluidization region decreased, indicating that the fluidization quality became poor, meanwhile, the initial fluidization velocity first increased and then decreased, the complete fluidization velocity first decreased and then increased, the minimum fluidization velocity decreased gradually. Different characteristic velocities corresponded to the transition between different fluidization stages. Combining the fluidization gas velocity with the "Composition-characteristic velocity" phase diagram, the fluidization state for the BMPs can be pre-judged. The fluidization gas velocity can be adjusted according to the requirement of industrial application. The empirical formula obtained in this paper can predict the minimum fluidization velocity of the BPMs of torrefied biomass and pulverized coal well with errors ranging from -25% to +20%. © 2019, Editorial Department of the Transactions of the Chinese Society of Agricultural Engineering. All right reserved.