Flowability and discharge characteristics of mixtures with different ratio of torrefied biomass and pulverized coal

Abstract: Co-gasification of coal with biomass has been considered as a new research focus of clean coal technology, as well as a novel utilization technology of biomass energy. However, the processes of biomass collection, storage and transportation are difficult and costly. It is necessary to pretreat the raw biomass properly. Recently, torrefaction is a promising technique for biomass pretreatment. Substituting raw biomass with torrefied biomass to blend with pulverized coal can improve the flowability and discharge characteristics of gasification feedstocks, which may affect not only the composition of the synthesis gas, but also the long-period, continuous and stable operation of the gasifer. Compared with pulverized coal, torrefied biomass particles are different in shape, density and material properties, and the flowability and discharge characteristics of binary particle mixtures (BPMs) of torrefied biomass and pulverized coal have certain particularity, which needs further experimental investigation. In this paper, the experiments of flowability and discharge characteristics for the BPMs of torrefied biomass and pulverized coal were carried out with a BT-1000 powder comprehensive characteristics analyzer and a Plexiglas hopper discharge experimental system, respectively. The effect of torrefied biomass mass fraction on the flowability of the BPMs was investigated, and the differences between different mixtures of torrefied biomass and pulverized coal were compared. Meanwhile, the effects of torrefied biomass mass fraction and discharge outlet diameter of the hopper on the discharge characteristics of the BPMs were studied. An empirical formula for predicting discharge mass flow rate of the BPMs was proposed. The results show that as torrefied biomass mass fraction increases, the angle of repose and compression degree of the mixtures increase, while the Carr flowability index (FI) decreases, and the flowability of the mixtures becomes worse. Compared to the methods of angle of repose and compression degree, the FI method can be used to evaluate the flowability of the BPMs more detailedly and comprehensively. The sequence of the FI of the 4 kinds of raw material is as follows: Anthracite 1 > anthracite 2 > torrefied biomass 1 > torrefied biomass 2. The torrefied biomass mass fraction of the BMPs corresponding to the flowability turning point from the common flowability region to the poor flowability region (FI=60), is as follows: Anthracite 2 / torrefied biomass 2 (34%) < anthracite 1 / torrefied biomass 2 (52%) < anthracite 2 / torrefied biomass 1 (70%) < anthracite 1 / torrefied biomass 1 (85%). As torrefied biomass mass fraction increases, the discharge mass flow rates of different BMPs all decrease. The successful discharge of the BPMs mainly depends on the amount of torrefied biomass. The extreme amount of torrefied biomass for discharge process can be predicted and determined by measuring the FI of the BPMs. When the FI of the BPMs is less than 55, the flowability of the BPMs is in the lower region of poor flowability, which is close to the region of bad flowability or even in the region of bad flowability, and the BMPs cannot be discharged successfully. As the discharge outlet diameter increases, the discharge mass flow rates of different mixtures increase. The empirical formula obtained in this paper can predict the discharge mass flow rate of the BPMs of torrefied biomass and pulverized coal well with errors ranging from -15% to 25%.