CHARACTERIZATION OF THE FLOWABILITY OF BIOMASS-COAL BLENDED POWDERS

摘要

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 [1], However, the processing and handling problems related to biomass-coal blended powders, such as arching and blocking phenomena in feeding and pneumatic conveying systems, have severely hampered the widespread applications of the co-gasification technology [2, 3]. Most of the published investigations have been focused on thermo-chemical conversions of biomass-coal blends due to the economic and environmental goal, but little attentions have been paid to the flowability properties of biomass-coal blended powders, and the fundamental understanding of the underlying mechanisms which cause those above-mentioned annoying problems is still lacking [4].The flowability is the ability of a powder to flow under specified conditions. It is a complex characteristic of a given powder, which is not only an inherent property of the powder but also is highly dependent on its physical properties, such as particle size, particle shape, moisture content and surface roughness of particle material, etc [5-7]. Characterisation of powder flowability has great importance in the consistent operating of powder handling processes and the reliable designing of equipment involved, especially for those mixtures of binary particles whose flowability has not been fully understood. There are a quantity of characterization parameters to evaluate powder flowability, such as angle of repose, Hausner ratio, compressibility index, Carr flowability index and flow function, etc [8-10]. At present, researches on powder flowability are mainly focused on food and pharmaceutical powders, and there are less investigations on the flowability of pulverized coal, as well as biomass powders [11-13]. Furthermore, the researches on flowability of biomass-coal blended powders are basically rarely reported. Qian et al. [14] investigated the differences between biomass powder and pulverized coal, and found that the flowability of biomass-coal blended powders is more complicated than that of pulverized coal. They also found that the pulverized coal particles move faster than biomass powder, as the mass fraction of biomass powder increases, the mean velocity of the particles of blends decreases and the flow stability of the blends decreases. Cui et al. [15] thought that the flow of biomass particles is significantly different from that of smooth spherical particles due to the complex physical properties of biomass, which is worthy of further research. Guo et al. [10, 16, 17] carried out a series of research on the flowability of biomass-coal blended powders and found that needle-shape particles of biomass powders can improve the flowablity of viscous pulverized coal in a certain range. They also found that the factors affecting the flowablity of the blends are length-width ratio and surface roughness of biomass particles. Guo et al. [18] studied the on the flow properties of the blended particles of rice straw and coal, their research mainly focused on the effects of particle size and mass fraction of biomass powders in the blends. Zulfiqar et al. [4] investigated flow properties of Australian coal and biomass as well as their blends systematically, they found that the flow properties of the coal-biomass blends were dependent upon the form of biomass, the blends of sawdust and coal could reduce the possibility of flow stoppage, while the blends of woodchips and coal were more susceptive to flow stoppage. It is now commonly accepted that compared with pulverized coal, biomass particles are different in shape, density and material properties, and the flowability of biomass-coal blended powder has its certain particularity. However, the flowability of biomass-coal blended powders plays an important role for reliable dense-phase pneumatic conveying in the co-gasification process, which may not only affects the composit