Characteristics of chars formed during rapid pyrolysis of biomass model components at high temperature

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This paper reports char formation and inherent inorganic transformation during rapid pyrolysis of various biomass model components under simulated pulverized fuel (PF) conditions at 1300 °C. A drop-tube furnace with a novel double-tube configuration was deployed to achieve direct determination of char yield. The results show that rapid pyrolysis of xylan and water-washed lignin (W-L) under the conditions results in char yields of 3.4 wt.% and 12.6 wt.%, respectively, while no char was founded during rapid pyrolysis of water-washed cellulose (W-C). After loading K2CO3 into the W-C (i.e. KW-C) and W-L (i.e. KW-L), the char yields increase to 2.1 wt.% and 15.6 wt.%, respectively. The retentions of Na and S are low in chars after pyrolysis. After rapid pyrolysis, W-L and KW-L chars have higher retentions of AAEM species than xylan, W-C and KW-C chars. Micromorphology analysis shows char particles formed after rapid pyrolysis of all biomass components have a cenospheric structure and a rough surface with many bubbles and pores, demonstrating strong melting processes. For xylan and KW-L, the abundant inorganics accelerate char formation with swelling and reduce the extent of particle shrinkage, resulting in char particles with apparent sizes bigger than the parent feedstock particles. Oppositely, for KW-C and W-L that have low contents of inorganic species, the pyrolyzing particles experience significant shrinkage, resulting in formed char particles with apparent sizes that are much smaller than feedstock particles.