Effects of torrefaction and water-washing on the mineral transformation behavior during co-combustion of straw and coal: A CCSEM analysis

Abstract Biomass can be torrefied to improve the fuel quality prior to its utilization. In addition, a large number of soluble alkali metals can be removed by water washing, to solve slagging and other ash-related problems. In this study, computer-controlled scanning electron microscopy (CCSEM) was used to analyze the mineral composition and elemental distribution of straw, torrefied straw char, and torrefied-water washed straw char. The mineral transformation behavior and the distribution of main elements, when these biomass species were individually co-combusted with coal, were also studied in detail. The comparison of the mineral composition of bulk ash obtained from torrefied straw combustion and straw combustion revealed that the quartz phase decreased and the aluminosilicates of K-, Na-, and Ca–Fe–K increased in the former compared to that in the latter. After the torrefied straw was washed with water, the quartz and Si-Rich phases increased. K aluminosilicate decreased and the quartz phase increased when the torrefied sample was co-combusted with coal at both mixing ratios of 1:1 and 1:4 compared to straw/coal co-combustion. When the torrefied-water washing sample and coal were co-combusted at a 1:1 ratio, the quartz phase decreased, and the Si-Rich and mullite phases increased. When the straw/torrefied char/torrefied-washed char was co-fired with coal at the 1:4 ratio, the amount of K aluminosilicate was less compared to that observed at the 1:1 ratio. It can be concluded that both torrefaction and torrefaction-washing can reduce the content of low-melting-point aluminosilicates in the mineral composition of the studied biomass after co-combustion, which can be beneficial to alleviate the ash slagging problem.