The influence of residence time during hydrothermal carbonisation of miscanthus on bio-coal combustion chemistry

Abstract

Miscanthus was treated by hydrothermal carbonisation in a 2-L batch reactor at 200 ◦ C and 250 ◦ C with residence times ranging between 0 and 24 h to understand the impact of residence time has on the resulting bio-coal combustion chemistry. Increasing the residencetime results in dehydration of the bio-coal and increased repolymerisation; however, temperature has the greatest influence on bio-coal properties. After 24 h at 200 ◦ C, bio-coal has similar properties to that of the 250 ◦ C + 0 h bio-coal. After 1 h at 250 ◦ C, the cellulose present in the raw biomass appears to be largely removed. The removal of cellulose and the associated dehydration and repolymerisation results in bio coalhaving a ‘coal like’ combustion profile, which exhibits a decreasing reactivity with increasing residence time. At 200 ◦ C + 0 h, 75% of the alkali metal is removed, increasing to 86% with increasing residence time. Further extraction is seen at 250 ◦ C. Phosphorus and sulphur appear to undergo substantial extraction at 200 ◦ C + 0 h but then are reincorporated with increasing residence time. The calcium content increases in the bio-coal with increasing residence time at 200 ◦ C but then reduces after 1 h at 250 ◦ C. Increasing temperature and residence time has been shown to decrease the fuels’ fouling and slagging propensity.