Entrained flow gasification of biocoal from hydrothermal carbonization

Hydrothermal carbonization (HTC) converts biomass into a coal-like substance, here referred to as biocoal. Beech wood chips are treated hydrothermally at 210 °C for 3 h in a 250 L pilot scale reactor. The carbon content (daf) thereby increases from 48 wt.% to 57 wt.% and the volatile matter (wf) decreases from 79 wt.% to 69 wt.%. The carbon and energy yields are both 80% on a mass and HHV basis, respectively, with the major loss being solubilized substances. Pulverization of biocoal is found to be much less energy intensive compared to wood. Moreover, pulverized particles show a spherical shape which may facilitate fluidization. Gasification experiments are carried out at temperatures of 1000 °C, 1200 °C and 1400 °C at atmospheric pressure in a laboratory scale entrained flow reactor. The standard particle diameter used in the experiments is 80-160 μm. During each experiment char samples are collected and the synthesis gas concentration is measured. Carbon conversion at a residence time of 1.0 s is 84%, at both 1000 °C and 1200 °C. A significant increase in carbon conversion to 88% at 1400 °C is observed. Comparable gasification experiments are carried out with lignite as the fuel. Rhenish lignite is gasified using the same stoichiometry and particle size. The conversion of lignite is slightly lower indicating the high reactivity of biocoal. An increase of particle size would be beneficial due to the lower power consumption for pulverization. In experiments with biocoal at a particle size of 160-250 μm, a slightly lower conversion is measured due to internal and external mass transfer limitations at the particles. The high conversion at short reaction times indicates a high reactivity of biocoal under entrained flow gasification conditions.