Influence of Torrefaction and Hydrothermal Carbonization on Fuel and Ash Properties

Abstract

In this work, torrefied as well as hydrothermal carbonised biomass have been examined. To evaluate the torrefaction process, grindability, ash melting behaviour and fuel as well as ash composition of untreated and torrefied woods were analysed. For that purpose populus, alder, spruce and birch have been used. Significant differences in heating values and particle size distributions could be observed. For spruce and birch a mean increase in the heating value of 15 % has been observed. For poplar and alder the heating value slightly increased by 7-9 %. The increase in heating value can be mainly explained with the decrease in oxygen and hydrogen containing compounds. The median values of the ground fractions for the torrefied woody biomass were within the range from 80 to 95 µm, whereas the non-torrefied woody biomass showed median values ranging from 180 to 230 µm. Main components of the ash of all analysed wooden biomass are potassium oxide, silicon oxide and calcium oxide. Torrefied biomass shows a broader melting range and a more pronounced melting peak. Analysis of ash melting, ash characteristic numbers and FactSage simulations of the untreated and torrefied wood, show a lower melting range at lower melting temperatures reffering to the untreated biomass. The H/C and the O/C ratio of the HTC coals show similar results as lignite. Except for the hydrothermal carbonised green waste, the HTC-coals of spent grains and compost show a high melting range in the field of the torrefied woods.