Impact of water leaching of biomass on the composition of torrefaction condensates

Abstract

This work has been carried out to demonstrate the effect of water-leachable biomass components on the liquid product distribution of torrefaction. For the partial removal of the mineral matter, each biomass feedstock was leached with hot water. The original and leached wheat straw, rape straw and black locust wood as non-food agricultural and forestry waste samples were torrefied within the temperature range of 200–300 °C. The composition of the collected torrefaction condensates as liquid products was determined from solution by GC/MS and the compositional differences were evaluated. The analysis of the torrefaction condensates allowed us to reveal the effects of water-soluble inorganic contents on the thermal decomposition processes, the yield, and the compositional changes as a function of the applied temperature. Formation of 5-hydroxymethylfurfural (HMF) was detected from the unleached black locust wood and rape straw samples at as low as 200 °C apparently formed by dehydration of the extractable sugar content. Aromatic compounds derived from lignin were released in a significant amount at and above 225 °C. The formation of these decomposition products was hindered from the water-leached samples below 250 °C. In the cases of leached samples, depolymerization of both cellulose and hemicellulose was favored contrary to fragmentation processes within the temperature range of torrefaction similarly to pyrolysis at higher temperatures. The influence of water-leachable inorganic content was also shown on the formation of lignin decomposition products with different side-groups. Interaction between the organic components of biomass was exemplified by the thermal reaction of the lipid content with the methoxy groups of lignin resulting in the formation of methyl palmitate.