The role of lignin in the densification of torrefied wood in relation to the final product properties

Abstract Biomass properties can be improved for heat and power applications through combined torrefaction and pelleting. Good pellet quality in terms of durability, density, moisture absorption, fines production and heating value paired with a low power consumption in the pellet mill render the densification process of torrefied materials challenging. The aim of this study is to identify the lignin components/intermediates and the corresponding mechanisms during torrefaction of wood that play a role in pelleting behaviour and pellet quality. The importance of lignin lies in its ability to act as a natural binder during densification. Structural differences caused by torrefaction of spruce and poplar (270 °C, 32–45 min) were studied by using NMR and TD-GC/MS as well as by pressing single pellets. Spruce chips were torrefied (280 °C, 35–45 min) and conditioned in steam in a pilot plant. The products were ground on a 4-mm sieve and densified in a single-pellet press, where differences in the measured responses were explained on basis of their lignin properties. The lignin was isolated from the spruce samples by organosolv fractionation and characterised in terms of amount, molecular weight distribution (SEC) and glass-transition temperature (DSC). The results of the tests and analyses indicate that torrefied softwood should be densified immediately after production. Furthermore, pellet quality of the torrefied material was found to depend on the binding ability of its lignin in the presence of moisture during densification. Additionally, storage of torrefied spruce prior to densification causes reduced binding ability of its lignin leading to pellets of lower quality.