Torrefaction of biomass : a comparative and kinetic study of thermal decomposition for Norway spruce stump, poplar and fuel tree chips

Abstract: Stump biomass is energy rich and stump harvesting for use as fuel become more and more interesting in Sweden. Swedish Forest Agency (2009) has estimated that stump harvesting in Sweden would respond to an annual energy supply of 57 TWh/year. However, stump has not been recognized as a bioenergy resource in Sweden. Suitable methods for pre-treatment of stump are probably of great importance to make it accepted as fuel. It is therefore rewarding to carry out an investigation in this area for stump. This report represents results from a diploma project, which was aimed to develop a fixed bed reactor for experimental study of biomass torrefaction, followed by TG analysis and kinetic modelling employing Ozawa method and different kinetic models including one-step and three-pseudo-component models. The focus was on Norway spruce stump. Two other types of biomass, poplar and fuel tree chips were also included in the study for comparison. The study has demonstrated that fuel properties, including heating values and grindability of the investigated biomasses, were improved by torrefaction. The heating rate affects the thermal decomposition process. In addition, SEM analysis indicated that the wood surface structure was broken and destroyed by torrefaction process. Activation energy of untreated and torrefied biomass were determined, being in good agreement with data in the literature. Among the kinetic models tested, three-pseudo-components model with reaction order n ≠ 1 appears to be the best for simulating pyrolysis of untreated and torrefied biomass.