Abstract
The heterogeneous nature of biomass makes combustion a substantial challenge compared to the more traditional fuel types such as fossil fuels and natural gas. Torrefaction is a thermal pre-treatment process that brings many of the biomass fuel properties closer to coal. This is quite advantageous because conventional technologies that are developed for coal can be employed on biomass. Many of the studies found in the literature attempt at identifying enhancements in fuel properties although few have looked on the combustion behaviour of such fuels. This work aims at optimizing the torrefaction process parameters for the production of an upgraded fuel suitable for use in bioenergy plants. The “upgraded” fuel will give more stable plant operating conditions which will result in higher efficiencies and lower emissions. The torrefaction of typical Norwegian biomasses (spruce and birch) have been studied in a macro-TGA reactor. Important torrefaction parameters such as temperature, holdup time and particle size have been varied. The torrefaction behaviour of the samples in the form of weight loss rate is reported as a function of the above mentioned variations. The main product distribution between the liquid, gas and solid fractions is also presented. In order to investigate the water repelling property of torrefied fuels, hydrophobicity tests were performed. In addition, both the energy requirements for milling the samples and the particle size distribution after the milling is reported. Finally, the combustion kinetics of some of the torrefied samples including raw spruce and birch are presented. The raw fuels were used as a reference for the comparison with the other varied torrefaction parameters.