Torrefied Biomass for Large-Scale Electricity Generation

Bioenergy is increasingly being used a means of combatting the effects of anthropogenic climate change in sectors such as the power industry. Problems can exist in utilising biomass fuels however such as high moisture contents and low calorific value when compared with fossil fuels. Torrefaction is a pre-treatment process that aims to address some of these issues. In this pre-treatment step, solid biomass is heated in an inert atmosphere to between temperatures between 200 and 300°C resulting in the loss of low-energy volatiles and moisture improving fuel chemical and physical properties and is the focus of this thesis work. Firstly, the effect of changing torrefaction temperature and residence time is investigated. Results show that torrefaction increases the calorific value of fuels via reduction in moisture and volatiles contents as a result of degradation of some of the lignocellulosic components- this also improving the grindability characteristics of torrefied materials- with these change more pronounced as conditions become more severe. Results further show that with increasing torrefaction severity, the solid product yields decrease while the liquid and gaseous products increase. The combustion properties of torrefied biomass is also investigated, with results showing that torrefaction reduces the reactivity of biomass fuels and that upon rapid devolatilisation, chars from torrefied fuels differ morphologically to those of untreated biomass and undergo a lesser degree of burnout. Results also show that promotion of nitrogen to the gas phase during rapid devolatilisation may be fuel dependent after torrefaction has been performed. Finally, the effect of torrefaction of supply chain GHG emissions is investigated where it was found that GHG savings can be made as a result of increased calorific value in torrefied materials. Emissions savings are maximised where heat integration of the combustion of the volatiles gases evolved during torrefaction is implemented and wood chips are used as utility fuel. Results also indicate that torrefaction favours long distance transport as a result of improved fuel properties.