Characterization of wood pellets combustion for clean energy generation

As the 4th largest global primary energy carrier following fossil fuels (coal, oil and natural gas), biomass is the most abundant renewable energy source on earth. Among all the biomass conversion technologies, the direct burning of biomass for heat and power generation is the most common and cost-effective route to turn biomass into energy. Natural wood biomass is widely used as an alternative to fossil fuels to reduce atmospheric emissions from heat and power generation in the UK, France and the rest of the world. Meanwhile, black pellets are recently produced from various woody biomass or agricultural residues through a thermal process, called torrefaction. Black pellets are regarded as high-grade solid biofuels with stable, homogeneous and greater energy density and calorific value than the original biomass. It is believed that torrefied biomass offers a cost-effective route to significantly increase the use of biomass as a renewable energy source and at the same time to lower greenhouse gas and particulate emissions from heat and power generation.

However, there are very limited systematic and comprehensive evaluations of the combustion characteristics and environmental benefits of these two types of biomass fuels.

The aim of this thesis is to quantify the combustion characteristics and associated pollutant emissions of natural wood pellets. To achieve this goal, A modified 10 kW boiler was used as the burner in our research. The characterization of the combustion of natural wood pellets is carried out using advanced equipment (measurement of gaseous emissions by analyzers and study of particulate emissions by LII and SMPS). In addition, a calibration method has been improved to directly deduce the soot volume fraction from the LII signal using the absolute radiance emitted from a light source having black body. In parallel, SMPS technology is also used to measure the particle size distribution of particulate matter emissions. Emissions of gaseous and particulate pollutants were studied for two conditions, with and without secondary air.