Performance of a Residential Pellet Combustor Operating on Raw and Torrefied Spruce and Spruce-Derived Residues

The heterogeneous nature of solid biomass fuels makes their combustion a substantial challenge compared to the more traditional fuel types, such as fossil fuels and natural gas. Many studies found in the literature attempt at identifying enhancements in fuel properties of biomass after a thermal pretreatment step, such as torrefaction, but only few investigate specifically the combustion behavior of these fuels. In this study, pellet combustion of raw and torrefied spruce and spruce tree tops and branches (T&B) has been investigated with regard to the emissions of gaseous pollutants and particulate matter (PM). The combustion was performed in a residential pellet stove, where a total of six different feedstocks, with and without pretreatment, were tested. The wide range of the feedstock properties was shown to go beyond the design limitations of the pellet stove. This could be seen as combustion instability for the T&B torrefied at 275 °C. Technology adjustments might be needed in terms of combustion air distribution and chamber design for these fuels. Mild torrefaction, in general, reduced the emissions of CO, unburned hydrocarbons, and the organics in particles smaller than 1 μm. Combustion at a low load (low thermal input) resulted as expected in increased emissions of organic compounds, which was again reduced substantially for the mildly torrefied feedstocks. In comparison to raw spruce at low load, a reduction by a factor of 3 from the organic share of the PM1.0 particles is obtained. For the same experiments, CO in the flue gas is reduced by 150%. For T&B, similar trends were obtained for organic particles; however, torrefaction resulted in an increase in the total PM1.0 emissions. The decrease in the organic share was more than offset by a substantial increase in the inorganic share of the PM1.0 emissions. For this reason, torrefaction might not be a viable pretreatment solution for feedstocks with high ash content for use in stoves for residential heating, without combustion technology adjustments.