Release of NH₃, HCN and NO during devolatilization and combustion of washed and torrefied biomass

The influence of two different biomass pre-treatment methods (torrefaction and combined treatment of washing and torrefaction) on the formation of NO and its precursors, NH₃ and HCN, during combustion has been studied. Experiments were conducted in an electrically heated single particle reactor under well-controlled conditions in N₂ and O₂/N₂ at 850 °C. NO concentrations of the product gases were measured during devolatilization and char oxidation, and the total amounts of released NH₃ and HCN were determined during devolatilization. The results show that the conversion of vol-N and char-N to NO changed due to the pre-treatments, influencing the NO emission factor (released NO/energy content) of the fuel. For torrefied straw, the NO emission factor was significantly higher than for raw straw. The change in NO formation can partly be explained by how the nitrogen contents change due to the pre-treatments: both for bark and straw, around 20% of the initial fuel-N was removed during the torrefaction; for straw, 50% of the initial fuel-N was removed during the combined torrefaction and washing. In addition, the changes in the NO formation can be explained by the release of NH₃ and HCN during devolatilization. The HCN/NH₃ ratio increased with increasing N content and decreasing H/N ratio due to the pre-treatments. The conversion of vol-N to NH₃ increased with increasing Ca content and decreasing K content. In general, the devolatilization of the pre-treated biomass samples resulted in a higher conversion of vol-N to NH₃ and HCN as compared to the raw biomasses.