Influence of pre-treatment of straw biomass and additives on the release of nitrogen species during combustion and gasification

The aim of this study is to investigate how pre-treatment of herbaceous straw biomass for ash control affects the release of nitrogen species during combustion and gasification. To comprehend the formation of NO and its precursors, NH₃ and HCN, the release of these species was investigated and compared under both combustion and gasification-like conditions at 950 °C. The effects of various upgrading methods, such as torrefaction, water-leaching, a combination of leaching and torrefaction, and CaCO₃ addition, were studied. The assessment of nitrogen release was divided into two consecutive conversion steps – devolatilization/pyrolysis and ash/char reactions. The release of nitrogen is highly dependent on the reaction conditions. For instance, the emissions of NO from the combustion conditions (3 vol% O₂) for all fuel samples were, on average, six times higher than under gasification conditions (14.5 vol% H₂O and 5 vol% CO₂). The emissions of NO from the combustion and gasification of torrefied biomass were, on average, 20 % higher than those from raw biomass. Water-leaching had a suppressing effect on NO formation during char conversion. Approximately 62 % of the char-N formed NO for raw and torrefied material, whereas only 26 %–35 % was formed for pre- or postwashed samples. The effect of the applied pre-treatment approaches on the release of nitrogen was particularly significant during char conversion. Increasing calcium and decreasing potassium content had catalytic effects, mainly on the conversion of volatile-N to NH₃. The Ca-doped biomass feedstock showed approximate 10 % increase in volatile-N to NH₃ conversion compared to the source material.