Wood pellets have become a solid biomass fuel of choice because they are a standardized product with known quality properties. However, the self-heating and off-gassing tendencies during storage of wood pellets threatens the consistency of the pellets’ quality properties. The aim of this thesis was to increase the body of knowledge towards understanding of the off-gassing of wood pellets. The effects of total wood extractive content and types of extractives in the raw material on off-gassing of wood pellets were investigated through two separate studies. In the first study, the pellets were produced from fresh and pre-treated Scots pine sawdust. The pre-treatments involved storing, extraction and adding additive oils. The second study used synthetic pure cellulose that was pre-treated by adding different additive oils. The pellets were subjected to off-gassing tests under controlled conditions.

The results from the first study showed that the total amount of extractives in the raw material has little effect on off-gassing. While gas emissions were reduced for stored and acetone extracted pine pellets, the coefficients of determination (R2) from the linear correlation analysis between off-gassing and the total extractive content of the raw materials were below 0.5 for all the three off-gasses indicating no correlation. The results of cellulose pellets with added additive oils in the second study showed that the off-gassing is highly dependent on the type of extractives in the raw material. The highest mean concentrations of the carbon oxides and methane were recorded from cellulose pellets with added linseed oil. Pellets with added linseed oil had higher off-gas emissions due to the high content of unsaturated fatty acids of 73.9% linolenic and 7.6% linoleic. Based on these results, it was concluded that one of the main causes of off-gassing during storage of wood pellets is the α-oxidation of unsaturated fatty acids. The other notable effect was that methane formation is dependent on anaerobic conditions, whereas formation of carbon oxides can occur both under aerobic and anaerobic conditions.

Wood pellets have over the years become a solid biomass fuel of choice for heat and power generation because they are a standardized product with known quality properties. However, the self-heating and off-gassing tendencies of wood pellets threatens the consistency of the quality properties. Self-heating and off-gassing can cause disintegration of the pellets resulting in dry matter losses and reduction in quality properties. Additionally, self-heating may lead to fires while off-gassing of toxic gasses such as carbon monoxide is a human health and environmental hazard.

The aim of this thesis was to increase the body of knowledge towards the understanding of off-gassing of wood pellets. The results showed that total amount of extractives in the raw material has little effect on off-gassing. One of the main causes of off-gassing of carbon oxides by wood pellets is the α-oxidation of unsaturated fatty acids and other triglyceride oils that are chemically unstable. The other notable effect was that methane formation is dependent on anaerobic conditions, whereas formation of carbon oxides can occur both under aerobic and anaerobic conditions.