Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing during storage. This poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets, and offer solutions for producing wood pellets from freshly generated sawdust, with reduced self-heating and off-gassing tendencies.

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 (papers I and II). The results from paper I showed that the total amount of extractives in the raw material has little effect on off-gassing. While gas emissions were reduced for pellets produced from Scots pine sawdust that had low amounts of extractives (stored and acetone extracted), 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 low or no correlation. The results of cellulose pellets with added additive oils (paper II) 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.

The effects of raw material type and pre-treatments on self-heating and off-gassing of wood pellets were also investigated through two separate studies (papers III and IV). The results from both studies indicated significant influences of both raw material type and drying temperature. There was a strong linear correlation between off-gassing and sapwood content, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases (paper III). An increase in sapwood content of the raw material (fresh Scots pine sawdust) led to a significant increase in off-gassing of CO₂, CO and CH₄, and O₂ consumption. Storing of sawdust for over six months prior to pellet production, and increasing the temperature of drying the sawdust led to significant reduction of off-gassing for sapwood pellets. For heartwood pellets, increasing the drying temperature resulted in increased off-gassing and raw material storage had no effect. In the other study (paper IV), the pellets produced from Scots pine mature wood sawdust were more prone to self-heating and off-gassing compared to those produced from juvenile wood sawdust. Steam drying the sawdust at high temperature and pressure led to a significant reduction in heat and gas generation for both materials. Furthermore, the study established a notable connection between self-heating and off-gassing, the storage piles with high temperature increase also exhibited high concentrations of off-gases.

The overall results indicated that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets. 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. While storing fresh sawdust for a period of time prior to pellet production remains the most effective method for mitigating self-heating and off-gassing during pellet storage, sorting and separating the raw materials at source can facilitate the development of storage schedules tailored to specific raw materials, thereby reducing on the raw material storage time.

The global wood pellets production increased from about 18 million tonnes in 2012 to about 46 million tonnes in 2022. Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing and this poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets, and offer solutions for producing wood pellets with reduced tendencies for self-heating and off-gassing using freshly generated sawdust.

The results showed that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets. While storing of fresh sawdust for a period of time prior to pellet production remains the most effective method for mitigating self-heating and off-gassing, sorting and separating the raw materials at source can facilitate the development of storage schedules tailored to specific raw materials, thereby reducing on the raw material storage time.

Wood pellets have become a preferred solid biomass fuel for heat and power generation due to their standardized nature, known properties, and consistent quality. However, pellets produced from fresh sawdust sometimes undergo self-heating and off-gassing and this poses a challenge for their transportation and storage. The aim of this thesis was to increase the body of knowledge towards understanding the mechanisms underlying the self-heating and off-gassing of wood pellets.

The results from paper I showed that the total amount of extractives in the raw material has little effect on off-gassing. Although pellets produced from Scots pine sawdust with lower extractive contents exhibited reduced concentrations of off-gasses; CO, CO2 and CH4, increasing the extractive content of the sawdust through the addition of additive oils did not result in increased off-gas emissions. The results of pure cellulose pellets with added additive oils showed that off-gassing is influenced by the type of extractives in the raw material (paper II). The highest concentrations of off-gasses were recorded from pure cellulose pellets with added linseed oil due to their high content of unsaturated fatty acids of 73.9% linolenic and 7.6% linoleic.

The results from papers III and IV indicated significant effects of both raw material and drying temperature on off-gassing and self-heating of wood pellets. There was a strong linear correlation between off-gassing and the sapwood content of the raw material, with correlation coefficient (R) values greater than 0.9 at p < 0.001 for all the off-gases (paper III). An increase in sapwood content led to a significant increase in off-gassing of CO2, CO and CH4, and O2 consumption. Storing of sawdust for over six months prior to pellet production, and increasing the drying temperature led to a significant reduction in off-gassing for sapwood pellets. For heartwood pellets, increasing the drying temperature resulted in increased off-gassing while raw material storage had no effect. In the other study (paper IV), the pellets produced from Scots pine mature wood sawdust were more prone to self-heating and off-gassing compared to those produced from juvenile wood sawdust. Steam drying the sawdust at high temperature led to a significant reduction in heat and gas generation for both materials.

The overall results indicated that a biological process, in combination with the chemical oxidation of unsaturated fatty acids lay behind the self-heating and off-gassing of wood pellets.