To reduce the use of fossil fuels, it is important to increase the use of renewable alternatives such as fuel pellets. Fuel pellets in Sweden are primary produced from various residual products from spruce and pine. Before the production of fuel pellets, the pine sawdust needs some storage time to achieve the right pelletizing properties. During the storing process self-ignition may occur, which is a known and common problem. The purpose of this work was to increase the knowledge about how pine shavings change during laboratory storage, and to demonstrate whether it is the sapwood or heartwood that causes self-ignition. Further, it investigates the differences between pine sapwood and heartwood when it is run through a mechanical dewatering process. This in order to increase the knowledge to industries that handle and stores pine-sawdust, about how and why the problem of self-heating occurs and in the long run can be counteracted. The aim was to prove, by temperature measurements, if the cause of self-ignition is greater in the sapwood or heartwood of the pine. The study will also show how the pine shavings changes during storage through color-, moisture- and NIR-analyses. A checkpoint in the study is to show if the organic compounds, the moisture content and the compression energy are different in the sapwood and heartwood, due to a dewatering process.  It was not possible to show if it is a certain part in the pinewood that causes self-ignition through temperature measurements, but during the storage process both color- and moisture differences occurred. The NIR-analyses showed that during both anaerobic and aerobic storage the content of extractives and other compounds in the pine shavings was reduced. The dewatering process did not show big differences between the organic compounds in the sapwood and heartwood. But the process where able to decrease the moisture levels in the shavings even if the heartwood needed more compression energy.  To sum up, the study showed that it occurs changes in pine shavings during a storage process even though there was not possible to prove if it is the sapwood or heartwood that causes self-ignition.

Self-heating in storage facilities of biomass pellet can lead to spontaneous combustion. This has resulted in many fires over the years, causing both financial and environmental losses. In order to assess the risk for spontaneous combustion of biomass pellets during storage, it is important to know how prone the fuel is to self-heating, i.e. to determine its reactivity. This thesis presents experimental work performed to develop a sensitive screening test procedure for biomass pellets, using isothermal calorimetry for direct measurement of the heat production rate. This method can be used to compare the reactivity of different biomass pellets. This could be useful for e.g. facility owners to gain better knowledge of their fuels propensity for self-heating and thereby facilitate safer storage. The screening test procedure can also be used for research purposes. Experiments have been performed with 31 different biomass pellet batches to investigate how the pellet composition, origin, etc. influence the reactivity of the pellets. The results from these experiments clearly show a significant difference in reactivity between different types of pellets. The results indicate that pine/spruce mix pellets are significantly more reactive than all other types of pellets tested, and that pellets consisting of 100 % pine are more reactive than pellets consisting of 100 % spruce. Pellets produced from winery wastes, straw, or eucalyptus, have low reactivity compared to pellets consisting of pine and/or spruce. The reactivity of the pellets was shown to be reduced by either introducing certain types of anti-oxidants into the pellets or by extracting lipids from the raw material of pellets. The screening test procedure is already being used today by some facility owners for assessing their fuels propensity for self-heating. The procedure is also one of the suggested test methods in “ISO/CD 20049 Solid biofuels — Determination of self-heating of pelletized biofuels”. / This thesis presents experimental work performed to develop a sensitive screening test procedure for biomass pellets, using isothermal calorimetry for direct measurement of the heat production rate. This method can be used to directly compare the reactivity, that is how prone the fuel is to self-heating, of different batches of biomass pellets. The results could be used for safety assessment by the industry or for research purposes to investigate how different factors influence the self-heating potential. Experiments were performed with 31 different biomass pellet batches to investigate how the pellet composition, origin, etc. influence the heat release rate. The results clearly show that there is a significant difference in reactivity between different types of pellets. Pine/spruce mix pellets are more reactive than the other types of pellets tested and pellets consisting of 100 % pine are more reactive than pellets consisting of 100 % spruce. Pellets produced from winery wastes, straw, or eucalyptus, have low reactivity compared to pellets consisting of pine and/or spruce. The results also show that the reactivity of the pellets can be reduced by either introducing certain types of anti-oxidants into the pellets or by extracting lipids from the raw material of pellets.