A commonly known problem with storage of biomass pellets is the risk for self-heating. The propensity for self-heating depends on several parameters e.g. type of pellets, humidity, ventilation, temperature, type of storage and handling prior to storage.

Within the framework of the research project SafePellets (Safety and quality assurance measures along the pellets supply chain) a medium-scale methodology to assess the propensity for self-heating has been developed. In addition, methods to study carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2) concentrations as well as different aldehydes have been tested and evaluated in this study.

Biomass pellets from three different sources, i.e. 100 % pine; a mixture of spruce and pine and a mixture of straw, seed residue and spruce, were tested in a 1 m3 test container. The test container and the pellets were pre-heated and kept at the nominal test temperature until self-heating occurred, or the test was terminated. Temperatures were measured at more than 40 different positions and gas samples were extracted from the test container and analysed.

Differences were observed as a function of pellet type, but also as a function of nominal test temperature and ventilation. Significant levels of CO and CO2 and a reduced level of O2 were observed direct after the pre-heating, indicating oxidation of the pellets. Ten different tests were made; ignition occurred in four of them. The higher the nominal test temperature, the higher propensity for self-heating. When ignition occurred, the concentrations of CO and CO2 increased rapidly. It was found that the ventilation conditions were important. In some of the tests, natural convection caused the pellet bulk to cool. In other tests, when the test container was closed, the oxygen concentration dropped, and self-heating was reduced.

Measurements of CO, CO2 and O2 contributed with information about the tests. However, the results from aldehyde measurements were unconcise and the values have only been used as indicative. Identified aldehydes were hexanal, butyraldehyde, valeraldehyde, formaldehyde, propionaldehyde, acetaldehyde and acrolein.