Effect of storage temperature on off-gassing and physical properties of wood pellets

Storage of wood pellets in warehouses results in emissions of off-gases like CO, CO₂, and CH₄. Exposure to these off-gases can cause adverse health effects to warehouse workers. The objective of the present work was to investigate the emission of these gases both at room and elevated storage temperatures. Six steel column reactors of 12" diameter and 24" tall were used in the present study. Thermocouples were arranged to measure the pellet and headspace temperature during storage. The storage temperatures under study were between 30-50°C. Commercial wood pellets procured locally from Fiberco Inc, Vancouver with 4.67 % moisture content (w.b) was used in present study. The wood pellet properties measured were % fines, pellet density, bulk density and durability index. The reactors were filled with wood pellets leaving a headspace of about 25 % and were further insulated to prevent any loss of heat during heating. The off- gases were collected using air tight gas syringes initially everyday for the first week of the storage thereafter the sampling was done based on the emission rate. The gases were further analyzed using gas chromatography. The emissions of CO (5000 ppm) and CO₂ (10000 ppm) were relatively high at room temperature, but these values increased exponentially when the storage temperature was >30°C. At a storage temperature of 50°C a maximum concentration of CO₂ of about 60000 ppm and CH₄ 3000 ppm was observed, whereas in the case of CO the maximum concentration of 18000 ppm was observed at a storage temperature of 40°C. The formation of CO and CO₂ during storage at room and elevated temperatures can be due to auto-oxidation of unsaturated free fatty acids in the wood pellets initiated by temperature. The pellet % fines had significantly increased after storage at elevated temperature where a maximum of 0.96 % was observed and whereas the loss of durability index has decreased by 32 % at 50°C. There were only marginal changes in other pellet properties like moisture content, pellet density and bulk density.