Self-heating and spontaneous combustion of wood pellets during storage

Abstract

Self-heating of wood pellets is a major concern during long term storage. Internal temperatures rose to 57℃ in 10 days in a wood pellet silo of 21.9 m diameter in Fibreco Inc. (Vancouver, Canada) after pellets (about 20℃) were loaded into the silo. Self-heating could lead to serious accidental fires, causing enormous damage and danger to workers. In this study, the self-heating rate at different temperatures was experimentally determined, and the thermal properties were measured. for wood pellets produced in British Columbia, Canada. The factors such as moisture content, pellet age and environment temperature were investigated and their impacts on the self-heating process were analyzed. Moisture content has a significant effect on effective thermal conductivity and specific heat capacity of packed pellets, but has no effect on the self-heating at the temperature range of 30℃ to 50℃. Pellets age and environment temperature are two major factors impacting the self-heating and off-gassing process. The self-heating rate is significantly increased at higher a temperature and eventually will lead to a thermal runaway when the ambient temperature is high enough. Experimental results show that the critical ambient temperature for thermal runaway decreases as the reactor size increases. The reaction kinetics was studied at both low temperatures (30℃ to 50℃) and high temperatures (100℃ to 200℃) and kinetic parameters were extracted from experimental results and correlations were developed. Based on all measured properties data and kinetics data, a two-dimensional axi-symmetric self-heating model was developed to predict the self-heating process and thermal runaway in large wood pellet silo. The influences of cooling airflow rate, wall insulation, and dimension of the storage container, ambient temperature and wind condition were studied. The results show that air ventilation inside of the silo is a very effective approach for reducing self-heating and preventing thermal runaway at ambient temperatures lower than 330 K. The critical ambient temperature for a 21 m diameter silo can be as low as 36℃ in the absence of air ventilation. The current model can be used to safe guide the design and operation of large industrial wood pellets silos.