UBC Theses and Dissertations
Evolution and stratification of off-gasses in stored wood pellets Yazdanpanah, Fahimeh
Storage of wood pellets has resulted in several deathly accidents in connection with off-gassing and self-heating. The goal of the present study was to quantify off-gassing characteristics of white wood pellets when stored in an experimental silo. Wood pellets properties were characterized with respect to gas adsorption-desorption and spatial and temporal concentrations of off-gases and thermal conditions within the pilot storage were quantified. In the last part, the effectiveness of purging the silo in reducing off-gas concentration was evaluated. To assess the adsorption of off-gases by wood pellets in storage, Temperature Programmed Desorption was used. Highest CO₂ adsorption was seen by torrefied wood pellets while lowest uptake was showed to be for steam exploded pellets. Quantifying the uptake of CO was challenging due to chemical reaction and therefore strong bonds between the material and carbon monoxide. Studies on emission and stratification of off-gases showed higher emission factor compared to work done with white wood pellets in small scale. Some stratifications were observed for CO₂ and CH₄ over the first days of storage. However for CO the stratification was much clear and related to high uptake of CO by wood pellets over time. During the entire period of storage, maximum temperature in the silo was recorded on day 15 of storage (storage time was 63 days) at the elevation of 2.5 m (silo dimension was 1.2m diameter and 4.6m height). Measured temperature in the silo during 5.5 hour purging experiments with air at 18-18.5 °C, helped the temperature decrease in the lower parts and slightly middle parts of the silo after 200 minutes of purging. To evaluate the effectiveness of a purging system to sweep the off-gases from the experimental silo, multiple purging tests were done. Mixing experiments showed large deviations from plug flow and thus better mixing for all superficial velocities used. Predicted results showed the concentration model fitted best to the measured off-gas concentration at the bottom and in the middle of the silo while the model overestimated the exponential decay of the off-gases in the head-space of the silo.
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