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Steam explosion of biomass to produce durable wood pellets Lam, Pak Sui


Wood pellet is regarded as a clean fuel for combustion with low ash content (less than 1% by weight) and a high heating value around 21500 MJ/m³ compared to a heating value of 5400 MJ/m³ for dry wood chips. However, pellet is easily disintegrated into fines due to impact or moisture sorption during handling and storage. Fines may promote dust explosion during handling or self-heating of pellets in storage. The present study investigates the use of steam explosion pretreatment to improve the pellet durability in terms of mechanical strength and moisture sorption resistance. In this research, a batch steam explosion unit consisting of a steam generator, a steam treatment reactor, and control devices was developed. Steam explosion experiments were carried out on Douglas Fir at 2 temperatures (200°C and 220°C), 2 treatment durations (5 min, 10 min), and 2 particle sizes (0.4 mm and 0.9 mm). It was found that the bulk density and tapped density of steam treated wood increased with the treatment severity. The pellets made with biomass treated at different combinations of temperature-time were 1.4 to 3.3 times stronger than untreated pellets. The steam treated biomass required 12% to 81% more energy to form durable pellets than the untreated biomass. Energy input to produce 45000 metric ton regular pellets and steam exploded pellets was estimated. The input energy ranged from 2.80 to 3.52 MJ/kg. Producing pellets from untreated biomass consumed the least energy while pellets made from biomass treated with saturated steam at 220°C for 10 minutes consumed the highest. A kinetic model for pseudolignin formation during steam explosion was developed. Based on the experimental data in this research and published literature, it was postulated that the creation of pseudolignin is responsible for improved durability of steam exploded pellets. A reaction model was developed to predict the formation of pseudolignin and evaluate the optimized treatment condition for making durable and water repellent wood pellets.

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