Effect of in-situ torrefaction and densification on the properties of pellets from rice husk and rice straw.

The research on preparing high-quality pellets by combining torrefaction and densification of biomass has received widespread attention. This paper investigated the influence of torrefaction temperature on biomass and evaluated the quality of three kinds of pellets (raw pellets, ex-situ torrefied densified pellets and in-situ torrefied densified pellets). When the torrefaction temperature was raised to 300 °C, the energy yield of rice straw (RS) and rice husk (RH) quickly decreased to 71.08% and 77.62%, and the cellulose was decomposed significantly. The results proved that 250 °C was an optimum temperature for RS and RH torrefaction. The densities of RS and RH in-situ torrefied densified pellets were 1236.84 kg/m and 1277.50 kg/m under 150 MPa, respectively. The density, Meyer hardness, hydrophobicity, and mechanical specific energy consumption of the pellet increased with the increase of molding pressure. The in-situ pellets had higher Meyer hardness, hydrophobicity, and lower mechanical specific energy consumption under the same molding pressure than raw pellets and ex-situ torrefied densified pellets. In addition, the bonding mechanism was studied by using scanning electron microscopy and ultraviolet auto-fluorescence. In-situ torrefaction and densification facilitated the formation of self-locking and the migration of lignin between particles. Compared with RH pellets, RS pellets had higher quality due to the higher hemicellulose content, which was necessary for forming stable hydrogen bonds.