Effects of torrefaction on slow pyrolysis of the sorghum straw pellets: a kinetic modeling study

The slow pyrolysis of pretreated sorghum straw pellets (torrefied at 503, 533, 553 and 573 K) was investigated. The product yields of the torrefaction and slow pyrolysis and the total product yields of the two steps were measured separately. The results suggested that the increase in torrefaction temperature increased the carbon content and calorific value and decreased the hydrogen and oxygen content of the solid products. Compared to the raw sorghum straw pellets, the total yield of the gases obtained from slow pyrolysis of the torrefied sorghum straw decreased, while the char yield did not change significantly. It is demonstrated that torrefaction had little effect on the char yield of the biomass slow pyrolysis. The results of ultimate analysis showed that the carbon content (C) in bio-oil produced from torrefied sorghum straw pellets increased with the increased torrefaction temperature, while the oxygen (O) and hydrogen content (H) in bio-oil decreased. Based on the results of ultimate analysis and mass conservation calculation, a simplified one-step chemical equation was developed for the slow pyrolysis of raw and torrefied sorghum straw, and the stoichiometric coefficients of the equation were calculated by elemental balance and mass conservation. A two-step kinetic model was proposed for the pyrolysis of biomass, the apparent activation energy, the pre-exponential factor and the exponents of the two reaction models increased with the increased torrefaction temperature. The apparent activation energy and pre-exponential factor of the reaction increased proportionally with the increased C/O ratio of the reactants. The combination of the one-step chemical equation and the two-step kinetic model can effectively describe the volatiles release behavior of biomass pyrolysis and can be used to predict the relative yield of char, bio-oil and some major gas species.