Pretreatment of agricultural residues for co-gasification via torrefaction

Torrefaction is a main pretreatment technology for improving the properties of agricultural biomass in order to deal with such problems as high bulk volume, high moisture content and poor grindability. Two typical agricultural residues, rice straw and rape stalk were torrefied in a vertical reactor at 200 °C, 250 °C and 300 °C for 30 min, under inert atmosphere. The product distribution profiles of solid, liquid and gases were obtained. The grindability of the torrefied biomass was evaluated by the particle size distribution after being milled in a ball mill. It was found that temperature strongly affected the torrefied biomass and the type of feedstock influenced the conversion rate due to the different volatile content in raw biomass. An increase of torrefaction temperature leads to a decrease in solid bio-char yield and an increasing yield in the volatile matters including liquid and non-condensable gases. The maximum increase of the heating value of the torrefied residue compared with the raw material is 17% for the rice straw and 15% for the rape stalk, respectively. On the other hand, the torrefied residues are liable to be pulverized. A kinetic study on the generation of main non-condensable gases was accomplished, which shows that the gases are formed through parallel independent first-order reactions. The kinetic characteristic parameters for the generation of each gas were determined. A novel method which combined torrefaction with co-gasification to improve the efficiency of biomass utilization is promising.