Improvements of biomass physical and thermochemical characteristics via torrefaction process

Abstract

Various types of feedstocks such as straws and cotton gin waste are characterized by their low bulk density and relatively low energy content as well as irregular particle sizes and shapes. These factors impact the production of bioenergy from these fluffy feedstocks. The goal of this research is to improve the physical and thermochemical characteristics of biomass via torrefaction process. Torrefaction involves the heating of biomass to moderate temperature of 200 to 300°C in an inert atmosphere. Thus, the properties of the biomass undergo chemical and physical changes under torrefaction process. Several batches of wheat straw, rice straw, and cotton gin waste were torrefied at 260°C for 0, 15, 30, 45, and 60 min using a batch torrefaction system located at the bioenergy lab, Stuttgart, AR. Another set of experiments took place in which wheat straw was used as the feedstock. The effects of torrefaction temperature (200, 260, and 315°C) and residence time (60, 120, and 180 min) on the physical and thermochemical characteristics of wheat straw were studied. During 1 hour torrefaction at 260°C, moisture content was reduced by 70.5%, 49.4%, and 48.6% for wheat straw, rice straw, and cotton gin waste, respectively. Insignificant reduction (<1.9%) of the volatile solids was observed under the same torrefaction conditions. The heating value increased by 15.3%, 16.9%, and 6.3% for wheat straw, rice straw, and cotton gin waste, respectively. Rice straw showed the highest weight loss of 30.7% under the same torrefaction conditions. The second set of experiment showed that the torrefaction temperature have stronger effect compared with residence time. The highest heating value (22.75 MJ/kg) and the highest weight loss (54%) of wheat straw were observed at torrefaction temperature of 315°C and 3‐h residence time. Also, improvement of size reduction was visually observed, which has the favorable properties for application for gasification and/or pyrolysis. © 2009 American Institute of Chemical Engineers Environ Prog, 2009