Stump torrefaction for bioenergy application

A fixed bed reactor has been developed for study of biomass torrefaction, followed by thermogravimetric (TG) analyses. Norway spruce stump was used as feedstock. Two other types of biomass, poplar and fuel chips were also included in the study for comparison. Effects of feedstock types and process parameters such as torrefaction temperature and reaction time on fuel properties of torrefied solid product were investigated. The study has demonstrated that fuel properties, including heating values and grindability of the investigated biomasses were improved by torrefaction. Both torrefaction temperature and reaction time had strong effects on the torrefaction process, but temperature effects are stronger than effects of reaction time. At the same torrefaction temperature, the longer reaction time, the better fuel qualities for the solid product were obtained. However, too long reaction times and/or too higher torrefaction temperatures would decrease the solid product yield. The torrefaction conditions of 300 °C for 35 min resulted in the energy densification factor of 1.219 for the stump, which is higher than that of 1.162 for the poplar wood samples and 1.145 for the fuel chips. It appears that torrefied stump requires much longer time for grinding, while its particle size distribution is only slightly better than the others. In addition, the TG analyses have shown that untreated biomass was more reactive than its torrefaction products. The stump has less hemicelluloses than the two other biomass types. SEM analyses indicated that the wood surface structure was broken and destroyed by torrefaction process.