Optimization of woody biomass torrefaction in inert and oxidative atmospheres using combined experimental and modelling approach

Abstract

Torrefaction is a thermochemical pretreatment method to increase the energy density of biomass. The process is carried out at 200-300 ⁰C in an inert atmosphere. However, large-scale use of inert gas is neither realistic nor economical. More than 50% of industrial flue gas in Sri Lanka is greater than 200 oC and has less than 10% oxygen content which is suitable as a torrefaction medium. Lab-scale torrefaction experiments were conducted for Gliricidia and Rubberwood, at 250-300 oC temperature range and 30–60 minutes in nitrogen and 3%, 6%, 9% oxygen environments to understand the torrefaction behaviour. TGA and FTIR were used to characterize raw and torrefied biomass. In the lab-scale experiments in inert torrefaction, Gliricidia and Rubberwood showed more than 30% mass loss under the most severe conditions, resulting in a 22.8% and 11.6% volatile drop, respectively. The higher heating value of the torrefied product increased from 18.9 MJ/kg to 30.15 MJ/kg for Rubberwood and from 19.46 MJ/kg to 28.2 MJ/kg for Gliricidia under the most severe conditions. The severity factor was modified by finding the optimum fitted parameter ω, establishing a feedstock-specific relationship between torrefaction severity and operating conditions. The normalized severity factor shows a linear correlation with the properties of torrefied biomass, which could facilitate torrefaction modelling. Rubberwood shows its optimum oxidative torrefaction properties at 300 oC temperature and 31-min residence time and 9% oxygen, whereas Giricidia shows it at 286 oC temperature at a 60-min residence time and 9% of oxygen. A process model was developed for oxidative torrefaction, and results show that CO2 has the highest volume fraction, followed by CO and CH4 in the non-condensable product and water has the highest fraction in the condensable product. Acetic acid, formic acid, phenols and furfurals were other dominant components. Keywords: Gliricidia, Rubberwood, Torrefaction, Oxidative torrefaction, Process simulation, Aspen Plus