Oxidative torrefaction of densified woody biomass: Performance, combustion kinetics and thermodynamics

Abstract

Torrefaction in partially oxidative atmospheres is a non-traditional approach to the thermal pre-treatment of biomass. Despite the potential benefits arising from this compared to traditional torrefaction undertaken under costly inert atmospheres, the literature lacks data on the combustion and kinetic analyses of densified biomass torrefied under partially oxidative conditions comparable to those of (reusing) combustion flue gases. This paper, therefore, investigates the effects of partially oxidative atmosphere on the chemical functionality, combustion kinetics and thermodynamics of densified (Australian) woody biomass. Torrefaction was conducted at 250, 275, 300 °C for 30min under both inert and partially oxidative atmosphere (O2: 5 vol%, N2 balance). The kinetic parameters were derived from iso-conversional method over the entire conversion range. Results show that solid biofuel torrefied under partially oxidative atmospheres has superior chemical functionality (e.g., less oxygenates and higher thermal stability), but comparable physical properties to inert atmospheres. In addition, improved thermodynamic features and lower combustion activation energy for partially oxidative torrefied biomass was observed. Overall, the results suggest that the use of partially oxidative atmospheres for torrefaction not only produce a high quality solid biofuel but can also improve process efficiency by using waste heat and flue gases from other combustion processes to thermally pre-treat biomass.