Comparison between oxidative and non-oxidative torrefaction pretreatment as alternatives to enhance properties of biomass

Abstract

Oxidative and non-oxidative torrefaction pretreatments are the most employed in order to enhance some properties of biomass such as hygroscopicity, low energy density, or poor grindability and so biomass could be a more competitive renewable energy source. The aim of this paper is to compare oxidative and non-oxidative torrefaction processes applying to a pine wood sample. Torrefaction in the range of temperatures 200–250ºC was conducted in a tube furnace reactor and proximate and ultimate analyses and heating value determination were carried out in order to characterize the torrefied samples. Torrefaction kinetics were obtained at three different temperatures using a thermogravimetric analyser. Both mass and energy yields in oxidative cases were lower as torrefaction temperature was increased compared to non-oxidative treatment. Heating value data of oxidative option were slightly lower than those of non-oxidative one were, although they were higher than the heating value of raw biomass. According to the proximate analysis, the moisture content decreased from 7.63% in raw biomass to 3.46% and 3.15% in pine torrefied at 250°C in oxidizing and inert atmospheres, respectively. H/C and O/C ratios decrease was more drastic for oxidative torrefaction than for non-oxidative one. In addition, the van Krevelen diagram shows that oxidative torrefaction is more interesting than non-oxidative alternative, this circumstance suggests that any optimizing criterion has to take into account at least three variables, including ultimate analysis and mass and energy yields.