Comparative study of the fuel quality and torrefaction performance of biomass and its molded pellets: effects of temperature and residence time

Abstract

ABSTRACT Torrefaction is a promising pretreatment for upgrading biomass. Biomass pellets have been widely used in heating and power plants, but the pellets torrefaction has not been fully studied. The torrefaction performance of raw biomass samples and molded pellets of those biomass samples were compared in this study. The torrefaction was performed using a horizontal sliding tube furnace with different torrefaction severities (210°C, 240°C, 270°C, and 300°C; 15 min, 30 min, and 45 min). Three key results were obtained from this work. (i) The temperature and residence time had an obvious influence on biomass torrefaction performance. Compared to the untreated samples (rice husk particles, rice husk pellets, pine wood sawdust, and pine wood pellets), the oxygen content of the samples torrefied under the most severe torrefaction conditions (300°C and 45 min) decreased by 43.1%, 27.4%, 34.2%, and 25.3%, respectively. In contrast, the high heating value and hydrophobicity of the torrefied samples increased. (ii) The biomass source had important effects on the torrefaction results. Under the same torrefaction conditions, the mass yields and energy yields were highest for pine wood sawdust, followed by rice husk particles, pine wood pellets, and rice husk pellets. This is related to the composition and thermal decomposition of the different biomass samples. (iii) The loose raw biomass released more volatiles during torrefaction compared to biomass pellets, which were limited by heat and mass transfer, leading to complete thermal decomposition of hemicellulose and other components. Overall, the torrefaction performance of biomass varied with the temperature and residence time, and torrefaction had a greater effect on the raw biomass than on the biomass pellets.