Torrefaction of Woody and Agricultural Biomass: Influence of the Presence of Water Vapor in the Gaseous Atmosphere

Abstract

Biomass preheating in torrefaction at an industrial scale is possible through a direct contact with the hot gases released. However, their high water-content implies introducing moisture (around 20% v/v) in the torrefaction atmosphere, which may impact biomass thermochemical transformation. In this work, this situation was investigated for wheat straw, beech wood and pine forest residue in torrefaction in two complementary experimental devices. Firstly, experiments in chemical regime carried out in a thermogravimetric analyzer (TGA) showed that biomass degradation started from lower temperatures and was faster under a moist atmosphere (20% v/v water content) for all biomass samples. This suggests that moisture might promote biomass components’ degradation reactions from lower temperatures than those observed under a dry atmosphere. Furthermore, biomass inorganic composition might play a role in the extent of biomass degradation in torrefaction in the presence of moisture. Secondly, torrefaction experiments on a lab-scale device made possible to assess the influence of temperature and residence time under dry and 100% moist atmosphere. In this case, the difference in solid mass loss between dry and moist torrefaction was only significant for wheat straw. Globally, an effect of water vapor on biomass transformation through torrefaction was observed (maximum 10%db), which appeared to be dependent on the biomass type and composition.

Comparative study of solid biofuels derived from sugarcane leaves with two different thermochemical conversion methods: wet and dry torrefaction

“…During torrefaction, a part of the raw biomass is released as volatiles due to the thermal decomposition of hemicellulose and a bit of cellulose, thus a significant reduction in volatile content of the torrefied solid product was observed, while the relative amounts of fixed carbon and ash increased. This is consistent with solid yield, the yield is less due to the most volatile matter, which is about 30% of mass are lost as volatile and permanent gases, mainly CO 2 , and condensable species [57]. As it is well known, high volatile matter content of biomass is often used as an indication of the ease of its direct firing and coal co-combustion.…”

“…This indicates that the oxygen removal by the wet process can occur at relatively lower temperatures than the dry process. The decline of oxygen in both dry and wet-torrefied samples is due to decarboxylation and dehydration reaction of hydrolyzed products, which release H 2 O and oxygencontaining compounds like phenols and acids in the liquid phase and gaseous products such as CO 2 and CO [57]. The release of gaseous products contributed faster than dehydration reaction to decrease the oxygen content from biomass during torrefaction which is confirmed by analysis of gaseous products (using GC-FID/TCD) [61].…”

Utilization of used bleaching clay in pellet fuel production with torrefied oil palm fronds

“…Torrefied biomass in pellet fuel gave an increased carbon content (C) and a decreased oxygen content (O) from those of pellets with raw biomass. This fact seen in Table 6 can be attributed to decarboxylation and dehydration reactions, with volatilization of water and oxygen-containing compounds (González et al 2021). Consequently, the carbon content was elevated, leading to a higher heating value (Tumuluru et al 2016).…”