Efficient Fuel Pretreatment: Simultaneous Torrefaction and Grinding of Biomass

Abstract

Combining torrefaction and grinding of biomass in one reactor may be an attractive fuel pretreatment process. A combined laboratory torrefaction and ball mill reactor has been constructed for studies of the influence of temperature and residence time on the product yields and particle size reductions of Danish wheat straw, spruce chips, and pine chips. On the basis of initial experiments, which evaluated the influence of reactor mass loading, gas flow, and grinding ball size and material, a standard experimental procedure was developed. The particle size reduction capability of the torrefaction process has been evaluated by the relative change in d 50, and this method was compared to the Hardgrove grindability index (HGI), showing reasonably similar results. Significant differences in torrefaction behavior have been observed for straw and spruce chips torrefied at 270–330 °C. Torrefaction of straw for 90 min yielded a higher mass loss (27–60 wt %) and relative size reduction (59–95%) compared with spruce (mass loss of 10–56 wt % and size reduction of 20–60%). The two types of biomass investigated differ with respect to hemicellulose type, lignocellulosic composition, particle morphology, and ash composition, where straw has a higher alkali content. This and other studies indicate that the large difference in the alkali contents of the biomasses is the main cause for the observed difference in torrefaction characteristics. Experiments with separate particle heating and grinding showed a swift grinding of the torrefied biomass. This implies that the rate-limiting step in the laboratory reactor is the heat transfer and not the grinding process. Large pine particles (8–16 mm) showed a slightly higher mass loss than 4–8 and <4 mm particles. This could be the consequence of exothermic reactions in the particle core, which locally increase the temperature and conversion.

Advances in Bioprocess Technology

“…Various authors have found a similar trend with woody as well as herbaceous biomass (Deng et al 2009;Chen et al 2011a, b;Chew and Doshi 2011;van der Stelt et al 2011;Wang et al 2011;Agar and Wihersaari 2012;Shang et al 2012;Lu and Chen 2013;Ohliger et al 2013;Saleh et al 2013;Satpathy et al 2014;Mei et al 2015). However increasing the severity (temperature and time) also translates to more energy use for pretreatment, therefore an optimum level of severity should be developed and this is likely to depend on the type of biomass.…”

A review on various types of densification/briquetting technologies of biomass residues

“…Torrefaction [38,39] It is a process of slow heating of biomass residues materials to a temperature of near about 300 0 C. Due to this, volatile materials and smoke evolving products are reduces to make final product better.…”

“…Grinding [39] It is a process of grinding of the biomass raw/residues material at specific size depends upon the densification technology selected.…”

Thermochemical Processing of Biomass

“…Ibrahim et al found that increasing the severity of the thermal treatment improves the grindability of both softwoods and hardwoods (Ibrahim et al 2013) while Bridgeman et al found that the higher temperature severity caused torrefied Miscanthus and Willow to exhibit similar grindability as coal (Bridgeman et al 2010). Various authors have found a similar trend with woody as well as herbaceous biomass (Deng et al 2009;Chen et al 2011a, b;Chew and Doshi 2011;van der Stelt et al 2011;Wang et al 2011;Agar and Wihersaari 2012;Kim et al 2012;Shang et al 2012;Lu and Chen 2013;Ohliger et al 2013;Saleh et al 2013;Satpathy et al 2014;Mei et al 2015). However increasing the severity (temperature and time) also translates to more energy use for pretreatment, therefore an optimum level of severity should be developed and this is likely to depend on the type of biomass.…”