Enhancing biomass from palm wastes properties by using dry torrefaction

Abstract

Dry torrefaction is a mild pyrolysis process carried out in anoxic atmospheres between temperature ranges of 200ºC and 300ºC. It is used as a pretreatment of biomass as it improves its characteristics and reduces the biomass limitations of usage by reducing its hygroscopy, increasing its energy value, etc. Due to its favorable effects, a torrefaction process has been carried out using Malaysian oil palm wastes (empty fruit bunches, mesocarp fiber and kernel shell), at two different temperatures (250ºC and 300ºC) located before and after the holocellulosic decomposition temperature (aprox. 270ºC), and a residence time of 30 minutes. In order to create an anoxic atmosphere, an inert gas (such as nitrogen or carbon monoxide) was used. Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), proximate analysis and minimum ignition energy (MIE) analysis have been carried out prior and after torrefaction process, in order to observe improvements in the samples. The results show changes of moisture, specific heat capacity and minimum ignition energy; concluding that, after torrefaction, biomass presents higher fixed carbon content, enhanced calorific values and physical properties in all samples but one: palm empty fruit bunches (EFB). Proximate analysis shows that EFB presents higher volatile content and TGA evaluation shows no holocellulosic decomposition, but only heavy volatiles decomposition at 310ºC. Because of its composition, and its temperature of heavy volatiles decomposition, torrefaction process gave different results for this sample: EFB physical properties were enhanced but thermal properties were not affected by the treatment. On the other hand, improving energetic properties may involve industrial safety risks as by applying torrefaction, combustion becomes easier to happen. MIE analysis supported this scenario as the samples reduced its minimum ignition energy as the torrefaction process was applied.