Proximate and ultimate compositional changes in corn stover during torrrefaction using thermogravimetric analyzer and microwaves

Energy from biomass is considered carbon-neutral because the carbon dioxide released during its use is already part of the carbon cycle. Increasing the use of biomass for energy can help to reduce the negative CO₂ impact on the environment. There are many challenges in using biomass for energy applications, such as low bulk density, high moisture content, irregular size and shape, hydrophilic nature, and low calorific value. In commercial scale operations where large quantities of biomass are needed, these limitations will create problems associated with storage and transportation. Furthermore, grinding raw biomass with high moisture content is very challenging as there is no specific equipment, which can increase costs, and in some cases becomes highly impossible. All of these drawbacks led to the development of some pretreatment techniques to make biomass more suitable for fuel applications. One of these is torrefaction. Torrefaction is the heating of biomass in an inert or reduced oxygen environment. During torrefaction, biomass losses moisture, becomes more brittle, and increases energy density values. Several technologies exist for the torrefaction of biomass - fixed bed, bubbling sand bed, screw extruder, and moving bed are the most commonly used. The use of microwaves for the torrefaction of biomass has not been explored at present. In the present study, we looked into the torrefaction of biomass using the established methods as well as by using microwaves and their effect on proximate and ultimate composition. Studies indicated that microwave torrefaction is a good way to pretreat the biomass in short periods of time. A maximum calorific value of 21 MJ/kg is achievable at 6 mm residence time as compared to 15 mm using the dry torrefaction technique. Increasing the residence time also increased the carbon content where a maximum carbon content of 52.20% was achievable at a lower residence time. The loss of volatiles is comparatively lower as compared to the dry torrefaction technique. Moisture content of microwave torrefied samples was in between 2-2.5% (w.b).