Abstract
Torrefaction is a thermochemical pre-treatment process for upgrading the properties of biomass to resemble those of fossil fuels such as coal. Biomass properties of particular interest are chemical composition, physical property and combustion characteristics. In this work, torrefaction of beech wood and miscanthus (sinensis) was carried out to study the influence of torrefaction temperature (240–300 °C) and residence time (15–150 min) on the aforementioned properties of the biomass. Results of the study revealed that torrefaction temperature has a significant influence on mass and energy yields, whereas the influence of the residence time becomes more apparent for the higher torrefaction temperatures (>280 °C). Torrefied miscanthus resulted in higher energy densification compared to beech wood for a residence time of 30 min. A significant improvement in grindability of the torrefied beech wood was obtained even for lightly torrefied beech wood (at 280 °C and 15 min of residence time). Observation from the combustion study showed that the ignition temperature is slightly affected by the torrefaction temperature. As a whole, the torrefaction temperature determines the characteristics of the torrefied fuel compared to other process parameters like residence time. Furthermore, with optimal process conditions, torrefaction produces a solid fuel with combustion reactivity and porosity comparable to raw biomass, whereas grindability and heating value are comparable to low quality coal.
Highlights
Because of the environmental impacts of conventional energy sources, the world is moving to green energy options
A residence time of 30 min was selected based on our study of beech wood torrefaction
We found that 30 min can be considered the optimum residence time for miscanthus torrefaction
Summary
Because of the environmental impacts of conventional energy sources, the world is moving to green energy options. The Dutch government set a goal to replace fossil fuels with renewable resources by 16% in 2020 and 100% in 2050. Biomass is a CO2 neutral fuel for large-scale electricity production. The wide utilization of biomass is restricted due to its high moisture content, low calorific value, hygroscopic nature, low energy density and low combustion efficiency as a result of its larger particle size coupled with its high volatile matter content [1]. Thermochemical pre-treatment of biomass via torrefaction is a method of modifying the properties of biomass to more closely resemble those of coal. Torrefaction is a roasting of biomass in the absence of oxygen at a temperature of 220–300 °C
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