Torrefaction: a sustainable method for transforming of agri-wastes to high energy density solids (biocoal)

The depletion of fossil fuel reserves and greenhouse gas emissions led to limit the use of fossil fuels, including natural gas, coal, or petroleum, and demand a clean and sustainable source of energy. Many efforts are being made by the researchers to address these issues through the use of natural renewable resources (or lignocellulosic biomass), such as agricultural wastes and forest residues as a cleaner source of energy. But its poor properties like low energy density, high moisture content, irregular shape and size, and heterogeneity make it difficult to use in its natural form. Torrefaction, a simple heat treatment procedure, is widely employed to the natural bioresources to improve its thermal properties to be used as an energy source in the domestic power plants. The quality of the resultant torrefied solids (the so-called biocoal) is depending on the settings of heating conditions under the absence of oxygen, which can be improved by selecting and adjusting the processing conditions precisely. Typically, the process brings down the moisture content up to < 3 wt%, and increases the grinding energy up to 90%. Mainly, the calorific value and fixed carbon content of torrefied biomass increase by roughly 15–25 wt%, which makes it more appealing than non-torrefied biomass. The review emphasizes the available biomass torrefaction technologies, and it’s potential in the field of bioenergy generations. It also covers few case studies of biomass torrefaction and its application in the power generation sectors.