5.2.1 Drying

The biomass, in the forms of wood chips, sawdust, bagasse, grass, and agricultural residues is bio-origin material that initially contains moisture from 50 % to over 150 % (dry basis) in the fresh form. In order to increase energy efficiency, improve energy product quality, and reduce emissions in its thermochemical energy conversion, drying of the biomass to the required MC is important in the development of energy production systems [13, 14]. In addition, it was found that uniformity of drying also significantly affects the energy efficiency in a combined heat and power (CHP) plant [15]. Other issues needed to be considered in biomass drying include forms of feedstock, energy conversion technology, environmental impact, risk of fire and explosion, available energy source, and drying costs [13].

Three common types of dryers widely used in industry are packed moving bed (PMB) dryer [16, 17], rotary dryers [18-20], and pneumatic or flash dryers [19, 21]. For the development of biomass drying technologies, multistage drying, exhaust air recycling, heat recovery, and the optimization of the drying conditions have been explored [13, 19, 22]. In particular, superheated steam drying has attracted great interest in order to prevent the risk of fire, reduction of emissions and increase the energy efficiency in drying [13, 23, 24].