2 Biomass Densification As mentioned in last chapter, biomass can be used directly through traditional (combustion) and modern ways (CO-firing and CHP) to produce heat and electricity. Problems associated with direct application of biomass in its original form are discussed in this chapter. There are general solutions to address these problems suggested through physical and chemical upgrade of original biomass through a proper and optimized process. There are a number of technological options available to make use of a wide variety of biomass types (discussed in last chapter) as a renewable energy source to convert it to another form, such as upgraded solid biofuel through physical and thermo-physical conversion, liquid biofuel and combustible biogas through chemical, biochemi- cal and thermochemical conversion. While for some classes of biomass resource there may be a number of variety options, for others there may be only one appropriate technology. In this chapter biomass densification which falls into the physical conversion method is mainly discussed. General aspects and proper equipment to apply this tech- nique are also discussed in detail. 2.1 Problems Associated with Direct Application of Biomass Today there is a large production of biomass such as agricultural and forestry residues; how- ever, the biomass utilization proportion as a source of energy is low in comparison with other source of energies. Economics and availability of biomass and more importantly the necessary technology can be mentioned as important fac- tors which affect the utilization of the biomass as a source of energy [1]. Biomasses such as agricultural, forestry, crops residues and etc. are well known and the most common source of energy for both domestic and industrial consumption. But nowadays with the advent of new technologies and new sources of energy it is not economical to use them in tra- dition ways because of their low bulk density, high transportation cost and high mass loss dur- ing transportation [2]. The biomass application would be more inef ficient in the areas with lower availability since local availability of biomass sources is one the most important factor which influences the level of energy usage of these sources and also can decreases transportation cost [3]. Also, insuf ficient practical information about the combustion and emission characteris- tics of different types of biomasses (especially mentioned residues) which causes serious design and operation problems can be mentioned as another factor of low application of biomass as a source of energy. For example, agricultural residues have low ash melting point that cause problems such as agglomeration, fouling, slag- ging and corrosion. Also flying ash which is classified as a hazardous emission can be formed due to low particle density, high content of unburnt pollutants and high volatile matters. To © Springer International Publishing AG, part of Springer Nature 2018 M. A. Abdoli et al., Wood Pellet as a Renewable Source of Energy, University of Tehran Science and Humanities Series, https://doi.org/10.1007/978-3-319-74482-7_2 33