Drying, particle size reduction, and densification of lignocellulosic materials

Thermochemical processing units receive wood in the form of logs and forest wastes such as chips, and an important fraction of the agricultural waste is transported as bales. The size of these raw biomass particles is often several orders of magnitude larger than the optimal size required in the equipment for chemical or thermochemical conversion. Lignocellulosic materials received at thermochemical conversion facilities are often bulky, wet, and difficult to feed into the reactors (Cummer and Brown, 2002), thus requiring biomass pretreatment. Pretreatment includes reducing feedstock moisture (drying), reducing mechanical particle size, and reducing densification (pelleting or briquetting). Drying is essential to decrease energy consumption in the size reduction step and in the thermochemical reactor. Size reduction improves powder flow and accelerates feedstock handling and feeding into the reactors (Oyedeji et al., 2020). Despite requiring a lot of energy, particle size reduction is necessary. In some industries, energy for grinding accounts for up to one-third of overall power requirements (Bitra et al., 2009). This chapter describes the phenomena and equipment frequently used for biomass pretreatment operations (drying, particle size reduction, and densification). The size reduction unit operations that happen in the field to facilitate transportation and handling were discussed in Chapter 2. This chapter focuses on pretreatment strategies for dry thermochemical processes (i.e., pretreatment specifically used for wet thermochemical processes is not discussed). The diagram below shows a typical biomass pre-processing arrangement in which the feedstock is subjected to an initial size reduction step followed by drying.