1.3.1. Pelleting Process Variables

The pelleting process is influenced by various process variables, such as feedstock moisture, content particle size, residence time, length-to-diameter (L/D) ratio, preheating temperature, steam conditioning, and pellet die diameter [16,20]. Temperature is another important variable that can impact the quality of the produced pellets. Temperature influences the glass transition temperature of the biomass components and helps in particle bonding during pelleting. Pelleting studies conducted by Stelte et al. [31] indicated pelleting pressure decreased significantly at 100 ◦C for woody biomass. Compression pressure, which is dependent on the L/D ratio of the pellet die, also influences the quality of the produced pellets. Under steady state pellet production, the compression pressure equals to the extrusion pressure. If the compression is lower than the extrusion pressure, it generates back pressure and blocks the die. According to Holm et al. [32], pelleting pressure is dependent on the friction coefficient, the compression ratio, and the Poisson ratio. Their model does not take moisture content, particle size, and preheating temperature into account, but the Poisson ratio and friction coefficient for given biomass is dependent on the biomass type, temperature, and moisture content.

Feedstock composition is another important variable that impacts pellet quality and the consumption of energy during the process. In general, the degradation of hemicellulose produces adhesive products that result in natural bonding. Additionally, lignin in the biomass helps to form the solid bridges above the glass transition temperature and helps to form densified products. Van Dam et al. [33] noted that lignin above 140 ◦C acts as a binder and helps to form more durable pellets. Tumuluru et al. [16], Pradhan et al. [20], Serrano et al. [34], Mani et al. [35], Shaw et al. [36], Carone et al. [37], and Puig-Arnavat et al. [38] have all indicated that moisture plays a major role in the quality of the produced pellets. Most of the pelleting work reported in the literature is in the 10–15% (w.b.) moisture content. Higher moisture in the feedstock results in bulk density and durability loss of the produced pellets. Typically, moisture content necessary for pelleting depends on feedstock type. In general, grasses and crop residues need high moisture for pelleting as compared to woody biomass. Jackson et al. [39] noted that the pelleting of switchgrass, miscanthus, and wheat straw at 10% (w.b.) in a flat die pellet mill resulted in no pellet formation. These same authors also found that pellets with grasses formed when the moisture content was greater than 20% (w.b.). Understanding the impact of feedstock moisture content on pellet quality is very important to optimize the pelleting process in terms of pellet quality and cost.
