1.3.2. High-Moisture Pelleting Process

Idaho National Laboratory (INL) has developed a high-moisture pelleting process that can help to reduce pellet-production costs significantly. Tumuluru [21–23], Tumuluru et al. [40], Bonner et al. [41], and Hoover et al. [42] have all successfully tested this process on both woody and herbaceous biomass, as well as chemically pretreated biomass, in both pilot and commercial-scale pelleting systems. In this process, the biomass is pelleted at a higher moisture content of > 15% (w.b.), while the high moisture pellets that are produced are further dried in low-temperature and low-cost dryers, such as grain or belt dryers. That is, this pelleting process replaces a rotary dryer in the front end with a grain or belt dryer at the back end of the pelleting process.

In this process, the biomass loses some moisture during pelleting due to frictional heat developed in the pellet die. Roughly 5–10% (w.b.) moisture loss in the biomass is seen during high-moisture pelleting. Also, the amount of moisture lost from the biomass depends on the initial moisture content of the feedstock. Lamers et al. [25] and Tumuluru et al. [26] indicated that by performing this process, a 40% reduction in pellet production costs could be realized as compared to the conventional method followed by industry. Even though the high moisture pelleting process is relatively new, there are some studies in the published literature where researchers have used this process to densify wood, agricultural crop residue, straws, grasses, and the compost [20,21,39,43,44].
