14.8.1 Mechanical and Chemical Pretreatments

Communition of woody biomass through chipping, grinding and shredding increases its bulk density, which improves transportation efficiency by allowing trucks to carry heavier payloads. Such processing also improves handling and storage by reducing particle size and increasing homogeneity, allowing material to be more efficiently handled by loaders, conveyors and other equipment. In the context of woody biomass logistics, pretreatment generally includes additional processing that further improves the transportation, handling, storage and end use characteristics of biomass feedstocks beyond typical communition methods. Physical, chemical and thermal pretreatments are all technically possible but vary significantly in their operational characteristics and commercial potential.

When end users of woody biomass have feedstock specifications that are outside tradi­tional parameters for chips and hog fuel, additional drying, milling, chipping and screening can be used as pretreatments. For example, many distributed scale gasification systems require clean, dry, microchips as a preferred feedstock (e. g., low ash, bark-free chips less than 3 cm in size and 10% water by weight). The equipment to produce this high quality of feedstock from woody biomass is commercially available and widely deployed in industrial settings. More intensive debarking, chipping and screening are easily accomplished on a log landing, though these steps obviously incur additional costs. In-woods pelletization has also been explored as a pretreatment option, but remains difficult to do efficiently at distributed scales. Similarly, chemical pretreatments are widely used by cellulosic ethanol operations to reduce lignin content and improve sugar yields, but these techniques are not easily mobilized for field applications and typically involve liquid waste management and reprocessing that is almost impossible to do efficiently away from a large-scale facility. In contrast, there has been growing interest in using mobile thermal pretreatment technologies close to the harvest site to further improve transportation efficiency and produce renew­able high-value bioproducts that can be shipped efficiently to distant markets, especially in areas characterized by long transportation distances. Though discussed here as a pretreat­ment option, thermochemical pretreatments can also be classified as biomass conversion technologies, especially when deployed at larger centralized facilities (Chapter 2).