Physical Pretreatment
Physical pretreatment involves the breakdown of biomass size and crystallinity by milling or grinding. This pretreatment is the common first pretreatment in any multi-step pretreatment process, as it is essential to breakdown dried bulk biomass into smaller pieces as a prerequisite for other pretreatment techniques. Reduction in particle size will make the second pretreatment easier, as well as improve hydrolysis results due to the reduction in crystallinity and improved mass transfer characteristics. The energy requirements, machinery, and mechanics for physical pretreatments will depend on the final particle size and reduction in crystallinity of the lignocellulosic material. Chipping, grinding and milling can be used to reduce the particle size. Common lignocellulosic materials can be usually reduced to 10-30 mm by chipping and 0.2-2 mm by grinding or milling methods.
Energy required for this type of mechanical process of lignocellulosic material is directly related to the final particle dimension and the kind of feedstock. Softer materials require less energy, and can vary from 130 kWh per ton for hardwood, reduced to 1.60 mm, to 3.2 kWh per ton for corn stover, reduced to 9.5 mm [5]. The grinding type of method requires a higher energy amount compared to chipping and milling. It is therefore desirable that a selected pretreatment procedure minimizes or even avoids the need for size reduction to very small particle sizes by grinding-type methods. In some cases where the only option available for pretreatment is physical, the required energy is usually very high and may be higher than the theoretical energy content available in the biomass. The method of using only the physical pretreatment is not practically feasible and likely will not be used in a large-scale process.
Incorporation of a mild torrefaction is a more recent approach to biomass physical pretreatment, and this technique may well provide an energy saving approach in a multi-step pretreatment [17, 18]. Torrefaction is a mild thermal treatment of biomass, typically in the temperature range of 200 to 320°C under atmospheric pressure without oxygen. Among other effects, biomass torrefac — tion improves the grindability of fibrous materials, thus reducing the energy demand for grinding the feedstock, and makes the biomass more accessible to enzymes for hydrolysis.