Among thermal pretreatment options, torrefaction, or pyrolysis of biomass in the 200- 300°C temperature range, is closest to widespread commercial use [8]. Torrefaction produces a devolitalized, hydrophobic, high-carbon content product often referred to as torrefied wood. Several characteristics of torrefied wood make it more efficient to transport and store than untreated biomass, including lower water and oxygen content, higher energy density, hydrophobicity, resistance to decay, grindability, and relatively homogenous parti­cle size. Torrefied wood is generally considered a solid fuel product suitable for combustion applications, including utility boilers and co-flring with coal, but may also be used in gasi­fication and bioproducts manufacturing. Much attention has been paid to using torrefied wood as raw material in the manufacture of fuel pellets because low water content and high energy density are desirable for most energy applications. The sequence of processing can also be reversed, with wood pellets serving as the feedstock for torrefaction. However, this configuration is not a viable in-woods option due to the difficulty in efficiently down­scaling pellet manufacturing, which is strongly subject to economies of scale in production, handling and transportation. In most torrefaction systems, once pyrolysis is initiated with an application of heat, the process is exothermic and self-sustaining, meaning the chem­ical reactions required to produce the end product will proceed without net additions of energy, such as heat from combustion of propane, natural gas or combustible gases pro­duced by the reaction itself. This provides a deployment advantage for log landings that are close to the harvest site and typically distant from infrastructure. Another advantage is that torrefied wood can typically be handled by the same equipment used to handle and trans­port processed biomass, though initial cooling and additional dust control measures may be required.

Pyrolysis of biomass at higher temperatures (300-700°C) produces recalcitrant charcoal as well as volatile gases, a fraction of which can be condensed into liquid pyrolysis oil, also called bio-oil. Mobile pyrolysis systems have been examined as a pretreatment option for woody biomass but are not yet widely used in the forest sector [9]. The charcoal produced has most of the same favorable properties as torrefied wood and can be used in its raw form as solid fuel or as a feedstock for the production of other products, including chemicals, pellets, activated carbon and soil additives. The charcoal output of pyrolysis of biomass is commonly called biochar when it is used as an additive to improve the bulk density and nutrient and water holding capacity of soils. Pyrolysis oil can be used in its raw form as liquid fuel. However, because of its high oxygen and water content and low chemical stability, it is generally considered a crude product to be used in the production of refined (i. e., upgraded) biofuels and industrial chemicals.

Pyrolysis in this temperature range often produces residual tars, which can provide fuel for conversion, be sold as a commercial output, or handled as an undesirable waste by-product, depending on production objectives, equipment capabilities, and markets. Sys­tems operating at the low end of this temperature range may be exothermic, similar to torrefaction systems, but fast pyrolysis units operating at higher temperatures are charac­teristically endothermic and require net additions of energy to sustain the thermochemical reaction due to their high heating rate and the relatively short residence time of the feed­stock. Often this energy can be provided by combustion of producer gas generated by the system, which is generally composed of carbon monoxide, hydrogen, carbon diox­ide, methane and other non-condensable gases. Because of the high temperatures and smaller feedstock particle size, which facilitate rapid heat transfer, the pulverized char­coal from fast pyrolysis systems can require significantly different handling than wood chips or torrefied wood — most often a cooling phase followed by containerization in drums, closed trailers, or large industrial bulk bags. Compared to biomass, pyrolysis oil is energy dense, and thus has the potential to improve transportation efficiency, but as a liquid product it adds material handing requirements that are unusual for most forest oper­ations, including on-site liquid fuel storage, specialized trucking needs, and fire and spill containment preparations.

14.8.2 Locating Pretreatment Operations

As a component of woody biomass logistics, pretreatment can occur close to the harvest site, at intermediate processing and storage facilities such as concentration yards, or prior to use at the conversion facility. The location and timing of necessary pretreatment is highly dependent upon the end use and other components of the supply chain. However, several general considerations are worth mentioning here. In any logistics configuration, the value of pretreatment is likely to depend on the cost of the pretreatment weighted against the cost savings associated with increased transportation efficiency and the difference in delivered price between the treated and untreated materials. For example, when compared to green chips, torrefied wood produced from green chips at a harvest site may be cheaper to deliver on a cost per ton basis and may also command a higher delivered price attributable to its higher energy content. However, if the cost of the torrefaction operation is greater than the sum of transportation cost savings and new revenue, then the torrefaction preprocessing option is unlikely to be commercially viable.

Balancing the scale of operations is also important. Many existing pyrolysis and tor — refaction technologies that can be deployed to forest settings have much lower material throughput (e. g., 1 t h-1) than grinding and chipping systems, which can produce up to 50 th-1. When forest operations are bottlenecked through lower productivity preprocessing, gains in transportation and revenue may be erased by operational delays in the harvest­ing and processing components of the system. This is especially true of batch systems, where equipment may be idle during preprocessing periods. In addition, some technologies (e. g., refinery operations) benefit from clear economies of scale and cannot be effectively down-scaled for deployment to in-woods and concentration yard environments. Many of these challenges can be overcome with effective engineering, operations planning and logistics management, but others reflect the realities of preprocessing technology deployed in difficult operating environments.