Effect of Source on Feedstock Quality

Because logging residues are often laid or piled on disturbed, exposed soil during harvesting and processing, either in-woods or at the roadside, and may also be dragged along skid trails during extraction, the ash content tends to be high and affects the quality and value of this feedstock source. It is especially important that machine operators know if logging residues are going to be used as biomass rather than burned for disposal because they can work to minimize contamination in piling, especially on the landing. A number of post-harvest methods for reducing ash content in order to meet quality specifications of different biofuel and bioenergy processes exist. The most common methods are: (1) using rotary trommel screens to reduce the percentage of fine, inorganic materials that damage mill dies and increase ash content, and (2) downstream blending of feedstock from different material sources to meet quality specification standards. For example, if ash content of a residue feedstock is 5% and needs to be at or below 2% ash to meet quality specs for a particular conversion process, blending of 20% logging residue with 80% cleaner feedstock (for example, a one-pass agricultural residue, or clean pulp chips in pre-processing) can achieve a blended fuel with quality specification of 1.8 % ash content, though using higher quality feedstock in blending is likely to drive up costs.

Regulating moisture content of woody biomass feedstock from logging residues is an important research and development area. Depending on the season of the year, local cli­mate, time between harvest and delivery, timing of processing, and species, the moisture content of cut slash and tops may vary from 12 to 50%. High or low moisture content may be desirable in final material specifications, depending on the conversion process. For example, aviation biofuels produced with a wet, thermochemical process are ultimately digested at high moisture content. For this reason, wetting dry feedstocks after transporta­tion may be desirable for some conversion processes. In contrast, densification of uniform feedstock biomass into energy pellets requires dry material. Dried, ground biomass that is stored for subsequent use may actually regain moisture from ambient air prior to conver­sion, necessitating proper storage. Reduction in moisture content tends to reduce per unit transportation costs for biomass and may increase its value if end users pay for feedstock on a dry basis. From a technical standpoint, developing logistic supply chains that deliver feedstock with appropriate moisture content requires development and validation of pre­dictive models that integrate tree and wood physiology (e. g., evapotranspirative drying as a function of local climate) with forest operations to consistently deliver a final product at required quality standards to meet conversion requirements. However, it may be more economically efficient to meet narrow feedstock specifications by centralized processing and drying at the facility rather than trying to meet them in the field.

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