An initial techno-economic analysis for the NARA biojet scenario has been completed. With the premise of constructing totally new facilities and current feedstock availability and costs, we estimate the manufacturing cost to produce biojet fuel derived from woody residuals at 2-3 times above the current market price for petrochemical-based jet fuel. Our current efforts are now aimed at reducing this initial cost estimate on many fronts. In the analysis, feedstock cost, handling and transport represents up to 20% of the overall biojet manufacturing costs; opportunities to reduce these costs could significantly reduce the current price gap between petrochemical-based jet fuel and biojet fuel derived from wood residuals.
A recent publication, authored by John Sessions and his team and partially funded by NARA, entitled “Pricing Forest Biomass for Power Generation”, explores how moisture content in wood residuals contributes to feedstock and transportation costs and provides opportunities for suppliers to improve competitiveness.
The primary audiences for this publication are feedstock suppliers and customers involved with using woody biomass to generate power and heat. For this purpose, burning efficiency increases with lower moisture content, and a premium value is placed on dry biomass material. The paper provides useful data addressing how moisture affects feedstock value and methods used to verify the moisture content in a truckload.
Conversely, for the production of biofuels, woody biomass material is not being burned but converted to biofuels which actually needs water; so the moisture content of woody biomass upon delivery is not as critical as for the bioenergy production industry. Where NARA and the bioenergy industry do share interest, however, is the intent to maximize the amount of wood biomass per truckload so that transportation costs are reduced. The following summary addresses this topic.
Moisture and volume
The paper provides an equation that relates wood moisture content to the volume that can be transported in a typical 48-foot rectangular trailer given that the trailer has a weight maximum:
H=[(1-MC)/D*Wg]/(L*w)
H is average height of biomass in the trailer (ft), MC is moisture content (decimal percent), D is the dry density of the grindings (lb per ft3), Wg is the green weight of the trailer load (lb), L is length of the trailer (ft), and w is the inside width of the trailer(ft).
As moisture content increases, the height of the load decreases, given that the remaining variables are constant, and the trailer with load is weight limited. A decrease in moisture content allows for greater biomass volume (height); thus reduced moisture content allows for a greater volume of biomass per load.
Reducing moisture content
Tables in the paper indicate the average percent moisture content of sapwood and heartwood of common softwoods (moisture content in species can vary two-fold) and the average moisture levels of delivered forest residuals for each month of the year. Recently cut forest residuals can have moisture content above 60% (wet basis). If allowed to field dry, the moisture content can be reduced to below 30%. The paper points out that managing wood residual moisture content is a supply chain challenge dependent on harvest season, storage opportunities and road and equipment capacity.
Increasing biomass density
The denser the load, the more cost effective is the transport provided the load is volume limited and not weight limited. As wood residuals are transported, load consolidation occurs and can reduce the total volume by up to 20%. Addressing ways to simulate load consolidation before transport could provide a denser load. The paper also points out that chipped feedstock packs more densely than feedstock that is ground, and use of a blower increases load density over a conveyor system that drops the chips. Chipping opportunities are limited, however, upon the condition of the forest residuals. Rocks and dirt can damage the knives. The paper also notes that load density is affected by species with Douglas-fir chips weighing more per unit volume compared to species with lower specific gravity such as white fir and ponderosa pine.
Further Research
Under NARA, the team lead by John Sessions, first author of the paper and NARA Feedstock Logistics Team Leader, is tasked to develop moisture management strategies and models to be used to determine the best time to haul forest residuals; evaluate chipping and grinding strategies, and work with trailer manufacturers to increase load efficiencies and performance for transporting woody biomass. “Our goal”, says Sessions, “is to increase biomass per trailer load 20% or more above that commonly achieved today”. Published results from these efforts will contribute to reducing the cost of transporting biomass.