NARA Woody Forest Feedstocks-Total Polysaccharides. Image created by Gevan Marrs, Weyerhaeuser
NARA Woody Forest Feedstocks-Total Polysaccharides. Figure created by Gevan Marrs, Weyerhaeuser

Gevan Marrs at Weyerhaeuser generated this above figure. Packed with useful information, it is often used in NARA presentations. The purpose of this article is to define elements in the figure and provide some analysis. A bulk of the information presented in the figure relates to the amount of polysaccharides contained in various woody feedstocks. Polysaccharides are long molecules made up of simple sugars (monosaccharides) bound together. Within the NARA project, the simple sugars derived from wood polysaccharides are used to make isobutanol, which is then converted into biojet fuel. Simply put, biojet fuel in the NARA process is derived from wood polysaccharides. There are different types of polysaccharides found in wood and those are listed as mannan, xylan, glucan, galactan and arabinan.

The term “feedstock” in this case refers to the plausible residual softwood to be used in the wood residue to biojet fuel conversion process. Each feedstock sample has a designated code (FS-01, FS-02…) and is distinguished by either species/location or degree of cleanliness. All of the feedstock samples were chipped, screened and air-dried. Note that each feedstock (except FS-01) is divided into two segments: fines and accepts. Fines are small wood particles that go through a 1/8 inch gyratory screen, accepts are wood chips that do not pass through the screen.

Polysaccharides, fines and bark content

The figure illustrates that polysaccharide content in wood residuals varies significantly depending on species with Douglas-fir residuals containing up to 60% total polysaccharides in the accepts fraction. Polysaccharide type, represented by varied colors within the vertical bars, also varies. The percentage of chipped wood material that is reduced to fines when screened ranges from 34% of sample FS-08 (hog fuel is mill wood waste typically used as boiler fuel) to 9% in sample FS-10. The percentage of bark present in the “accepts” fraction is shown in red numbers and ranges from 30.1 % in sample FS-08 to 1.4% in sample FS-01.

Feedstock logistics, economic and downstream considerations

Note that those samples containing the least amount of bark content show the greatest value of total polysaccharides. Gevan Marrs draws the following conclusion relating bark content to feedstock costs: “the lowest bark content occurs in pulp-type chips, which carry a significantly higher price due to market competition for those chips. Dropping the cost tier significantly into the forest residuals category, but only compromising the eventual sugars yield by minimizing the bark content and fines losses seems to be the most promising feedstock supply strategy. Accordingly, a relatively low-bark (3-5%) Douglas-fir forest residuals type is desired as the near-term target feedstock.” He goes on to make the following recommendations based on the information presented in the figure:

  1. Forest residuals sampled to date, with existing chipping / grinding processes have a relatively high fines content, and those fines are also high in bark and ash. Some removal of these fines by screening is likely to be beneficial, but at a cost of losing part of the feedstock. Studying the optimization of this tradeoff is likely to be an important factor for overall feedstocks economic contribution to the process.
  2. Bark in the Pacific Northwest softwoods diminishes the total polysaccharides available for fermentation to isobutanol by having about 1/3 the starting amount compared to wood. This suggests investigating more closely how collection and harvesting practices could be modified, at minimal added cost, to achieve a reduced bark content to give a higher conversion yield. (None of the operations that have been sampled to date are sensitive to bark content, so productivity per hour driving down costs has been the goal.)
  3. The minus 1/8-inch fines screened out to date have very high ash contents—so high as to almost certainly be intolerable. Since screening out fines impacts feedstock cost, it suggests investigating how much harvesting practices can alter this without significantly reducing productivity and driving unit costs up. Again, until operators are aware of a need to pay attention to this aspect, productivity is likely to be the main goal.

These conclusions should help guide efforts to minimize overall conversion costs and provide valuable outreach information to stakeholders.feedstock-polysaccharide figure

The FS-01, FS-03 and FS-10 samples have been pretreated, enzymatically hydrolyzed and the hydrolysate has been supplied to Gevo for isobutanol conversion using their yeast biological catalyst. As further samples are evaluated in the conversion process, the data contained in this figure provide a valuable baseline used to compare downstream conversion results for each sample to original feedstock characteristics.