When it comes to evaluating an industry that uses post-harvest forest residuals (slash) to make bio-jet fuel and co-products, the primary components in softwoods, like Douglas-fir, receiving the most industrial interest are cellulose, hemicellulose and lignin. The cellulose and hemicellulose contain the simple sugars used to biofuel and other chemical products. The lignin is a byproduct of the fuel conversion process and can be used to make additional products like activated carbon and plastics. If water is excluded, these three components comprise up to 90% of the wood.
There are a number of other chemical components in softwoods found in much lower concentrations. A collective term for many of these chemicals is “extractives”. Extractives include waxes, flavonoids, isoprenoids, tannins and other molecules. These chemicals are located in all softwood tissue types with a higher percentage in bark and needles. Trees produce extractives primarily for biological signaling, defense and metabolism.
There are many reports that describe and quantify extractives in Douglas-fir and many other tree species. From a commercial perspective, there is a marketplace for many softwood extractives used to make solvents and pharmaceuticals. Extractives can, however, interfere with industrial processes used to make paper and chemical products like bio-jet fuel from wood and can also contribute to the toxicity of the waste stream.
In a recent peer-reviewed paper, supported by the USDA-NIFA through NARA, researchers Karl Oleson and Daniel Schwartz ,from the University of Washington, provide a review of Douglas-fir extractives and the first quantitative assessment of the extractives found in post-harvest forest residuals (slash), which is the material evaluated by NARA as a feedstock to produce bio-jet fuel and other products .
Using data presented in previous studies that quantify the amount of extractives found in various Douglas-fir tissues like bark, heartwood and sapwood, the authors compute the amount of extractives found in a post harvest forest residual sample (FS-03). FS-03 is used by NARA as a representative slash sample for a Douglas-fir harvest. They then quantify, using an ASPEN model simulation, the amount of extractives expected in product and waste streams generated using the sulfite/bisulfite pretreatment and enzymatic hydrolysis processes optimized by NARA.
Using data from previous analyses performed by NARA researchers, the authors calculate that the FS-03 sample (Douglas-fir forest slash) consists of 23% bark, 33% sapwood and 43% heartwood, and all tissues combined contain 8.08% extractives. Besides listing and quantifying the varied extractives expected in the FS-03 sample, additional components including water, protein, ash, lignin, poly- and monosaccharides are quantified. As the extractives move through the pretreatment and hydrolysis steps used to produce bio-jet fuel, the ASPEN simulation shows that of the ~8% extractives, 3.579% is retained in the spent sulfite liquor, 0.026% is in the evaporator waste, and 4.685% remains in the sugar stream that is sent to fermentation. The simulation also follows the likely chemical changes that occur as these varied extractives are subjected to the thermal and chemical conditions used during wood to biofuel processing.
Much of the paper reviews the unwanted affects that extractives have on processes used to make biojet fuel from post-harvest forest residuals. For instance, the extractive class proanthocyanidins hinder cellulase activity used to remove simple sugars from cellulose and hemicellulose. Another extractive class, diterpenes, can be toxic to yeast used in the fermentation step that converts simple sugars into isobutanol.
This analysis provides an excellent starting point for understanding how extractives found in post-harvest forest residuals can affect processing steps used to create bio-jet fuel and other chemical products. The information also assists NARA researchers who are optimizing these processes and providing estimates used to determine the amount of volatile and/or toxic emissions created in the supply chain.
The authors also point out knowledge gaps. Extractive quantities are not specified between the two prominent Douglas-fir species (var. menziesii and var. glauca). In addition, the thermodynamic parameters for many extractives molecules have yet to be determined.