Pretreatment is the process that breaks up major wood components (lignin and polysaccharides like hemicelluloses and cellulose) so that enzymes can degrade (hydrolyze) the polysaccharides into simple sugars. Effective pretreatment and hydrolysis are necessary to achieve high sugar quality and yield. The sugars derived from the pretreatment/hydrolysis process can serve as a feedstock to produce chemicals including bio-fuels.
NARA has evaluated multiple pretreatment technologies to develop a pretreatment strategy that is effective on post-harvest forest residuals (slash), adaptable to existing pulp mill infrastructure , and produces a sugar slurry with few inhibiting chemicals that could interfere with downstream fermentation. This effort has produced a sulfite–based pretreatment protocol that is being used at large scale to produce 1000 gallons of bio-jet fuel made from slash. It is anticipated that Alaska Airlines will use this bio-jet fuel in a commercial flight once the jet fuel receives ASTM International (ASTM) approval for commercial use.
The wood material used to produce the 1000 gallons of bio-jet came from post-harvest forest residuals sourced from multiple private forests in the Pacific Northwest and from reject fibers provided by Cosmo Specialty Fibers, Inc. (Cosmo). In a peer-reviewed paper, partially funded by the USDA-NIFA through NARA, NARA researchers evaluate how much sugar can be obtained from representative post-harvest forest residuals and from the Cosmo reject fibers.
The authors evaluated two feedstock samples for sugar production through enzymatic hydrolysis: sulfite mill rejects from Cosmo and post-harvest forest residuals. Cosmo produces high-grade dissolving pulp used to make a wide variety of products including textiles, food ingredients and chemicals. Cosmo joined NARA as an affiliate member in order to explore markets for the rejected fibers produced from their production processes. The mill rejects contain a high amount of moisture (up to 70%) which makes burning them for energy and heat a challenge. Weyerhaeuser, another NARA affiliate member, provided the post-harvest forest residuals obtained from a regeneration harvest in Lane County, Oregon. These residuals, labeled FS-10, have served as a representative feedstock sample used by NARA to optimize the pretreatment, hydrolysis and fermentation steps in the supply chain to make bio-jet fuel from slash.
Prior to enzymatic hydrolysis, the Cosmo fiber rejects were milled for effective hydrolysis. The FS-10 samples were pretreated using the sulfite-based pretreatment protocol for large-scale processing. Commercial enzymes (Cellic CTec3 and HTec2) were obtained from Novozymes and used to perform the hydrolysis step that converts polysaccharides into simple sugars.
Sixty-five percent of the Cosmos reject fiber sample consisted of carbohydrates (cellulose and hemicelluloses). Hydrolyzing enzymes were able to convert 80% and 85% of the cellulose and hemicelluloses respectively into simple sugars, which is considered a high rate of enzymatic saccharification. Previous hydrolysis experiments had been conducted on reject fibers; however, those fibers were obtained by ammonia sulfite pumping without ammonia recovery. The Cosmo reject fibers were processed using magnesium sulfite with a magnesium recovery step. The results presented in this study indicate that the magnesium ion did not adversely affect hydrolysis efficiency.
Approximately 80% of the carbohydrates contained in the pretreated FS-10 sample were hydrolyzed into simple sugars. The sulfite-based pretreatment process creates a liquid and solid fraction, and both fractions were evaluated for hydrolysis efficiency. The results not only show that hydrolysis efficiency is high when slash is pretreated with the optimized sulfite-based pretreatment protocol, but also that the solid and liquid fractions can be combined and that the solids fraction does not need washed for efficient hydrolysis. The ability to efficiently hydrolyze unwashed solids with the liquid fraction presents a simple one-step process for hydrolysis. The authors also evaluated the lignosulfonate generated as a byproduct of the sulfite-based pretreatment/hydrolysis process and determined that the lignosulfonate is a valuable and directly marketable co-product.
The data suggests that milled reject fibers from Cosmo and the sulfite-based pretreated FS-10 samples both provide high sugar yields after hydrolysis. In addition, both processes can be scaled and adapted to a pulp mill infrastructure.