Pretreatment is a process that breaks up wood fibers so that enzymes can access and release the simple sugars found in wood. For the supply chain being evaluated and developed by NARA, pretreatment is a highly critical step. It occurs before the enzymatic hydrolysis and fermentation steps and can therefore influence the success of those downstream events.
The wet explosion pretreatment process has been practiced by the Bioproducts, Sciences and Engineering Laboratory at WSU-Tri-Cities under the direction of NARA member Brigitte Ahring. This process uses steam and oxygen to break up the wood fibers so that the simple sugars can be released by enzymatic hydrolysis. Some attractive features assigned to this pretreatment process are that no hazardous chemicals are used and the pretreatment process can accommodate a wide range of lignocellulosic feedstocks.
Ahring’s team recently published the results of a pilot-scale evaluation of the wet explosion pretreatment performed on Douglas-fir forest residuals.
Pretreatment optimization and mass balance results
The publication, funded by the USDA-NIFA through NARA, describes how the wet explosion pretreatment method was optimized for use on Douglas-fir forest residuals and provides a mass balance for the optimized pretreatment and enzymatic hydrolysis results.
To optimize the pretreatment parameters, 17 experiments were conducted at high solids concentration of 30%. Each experiment differed by either the reaction temperature, amount of oxygen, or reaction time. An optimized wet explosion pretreatment protocol, based on maximum enzyme digestibility, was established at 190°C, oxygen loading at 7.5%, and a reaction time of 30 minutes. The enzymatic hydrolysis reaction was optimized at a pH of 5.5 to produce a glucose yield of 63.3% from the Douglas-fir residuals pretreated under optimal conditions. After enzymatic hydrolysis under optimal pretreatment conditions, the yields of major hemicellulosic sugars in the feedstock such as xylose and mannose were 64.4% and 71.3%, respectively, showing minimal loss of easily degradable sugars. Further, the lignin and cellulose recovery recorded in the mass balance was 96.3% and 99.9 % respectively.
It was also determined that the reaction temperature and time were conditions that affected the amount of degradation products produced; oxygen levels did not affect the amount of degradation products. Degradation products arise when simple sugars are converted into other chemicals (HMF and furfural for instance) due to the pretreatment conditions. They are generally unwanted because they lower the quantity of simple sugars produced and can potentially inhibit the downstream biological conversion processes like fermentation.
NARA evaluated multiple pretreatment protocols
NARA funded research to evaluate the wet explosion pretreatment process along with other pretreatment processes using the Douglas-fir residuals mentioned in this study. When comparing pretreatment processes, NARA considered sugar yields and fermentation results, energy and supply costs, environmental impact, effect on lignin and the pretreatment’s adaptability to existing pulp mill infrastructure. After a thorough review, the mild bisulfite pretreatment process was selected for use to produce biojet fuel for a demonstration flight and to incorporate in the life cycle assessment (LCA) and techno-economic analysis (TEA) being developed by NARA.
The results published in this paper provide researchers and industry with a through chemical analysis regarding the use of wet explosion pretreatment technology on forest residuals to produce simple sugars. The protocol allows for pretreatment without introducing harmful chemicals that interfere with downstream processes.