There are many ways to convert wood to chemical products. Some include a “pretreatment” step that is used to reduce the chemical bonds between three primary wood components: cellulose, hemicellulose and lignin.
The best pretreatment method to use will depend on many considerations including the financial cost of equipment and supplies; the environmental impact; the characteristics and quantity of the cellulose, hemicellulose and lignin after the pretreatment; the effects on downstream processes like enzyme hydrolysis and/or fermentation; and the intended final products.
NARA researchers have optimized and compared two pretreatment processes, mild bisulfite and wet oxidation, for use in converting forest slash into biojet fuel. A third pretreatment option called “wood milling” is also being evaluated.
In a recently peer-reviewed publication, funded by the USDA-NIFA through NARA, NARA researchers Rui Zhu and Vikram Yadama evaluate hot water extraction (HWE) as a pretreatment protocol for softwoods. This pretreatment method uses hot water (130-230°C) and time (seconds to several hours) to strip away the hemicellulose in softwood and expose the cellulose and lignin for further processing. Although a number of studies explore the effects of HWE on hardwoods, this study applies to softwoods, particularly Douglas-fir. Their research evaluates how temperature and retention time affect the amount of hemicellulose released and the chemical properties of Douglas-fir wood chips after pretreatment.
How does HWE work?
Softwoods like pines, firs and cedar have a varied chemical composition. About 40% is cellulose, 30% hemicellulose, 25 % lignin, and the remaining 5% is ash and other extractives. The cellulose and hemicellulose are polymers containing simple sugar molecules. When hot water is applied to softwoods, the hemicellulose becomes soluble and detaches from the cellulose and lignin. The hot water also causes some of the wood molecules to degrade and release functional groups that make the solution acidic. This higher acidity increases the rate of hemicellulose removal. As hemicellulose is released in solution, the lignin and cellulose components become more accessible to chemical and enzymatic treatments.
The data from this study indicate that as temperature or extraction time increase, more hemicellulose is removed from the Douglas-fir wood. However, at the more extreme temperature levels, the hemicellulose sugars degrade into unwanted chemicals. Within the experimental parameters used, the optimal conditions of time and temperature are 79 min. at 180 degrees. Under these conditions, 67.44% of the hemicellulose is extracted. The full range of hemicellulose extraction observed was 19.29% to 70.81%.
HWE pretreatment had other effects on Douglas-fir wood quality. It reduced the capacity for the wood to absorb moisture. HWE pretreatment increased the wood’s ability to maintain structure in high temperatures, and it was observed that higher temperatures darkened the wood color. These effects could provide a positive or negative quality to the final wood product desired.
Based on these results, the authors conclude that HWE could work well for supply chains that produce products like acetic acid, methane, fuel pellets, and simple sugars. In addition, the pretreated wood-chips, due to their lower ability to absorb moisture, could be used to make durable composite materials.
At today’s price for jet-fuel, providing bio-jet fuel from forest slash is an economic challenge unless other high-value products are produced to offset the cost of producing bio-jet fuel. Expanding the range of pretreatment options helps broaden the opportunities to provide a wide range of products from a forest slash feedstock.