Unpressed (left) and hot-pressed (right) pretreated lodgepole pine samples. Images taken at USDA Forest Products Laboratory.
Unpressed (left) and hot-pressed (right) pretreated lodgepole pine samples. Images taken at USDA Forest Products Laboratory.

There are multiple ways to envision a supply chain that uses forest residuals to make a variety of chemical products. One option would have all of the wood residuals transported to a single integrated biorefinery where the pretreatment, hydrolysis, fermentation and conversion steps would occur at a single facility.

Other options would consider a distributed depot strategy where conversion processes occur at multiple facilities. NARA is evaluating the economics, logistics and environmental sustainability of multiple supply chain scenarios that include a depot strategy. A depot strategy can provide economic and logistical advantages through use of existing facilities, improved transportation efficiency and by extending the range of biomass availability.

To learn more about potential biorefinery and depot locations in the NARA region visit here.

One potential depot scenario would have forest residuals pretreated and densified into pellets or mats at one facility and the downstream possessing occurring at a different location, presumably a centralized biorefinery. The densification process would shrink and dry the pretreated material and make it more cost efficient for transport. It is unknown, however, whether the process of making pellets or mats from pretreated material would change the wood chemistry in a way that negatively affects downstream processes used to produce simple sugars and isobutanol.

To address this question, NARA researchers from the USDA Forest Products Lab and Washington State University recently published a paper that evaluates how pressing sulfite-based pretreated wood chips into mats affects downstream sugar and alcohol yields.

To obtain a copy of Effect of Hot-Pressing Temperature on the Subsequent Enzymatic Saccharification and Fermentation Performance of SPORL Pretreated Forest Biomass click here.

Testing sugar yield from hot-pressed pretreated samples

A hot-press protocol, typically used to make plywood and particle board, was used to densify the pretreated residuals into mats. Multiple hot-pressed samples were made from pretreated lodgepole pine and poplar wood chips under a range of pressing temperatures from room temperature to 177°C. Turns out that when higher temperatures were used, the hot-pressed samples were less susceptible to enzyme hydrolysis. The heat changed the wood structure in a way that made it more difficult for the cellulase enzymes to penetrate the material and access the cellulose fibers. The most dramatic reduction in cellulase accessibility occurred with temperatures above 110°C. The increased temperature, however, did not apparently affect the cellulase’s ability to perform its function of cleaving off simple sugars once it did penetrate to the cellulose fibers. When the cellulase enzyme was allowed to incubate with the 110°C pressed pretreated residuals for 72 hours, there was no difference in sugar yield between the 110°C pressed and unpressed pretreated wood chips. These results suggest that a longer hydrolysis time can compensate for reduced cellulase accessibility to cellulose due to the 110°C temperature.

Testing hot-pressing effects on ethanol yield

The unpressed and pressed (110°C) poplar and lodgepole pine pretreated chips were hydrolyzed using cellulase enzymes, and the resulting sugar broth was fermented using yeast. Yeast fermentation converts the simple sugars into ethanol. The ethanol yield for the pressed pretreated poplar samples was similar to the unpressed pretreated sample. To account for the similarity, the authors suggest that the cellulase enzymes continued working during fermentation to make up for the reduced sugar yield due to the 110°C pressing. The ethanol yields were not similar, however, for the pressed and unpressed pretreated lodgepole pine samples. In this case, the pressed chips produced an ethanol yield 45% below the unpressed samples, and apparently, cellulase activity during fermentation did not compensate for the lack of accessibility induced by hot-pressing.

The results suggest that a hot-pressing temperature of 110°C is suitable for poplar pretreated material with little decrease in alcohol yield compared to unpressed samples. For lodgepole pine samples, however, the hot-pressing temperature should be below 110°C to avoid dramatic reductions in alcohol yields. Further studies are underway to optimize the process design and evaluate the economics associated with hot-pressing wood residuals.