Up to 75% of wood is made of simple sugars. For the NARA project, these simple sugars are the molecules used to make biojet fuel. Even though simple sugars are abundant in wood, making them available for further processing into alcohols and biojet fuel is a challenge. The simple sugars are bound together as polymers, and these polymers bind to other molecules to form lignocellulose, which does not break apart easily.
Energy and reactive chemicals are required to break apart lignocellulose in order to expose the simple sugar polymers. This process is called pretreatment, and the more efficient the pretreatment, the higher sugar yield, however, there is a catch. The trick is that as more energy and harsher conditions are applied, simple sugars degrade, and molecules termed “inhibitors” are formed that can interfere with the downstream processes that release the simple sugars (enzymatic hydrolysis) and convert them to alcohols (fermentation).
In order to increase the efficiency of pretreatment, while keeping the level of sugar degradation and inhibitor formation low, researchers at NARA’s affiliate USDA Forest Service, Forest Products Laboratory and their colleagues investigated how the timing of pH levels impact sugar degradation, inhibitor formation and pretreatment efficiency. There work, which was partially funded by the USDA-NIFA through NARA, was recently published in the journal Biotechnology for Biofuels.
View High titer and yield ethanol production from undetoxified whole slurry of Douglas-fir forest residue using pH profiling in SPROL here.
The effects of pH on pretreatment
The authors compared the simple sugar, lignin and inhibitor yields from Douglas-fir residuals using four sulfite-based pretreatment experiments that differed due to the timing of sulfuric acid introduced. Recall from chemistry classes that the pH of a solution is lowered when acid is added. One experiment introduced acid at the beginning of the pretreatment process and provided a baseline condition. The other three experiments had acid introduced at varied times (25, 35 and 45 minutes) during pretreatment.
The experiments show that acid injections made after 25 minutes into the pretreatment process could reduce inhibitor levels by up to 70%. The acid treatment improved lignin removal and decreased hemicellulose removal resulting in enzymatic efficiency (the ability for enzymes to remove simple sugars) that was similar to the pretreatment without acid injection.
The reduced inhibitor levels due to acid injections resulted in more simple sugars available to the fermentation yeast. As they consumed more simple sugars, they produced higher ethanol concentrations. The reduced inhibitor levels also allowed for a higher concentration of pretreated material (over 35%) to be loaded for fermentation without an additional detoxification step. Loading higher concentrations of pretreated material for fermentation provides higher ethanol concentrations and uses less water.
Improving conversion economics
Discovering ways to lower the cost and increase efficiencies of processing forest residuals into chemical products like biojet fuel is a central component to the NARA project. The results published in this paper show how the timing of pH adjustments to sulfite-based pretreatment can reduce inhibitor formation and improve simple sugar yields. These improvements can lead to cost savings as they are relatively inexpensive to implement and allow pretreatment facilities to feed a higher concentration of pretreated material to downstream processes without the need to detoxify the pretreated material.