Simplified cartoon of the mild bisulfite pretreatment process
Simplified cartoon of the mild bisulfite pretreatment process

When NARA was initiated in the Fall 2011, it was tasked to evaluate available pretreatment methods and ultimately select one as a NARA-preferred method to incorporate into a wood to bio-jet fuel conversion process. Pretreatment is the process that breaks up the wood fibers so that enzymes can access and release the simple sugars in wood. The simple sugars are used to make isobutanol, which is further processed to make a number of products including biojet fuel.

Initially, NARA considered the below pretreatment processes:

1. SPORL (sulfite pretreatment to overcome recalcitrance of lignocellulose): This pretreatment method was developed at the USDA Forest Service, Forest Products Laboratory, a NARA affiliate. The process relies on heat, chemicals (sodium bisulfite) and mechanical grinding.

2. Mild bisulfite (MBS): This process was developed at Catchlight Energy and USDA Forest Service, Forest Products Laboratory. The process is similar to SPORL.

3. Wet Oxidation (WO): Developed at Washington State University’s Bioproducts, Sciences and Engineering Laboratory (WSU-BSEL), this process relies on pressure and oxygen.

4. Dilute Acid (DA): This method uses sulfuric acid and heat and has been widely studied and used to pretreat crop residues like wheat straw and corn stover.

For a review of the pretreatment process view here.

A comparative analyses between DA and SPORL pretreatment methods on wood samples determined that SPORL pretreatment delivered higher sugar and ethanol yields and facilitates lignin removal better than DA pretreatment. NARA researcher Xiao Zhang uses the DA protocol to rapidly screen Douglas-fir seedlings for susceptibility to pretreatment; but as an industrial pretreatment protocol, SPORL was a superior choice.

Over the last two years, researchers at the USDA Forest Service Forest Products Laboratory and Catchlight Energy made modifications to the SPORL protocol to create a hybrid protocol termed the mild bisulfite (MBS) pretreatment. MBS differs from SPORL by employing calcium bisulfite instead of sodium bisulfite and a lower cook temperature, which were instrumental to improved conditions for downstream isobutanol production and for adoption into existing biorefinery infrastructure.

These developments left two pretreatment protocols under consideration, MBS and wet oxidation. Last September, after NARA’s annual meeting, the NARA Advisory Board recommended that NARA select a single pretreatment protocol. This action would establish a single wood to sugar conversion protocol and allow NARA teams investigating environmental and economic impacts, plus teams optimizing protocols to convert the sugars and lignin residues into products, to focus their analyses on a single wood to sugars conversion pathway. A single pretreatment protocol would also allow NARA’s feedstock sourcing team to specify feedstock moisture content and particle size that best accommodate the selected protocol.

A Phase Gate Analysis on MBS and wet oxidation

The Phase and Gate process is a well-known project management and decision support tool that improves project execution and promotes fact-based decision-making. Phase and Gate processes typically include “Phases”, where the project work is completed, and “Gates”, where decisions for continuation and next Phase objectives are set. NARA employs the Phase and Gate model, and the phase of optimizing and evaluating pretreatment protocols was wrapping up.

To learn more about NARA’s Phase and Gate model, read here.

In order to evaluate whether the MBS or wet oxidation pretreatment protocol are best suited for wood residual to bio-jet fuel and co-product industrial production, data and observations were collected and organized from multiple NARA teams. The pretreatment team evaluated both protocols for saccharification efficiency (the ability for enzymes to release simple sugars). The Gevo conversion team considered how well their specialized yeast converted the hydrolysate (sugar solution) into isobutanol. The co-products team compared activated carbon products developed from the lignin-rich residuals generated from both protocols. The techno-economics team compared the costs associated with each protocol while the life cycle assessment team considered the environmental impacts.

Linda Beltz, NARA executive committee member, summarized the detailed information from these teams into a Gate 1/2 Feasibility Analysis and Validation Packet along with conclusions and a summary. The data was reviewed by NARA team leaders and sent to the NARA Advisory Board. A conference call was held on March 28th, with NARA’s Executive Committee and Advisory Board, to discuss the details and provided a single pretreatment recommendation.

The mild bisulfite (MBS) pretreatment protocol was selected as the “preferred method”

Based on feedback from NARA’s Project Director Ralph Cavalieri and review facilitator Jim Reed, the NARA Advisory Board was impressed with the report’s quality and credibility and the decision to approve the MBS choice was unanimous.

The mild bisulfite protocol is still being optimized and work to scale the process and produce 1000 gallons of bio-jet fuel within the next two years is underway.

For a detailed description of the mild bisulfite pretreatment process, read TASK C-P-4 in the “Sustainable Biojet” section in the second NARA Cumulative Report here.