Schematic of mild bisulfite pretreatment process. Courtesy of Junyong Zhu at USDA Forest Service, Forest Products Laboratory.
Schematic of mild bisulfite pretreatment process. Courtesy of Junyong Zhu at USDA Forest Service, Forest Products Laboratory.

Building a biorefinery to convert wood residues into chemical products like biojet fuel is expensive. The projected capital investment exceeds $850 million. An alternative, and potentially less costly approach, is to retrofit existing wood-based facilities and establish a distributed supply chain where various stages of the conversion process are conducted at separate facilities.

Pulp mills are well suited for use in a wood residual-to-biojet fuel supply chain. They are generally located in areas rich with woody biomass and have the transportation, storage capacity, and permit assets established. In addition, pulp mills are engineered to break down large amounts of wood material using chemicals and heat to isolate cellulose for paper production. This process is similar to the pretreatment phase, a process that exposes cellulose for enzymatic hydrolysis to simple sugars, used to produce biojet fuel from wood biomass. Because of the similarities shared between these two processes, it is likely that a pulp mill’s infrastructure could be used for pretreatment with minimal modifications.

NARA evaluated multiple pretreatment processes including wet oxidation, dilute acid and mild bisulfite with the intent of selecting the best process for industrial use. Multiple considerations were investigated, and the mild bisulfite pretreatment protocol was selected.

The mild bisulfite pretreatment protocol was developed at the USDA Forest Service, Forest Products Laboratory and Catchlight Energy, both NARA affiliates. One of the advantages to the mild bisulfite process is that it uses sulfite chemistry, which is similar chemistry employed at pulp mills. Another advantage is that it was designed to operate at a relatively low temperature (145°C), which is compatible for a typical pulp mill’s plumbing.

Recently, NARA researchers conducted a 50 kg pilot scale evaluation of the mild bisulfite process using conditions most adaptable for a pulp mill. The results were published in the Bioresource Technology.

Read Using sulfite chemistry for robust bioconversion of Douglas-fir forest residue to bioethanol at high titer and lignosulfonate: A pilot-scale evaluation here.

A 70% ethanol yield

For this experiment, a 50 kg sample (oven-dried) of ground Douglas-fir residuals was used. This sample was taken from a larger sample collection labeled (FS-10) which is used as a “forest residual” reference sample for NARA’s conversion experiments. FS-10 was collected from roadside slash piles in southwest Oregon. The roadside slash was ground and screened with 1.75-inch mesh to remove oversize particles and a 1/8-inch mesh to remove fines. The screen accepted materials labelled FS-10 was then dried to a 15% moisture content.

The experiment described in this publication was designed to replicate a commercial-scale conversion process that could be performed at a typical wood pulp mill. The FS-10 forest residuals were loaded “as is” without additional processing. A dilute sulfite solution at low pH was produced containing calcium hydrogen sulfite (Ca(HSO3)2) and free sulfur dioxide (SO2). This was done by “bubbling” sulfur dioxide into a calcium hydroxide solution. The total SO2 loading was only approximately 25% of that used for sulfite pulping. This process, combined with the 145°C temperature maintained for four hours, provided the “pretreatment” step that could be conducted at a typical pulp mill. The entire slurry at the end of pretreatment had a solids content of ~25%.

The slurry was diluted to 16.7 weight percentage of total solids, brought to a pH of 6.2, and hydrolytic enzymes were added to release the simple sugars from the cellulose and hemicellulose. After pre-hydrolysis (~24 hours in laboratory due to lack of mechanical mixing), yeast was introduced to the hydrolysate and used to convert the simple sugars into ethanol.

As mentioned, this experiment was designed to replicate a commercial scale wood-to-alcohol conversion. It is important to note that no additional processing steps such as washing or detoxification were applied to the pretreatment slurry. The pretreated FS-10 whole slurry was directly used for enzymatic saccharification and fermentation after neutralization. The buffer used in this laboratory experiment can be eliminated for commercial practice. Under these conditions, an ethanol yield of 282 liters per ton of FS-10 forest residue was achieved which reflected a 70% theoretical yield based on the content of glucan, mannan and xylan in the FS-10 sample.

Commercial grade lignosulfonate

Alcohols are not the only commercial product generated by this process. A highly sulfonated lignin product (lignosulfonate) was produced as a byproduct. Total lignosulfonate yield was 130 kg per ton of forest residuals. Lignosulfonates are used for a wide range of applications. They are added to improve the quality of concrete and plasterboard and applied to dirt roads as a dust suppressant. In 2001, the U.S consumption of lignosulfonates exceeded 400,000 metric tons. The lignosulfonate generated from this experiment shared comparable properties with commercial lignosulfonate. This result suggests that commercially viable lignosulfonate can also be produced under these pilot scaled conditions.

From pilot test to real world

NARA is assessing the viability of retrofitting operational and idle pulp mill facilities as viable conversion sites. Tools have been developed to rank regional sites based on physical assets and social acceptance, and applied to the NARA region.

NARA’s Education and Outreach teams are working with partners and regional stakeholders to assess potential sites across the supply chain where retooling of existing facilities can occur to convert forest residuals to alcohols, and/or biojet fuel, and co-products like lignosulfonates.

The authors conclude their study by stating, “This study demonstrated that underutilized woody biomass such as forest residuals can be efficiently converted to biofuel and bio-products using mature pulping technologies with proven commercial scalability.” Their work provides a solid technical basis for potential retrofits.