(503e) Production of Low-Cost and Highly Fermentable Sugar from Corn Stover Via Chemical-Recovery-Free Deacetylation and Mechanical Refining (CRF-DMR) Process

The pioneer 2^nd generation cellulosic ethanol plants faced challenges due to the low fermentability of sugar hydrolysates produced by dilute acid pretreatment. This technique depolymerizes hemicelluloses, mainly xylan, in corn stover biomass, improving the digestibility of pretreated biomass solids and resulting in a mixed glucose and xylose sugar hydrolysate through enzymatic hydrolysis. However, the high temperature and low pH conditions of the reaction cause rapid sugar degradation reactions, leading to the formation of furfural from xylose and 5-hydroxymethylfurfural (HMF) from glucose, both of which are strong fermentation inhibitors even at low concentrations. Additionally, hemicelluloses in corn stover are heavily esterified with acetyl groups, and during dilute acid pretreatment, these groups are released, forming acetic acid, another potent inhibitor of ethanol fermentation using common organisms like Zymomonas and Saccharomyces cerevisiae. The resulting sugar hydrolysate from dilute acid pretreated corn stover (hydrolyzed at 20% total solids) contains roughly 10 g/L of acetic acid, 5 g/L of furfural, and 0.5 g/L of HMF. As a result, only 50% of the xylose is utilized after 50 hours of fermentation when glucose has already been completely consumed, leading to an overall ethanol yield of around 80% after 75 hours of fermentation.

The National Renewable Energy Laboratory (NREL) has developed a solution to this issue in the form of a scalable, atmospheric pressure, and low severity biomass deconstruction and fractionation process known as the Deacetylation and Mechanical Refining (DMR) process. The DMR process has exhibited exceptional performance in converting corn stover to high-concentration, low-toxicity sugars at high yields. The process involves a dilute NaOH deacetylation step to remove the acetate in biomass at atmospheric pressure (90°C), followed by mechanical refining of the deacetylated biomass using a disk refiner and a Szego mill, both commonly used in the pulping and milling industry. The entire process is carried out at low temperatures without an acid catalyst, effectively preventing the formation of furfural and HMF. Additionally, the removal of acetyl groups significantly reduces acetic acid formation during sugar hydrolysate production, resulting in high fermentation efficiency. The hydrolysate produced by DMR from corn stover (hydrolyzed at 25% total solids) resulted in an ethanol yield exceeding 98% with xylose utilization exceeding 95% in only 40 hours of fermentation due to zero furan content and very low acetic acid concentration of less than 0.5 g/L.

The high sodium hydroxide usage in DMR process is beneficial to increase sugar yields while reducing enzyme usage. However, the cost of sodium hydroxide can be significant at $0.24/lb, and the high loadings required in the process can lead to high greenhouse gas (GHG) emissions. While NaOH recovery is common in the pulp and paper industry, it is complex and capital intensive, requiring an expensive recovery boiler, lime kiln, and recausticization cycle. These processes account for approximately 50% of the total capital investment for a 2000 tonne biomass/day biorefinery plant.

This study developed a chemical-recovery-free pretreatment technology for the DMR process, which potentially addresses current challenges and decrease the minimum sugar selling price. In this modified DMR process, NaOH is replaced by ammonium or potassium alkali and salts, which results in post-fermentation spent liquor that can be used as a fertilizer without requiring chemical recovery. In fact, ammonium or potassium sulfite pulping spent liquor (with low pH) has been used as a fertilizer in various countries. Additionally, the modified DMR spent liquor has a lower sulfur content due to the reduced sulfite charge. By partially absorbing CO₂ from downstream fermentation, the alkaline spent liquor becomes near pH-neutral, which is more beneficial for the environment and soil. This method not only reduces GHG emissions but also provides a carbon source for plants. By converting the pretreatment spent liquor into fertilizer without the need for wastewater treatment or chemical recovery, we could save at least $150MM in capital investment, and the revenue from the fertilizer could cover the cost of pretreatment chemicals. The return of nutrients, lignin, and biogenic CO₂ back into the soil would help balance nutrient loss and maintain soil organic carbon that is removed during the corn stover removal process. Furthermore, the chemical recovery-free process reduces the energy needed for the GHG-intensive calcination process, which could potentially reduce the carbon footprint of sugar production.