|Chemical pretreatment methods applicable for switchgrass and corn stover were investigated to improve enzymatic hydrolysis and fermentation yields. Various pretreatment strategies were studied including acidic, alkaline and their combination.
Dilute acid pretreatment hydrolyses hemicellulose from biomass producing liquid rich in hemicellulose sugars and solid containing cellulose and lignin. Degradation products of biomass significantly inhibit the enzymatic hydrolysis reaction reducing the glucan digestibility by 63%, and xylan digestibility by 90% when the enzymatic hydrolysis was carried out with a 50/50 mixture of buffer and the pretreatment liquid. Glucose, cellobiose and acetate were identified as strong inhibitors to cellulase activity; each showing 10%+ reduction of activity at less than 1 g/L. Fermentation under the SSF scheme can eliminate the inhibitory effects of sugars by concurrent consumption of the sugars in the broth. The concentration of acetate in the pretreatment liquid was found to be 13 g/L, which makes it the most potent inhibitor in the system.
To overcome the inhibitory effects of acetic acid present in dilute acid prehydrolysate, a two-stage processing of alkaline deacetylation followed by dilute acid pretreatment was investigated. The main advantage of this scheme is that the acetate in the feedstock is removed early in the process and does not interfere with the dilute acid pretreatment. Deacetylation with 5% Na2CO3 at 30ºC for 6 h followed by pretreatment with 1% H2SO4 at 130ºC for 12 h reduced the final acetyl concentration in hydrolysate by 73%. This improved the enzymatic hydrolysis yield of corn stover by 30% over dilute acid pretreatment alone.
In contrast to acids, alkaline reagents selectively remove lignin from biomass. This method has an advantage over acid pretreatment that it does not require detoxification of hydrolysate from the pretreatment process. Three different alkaline reagents were studied, namely, sodium hydroxide, ammonia and sodium carbonate in a pretreatment process named Soaking in Aqueous Alkali (SAAL). The SAAL pretreatment applying each of the three alkaline reagents attained above 75% glucan digestibility for switchgrass. For corn stover, the digestibility was significantly higher at 85%. Each reagent has its own merits: Ammonia is easily recoverable; NaOH is a strong base, thus requiring low dosage; Na2CO3 is a weak alkali, but much less expensive than NaOH and easier to recover. Sodium carbonate was further studied as a pretreatment reagent for herbaceous feedstocks.
The SAAL process using Na2CO3 removed 50-70% of the lignin from biomass. Pretreatment on switchgrass applied under the conditions of 15% Na2CO3 at 90ºC for 24 h gave a glucan digestibility of 71% and xylan digestibility of 49% in the enzymatic hydrolysis using 30 mg total enzyme (cellulase + β glucosidase)/g glucan. Enzymatic digestibility of corn stover treated with SAAL under the conditions 15% Na2CO3 at 60ºC for 24h were 84% for glucan and 67% for xylan.
Sodium carbonate percolation (SCP) treatment was investigated in an attempt to improve the enzymatic hydrolysis of switchgrass and to reduce the loss of hemicellulose during pretreatment. The SCP treatment applied under 15% Na2CO3 at 160ºC for 20 minutes achieved a glucan and xylan digestibility of 76% and 57% while reducing the xylan loss by half to 25% in comparison to SAAL. Xylanase supplementation further improved the digestibility to 86% and 78% for glucan and xylan respectively. FTIR of SCP treated lignin has shown a pattern comparable to that of NaOH treated lignin indicating that the mechanism of delignification is similar for the two reagents.