2005 Progress Report: Pretreatment of Agricultural Residues Using Aqueous Ammonia for Fractionation and High Yield Saccharification

EPA Grant Number: R831645
Title: Pretreatment of Agricultural Residues Using Aqueous Ammonia for Fractionation and High Yield Saccharification
Investigators: Lee, Y. , Elander, Richard
Institution: Auburn University Main Campus , National Renewable Energy Laboratory
EPA Project Officer: Richards, April
Project Period: June 1, 2004 through May 31, 2006 (Extended to December 31, 2006)
Project Period Covered by this Report: June 1, 2005 through May 31, 2006
Project Amount: $190,156
RFA: Technology for a Sustainable Environment (2003) RFA Text |  Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , Sustainability

Objective:

The overall objective of this research project is to develop a pretreatment process suitable for enzymatic conversion of agricultural residues into fermentable sugars. The proposed process uses aqueous ammonia (a nonpolluting substance) as the pretreatment reagent. Use of ammonia offers significant economic and environmental merits because it is easily recovered and leaves no residual effect on the environment. The proposed pretreatment is a part of the integral biomass-to-fuels process that does not generate net CO2 (a green energy process). When it is incorporated into the biomass saccharification processes, it can accomplish a near complete fractionation of biomass into the three major constituents (cellulose, hemicellulose, and lignin). The goal of this project is to expand the fundamental knowledge base of this method and advance it to a point where it can be evaluated as a process technology.

A pretreatment process is applied before the biomass is subjected to the biological processing. The proposed method has been proven to be highly effective in delignification of agricultural residues and herbaceous feedstocks. Delignifying biomass at the early phase of the process is beneficial for a number of reasons. Low lignin in the solid substrate improves the digestibility and the overall enzyme efficiency thus lowering the enzyme dosage. The low-lignin carbohydrates are less toxic to microorganisms. Early removal of lignin also eliminates the complications in the downstream processing, including the cell separation in the bioreactor and the distillation. The lignin separated by the proposed process is clean and free of contaminants. It is a high-grade fuel with no known environmental problems. It is also a potential feedstock very much amenable for further conversion into value-added chemicals. In addition to the clean lignin, the proposed process can produce high-grade cellulosic material that has a broader market than the saccharification feedstock. It is a short chain cellulose fiber with high α-glucan content. Its potential market includes filler-fiber in papermaking and microcrystalline cellulose.

Progress Summary:

During this project period, effective utilization of hemicellulose was explored by investigating the production of value-added chemicals other than ethanol from the pretreated corn stover. The end-products sought were xylooligosaccharides (high-value food additive) and lactic acid. Soaking in aqueous ammonia (SAA) treatment of corn stover and corn cobs resulted in clean and xylan-rich substrates that were susceptible for xylanolytic hydrolysis but poorly reactive for autohydrolysis. The products from the enzymatic hydrolysis using endoxylanase consisted of primarily xylooligosaccharides. Fractionation and refining of xylooligosaccharides were accomplished by charcoal adsorption followed by ethanol elution. An effective product purification method was also developed whereby all xylose monomer and color-forming substances were removed, yet the majority of xylooligosaccharides was recovered. The digestion of xylan in SAA-treated corn stover caused a slight decrease in the cellulose digestibility but still achieved above 80 percent digestion.

Corn stover treated by SAA was further investigated as the substrate for lactic acid production by simultaneous saccharification and co-fermentation (SSCF). A commercial cellulase (Spezyme-CP) and Lactobacillus pentosus ATCC 8041 (CECT-4023) were employed in the SSCF. In batch operation of the SSCF, the carbohydrates in the treated corn stover were efficiently converted to lactic acid. The maximum lactic acid yield reached 92 percent of the stoichiometric maximum based on total fermentable carbohydrates (glucose, xylose, and arabinose). A small amount of acetic acid also was produced in the process from pentoses through the phosphoketolase (PK) pathway. Among the major process variables of the SSCF, the enzyme loading and amount of yeast extract were found to be the key factors affecting lactic acid production. Further tests on nutrients indicate that corn steep liquor could be used as a nitrogen source in place of yeast extract without adversely affecting the lactic acid yield. Fed-batch operation of the SSCF was beneficial in raising the concentration of lactic acid, the maximum value reaching 75 g/L.

Conclusions

Bioconversion of hemicellulose into value-added chemicals other than ethanol was investigated using SAA-treated corn stover as the feedstock. The end-products sought were xylooligosaccharides (high-value food additive) and lactic acid. SAA treatment of corn stover and corn cobs resulted in clean and xylan-rich substrates that were susceptible for xylanolytic hydrolysis by xylanase enzymes but poorly reactive for autohydrolysis. Enzymatic hydrolysis-treated corn stover and cobs by endoxylanases produced primarily xylooligosaccharides. Fractionation and refining of xylooligosaccharides were accomplished by charcoal adsorption followed by ethanol elution. The digestion of xylan in SAA-treated corn stover caused a slight decrease in the digestibility of the remaining cellulose yet achieved above 80 percent digestion.

Corn stover treated by SAA was further investigated as the substrate for lactic acid production by SSCF. A commercial cellulase (Spezyme-CP) and L. pentosus ATCC 8041 (CECT-4023) were employed in the SSCF. From this process, the carbohydrates in the treated corn stover were efficiently converted to lactic acid. The maximum lactic acid yield reached 92 percent of the stoichiometric maximum based on total fermentable carbohydrates (glucose, xylose, and arabinose). A small amount of acetic acid was also produced in the process from pentoses through the PK pathway. Fed-batch operation of the SSCF was beneficial in raising the concentration of lactic acid, the maximum value reaching 75 g/L.

Future Activities:

The investigators did not report any future activities.


Journal Articles on this Report : 2 Displayed | Download in RIS Format

Other project views: All 9 publications 6 publications in selected types All 6 journal articles
Type Citation Project Document Sources
Journal Article Zhu Y, Lee YY, Elander RT. Optimization of dilute-acid pretreatment of corn stover using a high-solids percolation reactor. Applied Biochemistry and Biotechnology 2005;124(1-3):1045-1054. R831645 (2005)
R831645 (Final)
  • Abstract from PubMed
  • Abstract: Springer-Abstract
    Exit
  • Journal Article Zhu Y, Kim TH, Lee YY, Chen R, Elander RT. Enzymatic production of xylooligsaccharides from corn stover and corn cobs treated with aqueous ammonia. Applied Biochemistry and Biotechnology 2006;130(1-3):586-598. R831645 (2005)
    R831645 (Final)
  • Abstract from PubMed
  • Abstract: Springer-Abstract
    Exit
  • Supplemental Keywords:

    pretreatment, corn stover, aqueous ammonia, simultaneous saccharification and fermentation, simultaneous saccharification and co-fermentation, SSF, SSCF, xylo-oligosaccharides, ethanol, lactic acid,, RFA, Scientific Discipline, INTERNATIONAL COOPERATION, Sustainable Industry/Business, cleaner production/pollution prevention, Environmental Chemistry, Sustainable Environment, Technology for Sustainable Environment, Chemicals Management, fermentation of sugars, environmentally friendly transportation fuel, agricultural byproducts, alternative materials, biomass, enzyme transformations, aqueous ammonia, feedstocks, alternative fuel, biowaste, alternative energy source, pollution prevention

    Progress and Final Reports:

    Original Abstract
  • 2004 Progress Report
  • Final Report