2004 Progress Report: Pretreatment of Agricultural Residues Using Aqueous Ammonia for Fractionation and High Yield SaccharificationEPA 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, 2004 through May 31, 2005
Project Amount: $190,156
RFA: Technology for a Sustainable Environment (2003) RFA Text | Recipients Lists
Research Category: Pollution Prevention/Sustainable Development , Sustainability
The 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 non-polluting substance) as the pretreatment reagent. Use of ammonia offers significant economic and environmental merits because it is recycled easily 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). It is a pretreatment method of our own invention. When it is incorporated into the current biomass saccharification processes, it can accomplish a near complete fractionation of biomass into the three major constituents (cellulose, hemicellulose, and lignin). We intend to expand the fundamental knowledge base of this method and advance it to a point where it can be evaluated as a process technology.
The first year of this project was focused on fractionation of corn stover by treatments with aqueous ammonia. Utilization efficiency of lignocellulosic biomass can be improved significantly by fractionation of the biomass. A two-stage percolation process was investigated for pretreatment and fractionation of corn stover. This process is composed of hot water treatment followed by treatment with aqueous ammonia, both applied in a flow-through (percolation) reactor. The first stage processing is intended for hemicellulose removal, whereas the second stage is intended for delignification. Ammonia is easily recoverable, and it poses no environmental problem in aqueous solution. The treated end product was found to be very high in cellulose content and also highly susceptible to enzymatic digestion. The conditions that achieve satisfactory level of biomass fractionation and acceptable enzymatic hydrolysis were identified in terms of reaction temperature, flow rate (retention time), and reaction time for each stage. With proper operation of two-stage treatment, fractionation of biomass was achieved to the extent that the xylan fraction is hydrolyzed with 92 to 95 percent conversion and recovered with 83 to 86 percent yields; the lignin removal is 75 to 81 percent. The remaining solid after two-stage treatment contained 78 to 85 percent cellulose. The two-stage treatments enhanced the enzymatic digestibility to 90 to 96 percent with 60 filter paper units (FPU)/g of glucan, and 87 to 89 percent with 15 FPU/g of glucan. The composition and digestibility data of the treated samples indicate that the lignin content in the biomass is one of the major factors controlling the enzymatic digestibility.
In the second phase of this work, a new pretreatment method, soaking in aqueous ammonia (SAA), was investigated. In this method, a feedstock is soaked in aqueous ammonia over an extended period (1-10 days) at room temperature. This is done without agitation under atmospheric pressure. Treatment of corn stover by SAA removes 55 to 74 percent of the lignin, but retains nearly 100 percent of the glucan and 85 percent of the xylan. The SAA treatment of corn stover achieved enzymatic digestibilities comparable to those of high temperature aqueous ammonia treatments such as ammonia recycle percolation. The xylan remaining in the corn stover after SAA was hydrolyzed, along with glucan, by cellulase enzyme because of the presence of xylanase in “cellulase.” The SSA treated corn stover was evaluated further by simultaneous saccharification and fermentation (SSF) and by simultaneous saccharification and co-fermentation (SSCF). In the standard SSF test using Saccharomyces cerevisiae (NREL-D5A), an ethanol yield of 73 percent was obtained on the basis of the glucan content in the treated corn stover. Xylose accumulation in the SSF appears to inhibit the cellulase activity of glucan, limiting the yield of ethanol. In the SSCF test using recombinant Escherichia coli KO11, both the glucan and xylose were utilized effectively, giving an overall ethanol yield of 77 percent of the theoretical maximum, based on glucan and xylan. With the SSCF results, the fact that the xylan fraction is retained is a desirable feature in pretreatment because the overall bioconversion can be carried out in a single step without separate recovery of xylose from the pretreatment liquid.
To reduce the treatment time in the SAA, increase of treatment temperature was attempted. In the modified SSA, corn stover was soaked in 15 to 30 weight percent (wt %) aqueous ammonia at 40-90°C for 6 to 24 hours. The optimum treatment conditions of this process were 15 wt % of NH3, 60°C, 1:6 solid-to-liquid ratio, and 12 hours of treatment time. The modified SAA retained 85 percent of the xylan and removed 62 percent of the lignin. Under optimum treatment conditions, the enzymatic digestibility of glucan was enhanced from 17 to 85 percent for glucan with 15 FPU/g-glucan enzyme loading, and 78 percent of the xylan in the treated biomass was hydrolyzed by cellulase enzyme. The SSCF test of the modified SAA samples (3% weight/volume glucan loading, recombinant E. coli KO11) has shown highly effective glucan and xylan utilization for eventual conversion into ethanol. The overall ethanol yield of the SSCF was 77 percent of the theoretical maximum, based on glucan and xylan. The maximum ethanol concentration reached 19.2 g/L, and it occurred at 96 hours.
In the second year of this project, we plan to investigate the process aspects of the pretreatment method we developed in the first year. This will involve development of a laboratory-scale continuous reactor that can simulate the ammonia recycle percolation process.
Additionally, we plan to investigate the production of value-added chemicals other than ethanol from the pretreated corn stover. The products under consideration are xylooligosaccharides (high-value food additive) and lactic acid.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
|Other project views:||All 8 publications||5 publications in selected types||All 5 journal articles|
||Kim TH, Lee YY. Pretreatment and fractionation of corn stover by ammonia recycle percolation process. Bioresource Technology 2005;96(18):2007-2013||
||Kim TH, Lee YY. Pretreatment of corn stover by soaking in aqueous ammonia. Applied Biochemistry and Biotechnology 2005;121(Spring):1119-1131||