Bioethanol from Agricultural Waste Residues

Transcription

1 Bioethanol from Agricultural Waste Residues Pascale Champagne Department of Civil Engineering, Queen s University, Kingston, ON K7L 3N6, Canada; champagne@civil.queensu.ca (for correspondence) Published online 3 December 2007 in Wiley InterScience ( DOI /ep Ó 2007 American Institute of Chemical Engineers Under the Kyoto Protocol, the Government of Canada has committed to reducing its greenhouse gas emissions by 6% from 1990 levels between 2008 and Ethanol-blended gasolines have the potential to contribute significantly to these emission reductions. Ethanol is derived from biologically renewable resources and can be employed to replace octane enhancers and aromatic hydrocarbons or oxygenates. To date, the ethanol production industry in Canada is comprised mainly of small-scale plants producing ethanol primarily from agricultural crops as feedstock. Research interests in the area of bioethanol production from organic waste materials emerged in the late Significant advances in lignocellulosic material extraction and enzymatic hydrolysis have been reported in the last decade, however, continued research efforts are essential for the development of technically feasible and economically viable largescale enzyme-based biomass-to-ethanol conversion processes. This research aims to develop and test an enzyme-based biomass-to-ethanol conversion process, which employs organic waste materials, such as livestock manures, as alternative sources of cellulosic material feedstock. The source of the livestock manure, manure management practices and cellulose extraction procedures have a significant impact on the quantity and quality of the cellulosic materials derived. As such, raw feedstock materials must be carefully characterized to assess the impact of these factors on the yield of bioethanol and residual end products. The success of cellulose-to-ethanol conversion processes for cellulose extracted from these waste materials as feedstock is generally a function of cellulose fiber pretreatment, enzyme selection and operating conditions. These will differ depending on the source of the waste material feedstock. The long-term benefits of this research will be to introduce a sustainable solid waste management strategy for a number of livestock manure and other lignocellulosic waste materials; contribute to the mitigation in greenhouse gases through sustained carbon and nutrient recycling; reduce the potential for water, air, and soil contamination associated with land disposal of organic waste materials; and to broaden the feedstock source of raw materials for the ethanol production industry. Ó 2007 American Institute of Chemical Engineers Environ Prog, 27: 51 57, 2008 Keywords: bioethanol, manure, enzymatic hydrolysis, agricultural waste INTRODUCTION Under the Kyoto Protocol, the Government of Canada has committed to reducing its greenhouse gas emissions by 6% from 1990 levels between 2008 and Ethanol-blended gasolines have the potential to contribute significantly to these emissions reductions. Ethanol is an alternative fuel derived from biologically renewable resources and can be employed to replace octane enhancers such as methylcyclopentadienyl manganese tricarbonyl (MMT) and aromatic hydrocarbons such as benzene or oxygenates such as methyl tertiary butyl ether (MTBE). A potential source for low-cost ethanol production is to utilize lignocellulosic materials (crop residues, grass, sawdust, woodchips, sludges, livestock manure), but the feasibility of using these materials as a feedstock is often limited by the cost of bioethanol production, which is relatively high based on current technologies. The challenges are generally associated with the low yield and the high cost of the hydrolysis process. Research involving bioethanol production from lignocellulosic waste materials have included crop residues [1 4], municipal solid wastes [5 8], forest products wastes [9 11], leaf and yard wastes [12], municipal sludges [13], as well as a few studies involving dairy and cattle manures [14 16]. The current bioethanol production industry in Canada Environmental Progress (Vol.27, No.1) DOI /ep April

2 Figure 1. Potential value-added products recovery from lignocellulosic waste materials. from lignocellulosic materials is comprised of only a few small-scale plants, producing ethanol mainly from agricultural crops as feedstock materials. Iogen Corporation has developed a demonstration plant for the conversion of wood, hay, straw and other agricultural crop residuals to ethanol. Other ethanol production plants currently in operation in Canada include: Mohawk Oil Canada (wheat-based), Pound-Maker Agventures (wheat-based), Commercial Alcohols (corn-based), API Grain Processors (wheat-based), and Tembec (forestry product-based). In 1995, Dinel and Dumontet [17] introduced a concept of sustainable waste management termed Biological and Chemical Integrated Recycling System. They proposed a series of possible extraction and fractionation procedures which would allow for the recovery of a number of value-added products, including cellulose, from various lignocellulosic waste streams (see Figure 1). Livestock manures and sludges contain large quantities of lignocelluloses, polysaccharides, proteins and other organic materials. The conversion of these materials to value-added products has been recognized as an attractive waste management approach. In addition, the recovery of raw materials from organic wastes such as manures and sludges, and their conversion to value-added products has the potential to reduce the microbial production of CO 2 and CH 4, thereby minimizing potential environmental impacts and the strain on nonrenewable resource reserves. However as feedstocks, organic wastes often have a complex physical and chemical composition which has made it difficult to utilize, and the limited research efforts to convert these materials to higher-value chemicals and energy on a commercial scale have been unsuccessful [14]. Bioethanol production utilizing lignocellulosic materials generally takes place in three phases. First, delignification to liberate cellulose and hemicellulose from lignin. Second, depolymerization of the carbohydrate polymers (cellulose and hemicellulose) to produce free sugars. Third, fermentation of mixed hexose and pentose sugars to produce ethanol. In addition, a preliminary step is often required when employing livestock manure and sludges as feedstock to obtain a better solids/liquid separation and to solubilize and remove metals. Factors affecting the acid and enzymatic hydrolysis of cellulose include porosity of the waste materials, cellulose fiber crystallinity, and lignin and hemicellulose content. Hydrolysis can be significantly enhanced by introducing physical and chemical fiber pretreatments including: grinding, pyrolysis, steam explosion, ammonia fiber explosion (AFEX), CO 2 explosion, ozonolysis, acid hydrolysis, alkaline hydrolysis, oxidative delignification, Organosolv process involving extraction with hot aqueous ethanol and biological pretreatment of the lignocellulosic materials to remove lignin and hemicellulose and reduce fiber crystallinity. Enzymatic hydrolysis can be improved through the optimization of substrate concentration, cellulase dosing, use of surfactants, enzyme mixtures, and enzyme recycling. Simultaneous saccharification and fermentation (SSF) is an effective process configuration which removes glucose, an inhibitor to cellulase activity, thereby increasing the yield and rate of cellulose hydrolysis [18]. DELIGNIFICATION OF AGRICULTURAL WASTE MATERIALS Rivers and Emert [3] studied the effects of substrate composition, cellulose crystallinity, and particle size on the yields of enzymatic hydrolysis using bagasse and rice straw. They concluded that each type of lignocellulosic feedstock requires a specific pretreatment in order to optimize enzymatic hydrolysis. The 52 April 2008 Environmental Progress (Vol.27, No.1) DOI /ep

3 Table 1. Percent metal reduction in hog manure after HCl pretreatment. Metal removal efficiency (%) Metal 0.1 N HCl 1.0 N HCl Al Ca Cu Fe Mg Zn Na P Table 2. Total solids recovery in the HP and NHP after pretreatment and alkaline hydrolysis. HCl/KOH strength (N) NHP (g TS NHP / 100g TS OM ) HP (g TS HP / 100g TS OM ) 0.0/ / / / / / change in the crystallinity of rice straw following pretreatment appeared to correlate well with conversions to either glucose or ethanol. Modification of the lignocellulose matrix from its native state by alkaline pretreatment resulted in the most significant increases in enzymatic hydrolysis reinforcing the concept that the nature of the lignocellulose matrix is a significant limiting factor in the enzymatic hydrolysis of bagasse and rice straw. In the research conducted at Carleton University, (Ottawa, Canada) general laboratory-scale chemical extraction and fractionation procedures and protocols to be applied to agricultural crop residues, municipal sludges and livestock manures were developed. Henderson et al. [19] elaborated an alkaline delignification technique and reported that the alkaline delignification technique was effective in separating cellulose as the nonhydrolyzable product (NHP) from the lignin and hemicellulose as the hydrolysable product (HP) of organic crop residues. Experimental variables included the type and strength of hydrolyzing base, temperature, and type of acid used for final neutralization of the HP. The efficiency of separation was assessed based on material recovery, and the carbon, nitrogen, phosphorous and ash recovered in each of the two fractions. Using bagasse and corn stovers as feedstock, the most efficient separations from the bagasse (70% NHP) and corn stovers (50% NHP) were obtained by using two alkaline hydrolysis cycles of 0.5 N KOH at a temperature of 708C. The alkaline delignification technique was then adapted to extract and fractionate hog manure, a more complex waste material, into NHP and HP. Levy et al. [20, 21] and Champagne et al. [22] also demonstrated N acid pretreatments were effective in separating metals from the hog manure. Although a 1.0 N solution showed a higher metal removal efficiency (Table 1), it also had a negative impact on NHP and HP recovery compared to the dilute 0.1 N HCl pretreatment, which implied that organic matter and nitrogen recovery could also be impacted (Table 2). Aside from Al (5%), more than 71% of metals were separated from the hog manure using the 0.1 N HCl pretreatment. It was suggested that better metal separation could be achieved through coagulation with the removal the colloidal matter. Upon further investigation it was noted that nonionic polymers Figure C NMR scan of HP resulting from 0.1 N HCl pretreatment and 0.5N alkaline hydrolysis. were most effective in removing the colloidal matter from suspension. A separation of 61% NHP and 11.5% HP were observed using a 0.1 N HCL pretreatment solution and a 0.5 N KOH alkaline hydrolysis solution. The NHP accounted for the largest volatile solids (62.4%) and carbon (60.1%) recovery, while the HP accounted for the largest nitrogen (45.5%) recovery. FTIR and 13 C NMR [22] spectra of the products suggested that the HP had a more complex structure than the NHP. It was shown that the HP possessed humic-like characteristics with the presence of alkenes and alkanes, proteinaceous carbon and nitrogen, as well as aromatic carboxylic and phenolic structures. The spectra for the NHP showed mainly aliphatic chemical structures with the presence of carbohydrate carbon as would be observed for cellulose (Figures 2 and 3). BIOETHANOL PRODUCTION FROM AGRICULTURAL WASTES A research group at Washington State University conducting research on the extraction of value-added products from dairy and cattle manures [14 16], developed a process for hydrolyzing lignocellulosic materials from manure into fermentable sugars. They reported that when raw dairy manure was pretreated with 3% sulfuric acid at 1108C for 1 h, hemicellulose was completely degraded to arabinose, galactose and xylose. The pretreated materials were then treated with cellulolytic enzymes to hydrolyze the cellulose. The optimal enzyme loadings were identified as 13 FPU cellulose/g substrate and 5 IU b-glucosidase/g Environmental Progress (Vol.27, No.1) DOI /ep April

4 Figure C NMR Scan of NHP resulting from 0.1 N HCl pre treatment and 0.5 N alkaline hydrolysis. Figure 4. Pretreatment of poultry manures cellulose material fibers. Figure 5. Glucose yields from poultry manure using different feedstock and fiber pretreatments. substrate. The optimal temperature and ph were determined to be 468C and 4.8, respectively. A substrate concentration of 50 g/l favored both glucose concentration and glucose yield. It was also found that a reduced particle size of 590 lm resulted in a high glucose yield and further decreases in particle size did not increase the yield which supports findings by our research group. For each particle size tested, the addition of a surfactant resulted in at least 20% improvement in glucose yield. The optimized hydrolysis process achieved a glucose yield of 11.3 g/ 100 g manure, which corresponded to 40% cellulose. The feasibility of producing ethanol from waste materials comparing crop residues, poultry manure and municipal sludges as low-cost lignocellulosic feedstock was examined by Li and Champagne [23, 24]. The enzymatic hydrolysis of crop residues achieved relatively high glucose yields. At 408C, with an enzyme loading of 800 units/g substrate, the percentages of conversion in 24 h were 65.4% and 51.1% for KOH-treated corn stovers and bagasse respectively. The study showed that physical and/or chemical NHP fiber pretreatments (grinding, drying and phosphorylation) had a great impact on the glucose yields from the saccharification process, primarily because it affected the enzyme access to the cellulose material. It was concluded that higher glucose yields might be achieved by tailoring the sequence of fiber pretreatment processes depending on the feedstock from which the NHP fibers were derived. The sequence of fiber pretreatments affected the enzymatic hydrolysis rate, where crop residues ground before alkaline treatment and the use of a wet substrate yielded higher conversion rates, compared to trials using dry substrate which was first treated with an alkaline solution followed by grinding. The hydrolysis rate was also found to increase when half of the enzyme dose was added each at the beginning and midpoint of hydrolysis rather than entirely at the beginning. In a study using poultry manure as a lignocellulosic feedstock [23, 24] it was found that the alkaline KOH delignification technique was effective in improving enzymatic hydroloysis to glucose. In addition, physical pretreatments such as drying and grinding, also exhibited an impact on the hydrolysis (see Figure 4). The glucose yields for poultry manure with various pretreatment are shown in Figure 5. At 408C, with an enzyme loading 400 units/g substrate, the glucose yield for raw poultry manure was 7.1, and this increased to 10.2 when KOH pretreatment was employed. These results were quite different to that observed with crop residues, where a 10-fold increase in the glucose yield was noted as a result of KOH treatment. This would suggest that in the poultry manure, most of the cellulose was not as easily extracted from other organic matters. Although the integrated structure of cellulosic material might be broken up during the KOH treatment, the broken structure could still not provide enough access for the enzymes to act on the cellulose molecules. In the case of KOH-treated manure, dried before enzymatic hydrolysis, the glucose yield increased to These results demonstrated that drying the KOH-treated manure did improve the glucose yield from enzymatic hydrolysis. It could be inferred that some of the organic matter might be degraded at high temperature (up to 708C), thus more cellulose could be uncovered during the drying process; or that the physical struc- 54 April 2008 Environmental Progress (Vol.27, No.1) DOI /ep

5 Figure 6. Pretreatment of primary sludge cellulosic material fibers. Figure 7. Glucose yields from primary sludge using different feedstock and fiber pretreatments. ture of substrate became more accessible to the enzyme due to the drying. The highest glucose yield was noted for KOH-treated and ground poultry manure. With a conversion of 27.6, this represented a twofold increase in yield compared to that of unground manure. These results suggested that by grinding the poultry manure, the substances that covered the cellulose were broken up, thus, exposing the surface of the cellulose. In addition, reducing the particle size of the substrate also created a larger surface area for reaction. Thus, the substrate became much more accessible to the enzyme. As a result, the glucose yield increased significantly. The cellulose content in primary sludge is mainly from waste paper, however, the cellulosis fraction must be separated from the noncellulosic components prior to the enzymatic hydrolysis to maximize glucose yield [23, 24]. Pretreatment approaches for primary sludge include KOH, HCl, and HCl followed by KOH alkaline delignification (see Figure 6). Figure 7 presents the glucose yields from primary sludge following various feedstock material and fiber pretreatments. Both wet and dry substrates were tested for each of them. All the trials were conducted at 408C for a period of 24 h. Results indicated that when KOH pretreatment was performed on primary sludge, the conversion for wet substrate increased from 31.1% to 35.4%, which represented a 4.3% increase in glucose yield. However, compared to that of crop residues, which achieved an almost ten-fold increase in the conversion percentages, the KOH pretreatment was not as effective on the primary sludge. Similarly, when the primary sludge was treated with HCl, the glucose yield increased to 42.6% for wet substrate, which was 11.5% higher than that observed without acid and alkaline treatment. This suggested that through HCl pretreatment, which would remove metals, enzyme inhibition due to metals may have been reduced, resulting in a higher glucose yield from the enzymatic hydrolysis. When both acid and alkaline pretreatments were performed on the primary sludge, the glucose yield reached 54.2% and 37.0% for wet and dry substrates, respectively, which supported the value of acid and alkaline pretreatments for effective glucose conversion from enzymatic hydrolysis. Moreover, the results implied that considerable cellulose is contained in the primary sludge on a dry mass basis, and cellulosic material recovery and conversion to ethanol might present a valuable waste management alternative when employed as a wet feedstock. This is contrary to what was noted with poultry manure, which demonstrated a higher glucose conversion when dry substrate was used. This might indicate that the cellulose in primary sludge was more readily accessible to the enzymes and that drying and grinding might not be required to improve the enzymatic hydrolysis as it was using with poultry manure. CONCLUSIONS Preliminary research using organic waste materials as lignocellulosic feedstock for ethanol production has shown great promise to date, however, further research is essential to investigate its application beyond the laboratory-scale. For instance, cellulose extraction procedures and fiber pretreatments should be optimized as a function of feedstock to maximize the enzymatic hydrolysis and fermentation processes. Enzyme recycling and the use of simultaneous saccharification and fermentation to integrate the ethanol production process and achieve higher reaction rates at lower costs would also be beneficial to the development of this waste management strategy. FUTURE RESEARCH NEEDS AND ANTICIPATED SIGNIFICANCE OF RESEARCH The research will result in the development of an innovative waste management approach that uses agricultural waste materials as a renewable resource for the extraction of a value-added product, cellulose, and its conversion to bioethanol. This alternative strategy would be of great benefit to the agricultural industry where traditional manure management prac- Environmental Progress (Vol.27, No.1) DOI /ep April

6 tices have begun to inhibit rural development and limit the establishment of livestock facilities due to potential health risks for the general public. Municipalities and industries that rely heavily on landfilling as a disposal strategy for their biosolids and process sludge may be benefited as well. The long-term benefits will be to reduce the potential for water, air, and soil contamination associated with land disposal of organic residuals; to broaden the feedstock source of raw materials for the ethanol production industry; and to contribute to the mitigation of greenhouse gases through sustained carbon and nutrient recycling. In order to develop efficient and economically viable bioethanol conversion processes for waste materials, future research needs to include the elaboration of existing laboratory procedures and protocols, design recommendations, as well as economic and environmental assessments generated from ongoing research on lignocellulosic materials. Processes could then be adapted and applied to other waste materials which could be employed as lignocellulosic feedstock for the production of bioethanol. These could include municipal and industrial sludge, biosolids, leaf and yard wastes, as well as other organic residues. As a secondary research objective, the extraction of other value-added products from various organic residues could be investigated to broaden the biomass feedstock pool relevant to the emerging bioproducts industry. For instance, the NHP could be used as a substrate for microbial synthesis to produce ethanol, as well as organic acids or complex proteins in the form of live biomass. Products useful for agriculture could be generated from the nitrogen and carbonrich HP, which would involve using the HP and the residual fraction of the NHP that contribute effectively in developing a sustainable waste management approach. LITERATURE CITED 1. Cuzens, J.C., & Miller, J.R. (1997). Acid hydrolysis of bagasse for ethanol production, Renewable Energy, 10, Kim, S., & Dale, B.E. (2004). Global potential bioethanol production from wasted crops and crop residues, Biomass and Bioenergy, 26, Rivers, D.B. (1988). Factors affecting the enzymatic hydrolysis of bagasse and rice straw, Biological Wastes, 26, Zayed, G., & Meyer, O. (1996). The single-batch bioconversion of wheat straw to ethanol employing the fungus Trichoderma viride and the yeast Pachysolen tannophylus, Applied Microbiological Biotechnology, 45, Green, M., & Shelef, G. (1989). Ethanol fermentation of acid hydrolysate of municipal solid waste, Chemical Engineering Journal, 40, B25 B Green, M., Kimchie, S., Malester, A.I., Rugg, B., & Shelef, G. (1988). Utilization of municipal solid wastes (MSW) for alcohol production, Biological Wastes, 26, Lark, N., Xia, Y., Qin, C.-G., Gong, C.S., & Tsao, G.T. (1997). Production of ethanol from recycled paper sludge using cellulase and yeast, Kluveromyces marxianus Biomass and Bioenergy, 12, Mtui, G., & Nakamura, Y. (2005). Bioconversion of lignocellulosic waste from selected dumping sites in Dar es Salaam, Tanzania, Biodegradation, 16, Duff, S.J.B., & Murray, W.D. (1996). Bioconversion of forest products industry waste cellulosics to fuel ethanol: A review, Bioresource Technology, 55, Fan, Z., South, C., Lyford, K., Munsie, J., van Walsum, P., & Lynd, L.R. (2003). Conversion of paper sludge to ethanol in a semicontinuous solids-fed reactor, Bioprocess Biosystem Engineering, 26, Kadar, Z., Szengyel, Z., & Reczey, K. (2004). Simultaneous saccharification and fermentation (SSF) of industrial wastes for the production of ethanol, Industrial Crops and Products, 20, Lissens, G., Klinke, H., Verstraete, W., Ahring, B., & Thomsen, A.B. (2004). Wet oxidation pre-treatment of woody yard waste: Parameter optimization and enzymatic digestibility of ethanol production, Journal of Chemical Technology and Biotechnology, 79, Cheung, S.W., & Anderson, B.C. (1997). Laboratory investigaton of ethanol production from municipal primary wastewater solids, Bioresource Technology, 59, Chen, S., Liao, W., Liu, C., Wen, Z., Kincaid, R.L., Harrison, J.H., Elliot, D.C., Brown, M.D., Solana, A.E., & Stevens, D.J. (2004). Value-added chemicals animal manures (135 p), Northwest Bioproducts Research Institute Technical Report, US Department of Energy Contract DE-AC06 76RLO Chen, S., Liao, W., Liu, C., Wen, Z., Kincaid, R.L., & Harrison, J.H. (2003). Use of animal manure as feedstock for bio-products. In Proceedings of Ninth International Animal, Agricultural and Food Processing Wastes Symposium (pp ), Research Triangle Park, NC. 16. Wen, Z., Liao, W., & Chen, S. (2004). Hydrolysis of animal manure lignocellulosics for reducing sugar production, Bioresource Technology, 91, Dinel, H., & Dumontet, S. (1995). Organic wastes management: Organic leftovers, compost and soil structure, Fresenius Environmental Bulletin, 4, Sun, Y., & Cheng, J. (2002). Hydrolysis of lignocellulosic materials for ethanol production: A review, Bioresource Technology, 83, Henderson, B., Champagne, P., & Tudoret, M.J. (2003). Chemical separation of cellulose from lignin in sugarcane bagasse. In Eighth Specialty Conference: Environment and Sustainable Engineering and 31st Annual CSCE Congress Proceedings, Moncton, New Brunswick. ENK April 2008 Environmental Progress (Vol.27, No.1) DOI /ep

7 20. Levy, T., Champagne, P., Tudoret, M.J., & Dinel, H. (2003). Bio-chemical integrated recycling of hog manure. In Eighth Special Conference: Environment and Sustainable Engineering and 31st Annual CSCE Congress Proceedings, Moncton, New Brunswick. ENK Levy, T., Champagne, P., Tudoret, M.J., & Dinel, H. (2003). Feasibility study on the recovery of commodity chemicals and agri-products from hog manure. In 18th International Conference on Solid Waste Technology and Management, Philadelphia, PA. 22. Champagne, P., Levy, T., & Tudoret, M.J. (2005). Recovery of value-added products from hog manure A feasibility study, Journal of Solid Waste Technology and Management, 31, Li, C., & Champagne, P. (2005). Feasibility of using waste materials as feedstocks for ethanol production, International Journal of Solid Waste Technology and Management, 31, Li, C., & Champagne, P. (2005). Enzymatic hydrolysis of cellulose from various waste sources and their feasibility as feedstocks for ethanol production. In 20th International Conference on Solid Waste Technology and Management, Philadelphia, PA. Environmental Progress (Vol.27, No.1) DOI /ep April