Author : Xiguang Chen
Release : 2012
Genre :
Kind : eBook
Book Rating : 671/5 ( reviews)
Book Synopsis Development of Effective Pretreatment and Bioconversion Systems for Converting Organic Residuals to Bioenergy by : Xiguang Chen
Download or read book Development of Effective Pretreatment and Bioconversion Systems for Converting Organic Residuals to Bioenergy written by Xiguang Chen. This book was released on 2012. Available in PDF, EPUB and Kindle. Book excerpt: Food processing industry wastes and agricultural residues are two major categories of organic wastes available in California. Treatment and disposal of such waste streams in a cost-effective way without causing social and environmental problems has long been an issue for the State. Meanwhile, as a result of rapidly escalating energy prices and increasing concerns about environmental pollution by refining fossil-based fuels, converting organic wastes to renewable energy and other valuable products can bring many economic and environmental benefits to the public. Anaerobic digestion is an in-vessel biological conversion technology that operates in the absence of oxygen that achieves waste treatment and renewable energy production all at once. However, lignocellulosic materials in such waste streams become the obstacle for developing and implementing the anaerobic digestion processes because the complex structural matrix in the lignocellulosic biomass limits the accessibility of microorganisms and enzymes. Pretreatment can alter the structure of the lignocellulosic materials and make the biomass more accessible to the microorganisms, consequently increasing the rate and the yield of conversion, and improving the feasibility of bioenergy production. Alkaline pretreatment NaOH was applied to grape pomace at different NaOH loadings and temperatures to determine the optimum pretreatment conditions for enhancing anaerobic digestibility of grape pomace. The results indicated that grape pomace treated with 6% NaOH at 20°C had the highest biogas yield of 544 mL/g VS after 20 days of anaerobic digestion, which was on average 31% higher than raw pomace. High temperature at 121°C indicated less pretreatment effectiveness than low temperature (20°C) counterpart and also inhibited anaerobic digestion test at the early stage of anaerobic digestion. Energy and mass balance, and economic analysis for the proposed integrated pretreatment and anaerobic digestion system showed that the optimum NaOH loading were 1.9, 3.3, and 4.4% for 50, 150, and 460 ton/day feedstock throughputs, respectively, and 164 ton/day is the minimum facility throughputs to make the project financially attractive. Woody biomass was considered as a supplemental feedstock for the grape pomace digestion facility in order to overcome the seasonal production of pomace. This study revealed that the alkaline pretreatment can effectively increase the biogas yield from woody biomass by 52% and inhibitors removal prior to anaerobic digestion is not necessary for biogas production. Co-digestion of grape pomace and woody biomass presented no synergetic effects, probably because the unbalanced carbon and nitrogen in the co-digestion mixture. Additional nitrogen source for continuously co-digesting grape pomace and woody biomass is expected and recommended for robust anaerobic microorganism consortium and optimum biogas production. Two-step NaOH pretreatment using fresh water as reaction solvent was designed to further increase the biodegradability of rice straw, which structure is more recalcitrant than grape pomace. The results demonstrated that two-step pretreatment improved the lignin removal and glucose production by 11% and 18%, respectively. Moreover, with the purpose of reducing water and chemical consumptions, black liquor separated after pretreatment was reused in substitution of fresh water. The results indicated two-step pretreatment using black liquor had lower (11%) bioconversion efficiency than two-step pretreatment using fresh water, but still improved 5% and 11% from one-step pretreatment using fresh water and one-step pretreatment using black liquor, respectively. Given the significant cost and environmental advantages of black liquor recycle and reuse, a semi-continuous two-step NaOH pretreatment experiment using recycled black liquor was conducted for downstream biogas production through anaerobic digestion. Lignin removal and enzymatic hydrolysis results suggested that the pretreatment effects decreased by replacing fresh water with black liquor but reached steady state after three recycling times using black liquor. However, sodium concentration limited the black liquor recycle times to three, otherwise anaerobic digestion processes could be severely inhibited. Therefore, it could be concluded that four-round two-step pretreatment using recycled black liquor could save 20% NaOH and 58% fresh water addition to the system while no significant decreases in biogas production. Economic analysis showed two-step NaOH pretreatment using black liquor and anaerobic digestion system had 53% higher net present worth than conventional one-step pretreatment and digestion system. However, both proposed projects were not financially viable (negative net present worth) under the assumptions in this study. Post-processing biogas treatment and marketable by-products development are two important factors that can improve the project feasibility based on sensitivity analysis. The recommended further studies based upon the results from this research include investigate mechanisms of two-step NaOH pretreatment and inhibition by lignin and its derivatives, conduct continuous pretreatment and anaerobic digestion test to determine operational parameters, develop digester effluent treatments to produce valuable by-products and release environmental burdens of disposal, and study Federal, State and local regulations that promote renewable energy projects to improve the feasibility of the project.
