Developing models for analyzing cost structures in fermentative bioprocesses

The main objective for this thesis was to develop techno-economic modeling tools to analyze two bioprocessing systems: anaerobic digestion and industrial fermentation. While both of these processes can be used to produce products that ultimately offset fossil fuels, there are fundamental differences between each process. Anaerobic digestion is a mixed culture process used for waste treatment - it uses a feedstock of low or sometimes negative value. In contrast, industrial fermentation is a pure culture process typically used to create high value products, requiring relatively expensive feedstocks and typically higher-technology infrastructure to support. Understanding the cost structures of different bioprocesses helps engineers and scientists identify critical variables that should be targeted for reducing production costs. This thesis is prepared in the journal paper format and includes two papers that have been prepared for submission to a journal. The objective for the first paper in this thesis was to develop a model to analyze farm-scale anaerobic digestion. Anaerobic digestion is a biological process that can be used to treat animal waste, producing biogas and a nutrient-rich digestate. A spreadsheet model was developed to analyze economic and technical barriers to this technology, using operation size as the primary input and the cost for producing methane as the primary output. Trends in the methane cost ratio, or the ratio of the production cost for methane to the market value of natural gas, as a function of different process variables were evaluated, and recommendations for improving deployment rates were discussed. Results showed that moderate reductions in the interest rate are capable of making 1000-cow digesters economically feasible if high carbon credits values, high natural gas prices, and low gas clean-up costs can be achieved; however, if carbon credit values are low as they currently are, more extreme modifications to the digester cost and interest rate, combined with increases in digester life and/or natural gas prices are required for smaller dairies to break-even when using anaerobic digestion. The objective for the second paper was to develop a model to analyze the cost structure of industrial fermentation processes for producing biorenewable chemicals. As metabolic engineers develop improved microbial strains for industrial fermentation, understanding the tradeoffs between different kinetic parameters on the production cost is vital. A spreadsheet model was developed to provide an order-of-magnitude estimate of chemical production cost based on kinetic and operating parameters. Results showed that production cost is most sensitive to yield, fraction of product in the biomass, and substrate concentration. Feedstock cost makes up the largest portion of the production cost except under the most pessimistic fermentation conditions. Minimizing fermentation costs is critical to making biorenewable chemicals cost competitive with their well-established petroleum-derived competitors

Effect of Ammonia Soaking Pretreatment and Enzyme Addition on Biochemical Methane Potential of Switchgrass

This article presents the biochemical methane potential (BMP) results from the anaerobic digestion (AD) of switchgrass. Triplicate BMP assays were performed on: untreated switchgrass, aqueous ammonia soaking (AAS) pretreated switchgrass (soaked in 29.5% reagent-grade aqueous ammonia at 5 L kg-1 switchgrass for 5 d), and AAS-pretreated switchgrass plus cellulytic enzymes at 12.5, 25, 62.5, and 125 filter paper units (FPU) enzyme g-1 volatile solids (VS). Biogas production and biogas methane content were measured daily in all treatments for 21 d. Both biogas and corrected methane production varied significantly among treatments, especially during the first 7 d of the BMP period. Total methane production at 21 d was corrected for enzyme degradation, and methane yields ranged from 0.15 to 0.36 m3 CH4 kg-1 VS. We compared the corrected energy yield of biogas from switchgrass to prior reports of the energy yield of ethanol from switchgrass via simultaneous saccharification and fermentation (SSF). The AD of AAS-pretreated switchgrass at the highest enzyme loading rates resulted in a 120% increase in energy extracted as compared to AAS-pretreated switchgrass converted to ethanol via SSF. Overall, the addition of enzymes to AAS-pretreated switchgrass greatly accelerated the rate of methane production over the untreated switchgrass and AAS-pretreated switchgrass without enzymes. However, the process economics are not clear, and additional work is needed to determine whether pretreating switchgrass with aqueous ammonia and/or enzymes before AD is economically advantageous

Developing models for analyzing cost structures in fermentative bioprocesses

The main objective for this thesis was to develop techno-economic modeling tools to analyze two bioprocessing systems: anaerobic digestion and industrial fermentation. While both of these processes can be used to produce products that ultimately offset fossil fuels, there are fundamental differences between each process. Anaerobic digestion is a mixed culture process used for waste treatment - it uses a feedstock of low or sometimes negative value. In contrast, industrial fermentation is a pure culture process typically used to create high value products, requiring relatively expensive feedstocks and typically higher-technology infrastructure to support. Understanding the cost structures of different bioprocesses helps engineers and scientists identify critical variables that should be targeted for reducing production costs. This thesis is prepared in the journal paper format and includes two papers that have been prepared for submission to a journal. The objective for the first paper in this thesis was to develop a model to analyze farm-scale anaerobic digestion. Anaerobic digestion is a biological process that can be used to treat animal waste, producing biogas and a nutrient-rich digestate. A spreadsheet model was developed to analyze economic and technical barriers to this technology, using operation size as the primary input and the cost for producing methane as the primary output. Trends in the methane cost ratio, or the ratio of the production cost for methane to the market value of natural gas, as a function of different process variables were evaluated, and recommendations for improving deployment rates were discussed. Results showed that moderate reductions in the interest rate are capable of making 1000-cow digesters economically feasible if high carbon credits values, high natural gas prices, and low gas clean-up costs can be achieved; however, if carbon credit values are low as they currently are, more extreme modifications to the digester cost and interest rate, combined with increases in digester life and/or natural gas prices are required for smaller dairies to break-even when using anaerobic digestion. The objective for the second paper was to develop a model to analyze the cost structure of industrial fermentation processes for producing biorenewable chemicals. As metabolic engineers develop improved microbial strains for industrial fermentation, understanding the tradeoffs between different kinetic parameters on the production cost is vital. A spreadsheet model was developed to provide an order-of-magnitude estimate of chemical production cost based on kinetic and operating parameters. Results showed that production cost is most sensitive to yield, fraction of product in the biomass, and substrate concentration. Feedstock cost makes up the largest portion of the production cost except under the most pessimistic fermentation conditions. Minimizing fermentation costs is critical to making biorenewable chemicals cost competitive with their well-established petroleum-derived competitors.</p

Gravimetric analysis of methane adsorption in activated carbon (Faulhaber) [abstract]

Abstract only availableFaculty Mentor: Dr. Peter Pfeifer, PhysicsALL-CRAFT has an interest in using activated carbon to store high-capacities of natural gas at low pressures for use in natural gas vehicles.  Activated carbon made from waste corncob, contains fractal pores that are favorable in the storing natural gas.  To further investigate the storage capacity of carbon, an analysis was done on the equilibration times for a sample of carbon and several carbon samples were tested to determine the sample with the highest storage capacity.  A sample of carbon was exposed to methane and several gravimetric measurements were taken at different equilibration times.  I also tested the amount of methane adsorbed by several samples of carbon, taking gravimetric measurements.  Results showed that 1 hour of methane uptake was the minimum equilibration time needed for reasonable amounts of methane to be adsorbed by the carbon.  Future studies hope to test more samples of carbon to determine how the pore size distribution and activation process of different samples impact a sample's storage capacity

Techno-economic Analysis of Farm Scale Plug-flow Anaerobic Digestion

Treating animal wastes through anaerobic digestion (AD) yields methane-rich biogas that can be used for power generation or heating, and a nutrient-rich digestate that can be land applied as fertilizer. Anaerobic digestion also reduces odors from stored and land applied manures. Despite these benefits, AD deployment rates in the United States (US) are only 5% for dairy farms identified as being suitable for AD by the US Environmental Protection Agency. The objective of this study was to analyze the economic and technical limitations of farm-scale plug-flow anaerobic digesters using a simple model permitting insight into the fundamental constraints on the technology. A model was developed to determine the cost of methane produced via AD based on operation size. For context, the cost of ADmethane was then compared to commercial methane costs (i.e., natural gas). The analysis shows how critical farm size is to making AD-methane cost-competitive with natural gas. At low herd sizes (below 400 animals), carbon credits and odor reductions alone appear insufficient to overcome the relatively low commercial energy rates in the US. However, moderate reductions in digester cost and interest rate, coupled with moderate increases in amortization period, and/or natural gas prices appear could make AD more competitive with commercial energy in the US even at relatively small herd sizes (ca. 200 animals).</p

An Engineering-Economic Model for Analyzing Dairy Plug-Flow Anaerobic Digesters: Cost Structures and Policy Implications

Treating animal wastes through anaerobic digestion (AD) yields methane-rich biogas that can be used for power generation or heating, and a nutrient-rich digestate that can be land-applied as fertilizer. Furthermore, AD reduces odors from stored and land-applied manures. Despite these benefits, AD deployment rates in the U.S. are only 5% for dairy farms identified as suitable for AD by the U.S. Environmental Protection Agency. The objective of this study was to analyze the economic and technical limitations of farm-scale anaerobic digesters using a simple model permitting insight into the fundamental constraints on the technology. A model was developed to determine the cost of methane produced via AD based on operation size. Dairy plug-flow systems were modeled because of their well-documented economic performance, and model validation used data from AgSTAR's FarmWare program. The analysis shows that farm size is critical to make digestion-derived methane cost-competitive with natural gas. At low herd sizes (animals), carbon credits and odor reductions alone appear insufficient to overcome the low commercial energy rates in the U.S. However, moderate reductions in digester cost and interest rate, coupled with moderate increases in amortization period and/or natural gas prices, could make AD more competitive with commercial energy in the U.S. even at relatively small herd sizes (approx. 400 animals).This article is from Transactions of the ASABE, 55, no. 1 (2012): 201–209.</p

Effect of Ammonia Soaking Pretreatment and Enzyme Addition on Biochemical Methane Potential of Switchgrass

This article presents the biochemical methane potential (BMP) results from the anaerobic digestion (AD) of switchgrass. Triplicate BMP assays were performed on: untreated switchgrass, aqueous ammonia soaking (AAS) pretreated switchgrass (soaked in 29.5% reagent-grade aqueous ammonia at 5 L kg-1 switchgrass for 5 d), and AAS-pretreated switchgrass plus cellulytic enzymes at 12.5, 25, 62.5, and 125 filter paper units (FPU) enzyme g-1 volatile solids (VS). Biogas production and biogas methane content were measured daily in all treatments for 21 d. Both biogas and corrected methane production varied significantly among treatments, especially during the first 7 d of the BMP period. Total methane production at 21 d was corrected for enzyme degradation, and methane yields ranged from 0.15 to 0.36 m3 CH4 kg-1 VS. We compared the corrected energy yield of biogas from switchgrass to prior reports of the energy yield of ethanol from switchgrass via simultaneous saccharification and fermentation (SSF). The AD of AAS-pretreated switchgrass at the highest enzyme loading rates resulted in a 120% increase in energy extracted as compared to AAS-pretreated switchgrass converted to ethanol via SSF. Overall, the addition of enzymes to AAS-pretreated switchgrass greatly accelerated the rate of methane production over the untreated switchgrass and AAS-pretreated switchgrass without enzymes. However, the process economics are not clear, and additional work is needed to determine whether pretreating switchgrass with aqueous ammonia and/or enzymes before AD is economically advantageous.This article is from Transactions of the ASABE, 53, no. 6 (2010): 1921–1927.</p

Gas Storage Capabilities and Structure of Nanoporous Carbon

Networks of fractal nanopores in activated carbon have recently been discovered (Pfeifer et al., Phys. Rev. Lett. 88, 115502 (2002). These networks have shown promise in the storage of methane and hydrogen for use as alternative fuels. Our group produces activated carbon made from Missouri corn cob, figure 1. Analysis of the pore structure of these carbons is required in order to optimize the storage capabilities. A pore width of 1.1 nanometers is ideal for methane storage. Other properties, such as surface area, are also studied for use in the development of larger storage capacities