Energy and Greenhouse Gas Profiles of Polyhydroxybutyrates Derived

Sep 18, 2008 - Most of energy used in the corn wet milling and PHB fermentation and recovery processes is generated in a cogeneration power plant in w...
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Environ. Sci. Technol. 2008, 42, 7690–7695

Energy and Greenhouse Gas Profiles of Polyhydroxybutyrates Derived from Corn Grain: A Life Cycle Perspective SEUNGDO KIM* AND BRUCE E. DALE Department of Chemical Engineering & Materials Science, Michigan State University, East Lansing, Michigan 48824-1226

Received February 11, 2008. Revised manuscript received July 29, 2008. Accepted August 13, 2008.

Polyhydroxybutyrates (PHB) are well-known biopolymers derivedfromsugarsorvegetableoils.Cradle-to-gateenvironmental performance of PHB derived from corn grain is evaluated through life cycle assessment (LCA), particularly nonrenewable energy consumption and greenhouse gas emissions. Sitespecific process information on the corn wet milling and PHB fermentation and recovery processes was obtained from Telles. Most of energy used in the corn wet milling and PHB fermentation and recovery processes is generated in a cogeneration power plant in which corn stover, assumed to be representative of a variety of biomass sources that could be used, is burned to generate electricity and steam. County level agricultural information is used in estimating the environmental burdens associated with both corn grain and corn stover production. Results show that PHB derived from corn grain offers environmental advantages over petroleumderived polymers in terms of nonrenewable energy consumption and greenhouse gas emissions. Furthermore, PHB provides greenhouse gas credits, and thus PHB use reduces greenhouse gas emissions compared to petroleum-derived polymers. Corn cultivation is one of the environmentally sensitive areas in the PHB production system. More sustainable practices in corn cultivation (e.g., using no-tillage and winter cover crops) could reduce the environmental impacts of PHB by up to 72%.

Introduction Worldwide production of plastics was about 230 million metric tons in 2005 (1). Most plastics are derived from petroleum. Plastics in the municipal solid waste stream in the United States in 2005 accounted for 11.8% of municipal solid waste (about 26.2 million tones), and the plastics recycling rate was about 5.7% (2), which is quite small compared to total municipal solid waste (recycling rate 32.1%). Thus both environmental concerns and high crude oil prices increase the demand for more sustainable materials, such as biopolymers. Biopolymers are biodegradable and are made from renewable feedstock. Polyhydroxybutyrates (PHBs), well-known biobased thermoplastic polyesters, are produced via bacterial fermentation of sugar or vegetable oil. Its performance characteristics are similar to those of polypropylene (3). PHB is used in a wide * Corresponding author phone: (517) 355-4621; fax: (517) 4321105; e-mail: [email protected]. 7690

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range of applications: from packaging materials, appliances, automotive components to adhesives. Thus PHB can replace various conventional petroleum-derived plastics. For example, recently, gift cards made from PHB have become available (4), resulting in replacing polyvinyl chloride acetate. Currently, Telles commercializes the production of PHB polymers as Mirel natural plastics (4). Several studies implement life cycle assessment (LCA) to estimate energy consumption, greenhouse gas emissions, and other environmental impacts associated with the biopolymer production system and to compare its environmental performance to that of petroleum-derived polymers (5-14). Most studies show that biopolymers have environmental advantages over conventional petroleum-derived polymers, whereas some of the older studies draw contrary conclusions (5, 6). As biopolymer production technologies have matured, their environmental performance has improved. Process information for the PHB fermentation and recovery processes in all of the previous LCA studies on PHB is from simulations based on laboratory-scale or pilot-scale data. This study estimates, through LCA, cradle-to-gate nonrenewable energy consumption and greenhouse gas emissions associated with corn-starch-based PHB to be produced in an actual PHB facility located in Iowa. Site-specific process information on the corn wet milling and PHB fermentation, and recovery processes is used in the analysis. Corn farming sites are specified to estimate county level soil organic carbon and nitrogen dynamics. Greenhouse gas emissions associated with the composting process and application of compost are also discussed.

Methodology Goal. The goal of this study is to estimate the cradle-to-gate environmental performance of PHB derived from corn grain produced in specific counties, particularly nonrenewable energy consumption and greenhouse gas emissions. Scope Definition. The functional unit in this study is one kg of PHB. The system boundary includes processes from agricultural production through the PHB fermentation and recovery process: corn cultivation, transportation of biomass from corn farm to wet milling, corn wet milling, PHB fermentation and recovery process, and upstream processes (e.g., fertilizers, agrochemicals, fuel, chemicals, etc.). The use phase of PHB and its waste management phase are not included in the system boundary because of the wide variety of PHB applications. However, greenhouse gas emissions associated with a composting scenario are discussed. Corn farming counties are specified to estimate county level environmental burdens associated with corn production. Four counties in Iowa (i.e., Boone, Cedar, Clinton, and Jones counties) supply corn grain to corn wet milling and PHB fermentation and recovery processes, which are located in Clinton County, Iowa. The corn grain supply rate of each county is provided by Telles. County level agricultural information (e.g., corn yield, tillage practices, soil properties, climate information, etc.) is used in estimating the environmental burdens associated with corn production (15-19). For application rates of fertilizers, agrochemicals, and fuels consumed during corn cultivation, state level information is used because of lack of county level information (15, 20). County level soil organic carbon and nitrogen dynamics are predicted by the DAYCENT model, a daily time step version of the CENTURY model (21-23). The DAYCENT model simulations predict corn yield, soil organic carbon sequestration rate, and nitrous oxide emissions from soil. 10.1021/es8004199 CCC: $40.75

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Current tillage practices are applied to the simulations of corn cultivation. Site-specific process data on the corn wet milling and PHB fermentation and recovery processes are obtained from Telles. Most of the energy used in the corn wet milling and PHB fermentation and recovery processes is generated in a cogeneration power plant, in which corn stover is burned to generate electricity and steam. Note that corn stover refers to all of the above ground parts of the corn plant except grain (e.g., cob, leaves, stalks, etc.). Corn stover was used as a representative biomass source recognizing that the cogeneration facility can use a variety of biomass sources. Approximately equal masses of stover and grain are produced. The above three counties, except for Boone County, also supply corn stover to the cogeneration power plant. It is assumed that corn stover is collected only in corn cultivation under no-tillage practice and that only 50% of total corn stover produced is collected in order to maintain soil erosion at a tolerable level (24, 25). Corn stover is assumed to be harvested in a second pass (24, 25). Energy generated from the cogeneration power plant is sufficient in the corn wet milling process so that no off-site energy is required. The PHB fermentation and recovery process consumed a small amount of off-site power, which is purchased using renewable energy credits (RECs). Thus nonrenewable energy is not directly involved in producing glucose, its coproducts and PHB. Life cycle inventory information on chemicals, fuel and other processes is obtained from literature and commercial LCA database (26-37). Transportation of corn from the farm to the corn wet milling plant is divided into two modes: internal transportation and intercounty transportation. The internal transportation is transportation of corn within a county from the farm to a local elevator. Because of lack of information on traveling distance, it is assumed that the shape of the county is a circle and that one and half-times of the radius of a county is the traveling distance for internal transportation. Intercounty transportation is transportation from a local elevator to the corn wet milling plant via rail. Fermentation residues from the PHB fermentation and recovery process are used as fuel in a cogeneration power plant. Thus the PHB fermentation and recovery process is a multioutput process. Allocating the environmental burdens associated with the PHB fermentation and recovery process to PHB and fermentation residues is done by introducing an alternative product system for fermentation residues, the system expansion approach (38). Fermentation residues are assumed to replace coal in a hypothetical cogeneration power plant which is not involved in the PHB production system. The alternative product system for fermentation residues is a coal-fired boiler system. Combustion process for fermentation residues and pollution control systems are included in the system boundary to fulfill an equivalent function. There are two more multioutput processes in the foreground system: corn stover production and the corn wet milling process. The system expansion approach is also used in both multioutput processes. The environmental burdens associated with corn stover are only the incremental effects of harvesting corn stover. The effects include changes in soil organic carbon level, nitrous oxide emissions from soil, yield reduction in the subsequent growing season, and fuel consumption in harvesting corn stover. Corn wet milling produces glucose, corn gluten meal (CGM), corn gluten feed (CGF), and corn germ. CGM and CGF are used as animal feeds, and corn germ is used as a feedstock in producing corn oil. The alternative products for CGM and CGF are corn grain and nitrogen in urea, with their appropriate replacement factors (39). The alternative product system for corn germ is soybeans based on their relative oil production rates. It is assumed that corn germ will replace soybeans produced

in Iowa. The environmental burdens associated with soybean production in Iowa are calculated. A sensitivity analysis on the allocation procedures is conducted to determine the effects of the allocations on the overall results. Monte Carlo simulations are performed to determine the uncertainties of the input parameters (e.g., PHB yield, glucose yield, application rate, etc.) and identify the environmentally sensitive factors. No-tillage and winter cover crop practices are explored in scenario analyses to evaluate the environmental performance of these scenarios. Planting winter cover crops is a practice to protect and improve soil quality and to reduce nutrient losses (40). Winter cover crops are usually planted after harvesting the cash crop and are then killed by herbicides prior to planting the cash crop for the subsequent growing season. Nonrenewable energy consumption is defined as a sum of nonrenewable energy used in the process (i.e., fossil energy, nuclear power and electricity) and feedstock energy. Greenhouse gas emissions (GHG) include carbon dioxide, methane, nitrous oxide (N2O), and other greenhouse gases. Greenhouse gas emissions also include carbon sequestration due to increasing (or decreasing) soil organic carbon and N2O releases from soil during corn cultivation. The carbon content (55.8%) of PHB is taken into account as an environmental credit in greenhouse gas analysis. The 100-year time horizon global warming potentials (41) are used to estimate greenhouse gas emissions.

Results and Discussion Nonrenewable Energy. The PHB production system consumes 2.5 MJ kg-1 of nonrenewable energy. Nonrenewable energy consumption is most sensitive to the corn grain and PHB fermentation and recovery process. Natural gas is the primary fossil energy in the PHB production system. About 70% of nonrenewable energy consumption in corn grain production is associated with fertilizers and agrochemicals. Nitrogen fertilizer accounts for about 44% of the nonrenewable energy consumption in corn grain production, and phosphorus fertilizer is the second largest input, accounting for 18.5%. More than 50% of nonrenewable energy consumption in the PHB fermentation and recovery process is associated with ammonia, which is consumed mostly in fermentation. Sodium hydroxide is the most important chemical contributor to nonrenewable energy consumption in the corn wet milling process. In the system expansion approach, glucose produced in the corn wet milling process offers energy credits because of its coproducts and renewable energy used as process energy. More than 50% of the energy credits of coproducts from the corn wet milling process (referred to as System A in Figure 1) are associated with corn gluten feed. The largest energy credits (∼9.2 MJ kg-1) are associated with the alternative product system for fermentation residues, a coal-fired boiler system. The PHB fermentation and recovery process consumes about 80% of nonrenewable energy associated with corn stover that is used in the cogeneration facility. Greenhouse Gas Emissions. PHB offers greenhouse gas credits, approximately -2.3 kg of CO2 eq. kg-1. Thus using PHB would reduce overall greenhouse gas emissions. The primary reasons for the greenhouse gas credits are renewable feedstock, renewable energy used in the corn wet milling and PHB fermentation and recovery processes, and utilization of fermentation residues as fuel. Carbon dioxide is the primary greenhouse gas, followed by nitrous oxide. The PHB production system produces greenhouse gas credits because of the utilization of fermentation residues as fuel. Corn production releases most of nitrous oxide emissions associated with the PHB production system, VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Nonrenewable energy consumption associated with one kg of PHB [System A, alternative systems for coproducts in the wet milling process; PHB, PHB fermentation and recovery process; System B, alternative systems for fermentation residues in the PHB fermentation and recovery process].

FIGURE 2. Greenhouse gas emissions associated with one kg of PHB [System A, alternative systems for coproducts in the wet milling process; PHB, PHB fermentation and recovery process; System B, alternative systems for fermentation residues in the PHB fermentation and recovery process; C-credit, carbon dioxide credit due to carbon content in PHB]. Corn grain production is the largest greenhouse gas source in the overall PHB production system. This illustrated in Figure 2. Nitrous oxide emissions from soil account for about 43% of total greenhouse gas emissions associated with corn grain production, and about 24% of total greenhouse gas emissions of corn grain are associated with nitrogen fertilizer. Thus nitrogen fertilizer application is the most important factor in greenhouse gas emissions associated with corn grain production because N2O emissions from soil depend strongly on the quantity of nitrogen fertilizer applied. Greenhouse gas emissions associated with fuel used in corn grain production are about 24% of the total greenhouse gas emissions of corn grain production. Like nonrenewable energy consumption, glucose also offers greenhouse gas credits because of the alternative product systems for coproducts in corn wet milling. Sodium hydroxide is the largest greenhouse gas source in the corn wet milling process. Ammonia is the largest greenhouse gas source in the PHB fermentation and recovery process, followed by potassium hydroxide. The utilization of fermentation residues offers 1.4 kg of CO2 eq. kg-1 of greenhouse gas credits. The greenhouse gas emissions of the alternative product system for fermentation residues include greenhouse gas emissions associated with chemicals used in a pollution control facility in a fermentation residue-fired boiler system and greenhouse gas emissions produced from a coal-fired boiler system. Carbon dioxide released in a fermentation residue-fired boiler system is not included in the analysis because of its biological origin. 7692

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FIGURE 3. Effects of allocation methods on nonrenewable energy consumption and greenhouse gas emissions associated with one kg of PHB [Baseline, system expansion approach; Mass_stover, output mass allocation between corn grain and stover; Mass_wet, output mass allocation between glucose and its coproducts; Mass_PHB, output mass allocation between PHB and fermentation residues in the PHB fermentation and recovery process]. Increased soil organic carbon levels in corn grain production provide 64.5 g of CO2 eq. kg-1 of greenhouse gas credits because about 50% of corn in these counties is grown under conservation tillage practices. Simulations from the DAYCENT model predict a reduced soil organic carbon sequestration rate due to corn stover removal. The DAYCENT model also predicts that removing corn stover from soil could reduce N2O emissions from soil because fewer nitrogen containing species from corn stover would be converted into N2O emissions. Greenhouse gas emissions associated with the decrease of soil organic carbon levels due to corn stover removal are about 192 g of CO2 eq. (kg of PHB)-1. Overall, the PHB production system reduces soil organic carbon levels by about 40 g of C (kg of PHB)-1 when corn grain grown under current tillage practices is used as feedstock. The value includes the changes of soil organic carbon level in the alternative product systems for coproducts in the corn wet milling process. The effects of no-tillage and winter cover crops on soil organic carbon levels are investigated in scenario analyses given below. PHB versus Petroleum-Derived Polymers. Nonrenewable energy consumption in most petroleum-derived polymers is 69-101 MJ kg-1, and greenhouse gas emissions of petroleum-derived polymers range from 1.9 to 5.4 kg of CO2 eq. kg-1 (42). Therefore, PHB produced in a process and manner as described in this study has better environmental characteristics than conventional petroleum-derived polymers with regard to nonrenewable energy consumption and greenhouse gas emissions. Sensitivity Analysis. The allocation procedure is a key factor affecting LCA results. Three different allocations are involved in foreground system of the PHB production system: (1) allocation in corn stover production (between corn stover and corn grain), (2) allocation in the corn wet milling process (between glucose and its coproducts), and (3) allocation in the PHB fermentation and recovery process (between PHB and fermentation residues). The system expansion approach is used as a baseline method comparing all three allocation approaches. The output mass allocation method is applied to three cases, respectively, to determine the effects of the allocation method in the PHB production system. Results are shown in Figure 3. Changing the system expansion to the output mass allocation in corn stover production causes a 1.3-fold increase in nonrenewable energy consumption and 8% increase in greenhouse gas emissions associated with PHB. In the output mass allocation method,

the incremental effects of harvesting corn stover as well as the environmental burdens associated with corn grain are allocated to corn stover by its output mass fraction. For example, fuel used in harvesting corn grain is allocated to corn stover and corn grain. The output mass allocation in the corn wet milling process lowers nonrenewable energy consumption and greenhouse gas emissions. In the output mass allocation method, the environmental burdens associated with corn grain, wet milling, and corn stover used as fuel in the corn wet milling process are the subjects to be allocated to glucose and its coproducts. The allocation approach chosen for the PHB fermentation and recovery process is the most sensitive parameter for environmental performance. Energy content is not a satisfactory allocation approach because the function delivered by PHB is materials-oriented, whereas the function of the fermentation residues is energy-oriented. Thus, very few common functions between these two products could be defined. Perhaps economic value is a better candidate for an allocation factor rather than output mass. Market value for fermentation residues is not determined because these materials are used internally. Therefore, the system expansion approach is the most reasonable approach to allocate the environmental burdens associated with PHB fermentation and recovery process to PHB and fermentation residues. Although the output mass allocation would be a feasible allocation method in corn stover production and corn wet milling, ISO standards (38) recommend subdivision of the system or system expansion, wherever allocation can be avoided. The sensitivity analyses on the allocation approaches show that the allocation methods do not affect the conclusion that PHB produced in a process and manner as described in this study offers better environmental performance than do petroleum-derived polymers in terms of nonrenewable energy consumption and greenhouse gas emissions. Uncertainty Analysis. Monte Carlo simulation can determine uncertainties associated with the input parameters (e.g., fuel used, application rates of fertilizers, agrochemicals, glucose yield, PHB yield, etc.) and identify the most important environmentally sensitive areas. The Economic Research Service (ERS) of the USDA provides standard deviations of fuel consumption in corn agriculture (20). Diesel, gasoline, and liquefied petroleum gas have standard deviations equal to or less than 10% of given values. Electricity and natural gas have larger deviations; 40 and 53%, respectively. Fuel is assumed to have a log-normal distribution function. Standard deviations for other input parameters (e.g., application rates of fertilizers, agrochemicals, glucose yield, PHB yield, etc.) are not available. It assumed that these parameters have a log-normal distribution function with a standard deviation equal to 10% of the given value. Although corn yield, soil organic carbon sequestration rate and N2O emissions from soil are functions of various variables, these factors are assumed to be only a function of the application rate of nitrogen fertilizer. Several DAYCENT model simulations are run with various application rates of nitrogen fertilizer to estimate functions for corn yield, soil organic carbon sequestration rate, and N2O emissions from soil. Results from the DAYCENT simulations show that corn yield, soil organic carbon sequestration rate, and N2O emissions from soil increase with the application rate of nitrogen fertilizer. But corn yield and soil organic carbon sequestration rate approach a steady state at higher application rates of nitrogen fertilizer. Monte Carlo simulations show that the uncertainties of the input parameters affect nonrenewable energy consumption more than greenhouse gas emissions because the effects of nitrogen fertilizer on greenhouse gas emissions associated with the PHB production system are both benign and adverse.

FIGURE 4. Mean values of nonrenewable energy consumption and greenhouse gas emissions associated with PHB production. Higher corn yield and higher soil organic carbon sequestration rate at higher application rates of nitrogen fertilizer are benign effects on greenhouse gas emissions, but higher N2O emissions from soil at higher nitrogen rates is an adverse effect. Mean nonrenewable energy is 2.6 MJ kg-1 with standard deviation of 1.6 MJ kg-1, ranging from -5.7 to 8.0 MJ kg-1. Mean greenhouse gas emissions are -2.5 kg of CO2 eq. kg-1 with standard deviation of 0.23 kg of CO2 eq. kg-1, ranging from -2.9 to -2.1 kg CO2 eq. kg-1. This is illustrated in Figure 4. The uncertainties associated with the input parameters do not affect the conclusion that PHB derived from corn offers better environmental performance than do petroleum-derived polymers. According to results from analyses for correlation coefficients of input parameters, PHB yield, glucose yield and the alternative system for fermentation residues in PHB fermentation and recovery process are the most environmentally sensitive factors. Ammonia used in the PHB fermentation and recovery process, stover used as an energy source in the PHB fermentation and recovery process, and nitrogen fertilizer in corn culture are also environmentally important factors. PHB and glucose yields are also important economic factors. More efforts to improve PHB and glucose yields, to reduce the amount of ammonia or energy consumed in the PHB fermentation and recovery process and to reduce the environmental impacts associated with nitrogen fertilizer are recommended to develop a more sustainable product system. In scenario analyses, a cropping management practice to reduce the environmental impacts associated with nitrogen fertilizer is also explored below. Scenario Analysis. As mentioned previously, corn grain production is one of the most environmentally sensitive areas in the PHB production system. Different cropping systems on corn cultivation are investigated in the scenario analyses including no-tillage practice and winter cover crop practice combined with no-tillage practice. These two scenarios are believed to improve the environmental performance of corn production. The DAYCENT model predicts that no-tillage practice would increase soil organic carbon levels. The simulations also predict that planting winter cover crops would increase soil organic carbon levels more than would the cropping systems alone without planting a winter cover crop. Adding fresh carbon from winter cover crops to soil increases soil organic carbon level. Planting winter cover crops would also reduce N2O emissions from soil because winter cover crops take up nitrogen from soil during the off season. Winter cover crops also increase corn yield in the subsequent growing season by about 3-17% because of additional nutrients from winter cover crops. The simulation results with DAYCENT VOL. 42, NO. 20, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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for winter cover crops are consistent with results from other studies, including field studies (40, 43-45). Increased soil organic carbon levels in corn grain production provide greenhouse gas credits of 284 g of CO2 eq. kg-1 in no-tillage practice and 384 g of CO2 eq. kg-1 in winter cover crop practice. Thus the PHB product system could increase soil organic carbon levels when corn is grown under no-tillage practice or when winter cover crop practice is applied. Converting the current tillage practice to no-tillage could reduce nonrenewable energy consumption in the PHB production system by about 9% and greenhouse gas emissions of PHB by about 13%. The primary reasons for the reductions due to no-tillage practice are (1) less diesel fuel used in the tillage operations and (2) high soil organic carbon sequestration rate in no-tillage practice. Even though additional fuel and herbicides are consumed in planting and killing winter cover crops, planting winter cover crops could significantly reduce nonrenewable energy consumption in the PHB production system by about 72% and greenhouse gas emissions of PHB by about 36% because of higher corn yield, higher soil organic carbon sequestration rate and lower N2O emissions from soil. No-tillage practice and planting winter cover crops could be attractive options to improve the environmental performance of PHB. Composting. PHB is biodegradable and a product made from PHB could be treated in a composting facility at the end of its life. The waste treatment of PHB is not included in the analysis because of wide variety of waste disposal options. Instead of detailed analyses of the waste treatment options, greenhouse gas emissions associated with a product made from PHB, which goes to a composting facility at the end of life, are estimated. The benefits of organic compost are (1) less nutrient consumption, (2) soil improvement, (3) sequestration of carbon in soil, (4) less water use, etc. (46, 47). Sequestration of carbon in soil is considered as the only benefit of organic compost in this study because the nutrient contents of PHB are irrelevant and other benefits are not well quantified. About 44% of initial carbon in PHB is assumed to be lost during the compost operation (46-50). Methane gas is not usually generated in well-managed compost operations (46). When compost is applied to soil, about 18% of carbon in the compost is sequestered in soil (46). Compost operation and the application of compost made from PHB offer 84 g of CO2 eq. kg-1 of greenhouse gas credits, which includes energy consumption in the composting facility and transportation of compost to land application. The overall greenhouse gas emissions of PHB from cradle to grave are -304 g of CO2 eq. kg-1 when waste PHB is treated in a composting facility. Therefore, PHB still offers greenhouse gas credits when it is disposed through composting.

Discussion PHB derived from corn grain offers environmental advantages over petroleum-derived polymers in terms of nonrenewable energy consumption and greenhouse gas emissions. Furthermore, PHB provides greenhouse gas credits; every kilogram of PHB reduces 2.3 kg of CO2 eq. of greenhouse gas emissions. Corn cultivation is an environmentally sensitive area in the PHB production system. More sustainable practices in corn cultivation (e.g., no-tillage and winter cover crops) could reduce the environmental impacts of PHB up to 72%. The results show that improving process technologies, using cellulosic biomass to provide process energy, and utilizing fermentation residues as fuel further improves the environmental performance of the PHB production system. Other environmental concerns (e.g., eutrophication, acidification, eco-toxicity, human toxicity, etc.) need to be fully 7694

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examined to evaluate the comprehensive environmental performance of PHB polymer.

Acknowledgments The authors gratefully acknowledge support provided by Metabolix, Inc., Archer-Daniels-Midland Company, U.S. Department of Energy, and U.S. Department of Agriculture.

Supporting Information Available Agronomic inputs and fuel consumption in corn production in Iowa, county-level information on corn production, agronomic inputs and fuel consumption in soybean production in Iowa, county-level information on corn soybean production, data sources for upstream processes, system expansion, nonrenewable energy consumption and greenhouse gas emissions, comparisons with other studies, absolute correlation coefficients of input parameters, composting process (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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