Policy Analysis pubs.acs.org/est
Cradle-to-Gate Life Cycle Assessment for a Cradle-to-Cradle Cycle: Biogas-to-Bioplastic (and Back) Katherine H. Rostkowski,* Craig S. Criddle, and Michael D. Lepech Civil and Environmental Engineering, Stanford University, Stanford, CA ABSTRACT: At present, most synthetic organic materials are produced from fossil carbon feedstock that is regenerated over time scales of millions of years. Biobased alternatives can be rapidly renewed in cradle-to-cradle cycles (1−10 years). Such materials extend landfill life and decrease undesirable impacts due to material persistence. This work develops a LCA for synthesis of polyhydroxybutyrate (PHB) from methane with subsequent biodegradation of PHB back to biogas (40−70% methane, 30−60% carbon dioxide). The parameters for this cradle-to-cradle cycle for PHB production are developed and used as the basis for a cradle-togate LCA. PHB production from biogas methane is shown to be preferable to its production from cultivated feedstock due to the energy and land required for the feedstock cultivation and fermentation. For the PHB-methane cycle, the major challenges are PHB recovery and demands for energy. Some or all of the energy requirements can be satisfied using renewable energy, such as a portion of the collected biogas methane. Oxidation of 18−26% of the methane in a biogas stream can meet the energy demands for aeration and agitation, and recovery of PHB synthesized from the remaining 74−82%. Effective coupling of waste-to-energy technologies could thus conceivably enable PHB production without imported carbon and energy.
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received increasing attention since the 1980s.13 Under growth limiting conditions, many microorganisms produce PHAs as intracellular storage granules made of one or more PHA polymers.8,14 Over 100 PHA molecules have been identified,15 containing long polyester chains, with 100−30,000 repeated monomer units (n). This structure confers thermoplastic properties needed for molding and extrusion, making them suitable replacement options for synthetic plastics in many applications.13,16 Details of the properties of specific PHAs are extensively described elsewhere.8,10,14,17 Different microorganisms can produce different types of PHAs, and the nature of the carbon feedstock affects the type of PHA produced.10,15,16,18 The PHA is stored within the cell as cytoplasmic granules 0.3− 1.0 μm in diameter19 (Figure 1). When the stored PHA is needed to meet carbon or energy requirements, it is degraded to acetyl-CoA, a key metabolic intermediate.20 In soil, sludge, and seawater, PHA resins degrade rapidly,21 with aerobic mineralization to carbon dioxide and anaerobic biodegradation to biogas.19 Both PHAs and biocomposites containing PHAs rapidly degrade to biogas in methanogenic bioreactors.22 The market for PHAs has expanded from initial applications in packaging23 to industrial and agricultural applications24 to medical applications, where they are now marketed at lowvolume and high-price as premium biocompatible bioplastics.6a Further expansion of the market is limited by production cost.8,10 Major factors affecting production cost are the use of cultivated feedstock, such as corn and sugar cane, with the
INTRODUCTION Globally, more than 140 million tons of plastics are produced each year.1 The benefits of these materials have come at a significant cost. Including both material and energy inputs for plastic production, conventional plastics account for nearly 10% of the oil and gas produced and imported in the United States.1 This market is expected to grow at a rate of 15% per year.1 Plastic production facilities consume approximately 270 million tons of oil and gas annually worldwide to supply power and raw materials,2 resulting in high greenhouse gases emissions.3 Franklin Associates quantified the global warming potential for several plastic resins, with values ranging from 1.477 kg CO2 equiv/kg resin for the production of low-density polyethylene (LDPE) to 3.149 kg CO2 eq/kg resin for the production of acrylonitrile butadiene styrene (ABS).4 Most plastic products are recalcitrant and accumulate in landfills5 and the marine environment.6 In 2010, only 8% of the 31 million tons of the plastic discarded in the United States was recovered and recycled.7 Meanwhile, the estimated accumulation rate of plastics in 1996 was 25 million tons per year.8 Municipal solid waste (MSW) contains plastics ranging from 13.2 to 15.8% by wet mass.9 Plastics occupy about 20% of landfill volume 10 and can persist for over 2,000 years.1 Other concerns include in-use leaching of potentially harmful additives,6a such as bisphenol A (BPA) and phthalates,11 and release of unwanted residues during incineration (e.g., dioxins, sulfur oxides, hydrogen chloride, cadmium, lead, zinc, and arsenic).6a,12 One way to address the challenges that arise from the widespread use of synthetic plastics, without compromising convenience and disposability, would be to replace them with functionally equivalent materials that are biodegradable and biocompatible, such as polyhydroxyalkanoates (PHAs). Such materials have © 2012 American Chemical Society
Received: Revised: Accepted: Published: 9822
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provide market incentives for more efficient capture of such emissions. While energy production is currently a market for biogas, biogas often contains trace contaminants including hydrogen sulfide, halides, and siloxanes that can damage combustion engines and result in expensive repairs and service interruptions.32 Current biogas production facilities could theoretically sustain PHB production levels of approximately 27−41 Tg/yr, assuming theoretical yields of 0.45−0.67 g PHB/ g methane33 and a 0.5% fugitive loss.34 These values suggest that PHB from existing landfills and anaerobic digesters could theoretically replace 20−30% of the total plastics annual market.1 In step 1 of Figure 1, methanotrophic bacteria are used to produce PHB. Methanotrophs are the major terrestrial sink for methane, obtaining both energy and carbon from it,35 and are a subset of the methylotrophs, bacteria that metabolize onecarbon compounds.35b,36 Large and active methanotrophic populations naturally assemble when methane and oxygen are simultaneously present.36a,37 As early as 1970, researchers discovered that some methanotrophs could make PHB under nutrient-limiting conditions.38 The subset that can produce PHB are known as the “type II methanotrophs”.39,40 When diverse type II methanotrophs were screened for PHB production, levels of PHB produced ranged from 9 to 44% by dry mass.39 Others have reported levels of 51%40a,41 and 52%,42 under optimized conditions. After PHB is extracted from cells and purified (step 2), the resulting Bioplastic resin can be used to make a wide range of products (step 3). At end of life, these products are ideally either recycled directly or returned to a controlled anaerobic environment, such as a landfill with efficient biogas capture or an anaerobic digestion facility (step 4). In such environments, waste products containing PHB are broken down and the resulting biogas becomes feedstock for PHB production (step 1). The PHB can be of high molecular weight (>1 million Da), enhancing its value compared to PHB from other sources.41 Use of biogas methane for PHB production is thus an example of waste valorization,43 and results in environmental benefits from the greater goal of “loop closing”.44 Opportunities for industrial symbiosis become apparent.45 For example, a landfill or wastewater treatment plant (or both, since they are often located in close proximity) could be colocated with a PHB production facility, allowing use of a continuous and stable supply of biogas methane, or treated wastewater effluent for cooling. Several studies have evaluated cradle-to-gate processes for the production of PHB,12,46 but most only consider energy
Figure 1. Cradle-to-cradle feedstock cycle for PHB and biogas methane.
associated water, chemicals, and energy required for growth, harvesting, transport, and feedstock processing, and the energy and chemical costs of PHA synthesis, extraction, and purification.6a 25 According to one report, the cost of cultivated PHA feedstock accounts for 40−50% of total production costs.26 The volatile and increasing price of crude oil27 and increasing awareness of the adverse environmental impacts of petrochemical-based plastics have revitalized research in PHA production6a and the potential use of organic waste as a feedstock opens the door to new methods of production that do not rely upon cultivated feedstock and imported energy. As shown in the feedstock life cycle of Figure 1, synthesis of PHB from methane provides a clear path whereby carbon from methane, a potent greenhouse gas, can be sequestered in a valuable product using design for environment principles.28 The cradle-to-cradle feedstock cycle for PHB and biogas methane (Figure 1) takes advantage of the abundant biogas (typically consisting of 40−70% methane and 30−60% carbon dioxide) that is often flared or allowed to escape to the atmosphere by the waste sector, the third largest contributor to global emissions of noncarbon dioxide greenhouse gases (accounting for 15% of these emissions).29 Methane (CH4) is a greenhouse gas with a global warming potential (GWP) of 25 over a 100-year period.30 The two largest sources within the waste sector are solid waste landfills and wastewater treatment plants.29 The US EPA estimated global methane emissions for 2005 at ∼36 Tg from landfills and ∼26 Tg from wastewater treatment facilities,29 and these emissions are expected to double by 2030.31 Demand for biogas as a feedstock could Table 1. Life Cycle Stages for Petrochemical Plastics and PHB
life cycle stage description life cycle stage raw material acquisition material processing manufacture and assembly use and service retirement and recovery treatment and disposal
petrochemical plastics (e.g., polypropylene) fossil fuel feedstock (oil and natural gas) extraction cracking polymerization
PHAs from cultivated feedstock
PHB from waste methane
feedstock cultivation (e.g., corn, sugar cane) renewable feedstock acquired from and fermentation (e.g., corn to glucose) waste (e.g., landfill waste methane) microbial synthesis of PHB granules and recovery and purification of granules
extrusion, injection molding, or stretch blow molding filling (if applicable), retail, and use reuse, collection for remanufacture, closed-loop recycling, open-loop recycling, or landfilling/incineration landfilling/incineration (persistent)
reuse, collection for landfilling/incineration
reuse, collection for reconversion into bioplastic PHB biodegradation in a landfill, anaerobic bioreactor, or composting facility
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Figure 2. Process flow diagram (PFD) of LCA system.
Figure 3. Methane−PHB cycle.
We have used the findings to identify research areas that will be critical for industrial scale production of PHB generally and, more specifically, for the use of waste biogas as a feedstock. Goal and Scope Definition. The goal in this study was to anticipate the environmental impacts of PHB production from waste biogas by extrapolation from laboratory scale studies.39,40,40d,e,41,42,48 LCA is used as an early stage design tool49 to identify opportunities for pollution prevention, reduce resource consumption,49 guide environmental performance improvements,50 and identify research needs. The LCA also enables comparison with published LCAs for PHB produced from other feedstocks. This study considers 9 environmental impact categories using the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) 2.0 V 3.01 impact assessment method51 developed by the U.S. Environmental Protection Agency. This tool was designed specifically for the US using input parameters consistent with US locations. It is a midpoint oriented LCIA method including the following impact categories: global warming, acidification, carcinogenics, noncarcinogenics, respiratory effects, eutrophication, ozone depletion, ecotoxicity, and smog. Using SimaPro software, the study considers Cradle-to-resin production of PHB from waste biogas. Cradle-to-resin production is used as a boundary to
requirements and global warming impacts rather than undertaking a more thorough environmental impact assessment. In addition, the results show high variability.2,12,46a,b,e,47 Literature evaluating the use of corn as a feedstock ranges in energy requirements from 2.546e to 81.0 MJ/kg PHB.2 Often, data is not available or is provided by industry and may be inaccurate or biased. There have been no studies evaluating PHB beyond the resin phase and no studies considering the use of waste methane as a feedstock for PHB production. Because no full life cycle assessment exists, the full environmental benefits of PHBs are unknown.12 There are several ways in which the PHB production method from waste biogas differs from the production of commercially available plastics and bioplastics. Table 1 summarizes these stages, raw material acquisition, material processing, retirement and recovery, and treatment and disposal, for polypropylene produced from fossil fuels, PHA produced from cultivated feedstock, and PHB produced from biogas methane.
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METHODOLOGY
This life cycle assessment (LCA) evaluates the production of PHB by methanotrophs from waste biogas. The analysis consists of four components: goal and scope definition, inventory analysis, impact assessment, and interpretation.34 9824
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Table 2. Parameterization of PHB Production from Methane parameter
value
reference
% PHB achieved yield of PHB on methane growth yield oxygen requirement nitrogen requirement PHB recovery by extraction methane recovery from biomass waste methane recovery from carbohydrate organic waste methane recovery from PHB
50% (0.5 g PHB/g total mass) 0.55 g PHB/g methane 0.345 g biomass/g methane 4 g oxygen/g methane 0.12 g nitrogen/g biomass 90% 0.32 g methane/g biomass 0.27 g methane/g organic waste 0.38 g methane/g PHB
measured value52 lumped average of measured and theoretical values33,40a,53 average value54 thermodynamic estimate typical 12% N in microbial biomass. reported value13,55 calculated from the empirical formula for biomass waste56 calculated from the empirical formula for carbohydrate organic waste56 calculated from the empirical formula for PHB56
Table 3. Life cycle inventory of LCA system flow to next process
inputs methane oxygen water chemical energy
5.26 21.04 224.44 0.43 57.60
kg kg L kg MJ
cell culture containing PHB
222.22
L
new solvent recovered solvent ethanol energy
4.89 40.00 448.89 6.13
L L L MJ
cell culture containing PHB surfactant hypochlorite energy
1.00 0.62 25.92 1.45
L kg kg MJ
cell culture containing PHB acid base hypochlorite water
0.00 0.25 0.21 2.96 30.46
L kg kg kg kg
energy
1.47
MJ
excess cell material
1.22
kg
PHB production cell culture containing PHB cell material PHB
PHB recovery: all methods PHB excess cell material solvent extraction
outputs 222.22 1.11 1.11
L kg kg
methane losses oxygen losses water losses
0.03 0.11 2.24
kg kg kg
1.00 1.22
kg kg
wastewater
219.44
L
waste solvent waste ethanol solvent losses ethanol losses
4.89 444.44 0.45 4.49
L L L L
waste surfactant waste hypochlorite surfactant losses hypochlorite losses
0.61 25.66 0.01 0.26
kg kg kg kg
waste acid waste base waste hypochlorite hypochlorite losses acid losses base losses hypochlorite losses water losses
0.25 0.21 2.96 0.00 0.00 0.00 0.03 0.30
kg kg kg kg kg kg kg L
carbon dioxide
0.64
kg
PHB recovery: surfactant digestion
PHB recovery: selective dissolution
excess cell material use: all options excess cell material use: combustion energy 20.21 excess cell material use: anaerobic degradation methane 0.39
MJ kg
highlighting the processes and flow of materials for PHB production from methane (also shown in Figures 1 and 2). Figure 3 illustrates the methane−PHB cycle for the production of 1.0 g of PHB. Table 2 defines the parametrization of PHB production from methane, the values that were used, and the associated references. To produce 1.0 g PHB, ∼5.2 g of methane (this value is conservative and would be
