The Greenhouse Gas Emissions and Fossil Energy Requirement of

Aug 16, 2008 - Nicola Frison , Evina Katsou , Simos Malamis , Adrian Oehmen , and Francesco Fatone. Environmental Science & Technology 2015 49 (18), ...
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Environ. Sci. Technol. 2008, 42, 6961–6966

The Greenhouse Gas Emissions and Fossil Energy Requirement of Bioplastics from Cradle to Gate of a Biomass Refinery JIAN YU* AND LILIAN X. L. CHEN Hawaii Natural Energy Institute, University of Hawaii, 1680 East-West Road, POST104, Honolulu, Hawaii 96822

Received December 21, 2007. Revised manuscript received June 13, 2008. Accepted July 8, 2008.

Polyhydroxyalkanoates (PHA) are promising eco-friendly bioplastics that can be produced from cellulosic ethanol biorefineries as value-added coproducts. A cradle-to-factorygate life cycle assessment is performed with two important categories: the greenhouse gas (GHG) emissions and fossil energy requirement per kg of bioplastics produced. The analysis indicates that PHA bioplastics contribute clearly to the goal of mitigating GHG emissions with only 0.49 kg CO2-e being emitted from production of 1 kg of resin. Compared with 2-3 kg CO2-e of petrochemical counterparts, it is about 80% reduction of the global warming potential. The fossil energy requirement per kg of bioplastics is 44 MJ, lower than those of petrochemical counterparts (78-88 MJ/kg resin). About 62% of fossil energy is used for processing utilities and wastewater treatment, and the rest is required for raw materials in different life cycle stages.

1. Introduction With increased concerns on global warming and peak oil, biobased fuels, chemicals, and materials derived from renewable resources have attracted great interest. Cellulosic biomass such as corn stover is a renewable inexpensive feedstock for biobased products (1). Compared to corn or other grains, cellulosic biomass contains much less glucan, and more hemicellulose, lignin, and other plant components. A substantial amount of plant components is left in the process water of cellulosic ethanol production, posing a high treatment cost or environmental liability to ethanol biorefineries (2). In moving toward biobased manufacturing, a concept of biorefinery, analogous to petroleum refinery, has emerged, where processing facilities based on different technologies are used to convert biomass into various products (3). By producing multiple products, advanced biorefineries can take advantage of the natural complexity and differences in biomass components and maximize the value derived from the feedstock. Polyhydroxyalkanoates (PHAs) are potential value-added coproducts for advanced biomass refining (4). PHAs are natural polyesters synthesized from carbonaceous substrates by some bacterial species as carbon and energy reserve under growth-limited conditions. The biopolyesters exhibit a broad range of mechanical properties from rigid plastics to ductile elastics (5). PHA production was commercialized in the early 1990s via bacterial fermentation of glucose and organic acid * Corresponding author tel: (808) 956-5873; fax: (808) 956-2336; e-mail: [email protected]. 10.1021/es7032235 CCC: $40.75

Published on Web 08/16/2008

 2008 American Chemical Society

(6). Early cradle-to-factory-gate studies, however, indicated that PHA production based on microbial fermentation of corn sugar was not sustainable because more fossil energy was required for PHA biopolymers than for petrochemical counterparts (7, 8). The high energy requirement is attributed not only to the bioprocess itself, but also to a large amount of energy and materials consumed in corn farming and wet milling. It is interesting to note that recent life cycle assessments show positive impacts of PHA bioplastics on the environment such as less greenhouse gas (GHG) emissions and lower total fossil energy requirement than those of petrochemical counterparts (9, 10). The contradictory conclusions might arise from different inventory analysis, feedstocks, and technical assumptions in those studies. Coproduction of PHA bioplastics in cellulosic ethanol biorefinery is a new technology that needs careful examination of its environmental impacts.

2. Methodology and Technologies 2.1. Life Cycle Assessment. Life cycle assessment (LCA) is a standardized method to quantify environmental impacts of various products (11). In this study, two categories of increasing importance today are analyzed: fossil energy consumption and greenhouse gas (GHG) emissions. The use of fossil energy resources is an important component of sustainability as global resources are limited. An equally important problem with the use of fossil energy is the huge translocation of carbon from underground reservoir into the atmosphere accompanied by emissions of nitrogen oxides, hydrocarbons, and heavy metals. Carbon dioxide emissions represent about 84% of total GHG emissions in the United States, and 98% of carbon dioxide is emitted as a result of fossil fuel combustion (12, 13). The anthropogenic greenhouse gases are widely understood to drive global warming. Cradle-to-factory-gate analysis is a popular life cycle assessment for polymer production (7-10, 14). The preliminary focus on polymer resins has the advantage that the analysis provides the first impression about the environmental advantages or disadvantages of the materials. Emission factors are useful tools for the analysis, which avoids the need for detailed calculations of emissions. An emission factor is a quantity of GHG released to the atmosphere from production of 1 kg of product. Methane and N2O are two greenhouse gases that have higher impact on climate change per molecule released than does CO2 (15). Their emissions are often counted as equivalent CO2 emission (CO2-e) per kg of product. Corn stover is chosen as the target biomass because of quantitative information available for inventory analysis. As shown in Figure 1, the system of this study includes bioplastic production and wastewater treatment. It analyzes the GHG emissions and fossil energy requirements within the system as well as those associated with the material inputs that involve broad activities in farming, transportation, and manufacturing industries. 2.2. Biomass Refining for Ethanol. This analysis uses the data of a simulated ethanol plant that are based on laboratory and pilot-plant results (16). The model plant uses cocurrent dilute acid prehydrolysis and enzymatic saccharification to make sugars from corn stover, followed by ethanol fermentation and distillation. Under the optimal conditions, one gallon of ethanol generates about 12 gallons of stillage that contains about 4% wt of soluble organic residues and 6% wt of insoluble solids. The insoluble solids, mostly lignin, are conveniently recovered and burned to produce steam and electricity. The content of soluble solids in aqueous solution is further increased to about 26 wt % via VOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic inputs, outputs, and system boundary of PHA LCA for a mini-biorefinery of corn stover. The annual output of stover is 105,000 MT, and 63,000 MT is actually removed and transported to a biorefinery. The rest (42,000 MT) is left on the field for soil conditioning. multieffect evaporations to yield black syrup (BS). The syrup contains 4.4% wt of acetic acid, 9.2% wt of soluble sugars, and other organic byproducts and biomass residues (16). Currently, it has no commercial value and is disposed of via combustion. Along with 5 million gallons of fuel ethanol being produced, the biorefinery generates about 26,000 t of black syrup, which is used as the feedstock for bioplastics production in this study (Figure 1). It is assumed that the organic carbons in black syrup left or derived from stover have the same GHG emissions and fossil energy requirements of raw biomass. 2.3. PHA Production Technology. Many organic compounds in black syrup such as acetate, furfural, phenols, and glucose can be directly utilized by bacteria such as Ralstonia eutropha, to produce PHA biopolymers (4). The microbial cells may not utilize other carbonaceous residues such as xylose and oligosaccharides. Pretreatment under anaerobic conditions could effectively convert the black syrup solids into the substrates for PHA biosynthesis (17). The overall stoichiometric synthesis of PHA (CH1.5O0.5) and cell growth (CH1.8O0.5N0.2) from the mixed substrates (CH2O0.7) is CH2O0.7 + 0.654O2 + 0.027NH3 f 0.316CH1.5O0.5 + 0.133 CH1.8O0.5N0.2 + 0.551CO2 + 0.683H2O Figure 2 is a process flow sheet consisting of seed preparation, black syrup pretreatment, aerobic PHA fermentation, and PHA recovery (18). Process operation is simulated with SuperPro Designer (V4.9 Intelligen Inc., Scotch Plains, NJ) to give rigorous material balance and energy consumption. Table 1 is a list of process conditions for BS pretreatment and PHA biosynthesis. In addition to black syrup, other raw materials and utilities are also required. Their emission factors and fossil energy consumption are analyzed as follows. 6962

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3. Inventory Analysis and Emission Factors 3.1. Farming Inputs and Outputs. The energy consumption in corn farming is based on the 1996 Agricultural Resource Management Survey (ARMS), which included different fossil fuels, electricity, fertilizers, and other chemicals for grain output (19). The average corn yield of nine states in this survey was 7,889 kg/ha and the average total energy use for farming was 2.37 MJ/kg corn. By using the carbon emission coefficients of fossil fuels (12), the CO2 emission for sole grain output is 0.15 kg CO2/kg corn including those from seed, fertilizer, and chemicals (9). The estimation accounts only for the anthropogenic emissions, not including the carbon flux in the natural carbon cycle such as CO2 release from organic matter in soil. In the ARMS study, corn stover was considered to have no commercial value and left on the field to prevent soil erosion. With a proportion of stover being harvested as feedstock, the impacts associated with farming should be redistributed to all products. In a partitioning approach, the environmental burdens are distributed between the grain and stover based on their respective dry mass removed from the field. Since good farming practice requires at least 40% of stover left on the field, only 60% of stover is removed (20). Taking a ratio of grain to stover (55/ 45 dry mass) (21), the CO2 emission and fossil energy requirement per kg of stover removed are 0.1 kg and 1.59 MJ, respectively (Table 2.) The amount of CO2 assembled by plant growth is estimated from a representative carbon content of stover (40 wt % C), or -1.47 kg CO2/kg stover (22). The negative carbon emission is a net result of plant growth because the organic carbon of biomass is converted from CO2 via photosynthesis. A major source of nitrous oxide emissions is the microbial transformation of nitrogen fertilizer. In the ARS study, an average of 0.018 kg of fertilizer nitrogen was applied for 1 kg of corn output, or 0.012 kg N/kg stover removed. The current IPCC method assumes

FIGURE 2. Schematic process flow sheet of PHA production on black syrup. The medium sterilization is not shown.

TABLE 1. Process Conditions for Conversion of Black Syrup Solids into PHA Polyesters in Two Stages pretreatment: BS solids f substrates temperature (°C) duration (hours) aeration (vvm) mechanical agitation (kW/m3) substrates/BS solids (w/w) cell growth/BS solids (w/w) CO2 emission/BS solids (w/w)

PHA biosynthesis: substrates f PHA 35-40 80 0 0.2 0.8 0.12 0.08

TABLE 2. GHG Emissions and Fossil Energy Consumption for Corn Stover Production and Transportation outputs and inputs

CO2-e kg/kg product

MJ/kg product

grain only grain and stover CO2 fixation in stovera N2O emissionb stover transportation stover subtotal

0.15 0.10 -1.47 0.14 0.01 -1.22

2.37 1.59 0.12 1.71

reference 9, 12, 19 this study this study 15, 19 12

a Based on a stover carbon content of 40% w/w (22). The radiative forcing potential of N2O relative to CO2 is 296 (15).

b

that on average 1.25% of the nitrogen applied is lost as N2O (15), which is equivalent to 0.14 kg CO2 /kg stover output (Table 2). The stover containing 20% moisture is transported from farms to the biorefinery, for instance, by a typical class 8b tractor/trailer combination with a maximum load of 25 t. Assuming an average fuel economy of 4.8 miles per gallon of diesel (107,202 Btu/gallon) for a return trip of 50 miles one-way, the energy consumption is 0.12 MJ/kg dry biomass

temperature (°C) duration (hours) aeration (vvm) mechanical agitation (kW/m3) PHA/substrates (w/w) cell growth/substrates (w/w) CO2 emission /substrates (w/w) final cell density (g/L) PHA content (% wt)

30-35 40 0.5-1.0 0 0.27 0.38 0.96 75 67

(Table 2). The carbon emission of diesel is equivalent to 0.008 kg CO2/kg dry stover (12). Biomass storage in the biorefinery is not considered in this study. The subtotal GHG emissions and fossil energy consumption of biomass feedstock are -1.22 kg CO2-e and 1.71 MJ per kg of dry stover, respectively (Table 2). 3.2. Chemicals and Auxiliary Materials of Bioprocessing. In addition to the black syrup derived from corn stover, other materials and chemicals are also consumed in bioplastic production. Their emission factors and fossil energy consumptions are listed in Table 3. Glucose is used for cultivation of microbial inoculum of PHA fermentation and produced from corn via wet milling. One kg of glucose has GHG emissions and fossil energy requirement of -0.97 kg CO2-e and 7.5 MJ, respectively, calculated in a LCA study when corn grain is the sole output from farming (9). The results include those of corn farming, i.e., 0.15 kg CO2 and 2.4 MJ per kg of corn. As discussed above, these two values can be reduced by about 33% when a proportion of stover is also harvested as an output. The GHG emissions and fossil energy consumption are therefore -1.02 kg CO2-e and 6.6 MJ per kg of glucose (Table 3). Ammonia is a popular nitrogen source in industrial microbial fermentation. The synthesis of ammonia is an energy-demanding process, with the current fertilizer manuVOL. 42, NO. 18, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3. GHG Emissions and Fossil Energy Consumption for Supplies of Raw Materials (RM) and Production of PHA Bioplastics (PHA) substance

CO2-e (kg/kg RM)

fossil energy (MJ/kg RM)

CO2-e (kg/kg PHA)

fossil energy (MJ/kg PHA)

reference

black syrup glucose ammonia salts (P, K, Mg) sulfuric acid sodium hydroxide hypochlorite process water subtotal

-1.22 -1.02 1.26 0.97 0.46 0.96 0.96 0.0002

1.71 6.6 30 24.8 5 18.4 18.4 0.0038

-5.39 -0.36 0.13 0.04 0.05 0.01 0.08 0.004 -5.44

7.55 2.34 3.14 0.93 0.59 0.28 1.62 0.09 16.54

this study 9 23, 24 13, 26 25, 27 9 9 9

TABLE 4. GHG Emissions and Fossil Energy Consumption of Utilities in Process Sections and Operations sections (operations) inoculum preparation (medium sterilization) (aeration) (bioreactors operation) PHA fermentation (aeration) (other operations) PHA recovery unlisted equipmentc total

GHG emissions

fossil energy

CO2-e kg/kg PHAa 0.17 (0.04) (0.06) (0.07) 0.85 (0.59) (0.26) 0.67 0.04 1.73

MJ/kg PHAb 2.53 (0.49) (1.06) (0.98) 13.87 (9.75) (4.12) 9.42 0.70 26.52

CO2-e kg ) (0.61 kg/kWh)(kWh) + 0.25 (kg/kg)(kg steam). b Fossil energy MJ ) (10 MJ/kWh)(kWh) + (3.44 MJ/kg)(kg steam). c Unlisted equipment powered by 5% of total electrical power of the listed equipment (29). a

facturers typically consuming 25-35 MJ/kg ammonia (23). An average of 30 MJ/kg ammonia is used in this study. CO2 emission arising from fossil fuels dominates the GHG emissions of 1.26 kg CO2-e/kg ammonia with minor contributions from other sources (24). A small amount of salts containing potassium, magnesium, phosphate, and other mineral nutrients are also needed in PHA fermentation. Production of the mineral salts uses technologies similar to those of fertilizers (25). The GHG emissions are approximately 0.97 kg CO2-e per kg of mineral salts (26). Because most GHG emissions in the chemical industry arise from fossil energy consumption, the fossil energy requirement of 24.8 MJ/kg salts is estimated with an overall ratio of energy to GHG emissions (25.6 MJ/kg CO2) based on average performance of chemical industry in the United States (13). Sulfuric acid is a common agent in biomass refining for pH control. It is usually made from an oxidation process based on burning of elemental sulfur (brimstone). Its conversion process is highly exothermic, often resulting in a net export of energy from the more efficient sulfuric acid plants. On industrial average, its GHG emission and fossil energy requirement are 0.46 kg CO2-e and 5 MJ per kg of sulfuric acid, respectively (25, 27). Other chemicals and materials used in the bioprocess include sodium hydroxide, hypochlorite, and process water. According to a study by Akiyama and co-workers (9), the GHG emissions are 0.96, 0.96, and 0.0002 kg CO2-e and fossil energy consumptions 18.4, 18.4, and 0.0038 MJ per kg of materials, respectively. 3.3. Bioprocessing Utilities. Bioprocess is often an energy intensive process, which consumes a substantial amount of electrical power and steam. Although a portion of the energy requirement can be compensated by combustion of residual biomass, the energy credit is not claimed in this study. It is assumed that electrical power is generated from coal combustion and purchased from the grid. It is also assumed that the high temperature steam in bioprocessing is generated 6964

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onsite from natural gas. These assumptions make the results of this assessment comparable with different feedstocks such as glucose and vegetable oil that have little biomass residues for energy credit. According to a recent report (28), 1 kWh of electric power (3.6 MJ) is equivalent to 10 MJ of coal energy. At a thermal efficiency of 80% in steam generator, one kg of steam (152 °C, 2.75 MJ) is equivalent to 3.44 MJ of natural gas energy. The GHG emissions from fuel combustion are 0.61 kg CO2-e /kWh and 0.25 kg CO2-e/kg steam, respectively (9, 28).

4. Results and Discussion 4.1. Raw Materials. Material balance of the PHA process shown in Figure 2 provides the annual inputs of raw materials for 1500 t of bioplastics. By using the emission factors and fossil energy requirement of individual materials, their contributions to GHG emissions and fossil energy requirement for 1 kg of PHA bioplastics are calculated and shown in Table 3. Because of the large amount of CO2 fixed during plant growth and relatively small amount of chemicals used in the process, the GHG emissions from the raw materials are actually negative, -5.44 kg CO2-e /kg PHA. The subtotal fossil energy requirement for raw material supplies is 16.54 MJ/kg PHA, determined mainly by black syrup, ammonia, glucose, and hypochlorite. 4.2. Process Utilities. Process simulation also gives annual consumption of electrical power and steam of individual equipment, which are converted to GHG emissions and fossil energy by using the conversion factors above. As shown in Table 4, the process emits 1.73 kg CO2-e and consumes 26.52 MJ of fossil energy for production of 1 kg PHA. About 62% of fossil energy is spent on microbial fermentation including inoculum preparation and PHA biosynthesis, and the rest is used in the operations of polymer recovery (Figure 2). Aeration of PHA fermentation is the biggest single energy user (9.75 MJ/kg PHA). In addition to the major equipment shown in Figure 2, unlisted equipment such as pumps and fans for auxiliary operation and main-

TABLE 5. GHG Emissions and Energy Consumption of Wastewater Treatment item mineral nutrients (kg) electricity (kWh) respiration CO2 (kg) sludge incineration CO2 (kg)d subtotal

water-based (1000 kg)

PHA-based (1 kg PHA)

0.6 3.24 7.73 3.87 -

GHG (CO2-e kg/kg PHA)

0.012 0.065 0.14 0.08 -

a

0.012 0.039b 0.14 0.08 0.27

fossil energy (MJ/kg PHA) 0.30a 0.65b 0 0 0.95

a 0.97 kg CO2-e and 24.8 MJ per kg of mineral salts (Table 3). b 0.61 g CO2-e and 10 MJ fossil energy per kWh of electrical power. d A typical observed sludge formation is 0.4 kg per kg of BOD removed (30).

tenance are powered by 5% of total electrical energy of the listed equipment (29). 4.3. Wastewater Treatment. After PHA production, the organic content in the residual broth is significantly reduced. Combined with wastewater from PHA decolorization and washing, it is suitable to conventional biological wastewater treatment (30). The sludge formed with removal of biological oxygen demand (BOD) is incinerated with lignin solids in the biorefinery and no energy credit is claimed in this study. The requirements of mineral nutrients, electrical power, CO2 respiration with BOD removal, and CO2 release from sludge incineration in Table 5 are based on average industrial performance (16, 30). With the amount of wastewater generated from bioprocess (Figure 2), the GHG emissions (0.27 kg CO2-e) and fossil energy consumption (0.95 MJ) per kg of bioplastics are calculated as shown in Table 5. 4.4. Overall Results and Sensitivity Analysis. In addition to the GHG emissions above, a large amount of CO2 is released from microbial respiration, particularly under aerobic conditions. PHAs are polyesters of high energy content and the microbial cells need a substantial amount of reducing power (NADPH) and bioenergy (ATP) in PHA biosynthesis (7). Aerobic respiration is an efficient way for the cells to obtain energy from carbonaceous substrates. The total release of respiration CO2 is 3.93 kg CO2/kg PHA, about 90% from aerobic PHA biosynthesis and the rest from anaerobic pretreatment (Table 1). The overall GHG emissions from cradle to factory gate are 0.49 kg CO2-e/kg PHA, including the contributions from raw materials (-5.44 kg), cell respiration (3.93 kg), and a combination of bioprocessing and water treatment (2 kg) (Tables 3-5). The total fossil energy requirement is 44 MJ per kg of PHA produced, in which the process utilities and water treatment account for 62.4% (27.47 MJ), and the rest is for raw materials (16.54 MJ) (Tables 3-5). The results of LCA analysis depend on data quality of the impact categories that are inventoried (11). In this study, industrial averages are used for assessment of general performance of biorefineries. High deviations of 10-20% from the averages, however, are observed. For example, in the ARS study, corn farming in Nebraska consumed the largest amount of fossil energy, about 19% more than the average (19). This would cause a 2% increase in GHG emissions and 9% increase in fossil energy requirement for the raw materials. Correspondingly, the overall GHG emissions from cradle to factory gate would be increased from 0.49 to 0.58 kg CO2-e/kg PHA, an increase of 18%, and the overall fossil energy consumption would increase from 44 to 45.4 MJ/kg PHA, an increase of 3%. The significant percentage change of the overall GHG emission is attributed to its small value. 4.5. Comparison with Petrochemical Plastics. The GHG emissions and fossil energy requirements of petrochemical plastics are obtained from a study in Europe and compared with those of bioplastics in Figure 3 (9, 14, 31). The three PHA polymers in Figure 3 are produced from different carbon sources: PHA-G on glucose, PHA-O on vegetable oil (9), and PHA-BS on black syrup in this study. The GHG emissions from production of 1 kg PHA bioplastics range from 0.25 to

FIGURE 3. Comparison of GHG emissions and fossil energy requirements of representative petroleum- and biobased polymers based on 1 kg of resin produced. Symbols: polystyrene (PS), low-density polyethylene (LDPE), polyethylene terepthalate (PET), polypropylene (PP), polylactide (PLA), polyhydroxyalkanoates on glucose (PHA-G), PHA on oil (PHA-O), and PHA on black syrup (PHA-BS). 0.5 kg CO2-e and are much lower than those of petrochemical counterparts (2-3 kg CO2-e). Polylactide (PLA) is also a biobased polyester, but has relatively high GHG emissions in comparison with PHA bioplastics (14). The low GHG emissions of PHA bioplastics are determined by the feedstock biomass that fixes CO2 from the atmosphere and the cell respiration under aerobic fermentation. The combined emissions from bioprocessing and water treatment are 2 kg CO2-e /kg PHA in this study, close to the levels of polypropylene and polyethylene (Figure 3). The total fossil energy requirement (80-90 MJ/kg resin) of petrochemical plastics is higher than those (44-60 MJ/kg resin) of bioplastics (Figure 3). In production of petroleumbased polymers, a proportion of petroleum such as ethylene is converted into the polymers, and the reserved energy can be estimated with the heat of combustion of polymers (23-48 MJ/kg) (32). It has been argued that this part of fossil energy is not consumed during the production, and may be recovered after material use (8). If this amount of energy is excluded, the consumed fossil energy of commodity plastics, except PET, is less than those of bioplastics as shown in Figure 3. Because biopolymers are converted from CO2, the total fossil energy requirement is consumed directly or indirectly during the conversions in different life stages. The energy of biopolymers, however, can also be recovered like petrochemical counterparts. For a representative PHA, polyhydroxybutyrate (PHB) with heat of combustion of 24.1 MJ/kg (32), the net fossil energy requirement of PHA could be reduced to 19.9 MJ/kg PHA, which is much lower than that of energy efficient polyethylene (34 MJ/kg LDPE).

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(17) (18) (19) (20) (21)

(22)

(23) (24) (25) (26) (27) (28)

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