Life Cycle Assessments of Ethanol Production via Gas Fermentation

Article pubs.acs.org/IECR

Life Cycle Assessments of Ethanol Production via Gas Fermentation: Anticipated Greenhouse Gas Emissions for Cellulosic and Waste Gas Feedstocks Robert M. Handler,*,† David R. Shonnard,† Evan M. Griffing,‡ Andrea Lai,§ and Ignasi Palou-Rivera§ †

Sustainable Futures Institute, Department of Chemical Engineering, Michigan Technological University, Houghton, Michigan 49931, United States ‡ Environmental Clarity, 2505 Fauquier Lane, Reston, Virginia 20191, United States § LanzaTech, 8045 Lamon Avenue Suite 400, Skokie, Illinois 60077, United States ABSTRACT: LanzaTech has developed novel microbial bioreactor systems capable of direct gas fermentation to produce ethanol from carbon-containing gases. In this study, a life-cycle assessment method is used to quantify the global warming potential of several scenarios for producing renewable ethanol with the LanzaTech process. Scenarios considering ethanol produced from steel mill waste gases or biomass (corn stover, forest residue, or switchgrass, via gasification) have been considered, using input data from peer-reviewed literature, government reports, life cycle inventory databases, and LanzaTech process engineering estimates. Using standardized life-cycle assessment methods, ethanol produced via LanzaTech fermentation appears to result in greenhouse gas emissions that are at least 60% lower than that of conventional fossil gasoline, with biomassbased ethanol achieving close to 90% emission reductions. Results indicate that the LanzaTech gas fermentation technology can be a viable alternative for producing next-generation biofuels that satisfy United States Renewable Fuels Standard policies concerning fuels with a reduced greenhouse gas emissions footprint.

■

INTRODUCTION The United States continues to depend on petroleum for transportation fuels, which accounted for roughly 28% of the country’s energy-related CO2 emissions in 2012.1 Roughly onethird of petroleum fuels used in the United States are imported, warranting a continued focus on domestic energy production as a means to improve domestic economic and energy security.2 Several solutions are being developed to deal with the environmental, economic, and social challenges caused by continued use of polluting fossil transportation fuels from imported and domestic sources. The most widely adopted emissions reduction strategy is the production of alternative liquid transportation fuels (ethanol and biodiesel, among others) made from renewable feedstocks. These fuels are compatible as blendstock with existing transportation infrastructure as minor components of the final fuel mix, generally 10 or 15% for ethanol, and have benefitted from existing agricultural production systems to generate large quantities of feedstock. Ethanol is the predominant alternative liquid transportation fuel. It has successfully been integrated into the national fuel system and is offered in a range of blend ratios with petroleum gasoline. Corn-based ethanol has been touted as a domestic energy success story, with measurable impacts on environmental metrics and rural economies.3,4 While corn-based ethanol currently comprises the large majority of domestic renewable fuel production, scientists and policy makers continue to develop plans for transitioning to a new array of renewable transportation fuels that improve upon the environmental benefits of corn ethanol while reducing concerns about the environmental impact of large-scale biofuels production, © XXXX American Chemical Society

such as competition for land and water resources, especially for irrigated crops, the impact on the price of food; and the resulting decline in genetic diversity, among others.5−8 Biofuels relying on nonfood crops, agricultural residues, or other wastes would alleviate some of these concerns. An increasing portion of the renewable fuels requirement in the United States is due to come from nonstarch sources and qualify as advanced (50% reduction) or cellulosic (60% reduction) biofuels, depending on their level of greenhouse gas (GHG) emissions reductions compared to petroleum sources.9 These “second-generation” biofuels based on agricultural residues promise to lower GHG emissions associated with liquid transportation fuels, but ongoing life cycle studies will be required to ensure that environmental concerns associated with expansion of conventional agriculture are also mitigated.10 One example of an advanced, second-generation biofuel is that produced by LanzaTech. LanzaTech has developed novel fermentation processes to convert carbon monoxide and hydrogen-containing gases into valuable fuel and chemical products, including ethanol, 2,3-butanediol, acetic acid, isopropanol, acetone, butanol, succinic acid, and isoprene. Process inputs can be low-value or waste gases from industries such as steel manufacturing, oil refining, and chemical production, as well as gases generated by gasification of forestry and agricultural residues, municipal waste, natural gas, and coal. Special Issue: Sustainable Manufacturing Received: September 1, 2015 Revised: December 3, 2015 Accepted: December 7, 2015

A

DOI: 10.1021/acs.iecr.5b03215 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 1. LanzaTech process block diagram. Light gray areas indicate potential coproduct unit operations, which are possible but not included in this LCA study.

■

GOAL AND SCOPE The goal of this study is to determine overall greenhouse gas emissions from fuel ethanol production using the LanzaTech process and to compare its environmental burdens to the petroleum-derived gasoline life cycle GHG emissions. Our functional unit for this analysis will be one megajoule (MJ, lower heating value) of final ethanol (EtOH) fuel product, because energy content is a reasonable predictor of performance in engines and this is a commonly used basis of comparison for different fuels. To arrive at a comprehensive GHG emissions assessment of ethanol production by the LanzaTech process, our analysis takes a cradle-to-grave approach. A detailed inventory of processing inputs was developed from LanzaTech’s commercial plant design in collaboration with LanzaTech process engineers. Production of all required inputs, including chemicals and energy, are included in the analysis, in addition to transport of ethanol product prior to use. All emissions of gases from the bioreactor and anaerobic digestion of settled solids are included, assuming that any methane in the emissions is converted to CO2 through flaring. Final combustion of the ethanol product is also included. Scenarios. Four different fuel ethanol production pathways are examined as part of this study. The BOF gas scenario uses carbon-containing BOF gas as the carbon-containing fermentation feed gas. Three biomass-based scenarios are also included, which use corn stover, switchgrass, and forest residue as carbon-containing feedstocks, and are labeled accordingly. In the biomass-based scenarios, solid biomass feedstock materials are converted to syngas via a gasification process, and the syngas is used as the fermentation feed gas. The resulting different fermentation feed gases in each scenario are utilized in the LanzaTech process for the production of ethanol.

In doing so, waste carbon streams are captured and functionally recycled into new, useable products. Fermentation products can be used directly or thermochemically converted to drop-in fuels and chemical products, such as jet fuel, olefins, and other commodity chemical intermediates. These processes have the potential to produce fuels meeting the EPA emissions criteria for next-generation biofuels, while utilizing a diverse range of feedstocks. LanzaTech has successfully demonstrated its gas fermentation technology at a 300 TPA demonstration plant with Baosteel in Shanghai, China, and is currently operating a second demonstration plant with Shougang Steel at Caofeidian, China. These plants can utilize a range of mill gases including basic oxygen furnace (BOF), blast furnace, COREX, and coke oven gas. Both demonstration plants have exceeded nameplate capacity and met performance targets. LanzaTech also owns and operates the Freedom Pines Biorefinery in Soperton, Georgia. The facility has biomass handling and gasification equipment, and pilot-scale testing of biomass-based fuel and chemical production is expected by the end of 2015. Initial life-cycle assessment (LCA) studies on the environmental impact of steel mill waste gas conversion to ethanol in China have been described in the literature,11 indicating the potential for a low-emission fuel product. This article updates data for steel mill gas conversion to ethanol, based on design improvements from LanzaTech’s scale-up work and application of the technology to a domestic United States market, and presents initial LCA results for ethanol produced from biogenic (corn stover, forestry residues, and switchgrass) feedstocks. GHG emissions are tabulated for each stage of the ethanol life cycle and are compared to conventional transportation fuels and other ethanol production pathways. B


Article


percentage is flared, and this percentage is highly variable by region. In the United States, recent surveys indicate that BOF steel mill exhaust gases are not currently utilized by any United States mills, but they may represent a significant opportunity for heat and fuel recovery, offsetting significant portions of the cumulative GHG emissions burden for steelmaking.15 LanzaTech is focusing commercial development of this technology on situations where BOF gas is underutilized because these scenarios present the greatest opportunity for adding value to current steel mill operations by turning this low-value gas stream with variable composition and flow rates into a consistent source of high-value fuels and chemicals. For the purposes of this study, the prior fate of BOF exhaust gas is assumed to be flaring, where all carbon contained in the BOF gas is converted to CO2 and released to the environment. Due to this assumption, the input gas stream appears in the cradleto-gate LCA as a GHG emissions credit in the LanzaTech process, as it is being removed from this prior fate of immediate flaring release to the environment. Quantitative results of this assumption are illustrated in Table 2. Emissions of all carboncontaining gases are counted in the GHG emissions profile of the fuel at every downstream processing stage. In this scenario, the LanzaTech process is envisioned to be an independent plant colocated with an existing steel mill. This colocation eliminates any need for transporting feedstock gas to the LanzaTech bioreactor. The other three scenarios involve the production and use of carbon-containing gases from biomass sources (corn stover, switchgrass, and forest residue). Data surrounding the production of corn stover comes from the GREET model developed by Argonne National Laboratory16 and includes inputs of materials and energy related to fertilizer use, collection, 50 miles of truck transport, and carbon stock changes associated with direct and indirect land-use change. In this case, the GREET model assumes that using corn stover as a feedstock for transportation fuels would lead to small increases in carbon stocks on the landscape (negative CO2 emission, or CO2 credit) as corn-growing land becomes more productive and less land is utilized for corn production.17 Switchgrass production data also comes from the GREET model16 and includes inputs related to cultivation, harvesting, 50 miles of transport, and any land-use change effects. Switchgrass involves larger assumptions of fertilizer use compared to corn stover and also contains a small CO2 emission factor for land-use change as a result of increased switchgrass cultivation. Life-cycle input data for forest biomass has been incorporated from a detailed study of the Consortium for Research on Renewable Industrial Materials (CORRIM) group, which focuses on whole tree thinning operations in the Southeast region of the United States and residue grinding data from the Pacific Northwest.18 Biomass from forest residues is generally produced from thinning operations in managed forests or when nonmerchantable timber (tops and branches) are separated in a commercial harvest. Unlike the agricultural biomass scenarios, no land-use change effects or additions of fertilizer to increase biomass productivity are assumed in this study for forest biomass. Biomass collection, processing, and 90 miles of truck transport are assumed to occur using machinery typical to the increasingly mechanized forest products industry in the specific region. Gasification of biomass streams in all cases was modeled according to parameters outlined in a National Renewable

LanzaTech Process Overview. Key steps in the LanzaTech process are gasification, gas handling, fermentation, and product recovery. A graphical depiction of the LanzaTech process is shown in Figure 1. First, in the case of biomass feedstocks, gasification produces an input gas and excess heat is used to generate steam and electricity. Next, in the gas handling step, carbon monoxide-containing gas streams, such as waste or byproduct gases from industrial processes or gasified biomass residues, are deoxygenated and compressed for use as the primary input to the fermentation. The treated gas is fed to a biological reactor for fermentation in which proprietary microorganisms suspended in a liquid nutrient solution utilize CO as both a carbon and energy source. If present in the input gas blend, H2 can also be utilized by the microorganisms as a supplemental energy source. The microbes secrete fermentation products such as ethanol, 2-3 butanediol, and acetic acid into the surrounding broth. Fermentation broth continuously withdrawn from the bioreactor is sent through a steadfast distillation-based separation system for product and coproduct recovery. The fermentation can be tailored to produce a range of valuable chemical coproducts which, if recovered, processed, and transported to market, would be considered in the overall life cycle as well. For the purposes of this study, the process under analysis is one in which production of ethanol is maximized, with minimal coproduct creation and no coproduct recovery. Waste streams are minimized and recycled internally within the process as much as possible. Biological solids (spent microbial biomass) are filtered out of the distillation waste stream and sent to anaerobic digestion, and a portion of the filtered stream is used to make up fresh media (a water and nutrient mixture) that is fed continuously to the LanzaTech microbes. In this way, process water within the system is recycled back to the fermentation bioreactor. The remaining liquid wastes are treated on-site via anaerobic digestion. The biogas from anaerobic digestion is mixed with a portion of the reactor vent gas to increase the gas energy density, enabling the use of the mixed gas for internal steam or power generation. Vented gas from the fermentation bioreactor is scrubbed, oxidized, and released to the atmosphere with a GHG emissions penalty. Generated utilities are mostly used to offset utility needs; however, in the biomass-based scenarios involving biomass gasification, excess electricity is exported to the grid. Fermentation Gas Generation. The scenario using industrial waste gas (BOF gas scenario) is based on steel mill basic oxygen furnace gas. In 2011, global production of steel was 1.5 billion tons/yr. Of this, 1.1 billion tons were from the blast furnace/BOF route.12 In steel-making, carbon from coal, natural gas, and/or oil is used to reduce iron ore into iron metal according to the following reaction: Fe2O3 + 3C → Fe + 3CO

This reaction takes place in the blast furnace (BF) and produces molten iron or pig iron with high carbon content (typically 3.5−4.5%). This iron is then processed in a basic oxygen furnace. The BOF controls the amount of carbon left in the final steel product by blowing pure oxygen over the hot metal.13 The oxygen reacts with carbon in the pig iron and carries it away as a carbon-rich gas residue that is 50−60% CO, 10−20% CO2, and 20−30% N2. A portion of these gases may be used on-site to generate heat. In Europe, about 25% of all BOF steel mill gases are flared in lieu of being used for heat or power generation.14 On a global basis, a much higher C


Article

Industrial & Engineering Chemistry Research Energy Laboratory study.19 Gasification of biomass results in a stream of hot syngas (CO and H2 gases), which is cooled via heat integration with downstream unit operations that require heat inputs. After cooling, the syngas can be fed into the LanzaTech fermentation system. This syngas stream typically has a composition of 30−35% CO, 40−45% H2, 10% CO2, and other trace gases. Gasification may require a small amount of input natural gas to heat incoming biomass to gasification temperatures as the process begins, but at steady-state the process is self-sustaining, where gasification of complex molecules into smaller molecules liberates enough energy to heat incoming biomass and maintain the process. Gas Fermentation. The LanzaTech process considered here converts carbon-containing input gas into ethanol. The fermentation process can accommodate a range of input gas compositions and is tolerant of typical gas contaminants such as sulfur, which minimizes pretreatment requirements. The LanzaTech microbes utilize CO as both a carbon and energy source. If present in the input gas blend, H2 can be utilized by the microorganisms as a supplemental energy source. In addition to the input carbon as gas, a fermentation media including macro- and micronutrients for the organism is fed into the bioreactor. The LanzaTech process is a continuous fermentation, meaning that media is continuously fed into the bioreactor while fermentation broth (containing ethanol, fermentation coproducts, and spent biomass) is removed at an equal rate. Trace amounts of other coproducts, besides ethanol, are also produced in the fermentation process. The ratio of ethanol to coproduct and identity of the coproducts can be varied substantially by modifications to the process. Product Separation. Product separation proceeds through a distillation process that has been modified to suit specific LanzaTech operating conditions. Distillation energy requirements are in good agreement with standard models of the conventional distillation process in an integrated distillation sequence followed by dehydration, which require 2.0−2.5 tons of steam per ton of ethanol, given the specific titer of the LanzaTech fermentation broth.20−22 Settled solids, composed of biomass from the fermentation organism, and other organics from the fermenter are separated from the product stream and sent to an anaerobic digestion unit. The LanzaTech process considered here does not make any attempt to recover the trace amount of chemical coproduct, although modified product recovery units designed to separate ethanol from representative coproducts such as butanediol have been utilized successfully in separate LanzaTech trials. Product Transport and Use. Ethanol is assumed to be transported 100 km by truck prior to use, based on prevalence of steel mill facilities and gasoline blending terminals in the eastern half of the United States. Ethanol will be used as a blended fuel component, with CO2 emissions on combustion as predicted by stoichiometry (1.91 kg CO2/kg EtOH, 71.4 g CO2eq/MJ EtOH). Utility Consumption. Specific inputs of electricity and steam are withheld here to protect LanzaTech confidentiality, but resulting GHG emissions are aggregated together as “utilities” in the LCA results. Utility inputs for all scenarios are electricity and medium-pressure steam, though specific requirements are slightly different in the BOF gas scenario versus biomass feedstock scenarios. Though utility inputs are important sources of environmental impacts, the LanzaTech process takes advantage of many opportunities for utility

integration within the host site and opportunities to take advantage of internally produced steam and electricity. Within the context of steel mill operations, there will likely be an excess of low-quality steam suitable for the LanzaTech process available onsite, from coke oven gases, blast furnace gases, or BOF gases.23 In the BOF gas scenario, biogas from the anaerobic digestion of bioreactor soluble and solid metabolites and vented fermentation reactor gas are combined and combusted to produce steam, which supplies steam in sufficient quantities to satisfy the needs of ethanol distillation and gas treatment. Required electricity comes from the off-site electricity grid. The mix of generation sources for the United States electricity grid is variable between regions and over time, and for this assessment the default United States grid assumptions from Ecoinvent life cycle inventory database version 2 were used,24 which includes a combination of coal (49%), nuclear (19.6%), natural gas (17.3%), oil (3.3%), and renewables (9.6% including hydropower) as the primary sources of electricity generation. This grid mix results in a conservative electricity generation profile, with emissions that could be improved or worsened based on changing the grid mix to reflect the most current situation in regions of the United States or other countries, as discussed in Results and Discussion. Biomass gasification produces a stream of hot syngas (∼900 °C).19 The sensible heat from this gas stream can generate steam, ∼0.5 tons steam per ton syngas, which can be used directly in industrial processes or used to make electricity. This source of utilities is a significant benefit to the biomass-based scenarios, where all utilities can be generated internally with the productive use of sensible heat, including both steam heat (gasifier, gas treatment, ethanol distillation units) and power (gasifier, fermentation reactor units). The LanzaTech process is currently modeled assuming conversion efficiencies in a combined heat and power (CHP) system of 85% from syngas to steam and 35% from steam to electricity. Of the total amount of steam produced in the biomass-based scenarios from biomass gasification, 30% of the steam is used as process heat, and the rest is used to generate power for internal process use and to export as excess electricity. Exported electricity is assumed to displace the need for electricity from the United States grid and a credit for displaced electricity use is attributed to the process. Roughly 20% of the power that is produced is in excess of internal use and can be exported to the grid in the biomass-based scenarios. Electricity requirements for gas compression are roughly 50% lower in the biomass feedstock cases because biomass syngas is produced at pressure, while the BOF exhaust gas is assumed to enter the system close to atmospheric pressure. In all scenarios, the combustion of biogas produced through anaerobic digestion and bioreactor vent gas is assumed in this study to be used on-site to generate steam and/or electricity, using the same CHP efficiency assumptions as stated previously for biomass gasification-derived steam and electricity. This gasbased generation also results in a small displacement credit for excess electricity in all analyzed scenarios.

■

METHODS AND INVENTORY Input Data. Inputs for the model LanzaTech facility have been tabulated on the basis of 1000 kg of EtOH production, which is then normalized by the ethanol production from the facility and the energy content per unit of ethanol (26.8 MJ/ kg). Input data is summarized below, along with comments on D


electricity, medium voltage, United States Grid, Steam, for chemical processes

utilities (steam and electricity)

BOF gas scenario

E

1.91 × 103 kg

1.91 × 103 kg

CO2 equivalents (CO2eq)

switchgrass scenario

1.91 × 103 kg

54.8 kg C converted to CO2 and CH4; 200.9 kg CO2eq 100 km

1470 kg

1.86 kg

1250 kg

provided to account for stoichiometric requirements of bioreactors 7340 kg

2.15 × 103 kg

2870 kg 2.19 m3

generated internally

forest residue scenario

1.91 × 103 kg


1450 kg

1.86 kg

1250 kg


2.15 × 103 kg

2670 kg 2.03 m3

generated internally

comments

based on stoichiometry

makeup water assuming 1% loss due to evaporation and blowdown makeup water due to reaction losses, media outflow with solids stand-in for mixed-metal oxide sorbent (sulfur removal) stand in for treatment of waste from anaerobic digestion, comparable organic carbon content LanzaTech internal process model to account for anaerobic digestion of settled solids, and power generation from anaerobic digestion transport of EtOH 100 km to blender

Assuming all CO and CH4 converted to CO2 via flaring stand-ins for chloride salts, metal chlorides, P requirement, N requirement, vitamins

Dry weight basis

generated from cooling hot syngas in biomass scenarios; accounting for internal credit from biogas produced through anaerobic digestion

Ecoinvent database24 unless otherwise noted. bRefers to BOF gas or gasified biomass input stream, minus bioreactor vent gas emissions. cNutrient inputs are used to create optimal production in the reactor. These are used to control environmental conditions or used by the microorganism as nutrients.

a

transport of EtOH combustion of Ethanol



anaerobic treatment of solid, liquid waste (biomass, ethanol, etc.), conversion of biogas to CO2 upon combustion; released in anaerobic digestion emissions transport, lorry >32 t, EURO5

1.86 kg 1470 kg

magnetite, at plant

1250 kg

1293 kg

1.86 kg

tap water, at user

fermentation process water sulfatreat

treatment, sewage whey digestion, class 4

2554 kg

water, decarbonised, at plant

cooling water



2870 kg 2.19 m3 2.15 × 103 kg

calcium chloride, iron(III) chloride, superphosphate, ammonia, organic chemicals

wastewater treatment treatment of biosolids

corn stover scenario generated internally

2.12 × 103 kg

provided to account for process requirements

bioreactor nutrient inputsc

Gasification Inputs biomass input process water Fermentation Inputs CO2 equivalents (CO2eq) net gas inputb

item used for LCAa

requirement

Table 1. Input LCA Data, on the Basis of 1000 kg/h of EtOH Production

Industrial & Engineering Chemistry Research Article


Article


nature but have a similar impact on the ethanol life cycle. As biomass grows it removes carbon from the atmosphere, and the biomass feedstock is given credit for this carbon removal. Based on the assumed carbon content of the biomass, this translates to (461 kg C/ton corn stover) × (44 kg CO2/12 kg C) = 1690 kg CO2 per ton of corn stover, which appears in the life cycle as a GHG emissions credit for removing carbon from the atmosphere (Table 2). As in the BOF gas scenario, subsequent emissions of carbon throughout the process are accounted for in the life cycle, throughout all stages of the process. In all scenarios, a small amount of carbon exits the system boundary in wastewater treated off-site, as well as CO2 from combusted anaerobic digestor biogas, which are both accounted for as well. A large majority (>95%) of the carbon entering the system as BOF gas or biomass exits the system either as carbon in ethanol or combusted vent gas from the bioreactor, similar to current sugar fermentation reactors. The apparent carbon-use efficiency of the LanzaTech process varies from the BOF scenario to the biomass-based cases because of the composition of carbon entering the reactor, but in both cases between 20 and 40% of the carbon entering the system as feedstock exits in the ethanol product. Implications of Feedstock Choice. The LanzaTech fermentation process is robust and can use a variety of carbon-containing gas feedstocks, and we explore the implications of feedstock choice in this paper. In Table 2 we

the source and structure of the data (Table 1). For the biomassbased scenarios, slightly less process water is required for the forest residue scenario compared to the other scenarios because of differences in feed gas composition resulting from biomass stoichiometry. Process water and cooling water are differentiated in the fermentation inputs (Table 1) because of different levels of pretreatment assumed for the water streams, as more dissolved solids are removed from the cooling water prior to use. Wastewater treatment of liquids from the anaerobic digestor is modeled using a template from the Ecoinvent database (sewage whey digestion) that most closely matches the organic carbon content of the LanzaTech bioreactor, in order to accurately reflect emissions from the wastewater treatment process. Settled solids from the LanzaTech fermentation bioreactor are sent to anaerobic digestion, where biogas is produced with a 40:60 composition of CO2:CH4. After biogas conversion, the biogas stream is combusted to produce energy, and complete conversion of all CH4 to CO2 is assumed before the biogas is released to the atmosphere. Transportation of ethanol is assumed to be performed using a 32 t truck with updated emissions controls (class 4 in European system used in Ecoinvent) and a payload of roughly 7 t. Impact Assessment. Environmental impacts of the production and use of material and energetic inputs listed in Table 1 were included as part of this assessment through use of the Ecoinvent 2.1 database24 or other literature sources as stated in Table 1. Global warming impacts were calculated using the IPCC 2007 GWP 100a method in SimaPro LCA software, version 7.3.25 In this method, CO2 has a global warming potential (GWP) of 1, CH4 = 25, and N2O = 298. A full accounting of the GWP of all climate-active chemicals, including solvents and refrigerants used in upstream processes, is included in the impact assessmemt method using inventory elements from the Ecoinvent database in SimaPro 7.3, and these impacts are combined into CO2-equivalents (CO2eq) as a single metric of greenhouse gas emission impacts. As a comparison standard, results are presented alongside equivalent environmental impact assessments for conventional gasoline. Data for petroleum gasoline greenhouse gas emissions is based on national inventories from Argonne National Laboratory, which report average GHG emissions of 94 g CO2eq/MJ.26

Table 2. GHG Credits and Emissions Associated with Feedstock Choice item

BOF gas

corn stover

switchgrass

forest residue

carbon content (percent carbon by weight) GHG emissions credit from feedstock gas incorporation into LanzaTech process (kg CO2eq/ton feedstock) GHG emissions due to feedstock procurement (kg CO2eq/ton feedstock)

32.4%a

46.1%b

46.1%b

49.6%b

−1188

−1690

−1690

−1819

0

94.3c

124.8d

59.7e

a

BOF gas parameters from LanzaTech customer surveys, dry weight basis. bBiomass carbon content from LanzaTech processing estimates. c Corn stover procurement burdens (including land-use change) from GREET model estimates.16 dSwitchgrass procurement burdens (including land-use change) from GREET model estimates.16 eForest residue procurement burdens from CORRIM research study.18

■

RESULTS AND DISCUSSION Carbon Accounting. The LanzaTech process model described above relies on previously accepted process engineering assumptions for gasification, product recovery, and waste treatment. Carbon mass flows and other material flows are fully accounted for in the process model, and emissions of carbon are accounted for in all stages of the process. As mentioned previously, the BOF gas scenario assumes that the BOF gas utilized in this process would have been flared, with oxidation of all carbon to CO2, before being released to the atmosphere. The quantitative effect of this assumption is that BOF gas modeled in our process, with a carbon content of 32.4%, would be producing a GHG emission penalty of (324 kg C/ton BOF gas) × (44 kg CO2/12 kg C) = 1188 kg CO2/ton BOF gas (Table 2). This level of GHG emissions is now credited to the feedstock for avoiding flaring emissions and entering the LanzaTech process, with subsequent emissions attributed to the process for carbon emissions that occur throughout the process, up to and including ethanol combustion. The assumptions regarding biomass-based scenarios are somewhat different in

compare the GHG emissions credits for incorporating each feedstock into the LanzaTech process (on the basis of carbon contents) and the GHG emissions associated with supplying the particular feedstock. There are clear differences among all four modeled scenarios on the assumed burdens associated with supplying feedstock to the process. As mentioned above, it is assumed that LanzaTech operations colocated with steel production can acquire and incorporate BOF exhaust gas with no additional effort compared to current steel mill operations; therefore, no GHG emissions are attributed to feedstock collection and transportation. In the biomass feedstock scenarios, there are differences in the GHG emissions burden of biomass feedstocks, most notably for switchgrass which has a larger associated GHG emissions burden due primarily to the assumption of increased fertilizer application and resulting soil N2O emissions.16 It is clear that in all cases, however, that the embodied GHG emissions associated with feedstock procureF


Article


biomass-based scenarios, because feedstock GHG emissions credits and bioreactor vent gas are the two largest flows of carbon in the process and their magnitude far outweighs other GHG emissions shown in the life cycle. In the biomass-based scenarios, biomass feedstocks entering the process are credited for removing carbon from the atmosphere, but subsequent emissions of carbon are accounted for at later stages of the process. Utility consumption (electricity and steam) accounts for a large portion of GHG emissions in the ethanol life cycle for the BOF gas scenario, while other inputs to the process (nutrients, chemicals, water) result in a relatively minor contribution (Table 3). Production and combustion of biogas from anaerobic digestion of organic bioreactor waste results in a small amount of electricity that can be used on site, but combustion of the biogas also results in GHG emissions that are accounted for. Accounting for all inputs to the BOF gasbased life cycle in the BOF gas scenario, we estimate a total GHG emissions result for LanzaTech ethanol of 31.4 g CO2eq/ MJ. This emissions total is small enough relative to the life cycle GHG emissions for petroleum gasoline (67% emissions reduction) that this scenario would produce ethanol that meets the same emissions reductions targets given to cellulosic biofuel according to EPA guidelines.9 Compared to the prior LCA study results of the LanzaTech process,11 GHG emissions for the BOF are lower in this case than in the prior study, 31.4 g CO2eq/ MJ in this case compared to 40.7 g CO2eq/ MJ when steam generation was provided internally. Several methodological differences exist between these two studies to explain the different LCA results, most notably the reliance of the prior work on electricity provided from the GHG-intensive Chinese electricity grid and reliance on older LanzaTech process models with lower assumed ethanol yields. Specific aspects of the life cycle change for the biomass-based scenarios, but the overall result is quite encouraging. As mentioned in Table 2, GHG emissions associated with feedstock procurement are incorporated into these scenarios, resulting in emissions ranging from 6.6−14.9 g CO2eq/MJ ethanol. The burdens associated with feedstock procurement are more than made up for by the benefits associated with biomass gasification, namely, the on-site coproduction of heat and power that eliminates utility consumption throughout the entire process and even results in a small excess of electricity which is credited as displacing the use of United States grid

ment activity are much less than the GHG emission credits associated with incorporating this renewable or waste carbon in the LanzaTech process, as described in the previous section. This analysis of the feedstock procurement stage alone does not tell the entire story of ethanol produced from these feedstocks, as carbon is released at later stages of the ethanol production process. Therefore, a thorough carbon accounting of the full process was completed to track inputs and emissions of carbon from the process at all stages of the life cycle. LCA Results. Greenhouse gas emissions data for the entire ethanol life cycle are presented in Table 3. In the BOF gas Table 3. GHG Emissions for LanzaTech Scenarios GHG emissions (g CO2eq/MJ ethanol) scenario net gas useda feedstock procurement utilities (heat and power)b other inputsc anaerobic digestor emissions waste treatmentd ethanol transport ethanol combustion net EtOH GHG emissions % reductione

BOF gas

corn stover

switchgrass

forest residue

−79.0 0.0 28.8 1.8 6.8

−80.2 11.2 −6.3 1.3 7.5

−80.2 14.9 −6.3 1.3 7.5

−80.2 6.6 −8.1 1.3 7.5

0.9 0.7 71.4 31.4 67%

1.1 0.7 71.4 8.0 92%

1.1 0.7 71.4 11.7 88%

1.0 0.7 71.4 1.5 98%

a

Net gas used refers to the net GHG emissions of the credits associated with utilizing different feedstocks, combined with the GHG emissions of vent gas that is released in the fermentation reactor (Figure 1). bIncludes credits for electricity and steam generated through combustion of anaerobic digestion biogas. cRefers to nutrients, water, chemicals, etc. outlined in Table 1 dIncludes wastewater treatment and solid waste disposal. eCompared to petroleum gasoline standard reference defined above (94 g CO2eq/ MJ fuel)

scenario, a large credit is granted for removing BOF gas from its assumed prior fate, flaring and release into the atmosphere. This credit is in part negated by large emissions observed in the system from the bioreactor vent gas, but the combination of these two items, presented as “net gas used” in Table 3, is still quite large. This type of carbon accounting is repeated for the

Table 4. Sensitivity Analysis Results for Key Parameters in LanzaTech Process percent change in overall life cycle GHG emissionsa BOF gas parameter ethanol transport distance electricity grid emissions biogas methane emissions utility consumption biomass procurement impacts media requirements

base case

low case

100 km 214 g CO2eq/MJ elec.

high case

50 km b

190 g CO2eq/MJ elec.

500 km c

290 g CO2eq/MJ elec.

d

corn stover

% change (low)

% change (high)

% change (low)

% change (high)

−1%

9%

−4%

35%

−9%

33%

8%

−28%

0%

−

2%

−

5%

−

23%

current current

10% reduction 10% reduction

10% increase 10% increase

−9% −

9% −

−20% −14%

49% 14%

current

10% reduction

10% increase

−1%

1%

−2%

2%

a

As compared to 31.4 g CO2eq/MJ (BOF gas) and 8.0 g CO2eq/MJ (Table 3). bCorresponds to United States average medium voltage electricity grid in Ecoinvent v2 database. cCorresponds to most current U.S. EPA assessment of current generation mix.27 dCorresponds to Chinese electricity grid GHG emissions as reported in 2011.11 G


Article


according to the low and high case assumptions. The overall LCA result was then recalculated, and the new result was compared to the base case result from Table 3 in terms of percent change to the overall GHG emissions. This process was repeated for the BOF gas and corn stover scenarios. Importantly, none of the scenarios explored resulted in life cycle GHG emissions that would make the final ethanol product miss qualifying as an advanced biofuel under U.S. EPA guidelines. However, results of the sensitivity analysis can provide a few insights on the developments of this process and provide commercial developers with a sense of the important factors that should be considered. Excessive transport distance for ethanol product can influence the overall GHG emissions result, but only by ∼5 g CO2eq/MJ in absolute terms. Changes in the electricity grid emissions profile had opposite impacts in the BOF and corn stover scenario, because in the BOF gas scenario the process is relying on inputs of imported electricity, while in the corn stover case the excess electricity is being supplied to the grid, making it more favorable to displace the prior use of electricity from a dirtier grid. Small amounts of fugitive methane emitted from the anaerobic digestor can have an outsized influence on overall GHG emissions due to the relatively large global warming potential of methane compared to CO2, so this process should be designed and maintained with proper attention. Utility consumption had a surprising effect on the corn stover case, because an increase in utility consumption ultimately reduces the amount of excess electricity that may be exported to the grid for a displacement credit in this process. The goal of this life cycle assessment was to understand the life-cycle greenhouse gas emissions of ethanol produced from the LanzaTech process. This analysis was based on estimates provided by LanzaTech of a model facility utilizing either BOF exhaust gas or one of several biomass streams as the incoming feedstock. Because input feedstocks were treated as avoided waste streams (BOF gas) or renewable sources of carbon, large credits were granted for carbon entering the system boundary. Gasification of biomass into syngas for use in the LanzaTech process has the added benefit of colocated utility production, which results in more favorable LCA results for the biomass scenarios, despite the small added burdens associated with growing and procuring biomass feedstocks. As a result of this analysis, several recommendations can be made to guide future study and development of LanzaTech fermentation technology. The large volumes of gas entering and exiting the bioreactors will require careful monitoring, as these two gas streams are significantly larger than all other sources of greenhouse gases in this life cycle analysis. Several assumptions surrounding biomass procurement have been made, using thorough yet generalized assumptions. As with any biofuels study, site-specific feedstock studies to verify biomass yields, growth inputs, harvest efficiency, and transport burdens will strengthen the current analysis and the overall field of study. Important assumptions have been made surrounding utility generation and use, and deviations from these standard assumptions could affect the resulting greenhouse gas balance in a negative manner (e.g., using electric power exclusively from coal) or a positive manner (use of cleaner alternatives such as wind or hydropower, or system integration to reduce steam use). These variables will likely change from situation to situation and will require specific LCAs for the different operating conditions to verify GHG savings. The LanzaTech process can be tailored to produce different coproducts alongside ethanol at different ethanol:coproduct ratios, which

electricity. Demands of other inputs including bioreactor nutrients and process water are similar compared to the BOF scenario, reflecting the minor changes associated with different feedstock composition. The resulting life-cycle GHG emissions for ethanol produced from biomass gasification in all biomassbased scenarios are quite low, achieving 88−98% reductions in GHG emissions compared to fossil petroleum gasoline. Many LCA studies exist for ethanol production from biomass feedstocks, which all rely on different assumptions that make direct comparison challenging. Life-cycle GHG emissions results within the well-described GREET model14 for ethanol produced by fermenting corn stover (33 g CO2eq/MJ), switchgrass (36 g CO2eq/MJ), and forest residue (15 g CO2eq/MJ) are all higher than corresponding LCA results for scenarios described here, suggesting that LanzaTech biomassbased ethanol can achieve a low GHG emissions profile relative to common ethanol production technologies assumed for these feedstocks. Additionally, combined process and cooling water consumption within the LanzaTech combined gasification− fermentation process for corn stover (0.40 kg/MJ ethanol), switchgrass (0.40 kg/MJ ethanol), and forest residue (0.39 kg/ MJ ethanol) are also lower than water consumption estimates at the ethanol production stage reported in the GREET model (0.65, 0.65, and 0.50 kg/MJ ethanol, respectively).14 Water consumption within the BOF gas scenario are lower than all biomass-based scenarios, 0.34 kg/MJ ethanol. Sensitivity Analysis. To illustrate the extent to which the LCA results presented here are dependent on certain key assumptions, a sensitivity analysis was performed on the life cycle inputs to the BOF gas and corn stover scenarios. Key input assumptions in the BOF gas and corn stover scenarios were varied to illustrate the impacts of potential variability in the life cycle. The sensitivity analysis results are presented in Table 4. Parameters included in the sensitivity analysis are (1) ethanol transport distance, (2) GHG emissions associated with the electricity grid, (3) fugitive methane emissions from biogas production, (4) consumption of utilities, (5) impacts associated with biomass procurement, and (6) consumption of fermentation reactor media inputs. Transport distance was reduced to 50 km for the low cases but increased to 500 km in the high cases to represent long-distance transport from relatively remote ethanol facilities. The GHG emissions profile associated with the United States electricity grid represented in Ecoinvent is calculated to be 214 g CO2eq/MJ electricity using the IPCC 100a method used in this study. The Low case for this parameter represents the GHG emissions for a recent electricity generation mix using data from the U.S. EPA,27 which most notably involves a reduction in coal usage (from 49 to 37%) and an increase in natural gas (from 17% to 30%) usage compared to the Ecoinvent grid used in the base case scenarios. A high case for this parameter was established by using data relating to the Chinese electricity grid, which uses a larger proportion of GHG emissions-heavy fuels like coal and oil. A high case was created in regards to fugitive methane emissions from biogas production by assuming that 2% of methane produced in anaerobic digestion escapes into the atmosphere without being converted to CO 2 during combustion. For parameters 4−6, the low and high cases were created by assuming a 10% reduction or increase in the parameter, respectively, as an illustration of uncertainty in these areas. For each scenario that was created in the sensitivity analysis, the parameter under consideration was modified H


Article


(13) Stubbles, J. The Basic Oxygen Steelmaking (BOS) Process. www. steel.org/Making-Steel/How-Its-Made/Processes/Processes-Info/ The-Basic-Oxygen-Steelmaking-Process.aspx (accessed March 2015). Steelworks, Online Resource for Steel; American Iron and Steel Institute. (14) Boston Consulting Group (BCG) and the Steel Institute VDeh. Steel’s Contribution to a Low-Carbon Europe 2050: Technical and Economic Analysis of the Sector’s Abatement Potential. http://www. eurometal.net. 2013. (15) U.S. Environmental Protection Agency. Available and Emerging Technologies for Reducing Greenhouse Gas Emissions from the Iron and Steel Industry. http://www3.epa.gov/nsr/ghgdocs/ironsteel.pdf. Office of Air Quality Planning and Standards, Sector Policies and Programs Division, 2012. (16) Argonne National Laboratory. GREET 2014 Life-cycle Model. Center for Transportation Research, Energy System Division. 2014. greet.es.anl.gov. (17) Taheripour, F.; Tyner, W. E.; Wang, M. Q. Global Land Use Changes due to the U.S. Cellulosic Biofuel Program Simulated with the GTAP Model. https://greet.es.anl.gov/publication-luc_ethanol. Argonne National Laboratory, Argonne, IL, 2011. (18) Johnson, L.; Lippke, B.; Oneil, E. Modeling Biomass Collection and Woods Processing Life-Cycle Analysis. Forest Products Journal 2012, 62 (4), 258−272. (19) Dutta, A.; Talmadge, M.; Hensley, J.; Worley, M.; Dudgeon, D.; Barton, D.; Groenendijk, P.; Ferrari, D.; Stears, B.; Searcy, E. M.; Wright, C. T.; Hess, J. R. Process Design and Economics for Conversion of Lignocellulosic Biomass to Ethanol: Thermochemical Pathway by Indirect Gasification and Mixed Alcohol Synthesis; NREL/TP-5100-51400; National Renewable Energy Laboratory: Golden, CO, 2011; p 187. (20) Ingledew, W. M. The Alcohol Textbook; A Reference for the Beverage, Fuel and Industrial Alcohol Industries; Ethanol Technology Institute, Nottingham University Press: United Kingdom, 2009. (21) Cardona, C. A.; Sánchez, O. J. Fuel Ethanol Production: Process Design Trends and Integration Opportunities. Bioresour. Technol. 2007, 98 (12), 2415−57. (22) Madson, P. W.; Monceaux, D. A. Fuel ethanol production. In The Alcohol Textbook, 3rd ed.; Jacques, K. A., Lyons, T. P., Kelsall, D. R., Eds.; Nottingham University Press: Nottingham, U.K., 1999; Chapter 17. (23) Johnson, I.; William, T.; Choate, W. T.; Davidson, A. Waste heat recovery: Technology and opportunities in US industry. http:// www1.eere.energy.gov/manufacturing/intensiveprocesses/pdfs/ waste_heat_recovery.pdf. US Department of Energy, Office of Energy Efficiency and Renewable Energy, Industrial Technologies Program; Prepared by BCS, Inc., 2008. (24) Frischknecht R.; Althaus H. J.; Doka, G.; Dones, R.; Heck, T.; Hellweg, S.; Hischier, R.; Jungbluth N.; Nemecek, T.; Rebitzer, G.; Spielmann, M. Overview and Methodology, Final report Ecoinvent, v2.0 No. 1. http://www.ecoinvent.ch/. Swiss Centre for Life Cycle Inventories: Duebendorf, Switzerland, 2004. (25) PRé - Project Ecology Consultants. SimaPro, version 7.2. http:// www.pre.nl. (26) Elgowainy, A.; Han, J.; Cai, H.; Wang, M.; Forman, G. S.; DiVita, V. B. Energy efficiency and greenhouse gas emission intensity of petroleum products at US refineries. Environ. Sci. Technol. 2014, 48 (13), 7612−7624. (27) U.S. Environmental Protection Agency. Emissions and Generation Resource Integrated Database (eGRID), eGRID 2012 Summary Tables. http://www2.epa.gov/energy/egrid.

can be optimized according to market conditions or other factors. This flexibility will require additional study to determine the implications of simultaneous chemical production on the environmental footprint of the LanzaTech process, under different schemes for allocating environmental burdens between products. The LanzaTech process can enable fuel production with substantially reduced GHG emissions from either BOF exhaust gas from steel mills or gasified biomass, as this study demonstrates. This offers a substantial opportunity to both produce low-carbon fuels and reduce global GHG emissions.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: 906-487-3612. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A portion of this work was funded by a 2011 award from the U.S. Department of Energy to LanzaTech and Michigan Technological University (Award DE-EE0005-356).

■

REFERENCES

(1) U.S. Environmental Protection Agency. Sources of Greenhouse Gas Emissions. http://www.epa.gov/climatechange/ghgemissions/ sources/transportation.html. 2014. (2) U.S. Energy Information Administration. Annual Energy Outlook 2015. http://www.eia.gov/forecasts/aeo. U.S. Department of Energy, Washington, D.C, 2015. (3) Wang, M. Q.; Han, J.; Haq, Z.; Tyner, W. E.; Wu, M.; Elgowainy, A. Energy and greenhouse gas emission effects of corn and cellulosic ethanol with technology improvements and land use changes. Biomass Bioenergy 2011, 35 (5), 1885−1896. (4) Swenson, D.; Eathington, L. Determining the regional economic values of ethanol production in Iowa considering different levels of local investment. Part A: Developing a modeling and measurement structure; Iowa State University, Department of Economics: Ames, IA, 2006. (5) Fargione, J. E.; Cooper, T. R.; Flaspohler, D. J.; Hill, J.; Lehman, C.; Tilman, D.; McCoy, T.; McLeod, S.; Nelson, E. J.; Oberhauser, K. S. Bioenergy and Wildlife: Threats and Opportunities for Grassland Conservation. BioScience 2009, 59 (9), 767−777. (6) Pimentel, D.; Marklein, A.; Toth, M. A.; Karpoff, M. N.; Paul, G. S.; McCormack, R.; Kyriazis, J.; Krueger, T. Food Versus Biofuels: Environmental and Economic Costs. Human Ecology. 2009, 37 (1), 1− 12. (7) Searchinger, T.; Heimlich, R. Avoiding Bioenergy Competition for Food Crops and Land.Working Paper, Installment 9 of Creating a Sustainable Food Future. World Resources Institute, Washington, DC. 2015. http://www.worldresourcesreport.org. (8) Solomon, B. D. Biofuels and sustainability. Ann. N. Y. Acad. Sci. 2010, 1185, 119−134. (9) U.S. Environmental Protection Agency. Regulation of Fuels and Fuel Additives: Changes to Renewable Fuel Standard Program [EPAHQ-OAR-2005-0161; FRL-8903-1] Fed. Reg. 2009, 74 (99), 24908. http://www.epa.gov/otaq/fuels/renewablefuels/regulations.htm. (10) Lark, T. J.; Salmon, J. M.; Gibbs, H. K. Cropland expansion outpaces agricultural and biofuel policies in the United States. Environ. Res. Lett. 2015, 10 (4), 044003. (11) Ou, X.; Zhang, X.; Zhang, Q.; Zhang, X. Life-cycle analysis of energy use and greenhouse gas emissions of gas-to-liquid fuel pathway from steel mill off-gas in China by the LanzaTech process. Frontiers in Energy 2013, 7 (3), 263−270. (12) World Steel. Iron Production 2011; World Steel Association, 2012. www.worldsteel.org. I


Life Cycle Assessments of Ethanol Production via Gas Fermentation

Recommend Documents