CRADLE-TO-GRAVE LIFE CYCLE ASSESSMENT OF SYNGAS ELECTRICITY FROM WOODY BIOMASS RESIDUES 1 Hongmei Gu*† Research Forest Products Technologist E-mail: [email protected]Richard Bergman† Project Leader USDA Forest Service, Forest Products Laboratory One Gifford Pinchot Drive Madison, WI E-mail: [email protected](Received August 2016) Abstract. Forest restoration and fire suppression activities in the western United States have resulted in large volumes of low-to-no-value residues. An environmental assessment would enable greater use while maintaining environmental sustainability of these residues for energy products. One internationally accepted sustainable metric tool that can assess environmental impacts of new bioenergy conversion systems is the life cycle assessment (LCA). Using the LCA method, this study evaluated the synthesis gas (syngas) electricity produced via a distributed-scale biomass thermochemical conversion system called the Tucker renewable natural gas (RNG) system. This system converts woody biomass in a high-temperature and extremely low-oxygen environment to a medium-energy syngas that is burned to generate electricity. The system also produced biochar as a by-product and tar as a waste. Results from the life cycle impact assessment included an estimate of the global warming (GW) impact from the cradle-to-grave production of syngas for electricity. When the carbon sequestration effect from the biochar by-product was included, GW impact value (0.330 kg CO 2 -eq/kWh) was notably lower compared with electricity generated from bituminous coal (1.079 kg CO 2 -eq/kWh) and conventional natural gas (0.720 kg CO 2 -eq/kWh). Other environmental impacts showed that syngas electricity ranged between the direct-biomass-burned electricity and fossil-fuel-combusted electricity for different impact categories. This occurred because, although the woody biomass feedstock was from a renewable resource with less environmental impact, propane was consumed during the thermochemical conversion. Specifically, the evaluation showed that the highest greenhouse gas (GHG) emissions contribution came from burning propane that was used to maintain the endothermic reaction in the Tucker RNG unit. If the tar waste from the system were converted into a low-energy syngas and used to supplement propane consumption, a further decrease of 41% in GHG emissions (ie fossil CO 2 ) could be achieved in this cradle-to-grave assessment. Keywords: Bioenergy, woody biomass, syngas electricity, life cycle analysis, environmental assessment. INTRODUCTION There has been a great demand on management of US western forests to decrease threats from insect and disease outbreak, invasive species, and in particular, forest fires. Restoration treat- ments on western US forests produce large quan- tities of woody biomass that can be used as feedstock for production of biofuels and other bioproducts (Tilman et al 2009). Producing bioenergy and bioproducts from such forest thinning or timber harvest by-products could contribute to reaching broad national energy objectives, including the nation’s energy secu- rity and lowering greenhouse gas (GHG) emis- sions from fossil fuels, a major cause of climate change, according to the International Panel on Climate Change (IPCC 2014). The US Department of Energy (DOE) and the US Department of Agriculture (USDA) are both strongly committed to increasing the role of * Corresponding author † SWST member 1 This article was written and prepared by the US Govern- ment employees on official time, and it is therefore in the public domain and not subject to copyright. Wood and Fiber Science, 49(2), 2017, pp. 177-192 # 2017 by the Society of Wood Science and Technology
Transcript
CRADLE-TO-GRAVE LIFE CYCLE ASSESSMENT OF SYNGASELECTRICITY FROM WOODY BIOMASS RESIDUES1
Abstract. Forest restoration and fire suppression activities in the western United States have resultedin large volumes of low-to-no-value residues. An environmental assessment would enable greater usewhile maintaining environmental sustainability of these residues for energy products. One internationallyaccepted sustainable metric tool that can assess environmental impacts of new bioenergy conversionsystems is the life cycle assessment (LCA). Using the LCA method, this study evaluated the synthesis gas(syngas) electricity produced via a distributed-scale biomass thermochemical conversion system called theTucker renewable natural gas (RNG) system. This system converts woody biomass in a high-temperatureand extremely low-oxygen environment to a medium-energy syngas that is burned to generate electricity.The system also produced biochar as a by-product and tar as a waste. Results from the life cycle impactassessment included an estimate of the global warming (GW) impact from the cradle-to-grave productionof syngas for electricity. When the carbon sequestration effect from the biochar by-product was included,GW impact value (0.330 kg CO2-eq/kWh) was notably lower compared with electricity generated frombituminous coal (1.079 kg CO2-eq/kWh) and conventional natural gas (0.720 kg CO2-eq/kWh). Otherenvironmental impacts showed that syngas electricity ranged between the direct-biomass-burned electricityand fossil-fuel-combusted electricity for different impact categories. This occurred because, although thewoody biomass feedstock was from a renewable resource with less environmental impact, propane wasconsumed during the thermochemical conversion. Specifically, the evaluation showed that the highestgreenhouse gas (GHG) emissions contribution came from burning propane that was used to maintainthe endothermic reaction in the Tucker RNG unit. If the tar waste from the system were converted intoa low-energy syngas and used to supplement propane consumption, a further decrease of 41% in GHGemissions (ie fossil CO2) could be achieved in this cradle-to-grave assessment.
There has been a great demand on managementof US western forests to decrease threats frominsect and disease outbreak, invasive species,and in particular, forest fires. Restoration treat-ments on western US forests produce large quan-tities of woody biomass that can be used as
feedstock for production of biofuels and otherbioproducts (Tilman et al 2009). Producingbioenergy and bioproducts from such forestthinning or timber harvest by-products couldcontribute to reaching broad national energyobjectives, including the nation’s energy secu-rity and lowering greenhouse gas (GHG) emis-sions from fossil fuels, a major cause of climatechange, according to the International Panel onClimate Change (IPCC 2014).
The US Department of Energy (DOE) and theUS Department of Agriculture (USDA) are bothstrongly committed to increasing the role of
* Corresponding author† SWST member1 This article was written and prepared by the US Govern-ment employees on official time, and it is therefore in thepublic domain and not subject to copyright.
Wood and Fiber Science, 49(2), 2017, pp. 177-192# 2017 by the Society of Wood Science and Technology
biomass as an energy source. Both agencies hopeto replace 30% of the current US petroleumconsumption with biofuels by 2030 (Perlacket al 2005). Biomass fuels and products are oneway to lower the requirement for oil and gaso-line imports while supporting the growth ofagriculture, forestry, and rural economies (Naiket al 2010; USDOE 2016). Also, increasing bio-fuel and bioproduct production from biomasshas the potential to decrease net GHG emissionsand improve local economies and energy secu-rity. The Biomass Research and DevelopmentInitiative (BRDI) was formed by USDA and DOEas an interagency program to support the creationof a biomass-based industry in the United Statesfor energy production and environmental safety.This study was part of one of these BRDI projectsin which the team conducted an integrated evalua-tion of biomass feedstock production, logistics,conversion, distribution, and end use focused onan innovative thermochemical conversion sys-tem for existing forest industry operations (Milleret al 2014, 2015) which life cycle assessment(LCA) research was part of.
LCA research was conducted to evaluate the netlife cycle GHG emissions and energy balance forwoody biomass residue to bioenergy and bio-product conversion and then compare the lifecycle impact assessment (LCIA) outcomes tofossil-based alternatives. LCA is a well-establishedand internationally accepted method for catego-rizing life cycle GHG and environmental perfor-mance metrics (ISO 2006a, 2006b). Thus, it isoften used as a science-based tool to evaluateassertions that increasing bioenergy productionfrom woody biomass can lower net GHG emis-sions. Within the LCA framework, to quantifythe various impacts from air and water emis-sions released to the atmosphere during productproduction, the categorized life cycle inven-tory (LCI) flows are characterized into com-mon equivalence units that are then summedto provide an overall impact category total. Dif-ferent LCIA impact categories cover differentemissions (ie LCI flows).
The use of LCA to evaluate the environmentalimpacts from converting biomass to bioenergy,
including electricity, has been studied inten-sively in recent years (Cherubini and Stromman2011; Sebastian et al 2011; Steubing et al 2011;Field et al 2013; Hertwich et al 2013; Pierobonet al 2014; Stephenson and MacKay 2014). Inparticular, Stephenson and MacKay (2014) fromthe UK Department of Energy and ClimateChange performed a scenario analysis usingNorth American woody biomass for the UnitedKingdom’s electricity in 2020. They found thatthe lowest GHG impact can be achieved byusing forest or mill residues or trees killed fromnatural disturbance, which is the feedstock thatwould otherwise be burned as waste (<100 kgCO2-eq/MWh). Pierobon et al (2014) used radi-ative forcing analysis to evaluate the environ-mental impact of woody-biomass-based bioenergyconversion. Pierobon et al (2014) incorporated thedynamics of carbon sequestration, decompositionof residues, and biomass processing in the lifecycle analysis framework of bioenergy and con-cluded that the adverse global warming (GW)impact associated with biomass collection andburning from industrial forests can be fully offsetby the carbon sequestration during forest growthwithin about 18 yr. To put biomass (wood) elec-tricity production into context, according to theUS Energy Information Administration (USEIA),wood-powered electricity is estimated to be43.4 million MWh for the year 2016 which is aconsiderable value for many countries but it isonly about 1% of the US electrical grid (USEIA2016). Regardless of the current value for theUnited States, there is huge potential in increas-ing wood-powered electricity (USDOE 2016).Thus, as it is in this case, it is critical that LCAscontinue to evaluate new technologies as they aredeveloped. LCA can focus on parts of the lifecycle of the technology that may not be consid-ered once a process becomes commercialized. Itcan do this while the process is still in the devel-opment phase to evaluate what-if scenarios.
From the review on LCA work for bioenergysystems by Cherubini and Stromman (2011), itwas found that the net GHG emissions frombiomass-generated electricity are usually 5-10%of those from fossil-fuel-based electricity, and
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GHG emissions could be lower if the feedstockbiomass is derived from residue streams such aslogging slash and small-diameter trees. All thestudies reviewed by Cherubini and Stromman(2011) assumed neutral climate impact frombiomass combustion in terms of CO2 emissions.Steubing et al (2011) conducted a cradle-to-graveLCA of a polygeneration unit that producedsynthesis gas (syngas) for heat, electricity gen-eration, and transportation fuel. They comparedthe results with a fossil-fuel-based system. Theirstudy showed substantially less contribution toclimate change when syngas was substituted forfossil fuel, but these benefits were partially off-set by other environmental effects related tohuman health and eutrophication. They consid-ered syngas from wood used for transportationfuel as a promising technology in light of grow-ing demand for renewable transportation fuels.Field et al (2013) did a case study on a Coloradoregional coproduction of biochar and bioenergyfrom biomass residue feedstock. Their financialanalysis suggested that the returns were gener-ally greater when biochar was used for energy(biocoal) than when used for soil amendment(biochar), whereas biochar application had greaterGHG mitigation value than did biocoal.
The goal of this study focused on the down-stream process of burning syngas produced froma distributed-scale advanced biomass pyrolysissystem which will be referred to as the Tucker(developed by Tucker Engineering Associates,Inc., Locust, NC) renewable natural gas (RNG)unit to generate electricity. The authors willanswer the question of how much environmentalimpact can be decreased if woody-biomass-derived syngas electricity is substituted for fossil-fuel-based electricity. Applying LCA can helpto compare the processes or technologies forenergy and environmental benefits and identifythe environmental “hot-spots” (highest points) ofthe various impact categories.
METHODS
For this study, the LCA assessed electricity gen-erated from the syngas produced from the Tucker
RNG unit. The LCI model was constructedin three parts: 1) upstream model, including for-est management, log extraction, transportation,and feedstock processing; 2) mainstream model,thermochemical conversion including TuckerRNG unit process; and (3) downstream model,including generation of electricity from theprimary product syngas and the impacts fromcarbon sequestration by biochar, a by-productfrom the system (Fig 1). For further refer-ence, Gu and Bergman (2016) detailed mostof the first two parts of the LCI includingfeedstock processing and thermochemical con-version and their data were used in this analy-sis. For the Tucker RNG unit, the productproduced by mass by far was syngas with somebiochar generated. Because this study focusedon the generation of electricity from syngas, allenvironmental burdens were assigned to thesyngas as the product of interest. Thus, biochartook zero environmental burden from the LCAoutput as a by-product from the system, but itsrole for long-term carbon storage in the soilwas analyzed for carbon sequestration benefitsin the LCA in offsetting the syngas electricityenvironmental impacts.
Primary data were collected from the sawmillchip operation in St. Regis, MT, and a single1-h continuous run of the Tucker RNG unit.The feedstock was wood chips processedfrom under-used small-diameter logs extractedfrom Rocky Mountain National Forest with amix of conifer species dominated by lodgepolepine (Pinus contorta), Douglas fir (Pseudotsugamenziesii), and ponderosa pine (Pinus ponderosa).Before feeding into the Tucker RNG unit, thechips were force-dried to 12% MC by a saw-dust drier to improve the chips’ performancein the Tucker RNG unit. Secondary data weredrawn from the US LCI Database (NREL 2012)and peer-reviewed literature. With the materialand energy inputs and reported emissions, thecradle-to-grave LCI model for the Tucker RNGsyngas electricity was built in SimaPro 8 to esti-mate the environmental impacts and cumulatedenergy consumption (PRé Consultants 2016).Within the SimaPro software, the inventory data
179Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
were compiled into the impact category indi-cators of interest, such as GW, acidification,eutrophication, etc.
For assessing the environmental impacts ofelectricity production, the Tool for the Reduc-tion and Assessment of Chemical and otherenvironmental Impacts (TRACI) method wasused. TRACI is a midpoint-oriented LCIAmethod developed by the US EnvironmentalProtection Agency (EPA) specifically for theUnited States using input parameters consistentwith US locations (Bare 2011). TRACI is avail-able through LCA software modeling such asSimaPro used in this analysis (PRé Consultants2016). This study included the LCIA impactcategories of fossil fuel depletion (MJ), GW(kg CO2-eq), acidification (kg SO2-eq), eutro-phication (kg N-eq), ozone depletion (kg CFC-11-eq), smog (kg O3-eq), carcinogens (CTUh),noncarcinogens (CTUh), respiratory effects(kg PM2.5-eq), and ecotoxicity (CTUe). Otherimpact measures as cumulative (total) energydemand (primary energy) (MJ-eq), includingthe contributions from both nonrenewable sourcessuch as fossil fuel and renewable sources such
as biomass, hydropower, and wind power, weresummarized from the LCI flows.
Scope
This study covered the cradle-to-grave LCA ofelectricity generated from syngas derived frompyrolyzing woody biomass. LCI and LCIA datafor producing syngas from the Tucker RNGpyrolysis unit were already constructed by Guand Bergman (2016) and were incorporated intothe model. In addition to the LCA on syngaselectricity, data from LCI databases for electric-ity generated from other sources including bio-mass, bituminous coal, lignite coal, anthracitecoal, natural gas (NG), and a regional eGrid,were drawn and analyzed for a comparativeLCA to examine the marginal GHG effects onthe electricity grid (NREL 2012). The electricitygrid is comprised of many regions with variousenergy sources (USEPA 2015). The EPA hasbroken the US electricity grid into “eGrids”.The eGrid system from the northwest (NWPP)region was included in the analysis. The eGridNWPP is representative of the mix of fuelsused for electric utility in the northwestern
Figure 1. System boundary for the life cycle of generating synthesis gas electricity.
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United States in 2008. Fuels include coal, bio-mass, petroleum, geothermal, NG, nuclear, hydro-electric, wind, and other energy sources. TheNWPP electricity grid covers an area includ-ing Washington, Oregon, Idaho, Utah, mostof Montana, Wyoming, Nevada, northern partsof California, Arizona, and New Mexico.
Functional Unit
Functional unit is the reference unit used toquantify the environmental performance of aproduct or a system. It is also a referencerelated to the inputs and outputs. Because thegoal of this research was to compare the envi-ronmental performances of electricity generatedfrom Tucker RNG syngas to those of electric-ity generated from other sources, the func-tional unit was defined as production of 1 kWhof electricity. Material flows, energy use, andemission data were standardized based on thisfunctional unit and then compiled within the sys-tem boundaries described subsequently in theSimaPro model to quantify the GHG emissionsand other environmental outputs. The input andoutput data for the SimaPro model are shown inTable 1.
System Boundary
Defining the system boundary selects the unitprocesses to be included in the system. Basedon the goal to determine the environmentalimpacts of syngas electricity, the system bound-ary included the upstream feedstock processing,mainstream thermochemical conversion processwith the Tucker RNG unit, and the down-stream syngas electricity production (Fig 1).The Tucker thermochemical process includedfeedstock conveyance, active reacting, passivereacting, condensing, tar cracking, cooling,collecting, and storing. The cumulative systemboundary included both on- and off-site emis-sions for all material and energy consumed.Fuel and electricity consumed for the upstreamand mainstream processes were included inthe cumulative boundary (solid line) to calcu-late the total emissions. The on-site emissions
included the processes within the dotted line.The off-site emissions included the grid elec-tricity production, transportation, and fuels pro-duced off-site but consumed on-site.
Syngas Combustion Unit Processes
To conduct the LCI, the syngas electricitysystem was built from several unit processeswithin the upstream, mainstream, and down-stream models. For the upstream model, theunit processes of Inland West forest manage-ment and forest residue (log) extraction in theUS LCI Database were used (NREL 2012),whereas the feedstock (chipping and screen-ing) process was modeled using the specificoperational (primary) data collected from anoperating sawmill in western Montana. Themainstream model of this study was thermo-chemical conversion (ie Tucker RNG unit).The downstream electricity generation processwas modified from the US LCI NG electric-ity generation process for our specific syngaselectricity. The input and output data for the
Table 1. Gas composition and heating value for TuckerRNG syngas from gas chromatography (Source: Gu andBergman 2016).
181Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
downstream model are shown in Table 2.Using wood chips as the feedstock, the TuckerRNG unit must produce about two times thevolume of syngas to generate the same elec-tricity as NG, because the higher heating value(HHV) of the produced Tucker RNG syngasis 19.70 MJ/m3, about half of the NG HHVat 38.3 MJ/m3. The main components by vol-ume of the syngas are carbon monoxide (48%),hydrogen (17%), and methane (15%), as shownin Table 1.
Starting with the functional unit of 1-kWh elec-tricity generated, fuels and equipment use, andtransportation requirements were compiled inthe SimaPro model to quantify the GHG emis-sions and other environmental outputs. Afterrunning the model in SimaPro, the LCI flows
were used to find the 100-y GW impact andLCIA outcomes from other impact categoriesaccording to the TRACI method (Bare 2011;IPCC 2014). TRACI 2.1 method incorporatedin SimaPro 8.1 was used.
Project Limitations
Human labor and the manufacturing of machin-ery and infrastructure were outside the systemboundaries and therefore were not modeled inthis analysis.
Because the Tucker RNG syngas was similarto NG, the NG combustion emission profileswere assumed for Tucker RNG syngas electric-ity generation (Tucker 2016; Morris 2016).
For SimaPro modeling, liquid petroleum gas(LPG) was used as the proxy for propane. TheUS LCI Database indicates LPG is 100% pro-pane. However, this is not the case because theemission profiles do not match when convertingfrom mass to volume (Channiwala and Parikh2002). Therefore, to convert the measured vol-ume of propane gas consumed in the main-stream process to mass then to the volumeof LPG (proxy for propane in the SimaPromodel) based on the stoichiometry, a liquid den-sity of 0.573 kg/L for LPG was calculated fromthe US LCI database and used in our model(Johnson 2016).
Cutoff Rules
If the mass or energy of a flow is less than 1%of the cumulative mass or energy of the entiremodel flow, it may be excluded, provided itsenvironmental relevance is minor. This analysisincluded all the energy and mass flows for pri-mary data.
RESULTS AND DISCUSSION
The environmental impact assessment for pro-ducing 1 kWh of syngas electricity from anadvanced thermochemical converting technologyusing wood residues was carried out using LCA,and the results are described subsequently.
Table 2. Input and outputs for combusting syngas to gen-erate 1-kWh electricity.
a RNG, renewable natural gas.b VOC, volatile organic compounds.
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Life Cycle Inventory
Within the LCA method, the LCI phase mea-sures all the raw materials and energy inputs forproducing 1-kWh electricity from the syngasproduced by the Tucker RNG unit within thedefined system boundary (Fig 1). The emissionprofiles included activities associated with forestresource extraction and transportation of logs,chip production, and drying at the sawmill, ther-mochemical conversion with the Tucker RNGunit, and finally the syngas combustion forgenerating electricity. Major air and water emis-sions from the LCI flows are presented inTable 3. The GW impacts were derived pri-marily from fossil CO2, CH4 (mainly fossil),and N2O emissions. The total fossil CO2 emis-sions in the LCI flow (Table 3) calculated bySimaPro were 0.704 kg/kWh. The total fossilCH4 emissions were 0.0012 kg/kWh and N2Oemissions were 0.0057 kg/kWh. CH4 and N2Oemissions were much smaller in quantity buthad a much greater GW impact by mass thandid fossil CO2 (IPCC 2014).
To ensure data quality, the material flow fromthe forest to syngas electricity generation wasdeveloped to produce 1 kWh of electricity. Fromthe cradle-to-grave model built in this study, wesummarized about 0.537 m3 of syngas wouldbe needed from pyrolyzing 0.888 kg of ovendry wood chips (mixture of several softwoodspecies from West Inland National Forests) bythe Tucker conversion unit. The wood chips areequated to about 0.0017 m3 of whole tree logsextracted from the forest.
Life Cycle Impact Assessment
LCIA indicators from the three modeled pro-cesses along the whole life cycle are presentedin Table 4 and Fig 2. The GW impact fromcradle-to-grave LCA for syngas electricity was0.748 kg CO2-eq/kWh without considering bio-char’s potential for carbon sequestration (Table 4).After considering carbon sequestration frombiochar, the value was decreased to 0.330 kgCO2-eq/kWh for GW impact. Carbon sequestra-tion by biochar is discussed subsequently. The
GW impact results were divided into threestages: upstream feedstock processing, main-stream syngas production, and downstreamsyngas electricity. Syngas production releasedabout 60.8% of the total GHG emissions (Fig 2and Table 4). Feedstock processing contrib-uted the second highest emission and includedextraction of forest thinning materials, transpor-tation, and size reduction and pretreatment ofthe feedstock. About 38.3% of the total GHGemission was from this upstream feedstockprocessing stage, which leaves only 0.93% ofGHG emission associated with the downstreamsyngas electricity generation process. For com-parison, Steubing (2011) reported a GW impactof 0.103 kg CO2-eq/kWh for a case in whichthe syngas was primarily composed of CH4
and very little fossil fuel (ie gas) was consumedin the production of syngas, unlike the TuckerRNG unit.
Table 3. Cradle-to-grave emissions for 1-kWh syngaselectricity.
Emission to waterChloride 0.026Solved solids 0.021Suspended solids, unspecified 0.013Sodium, ion 0.0048Calcium, ion 0.0015Barium 0.0008Calcium 0.00078Magnesium 0.00045COD 0.00062DOC 0.00034TOC 0.00034NMVOC, nonmethane volatile organic compounds; VOC, volatile organic
compounds; COD, chemical oxygen demand; DOC, dissolved organic car-bon; TOC, total organic carbon.
183Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
Other impacts besides GW from each stageare shown in Table 4 and Fig 2. The syngaselectricity stage generally had very little impactsin all the categories from the cradle-to-gravechain. Syngas production by the Tucker RNGunit imposed significant impacts in most ofthe categories, ie ozone depletion (99.9%), GW(60.8%), carcinogenics (70.1%), noncarcino-genics (65.2%), ecotoxicity (68.2%), and fossilfuel depletion (66.8%). This was because of thesignificant amount of propane consumption for
thermochemical reaction heating. The feedstockprocess stage made significant impacts in smog(66.5%), acidification (69.3%), eutrophication(56.7%), and respiratory effects (69.5%). Thesewere mainly from transportation fuel and elec-tricity consumption.
Cradle-to-grave cumulative energy consumptionwas calculated from the LCI output from theSimaPro model for 1-kWh syngas electricityand other fossil or biomass electricity. The
Table 4. Life cycle impact assessment results for cradle-to-grave syngas electricity at various life cycle stages (withoutconsidering carbon sequestration by biochar).
Figure 2. Contribution analysis for three stages within the cradle-to-grave system of the syngas electricity.
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estimated cumulated energy demand values areshown in Table 5. An approximate 16.64 MJ ofenergy was needed to produce 1 kWh of elec-tricity with the technology and material sourcestudied in this project. This is at the high endof energy consumed to produce electricity withdifferent technology and an alternative resource(Table 5). However, within the 16.64 MJ energyconsumed, 38.7% was from a renewable energysource, such as biomass, wind, solar, or hydroand the rest (61.3%) was from nonrenewablefossil energy sources. In terms of the type ofenergy consumed in each of the three stages,more renewable biomass energy was consumedin the feedstock processing stage than boththe syngas producing and electricity generationstages because it used woody biomass heatingfor feedstock drying and some processing. Car-bon dioxide (ie biogenic CO2) emissions fromburning woody biomass were tracked but notconsidered in estimating the GW impact. Theneutrality assumption for biogenic carbon fromwood is valid for the United States, becausethe national-level inventory reporting showsoverall increasing and/or neutral forest carbonstocks in recent years (USFS 2011; Woodallet al 2015; USEPA 2016a). Feedstock dryingand processing took place at the sawmill witha wood boiler producing process heat for dry-ing. The endothermic reaction of the TuckerRNG unit was sustained by propane combus-tion; therefore, the thermochemical conversionwas identified as the major fossil fuel energyconsumption (ie environmental hot spot) for thewhole system.
Carbon Sequestration Effect from Biochar
In this analysis, biochar was produced from theTucker RNG unit as a by-product, thus takingno environmental burden from the process. Thestudy allocated all environmental burdens to thesyngas product. However, in the case of bio-char, the resultant product is highly stable andrecalcitrant, with high carbon content. There-fore, decomposition can be delayed for hun-dreds to thousands of years, beyond currentGHG accounting time frames (Cowie andCowie 2014). Thus, it is important to model thisdelay in the emissions to demonstrate direct cli-mate change impacts from biochar in the stud-ied system.
Biochar is characterized by stable aromatic Cstructures and low bulk density with high ashcontent. The stable storage of biochar in soilsrepresents a long-term removal of atmosphericC, ie C sequestration (Sohi et al 2010). Thereare two types of carbon movements in the eco-system. The movement of C from one reservoirto another is called carbon accumulation. Themovement of C from the atmosphere into a res-ervoir is called carbon sequestration. Accordingto Ciais et al (2013), carbon sequestration as aCO2 removal method can be defined as theuptake of C-containing substances, and in par-ticular CO2, into another reservoir with a longerresidence time. Biochar produced from thisstudy will either be applied to the ground as soilamendment or used as a precursor for activecarbon that can sequester carbon for hundredsof years with little degradation (ie C emissions).
Table 5. CED for 1-kWh electricity generated by different technologies.a
Type Cumulated energy (MJ)
Electricity, biomass, at power plant/US 0.032Electricity, Tucker RNG syngas, at eGrid, NWPP 16.64Electricity, bituminous coal, at power plant/US 14.13Electricity, lignite coal, at power plant/US 20.59Electricity, anthracite coal, at power plant/RNA 17.12Electricity, natural gas, at power plant/US 12.69Electricity, at eGrid, NWPP, 2008/RNA U 7.28
CED, cumulated energy demand; NWPP, northwest power pool.a RNG, renewable natural gas.
185Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
If the biochar produced from the Tucker RNGunit as a by-product is intended to be applied asa soil amendment, the benefit of C sequestrationto slow or even reverse the increase in atmo-spheric concentration of CO2 should apply tothe GHG emission accounting. From the mate-rial ultimate chemical analysis, biochar fromforest thinning residue had a fixed carbon con-tent as high as 90% on a dry weight basis(Gu and Bergman 2016). Based on Wang et al(2014), a carbon stable factor of 85% was cal-culated for the biochar generated from theTucker RNG unit. With this, the total C in thebiochar produced as a by-product for generating1-kWh syngas electricity can be calculatedand converted to CO2-equivalent weight, as adecrease in the total GHG emission accountingfor the entire process. The C sequestration bythe biochar directly decreased the GW impactas shown in Fig 3. However, transportation ofbiochar, biochar spreading, and soil manage-ment practices and their associated environmen-tal impacts were not included in this studybecause it is outside the boundary defined in
this analysis. The GHG emissions from burningfossil fuels in these activities would probablydecrease the benefits of biochar’s carbon seques-tration (Gaunt and Lehmann 2008; Bergmanet al 2016).
Comparing GHG Emissions of SyngasElectricity with Other ElectricityTechnologies
LCA for coal electricity, NG electricity, biomassdirect-combustion electricity, and the NorthwesteGrid profile electricity were performed usingSimaPro modeling software with the data inthe US LCI Database. Figure 3 shows thecradle-to-grave results of GHG emission fromthe LCA output. For 1-kWh electricity generatedby the Tucker syngas produced from forest resi-due chips, the cradle-to-grave GHG emissionswere estimated to be 0.748 kg CO2-eq/kWhwithout taking biochar carbon sequestrationinto consideration. When the carbon sequestra-tion from the biochar by-product was applied,the GHG emissions were decreased by 56% to
Figure 3. Global warming potential (GWP) impacts for various electricity sources and technologies with and withoutcarbon sequestration accounting (RNG, renewable natural gas).
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0.330 kg CO2-eq/kWh. Thus, a notable influ-ence was discovered from carbon sequestrationby the by-product biochar when included andshould be emphasized in future analysis ofbio-based renewable electricity-generating tech-nologies. Coal and NG electricity GW values of1.079 and 0.72 kg CO2-eq/kWh, respectively,were substantially higher than that of the syn-gas electricity studied here. Electricity gener-ated from biomass direct combustion had amuch lower GW impact (0.046 kg CO2-eq/kWh) because of little fossil fuel consumptionand neutral impact to the environment regard-ing biogenic CO2 emission. To put these resultsin context, Schreiber et al (2012) did a meta-analysis of LCA for electricity generation fromdifferent regions (Europe, United States, Japan,and global) and different fuels (hard coal, lig-nite, and NG). They provided an absolute GWpotential of the pulverized hard coal combus-tion technology without any carbon capturefrom 0.765 to 1.092 kg CO2-eq/kWh. Then,in a systematic review (Whitaker et al 2012),an LCA of utility-scale coal-fired electricitygeneration systems reported 0.675 to 1.689 kgCO2-eq/kWh.
GHG Performance Indicator
To compare GHG performance of the TuckerRNG syngas electricity to fossil or other basedelectricity, the GHG performance indicator fromSebastian et al (2011) is used here and definedas the following:
GHGfossil or other � GHGsyngas� � �
GHGfossil or other
¼ GHGperformance inpercentð ÞThis GHG performance indicator represents theGHG improvement of syngas electricity com-pared with fossil or other source equivalents.The GHG emission for syngas electricity with
biochar carbon sequestration was used in thecalculations. The performance indicators for elec-tricity of various sources are shown in Table 6.The GHG performance of the studied syngas elec-tricity demonstrated approximately 70% improve-ment compared with coal-based electricity,greater than 50% improvement compared withNG electricity, and even 34% improvementcompared with commercial eGrid electricity forwestern Montana where a Tucker RNG unitwas in the plans. However, this conversion-technology-produced syngas electricity cannotbe compared with biomass direct-combustionelectricity because the indicator was negative,as one would expect. This was because the bio-mass electricity consumed woody biomass, acarbon-neutral fuel as defined in the LCA, andconsumed little fossil fuel in the process exceptfor woody biomass processing and transporta-tion. More significantly, no fossil fuel use wasrequired to keep the reaction going for directcombustion unlike the thermochemical conver-sion mainstream process of the Tucker RNGunit. Thus, it performs much better for GHGemission reduction potential than the studiedsyngas electricity system. This particular resultought to be taken in context because the overallBRDI project is driven by exploring the oppor-tunities and the economics of generating newbioenergy and bioproducts such as AC.
Although not common, relatively high GHGemissions for biomass energy production dooccur. In some cases, the process of producingbioelectricity from biomass feedstock is energy-intensive and therefore performs even worse forGHG emissions than does fossil fuel electricity(Sebastian et al 2011). Turconi et al (2013)thoroughly reviewed LCA research for variouselectricity generation technologies and com-pared environmental impacts for these technolo-gies. Figure 4 shows the range of data collected
Table 6. GW reduction from Tucker RNG syngas electricity compared with the fossil fuel-based electricity.
Coal Natural gas Northwest US eGrid Tucker RNG syngas
187Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
by Turconi et al (2013) and the studied syngaselectricity GW impact value. The ranges of theGW values discovered from the literature foreach type of electricity technology were drawon the plot and with the midpoint on each barshowed as the averages. The Tucker syngaselectricity GW was between that for renewable-energy-generated and fossil-fuel-based electric-ity technologies. Regardless of these outcomes,it is important to remember, one of the project’soriginal goals was to produce a high-qualitybiochar (ie low variability in properties) as aprecursor to activated carbon (AC), a high-value product, and not biochar as a soil amend-ment. This endeavor requires greater control ofthe thermochemical conversion process. There-fore, one could expect to have a GW impact
higher than the one for biomass direct-combustionelectricity seen in this study.
Table 7 reveals the cradle-to-grave LCA com-parison of syngas electricity with other sourcesof electricity. The other environmental impactsof syngas electricity ranged between biomass-produced electricity and fossil-fuel-producedelectricity in different impact categories because,although syngas electricity used woody biomassfor its feedstock, propane gas was consumedduring thermochemical conversion. Specifically,the LCA for syngas electricity showed the hot-spot of the overall environmental impacts wasfrom LPG (proxy for propane gas) combus-tion to maintain the thermochemical reactionin the mainstream process. Thus, a scenariowith substituting the low-heat value waste tar
Figure 4. Global warming (GW) impact values for various electricity-generating technologies and the syngas electricityestimated in this study (RNG, renewable natural gas). Each bar represents the range of the GW values; midpoint on eachbar represents the average represented GW.
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from the process for LPG was analyzed andpresented next.
Scenario Analysis
Quantifying GW showed both the C benefitsof sequestering biochar and the C hot-spotssuch as from burning propane to maintain theendothermic reaction in the Tucker RNG unit.If decreasing or substituting propane usage inthe Tucker RNG unit is possible, GW impactcould be further decreased. During the thermo-chemical conversion process in the Tucker RNGsystem, low-energy (waste) syngas was producedwithout being collected for use. Collecting andusing this low-energy (waste) syngas to sup-plement propane usage could further decreasethe overall GHG emissions (ie fossil CO2)associated with the cradle-to-grave LCA ofsyngas electricity. Therefore, a scenario analy-sis was conducted with a 30% propane usereduction by the substitution of now-unusedlow-energy syngas produced from the TuckerRNG unit. The GW impact was decreased by41% in total from the cradle-to-grave LCA resultfor syngas electricity (from 0.330 to 0.195 kgCO2-eq/kWh).
Another scenario analysis on the fuel sourcefor the Tucker RNG unit was conducted. Pro-pane was used for the thermochemical reactionheating in the system because of the limit ofavailable fuel on-site. Therefore substituting
propane with NG for reaction heating is assumedand modeled for the LCA analysis. To keepenergy equivalence of 1708 MJ/hr, the requiredamount of NG for the same conversion was cal-culated based on the HHVs of propane and NG.Because NG (consisting primarily of methane)generates fewer CO2 emissions per BTU thanpropane when burned, thus the LCA resultdemonstrated that the GW impacts for this sce-nario was reduced about 19% from the basecase of propane heating.
Intention of this Tucker technology was to con-vert forest residues from restoration manage-ment to generate renewable energy to substitutefor fossil fuel energy. Therefore, only when therenewable energy credit (REC) is gained for thebioelectricity generated from the Tucker RNGsyngas, can profits be generated for investorswho are interested in the Tucker technology.When no REC is applied for the Tucker syngaselectricity studied here, the economy of suchapplications would hinder its push for theshare on the electricity grid. Then the scenarioof applying the bioenergy back to feed its ownsystem to produce more valuable bioproductswould be a case for study. The LCA modelwas modified for this scenario and analysiswas run for comparing the GW potential again.Results showed a 25% reduction in GW impact(in kg CO2-eq) if recycling 50% of the out-put syngas back to the Tucker system forthe thermochemical conversion process while
Table 7. Life cycle impact assessment result for cradle-to-grave syngas electricity and comparison with other electric-ity types.
189Gu and Bergman—CRADLE-TO-GRAVE LCA OF SYNGAS ELECTRICITY FROM WOODY BIOMASS
simultaneously replacing propane with NG forthe remainder.
CONCLUSIONS
Generating electricity from renewable sourcessuch as woody biomass from sustainable forestscan have relatively low GW impacts comparedwith electricity generated from coal and NG. Inthis study, generating electricity from the syn-gas through thermochemical conversion tech-nology such as the Tucker RNG unit resulted ina notable GHG reduction compared with fossil-fuel-based electricity, especially considering thecarbon sequestration effect of biochar by-product. Because of its long-term stability inthe biochar, carbon stored in the biochar equatesto CO2 removed from the atmosphere. In partic-ular, energy from woody biomass rather thanfossil fuels leads to avoidance of fossil CO2
emissions, which are a substantial contributionto climate change.
The sum of these two effects associated withsyngas electricity, a renewable carbon-neutralresource and a C sequestration effect, notablylowers the GW impact (ie GHG emissions). Itis known that burning fossil fuels for electricitygeneration is the main contributor to climatechange (Hertwich et al 2013); thus, the con-sumption of biomass (directly combusted orindirectly derived) for bioelectricity is assumedto be carbon neutral. However, carbon neutral-ity for the biomass burned to generate electric-ity continues to be questioned (USEPA 2016).Regardless of biogenic C neutrality, GHG(ie fossil CO2) emissions are generated from cul-tivation, harvesting, processing, and transporta-tion processes that contribute to climate change.This study tracked these GHG emissions includ-ing fossil and biogenic CO2 and were includedin the analysis. In addition, consuming woodharvested from sustainably managed forests pro-vides substantial air quality benefits by avoidingparticulate matter and CO2 emissions related toburning through forest fires or natural decompo-sition of forest thinning residues. As the mostrecent US Forest Carbon Accounting Framework
(Woodall et al 2015) reported, the forest hasnumerous carbon pools that emit carbon throughdecay and combustion, but it serves an evenmore active role as a sink of carbon, in contrastto fossil fuels, which only serve as a carbonemission source.
Recommendations for future work for the broaderproject include using the biochar as a coproductinstead of a by-product in the LCA frameworkand then evaluating the additional life cyclestage of producing AC. The reason is that AChas a higher market value than biochar as asoil amendment. However, it takes processingin tightly controlled environments such as theTucker RNG unit to generate the physical prop-erties required, which means additional energyand materials are required to make it, as thisstudy has shown.
ACKNOWLEDGMENT
We thank Richard Tucker and David Barbee(Tucker Engineering Associates), Van Morris(US Microgrid), for technical assistance onTucker RNG unit operation process. We grate-fully acknowledge financial assistance for thisresearch project provided by the U.S. Depart-ment of Agriculture (USDA) National Instituteof Food and Agriculture Biomass Research andDevelopment Initiative (BRDI) award no. 2011-10006-30357. BRDI is a joint effort betweenthe USDA and the U.S. Department of Energy.
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