‘Woods-to-Wake’ Life Cycle Assessment of residual woody biomass based jet-fuel using mild bisulfite pretreatment

Biomass and Bioenergy 108 (2018) 207–216

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Research paper

‘Woods-to-Wake’ Life Cycle Assessment of residual woody biomass based jetfuel using mild bisulfite pretreatment

T

Indroneil Gangulya, Francesca Pierobona,∗, Tait Charles Bowersa, Michael Huisengab, Glenn Johnstonc, Ivan L. Eastina a

Center for International Trade in Forest Products, School of Environmental and Forest Sciences, University of Washington, Seattle, WA 98195, United States Sustainability and Energy, WSP Parsons Brinckerhoff, Boulder, CO 80302, United States c Regulatory Affairs, Gevo, Inc., CO 80112, United States b

A R T I C L E I N F O

A B S T R A C T

Keywords: LCA Lignocellulosic feedstock Biofuel Iso-paraffinic kerosene Harvest slash Avoided impacts

The residual woody biomass (a.k.a. harvest slash) produced during forest harvest operations in the Pacific Northwest is generally collected into piles and burned and/or left on the forest floor to decompose. Producing drop-in biofuels from this residual cellulosic feedstock can provide an alternative use for this unused resource while simultaneously displacing petroleum based fuels. Utilizing a ‘Woods-to-Wake’ (WoTW) Life Cycle Assessment (LCA) approach, which is comparable to a Well-to-Wake (WTW) LCA for petroleum based aviation fuel, this paper assesses the environmental implications of feedstock recovery, production, and utilization of residual woody biomass based biojet fuel. This paper also presents a comparative assessment of the environmental implications of substituting petroleum based jet fuel with that of residual woody biomass based biojet fuel. The bioconversion process presented in this paper uses a mild bisulfite pre-treatment of the feedstock to liberate the C5 and C6 sugars which then go through enzymatic hydrolysis and saccharification to produce fermentable sugars. The sugars are then converted to bio-jet fuel (iso-paraffinic kerosene, IPK) using a proprietary biocatalytic fermentation and oligomerization process. The WoTW environmental implications of producing woody biomass based jet-fuel are then compared to the WTW environmental impacts of producing petroleum based jet-fuel. The results reveal that the WoTW global warming impact of residual wood based biojet fuel represents a 78% improvement over the WTW global warming impact of petroleum-based jet fuel.

1. Introduction Interest in the biochemical conversion of non-merchantable woody biomass (especially harvest slash) into biofuels and other usable energy products is continually growing [1–3]. One of the reasons for this increased interest is that, as countries seek ways to reduce GHG emissions, renewable forest based bioenergy is seen as an appealing alternative to fossil fuels [4]. In the US, the significance of converting residual forest biomass into biofuels goes beyond fossil fuel displacement. Typical forest harvest operations in the US, especially along the western part of the country, leave a considerable volume of unused woody biomass in the forest in the form of treetops and branches [5]. Given the limited economic feasibility of extracting these harvest residues, the residual woody biomass is typically piled and burned in the forest as prescribed burns [6–8]. Moreover, the lack of a reliable market for woody biomass has reduced the economic incentives to conduct

forest thinning activities, resulting in the steady accumulation of woody biomass in forests which have reached hazardous levels [9,10]. This increasing volume of woody biomass has contributed to the increase in the number of catastrophic wildfires that have occurred in the western US over the past decade. The burning of woody biomass in forests (both prescribed burns and wildfires) is a major source of greenhouse gas emissions in the western US [11,12]. Woody biomass burning results in emissions of various compounds, including carbon dioxide (CO2), carbon monoxide (CO), nitrogen oxides (NOx), volatile and semivolatile organic compounds (VOC and SVOC), particulate matter (PM10 and PM2.5), ammonia (NH3), sulfur dioxide (SO2) and methane (CH4) [13]. While some of the emitted chemicals, such as CO2 and CH4, are potent agents of global climate change [14], biomass burning also adversely affects local and regional air quality, with acute negative impacts on human health at the local and regional levels [15]. One strategy for the increased utilization of thinning and post-

∗

Corresponding author. E-mail addresses: [email protected] (I. Ganguly), [email protected] (F. Pierobon), [email protected] (T.C. Bowers), [email protected] (M. Huisenga), [email protected] (G. Johnston), [email protected] (I.L. Eastin). https://doi.org/10.1016/j.biombioe.2017.10.041 Received 15 September 2016; Received in revised form 5 April 2017; Accepted 28 October 2017 0961-9534/ © 2017 Elsevier Ltd. All rights reserved.

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Abbreviations ASTM CO CO2 FRS GHG GREET GW GWP iBuOH IPK ISO

LCA LCI MBS NH3 NOx PM SO2 SSL TRACI

American Society for Testing and Materials Carbon monoxide Carbon dioxide Fermentation residual solids Greenhouse gas Greenhouse gases, regulated emissions, and energy use in transportation Global warming Global warming potential Isobutanol Iso-paraffinic kerosene International Standards Organization

VOC WoTW WTW

Life cycle assessment Life cycle inventory Mild bisulfite Ammonia Nitrogen oxide Particulate matter Sulfur dioxide Spent sulfite liquor Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts Volatile organic compounds Woods-to-Wake Well-to-wake

gas emissions of cellulosic bio-fuel must be 60% lower than the carbon emissions generated during the production of petroleum-based jet fuel in order to meet public procurement guidelines [23]. It has been suggested that the replacement of fossil fuels with biofuels produced from forest residues and forest thinning could substantially reduce greenhouse gas emissions [24]. To estimate the overall environmental impact associated with recovering residual woody biomass to produce bio-jet fuel, as well as any net reduction in emissions to the atmosphere achieved by displacing fossil fuels, the Life Cycle Assessment (LCA) method is used. Life Cycle Assessment (LCA) is an internationally recognized methodology to assess the environmental impacts of a product or activity (a system of products) over its entire life cycle. LCA has evolved into an internationally accepted method for analyzing the complex environmental impacts of a product [25]. This paper presents a comprehensive Life Cycle Assessment (LCA) of forest residue based aviation fuel using a ‘Woods-to-Wake’ (WoTW)

harvest residual woody biomass is its conversion to advanced second generation drop-in fuels. Conversion of the residual biomass into biofuel will not only remove slash piles from forests which mitigates the negative environment impacts of decaying or burning [16,17], but it will displace fossil fuels, thereby reducing our reliance on petroleumbased fuels. Renewable second generation (from non-food sources) jet fuel is a practical solution for the aviation industry looking for an alternative carbon-reduction strategy that requires no aircraft or engine modifications [18,19]. Despite the potential environmental benefits of using forest residuals, the economic feasibility of extracting residuals from the forest is limited due to a combination of low market demand and high collection and transportation costs [20,21]. Recently, the use of clean renewable fuels has been encouraged since the Energy Independence and Security Act was signed into law in 2007, providing meaningful economic opportunity for the reduction of foreign oil dependence and greenhouse gas emissions [22]. The US Energy Information Administration (EIA) requires that the overall greenhouse

Fig. 1. System boundary of the residual woody biomass based jet-fuel LCA.

208

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scattered around the forest floor during the harvest and skidding operations. Based on empirical time-motion studies, it is estimated that approximately 65% of the residual biomass gets accumulated into slash piles at the primary harvest landings [29], the remaining 35% gets scattered on the forest floor. This research further assumes that 10% of the biomass in the slash pile is inadvertently left behind at the landing during the loading, chipping, and transporting of the biomass from the landing site to the biomass processing facility [28]. The residual woody biomass (a.k.a harvest slash) produced during commercial harvest operations in the Pacific Northwest is generally burned in the forest or left on the forest floor to decompose. Based on these conditions, it is estimated that only 58.5% of the total harvest residuals generated during the timber harvest operation are delivered to the pre-treatment facility for conversion into biofuel. Furthermore, according to the preliminary assessment of the feedstock quality, it is estimated that approximately 91% of the delivered feedstock material will pass through the screen and be sent to the pretreatment chamber, while 9% will be rejected and be used as fuel in the boiler [30]. Based on the forest management practices, topography and existing road network in the inland west region, this paper adapts the feedstock logistics model developed in ‘Modeling the Processing and Transportation Logistics of Forest Residues Using Life Cycle Assessment’ [31], as presented in Tables 1 and 2. The harvest system and in-woods feedstock handling benchmark scenario data are presented in Table 1. The transportation distance of the woody biomass from the harvest site to the processing facility on different types of roads is presented in Table 2. The loose residues are transported from the primary landing to the secondary landing in a 30 m3 bin truck, where they are processed using a stationary grinder [28,32]. In this scenario, the residuals are transported from the primary landing to the secondary landing where the grinder and direct loader are located because a 100 m3 chip van cannot navigate the tight curves of the forest spur roads. The processed residues are directly loaded into the 100 m3 chip van at the secondary landing and transported to the pretreatment facility. Based on the biomass availability conditions and the total biorefinery capacity, the average transportation distance from the primary landing to the biomass processing facility is 80 km. The feedstock is unloaded, screened and stored in metering bins at the pretreatment facility.

approach which is aligned to the Well-to-Wake (WTW) analysis used in the Argonne Lab's GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) analysis [26,27]. One of the significant potential benefits of converting residual woody biomass to biofuel is to avoid burning them in prescribed burns. This study incorporates the benefits of the avoided environmental burdens associated with biomass burns, as a result of biomass conversion to biofuels, using the LCA approach. Hence, the overall objectives of this paper can be summarized as follows:

• Perform a detailed Life Cycle Assessment (LCA) to evaluate the en• •

vironmental impacts of using woody biomass as a feedstock for conversion into bio-jet fuel. Incorporate the avoided environmental impacts associated with piling and burning the woody biomass within the forest into the LCA calculations. Compare the LCA-based environmental impacts of the Woody Biomass-Based Jet Fuel to Petroleum-Based Jet Fuel.

2. Materials and methods 2.1. System boundary A comprehensive Life Cycle Assessment (LCA) of forest residue based aviation fuel is performed using a ’cradle-to-grave’ approach where ‘cradle’ is defined as forest residues collected into slash piles and ’grave’ is defined as the fuel combustion during flight in an aircraft. The product system is woody biomass based bio-jet fuel whose function is to fuel an aircraft during flight. The functional unit of the system is 1 GJ of energy produced by fuel combustion. The study is based on a production facility which is scaled to produce 113 Gg of IPK (bio jet fuel) using 700 Gg of screened woody biomass (on an oven dry basis). A simplified diagram of the system boundaries associated with the biofuel production process is shown in Fig. 1. As shown in Fig. 1, the overall system boundary for developing the LCA of the bio-jet-fuel consists of the following components: (i) feedstock: (ii) biomass conversion: (iii) combustion in the jet engine. The individual components of the flow chart presented in Fig. 1 are explained in greater detail in the following sections. A mass allocation, between logs and residual biomass (tops and branches), is used for evaluating the upstream environmental burdens associated with the piled woody biomass at harvest landing. 100% of the burdens associated with all activities beyond the primary harvest landing are assigned to the biofuel processes.

2.1.2. Biomass Conversion and Biofuel Refinery The analysis presented in this paper assumes an integrated biomass conversion facility, where the biomass storage, extraction of sugar from woody biomass and conversion of sugar into bio-jet-fuel, are all undertaken at the same location. This paper uses a mild bisulfite pretreatment of the feedstock to liberate the C6 sugars and break down the lignocellulosic material which then is mixed with a cellulase enzyme and hydrolyzed to produce a fully saccharified sugar stream [33]. The fermentable sugars are then converted to isobutanol (iBuOH) using a proprietary bio-catalytic fermentation and oligomerization process to produce bio-jet fuel (iso-paraffinic kerosene, IPK) [34]. Therefore, in this study the overall conversion of residual woody biomass to aviation

2.1.1. Feedstock This study assumes collection of residual biomass from the harvest landings to be used as the feedstock for bio-conversion to bio jet fuel. The overall biorefinery is scaled at 700 Gg of screened woody biomass per year. Assuming 9% rejects the yearly feedstock demand is modeled for 763 Gg of residual woody feedstock per year, delivered to the screening facility. Given the logistical challenges in the case of forest residue based bio-fuel, feedstock collection and in-woods handling play a significant role in the overall environmental impact of the biofuel [1]. Geographical location, regional vegetation, and topographical characteristics significantly affect the environmental impacts associated with collecting and transporting the woody residues from the forest landings to the biomass processing facility [28]. This paper uses the environmental implications of producing woody biomass in the Western Washington region. A mass allocation approach is used to account for upstream burdens associated with the feedstock (includes harvesting, forwarding and skidding operations). The assumptions presented in the following paragraph details the proportion used for mass allocation. Woody slash piles at harvest landings are generated during harvest operations, with a significant portion of the residual biomass being

Table 1 Feedstock logistic equipment configuration. Source: ‘Modeling the Processing and Transportation Logistics of Forest Residues Using Life Cycle Assessment’ [31].

209

Equipment

Capacity

Productivity [Mg h−1]

Fuel consumption [l h−1]

Forwarder Escavator loader Grinder Post-grinder loader Bin truck Chip van

130 kW 30 kW 560 kW 105 kW

31.4 36.2 45.4 45.4

29.9 25.6 96.5 21.3

30 m3 100 m3

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calcium bisulfite (Ca(HSO3)2) at 145 °C for a residence time of 4 h to liberate the hemicellulose (C6) sugars and break down the lignocellulosic material in preparation for enzymatic hydrolysis [33]. The pretreatment stage results in two streams being produced: the first stream, the pretreated pulp, contains the pulp solids and proceeds to the enzymatic hydrolysis department; the second stream, the spent sulfite liquor (SSL), contains primarily soluble sugars, extractives and lignosulfonate, and proceeds directly to fermentation.

Table 2 Feedstock logistic benchmark scenario for road-type specific transportation distances. Source: ‘Modeling the Processing and Transportation Logistics of Forest Residues Using Life Cycle Assessment’ [31]. Road type Avg. speed

Dirt (8 km h−1)

Gravel (24 km h−1)

Highway (72 km h−1)

Total

One way haul km

1.6

10

68.4

80

2.1.2.2. Enzymatic hydrolysis. Microorganisms used to convert cellulose into biofuels have limited efficiency in digesting the material because of the complex structure of the polysaccharides constituting the cellulose stream. In order to improve the efficiency of the biofuel production, hydrolysis is generally applied at an industrial scale prior to fermentation [41]. In the enzymatic hydrolysis the lignocellulosic material is mixed with a cellulase enzyme and hydrolyzed to decompose the macromolecules of polysaccharides contained in the pretreated pulp into simple monosaccharides units producing a fully saccharified sugar stream ready for fermentation. The enzymes are produced using purchased glucose and a filamentous fungi strain (Trichoderma reesei), induced with sophorose. The pulp stream from pretreatment (80 °C, pH 1.8) is cooled to 50 °C and pH is adjusted by lime to a pH of 5.0. The hydrolysis takes place over 72 h and the released sugars are then used for fermentation into isobuthanol.

biofuel is separated into four different sub-processes; (i) pre-treatment of residual woody biomass, (ii) enzymatic hydrolysis, (iii) fermentation and oligomerization to produce iso-paraffinic kerosene (IPK), and (iv) boiler, turbogenerator, wastewater treatment and utilities. Major mass input and output data, in aggregated form, for the biorefinery units are summarized in Table 3. The overall distribution of energy input and output data is presented separately in Table 4. It may be noted, that the overall energy output in the turbogenerator unit of Table 3 is carried over to Table 4 and is then distributed among the other biorefinery units. Further details regarding process level inputs and outputs data can be found in ‘Process Design and Economics for Biochemical Conversion of Softwood Lignocellulosic Biomass to Isoparaffinic Kerosene and Lignin Co-products’ [35]. 2.1.2.1. Pretreatment of biomass. The complex structure of lignocellulosic biomass (i.e. wood, wheat straw and corn stover) makes it difficult to break down the lignocellulosic material and liberate the C6 sugars to utilize the material in biofuel production [36,37]. The composition of softwood harvest residue, which primarily includes tops and branches, constitutes of a mass fraction of 40–45% cellulose, 20–25%hemicellulose (including xylan, arabinan, galactan and mannan), 25–30%lignin (soluble and insoluble) and 8–10%bark and extractives. To dissociate the plant cell wall and remove the lignin in order to improve the accessibility of the enzymes and enhance the digestibility of the cellulose in the hydrolysis process, a pretreatment stage is necessary [38–40]. Accordingly, the feedstock is treated with

2.1.2.3. Fermentation and oligomerization. The fermentable sugars are then converted to isobutanol (iBuOH) using a proprietary fermentation biocatalytic process; the Gevo, Inc. patented GIFT® process [34]. The saccharified pulp biomass and the SSL stream are fermented separately. The fermentation process requires up to 48 h at 34 °C and a pH of 4.3. Beer produced during the fermentation process is then distilled to produce isobutanol. The separated and concentrated isobutanol is dehydrated using a catalyst to produce isobutene. The isobutene undergoes an oligomerization process using another catalyst and is fully saturated with hydrogen to produce iso-paraffinic kerosene (IPK).

Table 3 Aggregated inputs and outputs for the biorefinery units. Source: adapted from ‘Process Design and Economics for Biochemical Conversion of Softwood Lignocellulosic Biomass to Isoparaffinic Kerosene and Lignin Co-products’ [35]. Feedstock preparation and pretreatment Inputs [kg h−1] Forest residual

Enzymatic hydrolysis

450 018.3 Inputs [kg h−1] 91 372.9 Pretreated pulp

404 513.6 65 507.8

Sulfur

2 993.7 Corn steep liquor

199.6

Calcium carbonate

2 948.4 Glucose

Sodium hydroxide

113.4 Lime

3 401.9 471.7

Water* Process water**

60 920.9 NH3 291 669.0 SO2 HTEC Enzyme Water* Process water**

136.1 18.1 263.1 129 319.2 205 196.1

Outputs [kg h−1] Pretreated pulp Spent sulfite liquor

450 022.6 Outputs [kg h−1] 65 507.8 Hydrolysate 24 604.7 Saccharification vent

404 514.4 69 662.7 1 711.7

Pretreatment combined vent Forest residual fines Water*

850.9 Water* 8 223.6 350 835.6

333 140.0

Fermentation and upgrading

Waste water treatment

Inputs [kg h−1] Hydrolysate

Inputs [kg h−1] Spent sulfite liquor stillage Fermentation residual stillage Chemicals

656 779.9 21 191.8

Water*

625 421.6

710 498.0 69 662.7

Spent sulfite liquor

24 602.9

Proprietary inputs [34] Water*

29 057.1

Outputs [kg h−1] IPK Spent sulfite liquor stillage Solids (57% lignin) Fermentation residual stillage Water*

*Total water content of the inputs/outputs. **Process water added to the system (recycling water from wastewater treatment).

210

587 175.3

710 500.0 12 201.6 21 191.8 28 901.6 9 063.6 639 141.4

Outputs [kg h−1] Waste water Biogas (30% methane) Sludge Emissions to air Water* Recycling water**

Boiler turbogenerator (Extraction condensing)

9 063.6 1 102.9

656 782.8 174.4 24 316.0

Inputs [kg h−1] Solids (57% lignin) Sludge

435 023.3 28 901.6

Biogas (30% methane) Forest residual fines Water* Process water** Steam condensate Make up water

24 316.0

34 99 194 42

Outputs [kg h−1] Electricity [GJ] Steam

435,023.1 183.6 194 854.1

2 497.7

Waste

4 636.2

Emissions to air

28 818.5 596 340.0

Water* Cooling tower evaporation

2 497.7

8 223.6 036.0 474.9 854.2 719.3

2 122.8 61 816.0 34 036.0 142 194.2

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Table 4 Aggregated energy inputs and outputs for the biorefinery units. Source: adapted from ‘Process Design and Economics for Biochemical Conversion of Softwood Lignocellulosic Biomass to Isoparaffinic Kerosene and Lignin Co-products’ [35]. Feedstock preparation and pretreatment

Enzymatic hydrolysis

Fermentation and upgrading

Waste water treatment

Boiler turbogenerator (Extraction Condensing)

Energy Inputs Steam [kg h−1] Electricity [GJ]

Steam [kg h−1] Electricity [GJ]

444.5 0.7

Steam [kg h−1] Electricity [GJ]

122 469.9 87.8

Steam [kg h−1] Electricity [GJ]

0.0 42.1

Steam [kg h−1] Electricity [GJ]

Steam [kg h−1] Electricity [GJ]

0.0 0.0

Steam [kg h−1] Electricity [GJ]

0.0 0.0

Steam [kg h−1] Electricity [GJ]

0.0 0.0

71 939.7 16.9

Energy Outputs (imported from Table 3) Steam [kg h−1] 0.0 Electricity [GJ] 0.0

Steam [kg h−1] Electricity [GJ] (To biorefinery [GJ] Excess to Grid [GJ])

0.0 7.2

194 854.1 183.6 154.7 28.9

electricity grid and an energy credit is attributed to IPK.

The IPK produced by this process meets the requirements of ASTM D7566-17a [56] for hydro-processed synthesized paraffinic kerosene (SPK), a blendstock used in jet fuel [34]. The fermentation residual stillage (FRS) resulting from the fermentation of the saccharified pulp contains the unconverted insoluble solids. The stillage is conveyed to a lignin separator to separate and dewater the solids (mainly lignin) that are sent to the combustor. The SSL stillage is conveyed to the wastewater treatment.

2.1.4. IPK combustion The model assumes that 6.8kg of clean woody biomass (after screening out ash and fines) produces 1 kg of IPK. In the analysis we assumed a calorific value of 43.1 MJ kg−1 for the petroleum based jet fuel of and of 43.2 MJ kg−1 for the bio-jet fuel [42]. Combustion emissions were estimated using the Ecoinvent database for intercontinental air freight since primary data for IPK combustion are not available [43].

2.1.3. Utilities: boiler, turbogenerator and wastewater treatment The wastewater treatment process used in this analysis is a closed loop system consisting of anaerobic digestion, aerobic treatment, membrane bioreactor and reverse osmosis. The SSL stillage from fermentation and the water stream from lignin separation of the FRS stillage feed into the anaerobic digestion chamber. In anaerobic digestion, a mass fraction of 91%of the organic component is destroyed and a mass fraction of 86%gets converted to biogas (methane and carbon dioxide), while the remaining is converted to cell mass [36]. Methane and carbon dioxide are produced at equimolar rate, such that the biogas from the digester is 50% CH4 and 50% CO2. In the aerobic treatment a mass fraction of 96%of the remaining soluble organic matter is removed. The digested material is conveyed to a membrane bioreactor which removes additional organic matter and to a reverse osmosis system for salt removal. The treated water from the waste water treatment is then recycled back to the biorefinery units and used as process water for the pretreatment, enzymatic hydrolysis and fermentation/upgrading units. Biogas produced in the waste water treatment (which constitutes of a mass fraction of 30%methane) along with dewatered sludge from the anaerobic and aerobic treatment processes, is then delivered to the boiler (Tables 3 and 4). A grate stoker-fired boiler burns the biogas, sludge, screen rejects, lignin and unconverted solids to generate high pressure steam which is sent to a steam turbine. The boiler efficiency is assumed to be 80% [36]. The system produces about 350 000 kg h−1 of steam at 454 °C and 6.2 MPa. The turbine has two controlled extractions to deliver process steam with excess steam being condensed to produce electricity. Of the superheated steam produced, a mass fraction of 21% is extracted from the turbine as high pressure steam at 1.4 MPa and 272 °C to be used in the pretreatment stage and a mass fraction of 54% is extracted as low pressure steam at 0.9 MPa and 236 °C to be used in the remaining processes. The rest of the steam is condensed at −0.09 MPa and 46 °C and pumped back to the boiler. A cooling tower supplies cooling water to the condenser and other processes. As is presented in Tables 3 and 4, a total of 183.6 GJ of electricity is produced by the turbogenerator unit. The total biorefinery electricity consumption is estimated to be 154.7 GJ, of which 56.7% is used in the fermentation and oligomerization unit, 27.2% in the wastewater treatment unit, 10.9% in the feedstock preparation and pretreatment unit, 4.7% in the turbogenerator and 0.5% in enzymatic hydrolysis units. After accounting for the overall electricity consumption within the biorefinery units, 28.9 GJ of excess electricity is sent to the local

2.2. Location of the biorefinery The location of the biorefinery plays a significant role in the overall LCA analysis. There are a number of factors used in the analysis that are location specific, others are specific to the region. The annual feedstock demand for the facility is scaled at 700 Gg of screened woody biomass to produce 113 Gg of IPK per year. The overall impact of feedstock collection, in-woods processing and transportation to the biorefinery is heavily dependent on the location of the facility. The LCI data associated with electricity grid, diesel at the pump, baseline jet fuel, etc., are region specific (e.g., for electricity we used the ‘Electricity, at eGrid, NWPP’, which is recommended for PNW). For the analysis presented in this paper, we used a hypothetical location in the Grays Harbor County, in Western Washington. This site is identified based on its proximity to the feedstock, sufficient for the bioconversion facility of the stated magnitude, and the availability of the support infrastructure and site suitable for building a facility of the proposed scale. 2.3. Evaluation methods and model assumptions This study follows the ISO 14040 and 14044 standard [44,45] for the overall LCA framework. The environmental impacts were assessed using TRACI 2.1 (Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts) [46]. The following impact categories were included: global warming, smog, acidification, eutrophication, carcinogenics, non carcinogenics, respiratory effects and ecotoxicity. The life cycle inventory analysis and impact assessments were conducted using SimaPro 8. As per the IPCC Fifth Assessment Report, this paper reports the 100 year impact for the global warming potential [14]. To estimate the environmental burdens from the residual woody feedstock collection and pre-processing, the study uses the Consortium for Research on Renewable Industrial Materials (CORRIM) guidelines and format [25,47] with some project-specific modifications. For the pretreatment of the biomass, a process developed by the USDA Forest Product Laboratory is used to convert forestry residues to fermentable sugar [33]. This simulation is undertaken using ASPEN plus 8.6 using the high-level process area inputs and outputs from the NREL model [36] and applied to the integrated wood based bio-jet fuel process to produce mass and energy balances for the life cycle 211

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simulate the production of electricity and process steam, cooling water, and electricity from the biomass materials which are available for energy conversion. These three models are not directly linked, so the results were combined in an Excel spreadsheet and there may be additional opportunities for heat integration and energy savings that are not reflected in the current energy balance. Environmental burdens from the production of capital equipment, including the fermentation plant, the combined heat and power plant (CHP), the wastewater treatment plant, chemical production and IPK storage equipment were included in the LCA by amortizing their production over the lifetime production volume.

assessment. Process cooling, steam and electrical power loads for the distillation, separation of solids and oligomerization processes were added in so that the total biorefinery (and supporting auxiliary systems) loads would more accurately reflect the differences in feedstock, the different products of fermentation and the differences in the back-end chemistry. The results obtained from the ASPEN plus simulation for the Biomass Conversion and Biofuel Refinery were then used to build the LCA model. The LCA specified that the process chemicals and cellulase enzyme were purchased from ancillary industries and the environmental impacts associated with the production of each of these chemicals were incorporated in the LCA analysis. A mass and energy (M&E) balance was obtained from the Aspen simulation results for the biofuel conversion processes. The M&E balances were combined to create a total M&E balance for the integrated biorefinery. Finally, an energy balance for the combined heat and power (CHP) plant was developed to

2.3.1. Avoided slash pile burn credit framework The feedstock used for producing bio-jet fuel, as mentioned in section 2.1.1, is residual harvest slash. Recovering harvest residues to produce IPK results in avoided emissions from the burning of slash piles

100%

80%

4.3%

10.6%

9.1% 47.9%

9.5%

34.1%

17.7%

60%

40%

17.4% 15.3%

14.8% 6.4%

20%

10.4%

7.3% 3.4%

37.7%

34.9%

28.3%

80.7%

66.8% 58.4%

11.4%

29.1%

8.3% 9.3% 20.4%

24.1%

6.3% 6.6%

0%

-7.3%

5.4%

1.3%

15.3%

10.4% 7.17%

-9.7%

-22.3% -20%

-44.1% -15.6%

-39.0%

-40% -100.0% -60%

-80%

-100.0%

Biomass harvesƟng and logisƟcs Pretreatment EnzymaƟc hydrolysis FermentaƟon and upgrading Wastewater treatment IPK combusƟon in aircraŌ Boiler and turbogenerator Avoided emissions (50% slash pile burn)

-100%

Bi oma ss ha rvesƟ ng a nd logi s Ɵ cs

kg CO2 eq

kg O3 eq

kg SO2 eq

kg Neq

CTUh

CTUh

kg PM2.5 eq

CTUe

Global Warming

Smog

Acidifica on

Eutrophica on

Carcinogenics

Non carcinogenics

Respiratory effects

Ecotoxicity

10.90

4.79

0.15

9.02·10

-3

1.50·10

-7

1.47·10

-6

3.10·10

-3

6.44·10

-9

29.60

1.84·10

-7

3.57·10

-3

0.57

Pretrea tment

0.45

0.12

0.06

4.75·10

-4

Enzyma Ɵ c Hydrolysi s

4.78

0.33

0.06

8.55·10

-3

1.81·10

-7

5.58·10

-6

6.72·10

-3

15.35

FermentaƟ on a nd upgradi ng

11.80

1.08

0.08

1.27·10

-2

1.10·10

-7

9.98·10

-7

4.90·10

-3

9.04

2.10·10

-8

9.07·10

-7

2.88·10

-3

229.40

-3

1.90

Wa s te wa ter treatment

2.85

2.50

0.13

9.10·10

-2

IPK combusƟ on i n a i rcraŌ

0.48

8.11

0.25

1.45·10

-2

4.52·10

-8

4.14·10

-7

4.73·10

Boi l er a nd turbogenerator

-6.97

-0.30

-0.05

-1.43·10

-4

4.39·10

-9

7.15·10

-9

-2.85·10

-3

-1.57

-1.32·10

-2

-3.62·10

-1

-1.25

Avoi ded emi s s i ons (50% s l a sh pi l e burn)

-4.87

-7.46

-0.29

Total

19.41

9.17

0.39

1.12

-6.22·10

-7

-8.22·10

-1.04·10

-7

9.55·10

-9

-6

-0.34

283.03

Fig. 2. Contribution of the LCA phases to the overall impacts for 1 GJ of energy produced by IPK (50% avoided emissions from slash pile burning).

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scope of analysis to only greenhouse gas emissions. The results for global warming potential were also compared against the baseline for the life cycle GHG emissions from conventional petroleum-based transportation fuels sold or distributed in the United States, as determined by the Energy Independence and Security Act of 2007 [22].

that would otherwise occur. Existing slash treatment options include burning of slash pile or collecting, chipping and selling it as hog-fuel or pulpwood. Based on the WA biomass calculator [48], given the existing demand of residual biomass, a conservative estimate of the biomass disposed in the slash piles (excluding the biomass scattered on the forest floor) through non-burn alternatives, ranges between 20 and 40%, depending on the location of the slash piles, which indicates that, given this low demand of hog-fuel or pulpwood in the region, 60–80% of the biomass in the slash pile burning is disposed by burning. Hence, based on ISO 14044 guidelines [45], the avoided environmental impacts of slash pile burning in the region were incorporated in the LCA as a credit. Taking a conservative approach, in this paper we assumed a 50% burn scenario for the avoided credit, which indicates that only 50% of the biomass in slash piles would have been disposed by burning, had we not collected it for bio-conversion and the remaining 50% would have been disposed through other non-burn options. The data for slash pile burn related emissions were developed from published studies that documented and modeled emission rates from both prescribed burns and wildfires [49–52].

3. Results and discussion 3.1. Contribution analysis A complete contribution analysis for the woody biomass based biofuel sub-processes is presented in Fig. 2, for the eight TRACI Life Cycle Impact Assessment (LCIA) metrics. The contribution analysis is presented in a percentage scale along the y-axis, which runs from (+) 100% to (−) 100%. The individual TRACI impact categories are presented along the x-axis. To be able to have a perspective on the scale of the emissions, the actual values are also presented in the lower portion of figure. As can be observed from the figure, some of the sub-processes have a net negative contribution to specific LCIA categories and some net positive. For example, the ‘boiler and turbo-generator’ process produces excess electricity, after feeding all of the steam and electricity requirements, resulting in a net negative impact in multiple categories, with significant net beneficial (i.e., net negative) impacts in global warming and acidification categories. To calculate the electricity credit, the ‘Electricity, at eGrid, NWPP’ LCI data from the US-LCI database is used. The other sub-process with net beneficial environmental impact, with negative impact values in multiple categories, is the avoided emissions which will be discussed later in this section. Two of the LCIA categories, carcinogenics and respiratory effects,

2.3.2. Comparative assessment framework The overall environmental impact associated with the production of bio-jet fuel is then compared against the emissions associated with petroleum based aviation fuel. For this purpose, the LCA emissions associated with 1 GJ of energy produced using bio-jet fuel (iso-paraffinic kerosene, IPK) were compared with those emitted using fossil fuel (kerosene) by means of the Ecoinvent database. The Ecoinvent database is one of the only sources containing a full life cycle inventory for jet fuel used by commercial aircraft, whereas other studies have limited the

Fig. 3. Comparison of the LCA results for IPK with 50% avoided emissions from slash pile burning and petroleum based jet fuel for a functional unit of 1 GJ.

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with aircraft operation, the aircraft emission profile from the Ecoinvent database, a depository of European data, is used.1 For developing an LCI of the petroleum based fuel used in aircraft operation, the inventory data of Jet-A (the predominant petroleum based jet fuel) for Pacific Northwest is created by adapting the Ecoinvent processes to the PNW region, by using US-LCI data for all the sub processes and modifying the supply chain accordingly. The resultant process associated with petroleum based aircraft operation, for PNW, is modeled using Simapro 8.1. It may be noted, that though the U.S. Department of Energy's GREET (Greenhouse gases, Regulated Emissions, and Energy use in Transportation) software has a life-cycle model associated with aircraft operations, for the US, it focuses only on the greenhouse gases and the GWP impact assessment. The GREET software does not provide the necessary data associated with the non-GWP LCIA categories as identified in TRACI. The GWP impact category results obtained from Simapro are included along with the results obtained from GREET. In Fig. 3 the baseline petroleum based jet-fuel is indicated as ‘DOE baseline’, corresponding to GREET 1.3 GWP associated with jet-fuel. It can be observed that the DOE baseline GWP number is similar (within 3%) of that modeled in Simapro using Ecoinvent data. As compared to the DOE baseline, the overall GWP impact associated with woody biomass based jet fuel is 22.1% of that of the DOE baseline, which represents a 77.9% reduction in overall GHG emissions, using a 50% avoided slash burn scenario. From the contribution analysis it can be observed that the GWP reduction associated with the slash burn credit contributes to a reduction of 15.6% of the overall GWP impact associated with the WoTW assessment of the biofuel. This relatively low value of GWP credit is due to the fact that the avoided burdens associated with the large quantity of biogenic CO2 emitted during the slash pile burning is not included in the analysis as per ISO and EPA guidelines. This reduction in the overall global warming potential is primarily due to the avoided CH4 emissions, which are emitted during the slash pile burn. Hence, even without considering the avoided impacts associated with slash pile burning (i.e. 0% avoided burn scenario), the GWP associated with bio-jet fuel is 26.5% of the petroleum based option, which would represent a 73.5% reduction in GWP. Hence, the overall reduction in GHG emissions associated with the residual biomass based jet fuel would meet the EISA mandated 60% reduction with or without the avoided slash burn credit. Compared to fossil fuel, the use of IPK as jet-fuel is not only beneficial in terms of global warming, but also in terms of smog, acidification, and non carcinogenics. Compared to fossil fuel, there is a net benefit (negative impact) for carcinogenics and respiratory effects. However, the impact associated with the use of IPK is substantially higher on the eutrophication and ecotoxicity impact categories. The conversion process from biomass to IPK releases phosphates to the water which contribute to eutrophication. Furthermore, the conversion process also releases HCFC 140, dichlorobenzene, ethyl acetate, propanol and butanol into the air. Both the feedstock and the conversion phases are responsible for the release of heavy metals to the water, contributing to ecotoxicity.

resulted in net negative value, even after factoring in all the emissions associated with biomass collection, in-woods processing and bioconversion. This is due to the significant negative values associated with the ‘avoided emissions’ process and highlights the net beneficial effect of avoiding the slash piles in the forests for developing value added impacts. One of the most important differences associated with open burns emissions (like, emissions from pile burns) and emissions from the tail pipe on an aircraft is the PM2.5 concentration. As can be observed in the ‘respiratory effects’ column of the table associated with Fig. 2, the contribution of the total process related PM2.5 emissions, including that of emissions from aircraft operation, operating machineries and transportation of the feedstock, is less than 5% than the avoided PM2.5 emissions associated with not burning the slash piles. This specific beneficial effect is characteristic of ‘residual’ woody biomass based biofuel, and cannot be achieved in energy crop based biofuels. The results show that fermentation and upgrading phase is the primary contributor to global warming potential, contributing to 37.4% of the GWP, whereas the feedstock collection and in-woods processing contributes 34.5% of the total GWP and enzymatic hydrolysis contributes 15.1%. This relatively high contribution associated with feedstock collection and processing, as compared to other biofuel LCAs [53], highlights the logistical challenges and higher impact associated with collection and in-woods processing of the residual feedstock. All of the energy needs, steam and electricity, associated with the bioconversion process is internally produced within the ‘boiler and turbo-generator’ process. After producing enough steam and electricity to power the biorefinery, the residual woody biomass to IPK production process produces 15% excess electricity (which is transferred to the grid), which plays a significant role in achieving lower GWP impact. Here it may be noted that the relatively low GWP impact associated with pretreatment can be attributed to the fact that all of the steam required for the pretreatment process is internally produced by the ‘boiler and turbo-generator’ process, and these energy flows are contained within the system boundary. According to international LCA standards and guidelines, the release of biogenic carbon dioxide is treated as carbon neutral and does not impact the GWP assessment [23,44,45,54,55]. In case of woody biomass based bio-jet fuel, woody feedstock is the only source of carbon in the fuel, which is renewable, and is considered carbon neutral. The carbon neutrality assumption of biogenic carbon resulted in low contribution of bio-jet combustion in the overall GWP. After factoring in the net negative GWP impacts associated with the credit from the ‘avoided slash pile burns’ and the excess electricity, the overall GWP impact associated with the WoTW of 1 GJ of energy produced by fuel combustion is equal to 19.41 kg of CO2e, not including the biogenic CO2 emissions. As can be observed from the figure, ‘eutrophication’ LCIA and ‘ecotoxicity’ LCIA are mostly caused by the wastewater treatment process which releases several compounds in water, such as phosphates, nitrates, furfural, acetic acid, lignosulfonic acid soluble sugars and dissolved gases (nitrogen dioxide and sulfur dioxide). Combustion of IPK in the airplane engine is the main contributor to the ‘smog’LCIA criterion (47.4%) and is also an important contributor to the ‘acidification’ LCIA (33.7%). The other impacts associated with the combustion of biojet fuel include, ‘eutrophication’ impact (10.6%), ‘carcinogenics’ impact (7.3%) and ‘non carcinogenics’ impact (4.3%).

4. Conclusions The WoTW/WTW comparative analysis of petroleum and residual biomass based jet fuel reveals that a 78% reduction of global warming potential can be achieved by substituting petroleum 100% based jet fuel with residual woody biomass based jet fuel. Highlighting the intensity of the feedstock collection process, the residual woody biomass recovery and in woods processing played a significant role in the overall carbon footprint of the bio-jet fuel, contributing to a third of the overall GWP of bio-jet fuel production process. It may be noted that all the

3.2. Comparative analysis: environmental implications of the bio-jet fuel vs petroleum based jet-fuel For the purpose of the comparative assessment, WoTW LCIA of biojet fuel is compared to WTW LCIA of petroleum based jet-fuel (Fig. 3). In order to undertake the comparative assessment, a WTW LCA of petroleum based jet-fuel is undertaken using two different data sources. Given the US-LCI database does not have the inventory data associated

1 The assumption used in this case is the emission profile of a jet aircraft should remain same independently from the source of the data.

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steam and electricity needs associated with the bio-conversion process is internally produced by utilizing the waste-stream, reducing the net GWP impact associated with the bio-conversion process. Apart from the GWP, the key environmental benefits associated with residual biomass based bio-fuel are the avoided slash pile burns which result in beneficial local air quality impacts resulting from a significant reduction of the harmful carcinogens and pollutants from the air resulting in local health benefits. However, not all LCA indicators favor the biojet fuel. Some of the LCA impact categories, including ‘eutrophication’ and ‘ecotoxicity’ impacts, were worse for the biojet fuel. Further research is needed for modifying the biomass pretreatment and waste stream handling and treating procedures in order to minimize these emissions.

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