1 Document word count (including fig captions): 7944
1
Guayule as an Alternative Source of Natural Rubber: A Comparative Life Cycle 2
Assessment with Hevea and Synthetic Rubber 3
4
Kullapa Soratana1,2, Daina Rasutis2, Habib Azarabadi2, Pragnya L. Eranki3,*, and Amy E. Landis3 5
1School of Logistics and Supply Chain, Naresuan University, Phitsanulok, Thailand 6
2School of Sustainable Engineering and the Built Environment, Arizona State University, USA 7
3Glenn Department of Civil Engineering, Clemson University, USA 8
*Corresponding author: Glenn Department of Civil Engineering, Clemson University, 1 Lowry Hall, 9
Clemson, SC 29634, USA. E-mail: [email protected], Tel.: +1 864-656-3000.
10
Abstract 11
A comparative cradle-to-gate life cycle assessment (LCA) including raw material acquisitions, rubber 12
production and transportation of synthetic rubber (SBR) and two natural rubbers, namely imported Hevea 13
and US-grown guayule, was conducted to quantify environmental impacts and to identify area(s) of 14
environmental improvement. The use of co-products from the two natural rubbers as a source of energy 15
was explored for their impacts and energy offset potential. Three environmental impact categories- ozone 16
depletion (ODP), global warming (GWP), and acidification potential (AP)- and net energy were evaluated.
17
Results indicated that guayule rubber had a lower ODP impact compared to SBR, and lower GWP and AP 18
impacts compared to Hevea rubber. The Hevea rubber production with biodiesel co-product from seed oil 19
scenario had the highest GWP and AP impacts, whereas guayule rubber production with bio-oil co-product 20
from bagasse had the highest ODP impact. Only the scenario of Hevea rubber production with biodiesel 21
production indicated an overall energy production of 3.25 MJ/kg rubber, whereas all other scenarios 22
indicated a net energy consumption. However, co-products contributed to reduction in total energy 23
consumed in every scenario that was investigated. The most promising guayule rubber scenario among the 24
six examined was that with electricity production from bagasse, as it contributed the least to all three 25
impact categories and yielded the most net energy. Sensitivity analysis on this most promising guayule 26
scenario indicated rubber yield, water use and electricity for water pumping in guayule irrigation as the key 27
parameters that have significant impacts on results.
28
Keywords: Guayule; Hevea; Natural Rubber; Comparative LCA; Co-products; Energy Footprint 29
1. Introduction 30
The US is in need of a domestic and sustainable source of natural rubber (cis-1,4-polyisoprene) in order for 31
the country to increase rubber independence and reduce reliance on both petroleum polymers and imported 32
non-petroleum polymers. Factors that have potentially contributed to the price rise and demand of natural 33
rubber include a higher rubber demand from developing countries, such as China and India, and a lower 34
production by major producers, such as Thailand, due to an increasing domestic demand and the 35
replacement of rubber plantations with the more profitable palm-oil plantations used for the biofuel market 36
(FAO, 2003; Jawjit et al., 2010; van Beilen and Poirier, 2007a). In 2013, world production of natural rubber 37
reached 12 million metric tons (ISRG, 2014). The US imported approximately 4.6×104 metric tons of 38
natural rubber (United Nations Statistics Division, 2014), mainly from Thailand, Indonesia and Vietnam, 39
respectively (UN comtrade, 2013). The three countries are the world’s largest producers of natural rubber 40
from Hevea brasiliensis (Hevea), which is a tropical tree originally from Brazil but is currently commonly 41
grown in Southeast Asia. Hevea agriculture relies mainly on an annual rainfall of at least 1,350 mm, 42
fertilizers (nitrogen (N), phosphorus (P) and potassium (K)) and pest management (using e.g. cover crops 43
and pesticides) (DOAE, 1993). The natural rubber can be collected in a fresh latex liquid form by tapping 44
the tree bark after growing for seven years, which is the time needed for Hevea tree to be mature and ready 45
for extraction (Jawjit et al., 2010). Hevea trees are normally established from bud-grafted trees and are 46
replanted every 25-30 years; the average lifetime of Hevea trees depends on the economics (Priyadarshan, 47
2011; Teoh et al., 2011). For Hevea rubber processing, latex, the main input, is blended and cleaned, 48
coagulated, crumbed, rolled, shredded, cut and dried to produce Hevea rubber with rubberwood residues 49
and seed oil as co-products. Other process inputs are electricity, diesel, LPG and chemicals, such as 50
ammonia (NH3) to prevent fresh latex from coagulating before reaching the mill (Jawjit, 2012).
51
Parthenium argentatum Gray, commonly called guayule (pronounced ‘why-you-lee’), is another plant that 52
produces a sufficient quantity of natural rubber for commercial rubber production (Benedict et al., 2008).
53
Guayule is a perennial shrub native to the Chihuahuan desert of Northern Mexico and the Southwestern Big 54
Bend area of Texas (Benedict et al., 2008; Coffelt and Ray, 2010). The agricultural process of guayule 55
consists of planting, harvesting and baling for guayule biomass (e.g. stems and bark) that contains rubber, 56
latex and resin. Guayule is a woody shrub that can be established from direct seeding and the whole plant 57
can be harvested for rubber after growing for 2-3 years (van Beilen and Poirier, 2007b). Guayule 58
agriculture relies on irrigation systems, such as flood, drip, furrow or sprinkler, for biannual water 59
requirements of approximately 2,850 mm, N and P fertilizers and weed control (using e.g. oil sprays and 60
herbicides) (Foster and Coffelt, 2005; Nakayama et al., 1991b). Guayule rubber contained in bark 61
parenchyma cells of the plant can only be collected by harvesting the whole shoot system above ground 62
(including stems and leaves). Guayule rubber can be extracted from tissues of ground branches and stems 63
of 2-3 year old guayule plants, using either water to form a colloidal latex suspension of rubber and non- 64
rubber substances, or using organic solvents e.g. acetone and hexane, to extract rubber in solution 65
(Nakayama, 2005; Schloman Jr, 2005; Sfeir et al., 2014). For guayule rubber processing, guayule biomass 66
is the main input, which includes leaves, stalks and stems. The biomass goes through several processes 67
consisting of pre-grinding, wet milling, filtration, clarification, separation, concentration and creaming 68
(Cornish et al., 2011). Other inputs for guayule rubber processing are electricity and chemicals, such as 69
ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), sodium bicarbonate (NaHCO3) and potassium 70
hydroxide (KOH) as a buffer (Cornish et al., 2011). KOH is selected as the buffer in this case study.
71
Guayule rubber processing products are comprised of 10% guayule rubber, 20% bagasse with resin and 72
70% bagasse without resin (Bedane et al., 2011). Heating value of bagasse with resin is higher than that of 73
bagasse without resin by 2 MJ/kg. However, resin, which makes up 10% of bagasse material, is generally 74
extracted and used to produce wood preservatives, paints and adhesives (Nakayama, 2005). Guayule does 75
not contain the allergy-causing proteins that are found in Hevea latex, therefore may be suitable for medical 76
products, such as gloves (Coffelt and Ray, 2010; Jawjit et al., 2010). Hypoallergenic products and other 77
general rubber goods, such as hoses, soles, belts and footwear, are other market shareholder of natural 78
rubber; however, these materials are not major consumers of natural rubber. Automobile tires, on the other 79
hand, have the highest market share of over 60% (FAO, 2003). Tires are made up of 31% synthetic rubber 80
and 16% natural rubber by weight (Fiksel et al., 2011). Natural rubber is important in tires due to its 81
chemical structure providing outstanding resilience and strength properties (Herman, 2004; Miyamoto and 82
Bucks, 1985). The natural rubber demand in tire manufacturing is driven by the expansion of radial and 83
heavy-duty tire consumption (Hirata et al., 2014).
84
The most common type of synthetic rubber consumed in the US is styrene butadiene rubber (SBR). SBR 85
can be made in two different processes: Emulsion SBR (E-SBR) and Solution SBR (S-SBR). The 86
ingredients in producing SBR are styrene, butadiene, water, soap, modifier and activation components, 87
which are pumped into series of 6-12 well-agitated reactors at a controlled rate before being pumped again 88
to coagulation process. During the coagulation, the rubber is precipitated for rubber crumbs, which are then 89
baled and packed.
90
Although guayule can be domestically cultivated, this does not ensure that guayule will be an 91
environmentally sustainable source of natural rubber over the common source of natural rubber i.e., Hevea.
92
Guayule rubber may require either more or less inputs during its agricultural process (e.g. the guayule plant 93
can survive in the drier southwest region of the US, however, compared to Hevea, the guayule plant 94
requires more water in order to achieve maximum rubber production (Foster and Coffelt, 2005)) and/or 95
harvesting processes compared to Hevea rubber, which has to be transported overseas from countries in 96
Southeast Asia. The objectives of this study are to evaluate and compare life cycle environmental impacts 97
of the US-produced guayule and imported Hevea natural rubbers, and petroleum-based synthetic rubber, 98
and to identify areas of environmental improvement of natural rubber production. Life cycle assessment 99
(LCA), which is a tool to quantify environmental impacts over the entire life cycle of a product 100
(International Organization for Standardization, 2006), is used to investigate the environmental impacts and 101
tradeoffs of Hevea and guayule rubbers and their co-products.
102
2. Methods 103
A cradle-to-gate LCA was performed for the three rubbers 1) petroleum (i.e. synthetic), 2) Hevea and 3) 104
guayule. The rubbers investigated in this study were ‘tire-grade’ and pertinent to tire manufacture, therefore, 105
other rubbers (such as recycled rubber) were considered to be unsuitable to tire manufacture and were not 106
included in the scope of this study. The life-cycle inventory data of the two natural rubbers were evaluated 107
through the use of peer-reviewed literature, while that of the synthetic rubber, or SBR, was collected from 108
the ecoinvent database (Weidema and Hischier, 2010). The environmental impact categories quantified in 109
this study were ozone depletion potential (ODP), global warming potential (GWP) and acidification 110
potential (AP) using the Tool for the Reduction and Assessment of Chemicals and other environmental 111
Impacts (TRACI), which is a life cycle impact assessment (LCIA) tool developed particularly for the US 112
(Bare, 2012). Eutrophication potential was not included due to lack of inventory data for both crops as well 113
as lack of regionally appropriate LCIA methodology. The energy profile over the entire life cycle of the 114
natural rubber production and potential energy offsets from co-products were examined. For Hevea rubber 115
production, rubberwood residues and seed oil were assessed as energy offsets for electricity and biodiesel, 116
respectively; for guayule rubber production, bagasse was examined as an offset for bio oil and biochar.
117
Areas of improvement for guayule rubber were identified.
118
2.1 System boundaries 119
Cradle-to-gate LCAs of the two natural rubbers considered in this study included the agricultural process, 120
rubber production process and transportation, i.e. from field to processing, from processing to port by ship 121
(for Hevea) and to rubber manufacturing in the US by truck or rail. The distances from processing facility 122
located in Southwestern US to manufacturing facility located in Southern US were assumed to be 2,538 km 123
for truck and 2,317 km for rail. A summary of assumptions for distances is presented in Table 1. System 124
expansion and substitution were conducted to handle co-products from the two natural rubbers. The 125
production of energy from energy-rich co-products, such as rubberwood residues and seed oil from Hevea 126
and bagasse from guayule generated from the production of 1 kg of natural rubber, was included. As a 127
result, there were a total of eight scenarios investigated in this study, i.e. Hevea rubber and Electricity from 128
rubberwood residues (HE), Hevea rubber and biodiesel from seed Oil (HO), Guayule rubber transported by 129
Truck and Electricity from bagasse (GTE), Guayule rubber transported by Rail and Electricity from 130
bagasse (GRE), Guayule rubber transported by Truck and bio-Oil from bagasse (GTO), Guayule rubber 131
transported by Rail and bio-Oil from bagasse (GRO), GTO and bioChar (GTOC) and GRO and bioChar 132
(GROC), as illustrated in Figure 1. All results were compared on the same unit basis, also known as a 133
functional unit, which was one kg of rubber, assuming that one kg of different types of rubbers are 134
functionally equivalent.
135
Table 1 Transportation details for Hevea and guayule (Google, 2014; Jawjit, 2013; Sfeir et al., 2014) 136
Material From To By Capacity per one load
(kg)
Distance (km)
Hevea Latex Field Processing Truck 14,960 60
Guayule Biomass Field Processing Truck 14,960 150
Hevea Rubber Processing Tire
Manufacturing
Truck and Ship
14,960 402
28,000 27,412
Guayule Rubber Processing Tire
Manufacturing
Truck or Rail
14,960 2,538
60,000 (per wagon) 2,317
137
(Placeholder for system boundary – Figure 1) 138
139
The production of electricity and biodiesel from Hevea rubberwood residues and seed oil, respectively, and 140
electricity and bio-oil from guayule bagasse were included in the system boundaries. Electricity production 141
considered in this study was from direct combustion of biomass, while biodiesel and bio-oil were produced 142
via trans-esterification and fast pyrolysis processes, respectively (Boateng et al., 2009; Tsukahara and 143
Sawayama, 2005). This study did not include wastewater and runoff from Hevea and guayule agriculture 144
and rubber processing, and carbon capture sequestration in soils, due to the lack of data in the peer- 145
reviewed literature. It did not include water consumption for processing and irrigation because no data was 146
available for Hevea. Hevea agriculture relies on rainfall and requires approximately 3,515 mm of rain per 147
year (Rasutis et al., 2015). However, guayule has been reported in literature as requiring 1,000-1,425 mm 148
irrigation water per year (Bucks, 1985a; Nakayama, 1991a; Rasutis et al., 2015). An LCA on drip vs. flood 149
irrigation for guayule showed that it could require as much as 2500 mm of water (Eranki, et al. 2017).
150
Impacts included in this study were from the production phase of those raw materials specified in the 151
system boundary. The manufacturing of capital equipment such as machinery was excluded since, 152
generally, its impact contribution is considered negligible compared to other processes (Sheehan, 1998). In 153
addition, this model did not include any direct or fugitive emissions from processing (i.e. emissions from 154
use of solvents during processing) due to lack of data, but included direct and indirect field emissions under 155
the GWP category. Direct and indirect field emissions were not included under the AP impact category, 156
due to lack of data.
157
2.2 Life cycle inventory (LCI) 158
Inventories for each rubber production process were primarily collected from peer-reviewed literature, 159
patents, government and corporation reports and life-cycle databases, such as USLCI and ecoinvent 2.2 160
(National Renewable Energy Laboratory, 2012; Weidema and Hischier, 2010). Different databases were 161
used to best represent the region where the process occurs e.g., the USLCI database to represent guayule 162
grown in the US, while the appropriate or most suitable region from the ecoinvent database to represent 163
Hevea cultivated in Asia. The model included data and parameters for Hevea and guayule agriculture, 164
processing and transportation processes, i.e., factors such as biomass and rubber yields, energy required for 165
equipment (e.g. pumps, conveyor belt and grinder), chemical consumed during processing, (e.g. NH3, KOH 166
and sulfuric acid), co-products generated and transportation distances that are described in detail in this 167
section. Wherever applicable, all input data such as on irrigation, fertilizer and herbicide application 168
collected from literature were converted from the original units in which they were reported in the literature 169
to suit the functional unit used in this study. The model also took into account that Hevea and guayule are 170
perennial crops - guayule is replanted every 3-5 years, while the lifetime of Hevea is approximately 25-30 171
years. Inputs for Hevea were calculated based on its average lifetime of 27 years, while inputs for guayule 172
were calculated using data to represent the inputs required over the crop’s lifetime (while it’s both 173
immature and mature) based on a literature study that describes the methodology to account for perennial 174
crops in LCAs (Bessou et al., 2013). The full LCI including data sources is provided in the Supplementary 175
Material. Average values, obtained either as single point estimates, or calculated from a mean data, or as 176
averages from the minimum and maximum values (shown in the SI), were used to construct the inventory 177
for this LCA, in conjunction with ecoinvent and USLCI databases.
178
2.2.1 Hevea agriculture, processing, and transport 179
Hevea agricultural and processing data was collected from a study performed by Jawjit et al. (2010) on 180
greenhouse gas (GHG) emissions of Hevea rubber production. The Hevea rubber yield value used in this 181
analysis was 1,600 kg/ha/year (DOAE, 1993; Jawjit et al., 2013; OAE, 2012). Agricultural field emissions- 182
CO2, N2O and NO - were also accounted for. Field emissions as obtained from literature for Hevea 183
included 976.53 kg C/ha/yr (4.48 kg CO2 eq/kg rubber) from CO2 (Podong, 2014), direct emissions of 0.24 184
kg N2O/ ton latex (0.164 kg CO2 eq/kg rubber) from the cultivation of mineral forest soils, indirect 185
emissions following N leaching and runoff, and those following all direct fertilizer emissions of 0.06 kg 186
N2O/ ton latex (0.041 kg CO2 eq/kg rubber) (Jawjit, 2010). Agricultural electricity, energy and chemical 187
inputs, and processing yields were specific for bale rubber (i.e. tire-grade rubber). The major source of 188
Hevea rubber in the US was from Indonesia; therefore an Indonesian electricity mix, with 44% coal, 31%
189
natural gas, 13% oil, 7% hydropower, 5% geothermal, 1% renewables, was used to model Hevea 190
processing electricity (UN comtrade, 2013). Although the study by Jawjit et al. (2010) was conducted 191
based on Thailand data, it was assumed to be similar to that conducted in Indonesia. Transportation 192
considered in this study for Hevea included from field to rubber processing facility by truck, from the 193
processing facility to a port in Indonesia by truck, from the Indonesian port to the US port by ship and from 194
the US port to a rubber product manufacturing gate by truck. More details on distance, loading capacity and 195
fuel efficiency for truck and distance, payload and fuel consumption for ship transport are listed in 196
Supplementary Material.
197
2.2.2 Guayule agriculture, processing, and transport 198
Guayule agricultural and processing data was collected from several peer-review studies, as indicated in the 199
Supplementary Material. Guayule rubber yield used in this analysis, 1,400 kg rubber/ha/year, was obtained 200
as an average from published literature, including (Bedane et al., 2011; Estilai, 1991b; Foster and Coffelt, 201
2005; Thompson and Ray, 1989; van Beilen and Poirier, 2007b). As mentioned before in this ection 202
guayule is a perennial crop- its planting and field preparation are only conducted once every 3-5 years 203
(Personal communication with Hunsaker, 2014). However, literature data for fuel consumption of field 204
preparation and cultivation were reported based on a 2 year lifetime (data shown in SI); therefore, we 205
annualized these data for use in this study. Inputs were collected for several years representing different 206
plantation phrases (immature to mature) and were then used to calculate average values. Data on field 207
emissions from guayule agricultural field were absent in literature. We used IPCC guidelines to calculate 208
direct and indirect N2O emissions for managed soils (IPCC, 2006) contributing to GWP (0.231 and 0.0231 209
kg CO2 eq/kg rubber, respectively); however, there were no data on direct CO2 field emissions for guayule 210
(whereas this value was included in Hevea field emissions). Agricultural runoff in the case of guayule was 211
assumed to be absent since the surface runoff in arid landscapes is zero (Personal communication with 212
Bronson, 2015; Teixeira, 2001).
213
Guayule processing data mainly included electricity consumed by conveyor belt, crusher, scale, grinder, 214
press filter, centrifuge and creaming tank (Sfeir et al., 2014). Although the Sfeir et al. study was conducted 215
in Mediterranean Europe, it was the only study that provided data particularly for guayule latex processing.
216
Transportation distances were assumed to represent possible harvesting and processing locations. The 217
round-trip travel distance used for the study was 96 km (60 miles) as an estimate of the distance between 218
agricultural areas and possible processing locations e.g. in Maricopa County in Arizona - a region of 219
interest for guayule agriculture in Southwestern USA (Benedict et al., 2008; Coffelt and Ray, 2010).
220
Electricity used for pumping agricultural water and guayule rubber processing was modeled for the 221
Southwest Energy Efficiency Project’s Arizona Electricity Mix (36% coal, 29% nuclear, 27% natural gas, 222
8% renewable) (SWEEP, 2014).
223
2.2.3 Co-products from Hevea and guayule rubber productions 224
Eight co-product scenarios were considered in Hevea and guayule rubber production. The eight scenarios 225
included: electricity from Hevea rubberwood residues, biodiesel from Hevea seed oil, and c. electricity, 226
bio-oil and biochar from guayule bagasse with two modes of guayule rubber transportation, via either truck 227
or rail. The quantity of Hevea rubberwood residues approximately accounted for 2% of the rubberwood 228
biomass dry weight (DEDE, 2007; DOAE, 1993; Krukanont and Prasertsan, 2004; Teoh et al., 2011).
229
Hevea rubberwood residues consisting of small branches left on an agricultural field, with an approximate 230
high heating value (HHV) of 10.5 MJ/kg, were assumed as being used to produce electricity via 231
combustion in a power plant with a heat rate of 10,498 BTU/kWh or 33% efficiency (U.S. Energy 232
Information Administration, 2012). Biodiesel from Hevea seed oil was assumed to be produced through a 233
two-step transesterification process- acid catalyzed followed by alkaline catalyzed transesterification 234
(Ramadhas et al., 2005a). Guayule bagasse was assumed to produce one of the following: either electricity 235
via combustion, or 60% bio-oil and/or 30% biochar (by weight) via fast pyrolysis in a fluidized bed reactor 236
(Deshmukh et al., 2013). The energy consumption for the fast pyrolysis process of converting guayule 237
bagasse was modeled based on the process of sugarcane bagasse due to the lack of data for the guayule 238
bagasse process. Energy content of the Hevea rubberwood residues and guayule bagasse generated from the 239
production of one kg of rubber were considered to offset energy and impacts from local electricity 240
production (Indonesia and Arizona, respectively), while the Hevea seed oil was considered to offset energy 241
and environmental impacts from petroleum diesel production. More details on the LCI data of Hevea and 242
guayule rubber co-products are provided in the Supplementary Material.
243
2.3 Life cycle impact assessment and energy offset analysis 244
TRACI 2.1 V1.01/US 2008 was used to quantify three environmental impact categories, ODP, GWP, and 245
AP, throughout the life cycle of natural rubbers and SBR. ODP and GWP represent impacts applicable to 246
the atmosphere, whereas AP represents impacts applicable to air and soil/water. In this analysis, the 247
aqueous impacts included in AP are not considered due to lack of data. In the energy-offset analysis, the net 248
energy of the five co-product scenarios (electricity from Hevea rubberwood, biodiesel from Hevea seed oil, 249
electricity from guayule bagasse, bio-oil from guayule bagasse and bio-oil and biochar from guayule 250
bagasse), were evaluated and compared to two additional scenarios without co-products. The net energy 251
was calculated by subtracting the total energy consumed from the total energy produced. The net energy 252
results were reported on a functional unit basis as MJ per one kg of rubber.
253
3. Results and discussion 254
The results of this analysis helped identify processes with major environmental impact contributions, and 255
subsequently assisted in identifying environmental hotspots. The life cycle environmental impacts of ODP, 256
GWP and AP of Hevea and guayule were evaluated and the results were compared to those from SBR.
257
Different co-product scenarios from Hevea and guayule rubbers were quantified to indicate their potential 258
to offset impacts and energy required for rubber production. Since the focus of this study is guayule rubber 259
as an alternative for existing natural rubber, processes related to guayule rubber production with the highest 260
contribution to each impact category were identified and compared to Hevea and synthetic rubbers.
261
3.1 A comparison of impacts of the two natural rubbers, and synthetic rubber 262
Synthetic rubber (SBR) had the lowest GWP and AP impact contributions compared to Hevea and guayule, 263
as presented in Figure 2. The total GWP impact from SBR was approximately 2.7 kg CO2 eq/kg rubber;
264
this was equal to 8% and 21% of the total GWP impacts from Hevea and guayule rubbers, respectively.
265
Similarly, the total AP impact of SBR was approximately 0.01 kg SO2 eq/kg rubber, which was equal to 266
13% and 14% of the total AP impact from Hevea and guayule rubbers, respectively. Approximately 66% of 267
the total AP impact from SBR production was from sulfur dioxide (SO2) and from the production of 268
sulfuric acid (H2SO4) used in the vulcanization process. However, because these results were based on 269
literature data for a guayule lifetime of 2 years, they may have slightly overestimate certain impacts of 270
guayule as its actual lifetime is 3-5 years. The planting and cultivation of guayule never contribute more 271
than 16% to any LCIA category.
272
(Placeholder for LCA of rubbers without co-products credits – Figure 2) 273
A comparison of impacts of the two natural rubbers showed that Hevea had higher GWP and AP impacts, 274
whereas guayule rubber had higher impacts in the ODP category. The primary hotspot that resulted in GWP 275
and AP impact contributions in Hevea was from the liquefied petroleum gas (LPG) consumption for drying 276
during the milling process. The LPG consumption contributed 27.6 kg CO2 eq/kg rubber and 0.08 kg SO2 277
eq/kg rubber- equivalent to 84% of the total GWP and 96% of the total AP impacts, respectively. As an 278
alternative, diesel could be used in place of LPG (diesel used to be burnt in lieu of LPG in Thai rubber 279
milling in the past; however, Jawjit et al. (2010) reported that LPG was adopted in response to rising diesel 280
prices). Results also pointed to field emissions as a hotspot, with CO2 and N2O, from Hevea agriculture 281
accounting for 14% of the total GWP impact from Hevea rubber.
282
In guayule rubber on the other hand, GWP and AP impacts mainly resulted from energy use for water 283
pumping in guayule agriculture. This energy use for water pumping accounted for 56% of the total GWP 284
and 66% of the total AP in guayule rubber. Since multiple modes of transport were considered in the case 285
of guayule, these choices showed that the GWP and AP impacts from the transportation of guayule rubber 286
from processing to manufacturing unit by truck were 83% higher than those by rail (with values of 3.6×10-2 287
kg CO2 eq/kg rubber and 4.8×10-4 kg SO2 eq/kg rubber, respectively). However, the impacts from any 288
mode of transport only accounted for 1% of the total GWP and 2% of the total AP results, making them a 289
non-significant emission category. The ODP impacts of guayule rubber were 90% greater than those of 290
Hevea rubber, but were equal within both guayule scenarios with different domestic modes of 291
transportation. Similar to the other impact categories in guayule, the primary ODP contributor was the 292
energy consumption for water pumping in guayule’s agriculture stage, which resulted in 3.7×10-7 kg CFC- 293
11 eq/kg rubber, i.e., 54% of the total ODP impact from guayule rubber. On the other hand, Hevea rubber 294
had the lowest ODP impacts out of all rubbers at 6.9×10-8 kg CFC-11 eq/kg rubber, and 78% of this total 295
ODP from Hevea rubber, equals to 5.36×10-8 kg CFC-11 eq/kg rubber, came from N fertilizer (urea) used 296
in its agriculture.
297
3.2 Impacts avoided through the use of co-products 298
Figures 3, Figure 4, and Figure 5 show that considerations of co-products generated from the production of 299
one kg of each rubber change the results in each category in comparison to Figure 2 that does not consider 300
co-product scenarios. Electricity produced from either Hevea rubberwood residues or guayule bagasse (i.e.
301
HE, GTE and GRE scenarios) aided in impact reductions under all impact categories (and net energy that is 302
discussed in a subsequent section), while bio-oil and biochar produced from guayule bagasse reduced only 303
AP impacts. Biodiesel produced from Hevea seed oil did not form a factor in reducing any impacts. When 304
electricity was included as a co-product from Hevea rubberwood residues, its ODP, GWP, and AP impacts 305
were lowered compared to the baseline (i.e., in Figure 2 with no co-products) by 3035%, 22%, and 481%
306
respectively. Similarly when electricity from guayule bagasse was considered its ODP, GWP, and AP were 307
reduced by 129%, 132%, and 155% respectively. Producing bio-oil from guayule bagasse as a co-product 308
lowered its AP impacts by 96%, whereas producing bio-oil and biochar from guayule bagasse lowered its 309
AP impacts by 134%.
310
Hevea with electricity (HE) from rubberwood residues had the lowest ODP (-1.5×10-6 kg CFC-11 eq/kg 311
rubber) because of the electricity production offset from Hevea rubberwood. On the other hand, the two 312
scenarios of guayule with bio-oil as co-product (GTO and GRO) had the highest ODP (3.3×10-6 kg CFC-11 313
eq/kg rubber), mainly due to high energy consumption during the bagasse bio-oil production process.
314
Similarly, the guayule scenarios with bio-oil and biochar, i.e., GTOC and GROC contributed significantly 315
to ODP impacts as well, due to the high energy demand during the bio-oil and biochar production process 316
form bagasse. The guayule scenarios with the production of electricity from bagasse (i.e., GTE and GRE), 317
however, were the most promising pathways as they substantially reduced ODP impacts due to electricity 318
offsets; in this case, the electricity required for the direct combustion system accounted for only 2% of the 319
energy content (yield) of guayule bagasse (Boateng et al., 2009; Deshmukh et al., 2013).
320
(Placeholder for ODP results – Figure 3) 321
322
The GWP impacts of all six guayule scenarios were lower than GWP from scenarios of Hevea rubber.
323
However, GTE and GRE were the only two scenarios with negative GWP among all guayule scenarios, as 324
indicated in Figure 4. The GWP impact mitigation in these scenarios resulted from the production of 325
electricity from guayule bagasse that offset (or avoided) approximately 17 kg CO2 eq/kg rubber from the 326
Arizona electricity mix production (SWEEP, 2014) equivalent to a 400% reduction of total GWP. The HO 327
scenario (i.e., biodiesel co-product from Hevea seed oil) had the highest GWP impact at 32.8 kg CO2 eq/kg 328
rubber- this was higher than the Hevea baseline by 0.03 kg CO2 eq/kg rubber. The GWP impact from HE 329
was 78%, GTO and GRO 45% each, SBR 8% and GTOC and GROC 32% each, compared to the GWP 330
from the HO scenario. The co-products that did not offset instead increased the total GWP impact were 331
biodiesel from Hevea seed oil in HO scenario and bio-oil from guayule bagasse in GTO and GRO 332
scenarios; this resulted from a higher GWP contribution from the resources required to produce the co- 333
products than the impacts avoided by the same co-products. While the focus of the present study was on 334
bioenergy co-products, after felling Hevea rubberwood could be used to produce furniture- an alternate co- 335
product that might potentially sequester higher amounts of carbon and thus avoid GWP impacts; however, 336
this sequestration credit was not added in the present LCA. The carbon sequestration of hardwood trees can 337
be estimated as follows: assuming a moderate growth rate, i.e., at the tree age of 27 years a survival factor 338
by growth rate of 0.398, annual sequestration rates would be 32.5 lbs C/tree/year; therefore the C 339
sequestered per tree would be 12.935 lbs or 5.87 kg, i.e., approximately 21.5 kg CO2 eq/tree/year (Teoh et 340
al., 2011; U.S. Department of Energy, 1998). Assuming only 50% of the rubberwood biomass could be 341
used to produce furniture, the CO2 eq sequestered in rubberwood furniture would be approximately 11 kg 342
CO2 eq/furniture/tree. However, this estimate of C-sequestration did not consider GWP generated from the 343
furniture manufacturing process or the lifetime of furniture. Considering Hevea rubberwood’s potential for 344
furniture production, including the furniture manufacture process could be factored into future LCA studies.
345
(Placeholder for GWP results – Figure 4) 346
347
The HO scenario was a major contributor to the AP impacts (0.08 kg SO2 eq/kg rubber), whereas the HE 348
scenario contributed the least to AP impacts, as presented in Figure 5; 91% of the total AP impact from the 349
HO scenario was from the LPG consumed during the Hevea rubber milling process. On the other hand, the 350
production of electricity from Hevea rubberwood significantly offset AP impacts by 0.38 kg SO2 eq/kg 351
rubber. SBR was the second highest AP impact contribution with 0.01 kg SO2 eq/kg rubber (equivalent to 352
12% of the total AP impact from the HO scenario). AP impacts from the six guayule rubber scenarios (all 353
negative values as shown in Figure 5) had the following order: GTO > GRO > GTOC > GROC > GTE >
354
GRE. AP impacts from transportation were negligible compared to other processes of the guayule rubbers 355
and their co-products. The avoided AP impacts in these scenarios and their equated transportation 356
counterparts were 4%, 31% and 51% respectively of the total AP impacts from the HO scenario. Compared 357
to the baseline results in Figure 2, the production of co-products of guayule rubber, i.e., electricity, bio-oil 358
and biochar or only bio-oil, can offset AP impacts by 0.08, 0.05 and 0.03 kg SO2 eq/kg rubber, respectively.
359
Another case that is not included is a scenario where both the rubberwood residues and seed oil from Hevea 360
could be used as co-products to generate electricity and biodiesel, respectively. However, the results of 361
such a scenario were calculated by combining respective individual results and also indicated a potential for 362
impact reduction. ODP, GWP, and AP impacts contributions from the two Hevea’s co-products combined 363
(without the impacts from Hevea rubber production) were -2.1×10-6 kg CFC-11 eq/kg rubber, -7.2 kg CO2
364
eq/kg rubber, and -0.37kg SO2 eq/kg rubber, respectively.
365 366
(Placeholder for AP results – Figure 5) 367
3.3 The energy footprints of Hevea and guayule rubbers, and their co-products 368
Figure 6 shows the life-cycle energy footprints of Hevea and guayule rubbers and their co-products; any 369
energy produced is indicated as a positive value, whereas energy consumed is indicated in a negative value.
370
The net energy of all scenario except HO was negative (i.e. energy was consumed instead of produced); the 371
HO scenario had a positive net energy value of 3.25 MJ/kg rubber. Although overall, other scenarios did 372
not yield a positive result, the co-products still showed the potential to reduce the total energy consumption 373
in all scenarios. For example, GRE and GTE were the two cases with the highest energy avoidance- with 374
the inclusion of co-products both scenarios could avoid approximately 99 MJ/kg rubber, accounting for 375
55% and 54% of the total energy consumed, respectively. The co-products in GROC and GTOC scenarios 376
and GRO and GTO scenarios could offset approximately 112 and 89 MJ/kg rubber (30% and 24% of the 377
total energy consumed) respectively. The most energy intensive process in all six guayule scenarios was the 378
consumption of diesel for transportation from field to processing unit. The diesel consumption contributed 379
85% and 84% of the total energy consumption from GRE and GTE scenarios, respectively, and 380
approximately 41% of the net energy from all the other scenarios of guayule rubber. The production of bio- 381
oil and/or biochar was also an energy intensive process contributing to 52% of the net energy from GROC, 382
GTOC, GRO and GTO scenarios.
383
(Placeholder for energy results – Figure 6) 384
3.4 Model limitations and data quality discussion 385
Secondary data from literature was mainly used in the development of this LCA. However, there is an 386
absence of LCA studies especially in the context of guayule rubber production and use, and additionally 387
those that compare these natural rubbers. Thus, this study serves as a preliminary assessment in the 388
evaluation of these rubbers and the potential of their co-products. Moreover, some data was collected as a 389
substitute for guayule, such as bagasse from sugarcane, which was considered to have a similar conversion 390
process as bagasse from guayule. However, the data and inventories used were considered the most recent 391
and/or most appropriate sources (e.g. journal publication, patents and reports) at the time when this study 392
was conducted. Furthermore, the data used in this study was collected from various existing research 393
studies conducted under different conditions; thus, we acknowledge the incidence of uncertainty. This 394
study, however, did not include an uncertainty (Monte Carlo) analysis due to limited and inaccessible data;
395
instead we performed a sensitivity analysis (shown in the subsequent section) on the GRE scenario- the 396
most promising scenario for guayule rubber (the product of focus in this study) to evaluate the sensitivity of 397
results to various key parameters. The sensitivity analysis was conducted using the minimum, average, and 398
maximum values for three key parameters for the GRE scenario in the model.
399
3.5 Sensitivity analysis 400
In the GRE scenario, the energy used for water pumping during the agricultural process had the highest 401
impact contribution in all three impact categories of ODP, GWP and AP, and it also had the highest net 402
energy produced. The energy used for water pumping depends on several parameters, such as the volume 403
of water used for drip irrigation and pump efficiency. Thus, we identify these as the key parameters of 404
improvement that should be further investigated. For example, different irrigation methods could alter the 405
energy needed for water pumping; by changing the energy input by ±83% in GRE scenario, all impacts 406
changed by ±21-22%. Increasing the pump efficiency from 68% to 80% (by 12%) and to 90% (by 22%) 407
reduced GWP impacts by 26% and 42%, respectively. A specific sensitivity analysis constituted examining 408
the factors of: i) rubber yield, ranging from 800 to 2000 kg/ha/yr (an average value of 1400 kg/ha/yr was 409
used in the previous results (Bedane et al., 2011; van Beilen and Poirier, 2007b)), ii) drip irrigation rate 410
ranging from 1000 to 2850 mm/yr (average value of 1925 m/yr (Bucks, 1985a; Nakayama, 1991a)), and iii) 411
energy usage of water pump ranging from 1.0×10-3 to 1.5×10-3 kWh/gallon (average value of 1.25×10-3 412
kWh/gallon (Great River Energy, 2014)). Results of the sensitivity analysis are shown in Figure 7. Using a 413
rubber yield of 2000 kg/ha/yr of rubber yield, 1000 mm/yr of drip irrigation rate and 1.0×10-3 kWh/gallon 414
of energy use for water pumping, the total ODP, GWP and AP impacts improved from the case using 415
average values by 300%, 270%, 164% and 3,129%, respectively (i.e., 6×10-7 kg CFC-11 eq, 11 kg CO2 eq 416
and 7×10-2 kg SO2 eq per kg rubber, respectively). The improvements were primarily from the increased 417
energy yield of co-products. An important change that occurred due to the changes in the key parameters 418
was diesel use for agricultural activities, which was higher than the results from the average-value scenario 419
by 5%. On the other hand, by reducing the rubber yield to 800 kg/ha/yr of rubber yield, increasing 420
irrigation to 2850 mm/yr and using 1.50×10-3 kWh/gallon of energy for water pumping, the total ODP, 421
GWP and AP impacts increased by 460%, 730% and 230% compared to the average-value scenario. The 422
exacerbation of impacts was mainly due to the increase in quantity of water used in drip irrigation.
423
(Placeholder for sensitivity analysis results – Figure 7) 424
425
The green dot in Figure 7 represents the average-value results. The upper bars show the worst-case 426
sensitivity parameters results in the GRE scenario, i.e., 800 kg/ha/yr of rubber yield, 2850 mm/yr of drip 427
irrigation rate and 1.50×10-3 kWh/gallon of energy use for water pumping. The lower bars show the best- 428
case parameters results in the GRE scenario, i.e., 2000 kg/ha/yr of rubber yield, 1000 mm/yr of drip 429
irrigation rate and 1.00×10-3 kWh/gallon of energy use for water pumping. The environmental impact 430
results of GRE using best parameter values suggested that this scenario could be a sustainable alternative in 431
replacing Hevea and SBR. Using average parameter values however, GRE had higher ODP and AP than 432
HE, but lower than HO and SBR; GRE also showed lower GWP than Hevea and SBR. Using worst-case 433
scenario parameter values, GRE showed higher ODP than Hevea but lower than SBR; it had higher GWP 434
than SBR but lower than Hevea, and had higher AP than HE and SBR but lower than HO. A comparison of 435
sensitivity analysis results is summarized in a table in the SI.
436
4. Conclusion 437
There were tradeoffs between synthetic, Hevea and guayule rubbers. Guayule rubber contributed lower 438
ODP compared to SBR, and lower GWP and AP compared to Hevea rubber. One area of improvement for 439
guayule agriculture is irrigation energy; the energy used for water pumping depended on several parameters, 440
e.g. volume of water for drip irrigation, pump efficiency and electricity consumed per m3 of water.
441
Different irrigation methods or increasing pump efficiency could alter the energy needed for irrigation and 442
improve ODP, GWP and AP impacts. Future studies should focus on field data collection for a better 443
estimate of LCA results and availability of primary data. For example, direct emissions from agricultural 444
activities (i.e. particulate matter from tillage, runoff from agriculture), direct emissions from processing (i.e.
445
emissions from use of solvents during processing), as well as irrigation and water consumption data should 446
be included in LCAs. In addition, new guayule agricultural practices should be included, such as direct 447
seeding and drip irrigation. The use of co-products can reduce the environmental impacts for both guayule 448
and Hevea rubbers. This study showed that the most promising co-products include electricity from 449
biomass residues, i.e. guayule bagasse and Hevea rubberwood residues, which reduced ODP, GWP and AP 450
impacts. The production of bio-oil and biochar from guayule bagasse reduced AP impact. The most 451
promising scenario of guayule rubber was using its bagasse as a feedstock to produce electricity, i.e.
452
scenario GRE where guayule is transported by rail, however transportation had little to do with the 453
environmental impacts. The GRE scenario contributed the least to all three impact categories investigated, 454
and yielded the greatest amount of net energy among the six guayule scenarios examined.
455
Acknowledgements 456
This work was supported by the Biomass Research and Development Initiative (BRDI) grant (USDA-NIFA 457
2012-10006-19391 OH). The authors are very grateful to Dr. Kevin Bronson for field data and input on 458
guayule agriculture. Any opinions, findings and conclusions or recommendations expressed in this material 459
are those of the authors and do not necessarily reflect the views of USDA.
460
References 461
462
Bare, J., 2012. TRACI User's Manual, Software Name and Version Number: TRACI version 2.1. U.S. EPA.
463
Bedane, G.M., Gupta, M.L., George, D.L., 2011. Development and evaluation of a guayule debarker.
464
Industrial Crops and Products 34, 1256-1261.
465
Benedict, C.R., Greer, P.J., Foster, M.A., 2008. The physiological and biochemical responses of guayule to 466
the low temperature of the Chihuahuan Desert in the biosynthesis of rubber. Industrial Crops and Products 467
27, 225-235.
468
Bessou, C., Basset-Mens, C., Tran, T., Benoist, A., 2013. LCA applied to perennial cropping systems: a 469
review focused on the farm stage. The International Journal of Life Cycle Assessment 18, 340-361.
470
Boateng, A.A., Mullen, C.A., Goldberg, N.M., Hicks, K.B., McMahan, C.M., Whalen, M.C., Cornish, K., 471
2009. Energy-dense liquid fuel intermediates by pyrolysis of guayule (Parthenium argentatum) shrub and 472
bagasse. Fuel 88, 2207-2215.
473
Bucks, D.A., Roth, R.L., Nakayama, F.S., Gardner, B.R., 1985a. Irrigation water, nitrogen, and 474
bioregulation for guayule production. ASAE 28, 1196–1205.
475
Coffelt, T.A., Ray, D.T., 2010. Cutting height effects on guayule latex, rubber, and resin yields. Industrial 476
Crops and Products 32, 264-268.
477
Cornish, K., McCoy, R.G., Martin, J.A., Williams, J., Nocera, A., 2011. Biopolymer extraction from plant 478
materials.
479
DEDE, 2007. Biomass Energy. Department of Alternative Energy Development and Efficiency, Thailand.
480
Deshmukh, R., Jacobson, A., Chamberlin, C., Kammen, D., 2013. Thermal gasification or direct 481
combustion? Comparison of advanced cogeneration systems in the sugarcane industry. Biomass and 482
Bioenergy 55, 163-174.
483
DOAE, 1993. Hevea Plantation. Department of Agricultural Extension.
484
Eranki, P. L., El-Shikha, D., Hunsaker, D. J., Bronson, K. F., & Landis, A. E., 2017. A comparative life 485
cycle assessment of flood and drip irrigation for guayule rubber production using experimental field 486
data. Industrial Crops and Products, 99, 97-108.
487
Estilai, A., Ray, D. T. , 1991b. Genetics, cytogenetics, and breeding of guayule, in: Whitworth, J.W., 488
Whitehead, E.E. (Eds.), Guayule Natural Rubber, Office of Arid Lands, Univ. of Arizona, Tucson.
489
FAO, 2003. Medium-term prospects for agricultural commodities - Projections to the year 2010, p. 181.
490
Fiksel, J., Bakshi, B., Baral, A., Guerra, E., DeQuervain, B., 2011. Comparative life cycle assessment of 491
beneficial applications for scrap tires. Clean Technologies and Environmental Policy 13, 19-35.
492
Foster, M.A., Coffelt, T.A., 2005. Guayule agronomics: establishment, irrigated production, and weed 493
control. Industrial Crops and Products 22, 27-40.
494
Great River Energy, 2014. Energy Guide, in: Wise, E. (Ed.), Farm Electricity Costs.
495
Herman, M.F., 2004. Encyclopedia of Polymer Science and Technology : Rubber, Guayule, in: Cornish, K., 496
Schloman, W. W. (Ed.), 3rd Edition ed. John Wiley & Sons, Inc. . 497
Hirata, Y., Kondo, H., Ozawa, Y., 2014. 12 - Natural rubber (NR) for the tyre industry, in: Kohjiya, S., 498
Ikeda, Y. (Eds.), Chemistry, Manufacture and Applications of Natural Rubber. Woodhead Publishing, pp.
499
325-352.
500
International Organization for Standardization, 2006. ISO 14040:2006, Environmental Management - Life 501
cycle assessment - Principles and framework, Geneva, Switzerland, p. 20.
502
ISRG, 2014. Statistical Summary of World Rubber Situation, Natural Rubber Production International 503
Rubber Study Group 504
Jawjit, W., Kroeze, C., Rattanapan, S., 2010. Greenhouse gas emissions from rubber industry in Thailand.
505
Journal of Cleaner Production 18, 403-411.
506
Jawjit, W., Pavasant, P., Kroeze, C., 2013. Evaluating Environmental Performance of Concentrated Latex 507
Production in Thailand. Journal of Cleaner Production.
508
Jawjit, W., Pavasant, P., Kroeze, C., 2012. Evaluating Environmental Performance of Concentrated Latex 509
Production in Thailand, The 18th Greening of Industry Network Conference, Linköping Congress Centre, 510
Sweden.
511
Krukanont, P., Prasertsan, S., 2004. Geographical distribution of biomass and potential sites of rubber 512
wood fired power plants in Southern Thailand. Biomass and Bioenergy 26, 47-59.
513
Miyamoto, S., Bucks, D.A., 1985. Water quantity and quality requirements of guayule: Current assessment.
514
Agricultural Water Management 10, 205-219.
515
Nakayama, F.S., 2005. Guayule future development. Industrial Crops and Products 22, 3-13.
516
Nakayama, F.S., Bucks, D.A., Roth, R.L., Gardner, B.R., 1991b. Guayule biomass production under 517
irrigation. Bioresource Technology 35, 173-178.
518
Nakayama, F.S., Bucks, D.A., Gonzalez, C.L., Foster, M.A., 1991a. Water and nutrient requirements of 519
guayule under irrigated and dryland production, in: Whitworth, J.W., Whitehead, E.E. (Ed.), Guayule 520
Natural Rubber. Office of Arid Lands Studies, University of Arizona Tucson, AZ, pp. 145-172.
521
National Renewable Energy Laboratory, 2012. U.S. Life Cycle Inventory Database.
522
OAE, 2012. Thailand Agriculture Statistics, Office of Agricultural Economics, Ministry of Agriculture and 523
Cooperative, Bangkok, Thailand.
524
Personal communication with Bronson, K., 2015. Personal Communication at U.S. Arid-Land Agricultural 525
Research Center.
526
Personal communication with Hunsaker, D., 2014. Personal Communication at U.S. Arid-Land 527
Agricultural Research Center.
528
Podong, C., 2014. Estimate of soil organic carbon and greenhouse gas emissions in para rubber (Hevea 529
brasiliensis Mull. Arg) plantation by DNDC model in Upland Area Northern, Thailand. Computational 530
Ecology and Software 4, 269.
531
Priyadarshan, P.M., 2011. Biology of Hevea Rubber. CABI Publishing, Wallingford, Oxon, GBR.
532
Ramadhas, A.S., Jayaraj, S., Muraleedharan, C., 2005a. Biodiesel production from high FFA rubber seed 533
oil. Fuel 84, 335-340.
534
Rasutis, D., Soratana, K., McMahan, C., Landis, A.E., 2015. A sustainability review of domestic rubber 535
from the guayule plant. Industrial Crops and Products 70, 383-394.
536
Schloman Jr, W.W., 2005. Processing guayule for latex and bulk rubber. Industrial Crops and Products 22, 537
41-47.
538
Sfeir, N., Chapuset, T., Palu, S., Lançon, F., Amor, A., García García, J., Snoeck, D., 2014. Technical and 539
economic feasibility of a guayule commodity chain in Mediterranean Europe. Industrial Crops and 540
Products 59, 55-62.
541
Sheehan, J., Camobreco, V., et al., 1998. Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in 542
an Urban Bus. National Renewable Energy Laboratory, U.S. Department of Energy and U.S. Department 543
of Agricultural, Golden, CO.
544
SWEEP, 2014. Arizona Energy Fact Sheet: Energy Efficiency & Energy Consumption. Southwest Energy 545
Efficiency Project.
546
Teixeira, W.G., 2001. Land use effects on soil physical and hydraulic properties of a clayey Ferralsol in the 547
central Amazon.
548
Teoh, Y., Don, M., Ujang, S., 2011. Assessment of the properties, utilization, and preservation of 549
rubberwood (Hevea brasiliensis): a case study in Malaysia. J Wood Sci 57, 255-266.
550
Thompson, A.E., Ray, D.T., 1989. Breeding Guayule, Plant Breeding Reviews. John Wiley & Sons, Inc., 551
pp. 93-165.
552
Tsukahara, K., Sawayama, S., 2005. Liquid Fuel Production Using Microalgae. Journal of the Japan 553
Petroleum Institute 48, 251-259.
554
U.S. Department of Energy, 1998. Method for Calculating Carbon Sequestration by Trees in Urban and 555
Suburban Settings, p. 16.
556
U.S. Energy Information Administration, 2012. Electric Power Annual.
557
UN comtrade, 2013. Natural rubber latex, including prevulcanised.
558
United Nations Statistics Division, 2014. UN Comtrade Database.
559
van Beilen, J.B., Poirier, Y., 2007a. Establishment of new crops for the production of natural rubber.
560
Trends in Biotechnology 25, 522-529.
561
van Beilen, J.B., Poirier, Y., 2007b. Guayule and Russian Dandelion as Alternative Sources of Natural 562
Rubber. Critical Reviews in Biotechnology 27, 217-231.
563
Weidema, B., Hischier, R., 2010. ecoinvent data v2.2, the 2010 version of the most comprehensive and 564
most popular public LCI database. Swiss Centre for Life Cycle Inventories (ecoinvent Centre).
565 566
Figure 1: System boundaries of Hevea and guayule rubbers-include processes of Hevea and guayule rubber production, co-products and energy from co-products
The dashed line defines system boundary consisting of agriculture, rubber processing &
transportation processes, and inputs for each process. The eight scenario pathways analyzed in this study are highlighted with a bold, capital first letter, including HE, HO, GTE, GRE, GTO, GRO, GTOC, and GROC.
Figure 2: Normalized lifecycle impacts without co-product credits per kg natural and synthetic rubbers
The inset legend illustrates most significant impacts and the detailed legend indicates all impacts. Impacts from each scenario were normalized to the highest impact in each category. Co-product credits are not included.
Figure 3: Ozone depletion potential (ODP)
Ozone depletion potential impacts per kg natural and synthetic rubbers for different transportation modes and co-product scenarios
Figure 4: Global warming potential (GWP)
Global warming potential impacts per kg natural and synthetic rubbers for different transportation modes and co-product scenarios
Figure 5: Acidification potential (AP)
Acidification potential impacts per kg natural and synthetic rubbers for different transportation modes and co-product scenarios
Figure 6: Lifecycle energy footprint per kg Hevea and guayule rubber production and co-products
Positive values represent energy produced and negative values represent energy consumed; black bars represent net energy in each scenario
Figure 7: Sensitivity Analysis
Sensitivity Analysis in the scenario with guayule rubber transported by Rail and
electricity from bagasse (GRE)
Tire product manufacturing in the U.S.
Hevea Agriculture
Guayule Agriculture
Latex
Transportation Transportation
Processing Processing
4
3
Rubber
3
2 4 Rubber
Transportation - Truck - Ship
Transportation:
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
Hevea: LPG use Guayule: Water for drip Guayule: Water Agricultural field Hevea: N Fertilizer
Guayule: Centrifuge Guayule: Crusher SBR
SBR Production: At plant
GuayuleTransportation: Truck (processing to manufacturing) Guayule Processing: Centrifuge
Guayule Processing: Grinder Guayule Processing: Crusher
Guayule Agriculture: Diesel use for transport (field to processing) Guayule Agriculture: Diesel for forage harvester
Guayule Agriculture: Diesel for cultivator
Guayule Agriculture: Energy use for water pumping
Hevea Transportation: Diesel use by truck (port to manufacturing) Hevea Transportation: Diesel use by truck (processing to port) Hevea Processing: LPG use
Hevea Processing: Ammonia use
Hevea Agriculture: Diesel use for transport (field to processing) Hevea Agriculture: P fertilizer
GuayuleTransportation: Rail (processing to manufacturing) Guayule Processing: Creaming tank
Guayule Processing: Press filter Guayule Processing: Sclae Guayule Processing: Conveyor belt Guayule Agriculture: Diesel for baler Guayule Agriculture: Diesel for planter Guayule Agriculture: N fertilizer
Guayule Agriculture: Water for drip irrigation Hevea Transportation: Diesel use by ship (port to port) Hevea Processing: Diesel use in rubber mills
Hevea Processing: Electricity use Agriculture: Field emissions
Hevea Agriculture: Diesel use in tillage Hevea Agriculture: N fertilizer
-3.0E-6 -2.0E-6 -1.0E-6 -1.0E-21 1.0E-6 2.0E-6 3.0E-6 4.0E-6 5.0E-6
Hevea rubber and electricity
from rubberwood
Hevea rubber and biodiesel from seed oil
Guayule rubber, truck and electricity
from bagasse
Guayule rubber, truck
and bio-oil from bagasse
Guayule rubber, truck and bio-oil and
charcoal from bagasse
Guayule rubber, rail and electricity
from bagasse
Guayule rubber, rail and bio-oil from bagasse
Guayule rubber, rail and bio-oil and
charcoal from bagasse
Styrene Butadiene
Rubber
kg CFC-11 eq/kg rubber
Agriculture: Agriculture: Agriculture: Agriculture: Agriculture: Processing: Processing: Processing:
Processing: Transportation: Transportation: Transportation: Co-products: Co-products: Co-products: Co-products:
Co-products: Co-products: Co-products: Agriculture: Agriculture: Agriculture: Agriculture: Agriculture:
Agriculture: Agriculture: Agriculture: Processing: Processing: Processing: Processing: Processing:
Processing: Processing: Transportation: Transportation: Co-products: Co-products: Co-products: Co-products:
Co-products: Co-products: Co-products: SBR Production: Total GTE
HO
GTO
GTOC
GRE GRO
GROC
SBR HE
0
-80 -60 -40 -20 0 20 40 60 80 100
Hevea rubber and electricity
from rubberwood
Hevea rubber and biodiesel from seed oil
Guayule rubber, truck and electricity
from bagasse
Guayule rubber, truck
and bio-oil from bagasse
Guayule rubber, truck and bio-oil and
charcoal from bagasse
Guayule rubber, rail and electricity
from bagasse
Guayule rubber, rail and bio-oil from bagasse
Guayule rubber, rail and bio-oil and
charcoal from bagasse
Styrene Butadiene
Rubber kg CO2eq/kg rubber
Agriculture: Agriculture: Agriculture:
Agriculture: Processing: Processing:
Processing: Processing: Transportation:
Transportation: Transportation: Co-products:
Co-products: Co-products: Co-products:
Co-products: Co-products: Agriculture:
Agriculture: Agriculture: Agriculture:
Agriculture: Agriculture: Agriculture:
Agriculture: Processing: Processing:
Processing: Processing: Processing:
Processing: Processing: Transportation:
Transportation: Co-products: Co-products:
Co-products: Co-products: Co-products:
Co-products: Co-products: SBR Production:
Co-products: Total energy for esterification Agriculture: Field emission Total GTE
HO
GTO
GTOC
GRE GRO
GROC
SBR HE
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2
Hevea rubber and electricity
from rubberwood
Hevea rubber and biodiesel from seed oil
Guayule rubber, truck and electricity
from bagasse
Guayule rubber, truck
and bio-oil from bagasse
Guayule rubber, truck and bio-oil and
charcoal from bagasse
Guayule rubber, rail and electricity
from bagasse
Guayule rubber, rail and bio-oil from bagasse
Guayule rubber, rail and bio-oil and
charcoal from bagasse
Styrene Butadiene
Rubber kg SO2eq/kg rubber
Agriculture: Agriculture: Agriculture:
Agriculture: Processing: Processing:
Processing: Processing: Transportation:
Transportation: Transportation: Co-products:
Co-products: Co-products: Co-products:
Co-products: Co-products: Agriculture:
Agriculture: Agriculture: Agriculture:
Agriculture: Agriculture: Agriculture:
Agriculture: Processing: Processing:
Processing: Processing: Processing:
Processing: Processing: Transportation:
Transportation: Co-products: Co-products:
Co-products: Co-products: Co-products:
Co-products: Co-products: SBR Production:
Co-product: Total energy for esterification Total GTE HO
GTO
GTOC
GRE GRO
GROC
SBR HE
