Environmental life cycle assessment of bioethanol production from wheat straw

Environmental life cycle assessment of bioethanol production from wheat straw

b i o m a s s a n d b i o e n e r g y 4 7 ( 2 0 1 2 ) 9 e1 9 Available online at www.sciencedirect.com http://www.elsevier.com/locate/biombioe Envi...

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b i o m a s s a n d b i o e n e r g y 4 7 ( 2 0 1 2 ) 9 e1 9

Available online at www.sciencedirect.com

http://www.elsevier.com/locate/biombioe

Environmental life cycle assessment of bioethanol production from wheat straw Aiduan Li Borrion*, Marcelle C. McManus, Geoffrey P. Hammond University of Bath, Department of Mechanical Engineering, Bath BA2 7AY, UK

article info

abstract

Article history:

Ethanol produced from lignocelluloses is expected to make a major contribution on

Received 18 January 2012

transportation fuel markets. In this paper, a life cycle assessment was carried out to assess

Received in revised form

the environmental burdens of ethanol production from wheat straw and its use as ethanol

3 August 2012

blend fuels. Two ethanol based fuel E15 (a mixture of 15% ethanol and 85% petrol by

Accepted 18 October 2012

volume) and E85 (85% ethanol and 15% petrol by volume) were assessed and results were

Available online 10 November 2012

compared to those of conventional petrol (PT) in 1 km driven by an equivalent car.

Keywords:

global warming, ozone depletion, photochemical oxidant formation, acidification, eco-

Life cycle assessment

toxicity, eutrophication, water depletion and fossil depletion. The results show that,

Environmental impact

compared to petrol, life cycle greenhouse gas emissions are lower for ethanol blends, with

Ethanol

a 73% reduction for an E85-fuelled car and 13% reduction with E15. A modest savings of 40%

Wheat straw

in fossil depletion was also found when using E85 and 15% when using E15. Similar results

Lignocelluloses

are also observed for ozone depletion. The findings highlight a number of environmental

Climate change

issues such as acidification, eutrophication, ecotoxicity and water depletion for which

The environmental performance was studied using ReCiPe methodology and includes

areas ethanol blend use does not offer any advantages compared with petrol. A further analysis of ethanol production at well to gate level helps identify the key areas in the ethanol production life cycle. The results indicate where effort needs to be placed to improve the technology performance and process design which can help in lowering the environmental impacts in the whole life cycle. ª 2012 Elsevier Ltd. All rights reserved.

1.

Introduction

In a context where climate change and energy security have become two of the greatest challenges for many nations, the production of renewable energy has been increased with the aim to reduce our current dependency on fossil fuels. Bioenergy, often with the benefits of little additional point of use infrastructure and non immediate dependency on weather, offers a unique source of renewable energy. In particular, biofuel from lignocellulosic biomass offers the potential to provide a significant source of clean, low carbon and secure

energy. Although often regarded as carbon neutral process, concerns of environmental impacts have been raised for lignocellulosic fuel, for example, the system production and the transportation and growth of feedstock. As a result, sustainability assessment is now acknowledged to be an important element of the development of bioenergy from lignocellulosic material. Life cycle assessment (LCA) is a methodological tool used to quantitatively analyse the life cycle of a product or an activity with a generic framework provided by ISO 14040 and 14044 [1,2]. When analysing environmental impacts, LCA takes into

* Corresponding author. Tel.: þ44 1225385164. E-mail address: [email protected] (A.L. Borrion). 0961-9534/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biombioe.2012.10.017

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account the complete life cycle of a product or an activity delivering a functional unit. It is, therefore, well suited to examine the environmental impact of lignocellulosic biofuel. By examining the system of interest, quantifying the material and energy inputs and outputs to air water and soil, LCA can assess how the system can potentially impact on the environment. Several studies [3e6] have examined the environmental impact of bioethanol, with a particular focus on two main categories: greenhouse gas (GHG) emissions and fossil energy efficiency. These studies show, to a varying degree, reduction of fossil fuel use and of GHG emissions, in comparison with the use of conventional energy such as gasoline. Biomass is often considered a carbon neutral feedstock but there is a significant amount of GHG emissions that are released during the biomass production and conversion to the biofuels. Additionally, comprehensive sustainability assessment of biofuel is urgently needed to assess economic, social and environmental impacts of biofuel production and consumption [7]. Yan and Lin [8] revealed that the interactions among various sustainability issues make the assessment of biofuel development difficult and complicated. The complexity during the whole biofuel production chain generates significantly different results due to the differences in input data, methodologies applied, and local geographical conditions. A useful tool for addressing environmental sustainability issues is LCA. A typical LCA study of biofuel includes both the feedstock growth at farming level, and biofuel conversion process and fuel use in transportation stage. Since the carbon dioxide uptake in agriculture is counteracted by the nitrous oxide emitted in agriculture and the CO2 emissions generated in other parts of the life cycle, the reduction of GHG emissions depends on the greenhouse gases emitted in the whole chain, which may be substantial in relation to the emissions at final use. Wheat straw is currently under study as potential source for ethanol production. This feedstock is a non-food biomass and has 42% of cellulose and 25% if hemicelluloses which can be converted to ethanol production. This study aims at addressing this topic by performing a thorough LCA of ethanol production from wheat straw and its use as a blend fuel. The assessment takes into account biomass cultivation, processing, transport, conversion and final use of products, along with the use of chemicals, enzymes and nutrients as well as manufacture infrastructure. This paper considers a wide range of environmental issues such as GHG, fossil depletion, acidification, eutrophication as well as water depletion. Results of the ethanol blend fuel from wheat straw are compared with the conventional petrol. This comparison of wider environmental issues beyond GHG and energy consumption provides a better picture of the environmental performance of ethanol fuel blend and hence supports the choice of blend (E15 or E85) from an environmental point of view. The further investigation of the ethanol production at well to gate level shows how much each unit process of conversion contributes to the environmental burdens thus suggest where efforts should be placed in order to minimise the environmental impact. This will help decision makers identify where the environmental hot spots are and hence to improve their processes in the early research stage.

The work presented here is part of the ongoing BBSRC Sustainable Bioenergy Centre Project e Lignocellulosic Conversion to Ethanol programme. The conversion of the wheat straw to ethanol is based on the laboratory data of wheat straw from the ongoing research programme and the National Renewable Energy Laboratory (NREL) large scale simulation process [9]. The study covers the whole life cycle of ethanol fuel: the wheat straw growth and harvest at farm level, ethanol conversion process, and the amount of fuel to power a small passenger vehicle.

2.

Methodology

The environmental management tool, LCA, is used in this study. The software package SimaPro (version 7.2 [10].) is used to help build the inventory and undertake the impact assessment analysis. Databases included in the package such as Ecoinvent database developed by the Swiss Centre for Life Cycle Inventories [11] are also used in this study; in the case of different data available in different databases, preference is given to Ecoinvent library. In this study, the life cycle impact assessment was conducted using ReCiPe Midpoint methodology [12]. The primary objective of the impact assessment stage is to transform the long list of Life Cycle Inventory results into a limited number of indicator scores. These indicator scores express the relative severity on an environmental impact category. These indicators are then classified into categories according to their potential long term damage. A midpoint approach was taken; midpoint methods convert the emissions of hazardous substances and extractions of natural resources into impact category such as climate, acidification and ecotoxicity. The method calculates the impact towards 18 impact categories. This provides information about the amount of input or output of, for example, chemicals or materials from the product or system that can impact on a particular issue. Within this study the following midpoint impact categories are analysed: global warming, ozone depletion, photochemical oxidant formation, acidification, freshwater eutrophication, marine eutrophication, terrestrial ecotoxicity, freshwater ecotoxicity, marine ecotoxicity, water depletion, and fossil depletion. Detailed explanations of these impact categories can be found in the Result and Discussion Section. Only classification and characterization are applied; weighting and normalisation are not considered in this paper. The analysis was applied to both well to wheel and well to gate life cycle as presented in the Function Unit section.

2.1.

Goal and scope

The goal of the study is to quantify the environmental impacts of the bioethanol from wheat straw conversion processes and compare ethanol blend fuels with conventional petrol. Blends of 15% (by volume) ethanol with petrol (E15) and 85% with petrol (E85) are considered for the purpose of comparison with petrol. In this study, the use of a passenger car was considered with the assumption that the vehicles can use both conventional petrol and blends of ethanol and petrol. The scope of the study includes the life cycle of ethanol use from a well to

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wheel perspective, including wheat straw, ethanol conversion and transport to blending refinery, ethanol blending with petrol and storage and, burning of fuel in a small passenger car. It does not include any specific differences regarding vehicular emissions at different blend rates.

2.2.

Functional unit

Ethanol is currently used as vehicle fuel mainly in two ways. Firstly it can be blended with petrol in 5e20% by volume for its use in vehicles without engine modifications. Alternatively it is possible to use ethanol almost in its pure form (85e100%) in vehicles with modified engines. In this paper, two types of ethanol blend are assumed: E15 (15% ethanol and 85% petrol by volume) and E85 (85% ethanol and 15% petrol by volume) in small passenger car. LCA methodology was used to compare the environmental performance of these fuels in a “well to wheel” analysis with the use of conventional petrol (PT). The function of the study is to drive a small passenger car. The functional unit chosen to

compare the life cycle flows is the amount of fuel to drive 1 km distance by a small passenger car. Under these conditions, the amount of fuel required for travelling 1 km is calculated to be 60 g for PT, 70 g for E10, and 94 g for E85 [13]. A second functional unit, 1 kg of ethanol produced from wheat straw, is also used in this study to further study the contribution of environmental burdens from the ethanol conversion process. For this function unit, a “well to gate” analysis is applied.

2.3.

System boundary and data source

Fig. 1 shows the boundary of the ethanol fuel systems. Machinery, transportation, electricity and fuel used as well as waste management are included in the system. The production and disposal of the car, construction and maintenance of road are outside of the system boundaries. Table 1 summarises the data sources for the subsystems that are used for this study. Detailed descriptions of each subsystem including wheat straw growth subsystem, ethanol production subsystem, fuel

Production and transportation Machinery, Fuel, Fertiliser, Irrigation, Agrochemicals

S1 Wheat straw growth Raising seeding Soil preparation Sowing Raising seeding

Field preparation Soil cultivation Mulching Transplanting Training Fertiliser application Pesticide application Harvest

Wastewater treatment

Ethanol recovery

Straw for incorporation

Straw for Biofuel

S2 Ethanol conversion process Prehydrolysis & Hydrolysis & conditioning Co-Fermentation Utilities: Fuel Electricity Steam Water

Wheat

Production treatment Grading Packing

Feedstock handling (straw) Waste to landfill Chemical production Enzyme production

S3 Fuel distribution Petrol Storage

Ethanol Storage Blending

E15 at service station

E85 at service station

S4 Fuel use Use E15

Use E85

Fig. 1 e System boundary (dot line indicates subsystem boundary).

Petrol production

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Table 1 e Data sources for the studied system.

Table 2 e Wheat straw composition.

Subsystem

Composition

Data required

S1 wheat straw Soil cultivation growth Sowing Fertilisation Weed control Pest and pathogen control Harvest Grain drying Diffusion emissions Machinery Fuel use S2 ethanol Production scale conversion Enzyme use process Chemical use Nutrient use Industrial equipment use Transport Waste and water treatment Energy use S3 fuel Petrol production and distribution transport Bioethanol transport Fuel storage S4 fuel use Fuel use Emission

Data source EcoInvent database [11]

2.3.2. Laboratory results, NREL simulation [9], EcoInvent database [11], Literature [14]

EcoInvent database [11]

Literature [13], EcoInvent database [11]

distribution subsystem and fuel use subsystem are presented in the following sections.

2.3.1.

Cellulose Xylan Arabinan Galactose Mannose Lignin

Wheat straw growth subsystem

A generic wheat straw from the European region was considered. The inventory at farm level includes the processes of soil cultivation, sowing, weed control, fertilisation, pest and pathogen control, harvest and grain drying. Machine infrastructure and a shed for machine sheltering are also included. Inputs of fertilisers, pesticides and seed as well as grain transports to the regional processing centre (10 km) are included. The direct emissions from the field (such as agrochemicals emissions and fuel combustion emissions) are also calculated. Data were obtained from the Ecoinvent database. From within this inventory data for the production of 1 kg wheat grains at farm with a moisture content of 15% has been selected. The product yield for wheat grain is 5305 kg yield/ha and 3232 kg yield/ha for wheat straw. It was assumed that two thirds of straw produced remains in the field to maintain soil quality. Mass allocation was used to assign the environmental burdens for wheat grain, straw for field incorporation and straw taken for biofuel production as a base case. Table 2 shows the chemical composition of wheat straw; the results are supplied by the research laboratory as part of the Lignocellulosic Conversion to Ethanol programme (LACE) in University of Nottingham. The content presented in Table 1 is used to calculate the ethanol yield and chemical inputs. The study also takes in account of the binding of carbon from the atmosphere and estimates the C-content in the dry matter multiplied by the stoichiometric factor 44/30, based on the assumption that the carbon in the biomass is completely taken from the air (1.47 kg CO2 per kg dry biomass).

Unit % Dry % Dry % Dry % Dry % Dry % Dry

basis basis basis basis basis basis

Amount 42 22 3 1.0 0.4 22

Ethanol production subsystem

The particular aim of this study is to evaluate the environmental impact of lignocellulosic ethanol conversion process from wheat straw. This paper takes into account the machinery used for ethanol conversion process. With the division of the conversion process into five sub-processes: feedstock handling, prehydrolysis, saccharification & fermentation, ethanol recovery and wastewater treatment, this study also assesses the environmental burdens from these sub-processes; the results indicate how much of each process’s contribution to environmental impacts, thus suggest where efforts shall be placed in order to improve the environmental performance. Data for the bioethanol conversion process in this study are obtained from various sources including the available databases and literature. The report from the National Renewable Energy Laboratory (NREL) of the US department of Energy [9] is the main source for the ethanol conversion process (such as conversion rate and reaction condition). Material flow and energy flow are collected from the report; equipment information and chemicals are collected through manufacture websites, estimation and life cycle inventory databases, predominantly eco-invent. Building on the NREL corn stover to ethanol process, a simulation system was developed for the wheat straw to ethanol process. The same industrial plant, conversion rate and reaction condition was assumed. Table 3 shows the material input and output during the conversion plant of wheat straw to ethanol. Typical input/output values obtained by NREL were adapted to a wheat straw to ethanol simulation process have an input rate of 105,968 kg/h wheat straw with 15% moisture content and output rate of 27,984 kg/h ethanol was modelled. The process is assumed to have 8406 h of operation time, which is equal to an annual production scale of 757,152 tons of wheat straw on a dry basis. The process begins as wheat straw is delivered to the feedstock handling area where it is washed and shredded. In the following step, the hemicelluloses sugars are released using dilute sulphuric acid hydrolysis and steam in the prehydrolysis area. After the prehydrolysis step, the hydrolyzated stream goes to the saccharification and fermentation area where the stream is split to the fermentation reactor and the cellulase enzyme production reactor where cellulase enzymes are formed. The cellulase enzymes are then sent to the saccharification reactor where sugars are formed; after saccharification, the enzymes are recycled back to cellulose enzyme the production reactor whilst the sugars are sent to fermentation reactor where ethanol is formed. Ethanol is then purified by distillation and dehydration in the ethanol recovery area, and then sent to

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Table 3 e Material flow of ethanol conversion process (kg/hr). Materials

Value

Inputs from the technosphere Wheat straw Sulphuric acid Lime Corn steep liquid Diammonium phosphate Polymer feed Enzymes Nutrient feed High pressure steam Low pressure steam

105,968 3551 2586 1410 177 30 7369 63 40,212 17,672

Inputs from the environment Freshwater

46,458

Outputs to the technosphere Ethanol Sludge to landfill Gypsum to landfill

27,984 2000 7794

Outputs to the environment Ethanol Methane VOC Carbon dioxide Sulphuric acid Vapour Water evaporated

196 279 5286 25,282 2 43,420 12,312

product storage. The liquid flow from the distillation process is sent to wastewater treatment where they are treated. The recovered water is sent back to the process as recycled water. The solid residuals are sent to landfill. Lignin from biomass can be combusted using a combined burner/boiler/turbogenerator to produce electricity to supply the ethanol plant; however this is not included in the system boundary but could be expanded in a future study.

2.3.3.

Fuel distribution subsystem

Bioethanol is distributed to service stations (transport distance assumed: 100 km) where it is used to fuel passenger cars at a specific consumption of 2.45 MJ/km. Emissions occurring during the distribution were modelled using the Ecoinvent inventory database. The processes include blending, transportation to the regional storage and service station.

2.3.4.

Fuel use subsystem

Average emissions for combustion of bioethanol in cars are calculated using the data from Ecoinvent. The average fuel economy considered in the passenger cars under study running with PT, E15, and E85 was 11.91 km/L, 11 km/L, and 8.29 km/L respectively [13].

3.

Environmental LCA results

Table 4 summarizes the LCA characterization results for PT, E15 and E85 considered in this study. Change represents

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impacts of substituting any of the two alternatives for conventional petrol. A positive value implies an increase in the environmental load compared to conventional petrol and negative value implies a reduction in the environmental load. Table 5 shows the LCA characterisation results of ethanol production for the well to gate life cycle. To better understand the contribution of environmental burdens from the ethanol conversion process, the bioethanol conversion process was broken down into five sub-processes: feedstock handling, prehydrolysis, saccharification and fermentation, ethanol recovery and wastewater treatment. This method allows comparing each sub-process’s contribution to environmental burden; from which one can identify where the environmental performance can be improved. The results show that the levels of impacts in global warming, ozone depletion and fossil depletion are considerably reduced when shifting from petrol to ethanol blend fuels. According to these results, the higher the ethanol ratio in the blend is, the higher the change in all the above categories. These results are mostly due to the replacement of petrol by ethanol and the higher contribution from activities related to feedstock production. Reductions up to 73%, 50% and 40% are achieved when E85 is used as transport fuel in terms of global warming, ozone depletion and fossil depletion respectively. For the ethanol production at well to gate level, prehydrolysis, also called as pretreatment process, contributes significant to environmental burdens compared with other sub unit processes such as saccharification & fermentation and ethanol recovery.

3.1.

Climate change

Table 4 shows that in terms of climate change potential it is more advantageous to use ethanol blend fuels than conventional petrol. At a well to wheel level, the use of E85 can avoid 242 g CO2 eq per km, which corresponds to a 73% GHG reduction; and the use of E15 can avoid 43 g CO2 eq per km corresponding to a reduction of 13%. The cause of this reduction is due to the influence of the carbon sequestration during crop growth which contributes to offset the GHG emissions. However, despite the overall reduction of GHG emissions, N2O emissions increase when ethanol blend fuels are used (Fig. 3); this is due to the application of nitrogen based fertilizer during the biomass growth phase which contributes to the N2O emissions. CH4 emissions are slightly increased but large differences are attributed to CO2 emissions. From Fig. 2, one can observe that majority of CO2 emission from the petrol life cycle comes from the fuel use in the vehicles, with less than 15% coming from the petrol production and distribution. For ethanol based fuel, a significant contribution of CO2 emissions is from the ethanol conversion subsystem followed by fuel use subsystem. The capability of the carbon fixation during biomass growth offsets the majority of the total GHG emissions, which leads to high level of GHG reduction of 73%. Approximately 85% of total GHG emissions throughout the life cycle are cancelled by CO2 uptake in biomass subsystem as shown in Fig. 2. For better understanding of the contribution of GHG during ethanol production, GHG emissions are also analysed at a well to gate level as shown in Fig. 4. Within the biomass growth

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Table 4 e Comparison of environmental burdens from petrol, E15 and E85 (per km driven). Categories

Global warming Ozone depletion Photochemical oxidation formation Acidification Freshwater eutrophication Marine water eutrophication Terrestrial ecotoxicity Freshwater ecotoxicity Marine ecotoxicity Water depletion Fossil fuel depletion

Unit

Petrol

g CO2 eq mg CFC-11 eq g NMVOC g SO2 eq g P eq g N eq g 1,4-DB eq g 1,4-DB eq g 1,4-DB eq L g oil eq

E15

330.09 0.11 1.19 1.01 0.02 0.20 0.04 0.58 0.58 0.84 241.29

and handling stage, more than 70% of GHG are attributed to the N2O emissions. For both prehydrolysis and saccharification & fermentation sub-processes, around 98% of the GHG comes from both CO2 and N2O respectively. However, in terms of the overall GHG emissions, the results show that the main contribution of GHG emissions comes from the prehydrolysis process, followed by the saccharification and fermentation process, and then by the wastewater treatment process. Within the prehydrolysis process, process steam, sulphuric acid and electricity are identified as the main sources for generating greenhouse gas for the studied system. The reason that wastewater treatment process contributes significantly to GHG emissions is due to large amount of waste usage thus resulting in large amount of wastewater needed to be treated. Large amount of water use in ethanol production process has been reported in some studies (e.g. [15].). In the case presented, for every kilogramme of ethanol produced, nearly 4 kg of wastewater is produced. Additional contributions come from ethanol recovery process; the carbon dioxide was produced during the fermentation process but was not released until ethanol recovery process. This contribution is also suggested by Buratti et al. [16]. GHG emissions in agriculture are largely determined by the emissions of nitrous oxide, whereas in the ethanol production process they are largely determined by CO2 due to electricity generation and fermentation. The production of the enzyme used for saccharification requires a substantial amount of

E85

E15

Change

E85

Change

287.13 0.09 1.12 2.43 0.03 1.80 1.49 1.21 0.66 1.08 206.27

0.13 0.19 0.05 1.41 0.83 7.99 32.67 1.07 0.13 0.30 0.15

88.10 0.05 2.86 12.96 0.15 12.80 10.72 6.06 1.62 4.02 146.88

0.73 0.55 1.41 11.85 7.53 62.82 241.95 9.38 1.78 3.81 0.39

fossil or combustion electricity for air compression which also generates a considerable amount of CO2 emission in the chain [17]. Therefore, research must be focussed on these processes to improve overall environmental performance of lignocellulosic ethanol.

3.2.

Ozone depletion

Similar to GHG, using ethanol blend fuel offers advantage in terms of ozone depletion potential. Table 4 highlights that a 55% reduction can be achieved when using E85, and near 20% of reduction is possible when using E15 in comparison to the use of petrol. For the ethanol production, prehydrolysis step is also identified as the main unit with significant contribution to the ozone potential. The main sources are steam used and sulphuric acid used during prehydrolysis process. Another main contribution is wheat production.

3.3.

Photochemical oxidants

The LCA results suggest that the photochemical oxidant potential increases when shifting from conventional petrol to ethanol blend fuels. Slight reductions can be achieved in photochemical potential (5%) when E15 is used. The sources of the increases are from ethanol production subsystem and wheat straw growth subsystem as shown in Tables 4 and 5. In

Table 5 e Contribution of environmental burdens from ethanol conversion process (per kg ethanol produced). Categories Global warming Ozone depletion Photochemical oxidation formation Acidification Freshwater eutrophication Marine water eutrophication Terrestrial ecotoxicity Freshwater ecotoxicity Marine ecotoxicity Water depletion Fossil fuel depletion

Unit

Feedstock handling

Pre-hydrolysis

Saccharification & fermentation

Ethanol recovery

Wastewater treatment

g CO2 eq mg CFC-11 eq g NMVOC g SO2 eq g P eq g N eq g 1,4-DB eq g 1,4-DB eq g 1,4-DB eq L g oil eq

963.89 0.04 3.52 40.90 0.30 44.86 0.79 7.73 1.94 1.02 95.49

1532.47 0.07 5.44 44.91 0.38 46.79 0.84 9.42 3.50 13.60 294.96

841.69 0.04 3.00 25.23 0.22 26.28 0.48 5.42 2.10 7.87 160.25

595.95 0.02 2.06 15.57 0.13 16.10 0.29 3.29 1.27 4.75 125.31

269.59 0.002 1.06 1.05 0.005 0.39 0.002 0.08 0.08 0.23 101.63

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0.600

100% 90%

0.400

80% C sequestration

0.200

70%

Ethanol converion Agricultural activities

0.000

Distribution

40%

Petrol production

30%

-0.400

N20 CO2

20% 10%

-0.600 Petrol

E15

E85

0% FH

Fig. 2 e Comparison of global warming potential from petrol, E15 and E85 unit: g CO2 equivalent per km.

particular, there is an increase of CO and NMVOC emissions from the use of fuel in agricultural machinery ethanol conversion plant. Diffused emissions of ethanol from the conversion process also contribute to the increase of photochemical oxidant. Moreover, although the combustion of ethanol based fuels produces lower emissions of CO, there are higher emissions of acetaldehyde which also contribute to photochemical oxidant. As a result, contributions to this from the blend use increase with the ratio of ethanol in the blend. Similar to other impact categories, the major contributions are from prehydrolysis process, saccharification and fermentation as well as wastewater treatment process. Heat for the process is produced from natural gas and electricity from coal and oil, and this contributes to the photochemical oxidation.

3.4.

CH4

50%

Fuel use -0.200

60%

Acidification

Similar results for acidification potential are obtained. Increasing the ratio of ethanol in the blend produces more emissions contributing to acidification. Results presented in Tables 4 and 5 suggest that upstream activities are the main contributors, in particular the biomass growth and prehydrolysis step (with about 70% of the total). The source of these

0.4 0.3 0.2 0.1

CH4 N2O

0.0

CO2 -0.1 -0.2 -0.3 Petrol

E15

E85

Fig. 3 e GHG emissions from petrol, E15 and E85.

PH

HF

ER

WWT

Fig. 4 e Contribution to different types of GHG emissions from different stages of ethanol conversion process (percentage of total GHG emissions during each stage). Note e FH: feedstock handling, PH e prehydrolysis, HF e hydrolysis and fermentation, ER e ethanol recovery; WWT e wastewater treatment.

emissions during biomass growth comes from the application of fertilizers and the use of agricultural machinery, while for the prehydrolysis step, significant contributions come from the use of sulphuric acid for treatment purposes. These two steps generate significant amount of SO2 and NOx which are emitted to the atmosphere. The emissions derived from the application of fertilizers are another important contributor in terms of acidification mainly due to NH3 emissions. Wheat cultivation is the principal processes that contribute to this impact category, followed by sulphuric acid and steam usage during the production process. That is a consequence of the high use of ammonia as a fertilizer for the wheat production. Production of petrol, sulphur for the fermentation and process heat contributes only to a small percentage.

3.5.

Eutrophication

Similar results were also observed for eutrophication as acidification. Increasing the ratio of ethanol in the blend produces more emissions contributing to eutrophication. Eutrophication occurs when fertilizers move from land to surface waters causing an increase in the aquatic plant growth. The nutrient causing eutrophication is often phosphorus for freshwater systems and nitrogen for marine systems, and thus the location of the release often makes a significant impact on the relative potential for damage. Results show that activities related to wheat growth are the main source of these emissions contributing to eutrophication. This high contribution is due to NH3 and NOx emissions from nitrogen based fertilizer production and application and diesel use in agricultural machinery and tractors. From Table 4, one can observe that the increases of marine eutrophication when using ethanol blend fuels are much more significant in comparison with the increases of freshwater eutrophication as marine eutrophication is affected by nitrogen based nutrients.

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Marine eutrophication is caused by a number of different nitrogen compounds to different compartments. In this study, the main nitrogen compounds include nitrogen oxide and nitrogen dioxide emitted from feedstock production, sulphuric acid and production of process steam. For the first source, this is mainly due to the use of fertilizers during the production of the corn, containing nitrate, ammonia and phosphate; for the second source, contributions come from direct use of acid, transport, combustion, production of petrol (all of them releasing NOx emissions). Another factor accounting to the impact on eutrophication in particular freshwater eutrophication is the use of diammonium phosphate (DAP) for the fermentation of the wheat straw i.e. the chemical process to convert the cellulose to ethanol, which pollutes the wastewater with amounts of phosphate.

3.6.

Ecotoxicity

In the ReCiPe assessment method used, three types of ecotoxicity are considered including terrestrial, freshwater and marine ecotoxicity. Results presented in Table 4 show that the contributions increase when fuel blends with a high ratio of ethanol are used as transport fuel. In all these impact categories, higher impacts from the upstream activities of ethanol blends govern the impacts of the fuel life cycle respect to petrol. In particular, a significant increase has been observed for the terrestrial potential. At the well to gate level as shown in Table 5, it can be observed that both feedstock growth and prehydrolysis with the use of acid contribute to majority of the emissions (around 70%) to all categories of ecotoxicity. The sources of terrestrial potential are dominated by pesticide emissions to agriculture soil as well as the use of both sulphuric acid and steam during the conversion process. Marine ecotoxicity is fully dominated by emissions of heavy metals and sulphuric acid, largely to air.

3.7.

Water depletion

Similar to acidification, eutrophication and ecotoxicity, ethanol blend fuels do not offer advantages in the water depletion when compared with conventional petrol. The impact on water depletion increases when using higher ratio of ethanol in the fuel blends as shown in Table 4. Within the ethanol conversion of well to gate level, Table 5 highlights the water consumption during the ethanol conversion system. The main cause of the water depletion during ethanol conversion process can be attributed to the following two processes: 1) agriculture irrigation for wheat production and 2) the use of water during both prehydrolysis and saccharification and fermentation process.

easily be seen that crude oil contributes largely to the fossil depletion, followed by natural gas and coal, respectively. The high reduction of crude oil in blends with high ratio of ethanol is related to the lowest ratio of petrol in them. For the ethanol conversion process, the highest contribution to fossil depletion comes from the prehydrolysis step as shown in Fig. 5. This is because the treatment process requires large amount of steam which requires heat from natural gas.

4.

Discussion

It is widely recognised that lignocelluloses can be converted to bioethanol, and thus may have potential environmental benefits and greenhouse gas savings. Lignocellulosic ethanol offers the potential to provide a significant source of clean, low carbon and secure energy. Lignocellulosic materials have advantages such as being able to be sourced locally and in large quantity. Hence, second-generation bioethanol is expected to make a major impact on transportation fuel markets. However, despite research attention on the technology development, the industrial scale up of this process appears to be still hindered by technological issues or by the lack of a biomass refinery approach [5]. The lack of industrial scale and technology uncertainties contributes to the difficulties and challenges of LCA studies. Concern about sustainability and security of fossil fuel use, along with advances in biomass conversion technology, has simulated the interested in ethanol production from lignocelluloses. Wider environmental issues beyond carbon and energy assessment need to be addressed to provide overall sustainability pictures of lignocellulosic conversion to ethanol. The LCA results suggested a reduction of 73% in GHG emission when using E85 and a reduction of 15% with the use of E15 produced from wheat straw. The finding is in line with other studies, for example Cherubini and Jungmeier [18] reported up to 60% of savings when using from crop residues including wheat straw and corn stover; Gonzalez-Garcia et al.

40% 35% 30% 25% Climate change

20%

Fossil depletion

15% 10% 5%

3.8.

Fossil depletion 0%

As shown in Table 4, a reduction of non-renewable fuel requirements can be achieved when changing from petrol to ethanol blends. Although shifting from petrol to ethanol blends increases the consumption of liquid fuel by agricultural machinery, less petrol is necessary to propel the car. A reduction of 40% of total fossil fuel consumption when E85 is used and a reduction of 15% when using E15 is shown. It can

FH

PH

HF

ER

WWT

Fig. 5 e Contribution to climate change potential and fossil depletion from different stage of ethanol conversion process (percentage of total contributions from the conversion process). Note e FH: feedstock handling, PH e prehydrolysis, HF e hydrolysis and fermentation, ER e ethanol recovery; WWT e wastewater treatment.

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[19] reported GHG savings of 88% when using E85 from Alfalfa stems. More than 100% of GHG saving has also been reported by some [20] using corn stover as a biomass source. Although E85 from wheat straw does not offer 100% of GHG savings due to the significant contribution from ethanol conversion process; a further reduction can be achieved if the improvement in the technology performance is made, in particular the prehydrolysis step. Less energy and chemical intensive deconstruction process could offer savings of GHG potential. This paper also highlights that GHG emissions in agriculture stage are largely determined by the emission of nitrous oxide whereas in ethanol conversion production stage by CO2 due to electricity generation and fermentation. The substantial amount of fossil or combustion electricity required for enzyme production also generates a considerable amount of CO2 emission in the chain [17]. Therefore, research should be focussed on these processes to improve overall environmental performance of lignocellulosic ethanol. A reduction of 55% for ozone depletion can be achieved when using E85 produced from wheat straw, but an increase of more than 100% for photochemical oxidation is also observed using same type of fuel. Similar finding on photochemical oxidation has also been reported when using black locust as biomass [20]. In contrast, when using E15, although less ozone depletion can be achieved (17%), a slight decrease in photochemical oxidation is obtained. This is because the source of emissions contributing to photochemical oxidation potential come from the upstream activities; less ethanol in the blend fuel receive much less influence from both biomass and ethanol conversion systems. Despite advantages in GHG savings, E85 does not offer benefits in terms of acidification, eutrophication and ecotoxicity. Similar findings have also been reported when other biomass such as poplar, alfalfa stems are used [13,19]. However, some studies [17,21] have also reported a decrease in both acidification and eutrophication using black locust and corn stover as feedstock respectively. The increases of these indicators are mainly due to N fertilization, which causes leaching of nitrates to groundwater. Use of sulphuric acid and steam during the conversion step also make significant contribution to these impact categories. With the conflicting results from different studies, it will be necessary to investigate further these issues for the lignocelluloses-based ethanol and to have more concrete evidence and conclusions. The LCA results also suggest that fossil fuel extraction decreases when ethanol based fuels are used as transport fuel. The findings are consistent with other studies with different kinds of feedstock [19]. Sheehan et al. [20] reported a comparison of non-renewable energy consumption for E85 from different feedstock (corn grain, grasses, trees and corn stover) and stated that fossil fuel savings for corn stover and trees based ethanol are considerably high (80%), and significantly better than the savings associated with corn grain based ethanol (33%). Kaufman et al. [22] reported up to 80% of energy savings in biorefinery ethanol production in comparison to reference fossil fuel system. Gonzalez-Garcia et al. [23] reported the reduction of fossil fuel when using pure ethanol can reach 70e80%, which appear different from some other studies: Wang et al. [24] showed ethanol from lignocelluloses can achieve reductions in fossil energy consumption over 90%

17

which is much higher than corn ethanol between 28% and 50% reduction. Similarly, Sheehan et al. [20] conducted a life cycle assessment of ethanol production from corn stover in Iowa and discovered that E85 reduces petroleum consumption 95% per kilometre travelled compared to conventional gasoline, and reduces total fossil energy consumption and greenhouse gas emissions by 102% and 113%, respectively. The results from this paper suggest that a 40% of reduction is possible when using wheat straw as a biomass. The different between the findings from this study and other studies is because most of other studies assume that electricity can be supplied by the combustion of lignin. Our future study will incorporate the use of electricity generated from lignin combustion; hence a further reduction is expected by reducing the fuel use in the biomass growth stage as well as in the ethanol conversion process. Although the energy savings are different in the reported studies, it appears that energy savings can be reached in all cased studied. Swana et al. [25] shows lignocellulosic-based ethanol has a significant increase in net energy produced comparing with the corn-based ethanol. Brehmer and Sanders [26] also agrees a rather modest improvement by expanding ethanol facilities to include lignocellulosic technology. Luo et al. [17] stated that when all the co-product are taken into account in the corn stover-based ethanol, the net energy value becomes much higher than the literature covering the corn cases, which shows ethanol production from cellulosic feedstock is more energy efficient than corn-based ethanol. The LCA findings also highlight an increase in water depletion when shifting from petrol to ethanol blend fuel. A very few studies [27e29] have reported the issue of water depletion between 1 L to nearly 14 L per the amount of E100 from lignocelluloses for 1 km. Our findings suggest 4 L water consumption for the amount of fuel E85 for travelling 1 km, which is in line with the findings reported between 1 L and 14 L. Harto et al. [29] pointed out that there is significant water consumption for some alternative transport fuels; in particular, agricultural crop based biofuels. Sheehan [30] also argues that the problem for biofuels and water is that the impact of increased biofuels production on water quality, mainly with regard to nutrient leaching and eutrophication from farming operations is poorly understood. For a sustainable development of crop based biofuel, water consumption cannot be ignored. Further studies are required to evaluate the impacts on water consumption as well as water quality with different types of lignocellulosic feedstock in particular crop and plant based biomass.

5.

Conclusions

This study focused on identifying the environmental impacts associated with the production and use of ethanol blend fuels (E15 and E85) from wheat straw taking into account two functional units based on the mass of ethanol and the travelling distance. Although several impact categories have been analysed such as acidification, eutrophication, photochemical oxidants formation, global warming as well as fossil fuels extraction, further studies are required to investigate other types of impact such as toxicity categories and water

18

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consumption in order to achieve a more complete scope of the impacts of ethanol fuel production. This paper highlights a number of environmental issues of ethanol production from wheat straw and its use as ethanol blend fuel. E15 and E85 applications reduce the dependence on fossil fuels, ozone depletion as well as the GHG emissions, but increase the contributions to photochemical oxidants formation, acidification, ecotoxicity, eutrophication and water depletion. E85 seems to be a better option than E15 in terms of global warming, fossil fuels extraction and ozone depletion. However, E15 offers better environmental performance than E85 in photochemical oxidants formation, ecotoxicity, eutrophication and water depletion. Therefore, the choice of the better blend (E15 or E85) from an environmental point of view depends greatly on what environmental impact categories are preferred. The LCA of wheat straw to ethanol at well to gate level was carried out considering the overall process, characterised by feedstock harvesting and handling, prehydrolysis, saccharification and fermentation, ethanol recovery and wastewater treatment. The assessment showed that the most significant environmental impacts occur during the prehydrolysis phase, followed by the saccharification and fermentation process and wastewater treatment. Another main finding is that the use of sulphuric acid and process steam as well as electricity is identified as the main source of major environmental impact categories such as climate change and ozone depletion. Further analysis should focus on other issues such as water, biodiversity and land use change which are important issues to be considered in the future studies. A wider range of environmental issues should be considered to provide an overview of the sustainability of lignocellulosic ethanol conversion.

Acknowledgements The research reported here was supported in full by the Biotechnology and Biological Sciences Research Council (BBSRC) Sustainable Bioenergy Centre (BSBEC), under the programme for ’Lignocellulosic Conversion To Ethanol’ (LACE) [Grant Ref: BB/G01616X/1]. This is a large interdisciplinary programme and the views expressed in this paper are those of the authors alone, and do not necessarily reflect the views of the collaborators or the policies of the funding bodies. The authors also acknowledge Dr Sanyasi Gaddipati from University of Nottingham for providing the wheat straw composition data. The authors would also like to thank the anonymous reviewers.

references

[1] International Standard Organisation 14040. Environmental management e life cycle assessment e principles and framework; 2006. [2] International Standard Organisation 14044. Environmental management e life cycle assessment e requirements and guidelines; 2006. [3] Kim S, Dale B. Life cycle assessment of various cropping systems utilized for producing biofuels: bioethanol and biodiesel. Biomass Bioenergy 2005;29:426e39.

[4] Kemppainen A, Schonnard D. Comparative life-cycle assessments for biomass-to-ethanol production from different regional feedstocks. Biotechnol Prog 2005;21: 1075e84. [5] Spatari S, Bagley DM, MacLean HL. Life cycle evaluation of emerging lignocellulosic ethanol conversion technologies. Bioresour Technol 2010;101:654e67. [6] Gonzalez-Garcia S, Luo L, Moreira MT, Feijoo G, Huppes G. Life cycle assessment of flax shives derived second generation ethanol fueled automobiles in Spain. Renew Sustain Energy Rev 2009;3:1922e33. [7] Halog A. Models for evaluating energy, environmental and sustainability performance of biofuels value chain. Int J Global Energy Issues 2009;32:83e101. [8] Yan J, Lin T. Biofuels in Asia. Appl Energy 2009;86:S1e10. [9] Aden A, Ruth M, Ibsen K, Lechura J, Neeves K, Sheehan J, et al. Lignocellulosic biomass to ethanol process design and economics utilizing co-current dilute acid prehydrolysis and enzymatic hydrolysis for corn stover. Technical report from National Renewable Energy Laboratory, Department of Energy, USA; 2002. [10] Pre Consulants, LCA software, SimaPro version 7.2. [11] EcoInvent Database, Swiss centre for life cycle inventories. [12] Goedkoop MJ, Heijungs R, Huijbregts M, De Schryver A, Struijs J, Van Zelm R, ReCiPe. A life cycle impact assessment method which comprises harmonised category indicators at the midpoint and the endpoint level; 1st ed. report I: characterisation; 2008. [13] Gonzalez-Garcia S, Gasol CM, Gabarrell X, Rieradevall J, Moreira MT, Feijoo G. Environmental profile of ethanol from poplar biomass as transport fuel in Southern Europe. Renewable Energy 2010;35:1014e23. [14] Luo L, van der Voet E, Huppes G. An energy analysis of ethanol from cellulosic feedstockecorn stover. Renew Sustain Energy Rev 2009;13:2003e11. [15] Li A, Khraisheh M. Bioconversion of municipal solid waste to ethanol and its related environmental issues. Int J Soil Sediments Water Documenting Cutting Edge Environ Stewardship 2008;1:5e10. [16] Buratti C, Barbanera M, Fantozzi F. Environmental balance of bioethanol from corn grain: evaluation of different procedures of co-products allocation. In: Proceeding of the 16th European biomass conference & exhibition; 2008. [17] Luo L, van der Voet E, Huppes H, Haes H. Allocation issues in LCA methodology: a case study of corn stover-based fuel ethanol. Int J Life Cycle Assess 2009;14:529e39. [18] Cherubini F, Jungmeier G. LCA of a biorefinery concept producing bioethanol, bioenergy, and chemicals from switchgrass. Int J Life Cycle Assess 2010;15:53e66. [19] Gonzalez-Garcia S, Moreira MT, Feijoo G. Environmental performance of lignocellulosic bioethanol production from Alfalfa stems. Biofuels, Bioprod Biorefin 2010;4:118e31. [20] Sheehan J, Aden A, Paustian K, Killian K, Brenner J, Walsh M, et al. Energy and environmental aspects of using corn stover for fuel ethanol. J Ind Ecol 2004;7:117e46. [21] Gonzalez-Garcia S, Moreira MT, Feijoo G, Murphy RJ. Comparative life cycle assessment of ethanol production from fast-growing wood crops (black locust, eucalyptus and poplar). Biomass Bioenergy 2012;39:378e88. [22] Kaufman AS, Meier PJ, Sinistore JC, Reinemann DJ. Applying life-cycle assessment to low carbon fuel standards e how allocation choices influence carbon intensity for renewable transportation fuels. Energy Policy 2010;38:5229e41. [23] Gonzalez-Garcia S, Gasol CM, Moreira MT, Gabarrell X, Pons JR, Feijoo G. Environmental assessment of black locust (Robinia pseudoacacia L.)-based ethanol as potential transport fuel. Int J Life Cycle Assess 2011;16:465e77.

b i o m a s s a n d b i o e n e r g y 4 7 ( 2 0 1 2 ) 9 e1 9

[24] Wang M, Saricks C, Santini D. Effects of fuel ethanol use on fuel-cycle energy and greenhouse gas emissions. ANL/ESD38. Argonne, IL: Argonne National Laborator; 1999. [25] Swana J, Yang Y, Behnam M, Thompson R. An analysis of net energy production and feedstock availability for biobutanol and bioethanol. Bioresour Technol 2011;102:2112e7. [26] Brehmer B, Sanders J. Implementing an energetic life cycle analysis to prove the benefits of lignocellulosic feedstocks with protein separation for the chemical industry from the existing bioethanol industry. Biotechnol Bioeng 2009;102:767e77.

19

[27] Scown CD, Horvath A, McKone TE. Water footprint of U.S. transportation fuels. Environ Sci Technol 2011;45: 2541e53. [28] Wu M, Mintz M, Wang M, Arora S. Water consumption in the production of ethanol and petroleum gasoline. Environ Manage 2009;44:981e97. [29] Harto C, Meyers R, Williams E. Life cycle water use of lowcarbon transport fuels. Energy Policy 2010;38:4933e44. [30] Sheehan JJ. Biofuels and the conundrum of sustainability. Curr Opin Biotechnol 2009;20:318e24.