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A life cycle assessment comparison of rapeseed biodiesel and conventional diesel
A life cycle assessment comparison of rapeseed biodiesel and conventional diesel M Stow 1, M C McManus 1, C Bannister 2 1 Sustainable Energy Research Team 2 Powertrain and Vehicle Research Centre Department of Mechanical Engineering, University of Bath, UK
1
ABSTRACT
Biodiesel is often considered to improve energy security and reduce the impact of fuel on climate change. However there are concerns about the impact of biodiesel when its life cycle is considered. The potential impact of using biodiesel rather than conventional diesel was investigated using a life cycle assessment (LCA) of rapeseed biodiesel. Biodiesel leads to reduced fossil fuel use and is likely to reduce the impact of transport on climate change. However it was found that the impact of biodiesel towards other categories, i.e. land use and respiratory inorganics, was greater than petroleum diesel. Therefore biodiesel production should be carefully managed to mitigate its impact on the environment. Keywords: Life Cycle Assessment; Biodiesel; Rapeseed 2
BACKGROUND
Biodiesel is considered to have a number of advantages over diesel. Biodiesel can be produced from an array of feedstocks and, unlike diesel, feedstock sources are highly distributed around the world. This means that an increase in biodiesel use should lead to an increase in energy security(1). Biodiesel is often thought, incorrectly, to be carbon neutral on the basis that any carbon released during combustion had previously been absorbed from the atmosphere during crop growth(2). Biodiesel is compatible with diesel. Blends of biodiesel and diesel are labelled Bη, where η is the proportion of biodiesel as a percentage. They can be blended together in any proportion, the same distribution infrastructure can be used and at low blend levels, used in diesel engines with no modification(3). These apparent advantages have led to legislation being introduced to increase the use of biofuels(4). The European parliament has set a target that 10% of fossil fuel consumption for transportation must be replaced by biofuels by 2020 in all Member States(5). However, there are concerns about biodiesel’s sustainability(4). The impact biodiesel has on land use change, both direct and indirect, is of major interest(1). Previous biodiesel LCAs have highlighted the agricultural stage to have a large effect on the impact of biodiesel, due mainly to the nitrogen fertiliser used(6). There is consensus that tailpipe emissions of NOx increase with biodiesel(7) and evidence that CO also increases(8). Due to these concerns, a LCA was carried out to investigate the environmental impact of the production and use of biodiesel in the UK using rapeseed as a feedstock.
_______________________________________ © The author(s) and/or their employer(s), 2012
23
3
INTRODUCTION TO LCA
The impact of a product on the environment can be investigated using LCA, as shown in Figure 1(9). LCA examines the environmental impact of a product or system over a range of environmental issues, such as greenhouse gases, fossil fuel use, and ozone depleting substances etc. LCA is analysed in terms of a functional unit, chosen because it reflects the function of the product. This allows comparison between products fulfilling the same function. When comparing multiple products, it is unlikely that one will perform better than another in all impact categories investigated. Thus, which product is considered to have the smallest overall impact on the environment will be decided based on the value the assessor places on each individual category.
Figure 1 LCA methodology: adapted from ISO 14040: 2006(9) In processes where more than one product is produced, the inventory should be apportioned between the co-products. According to a hierarchy set out in ISO 14044:2006 the first option is system expansion to include processes relating to the co-product. If this is not suitable, then the inputs and outputs should be partitioned between the co-products in a ratio which reflects some physical property of the products, this is the burden of that product(10). 4
LCA OF BIODIESEL
Figure 2 Rapeseed biodiesel production and use
24
Data to model the lifecycle of rapeseed biodiesel was obtained from published literature, which matched the data quality requirements, and used to build an inventory. This inventory was modelled in SimaPro 7(11), primarily using the Ecoinvent database, which contains inventory data for many materials and processes. The inventory was analysed using the Eco-indicator 99H impact assessment method. The diesel LCA was modelled using the same method. Table 1 Inventory of B100 for 1km with a fuel consumption of 68g Inputs
Amount
Nature Carbon Dioxide Arable land
Output
Amount
Emissions to air 353.7 g
Hexane
0.25 m2
Technosphere
0.0065 g
Carbon Monoxide
0.91 g
Carbon Dioxide
43.2 g
Methanol
10.2 g
Nitrogen Oxide (NOx)
0.65 g
Pesticide
0.23 g
Nitrous Oxide
0.45 g
Potassium Hydroxide
0.60 g
Methane
0.11 g
Ploughing
0.49 m2
Ammonia
0.50 g
Transport
0.068 tkm
Sowing Nitrogen fertiliser
0.49 m2 9.1 g
Hydrocarbons
0.055 g
Particulates
0.037 g
Emissions to water
Fertilising
0.49 m2
Nitrate
Phosphorous fertiliser
0.98 g
Phosphorus
Application of pesticides
3.4 m
Potash
1.2 g
Combine harvesting Sulphur Lime Electricity Heat
2
0.49 m2
Potassium
2.50 g 0.017 g 0.98 g
Emissions to soil Methane
0.032 g
3.9 g 0.16 kg 0.023 kWh 0.20 MJ
4.1 Goal and scope The purpose of this LCA was to investigate how a transition towards biodiesel might affect the impact of transportation by comparing it to a diesel LCA. It was also used to investigate the potential for reducing this impact, by identifying which processes contributed significantly and then to model alternative methods of production. Rapeseed biodiesel was considered to be a transportation fuel in this LCA. To allow a comparison between the biodiesel LCA and the diesel LCA, the functional unit was 1km of distance travelled. During biodiesel production (See Figure 2 for details of production method, inputs and outputs), three co-products are formed: straw, meal cake (what remains of the seed after oil extraction) and glycerol(12). Although system expansion is the preferred method of burden allocation(10), this would increase the uncertainty in the result as it is not clear what product the co-products would offset(13). Therefore, burdens were partitioned proportionally, according to the desired characteristic of the co-product, as outlined below.
25
Rapeseed straw is generally not harvested, consequently no burdens were allocated to straw(14). However, studies have shown that leaving straw in situ reduces soil carbon emissions: this was reflected in the inventory(6). Meal cake is commonly used as a feed for livestock and so burdens were allocated according to energy content(13). Burdens were allocated by mass for glycerol, as it can be used as a feedstock for a wide range of products(15). Due to the controversy surrounding allocation methods(16), a sensitivity analysis was undertaken. The outcome of the LCA is dependent on the allocation method: using mass allocation, biodiesel has a lower impact than diesel, whereas using the allocation method in this LCA, the impact of biodiesel is higher. 4.2 Inventory The inventory of inputs and outputs to travel 1km using B100 is presented in Table 1. The inventory is not exhaustive, however, the principal substances and processes contributing to the impact are included. The biodiesel production inventory was built up using data from multiple sources(6, 12, 14, 17-23). This was then combined with data on biodiesel use to create inventories for biodiesel blends up to B50(8). It was assumed that the linear relationships reported on fuels up to B50 would remain true for B100. 5
RESULTS AND DISCUSSION
5.1 Comparison of biodiesel and diesel Table 2 shows the embodied energy and global warming potential of diesel and biodiesel. The impact transportation has on climate change is likely to be reduced by switching to biodiesel. Although the process of producing and using biodiesel emits greenhouse gases, much of the carbon dioxide from the fuel has been absorbed during growth. However, fossil fuels are also used during processing and transport and so the fuel is not “carbon neutral”. The extent of the impact that biodiesel has on climate change has a high level of uncertainty attached to it as agricultural carbon cycles are difficult to model accurately. Soil carbon emissions are linked to prior land use(24) and in countries where crop rotations are common, they can’t be associated with a particular crop(6). Therefore the result presented here is only applicable to biodiesel produced from existing arable land and further work is required to investigate the uncertainty and impact of land use change. Table 2 Embodied energy and GWP100 data for diesel and biodiesel per km Category
Units
Diesel
Biodiesel
Cumulative Energy Demand
MJ equivalent
3.16
-1.24
Global Warming Potential
kg CO2 equivalent
0.0301
-0.0488
100
Figure 3 shows the impact of a person using a vehicle for a year, as a proportion of their total annual impact, for different biodiesel blends. As the amount of biodiesel in the blend increases, resource use falls, due to a reduction in fossil fuel use. By reducing the reliance of the transportation system on diesel, energy security in the UK has the potential to improve, as biodiesel feedstocks are a widely distributed commodity. However the use of biodiesel does not end reliance on fossil fuels, as natural gas is used as a feedstock for the methanol and ammonium nitrate fertilisers used in its production. Although resource use decreases, the impact on the eco system increases. The overall impact on human health does not significantly change, because whilst some categories increase, others decrease.
26
Impact as a proportion of the total annual impact of a European citizen %
Land use is the main driver behind the impact on the eco system and is likely to be the primary problem encountered with an increase in biodiesel adoption. Issues surrounding the use of land for biodiesel feedstock production are complex: displacement of food crops, land conversion & loss of biodiversity. Any increase in crops grown for biofuels will have to be carefully managed to prevent these issues becoming more problematic than the diesel they replace. For example, if legislation means that biofuels are required to come from a variety of feedstocks, the impact on biodiversity could be reduced. However if there is a large increase in the amount of farmed land due to government targets, then problems associated with farming, such as eutrophication (an excess of nutrients in waterways), would increase(1, 3, 17). Resource Use
30
Eco System
25
Human Health
20 15 10 5 0 0
10
20
30 40 50 60 Amount of Biodiesel %
70
80
90
100
Figure 3 The comparative impact of different fuel blends, when used to drive the annual average distance(25), as a proportion of an average European citizen’s total annual impact 5.2 Impact of biodiesel Figure 4 shows how the three stages of biodiesel production, rapeseed growth, oil extraction and transesterification (a process which alters the viscosity to more closely match that of diesel) and the stage of biodiesel use contribute to biodiesel’s impact. Growing rapeseed has a significant contribution across all categories whereas oil extraction has only a small contribution. Transesterification and biodiesel use have substantial contributions in a few categories such as fossil fuel use and respiratory inorganics. Production of ammonium nitrate fertiliser, soil emissions and land use were found to be mainly responsible for the impact attributed to rapeseed growth. Ammonium nitrate fertiliser is produced using natural gas as a feedstock and the process produces chemicals which are potentially harmful to the ecosystem. There is evidence that excess fertiliser is often applied to crops and that this causes significant damage due to nitrogen associated pollution(26).Leaving the straw in the field, can improve the soil quality, which may reduce the amount of fertiliser it is necessary to use(27). This indicates that straw should not be considered to be a waste product. Soil emissions can lead to an increase in the quantity of toxic substances in the ecosystem, climate change and eutrophication, and are linked to soil quality and fertiliser application(6). Transportation from the field to the biodiesel plant by road is the main cause for the impacts of oil extraction. The fuel used by lorries (and tractors) was modelled as diesel. As the proportion of biodiesel used increases, the impacts from transportation will change, although because it only accounts for a small portion of the overall impact of biodiesel the main findings of this LCA are unlikely to be significantly different. Methanol is used as a reactant in the transesterification process, which reduces the viscosity of the oil, to create rapeseed methyl ester (RME biodiesel). The most
27
Fossil fuels
Minerals
Land use
Ecotoxicity
Ozone layer
Radiation
Climate change
Resp. inorganics
Acidification Eutrophication
100%
Resp. organics
Carcinogens
common feedstock for methanol production is natural gas. This, along with the transport cost of distributing the biodiesel, accounts for most of the contribution this stage has on fossil fuel use.
80% 60% 40% 20% 0% -20% -40%
Rapeseed Growth
Oil Extraction
Transesterification
Vehicle Emissions
-60% -80%
Figure 4 Each processes contribution towards the total impact of biodiesel A number of studies have measured the emissions from an engine running on biodiesel and reported reductions in CO, hydrocarbons and particulate matter but an increase in NOx and fuel consumption(7). However in a study using a modern 2.0 litre common rail diesel engine examining emissions post catalyst, CO emissions were found to rise, due to a decrease in catalyst conversion efficiency(8) caused by a reduction in exhaust gas temperature when using biodiesel blends. This study was used because post catalyst emissions are most relevant to the environment. NOx emissions are known to cause photochemical smog and lead to the production of tropospheric ozone, both of which can have adverse effects when inhaled. Similarly, hydrocarbon and particulate emissions can lead to smog and contain carcinogenic compounds leading to respiratory diseases and cancer(28). Vehicle emissions of nitrogen oxides, hydrocarbons and particulates account for a large portion of the impact on respiratory effects. It should be noted that further aftertreatment systems, such as NOx traps and particulate filters, can be employed to significantly reduce NOx and particulate emissions respectively, thus mitigating the impact of transport on respiratory effects. Whilst switching to biodiesel appears to reduce the impact of transportation on global environmental issues, such as climate change and depleting resources, localised environmental issues, such as eutrophication of waterways and respiratory problems are likely to increase. However as a large proportion of the impact of biodiesel can be attributed to producing the crop, these localised environmental issues won’t necessarily be felt in the same area as the biodiesel is used. This is
28
due to the potential for direct and indirect land use change, for example, food crop displacement. Hence transportation will continue to impact at a global scale. 5.3 Reducing the impact of biodiesel Three aspects of biodiesel production and use: fertiliser, methanol and vehicle emissions were found to contribute significantly to the impact categories investigated. Therefore these aspects were investigated further to see if their impact could be reduced.
Improvement %
Figure 5shows how the changes made in each of the three scenarios affected the impact of biodiesel when compared to the standard scenario. A fourth scenario was set up to investigate the impact of biodiesel when all three of the alternative scenarios were combined. 220%
Fertiliser scenario
180%
Transesterification alcohol scenario
140%
Engine timing scenario
100%
Combined scenario
60% 20%
Fossil fuels
Minerals
Land use
Acidification Eutrophication
Ecotoxicity
Ozone layer
Radiation
Climate change
Resp. inorganics
Resp. organics
-60%
Carcinogens
-20%
Figure 5 Comparing the scenarios with respect to the standard case 5.3.1 Fertiliser scenario Due to the high impact ammonium nitrate fertiliser has on the environment, the use of an alternative fertiliser, meal cake, was investigated. Meal cake is a coproduct formed during the oil extraction process, which is commonly used as animal feed, but can also be used as a fertiliser. The amount of meal cake applied was calculated to maintain the level of macronutrients the crop received. Overall, the impact of rapeseed biodiesel was improved by using the alternative fertiliser, meal cake. Climate change impacts improve significantly. This is due to the larger amount of rapeseed crop modelled within the system, which is used to produce the meal cake. However land use increases with this change in fertiliser which has already been identified as a major issue with biodiesel use. 5.3.2 Transesterification alcohol scenario The use of methanol produced from natural gas in the transesterification process is a significant contribution towards fossil fuel use. Since a major driver for biofuels is for a reduction in fossil fuel use, the impact of substituting methanol produced from biomass was investigated.
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As expected, this change reduced fossil fuel use, impact on the ozone layer and contribution towards climate change. The effect on climate change is likely to be due to the increase in crops within the system rather than an actual reduction. However this change worsens the impact of biodiesel across all other categories. As access to agricultural land for biofuels is likely to become an increasing problem as production increases other ways of reducing the impact of methanol should be investigated in future studies. It has been shown that if ethanol is used, the resulting biodiesel produces lower exhaust emissions which could potentially lower the life cycle impacts of biodiesel(29). 5.3.3 Engine timing scenario Studies have shown that emissions can be reduced by optimising the engine operation, by adjusting fuel injection timing, to run on biodiesel(30). The scenario presented here is a theoretical scenario based on this study as exact reductions in tailpipe emissions were not available. It is included to investigate how changes would affect the LCA of biodiesel. Figure 6 shows measured tailpipe emissions comparing biodiesel and diesel, with the bar indicating the theoretical potential for reductions in biodiesel emissions with engine timing optimisation. In the theoretical engine timing optimisation, fuel consumption is reduced by 4g/km. This change leads to an improvement in all categories with the exception of climate change. These small changes can be attributed to the reduction in fuel consumption, and the corresponding reduction in rapeseed crop produced. Vehicle emissions of compounds which cause respiratory problems such as NOx and particulates were also reduced. This would be particularly beneficial for urban environments.
Emissions g/km
Optimising the engine to reduce fuel consumption is desirable to reduce the impact of biodiesel. However, during this transitional period it is unlikely that engines could be optimised correctly as the diesel standard, EN590, currently allows the addition of up to 7% biodiesel with no additional labelling(31). Furthermore part of the appeal of biodiesel is that it can be used in unmodified cars. 1 0.8
Biodiesel
0.6
Diesel
t
0.4 0.2 0 CO
NOx
Hydrocarbons
Particulates
Figure 6 Biodiesel emissions compared with diesel showing the potential reduction in emissions due to engine timing optimisation 5.3.4 Combined scenario A scenario which combined the fertiliser, transesterification alcohol and engine timing scenarios was used to investigate how multiple changes affected the life cycle. Although some of the individual changes worsened the impact of biodiesel in some categories, when combined together, all categories (with the exception of radiation and land use) show an improvement over the standard biodiesel scenario. Results from the current LCA indicate that there are ways in which the impact of biodiesel on the environment can be reduced. However these improvements are not always clear cut; they reduce impacts in one area, whilst increasing those in another. Therefore whether or not they are considered improvements will depend on the value judgment of the assessor.
30
6
CONCLUDING REMARKS
This LCA was carried out to assess the potential impact of transforming the transport system from diesel to biodiesel following biofuel targets implemented by the European parliament. Rapeseed can be used as a feedstock for first generation biodiesel, other feedstocks include soy and palm. Each feedstock will produce a different LCA, however many of the issues will be similar. Biodiesel can also be produced using second generation feedstocks, such as jatropha which may have different issues. Biodiesel reduces, although it doesn’t eliminate, the use of fossil fuels. Therefore energy security may improve due to the distributed nature of biodiesel feedstocks. Switching to biodiesel is likely to reduce the impact the UK has on climate change, assuming that, as a result of increased biodiesel use, there is no increase in farmed land. Land use has the potential to become a serious issue as the use of biodiesel increases. Either food crops will be displaced or there will be an increase in the amount of farmed land. The potential for rapeseed to displace food crops is limited as it should be grown as a break crop in a cereal crop rotation. However, rapeseed oil is currently used in commercial food production and this has the potential to be displaced by an increase in biodiesel use. If this transformation leads to an increase in farmed land it will result in loss of biodiversity and an increase in soil and water pollution due to the impact of growing the rapeseed. The use of ammonium nitrate fertiliser is a particular problem. The increased production of biodiesel will have to be carefully managed to prevent land use becoming a serious issue. Although the impact of biodiesel can be reduced by substituting substances used in the production process, such alterations do not normally lead to a reduction in impact for all categories considered. The value judgement of the assessor will therefore dictate whether they are considered improvements. A theoretical scenario was set up in which optimising the engine to run on biodiesel led to a reduction in tailpipe emissions and fuel consumption. This scenario was successful at reducing the impact of biodiesel in all categories, largely due to the reduced fuel consumption. However engine optimisation is unlikely to be feasible during any transitional period. When setting future biodiesel targets, the European Parliament will need to ensure that they do not pass legislation that is potentially more damaging to the environment than the current situation. Life cycle assessments are one of a number of tools which should be used to make such decisions. There are several areas which strongly contribute to uncertainty in the life cycle assessment. Further testing is required to reduce the uncertainty in these areas: measurement of tailpipe emissions with optimised engine timing and measurement of emissions associated with crop growth. The latter of which is a limitation common to all bioenergy. 7
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Lin, L., Z. Cunshan, S. Vittayapadung, S. Xiangqian, and D. Mingdong, Opportunities and challenges for biodiesel fuel. Applied Energy, 2011. 88(4): p. 1020-1031. Johnson, E., Goodbye to carbon neutral: Getting biomass footprints right. Environmental Impact Assessment Review, 2009. 29(3): p. 165-168.
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Luque, R., J.C. Lovett, B. Datta, J. Clancy, J.M. Campelo, and A.A. Romero, Biodiesel as feasible petrol fuel replacement: a multidisciplinary overview. Energy & Environmental Science, 2010. 3(11): p. 1706. Gallagher, E., The Gallagher Review of the indirect effects of biofuels production. 2008, Renewable Fuels Agency,. European Parliament and Council, Directive 2009/28/EC of the European Parliament and of the Council of 23 April 2009 on the promotion of the use of energy from renewable sources, in Directive 2009/28/EC. 2009: Official Journal of the European Union. Reijnders, L. and M.A.J. Huijbregts, Biogenic greenhouse gas emissions linked to the life cycles of biodiesel derived from European rapeseed and Brazilian soybeans. Journal of Cleaner Production, 2008. 16(18): p. 1943-1948. Lapuerta, M., O. Armas, and J. Rodriguezfernandez, Effect of biodiesel fuels on diesel engine emissions. Progress in Energy and Combustion Science, 2008. 34(2): p. 198-223. Bannister, C.D., J.G. Hawley, H.M. Ali, C.J. Chuck, P. Price, S.S. Chrysafi, A. Brown, and W. Pickford, The impact of biodiesel blend ratio on vehicle performance and emissions. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 2010. 224(3): p. 405421. British Standards Institution, BS ISO 14040: 2006 Environmental management — Life cycle assessment — Principles and framework. 2006. British Standards Institution, BS ISO 14044: 2006 Environmental management — Life cycle assessment—Requirements and guidelines. 2006. PRé Consultants. About SimaPro. 2011 Accessed on:[20.11.10]; Available from: http://www.pre.nl/ McManus, M.C., G.P. Hammond, and C.R. Burrows, Life-cycle assessment of mineral and rapeseed oil in mobile hydraulic systems. Journal of Industrial Ecology, 2003. 7(3-4): p. 163-177. Mortimer, N.D., P. Cormack, M.A. Elsayed, and R.E. Horne, Evaluation of the comparative energy, global warming and soci-economic costs and benefits of biodiesel, Department for Environment Food and Rural Affairs, Editor. 2003. Williams, A.G., E. Audsley, and D.L. Sandars, Determining the environmental burdens and resource use in the production of agricultural and horticultural commodities. 2006, Cranfield University and Defra: Bedford. Pagliaro, M., R. Ciriminna, H. Kimura, M. Rossi, and C. Della Pina, Recent advances in the conversion of bioglycerol into value-added products. European Journal of Lipid Science and Technology, 2009. 111(8): p. 788-799. Weidema, B., Market information in life cycle assessment. 2003, Danish Environmental Protection Agency: Denmark. Brandão, M., L. Milà i Canals, and R. Clift, Soil organic carbon changes in the cultivation of energy crops: Implications for GHG balances and soil quality for use in LCA. Biomass and Bioenergy, 2011. 35(6): p. 2323-2336. Peterson, C.L. and T. Hustrulid, Carbon cycle for rapeseed oil biodiesel fuels. Biomass and Bioenergy, 1998. 14(2): p. 91-101. Meher, L., D. Vidyasagar, and S. Naik, Technical aspects of biodiesel production by transesterification—a review. Renewable and Sustainable Energy Reviews, 2006. 10(3): p. 248-268. Worldwatch Institute, Biofuels for transport: global potential and implications for energy and agriculture. 1:3 ed. 2007, London: Earthscan. Thomas, M., The British survey of fertiliser practice - use on farm crops for crop year 2009, Department for Environment Food and Rural Affairs, Editor. 2010: London. Proforest, Agricultural production models and methods for UK biofuels, in Renewable fuels agency research programme. 2010. Evans, G., Oilseed rape workbook. 2010, NNFCC.
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Murty, D., M.U.F. Kirschbaum, R.E. McMurtrie, and M. H., Does conversion of forest to agricultural land change soil carbon and nitrogen? A review of the literature. Global Change Biology, 2002 8: p. 105-123. National Statistics, Transport trends: 2009 edition, Department for transport, Editor. 2010. Sutton, M.A., C.M. Howard, J.W. Erisman, G. Billen, A. Bleeker, P. Grennflet, H. Van Grinsven, and B. Grizzetti, eds. The European Nitrogen Assessment: Sources, effects and policy perspectives. 2011, Cambridge University Press: Cambridge. Wilhelm, W.W., J.M.F. Johnson, J.L. Hatfield, W.B. Voorhees, and D.R. Linden, Crop and soil productivity response to corn residue removal: A literature review. Agronomy Journal, 2004. 96(1): p. 1-17. Colvile, R.N., E.J. Hutchinson, J.S. Mindell, and R.F. Warren, The transport sector as a source of air pollution. Atmospheric Environment, 2001. 35: p. 1537-1565. Makareviciene, V. and P. Janulis, Environmental effect of rapeseed oil ethyl ester. Renewable Energy, 2003. 28(15): p. 2395-2403. Kegl, B., Effects of biodiesel on emissions of a bus diesel engine. Bioresource Technology, 2008. 99(4): p. 863-873. British Standards Institution, BS EN590: 2009 Automotive fuels - Diesel Requirements and test methods. 2009.
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1
ABSTRACT
Biodiesel is often considered to improve energy security and reduce the impact of fuel on climate change. However there are concerns about the impact of biodiesel when its life cycle is considered. The potential impact of using biodiesel rather than conventional diesel was investigated using a life cycle assessment (LCA) of rapeseed biodiesel. Biodiesel leads to reduced fossil fuel use and is likely to reduce the impact of transport on climate change. However it was found that the impact of biodiesel towards other categories, i.e. land use and respiratory inorganics, was greater than petroleum diesel. Therefore biodiesel production should be carefully managed to mitigate its impact on the environment. Keywords: Life Cycle Assessment; Biodiesel; Rapeseed 2
BACKGROUND
Biodiesel is considered to have a number of advantages over diesel. Biodiesel can be produced from an array of feedstocks and, unlike diesel, feedstock sources are highly distributed around the world. This means that an increase in biodiesel use should lead to an increase in energy security(1). Biodiesel is often thought, incorrectly, to be carbon neutral on the basis that any carbon released during combustion had previously been absorbed from the atmosphere during crop growth(2). Biodiesel is compatible with diesel. Blends of biodiesel and diesel are labelled Bη, where η is the proportion of biodiesel as a percentage. They can be blended together in any proportion, the same distribution infrastructure can be used and at low blend levels, used in diesel engines with no modification(3). These apparent advantages have led to legislation being introduced to increase the use of biofuels(4). The European parliament has set a target that 10% of fossil fuel consumption for transportation must be replaced by biofuels by 2020 in all Member States(5). However, there are concerns about biodiesel’s sustainability(4). The impact biodiesel has on land use change, both direct and indirect, is of major interest(1). Previous biodiesel LCAs have highlighted the agricultural stage to have a large effect on the impact of biodiesel, due mainly to the nitrogen fertiliser used(6). There is consensus that tailpipe emissions of NOx increase with biodiesel(7) and evidence that CO also increases(8). Due to these concerns, a LCA was carried out to investigate the environmental impact of the production and use of biodiesel in the UK using rapeseed as a feedstock.
_______________________________________ © The author(s) and/or their employer(s), 2012
23
3
INTRODUCTION TO LCA
The impact of a product on the environment can be investigated using LCA, as shown in Figure 1(9). LCA examines the environmental impact of a product or system over a range of environmental issues, such as greenhouse gases, fossil fuel use, and ozone depleting substances etc. LCA is analysed in terms of a functional unit, chosen because it reflects the function of the product. This allows comparison between products fulfilling the same function. When comparing multiple products, it is unlikely that one will perform better than another in all impact categories investigated. Thus, which product is considered to have the smallest overall impact on the environment will be decided based on the value the assessor places on each individual category.
Figure 1 LCA methodology: adapted from ISO 14040: 2006(9) In processes where more than one product is produced, the inventory should be apportioned between the co-products. According to a hierarchy set out in ISO 14044:2006 the first option is system expansion to include processes relating to the co-product. If this is not suitable, then the inputs and outputs should be partitioned between the co-products in a ratio which reflects some physical property of the products, this is the burden of that product(10). 4
LCA OF BIODIESEL
Figure 2 Rapeseed biodiesel production and use
24
Data to model the lifecycle of rapeseed biodiesel was obtained from published literature, which matched the data quality requirements, and used to build an inventory. This inventory was modelled in SimaPro 7(11), primarily using the Ecoinvent database, which contains inventory data for many materials and processes. The inventory was analysed using the Eco-indicator 99H impact assessment method. The diesel LCA was modelled using the same method. Table 1 Inventory of B100 for 1km with a fuel consumption of 68g Inputs
Amount
Nature Carbon Dioxide Arable land
Output
Amount
Emissions to air 353.7 g
Hexane
0.25 m2
Technosphere
0.0065 g
Carbon Monoxide
0.91 g
Carbon Dioxide
43.2 g
Methanol
10.2 g
Nitrogen Oxide (NOx)
0.65 g
Pesticide
0.23 g
Nitrous Oxide
0.45 g
Potassium Hydroxide
0.60 g
Methane
0.11 g
Ploughing
0.49 m2
Ammonia
0.50 g
Transport
0.068 tkm
Sowing Nitrogen fertiliser
0.49 m2 9.1 g
Hydrocarbons
0.055 g
Particulates
0.037 g
Emissions to water
Fertilising
0.49 m2
Nitrate
Phosphorous fertiliser
0.98 g
Phosphorus
Application of pesticides
3.4 m
Potash
1.2 g
Combine harvesting Sulphur Lime Electricity Heat
2
0.49 m2
Potassium
2.50 g 0.017 g 0.98 g
Emissions to soil Methane
0.032 g
3.9 g 0.16 kg 0.023 kWh 0.20 MJ
4.1 Goal and scope The purpose of this LCA was to investigate how a transition towards biodiesel might affect the impact of transportation by comparing it to a diesel LCA. It was also used to investigate the potential for reducing this impact, by identifying which processes contributed significantly and then to model alternative methods of production. Rapeseed biodiesel was considered to be a transportation fuel in this LCA. To allow a comparison between the biodiesel LCA and the diesel LCA, the functional unit was 1km of distance travelled. During biodiesel production (See Figure 2 for details of production method, inputs and outputs), three co-products are formed: straw, meal cake (what remains of the seed after oil extraction) and glycerol(12). Although system expansion is the preferred method of burden allocation(10), this would increase the uncertainty in the result as it is not clear what product the co-products would offset(13). Therefore, burdens were partitioned proportionally, according to the desired characteristic of the co-product, as outlined below.
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Rapeseed straw is generally not harvested, consequently no burdens were allocated to straw(14). However, studies have shown that leaving straw in situ reduces soil carbon emissions: this was reflected in the inventory(6). Meal cake is commonly used as a feed for livestock and so burdens were allocated according to energy content(13). Burdens were allocated by mass for glycerol, as it can be used as a feedstock for a wide range of products(15). Due to the controversy surrounding allocation methods(16), a sensitivity analysis was undertaken. The outcome of the LCA is dependent on the allocation method: using mass allocation, biodiesel has a lower impact than diesel, whereas using the allocation method in this LCA, the impact of biodiesel is higher. 4.2 Inventory The inventory of inputs and outputs to travel 1km using B100 is presented in Table 1. The inventory is not exhaustive, however, the principal substances and processes contributing to the impact are included. The biodiesel production inventory was built up using data from multiple sources(6, 12, 14, 17-23). This was then combined with data on biodiesel use to create inventories for biodiesel blends up to B50(8). It was assumed that the linear relationships reported on fuels up to B50 would remain true for B100. 5
RESULTS AND DISCUSSION
5.1 Comparison of biodiesel and diesel Table 2 shows the embodied energy and global warming potential of diesel and biodiesel. The impact transportation has on climate change is likely to be reduced by switching to biodiesel. Although the process of producing and using biodiesel emits greenhouse gases, much of the carbon dioxide from the fuel has been absorbed during growth. However, fossil fuels are also used during processing and transport and so the fuel is not “carbon neutral”. The extent of the impact that biodiesel has on climate change has a high level of uncertainty attached to it as agricultural carbon cycles are difficult to model accurately. Soil carbon emissions are linked to prior land use(24) and in countries where crop rotations are common, they can’t be associated with a particular crop(6). Therefore the result presented here is only applicable to biodiesel produced from existing arable land and further work is required to investigate the uncertainty and impact of land use change. Table 2 Embodied energy and GWP100 data for diesel and biodiesel per km Category
Units
Diesel
Biodiesel
Cumulative Energy Demand
MJ equivalent
3.16
-1.24
Global Warming Potential
kg CO2 equivalent
0.0301
-0.0488
100
Figure 3 shows the impact of a person using a vehicle for a year, as a proportion of their total annual impact, for different biodiesel blends. As the amount of biodiesel in the blend increases, resource use falls, due to a reduction in fossil fuel use. By reducing the reliance of the transportation system on diesel, energy security in the UK has the potential to improve, as biodiesel feedstocks are a widely distributed commodity. However the use of biodiesel does not end reliance on fossil fuels, as natural gas is used as a feedstock for the methanol and ammonium nitrate fertilisers used in its production. Although resource use decreases, the impact on the eco system increases. The overall impact on human health does not significantly change, because whilst some categories increase, others decrease.
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Impact as a proportion of the total annual impact of a European citizen %
Land use is the main driver behind the impact on the eco system and is likely to be the primary problem encountered with an increase in biodiesel adoption. Issues surrounding the use of land for biodiesel feedstock production are complex: displacement of food crops, land conversion & loss of biodiversity. Any increase in crops grown for biofuels will have to be carefully managed to prevent these issues becoming more problematic than the diesel they replace. For example, if legislation means that biofuels are required to come from a variety of feedstocks, the impact on biodiversity could be reduced. However if there is a large increase in the amount of farmed land due to government targets, then problems associated with farming, such as eutrophication (an excess of nutrients in waterways), would increase(1, 3, 17). Resource Use
30
Eco System
25
Human Health
20 15 10 5 0 0
10
20
30 40 50 60 Amount of Biodiesel %
70
80
90
100
Figure 3 The comparative impact of different fuel blends, when used to drive the annual average distance(25), as a proportion of an average European citizen’s total annual impact 5.2 Impact of biodiesel Figure 4 shows how the three stages of biodiesel production, rapeseed growth, oil extraction and transesterification (a process which alters the viscosity to more closely match that of diesel) and the stage of biodiesel use contribute to biodiesel’s impact. Growing rapeseed has a significant contribution across all categories whereas oil extraction has only a small contribution. Transesterification and biodiesel use have substantial contributions in a few categories such as fossil fuel use and respiratory inorganics. Production of ammonium nitrate fertiliser, soil emissions and land use were found to be mainly responsible for the impact attributed to rapeseed growth. Ammonium nitrate fertiliser is produced using natural gas as a feedstock and the process produces chemicals which are potentially harmful to the ecosystem. There is evidence that excess fertiliser is often applied to crops and that this causes significant damage due to nitrogen associated pollution(26).Leaving the straw in the field, can improve the soil quality, which may reduce the amount of fertiliser it is necessary to use(27). This indicates that straw should not be considered to be a waste product. Soil emissions can lead to an increase in the quantity of toxic substances in the ecosystem, climate change and eutrophication, and are linked to soil quality and fertiliser application(6). Transportation from the field to the biodiesel plant by road is the main cause for the impacts of oil extraction. The fuel used by lorries (and tractors) was modelled as diesel. As the proportion of biodiesel used increases, the impacts from transportation will change, although because it only accounts for a small portion of the overall impact of biodiesel the main findings of this LCA are unlikely to be significantly different. Methanol is used as a reactant in the transesterification process, which reduces the viscosity of the oil, to create rapeseed methyl ester (RME biodiesel). The most
27
Fossil fuels
Minerals
Land use
Ecotoxicity
Ozone layer
Radiation
Climate change
Resp. inorganics
Acidification Eutrophication
100%
Resp. organics
Carcinogens
common feedstock for methanol production is natural gas. This, along with the transport cost of distributing the biodiesel, accounts for most of the contribution this stage has on fossil fuel use.
80% 60% 40% 20% 0% -20% -40%
Rapeseed Growth
Oil Extraction
Transesterification
Vehicle Emissions
-60% -80%
Figure 4 Each processes contribution towards the total impact of biodiesel A number of studies have measured the emissions from an engine running on biodiesel and reported reductions in CO, hydrocarbons and particulate matter but an increase in NOx and fuel consumption(7). However in a study using a modern 2.0 litre common rail diesel engine examining emissions post catalyst, CO emissions were found to rise, due to a decrease in catalyst conversion efficiency(8) caused by a reduction in exhaust gas temperature when using biodiesel blends. This study was used because post catalyst emissions are most relevant to the environment. NOx emissions are known to cause photochemical smog and lead to the production of tropospheric ozone, both of which can have adverse effects when inhaled. Similarly, hydrocarbon and particulate emissions can lead to smog and contain carcinogenic compounds leading to respiratory diseases and cancer(28). Vehicle emissions of nitrogen oxides, hydrocarbons and particulates account for a large portion of the impact on respiratory effects. It should be noted that further aftertreatment systems, such as NOx traps and particulate filters, can be employed to significantly reduce NOx and particulate emissions respectively, thus mitigating the impact of transport on respiratory effects. Whilst switching to biodiesel appears to reduce the impact of transportation on global environmental issues, such as climate change and depleting resources, localised environmental issues, such as eutrophication of waterways and respiratory problems are likely to increase. However as a large proportion of the impact of biodiesel can be attributed to producing the crop, these localised environmental issues won’t necessarily be felt in the same area as the biodiesel is used. This is
28
due to the potential for direct and indirect land use change, for example, food crop displacement. Hence transportation will continue to impact at a global scale. 5.3 Reducing the impact of biodiesel Three aspects of biodiesel production and use: fertiliser, methanol and vehicle emissions were found to contribute significantly to the impact categories investigated. Therefore these aspects were investigated further to see if their impact could be reduced.
Improvement %
Figure 5shows how the changes made in each of the three scenarios affected the impact of biodiesel when compared to the standard scenario. A fourth scenario was set up to investigate the impact of biodiesel when all three of the alternative scenarios were combined. 220%
Fertiliser scenario
180%
Transesterification alcohol scenario
140%
Engine timing scenario
100%
Combined scenario
60% 20%
Fossil fuels
Minerals
Land use
Acidification Eutrophication
Ecotoxicity
Ozone layer
Radiation
Climate change
Resp. inorganics
Resp. organics
-60%
Carcinogens
-20%
Figure 5 Comparing the scenarios with respect to the standard case 5.3.1 Fertiliser scenario Due to the high impact ammonium nitrate fertiliser has on the environment, the use of an alternative fertiliser, meal cake, was investigated. Meal cake is a coproduct formed during the oil extraction process, which is commonly used as animal feed, but can also be used as a fertiliser. The amount of meal cake applied was calculated to maintain the level of macronutrients the crop received. Overall, the impact of rapeseed biodiesel was improved by using the alternative fertiliser, meal cake. Climate change impacts improve significantly. This is due to the larger amount of rapeseed crop modelled within the system, which is used to produce the meal cake. However land use increases with this change in fertiliser which has already been identified as a major issue with biodiesel use. 5.3.2 Transesterification alcohol scenario The use of methanol produced from natural gas in the transesterification process is a significant contribution towards fossil fuel use. Since a major driver for biofuels is for a reduction in fossil fuel use, the impact of substituting methanol produced from biomass was investigated.
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As expected, this change reduced fossil fuel use, impact on the ozone layer and contribution towards climate change. The effect on climate change is likely to be due to the increase in crops within the system rather than an actual reduction. However this change worsens the impact of biodiesel across all other categories. As access to agricultural land for biofuels is likely to become an increasing problem as production increases other ways of reducing the impact of methanol should be investigated in future studies. It has been shown that if ethanol is used, the resulting biodiesel produces lower exhaust emissions which could potentially lower the life cycle impacts of biodiesel(29). 5.3.3 Engine timing scenario Studies have shown that emissions can be reduced by optimising the engine operation, by adjusting fuel injection timing, to run on biodiesel(30). The scenario presented here is a theoretical scenario based on this study as exact reductions in tailpipe emissions were not available. It is included to investigate how changes would affect the LCA of biodiesel. Figure 6 shows measured tailpipe emissions comparing biodiesel and diesel, with the bar indicating the theoretical potential for reductions in biodiesel emissions with engine timing optimisation. In the theoretical engine timing optimisation, fuel consumption is reduced by 4g/km. This change leads to an improvement in all categories with the exception of climate change. These small changes can be attributed to the reduction in fuel consumption, and the corresponding reduction in rapeseed crop produced. Vehicle emissions of compounds which cause respiratory problems such as NOx and particulates were also reduced. This would be particularly beneficial for urban environments.
Emissions g/km
Optimising the engine to reduce fuel consumption is desirable to reduce the impact of biodiesel. However, during this transitional period it is unlikely that engines could be optimised correctly as the diesel standard, EN590, currently allows the addition of up to 7% biodiesel with no additional labelling(31). Furthermore part of the appeal of biodiesel is that it can be used in unmodified cars. 1 0.8
Biodiesel
0.6
Diesel
t
0.4 0.2 0 CO
NOx
Hydrocarbons
Particulates
Figure 6 Biodiesel emissions compared with diesel showing the potential reduction in emissions due to engine timing optimisation 5.3.4 Combined scenario A scenario which combined the fertiliser, transesterification alcohol and engine timing scenarios was used to investigate how multiple changes affected the life cycle. Although some of the individual changes worsened the impact of biodiesel in some categories, when combined together, all categories (with the exception of radiation and land use) show an improvement over the standard biodiesel scenario. Results from the current LCA indicate that there are ways in which the impact of biodiesel on the environment can be reduced. However these improvements are not always clear cut; they reduce impacts in one area, whilst increasing those in another. Therefore whether or not they are considered improvements will depend on the value judgment of the assessor.
30
6
CONCLUDING REMARKS
This LCA was carried out to assess the potential impact of transforming the transport system from diesel to biodiesel following biofuel targets implemented by the European parliament. Rapeseed can be used as a feedstock for first generation biodiesel, other feedstocks include soy and palm. Each feedstock will produce a different LCA, however many of the issues will be similar. Biodiesel can also be produced using second generation feedstocks, such as jatropha which may have different issues. Biodiesel reduces, although it doesn’t eliminate, the use of fossil fuels. Therefore energy security may improve due to the distributed nature of biodiesel feedstocks. Switching to biodiesel is likely to reduce the impact the UK has on climate change, assuming that, as a result of increased biodiesel use, there is no increase in farmed land. Land use has the potential to become a serious issue as the use of biodiesel increases. Either food crops will be displaced or there will be an increase in the amount of farmed land. The potential for rapeseed to displace food crops is limited as it should be grown as a break crop in a cereal crop rotation. However, rapeseed oil is currently used in commercial food production and this has the potential to be displaced by an increase in biodiesel use. If this transformation leads to an increase in farmed land it will result in loss of biodiversity and an increase in soil and water pollution due to the impact of growing the rapeseed. The use of ammonium nitrate fertiliser is a particular problem. The increased production of biodiesel will have to be carefully managed to prevent land use becoming a serious issue. Although the impact of biodiesel can be reduced by substituting substances used in the production process, such alterations do not normally lead to a reduction in impact for all categories considered. The value judgement of the assessor will therefore dictate whether they are considered improvements. A theoretical scenario was set up in which optimising the engine to run on biodiesel led to a reduction in tailpipe emissions and fuel consumption. This scenario was successful at reducing the impact of biodiesel in all categories, largely due to the reduced fuel consumption. However engine optimisation is unlikely to be feasible during any transitional period. When setting future biodiesel targets, the European Parliament will need to ensure that they do not pass legislation that is potentially more damaging to the environment than the current situation. Life cycle assessments are one of a number of tools which should be used to make such decisions. There are several areas which strongly contribute to uncertainty in the life cycle assessment. Further testing is required to reduce the uncertainty in these areas: measurement of tailpipe emissions with optimised engine timing and measurement of emissions associated with crop growth. The latter of which is a limitation common to all bioenergy. 7
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