Energy Fuels 2010, 24, 2489–2499 Published on Web 03/21/2010
: DOI:10.1021/ef100051g
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Global Warming Potential and Fossil-Energy Requirements of Biodiesel Production Scenarios in South Africa A. L. Stephenson,*,† H. von Blottnitz,‡ A. C. Brent,§ J. S. Dennis,† and S. A. Scott †
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Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom, ‡Department of Chemical Engineering, University of Cape Town, Rondebosch 7701, South Africa, § Centre for Renewable and Sustainable Energy Studies, School of Public Management and Planning, Stellenbosch University, Lynedoch, Stellenbosch 7603, South Africa, and Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, United Kingdom Received January 16, 2010. Revised Manuscript Received March 5, 2010
Life cycle assessment has been used to investigate the global warming potential (GWP) and fossil-energy requirements of the production of biodiesel from canola (oilseed rape), soybean, and sunflower oils in South Africa. The effect of scale and transportation of raw materials and products was investigated, as well as the effect of ploughing grassland and using irrigation to grow oil crops. This research shows that the GWP and fossil-energy requirements of biodiesel produced in South Africa vary widely, depending upon predominantly the crop yield, the requirement for irrigation, and the ploughing of uncultivated land. For the best case scenario, where no uncultivated land is newly ploughed and irrigation is not required, biodiesel has a GWP 20-36% lower than that of the fossil diesel mix currently used in South Africa and a fossil-energy requirement 50-62% lower. However, in the worst case scenario, where oil-seed crops are grown on newly cultivated land requiring substantial irrigation, this paper concludes that biodiesel can have a GWP significantly higher than South African fossil diesel. The scale of operation and transport distances involved are shown to have little influence on the GWP and fossil-energy requirement of biodiesel produced in South Africa.
country, some companies in South Africa are planning to export their fuels to Europe.5 Accordingly, many new biofuel production facilities are being planned in South Africa, at different scales and using a variety of feedstocks. It is therefore timely to study the sustainability of the different supply chains that could be used to produce biofuels in South Africa. Biodiesel is generally produced by the transesterification of a triglyceride (vegetable oil) with an alcohol (methanol or ethanol) in the presence of a base catalyst (usually sodium hydroxide or potassium hydroxide) to produce the respective fatty acid alkyl ester (biodiesel) and glycerol.6 Transesterification involves three reversible reactions, whereby the triglyceride is converted successively to diglyceride, monoglyceride, and glycerol, consuming 1 mol of alcohol in each step and liberating 1 mol of ester. The glycerol co-product can be refined and sold to the pharmaceutical industry; however, this market is currently saturated, and the refining process is complicated and energy-intensive. In South Africa, there is significant research into alternative uses for glycerol, e.g., to produce biogas by anaerobic digestion;7 however, at present, it is most commonly sold for use as a fuel in industrial furnaces.8 The four principal oils used by the biodiesel industry are canola (rapeseed), sunflower, soybean, and palm oils,6 while attention is also currently turning to the use of the nonfood feedstock, Jatropha curcas.9 The warm, temperate climate in
1. Introduction The global use of biofuels as an alternative to fossil-derived transport fuels is increasing. In 2003, the European Union (EU) released the EU Biofuels Directive, which set a target for member states to achieve a 5.75% market share of biofuels by 2010, calculated on the basis of the energy content of all petrol and diesel used for transport.1 In 2009, this target was revised in the EU Renewables Directive, which calls upon each member state to ensure 10% of the energy used by its transport industry is produced in a renewable manner by 2020.2 The federal government of the United States has recently set a commitment to increase the use of bioenergy 3-fold in the next 10 years.3 The biofuels industry is well-developed in some countries, such as Germany and Brazil; however, other countries are just embarking on new biofuel strategies. For example, the Biofuel Industrial Strategy of the Republic of South Africa,4 published in December 2007, aims at achieving a 2% penetration of biofuels in the national liquid fuel supply, calculated on the basis of the total volume of all road transport fuels used per year, by 2012. In fact, as well as supplying fuel for use within the *To whom correspondence should be addressed: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Telephone: þ44-(0)1223-748199. Fax: þ44-(0)1223-336362. E-mail:
[email protected]. (1) Directive 2003/30/EC of the European Parliament and of the Council. Off. J. Eur. Union 2003, 123, 42-46. (2) Directive 2009/28/EC of the European Parliament and of the Council. Off. J. Eur. Union 2009, 140, 16-62. (3) Demirbas, A. Energy Convers. Manage. 2008, 49 (8), 2106–2116. (4) Department of Minerals and Energy. Biofuels Industrial Strategy of the Republic of South Africa. 2007. (5) Phytoenergy. Environmental Impact Assessment and Environmental Management Plan for the Proposed Biodiesel Plant in East London and Associated Activities. 2008. r 2010 American Chemical Society
(6) Mittelbach, M.; Remschmidt, C. Biodiesel;The Comprehensive Handbook, 1st ed.; Graz University: Graz, Austria, 2004; pp 6-9. (7) Verster, B. Private communication. University of Cape Town, South Africa, 2008. (8) Murray, N. Private communication. Biodiesel Centre, Bellville, South Africa, 2008. (9) Achten, W. M. J.; Verchot, L.; Franken, Y. J.; Mathijs, E.; Singh, V. P.; Aerts, R.; Muys, B. Biomass Bioenergy 2008, 32, 1063–1084.
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South Africa is suitable for growing canola, sunflower, and soybean crops. Therefore, these crops are considered to be most appropriate feedstocks for biodiesel production in South Africa10 and are investigated in this study. Canola and sunflower crops have the potential to produce more biodiesel per hectare than soybeans, owing to their higher content of oil; however, factors specific to the land and climate (e.g., availability of water) determine the most appropriate feedstock to be grown in a particular region. The Biofuel Industrial Strategy aims at growing these crops on land classified as under-used but with high potential; significant areas of the former homeland regions of South Africa fall in this category.4 (Any of the 10 regions designated by South Africa in the 1970s as semi-autonomous territorial states for the black population. The black homelands were dissolved and reincorporated into South Africa as part of the 1994 transition to democracy.) This area is predominantly grassland and woody savannah, and it currently has three main uses: communal arable land, agricultural land, and arable state land.11,12 Communal arable land is mostly used for subsistence farming, animal grazing, and grassland. The agricultural land has recently been acquired by new farmers under the Land Reform program;12 most of this land is left uncultivated and used for grazing. The arable state land has not yet been cultivated for agricultural use and is currently either used for grazing or left for grass.12 The Biofuel Industrial Strategy is unclear about what proportion of the land is already used as farmland; however, it is evident that significant proportions of grasslands and grazing lands will require cultivation to provide the land required to grow the oilseed crops. For example, in the Eastern Cape province, there are plans to use 150 000 ha of underdeveloped land to grow canola crops and 100 000 ha to grow soybean crops specifically for the production of biodiesel, as part of an integrated rural development program.13 Over 95% of this land is currently uncultivated;13 therefore, it is important to quantify the effect on the global warming potential (GWP) of the resulting biodiesel of converting this land to arable land for the cultivation of oil-seed crops. This paper uses life cycle assessment (LCA) to investigate the GWP and fossil-energy requirements of biodiesel production in South Africa, at each stage, from the production and supply of the raw materials to the point of supply of the fuel, for the three oilseed crops. The effect of the scale of production on GWP and fossil-energy requirements has been studied, as well as the effect of ploughing grassland and using irrigation to grow oil crops. The use of the biodiesel in both South Africa and the U.K. is considered.
producing more than 100 000 tons/year of biodiesel; albeit, no such plant currently exists in South Africa. “Medium scale” was defined as a plant producing between 10 000 and 100 000 tons/year of biodiesel, with “small scale” production being between 1000 and 10 000 tons/year of biodiesel. Finally, “microscale” production was deemed to be a plant producing less than 1000 tons/year of biodiesel. Operating conditions for medium-, small-, and microscale plants in South Africa were obtained during site visits. 2.2. LCA. A LCA was undertaken according to the International Organization for Standardization (ISO) standards ISO 14040:200615 and ISO 14044:200616 via the sequential stages of (i) goal and scope definition, (ii) inventory analysis, (iii) impact assessment, and (iv) interpretation and reporting, as described below. The actual analysis was undertaken using the GaBi 4.3 LCA software package.17 2.2.1. Goal and Scope Definition. In this paper, two functional units (the basis for comparison) are defined: (i) 1 ton of biodiesel that has been delivered to a South African customer by road, blended with fossil-derived diesel to the desired fractional volume, and combusted in a typical, compact-sized car engine and (ii) 1 ton of biodiesel that has been delivered to the U.K. by sea, blended to the desired fractional volume with conventional, fossil-derived diesel, delivered to a filling station, and combusted in a typical, compact-sized car engine. The results are based on information gathered for the time period of 2006-2009. Process chains have then been used to summarize the consequent main activities in the production of this functional unit. These are shown in panels a and b of Figure 1 for the respective sizes of plants and are discussed below. The “control volume” in this study encompasses all of the stages directly used to produce biodiesel (i.e., the foreground system, including crop production, oil extraction, and esterification) and also the background system, which provide the materials and energy used by the foreground system. 2.2.2. Inventory Analysis. Quantitative mass and energy balances were performed over each control volume. Information regarding the agriculture of canola crops was gathered from a farm in the Western Cape province of South Africa. Site visits to a South African fertilizer manufacturer and the Elsenburg Agricultural College, Stellenbosch (Western Cape province), were also used to obtain necessary information on the agronomy of canola, soybean, and sunflower in South Africa. Quantitative information on both large- and small-scale oil extraction processes was required for the study. At the time of the data collection, most of the biodiesel produced in South Africa used waste cooking oil as the feedstock. From discussions with biodiesel producers, it was decided that solvent extraction methods would most likely be used to provide the oil feedstock for the medium-scale plants, while cold-pressing techniques would be used for small- and microscale generation. Operating information from oil extraction facilities situated in South Africa was not available for this study. However, information from standard large-scale, solvent extraction and small-scale, cold-pressing facilities in the U.K. was available.14 After the available extraction facilities in South Africa were investigated, it was decided that the processes would be similar to those employed in the U.K.; therefore, this process information was incorporated in the study. Process information from three, anonymous, biodiesel production plants operating at different scales was considered. These were (i) plant A, which has the capacity to produce ∼60 000 tons of biodiesel per year, (ii) plant B, with a capacity of ∼8000 tons of biodiesel per year, and (iii)
2. Materials and Methods 2.1. Definition of Scale. The scale of production was defined in terms of the production capacities of typical process plants across the world.14 “Large scale” was defined as a plant (10) Nolte, M. Commercial biodiesel production in South Africa: A preliminary economic feasibility study. Master’s Thesis, University of Stellenbosch, Stellenbosch, South Africa, 2007. (11) Kingwill, R.; Sapsford, P.; Barnard, J.; Cartwright, A. Land Issues Scoping Study: Communal Land Tenure Areas: Key Issues. Department for International Development Southern Africa (DFIDSA). 2003. (12) Letete, T. Private communication. University of Cape Town, South Africa, 2008. (13) Council for Scientific and Industrial Research (CSIR). Internal Discussions and Documentation, Pretoria, South Africa, 2008. (14) Stephenson, A. L.; Dennis, J. S.; Scott, S. A. Process Saf. Environ. Prot. 2008, 86, 427–440.
(15) International Organization for Standardization (ISO). ISO 14040:2006. Environmental Management. Life Cycle Assessment. Principals and Framework. 2006. (16) International Organization for Standardization (ISO). ISO 14044:2006. Environmental Management. Life Cycle Assessment. Requirements and Guidelines. 2006. (17) PE International. GaBi 4. Product Sustainability. LeinfeldenEchterdingen, Germany, 2008.
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Figure 1. Process chains for the production of biodiesel from the oil seeds, canola, soybean, and sunflower, in South Africa: (a) medium scale and (b) small- and microscale. Sun = sunflower (a and b denote current and recommended agricultural practices, respectively).
plant C, with a capacity of ∼300 tons of biodiesel per year. An inventory table was generated for each plant using the collated information, showing the resource usage and all of the emissions associated with the production of 1 ton of biodiesel. 2.2.3. Impact Assessment and Interpretation. Using the LCA software, it was possible to formulate the inventory table into a set of environmental themes based on the EDIP 2003 methodology18 using estimates of how much each input and emission contributes to certain environmental impacts. The EDIP 2003 methodology was chosen because it was the most up-to-date methodology available for the study. Moreover, it was developed in concert with the ISO standards ISO 14040:200615 and ISO 14044:200616 and is considered to be one of the most complete and consistent methodologies available.19 This paper reports on the GWP category (in kg of CO2 equiv) and fossilenergy requirement (in GJ). The LCA software included little data that was specific to South Africa. To overcome this problem, major inputs were identified using sensitivity analysis and adapted to suit the South African context. Specifically, the environmental burden of electricity was calculated by assuming the South African electricity mix to be 91% coal, 4% hydroelectricity, and 5% nuclear,20 on an energy basis. Further, the
diesel used for transporting the raw materials and products was assumed to consist of 65 vol % refined crude oil and 35 vol % synthetic fuel, with 15 vol % being produced by Sasol’s coal to liquid (CtL) plant in Secunda and 20 vol % from the gas to liquid (GtL) plant operated by PetroSA company in Mossel Bay.21,22 2.3. Reference System. It is important to use reference systems for any part of the process chain that would have an alternative use and a consequent different environmental burden if it were not used in the process under assessment. In the production of biodiesel, a key issue is the alternative use of the land used to grow the crops required for biodiesel; it is particularly important to know whether the land would otherwise be left uncultivated if it were not used to produce energy crops, because changing uncultivated land, such as grassland, to manage arable land growing annual crops results in the carbon content of the soil decaying at an exponential rate, toward a new, lower carbon content, characterized by a time constant of around 10-20 years,23 and therefore releasing substantial quantities of carbon that were previously stored in the soil. As noted above, it is likely that a significant proportion of the land required to grow the oil seed for biodiesel production will be grazing land or grassland, which will require new cultivation. In this work, both the best
(18) Hauschild, M.; Potting, J. Spatial Differentiation in Lifecycle Impact Assessment;The EDIP 2003 Methodology. Guidelines from the Danish Environmental Protection Agency, Copenhagen, Denmark, 2004. (19) Bare, J. C.; Gloria, T. P. J. Clean Prod. 2008, 16, 1021–1035. (20) Winkler, H. Energy Policies for Sustainable Development in South Africa. Options for the Future. Energy Research Centre, University of Cape Town, Cape Town, South Africa, 2006.
(21) Fitton, J. Private communication. Sasol, Johannesburg, South Africa, 2008. (22) South African Petroleum Industry Association (SAPIA). Annual Report. 2006. (23) Thomson, A.; Mobbs, D. Land Use Change and Soil Carbon in the U.K.: Current and Future Modeling Approaches. Centre for Ecology and Hydrology. Natural Environment Research Council. Cost 639/ v Workshop, Copenhagen, Denmark, 2008.
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assuming the thermal energy produced displaced the energy produced from combusting heavy fuel oil. As well as allocating the environmental burden between biodiesel and its co-products, the burden associated with the usage of land must also be allocated to the different agricultural products produced on the land, as crops are rotated each year. For example, in the Eastern Cape, canola can be grown on the same land every 4 years while being rotated with other, nonbiodiesel crops.13 In this study, if a burden was due to a nonannual treatment of or an emission from the land, the total burden was split equally among the crops grown on the land during that time period. For example, ∼4 tons of limestone is applied to South African arable soils approximately every 5 years,28 releasing CO2 when it neutralizes acidic soils. The total burden from the addition of the 4 tons of limestone was divided by 5 and allocated to each annual crop. As already mentioned, the impact of the conversion of grassland to cultivated land is investigated in this paper. Because the energy crops require rotation with non-energy crops, additional land as well that used to grow the energy crops may require cultivation to provide adequate areas of arable land for the rotation of the crops. The GWP burden caused by ploughing the land allocated to the biodiesel was calculated from the land area that the biodiesel crop occupies; the burden caused by any additional land requiring ploughing because of the crop rotation requirement was allocated to the non-energy crops. 2.5. Indirect Land-Use Change. Indirect greenhouse gas emissions can also be attributed to biofuels if the production of conventional agricultural commodities is displaced by the cultivation of bioenergy crops. The reduction in the production of the agricultural commodities must be met by increased production elsewhere or by the use of alternative products; accordingly, this may lead to the change of land-use elsewhere, which may have a considerable environmental burden associated with it (e.g., deforestation). For example, if grazing land in South Africa were converted to arable land to grow bioenergy crops, land elsewhere may be required for grazing, causing a change in land use. In a LCA, it is difficult to quantitatively account for such situations; therefore, indirect gashouse emissions were not included in this study. 2.6. Nitrous Oxide (N2O) from Soils. Nitrogenous fertilizers contribute to the GWP of biodiesel because (i) their production is energy-intensive, (ii) their production releases significant quantities of nitrous oxide, and (iii) a proportion of the nitrogen added to agricultural soils, in the form of fertilizer, is converted to N2O, a potent greenhouse gas, and released to the atmosphere.29 There is considerable uncertainty associated with the values used for the emissions of N2O from soils used for growing oil crops.30 These emissions vary widely and depend upon a number of factors, such as soil type, climate, tillage, fertilizer rates, and crop type. It was decided to use the 2006 IPCC Guidelines for National Greenhouse Gas Inventories to determine nitrous oxide emissions,31 which consider direct nitrous oxide production from increased nitrification and denitrification rates in soils, as well as indirect production from nitrate leaching and runoff, and the volatilization of N as NH3 and NOx followed by their accumulation in soils, lakes, and other waters. The 2006 IPCC Guidelines suggest a value of the conversion factor (CF) of 1.1 wt % of synthetic nitrogen inputs to N2O-N
and worst case scenarios have been investigated. The best case scenario corresponds to using idle arable land, which is already cultivated, therefore using it to grow energy crops would not release any significant carbon emissions from the soil. The worst case scenario would mean the land required for the energy crops would come from ploughing uncultivated grassland. The Intergovernmental Panel on Climate Change (IPCC) Guidelines for National Greenhouse Gas Inventories were used24 to determine the quantities of carbon dioxide emitted from soils because of the conversion uncultivated grassland in the homeland regions to arable land growing annual crops. Despite the exponential release of carbon, in this study, the total emission was split evenly over 20 years, as recommended by the IPCC for the conversion of uncultivated land to arable land,24 to determine an average environmental performance of the biofuel over the time frame. When converting from grassland with light to highly weathered soils, typical of soils in the homelands,25 to long-term cultivated land, in a warm, dry, and temperate climate, a total CO2 emission of 18 tons of CO2/ha was assumed, corresponding to 0.9 tons of CO2 ha-1 year-1 over 20 years.24 There is, however, much uncertainty associated with the values used because the carbon emissions are highly dependent upon the type of soil; therefore, further research is required for the carbon dioxide emissions from ploughing the specific land to be cultivated for energy crops. 2.4. Allocation Methods. The production of biodiesel generates the co-products seed meal and glycerol; one purpose of allocation is to determine, rationally, how a particular environmental burden, e.g., GWP, should be shared among the biodiesel and co-products. A preferred method of allocation is direct substitution.26 However, to use direct substitution, the product being replaced must already be satisfied by other processes. Thus, this approach cannot be taken when the product being replaced is always regarded as a co-product, byproduct, or waste. If direct substitution cannot be used, simpler allocation methods can be applied, including allocation by economic value, calorific value, or mass. In these cases, it is preferable to allocate burdens on the basis of economic value because economic relationships reflect socioeconomic demands.27 The allocation procedures adopted in this study are described below. In South Africa, seed meal is generally used as an animal feed. Animal feed is usually produced as a co-product of other processes, making allocation by substitution difficult. Therefore, the allocation of environmental burdens for the meal was calculated using quoted market prices, as shown in the following equation: Aγ allocation ¼ ð1Þ Aγ þ Bε Here, A is the market price of meal when used for animal feed, B is the market price of seed oil, γ is the proportion of the seed by mass converted to mass of meal, and ε is the proportion of the seed by mass converted to mass of oil. At present, significant quantities of animal feed are imported by South Africa; however, in the future, the animal feed market may become flooded as increasing quantities of seed meal are produced as a byproduct of biodiesel production. In this case, the meal may be exported, or alternative uses, such as its combustion for energy generation, may be investigated. It has been assumed that glycerol is used as a fuel in industrial furnaces. Allocation by substitution has been employed,
(28) Murray, N. Private communication. Biodiesel Centre, Bellville, South Africa, 2007. (29) Crutzen, P. J.; Mosier, A. R.; Smith, K. A.; Winiwarter, W. Atmos. Chem. Phys. Discuss. 2007, 7, 11191–11205. (30) Mortimer, N. D.; Elsayed, M. A. North East Biofuel Supply Chain Carbon Intensity Assessment. North Energy Associates Ltd., Stocksfield, U.K., 2006. (31) Intergovernmental Panel on Climate Change (IPCC). N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application, IPCC Guidelines for National Greenhouse Gas Inventories. 2006; Vol. 4.
(24) Intergovernmental Panel on Climate Change (IPCC). Agriculture, Forestry and Other Land Use. IPCC Guidelines for National Greenhouse Gas Inventories. 2006; Vol. 4. (25) Nagle, G. Development and Underdevelopment: Focus on Geography; Nelson Thornes: Cheltenham, U.K., 1999. (26) Department for Environment, Food and Rural Affairs (DEFRA). Evaluation of the comparative energy, global warming and socio-economic costs and benefits of biodiesel. Report 20/1. London, U. K., 2003. (27) Clift, R. Inst. Chem. Eng. Environ. Prot. Bull. 1998, 53, 9–13.
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irrigation pumps are highly variable, depending upon the water source, method of irrigation (e.g., central pivot system and sprinkler system), and the type of energy used by the pump (viz. diesel or electricity). In South Africa, groundwater, surface water, and recycled water are all used to irrigate crops.38 This work investigated two systems: (A) using groundwater pumped from boreholes, using electrically powered pumps, and (B) surface water pumped from a nearby river, using diesel-powered pumps. The system of irrigation assumed here is based on a method that is typically employed in South Africa, because of its low capital costs. This system uses ∼4 aluminum lines, which are moved across the land being irrigated by hand.39 It was assumed that the lines are spaced 20 m apart and each line is 300 m long, with sprinklers set every 18 m, each supplying 0.92 10-3 m3/s of water. The cycle involves irrigating the soil with 50 mm of water (taking approximately 5.7 h), to allow the soil to reach its full retention capacity, before moving the sprinkler lines to a different area (allowing 3 moves/day) and then returning to the same spot ∼10 days later, for further irrigation. This system irrigates ∼7.2 ha/day with 50 mm of water and requires 58.6 10-3 m3/s of water to be pumped during operation (17 h/day).39 When the lines are being moved, the pump still runs, albeit at a reduced load; this fuel consumption is accounted for by adding another 25% to the final value of the power required for pumping during 17 h/day.39 For irrigation from a borehole, the groundwater is pumped from the level of the water table. The level of the water table varies; for example, the water level of the Dendron aquifer in the Northern Province varies between 50 and 100 m below ground.40 In this work, it was assumed that the water requires pumping a height of 75 m. The total dynamic head of the pump was assumed to be 150 m, when accounting for total friction losses of 41 m and an exit pressure of 3.4 bar.41 When considering river water as a source, a total dynamic head of 65 m was assumed, with friction head losses of 28 m and the same exit pressure of 3.4 bar.41 The overall efficiencies of the pump and power unit, defined as the proportion of the supplied power that is transferred to the fluid, were taken to be 35% for a diesel-driven pump and 50% for an electrically powered pump.41 2.9. Emissions from the Combustion of Biodiesel. The emissions associated with the combustion of biodiesel must be considered when determining their overall environmental impacts. Because GWP is considered in this paper, the amounts of nitrous oxide, methane, and fossil-derived CO2 emitted during the combustion of biodiesel in a typical engine were estimated. For these calculations, the lower calorific value of biodiesel was assumed to be 37.2 MJ/kg.42 Because biodiesel is produced from the esterification of triglycerides with methanol, a small proportion of CO2 released during combustion is from methanol, which is usually derived from fossil fuels. In this work, it was assumed that each biodiesel molecule contains 19 carbon atoms, with 1 carbon atom originating from fossil methanol, resulting in fossil-derived CO2 emissions of ∼4 10-3 kg/MJ. Nitrous oxide and methane emissions released by the combustion of biodiesel assumed in this study were adapted from results detailed in the CONCAWE and EUCAR report,43 where emissions were
(N content of N2O) for dry climates, where leaching is unlikely to occur. The nitrous oxide release (kg ha-1 year-1) because of the addition of fertilizer MF (kg of N ha-1 year-1) can therefore be calculated according to 44 N2 O ¼ M F C F ð2Þ 28 The IPCC conversion values can also be applied to any crop residues recycled to agricultural soils, to determine nitrous oxide emissions because of the decomposition of such residues. Default values from the 2006 IPCC Guidelines31 were used to estimate the quantities of residual biomass produced during the cultivation of oil-seed crops and their contents of nitrogen. Oilseed rape and sunflower crops were assumed to fall under the category “root crops, other” in these guidelines, because information specific to these crops was not available, apart from the nitrogen content of oil-seed rape residues, which was taken to be 1.3 wt %.32 2.7. Farm Machinery. The volume of diesel fuel used per hour by each farming operation was calculated using the dimensional eq 3,33,34 while typical operating times per hectare35 were used to determine the diesel requirement per hectare. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi! PT PT Qdiesel ¼ PT 2:64 þ 3:91 - 0:203 738 þ 173 ð3Þ Pmax Pmax Here, Qdiesel is the diesel consumption (h-1); PT is the power required for an operation (kW); and Pmax is the maximum available power take off (PTO) from the tractor (kW). To determine the power requirement for farming operations requiring significant draft force (e.g., ploughing), eq 4 was used where detailed experimental results were available.33,34 F2 WTdepth S PT ¼ ðA þ BS þ CS 2 Þ ð4Þ 3:6Em Et Here, F2 is a dimensionless soil texture parameter for mediumtextured soils; W is the width of the machine (m); Tdepth is the depth of tillage (cm) (if operation does not involve tillage, this parameter equals 1); S is the field speed (km/h); Em is the mechanical efficiency of transmissions and power train (taken to be 0.96 for tractors with gear transmissions); Et is the traction efficiency; and A, B, and C are machine-specific parameters. For farming operations where such information was not available, estimates of power usage were determined after a discussion with a tractor manufacturer36 and used in eq 3. The amount of lubricating oil, Qoil (h-1), used for running farm machinery was determined using eq 5. The materials and energy required for the manufacture of the machinery were taken from Heller et al.37 and included on a field-hour basis, distributed over the estimated lifetime of the tractors (12 000 h) and implements (1500 h). Qoil ¼ 0:00059Pmax þ 0:02169 ð5Þ 2.8. Irrigation. The effect of irrigating land on the environmental performance of biodiesel produced in South Africa is investigated in this study. However, the energy requirements of
(38) Yokwe, S. Agric. Water Manage. 2009, 96, 1223–1228. (39) Hawthorn, W. Private communication. 2iC Consultants Ltd., Tullibody, U.K., 2008. (40) Masiyandima, M.; Van der Stoep, I.; Mwanasawani, T; Pfupajena, S. C. Phys. Chem. Earth 2002, 27, 935–940. (41) Phocaides, A. Handbook on Pressurized Irrigation Techniques, 2nd ed.; Food and Agricultural Organization of the United Nations: Rome, Italy, 2007. (42) Department for Transport (DfT). Carbon and Sustainability Reporting Within the Renewable Transport Obligation. Requirement and Guidance Government Recommendations to the Office of the Renewable Fuels Agency, London, U.K., 2008. (43) CONCAWE and European Council on Automotive Research and Development (EUCAR). Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context. Tank to Wheel Report Version 3. 2008.
(32) Trinsoutrot, I.; Recous, S.; Mary, B.; Justes, E.; Nicolardot, B. C and N mineralisation of oilseed rape crop residues in soil. 10th International Rapeseed Conference, Canberra, Australia, 1999. (33) American Society of Agricultural Engineers (ASAE). ASAE Standards: Agricultural Machinery Management. Report ASAE EP496.2 DEC98. 1999. (34) American Society of Agricultural Engineers (ASAE). ASAE Standards: Agricultural Machinery Management. Report ASAE EP497.4 JAN98. 1999. (35) Nix, J. Farm Management Pocketbook, 37th ed.; Wye Campus, Imperial College London: London, U.K., 2007. (36) AGCO Corporation. Tag Tractors. Private communication. Duluth, GA, 2007. (37) Heller, M. C.; Keoleian, G. A.; Volk, T. A. Biomass Bioenergy 2003, 25, 147–165.
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calculated on the basis of combustion in a typical European compact-size, five-seater car, using a direct-injection compression-ignition (DICI) engine. 2.10. Comparisons to Fossil Diesel. The results from this study have been compared to the environmental impacts of fossilderived diesel, used in either South Africa or the U.K., using results from the literature.42,44 Comparisons here are calculated on a basis of equivalent net energy content of biodiesel and diesel, assuming that the lower calorific value of biodiesel is 37.2 MJ/kg and that of fossil diesel is 43.1 MJ/kg.42 In comparison to fossil-derived diesel used in the U.K., biodiesel was compared to 100% fossil-derived diesel, with overall fossil-energy and GWP burdens of 1.14 and 86 10-3 kg of CO2 equiv/MJ, respectively. In the case of the comparison of biodiesel to fossil-derived diesel in South Africa, it has been assumed that 65 vol % of diesel used in South Africa is produced by refining crude oil, while the remaining 35 vol % was synthetic fuel, with 15 vol % being produced by Sasol’s CtL plant in Secunda and 20 vol % from the GtL plant operated by PetroSA company in Mossel Bay,45,46 resulting in average overall fossil-energy and GWP burdens of 1.39 and 104 10-3 kg of CO2 equiv/MJ, respectively. 2.11. Details of the Process Chains. The process chains shown in panels a and b of Figure 1 have been generated using the assumptions described below. Assumptions regarding the distances involved in the transport of the product and raw materials were formulated after discussions with farmers and biodiesel producers during the collection of information. Where details regarding the delivery of a chemical to the farm or process plant (e.g., N fertilizer and methanol) are not specified, it is implicitly assumed that it is transported an average distance of 100 km by bulk road carrier transport. There are considerable uncertainties associated with these distances; therefore, their influence on the total GWP of biodiesel have been investigated. Detailed information regarding the performance of trucks typically used in South Africa was not available; therefore, it was assumed that their performance would be similar to trucks used in Europe. However, emissions specific to the combustion of diesel used in South Africa, which contains ∼35 vol % synthetic fuel (as detailed above), were used. 2.12. Details of the Agricultural Procedures. Because this work investigates the typical production of biodiesel in South Africa rather than at a specific location, average agricultural information for the country has been used. The main inputs associated with the agricultural operations required for the growth of the oil crops, canola, sunflower and soybean, are shown in Table 1. 2.12.1. Canola. At present, canola is generally grown in the Western Cape province, and typically, a yield of around 1.5 tons/ha is realized.47 Most canola is not irrigated in South Africa; therefore, no irrigation is included in the base scenario. However, if canola were to be grown in drier regions of South Africa, irrigation would be required; for example, canola grown in the northwest of South Africa would require ∼450 mm per season.48 Before sowing the seeds, the soil undergoes stubble cultivation to help eliminate potential weed problems, requiring ∼10 10-3 m3/ha of diesel. Subsoiling, using ∼20 10-3 m3/ha of diesel is also required approximately every 7 years, to break up clods of earth.47 After cultivation, the seeds are sown at a rate of 5 kg/ha47 using a cereal drill, and the soil is rolled to ensure
adequate contact between it and the seeds. Both operations require ∼5 10-3 m3/ha of diesel. Fertilizer requirements of 57.5 kg of N ha-1 year-1 and 41 kg of phosphate ha-1 year-1 were based on typical usage in the region, assuming the rape straw is ploughed back into the field.47 South African soils generally have adequate quantities of potassium;49 therefore, it was assumed that no potash fertilizer was applied to the soils. Limestone (CaCO3) is applied to most soils to maintain the appropriate pH, with application frequency and quantity being specific to the soil type; typical values have been used, with ∼4 tons being applied per hectare every 5 years.28 A windrower is used for harvesting; this cuts the crops, which are then left in rows for 7-10 days to dry, before being picked up. This method, known as swathing, uses ∼23 10-3 m3 of diesel/ha. Swathing is advantageous because (i) the seeds do not require drying, (ii) fewer seeds are lost to the wind, (iii) uniform ripening of the seed can be ensured, and (iv) it allows for an earlier harvest, if necessary.47 2.12.2. Soybean. The soybean yield achieved in South Africa during the 2006/2007 season was 1.2 tons/ha,50 and this yield was assumed in this work. Some soybean crops are irrigated in South Africa, while others are reliant on prevailing rainfall. For example, in the Mpumalanga province, it is estimated that 55% of soybean crops are irrigated, while 15 and 13% are irrigated in the Kwazulu-Natal and Free State provinces, respectively,51 requiring in each case up to 700 mm irrigation per season.48 The agricultural procedure employed in South Africa to produce soybeans requires annual deep ploughing, using 42 10-3 m3/ha of diesel, as well as stubble cultivation. Seeds are sown at a rate of 90 kg/ha17 using a cereal drill. Fertilizer requirements were based on the 2004 South African average, where 40% of the land used to grow soybeans was fertilized at a rate of 7.3 kg of N/ha, 11.0 kg of P/ha, and 7.3 kg of K/ha, while the remaining 60% was not fertilized.52 As with canola, limestone is applied to most soils to maintain the appropriate pH and a typical application rate of 4 tons/ha every 5 years was assumed. A combine harvester is used to harvest the soybeans, requiring ∼27 10-3 m3/ha of diesel. 2.12.3. Sunflower. Generally, potential sunflower seed yields (1.5-2.5 tons/ha)53 are not achieved in South Africa; the average yield for the 2006/2007 season was 0.95 ton/ha.50 One reason for low yields is that sunflower is often grown as a catch crop in South Africa, thus not realizing its full potential. Other limiting factors include poor and uneven stands, poor weed control, late planting, insufficient fertilization, and wrong choice of cultivar.53 In this work, both current and recommended agricultural practices were considered, referred to as sunflower a and b, respectively. As with soybean crops, some sunflower crops are irrigated in South Africa, while others are not. In the Free State province, it is estimated that 51% of sunflower crops are irrigated, while 35% are irrigated in the North West province,51 using up to 500 mm irrigation water per season.48 The cultivation of sunflower crops involves annual deep ploughing to break up limiting layers and stubble cultivation for weed control. Seeds are sown at a rate of 2 kg/ha49 using a cereal drill, which is equipped with press wheels, to ensure adequate contact between the seed and the soil. The recommended fertilization of sunflower seeds differs from current
(44) CONCAWE and European Council on Automotive Research and Development (EUCAR). Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context. Well to Tank Report Version 2b. 2006. (45) Fitton, J. Private communication. Sasol, Johannesburg, South Africa, 2008. (46) South African Petroleum Industry Association (SAPIA). Annual Report. 2006. (47) Strausse, J. Private communication. Elsenburg College, Stellenbosch University, Stellenbosch, South Africa, 2007. (48) Water Research Commission. SAPWAT version 2.6.0. 2008 (http://www.sapwat.org.za/).
(49) KZN Agriculture and Environmental Affairs. Sunflower Production: A Precise Guide. 2008 (http://agriculture.kzntl.gov.za/portal/ Home/tabid/56/Default.aspx). (50) South African Crop Estimates Committee. Area planted and forth production estimate of summer crops: 2006/2007 season. 2007. (51) Food and Agriculture Organization of United Nations. Fertilizer use by crop in South Africa. Rome, Italy, 2005. (52) Fertilizer Society of South Africa FSSA-MVSA. Fertilizer Use by Crop in the Republic of South Africa Report. Pretoria, South Africa, 2004. (53) Fertilizer Society of South Africa (FSSA-MVSA). FSSA Fertilizer Handbook, 5th ed.; FSSA-MVSA: Pretoria, South Africa, 2003.
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Table 1. Main Inputs Required for the Growth of (i) Canola, (ii) Soybean, and (iii) Sunflower Crops (a, Current Practice; b, Recommended Practice) in South Africa agriculture
unit
canola
soybean
sunflower a
sunflower b
yield of seed at 9 wt % moisture seed oil quantity oil yield seed sowing rate nitrogen fertilizer requirement phosphate fertilizer requirement potash fertilizer requirement sulfate fertilizer requirement pesticide requirement number of pesticide sprays number of fertilizer spreads limestone requirement diesel requirement nitrogen in crop residues
tons/ha % tons/ha kg/ha kg of N/ha kg of P2O5/ha kg of K2O/ha kg of SO3/ha kg/ha year-1 year-1 kg of CaCO3/ha m3/ha kg of N ha-1 year-1
1.5 40-42 0.62 5 57.5 41 0 38 1.4 2 3 800 0.068 46.8
1.2 20 0.24 90 3 10 4 0 1.4 2 1 800 0.097 22.5
0.95 44 0.42 2 13 17 2 0 1.4 2 3 800 0.11 46.3
2.0 44 0.88 2 54 47 0 0 1.4 2 3 800 0.11 65.6
Table 2. Main Inputs Required for Medium-, Small-, and Microscale Biodiesel Production (Information from Site Visits to Plants A, B, and C) reactor type reaction temperature reaction time reaction pressure methanol input direct electricity requirement direct heating requirement (natural gas) catalyst requirement sulphuric acid requirement hydrochloric acid requirement magnesium sulfate requirement mass conversion of oil to biodiesel
units
medium-scale
small-scale
microscale
°C min bar kg/ton of biodiesel MJ/ton of biodiesel MJ/ton of biodiesel kg/ton of biodiesel kg/ton of biodiesel kg/ton of biodiesel kg/ton of biodiesel %
stirred batch 60 105 1.01 115 974 602 NaOH, 5.6 0.06 0 0 98
stirred batch 60 135 1.01 179 1286 0 KOH, 12.2 0 2.8 0.4 90
stirred batch 60 540 1.01 193 1365 0 NaOH, 3.9 0 0 0 90
recycled in the process. A waste stream consisting mainly of glycerol (∼80 wt %) is produced at a rate of 130 kg/ton of biodiesel produced. At the time of the site visit to plant A, this waste was stored on site, but the aim was to sell it as a fuel for industrial furnaces. For this work, it was assumed that the glycerol waste stream would be transported 500 km, by truck (27 ton capacity), to an industrial furnace, with an efficiency (defined as the proportion of the lower calorific value, which is converted to useful heat) of 60%.56 The use of the biodiesel in both South Africa and the U.K. was investigated. For the case of using the biodiesel in South Africa, it was assumed that the biodiesel product would be transported by truck (27 ton capacity) to the customer, an average distance of 200 km from the production plant. For its use in the U.K., it was assumed that the biodiesel would be first transported by truck (27 ton capacity), 200 km in South Africa, then by an average sea tanker, fueled by heavy fuel oil, a further 13 600 km to a blending site in the U.K. (distance between ports in Cape Town and London). It would then be transported a further 140 km to a filling station. 2.13.2. Small Scale. The small-scale system differs from the medium scale in that it was assumed that the storage and crushing of the seed would occur at the farm rather than at a large-scale, seed-crushing facility. It was assumed that the dried seed would be stored for 250 h in a storage facility, holding ∼500 tons of seeds, while it is aired using a 7.5 kW fan, requiring ∼14 MJ electricity/ton of seeds.57 The oil would then be extracted using a cold screw press, modeled on a 22 kW press capable of treating 400 kg of seed per hour and requiring approximately 227 MJ electricity/ton of seeds.58 If cold pressing is used to extract the oil from the seeds, a lower quantity of phospholipid is usually extracted than if solvent extraction is employed;6
practice. The current fertilization practice assumed in this work was based on that employed in 2004, where 85% of sunflower crops were fertilized with 15.1 kg of N/ha, 8.5 kg of P/ha, and 2 kg of K/ha, while the remaining 15% received no fertilizer.52 Recommended nitrogen and phosphorus additions are higher, at 54 and 20.5 kg/ha, respectively.53 A combine harvester is used to harvest sunflower seeds, using ∼27 10-3 m3/ha of diesel. 2.13. Details of the Biodiesel Production Processes. The main inputs required for the oil extraction and biodiesel production of medium-, small-, and microscale biodiesel production are shown in Table 2. 2.13.1. Medium Scale. It was assumed that the oil seed would be harvested with a suitable moisture content for the oil-extraction facilities (e9 wt %), so that drying would not be required. After harvesting, it was assumed that the oil seed would be transported 200 km by truck (9 ton capacity) and then taken to storage, where it would be cooled using 220 kW electric fans.30 The oil would next be extracted from the seeds by solvent extraction with hexane. Seed meal is produced as a co-product of the oil extraction, with an oil content of 1-2 wt %. After extraction, the oil would be refined at the extraction plant to remove phospholipids, fatty acids, and pigments, such as chlorophyll, before being transported 500 km by truck (27 ton capacity), from the crusher to the biodiesel production plant, which has been based on plant A, with a capacity to produce 60 103 tons of biodiesel per year. The process in plant A uses ∼1020 kg of vegetable oil and 115 kg of methanol per ton of biodiesel; 109 kg of methanol is converted into biodiesel during the esterification reaction, and it was assumed that the remaining 6 kg is split in the mass ratio of 6:4 between the product and glycerol streams.54 It was assumed that the methanol would be produced from natural gas.55 The methanol residing in the product phase is removed under reduced pressure; however, methanol is not
(56) Trinks, W.; Mawhinney, M. H.; Shannon, R. A.; Reed, R. J.; Garvey, J. R. Industrial Furnaces, 6th ed.; John Wiley and Sons: New York, 2004. (57) Armitage, D.; Prickett, A.; Norman, K.; Wildev, K. Survey of current harvesting, drying and storage practices with oilseed rape. Project Report 371. Home Grown Cereals Authority (HGCA), Kenilworth, U.K., 2005. (58) Dave Cripps Agricultural Ltd. Private communication. 2007.
(54) Zhou, W.; Boocock, D. J. Am. Oil Chem. Soc. 2006, 83 (12), 1047–1052. (55) Aasberg-Petersen, K.; Stub Nielsen, C.; Dybkjaer, I.; Perregaard, J. Large scale methanol production from natural gas, Haldor Topsøe, Lyngby, Denmark.
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Table 3. LCA Results for Biodiesel Produced and Used in South Africa: (i) Total Burden per Ton of Biodiesel, in kg of CO2 equiv for GWP and GJ for Fossil-Energy Requirement and (ii) Savings When Compared to Fossil-Derived Diesel Used in South Africa (65% from Crude Oil, 20% GtL, and 15% CtL)21,22 a medium scale GWP
canola soybean sunflower a sunflower b a
small scale
fossil energy
GWP
microscale
fossil energy
GWP
fossil energy
i
ii (%)
i
ii (%)
i
ii (%)
i
ii (%)
i
ii (%)
i
ii (%)
2893 3002 3082 2556
25 22 20 34
21.6 25.8 24.6 20.7
58 50 52 60
2892 3003 3071 2470
25 22 21 36
20.9 25.5 24.3 19.8
60 51 53 62
2912 3023 3091 2491
25 22 20 36
21.2 25.7 24.5 20.3
59 50 53 61
Here, CtL and GtL are fossil-derived fuels from the Fischer-Tropsch processing of coal and natural gas, respectively.
therefore, it was assumed that the oil would not undergo a refining process. The oil would be transported by truck (9 ton capacity), from the farm to the biodiesel production plant, an assumed average distance of 200 km, as shown in Figure 1b. The plant was modeled on plant B, which has the capacity to produce 8000 tons of biodiesel/year. The process uses ∼1110 kg of vegetable oil and 179 kg of methanol per ton of biodiesel; 109 kg of methanol is converted into biodiesel during the esterification reaction, and the remaining 70 kg is split in the ratio by mass of 6:4 between the product and glycerol streams.54 The methanol residing in the product phase is removed; however, it was assumed that no methanol would be recycled because, at the time of the site visit, high water levels in the methanol prevented its reuse in the process. A waste stream consisting mainly of glycerol (∼46 wt %), unconverted oil (∼25 wt %), and methanol (∼19 wt %) is produced at a rate of 177 kg/ton of biodiesel produced, which is sold for use as a fuel in industrial furnaces (assumed efficiency of 60%56). It was assumed that the biodiesel would be used in South Africa, transported by a road tanker (17 ton capacity) to the customer, an average distance of 100 km from the production plant. 2.13.3. Microscale. As with the small-scale production process, it was assumed that the storage and crushing of the seed would occur at the farm, where a cold screw press would be used to extract the oil. It was assumed that the oil would not undergo a refining process and would be transported by a road tanker (9 ton capacity), from the farm to the biodiesel production plant, an assumed average distance of 10 km. The plant was based on plant C, which has the capacity to produce 300 tons of biodiesel per year. This plant uses ∼1110 kg of vegetable oil and 193 kg of methanol per ton of biodiesel, and no methanol is recovered. The reaction time of this process is ∼9 h, significantly longer than that of both the medium-scale reaction (1.75 h) and the small-scale reaction (2.25 h). A waste stream consisting mainly of glycerol (∼50 wt %), methanol (∼25%), and oil (∼25%) is produced at a rate of 221 kg/ton of biodiesel produced. As with plant A, at the time of the site visit to plant C, this waste was stored on site; this is not viable in the long term; therefore, for this work, it was assumed that the glycerol waste would be transported 500 km, by truck (9 ton capacity), to an industrial furnace with an efficiency of 60%56 for the generation of thermal energy. It was assumed that the biodiesel would be used in South Africa, transported by a road tanker (9 ton capacity) to the customer, an average distance of 10 km from the production plant.
Figure 2. GWP and total fossil-energy savings of using biodiesel, produced in South Africa at a medium scale, in either South Africa or the U.K. Comparisons are made based on the average burden of fossil-derived diesel used in each country.
results were used to compare the use of biodiesel to the current fossil-diesel mix of South Africa, using results from the literature, and it can be seen that savings in fossil-energy requirement and GWP can be achieved for these scenarios. When considering using the biodiesel produced at a medium scale for use in the U.K., the total burdens are not significantly different from those shown in Table 3. For example, the total GWP of biodiesel produced from canola at a medium scale is 2893 kg of CO2 equiv/ton when used in South Africa and 2937 kg of CO2 equiv/ton after transport by a sea tanker to the U.K. and delivery to a filling station. However, the savings achieved by using this biodiesel rather than fossilderived diesel reduce if it is used in the U.K., as shown in Figure 2. This is because fossil-derived diesel used in South Africa includes ∼35 vol % syn-diesel, with 15 vol % being produced from coal (CtL) and 20 vol % from gas (GtL),21,22 while in the U.K., conventional diesel from the refining of crude oil is predominantly used. Syn-diesel from coal or natural gas has a significantly higher fossil-energy requirement and GWP than diesel derived from the conventional refining of crude oil; therefore, greater savings can be achieved using biodiesel in South Africa rather than the U.K. When the results from this paper are compared to previous work in the literature, biodiesel produced in South Africa, when no land requires to be newly ploughed and no irrigation is required, has a higher GWP to biodiesel produced from both oilseed rape grown on arable land in the U.K. (∼2000-2400 kg of CO2 equiv/ton of biodiesel14,30,42) and soybeans grown on arable land in the United States of America (∼2200 kg of CO2 equiv/ton of biodiesel42). However, this biofuel has a significantly lower GWP to biodiesel produced from palm oil grown on peatland forest in Malaysia, which has a GWP of ∼14 500 kg of CO2 equiv/ton of biodiesel.59
3. Results and Discussion 3.1. Base Cases. The inputs displayed in Tables 1 and 2 and discussed above have been used to produce base scenarios for the production and use of biodiesel in South Africa from canola, soybean, and sunflower oil-seed crops, at medium-, small-, and microscales. The base cases assume that no irrigation would be required and no grassland would be ploughed. Variations from these scenarios were investigated. Table 3 shows the LCA results for the base scenarios; these
(59) Wicke, B.; Dornburg, V.; Junginger, M.; Faaij, A. Biomass Bioenergy 2008, 32, 1322–1337.
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Figure 3. Fossil-energy requirement of the key stages in the life cycle of the production of biodiesel from canola oil in South Africa, at medium-, small-, and microscale, including the option of delivering to the U.K.
Figure 5. Fossil-energy requirement, in GJ/ton of biodiesel, of the key stages involved in the agricultural practices used to produce the oil seeds, canola, soybean, and sunflower (current and recommended practice), for the production of biodiesel at a medium scale.
Figure 4. GWP of the key stages in the life cycle of the production of biodiesel from canola oil in South Africa, at medium-, small-, and microscale, including the option of delivering to the U.K.
Table 3 also shows that the scale of operation has little influence on the GWP and total fossil-energy requirements of biodiesel production in South Africa. It is also evident that biodiesel produced from either canola, soybean, or sunflower seed oil currently saves roughly the same GWP when compared to fossil-derived diesel, while the production of biodiesel from canola uses slightly less fossil energy. However, if the yield of sunflower seeds could achieve its potential using recommended agricultural practices, the GWP of biodiesel could be lowered significantly. Figures 3 and 4 show the fossil-energy requirement and GWP, respectively, for the main stages involved in the base case production of biodiesel from canola, at each scale, along with the burdens associated with delivering the biodiesel to a filling station in the U.K. It can be seen from Figure 3 that the main fossil-energy requirements arise from the esterification and agricultural stages of the process. When biodiesel is produced at a medium scale, the esterification step uses less fossil energy than when produced at a small or microscale, because the process requires less methanol, while the lower agricultural burden observed in Figure 3 is attributed to a higher conversion of oil to biodiesel. However, the waste glycerol stream produced by the medium-scale production process has a lower content of oil and methanol; therefore, the energy saved by combusting this stream is lower than for small- and microscale production. Solvent extraction and
Figure 6. GWP, in kg of CO2 equiv/ton of biodiesel, of the main stages involved in the agricultural practices used to produce the oil seeds, canola, soybean, and sunflower (current and recommended practice), for the production of biodiesel at a medium scale.
refining are also more energy-intensive than cold pressing; therefore, overall, these effects cancel and the total fossilenergy requirements for biodiesel production at each scale barely differ from each other. It can be seen from Figure 4 that the main source of GWP is from the agricultural stage, for each scale of operation. The same patterns in the differences in burdens between each scale of operation as those observed from Figure 3 for the fossil-energy requirement, are observed here. The fossil-energy requirement and GWP associated with the agricultural stage of the production of biodiesel at a medium scale, using each feedstock, are analyzed in greater detail in Figures 5 and 6. These results display the burdens associated with the biodiesel only, using the allocation methods detailed above. For each oil crop, the most significant input contributing to the fossil-energy requirement of the agricultural stage is seen in Figure 5 to be the use of farm machinery. When considering current agricultural practices, the cultivation of canola in South Africa for use as a biodiesel feedstock is shown to be less energy-intensive than soybean or sunflower, owing to a lower fuel requirement from farm operations; however, 2497
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Figure 7. GWP of biodiesel produced at a medium scale in South Africa, when the previous land use was either idle arable land, subject to regular ploughing, or grassland, requiring ploughing.
Figure 8. GWP of biodiesel produced at a medium scale in South Africa, when the quantity of irrigation is varied, using systems A and B, assuming a constant crop yield.
there is significant scope to reduce the fossil-energy requirement for the production of sunflower seeds, if recommended practices are employed. Figure 6 shows that emissions of CO2 from the neutralization of soils by limestone, fuel from the use of farm machinery, and emissions of N2O from both nitrogenous fertilizers and crop residues contribute the most to the overall GWP of the agricultural stage of biodiesel production. The contribution from the manufacturing of fertilizers is shown to be small for biodiesel produced from soya beans and sunflower a (current practice); however, this burden was found to represent ∼24% of the agricultural burden associated with biodiesel from canola (corresponding to 435 te CO2-equivalent/te biodiesel). It can also be seen from Figure 6 that the total GWP associated with the cultivation of each crop, using current practices, is very similar; however, sunflower seeds have the potential to reduce the GWP of the biodiesel feedstock by ∼20-25%, if recommended practices are employed. 3.2. Ploughing Grassland. The savings in GWP from using biodiesel rather than fossil fuels in South Africa, detailed in Table 3, were calculated assuming all of the land used to grow the crops had previously been idle crop land and had been fully cultivated. However, as noted earlier, it is likely that significant quantities of grassland will require cultivation to provide the land required for the oil crops. Figure 7 shows the GWP of biodiesel, produced at a medium scale from canola, soybean, and sunflower seeds, when the previous land use was either idle arable land, which had already been ploughed, or grassland, which would require ploughing. Figure 7 also shows the GWP of biodiesel that would result in zero GWP savings when compared to fossil-derived diesel used in South Africa and the U.K., on an energy basis. It can be seen that, if the oil-seed crops are grown on land that had previously been grassland and current agricultural practices were employed, the GWP of biodiesel would be greater than fossil diesel used in both South Africa and the U.K. 3.3. Irrigation. The results in Table 3 also assume that no irrigation would be used to grow the oil crops; however, in some cases, irrigation may be required. Figure 8 shows the increase in GWP of biodiesel, produced at a medium scale, when irrigated using systems A and B and assuming that the crop yield would not be affected. As noted earlier, system A uses an electric pump to retrieve groundwater from 75 m below the surface, while system B uses a diesel-powered pump to convey local river water. Irrigation is shown to have a dramatic effect on the GWP of the biodiesel, particularly if system A is used. For example, soybean crops can require up to 700 mm of irrigation water per season in South Africa;48 if soybeans were used to produce biodiesel, the
Figure 9. GWP of biodiesel produced in South Africa at a medium scale (i) if no irrigation were used and (ii) if irrigation systems A and B were used, causing the crop yield to double.
GWP of biodiesel would be ∼4.7 and 1.4 times greater than fossil diesel used in South Africa for irrigation systems A and B, respectively. The observed difference in GWP between systems A and B arises from the difference in the total dynamic head of the pumps (150 m for A and 65 m for B) and the fuels employed to power the pumps; to provide 1 MJ of energy, farm diesel releases ∼77 10-3 kg of CO2 equiv,60 while electricity from the South African grid (of which ∼90% is provided by coal-fired power stations20) releases ∼285 10-3 kg. It is therefore significantly more detrimental to use electrically powered pumps than those powered by diesel. The average oil-seed yield in South Africa is significantly lower than in other parts of the world,51 in part because the crops do not receive enough water for optimal growth. Figure 9 investigates whether an increase of 100% in the current yield would compensate for the increased GWP burden from irrigation using systems A or B, at levels recommended for dry land areas (450 mm/season for canola, 700 mm/season for soybean, and 500 mm/season for sunflower48). It can be seen from Figure 9 that, even if yields were doubled, irrigation at these levels using system A would cause an increase in the GWP of biodiesel produced from canola, soybean, and sunflower seeds, resulting in the biodiesel having a much higher GWP than fossil-derived diesel used in either South Africa or the U.K. (60) Department for Environment, Food and Rural Affairs (DEFRA). Statistical Analysis Database for DEFRA Project NF0614. Environmental tools for biomaterials. Primary Energy and Greenhouse Gas Multipliers for Fuels and Electricity. London, U.K., 2004.
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more accurate results, because these emissions are highly dependent upon soil type.
However, if irrigation using system B was used, the increase in GWP because of the energy requirement of the irrigation pump would be offset by the savings in agricultural burden because of the increase in yield, resulting in an overall GWP lower than that of fossil-derived diesel used in South Africa or the U.K. It is therefore extremely important to determine the method of irrigation and possible improvements in yield before deciding whether to use irrigated oil seeds for the production of biodiesel. 3.4. Transport. The impact of transporting feedstock and finished products has been examined by determining how varying the distances involved in medium-, small-, and microscale production changes the fossil-energy requirement and GWP of the process chains. For medium-scale production from canola and sunflower seeds, the distance to which these impacts would be most sensitive is that between the oilextraction facility and the biodiesel production plant, owing to the relatively large distances assumed. For medium-scale production from soybeans, the GWP and fossil-energy requirement were more sensitive to the distance between the farms and the crushing facility, owing to the low content of oil in soybeans. It was found that transport has little impact on the overall fossil-energy requirement and GWP; for example, doubling each of these distances would increase the overall fossil-energy requirement and GWP by less than 3% each. Because small- and microscale production involve shorter distances than production at a medium-scale, the transport of the feedstock and finished products would make an even smaller difference to the overall fossil-energy and GWP; doubling each distance involved would increase these burdens by less than 1% each. The transport of chemicals to the farm or process plant would have a negligible impact on the overall environmental burden of the processes. 3.5. Case Study: Biodiesel Production in the Eastern Cape for Export to Europe. At present, there are plans to build a biodiesel production plant in the Eastern Cape Province, to produce biodiesel for the European market. The plant would manufacture 400 000 tons of biodiesel/year and use canola as its feedstock, grown on currently uncultivated land in the Mzimvubu region of the Eastern Cape province.5 Canola crops in this region require ∼210 mm irrigation per season.61 Using the assumptions that (i) the yield of canola achieved would be similar to the South African average, (ii) irrigation would be supplied from the local Mzimvubu river using diesel-powered water pumps (system B), and (iii) the biofuel product would be transported in an average sea tanker, fueled by heavy fuel oil, from Cape Town to London, a distance of 13 600 km, the model used in this paper predicts the GWP to be ∼4504 kg of CO2/ton of biodiesel and the total fossil-energy requirement to be 29.4 GJ/ton of biodiesel. These results correspond to a GWP ∼41% larger than fossil-derived diesel used in the U.K. and a fossil-energy requirement ∼32% lower. Further research into the CO2 emissions from cultivating this land is required to achieve
Acknowledgment. The authors are grateful to the following for the information and advice that they provided: Stephen Arundell (Cargill), Thapelo Letete and Bernelle Verster (University of Cape Town), Neville Murray (Biodiesel Centre, South Africa), and Johan Strausse (Elsenburg College, Stellenbosch, South Africa). The support of the Engineering and Physical Sciences Research Council (EPSRC) is gratefully acknowledged.
(61) van Heerden, P. , Private communication. Department of Agriculture: Western Cape, Elsenburg, South Africa, 2008.
(62) South African Petroleum Industry Association (SAPIA). Annual Report. 2008.
4. Conclusions This work shows that the GWP and fossil-energy requirements of biodiesel produced in South Africa vary significantly, depending upon predominantly the quantity of irrigation used, the yield of the crop, and whether or not grassland is converted to arable land to make way for the oilseed crop. The scales at which the biodiesel conversion plants operate and the transport distances involved have little influence. When considering current agricultural practices, there is little variation in the GWP and fossil-energy requirement of biodiesel produced from soybean, canola, or sunflower seeds; however, if recommended practices were employed and target yields of sunflower seeds were reached, the GWP and fossilenergy requirement of biodiesel produced from sunflower seeds could be significantly reduced. For the best case scenario, where no irrigation is used and the land used to grow the crop had previously been idle arable land that was already in cultivation, biodiesel currently produced in South Africa has a GWP between 20 and 25% lower than the fossil-diesel mix currently used in South Africa and a fossilenergy requirement ∼50-60% lower. If the biodiesel were shipped by an oil tanker for use in the U.K., the savings in GWP would reduce to ∼0-10%, while the total fossil-energy savings would be 40-50%. It would therefore be more environmentally beneficial to use the biodiesel to satisfy home demand rather than exporting it to the U.K. For example, if ∼170 000 tons of biodiesel were produced in South Africa each year (representing ∼2 vol % of diesel usage in 200862), for the best case scenario, this would result in a savings of ∼160 000 tons of CO2/year if displacing diesel used in South Africa but only ∼40 000 tons of CO2/year if displacing diesel used in the U.K. If the grassland required cultivation, the GWP and fossilenergy requirement of biodiesel produced in South Africa would increase dramatically, resulting in its use having a higher GWP than fossil-derived diesel used in either the U.K. or South Africa. Irrigation has been shown to be very energy-intensive, and its use would significantly affect the overall GWP and fossil-energy requirement of biodiesel, particularly if borehole irrigation using electrically powered pumps was employed. A high yield of crop is, however, important; therefore, the trade-off between possible increased yields from using irrigation and the environmental burden of the system must be considered for each individual case.
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