Environmental systems analysis of biogas systems Part II: The environmental impact of replacing various reference systems

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1 Biomass and Bioenergy 31 (27) Environmental systems analysis of biogas systems Part II: The environmental impact of replacing various reference systems Pa l Bo rjesson, Maria Berglund Environmental and Energy Systems Studies, Department of Technology and Society, Lund University, Gerdagatan 13, SE Lund, Sweden Received 23 September 25; received in revised form 12 January 27; accepted 15 January 27 Available online 19 March 27 Abstract This paper analyses the overall environmental impact when biogas systems are introduced and replace various reference systems for energy generation, waste management and agricultural production. The analyses are based on Swedish conditions using a life-cycle perspective. The biogas systems included are based on different combinations of raw materials and final use of the biogas produced (heat, power and transportation fuel). A general conclusion is that biogas systems normally lead to environmental improvements, which in some cases are considerable. This is often due to indirect environmental benefits of changed land use and handling of organic waste products (e.g. reduced nitrogen leaching, emissions of ammonia and methane), which often exceed the direct environmental benefits achieved when fossil fuels are replaced by biogas (e.g. reduced emissions of carbon dioxide and air pollutants). Such indirect benefits are seldom considered when biogas is evaluated from an environmental point of view. The environmental impact from different biogas systems can, however, vary significantly due to factors such as the raw materials utilised, energy service provided and reference system replaced. r 27 Elsevier Ltd. All rights reserved. Keywords: Anaerobic digestion; Energy crops; Organic waste; Fossil fuel systems; Life cycle perspective 1. Introduction Anaerobic digestion and biogas production are promising means of achieving both global and local environmental benefits. Biogas is a renewable energy carrier, and the introduction of anaerobic digestion of farm residues and municipal organic waste may reduce potentially negative environmental impact of current agricultural practices and waste handling procedures. The development and implementation of biogas systems in Sweden is being stimulated by both existing and coming governmental incentives. Today, biogas is exempted from energy and environmental taxes, which improves the competitiveness of biogas compared with fossil fuels. Several on-going and planned biogas projects in Sweden have obtained governmental investment grants that aim at speeding up the transition of Sweden to an ecologically sustainable society. In addition, the new national waste handling policy Corresponding author. Tel.: ; fax: address: Pal.Borjesson@miljo.lth.se (P. Börjesson). includes a ban on landfilling with organic waste from the year 25 and an obligation to use biological treatment methods (e.g. anaerobic digestion or composting) of wet organic waste. Previously almost all biogas produced in Sweden was used for heat production in small-scale boilers or in largescale district heating plants. However, there is an increasing interest in the use of biogas as a transportation fuel. One reason is the increasing price of petrol and diesel due to increased raw oil prices and national taxes, e.g. the tax on carbon dioxide, whereas the cost of alternative fuels for heat production, mainly wood fuels used in district heating systems, has not increased to the same extent. The EU directive on alternative transportation fuels is likely to increase the interest in biogas as a transportation fuel further. Today, the total biogas production in Sweden amounts to approximately 5 PJ/year, of which 3 PJ comes from digestion of sewage sludge at waste water treatment plants. Of this total, some.5 PJ is up graded and used for transportation services (e.g. in busses, trucks, taxies and private cars) [1] /$ - see front matter r 27 Elsevier Ltd. All rights reserved. doi:1.116/j.biombioe

2 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) Biogas can also be used for electricity production, preferably in combined heat and power plants with high total conversion efficiency. Combined heat and power production provides higher profitability than stand-alone power production [2]. During 23, an electricity certificate system was introduced in Sweden that aims at increasing the production of renewable electricity, e.g. wind power and biomass-based electricity. The additional revenue from the trade of these certificates increases the profitability of, and thus the interest in the production of biogas-based electricity. The biogas potential in Sweden is estimated to be some 5 PJ/year, which is ten times higher than the current production and corresponds to 3 4% of the current energy consumption in Sweden. Of this total, slightly less than 4 PJ originates from agricultural biomass resources, such as manure, crop residues and dedicated ley crops [1,3]. Digestion of municipal organic waste and organic waste from the food industry could contribute to some 8 PJ, and sewage sludge to 4 PJ/year. The big differences among biogas systems make them complex to study from an environmental point of view. The environmental impact of each system is more or less unique because of the great variety of potential raw materials, digestion technologies, and fields of application for the biogas and digestates produced [4 6]. Furthermore, the total environmental impact of the introduction of a biogas system depends largely on the reference system replaced, concerning energy supply, waste handling and farming practice. Today, there is a lack of broad environmental systems analyses of biogas production systems that include the variation between different biogas fuel chains and the indirect environmental impact of replacing different reference systems. Our research project Energy and Environmental Systems Analyses of Biogas Systems was initiated to fill this gap. The results of the project are presented in two reports (in Swedish) [4,7] and three papers, of which this is the third, analysing the total environmental impact of replacing various reference systems by biogas production systems. The first paper assesses the energy performance of various biogas systems [8], and the second the fuel-cycle emissions from these systems [5]. The aim of this paper is to assess the total environmental impact of the introduction of various biogas systems and the replacement of different reference systems. The analysis includes both the direct environmental effects of replacing various energy carriers and energy systems, and indirect effects of changed handling of raw materials, e.g. waste management and farming practice. The purpose is also to identify factors of major importance for an accurate comparison between a biogas and a reference system, and to identify the most promising application of a biogas system from an environmental point of view. The calculations are aimed to be as transparent as possible in order to make the results useful in future analyses and to enable the reader to make her/his own calculations based on specific local conditions. 2. Methods and assumptions The analysis includes six different end-use technologies, namely large- and small-scale boilers for heat production, large- and small-scale gas turbines for co-generation of heat and power, and heavy- and light-duty vehicles. The biogas production includes six raw materials that are digested in large-scale biogas plants, or in farm-scale biogas plants in the case of agricultural raw materials (see [5,8] and Section 3). The reference systems chosen are assumed to be realistic alternatives to the biogas systems studied, based on current Swedish conditions. The emissions are expressed per energy service unit, that is, heat, heat and power, and kinetic energy for transportation, in order to allow for variations in the conversion efficiency among the final energy services. The functional unit used is 1 MJ of heat, 1 MJ of heat and power and 1 MJ of kinetic energy. This functional unit is chosen in order to account for variations in conversion efficiencies between the biogas system and the reference system compared. The analysis is based on literature reviews and refers mainly to Swedish conditions and state-of-the-art technologies. An extensive description of our calculations on fuelcycle emissions from various biogas systems and the system boundaries applied is given in [4,5,7,8]. Data on fuel-cycle emissions from the reference systems are based on previously published studies; a summary of the emissions assumed is given in the Appendix. The analysis includes both fuel-cycle emissions and indirect environmental effects. Fuel-cycle emissions are defined as emissions from the production and final use of energy carriers. The indirect environmental effects are here defined to be caused by emissions that are not directly related to the energy conversion in the systems, for example, changed emissions of ammonia and nitrous oxide from arable land and leakage of nitrate due to changed farming practice, or emissions of methane, ammonia and nitrous oxide from the storage of manure (see Section 4). The electricity used in the systems studied is assumed to be produced in condensing plants using natural gas, reflecting the estimated, long-term, marginal production of electricity in the Scandinavian countries [9]. Thus, the fuel-cycle emissions from the reference systems have been recalculated when other sources of electricity have been assumed in the studies quoted. The heat used in the biogas plants is assumed to be produced from biogas, based on present conditions at Swedish biogas plants. Uncontrolled losses of methane from the production of biogas is here assumed to correspond to 1% of the biogas produced when the biogas is used for heat or combined heat and power production, and 2% when the biogas is upgraded and used as a transportation fuel [1]. Uncontrolled losses of methane will also increase the emission of other pollutants due to the corresponding reduction in energy efficiency in the biogas systems. Energy crops are assumed to be cultivated on set-aside arable land, and hence replacement of food or fodder

3 328 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) kg raw material or hectares of arable land Biogas system Reference system based on fossil fuels Energy input Energy input Prod. of chemical fertilizers Alternative handling of the raw materials or arable land lies fallow Prod. of fossil fuels System enlargement System boundary System boundary Energy conversion kg plant nutrients 1 MJ of energy service kg plant nutrients 1 MJ of energy service Fig. 1. Comparing biogas systems and reference systems based on fossil fuels. production is not included in the study. Other raw materials are assumed to be waste products. Consequently, the analysis includes only the additional energy input and emission associated with the handling and transport of these waste products, and none of the input used in the production of the main product. The potential need to expand the system boundaries and system enlargement is taken into account in the analysis. is here assumed to be required when there are differences between the biogas system and reference system studied as regards total energy output per tonne of raw material or hectare of arable land, or reuse of plant nutrients (see Section 5). When the alternative handling of the raw materials does not generate any usable energy service or arable land lies fallow, fossil fuels are assumed to be used in the reference system to provide the same energy output as in the biogas system (Fig. 1). The analysis also includes reference systems that are based on bioenergy (e.g. combustion of raw materials or cultivation of willow, (Fig. 2). In these reference systems, less raw materials or arable land is generally needed to provide the same energy service as in the corresponding biogas system due to differences in conversion losses. This difference is assumed to be compensated for by an additional use of fossil fuels in the biogas system. Any difference regarding output of plant nutrients between two systems compared is assumed to be compensated for by an additional production and utilisation of chemical fertilisers in the system that provides less organic fertiliser available for recirculation. The analysis does not comprise other potential benefits of recycled organic matter. The analysis includes emissions of carbon dioxide of fossil origin (CO 2 ), carbon oxide (CO), nitrogen oxides (NO x ), sulphur dioxide (SO 2 ), hydrocarbons, except for methane (HC), methane (CH 4 ), particles, ammonia (NH 3 ) and nitrous oxide (N 2 O). Emissions to water include nitrate (NO 3 ). These emissions are classified into the following environmental impact categories: global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), and photochemical oxidant creation potential (POCP). The category indicators used are given in the Appendix. 3. Comparison between different biogas and reference systems An overview of the biogas and reference systems compared is given in Table 1. The reference systems included are assumed to represent an efficient and an inefficient system, respectively, from a land use and resource utilisation perspective. Methanol from lignocellulosic biomass (willow) is assumed to represent a liquid biofuel produced by gasification from which also other similar transportation fuels could be synthesised Ley crop-based biogas is compared with reference systems based on willow or fossil fuels, the latter implying that the arable land lies fallow. The fossil fuelbased reference systems are assumed not to include any extra energy input for the management of the

4 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) Biogas system kg raw material or hectares of arable land Reference system based on bioenergy Energy input Prod. of additional fossil fuels Energy input Prod. of chemical fertilizers Alternative handling of the raw materials or cultivation of energy crops System enlargement System boundary Energy conversion System boundary Energy conversion kg plant nutrients 1 MJ of energy service kg plant nutrients 1 MJ of energy service Fig. 2. Comparing biogas systems and reference systems based on bioenergy. Table 1 Overview of the comparisons carried out between biogas systems and reference systems a Biogas system Heat Heat and electricity Heavy- and light-duty vehicles Reference system b Fallow land & fossil fuel L & F L & F L Willow c L L Not recovered & fossil fuel L & F L & F L L & F Tops and leaves of sugar beets (s-b tops) Not recovered & fossil fuel L & F L & F L Conventional storage & fossil fuel L & F L & F L Municipal organic waste (MOW) and food industry waste (FIW) Composting & fossil fuel L L L L Letters indicate whether large-scale ( L ) and/or farm-scale ( F ) biogas systems were investigated. a Fossil fuel refers to fuel oil (concerning heat production), natural gas (combined heat and power production), petrol (light-duty vehicles) and diesel (heavy-duty vehicles). b Acronyms within parentheses are also used in Figs c Wood chips from willow are assumed to be used in the heat reference system, whereas methanol from willow is assumed to be used in the transportation alternatives. fallow land. Emissions from the reference systems based on willow are calculated for the cultivation and harvesting of the crop and transportation to the incineration or methanol production plant. The comparisons also include differences in field emissions of NO 3 (see Section 4) based biogas systems are compared with reference systems that use straw for heat production or fossil fuels. The same collection and transportation modes are assumed to be used in both the biogas systems and the

5 33 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) biomass-based reference systems. is not collected in the fossil fuel-based systems. Leaving straw in the field is not assumed to increase the diesel consumption for ploughing, or to significantly affect the nutrient leaching or the soil organic matter content compared to the biogas system Tops and leaves of sugar beets Biogas systems based on tops and leaves of sugar beets are compared with reference systems based on fossil fuels. The reference systems do not include harvesting or handling of the cropping residues. Leaving the residues in the field is assumed not to increase the diesel consumption for ploughing etc. The comparisons include field emissions of NH 3 and NO 3 (see Section 4) Biogas systems based on liquid manure from pigs are compared with reference systems based on fossil fuels, which also include spreading of the manure on arable land. The diesel consumption and fuel-cycle emissions are assumed to be the same for the spreading of 1 tonne of manure as for the spreading of 1 tonne of digestate. The comparisons also include field emissions of NH 3,N 2 O and NO 3, and emission of NH 3 and CH 4 from the storage of the manure (see Section 4) Municipal organic waste and food industry waste Biogas production from municipal organic waste and food industry waste is compared with reference systems based on the combustion of the waste in large-scale incineration plants, or with reference systems based on fossil fuels for energy production purposes and largescale composting of the waste. All reference systems include the collection and transportation of the waste to the incineration plant or composting facility. The same mode for collecting and transporting the waste is assumed to be used in the biogas systems as in the reference systems. Ash produced in the combustion of the waste is assumed to be used as landfill. However, the analysis does not include emissions from the transportation and landfilling of the ash or leakages from the landfill. The composting includes the energy used in the mixing of the compost and the emission of NH 3,N 2 OandCH 4 from the biological decomposing processes (see Section 4). The diesel consumed for tractor operations at the composting site is estimated to be, on average, 15 MJ/ tonne of waste [11]. The fuel-cycle emissions from these operations are given in the Appendix. The compost produced is assumed to be used as fertiliser. 4. Indirect environmental effects of replacing various waste handling and cropping systems An introduction of biogas systems may lead to indirect environmental effects that are not direct results of the replacement of other energy systems. Such indirect effects are seldom considered in environmental analyses of biogas systems, though they can affect the results significantly. The potential indirect effects considered here are divided into the following categories: (i) changed emissions from the handling and storage of raw materials and digestate, and (ii) changed nutrient leaching due to changed cropping practices Changed emissions from the handling and storage of raw materials and digestate Liquid manure Storage of liquid manure leads to spontaneous emissions of methane and ammonia, which can be reduced when the digestion of manure and the collection of the biogas produced replace conventional storage systems for manure. Calculations based on Danish conditions show that the emissions of CH 4 may be reduced, on average, by 1.6 kg CH 4 /tonne of pig slurry, or from 3.1 to 1.5 kg [12]. The manure is here assumed to be stored in the pig house for 15 days, based on current, average conditions. Reducing the storage time to 1 day would reduce the emissions to approximately 15% of the current levels. The emission of CH 4 is also affected by the temperature, duration of the storage, precipitation, content of straw etc. [13,14]. The reduction in NH 3 emissions when introducing biogas production is here estimated to amount to, on average, 1 g of NH 3 /tonne of manure [15]. This is equivalent to about 2% of the emissions from an uncovered storage tank. However, the conventional storage of manure is here assumed to be in tanks using semi-permeable cover materials, such as straw, Leca granules etc., reducing the emissions of NH 3 by 7 85%. Support for the assumption that N 2 O emissions are reduced by this type of storage has not been found in the literature. The emissions of CH 4 will also be reduced from manure tanks with semi-permeable cover sheets, but mainly temporarily, since a large part of the CH 4 produced during storage is released when the manure is pumped and spread on the fields. About 3 4% of the methane produced during storage may be oxidised into carbon dioxide when passing through the semi-permeable cover sheet [16]. Spreading of digested manure is here assumed to increase the emission of NH 3 slightly, and decrease the emission of N 2 O, compared with undigested manure [17]. Digested manure contains less easily decomposed organic matter than conventional manure, and thus less energy for the nitrous oxide-forming microorganisms, leading to an estimated reduction, on average, from 4 to 25 g N 2 O/ tonne of manure [16,18]. The increased emission of NH 3,

6 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) here assumed to be, on average, from 25 to 31 g NH 3 / tonne when using efficient spreading technology, is due to the higher content of ammonium in digested manure, which potentially can be converted into NH 3 [19] Tops and leaves of sugar beets Tops and leaves of sugar beets left on the field after harvest contain approximately 1 16 kg N/hectare. Approximately 2 4% of the nitrogen can be lost to the next cropping season through emissions of ammonia and nitrogen gas to the atmosphere, and leakage of nitrate into the ground water [2]. On average 3 tonne dry matter of tops and leaves of sugar beets are estimated to be available to be harvested for biogas production per hectare and year. This harvest is estimated to reduce the nitrogen losses by 3 kg N/hectare and year, of which one-third is estimated to be in the form of ammonia, equivalent to 77 g NH 3 / tonne of tops and leaves of sugar beets Composting of organic waste Composting of organic waste causes biological emissions of NH 3, N 2 O and CH 4 (Table 2). The total losses of nitrogen are estimated to be, on average, 35% of the nitrogen content in the organic waste, based on current composting technology, and excluding gas-cleaning equipment [18]. Emissions of ammonia are assumed to contribute to 93% and nitrous gas to 5% of the total nitrogen emissions. The emissions of methane are assumed to correspond to.35% of the total emissions of carbon dioxide. Table 2 Biological emissions from large-scale composting of organic waste a NH 3 (kg/tonne) N 2 O (g/tonne) Municipal organic waste Food industry waste a Based on [11,18]. Excluding external gas cleaning. CH 4 (g/tonne) 4.2. Changed nutrient leaching from changed cropping practices The potential nutrient leaching from arable land depends on various factors, such as cropping systems, fertilisation strategies, precipitation and soil type. An introduction of biogas systems is here assumed to affect the nutrient leaching in the following cases: (i) changed cropping systems, including land lying fallow, cultivation of ley crops or of willow, (ii) recovery of tops and leaves of sugar beets, and (iii) replacement of undigested liquid manure by digested manure as a fertiliser. Recovery of straw is here assumed not to affect the nutrient leaching since straw contains little nitrogen. Further, long-term field trials show small variations in nitrogen leaching between recovery and non-recovery of straw. Land lying fallow is here estimated to cause the lowest nitrogen leaching, followed by ley crop cultivation and thereafter willow cultivation (Table 3). Nitrogen leaching from fallow land refers to long-term fallow periods (several years) without soil tillage. The figures on nitrogen leaching are based on average conditions in southern Sweden, where the leakage may vary from about 1 up to 6 kg N/hectare and year depending on local conditions [21]. Thus, the assumptions made here about changes in nitrogen leaching are rough, but represent modest changes. Recovery of tops and leaves of sugar beets is estimated to reduce the nitrogen leaching by 25 3%, based on results from field trials in southern Sweden [22]. Digestion of liquid manure increases its quality as a fertiliser since organic-bound nitrogen is converted into ammonium available to plants. Increased concentration of ammonium allows for a higher precision in fertilisation and thereby reduced risk of nitrogen leakage. Normally, the amount of ammonium corresponds to 7% of the total nitrogen content in undigested manure, whereas it is about 85% in digested manure [12]. Results from field trails in southern Sweden show that the nitrogen leaching may be reduced by about 2% when digested manure replaces undigested manure [23]. Table 3 Estimations of changed nitrogen leaching from changed cropping practices Changed nitrogen leaching (kg N/hectare, year) ðgno 3 =tonneþ replace fallow land a replace willow a 5 75 Tops and leaves of sugar beets are recovered instead of left on the field b 1 3 Digested liquid manure replaces conventional liquid manure c Increased leaching is indicated by (+), and decreased leaching by ( ). a Based on [21] (referring to fallow) and [24] (willow). The ley crop yield amounts to 3 tonne/hectare and year [8]. b Based on [21,22]. The yield of tops and leaves of sugar beets amounts to 16 tonne/hectare and year [8]. c Based on [23]. The application rate of manure amounts to 3 tonne/hectare and year [8].

7 332 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) Table 4 Additional need of fossil fuels in biogas systems to compensate for a lower output of energy services per hectare of arable land or per tonne of raw material than in the biomass-based reference systems a Table 5 Additional production of commercial fertilisers in the reference systems to compensate for improved utilisation of plant nutrients in the biogas system due to utilisation of the digestates as fertilisers a Biogas system/reference system Proportions of energy carriers for the end-use in the biogas system Nitrogen (kg/ tonne raw material) Phosphorus (kg/tonne raw material) End-use application 5. Biogas (%) /willow b Heat, large-scale Light-duty vehicles 7 3 Heavy-duty vehicles /incineration of straw c Heat, large-scale Heat, small-scale 5 5 Food industry waste/incineration of the waste d Heat, large-scale 8 2 Municipal organic waste/incineration of the waste d Heat, large-scale 6 4 Fossil fuel (%) a Fuel oil is assumed to be used for the additional heat production, and petrol and diesel for the additional transport in light-duty and heavy-duty vehicles, respectively. Conversion efficiencies and fuel-cycle emissions from the use of fossil fuels are given in the Appendix. b The biomass yield is assumed to be 9 tonne dry matter/hectare of willow, and 6.8 tonne dry matter/hectare of ley crops. The heat value of willow is 18.7 GJ/tonne dry matter, and the methanol production is assumed to correspond to 9.4 GJ/tonne dry matter. The biogas yield from ley crops is 1.6 GJ/tonne dry matter [8,25,26]. c The heat value of straw is 17.6 GJ/tonne dry matter, and the biogas yield 7.1 GJ/tonne dry matter [8,25]. d The heat value of organic waste is estimated to be 21 GJ/tonne dry matter, and the biogas yield to 16 and 12.4 GJ/tonne dry matter of food industry waste and municipal organic waste, respectively [8,11]. It is crucial that the system boundaries are determined in a coherent way in order to make the comparisons between energy systems accurate. An expansion of the system boundaries is required when there are differences between the systems compared concerning the efficiency in land use or energy output per tonne of biomass, and differences in the utilisation of plant nutrients Differences in land use and in energy output efficiency The willow yield (dry matter) is estimated to be, on average, 3% higher per hectare than the ley crop yield [25]. In addition, the heat output is estimated to be approximately 7% higher per dry tonne of willow than per dry tonne of ley crops used for biogas production. of straw and organic waste is also assumed to provide high heat output compared to the digestion of the raw materials. The difference between the systems is comparatively small when willow is used for methanol Willow or fallow Tops and leaves of sugar beets Not recovered 1.3 Conventional storage.51 Food industry waste Composting a Municipal organic waste Composting a a Refers to large-scale composting without gas cleaning. All of the phosphorus in the compost is assumed to be available for recycling, whereas the nitrogen losses are significant from composting and a comparatively low share of the nitrogen is plant available (see the Appendix). production since the methanol production route is less energy efficient than the biogas production route. Difference in energy output per hectare or tonne of raw material is assumed to be compensated for by an additional use of fuel oil in the system that provides less energy output (see Table 4) Differences in the utilisation of plant nutrient Improved efficiency in the utilisation of plant nutrients due to digestion of raw materials and utilisation of the digestates as fertilisers is here assumed to be balanced by an additional production of commercial fertilisers in the reference systems, presupposing the same output of plantavailable N and P in both systems (Table 5). Energy use and emissions from the production of commercial fertilisers are given in the Appendix. All phosphorus in the digestate is assumed to be plant available, whereas the amount of plant-available nitrogen is assumed to be 85% in digestates from manure, and 7% in the digestates from the other raw materials. 6. Changed environmental impact of introducing various biogas systems The following (Figs. 3 8) gives an overview of the results of the comparisons carried out. Each group of two columns represents one comparison, the left column expressing the emission from the biogas systems, and the right the emissions from the corresponding reference system. Comparisons that include reference systems based on fossil

8 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO 2 -eq./mj heat) Reference systems based on oil Suger beet tops Reference system based on bioenergy EP (g PO 4- eq./mj heat) Suger beet tops AP(g SO 2 -eq./mj heat) Leycrops POCP (mg C 2 H 2 -eq/mj heat) Leycrops Suger beet tops Particles (mg particles/mj heat) Suger beet tops Biogas system: Production of biogas Indirect environmental effects Reference system: Production of energy carriers Additional handling of raw materials Fig. 3. Life-cycle emissions from small-scale heat production. fuels are given to the left in the diagrams, and the bioenergy-based comparisons to the right. The category system enlargement refers in the biogas systems to emissions from the production and final use of additional fossil fuels needed to achieve an equivalent energy output per tonne of raw materials or per hectare. In the reference systems it refers to emissions from additional production of commercial fertilisers needed to achieve an equivalent output of plant nutrients from the systems. The category Additional handling of raw materials is used in the reference system to account for emissions from the handling of raw materials that are not used for energy production purposes, for example, emissions from the spreading of manure and composting of waste products. Emissions that are defined as causing indirect environmental effects are given as the difference between the systems compared. These differences are always assigned to the system that causes the highest indirect environmental impact Heat The emissions of greenhouse gases per MJ heat is calculated to decrease by approximately 75 9% when biogas-based heat replaces fossil fuel based, the highest reductions being from the replacement of handling manure

9 334 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO 2 -eq./mj heat) AP (gso 2 -eq./mj heat) Reference systems based on oil Reference systems based on bioenergy Willow EP (g PO 4- eq./mj heat) POCP (mgc 2 H 2- eq/mj heat) Willow Willow Willow Particles (mg particles/mj heat) 2 1 Willow Biogas system: Production of biogas Indirect environmental effects Reference system: Production ofenergy carriers Additional handling of raw materials Fig. 4. Life-cycle emissions from large-scale heat production. and food industry waste. On the other hand, replacing biomass-based heat production is calculated to increase the emissions of greenhouse gases by approximately 5 5% when replacing combustion of willow and organic waste, or even up to 4 times when biogas replaces large-scale combustion of straw. This is mainly due to the decreased heat output per tonne of raw materials which require additional use of fossil fuels for the heat production in the biogas system. In the biogas systems, emissions of CO 2 contribute to about 6 75% and CH 4 to 25 4% of the life-cycle emissions of greenhouse gases. In the fossil fuel-based reference systems, CO 2 contributes to almost all emissions of greenhouse gases, except for systems including storage of manure in which losses of CH 4 and N 2 O contribute to about 45% of the life-cycle emissions. Emissions of N 2 O can also be of great importance when chemical fertilisers are used in the cultivation of energy crops or when additional production of chemical fertilisers is needed in the reference systems, for example, combustion of willow or organic waste. The calculations show large variations in the AP and EP among the comparisons carried out, especially when the indirect environmental effects are included. The indirect environmental effects are mainly caused by emissions of NH 3 from the composting of waste, field emissions of NO 3 from the use of manure, and field emissions of NO 3 and NH 3 from the handling of tops and leaves of sugar beets.

10 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO 2- eq./mj heat & power) AP (g SO 2- eq./mj heat & power) Reference systems based on natural gas Fallow+ Fallow+ EP (g PO 4- eq./mj heat & power) POCP (mgc 2 H 2- eq/mj heat & power) Fallow+ Fallow+ Particles (mg particles/mj heat & power) Fallow+ Biogas system: Production of biogas Indirect environmental effects Reference system: Production ofenergy carriers Additional handling of raw materials Fig. 5. Life-cycle emissions from small-scale cogeneration of heat and power. Using these raw materials for biogas production and to replace fossil fuel-based reference systems is estimated to reduce the AP and EP by up to 95%. The differences between the biomass-based reference systems and their equivalent biogas systems are not as large, or typically less than 75%. The POCP increases typically by 2 7% when biogasbased heat replaces combustion of waste or fossil fuelbased heat production. However, the emissions of POCP are reduced when the incineration of willow and straw, in which incomplete combustion causes high emissions of CO and HC, is replaced or when conventional storage of manure and composting of waste, which cause high emissions of CH 4, are replaced. Biogas-based heat is calculated to always decrease the emissions of particles in the comparisons carried out, typically by between 3% and 7%. Replacing the combustion of straw is calculated to decrease the emissions of particles even further, or by up to 9% Heat and power The environmental effects of introducing biogas for the cogeneration of heat and power are similar to those achieved when biogas is used for stand-alone heat production from fossil fuel (see previous section). However, the benefits of this introduction is generally somewhat smaller since the end-use emissions are normally lower

11 336 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO 2- eq./mj heat & power) 15 Reference systems based on natural gas 1 5 Fallow+ Compost.+ Compost.+ EP(g PO 4- eq./mj heat & power) Fallow+ Fallow+ Compost.+ Compost.+ Fallow+ Compost.+ Compost.+ Compost.+ Compost.+ AP (g SO 2- eq./mj heat & power) POCP (mgc 2 H 2- eq/mj heat & power) Particles (mg particles/mjheat & power) Fallow+ Compost.+ Compost Biogas system: Production of biogas Indirect environmental effects Reference system: Production ofenergy carriers Additional handling of raw materials Fig. 6. Life-cycle emissions from large-scale cogeneration of heat and power. from natural gas-based heat and power production than from corresponding heat production based on fuel oil Transportation The contribution to the GWP will normally be reduced by between 5% and 8% when biogas replaces petrol and diesel as a transportation fuel in light- and heavy-duty vehicles. On the other hand, using ley crop-based biogas to replace methanol from willow is calculated to increase the greenhouse gas emission by 3 5%, mainly due to the need for additional petrol or diesel in the biogas system to compensate for the lower energy output per hectare. Concerning the contribution to the AP and EP, the effects of introducing biogas as a transportation fuel are similar to those achieved when biogas is used for heat or combined heat and power production (see previous sections). The life-cycle emissions are substantially reduced when biogas from tops and leaves of sugar beets, manure and organic waste are utilised, mainly due to the reduced emissions of NO 3 and NH 3. Biogas from ley crops and straw will also lead to a reduced contribution to the AP and EP when diesel is replaced in heavy-duty vehicles, whereas using biogas from these raw materials to replace petrol in light-duty vehicles may lead to an increased contribution.

12 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO 2- eq./mj kinetic 8 4 Fallow+petrol Reference systems based on petrol Compost.+petrol Compost.+ petrol Reference system based on bioenergy EP (g PO 4- eq./mj kinetic Fallow+petrol Compost.+petrol Compost.+ petrol AP (gso 2- eq./mjkinetic 1 5 POCP (mg C 2 H 2- eq/mj kinetic Fallow+petrol Compost.+petrol Compost.+ petrol Particles (mg particles/mj kinetic Fallow+petrol Compost.+petrol Compost.+ petrol Fallow+petrol Compost.+petrol Compost.+ petrol Biogas system: Production of biogas Indirect environmental effects Reference system: Production ofenergy carriers Additional handling of raw materials Fig. 7. Life-cycle emissions from transportation fuels used in light-duty vehicles. The contribution to the POCP will be reduced by approximately 5 7% when biogas is used in light-duty vehicles and by about 2 65% when biogas is used in heavy-duty vehicles. The reduction will be even greater, about 8%, when biogas from manure is utilised. The life-cycle emissions of particles will be reduced further when biogas replaces diesel in heavy-duty vehicles, about 5 8%, than when biogas replaces petrol in lightduty vehicles, about 15 6%. However, when ley cropbased biogas replaces methanol from willow, the life-cycle emissions of particles may increase by 1 3%. 7. Sensitivity analysis Several factors included in this environmental analysis have been identified to significantly affect the result. Some of these factors are based on rather uncertain input data, mainly due to differences in local conditions or limited knowledge due to lack of data from monitoring and measurements, experiments, field trials etc. In the following section the importance of various critical factors in different environmental impact categories is analysed further.

13 338 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) GWP (g CO2-eq./MJ kinetic Reference systems based on diesel Reference system based on bioenergy EP(gPO4-eq./MJkinetic 1.5 Fallow+diesel Fallow+diesel Fallow+diesel Fallow+diesel AP(gSO2-eq./MJkinetic POCP (mg C2H2-eq/MJ kinetic as Fallow+diesel Particles (mg particles/mj kinetic Biogas system: Production of biogas Indirect environmental effects Reference system: Production ofenergy carriers Additional handling of raw materials Fig. 8. Life-cycle emissions from transportation fuels used in heavy-duty vehicles GWP losses of methane Biogas production causes uncontrolled losses of methane, which are normally of small magnitude in well functioning biogas systems, but can be significant in systems with defective technology. Even moderate losses of CH 4 can affect the GWP significantly since CH 4 is a 21-times more potent greenhouse gas than CO 2. In our calculations the uncontrolled losses of CH 4 are estimated to be 1% of the biogas produced when used for heat or combined heat and power production, and 2% when the biogas is used as a transportation fuel (see Section 4). The biogas produced in the storage of digestates is reported to correspond to 5 1%, or even up to 2%, of the total amount of biogas produced at biogas plants. Recent measurements of emissions from some Swedish biogas production plants indicate that the uncontrolled losses of methane may amount to about.5 1% of the total flow of methane at the plant. Losses of CH 4 during upgrading and pressurisation of the biogas have been reported to typically correspond to.2 2%, in some cases and even up to 13% of the total amount of gas treated at the upgrading plant.

14 P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) Table 6 The amount of methane that has to be lost from biogas systems to result in an equivalent contribution to the GWP as from the reference system a Small-scale boilers (%) Large-scale boilers (%) Microturbine (%) Large-scale turbine (%) Light-duty vehicles (%) Heavy-duty vehicles (%) Fossil fuel Willow Fossil fuel of straw Tops and leaves of sugar beets Fossil fuel Fossil fuel Food industry waste Fossil fuel of the waste Municipal organic waste Fossil fuel of the waste a indicates that the emissions of greenhouse gases are higher from the biogas system than from the reference system, even without losses of methane from the biogas systems. The degree of losses can be due to the upgrading technology, the required CH 4 content of the upgraded gas, and occasional uncontrolled leakages [1,27]. The need for biogas for energy purposes might occasionally be limited, leading to deliberate release of CH 4. For instance, experiences from Sweden show that the production of biogas may exceed demand by 3% during the summer season due to limited needs for heat in the residential sector during this period. The environmental impact of deliberate release of CH 4 can be reduced by flaring the excess biogas. The effects of increased losses of CH 4 from biogas systems are shown in Table 6, expressed as the point at which losses of CH 4 cause an equivalent contribution to the GWP as the reference system. The losses of CH 4 may typically amount to about 1 2% of the biogas produced, or even up to 3% for manure-based biogas, before the emissions of greenhouse gases from the biogas systems exceed those from the reference systems. The emissions of greenhouse gases are always lower from the biomass-based reference systems than from the corresponding biogas systems, even without losses of CH 4 from the biogas systems Eutrophication and acidification The eutrophication and acidification are significantly affected by the variations in indirect emissions of nitrate (NO 3 ) to water and ammonia (NH 3) to air caused by changed cropping practices and handling of the wastes and residues. The assumptions made here about these indirect emissions are uncertain (see Section 4.2). The actual nitrate leakage can vary greatly depending on location, and reliable input data are limited due to the lack of long-term field trials dedicated to monitor the specific aspects analysed here. The estimations on the emissions of ammonia are also uncertain, as the assumptions are based on limited input data and specific technologies (see Section 4.1.2). For example, the emission of ammonia from composting may be significantly reduced, by up to 8%, if the composting facility uses gas-cleaning equipment [18]. On the other hand, defective composting technology without gas cleaning in combination with a decreased carbon/nitrogen-ratio in the waste may lead to increased emission of ammonia (see Fig. 9) [28]. Fig. 9 shows also the effects on the eutrophication and AP of assuming half or twice the difference in nitrate leakage, and emissions of ammonia from tops and leaves of sugar beets, as assumed in the base case for large-scale heat production Photochemical oxidants and particles The production phase is normally the main contributor to the POCP in biogas systems, except when biogas is used to replace petrol as a transportation fuel in light-duty vehicles. HC are normally the major contributor to the POCP, by about 5 7%, whereas the contribution from CH 4 is about 15 4%. Most of the HC are emitted by tractors and trucks used in the biogas production chain. However, there are large variations among different types of tractors as well as between different tractor operations regarding emissions of hydrocarbons. Concerning the reference systems, the end-use emissions are normally the main contributor to the POCP of which HC often stands for 6 9% of the emissions, except for combustion of straw where CO is the main contributor. Thus, the emissions of POCP can be reduced significantly by

15 34 ARTICLE IN PRESS P. Börjesson, M. Berglund / Biomass and Bioenergy 31 (27) AP (g SO 2- eq./mj heat) NH 3 NO 3 - EP (g PO 4- eq./mj heat) NH 3 NO 3 - Not recovered Composting Composting Fallow Willow Not recovered Conv. storage Not recovered Composting Composting Fallow Willow Not recovered Conv. storage Biogas system Indirect environmental effects Fuel-cycle emissions Reference system Indirect environmental effects Fuel-cycle emissions Fig. 9. Effects on the contribution to the eutrophication and acidification potentials of assuming half or twice the difference in nitrate leakage, and emissions of ammonia from tops and leaves of sugar beets, as assumed in the base case of large-scale heat production. Corresponding changes in emissions of ammonia from the composting of organic waste are 8% decrease and 4% increase. improved end-use and exhaust gas cleaning technologies. In reference systems including manure, this contributes to the emissions of CH 4 by 35 9% of the POCP. The level of emission of CH 4 from the handling and storage of the manure thereby significantly influences the contribution to the POCP in these systems (similar to the GWP discussed in Section 7.1). Most of the emissions of particles from biogas systems normally originate from the production phase. As in the case of POCP, the life-cycle emissions of particles are relatively sensitive to assumed emissions from tractors and trucks used in the biogas production chain. In the reference systems, the main contributor is normally the end-use, except for combined heat and power production based on natural gas. Concerning heavy-duty vehicles using diesel, for example, up to 9% of the life-cycle emissions of particles derive from the end-use. The emission levels may be reduced significantly compared to the state-of-the-art technology assumed here by improved exhaust gas cleaning equipment, such as particle filters. 8. Conclusions and discussion Promotion of biogas systems is here shown to have the potential to be an effective strategy in combating several of today s serious environmental problems, not only climate change but also eutrophication, acidification and air pollution. An introduction of biogas systems may lead to direct benefits such as reduced emissions of air pollutants when fossil fuels are replaced, but also indirect benefits from changed land use and handling of organic byproducts. The indirect benefits can be the most important, for example, the reduced nitrogen leaching and emissions of ammonia and methane achieved when manure, crop residues, and organic waste are utilised for biogas production. However, when biogas systems replace other bioenergy systems, such as willow or straw for heat or methanol production, or combustion of organic waste for heat recovery, the emissions of, for example, greenhouse gases may increase. Thus, in order to maximise the various potential benefits, and to minimise potential negative effects, it is crucial that biogas systems are designed and located wisely. This paper shows that the environmental impact of introducing biogas can vary significantly due to the raw materials digested, the energy service provided and the reference systems replaced. This complexity calls for special attention to the methodology employed, to reach fair and consistent comparisons, that is, in setting the system boundaries. For example, the results are affected strongly by the assumptions made about the need for systems enlargement to attain equivalent outputs of energy services and plant nutrients. Another conclusion is that there are various uncertainties regarding the availability of accurate input data, assumptions about technologies and geographical conditions. In order to achieve more secure results in future environmental studies of biogas systems, these need to be based on data referring to the specific local conditions valid for the actual biogas system. Acknowledgement We gratefully acknowledge the economic support provided by Go teborg Energi AB. Appendix See Tables A1 A6.