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Assessment of energy performance in the life-cycle of biogas production


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Biomass and Bioenergy 30 (2006) 254–266 www.elsevier.com/locate/biombioe

Assessment of energy performance in the life-cycle of biogas production
? Maria Berglund?, Pal Borjesson ¨
Environmental and Energy Systems Studies LTH, Lund University, Gerdagatan 13, SE-223 62 Lund, Sweden Received 20 September 2004; received in revised form 18 November 2005; accepted 21 November 2005 Available online 19 January 2006

Abstract Energy balances are analysed from a life-cycle perspective for biogas systems based on 8 different raw materials. The analysis is based on published data and relates to Swedish conditions. The results show that the energy input into biogas systems (i.e. large-scale biogas plants) overall corresponds to 20–40% (on average approximately 30%) of the energy content in the biogas produced. The net energy output turns negative when transport distances exceed approximately 200 km (manure), or up to 700 km (slaughterhouse waste). Large variations exist in energy ef?ciency among the biogas systems studied. These variations depend both on the properties of the raw materials studied and on the system design and allocation methods chosen. The net energy output from biogas systems based on raw materials that have high water content and low biogas yield (e.g. manure) is relatively low. When energy-demanding handling of the raw materials is required, the energy input increases signi?cantly. For instance, in a ley crop-based biogas system, the ley cropping alone corresponds to approximately 40% of the energy input. Overall, operation of the biogas plant is the most energy-demanding process, corresponding to 40–80% of the energy input into the systems. Thus, the results are substantially affected by the assumptions made about the allocation of a plant’s entire energy demand among raw materials, e.g. regarding biogas yield or need of additional water for dilution. r 2005 Elsevier Ltd. All rights reserved.
Keywords: Energy analysis; Biogas; Biogas production system; Anaerobic digestion; Energy input; Energy balance

1. Introduction Anaerobic digestion and biogas production are promising means of producing an energy carrier from renewable resources and of achieving multiple environmental bene?ts. The net biogas potential in Sweden, for example, is estimated to be some 17 TWh per annum. Of this total, 14 TWh originates from agricultural raw materials, primarily straw, ley crops, and manure [1]. Using the digestate as a fertiliser on arable land can improve the utilisation of plant nutrients since (i) it makes plant nutrients in waste from urban areas available for recirculation and (ii) digestion of agricultural residues facilitates a more ef?cient use of their plant nutrients. In addition, the coming ban in Sweden on land?lling organic waste implies a need for the development of alternative waste management systems such as anaerobic digestion. There are also targets within
?Corresponding author. Tel.: +46 46 2229840; fax: +46 46 2228644.

E-mail address: Maria.Berglund@miljo.lth.se (M. Berglund). 0961-9534/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2005.11.011

the EU on reducing the amounts of biodegradable waste going to land?lls [2]. Several on-going and planned biogas projects in Sweden have obtained grants from Sweden’s Local Investment Programmes (LIP), which aim at speeding up the transition to an ecologically sustainable society in Sweden. LIP has now been replaced by the Climate Investment Programmes (KLIMP), which aim at reducing the Swedish emissions of greenhouse gases [3]. In energy and environmental system analysis, all energy and material input and outputs in a product’s life-cycle are identi?ed and quanti?ed. However, from a system analysis perspective, production systems for biogas are complex to study. The number of possible biogas systems is large due to the variety of available raw materials, digestion technologies and ?elds of application for the digestate and the biogas produced. Among the raw materials available are organic waste from the food industry, municipal organic waste, agricultural harvesting residues, manure, and ley crops. Only a few of them are currently being used for energy production. Biogas can be used for

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Industrial organic waste, food waste Municipal organic waste

heating, electricity generation, as a vehicle fuel or it can be distributed on the natural-gas grid. Furthermore, indirect energy or environmental impacts can be of great importance, for instance if an introduction of digestates as fertiliser on arable lands reduces the need for chemical fertilisers. Several Swedish systems analyses have been carried out that include biogas production. For example, studies have been done to compare various means of waste management, mainly incineration, land?lling, composting and anaerobic digestion [4–7]. Other studies have focused on speci?c biogas plants or raw materials, such as energy crops or common reed [8–11]. However, so far no general systems analysis of biogas production has been undertaken that combines all the various kinds of raw materials, digestion technologies and ?elds of application for the biogas produced. Therefore, we addressed this issue in our project ‘‘Energy and Environmental Systems Analyses of Biogas Systems’’. This paper is based on the results of the ?rst part of the project; a more detailed report (in Swedish) has been published [12]. The second part of the project focuses on environmental aspects of biogas production [13–15]. The aim of this paper is to assess the energy balance in biogas production, and to describe how the net energy output from the biogas systems is affected by the raw materials digested, the system design, and the allocation method chosen. The calculations are also meant to be as general and transparent as possible to make the results transparent and to enable the reader to make her/his own calculations.

Energy crops – ley crops

Harvest residues – tops and leaves of sugar beet; straw

Manure

Cultivation Harvesting Transport of raw materials Energy
System boundary

Recovery

Collection

Anaerobic digestion process Transport of digestate Spreading of digestate (Upgrading of the biogas) Gas Transport and final use of the biogas

Fig. 1. Overview of the biogas system studied. The arrows represent material or energy ?ows in the system.

2. Method and assumptions In order to make the results and analyses as transparent as possible, the calculations are performed for individual raw materials, and not for speci?c biogas plants. Hence, the results mirror the variations among raw materials more clearly than if speci?c biogas plants were analysed. In a real application, different raw materials are usually co-digested to establish a well-functioning biogas process. The raw materials studied in this analysis are also assumed to be codigested if necessary to obtain the pre-set biogas yield or to ful?l the conditions set by the design of the system. When the raw materials are co-digested there is a need to allocate the energy input at the co-digestion plant among the raw materials studied. Several allocation methods are investigated (see Sections 3.5 and 4.2). All energy ?ows in the biogas systems were identi?ed and summarised from a life-cycle perspective, and compared with the biogas yield. The raw materials, recovery technologies, conversion technologies, and transportation demands included, as well as the system boundaries applied, are shown in Fig. 1. The arrows indicate energy or material ?ows in the biogas systems studied. All calculations are based on data from literature reviews (a more detailed summary of the reviews is given in [12]). The analysis focuses on raw materials and biogas systems

suitable for Swedish conditions, but the results may be valid also for other regions with similar conditions and a similar availability of raw materials. The raw materials studied include those having the largest biogas potential in Sweden [1]. However, mainly due to the controversy concerning the spreading of sewage sludge on arable land, this raw material is not included despite a relatively high biogas potential [16,17]. In order to evaluate the energy balance in various biogas systems, energy needs are determined for all operations required to run the systems. Therefore, the energy needed for the cultivation and harvesting of energy crops is included since these crops are cultivated primarily for biogas production in the systems studied. Energy crops are assumed to be cultivated on set-aside arable land; consequently, no extra energy costs associated with the production of food or fodder replacements are included. The remaining raw materials are considered to be waste products. Consequently, only the additional energy input associated with the handling and transport of these waste products is determined, but none of the energy required in the production of the main product. All calculations of energy inputs are based on primary energy inputs; that is, all energy ?ows are calculated as unconverted and untransformed natural resources. Thus, the calculations include the energy required in the production of energy carriers, vehicles, fertilisers, etc., as well as the energy embodied in these products. The production and distribution of diesel fuel is estimated to correspond to 10% of the energy content in the fuel [18,19]; thus, 1 l of diesel corresponds to 42.6 MJ of primary energy. Manufacturing and maintenance of trucks and tractors intended for the transport of raw materials and digestate are assumed to correspond to 8% and 20%, respectively, of the energy content in the diesel fuel [18,20]. Using 1 l of diesel for transportation by truck or tractor then corresponds to 46 and 50 MJ of primary energy, respectively. Electricity is assumed to be based on natural gas with a conversion ef?ciency of 50%. One MJ of electricity then corresponds to 2.2 MJ of primary energy,

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including distribution losses in the electricity grid and energy requirements in the production and distribution of natural gas. The heating requirements in the biogas plants are met by biogas produced in the biogas system, assuming that 1 MJ of biogas corresponds to 1.3 MJ of primary energy. The energy used for production of chemical fertilisers is estimated to be 48 MJ/kg N, 7.9 MJ/kg P, and 4.8 MJ/kg K [18,21]. The energy embodied in the biogas plants is considered to be negligible compared to the net energy ?ows in the plants, and is therefore not included. A base scenario was developed to evaluate the energy ef?ciency in the entire life-cycle of the biogas systems. This scenario is based on the ?gures presented as ‘‘best estimate’’ and on the assumptions presented in the following sections. The base scenario comprises biogas production at large-scale, co-digestion biogas plants and does not take into account upgrading of the biogas or dewatering of the digestate. 3. Calculation of primary energy ?ows 3.1. Energy demand in the cultivation and harvesting of ley crops and recovery of harvest residues Primary energy inputs were determined for the cultivation and harvesting of ley crops and the recovery of harvest residues (Table 1). Harvesting and storage losses were taken into consideration, as well as the energy embodied in the tractors and machinery. The cultivation and harvesting of ley crops includes the production of fertilisers and diesel consumption for cultivation, harvesting, transport from ?eld to storage, and storage of ley crops in a bunker silo. Recovery of harvest residues includes diesel consumption for the recovery of the residues and transport from ?eld to storage. Straw is baled in the ?eld and tops and leaves of sugar beet are stored in a bunker silo. 3.2. Energy demand in the collection of organic waste

collecting area and the biogas plant. This distinction was made to account for the higher fuel consumption per km during collection due to the frequent stops necessary (Table 2). However, the fuel consumption can also vary greatly among collection areas due to the traf?c situation, the number of stops and households per stop, the driving distances, etc. The ?gures in the table are given per tonne of collected waste. Any dry matter losses related to pre-treatment at the biogas plant or decomposition of the waste are considered to be negligible. In addition, any extra energy input required for collecting or loading of other wastes included in this study are considered to be negligible. 3.3. Transport of raw materials and digestate The primary energy inputs were determined for the transport of raw materials and digestates (Table 3). The ?gures given per kilometre refer to the distance between the biogas plant and the location of the raw materials, or the delivery point for the digestates. The column ‘‘excl. empty return transport’’ refers to the cases when return transport is used, for example, when manure is transported to a large-scale biogas plant and the return transport is used to return digestate to the farm. When the return transport is empty, the ?gures in the column ‘‘incl. empty return transport’’ are used. In the base scenario, trucks are used for the transport of both raw materials and digestates. The digestates originating from the agricultural sector are returned to their farm of origin. Digestates originating from other sectors are transported 10 km, regardless of the transport distance of the raw materials. Digested manure is returned to the farm by return transport, all other return transports are empty. In the base scenario, the amount of digestate produced is assumed to be determined by the dry matter content of the raw materials (see Section 3.5). 3.4. Spreading of the digestates

In order to compare the energy input for waste collection in compacting trucks, the input was calculated for (i) the collection route and (ii) transportation between the

The primary energy inputs were determined for the handling and spreading of liquid and solid digestates

Table 1 The primary energy input for cultivation and harvesting of ley crops, and for recovery of harvest residues Raw material/handling operation Dry matter content (%) Energy input (GJ/dry tonne)a Best estimate Clover grass ley/cultivation and harvestingb Straw/recoveryc Tops and leaves of sugar beet/recoveryd
a

Low value 1.8 0.24

High value 2.8 0.59

23 82 19

1.9 0.28 0.54

‘‘Best estimate’’ represents the system design and energy input assumed in the base scenario. ‘‘Low value’’ and ‘‘high value’’ indicate the interval of energy input found in the literature. b In the cultivation and harvesting of ley crops, the largest part of the energy input is the fuels used in tractors and the production of fertilisers. The ley crop yield is assumed to correspond to 7–8 tonnes dry matter per hectare. Based on [18,22,23]. c The straw yield is assumed to be 2 tonnes per hectare and year. Based on [18,20,23]. d The amount of silage from tops and leaves of sugar beet is assumed to correspond to approximately 2 tonnes dry matter per hectare [12].

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M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266 Table 2 The primary energy input into the collection of municipal organic waste in various types of collection areasa Collection area/activity City Collection route Transportation Suburb Collection route Transportation Rural Collection route Transportation
a

257

Energy input (MJ/dry tonne km)b

Distance driven (km)c

Energy input (GJ/dry tonne)

40 (23–51) 8 30 (19–30) 8 15 (12–(30)) 8

10 (5–13) 10 (5–15) 35 (30–90) 20 (15–35) 70 (65–140) 40 (30–50)

0.4 (0.2–0.6) 0.08 (0.04–0.1) 1.1 (0.5–1.2) 0.16 (0.1–0.3) 1.1 (0.9–3) 0.3 (0.2–0.4)

Values within parentheses indicate the interval found in the literature. However, there are examples of collection routes and transports that are more than twice the distance given in these intervals. b ‘‘km’’ denotes the distance driven. Based on [24–27]. c Based on [26,28,29].

Table 3 The primary energy input for the transport of raw materials and digestate Vehicle Load (tonne) Energy input (MJ/tonne km)a Incl. empty return transport Truckb Truckb Truckc Truckd Tractore Tractore 16 30 16 8 15 1.6 (1.5–2.3) 1.1 (1.0–1.2) 2.9 3.2 3.5 2.5 Excl. empty return transport 1.0 (0.8–1.3) 0.7 (0.6–0.8) 1.7 Manure, digestate, slaughterhouse waste Ley crops, tops and leaves of sugar beet Straw Grease separator sludge Solid digestate Liquid digestate Raw material

a Values within parentheses indicate the interval found in the literature. ‘‘km’’ denotes the distance between the biogas plant and the location of the raw materials, or the point of delivery of the digestates. b Based on [24,30]. c The energy input per tonne of straw is comparatively high because of its low density [20]. d The energy demand for the transport of grease separator sludge is assumed to be relatively high due to the large number of stops on the collection route and driving in city areas [31]. e Based on [18,32–34].

(Table 4). The dry matter content of the digestates depends on the digestion technology used and whether dewatering of the digestate is applied. Hence, the digestates are assumed to be handled and spread in the same way as other liquid and solid organic fertilisers. The amount of digestate to be spread per hectare depends on its content of plant nutrients and heavy metals, and on the plant nutrient requirements per hectare. The average spreading dosage is assumed to correspond to 30 tonnes of liquid digestate or 25 tonnes of solid digestate per hectare. The weight losses due to the degradation of organic matter in the anaerobic digestion process are not considered to affect the amount of digestates. 3.5. Operation of the biogas plant The electricity and heating requirements were determined for the operation of large-scale and farm-scale biogas plants (Table 5). The ?gures given in the table represent the conditions in continuous, single stage, mixed

tank reactors operating at a mesophilic temperature. The energy inputs are expressed both as MJ per tonne added to the digester and as per cent of the biogas produced. The base scenario uses the unit MJ per tonne, assuming that ‘‘tonne’’ refers to a mixture of substrates with a 10% dry matter content. The required dry matter content is obtained by adding fresh water or by mixing raw materials that have different dry matter contents. When calculating the energy demand for individual raw materials in this scenario, the surplus water from wet raw materials are allocated to drier raw materials. Thus, 1 tonne of manure (8% dry matter) is assumed to correspond to 0.8 tonnes of mixture, generating 0.8 tonne of digestate, whereas 1 tonne of ley crops (23% dry matter) corresponds to 2.3 tonnes of mixture, generating 2.3 tonnes of digestate. The energy requirements vary among biogas plants and raw materials. For instance, the heating requirements for farm-scale biogas plants may be higher per tonne of raw material than for large-scale biogas plants, due to poorer insulation and restricted possibilities to use heat

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258 M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266 Table 4 The primary energy input for loading, transport and spreading of digestate on arable land Digestate Energy input Loadinga (MJ/tonne) Liquid phase Solid phase 2.5 7 Transportb (MJ/tonne) 5 7 Spreading (GJ/ha)c 0.5 (0.3–0.6) 0.35 (0.2–0.4) Spreading (MJ/tonne) 17 14 Total (MJ/tonne) 25 28

a The energy used for loading of the digestates is assumed to equal the energy input for the transport of solid digestates and half of the energy demand for the transport of liquid digestates. b The transport distance between storage and ?eld is assumed to be 2 km (see Table 3). c Values within parentheses indicate the intervals found in the literature. Based on [33,35,36].

Table 5 The primary energy input in farm-scale and large-scale biogas plantsa Heat Farm-scale (MJ/tonne)b Large-scale (MJ/tonne)c Large-scale (% of the biogas produced)c
a

Electricity 33d 66 (55–80) 11 (8–24)

250 110 (70–180) 13 (6–17)

The ?gurers refer to continuous, single stage, tank reactors, operating at a mesophilic temperature. No dewatering of the digestate or upgrading of the biogas. Values within parentheses indicate the interval found in the literature. b Farm-scale biogas plants typically digest less than approximately 10,000 tonnes of raw materials a year. Based on [10,40]. c Additional pre-treatment of dry raw materials (410% dry matter content) is assumed to require an extra electricity input corresponding to 33 MJ per tonne of raw material. Large-scale biogas plants typically digest some 20,000–60,000 tonnes of raw materials a year. Based on [8,10,35,37,38,41]. d Calculated from data on the electricity input expressed as per cent of the biogas produced.

exchangers. The heat input can also be affected by variations in the hygiensiation requirements for different raw materials. However, such variations are not assumed to affect the heat input since the heat used for hygienisation can be recovered and used to heat the biogas reactor. The electricity demand is affected by variations in, for example, maceration, pumping, and mixing requirements among raw materials and biogas plants. A base electricity demand is given in Table 5, valid for the digestion of wet raw materials without dewatering of the digestate or upgrading of the biogas produced. Dewatering of the digestate and recycling of some of the liquid phase in the digester can reduce the amount of fresh water needed and the amount of digestate to be distributed. Dewatering is assumed to require an electricity input corresponding to 10 MJ of primary energy per tonne of digestate [37,38]. Upgrading of the biogas is required if the gas is used as a vehicle fuel or added to the natural gas grid. Upgrading implies removal of undesirable gases, such as CO2 and H2S, and compression of the puri?ed gas. The primary energy input in large-scale upgrading plants is assumed to correspond to 11% of the energy content in the biogas produced [5,19,39]. The methane losses from the upgrading process are considered to be small and thus not to affect the net energy output from the biogas systems. 3.6. Biogas yields There are large variations in biogas yields and composition of the gas among the raw materials studied (Table 6)

due to the variation in the composition of the raw materials, and in digestion technologies and digestion conditions. Concerning the composition of the raw materials, high fat content (e.g. grease separator sludge) provides a biogas with high methane content. In addition, the composition of a speci?c raw material can vary markedly among sites, years etc. For instance, the time of harvesting will affect the content of various carbohydrates in ley crops, and thus affect the degradability of the crop. As to digestion technology, the gas yield can be affected by digestion temperature, retention time, load, digestion technology (co-digestion; batch or continuous; one or two phase digestion), pre-treatment of the raw materials, etc. 3.7. Energy savings from improved management of plant nutrients Indirect energy savings can be achieved when anaerobic digestion replaces other systems of handling the raw materials (Table 7) (a more extensive description of indirect energy savings is given in Borjesson and Berglund [15]). ¨ The indirect energy savings considered in this study are based on an improved utilisation of plant nutrients. Introducing ley crops, for example for biogas production, in a conventional crop rotation system can in the long-term reduce the need of nitrogen fertilisers by approximately 40 kg N per hectare and year [36]. The reduction is in part due to the increase of organic matter in the soil and in part due to an increased mineralization of the organic bound nitrogen in the soil. Introducing recovery of tops and leaves

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M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266 Table 6 The biogas yield from different raw materialsa Raw material Estimated dry matter content (%) Biogas yield (GJ/dry tonne) Best estimate Manure—cow Manure—pig Grease separator sludge Ley crops Municipal organic waste Slaughterhouse waste Tops and leaves of sugar beet Straw 8 8 4 23 30 17 19 82 6.2 7 22 10.6 12.4 9.4 10.6 7.1 Low value 5 5.6 20 5.3 10 7.8 5.6 High value 8.5 8.5 27 13 14 14 8.5 259

a Based on [8,40,42–47]. ‘‘Best estimate’’ represents the biogas yield assumed in the base scenario. ‘‘Low value’’ and ‘‘High value’’ represent the intervals found in the literature.

Table 7 Energy savings achieved when the introduction of anaerobic digestion reduces the need of chemical fertilisers compared to other handling systems Reduced need of chemical fertilisers (kg N/dry tonne) Introducing ley crops in a conventional crop rotation system Recovery of tops and leaves of sugar beet Digested pig manure replaces undigested Anaerobic digestion of municipal organic waste replaces Composting Incineration 18 10 2.8 6.7 19 (kg P/dry tonne) — — — — 4 Energy savings (MJ/dry tonne) 800 450 120 300 970

of sugar beet can reduce the nitrogen leakage, and reduce the losses of ammonia and N2 from the ?elds. These reductions are estimated to correspond to approximately 30 kg N per hectare and year, providing that there are negligible nitrogen losses from the storage of the silage and from the digestion process [13]. Anaerobic digestion, and thus mineralization of the organic matter, increases the amount of nitrogen available to plants in manure. Applying digested in place of undigested pig manure on arable land can thus reduce the nitrogen leakage. This reduction is estimated to correspond to approximately 7.5 kg N per hectare and year. Anaerobic digestion and composting of organic wastes facilitates the recirculation of plant nutrients in municipal organic waste. However, less nitrogen is assumed to be recycled when the waste is composted in stead of being digested, due to higher ammonia and N2O losses from the composting process. Incineration of organic waste without recycling of the ash implies that all plant nutrients in the waste have to be replaced by chemical fertilisers. 4. Energy balances To evaluate the energy balance and energy ef?ciency in biogas systems based on various raw materials, an energy input/output ratio was de?ned. The energy input/output ratio was calculated as the sum of primary energy input into a biogas system divided by the energy content in the

biogas produced. The higher the ratio, the less energy ef?cient is the biogas system. When the ratio exceeds 100%, the energy balance turns negative. 4.1. Base scenario The results show that the energy input required in the base scenario typically corresponds to 25–40% of the energy content in the biogas produced, depending on the raw material studied (Fig. 2). Operation of the biogas plant is generally the most energy demanding process in the biogas system, corresponding to 50–80% of the energy input. The ?gure also shows that the raw materials that have high biogas yields bene?t if the energy input at the biogas plant is expressed as MJ per tonne of substrate mixture and if surplus water is allocated to dry raw materials. For example, operation of the biogas plant corresponded to 8% of the energy content in the biogas produced from grease separator sludge while it corresponded to 26–28% of that from manure and straw. The biogas yield from grease separator sludge was three times higher per tonne dry matter than that from manure and straw. The results show that differences in required handling of the raw materials can increase the energy input in the biogas systems signi?cantly. Ley cropping was the handling process requiring the most energy, corresponding to 45% of the total energy input. Recovery of straw, tops and

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260
50
Biogas plant heating Biogas plant electricity Transport of digestate Spreading of digestate Transport of raw materials

M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266

Manure - cow
Handling of raw materials

Straw Ley crops Municipal org. waste

Manure - pig Tops & leaves of sugar beet

Energy input/output ratio (%)

40

Grease sep. sludge Slaughterhouse waste 120 100 80 60 40 20 0 0

30

20

10

Slaughterhouse waste

Manure - cow

Municipal org. waste

Tops & leaves of sugar beet

Fig. 2. The energy input/output ratio from digestion of various types of raw materials in the base scenario. The transport distance between the location of the raw materials and the biogas plant is 10 km.

Grease sep. sludge

Ley crops

0

Energy input/output ratio (%)

Manure - pig

Straw

50 100 150 Transportation distance (km)

200

leaves of sugar beet, and municipal organic waste corresponded to 10%, 20% and 25%, respectively, of the energy content in the biogas produced. The entire energy input for ley cropping is allocated to the biogas system since these crops are primarily produced for biogas production. The other raw materials are assumed to be waste products. Hence, the calculations only include the energy required for the recovering of these products. There are differences in energy input between the transport of a raw material and its digestate (Fig. 2). These differences depend mainly on the differences in transport ef?ciency and the amount of digestate per tonne of raw material. For example, the transport of grease separator sludge is signi?cantly more energy demanding than the transport of its digestate. This is explained by the energy-demanding transport of the sludge and by the allocation of surplus water from the sludge to drier raw materials, resulting in reduced amounts of digestate to be handled and transported. Transport of straw, ley crops, and tops and leaves of sugar beet requires less energy than the transport of their digestate due to energy-ef?cient transport of the raw materials and the considerable dilutions required. Fig. 3 shows how variations in transport distances affect the energy input/output ratio. The digestates originating from the agricultural sector are assumed to be returned to their farm of origin, thus the transport distance for the agricultural raw materials equals that for their digestates. The digestates originating from other sources are assumed to be transported 10 km. Furthermore, digested manure is returned to the farm by return transport, all other return transports are assumed to be empty. Our study shows that the transport distance could be up to approximately 580 and 750 km for municipal organic waste and slaughterhouse waste, respectively, before the energy balance turns negative. These raw materials have a comparatively high biogas yield and their transport is

Fig. 3. The energy input/output ratio for the base scenario as a function of the transport distance.

energy ef?cient. As to manure and straw, the energy balance turns negative when the transport distance exceeds 200 and 240 km, respectively. These raw materials have a low biogas yield and the transport of digestates is increasingly energy demanding as the distance between the biogas plant and the farm increases. Concerning straw, its digestate is highly diluted thus resulting in energy demanding transports. 4.2. The importance of choosing the appropriate allocation method Several methods were investigated to determine how to allocate the diluting requirements and energy input at the biogas plant among the raw materials. Each method resulted in a different way of expressing the mean electricity and heat input. The following approaches were used:

 



MJ per tonne of substrate mixture (denoted ‘‘MJ’’ in Fig. 4). This is the unit convention used in the base scenario. Per cent of the biogas produced (denoted ‘‘%’’ in the ?gure). As in the base scenario, the energy requirements for the transport and spreading of digestate depend on the need to dilute dry raw materials. Consequently, 1 tonne of manure (8% dry matter) corresponds to 0.8 tonnes of digestate, while 1 tonne of ley crops (23% dry matter) corresponds to 2.3 tonnes of digestate. MJ per tonne raw material (denoted ‘‘1-1’’ in the ?gure). This allocation method does not consider the energy requirements associated with the need for extra water. Thus, 1 tonne of raw material corresponds to 1 tonne of

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M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266
50 Biogas plant heating Handling of raw materials Biogas plant electricity Transport of digestate Spreading of digestate Transport of raw materials

261

40

Energy input/output ratio (%)

30

20

10

0
(%) (%) (%) (%) (%) (%) (%) (MJ) (MJ) (MJ) (MJ) (MJ) (MJ) (MJ) (MJ) (%) (1-1) (1-1) (1-1) (1-1) (1-1) (1-1) (1-1) (1-1)

Slaughterh. waste

Grease sep. sludge

Municipal org. waste

Straw

Residues - sugar beet

Manure - pig

Manure - cow

Ley crops

Fig. 4. The effects on the net energy output of using different methods for the allocation of diluting requirements and energy input at the biogas plant for the various types of raw materials. ‘‘MJ’’ illustrates the base scenario. ‘‘%’’ denotes a scenario in which the mean energy input at the biogas plant is given in per cent of the biogas produced. In ‘‘1-1’’, 1 tonne of raw material is assumed to correspond to 1 tonne of substrate added to the digester, generating 1 tonne of digestate.

substrate added to the digester, generating 1 tonne of digestate, regardless of the dry matter content of the raw material. There is a clear in?uence of the allocation method chosen on the net energy output for individual raw materials. Expressing the mean energy demands at the biogas plant in per cent of the biogas produced (denoted ‘‘%’’ in the ?gure) implies that these demands correspond to a constant part of the energy input/output ratio, independent of the raw material digested or its biogas yield. Using this allocation method will disadvantage those raw materials that have a high biogas yield per tonne of dry matter. This affects grease separator sludge in particular because of its high biogas yield, and because no surplus water is allocated to drier raw materials. Changing allocation method doubles the energy input calculated for grease separator sludge. Ignoring differences in dilution demands (denoted ‘‘1-1’’) affects in particular the net energy output for raw materials that have very high or very low dry matter contents. Using this allocation method means that the energy input in a biogas system based on straw decreases by 75% compared to the base scenario since the biogas system no longer carries the energy demand associated with the handling of the water used for dilution. On the other hand, the energy input for grease separator sludge will double since no surplus water is allocated to drier raw materials. The effects of altering the allocation methods are clearly important when calculating the net energy output for speci?c individual raw material. When digestion of dry raw materials requires an extra water supply (as assumed in the

base scenario), it is necessary to include the energy demand associated with the handling of this water. In contrast, if more raw materials with low dry matter content are added to a biogas plant without a corresponding increase of drier raw materials, it might not be possible to allocate any surplus water to the drier raw materials. 4.3. Sensitivity analysis The variations in input data (Tables 1–6) were used to evaluate how these variations affect the energy balance in the biogas systems studied (Fig. 5). The alternatives ‘‘high biogas yield’’ and ‘‘low biogas yield’’ are based on the highest and lowest values, respectively, given in Table 6, while all energy inputs equal those in the base scenario. In the alternatives ‘‘ef?cient system’’ and ‘‘inef?cient system’’, the energy inputs are chosen among the lowest and highest values, respectively, found in the literature, while the biogas yield equals that in the base scenario. The alternative ‘‘high’’ combines a high biogas yield with the energy inputs in an ef?cient system. The alternative ‘‘low’’ combines a low biogas yield with the energy inputs assumed in the inef?cient system. The results show that the energy output from the biogas systems is signi?cantly affected by the input data chosen. The variations found in input data are partly explained by differences in assumptions made about the system design. For instance, municipal organic waste is assumed to be collected in city areas in the ef?cient system, while the inef?cient system represents collection in rural areas. Input data on the digestion process are based on experiences of operating different biogas plants as well as on theoretical

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120 Biogas plant heating Handling of raw materials 80 Ley crops 60 Manure – pig 40 Municipal organic waste Biogas plant electricity Transport of digestate Spreading of digestate Transport of raw materials

M. Berglund, P. Bo ¨rjesson / Biomass and Bioenergy 30 (2006) 254–266

100 Energy input/output ratio (%)

20

0 Inefficient system Inefficient system Inefficient system Efficient system Low biogas yield High biogas yield High biogas yield Low biogas yield Low biogas yield High Efficient system High Efficient system Base scenario High High biogas yield Base scenario Base scenario Low Low Low

Fig. 5. Effects on the energy input/output ratio of changing the biogas yield and energy ef?ciency of the biogas system.

Reduced use of chemical fertilisers Handling of raw materials 60

Biogas plant heating Transport of digestate

Biogas plant electricity Transport of raw materials

Spreading of digestate

50 Manure - pig 40 Energy input/output ratio (%) Municipal organic waste Ley crops

30

20

10

0 15% dry matter Dewatering Upgrading Indirect: comp. Base scenario Base scenario Base scenario 8% dry matter 8% dry matter Empty return Dewatering Upgrading Farm-scale Upgrading Indirect

Indirect: inc.

Fig. 6. Variations in the energy input/output ratio due to changes in the system boundaries and assumptions made about the systems. (For explanation, see text.)

calculations. However, the differences in energy input levels might not be applicable for all raw materials due to the differing properties and pre-treatment demands. Furthermore, high biogas yields may require higher energy input than suggested in the ef?cient system. The sensitivity analysis also includes an evaluation of how the net energy output is affected by changes in system

boundaries and assumptions made about the systems (Fig. 6). The evaluation included the following changes to the base scenario:



Including the energy savings from improved management of plant nutrients (denoted ‘‘indirect’’ in the ?gure). The indirect energy savings corresponds to 2–8% of the

Indirect

-10

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energy content in the biogas produced. The range of the saving depends on the raw material studied and the reference system compared (see Section 3.7). Changed dry matter content (denoted ‘‘8% dry matter’’ and ‘‘15% dry matter’’). If the dry matter content in the material added to the digester decreases to 8% (keeping all other variables ?xed), the energy input is estimated to increase by 15% (municipal organic waste) and 23% (manure). This decrease means that more water has to be handled in the biogas plant and as digestate. Increasing the dry matter content to 15% is estimated to decrease the energy input by 18% (ley crops) due to reduced dilution requirements and amounts of digestate to be handled. Dewatering of the digestate (denoted ‘‘dewatering’’). Dewatering of the digestates from 1 tonne of ley crops and 1 tonne of municipal organic waste is assumed to provide 0.3 and 0.35 tonnes of solid digestate and 1.4 and 1.7 tonnes of liquid digestate, respectively, to be spread on arable land. The remaining liquid digestate is recirculated within the digester. Compared to the base scenario, more recycled liquid is used to achieve the appropriate dry matter content. 1 tonne of ley crops and of municipal organic waste is thus assumed to correspond to 2.6 and 3.5 tonnes of substrate mixture, respectively. The heat demand is estimated to increase by 15%, whereas the electricity demand increases by 25–30%. On the other hand, dewatering decreases the amounts of digestates to be transported and spread on arable land. The results show that dewatering is advantageous in these systems when the transport distance exceeds 60 km. However, the differences are not large between the base scenario and the dewatering alternative. Upgrading of the biogas produced (denoted ‘‘upgrading’’). Upgrading of the biogas is estimated to increase the electricity input by 120–180%, depending on the biogas yield per tonne raw material. The higher the yield, the higher the increase. Farm-scale biogas system (denoted ‘‘farm-scale’’). The energy input is estimated to increase to 55% of the energy content of the biogas produced from manure. Of this total, 80% is used to heat the digester. In the case of manure, the farm-based system is estimated to be more energy ef?cient than the large-scale system if the transport distance to the large-scale biogas plant exceeds 75 km. Empty return transport (denoted ‘‘Empty return’’). If the return transport is not used for the transport of the digested manure, the energy balance turns negative when the transport distance exceeds 120 km (compared to 190 km in the base scenario).

output it is important to consider how the different energy inputs are summarised.

5. Discussion The aim of this study was to analyse how the energy performance in biogas production is affected by the raw materials digested, the system design, and the allocation methods chosen. Consequently, the results are useful to identify the factors affecting the energy output and as a guideline for future assessments, rather than to rank or give exact ?gures on the net energy output from different existing, or potential, biogas systems. The results show that the energy input required to run the systems in most cases are substantially lower than the energy output. However, it is dif?cult to draw any far-reaching, general conclusions on the average energy performance in biogas production since the results are signi?cantly affected by the system design and the raw materials digested. There are large variations among the raw materials studied regarding handling and pre-treatment demands, origin, and degradability. In the case of the handling of the raw materials, there are important differences between energy crops (i.e. ley crops) and waste products. Handling of energy crops is assumed to include the entire energy input into the cultivation and harvesting of the crop, whereas handling of waste products includes only the additional energy input used for the collection of the waste, and none of the energy input used for the manufacture of a main product. In the case of energy input for the pretreatment and digestion of the raw materials, it has not been possible to give full consideration to the differing demands of the various raw materials, the main reason being the dif?culty in ?nding non-site-speci?c data concerning the electricity and heating demands of each type of raw material. When evaluating the energy performance in real biogas applications, site-speci?c data should be used. Regarding the biogas yield and net energy output, grease separator sludge is shown to give a high net output. These high biogas yields are obtained only by co-digestion with other raw materials. Differences in the assumptions made about the allocation method, the system design (e.g. large-scale or farmscale biogas plant), and the system boundaries chosen (e.g. whether to include upgrading of the biogas or indirect energy savings) were shown to affect the net energy output signi?cantly. To enable the reader to draw his/her own conclusions and to make comparisons with other system studies, the results and assumptions made are presented as clear as possible and several options are given. Since the calculations refer to individual raw materials, the energy input into the co-digestion systems must be allocated among the raw materials digested. In contrast to other similar system studies (e.g. [8–11]), we have investigated more than just one allocation method. Our results show the importance of how the allocation is done. In particular this

In most cases described above, each change affects the demand for a speci?c energy carrier, for example electricity or vehicle fuel, more than the net energy demand. Hence, when comparing how these changes affect the net energy

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is true for the allocation of the heat and electricity demand at a biogas plant. In contrast to some other energy systems analyses (e.g. [19]), this analysis does not consider the heating values of the raw materials. One reason is that this analysis does not include a comparison among various energy carriers than biogas or other ?elds of application (e.g. combustion) for the raw materials. However, when comparing different ways of using a raw material for energy production purposes, any differences in energy conversion ef?ciency among the systems must be compensated. Further, many of the raw materials studied (e.g. liquid manure, tops and leaves of sugar beet) would not be used for energy production if they were not digested. In order to be able to aggregate the energy inputs, they are all given as primary energy input. The sum depends on the energy carriers chosen and the conversion ef?ciency assumed. The energy input into each process described in Figs. 2–6 consist mainly of one kind of primary energy resource, thus a change in conversion ef?ciency can be done without great dif?culty. For instance, the main part of the electricity input is speci?ed as electricity used in the biogas plant. Changing the conversion ef?ciency for electricity affects the net energy output from all biogas systems similarly, since the proportion of electricity used in the systems does not differ greatly. High net energy output is not always the main objective when implementing new biogas plants. It may be equally important to reduce the plant nutrient leakage from arable land by digesting animal manure, to improve the recycling of plant nutrient, or to ?nd new waste management systems to replace land?lling [41]. Even though an energy-rich raw material could improve the biogas yield, it might not be added if it makes the digestate unsuitable as fertiliser. However, an improved system design and more energyef?cient processes can reduce the energy input. The input data used for the operation of the biogas plant, for instance, are based on existing biogas plants and technologies used today, and do not necessarily re?ect the future developments. Biogas production is a relatively new technology and it is likely that there is a considerable potential for improvement in the energy performance. 6. Conclusions The overall conclusions of the energy systems analysis are:





Operation of the biogas plant is generally the most energy-demanding process in the biogas systems, corresponding to approximately 40–80% of the net energy demand. Thus, assumptions made about how to allocate the energy input in a biogas plant among the raw materials digested, for example based on either biogas yields or dry matter content, substantially affect the results. In cases where production or extensive handling of the raw materials is required, a considerable part of the energy input can be used for these operations. For instance, ley cropping is estimated to correspond to some 40% of the energy input in a biogas system based on ley crops.

Acknowledgements We gratefully acknowledge the economic support provided by Goteborgs Energi, Sweden. ¨ References
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ASSESSMENT OF BIOGAS USE AS AN ENERGY SOURCE FROM ANAEROBIC DIGESTION OF ...ltration (UF) membrane unit employed in order to improve the performance of...
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