energies

Article

Methodology for Analysing Energy Demand inBiogas Production Plants—A Comparative Study ofTwo Biogas Plants

Emma Lindkvist 1,2, Maria T. Johansson 1,2,* ID and Jakob Rosenqvist 3

1 Department of Management and Engineering, Division of Energy Systems, Linköping University,SE-581 83 Linköping, Sweden; emma.lindkvist@liu.se

2 Biogas Research Center, Linköping University, SE-581 83 Linköping, Sweden3 Tranås Energi, SE-573 24 Tranås, Sweden; jakob.r@telia.com* Correspondence: maria.johansson@liu.se; Tel.: +46-13-281-185

Received: 30 September 2017; Accepted: 8 November 2017; Published: 10 November 2017

Abstract: Biogas production through anaerobic digestion may play an important role in a circulareconomy because of the opportunity to produce a renewable fuel from organic waste. However,the production of biogas may require energy in the form of heat and electricity. Therefore,resource-effective biogas production must consider both biological and energy performance. For theindividual biogas plant to improve its energy performance, a robust methodology to analyse andevaluate the energy demand on a detailed level is needed. Moreover, to compare the energyperformance of different biogas plants, a methodology with a consistent terminology, systemboundary and procedure is vital. The aim of this study was to develop a methodology for analysingthe energy demand in biogas plants on a detailed level. In the methodology, the energy carriersare allocated to: (1) sub-processes (e.g., pretreatment, anaerobic digestion, gas cleaning), (2) unitprocesses (e.g., heating, mixing, pumping, lighting) and (3) a combination of these. For a thoroughenergy analysis, a combination of allocations is recommended. The methodology was validated byapplying it to two different biogas plants. The results show that the methodology is applicable tobiogas plants with different configurations of their production system.

Keywords: biogas production; energy demand; methodology; anaerobic digestion; energyperformance; renewable fuel; waste management

1. Introduction

Biogas production through the anaerobic digestion of organic waste is an important part ofa sustainable energy system, as biogas is a renewable energy carrier that can replace fossil fuels,e.g., in the production of heat and electricity and as vehicle fuel. Moreover, biogas production fromorganic waste has an important role as a waste management system. For example, a ban on organicwaste in landfills has been in effect in Sweden since 2005, regulated by Swedish decree 2001:512on the landfilling of waste [1]. In addition, anaerobic digestion of organic waste enables recyclingof nutrients because the digestate from the biogas process can be used as fertiliser, substituting forartificial fertilisers on arable land. In summary, biogas production from organic waste may playan important part in both circular economy and bioeconomy.

There is a range of biogas production systems available, from small household digesters tolarge-scale industrial biogas plants. Domestic biogas systems produce biogas from human excrement,manure and kitchen waste at household level (see e.g., [2–5]) with no additional energy input.Domestic biogas systems are widespread in Africa and Asia. In contrast to domestic biogas productionsystems, larger, more complex biogas plants such as farm-scale plants (see e.g., [6]), biogas systems at

Energies 2017, 10, 1822; doi:10.3390/en10111822 www.mdpi.com/journal/energies

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waste-water treatment plants (see e.g., [7]) or biogas plants treating source-sorted organic municipalwaste (see e.g., [8]) require the input of additional energy, such as electricity and heat used for pumping,stirring, substrate heating, etc. The constitution of different biogas production plants varies widely.For example, several technologies for the pretreatment of the substrate are in use: the process canbe with or without hygienisation; the anaerobic digester can operate at different temperatures; gascleaning can be constructed in different ways; and the plant can include upgrading of the biogas tohigh methane content. Hence, to determine the overall performance of a biogas production plant,not only biological performance but also energy performance must be determined and evaluated.For individual biogas plants to improve the efficiency of their energy systems, detailed energy dataof the processes are needed, as well as an analytical methodology to compile and explain the data.Moreover, to benchmark the energy performance of biogas production plants, the system boundaries,terminology and analytical procedures must be consistent.

The energy performance of various biogas production systems has been studied in a numberof scientific articles. As an example, Terradas-Ill et al. [9] analysed a household-scale biogas systemand developed a heat transfer model that can estimate digester temperature and biogas productionin a small, unheated, fixed-dome digester buried in the ground. Moreover, Perrigault et al. [10]investigated the thermal performance of an experimental tubular digester and developed a heattransfer model. Hreiz et al. [11] developed a model to investigate heat transfer in a farm-scalesemi-buried agricultural digester. Energy performance of more complex biogas systems has beenstudied by for example Berglund and Börjesson [12], who studied energy performance in the lifecycle of biogas production. They analysed the effects of different digested raw materials, the designof the system and the chosen allocation method. Energy use was shown on an aggregated level,with total heat and electricity demand for a farm-scale and large-scale biogas plant, respectively.Pöschl et al. [13] evaluated the impact of different feedstock and process chains of biogas systems. Theyconcluded that there could be significant variation in energy efficiency depending on the feedstock andprocess adopted (single feedstock versus co-digestion), conversion technology applied, and digestatemanagement technique.

Other scientific articles have studied energy balances for specific substrates and specific processes.For example, Bohn et al. [14] studied the digestion of crops at low temperature; Gerin et al. [15]examined the digestion of maize and grass; and Lübken et al. [16] evaluated the digestion of cattlemanure and crops in a mesophilic biogas plant. Meanwhile, Banks et al. [8] studied the digestion ofdomestic food waste in a mesophilic biogas plant.

Hijazi et al. [17] reviewed 15 life cycle assessment (LCA) studies of European biogas systems.They concluded that regardless of the selection of feedstock, one measure to reduce environmentalimpact of biogas systems is to minimize electricity demand of the biogas production process. To findmeasures to improve electricity efficiency, an energy audit of the biogas plant must be performed.

Havukainen et al. [18] compared different methods for estimating the energy performance ofbiogas production. They reviewed three main approaches and used data from an existing biogasplant in Finland to evaluate the methods. Their review included 16 different studies using methodsfor evaluating efficiency of biogas production. The reviewed studies used different approaches anddifferent system boundaries, which makes their results hard to compare. That said, taken together,they concluded that to be able to compare different biogas plants in the future, it is essential that energyperformance is calculated in a more consistent way.

The aim of this study was to develop a methodology for analysing the energy demand in biogasplants on a detailed level. The goal is that the methodology shall be useful for biogas companies toaudit their energy systems and identify measures for improvement as well as for researchers, e.g., toserve as input to detailed LCA studies of biogas systems. The study is performed in a European contextand domestic biogas systems are excluded, since they do not have any additional input of energy.Moreover, biological performance of the biogas process is excluded, i.e., the conversion efficiency withregard to energy content in methane output in relation to energy content in substrate input. Hence, the

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substrate, biogas and digestate are considered process streams and energy analysis is performed onthe energy needed to operate the equipment in the biogas plant. The developed methodology shall beconsistent and robust, as well as applicable to all types of biogas plants with input of energy. Further,the methodology was validated by applying it to two Swedish biogas plants.

2. Energy Audit

An energy audit consists of a detailed analysis of an organisation’s energy performance and isbased on observations and measurements of energy supply and energy end-use [19]. The auditalso identifies and ranks opportunities for improving energy performance. An energy auditcan be performed on different system levels, e.g., a single production line or a whole company.The international energy audit standard ISO 50002 [19] was released in June 2014 and specifies theprocedures for carrying out an energy audit and facilitating its deliverables. A methodology forperforming energy audits in the manufacturing industry was described by Rosenqvist et al. [20].According to them, an energy audit can be divided into three phases:

• Collection of information and creation of an energy balance• Analysis of energy data• Identification and formulation of measures to improve energy efficiency

Using a top-down perspective, an energy audit can be conducted as an iterative process wherebythe analysis is performed in parallel with the collection of information [20]. During energy analysisand the formulation of measures for improvement, additional measurements of energy end-use maybe necessary in order to increase the accuracy and detail level of the energy balance. The iterativeprocess is stopped when the purpose is accomplished, e.g., enough data are available to suggestrelevant improvements.

Energy use can also be mapped with a bottom-up perspective whereby as much detailed dataabout as many processes and sub-processes as possible are collected. When all possible data arecollected, the analysis can begin.

To create a detailed energy balance, the utilised energy must be allocated to different energy-usingprocesses. ISO 50002 does not specify how energy use should be allocated, but leaves it up toindividual energy auditors to decide how to divide the energy system into different processes andentities. However, Söderström [21] and Rosenqvist et al. [20] included instructions on how to allocateenergy use in manufacturing industry. According to them, the smallest parts (or building blocks) ofan industrial energy system are the unit processes. The taxonomy for industrial unit processes used byRosenqvist et al. [20] has been developed by researchers at Linköping University, who categorised theunit processes into production processes, which are processes needed to manufacture the products,and support processes, which are processes needed to support the production processes but notneeded in actual production. The identified unit processes are believed to be universal for the wholemanufacturing industry, enabling comparisons of a given unit process. In this way, companies canbenchmark their energy performance with respect to that of other companies. Figure 1 presents thetaxonomy for unit processes used by Rosenqvist et al. [20].

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methodology shall be consistent and robust, as well as applicable to all types of biogas plants with input of energy. Further, the methodology was validated by applying it to two Swedish biogas plants.

2. Energy Audit

An energy audit consists of a detailed analysis of an organisation’s energy performance and is based on observations and measurements of energy supply and energy end-use [19]. The audit also identifies and ranks opportunities for improving energy performance. An energy audit can be performed on different system levels, e.g., a single production line or a whole company. The international energy audit standard ISO 50002 [19] was released in June 2014 and specifies the procedures for carrying out an energy audit and facilitating its deliverables. A methodology for performing energy audits in the manufacturing industry was described by Rosenqvist et al. [20]. According to them, an energy audit can be divided into three phases:

• Collection of information and creation of an energy balance • Analysis of energy data • Identification and formulation of measures to improve energy efficiency

Using a top-down perspective, an energy audit can be conducted as an iterative process whereby the analysis is performed in parallel with the collection of information [20]. During energy analysis and the formulation of measures for improvement, additional measurements of energy end-use may be necessary in order to increase the accuracy and detail level of the energy balance. The iterative process is stopped when the purpose is accomplished, e.g., enough data are available to suggest relevant improvements.

Energy use can also be mapped with a bottom-up perspective whereby as much detailed data about as many processes and sub-processes as possible are collected. When all possible data are collected, the analysis can begin.

To create a detailed energy balance, the utilised energy must be allocated to different energy-using processes. ISO 50002 does not specify how energy use should be allocated, but leaves it up to individual energy auditors to decide how to divide the energy system into different processes and entities. However, Söderström [21] and Rosenqvist et al. [20] included instructions on how to allocate energy use in manufacturing industry. According to them, the smallest parts (or building blocks) of an industrial energy system are the unit processes. The taxonomy for industrial unit processes used by Rosenqvist et al. [20] has been developed by researchers at Linköping University, who categorised the unit processes into production processes, which are processes needed to manufacture the products, and support processes, which are processes needed to support the production processes but not needed in actual production. The identified unit processes are believed to be universal for the whole manufacturing industry, enabling comparisons of a given unit process. In this way, companies can benchmark their energy performance with respect to that of other companies. Figure 1 presents the taxonomy for unit processes used by Rosenqvist et al. [20].

Figure 1. Taxonomy for energy unit processes in the manufacturing industry according to Rosenqvist et al. [20]. Figure 1. Taxonomy for energy unit processes in the manufacturing industry according to Rosenqvistet al. [20].

3. Methodology

The methodology for analysing the energy demand in biogas plants developed in this paperwas inspired by the energy audit methodology for the manufacturing industry described inRosenqvist et al. [20]. However, our methodology focuses only on allocating and analysing energydata. The procedure of collecting energy data is straightforward and can be found in the standard ISO50002 [19] and in Rosenqvist et al. [20], and the identification and formulation of measures to improveenergy efficiency are beyond the scope of this paper.

Developing the methodology involved the following steps:

1. Define the system boundary2. Identify and describe the sub-processes and which energy flow was part of which sub-process3. Identify and describe unit processes (divided into production processes and support processes)4. Validate the methodology by applying it to two biogas plants5. Evaluate the methodology

The development of the methodology and the data collection at the biogas plants studied werecarried out simultaneously, hence, an iterative process was used. The energy audits at the biogasplants served as inputs to the methodology and vice versa. Identification and description of unitprocesses departed from the unit processes defined for manufacturing industry by Rosenqvist et al. [20]and their applicability to biogas plants were evaluated. In addition to the concept of unit processes,we added the dimension of sub-processes. In our paper, a sub-process is defined as an entity in theproduction flow, i.e., the production chain, from reception of the substrate to the production of biogasand digestate. The energy demands in the biogas plants were analysed in three different ways:

• Energy is allocated to sub-processes• Energy is allocated to unit processes• Combination of 1 and 2

To be able to allocate energy to both unit processes and sub-processes, a more thorough anddetailed mapping of energy use than that described in Rosenqvist et al. [20] has been used.

Before data collection begins, it is important to define the system boundary of the object to bestudied. What processes and flows should be included in the energy analysis, and what should beleft outside the system boundary? In this study, the requirement for the boundary was to includeall general processes for all types of biogas plants. Two system levels were chosen: (1) gas cleaningand upgrading not included and (2) gas cleaning and upgrading included. This enabled a robustcomparison of energy performance of different biogas plants.

For the methodology to be transparent, unambiguous and standardised, a proper taxonomy ofsub-processes and unit processes is required. When developing the taxonomy in this paper, activities

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and energy flows were identified and analysed in order to establish a clear and concise sectioning,wherein activities and flows were grouped and allocated to different sub-processes and unit processes.

The developed methodology with a defined system boundary and taxonomy of sub-processes(with associated activities) and unit processes was applied to two Swedish biogas plants. In this casestudy, the biogas plants’ total energy demand, divided into different energy carriers, was quantifiedin the first step, and thereafter the energy flows were allocated to different sub-processes and unitprocesses. Information about the utilisation of electricity, fuel, district heating, etc., was provided bythe employees at the biogas plants (e.g., invoices, process statistics). More detailed data were collectedfrom technical documentation, process monitoring systems (e.g., current, power, energy and operationhours), meters already installed by the company, and instantaneous and logging measurements ofelectricity use.

The monitoring systems were mainly used to determine the operating hours for the studieddevices. Variable speed drives were used to determine the power of the devices, and in some cases,measurements were needed to facilitate data collection.

The analysis of energy data collected at the biogas plants followed the procedure described in thedeveloped methodology. Energy flows were first grouped according to unit processes, and the energyuse of the two biogas plants was analysed and compared. Thereafter, energy flows were allocated tosub-processes and the resulting energy demand was analysed. Finally, the energy flows were allocatedto the different sub-processes, and within each sub-process, energy was grouped according to unitprocesses. The different approaches were compared and evaluated.

4. The Biogas Plants Studied

4.1. Case A

Biogas plant A is owned by a commercial company. The plant has an annual output of6,000,000 Nm3 biomethane and treats about 60,000 tons of substrate per year. The substrate consistsmainly of waste products from the food industry in the area and is transported to the plant by trucks.The substrate is digested in two separate lines, each with two digesters of 4100 m3 and 2500 m3.The digesters are operated continuously. The digestate is stored in a post-digester (5000 m3) in whichgas is collected as well. The process is mesophilic and performed at 38 ◦C. The biogas producedis upgraded to vehicle fuel and compressed. The upgraded biogas is then sold to a neighbouringcompany that distributes the gas to customers. A portion of the biogas produced at the plant can alsobe used in a boiler to deliver heat to the processes. If the upgrading process, or the neighbouringcompany, is unable to accept all the gas produced, the excess gas can be flared. The data collection atplant A was performed during autumn 2016 (October–November).

4.2. Case B

Biogas plant B is owned and run by a municipal waste, water and energy management company.The plant has an annual output of 1,367,000 Nm3 biomethane and treats about 28,000 tons of substrateper year. The main substrate used at Biogas plant B is source-sorted, biodegradable household waste.Solid and liquid waste from a food industry supply chain is also used. The substrate originates fromthe nearby city and villages.

The substrate is digested in a thermophilic process working at 52 ◦C. A batch process is used andthe digester is used as a hygienization tank. In addition, a separate hygienization tank pasteurizessubstrates at 72 ◦C for at least 60 min, and delivers the hygienized substrate to a buffer tank. When thegas production from a batch in the digester is dropping, already hygienized material from the buffertank is fed into the digester to maintain the gas production. The total capacity of the digester at theplant is about 3200 m3.

The raw gas produced is upgraded and compressed for use as vehicle fuel, locally in trucks forgarbage collection and for buses for public transport. Some gas is used directly at the plant in a boiler

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to produce heat for the biogas production process. A small portion is also used to heat buildings at thewaste treatment site where the biogas plant is located. Data for the energy use at Biogas plant B werecollected during the spring of 2017 (March–May).

5. Results

The results are divided into two parts: description of the developed methodology, and resultsfrom the case studies.

5.1. The Methodology

The system boundary was set around the core processes of the biogas plant (see Figure 2). As canbe seen, two different system levels were studied, Level 1 and Level 2. The biogas plant was dividedinto a number of general, functional sub-processes, including all equipment used to achieve specificfunctions. Together, these sub-processes represent the whole biogas plant, from reception of thesubstrate to production of raw biogas and digestate (Level 1), and from reception of the substrate toproduction of the digestate and upgraded and compressed biogas (Level 2). The energy required fortransferring material (substrate, chemicals, biogas, etc.) through pipelines, conveyors etc. (vehicletransport excluded) between two sub-processes was designated to the sub-process that delivers thematerial. The sub-processes included in the study were Reception, Pretreatment, Hygienization,Anaerobic digestion, Digestate storage, Gas cleaning, Gas upgrading, Compression of upgraded gas,and Vehicle transports (internal). Distribution and utilisation of raw biogas for Level 1 and distributionand utilisation of upgraded biogas for Level 2 were excluded from this study, as were the distributionand utilisation of bio-fertilizer for both system levels. Production, collection and transportation ofsubstrate and production of energy carriers (heat, electricity, etc.) for the biogas process were alsoexcluded from the system since these are not considered core processes.

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for transferring material (substrate, chemicals, biogas, etc.) through pipelines, conveyors etc. (vehicle transport excluded) between two sub-processes was designated to the sub-process that delivers the material. The sub-processes included in the study were Reception, Pretreatment, Hygienization, Anaerobic digestion, Digestate storage, Gas cleaning, Gas upgrading, Compression of upgraded gas, and Vehicle transports (internal). Distribution and utilisation of raw biogas for Level 1 and distribution and utilisation of upgraded biogas for Level 2 were excluded from this study, as were the distribution and utilisation of bio-fertilizer for both system levels. Production, collection and transportation of substrate and production of energy carriers (heat, electricity, etc.) for the biogas process were also excluded from the system since these are not considered core processes.

Figure 2. The biogas system with the sub-processes. The sub-processes in green are included in the first system level: Level 1, the inner system boundary. The sub-processes in green and blue are included in the larger system level: Level 2, the outer system boundary. The sub-processes outside the system boundaries, in grey, are excluded from the systems studied.

• Reception—includes all processes associated with substrate reception, e.g., tipping hall, loaders, cranes, dump pocket, feed screws for solid materials, reception equipment and pumps for liquid materials and reception tanks with mixers and pumps.

• Pretreatment—includes all equipment that crushes, grinds and tears the substrate and removes unwanted materials (including plastic, textile, metal and gravel) with cyclones, strainers, screw presses, magnetic separators, or by sedimentation. This can involve physical, chemical, biological or combined processes.

• Hygienization—includes heat exchangers and tanks intended to sterilise the substrate and pumps, mixers or other equipment installed for the hygienization. In thermophilic digesters, hygienization can take place inside the anaerobic digester.

• Anaerobic digestion (AD)—anaerobic microorganisms digest the organic material into methane. This sub-process includes all equipment necessary to maintain an optimal environment for the microbial community as well as equipment necessary to capture the produced biogas.

Figure 2. The biogas system with the sub-processes. The sub-processes in green are included in the firstsystem level: Level 1, the inner system boundary. The sub-processes in green and blue are includedin the larger system level: Level 2, the outer system boundary. The sub-processes outside the systemboundaries, in grey, are excluded from the systems studied.

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• Reception—includes all processes associated with substrate reception, e.g., tipping hall, loaders,cranes, dump pocket, feed screws for solid materials, reception equipment and pumps for liquidmaterials and reception tanks with mixers and pumps.

• Pretreatment—includes all equipment that crushes, grinds and tears the substrate and removesunwanted materials (including plastic, textile, metal and gravel) with cyclones, strainers, screwpresses, magnetic separators, or by sedimentation. This can involve physical, chemical, biologicalor combined processes.

• Hygienization—includes heat exchangers and tanks intended to sterilise the substrate and pumps,mixers or other equipment installed for the hygienization. In thermophilic digesters, hygienizationcan take place inside the anaerobic digester.

• Anaerobic digestion (AD)—anaerobic microorganisms digest the organic material into methane.This sub-process includes all equipment necessary to maintain an optimal environment for themicrobial community as well as equipment necessary to capture the produced biogas.

• Digestate storage—includes digestate storage with stirrers and pumps and, in some cases,equipment necessary to capture produced biogas.

• Gas cleaning (Level 2)—includes all equipment that removes contamination from the raw biogas,such as sulphur and other impurities.

• Gas upgrading (Level 2)—includes all equipment needed to increase the methane content of thecleaned biogas.

• Compression (Level 2)—includes all equipment used to compress the biomethane toa desired pressure.

• Vehicle transports (Level 1 and 2)—includes vehicles used to transport substrate, digestate, etc.

The unit processes have been compiled with inspiration from Rosenqvist et al. [20]. The unitprocesses applied in this study are found in Table 1.

Table 1. Taxonomy of unit processes associated with biogas production (inspired by Rosenqvist et al. [20]).

Category Unit Process Examples of Applications

Production processes

Disintegrating Crushing, grinding and tearing of substrate in pretreatment

Mixing Mixing of substrate in reception, pretreatment and digester

Heating Heating of substrate in hygienization and digester

Cooling Cooling of substrate, digestate and biogas

Concentrating Concentrating substrate and digestate by removing watere.g., centrifuging. Compressing gas.

Diluting Diluting substrate by adding fresh water or process water

Packing Filling gas into gas cylinders

Support processes

Administration Control systems, computers

Cooling Cooling of equipment and space cooling

Lighting -

Compressed air Production of compressed air to the biogas productionprocess

Ventilation Ventilation of production area

Pumping Pumping of substrate, water, biogas, etc.

Tap water heating -

Internal transports Vehicles and conveyors (screw, scraper, conveyor belt,elevator)

Space heating -

Steam Production of steam to the biogas production process

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5.2. Results from the Case Study

The methodology was used to analyse energy demand in two Swedish biogas plants. System Level1 (see Figure 2) was analysed for biogas plant A and B. For biogas plant A, the sub-processes found atthis level were Reception of substrate, Pretreatment, Anaerobic digestion, Digestate storage and Vehicletransports. The sub-processes included for biogas plant B were Reception of substrate, Pretreatment,Hygienization, Anaerobic digestion, Digestate storage and Vehicle transports. In addition, systemLevel 2 was analysed for biogas plant B and the sub-processes included were Reception of substrate,Pretreatment, Hygienization, Anaerobic digestion, Digestate storage, Gas cleaning, Gas upgrading andVehicle transports. At system Level 1, the applicability of the methodology to biogas plants of differentsizes and configurations was evaluated, whilst the two system levels were evaluated by comparing theresults for Level 1 and Level 2 for biogas plant B.

The connection between the sub-processes and unit processes for the two plants studied areshown in Sections 5.2.1–5.2.9 below, and the allocation and analysis of energy demand are presentedin Sections 5.2.10 and 5.2.11.

5.2.1. Reception of Substrate (Level 1)

At plant A, the collected substrate was delivered to a tipping hall and silos, where it was storedtemporarily. From the temporary storage, the substrates were pumped or transported by conveyersfor pretreatment. Some lighting was connected to the tipping halls at plant A.

The most common substrate at plant B is solid household waste, received in a tipping pocket andtransported further on conveyer belts in the reception hall. Liquid waste from the food industry isreceived at a pumping hall. The buildings are illuminated and ventilated. The included unit processesfor the sub-process Reception are shown in Figure 3 for biogas plant A and B, respectively.

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methodology to biogas plants of different sizes and configurations was evaluated, whilst the two system levels were evaluated by comparing the results for Level 1 and Level 2 for biogas plant B.

The connection between the sub-processes and unit processes for the two plants studied are shown in Sections 5.2.1–5.2.9 below, and the allocation and analysis of energy demand are presented in Sections 5.2.10 and 5.2.11.

5.2.1. Reception of Substrate (Level 1)

At plant A, the collected substrate was delivered to a tipping hall and silos, where it was stored temporarily. From the temporary storage, the substrates were pumped or transported by conveyers for pretreatment. Some lighting was connected to the tipping halls at plant A.

The most common substrate at plant B is solid household waste, received in a tipping pocket and transported further on conveyer belts in the reception hall. Liquid waste from the food industry is received at a pumping hall. The buildings are illuminated and ventilated. The included unit processes for the sub-process Reception are shown in Figure 3 for biogas plant A and B, respectively.

Figure 3. The connection between the sub-process reception and unit processes at biogas plant A and B, respectively.

5.2.2. Pretreatment of Substrate (Level 1)

During the pretreatment process in plant A, the substrate is decomposed in a grinder. However, during the measurement period, the grinder was out of order, and hence no substrate in need of decomposition was digested at the plant. The substrate was mixed before being transported to the digester by pumps. The mixer tanks were ventilated, and some lighting was connected to the pretreatment locations.

In plant B, the first step of solid waste pretreatment consists of optical sorting, where black plastic bags with biodegradable waste are automatically separated from bags of other colours. The

Figure 3. The connection between the sub-process reception and unit processes at biogas plant A andB, respectively.

5.2.2. Pretreatment of Substrate (Level 1)

During the pretreatment process in plant A, the substrate is decomposed in a grinder. However,during the measurement period, the grinder was out of order, and hence no substrate in need ofdecomposition was digested at the plant. The substrate was mixed before being transported to

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the digester by pumps. The mixer tanks were ventilated, and some lighting was connected to thepretreatment locations.

In plant B, the first step of solid waste pretreatment consists of optical sorting, where black plasticbags with biodegradable waste are automatically separated from bags of other colours. The black bagscontaining the substrate are further transported on conveyer belts to a mixing hall, where the waste isdiluted with water and then mixed. A screw press separates the solid parts, such as plastic bags, fromthe substrate. The pretreatment products comprise solid waste for combustion and a substrate slurry(Meal) pumped to the Hygienization, Anaerobic digestion or buffer tanks. The included unit processesfor the sub-process Reception are shown in Figure 4 for biogas plant A and B, respectively.

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black bags containing the substrate are further transported on conveyer belts to a mixing hall, where the waste is diluted with water and then mixed. A screw press separates the solid parts, such as plastic bags, from the substrate. The pretreatment products comprise solid waste for combustion and a substrate slurry (Meal) pumped to the Hygienization, Anaerobic digestion or buffer tanks. The included unit processes for the sub-process Reception are shown in Figure 4 for biogas plant A and B, respectively.

Figure 4. The connection between the sub-process pretreatment and unit processes at biogas plant A and B, respectively.

5.2.3. Hygienization (Level 1)

Biogas plant A does not have any hygienization process since the substrate treated at the plant does not need it. In plant B, all of the substrate is hygienized. As the anaerobic digestion is thermophilic, the major part of the substrate is hygienized by maintaining the retention time in the digester for more than 30 days. However, a minor part of the substrate is treated at relatively high temperatures (72 °C) for at least 60 min in a hygienization tank. The substrate from the hygienization tank is further transported with pumps into the anaerobic digester. The included unit processes for the sub-process Hygienization are shown in Figure 5 for biogas plant B.

Figure 5. The connection between the sub-process hygienization and unit processes at biogas plant B.

Figure 4. The connection between the sub-process pretreatment and unit processes at biogas plant Aand B, respectively.

5.2.3. Hygienization (Level 1)

Biogas plant A does not have any hygienization process since the substrate treated at the plantdoes not need it. In plant B, all of the substrate is hygienized. As the anaerobic digestion is thermophilic,the major part of the substrate is hygienized by maintaining the retention time in the digester formore than 30 days. However, a minor part of the substrate is treated at relatively high temperatures(72 ◦C) for at least 60 min in a hygienization tank. The substrate from the hygienization tank is furthertransported with pumps into the anaerobic digester. The included unit processes for the sub-processHygienization are shown in Figure 5 for biogas plant B.

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black bags containing the substrate are further transported on conveyer belts to a mixing hall, where the waste is diluted with water and then mixed. A screw press separates the solid parts, such as plastic bags, from the substrate. The pretreatment products comprise solid waste for combustion and a substrate slurry (Meal) pumped to the Hygienization, Anaerobic digestion or buffer tanks. The included unit processes for the sub-process Reception are shown in Figure 4 for biogas plant A and B, respectively.

Figure 4. The connection between the sub-process pretreatment and unit processes at biogas plant A and B, respectively.

5.2.3. Hygienization (Level 1)

Biogas plant A does not have any hygienization process since the substrate treated at the plant does not need it. In plant B, all of the substrate is hygienized. As the anaerobic digestion is thermophilic, the major part of the substrate is hygienized by maintaining the retention time in the digester for more than 30 days. However, a minor part of the substrate is treated at relatively high temperatures (72 °C) for at least 60 min in a hygienization tank. The substrate from the hygienization tank is further transported with pumps into the anaerobic digester. The included unit processes for the sub-process Hygienization are shown in Figure 5 for biogas plant B.

Figure 5. The connection between the sub-process hygienization and unit processes at biogas plant B. Figure 5. The connection between the sub-process hygienization and unit processes at biogas plant B.

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5.2.4. Anaerobic Digestion (Level 1)

As mentioned above, biogas is produced in two parallel lines at biogas plant A, both with twodigesters. The meal can be heated or cooled by heat exchangers in a separate loop. There are two heatexchangers, and hence a loop for each line. This results in the possibility of heating the meal in one linewhile cooling the meal in the other. The meal is transported from the digester to the heat exchangerand can be transported back to the same digester or to the other one in the same line. The meal inthe digesters is mixed continuously. The digesters are ventilated and the raw biogas is transportedby blowers.

The digestion in plant B occurs in a single digester. The digestion is operated on a batch basis,whereby the meal is kept in the digester at 52 ◦C for 30 days. The meal is continuously ground andmixed in the digester. The digester is also heated to maintain the desired temperature. The digestate ispumped to the Digestate storage. The raw biogas is further transported by a blower.

The included unit processes for the sub-process Anaerobic digestion are shown in Figure 6 forbiogas plant A and B, respectively.

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5.2.4. Anaerobic Digestion (Level 1)

As mentioned above, biogas is produced in two parallel lines at biogas plant A, both with two digesters. The meal can be heated or cooled by heat exchangers in a separate loop. There are two heat exchangers, and hence a loop for each line. This results in the possibility of heating the meal in one line while cooling the meal in the other. The meal is transported from the digester to the heat exchanger and can be transported back to the same digester or to the other one in the same line. The meal in the digesters is mixed continuously. The digesters are ventilated and the raw biogas is transported by blowers.

The digestion in plant B occurs in a single digester. The digestion is operated on a batch basis, whereby the meal is kept in the digester at 52 °C for 30 days. The meal is continuously ground and mixed in the digester. The digester is also heated to maintain the desired temperature. The digestate is pumped to the Digestate storage. The raw biogas is further transported by a blower.

The included unit processes for the sub-process Anaerobic digestion are shown in Figure 6 for biogas plant A and B, respectively.

Figure 6. The connection between the sub-process anaerobic digestion and unit processes at biogas plant A and B, respectively.

5.2.5. Digestate Storage (Level 1)

The raw digestate at plant A is stored in a digestate storage tank in which it is continuously mixed. Gas is collected in this storage as well, but it was not possible to measure how much energy was used to transport the raw biogas from the digestate storage in relation to the gas transport from the anaerobic digester. Hence, all energy use connected to the transportation of raw gas is allocated to anaerobic digestion.

At plant B, the raw digestate is stored in two tanks. To avoid sedimentation in the tanks, the digestate is circulated between the two tanks and a stirrer is used to mix the digestate within each

Figure 6. The connection between the sub-process anaerobic digestion and unit processes at biogasplant A and B, respectively.

5.2.5. Digestate Storage (Level 1)

The raw digestate at plant A is stored in a digestate storage tank in which it is continuously mixed.Gas is collected in this storage as well, but it was not possible to measure how much energy wasused to transport the raw biogas from the digestate storage in relation to the gas transport from theanaerobic digester. Hence, all energy use connected to the transportation of raw gas is allocated toanaerobic digestion.

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At plant B, the raw digestate is stored in two tanks. To avoid sedimentation in the tanks,the digestate is circulated between the two tanks and a stirrer is used to mix the digestate within eachtank. No gas is retrieved from this sub-process. During the measurement period, the stirrer was notused because the circulation pump provided sufficient stirring.

The included unit processes for the sub-process Digestate storage are shown in Figure 7 for biogasplant A and B, respectively.

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tank. No gas is retrieved from this sub-process. During the measurement period, the stirrer was not used because the circulation pump provided sufficient stirring.

The included unit processes for the sub-process Digestate storage are shown in Figure 7 for biogas plant A and B, respectively.

Figure 7. The connection between the sub-process digestate storage and unit processes at biogas plant A and B, respectively.

5.2.6. Vehicle Transports (Level 1 and 2)

At plant A, the substrates are delivered directly to the tipping hall and silos. A wheel loader performs the only Vehicle transports at the plant.

The included unit processes for the sub-process Vehicle transports are shown in Figure 8 for biogas plant A.

For biogas plant B, information about Vehicle transports was not available during the data collection period.

Figure 8. The connection between the sub-process vehicle transports and unit processes at biogas plant A.

Figure 7. The connection between the sub-process digestate storage and unit processes at biogas plantA and B, respectively.

5.2.6. Vehicle Transports (Level 1 and 2)

At plant A, the substrates are delivered directly to the tipping hall and silos. A wheel loaderperforms the only Vehicle transports at the plant.

The included unit processes for the sub-process Vehicle transports are shown in Figure 8 forbiogas plant A.

For biogas plant B, information about Vehicle transports was not available during the datacollection period.

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tank. No gas is retrieved from this sub-process. During the measurement period, the stirrer was not used because the circulation pump provided sufficient stirring.

The included unit processes for the sub-process Digestate storage are shown in Figure 7 for biogas plant A and B, respectively.

Figure 7. The connection between the sub-process digestate storage and unit processes at biogas plant A and B, respectively.

5.2.6. Vehicle Transports (Level 1 and 2)

At plant A, the substrates are delivered directly to the tipping hall and silos. A wheel loader performs the only Vehicle transports at the plant.

The included unit processes for the sub-process Vehicle transports are shown in Figure 8 for biogas plant A.

For biogas plant B, information about Vehicle transports was not available during the data collection period.

Figure 8. The connection between the sub-process vehicle transports and unit processes at biogas plant A.

Figure 8. The connection between the sub-process vehicle transports and unit processes at biogasplant A.

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5.2.7. Gas Cleaning (Level 2)

To remove water vapour and other impurities from the raw biogas, the gas is cooled. The pressureis increased and the gas is transported to either the upgrading unit or the boiler. The included unitprocesses for the sub-process Gas cleaning are shown in Figure 9 for biogas plant B.

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5.2.7. Gas Cleaning (Level 2)

To remove water vapour and other impurities from the raw biogas, the gas is cooled. The pressure is increased and the gas is transported to either the upgrading unit or the boiler. The included unit processes for the sub-process Gas cleaning are shown in Figure 9 for biogas plant B.

Figure 9. The connection between the sub-process gas cleaning and unit processes at biogas plant B.

5.2.8. Gas Upgrading (Level 2)

At biogas plant B, the raw biogas is upgraded to natural gas quality (biomethane) with a methane content of 92–99%. In the upgrading process, contaminants such as carbon dioxide, sulphuric acid and water are removed. The cleaned biogas from the gas cleaning is compressed under high pressure before entering the scrubber column. The contaminants removed in the scrubber are pumped to a stripper flash unit for further cleaning. The gas is transported from the scrubber to two gas dryers before being transported to Compression.

The included unit processes for the sub-process Gas upgrading are shown in Figure 10 for biogas plant B.

Figure 10. The connection between the sub-process gas upgrading and unit processes at biogas plant B.

5.2.9. Gas Compression (Level 2)

At biogas plant B, the upgraded gas is compressed and kept under high pressure (over 220 bar) before being stored and delivered to customers.

Figure 9. The connection between the sub-process gas cleaning and unit processes at biogas plant B.

5.2.8. Gas Upgrading (Level 2)

At biogas plant B, the raw biogas is upgraded to natural gas quality (biomethane) with a methanecontent of 92–99%. In the upgrading process, contaminants such as carbon dioxide, sulphuric acid andwater are removed. The cleaned biogas from the gas cleaning is compressed under high pressure beforeentering the scrubber column. The contaminants removed in the scrubber are pumped to a stripperflash unit for further cleaning. The gas is transported from the scrubber to two gas dryers before beingtransported to Compression.

The included unit processes for the sub-process Gas upgrading are shown in Figure 10 for biogasplant B.

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5.2.7. Gas Cleaning (Level 2)

To remove water vapour and other impurities from the raw biogas, the gas is cooled. The pressure is increased and the gas is transported to either the upgrading unit or the boiler. The included unit processes for the sub-process Gas cleaning are shown in Figure 9 for biogas plant B.

Figure 9. The connection between the sub-process gas cleaning and unit processes at biogas plant B.

5.2.8. Gas Upgrading (Level 2)

At biogas plant B, the raw biogas is upgraded to natural gas quality (biomethane) with a methane content of 92–99%. In the upgrading process, contaminants such as carbon dioxide, sulphuric acid and water are removed. The cleaned biogas from the gas cleaning is compressed under high pressure before entering the scrubber column. The contaminants removed in the scrubber are pumped to a stripper flash unit for further cleaning. The gas is transported from the scrubber to two gas dryers before being transported to Compression.

The included unit processes for the sub-process Gas upgrading are shown in Figure 10 for biogas plant B.

Figure 10. The connection between the sub-process gas upgrading and unit processes at biogas plant B.

5.2.9. Gas Compression (Level 2)

At biogas plant B, the upgraded gas is compressed and kept under high pressure (over 220 bar) before being stored and delivered to customers.

Figure 10. The connection between the sub-process gas upgrading and unit processes at biogas plant B.

5.2.9. Gas Compression (Level 2)

At biogas plant B, the upgraded gas is compressed and kept under high pressure (over 220 bar)before being stored and delivered to customers.

The included unit process for the sub-process Compression is shown in Figure 11 for biogasplant B.

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The included unit process for the sub-process Compression is shown in Figure 11 for biogas plant B.

Figure 11. The connection between the sub-process compression and unit processes at biogas plant B.

5.2.10. Allocation and Analysis of Energy Demand at System Level 1

The energy use for the two biogas plants studied is presented (1) allocated to sub-processes, (2) allocated to unit processes, and (3) as a combination of the two. In Figures 12–17, only the unit processes and sub-processes that were active at the time of the measurements at the plants are included. The energy use at system Level 1, at biogas plant A and B, allocated to unit processes is presented in Figure 12.

Figure 12. The total energy use at system Level 1, allocated to unit processes for plant A and B, respectively. For plant A, the only energy carrier used during the measurement period was electricity. For plant B, the energy carriers used were electricity (37%) and heat (63%).

As can be seen in Figure 12, the unit process associated with the highest energy use at plant A is Mixing (65%), whilst at plant B it is Heating (63%). It is important to note that at plant A, there was no heating demand during the studied period. Since the main substrate at plant A is rich in carbohydrates and the process is mesophilic, there is a higher demand for cooling than heating in the digester during most of the year. There is also no need for hygienization at plant A. The energy use at biogas plant A and B allocated to sub-processes is presented in Figure 13.

As can be seen in Figure 13, the sub-process associated with the highest energy use at both plant A and B is Anaerobic digestion. For plant A, the sub-process with the second-highest energy use at plant A is Digestate storage. This could be because of the mixing in this sub-process. For plant B, the sub-process with the second-highest energy use is Hygienization, followed by Pretreatment of substrate.

Figure 11. The connection between the sub-process compression and unit processes at biogas plant B.

5.2.10. Allocation and Analysis of Energy Demand at System Level 1

The energy use for the two biogas plants studied is presented (1) allocated to sub-processes,(2) allocated to unit processes, and (3) as a combination of the two. In Figures 12–17, only the unitprocesses and sub-processes that were active at the time of the measurements at the plants are included.The energy use at system Level 1, at biogas plant A and B, allocated to unit processes is presented inFigure 12.

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The included unit process for the sub-process Compression is shown in Figure 11 for biogas plant B.

Figure 11. The connection between the sub-process compression and unit processes at biogas plant B.

5.2.10. Allocation and Analysis of Energy Demand at System Level 1

The energy use for the two biogas plants studied is presented (1) allocated to sub-processes, (2) allocated to unit processes, and (3) as a combination of the two. In Figures 12–17, only the unit processes and sub-processes that were active at the time of the measurements at the plants are included. The energy use at system Level 1, at biogas plant A and B, allocated to unit processes is presented in Figure 12.

Figure 12. The total energy use at system Level 1, allocated to unit processes for plant A and B, respectively. For plant A, the only energy carrier used during the measurement period was electricity. For plant B, the energy carriers used were electricity (37%) and heat (63%).

As can be seen in Figure 12, the unit process associated with the highest energy use at plant A is Mixing (65%), whilst at plant B it is Heating (63%). It is important to note that at plant A, there was no heating demand during the studied period. Since the main substrate at plant A is rich in carbohydrates and the process is mesophilic, there is a higher demand for cooling than heating in the digester during most of the year. There is also no need for hygienization at plant A. The energy use at biogas plant A and B allocated to sub-processes is presented in Figure 13.

As can be seen in Figure 13, the sub-process associated with the highest energy use at both plant A and B is Anaerobic digestion. For plant A, the sub-process with the second-highest energy use at plant A is Digestate storage. This could be because of the mixing in this sub-process. For plant B, the sub-process with the second-highest energy use is Hygienization, followed by Pretreatment of substrate.

Figure 12. The total energy use at system Level 1, allocated to unit processes for plant A and B,respectively. For plant A, the only energy carrier used during the measurement period was electricity.For plant B, the energy carriers used were electricity (37%) and heat (63%).

As can be seen in Figure 12, the unit process associated with the highest energy use at plant A isMixing (65%), whilst at plant B it is Heating (63%). It is important to note that at plant A, there was noheating demand during the studied period. Since the main substrate at plant A is rich in carbohydratesand the process is mesophilic, there is a higher demand for cooling than heating in the digester duringmost of the year. There is also no need for hygienization at plant A. The energy use at biogas plant Aand B allocated to sub-processes is presented in Figure 13.

As can be seen in Figure 13, the sub-process associated with the highest energy use at both plantA and B is Anaerobic digestion. For plant A, the sub-process with the second-highest energy useat plant A is Digestate storage. This could be because of the mixing in this sub-process. For plantB, the sub-process with the second-highest energy use is Hygienization, followed by Pretreatmentof substrate.

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Figure 13. The total energy use at system Level 1, allocated to sub-processes for plant A and B, respectively. For plant A, the only energy carrier used during the measurement period was electricity. For plant B, the energy carriers used were electricity (37%) and heat (63%), where the heat was divided as follows: 62% in anaerobic digestion, 37% in hygienization, and 1% in pretreatment.

In Figure 14, the energy use for 30 days at plant A and B is presented, in a combination of sub-processes and unit processes. The combination shows that for plant A, mixing is the highest energy user in the sub-process Anaerobic digestion as well as in Digestate storage. For the sub-process Pretreatment, the main energy user for plant A is pumping. For plant B, the main energy user in the sub-process Pretreatment is disintegration, followed by mixing and ventilation. For the sub-processes Hygienization and Anaerobic digestion, the main energy user is heating, which was the unit process with the highest total energy use.

Figure 13. The total energy use at system Level 1, allocated to sub-processes for plant A and B,respectively. For plant A, the only energy carrier used during the measurement period was electricity.For plant B, the energy carriers used were electricity (37%) and heat (63%), where the heat was dividedas follows: 62% in anaerobic digestion, 37% in hygienization, and 1% in pretreatment.

In Figure 14, the energy use for 30 days at plant A and B is presented, in a combination ofsub-processes and unit processes. The combination shows that for plant A, mixing is the highestenergy user in the sub-process Anaerobic digestion as well as in Digestate storage. For the sub-processPretreatment, the main energy user for plant A is pumping. For plant B, the main energy user in thesub-process Pretreatment is disintegration, followed by mixing and ventilation. For the sub-processesHygienization and Anaerobic digestion, the main energy user is heating, which was the unit processwith the highest total energy use.

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Figure 13. The total energy use at system Level 1, allocated to sub-processes for plant A and B, respectively. For plant A, the only energy carrier used during the measurement period was electricity. For plant B, the energy carriers used were electricity (37%) and heat (63%), where the heat was divided as follows: 62% in anaerobic digestion, 37% in hygienization, and 1% in pretreatment.

In Figure 14, the energy use for 30 days at plant A and B is presented, in a combination of sub-processes and unit processes. The combination shows that for plant A, mixing is the highest energy user in the sub-process Anaerobic digestion as well as in Digestate storage. For the sub-process Pretreatment, the main energy user for plant A is pumping. For plant B, the main energy user in the sub-process Pretreatment is disintegration, followed by mixing and ventilation. For the sub-processes Hygienization and Anaerobic digestion, the main energy user is heating, which was the unit process with the highest total energy use.

Figure 14. Cont.

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Figure 14. The total energy use for 30 days for plant A and B, presented in a combination of sub-processes and unit processes. The first two diagrams show the energy use at plant A, one with a linear scale and the other with a logarithmic scale. The last two diagrams show the energy use at plant B, one with a linear scale and the other with a logarithmic scale.

Figure 15. The unit processes for plant B with system Level 2.

Figure 14. The total energy use for 30 days for plant A and B, presented in a combination ofsub-processes and unit processes. The first two diagrams show the energy use at plant A, one witha linear scale and the other with a logarithmic scale. The last two diagrams show the energy use atplant B, one with a linear scale and the other with a logarithmic scale.

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Figure 14. The total energy use for 30 days for plant A and B, presented in a combination of sub-processes and unit processes. The first two diagrams show the energy use at plant A, one with a linear scale and the other with a logarithmic scale. The last two diagrams show the energy use at plant B, one with a linear scale and the other with a logarithmic scale.

Figure 15. The unit processes for plant B with system Level 2. Figure 15. The unit processes for plant B with system Level 2.

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Figure 16. The sub-processes for plant B with system Level 2.

Figure 17. The energy use for 30 days for plant B, presented in a combination of sub-processes and unit processes. The lower diagram has a logarithmic scale.

Figure 16. The sub-processes for plant B with system Level 2.

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Figure 16. The sub-processes for plant B with system Level 2.

Figure 17. The energy use for 30 days for plant B, presented in a combination of sub-processes and unit processes. The lower diagram has a logarithmic scale.

Figure 17. The energy use for 30 days for plant B, presented in a combination of sub-processes andunit processes. The lower diagram has a logarithmic scale.

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5.2.11. Allocation and Analysis of Energy Demand at System Level 2 (Plant B)

System Level 2 includes all sub-processes included in system Level 1, but also the sub-processesGas cleaning, Gas upgrading and Compression of upgraded gas. The results for the energy use atLevel 2 at biogas plant B, allocated to unit processes, are found in Figure 15. As can be seen, Heating isstill the unit process with the highest energy use. However, the share of total energy use has decreasedfrom 63% for system Level 1 to 52% for system Level 2. Other changes between the system levels arethat the unit process Vehicle transports has increased from 6% to 11%, and the unit processes Cooling,Concentration and Compressed air have appeared for system Level 2.

The energy use at biogas plant B, allocated to sub-processes, is presented in Figure 16. The totalenergy use is presented in the diagram at the top and in the two lower diagrams, the energy use hasbeen separated depending on energy carrier (heat or electricity). The sub-processes with the highestenergy use is the same for system Level 1 and 2: Anaerobic digestion, Hygienization and Pretreatment.However, the share of total energy use has decreased for all three. Of the sub-processes not includedin system Level 1, Gas upgrading has the highest energy use, and Compression the lowest.

When separating the energy depending on energy carrier, Pretreatment is shown to be the mainuser of electricity (37%), followed by Gas upgrading (26%). For heat, the main user is Anaerobicdigestion (62%), followed by Hygienization (37%). The other two sub-processes using heat are Gasupgrading and Pretreatment.

In Figure 17, the energy use for 30 days is presented for plant B, at system Level 2, in a combinationof sub-processes and unit processes. The results do not differ from those for system Level 1, but thethree sub-processes Gas cleaning, Gas upgrading and Compression of upgraded gas have beenincluded. As can be seen, the main energy user for the sub-processes Gas cleaning and Gas upgradingis Vehicle transports. For Compression, the unit process with the highest energy use is Concentration.

6. Concluding Discussion

The methodology describes the procedure of analysing energy demand in a biogas plant byallocating the energy to different sub-processes and unit processes. Unit processes have previouslybeen presented in studies of the manufacturing industry, e.g., [22]. However, our paper is the firstattempt to use this concept on biogas production plants. It was shown that the support processespresented by Rosenqvist et al. [20] and Söderström [21] could be used without modification sincethey are universal for all sectors. However, the production processes were modified in the sense thattwo unit processes (Concentrating and Diluting) not found in Rosenqvist et al. [20] were added tobetter represent the processes found at a biogas plant. Moreover, compared to previous methodologiesfor evaluating energy demand of biogas production systems, our methodology analyses energy dataon a higher detail level. It was shown that the developed analytical methodology is applicable fordifferent sizes and configurations of biogas production plants.

As mentioned earlier, the methodology does not analyse biological performance of the biogasprocess in terms of energy content in biomethane output in relation to energy content in substrateinput. However, the results from the analysis of energy demand could easily be correlated to amountof substrate treated and biomethane produced at the specific biogas plant.

To be able to compare the results for two or more biogas plants, the energy data should be collectedduring a full year to reflect seasonal variations in energy use. This is also true when evaluating energyuse at a single biogas plant, since energy demand between different processes in the plant can differthroughout the year. It may be that some of the unit processes are not in use during some periodsof the year, and hence improvement measures cannot be identified. The measurement periods forthe biogas plants studied here are not the same, which could explain some of the differences in theresults. Yet, the main differences depend on the configuration of the biogas plants, e.g., the substrates,pretreatment methods and digestion technologies (mesophilic and thermophilic) used. However, it isimportant to point out that the aim of our study was not to evaluate the energy performance andidentify measures for improvements at the biogas plants studied, but to test the applicability of the

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developed methodology. In line with this, the results from the energy analysis of biogas plants A andB are presented without uncertainties. We want to stress that when using the methodology in realcases in the future (e.g., as basis for new investments), it is important to include uncertainties in theenergy analysis, e.g., measurements contain uncertainties due to the type of equipment used and thecontext in which the measurements are made.

The developed methodology can be used by individual biogas companies to identify measuresfor improved energy efficiency or evaluate implemented measures. Furthermore, the clearly definedsystem boundaries, sub-processes and unit processes make the methodology suitable for benchmarkingone plant with similar biogas plants, for example, plants which digest the same substrate and use thesame digestion technology. The methodology could also be used when comparing energy systems ofbiogas plants with different configurations. In this case, the energy performance of different biogassolutions could be compared and evaluated. As an example, different pretreatment methods can becompared and evaluated. However, when comparing the energy performance of different biogas plants,it is important to understand and take into consideration the differences in substrate input, locationof the biogas plants, configuration of the plants, etc., and include these in the evaluation. The resultsfrom such a comparison could be used when planning the construction of new biogas plants.

Three different approaches were used to analyse the energy demand in biogas plants. Each ofthese have their virtues and drawbacks. Allocation of energy use to sub-processes could imply fewerpoints of energy measurements, especially if each sub-process has its own switchgear. This allocationapproach can identify the most energy-intensive sub-processes, but does not provide informationabout how the energy is used within each sub-process. However, this could be an indicator of whichsub-process to study in more detail in order to find measures for improved energy efficiency. Moreover,the results can be used to compare how the energy input is divided between sub-processes in differentbiogas plants.

Allocation of energy use to unit processes usually means more points of data collection as theenergy demand of each pump, conveyor, stirrer, etc., is determined. This approach can identifywhether the support processes or production processes are the most energy demanding, and whichkind of equipment, e.g., pumps or stirrers, uses the most energy. This can provide information aboutthe performance of the heating system, ventilation system, pumps, etc., and serve as input whenidentifying inefficiencies.

The approach of combining the two allocation methods is the most time consuming, but givesa deeper understanding of the biogas plant’s energy system. Therefore, this approach should beused when analysing energy performance and identifying measures for improved energy efficiency atindividual biogas plants.

Analysis of energy demand is an important part of efficient energy management at biogas plantsand serves as a basis for investment decisions as well as other measures to improve energy efficiencyof biogas production. In addition, the energy use can be compared with energy use from previousyears in order to evaluate implemented measures. Due to the high detail level of the energy analysisin our methodology, the results are relevant and useful for managers at individual plants. Moreover,the results from the analysis of energy demand can be used by researchers as input to a detailed LCAstudy of biogas production systems.

In conclusion, the developed methodology for analysing energy demand in biogas plantsconsists of a system description with well-defined system boundaries and sub-processes as wellas unit processes, which makes the methodology consistent and robust. The two system levelspresented makes the methodology flexible and applicable to biogas plants with different configurations.We recommend using the methodology in the data analysis phase when performing energy audits atbiogas plants.

Acknowledgments: This work was carried out within the Biogas Research Center (BRC). BRC is funded by theSwedish Energy Agency, Linköping University, and participating organisations. The authors want to thank thepersonnel at the biogas plants for providing information and answering our questions during the data collection.

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Additionally, we want to thank Manuel Cañaveras Sola, master’s student at Linköping University, for assistingwith the collection of energy data from biogas plant B. We also want to thank Magnus Karlsson at LinköpingUniversity for valuable comments on the manuscript.

Author Contributions: The methodology for analysing energy demand in biogas plants was developedcooperatively by Maria T. Johansson, Emma Lindkvist and Jakob Rosenqvist. Collection of energy data atbiogas plant A was performed by Emma Lindkvist and Jakob Rosenqvist. Energy data collection at biogas plant Bwas completed by master’s student Manuel Cañaveras Sola together with Jakob Rosenqvist, and the work wassupervised by Maria T. Johansson and Emma Lindkvist. The analysis of the energy demand of the two biogasplants was completed by Maria T. Johansson, Emma Lindkvist and Jakob Rosenqvist. The paper was writtenjointly by Maria T. Johansson, Emma Lindkvist and Jakob Rosenqvist.

Conflicts of Interest: The authors declare no conflict of interest.

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