Methane losses in the biogas system

Transcription

1 Methane losses in the biogas system This publication has been produced with the assistance of the European Union ( The content of this publication is the sole responsibility of Baltic Biogas Bus and can in no way be taken to reflect the views of the European Union.

2 The Baltic Biogas Bus project will prepare for and increase the use of the eco-fuel Biogas in public transport in order to reduce environmental impact from traffic and make the Baltic region a better place to live, work and invest in. The Baltic Biogas Bus project is supported by the EU, is part of the Baltic Sea Region programme and includes cities, counties and companies within the Baltic region. Author: Katarina Jonerholm, Hans Lundborg, Sweco Environment AB Project Manager: Lennart Hallgren, Stockholm Public Transport Date: Reviewed by: Sara Anderson, Lennart Hallgren, Anneli Waldén, Stockholm Public Transport Pictures: Digester chamber and buffer tank at Växtkraft biogas production plant, Västerås (upper left); Water scrubber upgrading unit (upper right); Ground work for piping of biogas distribution system, Stockholm Gas (lower left); Fuelling of biogas bus, SL, Stockholm (lower right) 2

3 TABLE OF CONTENTS Summary Abbreviations Introduction Biogas as a sustainable bio fuel Methane as a grnhouse gas The Swedish voluntary initiative Biogas in the Stockholm Region a general overview of the system Biogas production General distribution techniques for biogas Use in biogas buses Methane losses related to the biogas system Limitations Biogas production plants Reception of substrate or sludge Pre-treatment Hygienisation tanks Digestion chamber Digestate tank Dewatering unit Bio-fertilizer storage/sludge silos Ventilation Gas equipment and gas pipes Gas holder Flare Gas analysis instruments Summary Upgrading units Off-gas Ventilation Gas equipment and gas pipes Summary Distribution Pipeline distribution Mobile gas storage distribution Summary Fuelling at biogas bus depots Technical buildings Gas storage Exhaust air chimneys and exhaust ventilation ducts Filling dispenser Nozzles (mouthpiece) of the filling hose Summary Biogas buses Environmental work and measures Methane leakage identification and measuring Identifying Leakages

4 Gas sniffers Leak detection with fluids Gas alarms Odorisation of upgraded biogas Methods of measuring Flame ionisation detection, FID Gas chromatography Infrared photo acoustic spectrometer, IR Mass spectrometry Sampling Calculations and quantifications Preventative actions Prevention Ventilation air treatment Gas extraction from the digestate tank Temperature control Design of overflow safeguards Careful consideration of gas measuring methods Flaring Off-gas treatment Minimisation Planning and logistics Process control Pressure control Gas leakage control Controlled methane destruction Safe operation and education of personnel Practical guidelines Ask for requirements and assurances from suppliers Supervise the gas pressure in the biogas system Regular leakage control Regular gas safety valve checks Regulations Swedish Environment Code Health and Safety, HSE, Swedish regulations and instructions Swedish law about flammables and explosives Conclusions and comments References and contacts

5 Summary Methane is estimated to be the second largest contributor to global warming of the long lived greenhouse gases after carbon dioxide. Emitted methane will remain in the atmosphere for several years and mainly originates from anthropogenic sources like rice agriculture, livestock, biomass burning and fossil fuel mining and burning. Biogas in itself is also mainly methane, but combustion of biogas converts methane to carbon dioxide (CO 2 ) and there is no or little net increase in greenhouse gases as long as the substrate used for the methane production is renewable. However, if the methane is not combusted and directly emitted into the atmosphere, it is indeed contributing to the global warming. As such, it is essential that biogas systems are designed and controlled to minimise any methane leakage. This report addresses methane losses in the whole biogas system chain, i.e. during production, distribution and consumption. The report identifies sources of methane emissions from major components, presents a number of available methane leakage instruments and summarises the most important actions for prevention and minimization of methane losses. The report covers methane losses that are directly related to the biogas system. Emission sources, preventative actions and minimization are based on contacts with biogas producers, distributors and bus depot operators, as well as prior studies with thorough evaluations of whole biogas plants, plant components during operation and interviews with plant owners and operators. In Sweden, the stakeholder and trade association Swedish Waste Management Association started the so called Swedish voluntary initiative in year The initiative means that biogas plant and upgrading unit owners commit to work in a systematic way by leakages inventory, identify and decrease methane emissions in their facilities. The initiative is one of many important measures that the Swedish biogas industry and Swedish authorities have done together in order to minimize environmental impact and improve the biogas system. Other factors that has had a major contribution in the systematic approach to methane leakages identification and minimization, is work performed by the Swedish stakeholder and trade association Swedish Gas Association, which publishes a great number of guidelines, statistics and feasibility studies within the biogas industry. For example, the Swedish Instructions about biogas plants, BGA 2012 gives practical information about design and construction as well as examples about what to include in a Maintenance Plan including checking for methane leakages. There are a great number of actions that can be taken to minimise and altogether prevent methane emissions from the various emission sources identified in the biogas system chain. This is possible through careful consideration of these aspects already in the early phases of conceptual and detailed design, construction and procurement of suppliers of components/facilities, as well as during operation. 5

6 Essential measures for reducing methane losses are to ensure proper operation without major fluctuations of production/supply/fuelling of biogas and hence fluctuating gas pressures, regular controls and gas leakages checks. Every nation engaged in production, distribution and utilisation of biogas are recommended to should establish national instructions and guidelines to set standards and time intervals for controls and monitoring of methane emissions. Here, Sweden can definitely be seen as a good example, with stringent environmental regulation for air emissions together with a collective approach in the biogas industry to develop guidelines for support to the relevant stakeholders. In-depth knowledge of methane emissions within the biogas industry can be a powerful tool of information and dissemination to other stakeholders. How to tackle these problems can answer to questions of concern or scepticism from opponents or the general public regarding any doubts of biogas as a true environmental sound and sustainable option to conventional fossil fuels or other renewable alternatives. 6

7 Abbreviations BSR CBG CNG LBG LNG WWTP Baltic Sea Region Compressed Biogas Compressed Natural Gas Liquefied Biogas Liquefied Natural Gas Waste water treatment plant 7

8 1. Introduction The Baltic Biogas Bus project prepares for and increases the use of the renewable fuel biogas in public transport secotr. The aim is to reduce environmental impact from traffic and make the Baltic region a better place to live, work and invest in. The Baltic Biogas Bus project is partly funded by the European Regional Development Fund within the Baltic Sea Region programme. Twelve partners from eight Baltic Sea countries are directly involved in the project; SL Stockholm Lokaltrafik (Stockholm Public Transport Company), Sweden Biogas Öst/ Energikontoret i Mälardalen (Energy Agency Malardalen), Sweden Ruter (Public Transport Company of greater Oslo), Norway HOG Energy, fuel interest organisation for the region around Bergen, Norway Hordaland fylkeskommun (Hordalen County Council), Norway VTT Technical Research Centre of Finland Tartu, the second city of Estonia Riga, the capital of Latvia Kaunas, the second city of Lithuania Motor Transport Institute of Poland ATI erc Education, research and furtherance of cooperations, Germany ITC Innovations and Trendcenter, Germany Extended use of biogas for city buses will lower emissions, improve inner city air quality and strengthen the role of public transport in an efficient strategy to limit the impact from traffic on climate change. The project generates strategies and policies to introduce biogas as well as analyses necessary measures in biogas production, distribution and bus operations. Activities are executed to facilitate further expansion. A pan-baltic network of partners forms a show room to demonstrate a sustainable transport system as a step towards reaching EU s climate goals. The partnership offers an ideal platform for cooperation, exchange and dissemination of knowledge, experience and technology. The partnership makes it possible to obtain a better position to negotiate with infrastructure and bus suppliers and at the same time raise the visibility of biogas buses. There are good examples of the use of biogas buses in public transport, but wide acceptance and introduction in BSR cities has not taken place yet. Cities are unaware or have incorrect information of the benefits of biogas buses. Furthermore, shifting to biogas buses from fossil fuel buses is complicated and a long-term approach is needed. Biogas can be produced from a range of sources and biogas buses can be ordered from several bus manufacturers. Still the missing link for most cities is an integrated long-term strategy to work towards introduction of biogas buses. 8

9 Further information about the project is found at The content in this report is based on a number of literature studies, real case analyses and experiences from operational personnel at existing biogas production plants, upgrading units and biogas bus depots in Sweden Biogas as a sustainable bio fuel Biogas is considered a sustainable bio fuel that can replace fossil fuels as energy sources for heat and/or electricity production or as fuel for vehicles. Sweden is at the forefront in the business of biogas production and upgrading for utilisation as vehicle fuel, particularly for use in heavy duty vehicles and the public transport sector. One of the largest consumers of biogas as vehicle fuel in Sweden, and the largest in the Stockholm region is Stockholm Public Transport, SL. Today, in December 2011, biogas is considered SL s first hand choice in the company s aim to fulfil its environmental targets of: 1 At least 50 % of the bus traffic within SL will by the end of 2011 be running on renewable fuels, % of the bus traffic within SL will be running on renewable fuels no later than 2025 To meet these goals, a combination of different renewable fuels in the bus fleet is required Methane as a greenhouse gas Methane is estimated to be the second largest contributor to global warming of the long lived greenhouse gases after carbon dioxide. Methane has a high Global Warming Potential, 21 times higher than carbon dioxide, or so called 21 CO 2 - equivalents 1. This includes indirect effects from e.g. changes in ozone and stratospheric water vapour which are global cooling indirect effects associated with methane. Emitted methane will remain in the atmosphere for approximately 8.4 years before the molecule is deteriorated through chemical oxidation in the troposphere. Methane emissions can be divided into natural and anthropogenic sources. Natural sources include emissions from wetlands, ocean floors, forests, wildfire and geological sources. Anthropogenic sources include rice agriculture, livestock, landfills and waste treatment, biomass burning and fossil fuel mining and burning (for all types of hydrocarbon fuels). At present, these anthropogenic sources are estimated to account for more than 60 % of total global emissions to the atmosphere. Combustion of biogas as source of energy will emit carbon dioxide just like combustion of other hydrocarbons. The difference is there is no net increase in 1 CO 2 -equivalent is the amount of emitted carbon dioxide that would cause the same impact on the climate, over a given time horizon, as an emitted amount of the greenhouse gas in question. 9

10 greenhouse gases as long as the substrate generated is renewable. While for example natural gas, which also mainly consists of methane, requires millions of years to materialize, biogas can be produces from waste/manure/agricultural byproducts in approximately days. However, emissions of non combusted methane from the biogas system can be considered negative from an environmental point of view as it is such a powerful greenhouse gas, although to some extent it depends on what the alternative management of the substrate is and hence what the biogas production is replacing The Swedish voluntary initiative The Swedish Waste Management Association 2 started the so called Swedish voluntary initiative in year Through this initiative biogas plant and upgrading unit owners commit to work in a systematic way leakages inventory and thereby identify and decrease emissions in their facilities. Leakage control shall be conducted at least once per year by the plant staff. Additionally, a thorough inventory with emission measuring is to be conducted every third year by an independent consultant and leakages to be corrected and sealed, or quantified and a plan put forward for measures to be taken to minimize and prevent. Total emissions from the plant are summarized and goals set for emission levels after measures are to be installed. To perform the leakage controls, quantification and reporting the initiative has worked out specific protocol templates and check lists for leakages and deviations, measuring methods and calculations to be used, as well as protocols for reporting back to the Swedish Waste Management Association. The association summarizes the results and returns it to the plant owners. The reporting is also gathered centrally to provide public information about average emissions and preventative measures for capacity building within the biogas industry. Any plant owner who wishes to get involved in the initiative will have an agreement with the association. 2 The Swedish Waste Management Association, founded in 1947, is a stakeholder and trade association and the largest area in Sweden for private and public stakeholders involved in waste management and recycling. Swedish Waste Management Association has almost 400 members including municipalities, municipal owned waste management companies, manufacturers, consultants and public leasing contractors. ( 10

11 2. Biogas in the Stockholm Region a general overview of the system 2.1. Biogas production Biogas used as fuel in the SL bus traffic, is produced at two waste water treatment plants (WWTP) in the Stockholm Region. Henriksdal WWTP is situated in the City of Stockholm and one plant is situated in the City of Lidingö (Käppala). Raw biogas is generated in digesters and there after the gas is dried and treated to fulfil the vehicle gas standards. There are different techniques used at each waste water treatment plant. At Henriksdal biogas is compressed and stored in high pressure storages but at Käppala no biogas is stored. The biogas fuel from Käppala is stored after pipeline distribution at bus depots. At the biogas production plant raw biogas is treated and purified to fulfil the Swedish gas vehicle fuel standard i.e. including more than 97 % vol. of methane. There are several techniques to upgrade the raw biogas, one is water scrubber and another is PSA systems (Pressure Swing Absorption). When the biogas is upgraded to vehicle fuel it may be transferred from the site or compressed and stored in high pressure bottles at the site. Figure 1. Biogas plant close to the Henriksdal WWTP in Stockholm. (Photo Scandinavian Biogas Fuels AB) General distribution techniques for biogas There are several different techniques to transport and distribute biogas. Compressed biogas (CBG) may be transported by pipeline or in bottles by truck. 11

12 Pipeline network Biogas distribution via pipelines to bus depots is SL s preferred option. Pipelines offers low operational maintenance costs, high dependability for delivery, low environmental impact during operation and is by SL today considered the long-term solution for effective biogas distribution. Biogas distribution can be done in a so-called low-pressure pipeline with a maximum gas pressure of 4 bars, alternatively in pipelines with higher pressure, normally 10 bars or higher. 4 bar systems in Sweden and in the SL area are normally designed operated in accordance with the Swedish norm EGN 2009 (soon EGN 2011). The gas pipeline is usually made of PE material (polyethylene, which is a plastic material) but can also be made of stainless steel. The underground pipeline is laid in a line trench below the surface, often in combination with other types of wires and cables. How the pipeline should be fixed in the trench and what the acceptable distances to other pipelines and wires are, depend on the pressure in the system. Therefore, it is important that the planned location and specifics for the piping are taken into consideration when making the choice of pressure in the system. In some cases it may be necessary to place part of the pipeline in hard ground or places where it is difficult to dig a line trench, for example in a narrow street in a congested area. In this case controlled directional drilling may be a solution. Directional drilling means drilling non-vertically. The drill head can move in bends, for example under roads, railway tracks or watercourses. The pipeline, plastic or steel, is then placed inside the drilling hole. Biogas may also be transported in gas pipelines located in water, either in PE or stainless steel material. An underwater pipeline is supplied with weights to place the pipeline on a levelled bottom of the sea or lake. Some parts of SL s biogas distribution pipeline pass through the islands of Lidingö and Värmdö, neighbouring municipalities to the City of Stockholm, meaning the biogas pipeline at some parts have been placed under water. The biogas pipeline distribution network in the Stockholm area is illustrated in Figure 2. The pipeline grid is partly owned by SL and partly owned by the Stockholm Gas Company (SGAB). 12

13 Figure 2. The expected final result of the planned biogas grid system in the SL region 3. Compressed gas in bottles If there is no appropriate gas pipeline network available, accessibility to biogas can be made possible by distribution via mobile gas storages. This form of distribution means Compressed Biogas (CBG) is transported in bottles on trailer trucks by road. Distribution of compressed biogas (CBG) is usually done via steel gas cylinders placed in gas storages called swab-body containers, see Figure 3. (Another name for swap-bodies is mobile gas stocks.) The swap-body is loaded onto a truck and transported on road to a biogas fuelling station. At the biogas station the container is unloaded and the truck switches over and instead loads and leaves with an empty container (bottles with low gas pressure). Figure 3. A sketch of a swap-body with steel cylinders (AGA Gas). 3 Stockholm Gas,

14 The geometric volume in a swap-body container is commonly 7 m 3. Each swap-body can accommodate up to Nm 3 of biogas and is divided into six sections. Sectioning lowers the risk of large emissions of gas in case of accidents. If one section starts leaking in case of an accident the probability is that leaking only occurs of approximately 350 Nm 3 of biogas. In practice, the gas cylinders are normally not filled to a maximum, and a common load is about Nm 3 in total for a swap-body. An alternative to steel gas cylinder transport is using composite cylinders. Composite is a light and strong material and the bottles are then made of fibreglass reinforced plastic, a material often used where there are specific requirements for strength, heat resistance and low weight. The transported volume of gas can then be increased Use in biogas buses In Stockholm there are over 230 biogas buses running on the streets since the introduction started in The buses are fuelled at biogas bus depots either with slow or fast filling systems. There are currently four biogas bus depots up and running located centrally, North and South of Stockholm City Centre. The bus tank can store up to 400 Nm 3 biogas at 250 bar gas pressure which is sufficient for the daily consumption, although the daily consumption per bus varies depending on the traffic route and in which commuting area the bus operates. In year 2011 a total of 6.3 million Nm 3 biogas was utilised in SL s biogas bus traffic. 14

15 3. Methane losses related to the biogas system Methane losses can occur in every step of the biogas chain, i.e. during production, distribution and consumption. Emissions can be categorised into three different types of emissions: 1. Continuous emission sources 2. Occasional emission sources 3. Diffuse emission sources Some of these emissions are inevitable, some are planned and some are unwanted or unknown and could be avoided. In the following chapter these different types of methane emission sources are identified and explained in more detail. For easier readability the chapter is divided into the main general parts of the biogas system chain, see Figure 4. Biogas Production Upgrading Distribution Fuelling stations at biogas bus depots Biogas buses Figure 4. Schematic picture of the main steps of the biogas chain. The methane emission sources described are based on previous reports published by the Swedish trade organisations Swedish Waste Management Association, Swedish Gas Centre, Swedish Gas Association and other, which have been involving a number of biogas plants and upgrading units in Sweden. Emission sources are also identified via classification plans. In classification plans, i.e. compulsory permit documentations for identifying areas in a facility/plant where explosive gas mixtures can occur; the emission areas are categorised into zones depending frequency and durability of explosive gas mixtures: Zone 0 an area in which an explosive gas atmosphere is present continuously or for long periods or frequently Zone 1 an area in which an explosive gas atmosphere is likely to occur in normal operation occasionally Zone 2 area in which an explosive gas atmosphere is not likely to occur in normal operation but, if it does occur, will persist for a short period only In this study, contacts have been made with owners, operators and responsible managers at biogas production plants, upgrading units, distribution systems and biogas bus depots. Contacts are presented in the texts and in the references chapter. 15

16 3.1. Limitations This report cover methane losses that are directly related to the biogas system, and limitations are made for any other indirect methane losses that may occur as a result of biogas production. This is to avoid a life cycle analysis (LCA) discussion. LCA analyses can quickly grow to become very detailed and is not the intended scope of this report. The management of waste, sludge or crops used as substrates for biogas production prior to reception at the biogas production plant are only partly covered and discussed. Any methane losses related to waste water treatment, transports, food processing, waste collection, cultivation, harvest or similar precedent to the point of reception at the biogas plant are not considered. Also, any methane losses related to the unloading, transport and long-time storages of bio-fertilizer or sludge at the end user are not considered. Storage times vary depending on season and the plant owners have little or no means to affect the management of bio-fertilizer or sludge further down the chain. Also, any methane losses related to mobile gas transports, other than transports regarded as consumption of biogas as vehicle fuel are not considered Biogas production plants Biogas can be produced from a wide range of organic material. Examples include sewage sludge at WWTP, source separated household food waste, industrial food waste, slaughterhouse waste, agricultural by-products, energy crops, cattle manure. The type of substrate used in the biogas production will affect the overall process design and plant layout. In this section, the objective is to highlight possible methane losses at the point of biogas production in general. Therefore, common units of a complex biogas production plant are evaluated. In reality, some biogas production plants do not include all units (e.g. at many WWTP there are no hygienisation tanks or pre-treatment of sludge required). For simplicity and better understanding of the production steps, the units evaluated in this section are schematically presented in Figure

17 incoming substrate Reception Gas storage gas utilisation Upgrading Boiler Engine Pre-treatment Digestion (Flaring) Hygienisation digestate mngt Bio-fertiliser storage Dewatering unit Digestate tank Figure 5. Schematic picture of common main units in a biogas production plant Reception of substrate or sludge Incoming substrates are a potential source of methane emissions, if the waste/crop/sludge by some reason has been stored for a longer time period without aeration, due to delays in delivery or poor management. In normal circumstances this should be avoided all together. There are financial benefits both for the substrate supplier, to get rid of or sell the substrate without longer storage time periods, and the biogas producer, to avoid any loss of material for digestion and biogas production. Any accidental anaerobic digestion that may occur during this first time period before delivery to the biogas plant could be regarded as minimal in normal cases. The methane producing bacteria only work under moist anaerobic conditions and it normally takes several days before the methane production start from the point of loss of access to oxygen. This is because the methanogenesis step (methane forming step) in the anaerobic digestion process follows the previous hydrolysis, acidogenesis/fermentation and acetogenesis steps, see the schematic process description in Figure 5. The time period for these initial steps depend on the properties of the substrate(s) in the process. A recent report from SGC (Swedish Gas Centre) studying a large number of reports on hydrolysis experiments, presents an estimation of approx. 2-4 days for hydrolysis (4-7 days for sewage sludge) (SGC, 2010). 17

18 Figure 6. Schematic picture of the biological breakdown process. When the incoming substrate is delivered to the biogas plant it is normally more or less directly fed into the biogas plant digester or pre-treatment system. If the biogas plant has a substrate storage (not available at all biogas plants) it is often dimensioned for the planned receiving volumes. Both to keep the waiting periods before treatment to a minimum, and to keep an even flow into the system as a buffer storage. Therefore, it is estimated anaerobic digestion at the substrate reception is negligible under normal circumstances. Air emissions during reception of substrate often constitute of Volatile Organic Compounds (VOCs), hydrogen sulphides and ammonia. Measured methane emissions at the stage of reception in existing biogas production plants amounts to % or Nm 3 /y (RVF, 2005a) Pre-treatment Solid substrates require pre-treatment before digestion in a wet digestion process. How complex the pre-treatment depends on the substrate homogeneity. It is easier to grind wheat crops than to crush, sort and sieve municipal source separated food waste. The pre-treatment process is often not a completely closed system and anaerobic conditions are avoided. At some biogas plants there is a separate mixing and buffer tank before feeding into the digester. At disturbances during operation there is a risk of methane 18

19 production due to a prolonged retention time in the mixing tank (Avfall Sverige, 2007, Rev 2009). Air emissions during pre-treatment of substrate often constitute of VOCs, hydrogen sulphides and ammonia (RVF, 2005a) Hygienisation tanks At some biogas plants there is a legal requirement for hygienisation of the substrate to avoid risks of spreading diseases, mainly Salmonella bacteria. Hygienisation can be performed in different ways as long as the total retention time and temperature are correlated. In Sweden, hygienisation is commonly performed in batch in tanks with retention time minimum 1 hour at 70 C, but other combinations are acceptable. The high temperature inhibits methane producing bacteria. However, before heating of the incoming batch has reached the optimal temperature, there is a risk of methane production, especially if reject water from the digestate after the digestion chamber is used for dilution of incoming substrate. The air ventilation from the hygienisation tanks should be collected and monitored (RVF, 2005a). Measured methane emissions at the stage of hygienisation, mixing and buffer tanks in existing biogas production plants amounts to % or Nm 3 /y (RVF, 2005a) Digestion chamber After pre-treatment (in solid/liquid substrate fed biogas production plants) and possibly hygienisation, alternatively after the sedimentation of sludge in the waste water treatment process in WWTP, the slurry is fed into the digestion chambers where the methane production takes place. This is a closed process to maintain an oxygen free environment; hence normally no losses of methane should occur. Well equipped digestion chambers are equipped with an overflow safeguard to prevent overflowing and pressure build-up. The overflow safeguard can be considered as a continuous emission source if it is designed as a vertical open tube, or at least as an occasional emission source if they re opened regularly. It is important that the detailed design of the overflow safeguard is made with risks of methane emissions taken into consideration. Digestion chambers in Sweden are also equipped with gas safety valves on top of the chambers (gas domes) to avoid high pressure build-up. The safety valves are commonly designed as a hydraulic seal. The valves must be calibrated to manage normal chamber pressure, normally mbar. If they re not correctly calibrated and monitored regularly they can become occasional emission sources, where methane losses occur even at very low pressure build-up which the chambers can sustain. In e.g. Germany and Denmark, it is more common to use sealed membranes as top cover of the digestion chambers. These covers have in some cases been identified 19

20 as potential occasional or even continuous emission sources, as leakage may occur in the seams and edges (Brolin, pers. comm, 2011). Occasional methane emissions may also occur in the seal of the propeller/stirring device. Measured methane emissions at the stage of digestion in existing biogas production plants amounts to % or Nm 3 /y, where the single largest single identified source of leakage was the overflow safe guards (RVF, 2005a). During maintenance and inspections of the chambers they need to be emptied. Methane emissions can at these moments attain the same gas volume as the chamber size. Although inspections are expected and planned for, they only occur at extra ordinary circumstances and are not considered part of the normal operation. Disturbances during operation, for example foaming in the digester, are common sources of malfunctions and needed inspections that may cause inevitable methane emissions to the atmosphere Digestate tank After the digestion chamber the digestate is pumped to a digestate tank, at some plants also called degassing tank. The potential of methane losses after the digestion chamber is related to the degradation ratio. In some biogas production plants gas extraction also takes place in the digestate tank to collect methane from remaining material not digested during the retention time in the digester. At some plants the digestate tank functions as dewatering unit/thickener and may in some cases, particularly in larger WWTP, be unsealed to the atmosphere. In such cases the tanks constitutes a high potential risk as continuous methane emission source. Cooling of the digestate will subsequently lower the methane producing bacteria activity Dewatering unit The dewatering unit increases the dry solid (DS) content of the digestate to create a solid bio-fertiliser or WWTP sludge. The main purpose is to remove excess water and thereby reduce transportation/distribution costs of the bio-fertiliser or sludge. Typical dewatering units are centrifuges, screw presses or belting presses. Not all biogas plants dewater the produced digestate. The dewatering units are not completely sealed to the atmosphere. Any diffuse methane emissions occurring due to still active methane producing bacteria is unavoidable here Bio-fertilizer storage/sludge silos The last step in the plant for the generated bio-fertilizer or sludge is the storage. From here, the material is loaded onto trucks and further transport to the designated final destination. Bio-fertilizer is commonly transported to satellite storages by the farmlands. 20

21 The storages vary in design depending on the type of digestate. Dewatered solid bio-fertilizer or sludge is usually stored in open containers. Liquid bio-fertilizers are in Sweden most commonly stored in concrete wells, where the bottom is casted and the side walls installed in modules. The fertilizer is then also usually covered with a roof or floating tarpaulin to minimize losses of ammonia and avoid dilution from rain. Sizes vary from m 3. Also here the potential of methane losses is related to the degradation ratio in the digestion chamber. However, if the dewatering process has aerated the material thoroughly, the probability of methane emissions will be lower. Also, as the digestate is cooled down the methane production will gradually stop completely. Scandinavian Biogas Fuels has performed lab tests on dewatered digestate from a pilot biogas plant during year 2009, with results indicating a maximum of 1 Nm 3 methane loss/ton dewatered digestate. This figures should however not be directly seen as true for large-scale conditions, but rather as an indication of the order of magnitude (SBF, 2009). Methane production can be regarded as negligible at temperatures below 20 C (SNV, 2003) Ventilation Some biogas production plants connect all parts of the exhaust air ventilation per plant section to one general ventilation system. Often this exhaust air is led via an odour reduction unit (e.g. bio filters, water och chemical scrubbers, ozone or carbon filters) before release to the atmosphere. In principle, all major plant sections are ventilated: pre-treatment area, buffer and hygienisation tanks, dewatering units and digestate storage compartments and halls. The ventilation system is a continuous emission source of methane Gas equipment and gas pipes All gas equipment and gas pipes networks are potential sources of diffuse or accidental emissions due to leaking seals, flanges, safety valves or manual valves. Regular leakage controls of the equipment and monitoring the air quality in the equipment room area and ventilated exhaust air from the room are important. After the digestion chamber the produced biogas (normally containing % methane, depending on the substrate) is led to a gas holder. In some gas systems the gas is also passed via safety pressure regulation compartments, one or several steps of condensation traps and gas filters in order to remove impurities and reduce the water content. Gas safety equipment or safety valves will release biogas into the atmosphere if the pressure build-up is too high. 21

22 Gas holder Gas holders will help to maintain the right pressure in the gas system and function as buffer storage to even out fluctuations in production and consumption. Gas holders can be constructed in various ways. Some are constructed as a protective outer construction of concrete or steal, with inner membranes to contain the biogas, while others function as a floating dome, where the steel roof of the gas holder chamber fits like a cap and is submerged in water in a ledge around the gas holding compartment. Gas holder constructions and membranes must be checked regularly as they are also potential continuous emission sources with risks of leakage from seams or edges. Gas holders are also equipped with mechanical and/or hydraulic high pressure safety valves, which will release biogas into the atmosphere if the pressure build-up is too great. During normal operation methane losses from the gas holders should be regarded as negligible Flare The flare is utilized for destruction of excess biogas when applicable, for example during operational stops or if the upgrading unit, gas engines or boilers temporarily do not run or not at expected capacity. Risk of occasional methane emissions from the flare exist if the combustion efficiency is low, as some methane then will be emitted uncombusted to the atmosphere. The efficiency is related to the model and age of the flare. Open flares that are commonly used at biogas production plants offers less control over the airflow and sometimes lower temperatures. However, more technologically advanced flares can offer both good airflow control and surveillance of the flame via UV-sensors. This way the expected occasional methane emissions can be reduced to %, according to suppliers (RVF, 2005b). Figure 7. Flare at Växtkraft biogas plant, Västerås, Sweden (Sweco, 2011). 22

23 Accidental emissions from the flare may occur if the flare accidentally malfunctions and for some reason do not ignite or extinguish before all the airflow has been combusted Gas analysis instruments The biogas volumes produced is continuously monitored with flow meters. There are also possibilities to continuously measure gas quality in specific analysis instruments where a small tributary flow is extracted from the gas system and then emitted to the atmosphere, thus constituting a continuous emission source. 23

24 Summary The following main emissions sources can be identified at biogas production plants: Table 1. Main methane emissions sources at biogas production plants. Continuous emission sources Overflow safeguards in the digestion chambers Unsealed digestate tanks and tank agitator shafts Dewatering units (centrifuges, presses) Bio-fertiliser storage/sludge silos Ventilation system connected to the process plant sections Gas analysis instruments Occasional emission sources Overflow safeguards in the digestion chambers Gas safety valves Emptying of digestion chamber during inspections Emptying of digestate tank during inspections During flaring Diffuse or accidental emission sources At operational errors, for example prolonged retention time in the mixing and buffer tank, prolonged high pressure build-up causing safety valves to open for a long time period, no ignition in the torch during flaring, accidental intake of substrate with unknown properties causing the digestate to go sour or other, demanding further actions and in worst-case emptying of the whole chamber, other Because of poor maintenance and control, for example: leakage from seals, flanges, valves, leakage from membranes on the digestion chamber roof or membrane gas holder, leakage from gas safety valves that are not correctly calibrated, other It is important to note that suppliers of equipment for biogas production plants and gas systems sometimes offer guarantees of maximum methane losses. How this is monitored, calculated or attained post delivery during operation is not always defined. Within the Swedish voluntary initiative measures of methane emissions have been performed at a number of biogas production plants. Methane losses calculated as a percentage of produced methane per biogas production plant from measures performed 2007, 2008, 2009, indicate the following: 24

25 Table 2. Percentage of methane emissions from biogas production based on measurements performed within the Swedish voluntary initiative (Avfall Sverige, 2007, Rev 2009). at biogas production plants at WWTP at biogas production plants for source separated household waste at biogas production plants for organic industrial waste Average methane losses 3.1 % 2.1 % 1.7 % 1.1 % 0.2 % 0.2 % Median methane losses 3.3. Upgrading units There are several different methods of upgrading the raw biogas to vehicle fuel quality, but the main purpose for all of them is to remove the carbon dioxide and other gases present in the raw biogas. The most common techniques in Sweden are water scrubber, pressure swing adsorption (PSA) and chemical scrubbers with amine. Upgrading units are commonly delivered turn-key from the suppliers, which either may present methane losses as mass flow chart analyses, or they provide guarantees of percentage of methane loss (calculated, not measured). There are suppliers that give guarantees of 1-2 % methane loss (2005b). Figure 8. Upgradning unit at Bromma WWTP, Stockholm (Sweco, 2010). 25

26 Off-gas The single most important source of methane losses from upgrading units is the offgas. Off-gas is the excess gas left when the methane has been separated, mainly containing carbon dioxide, but also to some extent methane that has been adsorbed to the water, chemicals or active carbon. The quantity of methane in the off-gas depends on the upgrading technique. Older upgrading techniques, for example water scrubbers without regeneration of water for absorption have higher amounts of methane in the off-gas than chemical upgrading techniques, as the binding capacity of water is lower compared to chemicals. This is however a prioritised issue, and as advances in the technology are made, new water scrubbing upgrading units are designed to minimize methane slip. The off-gas can be thermally oxidized in a separate gas treatment facility (with thermal, catalytic or biological oxidation) or mixed with other air streams at the biogas production plant, if the facilities are built and designed to fit together, and flared. Separate gas treatment facilities for treating off-gas normally require another energy source for combustion. At some plants these are supplied with a raw biogas stream from the production Ventilation Exhaust air in the ventilation from the process rooms and other ventilated equipment areas may contain low levels of methane, as diffuse emissions may occur from compressors, gas pipe seals, flanges, safety valves or manual valves Gas equipment and gas pipes All gas equipment and gas pipes networks are potential sources of diffuse or accidental emissions due to leaking seals, flanges, safety valves or manual valves. Regular leakage controls of the equipment and monitoring the air quality in the equipment room area and ventilated exhaust air from the room are important. 26

27 Summary The following main emissions sources can be identified at biogas upgrading units: Table 3. Main methane emissions sources at biogas upgrading units. Continuous emission sources Off-gas (inevitable losses of methane as off-gas is released into the atmosphere) Ventilation system connected to a process room Gas analysis instruments Occasional emission sources Gas safety valves Diffuse or accidental emission sources At operational errors Ventilation Because of poor maintenance and control, for example: leakage from seals, flanges, valves, leakage from gas safety valves that are not correctly calibrated other Within the Swedish voluntary initiative measures of methane emissions have been performed at a number of biogas upgrading units. Methane losses calculated as a percentage of produced methane per biogas production plant from measures performed 2007, 2008, 2009, indicate the following: Table 4. Percentage of methane emissions from biogas upgrading units based on measurements performed within the Swedish voluntary initiative (Avfall Sverige, 2007, Rev 2009). Average methane losses Median methane losses Chemical scrubbers 0,4 % 0,4 % PSA 1,5 % 1,4 % Water scrubber 3,1 % 2,1 % It should be noted that that the age of the unit has a great impact on the size of the measured methane emissions. If only new upgrading units were to be measured and tested the figures would be expected to be lower Distribution Pipeline distribution As mentioned in chapter SLs idea is to supply all their biogas bus depots via pipeline biogas distribution. Pipelines offers low operational maintenance costs, high dependability for delivery, low environmental impact during operation and is by SL today considered the long-term solution for effective biogas distribution. 27

28 Biogas distribution pipes used are in PE material (polyethylene). Older gas pipes are made of steel or cast iron, such as parts of the Stockholm City town gas grid (natural gas), but PE-pipes are more cost efficient, corrosion free and require minimal maintenance of the pipe itself. Life time of the gas pipes are normally determined and tested to at least 50 years (SGC, 2001). Figure 9. PE pipes for biogas distribution in Stockholm In Sweden, it is the distributor who is responsible for the network, and an independent inspector approved by the Swedish Gas Association, carry out inspections and controls of construction and manufacture, commissioning and recurrent controls. At the recurrent controls corrosive damages or other damages, density, marking of pipes and valves amongst others are checked. The pipeline inspections, leak detection of the pipe network etc. shall take place one year after operations start and then at least every 6th year, but in some cases more frequently for biogas pipelines. Due to recurrent controls and durable pipe material, methane emissions from well maintained pipe networks are rare. Plastic pipes can however be wrecked mechanically through digging when for example street works are performed. Other cases where the pipe can cause leakages is due to cracks in defect pipe wall material or that the pipe is chemically degraded due to high temperatures and the properties of surrounding packing material (SGC, 1999) Mobile gas storage distribution Gas storages have in other reports been identified as diffuse methane emission source due to leaking seals at the mouthpiece of the gas cylinders (RVF, 2005a). At filling and emptying of the cylinders there is a quick increased pressure during release before the valves are closed. These so called gas puffs are inevitable, but should be minimized with proper control and check-ups of the valves. 28

29 Summary The following main emissions sources can be identified during distribution: Table 5. Main methane emissions sources during gas distribution. Occasional emission sources Gas puffs at filling and emptying of gas cylinders on mobile gas storages Diffuse or accidental emission sources Leaks at the mouthpiece of the gas cylinders on mobile gas storages Mechanical or chemical impact on gas distribution pipes Because of poor design, operation, maintenance or control, for example: leakage from seals, flanges, valves 3.5. Fuelling at biogas bus depots The design of a biogas bus depot varies depending on the biogas supply, but typically comprises of: compressor unit gas storage priority panel fuelling system (slow or fast filling system) LNG-back up storage for redundancy The filling system must manage a filling pressure of at least 200 bars at 15 C and the ramp is dimensioned to avoid drop in gas pressure. More details on design, function and operation of biogas bus depots are described within WP 5 and the report WP 5.2 Innovative biogas fuelling system alternatives for buses (2012). The overall design of a biogas bus depot may vary depending on the location, climate etc. There are several national requirements, regulations and European standards in place for biogas and natural gas vehicle filling systems to assure proper gas management and high level of safety. Emission sources identified at biogas bus depots are based on SLs experiences and classification plans Technical buildings At SLs biogas bus depots, compressors and booster compressors are placed inside a building for wind and weather protection. At normal operation methane leakage from compressors should not occur. However, compressors contain many moving parts and are powered by electric motors. They will eventually reach very high engine speeds that cause vibrations, leading to wear and tear of materials and affecting connections and piping, thus causing emissions (Wåhlin, 2011). During maintenance or malfunction methane emissions are also likely. Therefore, the building must fulfil high fire protection standards, have high-pressure discharge surfaces and be well ventilated. 29