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Biogas upgrading - Technical Review

Hoyer, Kerstin LU ; Hulteberg, Christian LU ; Svensson, Mattias LU ; Jernberg, Josefina LU and Nörregård, Öyvind (2016)
Abstract
Biogas produced by anaerobic digestion is often used in gas turbines to produce
electricity. In order to increase the value of the gas and to enable utilization of the gas in other applications, it may be advantageous to upgrade the biogas. In this way, the carbon dioxide as well as various impurities are removed and biomethane is produced. Biomethane is similar to natural gas and can be used in similar applications, e.g. fed into the natural gas grid, or as vehicle fuel. Several different biogas upgrading techniques are on the market today. Some of them
make use of the fact that carbon dioxide and methane have different solubility in
different solvents. By choosing a solvent which has a high solubility for carbon dioxide but... (More)
Biogas produced by anaerobic digestion is often used in gas turbines to produce
electricity. In order to increase the value of the gas and to enable utilization of the gas in other applications, it may be advantageous to upgrade the biogas. In this way, the carbon dioxide as well as various impurities are removed and biomethane is produced. Biomethane is similar to natural gas and can be used in similar applications, e.g. fed into the natural gas grid, or as vehicle fuel. Several different biogas upgrading techniques are on the market today. Some of them
make use of the fact that carbon dioxide and methane have different solubility in
different solvents. By choosing a solvent which has a high solubility for carbon dioxide but lets methane pass through unchanged, the carbon dioxide can be separated from the methane in biogas efficiently. Common solvents used for biogas upgrading are water, amines as well as organic solvents such as Genosorb®. The difference in adsorption behavior of carbon dioxide and methane on a surface at different pressures is used in pressure swing adsorption (PSA), which can be used to effectively separate carbon dioxide from methane. Another common biogas upgrading technique uses the fact that carbon dioxide is more likely to pass through a semi permeable barrier, e.g. a membrane, than methane. By letting biogas pass through such a membrane, the carbon dioxide can thus be removed from the gas, leaving concentrated methane in the product stream. Finally, the difference in boiling point between methane and carbon
dioxide may be used to separate the gases in cryogenic distillation. For this report, data on the specific investment cost was collected from companies supplying biogas upgrading plants using the above described processes. The data shows a span of investment costs, but it also shows that there is no significant general difference in investment cost between the different techniques when considering a given standard project. Also the energy consumption is rather similar for the different upgrading techniques. When deciding on a suitable biogas upgrading process, it is therefore important to rather consider other aspects. These may include the ability of the different processes to handle specific impurities present in the actual project or
specific requirements in product gas quality. Also the need for consumables such as anti-foam, chemicals for pH regulation as well as operational costs for any needed pretreatment differs between biogas upgrading processes but is of course also dependent on the pretreatment needed in a project. It is important to remember that the conclusion regarding specific investment cost in this report is related to a standard case. In a real project, where more or less pre- and posttreatment will be needed depending on the choice of upgrading technique, the investment cost for different biogas upgrading techniques will most likely differ. Biogas produced from various substrates such as agricultural residues, biological waste or sewage sludge contains low concentrations of unwanted substances, e.g. impurities, such as H2S, siloxanes, ammonia, oxygen and volatile organic carbons (VOC). H2S is separated from the methane in most biogas upgrading techniques. How efficient this removal is and thus whether it is enough to meet product gas requirements differs between the different techniques. Scrubbers using absorption in water, amines or organic solvent usually remove most of the H2S, while polishing filters are needed for membrane upgrading and PSA. When separated from the methane gas, H2S, however, ends up in a CO2 rich side stream such as stripper air where it usually needs to be
removed due to environmental legislation. If the CO2 stream is utilized, the necessity to remove H2S depends on what the gas is used for. H2S thus needs to be removed from the gas at some point in most cases, but depending on the biogas upgrading technique used, this may need to be done in the raw biogas or there may be a choice regarding where in the process to remove H2S. Siloxanes may be harmful to process equipment when present at too high concentrations. In scrubber systems the produced biomethane usually needs further drying and the main part of siloxanes are removed in the dryers. Ammonia is soluble in water and the concentrations commonly found in biogas are usually removed in the condensation which is usually part of a biogas upgrading system in order to protect the upgrading system from liquid water. Ammonia is not usually a problem in biogas upgrading systems. However, when H2S and ammonia are
present simultaneously, it is important to prevent precipitation of compounds formed when these two react with each other. Since anaerobic digestion occurs under anaerobic conditions, e.g. with no oxygen present, the concentration of oxygen in biogas is usually low. Improper adjustment of oxygen injection systems used in order to biologically remove larger concentrations of H2S may increase the oxygen levels of the raw biogas. However, the oxygen concentration is commonly monitored carefully in biogas systems in order to minimize the explosion risk. Biomethane quality requirements when the gas is fed into a natural gas grid are currently limiting the oxygen content in biomethane to almost zero, especially in gas grids which include gas storage systems. It may therefore be necessary to remove oxygen from the product gas, or raw biogas if preferred, if the oxygen present in the raw biogas is passed to the produced biomethane. This is valid for scrubber techniques except membrane and PSA
systems which remove a significant amount of the oxygen. The product gas leaving the plant must uphold certain gas quality criteria, either set as a bilateral agreement with the transporter and/or buyer of the biomethane, which currently are based on national specifications. A new CEN standard on biogas injection of H gas quality has recently been sent to formal vote, regulating levels of minor impurities such as siloxanes and ammonia, and major ones such as hydrogen and oxygen. The minimum calorific content is specified, but the wobbe index is not. Allowed sulfur levels are still relatively high, and not including the contribution of odorization, which is still an issue handled nationally in Europe. Biomethane and compressed natural gas (CNG) delivered at the point of retail is also under standardization. There are efforts to introduce a second dedicated non-grid based grade, which will be beneficial to the sales of biomethane, since most of the parameters will easily be upheld by normal upgrading, with the exception of raw biogas containing larger amounts of siloxanes. In biogas upgrading with membrane separation, amine scrubbers and PSA, very pure
CO2 can be produced. In biogas upgrading using these techniques, besides biomethane, CO2 can be produced and utilized. The most common ways to use CO2 are for the use in greenhouses, in the food and cooling industry or to utilize access electricity to let the CO2 react with H2 to produce methane, co-called power-to-gas. Power to gas constitutes a way to store access electricity in the form of gas which is gaining increased interest during recent years. (Less)
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Energiforsk
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978-91-7673-275-5
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@techreport{9e1c64bd-efe6-4cc4-88d5-c79eab06fcc5,
  abstract     = {Biogas produced by anaerobic digestion is often used in gas turbines to produce<br/>electricity. In order to increase the value of the gas and to enable utilization of the gas in other applications, it may be advantageous to upgrade the biogas. In this way, the carbon dioxide as well as various impurities are removed and biomethane is produced. Biomethane is similar to natural gas and can be used in similar applications, e.g. fed into the natural gas grid, or as vehicle fuel. Several different biogas upgrading techniques are on the market today. Some of them<br/>make use of the fact that carbon dioxide and methane have different solubility in<br/>different solvents. By choosing a solvent which has a high solubility for carbon dioxide but lets methane pass through unchanged, the carbon dioxide can be separated from the methane in biogas efficiently. Common solvents used for biogas upgrading are water, amines as well as organic solvents such as Genosorb®. The difference in adsorption behavior of carbon dioxide and methane on a surface at different pressures is used in pressure swing adsorption (PSA), which can be used to effectively separate carbon dioxide from methane. Another common biogas upgrading technique uses the fact that carbon dioxide is more likely to pass through a semi permeable barrier, e.g. a membrane, than methane. By letting biogas pass through such a membrane, the carbon dioxide can thus be removed from the gas, leaving concentrated methane in the product stream. Finally, the difference in boiling point between methane and carbon<br/>dioxide may be used to separate the gases in cryogenic distillation. For this report, data on the specific investment cost was collected from companies supplying biogas upgrading plants using the above described processes. The data shows a span of investment costs, but it also shows that there is no significant general difference in investment cost between the different techniques when considering a given standard project. Also the energy consumption is rather similar for the different upgrading techniques. When deciding on a suitable biogas upgrading process, it is therefore important to rather consider other aspects. These may include the ability of the different processes to handle specific impurities present in the actual project or<br/>specific requirements in product gas quality. Also the need for consumables such as anti-foam, chemicals for pH regulation as well as operational costs for any needed pretreatment differs between biogas upgrading processes but is of course also dependent on the pretreatment needed in a project. It is important to remember that the conclusion regarding specific investment cost in this report is related to a standard case. In a real project, where more or less pre- and posttreatment will be needed depending on the choice of upgrading technique, the investment cost for different biogas upgrading techniques will most likely differ. Biogas produced from various substrates such as agricultural residues, biological waste or sewage sludge contains low concentrations of unwanted substances, e.g. impurities, such as H2S, siloxanes, ammonia, oxygen and volatile organic carbons (VOC). H2S is separated from the methane in most biogas upgrading techniques. How efficient this removal is and thus whether it is enough to meet product gas requirements differs between the different techniques. Scrubbers using absorption in water, amines or organic solvent usually remove most of the H2S, while polishing filters are needed for membrane upgrading and PSA. When separated from the methane gas, H2S, however, ends up in a CO2 rich side stream such as stripper air where it usually needs to be<br/>removed due to environmental legislation. If the CO2 stream is utilized, the necessity to remove H2S depends on what the gas is used for. H2S thus needs to be removed from the gas at some point in most cases, but depending on the biogas upgrading technique used, this may need to be done in the raw biogas or there may be a choice regarding where in the process to remove H2S. Siloxanes may be harmful to process equipment when present at too high concentrations. In scrubber systems the produced biomethane usually needs further drying and the main part of siloxanes are removed in the dryers. Ammonia is soluble in water and the concentrations commonly found in biogas are usually removed in the condensation which is usually part of a biogas upgrading system in order to protect the upgrading system from liquid water. Ammonia is not usually a problem in biogas upgrading systems. However, when H2S and ammonia are<br/>present simultaneously, it is important to prevent precipitation of compounds formed when these two react with each other. Since anaerobic digestion occurs under anaerobic conditions, e.g. with no oxygen present, the concentration of oxygen in biogas is usually low. Improper adjustment of oxygen injection systems used in order to biologically remove larger concentrations of H2S may increase the oxygen levels of the raw biogas. However, the oxygen concentration is commonly monitored carefully in biogas systems in order to minimize the explosion risk. Biomethane quality requirements when the gas is fed into a natural gas grid are currently limiting the oxygen content in biomethane to almost zero, especially in gas grids which include gas storage systems. It may therefore be necessary to remove oxygen from the product gas, or raw biogas if preferred, if the oxygen present in the raw biogas is passed to the produced biomethane. This is valid for scrubber techniques except membrane and PSA<br/>systems which remove a significant amount of the oxygen. The product gas leaving the plant must uphold certain gas quality criteria, either set as a bilateral agreement with the transporter and/or buyer of the biomethane, which currently are based on national specifications. A new CEN standard on biogas injection of H gas quality has recently been sent to formal vote, regulating levels of minor impurities such as siloxanes and ammonia, and major ones such as hydrogen  and oxygen. The minimum calorific content is specified, but the wobbe index is not. Allowed sulfur levels are still relatively high, and not including the contribution of odorization, which is still an issue handled nationally in Europe. Biomethane and compressed natural gas (CNG) delivered at the point of retail is also under standardization. There are efforts to introduce a second dedicated non-grid based grade, which will be beneficial to the sales of biomethane, since most of the parameters will easily be upheld by normal upgrading, with the exception of raw biogas containing larger amounts of siloxanes. In biogas upgrading with membrane separation, amine scrubbers and PSA, very pure<br/>CO2 can be produced. In biogas upgrading using these techniques, besides biomethane, CO2 can be produced and utilized. The most common ways to use CO2 are for the use in greenhouses, in the food and cooling industry or to utilize access electricity to let the CO2 react with H2 to produce methane, co-called power-to-gas. Power to gas constitutes a way to store access electricity in the form of gas which is gaining increased interest during recent years.},
  author       = {Hoyer, Kerstin and Hulteberg, Christian and Svensson, Mattias and Jernberg, Josefina and Nörregård, Öyvind},
  institution  = {Energiforsk},
  isbn         = {978-91-7673-275-5},
  language     = {eng},
  month        = {06},
  pages        = {76},
  title        = {Biogas upgrading - Technical Review},
  year         = {2016},
}