Biogas upgrader

Under biogas processing method to be understood that where the biogas is cleaned, it can then be fed to an energy or material use. The raw biogas is produced in biogas plants and contains a mixture of different gases. Also, sewage gas and landfill gas must be treated before recycling.

Objectives and variants

Distinction can be drawn between the basic treatment of the raw biogas, which is necessary for example for utilization in a biogas cogeneration plant, and the more complex processing to natural gas quality ( biomethane or biogas ). The adjacent table shows the composition of raw biogas after primary treatment and biomethane. The fractions of crude biogas can, depending on the substrate, plant design, and other factors vary greatly. The nature of the biomethane is adapted to the corresponding qualities of natural gas.

Biogas is utilized mostly directly to the biogas plant in a combined heat and power (CHP). For this reason, in the course of treatment, the desulfurization and drying is necessary to avoid corrosion in the CHP. In order to feed biogas into the natural gas grid or use as a motor vehicle fuel a more comprehensive treatment is necessary. Also in this case there will be a drying and desulfurization. In addition, the separation of carbon dioxide take place, and the conditioning of the treated biogas into biomethane, which then have comparable natural gas properties. The biogas upgrading to natural gas quality has the advantage that the biomethane through the natural gas network led eg can be converted to electricity in CHP there, where the heat produced is required. Thus, the coal that occur, for example, by a CHP on a swimming pool, year-round has a high heat demand. It is also contain a certain proportion of biomethane gas for private customers the ability to choose gas supply contracts.

Process steps

The complete processing of raw biogas to biomethane usually comprises four steps: The biogas desulphurisation, the gas dehydration, the separation of methane and carbon dioxide and before feeding into the natural gas grid conditioning. These steps often rely on each other, but can be partially combined.

Desulfurization

At low hydrogen sulfide loads ( H2S) of the raw gas as they are likely to occur in anaerobic digestion of energy crops, is usually sufficient sulphide precipitation for coarse removal of H2S. Biogas from protein- rich substrates or sewage significantly higher concentrations of H2S may occur. In such cases, or even at high gas flow rates biogas scrubbers are used. In part, the desulfurization is performed in parallel for the separation of the gases contained in the raw biogas. Then often occurs a desulfurization, wherein said gas is passed through a plurality of series-connected active carbon filter.

Among the practical conventional methods of desulfurization of biogas

• Cleaning after gas production by Entschwefelungsfilter: Here, the gas containing iron filter material ( bog iron, steel wool ) is passed. The filter material must be replaced or regenerated by heating at saturation.
• Purification by addition of atmospheric oxygen (biological desulfurization, Gasraumentschwefelung ): The H2S is converted by bacteria that grow on surfaces in the gas space, with the added oxygen in the fermentation chamber to elemental sulfur and water. The sulfur is deposited, can accumulate in the system. This method is frequently used in practice and is usually sufficient so as not to exceed the maximum recommended for CHP concentrations of H2S. Upon subsequent treatment to biomethane but nitrogen and oxygen radicals shares of the added air can be problematic. An advantage of the method is that the sulfur predominantly found in the fermentation residue, thus increasing its fertilizer value. Instead in the gas chamber, air or another oxidizing agent may also be given directly into the reactor liquid ( Linde patent).
• Caustic wash: The biogas is washed in a packed column in countercurrent with water. The brine must be disposed of subsequently. This method reduces the sulfur content in addition to the CO2 concentration in the biogas. In caustic wash with biological desulfurization ( Paques patent) the liquor is regenerated in a second aerobic reactor in half. In addition to in comparison to normal caustic wash reduced, sulfur-free wastewater stream produced an elemental sulfur mud.
• Addition of iron ions: At high protein moieties in the starting substrate, the concentrations of H2S can exceed 20,000 ppm. This is beyond the capacity of available filter types. The addition of iron ions to help to prevent the formation of hydrogen sulfide in the digester. Iron has a high affinity to sulfur and connects to this to form insoluble iron sulfide ( FeS ). The iron sulfide remains as a solid digestate.
• Irreversible adsorption on activated carbon: Activated carbon as a filter medium is partially iodinated to increase its loading capacity. This method is suitable only for very low concentrations of H2S, for example, than cleaning.
• Recirculation of partially desulfurized biogas in the reactor liquid: This method improves the expulsion of the remaining dissolved in the liquid H2S. Since hydrogen sulfide is toxic and in high concentrations inhibits the biological processes in the fermenter of a biogas plant, this method is also used to process stability.

Dehydration

When drying the raw gas, water vapor is withdrawn. He must be removed from the biogas prior to its utilization, thus natural gas qualities are respected. Even before use in biogas cogeneration plants, the fuel gas is dried. Thus, the formation of pockets of water by condensation (precipitation ) and the corrosion of biogas engines and line facilities can be avoided.

Biogas is dehumidified by the cooling of the gas in the ground or by compressor cooling. The drop below the dew point of water vapor can condense the water. The condensate is collected at the low points of the most buried biogas line and derived ( condensate ). In a cooling by chillers the water falls to (condensate ) to the cold registers and can be collected and discharged there. Because ammonia is highly soluble in water, it can be removed in the drying out of the gas is discharged with the condensate.

Separation of gases

For the separation of carbon dioxide and methane, the following methods are available: pressure water wash, cryogenic processes, membrane separation processes, as well as pressure swing adsorption and other procedures for the separation of carbon dioxide by absorption (such as amine scrubbing or Selexol process ).

The washing solution under pressure, and binds at low temperature of carbon dioxide, hydrogen sulfide and water vapor from the biogas. Through relaxation and heating of the wash solution, the gases are desorbed again. In addition, a biological separation of sulfur by Bioscrubber or Biotropfkörper is possible. For example, lead amine scrubbing to a very high quality product gas, but require the use of process heat at a high temperature level. In most processes the raw gas must already roughly be free of suspended solids, drops of water and hydrogen sulfide.

The compressed gas scrubbing separating the methane from carbon dioxide and methane concentration required to 95-99 % by volume on. Then it is dried by cooling to the dew point at the respective application pressure.

Another common market practice for carbon dioxide capture with integrated desulfurization and dehumidification is the procedure BiogasUpgrader. This organic- physical process operates with an organic wash solution is regenerated in the process. It therefore falls to no process water for disposal. The washing solution under pressure, and binds at low temperature of carbon dioxide, hydrogen sulfide and water vapor from the biogas. Through relaxation and heating of the wash solution, the gases are desorbed again.

Conditioning

During conditioning, the conditioned biogas is set to the quality parameters for natural gas in relation to drought, pressure and condensing Wobbeindex before being introduced into the gas network. The quality and composition of methane to be injected has controlled the German Association for Gas and Water (DVGW ). Because there is gas networks for natural gas of different qualities in Germany (H- gas and L- gas ), the quality standards of the biomethane can be different. For conditioning includes the odorisation.

Dissemination and economy

Because of the required investment, the biogas processing is currently regarded as a capacity of about 250 to 500 m3 of biomethane per hour to make economic sense. This corresponds to a plant capacity of about 1 to 2 megawatts of electricity ( MWe ) under direct from biogas in the CHP. In particular, biogas plants, which can not settle on the ground, the waste heat from the biogas into electricity, suitable for a gas processing and feed-. Also for electricity from biomethane the basic compensation is paid pursuant to the Renewable Energy Sources Act ( EEG). In addition, the law provides a technology bonus for the gas supply and a combined heat and power bonus ( CHP bonus ). Together with the proceeds through the sale of heat more complete the complicated gas processing is so economical. In Germany there are currently (April 2010) about 33 plants for biomethane production capacities ranging from 148 m3 to 5000 m3 per hour in operation. At year-end is expected that around 50 plants total estimated 50,000 m3 of biomethane feed every hour in the network.

The stated goal of the federal government is that about 6 billion m3 of biomethane produced per year in Germany by the year 2020. This corresponds to the capacity of around 1200-1800 biomethane plants.

As the first wastewater treatment plant in Switzerland fed since January 2005, the ARA region Lucerne for recycled for natural gas parameters sewage as biofuel into the natural gas grid. In Switzerland, more large plants for biogas upgrading plants in the Bern region are built. In Austria, a prototype of the German Biomass Research Centre (DBFZ ) for large-scale production of synthetic biogas from wood chips into operation. In Sweden, the processing and feed is already done.