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Plant Bruck/Leitha

With the experience of extensive tests on lab scale and pilot plant scale the principle of gaspermeation for biogas upgrading has been realised for the first time in Austria on an industrial scale in Bruck an der Leitha in Lower Austria (40km to the east of Vienna). The planning and tendering phase for this plant began together with the Kick-off of the project “Virtual Biogas” in summer 2006. During spring 2007 the upgrading plant has been assembled by the plant constructor Axiom Angewandte Prozesstechnik at their company location. The plant has been assembled in a standardised 30-feet-container and has been transported to its final plant site at the cofermentation-biogas-plant in Bruck an der Leitha in May 2007. The official commissioning has been celebrated on 25th of June 2007. The regular feed-in operation implying several partial load and full load scenarios started in January 2008. Since then, the upgrading plant is in permanent operation and works as a technology-demonstrator. That means, numerous visitations, excursions and presentations for information transfer are being held.

Figure: left: Exterior view of the biogas upgrading container; right: Interior view of the biogas upgrading plant showing compressor, heat exchangers and membrane modules

The upgrading plant has been designed to produce a biomethane volume flow of 100m³STP/h which corresponds to approximately 180m³STP/h of raw biogas. The product gas biomethane corresponds to the applicable laws in every respect. The law applicable in Austria are the guidelines of the “Österreichische Vereinigung für das Gas- und Wasserfach" ÖVGW G31 (Erdgas in Österreich - Gasbeschaffenheit) and G33 (Regenerative Gase - Biogas). Therefore, the produced biomethane is a fully-fledged natural gas substitute and it is allowed to inject this gas into the public natural gas grid. Parallel to this grid injection two CHP-gas engines (830kWel each) are operated at the biogas plant in Bruck an der Leitha producing electric power and district heat.

The upgrading plant contains a two-stage gaspermeation step. The raw biogas is mixed with the permeate flow of the second membrane stage (recycle) and conjointly compressed and dried by cooling to gas temperatures of lower than 7°C. Subsequently, the gas is reheated (using a part of the compressor’s waste heat) to the optimum temperature for the successive process steps. After a final desulphurisation by adsorption the gas is transported to the two-stage gaspermeation for final upgrading. The two-stage layout has been implemented to assure a minimisation of the methane-slip of the upgrading plant. The methane-slip is defined as the fraction of methane contained in the raw biogas that is not supplied to the grid but is leaving the plant together with the offgas flow. Using this plant layout, the permeate of the second membrane stage (having significantly higher methane content compared to permeate of the first membrane stage) is recycled and recompressed. The permeate flow of the first membrane stage acts as a sink for carbon dioxide and leaves the upgrading plant as offgas. As with any other separation technique, it is not possible to transfer the whole methane contained in the raw biogas to the product gas flow. A certain part of the methane is also separated from the product gas and ends up in the carbon-dioxide-rich offgas, causing a certain low methane content in this gas stream (usually 2 to 3% of the produced biomethane flow). In order to achieve a zero-emission-operation regarding methane, the offgas flow is not released to the atmosphere but is transported to the already existing gas engines (CHPs). Thus, the remaining chemical energy content of this gas flow is used to produce heat and power.

Figure: Flowsheet of the biogas upgrading plant applying Gaspermeation in Bruck an der Leitha

The methane content of the product gas flow is controlled using a proportional valve in the piping of the second-stage retentate. The aperture of this valve is controlled via a PID-controller and influences the gas pressure of the high pressure side of the membrane and therefore the separation behaviour of the membrane. This control strategy allows for almost arbitrary methane contents in the product gas (from almost raw biogas composition with 70% methane up to 99% methane and even more). During standard operation in Bruck an der Leitha, the methane content of the product gas is adjusted to 98,0% and the carbon dioxide content is adjusted to 1,8%. Oxygen, nitrogen, hydrogen sulphide, ammonia and humidity are also reduced to values far below the limits given by law. Additionally, the volume flow of the produced biomethane is controlled via an enhanced PID-controller affecting the frequency converter (and therefore the rotational speed) of the piston compressor.

Figure: Composition of the biomethane product gas flow; left: major components methane and carbon dioxide; right: minor and trace components oxygen, water and hydrogen sulphide

The drying of the gas is done in two different steps in this upgrading plant. Firstly, the mayor amount of water is condensed and discharged by cooling of the gas to temperatures of lower than 7°C. Secondly, the final drying of the gas is accomplished with membrane separation, because of the high drying-potential of the applied polyimide membranes. As a result, the product gas reaches extremely low pressure dew points, significantly lower then prescribed by law.

Figure: Process integration of the biogas upgrading plant Bruck an der Leitha

Another important upgrading step at the site Bruck an der Leitha is the separation of hydrogen sulphide. Because of its toxicity and corrosivity only very low contents of this substance are allowed by law. The herein presented process realisation currently implies three steps of desulphurisation. The first step is the in-situ-desulphurisation by addition of special chemical substances (liquid mixtures of various metal salts) directly into the fermenter. This results in a reduction of hydrogen sulphide and ammonia by chemical bonding as well as in an improvement of the liquid milieu for the involved bacteria reducing the generation of these toxic substances and increasing the methane yield. The produced raw biogas at the exit of the gas storage tanks typically contains between 100 and 150ppmv of hydrogen sulphide for this plant (without dosing up to 3000ppmv possible. The second step is the biological desulphurisation by application of the chemoautotrophic bacteria thiobacillus in the form of a immobilised slime mould within a packed column. These microorganisms oxidise H2S with molecular oxygen and convert the unwanted gas compound to water and elemental sulphur or sulphurous acid which is discharged together with the column’s waste water stream. At the end of this step the gas usually contains up to 50ppmv of hydrogen sulphide. Prior to the commissioning of the biogas upgrading plant, the biological desulphurisation has been operated with air as an oxidiser. Due to the fact, that the biggest part of air is nitrogen which cannot be separated from the gas with the applied gaspermeation (as already discussed), this desulphurisation step has been refitted to use pure oxygen as an oxidiser. Currently, an advanced control algorithm is implemented to minimise the oxygen demand with an unchanging desulphurisation performance even during fluctuating gas flow rates. Operational experiences of more than two years prove the constant desulphurisation performance after the switch from air to pure oxygen as an oxidiser. The third step of desulphurisation is the final adsorptive desulphurisation using iron-oxide pellets usually reducing the H2S-content to values of lower than 1ppmv.

Figure: Steps of desulphurisation; left: In-situ-desulphurisation; mid: Biological desulphurisation; right: Final adsorptive desulphurisation

The downstream handling of the produced biomethane stream is also very interesting. After a detailed online-gas-analysis of all relevant gas components (methane, carbon dioxide, oxygen, hydrogen sulphide, moisture) the produced gas is transported to the gas distribution station via a biomethane pipeline of 2,8km length. If the gas quality is not complying to the Austrian standards the supply to the grid is immediately stopped and the gas is transported back to the gas engines of the biogas plant. The automated control system adjusts in this case the operational parameter to reach the setpoints again and automatically restarts the grid injection if the gas quality is sufficient again.

Figure: left: Cumulated monthly amount of delivered biomethane; right: Volume flow of delivered biomethane, both for February 2009

The supplied biomethane is transported via the local natural gas grid (up to 3bar) to the nearby city of Bruck an der Leitha (population about 7600). The total produced biomethane is consumed in the city during the winter months; additional fossile natural gas is needed to supply the city to 100%. During the summer months the local consumption of Bruck an der Leitha is significantly lower than the produced biomethane; therefore, the surplus biomethane is compressed to 60bar and fed to the regional gas grid (grid level 2). As a result, a constant operation of the biogas upgrading plant over the whole year is possible, assuring optimum plant utilisation and cost structure.

Figure: Gas distribution station and high pressure compressor for the supply to the local as well as to the regional gas grid