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DVGW Technology Report No. 1/10
As biogas originates from renewable natural resources and is therefore considered an eco-friendly alternative to fossil fuels, the Federal Government subsidises the generation of biogas as part of its promotion programme that encompasses all measures designed to exploit renewable sources of energy. When admixed to natural gas, biogas can help reduce CO2 emissions. Since an in-depth knowledge of the composition of biogas is indispensable for the treatment and later injection of biogas into the gas grid, researchers with the DBI Gas- und Umwelttechnik and the DVGW Research Centre at the Engler-Bunte-Institut have recently investigated the gas composition at injection points and biogas plants.
Biogas plants are proliferating, currently numbering over 4,500 in Germany. While most of these plants are used regionally in the generation of electricity, biogas is also increasingly injected into the natural gas grid.
The wide variety of technical concepts and sources employed in the generation of biogas makes it indispensable to determine the composition of the biogases produced from different substrates in order to optimise the use of the gas. When injecting biogas into a natural gas grid, care must be taken to maintain grid integrity, ensuring at the same time that downstream gas applications will not be compromised (e.g. by corrosion). So far, the body of data on raw biogas has been insufficient, while no measuring data have been publicly available at all on the quality of treated biogas. A research project sponsored by the DVGW therefore aimed to provide answers to all questions by means of a measuring programme.
New methane emission limits to apply from 2011 onwards
Gas constituents like oxygen, hydrogen sulphide, ammonia and other trace components as well as the moisture content of the treated biogas intended for injection into natural gas grids are critical factors to be considered when planning the design and economic efficiency of a gas purification plant. In addition to DVGW Codes of Practise G 260 and G 262, further regulations must be observed when injecting biogas into a natural gas grid. For instance, in accordance with Code of Practise G 685 the calorific value of the produced biogas may deviate from the billing calorific value by 2 per cent max. As from 2011 onwards, the Gasnetzzugangsverordnung [Ordinance on Access to Gas Supply Grids; GasNZV] stipulates a stricter limit value for methane released into the environment: the methane slip – i.e. the ratio between the amount of methane released into the environment and the total amount of methane produced – must not exceed 0.5% after that date. What is more, the Technische Anleitung zur Reinhaltung der Luft [Technical Instructions on Air Quality Control, TA Luft] limits the emission of hydrogen sulphide into the environment to 2.9mg/m3.
The research project involved 14 biogas plants located in the federal states of Saxony, Saxony-Anhalt, Baden-Württemberg, Bavaria, Hesse and Lower Saxony, seven of which were facilities designed to treat gas for injection into the natural gas grid, and seven locally converted the biogas into electricity in CHPs.
Limit values met
As biogas plants have not been around for a very long time yet, only a small body of data was available on the operation of treatment plants. The research project has been able to bridge this data gap and demonstrated that the biogas produced in the treatment plants under review met all limit values specified in DVGW Codes of Practise G 260 and G 262, with variations found only among the trace substances present in the gas. Plants relying on food residue as a substrate showed higher residual hydrocarbon, mercaptan, chlorine and fluorine contents which, however, did not exceed the allowable limit values or impact the injection process.
DVGW Code of Practise G 262 limits the carbon dioxide content to 6 per cent by volume max., while pursuant to G 260 the oxygen content shall not exceed 3 per cent by volume in dry gas grids and 0.5 per cent by volume in wet gas grids on account of the risk of corrosion. The measuring programme demonstrated that the oxygen content in the biogas of the plants under review did not exceed these limit values.
Post-treatment required for lean gas
The maximum hydrogen content of 5 per cent by volume specified in G 262 is generally not achieved in the generation of biogas. In contrast, the hydrogen sulphide content of raw biogas exceeds the specified maximum limit values by a wide margin, making it necessary to reduce it markedly prior to injection. Lean gas, too, has sometimes been found to have excessively high H2S contents when subjected to water pressure scrubbing, which calls for a downstream desulphurisation process. Save for this exception, lean gases do not show any major concentrations either of H2S or of any other trace substances. However, methane loads in lean gas were found to distinctly exceed the limit value of 0.5% in the treatment methods that had been analysed and therefore require lean gas post-treatment systems.
So far, all studies of raw biogases produced from renewable sources deemed carboxylic acids, alcohols and residual hydrocarbons to be unproblematic as only minimal traces of these substances were found. Small quantities of ammonia were found in all plants, the highest concentrations occurring in manure plants. Only the merest traces of ammonia were found in H/L biogas.
The raw biogas in some biogas plant types was found to contain silicon which probably does not originate from the raw materials used but rather from the use of anti-frothing or cleansing agents. As siloxanes create problems through increased system wear, it is important from an operating point of view to investigate the origin of any silicon present.
Another measuring programme intends to analyse other details as well as innovative methods of treatment (chemical scrubbing, membrane method).
Further information:
Biogas is produced from renewable raw materials, manure and bio-waste that are all used as substrates. The biomass is fermented under anaerobic conditions by microorganisms in a digester at temperatures between 25°C and 60°C approx. Biogas is a metabolic product that results in the process; it consists mainly of methane and carbon dioxide as well as a variety of trace substances such as, for instance, hydrogen sulphide and ammonia. When designing a biogas plant, care must be taken to ensure that it is leak proof as H2S has an unpleasant smell and is toxic in higher concentrations. Attention must also be paid to the composition of all exhaust flue gases.
Currently, the raw materials predominantly used in biogas plants are maize silage and manure, one tonne of maize silage yielding approximately 200m³ of biogas.
In the digester, microorganisms break down the biomass under anaerobic conditions, producing methane and carbon dioxide in the process. The use of substrates containing meat or waste residue requires the upstream installation in the digester of a hygiene system in order to kill any pathogens. Hydrogen sulphide and ammonia are hazardous metabolic by-products that occur during the fermentation process. The biomass that remains in the digester is conveyed to a post-digester where more biogas is generated. Any fermentation residue remaining in the post-digester may be used as fertiliser.
The biomass is first hydrolysed by different microorganisms and then transformed step by step into methane and carbon dioxide. In the process, the specialised microorganisms use the previously created intermediate products as suppliers of energy and carbon for their own reproduction. It is important to note in the production of biogas that e.g. the microorganisms involved in hydrolysis prefer conditions that differ from those preferred by microorganisms employed in methanogenesis. Complete biomass conversion is predicated on optimal conditions, i.e. all trace nutrients must be present in ratios that can optimally sustain the microorganisms involved, or else trace substances will remain in the biogas and will have to be removed prior to injection.
