New fuels: Highly selective polymer membranes purify biogas

Biogas as an energy source can provide a high energy yield per square metre of land. However, an extensive upgrading and purification process is required before biogas is fed into the natural gas grid.Evonik reports that new and highly selective polymer membranes can now convert raw biogas simply and efficiently into highly pure biomethane. This increases yield and conserves valuable resources.‘Green energy’ is becoming increasingly attractive to industry and commerce by offering different routes towards sustainability. According to the Renewables Global Status Report (GSR) 2011, renewable energies today account for about 16 percent of global energy consumption; by the year 2050, this figure could rise to more than 50 percent, as predicted in a scenario of the World Climate Council in its Special Report on Renewable Energy Sources and Climate Change Mitigation (SRREN).Evonik adds that with the major energy producers focusing mainly on wind, water and sun, biogas as an alternative energy source appears to have been somewhat overshadowed - unjustifiably, because it is a highly efficient energy source and an important component of decentralised supply structures.

Efficient biogas upgrading

Biogas is produced by microbial fermentation of sustainable raw materials known as biomass - an organic substance consisting of, for example, plants such as corn, liquid manure, or effluent sludge. But in addition to the methane energy source, raw biogas also contains carbon dioxide (CO₂) and various minor components like water vapour and hydrogen sulphide. Because CO₂ is not combustible, it lowers the calorific value of the gas and musttherefore be separated out. The biogas can then be used in the gas grid, in combined heat and power plants, and at filling stations.The common separation methods such as pressurised water scrubbing, pressure swing adsorption, and amine scrubbing have considerable disadvantages. For example, they need comparatively large amounts of energy as well as auxiliary materials and chemicals.Wastes and wastewater are generated that must be treated and disposed of. Further, the biogas after upgrading is usually at low pressure. Before it is fed into a medium-pressure grid, it needs to be compressed to 15-20 bar by, for example, an additional compressor.Conventional upgrading plants are therefore usually cost effective only for raw biogas quantities significantly in excess of 500 standard cubic metres per hour (Nm³/h).This usually makes them unsuitable for decentralised energy supply with a large number of relatively small plants.Evonik Industries has developed a technology for cost-effective and energy-efficient separation of CO₂ - bundles of highly selective membranes made up of multiple cylindrical polyimide hollow fibres. These are used in the new hollow fibre membrane modules of SEPURAN Green.

Highly selective membranes

Polyimides are high performance polymers with high pressure and temperature resistance. “For SEPURAN we rely on a specially optimised form of Evonik’s proven and tested polyimide family. The membranes have consistently high selectivity and are particularly suitable for separation of CO₂ and methane,” says Dr. Goetz Baumgarten of the Fibres and Membranes growth line of Evonik’s High Performance Polymers Business Line.How does the membrane work? Gas molecules are of different sizes and have different solubilities in polymers. The biogas to be cleaned is introduced under high pressure at one end of the membrane.“The CO₂ molecules are smaller than the methane molecules and also more soluble in polymers. As a result, they pass through the micropores of the membrane much faster and are separated from the methane,” explains Dr. Baumgarten.CO₂, water vapour, and traces of ammonia and hydrogen sulphide are drawn off at the low pressure side (from the lateral opening in Figure 4), while the methane collects at the other end of the membrane, the high-pressure side. The methane-rich gas is directly drawn off at the high-pressure side and needs no further compression for feeding into the grid. This saves the costs of an additional compressor. Further, the biomethane produced in this way can be used even by small plants and therefore allows decentralised supply of energy.In a test plant in Neukirchen an der Vöckla in Austria, Evonik’s experts have been rigorously testing the production modules since early 2011.“Our experience so far is that the new SEPURAN Green membrane modules are a robust and simple tool for gas purification. The methane of the raw gas can be cleaned to a purity higher than 99 percent. In use, the modules are distinguished by higher plant availability, reduced energy requirements, and lower maintenance costs than in alternative processes,” says Dr. Baumgarten. “These are compelling arguments in view of the globally increasing interest in green energy.”