Green is in, with bicycles preferred to cars, organic produce to fast food, and energy class A+++ to A: In our modern society, decisions are increasingly being influenced by ecological considerations. Industry and business are also reacting to the trend towards sustainability and offering more and more "eco" products. With the major energy producers focusing mainly on wind, water and sun, biogas as an alternative energy source appears to have been somewhat overshadowed - quite unjustifiably, because it is a highly efficient energy source and an important component of decentralized supply structures.

Biogas is produced by fermentation of biomass, an organic substance consisting of, for example, plants, liquid manure, or effluent sludge. But in addition to the methane energy source, raw biogas also contains carbon dioxide (CO₂) and other trace gases. Because CO₂ is not combustible, it lowers the calorific value of the gas and must therefore be separated out.

The common separation methods such as pressurized water scrubbing, pressure swing adsorption, and amine scrubbing have considerable disadvantages: 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 meters per hour (Nm³/h). This usually makes them unsuitable for decentralized energy supply with a large number of relatively small plants.

Evonik Industries has developed a technology for cost- and energy-efficient separation of CO₂. What appears at first sight to be a bunch of spaghetti strands or a paint brush is in fact a bundle of highly selective membranes made up of multiple cylindrical polymer hollow fibres. These are used in the new hollow fibre membrane modules of SEPURAN® Green.

"SEPURAN® membranes are made from an internally developed high-performance polymer with very high temperature and pressure resistance. This plastic gives the membrane the property of distinguishing particularly effectively between methane and CO₂, allowing the raw gas to be purified to more than 97 percent 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 Baumgarten. CO₂, water vapor, and traces of ammonia and hydrogen sulfide are drawn off at the low-pressure side, 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.

Evonik's membrane-based biogas upgrading offers particularly high plant availability, and has very low energy requirements and low maintenance costs. Moreover, the upgrading generates neither wastes nor emissions; nor are auxiliary materials such as water or sorbents required. All these pluses are directly reflected in the form of cost advantages. In addition, the membrane technology is applicable for small and large plants due to the high flexibility of the process. And the technology can easily be adapted for changing flow volumes and gas compositions.