The revolution in biogas upgrading: Utilising green gas to its full potential using highly selective gas separation membranes

Lems, R., Langerak, L. and Dirkse, E.H.M., DMT Environmental Technology, The Netherlands



More and more effort is put into the utilization of bio-solids. That can be an important source for useful products such as fertilizers and biogas. DMT has been developing biogas treatment plants for over 20 years, closely following market developments.

Biogas was first seen as a nuisance at e.g. landfills, creating odour problems and methane emissions. Flaring has always been a cheap and simple solution. With time, more and more biogas was produced intentionally from bio-solids to generate energy. First in the form of heat and power, but now with the new renewable heat (energy) incentive, it is becoming increasingly attractive to upgrade biogas to natural gas quality and inject it into the natural gas grid. In this way, the gas can be utilized with the best energy efficiency.

There are several biogas upgrading technologies, ranging from old-fashioned water scrubbing to  highly sophisticated cryogenic techniques. Each process has its advantages and disadvantages,  depending on the biogas origin, composition and plant location. However, with the latest developments in membrane separation, DMT has developed the Carborex® MS system. This system, based on an ingenious, multi-stage, highly selective membrane system, is a perfect fit for almost all situations, especially for plants up to 750 Nm3/h.

In this article it is shown that the DMT Carborex® MS is a compact modular unit built into a container. The biogas upgrading is performed with highly selective gas membranes. The upgraded gas with a methane concentration of 97-99% CH4 can be used in the local gas grid, or can be further compressed to 220 bar and used as vehicle fuel (known as compressed biogas or CBG). The system has the highest energy recovery on the market (>98%) with only 0.15-0.20 kWh/Nm3 energy consumption and <0.5% methane loss. The CO2 is recovered as >99,5% pure. After an optional liquefaction step, the CO2 can be obtained as a liquid at food grade purity (>99.9998%). Moreover, due to the liquefaction step the methane loss will be reduced to virtually zero.

Key words

Carborex® MS, CO2 liquefaction, Biogas, Bio-methane, CBG, Car fuel, Gas separation, Green gas, Highly selective gas membrane, Membrane separation, Upgrading


The transition from fossil to renewable fuels is on its way! Biogas produced at landfills and/or digesters can be considered as renewable fuel since it is produced from organic waste. Most commonly the biogas is converted to electrical energy by gas engines with an efficiency of around 40%. Increasing efficiency to levels near 100% will require upgrading of the biogas. This can be done by various processes. Upgraded biogas can be used as vehicle fuel or injected into the gas grid (Figure 1). Biogas used as vehicle fuel is one of the cleanest possible fuels, with hardly any CO2 emissions and very low local pollutants.

Upgrading of biogas mainly involves the removal of CO2, H2S and H2O from the raw gas. The CO2 is removed to increase the energy content of the gas. For vehicle fuel this is important, because it increases the mileage of vehicles. When injecting biogas into the gas grid, a similar energy content will be required as that of the gas already present in the grid. The CO2 concentration is also important to ensure flame stability and energetic value for the end users. H2S needs to be removed to prolong the life time of the equipment, piping and burners since it is a very corrosive gas. If H2O is present in a gas stream, condensation can occur, which is highly able, and therefore, should be completely avoided. Table 1 shows the composition of raw biogas and the requirements of upgraded gas.

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