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Removal of methane from the offgas

As mentioned before, the offgas produced during biogas upgrading still contains a certain amount of methane depending on the methane recovery of the applied gas separation technology. As methane is a strong greenhouse gas, it is of vital importance for the overall sustainability of the biomethane production chain to minimise the methane emissions to the atmosphere. It has to be mentioned, that the emissions of methane from biogas processing plants is limited in most countries. Additionally, higher amounts of methane in the offgas increase the specific upgrading costs and could inhibit an economic plant operation. But it’s not that simple as there is a trade-off in selecting a certain methane recovery value because a higher methane recovery always increases investment and operational costs of a certain upgrading technology. As a result, the most promising plant layout in terms of economics usually accepts a certain amount of methane left in the offgas and applies a certain treatment of the gas prior to venting it to the atmosphere.

The most common technique of removing the methane content in the offgas is the oxidation (combustion) and generation of heat. This heat can either be consumed at the anaerobic digestion plant itself (as this plant often has a heat demand), it can be fed to a district heating system (if locally available) or it has to be wasted by cooling. Another possibility would be to mix the offgas with raw biogas and feed it to an existing CHP gas engine. Either way, the layout of the plant has to be planned carefully as the offgas of a modern biogas upgrading plant seldom contains enough methane to maintain a flame without addition of natural gas or raw biogas.

Alternatively, the methane in the offgas can be oxidised by a low-calorific combustor or by catalytic combustion. A number of manufacturers already provide applicable technologies on a commercial basis. These systems provide stable combustion even at methane contents of as low as 3% in the combustion mixture with air. The treatment of offgas containing even less methane is increasingly difficult as not enough energy is provided during the combustion of this gas and raw biogas or biomethane have to be added in order to reach a stable oxidation. This is why it does not make sense to choose an upgrading technology with a methane recovery as high as possible because you always have to deal with the offgas. The integration of the upgrading plant into the biogas production facility and the overall concept of the biomethane production site are much more important. Only very few upgrading technologies with extremely high methane recoveries provide an offgas that is permitted to be directly vented to the atmosphere.