Foto: Martin Dam Kristensen

Upgrading of biogas at full speed

Friday 24 Mar 17
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by Morten Andersen
Biogas is currently not as valuable as natural gas. But it could be.

In recent years, a number of plants have been designed to convert manure, food waste, and other organic substances to biogas. Unfortunately, however, biogas does not have the same high quality as natural gas. Therefore, biogas can neither be sent directly to consumers nor stored together with natural gas in the large underground gas storage facilities.

The two Danish concepts in which DTU researchers play a key role are intended to change this situation. Each in their own way, they will upgrade biogas to a quality equivalent to that of natural gas.

The chemical name of natural gas is methane (CH4). Biogas consists of 60-65 per cent methane and 35-40 per cent CO2. In other words, you only need to ‘remove’ the CO2 to send the gas into the grid. Current techniques to remove CO2 are, however, expensive and energy-consuming. But combined with biogas, there is a more efficient way. Instead of capturing CO2, hydrogen (H2) can be added to the biogas. When the CO2 content of biogas reacts with hydrogen, methane and water are generated. Water can be condensed quite easily. You are then left with pure methane. This way, more methane is produced—up to 54 per cent more—from the same amount of biomass.

The necessary amount of hydrogen to be added can be produced in several ways. In a future sustainable energy system, hydrogen can be produced by means of electrolysis, using either wind power, hydropower, or solar power.

In this context, the fact that the hydrogen production does not have to remain constant is practical. It may be sufficient to produce hydrogen in periods with ample inexpensive, sustainable power. The upgrading of biogas by adding hydrogen can actually be regarded as a way of storing excess wind power as chemical energy in the form of methane.