Biogas gets greener

 By Robin Wylie

A new study has found that the CO2 footprint of biogas could be reduced to “extremely low” levels using the emerging technology of pressurised anaerobic digestion (PAD). Robin Wylie explains this discovery and discuss its importance for the renewables industry…

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Biogas is one of the greenest hydrocarbon fuels on the market. But a new high-pressure production technique could help make its carbon footprint even smaller, a new study has found.

Biomethane, the primary constituent of biogas, can be used as a low carbon replacement for fossil hydrocarbons fuels. For example, in the transport industry, CO2 emissions from biomethane are up to 200 percent less than those from conventional diesel.

The lowest carbon footprint available from most of today’s biogas is around 50 kilograms of CO2 per megawatt-hour (MWh) of power produced, which is significantly lower than that of most non-renewable hydrocarbon fuels (which tend to have carbon footprints closer to 200 kilograms per MWh). But new research from Poland suggests that it could be possible to reduce the CO2 emissions of biogas even further.

Most of today’s biogas is produced using a process called anaerobic digestion (AD), in which organic material is broken down by microbes in the absence of oxygen at normal (atmospheric) pressures.

But in the new study, Wojciech Budzianowski, a researcher at Wrocław University of Technology, and his student Karol Postawa, showed that by increasing the pressure under which AD is performed, the carbon footprint of the resulting biogas could be slashed to as little as 13 kilograms of CO2 per MWh (assuming that the fuel was used in the transport industry, rather than for electricity generation).

Biogas production facility in Bavaria, Germany. Credit: Biogastour 2013

Renewable energy researchers have been interested in PAD for several years, but the Polish study is the first to scientifically investigate its potential in a real-world scenario.

In the study, the researchers performed a variety of sophisticated numerical simulations to try to predict how using PAD, instead of AD, would effect the total carbon emissions of the produced biogas.

The simulations showed that when PAD was performed at 30 times atmospheric pressure (3 MPa/435 psi), the resulting biogas would have a carbon footprint of 32 kg CO2 per MWh, or 13 kg CO2 per MWh if the process was carried out at 50 times atmospheric pressure (5 MPa/725 psi). The pair’s conclusions suggest that if PAD could be implemented on a wide enough scale, the carbon footprint of biogas could eventually rival that of wind power (which with a footprint as low as 10 kg CO2 per MWh, is one of the greenest renewables on the market).

These figures may still only be hypothetical. PAD has not yet been used to produce biogas on an economic scale, but with their study, Budzianowski and Postawa suggest that the full potential of biogas as a low-carbon fuel has yet to be attained.

Maintaining high pressures in a biogas reactor could be challenging, however most biogas production today already requires pressurization to around 200 times atmospheric pressure (20 MPa/2900 psi) in order to separate out the useful biomethane from the CO2 and various unwanted trace gases which are present in raw biogas.

We have the theoretical know-how and the technological capacity to make biogas into a much greener fuel. If the computer simulations carried out in Poland could be repeated in the real world, the benefits to our planet could be significant.

SEE MORE: The cleantech burn by Michelle Leslie


about the author
Robin Wylie
Freelance earth/space science journalist. Currently finishing off a PhD in volcanology at University College London.