Using biogas from anaerobic digestion (AD) to generate electricity has been questioned by Ann Ballinger, a consultant at Bristol-based Eunomia, who says it creates fewer carbon savings than other end uses.

Speaking at the Association for Organics Recycling's (AfOR) annual conference and awards last week (February 11), Ms Ballinger explained that Eunomia had analysed a range of different uses for biogas.

The research examined the carbon impact of various uses of the biogas produced by anaerobic digestion

And, it found that using biogas to power vehicles had the lowest carbon footprint, followed by the use of biogas on-site in a combined heat and power (CHP) plant. Pumping gas into the grid was the next most efficient.

Ms Ballinger's research found that when the carbon impacts of treating one tonne of material using AD are analysed, the following carbon savings could be made:

• Using biogas to generate electricity has a carbon saving of -62kgCO²equivalent
• Using biogas on-site has a carbon saving of -86 kgCO²eqv
• Using biogas as vehicle fuel has a carbon saving of -97 kgCO²eqv
• Pumping biogas straight to the grid has a carbon saving of -85 kgCO²eqv

However, Ms Ballinger explained that, despite it being the most carbon efficient way of using the biogas, and therefore having the most positive impact on climate change, there was no government support for using biogas as a vehicle fuel.

She said: "Of the four different ways of using biogas there is no support for use of biogas as a vehicle fuel."

ROCs

In contrast, she said that the least efficient method - using biogas to generate electricity - was supported by Renewables Obligation Certificates (ROCs) and also, from April, Feed-in Tariffs.

The Renewable Energy Strategy 2009 supports biogas-to-grid and Ms Ballinger said that, in the future, schemes that utilised heat generated by AD would benefit from the proposed Renewable Heat Incentive from April 2011 (see letsrecycle.com story).

She commented: "The transport sector accounted for 21% of the UK's energy needs in 2008 and transport is a neglected sector in terms of impacts."

She explained that gas-fuelled vehicles emitted less CO² and used less fuel than traditional fuels such as petrol and diesel. Other advantages of gas fuel include less noxious emissions and quieter vehicles.

Pumping gas into the grid is another positive way of using the biogas, she explained, however, Ms Ballinger noted that a lot of work would have to be done to the biogas to make it good enough to be pumped in to the UK's main grid.

She said this work would involve adding propane to the biogas to improve its calorific value and reducing oxygen levels from 2% to 0.2%.

Eunomia expects to publish its full research into biogas in a report looking at the best way to treat organic waste across Europe, which is set to be published later this month.