Biogas – Sustainable energy or just hot air
Biogas seems to have been in the news a fair bit recently, with a major feature on Sunday’s Countryfile as well as George Monbiot writing for the Guardian. These features focused primarily on the food versus fuel debate but that’s only one of the questions around the use of biogas.
The role of biogas in gas transitions has generally been seen as pretty important.The simple concept being that you gradually increase the volume of biogas in the gas network to reduce the carbon emissions of heat use while simultaneously reducing demand through energy efficiency measures.This does sound like a very sensible strategy for gas networks, potentially cheaper as you continue using the gas network, easier for consumers as they don’t need to switch and better for the politicians and policy makers as it keeps existing interests happy.
But there are three really important issues about this which no-one has a clear answer to.These are around economics, availability of feedstocks and life-cycle emissions.
So on economics, biomethane grid injection is currently one of the highest cost per kWh in the RHI and it is expected to take up a major share of the budget. The RHI provides a flat rate per kWh injected and as such its been recognised that the largest scale projects are likely to be overcompensated hence the recently announced emergency review into biomethane tarriffs. The variability in types of feedstocks, some waste, some by-products, some energy crops, some sewage means that appropriately rewarding projects is very difficult.
The economics of using biogas for electricity generation are also slightly tricky. Current tariffs vary from 9.5-13.5p/kWh depending on project scale and although this is higher than for biomethane injection, it needs to be remembered that the value of a unit of electricity is significantly higher that a unit of gas and overall a kWh of electricity from biogas is cheaper than a kWh of biomethane. It’s also important to bear in mind that based on current grid conditions, electricity generation saves more carbon than biomethane (Carbon Trust, 2010) though this may change if the power grid decarbonises.
The question of feedstocks is equally complicated. Anaerobic digesters can basically take anything which is organic but non-woody from sewage to seaweed to maize. Clearly using non-crop feedstocks is preferable from a carbon and food growing perspective however, purpose grown maize is very popular across Germany and is growing in popularity in the UK. In order to make plants simple and profitable energy crops are simply easier than using waste which generally has a lower energy content and much more complex permitting requirements. The Farmer’s Guardian however suggests that 450-500 hectares of land is needed for each MW of power capacity when using maize. That’s about 650 football pitches for each MW – to me that sounds like a lot especially when you consider the footprint of a 1MW wind turbine is just a few square metres and they are cheaper per MWh and you can still grow food on the land too!
But it’s not always this simple as often break crops which are used to restore soil quality and nutrients can be used for AD.These can be things like grasses which can also be used for anaerobic digestion and so there can be benefits to using purpose grown crops.
One of the bigger questions is around how much gas you could ever actually get from biogas.ADBA suggest around 10% of the UK’s household gas demand could be supplied which although significant is hardly ever going to be a like for like replacement.National Grid’s widely used 50% of UK gas demand figure is no longer supported.
As for life-cycle emissions again this is really a question of feedtsocks. Generally it is the wastes: food wastes, slurries, organic process wastes which are the lowest carbon because not dealing with them tends to deal in emissions of methane to the atmosphere for example, food-waste in landfill just rots and gives off methane. Therefore capturing the methane and using it has significant benefits. These benefits are not the same for purpose grown crops as the growing process itself can be fairly carbon intensive and the same waste management benefits are not just there. There is currently no sustainability criteria for biomethane injection however one is going to be introduced by the end of this year which will require a 60% saving on the EU fossil fuel average alongside some land management criteria. Interestingly DECC has decided not to grandfather this criteria which says something about their concerns around biogas. For electricity generation using biogas the sustainability criteria are in place but are fairly week.
So biogas is not cheap, not always green and probably not going to be that significant a part of the future energy system and overall there isn’t a very clear picture around the policy owing to varied feedstocks and varied uses.
There is a clear benefit in using waste to produce biogas (bear in mind it’s still preferable not to produce the waste at all) however there appears to be little benefit in using purpose grown crops for biogas production because of the required land required and lower carbon savings.If Government do ever get to a point where they want to stop purpose grown crop AD (which is probably not that far away) they will need to be careful to ensure that the really great parts of the industry focusing on waste management and sewage management are protected from any potential changes.There are huge benefits to using biogas in the correct way but huge potential problems if regulations and policy support the use of purpose grown energy crops.