10 September 2012

More on the end of gas as a domestic fuel

I was asked last night why we couldn’t use gas from agriculture to heat our homes – so I thought I’d add a few numbers to illustrate the issue.

Roughly speaking – a dry matter tonne of plant biomass contains 18GJ – more or less no matter what sort of plant it comes from.  Converting this to energy can be done by burning – but in which case it has to be dried, gasifying, or by fermentation to biogas – a mix of methane and carbon dioxide.  For a variety of reasons gasification and anaerobic digestion are less than 100% efficient.  Thus Anaerobic Digestion (AD)  – releases about 10GJ of energy per dry tonne of feedstock in the form of methane.  This is about 3MWh of heat – or when converted to electricity, about 1.2MWh of electricity.

Maize is the most productive biomass crop for making AD gas – witness the huge spread of maize in Germany for AD use.  In the UK maize grows at the rate of about 12 dry matter tonnes per year per hectare – so giving us 36MWh of gas per hectare.

In 2010 the UK used 440 TWh of gas, coal and solid fuel in the domestic sector alone – for space heating, water heating and some cooking.  To generate this much gas would need 147 million dry matter tonnes of maize a year.  At 12 tonnes per year per hectare we would need 12 million hectares dedicated to energy crops alone.  The UK total cereal crop land is about 3 million hectares – so we would need four times as much land as our total crop area to generate just the household gas needs for our current population.  Add the 20% increase expected by 2050 and you have something around 5 times our current total cereal crop area being devoted to biogas.  This is clearly not on.  There are wastes from our existing agriculture, but they are all largely used already as animal feeds or else they are very difficult and expensive to collect.

Of course there is animal waste available – but a cow is a fairly efficient animal and an anaerobic digester in its own right.  It has already robbed most of the available energy from its feedstocks – so there is little left in the manure.  Dealing with animal waste is more of a waste disposal issue than a power generation opportunity.  Domestic waste is of course a source of potential methane too – but we would need to produce 10 tonnes of vegetable waste per person per year – or about half a tonne a week per average household!  That would need some wheelie bin!

The National Grid has made bold claims for the potential of biomethane to replace natural gas, citing up to 50% of current demand being provided by biomethane.  This is clearly at odds with my assertion that we need to phase gas out from domestic heating altogether.

So – where does the National Grid get its figures from?  Firstly, it assumes that people will want gas enough to generate it from wood waste.  Anaerobic digestion of wood is a very poor way to go – but you could always gasify it.  However you get much higher energy yields by burning wood waste in an efficient power station, and using the electricity in a heat pump – than you do trying to gasify it, cool it, clean it, and burn it in a home.  And even if you did do that you would create producer gas, a mix of hydrogen, methane and carbon monoxide.  Re-introducing Carbon Monoxide to the gas grid is unlikely to be practical or safe.

Miscanthus is another major contributor to the National Grid numbers.  1 tonne of maize silage produces about 330m3 of biomethane. To get 1.8 Billion m3 of methane would need about 5.5 million tonnes of maize.  Maize is a more productive crop than miscanthus, and easier to digest (or else the Germans would be growing miscanthus instead of maize in their AD operations).  Assume you need about 6 million tonnes of dry miscanthus, and assume the same yields as maize – 12 dry tonnes per year per hectare.  This still needs 500,000 hectares – about 20% of UK arable land – to produce 1.6% of our total gas demand.

Take out wood and miscanthus, and even if you accept the somewhat dubious figures for ‘biodegradeable waste’ you end up with 2020 production of only 2.5% of our national gas demand – or, if it goes 100% to the domestic sector – 7.5% of domestic gas demand in 2020.  The 2020 ‘stretch’ numbers need much more data to make them believable.  At the end of the day you can only get so much energy in the form of gas from a hectare of land.

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