I was drawn back into the muddy world of Biofuel Maize by reading an article in the Western Morning News entitled planting right variety of maize boosts AD Output.
The piece was a bit of puff for the AD industry, and while packed with statistics, the one piece of information I was looking for was missing. And it was this – how much methane does a hectare of biogas maize produce? It’s not a difficult question, but one that I had to do a fair bit of delving to find out.
Here is where I started. Burning a tonne of methane releases 2.75 tonnes of CO2 so every tonne of methane produced by biogas maize “saves” a tonne of natural gas (which is methane) from being burnt. Methane weighs 0.8 KG per cubic metre, so a tonne of CH4 is 1250 M3. So for every 1250 M3 of methane, 2.75 tonnes of CO2 is released. From the WMN article we learn that the rule of thumb is that each tonne of dry matter (DM) maize produces 210 M3 of CH4, so each DM tonne of Maize “saves” 0.46Tonnes of CO2.
A hectare of biogas maize produces 9 – 15 tonnes of DM per hectare (at 30% Dry Matter) so a hectare would save 4.14 – 6.9 tonnes of CO2. This is equivalent to (divide by 3.67) 1.1- 1.9 tonnes of Carbon per hectare.
Now on the plus side, Maize absorbs CO2 as it grows. Again it’s a difficult figure to track down but from what I have read it appears that a tonne of live (wet) maize absorbs half a tonne of CO2; and biogas Maize produces between 30 and 5o Tonnes of fresh Maize per Hectare, giving 9-15 Tonnes of Dry Matter per hectare. So for every hectare of biogas Maize produces, there is a saving of between 15 and 25 Tonnes of CO2, or between 4 and 7 Tonnes of Carbon.
But there are signficant costs too. Maize is a very hungry crop and a hectare can easily need 250kg of NPK fertiliser or equivalent. Producing this fertiliser takes a lot of energy, but the main negative factor is that it produces the highly potent greenhouse gas Nitrous Oxide. 1kg of NPK fertiliser produces the equivalent of 6kg of CO2. Added together the footprint of fertiliser for a hectare is about 12kg per hectare. So a 250kg dose has a cost of about 3 Tonnes of CO2 per hectare.
You may be surprised to know that cultivating a field has a cost of 150kg CO2equivalent per hectare, but this is relatively small compared to the other big cost. The other big cost associated with biogas maize (as a very low density product) is transport, from the field to the clamp (where it has to dry) then from the clamp to the AD plant.
A tractor towing a trailer over public roads has a high carbon footprint. I have calculated that it could be 1.5kg CO2 per kilometre towing 15 tonnes of DM maize, ie 100g CO2 per tonne per kilometre. If the total journey from field to AD via the farm (plus the return journey) was 50km that would add 50kg CO2 per Tonne and up to 750kg CO2 per hectare.
At this point the cost is up to 3.75 tonnes of CO2 per hectare. If you added in all the little carbon footprints of other things that are part of the process of producing AD Maize (eg transporting the seed, the surprisingly large footprint of pesticides and herbicides, including the fuel used to deliver them to the farm, then spray them on the fields) I think we could easily see that “cost” figure head towards 4 tonnes CO2e per hectare.
So going back to where we started, the article in WMN comes into focus. Because at the lower end of AD Maize production efficiency, the net CO2 saving is only around 11 tonnes CO2 per hectare per annum. This is equivalent to 3 Tonnes of Carbon per hectare per annum.
When I was at the Grasslands Trust I was very keen to promote an unrealised value of species-rich grasslands: they are very good Carbon sinks. The general figures bandied around for Carbon sequestration when converting arable land (a Maize field) to grassland was around a Tonne of Carbon per hectare per annum. That is, left to its own devices, with a bit of light grazing, an arable field on its own will absorb a tonne of Carbon per hectare, as it naturally develops into grassland. But some research has indicated that far higher rates of sequestration can be achieved. One particular experiment found rates as high as 3 Tonnes of Carbon per hectare absorbed. And Species-rich grassland have around twice as much Carbon in their soils as arable fields do. So a grassland will continue to absorb Carbon for decades after it has reverted from arable.
In other words, some biogas Maize production is no more efficient at reducing Greenhouse Gas emissions than letting an arable field revert to species-rich grassland. And the more intensive the production, the more the cost increases – at 500kg of fertiliser per hectare that carbon footprint shoots up to 6 tonnes of CO2 per hectare.
And that’s not even taking into account the external environmental costs of Maize production, which I have written about before. Maize is simply the most environmentally damaging crop grown in Britain and possibly in Europe. And while farmers and AD plant owners may are generating a healthy return on their investments, not only are we paying them through the subsidy, but we are also paying through damage to watercourses, increased flooding, contamination of drinking water supplies and the continuing loss of wildlife from the countryside.
It’s another example of how serious can be the unintended consequences of subsidies for supposedly good causes. In this case, the Feed in Tariff for biogas is so generous and has so few restrictions on it, that AD plants are paying as much as £500 a hectare to farmers to produce biogas Maize. No wonder the area of farmland under AD Maize is growing exponentially.
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One assumes that those clever civil servants in Defra and DECC had already done some kind of cost benefit analysis and risk assessment on unintended consequences, so were or should have been aware of figures like these you offer? If not why not?
If not, then what are the politicians responses to these kinds of figures?
I suspect that like most situations when they are appraised of reality they plead for ‘a broad mix of energy types needed’? But this one also takes out agricultural land which we are constantly told is needed to grow crops to feed the every growing population of the the UK espousing food security or self sufficiency – ha, we can dream?
Thanks mud-lark.
I think it’s fair to say that Defra get lobbied most vociferously on this kind of thing by the industry. Very little work is being done in the UK on biogas apart from by Friends of the Earth. They will be one small voice against a chorus from the Biogas industry.
Your other point is the one I didnt bother to make, but it applies to every biofuel crop. If we have to feed the world, why on earth are we using agricultural land to grow fuels. One answer that comes back from the industry is that with better Maize varieties specifically grown for biogas, these can occupy lower grade agricultural land, even “marginal” land. And of course this marginal land is where what’s left of England’s wildlife, archaeology and historic heritage still clings on by its fingertips.
It beggars belief that this situation is allowed to exist. AD was dreamt up as a way of making something out of waste. This is just another example of how subsidy ruins so much. It should be illegal to grow a crop for the primary purpose of energy. There are plenty of ways to use a crop’s by product for additional benefit. We know of a farmer who can bale enough OSR residue in an afternoon to fuel the whole farm boilers for a year. He has already harvested the primary crop. The secondary crop still attracts RHI and so he has 2 chances of profit for a minor additional input.
thanks Jonathan. Yes AD would be better if it was just run on crop residues and waste. It’s another example of the perverse outcomes of subsidy payments. Agriculture in the UK is littered with such examples.
So, there needs to be a ‘conservation party’ voice raising all these issues? FOE you suggest are the lone voice at the moment, I would have thought the RSPB would have picked this up but maybe they have enough on their plate responding to the ‘bully beef’, Howatt and Scott attack?
Never mind agri-industrial subsidies, we would save money if we stopped bailing out bankers and cramming sardines in the over crowded Westminster village?
Miles’s analysis omits some further costs. The maize growing cycle doesn’t fit well with most arable rotations. Soil structure in fields in my area (Cambridgeshire) were so damaged when the crop was harvested in a wet autumn 2013 that they could not be cultivated in spring 2014. Waterlogging could be seen well into late spring. The unused fields were sprayed to suppress weeds and cultivated in two passes to get ready for an autumn 2014 sown crop.
The additional costs (in terms of £ and carbon) of herbicide spraying and deep cultivation in summer 2014 need to be added. The loss of food production in the 2014 growing season is criminal and the loss of carbon sequestration by that crop has to be added to the carbon balance.
The longterm consequence of maize in wheat rotations needs to be thought about. Maize trash lying on the surface of the soil is a reservoir for Fusarium species that have the potential to infect wheat and raise mycotoxin levels.
As Jonathon Lodge points out, AD was originally suggested as an effective way of using secondary crop wastes. We’ve moved on from that useful idea to a position where we’re burning food from the grain-bowl of the poor.
Huw Jones is Labour PPC for SE Cambridgeshire
many thanks Huw, for those very helpful comments.
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