Maize in anaerobic digesters: Is Monbiot right?

  George Monbiot points his critical attention to the increasing use of food crops in the UK’s anaerobic digesters (AD). These huge green cylinders, usually on farms, take organic matter, expose it to bugs that have excrete enzymes that eat cellulose and starch in the absence of air. The bugs produce a mixture of methane and carbon dioxide as an output. This ‘biogas’ that comes out of AD plants is burnt in an engine to produce electricity.

Many digesters use the human waste from water treatment plants or from animal slurry while others take waste from food factories or from doorstep collections. But increasing number of AD plants are using maize and other food crops because the simple starches in these ingredients break down very well, creating more cubic metres of  valuable methane gas than, for example, the more complex molecules in cow manure. Many UK AD plants – built to digest municipal waste, for example – are now boosting their yields by mixing in maize that would otherwise have been used as food for animals or people.

Does it make sense in energy terms to grow maize (or even wheat) as a feedstock for a digester? No. The energy value of the methane that is produced in an AD plant, converted into electricity via a gas engine, is about 0.4 megawatt hours per tonne. This is approximately a tenth as much as the calorific value of maize to a human being.

This isn’t the whole story, since the digestate left behind after the energy has been extracted in an AD plant does have some value as a replacement fertiliser when it is reapplied to the fields. Nevertheless, putting maize into an AD plant to make energy involves a huge loss of calorific value. And the climate change implications also need considering: as well as the energy used in the Haber Bosch process the high levels of nitrogen fertiliser used on maize land produce large amounts of nitrous oxide, a powerful warming gas.

Monbiot has also recently shown the other cost of growing maize for AD: land used for maize has low water retention capacity in winter. The recent floods on the Somerset  Levels were exacerbated by the large areas of adjacent land given over to maize. If, instead, these hectares had been planted with short rotation coppice, such as hazel or willow, more water would have been stored in the soil. And, second, the energy value of the harvested wood, converted into pellets for use in domestic wood burners would have been about twice as great as the energy captured from the same area given over to maize for anaerobic digestion.

There are no good arguments for using productive food land for maize that is then pumped into an AD plant. (AD plants may get more effective at conversion of cellulose in the future and this might affect the universality of this assertion).

My calculations are as follows. (Comments *very* welcome indeed).

Maize in AD

(Figures taken from Farmers’ Guardian and used by Monbiot in the other Guardian).


Raw material needed by an AD plant creating 1 MW of electricity 20,000 tonnes of maize a year*
Annual electricity production from a 1 MW plant operating 8,000 hours a year 8,000 megawatt hours a year
Therefore, electricity output per tonne of maize 0.4 megawatt hours
Calorific value of maize in human diet per tonne About 4 megawatt hours
Food value compared to electricity production value Therefore maize’s food value is about 10 times its value in an AD plant

*Farmers’ Guardian says ’20,000-25,000 tonnes’ needed


Maize versus short rotation coppice

Energy value of electricity per hectare generated by maize in AD plant 17.8 MWh**
Tonnes of SRC per hectare (oven dried equivalent)*** 10 tonnes
Energy value of SRC per tonne 4.5 MWh
Efficiency if burnt in a biomass pellet stove in a domestic/small commercial property 80%
Usable energy value per hectare of SRC 36 MWh
Energy value of SRC versus maize digested in an AD plant Therefore SRC (36 MWh) about twice as good as maize (17.8 MWh) per hectare

** Farmers’ Guardian says 450 hectares produces 20000 tonnes of maize that is enough to provide the fuel for a 1 MW plant (therefore about 8,000 MWh per year).

*** To get this yield requires good husbandry but would be perfectly possible on the Somerset Levels.