Farmer’s wife Suzy Stennett studies the potential for biocrops and anaerobic digesters
FARMING seems to be an industry that is romanticised more than any other, harking back to the bygone days of heavy horses working the fields, when an entire community would be involved with the farm, from teams harvesting the fields, no doubt alive with banter and camaraderie, to blacksmiths and wheelwrights filling the village with their associated smells and noises.
Then came the agricultural revolution, and the introduction of tractors. The likes of the little Grey Fergie and the David Brown are now collectors’ items, as enthusiasts bag themselves a bit of history to lovingly polish. Every industry must move with the times, and farming is no exception. As world powers struggle to find the answer to global warming and alternative energy sources, farmers find themselves presented with new options for innovative energy production, from fields of solar panels and wind turbines, to growing crops for fuel.
Maize has historically been grown in the west country for animal feed, and in Suffolk has only really featured in small strips of pheasant cover. But over the last five years the agricultural landscape of Suffolk has gradually been transformed, as this crop, growing seven to eight feet high and resembling something out of John Wyndham’s The Day of The Triffids, is increasingly grown for biofuel. Its high energy content and relatively low input requirements, mean maize (also known as sweetcorn) meets the sustainability criteria set by energy companies. It produces more energy than is required to grow, harvest and convert it.
A SUBTROPICAL JUNGLE
Requiring a relatively fine seed bed with a good soil structure in order to set its roots, maize is drilled in early May once the land has been suitably cultivated. Seeds are spaced every 15cms down the row, with 50cms between each row, generating a jungle of 100,000 – 105,000 plants per hectare, ensuring each plant receives optimum sun exposure and is not shaded by the leaves of its neighbours.
Maize is a subtropical plant and doesn’t need irrigation, and, true to its low
maintenance crop criteria, it requires only four passes of herbicide and fertiliser as the plants shoot up. First leaves form around the stalk, followed by a tassel at the top, which is effectively the plant’s flower. The cobs then develop within the heart of the plant, forming the grain ready for harvest in September and October. Farmers hope for two large cobs of corn per stem for optimum energy production. Any more and the cobs would be smaller with a lower energy output.
Thea early autumn harvest means maize is cut at a dry matter of 32%, which means that, unlike the cereal harvest, there’s no waiting around in the morning for the dew to dry off, and a rain shower is not necessarily going to stop play. Although similar in appearance, a forage harvester used to cut the maize has a
completely different style to a refined cereal combine harvester. Rather like a hot headed younger sibling who enters the field to cause total destruction, the forage harvester has two objectives – to break open every grain on the maize cobs in order to help release the energy inside later in the process (remember, sweetcorn can pass through the body completely entire if not thoroughly chewed), and to chop the giant stalks and leaves into 7mm-10mm slices. This prevents them getting tangled around pumps and mixers.
The forage harvester chops the maize stalks 7cm-10cm from the ground and passes them through its feed rollers into a drum resembling one of the many intended destinies for James Bond. There it meets a set of knives which slash it against a shear bar, then continues through the James Bond-style torture chamber onto a corn cracker, where two rolls split and crack the grain, passing it into the accelerator. The accelerator disposes of the remains by blowing the chopped and cracked maize up through a shoot, expelling it into a trailer running alongside. The flow from the forage harvester’s spout is continuous, so drivers need to work well together to ensure that all the harvested maize is collected. To ensure the forage harvester can work undisrupted, thrashing through the field, there’s normally a team of about four tractors and trailers, relaying to and fro the yard. Other tricks in place to ensure the chopped maize reaches the trailers at all times include an ability to swing the spout around the back of the forage harvester by 180°, so the trailer can be positioned behind the forage harvester when necessary.
There’s normally a camera fitted to the end of the spout, with a feed to a screen in the cab of the forage harvester, so the driver can keep his aim accurate. If he also has a row finder fitted to the front of the forage harvester, to guide his machine straight down the rows, he can stay on course without having to look up from the screen.
In the yard the chopped maize is weighed and tipped onto a large walled concrete pad where the clamping tractor takes over, pushing it up into a clamp up to five metres high, constantly rolling it to squeeze out the air and start the fermentation process. Silage sheets, like thick black plastic bin liners, seal the unit, preventing air entering and enabling fermentation to turn the harvested maize into silage. It’s ready to use for energy production after six or seven weeks, but it can be stored this way for up to two years.
This silage is the food that feeds the Anaerobic Digester (AD), commonly known in the industry as a big concrete cow – fed at one end and releasing gas at the other, which is now helping to fuel the nation.
An AD plant is like a huge stomach. The insulated concrete tank houses methanogens, micro-organisms that produce methane as a metabolic byproduct in anoxic conditions. The tank, with its heaters and mixers, are all covered by a soft double-membrane dome.
Fed every hour by a programmed feed hopper, the micro-organisms break down and digest the silage. The mixers not only help expose the silage to the micro-organisms, but also help release the biogas produced in the digestion process (effectively wind!). The outer membrane of the AD plant is constantly blown out by a continuous flow of air, while the inner membrane rises and lowers according to the gas produced within the digester.
The anaerobic digester passes out the biogas, which consists of 52% methane, 46% carbon dioxide and 2% other gas composites. It’s then chilled and fed to a gas upgrade building, which strips out the carbon dioxide. The recovered carbon dioxide can then be chilled to -35°C and sold in liquid form to the food industry, perhaps reappearing as bubbles in fizzy drinks, or used to keep prepacked sandwiches fresh.
The remaining bio-methane has calorific value of 37 mega joules. It’s enriched with propane to boost that to 39 mega joules, to meet national grid specifications, then compressed and given an odorant (biomethane is odourless). The gas then leaves the farm and enters the national grid.
So are we now in the throes of another agricultural revolution, this time green and striving to fuel the nation as well as feed it? Will we look back in years to come and fondly remember farm yards devoid of silage clamps and the obligatory domed head of the AD plant? Only time will tell.
“Maize (also known as sweetcorn) meets the sustainability criteria set by energy companies. It produces more energy than is required to grow, harvest and convert it”
Above, the maize harvest. Below, the anaerobic digester plant
Above, food for the anaerobic digester