Wednesday, 27 May 2015

What's so good about turning food waste into biogas?

Yesterday I visited a plant that turns food waste into electricity (via biogas) at Bygrave, near Baldock. This is the newest of Biogen's anaerobic digestion (AD) plants, having been commissioned only last September. I was very impressed with the operations. It wasn't even particularly smelly except in the reception area where the food waste is delivered. I have endured much worse in a garden next to a composting facility. Our tour was only supposed to take an hour but we asked so many questions it took an extra 30 minutes. Fortunately our guides, plant manager Kevin and Stefan his chief engineer, did not seem to mind.

I have just two issues with this plant. Given that we have a lot of food waste to get rid of, AD is a great way to do this sensibly. You put food waste in and you get energy and farm fertiliser out. On the other hand, it would be a lot better if we didn't waste so much food in the first place. (France has just ruled that supermarkets must ensure waste food goes to charity or animal feed.) Secondly, this plant would be even better with more energy storage capability. At the moment, the gas from the AD digesters goes straight into electricity generators. It runs reliably, and more or less consistently depending on food supply - unlike wind and solar power that are both intermittent and unreliable. But with more facilities this sort of plant could fill in the gaps when other renewables aren't delivering, instead of generating all the time whatever the current balance of supply and demand.

An AD digester works like a cow's rumen to generate methane
The main part of the AD process is a big digester full of bugs that munch the food and produce gas that is about 65% methane. It is quite like what happens in a cow's rumen, where there are lots of microbes that are good at digesting grass. Unfortunately the methane that those microbes generate leaks out into the atmosphere (mainly from the front end I believe, but also from the rear). This is a big problem because methane is a very powerful greenhouse gas and so cattle contribute significantly to climate change. However, in an AD plant the methane is captured and used. At Bygrave it is used to generate electricity in a combined heat and power plant. It is also possible to inject it into the gas grid, though it has to be cleaned up first. There is at least one commercial plant doing that now at Lyndholme near Doncaster.

Food waste delivered to the AD plant at Bygrave. The fat grey pipe along the top of the rear wall with vents is sucking the smelly air out and through an activated carbon filter. This is amazingly effective at reducing odours

The biggest cylinders you can see here are the digesters, then the storage tank which feeds them and the smallest ones are the pasteurising tanks. There are also some enormous tanks for storing the digestate, before it is delivered to farms but they are not in this picture.
How it works in more detail - from removing the knives and forks to power on the grid
For those of you who are interested  in the process in more detail, it goes like this.

  1. The food to be delivered is inspected at source. It has to be food that was fit for humans - so no brain or spinal cord tissue for example. Also it is supposed to be food. Stefan said on one occasion they were given a sofa!
  2. Food is delivered either loose or in packaging. The first thing is to pull out as many contaminants as possible. There is a magnetic separator for metals which picks out a surprising number of knives and forks. Then it goes through a macerator that grinds the food into small pieces and at the same time it separates out plastic and paper packaging materials. The plastic and paper goes to another energy from waste facility so there is nothing going to landfill.
  3. Next the food goes into a storage tank where it is thoroughly mixed together. The AD bugs like their food to be boringly the same from one day to the next - they don't want meat today and oranges tomorrow. It also has enough water added so that it can be pumped easily.
  4. Then it goes into the digesters where it is gently stirred with gas and liquid jets. On average it spends 40 days in the digesters and gas is collected continuously.
  5. Digestate is removed in batches and run through one of the pasteurising tanks, to kill the bugs off and ensure that no more gas is produced in storage. Also this kills any pathogens that happen to be in the food.
  6. There is one final sieving process to remove any particulate matter such as egg shells, grit or bits of packaging that have slipped through.
  7. There are two 12,500 m3 tanks for storing the digestate. This is pumped into lorries and taken to farms for spreading on the soil. The Bygrave plant does not have its PAS110 certification yet; it takes about a year to go through all the checks. With certification the digestate will be considered a product that can be sold as bio-fertiliser. In the meantime it is waste and the Environment Agency must be consulted about how, when and where it is disposed of.
  8. The gas from the digesters goes into a big gas bag that feeds the CHP engines that generate electricity. They have two engines together generating 2.7 MW. They could generate more if there was grid capacity to take it. Heat from the engines is used in the digesters and the pasteurisation process. The gas bag can store about 5 hours production, enough to smooth out fluctuations in supply or handle temporary hiccoughs in the engines. If there is a serious problem with the engines and they have too much gas they have to flare it off as it is better to burn it and release carbon dioxide than to release methane.
What would happen if a tank ruptured?
The nightmare scenario for an AD plant is if a tank ruptures. This is rare of course, but not unknown. For example there was a nasty accident at Harper Adams University in Shropshire last year. The area around the tanks at Bygrave is protected by a concrete bund. Also the area has been landscaped so the tanks are in a dip that is lined with a waterproof membrane. If there is a spill it can't get into ground water.

AD can supply at most 2.5% of our domestic electricity. It should be used when we really need it, not just all the time.
Anaerobic Digestion is never going to supply a significant proportion of our energy needs. This facility takes about 45,000 tonnes of food waste each year and generates enough electricity for 4000 homes. According to WRAP, we throw out 7 million tonnes of food and drink each year. Putting all that through AD systems like Bygrave - if it were possible - would presumably power 620,000 homes - out of 27 million. That is just 2.5% of domestic electricity use. There are other potential feedstocks for AD but food is best as it is so nutritious. Given the limited supply it would be much more useful to store this gas and use it at peak times or to fill in when other renewables are not available. However that would mean either more gas storage, bigger generators and a stronger electricity grid connection; or gas cleaning equipment and a gas grid connection to inject pure methane. Either approach requires more investment. At the moment it is cheaper to supply peak demand from fossil fuel in existing power stations.

Composting is a cheaper alternative way to handle food waste but biogas is going to become more and more valuable.
AD is not the only way to handle food waste avoiding landfill. AD digestate is very good fertiliser as it still has some carbon and all the other nutrients including important supplies of phosphorus. However the advantage of composting is that it retains all of the carbon, not just some of it. Also, composting facilities, even with smell reducing equipment, are relatively cheap to build. None the less, I do think there is a strong case for AD because biogas is likely to be vital for our energy security. Plants like this can be upgraded to satisfy that need when the time comes.

AD will be a flash in the pan if we reform our food supplies to avoid waste
On the other hand, given that energy storage needs more capital investment, it is never going to be viable without secure supplies of feedstock. Will we keep throwing away lots of food for  the forseeable future, or will we learn to be less wasteful? If so, then AD will be just a flash in the pan.

Thanks to Transition Letchworth especially John Webb for organising the trip to Bygrave and thanks also to Kevin and Stefan at Bygrave for being so patient with our many questions.

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