Learning The Rules of Making Forage For Anaerobic Digestion

The UK’s largest lime manufacturer has turned its hand to anaerobic digestion and discovered that the best way to maximise energy output from its feedstock is through the use of the preservative Safesil.

Producing feedstock for energy generation requires all the same processes as producing forage for livestock production, so when AD plant manager, Dean Bingley, took on the task, he consulted widely amongst farmers and growers.

Running three anaerobic digestion plants in north Lincolnshire, whose energy output totals 3MWh, he embarked on a steep learning curve when he first entered the energy business in 2014. 

Today, the facilities he runs for Birch Energy – an offshoot of the UK’s largest independent lime manufacturer, Singleton Birch – provide an object-lesson in forage production with efficiencies brought into the system at every level. Standards of production have increased every year and local growers who have been unwilling or unable to meet the business’s increasingly exacting demands have fallen by the wayside.

Two key agricultural contractors – David Fox and Matt Pickering – are now the mainstay of the plants’ forage production and four growers, collectively working as North Lincs Energy Growers, supply much of the feedstock. Two nearby pig units provide manure as an additional feedstock source, while chicken manure is added to the mix, so broadening the plants’ supply base.

“One of the first things we had to decide was which feedstocks we would use and we decided early on that we would not enter into household waste,” says Mr Bingley. “Food waste would have to be pasteurised and needs a licence, but more to the point, it just isn’t good feedstock.

“Rotten food has lost much of its energy value and if you put rubbish in, you get rubbish out,” he says. “It also contains high sulphur which is corrosive and takes its toll on every piece of metal in the plant.”

Inconsistencies with household waste are also an inherent problem as anaerobic digestion relies on a stable population of bacteria to break down the incoming organic material.

“We call these digesters our concrete cows,” he says. “They are not just machines but living things. Just like a cow, if you change their diet too quickly, they get belly-ache and their yield goes down.

“It’s for this reason that when we make changes, we only ever do so at a rate of one tonne a day.”

Mr Bingley has learnt during almost three years of operation that the quality of forage going into the plant is directly related to its performance – or the amount of gas produced – which in turn influences the amount of electricity generated through the plants’ 12 combined heat and power units (CHPs).

“We test our feedstock through the labs of our plant manufacturer, Plan-ET, and this allows us to measure gas yields precisely,” he says.

Feedstocks tested range from sugar beet and maize silage at around 200m3 gas per tonne down to the lowest yielding of the farmyard slurries at 70m3/t (see table below).

Gas yields from feedstocks used by Birch Energy

However, producing high yielding silages has not been plain sailing and after the first year, when maize silage heated up and lost substantial amounts of energy and dry matter in the clamp, further research was undertaken at the plant.

“We started to work closely with the two contractors and sought their advice on the best methods of forage preservation,” says Mr Bingley. “They also referred us to forage preservation specialists, Kelvin Cave Ltd, who shared trial results with us on different methods and products.”

With so much at stake, Mr Bingley (pictured right) was not prepared to jump headlong into any new techniques so he set up a trial at the company’s main plant.

“We wanted to try a silage additive which was recommended by the contractors and which actually has its origins in human food preservation,” he says, referring to Kelvin Cave’s silage preservative, Safesil, whose key ingredients are sodium nitrite, sodium benzoate and potassium sorbate.

Michael Carpenter, Kelvin Cave’s northern manager, explains: “Safesil gives a clean and rapid lactic fermentation which results in more sugar being retained in the clamp. It also destroys undesirable bacteria which are responsible for producing large amounts of carbon dioxide and water, which would result in significant dry matter loss.

“Equally important is the destruction of yeasts and moulds as it is these above all, which cause the silage to heat up, further cutting its energy value,” he adds.

The trial got under way with the 2014 maize harvest and involved two of Birch Energy’s identical silage clamps being treated in exactly the same way in every respect bar one.

“We treated one with the preservative and had no additive with the other, and once the clamps were opened, we monitored the temperature at 19 different points across the face every week for eight months,” says Mr Bingley.

“The temperature of the treated clamp was far lower throughout, which was particularly pleasing since it was used at a hotter time of year, when spoilage is usually at its worst,” he says (see box at foot of page). “Compared with ambient temperature, the untreated clamp was 19.8°C higher but the treated was only higher by 8.4°C.”

The trial was not scientific but it provided the indicator needed by Mr Bingley, who felt the case for the preservative was overwhelming.

“We could see the heat from the untreated clamp and that’s when you know you have a problem,” he says. “Feedstock is our biggest cost, and if you don’t preserve and clamp it properly, you can easily lose 20% of your stock – for us that means losing tens of thousands of pounds.”

Other procedures relating to the clamping process were also tightened and the company’s keen regard for health and safety saw a switch made in the second year to an easier form of sheeting.

Feedstock quantities used on the main Singleton Birch AD site

“In our first year we used two separate sheets on each clamp but this was simply too much to handle,” he says, referring in particular to the scale of the clamps, with three having a 7,000 tonne capacity at 75m x 25m and a further six measuring either 50 or 60 x 25m.

“Using two sheets involved twice as many visits to the top of the clamp than really needed, more damage to the sheeting, and the final straw was when someone broke their wrist dragging the second sheet over the first,” he says.

It was after this incident that O2 Barrier 2in1 sheeting was introduced, again on the recommendation of the silagecontractor.

“Sheeting really is the worst part of the clamping job, so anything you can do to make the operation easier is worth going for,” says contractor, David Fox. “A standard black sheet will trap oxygen and you can see exactly where when you open the clamp and see the rotting material.

“With this two-layered product, the lower polyamide layer seals itself to the silage, and you don’t get these losses,” he says.

“Having the two contractors who have been honest from day one has made a massive difference,” adds Mr Bingley. “The recommendations they make are in their interest as well as our own, and this is one that has worked really well.

“It took 14 men seven hours to sheet each large clamp in the first year, but now we only use five men and it takes us five hours,” he says.

Other improvements have been made at feed-out, including a switch to shear grabs which are said to be safer than a rotary facer, and a move from telehandlers to Volvo L110s.

“The telehandler would only lift one tonne in the bucket, so we moved to the L110 which lifts five tonnes at a time,” he says. “It also has a quick release so changing the bucket for a shear grab takes a matter of moments.”

Everything used at the Birch Energy AD plants has been introduced on the strength of careful research and proven results, and the whole of the company’s management team has been delighted with the outcome.

Feedstock quantities used on the main Singleton Birch AD site

“Of course, we are still learning,” says Mr Bingley. “But you develop a sixth sense with these plants and can soon see what’s wrong. This applies to the silage as much as anything else - if it looks right, you normally find it is right; you can see and smell the difference and feel it in the temperature.”

Today, Singleton Birch has cut its £2 million annual electricity bill in half; it supplies all of the energy for two neighbouring pig units and it is considering expansion into a fourth plant.

“When the first plant was commissioned in March 2014, the feed-in tariff for electricity generated was 14.5p/KWh,” says Mr Bingley. “Today, it has dropped and the case for AD has weakened – but we are still costing our fourth plant although the decision is yet to be made over whether it will go ahead.” 

Two Claas Xerions at work on the maize silage clamp at Singleton Birch. The one at the top of the clamp is fitted with a SilaPactor from Kelvin Cave Ltd.

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