Biogas at Wighton

[sjt01]

My initial exposure to the biogas experience is when I was running the Ag Eng workshop at Bunda College, University of Malawi. In 1977, the then farm manager, Felix Pereira, a Goan, wanted to build a Gobar digester for the dairy manure to make gas for cooking staff food. I was roped in to get the roof/gasholder made:

We also made a mini-digester for a student project

On my return to the farm in 1998, we were faced with NVZ regs requiring a better way of handling the dairy slurry, so I investigated AD. It turned out that to run a CHP that was available then, we would need slurry from 750 cows.

In 2006 we visited 2 digesters, one near Church Stoke

and one near St Davids

Then in 2007 I went on a renewable energy tour of Germany and Austria led by Tim Evans, with, among others, Winston Reed.

I realised that, with some crop input, it might be viable. I could see the sugar beet price coming down, so could easily replace that in our cropping with more maize.

I went on various visits and to meetings, and was intrigued by the Portagester which extracted juice from FYM for digestion

I got a quote from Tim Evans, then with Renewable Zukunft (sp??) which seemed hopelessly optimisitic, and one from Michael Chesshire who was then running Greenfinch. We went to see the Ludlow food waste digester

Also went to see the Kemble site they were then commissioning

 

[sjt01]

I decided on the Greenfinch proposal, with me organising the groundworks to their design, and basic electrical installation. I did the hot water plumbing myself. They were to organise the plant and equipment, connecting up and commissioning.
We were very fortunate in not needing outside finance, as since my Father's demise and giving up the Wells land, we had a good farm sale, and could call on loans from other family members. By doing as much as possible ourselves, and utilising existing equipment, we could cut the cost. For example, we used the biomass boiler grain dryer radiator as the CHP heat dump radiator.

Jan 2008 we got our initial designs from Greenfinch.

March 2008 put in for planning permission

April 2008 Quotations from groundworks contractors

May 2008 planning approved with conditions including "the anaerobic digestion plant hereby permitted shall only process material produced on Copys Green Farm only unless otherwise first agreed ...."

June 2008 Permission from DNO to connect (EDF at the time)

Sept 2008 formally placed order with Greenfinch

October 2008 Trell Contractors of Watton started on groundworks. They were to be with us for about 6 months.

December 2008 Greenfinch install gas mixing pipes in base of digester, underground pipework and ducting.

Jan 2009 Digester tank starter ring installed

April/May 2009 most of the equipment installed

August 2009 started trial feeding, using dirty water and slurry to start. Solids level too low to carry straw etc. so frequent blockages.
Decided to get starter in from Anglian Water sewage works digester at Kings Lynn. Much paperwork with AW and Environment Agency!

Dec 2009 15 tanker loads of starter digestate from AW arrives, start feeding. First good gas.

Feb 2009 Beaver Power start trying to commission first CHP

June 2009 Beaver finally get it running long enough to commission and G59 witness test.

Nov 2009 After much grief, and intermittent running, decide to take out Beaver CHP, recompensed by supplier (not Beaver). Order CHP of my own selection.

end Jan 2010 New IET CHP arrives

Feb 2010 IET commissioned and witnessed in less than a day - finally we are up and running!

 

[sjt01]

Feeding the beast - we initially designed the digester to take cow slurry, FYM, maize silage, chopped beet and whey from our cheesemaking. The maize silage was not made very short chop for the digester, we take the top, sides and bottom of the cow feeding clamp, leaving the best for the cows.

To accomodate this variety of feedstock, there is a liquid feed loop. The feedstock was originally all tipped into a Trioliet single auger mixer. A Boerger power feeder fed the feedstock into the liquid stream, which then went to a Vogelsang macerator to chop it up. Initially there was a Boerger rotary lobe pump pushing lliquid past the end of the power feeder, through the macerator and back into the tank.

When the power feeder got a slug of slurry, the plug of material would be lost and the digester contents start to flood the area. We added a second pump between the macerator and power feeder, and reduced the amount of shitsplosions. However it was still occasionally happening, so I added a couple of pneumatic valves to close off the feed loop if the liquid level got too high.

We soon dropped the FYM, as it seemed to take more energy to get it into the digester than it yielded.

The rotary lobe pumps did not like our flints, and I had to change lobes every few days. This got expensive so I started refurbishing the lobes with Devcon Flexane from RS Components, and researching better pumps. We ended up with a GEA Houle 2 cylinder piston slurry pump, operated by hydraulic rams. This is nice and simple and easy to maintain, but the reversing mechanism was not too good, so I designed and implemented an electronic controller to do the reversing a bit more intelligently. The pump needs remarkably little maintenance, and is farmer friendly! Hammer and welder do a lot of the maintenance.

By utilising gravity and gas lift principles, we were able to do away with all 5 Boerger rotary lobe pumps. Their power feeder did not like liquids in the hopper, and needed a new gearbox every year for £1800 - cheaper to replace than refurbish. I designed and built a single auger feeder with pulsed liquid mixing, no plug, in stainless, for less than the cost of the gearbox. It used the original geared motor. After a bit of tuning it works well, but I still do not have a shaft sealing solution that works to my satisfaction. Each attempt results in an improvement, but we are not there yet.

 

[chaffcutter]

Your technical ability amazes me, are you mostly self taught?

 

[sjt01]

Schematic of the digester, sorry its a bit small

 

[sjt01]

Your technical ability amazes me, are you mostly self taught?

Did a first degree in Ag with Ag Eng final year option at Reading, then 2 years at Newcastle, Masters in Ag Eng. Most of the practical stuff picked up on the way. Finally did a PhD in Control Engineering at Wageningen during the final years of my work at Tea Research in Malawi. Thesis "Control of Fluidised Bed Tea Drying".
Picking it up in small doses is easy, over the period of many years. It would be horrendous to contemplate a fresh graduate having to master it all.

 

[Crofter64]

Schematic of the digester, sorry its a bit small

What is the objective of the digester? It seems like a massive effort, initially and ongoing.

 

[Suffolk Serf]

Love the word “shitsplosion” is that a technical term in AD circles?

 

[sjt01]

What is the objective of the digester? It seems like a massive effort, initially and ongoing.

Take low value materials, generate methane which can either be upgraded and pumped into the gas grid, or burnt in an engine to produce electricity and heat. We follow the latter route, and can produce 170 kW electrical and 200 kW heat. We make as much use as possible of the heat but many installations just dump it.
The leftover material (digestate) is brilliant fertiliser.

 

[The Son]

Really interesting, I too went to Church Stretton as part of my fact finding, fascinating chap, a bit heath robinson, but it worked.

 

[sjt01]

Love the word “shitsplosion” is that a technical term in AD circles?

It certainly has become so here. This cartoon from New Scientist a few years ago also applies

 

[Crofter64]

Take low value materials, generate methane which can either be upgraded and pumped into the gas grid, or burnt in an engine to produce electricity and heat. We follow the latter route, and can produce 170 kW electrical and 200 kW heat. We make as much use as possible of the heat but many installations just dump it.
The leftover material (digestate) is brilliant fertiliser.

Do you aerate the digestate before spreading?

 

[sjt01]

Do you aerate the digestate before spreading?

No, why would you? We apply with a trailing shoe spreader for liquid, muck spreader for solid

 

[sjt01]

Really interesting, I too went to Church Stretton as part of my fact finding, fascinating chap, a bit heath robinson, but it worked.

Sadly they have had to close down. I think it was to do with being on ROCs and not being able to get RHI. A lovely family.

 

[sjt01]

One thing I like about the design of our system is that the tank does not contain any mechanism or equipment, everything is external. The heat exchanger is a shell and tube, and sludge was originally circulated by a Boerger pump. We now use compressed biogas from the gas mixing system to lift the heated liquor back to the tank.

The tank is gas mixed. There are a couple of skids, each with a vane compressor. Then a rotary valve sends the compressed gas to each of 18 ports in turn, with gas pipes running to ports in the base of the tank.

We can draw a small amount of air into the gas mixing system, to maintain an oxygen level of around 0.2-0.3%. This keeps the hydrogen sulphide levels down to around 200 ppm, which is acceptable for the CHP. This level does not affect the methane percentage significantly. If the H2S rises above this, we throw in a sack of ferric hydroxide once a week.

The small oxygen content allows some bugs to digest the hydrogen sulphide down to sulphur. We have just had to clean out the gas pipe from the top of the digester down to the gas mixing skids and the gasholder, as the sulphur buildup was causing backpressure and lifting off of the gas safety valve. We got 4 or 5 kilos of sulphur out.

 

[sjt01]

Digester tank: Permastore vitreous coated steel, approx 10 m diam and 10 m high, giving 800 m3 total volume. Running at 90% fill gives us 720 m3, and an average hydraulic residence time of 50 days. Insulated with 100 mm rockwool, with IBR cladding. Roof uninsulated, as no liquid there and we want to condense water out of the gas.

Gasholder: Unlike many, we have a separate gas holder, ground mounted on a concrete plinth. The inner bag has a volume of 350 m3 when full. The outer is pressurised by air at 20 mBar by a blower (now been running for 90,000 hours), with a deliberate air leak and over pressure valve to maintain the 20 mBar.
Originally the gasholder fill level was supposed to be indicated by an ultrasound sensor on top, looking at a wooden target board on top of the inner bag. There were times when the target tipped up, and we got a reading of zero gas, cutting the engine, although there was a 2/3 fill. We tried replacing with a laser distance sensor, with the same result, so I went down the local chandlery and got a stainless steel wire rope setup, fixed to the top of the inner bag, over a pulley on top of the outer bag. The wire then goes across to the side of the digester tank, over another pulley and down to a chain and sprocket (connected to the instrument system to measure level) ending up with a lump of concrete to keep the wire taut. The original potentiometer was susceptible to moisture down the shaft after 3 years, so now replaced by a gravity sensor (Pepperl + Fuchs Analogue, PNP Inclination Sensor switching current 100 mA supply voltage 10 → 30 V dc RS Stock No. 814-0204) on a reduction gearbox.

 

[KennyO]

Very interesting. Thank you.

 

[sjt01]

Today we have just cleaned out one of the stone dropouts, reminding me to talk about this. Originally there was a dropout for stones and grit, after the macerator which traps anything that will not go through its screen.
So far the macerator has collected a good heap of tramp metal including gate hooks, the vet's stethoscope, nuts and bolts, and a variety of tramp metal from unknown sources. It collects flints up to about 6" across.
The original dropout did not collect very much, but as grit and stone has a mind of its own, it soon started accumulating in the pipework and causing blockages. We moved the traps to where the blockages occur, and clean them out occasionally. The first one gets about 20mm stone, the second pea shingle.

Stuff from macerator

Then, when the liquor reaches the digester tank and settles out, it collects in the middle of the tank as it has a shallow hopper bottom. There is a pipe from the centre of the tank to a sump, valved so whatever collects can be flushed out to the sump without risking emptying the digester contents. We find doing this once a month is enough. The sump should collect the heavy stuff before it reaches the digestate lagoon.

We also get sand and silt settling out of the liquid after the separator. After an 8" pipe blocked solid, we replaced it with a concrete open channel which needs desilting a couple of times a year. After all that, the digestate lagoon still needs occasional desilting.

Despite all this, we still get grit in the lagoon

 

[upnortheast]

Now that was an interesting read, Lot of it waaay over my head.
Clearly @sjt01 is a man who relishes finding a solution to every problem
Thank you

 

[New Puritan]

Thank you very much for this thread @sjt01 - it's really informative.

What informed the design you went with - especially having a separate gas holder? That's fairly unusual.

Just in basic terms, what sort of tonnage of feedstock do you get through per year?

I'm only a small tenant farmer currently, but I can dream When I own my own farm I would like to put in a small digester. If you add a pasteuriser it opens up a few options - I've even heard of people putting hand-rogued weeds through them. I really like the closed-loop-ness of farm AD plants. I work in renewables in my other job but not generally with AD, so this thread is fascinating to me.