Biogas at Wighton

The original design came from Greenfinch. I think the separate gasholder was standard for them, it ties in with using gas mixing, for which a tank about the same height as diameter works. The ones with the gasholder in the roof are generally much greater diameter than height.
Don't worry about pasteurising for weeds, a week at digester temperature will kill blackgrass seeds ( see reports from Wrap - I think it might be in http://www.wrap.org.uk/content/digestate-and-compost-agriculture-dc-agri-reports ). You only need pasteurisation if you are using animal byproducts or food waste which might contain meat.

Separators.
When the digester is over 90% full, we run the separator. We have both an analogue pressure sensor for level, a fixed "working level" probe and a "danger high" probe. If we reach danger high through the separator not working, we stop feeding.

The original separator was a Carier screen type, the screen rotating between 3 rollers and the whole liquor going into a trough on one side, The separated liquid came out the bottom, and the solids scraped off the outside of the screen. This worked well until the screen clogged up with limescale - we are on chalk, a very hard water area. Then changing the screen was a nightmare, as the machine was on a stand above a trailer, and needed 3 skyhooks to hold the rollers while changing the screen.

We were recommended to change to a Sepcom screw separator, which suited us as the dealer is only 15 miles away. We modified the liquid hopper to increase capacity, and fitted a low and high level sensor to operate the discharge pump as it was not feasible to run a returns system. We lost the plug a few times, which was another shitsplosion or two, so I designed and built a plug loss sensor and a plug movement sensor. The control system now throws errors, we hope before too much liquid ends up in the trailer, and stops discharge.

We started by running 0.75 mm main screen and 0.9 mm extension, and suffered quite a bit of wear and breakdowns, and had to feed a water softening chemical into the discharge. One time, they were not able to supply a 0.75 screen, just a 0.9 which we could borrow until we got the 0.75. This was the best thing - now both are 0.9 mm, it runs without chemical, far less cleaning and far less wear. The solids in the trailer are slightly wetter, but that is not a real issue.

We removed the discharge pump, and use gravity assisted by air lift - we just run a 6 mm air pipe into the bottom of the 100 mm pipe to the separator, and the lower density due to the air increases flow rates dramatically. The fewer moving parts I can have in contact with the slurry the better.

Feeding system:
We put all the slurry in the Shelbourne mixer tub, max 4 tonne at a time. If it is very sloppy, we might add something a bit more solid to help regulate the flow. If the control system sees more than 1 tonne in the tub, it feeds it all without a pause, as it will be low gas potential.

The whey from the cheese goes to a buffer tank as it is produced, and whenever feeding takes place and there is whey available, it gets pumped in after the solids are mixed in.

The Herbert solid feed bunker is loaded with the ration for beet for the day (max 2.5 tonnes, only in winter), any wholecrop available and topped up with maize. This is transferred to the tub in 200 kg batches.

When the tub holds less than a tonne, then it is fed every half hour. The size of the feed will depend on the level in the gasholder. There is an amount proportional to the level of gas, and this is increased if gas level is falling, reduced if rising. Below a certain level it feeds continuously. Above 90% fill it feeds just a minimal maintenance amount, to avoid having to flare off excess gas.

The CHP output and gas consumption is proportional to the gas level, between limits.

If the methane percentage falls below 52%, the feed is reduced to minimal maintenance although the gasholder level might be low. This is to prevent killing the digester when it has an upset stomach.

If the total solids percentage in the digester falls below 8%, then it is difficult to keep straw and the like in suspension, so we return separated solids in the feed, about a tonne a day, or the rate at which it is produced. This is necessary when it is wet and the slurry is dilute, also when we fed fodder beet (16-18%DM) rather than energy beet (24-25%DM). When we were growing fodder beet, we re-fed all the separated solids for 9 months; I think that by exposing the solids to the air, a fungal digestion started lignin breakdown which could then continue in the digester.

If the total solids percentage is too high, above 9.5%, normally in summer when there is not much slurry, it is difficult to pump the sludge so we pump in dirty water from the dirty water lagoon.

The gas from the gasholder is pressurised from 20 mBar to about 100 mBar by a direct drive side channel gas booster, replacing the belt drive multistage booster we used to use. The belt drive one ran at 10,000 rpm and needed new bearings twice a year, it is still kept as a backup. Because we up-sized from the original 140 kW CHP to the new IET 170 kW, the gas pipe is slightly undersized so we have to provide a bit of excess pressure to get the gas to the CHP, 60 metres away.

The CHP is located centrally in the farmyard, for ease of heat distribution and shorter electrical cable.

The IET has been a fantastic engine. We have had a few faults, but considering it has now been running well over 75,000 hours it is not bad at all. We service it at 1200 hours oil change, 2400 hours tappets and plugs. Cooper Ostlund do the major services, turbo service exchange at 14,000 hours, new cylinder liners, pistons, heads, con rods. Full service was at 56,000 hours when the engine was sent to their workshop in Northampton, where things like crankshaft and camshaft ovality was checked and found OK.

When the engine is being serviced, we have an Ideal (Hoval) biogas boiler which provides the heat we need, and avoids flaring too much gas. Getting the boiler and flare to light correctly is not easy, as normal flame detectors do not work reliably on biogas. We now use thermocouples, and a big spark transformer to ensure a good spark.

Heat use - one of the advantages of biogas over other systems - both electricity and heat

The CHP hot water circuit is connected to the digester heat exchanger, the cheese/house/dairy, and the workshop by Uponor and Rehau twin insulated underground pipes, 63 mm and 40 mm od. There is also a pipe from the cheese heating under the road to the 3 farm cottages.
We have 11 heat meters altogether. The older ones are gradually dying, and changing them involves a bit of a delay in Ofgem RHI payments. Without the RHI we would not have put the pipe under the road, but now we have the duct in, as well as heat we are supplying electricity and broadband from our FTTP connection.

Controls.
Digester operation is controlled by a PLC, originally Rockwell provided by Greenfinch but we have taken it out and replaced by Mitsubishi, as it took far too long and too much trouble to move the licensing from a dead computer to a new one. During that time we were operating blind, and there were many settings we could not change.
This was the original panel going in.

This is it now with lots more controls and indicator lamps, and the new PLC on top


PLC hardware

We have added lots of safety relays, to stop a manual operation going too badly wrong, or in case the PLC goes haywire.
We have re-written the ladder code for the Mitsi, and we often update and improve the code. The HMI screen is also of our own devising, as we like to follow trends, rather than have pretty pictures of tanks with a single temperature reading.


The CHP also has a stand alone control panel, driven by a Mitsubishi Alpha (we use a lot of these), controlling the CHP, boiler, hot water pumps, and heat dump fans.


The CHP itself has a control panel with a Deif controller,

We are on quite low power today as I took the photos, as we have had a lot of slurry to feed following all the rain, and slurry does not provide too much energy.
I can get the HMI screens on my mobile phone using Any Desk (better than Team Viewer which we used to use), and control the system from anywhere I get a 3G or better phone signal.

A couple of years ago we had a minor gas leak on the roof of the digester. Then we notice more corrosion on stainless steel roof hatch and sensor flanges. We had to get a breathing apparatus trained crew in to do the change for new components


This is some of the corrosion after cleaning up


That was 304 stainless steel, 3 mm thick. The replacements are in 316, and 6 mm thick!

Research
We have been involved with various research projects, supplying samples for analysis. The most interesting one has been the UK AD Microbiome Monitoring Project from the University of Warwick. They sampled the digester every week for a year, and did a DNA analysis (actually 16s RNA) on the bugs. The results are still being analysed, but they found many unidentified species, averaging 933 species in the digesters they looked at.
Ours is AD07 in the web report http://anaerodynamics.com/. I also had a sample of our cow slurry analysed, there were 1768 species in that. They also think there are several rare species that they did not pick up.

Digestate ammonia emissions.
From my crude measurements, I think that we lose at least 25% of the nitrogen in our digestate between the digester tank and the storage lagoon, let alone what is lost from the lagoon and during field application. If we can lock that ammonia into the liquid as ammonia ions, we can save ourselves a lot of purchased AN fertiliser.

The way to do this is to drop the pH from about 8 down to 5.5, as shown on the graph
The top right arrow is where it is in the digester, at about 40°C and pH 8. It is well over 10% ammonia which is quickly lost (notice the Y axis is a log scale). By reducing the pH to 5.5 we get it from 10% to 0.05%, and cooling to 10°C brings it to 0.005%.

We can achieve this by adding concentrated sulphuric acid (very carefully, and with full health and safety considerations!). Second user conc sulphuric is quite cheap - 10 tonnes cost us just over £1000 delivered, and we have to buy sulphur fertiliser anyway, so this is the cheapest source.

I have had a quote from a commercial supplier to install a system, but the payback on our small digester is far too slow. The only way I can make it work is to do it myself, utilising existing equipment (tanks, controls, pipework etc.) with minimal additions.

I have done some preliminary tests at bucket scale, and have made the necessary modifications to the digestate system. All I am waiting for now is some pump modifications and I can test manually, then put it in the control system.

There is quite a bit of foaming as the acid is added, so this needs to be taken account of in tank sizing and level sensing

I think by doing this we can save ourselves buying nitrogen fertiliser, and keep DEFRA happy that we are controlling ammonia emissions.

That's about it for our digester. I am happy to try to answer questions and comments

Just as a footnote, some performance figures

chaffcutter said:

You have basically redesigned and equipped your plant from its original supplied and installed state if I read this correctly. How on earth does the average farmer get on, when they would have nowhere near your technical ability to solve all the various problems that occur? Would they have to keep paying the installers to troubleshoot and upgrade continuously? If so it must be very costly.
I just can’t imagine the average bod on farm being capable of running this stuff!

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I would hope that the designs have been sorted out now and it will run much smoother. I have suffered from being an early adopter, but it is very different technology to solar or wind in terms of maintenance and commitment of time

Exfarmer said:

Stephen, An honest question from someone else in the renewables industry relying on FIT income from solar.
Do you think it will still be a viable proposition to run after the FIT and RHI income have ended?
I suspect from its age you are on a 25 year contract so 15 years to run.
My own feeling looking at your figures would be very marginal

please feel free to ignore this question.

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After the FIT and RHI, I think we will probably run with a smaller CHP, basically supplying our own needs of electricity and heat, and running mainly off slurry. The business of non-FIT grid generation is not a goer. Still, what will happen to the electricity market in the next 10 years??

Looking at the yield of solar in terms of GWh/hectare, it is far better than pure electrical generation from AD, and even including batteries for night time export probably cheaper than AD. I think the future of AD is to utilise waste only, no crops grown specially, and to utilise heat as much as possible.