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Discussion in 'Renewable Energy' started by Chippy, Apr 12, 2018.
There is your problem
Yeh I suspected temp would be a main reason. Will get oil boiler hooked up
Out of interest how bad is it to put formalin in the slats which will eventually enter the digester? Im talking 40 litres/week into a slatted tank that’s got 150,000 gallon of slurry in roughly ?
Great to hear from someone with experience, some questions.:
1) Was gas mixing part of the original design?
2) Is it a landia system?
3) What are the annual maintenance costs?
4) What is the annual energy consumption?
When you say Gas mixing has worked well for us for 9 years what do you actually mean ?
Chippy, before you do anything, get rid of the floating layer, it is pointless re-energising the bugs until this is done.
Possible inhibiting limits for formaldehyde are 100-400 mg/ltr, toxic around 500-100 mg/ltr.
Given your volumes of slurry, it should be OK... BUT !!!!!! how can you guarantee it is mixed perfectly ?
It is the original Greenfinch design
This is the base of the tank being poured. When the concrete had gone off, the pipes were trimmed to just above the concrete level.
This is maintenance records since installation in 2009 (I increased pulley size so they no longer eat Vee belts)
Apart from the Mikuni oiling pump, spares costs were trivial (<£20 a time), and that pump just needed cleaning up so the old one is now a spare.
Running costs - normally run for gas mixing about 20 minutes in the hour, but also use the compressed biogas as gas lift in the heating circuit so run 60 mins in the hour when heating. Compressor motor is 3 kW drawing about 6 A, rotary valve motor 0.14 kW drawing 0.5A all three phase
I visited the Ludlow Plant in 2009, which was working well on food-waste (from Somerset I believe). My only real concern for this type of mixing is when slow digested material is added such as grass and chopped straw and the risks of a floating layer.
What are your views.
We feed 30-50% slurry from cubicles bedded with straw, its generally chopped better than in the photo. The only time we had issues with a floating mat was at startup when the total solids level was too low. We started on dirty water with added slurry, but this did not work so had 15 tanker loads from the local Anglian Water AD Monsal stage,
What temp do you operate at ?
Normally 39-41°C, although when the pipes froze up recently we dropped to 33. It did not appear to hit gas production as much as I had feared, down about 1/3 but hard to estimate as we stopped feeding slurry when it was frozen.
Operating temp and HRT have a bearing on the ' speed of incorporation ' into the digestion process. Shock-loading (cold feed-stock) also has an impact on gas production. Prior to feeding, I think pre-heated well homogenised feed-stock is the best solution, and contributor to fast incorporation. But for this, the technology requires economy of scale.
This also reduces parasitic loading for the process agitators.
If you have a thick crust and are at 27 degrees you are wasting your time looking at alternative feedstocks. Get the thing mixed and brought back up to temperature. If your mixer can’t mix dairy slurry then it must be pretty bloody useless. What is a whisk, do you mean a submersible? That should make mincemeat of a little tank like that.
FWIW, maize meal could be a possibility when you do get back to full power. Some 500kw digesters here are using it in their ration and I am told it should completely digest so can’t see it being too hard to manage
Crust finally gone using enzymes and stirrer back to circulating store for now. I was hoping whey would create more gas and therefore speed up heating. Currently at 30 degrees but don’t think bugs really work at that temp
More feedstock will only produce more gas if your bugs are happy. You would need to be high 30s before I would make any changes. If you can’t maintain that temperature then you will never get on top of things
We had an issue approximately 2 weeks ago! We lost our output! Went from 60kw/hr to zero in just a few days! Not sure if it was crusting or we had given it bellyache !! It had dropped to about 26 deg! And as we were contemplating reheating! It started producing gas slowly but surely! Running more and more each day now ,and only up to 28deg! Ran all day today at about 22kw!
It's a good job we are not reliant on these systems for base-load
It is very interesting that each seem to be having so many problems, and no real reasons for the cause, other than Chippy indicating the stirrer is not up to the job.
There really could not be anything simpler than these small scale plants, operating may be the key issue here. If you were to calculate the lost kwhr's, you may soon find economy (capital cost) may have been a bit of a red-herring.
Criticism hurts, and it is not my intention to offend anyone, but if any of the lenders to agriculture get wind of all the issues for small scale Biogas, it may die before it really takes off.
It doesn't seem to be unexpected that these plants are struggling.
I am not sure what temp slurry is, but the amounts that have to be added each day, to not a very large tank, especially in the winter, will inevitably decrease the temp, and it becomes a vicious circle, as engine outputs fall, so does heat output.
Would it be more effecient to heat the slurry to 25 degrees before it enters the tank so its not a such a shock?
I hope that this is nothing more than teething problems! I think we as farmers can be a little gun-ho with things!! I think in my lack of experience we added maize silage strippings which could have given it belly ache! Got to learn to keep things simple, As it is designed to be! I think that the concept is brilliant as we are producing green energy from waste, reducing slurry smells,gases, improving nutrient availability and able to use a lot of the heat in dairy boilers! However as in all new ventures, there will be lessons to be learnt!! OMNI are replacing our tank with a concrete tank, which should be a better job! As to downtime! I am generally over cautious with sales men’s figures and built in plenty of contingency! Glad to say that it looks as though it will exceed even the salesmen’s figures!!
I have wondered whether such an idea of a pre heat tank was a good idea! However we are working with a low cost digester as we are feeding it with only cow slurry, so have to watch costs! We have noticed that it likes the same amount of slurry every day! When we are spreading digestate and use the reception pit, it seems to have a knock on effect on the output! As for the tank, ours is 1000m3 and we were adding 35m3 per day without any problems as long as it was consistent!
Enzymes will help break down the floating layer - we have trial results from the UK and they have been mixed (TBH) but when they work they have been excellent.
We have spent a lot of time with DuPont working on this in our former guise as Bock UK (MBO forced name change) -
Happy to explain more if needed. Could be a simpler short term solution and we have stock.