AD Feeding with Auger or Pumped System?
- Thread starter GreenerGrass
- Start date
- Location
- North Norfolk
Straw goes into our digester with slurry, but we do not feed straw as a feedstock in its own right.
Digester Man
Member
I have put straw dung into ours...
Straw goes into our digester with slurry, but we do not feed straw as a feedstock in its own right.
How does the straw break down in your system? Do you have the machine (I forget the name) that breaks the straw down into a mush before it reaches the digester!? Thanks.
Lazy Sod
Member
- Location
- Warminster Wiltshire
I know nothing about the dark art of biodigestion, but I was just wondering if digesters eventually get a build up of solids and sludge in the bottom? If so, how would you get it out?
- Location
- North Norfolk
Much of it comes out with the separated solids at the discharge. We have a macerator in the feed loop which cuts everything up. When we were running at low solids in the digester, we fed back the separated solids, and ran the plant for about 9 months with no straw taken away. I think the reason it did not build up was after separation, it was left in a heap for a few days and developed some fungal growth before being fed back to the digester. I suspect the fungal attack started to break the lignin down so the digester bugs could then attack it.
Stephen
- Location
- North Norfolk
Some digesters require periodic emptying (like every 5-10 years), and digging out with a small skid steer. Ours was designed for slurry and beet with associated grit and stones, so has a hopper bottom which we can flush out every month. We have been feeding for nearly 8 years with no build up. It is all down to design.
Stephen
- Location
- Cumbria
When designing an AD Plant insufficient time is spent looking closely at design and long-term op costs. Of course heavy solids separate out and sink to the bottom of the tank. The larger the dia of the tank, the greater the chance, due to surface area.
This can be ' managed ' to a degree by looking carefully at tank dia/design, agitation, grit removal and access. When the day comes that the plant or digester is to shut down to be cleaned out, the clock is ticking as every minute is costing. Kwhr's cannot be won back (if your plant is operating efficiently) so, how long the task takes to do v better design etc is a feasibility in itself.
sjt01 Dropping in a skid-steer will on shift material to a place, not remove it, for this you will require a industrial tank cleaner.
In addition (as I am sure you know) making a conical tank bottom affects stirring efficiency, by absorbing some of the energy generated by the agitators in the liquid (the kinetic energy)
However, I have seen very old AD Plants (over 10 years operating) very efficiently, having the roof of a tank blown off in a storm to find only around 30% of the tank was operational as the other 70% was totally built up with sludge/grit/solids.
- Location
- North Norfolk
As our digester is gas stirred, no extra energy is required in the conical base, and the gas mixing helps move the crud to the middle. As you can see by the photo when we were pouring the base, the slope is small but adequate.
The gas pipes are cut off close to the concrete level when it is cured. They connect to the pipes on the bottom left of the picture, where the gas is pumped in one pipe at a time
The Son
Member
- Location
- Herefordshire
We have a conical base, and a tanker valve so i can suck out a couple of loads every month from the base of the tank, seems ok at the moment.
Had a bad day today, alarm at 3am motor had fallen off the large paddle giant mixer, fortunately its external but has taken all day to sort, and lost a lot of output as the paddle giant takes the solids from from the dry feeder.
The silentone is correct, if i were to build again i would definately do things differently in the tank.
Had a bad day today, alarm at 3am motor had fallen off the large paddle giant mixer, fortunately its external but has taken all day to sort, and lost a lot of output as the paddle giant takes the solids from from the dry feeder.
The silentone is correct, if i were to build again i would definately do things differently in the tank.
- Location
- Cumbria
Conical base is OK, IF you have an internal mechanism to drag all the grit to the centre. The grit, sludge and heavy solids build up mainly round the outside, not in the middle.
The Son, the paddle giant is a very effective unit, but very energy demanding and (as you may be experiencing) there is lateral movement due to it's mass and size. This movement inevitably leads to a frequency failure of some kind unless regular checks are made of all drive couplings / nuts-bolts/ alignment etc.
Looking ahead, it may be worth costing the parasitic loading of the Paddle Giant + Maintenance + Down Time v pre-homogenisation and less aggressive / energy hungry mixing in the digester's. Maybe low level submersibles or side wall (like the Landia)
The Son, the paddle giant is a very effective unit, but very energy demanding and (as you may be experiencing) there is lateral movement due to it's mass and size. This movement inevitably leads to a frequency failure of some kind unless regular checks are made of all drive couplings / nuts-bolts/ alignment etc.
Looking ahead, it may be worth costing the parasitic loading of the Paddle Giant + Maintenance + Down Time v pre-homogenisation and less aggressive / energy hungry mixing in the digester's. Maybe low level submersibles or side wall (like the Landia)
The Son
Member
- Location
- Herefordshire
I already have two submersibles, which I keep on the bottom (the ropes have snapped) so I hope they are keeping grit in suspension, or moving it towards the centre.
I would question the pre-homogenasation idea, I know of a number of marches plants locally, and all spend a considerable amount of time unblocking the systems, and changing blades.
The way I see the plant is I have to get the most out of it until the fit payment runs out in 2013, so I am unwilling to make any more large capital investments in it, as I have to maximise my return.
I would question the pre-homogenasation idea, I know of a number of marches plants locally, and all spend a considerable amount of time unblocking the systems, and changing blades.
The way I see the plant is I have to get the most out of it until the fit payment runs out in 2013, so I am unwilling to make any more large capital investments in it, as I have to maximise my return.
- Location
- Cumbria
Interesting, do you know how to save the submersibles ? (how the Germans do it) Diver 
So get the snorkel out..!
My ideal design is alot different to Marches, and it works...
So get the snorkel out..!My ideal design is alot different to Marches, and it works...
- Location
- North Norfolk
Our design was Greenfinch/Marches, and it now works well! Apart from the tank itself, we have modified pretty much everything and it now works well. Most of the changes were not major, but combined they have had a dramatic effect on the amount of time taken to look after the system.
If ever we have a blockage or problem more than once, the next move is to sort the system so it does not happen again. If maintenance costs are too high, then modify.
Stephen
The Son
Member
- Location
- Herefordshire
Interesting, do you know how to save the submersibles ? (how the Germans do it) Diver
My ideal design is alot different to Marches, and it works...
What is your ideal design, and have you got one working?
- Location
- Cumbria
My ideal design would include pre-homogenisation as well as biological cleaning for both H2S and odour / exhaust.
Max efficiency for stirring in the tank/digester/fermenter with bespoke design, based on the main feed-stock.
A balance of cost v efficiency, what appears to be low cost in may cases turns out to be expensive when failure/breakdown occurs, you only get what you pay for.
Long-term reliability and plant efficiency mean taking away the human from operation, therefore a control system designed for full automation by a company with extensive experience in AD and high DM feed-stocks.
We are building a plant to generate energy, therefore max efficiency is No1
We are building a plant that is environmentally beneficial,and can prove this.
We are building a plant sponsored by the tax payer, so it must offer best value.
We are only building once, so build well.........
That will do for starters.............
Max efficiency for stirring in the tank/digester/fermenter with bespoke design, based on the main feed-stock.
A balance of cost v efficiency, what appears to be low cost in may cases turns out to be expensive when failure/breakdown occurs, you only get what you pay for.
Long-term reliability and plant efficiency mean taking away the human from operation, therefore a control system designed for full automation by a company with extensive experience in AD and high DM feed-stocks.
We are building a plant to generate energy, therefore max efficiency is No1
We are building a plant that is environmentally beneficial,and can prove this.
We are building a plant sponsored by the tax payer, so it must offer best value.
We are only building once, so build well.........
That will do for starters.............
Digester Man
Member
That's a nightmare scenario and one I always have a worry about deep down. I am fussy about what goes into our plant, for example I am now stopping using beet because I cannot get the stuff bought in clean enough. I hope that minimising crud on the way in will help manage this risk. But if the worst happens, it means a lot of down time and a dirty horrible job to clean out the tank before bringing it back online
- Location
- North Norfolk
Come on, that's hardly a design!
Would you use single or multi stage?
Fully mixed (CSTR), semi-plug flow or plug flow?
Liquid loop feed or auger/pump direct feed?
Mechanical or gas mixing?
Progressive cavity, rotary lobe, piston pumps or gas lift?
Routine degrit facility without stopping operation?
Internal or external heat exchanger?
Feedstock treatment - some sort of zapping, or maceration?
Gasholder built into digester tank (primary or secondary) or separate?
H2S control by air bleed or treatment dosing?
Apart from pre-homogenisation, the design goals you mention have been attempted by almost all the plants I have visited, so not much new there. The degree of success will differ however.
Stephen
- Location
- Cumbria
Come on, that's hardly a design!
Would you use single or multi stage? having carried out many a costing for large single / long retention v multi stage, multi stage wins every time.
Fully mixed (CSTR), semi-plug flow or plug flow? Fully mixed CSTR, if good degradable material is used, lots of attention paid to digester dia, material, insulation and parrasitic loading.
Liquid loop feed or auger/pump direct feed? Pre-homogenisation with liquid loop and parallel system to digesters
Mechanical or gas mixing? Sorry, I am not sold on gas mixing, however it can be beneficial for older systems.
Progressive cavity, rotary lobe, piston pumps or gas lift? Good question, it depends very much on the location and application. Primary pump after pre-tank would be progressive cavity. Rotary lobe wear to much, however they are easy to work on.
Routine degrit facility without stopping operation? Digester design would negate the need for any degrit system.
Internal or external heat exchanger? ............secret
Feedstock treatment - some sort of zapping, or maceration? Heavy duty shredder, hammer mill, macerater or extruder depending on the task.
Gasholder built into digester tank (primary or secondary) or separate? Built in, in the secondary or both depending on the time and volumes required.
H2S control by air bleed or treatment dosing? O2 injection and biological.
Apart from pre-homogenisation, the design goals you mention have been attempted by almost all the plants I have visited, so not much new there. The degree of success will differ however.
Stephen
......any more detail will cost you, but I am happy to answer the questions
- Location
- North Norfolk
......any more detail will cost you, but I am happy to answer the questions
Questions etc
Would you use single or multi stage? having carried out many a costing for large single / long retention v multi stage, multi stage wins every time.
Analysing the digestate from our single stage, there is virtually no gas potential left in the material (<3%) at 50 days retention. I looked at adding a second stage but could not justify it.
Fully mixed (CSTR), semi-plug flow or plug flow? Fully mixed CSTR, if good degradable material is used, lots of attention paid to digester dia, material, insulation and parrasitic loading.
The consensus of plant operators and designers I have spoken to agree.
Liquid loop feed or auger/pump direct feed? Pre-homogenisation with liquid loop and parallel system to digesters
Similar to ours, works well with slurry straw mix and other miscellaneous feedstocks
Mechanical or gas mixing? Sorry, I am not sold on gas mixing, however it can be beneficial for older systems.
Having heard the woes of several operators with mechanical mixing, gas mixing for me every time. Mind you, it works best in a taller, narrower tank.
Progressive cavity, rotary lobe, piston pumps or gas lift? Good question, it depends very much on the location and application. Primary pump after pre-tank would be progressive cavity. Rotary lobe wear to much, however they are easy to work on.
With flinty grit, progressive cavity are better than rotary lobe but worse than piston. Our piston pump is nice and simple, low maintenance, pumps gate hinges, bits of chain, vet's stethoscopes and other junk that lands in the slurry.
Routine degrit facility without stopping operation? Digester design would negate the need for any degrit system.
How would you separate the grit? Or would you try to keep it in suspension until the digestate emerges?
Internal or external heat exchanger? ............secret
Must be internal or you could not keep it a secret!
Feedstock treatment - some sort of zapping, or maceration? Heavy duty shredder, hammer mill, macerater or extruder depending on the task.
Heavy duty shredders seem to absorb huge amounts of power.
Gasholder built into digester tank (primary or secondary) or separate? Built in, in the secondary or both depending on the time and volumes required
Works if you have a large shallow digester, taller ones better with separate
H2S control by air bleed or treatment dosing? O2 injection and biological.
Agree
Stephen