AD Feeding with Auger or Pumped System?

[thesilentone]

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

Looking to your RBT at 3%, this figure looks high if you consider the PAS110 accreditation figure is 0.5 g/ltr VS.

3% could be costing you £9k a year....

Gas mixing would appear to have problems with taller tanks as it appears to contribute to the formation of a floating layer.

Grit removal/management is a compromise. Tank design/dia, location of agitators/type/speed and retention time all play a part. Long-retention times contribute to build up, however short retention and to regular feeding contribute to wash through, therefore two stage is essential. The ability to monitor internal build-up and what action to take if it occurs are all part of the action required.

Heating the biomass and keeping the tanks at a even temp throughout is paramount. The ability to manage this and take action as required is important. Ambient temp has a big impact on performance, however clever designers locating sensors where this difference has the least impact simply cover over problem.
The ability to (automatically) control digester temp is a critical design component. Then the question of what type of heat exchanger is used? what are the likely challenges? how quickly can changes be made?

Food for thought....

 

[sjt01]

Looking to your RBT at 3%, this figure looks high if you consider the PAS110 accreditation figure is 0.5 g/ltr VS. 3% could be costing you £9k a year....

Its less than 3%, that is worst case and £9k/year will not justify building a second stage tank. Most of the remaining VS is in the separated solids, virtually non in the liquor.

Gas mixing would appear to have problems with taller tanks as it appears to contribute to the formation of a floating layer.

Formation of a floating layer is only a problem during startup when the digester total solids are too low.

Grit removal/management is a compromise. Tank design/dia, location of agitators/type/speed and retention time all play a part. Long-retention times contribute to build up, however short retention and to regular feeding contribute to wash through, therefore two stage is essential. The ability to monitor internal build-up and what action to take if it occurs are all part of the action required.

Our system eliminates grit build up, taking 5 mins once every month. Stone traps are another issue! None of the stones dropped out where expected, apart from the big ones that would not go through the macerator. We had to move them to where we found the accumulations.

Heating the biomass and keeping the tanks at a even temp throughout is paramount. The ability to manage this and take action as required is important. Ambient temp has a big impact on performance, however clever designers locating sensors where this difference has the least impact simply cover over problem.
The ability to (automatically) control digester temp is a critical design component. Then the question of what type of heat exchanger is used? what are the likely challenges? how quickly can changes be made?

Internal heat exchangers sound good until a leakage occurs! Our external shell and tube heat exchanger works well, and is easy to maintain. Are there any recent digesters without automatic temperature, level, feed and discharge rate and other controls? If so they are unjustifiably cutting corners. (I speak as a control engineer!).

The one design fault I swear about every day on our digester is that the designers rejected my request for a decent fall on the concrete round the feed area. That makes cleaning up after maintenance much harder work than it need be.

Stephen

 

[thesilentone]

The one design fault I swear about every day on our digester is that the designers rejected my request for a decent fall on the concrete round the feed area. That makes cleaning up after maintenance much harder work than it need be.

Yes, I have seen several plants where vegi cuttings/outsize is used @ 19% DM through one dry feeder unit alongside another unit feeding maize @ 35% DM, guess which one had all the liquid and mess underneath

 

[sjt01]

We seem to have had no feedback from the OP on whether he has found the thread useful, but it forms quite a discussion on digester design and operation.

 

[Will Wilson]

Digester design, that would be a book I could write a few paragraphs about.

 

[sjt01]

Digester design, that would be a book I could write a few paragraphs about.

Perhaps we should make this a TFF project!

See you at the MGA Conference in 10 days time

Stephen

 

[thesilentone]

I gave a presentation to them in the 80's on the benefits a corn cracker made to digestion, things have moved on a bit since then....

 

[GreenerGrass]

We seem to have had no feedback from the OP on whether he has found the thread useful, but it forms quite a discussion on digester design and operation.

I was scared of jinxing the topic, as it was going so well... very informative and helpful.

Think we are going to go down the auger route despite the misgivings of thesilentone, I was very interested to learn all these things contained within. I can certainly see the merits but for us the plant has to fit in holistically and to a limited budget. We are going into its eyes open, and aware of potential weaknesses but at the same time, if we need to make changes in future funded from cash flow then whilst not perfect it will be manageable.

 

[sjt01]

I was scared of jinxing the topic, as it was going so well... very informative and helpful.

Think we are going to go down the auger route despite the misgivings of thesilentone, I was very interested to learn all these things contained within. I can certainly see the merits but for us the plant has to fit in holistically and to a limited budget. We are going into its eyes open, and aware of potential weaknesses but at the same time, if we need to make changes in future funded from cash flow then whilst not perfect it will be manageable.

I hope you have visited and spoken to as many operators as you can. You are welcome here (PM me). Do not just listen to the designers, but the people who have to run the plants from day to day, and deal with the blockages and breakdowns. We have had our fair share, but have engineered most out now. I have seen and know of auger feed systems that have had feed disasters, as well as others which end up as composting at the front end rather than digesting.

Stephen

 

[thesilentone]

I was scared of jinxing the topic, as it was going so well... very informative and helpful.

Think we are going to go down the auger route despite the misgivings of thesilentone, I was very interested to learn all these things contained within. I can certainly see the merits but for us the plant has to fit in holistically and to a limited budget. We are going into its eyes open, and aware of potential weaknesses but at the same time, if we need to make changes in future funded from cash flow then whilst not perfect it will be manageable.

If you have simple feed-stocks (slurry/energy crop) augers are fine, however I don't think there is much difference in the price if using per-homogenisation tank instead. The real interest and cream is in the small percentages that are lost through compromise, so of course challenging and looking deeper is all worthwhile. Good luck with your project, keep us posted, and insist on what YOU want.

 

[GreenerGrass]

Thanks both - yes simple I think. Predominantly slurry 60%+, remainder wholecrop, maize and grass silage. Understand your thoughts on efficiency and certainly happy to keep learning and tweaking. Thanks for the offer sjt - I have been to a few plants including one by a contributor to this topic, and all have been worthwhile and left me with something to reconsider or new ideas.

 

[Will Wilson]

See you at the MGA Steven,

If anyone needs any advice on silage clamp design or management get in touch- Please do not ignore the clamps - they are the single most important investment on your AD Plant. *I would say that wouldn't I*

It's true.

Will
[email protected]

 

[Birch Solutions]

To clarify the point between cooling and shock loading. When new material is fed to the process, the gas production immediately go's down below it's current level.However, this is only temporary and increases again quickly, not only back to it's existing level, but above this by the new VS added. A cold biomass temperature increases that curve and time. Pre-homogenised, per-heated biomass has a similar effect, just the curve is less, as is the time. Shock loading in terminology for new and fresh biomass added. (A bit like you having a very cold beer when very hot)

Contact Birch Solutions for supply of parts of any kind of feeding system, whether, auger or pump fed. [email protected]

 

[Birch Solutions]

Depends who's eye's your looking through and what the application requires. Sadly the average efficiency of on-farm AD plants in the UK is very low, the reasons for this are mainly attributed to management, the incidents of high risk category heath and safety incidents and risks to life on AD Plants in the UK is very high, this is mainly attributed to management, the environmental incidents at AD Plants in the UK is high, with 'serious risks to the environment' classification, this is mainly attributed to management - their is recurring theme here !!

Pre-feeding is ONE of the areas where automation is also a benefit.