cavitation in AD

[sjt01]

Hi Stephen, I am a bit surprised by your comments on the need for heat. We really only use it in the winter months to any extent, and it is the same for both plants, our primary tanks have 5 mixers each and move up and down through the tanks. Our plant with the long retention is giving us an average gas yield of 268sm3 per tonne of fresh feed based on the 60 40 split of maize and rye silage. when we do residual gas yield on the separated solid fraction there is very little. it is very fibrous which is not that surprising given how dry both crops were last harvest. If the fibre could be broken down I feel there has to be more gas/energy to be had, hence the thoughts on the cavitator.

Our biologist has not questioned the lack of heat requirement, but I will raise it with him. Thank you for the advice.
In terms of our location we are in the far south east between Canterbury Margate and Dover.
regards

Jim

To quote from "Biogas from waste and renewable resources" by Deublein & Steinhauser, Wiley 2011,
"As shown by the energy balances, very little heat is released with the anaerobic reaction. Therefore, bioreactors must be heated and well insulated" (page 100)

Stephen

 

[sjt01]

is that because you get aerobic bacteria breaking down the fibres that then allows it to be accessed by the anaerobic bacteria when you feed it through the system the second time. There is still plenty of energy as evidenced by the temperature it gets to in the heap. Are you therefore saying you feed the separated material around the system again once it has been in a field heap for a period? if so, do you know what it is giving you as a gas yield the second time round?

regards

Jim

I don't have proof, but I am pretty sure it is the fungal attack. The heap is covered with a fine white mycelium.
We then feed it back to boost the total solids level if required.
No idea what the energy yield is, but I know the liquor output has virtually no residual biogas potential

 

[Jim Pace]

Interesting, it looks like a 'posh' Extruder.

Many claims are questionable, eg reducing viscosity will reduce the energy required to stir of course may be true, however it will also require more stirring as the binding effect of a higher viscosity will be lost.

• Reduce feedstock costs or increase biogas production by 20% or 4 times depending on process and feedstock combination

Very ambitious claim, and what happens if biogas production does not increase by 20% ?

It looks like they have a hire before you buy scheme, so depending on hire costs there is not allot at risk.

High lignin feed-stocks must offer a strong future especially straw and wood, but the Jury is out on the right solution.

I am aware one of these has been installed in the UK.

http://www.biogas-systems.com/economizer_EN.html

evening the Silent one

I have been out to Austria to see the Economiser or at least their test one. Future biogas have also put one in, and have a second on the way. They are very good or certainly gave the impression of being so but they are also very expensive. I think they work where people are buying feedstock at higher prices or lack the land to grow it, but we can grow maize cheaper (provided it rains soon), and we need to stop investing for a while and just optimise what we have. A simple way of feeding in straw and making the most of it would be fantastic. Perhaps 10% of the feedstock by volume would be perfect

Jim

 

[Jim Pace]

Hi
The rye / grass based solids don't digest down at all from my experience.
They can get tipped in fields and 6months later they are in much the same state. Even if spread on grass the fibre kicks around for a long while.
So for rye/grass etc fed plants my point is valid.
Sure these machines will possibly have some side benefits but without extra gas output from the spent solids there is no return on capex/opex.
Obviously if we could all harvest rye at later growth stage when grains have filled and the grains gas yield is available and still get good gas yield from the higher dm straw then these cavitation machines could be abit of a game changer.....

well we can live in hope! the winter rye looks very good at the moment. it will handle the very dry spring much better than the maize at this rate. lets hope it rains soon

Jim

 

[Jim Pace]

I don't have proof, but I am pretty sure it is the fungal attack. The heap is covered with a fine white mycelium.
We then feed it back to boost the total solids level if required.
No idea what the energy yield is, but I know the liquor output has virtually no residual biogas potential

That is really interesting, thank you for this. We will do a bit of research. Perhaps I will send some of our separated and field stored digestate away for testing.

regards

Jim

 

[Jim Pace]

A residual biogas test should be part of routine maintenance, or at least quarterly, why put material through a plant if it is not being fully digested ? (this makes a mockery of the investment)
However, it is correct to say the separated dry fraction will digest further by aerobic bacteria, however some movement of the stack is usually required for the best results. What is left is usually a very friable growing medium.
Getting the best from any plant is down to attention to detail and of course having a design that is able to be flexible (single or parallel pipework etc).
It is a good operational procedure to be able to re-circulate from one tank to another during the process. This enables un-digested material to be re-digested, (as well as reduce the need for liquid) however this should be part of a mass balance to ensure there is not an excess of N.
However, the best and most efficient way is to pre-treat feed-stock at the front end as has been discussed on may other threads. A cavitiator is just one of a multitude of ways.

Edit: I should have added, the most efficient plant I have seen (not in the UK) put heating pipes into the front end liquid and fat storage tanks. The material is put through the process with some being re-circulated back to the front end storage tanks.

Go on then Silent one give me a clue if there is an obvious winning pre treatment!

 

[thesilentone]

Go on then Silent one give me a clue if there is an obvious winning pre treatment!

The pre-treatment directly relates to the feed-stock, each being relevant to the problem at hand. Cell Rupture, Hydrolysis, Mechanical Treatment, Steam Explosion, etc, etc, etc.

All have been driven by a problem or regulation, each adding to the overall efficiency of the plant.

EG: Grasses suffer from lignin content, high N and low batch density.

To overcome some of the above could be carried out in several ways.

Grasses take time to be incorporated into the digestion process, in addition you are left with allot of ' cork ' (fibre) that usually ends up floating on the top - big problem !

So, the first challenge is to speed up the incorporation time, this can be aided by breaking the fibres before the grass enters the digester with an extruder or cavitator or similar. More ends= faster incorporation. Reducing retention time and aggressive agitation is also a benefit with grass, so the ability to increase operation temp (within N limits) helps, as does the ability to pre-heat before incorporation.

Hydrolysis is another way to allow aerobic bacteria to work on the lignin, the issue is the time it takes. so more than once tank is required.

Roots need cleaned and shredded, low density fibre need compacted (briquettes)

Anything and everything to open the cellulose so the bacteria can get to work quickly.

 

[thesilentone]

Depending on the system, and available energy, a hammer mill is also a possibility.

 

[whitefox]

so was the cavitator just poorly made or was the damage due too extraneous/foreignobjects?

 

[Birch Solutions]

I have been reading about the advent of cavitation as a technique. It claims to offer a number of advantages to biogas plants, but at the moment there is very little independent research material.

The claims range from reducing viscosity and therefore reducing the parasitic load of the plant by cutting back the amount of stirring, to breaking down the cellulose structure of rye silage and even straw. The claim is that as a result the gas yield per tonne of feedstock increases by up to 15%, and the retention time is reduced, presumably as a result of the surface area increasing.

The catch is that the cavitator uses a fair amount of power, and it also produces heat. In the summer months this is a concern as we don't want the temperature getting much over 40-42 degrees in the tanks.

Has anyone installed one yet.

Our second challenge is that we want to find a way of increasing our CH4 in the biogas. Our Feedstock is 65% maize silage and 35% rye silage, the resulting gas is anywhere from 51.5 to 52.5% CH4. Our aim is to increase this to 60%, which will enable our gas upgrade plant to process more and so inject more bio-methane into the grid. One option is to install some kind of pre treatment of the feedstock to drive off some of the CO2. Has anyone installed a hydrolyser, and what are the advantages and drawbacks. Can anyone recommend someone to speak to. Many thanks.

Can we help? Danielle Stead is our resident biologist and would be able to advise you on the above: [email protected] As a farmer as well as a biologist she knows first hand the practicalities of managing your AD plant with the feedstock available to you. Luke Mole [email protected] is our AD specialist and would also be able to advise on everything to do with your tanks!