cavitation in AD

[Jim Pace]

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.

 

[sjt01]

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.

Is this what you are talking about:
http://www.cavimax.co.uk/node/36

If so I would guess the results are similar to the Vogelsang Biocrack http://www.vogelsang.info/product/disintegration/operating-principle-biocrack/

Surprised at the concern at extra heat, especially on a biomethane plant where you will not have much CHP heat. If the digester does not need heating, then you must be doing aerobic digestion at some stage. What oxygen level are you running at in the gas, as this will influence your methane/carbon dioxide ratio. If you are truly anaerobic then the digester will not be exothermic, but need a significant amount of heat to keep it at operating temperature.

 

[Jim Pace]

Is this what you are talking about:
http://www.cavimax.co.uk/node/36

If so I would guess the results are similar to the Vogelsang Biocrack http://www.vogelsang.info/product/disintegration/operating-principle-biocrack/

Surprised at the concern at extra heat, especially on a biomethane plant where you will not have much CHP heat. If the digester does not need heating, then you must be doing aerobic digestion at some stage. What oxygen level are you running at in the gas, as this will influence your methane/carbon dioxide ratio. If you are truly anaerobic then the digester will not be exothermic, but need a significant amount of heat to keep it at operating temperature.

many thanks SJT01-
yes cavimax is one of the applications I have come across. There is another from Italy by a company called Biobang. we went last year to Milan to look at plants using the system. It looked most interesting, but there was a lack of pier review data, and the cost was just too much for us without more than anecdotal evidence.

I have not heard of the biocrack, and I will do some more digging. There is as usual there is little by way of detail on the website.

on the tank temperature, we have found on both our plants that there is very little call for heat outside the cold summer months. I have heard that some people with high organic loading actually need some form of chilling in the summer months. We have not had this problem, but we now have about 110 days retention on one site and over 150 days on the other. regards Jim

 

[sjt01]

many thanks SJT01-
yes cavimax is one of the applications I have come across. There is another from Italy by a company called Biobang. we went last year to Milan to look at plants using the system. It looked most interesting, but there was a lack of pier review data, and the cost was just too much for us without more than anecdotal evidence.

I have not heard of the biocrack, and I will do some more digging. There is as usual there is little by way of detail on the website.

on the tank temperature, we have found on both our plants that there is very little call for heat outside the cold summer months. I have heard that some people with high organic loading actually need some form of chilling in the summer months. We have not had this problem, but we now have about 110 days retention on one site and over 150 days on the other. regards Jim

Vogelsang are a well established company, we use their macerator but I am not particularly impressed by any of the magic systems on offer.

If you are not needing to heat, you are losing biogas potential as you must be digesting aerobically, at least in the early stages of the process. Are you fully mixed single stage, multi stage or horizontally mixed with little vertial mixing? There is an exothermic Agriferm installation near us, or at least last time I went there. It would certainly pay you to sort this out before trying anything else.

You do not say where you are; if you are in East Anglia drop by us and have a chat.

Stephen

 

[thesilentone]

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

 

[thesilentone]

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.

Jim, to respond to this, more information is needed about your current system and what the process entails.

 

[sjt01]

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

Future Biogas often talk about using them, but I have not heard about the performance. There was research on the Norwich Research Park about steam explosion for breaking down lignin, at the Food Research Institute if I remember correctly

 

[thesilentone]

Everything is possible, the question is cost. Steam explosion is available from several sources, however there is a huge demand for heat, so of course ' feed-stock ' treatment can claim the RHI.

Then of course it is CAP-X v ROI, I see the potential for a Regional Plant treating straw using steam, then selling on the feed-stock to localised plants in a liquefied format maybe mixed with Pig Slurry.

Wouldn't it be great if you could do this as a active material, in heated tankers with the truck running of it's own payload....( you may need pedals to get home)

 

[DGC1]

cavitation machines - the only way to tell if they are capable of 'bursting gas' from lignin in rye would be to mix pressed fibre/solids that have just came out of the digester, mixing with pure water and putting back through the cavitation kit and then gas testing..... mmm..... I wonder.....

 

[sjt01]

cavitation machines - the only way to tell if they are capable of 'bursting gas' from lignin in rye would be to mix pressed fibre/solids that have just came out of the digester, mixing with pure water and putting back through the cavitation kit and then gas testing..... mmm..... I wonder.....

If you take separated solids out of a digester and leave it for a few days, it will then digest down completely. Our slurry is straw mixed, and we ran for about 9 months with no solids taken away when feeding low DM fodder beet.

 

[DGC1]

If you take separated solids out of a digester and leave it for a few days, it will then digest down completely. Our slurry is straw mixed, and we ran for about 9 months with no solids taken away when feeding low DM fodder beet.

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.....

 

[thesilentone]

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.

 

[thesilentone]

Part of the agenda at the forthcoming Bio-methane day.

 

[DGC1]

assuming that we are already running an efficient ad process ie
fed accurately
multi stage digestion process at correct dm's
proper ret times to get max gas yield
low N in the final digestate
etc etc

and without the need to pre treat feedstock in any way other than proper mixing....

and that all of the above result in above average gas yield from the given feedstock, the claim of these cavitators is that they can 'unlock' more gas potential by breaking down the lignin.... so the only place that this 'increased yield' can come from is the solid fraction
the only way to test this accurately would be to take digestate that is effectively spend and put it through the cavitator priotr to it being pressed and then feed a trial digester with this cavitated digestate

I'm very sceptical of claims of increased gas yield, yes these bits of kit will help an AD plant that is running an inefficient setup that's got issues but will they unlock more gas from the solid fraction in an ad plant that's on tune?
I would say that a lot of the claims are over stated but I would love to be proved wrong and that a cavitator could unlock more gas from high dm rye or older type grasses

 

[thesilentone]

assuming that we are already running an efficient ad process ie
fed accurately
multi stage digestion process at correct dm's
proper ret times to get max gas yield
low N in the final digestate
etc etc

and without the need to pre treat feedstock in any way other than proper mixing....

and that all of the above result in above average gas yield from the given feedstock, the claim of these cavitators is that they can 'unlock' more gas potential by breaking down the lignin.... so the only place that this 'increased yield' can come from is the solid fraction
the only way to test this accurately would be to take digestate that is effectively spend and put it through the cavitator priotr to it being pressed and then feed a trial digester with this cavitated digestate

I'm very sceptical of claims of increased gas yield, yes these bits of kit will help an AD plant that is running an inefficient setup that's got issues but will they unlock more gas from the solid fraction in an ad plant that's on tune?
I would say that a lot of the claims are over stated but I would love to be proved wrong and that a cavitator could unlock more gas from high dm rye or older type grasses

The objective is to have more CH4 available for export, so there is potential to achieved this in two ways. One is to increase the m3 Biogas, one is to improve the overall CH4 quality. These need attacked separately, and in a step-by-step process so you know where the results (if any) come from.

1) Do a RBT test, this is the only way to see if anything is worth trying. You are right to be sceptical about claims of increased gas yields - you can only get a pint out of pint pot - once that VS is degraded, that's it.
2) Increase process temperature - This will create more activity from the bacteria and reduce retention times. Increase gradually, keeping an eye on ph and N loading
4) Improve pre-treatment, by breaking down feed-stocks further opens it up to faster incorporation into the digestion process
5) Keep agitation just right, no over or under mixing. If possible create a small floating layer on the surface SMALL MEANS SMALL as this is a great environment for H2S bacteria to grow.
6) Please remember the primary objectives of agitation - to avoid separation and a floating layer. They are not really for mixing, this should be carried out outside the digester !!!
7) If you have low N, why is your CH4 value so low, are you injecting O2 ? If so, stop it and buy a cleaner before the upgrading (you do not offer this detail)

 

[DGC1]

hi
7) I think you maybe mix up my post with a prev post from someone else.... as I didn't mention a ch4 issue

so... do you feel that in an efficient ad process that's on tune a cavitation machine could unlock gas yield potential in the rye lignin or do you think that this claim is very debatable?
wont hold you to it! : ))

I'm not looking at steam cracking as my gut feel is telling me that its simply not worth the capex/opex unless there was a desire to process pure straw through it
i don't think it would be worth doing to only process high dm rye but i may be wrong

 

[thesilentone]

hi
7) I think you maybe mix up my post with a prev post from someone else.... as I didn't mention a ch4 issue

so... do you feel that in an efficient ad process that's on tune a cavitation machine could unlock gas yield potential in the rye lignin or do you think that this claim is very debatable?
wont hold you to it! : ))

I'm not looking at steam cracking as my gut feel is telling me that its simply not worth the capex/opex unless there was a desire to process pure straw through it
i don't think it would be worth doing to only process high dm rye but i may be wrong

Sorry about that, my response was to Jim Pace post above............

 

[thesilentone]

so... do you feel that in an efficient ad process that's on tune a cavitation machine could unlock gas yield potential in the rye lignin or do you think that this claim is very debatable?
wont hold you to it! : ))

On this point, the lignin is simply slowing down the process, so any way to remove or break-down is good. So, an easy low cost solution is - don't use high lignin feed-stock

However, with grasses, it is cost (pre-treatment cap-x and energy consumption) v result.

Straw is a different and more difficult challenge.

 

[Jim Pace]

Vogelsang are a well established company, we use their macerator but I am not particularly impressed by any of the magic systems on offer.

If you are not needing to heat, you are losing biogas potential as you must be digesting aerobically, at least in the early stages of the process. Are you fully mixed single stage, multi stage or horizontally mixed with little vertial mixing? There is an exothermic Agriferm installation near us, or at least last time I went there. It would certainly pay you to sort this out before trying anything else.

You do not say where you are; if you are in East Anglia drop by us and have a chat.

Stephen

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

 

[Jim Pace]

If you take separated solids out of a digester and leave it for a few days, it will then digest down completely. Our slurry is straw mixed, and we ran for about 9 months with no solids taken away when feeding low DM fodder beet.

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