AD Feeding with Auger or Pumped System?

GreenerGrass

Member
Location
Wilts
We are going to be running primarily slurry but with grass silage/maize/wholecrop etc in a 250kW digester.

The problem lies in that our tank (like many) is above ground, and to feed with an auger it will need to take things up 8m. We are aware this is a weak point in the design, and are interested in other options, and specifically we have considered Vogelsang which I understand conditions and mixes the feedstock with recirculated digestate, so that it can be pumped into the tank. However, I know pumps also have their own problems, and it's probably a fair bit messier to clean/fix than the auger.

Just wondering if we are missing anything, or if anyone has experience of either of these systems and could offer opinions/advice. An auger will block from time to time, and I think a pump will also. Ideally we'd want something with maximum uptime and reliability.
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
If there is any chance of stones or grit in the feedstock (eg from cows hooves or straw) then avoid rotary lobe pumps like Borger or Vogelsang. At one point I was changing rotors every 3 days at £320 per pair. We also started off with a Borger Power Feeder, which is now scrapped as it used to eat a gearbox a year when slurry got past the seals.
We now have a liquid loop feed system with a home made auger mixer, using a GEA Houle slurry pump which is nice and agricultural, and cheap and easy to fix.
We have been running our digester for almost 8 years now and have at long last got more reliability into the feed system. If you would like to have a look at what we are doing, PM me. We are in North Norfolk. Also happy to chat on the phone.

We are a similar scale to you at 170 kW, feeding slurry, maize silage, chopped beet and whey.

Stephen
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
GreenerGrass, take time on this as it is very important. Pre-feeding needs to be low cost, unless there are other benefits by design. Then cost v benefit feasibility.
Augers are cheap, generally reliable and require energy (that could be exported or used elsewhere). However, how else can you get dry material into the system ? But they do wear and will need repair/replacement in future. These guys have been round the block and know a bit: https://www.huning-maschinenbau.de/28-1-Conveyingtechnology.html
However feeding cold biomass creates digester shock loading, and affects biogas production and quality. Good quality Biogas means good quality CH4, that is what we want. Good quality saves money in cleaning and CHP wear and tear - money in the bank !
However, personally I like a pre-homogenisation tank adding both liquid and dry material - fitted with a chopper pump and stone trap. Mix using re-circulate and heat before feeding.
You start to break down cellulose and reduce shock loads, and pre-mix (thus reducing energy requirement of the digester) which all aid gas production (volume) and quality.
Food for thought..........
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
pre-homogenisation tank adding both liquid and dry material - fitted with a chopper pump and stone trap

Sounds an interesting approach. If chopper pump wear is not too much, then it could be lower running costs than a mixer wagon type feeder or walking floor supplying a Quickmix/Power Feeder type. How do you get feed into the tank? Some photos would be good. Any visitable sites?

To the OP - visit as many sites as possible before deciding, and speak to operators, especially about breakdown frequency and repair costs.

I might even consider changing my own system next time we are due for major repairs.

Stephen
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
Several AD companies use this system, and you are correct, you have to get the material into the tank. So, for liquid it is easy, pump and liquid connection point. For dry, you still need a ' box ' of some type for the grass (simple and low cost) feeding the tank via a conveyor.
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
So it seems to add an extra stage, to no particular benefit. Unless the residence time is very low (i.e. feed a significant proportion of total volume), and feeding is not little and often (we feed every 30 mins), then there will be no real thermal shock, even with frozen feedstock.
If you have a fully mixed system (CSTR) , which appears to give best performance, then premixing is of no benefit.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
So it seems to add an extra stage, to no particular benefit. Unless the residence time is very low (i.e. feed a significant proportion of total volume), and feeding is not little and often (we feed every 30 mins), then there will be no real thermal shock, even with frozen feedstock.
If you have a fully mixed system (CSTR) , which appears to give best performance, then premixing is of no benefit.

Less is more, more is less. Pre-homogenisation means less shock loading, less parasitic loading in the main digester, more biogas production and less contamination. In a industry of big numbers, small percentages make a big difference to the balance sheet.

In the case of GreenerGrass he is using his names-sake, grass, which is notoriously difficult to manage. Even hydrolysis may be worth consideration,, anything that ensures the grass is incorporated into the process quickly is a benefit. When using grass, a mixer/shredder pre-digestion is a must whatever system you decide on.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
You boys don't half complicate things, walking floor and a couple of augers works fine for me.

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.
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
You boys don't half complicate things, walking floor and a couple of augers works fine for me.
Doubt it would work for our mix of slurry and straw, 19 mm chop maize silage (top of the cows' clamp), beet and whey. Our digester runs on a wet feed loop, with a macerator. If I just augered it in, there would be a floating straw mat and problems.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
Doubt it would work for our mix of slurry and straw, 19 mm chop maize silage (top of the cows' clamp), beet and whey. Our digester runs on a wet feed loop, with a macerator. If I just augered it in, there would be a floating straw mat and problems.

You have an interesting mix, do you intentionally cut the top of the clamp silage a little longer to seal it ? 19.0 mm is quite long (even if you are making silage for cows) and more difficult to consolidate.

A pre-homogenisation mix, pre-heated should incorporate into the process faster, this reducing the chance of a floating layer. In addition, the tank can also be used for re-circulating and re-heating should process temp drop on cold days (this is a problem)
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
Seeing as we only feed 2% of digester volume per day (50 day residence time) in 48 portions, there is no real problem incorporating the new feedstock.

Even with our reduced heating capacity after scrapping pumps and moving to gas lift, it is rare to lose more than 0.1°C temperature from cold weather.

We use shorter chop right on top of the clamp (last few loads) to seal but the cows do better on the longer chop (scratch factor was the in phrase a few years ago). Currently 4.5% BF 3.7% protein from Brown Swiss.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
Seeing as we only feed 2% of digester volume per day (50 day residence time) in 48 portions, there is no real problem incorporating the new feedstock.

Even with our reduced heating capacity after scrapping pumps and moving to gas lift, it is rare to lose more than 0.1°C temperature from cold weather.

We use shorter chop right on top of the clamp (last few loads) to seal but the cows do better on the longer chop (scratch factor was the in phrase a few years ago). Currently 4.5% BF 3.7% protein from Brown Swiss.

The operating temp heat loss is much greater, (dependent on ambient) the key to reduced losses is the location of the sensor :) If you check the temp at the centre of the tank compared to beside the heating pipes or just above, you will see up to 5 degrees difference.
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
Our heat exchanger is an external shell and tube type, and the heated slurry is mixed with the incoming feedstock. As our tank is fully mixed (CSTR) the temperature sensor is only affected when the mixing is on the other side of the tank, once every two or three minutes.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
Our heat exchanger is an external shell and tube type, and the heated slurry is mixed with the incoming feedstock. As our tank is fully mixed (CSTR) the temperature sensor is only affected when the mixing is on the other side of the tank, once every two or three minutes.

Gotcha, you have to feed regularly to maintain temp as you have no heating in the tank ? So, you have a settlement temp just below feeding temp ?
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
We can heat without feeding or vice versa, but they tend to coincide and use the same pipe into the digester tank. What do you mean by settlement temp? We run the digester between 38 and 39° C (used to be 42°C but recommended to drop after a methane crash). Its 38.8 now.

We measure digester temperature, heat exchanger water temp in and out, but that's it at the digester.
 

thesilentone

Member
Livestock Farmer
Location
Cumbria
We can heat without feeding or vice versa, but they tend to coincide and use the same pipe into the digester tank. What do you mean by settlement temp? We run the digester between 38 and 39° C (used to be 42°C but recommended to drop after a methane crash). Its 38.8 now.

We measure digester temperature, heat exchanger water temp in and out, but that's it at the digester.

Have you had much hassle with the heat exchanger blocking ?
 

sjt01

Member
Mixed Farmer
Location
North Norfolk
Have you had much hassle with the heat exchanger blocking ?
Once during commissioning when our TS was very low. When we pumped the slurry round I dismantled a few times suspecting blockage as a cause of poor performance, but always clean as a whistle. The problem was the pump, now replaced by gas lift. We used to have 5 rotary lobe pumps, now none, just a simple 2 cyl piston pump, gravity and gas lift. Rotary lobes do not like flint or flint grit.
 
You boys don't half complicate things, walking floor and a couple of augers works fine for me.

Same here, hydraulic ram pushes our feed to one end of the hopper and then a single auger puts it direct into the digester. Grass whole crop beet and hen litter, no issues incorporating into the digester and no shock load? The tank is nearly a million gallons so 600kg every half hour doesn't get much of a chance to cool it!
 

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