Help - My gas won't burn.

[JMALW]

Does anyone know what I'm doing wrong?
I constructed an anorobic digester to trial the process on a small scale. It is a typical 210lt steel drum that I installed a paddle and heater coil in. It has two discharge valves on the side 1 x 25mm and 1 x 50mm. The feeder is a simple 100mm pipe taken down to within 100mm of the bottom and I seal it with a std drain test plug.  It has a gas feed pipe to a tub filled with steel wool to scrub any H2S then through a small drum filled with lime at the moment to try and scrub some CO2 out. From there it passes through a water trap then to two IBCs which I constructed to suck the gas in when water drains out, then alternate this between two IBC's - as water is filled in one it pushes the gas out to a third IBC for storage and use using a series of valves, the water that is used in this process is stored via a sand filter for use in the property and irrigation. It all works very well indeed and since I only expected to use a pressure of 30mb I have more pressure than I need without over pressurising the IBCs since they simply use the weight of the water which they are designed to do. The problem I have is that the gas DOES NOT BURN.  I expected something like 50% methane 40% CO2 and other misc gases. My methane sensor screams when exposed to the gas. I've adjusted the pressure up and down to no avail. I keep the temperature between 20 and 30 degrees C via the heating coil and it all bubbles away merrily. I use food waste,grass cuttings etc and have seeded the thing with those little blighters call methanogens. I don't know what else to do except scrap it.

[Tombo]

I am not an expert, but I'd say its too cold. 35 degrees is the tempreature commonly used. How long have you been running it for? Its not unusual for digestors to improve slowly over 9 months of so, as the bacteria that like your conditions grow and thrive (Assuming some digestate is retained in the digestor to populate the new batch) .
Grab a few cow pats! I think most digestors have some farm slurry in the first batch, even if they are going to run on supermarket food waste.
Be careful! Its not just the methane & explosion you need to worry about, the other gases produced are asphyxiates. It is suggested that a leak of these gases that claimed the life of man working at a Dorset AD plant this summer.

[daftlad]

JMALW, Welcome to the forum.
I have a book with a chapter on methane digesters (Technological self sufficiency by Robin Clarke) and it suggests 36 degrees c, it also states that there are also some bugs that like it even hotter but none that like it cooler.
This sounds like an intresting project, I, and probably lots of people, would like to see pictures and more information.
ta ta

[Ted]

My book covering AD is American so all temps are in Fahrenheit - 90 to 95 is quoted as being ideal which is 32.2 to 35 Celsius.

Also your raw material needs to be fed in with a carbon:nitrogen ratio of 30:1.  Starting with too much carbon should be OK as the fermentation will re-balance it but starting with too much nitrogen will result in incomplete fermentation.

[Ivan]

I think you'll find very few people with working experience of anaerobic digesters - most, like me, have only read about them. So I can only offer suggestions/observations:

1)Are you absolutely sure it's airtight? If you have even a small amount of air getting into the digester, it will produce much too much CO2 to burn

2)Digester bugs are divided into two classes - those which like to live at approx 37C (having developed in the guts of animals), and those that like to live at around 50C - thermophilic (though goodness knows why these should have developed!). So most digesters are run around the 37C mark. However, digesters will run at lower temperatures, although yields will be lower and gas production slower. If the temperature goes too low, the digester will stop altogether, but pick up again when the temperature increases. I know someone with a digester that is run around 25C in the winter. It still produces methane, albeit a lot slower.

3)Your methane sensor will bleep at the slightest trace of methane. I had a gas sensor which was designed for locating gas leaks on LPG conversions. It bleeped at very low concentrations - 1 in 1000 or lower. You'd never ignite this kind of mixture, even though the bleeper is going frantically.

4)Someone remarked on this forum (in response to my proposal to use an IBC as a digester) that methane will diffuse through a thin layer of plastic. I see this as entirely plausible (although I can't say if it's true or not) - so maybe you're losing your methane during storage?

5)How much gas have you produced. My books on AD, suggest that the first two reactor volumes of gas produced should be vented as it will contain too much CO2 (as the aerobic bugs consume any available oxygen).

6)On the same line as the above comment, have you produced enough gas to flush the air out of your system. If you have an air-filled IBC as your gas storage tank, you'll need to generate 1000litres of gas simply to displace the air. Of course, there will be considerable mixing, so in order to get your methane concentration to a high enough level to burn (you need to have at least 5% methane concentration in air. Probably significantly higher in CO2), you'll need to flush rather more than 1000litres through before you can start burning. Biogas is a VERY lazy flame (slow propagation speed) and that's at around 50-60% methane. I imagine 25% methane/75%CO2 would be very difficult to light.

7)How did you seed with methanogens?

8)Don't give up! It sounds a very interesting experiment. You probably need a little refinement/patience.

9)PLEASE POST SOME PHOTOS!

[JMALW]

Thank you all for responding. From what you say getting the balances just right is key to the problem.  To address some of your queries further - I am aware that it was most important to get all the air out of the system  mainly because no gas will burn without oxygen and having air in the system would be highly explosive if any leaks were evident. So I purged the system many times to be sure (wish I could afford fancy gas analysers) The storage tank and the two IBC pumps start off full of water completely and only then displaced with gas coming out of the digester.  There is a small risk of vacuum pulling air in from the suction side I know so I try to keep the suction in balance with the gas output - enough to overcome any pipe resistance etc.  It must be as you say - temperature and or chemistry. It was my plan to heat it with any gas production once started but so far I have to feed my homemade boiler with whatever will burn.  The good thing about the system so far is I have not had a dustbin collection for 4 months and the stuff coming out of the bottom feeds my plants - and they seem to like it. Prior top this I had been using Aerobic fermentation but it stunk to high heaven - nearly a divorce.

Love your forum.

[renewablejohn]

JMALW

I would suggest your problem is the IBC containers used for storage. I have heard that the orange juice containers are better for long term gas storage but the best is a 45 gallon steel drum upturned in a half water filled open top IBC container with weights applied to get a suitable pressure.

[JMALW]

JMALW

I would suggest your problem is the IBC containers used for storage. I have heard that the orange juice containers are better for long term gas storage but the best is a 45 gallon steel drum upturned in a half water filled open top IBC container with weights applied to get a suitable pressure.

Thanks for your comment - I had planned, and have a design, for a bespoke gasometer using two Poly tanks - one in the ground and one upturned with a nice sliding fit, sealed with water as you described, but a 7 cu mtr tank size cost quite a bit of money and don't feel like any more investment is useful until I get the prototype working properly ie the gas to burn.  I don't have a problem with pressure since the IBC design only sucks and pumps the gas in the short term - as soon as the water has drained from one pulling the gas in, it is filled with water thus displacing the gas to the storage - this so far cycles at about 1000 lts over two days. If I can get the gas to burn I would increase the size of the digester to produce more. I have modified a generator to accept the gas when cleaned up and also to use some of the heat it produces but none of this is any use if the gas is not pure enough or should that be "strong enough" - bit of a chicken and egg situation.  Have any of you got this digestion to work?
PS I bought a natural gas bunsen burner to test it with - just a few quid.

[Ivan]

Do we get to see some pictures? It sounds so interesting, I'd love to see what it looks like.


How are you trying to burn the gas? If you are using a burner of any kind you'll need to have much bigger jets that for LPG or natural gas.

[Justme]

I think the problem hinted at with the IBC's is that the combustable gas will pass through the walls & be lost & you will end up with just the non combustables.

[desperate]

Hang on though,

All that new yellow gas pipe they are busy installing everywhere is MDPE, I don't know what IBC's are made of, but I would have thought Methane molecules are far to big to migrate through any MD plastic.

Desperate

[desperate]

I've just thought of a little sperryment you could try. Take a sample of your gas and expose it to dry sodium hydroxide, this should remove all the co2 and see if what is left is flammable. I have a gut feeling that you have too much dilution with co2.   2NaOH+CO2= Na2CO3+H2O be careful with the jar/tube, the reaction will release quite a lot of heat, and the sodium hydroxide is lovely and dangerous, try sticking a little bit of aluminium in a strong solution

Desperate

[Fintray]

I used to work in a papermill that had both anaerobic and aerobic digester systems. the gas from the anaerobic system was collect in a gas holder, this was basically a steel tank in two halves bolted together about the mid point horizontally. Between the two halves was fitted a heavy duty neoprene type membrane (that effectively separated the tank) this had concrete blocks attached to the top of it. As the gas filled the tank the membrane was pushed upwards but maintained a pressure on the gas due to the concrete blocks.

[JMALW]

Hi fellas
I'm sorry but I boxed and wrapped everything up in insulation for the winter so the only thing visable is box 2.4x1.2x1.2. The only thing is the digester tank but I'm sure you seen plenty of them. I am so disappointed with the gas content so far - as you say it must be too diluted with CO2. However, the compensation for my original investment has been repaid somewhat by a two thirds reduction in my water bills - shame about the gas though.  I went to a great deal of trouble designing the valve system - one way and divertor valves to switch the the IBC filling and discharge automatically and the control system to do it - that's wasted if I can't use the gas - grrrrr. I'm currently running the genny on LPG just to develop it a bit more if I can produce as much gas as this propane uses I can cut my power bill by two thirds - just use mains when wife does her chores - I installed an AMF system to switch the power over quickly.

[JMALW]

I've just thought of a little sperryment you could try. Take a sample of your gas and expose it to dry sodium hydroxide, this should remove all the co2 and see if what is left is flammable. I have a gut feeling that you have too much dilution with co2.   2NaOH+CO2= Na2CO3+H2O be careful with the jar/tube, the reaction will release quite a lot of heat, and the sodium hydroxide is lovely and dangerous, try sticking a little bit of aluminium in a strong solution

Desperate

That sounds Scary - tell me how to do this safely.