Help - My gas won't burn.

[JMALW]

Thanks for the compliments guys.
Ivan - Your absolutely right, I read this about CO2 seperation too.  I realised just bubbling through water absorbs some  but adding lime also helps, however obviously not enough unless one uses a huge columb of water which I don't think even my unique pump arrangement is enough to push through.  I read that some bright chaps had deveoped a specialised plastic membrane that could allow Methane through quickly thus purifying the gas. My guess is they will want more than I could afford to pay at this stage in their development since I'm getting the message that it is a global problem right now.  Well we humans will undoubtedly win through sooner or later. Oh the burner I test with is a natural gas bunsen burner - cost me just £4.00 - I can play with the jetting and pressure using my lathe to cut the jets with - it has not worked so far except I got a flicker when I passed the gas through some household bleach - a start.

[daftlad]

My book suggests that the gas produced from a digester is only about 60 percent methane. It suggests that the gas is bubbled through lime water to remove the carbon dioxide and amonia, then passed through beds of calcium chloride to remove water vapour and then through iron filings to remove hydrogen sulphide.
The picture of the scrubbing unit shows the gas being bubbled under upturned plate things which get bigger further up the scrubber, presumably this increases contact between the gas and the water.
Hope this helps, or at least gives you some Ideas.
Keep up the good work, we all look forward to seeing pictures of that first poo powered cup of tea.
ta ta
PS PM me if you want me to post a picture of the scrubber.

[Ivan]

I think there's nothing wrong with your gas - you're just using a totally inappropriate burner. The gas burner I've seen was a piece of copper pipe bent into a ring and soldered into a T-piece (the vertical of the 'T' used to feed the gas into the ring). The holes were large - much larger than what you'd use for natural gas.

Just done a quick search. This reference says 2.25mm for the jet size (I told you it was big!)

http://books.google.co.uk/books?id=QfLDbf3qbcEC&pg=PA236&lpg=PA236&dq=biogas+jet+size&source=bl&ots=rW0UasSr74&sig=sdbsr-dglqVadxGsFkPZLMfn44w&hl=en&ei=gQkjS6jYLJrQjAeDhoXYBw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CBwQ6AEwBQ#v=onepage&q=biogas%20jet%20size&f=false

As long as you get the jet size right, it doesn't really matter if there's a lot of CO2 there. If you can avoid the need for excessive gas scrubbing, it'll make your life a whole load easier.

[JMALW]

I think there's nothing wrong with your gas - you're just using a totally inappropriate burner. The gas burner I've seen was a piece of copper pipe bent into a ring and soldered into a T-piece (the vertical of the 'T' used to feed the gas into the ring). The holes were large - much larger than what you'd use for natural gas.

Just done a quick search. This reference says 2.25mm for the jet size (I told you it was big!)

http://books.google.co.uk/books?id=QfLDbf3qbcEC&pg=PA236&lpg=PA236&dq=biogas+jet+size&source=bl&ots=rW0UasSr74&sig=sdbsr-dglqVadxGsFkPZLMfn44w&hl=en&ei=gQkjS6jYLJrQjAeDhoXYBw&sa=X&oi=book_result&ct=result&resnum=6&ved=0CBwQ6AEwBQ#v=onepage&q=biogas%20jet%20size&f=false

As long as you get the jet size right, it doesn't really matter if there's a lot of CO2 there. If you can avoid the need for excessive gas scrubbing, it'll make your life a whole load easier.

I agree with you Ivan and I also read elswhere that as little as 10% methane should burn - then I read or hear that they use CO2 as a fire supressant on ships - arrrrrg. I have tried different gas size jets in the bunsen burner - made little difference --- arrrrrg.  I'm going to kick my ball around the yard now to blow off some frustration.  Thanks - just to say I appreciate your input. In your link it states that "if a lighted match is applied to the end of a feed pipe the gas will burn without any burner - mine does not. For proper air mixing a burner will make the flame more efficient - I agree with this but it needs to burn first. 

[Ivan]

In that case I would start looking for airleaks. Are you introducing air when you add substrate to the digester, or when you stir? Is there an airleak on one of the connections? Even a small amount of oxygen poisons the methanogens bacteria





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[JMALW]

You may have a case for air taken in with the digestate - I'll work on that one Ivan. The other little bugs may be having a ball with that and producing more CO2 mmmmm,

[JMALW]

My book suggests that the gas produced from a digester is only about 60 percent methane. It suggests that the gas is bubbled through lime water to remove the carbon dioxide and amonia, then passed through beds of calcium chloride to remove water vapour and then through iron filings to remove hydrogen sulphide.
The picture of the scrubbing unit shows the gas being bubbled under upturned plate things which get bigger further up the scrubber, presumably this increases contact between the gas and the water.
Hope this helps, or at least gives you some Ideas.
Keep up the good work, we all look forward to seeing pictures of that first poo powered cup of tea.
ta ta
PS PM me if you want me to post a picture of the scrubber.

This does sound interesting - please post the picture.
John

[Rupert]

Your gas would burn at the end of a pipe unassisted.

Your gas (methane) may have not a good calorific value so will not burn, Natural gas is mixed as different fields produce different calorific valued gas to attain a standard caloric value to suit the gas burners it is being delivered to at those set pressures.

[Eleanor]

Well done JMALW, looks like a good system 

I've been thinking along the lines of a membrane which is selective to CO2. When we used to do infra red analysis on flue gases we often had to remove the water vapour as it interferes with the measurement of some gases. One approach was to cool it which is fine for high concentrations of not very soluble gases eg CO2 or lower concentrations of non soluble gases eg NO. This approach doesn't work for soluble gases eg SO2 and one method of removing the water is to use a selectively permeable membrane. The wet gas, kept above the dewpoint, flows in the sample tube on one side of the membrane and dry gas (N2 or air) flows the other side in the opposite direction and picks up the water vapour. We just need the right membrane for CO2!

I wonder if you could interest someone enough in the Chemistry Dept of a local college to get a sample in a gas tight syringe and put it through a gas chromatograph? It only takes a few minutes and would at least tell you what the CO2/CH4 content is. Having only read about it, it does seem that biogas needs a lot more air for combustion than natural gas.

Also, if you are thinking about sodium hydroxide I wonder if potassium hydroxide could be a better choice as I think you can use potassium carbonate for treating plants for powdery mildew and early blight. It's supposed to be better than sodium carbonate but I haven't tried either! 

I'm probably talking a load of anaerobic digester feedstock so I wouldn't take too much notice! 

[JMALW]

Thanks Eleanor
I come across some things called Dragur tubes, or something like that, which can determine parts per million of various gases - I think they are expensive but may not be to bad if they can be reused many times - I'll try and get some more info and also try some colleges to see what they say. I'm  only a simple whacky engineer not a chemist that's why I'm poor - should have gone into politics.

[Tombo]

A very  rough and ready technique that a professional would never use to test the flammability of biogas is to bubble it through some soapy water. Then put a match to the resultant bubbles. 
Disclaimer
Of course if you are stupid unwise and wait until you have several cubic metres of bubbles before putting a match to them it ...

[Alan]

Quote " I come across some things called Drager tubes "



Downside they are very expensive and can only be used once.

Was working on an industrial digester today that had loads of pressure, but the flare stack could not ignite the gas until the temperature in the tanks was increased.

Regards

Alan

[daftlad]

Picture of scrubber, ooooo errrr 
ta ta

[JMALW]

Thanks for the picy
I see what the idea is; I have a pipe into mine at the bottom in which I drilled .5mm holes in so that when the gas is pushed in it comes through similar to those things that are used in fish tanks very very fine bubbles - I may try to add some more of these plates to it - who knows it may just be enough to take a bit more CO2 out. So far I'm designing a macerator to munch up the digestate and seperate any air out, building a scrubber that will be good enough for NASA, breeding plants to eat any captured CO2, designing a burner to make Corus jealous and generating system to really p---s off the national grid - wish me luck folk.

PS When you take on a project like this it really humbles one against the effort the water companies and power companies go to to get what we have at our disposal.  I'll never waste a drop of potable water again and if you can afford the power don't waste it.

[Ivan]

Two more ideas:

1)have you got excessive pressure? Not sure what the recommended pressure for biogas is - but it's low - a couple of bricks on top of your gasometer will be enough. If the pressure is too high, you could be blowing your flame out

2)Is the gas too wet? I'm not sure to what extent moisture will adversely affect the flammability of the gas, but I'm sure it won't improve things. Chemists bubble through concentrated sulphuric acid to remove moisture. Not sure what the bodger uses.