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Biogas analysis in my kitchen


I know I’m onto a great thing by running my stoves on biogas, it’s cheap, easy and good for the environment.

Here are some facts about biogas from cow dung:

Cow dung gas is 55-65% methane, 30-35% carbon dioxide, with some hydrogen, nitrogen and other traces.

One cow produces approximately 36 – 68 kg of dung per day!

About one cubic meter of biogas can be generated from 16 kg of cow manure at around 28°C. This is enough gas to cook for a few hours.

One cow produces approximately 32 kg of dung per day – enough to feed a biogas system for an entire family!

We put two massive buckets of dung into our digester and have produced about 4 cubic meters of gas over a period of 3 days. Why is it acting so slow?

There are two things I need to do to improve my biogas system.

1. Find out what inputs are optimal for biogas production – does kitchen waste help?

2. Find out whether temperature or acidity is affecting the production of methane.

I set out to ask if my biogas is really operating at optimal temperatures, and how much methane is actually in the biogas?

My condom experiment yielded gas only from the mashed beans and dung so I tested that to find out if it was producing methane and not some other noxious gas .

After scouring Nairobi’s school lab suppliers I got a thermometer (-10 deg – + 60 deg Centigrade) for a whopping 750/- (almost 10$), a measuring cylinder for Ksh 250/- or 4$ (made in China – Pyrex one was 10 x the price!). I also bought some litmus ph paper (which cost me Ksh1,150 for  a reel (US 20$). I also bought a 50 ml syringe at the school shop for a whopping Ksh 150/- (2) I feel so cheated).

I’m so annoyed because you can use red cabbages to test acidity for free! (if you have red cabbages)

Determination of methane concentration

I found a website (but can’t remember where ) that provided a protocol for testing the concentration of biogas (methane) produced. Sodium hydroxide (caustic soda) reacts with carbon dioxide to produce carbonates.

So simple…

1. Dissolve some caustic soda crystals in 100ml water (watch out the stuff burns – add caustic soda grains to water not the other way round and don’t make it too concentrated – it really burns (experience)

2. Fill syringe with water then squirt most out to remove all gas

3. Put syringe end into the jet of biogas and draw in about 10 – 20 cc gas. Record the amount of gas.

4. Place into the caustic soda solution and draw up another 20 cc. Shake but keep the end of the syringe in the caustic soda solution. Or use a gloved finger to seal the end. Caustic soda burns like a bitch.

5. Now calculate the volume of the gas remaining. The NaOH absorbs all the carbon dioxide so you are left with only methane (in theory).

For our biogas we had 26/30 cc of gas was methane = that is a concentration of 86.7% methane. Not bad eh?

I then took the beans/dung condom, broke it under water, trapped the gas and repeated the experiment. 14/16 cc remained – that’s 87.5% methane. No real difference.

Out door temperature

The bacteria responsible for creating methane don’t like low temperatures. They operate optimally at 20 degrees and higher to 40 degrees. Lower than 10 degrees C and will virtually stop functioning.

Well, its’ the cold season here in Nairobi and our bag is above ground so it’s taking on the air temperature, especially at night.  I took 4 temperatures including waking up at at 3 am and 5 am to measure  the temperature – that’s dedicated!

10 am – 20.5

4 pm – 25

3 am – 15

5 am – 11.5

The generally low temperatures and daily range of 10 degrees centigrade may be pissing our bacteria off…but its the daily fluctuations that are the real problem. The bacteria’s really don’t like temperature shifts of more than 1 degree – no wonder the biogas production has slowed down.

Now I have to find a way of raising the temperature of the biogas plant and keeping it stable. My sister suggested I cover it with hay and sprinkle with water and effective microorganisms so that they start composting and producing heat…another stinky thought to consider. Or, I could simply dig a hole and partially bury the thing……

The problem of acidity

According to people who know, methane producing bacteria prefer neutral or slightly alkaline conditions. Like us humans, they don’t tolerate acidity! To find out hat’s happening to the acidity in the biogas digester I used the super expensive Ph paper that I bought in Nairobi at a swindlers shop on Kijabe Street (if you want home lab supplies don’t go to lab supplies, go to Kijabe street there are tons of stores but avoid the industrial suppliers and head to the school suppliers near Longonot Place – they have very cheap Chinese alternatives).

Well the tests came out in favour of methanogenic bacteria – the contents of the digester are living at a healthy pH of 9 which is rather alkaline.

Or maybe I just need more dung …

To find out more about my biogas installation check out my latest condom experiment on substrates with and the super cheap and very effective flexibag system designed by Dominic Wanjihia – and the motorbike trailer he designed to get dung to my house

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July 9, 2010 - Posted by | biogas, Kenya | , , , , ,

6 Comments »

  1. Good going Paula!

    Great concentration test – and an excellent result for you. Maybe bubbling the gas through a water column will reduce the CO2 even further.

    T.H. has been experimenting with using heat from an aerobic compost pile to heat the anaerobic digester via a heat exchanger. (posted pics on facebook)
    http://solarcities.blogspot.com/

    Q. How much was your bag? Is it designed for this purpose? How available is it?

    I know in Vietnam and many other places they’ve been doing this with generic polythene tubing (two layers).

    [m]

    Comment by Marcel | July 9, 2010 | Reply

    • HI Marcel,

      The bag is designed by my brother and he is making them to order – different sizes for different users. Want one? It costs Ksh 28,000 per unit (about USD 400 – vs constructed biogas which starts at USD 1,500) (minus piping to the stove, stove etc). I looked at the PVC systems – they just wouldn’t last in these conditions — they have been tested here and have a life of 2 years. This bag is hard wearing – one has been going for more than 10 years now.

      Comment by paulakahumbu | July 9, 2010 | Reply

  2. Thank you Paula for posting your observations on your biogas digester.

    I made a biogas digester 2 weeks ago using a 1000 guage polythene tube, similar to the one used in making silage. Its about 12 meters in length. It has so far generated gas very slowly and the main reason I think is because of the cold weather we are now experiencing in July (about 9 to 23 degrees celcius) here in Kiambuu, Kenya.

    Its good to see that your sister suggested that you apply hay on top then sprinkle it with water to help the bacteria generate some heat as the hay decompose. I think this can work as I had thought of using it (in a plastic drum) to warm up a tilapia pond so that the temperatures can be favourable during cold seasons.

    I will now give it a try on my biogas tubal digester.

    Well, before seeing your comments, I have been thinking of adding some Caustic Soda to the manure inside the tubal digester to generate heat so that the bacterias inside can be active.

    What do you think? Can it work? Will the caustic damage the polythene bag?

    Hope to hear you views.

    Thanks.

    Comment by Nicholas Mbugua | July 9, 2010 | Reply

  3. […] Biogas analysis in my kitchen […]

    Pingback by Biogas analysis in my kitchen « Wild About Africa | Galaxy Shopping | July 9, 2010 | Reply

  4. Hi Paula,

    I’ve used the same syringe protocol (credits to Paul Harris btw). One thing to be careful of in drawing your sample that way (from a burner) is that you may have air mixed in to your sample. Besides the ambient air, many burners pre-mix air with the gas ahead of the burner openings.

    If you do get air in the mix, your methane readings will be unusually high, because you are actually reading CO2, not methane. Air has only a small amount of CO2, so if you do the test on a syringe full of air you will get fantastic “methane” content, even though there isn’t any in it.

    Likewise, if there is some air mixed in with your biogas sample, it will read higher than it should.

    The way I do it is to take a gas hose or tube directly from the biogas source and bubble it up into an inverted 50 cc test tube filled with water and maintained in pan of water. When the test tube is full, I insert a tube attached to the syringe into the test tube and draw up a sample. I usually draw up about 12 cc. Then I push it back out until I have 9.5 cc (10cc – .5 cc for the amount in the tube. Then I draw the caustic solution back up to the 12 cc mark and agitate. Then I get rid of enough caustic to bring the plunger to 10cc and read off directly the percentage of CO2 on the cc scale (1cc=10%).

    Hope this helps.

    Comment by steve | July 28, 2010 | Reply

    • Thanks Steve, great advice, I actually stuck the syringe nozzel into a hose that was pouring out gas but yes, air could have gotten in. I’ll use your method next time. I burned my hands on the caustic soda!

      Comment by paulakahumbu | July 29, 2010 | Reply


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