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family size biodigester

Posted: Mon Sep 16, 2013 8:08 am
by Dutch Dave
Trying to be a little self sufficient in the kitchen usually means cooking home grown food on a wood stove. We did this for a while but I found some downsides tot cooking on a wood stove, it takes more time to cook on a wood stove, the kitchen heats up beyond a comfortable temperature in the warmer seasons and in those seasons a lot of fuel is “wasted” on cooking.
So I decided it was time to solve this problem, I found another fuel to cook on which I can make myself form garden and kitchen scraps, the extra benefit would be a first class fertilizer as a by-product of the biogas process. I wanted a digester that would make the most out of the organic matter which I fed it, I wanted it to be made out of recycled parts as much as possible and I wanted it to be a closed system (no nasty smells and dangerous H2S venting out)
I did some research and found that it takes between a hundred and 2 hundred litres of gas to cook a meal. So I built very simple biodigester which could provide that gas daily. Using kitchen scraps the biodigester makes about 150 litres per kilo.
If the temperature is around 35 to 40 degrees C it takes about 30 days to convert organic matter in to gas. This time can be shortened by a higher temperature or by preparing the digester feed.
To be sure the organic matter would be fully converted I chose to design a biodigester which can hold the organic matter for 70 days at 33C. This way the full “gas potential” of the organic matter is converted making it more efficient and providing an even better fertilizer for my veggie patch.
The produced gas is stored in 2 “floating barrels” that are weighed down with some bricks. This makes an excellent low pressure gas storage and can be made using recycled parts.

The digester before it got wrapped in insulation.

After wrapping and making a little control panel.

The double floating barrel gas storage.

So I started the digester up with some active slurry from a commercial digester and gently started feeding is my kitchen waste. After two weeks of acclimatising the bacteria to their new feed and temperature I started to gently increase production.

So now we made enough gas to cook on but we didn’t have a stove. Biogas doesn’t burn that well in a normal stove, the energy content is less then natural gas (which has around 90% methane vs the 65% in my biogas) and there isn't a lot less pressure on the biogas system. So I bought a little 2 burner camping stove and pulled it apart looking for a way to adjust it to the new fuel. I soon found the “injectors” and drilled them out to 2,5mm (they were 0.62mm). The bigger holes make for an easier flow through the system, which in turn causes more gas to flow to the burner, altering the fuel/air ratio. Much to my own surprise this was all I had to do to the stove to make it work well on biogas.

The biogas burns in a clear blue flame, invisible in daylight and without any smells.

I have been cooking on biogas for over a month now and i like it better then cooking on a wood stove. The total investment in the system was 50 euros. A lot cheaper then a woodstove with chimney. It doesn't produce any smoke and works just as well as a natural gas stove.

I think biogas is a good cooking fuel for those who want to save their wood to heat the house or people who don't have a woodstove in the kitchen and don't want to pay thousands of euros to get one and put a chimney in.

Spend 50 bucks once and make the cleanest biofuel at home for free and get the best biological liquid fertizer with it.

I thought this would be something you'd like :)

Re: family size biodigester

Posted: Mon Sep 16, 2013 10:36 am
by Odsox
That's fantastic.
I saw a program on TV a while back that caught my imagination about producing biogas from grass.
I have always thought it a waste to dump the several tons of grass mowings that I get every year, I can't compost it as I have no coarse "brown" material or paper to mix with such a large volume.
So maybe making a digester is something to think about for my next project.

Re: family size biodigester

Posted: Mon Sep 16, 2013 11:51 am
by Dutch Dave
Freshly cut grass had a "gas potential" of more then 300 litre per kg..

If you have enough it would even be possible( with a little tinkering) to heat the house and/or run a fridge or freezer on the gas :)

A friend of mine converted his car to run on woodgas, he has enough trees to provide for the fuel. Seeing his self sufficient car in action made me want one too. The only problem is that i don't own any woodland and that i rarely use a car.

So this winter i would like to make a bigger digester to produce biogas to use in my scooter as fuel. I converted my piaggio four stroke scooter to run on lpg (a mixture of propane and butane) a while back. If i get enough production i want to tinker a little more with the scooter to make it run on compressed biogas aswell.

Biofuel is addictive :P

Re: family size biodigester

Posted: Mon Sep 16, 2013 12:01 pm
by doofaloofa
I like it Dave

I have been considering capturing the gas from my septic tank


Re: family size biodigester

Posted: Mon Sep 16, 2013 12:25 pm
by Dutch Dave
That could work. I would advise you to put in a waterlock as a flame arrester, if there is enough oxygen in the gas mixture it wil explode if you light it. If it's just a floating plastic barrel exploding in the yard it's a nice story to tell in the pub. If the explosion happens in your septic tank you won't go to the pub, you'll be cleaning the sh*t of your walls and ceilings.. :)

Re: family size biodigester

Posted: Mon Sep 16, 2013 2:02 pm
by doofaloofa
Thanks for the tip

Re: family size biodigester

Posted: Mon Sep 16, 2013 2:23 pm
by Green Aura
I think this is brilliant - just the sort of stuff someone ought to write a tutorial about. Just a suggestion :wink:

Re: family size biodigester

Posted: Mon Sep 16, 2013 8:30 pm
by trappa
Bravo. A first class system!!!

Re: family size biodigester

Posted: Mon Sep 16, 2013 8:49 pm
by Zech
Wow! Yes, this is something I like very much.

I would love to do that, but I know my limitations (too many projects). Maybe after I've made my solar panels from old radiators, then I'll start badgering you for more information on this :wink: