I would like to use biogas for cooking in the house, so I built a pilot project to check if and how it works. We have the feedstock, being goat manure so I decided to give it a try.
I wasn’t going to spend any money on the project, so I built a crude digester out of junk lying around on the farm. It comprises a 220 litre drum filled with goat manure slurry, a 100 litre drum upended in the larger drum with a valve and a rock on top, and a piece of gas pipe, wedged into the valve with a plastic bag. Behold its precision engineered glory!
I filled the big drum about one third full with goat manure, topped up the drum with water and mixed it up into a nice slurry. I then put the small drum upside down inside, covered it with black plastic and waited.
For a week nothing happened.
In the next week the smaller drum started to lift a little, but the gas produced did not ignite.
In the following week a foul stench was noted.
And after about 3 weeks the contraption was producing gas every day, and I could make flares of gas flame out of the pipe.
Apparently the bacteria first ferment the mixture to produce CO2, then H2S is produced and finally methane production commences as the methane producing bugs establish themselves.
The next test was to see if anything useful could be done with the gas, so I connected up a camping gas cooker, but it wouldn’t light. It took a lot more google research to figure out that biogas (being mainly methane and carbon dioxide), behaves quite differently to your normal propane or butane gas that you buy in cylinders.
An important difference is that propane and butane liquify under pressure, so they occupy a lot less space as a liquid than as a gas. You can’t compress methane into a liquid, so you need relatively large storage volumes. Methane’s burning characteristics are different so you can’t just use a normal gas burner, it needs some tweaking.
I then bought a small one ring cooker that I could modify. What needs to be done is the jet needs to be opened up to much larger than for normal gas, and the amount of air restricted. I opened up the jet in the stove from about 0.5mm to 1.5mm, and blocked up the air intake with a piece of plastic with one 6mm hole drilled in it.
So, I opened up the gas, applied a match and hey presto! It works!
Next thing is to boil a kettle, which I duly did (OK, only 1cm of water in it, but it boiled nonetheless). You can’t see the flame or the steam in the pic, but it is boiling, trust me….
So I’m quite pleased that the experiment worked. The digester is probably making about 30 litres of gas per day, probably quite a lot is lost due to the small drum not filling nicely in the big drum.
My next step will be to make a 1000 liter digester so that we can use the single ring burner in the kitchen to make a cup of coffee in the morning. I am going to use the design from this website and see how it goes : Solar Cities IBC Digester
I will keep this digester bubbling as the bacteria are established in this system, so you can take this material to quick start the new system, without waiting for the whole process to establish from scratch again. Seeing as IBCs are fairly cheap, I will probably be able to get a small working system going for less than $50, hopefully enough to do some of our cooking on, but we’ll keep our normal gas stove as well. Based on the experience of this pilot system I would anticipate about 100 litres of gas per day from the 1000 liter system.
There are many variables that dictate how much gas a biogas system will produce, like type of feedstock, ambient temperature, water hardness and so on, which is why I wanted to see it work on a small scale first, but I am very pleased with the results!
I found these websites quite helpful in this process :
ATTRA – Sustainable Agriculture
Biogas Plant in a Plastic Drum
Modifying a Gas burner for Biogas