Operating a wood gas stove

I have created a better video of the MIDGE stove in operation. I'm no Orson Welles but I do come in on budget.

The stove is a particular type of gasifier known as an 'inverted downdraught gasifier'. Essentially, that makes it an upside-down downdraught gasifier. The difference being that the wood gas is immediately burnt in the inverted gasifier but in the normal configuration it is tapped off for running an internal combustion engine.

The inverted downdraught gasifier in its most basic form consists of one can suspended inside another. The outer can acts as a cowling permitting air to enter its primary air holes where it can pass into the inner can. The inner can has holes at the bottom, in the form a grating which aids combustion of the fuel, and a ring of secondary air holes at the top which create an air/fuel mixture with the wood gas.

Fuel consists of wood chips. This permits easy fuel collection from broken tree boughs and the sort of waste that owners of log burning fires reject. The chips are packed down into inner combustion can and a small amount of some flammable liquid (spirit or paraffin) is dripped on top so as to get the fuel to sustain a hot flame long enough for char formation.

A fire is lit on top of the fuel, which results in a char layer forming. Beneath this layer is the pyrolysis layer where wood is converted into hot wood gas. Combustion continues down through the wood chips, converting the chips into charcoal which is then reduced to wood gas. The wood gas rises up, meets hot air at the secondary air holes and ignites.

You can think of a MIDGE stove as a device for making charcoal, which is then reduced to various volatile gases and a small amount of ash. The operation is very efficient, is virtually smokeless and yields little ash.

YouTube - Wood Gas Stove

11 comments:

Anonymous said...

I made a stove as you taught in MIDGE doc. and I tried this evening, I found that the smoke is heavy....I wonder if there is something wrong with my stove. Maybe the hole in the cowling is too small? or the fuel(I use some twigs and a few sticks shaped like pencils), pls give me some advice.
Matthew, from Taiwan.

James said...

It's hard to say without seeing the stove. If you can put a picture on Picassa or some other site then I will have a look.

However, it does sound as though there is not enough air getting into the primary and/or secondary air intakes.

Tsong-ui said...

Thank you,James. I just uploaded photos taken after the stove was done. The site is http://picasaweb.google.com/angzonwi/MIDGEWoodGasStove

The Burner:
Diameter 12.5 cm
Height 20 cm.
up: sixteen holes about 0.8 cm diameter
down: four holes about 1 cm diameter
The wire cloth is 5 cm from the bottom.

The Cowling
diameter 16.5 cm
Height 21 cm
Six holes about 1.3 cm diameter

James said...

Thank you for the photographs. Now, I can see clearly what the problems are.

1) At the top there should be no air gap. I see you have pegs to hold the combustion chamber inside the cowling. You need to find another way. Cold air from outside is getting to the secondary air holes from the top rather than getting in from the bottom.

2) You are loading it up with too much wood. The top of the wood pile should be below the secondary holes. Remember the charcoal will give off the wood gas, which rises to meet air at the secondary holes, whereupon it combusts. You could also make the twigs of wood smaller so they pack down more closely together.

I would remove those clips and cut tabs into the combustion tin and fold them over to hold the combustion chamber on the lid. I would then use fire cement to seal any gaps. Air should only enter via the primary holes in the cowling. And, from there go to the grating at the bottom of the combustion chamber and to the secondary holes.

Alternatively, get another lid for the cowling and cut a hole smaller than the combustion chamber. Then cut tabs in the lid so can just squeeze the combustion chamber through the lid. That is what I did.

I forgot to upload the photos of my improvements. What I am saying will be a lot clearer. I will upload them in a while.

Other than that it looks very good and will make a fine wood gas stove.

Good luck!

Tsong-ui said...

Thank you for the detailed explanation and solution about the improvement of the stove i made.
Two questions:
1. The clips are for the potstand, Is it too high? or maybe I can try long-tailed clips, since they are thinner and can leave smaller space between the pot and the burner?
2. Will the burning be better if I put away the wire cloth?

I'll try to buy a small can of fire cement, if there's any downtown. Still, tomorrow I'll phone the fire cement manufacturer in Taipei.(I found one via Internet)but I'm afraid that they only sell big cans of that.

Actually this afternoon i fired the stove again and it is so windy that the burning isn't efficient. Just what you told me --- The cold air enters in the upper holes and ruins everything. I saw charcoals, but no good burning....no gas at all. What a shame and waste! It seems that in windy days, the stove cannot make it, can it?

Thank you for your prompt and considerate answering.

Since I live by countryside, There are a lot of stcks, twigs, pellets for me to pick up and bring them home.....

After the improvement, if everything is ok, I will share this good experience to my friends and they can save a lot.

What an efficient MIDGE stove!

James said...

1) The clips are okay for a pot stand.

2) I don't have a wire cloth in my stove. The wood rests on the holes at the bottom of the combustion can.

Wind is a problem for wood gas stoves. The answer is to keep it shielded outdoors. If you are out camping or similar then bring one of those cloth wind-breakers with stakes for pushing into the ground. Surround the wood stove on three sides. Not too close though as you don't want to set fire to it.

I have published a new post on the site now and it details the improvement I made to my stove.

Tsong-ui said...

Today I called for the fire cement, sadly they only sell 40kg pack, and it cost about $40 pounds UK dollars. I only need 500-1000g. I'll find another way.
Just now i found one in the Internet.
https://vault1.secured-url.com/chimney/acatalog/Index_Page_Fire_Cements_144.html
How do you think about that? Or any brand recommendation ?

James said...

That looks perfect. Any brand will do so long as it's fire cement.

Tsong-ui said...

James, I took 2 photos when burning. It's a waste for me to buy fire cement from other countries, I found one factory selling motar, 25 kg about 6 pounds UK dollars. And, excitingly, they can sell me 2kg about 0.5 pound (or just give me!!!:-) Can motar work as fire cement? I can't wait to seal the leaks, so I seal them with some A-B glue (sort of)...It cannot resist burning, and it spreaded bad smell when burning, whereas i have to burn them out...The pictures show the burning, This time I took away the wire cloth, It burns well I think. After I get motar, I will seal tem and try it next time, until then, the wood I picked must be very dry to be fuel.
Please take a look at them and tell me some advice. Thank you very much.
http://picasaweb.google.com/angzonwi/MIDGEWoodGasStove

btw, I found you are in Irland...U2 is my favorite band. and James Joyce is my favorite novelist as well.
Rural and natural life is what I am pursuing!

James said...

That looks like a good burn.

You have the tabs holding the combustion can. When they are sealed then everything will be perfect.

Do you have metal working foundries near where you are? You might be able to get hold of stuff called refractory. That is a heat resistant cement too.

You can use ferrocement to make wood gasifiers. Read my article at http://wood-gas.blogspot.com/2008/05/ferrocement-resources.html

Yes, my origins are in Ireland though I am not there at the moment. If you want to live in Ireland then I know a lonely Chinese woman with a big house.

Tsong-ui said...

I think I can get is High-temperature Refractory Mortar. hope it works....

I am not interested in chinese stuffs; besides, i am not a chinese but a Taiwanese.

So where are u now at the moment?