Saturday, 14 August 2010

Gasifier information - at last!

Woodgas rocket stove

Sorry for all you readers who have been searching to find this and haven't, I know I have been rubbish at putting updates on but life gets in the way. This update describes the building of a wood burning stove sometimes described as a rocket stove due to the forced air for combustion. It uses the principles of initially burning the wood in a reduced oxygen environment which causes the release of woodgas. This gas is then burnt elsewhere (in this case slightly higher in the stove) in the presence of plentiful oxygen. This allows a much cleaner more efficient burning process with next to no smoke and also breaks down virtually all the wood to charcoal. If you follow this you should be able to build one of these yourself but as usual I will not be responsible for anything that happens to you as you build it or operate it or whatever you burndown!
When I started investigating woodgas stoves and decided that wanted to make one I didn't want it to be two complicated. There are some fantastic designs and builds in many places on the web but I would suggest you check out for loads of background to the process and some very professional builds. I thought about the process for a while and the resources that I have to hand. That basically equates to things I could buy from local diy and household superstores all built with simple tools. So if your looking to build one of these what you will need it the following :-

Parts list

One all stainless steel flask (which I think is a one litre one) purchased from Poundstrecher for £5 but similar are in Tescos.
One standard can of Wests tuna (I got from Tescos)
One 40mm computer case fan or similar(I had one lying around but Maplin have them
Power for the above fan. As mines a case fan its 12v so I power it off a jump starter battery.
Four self tapping screws about 25mm but any length about that should do.
Four nut and bolts to hold the fan in position. I think these where 4mm but whichever fan you get the bolts need to fit the mounting holes.
One long point nail or preferably a centerpunch


A drill (cordless is easier but corded would work)
Drillbits 3.5mm, 6mm, 10mm
Screwdriver to fit the screws your bought
A set of pliers and spanner to fit the nuts and bolts you bought. Two spanners or two sets of pliers will work.
Flat file
Permanent pen.

Stage One - Prep the flask
Remove the cup on the top of the flask and put it to one side.
Unscrew and remove the plastic pouring cap and throw into your spare parts bin :)
Turn the flask upside down and inspect the base section. The flasks are made of an inner and outer skin with a vacuum between them. To create the vacuum the air is expelled through a hole in the base which is then sealed and then covered with an endcap. That endcap needs to come off for what we need to do with the flask and I found two methods of doing it.
The first involved drilling a small hole in the base and then blasting an air duster can into the hole. The increase in pressure pops the base off with a satisfying bang. Beware though if using this method as the base will fly across the room will considerable force and do damage to what/whoever it hits.
The second and safer method involves drilling a hole 10mm up the side from the base. Mark where you are going to drill the whole, 10mm up from the base which should be about 5mm down the flask from the join of the base to flask. Use the hammer and centerpuch (or nail) and make a nice dent where you have marked. This will help stop the drill from sliding around. Place the flask in a vice if you have one, if not place on the floor and hold at one end whilst using the drill with the other hand. Keep your hands and anything else fleshy well clear - if the drill slips and you drill yourself it will hurt! Carefully drill through with the 3.5mm, you only want to go through the outside base of the flask. Next expand the hole using the 6mm. Have a look inside the hole, you should see the inside edge of the base of the flask where the inner and outer halfs are joined. Use the screwdriver or the centerpunch (or nail) poked through the hole and pressed towards to bottom to lever off the base.
With the base off you should be able to see the copper valve roughly in the center of the flasks base where the vacuum was done.

Stage Two - Drill lots of holes
Look inside the flask from the top. At the very top the inside and outside of the flask meet, about 3.5cm down the inside of the neck there is a ridge all the way round. Use the marker pen and mark a dot on the top edge of that ridge. This will be location of the first of the set of secondary air injection points. Mark the second mark directly opposite the first then the third and fourth half way between each of them. Take two of the marks and mark another between them, keep dividing and marking until you have sixteen marks at equal spacing. If you fail you can always wipe the marks off and start again.
That done, flip the flask over to so you can place marks of the base. The base has ridges on which makes things a little easier for getting spacings but you don't have to be two precise. Mark four points around the center vacuum valve at equal spacing. Your going to drill 10mm holes here so make sure you have enough space from where the edge of the 10mm hole will be to the vacuum  - if in dought check with the drillbit that your going to have enough clearance. Next mark another eight at equal spacing slightly further away from the center making sure (using the 10mm drill) the holes are not going to overlap. Check the pictures further down to see the spacing I used.
Use the centerpunch again and dent each of the marks you have. Now drill the first hole with the 3.5mm drill. Careful here, it is a vacuum flask and that first whole will destroy that if your denting process hasn't already holed the flask. Drill through the outer flask then push the drill in about another 8mm and drill straight through the inside base of the flask. Repeat the process for the other eleven dents you have made.
Flip the flask and drill the sixteen dents in the inside of the neck you marked out earlier. Note this section you are only drilling through one wall of metal with your 3.5mm drill.
Flip the flask back to the base again and change up to the 6.5mm drill. Expand the twelve holes all the way through both skins. Now switch up to the 10mm drill bit and carefully expand ONLY the twelve OUTER holes.

Stage Three - Have something to eat
Next open the can of tuna and have a break eating the contents. When opening the can use the can opener all the way round completely removing the lid. Remove the label from the tin then mark a point about 15mm down the side from the top. Mark another again at 15mm down on the opposite side then again between the two marks and again opposite that. Keep marking until you have lots of marks all round about the side of the can 15mm from the top but again be careful not to overlap the 10mm zone. Turn the can over - the sealed end is now the new top. Place your fan in the middle on the top of the can and carefully hold it in place whilst using the centerpunch (or nail) through each of the fans mounting points to dent the can top. I know you need more hands than you have for this task so getting some help may be in order here. Whilst the fan is still in position poke the marker pen between the fan blades and drag it around the outer of the inside of the fan housing. When done remove the fan and you should be left with four dents with a large circle in the center marked in pen. You want to remove as much of this circle as possible so use the centerpunch to dent within this circle remembering the 10mm overlap rule but try to make as many areas to remove as possible. If you manage to remove the whole circle section then great as it will improve airflow.
Use the 3.5mm drill to drill out the four fan mount wholes and all the wholes round the edge of the can you marked earlier (now 15mm up from the bottom edge) Be very careful when doing this as the cans edges and the edges on the holes your drilling are very sharp and will produce very effective deep cuts! Carry on drilling by drilling out all the dent you made in the circle on the top. Switch over to the 10mm and expand out all the edge hole round the can and the circle. DO NOT drill out the fan mounting point holes. As I said earlier remove as much metal as you can from the inside of the area you marked on the top of the can in the circle. Take your file and file down all the rough edges around the holes you have drilled.

Stage Four - Assembly
Take you fan and now place it inside the can with the flow allow on the side of the fan pointing up and lining up the holes you drilled in the top with the mounting holes in the fan. Push through you four bolts in turn and add the nuts on below the fan. Tighten with the spanners or pliers. Stand the flask upside down and place the tuna can with the open section upwards on the top (which is the base!) Use the centerpunch and dent four marks out between the inside edge of the tuna tin and the outside edge of the fan (along the flat edges) Carefully hold the flask and tin together and drill through the tin and bottom of the flask with the 3.5mm drill. Again a vice or helping hand would be good here. After drilling screw the four self tapping screws through the tin into the flask to join the two together. Note where the wires are coming out the side of the fan and estimate the height and position on the outside of the tuna can. Drill a late 3.5mm hole through the can at that point (you know how to mark and dent by now!) Thread the cables through you new hole from the inside to the outside.

That completes your build, well done!

You can use twigs and other small burnable material to fire this up but what you really need it a regular sized and shaped fuel of a regular low moisture content. I wont explain the ins and outs of moisture and bridging here but please refer to the info on to find out more. After trying various fuel sources I eventually settled on organic cat litter. Its the right size and shape and has a regulated moisture content. Its also not to expensive and readily available. Its basic compressed wood pellets which is ideal :)

You need something to start the whole process going and I recommend Tescos bbq fire lighters as they have phosphorus embedded on the end which makes for easy lighting. Other people fire up their versions of toplit gasifiers using lighting gel. Whatever you use be careful.

Firing it up - The fun bit :)
Ok heres the fun (but potentially dangerous bit). Place the rocket stove on a flat fireproof surface and cable up the battery to the fan and test the fan fires up. At this point you should be able to feel a nice little draft getting pushed out the top of the stove. If your getting no draft and the fan isnt spinning odd are the blades are jammed on some part on the tincan. Disconnect the fan, take the tin off the flask and check it all out, filing, bending the can until the fan spins. Reassemble and recheck.
Once the fans spinning and you have confirmed draft disconnect the fan. Fill the stove with cat litter pellets until its about 4cm under the lip which you drilled holes in in the neck. Light up a lighter block and hold onto it long enough for it to burn well but not long enough to burn you. Once its going drop it into the neck of the stove and let it burn for a few seconds more on its own. Now connect the fan up and observe the increase in flames due to the forced air blast. After a few minutes the lighter block will burn down and set light to the top layer of woodchips (cat litter) As its burning in a reduced oxygen area the gas float off the fuel and gets pushed up to the neck of stove where it burns in the second forced air blast. You will see in the neck the flame will appear to float as the fuel burns down inside the stove. Eventually (mine runs for about an hour) the fuel burns to near nothing and the flame turns a blueish purple and is less aggressive for a time before it goes out. Once the flames out you can disconnect the fan power feed and leave the stove to cool. DO NOT attempt to pick the stove up or touch it it gets very very hot! Leave it for half a hour or so then carefully check to find out if its safe to touch. Tip the dead charcoal out (excellent for your plants) put the flask lid on and then pack up - job done.

General Musings
With the addition of a stabiliser to keep the stove upright and incorporating a pot stand this would be a very effective camping stove. A feed for the power could be easily taken off the cars lighter socket. Fuel can be left in the stove with the lid on and as long as the environment its stored in isn't damp. If anyone makes one of these with such a stabiliser and pot stand I would be most interested in seeing it.

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