[Stoves] Development of a refractory combustion chamber in Mongolia

Sat Aug 26 12:42:39 CDT 2017

Dear Friends

Tom Miles sponsors this list (with Alex and All) for the purpose of sharing technical information for the production of much improved stoves.  Therefore I am reporting on further progress in Mongolia where some of the most interesting developments in stoves have taken place over the past 10 years.

The current project is under the management of the Ulaanbaatar Clean Air Programme (UB-CAP). The current task is to create a combustion chamber that is very durable, that can be made locally, and which uses a flexible material such that it can be used by ordinary producers. In this case, these producers are mostly in the informal sector. They make cooking, heating and combined application stoves, as well as low pressure boilers for home heating that can also cook.

I recently asked the recently graduated engineer Altanzul (MSc from CAU) to provide information on the production of half-cylinders made using a material called 'plastic refractory'.  This is one of the most promising things I have ever come across. It consists of a high alumina set of minerals mixed (just before use) with aluminum dihydrogen phosphate (ADP).

I secured a couple of tons of this material and plastic drums of ADP. After a week of experimentation Altanzul and I arrived at the following mix:

9.7 kg of 'plastic' material (powder)
12.7% of ADP (by mass)
100 g of accelerant

After adding the ADP (photo) at medium speed, it is mixed at high speed using a cake mixer for 4 minutes. It is used immediately.
[cid:image002.png at 01D31DFC.9EED6B00]

This components yield 10.7 kg of slightly wet sticky material that is quickly pressed by hand into a rectangular form and rolled with a 2" pipe (by hand). The frame is a bit narrower than the required part, with the length suitable to contain a total volume of material equal to the volume of the final consolidated part, which in this case is a 320mm diameter half-cylinder, 300 mm high and 30 mm thick.

Here, Altanzul lifts the frame away from the rectangle of material.
[cid:image001.jpg at 01D31DFA.2057C990]

After the form is lifted off is it set aside, but cleaned with a scraper soon after.

The rectangle of material is lifted on a sheet of plastic and placed into a mould with demountable sides. The mould is made from 3mm mild steel and was made in less than a day. It is within 1mm of the required dimensions.
[cid:image017.jpg at 01D31E03.4A569600]
Note that the sides are higher than the required part. The material that is above the required size will be driven down into the mould. The 180 degrees 'coverage' is set by controlling by the volume of material.

Altanzul places the plastic sheet into the mould:
[cid:image002.jpg at 01D31DFA.2057C990]

Then the sides are pushed into place providing the space to be filled. The point makes a hole through the plastic sheet, if there is not one already there. To start with, the material does not touch the side walls.

[cid:image020.jpg at 01D31E03.4A569600]

After both sides are in place, a second plastic sheet is placed on top to protect the vibrator which is pushed down into the hollow centre.  Someone stands on it, and it is vibrated for 30 seconds. Using feet, the vibrator is gently rocked back and forth to ensure the compaction of the material on the very bottom.

The vibrator with side 'wings' (greyish strips left and right) form the jointing surfaces:
[cid:image012.png at 01D31DFC.9EED6B00]
The steel strip is used to trim very slightly the height of the flat surfaces to make it exactly 180 degrees, a dimension set by the top lip of the curing form.

After compaction, the vibrator is removed, the top plastic sheet peeled away and the sides of the mould removed. A receiving 'form' is located beside and on the lip of the mould, and the finished part is slid onto the form where it will remain for at least 12 hours.

[cid:image014.png at 01D31E01.22E4D8E0]
The mould is at the back (black), the side is on the floor behind it, the form is located on the edge of the mould, and the part is dragged onto the form using the plastic sheet. A square is placed on the side of the form to see that the two sides of the 'part' are exactly square (within 1mm).

Each form holds two parts. We started with a dozen of them.

[cid:image023.png at 01D31DFC.EF1E49A0]

After 12 hours the part can be removed from the form and the parts placed in a warming cabinet: 200 C for a day or stored for more than a week (to dry naturally). The reaction of one ADP molecule releases three water molecules each. When dry, it weighs a less than the initial mass.

The result is a very hard, dense, strong, heat shock resistant part that does not need firing. After fully drying it is ready to take temperatures up to 1300 and has a very high thermal shock resistance.

Held up with one hand, the part rings like a bell when struck with a hammer. It can be hand moulded and even hammered into shape with a rubber mallet. We tried all sorts of things. Rolling with a 2" pipe is very effective. So is the hammer.

It will cure in cold weather using more of the accelerant. A small bag with 3 kg is provided for each 22 kg of material, but we only use 250 g of that. The cost of the material is about 50 cents per kg plus transport and duties. The cost of production (materials) in remote Ulaanbaatar of one 10 kg part is about $9, the price of 6 refractory bricks.

Half-cylinders made from phosphate-bonded Plastic Refractory material:
[cid:image005.jpg at 01D31DFA.2057C990]

The parts can be handled after 12 hours so for practical reasons, a day's production is left overnight and then transferred to a warm drying cabinet for the rest of the next working day. This provides fully hardened results in 48 hours from mixing.

The density is 2.4 (like concrete). It has a high thermal capacity, high heat conduction, (1.5, metric) and is about as strong as cement bricks (14 MPa). It is dense to look at and unused, appears to be concrete. It turns white when used.

[cid:image006.jpg at 01D31DFA.2057C990]

Left alone unheated, the material should be left for 10-12 days to evaporate the water from the reaction - so we were told. In Ulaanbaatar it doesn't take that long.  It is really easy to make holes or channels by putting strips of wood under the lower plastic sheet. It can also have grooves to have parts hold each other in position.

Here is a simple stove with a square chamber lined with 4 flat places made in a very simple angle iron frame than makes beveled ends.

[cid:image007.jpg at 01D31DFA.2057C990]

The 'plastic mix' was rolled with a pipe and 'struck off' with the straight steel strip. Keep the edge of the steel sharp.

Here, the 4th side of a large square combustion chamber is dropped into position. The producer reported that the heat radiation from the stove was better than with other materials, and the stove continued to give off heat several hours after the fire appeared to be out.

[cid:image008.jpg at 01D31DFA.2057C990]

Chamber in place, before the grate is installed.
[cid:image009.jpg at 01D31DFA.2057C990]

The plates are very hard, strong  and difficult to grind using a metal (red label) disk. It is like grinding stone.  Users can be cut using a green label or diamond cutting disk.

Altanzul reported that she and two helpers made 158 pieces in the last week. The helpers are being trained in the mixing and forming.  We expect to be able to make 75 per day with the simple equipment we have, maybe 100 (1 ton).

We running a trial of 100 stoves to see how they perform. The cost is about the same as making a chamber from refractory bricks (SK32 for example) that have been cut in half thickness-wise - also about 30mm thick. But this material is far faster to produce, much easier and less fiddly to install, and much more durable. It can also form any shape, filling in corners and shaping outlets around chimneys and so on.  Cut-outs for fuel-filling, exhaust etc can be formed easily in the initial stage using a cut-out shape or an appropriate wooden block.

There is every reason to believe that it will make an excellent lining for small cooking stoves that require protection of thin metal, charcoal stoves that have 'inserts' and high powered devices like water boilers. I do not know how thin the material can be, but my guess, based on small samples, is 10-12mm; 8mm might be too thin. But, it is very strong so let me not limit what is possible.  It can be made into any shape like a Jiko chamber or a fire bowl for a POCA. For the POCA I would definitely try 8mm to see how it fares.

Regards
Crispin
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