[Stoves] Dushanbe Stove

Sat Dec 5 20:20:03 CST 2015

Dear Frank

When you do not have much control over the air supply to the fuel, changing the surface-to-volume ratio is one approach to extending the burn rate, or increasing the firepower.

>I have wondered if pellets are made of different sizes? From the ones I have worked with they all seem to be the same size. Reason? If adding larger pellets having a bulk mass with less surface area might work for night.

If the fuel is all the same size and mass, then control has to be by managing the heat getting into the fuel (not roasting it with stored heat) and then only letting it have as much air as is required at the time for the desired burn rate.

>Also; water is used a lot to store heat and release it slowly. A large (try large) pot filled with water on top would heat up during the day (perhaps insulated) and slowly give up its stored heat into a room at night (insulation removed). 

This is widely used with and without realising it. Water is in the radiators of low pressure boilers and the volume of the pipes. Another method is to store it in the mass of bricks in a specially built wall. This is used in the ‘double bell masonry heaters’ becoming popular now in North America. A brick doesn’t hold much heat but they are easy to add. Water holds a lot of heat but it tends to boil and leak.

Saturday we tested two improved stoves to see how improved they were. The field testing was extensive already in terms of observational tests with virtually no equipment. The two stoves were completely different and achieved real savings through two completely different mechanisms.

One added a water heating pipe with a thermosiphon plus a 40 litre tank.  It also had an add-on oven which was placed in the chimney. It also acted as a heat exchanger because of the large metal surface. The oven temperature reached 200°C. We fired it with wood+dung, then wood, then coal.

Some effort had gone into trying to limit the air flow through the grate and controlling leakage into the combustion area. The stove was tested with wood (apricot) and coal from Kyrgyzstan. The performance was quite different with the two fuels, and varied depending on the firepower at the time. 

This was the same result as with the stove we observed all over the bazaar and field tested earlier: high variability. The savings were reported to be 40% and that was confirmed, but immediately have to add that it depends on the fuel and the fueling rate.

The second improved stove also had been carefully tested in the field and tuned to the heating requirements of a particular region. It had a chunkier, cubic shape and was made from tread-plate which has a higher surface area than sheet steel. Internally there was some ‘gas path’ thinking. It turned out to be quite a bit better in terms of combustion efficiency but not heat transfer efficiency which was constrained by the heated surface area. In other words if it had a small wood fire, the heating efficiency was quite good (72%) because it could transfer the heat to the room, but the combustion efficiency was not acceptable at all. If the fire power was increased the efficiency dropped largely because of a high chimney gas temperature when it left the room. I put the sensor at 2.4m from the floor.

With coal the combustion efficiency was the best seen all day, provided the fire was large enough: CO/CO2 <1.3%.  OK so far, but the stove couldn’t get the heat into the room and the stack temperature (at the point at which the chimney passed through the ceiling) was very high taking more than half the heat away. With wood, the stove could not cook (gases too cool) and with coal it could cook well: high gas temperature but then high loss up the chimney.

The conclusion about stove 2 was that it was a pretty good coal stove when the firepower was up in terms of burning the fuel well, but it needed a lot of additional chimney around the room to get the heat ‘back’ so to speak. The good combustion was only under certain conditions, indicating it was probably accidental design, not intentional. The main reason why it worked under only one set of conditions (fuel rate + fuel type) was because the leakage of air into the chamber pretty much fixed the secondary air supply. If the secondary air flow is constant, then the fuel available and the rate of burn (primary air) has to match the ‘fixed component’. Then everything is balanced.

Fixing the fuel mass (which happens a lot in a batch loaded stove) or the primary or secondary air flow (ditto) means it will only run optimally at one firepower. Controllable power requires a controllable burn rate while retaining a balanced air supply above and below. That facility still has to be introduced.

So both stoves, when used in the context for which they were designed, saved a substantial amount of fuel which is the primary mission. One did it be connecting additional devices that drew heat from the fire. The other did it by improving combustion and providing the possibility of adding external radiating surface are (horizontal chimney length).

Both can be improved by applying additional engineering to the combustion area and matching the heat shedding area to the available power.

So…add this to the mix of information received so far. What are the next steps? It is clear that people are using three fuels in the same stove. What to do about that? The conditions required are really different, particularly the difference between dung and the coke remaining after the flaming pyrolysis of the coal has finished.

Regards

Crispin

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