YOu are really hitting the nails on the heads with that post. Congrats.
>Focusing on the original question about burning coal in an RMH, if one retained the J-tube Rocket configuration, what adaptations must be made to deal with burning coal?
The original downdraft stove was a J stove. With a flat J Iguesss you could say. So far there is not an available downdraft stove though I taught several artisans to make them. One surprised me by calling it a Russian name and he knew how to light it, andreally loved the performance. He got a 13 hours burn from a single load by putting a choke ring in the chimney. So they are known in Mongolia.
The huge advantage of a downdraft is that when burning high volatiles coal like Nalaikh (which is about 2/3 of domestic consumption in UB) it deals really well with the volatiles and the refuelling emissions which are horrificon a traditional stove. Remember the trad stove is a wood stove with bricks in it.
>Would one keep the downdraft configuration, or would a crossdraft or a dasifier/pasifier work better?
I have seen some pretty wild things tried in UB so I remain open to anything that will burn. I put a video on YouTube about the stove with the rotating grate - that was off-the-wall crazy and partly worked. Would have been better if it was made to tighter tolerances. Thermal eff was 35% tho.
>Is there still a risk of runaway burning?
I really think not. The coal burns pretty much like wood - you have to restrict the primary air to slow it down which is a problem with the trad stoves because of the poor fit of the ash drawrs, as I said.
>Would one need to inject secondary air at some point, heated or otherwise?
That is really likely. As the primary air is necessarly restricted because of the high volatiles, secondary air is needed. With hte GTZ 7 series stoves this was provided by running it through the grate at the low-level end where the coke is. It is an odd way to do it but it sure worked well. Very little additional secondary air is needed on top of that. As others have suggested above, restricting hte primary air at the entrance of the RMH will provide negative pressure in the firebox limiting the chance that anything will leak out.
>I would guess that a refractory combustion chamber would be a requirement. Would cast refractory be sufficient, or would kiln fired material be necessary?
To avoid a general discussion, yes and in the case of Ulaanbaatar, I suggest using the widely available (meaning three outlets or more) of the Chinese boiler liners. They are about $1.25 each and are really well made, can take tremendous thermal shock and are cheaper than the Russian ones. Mentioned above was the need to deal with higher temperatures. Quite right. Metal does not last when exposed to the coal fires at high revs, so to speak.
>Is the startup time for coal significantly longer than wood?
Definitely, especially as the fuel is 25% moisture. It is often frozen like a popsicle when it put in.
>If so, would some sort of draft inducer be necessary to maintain draft until the system is heated up?
In short yes, but what kind? The nest answer seen so far is the heat exchanger bypass which is a hole approx 40 x 50mm leading directly to the chimney bypassing the heating box. The box is made slightly larger to compensate for the heat bleed. It brings the stoves up to temp quickly and has been adopted by several manunfacturers. This is of course not necessary on the RMH.
>Addressing the issue of condensation, would a feasible solution be to make sure the flue in the bench, or whatever the mass is shaped like, is sloped to drain with a trap placed inside the heated structure at the bottom of the chimney, or before the exhaust vent perforates the exterior wall?
I was hoping Ianto or someone with experience would comment on this aspect. How is the condensation dealth with? Is it drained outside or pooled and evaporated later?
>Addressing cooking, I get the feeling that making a single combo stove for the cold extremes of Mongolia may become a case of "Jack of all Trades, Master of Nothing."
Yeah, a serious possiblity. The thing is, the population is in a state of flux and there are definite market segments for specialised products which the RMH might be one. In its favour is that they are used to mass walls which they call heating walls. I have heard one report of a fixed heating wall in a portable ger.
>While the barrel over the internal chimney of a heating stove may be used to make or warm tea, or slow cook, I don't know that it will be able to cook a meal in a 20 liter wok without some major alterations.
The cooking height is a non-negotiable so this has to be considered. Possible cooking locations are the horizontal portion between the firebox and the vertical hot chimney, or on top of the chimney which is too high for short women to put 10 litres of soup. Woks slosh easily.
>Could a cooking top be fabricated to fit the barrel, something that would lift the wok above the chimney to maximize heat at the top of the barrel, and make a good airtight seal at both the barrel and the wok?
The problem is still going to be the height. Making tea might fly.
>Would it be safer to put the cooking unit ahead of the internal chimney?
Exactly. Possible, as they say.
>Would it be simpler to just buy one stove to cook with and another to heat with?
We have available at least some social anthropoligical survey material (American, Cecil Cook) indicating that above a certain income level there is a strong preference for electric hotplate cooking, like a rice cooker and so forth. People also cook with bottled gas if they can afford it. This is also the group that a) has more money and b) probably lives in a house that the RHM could heat.
One must be careful when interviewing because if you ask about stoves, they tell you things that relate to the importance of 'storing heat' (which most stoves do poorly) and portability, even if it is in a fixed installation. It is a hangover from being nomads. We found the same in South Africa. What people think about stoves (which drives their purchases) is different from their use of them and the cost of using them.
People mistake a small smouldering fire as the stove 'storing heat'. The thermal mass of the stove is not large at all but this capacity rates highly on surveys.The reason is they have such a flash-in-the-pan massive fire followed by some smouldering coals. They mostly have never seen a stove that has continuous heat for a long time like Roger Lehet's vertical masterpiece. Have you seen it BTW? Mother Earth has picked it up as a promo technology through their fairs, somehow. He deserves the victory.
Incidentally I met with Adam Perry of Cob House fame together with Cecil Cook at the East London airport on Wednesday. We didn't talk RMH - no time - but got to see each other. He is examining the spread of soil-cement building technology in the old Transkei where is has displaced cinder blocks in low income housing in some towns. Cecil is an adjunct prof at the Univ of Fort Hare and Adam is studying there. There is a strong AT connection in there too I expect.