[Stoves] Heat / cook stove - proposed design

Sun Nov 27 01:30:15 CST 2011

Dear John D

>From private mails and news on the stove site I would like to share that
the development of this stove was a major accomplishment, taking many hours
of development and testing.  

The major development was not the hopper and combustion chamber. It was the
way the fire develops under the hopper. It does not drop fuel onto a grate
where it burns. There are British designs like that going back yonks. The
grate extends under the hopper, most of it, and the pyrolysis of the fuel
starts there in a poorly oxygenated environment. The gases cross through the
pyrolysed fuel at the side of the hopper nearest the ceramic bridge, then
join the fire that is burning coke sitting on the lower part of the grate.
There is a very small amount of secondary are entering the fire near the
base of the combustion chamber which turns out to be necessary, but is not
varied when the primary air is changes (to control the firepower). It is
really a small amount - two 12mm diameter holes (very high preheat).

This stove needs some attention if you want to maintain a high power all the
time. You must shake the grate perhaps once per hour. It is intended to cook
and then space heat at a lower power level. The grate angle is only 10
degrees because of this dual role. I would make it steeper if it was to
self-feed and maintain a power level of perhaps 75% of max. Most of the
secondary air passes through the coke bed at the back of the grate where
there is not enough fuel depth to choke it. Kinda breaks a lot of rules.

So after a time there is a relatively thin layer of coke burning on the
lower grate (at the back) getting progressively deeper towards the front. It
turns out that the distance from the grate to the bridge (75mm in a straight
line, not vertical) is key to getting extremely low emissions and this is
bound to be fuel-specific. We are using a wet (25%) moisture lignite which
measures (Dry) 50% volatiles. That is comparable to wood which is why I
believe it will burn wood, even damp wood, well in that configuration,
perhaps large pellets, small briquettes or chopped and split.  

Professor Lodoysamba suggested and then tried filling the combustion chamber
with coal then wood and top-lighting. It was very clean burning on ignition
(which has been the main problem all along in the city) and it can cook
immediately. The fire developed downwards into the grate area then lit the
fuel under the hopper. The combination means there is virtually no PM2.5
emissions at all, better than three nines reduction (>99.9% less) compared
with the baseline.

Made from a combination of cast iron in the main heat areas, coal boiler
brick lining and sheet metal for the rest (plus a cast iron top) it can last
for at least 5 years which is our target service period. The cost in sheet
metal (which is not ideal) is about $130-150 without a subsidy. 

At this time there are 4 more editions (7.2-7.5) each of which looks more
and more conventional in appearance, the 7.4 being the cleanest (PM2.5 <1.0
mg/net MJ delivered including standard ignition). We don't yet know the
emissions for the TLUD-ignition approach but it appears to be even less. The
baseline stove is 780 mg/net MJ. 

For comparison, a South African mbaula is also in the high 700's. The use of
the Basa Njango Magogo TLUD ignition reduces this to about 90mg/net MJ
though there are problems defining efficiency because it is ignited outdoors
(100% inefficient) and use indoors (98% efficient). Efficiency is calculated
into the net MJ number so it matters.

John, it was interesting to see that the SeTAR Centre tests of the mbaula
burning Witbank D get basically an identical value as the SEET Lab tests of
a traditional stove, which you called 'an mbaula in a box with a chimney'.
You were quite correct! It is absolutely dreadful. If Johannesburg were in a
valley with nightly inversions, it would be as polluted as Ulaanbaatar.

James Robinson and Vinny and Taffy and Harold and others (I think 17
altogether) are driving to Durban today for COP17 and will demonstrate the
RSA version of the BLDD stove, a parallel development working on very
different combustion principles. It should be able to burn coals with a high
hydrocarbon content and heavy tars. They need a long residence time which is
a serious challenge for a small stove. It is done inside a pipe. I am
getting flames as long as 2 feet with the version I have in Waterloo (v5.5
and v6.0). That is when burning a bituminous coal from Virginia. Horrible
stuff and makes flakes, not nuggets.

I am considering a blended version with aspects of the DD stove and the
crossdraft in order to gain the benefits of both. The DD stove has an
entirely contained gas path which makes things very simple.

For those who have not seen John D's stove the burn is virtually identical
to the BLDD stove from SeTAR but upside down! We knew that long ago and have
pursued the two paths in parallel. Both work really well.

John if you send me a set of drawings I will put it in the library as well.

Regards

Crispin

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