[Stoves] Ulaanbaatar Air Quality

Fri Nov 8 09:42:16 CST 2013

Dear Crispin,

In China burning coal instead of wood was established by Mao and it
continues today. We looked at the honey combed briquette makers in
Shengzhou, a town of about 800,000, and about 185 tons per day of honey
combed briquettes were made daily and sold. As you point out, coal burns
with very little PM once its lit. However, I hear that there are other
types of harmful pollution made by coal, like heavy metals. And it smells
bad.

Inexpensive electric lighters are sold in China to replace the starting
with wood although it's common to see incredible amounts of smoke in the
morning as the restaurants start up their coal cook stoves with wood.
Adding secondary air can burn up the CO and added 10% to thermal efficiency
because the burning CO no longer escaped unburned!

Like charcoal, once lit coal burns with very little PM. For some reason,
the theoretical ability of coal and charcoal to burn with very little PM is
often actively ignored and rejected. Perhaps it's good to concentrate on
how to accomplish the clean burning of wood. When we have stoves that can
burn wood with very low PM we have added another low PM resource for solid
fuel users joining coal and charcoal.

All Best,

Dean

On Fri, Nov 8, 2013 at 4:52 AM, Crispin Pemberton-Pigott <
crispinpigott at gmail.com> wrote:

> Dear Alex
>
>
>
> Very good question and complex to answer so I will have to put down some
> rules.
>
>
>
> Let’s define ignition as the first 30 minutes and the pyrolysed state
> (poorly defined) as the second important point in the measurement.
>
>
>
> The 30 minute window is the time taken to get the thing lit well and
> running at an even power level.
>
>
>
> Let’s take the traditional stove as the baseline. These pics are quite
> high resolution so you can zoom them.
>
>
>
>
>
> The purple line is the burn rate so when it stabilises at a constant rate
> (straight, angled line) the power level is constant.
>
>
>
> The yellow line shows the accumulated PM to that point. So after 30
> minutes it has not quite stabilised but is at 35 minutes. It does not last
> long before the burn rate starts to drop. This is typical of the product.
>
>
>
> The initial smoke for 7 minutes is the wood lighting with maybe or maybe
> not a couple of pieces of coal being affected. The huge blast of PM is the
> coal being put on top of the wood fire (roasting it) or else it is being
> lit crossdraft meaning the fire is hitting a pile of coal next to it. This
> is the main portion of the total PM.
>
>
>
> After it gets going, there is very little smoke. The second ‘puff’ is the
> refuelling of a hot stove at 70 minutes. It has a much shorter
> stabilisation time and a similar burn rate profile.
>
>
>
> I would say that the ignition is responsible for more than 90% of total
> emissions, if you include refuelling.
>
>
>
> Now let’s look at a clean igniting stove (this is not a TLUD).
>
>
>
> First note that the vertical scale is not quite the same.
>
>
>
> This is a crossdraft stove with a hopper. The wood ignition can be seen
> starting at minute 4 then dropping to nearly zero then the coal starts and
> is burning very well by minute 25. The burn rate is constant by 30 minutes
> indicating that the fire is established and constant. At minute 78 when the
> traditional stove needed refuelling, it was only necessary to shake the
> grate which made the spike in PM (ash, maybe a little BC).
>
>
>
> This particular stove went through 6 more iterations until the smoke was
> pretty much undetectable after 12 minutes. I would say that the ignition
> period was responsible for more than 95% of total emissions.
>
>
>
> Finally a TLUD (I have to hide the product name in this case) :
>
>
>
>
>
> First note that the reported emissions rate is per net MJ, not per MJ as
> is the case in the older tests above.
>
>
>
> The ignition goes well with the wood (almost always larch or in some cases
> pine). There is some smoke as the coal lights on top, largely due to the
> cold combustion chamber and an excess of air in the chamber.
>
>
>
> The increase in smoke at 55 minutes is not caused by someone fiddling or
> refuelling. It is a problem found with many TLUD’s that are created without
> adequate testing. There is a lot of heat generated in the combustion
> chamber and the primary air supply is not controlled well enough to limit
> the power adequately. It heats the stove body and chamber (cast iron
> typically) so much that at some point the pyrolysation rate exceeds the
> available air supply. This took 55 minutes with this stove. As the gas
> production rate starts to overwhelm the air supply combustion quality
> deteriorates and it starts to make more smoke than it did during ignition.
> This is a common problem and nearly no one is listening to us so we are
> starting a stove design training centre to bring real time measurements to
> the attention of the producers.
>
>
>
> I would say that in the case of TLUD’s the major part of the emissions
> comes when the pyrolysation reasons the bottom of the pile and it flashes
> the remaining volatiles into gas over a 15 minute period. The effect is
> common in all TLUD’s that burn to the bottom. There is no new raw fuel
> below to cool the hot zone with new moisture. So it becomes in effect a
> self-heating retort with thermal runaway.
>
>
>
> Only a few TLUD’s have proper and controllable secondary air. Some have it
> but it is not preheated. The TLUD’s with a cold supply of air at the top
> blowing across the top of the chamber have the worst results of those with
> secondary air. There is still a chance that the effect above will emerge.
> The problem at root is a runaway pyrolysation effect caused by trying to
> have the fuel be part of the air control system. If the fuel is not exactly
> right there will inevitably be negative consequences. In other words the
> stoves are too simple to work properly except in ‘certain circumstances’.
> The call to chip fuel to a particular size and density is in part to keep
> the stove simple. It is OK if you can organise it.
>
>
>
> So what does a really clean lighting stove look like?
>
>
>
> This is again a cross draft stove with a hopper:
>
>
>
>
>
> This is the result of 18 months of work on ignition and fuel feeding
> processes. It has a small amount of wood in the combustion chamber that is
> lit on top to get the initial smoke out of the way and then it lights to
> the side and runs constantly thereafter. It can be refuelled at will
> without consequences and in fact this stove was refuelled. You can’t see
> where. The change in power was caused by shaking the grate.
>
>
>
> It is with some annoyance that I see these stoves were specifically
> excluded from the Berkeley comparative assessment of stoves they wrote for
> the GACC on the excuse that people should not burn coal (which is what is
> says in the introduction). There is a whole group of stoves which would all
> sit in the bottom left corner of the stove performance chart deep into the
> Tier 4 section – cleaner than anything burning wood. In fact some of these
> stoves can burn wood very well too.
>
>
>
> The final example (remembering to correct for the different metrics) is
> 1293 times cleaner than the baseline for a +99.9% reduction in PM2.5. That
> is something to be praised, not hidden. What is the problem with letting
> people know that coal can be burned *this* cleanly? A modern power
> station cannot deliver electric heating and match the PM or CO numbers per
> net MJ. Those producing stoves of this calibre are in Turkey, Mongolia and
> China. There is a Japanese stove I have not seen which may also be in this
> league.
>
>
>
> Incidentally, all the best stoves (the four approved for subsidy this
> year, for example) produce no smoke after a certain time of running and
> starting removing ambient PM, meaning they have negative emissions for at
> least part of the time. The best ones, perhaps half, are able to scrub the
> air entering the stove completely of all particles even if the ambient air
> has 200 or more µg/m3. They literally clean the air to zero PM while
> burning wet lignite in a natural draft 10 kW fire.
>
>
>
> So far three types of combustors have achieved this: downdraft (BLDD),
> end-lit down-angled crossdraft (ELCD) and updraft (TLUD).
>
>
>
> Regards
>
> Crispin watching the early morning snowfall
>
>
>
>
>
>
>
> Crispin,
>
> This may be an affront to your technical cred, but if you could humour me
> with an educated guess, I have a question. How much of the
> improvement/reduction in emissions is due to 'top ignition', better start
> up and how much is due to improved steady state combustion?
>
> Alex
>
>
>
>
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