[Stoves] WorldStove replies to BioFuelWatyche's latestimprecisereporting of facts.

Fri Jul 29 07:00:54 CDT 2011

Dear Roger

As you are planning to get EPA certification for your stove I suggest that
you look carefully at the way they are going to run the test. They are very
particular about loading, lighting and operating the stove in 'a certain
way' that they wrote down. It may well be that the way you fuel and or
operate the stove is different from how they are going to operate it when it
is tested. It is blindingly obvious that their method may not suit your
stove, or anyone's stove. It is just how they are doing things and that is
that.

The problem is of course that if you have a novel technology and a good way
of operating that technology, and it is quite different from the EPA test
procedure, there are going to be long faces when the results come back,
maybe. So be prepared by looking at the test procedure and follow it to see
how your stove performs when loaded with that wood type and size and
operated in their manner. You might ask in advance if they will operate it
according to your instructions (or not). The answer might make you happy,
might not.

>The average stove at the time was reportedly dumping 60 to 70 grams pm10
particulate emissions per hour, per stove, into our airshed. 

There is an assumption by the EPA that the stoves for domestic use are
pretty much the same power, meaning the total heat delivered in MJ into the
home is about the same for all of them. If you build a stove with a higher
combustion efficiency, then there will be PM10 savings at the same power
level which will translate into reduced fuel use, as you noted. 

If you build a stove with a lower power level and the same combustion
efficiency, there will be a reduction in emissions not because of better
burning, but because of reduced fuel, as you noted. It would of course be
better to have both efficiencies: better combustion efficiency and better
thermal efficiency. One will produce less PM10 per kg burned and the other
will reduce the number of kg that need to be burned. Double plus!

As you mentioned it, I have attached a graph of the thermal efficiency of a
Mongolian 'ger stove' (they call it a ger, pronounced 'gare' like 'tare').
As you can see it varies a great deal from 100% when the stove body is cold
to a negative number at minute 140. The stove was fuelled at minute 12 and
refuelled at minute 60. The steep drops in efficiency are the top of the
stove being opened to add coal. When the top is opened huge amounts of air
pass into the stove, cooling the stack gases with sheer volume and carrying
away all the heat (up the chimney) at a lot temperature.

There are three lines on the graph. The Green one is the instantaneous
thermal efficiency which displays the efficiency as it changes in real time.
It is titled the 'interval efficiency' and is calculated every 10 seconds.
The Orange line is the power output in kilowatts and that number is the
'cumulative' or 'average so far' power during the test. The Blue one is the
thermal efficiency, also calculated cumulatively. The reason for doing this
is that we want to see the average heat output. We know in advance what the
heat demand is for a building, say a very small two room house. So we test
the stoves to see what the average power level is assuming that on average,
the house will be warm. The stoves are produced in three power ranges (for
the most part). This is a small one at 5 kW avg.

As you will see on the chart, the efficiency after ignition of this
traditional Mongolian stove is about 65% (the cumulative average) until
minute 100 and is still above 60% after 2 hours, which included a
refuelling. By minute 210 (3 hours) the average efficiency had dropped to
50% because of the very low efficiency during the third hour. The stove can
be made to operate at a higher efficiency with a small fire by watching the
stack temperature and oxygen level in the chimney then closing the damper,
but an operator would not have the equipment to do that, so they usually
guess at a damper setting and see what they get. In the USA and Europe
commercially produced domestic burners have a device that measures the
oxygen (a Lambda controller) that operates the damper automatically to
maintain the thermal efficiency. Nothing like that in Mongolia.

If the stack temperature is low, it could be because the heat exchanger (all
the parts of the stove that warm the room) is really efficient, or it could
also be because there is a lot of room air entering and passing through the
stove and cooling the chimney (or some combination). Obviously there is no
gain from such a waste which is why people put on a damper (to prevent a lot
of excess air going through). If you know how much air is going through
without helping the fire, you can determine the amount of dilution taking
place (with room air). The Mongolian traditional stove at high power is
quite efficient but it makes a lot of smoke when it is ignited and
refuelled. At other times it is pretty good.  

Note that the thermal efficiency is above 70% when the power level is above
7.5 kW at minute 40. But later it lets way too much air in at low power,
partly because the chimney is oversized and because they are generally
leaky, letting in air all over the place which dilutes the stack gases,
cools them, and carries the heat up the chimney. The note under the chart
says that the thermal efficiency (at low power) drops to -250%. The power
level at which this happens is on a different chart so you can't trace it on
this one. The point is in that condition, the stove is cooling the room
because it is pulling in air from the room which has to be replaced by cold
air from outside coming under the door (etc). The heat lost up the chimney
is not enough to warm the cold air entering so the efficiency goes negative.
Almost all stoves reach this condition but usually very late in the burn
when the CO is high (chemical loss) and the excess air level is high (heat
transfer efficiency loss).

If the EPA is going to test your stove in a way that does not include
operating the damper in the way you recommend, it is likely to have negative
consequences. 

The producers for the American market do not necessarily make stoves that
are optimized for clean burning and thermal efficiency. They build stoves
that will pass the EPA test as the EPA chooses to test them. If your stove
is highly innovative and uses shall we say, non-standard control methods, it
may not give good results when operated the way they are going to. You may
have to make some changes to get in as much fuel as they are planning to
load, for example. When you explain that your stove doesn't need as much
fuel they will tell you the regulation says 'load X-amount of split oak'.
And then they will do it.

Forewarned is forearmed. Do you know which EPA test (the number) will be
used in your area?

Regards

Crispin

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