[Stoves] re-kindling stoves

[Crispin Pemberton-Pigott]

Dear Andrew

>It would be interesting to see how this secondary air was contributing to
the effect. What if the secondary air were preheated to >800C?

Exactly - a most relevant question. What I have done is make most of the air
pass through the lowest part of the grate where the coal has difficulty
falling in (it is angled 15 degrees to assist coverage not but to completely
choke the grate). Then I have 2 x 12mm secondary air holes supplying
preheated air into the flame as it passes into the combustion chamber lined
with 4 ceramic flat plates.

The preheating is large, as the grate is really hot, and the preheating
effect of the ceramic plates (about 20mm thick) is good.

The next issue is the excess air quantity.

Testing two rather different stove this week we had good results from them
both. One is a TLUD in the classic sense, using the same wet lignite lit
with wood (about 800 g).

The way the operator ran it (without advice from us) was tested and it
eventually became a self-roasting retort with huge internal heat. The excess
air dropped lower than I have ever seen before - the O2 was less than 0.3%
in the stack. (Lambda 1.03) The CO level was of course high in that choked
environment, but the PM was pretty low! Remarkable. It was a very hot
environment with extremely low excess air.

After a while the coal was coked and the EA increased to something more
reasonable like 50% (Lambda 1.5). The CO vanished in the better environment
and the PM dropped to well below ambient. As the burn progressed the CO fell
to as low as 4 ppm in the stack with a CO(EF) [which is CO ppm x Lambda] of
less than 20. That is very difficult to achieve at all, let alone routinely
burn after burn.

That stove was from Turkey. It needs a little adjusting of the design to
prevent the roasting issue and more secondary air cured the EA problem (with
its concomitant CO issue). The other was the GTZ 7.5 cross draft stove which
also reached similar levels of PM and CO. It has had better on-bench
development so the overall result was about 1/8th of the PM per MegaJoule
but we are talking here of a 99% reduction by both so the relative values
are meaningless. We are wa-ay inside the territory of large computer
controlled burners.

>From these discussions, recent research on the morphing of aliphatic (
chainlike) molecules to >phenol type ( carbon rings) compounds as pyrolysis
temperature increase 

The discussion here has moved from what to do about them to what to do to
stop them forming in the first place - i.e. get the combustion right and
there isn't anything to control.

The flip side of no PM is that they are there, but that they are really
really small and can't be seen by the instrument. That is pretty much the
history of particle measurement.

A chemical analogy is the popular belief that there are 'low SO2 emission'
stoves. What people are measuring is SO2 and when they don't find it, they
think it is 'not being produced'. Well, that is sort of true, but if S is in
the coal, it is going to be H2S instead of SO2 which far worse! SO2 is the
desired product.

We have a long way to go...

It was -31 C this morning and it is very tough on the poor who have to
choose between food or heat. The brutal winters the UK is feeling are also
affecting us. Three in a row. Last year 12 C below normal. Eight million
animals froze last winter and this year it came early....

Regards
Crispin