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 _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address Stoves mailing list to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/ [email protected] http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org
