On 21 August 2013 19:07, Crispin Pemberton-Pigott <[email protected]> wrote:
> > Interestingly, for the same level of O2 in the stack, the EA is different > under different chemical conditions. Complex pyrolysing conditions provoke > all sorts of strange chemistry so the chemically balanced approach is > required, actually. For a given device, the best combustion tends to take > place under a certain EA level, but this varies (a lot) between devices. The > BLDD6 coal stove works best with 30% EA which is a very low value and really > surprised me. I would have guessed that was not possible. OK we're talking at slightly cross purposes. I think where you are coming from is to do more with the batch nature of most cook stoves. With a premixed fuel:air system, like a gas flame, the reactions are for all intents and purposes instantaneous, the oxygen disociates in the flame and immediately grabs a couple of hydrogen and then soon after another oxygen reacts with the left over carbon. When you burn wood the sequence of necessary events is longer and unle,s the wood is very small, like sawdust in a cyclonic burner, the processes all overlap. So a stick may be completely burning at its surface whilst the middle is heating up. As wood heats up and pyrolyses it evolves an offgas whose constituents change, early species emitted are things like acetic acid and this has a high oxygen content, later the major constituents are things like carbon monoxide, hydrogen and methane.At the end when just char is burning there is no contribution of oxygen from the fuel in species in the exhaust. Clearly these changes in offgas require differeing amounts of oxygen to burn out, so stoichiometric air changes with time. The thing about burning coal is that it contains barely any oxygen and hence this does not have any contribution to to the ultimate analysis of the exhaust. In fact burning coal with very little excess air is necessary because coal burns hotter than wood and hot enough to form nitrogen oxides if oxygen is present. This is why air supply is cascaded into the reaction after cooling the initial combustion to keep temperatures below the point at which NOx would form (1500C??). This quenching is the very thing we wish to avoid with a flame in a cook stove because we need to maintain high flame temperature to allow carbon particles to burn out. So yes stoichiometric air does vary as wood burns and excess air mirrors this but it is not reasonable to mix this up with the fact that oxygen from combustion air and from the fuel is conserved in the exhaust gases. AJH _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] 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://stoves.bioenergylists.org/
