[Default] On Wed, 21 Aug 2013 21:33:06 +0000,"Crispin Pemberton-Pigott" <[email protected]> wrote:
> >I followed everything until the end. The last paragraph. "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." I'll try and rephrase that: for a given dry lump of wood although the surface may be simultaneously pyrolysing and the nascent char burning to ash while the evolved offgas burns the centre is also heating up in the absence of combustion air, the pyrolysis products are also reacting within the mass, so pyrolysis offgas is diffusing out of the lump. This offgas initially has species, like acetic acid that are already moderately oxidised and react with little combustion air to release little heat. Later, as the lump gets hotter, more fuel rich gases like methane, hydrogen and carbon monoxide as well as tar vapours are evolved and these require more oxygen for the secondary combustion. As excess air is always necessary to increase the chance of an oxygen molecule reaching fuel molecules as stoich air varies so must excess air to maintain the same level of free oxygen in the exhaust. It is free oxygen which we are measuring with a wide band lambda sensor (I think lambda being the accepted character in internal combustion for equivalence ratio even though these sensors cannot measure below zero oxygen as they are cells that generate a voltage dependent on, but not directly, the proportion of oxygen in the flue gas on one side and air on the other. e.g. they will still measure an unvarying zero oxygen even if the fuel:air mixture is so rich CO is being exhausted.) >Re coal and Oxygen, there is not so much in the coal but wet coal fires can >make O2 during ignition. What reaction do you surmise releases this oxygen? 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/
