Dear Frank and Crispin, the main DIRECT product in the reaction of carbon with oxygen is CO but also some CO2 is directly formed. CO that is formed can be oxidized to CO2 (CO+½O2=CO2) at high enough temperatures in the vicinity of carbon particle or in later stage in the gas flow. So there are different zones along the gas flow in carbon particle layer when air (or gas) is flowing upwards through it:
I) Zone where exothermic reaction C+½O2=CO takes place (but some CO2 is also formed). Here the gas temperature is rather low because it is close to the inlet and the gas has not heated up enough. So CO is not burning well. II) Zone where the gas temperature becomes high enough so that also the exothermic reaction CO+½O2 =CO2 (enhanced by H2O) takes place in the gas. This leads to even higher local temperature along the gas flow so that exothermic reactions (C+½O2=CO and CO+½O2=CO2) take place at even higher rate. Here also the endothermic reaction CO2+C=2CO takes place because the char temperature is high enough. This rrwaction adjust the temperature level preventing it to increase very high. Then at the location, where all oxygen is consumed, the gas temperature and CO2 concentration reach the maximum values. III) After that the carbon reacts with CO2 producing carbon monoxide in endothermic gasification reaction CO2+C=2CO. The gas is cooled due to this endothermic reaction. If the layer is very thick, the gas is cooled to a temperature at which the reaction rate C+CO2=2CO becomes very low. So some conclusions: 1. For a thin layer, there is only zone I producing CO. 2. For thicker layer (zone II), the exit gas contains much CO2 and some CO. The exit gas is hot and CO may be burned introducing (preheated) secondary air. The exit gas is hottest if the thickness of the layer is just in the intermediate transfer regime from zone II to III. 3. For a thick layer, the gas contains much CO and some CO2. Its temperature is low and it is difficult to burn CO without highly preheated secondary air. The reaction CO+½O2=CO2 can take place also in the other direction. This reverse reaction (dissociation) of carbon dioxide is not significant at temperatures <1800 K. Regards Jaakko From: Stoves [mailto:[email protected]] On Behalf Of Frank Shields Sent: 18. syyskuuta 2013 0:55 To: 'Discussion of biomass cooking stoves' Subject: Re: [Stoves] Charcoal stove design Dear Crispin, So the CO2 > CO is endothermic. But still needs carbon. As I see it the only difference is the temperature changing as the gases move around the pot. Wondering if the reading could be affected by temperature? Perhaps amount of gas entering the instrument or something? Interesting. Could hydrogen react with one of the oxygen in CO2 making water leaving CO? Frank From: Stoves [mailto:[email protected]] On Behalf Of Crispin Pemberton-Pigott Sent: Tuesday, September 17, 2013 2:01 PM To: 'Discussion of biomass cooking stoves' Subject: Re: [Stoves] Charcoal stove design My conclusion is the CO splits endothermically. It does not happen in stoves with high EA. Crispin Dear Crispin, <snip> I found that directly above the middle of a good stove (under the pot) there is zero CO - I was amazed. But nearer the edges there is more. [Frank >] How is that possible? I thought going from CO2 > CO could only occur be in a bed of char. This is very strange. Regards Frank
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