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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The mix of colors from both carbon and volatile combustion could be playing optical tricks on us as we look down through the burn. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If the carbon/char is burning then the local temperature at the carbon is now about 2500 F (1370 C) instead of a gas combustion with high excess air at 1500-1800 F (820-980C). This is enough to volatilize some of the ash (potassium) in the char which would give you the purple hue. We see this when burning crop residues, leaves and branches that have high concentrations of potassium in the fuel. It takes some very sophisticated instrumentation to measure the potassium concentrations so I guess we’re stuck with speculation. <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Tom <o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> stoves-bounces@lists.bioenergylists.org [mailto:stoves-bounces@lists.bioenergylists.org] <b>On Behalf Of </b>Crispin Pemberton-Pigott<br><b>Sent:</b> Monday, September 24, 2012 6:59 AM<br><b>To:</b> 'Discussion of biomass cooking stoves'<br><b>Subject:</b> Re: [Stoves] Purple flame<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Dear Dean<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I think it has something to so with the rate of the pyrolysis of the final portion of the fuel. When the zone reaches the bottom, and there is metal there, the pyrolysis rate of the final portion of fuel is at a higher rate than any equal mass during the rest of the burn.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>As a result, the composition of the gas changes because the reaction rate is different. There is no fuel below the ‘final bit’ to absorb heat (esp from drying the fuel below) and also there is no rising moisture from the fuel below. Hot water vapour is highly reactive and changes the gas composition by changing the reaction speed.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>I have also notice the change in colour and have a few photos. Do you suppose it can be used as an indicator to the cook that the ‘end is near’?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Early fire, highest moisture level<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA><img width=416 height=447 id="Picture_x0020_1" src="cid:image001.jpg@01CD9A29.E2B594E0"></span><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Middle fire, constant moisture level<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA><img width=416 height=422 id="Picture_x0020_2" src="cid:image002.jpg@01CD9A29.E2B594E0"></span><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Late fire, low moisture level, highest pyrolysation rate, higher carbon burning portion<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA><img width=416 height=434 id="Picture_x0020_3" src="cid:image003.jpg@01CD9A29.E2B594E0"></span><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Notice that the purple flame is always there, and it is always at the point of secondary air entry. This indicates that is may be more related to the carbon burn (CO) than any of the speculation above. The overall impression is influenced by the bright white emissions from particles. As the volatiles are burned away, there are very few particles and the white disappears (or yellow if it is not a very hot fire). Thus the ‘change in colour’ might be more because the PM disappeared along with the light, than the presence of a new colour. The impression on the eyes would be the same in both cases.<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Best regards<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Crispin<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-CA style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA>Hi Stovers!<o:p></o:p></span></p><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA><o:p> </o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA>I've been noticing a shift from blue flames above the burning bio-char in a TLUD to purple flames nearer the end of the burn when the bottom of the pile is glowing bright orange. I suppose that the blue flame is caused by burning CO but I wonder why the purple forms after the blue? The blue self sustaining flame doesn't heat the water in the pot as well as the purple flame phase although the pile of bio-char is fully ignited at that time and may account for the more rapid rise in water temperature.<o:p></o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA><o:p> </o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA>Any ideas?<o:p></o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA><o:p> </o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA>Best,<o:p></o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA><o:p> </o:p></span></p></div><div><p class=MsoNormal style='mso-margin-top-alt:auto;mso-margin-bottom-alt:auto'><span lang=EN-CA>Dean<o:p></o:p></span></p></div></div></body></html>