<html><head></head><body lang="en-GB" style="background-color: rgb(255, 255, 255); line-height: initial;"> <div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">"If you suddenly close the primary air control at normal operation in a TLUD stove, the flames turns more yellow/orange, and if you open the primary air control again after a few of seconds, the flames turns completely blue, before it stabilizes again. ¿Why?"</span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">Dear Roberto</span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">Because when you increase the primary air flow above a certain value, it gasifies the charcoal. The flame is the colour of CO burning in reasonable excess air. </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">When the CO is not being generated, there are more evaporated volatiles which condense to create sticky particles which glow orange and yellow as they decompose in the fire. </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">At the end, when there is mostly char above and only a little raw fuel and the last raw fuel is drying rapidly, there is hydrogen production from the water (water gas shift reaction) and the flame turns pink and blue. The pink I attribute to the hydrogen. It might instead by water vapour heated to a high temperature. The colour changes noticeably when the volatiles start to run out. </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">Regarding your six hour burn, you may have to decrease the area and increase the height in order to have much char gasification (which contains the greater portion of the heat energy). If you slow the pyrolysis rate too low it will probably have stability problems. At a minimum, you would have to reduce the burner diameter. </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">I agree with your idea about the flame velocity and the gas velocity. At low hydrogen the velocity is lower. Maybe you can do something clever with that fact. </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';"><br></span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">Regards </span></div><div style="width: 100%; font-size: initial; font-family: Calibri, 'Slate Pro', sans-serif, sans-serif; color: rgb(31, 73, 125); text-align: initial; background-color: rgb(255, 255, 255);"><span style="color: rgb(0, 0, 0); font-family: 'Slate Pro';">Crispin </span></div></body></html>