<html><head><meta http-equiv="Content-Type" content="text/plain;"><style> body { font-family: "Calibri","Slate Pro","sans-serif"; color:#262626 }</style> </head> <body data-blackberry-caret-color="#00a8df" style=""><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Dear Huck</span></div><div><span style="color: rgb(31, 73, 125); font-family: Calibri, sans-serif; font-size: 11pt; background-color: rgb(255, 255, 255);"><br></span></div><div><span style="color: rgb(31, 73, 125); font-family: Calibri, sans-serif; font-size: 11pt; background-color: rgb(255, 255, 255);">What puzzled me about the TLUD is that there is heat to create the gases which are then burnt with the secondary air. Why isn’t this heat enough to burn the carbon (charcoal)? I can see that you can slow burning by controlling the primary air but I’m still puzzled by why, if there is enough primary air to support a fire to produces gases why isn’t it enough to burn carbon?</span></div><p style="margin: 0in 0in 0.0001pt; font-family: 'Times New Roman', serif; color: rgb(0, 0, 0); background-color: rgb(255, 255, 255);"><span style="font-size: 11pt; font-family: Calibri, sans-serif; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>The reason is this: wood (or virtually all biomass that is not processed) has something like 40 percent oxygen content. The primary air is enough to maintain a pyrolysis front, and no more. That air is enough to burn the volatiles above 280 C and that releases oxygen from the fuel. <div><br></div><div>If the amount of hydrogen and oxygen is calculated to make water, there is Almost enough O2 in wood to burn 100 percent of the H2. But not quite. </div><div><br></div><div>If there <span style="font-family: Calibri, 'Slate Pro', sans-serif;">was just a bit more, pyrolysis could be initiated and it would continue without any air from outside the chamber. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">In real life the carbon is not left untouched but you can generalise that the more primary air, the more carbon will be burned together with 100 per cent of the hydrogen. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Once the O2 is gone, the char remains. To burn charcoal takes just about the inverse in the primary/secondary air split. Again generalising, the split for pyrolysis is 1:3 primary:secondary. Burning the char is needs about. 3:1. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">So the char can continue to burn at the bottom where the air entrance holes are, but not at an acceptably high rate. Further, the heat being generated so far from the pot and under the pile of char makes for a terrible heat transfer efficiency even if the excess air level is low. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Acceptable power requires opening something. The easiest way to automatically open holes is:</span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Make the holes you need to burn the char at an adequate rate. Cut a piece of paper that covers the bottom, closing the holes. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Before you place the paper in the bottom make enough holes of appropriate size to supply the primary air. Put it in place. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Fill the chamber and operate normally. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">When the pyrolysis reaches the paper it will burn and open the second set of holes. </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;"><br></span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Regards </span></div><div><span style="font-family: Calibri, 'Slate Pro', sans-serif;">Crispin in Atlanta</span></div></body></html>