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Page history last edited by Philip Small 15 years, 4 months ago

For a cleaner burning configuration, consider a Top Lit Updraft (TLUD) pyrolysis technique, which takes advantage of some of the same principles which makes inverted downdraft gassification stoves so efficient.  In both, the pyrolysis front advances upwind, in the opposite direction of the draft.  This places charcoal between the smoke generation point and the flame.


I prefer TLUD over downdraft for 3 reasons: 1) a simpler design, it is easier to implement, 2) it yields less particulate (less blue smoke) and 3) the gas produced burns more readily. This last is my main reason for using a TLUD approach over a simple barrel configuration, as flaring off the smoke produced is a major goal.  Flaring is more neighbor friendly and it eliminates the methane component, a powerful GHG. 


A disadvantage of TLUD over downdraft may be that more of the bio-oil condensates are driven off.  An advantage of downdraft gasification is that it lends itself better to continuous fee.  TLUD is primarily a batch approach, but then most charcoal production is in batches.


The TLUD technique looks and works simply, however getting optimum performance involves some fairly sophisticated principles, especially throttling the superficial velocity down (PDF) and grading the fuel to achieve a distinct pyrolysis front. That doesn't prevent success using common materials and dead simple design. Take a simple open barrel configuration, tweak the design per the aforementioned sophisticated principles: proper sizing of the air intake is critical. Now light it from the top instead of the bottom.  This takes a different skill set than lighting from the bottom but its also not that difficult to master.  A little vaseline or ethanol on a cotton ball can work wonders for starting up. Once the fire gets going, the top layer of wood burns, creating charcoal, naturally. The heat from the top layer burning warms the wood below it releasing combustible and noncombustible gases which flow up into the charcoal layer. Glowingly hot charcoal has a wondrous ability to strip oxygen molecules from the smoke that passes over it, so it converts some of the water into hydrogen, and some of the carbon dioxide into carbon monoxide. These two gases are flammable. They join with the other flammable gases released from the fuel. These ignite as they mix with air coming into the top of the open barrel above the charcoal layer. it. The result is a scrubbed gas-fed flame that is much more controlled, and which burns substantially cleaner and hotter than can be achieved with the bottom lit burn barrel. (Source). Insufficient oxygen below the combustion zone impedes loss of the charcoal despite the high temperature flame immediately above it.  This allows charcoal to build up faster than it is consumed, at least until the pyrolysis zone reaches the bottom of the fuel column. The downside is that, while wondrously clean burning in even the simplest configurations, a top lit updraft pyrolysis device won't achieve its 20-30% charcoal-to-fuel yield potential without tricking it out to pre-heat and damper the intake.

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