[Stoves] TLUD theory -- Fuel Bed Temperatures

[ajheggie at gmail.com]

[Default] On Mon, 17 Jun 2013 12:44:53 -0400,Josh Kearns
<yeah.yeah.right.on at gmail.com> wrote:

>Using a 1-gal TLUD "Toucan" model in natural and forced air modes, we have
>seen, pretty consistently, ND peak temps. 600-650 C, and FD peak temps
>900-950 C. This has been across feedstocks - wood pellets, sugarcane
>bagasse pellets, bamboo chips, jatropha presscake pellets, pecan shells,
>cherry pits.

I saw much the same and my take is that the primary air supply during
natural low draught burning restricts the burn to a narrow pyrolysis
depth, there is insufficient oxygen to do more than provide heat for
downward conduction to a lower layer of fuel. Any char that was
burning in the pyrolysis front is rapidly snuffed out in the stream of
oxygen free offgas and the heat of combustion is exhausted in the flue
gas.

With forced draught there is more oxygen and the fuel bed below the
pyrolysis front is cooled, the depth of the pyrolysis front increases
and free oxygen enters the thicker bed of freshly formed char, char
combustion increases but as the offgas now contains some residual
oxygen and the bed temperature has increased some normal updraught
gasification takes place, the CO2 initially formed is reduced to CO, a
highly endothermic reaction which limits bed temperature but it's
still hot enough to cause problems with steel.

In the past it has been posted that the CO2 reduction is favoured when
the temperature exceeds 800C, if the char bed is 20 particle diameters
thick and there is sufficient primary air then the equilibrium moves
to reducing all the CO2.

So as the temperature of the pyrolysis front rises I would expect char
yield to decrease fro two reasons:

1) Char is burned to maintain the higher temperature
2) Char formed earlier above the pyrolysis front is cooked in a higher
temperature offgas flow.

AJH