[Stoves] Pyrolysis: No Air?

Julien Winter winter.julien at gmail.com
Sun May 17 10:50:40 CDT 2015


Hi Stoves;

Some terminology distinguishes pyrolytic processes by the origin of the
heat needed to break organic bonds: "allothermic" and "autothermic"
pyrolysis.  With allothermic pyrolysis the heat is supplied from outside
the system.  With autothermic pyrolysis, heat is supplied by exothermic
oxidation of some of the pyrolytic products.

The major thermal breakdown of organic matter is much the same in both
processes, when the fuel particles are "thermally thick" (i.e. there is a
substantive temperature gradient between the outside and inside of the
particle as it is heated), such as in a TLUD.  As particles heat up from
the outside, a thermal 'wave' advances toward the center of the particles,
raising the temperature of the organic matter through pyrolytic
temperatures in the absence of non-fuel oxygen, because the atmospheric
oxygen has been consumed outside the particle.  Thus, the thermal front
advancing into the particle is accompanied by a pyrolytic front that leaves
the outside of the particle charred.  The outer shell or rind of char does
not burn off, because the necessary oxygen is being consumed outside the
particle by burning pyrogas, and rate of char oxidation is 100 times slower
than the rate of pyrolysis and the rate of pyrolytic gas oxidation.

Therefore, the major pyrolytic breakdown of organics is basically the same
under allothermic or autothermic conditions with thick fuels. Under both
circumstances, the oxygen present is only from the biomass.  That is the
case for fuel particles > 1 mm thick, and phenomenon increases with
particle thickness, such that the center of large particles can be at
ambient temperature (ca. 25 °C), while the outside is very hot.

Of course, the big difference between allothermic and autothermic pyrolysis
of thick fuel is what happens to the pyrolytic gases as they exit the
particle through the rind of char.  The surface temperature of the
particles in a gasifier are in the range of 600 to 1200 °C (vs. 450 - 600
°C in a common pyrolysis retort), so the vaporized organics experience more
cracking and secondary char formation on the way out of the particle.  Once
outside the particle, some pyrolytic gases and some char are oxidized, and
secondary reactions continue.

When a particle is fully pyrolyzed, the reactions will move to the particle
surface were we will have char combustion, providing that there is a fast
supply of air, such as we find at the grate of a TLUD, or in a rocket stove.

'Autothermic pyrolysis' may be a more scientific term to use than
'gasification', but 'gasification' is in common use.

Kirk mentioned that you can get allothermic pyrolysis in a TLUD.  I have
seen the same when I measured the vertical temperature profile in a ND-TLUD
burning wood chips (screened of fines through a 9 mm hardware cloth mesh).
The ignition front almost always channelled down the sidewalls of the
reactor and hit the grate leaving a large volume of raw fuel above.  High
temperatures (> 800°C) developed at the grate in a charcoal fire that
comsumed most of the oxygen in the primary air.  Some flaming pyrolysis
advanced horizontally from the channels that had been followed by the
ignition front, but a substantive part of the fuel experienced no flame,
and underwent allothermic pyrolysis.

In order to stop channeling of the ignition front down the reactor
sidewalls with my wood chips, I had to insert fire-breaking metal rings
into the reactor.  Another approach would have been to have use a coarser
screen when I removed the fines from the fuel.

I have never succeeded to get channeling with wood pellets.  A pellet fuel
bed seems to have no spatial bias in gas conductivity (resistivity).

Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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