[Stoves] Three fuel combustion/pyrolysis papers

[Andrew Heggie]

On 20 May 2017 at 20:39, Paul Anderson <psanders at ilstu.edu> wrote:

> 2.  FLAMING pyrolysis.   This is down INSIDE the fuel bed.   It is flaming
> because O2 is present as the pyrolytic gases are released from the fuel
> particle.(actually sort of glowing because the flames are VERY small.

I would still like to see a video of these flames, Perceived wisdom is
that this flaming pyrolysis only occurs at a low superficial velocity,
from my limited observations most TLUD burns  are the complete
combustion of a portion of the nascent char at the pyrolysis front and
glow red rather than exhibit a flame. This is related to the business
of smouldering see below


> Contrast this with NON-flaming pyrolysis which is pyrolysis as it occurs
> inside of a retort (where there is sufficient heat but no entry of O2).
> Note that this is NOT referring to the flaming of the gases at the top of
> the gasifier (or even further away if through a pipe, etc.)

This is different in that no combustion is taking place in the retort
and all the heat is supplied through the retort walls. In TLUD
combustion does take place at the pyrolysis front and it is the heat
from this which drives the descending pyrolysis front.


>
> 3.  CHAR-gasifification.    (why call this "smouldering"?)    Note that the
> realtively small amount of gases being created are CO and they will cause

Well it is neither smopuldering nor gasification in my eyes unless the
superficial velocity of the air is high enough to cause gasification.
Rather it is char combustion. Normally this char combustion will
produce some CO but mostly CO2 and will glow red. If the fuel has not
completed its pyrolysis then the heat from the char burning will
volatilise and crack pyrolysis products which in the absence of a
flame or temperatures above the auto ignition point of the offgas will
rise as smoke. To my mind this is smouldering, the combustion of
nascent char without the combustion of pyrolysis offgas.

It is so easy to demonstrate this by taking a smouldering log and
raising the local temperature by using a small blowpipe, once the
invigorated char combustion reaches the auto ignition temperature of
the offgas the offgas flares.

In the char  burning case if the superficial velocity of the primary
air is increased and the fire bed is deep enough (Tom Reed suggested
20 particle diameters deep) then the initial reaction at the bottom of
the heap is to convert most of the char to CO2 and the temperature
rises to 2000C, As this hot CO2 rises through the bed it reacts with
hot char and  is reduced to CO but this is an endothermic reaction so
the gas and char cools . As this producer gas reaction is an
equilibrium reaction  once the bed falls below about 800C no more CO
is formed. However if the bed was deep enough and the primary air
controlled all the CO2 has been reduced so only CO and nitrogen
survive.

Andrew