[Stoves] RE: Henson Center Fiure Burner System; Was: Re: improving charcoal stoves

[Tom Miles]

Crispin,

I would have to agree with your two stage combustion. Think of the charcoal
as "coal" for a moment (high carbon, low volatile). A coal boiler will use
80% of the combustion air through the grate and 20% above the grate. It
takes a vigorous flame above the grate to help burnout the CO. In wood
boilers we use the grate primarily to generate combustible gas which is
burned out above the grate, either by excess air coming through the grate
but mostly by high velocity air that we supply to the combustible gas above
the grate. It is important to keep the temperature of the combustible gas up
to ensure ignition. So with wood we use 20%-40% of the combustion air
through the grate and up to 80% above it. 

A nice combination is 25% wood and 75% coal. When you burn them together on
a grate the tars and CO generated by the coal (read charcoal) have to pass
through a flame bath of volatile combustion provided by the wood. The result
is that you get very good burnout of the CO from the coal. We try to
maintain a fireball of 1500-1800F above the bed.

Mixing air with CO is the #1 challenge for burning out CO. CO and air have
about the same density and don't want to mix by themselves. One has to be
forced into the other. The best use of a fan is to promote mixing of the gas
with the combustion air. 

Tom

-----Original Message-----
From: stoves-bounces at listserv.repp.org
[mailto:stoves-bounces at listserv.repp.org] On Behalf Of Crispin
Pemberton-Pigott
Sent: Friday, June 02, 2006 3:47 AM
To: Stoves
Subject: [Stoves] RE: Henson Center Fiure Burner System;Was: Re: improving
charcoal stoves

That is my point today.  CO could be ignited in the hot space between
burning coals, however that heat is not really used for ignition, it is used
for making more and more CO because that heat is next to the charcoal.  From
what I see it is important to get a hot zone away from the charcoal in order
to light the CO without further increasing the production of gas.

I have not read anything about this so I am going strictly on what I measure
and understand.  The temperature at which the charcoal starts to 'burn'
(evolve gases) is as low as 300 to 400 C.  Running the temperature up to
700+ doesn't really lower the CO by lighting it, the heat just produces more
CO.

When there is some volatile material in the charcoal it runs out and carries
a flame which ignites the CO _above_ the charcoal.  This means that the key
to burning charcoal cleanly is to have a two stage process, a cool one in
which the CO is generated at a variable rate according to the cooking
demand, and a high temperature stage in which is it burned with or without
volatiles.  The flames seen at the beginning of a BBQ are misleading.  It is
when they disappear (volatiles now gone) that the poor combustion of an open
system is evident.