Re: Refractories and Insulation

[AJH <andrew.heggie@dtn.ntl.com>]

On Wed, 31 Oct 2001 01:30:49 -0600, Harmon Seaver
<hseaver@cybershamanix.com> wrote:

>     Also -- am I correct in thinking that if there was no secondary
>air, there would be no ignition of the producer gas. 

Yes if the primary air is completely used up in gasifying the fuel
offgas passed to the secondary chamber is pyrolysis products, hydrogen
and CO with as little CO2 as possible. If temperatures are less than
ideal steam and CO2 are carried over rather than reacted with the
char.
>I'm wondering how
>far away from the actual flame front can the secondary air be introduced
>and still have auto ignition of the gases?

I am not sure you mean auto ignition, this will only occur if the
offgas+secondary air temperature remains above the temperature at
which spontaneous combustion of one of the component gases occurs. The
more heat losses that occur the less likely this becomes, plus
volatile components condense out of the gas.

What generally occurs is that the "flare" of the secondary gases is
maintained by the flame "holding" on the burner, this means that there
is always a "spark" to keep the flame propagated. It is not at all
unusual to have the secondary flame blow out and give copious clouds
of whitish yellow noxious fumes, at which point a match or lighter
reignites the secondary burn.

In a downdraft gasifier the design is aimed at cracking or oxidising
all larger molecules and then passing the product (steam and CO2)
through a deep bed of very hot char to react them to syngas (CO+H2)
this can then be cooled and piped to further processes. With IDD
stoves the necessary temperature are not met so the offgas consists of
a lot of volatile pyrolysis chemicals.
AJH


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