[Stoves] Retted Switchgrass Fireball Test

['AJH']

On Sat, 2 Sep 2006 12:58:40 -0700, Tom Miles wrote:

>We find that producer gas flames can
>be very unstable below 4% O2. 

One of the things about the offgas from low temperature pyrolysis,
such as a smouldering fireball at, say, 300C is that the fire will not
get into the regions where the smaller molecules are favoured, so the
char will burn directly to CO2 and the offgas will be an aerosol of
the early pyrolysis products, tars and pics, in the effluent of N2 and
CO2. I imagine this mix will have very low flame speed, so the flame
will not attach and hold even if the air:fuel ratio were suitable.

Once you poke the combustion chamber temperature up to above 750C then
you probably start generating true fuel gas species, CH4, CO and H2.
We know CH4 is very particular about air:fuel ratios but it does have
good flame speed. CO and H2 are far more tolerant of air ratio, so I
surmise a hotter primary zone will be more likely to produce gases
that can sustain a secondary burn whilst still below their
autoignition temperature.

In fact you 4% Oxygen figure for stability looks like an a relatively
easy target to reach, I wonder what the top end would be?

It makes me think that to burn the fireballs Jeff had trouble with you
will need to up the temperature in the primary zone of the tlud, which
is going to mean increasing air supply and burning a bit of char. This
off course has major implications for the life of the metal parts.

AJH