[Stoves] Some Basic Properties of ND-TLUD Gasification -- pdf

[Julien Winter]

Hello all;

I have attached a handout that I prepared for a recent demonstration of
natural-draft, top-lit updraft gasifiers.  I think you may find it
interesting.  The content of Figure 1. I have shared with this list before
in some form or another.

Figure 2., however is new.  It shows the linear relationship between the
yield of char as a % of dry fuel, and the maximum temperature measured in
the TLUD reactor.  The purpose of the regression is to estimate what the
maximum temperature could have been, if you don't have a thermocouple, but
you do have an inexpensive banance.

The explanation for the linear relation is based on the how more biomass is
pyrolyzed to volatiles rather than char, as the rate of heating and maximum
pyrolysis temperature increases.  While this phenomenon has been know for a
long time (e.g. the Broido-Shafizadeh model), the chemical mechanism for
partitioning between volatiles and char still remains obscure.

However, I have two linear relations for the fuels that I used.  The fuels
are distinguished by their size, or the size of the pore space between
particles.  I think we have to remember that when thermocouples are
inserted into a fuel bed, they measure the radiant, convective, and
conducted heat in the pore space and not the actual temperature within the
particles, where pyrolysis occurs.  With large particles of cm in
thickness, the center of the particle can be at ambient 25°C, until heat
from the outside arrives.  A pyrolytic front enters the particle from the
outside as it heats up, so the major loss of volatiles will occur at a
lower temperature than outside the particle in the pore space. When the
pores are large, as between sticks, pore space is more concentrated in the
fuel bed.  With larger pores, there is more space for flaming, and the
thickness of the pyrolytic front increases with particle thickness (because
it takes longer for pyrolysis to reach the interior of larger particles
than smaller ones).  So, if flaming is less evenly distributed in fuel beds
of large particles, the pore space temperature should be higher.  Following
from the above, the reason why there are two regression lines could be
largely due to differences in pore space temperatures, and not so much
pyrolysis temperatures inside the particles.

Fuel moisture content didn't have a substantive effect on the Tmax vs.
%Char relationship.  I did not see a large effect of moisture on the
maximum temperature of flaming pyrolysis, and read a paper where there were
similar finding. ... but I other studies to read.

Cheers,
Julien.

-- 
Julien Winter
Cobourg, ON, CANADA
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