[Stoves] Increasing ND-TLUD riser height accelerates gasification and increases bed temperature

[Julien Winter]

Dear Crispin et al;

It would be nice if I had some good gear for my experiments, but what I
used was sufficient for my purpose.  I need to get some very basic
performance characteristics for a natural draft, top-lit updraft gasifier
(ND-TLUD).

The take-home message from my graphs is just how sensitive the reaction is
to small alterations of primary air at low rates.  Anybody designing a
ND-TLUD will see that the knob that adjusts primary air should move a
manageable distance to regulate primary air at low to nil rates.  As for
people who want to make biochar, or use the reactor as manufacturing
furnace, for the first time there is publically available observations on
what temperatures to expect when gasifying softwood pellets.

What I used for this research is (1) a scale to measure fuel and char, (2)
an overhead scale to measure mass loss during gasification, (3) a four
channel thermocouple data logger, and (4) an oven I can hold to 105°C to
measure fuel moisture content.  For unfunded, recreational research, that
is not bad.

What would have been the biggest improvement for my experiment would be to
set-up a more sophisticated method of feeding primary air, and measuring
the primary air superficial velocity (SV).  Expressing my results in terms
of grate aperture is very useful for TLUD design, but SV would have allowed
me to  compare directly my results to published papers on FD-TLUDs.  If I
had SV, for the first time, we would have known exactly were ND-TLUDs lie
on the range of primary air flow rates for top lit gasifiers and
combustors.  For research on gas burners, the flow rate of secondary air
and monitoring emissions are important.  Gear like that needs a full-time
laboratory.

As for trying to explain my observations, just about all explanations for
ND-TLUDs are speculative, because ND-TLUD SCIENCE DOES NOT EXIST.  There
are at least 50 peer-reviewed papers on 'FD-TLUDs' dating back to 1947.  No
all of them are appropriate to cookstoves, but a lot of them are.  For
ND-TLUDs there is nothing.

For the moment, that means that my graph showing the regression of
temperature on specific gasification rate is there mostly for "gee-wiz,
that is a high R²."  If someone was burning softwood pellets, and knew
their gasification rate, they could estimate the temperature.   If this
kind of regression is common across fuels, then it will beg an
explanation.

For now what we need are some very basic empirical studies on ND-TLUD
performance, so that builders of stoves can make more deliberate decisions,
so that rural extension workers can have confidence that their advise is
well-grounded, and so that both can respond effectively to unexpected
situations.

We need to start building ND-TLUD science at a very basic empirical level.
Nobody has actually measured primary and secondary air flow, so nobody
really knows the their optimum proportions for low emissions and how that
changes with gasification temperature, types of burner and fuel.  Nobody
has actually proven that preheating secondary air is effective; it is just
assumed to work, even though we are taking heat from the reactor to create
more viscous and less dense air.  If preheating makes no difference, then
it becomes easier to design the gas burner and the gasification reactor as
separate modules, that are mixed and matched according to need.  Nobody has
shown that TLUD biochar has safe levels of polycyclic aromatic hydrocarbon
and dioxins.  All this, and more, needs clarification in replicated,
controlled experiments ... and open access publication.

Cheers,
Julien.
-- 
Julien Winter
Cobourg, ON, CANADA
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