Hi Tom,
This reads like a trick question, as you obviously have something specific in
mind.
To what extent can you clean today's gas by passing it (cooled to about 150 C)
through a barrel of tomorrows REASONABLY DRY fuel, thus permitting you to burn
all the tars AND finish drying the fuel?
Firstly, I wouldn't do it, as once gas is made, I don't like to bring it
anywhere near filter material that can continue to discharge humidity into the
gas, just to dry the next days fuel. Yes the fuel might collect some of the
tars, but how to keep the operational temperatures of the filter mass stable,
can be severely affected by environmental and gas output conditions. Gas
humidity also carries sub-micron particulates, plus being inherently acetic in
the presence of tar, the condensate ingested into the engine will shorten the
oil and engine life.
Of the gasifiers I have seen using incoming fuel as the filter for dirty tar
laden gas:
1. Thailand using rice husk in large tanks, needing to be dug out by hand.
2. Wood blocks used to catch tars in the first Biomass Engineering gasifier
they inherited as a project in N.Ireland as a first stage filter, but quickly
replaced as their own engineering expertise recognised it's short comings for
later projects.
3.Then one you already know, wood chips are used as a filter for the FEMA
emergency gasifier.
4. Fluidynes own early experimental filtration systems used wood shavings and
wood blocks, very nasty to clean out . Good reason to learn how to make the gas
tar free ):-)
-----------
I don't recommend anything that filters tar, which in any way might bring the
operators into handling and breathing vapours from tar contaminated wood.
While a moisture content less than 20% is desirable for gasifier fuel, I
suspect it should not be bone dry for long runs because it becomes
autopyrolytic and won't retain a level pyrolysis zone.
I will show my ignorance as never seen written or used the term
"autopyrolytic". All I can say is, that what ever it means to you, I have never
seen any evidence that oven dried fuel is detrimental to gas making in the way
that I understand in downdraft gasification. The fact that you refer to a level
pyrolisis zone, suggests that you are linking into either stoves or open core
gasifiers as the ones quoted affected by this terminology.
The only thing that affects the zones in packed bed downdraft gasifiers with
fixed air nozzles using appropriately dried fuel, is excessive gas output rate,
which can see the pyrolysis zone shallow out as more endothemic heat is
consumed in reduction, and less radiant exothermic heat/time is available to
the incoming fuel for pyrolysis. Even using oven dry, roughly 20% of the fuel
weight is bound moisture, and H2 levels remain fairly constant, but less or no
condensate results.
Hope this may be of assistance, and look forward to any other contributions on
the subject as you describe.
Doug Williams.
Fluidyne.
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