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Dear Gasification Group:
The mineral "dolomite" (nominally CaMg(CO3)2 is
widely acclaimed in Europe as a tar cracking catalyst (Ekstrom, D'Eglise, Black,
...) . Knowing this, about 1985 we tested some U. S. dolomite, some
olivine, a Si-Al catalyst from Davison Chemicals and a Silicalite (molecular
sieve) catalyst from Union Carbide for tar cracking ability (reported in
Fundamentals, Development and Scaleup of the Air-Oxygen Stratified Downdraft
Gasifier (Reed, Graboski, Levie, available BEF Press).
We were puzzled to find that the Si-Al catalyst and
Silicalite catalysts were significantly better than the dolomite and only
olivene was less effective ( probably inert, cracking due to high temperature
only).
With this background and interest, I have kept my eyes and
ears open for any reports on dolomite cracking catalysts. I continue to
hear that dolomite works very well in Europe, and not in the U.S. A
Puzzle! Can European dolomite be different from U.S.? Are the tests
sufficiently different to give these different results?
~~~~~~~~
Last Friday, at the Colorado School of Mines Faculty club I
met a geologist, John Humphrey, who knew more about dolomite in 5 minutes than
the dozens of users I have queried over 17 years since then.
The name "Dolomite" covers a wide variety of minerals,
depending on origin and age. The Ca/Mg ratio is ideally 1, but varies all
over the map. Older dolomites tend to leach excess Ca and move toward 1, but
many other dolomites will have larger excesses of Ca over Mg, seldom is Mg
larger than Ca.
~~~~~
CaCO3 loses CO2 and becomes CaO at a temperature of (825C
(Merck), 900 C (CRH)) depending on the CO2 pressure existing in the surrounding
gases, a classical equilibrium problem. John and I believe that the MgCO3
is more stable and probably various dolomites are more stable than
limestone.
When dolomite is used as a cracking catalyst in a fluidized
bed there are two important properties to consider. A low
decomposition temperature will cause
conversion to CaO and MgO which are very weak minerals and the
particles can break up.
However, the possibility of producing "labile" CO2 may also
aid catalytic activity and will certainly promote removal of sulfur from the
gas.
So, a wide range of decomposition temperatures and pressures
from a range of uncharacterized "dolomite" used in tar cracking experiments
could certainly cause a wide range of positive and negative results which is
what we observe.
Sounds like a great PhD topic for some student.
~~~~~
I hope I have quoted John correctly and welcome his and all
other comments on this.
Yours truly,
TOM REED The
Biomass Energy Foundation
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