----- Original Message -----
Sent: Monday, February 11, 2002 7:02
AM
Subject: GAS-L: Pieces of Dolomite
Puzzle
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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