S-EPA
United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-096 July 1981
Project Summary
Vapor-Phase Cracking and
Wet Oxidation as Potential
Pollutant Control Techniques
for Coal Gasification
W. J. McMichael, S. K. Gangwal, D. A. Green, and F. 0. Mixon
This research program was initiated
with the overall objective of
investigating the suitability of two
techniques for pollutant control in
coal gasification processes. These
techniques are hydrocracking of
heavy organics in the raw gas prior to
quench, and wet oxidation of the
gasifier condensate.
Experiments were carried out in
bench-scale equipment to determine
rates of hydrocracking and wet
oxidation as a function of process
conditions and the catalyst used. A
microreactor system, capable of
holding 1.0 cm3 of material to be
screened for catalytic activity, was
used for the hydrocracking studies.
Benzene and alkylated benzene
compounds were used as model
compounds in determining the
potential of various materials as
cracking catalysts. Thiophene was
used as a model compound for
catalyst poisoning studies. Cracking
rates were determined for materials
produced from bituminous and
subbituminous coals and materials
containing iron and nickel.
Experiments were carried out at 300°
to 800°C and approximately
atmospheric pressure. It was found
that the most promising material
screened was a triply-promoted iron-
oxide-based ammonia-synthesis
catalyst. It had the greatest activity at
temperatures of practical interest for a
control device and showed more
sulfur resistance than the other
materials screened. However, the
cataiyst was poisoned by sulfur, and
attempts to regenerate it by air
oxidation were not successful.
Wet oxidation experiments were
carried out in a 1-liter autoclave
system using gasifier quench water
and a 10,000 ppm phenol solution at
150° to 200°C. In several of the
experiments, gasifier char was added
to the autoclave system. It was found
that no measurable oxidation took
place at 150°C. However.substantial
oxidation occurred at 200°C. Coal
char was found to be approximately
double the rate of wet oxidation in the
phenol solution, but had no apparent
effect on the rate of oxidation of the
components in the gasifier quench
water. The rates of oxidation
measured in the experiments were
found to be in the range which would
make wet oxidation of gasifier
condensate competitive with other
control techniques.
This Project Summary was
developed by EPA's Industrial
Environmental Research Laboratory.
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
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Introduction
In coal gasification systems where
the blast-coal contacting is counter-
current in nature, high loadings of
undesirable, heavy organic compounds
in the raw gas from the gasifier are
common Typically the raw gas is
quenched, giving rise to an aqueous
condensate stream which is
characterized by a heavy organic
loading and, in some instances, may
contain emulsified tars and oils
Treatment of the aqueous condensate
for reuse in the gasification process
presents expensive and formidable
problems to the widespread
commercialization of coal gasification
processes For example, the cost of the
equipment necessary for treatment of
the condensate in a Lurgi high-Btu
gasification plant represents a
mini mum of 8 percent of the total capital
investment.
The processing steps used in
condensate treatment include solvent
extraction, biological oxidation, and
adsorption. However, temperature and
pressure conditions of the raw gas and
condensate make two other processes
look technically attractive for control of
undesirable components These
processes are (1) vapor-phase cracking
of undesirable components prior to
quenching the raw gas, and (2) wet
oxidation of the condensate.
Vapor-phase cracking could be
carried out by placing a catalytic reactor
between the gasifier and the quench
system. However, an appropriate
catalyst must be found that can
withstand the severe conditions and
poisons to which the catalyst would be
subjected. Also, to be economically
feasible, the cracking would have to
take place at temperatures close to the
outlet gas temperatures of the gasifier,
or about 400° to 500°C A successful
vapor-phase cracking process would
have advantages over the traditional
quench-then-treat approach since
eliminating heavy organics from the
raw gas at high temperature would.
reduce operational problems
downstream of the gasifier, permit the
recovery of high quality waste heat, and
reduce the cost of condensate cleanup
for reuse in the gasification process.
The other approach, wet oxidation, is
a process in which dissolved or
suspended organic material is oxidized
in the liquid phase at an elevated
temperature and partial pressure of
oxygen. It is currently used as an
industrial waste treatment method in a
number of installations around the
world. In gasification processes which
utilize a shift converter, the desirable
level of steam in the feed to the shift
converter is on the order of 40 percent.
In order to maintain this level, the
quench system must be operated at
210° to 230°C and 600 to 1000 psig
These are ideal conditions at which to
feed oxygen into the condensate and
carry out wet oxidation of the organic
material.
To investigate these control
techniques, the Research Triangle
Institute (RTI) has been carrying out an
experimental program Cracking studies
were performed m small bench-scale
equipment (microreactors). Based on
previous studies in the literature, model
aromatic and sulfur-containing
compounds were selected and cracked
over coal-derived materials, various
metals, and commercial catalysts. In the
wet oxidation experiments, phenol in
water solutions and coal gasifier
quench waters were oxidized in the
presence of coal-derived materials. This
report presents the results of these
studies
Conclusions
The results of the experimental
studies on the cracking of selected
aromatic compounds over coal-derived
materials showthatthese materials can
increase the rate of cracking of the
aromatics by three orders of magnitude
over the homogeneous vapor-phase
rate. Gasification char produced from a
subbitummous coal showed a greater
enhancement of cracking rates than
chars produced from bituminous coals
Several iron- and nickel-containing
materials were evaluated for activity
toward enhancing hydrocracking of
benzene (the most stable ring
compound). It was found that an iron-
oxide-based ammonia-synthesis
catalyst completely hydrocracked
benzene to methane at temperatures
near 450°C, which is a temperature of
practical interest for the Lurgi gasifier.
However, the iron catalyst was found to
be poisoned by sulfur compounds
Based on work in the literature on hot
gas cleanup (sulfur removal) with iron
oxide, regeneration of the poisoned
iron-oxide catalyst may be possible.
However, attempts made at RTI to
regenerate the catalyst were
unsuccessful.
The results from the experimental
studies on wet oxidation of phenol-
water solutions and gasifier quench
waters showed that practical oxidation
rates were obtainable. The presence oi
coal char in the wet oxidation system
appeared to approximately double the
rate of oxidation of the phenol, but had
no effect on the rate of oxidation of the
organic constituents in the gasifier
quench water
Based on the experimental studies,
both vapor-phase cracking of heavy
organics and wet oxidation of quench
water appeared to have technical
promise as control technologies in coal
gasification processes. However, more
information is needed in order to fully
evaluate the technical and economical
potential of the two control techniques
Recommendations
Preliminary experimental results
obtained on the cracking activity of an
iron-oxide catalyst show it to be a
promising hydrocracking catalyst in
applications of controlling heavy
organics in gasifier effluents. Catalysts
composed of iron oxide are known to be
regenerable from sulfur poisoning.
Further work needs to be done in order
to properly evaluate activation and
regeneration steps for the catalyst and
to investigate the effect of gas
composition, pressure, temperature,
and space velocity on the conversion of
heavy organics in the presence of iron-
oxide catalysts. Based on this
information, actual tests in a coal
gasification system should be planned
and carried out in order to demonstrate
the feasibility of cracking heavy tars in
coal gasifier product gases. A
preliminary economic ana lysis of vapor-
phase cracking of undesirable
components produced during coal
gasification should be performed to
establish the commercial potential of
the control technique.
The work performed on wet oxidation
of gasifier condensate and the use of
coal char as an oxidation catalyst was a
preliminary study and not planned to be
exhaustive. Future work is needed so
that a thorough technical and economic
evaluation can be made of wet oxidation
as a gasifier condensate and cleanup
process. Experiments should be
performed using (1) a wider and higher
temperature range, (2) a range of
oxygen partial pressures, (3) gasifier
condensates with a wider range of
CODs than used in this work, and (4) a
variety of catalysts, including coal char
or ash.
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I/I/. J. McMichael, S. J. Gangwal, D. A. Green, andF. 0. Mixon are with Research
Triangle Institute, P.O. Box 12194. Research Triangle Park, NC 27709.
N. Dean Smith is the EPA Project Officer (see below).
The complete report, entitled "Vapor-Phase Cracking and Wet Oxidation as
Potential Pollutant Control Techniques for Coal Gasification," (Order No.
PB 81-219 594; Cost: $8.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
* US. OOVERNMENT PRINTING OFFICE. 1«61 -757-OU/7Z65
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Environmental Protection Information Environmental
Agency Cincinnati OH 45268 Protection
Agency
EPA 335
Official Business
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