United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-81-098 Aug. 1981
Project Summary
Coal Gasifier Parameters
Influencing Environmental
Pollutant Production
D. A. Green, J. G. Cleland, D. P. Daugherty, W. J. McMichael, F. 0. Mixon,
and R. S. Truesdale
A series of fixed-bed coal gasification
and pyrolysis tests have been per-
formed in a laboratory-scale reactor of
6:6 cm inside diameter. Chemical
analyses were conducted on the pro-
duct gas, aqueous condensate, tar,
and solid residue from the tests. The
effects of process variables, such as
feed mode, catalytic treatment, pres-
sure, mesh size, and coal type, by-
product and pollutant yields are de-
scribed. The production of gaseous
sulfur compounds, benzene and de-
rivatives (BTX), phenolics (phenol,
cresols, and xylenols), and tar has
been measured, and the fate of trace
elements such as arsenic, selenium,
and lead has been determined. By-
product production associated with
the pyrolysis phase of gasification has
been investigated, with emphasis on
the effects of particle size, residence
time, and atmosphere.
Experiments with six U.S. coals
ranging from lignite to anthracite are
described. Continuous operation re-
sulted in greatly reduced tar and
phenolic production. When the entire
mass of coal was introduced at the
beginning of the test, effluent stream
compositions more closely approached
those of larger scale, fixed-bed gasi-
fiers. Initial work on this project was
reported in EPA-600/7-78-1 71
(NTIS PB287916); three more recent
reports were EPA-600/7-79-200
through EPA-600/7-79-202 (NTIS
PB80-182769, 80-104656, and 81-
114308).
This Project Summary was devel-
oped by EPA's Industrial Environ-
mental 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).
Introduction
Technology for the conversion of coal
to gaseous fuel has been available for
many years. Renewed interest in coal
gasification processes, motivated by
greatly increased petroleum prices and
anticipated shortages of oil and natural
gas, has stimulated considerable re-
search into process improvements.
Gaseous fuels are inherently easier to
transport and distribute than coal. With
increasing constraints on the discharge
of oxides of sulfur and nitrogen, gaseous
fuels may require less expensive pollu-
tion control systems than coal and oil.
Additionally, indirect liquefaction of
coal via catalytic reaction of coal-
derived gas to produce oil substitutes is
approaching economic feasibility in the
U.S.
The increasing likelihood of large:
scale commercial development of coal
gasification in the U.S. has generated
concern over the pollutants produced in
these processes. The Research Triangle
Institute has performed a 5-year study
directed toward quantifying the produc-
tion of by-products and pollutants from
coal gasification processes and investi-
gating the influence of processing
-------
conditions on these materials. The
project was sponsored by the Industrial
Environmental Research Laboratory of
the U.S. Environmental Protection
Agency at Research Triangle Park, NC.
The experiments were conducted in a
laboratory-scale coal gasification system
designed and constructed at RTI. The
reactor is a 100-cm length of nominal 3-
in. Schedule 160, type 310 stainless
steel pipe, fitted with ring joint flanges
at both ends. Aflowdistributor, a porous
ceramic disk mounted on a stainless
steel plenum chamber, is inserted into
the reactor to a height of approximately
30 cm from the bottom reactor flange.
The flow distributor supports the coal
bed. The reactor is enclosed in a vertical
tube furnace with three independently
controlled heating zones. Superheated
steam is generated by pumping deioniz-
ed water with a positive-displacement,
packed, plunger-type metering pump
through stainless steel tubing enclosed
in a series of three electrically heated
furnaces. Nitrogen, oxygen, or air from
standard compressed gas cylinders are
mixed with the superheated steam and
enter the reactor through a tube con-
necting the bottom flange and the flow
distributor.
Coal was added to the reactor from
the top. Usually coal from a pressurized
feed hopper was added batchwise
through a pneumatically operated ball
valve. In some cases a rotating-disk coal
feeder was used to continuously supply
coal to the reactor. Gases flowed out of
the top of the reactor through a tube in
the upper flange into a water-jacketed
condensate trap. A two-phase mixture
collected in this trap: a tar phase and an
aqueous condensate phase made up of
moisture from the coal and unconverted
reactant steam. The condensate trap
was emptied periodically through a
pressure lock arrangement using two
remotely operated ball valves. The gas
flowed from the condensate trap through
an adjustable back pressure regulator
where the pressure was reduced to
slightly above atmospheric. The gas
then entered a sampling manifold with
provisions for discrete sampling in glass
bulbs, resin adsorption, and an impinger.
Finally, the gas passed through a dry
test meter and was vented.
The reactor temperature was deter-
mined using a multipoint thermowell
containing up to 12 chromel-alumel
thermocouples. This thermowell was
inserted through the upper flange of the
reactor to a point slightly above the flow
distributor. The thermocouples initially
were connected to digital displays. For
the more recent tests, the thermocouples
were also connected to analog-to-
dig ita I converters, with readings auto-
matically stored on magnetic disks.
Additional thermocouples were used to
measure the temperature of the inlet
steam and condensate trap. The pres-
sure at the steam-air inlet and at the exit
of the condensate trap was measured
both by transducers and by bourdon
tube gauges. The transducer outputs
were displayed digitally and stored.
A variety of fuels have been gasified
in the laboratory system. This report
deals with experiments made with the
following fuels:
1. North Dakota Beulah/Zap lignite.
2. Montana Rosebud/McKay sub-
bituminous coal.
3. Wyoming Smith/Roland subbi-
tuminous coal.
4. Illinois No. 6 bituminous coal.
5. West Kentucky No. 9 bituminous
coal.
6. Pennsylvania Bottom Red Ash
anthracite coal.
Conclusions
Operation of the laboratory gasifier in
the continuous mode resulted in a
higher ratio of C02 to CO, and a greater
yield of benzene, toluene, and xylenes
(BTX) than operation in the semicontin-
uous mode. The continuous gasification
mode also generated less tar and less
phenolic material than the semicontin-
uous mode, and the tar that was pro-
duced in continuous testing was lower
in tar acids than that produced in
semicontinuous testing. The tar pro-
duced in semicontinuous testing was
quite similar to that produced in larger
scale, intermittently fed, fixed-bed
gasifiers in both composition and yield.
Gasification of Illinois No. 6 coal that
had been treated with alkali metal
compounds indicated that the catalytic
treatment did not increase reaction
rates, lower the reaction temperatures,
or decrease caking and swelling tend-
encies in the laboratory reactor. Treat-
ment with KzCOa reduced H2S produc-
tion and increased sulfur and mercury
retention in the solid residue but had no
effect on nitrogen retention. Treatment
with NaOH at a much lower level reduced
tar production but did not change the
elemental composition of the tar; treat-
ment with NaOH did not affect nitrogen
or sulfur retention but did increase the
retention of mercury in the solid residue.
When the particle size of the coal fed
to the gasifier was doubled, the product
gas was higher in CO relative to CO2. For
Illinois No. 6 coal, larger particles
produced less BTX and a tar that was
higher in acids. Doubling the particle
size of North Dakota lignite increased
the tar yield.
Lower pressure gasification produced
more CO relative to CO2 and tar with a
decreased polynuclear aromatic content.
With Illinois No 6 coal, lower pressure
operation resulted in a product gas
higher in CO relative to hydrogen and a
greatly increased tar yield.
The effect of rank on by-product
production is shown in Figure 1. Both
tar and BTX yields increased with rank
from lignite to bituminous. Anthracite
coal produced no tar and very little BTX.
Phenol, cresols, and xylenols (PCX)
production generally decreased with
increasing coal rank. Lignite and sub-
bituminous coal produced tar with a
higher proportion of acids and a lower
proportion of bases than the tar from
bituminous coals. Consistent with be-
havior in combustion processes, more
sulfur was retained in the solid residue
from lignite and subbituminous coals
than in the solid residue from bituminous
coal. Nitrogen retention in gasification
residue did not vary consistently with
rank.
In tests with Illinois No. 6 coal, arsenic
volatilization was strongly correlated
with carbon and sulfur conversion.
These relationships are shown in Figure
2. Selenium volatilization was signifi-
cantly correlated with carbon conver-
sion but not with sulfur conversion.
Mercury was completely volatilized in
all tests for which data is available
except for tests where the feed coal was
treated with alkali metal compounds;
treatment with NaOH did, however,
increase the volatility of beryllium.
Pyrolysis experiments in an inert
atmosphere showed that particle size
had no effect on BTX production or rate
of devolatilization, but did influence
drying time. When Wyoming subbitu-
minous coal was heated in a hydrogen/
CO atmosphere, initial H2S production
equaled that of pyrolysis in a nitrogen
atmosphere. Additional H2S was pro-
duced after pyrolysis was complete
when the surrounding atmosphere
contained hydrogen. At no stage of the
pyrolysis experiment was H2S in equi-
librium with carbonyl sulfide; it was
concluded that HzS evolved from the
-------
50
40
30
20
10
Tar yield (mg/g MAF feed)
(a^ mean of two tests
(c) mean of four tests
72"
25
23"
47'
33
zero
ND
MONT WYO
ILL KEN
PA
ouuu
4000
3000
2000
1000
0
Tola
lm
1 phenol, cresol, xylenof yield {ug
4100
(a)
1300
4200
(b)
/gMA
(a) n
(b) n
(d) e
2000
(a)
F
lea
tea
xcl
feed)
n of two tests
n of three tests
uding gaseous xylenols
2600
(d)
61
ND
MONT WYO
ILL
KEN
PA
15000
10000
5OOO
0
benzene, toluene, xylene yield (ug/g MAF feed)
(c) mean of four tests
.
m
5400 \5600
8800
(c)
V4OOI
13000 56
I
ND MONT WYO
Lignite Subbituminous
ILL KEN PA
Bituminous Anthracite
Figure 1.
Effect of rank on byproduct production from airblown, fixed bed, semi-
continuous gasification of 8x16 mesh coal at 200 psi.
-------
D. A. Green, J. G. Cleland, D. P. Daugherty, W. J. McMichael, F. O. Mixon. and
R. S. Truesdale 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 "Coal Gasifier Parameters Influencing Environ-
mental Pollutant Production," (Order No. PB 81-221 301; Cost: $9.50, 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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
5 LIBRARY
230 S DEARBORN STREET
CH1CAGU IL 60604
------- |