United States
Environmental Protection
Agency
Industrial Environmental
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S7-84-082 Sept. 1984
v°/ERA Project Summary
Analysis of Volatile Products
from the Slow Pyrolysis of Coal
R.M. Felder and F. D. Gilman
The evolution of volatile matter was
studied for a subbituminous coal
pyrolyzed in a bench-scale fixed-bed
reactor, and for both a lignite and
subbituminous coal pyrolyzed in a
bench-scale fluidized-bed reactor. The
pyrolyses were carried out under inert
gas atmospheres, at a pressure of 135
kPa (5 psig), temperatures ranging from
400 to 1000°C, and heating rates
ranging from 1.45 to 6.0°C/s. The
pyrolysis products—tar, water, char,
and gases—were separated and anal-
yzed. Primary gas, aliphatic hydro-
carbon, aromatic hydrocarbon, and sul-
fur gas species were quantified by gas
chromatography. The effects of equili-
brium temperature, heating rate, coal
rank, and reactor design on asymptotic
weight loss, elemental volatilization,
gas species production and product
composition, and tar/gas production
ratios were examined.
Plots of asymptotic (long-time) weight
loss vs. temperature for the slow
pyrolysis (<45°C/s) of the coals pyro-
lyzed in this study and three other coals
ranging in rank from lignite to bitumin-
ous fall very close to a single curve
over a temperature range of 400-
1000°C. Similar plots based on pub-
lished data for the rapid pyrolysis (103-
105 °C/s) of three coals covering the
same range in rank also fall close to a
single curve, which lies below the slow
pyrolysis curve and approaches it at
temperatures of about 900°C.
Equilibrium weight loss and release of
elemental oxygen, carbon, and hydro-
gen increased with temperature. In the
fixed-bed pyrolysis of subbituminous
coal, the volatilization of sulfur and
yields of hydrogen, carbon oxides,
methane, benzene, thiophene, and
carbon disulf ide increased with temper-
ature, while yields of toluene, xylene,
C2-C4 aliphatic hydrocarbons, hydro-
gen sulfide, carbonyl sulfide, and
methyl mercaptan exhibited maxima at
temperatures between 700 and 900°C.
In the fluidized-bed pyrolysis of both
lignite and subbituminous coal, yields
of hydrogen, carbon monoxide, meth-
ane, ethylene, benzene, toluene, xylene,
and thiophene increased with tempera-
ture, while volatilization of sulfur and
yields of carbon dioxide, pyrolytic
water, ethane, C3-C4 aliphatic hydro-
carbons, hydrogen sulfide, carbonyl
sulfide, and methyl and ethyl mercap-
tans were maximized at temperatures
between 700 and 900°C. The data on
sulfur release suggest that the devola-
tilization of this element is affected at
high temperatures by decreases in
internal surface area and porosity of the
coal particles.
Carbon disulfide was produced from
the pyrolysis of subbituminous coal in
both the fluidized- and fixed-bed reac-
tors at temperatures above 800°C. It
was not produced in the fluidized-bed
pyrolysis of lignite. Conversely, ethyl
mercaptan was produced in the fluid-
ized-bed pyrolysis of lignite, but did not
appear in the products of either the
fixed- or fluidized-bed pyrolysis of
subbituminous coal. Yields of ethyl
mercaptan were lower than yields of
methyl mercaptan.
For all systems studied, tar-to-gas
production ratios dropped with increas-
ing temperature. For slow pyrolysis
over the temperature range of 400 to
900°C, tar production and tar-to-gas
ratios were lower and weight losses
greater than for rapid pyrolysis. As
noted above, coal rank had no signifi-
cant effect on asymptotic devolatiliza-
tion. Tar yield increased and carbon
-------
monoxide, carbon dioxide, and pyrolyt-
tic water yields decreased with increas-
ing coal rank. The yields of the individual
aliphatic hydrocarbons in the fluidized
bed were greater for subbituminous
coal than for lignite.
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 docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The principal obstacle to using coal as
an energy source is environmental. Large
quantities of potentially hazardous
organic and inorganic species are formed
during coal conversion, and must be cap-
tured and removed somewhere between
the point of formation and the point of
of release of effluents to the environ-
ment. If coal conversion is to be designed
to be environmentally sound, the chemi-
stry—thermodynamics and kinetics—of
the formation of pollutants must be
understood.
The first step in any coal conversion
process—combustion, gasification, or
liquefaction—is pyrolysis. The coal fed to
the process is brought to a high tempera-
ture, leading to the formation and release
of volatile species, including hydrocarbon,
sulfur, and nitrogen gases, tars, and
volatile trace metals. While much atten-
tion has been given in the literature to the
principal reactions of pyrolysis, little has
been done regarding the formation of
minor but potentially hazardous species.
In the present study, several coals were
pyrolyzed in laboratory-scale fixed- and
fluidized-bed reactors, and the volatile
products were quantitatively analyzed.
The extents of total devolatilization,
elemental release, and individual product
species formation were correlated with
the pyrolysis temperature, the heating
rate, and the rank of the feed coal. This
report describes the experimental sys-
tems used, gives the results obtained,
and discusses the implications with respect
to coal conversion processes. Information
and results beyond those given in the re-
port are available.
Experimental
Research was performed using bench-
scale, batch fixed- and fluidized-bed
reactors. The coal feed to both reactors
was set at 5 g, which provided adequate
samples of the pyrolysis products for
analysis. The fluidized-bed reactor was
found to provide more consistent and
reproducible results, and was therefore
used for most of the study.
The fluidized-bed reactor consists of a
stainless steel tube, 5/8-in. (1.59 cm)
I.D., with top and bottom stainless steel
frits. A Lindberg Model 55035 furnace is
used as a preheater, heating the sweep
gas from room temperature to 300°C. The
reactor is contained in a Lindberg Model
54032 single-zone tube furnace with a 1
ft (30.48 cm) long heating zone. The
reactor temperature can be varied
between 200 and 1200°C, with heating
rates ranging from 2.22°C/sec at 400°C
to 2.38°C/sec at 1000°C. Helium and
argon have been used as sweep gases.
Results have not shown any noticeable
dependence on which gas was used.
Water and tar in the reactor effluent are
captured in a packed ice-water-cooled
trap downstream from the reactor. The
tubing from the fluidized bed to the cold
trap is heat-traced to avoid condensation
prior to the trap. The noncondensing
volatiles are collected in an evacuated 1-
liter stainless steel bomb, or in a glass
bomb coated internally with hexamethyl-
disilizane to reduce the adsorption of
trace sulfur and hydrocarbon species.
Pyrolysis experiments were performed
on samples of 40x100 mesh New Mexico
Navajo Mine (NMNM) subbituminous
coal, and on samples of 40x100 mesh
Montana lignite. Ultimate and proximate
analyses of these coals are given in
Tables 1 -4.
Two replications were performed per
temperature per reactor. The tar, water,
gases, and char were physically sepa-
rated, collected, and analyzed. The con-
densed species were analyzed for carbon
and hydrogen using a standard ASTM
method, and for sulfur using a Fisher
Model 470 sulfur analyzer. The gases were
subjected to chromatographic analysis
using thermal conductivity detection,
flame ionization detection, or flame
photometric detection, depending on the
species analyzed. Complete descriptions
of the analysis are given in the report.
Table2. Proximate Analysis of As-Re-
ceived NMNM Coal*
Moisture
Volatile Matter
Fixed Carbon
Ash
9.6
33.3
37.7
19.4
"All units are Wt. %.
Pyrolytic Weight Loss
For both coals in both reactors, no
devolatilization occurred at temperatures
below 400°C, except for the moisture in
the feedstock. Between 400 and 700°C, the
major fraction of volatile material was
released. Increases in temperature above
700°C produced marginal increases in
asymptotic weight loss. A greater extent
of devolatilization was found in the
fluidized bed than in the fixed bed,
probably attributable to readsorption of
volatiles on the particles in the fixed bed.
Asymptotic weight loss data obtained
for slow pyrolysis in this study, and for
slow and rapid (103-105°C/s) pyrolysis for
several other published studies, are
plotted versus temperature in Figure 1.
The data for each heating regime show a
surprising independence of coal rank;
apparently, the fractional asymptotic
weight loss depends only on temperature
and whether the heating is slow or fast,
but not on the particular coal being
pyrolyzed. Moreover, the curves for the
two regimes are distinctly different:
apparent weight losses for fast pyrolysis
are lower than those for slow pyrolysis at
temperatures below 900°C; but, between
900 and 1000°C, apparent weight losses
are similar.
Linear regression was used to obtain a
polynomial fit of the slow pyrolysis data
shown in Figure 1, with the following
result:
Weight Loss (d.a.f) =
-49.0 + 0.20T - 1.066 x 10"4T2 (1)
where T = temperature in degrees
Celsius. The data for rapid pyrolysis
shown in Figure 1 were fitted by nonlinear
regression to obtain the following for-
mula:
Table 1. Ultimate Analysis of NMNM Subbituminous Goaf
As-Received Dry Basis
Dry Ash-Free Basis
Carbon
Hydrogen
Sulfur
Oxygen
Ash
Moisture
54.4
3.5
0.8
12.3
19.4
9.6
60.1
3.9
0.9
13.6
21.5
—
76.6
4.9
1.1
17.4
—
'All units are Wt. %.
-------
Table3. Ultimate Analysis of Partially Dried Montana Knife River Deposit Lignite"
As-Received Dry Basis Dry Ash-Free Basis
Carbon
Hydrogen
Sulfur
Oxygen
Ash
Moisture
57.1
2.7
0.6
18.7
11.0
9.9
63.3
3.0
0.6
20.9
12.2
---
72.1
3.4
0.7
23.8
—
---
Table 4. Proximate Analysis of Partially
Dried Montana Knife River De-
posit Lignite"
"All units are Wt.
Moisture
Volatile Matter
Fixed Carbon
Ash
99
41 0
38.1
11.0
"All units are Wt. %.
Weight Loss (d.a.f) = 165 exp(-000714T)
-846exp(-0.00310T) +
1350 exp(-0.00587T) (2)
Elemental Release
The fractional release of both oxygen
and hydrogen increased with tempera-
ture over the range 400-1000°C; 80% of
the elements were volatilized at 800°C,
and 90% were volatilized at 900°C. A
lower fractional volatilization of carbon
was found than was observed for either
hydrogen or oxygen. The maximum
volatilization of carbon, approximately
35% by weight of the feed carbon, was
observed at 1000°C for the subbitumin-
ous coal. At slow heating rates, a greater
fraction of the feed carbon was volatilized
from subbituminous coal than from
lignite. Comparison of the results with
literature data shows that rapid pyrolysis
produced lower volatilization of carbon
than did slow pyrolysis for similar lignite
feedstocks.
Plots of fractional sulfur volatilization
in the fluidized-bed pyrolysis of lignite
and subbituminous coal exhibit maxima
in the temperature range 600-800°C. This
result is consistent with an earlier
observation that the sulfur content of
coke reaches a minimum around 700 to
800°C. The production of hydrogen
sulfide, the major volatile sulfur species,
recommences only around 1100°C.
Several factors might account for the
observed maxima in the sulfur volatiliza-
tion curves for the fluidized bed. At
temperatures above about 750°C, inter-
nal sintering of coal begins to take place,
leading to a lowered porosity and a
decreased ability of the hydrogen sulfide
evolved to escape. At the same time, the
evolved hydrogen sulfide reacts to an
increasing extent with such mineral
constituents of coal as calcium, iron, and
magnesium oxides and carbonates,
forming sulfides and thereby retaining
the sulfur in the char.
In the pyrolysis of subbituminous coal
in the fixed-bed reactor, the asymptotic
sulfur volatilization did not exhibit a
70.
60-
50-
8
40-
.
30'
20.
JO
Montana Lignite A
Montana Lignite B
Montana Lignite C
New Mexico Subbituminous
Montana Rosebud Subbituminous
Pittsburgh Bituminous A
Pittsburgh Bituminous B
Slow
O
V
V
Rapid
e
Ci
400
500
600
l
700
800
900
1000
1100
Temperature, °C
Figure 1. Fractional devolatilization in the slow and rapid pyrolysis of several coals.
-------
maximum, but increased monotonically
with temperature, approaching a release
of 50% above 900°C. The same behavior
was observed earlier for the crucible
pyrolysis of lignite and bituminous coals.
Further experiments to clarify the differ-
ences in behavior between the fixed- and
fluidized-bed reactors are currently under
way.
Production of Tar and Water
The yields of tar and the principal
pyrolysis gas species are shown in Figure
2. For clarity of presentation, data points
are omitted in this and subsequent
figures. They are in plots in the report.
As Figure 2 shows, tar was the major
volatile species formed in the pyrolysis of
the subbituminous coal, with production
reaching a maximum at 600-700°C. Sig-
nificantly less tar was evolved from the
lignite. Above 700°C, the drop in tar
production due to the cracking of the tar
resulted in an increase in the gas fraction
of the volatiles. The relatively low tar yield
from the lignite, taken in conjunction with
the relatively high oxygen content of this
coal, supports an earlier observation that
the oxygen content of the feedstock sup-
presses tar yields by a phenolic conden-
sation mechanism.
The production of water from the
pyrolysis of lignite in the fluidized bed
was found to be greater than that from
subbituminous coal. A similar result was
obtained earlier in the rapid pyrolysis of
lignite and bituminous coal. The produc-
tion of pyrolytic water for the fluidized-
bed pyrolysis of lignite decreased at
temperatures above 800°C, paralleling
an earlier result for the rapid pyrolysis of a
lignite.
Overall mass, oxygen, and hydrogen
balances for fluidized-bed pyrolyis of
lignite support the observation that
volatilized oxygen shifts from pyrolytic
water to carbon monoxide as the tem-
perature increases, reflecting the in-
creasing rate of the steam/char reaction
yielding carbon monoxide and hydrogen.
Pyrolytic Gases
Plots of asymptotic yields of major
pyrolytic gas species vs. temperature for
the fluidized-bed pyrolysis of subbitumi-
nous coal and lignite are given in Figure 2.
For both coals, asymptotic yields of
hydrogen, carbon monoxide, and methane
increased with temperature. Carbon
dioxide was the principal gaseous pro-
duct at most temperatures studied. The
carbon dioxide production increased with
temperature up to 900°C,and then
leveled off for subbituminous coal and
decreased for lignite. Carbon monoxide
was the most abundant gas species
above 900°C.
Comparing carbon oxide yields from
the fluidized-bed pyrolysis of the two
feedstocks shows that greater yields of
carbon monoxide and carbon dioxide were
obtained from lignite than from subbitu-
minous coal. Earlier, similar results were
reported for rapid pyrolysis. Heating rate
apparently has little effect on carbon
monoxide formation, while more carbon
dioxide is obtained in slow pyrolysis.
Yields of aliphatic hydrocarbons are
shown in Figure 3. Methane was the
predominant aliphatic hydrocarbon pro-
duced from the fluidized-bed pyrolysis of
subbituminous coal and lignite. The
production of methane in the fluidized
bed increased with temperature, with
greater yields from subbituminous coal
than from lignite. The yields of methane
8
•£
>•
New Mexico Subbituminous
400 500 600 700 800 900 WOO 1100
Temperature, °C
Figure 2. Yields of tar and gases from the pyrolysis of two coals.
4
500
600 700 800 900
Temperature, °C
1000 1100
-------
from slow pyrolysis were greater than
reported yields from fast pyrolysis.
The results shown in Figure 3 indicate
that the asymptotic yields of aliphatic
hydrocarbon species were greater for
subbituminous coal than for lignite, and
the yield of each alkene was greater than
that of the corresponding alkane for the
same coal. Ethylene yields increased with
temperature for fluidized-bed pyrolysis of
subbituminous coal, but decreased for
lignite at temperatures above 900°C.
Yields of all aliphatics larger than
ethylene were maximized between 700
and 800°C.
At temperatures below 700°C, yields of
benzene, toluene, and xylene were
similar for both subbituminous coal and
lignite feedstocks. (See Figure 4.) The
production of benzene increased drama-
tically at temperatures above 700°C. For
both feedstocks, the yields of toluene and
xylene at temperatures above 700°C
showed marginal increases relative to
the benzene yields. At 1000°C the yield of
benzene was about 10 times greater than
that of xylene and 3 times greater than
that of toluene.
Sulfur gas species found in the
pyrolysis products include hydrogen
sulfide, carbonyl sulfide, methyl and ethyl
mercaptans, and carbon disulfide. Yields
of these species are shown in Figures 5
and 6. Hydrogen sulfide was by far the
predominant sulfur species formed. The
decrease in the hydrogen sulfide yield for
the slow pyrolysis of both coals at
temperatures above 800°C is consistent
with an earlier view that the volatilization
of sulfur at these temperatures is limited
by mass transfer.
Yields of carbonyl sulfide were two to
three times greater for the subbitumin-
ous coal than for the lignite. At tempera-
tures above 700°C, further carbonyl
sulfide production was marginal. Yields
of thiophene were similar for both
feedstocks. Yields of methyl mercaptan
for both coals showed maxima at about
700°C.
Carbon disulfide was produced from
the pyrolysis of subbituminous coal in
both the fluidized- and fixed-bed reactors
at temperatures above 800°C. It was not
produced in the fluidized-bed pyrolysis of
lignite. Conversely, ethyl mercaptan was
produced in the fluidized-bed pyrolysis of
lignite, but did not appear in the products
of either the fixed- or fluidized-bed
pyrolysis of subbituminous coal.
s*
i^
II
S
New Mexico Subbituminous
* *
Is
400 500 600 7OO BOO 900 10OO JJOO
Temperature, °C
Figure 3. Yields of aliphatic hydrocarbons from the pyrolysis of two coals.
500
600 7OO 800 900 1000 1100
Temperature, °C
-------
0.25
0.20-
!5 0.15.
*
0.10-
0.05.
0.00
New Mexico Subbituminous
0.25
0.20
0.15
? 3 0.10
0.05
0.00
400 500 600 700 800 900 1000 1100
Temperature, °C
400 500 600 700 800 900 1000 1100
Temperature, °C
Figure 4. Yields of aromatic hydrocarbons from the pyrolysis of two coals.
-------
0.20
New Mexico Subbituminous
0.040'
0.030'
8
35
.01 IB
8-a 0.020-
0.010'
0.00 , . .
400 500 600 700 800 900 1000 1100
Temperature, °C
Figure 5. Yields of sulfur gases from the pyrolysis of two coals.
0.000
New Mexico Subbituminous
Thiophene
400 500 600 700 800 900 1000 1100
Temperature, °C
R. M. Felder and F. D. Oilman are with North Carolina State University,
Department of Chemical Engineering, Raleigh, NC 27650.
N. Dean Smith is the EPA Project Officer (see below).
The complete report, entitled "Analysis of Volatile Products from the Slow
Pyrolysis of Coal," (Order No. PB 84 -230 036; Cost: $11.50. 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
•&U. S. GOVERNMENT PRINTING OFFICE: 1984/759-102/10693
-------
0.040
0.030 -
8
g-xj
<3 $ 0.020
0.010'
0.000
400
500
Figure 6.
600 700 800
Temperature. °C
Yields of sulfur gases from the pyrolysis of lignite.
900
t
1OOO
1100
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Environmental Protection
Agency
Center for Environmental Research
Information
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Penalty for Private Use $300
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