-------
91
accounted for 73% of the carbon in the pyridine extract, 35% of
the hydrogen was aromatic, the o aliphatic hydrogen accounted for
26% and B hydrogen accounted for 36% of the total hydrogen.
Hydroxyl hydrogen accounted for only 3% of the total hydrogen.
The values derived from the carbon disulfide extract were similar
although a higher percentage of aliphatic compounds seem to be
present in the carbon disulfide extract.
The average ring size of a high volatile bituminous coal
with 83% carbon content may be 5-8; the distribution of the
carbon may be about 70% aromatic, 30% aliphatic, with only 6% of
the carbon in methylene bridges; the distribution of the hydrogen
may be 20% aromatic, 75% aliphatic and about 5% in the phenolic
OH; the oxygen may be split equally between hydroxyl and carbonyl
oxygen with negligible amounts in carboxyl and ether groups.
-------
92
IV-3.0 PHYSICAL PROPERTIES OF COAL
Estimates of some of the physical properties of coal have
been used in this study. These values are not very accurate and
have been used only for crude calculations. The values of interest
are specific gravity, thermal conductivity, and specific heat. The
interpretation of these values for a substance such as coal that
is very heterogenous and changes both chemically and physically
when mildly heated is questionable. Even when no changes occur
in the coal the values are uncertain because the values strongly
depend on coal rank, mineral matter content, and moisture content.
Average values of the physical properties needed have been
taken from McCabe and Boley (1945), Badzioch et al^ (1964) and
Kirov (1965). Specific gravity will increase roughly 0.01 unit
for a percent increase in ash content. Thermal conductivity of
—4 -4
raw coals below 400°C varies from 5 x 10 to 8 x 10 cal/g-em-sec-K
Above about 300-400°C, where carbonization reactions begin, the
thermal conductivity rises rapidly until at 900°C (the maximum
temperature in Badzioch e_t al's study) it has a value of about
-4
60 x 10 cal/g-cm-sec-K. This high value in all probability repre-
sents conductive heat transfer augmented by radiative transfer
through pores and conveotive transfer by volatile products flowing
through cracks and pores. The latter two effects are difficult to
separate from the former experimentally. The thermal conductivity
of the particle is needed for purposes of calculating the rate of
temperature rise in the particle. Since the resistance to heat
transfer by conduction within the particle is negligible relative
-------
93
to the external resistance, uncertainty in the value of the thermal
conductivity is of little significance.
Measurements of specific heat will also be effected by the
heterogeneous nature of coal and by the physical and chemical
changes coal goes through as it is heated. The specific heat of
raw coal increases with increasing volatile matter and increasing
moisture content and decreases with increasing ash content. The
specific heat of raw coals with less than 10% volatile matter
(d.a.f.) will be between 0.2 and 0.25 cal/g-K, while coals with
volatile matter (d.a.f.) between 10 and 50% will have specific
heats between 0.25 and 0.35 cal/g-K. Coals with volatile matter in
the range of 40% (d.a.f.) will have specific heats around 0.3 cal/g--K.
Char residuals (based on weight of raw coal) could have a
maximum specific heat of 0.42 cal/g-K at 320°C for a char from a
coal with 50% d.a.f. volatile matter. The specific heat of chars
produced from coals with volatile matter (d.a.f.) greater than 30%
have a specific heat maximum in the region of rapid devolatilization,
The specific heat then decreases towards an asymptotic value at
temperatures above 1100°C. The position of the specific heat maxi-
mum is displaced towards higher temperatures as the volatile matter
of the coal decreases. Chars produced from coals with d.a.f. vola-
tile matter in the range of 30-40% would be expected to have a
specific heat value of about 0.33 cal/g-K based on the weight of
the raw coal.
-------
94
IV-4.0 CHARACTERIZATION OF OTHER COALS AND CHARS
Most experiments were run using the carefully characterized
lignite and bituminous coals described in Sections IV-1.0 and
IV-2.0. Several additional experiments were performed using a
Montana sub-bituminous coal. Char oxidation experiments utilizing
chars prepared from the Montana Lignite were also performed.
Proximate and ultimate analyses of these additional samples are
shown in Table IV-7.
-------
TABLE IV-7.CHARACTERIZATION OF COALS AND CHARS
^^••••M^MMiHflVMM^HHMBMnMI^^ManBI^MMHMH
Type*
Montana Lignite-A
Montana
Sub-bituminous**
Proximate Ultimate
Analysis Analysis
A.R. WtZ A.R. WtZ
Specification VM H,0 Aali C U M S 0
z (by Diff .)
•
Savage 36.20 13.60 7.80 54.40 4.96 0.88 0.84 17.42
Powder 35.16 21.23 9.34 53.26 3.35 0.87 0.78 11.17
River
Region
1250 K
Lignite Char
1750 K
Lignite Char
1750 K
Lignite Char***
MM* 4.20 .12.08 66.94 2.09 1.02 0.56 13.11
MM 2.02 18.03 76.42 0.69 "0.55 1.06 1.23
MM 2.10 19.00 76.11; 0.36 0.58 1.11 0.74
"MM - Mot Measured
**
Supplied by Aero therm Division of Acurex Corporation
***
"Used in time resolved measurements.
vo
in
-------
96
SECTION V
COAL RESULTS
This section reports the results obtained on evolution and
oxidation of nitrogen contained in coal using the equipment and
procedures described in Section III. The results described
include nitrogen loss from coal held in crucibles, nitrogen loss
from coal rapidly heated in the dispersed phase, and conversion
of coal and char nitrogen to nitric oxide when burned in a small
coal flame.
V-1.0 THE COAL FLAME- MOTIVATION FOR THE EXPERIMENTS
Coal supplies slightly over half of the fossil fuel burned
to generate electric power. Its use can be expected to increase
as the other fossil fuels become less abundant and more costly.
Most of the coal burned for power generation is burned in pul-
verized coal flames in large units (hundreds of megawatts
electrical capacity). Combustion of coal yields higher emissions
of sulfur dioxide, particulate matter, and nitrogen oxides than
other fuels. The data reported in this section were taken in an
effort to understand some of the complex aspects of the oxidation
and reduction of the nitrogen contained in the coal with the
ultimate goal of investigating strategies for control of the
fuel nitrogen contribution to total nitrogen oxide emissions.
A schematic of a pulverized coal flame is shown in Figure V-l
Coal is ground to less than 200 micron particles (most particles
-------
COAL DSVOLATILIZATION AND COMBUSTION
HEATING DEVOLATILIZATION VOLATILE BURNING HCTEROGENOUS COMBUSTION
SECONDARY AIR
CO,.H,0
PRIMARY AIR
COAL
COAL
PARTICLE
SECONDARY AIM
TIME SCALE 2MSEC
EXPERIMENTS INERT
DCVOLATILIZATION
DIFFUSION
S FLAME
100 MSEC
COAL OXIDATION
300 MSEC.
CHAR OXIDATION
FIGURE V-l. Schematic of a single coal particle burning in a pulverized
coal flame.
-------
98
are less than 100 microns), and fed through the primary injection
nozzle. A fraction of the total air (usually 15-20% of the
stoichiometric air requirements) is used to carry the coal into
the flame region. A large 500 mw utility boiler typically feeds
about 200 tons/hr coal and 3300 tons/hr air. If four burners
are used, each burner will be fed 50 tons/hr coal and about
50 tons/hr air through the primary nozzle. The remaining air is
preheated and fed through swirl vanes surrounding the primary
nozzle.
The coal particles exit the nozzle in a slightly fuel rich
atmosphere. The reducing nature of this atmosphere is increased
further by recirculation of partially combusted gases from the
flame region. The coal particles are heated, possibly as high
as 2200°K, by the hot recirculating gases in about one millisecond.
The coal particle rapidly loses volatile matter over the next tens
of milliseconds. This volatile matter, depending on the velocity
and uniformity of ejection, can exclude oxygen from the particle
surface and burn as a diffusion flame surrounding the particle.
The volatile matter may not be able to form an effective oxygen
shield in all cases, and the particle will simultaneously undergo
gas phase volatile matter burning and heterogeneous solid phase
combustion. Burning of the volatile matter is usually complete
in about 100 milliseconds.
Volatile burning will leave a char residue, even if the solid '
has been burning simultaneously with the volatile matter. The
char will be heterogeneously oxidized in about 300 milliseconds.
-------
99
Experiments that might be used to isolate the different
regions of coal combustion are listed at the bottom of Figure V-l.
Volatile matter removal can be investigated by devolatilizing coal
under conditions of reduced oxygen concentration which supress
ignition of the particle. Devolatilization of coal particles
under inert atmospheres will be reported in this section. Coal
oxidation under the condition when the coal particle is surrounded
by a diffusion flame of the volatile matter can be investigated
by burning the coal particle in a dilute phase so that particles
have little interaction with each other. Only overall oxidation
experiments where the coal particles burn as small diffusion
flames are reported in this section. Char oxidation kinetics can
be investigated by manufacturing the char in an atmosphere where
the coal particles will not ignite and then reinjecting the coal
particles into an oxidizing atmosphere where the particles burn
heterogeneously as individual particles. Some results from char
oxidation are reported in this section. The gas phase reactions
of the products produced from coal devolatilization and partial
combustion, descriptions of which are needed to complete the
picture of the oxidation of the nitrogen content in coal, are best
studied in simplified systems such as flat flame burners, and do
not constitute part of the present study.
-------
100
V-2.0 DEVOLATILIZATION RESULTS
Coal pyrolysis studies include asymptotic elemental and
total weight loss from crucible held samples as well as time-
resolved weight loss experiments conducted in the flow reactors
for residence times up to one second. Limited analyses of gas-
phase pyrolysis products are also presented in an attempt to close
the carbon and nitrogen material balances.
V-2.1 DEVOLATILIZATION RESULTS FROM CRUCIBLE EXPERIMENTS
The purpose of the crucible experiments is to investigate
the effect of temperature on the behavior of coal devolatilization
in inert atmospheres at long residence times. The nature of the
experiments precluded the attainment of the rapid heating con-
ditions typically found in pulverized coal flames. Heating rates
were typically on the order of several degrees Kelvin per second.
The data reported here are for times at which the coal weights
had reached their asymptotic values.
V-2.1.1 PITTSBURGH SEAM #8 hvA-BITUMINOUS COAL
Figure V-2 shows the retention of various elements in the
coal and ultimate weight loss for crucible held coal as the
temperature increases from 600 to 2200°K. The chars were analyzed
after times considered to represent asymptotic weight loss and
are represented on a dry basis. The weight loss for temperatures
above about 1200°K is asymptotic at about 40%, below 1200°K the
weight loss drops. The weight loss observed after 12 hours at
600°K is only about 10%. Carbon loss levels off at 1200°K, when
the weight loss is about 30%. Hydrogen is initially removed more
slowly than oxygen but after both oxygen and hydrogen contents of
-------
100 -
z
o
I-
z
UJ
»—
UJ
o:
I 1 1
HVA Bituminous Cool
ASTM Prox. Char
600 800
1000 120O 1400 1600 1800 2000
MAXIMUM TEMPERATURE ( *K )
FIGURE V-2. Crucible devolatilization of Pittsburgh Seam #8hv A-bituminous
ooal.
-------
102
the char have dropped to about 40% of their original values, the
rate of fractional loss of hydrogen and oxygen become
approximately equal. Oxygen, hydrogen, and nitrogen are com-
pletely removed from the char at high temperatures, under the
conditions of asymptotic weight loss. Nitrogen is much more
stable in the char than either hydrogen or oxygen.
V-2.1.2 MONTANA LIGNITE-A
Element loss from the lignite samples held in crucibles is
shown in Figure V-3. The retention trends of the various elements
are very similar to the trends observed for similar experiments
using the bituminous coal. The proximate analysis volatile matter,
dark symbols, agrees well with the ultimate weight loss obtained
by slowly heating coal samples. Carbon loss and total weight loss
increase only slightly above the proximate analysis temperature of
1223°K. The asymptotic carbon loss was about 20% although the
carbon loss appeared to increase slightly as the devolatilization
temperature increased above 1700°K. The char showed no additional
weight loss above about 1600°K. The asymptotic weight loss was
44% which is only slightly greater than the proximate volatile
matter of 41%. Little weight loss of carbon occurs at low temper-
atures .
Oxygen, hydrogen, and nitrogen are completely removed at
high temperatures. Hydrogen and oxygen loss as the percent of the
element originally present are very nearly the same. Initially
there is approximately five times as much hydrogen present as
oxygen on a mole basis. Hydrogen and oxygen contents are reduced
-------
—i 1 r
LIGNITE
ASTM PROX CHAR
100
-7 80
*
O 60
LJ
UJ
o:
20
T
D
1
D
a
I
1
600 800
1000 1200 1400 1600 1800 2000
MAXIMUM TEMPERATURE (*K)
FIGURE V-3. Oruoible devolatilization of Montana lignite-A.
o
LJ
-------
104
to low values at temperatures in the range of 1600°K and are both
reduced to zero above 1800°K.
Nitrogen is, again, the most refractory of the elements that
••
are completely removed from coal. The lignite coal used in this
study has approximately 28% of the original nitrogen removed
under proximate analysis conditions. This value agrees well with
the amount of nitrogen removed under the crucible tests of this
study.
-------
105
V-2.2 DEVOIATILIZATION OF DISPERSED COAL PARTICLES
Experiments were conducted on devolatilization of coal
falling freely through an inert gas surrounded by hot walls. The
purpose of this get of experiments was to obtain devolatilization
data for coal that is rapidly heated (^10 °K/sec) in a dispersed
phase for relatively long residence times, on the order of one
second. A slight modification of this experiment was used to
gather data on rapidly heated coal devolatilization at longer
residence times by dropping the coal into a crucible where
further devolatilization of the char occurs on a sample held at
the maximum temperature of the experiment.
V-2.2.1 PITTSBURGH SEAM »8 BITUMINOUS COAL
Data gathered on element loss from the bituminous coal under
a variety of experimental conditions, including free fall condi-
tions, is shown in Figure V-4. The free fall data has a residence
time of approximately one second and the free fall into a crucible
data has residence times varying from 3 x 10^ to 6 x 105 seconds.
The laminar flow experiments gave devolatilization residence time
from several milliseconds to a maximum of about 300 milliseconds.
The long residence time data is replotted from the crucible exper-
iments discussed in Section V-2.1.1.
The retention of the various elements are plotted in
Figure V-4 versus time in milliseconds with furnace temperature
as a parameter. A log time scale is used so that the data can be
represented on one plot. The retention of each element (weight
percent of the original element retained after devolatilization)
-------
106
100
O 50
TOO
X
© 50
* 0
too
u
*
50
.0
100
*
or
50
0
10O
g 50
§0
PERCENT OF ELEMENT RETAINED IN CHAR
PITTSBURG .SEAM & 8 BITUMINOUS COAL
JL
JL
r
MQK. Tamp,
Y610*K 4
JLL
ir
JL
JL
ir
JL
JL
1O 1O2
Lominer Flow Free Foil . Crucible
RESIDENCE TIME MILLISECONDS
PIGURE V-4. Element loss with devolatilization of a
Pittsburgh Seam #8 hvA~bituminous coal.
-------
107
is plotted with a common time axis to facilitate comparisons of
the stability of the different elements in the coal matrix. The
solid lines extended past one second represent retention of the
element for coal that has been dropped through the furnace, caught
in a crucible, and held at furnace temperature for the time
indicated. The dotted lines connecting two points represent the
change in retention with additional devolatilization when the
char produced at the shorter residence time is reinjected into
the furnace and held at the same temperature for additional time.
An asymptotic carbon retention of 75% was approached at
300 milliseconds for 1260°K, of 65% at 100 milliseconds for
1510°K, and of 60% at 50 milliseconds for 1740°K. Although the
total weight loss appeared to attain an asymptotic value of 52%
dry at 1940°K and 32 milliseconds and 57% dry at 2100°K and
21 milliseconds, the carbon loss does not appear to be complete.
This implies that further slow carbon loss might be obtained at
longer residence times at these high temperatures. The carbon
retention was about 50% at 32 milliseconds and 1940°K and about
40% at 21 milliseconds and 2100°K. Kobayashi (1976) presents a
more complete discussion of the same data.
Hydrogen is relatively easy to remove from coal during
pyrolysis. Below 1260°K, hydrogen loss is relatively rapid until
about 50% of the hydrogen has been removed; the remaining hydrogen
is removed much more slowly. However, hydrogen can be almost
completely removed at 1260°K for residence times on the order of
ten minutes. At high temperatures the complete removal of hydrogen
-------
108
is rapid, 21 milliseconds at 2100°K. Large quantities of hydrogen
are still retained in the char after pyrolysis at 740°K for 22
hours. Slow evolution of hydrogen probably continues for even
longer periods at 740°K, unless further evolution of hydrogen is
constrained by thermodynamics.
Nitrogen is the most difficult element to ultimately remove
from the coal matrix after carbon. At low temperatures, around
700°K. nitrogen loss is very small; at 1260°K nitrogen loss is
also small but can be seen to continue at a slow rate by the two
square points joined by dotted lines. Nitrogen removal is com-
plete at temperatures above 1500°K at long, but reasonable
residence times, as indicated by the data around five minutes
residence times. Nitrogen removal by pyrolysis and oxidation is
the major concern of this study and will be discussed in more
detail from an expanded plot of nitrogen removal.
Sulfur is not as difficult to remove from coal as carbon and
nitrogen, but is more difficult to remove than hydrogen and oxygen.
The data indicates that at temperatures below about 1900°K sulfur
removal is slow once 50% of the sulfur has been removed. The
amount of sulfur easily removed from coal is apparently tempera-
ture dependent. The asymptotic retention of sulfur in the char
at 1260°K is about 70% but at 1510 and 1740°K is about 40%. Sulfur
was almost completely removed at 2100°K and 21 milliseconds.
Oxygen appears to be slightly easier to remove than hydrogen
at higher temperatures but somewhat more difficult to remove at
lower temperatures. All the oxygen can be removed at 1260°K and
-------
109
ten minutes residence time or at times less than 7 milliseconds
at 1940°K. Slow evolution of oxygen at 1260°K is evident from
the decrease in oxygen concentration between the two points
connected by a dotted line.
-------
110
V-2.2.2 MONTANA LIGNITE
Data on the element retention of devolatilized lignite
(shown in Figure V-5) used in this study versus residence time
at different furnace temperatures is similar to the results for
the bituminous coal just presented. The release of hydrogen
appears to be more rapid under certain conditions for lignite
than bituminous coal; all the other elements showed a similar
behavior for bituminous coal and lignite. The curves for element
retention are, in general, similar in shape and magnitude to the
retention curves developed for bituminous coal.
Carbon loss appears to become asymptotic for all temperatures
at about 100 milliseconds. The asymptotic carbon retentions are:
75-80% at 1260°K, 65% at 1510°K, 55-60% at 1740°K, -^50% at 1940,
and 'x»40% at 2100°K. Little additional carbon appears to be re-
moved under conditions of low temperature and long residence times.
Hydrogen loss is complete at either moderate temperatures and
long residence times or high temperatures and short residence times.
For example, hydrogen was completely removed from char devolatilized
at 1750°K for five minutes or at 2100°K for 30 milliseconds.
Hydrogen is more easily removed from lignite than from bituminous
coal at temperatures below 1510°K; above this temperature the rate
of hydrogen removal from the two coals appears to be approximately
equal.
Nitrogen, as for bituminous coal, is the element most resistant
to ultimate devolatilization after carbon. The rates of nitrogen
removal from bituminous coal are slightly faster than from lignite
at all temperatures. The difference is, however, not great. The
-------
111
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8 50
u
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8
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50
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100
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100
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100
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i.
PERCENT OF ELEMENT RETAINED IN CHAR
MONTANA LIGNITE
O——Q
J I
i i
Mpx. Temp.
io'K
4720-740*K O2«00*K
OlOOO'K
OI220-I26O*K
O«740-1760*K
J I CS
0-8
I I
1 I
fi
Cructbl>
10'
RESIDENCE TIME MILLISECONDS
10'
FIGURE V-5. Element loss with devolatilization of a
Montana lignite-A.
-------
112
nitrogen removal from both coals is discussed in more detail
later.
The percent removal from the two coals of sulfur show similar
behavior, with the exception of the greater sulfur removal at
1260°K from the bituminous coal. As for bituminous coal, sulfur
removal is relatively slow. Indications are that sulfur can be
completely removed at high temperatures but can not be effectively
removed at low temperatures. Asymptotic retention of sulfur at
temperatures below 1510°K is approximately 50%.
Percentage oxygen removal from lignite is approximately equal
to the removal for bituminous coal at temperatures below 1510°K,
but oxygen removal from bituminous coal is considerably faster at
temperatures above 1940°K. Oxygen can be completely removed within
reasonable times from lignite char at all temperatures above 1260°K.
-------
113
V-2.2.3 NITROGEN PYROLYSIS FROM LIGNITE AND BITUMINOUS COAL
The nitrogen retention data from Figures V-4 and 5 for the
fast flow and free fall experiments is replotted in Figure V-6 with
expanded scales. The retention data at 1260°K for the bituminous
coal shows some scatter.
There are two features of the retention curves for lignite
that also should be noted. One datum point has higher retention
than a smooth curve through the rest of the data would indicate.
This point was obtained in an attempt to increase the residence
time of the fast flow furnace past the residence times that the
furnace yielded when operated in the normal mode. The plasma gun
could not be used to heat the large quantity of gas used to carry
the particle at the low flow rate required; it is, therefore, likely
that this point had a significantly lower temperature than the rest
of the data at 1510°K. Two data points taken at a temperature of
1740°K and short residence times show approximately the same small
nitrogen loss. The nitrogen content of the original coal has been
checked many times but, the nitrogen content determined by the
Dumas method gave consistantly lower results than the nitrogen
content determined by the Kjeldahl method. This can only be caused
by incomplete reduction of nitrogen oxides generated during the
burning of coal in the Dumas method. The Dumas method gave nitrogen
results about 20% lower than the Kjeldahl method for lignite, di-
gestion of the nitrogen compounds from butuminous coal is known to
be more difficult (Jodidi (1910) and Shacklock and Drakeley (1927))
so the nitrogen content of the bituminous coal was measured using
-------
X
0
o
lit
z
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a:
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IU
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oc
RETENTION OF NITROGEN IN PYROLIZED
CHARS
100
90
60
70
60
50
40
30
20
10
0
I
LI6NI
\
_L
BITUMINOU
I
I
10 100 1000
1 10 100
EXPOSURE TIME (MILLISECONDS)
1000
FIGURE V -6. Retention of nitrogen in devolatilized lignite and bituminous
coal chars.
-------
115
a 24 hour Kjeldahl digestion. Use of the nitrogen contents
determined by the Dumas method gave inconsistent results for
nitrogen in the early stages of devolatilization, therefore the
higher Kjeldahl results for nitrogen content of the original coal
have been used. Even when the Kjeldahl results are accepted there
is a residual uncertainty in the nitrogen measurements. This
uncertainty could indicate that the two points showing nitrogen
retention of approximately 97% actually represent conditions where
no nitrogen evolution occurs; this explanation agrees with the
hypothesis on nitrogen devolatilization to be presented in the
discussion section.
Figure V-6 shows more clearly that nitrogen is a very refrac-
tory element in coal. At the most extreme conditions of the
experiments presented here, no more than 70% of the nitrogen has
been removed after one second. Additionally nearly all the nitrogen
is retained in the char for an induction period that depends on
temperature. Bituminous coal has an induction period of about
100 milliseconds at 1260°K, 60 at 1510°K, ^20 at 1740°K, 12 at
1940°K, and near zero at 2100°K. The induction times for lignite
are: 100 milliseconds at 1260°Kf 30 at 1510°K, 20 at 1740°K,
10 at 1940°K, and 2 at 2100°K. The induction times for lignite
and bituminous coal appear to be consistent considering that some
extrapolations were necessary.
The nitrogen retention for lignite at 1510°K levels off at
67% and about 100 milliseconds. The shape of the curve appears
different when compared with the rest of the data. However, the
asymptotic value at one second is very well established so the
-------
116
shape of the curve must be nearly correct.
-------
117
V-2.3 ANALYSIS OF VOLATILE PRODUCTS OF COAL PYROLYSIS
A set of experiments was conducted in which the gas-phase
products of coal pyrolysis were partially analyzed in an attempt
to determine the fate of the carbon and nitrogen evolved from the
coal. The lignite coal in a 38-44 micron size cut was fed into the
free-fall reactor under an inert atmosphere of helium, and with a
residence time in the hot zone of about one second. Off-gases from
the furnaces were collected in a gas sampling bag for gas chromatog-
raphic analysis, or bubbled through scrubbers for electrochemical
analysis. The reader is referred to Section III-4 for a review
of the analytical procedures. Results pertinent to the carbon and
nitrogen material balances are shown in Figures V-7 and V-8 respect-
fully and summarized in Tables V-l and V-2.
The carbon material balance cannot, of course, be closed by
the limited analyses performed, insofar as hydrocarbons other than
methane are not accounted for and tars, known to form under these
conditions are, likewise, not detected. However, in view of
Suuberg's recent results (1977), tar and heavier hydrocarbon con-
tributions can be roughly estimated, as shown in Table V-l, yielding
a generally satisfactory closure of the carbon material balance.
HCN contributes negligibly.
Closure of the nitrogen balance is somewhat less certain,
however, because of large instrumental uncertainties in the analysis
for N2 as well as the possiblity of evolution of nitrogen bearing
species other than those indicated (N2, HCN, NH3). No NO was de-
tected (detection limit: -2.5 ppm) under the conditions of the
-------
118
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-------
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' TABLE V-l
CARBON DISTRIBUTION DURING COMBUSTION OF
LIGNITE AT T=1750 K and =3~4
Combustion Weight % of the Original
Products Carbon in Lignite
CO 72 - 75
C02 11 ~ 16
CH4 1-2
Other CH Not Measured
TAR Not Measured
Char - ' 4-5
Total 88-98
-------
121
TABLE V-2
NITROGEN DISTRIBUTION DURING COMBUSTION OF
LIGNITE AT T=1750 K AND =3~4
Combustion Weight % of the Coal-Nitrogen
Products in Lignite
N2 71 - 75
HCN 6-8
NH3 Trace
N0y 1~3
^\
Char 3~5
Total 81 ~ 91
-------
122
experiment. Furthermore, reported HCN levels may not strictly
represent primary yields of HCN as additional experiments
provide indication of thermal decomposition of HCN at the highest
temperature conditions in the furnace. The results in this section
show the consequence of both primary pyrolysis of the coal and
secondary pyrolysis of the volatile products. They are reported
to provide a measure of confidence in the data on the char through
an ability to close a material balance.
-------
123
V-3.0 OXIDATION RESULTS
In order to investigate the regime of volatile burning and
heterogeneous combustion depicted in Figure V-l, the burning and
conversion of coal nitrogen to nitric oxide for both raw and
devolatilized coal were investigated. The premise of conducting
oxidation experiments on both char and coal was that if volatile
burning and char burning could be treated as nearly independent,
then the contribution of the volatiles to total nitric oxide could
be obtained by subtracting the contribution of char oxidation from
the total nitric oxide emission obtained form oxidation of the raw
coal.
V-3.1 OXIDATION OF LIGNITE AND BITUMINOUS COAL-
FIXED RESIDENCE TIME
Lignite and bituminous coal were burned in 21% oxygen in helium
mixtures at varying percentages of the stoichiometric oxygen re-
quirement at three different temperatures according to the procedure
outlined in Section III-2. The unburned char was recovered and
residual combustibles and nitrogen were determined. Nitrogen oxide
production was determined by measuring the concentration of the
gas effluent from the furnace.
The results obtained from oxidizing raw coals at various fuel
equivalence ratios in a 1500°K furnace are shown in Figure V-9.
Little difference was apparent in nitrogen converted to nitric oxide
and nitrogen retained in the char for the two coals and the limited
number of particle sizes studied. There is some indication that the
burnout of bituminous coal may be more difficult than the burnout
of lignite.
-------
CONVERSION OP COAL NITROGEN
g 100
K 80
60
O
hi
II
go
o
z
O
V)
U
8
#
60
40
20
O
100
90
80
70
60
50
40
30
20
10
0
Wall Tamp. 1500 *K
Floma Tamp 1600 *K
A Llgnlta 75-90 pM
A Llgnita 38-45 /j M
a Bituminous 75-90/jM
• Bituminous 38 -
J.
234
FUEL EQUIVALENCE RATIO
FIGURE V-.9, .Conversion of coal nitrogen as a function of fuel equivalence
ratio in a 1500°K furnace.
N>
-------
125
As expected, the char burnout is near 100% at lean conditions
but residual char remains as the fuel equivalence ratio approaches
stoichiometric and burnout continues to decrease until an asymp-
totic value of about 70% at a fuel equivalence ratio of 1.5 is
obtained. Nitrogen retained in the char follows a curve compli-
mentary to the burnout curve. When char burnout is complete there
is obviously no nitrogen retained in the char; however, when char
oxidation is incomplete nitrogen is retained in the residual char.
Nitrogen retained in the char reaches an asymptotic value of 45% at
fuel equivalence ratios above 1.75.
The conversion of fuel nitrogen to nitrogen oxides approaches
60% at very lean fuel equivalence ratios but decreases as fuel
equivalence ratio increases. At a fuel equivalent ratio of 0.7
the conversion of fuel nitrogen to nitrogen oxides is about 30%, at
1.2 about 15% and above 2 the conversion is asymptotic at about 5%.
Similar conversion of lignite in a 1750°K furnace is shown in
Figure V-10. Several differences are apparent between the oxidation
of coals at 1500°K and at 1750°K:
1) Burnout at the higher temperature is complete even at
a fuel equivalence ratio of 1.5, and even at very rich
conditions the burnout is still 95%. This may be
" indicative of an oxidative pyrolysis mechanism and
2) The conversion efficiency of fuel nitrogen to nitrogen
oxides is marginally lower: about 5 absolute percent.
The conversion of fuel nitrogen to nitrogen oxides was about 60% at
very lean fuel equivalence ratios, 22% at 0.7, 10% at 1.2, and
reaches an asymptotic conversion efficiency of about 1% for fuel
equivalence ratios above 2.
-------
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80
60
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0 2 £ 20
X
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5* O
70
60
50
40
30
20
10
Furnace Temperature 1750 K
Lignite Coal 38~44jj
A f..A
1 2 3
•
Fuel / Oxygen Equivalence Ratio
Figure V-.10. Fate of Fuel Nitrogen During Oxidation: Conversion
to Nitric Oxide (Bottom), Retention by Unburned Char
(Middle), and Combustion Efficiency Defined by Solid
Weight Loss (Top). Montana Lignite at 1750 K.
-------
127
Similar conversion of lignite in a 1250°K furnace is shown
in Figure V-ll. Burnout, of course, is less efficient at the
lower temperature. Conversion efficiency of fuel nitrogen to
nitrogen oxides is, however, quite similar to that at 1500°K.
-------
1
•
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• ••
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80
60
60
40
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o v
#.
70
60
50
40
30
20
10
Furnace Temperature 1250 K
Lignite Coal 38~44
1 2 3
Fuel / Oxygen Equivalence Ratio
Figure V-ll.Fate of Fuel Nitrogen During Oxidation: Conversion
to Nitric Oxide (Bottom), Retention by Unburned Char
(Middle), and Combustion Efficiency Defined by Solid
Weight Loss (Top). Montana Lignite at 1250 1C.
to
00
-------
129
V-3.2 OXIDATION OF LIGNITE CHAR- FIXED RESIDENCE TIME
Lignite coal was devolatilized at 1500°K for one second and
then oxidized in a 1500°K furnace supplied with varying amounts
of oxidizer. Char produced in this manner has about 47% (d.a.f.)
weight loss and 0.67% nitrogen. The results are shown in
Figure V-12 and show similar trends, but with some differences,
to oxidation of the raw coal. Asymptotic burnout in one second
was not apparent for oxidation of char as it was for oxidation
of coal at the highest fuel equivalence ratio of 2.6. This may
result because the char has already been devolatilized at 1500°K
and has very little volatile matter left to lose . For the case
of raw coal , however, about 50 percent of the initial weight may
be lost by pyrolysis in addition to any oxidation that occurs.
The nitrogen retained in the char is higher than that found
for raw coals but there is more char remaining at a given con-
dition. The fractional retention of the nitrogen in the char is
expected to increase as the fuel equivalence ratio is increased,
with a consequent increase in the amount of residual char.
The conversion of char nitrogen to nitrogen oxides follows a
trend similar to the one observed for oxidation of raw coals. One
striking difference between the conversion of coal nitrogen from
chars and from coals is that the conversion for chars is signifi-
cantly lower. Under fuel-rich conditions, the conversion of the
nitrogen in the char reaches an asymptotic value of about 4% but
about 50% of the nitrogen is retained in the solid residual, whereas
at the same conditions, the conversion of the nitrogen in the raw
coal is about 15% and the nitrogen retained in the char from the
-------
.130
^
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5O
100
5O
40
30
20
10
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I
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1.O 2.0 3.O
(FUEL EQUIVALENCE RATIO BASED ON CHAR)
FIGURE V-12_ Conversion of nitrogen vs fuel equivalence ratio
in a 1500°K furnace for a lignite char previously
devolatilized at 1500°K for 1 second.
-------
131
coal is about 36%.
The residual unburned material from char oxidation is greater
than from coal oxidation at similar conditions and the nitrogen
retention of the residual from the char is correspondingly higher.
However, even under conditions where no residual is left for either
char or coal oxidation/ the conversion efficiency of the
char is significantly lower than that for the raw coal.
Similar results are obtained for oxidation of chars at two
other temperatures. A lignite coal was devolatilized for one
second at 1750°K and then oxidized in a 1750°K furnace. The results
are shown in Figure V-13. Similarly, a lignite coal was devola-
tilized for one second at 1250°K and then oxidized in a 1250°K
furnace. These results are shown in Figure V-14.
-------
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£ 60
.£ 60
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70
60
50
40
30
20
10
1
FUrnoce Temperature 1750 K -
1750 K Lignite Char
Fuel /Oxygen Equivalence Ratio
F1gureV-l3.
Fate of Char Nitrogen During Oxidation: Conversion to Nitric Oxide (Bottom): Retention by Unbumed Char
(Middle); and Combustion Efficiency (Top). Char from Montana Lignite Pyrolyzed and Oxidized at 1750 K.
Ul
to
-------
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£ 60
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c u 40
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70
60
50
40
30
20
10
Furnace Temperature 1250 K
1250 K Lignite Char
1 2 3 4
Fuel / Oxygen Equivalence Ratio
Figure V~14.
Fate of Char Nitrogen During Oxidation: Conversion to Nitric Oxide (Bottom); Retention by IJnbumed Char
(Middle); and Combustion Efficiency (Top). Char from Montana Lignite
Pyrolyzed and Oxidized at 1250 K.
U)
CO
-------
134
V-3.3 OXIDATION OF SUB-BITUMINOUS COAL
FIXED RESIDENCE TIME- EFFECT OF TEMPERATURE
In an attempt to elucidate the effect of furnace temperature
on the conversion of fuel-nitrogen to NO , an additional series
X
of experiments were performed in the flow reactor. A Montana
sub-bituminous coal in a 38-44 micron size cut was fed into the
reactor for a 1 second residence time at three temperatures and
a series of fuel/oxygen equivalence ratios varying from about
0.8 to 1.2. The results are shown in Figure V-15. Consistent
with results of Section V-3.1, it can be seen that conversion
efficiency increases with decreasing fuel/oxygen equivalence ratios
i
at a fixed temperature. Similarly, at a fixed fuel/oxygen
equivalence ratio, increasing the temperature from 1250°K to
1500°K renders little change in conversion efficiency, but a further
temperature increase to 1750°K results in a decrease in conversion
efficiency. A more detailed discussion of the effect of temperature
on NO production is presented in Section VI.
-------
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o
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o
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O1
o
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30
20
•S 10
o
9
T^
I T
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D 1500K
A 1750K
I
I
J
.0.4. 0.8 1.2 1.6
Rjct/ Oxygen Equlvolancc
Ratio
Co)
(b)
(c)
I
I
«0.8
« 1.0
• 1.2
1
t
I
1200 1400. 1600 1800
T«mpcrotupc*K
Figure V-15.
Effect of Temperature on Conversion of
Coal Nitrogen to Nitric Oxide. Montana
Sub-bituminous Coal.
UJ
.tn
-------
136
V-3.4 TIME-RESOLVED OXIDATION OF CHAR
As can be seen from Figures V-10 and V-ll, with increasing
fuel/oxygen equivalence ratio, the percent conversion of coal
nitrogen to NO decreases. Under the same fuel-rich conditions,
Ji
however, an increasing amount of the coal nitrogen may escape
the first stage of a staged combustor with the unburned char.
The kinetics of char oxidation have, therefore, been investigated
in order to establish residence time requirements for completion
of the char combustion and to determine the fate of the char-
nitrogen during oxidation.
A lignite char was prepared by feeding the lignite coal into
a 1750°K furnace for a residence time of one second. The char,
so prepared, was then fed into the furnace at temperatures of
1250°K, 1500°K, and 1750°K. Total weight loss and nitrogen and
carbon elemental weight losses were then measured for two different
oxygen partial pressures, and for residence times varying from
about 200 milliseconds to about 600 milliseconds by positioning
the collector probe at varying positions relative to the feeder
according to the procedure outlined in Section III-2. The data
are shown in Figures V-16 and V-17. As can readily be seen, total
weight losses and initial weight loss rates increase with tempera-
ture and oxygen concentration. Nitrogen to carbon ratios, similarly,
remain essentially unchanged or decrease only slightly with time
at the lowest temperatures, but decrease significantly with
increasing temperature. The decrease in N/C ratio is more pro-
nounced at lower oxygen concentrations. These data are consistent
with the contention that nitrogen is removed from the char both by
-------
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1 80
O*
~ 70
§ 60
50
«•*
01 40
I 30
S 20
5. 10
7T
1250K
O • 1500K
A A1750K
Op2n Symbols Po2«0.2 otm
Shod«d Symbols P0 aQ.4 otm
2
III!
1 2 34 5678
Distance Between. Feeder And Collector; inch
Figure V-16.
Weight Loss of Char Prepared from a Montana
lignite as a Function of Distance to the
Collector During Oxidation.
Ul
-------
# 140
J~ 120
£a 100
z o SO
*£'60
01
40
20
T
T
T
T
T
Open Symbols
Shaded Symbols PO2«0.4otm
I
I
I
I
I
1
I I
O0 1250 K
D • 1500K
A 17 50 K
I
I
1-2 3 4 5 6-7.8
Distance Between Feeder And Collector: Inch
Figure V-17.
Nitrogen/Carbon Ratio as Percentage of
that in the Original Char as a Function
of Distance to the Collector During
Oxidation.
u>
00
-------
139
continued devolatilization and heterogeneous oxidation, whereas
carbon, thought to form more stable structures in the char, is
further removed only by oxidative attack.
-------
140
SECTION VI
DISCUSSION OF EXPERIMENTAL RESULTS
VI-1.0 DEVOLATILIZATION RESULTS
At the start of this period, no data existed in the open
literature on devolatilization of nitrogen from coal at conditions
applicable to pulverized coal flames. A relatively large body of
data is available on nitrogen behavior in coal subjected to coking
conditions, i.e. low temperatures, 950°C; slow heating rates,
°C/min.; and in packed beds (Konle, 1928, Fielder and Davis, 1934,
Davis and Parry, 1939, Kirner, 1947, Hill, 1945, Lowry, 1942, and
Bronshtein, 1970). With the growth in interest in control of
conversion of coal nitrogen to nitric oxide in pulverized coal
flames, the data available on nitrogen behavior under coking
conditions has been extrapolated to the conditions of pulverized
coal flames and used to construct speculative models. Such
extrapolations of coking data have led to misinterpretation of
coal nitrogen behavior in pulverized coal flames.
Insofar as devolatilization can be viewed mechanistically as
the first step in coal combustion (See, again, Section V-1.0),
development of a strategy for control of NOV emissions based upon
Jv
combustion modification requires an accurate knowledge of the time/
temperature history of devolatilization under conditions appro-
priate to pulverized coal combustion. Such experiments are also
necessary for future development of a more detailed mechanistic
-------
141
model of coal pyrolysis which would hopefully relate evolution of
volatile species to thermal rupture of specific bonds or functional
groups in the complex matrix which comprises coal.
All of the pyrolysis data in this study were obtained from
coal devolatilized in an inert atmosphere. In a real pulverized
coal flame, of course, coal devolatilization occurs in a rich but
not oxygen-free atmosphere. The pyrolysis data of this study were
taken in an oxygen-free environment because the inert atmosphere
allowed the effects of thermal decomposition to be clearly separated
from the effects of oxidation. It is realized that thermal and
oxidative pyrolysis, as well as oxidation, may not proceed by
entirely independent mechanisms,but the experimental division out-
lined here represents the clearest, first-order approach to the
problem. Future extensions of this work might involve a more
detailed exploration of the coupling of the mechanisms of devola-
tilization and oxidation.
-------
142
VI-1.1 ASYMPTOTIC CRUCIBLE RESULTS
Crucible results (Figures V-2 and V-3) in this study extended
the maximum temperature of available data from about 1500°K to
2200°K. The nitrogen retention in the char and weight loss (dry
basis) are shown versus maximum temperature in Figure VI-1. The
nitrogen retention and weight loss are reported at the times at
which weight loss approached an asymptotic value. Under conditions
of asymptotic weight loss the chars may still undergo slow nitrogen
loss.
In Figure VI-1, the low temperature portion of the nitrogen
retention curve for lignite shows little nitrogen loss at tempera-
tures below 750°K; for bituminous coal, 5% of the nitrogen is lost
at the same temperature, even though the dry weight losses are about
equal. In general, the nitrogen loss from the bituminous coal under
coking conditions seems to be slightly greater than from the lignite.
This slight difference might be explained by some small amount of
the coal nitrogen being bonded in exinite resinous bodies that can
distill without significant thermal degradation.
The retention of nitrogen in lignite and bituminous coal char
is proportional to temperature between 80 and 20% retention in
agreement with other data. The first and last portion of the
nitrogen removal appears to be slower. The slow removal of nitrogen
from coals implies that nitrogen is contained in coal in a relatively
stable form, probably predominantly in heterocyclic nitrogen com-
pounds .
-------
A Lignite Nitrogen Retension
A Bituminous Coof Nitrogen Retension
O Lignite Weight Loss
O Bituminous Coal Weight Loss
ASTM WL Bituminous Cool
_
ASTM WL Lignite '
ASTM N Lignite
ASTM N Bituminous Cool
I I I I
500
100O
1500
TEMP °K
2000
Asymptotic Nitrogen Retension ond Weight Loss Crucible
Experiments
FIGURE Vi-1. Coal nitrogen retention in the char under conditions of asymptotic
weight loss.
U)
-------
144
Some of the data at 1223°K were obtained using the ASTM
Proximate Analysis test and some were obtained by heating to the
ASTM test temperature in the furnace. The nitrogen retention under
ASTM Proximate Analysis conditions is 72% for the lignite and 63%
for the bituminous coal. There appears to be little difference
between nitrogen retention for either coal between actual and
simulated ASTM Proximate Analysis tests. There was no obvious
correlation between ASTM nitrogen retention and the nitrogen
retention of chars in pulverized flames.
The nitrogen loss from both coals is complete at high tempera-
tures. Very little nitrogen (less than 10%) is retained in the char
at temperatures above 1750°K. Nitrogen is completely removed for
both coals at a temperature of 2200°K, even though the weight loss
is only 44% dry, and char has retained about 70-75% of the original
carbon. The carbon retained in the crucible experiments is much
higher than the carbon retained under simulated pulverized coal
flame conditions. The conclusion is that under slow heating in
packed beds, carbon can form complexes that are stable to at least
2200°K but that nitrogen compounds which may form higher molecular
weight complexes are not stable at pulverized coal flame conditions.
The total weight loss at 2200°K is only marginally greater than
the ASTM volatile matter, 44% compared to 42% dry. The ASTM volatile
levels indicated by the dashed lines are well established by repeated
tests from different laboratories. The weight loss shown at 1223°K,
the ASTM temperature, was slightly lower for both lignite and
bituminous coal but the difference is not thought to be significant.
-------
145
VI-1.2 TIME-RESOLVED FLOW FURNACE RESULTS
The pyrolysis results of greatest interest for purposes of
developing emission control strategies are those obtained under
conditions most nearly approximating those found in pulverized coal
combustors, namely, high heating rates, 104-105°C/sec, and high
temperatures, 1500-2200°K. It is expected that high heating rates
will result in weight losses differing somewhat from those found
in the crucible experiments. As will be seen in a later section,
these pyrolysis data are necessary for elucidating the mechanism
of NO formation during coal burnout, in particular, for distin-
guishing NO formed through homogeneous gas phase oxidation from
that formed via heterogeneous burnout of char. This matter is
returned to in detail in Section VI-3. Since the amount of nitrogen
devolatilized from coal under different conditions may have a large
effect on control strategies, it is desirable, as indicated above,
to develop a predictive model for nitrogen pyrolysis from coal that
will be applicable to conditions in a pulverized coal flame.
Empirical models can be developed that adequately represent
pyrolysis data. Little physical significance can, however, be
ascribed to these models nor can they reliably be extended beyond
the rartge of experimental conditions employed or coals studied.
This, of course, severly limits the usefulness of these models.
However, where fundamental models are difficult to develop, empirical
relations do, often, serve useful engineering functions.
Coal pyrolysis has, in the past, generally been considered too
complicated to model except empirically. New information developed,
-------
146
however, on the chemical and physical structure of coal does allow
some physically realistic models of complex coal processes to be
proposed. While the pyrolysis results presented here do lead to
some conclusions concerning the nature of devolatilization and
condensation reactions likely to predominate in coal pyrolysis,
proposal of a full mechanistic pyrolysis model is considered beyond
the scope of this work and will not, therefore, be attempted. We
do, however, in the tradition of coal kineticists, present an
empirical interpretation of coal nitrogen pyrolysis.
The usual first order treatment of nitrogen pyrolysis from
coal, except that the rate is restricted to the first 10% of
nitrogen loss, is compared with fundamental first-order rate con-
stants in Figure VI-2. The rate constants were taken from a com-
pilation by Benson and O'Neal (1970) and represent reliable
experimental values which have been heavily weighted by absolute
rate theory considerations. Also shown on the plot are some
pyrolysis data taken on nitrogen compounds that might be considered
to represent the form of fossil fuel nitrogen. The nitrogen com-
pound pyrolysis data was taken from Kurd and Simon (1962) and
Axworthy (1975) .
The pseudo-first order rate constant derived from the initial
slopes of the data presented in Figure VI-2 yields a value of
kN = 9.3 x 103 exp(-22,700/RT) sec'1
for the Montana lignite-A and Pittsburgh Seam #8 hy-A bituminous
coals studied here. It is apparent that the rate constant derived
in this study does not resemble elementary first order reaction
-------
10"
10'
10"
a '°
-------
148
rate constants. Rate constants derived from pyrolysis of model
nitrogen compounds are reasonably in agreement with the rate con-
stants derived for nitrogen removal from the coals of this study
but are in poor agreement with fundamental first order rate
constants.
Kobayashi (1976) has derived rate constants from the same
data in a parallel study and found an overall weight loss constant
for both coals of about:
= 6.6 x 104 exp(-25,000/RT) sec"1
The rate constants derived for loss of hydrogen, carbon, and oxygen
were also very similar to the constants for nitrogen loss and over-
all weight loss. Kobayashi has tabulated previously derived coal
pyrolysis rate constants and found the constants, within a range
of scatter, to be consistent with each other and to be consistent
with rate constants derived similarly in this study. The rate
constants do not agree with the theoretical values that would be
predicted if the kinetic models used to interpret the data were
physically realistic. The conclusion to be drawn is that first
order kinetic models can explain individual sets of data, provided
enough parameters are used, but cannot predict consistent rate
constants for coal pyrolysis.
It is probably obvious that coal pyrolysis involves an entire
spectrum of reactions of different orders, including parallel,
sequential, and competitive paths. As pyrolysis is traditionally
interpreted as occurring in a single reaction which is first order
-------
149
in material remaining to be reacted, it is not surprising that rate
parameters so derived do not resemble rate constants for elementary
reactions. It is extremely difficult, therefore, to derive physi-
cally meaningful pyrolysis rate parameters for a substance as
complicated as coal.
Perhaps a more useful analysis of the coal pyrolysis data,
from the viewpoint of development of a mechanistic model, is the
correlation of elemental nitrogen loss with total volatile loss
under conditions applicable to pulverized coal flames. This is
demonstrated in Figure VI-3.
The rate of nitrogen loss during pyrolysis is not independent
of the loss of other elements during pyrolysis because C, H, 0, N,
and part of the S are bonded together in intricate chemical struc-
tures. Kobayashi (1976) has demonstrated the similarity between
individual element removal rates and the rate of total weight loss
Gray, et a_l (1975)have also produced empirical correlations between
the extents of various element loss during hydrogasification of coal.
Figure VI-3 shows nitrogen retention versus d.a.f. weight loss
for the two coals of this study subjected to pyrolysis in flow
experiments (open points), free fall into crucible (half-open points),
and crucible results (solid points). The correlation between
nitrogen loss and total weight loss is good for both coals when only
the laminar flow data is considered. The data for pyrolysis of the
coals in crucibles show a much higher rate of nitrogen loss than
carbon loss.
-------
NITROGEN LOSS V3 WEIGHT LOSS
LIGNITE
BITUMINOUS
x
u
o
LJ
U
a:
LJ
o
o
QL
80 60 40 20 0 80 60 40 20 0
•/. WEIGHT L6SS ( d.O.f.)
FIGURE vi-3. Correlation between nitrogen loss and total weight loss during
pyrolysis of coal.
en
o
-------
151
In the long residence time crucible experiments reported in
Section V-2.1 it was demonstrated that the total weight loss
reached an asymptotic value only slightly higher than would be
predicted by the ASTM Proximate Matter Test, but that elements
other than carbon were completely removed. This can be explained
because carbon is the only element that can form a highly con-
densed substance that is stable at pulverized coal flame tempera-
tures. Nitrogen is not thought to have sufficient bonding electrons to
participate in high order condensation reactions with carbon.
Nitrogen must, therefore, be present only on the periphery of the
char structure where it can be removed under the conditions of a
pulverized coal flame.
Gray, e_t al. (1975) observed similar behavior in his experiments
on hydrogasification of coal. Under conditions similar to those
used in this work Gray found that nitrogen loss and carbon loss were
related linearly with a slope of 1.8. However, when coal was
pyrolysized in his "hot rod" reactor with heating rates and
environmental conditions more nearly like those encountered in
the crucible experiments of this experiment, nitrogen loss was
no longer linearly related to carbon loss. Nitrogen loss was more
rapid than carbon loss and nitrogen could be completely removed
with removal of as little as 30% of the carbon.
Nitrogen loss does not commence with carbon loss but is
delayed until approximately 10-20% of the carbon is lost. This
is consistent with the belief that some of the carbon is contained
in alphatic structures that are easy to remove compared to destruc-
tion of aromatic stuctures, while most of the nitrogen is probably
-------
152
held in heterocyclic ring stuctures.
Nitrogen is removed from coal more rapidly than total weight
loss occurs, after the initial delay in nitrogen removal, as in
evidence from the slope of the curves; the ratio of nitrogen loss
versus weight loss is 1.25 for the lignite and 1.5 for the
bituminous coal.
Nitrogen loss from different coals at pulverized coal flame
conditions in an inert atmosphere can then be crudely estimated,
probably within 10 to 20 percent, from total weight loss data by
use of the shape and nature of Figure VI-3. No nitrogen loss should
be assumed until approximately 15% of the total weight has been
lost, then nitrogen loss can be estimated as approximately 1.25-
1.5 times the total weight loss until approximately 60% d.a.f.
weight loss has occurred. Extrapolation of this relationship
beyond 60% weight loss is uncertain and would predict that, for
pyrolysis of coals under inert pulverized coal flame conditions,
complete nitrogen loss will occur at 80-90% total weight loss.
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153
VI-1.3 PRIMARY NITROGENEOUS PYROLYSIS PRODUCTS
It is possible at this juncture to draw some educated
conclusions as to the primary fate of fuel-nitrogen released as
volatiles from coal.
Pyrolysis data of this study indicates an induction period
after heating prior to release of fuel nitrogen from coal. This
is though to reflect the common speculation that most of the fuel
nitrogen is contained in reasonably stable, heterocyclic structures,
rather than in labile side chains. Consistent with this belief
is the recent observation of Blair, e_t a_l (1976) that under pyrolysis
conditions most of the coal nitrogen is, in fact, evolved as tars
and heavy, hetrocyclic molecules. These large molecules are then
thought to undergo secondary pyrolysis to yield HCN as the major
nitrogen-bearing species. Studies by Axworthy (1976) on the
pyrolysis of model compounds indicate rapid rates for this pyrolysis
step which, when extraplated to temperatures pertinent to pulverized
coal combustion, suggest that formation of HCN from these heavier
molecules is very rapid and certainly not rate controlling.
The results of this study seem to confirm this mechanism?
closure of the nitrogen material balance at high temperatures
(See Section V-2.3) resulting in HCN and N2 showing up as the major
stable species. The N2 so observed, however, is almost certainly
a product of the secondary pyrolysis of HCN, and not a primary
pyrolysis product. This is easily understood from the complementary
observation that under fuel-lean, low-temperature conditions (See
Figure VI-4) , conversion of volatile fuel-nitrogen to NOx is
-------
154
essentially complete. Insofar as, under these conditions, N~ is
not thought to be a reasonable precursor to NO, N2 cnnot be a
primary pyrolysis product of coal.
-------
155
VI-2.0 OXIDATIVE RESULTS
The primary objective of this work is the elucidation of the
mechanism of NOx formation during pulverized coal combustion with
a specific view towards development of a strategy for the minimi-
zation of NO emissions. As will be seen, NO can originate both
J\.
from gas phase oxidation of nitrogen bearing volatiles emitted from
the initial pyrolysis of the rapidly heated coal as well as from
heterogeneous oxidation of the devolatilized char. Insofar as the
chemical mechanisms of these two processes are rather different,
variation of combustion conditions by the standard techniques
available to the combustion engineer, e.g. change of temperature,
fuel/air equivalence ratio, degree of mixedness, staging of com-
bustion, etc., may effect the two processes quite differently; and
it is certainly not immediately obvious what combinations of
combustion conditions will lead to a minimum in NO emissions. In
Ji
an attempt to gather at least an empirical understanding of the
factors affecting the split of fuel-nitrogen between volatiles and
char and the oxidation of each, a series of independent coal and
char burnout experiments were undertaken. These results, combined
with the pyrolysis data of the previous section, will be seen to
throw valuable light on the problem.
All experiments were run in the laminar flow reactor described
above in Section III. As such, aerodynamic conditions were not
varied. In a somewhat parallel study, however, Pershing and Wendt
(1976) investigated the effect of mixing in a 6 Ib/hr unit and
found that efficiency of fuel nitrogen conversion to nitric oxide
was affected by burner aerodynamics but not affected by flame
-------
156
temperature, provided the flame temperature was not too high. The
results of Pershing and Wendt on fuel nitrogen conversion on a
similar coal agree with the conversions of this study. For instance,
at 15% excess air a Pittsburgh coal with 1.20% nitrogen showed
about 28% conversion; approximately the same value was obtained
for a similar coal in this study -
Pershing and Wendt injected coal through a divergent nozzle
and an axial injector and found, when everything else was equal,
about a 10% absolute difference in conversion efficiency between
the two nozzles for a Western Kentucky hvC bituminous coal and
Montana sub-bC coal. The injector system used in this study is
close to the axial injector system used by Pershing and Wendt.
The burner design had no effect on char oxidation. The injector
used in this study can not match the mixing conditions encountered
in a pulverized coal flame and so represents a different flame.
Data, however, can be compared on a relative basis for this mixing
condition and valid conclusions can be drawn about the effects of
variables other than mixing for systems in general.
Pershing and Wendt used adiabatic flame temperatures as a
comparison of temperature effects on fuel nitrogen behavior since
they had no direct measure of the flame temperature. Comparison
of the effects of flame temperatures on thermal and fuel nitrogen
emission showed that thermal nitric oxide emissions increased
greatly at higher temperatures but fuel nitrogen conversion did
not increase until the adiabatic flame temperature reached about
2400°K. The stoichiometric adiabatic flame temperatures in helium
for the coals of this study, disregarding dissociation, were about
-------
157
2100°K for the lignite and about 3000°K for the bituminous coal;
corrections for dissociation will lower the lignite temperature
about 200°K and the bituminous coal 300°K.
-------
158
VI-2.1 COAL OXIDATION
In this study, coal particles, nominally in the size range of
38-45 microns,or,occasionally»75-90, were injected with an equal
mass of gas through a 1-2 mm nozzle into a hot wall furnace.
Oxygen content of the helium/oxygen carrier stream corresponded to
15-25% of the stoichiometric requirement which corresponds well
with the values of the primary air utilized in industrial practice.
The secondary air (again stimulated by a helium/oxygen mixture)
was injected through a honeycomb flow straightener surrounding the
coal-feed nozzle. In view of the difference between the mode
of operation in this study and practical systems, the gross agree-
ment in the data is remarkable. The conversion of fuel nitrogen to
nitric oxide in our system for a fuel equivalence ratio of 0.7 is
about 30 percent which is towards the upper end of the range of
estimates for field units.
Comparison of the results in Figures V-9 and V-10 suggest tnat
higher temperatures appear to have lower fuel nitrogen conversions.
At a normal operating fuel equivalence ratio of 0.7, the percentage
of fuel nitrogen conversion to nitric oxide decreased by several
percentage points absolute when the furnace temperature was increased
from 1500 to 1750°K. At the higher temperature, as would be expected,
the burnout was markedly improved and the amount of nitrogen retained
in the char drastically reduced.
Because the data of Figures V-9 and V-10 were gathered over a
long time span, and because of the current interest in the effect
of temperature on NO emissions from pulverized coal flames, the
-------
159
effects of temperature were rechecked in an independent series of
experiments using a Montana sub-bituminous coal. These results,
shown in Figure V-15, confirm the tendency towards decreasing
conversion efficiencies with increasing temperature. The effect
of temperature on controlling NO emissions is returned to in
X
Section VI-3.
-------
160
VI-2.2 CHAR OXIDATION
Essential to elucidating the mechanism of fuel nitrogen
conversion to NO is an understanding of the split of fuel
Ai
nitrogen between volatiles which are homogeneously oxidized to
NO and char which is heterogeneously oxidized. Such information
can be gleaned from a comparison of coal oxidation results, which,
inherently, include contributions from both processes and char
oxidation results, which, necessarily, involve only the latter
process.
The conversion efficiency of fuel nitrogen to nitric oxide
for a char was lower by about 10% absolute than the corresponding
value for a coal at a fuel equivalence ratio of 0.7. The effect
of fuel/oxygen ratio on conversion of the fuel nitrogen to nitric
oxide in the coal was similar to that for the char (See Figures
V-9 through V-14). The fractional burnout of the char was lower
than the corresponding values for coal at fuel rich equivalence
ratios. In addition, more nitrogen was retained by the unburned
char than the unburned coal.
Pershing and Wendt (1976) report values for the conversion
efficiency of the nitrogen in a char of about 13 percent at a fuel
equivalence ratio of 0.7 for both a divergent nozzle and an axial
injector. This conversion efficiency agrees well with the value
found in this study using a char devolatilized at 1500°K for one
second. It seems reasonable that mixing should have much less
effect on the slow heterogeneous oxidation reactions than on the
oxidation of volatiles.
-------
161
Figures V-9 through V-ll demonstrate that with increasing fuel
equivalence ratios, the percent conversion of coal nitrogen to
nitric oxide decreases. Under the same fuel rich conditions,
however, an increasing amount of the coal nitrogen may escape the
first stage of the combustor with the unburned char. The study
of the kinetics of oxidation of char was therefore undertaken with
the objective of establishing the residence time requirements for
completing the combustion of the char and determining the fate of
the char-nitrogen during oxidation. The data in Figures V-16 and
V-17 are plotted as a function of distance. For determining kinetic
parameters, residence time measurements are necessary. Such mea-
surements were made by Kobayashi (1976) who used essentially the
same system and showed that the average particle velocity may be
approximated by 1.4 times the average main gas velocity. For the
interpretation of results of the present study, distances were con-
verted to residence times, using Kobayashi's approximation. The
results with times reported in milliseconds are summarized below:
^*\^ Distance, inch
Temperature, K ^^-^^^
1250
1500
1750
3
256
213
183
4
340
283
243
5
427
356
305
6
512
426
365
7
599
500
428
It has been previously shown (Figures V-4 and V-5) that
during coal pyrolysis the carbon loss appears to become asymptotic
-------
162
at times exceeding 100 milliseconds at 1750°K, the temperature used
to produce the char in this study, but that nitrogen release con-
tinues for longer times. The rationalization of the results was
that while carbon is present in relatively stable compounds in the
char, no comparable stabilized nitrogen structures are formed;
consequently, char-nitrogen continues to be released until it is
completely eliminated from char. Therefore, when a char particle
is oxidized, the nitrogen loss will be due to both devolatilization
and oxidation, but the carbon loss will be exclusively due to
oxidation.
Assuming the fuel nitrogen is uniformly distributed throughout
the char, the oxidation rate of the nitrogen should therefore equal
the product of the oxidation rate of the carbon and the mole ratio
of the fuel nitrogen to carbon in the char (Wendt and Schulze, 1976),
Since the pyrolysis loss and oxidation loss of char-nitrogen are
additive, the consumption rate of char-nitrogen can be written as:
Total Consumption-Rate l
of Char Nitrogen J
OR
Since
dN _ /3N
dT \3t / Pyrolysis
dC = /dC
dt Idt } Oxidation
Consumption-Rate of
Char-Nitrogen Due
to Pyrolysis
Consumption-Rate of
Char-Nitrogen Due
to Oxidation
3t } Oxidation
-------
163
Therefore,
— - — 1 + (dc\ (®\
dt ^ty Pyrolysis \dt/ Oxidation \C) Following Pyrolysis
This model indicates that for low oxygen pressure and high tempera-
ture char oxidation, the nitrogen to carbon ratio in the char will
decrease as a function of reation time. As the oxygen pressure
increases and temperature decreases, the oxidation process tends
to dominate, resulting in a nitrogen/carbon ratio which is approx-
imately constant during the course of the combustion process. The
experimental findings support this conclusion. The data can be
used to derive kinetic parameters for the oxidation and pyrolysis
of the char nitrogen. The experiments were carried out with large
excess of oxygen so as to maintain a constant oxygen partial
pressure and thus facilitate interpretation of the results. The
basic kinetic parameters derived from the data can be used to
analyze char burnout under the varying oxygen concentrations that
are encountered in practical combustors.
-------
164
VI-3. THE CONTRIBUTIONS OF VOLATILE AND CHAR NITROGEN
TO NITRIC OXIDE
Very little information exists on the contribution of nitric
oxide produced from the oxidation of char and from the oxidation
of volatiles. Pereria et al_ (1975) have shown the volatile
contribution in a fluidized bed to be negligible at temperatures
below 1000°K but to increase until the nitric oxide produced from
volatile matter accounted for 2/3 of the total emission at 1200°K.
These figures agree qualitatively with the results of this study,
although the systems, temperatures, and mixing patterns must be
considerably different.
The data from Figures V-9 through V-14 can be used to obtain
preliminary estimates of the fractions of the NO formed in pulver-
X.
ized coal flames that are contributed by the volatiles and char.
As a first order approximation it will be assumed that the conver-
sion efficiencies to nitric oxide of the nitrogen in the char and
volatiles are independent. The fate of the fuel nitrogen during
combustion of coal can be represented by the following simplified
scheme:
1-a
VOLATILES
CHAR
i_n —N0 + OTHER
11 ~
BURNED CHAR
1-n
1-Y
UNBURNED CHAR
+ OTHER
-------
165
where
a = fraction of the coalTN that is released as volatiles
1 - a = fraction of the coal-N that is retained in the char
n, = fraction of the volatile-N that is converted to NO
x
Y = fraction of the char-N that is consumed
Yr|2 = fraction of the char-N that is converted to NO
J\
The overall fractional conversion, n*, of coal-nitrogen to nitric
oxide is then:
n = an, + (1 - a) y
n
n2
Overall Volatile Char Contribution
Conversion Combustion
to NO
The value of (1 - a) as a function of temperature is obtained from
*
the pyrolysis data of Figure V-6 . Values of n and Yn2 as functions
of temperature and fuel equivalence ratio are obtained from the coal
oxidation data of Figures V-9 through V-ll and from the char oxida-
tion data of Figures V-12 through V-14. From this information and
the above scheme for fuel-nitrogen partitioning, it is possible to
derive values for both the efficiency of volatile nitrogen conver-
sion to NO , HI , and the function of total NO contributed by
x J. x
volatiles (ann/n) - These values are plotted as a function of fuel/
oxygen equivalence ratio for the two temperature extremes of these
experiments, 1250°K and 1750°K, in Figure VI-4 . The efficiency of
conversion to NO of nitrogenous species evolved as volatiles (r^)
li
and the fraction of overall conversion to NOx which originates from
-------
166
o
o 90
}| 8O
T. 7O
6O
^ 50
O 2
= O 40
S~ 30
> 20
° 10
•o
o
8
o o
I"
9O
80
70
60
5O
40
30
2O
10
^^-^^
Fuel / Oxygen Equivalence Ratio
Figure VI-4.
Contribution of Volatiles to NO Emission:
Total N0x Contributed by Volatiles (Top), and
Conversion of Volatiles to NO (Bottom).
-------
167
( \
I ) are both seen to decrease monotonically with
H* /
increasing fuel/oxygen equivalence ratio. The former result is
entirely consistent with previously reported data on the conversion
of volatile fuel-nitrogen to NO in both premixed (Fenimore 1972,
J\
Sarofim,et al 1975) and laminar diffusion flames (Sarofim, e_t al_ 1975)
and, in fact, can be shown to fall between the two extremes. The
latter result, however, would not have been obvious, a priori, and
represents the fact that though c; .litrogen conversion efficiency
to NOx also decreases with increasing fuel/oxygen equivalence ratio
(See Figures V-13 and V-14), its rate of decrease is not as rapid
as that of the volatiles.
Also of great interest is tho effect of temperature on the
volatile/char nitrogen partitioning and conversions to NO of fuel
J\
nitrogen. Comparison of Figures V-13 and V-14 for char oxidation
at 1750°K and 1250°K suggests that the fractional conversion of char
nitrogen to NO is independent of temperature. (The data of Figure
J^
V-12 at 1500°K, obtained much earlier in the studies, is thought
to be of lower reliability and to exhibit more scatter.) The results
of Figure VI-4, however, indicate that the fractional conversion to
nitric oxide of the volatile nxLr jen (r^) decreases with increasing
temperature. This result is consistent with the general observation
that increases in fuel nitrogen concentration result in a decrease
in conversion of fuel nitrogen to nitric oxide. As the temperature
is increased, the fraction of the fuel nitrogen devolatilized
increases and the concentration of fuel nitrogen in the gas phase
must also increase. Interestingly, however, the results of
-------
168
Figure VI-*4 also indicate that with increasing temperature the
fractional contribution to total NO by volatile I ttnA\ increases.
X I " I
V n* )
This, as inspection, again, of Figure V-6 reveals, is not incon-
sistent with the observations above in that with increasing
temperature, a much larger fraction of the fuel nitrogen is evolved
as volatiles. The results of Figure VI-rA merely imply that the
rate of increase of volatile evolution with increasing temperature
exceeds the complementary rate of decrease in conversion efficiency
to NOX. These two effects tend to compensate and partially explain
the small dependence on temperature of the net conversion of coal
nitrogen. In this study, a small net decrease is observed (See
Figure V-15). In a separate study, Pershing (1976) has found
practically no effect of temperature on the conversion of fuel
nitrogen to nitric oxide.
-------
169
VI-4. CONCLUSIONS AND IMPLICATIONS FOR CONTROL OF NO EMISSIONS
Major conclusions of this work may be summarized as follows
(1) Evolution of nitrogen from coal is kinetically
controlled for conditions encountered in typical
pulverized coal flames. Although carbon forms a
stable char through condensation reactions, no com-
parable stabilized nitrogen structures are formed.
Consequently, nitrogen can he completely eliminated
from the char at temperatures exceeding 1750°K.
(2) When coal is slowly heated under an inert
atmosphere in a crucible, most of the volatile matter
evolution occurs at temperatures below that of the
ASTM Proximate Analysis test (1023°K), whereas most
of the nitrogen is evolved at higher temperatures.
(3) The early stages of pyrolysis of nitrogen from
coal can be empirically correlated, for the two coals
studied, by a pseudo-first order rate constant of
value:
1; - 9.T x 103 -22,700/RT) sec"1
(4) Little nitrogen is lost, under pyrolysis conditions,
until 10-15% of the weight of the coal has been devola-
tilized. After this initial induction period, most
likely associated with loss of side chains and scission
of aliphatic bridges in coal, the rate of fractional
nitrogen loss is found to be proportional to the rate
-------
170
of fractional total weight loss.
(5) Conversion of fuel nitrogen to nitric oxide in
pulverized coal flames decreases with increasing
fuel/oxygen ratio, falling to negligible values at
equivalence ratios greater than 1.5. However, at
temperatures of 1500°K or lower, some of the coal
persists as unburned char which may contain a
significant fraction of the original fuel nitrogen.
(6) Conversions to nitric oxide of fuel nitrogen
during char burnout are 2-3 times lower than the
corresponding values for coal.
(7) During coal burnout, fuel nitrogen is partitioned
between volatiles and char, higher temperatures
favoring a larger fraction of volatiles. The effi-
ciency of conversion of the volatile nitrogen to
nitric oxide decreases with increasing fuel/oxygen
ratio and decreasing flame temperature. The effi-
ciency of conversion of char nitrogen to nitric
oxide also decreases with increasing fuel/oxygen
ratio, albeit, more slowly than does volatile
nitrogen, but is, however, rather insensitive to
temperature. Carbon burnout, of course, is enhanced
at higher temperatures.
(8) Oxidation experiments on chars support the view
that there is no selectivity between nitrogen and
carbon loss during oxidation but that the char
-------
171
nitrogen may undergo further pyrolysis in parallel
with the oxidation.
The above results may be used to guide the development of
control strategies. In a single stage combustor, conversion of
nitrogen to nitric oxide may be reduced by the increase in the
fuel/air ratio, to the limit imposed by the emission of unburned
carbon, CO, or soot. Because of the compensating effects discussed
above, little change in fuel nitrogen conversion to nitric oxid<=-
can be expected with changes in combustion temperature but the
contribution of thermal fixation of nitrogen will decrease with
decreases in combustion temperature.
In a staged combustor, the conversion of fuel nitrogen to
nitric oxide can be decreased by increasing the fuel/air ratio,
which results in decreases in conversion efficiency of the fuel
nitrogen to nitric oxide for the nitrogen content of both volatiles
and char. Since char oxidation is slow and nitrogen retained in
the char may be oxidized to nitric oxide in a second stage, it is
advantageous to operate the first stage at high temperatures to
devolatilize the coal nitrogen in the locally fuel rich environment.
Although these strategies may be evident, the result of the present
study provides a quantitative framework to guide the development of
contro-1 strategy. For example, the kinetic data on the pyrolysis
of coal can be used to estimate the temperature-time combinations
required to devolatilize the nitrogen in the first stage of a staged
combustor. In addition, the kinetic data on char oxidation may be
used to size the second stage of the combustor. The staging, can
of course, be achieved either by introduction of the combustion
air at different positions in a furnace or by aerodynamic means.
-------
172
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177
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-600/7-78-165
2.
3. RECIPIENT'S ACCESSION NO.
4.T.TLE ANO SUBTITLE Combustion Research on the Fate of
Fuel-Nitrogen Under Conditions of Pulverized Coal
Combustion
5 REPORT DATE
August 1978
•ERFORMING ORGANIZATION CODE
7. AUTHORIS)
Y.H.Song
J.M.Levy, J.H.Pohl, A.F.Sarofim, and
P. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Massachusetts Institute of Technology
Department of Chemical Engineering
Cambridge, Massachusetts 02139
10. PROGRAM ELEMENT NO.
EHE624A
11. CONTRACT/GRANT NO.
Grant R803242, Task 2
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Industrial Environmental Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PER.OD COVERED
Task Final; 8/74-8/77
14. SPONSORING AGENCY CODE
EPA/600/13
. SUPPLEMENTARY NOTES T£RL-RTP project officer is John H. Wasser, Mail Drop 65,
919/541-2476.
16. ABSTRACT
The report gives results of an experimental investigation of coal pyrolysis
and oxidation, and char oxidation to determine the effects of temperature and fuel/
oxygen equivalence ratio on the conversion of coal-nitrogen to NOx. Experiments
involved a laboratory laminar flow furnace under conditions representing pulverized
coal combustors; i.e. , heating rates of 1000 to 1 million K/sec, temperatures of 1000
to 2100 K, and residence times of 2 to 1000 milliseconds. The devolatilization experi-
ments showed that no nitrogen loss occurred until 10 to 15% of the coal had been devol-
atilized, consistent with the hypothesis that coal-nitrogen is contained mostly in het-
erocyclic rings. Nitrogen was completely removed from the char by prolonged heat-
ing at above 1750 K, implying that nitrogen does not form condensed stable structures
at pulverized flame temperatures, as does carbon. After initial fracture, loss of
nitrogen and total weight loss are linearly correlated with a nitrogen-to-carbon
slope of 1.25 to 1. 5. Volatile nitrogen compounds accounted for the major fraction of
NOx produced from coal-nitrogen, especially at high temperatures and low fuel/oxy-
gen equivalence ratios. The results suggest that low NOx emissions from pulverized
coal combustion are favored by a two-stage design: the first stage operated fuel-rich
at high temperature; the second, fuel-lean at low temperature.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Held/Group
Pollution
Combustion
Research
Coal
Pulverized Fuels
Nitrogen
Pyrolysis
Oxidation
Nitrogen Oxides
Pollution Control
Stationary Sources
Fuel-Nitrogen
Char
Devolatilization
13B
21B
14B
21D
07B
07D
07C
18. DISTRIBUTION STATEMENT
Unlimited
19. SECURITY CLASS (ThisReport)
Unclassified ______
2O. SECURITY CLASS (This page)
Unclassified
187
22. PRICE
EPA Form 2220-1 (9-73)
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