Environ. Sci. Technol. 2008, 42, 4771–4776
Effect of Volatile-Char Interaction
on the NO Emission from Coal
Combustion
M I N G Y U Y A O , † D E F U C H E , * ,†
YANHUA LIU,‡ AND YINHE LIU†
State Key Laboratory of Multiphase Flow in Power
Engineering, School of Energy and Power Engineering, and
School of Human Settlements and Civil Engineering, Xi’an
Jiaotong University, Xi’an 710049, China
Received August 4, 2007. Revised manuscript received
January 18, 2008. Accepted April 15, 2008.
To clarify the effects of volatile-char interaction on the
redistribution of fuel-N to N2 during devolatilization and the
reduction of NO through gas-solid reactions during combustion,
two types of experiments were performed on a novel reactor.
The separate combustion of volatile and char and the
combustion of entrained pulverized coal, and the formation of
NO was examined between 800 and 1100 °C by using four
typical Chinese coals with different ranks. The effect of
volatile-char interaction on fuel-N conversion to NO during
combustion was elucidated through comparing the NO emissions
from the two types of combustion experiments. The results
show that the volatile-char interaction is more important in the
redistribution of fuel-N to N2 during devolatilization than in
the reduction of NO over 900 °C, and a contrary conclusion is
obtained below 850 °C for all used coals. A specific parameter
has been proposed to characterize the relative importance of the
volatile-char interaction in the redistribution of fuel-N to N2
during devolatilization to the interaction in the reduction of NO
to N2 during simulataneous combustion of volatile and char.
The results are of significance for minimizing the NO formation
in industrial combustion processes.
1. Introduction
NO emissions from coal combustion are derived from both
the coal nitrogen and the nitrogen in the combustion air.
Three types of NO formation, thermal, prompt, and fuel,
have been identified (1, 2). Coal contains 0.5-2 wt % nitrogen
integrated in the organic coal structure. This nitrogen, known
as fuel-N, is partly converted to NO during combustion, and
becomes the major source of NO in pulverized coal combustion, accounting for more than 80% of the total NO
emission (3, 4).
To optimize combustion and minimize NO formation, a
good understanding of the fuel-N conversion is required.
The combustion of coal can be conceptually divided into
two steps, that is, the devolatilization of coal followed by the
reaction of the volatile and char with oxygen (1). The
devolatilization is the thermal decomposition of the fuel
resulting from the heating of the coal particles. The products
* Corresponding author phone: +86-29-82665185; fax: +86-2982668789; e-mail: [email protected].
†
State Key Laboratory of Multiphase Flow in Power Engineering,
School of Energy and Power Engineering.
‡
School of Human Settlements and Civil Engineering.
10.1021/es071945k CCC: $40.75
Published on Web 06/04/2008
 2008 American Chemical Society
of devolatilization are char and volatile. Part of the nitrogen
escapes as volatile-N in gas phase such as cyanic (CN) and
amino (NH) compounds. The nitrogen retained in the residual
char is called char-N. Thus, NO finally formed during coal
combustion comes from the combustion of both volatile-N
and char-N (5–7). However, char-N differs from volatile-N
in the mechanism of NO formation during combustion. The
NO from the combustion of volatile is formed through
homogeneous reactions, whereas the NO formation from
char-N involves a gas-solid heterogeneous oxidation and
an important reduction step of NO back to N2 (8).
One of the important objectives in coal combustion
research is to develop a comprehensive model of NO
formation. To do so, most investigators developed individual
models for volatile-N and char-N and then their relative
contributions to the total NO emission were evaluated
(1, 9–11). Therefore, it is a common practice that volatile and
char are combusted separately to investigate their relative
contributions to the total NO emission (2, 9). However,
separate combustion of volatile and char cannot represent
the real process in industrial furnaces. Previous work has
shown that volatile-char interaction can lead to drastic
changes in char structure and properties during the pyrolysis
of coal (13–15), and throughout the process of coal-N
oxidation there are significant gas-solid reactions between
volatile and char if they are not separated from each other.
Therefore it is necessary to take into account the effect of
volatile-char interaction to develop an even more exact
prediction model of NO formation during coal combustion
for the minimization of NO emission (16, 17).
Currently, little is known about the effect of volatile-char
interaction on the conversion of fuel-N to NO. Using a novel
reactor, this study is focused on the relative contributions of
volatile-N and char-N to the total NO emission and the effect
of volatile-char interaction during the coal combustion. The
results demonstrate that the volatile-char interaction has a
drastic effect on the conversion of fuel-N during combustion
through two ways: the fuel-N redistribution during devolatilization and the reduction of NO during the combustion of
volatile and char.
2. Experimental Section
Radicals formed during devolatilization surrounding the char
can greatly affect the reactions of char-N. Recent studies
(13–15) have confirmed that volatile-char interaction during
pyrolysis at elevated temperatures leads to a drastic change
in the releases of fuel-N. It was believed that the volatile-char
interaction would make more fuel-N release as N2 during
devolatilization through the gas-solid reactions (4, 16–20).
In addition, the reduction of NO could be enhanced by the
volatile-char interaction during the simultaneous combustion of volatile and char (22, 23). The fate of the fuel-N is
schematically illustrated in Figure 1.
Figure 1 implies that the volatile-char interaction could
help reduce the NO formation through the redistribution of
fuel-N to N2 during devolatilization and the reduction of NO
during the combustion that follows.
The conversion of fuel-N to NO without volatile-char
interaction is assumed to be X0. The conversion of fuel-N to
NO with volatile-char interaction taking effect only during
devolatilization is expressed by X1, and the separate combustion of volatile and char can exemplify this conversion.
X2 is assumed to denote the conversion of fuel-N to NO with
volatile-char interaction taking effect only during combustion, and the combustion of entrained pulverized coal can
exemplify this conversion.
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FIGURE 1. Simplified scheme to represent the fate of fuel-N in
theory.
x1 and x2 are defined as the volatile-char interaction
coefficients to describe the effect of the interaction on NO
abatement:
x1 )
X0
;
X1
x2 )
X0
X2
(1)
Two types of experiments were designed in order to clarify
the relative contributions of char-N and volatile-N to NO
emission from the combustion of coal and the effect of
volatile-char interaction on fuel-N conversion.
In the first type of experiment, NO emission from char-N
and volatile are quantified separately. A novel setup was
established for the experiments, as shown in Figure 2. The
quartz reactor was located in the furnace. The electrical
heating created approximately isothermal conditions. Temperature could be adjusted continuously. A 0.1 g coal sample
was used throughout all experiments. The sample was loaded
in the microfeeder (5) in advance, and the helium gas flow
(800 mL/min) entrained the coal particles into the reactor
that had been heated to a predetermined temperature within
two minutes, as shown in Figure 2. The exiting tube was
fixed and the sample tube could be pushed forward by a
stepping motor to keep a given distance between the bottom
of exiting tube and the surface of sample bulk, so that a
constant feeding rate could be maintained. Quartz cotton of
3 cm thick was placed in the inner tube (9) at about 10 cm
to its exit. Once coal particles entered the reactor volatiles
were released and char was formed. The volatiles were able
to pass through the quartz cotton and entered the outer tube
of reactor, but the char was retained on the quartz cotton.
When an experiment was started, the O2 stream (200 mL/
min) was switched to Line b in Figure 2 to oxidize the volatiles.
The O2 stream was switched to Line a in order to oxide the
char formed as soon as devolatilization had been finished.
Experiments continued until the complete combustion of
char.
The purity of the O2 and He used was more than 99.999%.
The concentration of NO from the combustion of volatile
and char was measured in succession by a GASMETTM
DX4000 FT-IR gas analyzer.
The profile of NO emissions from the separate combustion
of volatile and char is shown in Figure 3a. Ts represents the
time when O2 is switched and char-N began to combust. The
two peaks in the NO profile suggest that there should be two
NO sources, the first arises from the nitrogen in the volatile
matters, and the second arises from the fixed nitrogen in
char. The char came from the same coal sample as volatiles,
and it was combusted intactly as it was prepared to keep the
activity of char until being oxidized. Obviously, an interaction
between volatile and char would exist in the inner tube during
pyrolysis. However, since the volatile was separated from
char and oxidized in a different place, there would be no
interaction between the volatile and the char during this
period. So the total NO emission in this type of experiments
can be represented as
X1 )
X0
x1
(2)
X1 is the total conversion of fuel-N to NO from both volatile
and char.
Experiments of the second type involved the combustion
of entrained pulverized coal. The gas for feeding coal particles
was switched from pure He to He-O2 mixture. Volatile and
char undergo simultaneous combustion. The profile of NO
emission for the second type of experiments is shown in
Figure 3b. The volatile was oxidized as soon as it was released
from coal particles because of sufficient oxygen. But, there
FIGURE 2. Schematic experimental setup: 1 compressed gas; 2 steady flow valve; 3 pressure valve; 4 flow meter; 5 microfeeder; 6
three-way valve; 7 electrical furnace; 8 feeding tube; 9 inner tube; 10 outer tube; 11 quartz cotton;12 gas analyzer.
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FIGURE 3. NO emissions from the separate combustion of volatile and char (a) and the combustion of entrained pulverized coal (b).
TABLE 1. Fuel Characteristics
proximate analysis [wt%, dry]
ultimate analysis [wt%, daf]
coal
code
ash
volatile matter
fixed carbon
C
H
N
S
Oa
Shenmu
Hejin
Tongchuan
Jiaozuo
SM
HJ
TC
JZ
7.9
24.09
27.00
19.04
33.6
17.10
14.43
5.86
58.5
59.00
58.57
75.24
79.14
65.92
62.65
75.61
4.56
3.32
3.05
2.25
1.65
0.87
1.10
0.73
0.90
0.44
2.26
0.73
13.75
5.36
3.93
1.64
a
O determined by difference.
was little chance for the volatile to react with char under this
condition, and the effect of volatile-char interaction on the
fuel-N redistribution during devolatilization could be ignored. Nevertheless, the NO formed from volatile-N could
be reduced by char through the gas-solid heterogeneous
reactions, so that the volatile-char interaction was ineluctable during the combustion of volatile and char, and the
NO emission can be represented as follows:
X2 )
X0
x2
XNO )
t0+∆t
t0
F × CNO × 10-6 × dt ⁄ 22.4
M × Nd ⁄ 14
metal content, wt% (dry)
sample
ash (wt%,
dry)
Si
Al
Fe
Ca
Mg
K
Na
Ti
SM
HJ
TC
JZ
7.9
24.09
27.00
19.04
4.18
11.69
13.21
9.52
1.31
8.44
8.74
6.26
0.46
0.88
1.81
0.81
1.06
1.08
0.94
0.89
0.10
0.18
0.15
0.15
0.02
0.20
0.04
0.24
0.23
0.22
0.76
0.19
0.06
0.32
0.32
0.27
(3)
The total NO emission during experiments is the integral
of NO concentration within a certain length of time. If XNO
is used to represent the ratio of nitrogen converted to NO
during combustion process within ∆t to the nitrogen
contained in the sample, then
∫
TABLE 2. Ash and Metal Contents
× 100%
(4)
where F ) volumetric flow, L/s; t0 ) time at which
measurement begins; ∆t ) measurement duration, s; M )
sample weight, g; Nd ) nitrogen content in sample, % db;
and CNO ) NO concentration in flue gas, µL/L.
Four Chinese coals used for experiments were Jiaozuo,
Tongchuan, Hejin, and Shenmu, which are denoted respectively as JZ, TC, HJ, and SM hereinafter. All the coal samples
were air-dried at 105 °C for 24 h, then ground and sieved to
a size of less than 150 µm. The ultimate and proximate
analyses are listed in Table 1. Table 2 shows the ash and
metal contents in the used coals.
3. Results and Discussion
3.1. NO Emissions from the Separate Combustion of
Volatile and Char. Figure 4 shows the NO emissions from
the separate combustion of volatile and char, that is, the
emissions from the experiments of the first type, and the
total conversion of fuel-N to NO (expressed by X1) is XNO
from volatile plus that from char. The results show that the
total NO emission from separate combustion of volatile and
char is greater at lower temperatures, and reaches the highest
at 800 °C for all samples. Some previous investigators reported
that the total NO emission increased with increased temperature (22, 23), but their experiments were performed under
the condition of simultaneous volatile and char combustion.
It has been reported that there was abundant N2 release from
coal during devolatilization, and the conversion of fuel
nitrogen to N2 increased almost linearly with temperature
(16–20). As such, the total NO emission in this study inevitably
decreases because of the more conversion of fuel nitrogen
to N2 at higher temperatures. It seems that the volatile-char
interaction has more significant effect on the redistribution
of fuel-N to N2 during devolatilization at higher temperatures,
resulting in less NO emission. It can also be seen from Figure
4 that there is an obvious dependence of X1 on XNO from
char, since XNO from volatile varies very insignificantly with
temperature, especially when the temperature is over 1000
°C. It has been indicated that higher conversion of nitrogen
to N2 is always associated with lower conversion to char-N,
and XNO from volatile is independent of temperature when
the temperature is high enough (19, 20, 24). The experimental
results show that for HJ and TC coals the total NO emission
X1 and XNO from char is increased remarkably at 1100 °C
relative to the values at 1000 °C. As we know, various minerals
in coal can exert an influence on the conversion of char-N
to NO (25–27). It is assumed that the catalytic effect of
minerals in HJ and TC coals on the conversion of char-N to
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FIGURE 4. NO conversion from the separate combustion of volatile and char.
FIGURE 5. Contribution of Char-N to NO emission.
FIGURE 6. NO conversion from the combustion of entrained
pulverized coal.
NO becomes predominant at 1100 °C, resulting in more XNO
from char, hence more X1.
Figure 5 shows the contribution of char-N to total NO
emission, i.e., the ratio of the NO emission from char to the
total NO emission. Obviously, the NO emission from char of
SM coal (bituminous) is less than the NO emission from
volatile during 800-1100 °C, since it is always less than 35%.
As for JZ coal (anthracite), char-N becomes the primary source
of NO emission and the contribution of char-N is greater
than 56% at all temperatures. Both HJ and TC are lean coals,
and the contributions of char-N for these two coals are greater
than SM but less than JZ. It can be concluded that the
contribution of char-N to NO emission is greater for higher
rank coals.
In addition to coal rank, temperature can have a profound
effect on the contribution of char-N to NO emission. At low
temperatures (900 °C or below), char-N is the main source
of NO emission, except for the coal with very high volatile
content (SM coal for instance). As temperature increases,
volatile-N becomes more important. At high temperatures,
volatile nitrogen conversion is generally greater than that of
char nitrogen, which is in agreement with previous work. It
has been reported that the NO emission from volatile can be
as high as 60- 80% of the total fuel NO emission under the
conditions typical for pulverized coal systems (2). Besides,
mineral matters in coal can change the redistribution of
fuel-N between char and volatile and catalyze the oxidation
of char, which has been verified by our demineralized
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FIGURE 7. Conversion of fuel-N to NO from the two types of experiments.
FIGURE 8. Values of r at different temperatures.
experiments. It is suggested that the increase of char-N
contribution to NO emission at 1000 °C or above for HJ and
TC coals should be caused by the minerals in the two samples.
3.2. NO Emission from the Combustion of Entrained
Pulverized Coal. It is well-known that the oxidation of fuel-N
to NO and the reduction of NO occur simultaneously during
coal combustion. Consequently, the NO emission is determined by the competition of the two processes. NO emission
increases if the formation rate of NO is greater than its
reduction rate, and vice versa.
The conversion of fuel-N to NO during the combustion
of entrained pulverized coal, i.e., the second type of experiment, is shown in Figure 6. It can be seen that the fuel-NO
emission does not increase with increased temperature
monotonously. The conversion of fuel-N to NO reaches its
maximum at 900 or 1000 °C, and decreases beyond that
temperature, which is attributable to more NO reduction
over 1000 °C (28). In this type of experiment, the volatile-char
interaction affects the NO formation only through enhancing
the gas-solid reaction on the surface of char. The experimental results show that the effects of volatile-char interaction on the NO reduction can be significantly elevated at
1000 °C, so that the reduction rate of NO increases more
quickly than the oxidation rate of fuel-N to NO.
The comparison between the conversion of fuel-N to NO
from the combustion of entrained pulverized coal (expressed
by X2) and that from the separate combustion of volatile and
char (X1) is shown in Figure 7. It can be seen that X2 is less
than X1 at 800 °C and greater than X1 at 900 °C or above. That
is to say, more NO release from the combustion of entrained
pulverized coal than from the separate combustion of volatile
and char when the temperature is greater than 900 °C. The
effects of temperature on X1 and X2 are different. X1 decreases
with increased temperature below 1000 °C, and increases
beyond 1000 °C for some coals. X2 increases with increased
temperature below 1000 °C, and decreases beyond 1000 °C
for all used coals. X1 may be equal to X2 between 800 and 900
°C. The effect of volatile-char interaction on the reduction
of NO is least at 1000 °C and elevated at 1100 °C. The effect
of volatile-char interaction on fuel-N redistribution to N2
during devolatilization increases with increased temperature
when the temperature is lower than 1000 °C, but it might
decrease beyond 1000 °C.
3.3. Effect of Volatile-Char Interaction. As mentioned
above, x1 ) X0/X1 and x2 ) X0/X2. We define
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r)
x1 X 0 X 0 X 2
) ⁄ )
x2 X1 X 2 X1
(5)
r indicates the relative importance of the volatile-char
interaction in the redistribution of fuel-N to N2 during
devolatilization to the interaction in the reduction of NO to
N2 during the combustion of volatile and char.
The calculated results of r are shown in Figure 8. Obviously,
coal rank has a profound effect on the NO formation. It seems
that the volatile-char interaction during devolatilization is
stronger as the coal rank increases between 900-1000 °C,
but JZ coal is an exception. Beyond that temperature range,
coal rank has a more complex effect on the value of r. It can
be deduced that nitrogen content, nitrogen functionalities,
mineral matters, etc., could have an effect on the interaction.
More work is needed to clarify these effects.
While r is greater than 1.0, the volatile-char interaction is
more significant in the fuel-N redistribution to N2 during
devolatilization than in the NO reduction through gas-solid
reactions during combustion. A greater value of r means a more
significant interaction during devolatilization as the first step
of coal combustion. In fact, NO emission would decrease when
combustion is conducted under conditions of departure from
stoichiometry, for example in the case of bias combustion or
staged combustion. Theoretically, while r is greater than 1.0,
NO emission would be decreased to the greatest extent if the
two steps of coal combustion can be distinctly separated and
an adequate devolatilization process can be ensured. For
example, NO emission can be decreased if coal is gasified first
and the formed gas and residual char are sent to furnace at
different points to complete combustion separately.
If X2 is less than X1, i.e., r is less than 1.0, the volatile-char
interaction is more significant in the reduction of NO during
combustion than in the redistribution of fuel-N to N2 during
devolatilization. So that, combustion was expected to be
performed under stoichiometric conditions to decrease the
conversion of fuel-N to NO when r is less than 1.0. In practice,
stoichiometric excess and uniform air distribution are
necessary to control NO emissions, and bias combustion or
staged combustion are not recommended to decease NO
emissions in this case.
According to the above discussions and Figure 8, it can
be concluded that 1000 °C is the most suitable temperature
for controlling the NO emission from coal combustion under
conditions of departure from stoichiometry (oxygen deficiency) for these four coals. For example, bias combustion
or staged combustion technology can be applied over 900
°C. Stoichiometric excess and uniform air distribution are
necessary to control NO emission below 850 °C because r is
less than 1.0 for all coals.
Acknowledgments
The financial support from the Natural Science Fund of China
(50676076, 50576071) is gratefully acknowledged.
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