The fate of fuel nitrogen during combustion of char: A
review
Contents
1Contents
Abstract 2
31. Introduction
62. Experimental studies of the fate of char-N during
combustion
62. 1 Experimental technique
112. 2 Effect of operation conditions on the formation of NO
during char combustion
212. 3 The formation of other N-containing gas species during
char oxidation
243. Mechanistic studies of char-N conversion
253.1 The mechanisms of the conversion of NO during char
combustion
293.2 NO reduction over char with oxygen and effect of other
reacting gases
344. Model studies of nitric oxides formation from char
combustion
344.1 Single particle model
424.2 The SKIPPY (Surface Kinetics in Porous Particles)
approach
455. Concluding remarks
46References
The fate of fuel nitrogen during combustion of char: A
review
Zhou H., Jensen A., Glarborg P.,
Abstract:
The conversion of char-N to NO is the main contribution of the
formation of nitric oxides and is the most unsolved modeling
problem during the char combustion. This literature review
summarizes the current understanding of the fate of fuel-N during
combustion of char. The review focused on the following three
topics: (1) Experimental findings of the split of the fuel-N under
single and batch char particle combustion conditions. Approximately
100 % of fuel-N was found to be converted to NO for fine particle
under single particle conditions for fixed bed, fluidized bed, and
pulverized combustion systems. The finding implies that the
formation of minor N-containing species such as NH3, HNCO, and HCN
may be caused by the secondary reaction after the release of NO.
The operation conditions such as the reactor temperature, the
particle size, and the coal rank may affect differently the
formation and destruction of NO at different combustion systems;
(2) The most useful mechanism for describing the formation and
destruction of the NO and N2O during char combustion was proposed
by De soete (1990). The effects of major and minor species such as
O2 and H2 on the NO reduction of char have not been well elucidated
both experimentally and theoretically; and (3) Different simplified
models have been proposed at single particle level for describing
the fuel-N conversion during char combustion.
Keywords: char, combustion, nitrogen oxides, emission, model
1. Introduction
Nitrogen oxides (NO, NO2, and N2O) emission has drawn much
attention by researchers and public health authorities due to the
serious environmental impact of NOx (Kraushaar and Ristinen, 1993).
Combustion systems are one of the major sources of pollution due to
NOx. Fossil fuels such as coal, natural gas, petroleum, as well as
renewable bio-fuels and municipal solid waste for instances
agricultural residuals (straw, cotton stalk), and forest debris are
commonly used in the systems (Smoot, 1993, Saxena, 1994, Samuelsson
et al. 2004, Zhou et al. 2006). The fuels, particularly solid
fuels, as listed in Table 1, contain significant amounts of
nitrogen and give rise to the NOx emission during combustion.
Table 1. Typical nitrogen contents of various fuels
Fuel
Nitrogen (% w/w)
Natural gas
Crude oil
Heavy fuel oil
Light fuel oil
Coal
Biomass
May contain 1-5 % molecular N2
0.1 – 0.8
0.2 – 0.5
0.003 – 0.01
0.5 - 2.0
0.2 – 2.1
Three mechanisms have been identified for the production of NOx
in combustion systems: (a) Thermal NO - significant NO may be
formed by this route when the temperature is in excess of 1573 K.
Therefore control of the thermal NO can be achieved by decreasing
either the combustion temperature or the residence time at high
temperature; (b) Prompt NO - hydrocarbon radicals produced when the
fuels being burned react with N2 to produce HCN or CN which
subsequently be oxidized to NO. Under most practical combustion
conditions, the contribution of the prompt NO to total NO formation
is small; and (c) Fuel NO - the fuel NO is formed by combustion of
fuel nitrogen. In modern combustion systems, fuel NO contributes
more than 80 % of the total emissions. N2O may also be produced
during fuel nitrogen conversion particularly at low temperature as
in fluidized bed combustors.
The nitrogen functionalities present in coal and in char are:
pyrrolic (50-80 %), pyridinic (20-40 %), and quaternary nitrogen (0
– 20 %) (Thomas, 1997). The group may rearrange during pyrolysis
and at higher temperature the formation of quaternary nitrogen in
graphene layer has been postulated (Wojtowicz et al. 1995). The
effect of the nitrogen functionalities on the rate of NO formation
is believed to be small (Pels, et al. 1995, Stanczyk, 1999).
Previous studies have shown that the effect of volatile and char
nitrogen on the transformation of fuel-N to NO is significant.
Volatile nitrogen has been identified to form HCN, soot-bound
nitrogen, and NH3 as intermediate species during the combustion of
pyrolysis products. A large fraction of char-N may form NO
directly. It is believed that char nitrogen is a greater
contributor to NO and N2O formation than the volatiles (Tullin et
al., 1993, b). Technologically, the released volatile nitrogen is
more amenable to control by modification of combustion zone
aerodynamics than is char nitrogen (Wendt, 1993). The fate of the
nitrogen that remains within the char is crucial when determining
the ultimate NO emissions (Coda et al. 1998). Phong-Anant et al.
(1985) identified that the contribution to total NOx from char-N in
low NOx burners is greater than 60 %. Wendt (1993) further
described that the formation of NOx from coal char as the most
important unsolved NO modeling problems. An improved knowledge of
the parameters that determine NOx formation from the coal char is
imperative in order to generate efficient control methods as more
stringent emission regulations to be applied.
Reviews of the understanding of nitrogen conversion are given by
Miller and Bowman (1989), Johnsson (1994), Thomas (1997), Aarna and
Suuberg (1997), Molina et al. (2000), and Glarborg et al. (2003).
Different authors restricted to specific topics of NO formation and
destruction during solid fuel combustion. Miller and Bowman (1989)
discussed mechanisms and rate parameters for the gas-phase
reactions of nitrogen compounds. Johnsson (1994) tabulated data on
the fraction of char-N to NO and N2O in fluidized bed combustion.
He concluded that the conversions of char-N to NO vary between
20-80 %, largely because of wide variations in experimental
techniques and the influence of devolatilization temperature,
combustion temperature, coal type, particle size, and nitrogen
content of the char. Thomas (1997) reviewed the influence coal and
char structural characteristics on the release of nitrogen oxides
during the combustion of chars. Aarna and Suuberg (1997) elegantly
summarized the kinetics of the carbon-NO reaction, and they pointed
out that this reaction may involve the possible initial
chemisorption of NO and also the reaction of surface complexes. The
reduction reaction with pure NO is generally found to be first
order with respect to the NO concentration. The reaction is known
to be enhanced in the presence of CO and inhibited in the presence
of water. Molina et al.(2000) focused their review on the mechanism
and model of nitrogen release from the char to the homogeneous
phase and its further oxidation to NO, and the reduction of NO on
the surface of the char. Glarborg et al. (2003) gave a general
review of solid fuel (coal and biomass) nitrogen conversion in
fired systems. They emphasized on discussing the homogeneous and
heterogeneous pathways in fuel NO formation and destruction and
evaluating the effect of fuel characteristics, devolatilization
conditions and combustion mode on the oxidation selectivity towards
NO and N2.
To further elucidate the mechanism of NO formation during the
char combustion, the following three topics are reviewed below: (1)
The experimental findings of the split of fuel nitrogen at single
particle condition and during batch combustion for different
combustion systems; (2) The mechanisms of the conversion of fuel-N
proposed by different researchers. We will focus on the review of
the formation of HCN during the char combustion and the reduction
of NO on the char surface in the presence of oxygen; (3) The
modeling work of the fate of fuel-N during char combustion at
single particle condition. The main purposes of the review are to
present how the fuel-N is split during the char combustion at
different reaction systems, and how to explain the experimental
findings from the mechanisms and the model point of view.
2. Experimental studies of the fate of char-N during
combustion
The heterogeneous char-oxygen reaction, the complex char
structure, the surrounding gas species and particles, as well as
the very uncertain fluid dynamics in the practical combustion
system make it very difficult to elucidate the pathway for char-N
conversion during the char combustion. Therefore, different
experimental techniques for different combustion systems have been
used by researchers in order to understand the conversion of fuel-N
to NO during the char combustion.
2. 1 Experimental technique
Table 2 summarizes the experimental studies of char-N conversion
during combustion. Test rigs include pulverized coal combustor
(PC), fixed bed reactor (FB) including Thermogravimetric Analysis
(TGA), and fluidized bed reactor (FBC). It can be observed that the
percents (W/W) of fuel-N converted to NO, N2O, and HCN are in the
ranges of 3-90% , 0.2-5.7 %, 0.5-10 % depending on coal type, char
preparation, residual volatile matter content, temperature, reactor
type, stoichiometry, and particle size. The NO is the major product
of fuel-N converted and the ratio of fuel-N conversion to NO is
remarkably different for different researchers.
Traditional experimental method, for instance, a steady-state
technique (de soete, 1990), is commonly employed to determine the
global rates of char oxidation, NO and N2O formation during char
oxidation and heterogeneous NO and N2O reduction on the char
surface.
Some new and promising techniques have been applied and are very
helpful to improve the understanding of the pathway of fuel-N
conversion. Winter et al. (1996) applied an approach called an
iodine addition technique, in which the purpose of the addition of
iodine is to reduce radical concentrations to equilibrium levels
and therefore to suppress homogeneous reactions, but not affect the
heterogeneous. Their experiments provided strong evidence that N2O
may be only produced by homogeneous oxidation of HCN in fluidized
bed combustion conditions. However, the iodine addition technique
has not been used by other researchers mainly due to the influence
of the iodine chemistry on the combustion process is not known yet.
Miettinen (1996) used tracer technique to trace the path of fuel-N
during the char combustion. The advantage of the technique is that
it may distinguish the source of N. A 15N-isotope-marked NO was
used in the inlet gas by to investigate N2O formation during
fluidized bed char combustion. Her experiments confirmed three
paths of the possibilities of N2O formation during char combustion,
i.e. (1) heterogeneous formation of N2O (one nitrogen atom from the
char and the other from NO); (2) homogeneous formation of N2O (two
nitrogen atoms from the inlet gas containing NO), and (3) primary
formation of a cyano compound (HCN or HNCO) then further reacts
with O2 and / or NO to form N2O (
H
NCO
O
HCN
+
®
+
,
CO
O
N
NO
NCO
+
®
+
2
).
The NO formed from a particle can be subsequently reduced by
surrounding char particles or gas species which may make the
reaction system very complicated and very hard to elucidate the NO
formation during char combustion. To reduce the possible secondary
reaction, a few special techniques were developed. To minimize the
temperature fluctuation and the second reaction of NO with char, a
pulse technique was adopted by Orikasa and Tomita (2003). In their
experiments, a small amount of char sample (about 2.5 mg of char,
roughly 10-15 particles) was used to minimize the heat generation
and a series of a little O2 pulse was fed to the sample bed.
Furusawa et al. (1982), Aihara et al. (2000), and Jensen et al.
(2000) adopted a single particle technique, in which very small
amount of char was placed into fixed bed reactors during char
combustion. In this case, the char particles situated on the
reactor distributor is only one layer. Therefore, the interactions
between particles can be neglected. The experiments thus may be
regarded as single particle experiments. Their experiments showed
that most of fuel-N is converted to NO, and in some cases the
conversion is near 100 %.
Table 2. Summary of experimental char-N conversion studies
Reactor
Parent coal
Char preparation
Nitrogen
(N/C, w/w)
Fraction of char-N to NOx, N2O, and HCN
Operation conditions
Reference
1
PC
FMC coal char
From FMC-COED coal gasification process
0.0136
35
.
0
05
.
0
-
=
NO
f
150.0
m
m
, stoichiometric 1.02-1.25, O2/Ar
Pershing and Wendt (1976)
2
PC
Montana lignite
Char produced at temperatures of 1250 and 1750 K
0.0072-0.0162
35
.
0
05
.
0
-
=
NO
f
21%O2/He, 1250 K
Song et al. (1982)
3
FB
2 coals
Char produced at 1273,1373,and 1473 K
0.008-0.022
82
.
0
1
.
0
-
=
NO
f
65-1125
m
m
, 0.02-5 g, 5%O2/Ar, 1073 and 1173K
Ninomiya et al. (1989)
4
FB
3 coals
Char produced in argon at 1450 K
0.0145-0.0178
674
.
0
347
.
0
-
=
NO
f
EMBED Equation.3
057
.
0
006
.
0
2
-
=
O
N
f
35-500
m
m
, 50-100 mg, 2-21 %O2/Ar
De soete (1990)
5
FB
9 coals
Char produced in fixed and fluidized bed at 1173 K
0.00758-0.02275
5
.
0
24
.
0
-
=
NO
f
500-590
m
m
, 5-10 mg, 1163 K, 3.15O2/Ar, single particle level
Shimizu et al. (1992)
6
FBC
Kentucky No. 9 bituminous coal
Char produced in the FBC at 1223 K
0.021
58
.
0
18
.
0
-
=
NO
f
1.02-4.48 mm, 0.2-1.2g, 10%O2, 753-1003 K
Yue et al. (1992)
7
FB
2 subbituminous coal 1 lignite
Char produced in the FB up to 1223 K
0.0144-0.0188
3
.
0
03
.
0
-
=
NO
f
EMBED Equation.3
036
.
0
009
.
0
2
-
=
O
N
f
90-106
m
m
,0.2 MPa, 6-20 % O2, 0-50 %CO2
Croiset et al. (1995)
8
FBC
3 coals – Tilmanstone, Holditch, and Baddesley
coals were put into the FBC
Not available
33
.
0
06
.
0
-
=
NO
f
EMBED Equation.3
025
.
0
002
.
0
2
-
=
O
N
f
1.0/2.5 g (1-2 cm), single particle level and
1.0g batches of small particles (1.4-1.7 or 2.0-2.4 mm)
1073 K/1173 K
Hayhurst and Lawrence (1996)
9
FBC
Bituminous and sub-bituminous coal
coals were put into the FBC
0,0126-0.018
18
.
0
16
.
0
-
=
NO
f
005
.
0
014
.
0
-
=
HCN
f
EMBED Equation.3
035
.
0
015
.
0
2
-
=
O
N
f
5,10,15 mm, 600-1173 K, 5-21%O2/N2, at a particle level
Winter et al. (1996)
10
FB
(TGA)
20 coals from anthracite to bituminous coal
Produced in entrained flow reactor at 1273 K
0.0124-0.031
03
.
0
04
.
0
-
=
NO
f
37-75
m
m
, 5 mg, 20%O2/Ar, 873-1323 K
Harding et al. (1996)
11
FBC
Newland coal
Produced in fluidized bed at 1123 K
Not available
035
.
0
31
.
0
-
=
NO
f
04
.
0
2
=
O
N
f
200 mg(4-5 particles),1092 K, 8% O2/He
Goel et al.(1996)
12
Fluidized bed
Anthracite and bituminous coals
Produced by rapid pyrolysis at 1123 K
0.0157-0.017
54
.
0
25
.
0
-
=
NO
f
04
.
0
2
=
O
N
f
0.25-1.5mm, 1123 K, 2.5-21%O2/N2, single or a few particles
Klein and Rotzoll (1999)
13
FB
(TGA)
Bituminous coals
Produced in a quartz reactor up to 1123 K
022
.
0
0145
.
0
-
063
.
0
05
.
0
-
=
NO
f
25 mg, 20%O2/Ar, 773-1473 K.
Arenillas et al. (1999)
14
FB
Blair Athol and bituminous coals
Produced in fluidized bed at 1273 K
018
.
0
22
.
0
11
.
0
-
=
NO
f
70-150
m
m
, 50-250 mg, 0.55-2.0 %O2/He, up to 1273 K
Aihara et al. (2000)
15
FB
(TGA)
Bituminous coal
Produced in entrained flow reactor
0244
.
0
0109
.
0
-
22
.
0
=
NO
f
EMBED Equation.3
12
.
0
=
HNCO
f
06
.
0
=
HCN
f
EMBED Equation.3
01
.
0
2
=
O
N
f
45-75
m
m
, 60 mg, 2 % O2/He, 873 K
Ashman et al. (2000)
16
FB
bituminous and anthracite coals
Produced in fixed bed at 1123 – 1423 K
016
.
0
09
.
0
-
90
.
0
06
.
0
-
=
NO
f
10-20
m
m
, 0.1-20 mg, 10 % O2/N2, 1123, 1323, 1423 K, experiments were
performed at particle level
Jensen et al. (2000)
17
FB
bituminous and lignite coals
Produced in fluidized bed at 1273 K
0075
.
0
0067
.
0
-
68
.
0
06
.
0
-
=
NO
f
0.25-0.5 mm, 25 mg, 5 %O2/N2
Lin et al. (2002)
18
PC
4 coals
Produced in an entrained flow reactor
0,017-0.021
8
.
0
01
.
0
-
=
NO
f
45-75
m
m
,CO2/H2O/O2, 1473 K
Nelson et al. (2002)
19
PC
5 coals of various rank – bituminous, subbituminous, and lignite
coals
Produced in the pc combustor at temperatures of 1700-1820 K
0212
.
0
0097
.
0
-
6
.
0
~
4
.
0
=
NO
f
~200
m
m
, 64%Ar/15%CO2/21%O2, 1800-2020 K
Spinti and Pershing (2003)
20
FB
(TGA)
9 coals
Produced in drop-tube reactors at 1273 and 1623 K
0.0174-0.0206
0
.
1
~
2
.
0
=
NO
f
Ar/20%O2,
750-1250 K
Jones et al. (2004)
21
FB
Australian bituminous coal
Produced in an entrained flow reactor
0.021
2
.
0
=
NO
f
,
1
.
0
=
HCN
f
0.1 ~ 1.0MPa,
m
m
75
45
-
,
mg
60
, 873 K, 2% O2
Park et al. (2005)
1
®
NO
f
when loading is close to zero
0.1 MPa, 1-120 mg
2. 2 Effect of operation conditions on the formation of NO
during char combustion
Table 2 provides an overview This section summarizes
experimental investigation of the effect of the type of combustion
system, the properties of char, the operation conditions such as
reactor temperature, stoichiometry, particle size, and the amount
of char used on the NO formation during char combustion.
2.2.1 Fuel-N conversion to NO at a single particle condition
(a) – Experimental findings under fixed bed conditions
The fuel-N conversion to NO at single particle condition
provides important information on the mechanisms of NO formation
and reduction during char combustion. However, only a few
researchers have paid attention to this point mainly due to the
measurement difficulty. Figure 1 plots the fraction of the
conversion from fuel-N to NO against char loading. It is observed
that more fuel-N is converted to NO with the decreasing of the char
loading. In the case of the single particle condition, i.e. the
char loading is close to zero, most of fuel-N is converted to
NO.
General agreement reached by different researchers is that the
fraction of fuel-N to NO is independent of O2 concentration (Jensen
et al. 2000, Park et al. 2005) at single particle conditions, and
the smaller particle size used, the more fuel-N is converted to NO.
The effects of parent coal rank and the temperature on the fuel-N
to NO reported by different researchers are inconsistent. Jensen et
al. (2000) found that the fraction of fuel-N to NO depends on the
reactor temperature. They estimated that around 65 % of nitrogen in
the char evolved as NO at 850 °C and it was almost 100 % at 1050
and 1150 °C under single particle condition. More fuel-N is
converted to NO at the higher reactor temperature. However, as
shown in Figure 2, experiments by Ninomiya et al. (1989) indicated
that the effect of temperature on the conversion of fuel-N to NO is
small. The different findings by Jensen et al. (2000) and by
Ninomiya et al. (1989) may be explained as following: the char-O2
reaction becomes diffusion limited and the surface reaction
dominates the char-O2 reaction at the high reactor temperature. The
formed NO at the char surface may escape the char surface rapidly
and the contribution of the secondly reaction of NO reduction over
char surface is negligible. Almost all fuel-N is therefore
converted to the NO. While O2 may diffuse to the internal side of
the char particle, part of the formed NO inside the particle may be
reduced by the char when the NO diffuses from inside to the out
side at the low temperature. Less fuel-N is therefore converted to
NO. In Jensen’s experiments, the applied temperatures are range
from 1123 to 1423 K, the control reaction may change from kinetic
to diffusion, while in Ninomiya’s experiments, the reaction is only
in either kinetic control or diffusion control in the temperature
range of 1073 to 1173 K..
Jensen et al. (2000) found that the fuel-N to NO is independent
of the parent coal for making coal char. Very similar tendencies of
the fuel-N to NO as functions of char loading were obtained when
bituminous coal char or anthracite char was applied. While
experiments by Ninomiya et al. (1989) showed that the lower rank
the parent coal was, the higher conversion of fuel-N to NO was
obtained as shown in Figure 1. In their experiments, the applied
Taisei coal has much higher volatiles than that of the Taiheyo. It
should be noted that the loading shown in Figure 1 by Ninomiya et
al. (1989) is estimated on the particle size and the projected area
provided by Ninomiya et al. (1989).
0
50
100
150
200
250
300
350
0,0
0,2
0,4
0,6
0,8
1,0
Park et al. (2005)
Jensen et al. (2000)
1123 K
1323 K
1423 K
Aihara et al. (2000)
Ninomiya et al. (1989)
Taisei
(460)
Taiheyo
(180)
(460)
(1125)
Char-N to NO / %
Char loading / mg
Figure 1: The conversion of char-N to NO as function of the char
loading in the fixed bed reactor
0
200
400
600
800
1000
1200
0,0
0,2
0,4
0,6
0,8
1,0
1073 K
1173 K
Char-N to NO / %
Particle size (10
-6
) / m
Figure 2: The conversion of char-N to NO as function of the
particle size under single particle condition at two different
reactor temperatures (Ninomiya et al. 1989)
(b) – Experimental findings under fluidized bed conditions
Fuel-N conversion to NO under fluidized bed conditions has been
found to be less than 60 % at single particle conditions (Johnsson
et al. 1994). The particle size used in these experiments was
always at mm level and the reactor temperature was at around 1123
K, which means the char-O2 reaction is always under both kinetic
and diffusion controls. The internal secondary reaction of NO+C
therefore may be the main reason of the low fuel-N conversion to
NO. Moreover, due to the measurement accuracy, generally a few
numbers of char were used in the experiments, the well mixed char
particle in the fluidized bed reactor may also lead to the external
secondary reaction.
Experiments indicated that the fuel-N conversion to NO decreases
monotonically with increasing the particle size (Tullin et al.
1993, a). The larger the particle size, the more formed NO during
C(N)–O2 reaction inside the particle may be reduced by C-NO
reaction when the NO diffuses from the inside of the particle to
the surrounding gas. The fuel-N conversion to NO increases with the
increasing of the bed temperature as shown in Figure 3. With the
increase of the bed temperature, the C-O2 reaction transfers from
the kinetic control to the diffusion control. Under diffusion
control, the surface reaction may dominate the C-O2 reaction. That
is, most of the NO formed by the fuel-N-O2 reaction may released
directly to the surrounding gas which leading to the higher fuel-N
to NO.
The effects of O2 concentration on the fuel-N to NO reported by
different authors are inconsistent as shown in Figure 4. Klein and
Rotzoll (1999) reported that the fuel-N to NO decreases with the
increasing the inlet oxygen concentration which may be attributed
to the high char particle temperature leading to the fast NO-C
reaction. However, Winter et al. (1996) found that the conversion
decreases initially when the inlet O2 concentration increases from
5 to 10 %, and then the fuel-N conversion increases again when the
applied O2 concentration is 21 %. Experiments by Hayhurst and
Lawrence (1996) indicated that less NOx and N2O are released during
burning of char when the concentration of O2 in fluidizing gas was
reduced. Moreover, Hayhurst and Lawrence (1996) also reported that
coals of higher rank yiel more N2O and more NOx during char
combustion. Therefore, more experiments should be conducted to
determine the effect of inlet O2 concentration on the fuel-N
conversion in the fluidized bed combustion of char.
500
600
700
800
900
1000
0,0
0,2
0,4
0,6
0,8
1,0
Winter et al. (1996)
Hayhurst and Lawrence (1996)
Klein and Rotzoll (1999)
Fuel-N to NO / %
Bed temperature /
o
C
Figure 3. The effect of bed temperature on the conversion of
char-N to NO
0
5
10
15
20
25
0.0
0.2
0.4
0.6
0.8
1.0
Winter et al. (1996)
Hayhurst and Lawrence (1996)
Klein and Rotzoll (1999)
Fuel-N to NO / %
Oxygen concentration in inlet gas / %
Figure 4. The effect of oxygen concentration on the conversion
of char-N to NO
(c) – Experiments findings under pulverized conditions
In the case of pulverized coal combustion, Haussmann and Kruger
(1990) are the sole authors who reported that for Rosebud coal
(Proximate analysis: volatile matter: 42.4 %, 10.1 % ash; Ultimate
analysis: 67.2 % C, 4.4 % H, 0.78% N, 0.52 % S, and 17.0 % O2 )
under single particle conditions at oxygen concentrations as low as
0.5 %, nearly 100 % of the released fuel nitrogen is converted to
NO during combustion. They inferred that under such conditions,
individual particles remain surrounded by an oxygen environment,
with nitrogen reduction chemistry confined to the volatile cloud
surrounding individual coal particles. This suggests that for the
Rosebud coal, fuel nitrogen conversion to molecular nitrogen
requires conditions under which released volatile matter is able to
form aggregate clouds, in regions devoid of oxygen. In contrast,
their experiments indicated that only 55 % evolved fuel-N is
converted to N2 during combustion of Pittsburg # 8 coal under
similar conditions. The low fuel-N conversion to NO may be
attributed to the secondary heterogeneous of NO on tar/soot due to
the high yield of tar and soot during the Pittsburgh # 8 coal
combustion. Experiments by Haussmann and Kruger (1990) also confirm
100 % fuel-N may be converted to NO under the condition of without
any secondary reactions.
(d) - Implications of the experimental results
Experiments indicate that the NO/N2 selectivity strongly depends
on the extent of the secondary reaction of NO with char, in other
word, all fuel-N may be converted to NO and then the formed NO
reacts with its surrounding species or solid by homogeneous and
heterogeneous reactions to form N-containing species such as HCN,
HNCO, and NH3 etc. If the conclusion can be further verified
experimentally under fine and single particle combustion
conditions, then the existing mechanisms of the formation of
N-containing species during char combustion should be
reconsidered.
2.2.2 Fuel-N conversion to NO at batch particle condition
Table 3 lists the effect of the operation conditions such as the
inlet oxygen concentration, the particle size, the coal rank, the
reactor temperature, and the reactor pressure on the fuel-N
conversion to the NO under batch coal particle conditions.
Obviously, in most cases the changes of the fuel-N conversion to NO
reported by different authors are consistent.
Table 3. The effect of operation conditions on fuel-N to NO
Oxygen
Concentration
Particle
Size
Coal
rank
Rector temperature
Reactor pressure
Fixed bed
Consistent
Inconsistent
Consistent
consistent
↔
↔
↑ ↑
↑ ↓
↑ ↓
↑ ↓
De soete (1990)
Ashman et al. (1999)
De soete (1990)
De soete (1990)
Harding et al. (1996)
Harding et al. (1996) Ashman et al. (1998)
Orikasa and Tomita (2003)
Jones et al. (2004)
Croiset et al. (1995) Lin et al. (2002)
Park et al. (2005)
Fluidized
Bed
Consistent
Consistent
Consistent
Consistent
Consistent
↑ ↓
↑ ↓
↑ ↑
↑ ↑
↑ ↓
Tullin et al. (1993)
Gao (2003)
Hayhurst and Lawrence, 1996,
Yue et al. 1992
Hayhurst and Lawrence (1996)
Shimizu et al. (1992
Hayhurst and Lawrence, 1996,
Yue et al. 1992, Amand and Leckner, 1991
Laughlin et at. (1994)
Shen and Yoshizo (2001)
Pulverized combustor
Consistent
Consistent
Consistent
↑ ↓
↑ ↓
↑ ↑
↑ ↓
Pershing and Wendt (1976)
Song et al. (1982)
Nelson et al. (2002)
Haussmann et al. (1990)
Chen et al. (1982)
Song et al. (1982)
Spinti and Pershing (2003)
Note 1. ↑ ↑ represents the conversion of fuel-N NO increase with
increasing of the parameter;
2. ↑ ↓ represents the conversion of fuel-N NO increase with
decreasing of the parameter;
3. ↔ no noticeable tendency
Many experiments have been performed as shown in Table 2 in the
cases of batch coal particles were used in the fixed bed reactor.
Both de Soete (1990) and Ashman et al. (1999) found that changes of
the inlet oxygen concentration have little effect of the fuel-N
conversion to NO. Contradictory trends were reported by de Soete
(1990) of the effect of particle size on the fuel-N conversion to
NO. He found that the fuel-N conversion may either increase or
decrease with the increase of particle size depending on different
char used. 20 coals, covering a wide range of rank from anthracite
to high volatile bituminous coal, were used by Harding et al.
(1996). A lower conversion of char-N to NO was observed for coal
chars produced from high ranks coals. They attributed to the
finding were resulted from the low surface area and low reactivity
of char produced from low rank coals. Consensus results have been
obtained on the effect of reactor temperature and the bed pressure.
Less fuel-N is converted to NO with the decreasing of the reactor
temperature and increasing the reactor pressure. Furthermore, Jones
et al. (2004) found that the conversion of fuel-N to NO decreases
to an approximate constant value around 20 % at high bed
temperature when diffusion becomes important. They further
concluded that the quite constant conversion of fuel-N to NO may be
applicable to a wide range of combustion systems.
Quite consensus results have been obtained under the fluidized
bed combustion conditions as illustrated in Table 3. The fraction
of char-N released as NO decreased when the particle size, the O2
concentration, and the reactor pressure were increased. It was
found that the increase of operation pressure remarkably decreased
the NO emission. The higher the rank of the coal and the reactor
temperature, the greater is the fraction of char-N that was
released as NO. However, some experiments of Hayhurst and Lawrence
(1996) showed the formation of NO is not sensitive to the
temperature and so it appears that no clear trend can be
observed.
Under pulverized coal combustion conditions, the char-N to NO
conversion decreases with increasing the fuel equivalence ratio as
shown in Figure 5. The fractional conversion of fuel-N to NO are in
the ranges of 10-70 % under fuel-lean conditions, and it is 5-10 %
under fuel-rich conditions. It seems that the effect of increasing
O2 on fuel-N to NO in the fuel lean zone is stronger than that in
the fuel rich area, for example, about 68 % fuel-N is converted to
NO at Φ=0,3, it is 0.39 at Φ=1, and 0.32 at Φ=3.6, where Φ is the
equivalence ratio.
0
1
2
3
4
5
0.0
0.2
0.4
0.6
0.8
1.0
Nelson et al. (2002)
Spinti and Pershing (2003)
Song et al. (1982)
Pershing and Wendt (1976)
Fraction conversion of fuel-N toNO / %
Fuel/oxygen equivalence ratio
Figure 5. Effect of equivalence ratio on the conversion of
char-N to NOx
In actual coal flame, more or less NO exists in the gas phase
before the onset of char oxidation mainly formed from volatile-N. A
few experiments have been performed to study the effect of the
initial NO concentration in the gas phase on the char-N conversion
during char oxidation. Experiments by Spinti and Pershing (2003)
indicated that the conversion of char-N to NO depends strongly on
the initial NO concentration regardless of char rank and nitrogen
content. The apparent conversion (defined as the ratio of formed NO
to char-N plus N in added NO) decreases dramatically as the initial
NO rises. Different from the observations of Spinti and Pershing,
Nelson et al. (2002) found that the conversions are quite similar
in the cases of with or without doping NO in the gas phase.
Experimental findings of the effect of fuel-N content on the
conversion are different. Pershing’s data (1990) exhibits a slight
decrease in fuel-N conversion to NO with increasing fuel-N while
Spinti and Pershing’s findings (2003) show opposite tendency.
The effects of individual operation conditions on the fuel-N
conversion to NO are not always uniform for different reactors as
shown in Table 3. Under fluidized bed combustion conditions, more
fuel-N was found to be converted to NO when the bed temperature was
increased, while contrary tendencies were found under fixed bed and
pulverized combustion conditions. The effect of oxygen
concentration on fuel-N conversion is insignificant in the fixed
bed combustion, while less fuel-N is converted to NO in the cases
of fluidized bed and pulverized combustion conditions, and so
on.
2. 3 The formation of other N-containing gas species during char
oxidation
Char-N is released predominately as N2 and NO with small amounts
of N2O, HCN, NH3, and HNCO. The formed minor species as N2O may
significantly impact the environment. The experimental
determination of the formation these species are summarized as
following:
2.3.1 Formation of N2O
N2O is regarded as one of greenhouse gases, which plays an
important role in destruction of the ozone layer and gives rise to
the greenhouse effect. The fuel-N conversion to N2O is important
only when the temperature is low in the ranges of 1123-1273 K, for
example when the fluidized bed combustor is used. The N2O emission
may reach as high as 20 – 250 ppm. The fuel fed into the fluidized
bed combustor undergoes drying, devolatilization, and char
combustion. The species evolved during these processes react in a
complex homogeneous and heterogeneous reaction system. Moreover,
the different solids in the bed material act as effective
catalysts. Thus, it is difficult to explain the whole mechanisms of
formation and reduction of N2O. Detail reviews of the mechanism of
formation and destruction of the N2O was given by Johnsson
(1994).
2.3.2 Formation of HCN
In literature, strong evidence exists that when char-nitrogen is
oxidized the main products are not only NO, N2, and N2O but
significant amounts of HCN is also formed ranging typically from 1
to 10% of char-nitrogen. This HCN can be further oxidized to
nitrogen oxides and N2.
Kramlich et al. (1989) proposed that the devolatilization or
gasification of char nitrogen followed by gas-phase reaction could
lead to N2O formation. They suggested that HCN might be the species
that once released from the char is oxidized in the homogeneous
phase to produce N2O. A simple HCN release model was proposed
(Eqs.1 to 2)– but the mechanism is unknown.
[
]
HCN
N
C
®
(1)
[
]
[
]
N
kC
dt
N
dC
-
=
(2)
where
÷
ø
ö
ç
è
æ
-
=
RT
k
100
,
29
exp
5800
.
A similar hypothesis was proposed by Amand and Leckner (1993)
when explaining why the conversion of char nitrogen to N2O
increased during CH3CN addition to a fluidized bed reactor. De
Soete (1992) reported the formation of HCN during the reaction of
coal char with NO, but there were no further discussions about its
formation mechanism. It is not clear whether nitrogen in HCN was
from inherent nitrogen in char matrix or from gaseous NO. Jones et
al. (1995) reported HCN formation during coal char gasification in
the presence of 20 % O2 at above 500 oC. Winter et al. (1996)
conducted the char combustion at 750 oC. They added iodine to the
reactant gas stream to suppress the radical formation and found the
increment of HCN formation. They assumed the release of HCN is
proportional to the carbon conversion, it was arbitrarily taken as
4 % w-%. Ashman et al. (1998, 2000) reported HCN formation during
the oxidation of coal char at 600 oC. They suggested that a portion
of HCN may be formed through the secondary reaction of NO, which
were derived from the oxidation.
Orikasa et al. (2002, 2003) found that the formation rate of HCN
was greatly affected by the available hydrogen. They thus proposed
the following mechanism of HCN formation:
(
)
(
)
HCN
N
H
C
H
or
H
N
C
®
®
+
,
2
1
2
(3)
(
)
(
)
(
)
HCN
N
H
C
H
C
N
C
®
®
+
,
(4)
They further postulated that direct formation of HCN from C(N)
might also be possible, and that the exact nature and structure of
the C(H,N) species remains unclear. In this scheme, C(N) is formed
by reaction of NO with the char surface
(
)
(
)
O
C
or
and
CO
N
C
C
NO
+
®
+
. The fate of the
(
)
N
C
depends on the availability of hydrogen. Hydrogen may be
supplied by gas phase H2, a gas phase H atom, and/or a H atom on
the char surface. In this scheme, N2 formation becomes predominant
when there is insufficient hydrogen available, and results from the
reaction of C(N) with NO:
(
)
(
)
O
C
or
and
CO
N
NO
N
C
+
®
+
2
. This mechanism is in agreement with the results of isotopic
labeling experiments by Chambrion et al., (1997, a and b, 1998).
Their experiments indicated that the mixed isotopic composition of
N2, HCN, and HNCO were formed when 15NO is added to the gas phase.
Experiments by Park et al. (2005) showed that the presence of H2O
in the reactant gas also gave rise to HCN. Based on the
observation, they proposed that the source of H in Eqs. (3) and (4)
may also be provided by steam.
2.3.3 Formation of HNCO
FTIR measurements by Nicholls and Nelson (2000) indicated the
formation of detectable concentration of HNCO in a laboratory scale
quartz fixed bed reactor at temperatures of 600 and 900 oC. Ashman
et al. (2000) identified that 12 % fule-N may be converted to HNCO
at low temperature of 873 K in 2 % O2. They proposed that the
formation of HNCO may have similar process as that of the HCN.
2.3.4 Formation of NH3
Park et al. (2005) found that NH3 was produced in the presence
of H2O. The formation of NH3 increases by increasing of the
pressure and the concentration of H2O in the feed gas. They found
the major nitrogen-containing products are NH3, HCN, and N2. The
conversions of fuel-N to NH3, HCN, and N2 change slightly varying
with H2O content. The fractional conversion of char-N to NH3, HCN
and N2 are around 50, 25, and 20 %. No NO and HNCO was detected in
the presence of H2O alone. The results suggested that NH3 may be a
secondary product of the char-H2O reaction, and probably arises
from interactions of the initial product of HCN which reacts with
the char surface which is modified by the uptake of water in some
form. The mechanism of the formation NH3 is unknown.
3. Mechanistic studies of char-N conversion
The production of NOx from coal char combustion is a balance
between the oxidation of char-N and its subsequent reduction at the
char surface and its surrounding environments. Different mechanisms
have been proposed although there is no evidence to prove that a
given mechanism is valid each or is useful in the development of
rate equation.
3.1 The mechanisms of the conversion of NO during char
combustion
The most comprehensive mechanism of NO and N2O from coal was
proposed by De soete (1990). A steady-state technique was used to
determine the global rates of char oxidation, NO and N2O formation
during char oxidation and heterogeneous NO and N2O reduction on the
char surface. A transient-state technique, created by sudden
suppression of O2 in the reactant gases, was employed to determine
the rate of different adsorption and desorption reactions.
Consequently, the flowing mechanisms were proposed:
(a) Char oxidation
(
)
(
)
CO
C
O
-
¾
®
¾
-
+
2
2
1
2
(5)
(
)
fas
C
CO
CO
-
+
¾
®
¾
-
2
(6)
(
)
fas
C
CO
CO
-
+
¾
®
¾
-
2
3
2
(7)
(b) Formation of NO
(
)
(
)
(
)
(
)
CNO
CO
CN
C
O
-
+
-
¾
®
¾
-
+
-
+
4
2
(8)
(
)
fas
C
NO
CNO
-
+
¾
®
¾
-
5
(9)
(c) Formation of N2O
(
)
(
)
fas
C
O
N
CNO
CN
-
+
¾
®
¾
-
+
-
2
2
7
(10)
(d) Destruction of NO and N2O
(
)
CO
N
C
NO
fas
-
+
¾
®
¾
-
+
2
6
2
1
(11)
(
)
CO
N
C
O
N
fas
-
+
¾
®
¾
-
+
2
8
2
(12)
Reaction 6 is the mass balance of the following two reactions,
from which 9 is rate controlling:
(
)
(
)
N
C
CO
C
NO
fas
...
2
9
-
+
-
¾
®
¾
-
+
(13)
(
)
fas
C
N
N
C
-
+
¾
®
¾
-
2
...
2
2
10
(14)
where atoms or atom groups placed between brackets are solid
bound atoms.
The De soete approach (de soete 1990) is quite useful in that it
also incorporates the formation of N2O at the char surface and thus
it can be extended to the conditions of N2O is significant.
Table 4 lists the different mechanisms proposed by researchers
except by de soete (1990). Chan et al. (1983) proposed the possible
mechanisms of the enhancement of NO reduction over char in the
presence of CO. They thought that 2 reasons may be attributed to:
(1) NO-CO reaction may be catalyzed by the carbon surface; and (2)
CO may attack the chemisorbed oxygen, C(O), the subsequent release
of CO2 from the C surface provides active carbon atom for further
reaction with NO. Suzuki et al. (1994) tried to explain the role of
O2 in the C-NO reaction since their experiments indicated that the
presence of a certain amount of O2 may significantly increases the
rate of NO-C reaction. With the presence of O2, a number of
oxygen-containing complexes, C(O), may be produced. The
decomposition of COx may provide active site for NO-C reaction.
Additionally, the formed nitrogen-containing compound, C(N), on the
carbon surface may be attacked by either O2 or NO to form N2.
However, two important factors, i.e. the rise of char particle
temperature and the produced CO due to the presence of O2, on the
enhancement of C-NO reaction were not accounted for by Suzuki et
al. (1994). Besides the mechanisms of NO and N2O formation
suggested by de Soete (1990), Krammer and Sarofim (1994) proposed a
possible pathway of NO and N2O. They believed that the bounded C-N
must first be broken due to the attack of O2 to form intermediate
active bound nitrogen, -N, and the –N may further reacts with O2 or
NO to form NO or N2O. They also proposed the intermediate produced
by C-N-O2 reaction may be CNO, and the CNO may be attacked by
surrounding NO to form N2O. Their proposed mechanisms seem to be in
agreement with experiments of Tullin et al. (1993, a). Harding et
al. (1994) proposed that the mechanisms for NO formation and
reduction on the char surface are similar to those of carbon
oxidation. The main point in their mechanisms is that the surface
reaction by adjacent complex such as C(N) and C(NO) plays important
role in the NO and N2O formation and reduction. They believe that
the mechanisms are not reasonable for the following reasons: let do
a simple estimation, a char has ultimate analysis of the fixed
carbon 79.5 and N 1.25 % respectively. The numbers of molecular C
is 50 times higher than that of N. If we assume that elements C, H
and N distributed uniformly in the solid, then we may say the
elements H and N should be separated by C and the opportunity of
N-N in neighborhood is rare. Chambrion et al. (1998) and Aihara et
al. (2000) proposed that the NO may react with complex C(N) to form
N2O. More recently, Park et al. (2005) suggested that the formation
of HCN is due to the attacks of the surface C(N) by the presence of
the surrounding H2 or by the adjacent C(H) and C(N). The author
also believes the possibility of C(N) and C(H) is very low due to
the low very low H content in chars.
Table 4. Different mechanisms of char oxidation, and NO and N2O
formation and destruction from char-N, and the values of reaction
rate
Char oxidation
NO formation
N2O formation
HCN formation and destruction
NO and N2O destruction
Chan et al. (1983)
2
2
5
.
0
N
CO
CO
NO
char
+
¾
¾
®
¾
+
(
)
f
C
CO
CO
O
C
+
®
+
2
(
)
2
5
.
0
N
O
C
C
NO
f
+
®
+
Suzuki et al. (1994)
(
)
x
fas
CO
O
C
O
C
+
-
®
+
2
(
)
fas
C
CO
-
®
-
(
)
x
f
CO
N
C
NO
C
+
®
+
(
)
(
)
x
CO
N
C
NO
O
C
+
®
+
(
)
2
2
5
.
0
N
CO
O
N
C
x
+
®
+
(
)
2
N
CO
NO
N
C
x
+
®
+
Krammer and Sarofim (1994)
(
)
(
)
CO
N
O
N
C
+
®
+
2
5
.
0
(
)
NO
O
N
2
2
2
®
+
(
)
O
N
NO
N
2
®
+
or
(
)
(
)
CO
O
N
NO
C
NO
-
+
®
-
+
2
Harding et al. (1996)
(
)
2
2
O
C
C
O
f
®
+
(
)
(
)
(
)
lorm
lorm
fas
CO
CO
C
O
C
-
+
®
+
2
CO
C
O
C
f
lorm
+
®
)
(
EMBED Equation.3
(
)
2
2
CO
C
O
C
f
m
+
®
(◊)
(
)
(
)
(
)
NO
C
C
O
C
N
C
s
+
®
+
(
)
NO
C
NO
C
f
+
®
(
)
(
)
O
N
C
N
C
NO
C
fas
2
2
+
®
+
(
)
NO
C
NO
C
s
®
+
(
)
2
2
2
2
O
N
C
NO
C
f
+
+
®
-
(
)
2
2
2
N
C
N
C
f
+
®
Chambrion et al. (1998)
(
)
O
N
C
NO
N
C
fas
2
+
®
+
Aihara et al. (2000)
(
)
CO
NO
O
N
C
+
®
+
2
(
)
O
N
C
NO
N
C
fas
2
+
®
+
(
)
CO
N
NO
N
C
+
®
+
2
Park et al. (2005)
(
)
(
)
HCN
N
H
C
H
or
H
N
C
®
®
+
,
2
1
2
EMBED Equation.3
(
)
(
)
(
)
HCN
N
H
C
H
C
N
C
®
®
+
,
(
)
(
)
O
C
or
and
CO
N
C
C
NO
+
®
+
(
)
CO
N
N
C
NO
+
®
+
2
(
)
CO
N
C
O
N
fas
+
®
+
2
2
3.2 NO reduction over char with oxygen and effect of other
reacting gases
The chemistry of NO reduction by carbon or in the presence of CO
is excellently reviewed by Thomas (1997) and Aarna and Suuberg
(1997). It is generally recognized that the reaction of pure nitric
oxide with carbons is first order with respects to NO and the
NO-carbon reaction was enhanced significantly with the presence of
surrounding CO. Therefore, instead of repeating these aspects, we
hereafter summarize how O2, CO2, H2, and H2O present in practical
combustors will affect the NO-char reaction.
3.2.1 NO-O2-C system
In the char combustion process, NO gas produced during
combustion will further react with char. Generally there is some
oxygen in these practical processes. There is consensus that the
presence of oxygen alone promotes NO reduction during NO/char
reactions.
In a pioneering work of Smith and co-workers (1959), six
reaction (15-20) steps as following were proposed to explain
experimental observations that O2-preteatment increased the
NO-carbon reaction rate whereas H2 treatment played the opposite
role. They postulated that the first step was the formation of C-O
surface complexes, and these complexes were very important
intermediates in the reactions. Furthermore, they assumed that C-O
complexes formed by O2 were greater in quantity and somewhat
different in nature from those produced by NO. When the carbon was
treated with O2 prior to the addition of NO, the surface has more
the usual concentration of
(
)
O
C
'
and
(
)
O
C
'
'
, and thus the NO reduction rate is greater.
[
]
(
)
(
)
[
]
2
'
'
'
'
'
'
2
N
O
C
O
C
C
C
NO
+
+
®
+
+
(15)
(
)
[
]
(
)
[
]
'
'
'
'
'
'
'
'
C
ON
O
C
C
O
C
NO
+
×
×
×
«
+
+
(16)
(
)
[
]
(
)
2
2
'
'
'
'
'
'
'
'
N
O
C
O
C
C
ON
O
C
NO
+
+
®
+
×
×
×
+
(17)
(
)
[
]
(
)
[
]
'
'
'
'
'
'
'
'
'
C
ON
O
C
C
O
C
NO
+
×
×
×
«
+
+
(18)
(
)
[
]
2
2
'
'
'
'
'
'
'
'
'
'
N
O
C
O
C
C
ON
O
C
NO
+
+
«
+
×
×
×
+
(19)
2
2
'
'
'
'
2
'
'
'
'
2
2
N
O
C
O
C
NO
+
«
+
(20)
(In the figure : α represents the concentration ratio of
oxygen to NO)
Figure 6. Temperature dependence of reaction rate by Kunii et
al. (1980)
The addition of small amounts of oxygen increased the rate of NO
destruction over char as shown in Figure 6. Kunii et al. (1980)
found that the rate of NO reduction over char was significantly
enhanced by the presence of O2 in a range of lower temperatures
(< 1023 K) while the rate was not significantly reduced at 1073
K. At the low temperature, the desorption of C-O surface complexes
is regarded as a controlling reaction of the overall rate, and at
the high temperature, the reaction with the coadsorbed O atoms may
keep the carbon surface continuously rich in C-O complexes thus
maintaining the constant reaction rate. The mechanism was further
developed by Yamashita et al. (1993). They postulated that the C-O
complexes consists of reactive C(O) and stable C-O, and both of
them are increased by the presence of O2 caused by the
reaction:
(
)
O
C
or
O
C
O
C
fas
-
®
+
2
2
2
2
. The desorption of oxygen from C(O) produces highly reactive Cf
sites which is easily attacked by NO.
Chan et al. (1983) and Rodriguez-Mirasol et al. (1994) ascribed
the enhancement of the NO reduction over char to the action of CO
formed by C-O2 reaction. The rate of the O2-char reaction is
approximately two orders of magnitude greater than that of the
NO-char reaction. For this reason, when NO and O2 are passed over
carbon under conditions of no diffusional resistance, either the O2
will be completely consumed early in the reactor before any
appreciable NO reduction is achieved, or a negligible fraction of
NO reduction will take place with an appreciable fraction of the O2
escaping unreacted. In this case, O2 may both accelerate and
inhibit the NO-CO reaction. Under diffusional control conditions,
the O2 is consumed close to the particle surface, excepting for the
smaller particles of for low-temperature operation. Most of the NO
reduction will occur therefore in the porous of the solid where
negligible amount of O2 are present. The effect of O2 will then be
to produce CO which will codiffuse with NO and accelerate the rate
of NO reduction. The effect of O2 is to enhance NO reduction by
generating CO to inhibit the reaction. De soete (1992) found that
the effect of O2 in NO-C-CO2 system depends on CO2/CO ratio. The
oxygen inhibits NO reduction when the CO2/CO ratio in the oxidation
products is greater than one, and it promotes NO reduction when the
ratio is smaller than one. Chen and Tang (2001) reported that NO-C
reactivity decreases about 2 – 4 orders to magnitudes in the
natural logarithmic scale in typical reburning environments (CO2
and O2 in feed are 16.8 and 1.95 % respectively).
Suzuki et al. (1994) proposed that not only the formation of C-O
complex, but also the removal of nitrogen complex in the presence
of O2 as:
(
)
x
f
CO
O
C
O
C
+
®
+
2
,
(
)
2
2
2
N
CO
O
N
C
+
®
+
.
Another plausible role of O2 in NO-carbon reaction is the
formation of NO2 as intermediate. Ahmed et al. (1993) found that
NO2 was more readily reduced to N2 than was NO whatever in the
presence or absence of O2. They proposed more NO could be oxidized
to NO2 by increasing the gas-phase O2 concentration as:
2
2
2
1
NO
O
NO
®
+
, and
2
2
NO
C
C
NO
-
®
+
. The mechanism was confirmed by Kong and Cha (1999) that NO2 is
more readily chemisorbed than that of NO on the carbon surface.
However, experiments of Suzuki et al. (1994) indicated that the
major product of C-NO2 reaction is NO not N2 in the absence of
O2.
Contrarily, O2 inhibits N2O reduction with carbon or carbon
catalysts (Noda et al. 1999; Zhu et al. 2000). The opposite effect
of O2 on NO-C and N2O-C systems can be attributed to the different
manners of N2O and NO adsorption on the carbon surface (Zhu et al.
2001). Theoretical calculation indicated that in the case of excess
O2, NO is more likely to chemisorb in the form of NO2 and NO
chemisorption is more thermodynamically favorable than O2
chemisorption. N2 can form through –C-ONO-C or –C(N) complexes (the
former one is the main pathway). Oxygen remains on the carbon edge
site during the formation of N2 since it is not necessary to weaken
or break the adjacent C-C bounds. New active sites are quickly
produced through the formation of epoxy oxygen and consequently the
NO-C reaction is greatly improved. By contrast, it was found that
N2O chemisorption is much less thermally stable either on the
consecutive edge sites or edge sites isolated by semiquinone
oxygen. Although the presence of excess O2 produces a large amount
of new active sites, the produced active sites are occupied by O2
again. Therefore, the N2O-C reaction rate is depressed by the
presence of O2.
3.2.2 NO-CO2-C system
CO2 is assumed essentially inert in NO-C reaction system.
However, Suuberg and Aarna (1998) reported that the NO reduction
over graphite is strongly inhibited by the addition of CO2, and the
reaction rate in the NO/CO2 graphite system is, on average, lower
by a factor of 1.7. In the NO/CO2-wyodak coal char system, they
reported that the NO reduction over the char is not affected by the
presence of CO2 at supplying CO2 partial pressure below 2 kPa,
while the inhibiting effect of CO2 is obvious when the CO2 partial
pressure is around 101 kPa. They thus concluded that the extent of
inhibition of the NO-C reaction by CO2 depends on the natural of
carbon itself, as well as on CO2 partial pressure.
3.2.3 NO-H2O-C system
There has been no detailed study of the NO-C reaction in the
presence of water vapor. Levy et al. (1981) reported that the NO-C
reactivity is reduced by water vapor at high temperature above 1000
oC.
3.2.4 NO-H2-C system
The effect of the presence of H2 on NO-C reactivity is
inconsistent. Smith et al. (1959) found that the NO-carbon reaction
rate was reduced by the pretreatment of H2. They found that N2 was
formed instead of
O
C
'
and
O
C
'
'
(see Eq. 15) in which less C-O complexes appeared in the char
surface and thus leads to the lower NO-C reaction rate. Furusawa et
al. (1982) and Johnsson (1994) indicated that, in the presence of
H2, char catalyzes the NO reduction, and the consumption of carbon
in the char is nearly negligible.
In coal combustion system, NO-C reactivity is more or less
affected by the presence of O2, CO, CO2, H2, and H2O. The reached
consensuses results are: the NO-C reactivity is significantly
enhanced in the presence of CO, and the reactivity may be increased
with the O2 addition. The mechanisms of enhancement of NO-carbon
reaction in the presence of O2 have not been investigated
systematically yet as described above, and no global kinetic rate
regarding the influence of O2 on NO-carbon reaction has been
developed. The effect of the presence of H2O, H2 and SO2 on NO-C
reactivity has not yet been well elucidated theoretically and
experimentally.
4. Model studies of nitric oxides formation from char
combustion
4.1 Single particle model
Following the pioneering work of Wendt and Schulze (1976),
Sarofim and coworkers (Goel et al., 1994; Goel, Zhang et al., 1996;
Krammer and Sarofim, 1994; Tullin, Goel et al., 1993; Tullin,
Sarofim et al., 1993; Tullin et al., 1995), and Visona and Stanmore
(1996) used exclusively heterogeneous processes to explain and
model the main experimental findings on the conversion of char-N to
nitrogen oxides.
The formation of NO during a char particle includes the release
of char-N inside the particle and further reaction at the boundary
layer of the particle. The general transport equation for
description the process in symmetric, spherical coordinate (r terms
only) is used via:
(
)
(
)
i
i
i
i
i
S
r
r
r
r
v
r
r
r
t
,
2
2
2
2
1
1
f
f
f
r
rf
+
÷
ø
ö
ç
è
æ
¶
¶
G
¶
¶
=
¶
¶
+
¶
¶
(21)
where
r
is the density, t is the time,
f
is the process variable (energy, gas species concentration, ……),
r is the radial position, v is the velocity,
G
is the transport coefficient, S is the source term. The first
term on the right side represents the rate of change of
f
per unit volume, the second term is the convection flux of
f
, the first term on the right side stands for the diffusion flux
of
f
, and the last term is rate of generation of
f
. The
f
can have a positive or negative value.
To model the formation of nitric oxides, simplified mechanisms
of the production of major species of CO and CO2, and the formation
and destruction of minor species of HCN, NO and N2O have been
proposed by different authors as listed in Table 5. CO and CO2 are
two major products during char combustion. General, it is assumed
that CO is initially formed by surface reactions and then is
further oxidized to CO2 either in the pores of the particle or in
the gas phase. Modeling of char combustion was reviewed (Smith,
1982, Smoot and Smith, 1985) and modeling of CO oxidation around
the particle (Goel et al. 1996, Mitchell et al. 1990). Modeling of
char combustion, CO oxidation, the formation and reduction of NO
and other N-containing species both internal and external of a char
particle is complicated due to the high temperatures involved, the
complex coupling between the partial differential equations
governing mass and energy conservation, and stiff gas phase
chemistry as indicated in Eq. (21).
Table 5. Mechanisms used for modeling the release of NO from a
single char particle model
Char oxidation
NO formation
N2O formation
HCN formation and destruction
NO and N2O destruction
Wendt and Schulze (1976)
CO
O
C
®
+
2
2
1
2
2
2
1
CO
O
CO
®
+
NO
O
N
char
®
+
-
2
2
1
NI
NI
(
)
O
N
N
NO
+
®
+
2
Hahn et al. (1983)
(
)
2
2
1
1
1
1
2
2
CO
CO
O
C
+
+
+
®
+
+
+
a
a
a
a
a
(
)
NO
O
N
®
+
2
2
1
NI
NI
(
)
2
2
,
2
CO
CO
N
NO
C
+
®
+
Arai et al. (1986)
CO
O
C
¾
®
¾
+
1
2
2
1
O
H
O
H
2
2
2
1
4
1
®
+
(
)
(
)
g
NH
s
NH
®
NO
O
N
®
+
2
2
1
NI
NI
Shimizu et al. (1992)
CO
O
C
®
+
2
2
1
NO
O
N
char
®
+
-
2
2
1
where
(
)
[
]
NO
T
k
r
=
a
NI
NI
2
2
2
1
CO
N
CO
NO
char
+
¾
¾
®
¾
+
Tullin et al. (1993)
CO
O
C
®
+
2
2
1
(
)
(
)
char
C
N
CO
NO
a
2
1
-
=
(
)
char
C
N
CO
O
N
a
=
2
NI
2
2
2
1
CO
N
CO
NO
char
+
¾
¾
®
¾
+
CO
N
C
NO
+
®
+
2
2
1
CO
N
C
O
N
+
®
+
2
2
Goel et al (1993)
(
)
(
)
CO
C
O
-
¾
®
¾
-
+
1
2
2
1
(
)
(
)
C
CO
CO
-
+
¾
®
¾
-
2
(
)
(
)
2
3
2
2
1
CO
CO
O
-
¾
®
¾
-
+
(
)
(
)
C
CO
CO
-
+
¾
®
¾
-
2
4
2
(
)
(
)
NCO
or
CNO
CN
O
-
-
¾
®
¾
-
+
5
2
2
1
(
)
(
)
N
CO
NCO
-
+
¾
®
¾
-
6
(
)
(
)
C
NO
CNO
-
+
¾
®
¾
-
'
6
(
)
(
)
NO
N
O
-
¾
®
¾
-
+
7
2
2
1
(
)
(
)
C
NO
NO
-
+
¾
®
¾
-
8
(
)
(
)
O
N
N
NO
2
13
-
¾
®
¾
-
+
(
)
(
)
(
)
CO
O
N
NCO
NO
-
+
-
¾
®
¾
-
+
2
'
13
EMBED Equation.3
(
)
(
)
C
O
N
O
N
-
+
¾
®
¾
-
2
14
2
NI
(
)
(
)
CO
N
C
NO
-
+
¾
®
¾
-
+
2
9
2
1
(
)
(
)
NO
C
NO
-
¾
®
¾
-
+
10
(
)
(
)
CO
C
CO
-
¾
®
¾
-
+
11
(
)
(
)
(
)
C
CO
N
CO
NO
-
+
+
¾
®
¾
-
+
-
2
2
1
2
2
12
EMBED Equation.3
(
)
(
)
CO
N
C
O
N
-
+
¾
®
¾
-
+
2
15
2
Visona and Stanmore (1996)
CO
O
C
®
+
2
2
1
NO
O
HCN
®
+
2
(
)
NO
O
CN
®
+
-
2
2
1
NI
(
)
HCN
CN
®
-
2
N
NO
HCN
®
+
2
N
C
NO
®
-
+
Sarofim et al. (1999)
(
)
(
)
CO
C
O
-
®
-
+
2
2
1
(
)
NO
O
CN
®
+
-
2
2
1
NI
NI
(
)
CO
N
C
NO
-
+
®
-
+
2
* NI – not included
In general, the following approaches have been used to simply
the description of NO release during a single char particle
combustion: (1) NO formation: NO production is proportional to the
nitrogen/carbon atomic ratio of char and the char combustion rate
(Wendt and Schulze, 1976, Hahn et al. 1983, Shimizu et al. 1992,
Tullin et al. 1993, Goel et al. 1994, Sarofim et al. 1999). Arai et
al. (1986) assumed that char-N is converted competitively to NO and
NHi at the surface of the particles during char combustion. Tullin
et al. (1993) assumed that fraction of the formed NO may be further
converted to N2O; (2) NO reduction: Different approaches have been
applied to describe the NO reduction of the formed NO inside the
particle, on the particle surface, and with surrounding species.
Wendt and Schulze (1976) believed that the NO disappearance is
controlled by a reverse Zeldovich reaction, Hahn et al. (1983)
regarded that the NO reduction over char occurs both on the char
surface and inside the char particle and the effect of CO on NO
reduction is neglected, while the presence of CO on NO reduction of
the internal surface of char is included by Shimizu et al. (1992);
(3) The NO formation and destruction inside and outside of the char
particle was described separately (Wendt and Schulze, 1976, Visona
and Stanmore, 1996) or only the reactions inside the particle was
considered (Goel et al. 1994); (4) Mass transfer from the particle
surface to the external gas phase is driven by molecular diffusion
and convection; and (5) The pore size and surface area are
constants (Wendt and Schulze, 1976, Arai et al. 1986, Tullin et al.
1993, Goel et al. 1994, Sarofim et al. 1999) or change during char
combustion (Hahn et al., 1983; Shimizu et al. 1992).
The importance of physical properties of char on the selectivity
of fuel-N toward NO depends on combustion conditions (Wendt and
Schulze, 1976). Pore size is important at low temperature fluidized
bed combustion, while the particle size significantly affects and
the temperature has very weak impact on the NO formation. Hahn and
Shadman (1983) concentrated their work on investigating the effect
of the changes of porosity and surface area on the rate of NO
formation during char combustion. They found that better
predictions as compared with experimental data were given with the
char structure effects were included, and the model provided
possibility of explaining why fuel-N is only partially converted to
NO even when there is excess O2 in the bulk gas. Twenty-five
heterogeneous and homogeneous reaction steps have been adopted to
model the overall formation of char-NO during the combustion of a
single particle of coal char by Arai et al. (1986). Two distinct
features can be identified in the model: (1) char-N is converted to
both NO and NHi at the particle surface during the single particle
char combustion; and (2) homogeneous reactions after different
species released from char particle were described using detailed
chemical kinetic reactions (18 steps). Furthermore, the model was
validated by experimental data of the transient formation of
char-NO and for the overall fraction conversion of char-N into
char-NO for three kinds of chars. Shimizu et al. (1992) used a very
simple model to explain the observations of their experiments. In
their model, NO reduction on the internal surface of a single char
particle was included. In Visona and Stanmore’s (1996) model, the
intrinsic char combustion developed by Smith (1976) was employed
and the NO or HCN may be released directly from char-N. They
suggested that there are two possible mechanisms for the conversion
of char-N to NO at high temperature: (1) char-N may first go
completely to NO, and subsequently be reduced on the char surface.
The conclusion is in agreement with the latter experimentally
findings by Jensen et al. (2000); and (2) The char-N may go
completely to HCN, and is then be oxidized and reduced by secondary
reactions with surrounding atmosphere. The most detailed model
regarding the mechanisms of the formation and destruction of NO at
a single particle level has been developed by Sarofim and his
colleagues (Tullin et al. 1993, Goel et al. 1993, Sarofim et al.
1999). Based on Goel’s model, a simple energy balance model was
added and a modified partial differential equations solution method
was used by Kilpinen et al. (2002) to investigate the nitrogen
chemistry in fluidized bed char combustion conditions.
The Goel model (1993), includes 15 reaction steps which are
grouped into 4 main parts, i.e. (1) the reaction of O2 with the
carbon to form CO and CO2; (2) the formation of NO from char bound
nitrogen; (3) destruction of NO on char surface and subsequent
formation of N2O; and (4) destruction of N2O on the char surface.
The model is based on char-C oxidation and char-N oxidation and
reduction as illustrated in Figure 7.
Figure 7. Reaction network proposed by Goel et al. (1993)
The Goel’s model single particle model solves for the diffusive
transport and the kinetics of the reactants and products in the
shrinking char sphere. The main assumptions are:
1. The char particle is spherically symmetric,
2. The system is at pseudo steady state,
3. Ash falls from the particle surface during the oxidation,
4. The diameter of the particle decreases continuously while its
density remains constant.
The governing mass conservation equations of the single particle
oxidation are:
[
]
[
]
[
]
(
)
0
2
2
2
,
=
×
+
÷
÷
ø
ö
ç
ç
è
æ
¶
×
¶
+
¶
×
¶
i
i
e
R
r
r
r
D
(22)
The boundary conditions are
[
]
0
0
=
¶
×
¶
=
center
r
r
(23)
[
]
[
]
[
]
(
)
s
g
surface
i
e
k
r
D
R
r
×
-
×
=
¶
×
¶
=
¥
,
(24)
where
i
e
D
,
is the effective diffusion coefficient (m2/s).
[
]
¥
×
and
[
]
s
×
are the concentrations of species i in the bulk gas and at the
particle surface, respectively, kg is the external mass transfer
coefficient (m/s), and r is the radial position inside the particle
(m). The term Ri represents the rates of the chemical reactions
occurring inside the char particle: char-C oxidation and char-N
oxidation and reduction.
Figure 8. Comparison of the experimental and model predicted
fuel-N to NO and N2O (Goel et al. 1994)
Figure 8 is an example of the comparison of the experimental and
model predicted of fuel-N to NO and N2O. The model prediction is in
good agreement with the measurement which suggests that the kinetic
model is able to explain the experimental observations and that it
can be used as a predictive tool for other experimental
observations.
One of the main limitations of Goel’s model is that the energy
due to the C-O2 reaction was not taken into account. Kilpinen et
al. (2002) extended the Goel’s model with the energy balance and
tested different input parameters such as kinetic rate of C-O2 and
C-NO. Simulation by Konttinen et al. (2002) shows that the
temperature of char particles reaches around 24 K higher than its
surrounding gas temperature while the temperature profile inside
the particle is quite uniform except near the particle surface when
the applied O2 is 8 % (vol), char particle size is 4 mm, and the
surrounding gas temperature is 1123 oC. The NO-char reaction was
found to be around 25 % higher as the energy balance was accounted
when the C-NO reaction kinetic given by Goel was used, which
indicated that the temperature change due to the C-O2 reaction may
greatly affect the formation and destruction of NO. The prediction
obtained by the improved model of Kilpinen et al. (2002) matches
better with experimental data as compared with Goel’s model.
The mentioned model for describing NOx formation and destruction
during single particle char combustion can be ascertained as
stationary model since the effect of fluid dynamics on the
combustion process is not accounted for. With the development of
the computer capacity, a new approach to track the motions of
individual particles including chemical reactions in fluidized bed
combustion of char has been employed by Rong et al. (2000) and Zhou
et al. (2004). They found that the evolution of the release of
fuel-N during the char combustion is not only affected by the
physical properties of char but also the local fluid hydrodynamics
of gas and particle particularly their temperatures. The changes of
particle temperature greatly affect the NOx emission.
Unfortunately, the global kinetic rate instead of very detailed
mechanism model were applied by Rong et al. (2000) and Zhou et al.
(2004) to describe the char combustion and the formation and
destruction of NO and N2O. More detailed mechanisms for describing
the formation and destruction of the conversion of fuel-N to NO
together with Computational Fluid Dynamic (CFD) approach describing
the gas particle flow may be applied in the future.
4.2 The SKIPPY (Surface Kinetics in Porous Particles)
approach
The SKIPPY approach, developed by Haynes at The University of
Sydney (Ashman et al. 1999), may be applied to study fuel-N
conversion during coal combustion. The SKIPPY may calculate species
and temperature profiles for the reaction of a porous solid in a
reacting gaseous environment at pseudo-steady state, it accounts
for detailed finite-rate elementary chemical kinetics and molecular
transport. The relationship between the various components of the
program is depicted in Figure 9. The SKIPPY code employs data and
subroutines from the CHEMKIN package (Kee et al., 1989), the
SURFACE CHEMKIN package (Coltrin et al., 1990) and the Sandia
transport packages (Kee et al., 1986) to handle the chemical
kinetics, thermodynamics, and transport properties
calculations.
Ashman et al. (1998) reported their modeling work of fuel-N
conversion during char combustion in a TGA system using the SKIPPY
model. The network of the reaction of char oxidation and fuel-N
conversion chemistry are listed in 25 to 30:
2Csolid + O2 ( 2CO
(25)
2Nsolid + O2 ( 2NO
(26)
Ysolid + O2 ( HCN + 2CO
(27)
In accordance with accepted models for the reactions of NO with
chars, secondary heterogeneous reactions of this species were
modeled as
Nsolid + NO ( N2O
(28)
Nsolid + NO + CO ( N2 + CO2
(29)
Finally, the well known reduction of N2O by carbon is
represented as
Csolid + N2O ( N2 + CO
(30)
In Reaction 27 , Y represents the HCN precursor “Y” .
Chemkin
Interpreter
Gas Phase
Reactions
Thermodynamic
Data Base
Chemkin
Link File
Surface Data
File
Transport
Data Base
Transport
Fitting Code
Surface
Interpreter
Transport
Link File
Surface Link
File
Transport
Library
Surface
Library
Chemkin
Library
SKIPPY
Keyword
Input File
Restart
File
Printed
Output File
Binary
Save File
Figure 9. Schematic diagram showing the various components of
the SKIPPY program and the relationships between each of them.
Figure 10 compares the experimental and predicted distribution
of the char nitrogen into gas-phase products. The predictions are
in fairly good agreement with the experimental data although the
poor state of knowledge of the fundamental kinetics of the
heterogeneous reactions of carbon, no attempt has been made to
optimize the fit of the predictions to the experimental points.
The SKIPPY approach was adopted by Molina et al. (2002) to
investigate char-N conversion to NO, N2O, and HCN at the boundary
layer during chat combustion. They found that the predictions are
in agreement with most of the qualitative experimental trends. The
SKIPPY approach is a new method which can be used to predict fuel-N
conversion during char combustion even in the case of the
mechanisms of the formation and destruction of nitric oxides during
the char combustion are unclear.
0.00E+00
1.00E-01
2.00E-01
3.00E-01
4.00E-01
5.00E-01
6.00E-01
7.00E-01
8.00E-01
9.00E-01
1.00E+00
800
900
1000
1100
1200
1300
1400
Temperature (K)
Fraction of Char N
NO
N2O
HCN
N2
Figure 10. Comparison of experimental and predicted fate of char
N in TGA experiments
5. Concluding remarks
To develop more effective NOx control technology, a large amount
of experiments of NO formation and destruction during char
combustion at different operation conditions have been performed
and different mechanisms to describe the relevant reactions have
been proposed. Consensus observations as well as inconsistent
results were reported by different research groups. The following
conclusions can be reached after a review of these studies:
(1) NO is a primary nitrogen-containing product in the
char-nitrogen combustion. In the case of without secondary reaction
(small particles under single conditions), approximately 100 % of
fuel-N is converted to NO. Globally, less fuel-N is converted to NO
as the char particle loading, the particle size, or the bed height
increase which can be attributed to the occurrence of secondary
reaction on the char surface. The observations implies that all
fuel-N may be first converted to NO in the presence of O2, and then
the formed NO may further react with the char itself or surrounding
particles or gas to form N-containing species such as N2O, HCN and
NH3. The mechanisms of the fuel-N conversion during char combustion
should be re-accounted if all fuel-N is converted to NO applicable
for all chars without secondary reactions. More experiments should
be conducted to confirm the experimental observations.
(2) In the practical combustion system, gas species in the
reacting system are complicated which may contain major species of
CO, CO2, H2O, CO2 and O2, and minor species NO, N2O, HCN, and HNCO.
The presence of the major species might significantly affect the
kinetics of the reactions of C with N-containing species. The lack
of the systematical studies of such effects may inhibit the
understanding of the mechanisms and modeling the formation and
destruction of NO during char combustion.
(3) The effects of operation conditions on the changes of NO
formation in some cases depend on the reactor systems. For example,
the conversion of fuel-N to NO increases with increasing of the
fluidized bed temperature, while contrary tendencies were observed
for the fixed bed reactor and pulverized combustor.
(4) The presence of H2 and H2O seems important to give rise to
the formation of N-H containing species such as NH3, HCN and HNCO.
The mechanisms of the formation of such kind of minor N-containing
species are unclear.
(5) Models for describing the fuel-N conversion at single
particle level have been developed based on different assumptions.
The Goel’s model seems the best model to describe the fuel-N
conversion although there are some limitations for instance the
change of particle temperature during the char combustion is
neglected;
(6) The SKIPPY is a new approach to elucidate the fuel-N
conversion in the case of the lack of the fundamental kinetics of
the heterogeneous reactions of carbon and detailed mechanisms of
the formation and destruction of NO.
References
1. Aarna I., and Suuberg EM., (1997). A Review of the Nitric
Oxide-Carbon Reaction. Fuel, 76: 475-491
2. Ahmed SN., Baldwin R., Derbyshire F., McEnaney B., Srencel
J., (1993). Catalytic reduction of nitric oxide over activated
carbons. Fuel. 72: 287-292
3. Aihara T, Matsuoka K, Kyotani T, Tomita A. (2000) Mechanism
of N2 formation during coal char oxidation. Proc Combust Inst.;
28:2189–95
4. Åmand LE., Leckner B., (1991). Influence of fuel on
emissi
