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Fuel—Gas injection to reduce N2O emissions from the combustion of coal in a fluidized bed
Fuel-Gas Injection to Reduce N,O Emissions from the Combustion of Coal in a Fluidized Bed GREGORIO
MARBAN*,
FREEK KAFTEIJN*
and JACOB A. MOULLJN
Industrial Catalysis-CPT, De@ University of Technology, De@, The Netherlands
A laboratory-scale, fluidized-bed reactor (29 mm i.d.) has been used for experiments in which a stream of simulated combustion gases is passed through a countercurrent flame of either methane or propane. The effects of N,O concentration and bed temperature on N,O reduction have been analyzed. Additionally, the effects of NO, SO,, O,, and carrier gas (N, or He) in the inlet stream have been studied. An attempt to establish whether N,O decomposition in the flame proceeds uia radical or thermal mechanisms was carried out by assuming an ideal reaction model in the flame. Up to 99% N,O decomposition was achieved at a gas/oxygen equivalence ratio of 0.83 (12 vol.% 0,) and a total flow rate of 1 L/min, for both methane and propane injected into the reactor. The analyses indicate that NO, is formed in the flame mainly uia a “prompt NO” mechanism. Metallic surfaces can alter the N,O chemistry, either enhancing (empty reactor) or inhibiting (flame) N,O decomposition. Both NO and SO, play a minor role in the decomposition of N,O, and so does the carrier gas, though in this case, N, can produce considerable amounts of NO, under particular circumstances. Under the conditions used, thermal decomposition accounts for only around 10% of the high N,O conversions achieved in the flame, radical mechanisms playing a major rolk.
INTRODUCTIO‘N N,O has been identified as an active absorber
of the earth’s radiation in the troposphere, contributing (with the partial re-emission of this energy) to the global warming of the earth’s surface. This gas is especially harmful because of i,ts long lifetime in the atmosphere (= 150 y). Additionally, tropospheric N,O reaching the stratosphere is largely the main source of stratospheric NO [ll, which is thought to be responsible for about 70% of the global chemical destruction of stratospheric 0, [2]. However, it should be noted that between 62 and 81% of the total N,O emitted into the atmosphere is believed to be released by natural sources [2-41. Also, NO, (produced in the catalytic destruction of 0, by NO) could potentially have a beneficial effect in the ozone layer uia its reaction with ClO, which is another chain carrier involved in the catalytic destruction of 0, by chlorine, forming the inert product ClONO, [l]. Although net N,O emissions from fluidized-bed combustion of coal can be as high as 200 ppm, [5], data reported in a recent review by Kramlich and Linak [l] present this tech-
*Corresponding authors. COMBUSTIONANLI
FLAME
107: 103-113
nique as a minor contributor to the global anthropogenic N,O into the troposphere (6-10.7% of the total anthropogenic sources). However, this contribution would be higher than 50% if all the coal and peat currently cornbusted were burnt in a Fluidized Bed Combustor (FBC) [6]. This fact has recently initiated much research, mainly focused on N,O formation/destruction mechanisms in FBC [7-111 and control strategies [12-141. With respect to the latter, Wbjtowicz et al. [14] suggested three different approaches: (a) innovative combustor design to produce low-emission systems, (b) minimization of emissions through improvements in operating conditions and process control of boilers, (c) post-combustion measures easy to install or retrofit. The first approach seems comparatively expensive at present, although it could be an option if future legislation restricts N,O emissions. Approach (b) is highly recommended. However, it is difficult to apply due to the numerous trade-offs for the different gaseous species produced during combustion between operating variables (especially bed temperature) and combustion efficiency. Thus, an increase in bed temperature above standard (= 1,100K) brings a beneficial effect with respect to N,O emissions, but restricts SO, capture by limestone and causes an increase in NO, emissions
(1996)
Copyright 0 1996 by The Combustion Institute Published by Elsevier Science Inc.
OOlO-2180/96/$15.W PI1 SOOlO-2180(96KlOOl5-6
104 [151. Addition of limestone to the bed has a similar effect to temperature [16], although to a lesser extent. Decreased excess air ratio leads to reduced N,O and NO, emissions, but results in a lower combustion efficiency [17]. In this sense, approach (c) is a natural alternative to in-bed measures to reduce N,O emissions at low expense and to avoid undesirable chemical interactions in the bed. In the context of this approach, gas afterburning constitutes a recently developed “end-of-pipe” solution for N,O decomposition. This technique consists of the creation of a hot-temperature region in the free-board, cyclone, or further downstream, e.g., by injecting methane or another combustible gas. Under favorable conditions (i.e., good gas-mixing, no heat losses to the reactor’s walls or by the bed’s particles, etc.) N,O passing through this region may be destroyed uiu radical and/or thermal mechanisms. Rovy and Bowman [181 showed direct evidence for the poor survivability of N,O in fuel-rich regions. Kramlich et al. [19] used a tunnel furnace fired with high-volatile bituminous coal and demonstrated that gas-afterburning achieved a reduction in N,O emissions from 7 to below 3 ppm,, the kinetic analysis showing that N,O should be effectively destroyed in the fuel-rich zone by reaction with H atoms. However, only the work by Gustavsson and Leckner [12, 131 dealt with gas injection to reduce N,O under actual combustion conditions. They carried out full-scale experiments with fuel-gas injection (natural gas [91.1 vol. % methane] and propane 95 [95 vol. % propane]) in the cyclone of a 12 MW circulating fluidized-bed boiler, during combustion of an Australian bituminous coal, and verified the feasibility of N,O reduction without additional air. Nevertheless, the actual reduction and that calculated by computer simulation (of the homogeneous gas-phase chemistry in a well-stirred reactor, based on the set of reactions given by Glarborg et al. [20, 211) were, respectively, 90% and about 50%. The experiments were made at injection fuel ratios (energy in gas divided by energy in primary fuel) from 0 to 20%, bed temperatures from 1,073 to 1,173 K, and 0, concentrations in the reactor from 1.7 to 7 vol. %. In a specific case, fly ash recirculation was employed. These authors [12,
G. MARBAN ET AL. 131 attributed the discrepancy between experimental and theoretical results mainly to the heat losses to the cyclone’s walls and to the bed particles, which could be half the heat added by the fuel gas. They also considered factors such as gas mixing with the burner in the cyclone, residence time (which was l-2 s with the burner in the cyclone and 0.5 s in the refractory-lined outlet duct; sufficiently long relative to the residence time used in calculations, which was normally 0.5 s), and excess air ratio (whose reduction could lead to a reduced sulfur-capture efficiency by limestone, depending on the reactor design). Finally, these authors remarked that the influence of particles in the cyclone on the results from the homogeneous chemistry of the calculations is poorly understood, and they ended by pointing out that, among all the factors, heat loss remains the main cause for the unsatisfactory results obtained in N,O reduction during experimental performance. A more recent study in the same 12 MW Circulating FBC C (FBC) [22, 231 using LPG (liquefied petroleum gas), coal, fuel oil, pulverized wood, or sawdust as injected fuels through three nozzles in the cyclone’s entrance indicates that thermal decomposition is the main N,O destruction mechanism in the hot area. However, more research must be carried out to assess the relative importance of radical and thermal ways of destroying N,O in the hot area (considering factors such as temperature and residence time). It was also confirmed that this afterburning technology did not increase other emissions: CO emissions were reduced by increasing the cyclone’s temperature; also NO, emissions did not increase, irrespective of the nitrogen content of the additional fuel. Instead, NO, emissions decreased slightly due to a certain “reburning effect.” SO, emissions also did not increase, although it was suspected that calcium sulfate would decompose due to the high temperature and lead to an increase in SO,. Of course, when coal was used as an additional fuel, SO, was increased due to the sulfur content of the coal [22, 231. As stated above, the relative importance of radical and thermal mechanisms for both N,O decomposition and NO, formation in the flame at actual combustion conditions remains an
REDUCTION
105
OF N,O FROM FBC OF COAL
open question. A brief account of the main radical and thermal mechanisms involving formation/decomposition of N,O and NO, is presented here. Main radical reactions
The homogeneous destruction of N,O by radical attack is generally thought to proceed via the reaction [19, 24, 251 N,O + H + N, + OH,
(1)
and to a lesser extent via N,O + OH -+ N-, + HO,
(21
At low temperatures and under fuel-lean conditions, where the concentration of OH radicals remains much higher than the concentration of the H radical, Reaction 2 contributes to N,O destruction [45], although not as much as stated in previous work [26]. In addition, the other long-lived radical in a flame, i.e., the free oxygen atom, attacks N,O [7, 251 in: N,O + 0 --f N, + 0,
(3)
N,O + 0 * 2N0
(4)
Both paths, Reactions 3 and 4, are approximately equally favored over a wide temperature range [25]. In a flame, hydrocarbon radicals (CH,; 0 5 i I 3) are also involved. In this sense, it is worth noting that NO can be produced in a flame by the fixation of atmospheric N, from CH, radicals attacking N, in: N, + CH + HCN + N
(5)
N: + CH, + HCN + NH
(6)
with Eq. 5 being the dominant reaction under most combustion conditions [24]. The products HCN and free N atoms subsequently disappear to yield mainly NO at flame temperature [27]. This mechanism was termed “prompt NO” by Fenimore [28], since the rapid formation of NO was confined to regions near the flame zone. Typical levels of prompt NO range from a few ppm,, to more than 100 ppm, [24]. Although conditions prevailing in fluidizedbed combustion decrease NO emissions to a
large extent, there is always some NO carried upward by the flue gases through the flame. This “in-bed NO” together with the prompt NO, the latter being formed in the flame, can be “recycled” by hydrocarbon radicals uia the following reactions 120, 29, 301: NO+C+CN+O
(7)
NO + CH + HCN + 0
(81
NO + CH, + HCN + OH
(9)
NO + CH, --j HCN -I- H,O
(10)
It was said above that HCN oxidizes in a flame to form mainly NO. This statement should be analyzed in more detail. In fact, the conversion of HCN to NH, is favored by the flame conditions [31], fuel-rich or oxygen-lean, where OH becomes a dominant radical relative to 0. The reaction mechanism is as follows: HCN+OH-tHNCO+H
(11)
HNCO + H -+ NH, + CO
(12)
NH, + H,O + NH, + OH
(13)
This path, however, is very slow, especially Reaction 13, and only a modest conversion is expected. It is therefore assumed [32] that NH, observed in flames (in pulverized coal combustion) is either a consequence of heterogeneous processes or is a direct pyrolysis product in the case of sub-bituminous coals. The oxidation of HCN can proceed cia the intermediate NCO. At typical flame temperatures, this intermediate is mainly oxidized to NO by the radicals 0 and OH. However, when the flame temperature is not very high, the reduction of NC0 by NO to produce N,O and CO should not be dismissed [15]. Thus, we conclude that NO is the main product from HCN at typical flame temperatures. Taking this into account and considering Reactions 7-10, we can conclude that NO recycling finally yields NO, so that there is no major sink for NO in a flame. Thermal reactions
Thermal decomposition
of N,O occurs oia
N,O + M + N, + 0 + M,
(14)
where M stands for a gas molecule. The nature of this third body, M, must be taken into
106
G. MARBAN
account, since collision efficiencies vary significantly for different inert gases [33]. Reaction 14, which is increasingly relevant at higher temperatures, has been extensively studied [24, 34-391. The activation energies found for the pseudo-second-order decomposition of N,O vary from 215.7 kJ/mol [24] in the temperature range l,OOO-2,000 K to 251.2 kJ/mol 1381 in the range 1,190-1,370 K. Assuming a pseudo-first-order mechanism, Johnsson et al. [37] found activation energies of 234.5, 224.0, and 225.7 kJ/mol for M being Ar, N,, and He, respectively, in the temperature range l,OOO-1,350 K. Thermal NO is formed according to the Zeldovich mechanism [40], from molecular nitrogen, supplied primarily with the combustion air. Conversion starts at temperatures about 1,570 K and increases markedly with rising
ET AL.
temperatures. The degree of conversion is also proportional to the concentration of atomic oxygen. The objective of the present work is to assess the effect of fuel-gas injection on the different gaseous emissions (N,O, NO, and SO,) and to evaluate the relative importance of radical and thermal mechanisms for N,O decomposition and NO, formation. EXPERIMENTAL SECTION The experiments were carried out in a laboratory-scale, fluidized-bed combustor made of quartz (29 mm i.d.), operating atmospheric pressure; the main features are depicted in Fig. 1. The distributor consists of a quartzfritted plate. This supports a bed of Sic particles (d, = 115-150 pm) with a static height of
EbCtllCCYl reststance
\
\
Gas Injector (Quartz, 6x4 mm)
Bed of catborundum pClltiCl8S
t
QUARTZ REACTOR (3 cm LD.) Fig. 1. Experimental
set-up.
REDUCTION
107
OF N,O FROM FBC OF COAL
20 mm. The reactor is electrically heated along a length of 20 cm (7 cm preheating and 13 cm bed plus free-board). In this zone, the reactor behaves approximately isothermally, the temperature dropping sharply from 13 cm above the distributor, due to lack of thermal insulation along the nonheated zone. A thermocouple immersed in the bed allows the temperature in the reactor to be accurately controlled. The effective volume of the isothermal reactor is = 100 cm3. A gas injector made of quartz and ending in a fritted plate (6 X 4 mm) is used to create a homogeneous flame of either methane or propane. It is introduced from the upper part of the reactor at a distance of 10 cm from the distributor. To measure the flame temperature, a thin, high-temperature thermocouple (type S) is longitudinally stuck on the outer surface of the injector, with its tip immersed in the flame. Heat losses by radiation and conduction were estimated to be low (= 35 K at a measured flame temperature of 1,250 K), not affecting the conclusions drawn from this work. Visual observations of the flame at standard conditions (25 mL/min CH,, bed temperature Th = 1,123 K, total flow rate in the reactor [12 vol. % 0,] = 1 L/min) permit one to assign a volume to the flame of about 6 cm3, occupying approximately 80% of the cross-sectional area of the reactor. Under these conditions, the N,O residence time is = 1.6 s in the empty factor (Tb = 1,123 K) and = 0.3 s in the flame (25 mL/min CH,, Tf = 1,250 K). A set of mass-flow controllers allows a mixture of a known composition of different gases (O,, N,, He, CO,, N,O, NO, and SO,) to be introduced into the reactor from the lower part. Except for a few specific experiments, the total flow rate was kept at 1 L/min, corresponding to a fluidization velocity of = 0.1 m/s at 1,123 K (five times the minimum fluidization velocity). Values of 0, concentration in the inlet gases below 12 vol. % resulted in flame instability. In this sense, the 0, concentration used during the experiments had to be fixed at 12 vol. %, despite being higher than that existing at actual combustion conditions. The flue gas was dried before its composition was determined. Paramagnetic, non-dispersive infrared and chemiluminescence gas analyzers measured the concentrations of O,,
CO, and NO,, respectively. N,O analysis was performed using a gas chromatograph equipped with an electron capture detector (ECD). Separation was effected at 508 K on a 5 A-molecular sieve column (2 m long), with nitrogen as carrier gas. Different experiments were carried out both with and without a flame. In the following, we refer to those experiments carried out without the flame as being made with the “empty reactor.” For most experiments with the flame, the gas/O, equivalence ratio, 4, was kept at 0.42. This ratio is unity, when the reaction between the fuel-gas and 0, proceeds under stoichiometric conditions. The different inlet conditions used in the experiments are indicated in Table 1 and denoted by the letter in the final column of Table 1. Effect of the experimental decomposition
set-up on N,O
When carrying out experiments with the empty reactor, it was found that solid surfaces exerted a certain effect on N,O decomposition that was coupled with the thermal effect. Figure 2 shows the effect of different solid surfaces (quartz gas injector and flame thermocouple) on N,O decomposition (Tb = 85O”C, He to balance)‘. For obvious reasons, the bed’s thermocouple (sheath material: stainless steel 316) could not be removed to study its effect. It can be seen that the combined effects of the thermocouple (sheath material: INCONEL 600; approx. composition: 75% Ni, 15% Cr, 8% Fe, 1% Mn, 0.5% Cu, and 0.5% [Si, C, S]) and the quartz injector account for an increase by about 8% in the N,O conversion. The increase in conversion due to the flame’s thermocouple can be ascribed to catalytic effects by transition metals from group VIII (Ni and Fe), as well as by Mn, Cu, and Cr. Oxides of these metals are found to be highly active for N,O decomposition [41]. The increase in N,O conversion due to the quartz injector can be a result of an enhancement of the collision efficiency for thermal N,O decomposition. The flame’s thermocouple was found to exert a negative effect on N,O decomposition in the flame; thus, when the thermocouple was removed from a methane flame (gas/O,
108
G. MARBAN ET AL,. TABLE 1 Inlet Conditions for Different Experiments
0.42 (CH,) 0.42, 0.83 (CH,, C,H,) 0 0 0.42 (CH,) 0 0 0.42 (CH,) 0.42 (CH,) 0 0 0.42 (CH4, C,H,)
Tb (K)
N,O (ppm,.)
1,123 1,123 1,123 1,123 1,123 1,123 1,223 973 1,123 1,123 1,023-1,223 973
211 53-211 106-211 106 53-211 106 106 53-211 106 106 106 53-211
0, (vol. %o) CO, (vol. %o) N, (vol. %) 12 12 12 o-12 12 0 12 12 12 12 17.1-10s 12
equivalence ratio C$= 0.421, the N,O conversion increased from 71.4 to 74% (Ref. A in Table 1; flame temperature Tf = 1,290 K>. This can be a result of radical recombination on the thermocouple’s surface, leading to less N,O decomposition by radical attack [30]. The NO, formed in the flame also increased when the thermocouple was removed: from 15.3 to 17.2 ppm,,. This has an explanation similar to the increase in N,O decomposition, considering the importance of radicals in the prompt NO route. In the experiments described in the following section, the configuration of the experimental set-up was kept as shown in Fig. 1.
14 14 14 14 14 14 14 14 14 14 14 14
He (vol. %)
Reference
40 74 74 20 10-40 86,20 20 74 20 20 20 74
A B C D E F G H I J K L
34 0 0 66-54 64-34 0, 66 54 0 54 54 48.9-55.5 0
Only in a few cases, which are specified, was the flame thermocouple removed. The resulting experimental error was assumed not to affect the general conclusions drawn from this work. RESULTS AND DISCUSSION The flame temperatures achieved by the combustion of methane were 1,290 K at 4 = 0.42, and 1,333 K at 4 = 0.83; propane flame temperatures varied from 1,313 K at 4 = 0.42, to 1,340 K at C#J = 0.83 (Ref. B in Table 1). This corresponds to an increase in temperature of between 167 and 217 K with respect to the bed temperature. Figure 3 shows the N,O conver-
75 Injector
s
+
65
* -A-
1 .-b
55-
z” j45-
35
Yes
Yes NO
100
Thermocouple Yes NO No
1
$ =0.83
II
??
$ = 0.42 0............... ......
I
& ............... .:::::::::::~::::::::::::::::::::e .....&:::~;;~~
6
6
Empty re~c tor
+ 25
! 25
I 75
125
Inlet N20 concentration
175
225
(ppm”)
Fig. 2. Effect of gas-injector and flame thermocouple on the decomposition of N,O in the empty reactor (Tb = 1,123 K, inlet gas composition: 53-211 ppm, N,O, balance He).
0
1
,
25
75
I
I
125
175
Inlet N20 concentration
-I 225
(ppm,)
Fig. 3. Effect of equivalent gas/O, ratio on N,O conversion at different inlet N,O concentrations (Tb = 1,123 K, inlet gas composition: 53-211 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He).
REDUCTION
109
OF N,O FROM FBC OF COAL
sions achieved in the above-mentioned flames and in the entropy reactor (Ref. C in Table 1). N,O conversions up to 99% can be achieved with both flames (CH, and C,H,) at 4 = 0.83. The N,O conversion in the empty reactor has lower values than those plotted in Fig. 2 (with both gas injector and flame thermocouple). This is ascribed to the inhibiting effect of 0, on the catalytic decomposition of N,O by the flame thermocouple. This effect can be seen in Fig. 4, where the conversions of N,O in the empty reactor at different inlet 0, concentrations are shown (Ref. D in Table 1). Inhibition of N,O decomposition in catalysts by 0, has been reported by several authors [41-431. Nevertheless, a difference of N,O conversion of about 8% can be noticed between the points of 0 vol. % 0, in Fig. 4 (N,O conversion = 36.5%) and the point at 106 ppm,: N,O in Fig. 2 corresponding to the experiment carried out with both injector and flame thermocouple (N,O conversion = 44.7%). The only difference between them is the presence of 14 vol. % CO, in the inlet gas in the first case. An explanation for the negative effect of CO, could not be found. To prevent errors in the comparison of different results, most of the experiments were carried out with 14 vol. % CO, in the inlet gas stream. In the experiments with the flame described above (Ref. B in Table 1, Fig. 31, NO, could
4o1
16
12
8
4
25
75
125
! 0
Inlet N20 concentration
(ppm,)
only be formed from the N, released during N,O decomposition. In Fig. 5, these emissions of NO, are plotted against the different inlet N,O concentrations. Less than 16 ppm,, were formed in all cases (see Fig. 5). As can be seen in Fig. 6, the formation of NO, slightly increased when the carrier gas was replaced by mixtures of N/He (Ref. E in Table 1). The N,O conversion was not affected by the replacement of He as a carrier gas by N/He mixtures (see Fig. 6). Similarly, no difference was found regarding N,O decomposition in the empty reactor with either He or NJHe as the
100
I 4
Inlet
225
175
Fig. 5. NO, emissions in the flame at different inlet NT0 concentrations (T, = 1,123 K, inlet gas composition: 53-211 ppm,. N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He).
542 N2
20
1
0
1 a
02 concentration
I 12
I 16
(vol. Z)
Fig. 4. Effect of inlet 0, concentration on N,O decompe sition in the empty reactor (Tb = 1,123 K, inlet gas composition: 106 ppm, N,O, O-12 vol. % O,, 14 vol. % CO,, and balance He/N,).
342 Nz
442 N2
50 25
75
125
Inlet N20 concentration
17.5
225
-
(ppm,)
Fig. 6. Effect of carrier gas on N,O decomposition and NO, emissions in the flame CT,,= 1,123 K, +(CH,) = 0.42, inlet gas composition: 53-211 ppm,, N,O, 12 vol. % O,, 14 vol. % CO,, and either 74 vol. % He or N2/He mixtures).
110 carrier gas. Thus, for the mixtures referred to as F in Table 1, with either 86 vol. % He or 66 vol. % N, + 20 vol. % He, the conversion of N,O at Tb = 1,123 K was always approximately 36.8%. For the conditions used in this work, NO, was formed in the flame mainly via the prompt NO mechanism. Thus, in experiments where air was passed through a flame of methane (4 = 0.481, the flame temperature changed from 1,331 K at Tb = 1,023 K to 1,430 K at Tb = 1,223 K, but NO, emissions only increased by 99.7 to 125 ppm,. However, when the gas/O, equivalence ratio at Tb = 1,223 K was changed from 4 = 0.10 to C$= 0.48, NO, emissions spectacularly increased by 6.2 to 125 ppmu, though the increment in flame temperature was similar to that in the previous case (from 1,303 to 1,430 K). This fact reveals the low relative importance of thermal mechanisms for NO, formation in the flame. To further substantiate this statement, a stream of gas with 106 ppm, of N,O (Ref. G in Table 1) was introduced in the empty reactor at 1,223 K. In this case, no NO, was formed. However, when a flame of methane (4 = 0.42) was used to reduce N,O from a gas of varying N,O concentration (Ref. H in Table 1) passing through the reactor at Tb = 973 K, without using the flame thermocouple, the emissions of NO, increased by 3.2 ppm, (inlet N,O = 53 ppm,) to 10.5 ppm, (inlet N,O = 211 ppm,), although the flame temperature in this case was lower than the reactor temperature in the previous case (1,178 and 1,223 K, respectively). The conversion of N,, produced from the decomposition of N,O, to NO, was 4.9 + 0.3% in all cases. Figure 7 shows that NO introduced with the inlet gas (O-69 ppm,) has no effect on N,O decomposition in the flame of methane (Ref. I in Table 1). On the other hand, NO, formed by the prompt NO mechanism in the flame diminishes slightly from 14.7 to 8.4 ppm, when the inlet NO concentration increases from 0 to 69 ppm”. In this sense, NO seems to inhibit slightly the NO, formation in the flame. The effect of SO, on N,O decomposition both in the empty reactor and in the flame can be observed in Figs. 8A and 8B. In these experiments, a gas mixture containing O-
G. MARBAN ET AL. ‘; 18
E
::
16 ,5 t: 14 p 12
: 6
ti
5
20,
0
I
I
I
I
I,
10
20
30
40
50
Inlet NO concentration
IT
60
(ppm”)
70
3 ’
Fig. 7. Effect of inlet NO concentration on N,O decompe sition and NO, emissions in the flame (Tb = 1,123 K, 4 (CH,) = 0.42, inlet gas composition: O-69 ppm, NO, 106 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, 54 vol. % N2, and 20 vol. % He).
149 ppm, SO, was used at T, = 1,123 K with (Ref. I in Table 1) and without flame (Ref. J in Table 1). The presence of SO, in the empty reactor caused an increase in N,O reduction from 25% (0 ppm, SO,) to 40% (149 ppm, SO,). This positive effect could be a result of reaction of SO, with N,O to yield SO, and N,. However, more research should be carried out to confirm this. With a flame, the increase in N,O conversion with the inlet SO, concentration was not so marked (Fig. 8A). In fact, SO, resulted in an overall decrease in N,O conversion in the flame, as shown in Fig. 8B. This effect was similar to that exerted by SO, on NO, emissions from the flame (Fig. 8Bl. To explain this fact, radical recombination in the flame catalyzed by SO2 must be considered, according to the mechanism 1161 H + SO, -+ HSO, + M
(151
H + HSO, --) H, + SO,
(161
OH + HSO, + H,O + SO,
(17)
This mechanism can account for the decrease of N,O conversion and NO, formation observed in Fig. 8B, if thermal mechanisms are assumed to control both processes. Experiments at different total flow rates (from 0.75 to 1.15 L/min) were performed at three bed temperatures (1,023, 1,123, and 1,223 K) to calculate kinetic parameters of
REDUCTION
111
OF N,O FROM FBC OF COAL
20 -
B
r55
3 -
I
E" z
-1 11
, 140
30
0
60
90
Inlet SO2 concentration
li0
150
-
0
30
60
90
Inlet SO2 concentration
(ppm”)
120
t25 150
s x
(ppm”)
Fig. 8. Effect of SO, on N,O decomposition (Tb = 1,123 K, +(CH,) = 0.42, inlet gas composition: O-149 ppm, SO,, 106 ppm,, N,O, 12 vol. % O,, 14 vol. % CO,, 54 vol. % N,, and 20 vol. % He): (a) Comparison between N,O decomposition in the empty reactor and in the methane flame (4 = 0.42). (b) Effect of SO, on NO, emissions and N,O conversion in the flame.
thermal N,O decomposition in the empty reactor. To maintain a constant 0, flow rate of 120 mL/min, the 0, concentration in the inlet gas had to be varied for the different total flow rates experimented (Ref K in Table 1). The Arrhenius plot achieved by assuming an ideal plug-flow model for the reactor is shown in Fig. 9. The best fit was obtained for an apparent reaction order of 1. Thus, the experimental data can be interpreted in terms of the following pseudo-first order decomposition rate constant: k = 2.87 X 10” X exp(- 28573/T) s-l. The activation energy found in this work
3
Ref.U51;Nzasthirdbody
2
0.01
8.0
a:5
9Io 1 04/T
915
16.0
(K-' )
Fig. 9. Arrhenius plot for the kinetics of thermal decomposition in the empty reactor (flow rates: 0.75-1.15 L/min, Tb = 1023-1223”C, inlet gas composition: 106 ppm, N,O, 17.1-10.5 vol. % O,, 14 vol. % CO,, 48.9-55.5 vol. % N,, and 20 vol. % He). Results obtained from previous work [15] are plotted as a comparison.
(237.3 kJ/mol) is similar to those found by Johnsson et al. [37] in the range of l,OOO1,350 K (224.3 kJ/mol for Nz as third body and 225.9 kJ/mol for He). However, the preexponential factor found in this study is 3-7 times higher than those given by Johnsson et al. [37]. To assess the relative contributions to the N,O decomposition by radical and thermal mechanisms in the flame, different experiments were carried out with the reactor at Tb = 973 K. At that temperature, the conversion of N,O in the empty reactor was less than l%, according to the equation previously presented. Thus, the flame is responsible for almost all N,O decomposition achieved in the reactor. In these experiments, the flame thermocouple was removed for calculation of the N,O conversions. The inlet gas composition is indicated in Table 1 (Ref. L in Table 1). The flame temperatures achieved under these circumstances were 1,178 K (CH,, 4 = 0.42) and 1,212 K (C,H,, 4 = 0.42). Reaction rates for N,O decomposition were calculated assuming perfect mixing in the flame’s volume. These are represented in Figs. 10A (methane) and 10B (propane). Included there are the reaction rates for thermal N,O decomposition in the empty reactor evaluated at the flame temperatures (1,178 and 1,212 K> using the calculated rate constant. The reaction rates in the flame are between 9 and 15 times higher than those in the empty reactor. By assuming a more
112
G. MARBAN ET AL.
1
25
75
125
Inlet N20 concentration
175
-I 225
(ppm”)
-f 25
75
125
Inlet N20 concentration
175
2 15
(ppm,)
Fig. 10. Comparison between N,O decomposition in the empty reactor and in the flame: (a) methane (4 = 0.42), (b) propane (r#~= 0.42). Tb = 973 K, inlet gas composition: 53-211 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He.
realistic flame model (i.e., counterllow laminar diffusion flame [44]) instead of that of perfect mixing, the reaction volume would be smaller, and thus the reaction rates in the flame would be even higher. Additionally, error introduced by the uncertainty in the temperature measurement by the flame thermocouple was estimated to be low. Thus, by assuming a methane flame temperature 50 K higher than measured, the reaction rates in the flame still would be between 9 and 10 times higher than those achieved in the empty reactor at the same temperature. This strongly suggests that, at the conditions studied in this work, radical mechanisms are much more important than thermal mechanisms for N,O decomposition in the flame. CONCLUSIONS Reductions of N,O by up to 99% were achieved in the present work for both CH, and C,H, flames, confirming the feasibility of gas injection for N,O reduction purposes. The reaction rate for N,O decomposition in the flame was more than ten times higher than that achieved by the empty reactor at the same temperature. This corroborates the fact that radical attack on N,O plays an important role under the conditions used in this work. Metallic components of the thermocouples catalytically enhance N,O reduction in the empty reactor. This effect is the opposite of that in the flame due to radical recombination
at the solid surfaces, which reduces any decomposition of N,O. 0, inhibits the mentioned catalytic effects by radical attack. CO, unexpectedly disfavors any N,O reduction in the empty reactor. Prompt NO is the main mechanism for NO, formation in the flames used in this work. NO has no effect on N,O reduction in the flame. SO, slightly increases the N,O reduction in the empty reactor, but inhibits N,O reduction under flame conditions, due to SOZ catalyzing radical recombination. The reaction rate for thermal N,O decomposition in the empty reactor was rNZO = kc, o, where k = 2.87 X lOlo X exp(-28573/T\ s-l. The authors thank the Commission of the European Communities (Contract No. JOU2-Ci’920229) for financing this work. G. Marbcin is also thankful to the Spanish Ministry of Education and Science (MEC) for the award of a Postdoctoral Research Fellowship.
REFERENCES 1. Kramhch, J. C., and Linak, W. P., Prog. Energy Combust. Sci. 20:149-202 (1994). 2. Levine, J. S., 5th International Workshop on Nitrous Oxide Emissions, Tsukuba, Japan, 1992, KL-1. 3. Minami, K., 5th International Workshop on Nitrous Oxide Emissions, Tsukuba, Japan, 1992, KG5 4. Khalil, M. A. K., and Rasmussen, R. A., J. Geophys. Res. 97(D13):14, 651 (1992). 5. Hein, K., and Raible, L., EPA/IFP European Workshop on the Emission of Nitrous Oxide from Fossil Fuel
REDUCTION
6.
I. 8.
9.
10. 11. 12.
13. 14.
113
OF N,O FROM FBC OF COAL
Combustion, Rueil-Malmaison, France, 1988, pp. 77-78. De Soete, G. G., LNETI/EPA/IFP European Workshop on the Emission of Nitrous Oxide, LNETI, Lisbon, 1990, pp. 41-45. Hayhurst, A. N., and Lawrence, A. D., Prog. Energy Combust. Sci. 18:529-552 (1992). Johnsson, J. E., Fuel 73(9):1398-1415 (1994). Land, L.-E., and Leckner, B., 6th International Workshop on Nitrous Oxide Emissions, Turku, Finland, 1994. Collings, M. A., and Mann, M. D., Energy & Fuels 8:1083-1094 (1994). Bonn, B., Pelz, G., and Baumann, H., Fuel 74(2):165-171 (1995). Gustavsson, L., and Leckner, B., 11th International Conference on Fluidized Bed Combustion, ASME, New York, 1991, pp. 677-685. Leckner, B., and Gustavsson, L., .I. Inst. Energy 64:176-182 (1991). Wojtowicz, M. A., Pels, J. R., and Moulijn, J. A., Fuel 73(9):1416-1422
27.
Hayhurst,
28.
Cornbust. Sci. 6:35 (1980). Fenimore, C. P., I3th Sympos. (Int.) Combust., The
29.
Combustion Institute, Pittsburgh, pp. 373-379, 1971. De Soete, G. G., The Coal Research Forum, NO,: Basic Mechanisms of Formation and Destruction and Their Application to Emissions Control Technologies,
30.
31.
32. 33. 34.
15. W6jtowicz, M. A., Pels, J. R., and Moulijn, J. A., Fuel
Coal Research Forum, NO,: Basic Mechanisms of Formation and De.etruction and Their Application to Emissions Control Technologies, Imperial College of Sci-
ence, London, 1993. 17. Leckner, B., Int. J. EnergV Res. 16:351-363 (1992). 18. Rovy, R. J., and Bowman, C. T., Combustion and Flame 70:119- 123 (1987). 19. Kramlich, J. C., Cole, J. A., McCarthy, J. M., Lanier, W. S., and McSorley, J. A., Combustion and Flame ?7:375-384
(1989).
20. Glarborg, P., Miller, J. A., and Kee, R. J., Combustion and Flame 65: 177-202 (1986).
21. Glarborg, I’., Kampp, N. B., and Hadvig, S., Reduction of NO, Emissions from Natural Gas Fired Boilers (in Danish), Technical Report, University of Denmark, Laboratory for Heating and Ventilation, March 1987. 22. Leckner, B., LYTERFLUID Workshop, Nagoya, Japan, 1994. 23. Gustavsson, I~., and Leckner, B., 6th International Workshop on Nitrous Oxide Emissions, Turku/.&bo, Finland, 1994. 24. Miller, J. A., and Bowman, C. T., Prog. Energy Combust. Sci. 15:287-338 (1989). 25. Kramlich, J. C., and Linak, W. P., Prog. Energy Combust. Sci. 20:149-202 (1994). 26. Kilpinen, P., and Hupa, M., Combustion and Flame 85:94 (1991).
London, 1993. Leckner, B., and &and,
L.-E., Joint Meeting of the French, Italian and Swedish Sections of the Combustion Institute, Capri, Italy, 1992. Peck, R. E., Glarborg, P., and Johnsson, J. E., Combusr. Sci. and Tech. 7681-109 (1991). Endo, H., Glanzer, K., and Troe, J., J. Phys. Chem., 83:2083 (1979). Baulch, D. L., Drysdale, D. D., Home, D. G., and Lloyd, A. C., Evaluated Kinetic Data for High Temperature Reactions: Homogeneous Gas Phase Reactions of the Hz-Nz-0, System, Vol. 2, CRC Press, Cleveland,
(1993).
16. Wojtowicz, M. A., Pels, J. R., and Moulijn, J. A., The
Imperial College of Science, London, 1993. Hayhurst, A. N., and Lawrence, A. D., The Coal Research Forum, NO,: Basic Mechanisms of Formation and Destruction and their Application to Emissions Control Technologies, Imperial College of Science,
(1994).
Proc. Tech. 34:1-71
A. N., and Vince, I. M., Proc. Energy
1973. Hanson, R. K., and Salimian, S., Combustion Chemistq, W. C. Gardiner (Ed.), Springer-Verlag, New York, 1984. 36. Dindi, H., Tsai, H. M., and Branch, M. C., Combus35.
tion and Flame 87:13 (1991).
37. Johnsson, J. E., Glarborg, P., and Dam-Johansen, K., 24th Sympos. (Int.) Combust., The Combustion Institute, Pittsburgh, pp. 917-923, 1992. 38. Hulgaard, T., Nitrous Oxide from Combustion, Ph.D. dissertation, TU Denmark, Lyngby, Denmark, 1991. 39. Fujii, N., Sagawai, S., Sato, T., Nosaka, Y., and Miyana, H.. J. Phys. Chem., 93:5474 (1989). 40. Zeldovich, Y. B., Sadovnikov, P. Y., and FrankKamenetskii, D. A., Oxidation of Nitrogen in Combustion, translated by M. Shelef, Academy of Sciences of the USSR, Moscow, 1947. 41. Winter, E. R. S., J. Catalysis 19:32-40 (19701. 42. Ramanujachary, K. V., and Swamy, C. S., J. Catalysis 93:279-287
(1985).
43.
Christopher, J., and Swamy, C. S.. Indian J. Catalysis
44. 45.
Tsuji, H., Proc. Energy Combust. Sci. 8:93-l 19 (1982). Glarborg, P., Johnsson, J. E., and Dam-Johansen, K.,
29A:1106-1110
(1990).
Combustion and Flame 99:523-532 Received IO May 1995
(1994).
MARBAN*,
FREEK KAFTEIJN*
and JACOB A. MOULLJN
Industrial Catalysis-CPT, De@ University of Technology, De@, The Netherlands
A laboratory-scale, fluidized-bed reactor (29 mm i.d.) has been used for experiments in which a stream of simulated combustion gases is passed through a countercurrent flame of either methane or propane. The effects of N,O concentration and bed temperature on N,O reduction have been analyzed. Additionally, the effects of NO, SO,, O,, and carrier gas (N, or He) in the inlet stream have been studied. An attempt to establish whether N,O decomposition in the flame proceeds uia radical or thermal mechanisms was carried out by assuming an ideal reaction model in the flame. Up to 99% N,O decomposition was achieved at a gas/oxygen equivalence ratio of 0.83 (12 vol.% 0,) and a total flow rate of 1 L/min, for both methane and propane injected into the reactor. The analyses indicate that NO, is formed in the flame mainly uia a “prompt NO” mechanism. Metallic surfaces can alter the N,O chemistry, either enhancing (empty reactor) or inhibiting (flame) N,O decomposition. Both NO and SO, play a minor role in the decomposition of N,O, and so does the carrier gas, though in this case, N, can produce considerable amounts of NO, under particular circumstances. Under the conditions used, thermal decomposition accounts for only around 10% of the high N,O conversions achieved in the flame, radical mechanisms playing a major rolk.
INTRODUCTIO‘N N,O has been identified as an active absorber
of the earth’s radiation in the troposphere, contributing (with the partial re-emission of this energy) to the global warming of the earth’s surface. This gas is especially harmful because of i,ts long lifetime in the atmosphere (= 150 y). Additionally, tropospheric N,O reaching the stratosphere is largely the main source of stratospheric NO [ll, which is thought to be responsible for about 70% of the global chemical destruction of stratospheric 0, [2]. However, it should be noted that between 62 and 81% of the total N,O emitted into the atmosphere is believed to be released by natural sources [2-41. Also, NO, (produced in the catalytic destruction of 0, by NO) could potentially have a beneficial effect in the ozone layer uia its reaction with ClO, which is another chain carrier involved in the catalytic destruction of 0, by chlorine, forming the inert product ClONO, [l]. Although net N,O emissions from fluidized-bed combustion of coal can be as high as 200 ppm, [5], data reported in a recent review by Kramlich and Linak [l] present this tech-
*Corresponding authors. COMBUSTIONANLI
FLAME
107: 103-113
nique as a minor contributor to the global anthropogenic N,O into the troposphere (6-10.7% of the total anthropogenic sources). However, this contribution would be higher than 50% if all the coal and peat currently cornbusted were burnt in a Fluidized Bed Combustor (FBC) [6]. This fact has recently initiated much research, mainly focused on N,O formation/destruction mechanisms in FBC [7-111 and control strategies [12-141. With respect to the latter, Wbjtowicz et al. [14] suggested three different approaches: (a) innovative combustor design to produce low-emission systems, (b) minimization of emissions through improvements in operating conditions and process control of boilers, (c) post-combustion measures easy to install or retrofit. The first approach seems comparatively expensive at present, although it could be an option if future legislation restricts N,O emissions. Approach (b) is highly recommended. However, it is difficult to apply due to the numerous trade-offs for the different gaseous species produced during combustion between operating variables (especially bed temperature) and combustion efficiency. Thus, an increase in bed temperature above standard (= 1,100K) brings a beneficial effect with respect to N,O emissions, but restricts SO, capture by limestone and causes an increase in NO, emissions
(1996)
Copyright 0 1996 by The Combustion Institute Published by Elsevier Science Inc.
OOlO-2180/96/$15.W PI1 SOOlO-2180(96KlOOl5-6
104 [151. Addition of limestone to the bed has a similar effect to temperature [16], although to a lesser extent. Decreased excess air ratio leads to reduced N,O and NO, emissions, but results in a lower combustion efficiency [17]. In this sense, approach (c) is a natural alternative to in-bed measures to reduce N,O emissions at low expense and to avoid undesirable chemical interactions in the bed. In the context of this approach, gas afterburning constitutes a recently developed “end-of-pipe” solution for N,O decomposition. This technique consists of the creation of a hot-temperature region in the free-board, cyclone, or further downstream, e.g., by injecting methane or another combustible gas. Under favorable conditions (i.e., good gas-mixing, no heat losses to the reactor’s walls or by the bed’s particles, etc.) N,O passing through this region may be destroyed uiu radical and/or thermal mechanisms. Rovy and Bowman [181 showed direct evidence for the poor survivability of N,O in fuel-rich regions. Kramlich et al. [19] used a tunnel furnace fired with high-volatile bituminous coal and demonstrated that gas-afterburning achieved a reduction in N,O emissions from 7 to below 3 ppm,, the kinetic analysis showing that N,O should be effectively destroyed in the fuel-rich zone by reaction with H atoms. However, only the work by Gustavsson and Leckner [12, 131 dealt with gas injection to reduce N,O under actual combustion conditions. They carried out full-scale experiments with fuel-gas injection (natural gas [91.1 vol. % methane] and propane 95 [95 vol. % propane]) in the cyclone of a 12 MW circulating fluidized-bed boiler, during combustion of an Australian bituminous coal, and verified the feasibility of N,O reduction without additional air. Nevertheless, the actual reduction and that calculated by computer simulation (of the homogeneous gas-phase chemistry in a well-stirred reactor, based on the set of reactions given by Glarborg et al. [20, 211) were, respectively, 90% and about 50%. The experiments were made at injection fuel ratios (energy in gas divided by energy in primary fuel) from 0 to 20%, bed temperatures from 1,073 to 1,173 K, and 0, concentrations in the reactor from 1.7 to 7 vol. %. In a specific case, fly ash recirculation was employed. These authors [12,
G. MARBAN ET AL. 131 attributed the discrepancy between experimental and theoretical results mainly to the heat losses to the cyclone’s walls and to the bed particles, which could be half the heat added by the fuel gas. They also considered factors such as gas mixing with the burner in the cyclone, residence time (which was l-2 s with the burner in the cyclone and 0.5 s in the refractory-lined outlet duct; sufficiently long relative to the residence time used in calculations, which was normally 0.5 s), and excess air ratio (whose reduction could lead to a reduced sulfur-capture efficiency by limestone, depending on the reactor design). Finally, these authors remarked that the influence of particles in the cyclone on the results from the homogeneous chemistry of the calculations is poorly understood, and they ended by pointing out that, among all the factors, heat loss remains the main cause for the unsatisfactory results obtained in N,O reduction during experimental performance. A more recent study in the same 12 MW Circulating FBC C (FBC) [22, 231 using LPG (liquefied petroleum gas), coal, fuel oil, pulverized wood, or sawdust as injected fuels through three nozzles in the cyclone’s entrance indicates that thermal decomposition is the main N,O destruction mechanism in the hot area. However, more research must be carried out to assess the relative importance of radical and thermal ways of destroying N,O in the hot area (considering factors such as temperature and residence time). It was also confirmed that this afterburning technology did not increase other emissions: CO emissions were reduced by increasing the cyclone’s temperature; also NO, emissions did not increase, irrespective of the nitrogen content of the additional fuel. Instead, NO, emissions decreased slightly due to a certain “reburning effect.” SO, emissions also did not increase, although it was suspected that calcium sulfate would decompose due to the high temperature and lead to an increase in SO,. Of course, when coal was used as an additional fuel, SO, was increased due to the sulfur content of the coal [22, 231. As stated above, the relative importance of radical and thermal mechanisms for both N,O decomposition and NO, formation in the flame at actual combustion conditions remains an
REDUCTION
105
OF N,O FROM FBC OF COAL
open question. A brief account of the main radical and thermal mechanisms involving formation/decomposition of N,O and NO, is presented here. Main radical reactions
The homogeneous destruction of N,O by radical attack is generally thought to proceed via the reaction [19, 24, 251 N,O + H + N, + OH,
(1)
and to a lesser extent via N,O + OH -+ N-, + HO,
(21
At low temperatures and under fuel-lean conditions, where the concentration of OH radicals remains much higher than the concentration of the H radical, Reaction 2 contributes to N,O destruction [45], although not as much as stated in previous work [26]. In addition, the other long-lived radical in a flame, i.e., the free oxygen atom, attacks N,O [7, 251 in: N,O + 0 --f N, + 0,
(3)
N,O + 0 * 2N0
(4)
Both paths, Reactions 3 and 4, are approximately equally favored over a wide temperature range [25]. In a flame, hydrocarbon radicals (CH,; 0 5 i I 3) are also involved. In this sense, it is worth noting that NO can be produced in a flame by the fixation of atmospheric N, from CH, radicals attacking N, in: N, + CH + HCN + N
(5)
N: + CH, + HCN + NH
(6)
with Eq. 5 being the dominant reaction under most combustion conditions [24]. The products HCN and free N atoms subsequently disappear to yield mainly NO at flame temperature [27]. This mechanism was termed “prompt NO” by Fenimore [28], since the rapid formation of NO was confined to regions near the flame zone. Typical levels of prompt NO range from a few ppm,, to more than 100 ppm, [24]. Although conditions prevailing in fluidizedbed combustion decrease NO emissions to a
large extent, there is always some NO carried upward by the flue gases through the flame. This “in-bed NO” together with the prompt NO, the latter being formed in the flame, can be “recycled” by hydrocarbon radicals uia the following reactions 120, 29, 301: NO+C+CN+O
(7)
NO + CH + HCN + 0
(81
NO + CH, + HCN + OH
(9)
NO + CH, --j HCN -I- H,O
(10)
It was said above that HCN oxidizes in a flame to form mainly NO. This statement should be analyzed in more detail. In fact, the conversion of HCN to NH, is favored by the flame conditions [31], fuel-rich or oxygen-lean, where OH becomes a dominant radical relative to 0. The reaction mechanism is as follows: HCN+OH-tHNCO+H
(11)
HNCO + H -+ NH, + CO
(12)
NH, + H,O + NH, + OH
(13)
This path, however, is very slow, especially Reaction 13, and only a modest conversion is expected. It is therefore assumed [32] that NH, observed in flames (in pulverized coal combustion) is either a consequence of heterogeneous processes or is a direct pyrolysis product in the case of sub-bituminous coals. The oxidation of HCN can proceed cia the intermediate NCO. At typical flame temperatures, this intermediate is mainly oxidized to NO by the radicals 0 and OH. However, when the flame temperature is not very high, the reduction of NC0 by NO to produce N,O and CO should not be dismissed [15]. Thus, we conclude that NO is the main product from HCN at typical flame temperatures. Taking this into account and considering Reactions 7-10, we can conclude that NO recycling finally yields NO, so that there is no major sink for NO in a flame. Thermal reactions
Thermal decomposition
of N,O occurs oia
N,O + M + N, + 0 + M,
(14)
where M stands for a gas molecule. The nature of this third body, M, must be taken into
106
G. MARBAN
account, since collision efficiencies vary significantly for different inert gases [33]. Reaction 14, which is increasingly relevant at higher temperatures, has been extensively studied [24, 34-391. The activation energies found for the pseudo-second-order decomposition of N,O vary from 215.7 kJ/mol [24] in the temperature range l,OOO-2,000 K to 251.2 kJ/mol 1381 in the range 1,190-1,370 K. Assuming a pseudo-first-order mechanism, Johnsson et al. [37] found activation energies of 234.5, 224.0, and 225.7 kJ/mol for M being Ar, N,, and He, respectively, in the temperature range l,OOO-1,350 K. Thermal NO is formed according to the Zeldovich mechanism [40], from molecular nitrogen, supplied primarily with the combustion air. Conversion starts at temperatures about 1,570 K and increases markedly with rising
ET AL.
temperatures. The degree of conversion is also proportional to the concentration of atomic oxygen. The objective of the present work is to assess the effect of fuel-gas injection on the different gaseous emissions (N,O, NO, and SO,) and to evaluate the relative importance of radical and thermal mechanisms for N,O decomposition and NO, formation. EXPERIMENTAL SECTION The experiments were carried out in a laboratory-scale, fluidized-bed combustor made of quartz (29 mm i.d.), operating atmospheric pressure; the main features are depicted in Fig. 1. The distributor consists of a quartzfritted plate. This supports a bed of Sic particles (d, = 115-150 pm) with a static height of
EbCtllCCYl reststance
\
\
Gas Injector (Quartz, 6x4 mm)
Bed of catborundum pClltiCl8S
t
QUARTZ REACTOR (3 cm LD.) Fig. 1. Experimental
set-up.
REDUCTION
107
OF N,O FROM FBC OF COAL
20 mm. The reactor is electrically heated along a length of 20 cm (7 cm preheating and 13 cm bed plus free-board). In this zone, the reactor behaves approximately isothermally, the temperature dropping sharply from 13 cm above the distributor, due to lack of thermal insulation along the nonheated zone. A thermocouple immersed in the bed allows the temperature in the reactor to be accurately controlled. The effective volume of the isothermal reactor is = 100 cm3. A gas injector made of quartz and ending in a fritted plate (6 X 4 mm) is used to create a homogeneous flame of either methane or propane. It is introduced from the upper part of the reactor at a distance of 10 cm from the distributor. To measure the flame temperature, a thin, high-temperature thermocouple (type S) is longitudinally stuck on the outer surface of the injector, with its tip immersed in the flame. Heat losses by radiation and conduction were estimated to be low (= 35 K at a measured flame temperature of 1,250 K), not affecting the conclusions drawn from this work. Visual observations of the flame at standard conditions (25 mL/min CH,, bed temperature Th = 1,123 K, total flow rate in the reactor [12 vol. % 0,] = 1 L/min) permit one to assign a volume to the flame of about 6 cm3, occupying approximately 80% of the cross-sectional area of the reactor. Under these conditions, the N,O residence time is = 1.6 s in the empty factor (Tb = 1,123 K) and = 0.3 s in the flame (25 mL/min CH,, Tf = 1,250 K). A set of mass-flow controllers allows a mixture of a known composition of different gases (O,, N,, He, CO,, N,O, NO, and SO,) to be introduced into the reactor from the lower part. Except for a few specific experiments, the total flow rate was kept at 1 L/min, corresponding to a fluidization velocity of = 0.1 m/s at 1,123 K (five times the minimum fluidization velocity). Values of 0, concentration in the inlet gases below 12 vol. % resulted in flame instability. In this sense, the 0, concentration used during the experiments had to be fixed at 12 vol. %, despite being higher than that existing at actual combustion conditions. The flue gas was dried before its composition was determined. Paramagnetic, non-dispersive infrared and chemiluminescence gas analyzers measured the concentrations of O,,
CO, and NO,, respectively. N,O analysis was performed using a gas chromatograph equipped with an electron capture detector (ECD). Separation was effected at 508 K on a 5 A-molecular sieve column (2 m long), with nitrogen as carrier gas. Different experiments were carried out both with and without a flame. In the following, we refer to those experiments carried out without the flame as being made with the “empty reactor.” For most experiments with the flame, the gas/O, equivalence ratio, 4, was kept at 0.42. This ratio is unity, when the reaction between the fuel-gas and 0, proceeds under stoichiometric conditions. The different inlet conditions used in the experiments are indicated in Table 1 and denoted by the letter in the final column of Table 1. Effect of the experimental decomposition
set-up on N,O
When carrying out experiments with the empty reactor, it was found that solid surfaces exerted a certain effect on N,O decomposition that was coupled with the thermal effect. Figure 2 shows the effect of different solid surfaces (quartz gas injector and flame thermocouple) on N,O decomposition (Tb = 85O”C, He to balance)‘. For obvious reasons, the bed’s thermocouple (sheath material: stainless steel 316) could not be removed to study its effect. It can be seen that the combined effects of the thermocouple (sheath material: INCONEL 600; approx. composition: 75% Ni, 15% Cr, 8% Fe, 1% Mn, 0.5% Cu, and 0.5% [Si, C, S]) and the quartz injector account for an increase by about 8% in the N,O conversion. The increase in conversion due to the flame’s thermocouple can be ascribed to catalytic effects by transition metals from group VIII (Ni and Fe), as well as by Mn, Cu, and Cr. Oxides of these metals are found to be highly active for N,O decomposition [41]. The increase in N,O conversion due to the quartz injector can be a result of an enhancement of the collision efficiency for thermal N,O decomposition. The flame’s thermocouple was found to exert a negative effect on N,O decomposition in the flame; thus, when the thermocouple was removed from a methane flame (gas/O,
108
G. MARBAN ET AL,. TABLE 1 Inlet Conditions for Different Experiments
0.42 (CH,) 0.42, 0.83 (CH,, C,H,) 0 0 0.42 (CH,) 0 0 0.42 (CH,) 0.42 (CH,) 0 0 0.42 (CH4, C,H,)
Tb (K)
N,O (ppm,.)
1,123 1,123 1,123 1,123 1,123 1,123 1,223 973 1,123 1,123 1,023-1,223 973
211 53-211 106-211 106 53-211 106 106 53-211 106 106 106 53-211
0, (vol. %o) CO, (vol. %o) N, (vol. %) 12 12 12 o-12 12 0 12 12 12 12 17.1-10s 12
equivalence ratio C$= 0.421, the N,O conversion increased from 71.4 to 74% (Ref. A in Table 1; flame temperature Tf = 1,290 K>. This can be a result of radical recombination on the thermocouple’s surface, leading to less N,O decomposition by radical attack [30]. The NO, formed in the flame also increased when the thermocouple was removed: from 15.3 to 17.2 ppm,,. This has an explanation similar to the increase in N,O decomposition, considering the importance of radicals in the prompt NO route. In the experiments described in the following section, the configuration of the experimental set-up was kept as shown in Fig. 1.
14 14 14 14 14 14 14 14 14 14 14 14
He (vol. %)
Reference
40 74 74 20 10-40 86,20 20 74 20 20 20 74
A B C D E F G H I J K L
34 0 0 66-54 64-34 0, 66 54 0 54 54 48.9-55.5 0
Only in a few cases, which are specified, was the flame thermocouple removed. The resulting experimental error was assumed not to affect the general conclusions drawn from this work. RESULTS AND DISCUSSION The flame temperatures achieved by the combustion of methane were 1,290 K at 4 = 0.42, and 1,333 K at 4 = 0.83; propane flame temperatures varied from 1,313 K at 4 = 0.42, to 1,340 K at C#J = 0.83 (Ref. B in Table 1). This corresponds to an increase in temperature of between 167 and 217 K with respect to the bed temperature. Figure 3 shows the N,O conver-
75 Injector
s
+
65
* -A-
1 .-b
55-
z” j45-
35
Yes
Yes NO
100
Thermocouple Yes NO No
1
$ =0.83
II
??
$ = 0.42 0............... ......
I
& ............... .:::::::::::~::::::::::::::::::::e .....&:::~;;~~
6
6
Empty re~c tor
+ 25
! 25
I 75
125
Inlet N20 concentration
175
225
(ppm”)
Fig. 2. Effect of gas-injector and flame thermocouple on the decomposition of N,O in the empty reactor (Tb = 1,123 K, inlet gas composition: 53-211 ppm, N,O, balance He).
0
1
,
25
75
I
I
125
175
Inlet N20 concentration
-I 225
(ppm,)
Fig. 3. Effect of equivalent gas/O, ratio on N,O conversion at different inlet N,O concentrations (Tb = 1,123 K, inlet gas composition: 53-211 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He).
REDUCTION
109
OF N,O FROM FBC OF COAL
sions achieved in the above-mentioned flames and in the entropy reactor (Ref. C in Table 1). N,O conversions up to 99% can be achieved with both flames (CH, and C,H,) at 4 = 0.83. The N,O conversion in the empty reactor has lower values than those plotted in Fig. 2 (with both gas injector and flame thermocouple). This is ascribed to the inhibiting effect of 0, on the catalytic decomposition of N,O by the flame thermocouple. This effect can be seen in Fig. 4, where the conversions of N,O in the empty reactor at different inlet 0, concentrations are shown (Ref. D in Table 1). Inhibition of N,O decomposition in catalysts by 0, has been reported by several authors [41-431. Nevertheless, a difference of N,O conversion of about 8% can be noticed between the points of 0 vol. % 0, in Fig. 4 (N,O conversion = 36.5%) and the point at 106 ppm,: N,O in Fig. 2 corresponding to the experiment carried out with both injector and flame thermocouple (N,O conversion = 44.7%). The only difference between them is the presence of 14 vol. % CO, in the inlet gas in the first case. An explanation for the negative effect of CO, could not be found. To prevent errors in the comparison of different results, most of the experiments were carried out with 14 vol. % CO, in the inlet gas stream. In the experiments with the flame described above (Ref. B in Table 1, Fig. 31, NO, could
4o1
16
12
8
4
25
75
125
! 0
Inlet N20 concentration
(ppm,)
only be formed from the N, released during N,O decomposition. In Fig. 5, these emissions of NO, are plotted against the different inlet N,O concentrations. Less than 16 ppm,, were formed in all cases (see Fig. 5). As can be seen in Fig. 6, the formation of NO, slightly increased when the carrier gas was replaced by mixtures of N/He (Ref. E in Table 1). The N,O conversion was not affected by the replacement of He as a carrier gas by N/He mixtures (see Fig. 6). Similarly, no difference was found regarding N,O decomposition in the empty reactor with either He or NJHe as the
100
I 4
Inlet
225
175
Fig. 5. NO, emissions in the flame at different inlet NT0 concentrations (T, = 1,123 K, inlet gas composition: 53-211 ppm,. N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He).
542 N2
20
1
0
1 a
02 concentration
I 12
I 16
(vol. Z)
Fig. 4. Effect of inlet 0, concentration on N,O decompe sition in the empty reactor (Tb = 1,123 K, inlet gas composition: 106 ppm, N,O, O-12 vol. % O,, 14 vol. % CO,, and balance He/N,).
342 Nz
442 N2
50 25
75
125
Inlet N20 concentration
17.5
225
-
(ppm,)
Fig. 6. Effect of carrier gas on N,O decomposition and NO, emissions in the flame CT,,= 1,123 K, +(CH,) = 0.42, inlet gas composition: 53-211 ppm,, N,O, 12 vol. % O,, 14 vol. % CO,, and either 74 vol. % He or N2/He mixtures).
110 carrier gas. Thus, for the mixtures referred to as F in Table 1, with either 86 vol. % He or 66 vol. % N, + 20 vol. % He, the conversion of N,O at Tb = 1,123 K was always approximately 36.8%. For the conditions used in this work, NO, was formed in the flame mainly via the prompt NO mechanism. Thus, in experiments where air was passed through a flame of methane (4 = 0.481, the flame temperature changed from 1,331 K at Tb = 1,023 K to 1,430 K at Tb = 1,223 K, but NO, emissions only increased by 99.7 to 125 ppm,. However, when the gas/O, equivalence ratio at Tb = 1,223 K was changed from 4 = 0.10 to C$= 0.48, NO, emissions spectacularly increased by 6.2 to 125 ppmu, though the increment in flame temperature was similar to that in the previous case (from 1,303 to 1,430 K). This fact reveals the low relative importance of thermal mechanisms for NO, formation in the flame. To further substantiate this statement, a stream of gas with 106 ppm, of N,O (Ref. G in Table 1) was introduced in the empty reactor at 1,223 K. In this case, no NO, was formed. However, when a flame of methane (4 = 0.42) was used to reduce N,O from a gas of varying N,O concentration (Ref. H in Table 1) passing through the reactor at Tb = 973 K, without using the flame thermocouple, the emissions of NO, increased by 3.2 ppm, (inlet N,O = 53 ppm,) to 10.5 ppm, (inlet N,O = 211 ppm,), although the flame temperature in this case was lower than the reactor temperature in the previous case (1,178 and 1,223 K, respectively). The conversion of N,, produced from the decomposition of N,O, to NO, was 4.9 + 0.3% in all cases. Figure 7 shows that NO introduced with the inlet gas (O-69 ppm,) has no effect on N,O decomposition in the flame of methane (Ref. I in Table 1). On the other hand, NO, formed by the prompt NO mechanism in the flame diminishes slightly from 14.7 to 8.4 ppm, when the inlet NO concentration increases from 0 to 69 ppm”. In this sense, NO seems to inhibit slightly the NO, formation in the flame. The effect of SO, on N,O decomposition both in the empty reactor and in the flame can be observed in Figs. 8A and 8B. In these experiments, a gas mixture containing O-
G. MARBAN ET AL. ‘; 18
E
::
16 ,5 t: 14 p 12
: 6
ti
5
20,
0
I
I
I
I
I,
10
20
30
40
50
Inlet NO concentration
IT
60
(ppm”)
70
3 ’
Fig. 7. Effect of inlet NO concentration on N,O decompe sition and NO, emissions in the flame (Tb = 1,123 K, 4 (CH,) = 0.42, inlet gas composition: O-69 ppm, NO, 106 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, 54 vol. % N2, and 20 vol. % He).
149 ppm, SO, was used at T, = 1,123 K with (Ref. I in Table 1) and without flame (Ref. J in Table 1). The presence of SO, in the empty reactor caused an increase in N,O reduction from 25% (0 ppm, SO,) to 40% (149 ppm, SO,). This positive effect could be a result of reaction of SO, with N,O to yield SO, and N,. However, more research should be carried out to confirm this. With a flame, the increase in N,O conversion with the inlet SO, concentration was not so marked (Fig. 8A). In fact, SO, resulted in an overall decrease in N,O conversion in the flame, as shown in Fig. 8B. This effect was similar to that exerted by SO, on NO, emissions from the flame (Fig. 8Bl. To explain this fact, radical recombination in the flame catalyzed by SO2 must be considered, according to the mechanism 1161 H + SO, -+ HSO, + M
(151
H + HSO, --) H, + SO,
(161
OH + HSO, + H,O + SO,
(17)
This mechanism can account for the decrease of N,O conversion and NO, formation observed in Fig. 8B, if thermal mechanisms are assumed to control both processes. Experiments at different total flow rates (from 0.75 to 1.15 L/min) were performed at three bed temperatures (1,023, 1,123, and 1,223 K) to calculate kinetic parameters of
REDUCTION
111
OF N,O FROM FBC OF COAL
20 -
B
r55
3 -
I
E" z
-1 11
, 140
30
0
60
90
Inlet SO2 concentration
li0
150
-
0
30
60
90
Inlet SO2 concentration
(ppm”)
120
t25 150
s x
(ppm”)
Fig. 8. Effect of SO, on N,O decomposition (Tb = 1,123 K, +(CH,) = 0.42, inlet gas composition: O-149 ppm, SO,, 106 ppm,, N,O, 12 vol. % O,, 14 vol. % CO,, 54 vol. % N,, and 20 vol. % He): (a) Comparison between N,O decomposition in the empty reactor and in the methane flame (4 = 0.42). (b) Effect of SO, on NO, emissions and N,O conversion in the flame.
thermal N,O decomposition in the empty reactor. To maintain a constant 0, flow rate of 120 mL/min, the 0, concentration in the inlet gas had to be varied for the different total flow rates experimented (Ref K in Table 1). The Arrhenius plot achieved by assuming an ideal plug-flow model for the reactor is shown in Fig. 9. The best fit was obtained for an apparent reaction order of 1. Thus, the experimental data can be interpreted in terms of the following pseudo-first order decomposition rate constant: k = 2.87 X 10” X exp(- 28573/T) s-l. The activation energy found in this work
3
Ref.U51;Nzasthirdbody
2
0.01
8.0
a:5
9Io 1 04/T
915
16.0
(K-' )
Fig. 9. Arrhenius plot for the kinetics of thermal decomposition in the empty reactor (flow rates: 0.75-1.15 L/min, Tb = 1023-1223”C, inlet gas composition: 106 ppm, N,O, 17.1-10.5 vol. % O,, 14 vol. % CO,, 48.9-55.5 vol. % N,, and 20 vol. % He). Results obtained from previous work [15] are plotted as a comparison.
(237.3 kJ/mol) is similar to those found by Johnsson et al. [37] in the range of l,OOO1,350 K (224.3 kJ/mol for Nz as third body and 225.9 kJ/mol for He). However, the preexponential factor found in this study is 3-7 times higher than those given by Johnsson et al. [37]. To assess the relative contributions to the N,O decomposition by radical and thermal mechanisms in the flame, different experiments were carried out with the reactor at Tb = 973 K. At that temperature, the conversion of N,O in the empty reactor was less than l%, according to the equation previously presented. Thus, the flame is responsible for almost all N,O decomposition achieved in the reactor. In these experiments, the flame thermocouple was removed for calculation of the N,O conversions. The inlet gas composition is indicated in Table 1 (Ref. L in Table 1). The flame temperatures achieved under these circumstances were 1,178 K (CH,, 4 = 0.42) and 1,212 K (C,H,, 4 = 0.42). Reaction rates for N,O decomposition were calculated assuming perfect mixing in the flame’s volume. These are represented in Figs. 10A (methane) and 10B (propane). Included there are the reaction rates for thermal N,O decomposition in the empty reactor evaluated at the flame temperatures (1,178 and 1,212 K> using the calculated rate constant. The reaction rates in the flame are between 9 and 15 times higher than those in the empty reactor. By assuming a more
112
G. MARBAN ET AL.
1
25
75
125
Inlet N20 concentration
175
-I 225
(ppm”)
-f 25
75
125
Inlet N20 concentration
175
2 15
(ppm,)
Fig. 10. Comparison between N,O decomposition in the empty reactor and in the flame: (a) methane (4 = 0.42), (b) propane (r#~= 0.42). Tb = 973 K, inlet gas composition: 53-211 ppm, N,O, 12 vol. % O,, 14 vol. % CO,, and 74 vol. % He.
realistic flame model (i.e., counterllow laminar diffusion flame [44]) instead of that of perfect mixing, the reaction volume would be smaller, and thus the reaction rates in the flame would be even higher. Additionally, error introduced by the uncertainty in the temperature measurement by the flame thermocouple was estimated to be low. Thus, by assuming a methane flame temperature 50 K higher than measured, the reaction rates in the flame still would be between 9 and 10 times higher than those achieved in the empty reactor at the same temperature. This strongly suggests that, at the conditions studied in this work, radical mechanisms are much more important than thermal mechanisms for N,O decomposition in the flame. CONCLUSIONS Reductions of N,O by up to 99% were achieved in the present work for both CH, and C,H, flames, confirming the feasibility of gas injection for N,O reduction purposes. The reaction rate for N,O decomposition in the flame was more than ten times higher than that achieved by the empty reactor at the same temperature. This corroborates the fact that radical attack on N,O plays an important role under the conditions used in this work. Metallic components of the thermocouples catalytically enhance N,O reduction in the empty reactor. This effect is the opposite of that in the flame due to radical recombination
at the solid surfaces, which reduces any decomposition of N,O. 0, inhibits the mentioned catalytic effects by radical attack. CO, unexpectedly disfavors any N,O reduction in the empty reactor. Prompt NO is the main mechanism for NO, formation in the flames used in this work. NO has no effect on N,O reduction in the flame. SO, slightly increases the N,O reduction in the empty reactor, but inhibits N,O reduction under flame conditions, due to SOZ catalyzing radical recombination. The reaction rate for thermal N,O decomposition in the empty reactor was rNZO = kc, o, where k = 2.87 X lOlo X exp(-28573/T\ s-l. The authors thank the Commission of the European Communities (Contract No. JOU2-Ci’920229) for financing this work. G. Marbcin is also thankful to the Spanish Ministry of Education and Science (MEC) for the award of a Postdoctoral Research Fellowship.
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6.
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113
OF N,O FROM FBC OF COAL
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Combustion and Flame 99:523-532 Received IO May 1995
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