60
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 1, 1979
Relative Contributions of Volatile Nitrogen and Char Nitrogen to NO, Emissions from Pulverized Coal Flames D. W. Pershing and J. 0. L. Wendt" Depaament of Chemical Engineering, University of Arizona, Tucson, Arizona 8572 7
Special experimentation on a laboratory combustor allowed determination of relative contributions of volatile NO and char NO to total fuel NO emissions from self-sustaining pulverized coal flames. Volatile nitrogen conversion was deduced from data from coal flames both with and without addition of volatile nitrogenous compounds in the fuel jet. Char nitrogen conversion was measured from char combustion studies. It was concluded that under typical self-sustaining flame conditions about 50% of the coal nitrogen remains in the char, that volatile nitrogen conversion to volatile NO is the major contributor to NO, emissions, and that volatile NO is amenable to control by changes in burner aerodynamics while char NO is not. However, some combustion modifications do also cause changes in nitrogen distribution between char and volatiles. Changes in pyrolysis temperature alone are insufficient to cause drastic changes in NO emissions.
Introduction Recent research on NO, abatement from pulverized coal combustion has focused on two major areas: (i) determination of the importance of fuel nitrogen under typical combustion conditions and the influence of combustion variables thereon (Pershing and Wendt, 1977) and (ii) determination of the effect of temperature and heating rate on the fate of nitrogen in coal particles under well defined experimental conditions (Pohl and Sarofim, 1977; Blair et al., 1977). Pershing and Wendt (1977) found that the major portion of nitrogen oxide emissions in typical pulverized coal units results from fuel nitrogen oxidation and that this fuel NO was remarkably insensitive to flame temperature over a wide range of temperatures of practical interest. This latter result contrasts with data of Pohl and Sarofim (1977) and of Blair et al. (19771, who showed that an increase in pyrolysis temperature and in particle heating rate (Pohl and Sarofim, 1977) increased the fraction of coal nitrogen volatilized. The latter authors also inferred that a t a furnace temperature of 1500 K, 6O-80% of the fuel NO was contributed by oxidation of the nitrogen released with the volatiles to form volatile NO. In this paper, we seek first to expand the findings of Pohl and Sarofim to typical pulverized coal combustion conditions with and without changes in burner aerodynamics, secondly, to reconcile fundamental data on the effect of pyrolysis temperature with our pilot data, and thirdly, to develop a new experimental methodology for determination of volatile NO and char NO from selfsustaining pulverized coal flames. The overall goal is to determine in more detail how and why changes in burner aerodynamics influence fuel nitrogen conversion and to infer the level of NO, abatement possible through this approach. That conversion of gas phase nitrogenous species to NO during combustion is dependent on local oxygen concentration has been demonstrated by a number of researchers, both in a premixed (DeSoete, 1975; Sawyer, 1975; Merryman and Levy, 1975) and in a diffusion (Sarofim et al., 1975) flame environment. Furthermore, the degree of conversion is not strongly dependent on the speciation of the compound containing chemically bound nitrogen, although conversion to NO will decrease with increasing nitrogen level. During coal devolatilization, the total mass volatilized increases with increasing pyrolysis temperature (Badzioch and Hawksley, 1970) as does the total nitrogen volatilized 0019-7882/79/1118-0060$01.00/0
(Pohl and Sarofim, 1977). Furthermore, total nitrogen volatilized has a steeper dependence on pyrolysis temperatures than does total weight lost (Blair et al., 1977). Therefore it might be argued that an increase in pyrolysis temperature will lead to a decrease in volatile nitrogen conversion, although the total nitrogen evolved has increased. Other work (Wendt and Pershing, 1977; Pohl and Sarofim, 1977) indicated that the volatile nitrogen is released only after 10-15% of the coal had been devolatilized, and that therefore the primary air (required for pulverized coal transportation to the burner) does not participate directly in volatile NO formation. Rather, it is the mixing between the fuel jet and the secondary air that controls volatile nitrogen conversion in practical combustion systems, and it is this fact that provides optimism for volatile NO control by aerodynamic changes of pulverized coal burners. The situation with respect to char NO is different, for a number of reasons. First, coal char is oxidized over kinetic time scales longer than even some slow mixing times induced by injector design variations. This means that char NO formation may be essentially independent of secondary air mixing occurring near the injector tip, with the important exception that the early heating rate of the parent coal particle will influence the nitrogen content of its char progeny. Furthermore, the mechanisms of char NO formation from char nitrogen involve gas-solid reaction processes (Wendt and Schulze, 1976) and an important step may well be reduction of NO back to N2 (Pereira et al., 1975). Data on coal char combustion (Pershing and Wendt, 1977) indicate that char nitrogen conversion to NO is much lower than volatile nitrogen. This raises the question whether burner aerodynamics affect total NO emissions through changes of mixing between volatile nitrogen and air or through changes in volatile/char nitrogen distribution and ensuing differences between volatile nitrogen and char nitrogen conversion to NO. Certainly, in lifted flames, where high fuel NO levels have been observed, both particle heating and early mixing have been changed from those experienced in attached flames. Approach The combustion of pulverized coal particles can be visualized as the combination of two (partially overlapping) phenomenological parts: volatile combustion and char burnout. This conceptual picture was extended to explicitly include the nitrogen species and, in particular, to 0 1978 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 1, 1979
61
WV
homog volatiles
.
combustion
co2
+
coal p a r t l r l e
0
-0
* co2
he troqeneous
A+#
combustion
H2°
de v o l a t i 1i z a t ion
"r." yowo
\-*
/ No
CHAR NITROGEN--+
char combust ion
N-, L
YCWC
Figure 1. Phenomenological model.
develop a methodology for determining the importance of nitrogen evolved with the volatiles relative to that retained in the char. Since volatile combustion times are of the order of 10 ms, while char burnout generally requires more than 300 ms, it was assumed that the homogeneous conversion of nitrogen species evolved with the coal volatiles is not chemically coupled to the conversion of char nitrogen to NO during the char burnout regime. Figure 1 illustrates this conceptual picture of fuel NO formation. The four-part approach outlined below combined experimental results from fundamental and pilot-scale combustion studies with empirical modeling to obtain predictions of char and volatile NO formation under a variety of conditions. In particular this work consisted of: (1)An experimental determination of fuel NO formation as a function of excess air, initial mixing, and fuel composition for self-sustaining pulverized coal flames. An empirical model of total fuel nitrogen conversion resulted. (2) Measurement of the formation of NO during the combustion of a coal char under a variety of conditions. These data led to an equation for char nitrogen conversion as a function of the process variables. (3) An experimental study on the oxidation of simulated nitrogen volatiles in actual pulverized coal flames. This involved measurement of the change in NO caused by an increase of volatile nitrogen alone. (4) Numerical combination of the previously obtained correlations and data to predict the fraction of the fuel NO emissions which result from char and volatile nitrogen, respectively. Thus, in essence, the approach was to break the formation of fuel NO into the conceptual steps shown in Figure 1, experimentally study each step in a partially decoupled mode, and then use the results in conjunction with a simplified model to interpret data on actual coal flames. Experimental Studies Apparatus. The experimental furnace facility is illustrated in Figure 2. The vertical combustion chamber
C O ~ LD R U M
#COAL
BIN
Figure 2. Experimental furnace.
is 76 in. long and 6 in. in diameter inside. A t the full load firing rate of 85000 Btu/h (6.6 lb of coal/h) it provides a nominal residence time of approximately 1 s. Fuel and air enter the combustion chamber a t the top via a water-cooled, variable swirl burner. The burner and fuel injectors used in this study are shown in Figure 3. The divergent injector contains three holes angled t o distribute the coal away from the axis of the furnace and is characterized as a rapid mixing injector because it produces a short bulbous flame. It was designed to be similar to the "coal spreader" system employed in many commercial systems. The second injector contains a single, center hole with an area equal to that of the three holes in the divergent injector. It produces relatively slow mixing between the primary and secondary air streams and hence gives a long, very thin flame. The two injectors are thus somewhat representative of two different classes of coal combustion equipment-one with intense mixing common in wall fired units, the other with slow mixing common in tangentially fired units.
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Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 1, 1979 DIVERGENT
Table 11. Nitroeen Conversions t o NO
AXIAL
% conversiona
SWIRL
I
--
&
13/64
VANES
L/
OUARL
I
I
Figure 3. Burner and fuel injectors. Table I. Pulverized Fuel Compositions Western FMC Kentucky coal char ultimate analysis, % dry C
H
73.0 5.0
N
1.40
S
3.1 9.3 8.2
0 Ash
heating value, Btu/lb, wet
12450
proximate analysis, % wet volatile fixed carbon moisture ash
36.1 51.2 4.8 7.8
72.8 0.9
0.99 3.5 0.7 21.2
3.6 73.8 1.8 20.8
The analytical system was designed so that continuous monitoring of NO, NO2, CO, C02, Oz, and SOz could be achieved. NO-NO, measurement was by chemiluminescent analysis and all sample lines were Teflon with 316 stainless steel fittings. Full details of the experimental facility have been reported previously (Pershing and Wendt, 1977; Pershing, 1976). Fuels. The compositions of the Western Kentucky coal and the coal char used in this work are given in Table I. The coal char resulted from the FMC-COED coal gasification process. Pulverized Coal Combustion. Studies on the combustion of the Western Kentucky coal were conducted to establish typical emission characteristics. Fuel NO formation was determined by replacing the combustion air with an artificial atmosphere containing 21% 0 2 / 1 8 % COz/61% Ar. This mixture was selected to eliminate the oxidation of atmospheric nitrogen and yet not change the temperature of the coal flame. Table I1 lists these results in terms of percent conversion of fuel nitrogen to NO. As previously reported (Wendt and Pershing, 1977), it was also possible to correlate the actual emissions in terms of a dimensionless emission coefficient, $ ppm of NO $ E = 2.OSR - 1.32 ppm of NO @ S R = 1.25 where SR = stoichiometric ratio (air/fuel). This correlation was valid for a wide variety of fuels and a wide variety of flame aerodynamics. Determination of Char Nitrogen Conversion. Studies on the combustion of FMC coal char were conducted to provide data for an empirical correlation of char nitrogen conversion to NO as a function of combustion parameters. It was assumed that FMC char combustion behavior was similar to coal char combustion in the char
of nitrogen- % conversion stoichiometric Western of nitrogenratio Kentucky coal coal charb 1.05 19.3 10.0 1.10 22.1 11.2 1.15 24.4 12.2 1.20 26.1 12.9 _13.3 1.25 Based on fuel NO measurements, divergent injector, 650 "F. Based on fuel NO measurements, flame mode of char combustion, 550 "F preheat.
burnout regime. The char was burned in two modes: (1) the flame mode, in which a turbulent diffusion flame was attached to the injector with the help of a small quantity of methane (21 % of the total heat release) in the primary "air" and in which methane simulated nitrogen free volatiles; and ( 2 ) the reactor mode, in which pure char without methane burned far from the injector and which simulated the char burnout regime of coal after all volatiles had been consumed and after significant mixing had taken place. These two modes of char combustion represent extremes in the time/temperature history and local oxygen environment which an actual coal char particle might experience. Figure 3 shows the char NO emissions in both modes compared to the emissions from the Western Kentucky coal under similar conditions. With char the influence of combustion mode was relatively small; the reactor mode data are only 100 ppm higher than the flame mode emissions (corrected for dilution by methane combustion products). In contrast, the coal emissions increased by over lo00 ppm with the reactor mode, indicating the importance of volatile nitrogen in coal. The combustion characteristics of the two modes were markedly different, however. Visual observation indicated that in the flame mode combustion was essentially complete within 4 to 6 burner diameters from the fuel injector, while in the reactor mode the particles burned alone (rather than in a flame sheet) and ignition often occurred farther than 10 burner diameters from the fuel injector. Further, for a given mode, the type of fuel injector had no measurable effect on char NO; that is, in the flame mode, the divergent and axial injectors gave identical emissions. Thus, particle heating rates and local concentration environments appear to have little effect on char nitrogen conversion. Testing with Ar/OZ/COz oxidizer indicated that essentially all the NO emissions were the result of char nitrogen oxidation; thermal NO was insignificant in both modes. However, as Table I1 indicates, the percentage conversion of the char nitrogen is only approximately half that of a corresponding pulverized coal. The experimental char data shown in Table I1 were used to develop the following empirical equation for the conversion of char nitrogen to NO, xc,in terms of the char nitrogen content, Yc X,
= (2.0SR -
where 0,= 540. The stoichiometry dependence is identical with that for pulverized coal because although the dependence of char NO formation on stoichiometric ratio is relatively small (Figure 4), the emission levels are also lower and, hence, the relative increase with increasing excess air corresponds to that found for coal flames. A single conversion coefficient, Pc, was used because the experimental data on char
Ind. Eng. Chem. Process Des. Dev., Vol. 18, No. 1, 1979 83
t O
6 0 CI r
5 1 600
O L L - - i _ d 3
1 1
2
i t
U-L-.
I 3 0
2
3
S’O I C H I O V E T R 1 C 9 A T IO
Figure 4. Comparison of Combustion mode. Western Kentucky cod vs. FMC coal char.
demonstrated that the influence of burner parameters and temperature (Pershing, 1976) was slight. Its value (/3, = 540) was determined directly from the data in Table 11. The reciprocal-sum dependence was selected to be consistent with experimental evidence on total conversion limits (Martin and Berkau, 1972). It is similar to that recommended by Fenimore (1972) and in agreement with the mechanisms proposed by Wendt and Schulze (1976) for char nitrogen conversion. If the rate-controlling step in the NO vs. N2 mechanism is second order in NO, eq 1 becomes an exact representation. Determination of Volatile Nitrogen Conversion. Experimental data were obtained to quantify the conversion of volatile nitrogen to NO in an actual pulverized coal flame environment. “Typical” volatile nitrogen compounds were added to the primary air/coal stream just prior to the fuel injector. At present, there is no general agreement on volatile nitrogen speciation. Axworthy (1975) found significant amounts of HCN were evolved during pyrolysis. Blair et al. (1977) found that the nitrogen was evolved late in the devolatization process as heavy organics although these are almost certainly further pyrolyzed before oxidation. Since fundamental data indicate that the dependence of conversion on speciation is small, NH, and NO were chosen as representative species which were commercially available and could be metered accurately. In addition, NO was believed to represent an upper limit on volatile nitrogen conversion assuming that NO reduction mechanisms are controlling the overall rate. Both gases were CP grade. Flow was metered with a pre-caiibrated rotameter and maintained a t a rate corresponding to 300 ppm (STOICH) in the flue with total N to NO conversion. At each test condition, NO emission data were taken with and without the additive. The difference in emissions was assumed to arise from a difference in volatile nitrogen alone. Figure 5 shows typical data for the Western Kentucky coal with the divergent injector. The data confirm the insensitivity of the controlling mechanism to nitrogen speciation; both NO and NHJ gave essentially identical results. Although the difference between the upper and lower lines could be directly attributed to the incremental increase in volatile nitrogen content, the volatile conversion could not be calculated directly because in each case the total amount of volatile nitrogen was unknown. Instead, the NO emission data (in ppm) were used as input to the analysis described in the next section. The percentage conversions were determined for both the base and the additive cases in a coupled manner. Analysis Methodology The results of the experimental testing provided a generalized empirical correlation for the fractional conversion of char nitrogen to NO and specific data on fuel
200
t
PbSE
0
:ASE
h C bCSED
0 hns A C D E D
@+OO -
0I O
I 10
STOICHIOMETRIC
120
so
RATIO
Figure 5. Experimental results with and without additional volatile nitrogen-Western Kentucky coal, divergent injector.
NO emissions with and without an incremental increase in volatile nitrogen content. A methodology was developed to interpret these latter data which were specific to each particular set of test conditions (e.g., fuel injector, air preheat, swirl, excess air, etc.) It should be emphasized that the subsequent model is not a solution of fundamental equations, it is a tool for back-calculating mass distributions and NO formation mechanisms from experimental data specific t o each condition of interest. Figure 1 summarizes the parameters of interest for any set of combustion conditions, e.g., the relative masses, w i , the nitrogen mass fractions, y,, and the fractional conversions of nitrogen t o NO, x,, for the original coal, the volatiles, and the char. These parameters are related by six independent mass balance equations: three for the base coal combustion case and three for the case in which the incremental nitrogen has been added (denoted by primes). 1. total mass balance base: wt = w, + wc (2) base + added volatile: wt/= + U’,dd (3) 2. nitrogen balance base: ytwt = y,w, + yCwc (4) base + added volatile: y,‘w,‘ = yyl(w, + wa,dd) + ycwc (5) 3. NO mass balance base: xatio, = x b 3 \ w v + X > ~ U ‘ , (6) base + added volatile: xt’yt/wt/ = x
