Energy & Fuels 1988,2, 301-308
301
Coal Composition Effects on Mechanisms Governing the Destruction of NO and Other Nitrogenous Species during Fuel-Rich Combustion Arun C. Bose, Karin M. Dannecker, and Jost 0. L. Wendt* Department of Chemical Engineering, University of Arizona, Tucson, Arizona 85721 Received August 31, 1987. Revised Manuscript Received December 7, 1987
Mechanisms governing the destruction of NO in the fuel-rich stage of a pulverized-coal stagedcombustion process were investigated. Emphasis was on determination of the effects of coal rank, temperature, and stoichiometric ratio on the speciation and rates of destruction of NO, HCN, and NH3. Experiments on eight coals, burned in a 2 kg/h downflow combustor over a wide range of combustion conditions, yielded a large data base of timeresolved profiles of temperature, major species, and minor species. Fuel nitrogen speciation varied significantly from coal to coal and depended on stoichiometric ratio and temperature, which were varied independently. A general correlation describing the destruction rate of NO was derived from the data. In contrast to previous work, this rate, which was first order in both NO and NH3, was generally valid for all coals and all conditions examined. The dominant mechanism of NO destruction in these coal flames was found to be homogeneous rather than heterogeneous but arose from more than a single fundamental elementary NO destruction reaction. Reactions between NO and hydrocarbons were also found to play a role if levels of the latter were sufficiently high. Heterogeneous p r o ” involving the evolution of nitrogen, over long time scales in the fuel-rich primary zone, and the effect of coal composition thereon were also important in governing the temporal profile of nitrogenous species.
Introduction Oxides of nitrogen remain a major pollutant from pulverized coal combustion and are receiving increased international attention because of their role as precusors to acid rain and as contributors to premature forest damage. This paper contributes to the development of practical models that yield insight into kinetic limitations encountered in achieving extremely low levels of NO, emissions through the staged combustion of pulverized coal. In modeling the fate of coal nitrogen during staged combustion, a simple approach is to view the kinetic processes as occurring in three different sequential combustion zones. The first zone involves short time scales of roughly 100 ms. This is where most of the coal mass and coal nitrogen are devolatilized, the volatile pyrolysis products are partially oxidized, and HCN, NH3, NO, and char nitrogen are formed. Even under fuel-rich conditions, O2is not fully consumed until after the end of this zone. The second zone, under fuel-rich conditions, involves long time scales (1-2 s) in the post flame. Although concentrations of major species may not change significantly in this zone, much happens to those of the trace nitrogenous species,192 and a substantial fraction of the original coal nitrogen is converted to N2 The third zone commences a t the point of final air addition to render an overall fuel-lean environment. NO can both be formed or destroyed in this zone. Needed is a quantitative kinetic description of the entire process describing the fate of coal nitrogen from the beginning of the first zone to the end of the third zone. This paper focuses exclusively on the second zone and attempts to quantify and correlate the behavior of nitrogenous species for a wide variety of both low-rank and high-rank coals. Previous work1 has shown for a single Utah bituminous coal that char/NO heterogeneous reactions were not dominant, and that the destruction of NO
* To whom correspondence should be addressed.
could be modeled by a single reaction, hypothesized by Fenimore3i4to be NO
+ NH2
-
N2 + products
These conclusions were reached from data obtained a t fuel-rich stoichiometric ratios (SR)of 0.8 and 0.4 and with a model involving partial equilibrium assumptions for determination of NH2, together with global equilibrium assumptions for OH. More recent work5v6has indicated that these conclusions may not be generally valid and that heterogeneous char/NO reactions may be important. From his data on an isothermal furnace, Schulz5 derived heterogeneous reaction expressions, all of which depended strongly on coal composition but were first order in NO and 0.7 order in CO. Hill et al.’ successfully described NO formation in a wide range of pulverized coal turbulent diffusion flames, without recourse to a kinetic model that included NH3 as a critical intermediate. That coal composition effects are important has been shown in parametric studiesg which revealed that at the (1)Glass, J. W.;Wendt, J. 0. L. Nineteenth Symposium (International) on Combustion; the CombustionInstitute: Pittsburgh, PA, 1982; p 1243. (2)Wendt, J. 0.L.; Pershing, D. W.; Lee, J. W.; G l w , J. W . Seuenteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979; p 77. (3)Fenimore, C. P.Combust. Flame 1976,26, 249. (4)Fenimore, C. P. Seuenteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1979;p 661. (5)Schulz, W. Dr.-Ing Dissertation, Ruhr-Universit&tBochum, FRG, 1985. (6)Kremer, H.; Schulz, W., Paper presented at Twenty-First Symposium (International) on Combustion, Munich, FRG, August 1986. (7)Hill, S. C.; Smoot, L. D.; Smith, P. T. Twentieth Symposium (International)on Combustion;The Combustion Institute: Pittsburgh, PA, 1985;p 1391. (8) Glass, J. W. Ph.D. Dissertation, University of Arizona, 1981. (9)Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. Nineteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1983;p 1271.
0887-0624/88/2502-0301$01.50/00 1988 American Chemical Society
302 Energy & Fuels, Vol. 2, No. 3, 1988
1
Coal/TransDort -804cm-. Air
I
Secondary Air
I
’
4 CSOC~ FueVAir Premixing Chamber
I
Bose et al. nisms and rates from the data. The experimental approach described here attempts, therefore, to bridge the gap between the more fundamental, more specifically defied flat flame and drop tube experiments of others and those from pilot and full-scale coal combustion tests. In this way the mechanisms derived are both fundamentally correct and still directly applicable to general engineering practice. In them experiments,2 kg/h of pulverized dwere transported by primary air and subsequently mixed with preheated secondary air in a “premixed” burner, following the design of Beer and Thring.lo The 15.2-cm4.d. combustor itself was constructed of three concentric Zircar vacuum-formed alumina cylinders surrounded by Kaowool bulk fiber and contained in a 80.4-cm-0.d. steel shell. The radiant section was approximately 2 m long, allowing residence times of 1-3 s, depending on the coal and the stoichiometric ratio. Temperature Measurement. Axial gas temperatures were measured at various axial positions (porta), by using a bare Type R thermocouple supported by a water-cooled holder. The measured thermocouple temperatures were corrected for radiation heat loss, where the magnitude of this correction depends both on the measured gas temperature and on the wall temperature. Since measurements of the latter were sparse, it was calculated from a calibrated, one-dimensional, heat-transfer model of the combustor. Because of the insulated nature of the combustor, the wall temperatures were usually within 20 K of the corrected gas temperatures, and the maximum correction to the measured thermocouple temperature was always less than 35 K. The corrected temperatures reported here are, therefore, consistent with the measured axial temperature profile, with the few measured wall temperatures, with a local heat balance around the thermocouple, and with another heat balance down the furnace. Since previous work had shown that, under fuel-rich conditions, particle temperatures did not differ much from those of the gas, the multiwavelength infrared pyrometer utilized previously’ was not used in this work. Sampling and Analysis. Flue gas samples were withdrawn at the same locations (ports), by using a water-cooled, waterquenched probe. These samples were analyzed to yield timeresolved profiles of major and minor species. After passing through a refrigerated knockout pot, the sampled gases passed through in-line instruments for 02,CO, COz, and NO, analysis and also to a Perkin-Elmer Sigma 1B chromatograph outfitted with dual molecular sieve and Porapak T columns and thermal conductivity,nitrogenfphwphorous, and flame ionization detectors. Emphasis was on accurate H2 measurements, and for this, argon, rather than helium, was used as the carrier gas. Hydrocarbon analysis focused on CH,, CzH2,C&, and C,&, of which only the first three were observed in the gas phase. Approximately half of the HCN was dissolved in the (metered) quench water, which was subsequently analyzed for HCN (after S-precipitation) and for NHS, by using ion and gas electrodes respectively. These data were then converted to ppm (wet) values in the flue gas. Previous work8 had demonstrated that this procedure allowed NHS recoveries exceeding 95%. H 2 0 concentrations were calculated from a hydrogen element balance. Solid carbon reaidue was calculated from a carbon element balance and was also collected in the knockout pot. An independent oxygen element balance was closed, usually to within 5%, and this confirmed that all the data were consistent. Closure of the nitrogen balance was not attempted in this work, primarily because of difficulties both in operating an N2-free facility of this size and in accurately sampling and measuring the total nitrogen remaining in the char residue. Facility Validation. Radial NO and O2 and temperature measurements confirmed that the system was essentially onedimensional in concentrations and temperature. These checks for one-dimensionalitywere made routinely for the base case runs described below. However, during some special tests in which nitrogen dilution was used to lower temperature profiles, coal ignition problems were experienced and deviations from onedimensiontiIityappear to have occurrei3 when injection of N2 was
/ I tsuo“-
Figure 1. Experimental combustor.
end of the second zone, bituminous coals yielded higher HCN than NH3 values, while the converse was true for lignites. This is to be contrasted with other datae which indicated for a bituminous coal that NH3 was always higher than HCN and that both depended strongly on stoichiometric ratio but that temperature alone did not affect either HCN or NH3values at the end of the primary zone. In the former case, the furnace used was autothermal and therefore had an axial temperature decay in the primary zone, while, in the latter case, species concentrations were measured under isothermal conditions. This distinction is important since the values of nitrogenous species at the end of the second zone depend on the entire temporal history of devolatilization processes, homogeneous kinetic processes, and heterogeneous kinetic processes up to the sampling point. The purpose of the experimentation and analysis reported here was to help resolve some of the discrepancies outlined above. To this end, a large data base consisting of aerodynamically well-defined, time-resolved profiles of major and minor species measured during fuel-rich pulverized coal combustion, has been generated and analyzed. Effects of coal composition, stoichiometric ratio, temperature changes caused by O2enrichment and N2 dilution, and addition of trace quantities of NO and hydrocarbons are explored and compared to similar data from doped gas flames. Thus, the following specific issues are addressed: (1) To what extent do mechanisms governing the destruction of NO depend on coal composition? (2) Can time-resolved data from a very wide range of coals be correlated under the aegis of a simplified unified (coalindependent) mechanism? (3) To what extent do homogeneous and heterogeneous processes play an important role in the formation and destruction of nitrogenous species in the second zone? The data presented can also be used to further the development of more detailed coal combustion kinetic models and subsequently to promote their application to determine kinetic limits to NO, abatement through combustion modifications. Experimental Section Furnace. A complete description of the laboratory combustor shown in Figure 1 is available and only a brief summary is presented here. In essence, it was designed to allow self-sustaining combustion, with no external heating, in a configuration that was representative of practical units in terms of characteristic times and temperatures, yet still sufficiently well-defined aerodynamically to allow the extraction of mecha-
(10)Beer, J. M.; Thring, M. W .Proceedings of Anthracite Conference Bulletin 75; The Pennsylvania State University: University Park,PA, 1961.
Energy & Fuels, Vol. 2, No. 3, 1988 303
Coal Composition Effects on Mechanisms
no. 1 2
3 4 5
6 7 8 9 no. ~~
~
1
2 3 4 5 6 7 8 9
Table I. Pulverized Coal Compositions proximate anal., wt % coal volatile matter fixed carbon moisture Utah bituminous No. 1 39.2 47.4 4.5 Utah bituminous No. 2 46.6 43.1 1.8 Western Kentucky bituminous 41.8 46.9 3.0 Texas lignite 34.5 30.2 14.0 38.3 35.3 13.1 Beulah lignite low Na No. 1 37.1 37.1 17.8 Beulah lignite low Na No. 2 Beulah lignite high Na 36.3 33.9 22.5 RWE German brown coal 48.9 13.4 28.8 dried RWE German brown coal' 38.8 30.7 15.3 ultimate anal., w t % (dry with ash) coal carbon hydrogen nitrogen sulfur 70.87 5.11 1.64 1.03 Utah bituminous No. 1 Utah bituminous No. 2 5.19 1.29 0.45 71.59 Western Kentucky bituminous 73.22 4.55 0.82 3.00 Texas lignite 54.60 0.85 3.39 3.60 62.34 Beulah lignite low Na No. 1 0.92 3.48 3.07 Beulah lignite low Na No. 2 57.79 2.07 3.95 0.91 Beulah lignite high Na 59.80 1.02 3.90 1.08 RWE German brown coal 62.88 4.16 0.87 0.65 dried RWE German brown coal' 48.52 0.72 0.78 3.37
ash 8.9 9.3 8.6 21.3 13.3 8.0 7.3 8.8 15.2 oxygen (diff) 12.08 12.66b 12.53 16.21 16.90 25.57 24.78 20.92b 21.83b
From 74.4% moisture. *Analytically determined. made downstream of the primary ignition zone. These specific instances are noted below. Previous work8 had confirmed that neither NO nor NHBwas destroyed on the combustor wall under the hot fuel-rich environments experienced here. coal Compositions. Compositions of the eight different coals examined are shown in Table I. They consist of (1) three different U.S.bituminous coals, two of which were nominally similar but were supplied from different sources and a t different times, (2) one German brown coal, and (3) four U.S. lignites, two of which were again generically similar but supplied from the same source a t different times. The choice of these coals w a deliberate, ~ since it was desired both to span a range of coals of practical interest and also to determine how variations from coal to coal within one clam, or indeed within one type, compared to variations between coals of completely different classes. All coals were pulverized (75% through a 200 mesh screen) as they would be if burned in practical steam boilers.
Experimental Results Presentation of Results. Experimental data are presented in three parts. First, the base-line profiles of major and minor species of a Utah bituminous and a German brown coal at SR = 0.8 and SR = 0.6 are presented and discussed in detail. Residence times were calculated by using measured flue gas compositions, corrected temperatures, and calculated molar flue gas rates. All concentrations are reported on an actual wet flue gas basis. Similar data from the other coals are reported in tabular form. In the second part of this section, the effect of temperature, at constant stoichiometric ratio, on nitrogenous species profiles is examined for both the Utah and German brown coals. This also allowed coal composition effects to be investigated under similar temperature profile conditions. The third part of this section consists of data describing the effect of the addition of NO and hydrocarbons to the post flame. These experiments were conducted t o test specific hypotheses regarding the order of reaction with respect to NO and the possible role of NO reduction mechanisms involving hydrocarbon fragments. These tests have applicability to other NO, abatement methods such as reburning, in which NO is destroyed by secondary fuel injection." (11)Wendt, J. 0. L.; Stemling, C. V.;Matovich, M. A. Fourteenth Symposium (Internotional) on Combustion; T h e Combuetion Institute: Pittsburgh, PA, 1973; p 881.
SR.06
SR=O 8
14
t
t
Residence Time, s
Residence Time, s
Figure 2. Base case profiles for Utah bituminous coal: batch 1, open symbols; batch 2, shaded symbols.
Base-Line Profiles. Figure 2 shows time-resolved profiles for Utah No. 1coal (open symbols, two separate runs) and Utah No. 2 coal (shaded symbols, one run) that are representative of the bituminous coals. Coal No. 2 burned at higher temperatures than did the No. 1coal, and this, together with slight differences in coal composition, contributed to the differences in flue gas species concentrations. At SR = 0.8, O2was completely consumed at 0.5 s. Appreciable quantities of CH4 and C2H2(but no other gaseous hydrocarbons) were observed for the No. 2 coal but decayed towards low asymptotes. As expected, H2was a major species under these fuel-rich conditions. Nitrogenous species profiles are noteworthy in that large quantities of NO are formed in the early time zone where O2is still being consumed. This NO decays over long time scales spanning the second post-flame time region. At SR = 0.8, HCN and NH3 levels are low, between 10 and 30 ppm. At SR = 0.6, HCN was the dominant volatile nitrogenous species and, for r > 1.0 s, was present in greater amounts than either NO and NHB. If NH3 is formed homogeneously from HCN, then the lack of a decay for HCN indicates that there must be a source of that species along
Bose et al.
304 Energy & Fuela, Vol. 2, No. 3,1988 SR.08
y
SR.06
I
Y 1800l
1800r
1000
6 s
8
I0
6 2
L
annt
I
I
0
I
2
Residence Time, s
0
1
2
3
Residence Time, s
Figure 3. Base case profiles for German brown coal.
the combustor. This source, which has been observed previously,' may be caused either by post-flame devolatilization of coal nitrogen or by reactions of NO with hydrocarbons. Analogous profiles for the German brown coal are presented in Figure 3 and are somewhat representative of lower grade coals. Noteworthy are the low temperatures at which this coal burns and the persistance of O2 at SR = 0.8. Furthermore, hydrocarbons appeared to be "frozen in" at fairly high values. The German brown coal yielded relatively low HCN and NH3 values at both SR = 0.8 and SR = 0.6, even though ita nitrogen content was not significantly different from that of the bituminous coal. In contrast to the Utah coal, NH3 levels were significantly higher than HCN levels at SR = 0.6 while at SR = 0.8 NH3 and HCN values were of comparable magnitude. As with the Utah coal, HCN did not decay down the combustor length, indicating a continuous source for this species. Table I1 summarizes nitrogenous species data from all coals. Peak values of NO and temperature are presented, along with (interpolated) values for T,NO, HCN, and NH3 at 1-and 2-s residence times. Noteworthy is the range of values of HCN and NH3 observed, ranging from less than 10 ppm to several hundred ppm. One of the most important and most significant findings to be drawn from these data is that, in general, bituminous coals yield higher HCN values than NH3, while for low-rank coals the converse is true. This is in agreement with the parametric studies of Chen et al.gbut not with those of Kremer and Schuk6 In general, more rapid decay rates for NO occur a t the higher NH3 concentrations. As expected, all measured nitrogenous species concentrations were, in all cases, far above their equilibrium values. Major species profiles showed significant quantities of CO and H2 for all coals. At SR = 0.8, the Western Kentucky coal yielded values of 3.6% and 0.1 %, respectively while the lignites yielded values that ranged between 4% and 6% for CO and 1%and 2 % for Hp It was found that, except at the largest residence times, the water gas shift equilibrium did not hold, and could not follow the temperature decay in the post flame. Indeed, temperatures calculated from the measured species concentrations and the water gas shift equilibrium were significantly lower than those measured, indicating that this discrepancy was not due to a frozen equilibrium at a higher temperature. At SR = 0.6, the asymptotic values for CO ranged between 6% and 8% while those for H2 lay between 2% and 6%.
00
Residence Time, s
Figure 4. Effect of temperature on speciation and distribution of nitrogenous species for Utah bituminous coal, SR = 0.6.
00 02
n 0.6
b
Effect of temperature on the speciation and distribution of nitrogenous species for German brown coal, SR = 0.6.
Figure 5.
Again, the water gas shift equilibrium did not hold in general. Effect of Temperature. Additional experiments were performed to determine whether the distribution of nitrogenous species, especially the relative magnitudes of HCN and NH3, resulted from inherent differences in coal composition or from the different temperatures at which various coals burned. Temperature changes were achieved by O2enrichment and N2 dilution, and in order to account for simple dilution effects, results are reported in terms of fraction of coal nitrogen converted to the indicated species.
Coal Composition Effects on Mechanisms
Energy & Fuels, Vol. 2, No. 3, 1988 305
Table 11. Summary of Nitrogenous Species Profiles" peak values NO T
no.
coal
1
Utah bituminous No. 1 Utah bituminous No. 2 Western Kentucky Beulahlow sodiumNo. 2 Beulah high sodium German brown coal
930 lo00 1294 600 282 560
Utah bituminous No. 1 Utah bituminous No. 2 Western Kentucky Texas lignite Beulah low sodium No. 1 Beulah low sodium No. 2 Beulah high sodium German brown coal
800 947 1300 600 450 137 407 470
2 3 6 7 8
T
NO
1720 1745 1680 1550 1440 1482
atT=ls NO HCN NHS SR = 0.8 410 13 20 468 30 20 760 43 12 400 12 37 140 30 40 450 30 25
1450 1483 1540 1430 1360 1387
220 30 330
5 19 50
40 22 45
1330 1240 1298
1600 1695 1670 1450 1300 1530 1410 1502
SR = 0.6 180 340 300 270 410 120 100 60 180 100 90 60 40 140 370 40
1300 1398 1490 1350 1220 1330 1260 1425
100
280
220
1150
200 40 50 26 10 170
180 55 80 66 100 36
60 200 550 199 400 65
1346 1350 1150 1220 1140 1301
205 100 42 190 450 190 400 55
atT=2s HCN NH,
T
Species concentrations are in ppm; temperature is in K. Peak values of NO and T do not necessarily occur at the same time. Values are interpolated from data.
Figures 4 and 5 show data for the Utah bituminous No. 2 and the German brown coals, respectively. Comparison of the temperature profiles in these figures shows that some of the profiles overlap between the two coals. Total volatile fuel nitrogen (TVFN) comprises the sum of NO, N H , and HCN. The differences between this and unity is equal to the fraction of coal nitrogen remaining in the char plus that converted to NP In all cases, increasing temperature decreases TVFN. For the Utah coal, that quantity decays with time, confirming the practical importance of first-stage time and temperature in staged combustion. German coal results are similar if one neglects the three low-temperature data points at times greater than 1.6 s. These data points do not allow a nitrogen balance to be closed and thus are thought to be in error due to stratification down the furnace, which is more likely under the difficult ignition conditions encountered at the lower temperatures. The relative quantities of HCN and NH3 appear to be a property of the coal composition, rather than of combustion temperature, although both HCN and NH3 increase with decreasing temperature. At lower temperatures very large quantities of NH3 can be produced from the German but not from the Utah coal. Close examination of these and other data obtained at SR = 0.8, however, indicates that even in this case, HCN appears in the bulk gas phase before NH3. Therefore, the differences in N species distribution attributable to inherent differences in coal composition are most likely due to ways the latter affects either the temporal environment in the bulk gas phase or the evolution rate of nitrogenous species, rather than to important speciation differences in the primary nitrogenous compound evolved from the particle surface. Freihaut and Seery12showed that primary pyrolysis yields of HCN relative to tar nitrogen increase as % C (daf) of the coal decreases. NH3 was not a major primary pyrolysis product, even for lignite. Their results are consistent with our observations if one considers that, under oxidative pyrolysis conditions, NH3 is derived from HCN and HCN from tar nitrogen. Effect of NO and Hydrocarbon Addition. Special experiments in which NO was added to Utah bituminous No. 1 coal at SR = 0.8 and SR = 0.6 were conducted to confirm the order of reaction with respect to NO and to provide some insight into NO destruction mechanisms by (12) Reihaut, J. D.; Seery, D. J. Presented at InternationalConference on Coal Science, Sidney, Australia, October 1986.
Before NO Addition
' NOAddilion
1400 -
E
1200
SR = 0.8
-
.- 800 -
80
-
140; 60
a a io00
-E
i 0
0
600
200
400
8
OL I4O0
r
"oat
20
10
I
SR =0.6
E 1000 2
.e e
800
o
Residence Time, s Figure 6. Effect of NO addition on nitrogenous species for Utah bituminous No. 1 coal.
reburning. Results (Figure 6) indicate that NO destruction rates are approximately first order with respect to NO and that NO addition leads to small, but measurable, increases in HCN and NH3. In order to explore the importance of NO destruction mechanisms involving hydrocarbons, acetylene transported by argon was added into the post flame at port 4, and changes in NO, HCN, and NH3 concentrations were measured. Results (Figure 7) indicate a rapid destruction of NO and formation of HCN, NH3,and Nz (obtained from a nitrogen balance). Clearly, NO reacts rapidly with hydrocarbons to form HCN, which generates NH3 and Nz. However, this reaction appeared to occur rapidly at the point of acetylene addition where hydrocarbon concentrations are much higher than what was measured relatively far downstream. Thereafter, NO destruction occurred at a rate similar to that without acetylene addition. This suggests that although NO/HC reactions are important at high hydrocarbon concentrations, they account neither for the NO decay observed in the base-line tests nor for the continuous source of HCN discussed in the fmt
Bose et al.
306 Energy & Fuels, Vol. 2, No. 3, 1988
i
0 SR.061
BaseRun BoseRun A SR=077 0, Enrichment 0 SR.062 N, Dilution S R z 0 3 8 N, Dilution SR.084
C
bA.oeo
i
' 28-b 0
'\ kh
E
2 400
k;
I
,
I
I
0
2
3
Residence Time, I
3
5
Figure 7. Effect of C2H2addition at port 4 on NO, HCN, and NH8 profiles.
6
7 8 I / T X IO4 ( K - ' )
9
Figure 8. Homogeneous rate constants for Utah bituminous No.
part of this section. Therefore, the most likely source of this HCN is the continuous evolution of nitrogen from the coal residue in the post flame.
Data Analysis Comparison with the Fenimore Mechanism. The overall objective of thisresearch was to develop engineering models that can be used to predict the destruction of NO and other nitrogenous species in the post-flame zone of a fuel-rich pulverized-coalflame. Therefore, it is appropriate first to determine whether the previously published conclusions of Glass and Wendt' that NO and HCN destruction can be modeled by the gas-phase mechanism of Fenimore3p4extend to the different coals and wider temperature ranges explored here. According to that mechanism, HCN is destroyed through HCN
+ OH
kl
NHS + products
2 coal.
-----
+ NO -% N2 + H 2 0
(1)
Using the partial equilibrium NHi+l+ OH NH; H2O a global equilibrium for OH 20H + H2 + 2H20
+
40
(2)
--
IO 0
and NO is destroyed and N2 is formed by NH2
Bituminous # 2 W Kentucky Bituminous Texas Lignite Beulah Lignite Low No # I Beulah Liqnite Low Na X2 Beulah Liqnite Hiqh No -Glass Data, Utah Bituminous German Brown -Utah
t
30410
io toI / T X 1A0 4 (K-')8 1 0
9'0
lolo
Figure 9. Homogeneous rate constants for nine different coals.
(3) (4)
and a nitrogen balance in which NH2 is present a t concentrations much less than those of NH3 yield the global equations d(YNHa- YNo)/dt = ~I'CTYHCNYH~O/YH:'~( 5 )
It should be noted that the left-hand side of eq 5 is equal only to the destruction rate of HCN and not to its net formation rate and therefore is not equal to measured values of d(YHcN)/dt. The global rate coefficients k{ and k i consist of products of elementary rate coefficients and appropriate equilibrium constants and should be functions only of temperature, not of coal composition. Values of all variables in eq 5 and 6 can be obtained from the experimental data presented, leading to experimentally determined values for k{ and k i . All the Utah bituminous No. 2 coal data were so analyzed, resulting in Figure 8, which shows an Arrhenius plot
of k,' and k i . The data correlate well even though they span large variations in Y N ~YNHs, , and T. Similar Arrhenius plots were obtained for each of the other coals and showed that for each coal the data could thus be well correlated. However, if correlations for all coals (including those of Glass and Wendt') are combined (Figure 9),a disturbing fact emerges, namely that a common mechanism fails, and changes in both preexponential factor and activation energy occur from coal to coal. This should not be the case if the homogeneous gas-phase Fenimore mechanism is generally valid. Therefore, other ways of correlating the data were explored. Extended Mechanism. One of the more recent evaluations of detailed kinetics of NO formation from gaseous fuel nitrogen has been produced by Glarborg et al.13 This detailed reaction scheme incorporates the prior fundamental results of Morley,'* Haynes,16J8Branch et d.,"and (13)Glarborg, P.; Miller, J. A.; Kee, R. J. Combust. Flame, 1986,65, 177-202. (14)Morley, C.Eighteenth Symposi~m(International) on Combustion: The Combustion Institute Pittsburgh, PA, 1981;p 23. (16) Haynes, B. S. Combust. Flame 1977,28,31.
Energy & Fuels, Vol. 2, No. 3, 1988 307
Coal Composition Effects on Mechanisms
i-
Roose.l* Examination of this mechanism, together with a reexamination of Fenimore's mechanismF4 revealed that in the gas phase the following NO destruction mechanisms should be as important as reaction 1: N
+ NO
NH + N O
+0 N2 + OH N.7,
+
+
(7)
(8)
Retention of the partial equilibrium assumption for N, NH, and NH2 (eq 3) and global equilibrium for OH (eq 4), yields
(9) where the power of [H,] corresponds to (3 - i ) / 2 with 0 Ii I2 and i defined by the NO destruction reaction NO + NHj N2 + OHj
1
+
A plot of log [(1/[NH3)])d(1og [NO])/dt] versus log [H2] a t three different fixed temperatures was compiled from our data in order to determine if one single reaction of the three, (21, (71, or (81,dominates over the temperature region explored. No correlation exi~ted.'~Therefore, we concluded that if homogeneous gas-phase kinetics were paramount, then either no single reaction dominated NO destruction or the equilibrium assumption for OH was invalid. Subsequent comparisons of NO profiles predicted by eq 9 to those measured for all our coals showed that (1)the model predicts quenching at too high a temperature, (2) there is a strong effect of temperature on NO destruction rates, and (3) all three reactions are important at the various temperatures encountered; i.e., at the higher temperatures encountered the reverse Zeldovich reaction (eq 7) was important, while at lower temperatures Fenimore's reaction (eq 1)becomes significant but is quenched too soon. Figure 10 shows the result of these comparisons between the predictions of eq 9 and all the data for Utah bituminous No. 2 coal, The data showed conclusively that d[NO]/dt was first order in [NO] and first order in [NH,], over the range of stoichiometric ratios and temperatures tested. Therefore, Figure 10 compares the predicted and measured relationship between 1 d(log YNO) YNHa
dt
and 1/T. Shaded symbols correspond to the model (eq 9),open symbols correspond to the data, and differently shaped symbols represent different experimental runs as shown. The data correlate well but demonstrate a less severe temperature dependence than the model. The difference in apparent global activation energies cannot be attributed to errors in temperature measurement, which would merely tend to shift the experimental data laterally, but to discrepancies in the model. Also shown on Figure 10 are data and predictions from a series of testa in which temperature, nitrogenous species, and major species were measured in the post-flame zone of fuel-rich doped propane flames. These are denoted as (16) Haynes, B. S. Combust. Flame 1977,28, 113. (17) Branch, M. C.; Kee, R. J.; Miller, J. A. Combust. Sci. Technol. 1982,29, 147. (18) Rooae, T.R. Ph.D. Dissertation, Stanford University, 1981. (19) Bose, A. C.;Dannecker, K. M.; Wendt, J. 0. L. Presented at the Fall Meeting, WSS-CI, 1986.
Model
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Figure 10. Comparison between measured and predicted NO destruction rates for Utah bituminous No. 2 coal and doped propane gas runs.
open and shaded stars. It is clear that even for these nonheterogeneous systems the measurements exhibit a lower global activation energy than that predicted by the model, although the fmt-order behavior in NO and in NHB still holds. Although the gas data (and predictions) generally fall somewhat below those of the coal, this is due, we believe, to the higher levels of H2 in the gas runs. Because of similarity between the gas and coal data, we conclude that homogeneous gas-phase chemistry rather than heterogeneous chemistry controls the destruction of NO. A least-squares line (not shown in Figure 10) drawn through the combined data resulted in ? = 0.74. The coal data alone yielded ? = 0.82; the gas data along yielded ? = 0.86. Figure 11shows a composite plot for all the remaining coals, where each coal is denoted by a separate symbol, encompassing a range of temperatures and stoichiometric ratios. With the exception of data from the two-low temperature runs for the German coal, the data both correlate reasonably well and compare well to those for the Utah bituminous No. 2 coal and the gas data shown in Figure 10. In general, NO destruction is first order in NO and NH3 and exhibita a lower overall global activation energy than would be predicted from the partial equilibrium model (eq 9). Eight data points from the two low-temperature runs for the German coal, shown in Figure 5, fall close to the (OH equilibrium model) predictions and not on the general correlations for the vast majority of the data. If we attribute the general discrepancy between predictions and data to failure of the OH equilibrium assumption, then the surprising fit of the eight errant data points may be because stratification during these runs yielded local stoichiometries significantly richer than SR = 0.6, when global equilibrium may be reasonable for OH. It is noteworthy that Glass' SR = 0.8 data fib the general data trend but his SR = 0.4 data lie on the equilibrium model predictions. Consideration of the whole picture, therefore,
308 Energy & Fuels, Vol. 2, No. 3, 1988 r
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species depend not only on stoichiometric ratio and temperature but also specifically on coal composition. The latter variable is important primarily because it controls the environment and temperature history down the combustor, rather than because variations exist in the primary nitrogen species evolved. Analysis from a large data base containing results from eight coals burned under a wide variety of conditions yielded a general correlation describing the destruction of NO. In contrast to the conclusions of Schulz? these data strongly suggest that dominant mechanisms are gas phase and first order in both NO and NHB. Therefore, in order for this model to be used to predict NO destruction rates in combustion equipment, it must be expanded to allow prediction of NH3, and in contrast to Hill et al.,' our data show that NH, is a key intermediate that cannot be neglected. Moreover, these new results do not completely concur with the previous conclusions of Glass and Wendt,' since not one single elementary reaction controls NO destruction, and the global equilibrium assumption made for OH appears not to be generally valid at SR 1 0.6, even though times are long. Additional mechanisms identified to be important were the rapid reaction of NO and hydrocarbons to form HCN and the evolution of nitrogen to form HCN far in the second zone. This suggests that the arbitrary division of the process into a short-time-scale zone in which devolatilization and some reaction takes place and a longer time scale in which only gas-phase reactions occur is not valid. Future work is required to obtain direct experimental evidence for the slow release of nitrogen in the post flame, as suggested by the data reported here. From the nitrogen content of sampled char residue, Glass and Wendt4 concluded that between 10% and 20% of the original coal nitrogen entered the post flame, although precision in their data did not allow one either to confirm or deny that some devolatilization may have occurred thereafter. In order to predict the fate of coal nitrogen in staged combustion systems, future models must account for the interplay between heterogeneous devolatilization reactions to yield HCN and the subsequent conversion of HCN to NH, and the effects of temperature on all these processes. The data reported here, however, represent a solid data base that can be used to facilitate the development of more accurate detailed kinetic models which thus couple realistic coal devolatilization mechanisms with the nonequilibrium gas-phase chemistry gleaned from gas-phase experiments.
Acknowledgment. This work was supported by the Pittsburgh Energy Technology Center, U.S.Department of Energy, under Contract DE-AC22-84PC70771,administered by Energy and Environmental Research Corp. The Project Manager was John Kramlich of EER and the DOE Project Officer was James D. Hickerson of PETC. The authors would also like to acknowledge the assistance rendered by Terry Long and Robert Giattino in running the experiments and by Elizabeth Heddle in analyzing the data. Registry NO.NO,10102-43-9;HCN, 74-90-8; NHB, 7664-41-7.
