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Energy & Fuels 1998, 12, 1322-1327
Coal-Nitrogen Release and NOx Evolution in Air-Staged Combustion B. Coda,* F. Kluger, D. Fo¨rtsch, H. Spliethoff, and K. R. G. Hein Institute for Process Engineering and Power Plant Technology (IVD), University of Stuttgart, Germany
L. Tognotti Dipartimento di Ingegneria Chimica, Universita` di Pisa, Italy Received April 27, 1998
Experiments on an electrically heated entrained flow combustion reactor were carried out in order to test the air-staging behavior of four bituminous coals of industrial interest. Through measurements of gaseous nitrogen-containing species profiles (NO, HCN) and sampling of char particles at different conversion levels, a study was elaborated about the impact of process parameters and coal type on NO formation and reduction, as well as on the nitrogen fate during the course of combustion. While the air-staging abatement efficiency was observed to be correlated with the volatile-nitrogen release from the coal, the presented analysis reveals that the contribution of char-nitrogen release cannot be neglected. This study shows that nitrogen release rates change significantly during the various phases of combustion, also revealing the effect of the operating conditions on the release rates. A simple computational modeling has been carried out in order to estimate the relative influence of the process parameters on char-nitrogen conversion into NO in the burnout zone. The results exhibit the influence of the NO concentration level in the gas phase as one possible explanation of the differences exhibited by the coals. The comparison of experimental data and the computational modeling also displays the necessity of a more detailed kinetic approach to describe char-nitrogen evolution by computer codes for the optimization of staged combustion processes.
Introduction The development of NO control strategies for coalfired utility boilers has received increasing attention during recent years; new combustion technologies involving the setting of reducing conditions, like airstaging or reburning, have recently been investigated and tested on bench-scale facilities for many suites of coals.1-3 Since fuel-nitrogen represents the main source of NO from industrial boilers,4 several studies have focused on understanding the fundamental kinetic mechanisms describing the fuel-nitrogen conversion paths5-8 under combustion conditions. It is assumed that when a pulverized coal particle enters a flame, fuel-N is distributed between volatile-N (light gases and tars) and (1) Spliethoff, H.; Greul, U.; Ru¨diger, H.; Hein, K. R. G. Fuel 1996, 75 (5), 560-565. (2) Mareb, J.; Wendt, J. O. J. Fuel 1994, 73 (7), 1020-1026. (3) Liu, H.; Hampartsoumian, E.; Gibbs, B. Fuel 1997, 76 (11), 985993. (4) Chen, S. L.; Heap, M. P.; Pershing, D. W.; Martin, G. B. 19th Symposium (Int.) on Combustion, The Combustion Institute: Pittsburgh, PA, 1982; pp 1271-1280. (5) Niksa, S. 25th Symposium (Int.) on Combustion, The Combustion Institute: Pittsburgh, PA, 1994. (6) Miller, J. A.; Bowman, C. T. Prog. Energy Comb. Sci. 1989, 15, 287-338. (7) Axworthy, A. AiChE Symp. Ser. 1971, 148, 43-50. (8) De Soete, G. 15th Symposium (Int.) On Combustion, The Combustion Institute: Pittsburgh, PA, 1974; pp 1093-1102.
char-N. Many studies in the recent past dealt with the release of fuel-N and the distribution in the volatile products depending on heating rate, temperature, and atmosphere conditions.9-11 From fundamental studies and laboratory experiments, kinetic information on different coals is required since combustion modification strategies generally exploit some distinctive kinetic features of the conversion mechanisms, to be taken into account in predictive codes for the optimization of the process parameters. Previous studies have shown that the conversion of volatile-nitrogen to NO is the prime mechanism contributing to NO emissions in pulverized coal systems.4 Many investigations12,13 have focused on understanding the release of volatile-nitrogen: from a technological point of view, the amount of nitrogen released during devolatilization is an important parameter because volatile-nitrogen more likely responds to control by modification of combustion-zone aerodynamics than (9) Wang, W.; Brown, S.; Hindmarsh, C. H.; Thomas, K. M. Fuel 1994, 73 (9), 1381-1388. (10) Kambara, S.; Takarada, T.; Toyoshima, M.; Kato, K. Energy Fuels 1993, 7, 1013-1020. (11) Niksa, S.; Cho, S. Energy Fuels 1996, 10, 463-473. (12) Maier, H.; Spliethoff, H.; Kicherer, A.; Fingerle, A.; Hein, K. R. G. Fuel 1994, 73 (9), 1441-1452. (13) Takagi, T.; Tasumi, T.; Ogasawara, M. Combust. Flame 1979, 35, 17-25.
10.1021/ef980097z CCC: $15.00 © 1998 American Chemical Society Published on Web 10/24/1998
Coal-N Release and NOx Evolution
char nitrogen does. It has been shown that14 the distribution between volatile- and char-nitrogen is kinetically controlled, with increasing temperature and residence time in a pyrolysis zone at fuel-rich conditions, favoring the conversion of coal-nitrogen to volatilenitrogen. However, in the optimization of staged combustion, the behavior of char-nitrogen also plays a very important role. The main problem is that nitrogen in the char may persist after the reducing zone in staged combustion, being exposed to further oxidation when it is released during final burnout, thus provoking a further increase of the NO level in the fuel-lean, second stage. The prediction of the char-nitrogen level and its evolution may be significant in order to establish how much nitrogen is retained in the char after the “reburn zone” under reburning conditions or after the reducing zone under air-staging conditions;15 theoretically, this may increase the final NO emissions. The fate of N that remains within the char is crucial when determining the ultimate NO emissions. The release of char-nitrogen during unstaged combustion for coals of various ranks was studied by Baxter et al.,16 showing that at the onset of combustion, the nitrogen release rate is significantly higher than the carbon release rate whereas during the final stages of oxidation, the fractional loss of nitrogen is comparable to that of carbon. Song et al.17 pointed out that the problem in experimentally investigating the charnitrogen release due to oxidation is complicated by the parallel release of volatile-nitrogen that may occur. To the author’s knowledge, one single reason explaining the char-nitrogen loss has not been found yet, whether the release of char-nitrogen is due to oxidation or to thermally-induced nitrogen liberation or whether the two processes occur simultaneously. The lack of a complete, detailed understanding of the mechanism of char-nitrogen release has relevant implications for the development of kinetic reaction modeling of the combustion processes. It is generally assumed that18,19 the residual nitrogen in the char is released at a rate proportional to the char oxidation rate. Nitrogen may leave the char particle as active precursor species (NHi, HCN) or nitrogen oxidation can already occur in the char. Recently, the two approaches have been reviewed and tested against experimental results by Visona.20 However, the establishment of a correct kinetic model of the fate of char-nitrogen during combustion is still a challenging goal. This paper reports on the char-nitrogen release and the evolution of NO and HCN in the course of air-staged and unstaged combustion process. Experiments carried (14) Song, Y.; Pohl, J.; Beer, J.; Sarofim, A. Combust. Sci. Technol. 1982, 28, 31-39. (15) Fo¨rtsch, D.; Kluger, F.; Schnell, U.; Spliethoff, H.; Hein, K. R. G. 27th Symposium (Int.) on Combustion, The Combustion Institute: Pittsburgh, Pa, 1998. (16) Baxter, L. L.; Mitchell, R. E.; Fletcher, T. H.; Hurt, R. H. Energy Fuels 1996, 10, 188-196. (17) Song, Y.; Pohl, J.; Beer, J.; Sarofim, A. Combust. Sci. Technol. 1982, 28, 177-183. (18) Mitchell, J. W.; Tarbell, J. M. AiChE J. 1982, 28, 303-311. (19) Abbas, T.; Costa, M.; Costen, P.; Godoy, S.; Lockwood, F. C.; Ou, J. J.; Romo-Millares, C.; Zhon, J. Fuel 1994, 73 (9), 1423-1436. (20) Visona, S. P.; Stanmore, B. R. Combust. Flame 1996, 106 (3), 207-218.
Energy & Fuels, Vol. 12, No. 6, 1998 1323
Figure 1. Scheme of the electrically heated entrained flow reactor. Table 1. Experimental Combustion Parameters process parameters wall temperature (Tw) residence time in the reduction zone (τ) air-to-fuel equivalence ratio (λ) oxygen concentration in the flue gas
staged combustion
unstaged combustion
1300 °C 3s
1300 °C
0.75 3% vol
1.15 3% vol
out at the IVD’s entrained flow combustion reactor facility, while helping to evaluate the impact of process parameters on in-furnace DeNOX technologies efficiency, can at the same time provide useful data to achieve a better knowledge about the fate of fuelnitrogen under process conditions that are very similar to the industrial combustor ones. Experimental Section A scheme of the electrically heated entrained flow combustion reactor (57 kWel) is presented in Figure 1. The reaction ceramic tube has a length of 2500 mm and an internal diameter of 200 mm. This test facility provides a good environment in order to investigate staged-combustion conditions: burnout air can be injected at several locations along the whole reactor length with a vertically movable probe, thus allowing the setting of different residence times in the reduction zone. The reactor allows for feeding of gas or coal for reburn tests.15 The impact of the stoichiometry in the primary zone on the efficiency of NO reduction can be evaluated by regulating the relative ratios of primary combustion air and burnout air. Furthermore, the wall temperature can be set up to a level of 1400 °C in five different zones of the reactor, thus allowing one to investigate the effect of temperature in the various zones with respect to pollutant formation sepa-
1324 Energy & Fuels, Vol. 12, No. 6, 1998
Coda et al.
Table 2. Physical Properties of the Coals coal
Ashland (AS)
carbon hydrogen nitrogen sulfur oxygen fixed carbon volatile matter ash moisture LCV particle size (d50)
% dry % dry % dry % dry % dry % dry % dry % dry % MJ/Kg µm
75.46 4.52 1.43 0.75 6.11 57.66 31.51 10.83 1.77 29.5 25; 50 (AS25-50)
rately. The typical overall residence time in the reactor ranges from 4 to 7 s. The burner to inject the coal into the chamber is situated on the top of the furnace; the feeding system consists of a gravimetric screw conveyor. The pulverized coal is supplied to the burner by carrier air. The combustion air is injected through annular clearances and is divided into primary and secondary air. The medium heating rate is 104 K/s. Measurements of gas temperature performed in the recent past and confirmed by computational modeling21 show a temperature peak in flame of about 1500 °C at a wall temperature of 1300 °C; the gas temperature approaches the wall temperature later by the occurrence of plug-flow conditions. The injection of the burnout air through the three-hole probe can provoke the presence of backmixing zones,15 which can influence the gas concentrations. The maximum error on NO measurements in this region has been estimated at about 10%. Experimental Conditions and Analytical Characterization. During the experiments a constant wall temperature along the reactor was used. Unstaged combustion takes place under excess air with an air ratio of λ ) 1.15 (approximately 3% O2 in the flue gas). During air-staging trials, a residence time of 3 s in the fuelrich primary zone has been chosen and an air-to-fuel equivalence ratio λ ) 0.75. The coal feed rate was approximately 1 kg h-1 for all the tests. Gas samples were withdrawn isokinetically from the reactor using a stainless steel, water-cooled probe and analyzed using standard continuous instrumentation (chemiluminescent NO; paramagnetic O2; NDIR CO, CO2; infrared single-beam photometer HCN). Solid sampling was performed with the same probe through a glass fiber filter; proximate analysis and ultimate analysis (C, H, N, S content) of the char samples were obtained using a TGA 500-LECO and a Vario-Elementar analyzer, respectively, according to ASTM standardized procedures. Table 1 summarizes the experimental conditions. The physical properties of the investigated bituminous coals are listed in Table 2.
Go¨ttelborn (GB)
Koonfontain (KF)
Emil Mayrisch (EM)
73.80 4.90 1.42 0.80 9.10 57.60 32.40 10.00 2.80 29.7 25
71.68 3.90 1.98 0.62 4.80 59.15 27.21 13.64 3.00 26.7 25
82.23 3.83 1.64 0.92 3.20 77.19 14.65 8.16 1.24 32.2 18
Figure 2. NO, HCN, and O2 profiles under unstaged combustion. The data are referred to Ashland coal, coarse distribution (d50 ) 50 µm).
Figure 3. NO, HCN, and O2 concentrations under staged combustion. The data are referred to Ashland coal, coarse distribution (d50 ) 50 µm).
Results and Discussion Phenomenological Description: Comparison between Air-Staged and Unstaged Combustion. Figures 2 and 3 show the typical trend of NO, HCN, O2 concentrations during air-staged and unstaged combustion, respectively. The data are plotted vs the axial distance from the burner. The following simplified scheme can be useful in order to understand the different paths of coal-nitrogen conversion dominating under unstaged and staged combustion The scheme holds under the hypothesis that HCN is the most important precursor of NO, which has been verified for bituminous coals.5 Under excess air conditions, the nitrogen released as a volatile in the very first stages of combustion finds availability of oxygen and (21) Diplomarbeit Nr. 2585, IVD, University of Stuttgart.
the precursory species are converted into NO to a high extent. The observed profile under staged combustion is quite different: the NO increase due to the presence of O2 in the flame is then followed by a net decrease of the NO concentration as soon as the oxygen is consumed, because of the prevalence of the heterogeneous reduction. It is obvious that the setting of an airdeficient zone promotes N2 formation from the heterogeneous reduction of NO. Figure 4 shows the variation of NO profiles of the coals examined under the same operating furnace conditions. The NO measurements are referred to 0%
Coal-N Release and NOx Evolution
Figure 4. Effect of coal type on NO emissions under airstaging conditions. NO emissions are calculated at 0% O2 to take into account the effect of the dilution after the addition of burnout air in the burnout zone.
Figure 5. Relation between NO final emissions (normalized with respect to fuel theoretical NO) and coal volatile-matter content.
O2 to exclude the effect of dilution when passing from lower to higher O2 concentrations in the burnout zone. In the following description, the effect of the particle size has not been taken into account. By analyzing the NO profiles in the reduction zone, one may observe a variation of NO formation and subsequent reduction in the early-stage region depending on the coal type. One of the aims of these experiments is to provide simple relations between coal properties and observed NO levels. In Figure 5, the final NO levels (normalized with respect to the maximum fuel-NO, as an index of the coal-nitrogen content) are plotted versus the volatilematter content of the coal, both for staged and unstaged conditions. The plot is referred to the examined coals and other published data.22 The graph indicates that the NO levels are correlated to the volatile content in a way that a higher volatile-matter content results in reduced nitrogen oxide emissions, because of the reduction enhancement due to higher conversions of fuelnitrogen into precursory species for staged combustion. Under unstaged combustion, as described above, the NO conversion rate strongly increases with higher volatiles content, because of the small impact of heterogeneous (22) Kluger, F.; Fo¨rtsch, D.; Spliethoff, H.; Schnell, U.; Hein, K. R. G. Proceedings of the 23rd International Technical Conference on Coal Utilization & Fuel Systems, Clearwater, Florida, 1998.
Energy & Fuels, Vol. 12, No. 6, 1998 1325
Figure 6. Char-nitrogen to nitric oxide conversion net efficiency, η, as a function of NO in the gas phase. η is defined as follows: η ) (net mass of NO produced/mass of NO if all the char-nitrogen released from the particle is converted to NO).
reduction. The figure suggests higher abatement efficiencies in correspondence to higher volatile-matter content. The proposed interpretation of the data is very rough, surely valid under the chosen process conditions, but not accurate enough to quantitatively describe the impact of the thermal history on the nitrogen release rate during the various phases of combustion. This will be discussed in the following sections. NO Formation in the Burnout Zone. The critical issue about staged combustion is the fate of charnitrogen in the late, oxidizing region. Figure 4 also shows that the behavior of the four coals differs with respect to NO formation in the burnout zone. They exhibit a different rise in NO concentration near the injection point of the burnout air, caused by oxidation of char-nitrogen released during the final burnout of the char. From mass balances it has been calculated that the nitrogen content of the char leaving the reduction zone is high enough to theoretically ensure an increase in NO concentration of about 100-150 ppm. The NO formation in the burnout zone has been investigated by applying a mathematical modeling in order to study the influence of various parameters on the net production of NO from char-nitrogen. The model assumes the char-nitrogen loss rate to be proportional to the char burnout rate; nitrogen is modeled to be released as active precursors, which react with O2 or NO to form, respectively, NO and N2; the heterogeneous reduction of NO occurring on the char particle surface is also taken into account. Conversion of HCN to NO within the particle has been neglected, as suggested in ref 20. For calculation purposes, the surrounding gas is assumed to have a constant temperature [gas temperature ) 1573 K] and constant fixed composition [3% O2]. A detailed description of the model and the applied rate constants can be found in ref 15. However, for simplicity reasons, the comparison between the various coals do not take into account the different reactivity and only one NO-reduction rate has been used. The results of the computational modeling are viewed as a net nitrogen conversion efficiency to nitric oxide, η. In Figure 6, η is plotted versus NO concentration in the gas phase and compared to the experimental data for the various coals. The error bar of the experimental
1326 Energy & Fuels, Vol. 12, No. 6, 1998
Figure 7. Effect of the N/C mass ratio on char-N to NO conversation efficiency η. Parameter: NO in the gas phase.
Figure 8. Cumulative loss of nitrogen as a function of mass loss for the coals under staged combustion. Effect of the coal type.
points summarize the uncertainties due to NO measurements due to neglecting the backmixing flow and to the negligible contribution of homogeneous NO formation from HCN in the gas phase. The figure displays a good agreement between the predicted charnitrogen conversions and the experimental trends. In the calculation, a N/C mass ratio of 0.02 has also been assumed for the char particle as an average value of those found experimentally, also assuming that within a short residence time it remains constant. The effect of a variation of the N/C ratio of the char particle has been shown to have only a negligible effect on the conversion efficiency η, as shown in Figure 7. Under the simplified kinetic assumptions of the model, the impact of the heterogeneous reduction on the char particle is relevant with increasing NO concentration in the gas phase, even in the presence of oxidizing conditions, and that its role can still be very significant at high-temperature conditions such as those typical of pf systems. Nitrogen Release Rate. Figures 8 and 9 display the cumulative fraction of released nitrogen as a function of overall burnout (daf) occurring under staged and unstaged combustion, respectively. Figures 10 and 11 refer to differential changes in the nitrogen and carbon content, respectively, as a function of the residence time. The ratio (dN/N/dC/C) was previously proposed by Baxter et al.:16 the nitrogen release rate is viewed as a selective removal versus carbon and the differential
Coda et al.
Figure 9. Cumulative loss of nitrogen as a function of mass loss for the coals under unstaged combustion. Effect of the coal type.
Figure 10. Release rate of nitrogen relative to carbon as a function of residence time under staged combustion. Effect of the coal type. (Data are obtained by normalizing the differential changes in nitrogen and carbon composition by the instantaneous nitrogen and carbon values.)
Figure 11. Release rate of nitrogen relative to carbon as a function of residence time under single-stage combustion. Effect of the coal type.
basis gives a more direct indication of the relative reaction rate of carbon and nitrogen loss. Furthermore, this ratio be can easily correlated with a kinetic equation describing char-nitrogen release as a function of carbon loss.17 Values of the ratio (dN/N/dC/C) > 1 indicate that nitrogen is more reactive than carbon, i.e., more prone to devolatilization, to oxidation, or in general to leave the solid particle.
Coal-N Release and NOx Evolution
By means of this analysis, it is possible to obtain some useful parameters for optimizing the nitrogen kinetic paths under reburning or air-staged conditions. According to the high heating rate and residence times in the furnace, the experimental points are referred only to the intermediate and the latest stages of burnout, both under staged and unstaged combustion conditions. By analyzing Figure 8, referred to the staged-combustion trials, it is noticeable in the intermediate stages of the burnout process there is a substantial increase of the char-nitrogen release with respect to overall mass loss and that this also persists when reducing conditions occur. This is confirmed by the trend of Figure 10: the ratio (dN/N/dC/C) shows values higher than one, both in the first oxidizing and in the reducing zone, passing through a maximum value. The decrease of nitrogen release rate again in correspondence with the burnout air addition until the latest stages of burnout exhibits even a slight depletion of nitrogen with respect to carbon. Figure 9, with the results of an unstaged combustion trials plot, referring to higher conversion levels throughout the reactor, reveals that with the exception of Emil Mayrisch coal, the nitrogen-release rate becomes more similar to the overall mass-loss rate, displaying even a preferential loss of carbon at the latest stages of burnout. The main problem in analyzing the data is that nitrogen oxidation is complicated by the parallel devolatilization that may occur,17 especially in the first stages of heterogeneous char combustion. Under staged combustion trials, an enhancement of the nitrogen-release rate is observed both when oxidizing conditions still occur and also along the reducing zone, to later experience a “slowing-down”. Other studies16 show that fractional nitrogen-release rates relative to those of carbon pass through a maximum at the first stages of oxidation; Song et al.17 postulated a thermal effect on nitrogen loss associated with the high temperature occurring in char combustion. By means of our analysis, it is not possible to quantify the relative effect of the oxygen attack on char-nitrogen loss; furthermore, it is not possible to directly compare the data obtained at the two different process conditions because of the combined effect of higher particle temperature and higher oxygen concentration occurring under single-stage combustion. The low reactivity of Emil Mayrisch and its low conversion degrees displayed under oxidizing conditions allow a qualitative comparison between the two process conditions and indicate that, with respect to the enhancement of nitrogen release, there is hardly any effect of the oxygen concentration. Data for Emil Mayrisch agree quite well with other data published by Baxter et al.16 for another low-volatile bituminous coal. The observed nitrogen retention at the latest stages of combustion with increasing burnoff is due to a quasitotal consumption of nitrogen and the lack of availability of nitrogen atoms with respect to carbon atoms, which
Energy & Fuels, Vol. 12, No. 6, 1998 1327
are statistically more subject to oxygen attack at the latest stages of oxidation. Harding et al.23 also supported the nitrogen retention theory when explaining the increase of the NO/(CO+CO2) ratio with increasing burnoff experienced by chars during temperatureprogrammed combustion. Though the role of the process conditions needs to be further clarified, the presented analysis brings up some interesting topics, since it points out that it is restrictive to suppose that carbon and nitrogen evolve proportionally to their concentrations in the char matrix, a correlation adopted in many simulation codes: actually, the tests over the examined coals proved that a preferential release of nitrogen with respect to carbon exists. While the enhancement of nitrogen release occurring under reducing conditions is surely an important phenomena to be taken into account when modeling the kinetic paths of char-nitrogen under staged combustion,15 the retention of nitrogen with increasing burnoff is a process which needs to be further investigated, especially for the possible contributions to the optimization of computer codes applied to NO predictions. Conclusions Experiments carried out in an electrically heated entrained flow reactor on four bituminous coals allowed investigations of the dominant factor affecting NO formation and reduction under staged combustion conditions. The abatement efficiency of air-staging was related to volatile-nitrogen release from the coal. A mathematical model has been tested against the experimental results to investigate the influence of several parameters on NO formation in the burnout zone. It has been shown that the NO level in the gas phase has a major impact on the conversion of char-nitrogen to NO under oxidizing conditions. Furthermore, the study was focused on char-nitrogen release: the evaluation of experimental trends of the relative nitrogen-to-carbon release reveals that a selective removal of nitrogen exists, with reducing conditions enhancing nitrogen release from the char. Though further investigations are required in order to establish the influence of the operating parameters, especially oxygen concentration and temperature, the presented study suggests the necessity of a more detailed kinetic approach when applied to computer codes describing char-nitrogen release, which could have relevant effects for the optimization of final NO emissions under airstaging and reburning conditions. Acknowledgment. The Experimental Section of this work was performed under a collaboration between IVD and University of Pisa under the EU research Project No. JOF3-CT95-0005 EF980097Z (23) Harding, A. W.; Brown, S. D.; Thomas, K. M. Combust. Flame 1996, 107 (4), 336-350.
