Energy & Fuels 1996, 10, 197-202
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Laboratory Study of N2O Formation from NO While Burning Char Particles at FBC Conditions
15N-Isotope-Marked
Heije Miettinen Department of Inorganic Chemistry, University of Go¨ teborg, S-412 96 Go¨ teborg, Sweden Received May 2, 1995X
This paper is a continuation of the discussion of N2O formation during char combustion started in a previous paper (Energy Fuels 1995, 9, 10-19). The effect of different NO and CO concentrations in the inlet gas, as well as the effect of changing gas residence time within the fixed bed and after the fixed bed on the emission of N2O from char particles burning at single particle conditions, is reported. In order to distinguish between char nitrogen and NO from the inlet gas a 15N-isotope-marked NO was used in the inlet gas. The experiments were carried out in a fixed bed reactor containing quartz sand (SiO2) to support the coal particles. The bituminous coal particles were devolatilized in situ prior to the actual combustion in an argon flow. The inlet gas mixture contained 15NO, CO, O2, and Ar or just 15NO, O2, and Ar. The 15NO concentration was varied between 500 and 1950 ppm, and the CO concentration was varied between 0 and 2500 ppm. The inlet gas flow was varied between 297 and 1200 mL/min NTP (273 K, 1 atm). The bed temperature was 1073 K. The off-gases were analyzed for N2O. The addition of 15NO to the inlet gas led to the formation of 15N14NO and 15N2O, and the formation of these species increased with increasing concentration of 15NO in the inlet gas, especially the formation of 15N2O. The formation of 15N14NO was greater than the formation of 15N2O under all circumstances examined, but with increasing 15NO concentration in the inlet gas the 15N2O formation increased, and was almost as great as the formation of 15N14NO. The influence of gas residence time within the fixed bed and after the fixed bed was confirmed, and the N2O formation and formation of 15N14NO and 15N2O increased substantially with increasing gas residence time. The addition of CO to the inlet gas had almost no effect on the N2O formation.
Introduction Formation and destruction of N2O in fluidized bed boilers (FBB) is a matter of interest and of concern. In the last few years many experimental efforts have been performed in order to understand the formation and the destruction mechanisms of N2O during fluidized bed combustion. This work is a continuation of the general discussion and a continuation of the particular discussion of N2O formation in the work “Laboratory Study of N2O Formation from Burning Char Particles at FBC Conditions”.1 The issue is whether the formation of N2O from char combustion should be described by a homogeneous or a heterogeneous mechanism or by a combination of both. The N2O formation mechanisms were first introduced by Kramlich et al.2 and included the direct oxidation of char nitrogen to N2O (M1), the heterogeneous formation of N2O by NO (M2) and the homogeneous formation of N2O in the gas phase (M3). The M1 mechanism has been more or less abandoned and is not discussed in this paper. In the M2 mechanism NO reacts with a nitrogen atom fixed in the char matrix, or adsorbs onto the char surface where it subsequently reacts with a second NO molecule and forms N2O. In the M3 mechanism the char nitrogen undergoes devolatilization or gasification and the niAbstract published in Advance ACS Abstracts, November 1, 1995. (1) Miettinen, H.; Paulsson, M.; Stro¨mberg, D. Energy Fuels 1995, 9, 10-19. (2) Kramlich, J. C.; Cole, J. A.; McCarthy, J. M.; Lanier, W. S.; McSorley, J. A. Combust. Flame 1989, 77, 375-384. X
0887-0624/96/2510-0197$12.00/0
trogen containing compound reacts further in the gasphase to form N2O via e.g.:
HCN + O f NCO + H
(R1)
NCO + NO f N2O + CO
(R2)
There are several investigations proposing either of the two mechanisms (M2 or M3) or both, and a couple of them are presented below. A˙ mand and Leckner3 studied the formation of N2O in a full-scale circulating fluidized-bed combuster (12 MWth) and introduced a fourth mechanism, M4, which involves a primary formation of a cyano compound (HCN or HNCO for example) as a result of the reaction of the disrupted aromatic rings with O2 and/or NO. The HCN or HNCO then reacts further to N2O according to the reactions R1 and R2. The M3 and M4 mechanisms are both homogeneous and very similar. The only difference is that in M4 not only the char nitrogen can form a cyano species but also that NO can be reduced to a cyano species on the char surface. The simultaneous formation of HCN when NO is reduced over a char surface has been studied by de Soete.4,5 Moritomi et al.6 have measured increasing (3) A° mand, L.-E.; Leckner, B. Energy Fuels 1993, 7, 1097-1107. (4) de Soete, G. G. Report 36 752; Institut Francais du Pe´trole, France, 1989. (5) de Soete, G. G. Proceedings of the Fifth International Workshop on Nitrous Oxide Emissons; NIRE/IFP/EPA/SCEJ: Tsukuba, Japan, 1992, KL-4-(1-24).
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HCN concentrations at the bottom of the riser in a laboratory CFB combuster when NO was injected to the primary air. They also injected pyridine, and the injection of pyridine as well as the injection of NO resulted in increasing N2O concentrations pointing out the importance of cyanide compounds for the N2O formation (M3 and/or M4). A° mand and Leckner3 have injected CH3CN, NH3 or NO at the bottom, at a height of 5.5 m, and at the cyclone inlet in a 12 MWth CFB burning bituminous coal, wood chips, and coke. Increasing N2O concentrations were measured for all gases and fuels, although the increases were small in the cases where NH3 was added or when the fuel was wood chips. They conclude that cyanides show a greater tendency than ammonia to form N2O and that the concentration of NO and the char loading of the combustion chamber are important for the formation of N2O during combustion of char particles. As a result of their investigation they proposed the M4 mechanism. Klein and Rotzoll7 have reported results from combustion experiments in a laboratory FB reactor burning batches of anthracite and bituminous coals and their chars. They propose that HCN is formed as an intermediate during char combustion (M3 and/or M4). On the other hand, there are several papers in which experiments are presented that are assumed to support the M2 mechanism. For example, Goel et al.8 burnt in situ devolatilized coal particles in a laboratory FB combuster under single-particle conditions. They conclude that HCN is not released in significant amounts during char combustion and that nitric and nitrous oxides produced from char nitrogen are not formed, in the studied case, from the homogeneous oxidation reactions of HCN, but from the heterogeneous reactions between oxygen and NO and char nitrogen (M2). Tullin et al.9-11 and later Krammer and Sarofim,12 who both studied the formation of N2O during single-particle combustion of coal, added NO to the inlet gas and explained the increasing N2O yields with the heterogeneous formation mechanism (M2). They concluded that the formation of N2O increased with increasing NO concentration due to reaction between NO and char nitrogen as the char is combusted. In the present study in situ devolatilized coal particles were burnt with gas mixtures containing 15N-isotopemarked NO. In order to distinguish between char nitrogen and NO from the inlet gas, the 15N-isotopemarked NO was used in the inlet gas. The experiments were carried out in a fixed bed reactor made of quartz containing quartz sand (SiO2) to support the coal particles. The effect of different 15NO and CO concentrations in the inlet gas, as well as the effect of changing gas residence time within the fixed bed and after the (6) Moritomi, H.; Harada, M.; Fujiwara, N.; Hirama, T.; Okazaki, K. Proceedings of the Fifth China-Japan Symposium; Chemical Industry Press: Beijing, 1994; pp 289-296. (7) Klein, M.; Rotzoll, G. Proceedings of the Sixth International Workshop on Nitrous Oxide Emissons; ÅAU, HUT/VVT/NIRE/IFP/ EPA: Turku, Finland, 1994. (8) Goel, S.; Zhang, B.; Sarofim, A. F. Submitted for publication in Combust. Flame. (9) Tullin, C.; Sarofim, A.; Bee´r, J. J. Inst. Energy 1993, 66, 207215. (10) Tullin, C.; Goel, S.; Morihara, A.; Sarofim, A.; Bee´r, J. Energy Fuels 1993, 7, 796-802. (11) Tullin, C. J.; Sarofim, A. F.; Bee´r, J. M.; Teare, D. J. Combust. Sci. Technol., in press. (12) Krammer, G.; Sarofim, A. F. Combust. Flame 1994, 97, 118124.
Miettinen
Figure 1. Experimental setup.
fixed bed on the emission of N2O from char particles burning at single-particle conditions, is reported. The N2O reduction potential of the reactor and the fixed bed including the char particles, as well as the conditions for single particle combustion, has been reported earlier in ref 1. Experimental Section The Experimental Setup. The tests were run in a fixedbed reactor shown in Figure 1 and earlier described in ref 1. The reactor consists of a removable quartz tube (28 mm i.d. × 1000 mm) fitted with a sintered quartz frit used to support the solid particles of the fixed bed. The static bed height was 20 mm. The reactor is electrically heated and the bed temperature is controlled by a Pt-Rh thermocouple connected to a temperature control unit. The thermocouple tip was placed in the middle of the bed. Mass flow regulators (Brooks Model 5850E) were used to set the desired gas flow that could either pass through the reactor or pass by as directed by three-way taps. The N2O concentrations were measured in a continuous mode at room temperature with a quadropole mass spectrometer (BALZERS QMS 311). The formed N2O was measured by monitoring the m/e 45 (14N15NO) and m/e 46 (15N2O) signals. Since a lot of carbon dioxide, compared to N2O, is formed during combustion and since carbon dioxide has mass number 44, the carbon dioxide was taken away in order to avoid an overlap of the peaks (44 and 45). The CO2 was removed prior to the N2O analysis by means of a tube containing ascarite (asbestos + NaOH). The use of ascarite is common when working with mass spectrometers and does not affect N2O. It easily converts all CO2 to NaHCO3 while changing color from yellow to white, and there is no slip of CO2 as long as there is fresh ascarite. The mass spectrometer was calibrated daily before use and checked after every experiment. The Tests. The purpose of the experiments was to focus on the N2O formation from gas phase NO during char
N2O Formation from
15N-Isotope-Marked
NO
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Table 1. Fuel Characteristics of the Pennsylvanian Coal
proximate anal (%) combustibles ash moisture volatiles (% maf) ultimate anal (%maf) C H O S N
untreated
devolatilized (1073 K, 1 h)
85.0 12.2 2.8 31.9
81.5 17.0 1.5 6.3
80.9 5.0 11.8 0.42 1.90
94.2 1.1 2.18 0.40 2.12
combustion. In order to distinguish between NO from the char-N and NO from the inlet gas, a 15N-isotope-marked NO was used in the inlet gas. The char particles were prepared from a bituminous coal from Pennsylvania, which had been used earlier in ref 1. The considered parameters were gas residence time within the fixed bed and after the fixed bed, NO concentration, and CO concentration. The proximate and ultimate analyses of the coal and char are given in Table 1; the analyses were made by the Swedish National Testing and Research Institute. The coal particles were dried, ground, and sieved and a particle size fraction between 0.5 and 0.71 mm was used in these experiments. The fixed bed used was made of 16 g of quartz sand (pro analysi) mixed with 15 mg of coal particles. Quartz sand was used as bed material since the reactor was made of quartz glass. The particle size of SiO2 was 0.2-0.8 mm. The char particles were prepared in situ prior to the experiments by heating the coal particles at 1073 K for 1 h with an argon flow of 1000 mL/min through the reactor. The formula used to calculate the char nitrogen content (in grams) per gram coal was
[combustibles] × [N]1073-maf × {1 - ([vol]1173-maf - [vol]1073-maf)} ) mchar-N/mcoal where [N]1073-maf is the moisture and ash free (maf) nitrogen content in the Pennsylvanian coal devolatilized at 1073 K for 1 h as shown in Table 1c. The volatile content in the coal was determined by heating the coal for 7 min at 1173 K in an inert atmosphere. [vol]1173-maf is the moisture and ash free volatile content in the untreated coal (no prior devolatilization) and [vol]1073-maf is the moisture and ash free volatile content in the coal devolatilized at 1073 K for 1 h in an argon flow prior to the analysis. The produced char particles were then burnt at 1073 K directly after the devolatilization with a gas mixture containing NO, O2, and Ar or NO, CO, O2, and Ar. The NO concentration was varied between 500 and 1951 ppm and the CO concentration was varied between 0 and 2500 ppm. The oxygen concentration was either 3.5 or 3.2%. To obtain different gas residence times within the bed the gas flow was varied between 297 and 1200 mL/min NTP (273 K, 1 atm). The gas residence time within the fixed bed is defined as the time it takes for the gases to pass through the fixed bed, and the residence time is calculated by dividing the void volume of the fixed bed with the actual gas flow at 1073 K. The results from the experiments are reported as the molar ratio of the cumulative emission of 15N14NO or 15N2O in relation to the amount of fuel nitrogen in the char used in the experiments (mole 15N14NO/mole char-N or mole 15N2O/mole char-N). For convenience, and to simplify the comparison between the char nitrogen conversion to 15N14NO and the formation of 15N2O, the 15N2O results are also plotted as mole 15N O/mole char-N, although 15N O does not contain any char 2 2 nitrogen. The interaction between NO and CO in the gas phase and on the char particles in the fixed bed was studied in separate NO reduction experiments. First, 15 mg of the bituminous
Figure 2. Impact of varying gas residence times within the fixed bed and after the fixed bed within the hot zone of the reactor on the N2O yields (mole 15N14NO/mole char-N and mole 15N O/mole char-N). The inlet gas contained 500 or 1500 ppm 2 NO and 3.5% O2 in argon and the combustion temperature was 1073 K. coal was mixed with 16 g of SiO2; but due to the inertness of the char particles a second reduction experiment was performed with 100 mg of the bituminous coal in 16 g of SiO2. The NO concentration was 1000 ppm and the CO concentration was varied between 1000 and 4100 ppm. The residence time within the bed was varied between 60 and 240 ms.
Results The combustion of char particles leads to formation of N2O whether or not NO is present in the inlet gas. If no NO is present in the inlet gas both nitrogen atoms that are needed to form N2O have to come from the charN, and this leads to considerably less formation of N2O than if NO is present in the inlet gas.1 Our interest was to focus on the interaction between burning char particles and the NO in the inlet gas leading to N2O formation. The nitrogen atoms in the measured N2O can originate either from the char-N and the inlet 15NO (15N14NO) or just from the inlet 15NO (15N2O). The N2O formed with two nitrogen atoms from the char-N was not measured in this study due to the very low yields compared to the yields with NO present in the inlet gas. In Figure 2 the impact of varying gas flows through the reactor on the N2O yields, mole 15N14NO/mole char-N and mole 15N2O/mole char-N, are shown. Five different gas flows (1200, 791, 593, 474, and 297 mL/ min NTP) were used, and the residence times within the fixed bed were calculated to be 60, 90, 120, 150, and 240 ms, respectively. The temperature (1073 K) in the reactor was stable within a zone of approximately 5 cm after the fixed bed, and the gas residence times within this hot zone were calculated to be approximately 0.4, 0.6, 0.8, 1.0 and 1.6 s. The oxygen concentration was 3.5% and the 15NO concentration was 500 or 1500 ppm. The load of coal particles was 15 mg before devolatilization. The 15N14NO yield (mole 15N14NO/mole charN) increased with increasing gas residence time within and after the fixed bed up to 1.0 s. For gas residence times longer than 1.0 s the 15N14NO yield seems to level off. The 15N14NO yield was the same whether 500 or 1500 ppm 15NO was added to the inlet gas. If 500 ppm 15NO was added to the inlet gas the formation of 15N14NO was greater than the formation of 15N O for all 2 gas residence times, and when it comes to residence
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Miettinen
Figure 3. Influence of 15NO concentration on the N2O yields (mole 15N14NO/mole char-N and mole 15N2O/mole char-N). The inlet gas contained 15NO, 3.5% O2, and Ar and the gas residence time was 0.8 s. The combustion temperature was 1073 K.
Figure 4. Influence of CO concentration on the N2O yields (mole 15N14NO/mole char-N and mole 15N2O/mole char-N). The inlet gas contained CO, 750 ppm 15NO, 3.2% O2, and Ar and the gas residence time was 0.7 s. The combustion temperature was 1073 K.
times longer than 0.6 s the formation of 15N14NO was much greater than the formation of 15N2O. If 1500 ppm 15NO was added to the inlet gas, still the formation of 15N14NO was greater than the formation of 15N O for 2 all gas residence times but the difference decreased. As with 15N14NO the formation of 15N2O increased with increasing gas residence time, and the increase was substantial when 1500 ppm 15NO was added to the inlet gas mixture compared with when 500 ppm 15NO was added. In the case of 500 ppm 15NO the increase of 15N O with increasing gas residence time was consider2 ably less but still an increase could be noticed. In Figure 3 the influence of different 15NO concentrations on the N2O yields can be seen. The 15NO concentrations were 500, 1000, 1500, and 1950 ppm. The oxygen concentration was 3.5% and the gas residence time within the fixed bed and after the fixed bed within the hot zone of the reactor was 0.8 s. The load of coal particles was 15 mg before devolatilization. The 15N14NO yield was fairly stable within the examined interval. The formation of 15N14NO was greater than the formation of 15N2O for all 15NO concentrations, but the difference decreased with increasing 15NO concentration, and for high 15NO concentrations (1950 ppm) the formation of 15N2O was almost as great as the formation of 15N14NO. In Figure 4 the influence of different CO concentrations on the N2O yields can be seen. The CO concentrations were 0, 226, 453, 905, and 2500 ppm. The oxygen concentration was 3.2%, the 15NO concentration was 750 ppm, and the gas residence time within the fixed bed and after the fixed bed within the hot zone of the reactor was 0.7 s. The load of coal particles was 15 mg before devolatilization. The addition of CO to the inlet gas had a minor but decreasing influence on the 15N14NO yield, and for high CO concentrations (2500 ppm) in the inlet gas the 15N14NO yield decreased from 33 to 23%. The formation of 15N14NO was greater than the formation of 15N2O, and the magnitude of the difference is of the same size as when no CO was added in all cases except when 2500 ppm CO was added. High concentrations of CO (2500 ppm) seem to reduce the 15N14NO yield but preserve the 15N2O yield, but the differences are small and no clear effects of the CO addition can be seen. The 15NO reduction experiments were performed in order to obtain knowledge about the reduction potential
of the reactor and the fixed bed including the devolatilized coal particles. To see if reduction products such as H15NCO (mass 44) and 15NCO (mass 43) could be formed as a result of the 15NO reduction on the char surface a high reduction degree was sought. Due to the same masses of HC15N and CO (mass 28) and the high concentrations of CO, it was not possible to measure any HC15N. The 15NO concentration was 1000 ppm in all reduction experiments and the CO concentration was varied between 1000 and 4100 ppm. The residence time within the fixed bed was varied between 60 and 240 ms, and the temperature was 1073 K. The load of coal particles was 15 or 100 mg before devolatilization. If the fixed bed contained 15 mg of coal particles before devolatilization and the inlet gas contained 1000 ppm 15NO and 1000 ppm CO, the 15NO reduction was insignificant but increasing (0-4%) with increasing residence time (60-240 ms). If the CO concentration was increased to 3300 ppm, the 15NO reduction increased to 28% (4-28%) for a residence time of 240 ms. Due to the low reduction degree the amount of coal particles in the bed was increased. If the fixed bed contained 100 mg of coal particles before devolatilization and the inlet gas contained 1000 ppm NO and 1000 ppm CO, the 15NO reduction was 38% for a residence time of 120 ms. The 15NO reduction increased from 2 to 38% with the increase of coal particle load from 15 to 100 mg. The 15NO reduction was further increased by increasing the residence time and the CO concentration giving a 15NO reduction of 50% with 3300 ppm CO and a gas residence time of 240 ms. The measured reduction products were N2 and CO2, and neither H15NCO (mass 44) nor 15NCO (mass 43) was found among the reduction products in the examined interval. H15NCO and CO2 have the same mass number, 44, but if H15NCO had been formed it would also have given a peak on mass number 43, and since there was no peak on mass number 43 the conclusion that no H15NCO was formed can be drawn. If the NO concentration was decreased to 500 ppm and the CO concentration was further increased to 4100 ppm the NO reduction reached 85%. Discussion The 15N14NO yield (mole 15N14NO/mole char-N) increased with increasing gas residence time within and
N2O Formation from
15N-Isotope-Marked
NO
after the fixed bed (Figure 2). Miettinen et al.1 and Suzuki et al.13 have shown earlier that the bulk residence time after the fixed bed is of importance for the N2O formation when burning char particles. The enhancement of the 15N14NO yield as the residence time increases can be explained by the two-step mechanisms, M3 or M4, where the first step is the release of a CN containing compound. This first step is independent of the residence time. The second step, where the CNcontaining compound is oxidized and with the help of 15NO forms 15N14NO, ought to be residence time dependent. If the residence time is too short, the formation reactions may not have come to an end and the reactants will then flow out of the reacting zone before they have had a chance to react. The formation of 15N2O as well as the formation of 15N14NO is dependent on the gas residence time within the fixed bed and after the fixed bed (Figure 2). The formation of 15N2O requires two nitrogen atoms from the inlet gas, and a plausible mechanism is M4 where a primary formation of a cyano compound is the result of the reaction of O2 and/or 15NO with the char. The cyano compound then reacts further in the gas phase with 15NO to form 15N2O. The gas residence time dependence for 15N2O is a support for the M4 mechanism. The heterogeneous formation mechanism, M2, is independent of the gas residence time within and after the fixed bed. In the case with 1500 ppm 15NO in the inlet gas the formation of 15N2O was much greater than in the case with 500 ppm 15NO; this can be explained by the greater probability (1500 ppm, 3.5% or 1:23 compared to 500 ppm, 3.5% or 1:70) for the reaction between 15NO and the char relative to the reaction between O2 and the char. Experiments performed without NO present in the inlet gas give lower N2O yields than experiments with NO present1 and the N2O yields increased with increasing NO concentrations in the inlet gas1 but this is only true for the formation of 15N2O (Figures 2 and 3). The influence of 15NO concentration on the formation of 15N14NO was constant, the 15N14NO yield was about 25% within the examined interval, and even if the 15NO concentration was increased 400% the N2O yield increased less than 5% (Figure 3). A possible explanation could be that the release of char nitrogen as cyano compounds is the limiting factor. The formation of 15N2O increased with increasing 15NO concentration in the inlet gas and the increasing 15N2O yield can be explained by the two step mechanism, M4, where the first step is 15NO reduction on the char surface with simultaneous formation of cyano species. The 15NO acts as an oxidizing agent and together with O2 they oxidize the char particle. During this oxidization the char particle undergoes intensive bond breaking and as a result from this massive chemical and physical rupture the char particle releases not fully oxidized compounds (for example cyano compounds formed during NO reduction) which react further in the gas phase. The second step in this two-step mechanism would be the reaction between 15NO and the released and partly oxidized char compounds forming 15N O. Since both O and 15NO act as oxidizing agents, 2 2 although with different reactivity, the probability for 15NO reduction on the char surface increases with (13) Suzuki, Y.; Moritomi, H.; Kido, N. Proceedings of the Fifth International Workshop on Nitrous Oxide Emissions; NIRE/IFP/EPA/ SCEJ: Tsukuba, Japan, 1992; 5-4-(1-9).
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increasing 15NO concentration in the inlet gas as long as the O2 concentration is the same. The probability for the reaction between 15NO and the char surface compared to the reaction between O2 and the char surface increased from 1:70 for 500 ppm 15NO to 1:18 for 1950 ppm 15NO. The influence of CO concentration on the formation of 15N14NO and 15N2O was small (Figure 4). High concentrations of CO (2500 ppm) seemed to reduce the 15N14NO yield but preserve the 15N O yield compared to the case without CO, but the 2 differences were small. If the formation mechanism of N2O is the M3 or the M4 mechanism, the formation of 15N14NO is independent of the reduction of 15NO on the char surface and the decrease in 15N14NO yield can be explained by the increased homogeneous or heterogeneous reduction of 15N14NO by CO in the reactor. The 15NO reduction experiments showed that CO enhances the reduction of 15NO over char and the formation of 15N O, according to the M4 mechanism, is dependent 2 on the 15NO reduction. The reduction experiments were carried out without oxygen present, and the simultaneous presence of oxygen in the combustion experiments might of course diminish the influence of CO on the 15NO reduction. Nevertheless, the 15N O yield was 2 approximately the same as without CO, and since the 15N14NO yield decreased with increasing CO concentration in the inlet gas it is probable that the 15N2O yield also would have decreased if not counterbalanced by the increased formation of 15N2O. The reduction experiments showed that the 15NO reduction increased with increasing CO concentration, increasing gas residence time within the fixed bed and increasing amount of char particles in the fixed bed, but for conditions similar to the combustion experiments with CO the 15NO reduction was small. De Soete4,5 showed that the reduction of NO over char particles leads to the formation of HCN, but Goel et al.8 concluded that HCN is not released in significant amounts during char combustion and that nitric and nitrous oxides produced from char nitrogen are not formed, in the studied case, from the homogeneous oxidation reactions of HCN. A° mand and Leckner3 propose that any cyano compound, HNCO, for example, could be the precursor of N2O. In the present study, a high reduction degree was sought in order to see if measurable amounts of reduction products/intermediates such as H15NCO and 15NCO could be formed as a result of the 15NO reduction on the char surface. To obtain a high reduction degree, a long gas residence time within the reactor and a high CO concentration were needed which made the survival of possible reduction products/intermediates more difficult. The absence of measurable amounts of H15NCO and 15NCO could be due to a very small formation, or none at all, of these species without oxygen present or a very high reactivity that shortens their life time. It is also possible that other cyano compounds are the precursors of N2O. Since the formation of 15N2O increased with increasing gas residence time within the bed and with increasing 15NO concentration in the inlet gas, in accordance with the M4 mechanism, it is obvious that the 15NO reduction was increased during the combustion of the char particles. The oxidizing atmosphere and the rupture of the char particles seem to be important for the 15NO reduction and the 15N2O formation.
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Conclusions The following conclusions can be made from experiments burning char particles of a bituminous coal with gas mixtures containing 15N-isotope-marked NO in a fixed bed reactor: During combustion N2O can be formed in considerable amounts with one nitrogen atom from the char and one nitrogen atom from the inlet 15NO, 15N14NO, or with two nitrogen atoms from the inlet 15NO, 15N2O. The formation of 15N14NO as well as the formation of 15N O increased with increasing gas residence time 2 within and after the fixed bed. The enhancement of the 15N14NO formation as the residence time increases can be explained by the twostep mechanisms, M3 or M4, where the first step is the release of a CN-containing compound. This first step is independent of the residence time. The second step, where the CN-containing compound is oxidized and with help of 15NO forms 15N14NO, is residence time dependent. The enhancement of the 15N2O formation as the residence time increases can be explained by the twostep mechanism M4 where a primary formation of a cyano compound is the result of the reaction of O2 and/ or 15NO with the char. The released and partly oxidized char compound then reacts further in the gas phase with 15NO to form 15N O. 2
Miettinen
The 15N2O formation increased with increasing 15NO concentration in the inlet gas and this is consistent with 15N O being formed by the M4 mechanism. Both O and 2 2 15NO act as oxidizing agents, although with different reactivity, and the probability for 15NO reduction on the char surface increases with increasing 15NO concentration in the inlet gas as long as the O2 concentration is the same. The influence of CO on the 15N14NO and 15N2O yields was small and a high concentration of CO (2500 ppm) seemed to reduce the 15N14NO yield somewhat but preserve the 15N2O yield compared to the case without CO. The 15NO reduction increased with increasing CO concentration, increasing gas residence time within the fixed bed and increasing amount of char particles in the fixed bed. For conditions similar to the combustion experiments with CO but without oxygen the 15NO reduction was small. An oxidizing atmosphere is needed to release the cyano compounds from the char particles and to increase the 15NO reduction to 15N2O. Acknowledgment. The Swedish National Board for Industrial and Technical Development is gratefully acknowledged for their financial support. The author express her gratitude to Dan Stro¨mberg for support and helpful discussions. EF950084A
