Study on NO and N2O Formation and Destruction Mechanisms in a

NH3 and HCN oxidation, as well as NO and N2O destruction, was studied ... Figure 1 Formation rate unit (FRU) (Wartha, 1998): (1, 2) inlet of fluidizin...
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Energy & Fuels 2002, 16, 1024-1032

Study on NO and N2O Formation and Destruction Mechanisms in a Laboratory-Scale Fluidized Bed G. Loeffler,* C. Wartha, F. Winter, and H. Hofbauer Institute of Chemical Engineering, Fuel Technology and Environmental Technology, Vienna University of Technology, Getreidemarkt 9/159, A-1060 Vienna, Austria Received September 10, 2001. Revised Manuscript Received March 30, 2002

In fluidized bed combustors, the harmful pollutants NO and N2O are formed from fuel-nitrogen (fuel-N). The complex homogeneous and heterogeneous reaction mechanisms determine the tradeoff between N2, NO, and N2O from fuel-N conversion affected by the bed temperature, fuel characteristics, residence time, and many more factors. To obtain a better understanding of these mechanisms and to study the relative importance of homogeneous and heterogeneous catalyzed reactions, a study on the gas reactions in a laboratory-scale fluidized bed reactor was performed. NH3 and HCN oxidation, as well as NO and N2O destruction, was studied simulating the conditions of devolatilization and char combustion stages. The experimentally obtained results were analyzed with a detailed chemical kinetic model considering the two-phase structure of a fluidized bed and the quenching of radicals on the solids’ surface. The significance of homogeneous reactions depends on temperature and the presence of combustible gases. Heterogeneous catalyzed reactions oxidize HCN and NH3 to N2 and NO, while almost no N2O is formed. The CH4 addition increasing the radical level enhances the NO formation in NH3 oxidation and N2O formation in the case of HCN. The presence of NO increases the selectivity toward N2 and N2O for HCN and NH3 oxidation. NO and N2O destruction tests demonstrate that thermal reduction of NO and N2O is negligible under present conditions. The presence of CH4 reduces N2O emissions slightly due to reduction with the H radical. However, the presence of CH4 affects not only the formation and destruction paths of NO and N2O but NO also significantly influences CH4 combustion in fluidized beds by sensitizing its oxidation.

Introduction At the operating temperatures of a fluidized bed combustor (around 850 °C), NOx and N2O emissions are formed from the nitrogen in the solid fuel. The pathways for this transformation are complex and comprise homogeneous as well as heterogeneous reactions catalyzed by the bed material.1 The fraction of the fuel-N that is converted into NO, N2O, and N2, respectively, depends on the fuel, fuel characteristics, bed temperature, residence times, fluid dynamics, combustor geometry, heat, and mass transfer and so on (e.g., refs 1 and 2). Moreover, the nitrogen oxides may also affect the oxidation the volatile hydrocarbons.3-5 Because of these complex interrelations, it is advantageous to study these processes separately under simplified conditions. In this way, the relative importance of the processes can be analyzed, i.e., homogeneous versus heterogeneous catalyzed reactions and formation versus destruction reactions. To study the homogeneous formation and destruction paths of NO and N2O in fluidized bed combustors, (1) Johnsson, J. E. Fuel 1994, 73, 1398-1415. (2) Leckner, B. Prog. Energy Combust. Sci. 1998, 24, 31-61. (3) Wartha, C.; Winter, F.; Hofbauer, H. J. Energy Res. Technol. 2000, 122, 94-100. (4) Bendtsen, A. B.; Glarborg, P.; Dam-Johansen, K. Combust. Sci. Technol. 2000, 151, 31-71. (5) Lo¨ffler, G.; Wargadalam, V. J.; Winter, F.; Hofbauer, H. Fuel 2001, 81, 855-860.

different gases (HCN, NH3, NO, and N2O) were added into the laboratory-scale stationary fluidized bed. The conversion of these species is investigated with and without CH4 present to change the radical pool. The work is performed to study nitrogen chemistry related to devolatilization and char combustion, where the primary products released from the fuel particle are besides others HCN, NH3, NO, hydrocarbons, and CO.6,7 Moreover, the significance of the homogeneous versus heterogeneous catalyzed reactions for the NOx and N2O emissions in fluidized bed combustion can be demonstrated. The experiments were accompanied by modeling work, combining a two-phase reactor model for the fluidized bed with a detailed chemical kinetic mechanism including homogeneous as well as heterogeneously catalyzed reactions and radical quenching at the surface of the bed material. Experimental Section The tests were performed in a laboratory-scaled fluidized bed reactor called Formation Rate Unit (FRU, internal diameter 35 mm, static bed height about 40 mm, Figure 1). It is made of quartz glass to minimize possible catalytic effects of the reactor wall. The distributor is a quartz glass frit. A fluidized bed of silica sand (mean diameter 250 µm), containing (6) Winter, F.; Wartha, C.; Lo¨ffler, G.; Hofbauer, H. Proc. Combust. Inst. 1996, 26, 3325-3334. (7) Ashman, P. J.; Haynes, B. S.; Buckley, A. N.; Nelson, P. Proc. Combust. Inst. 1998, 27, 3069-3075.

10.1021/ef010228n CCC: $22.00 © 2002 American Chemical Society Published on Web 07/20/2002

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Figure 1. Formation rate unit (FRU) (Wartha, 1998): (1, 2) inlet of fluidizing gas, (3) mass-flow controllers, (4) display for the mass-flows, (5) preheating zone, (6) heating shells, (7) thermocouple, (8) path to the chimney, (9) heated filter, (10) heated sample lines, (11) heated pump, (120 heated gas cell, (13) FTIR spectrometer, (14 and 15) fuel inlet, (16) flask for iodine addition in a water-bath, (17 and 18) inlet of reaction gas (NH3, HCN, and NO), (19) inlet of combustible gas (CH4). also small quantities of ash from coal and wood combustion, was used. The FRU is heated with heating shells producing a uniform temperature profile, which was varied between 600 and 900 °C. Air/nitrogen mixtures were used to set the oxygen partial pressure of the fluidizing gas (pO2) at 10kPa. The superficial velocity v was kept constant at 0.68 m/s to keep the gas residence time constant (e100 ms in the hot zone). The reaction gas was added to the fluidizing gas (refer to Figure 1). The CH4 was added directly into the fluidized bed. In the flue gas CO2, CO, CH4, NO2, NO, N2O, HCN, and NH3 were measured continuously with a FTIR spectrometer (BioRad FTS 60A) in combination with a heated gas cell (accuracy of analysis, CO2, CO, CH4, and NO: 5 rel%; NO2, N2O, HCN, and NH3: 10 rel %). A more detailed description of the experimental setup is given by Wartha.8

Chemical Kinetic Modeling The chemical kinetic modeling was performed with the program FBRSim,9 which has been described briefly by Lo¨ffler et al.10 This model describes detailed homogeneous reaction mechanism in a two phase fluidized bed reactor and in the freeboard. Moreover, the heterogeneously catalyzed reactions on the bed material and the quenching of radicals on the solids’ surface are included. (8) Wartha, C. Ph.D. Thesis, Vienna University of Technology, Vienna, Austria, 1998. (9) Lo¨ffler, G.; Andahazy, D.; Winter, F.; Hofbauer, H. Report No. VTWS-99-FB-11. Institute of Chemical Engineering, Fuel Technology and Environmental Technology, Vienna University of Technology: Vienna, Austria, 1999. (10) Lo¨ffler, G.; Andahazy, D.; Wartha, C.; Winter, F.; Hofbauer, H. J. Energy Res. Technol. 2001, 123, 228-235.

The detailed homogeneous kinetic reaction mechanism is taken from Bowman et al.11 with the H/N/O reactions replaced by the mechanism of Glarborg and co-workers12,13 as supposed by Wargadalam et al.14 Moreover, the kinetics of the N2O destruction reactions were adapted as recommended by Lo¨ffler et al.,15 and to account for the NOx sensitized CH4 oxidation the changes proposed by Lo¨ffler et al.5 were included. Heterogeneously catalyzed reactions for NH3 oxidation and decomposition on bed material are included according to the recommendations of Johnsson16 and Johnsson and Dam-Johansen.17 Finally, the heterogeneous oxidation of HCN and CO is considered according to the kinetic rate expressions proposed by Lo¨ffler et al.18 An overview of the applied detailed kinetic reaction scheme is given in Table 1. (11) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, W. C.; Lissianski, V.; Smith, G. P.; Golden, D. M.; Frenklach, M.; Goldenberg, M. http://www.me.berkeley.edu/gri_mech/; 1996. (12) Glarborg, P.; Dam-Johansen, K.; Miller, J. A. Int. J. Chem. Kinet. 1995, 27, 1207-1220. (13) Kjaergaard, K.; Glarborg, P.; Dam-Johansen, K.; Miller, J. A. Proc. Combust. Inst. 1996, 26, 2067-2074. (14) Wargadalam, V. J.; Lo¨ffler, G.; Winter, F.; Hofbauer, H. Combust. Flame 2000, 120, 465-478. (15) Lo¨ffler, G.; Wargadalam, V. J.; Winter, F.; Hofbauer, H. Combust. Flame 2000, 120, 427-438. (16) Johnsson, J. E. CHEC Report No. 9003. Department of Chemical Engineering, Technical University of Denmark: Lyngby, Denmark, 1990. (17) Johnsson, J. E.; Dam-Johansen, K. In Proceedings of the 13th International Conference on Fluidized Bed Combustion; ASME: New York, 1995; 859-869. (18) Lo¨ffler, G.; Wargadalam, V. J.; Winter, F. Fuel 2001, 81, 711-717.

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Figure 2. NH3 conversion to N2, NO, N2O, and NO2 versus bed temperature with (a) and without (b) CH4 addition. (a) Inlet concentrations: [NH3] ) 200 ppm, [O2] ) 10 v %. (b) Inlet concentration: [NH3] ) 200 ppm, [CH4] ) 8000 ppm, [O2] ) 10 v%. Superficial velocity 0.68 m/s. Lines indicate modeling results, (2) NH3, (b) NO, (9) N2O, NO2 not measured. Table 1. Detailed Chemical Kinetic Reaction Mechanism system H/N/C/O

mechanism (by)

changes

no. of reactions

GRIMECH by Bowman et al.11 H/N/O reactions replaced by Glarborg mechanism12 three rates changed as recommended by Amano and Dryer32 six reactions added to include CH3O2 as recommended by Bromly et al.33 Glarborg et al.12

H/N/O

changes as recommended by Kjaergaard et al.13 N2O decomposition rates changed as recommended by Lo¨ffler et al.15 heterogeneous catalyzed reactions radical recombination

no. of species

277

Johnsson,16 Johnsson and Dam-Johansen,17 Lo¨ffler et al.18 Kim and Boudart,34 Kristensen et al.35

total

-42

+6

+1

+106 +2

+6

+9

+4 362

The thermodynamic data are taken from Sandia Thermodynamic Database19 with changes as recommended by Glarborg et al.20

In the following, the results of NH3 and HCN oxidation, the destruction of NO and N2O and the effect of the radical pool influenced by CH4 addition is discussed. NH3 and HCN Oxidation. Figure 2 shows the conversion of NH3 in the presence and absence of CH4 for different bed temperatures. Without CH4 addition, the conversion to NO is small (around 10%) and almost independent of bed temperature. On the other hand, the total conversion of NH3, mainly converted to N2, increases significantly with increasing temperature. Below 800 °C, in the presence of CH4, the conversion to NO decreases. This is explained by the conversion of (19) Kee, R. F.; Rupley, F. M.; Miller, J. A. Sandia National Laboratories Report SAND87-8215B. Sandia National Laboratories: Livermore, CA, 1993. (20) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1-27.

56

NO to NO2 in the presence of hydrocarbons via reaction 1 (ref 21).

NO + HO2 T NO2 + OH Results and Discussion

49

(1)

Above 800 °C, the conversion to NO increases strongly, due to the combustion of CH4 increasing the radical level. Thus, the formation of NO via the following reaction sequence is accelerated.

NH3 + OH T NH2 + H2O

(2)

NH3 + O T NH2 + OH

(3)

NH2 + HO2 T H2NO + OH

(4)

NH2 +O T HNO + H

(5)

H2NO + NH2 T HNO + NH3

(6)

H2NO T HNO + H

(7)

H2NO + O T HNO + OH

(8)

(21) Hori, M.; Matsanuga, N.; Malte, P. C.; Marinov, M. N. Proc. Combust. Inst. 1992, 24, 909-916.

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H2NO + OH T HNO + H2O

(9)

HNO + O2 T NO + HO2

(10)

HNO T NO + H

(11)

HNO + O T NO + OH

(12)

Because NH3 is a bad precursor for N2O, only insignificant amounts are formed under all conditions investigated. Generally, the model predictions are in good agreement with the experiments, though NO formation in the presence of CH4 above 800 °C is overestimated. The calculations show, that in absence of CH4 the NH3 conversion is almost exclusively determined by the heterogeneous reactions catalyzed by the bed material. With CH4 addition, homogeneous reactions gain significant importance above 800 °C. Figure 3 shows the relative conversion of NH3 at 850 °C in absence (a) and presence (b) of 8000 ppm CH4. Without CH4 addition NH3 is mainly oxidized directly to NO via

NH3 + 5/4O2 T NO + 3/2H2O

(13)

catalyzed by the bed material. The presence of CH4 increases the radical level and thus the rates of homogeneous reactions. Thus, NH3 is mainly oxidized by reaction with the OH radical (reaction 2) and only to about 29% via reaction 13. In Figure 4, the comparison between measurements and calculation for the HCN oxidation is given. Below 800 °C, the HCN conversion is not influenced by the presence of CH4. Measurements show a strong increase of the HCN conversion from 9% to 55%. Since there is no effect of CH4 on the HCN destruction, heterogeneous reactions may be responsible. This is also supported by Wargadalam et al.14 finding that HCN is not oxidized homogeneously in this temperature range within a residence time, which is even longer (around 0.3 s) than in the experiments here (around 0.1 s). The calculations show also that HCN is only converted by heterogeneously catalyzed reactions 14 and 15, besides the tests with CH4 addition and temperatures above 800 °C.

HCN + 5/4O2 T 1/2N2 + CO2 + 1/2H2O

(14)

HCN + 7/4O2 T NO + CO2 + 1/2H2O

(15)

There are significant deviations between the calculations and the experimental results in the HCN/O2 system (refer to Figure 4a). In the presence of CH4 (refer to Figure 4b), the agreement is fine above 800 °C, where the homogeneous oxidation of HCN becomes dominant. This indicates that the applied kinetics for the heterogeneous HCN oxidation catalyzed by the bed material is not appropriate for the present bed material. Because of the lack of accurate data on the kinetics for the reactions 14 and 15 in the literature, the rate expressions proposed by Lo¨ffler et al.18 assuming 0.5 wt % spruce wood ash in the bed material are used. However, as shown in that work,18 the kinetics of heterogeneously catalyzed HCN oxidation is strongly dependent on the type of ash. In the NH3 oxidation systems, the agreement between calculations and experimental results was fine indicating that the kinetics of the heteroge-

neous NH3 reactions fit well. These were taken from Johnsson16 and Johnsson and Dam-Johansen17 for bed material from bituminous coal combustion. From the experiments it can be concluded that, for the current bed material, the temperature dependence of the HCN oxidation is stronger and NO is the dominating product. This does not agree to the results obtained from spruce wood ash showing smaller activation energy. Similar to the NH3 oxidation, the presence of CH4 reduces the NO emissions below 850 °C significantly by converting them into NO2 as discussed above. Exceeding 850 °C NO2 is converted back into NO. At these temperatures, CH4 addition increases the significance of homogeneous reactions on HCN conversion to NO via

HCN + O T NCO + H

(16)

HCN + OH T CN + H2O

(17)

HCN + O T NH + CO

(18)

CN + O2 T NCO + O

(19)

NCO + O T NO + CO

(20)

NH + O T NO + H

(21)

NH + O2 T NO + OH

(22)

NO is partly reduced reacting with NH and NCO to N2O and N2

NCO + NO T N2O + CO

(23)

NCO + NO T N2 + CO2

(24)

NH + NO T N2O + H

(25)

NH + NO T N2 + OH

(26)

For this reason CH4 addition increases significantly the formation of N2O by enhancing homogeneous reactions as shown in Figure 4. Further calculations show that the rate of heterogeneous HCN conversion observed in the experiments exceeds clearly the rate of mass transfer between bubble and emulsion phase. The maximum HCN conversion for an infinite rate of the heterogeneous HCN oxidation in the emulsion phase is indicated by a thin solid line in Figure 4a. As discussed for instance in Grace22 and Lo¨ffler,23 the predictions of overall conversion for very fast reactions are strongly sensitive toward the assumptions for the hydrodynamic model of the fluidized bed (i.e., flow split, mass transfer, entrance and freeboard effects, and solids in the bubble phase, refer to Grace22). Thus, according to the recommendations of Kunii and Levenspiel,24,25 0.5 v % solids are assumed being suspended within the bubbles. Moreover, the rates of the heterogeneously catalyzed HCN oxidation (i.e., reactions 14 and 15) were adapted to obtain fine agreement between experimental and modeling results as shown in Figure 5. (22) Grace, J. R. Chemical Reactor Design and Technology; de Lasa, H. D., Ed.; Martinus Nijhoff: 1986; pp 245-289. (23) Lo¨ffler, G. Ph.D. Thesis, Vienna University of Technology, Vienna, Austria, 2001. (24) Kunii, D.; Levenspiel, O. Ind. Eng. Chem. Res. 1990, 29, 9(7) 1226-1234. (25) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Boston, 1991.

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Figure 3. Reaction analysis of NH3 conversion at 850 °C bed temperature with (a) and without (b) CH4 addition. (a) Inlet concentrations: [NH3] ) 200 ppm, [O2] ) 10 v %. (b) Inlet concentration: [NH3] ) 200 ppm, [CH4] ) 8000 ppm, [O2] ) 10 v %. Superficial velocity 0.68 m/s.

Figure 4. HCN conversion to N2, NO, N2O, and NO2 versus bed temperature with (a) and without (b) CH4 addition. (a) Inlet concentrations: [HCN] ) 180 ppm, [O2] ) 10 v %. (b) Inlet concentration: [HCN] ) 180 ppm, [CH4] ) 8000 ppm, [O2] ) 10 v %. Superficial velocity 0.68 m/s. Lines indicate modeling results, (2) HCN, (b) NO, (9) N2O, ()) NO2. Table 2. Kinetic Parameters of Heterogeneous HCN Oxidation per Mass Bed Material rate

k [mol(1-a-b)/ (kgbedmat cm3(a+b) s)]

Ea [J/mol]

a

b

rNO 2rN2

2.53 × 2.20 × 1020

2.68 × 2.20 × 105

0.867 1.795

0.138 -0.295

1017

105

Best fits to the experiments were obtained with the kinetic rate parameters given in Table 2, assuming the dependence from the HCN and O2 concentrations (i.e., a and b, refer to Table 2) as obtained by Lo¨ffler et al.18 for the spruce wood ash. A comparison between the rates of reactions 14 and 15 obtained for 0.5 wt % spruce wood ash in the bed and the current bed material fitted by assuming 0.5 v % solids to be present in the bubble phase is given in Figure 6. It can be clearly seen that the temperature dependence of NO formation for the current bed material is stronger than obtained for spruce wood ash. The HCN conversion to N2 has the same activation temperature in the range from 800 °C to 900 °C, where the rate expression for spruce wood ash was obtained. At 700 °C, however, the rate of N2 formation is significantly decreased. It has to be kept in mind that a quantitative comparison of the rates is not reasonable because the rates depend either on the assumed content of spruce

Figure 5. HCN conversion to N2, NO, N2O, and NO2 versus bed temperature without CH4 addition with fitted heterogeneously catalyzed HCN oxidation on bed material. Inlet concentrations: [HCN] ) 180 ppm, [O2] ) 10 v %. Voidage in the bubbles b ) 0.995. Superficial velocity 0.68 m/s. Lines indicate modeling results, (2) HCN, (b) NO, (9) N2O.

wood ash in the bed or on the assumed solids concentration in the bubble phase.

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Energy & Fuels, Vol. 16, No. 5, 2002 1029

Figure 6. Comparison of heterogeneously catalyzed HCN oxidation rates to NO (a) and N2 (b). Solid line indicates rate expression for 0.5 wt % spruce wood ash in silica sand. Dashed line indicates least-squares fit to experimental data. Experimentally obtained rate assuming 0.5 v % solids in the bubble phase (0).

Figure 7. NH3 and NO conversion to N2, NO, N2O, and NO2 versus bed temperature with (a) and without (b) CH4 addition. (a) Inlet concentrations: [NH3] ) 200 ppm, [NO] ) 200 ppm, [O2] ) 10 v %. (b) Inlet concentration: [NH3] ) 200 ppm, [NO] ) 200 ppm, [CH4] ) 4000 ppm, [O2] ) 10 v %. Superficial velocity 0.68 m/s. Lines indicate modeling results, (2) NH3, (b) NO, (9) N2O, ()) NO2.

The temperature for the maximum NO reduction is rather dependent on the presence of CO or hydrocarbons

affecting the radical pool29 and shifting the temperature window for optimum NO reduction to lower temperatures. Without CH4 addition, the conversion of NH3 increases from 60% at 700 °C to 90% at 900 °C bed temperature. Contrary, the conversion to NO shows a maximum of around 60% at 800 °C as shown in Figure 7a. Above this temperature, the reduction of NO by reactions 27 and 29 gains importance. Adding CH4 to the system, NH3 is already almost completely converted at 700 °C (refer to Figure 7b). NO sensitizes the CH4 oxidation5 so that, at 700 °C, also 90% of CH4 is converted mainly to CO. In the course of this, a significant part of NO is converted into NO2. As NO2 is converted back into NO after complete oxidation of the hydrocarbons, the conversion to NO increases with temperature, while NO2 decreases. Figure 8 shows the concentration profiles of the main nitrogen and carbon containing species at 700 °C and 800 °C bed temperature. At 700 °C, NH3 is heterogeneously oxi-

(26) Duo, W.; Dam-Johansen, K.; Ostergaard, K. Can. J. Chem. Eng. 1992, 70, 1014-1020. (27) Hemberger, R.; Muris, S.; Pleban, K. U.; Wolfrum, J. Combust. Flame 1994, 99, 660-668.

(28) Kasuya, F.; Glarborg, P.; Johnsson, J. E.; Dam-Johansen, K. Chem. Eng. Sci. 1995, 50, 1455-1466. (29) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1996, 107, 211-222.

Effect of NO. During char combustion heterogeneously formed NO is released, which is known to significantly affect the selectivity in HCN and NH3 oxidation (e.g., ref 14). NH3 and HCN act as precursor in NO formation as well as agent reducing NO similar to the thermal DeNOx process. The reaction mechanism of NO reduction by NH3 (referred as the selective noncatalytic reduction, SNCR, or thermal DeNOx) has been investigated widely (e.g., refs 26-28). The key step in the reaction mechanism of that system is the reaction between NH2 and NO, which has three product channels (i.e., reactions 27-29).

NO + NH2 T N2 + H2O

(27)

NO + NH2 T N2 + H + OH

(28)

NO + NH2 T NNH + OH

(29)

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Figure 8. Calculated concentration profiles of (a) NH3, NO, NO2, CH4, CO, and CO2 versus reactor height for the NH3/NO/CH4/ O2 system at (a) 700 °C and (b) 800 °C bed temperature. Inlet concentration: [NH3] ) 200 ppm, [NO] ) 200 ppm, [CH4] ) 4000 ppm, [O2] ) 10 v %. Superficial velocity 0.68 m/s. Bold lines indicate concentrations in the bubble phase and the freeboard; thin lines indicate concentrations in the suspension phase. Measured outlet concentrations (2) NH3, (b) NO, (9) N2O, ()) NO2.

dized to NO in the suspension phase. After a delay time this is converted into NO2 mainly in the bubble phase. With the presence of NO2 the CH4 oxidation starts via5

CH4 is poor. This again is caused by the lack of accurate kinetic data for the heterogeneously catalyzed oxidation of HCN for the current bed material. In the presence of CH4, where the homogeneous reactions gain significance, the agreement is fine. The results indicate that without CH4 present, HCN is oxidized to NO and no reduction of NO by NCO and NH occurs.

CH4 + OH T CH3 + H2O

(30)

CH3 + NO2 T CH3O + NO

(31)

CH3O T CH2O + H

(32)

NCO + NO T N2O + CO

(23)

Simultaneously, the homogeneous oxidation of NH3 starts and NO is partly reduced to N2 via reaction 27. With a bed temperature of 800 °C (refer to Figure 8b) ignition takes place earlier and all CH4 is consumed within the bed. A small peak in NO2 concentration can be seen during the oxidation of CH4 to CO. After all the CH4 is consumed, NO2 is almost completely converted back into NO. As homogeneous reactions starts earlier than in the case of 700 °C bed temperature, the homogeneous oxidation of NH3 and reduction of NO by NH2 successfully compete with the heterogeneously catalyzed oxidation of NH3. This explains the steep decrease in NH3 and NO concentrations in the bubble phase. In Figure 9, the conversion of HCN and NO with and without CH4 addition versus bed temperature can be seen. As already discussed above, the agreement between measurements and model results in absence of

NCO + NO T N2 + CO2

(24)

NH + NO T N2O + H

(25)

NH + NO T N2 + OH

(26)

Thus, there is no N2O formation in absence of CH4. Adding CH4 to the HCN/NO/O2 system, the NO levels are reduced. Below 800 °C, significant part of NO is converted into NO2, while above this temperature N2O formation and reduction to N2 becomes significant. NO and N2O Destruction. The oxidation of HCN and NH3 forms NO and N2O, which may be destroyed partly afterward. To study the destruction separately, NO and N2O were introduced into the formation rate unit (FRU) with and without addition of CH4. As shown in Figure 10a, there is no conversion of NO without CH4 present. Thermal destruction or oxidation

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Energy & Fuels, Vol. 16, No. 5, 2002 1031

Figure 9. HCN and NO conversion to N2, NO, N2O, NO2 versus bed temperature with (a) and without (b) CH4 addition. (a) Inlet concentrations: [HCN] ) 180 ppm, [NO] ) 260 ppm, [O2] ) 10 v %. (b) Inlet concentration: [HCN] ) 180 ppm, [NO] ) 260 ppm, [CH4] ) 8000 ppm, [O2] ) 10 v %. Superficial velocity 0.68 m/s. Lines indicate modeling results, (2) HCN, (b) NO, (9) N2O, ()) NO2.

Figure 10. (a) NO conversion versus bed temperature with and without CH4 addition. (b) CH4 conversion versus bed temperature in the NO/CH4/O2 system. Inlet concentrations: [NO] ) 260 ppm, [O2] ) 10 v% ([CH4] ) 8000 ppm). Superficial velocity 0.68 m/s. Lines indicate modeling results, experimental results: (b) NO, ()) NO2.

to NO2 are insignificant under present conditions. Adding CH4 to the system, a significant part of NO is converted to NO2. About 65% of NO is oxidized to NO2 and about 50% of CH4 is converted into CO and CO2. The calculations predict lower CH4 and higher CO emissions at 700 °C, but excellent agreement is obtained for the other temperatures. With increasing temperature and total oxidation of the hydrocarbons, NO2 is converted back into NO. Over the whole range of conditions investigated, the sum of NO and NO2 remains constant at the NO inlet level. A reduction of the NOx emissions by CH4 similar to the reburning process30 cannot be obtained under present conditions. This process, where part of the fuel is introduced above the main combustion zone in order to obtain a fuel-rich zone, reduces NO to N2 and HCN or NH3. It is operated at higher temperature (typically above 1500 K) and slightly reducing conditions (e.g., ref 31). Thus, it was to be expected, that under the present strongly oxidizing conditions and low temperatures no NOx reduction occurs. (30) Wendt, J. O. L.; Sterling, C. V.; Matovich, M. A. Proc. Combust. Inst. 1973, 14, 897-904.

To test the relative importance of radicals on the destruction of N2O, 200 ppm N2O and 10 v % O2 were introduced into the FRU with and without CH4 addition. In absence of CH4 it can be seen that thermal destruction of N2O (reaction 33) is negligible under present conditions, though calculations predict a little higher destruction than experimental obtained (refer to Figure 11). Adding CH4 to the reaction system increases the N2O destruction slightly mainly due to reduction with the H radical (reaction 34).

N2O + M T N2 + O + M

(33)

N2O + H T N2 + OH

(34)

(31) Alzueta, M. U.; Bilbao, R.; Millera, A.; Glarborg, P.; Ostberg, M.; Dam-Johansen, K. Energy Fuels 1998, 12, 329-338. (32) Amano, T.; Dryer, F. L. Proc. Combust. Inst. 1998, 27, 397404. (33) Bromly, J. H.; Barnes, F. J.; Muris, S.; You, X.; Haynes, B. S. Combust. Sci. Technol. 1996, 115, 259-296. (34) Kim, Y. C.; Boudart, M. Langmuir 1991, 7, 2999-3005. (35) Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. CHEC Report No. 9511. Technical University of Denmark: Lyngby, Denmark, 1995.

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toward NO is significantly reduced due to the reduction with NH2.

NO + NH2 T N2 + H2O

(27)

NO + NH2 T NNH + OH

(29)

Without CH4 addition, HCN is almost exclusively oxidized heterogeneously catalyzed on the bed material.

Figure 11. N2O conversion versus bed temperature with and without CH4 addition. Inlet concentrations: [N2O] ) 160 ppm, [O2] ) 10 v % ([CH4] ) 8000 ppm). Superficial velocity 0.68 m/s. Lines indicate modeling results, (b,O) N2O.

Conclusions To study the complex homogeneous and heterogeneous reaction mechanism governing the NOx and N2O emissions in a fluidized bed combustor, an experimental and modeling study of gas reactions in a laboratory-scale stationary fluidized bed (FRU) has been performed. The oxidation tests of HCN and NH3 show that both, heterogeneous and homogeneous, reactions contribute to the formation of NO. Their relative contribution depends on the presence of other combustible gases determining the radical level in the combustor. In the emulsion phase, heterogeneous reactions catalyzed by the bed material dominate, while homogeneous reactions are suppressed by the quenching of radicals on the solids’ surface. In NH3 oxidation, N2 is the main product, while no N2O is formed. From heterogeneous NH3 oxidation (reactions 13 and 35) similar amounts of NO and N2 are formed.

NH3 + 5/4O2 T NO + 3/2H2O

(35)

NH3 + 3/4O2 T 1/2N2 + 3/2H2O

(13)

The kinetics of Johnsson16 and Johnsson and DamJohansen17 for bed material of bituminous coal combustion fits very well to the experimental results. Above 800 °C homogeneous reactions gain importance. In the presence of CH4, these increase NO formation, while without CH4 addition the selectivity in NH3 oxidation

HCN + 5/4O2 T 1/2N2 + CO2 + 1/2H2O

(14)

HCN + 7/4O2 T NO + CO2 + 1/2H2O

(15)

Under the conditions investigated, NO is the main product of heterogeneous HCN conversion, while no N2O was found. The applied kinetics for these reactions catalyzed by bed material with 0.5 wt % spruce wood ash does not well describe the experimentally observed results. The calculations indicate that the heterogeneous reactions of HCN may be rather fast and the conversion is above the mass transfer limit between bubble and emulsion phase. Thus, the results are rather sensitive to the fluidized bed reactor model. Assuming 0.5 v % solids present in the bubble phase, it was possible to fit the experimental results. Adding CH4 to the reaction system, the homogeneous reactions again become dominant above 800 °C bed temperature. With that, significant formation of N2O was observed. Adding NO to the reaction systems did not affect the conversion of HCN and NH3 in absence of CH4, where the heterogeneous reactions dominate. In presence of CH4, significant reduction of NO occurs mainly to N2 via reactions 27 and 29 in the case of NH3 and to N2 and N2O via

NCO + NO T N2O + CO

(23)

NCO + NO T N2 + CO2

(24)

NH + NO T N2O + H

(25)

NH + NO T N2 + OH

(26)

for HCN oxidation, respectively. Destruction tests for NO and N2O demonstrate that thermal destruction is insignificant under conditions investigated. N2O is slightly reduced by reaction with the H radical (i.e., reaction 34) adding CH4 into the fluidized bed reactor. The importance of NO on the low-temperature conversion of CH4 was demonstrated. As discussed in Lo¨ffler et al.5 the onset of CH4 oxidation is lowered significantly due to the presence of NO, which in the course of this is converted into NO2. EF010228N