in Gasification Gas. 3. Experiments at Elevated Pressure - American

When the oxidizer consisted of NO2 and O2, large. NH3 conversion ... gas the highest NH3 conversion took place at 550 °C. The addition of O2 alone to...
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Energy & Fuels 1998, 12, 758-766

Selective Catalytic Oxidation of NH3 in Gasification Gas. 3. Experiments at Elevated Pressure Jukka Leppa¨lahti* VTT Energy. P.O. Box 1601, FIN-02044 VTT, Espoo, Finland

Pia Kilpinen and Mikko Hupa Department of Chemical Engineering, Åbo Akademi University, FIN-20520 Turku, Finland Received December 17, 1997

In this study the ability of aluminum oxide to catalyze the selective oxidation of NH3 in hot pressurized (20 bar) synthetic gasification gas was examined. When the mixture of NO and O2 was used as oxidizer, it was found that part of the NO may be converted to NO2 in the gas feeding system. It was found that the ratio of NO/NO2 in the oxidizer mixture can be controlled by controlling the residence time and the temperature of the gas feeding system. Increasing the pressure increased the rate of NO2 formation. When the oxidizer consisted of NO2 and O2, large NH3 conversion was attained below 500 °C. With NO and O2 addition into the hot gasification gas the highest NH3 conversion took place at 550 °C. The addition of O2 alone to the aluminum oxide bed at high pressure reduced also the NH3 content of the gas. It was shown that aluminum oxide catalyzes the reaction between NH3 and O2.

Introduction In the gasification of solid fuels, the fuel nitrogen is released into the hot gasification gas primarily as ammonia (NH3), hydrogen cyanide (HCN), organic compounds (tar-N), or as molecular nitrogen. Ammonia is usually the predominant compound in gasification. The share of other nitrogen compounds is small.1 It has been known for a long time that these reactive compounds form nitrogen oxide (NOx) emission when gasification gas is burned.2,3 In fact, when the product gas is burned from a gasification process in which the solid fuel is gasified with air (air gasification), the flame temperature is low enough to prevent the oxidation of molecular nitrogen in the combustion air. Thus, practically all of the NOx emission in these applications originates from fuel bound nitrogen.4 Even if advanced low-NOx burners are used in gas combustion, the formed NOx emission may be unacceptable. In air gasification applications the NOx emission can be reduced or almost totally eliminated by reducing the amount of fixed nitrogen compounds in the gas. Even partial reduction of nitrogen species would relieve the demands for the burner. However, at the moment there does not seem to exist suitable processes for hightemperature cleanup of gasification gas from nitrogen compounds. (1) Leppa¨lahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43 (1), 1-45. (2) Chomiak, J.; Longwell, J. P.; Sarofim, A. F. Combustion of Low Calorific Value Gases; Problems and Prospects. Prog. Energy Combust. Sci. 1989, 15, 109-129. (3) Leppa¨lahti, J.; Ståhlberg, P.; Simell, P. Nitrogen compounds in peat and wood gasification and combustion. In Proceedings of the Symposium Research in Thermochemical Biomass Conversion; Bridgewater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 711-724. (4) Woycenco, D. M.; Smart, J. P. Combustion of gaseous fuels in gas turbines. J. Inst. Energy 1993, 66, 150-158.

Catalytic high-temperature decomposition of ammonia has been studied as one option to solve this problem.5,6 One drawback of this system is the very high temperature (800-950 °C) needed to achieve a good ammonia conversion, which is not always advantageous or possible, for example, when biomass is gasified. High temperature imposes also severe requirements for the thermal stability of the catalyst. Catalysts, which have good activity, are usually based on transition or noble metals (e.g., nickel, iron, ruthenium). These metals are also very sensitive for poisoning caused by sulfur species. Also partly because of this, a very high temperature is needed for efficient NH3 decomposition. Hence, it would be preferable if the NH3 decomposition rate could be increased by some means at a lower temperature in the gasification gas. In selective catalytic oxidation (SCO) the NH3 decomposition rate is increased by adding a small amount of oxidizers to the gasification gas. Because the NH3 decomposing activity of the catalyst is based on the activity of the additive on the catalytic surface, catalysts other than noble or transitional metals could also be efficient and would not be limited by the same problems as these metals. The efficient oxidation catalyst should increase the reaction rate of additives more with NH3 than with other gas components. One advantage of this type of process compared with the traditional catalytic NH3 decomposition at high temperature is that the global gas equilibrium does not limit the achievable NH3 (5) Simell, P.; Kurkela, E.; Ståhlberg, P.; Hepola, J. Catalytic hot gas cleaning of gasification gas. Catal. Today 1996, 27. (6) Jothimurugesan, K.; Gangwal, S. Catalytic Decomposition of Ammonia in Coal Gas. In Proceedings of the 3rd International Symposium on Gas Cleaning at High Temperatures; Schmidt, E., Ga¨ng, P., Piltz, T., Dittler, A., Eds.; Institute fur Mechanische Verfahrenstechnik und Mechanik, Universita¨t Karlsruhe (TH): Karlsruhe, Germany 1996; pp 383-392.

S0887-0624(97)00229-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/19/1998

Oxidation of NH3

decomposition. In high-temperature catalytic decomposition the whole gas system is driven near the global equilibrium. In SCO at lower temperature, mainly NH3 decomposition reactions are catalyzed while otherwise the gas system remains far from global chemical equilibrium. This means that lower NH3 levels are attainable in SCO than in high-temperature catalytic decomposition. This work is the third part of a series of papers where the results from selective oxidation studies are reported. In the first part we reported the effect of iron sinter and dolomite on the reactions of NH3, NO, and O2 in gasification gas. In the second part results from experiments with aluminum oxides and aluminum silicates were reported. In these experiments selective NH3 decomposing reactions were initiated at atmosperic pressure in synthetic gasification gas flowing through aluminum oxide and aluminum silicate catalyst beds, when a small amount of the mixture of O2 and NO or O2 alone was added to the gas.7,8 The best conversion with simultaneous NO and O2 addition was achieved at a relative low temperature of 400-500 °C. When the temperature was increased, the NH3 conversion was always decreased. With O2 addition alone good NH3 conversion was attained in the temperature range 550750 °C. In these first experiments the goal was to find conditions where NH3 reacts selectively in otherwise fuel-rich fuel gas. Hence, the suitability of many catalysts were screened and only little attention was possible to devote to the study of the reaction mechanism behind the NH3 reduction. In this third part we report the results from the experiments conducted with aluminum oxide at high pressure. The purpose of this work was to study the effect of total pressure on the NH3 conversion using an aluminum oxide catalyst, which can at the moment be considered the most interesting candidate for the oxidation catalyst. The aim was also to study more closely the main features of the mechanism behind the NH3 reduction. As part of the study, kinetic modeling calculations were carried out to gain insight of the importance of the gas-phase reactions of the used oxidizer mixture and possible reaction mechanisms in the applied experimental conditions. Experimental Section The experimental equipment consisted of a pressurized high-temperature tube reactor shown in Figure 1. The composition and flow of the synthetic gasification gas mixture were controlled by using mass flow controllers. The gases were mixed and preheated to 300 °C before water was added with liquid chromatography pumps and vaporized into the gas flow. The preheated gas mixture was fed into the quartz reactor positioned inside the heating tube. The inner walls of the pressure vessel and the heating tube were flushed with nitrogen to protect the pressure vessel from overheating and from corrosive gases. The catalyst particles were supported with quartz wool in the central part of the reactor. The inner diameter of the sample bulp was 12 mm. (7) Leppa¨lahti, J.; Koljonen, T.; Hupa, M.; Kilpinen, P. Selective Catalytic Oxidation of NH3 in Gasification Gas. 1. Effect of Iron Sinter and Dolomite on the Reactions of NH3, NO, and O2 in Gasification Gas. Energy Fuels 1997, 11 (1), 30-38. (8) Leppa¨lahti, J.; Koljonen, T.; Hupa, M.; Kilpinen, P. Selective Catalytic Oxidation of NH3 in Gasification Gas. 2. Oxidation on Aluminium Oxides and Aluminium Silicates. Energy Fuels 1997, 11 (1), 39-45.

Energy & Fuels, Vol. 12, No. 4, 1998 759

Figure 1. Pressurized high-temperature tube reactor. Table 1. Average Analysis of Synthetic Gasification Gas CO (vol %)

CO2 (vol %)

H2 (vol %)

CH4 (vol %)

N2 (vol %)

H2O (vol %)

NH3 ppmv

13

13

12

1

50,5

10

5000

A thin (2 mm i.d.) quartz tube was installed at the top of the reactor for mixing the oxidizers with the main gas flow immediately in front of the catalyst bed or directly into the bed. This prevented the oxidizers from reacting in the gas phase during the gas heating before the catalyst bed. The catalyst bed temperature was monitored with K-type thermocouples placed inside the quartz shield and installed in the middle of the catalyst bed. The catalyst used was of commercial-grade aluminum oxide with a BET surface area of 155 m2/g. The catalyst was crushed and sieved to the particle size fraction of 0.29-0.35 mm. After the reactor the gas sample was drawn through a pressure let-down valve and led to an on-line FT-IR analyzer at atmospheric pressure and at 180 °C. The Gasmet FT-IR analyzer was used to analyze CO, CO2, CH4, H2O, NH3, HCN, NO, NO2, and N2O contents of the gas after the catalyst bed. In addition, H2, O2, and from time to time also other gases were analyzed with methods described previously.7 The experiments were always started by adjusting the flow and the composition of the main gas. As soon as the desired experimental gas composition and flow were reached, the oxidizers were flown to the gas through the quartz probe. The composition of synthetic gasification gas used in the experiments is presented in Table 1. The space velocity in the experiments was 15 000 L/h, calculated from the gas flow under inlet conditions (NTP). The reaction time (in seconds) calculated in the empty reactor volume is given as the ratio 1273/T (K), where T is the temperature in Kelvin. The pressure was 20 bar in all experiments. The reproducibility of all gas measurements was usually better than 10%. However, in the presence of H2O the detection limit was about 30 ppms for NO2, 50 ppms for N2O, and 10 ppms for HCN, which correspondingly limited the accuracy, when low concentrations of the these gases were measured with FTIR.

Kinetic Modeling Procedure In our experiments, where oxidizers are fed to the hot mixture of combustible gases, very rapid gas-phase

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reactions may take place before and in the catalyst bed. It is also possible that there are strong interactions between the gas-phase reactions and the reactions taking place on the surface of catalysts. The kinetics of gas-phase reactions may, for example, change the concentrations of oxidizers and other radical species that may diffuse to the catalyst surface. Hence, it is of utmost importance to be able to clearly identify and separate the effect of gas-phase reactions from the effect caused by the true catalytic reactions. For this purpose homogeneous chemical kinetic calculations were made with a detailed reaction scheme. The scheme has been developed for analysis of gas-phase nitrogen chemistry in combustion and was taken from Kilpinen et al.9 It is based on the mechanism published by Miller and Glarborg10 consisting of 133 Thermal deNOx reactions. To this mechanism, four oxidation reactions of CO were added from Glarborg et al.11 In the conditions where hydrocarbons were present 216 additional oxidation reactions of C1 and C2 hydrocarbons, HCN, and HNCO were included as well.12 Thermodynamic data were taken from Kee et al.13 and Glarborg et al.12 Codes based on the Chemkin subroutine library were used.14,15 Although the scheme has been validated against a body of experimental data and has been shown to describe the nitrogen reactions satisfactorily at a number of combustion conditions, it is important to notice that model calculations may contain uncertainties in the conditions of this work. For example, the pressure dependence of some rate constants are not well-known at the moment.

Leppa¨ lahti et al.

Figure 2. NH3 reduction and reaction products in the aluminum oxide bed. NO/NH3 ) 1 and O2/NH3 ) 4. Pressure is 20 bar. Inlet gases are the following: CO, 13 vol %; CO2, 13 vol %; H2, 12 vol %; CH4, 1 vol %; H2O, 10 vol %; O2, 2.0 vol %; NO, 5000 ppmv; NH3, 5000 ppmv.

Results and Discussion Experiments with the Mixture of NO and O2. When the synthetic gasification gas was flown through the aluminum oxide bed, no decomposition of NH3 took place even if the temperature was increased to 800 °C. Instead, when NO and O2 (NO/NH3 ) 1 and O2/NH3 ) 4) were added simultaneously into the hot gas, a large NH3 reduction took place (Figure 2). As can be detected from Figure 2, NH3 conversion was increased when the temperature was decreased. A small amount of NO was still present in the gas after the catalyst bed. At lowtemperature, NO2 and N2O were formed in the gas. When the temperature was increased to 500-600 °C, the amount of both NO2 and N2O was reduced to a (9) Kilpinen, P.; Hupa, M.; Aho, M. Selective non-catalytic NOx reduction at elevated pressures: Studies on risks for increased N2O emissions, Proceedings of the 7th International Workshop on Nitrous Oxide Emission, Cologne, April 21-23, 1997; Wiesen, P., Ed.; Bergische Universita¨t Gesamthochschule Wuppertal, 1997. (10) Miller, J. A.; Glarborg, P. In Gas phase chemical reaction systems: Experiments and modeling: 100 years after Bodenstein; Springer Series in Chemical Physics; Springer-Verlag: Berlin, 1996. (11) Glarborg, P.; Kubel, D.; Kristenssen, P. G.; Hansen, J.; DamJohansen, K. Combust. Sci. Technol. 1995, 110-111, 461. (12) Glarborg, P.; Dam-Johansen, K.; Kristensen, P. Reburning rich lean kinetics; Final Report, Gas Research Institute 5091-260-2126, Nordic Gas Technology Centre 89-03-11; 1993. (13) Kee, R. J.; Rupley, F. M.; Miller, J. A. The Chemkin thermodynamic data base; SAND87-8215B; Sandia National Laboratories: Albuquerque, NM, 1990. (14) Lutz, A. E.; Kee, R. J.; Miller, J. A. A Fortran program for predicting homogeneous gas phase chemical kinetics with sensitivity analysis;SAND87-8248; Sandia National Laboratories: Albuquerque, NM, 1988. (15) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin II: Fortran Chemical kinetics package for the analysis of gas phase chemical kinetics; SAND89-8009; Sandia National Laboratories: Albuquerque, NM, 1990.

Figure 3. NH3 reduction and reaction products as a function of NO/NH3 ratio. O2/NH3 ) 4, temperature is 400 °C, and pressure is 20 bar. Inlet gases are the following: CO, 13 vol %; CO2, 13 vol %; H2, 12 vol %; CH4, 1 vol %; H2O, 10 vol %; O2, 2.0 vol %; NO, 0-8 000 ppmv; NH3, 5000 ppmv.

negligible level. No HCN was formed in the experiments. Only a small amount of oxygen was consumed at low temperature. At 400 °C the oxygen content of the gas decreased from 2 vol % to about 1.4-1.6 vol %. When the temperature was increased over 500 °C, all added oxygen was probably mostly consumed in reactions with H2 and CO. The effect of changing the NO/ NH3 molar ratio, when the O2/NH3 molar ratio was kept constant ()4) at a temperature of 400 °C, is shown in Figure 3. As can be seen, increasing the amount of fed NO resulted in a steady increase in the NH3 removal. With the NO/NH3 molar ratio greater than 1, complete NH3 removal was achieved. However, the excess NO remained in the gas and was probably partly converted into NO2 during the process. The effect of changing the amount of O2 fed into the reactor with a constant molar ratio of NO/NH3 ) 1 is shown in Figure 4. Without oxygen there is no reaction of NO with NH3. The presence of even a small amount of oxygen (O2/NH3 ) 1) in the gas is sufficient to increase the reaction rate of NO with NH3 considerably. Increasing the ratio O2/NH3 over 1 further increases the

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Energy & Fuels, Vol. 12, No. 4, 1998 761

Figure 4. NH3 reduction in aluminum oxide bed as a function of the ratio O2/NH3. Temperature is 400 °C, NO/NH3 ) 1. Inlet gases are the following: CO, 13 vol %; CO2, 13 vol %; H2, 12 vol %; CH4, 1 vol %; H2O, 10 vol %; O2, 0-2 vol %; NO, 5000 ppmv; NH3, 5000 ppmv.

reaction rate and the fraction of NO that is converted into NO2 in the process. In our earlier study7,8 we concluded that the net rate of reaction 1

NO + NO + O2 f 2NO2

(1)

was too slow to explain the observed NO2 formation under the reactor conditions and suggested that at least part of the NO2 is formed in the oxidizer feeding line. The low temperature (room temperature) of the gas mixture before the pressurized reactor and the high concentrations of NO and O2 increase the NO2 formation according to reaction 1.7 As part of this study, kinetic calculations were performed to study various aspects of NO2 formation and to explain the observed high NO2 conversion. First, the formation of NO2 in the oxidizer feeding line was calculated at atmospheric pressure and at 20 bar pressure. According to calculation results, the formation of NO2 is very sensitive to temperature and pressure variations. At low temperature the calculations produced high NO2 concentrations in the gas (Figure 5). Measurements of the formed NO and NO2 in the oxidizer feeding line were conducted at atmospheric pressure by leading the oxidizer mixture instead of the pressurized reactor directly to the bypass line and to the gas analyzer. The temperature in this part of the feeding line was the room temperature. Immediately after entering the bypass line, this gas mixture was diluted with known N2 flow in order to adjust the concentrations to the range in which the analyzer could be operated (0-5000 ppm). The measurements showed that NO was completely converted to NO2 already in the oxidizer feed line at room temperature before the pressurized reactor. The NO2 content in the experiments was, however, higher than the model predicted at atmospheric pressure and at 25 °C (Figure 5a). One explanation to this might be the nonuniform gas temperature in the gas feeding line. Immediately before NO and O2 were mixed and the gas mixture fed into the feeding lines, the gas pressure was relieved from 20 to 1 bar. The gas expansion might have cooled the

Figure 5. Predicted formation of NO2 under oxidizer feeding line conditions and comparison with experimental points. Inlet gases are NO (8vol %), O2 (32 vol %), and the rest is N2. Pressure in part a is 1 bar and in part b is 20 bar.

gas temperature temporarily below room temperature, after which the temperature may have increased back to room temperature. This cold spot might have then increased the NO2 formation rate. Increasing the pressure in the calculations accelerated also the NO2 formation rate (Figure 5b). At high pressure in less than 1 s, the NO2 content increased to the final level, which probably was set by the thermodynamic equilibrium of the reaction 1. Hence, the NO2 in the reacted gas in Figures 2-4 is the residual NO2 that is formed in oxidizer feeding lines and is not consumed during the course of the reactions. The most important conclusions, however, from these calculations and experiments are that at low temperature and especially at high pressure the NO/NO2 ratio of the oxidizer can be controlled by controlling the conditions, e.g., reaction time and temperature of the oxidizer feeding system. Because the formed NO2 is not stable at high temperature, the reverse of reaction 1 will proceed and a mixture of NO and NO2 may be formed when the temperature is increasing in the hot parts of the oxidizer line and the reactor. The final NO/NO2 ratio of the oxider will then depend on the temperature and residence time of the gases in the feeding system. The NO/ NO2 ratio of the oxidizer can also be assumed to have an effect on the NH3 conversion. In the literature the reactions of NH3 with NO and NO2 on the catalytic surface are commonly described with global reactions

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2-5:16-18

4NO + 4NH3 + O2 f 4N2 + 6H2O

(2)

6NO2 + 8NH3 f 7N2 + 12H2O

(3)

NO + NO2 + 2NH3 f 2N2 + 3H2O

(4)

6NO + 4NH3 f 5N2 + 6H2O

(5)

These reactions represent formal stoichiometric relationships, in which each includes probably a number of fundamental reactions steps on the catalyst surface. In our earlier paper it was shown that NO without the presence of O2 does not react with NH3 on aluminum oxide (reaction 5). There is evidence in the literature that a different result may be expected depending on which of the reactions 2-4 will be dominating. According to Nakajima and Hamada,18 reaction 4 may be the fastest on the catalytic surface. It may be thus suspected that the formation of NO2 may be also essential for the good conversion (reactions 3 and 4) reached in our experiments. The NO/NO2 ratio of the oxidizer mixture under the experimental conditions of our study was next studied by carrying out experiments without the presence of NH3. The experiments of Figure 2 were repeated without NH3 and catalyst first in inert CO2, H2O, and N2 atmosphere and then in gasification gas atmosphere. Finally, gasification gas with the catalyst bed but again without NH3 was used. In an inert atmosphere, NO2 was dominating over NO after the reactor at low temperature. At high-temperature, NO was dominating (Figure 6a). This seemed to confirm the hypothesis that NO2 was first formed in the oxidizer feeding lines at room temperature and was later decomposed to the NO/ NO2 mixture, the composition of which depends on the experimental conditions. Adding CO, H2, and CH4 to gas accelerated the NO2 decomposition in the reactor, as may be detected from Figure 6b. In this case complete conversion of NO2 to NO had taken place already at 500 °C. By comparison of this situation with the NH3 conversion of Figure 2, it can be detected that good NH3 conversion was achieved in the temperature range 350-500 °C, where the mixture of NO and NO2 is present in the catalyst bed. Over the temperature of 500 °C NO2 was probably decomposing so fast that it was not available for reaction with NH3 on the catalyst surface for a sufficiently long time. Modeling calculations in fact showed that the NO2 decomposition rate is relatively slow at 400 °C and below but increases rapidly if the temperature is increased to 500 °C (Figure 7). Increasing the pressure from 1 to 20 bar again increases the NO2 decomposition rate. This may mean that achievable NH3 conversion with NO2 will be necessarily smaller at high pressure than under otherwise similar conditions at atmospheric pressure. Generally, the fit between modeling calcula(16) Thiemann, M.; et al. Nitric Acid, Nitrous Acid, and Nitrogen Oxides. In Ullmann’s Encyclopedia of Industrial Chemistry, Naphthalene to Nuclear Technology, 5th ed; Elvers, B., et al., Eds.; VCH: Germany, 1991; Vol. A17, pp 293-339. (17) Yamaguchi, M.; Matsushita, K.; Takami, K. Remove NOx from HNO3 tail gas. Hydrocarbon Process. 1976, August, 101-106. (18) Nakajima, F.; Hamada, I. The state-of-the-art technology of NO control. Catal. Today 1996, 29, 109-115.

Figure 6. Nitrogen compounds formed from the oxidizer in (a) empty reactor (NO (converted to NO2 in feed lines), 5000 ppmv; O2, 2 vol %; CO2, 13 vol %; H2O, 10 vol %; rest N2), (b) empty reactor (gases as in part a) and added CO at 13 vol %, H2 at 10 vol %, and CH4 at 1 vol %, and in (c) aluminum oxide bed (gases as in part b). Pressure is 20 bar. Reaction time (in seconds) in the empty reactor volume is 1273/T (K).

tions and experimental points in Figure 7 was good. Also, the calculations clearly verified that the mixture of NO and NO2 was formed from decomposed NO2 into the reactor and that the fraction of NO2 decreased rapidly with increasing temperature, which might partly explain the temperature dependence of the NH3 conversion shown in Figure 2. The presence of an alumina catalyst further accelerates the reduction of the oxidizer by CO, H2, and CH4 (Figure 6c), which further may explain the poor NH3 conversion at temperatures over 500 °C. No HCN was formed in these experiments.

Oxidation of NH3

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Figure 7. Predicted destruction of NO2 and comparison with experimental points. Inlet gases are the following: NO2, 5000 ppmv; O2, 2 vol %; CO, 13 vol %; H2, 12 vol %; CH4, 1 vol %; CO2, 13 vol %; H2O, 10 vol %; rest is N2. Pressure in part a is 1 bar and in part b is 20 bar.

Hence, if the formation of NO2 would be prevented in the oxidizer feeding system, different oxidizer composition would be created, and this probabaly would affect the NH3 conversion. In the next experiments the feeding of NO and O2 was arranged through separate feeding lines, and thus, the formation of NO2 in the feeding lines was easily restrained. In these experiments O2 was fed to the reactor as in the earlier experiments, but for NO feeding, a new connection was assembled before the main reactor. First, the behavior of separately added NO and O2 was studied without the presence of NH3 as in earlier experiments (Figure 8). In this case, in an inert atmosphere NO is now the dominating nitrogen oxide. However, part of the NO was still oxidized to NO2 (Figure 8a). The addition of reactive CO, H2, and CH4 gases only removes NO2 when the temperature is increased over 500 °C. Also, some NH3 tends to be formed from the reaction of NO and H2 at high temperature (Figure 8b). Finally, the addition of alumina catalyst accelerates the NO reduction at 600 °C and also increases the NH3 formation to some extent (Figure 8c). Anyhow, NO is now dominating over NO2 in the oxidizer mixture in the temperature range used in the experiments. Model calculations showed NO oxidation to be negligible under reactor conditions at atmospheric pressure (Figure 9). In the oxidizer feeding lines the high NO and O2 partial pressure ensured that NO oxidation

Figure 8. Nitrogen compounds formed from oxidizer (separate feeding of NO and O2) in (a) empty reactor (NO, 5000 ppmv; O2, 2 vol %; CO2, 13 vol %; H2O, 10 vol %; rest is N2), in (b) empty reactor (gases as in part a) and added CO at 13 vol %, H2 at 10 vol %, and CH4 at 1vol %, and in (c) aluminum oxide bed (gases as in part b). Pressure is 20 bar. Reaction time (in seconds) in the empty reactor volume is 1273/T (K).

took place (Figure 5) but in the reactor conditions the NO and O2 partial pressures were lower and practically no NO2 formation occurred at atmospheric pressure. At 20 bar pressure, however, partial pressures of these reactants were high enough in order to produce some NO2, which can be detected from Figure 9. Also in this case NO formation is very temperature sensitive. It may be detected from Figure 9 that measured values of NO2 conversion are lower than calculated. The natural reason for this discrepancy could be the difficulty of measuring the NO2 content, which reliably would

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Figure 10. NH3 reduction and reaction products in aluminum oxide bed for simultaneous O2 and NO addition from separate feeding lines. Inlet gases are the following: CO, 13 vol %; CO2, 13 vol %; H2, 12 vol %; CH4, 1 vol %; H2O, 10 vol %; NH3, 5000 ppmv; O2, 2 vol %; NO, 5000 ppmv; rest is N2. Pressure is 20 bar. Reaction time (in seconds) in the empty reactor volume is 1273/T (K).

Figure 9. Predicted oxidation of NO to NO2 and comparison with experimental points. Inlet gases are the following: NO, 5000 ppmv; O2, 2 vol %; CO2, 13 vol %; H2O, 10 vol %; rest is N2. Pressure in part a is 1 bar and in part b is 20 bar.

represent the NO2 content at a fixed temperature. Because the NO2 formation rate is increasing, when temperature is decreasing, some NO2 is probably formed during sampling and analyzing at lower temperature. This problem is pronounced under pressurized conditions of this reseach, where the partial pressures of reactants are high and the NO2 formation rate accordingly higher than, for example, in standard flue gas conditions. Measuring NO2 more accurately than in this study would require in situ measuring equipment to be used, which was not available during this research. Sampling lines of the reactor system were kept at 300 °C in order to prevent the formation of ammonium nitrate, and the FTIR analyzer worked at 180 °C. These lower temperatures make it possible that some NO2 could be formed even inside the analyzer. The experimental points in Figure 9 lie in fact on the conversion graph predicted by the calculations, which represent the NO2 formation at about 250 °C. The difference between measured and predicted values is not very large in ppm. When the starting NO concentration is 5000 ppm, the model predicts roughly 500 ppm more NO2 to be formed than what was measured. The conclusion from this part of the calculation is that some NO2 may be formed in the reactor even in the case where NO2 formation is supressed in the feeding lines. However, the NO2 level shown in Figure 8 is possibly about 500 ppm too high especially in the upper temperature range. Hence, the

effect of this NO2 formation on the NH3 conversion achieved in this case must be relatively small. When NH3 was added to the gas and the corresponding NH3 conversion was measured, it was clearly detected that at low temperature the conversion was decreased (Figure 10) compared to the situation where the oxidizers were fed to the gas from the common oxidizer line (Figure 2). Now instead, the NH3 conversion was increased when the temperature was increased. At 550 °C almost 80% of the NH3 had reacted synchronously with the NO. At higher temperature, the reaction stops and the NH3 concentration is suddenly increased. At this critical temperature the reaction rate of O2 is suddenly increased and O2 disappears from the gas. At low temperature O2 reacts slowly on the catalyst surface, but when the critical temperature is reached, the gas-phase reactions of O2 with H2, CO, and CH4 begin to control the reaction rate (ignition) and the NH3 conversion process becomes inefficient. The presence of O2 seems to be necessary for the NH3 conversion, which thus probably takes place according to reaction 2. At low temperature, the aluminum oxide catalyst is not, however, a very effective catalyst for this reaction and the NH3 conversion remains relatively poor. At the optimum temperature of 500-550 °C, no HCN was formed and only negligible amounts of NO2 and N2O were created during the reactions. Only some unreacted NO remained in the gas, which suggests that the NH3 conversion could be improved in this case by using a smaller space velocity in the reaction system. Use of O2 Alone as Oxidizer. The goal of the development of the selective catalytic oxidation (SCO) process is to find a relatively simple and cheap oxidizer in order to improve the economy of the process. In the catalyst screening studies conducted earlier at atmospheric pressure,7,8 it was found that relatively good NH3 conversion was also achieved when O2 alone was used as the oxidizer in the aluminum oxide and aluminum silicate catalyst beds.8 Relatively high NH3 conversion was also achieved now at high pressure when O2 alone was added to the synthetic gasification gas flowing through the aluminum oxide bed (Figure 11). No oxides of nitrogen could be found from the reacted

Oxidation of NH3

Energy & Fuels, Vol. 12, No. 4, 1998 765

Figure 11. NH3 conversion and O2 content in synthetic gasification gas after O2 addition to the aluminum oxide bed. Inlet gases: CO, 13 vol %; CO2, 13 vol %; H2, 12 vol %; CH4, 1 vol %; H2O, 10 vol %; NH3, 5000 ppmv; O2, 2 vol %; rest is N2. Pressure is 20 bar. Reaction time (in seconds) in the empty reactor volume is 1273/T (K).

gas, which points to the fact that propably all reacted fixed nitrogen in NH3 formed N2. Without the catalyst practically no NH3 reduction took place. Also, the modeling calculations showed that without the presence of the catalyst bed the O2 was consumed rapidly in the gas-phase reactions of CO and H2 at about 550 °C. However, in the catalyst bed the O2 and NH3 content of the synthetic gasification gas decreased simultaneously with increasing temperature. At 550 °C the O2 content had decreased from 20 000 to 14 000 ppm and correspondingly the NH3 content from 5000 to 2000 ppm. Some O2 was also consumed in the reactions with CO and H2 and in reactions with NH3. When the temperature was increased further, the O2 was consumed more rapidly by gas-phase reactions with combustible gases, the temperature was increased by these exothermic reactions, and NH3 conversion was decreased. The reactivity of combustible gases thus sets a limit to the highest temperature, where NH3 reduction may be accomplished. To gain more insight to the NH3 conversion process, the catalytic effect of aluminum oxide on the reaction of O2 and NH3 was now further studied without the presence of combustible gases H2, CO, and CH4. First, the NH3 reduction and reaction products of the homogeneous gas-phase (without the catalyst) reaction between NH3 and O2 were measured. Without the presence of the aluminum oxide bed the O2 addition to the inert gas reduced the NH3 content slowly when temperature was increased. At 900 °C roughly 50% of NH3 had reacted (Figure 12). It deserves to be noted that not any oxides of nitrogen were created during the NH3 oxidation. Catalytic oxidation of NH3 with O2 is generally assumed to take place according to the following steps: 16,17,19

3O2 + 4NH3 f 2N2 + 6H2O

(6)

5O2 + 4NH3 f 4NO + 6H2O

(7)

(19) Satterfield, C. N. Heterogeneous Catalysis in Practice; McGrawHill: New York, 1980; p 416.

Figure 12. Predicted and experimental oxidation of NH3. Inlet gases: NH3, 5000 ppmv; O2, 2 vol %; CO2, 13 vol %; H2O, 10 vol %. Reaction time (in seconds) is 1273/T(K). Pressure is 20 bar.

2O2 + 2NH3 f N2O + 3H2O

(8)

Because no oxides of nitrogen was found in our experiments, it may be assumed that N2 and H2O were the reaction products according to the global reaction 6, which describes the overall oxidation of NH3 on aluminum oxide. The results of the kinetic modeling calculations of gasphase reaction 6 are also given and compared with the experimental data in Figure 12. The results of kinetic calculations fit relatively well the experimental results, although the NH3 conversion is a little lower at low temperature and higher at high temperature in the calculations compared with the experimental results. The kinetic modeling results also confirmed the important result that no oxides of nitrogen are formed under these conditions in the gas-phase oxidation. Finally, the NH3 conversion without the presence of a catalyst in oxygen addition (data of Figure 12) is compared with the NH3 conversion reached with addition of oxygen to the aluminum oxide bed in Figure 13. When oxygen was added to the aluminum oxide bed, NH3 conversion was clearly increased. In this case, most of the NH3 had decomposed already at 800 °C. In the reactions products, again no NO, NO2, or N2O could be detected. On the basis of these results, it may be concluded that the aluminum oxide catalyst has catalytic properties that catalyze the global reaction 6 between NH3 and O2 and not global reactions 7 and 8. By comparison of these results (inert gas) with the results presented in Figure 11 (syngas), it may be detected that in the presence of combustible gases the NH3 content of the gas starts to decrese at lower temperature than in the inert gas atmosphere. This favorable effect may be due to the oxidation of CO and H2 gases, which produces radical species on the catalyst surface, thus increasing the rate of NH3 oxidation on the catalyst surface. As may be detected from Figure 13, aluminum oxide did not decompose NH3 without the O2 addition.

766 Energy & Fuels, Vol. 12, No. 4, 1998

Figure 13. Oxidation of NH3 in CO2, H2O, and N2 atmosphere without O2 adddition in aluminum oxide bed, with O2 addition in empty reactor, and with O2 addition in aluminum oxide bed. Inlet gases: CO2, 13 vol %; H2O, 10 vol %; NH3, 5000 ppmv; O2, 2 vol %; rest is N2. Pressure is 20 bar. Reaction time (in seconds) in the empty reactor volume is 1273/T (K).

Conclusions In this third part of a series of papers it was shown that NH3 may also be selectively reacted in highpressure gasification gas by adding a small amount of oxidizers to the gas. In the aluminum oxide catalyst bed three different reaction paths are available for NH3 reduction. If the mixture of NO2 and O2 is used, large NH3 conversion is attained when the temperature is under 500 °C. By use of NO and O2 as oxidizers, selective reactions of NH3 take place at higher temperature in the temperature range 500-550 °C. With O2

Leppa¨ lahti et al.

addition alone, the NH3 content of the synthetic gasification gas can also be reduced in the aluminum oxide bed in the temperature range 500-700 °C. The effectiveness of O2 addition is explained by the fact that aluminum oxide has catalytic properites, which increase the reaction rate between NH3 and O2. More information is needed especially from controlling factors, which affect the reactivity of small O2 content in hot gasification gas, in order to be able to optimize the NH3 conversion with O2 addition. NO2 can be produced at low temperature in the oxidizer feeding lines from NO and O2. The ratio of NO/ NO2 in the oxidizer mixture can be controlled by controlling the temperature and residence time in the oxidizer feeding lines. At room temperature and with a residence time of 3-5 s almost all of the NO can be converted to NO2. Increasing the temperature and decreasing the residence time will reduce the NO2 formation. In a practical gas cleaning process, this might offer a means to adjust the ratio of NO/NO2 in the oxidizer for the highest possible NH3 reduction. Acknowledgment. This study was part of the Finnish National Combustion and Gasification Research Program LIEKKI 2. The work was partially financed also by the European Commission through JOULE Programme under Contract JOR-CT97-0157, Imatran Voima Oy and Academy of Finland. Ms. Katja Heiskanen is acknowledged for her contribution in doing the experiments with the flow reactor. EF9702299