Energy & Fuels 1997, 11, 39-45
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Selective Catalytic Oxidation of NH3 in Gasification Gas. 2. Oxidation on Aluminum Oxides and Aluminum Silicates Jukka Leppa¨lahti*,† and Tiina Koljonen VTT Energy, P.O. Box 1601, FIN-02044 VTT, Espoo, Finland
Mikko Hupa and Pia Kilpinen Department of Chemical Engineering, A° bo Akademi University, FIN-20520 Turku, Finland Received December 26, 1995X
In this study the ability of various acid catalysts to selectively oxidize NH3 to N2 was examined in synthetic gasification gas, when O2 and NO simultaneously or O2 or NO alone were added to the gas. If no oxidizers were added to the gas, all catalysts were inert and no reduction of NH3 took place. With aluminum oxide 95% NH3 reduction took place, when O2 and NO were added simultaneously to the gas at 400 °C. Also aluminum silicates and montmorillonite were effective. Generally, NH3 conversion was always decreasing when the temperature was increasing. At low temperature, N2O formation was detected, which was retarded at high temperature. Only a small amount of HCN was formed, when temperature was above 600 °C. Adding NO alone to the gas did not reduce the NH3 content of the gas. With O2 addition alone, 50-60% NH3 reduction was achieved at a very narrow temperature window of 600-700 °C.
Introduction The condition for efficient and successful use of gasification technology in future is the environmentally acceptable operation. One advantage of gasification over traditional combustion technology is the possibility to clean the gas from fuel-bound impurities already before combustion. Removal of H2S from hot gasification gas in order to reduce the SO2 emission has been the object of several studies.1-3 Relatively little attention, however, has been devoted to the possibility to reduce the NOx emission by removing the NH3 and HCN from hot gasification gas. These fuel-bound impurities are the main NOx-forming precursors in gasification gas. Selective oxidation of nitrogenous compounds in gasification gas to N2 would reduce the NOx emission, because NOx formation from molecular nitrogen is not significant in low-Btu gas combustion.4,5 In the SCR process (selective catalytic reduction) molecular nitrogen is produced from flue gas NO by feeding NH3 to the flue gases before the catalyst bed, when NO is selectively reduced in oxidizing conditions according to the overall †
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[email protected]. Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Furimsky, E.; Yumura, M. Erdo¨l Kohle, Erdgas, Petrochem. 1986, 39, 163-172. (2) Nakayama, T.; Ito, S.; Matsuda, H.; Shirai, M.; Kobayashi, T.; Ishikawa, H. Development of fixed-bed type hot gas cleanup technologies for integrated coal gasification combined cycle power generation. Yokosuka Research Laboratory, April 1990, Report EW89015, p 34. (3) Gangwal, S. K.; Stogner, J. M.; Harkins, S. M. Environ. Prog. 1989, 8, 26-34. (4) Kelsall, G. J.; Smith, M. A.; Todd, H.; Burrows, M. J. Combustion of LCV coal derived fuel gas for high temperature, low emissions gas turbines in the British coal topping cycle. Paper presented at the International Gas Turbine and Aeroengine Congress and Exposition, Orlando, FL, June 3-6, 1991; ASME 91-GT-384; p 8. (5) Nakata, T.; Sato, M.; Ninomiya, T.; Yoshine, T.; Yamada, M. J. Eng. Gas Turbines Power 1994, 116, 555-558. X
S0887-0624(95)00266-0 CCC: $14.00
reaction 1.6
4NO + 4NH3 + O2 f 4N2 + 6H2O
(1)
With regard to gasification, an analogous situation exists: In the overall reducing gas atmosphere, NH3 should be selectively oxidized by feeding suitable oxidants to the hot gas and leading the gas through the catalyst bed. In the gasification case the oxidants could be NO and O2. In part 1 of our study, we explored this possibility and found out that the result from the catalytic oxidation, when NO and O2 were simultaneously fed to the hot gasification gas, was largely controlled by the reactions of H2 and CO.7 The reactions of H2 with NO may in fact lead to increased amounts of NH3 in certain conditions. Oxidation of CO may consume the O2 too rapidly and at too low temperatures thus making reaction 1 inefficient. Our conclusion from the earlier study was that suitable catalyst metals would be those that do not adsorb H2 and would not catalyze the oxidation of CO too significantly. The optimum catalyst should, however, readily adsorb NH3, NO, and O2. Because there is little relevant information in the literature concerning the adsorption of complex gas mixtures such as gasification gas on solids, the screening of catalysts was extended by including acid catalysts. Solid acids are widely used as catalysts in hydrocarbon cracking, hydration, and dehydration reactions.8 (6) Hjalmarson, A.-K. NOx control techologies for coal combustion. IEA CoalResearch, Report IEACR/24 1990, p 102. (7) Leppa¨lahti, J.; Koljonen, T.; Hupa, M.; Kilpinen, P. Selective Catalytic Oxidation of NH3 in Gasification Gas. Part 1: Effect of Iron Sinter and Dolomite on the Reactions of NH3, NO and O2 in Gasification Gas. Energy Fuels, preceding paper in this issue. (8) Emmet, P. H., Ed. Catalysis Reinhold: New York, 1960; Vol. 7, p 378.
© 1997 American Chemical Society
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Energy & Fuels, Vol. 11, No. 1, 1997
Leppa¨ lahti et al.
Table 1. Average Analysis (vol %) of Inlet Gas CO
CO2
H2
CH4
N2
H2O
NH3
13
13
12
1
50.5
10
0.49
Due to their acidic nature these catalysts adsorb bases such as NH3, which could make them promising candidates with regard to selective oxidation. In fact the controlled NH3 adsorption is used to measure the total acidity and qualitative distribution of the acidity as a function of acid strength.9 In this paper we present the results from the catalyst screening experiments, in which solid acids were used as catalysts in synthetic gasification gas atmosphere. Experimental Section The detailed description of the experimental apparatus, gas sampling, and analysis systems and experimental procedure has been given elsewhere.7 The reactor system consisted of an electrically heated furnace into which the quartz reactor (diameter 28 mm, volume 70 mL) was placed. The catalyst bed was supported in this reactor on a quartz wool in the bottom of the reactor. The synthetic gasification gas atmosphere was prepared by mixing gases from gas bottles and adding the needed steam from the evaporator. The average gas composition used in the experiments is shown in Table 1. The catalyst materials used in experiments are listed in Table 2. The quartz reactor was fitted with two capillary tubes for gas feeding, one for the synthetic gasification gas and the other for the oxidant. The oxidant was added to the main gas flow before the gases flowed through the catalysts bed. After the catalyst bed the NH3, NO, N2O and HCN content of the gas was measured in addition to the main gas components such as CO, CO2, H2, CH4, and O2. By measuring the total flow of the dry inlet gas, water consumption in the evaporator, and the dry outlet gas it was possible to calculate the water vapor content of the outlet gas. The total flow of gas was kept constant at each temperature. This means that the reaction time was changed with temperature, but the product of reaction time and temperature was constant. Reaction time is given as ratio M/T seconds, where M depends on the flow rate, temperature, and the reactor volume. T is the temperature in kelvin. Reactor volume is the unpacked volume and the flow rate is the flow rate at inlet conditions. Hence the reaction time in the experiments was between 1 and 1.9 s and the corresponding space velocity (SV) between 3600 and 1900 1/h in the experiments. All the gas analyses in this study are presented as concentrations in moist gas. The amount of NH3 remaining in the gas after reactions is given as a ratio of molar flows:
r (%) ) NH3(out)/NH3(in) × 100
(2)
The decomposition (conversion) of NH3 is then
c (%) ) 100 - r
(3)
Results and Discussion In part 1 of this series, we reported that the porous oxide layer formed on silicon carbide seemed to catalyze the NH3 oxidation at relatively narrow temperature range of 700-800 °C, when O2 or the mixture of O2 and NO was added to the gas. The oxidation was selective, which means that the conversion of NH3 was larger than the conversion of other combustible gases such as (9) Le Page, J. F., et al. Applied heterogeneous catalysis; Institute Francais due Petrole publications; Editions Technip: Paris, 1987; p 515.
CO, H2, and CH4. Dolomite and iron sinter instead seemed to catalyze the oxidation of CO and H2 and no oxidation of NH3 took place.7 A totally different behavior was observed in this study, when the effect of aluminum oxide on the NH3 oxidation was studied. When O2 and NO were added (O2/NH3 ) 4, NO/NH3 ) 1) simultaneously to the gasification gas before the catalyst bed, very large NH3 reduction was achieved (Figure 1). The conversion was increased dramatically, when the temperature was decreased. At the temperature of 400 °C the achieved NH3 reduction was about 95%. Without oxidizer addition this material was inert and did not show any catalytic activity for NH3 decomposition. Also NO addition alone was inefficient but when O2 was added, large NH3 conversion was again detected at 600 °C. However, if a high conversion between 350 and 600 °C was wanted, both oxidants had to be injected simultaneously to the gas. This indicates that the NH3 conversion was caused by a similar type of reaction as that described in reaction 1 above. The NH3 conversion with different aluminum oxide beds is compared in Figure 2. NH3 reduction took place on the oxide surface, which was demonstrated indirectly in the experiment with nonporous aluminum oxide C. When nonporous aluminum oxide was used as bed material, the NH3 reduction was only 20% at its best. Similar conversion was also achieved in the empty reactor without catalysts.7 Changing the particle size of the catalyst bed from 0.59-0.84 to 1-3 mm had negligible effect of the conversion. Generally, the results with different aluminum oxide grades were very similar. However, better NH3 conversion than shown in Figure 2 was achieved with aluminum oxide B using catalyst samples from another lot than with samples used in most of the experiments. With these samples almost complete NH3 conversion took place over a wide temperature range of 350-600 °C. These experiments could not be repeated, because the catalyst of that lot ran out. The reason for the better conversion is not clear at the moment. Aluminum silicate materials showed a roughly similar behavior as aluminum oxides (Figure 3). Aluminum silicate did not decompose NH3 without oxidizer additions. NO addition alone seemed to slightly increase the NH3 content of the gas. With oxygen addition, the same type of NH3 conversion seemed to occur at 700 °C as in the experiments with aluminum oxide at 600 °C. Simultaneous addition of O2 and NO produced again a large NH3 conversion (90%), which increased when temperature was decreased. The effects of two different aluminum silicate materials are compared in Figure 4. It can be seen that the conversion with material A was high even at 700 °C, after which the activity was suddenly lost. Although the detailed mechanism of NH3 reduction is not clear at the moment, important conclusions concerning the nature of the reactions can be drawn from the changes of concentrations of both NO and O2 that were fed to gas and combustible components CO and H2 and from the amounts of reaction products like HCN and N2O. From Figure 5 it can be seen that, when NO and O2 were added simultaneously to the gas before the aluminum oxide bed almost all NO reacted and disappeared from the gas in the catalyst bed. In the empty
Selective Catalytic Oxidation of NH3 in Gasification Gas
Energy & Fuels, Vol. 11, No. 1, 1997 41
Table 2. Catalyst Materials content (wt %)
material
surface area (m2/g)
particle size (mm)
aluminum oxide A (BDH Lab Supplies) aluminum oxide B (Engelhard) aluminum oxide C (BDH Lab Supplies) aluminum silicate A (Grace) aluminum silicate B (Engelhard) montmorillonite (BDH Lab Supplies)
330 155
