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Energy & Fuels 1997, 11, 30-38
Articles 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 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
Possibilities to selectively oxidize NH3 in gasification gas to N2 have been investigated. Either O2 or NO or both oxidizers simultaneously were added to the hot synthetic gasification gas, which was led through a catalyst bed positioned in the electrically heated furnace. In the empty reactor and in the nonporous quartz bed, only a slight NH3 reduction was achieved. When dolomite or iron sinter was used as catalyst, the added NO formed more NH3. The reactions of H2 with NO in fact may limit the use of certain catalysts in gasification atmosphere. In the experiments with silicon carbide, the silicon carbide surface was oxidized to porous oxide layer, which catalyzed the selective oxidation of NH3 to N2 at the temperature of 700-800 °C. Even with O2 addition alone large NH3 reduction was achieved. At the same time, significant NO formation was, however, noticed. The reason for the different behavior of the porous silicon dioxide layer, and dolomite and iron sinter, is probably their different capability to adsorb various gas species on their surface.
Introduction In the combustion of fossil fuels the principal source of nitrogen oxides is the nitrogen bound in the fuel structure. In gasification, part of fuel nitrogen forms N2 and part is converted to NH3 and to a lesser extent to HCN and other compounds into hot gasification gas. Especially in the fluidized bed gasification the conversion of fuel nitrogen to NH3 has been shown to be high.1 The NH3 content of the gas in different gasification tests has usually been within the range of 100-10000 ppm, depending on fuel quality and on process conditions. If the gasification gas containing NH3 is burned, high nitrogen oxide emission may be formed. Part of the NH3 can be decomposed to N2 by using low-NOx burners in gas combustion, which have the potential to efficiently reduce the NOx emission from combustion. The development of burners suitable for combusting gasification gas, for example, in gas turbine applications is underway.2-4 The NOx emission level †
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[email protected]. Abstract published in Advance ACS Abstracts, September 15, 1996. (1) Leppa¨lahti, J.; Koljonen, T. Fuel Process. 1995, 43 (1), 1-45. (2) Sato, M.; Abe, T.; Ninomiya, T.; Nakata, T.; Yoshine, T.; Hasegawa, H. Development of a low-NOx combustor for coal gasification combined cycle power generation systems. Paper presented at the Gas Turbine and Aeroengine Congress and Exposition, June 4-8, 1989, Ontario, Canada; ASME 89-GT-104, p 8. (3) Bevan, S., et al. Development and testing of low Btu fuel gas turbine combustors. Proceedings of the Coal-Fired Power Systems 94. Advances in IGCC and PFBC Review Meeting; DOE/METC-94/1008; DOE: Morgantown, 1994; pp 280-289. X
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achievable with low-NOx burners now under development will be seen as soon as these burners operate under realistic conditions connected to gasifiers producing the fuel gas. The use of flue gas NOx removal technologies such as SCR (selective catalytic reduction) is limited due to poisons, dusting, or temperature limitations. In SCR some of the injected NH3 remains unreacted (NH3 slip) and is vented to the atmosphere thus generating a new pollutant. Sulfur contamination, oxidation of SO2 to SO3, pore plugging, and erosion are potential catalyst problems. With the SO2 containing flue gas, ammonium sulfates may form in the catalyst bed, which cause fouling of downstream process equipment.5 An increase in N2O formation has also been detected during high temperature SCR.6 Hence, new options to reduce the NOx emission originated from fuel nitrogen in energy production technologies based on gasification would help to make these technologies more efficient and competitive. One way to fulfill the requirements of NOx emission legislation would be to reduce the NH3 content of the gasification gas already before combustion by cooling the gas and removing the NH3 by gas scrubbing. However, the handling of liquid streams resulting from (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) Armor, J. H. Appl. Catal. B: Environ. 1992, 1, 221-256. (6) Takeshita, M. IEA Perspectives 17 January 1995, 38.
© 1997 American Chemical Society
Selective Catalytic Oxidation of NH3 in Gasification Gas
Energy & Fuels, Vol. 11, No. 1, 1997 31
gas scrubbing may be complicated and costly. If the gasification gas could be cleaned at high temperature the formation of process streams with liquid solutions would be avoided, the process would be simplified, and the efficiency could also be increased in many cases. This seems to be true especially for the so-called simplified IGCC process, in which the hot gasification gas is burned in the gas turbine combustion chamber.7 Catalytic decomposition on noble metals has been suggested as one alternative to remove NH3 from the hot gasification gas.8,9 Usually, a high operating temperature (above 850 °C) is required for sufficient activity. Vaporization of noble metal, sintering, and various poisoning effects might limit the long-term durability of noble metal catalysts in the gasification atmosphere. No studies of other chemical means to remove NH3 from hot gasification gas are known to us. The decomposition rate of NH3 can, for example, be thought to be increased by adding suitable reactants to the gas, which on a suitable catalytic surface would react with NH3 and produce N2. This process could be called the selective catalytic oxidation (SCO) of NH3. The advantage of a process of this type compared with the traditional catalytic decomposition would be the possibility to achieve a high NH3 conversion on the catalyst surface at considerably lower temperature than when using traditional noble metals without additives. The NH3 decomposing activity being based on the activity of additives on the catalytic surface, catalysts of other type than noble metals could also be efficient and would not be limited by the same problems as the noble metal catalysts. The efficient oxidation catalyst should increase more the reaction rate of additives with NH3 than that of additives with other gas components such as CO, H2, and CH4. The SCR process uses catalysts and ammonia as a reductant to selectively remove NO in the presence of excess of O2 from postcombustion flue gases.10 The overall SCR reaction is best described by reaction 1:
CO + H2O f CO2 + H2
(2)
5H2 + 2NO f 2NH3 + 2H2O
(3)
4NO + 4NH3 + O2 f 4N2 + 6H2O
(1)
Several catalysts have been shown to enhance the rate of reaction 1. Hence, the same type of reaction could be used to remove NH3 from gasification gas. In this case NO and O2 should be added to the gasification gas before the catalyst bed. However, previous studies concerning the SCR process have been carried out in an oxidative atmosphere without the presence of large amounts of CO, H2, or CH4 and it is not known how these gases would affect the reaction between NO, O2, and NH3. Especially the role of hydrogen could be problematic. In the study by Taylor and Klimisch11 on the catalytic reduction of NO by carbon monoxide and hydrogen on ruthenium catalyst it was observed that NH3 could be formed through reactions 2 and 3 even if hydrogen were not initially present in the gas: (7) Lundqvist, R. Bioresour. Technol. 1993, 46, 49-53. (8) Krishnan, G. N.; Wood, B. J.; Tong, G. T.; McCarthy, J. G. Study of Ammonia Removal in Coal Gasification Processes; SRI International: Menlo Park, CA; DOE/MC/23 087-2667; p 81. (9) Leppa¨lahti, J.; Simell, P.; Kurkela, E. Fuel Process. Technol. 1991, 29, 43-56. (10) Kato, A.; Matsuda, S.; Kamo, T. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 406-410. (11) Klimisch, R. L.; Taylor, K. C. Ind. Eng. Chem. Prod. Res. Dev. 1975, 14, 26-29.
Hydrogen can also consume the added O2 and hence prevent N2 formation through reaction 1. It is also known that on some catalysts CH4 may reduce NO effectively to N2.12 In the gas cleaning process like SCO, some residual NH3 and NO may remain in the gas and some other reaction products like N2O may be formed during the reactions. Some amount of these residual oxides may be allowed to form, because the final emission to the atmosphere is determined only after the combustion step. However, their effect on NO emission formation in the gas burner should be studied and optimum content of these residual compounds in the fuel should be found out. Because the development of the SCO process for gasification applications is hampered by insufficient knowledge of high-temperature nitrogen chemistry on catalytic surfaces, a research program was initiated at VTT Energy with the aim to produce information on the reactions between oxidizers (NO and O2) and other gasification gas components (NH3, H2, CO, CH4) in the presence of different catalytic materials. Better understanding of the main reaction paths of fuel-bound nitrogen compounds in the gasification atmosphere is also needed to reduce the formation of these compounds already inside the gasifier. For example, NO can be formed from fuel nitrogen in gasifiers near the air inlets. Control of the reactions of this NO could offer one possibility to control the total conversion of fuel nitrogen to NH3. Very limited information is so far available concerning the main reaction paths of NO inside different types of gasifiers.13 The research was begun by examining the kinetics of gas phase reactions of NO and O2 in gasifications gas.14 It was noticed that to reach moderate NH3 conversion without changing the composition of gasification gas too much, high temperature (T ) 1000 °C) and long reaction time (4 s) were needed. These results suggested that the use of catalysts might make the process more efficient at lower temperature. Because earlier studies of catalysts in gasification atmosphere were lacking, the catalytic research had to be started with an explorative study, where numerous materials and their efficiency were to be screened. The aim was to identify the most effective ones for further studies. In this and in the accompanying paper the results from this explorative study are reported. In this paper the effect of silicon carbide, dolomite, and iron sinter on the NH3 content of the gasification gas in the presence of oxidizers (NO, O2) is reported. These results led to an idea of more efficient type of catalysts, which were included into the study. The efficiency of these catalysts is reported in the second (12) Li, Y.; Armor, J. N. Appl. Catal. B 1992, 1, L31. (13) Nichols, K. M.; Hedman, P. O.; Blackham, A. U. Fuel 1990, 1339-1344. (14) Leppa¨lahti, J.; Koljonen, T.; Kilpinen, P.; Hupa, M. Selective Non-catalytic Oxidation of Ammonium in Gasification Gas; VTT Publications: Espoo, Finland, 1994; p 45.
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Leppa¨ lahti et al.
Figure 1. Schematic diagram of the apparatus used in the experiments. Table 1. Sampling Systems and Analysis Methods component
N2O NO CO, CO2, CH4 H2 O2
online online online online
HCN
Figure 2. Catalyst reactor.
part of our paper.15 Together, these results form a basis for the development of new NOx reduction technology for gasification processes. Experimental Section Experiment Equipment. In Figure 1, the reactor system used in the experiments is shown. The composition and flow rate of the synthetic gasification gas were controlled using mass flow meters. NH3 and NO were fed into the system as mixtures of 5 vol % NH3 in nitrogen and 10 vol % NO in nitrogen. Steam was generated by pumping water with a peristaltic pump to the evaporator and the amount of water was detected with a balance. Preheated gas mixture was fed into the quartz reactor (Figure 2) positioned in an electric oven. The quartz reactor was fitted with two capillary tubes for gas feeding, one for synthetic gasification product gas and the other for the oxidizing agents. It is possible that part of the NO reacts with O2 forming NO2 in the feed line especially at low temperature. This possibility will be dealt together with the interpretation of the results. The gases were heated to the reaction temperature prior to mixing in order to perform (15) Leppa¨lahti, J.; Koljonen, T.; Hupa, M.; Kilpinen, P. Selective Catalytic Oxidation of NH3 in Gasification Gas. Part 2: Oxidation on Aluminum Oxides and Aluminum Silicates. Energy Fuels, following paper in this issue.
sampling absorption solution absorption solution gas bag
NH3
method Kjeldahl titration Orion 94-06 cyanide electrode AE-detector and gas chromatography Thermoelectron Model 10A Hartman & Brown Uras 3G Hartman & Brown Caldos 4T Hartman & Brown Magnos 3
an effective mixing of reactive gases with the main gas mixture at the temperature desired. After mixing the gases passed through the catalyst bed. Catalyst particles were supported on a quartz wool in the bottom of the reactor. The diameter of the bed was 28 mm and the bed volume 70 mL, when long reaction time was needed. When shorter reaction times were used the bed diameter was 23 mm and volume 46 mL. The temperature was measured in the middle of the catalyst bed with a K-type thermoelement positioned in the quartz tube through the bottom of the reactor. The temperature along the bed was usually in the range of (15 °C from the bed middle temperature. Sampling System and Analyses. The detection of NH3 and HCN was accomplished by an absorption system in which the product gas was passed through gas washing bottles immersed in an ice bath. The absorption solution was 5 wt % H2SO4 for NH3 and 5 wt % NaOH for HCN. Ammonia was analyzed by titrating according to the standard ASTM D 142679. The cyanide content of the solution was determined with an Orion 9406 electrode (Table 1). The gas was dried with calcium chloride (CaCl2) before leading to other sampling systems and analyzers. N2O samples were collected in tedlar bags and analyzed with HP 5890 Series II gas chromatography equipped with HP 5921 A atomic emission detector. The stability of N2O in the gas bags was studied by making aging tests with selected samples. As the results indicated that the amount of N2O may change in a couple of hours after sampling, N2O was analyzed within an hour. NO concentration of the gas was measured with an on-line chemiluminescence NO/NOx analyzer Thermo Electron Model 10 A. CO, CO2, and CH4 were analyzed with infrared spectrophotometry (Hartman & Brown Uras 3B, NDIR method). The methods for H2 and O2 measuring were based on thermal conductivity (Hartman & Brown Caldos T) and magnetic susceptibility (Hartman & Brown Magnos 3).
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Energy & Fuels, Vol. 11, No. 1, 1997 33
Table 2. Catalyst Materials and Their Compositions material dolomite iron sinter quartz silicon carbide a
content (wt %)
surface area (m2/g)
particle size (mm)
Fe
Ca
Mg
Al
Si
12.1 6.3
