2396
Znd. Eng. Chem. Res. 1991, 30, 2396-2404
Influence of Additives on Selective Noncatalytic Reduction of NO with NH3 in Circulating Fluidized Bed Boilers Bo Leckner* and Maria Karlsson Department of Energy Conversion, Chalmers University of Technology, 412 96 Goteborg, Sweden
Kim Dam-Johansen and Claus Erik Weinell Department of Chemical Engineering, Technical University of Denmark, Bygning 229,2800 Lyngby, Denmark
Pia Kilpinen and Mikko Hupa Department of Chemical Engineering, Abo Akademi University, 205 20 Turku, Finland
The application of selective noncatalytic reduction of nitric oxide with ammonia in circulating fluidized bed boilers is investigated. Special attention is directed to the use of additives t o the ammonia so that the efficiency of the NO reduction a t lower temperatures can be increased. Tests under realistic conditions in a research boiler and reaction kinetic calculations show that the type of additives used did not improve the process. On the other hand, it is shown that ammonia injection as such, when employed before the cyclone of the boiler, effectively reduces the NO emission to a level of 20-60 ppm, without significant negative effects such as ammonia bypass and an increased CO emission.
Introduction Low emission of NO (and a negligible amount of NOz) is one of the principal advantages of fluidized bed combustion. However, many countries have lowered their standards for emission limits to a level which more or less coincides with the emission level of circulating fluidized bed (CFB) boilers. Since the CFB boiler emissions, at the present state of knowledge, cannot be predicted with great enough precision to design a boiler that is certain to meet the emission standards, additional measures must be taken to ensure emissions below the limits established. Catalytic reduction of NO would lead to a large investment, which would even make the application of CFB doubtful. Process equipment for selective noncatalytic reduction (SNR), on the other hand, consists of uncomplicated injection nozzles and a feeding system. Since only a small reduction, if any, is required, the reduction of NO obtained with SNR would be satisfactory. Selective noncatalytic reduction of NO is commonly achieved by injection of ammonia or urea into the hot postcombustion flue gases. As urea has already been shown to increase the emission of N20somewhat (Mjornell et al., 1991; J ~ d a et l al., 1990), only ammonia will be considered here. It has been shown in a number of studies that the reduction of NO with NH, functions well in flame combustion boilers within a narrow temperature window (850-1100 "C) centered at about 975 "C. This temperature can be shifted to lower temperatures by radicals produced by additives (Lyon, 1975). Additives to ammonia may also be important for CFB boilers, where the commonly employed bed temperature, 850 "C, is just at the lower limit of the temperature window of the SNR process for relevant residence times. Furthermore, at low boiler loads, say below 70% of maximum load, the temperature in the upper part of the combustion chamber normally drops below the bottom bed temperature. In spite of the low temperatures of fluidized beds, however, good results have been obtained by injection of ammonia in the splash zone of a bubbling bed (Amand and Leckner, 1988) as well as in the upper part of the
* To whom
correspondence should be addressed.
0000-5005/91/ 2630-2396$02.50/0
combustion chamber of a CFB (Hiltunen and Tang,1988) at temperatures as low as 800 "C, but then at the cost of high ammonia bypass and increased CO emission. Radicals produced by the combustion process may act as an alternative to those formed by the additives injected with ammonia. This may explain the effectiveness of ammonia at the low temperatures, in agreement with what has been found in flame combustion by Chen et al. (1988). In CFB boilers the presence of high particle concentrations does not permit injection of NH3in zones where combustion reactions normally take place, primarily because the injected gas carrying the NH3does not penetrate far into the particle suspension but tends to rise in a "plume" which does not spread over the cross section of the combustion chamber. Also, the particle suspension in this plume converts most of the ammonia to N2, a conclusion which can be inferred from the experiments of Amand and Leckner (1988) where neither a change in the NO concentration nor any NH3 was observed in the flue gases in spite of considerable additions of NH3 to the lower parts of a CFB combustion chamber. Thus, the low temperatures, the presence of bed particles, and the inherently low NO concentration in the flue gases of a CFB boiler make the application of ammonia injection in a CFB different from that in a flame combustion boiler. It is not obvious how and where to apply ammonia injection to CFB boilers, and therefore the purpose of the present work is to investigate both the effect of ammonia alone and that of ammonia together with additives on the NO emission of CFB boilers. The influence of the location of injection is also studied. The NO reduction with ammonia is successful only if the negative consequences are small. Thus, when ammonia and additives are injected, special attention is paid to the bypass of ammonia to the stack, to increments in the CO emission, and to changes in the emission of nitrous oxide (N@). Test Plan A comprehensive laboratory study previously performed by Duo and co-workers (1990a) forms the basis of the investigation in the 12-MWh CFB test boiler at Chalmers University of Technology. The laboratory study included an investigation of the influence of additives to ammonia 0 1991 American Chemical
Society
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2397
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and showed how the temperature window of NO reduction by ammonia is shifted toward lower temperatures by hydrocarbons and other additives. The tests were carried out in a plug-flow reactor with an inlet concentration of about 500 ppm NO in a synthetic flue gas (4% O2 in NJ. On the basis of the results of the laboratory study, the following additives were selected for the present tests to be injected with ammonia in gaseous form: hydrogen (H2), methane (CH,), ethane (C2H&,and butane (C4HI0).When ammonia in water solution was used, hydrogen peroxide (H202)or ethanol (C2H,0H) served as additives. The tests were planned to be carried out as follows: The bed temperature before the cyclone should be varied between 800 and 900 OC. Molar ratios of NH3/N0 less than 6, and of additives/NO less than 4, based on the NO concentration measured before the stack, should be injected before or after the cyclone of the boiler. Tests should be performed with and without limestone addition (for sulfur capture) to a bed mainly composed of silica sand and fuel ashes.
Experiments Equipment. Only the cyclone and the gas exit duct of the 12-MW boiler, Figure 1, are of interest for this investigation, as the combustion chamber of the boiler simply serves as a producer of flue gases and NO to the cyclone. A description of the boiler has been presented by Leckner et al. (1991). The particle-laden flue gases from the combustion chamber pass the cyclone where most particles are separated from the gas and returned to the combustion chamber. After the cyclone the flue gases continue through the short refractory-lined exit duct before they are finally cooled in the convection pass. The temperature of the gases is measured with thermocouples located at the bottom and the top of the furnace, before and after the cyclone, and along the gas pass. At the boiler load maintained during most tests, 8 MW, the temperature drops were similar constant temperature in the furnace, a drop of about 50 O C over the cyclone, and then a further drop of about 25 OC over the exit duct. In the low load case, 5.5 MW, there was a temperature drop from the bottom of the furnace to the cyclone inlet of about 70 “C.The gas residence time in the cyclone, including the entrance, is
Figure 2. System for injection of ammonia and additives. Positions A and B were used during the present tests. Table I. The Gas Analyzers set of analyzers range name stack NO 0-100, 1-250 ppm Beckman 955, chemiluminescence 0-lo00 ppm Binos, IR co &lo% Magnos ST, 0 2 paramagnetic boiler operation 0-1000 ppm Hartmann & Braun, IR co &lo% Altech, zirconium 0 2 stack. cvclone inlet a n i outlet 0-1500 ppm Uras 3G,IR SO2 NO 0-100,0-250 ppm Beckman 955, chemiluminescence W.5% Uras 7N, IR co 0-100, 0-250 ppm Spectran 647, NDIR N20
1-2 s, and in the exit duct about 0.5 s, at 8-MW load. Ammonia and additives were injected at position A or B, Figure 1. There are six nozzles at each position. In the cyclone entrance there are three nozzles at each side. Gaseous ammonia was injected with a carrier gas, recirculated flue gas, to provide sufficient momentum, whereas the ammonia in water solution was injected directly through atomization nozzles surrounded by carrier gas. Preliminary tests showed that the u?e of all the nozzles (six) was beneficial for the NO reduction in the cyclone exit, but that in the cyclone entrance two nozzles were sufficient. The central nozzles (one from each side) in the cyclone entrance were used during the main tests. The arrangement is shown in Figure 2. The flow rates of ammonia and additives were measured by flow meters or by weighing the containers. Three sets of gas analyzers, listed in Table I, were used for gas analysis. One set was always connected to the stack (after the baghouse filter of the boiler). The analyzers employed for boiler operation were also continuously sampling at their position after the convection pass (before the baghouse filter), whereas the sampling of the third set of analyzers was switched at regular intervals among stack, entrance, and exit of the cyclone. The sampling probes were stainless steel tubes, uncooled except in the cyclone positions where the probes were water-cooled. At the gas exit of the probes there were heated ceramic filters. The probes in the dust-laden
2398 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991
positions around the cyclone were back-blown at regular intervals. From the filters, the gas was conducted through a heated Teflon tube to a refrigerator. After the refrigerator there was a paper filter, a pump, and a distributor to the various gas analyzers in the set. The recently developed Spectran 647 N20analyzer was carefully tested in the laboratory before use (Hulgaard et al., 1989). The analyzer proved to have a considerable cross-sensitivitytoward methane. Therefore, the methane content of the gases was checked regularly by gas chromatography. For this purpose, gas samples were taken in Tedlar sampling bags for subsequent analysis. The measurement of the N20 concentration was within an error limit of a few parts per million. The influence of the ammonia concentration in the range of concentrations concerned was within this error limit. It was also found that the CO analyzers used were strongly affected by the concentration of N20 in the flue gases. Laboratory tests were performed in order to provide correction data. (In the case of the Binos IR analyzer the correction was as large as 60 ppm at an N20 concentration of 150 ppm.) The gas concentration measurements as well as those of the temperatures were made in fixed positions in the gas ducts, However, before the tests the ducts had been traversed for O2 and for temperature to ensure representative measurement positions. All gas concentrations were converted into (volume) parts per million at 6% O2 (dry gas) by means of the local values of the O2concentration. The ammonia concentration in the flue gas was measured by absorption in acidic water solution with an ammonium ion selective electrode, and the cyanides, after absorption in basic water solution, were measured with argentometry (Liebig-Deniges method). The duration of each test was 1-2 h with the boiler in stationary operating conditions. The data presented are average values for each test period, except for the ammonia and cyanide concentrations, which were taken only once during each test. Fuel and Solid Materials. The fuel was an Australian bituminous coal with the following mass fractions: combustibles/ash/moisture = 0.77/0.13/0.10 and C/H/O/N/S = 0.84/0.05/0.09/0.019/0.005. A Swedish limestone from Ignaberga with an average size of 0.7 mm was used. The sand was of the brand Silversand 35 (mean size 0.35 mm). Boiler Performance. The boiler was run at a constant load of 8 MW with a fluidization velocity of about 6 m/s in the upper part of the furnace. The excess air corresponds to 5% O2in the flue gases. Low load conditions, 5.5 MW and 7% 02, were also briefly investigated. A few tests were performed with a secondary to total air ratio of 0.43,but otherwise 0.27 was maintained. The calcium to sulfur molar ratio, due to added limestone Ca/S, was 0 (sand/ash bed), 1 or 3.3. As a consequence of the very low sulfur content of the coal, it was difficult to adjust exactly the small limestone feed rates to get a molar ratio of 1. This ratio therefore varied somewhat from test to test (but it was constant during each individual test). Also, because of the low sulfur content of the coal, the molar ratio of the lime of the coal ash was relatively high, (Ca/S),,, = 0.3. The bed particle concentration (sand, lime, ash, and char) varied from about 1000 kg/m3 in the bottom of the furnace to about 1 kg/m3 just before the entrance to the cyclone. The particle distribution in the cyclone is not known, but it can be assumed that the particles gather at the walls of the cyclone, leaving the central parts with a
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Figure 3. (a) NO emission versus bed temperature without ammonia injection (NH,/NO = 0). (0) 5% 0,;secondary/total air = 0.27; Ca/S = 0. (+) 5% 0,;secondary/total air = 0.43; Ca/S = 0. ( 0 )7% 0,;secondary/total air = 0.21; Ca/S = 0; 5.5 MW. ( 0 )5% 0,; secondary/total air = 0.27; Ca/S = 3.3. (w) 5% 0,; secondary/total air = 0.27;Ca/S = 1.6. (A) 5% 0,; secondary/total air = 0.27; Ca/S = 0.5. ( X ) 7% 0,;secondary/total air = 0.27; Ca/S FS: 0.9; 5.5 MW. (A) 5% 0,; secondary/total air = 0.43; Ca/S = 0; fly ash recirculation. (b) NzO emission versus bed temperature without ammonia injection (NH,/NO = 0). For symbols, see (a).
rather low particle concentration. The cyclone acts as a reactor where the CO concentration is reduced from about 1000 ppm at the entrance to about 100 ppm at the exit. Furthermore some char is burned, and the total combustion in the cyclone corresponds to a decrease in the oxygen content of the gas of about 0.5% unit of O2 Also, without ammonia addition the NO concentration is reduced by about 30% and N20 by 1-2% in the cyclone. The gas concentrations after the cyclone differ very little from those measured before the stack. The furnace produces the NO for the ammonia injection experiments in the cyclone. This has advantages and disadvantages. The advantage is that the experimental situation is realistic, close to a commercial application with realistic NO levels. The disadvantage is that the NO concentration at the location of the NH3 injection is not a directly adjustable parameter, but depends on several boiler parameters, which is illustrated in Figure 3. This figure shows the resulting emissions of NO and N20, without ammonia injection, as a function of bed temperature. Also shown is the effect of a number of other influencing factors, such as limestone addition, secondary to total air ratio, and excess air. Similar results from systematic studies in a 40-MW CFB boiler have been presented by Mjornell et al. (1991). This relationship between the NO concentration and primarily the gas temperature in the cyclone inlet (at NHB/NO= 0)should
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be taken into account when judging the results from injection of ammonia and additives. In the presentation of the results, the amount of ammonia and additives is always related to the NO concentration just before the stack. This NO concentration is also called the NO emission.
Results Ammonia Injection without Additives. When ammonia is injected, the NO emission is reduced as shown in Figures 4 and 5. The following can be concluded. There is an influence of temperature on the reduction of NO by ammonia as well as on the initial concentration of NO. The data indicate a minimum NO concentration of 20-60 ppm, which is difficult to reduce in spite of an increased rate of ammonia injection. This minimum NO concentration is dependent on the temperature and perhaps on the residence time. The principal difference in NO reduction when comparing injection before and after the cyclone at similar temperatures in both cases before the cyclone was found to be a higher minimum NO concentration level when ammonia is injected after the cyclone. Direct injection of ammonia in water solution was somewhat less efficient than injection of gaseous ammonia. When ammonia is not injected (NH,/NO = 01,there is an influence on the NO emission from limestone addition, as seen in Figure 3a. The corresponding data for Ca/S = 0 and 3.3 can be compared in Figure 4a and Figure 4b for the cases of 890 and 830 "C. When ammonia is injected, the curves of Figure 4 still have approximately the same
2 NH3/N0
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relative path, at least for NH3/N0 I2, from which it is concluded that the lime addition has had only minor, if any, influence on the NO reduction by ammonia. When injection takes place after the cyclone, the ammonia bypass exceeds 10 ppm even at injection molar ratios NH3/N0 = 1.5 (Figure 6),and the CO emission is considerably increased even at NH,/NO = 1,whereas for injection before the cyclone the ammonia bypass is small and the CO emission is not affected until the molar ratio NH3/N0 exceeds 3 (Figure 7). No significant change in the N20 emission was observed during injection of ammonia (Figure 8).
2400 Ind. Eng. Chem. Res., Vol. 30, No. 11,1991 +
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The results obtained during low load operation did not contain much additional information since the initial NO concentration was only 60 ppm. This concentration level was slightly reduced by ammonia injection.
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0
2 NH3/N0
4 6 (molar ratio)
8
Figure 8. Increase of N20 emissions versus amount of ammonia 850 "C, (0) 890 "c,injection before the injection. (A)795 "C,(0) ).( 830 "c, injection after the cycyclone; (A)735 "C,( 0 )790 "C, clone.
Ammonia Injection with Additives. Against expectations, injection of ammonia with additives had no clearly positive effect in lowering the NO concentration, as seen from Figure 9. In most cases a slight increase in the NO concentration was observed (Figure 9a-c). A decrease in the NO concentration was found in the case shown in Figure 9d, which differs from the other cases only by a slightly higher initial NO concentration. The behavior of H202and CzH50Hadded with ammonia in aqueous solution was similar to that of the additives in Figure 9. No influence of the additives on the emission of N20was observed and no cyanides were emitted. Other Results. An investigation of the influence of particle concentration on the ammonia injection was planned. The particle concentration can be increased by recirculation of fly ash from the external fly ash separator back to the furnace. A test run with a recirculation ratio of 1 (recirculated ash flow equal to fuel flow rate) was not successful, since the NO emission without injection of ammonia decreased to below 30 ppm as a consequence of the recirculation (see Figure 3a). When the ammonia injection was tried, it had no observable effect. It should be mentioned that both the CO and the NzO emission increased during the fly ash recirculation (see Figure 3b).
Interpretation of the Results a n d Discussion General. Ammonia injection does have an effect in spite of the low temperature level of the boiler compared with the well-known temperature window of 850-1100 "C. Such a temperature window was obtained in the laboratory study of Duo et al. (1990a),where also a distinct influence of additives on the displacement of the temperature window was demonstrated, again in contrast to the present boiler tests where the additives mostly appeared to have an adverse effect on the NO reduction. In order to explain these differences and to interpret the results of the boiler investigation, homogeneous gas phase chemical kinetic calculations were made with a detailed reaction scheme consisting of 251 elementary reactions and 49 chemical species. Kinetic Reaction Scheme. The reaction scheme includes the oxidation reactions of C1 and C2hydrocarbons, NHB,and HCN, together with a subset describing the reactions between hydrocarbon-derived species (CHi, HCCO, etc.) and nitrogen species (Nz, NO, NHJ. The scheme has been developed for analysis of gas-phase nitrogen chemistry in combustion and is based on the recently published mechanisms by Glarborg and Hadvig (19891, Miller and Bowman (1989), and Glarborg et al. (1986). A more detailed description of the reaction scheme, that thermodynamic data, the computer programs, etc., is available elsewhere (Kilpinen and Hupa, 1991). The scheme and its subsets of reactions have been tested against experimental data and have been shown to give a satisfactory description of the nitrogen chemistry under different conditions; see, e.g., Miller and Bowman (1989). Also during the present work the reaction scheme was analyzed in a number of preliminary calculations. First a comparison with the plug-flow measurements of Duo et al. (1990a) was carried out (Kilpinen, 1990). The measured NO reduction by ammonia was well described by the calculations. In the presence of combustible additives (Hz,CzH6,and CHI were used) the calculated optimum temperatures were shifted to lower values in the same order as given by the measurements. The conclusion from this comparison is that the scheme performed well, but quantitative differences between measurements and calculations were found (Figure 10). The best agreement was obtained with ethane, which was chosen for comparison with the boiler results. The flow pattern in the cyclone and the exit duct is complex. After passing the entrance, the gas obtains a vortex motion which ideally moves downward in the cyclone and then returns in a central vortex, leaving the cyclone with a high swirl through the exit duct. This complex structure was to be modeled with a continuously stirred tank reactor (CSTR) or an ideal plug flow reactor (PFR). It was shown to be of small importance which one of the reactor models is chosen for the calculations; the results given below are obtained with a PFR. The PFR was tested in both an isothermal and a nonisothermal mode with a temperature variation similar to that of the exit duct. The difference between the isothermal and the nonisothermal case was less than lo%, which was considered acceptable for the present purpose, and the isothermal approximation was used. Inlet gas concentrations for the calculations are given in connection with the figures below. The choice of zero initial radical concentrations (0,OH, H, etc. = 0) was made
Ind. Eng. Chem. Res., Vol. 30, No. 11,1991 2401 a I4O
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differences between measurements and calculations before and after the cyclone. Without ammonia injection, the calculations indicate a reduction of N20 by 10-50% over the cyclone, whereas only a few percent reduction of N20 was actually measured. One (hypothetical) explanation is that N20 is produced in the cyclone and that production and destruction compensate each other. Likewise without ammonia injection, the calculations show a reduction of NO (somewhat smaller than that measured over the cyclone); however, this NO reduction corresponds to a calculated increase of NO2which has not been found by measurements. (Measurements have been made in the stack under normal operating conditions during a previous test, 8 MW, 850 "C bed temperature, 5% 02,without finding any NO2 emission exceeding a few parts per million.) I t is believed, instead, that NO is reduced by CO on char particles (perhaps partly to N20and partly to N2). In fact, this explanation is given for the reduction of NO and the simultaneous increase in N20, which is quite evident in the test with fly ash recirculation, Figure 3. The influence of particles on the NO reduction is further supported by the fact that no similar reduction of NO takes place between the cyclone outlet and the stack. As a consequence of the differences between the boiler conditions and the ideal homogeneous reaction calculations, a direct comparison between calculations and the few data points from the boiler measurements, like the comparison with the laboratory results (Figure lo), becomes very uncertain. For this reason, calculations will
2402 Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 250
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Figure 11. Calculated NO, reduction during ammonia injection in flue gases at two inlet NO levels, 50 or 200 ppm NO. Calculated maximum NOz concentration 10 ppm. Flue gas composition: 13% C02;4% HzO, 6% OZ,150 ppm NzO, 77% Nz, 200 ppm CO. Molar ratio NH,/NO = 1.5. Residence time 1.0 s.
mostly be used for qualitative explanations of the results. However, a comparison with measured data points has been tried for a few cases from the exit duct where the isothermal, homogeneous, plug-flow assumption would be most applicable. The calculated data are in the form of NO, = NO + NOz, whereas measured data are always expressed as NO only (for the reason explained above). Calculation Results. Figure 11shows calculated NO, reduction with ammonia. The optimum temperature for NO, reduction is shifted to significantly lower values than the 975 "C found in the laboratory study of Duo et al. (1990a), where the NO concentration was about 500 ppm (adapted to flame combustion) (cf. Figure 10). It is evident that the optimum is shifted to lower temperatures at lower NO concentrations. This is a significant feature of fluidized bed combustion, where the NO levels are lower than those of flame combustion. In addition to the NO concentration, a significant reason for the shift of the optimum shown in the curves of Figure 11 was found to be the presence of CO in the flue gas, whereas variations in the water vapor and oxygen concentrations yield only a small change, in agreement with the observations of Duo et al. (1990a). A change in the CO concentration from 200 to 0 ppm typically shifts the optimum to about 50 "C higher temperatures, whereas a change from 200 to lo00 ppm moves the optimum to about 50 "C lower temperatures than those of Figure 11. The presence of NzO also shifts the optimum toward lower temperatures. The residence time influences the NO reduction considerably on the lower side of the temperature window, whereas on the higher side the reactions are fast enough even for the shortest times considered here (0.5 s) (Figure 12). Similar results were obtained experimentally by Duo et al. (1990b). Consequently, an increase of residence time also shifts the temperature optimum to lower values. The calculated ammonia bypass and the rise in CO concentration range from their input values at low temperatures to 0. The zero concentration is reached at a temperature that is 50-100 "C on the higher side of the temperature window. From the temperature of zero CO concentration and higher temperatures, no rise in CO concentration and no NH3 bypass are found in the calculations. At the lower side of the temperature window, both the NH3 bypass and the CO concentration increase with increasing NH3/N0 ratio and with decreasing residence time. The calculated influence of ammonia injection on the N20 emission is complex but small. At temperatures below
600
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900 1000 1100
Temperature ("C)
Figure 13. (a) Comparison of calculated NO reduction (NO,) and boiler measurements (NO) during ammonia injection with CzH, as an additive. Calculated maximum NOz concentration 95 ppm. Conditions: NH3/N0 = 2. Flue gas: 13% COz, 6% Hz0, 5% 02, 60 ppm CO, 150 ppm NO, 120 ppm NzO, rest N2 Residence time 0.5 s. Boiler data from Figure 9d: Add/NO, 0 - 0.0; 0 - 0.07;A 0.14;+ - 0.20; X - 0.63. (b) Comparison of calculated NO reduction (NO,) and boiler measurementa (NO) during ammonia injection with CzH, as an additive. Calculated maximum NOz concentration 45 ppm. Conditions: NH,/NO = 2. Flue gas: 12% COz, 6% HzO, 7% Oz,180 ppm CO, 50 ppm NO, 250 ppm NzO, rest N2 Residence time 0.5 s. Boilder data from Figure 9 b Add/NO, 0 - 0.0; 13 - 0.12;A - 0.25; + - 0.50; X - 2.0.
800 OC,a slight increase is calculated for low NH3 injection rates, but a reduction of N 2 0 is found for most other conditions. Figure 13a and Figure 13b give examples of calculated NO reductions compared with the boiler measurementa from Figure 9b and Figure 9d for injection of ammonia with ethane as an additive. In both cases the measured
Ind. Eng. Chem. Res., Vol. 30, No. 11, 1991 2403 data points are above the calculated curves. Several reasons for differences between calculated and measured values have been mentioned above; for instance, a considerable change in the NO reduction would appear if the temperature curves are only slightly shifted to higher or lower temperatures; cf. Figure 10. The calculated curves, however, do provide an idea about the relative order of the measured points. It can be seen from the curves of Figure 13 that there is an intersection point (or rather, an intersection region) where the curve showing no addition of C2H, is crossed by the curves with addition. At this point there is no effect of the addition. For temperatures above this point, additives have a negative effect on NO reduction, whereas for temperatures below this point, the effect is positive. The measurement data of Figure 13a are on the low-temperature side and those of Figure 13b are on the high-temperature side. Although no calculations have been made for the cases of Figure 9a,c, it is obvious that they are on the higher side of the intersection temperature as well as that of the temperature window. This explains the marked decrease in NO reduction with an increased additive injection, seen especially in Figure 9c. A rough comparison between the data of Figure 4a with the calculated curves of Figure 11, taking into consideration a slight displacement of the curves due to a higher CO concentration in the cyclone, again yields only a qualitative result. However, the agreement is good enough to indicate that the particles in the cyclone could not have had a major influence on the results. The probable effect of char particles on NO reduction in the cyclone is the only significant influence of particles observed. The calculated bypass of ammonia is greater than the measured one; however, the calculated results qualitatively explain the measurements. The temperatures after the cyclone are generally on the lower side of the temperature window which, according to the calculations, results in bypass of ammonia and higher CO emission. The temperatures before the cyclone lay in the present tests on the high-temperature side of the optimum. Moreover, the residence time was three to four times as long as for injection after the cyclone. Both factors lead to a low ammonia bypass and low increase in the CO emission. In this case as well, a possible effect of particles on the reactions is not needed for the explanation of the results. Practical Aspects. Why bother with SNR when emission levels less than 50 ppm can be achieved without any ammonia injection? The answer to this question depends, principally, on legislation and the emission limits in various countries, at present and in the future. Assuming that a certain legislated standard of emissions exists, Figure 3 can illustrate part of the problem. If high limestone addition is necessary in order to fulfill the SO2 emission limit, then the NO emission increases compared with the case when no limestone addition is necessary and the NO emission limit may be exceeded. If restrictions are imposed on N20 emissions, one solution would be to increase the bed temperature, but then the NO emission increases, and again an NO emission limit may be exceeded which would necessitate additional reduction of NO, for instance with SNR. In short, the question is difficult to answer in a general way. The answer is different in different countries and at different times. Conclusions In general, the injection of ammonia can contribute to a significant reduction of NO in CFB boilers, at least down to a level in the order of 60-20 ppm of NO (Figures 4 and 5).
If ammonia is injected before a cyclone of a CFB boiler, negative effects such as an increase of the CO emission and ammonia bypass can be avoided, even at molar ratios of NH3/N0 as high as 3-4 (Figures 6 and 7). The injection of ammonia with or without additives caused no significant change in the emission of N20 (Figure 8). Direct injection of ammonia in water solution seems to be less efficient than injection in the form of gas. As there are no differences anticipated for chemical reasons, the result is probably caused by physical factors such as evaporation and dispersion of the medium in the flue gas (Figures 4 and 5 ) . In most cases, the introduction of additives to ammonia is not effective in reducing NO under CFB conditions (Figure 9). In the case of flame combustion, on the other hand, additives are of importance, as shown by the experiments of Duo et al. (1990a) and Jerdal et al. (1990). Acknowledgment This work was financed by the Swedish State Power Board, the Swedish National Energy Administration, and the Finnish Combustion Research Program LIEKKI. Among many contributions to the project which are gratefully acknowledged, we would like to mention the design and operation of the injection equipment by M. Mjornell (Gotaverken Generator AB) and of the measurement equipment by A. Zarrinpour (Department of Energy Conversion). Nomenclature CFB = circulating fluidized bed SNR = selective noncatalytic reduction MW, MWu, = megawatt thermal power NDIR = nondispersive infrared IR = infrared Registry No. NO, 10102-43-9 NH3, 7664-41-7; CO, 630-08-0;
H,,1333-74-0; CH,, 74-82-8; C2Hs,74-84-0;H(CHP)IH, 106-97-8; H202, 7722-84-1; EtOH, 64-17-5.
Literature Cited Amand, L.-E.; Leckner, B. Ammonia Addition for NO, Reduction in Fluidized Bed Boilers; First European Conference on Zndustrial Furnaces and Boilers; JNFUB Lisboa, March 1988. Chen, S. L.; Cole, J. A.; Heap, M. P.; Kramlich, J. C.; McCarthy, J. M.; Pershing, D. W. Advanced NO, Reduction Processes Using -NH and -CN Compounds in Conjunction with Staged Air Addition. Twenty-Second Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1135-1145. Duo, W.; Dam-Johansen,K.; 0stergaard, K. Widening the Temperature Range of the Thermal DeNO, Process, An Experimental Investigation. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990a; pp 297-303. Duo, W.; Dam-Johansen, K.; 0stergaard, K. Kinetics of the Gasphase Reaction between Nitric Oxide, Ammonia and Oxygen. CHEC Article no. 9002. Department of Chemical Engineering, Eng., The Technical University of Denmark. Presented at the First CIC Congress, 40th Canadian Chemical Engineering Conference, Halifax, 1990b. Glarborg, P.; Hadvig, S. "ReactionDatabase/The Chemical Kinetic Model";NGC-Report NGC89/FM/141; Nordic Gas Technology Centre; 1989 (in Danish with an English abstract). Glarborg, P.; Miller, J. A.; Kee, R. J. Kinetic Modelling and Sensitivity Analysis of Nitrogen Oxide Formation in Well-Stirred Reactors. Combust. Flame 1986, 65, 177-202. Hiltunen, M.; Tang, J. T. NO, Abatement in Ahlstrom Pyroflow Circulating Fluidized Bed Boilers. In Circulating Fluidized Bed Technology ZI; Basu, P., Large, J. F., Eds.; Pergamon Press: Oxford, 1988; pp 429-436. Hulgaard,T.; Dam-Johansen, K.; Karlsson, M.; Leckner, B. Evaluation of the Performance of an N20 Analyser; First Topic Or-
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iented Technical Meeting; International Flame Research Foundation: Amsterdam, 1989. Jodal, M.; Nielsen, C.; Hulgaard, T.; Dam-Johansen, K. Pilot-Scale Experiments with Ammonia and Urea as Reductants in Selective Non-Catalytic Reduction of Nitric Oxide; Twenty-Third Symposium (International)on Combustion; The Combustion Institute: Pittsburgh, PA, 1990;pp 237-243. Kilpinen, P. Kinetic Modelling of Gas Phase Nitrogen Chemistry in Combustion. Thesis of Techn. Lic., Combustion Chemistry Research Group, Abo Akademi University, Report 90-13, 1990. Kilpinen, P.; Hupa, M. Homogeneous N20 Chemistry at Fluidized Bed Combustion Conditions: A Kinetic Modelling Study. Combust. Flame 1991,85,94-104. Leckner, B.; Golriz, M. R.; Zhang, W.; Anderson, B.-A,; Johnsson, F. Boundary Layers-First Measurements in the 12 MW CFB
Research Plant a t Chalmers University. The Eleuenth Znternational Conference on Fluidized Bed Combustion; ASME: Montreal, 1991;pp 771-776. Lyon, R. K. US Patent 3900554,1975. Miller, J. A.; Bowman, C. T. Mechanism and Modelling of Nitrogen Chemistry in Combustion. Prog. Energy Combust. Sci. 1989,15, 287-338. Mjornell, M.; Leckner, B.; Karlsson, M.; Lyngfelt, A. Emission Control with Additives in CFB Coal Combustion. The Eleventh International Conference on Fluidized Bed Combustion; ASME Montreal, 1991;pp 655-664.
Received for review February 13, 1991 Revised manuscript receiued July 10,1991 Accepted July 29,1991
Some Experimental Liquid Saturation Results in Fixed-Bed Reactors Operated under Elevated Pressure in Cocurrent Upflow and Downflow of the Gas and the Liquid Faigal Larachi, Andre Laurent,* Gabriel Wild, and No61 Midoux Laboratoire des Sciences du Ggnie Chimique, CNRS-ENSIC-INPL, BP451-I, Rue Grandville, 54001 Nancy Cgdex, France
The effect of pressure (0.3 IP/MF'a 5 5.1) on the total liquid saturation (measured by RTD method) of a fixed bed operated with cocurrent gas and liquid in both upflow and downflow is investigated. For upward flows, the liquid saturation is greater than for downward flows regardless of the operating pressure. However, in the pulsing flow regime and a t high gas velocities, the same asymptotic value is observed for both flow directions. The liquid saturation increases with pressure, mass flow rates being constant, but decreases when the liquid viscosity is decreased, independently of the flow direction and the operating pressure. For low gas velocities (uG < 1cm/s), the total liquid saturation no longer depends on the pressure. For higher gas velocities (uG > 1-2 cm/s) and nonfoaming liquids, the drift flux can provide an acceptable estimation technique of the liquid saturation if experiments under high pressure could not be conducted.
Introduction Historically, refining industries and petrochemical processes have widely used trickle-bed reactors, i.e., fixed beds with gas and liquid flowing cocurrently downward throughout a catalyst bed. Nevertheless, in many processes, such reactors, compared with fixed beds with an upward flow of gas and liquid (flooded-bed reactors), present drawbacks from the point of view of the heat transfer and the wetting efficiency. Indeed, with an upward flow of the two phases (both gas and liquid), under similar conditions, because of the high liquid saturations encountered for fixed beds with this flow configuration, risks due to hot spots appearance may be avoided. Moreover, the following other advantages of the floodedbed reactors over trickle beds have been described: better fractional wetting (Goto and Mabuchi, 1984), higher conversions (Snider and Perona, 1974),higher selectivity and catalyst life and less residue production as in selective hydrogenation of diolefin compounds (Ragaini and Tine, 1984), and better residence time and liquid distribution throughout the catalyst bed (Montagna and Shah, 1975). Actually, only a few papers in the literature deal with the experimentaldetermination of the liquid saturation under high-pressure conditions in fixed-bed reactors with twophase gas-liquid upward and downward flow. This paper presents original data on the liquid saturation (measured from tracer injection in the liquid phase) up to 5.1 MPa in a fixed bed operated in both upflow and downflow. We describe here successively the influence of the operating pressure, the gas and liquid throughputs, the flow direction, and the nature of the gas phase on the liquid
saturation. The drift flux concept, initially proposed in the modeling of bubble columns and fluidized beds, is applied here to high-pressure fixed-bed reactors; the drift flux used can be seen as the superficial velocity of the gas phase relative to a frame represented by both fluid phases flowing into the section offered to the gas. A more detailed description of this drift flux is given in the discussion of the results. Even though the type of reactor investigated here is quite different from the bubble column or from the three-phase fluidized bed, the drift flux concept proves to be a potent tool also in the investigation of cocurrent fixed-bed reactors. By using simple correlations of the drift flux obtained at atmospheric pressure, it is possible to get rough estimates of the liquid saturation at high pressures without carrying out high-pressure experiments, as long as the liquid does not exhibit foaming behavior and the gas superficial velocities are larger than 1-2 cm/s.
Brief Literature Survey In a recent paper (Larachi et al., 1991), the few references concerned with two-phase pressure drop and liquid saturation measurements in pressurized trickle-bed reactors have been cited: one has to mention the excellent experimental work of the team of Twente University (Wammes et al., 1990,1991). Unfortunately, there is even less research dealing with the hydrodynamics of fixed beds with upward two-phase flow under high pressure, and to our knowledge, only one published work has compared liquid saturation results in fixed beds with both upflow and downflow (Turpin and Huntington, 1967). These authors showed that, at least for nearly atmospheric con-
0888-5885/91/ 2630-2404$02.50/0 0 1991 American Chemical Society
