Ind. Eng. Chem. Res. 1995,34, 4531-4539
4531
Experimental Study of the Influence of the Operating Variables on Natural Gas Reburning Efficiency Rafael Bilbao,*Maria U. Alzueta, and Angela Millera Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain
The influence of the main operating conditions in the reburning zone using natural gas on the NO, HCN, and NH3 concentrations obtained at the exit of this zone has been analyzed. Experiments with a wider range of variable values than those presented in the literature have been performed. The variables analyzed have been the temperature (1200-1500 "C),the natural gas concentration (1-4.5%) or reburning fuel fraction (0.08-0.50), the oxygen concentration at the inlet of the reburning zone (0-5%), the stoichiometric ratio in the reburning zone (0.751.05), the primary NO concentration (100-1200 ppm), and the gas residence time (94-280 ms).
Introduction The process of reburning with natural gas has been demonstrated to be an attractive technique that can be applied to reduce the NO, emissions in coal combustion systems. This process can be implemented with low investment costs and jointly with other NO, and/or SOZ reduction methods (Bartok et al., 1990; Bowman, 1992). The reburning process divides the coal boiler into three zones (Figure 1). In the primary zone the coal combustion is produced with a slight air excess, and the NO, and other typical combustion products are generated. Downstream the reburning fuel is added in the so-called reburning zone where the NO, products are partially reduced, by the action of hydrocarbon radicals, to NZ and other unwanted intermediate nitrogenous Finally, additional air species (mainly HCN and "3). is injected into the burnout zone in order t o eliminate any remaining fuel fragment. The efficiency of the reburning process depends appreciably on the use of suitable operating conditions in the reburning zone. The efficiency of the reburning zone depends on the values of different variables, mainly the temperature, the amount of natural gas introduced, the gas residence time, the mixing conditions, and the concentrations of oxygen and NO in the gases coming from the primary zone. Different studies have been carried out to analyze the influence of these variables on the NO, reduction by natural gas reburning. In general, most of the studies have been centered on specific operating conditions, which could be the reason for some of the discrepancies observed. It is accepted that temperature exerts a significant influence on NO, reduction. It affects the formation of the radical species (the concentration and the mechanism of generation) and the reaction rates involved. An increase in the temperature produces a higher formation of radical species, such as CHi (Thorne et al., 19861, and 0, OH, and H (Knill and Morgan, 19891, which influences respectively the reactions with NO, and the oxidation reactions of volatile fragments and the conversion of intermediate nitrogenous species t o Nz. There are some discrepancies concerning the temperature necessary to obtain a good reburning efficiency (Fujima et al., 1990; Lanier et al., 1986; Mereb and Wendt, 1994).
* To whom correspondence should be addressed.
Burnout Zone
Air 4
Coal
+ Air
4
' 7
w
I
Primary Zone
Figure 1. Reburning with natural gas in a coal boiler.
The amount of natural gas introduced is an important parameter for the efficiency of the process, and its influence is related to the amount of oxygen available in the reburning zone. Both factors can be taken together in the influence of the stoichiometric ratio in the reburning zone, SRZ. Different optimum values appear in the bibliography for SR2, such as lower than 0.9 (Kremer et al., 1990; Lanier et al., 1986; Miyadera, 1990;Toqan et al., 1987)or between 0.9 and 0.95 (Burch et al., 1991; Chen et al., 1986; Kolb et al., 1988; May and Krueger, 1992; Miyamae et al., 1986; Okigami et al., 1986). These discrepancies could be due to the effect of different process parameters, such as the injection method of the reburning fuel and therefore the degree of mixing reached, the air excess used in the primary combustion zone, the different characteristics of the fuel combusted in the boiler, and the reburning temperature. There are also some discrepancies with respect to the efficiency of the reburning process when the NO concentration proceeding from the primary zone, (NO),, is varied. While some authors (Chen et al., 1986; Lanier et al., 1986) consider this process ineffective for NO concentrations lower than 150 ppm, Fujima et al. (1990) have obtained significant reduction for lower (NO), concentrations. The gas residence time in the reburning zone is an important variable that, because of the dimension requirements, could be a limiting factor in the retrofit application of the reburning process to existing boilers. Different values of necessary residence times have been reported in the literature (Chen et al., 1986; Fujima et al., 1990;Kolb et al., 1988; Lanier et al., 19861, although it is necessary to take into account whether these values correspond to the time necessary only for chemical reaction, or if they also include the time necessary to reach a good gas mixing.
0 1995 American Chemical Society
4532 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
7-j~ HOWcontrollers
Mixing zone
HOWcontrollers
tt Natural gas
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8001
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ce
NO
0
A
Electric furnace
0
0
300
?
1200°C 1300'C 1400°c 1500OC
600 900 1200 Reactor length (mm)
1500
Figure 3. Temperature profiles inside the reburning reactor.
I Gaschromatograph I Figure 2. Experimental installation.
Taking into account the studies already existing in the literature, the aim of this work is to obtain a wider and more detailed knowledge of the influence of the above-mentioned variables on the efficiency of the reburning zone when natural gas is used as a reburning fuel. An experimental parametric study has been performed, extending the ranges of variable values with respect to those that appear in the bibliography.
Experimental Method The experimental installation (Figure 2) consists basically of a reaction system, a gas feeding system, and a gas analysis system. The reaction system includes a ceramic reactor heated by an electric furnace that allows us to reach temperatures up to 1500 "C. The reactor is an alumina tube of 20 mm inside diameter and 2500 mm in length. Some previous experiments were performed in order to determine the longitudinal temperature profiles inside the reburning reactor for different system temperatures and flow rates; t o determine the possible formation of thermal NO a t different temperatures, gas flow rates, and oxygen concentrations; and to establish the range of temperatures where the NO reduction is feasible in the reburning process. Experiments t o determine the longitudinal temperature profiles inside the reburning reactor were carried out at system temperatures between 1200 and 1500 "C and nitrogen flow rates ranging between 600 and 1200 NL/h (referring to 1 atm and 0 "C). An example of the temperature profiles obtained for a gas flow rate of 900 NL/h and different system temperatures is shown in Figure 3. The results obtained showed the existence of significant temperature profiles inside the reburning reactor, the shape of these profiles being very similar for all the system temperatures and flow rates studied. There is a central zone ( ~ 8 0 mm 0 in length) where the temperature approximates the system temperature and where the influence of the gas flow rate on the temperature is not significant. Experiments were also performed using gas mixtures of similar composition to those corresponding to some reburning experiments. In
these experiments, the temperature was measured at different points of the reburning reactor. When a low oxygen amount was used in the experiments, no significant different temperatures were observed compared with those obtained with nitrogen, although a small increase in the temperatures was produced in an oxidizing environment. Given these results, a length of 800 mm was considered as the reaction zone length and the temperature in this zone as the nominal temperature of the experiment. The thermal NO formation was analyzed considering system temperatures in the range between 1200 and 1500 "C and gas flow rates between 600 and 100 NL/h in dry basis. A gas mixture consisting of 0 2 (1, 2, and 5%) and N2 to fulfill the gas balance in dry basis was introduced into the reactor. Steam was added to the total gas flow rate, representing an extra 6%. It was observed that, in these conditions, the NO formation is only appreciable at temperatures higher than 1350 "C, being significant for temperatures close to 1500 "C. Moreover, the influence of the flow rate is not significant, merely obtaining a slight increase in the NO concentrations as the flow rate decreases. Similar concentrations of NO were obtained for oxygen concentrations of 1 and 2% at the different temperatures, and a small increase in these concentrations was observed for an oxygen concentration of 5%from 1350 "C. Thus, although a certain oxygen level is necessary for thermal NO formation, the temperature seems the most important variable that influences this formation. Finally, from experiments performed at different temperatures it was observed that a significant NO reduction is obtained for temperatures of 1200 "C and higher, although for temperatures close to 1100 "C some NO reduction is also achieved. The reburning experimentation was carried out by simulating the gas composition exiting from the primary combustion zone of a coal boiler and adding natural gas as a reburning fuel. Thus, in the reburning experiments, a gas consisting of 0 2 , COa, and Nz was prepared and bubbled into a water container, in order to reach a desired moisture at a given temperature. The gas was later mixed with NO and natural gas, and the mixture was fed into the reburning reactor. In all the experiments, a fixed concentration of C02 (20%) in dry basis was used, and the amount of steam added represented an extra 6% of the total flow rate in dry basis. These concentrations of C02 and steam were used because they are representative of those obtained in several Spanish coal power plants.
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4633 In order t o study the influence of the different variables, different value intervals of these variables have been chosen. The natural gas concentration at the inlet of the reburning zone was varied between 1 and 4.5% (dry basis). This concentration can also be referred to by the term reburning fuel fraction ( ~ R B )which is the fraction, in energetic content, of the reburning fuel with respect to the total amount of fuel used in the boiler. The values of natural gas concentrations used correspond t o a reburning fuel fraction between approximately 0.08 and 0.5, when a coal with a heating value of 3850 kcaVkg is used. The natural gas used was analyzed daily, showing an average composition of 90.5% CH4, 8.5% CzHs, 0.5% C3H8, 0.4% Nz, and 0.1% C, ( n = 4-7). The oxygen concentration at the inlet of the reburning zone was ranged between 0 and 5% (dry basis), values which correspond to the use of an air excess in the primary zone between approximately 0 and 30%. From the natural gas and oxygen concentration values considered, the value of the stoichiometric ratio in the reburning zone, SR2, varied between 0.75 and 1.05. Taking into account the possible NO amount produced in existing pulverized coal combustors, the primary NO concentration in the gas entering the reburning zone was varied between 100 and 1200 ppm. The reburning temperature used in the different experiments ranged between 1200 and 1500 "C. The gas flow rates used in the different experiments varied between 1500 and 600 NLh, which correspond to gas residence times in the reaction zone considered between 94 and 280 ms depending on the temperature. These residence time values have been calculated considering the length of the reaction zone mentioned above ( 4 0 0 mm) where the temperature can be considered as a constant. The residence times refer only to reaction times, because it is assumed that the gas enters the reaction zone already well mixed. Logically, some reburning reactions could to some extent be produced at both ends outside the considered reaction zone. However, the temperatures upstream and downstream the considered reaction zone are low, and the NO reduction obtained for these low temperatures is small. Therefore, the contribution of the reactions corresponding to those zones can be considered negligible. The analysis of the products at the outlet of the reburning zone was performed using different methods. The contents of NO, (NO and NOz), 0 2 , CO, and COZ were measured by continuous analyzers. 0 2 , H2, N2, CO, C02, CHI, and other hydrocarbons were determined by gas chromatography. The concentration of NH3 was determined by passing a known volume of gas through an aqueous acid solution, which was subsequently analyzed using the Nessler colorimetric method (Clesceri et al., 1989). The concentration of HCN was also determined by collecting a gas sample in an aqueous basic solution and analyzing it using the barbituricpyridine colorimetric method (Clesceri et al., 1989).
Results and Discussion The influence of the different variables on the efficiency of the reburning zone has been performed by taking into account the output concentration in this zone of NO, HCN, and "3. The concentration of NH3 in the outlet of the reburning zone was, in all the experiments carried out, negligible compared to the concentration of total fixed
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Figure 4. NO concentration versus natural gas concentration for different temperatures.
+
nitrogen, TFN (NO i-HCN "3). Therefore, the 3" results have not been included. Some authors agree with the fact that the NH3 concentration is not important with respect to the rest of the nitrogenous species, representing less than 5% of the total fixed nitrogen concentration when natural gas is used as reburning fuel (Kristensen et al., 1992; Miyadera, 19901,although it has also been mentioned that the amount of NH3 formed from NO could reach values close to 20% of the TFN concentration (Burch et al., 1991; Fujima et al., 1990). The reburning fuel fraction ( ~ R B ) used, or its corresponding natural gas concentration at the inlet of the reburning zone, is a very important parameter in order to analyze the efficiency of the process, as well as its economy. Moreover, this is an easy parameter to modify during the operation of a coal boiler. The influence of the natural gas concentration at the inlet of the reburning zone on the NO and HCN concentrations has been analyzed. Figure 4 shows the NO concentrations obtained at different temperatures and natural gas concentrations using an oxygen concentration of 2%. It can be observed that the NO concentration diminishes when the amount of natural gas introduced is increased, until a given value of this parameter. The value of this parameter will depend on the oxygen concentration existing in the reburning zone, being 1.7% for an oxygen concentration of 2%. For low natural gas concentrations, oxygen is in excess and the formation of CO and COz could be favored compared to the formation of the hydrocarbon radicals necessary to react with NO. In addition, the oxygen existing in the reaction zone could oxidize the nitrogenous intermediate species formed. Therefore, high NO concentrations are obtained. For high natural gas concentrations different trends are obtained depending on the temperature and the oxygen concentration. For the highest temperatures (2' 2 1400 "C), the NO reduction always remains high when the natural gas concentration increases, and it is practically independent of the oxygen concentration. This fact could be due to the large formation of hydrocarbon radicals which is obtained at these temperatures through the reactions of partial oxidation and thermal decomposition of natural gas (Back and Back, 1983; Myerson, 1974). For a temperature of 1200 "C and with an oxygen concentration of 2% (Figure 41, the amount of unreacted NO increases when the natural gas concentration is increased. The natural gas concentration value necessary to obtain the minimum NO concentra-
4534 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
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Figure 5. HCN concentration versus natural gas concentration for different temperatures.
Figure 6. NO concentration versus 02 concentration for different temperatures.
tion would correspond to those operating conditions in which the maximum natural gas decomposition is obtained t o give the highest concentration of hydrocarbon radicals (Bilbao et al., 1994). For a temperature of 1300 "C and an oxygen concentration of 2%, the tendency observed is similar t o those obtained for higher temperatures (Figure 4), but for lower oxygen concentrations ( ~ 1 . 5 %the ) ~NO reduction presents a minimum for given natural gas concentration values. From these results, it can be deduced that, in order to achieve a high NO reduction, it is necessary to introduce a certain minimum amount of natural gas, the amount depending on the oxygen concentration available. At the lowest temperature and again depending on the oxygen concentration, there is an optimum value of natural gas concentration that allows us to achieve the minimum NO concentration. With respect t o the HCN concentration, Figure 5 shows the results obtained for different temperatures and natural gas concentrations. The HCN concentration increases as the natural gas concentration increases. In this work, it has been experimentally observed that, in the operating conditions considered, the formation of HCN through the reactions corresponding to the fixation of Fenimore (1972) is not significant. The increase in the concentration of HCN when the natural gas concentration increases could be explained by taking into account that HCN is formed from NO. For a given oxygen concentration, an increase in the natural gas concentration originates a more reducing environment and a lower amount of oxygen is available for further HCN oxidation. For given natural gas and oxygen concentrations, an increase of the temperature causes a diminution of the HCN concentration, owing mainly to a higher rate of HCN oxidation reactions. The influence of the oxygen concentration in the inlet of the reburning zone has also been studied. The oxygen concentration that reaches the reburning zone from the primary combustion zone depends, logically, on the air excess used in the coal combustion. This is a very important variable that can influence appreciably the decomposition of natural gas by oxidation, and therefore the formation of hydrocarbon radicals. A certain oxygen concentration in the reburning zone seems to be necessary t o allow the natural gas decomposition to produce hydrocarbon radicals. In the absence of oxygen the decomposition would take place through pyrolysis reactions which are slow at low temperatures and can be significant at high temperatures (Lanier et al., 1986;
Myerson, 1974). Moreover, the presence of oxygen is also necessary to promote the reactions between oxygen and HCN or other nitrogenous intermediate species (originated by the reaction of hydrocarbon radicals with NO), finally producing molecular nitrogen (Glarborg, 1990;Miller et al., 1984). In this oxidation, NO can also be produced, and the product distribution in the output of the reburning zone will be dependent on the oxygen availability. Figure 6 shows the NO concentration obtained at different temperatures and oxygen concentrations for a given natural gas concentration of 1.7%. For temperatures of 1200 and 1300 "C and for low oxygen concentrations, the concentration of NO is high, and an increase in the oxygen concentration causes a sharp diminution of the NO concentration down to a minimum value. Therefore, it can be deduced that at these temperatures there is a minimum value of oxygen concentration that needs to be present to obtain a high NO reduction, because of the necessity of oxygen for the formation of hydrocarbon radicals. The amount of oxygen necessary depends on the reburning temperature, being lower for 1300 "C than for 1200 "C. For the highest temperatures, the minimum in the concentration of NO is not so clear, because even for low oxygen concentrations the output concentration of NO is low. This fact could be due to the high decomposition of natural gas observed even for very low oxygen concentrations (Bilbao et al., 1994). For all the temperatures studied, the NO concentration begins to increase from the same value of oxygen concentration (2%in this case, for a natural gas concentration of 1.7%),and the values are similar t o the inlet NO concentration for oxygen concentration values higher than 4%. In these conditions, a t a high oxygen level, the formation of NO is clearly favored as opposed t o its destruction, and for the highest temperatures the reactions of thermal NO formation can become significant. HCN is an intermediate product. It is formed from the reaction between NO and hydrocarbon radicals and can be oxidized afterward. For a given natural gas concentration, the HCN concentration will be dependent on the amount of oxygen existing in the reburning zone. Figure 7 shows the results obtained for different temperatures and oxygen concentrations. For temperatures of 1400 "C and higher, the HCN concentration always diminishes when the oxygen concentration is increased, because of the enhancement of HCN oxidation reactions. For temperatures of 1200 and 1300 "C, a maximum in the HCN concentration is obtained for oxygen concen-
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4535
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Figure 8. Ratio HCN/Reacted NO versus 02 concentration for different temperatures.
trations ranging between 1 and 1.5%. At these temperatures and for very low oxygen concentrations, few hydrocarbon radicals are generated and a low amount of NO reacts in the reburning zone, and therefore low HCN concentrations are obtained. For all the temperatures studied, the amount of HCN in the output of the reburning zone is very low for oxygen concentrations higher than 3%. In any case, for high oxygen concentrations the efficiency of the reburning process decreases considerably, because a significant amount of NO is obtained (Figure 6)and only a slight variation of the total fxed nitrogen concentrations is observed before and after the reburning process. The HCN concentration can be also analyzed from the ratio between the HCN concentration and the amount of NO that has disappeared. Figure 8 shows the selectivities to HCN for different temperatures and oxygen concentrations. It can be observed that for all temperatures, even 1200 and 1300 "C, the selectivity diminishes when the temperature and the oxygen concentration increase. These results confirm the fact that HCN is an intermediate product between the NO reduction and the HCN oxidation and that HCN oxidation is enhanced when the oxygen concentration and the temperature are increased. The influences on the NO and HCN concentrations of both the amounts of natural gas and of oxygen existing in the reburning zone are closely connected. A parameter that can be used t o analyze the influence of both variables is the stoichiometric ratio in the reburning zone, SRZ. This ratio quantifies the environment (oxidizing or reducing) existing in the reburning zone,
0.84
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0.92
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SR 2 Figure 10. NO reduction versus SR2 for a temperature of 1200 "C and different 0 2 concentrations.
and it is defined as the ratio between the amount of oxygen available for reaction and the amount of oxygen necessary to obtain the complete combustion of the species that enter the reburning zone. The influence of SRZon the NO reduction for different temperatures and oxygen concentrations has been analyzed. The results corresponding to the highest temperatures (2' 2 1400 OC)'are shown in Figure 9. It can be observed that a significant NO reduction is achieved until a given value of SRZof approximately 0.93. From that value, the NO reduction diminishes when SRZ increases, owing to the highly oxidizing environment existing in the reburning zone. The high NO reduction achieved in greatly reducing environments could be related to the fact that, at these temperatures, the formation of hydrocarbon radicals begins to depend only slightly on the oxygen availability, the reactions of natural gas pyrolysis being dominant (Bilbao et al., 1994). It has also been observed that the NO reduction only depends on the SRZvalue, being practically independent of the oxygen concentration. From these results it can be concluded that, for these high temperatures, SRZcan be used as a representative parameter that includes the influence of natural gas and oxygen concentrations on the NO reduction. At the lowest temperatures, the influence of SRz on the NO reduction depends on the oxygen concentration. A n example is shown in Figure 10 for a temperature of 1200 "C. For low oxygen concentrations, a maximum in the NO reduction is obtained for a SR2 value of approximately 0.95. When the oxygen concentration increases, the maximum becomes less clear and the
4536 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 500 400
100
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f T = 1300°C (NO) p = 900 ppm Q=900NVh t = 170 ms
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results approach those obtained with high temperatures. A similar tendency has been obtained for a temperature of 1300 "C, until the effect of the oxygen concentration becomes unnoticeable for values higher than 2%. These results corroborate the necessity, at these temperatures, for a certain oxygen level in the reburning zone in order to begin the natural gas decomposition and the subsequent formation of hydrocarbon radicals. With respect t o the influence of SRZ on the HCN concentration, similar trends are obtained for all the temperatures studied. As an example, Figure 11shows the results corresponding to a temperature of 1300 "C and different oxygen concentrations. The HCN concentration diminishes significatively when the SRZvalues increase. For the highest values of SRz, an increase of the oxygen concentration causes a decrease in the HCN concentration obtained. At temperatures of 1300 "C and higher, in general for low SRZvalues (reducing environment) most of the reactive nitrogen is HCN, and for high SRZvalues (a slight oxidizing environment) almost all the reactive nitrogen is NO, because in these conditions NO formation could be favored instead of its destruction (Bose et al., 1988). This effect could be due to the high oxygen level, which could be responsible for the inhibition of the formation of hydrocarbon radicals, the oxidation of the intermediate nitrogenous species coming from the primary zone being favored, and the thermal NO formation, especially for the highest temperatures studied. The influence of the NO concentration coming from the primary zone, (NO),, on the efficiency of the reburning zone has been also analyzed. Similar trends have been obtained for all the temperatures studied. With respect t o the NO reduction values, Figure 12 shows the results obtained for different temperatures and (NO), values for a SR2 value of 0.938. For high values of primary NO concentration, (NO), L 700 ppm, the NO reduction is almost independent of the (NO), value. This fact has already been mentioned by other authors (Fujima et al., 1990; Laksa et al., 1990). When the (NO),values decrease, the NO reduction diminishes. For low (NO), concentrations other reactions apart from the NO reduction can become important and produce a subsequent diminution in the efficiency of the reburning process. One example of such reactions could be the conversion of the nitrogen contained in the combustion air, which through reactions with hydrocarbon radicals and through the HCN intermediate could give NO
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(Lanier et al., 1986), although no evidence of these reactions has been experimentally observed in this work. Moreover, it is observed that, in general, as the temperature increases, the NO reduction also increases. This trend changes when the temperature exceeds 1450 "C, because less reduction is obtained. This fact could be due to the increasing relevance of the reactions of thermal NO formation at the highest temperatures. Although some authors (Chen et al., 1986; Lanier et al., 1986) consider that the reburning technique is not effective for (NO), values lower than 150 ppm, Figure 12 shows, as an example, that for a temperature of 1300 "C a NO reduction higher than 75% is obtained for a (NO), concentration of 100 ppm, these results coinciding with those obtained by Fujima et al. (1990). An example of the influence of the (NO), values on the concentration of NO and HCN obtained is shown in Figure 13 for a temperature of 1300 "C. It can be observed that, for given reburning operating conditions, the output NO and HCN concentrations increase when the inlet NO concentration increases. Similar results have been obtained for the other temperatures studied. The influence of (NO), on the efficiency of the reburning zone can be also analyzed using the concentration of total fixed nitrogen (TFN) obtained. The effect of the (NO), concentration on the concentrations of TFN is shown in Figure 14, using the ratio TFN/(NO),. For temperatures of 1200 and 1300 "C, this ratio presents a maximum. This maximum is obtained for lower (NO), concentrations when the temperature increases from 1200 t o 1300 "C. For higher temperatures and for the
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4537 100 0
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(NO), concentrations considered, the ratio TFN/(NO), diminishes as the (NO), concentration increases. The maximum reburning efficiency is obtained for high (NO), concentrations, although it is important to consider that in all cases the TFN values are low, ranging between 35 and 410 ppm. From the results obtained, the natural gas reburning process seems to be a good NO reduction method with a high reduction potential when the initial NO concentration is high, as usually occurs in pulverized coal combustion systems. Logically, in order to obtain a low concentration of nitrogenous species at the output of the reburning zone, it is advantageous to minimize the NO formation in the primary combustion zone and to start with values of (NO), which are not too high, because the final emission of nitrogenous species will depend on the concentration of (NO),. It is also important to analyze the influence of the gas residence time in the reburning zone on the efficiency of this zone. In a real boiler the residence time must be considered as the sum of the mixing time between the products coming from the primary combustion zone and the reburning fuel and the reaction time of those products. Taking into account that the mixing time will depend on the specific industrial case, in this work it has been assumed that the products arrive at the reaction zone already well mixed, and therefore the residence time can be considered as the reaction time. The mixing time should be considered and analyzed in each individual application of the reburning process to a given boiler. The necessary reaction time to obtain a high efficiency in the reburning process seems to be dependent on the local environment existing in the reburning zone (Glarborg, 1990). This environment is determined by the oxygen availability and by the level of hydrocarbon radicals existing, and therefore by the amount of reburning fuel added. Figure 15 shows the results of NO reduction for different temperatures and residence times. The NO reduction usually increases as the temperature increases, this effect being attenuated for the highest temperatures. The dependence of the NO reduction on the residence time is significant for a temperature of 1200 "C, but for higher temperatures the variation in the NO reduction values is small for the different residence times considered. The concentrations of HCN for different temperatures and residence times are shown in Figure 16. The HCN concentration diminishes slightly when the residence
0.14
0.18
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0.22
0.26
0.30
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Figure 15. NO reduction versus the gas residence time for different temperatures.
1.7 % natural gas 300 0
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1200oc 1300°C 1400T 1450 "C
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Figure 16. HCN concentration versus the gas residence time for different temperatures.
time increases. It also diminishes when the temperature increases, being almost negligible at a temperature of 1450 "C. The minor effect of the residence time on the concentration of nitrogenous species a t the output of the reburning zone could be due to the values of residence time considered in this work, being values which allow a good efficiency of the reburning process in the conditions studied. When the residence time decreases, a diminution in the NO reduction is observed only for a temperature of 1200 "C, which could be due t o the smaller amount of hydrocarbon radicals originating from the natural gas to react with the NO. In general, residence times of approximately 150200 ms allow the obtention of good results, NO reduction exceeding 80% and TFN concentration being less than 300 ppm.
Conclusions
A significant NO reduction can be obtained when natural gas is used as a reburning fuel and temperatures of 1200 "C or higher are used. The diminution in the concentration of NO can be accompanied by the formation of significant amounts of intermediate species such as HCN. The NH3 concentration obtained has been negligible compared to the total fixed nitrogen concentration. An increase in the reburning temperature produces a higher eKiciency of the reburning process, although a very high temperature could enhance the formation of significant amounts of thermal
4538 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995
NO. The optimum range for the temperature could be considered between 1300 and 1400 "C. At the lowest temperature, 1200 "C, a minimum value of oxygen concentration t o obtain a high NO reduction is necessary, because of the necessity of oxygen for the formation of hydrocarbon radicals. The minimum value diminishes when the temperature increases. For a given natural gas concentration and for all temperatures the NO concentration begins to increase from the same value of oxygen concentration. It is also necessary to introduce a minimum amount of natural gas to achieve a high NO reduction. This amount will depend mainly on the oxygen concentration at the inlet of the reburning zone. For high temperatures, the stoichiometricratio in the reburning zone, SR2, can be used as a representative parameter that includes the influence of natural gas and oxygen concentrations on the NO reduction. For these temperatures, a significant NO reduction is achieved until SR2 M 0.93, diminishing for higher SR2 values, while the HCN concentration decreases when SR2 increases. It can be concluded that, for high temperatures, 0.93 is an optimum SR2 value. For low temperatures, the influence of SR2 on NO reduction depends on the oxygen concentration. For low oxygen concentrations a maximum NO is observed for SR2 M 0.95, and this optimum value approaches 0.93 when the oxygen concentration and temperature increase. For high values of primary NO concentration, (NO), > 700 ppm, the NO reduction is almost independent of the (NO), value. For accurate operating conditions, the efficiency diminishes slightly when the (NO), value decreases, even though a relatively high NO reduction, 75%, is achieved for low (NO), values (5100 ppm). The efficiency of the reburning process increases as the residence time increases, although the influence of this parameter in the interval of residence times studied (94-280 ms) is not very significant. Reaction times of 120-200 ms seem to be suficient to obtain high NO reductions. Besides these values, the mixing time should be considered in each specific application of the natural gas reburning process to a given boiler.
Acknowledgment The authors express their gratitude to the ENAGAS, ENDESA, and SEVILLANA companies, to OCIGAS, OCIDE, and CICYT (Project AMB92-0888) for providing financial support for this work, and also to Ministerio de Educaci6n y Ciencia (Spain) for a research grant awarded t o M. U. Alzueta.
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Combustion;The Combustion Institute: Pittsburgh, PA, 1986; pp 963-977. Toqan, M. A.; Teare, J. D.; Be&, J. M.; Radak, L. J.; Weir, A. Jr. Reduction of NO by Fuel Staging. Proceedings of the Joint Symposium on Stationary Combustion NO, Control; EPRY EPA New Orleans, LA, 1987; Vol. 11, pp 35\1-3548. Received for review January 27, 1995 Revised manuscript received July 3, 1995 Accepted July 20, 1995@
IE950090U Abstract published in Advance ACS Abstracts, October 15, 1995. @
