2440
Ind. Eng. Chem. Res. 1997, 36, 2440-2444
Dilution and Stoichiometry Effects on Gas Reburning: An Experimental Study Rafael Bilbao,* Marı´a U. Alzueta, Angela Millera, and Lina Prada Department of Chemical and Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain
Gas reburning is a NOx reduction technique that can be applied to different combustion systems. The influence of stoichiometry and dilution effects on the efficiency of the gas reburning process has been studied from an experimental point of view at a temperature of 1100 °C. Methane, ethane, and natural gas have been used as reburning fuels. The results obtained show that both stoichiometry and dilution level are very important parameters for the performance of the process. Introduction Natural gas reburning is a technique that can be applied to different combustion systems in order to reduce NOx emissions. The reburning configuration divides the combustion chamber into three zones. In the primary zone, the combustion of the main fuel is produced and the typical combustion products as well as NOx are generated. Downstream, natural gas is injected into the so-called reburning zone, creating a fuel-rich environment where NOx are partially destroyed by the action of hydrocarbon radicals. In this zone, NO is converted to N2 and nitrogenous intermediate species, such as HCN and NH3. Finally, additional air is introduced into the burnout zone in order to complete the combustion of unburnt products. Depending on the operating conditions in this zone, the intermediate nitrogenous species can be converted to N2 or recycled back to NO. A number of large-scale reburning tests have been performed during recent years with different degrees of success (see, for example, Glarborg et al., 1994; Payne et al., 1995), and this technology has been demonstrated to be effective under a wide range of operating conditions. The reduction of NO by the action of natural gas taking place in the reburning zone is a complex process that has been extensively studied during recent years from both experimental and kinetic modeling points of view (e.g., Alzueta et al., 1997; Bilbao et al., 1994, 1995a,b; Burch et al., 1991; Glarborg et al., 1992; Kilpinen et al., 1992; Kolb et al., 1988; Miller and Bowman, 1989), and most of the key issues concerning the reduction of NO by the action of hydrocarbon radicals have been identified. However, the reburning process is known for its complexity and significant uncertainties still remain, in particular concerning the NO reduction potential by gas reburning at low temperatures. Few studies have considered the interactions between NO and hydrocarbons at low temperatures (Alzueta et al., 1997; Burch et al., 1991; Chen et al., 1986), and among them certain discrepancies are observed which are attributed to the different conditions under which the various authors conduct their experimental studies. Previous work by our group (Bilbao et al., 1994, 1995a-c) has emphasized the study of natural gas reburning at high temperatures, in the range 12001500 °C. Consequently, with the aim of acquiring a * Author to whom correspondence is addressed. Telephone: +34 976 761150. Fax: +34 976 762142. e-mail address:
[email protected]. S0888-5885(96)00610-0 CCC: $14.00
Figure 1. Experimental installation.
wider and improved knowledge of the reburn chemistry at relatively low temperatures (under 1200 °C), an experimental study concerning NO reduction by gas reburning has been performed in this work. Natural gas, methane, and ethane (as majoritary compounds in natural gas) have been used as reburning fuels, and the contribution to NO reduction of the main individual components of natural gas has been identified. The influence of the stoichiometry, both global and local, has been analyzed under low-temperature conditions. Dilution can play a key role in practical applications of gas reburning itself, as well as when other techniques such as flue gas recirculation are implemented in real installations. Furthermore, the effects of dilution on the results obtained have been analyzed. Experimental Method The experimental installation consists of a reaction system, a gas feeding system and a gas analysis system (Figure 1). Pure gases from gas cylinders and commercial natural gas were dosed by means of mass flow controllers and fed to the reactor. The reaction system includes a quartz tube reactor of 23 mm inside diameter and 1500 mm in length, heated by an electric furnace that allows us to reach temperatures up to 1500 °C. Experiments to determine the longitudinal temperature profiles inside the reburning reactor were performed at system temperatures between 700 and 1200 °C and nitrogen flow rates ranging between 600 and 1500 NL/h (referring to 1 bar and 0 °C). The results obtained showed that the shape of the profiles is very similar for all the system temperatures and that there is a central zone (approximately 600 mm in length) where the temperature can be considered constant. An example of the temperature profiles obtained for a system temperature of 1100 °C and different flow rates is shown in Figure 2. Given these results, a length of © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2441
was varied between 1 and 3% for natural gas and methane and between 0.5 and 2% for ethane. (ii) The effect of the dilution. While maintaining in the inlet gas the same ratio between the concentration of reburning fuel and oxygen, the absolute amounts of these compounds were varied. In these experiments, the percentages of natural gas studied ranged between 0.5 and 2.5% of the inlet total gas flow in dry basis. Since methane represents approximately 90% of natural gas, the same values of concentration as for natural gas were considered. In the experiments with ethane, percentages of 0.09-2% of the total flow rate were used. Results and Discussion
Figure 2. Temperature profiles inside the flow reactor for different flow rates and a system temperature of 1100 °C.
600 mm was considered as the reaction zone and the temperature in this zone as the nominal temperature in the experiments. The temperature profiles inside the reactor can be modified by the presence of other combustible gases besides nitrogen, due to combustion. In order to evaluate the increase in temperature due to the use of reactant mixtures, a number of thermocouples were placed inside the quartz tube at different locations during the reburning experiments. The temperature values measured during reaction were within 20 °C of those measured in a nitrogen atmosphere. In the reburning experiments, a flow of N2 was bubbled into a water container in order to reach a desired moisture at a given temperature. This gas was mixed with O2, CO2, NO, and reburning fuel, and the mixture was fed into the reactor. In all the reburning experiments, fixed concentrations in dry basis of CO2 (18%) and NO (900 ppm) were used, and the amount of steam added represented an extra 6% of the total flow rate in dry basis. The concentrations of oxygen and reburning fuel were varied. Methane, ethane, and natural gas were used as combustibles. The natural gas used was analyzed daily, showing an average composition of 90.7% CH4, 7.8% C2H6, 0.7% C3H8, 0.6% N2, and 0.2% CnHm (n ) 4-7). The reburning temperature was 1100 °C in all the experiments. A constant flow rate of 700 NL/h (referring to 1 bar and 0 °C, in dry basis) was used, which corresponds to gas residence times in the reaction zone of 220 ms for the temperature studied. The gas composition at the outlet of the reburning zone was determined using different analysis methods. The concentrations of NOx (NO and NO2), O2, CO, and CO2 were measured by continuous analyzers. O2, CO, CO2, CH4, and other hydrocarbons were determined by gas chromatography. The uncertainty of these measurements is lower than 1% of the concentration measured. The concentrations of HCN and NH3 were determined by passing a known volume of gas through an aqueous solution (basic for HCN and acid for NH3), which was subsequently analyzed using the colorimetric methods of barbituric-pyridine for HCN and Nessler for NH3 (Clesceri et al., 1989). An estimated uncertainty of these measurements is 20 ppm. An extensive experimental study was performed in order to analyze the following: (i) The influence of the stoichiometry in the reburning zone. Experiments with different values of the concentration of oxygen and reburn fuel were carried out. The oxygen percentage in the inlet gas was varied between 0.5 and 4%. The concentration of the reburning fuel
The influence of the different variables on the reburning zone efficiency has been studied, taking mainly into account the outlet concentrations of NO and HCN. The concentration of NH3 was, in all the experiments carried out, negligible compared to the concentration of the total fixed nitrogen, TFN (NO + HCN + NH3). Therefore, the NH3 results have not been included in this work. The low significance of NH3 with respect to the total concentration of nitrogenous species resulting from the interactions between NO and hydrocarbons has already been mentioned by different authors (Alzueta et al., 1997; Bilbao et al., 1994, 1995a; Kristensen et al., 1996; Miyadera, 1990). In the gas reburning process, the NO reduction takes place through reactions between NO and hydrocarbon radicals originated from thermal decomposition and/or oxidation of the reburning fuel (Bilbao et al., 1994; Kilpinen et al., 1992; Mereb and Wendt, 1994). The formation of hydrocarbon radicals by thermal decomposition or pyrolysis of hydrocarbons is significant at very high temperatures, higher than approximately 1300 °C (Bilbao et al., 1994; Billaud et al., 1993). Thus, at the temperature of 1100 °C used in this work, the pyrolysis process can be considered not to be significant, the oxidation of hydrocarbons being the main mechanism involved in the production of active hydrocarbon radicals which react with NO. The influence of both the amount of reburning fuel and of oxygen existing in the reburning zone on the NO and HCN concentrations can be analyzed using global or local stoichiometries. The global stoichiometry, SR2, is based on the total amount of fuel and oxidizer that enters the combustion system in the first two zones of the combustor (i.e., primary fuel, primary air, and reburn fuel), regardless of whether they have reacted or not. The global stoichiometry is usually referred to in pilot- and full-scale tests, where it is not easy to have a clear separation between the primary and reburn zones of the combustor. The influence of the global stoichiometry on NO reduction has been analyzed. As an example, Figure 3 shows the results of NO concentration obtained using natural gas as reburn fuel and different oxygen concentrations at the inlet of the reburn zone. It can be observed that a minimum in NO concentration is obtained and that the influence of SR2 on the NO reduction depends on the oxygen concentration, especially for low SR2 values. This is in agreement with previous observations at the temperature of 1200 °C (Bilbao et al., 1995a). For all the different oxygen concentrations studied, the HCN concentration diminishes as the stoichiometry becomes leaner, following the same trends as observed in previous works at higher temperatures (Bilbao et al., 1994, 1995a). The global stoichiometry to some extent includes both local stoichiometry and dilution effects in the reburning
2442 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
Figure 3. NO concentration versus SR2 for different oxygen concentrations using natural gas as reburning fuel.
Figure 4. NO concentration versus λ, using natural gas, methane, and ethane as reburning fuels.
zone. Therefore, it has been considered interesting to study the influence of these factors separately on NO reduction, which is possible in laboratory scale. Local stoichiometry, λ, can be used to quantify the environment (oxidizing or reducing) existing at the entrance of the reburning zone. This is defined as the ratio between the amount of molecular oxygen available for reaction in the reburning zone and the amount of oxygen necessary to obtain the complete combustion of the reburning fuel. In order to study further the influence of the reducing environment on the reburning process, some experimental results obtained with different λ values are shown. These results correspond to an oxygen percentage of 2% and different percentages of reburning fuel. The amount of fuel added as reburning fuel is a very important parameter for both the efficiency and the economy of the process. Furthermore, this is the easiest parameter to modify during the operation of combustion systems. Taking into account that methane is the major compound in natural gas, the results of NO and HCN obtained using both natural gas and methane are compared in Figures 4 and 5 for different local stoichiometries at the temperature of 1100 °C. With respect to the NO concentrations (Figure 4), no significant differences are observed for either fuel, and only for stoichiometries lower than 0.5 does the use of natural gas lead to higher NO reduction than methane. For both hydrocarbons, NO concentration presents a minimum for an approximate stoichiometry, λ, of 0.5, and a very poor NO reduction is observed when the environ-
Figure 5. HCN concentration versus λ, using natural gas, methane, and ethane as reburning fuels.
ment becomes leaner. The λ value corresponding to the maximum NO reduction is more or less independent of the hydrocarbon type when methane and natural gas are used (Alzueta et al., 1997). The concentration of HCN (Figure 5), diminishes as the stoichiometry increases because HCN oxidation is logically favored as the O2 availability increases. This fact could also explain the high NO concentration obtained for high λ values, because the HCN is oxidized in part to NO. Furthermore, high levels of O2 could be responsible for the inhibition of the formation of hydrocarbon radicals, favoring the formation of CO and CO2 (Bilbao et al., 1994). Calculations with the well-known detailed kinetic model of Miller and Bowman (1989) show that the NO reduction under the conditions of Figures 4 and 5 is mainly produced through the reaction of NO with CH3 radicals, which are the dominant hydrocarbon radicals originated from methane oxidation at low temperatures. Furthermore, these radicals are fairly unreactive and have a long lifetime under the conditions studied. The fact that for local stoichiometries lower than λ ) 0.5 the use of natural gas leads to higher NO reduction than methane can be explained by taking into account the presence of ethane in natural gas. The presence of ethane even in very low concentrations has been shown to enhance the oxidation of methane since ethane decomposition easily produces different radicals that react more readily than the CH3 radical formed in the first stages of methane oxidation (Dagaut et al., 1991; Tan et al., 1994). Therefore, at those low stoichiometries, where the availability of oxygen is limited, the presence of ethane (which is lower than 10% in natural gas) may have an effect favoring the oxidation of methane. Because the presence of ethane is shown to modify the results obtained when only methane is used in comparison with the use of natural gas, a number of experiments using ethane as reburning fuel were performed. Figures 4 and 5 also show the results of NO and HCN obtained with ethane for different stoichiometries at the temperature of 1100 °C and an oxygen percentage of 2%. Trends similar to those observed for methane are obtained in this case. However, lower NO and higher HCN concentrations are obtained when ethane is used. This effect could be due to the fact that ethane oxidation is faster than methane at the low temperature considered in this work. Therefore, the higher amount of hydrocarbon radicals generated leads to a higher amount of HCN because a higher amount of initial NO is converted. Furthermore, model calcula-
Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997 2443
Figure 6. TFN concentration versus λ, for different reburning fuels.
tions show that during ethane oxidation a high concentration of HCCO radicals is produced, which react with NO at lower temperatures than the stable CH3 radical. The influence of the local stoichiometry on the TFN values at a temperature of 1100 °C is shown in Figure 6, for natural gas, methane, and ethane. It can be observed that the TFN concentration presents a minimum for an approximate λ value of 0.5 for the three hydrocarbons studied. In the case of ethane, higher TFN values are obtained, mainly for low values of λ, due to the higher HCN concentrations obtained in these conditions. The local stoichiometry used is a very important parameter in order to analyze the efficiency of the process, as well as its economy. Furthermore, for a given O2 level, the reburning fuel concentration determines the oxidizing or reducing environment existing in the reburning zone and influences the distribution of nitrogenous species obtained at the outlet of this zone. Recent flow reactor results concerning the interactions between NO and hydrocarbons (CH4, C2H6, and CH4/ C2H6 mixtures) by Alzueta et al. (1997) show that the NO and HCN concentrations at the outlet of the reducing zone are influenced not only by the stoichiometry but also by the absolute levels of oxygen and hydrocarbons. This fact was observed using very low concentrations of hydrocarbons and oxygen (4500 and 6200 ppm, respectively). It could, therefore, be expected that dilution effects would be even more significant under the operating conditions existing in practical applications, where higher concentrations of the reactants are present. From a practical point of view, dilution could be an important issue in real installations where the mixing of the reburning fuel with flue gas coming from the primary combustion zone plays a key role in the effectiveness of the process. For example, bad mixing of reactants would lead to a low effective fraction of the reburning fuel available to react with NO in the reaction zone. Also, the use of flue gas recirculation together with the injection of the reburning fuel would modify the concentration level of the reactants. In order to assess the effects of dilution on the reburning process performance under conditions comparable to those existing in practical applications, a number of experiments with different dilution levels have been performed. In these experiments, the amounts of oxygen and reburning fuel were varied simultaneously, keeping the same value of local stoichiometry, λ. Furthermore, the reburning fuel and oxygen percentages in the inlet gas were identical.
Figure 7. Influence of the dilution on the NO concentration for different reburning fuels.
Figure 8. Influence of the dilution on the HCN concentration for different reburning fuels.
Figure 9. Influence of the dilution on the TFN concentration for different reburning fuels.
Figures 7-9 show respectively the results of NO, HCN, and TFN for different dilution levels, using different reburn fuels (methane, ethane, and natural gas), for a given value of local stoichiometry, λ, which is different for every hydrocarbon (e.g., approximately 0.5 for methane and natural gas and 0.3 for ethane). This shows that the different dilution levels come from the variation of the absolute concentrations of oxygen and reburn fuel together with the amount of N2 to balance. For every reburning fuel, the NO concentration diminishes as the dilution diminishes (Figure 7) down
2444 Ind. Eng. Chem. Res., Vol. 36, No. 6, 1997
to a determined dilution level, here called the dilution limit, for which an increase in the absolute concentrations of oxygen and reburn fuel does not produce any significant improvement of NO reduction. The dilution limit is dependent on the reburn fuel. Methane and natural gas show a similar behavior with respect to dilution. As is seen in Figure 7, the NO profiles for methane and natural gas are very similar even though lower NO concentrations are always obtained with natural gas, as previously discussed. However, the dilution limit observed with ethane is shifted to lower values of the absolute concentrations of ethane and oxygen. This is in agreement with the fact that ethane oxidizes easier and is more effective in producing active hydrocarbon radicals and therefore reducing NO at the temperature studied than are methane and natural gas. The effect of dilution is less significant for the concentrations of HCN obtained under the conditions studied (Figure 8). The dilution limit found for NO is not seen for HCN. The concentration of HCN increases as the absolute concentrations of oxygen and reburn fuel increase, this effect being more clear when ethane is used as the reburning fuel. In order to evaluate the effects of dilution on the global efficiency of the reburning zone, Figure 9 shows the outlet concentrations of TFN obtained. Again, the dilution limit is seen, from which a further increase in the concentrations of oxygen and reburning fuel does not produce any significant improvement in the reduction efficiency. The results obtained with different levels of dilution are important because they show that dilution has indeed a significant effect on the concentrations of NO and HCN, even for a similar value of λ used for each of the hydrocarbons. Conclusions The results concerning the influence of SR2 at 1100 °C, when natural gas is used as reburning fuel, show that NO concentration presents a minimum for a given value of SR2 and HCN concentration diminishes as the SR2 value increases. These trends are basically similar to those found in previous studies of reburn chemistry at higher temperatures. For a temperature of 1100 °C, the concentration of NO presents a minimum for a given value of λ for each reburning fuel studied and the concentration of HCN increases as the environment becomes leaner in the reburning zone. Similar results concerning NO reduction and HCN concentration are obtained when methane and natural gas are used as reburning fuels. However, the use of ethane produces a lower concentration of NO and a higher HCN concentration. The dilution level in the reburning zone is an important parameter that affects the results obtained, even for a similar value of λ. The efficiency of the process is increased as the dilution diminishes down to a certain dilution level, the dilution limit, which depends on the hydrocarbon considered and from which an increase in the absolute concentrations of the reburn fuel does not produce any further improvement with respect to the TFN concentration. Acknowledgment The authors express their gratitude to CICYT (Project AMB92-0888) for providing financial support for this work. L.P. expresses her gratitude to Ministerio de Educacio´n y Ciencia for the predoctoral grant awarded.
Literature Cited Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Low Temperature Interactions between Hydrocarbons and Nitric Oxide. An Experimental Study. Combust. Flame 1997, in press. Bilbao, R.; Millera, A.; Alzueta, M. U. Influence of the Temperature and Oxygen Concentration on NOx Reduction in the Natural Gas Reburning Process. Ind. Eng. Chem. Res. 1994, 33, 2846. Bilbao, R.; Alzueta, M. U.; Millera, A. Experimental Study of the Influence of the Operating Variables on Natural Gas Reburning Efficiency. Ind. Eng. Chem. Res. 1995a, 34, 4531. Bilbao, R.; Alzueta, M. U.; Millera, A.; Duarte, M. Simplified Kinetic Model of the Chemistry in the Reburning Zone Using Natural Gas. Ind. Eng. Chem. Res. 1995b, 34, 4540. Bilbao, R.; Alzueta, M. U.; Millera, A.; Cantı´n, V. Experimental Study and Modelling of the Burnout Zone in the Natural Gas Reburning Process. Chem. Eng. Sci. 1995c, 50, 2579. Billaud, F.; Broutin, P.; Busson, C.; Gue´ret, C. Coke Formation During Hydrocarbon Pyrolysis. Part Two: Methane Thermal Cracking. Re´ v. Inst. Fr. Pe´ t. 1993, 48, 115. Burch, T. E.; Tillman, F. R.; Chen, W. Y.; Lester, T. W.; Conway, R. B.; Sterling, A. M. Partitioning of Nitrogenous Species in the Fuel-Rich Stage of Reburning. Energy Fuels 1991, 5, 231. Chen, S. L.; McCarthy, J. M.; Clark, W. D.; Heap, M. P.; Seeker, W. R.; Pershing, D. W. Bench and Pilot Scale Process Evaluation of Reburning for In-Furnace NOx Reduction. In Proceedings of the Twenty-First Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; p 1159. Clesceri, L. S., Greenberg, A. E., Trussell, R. R., Eds. Standard Methods for the Examination of Water and Wastewater; APHA-AWWA-WPCE: Washington, DC, 1989. Dagaut, P.; Boettner, J. C.; Cathonnet, M. Methane Oxidation: Experimental and Kinetic Modeling Study. Combust. Sci. Technol. 1991, 77, 127. Glarborg, P.; Lilleheie, N. I.; Byggstøyl, S.; Magnussen, B. F.; Kilpinen, P.; Hupa, M. A. Reduced Mechanism for Nitrogen Chemistry in Methane Combustion. In Proceedings of the Twenty-Fourth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992; p 889. Glarborg, P.; Karll, B.; Pratapas, J. M. Review of Natural Gas Reburning. Initial Full Scale Results. Report of Committee F: Industrial and Commercial Utilization of Gases; Danish Gas Technology Centre: Hørsholm, Denmark, 1994. Kilpinen, P.; Glarborg, P.; Hupa, M. Reburning Chemistry: A Kinetic Modeling Study. Ind. Eng. Chem. Res. 1992, 31, 1477. Kolb, T.; Jansohn, P.; Leuckel, W. Reduction of NOx Emission in Turbulent Combustion by Fuel-Staging. In Proceedings of the Twenty-Second Symposium (Internationl) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; p 1193. Kristensen, P. G.; Glarborg, P.; Dam-Johansen, K. Nitrogen Chemistry During Burnout in Fuel-Staged Combustion. Combust. Flame 1996, 107, 211. Mereb, J. B.; Wendt, J. O. L. Air Staging and Reburning Mechanisms for NOx Abatement in a Laboratory Coal Combustor. Fuel 1994, 73, 1020. Miller, J. A.; Bowman, C. T. Mechanism and Modeling of Nitrogen Chemistry in Combustion. Prog. Energy Combust. Sci. 1989, 15, 287. Miyadera, T. Reaction of Nitric Oxide with Hydrocarbons in Two Staged Reactor. In Proceedings of the Reburning Workshop, O ¨ rena¨s Slott, Sweden, Nov 1990; Nordic Gas Technology Centre: Hørsholm, Denmark, 1990; p 225. Payne, R.; Folsom, B.; Sommer, T. Natural Gas Reburning Applied to Three Coal-Fired Utility Boilers. In Proceedings of the International Gas Reburn Technology Workshop, Malmo¨, Sweden, Feb 1995; GRI Report No. GRI-95/0287; Gas Research Institute: Chicago, IL, 1995; p D29. Tan, Y.; Dagaut, P.; Cathonnet, M.; Boettner, J. C. Oxidation and Ignition of Methane-Propane and Methane-Ethane-Propane Mixtures: Experiments and Modeling. Combust. Sci. Technol. 1994, 103, 133.
Received for review October 4, 1996 Revised manuscript received February 20, 1997 Accepted February 24, 1997X IE9606101
X Abstract published in Advance ACS Abstracts, April 15, 1997.
