Energy Fuels 2010, 24, 3936–3941 Published on Web 06/15/2010
: DOI:10.1021/ef100443s
Design and Test of a Selective Noncatalytic Reduction (SNCR) System for Full-Scale Refinery CO Boilers To Achieve High NOx Removal Wei Zhou,*,† Antonio Marquez,† David Moyeda,† Santosh Nareddy,† Jennifer Frato,† Guanghui Yu,† Sigbjørn Knarvik,‡ and Vidar Frøseth‡ †
General Electric Energy, 1831 Carnegie Avenue, Santa Ana, California 92705, and ‡Statoil ASA, Mongstad Refinery, N-5955 Mongstad, Norway Received April 8, 2010. Revised Manuscript Received May 29, 2010
Selective noncatalytic reduction technology (SNCR) is an effective and economical method of reducing NOx emissions from a wide range of industrial combustion systems. It is widely known that the SNCR process is primarily effective in a narrow temperature window, around 900-1000 °C, and that high CO concentrations can both shift the temperature window and limit its effectiveness. While the application of SNCR on a utility boiler is challenging because of a number of factors that can have negative impacts on SNCR NOx reduction performance, the implementation of SNCR technology on an industrial or refinery boiler has unique challenges intrinsic to these boiler designs and fuels fired. This paper describes the design and test of SNCR technology on two refinery CO boilers. The work presented consisted of (1) baseline testing, (2) process analysis, (3) computational fluid dynamics (CFD) simulations, and (4) optimization testing. A three-dimensional full-scale CFD model was constructed for the boiler and was calibrated using baseline test data to simulate the flow characteristics, temperature profile, and oxygen/combustibles distribution of the boiler. The CFD model was also used to design and optimize a reagent injection system that resulted in fast and effective distribution of the reagent in the flue gas of the boiler. The CFD model, which incorporates a reduced SNCR chemistry model, was further applied to predict NOx reduction efficiency and ammonia slip. After installation and commissioning of the SNCR system, a series of parametric optimization tests were performed on both units. The final performance tests showed that the application of SNCR technology could reduce NOx emissions by at least 50%, with less than 5 ppm of ammonia slip, at a nitrogen stichiometric ratio (NSR) of 1.5. The test results are shown in the paper for a comparison to the model predictions made during the design phase.
Although the optimum process temperature depends upon the agent and furnace quench rate, the accepted window for SNCR application is generally at temperatures between 900 and 1000 °C. The challenges for a SNCR retrofit of a utility boiler include (1) large temperature gradient and, therefore, short residence time of the reagent in the SNCR window, (2) dynamic and unsteady boiler conditions, (3) a wide range of loads that requires multiple layers of injections, (4) difficult to achieve good mixing between the reagent and the flue gas, especially at the center of the furnace, and (5) high flue gas temperature, forcing injectors to be placed near or between the tube bundles. A refinery or industrial boiler on the other hand typically has a process condition that favors SNCR technology. For example, a CO boiler often runs at a relatively steady load, and its flue gas temperature at the exit of the combustion chamber is usually closely controlled in the range of 860-1050 °C. Another example is for application on process heaters. The process heater often has low CO emissions, and its flue gas temperature is around 1000 °C. However, to design a system that can achieve optimal NOx reduction performance, careful use of design tools is needed to identify the best location for reagent injection, to maximize residence time and
1. Introduction Selective noncatalytic reduction (SNCR) technology has been applied to utility boilers, waste incinerators, and other stationary combustion systems for NOx control.1-4 It is a flue gas treatment process, in which a nitrogen-containing agent, such as ammonia (NH3) or urea [CO(NH2)2], is injected into the combustion gases to react with and reduce NOx formed during the combustion process.5 At the proper temperature window, NH2, NH, and N, generated from the decomposition of the injected reagent, react directly with NO to form N2. *To whom correspondence should be addressed. Telephone: 949-7942628. E-mail:
[email protected]. (1) United States Environmental Protection Agency (U.S. EPA). EPA 171-R-92-003. The use of SNCR as BACT for NOx control in boilers and municipal solid waste incinerators, http://nepis.epa.gov. (2) Nguyen, Q. H.; Zhou, W.; Moyeda, D. K.; Payne, R.; Suter, R. A successful SNCR design with CFD applications in a gas fired CO boiler. AIChE Annual Meeting, Cincinnati, OH, Nov 4, 2005, http://aiche. confex.com/aiche/2005/preliminaryprogram/abstract_32081.htm. (3) Himes, R.; Quartucy, G.; Muzio, L.; Cremer, M.; Sun, W. Evaluation of SNCR trim on a 185 MW tangential design coal-fired utility boiler. In Proceedings of 2002 DOE Conference on Selective Catalytic and Non-catalytic Reduction for NOx Control, Pittsburgh, PA, May 2002. (4) Horton, J.; Linero, A.; Miller, F. M. Use of SNCR to control emissions of oxides of nitrogen from cement plants. Cement Industry Technical Conference, Phoenix, AZ, 2006; Conference Record. Institute of Electrical and Electronics Engineers (IEEE): New York, p 29. (5) Lyon, R. K. Method for the reduction of the concentration of NO in combustion effluents using ammonia. U.S. Patent 3,900,554, 1975. r 2010 American Chemical Society
(6) Nguyen, T. D. B.; Yang, T.-H.; Lim, Y.-I.; Eom, W.-H.; Kim, S.-J.; Yoo, K.-S. Application of urea-based SNCR to a municipal incinerator: On-site test and CFD simulation. Chem. Eng. J. 2009, 152 (1), 36–43.
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Energy Fuels 2010, 24, 3936–3941
: DOI:10.1021/ef100443s
Zhou et al.
6,7
minimize the CO impact. In some cases, additives can be used to enhance the SNCR NOx reduction performance when flue gas temperatures are less than optimal.8-10 To improve the confidence level of a SNCR system design, a recently developed reduced SNCR mechanism model was incorporated into the commercial computational fluid dynamics (CFD) model.11 The model can be used to facilitate the design process and predict the SNCR NOx removal efficiency and the ammonia slip of a proposed design. The model includes 10 species and 6 steps of reactions to describe the SNCR chemistry. Validations show good agreement between the model results and both the pilot- and full-scale test data. The design presented in this paper used this model to optimize the design of the SNCR system and predict the NOx reduction performance for two refinery CO boilers. The Mongstad refinery is the largest refinery in Norway. A residue fluidized catalytic cracker (RFCC) unit at the refinery upgrades heavy hydrocarbons to lighter hydrocarbons via cracking and is a major producer of gasoline. The RFCC unit has two CO boilers located before the electric static precipitator (ESP) to burn off CO in the flue gas before it is released to the atmosphere. In May 2007, the pollution permit from KLIF (the Norwegian Climate and Pollution Agency) mandated new regulations regarding NOx emissions for the Mongstad refinery. To help Mongstad comply with the regulation, GE designed an ammonia-based SNCR system for the two CO boilers. The system achieved greater than 50% NOx reduction during the optimization tests, with less than 5 ppm ammonia slip, at a nitrogen stichiometric ratio (NSR) of 1.5. The design approach, performance prediction, and performance test data are discussed in this paper.
Figure 1. Schematic CO boiler. Table 1. Normal Boiler Flow Conditions stream
flow rate (tons/h)
pressure (mbarg)
temperature (°C)
fuel gas burner air quench air RFCC flue gas pilot gas total
1.6 59 32 164 0.046 256
280 50 50 50 500
34 27 27 27 657
RFCC Flue Gas Composition (vol %) H2 C1 H2O CO2 CO
0.2 0.2 7.4 12.8 ∼6
varies to meet the steam demand, and the quench air flow rate varies to control the combustion chamber temperature at 1025 °C. The hot flue gas from the combustion chamber flows through a round-to-rectangular transition duct and enters a refractorylined transition section that turns the flue gas upward into the convection section. In the convective section, the energy associated with the boiler flue gas is recovered and converted into steam. The nominal flue gas temperature exiting the convective section is approximately 220 °C. The cooled flue gas then flows to an electrostatic precipitator (ESP), where catalyst fines are removed, and then to a seawater scrubber for sulfur oxide removal. The clean flue gas is then emitted to the atmosphere through a stack. Input process streams and average conditions entering the CO boilers are shown in Table 1. These data represent averages taken from the distributed control system (DCS) during nominal boiler operation and show streamflow rates, pressures, and temperatures that were the basis for the system design. 2.2. Baseline Test. For a SNCR retrofit, it is critical to understand specific boiler process conditions, such as the temperature and CO distributions in the firebox. These data are acquired by performing a baseline test program. The test data can also be used to calibrate and validate the CFD modeling results. For this study, a baseline parametric test program was conducted that acquired all of the necessary process data across a range of boiler operating conditions. Because the two boilers are similar, baseline testing was only performed on one of the boilers. The test program included in-furnace traverse measurements of temperature, oxygen, and CO distributions. These measurements were performed using existing sampling ports located on the side and rear of the transition section. The gas sampling measurements were performed with a water-cooled suction pyrometer. The sampling probe was inserted to a specific depth inside of the furnace, and a gas sample was pulled over a shielded type-N thermocouple for measurement of the local gas temperature. The gas sample was cooled, filtered, and then input into an array of gas analyzers for measurement of O2, CO2, and CO concentrations.
2. SNCR Design Basis 2.1. CO Boiler Process Conditions. A schematic of the CO boiler is shown in Figure 1. The CO boiler consists of a combustion chamber, a transition section, and a convective section. In the combustion chamber, the refinery fuel gas is fed to four burners, which are supplied with combustion air from a common plenum. At the nominal firing condition, two of the four burners at a diagonal position are in service. Quench air is added to eight ports that are located on the face of the combustion chamber, distributed about the furnace centerline. A single forced draft (FD) fan supplies ambient air to both the combustion air and quench air plenum. The RFCC flue gas is introduced to a large plenum that feeds nine ports distributed around the periphery of the combustion chamber wall downstream of the combustion chamber face. In the combustion chamber, the RFCC flue gas is mixed with the quench air and the flames from the fuel gas burner to oxidize CO to CO2. The resulting flue gas exits the combustion chamber at a temperature on the order of 1025 °C. The fuel gas flow rate (7) Liu, G.-S.; Higgins, B. S. Computer simulation as a NOx reduction design tool. Power Magazine, Oct 15, 2008; http://www.powermag.com/ environmental/Computer-simulation-as-a-NOx-reduction-design-tool_ 1467.html. (8) Zamansky, V. M.; Lissianski, V. V.; Maly, P. M.; Ho, L.; Rusli, C.; Gardiner, W. C., Jr. Reactions of sodium species in the promoted SNCR process. Combust. Flame 1999, 117 (4), 821–831. (9) Tayyeb, J. M.; Nimmo, W.; Mahmood, A.; Irfan, N. Effect of oxygenated liquid additives on the urea based SNCR process. J. Environ. Manage. 2009, 90 (11), 3429–3435. (10) Sang, W. B.; Seon, A. R.; Sang, D. K. No removal by reducing agents and additives in the selective non-catalytic reduction (SNCR) process. Chemosphere 2006, 65 (1), 170–175. (11) Zhou, W.; Moyeda, D. K.; Lissianski, V.; Chen, J.-Y. Development and implementation of numerical simulation for a selective noncatalytic reduction system design. Ind. Eng. Chem. Res. 2009, 48 (24), 10994–11001.
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Figure 2. Traverse temperature measurements.
Figure 4. Traverse CO measurements.
Figure 3. Traverse oxygen measurements. Figure 5. Schematic of the SNCR system. Table 2. Gas Species Sampling species
technique
range
reference method
O2 CO2 CO
paramagnetic cell nondispersive infrared gas filter correlation infrared chemiluminescence
0-25% 0-20% 0-100 ppm
EN 14789 DS/ISO 12039 EN 15058
0-1000 ppm
EN 14792
NOx
SNCR injection location can be beneficial for high levels of NOx reduction. Overall, the baseline test program was an effective method to characterize process conditions and boiler emissions, and data proved to be a valuable asset during the design phase. 2.3. SNCR System. The major equipment of the Mongstad SNCR systems is shown schematically in Figure 5, including an air blower, a steam air heater, a steam ammonia vaporizer, and a static mixer. Aqueous ammonia is vaporized in the ammonia vaporizer and mixes with hot air from the air heater in the static mixer prior to injection into the boiler. The mixed ammonia vapor/air stream is injected through a series of injectors into the flue gas of the boiler. The specifications of the injection system will be discussed in the following section.
Gas samples were also extracted from the CO boiler flue gas duct via an existing sampling port at the ESP inlet. The flue gas sample at the ESP inlet was extracted from the duct, filtered, cooled, and then sent to an array of gas monitors for analysis of O2, CO2, CO, and NOx (NO and NO2). The sampling technique used in the measurements is summarized in Table 2. This table also shows the reference method for each of the measurements. The test data showed that the boiler is nominally operated with an exit O2 of about 2.5% (wet). CO emissions were less than 2 ppm on a dry basis, and NOx emissions were between 160 and 180 ppm (on a dry basis and corrected to 3% O2) at the ESP inlet. Figures 2-4 show traverse measurements of temperature and oxygen and CO concentrations obtained by the sampling probes inserted through the sidewalls of the transition section (Figure 1), respectively. The measured data are also compared to the predictions from a calibrated CFD model. The CFD modeling approach is discussed in the next section. The measured flue gas temperature in the transition section was evenly distributed and was around 1025 °C. The CFD predicted temperature is within 50 °C from the measurements. The oxygen concentration across the furnace width was relatively uniform. The measured O2 concentration near the wall is influenced by the presence of the aspirator air. Historically, accurate local CO concentration predictions are difficult to achieve. In this study, both measurements and CFD indicate that the CO levels at the measurement location were low. Low CO concentrations at the
3. CFD Modeling Facilitated SNCR Injection Design The CFD simulation tool was used to design and optimize the SNCR injection system. A three-dimensional full-scale model was developed for the Mongstad CO boiler. The model was used to (1) understand the baseline boiler flow field characteristics, the temperature profiles, and the CO distributions, (2) identify appropriate injection locations, (3) optimize injection configurations, and (4) predict the SNCR performance. The commercial CFD software, FLUENT,12 with the built-in reduced chemistry model for SNCR performance predictions,11 was used. Figure 6 shows the CFD model geometry for the CO boiler. The model domain consists of a CO plenum, four burner fronts, a combustion chamber, a transition duct, and the convective pass. To model the combustion phenomena in (12) FLUENT User’s Guide. ANSYS, Inc.: Canonsburg, PA, www. fluent.com.
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Figure 6. CO boiler CFD modeling domain.
the burner region, the burner details, such as pilot gas guns, center guns, and pokers, are also included. The computational domain is discretized in GAMBIT13 (a commercial mesh generation tool) with high-quality hexahedral and hybrid cells. The total number of cells for this model was about 500 000 in the baseline case and about 700 000 in the SNCR injection case. To simulate turbulent gas combustion in the boiler, the eddy-dissipation model14,15 was applied to account for the fast reaction of the hydrocarbons. The model assumes that the turbulent mixing rate dominates the combustion rate. The combustion rate in the model was calibrated against the baseline test data sampled from the stack for accurate predictions of CO emissions. Once the combustion rate was calibrated, it was used for the SNCR system design cases to predict CO for different firing conditions. A porous media model was applied to model the pressure drop across the tube banks in the convective pass. Heat sinks were defined in each tube banks to model the flue gas temperature drop. The pressure and temperature across each tube bank are calibrated against the data to best represent the real boiler conditions. Figures 7 and 8 show the contours of velocity magnitude and temperature in the baseline boiler model. The CO plenum has nine port exits to the combustion chamber. The flow areas of the ports are sized differently to allow even velocity distribution among the ports. However, because the flow areas are different, the resulting flow rates and the momentums are different among the nine ports. The CO ports near the bottom of the combustion chamber have the highest flow rates and, therefore, momentums, while the ports near the top of the combustion chamber have the lowest flow rates. The flow and momentum imbalance between the lower and upper ports forms a biased flow distribution in the combustion chamber, as predicted in the model. The predicted velocity and temperature distributions are biased from lower to upper regions of the combustion chamber. The biased profiles can be seen clearly in the crosssectional plane in the middle of the combustion chamber,
Figure 7. Velocity magnitude distribution.
Figure 8. Temperature distribution.
where the velocity varies from 8 to about 45 m/s. The velocity profile of the model also shows a low-velocity flow separation zone at the wake of the CO gas flow. The biased distribution and the flow separation are caused by the nonuniform CO gas flow momentum distributions at the exit of the CO plenum. The temperature is a critical parameter for the SNCR mechanism. Figure 9 shows the predicted temperature distribution of the model in the transition duct. The predicted average flue gas temperature was around 1029 °C, with the local temperature varying from 950 to 1080 °C. The SNCR temperature window is plotted in Figure 9 as iso-temperature surfaces, which show that the temperature window spans over the entire transition duct. The local CO concentration is another important parameter to be considered for SNCR performance. When local CO concentrations approach 500 ppm, NOx reduction can be
(13) GAMBIT User’s Guide. ANSYS, Inc.: Canonsburg, PA, www. fluent.com. (14) Zhou, W.; Moyeda, D. K.; Nguyen, Q.; Payne, R. Comprehensive process design study for layered-NOx-control in a tangentially coal fired boiler. AIChE J. 2010, 56 (3), 825–832. (15) Zhou, W.; Moyeda, D. K.; Payne, R.; Berg, M. Application of numerical simulation and full scale testing for modeling low NOx burner emissions. Combust. Theory Modell. 2009, 13 (6), 1053–1070.
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Figure 9. Temperature contours in the transition area.
Figure 10. CO contours in the transition area.
significantly degraded. The baseline data indicated that the CO emissions were extremely low in the Mongstad CO boilers. Figure 10 shows the predicted CO concentration profiles. The model indicated that higher CO levels were biased toward the upper region in the duct and that the maximum local CO concentration was less than 200 ppm. The model also suggests that most regions of the transition duct see zero CO concentration. Therefore, the relatively low CO concentration in the boiler flue gas is an indication of good combustion of the RFCC flue gas. Once the baseline conditions were analyzed and the modeling parameters were calibrated, the model was used to optimize the design of the SNCR injection system. The design objectives were to place the injectors (1) at the end of the combustion zone, where CO concentrations are at a minimum, and (2) upstream of the convective pass that allows for sufficient mixing and residence time for the reagent to achieve the optimal performance. After several iterations of design optimizations and improvements, the optimum injection location was identified to be at the end of the combustion chamber, as shown in Figure 11a. Figure 11a shows the reagent path lines colored by the flue gas temperature as predicted by the model. The dispersion of the non-reacting ammonia is illustrated in Figure 11b. The simulation of the optimum injection configuration showed that the mixing in the lower or rear part of the duct was relatively easy to achieve. In fact, the momentum provided from the lower injectors needed to be throttled to prevent overpenetration of the jets. On the other hand, dispersion in the upper-front part of the duct was more difficult to obtain. As shown in Figure 11b, the model suggests relatively low concentrations (dark blue) of ammonia in the center region. Better
Figure 11. Ammonia dispersion and path lines.
ammonia dispersion could be possible with higher momentum jets capable of penetrating the center of the furnace. The predicted NOx concentration in the CO boiler is illustrated in Figure 12. Initial or inlet NOx is assumed to be evenly distributed at an average of 184 ppm. Using the optimized injection system and assuming the maximum design condition with an ammonia to flue gas NOx molar ratio, i.e., NSR of 3, the model predicted a 42% reduction in NOx emissions (corrected to 3% O2) and zero ammonia slip. At the more normal operating conditions, the model would suggest 53% reduction, with no ammonia slip and with NSR of 3. 4. Full-Scale Test Results After the installation and commissioning of the SNCR systems at Mongstad, a series of parametric tests were conducted on each boiler to optimize and evaluate the SNCR system. The test program included evaluations of the following parameters on the SNCR performance: (1) the ammonia flow rate or the molar ratio of ammonia to flue gas nitrogen, i.e., NSR, (2) boost air to the injectors, (3) burner air distribution, and (4) the combustion chamber temperature. The actual boiler conditions varied considerably from those described earlier in the baseline test section. The excess O2 was found to increase from 2.5 to 3.8%, and the CO level in the RFCC flue gas increased from 6 to 7.5%. Nevertheless, the variable that has the most impact on SNCR NOx removal efficiency and ammonia slip was the ammonia flow rate. The other factors were found to be secondary in nature in the range of variation typical for the CO boilers. 3940
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: DOI:10.1021/ef100443s
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Figure 13. NOx emission data.
Figure 14. Ammonia slip data.
Further studies and modeling validations are needed to confirm and address the discrepancies. When the level of NOx reduction achieved at NSR of 1 in Figure 12 is compared to the stir tank reactor predictions from the detailed chemistry shown by Zhou et al.,11 the designed system at Mongstad delivers a performance close to that of a perfectly mixed system with no local CO concentration. When the test data are curve-fitted, the NOx emission (ppm) can be expressed as a function of initial or baseline NOx,0 and NSR, as shown below
Figure 12. NOx concentration for design maximum conditions.
Figure 13 shows a comparison between the NOx test data for the two CO boilers and the CFD prediction made during the design phase. Typical reductions from the field data indicated reductions of 45% with NSR of 1 and reductions up to 80% with NSR of 3. Ammonia slip was also measured during the parametric tests, and the results are shown in Figure 14. Ammonia slip typically increases with NSR. At Mongstad, ammonia slip can be controlled to less than 5 ppm, when NSR is less than 1.5. Ammonia injection rates between NSR of 2.5 and 3.0 resulted in ammonia slip increase. The field data indicate that the SNCR systems as installed achieved better SNCR NOx removal performance than was predicted by the models. The higher reduction rates may come from the fact that the boiler was operated at a higher boiler exit O2 than the design value used in the CFD analysis. It is likely that the ammonia reagent encountered less CO in the SNCR temperature window than was used in the model. In addition, the higher O2 level also resulted in higher initial NOx levels, which is known to result in higher reduction rates. Another possible reason is that the local flue gas temperature during the performance test may be lower than the CFD predictions made during the design phase partially because of the increase of the oxygen level. A lower flue gas temperature may result in a better NOx reduction and increased ammonia slip.
NOx ¼ NOx, 0 e- 0:6492NSR where NSR is from 0 to 3, with R2 of 0.97. 5. Conclusions The work presented in this paper presents the design analysis and test results for a SNCR system installed on two CO boilers. The study demonstrates that, when a thorough and systematic approach is applied to design and optimization, SNCR technology can be successfully implemented on refinery CO boilers and can achieve high levels of NOx reduction, with a minimum of ammonia slip. The study also increased the confidence of using CFD as an effective tool for design and prediction of SNCR NOx reduction performance. The SNCR system designed and installed for the Mongstad CO boilers achieved higher than 50% NOx reduction, with less than 5 ppm slip, at NSR of 1.5.
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