An Integrative Approach for Combustor Design Using CFD Methods

Mar 21, 2002 - When applicable, the choice for selecting a two-dimensional grid was made in order to decrease the complexity of the problem and to low...
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Energy & Fuels 2002, 16, 622-633

An Integrative Approach for Combustor Design Using CFD Methods V. Zarnescu and S. V. Pisupati* Energy and Geo-Environmental Engineering Department, The Pennsylvania State University, 110 Hosler Building, University Park, Pennsylvania 16802 Received July 25, 2001

A CFD-based integrative approach for combustor design is presented and applied to a pilotscale 147 kW down-fired combustor (DFC) unit for maximum NOx reduction. The approach is based on synergistic integration of several NOx control methods, including burner optimization, air staging, and reburning. The performance of coal, coal-water slurry, and biomass as reburn fuels was predicted using numerical simulations and compared with measurements. Reduction of NOx levels was targeted at every stage, the results being coupled with the optimized parameters for mixing and injection configuration. A sensitivity analysis was conducted in order to evaluate the variation of predictions with respect to model parameters. Modeling results show that improved mixing and burner optimization can contribute significantly to lowering the primary zone NOx levels. Validation results for natural-gas and coal-water-slurry reburn on optimized injection configurations are included and show important NOx emissions reduction. Different scenarios are discussed and recommendations are made for maximum NOx-reduction efficiency.

Introduction As the regulations to control emissions of nitrogen oxides from fossil-fuel-fired utility boilers become more stringent, boiler combustion systems have become more complex. This is due to the need to burn fuels more efficiently while reducing emissions to lower and lower levels for both new and existing units. Therefore, there is an increasing need for the development and application of cost-effective technologies for controlling these emissions. Numerical modeling can fill this need by providing the boiler designers with valuable quantitative and qualitative information to improve performance and prevent potential problems while keeping emissions to a minimum. The numerical design evaluation procedures can be used together with conventional methods to optimize system performance. An efficient combustion system should also be flexible and easy to modify in order to accommodate better NOx-control technologies, multifuel burners, and the switching to alternative fuels. The main objective of the boiler designers is to achieve minimum pollutant emissions without lowering the combustion efficiency or the standards for safe operation. To meet these demands, one needs to address a variety of issues regarding burner optimization and whole furnace design. Flow and combustion parameters, fuels and firing configurations, injection location, and mixing characteristics are important issues to be addressed and optimized for maximum NOx reduction performance. A combination of measures can be used to control NOx formation, burner optimization usually being the first option investigated. This can be coupled with other measures such as air1-3 or fuel4-6 staging, flue-gas recirculation, or low-NOx burners.7 However, although retrofitting a boiler with primary measures, such as air or fuel staging for NOx control, may be

difficult in terms of space availability and limitation in boiler operation, the implementation of these measures requires relatively little capital investment compared to postcombustion control methods, such as SCR or SNCR. The concept of an integrated combustor design evolved from the need of meeting the stringent emission standards and the challenges of finding solutions for a large variety of boilers. Reburning for NOx control stands out as a recognized, effective and mature technology that has been demonstrated on several coal-fired boilers in the United States8-11 and worldwide.12,13 In the past several years, various testing programs have shown that a wide range of fuels can be used effectively for reburning.14,15 Coal water slurry,11,16 biomass,17 and orimulsion18 are the (1) 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 21st Symposium (International) on Combustion; The Combustion Institute; Pittsburgh, PA, 1986; pp 1159-1169. (2) Mereb, J. B.; Wendt, J. O. L. Air Staging and Reburning Mechanisms for NO Abatement in a Laboratory Coal Combustor. Fuel 1994, 73, 1020. (3) Kluger, F.; Fortsch, D.; Spliethoff, H.; Schnell, U.; Hein, K. R. G. Comparison of Coals for Unstaged and Air Staged Combustion with Respect to NOx Emissions. In Proceedings of the 23rd International Technology Conference on Coal Utilization and Fuel Systems, Clearwater, FL, 1998. (4) Smart, J. P.; Knill, K. J.; Visser, B. M.; Weber, R. Reduction of NOx Emissions in a Swirled Coal Flame by Particle Injection into the iNternal Recirculation Zone. In Proceedings of the 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1988; pp 1117-1125. (5) Papadakis, G.; Bergeles, G. Prediction of Staged Coal Combustion in Three-dimensional Furnaces. J. Inst. of Energy 1994, 67, 132-143. (6) Smart, J. P.; Morgan, D. J. The Effectiveness of Multi-fuel Reburning in an Internally Fuel-staged Burner for NOx Reduction. Fuel 1994, 73, 1437. (7) Vatsky, J.; Howell, D. Pulverized coal NOx Control: Recent Experience with an Advanced Low-NOx Burner Retrofit. In Proceedings of the 23rd International Technology Conference on Coal Utilization and Fuel Systems, Clearwater, FL, 1998.

10.1021/ef010189f CCC: $22.00 © 2002 American Chemical Society Published on Web 03/21/2002

Combustor Design

ones receiving the highest interest due to their potential for replacing natural gas and coal. Basic reburning performance for coal fuels and biomass was found to approach that of natural gas, with over 70% NOx reduction available at reburn heat inputs above 20%.15 While reburning with alternative fuels has the potential to cause boiler impacts, such as increased slagging with biomass, these impacts are generally minimized because the reburn fuel represents only a small fraction of the total boiler heat input. Reburning techniques with these fuels are flexible since they can be readily applied to a wide variety of waste products and off-specification fuels having low cost. Therefore, alternative-fuel reburning can provide great economic benefits while offering the potential for high levels of NOx reduction for a wide range of new and retrofit power-plant applications. Although a wide range of fuels can effectively be used for reburning, including natural gas, coal, coal-water slurry, and biomass, optimization of the reburning process in terms of cost, combustion efficiency, operation, and pollutant-emission reduction proves to be an area where recent efforts have only scratched the surface. The integration of optimized reburning with the whole furnace functioning like a single unit with minimum emissions is highly desirable. The objective of the present investigation is to design an integrated optimized system, capable of a balanced operation which will burn fuels such as coal, natural gas, coal-water slurry, or biomass more efficiently, while maximizing NOx emissions reduction. This system integrates an optimized burner with reburning and air staging for maximum NOx reduction. Although the findings are based on a pilot-scale unit for which the model was designed, the results can be scaled up for bigger units. A CFD-based integrative approach for combustor design is presented and applied to a pilot-scale 147 kW (8) Chen, S. L.; Cole, J. A.; Heap, M. P.; Kranlich, J. C.; McCarthy, J. M.; Pershing, D. Advanced NOx Reduction Processes Using -NH and -CN Compounds in Conjunction with Staged Air Addition. In Proceedings of the 22nd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 1135-1145. (9) Sanyal, A.; Sommer, T. M.; Hong, C. C.; Folsom, B. A.; Payne, R.; Seeker, W. R.; Ritz, H. J. Advanced NOx Control Technologies. In Proceedings of the 10th Annual (International) Pittsburgh Coal Conference; Univ. of Pittsburgh: 1993; pp 676-683. (10) Liu, H.; Hampartsoumian, E.; Gibbs, B. M. Evaluation of the Optimal Fuel Characteristics for Efficient NO Reduction by Coal Reburning. Fuel 1997, 76, 985-993. (11) Ashworth, R. A.; Maly, P. M.; Carson, W. R. Results of CWS reburn tests on a 10 × 106 Btu/hr Tower Furnace and Its Impact on CWS Reburn Economics. In Proceedings of the 22nd of the Internatinal Conference on Coal Utilization and Fuel Systems; Coal and Slurry Technology Association: Clearwater, FL, 1997; pp 789-800. (12) Maier, H.; Spliethoff, H.; Kicherer, A.; Fingerle, A.; Hein, K. R. G. Effect of Coal Blending and Particle Size on NOx Emission and Burnout. Fuel 1994, 73, 1447. (13) Spliethoff, H.; Greul, U.; Rudiger, H.; Hein, K. R. G. Basic Effects on NOx Emissions in Air Staging and Reburning at a Benchscale Test Facility. Fuel 1996, 75, 560-564. (14) Jones, C. Reburn Technology Comes of Age. Power 1997, Nov/ Dec, 57-62. (15) Maly, P. M.; Zamansky, V. M.; Ho, L.; Payne, R. Alternative Fuel Reburning. Fuel 1999, 78, 327-334. (16) Huettenhein, H.; Chari, M. V. Premium Coal Water Fuel (CWF). In Proceedings of the 23rd International Technology Conference on Coal Utilization and Fuel Systems; Coal and Slurry Technology Association: Clearwater, FL, 1998; pp 1099-1108. (17) Kartak, J.; Pichal, M.; Benes, I. Co-Firing of Coal and Biomass. Advantages and Problems. In Proceedings of the 13th Annual International Pittsburgh Coal Conference; University of Pittsburgh: Pittsburgh, PA, 1996; pp 1487-1492. (18) Schimmoller, B. K. Orimulsion Rivals Gas as Reburn Fuel. Power Eng. 1998, February, 32-36.

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Figure 1. Integrated combustor design flow chart.

down-fired combustor (DFC) unit for maximum NOx reduction. The approach is illustrated in Figure 1 and is based on synergistic integration of several NOx-control methods, including burner optimization, air staging, and reburning. Separate studies were completed for burner optimization, NOx-reduction methods, and full-scale combustor modeling. The objective of the burneroptimization part was to analyze and computationally evaluate four burner designs for subsequent full-scale combustor tests. The main goal was to establish a design capable of attaining minimum NOx emissions, improving flame stability and char burnout. On the basis of the burner optimization results, the modeling of the entire furnace as an integrated system for NOx reduction was attempted. First, the numerical model was “tuned” to the experiments, based on a detailed experimental matrix completed for a variety of fuels and firing configurations.19 Once the validity of the numerical code was verified with sufficient grid resolution for various inlet conditions, the modeling of the entire furnace was initiated. The following issues were addressed: effect of mixing, residence time, air staging, reburning, and parameter optimization. The performance of coal, coalwater-slurry fuel, and biomass as reburn fuels was predicted using numerical simulations and compared with measurements. Reduction of NOx levels was targeted at every stage, the results being coupled with the optimized parameters for mixing and injection configuration. A sensitivity analysis was conducted in order to evaluate the variation of predictions with respect to model parameters. Modeling results showed that improved mixing and burner aerodynamics contributes significantly to lowering the primary-zone NOx levels. This fact coupled with optimized injection configuration and reburning parameters can result in important NOxemissions reduction. Experimental Section The facility used for this experimental program was the pilot scale, down-fired combustor (DFC), illustrated in Figure 2. The combustor has a 508 mm internal diameter, has a height of 3.683 m, and is designed for a thermal input of 350000 Btu/h (nominal), but this can be varied from 200000 to 500000 Btu/ h. The combustor is constructed in six modules, each of which can be removed to adjust the height if necessary. A divergent refractory cone is positioned on top of the combustor and is fitted with a multifuel burner, capable of firing coal, oil, and natural gas. The combustor is lined with 76 mm thick HYDRECON 3000 refractory material able to withstand temperatures up to 1648 °C (3000°F). It also contains a 203 mm thick lightweight high-alumina insulation layer to mini(19) Zarnescu, V.; Hill, M.; Clark, D.; Pisupati, S. V. Effect of Reburning Fuels and Firing Configuration on NOx Reduction in a Pulverized Coal Combustor. In Proceedings of the 15th Pittsburgh Coal Conference; University of Pittsburgh: Pittsburgh, PA, 1998; Paper 151.

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Figure 2. Schematic diagram of the down-fired combustor test facility. mize heat loss and maintain a flame temperature of 14501500 °C (2642-2732 °F). Flue-gas-sampling ports are located along the combustor for continuous on-line monitoring of the CO2, CO, O2, SO2, and NOx concentrations. The gas analyzers were calibrated several times during each test for accurate measurement. Sampling ports are numbered 1 through 6 starting at the top of the combustor. Flue gas samples were extracted isokinetically at each port of interest. A sampling probe was designed and used for collecting char samples during the tests at various axial locations to measure the char composition. Wall temperatures were monitored with type-R thermocouples at six locations. The temperature of the flue gas and water entering and leaving the heat exchanger were monitored with type-K thermocouples. A Texas Micro Systems computer with a Keithley metrabyte data acquisition system and ViewDac software was used for data recording. The primary fuel used during the baseline and reburning tests was pulverized coal (Bradford No. 2 fines from the Kittaning seam). A series of different reburning fuels, including coal-water slurry (CWS) made from Washington Energy Processing Cleaned Peptec filter cake, natural gas, propane, and pulverized coal, were evaluated in a pulverized coal combustor, under different conditions. A test matrix of 18 tests has been designed and carried out over a range of process conditions typical of utility boilers.19 The wall temperatures and gaseous species concentrations were continuously monitored and recorded. The NOx-reduction efficiency was then calculated on basis of the exit gas concentration and compared to the base-line results. The gas temperatures in the combustor were measured using suction pyrometry. Also, a combination of reburning and air staging was employed, in some tests all combustion air was introduced through the burner whereas in other tests makeup air was introduced downstream of the burner, at Port 3 (x/D ) 3). The reburn fuels were injected downward along the furnace axis at Port 2 (x/D ) 1.8). The

Table 1. Compositional Analysis of Coal, CWS, and Biomass (wt%, Dry Basis) pulverized coal coal water slurry fixed carbon volatile matter ash HHV (kJ/kg-dry) carbon hydrogen nitrogen sulfur ash oxygen

62.94 24.17 12.89 31,000 76.10 4.66 1.37 1.59 12.89 3.39

60.64 30.22 9.14 31,021 77.46 4.51 1.46 1.25 9.14 6.18

biomass 19.92 75.29 4.79 19,920 49.89 5.75 0.53 0.05 4.79 40.29

downward injection was chosen because of concerns that a radial-injected reburn-fuel jet, having a large momentum will impact on the opposite wall, thus disturbing the flame and the flow pattern. The compositional analysis of the coal, CWS, and biomass used in the experiments and simulations is summarized in Table 1. Base-line tests were performed in the beginning for “tuning“ of the unit to the desired test parameters. The overall stoichiometry of the system was maintained between 1.11 and 1.16 for all tests. For a firing rate of 500000 Btu/h, the pulverized-coal mass flow rate was 38.32 kg/h. The typical overall residence time in the combustor is in the range between 2 and 3 s. A base-line case was established with 0% reburn fuel and pulverized coal as the primary fuel. The NOx level for this case was of 850 ppm (5% stack O2). Test variables found to most directly impact NOx reduction include reburn heat input, reburn-zone residence time, and oxygen concentrations. For the validation tests, natural gas and coal-water slurry were used as reburning fuels.

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Figure 3. DFC nozzle detail and three-dimensional grid of the down-fired combustor.

Numerical Modeling Approach The computations for the burner optimization part were performed on a two-dimensional nonuniform 52 × 62 grid with higher concentration of grid lines in the near-burner region, where the flow is expected to change rapidly and steep gradients are present. The same approach was used for the full combustor simulations where two-dimensional as well as three-dimensional grids were generated and tested for accuracy and stability of numerical predictions. A 300000 grid point, three-dimensional unstructured mesh was also generated and used in the simulations. A commercial CFD package Fluent 5.020 was used. Since a two-dimensional model has difficulties providing an accurate description of complex three-dimensional phenomena, such as the recirculation vortex, special attention was given to the grid-generation procedure. When applicable, the choice for selecting a two-dimensional grid was made in order to decrease the complexity of the problem and to lower the large CPU usage associated with three-dimensional modeling. Grid accuracy tests were conducted in order to examine the influence of grid size and resolution on the numerical predictions. Simulations were conducted on a two-dimensional, nonuniform 425 × 62 × 1 grid as well as on the three-dimensional unstructured grid, which were selected as providing an optimum combination of grid independence and computational time. A detail of the nozzle section and that of the threedimensional unstructured grid of the DFC are shown in Figure 3. The k- model was chosen as the suitable turbulence model for these cases.21 The values for the constants in the k- model are the ones originally proposed and generally agreed upon in the literature.21 The required turbulent quantities at the inlet were (20) Fluent, Inc. FLUENT User’s Guide, version 5.0; 1998. (21) Launder, B. E.; Spalding, D. B. The Numerical Computation of Turbulent Flows. Comput. Methods Appl. Mech. Eng. 1974, 3.

selected based on the recommendations in the literature.22 The turbulent intensity, a measure of the timeaverage fluctuation of the velocity in the fluid, is in general function of the upstreamflow conditions and is about 10% for a free jet, confined jet, or flow in a pipe. The effects of turbulence model and of the turbulent intensity on the sensitivity of the recirculation predictions are discussed later in the sensitivity analysis section. A turbulent intensity of 10% was used in this work. Burner mixing is characterized by the turbulence intensity of the burner. The combusting flow is modeled using the mixturefraction (PDF) approach, coupled with the P-1 radiation model and the NOx module as a postprocessor. Chemical equilibrium is assumed to relate instantaneous mass fractions and temperature with the conserved scalar, taken as the mixture fraction. The P-1 radiation model is the simplest case of the P-N model, which is based on the expansion of the radiation intensity into an orthogonal series of spherical harmonics.23 This approach has the advantage of having only a simple diffusion equation, which is easy to solve with little CPU demand and also allows particulate (and anisotropic) scattering.24 Finite-rate chemical kinetics are incorporated in the Fluent NOx postprocessor. This is based on the solution of transport equations for NO and HCN. Due to the fact that the relationships among NOxformation rate, temperature, and species concentration are highly nonlinear, a time-averaged NO formation rate must be computed at each point in the domain using averaged flow-field information. The nitrogen in coal is assumed as equally divided between char and (22) Sloan, D. G.; Smith, P. J.; Smoot, L. D. Modeling of Swirl in Turbulent Flow Systems. Prog. Energy Combust. Sci. 1986, 12, 163250. (23) Siegel, R.; Howell, J. R. Thermal Radiation Heat Transfer; Hemisphere Publishing Corporation: Washington, DC, 1992. (24) Sazhin, S. S.; Sazhina, E. M.; Heikal, M. R. Modeling of the Gas to Fuel Droplets Radiative Exchange. Fuel 2000, 79,1843-1853.

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where the total enthalpy is defined as

Table 2. Typical Coal Particle Size Distribution diameter, d (µm) mass fraction (%)

200 0.5

130 5.0

90 79

70 11

50 4.5

H)

∑i mjHj

(4)

Table 3. Typical Operating Parameters parameter

NOx Control by Reburning

value

Typical Operating Parameters primary air (kg/h) 9.3 secondary air (kg/h) 74.0 tertiary air (kg/h) 94.9 atomization air (kg/h) 8.6 staged air (Port 3) 24.9 Coal Devolatilization Parameters R1 (s-1) 3.7 × 105 R2 (s-1) 1.5 × 1013 k1 (kJ kmol-1) 7.4 × 104 k2 (kJ kmol-1) 2.5 × 105 Coal Char Combustion Parameters A (kg/(m2 s Pa0.5)) 0.051 k (kJ kmol-1) 6.1 × 104

volatile matter. The PDF technique25 is used to model the mean turbulent reaction rate. The rate of NOx formation is especially sensitive to the temperature, therefore an accurate combustion solution is required for meaningful results. In addition to the selection of a turbulent model, prediction of the actual flow field is also heavily dependent upon the inclusion of realistic boundary conditions. The velocity, temperature, and mixture fraction are prescribed at the inlet, whereas the kinetic energy of the turbulence and its dissipation rate are estimated. The particle trajectories were predicted by integrating the force balance on the coal particles (which is written in a Lagrangian reference frame, from Fluent Inc.20). The size distribution of coal is separated into discrete particle groups, assuming a Rosin-Rammler distribution. A typical coal particle size distribution is presented in Table 2. Typical operating and coal combustion parameters are shown in Table 3. The particles were injected at four locations, with 10 track paths at each location, The maximum number of steps in each trajectory was 5000. Stochastic particle tracking incorporates instantaneous values of the fluctuating gas velocity components in the trajectory calculations. The inlet coal mass flow rate was 0.005 kg/s. The basic equations for the computation of the turbulent flow are summarized below. The continuity equation is given by

∂ (FUi) ) 0 ∂xi

(1)

The momentum conservation equation is

[

]

∂ ∂Ui ∂Uj ∂F ∂ ∂ (FUiUj) - µ + + + (Fu u ) ) 0 ∂xi ∂xi ∂xj ∂xi ∂xi ∂xi i j (2) The energy equation for the PDF model is given by

( )

∂ui ∂ ∂ kt ∂H ∂ (FH) + (FuiH) ) + τik + Sh (3) ∂xi ∂xi ∂xi cp ∂xi ∂xk (25) Janicka, J.; Kollmann, W. A Two-variables Formalism for the Treatment of Chemical Reactions in Turbulent H2 Air Diffusion Flames. In Proceedings of the 17th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1998; p 421.

The basic reburning process has been recognized for over 2 decades. First, it has been suggested that NO can react with hydrocarbon radicals, such as CH,26 and then the reburning principle was proved,27 by showing that NO can be reduced by adding methane or ammonia to the combustion products. This was followed by experiments at the semiindustrial scale. The first fullscale demonstration followed soon afterward.28 Since then, numerous studies have been carried out to determine the controlling parameters for the reburning process. These showed that mixing of the reburn fuel with primary-zone products, stoichiometry, residence time in the reburn zone, and temperature can greatly affect the efficiency of the NOx-reburn process. Mixing effectiveness can be controlled by location of the reburnjet injection, momentum, and the number of reburn jets. Good mixing as well as optimized reburn-zone residence time and temperature are essential for an efficient reburning process. The exact paths that control the fate of coal nitrogen under combustion conditions are not clearly established. However, the main reactions that approximately represent the complex reburning process are

reburnfuel f CHx + ... CHx + NOx f •CN + •NH2 + H2O NOx + •NH2 f N2 + H2O NOx + •CN f N2 + CO NOx + CO f N2 + CO2

(5)

Under rich conditions, HCN plays a critical role in driving the nitrogen cycle to form N229 and under natural-gas-reburning conditions the Fenimore N2fixation reaction,

CH + N2 f HCN + N

(6)

also produces HCN.30,31 It has been shown32 by sensitiv(26) Myerson, A. L.; Taylor, F. R.; Faunce, B. G. Ignition Limits and Products of the clames of Propane-nitrogen Dioxide Mixtures. In Proceedings of the 6th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1957; pp 154-163. (27) Wendt, J. O. L.; Sternling, C. V.; Matovich, M. A. Reduction of sulfur trioxide and nitrogen oxides by secondary fuel injection. In Proceedings of the 14th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1973; pp 897-904. (28) Takahashi, Y. 1983 Development of Mitsubishi “MACT” InFurnace NOx Removal Process. U. S.-Japan NOx Information Exchange, Japan, May. (29) Wendt, J. O. L. Mechanisms Governing the Formation and Destruction of NOx and Other Nitrogenous Species on Low NOx Coal Combustion Systems. Combust. Sci. Technol. 1995, 108, 323-344. (30) Lanier, W. S.; Mulholland, J. A.; Beard, T. T. Reburning Thermal and Chemical Processes in a Two-Dimensional Pilot-Scale System. In Proceedings of the 21st Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; p 1171. (31) Mereb, J. B.; Wendt, J. O. L. Reburning Mechanisms in a Pulverized Coal Combustor. In Proceedings of the 23rd Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; p 1273.

Combustor Design

Figure 4. Comparison between experimental and predicted axial temperature profiles for the base-line case.

Figure 5. Comparison between experimental and predicted midaxial temperature profiles for the base-line case.

ity analysis and simulations of premixed flames, that a significant reburning pathway in fuel-rich flames is the NOx “recycle reaction”: (NO + CH f HCN + ...). The Base-Line Case Model To establish a reference model case specific to the experimental unit and a basis for future predictions, the numerical model was “tuned” on the basis of data received from field tests. Boundary conditions, such as inlet parameters and wall temperature profiles, as well as experimental data, were incorporated into the code. The level of complexity was increased one step at a time, and sensitivity tests were conducted in order to assess the influence of various parameters on the numerical simulations. The results predicted the base-line experiments reasonably well, showing also little variation along the combustor axis. Figures 4 and 5 show the comparison between the numerical and experimental temperature profiles, along the axial and midaxial coordinate, respectively. Good agreement is reached for both cases, with the highest uncertainty close to the burner for the axial profile and in front of Port 4 (x/D ) 5.75) for the midaxial case. Here, x represents the (32) Chen, W.; Smoot, L. D.; Hill, S. C.; Fletcher, T. H. Global Rate Expression for Nitric Oxide Reburning. Part 2. Energy Fuels 1996, 10, 1046-1052.

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Figure 6. Comparison between experimental and predicted axial NOx profiles for the base-line case.

Figure 7. Comparison between experimental and predicted midaxial NOx profiles for the base-line case.

coordinate along the combustor axis and D the diameter of burner opening. The midaxial coordinate is the location halfway between the combustor axis and wall. Figures 6 and 7 show the axial and midaxial NOx predictions for the base-line case. The numerical model adequately predicts the experimental data, in particular in the first half of the combustor. Some degree of variation in the solution is observed downstream of Port 3 (x/D ) 3), and this is attributed to the uncertainties in experimental measurements and also to the asymmetry of the combustor. To test the robustness of the numerical model, a sensitivity analysis was conducted. The effect of wall emissivity, turbulence intensity, and fuel switching was investigated. The results are presented as follows. Figure 8 shows that axial gas temperatures are insensitive to a change in wall emissivity from 0.6 to 0.8. However, a value of 0.7 seems to match experimentally measured values the best. The turbulence intensity level was varied by factors of 0.5 and 1.5 of the initial values. The axial gas temperatures were relatively insensitive to these changes as shown in Figure 9. This suggests that the flow field is relatively stable and the choice of turbulence parameters is adequate. As many industrial boilers switch between using different fuels and firing conditions, simulations were

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Figure 8. Effect of wall emissivity on axial gas temperature profiles.

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alternative fuels can be accomplished without significantly affecting combustor performance and having the benefit of reduced NOx emissions. Figures 11 and 12 illustrate the comparisons between the two-dimensional and three-dimensional numerical modeling cases for the velocity vectors and temperature profiles, respectively. The flow solution for the baseline case shows that the main jet penetrates fairly far inside the combustor, establishing several recirculation spots, that is regions were the flow stagnates and the velocity is zero. However there is not enough recirculation on the side-walls to bring the coal particles back to the main combustion zone and consequently enhance the combustion. The flow velocity drops rapidly to about half of its burner value at approximately three burner diameters from the burner. An internal recirculation zone, which is large and intense enough for all of the coal particles to devolatilize, does not exist here, due to the geometric characteristics of the burner and of the quarl zone. The location of the recirculation zone where particles spend more time is shifted close to the wall in the upper part of the combustor, and another one is created close to the exit. The maximum predicted gas temperature was about 1800 K when firing pulverized coal at 500000 Btu/h. The overall shape of the flame was predicted relatively well, with the higher values of the temperature being located at the boundaries of the main jet, as it was issued from the burner. Mixing Optimization

Figure 9. Effect of turbulence intensity on axial gas temperature profiles.

Figure 10. Effect of fuel switching on overall NOx values for the base-line case.

conducted using coal-water slurry (CWS) and biomass with the same inlet conditions as for coal firing. This also served as a test for the numerical model sensitivity to fuel switching. The overall NOx values were reduced by approximately 34% for coal water slurry and 55% for biomass, as shown in Figure 10, while the overall flow configuration was relatively insensitive to fuel switching. These results indicate that switching to

Since mixing effects control the dispersion of the reburning fuel and hence its ability to reduce primary zone NOx, good mixing is a necessary condition for efficient reburning. The ability to obtain a good dispersion of the reburn fuel is often critical for the reburning efficiency. Often techniques such as flue-gas recirculation are employed to improve mixing. In an effort to optimize the dispersion of the reburn jet, two options were examined: one with axial injection and the other one with radial-wall injection. During the experiments, only the Port 2 (x/D ) 1.8) axial injection was used. However, indications of poor mixing suggested not only that radial injection might work better but also that a different injection location should be tested. Numerical modeling was used to evaluate the most efficient injection configuration. The recommended injection port utilization is intended to provide maximum jet penetration, enhanced mixing, and improved combustion. A single-jet configuration was simulated, because of concern that opposing jets interference inhibits jet penetration and leads to higher velocities in the center of the combustor. Both high momentum and low momentum jet cases were investigated. The results are shown in Figures 13 and 14, respectively. Here, Ymax is the maximum jet penetration inside the combustor, x is the earlier defined axial coordinate along the combustor axis, and D is the diameter of the burner opening. Reburning jets were injected with nozzle velocities of 150 and 75 m/s for high- and low-momentum cases, respectively. The injection location of the reburning jet was varied from Port 1, close to the burner, to Port 5 downstream. Since the objective was to determine the injection configuration that gives maximum jet penetration, the axial jet injection was compared to the radial

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Figure 11. Two- and three-dimensional comparisons of the velocity vectors in the DFC for the base-line case.

Figure 13. High-momentum jet penetration of the reburn jet injected at different ports.

Figure 12. Two- and three-dimensional comparisons of the temperature profiles in the DFC for the base-line case.

wall injection for the high- and low-momentum cases, as illustrated in Figures 13 and 14. The trend for both injection configurations was similar for the high- and low-momentum cases, a fact that was expected since the order of magnitude analysis showed that, for the highand low-momentum cases, mixing times are comparable. However, an order of magnitude difference exists between the radial and axial configurations, which shows that radial injection is a better injection choice in terms of jet penetration. This means that the radial injection configuration acts like a fast mixing promoter, fact that benefits the jet penetration and hence jet dispersion inside the combustor. The difference between Ports 2, 3, and 4 jet penetration is minimal for all cases, but a maximum is reached for Port 3 injection. The numerical

Figure 14. Low-momentum jet penetration of the reburn jet injected at different ports.

simulations clearly show that jet penetration for both the high- and low-momentum cases is better for the radial injection than for the axial case. Hence the

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Figure 15. Influence of primary-zone residence time on NOx reduction for air and fuel staging.

dispersion of the reburn jet inside the combustor is much improved for radial-wall injection. Furthermore, the injection at Port 3 (x/D ) 3.0) gives the maximum jet penetration in all cases and therefore is the preferred choice. These results demonstrate that the coupling of physical and numerical models with optimized mixing characteristics can be extremely helpful in the design and evaluation of pilot-scale reburn systems. The current predictions can be scaled up for the simulation of the full-scale boilers. However, due to the many uncertainties, this process has to be coupled with the use of full three-dimensional modeling and with more accurate models for coupling the complex interactions between turbulence and chemistry that occur in the turbulent mixing process. The Reburning Case Model Because of the complex inter-relationship between the key combustor operating parameters, emissions and combustion efficiency, it was necessary to establish an initial target operating range for key parameters, to be followed later by detailed fine-tuning. The choices for the reburning configuration case model were based on the above results for mixing and also on a previous burner optimization study.33 Also, separate numerical trials were conducted for different reburning and overfire air (OFA) configurations to determine the extent of their benefit alone or in combination. To make a comparison of the effects of air staging and reburning, it would be ideal to have identical residence time. However, due to the characteristics of the unit, the residence time was not identical but very close. In the present situation, the total residence time was approximately 2.2 s for a total heat input of 500000 Btu/ hr. The maximum individual NOx reduction was similar for air and fuel staging, and it was about 55% for the radial injection configuration. The effect of the primaryzone residence time on the NOx reduction is illustrated in Figure 15. For air-staged pulverized-coal combustion, an increase in residence time in the primary-zone reduces NOx emissions, reaching a maximum reduction (33) Zarnescu, V.; Pisupati, S. V. Numerical Predictions of Burner Performance during Pulverized Coal Combustion. In Proceedings of the International Joint Power Generation Conference and ASME-FACT; San Francisco, 1999; Vol. 23, p 245.

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for a residence time of about 1 s. This is rather high for the current DFC unit for which the total residence time is of the order of 2 s. Whereas NOx reduction is predominantly influenced by the residence time in the primary combustion zone, the burnout will be affected by the particle residence time inside the combustion chamber. A longer primary residence time will shift the burnout air addition toward the combustor outlet, in a lower temperature zone, shortening the residence time available for the final burnout. Hence, an increase in the primary-zone residence time will have an impact on the final burnout. An option to overcome this limitation is to stage the burnout air addition. In this way, the available combustion chamber can be better used for the burnout stage without increasing the NOx emissions. Also, a higher fineness of the coal grinding, i.e., a smaller particle size will decrease the necessary time for burnout and hence will also benefit the burnout. For fuel staging, temperature and residence time in the reduction zone play a key role. Figure 15 shows that increasing the primary-zone residence time beyond 0.4 s will have a negative effect on the NOx-reduction performance of the reburning system. This is due to the fact that, as the primary-zone residence time is increased, the reburn-fuel injection location is moved downstream, and into zones of lower temperature. This decreases NOx reduction because higher temperatures are effective for the nitrogen oxide conversion under reducing conditions.13 This happens because, at higher temperatures, increased reaction rates are encountered. Also, due to the geometry of the combustor, the reburnfuel injected at Ports 1, 2, and 3 will be mixed and dispersed faster because of the smaller cross-sectional area. Hence a certain amount of time is required for the hydrocarbon radicals in the reburn fuel to come in contact with the NOx species and react. From the above results it can be concluded that similar NOx reduction can be achieved by either reburning or air staging but under different conditions. By prolonging the primary residence time in air staging, the NOx reduction is comparable to reburning, although this would necessitate a longer total residence time. By prolonging the primary-zone residence time in fuel staging, the reburn zone is moved into zones of lower temperatures and this slows the nitrogen oxide conversion under reducing conditions. In principle, reducing the primary-zone residence time should help reburning effectiveness because it increases the fuel-rich zone temperature. To see the effect of the primary NOx concentration on the final NOx level for reburning at different injection ports and hence different primary-zone residence time, several cases were simulated and compared with available experimental data. The results are presented in Figure 16. The reburn fuel fraction was 30% in all cases. The experimental data are all for Port 3 injection of the reburn fuel, with data for coal, coal-water slurry, propane, and natural gas. The numerical cases are for coal reburning, with injection at Port 2 (primary zone residence time of 0.136 s) and Port 3 (primary zone residence time of 0.274 s), respectively. It can be seen that decreasing the primary zone residence time has a detrimental effect on the reburning efficiency. For instance, at 600 ppm initial NOx concentration, NOx reduction was 31% for Port 2

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Figure 17. Effect of reburn-zone residence time on NOx reduction. Figure 16. Influence of primary-zone NOx concentration on final NOx levels for different primary-zone residence times.

injection and 41% for Port 3 injection. This is due to the fact that insufficient primary zone residence time results in incomplete combustion in the primary zone. Consequently, there will be an increased oxygen concentration at the end of primary zone. Therefore, if the primary zone residence time is not adequate to ensure almost complete consumption of the available oxygen, the primary fuel will continue to oxidize through the reburning zone under fuel-rich conditions, and hence will decrease the NOx-reduction efficiency of the system. The numerical data predicts superior performance compared to the experimental data for the same type of fuel, i.e., coal. This is due to the injection configuration for the reburn fuel, which is axial for the experimental data and radial for the numerical simulations and also due to the variation in the reburn zone stoichiometry during experiments. Studies of chemistry involved in the process suggest that most fuels exhibit a maximum NOx reduction at a reburn-zone stoichiometry of 0.9 because this is the point at which the generation of CH radicals is highest. For fuels containing fuel bound nitrogen, the optimum reduction occurs at a stoichiometry of 0.9 due to tradeoffs between primary NOx destruction and formation of HCN from the fuel-bound nitrogen. While the reburn zone stoichiometry was maintained at 0.9 for the numerical cases, the experimental results with coal and coal-water slurry had reburn-zone stoichiometries of 1.07 and 1.11, respectively, which shows that they were actually operated under fuel-lean conditions. This explains the overprediction in NOx-reduction performance of numerical cases compared to the experiments. Also, this supports the fact that an optimized configuration plays a key role in the reburning process and can result in improved reburning performance even for a fuel which has significant fuel bound nitrogen, like coal. In addition to the effect of the primary-zone residence time, a key factor in the reburning performance is represented by the reburn-zone residence time. Figure 17 shows the influence of the reburn zone residence time on NOx reduction. The residence time includes the contribution of mixing and reaction times and can be a limiting factor in reburn efficiency. An optimized residence time is therefore highly desirable and represents an overall mixing time, which is the result of both micro

and macro-mixing processes. Hence, an increase in the mixing time requires a longer residence time in the reburn zone, and corresponds to a delay in the complete mixing of the reburn jet with the primary jet. This has a detrimental effect on the overall NOx reduction. Thus, once we pass the optimum mixing time, any increase in the residence time will either have no effect on the NOx reduction or will actually lower the NOx-reduction efficiency of the system. This is a direct result of the strong nonlinear interaction between mixing phenomena and chemical reaction processes. It can be seen that the greatest reduction is achieved in a time interval between 0.9 and 1.1 s with the maximum occurring for an optimum reburn zone residence time of 0.95-0.97 s. This residence time corresponds to Port 3 injection of the reburn jet. This finding corroborates the conclusion drawn from the mixing optimization that radial injection at Port 3 results in minimum NOx emissions. Although there is no agreement in the literature regarding an optimum reburn-zone residence time and temperature, the reburn-zone residence time found to be optimum in this work and its trend correlates with the residence time found to give maximum NOx reduction for a subbituminous coal.34 With all the information provided by mixing, air staging, fuel staging, primary-, and reburn-zone residence times, several choices were available for the optimization of the DFC unit. An optimum configuration should provide adequate mixing of the reburn fuel jet, as well as enough primary-zone and reburn-zone residence times to maximize NOx reduction. The results are shown in Figure 18 and indicate that some NOx reduction will be accomplished regardless of the configuration used. However, the degree of reduction will depend strongly on the reburn-jet and overfire-air (OFA) injection location and hence on the reburn jet dispersion and mixing and the reburn-zone residence time and temperature. On the basis of the mixing optimization, radial injection at Port 3 was chosen for the introduction of the reburning jet. This configuration was combined with OFA injection at Ports 4, 5, and 6, respectively. It can be seen that a significant reduction in NOx values is achieved immediately after the introduction of the (34) Moyeda, D. K.; Li, B.; Maly, P.; Payne, R. Experimental/ Modelling Studies of the Use of Coal-based Reburning Fuels for NOx Control. In Proceedings of the 12th Annual International Pittsburgh Coal Conference; University of Pittsburgh: Pittsburgh, PA, 1995; pp 1119-1124.

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Table 4. Validation Test Matrix Summary test no.

coal rate (MM Btu/h)

reburn fuel type and rate (MM Btu/hr)

NOx level (corrected, ppm)

comments

1 2

0.400 0.280 0.280 0.280 0.280 0.280 0.280 0.280 0.280 0.280

NG-0.120-LM NG-0.120-HM NG-0.120-LM NG-0.120-HM NG-0.120-LM NG-0.120-HM CWS-0.120 CWS-0.120 CWS-0.120

571 158 193 200 184 204 241 308 325 337

base line OFA at Port 4 OFA at Port 4 OFA at Port 5 OFA at Port 5 OFA at Port 6 OFA at Port 6 OFA at Port 4 OFA at Port5 OFA at Port 6

3 4 5 6 7

a Low-momentum jet velocity: 75 m/s. High-momentum jet velocity: 150 m/s. Primary fuel: Middle Kitanning. Reburn fuels: natural gas and coal-water slurry.

Figure 19. Effect of combined methods on NOx reduction for 30% natural gas (NG) and coal-water-slurry (CWS) reburn and for different overfire (OFA) air port (P-4, P-5, and P-6) configuration. Figure 18. Effect of combined methods on NOx reduction based on the reburning configuration.

reburn fuel, indicating fast mixing and efficient dispersion of the reburn jet. The addition of OFA adds to the NOx-reduction effect but varies in magnitude, depending of the location of the air staging port. Adding overfire air at Port 4 (x/D ) 5.75) or port 5 (x/D ) 7.65) gives significantly greater reduction that at Port 6 (x/D ) 9.55). This shows that residence time in the reburn zone is a controlling factor for the NOx reduction in the combustor. The difference in performance between Port 4 and Port 6 overfire air can also be explained by monitoring the HCN levels inside the combustor. As discussed earlier, HCN is an intermediate in the process of NOx reduction to molecular nitrogen. Hence, increased HCN levels close to the exit will show a better NOx-reduction effectiveness. A comparison between the cases of reburn at Port 3 + OFA at Port 4, and reburn at Port 3 + OFA at Port 6 shows that HCN levels at Port 6 (close to the combustor outlet) for OFA at Port 4 case are higher than for the OFA at Port 6 case. This means that the conversion of NOx to HCN for the OFA at Port 4 case is higher than for OFA at Port 6. Hence NOx is more effectively reduced with the OFA at Port 4 configuration. Model Validation To validate the model, a series of six reburning tests was designed and carried out over a range of optimized configurations. The primary fuel used in all the reburning tests was pulverized coal (Middle Kittanning) and the reburn fuel fraction was 30% of the total heat input in all cases. Natural gas and coal-water slurry (CWS) were used as reburning fuels. The test matrix is

presented in Table 4. The wall temperatures and gaseous species concentrations were continuously monitored and recorded. The NOx reduction efficiency was then calculated based on the exit gas concentration and compared to the baseline results. The reburn fuels were injected at Port 3, in all cases in the following fashion: radial injection for the natural gas with high-momentum and low-momentum jets, and axial injection for the CWS. The overfire air injection location was moved from Port 4 to Port 5 and Port 6, for both the natural-gas and the CWS tests. The purpose of this change was to show that the combined influence of mixing and reburn zone residence time, has the potential of reducing NOx significantly, as demonstrated by the numerical simulations. Two different nozzle sizes were used for the natural gas injection to simulate the two momentum jets: a 1/4 in. diameter nozzle was used for the low-momentum jet and a 1/8 in. diameter nozzle was used for the high-momentum jet. The switching from the low-momentum to the high-momentum cases was done during the testing. A Dalavan nozzle was used to atomize the coal-water slurry. The results for the natural-gas (NG) and coal-waterslurry (CWS) reburning on optimized configurations are presented in Figure 19. The degree of NOx reduction obtained with high- and low-momentum jets for 30% natural gas reburn injected radially at Port 3 and varying the overfire air location from Port 4 to Port 6 is illustrated. It can be seen that NOx reduction of up to 74% is obtained, which is an improvement of more than 50% over the maximum reduction (47%) obtained with 30% natural gas on nonoptimized configurations in previous experiments on the same unit (ref 19). It is also shown that in general the low-momentum and highmomentum jets reburning have comparable perfor-

Combustor Design

mance as predicted by the theoretical study of the mixing times and order of magnitude of the jets. It is worth noting that in general, a low-momentum jet provides enough mixing inside the combustor by effectively dispersing the reburn fuel. The hydrocarbon radicals are brought in contact with the NOx species immediately and efficiently. An adequate reburn zone residence time, provided by the OFA injection at Port 4, completes the reburning process and the effect is that NOx emissions are minimized. Another important thing that can be observed from Figure 19 is that the case of OFA at Port 4 is giving the highest NOx reduction, followed by the case of OFA at Port 5 and the OFA at Port 6. This behavior was also predicted by the numerical simulations and is directly related to the reburn zone residence time. The tests were repeated in different days and sets of data were taken in identical conditions, showing good repeatability (( 1%). This demonstrates the fact that the difference in NOx-reduction performance between the different OFA cases, is attributable to the effect of mixing as well as reburn-zone residence time because the differences between these separate cases are not within the experimental error. The results for the slurry reburning tests are also included in Figure 19. In the case of slurry reburning, the axial injection was chosen over the radial injection because of concern that the slurry jet will completely clog the opposite ports and will affect experimental conditions. The reburn-fuel fraction was again 30% and the injection was done at Port 3. Overfire air was again introduced downstream of the reburn fuel injection, at Port 4, Port 5, and Port 6, respectively. It can be seen that a NOx reduction of up to 48% was obtained. This again is better then the maximum reduction (39%) obtained with 30% slurry reburn on nonoptimized configurations. As for the natural-gas reburn experiments, the cases with OFA at Port 4 and OFA at Port 5 give greater reduction than the one with OFA at Port 6. However, it should be noted that the difference between the case with OFA at Port 5 and OFA at Port 6 is not significant enough (( 2%) to be caused by the effect of reburn zone residence time. Therefore, it can be concluded that only the case of OFA at Port 4 can clearly be labeled as an optimized configuration and the NOx-reduction effect attributed to the mixing inside the combustor and the reburn-zone residence time. Conclusions The tuning and optimization of the baseline and reburning case models for NOx emissions reduction was conducted on a down-fired combustor unit. The numerical simulation cases were conducted on two-dimensional and three-dimensional geometries that were tuned based on data received from field tests. The numerical model predicted the base-line experiments reasonably well, with some degree of variation in the second half of the combustor. A sensitivity analysis showed that axial gas temperatures are insensitive to a change in wall emissivity from 0.6 to 0.8 and also to a change in turbulence intensity level by factors of 0.5 and 1.5. With fuel switching, the overall NOx values were reduced by approximately 34% for coal-water slurry and 55% for biomass while the changes in flow configurations were minimal. The optimization of mixing and dispersion of the reburn fuel jet showed that

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radial injection is significantly better than axial injection for both the high- and low-momentum jets. Also, Port 3 injection of the reburn jet gives the maximum jet penetration in all cases and therefore is recommended as the best configuration. The mixing results are correlated with the optimum residence time by employing a combination of reburning at Port 3 with different over fire air configurations. A reburn-zone residence time of 0.95-0.97 s was found to be optimal for the current conditions. The concept of integrated combustor design was validated for a range of optimized configurations. The agreement between the model predictions and experimental data was very good in the case of using the recommended optimized configurations (including injection configuration, reburn-fuel fraction, jet momentum, and reburn-zone residence time). The NOx-reduction performance was significantly improved over the previous experiments using the same reburn fuel and reburn fuel fraction. An important result was that the lowmomentum and high-momentum jets reburning have comparable performance as predicted by the theoretical study of the mixing times and order of magnitude of the jets. It was noted that the model performed very well for the natural-gas-reburn tests, and reasonably well for the slurry reburn. This indicates that the model may have a more general applicability than for the studied cases. These results show that the reburning process in the DFC unit can effectively be optimized for NOx-emission reduction, by using an integrated approach based on CFD methods. Although these findings are specific to the down-fired combustor unit, for which the model was designed, the results could, with some degree of uncertainty, be scaled up for bigger units. The objective remains the same, regardless of the size of the unit and that is to design an integrated optimized system, capable of a balanced operation which will burn fuels more efficiently while maximizing emission reduction. Nomenclature DFC: down-fired combustor CFD: computational fluid dynamics CWS: coal water slurry OFA: overfire air A: external char surface area cp: specific heat d: particle diameter D: diameter of burner opening H: total enthalpy k, k1, k2: kinetic reaction rate kt: thermal conductivity mj′: mass fraction of species j′ Sh: source term in the energy equation T: temperature U: velocity ui: fluctuating velocity components Ymax: maximum jet penetration x: axial coordinate Greek Symbols R, R1, R2: mass stoichiometric factor F: density µ: molecular viscosity τik: stress tensor EF010189F