CFD Modeling Study of a 500 MW Gas-Fired Utility Boiler E.H. Chui, A. Runstedtler, A. Majeski CANMET Energy Technology Centre, Natural Resources Canada, Ottawa, Canada I. Leach OPG Lennox Generating Station, Bath, Ontario, Canada N. Macfadyen Union Gas Ltd., Chatham, Ontario, Canada Abstract The performance of a full-scale natural gas-fired utility boiler has recently been simulated using CFX- TASCflow. The boiler originally designed for oil-firing has been converted to have an additional naturalgas firing capability. This purpose of this study is to use CFD to determine detailed natural gas combustion characteristics within the unit in order to identify the most optimal operating condition. This paper describes a collaborative approach undertaken by the modeler, utility staff and other experts that has proven to be effective in generating insights and boiler information difficult to obtain from field measurements. Introduction A 500+ MW e utility boiler has recently been retrofitted to add natural gas firing to its original oil-firing capability. The motivation is to reduce CO 2 emissions by converting to a lower carbon fuel. The unit can currently be fully natural gas-fired or oil-fired or a combustion of both from quarter load to full load The introduction of natural gas firing to a unit not originally designed for such conditions generates new operational issues to resolve. Some have already been tackled by field trials but others may require a more in-depth understanding of the combustion process in order to determine the appropriate resolutions. A stateof-the-art computational fluid dynamics (CFD) modeling capability is utilized to simulate the detailed natural gas combustion and NO x formation characteristics in the unit. This paper presents the modeling approach adopted to perform the calculations inside the boiler and describes the collaborative efforts employed between the equipment operator, gas utilities personnel and the researchers to make the modeling exercise a cost-effective, integral component in an industrial performance enhancement program. The incremental improvements determined from the simulations will also be highlighted and discussed. Modeling Approach Comprehensive CFD modeling of full-scale furnaces has been in constant progress in the last two decades. Early studies like [1-3] demonstrated the potential of CFD in modeling the operations of utility boilers in electricity generating stations. Due to hardware limitations, these studies could only utilize fairly coarse computational grids (about 3000 to 4000 nodes) and the results were of a qualitative nature. With the advances and accessibility of modern computer hardware, recent studies could tackle more practical power plant issues. Some examples include slag reduction strategy [4], ash deposition [5], NO x formation [6], coal blending [7, 8] and uneven wall temperature distributions in crossover pass [9]. CANMET Energy Technology Centre (CETC) has been developing a CFD full-scale furnace modeling capability since the late 1980s [10]. This capability has been utilized with success in simulating the operations of power plant coal-fired utility boilers [8, 11]. In this study, CETC investigates the performance of a 500+ MW e natural gas-fired unit using a simulation technique different from what is normally employed for coal-firing units.
Modeling Natural Gas Combustion in a Tangentially Fired Utility Boiler In a tangentially fired utility boiler, the natural gas combustion typically involves two stages of mixing between air and natural gas: first in the fuel compartments before exiting the burners and subsequently in the boiler with air from the auxiliary and other compartments. Because of the change in pressure and temperature, natural gas generally enters the fuel compartment at a relatively high speed (close to or at supersonic velocity). It is expected that the natural gas and air can mix without burning prior to entering the boiler to avoid equipment degradation on burner face. To account for this partially premixed natural gas flame, a combination of the flamelet formulations for premixed and non-premixed combustion is used. Full details of this approach are available in [12]. Only a summarized description is provided here The flamelet concept for non-premixed combustion is based on the equation for the mixture fraction, Z, which characterizes the diffusion of reactants to the flame surface. On the other hand, the laminar flamelet model for premixed combustion uses a scalar, G, to track the flame front (G = G o ), the burnt (G > G o ) and the unburnt (G < G o ) regions. In a partially premixed turbulent condition, G is governed by the flow velocity in the unburnt region and the turbulent burning velocity, s T, which in turn is prescribed by the laminar flame speed, mixture fraction (Z), scalar dissipation (χ) and local turbulence level. By solving the G-equation, the propagation of the partially premixed flame front (G = G o ) is determined. Regions of the flow field in which G > G o (i.e. burnt) are described by the non-premixed diffusion flamelet concept. The '2 local Z, its variance Z and scalar dissipation χ specify the local species composition and heat release based on a flamelet library specific to the natural gas-air combination at the inlets. For regions with G < G o, the same three quantities specify the species composition but the mixture is presumed cold and combustion is suppressed. The standard k-ε method is used for turbulent fluid flow calculations. The effect of turbulence on Z is accounted for by integrating a beta-pdf (probability density function) derived from the local Z and Z '2 which in turn are obtained by solving their respective transport equations. Windbox Air Flow Analysis For a full-scale boiler, the total amount of air going into the windbox is controlled but the precise distribution of air over the various compartments is not generally known. However, the partially premixed natural gas combustion stated above is very sensitive to mixing. To ensure credible simulation results, it is important to first determine how air is introduced into the boiler. Figure 1 shows the front and side views of the burner assembly at each of the four corners of the tangentially fired unit. The pressure drop characteristics over each type of compartment are estimated by a CFD analysis taking into account the actual geometry and damper position. Figure 2 shows the sample results of the air velocity vectors at the outlet and within the auxiliary air compartment. By repeating the CFD analysis over a range of flow rates, the sensitivity of pressure drop over airflow is characterized over each compartment type. The precise distribution of airflow at each compartment is thus obtained by finding a common pressure drop (from these pressure drop versus flow rate curves) that yields the individual flow rates for the different types of compartments which total to the amount of air required for the windbox. This windbox airflow analysis essentially ascertains the inlet flow conditions of air and natural gas entering the boiler and allows the boiler simulation to proceed with reliable input information.
Figure 1. Burner assembly at the corner of a tangentially fired boiler
Figure 2. Velocity vectors at outlet (left) and inside (right) the auxiliary air compartment Boiler Simulation The boiler being investigated is a twin unit with two furnaces separated by a partition wall. To conserve computational expense, only half of the twin unit is modeled. Figure 3 shows the computational grid created for the furnace. It represents the radiant section of the unit from the hopper to the furnace exit by subdividing this enclosure into 1.04 million cells (ranging from 0.5 cm 3 to 0.5 m 3 in size). The fluid flow, natural gas combustion, and heat transfer characteristics are calculated by solving the set of differential equations that govern these physical processes, providing information on temperature, velocity vectors, heat transfer and 49 species in each cell. NO x formation from the natural gas flame [13] is computed subsequent to the combustion calculations to yield additional information on species NO, HCN and N over the entire domain.
Figure 3. Computational grid for boiler simulation. The red region represents the burner assembly EVALUATION OF MODEL RESULTS To ascertain the validity of the model results, a test case with actual performance data was chosen as the first target of the simulation exercise. This test case also represents the typical full load condition firing only natural gas and can be treated as a base case, from which future analyses of the unit can be derived. At the outset of the simulation exercise, only the input conditions of the test case were communicated to CETC and the measured results were purposely withheld to protect the integrity of the simulation. Base Case Results The flamelet combustion approach adopted for this study indeed indicates that natural gas does not start to combust until it enters the boiler. The iso-surfaces of CO (7% by volume), shown in Figure 4, provide a view of the near burner combustion characteristics. CO is generated significantly away from the natural gas injection locations, substantiating that natural gas combustion occurs away from the burner face.
Figure 4. Iso-surfaces of CO at 7% (vol.) in near burner region. Colour represents temperature Overall the model results compare fairly well with measured data. Unfortunately, they cannot be presented in a qualitative manner in order to protect proprietary operating information of the unit. At the furnace exit, the predicted CO level and gas temperature (furnace outlet temperature, FOT) are substantiated by the observed characteristics. The average NO prediction at furnace outlet (under 200 ppm_dry) is within 3% of the value measured at the outlet of the economizer. Typically, an accurate prediction of NO x levels is a good indicator of the overall convergence of measured and numerical findings because NO x production is very sensitive to combustion characteristics. In this case, the simulation results show some major features of the combustion characteristics inside the boiler that coincide very well with the operators actual experience. UTILIZATION OF MODEL RESULTS Once the model results have been confirmed realistic, they are used to identify possible modification strategies to improve the boiler performance. This process is performed collaboratively involving the original lead designer of the unit and representatives from the generating plant, natural gas supplier and CETC to assess risks and ensure that the new strategies are technically feasible, operationally practical and economically sound. Then the proposed improvement strategies are further assessed in a case-by-case manner using the modeling tool. After each modeling exercise, the results are again evaluated by a committee consisted of personnel from the plant, gas supplier and CETC in order to determine the next case to be studied. This approach has proven to be effective in controlling both the cost of the study and ultimately the retrofit cost to implement the most optimal improvement strategies. To illustrate the utilization of model results, Figures 5 and 6 show some of the sample plots used by the committee in the decision making process. Figure 5 compares the total heat transfer through the boiler
walls between the base case and two new hypothetical operating conditions (trials A&B). Based on the model results, both of these conditions would improve heat transfer. Figure 5. Total heat transfer to boiler walls Figure 6. NO distributions inside boiler
However, trial A would also cause the NO x level at the furnace exit to increase by 33%. On the other hand, the NO x penalty for trial B is only 2% but the gain in heat transfer is high (10% increase). The model results for these trials also provide new information on the sensibility of combustion characteristics to particular aspects of operating conditions, suggesting that further performance optimization can still be made. Additional simulations are being conducted as a result. SUMMARY REMARKS A modeling approach for a full-scale natural gas-fired utility boiler has been presented. The approach was implemented on a 500+ MW e unit in a test case prescribed by the generating plant and the model results were deemed realistic by the operators. Also, based on the model results a number of performance improvement strategies were identified and reviewed by the operators, natural gas suppliers and one of the original designers of the unit. Additional simulations were performed to investigate the potential improvement strategies and the results indicated that substantial performance gains could be obtained by modifying certain operating conditions. The search for the most optimal strategies is still on going. Overall the modeling tool has proven to be effective in generating insights to the complex combustion and NO x formation characteristics in the unit and also providing guidance to determine the best approach to improve boiler operations economically. ACKNOWLEDEGMENTS This work is supported by the Technology Early Action Measures (TEAM), Emerging Technology Program (ETP) and the Panel for Energy Research and Development (PERD) of the Federal Government of Canada. Additional support is also provided from OPG Lennox Generating Station, and Union Gas Limited. Also, the simulations in this study were completed using CFD software, CFX-TASCflow, supplied by ANSYS Canada Ltd. References 1. Robinson, G.F., A Three-dimensional Analytical Model of a Large Tangentially-fired Furnace. J. of the Inst. of Energy, 1985. LVIII(436): p. 116-150. 2. Boyd, R.K., Kent, J.H. Three-dimensional Furnace Computer Modelling. in Twenty-first Symp. (Int.) on Combustion. 1986: The Combustion Institute. 3. Abbas, A.S., Lockwood, F.C. Prediction of Power Station Combustors. in Twenty-first Symp. (Int.) on Combustion. 1986: The Combustion Institute. 4. Mann, A.P., Moghtaderi, B., Kent, J.H., Computational Modelling of a Slag-reduction Strategy in a Wall-fired Furnace. J. of the Inst. of Energy, 1995. 68: p. 193-198. 5. Lee, F.C.C., Lockwood, F.C., Modelling Ash Deposition in Pulverized Coal-fired Applications. Progress in Energy and Combustion Science, 1999. 25: p. 117-132. 6. Stanmore, B.R., Visona, S.P., Prediction of NO Emissions from a Number of Coal-fired Power Station Boilers. Fuel Processing Technology, 2000. 64: p. 25-46. 7. Su, S., Pohl, J.H., Holcombe, D., Hart, J. A., Techniques to Determine Ignition, Flame Stability and Burnout of Blended Coals in P.F. Power Station Boilers. Progress in Energy and Combustion Science, 2001. 27: p. 75-98. 8. Chui, E.H., LeBlanc, M.P. Modeling the Performance of a Full-Scale Utility Boiler Equipped with Multiple Low NOx Burners. in Sixth International Conference on Combustion Technologies for a Clean Environment. 2001. Porto, Portugal. 9. Yin, C., Rosendahl, L., Condra, T. J., Further Study of the Gas Temperature Deviation in Large- Scale Tangentially Coal-Fired Boilers. Fuel, 2003. 82: p. 1127-1137.
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