scr cfd
CFD for SCR Design
Jia Mi, Ph.D.
Southern Company Services, Inc.
Atlanta, GA 30308
Abstract
For the last several years, Computational Fluid Dynamics (CFD) modeling has been successfully used in Southern Company as one of the design and analysis tools to retrofit Selective Catalytic NO x Reduction (SCR) systems to existing coal-fired steam power plants. SCR technology is the most effective method to reduce NO x emissions by at least 75% ~ 90%. CFD has been integrated in many aspects of design and engineering processes and has provided values and benefits unattainable by either physical flow models or field tests. This paper will summarize two selected CFD applications that were proved to be essential in the SCR optimization processes.
Introduction
Nitrogen oxides (NO x) are one of the primary emissions released during combustion processes, which are highly regulated by federal, state and local environment protection agencies. NO x generally refers to combined emissions of nitric oxide (NO), nitrogen dioxide (NO2) and trace quantities of other species created during combustion [1]. Combustion of any fossil fuel generates NO x due to high temperatures and the availability of oxygen and nitrogen from both air and fuel. NO x emissions may be controlled by utilizing low NO x combustion technologies and post-combustion techniques. Low NOx technologies usually involve controlling the rate of fuel-air mixing, reducing oxygen concentration in the initial combustion zone and reducing peak flame temperatures. However, to achieve further reduction of NO x emissions, post-combustion control systems such as selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR) are required.
SCR technology involves the catalytic reaction of ammonia (NH3) which is injected into the flue gas containing NO x to produce molecular nitrogen (N2) and water vapor (H20). This technology is by far the most effective approach to reduce NO x emissions by 75% ~ 90%. Figure 1 illustrates a schematic description of a SCR system retrofitted to a coal-fired steam power plant. Hot flue gas leaving the boiler is ducted to the SCR reactor. Prior to entering the reactor, NH3 is injected into the flue gas at a distance sufficiently upstream to provide for complete mixing of NH3 and flue gas. Typically, NH3 is introduced into flue gas stream through an ammonia injection grid system (AIG). The quantity of NH3 can be adjusted as it reacts with NO x in the presence of the catalyst to remove NO x from flue gas. The catalyst is offered commercially in two basic geometric shapes, honeycomb and plate.Current formulations of SCR catalyst are patented by the Japanese and are typically comprised of Vanadium Pentoxide (V2O5) and Tungsten Trioxide (WO3) as the active materials deposited on or incorporated with a substrate, which is typically composed of Titanium Dioxide (TiO2). Provisions are made for ash removal before flue gas enters the reactor. For best NO x removal performance, flue gas is maintained at about 600 ~ 700 oF before entering the catalytic reactor. The SCR systems are operated primarily during the ozone season which lasts from the beginning of May to the end of September. During the none-ozone season, flue gas will bypass the SCR reactor through the bypass ductwork. Figure 2 shows an aerial picture of one SCR construction.
Flue gas flow distribution and mixing of ammonia with flue gas have significant impact on SCR’s performance. It is well recognized that trial-and-error attempts to design a SCR system can be cost prohibitive and not feasible. Physical flow modeling has historically been the dominant technique that uses scaled models to study the flow characteristics. When specified parameters achieve an acceptable value in the physical model, it is assumed that the field results will be similar according to fluid dynamics similarity laws. Figure 3 shows a picture of the 1/12 scaled physical model setup for a SCR system.
Air Heater
Figure 1. Schematic description of a SCR system
Figure 2. A SCR system in construction
CFD has been used as an engineering design and analytical tool in Southern Company for nearly a decade. The successful applications for a wide range of components and auxiliary equipment of a coal-fired steam power plant include pulverizers, burners, boilers, cooling towers, water intake structures, windbox and secondary air systems, fans, pumps, ductwork and piping, electrostatic precipitators and stacks. Both
physical models and CFD models have been employed for various SCR projects within Southern Company and each provides its unique values and benefits. CFD analyses were mainly used to evaluate the design of ductwork and gas flow control devices to meet established design criteria for velocity distribution, pressure loss, mixing of ammonia with flue gas, and large diameter particulate removal. It has been demonstrated that CFD modeling efforts provided valuable input to the SCR design and optimization processes with a timely and cost-effective manner.
Figure 3. A physical model setup of SCR system
Analysis
For three dimensional, incompressible and turbulent flows, the Reynolds-averaged governing equations can be described in terms of the continuity, momentum, and energy equations [2]. Continuity Equation:
0)(=+??xi ui t ?ρ?ρ
(1)
Momentum Equation:
Bj xj ui xi uj eff xi xj p xi uiuj t uj +??++?=+??))(()
()
(??μ?????ρ?ρ (2) Energy Equation:
)()()
(xi h Keff xi t p xi uih t h ???????ρ?ρ+=+?? (3)
where x i is the coordinate, u i is the time-averaged velocity component, p is the time-averaged pressure, h is the time-averaged static enthalpy, ρ is the constant flue gas density and Bi is the body force term, such as gravity force. Here, μeff is the effective viscosity which includes molecular viscosity μl and turbulent viscosity μt . The turbulent viscosity μt is determined from high Reynolds number form of standard two-equation k-ε turbulence model, which is based on eddy viscosity hypothesis. In this model, the turbulent viscosity is computed from local values of the kinetic energy of the turbulence k, and its dissipation rate ε. K eff is the effective thermal conductivity, which also includes laminar and turbulent effects, based on eddy diffusivity hypothesis. The heat conduction equation solved in the solid is presented as:
0)()
(=???????xi T s xi t sh λρ (4)
where h = C s T. Here, ρs , C s and λs are density, specific heat, and the possibly anisotropic conductivity of the solid.
ANSYS’s commercial CFD software CFX-4 was used for the flow simulation. The differential equations represented by Eqs. (1-4) are formulated using the “control volume’’ approximations. This involves subdivision of the computational domain into a large number of small control volumes, each associated with a grid point and integration of the governing equations over the computational domain. The grids were generated using CFX-BUILD, the pre-processor for CFX-4. This grid generator creates multi-block unstructured hexahedral meshes. The multi-block approach simplifies the grid generation process and allows better control over the quality of the grid.
The resulting grids usually comprise of 500,000 ~ 1,000,000 cells. All vanes, turning or splitter, were modeled as infinitely thin surfaces. The geometry of the AIG, the flow straighter and the catalyst layers were not modeled explicitly. Instead, a porous media assumption was made, and an appropriate pressure loss correlation was applied to each of these regions [3].
The boundary conditions were as follows:
?
A uniform velocity profile was specified at the inlet. ?
A static pressure boundary was specified at the exit of the domain. ? All walls were assumed to be smooth with a no slip velocity condition.
For each run, solutions were considered to be converged when both normalized residual and global imbalance of mass, momentum and energy were below 1.0E-3.
Analysis Results & Discussion
SCR General Arrangement
For an existing coal-fired steam power plant, SCR systems are usually retrofitted between the boiler’s
economizer outlet section and the air heater inlet to minimize the change of existing arrangement, as shown
in Figure 1. The layout of SCR general arrangement can directly impact system pressure loss and flue gas flow distributions as well as construction and scheduling. System pressure losses influence the net electric power output from a power plant. For a retrofitting of SCR, it also dictates if additional capital investment is required to increase the fan capacity. Flue gas flow distributions though Ammonia Injection Grid (AIG) and catalyst layers affect NO x removal, ammonia slip, SO3 conversion and catalyst life. Flow distribution through the air heater influences the performance of the air heater, which is important to the boiler performance. All these have to meet the established design criteria.
CFD analyses were used to redesign the originally proposed general arrangement of one SCR system to recover system pressure loss and improve flow distribution. This led to direct, tangible operation and maintenance savings.
Two different CFD models were built to describe two different operations:
?SCR operation (during ozone season) shown in Figures 4(A) and 5(A): Flue gas starts from the boiler’s economizer outlet, though the upturn where the AIG is located, then down through the
SCR catalytic reactor and into the air heater.
?Bypass operation (during none-ozone season) shown in Figures 4(B) and 5(B): Flue gas starts from the economizer outlet and directly into the air heater through bypass duct.
The magenta color in Figures 4 and 5 indicates the highest speed flow regions while dark blue color indicates the lowest speed flow regions. Although the originally proposed general arrangement met the design specifications, CFD analysis proved that there was room for improvement. There were also concerns that high values of localized velocity may lead to high erosion rate in the duct. Several likely modifications were identified and tried using CFD. The ductwork exiting the economizer outlet was modified to streamline the flow for both SCR operation and bypass operation. Cross section areas of some sections of the ductwork were enlarged to reduce the local velocity for lower erosion and more fly ash fallout. As a result of several iterations of CFD calculations, general arrangement and several internal flow devices were optimized, as illustrated in Figure 5.
When compared to the originally proposed general arrangment, system pressure losses for the modified arrangement were reduced by 40% and 60% for SCR operation and bypass operation, respectively. The modified arrangement improved flow distributions through AIG, catalyst layers and air heater with minimal pressure loss impact. The increased duct cross-section areas and improved duct bends reduced localized maximum speeds which led to decreased erosion rates. Most importantly, the modified arrangement accommodated the same construction sequence and schedule.
Results of predicted pressure losses from CFD simulations were compared with the test data from Pitot tube measurements on a 1/12 scaled physical flow model. The difference was less than 10%. It gave the design engineers the confidence when applying CFD modeling as a design and analysis tool during the optimization process.
Gas Flow
Gas Flow
(A) SCR Operation (B) Bypass Operation
Figure 4. Proposed Layout for SCR Operation and Bypass Operation
Gas Flow
Gas Flow
(A) SCR Operation (B) Bypass Operation
Figure 5. Modified Layout for SCR Operation and Bypass Operation Transient thermal history of SCR reactor and its components during
a start-up process
In order to bring the SCR into service, it is generally necessary to modulate the damper installed at the SCR reactor inlet over a period of time. The thermal shock to the SCR reactor and associated ductwork can cause immediate structural stress and damage or shorten the lifespan of the system. The system ramp
capabilities are limited by allowable differential temperature of the SCR reactor steel structure and allowable thermal ramp rate of the catalyst.
CFD was used to model the unsteady heat transfer characteristics between flue gas and SCR reactor structural components. Flow equations were solved for flue gas while energy equation was solved for both flue gas and reactor structures. The temperatures and heat fluxes at the interface between flue gas and SCR reactor were assumed to be continuous. The reactor model included steel structure as well as catalyst layers, flow straighter and their supports, as described in Figure 6. The light blue color represents the reactor structure, red color represents catalyst layers and flow straighter, and green color represents their supports. Of particular interest is the wall temperature differential that determines thermal stress across the structure. The temperature distribution can be used as input for a detailed structural analysis to determine these thermal stresses and thus the suitability of the design under these load conditions.
Figure 7 shows the results for a hypothetical test case. For this exercise, a constant volume of flue gas at a temperature of 650o F flows from the top of a SCR reactor. The reactor is maintained at 70o F initially. Wall temperature differentials, defined as the temperature difference between the steel wall inside temperature and steel wall outside temperature, have been evaluated at five different elevations. Elevation 1 is at the top of the reactor and elevation 5 is at the bottom. During the first hour, temperature differentials at all five elevations rose very rapidly, eventually reached a peak of 200o F. Gradually the differentials dropped and reached an equilibrium value of 50o F after about 12 hours. During the process, higher elevation in the reactor reached both the peak and equilibrium at a faster rate. The results here provided useful information such as the maximum values of wall temperature differential for a given flue gas flow rate and time duration needed to achieve wall temperature differential equilibrium. Excellent correlation was obtained between CFD model and independent calculations performed using ANSYS structural analysis code.
Figure 8. Schematic description of a SCR reactor
Figure 9. SCR reactor wall temperature differential
Other CFD Applications
CFD has also been used for some other areas of the SCR design processes:
?Predict the mixing behavior of ammonia with flue gas flow using geometric details of AIG.
?Size duct hoppers at the bottom of a vertical duct run up to the AIG and its ash handling system by estimating what percentage of ash particles will fallout into the hoppers.
?Analyze wind load on the internal flow control devices within the ductwork.
Conclusion
Southern Company has successfully applied CFD modeling for its Selective Catalytic NO x Reduction (SCR) design and optimization processes. It has proved that CFD is becoming an increasingly valuable design and analysis tool with substantial benefits of cost saving and quick turnaround time.
References
[1] Stultz, S. C., and Kitto, J. B., 1992, Steam, Its Generation and Use, 40th Edition, The Bobcock & Wilcox Company, Barberton, Ohio, USA.
[2] CFX-4.3, User Documentation, Volumes 1 to 5, AEA Technology plc, 1999.
[3] Idelchik, I. E., 1996, Handbook of Hydraulic Resistance, 3rd Edition, Begell House, Inc., New York, New York.
[4] Tabakoff, W., and Malak, M. F., 1987, “Laser Measurements of Fly Ash Rebound Parameters for Use in Trajectory Calculations”, Journal of Turbomachinery, 109, pp.535-540.