Optimized Air Mixing Improves Pulverizer Efficiency
Pulverizer CFD Modeling
Daniel A. Pitsko Jr., Southern Company Services Inc.
In 1996, Three of the Coal-Fired Power Plants Operated by the Southern Company`s Georgia Power Co. subsidiary
began optimization programs to improve plant efficiency following low-NOx burner installations. Plant operating personnel at the McDonough Generating Station, the Wansley Generating Station and the Yates Generating Station believed improved air mixing to the pulverizer mills would enhance performance, and engineers with Southern Company Services Inc. (SCS) quickly started investigating the use of thermal mixers and flow straighteners to improve mill flow characteristics. The project team relied heavily on the use of computational fluid dynamics (CFD) and computer simulations to accelerate development efforts and optimize mixer design.
At the plants, vertical spindle roller-type mills use air to dry, pulverize and transport the feed coal to the boiler, which is equipped with low-NOx burners. Air is supplied to the mills by two ducts: a cold air duct at about 100 F and a hot air duct at about 500 F. The hot air is heated by a rotating basket air heater and is always at a lower pressure than the cold air because of the additional pressure drop through the heater.
As the temperature of the air supplied to the mills increases, the amount of moisture that can be removed from the coal increases. At lower moisture contents, the coal can be more readily crushed and pulverized into smaller particles. This directly improves the efficiency of the generating plant by increasing the percentage of coal burned in the furnace. Mill capacity is increased and burner performance can be controlled to manufacturer`s specifications, which leads to improved NOx and thermal performance.
An unfortunate side effect of increased mill temperature, however, is the increased risk of fire. To reduce this risk without compromising the mill throughput, a small temperature stratification at the mill inlet is desired, enabling higher bulk temperatures to be maintained without producing localized hot spots. The objective of mixing optimization, therefore, is to reduce thermal gradients in the air supplied to the mill and improve the operating efficiency of the plant.
The hot air and cold air supplies at the plants are fed into a common air duct, called the mill inlet duct, prior to entering the pulverizer. Temperature gradients in the mill of more than 270 F frequently occurred at the plants, forcing operators to reduce the temperature of incoming air to avoid creating a fire hazard; reducing the temperature, however, had a concomitant negative impact on the plant operating efficiency. The mill manufacturer recommended that temperature stratification be limited to a maximum of 50 F to maximize the pulverizing capacity of the mill.
To address this problem and improve plant operation, SCS engineers began designing a mixer and flow straightener at the point where the hot and cold air ducts join. The original duct did not contain any special mixing devices. Plant operating personnel identified the underlying causes for the poor performance that needed to be considered in the new design:
flow reversal, which limited total airflow into the mill;
temperature stratification, which hampered proper mill operation and increased the fire potential; and
high pressure drop, which frequently forced the fan system delivering air to the mill to operate at its limit.
Because the building and testing of prototype mixing devices for evaluation at the mill would have required shutting down for an extended period, SCS engineers decided to simulate the system with CFD. CFD involves the solution of the governing equations for fluid flow, heat transfer and chemistry at many thousand discrete points on a computational grid representing the flow domain.
The accuracy of a CFD analysis, and the amount of time required to achieve it, are highly dependent upon the number of cells in the grid. The ideal mesh is finer in areas where there are large variations in the fluid flow and coarser in areas where variations are small. One of the most recent developments in CFD technology is the availability of pre-processors that automatically adjust the density of the mesh to achieve just this ideal balance.
The finite volume solution method is used in most commercial CFD programs. This approach consists of integrating the governing equations of fluid flow over all the control volumes of the solution domain, substituting finite-difference type approximations for the terms in the integrated equation representing flow processes, and solving the algebraic equations by an iterative method.
SCS engineers used the CFX software package from AEA Technology to model the pulverizer systems at the plants. One key feature of this software is the multi-block body-fitted grid capability that greatly facilitates the generation of geometrically complex models. Multi-block capabilities permit the user to create complex geometries with a minimum number of cells, while the body-fitted capabilities allow the grid to conform to the actual shape of the component.
The SCS project team`s first step in solving the air mixing problems involved modeling the original duct design. The model boundaries began at the mill inlet duct cold and hot air dampers and ended at the mill inlet. Engineers conducted several parametric runs, varying the cold and hot air damper positions. The CFD model output closely matched actual measured data for speed and temperature. The CFD model also confirmed that it was not possible to open the cold damper very far without the cold air tending to flow back into the hot air duct. This is the flow reversal problem predicted by plant operating personnel.
The first design modification concentrated on minimizing the potential for the cold air to flow back into the hot air duct. Engineers simulated such a modification by adding a simple baffle to the original CFD model. The results of several parametric studies with the baffle indicated that the cold air damper could be opened further before the cold air would reverse flow into the hot duct. Engineers also simulated the use of a swirling-type mixing device downstream of the baffle to improve thermal mixing. This proved to be too costly in terms of pressure drop and distance for mixing. Additional model simulations demonstrated that although the velocity profile exhibited reduced turbulence, temperature stratification persisted and had a much greater impact on mill performance and flow measurement capability than flow reversal. From this point on, engineers focused CFD simulation on temperature stratification.
Mortise and tenon mixer
Analysis of CFD results led the project team to conceive a new mixer design in which rectangular strips across the face of both the cold and hot air ducts divided the airflow into slices. Engineers strategically oriented the strips so that when the cold and hot air streams merged, the slices of air would fit together like a mortise and tenon joint. By increasing the area of surface contact between the hot and cold air, the turbulence that causes pressure drop is minimized and mixing is improved. CFD parametric studies, in which the number of slices was varied, revealed that more slices produced better thermal mixing, but also increased pressure drop. Engineers selected a design with five cold and six hot slices as an effective compromise.
The parametric studies identified one problem. The cold air mass flow would stratify around the outside of the turn where the hot air and cold air sections merged. SCS solved this problem by tapering the slices and adding turning vanes to the cold air section of the mixer. To optimize the design, engineers conducted many additional CFD simulations with different tapers, turning vanes and cold/hot air volume ratios in the mixer. The final design predicted a thermal stratification of 105 F and a pressure drop of 0.5 inches H2O, significant improvements over the 270 F temperature stratification and 1.5 inches H2O pressure drop associated with the original design. The mill inlet ducts were subsequently modified according to this design. Plant test results with the new mixer demonstrated improved flow profile, approaching a parabolic pattern, and a temperature stratification of only 73 F.
SCS installed several of the mixing devices at McDonough in 1996. Operating results were excellent, leading to the installation of additional units at Wansley and Yates in the spring of 1997. Success with the first mixer prompted Georgia Power personnel to ask for a similar device for a pulverizer mill of different design. This mill had a much shorter mixing section, requiring more aggressive mixing than in the first design. According to CFD simulation results, increasing the number of slices would not provide the desired level of mixing while maintaining pressure drop at acceptable levels. SCS engineers conceived a modification that added paddles to the exit section of each slice. These paddles alternate the flow pattern at short intervals along the length of the slices, causing a checkerboard pattern of hot and cold sections with more area for heat transfer. The approach proved successful, providing the same mixing and pressure drop values as the previous design in only one-third the mixing distance. This design has been installed (awaiting testing) and is expected to have at least as favorable impact on mill performance as the first design. S
Author–Daniel A. Pitsko Jr. has been a contract engineer with Southern Company Services Inc. for the past five years, installing low-NOx combustion technologies at Georgia Power`s coal-fired power plants. Previously, he worked for Westinghouse Electric as a nuclear plant engineer. Pitsko graduated from the U.S. Air Force Academy in 1974 with degrees in aeronautical and astronautical engineering.