By Bonnie Courtemanche, Riley Power and Chris J. Curow, Southern Company Generation
An Alabama Power generating plant faced the prospect of having to make substantial reductions in NOx emissions to meet new regulatory requirements. The company presumed that it would need to install new burners in order to achieve the required 30% reduction relative to the low NOx burners that had been installed in an earlier emissions upgrade. But engineers later determined a way to meet the emissions requirements at a lower cost through a series of modifications to their existing burners and installation of an overfire air (OFA) system. The modifications made it possible to achieve the required emissions levels at about half the cost of installing new burners and with much less impact on plant operating schedules. A key to the success of the modifications was the use of computer simulation to optimize the performance of the modified burner design while considering the constraints of the existing burner geometry.
Alabama Power, a Southern Co. subsidiary, owns the Greene County generating facility located in Demopolis, Alabama. Unit 2 at this facility was designed by Riley Power Inc. (RPI), a Babcock Power Inc. company, in 1967 to generate 1,800,000 pounds/hour of steam at an operating pressure of 1650 psig and temperature of 1,005 F. The unit was designed to burn eastern bituminous coal using 18 flare type pulverized coal burners. Alabama Power installed a low NOx combustion system in 1997 to meet the requirements of the Clean Air Act Amendment of 1990, replacing the original flare burners with Babcock & Wilcox (B&W) XCL low NOx burners. In 2003, Alabama Power placed an inquiry bid with Riley Power to discuss options to further reduce NOx emissions as required by new regulations.
A simpler approach to emissions reduction saves money
The initial thoughts on the project were to replace completely the existing burners with the latest low NOx burner technology and add an OFA system. Riley Power, however, proposed an approach involving fewer hardware modifications and much lower installation costs. This solution involved retaining the existing burners, replacing critical components within the existing burner with Riley Power CCV (controlled combustion venturi) burner technology, and adding a new OFA system. By performing regression analysis on a database of historical installations, Riley Power engineers predicted that the more economical approach would meet Alabama Power’s emissions requirements at about half the cost of installing new burners and an OFA system.
Optimizing the design of the burner modifications was critical to actually accomplishing the project goals. The design objectives for the modifications to the combustion system included operability over a wide load range and optimization of furnace conditions with reduced emissions. The basic design incorporated one OFA port located above each burner column and four wing OFA ports located between the sidewalls and the outermost burner columns. The basic idea behind the modifications was to stage the combustion by reducing the amount of air supply in the burner zone and adding air back above the burner zone through the OFA system. Spreading out the combustion over a greater area reduces temperatures in the primary combustion zone, which lowers the thermal NOx, without reducing efficiency. The NOx levels due to fuel-bound nitrogen are also reduced as a result of OFA since fuel nitrogen is released in a reducing environment.
This configuration allows for varying amounts of overfire air to be fed into the furnace depending on the unit load and the staging levels required for NOx control. Critical to the burner modifications is the design of the venturi coal nozzle and a flame stabilizer ring (FSR) that provide precise control over the primary air/fuel into the burner. The low NOx burner with venturi coal nozzle and FSR distributes incoming air into three streams: primary, secondary and tertiary. The primary stream delivers a mixture of coal and air directly to the burner. The secondary and tertiary airstreams are designed to meet up with the burning coal well after combustion has begun. In particular, in the CCV design, the secondary airflow surrounds a recirculation zone of primary air and coal, which delays the airstream, stretching out the combustion to a degree that provides the greatest possible emissions reduction.
Optimizing the design to account for the existing burner geometry
This sophisticated design, particularly the FSR, needs to be optimized to meet the specific characteristics for the burners onto which it is retrofitted in order to achieve optimal performance. The key to success is achieving just the right blend of primary, secondary and tertiary airflow and generating strong recirculation zones in the primary airflow. The geometry and operating conditions of every burner is different and some of these differences can have a major impact on airflows. For example, in this application the existing burner geometry restricted the CCV coal nozzle with FSR to a diameter that is one inch smaller than the standard design.
It would conceivably be possible to optimize the airflow by performing a series of physical tests on the burner, but this would be extremely expensive both in terms of unit downtime and the need to build a series of different FSRs for testing purposes. Riley Power engineers overcame this challenge by using computational fluid dynamic (CFD) modeling to simulate the old burner with the new modifications installed. CFD produces solutions for problems with complex geometries and boundary conditions. A CFD analysis provides fluid velocity, temperature, and the concentrations of chemical species throughout the solution domain. It also calculates trajectories of coal particles, which interact with the fluid. For coal particles, the complete cycle can be tracked and coupled to the fluid flow, from devolatilization through combustion to char burnout. The results of the analysis allow a designer or an engineer to optimize fluid flow patterns or temperature distributions by adjusting either the geometry of the system or the boundary conditions, such as the inlet velocity and temperature, or the wall heat flux, for example. CFD also can provide detailed parametric studies that can significantly reduce the amount of hardware experimentation necessary to develop a prototype and thus reduce design cycle times and costs.
Simulating the modifications
Riley Power engineers used FLUENT CFD software from Fluent Inc., Lebanon, New Hampshire, to simulate the burners. The software was selected because it has the complete range of mathematical models needed for simulating utility furnace combustion and has demonstrated the ability to simulate a wide range of systems with high fidelity.
The use of CFD made it possible to evaluate various alternative FSR designs and iterate to the design that provides the airflow distribution and recirculation pattern that provides the best results. Riley Power engineers evaluated nine different FSR designs and for each design, they viewed the predicted airflow through the burner in the form of color-coded graphics that helped them understand its performance by providing insight into why or why not the design performed the way it did. These insights provided by the early design changes made it possible to move much more quickly to an optimized design than would have been possible with physical testing, which provides far less airflow information at a much higher cost.
The burner components and OFA were constructed at RPI’s Erie, Pennsylvania manufacturing facility and installed by Southern Company Generation during a scheduled four-week outage, keeping the combustion system off the outage critical path. The installation of a new low NOx burner system would have required a much larger scope of plant site work. It would have included items such as removal of all existing burner equipment and support steel, installation of new support steel in the windbox, potential pressure part changes, installation of new burners, potential changes to the coal piping, installation of new igniters and flame scanners, and many other items. The lower cost burner modification approach selected significantly reduced the field installation scope and time. The actual field installation was limited to coal piping support, coal head replacement, primary CCV coal nozzle replacement, and installation of new secondary and tertiary air diverters. All modifications were completed from the furnace or burner deck.
Testing shows modifications meet all requirements
After completion of the unit outage, Alabama Power, Southern Company Generation, and Riley Power began the start-up, commissioning, tuning and testing. The combustion tuning was completed in nine days with five tests. During these tests, Riley Power met all guarantees at three different boiler loads while maintaining NOx emissions below 0.320 lb/Mbtu during all loads between full and low load conditions. The CFD modeling conducted earlier in the project had determined the initial burner settings that would produce the best burner near-field flow patterns. Utilization of these settings during startup significantly reduced the commissioning time needed before conducting acceptance testing.
The new burner modifications and OFA system achieved NOx emissions of 0.288 lb/Mbtu at full load and below the guarantee 0.320 lb/Mbtu over the load range (40-100 percent MCR) of the unit, reducing NOx emissions 30 percent compared to the previous low NOx burner. Testing results showed that the burner modifications alone resulted in a NOx reduction exceeding 12 percent without any optimization. The CO levels obtained during testing were less than 150 ppm. In addition the 90 percent reduction in superheat spray flow and the 2 percent reduction in reheat spray flow resulted in improved heating surface absorption. All in all, this project demonstrates that modifications to existing low NOx burners can often achieve near new low NOx burner performance at a much lower cost when state-of-the-art technology is properly applied.
For more information about CFD software, contact Fluent Inc., 10 Cavendish Court, Centerra Resource Park, Lebanon, NH 03766. Ph: 603-643-2600, Fax: 603-643-3967. Visit Fluent’s Web site at www.fluent.com.
For additional information about Babcock Power Inc., see the Web site www.babcockpower.com.