By Joe Estrada, Strategic Engineer, Fuels & Power Optimization and Robert Sisson, Crystal River–Plant Engineer, Progress Energy Corp.
Increases in federal and state regulation on power plant emissions and heat rate have significantly impacted the power generation sector in recent years. In the midst of all commercial technologies available to end-users, the fundamental solution of combustion improvement is often overlooked. Progress Energy has implemented combustion optimization projects at power plants in the Carolinas and Florida with the intent of reducing emissions and increasing plant efficiency. To date, varying forms of these projects have been installed on eight coal-fired plants, each producing favorable results.
A recent installation of the Continuous Combustion Management system (CCM) at Progress’ Crystal River Power Plant in Crystal River, Fla., was performed with the objective of balancing air:fuel (A/F) ratios at each burner by way of balancing coal distribution to all burners and secondary air distribution from compartmentalized windboxes to each burner. Doing so would theoretically balance burner flames across the furnace, yielding a homogeneous temperature profile and eliminating reducing atmospheres. The project consisted of integrating individual burner coal flow measurement, burner coal flow adjustment valves, burner secondary air measurement and actuation, CO measurement augmentation and O2 measurement improvement.
Progress Energy has previously installed combustion optimization/control equipment on several boilers in their system. These combustion upgrades have since been deemed continuous combustion management, or CCM system.
In the spring of 2010, the CCM system was installed on Crystal River Unit 4 at Crystal River. This unit is a Babcock & Wilcox opposed fired boiler with 54 DRB-4Z low-NOx Burners. The CCM System consisted of:
- Coal flow measurement installed on each of the 54 burner pipes
- The use of the integral burner pitot tubes to measure secondary air flow
- Linear pneumatic drives installed on each burner to control secondary air flow
- Self purging, primary air transmitters to accurately measure primary air flow
- Burner line diffusing coal valves installed on each coal pipe for coal flow balancing
- The O2 measurement system was replaced and relocated to a location that would improve accuracy and consistency
- New CO monitors were installed to monitor combustion efficiencies.
Airflow Measurement Technology
Air Monitor Corp.’s (AMC) IBAM technology was selected for these projects based on its ease of integration with the existing burners. To date, AMC’s IBAM technology has been integrated into all the CCM projects within the Progress Energy fleet. For the Crystal River project, the Babcock & Wilcox 4Z burner pitot ring was used instead of retrofitting AMC’s pitot tube. Installed during a previous outage, these burners were retrofitted with a continuous air purge system to maintain the pitot tubes free of debris and fly ash in anticipation of a future CCM project integration..
This decision resulted in a reduction in CCM project integration costs. To ensure accurate flow measurement, a full scale model replica of the Crystal River 4Z burners was constructed and tested at AMC’s wind tunnel at their headquarters in Santa Rosa, Calif. Characterization of the flow elements was performed and equations were developed for use in the plant distributed control system (DCS). At Crystal River, nine different fourth-order equations consisting of three operating parameters (inner spin vane position, outer spin vane position and windbox pressure) were developed. The result of the equation yielded a coefficient which is multiplied by the measured flow to produce a continuous final corrected flow rate.
Coal Flow Measurement Technology
AMC Power’s Pf-FLO coal flow measurement technology was selected for all for all CCM projects. Pf-FLO uses frequency shift microwave technology to measure the mass loading (density) of the coal in each pipe and electrostatic cross-correlation to measure the velocity of the moving coal particles within the pipe. The mass flow is the product of the density and the velocity. The velocity signal is also critical to combustion optimization as it assists with proper primary airflow control, ideal flame stability, and avoidance of problems like coal layout, pipe fires, mill fires and so on.
Balancing Coal Mass Flows and Velocities
Burner line diffusing valves were supplied by Combustion Technologies and installed in the burner pipes at the MPS-89 pulverizer turret discharge. The valves are perforated AR-500 steel butterfly-style manually operated valves with truncated ends. This design allows dynamic coal flow throttling across a wide range without the risk of completely choking flow.
During coal balancing, the valves were adjusted at the mill outlet by one person, while another monitored live coal flow data and communicated changes via radio. The use of the coal valves affects both the mass flow and velocity of the coal in the pipes. In some cases the valves were used to gain more uniform coal velocity in each of the pipes. This is not a consistent result due to the complexity of two-phase flow dynamics. The basic premise is to balance the coal flow to the extent possible within all nine burner lines from each pulverizer at the load range most generally operated.
Through efforts to reduce NOx at other Progress Energy stations, Progress Energy has developed a burner secondary air (SA) controls strategy that is flexible enough to allow for changes in load (mills in and out of service), changes in total air to the boiler and other operational changes.
The basic idea behind air:fuel ratio control is that each burner can have a programmable set point. The system evaluates the mass of secondary air (SA) to each burner and the mass of coal to each burner. If this actual A/F ratio does not equal the A/F set point, the burner SA damper opens or closes accordingly until the set point is achieved. The coal flow balancing serves as the rough tuning and the secondary air flow serves to fine-tune the combustion burner-to-burner.
Because of changes in load and total air due to varying O2 set points, a fixed A/F set point is not practical. O2 control sets the total quantity of secondary air to a boiler. The CCM logic analyzes and sums the total airflow to the operating burners (ignoring out out-of-service mills and their associated burners). This total yields the A/F ratio independent of O2 set point. Dividing by the number of in-service burners gives an average burner A/F ratio. This is used as the target set point by the DCS. If all burners can be adjusted to this set point, combustion should, theoretically, be at its optimum point.
Crystal River 4 has compartmentalized windboxes encapsulating a single burner row and a single mill. Therefore the CCM A/F ratio is based on total airflow to each windbox compartment. The air is then distributed to the burners within that compartment based on the target A/F ratio and the coal flow measured to each burner at that instant. The target A/F is therefore further continuously optimized for each compartment and accompanying burner row.
Using PI Historian data, Progress Energy engineering developed a dedicated CCM data dashboard that can be remotely accessed. From here a user can analyze CCM data off-site and make recommendations to the plant as needed. With this tool, CCM systems fleet wide can be analyzed and maintained from a central location.
O2 Relocation and CO Expansion
As part of the CCM project, Crystal River 4 received new Yokogawa O2 probes and cabinets to replace the old equipment. Understanding that the existing O2 probe location was less than representative, multi-point test grids were set up to determine a better location, including the probe depth most representative of a true duct average O2. The pre-outage probe location was in a 40-foot-deep section of boiler back pass at close proximity downstream of the economizer. The project relocated the seven-foot probes to a 15-foot flue duct at the economizer hopper discharge. This provided a more consistent reading for all eight probes due to the mixing effect associated with the hopper and 90-degree turn.
In addition to the O2 probe relocation, carbon monoxide (CO) monitors were added to the unit at the same location as the O2 probes. CO monitors provide a more direct manner of monitoring combustion efficiency. Elevated CO readings are indicative of incomplete combustion because they measure carbon molecules that have not been fully oxygenized to CO2 through proper combustion.
Project Results and Benefits
Combustion tuning and optimization consisted of a dual-phase process. The first phase consisted of commissioning the coal flow and air flow measurement systems and subsequent continual A/F balancing through automation of Secondary Air flow actuation systems. Through A/F balancing alone a boiler efficiency gain of 0.25 percent was gained.
The second phase involved developing a new O2 curve for the unit using the relocated O2 and new CO measurement probes. As a result of the O2 probe relocation project, the consistency of the O2 profile improved from probe to probe. Prior to this project, O2 readings were not reliable due to the disparity of readings among the installed O2 probes, forcing the unit O2 control to be operated manually. Using the new O2 and CO probes, a new O2 curve was developed by lowering O2 levels at numerous load points across the control range until CO began to rise, leaving some margin at each loads resulting in a new O2 control curve. The unit subsequently was monitored and tested upon integration of the new control curve. Traditional combustion principles recommend limiting minimum full load furnace O2 to levels in the 2.5 to 3.0 percent range to avoid unsafe elevated CO conditions.
With the improved combustion controls of the CCM system, O2 control can be safely reduced to levels as low as 1.4 percent while maintaining CO levels below 100 ppm at full load under steady state operation. To avoid unsafe CO generation during transient periods, the final control curve specifies full load O2 control levels of 2.0 percent.
The new O2 curve developed for Crystal River 4 resulted in lower O2 set points at all loads. Using less O2 for the same load resulted in several benefits including reduced fan loading. Boiler NOx generation decreased by as much as 25 percent at part load and 5 to 8 percent at full load. This presents savings in ammonia reagent as well as SCR catalyst savings. Furthermore, a 0.25 percent boiler efficiency gain was realized due to the reduced dry gas loss and the reduction of moisture in the air. An overall boiler efficiency gain of 0.5 percent was achieved due to combustion optimization.
Since implementation of the CCM project, loss on ignition (LOI) levels have become less erratic and levels have decreased by approximately 1.5 percent compared to pre-project levels and are consistently lower than its sister unit, Crystal River 5. Additional unquantifiable benefits include improved ESP performance, reduced erosion due to lower flue gas velocities at full load and reduced risk of slag causing boiler reduced atmospheres.
Online burner coal and air flow measurement combined with advanced control strategies have been successfully integrated on multiple units at Progress Energy to reduce carbon monoxide, NOx and LOI. Projects like this have proven that balancing coal and air to the burners can be automated to allow for continuous combustion optimization at all loads. The addition of these upgrades to the plants at Progress has provided invaluable tools to the unit operators, regional and plant engineering, as well as central engineering groups located in Raleigh, N.C. and St. Petersburg, Fla. Progress Energy feels the CCM systems allow for greater fuel flexibility while maintaining a high level of reliability.
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