By Randy Calhoun, Fairfield Engineering Co. and Frank Bielser, Dynegy Midwest Generation
Dynegy’s two-unit Hennepin Power Station recently completed a coal handling automation project to upgrade the 50-year old tripper system feeding coal to its bunkers. The project, which included various mechanical and electrical upgrades, has resulted in more optimum allocation of manpower, enhanced safety, improved process control, and better access to system data.
When Dynegy merged with Illinova, the parent company of Illinois Power, in early 2000, the merger added 3,800 MW of unregulated generating capacity to Dynegy’s existing portfolio. Dynegy Midwest Generation, a division of Dynegy, Inc., assumed responsibility for the operation and maintenance of Illinois Power’s generation facilities throughout Illinois. Included in the acquisition was the Hennepin Power Station on the Illinois River in northern Illinois. Hennepin is capable of producing 289 MW of electricity burning coal and/or natural gas. The first unit went into commercial operation in 1953 and the second unit went commercial in 1959.
In recent years, Hennepin personnel had identified the coal handling system, particularly the tripper system delivering coal to the bunkers, as in need of improvement. The upgrade was particularly timely because it also dovetailed with the plant conversion to Powder River Basin coal in 2000.
Tripper floor at Dynegy Midwest Generation’s Hennepin Station.
Hennepin receives almost all of its coal by barge. Although some coal can be sent from the barge to the stockpile, the typical route takes the coal directly from the barge to the breaker building before being conveyed to the tripper for distribution in the bunkers. The bunkers hold a one-day supply of coal for the two units. Three feeder points off the bottom of the bunker feed the pulverizers for Unit 1, while the larger Unit 2 requires eight feeder points.
The original tripper system was completely manual. An operator was stationed on the tripper floor to monitor the bunker levels and reposition the tripper to the next fill position when necessary. The tripper required the operator to be in close proximity to operate the mechanical controls to start, stop, and reverse motion. A different operator was required to walk the rest of the coal handling system and start the individual conveyors and position the gates. These duties typically took most of the day to perform. The original tripper system was equipped with an automatic trip mechanism to send the tripper back when it reached the end of its traverse, but the Hennepin personnel had lost confidence in the ability of this mechanism to perform consistently.
Health and safety concerns also motivated the project from the beginning. Significant dust levels could be produced when operators were working on the tripper floor, and the tripper occasionally jumped out of gear, endangering workers and requiring frequent maintenance.
Dynegy Midwest Generation had four main goals for the project:
- Reallocate manpower from the tripper floor to perform other critical tasks.
- Improve system functionality and performance through automation.
- Provide new and improved process monitoring and data collection.
- Provide automated reporting features.
Of course, the solution also had to be cost-effective.
Operator’s overview screen for new tripper system
Because the upgrade would involve both coal handling and controls automation, Dynegy evaluated several system integrators before contracting with the Fairfield Engineering Company. During the first phase of the project, Fairfield generated a “System Study” that defined the areas in question and provided workable solutions. Dynegy Midwest Generation engineers and plant personnel worked closely with Fairfield to set goals and objectives, describe operational criteria, and establish a central architecture for the coal handling automation.
To automate the entire coal handling system, a number of tasks were required:
- Reworking the tripper to include an electric drive motor for remote operation.
- Adding ultrasonic level monitoring to the coal bunkers.
- Adding graphical operator interfaces to support operations from three different locations (crane cab, coal yard group leader’s office, and the tripper floor).
- Replacing the existing relay-based control system with a small PLC and associated I/O.
- Providing an Ethernet-capable PLC to allow collected data for coal usage and barge information to be accessible from the plant LAN.
- Interfacing to existing drives for remote setpoint provisions from operator interfaces. This remote control was added to support the blending requirements of the plant.
- Interfacing to existing scales to collect coal usage and rate information.
- Adding tripper positioning technology.
A conveyor belt engaging a drum and gearbox assembly drove the original tripper. For the upgrade, Fairfield reworked the tripper to include a new motor and brake, drive chain, and chain guard, and also modified the chute to accommodate onboard ultrasonic sensors. The project team mounted ultrasonic sensors into the tripper floor to monitor bunker fill positions and ensure the tripper did not dump coal into a full bunker area. Level sensors were also installed in the bunker compartments themselves to communicate with the tripper system.
The control system for the upgraded tripper was re-designed to support three modes of operation: Automatic, Manual, and Local. The Automatic and Manual modes require the PLC and at least one operator interface to be functional. The Local mode enables the operators to operate the system outside of the PLC from the local control stations. This design gives the operator greater system control flexibility and provided a means for a smooth changeover from the old manual system to the new automated system without affecting operations.
The remote tripper filling operation was based on a unique algorithm that Fairfield has developed to provide for the needs of a variety of operational requirements. The control scheme algorithm not only handles starvation issues, but also addresses equal filling of selected bunker positions, and a top-off mode by means of a tripper multi-pass solution. At Hennepin, a six-pass solution best fit the bunker fill requirements. The first pass gets each of the bunker areas to a 50 percent level, and five successive passes bring the bunker compartments close to full.
The tripper positioning system relies on radio frequency (RF) technology. RF identification tags are embedded in the floor at each of the 11 bunker fill positions (three for Unit 1 and eight for Unit 2) and an antenna is installed on the tripper. Hennepin considered laser positioning technology, which can be cheaper in the long run, but opted for the RF system because it provides a positive indication of where the tripper is at a given point in time and does not require re-calibration.
Fairfield designed the graphical screens with functionality and ease-of-use in mind. The screens include System Overview, Coal Flow Path Selection, Barge Data Entry, Coal Type Data Entry, Tripper Unit No. Bunker Setup, Tripper Control, Feeder Rate Setup, Manual Controls, Motor Run Time, and Alarms. These screens enable Hennepin’s coal yard operators to make sure tonnages are flowing properly, to answer alarms, to monitor blending operations, and to take quick action when equipment trips off.
A Visual Basic program interrogates the PLC to gather collected data. When started, the program syncs the time and date of the PLC with that of the PC on which the program runs. The program takes the collected information and places it in a database stored on the plant’s server. It also supports the generation of reports by querying the database for the requested report information and displaying it for viewing and printing purposes. Reports cover everything from how much coal is loaded each day to which bunker area received a certain quantity of coal. Reports can be generated more quickly with the new control and data acquisition system, dropping it right into a spreadsheet for easy analysis and evaluation.
Testing and Installation
Fairfield designs their PLC systems to allow complete software simulation of the process in question. This allows the screens and PLC program to be fully tested and provides the opportunity to train operators before system installation ever takes place. Dynegy took advantage of Fairfield’s “Customer Acceptance” time period, in which plant representatives visited Fairfield’s offices to run the system through its paces. Operations personnel were allowed to set up and run transfers from all the allowable paths and run whatever exceptions they could imagine.
One exception the operators tested involved the viability of certain coal handling paths. The control system correctly determined that the handling network could not simultaneously send coal from the barge to the reclaim pile and from the reclaim pile to the bunkers because common equipment would be in use for both tasks. Similarly, the system prevented simultaneous coal transfer from the barge to the bunker and from trucks to the bunker.
During this testing, the Dynegy personnel also determined that Fairfield configured the system to permit the bunker level to fall to 20 percent. Since only one 10-hour shift is allotted to coal loading at Hennepin, it is possible that individual bunker areas could run dry before the next filling cycle if a 20 percent limit were used. Therefore, Hennepin requested a 50 percent setpoint for the level sensors.
Hennepin’s daily fueling operations had to be maintained while the tripper changeover took place, which required careful planning. Working together, Dynegy and Fairfield completely installed the new system during off shifts, which allowed Hennepin to unload the barges and feed the plant from the barge, storage, or trucks during the day shift. Dynegy would top-off the bunkers by the end of the day and allow Fairfield’s contractor to perform installation tasks during the night shift.
The real challenge with the changeover plan involved the tripper modifications. The required work on the tripper was more than could be accomplished in a single evening. Once the existing tripper drive was removed, there was no turning back. The new system had to be operational for the day shift. The tripper motor and electrical control panel were mounted one night and the motor was tested for the correct rotation. The following evening the old tripper drive system was removed, new tripper axles were mounted, and drive chains installed. The tripper was now functional under local control.
After the changeover was completed, the system was started under automatic control and the remote filling process was tested and fine-tuned.
Dynegy Midwest Generation’s Hennepin Station.
Hennepin realized a number of immediate benefits from the upgrade project.
- The automation of the tripper removed the operator from close proximity to the moving tripper and freed him for other critical tasks.
- The barge crane operator could now start, stop, and purge the system from within the crane cab.
- The Coal Yard Leader received data automatically for barge information along with coal usage, coal types, source, and destination information. Comments and other crucial information could be entered into the Access database through Visual Basic.
There has also been a savings in the start-up time required for the tripper system. With the original handling system, operators had to go to each conveyor and start them individually. With the new system, one person can do this from one location. Troubleshooting and problem-solving capabilities have also been improved. If the tripper shuts down, for example, the control system does a better job showing that a plugged chute or some other fault caused the problem.
Randy Calhoun is a Project Engineer/Software Team Leader in the Process Controls Group of the Fairfield Engineering Company, located in Marion, Ohio. He has been with the Fairfield Engineering Company since 1988 and holds a bachelor’s degree in Computer & Electrical Engineering from Purdue University.
Frank Bielser is a Project Engineer, Construction & Maintenance, for Dynegy Midwest Generation in Decatur, Ill. He started with Illinois Power Company in 1982 as an Engineer at the Baldwin Power Station and has held various positions in maintenance, safety and generation dispatch throughout Illinois in the Dynegy system. Bielser holds a bachelor’s degree in Mechanical Engineering from the University of Illinois, Urbana-Champaign.