Despite best efforts at dust collection and other improvements to the local environment, a coal bunker building is still an unpleasant place to work.
Some dust is inevitable, interior temperatures can be extreme, and slip-and-fall hazards make checking coal inventory levels manually a challenge. Dynegy sought to address these issues and improve operator safety by automating level measurements of coal in its bunkers.
An operator following the long-standing manual measurement procedure lifts up a roof section and peers into the bunker with the aid of a flashlight (Figure 1). This presents a host of problems including difficulties seeing through coal dust, accurately determining actual level and avoiding slip-and-fall hazards. These issues are exacerbated because the operator typically works alone.
Figure 1: Plant operators found it difficult to ascertain the remaining amount of coal by peering into the bunker.
An automated means of continuously and accurately measuring level was needed, but this would require an innovative solution capable of reading the characteristic peaks and valleys that form in coal bunkers. To address this and other level measurement issues, Dynegy turned to Emerson. Finding a solution began with examining the nature of the challenge.
Coal Bunker Operations
Dynegy’s 1,185 net MW coal-fired power plant in Baldwin, Illinois started operation in 1970. The bunkers for Units 1 and 2 are both about 186 feet long, 35 feet wide and 48 feet tall. Coal is fed into the top and then flows from the outlets into the coal feeders that transport it to the cyclones.
Coal from the coal yard is supplied to the unit via parallel conveyor belts. Each belt has a tripper car that takes coal from the conveyor belt and deposits it in the coal bunker. There are two parallel openings for each belt along the length of the bunker room floor (which is also the coal bunker roof) where the coal is gravity-fed into the bunkers. These openings are covered by a dust belt. Each tripper car is capable of fueling the entirety of the Unit 1 and 2 bunkers.
Limit switches are installed on the north and south ends of each bunker, and the tripper car continually transverses back-and-forth between these switches during fuel filling operations (Figure 2). The tripper car can be operated manually to top off or fill specific areas of the bunker as needed. Only one side needs to be running to maintain coal supply to the plant, so it is a redundant system in some ways because it allows the plant to maintain its fuel supply in the event that one of the conveyor belt systems is out of service.
Figure 2: The tripper car travels back and forth on a track, dumping coal into the bunker along the way.
The tripper car operates semi-automatically but the control room has no way of knowing where it is on the track, or if it is operating correctly. An operator has to be present at all times to verify tripper car operation and to monitor level visually. Due to the nature of this environment with its dust and possible elevated carbon monoxide levels, the operator is required to perform duties in a high hazard area for long periods of time. With the current procedures in place, it also makes it impossible to measure bunker level with any precision.
An operator must be working in the bunker room and in communication via radio with the coal yard and the control room. He or she determines the bunker level by peeling back the dust belt/mat to look inside the bunker with a flashlight, in effect acting as the level instrument. The operator determines which bunker needs coal and at what point the tripper car can be moved to the next bunker. The operator must also tell the control room personnel how long they need to keep supplying coal to the belt. Since the operator can reliably only see only about 15 feet into the bunker, he or she always fills the bunker to within a foot or so of the roof to provide a safety margin, which is often not the desired level.
Innovative Level Measurement Solutions
Most level instruments are designed to measure liquids which settle to the same level in a tank or vessel, so only a single point on the surface needs to be measured because that one location will be at the same level as at any other point within the vessel. There is no dust to contend with, so liquid measurements are relatively straightforward.
However, the same is not true of solids. Few solids flow readily enough to spread evenly, and their tendency to clump, pile and stick can change with atmospheric conditions. With filling and emptying cycles, a multitude of peaks and troughs can form and constantly change as product is added and removed. Depending on how much material can pile up before sliding and the width of the vessel, the difference between a peak and a trough can be as much as 100 percent. Coal, in particular, produces heavy dust, adding to the difficulties in measuring its level in bunkers.
After examining the issues in detail in conjunction with plant personnel, experts from Emerson and Experitec, a member of Emerson’s Impact Partner Network, recommended the application of Emerson’s Rosemount™ 5708 3D Solids Scanner instrument. The acoustic transmitters in this instrument use phased-array antennas to gather multiple points of measurement, which are used to generate a 3D scan of the product surface in a vessel (Figure 3).
Figure 3: Emerson’s Rosemount 5708 3D Solids Scanner provides continuous online volume measurement including visualization of the various peaks and valleys within vessels.
After sending signals using three different frequencies, the antennas receive multiple echoes from the walls and contents. Using phased-array antennas, the scanners continuously measure the direction and distance of each echoed signal and generate a coordinate of the echo inside the vessel.
Digital signal processing within the transmitter samples and analyzes the echoed signals, and produces accurate measurements of the level and volume across the entire surface within the wide beam angle of the device. Matching the received data with known vessel dimensions allows the instrument to calculate product volume, providing accurate values continuously.
In most applications, a survey is taken of the storage vessel or container to determine optimal placement of each transmitter. But in this case, the 13 existing openings in the bunker roof had to be used to mount the transmitters because it was too expensive to drill new holes in the thick concrete roof (Figure 4). This limitation forced Emerson to produce a custom design, using the output from each of the 13 transmitters as inputs to a software program hosted on a PC, referred to as the bunker scanner controller (BSC).
Figure 4: One transmitter was mounted in each of the 13 existing holes in the roof of the coal bunker.
Although the software program is a standard Emerson platform designed for use in conjunction with the 5708, it had to be customized for this particular application to produce the required 3D view of the coal levels in the bunker (Figure 5). Further customization was required to generate data required by the plant’s Ovation distributed control system (DCS).
Figure 5: The level measurement system provides a clear, graphical view indicating the level of coal in the bunker.
Connecting the signals from each transmitter to the BSC also presented a challenge (Figure 6). Each of the 13 transmitters was connected on an RS-485 digital communications network. This network cabling passed through an intrinsically safe (IS) barrier to limit the amount of power available to on the network, making it safe for installation in this Class II, Division 1 hazard area. A USB RS-485 to Ethernet adapter allowed direct connection to the BSC.
Figure 6: This system architecture diagram shows the connections between the 13 level transmitters and the bunker scanner controller.
Displaying level information on the BSC in the control room and transmitting this information to the DCS eliminated the need for an operator to measure level visually, but this was only the first step to improving operator safety.
As previously mentioned, the control room had no visibility of the tripper car or bunker filling actions. The operator transmitted all information regarding its operation verbally via radio.
To improve on this situation, plant personnel installed two cameras in the bunker room, one on the south end and the other on the north. Each camera (Figure 7) is connected to a root access point (RAP), with the two RAPs transmitting video to the bunker system Ethernet switch. This switch is hardwired to the fiber breakout panel, which is in turn hardwired to a second bunker system switch in the control room. This second switch is hardwired to the control room 3D level and camera view station, and also to the Ovation DCS. Once data is received by the DCS, it is integrated into the plant’s data historian.
Figure 7: This diagram depicts the overall system architecture for the upgraded bunker room control and monitoring system.
The plant also installed a laser position system to track the position of the tripper car. A stationary laser source is mounted at one end of the tripper car rails, and it is pointed at the back end of the tripper car where a reflector is placed. A receiver on the laser system reads the returning signal to determine the position of the tripper car. This data is then transmitted to the control room on the same newly installed fiber optic cables used by the scanner controller and the cameras.
Operators are now provided with mobile operator tablets connected to the bunker system via a wireless network under the direction of the wireless LAN controller, allowing them to direct filling operations from any location, to fill each bunker compartment to its optimum level.
Operator safety improved substantially by greatly reducing the amount of time they were required to spend in the bunker area. Without the need for operators to measure coal level in the bunker manually, or remain and observe operation of the tripper cars, having individuals in this dangerous area was no longer necessary.
All components and systems are now in place to facilitate the next project: fully automated filling of the bunkers from the tripper car. This is expected to take place in three steps, the first of which is already completed:
1. Tripper car position continuously supplied to the DCS (complete).
2. Tripper car controls updates to install retractable reel-type cables with higher data capacity, expected completion by the end of 2019.
3. Fully automated fueling, complete by end of 2020. The plant plans to completely automate their coal fueling process by combining information from the 3D level scanner system and the tripper car laser position system. These and other existing information will be used as inputs to the Ovation DCS, and the DCS will then use this data to control the tripper car via the new and upgraded tripper car cabling capabilities. This upgrade will allow the operation to run without putting personnel in harm’s way, and will only require a periodic check of the area to detect and deal with any maintenance issues.
Fully automated fueling will allow the current operators to be assigned other duties, thereby eliminating the need for possible overtime or additional labor costs.
Completing these steps will allow the plant to automate fully the transfer of coal from the tripper car to the bunker, which will be one of the first installations of this type in the world.
(Editor’s Note: Figures all courtesy of Luminant)
About the Author: Dave Wombacher is a Senior Engineer at Luminant’s Baldwin Power Station, responsible for plant operations and maintenance since joining the company in September 2014. Prior to his current position, he was the Mechanical Maintenance Supervisor at US Steel – Granite City Works from 2011 to 2014. Dave holds a BS in Mechanical Engineering from St. Louis University, granted in May 2009.