Dry system/modern controls upgrade Labadie ash handling
By Jon Shadduck and William Stillman, Union Electric Co.
One Union Electric power plant gained benefits in its switch from a wet to a dry flyash handling system featuring
Programmable logic controllers and video control panels linked by fiber optic communications highlight a new dry-handling system for flyash at Union Electric?s 2,400-MW Labadie Station near St. Louis, Mo. The dry system replaces the plant?s original wet sluice equipment, which was installed when the four 600 MW units of Labadie were completed in 1970. It was the largest coal-fired power plant in the United States at that time.
In 1991, air pollution compliance requirements and a need for normal upgrading of the Labadie ash handling systems prompted an evaluation of choices for the future. The alternatives studied involved whether to burn or not burn low sulfur coal, or to install scrubbers for compliance. An estimate indicated that low sulfur coal would be the best choice. The next step was to consider ash handling options, such as CO2 injection systems, wet or dry ash handling, or bottom ash scouring systems.
In the process, the engineers calculated the cost of switching to a dry ash handling system. That cost was high?several hundred million dollars through all of the power plants. A switch to low sulfur coal called for major changes in the coal handling system, modification of the coal pulverizers, added handling trains and much more. In total, a dry system showed a much higher capital cost than the alternatives. A wet ash handling system, for example, has a lower capital installation cost, but its high maintenance and problems with reliability offset that advantage in this case, especially when the plant is burning low sulfur coal.
It is worth noting that the existing wet ash handling system had created problems in the piping. Hence, a method of running bottom ash through the flyash transport pipes to scour out cementitious material buildup was an early consideration as an option.
With all of that, though, costs to install scrubbers or build new plants clearly would be higher. Hence, the Union Electric staff chose a dry ash handling system for Labadie at a cost that was roughly 25 percent lower than a wet system when considering capital, operating and maintenance costs, as well as flyash sales possibilities. Work began in August 1992.
Dry ash handling system advantages
Dry ash handling eliminates the problem of calcium scale buildup that developed in the piping of the wet system after the station switched from the high-sulfur Illinois coal to low-sulfur Powder River Basin (PRB) coal. PRB coal contains approximately 30 percent calcium compounds.
Ash now is blown approximately 3,000 feet through piping that travels from the precipitators to the silos, which are located at a new ash pond. System capacity is 220 ton/hr, considerably greater than the 48 to 68 ton/hr station output and more than a 2:1 safety factor over a 98 ton/hr maximum output required. This provides plenty of latitude for the system to burn Illinois coal again if needed.
The inherently higher cost and complexity of the new dry system will be offset in part by Union Electric?s recent contract with a major cement manufacturer to buy Labadie flyash. In a cooperative agreement, the cement company is building two additional silos and a loading facility on power plant property.
More extensive control
Overlaying this ash handling modernization project is a programmable logic controller-based (PLC) control system that replaces the original hardwired relay logic, which had minimal communications capability. The new control system features five Modicon 984-785 PLCs; one assigned to each of the four generating units plus another for an ash pond storage silo. The PLCs interface with an existing station-wide distributed control system (DCS) and its graphics terminals in the station control room. The DCS provides the primary operator interface.
Total input/output points handled by the PLCs is approximately 3,100, largely discrete. Approximately 1,100 logic networks per PLC were required to cover all of the interlocks and sequences required. Equipment in the process train is extensively duplicated, paralleled and interlinked to permit individual components to be serviced while ash transfer continues at full capacity.
The 984-785 PLC permits large amounts of data to be quickly gathered by the DCS and displayed for control room operators. The operators can set up, initiate and monitor process trains without leaving the control room.
A helpful and cost-saving feature of the new control system is a series of DCS display screens detailing equipment permissives gathered from the PLCs. Should a process train fail to start, the failed permissive within the PLC logic is immediately highlighted on the screen. A technician can then be directed to a particular valve that failed to open, for example. In the past, when the wet process failed to start, the technician often had to physically examine the entire train to find the problem. This effort could take an entire day.
Ash transferred long distances
Flyash removal from hoppers is continuous using duplicated, 100 percent capacity, 100 hp vacuum pumps, 1,600 cubic foot compartmented transfer tanks with airlocks and 500 hp blowers. Blower pressure of 16 psi is required to transfer the flyash 3000 feet; Union Electric knows of no other utility in the United States that is blowing flyash as far. To help equalize pressure throughout the pipeline, the first 1,500 feet is 12-inch diameter pipe and the remainder is 14-inch pipe.
Two 250-ton storage silos were built by Union Electric at a new 80-acre ash pond dedicated to flyash alone. Unsold ash is sluiced into this pond. The cement company is building two additional, interconnected 750-ton flyash silos.
Economizer ash is still wet sluiced, but in a different way. The hopper is batch-emptied into a storage tank using a 40-hp vacuum pump. Every two or three days, this ash is sluiced into bottom ash lines using a water injection pump. Although economizer ash sluicing cannot take place simultaneously with bottom ash transfer, no operating problems have arisen. An older pond receives bottom and economizer ash.
Video control panels for local operation
The ash handling system for each generating unit has five PanelMate Plus video control panels (VCPs) for local control?one outdoors on the grade at the vacuum/pressure transfer tank area, three in the precipitator hopper gallery for three flyash hoppers and one for the economizer hopper.
Each VCP has five or six custom graphics screens for monitoring and controlling hoppers, transfer tank airlocks, vacuum pumps, blowers, piping valving arrangements and others. Control is by means of membrane touchpads keyed to CRT-displayed templates containing control options and operating data. Video control panels permit service work to be accomplished without involving control room personnel in the details.
VCPs were selected instead of conventional pushbutton/pilotlight panels for this application because of their flexibility in configuring and altering control and data-
gathering options, extensive feedback to the local operator when troubleshooting, minimal wiring( a single coaxial cable from each VCP to its PLC), high reliability and lower cost. The VCPs are housed in NEMA 4X sealed enclosures having fan-cooled heat sinks. Instrument air purging moderates the temperature in the enclosure during weather extremes.
63,000 feet of fiber optics
Including the new ash and coal handling systems, 63,000 feet of fiber optic cables and redundant fiber optic modems were required to link PLCs, remote PLC I/O racks, DCS interfaces and industrial workstations over two 1.0 Mbps Modicon Modbus plus peer-to-peer local area networks. One network is dedicated to ash handling, the other to coal handling. The networks may mark the most significant use yet for fiber optic communications in a power station.
Reliability was the chief reason fiber optics were chosen. The electronic cables that formerly linked obsoleted control equipment proved to be highly sensitive to electromagnetic interference and moisture. This resulted in excessive failures in communication. For this reason, station management insisted on more reliable communications for the upgraded systems. END
Figure 1. Dry ash handling system communications network. Includes a bidirectional loop and redundant fiber optic modems for each of five PLCs.
Figure 2. Ash handling system control panel. They are located outdoors on grade adjacent to dry ash transfer tanks.
Coal handling control improved
To handle larger volumes of the low BTU PRB coal, station conveyor capacity was recently increased by widening and speeding belts. The Labadie Station?s coal handling system consists of rail unloading, two stacker towers, a traveling stacker and reclaiming equipment.
New coal handling controls were installed as well. Included are two sets of redundant 984-785 PLCs for overall logic. Twelve additional PLCs are dedicated to unit silo filling, stackers, coal sampling and dust suppression. Point count, all discrete except for a few feedrate PID loops, is approximately 3,600.
Six computer workstations that run process monitoring and control software provide for local control in coal receiving. Coal reclaim is controlled via the DCS only. Alarms and supplemental workman protection tagging (WPT)?through output point disabling?automatically appear on both DCS and coal receiving workstation screens. Supplemental WPT can be initiated through either system.