By Neal R. Hood and Kristi Simmons, Alabama Power
Plant engineers at Alabama Power recently decided to upgrade damper drives for each of two coal-fired units at the James H. Miller Jr. Electric Generating Plant. The two units, similar in design and having 25 years or more of service, each have a 720 MW capacity. High maintenance levels for the original drives, along with a growing scarcity of parts, provided the economic justification for replacing 45 damper drives for each boiler.
After a comprehensive engineering study, the plant chose Intelligent Contrac electric rotary actuators from ABB Instrumentation to replace the original drives. The replacement drives have smaller footprints and are relatively simple to retrofit. Replacements went smoothly during planned successive month-long outages of the two units.
Plant engineers expect the new drives, which offer more precise and responsive damper positioning, to substantially improve boiler performance and combustion efficiency. According to Electric Power Research Institute (EPRI) calculations, improving excess O2 control 0.5 percent with tighter forced draft/induced draft fan damper positioning can result in 0.15 percent heat rate improvement. For a 500 MW coal-fired boiler, fuel savings could be $120,000 per year. The replacements have also reduced maintenance costs at the plant for drives due to their 10-year service interval.
The sub-critical Babcock & Wilcox balanced-draft drum boilers for the generators burn sub-bituminous western coal to produce steam for the turbines. Seven pulverizers for each unit feed ground coal to the wall-fired furnaces.
As indicated in Figure 1, two damper drives (red bars) control heated and tempering primary air to a common duct. A third damper from the common feeds hot air to each pulverizer. A distributed control system (DCS) positions these 21 dampers to maintain a target temperature within the pulverizers and to develop the desired level of air flow to transport ground coal to the furnace burners.
Figure 1 Damper Drives
Damper drives (red bars) control the flow of air to the pulverizers and windboxes. They also provide a negative draft within the furnace and exhaust system.
Secondary air from the two forced draft (FD) fans runs to the seven windboxes that surround a row of burners. A pulverizer feeds ground coal to each of the seven rows, as shown. The DCS positions two dampers on each windbox to provide air for maximum combustion efficiency and clean emissions. It controls ID fan inlet dampers while fan speeds range between 17 percent and 24 percent of maximum. At higher fan speeds, the dampers are full open and the draft is strictly controlled by fan speed.
These windbox dampers, plus dampers on the outlets of the FD fans and inlets/outlets of the four induced draft (ID) fans, bring the total of new drives to 45 for each generating unit. The plant plans to add new drives to the two primary air (PA) fans at a later time.
Torque requirements vary, depending on drive function. All are capable of continuous modulating duty with positioning accuracy of 0.1 percent for optimum process performance.
- Primary heated and tempering air drives (14): 200 ft. lbs.
- Hot air drives to pulverizers (7): 1,900 ft. lbs.
- FD fan outlet drives (2): 1,900 ft. lbs.
- Secondary air drives for windboxes (14): 400 ft. lbs.
- ID fan inlet drives (4): 3,000 ft. lbs.
- ID fan outlet drives (4): 3,000 ft. lbs.
Each new drive comes equipped with field-mounted Contrac electronics units and HART protocol communications capability. The processor-controlled electronics unit receives 4-20 mA setpoint signals from the DCS and feeds back 4-20 mA signals representing actual actuator positions.
Retrofitting the new drives involved changeover at the DCS of the original signal range (-10 to +10 volts) to the Contrac signal range of 4 to 20 mA. In addition, ABB provided new linkage between the drives and the dampers. Alabama Power fabricated mounting plates where necessary.
The electronics unit drives the actuator’s very reliable asynchronous three-phase motor with cage rotor. Using advanced frequency converter technology, the unit can independently vary the torque or travel speed of the intelligent actuators. The actuator continuously follows the damper drive setpoint signal from the DCS. The stepless motor increases or decreases the torque smoothly and proportionally as the damper drives seek the desired position. The actuator force and the process counter force maintain a continuously balanced condition.
The electronics unit eliminates the need for torque or limit switches. If the voltage is cut off, the motor is de-energized and a brake at the motor’s rear shaft-end holds the damper drive in its latest position. Additionally, the electronics unit logs operating, maintenance and service information. It provides predictive diagnostics with advance warning of required maintenance service.
The drives are virtually maintenance-free. Depending on the load, the interval between maintenance attention can be up to 10 years.
An ABB startup engineer helped commission the drives following their installation to insure proper operation. The HART compatibility permitted commissioning of the drives via ABB’s FDT (Field Device Tool) AssetVision software and Contrac DTM (Device Type Manager) interface within a notebook computer. The notebook connects to the drive’s electronics unit via a frequency shift keying (FSK) modem during commissioning (see Fig. 2). Asset Vision’s graphical user interface simplified drive set up for the startup engineer. The software also comes into play for future diagnostics and maintenance.
Figure 2 Device connection schematic
The Contrac drives connect with the DCS via HART, permitting commissioning and diagnostics via ABB SmartVision software in a notebook computer.
Operators and maintenance specialists at Alabama Power have the option to set and alter basic actuator parameters via a small service panel at the electronics unit. In addition, by turning a handwheel on the actuator, operators can manually position a damper in case of an interruption of the control signal.
Authors: Neal R. Hood is a maintenance specialist and Kristi Simmons is an air and gas engineer at Alabama Power, a Southern Co. subsidiary.
Big Stone II coal-fired power plant cancelled
Montana-Dakota Utilities Co. (MDU), along with several other partner utilities behind the planned Big Stone II coal-fired power plant, said the company will not go on with the $1.6 billion project. The project required additional participants but none committed, according to a release from MDU in early November.
The planned 630 MW plant was to be built in South Dakota, but Otter Tail Power Co., the project’s lead developer, pulled out in September. Great River Energy also pulled out of the project in 2007. Both companies said they would not commit to their share of the project. The recession and uncertainty about federal carbon legislation also played a part in their decision.
The other Big Stone II Project participants were Central Minnesota Municipal Power Agency, Heartland Consumers Power District and Missouri River Energy Services.
“We have a purchased power agreement through 2015 that was to bridge us to Big Stone II going online; we still have that agreement in place,” Montana-Dakota President and CEO Dave Goodin said. “We will now look at other supply options that are reliable and cost-beneficial for our customers. We have plans to expand our wind production by 30 MW in 2010 and will review other generation options.”