By Douglas J. Smith IEng,
As more and more power plants switch to low sulfur and higher moisture coals, grinding capacities are often reduced, forcing plant owners to implement various equipment upgrade projects or accept a unit derate. Installing larger capacity equipment is one option, but smaller-scale component upgrades are often more economic.
Pulverizers in today’s coal-fired power plants must be capable of handling a wide range of coals with varying grindabilities, moisture levels and ash properties. Over the years all pulverizer manufacturers have improved the design of pulverizers to reduce wear, improve fineness, increase capacity and maximize efficiency. Some of the major design changes have been the change from static classifiers to dynamic classifiers and the replacement of spring-loaded rolls with hydraulic adjusted rolls.
High-efficiency fan wheel after 20 weeks of operation at a large mid-Atlantic power plant. Photo courtesy of Robinson Industries.
In most modern pulverizers, coal is fed into the top through a central coal inlet pipe, where it then falls by gravity to a rotating table. Centrifugal force moves the coal outward where it is pulverized between the ring and the grinding rolls. When the coal load increases or decreases, the rollers automatically adjust vertically.
A nozzle ring circumferential to the outside of the grinding ring feeds pre-heated primary air to the pulverizer. This stream of low velocity primary air carries the pulverized coal particles upwards into and through the classifier. In the classifier the finer particles are separated from the coarser particles. From the classifier, primary air transports the finer coal particles to the burners while the heavier particles are returned to the grinding ring for further pulverization. Any tramp metal or pyrites fall through the nozzle ring openings where they are rejected into a hopper mounted on the side of the pulverizer. In addition to transporting the coal through the pulverizer, the pre-heated primary air also dries the coal.
Whizzer Wheel Replacement
Some pulverizers use exhauster fans that incorporate a paddle or “whizzer” wheel to transport the air/pulverized coal mixture to the burners. Unfortunately whizzer wheels are inefficient and in operation are subjected to considerable wear from the abrasive coal. As a result the maintenance costs and downtime of these fans are quite high.
Grinding problems at one mid-Atlantic coal-fired power plant convinced plant management to award a contract to Robinson Industries to analyze and come up with solutions to improve fan performance. The utility established two objectives: the generating units should maintain the rated load throughout the year and the pulverizers’ capacity must be increased to handle harder coals.
After reviewing the problem, the project team decided that the best way to meet these goals was to increase the exhauster fan capacity. Because increasing the fan’s capacity would increase the volume and pressure in the fan, the project required a complete redesign and rebuild of the exhauster fan and casing. Instead of using the whizzer wheel design, Robinson proposed a backward curved fan design.
Robinson began with three objectives for redesigning the exhauster fan: (1) The new design had to include improved wear resistance; (2) There must be an increase in fan efficiency; and (3) The fan design must increase the overall fan system flow.
Modeling the pulverizer and piping system eventually led to the development of the curvature and angle for a backward curved blade that would optimize efficiency and minimize wear in the exhauster fan. The next phase was to design the fan and the blade so as to reduce abrasion from the coal.
Robinson enhanced the initial wheel design by adding sacrificial inlet dust deflectors between the main blades. The sole purpose of these sacrificial deflectors is to direct the heavy coal particles between the blades rather than letting them impinge on the structural members. The deflectors are also removable, thus allowing them to be replaced quickly and at low cost.
To improve wear resistance, the blades, shroud, web, dust deflectors and the housing were all lined with a lightweight ceramic tile. The high wear areas incorporated a higher-grade ceramic lining to ensure maximum wear life.
After final design and laboratory testing, Robinson fabricated the new high efficiency wheel and casing to retrofit it into one of the pulverizers at the mid-Atlantic plant. Because of space restrictions, the new wheel assembly had to fit into the limited space of the existing whizzer wheel footprint. During field performance testing, the high efficiency backward curved design produced 16,000 acfm of air at 28 inches wg. This compared with the old whizzer wheel design of 13,000 acfm at 21 inches wg.
Robinson is working closely with the mid-Atlantic power plant to monitor the wheels during service. Robinson has checked the wheel and casing for wear after 20 weeks and 50 weeks. After the first 20 weeks of operation, the assembly was inspected and no wear was found on the ceramic tile or the structural members (see photo). The excess glue was worn off, but the ceramic tile and structural members showed little to no wear. The subsequent 50-week inspection also showed little or no wear to the assembly.
The power plant has not yet reached the two-year inspection point, but a more recent inspection did reveal some wear, according to Deanna Clamidori, Robinson Senior Sales Application Engineer. “We’ve found that paint injection techniques are not enough in certain instances to pinpoint wear areas,” said Clamidori. “Using CFD modeling, however, we were able to locate the wear locations and make some modifications to the wear material that we believe will eliminate the problem.”
To reduce NOx emissions at its Frank E. Ratts generating plant, Hoosier Energy installed low-NOx burners and flame scanners on Units 1 and 2 in 1993 and 1994. In addition, the coal supply changed from a 3-percent sulfur, low moisture bituminous coal to a 1.3-percent sulfur bituminous coal with higher moisture.
After the fuel switch and low-NOx burner retrofit, the plant began experiencing loss of ignition (LOI) values well above 20 percent and the scanner frequently signaled loss of flame condition following any minor change in burner operation. As a result the plant operators were constantly fighting loss of coal pipe permissives, which invariably resulted in pulverizer and boiler trips. In addition, although the grindability of the low sulfur coal did not change appreciably, the higher moisture did reduce the capacity of the pulverizers.
To resolve the problem, Hoosier Energy worked with Burns & McDonnell to study the complete fuel delivery system comprising the feeder, primary air system, pulverizers, classifiers and coal supply piping. The study found that the bar-type classifiers being used with the original pressurized Riley Ball Tube Mills were unable to meet the tight coal balancing and coal fineness requirements necessary for optimizing the performance of the low-NOx burners.
The Ratts plant considered several options for increasing pulverizer capacity. Initially, Burns & McDonnell recommended increasing the coal supply to the pulverizers and upgrading the existing classifier to a centrifugal (static) classifier. Although these changes would have increased capacity of the pulverizer to 47,900 lb/hr of coal, the plant needed to increase the capacity to at least 53,800 lb/hr to meet full station capability.
Installation of an HEP Dynamic Classifier. Photo courtesy of FLS miljo Inc.
After further investigation, Burns & McDonnell recommended replacing the old classifiers with dynamic classifiers, improving the coal loading to the pulverizers, and optimizing the coal balance in the fuel discharge system.
Following Burns & McDonnell’s recommendations, the Ratts generating station retrofit FLS miljo HEP dynamic classifiers on each the plant’s double-ended pulverizers on Units 1 and 2. The classifiers are a two-stage design with louvers and rotors that combine the advantages of static and dynamic classifiers. Using adjustable guide vanes, the classifiers are also able to balance the air-coal flow to each coal pipe leaving the pulverizer.
Short- and long-term performance of the classifiers has been excellent. Immediately after the retrofit, product fineness at full load measured 80 percent passing 200 mesh and 99.8 percent passing 50 mesh. For Unit 2, the unit heat rate prior to retrofit at 80 percent capacity factor was 9,987 Btu/kWh; for the three-week period after the retrofit, the heat rate averaged 9,469 Btu/kWh, a five percent heat rate improvement. Over the long-term, the classifiers have considerably lowered the high LOI levels experienced after the fuel switch and low-NOx burner retrofit, according to Jerry Stuffle, maintenance manager and assistant plant manager at Ratts.
“Since their installation, the dynamic classifiers have provided the fineness we expect,” said Stuffle. “By using the flow vanes mounted on the classifiers, we have the capability to balance the flow to the individual coal pipes leaving the classifiers, but to take maximum advantage of this capability, we need to be able to measure the coal flow effectively. We are in the process of installing Air Monitor coal flow measuring devices on each individual coal pipe so that we can balance the flow to the pipes and improve plant performance.”
CEC Upgrades Pulverizers
To develop a viable emissions compliance plan for its J.H. Campbell plant, Consumers Energy Company (CEC) analyzed the effects of switching from a 70/30 blend of eastern/western coal to 100 percent western coal from the Powder River Basin. Maintaining Unit 1 at MCR (Maximum Continuous Rating) would increase total coal flow to the pulverizers by about 20 percent because of the coal’s lower heating value, thereby reducing pulverizer capacity. The tangentially fired boiler was originally designed to fire Midwestern bituminous coal with a heating value of 10,700 Btu/lb, a Hardgrove grindability index of 55, and 24 percent moisture. The unit had five pulverizers, only four of which were required to maintain MCR with the design coal, but CEC was concerned that the fuel switch would result in no excess grinding capacity.
CEC identified several factors that limited the capacity of the pulverizers, including insufficient airflow of the exhausters and inadequate primary air temperature. The electric drive motors were also limiting the amount of coal the pulverizers could process. To ensure acceptable unburned carbon levels it was also important that the coal fineness be maintained. After investigating the different options, CEC decided to rebuild and modify the existing pulverizers rather than invest in a complete pulverizer replacement.
The design modifications included:
- The pulverizer exhaust that serves as the air delivery system from the pulverizer to the boiler would be modified to increase both pressure and flow.
- Additional power would be required to grind the increased coal flow and drive the modified exhauster.
- Direct fired auxiliary primary air heaters would be added to supplement the heat input of the air heater and thus increase the drying capacity.
- An improved dynamic classifier would be required to lower internal recirculation load and increase grinding capacity.
With these modifications the plant expected to significantly improve the grinding and drying capacities of the pulverizers and improve the fineness of the coal going to the burners.
CEC competed all of the pulverizer modifications during a scheduled 15-week outage in the Spring of 2001. Prior to the modifications the spillage from the pulverizers had been a problem. However, none of the pulverizers have had any significant coal spillage under any operating conditions since the upgrade. Similarly, the plant can now generate full MCR boiler steam flows with one pulverizer out of service.
The higher-speed exhauster, as expected, increased clean airflow, from the maximum pre-modification value of 126 percent of design airflow, to 180 percent of design airflow. Product quality has also improved, exceeding the 70 percent minus 200 mesh goal. The pulverizers can maintain fineness levels of about 75 percent minus 200 mesh with less than 0.5 percent larger than 50 mesh.