Power plants burn powder river basin coal (PRBC) to benefit from its relatively low sulfur content and price. The use of PRBC allows a power plant to meet stringent requirements on S02 emissions without a major investment in pollution control equipment such as a scrubber. However, PRBC also has an elevated calcium content that creates equipment erosion problems — in particular, accelerated wear in primary air and induced draft fans. There are several techniques that can be used to either prevent or reduce this wear, depending on budget and plant specifics.
If the boiler system was originally designed so that the induced draft (ID) fan handled dirty gas, the ID fan can sometimes be repositioned downstream of the pollution control equipment. This is the most effective wear prevention strategy, but may not be cost effective if the pollution control equipment was not designed to handle negative pressure.
Another effective approach is to install a mechanical dust collector upstream from the ID fan. Although these devices cannot meet today’s stringent particulate emissions requirements, they do a very good job of removing large wear producing particles (those above 40 microns in size) from the gas stream. They can also reduce the load on more expensive baghouses and electrostatic precipitators downstream. There is a capital cost for purchasing the equipment and a continuing horsepower cost from the additional pressure drop required to operate the collector.
A third option is right-sizing. Many boiler systems have primary air (PA) fans that are oversized for the service they are required to perform. Testing and right-sizing these fans can lower the volume of air handled, and therefore the velocity of the air handled, which can make a noticeable difference in preventing wear.
Reducing Fan Wear
If the prevention strategies mentioned above are unavailable or too costly, the next best strategy is to find ways to reduce wear. Over the years, fan manufacturers have developed a variety of techniques and materials for resisting fan wear. The simplest, and often least expensive, is to increase the thickness of the fan’s structural components vulnerable to wear. The hardness of the structural components can be increased as well.
Manufacturers also use flow disruption techniques to keep dust-laden gasses away from structural components. These “spoiler beads,” when properly positioned, lift the flow off the structural surface and take most of the wear themselves. The materials used to make spoiler beads can range from the soft filler metal used to weld the structural components to a high-hardness tungsten or vanadium carbide alloy.
The most common strategy for resisting wear, though, is to add wear plates of various sizes and materials to the most wear-prone areas of the fan. The materials used for these wear plates vary widely, but most fall into one of three categories: an alloy identical to, or similar to, the alloy used for the structural components; quenched and tempered steels; or composite materials with a hard-wearing surface applied to a weldable or boltable structural base material.
Thermally sprayed coatings provide yet another category of wear resisting strategies. Almost any filler metal that can be weld applied can also be sprayed onto a surface. Some spray methodologies incorporate ceramics and other non-weldable materials.
Any of these wear resistance strategies raise concerns over the increase in rotary inertia that accompanies any addition of material to a rotating assembly; reduced shaft natural frequency; and decreased structural integrity. Operating companies should also evaluate the inevitable increase in maintenance costs and decrease in maintainability that come with implementing these wear-resistance techniques.
In summary, PRBC has been sought for its low sulfur content, but it has created other problems due to the abrasive nature of its ash. Awareness of the problems and careful monitoring of the equipment exposed to PRBC fly ash can prevent costly failures and downtime.
If you have additional questions about the effects of PRBC or any of these wear prevention and resistance strategies, please don’t hesitate to call us at 1-888-663-2028. We’re happy to review specific cases, as well as discuss how some of these techniques can be combined to further protect primary air and induced draft fans.