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Upon starting up, the result of this action was dramatic. The excessive slagging immediately stopped in the finishing superheater. Normal sootblowing was able to completely clean these tubes.
For the first time it was possible to see bare tube metal at the completion of a finishing superheater sootblowing cycle. The decision to weld the under-fire air ports was based on sending combustion air to where it is needed. With an uncontrollable source of air being fed in at the under-fire air ports, the burners were being starved for combustion air.
Having improved on the coal flow distribution and eliminating a source of tramp air, it was possible for the plant to make judgments about coal quality and its effect on the boiler. A separation of these important facets of boiler operation is essential to ferreting out combustion problems.
In the fall of 2008, the plant was again forced to shut down due to a slag fall. It had not switched coal suppliers yet, and because of the improvements made to the plant’s combustion process, plant personnel still felt this forced
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Note again Figure 9, and the difference in the normalized iron content. After having operated for three months under the new contract, the plant concluded that this difference in iron has led to an improvement in plant reliability and performance. This conclusion is supported by plant observations of its slag, bottom ash and the ease with which its sootblowers are maintaining boiler cleanliness.
The boiler continues to look clean, with ash loading below 6 lbs/mmBtu. The plant still experiences an occasional slag fall; however the results have not been catastrophic. The amount of hard slag in the bottom ash has been notably reduced. This difference in bottom ash quality is being attributed to the consistently low iron content of the coal currently being burned. Additionally, being able to correlate an abrupt change in boiler slag formation to a specific characteristic – ash loading – will enable the plant to make decisions regarding future coal contracts and immediate plans for coal usage.
Summary
This article has shown how Boardman approached problems encountered with the process of burning coal. These problems would not have been found if the plant staff had not been diligent and tenacious in their testing, daily boiler inspections and coal analysis. The pulverizer modifications performed by the plant improved the combustion process by equalizing the coal conduit flows of each pulverizer. Having made these improvements in the combustion process, the plant was able to associate specific coal attributes with a slagging event. With this understanding, the plant will be able to competitively negotiate with coal suppliers for future coal contracts.
To be able to identify such a characteristic, it is imperative to take the correct steps to ensure good combustion, i.e., eliminate any confusion about the cause of any combustion problems. This is essential to improving the reliability and efficiency of this important generation asset. ~
Editor’s note: This paper, PWR2009-81079, was printed with permission from ASME and was edited from its original format. To purchase this paper in its original format or find more information, visit the ASME Digital Store at www.asme.org.
Dave Rodgers is the Technical Services and Maintenance manager for the Boardman Coal Plant, which is part of Portland General Electric. He is a registered PE in Oregon and Washington, and an ASME member. He has a B.S. in Electrical Engineering from the University of Kansas and an M.S. in Electrical Engineering from the University of Portland.
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