Energy-Tech Magazine

As of this writing, Boardman installed a real-time coal flow monitoring system on all 8 of its pulverizers and modified 6 of its 8 pulverizers with diffusers and adjustable orifices.

Slagging
Three months after the plant changed its coal supplier it experienced its first slag induced forced outage, followed by another within one month. After the second forced outage, the plant raised its excess O₂ set point from 2.0 percent to 2.7 percent. This seemed to provide some relief, with the frequency of forced outages dropping off. The problem, however, remained.

Excessive slag was forming on the leading edges of the finishing superheater and the wing wall superheater. Slag from these areas was the source of plant’s forced outages. To counter this, plant operators began to closely monitor these sections of the boiler and adjust the plant’s burners based on their observations. Concurrently, with training provided by Rod Hatt of Coal Combustion Inc., plant engineers began tracking key ash mineral characteristics.

Boardman

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engineers felt that combustion improvements could be made to stop the excessive slagging, but at the same time they could not ignore the fact that shortly after changing coal suppliers, the plant experienced slag induced forced outages. The overwhelming feeling at the time was the coal from the new supplier was the source of the plant’s problem.

As taught by Rod Hatt, it is necessary to look at the different components of the ash mineral analysis. To make any sense of these numbers, it is necessary to normalize these values to the sample’s heating value. The following elements of a mineral analysis were plotted and closely monitored:

Normalized Iron = (%Fe) x (%Ash)/Heating Value (1)
       
Normalized Ash = %Ash/Heating Value (2)

       
Figure 8 is a plot of normalized ash as calculated using Equation (2) with noted slagging events.

With plant operators paying close attention to slagging conditions in the boiler, and plant engineers tracking the ash mineral analysis, the plant was able to pinpoint excessive slagging conditions to a specific coal characteristic. In this case, it was the ash loading. Figure 8 shows several trains with high ash loading. It was this series of train deliveries that caused the noted slagging event. The supplier was contacted and the high ash loading was remedied. Using this information, the plant was able to make a decision about future trains with high ash loading. The plant has stacked out a train with high ash rather than send it directly to the plant to burn. Stacking a load from a train enabled the plant to blend it with a more acceptable coal. What is most important, the plant now has specific information to use to judge coal quality.

This information was put to use when the plant negotiated its new fuel contract.
Plant personnel, along with the company’s fuel purchaser and Rod Hatt, collaborated on a fuel specification. Different from previous contracts was the addition of a more restrictive ash loading specification. The contract specifies the supplier is to meet a 5 train rolling average for ash loading.

Previous contracts did not have this specification.

With the plant battling the noted slagging problems and efforts to improve combustion, plant engineers started to look at its under-fire air ports. Boardman’s boiler was built with 4 under-fire air ports located in each of the 4 lower corners of the boiler. Previous experience with these air ports led to operating with these closed. Noting the slag formation around these air ports, it was decided that they were sources of tramp air into the furnace. The under-fire air ports were leaking excessively and robbing air from where it was needed. During the plant’s annual spring outage of 2008, the 4 under-fire air ports were welded completely shut.