The Problem

Northern States Power (NSP) has a 384 megawatt coal-fired power plant in Minneapolis, MN with coal bunkers built more than 30 years ago. After switching to a low sulfur sub-bituminous coal, NSP experienced several fires and one explosion in its unit 7 coal bunker. Immediately, a task force known as "Operation Cease Fire" was set up to investigate and eliminate the spontaneous combustion problem in all of NSP's plants.

Jenike & Johanson's Expertise

Jenike & Johanson (J&J) engineers determined the problem to be the funnel flow discharge pattern of sub-bituminous coal through the bunkers. The first-in last-out flow sequence allowed stagnant regions of coal to remain indefinitely. Such stagnant coal is highly susceptible to spontaneous combustion.

Samples of the PRB coal were sent to our lab for flow property tests, using the widely accepted Jenike Shear Tester. With cohesive strength and wall friction data now available, we were able to recommend modifications that would provide mass flow to the existing bunkers, where all of the material is in motion, which provides a uniform residence time for all of the material.

The Result

According to Jeri Huggins, Production Engineer at NSP, "It is a definite improvement to go an extra 6 hours between refills, but most important is the fact that no fires or explosions have occurred since the bunker was modified. The testing and design recommendations provided by Jenike & Johanson, as well as the engineering assistance from Quadrant Engineering Plastic Products on the UHMW installation, have been an unqualified success from start to finish."

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