Coal Handling System Modification Study
Tri-State G&T's 110 MWe Nucla Station
The Nucla Station originally consisted of three 1959 vintage, 36 (3 x 12) MWe, stoker-fired boilers. In 1988, the stokers were replaced by a new, dual combustor, 110 MWe AFBC boiler. A new 74 MWe topping steam turbine/generator, with extraction to the existing turbines, was installed at that time.
The in-plant coal handling system of the Nucla Station was altered in 1985. Pyropower Corporation of San Diego, California designed an extension to accommodate the AFBC combustors. Later modifications, the addition of a sampling system and revamping of the conveyor drives and platforms were also completed.
Tri-State engaged Energy Associates to investigate improvements for the in-plant coal handling system, which was part of the 1985 AFBC addition. This portion of the conveying system consisted of Stephens-Adamson Redler conveyors and tertiary crushers. The Redler conveyors featured an en-mass conveying design that allows these conveyors to elevate coal at a 66 degree incline. The plant was, however, experiencing wear and other problems with these conveyors. The conveyors were operating 20 hours per day. The plant considered conventional belt conveyors to be more reliable. Belt conveyors are used on the unloading, stacking, and reclaim portions of the coal handling system.
Several alternative configurations for new conveyors were investigated. The steep incline of the existing Redler conveyors dictated a configuration favoring high angle conveying technology. Accordingly, the substitution of high angle belt conveyors for the Redler conveyors was evaluated as a primary candidate improvement. Since belt conveyors require more space than the Redler conveyors, structural modifications to the building structures would be necessary. To minimize these modifications, improved en-mass conveyors were also considered. Since the tertiary crushers represented a bottleneck in the in-plant system, the ability to increase the capacity of the existing crushers and substitution of new higher capacity crushers were investigated. One alternative was selected by Tri-State as best meeting their operating objectives. These modifications were approved by Tri-State for a 1996 construction program. The modification utilizes high-angle conveyor technology. One of the steeply inclined en-mass conveyors is being removed and a "C" high-angle conveyor configuration is being added. Other changes include the elimination of a surge bin, addition of a splitter gate, and replacement of horizontal drag conveyors with belt conveyors.
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