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Manitowoc Overcomes Coal Firing Challenge

Published Tuesday, June 1, 1999
by Ann Chambers, Associate Editor
clientuploads/05 About Us/Case History/ManitowocPU.jpgManitowoc Public Utilities (MPU) is Wisconsin’s largest municipally owned generating utility. Located on the western shore of Lake Michigan, it provides electricity and water to the city of Manitowoc and provides low pressure steam to local customers. Total generating capacity of the power plant is about 89 MWe.
MPU has completed several projects t extend the life of the existing plant and to improve its operating efficiency and flexibility. One of these initiatives involved fuel switching to reduce operating costs.
The plant needed to use less expensive run-of-mine and/or western coals. The less expensive fuels caused the plant problems, however, because of the large percentage of fines. When using a conventional feeder, coal with a high percentage of fines may be unevenly distributed front to back, resulting in elevated grate temperatures in the rear due to intermittent ash cover. This will eventually result in reduced life of stoker firing equipment.
MPU uses three Detroit Stoker RotoGrate stokers at the plant, so the utility approached Detroit Stoker asking for a solution. Detroit designed and manufactured a new feeder that could handle lower quality coals that are typically friable and produce a lot of fines. Feeder problems of material bridging, binding, and poor front to rear fuel distribution in the stoker are minimized with the new design. The new feeders combine air assist with an underthrow rotor mechanism. The new feeder design also has fewer moving parts and offers more protection from overheating problems associated with other feeder designs.
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Detroit Stoker provided six new feeders for boiler unit No. 6. The feeders were designed to fit into the existing 18 in. opening with no modifications required to the stoker front plate or the boiler pressure parts. The feeders are capable of delivering more fuel that the old style reciprocating feeders so steam boiler capacity was never an issue. Four feeders would have been adequate, but it was more economical to place new feeders in the existing spaces than to close off two of the boiler openings.
The feeder design discharges the fuel in the underside of the rotor, enabling the blades to contact the coal for a longer period of time. This design feature, in conjunction with the pneumatic distributor plate, facilitates a more horizontal trajectory of the fuel, which distributes the fine particles more consistently onto the bed area.  This leads to better combustion efficiency and an LOI reduction.
“We have had Detroit Underthrow Feeders operating at Manitowoc Public Utilities since June 1997. Consequently, we have been able to switch from expensive eastern stoker coal to run-of-mine fuel with fines in excess of 60%,” said Ray Struzl, MPU electric production manager. “This has resulted in an appreciable fuel cost savings for MPU. The fuel and efficiency savings have provided us the flexibility necessary to compete in the emerging deregulated ‘power choice’ market. The coal feeders’ successful performance and operation have led to our purchase of six additional feeders for another boiler unit at our facility.” 

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