Upgrade Modification Study
for
Nucla Station's Coal Handling System

Client
Tri-State Generation & Transmission Association
Nucla Plant
P.O. Box 698
Nucla, Colorado

Completion Date
January, 1994

Project Description
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 coal handling system of the Nucla Station has been expanded and altered at different times. The coal handling system consists of the following phased modifications:

1. The original system was designed in 1957 and 1958 by Hack Engineering Company of Denver, Colorado.
2. In 1977, Thomas Systems Inc. of Fort Worth, Texas designed a major expansion for the coal handling system.
3. In 1985, Pyropower Corporation of San Diego, California designed an extension to accommodate the AFBC combustors.
4. The plant has implemented several modifications to alleviate handling problems and handle emergency breakdowns.

When the Nucla Station's MWe rating was nearly tripled in 1985, the capacity of the coal handling system was not increased. As a result, the coal handling system was operating 16 to 20 hours per day.

A number of problems plagued the coal handling system. These include oversized "boulders" (lumps larger than 30 inches) from the open pit mine; fines which cling to chutes, hoppers, and vibrating feeders; widely fluctuating feed rates; spillage; excessive conveyor trips; and high wear on the Redler conveyors installed with the Pyropower combustors.

Several improvements were evaluated and recommended. To eliminate the oversized "boulders" at the source, a sled mounted grizzly was recommended for the mine. The grizzly separates the "boulders" from the properly sized coal as an economical alternative to an in-pit crusher. To reduce the wide fluctuations, a combination of hopper level switches and belt feeders to replace the vibrating feeders was recommended. An evaluation of the belt conveyors determined that the older conveyors were underpowered. To eliminate conveyor problems, new upgraded drives and higher belt speeds were recommended. Changes in chute design for the secondary and tertiary crushers were recommended to provide a vertical drop of coal into the crushers. The plant completed the recommended unloading, stacking, and reclaim modifications in 1994. The truck hopper belt feeders are being installed in the photograph above.