PPL MONTANA generates power at two coal-fired generating plants, the J.E. Corette and the Colstrip facilities. Both of the plants are near Billings, Mont. Capacity at Corette is 185 MW. Colstrip includes two 330 MW units and two 803 MW units. Units 1 and 2 began commercial operation in 1975 and 1976, and Units 3 and 4 began in 1984 and 1985.
The Colstrip generating plant is the second largest coal-fired power plant west of the Mississippi. In addition, it is ranked as one of the lowest cost fuel plants in the Western Electricity Coordinating Council (WECC). WECC is a regional member of the North American Electricity Reliability Council that includes all the western states and the Canadian provinces of Alberta and British Columbia.
PPL Montana’s Colstrip coal-fired power plant.
Western Energy Company’s Rosebud mine supplies the Colstrip plant with low-sulfur, sub-bituminous Powder River Basin (PRB) coal. Although PRB coal is clean burning, the handling of the coal poses many challenges. Chief among these are the large quantity of fine particulates it contains. Although PRB has high moisture content, the fine particulates can dry out over time and increase the risk of spontaneous combustion.
PRB coal dust, which historically builds up on the walls of bunkers and silos and along conveyors and transfer points, can wreak havoc on dust collection systems. In addition, the dust build-up can be hazardous to staff and to plant equipment.
At the Colstrip plant, maintenance of the dust collection systems had been a continuing problem. As a result, PPL Montana sought a solution that would not only control the dust passively but would allow them to eliminate and remove the old dust collection systems.
In 2002, PPL Montana issued a request for proposal to design and construct a new transfer point from No. 6 conveyor to No. 7 conveyor in the Colstrip facility. Coal moves through this chute at 650 tons/hr onto a 30-inch belt. Similarly, in 2003, PPL Montana sought a similar solution at the mine conveyor head to the No. 60 belt conveyor transfer chute in the “C” transfer house. This chute transfers 1,800 tons/hr of coal onto a 48-inch conveyor belt.
Air Control Science, Boulder, Colo., was selected for both projects. A project manager, materials containment engineer and dust control engineer were given the responsibility for the design and fabrication of new chutes and transfer points to manage the dust passively.
Design of the new conveyor transfer points focused on controlling the coal dust by containing air movement within enclosures, eliminating spillage and minimizing coal carry-back. Smooth coal flow is critical as it minimizes dust generation. In addition, a modern chute design helps center the load on the belts near the belt’s speed, thus reducing wear and tear on the belt and the chute’s components. It also helps to prevent material degradation.
To develop a valid design for the chute and transfer point, and to verify the chute’s airflow, the engineers used Dust Analyst software and Discrete Element Modeling (DEM). The DEM creates a three-dimensional wire representation of existing conditions for modeling the chutes. 3-D simulation is verified against observed plant conditions before the wire frame model is converted to actual fabrication drawings.
Design of the No. 6 to No. 7 transfer point included a new 12-foot chute and a passive dust control re-circulation chamber. To reduce turbulence and noise in the load zone, and to prevent particle breakdown, dust, and belt and chute wear, the angle and curvature of the chute was matched to the speed of the conveyor belt.
To contain the dust at the point of transfer, the design incorporated a re-circulation chamber. The re-circulation chamber, a dustless transfer system, contains the dust at the transfer point. However, it still allows enough air volume for the dust to precipitate and return to the conveyor.
The same solutions were applied to the No. 60 belt conveyor transfer chute in the “C” transfer house. At that location the chamber and chute were adjusted to factor in the increased volume of the belt size and capacity. At this point the chute is 32-feet long.
Design, engineering, fabrication and installation of each chute and transfer point was completed in less than four months. During this period the chutes were only off-line for two weeks for the installation of the new chutes and transfer points.
Colstrip’s new dust control system.
Operation of the two chute transfer points has improved dramatically. Before the new design at each of the transfer points, coal dust and spillage was visible in the air and in the surrounding vicinity. Since installation of the new equipment more than one year ago, however, no dust has been observed.
With the success of the project, PPL Montana is now planning to replace the dust collectors with passive dust control systems at other transfer points. Re-engineering the coal-handling systems has helped the plant to eliminate dust and maintain a safe and healthy environment while allowing the plant to run at optimal capacity.