Complex Fuel Delivery System
Once the lignite is mined and separated (based on quality), the fuel is delivered by truck to the plant. As shown in Figure 4, the lignite-handling facility at Red Hills begins with a truck dump hopper/primary crusher (1), the 60-inch wide belt C1 conveyor with tramp metal detector, and 0.25% accurate scale. The 60-inch C2 conveyor delivers 1,200 to 1,600 tph of coal to Silo 1 or onto conveyor C3, which goes into Silo 2. The combined capacity of both silos is 40,000 tons. Conveyors C1 and C2 run at a constant 400 fpm belt speed.
4. Fuel life cycle. Fuel from Red Hills Mine is delivered by truck to the power plant’s truck dump hopper and will eventually make its way to the twin circulating fluidized bed combustion boilers. On this diagram 1 is the primary crusher, 2 is silo 2, 3 is the secondary crusher building, 4 is the boiler island, and 5 is the limestone building. Source: SABIA
Next, each of the 42-inch-wide C4A and C4B conveyors deliver about 500 tph of coal to the secondary crusher building. The size coming out of the secondary crusher is designed to be less than ¼ inch, although fragments up to 1½ inch are common. The plant can operate using either C5A or C5B, although normal operating practice is that one conveyor serves one of the two circulating fluidized bed (CFB) boilers, about 11,000 tons per day per conveyor.
Finally, the C5A and C5B conveyors deliver coal to the tripper deck and day silos on the boiler island. Contractually, this is the lignite sales point. The pay scales are located on C5A and C5B, as is the automatic sampling system. Because there are two belts, there are two pay scales and two primary cutters on the sampling system. Coal feed is from the day silos to feed the two boilers. Limestone used for SO2 control in the CFB is stored in the limestone building.
A Contractual Quandary
The mine must ship at least 3.6 million tons of lignite to the plant each year to meet the mine’s financial goals. Also, the original fuel supply contract required the ash content in the coal to be no greater than 23.5% by weight. However, coal with high ash variation can cause considerable boiler combustion problems that will show up as reduced plant reliability. Once commercial operation began, operators determined that the practical upper limit for coal ash content was 18%, causing rejection of many fuel deliveries. Any significant quantity of fuel with ash content greater than 18% caused combustion problems and required a plant output derate or, in extreme situations, caused a plant outage. Historically, this happened several times a year.
Thus the mine’s dilemma: When meeting the plant’s fuel specification, the plant often couldn’t burn the supplied coal, causing financial distress to both parties. The mine – power plant team decided that there must be a way to control the quality of the lignite so that the ash content never exceeds 18%. If that were the case, the mine could profitably sell its fuel and the plant could produce reliable electricity. It would be a true win-win proposition.
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