Although coal continues to fuel about 50% of U.S. electricity production, coal-fired generation has spent the past few years in the wilderness. Burning coal has become synonymous—at least in public discourse—with poisoning the air with SO
2, NO
x, and mercury and generating huge amounts of carbon dioxide, a greenhouse gas now considered a major contributor to global warming.
Recently, however, coal has regained popularity among utility resource planners. To comply with Clean Air Act and other mandates, billions of dollars are being spent to retrofit gigawatts worth of old coal-fired units with flue gas desulfurization systems—scrubbers. The first sizable new U.S. power plant fueled by pulverized coal in a decade has come on-line (see the cover story in POWER, September 2006). A few utilities are planning to build integrated gasification combined-cycle plants powered by coal (see this issue's story on IGCC), and others are sure to follow.
All types of coal-fired plants benefit from being able to blend and burn different kinds of coal. The rationale for blending may be cost reduction, higher plant output (mixing in coal with a higher heating value), or reducing emissions of SO2. The best example of the last rationale is the ongoing wholesale switch to low-sulfur Powder River Basin (PRB) coal by power plants in eastern states.
Most plants cannot switch from burning only high-sulfur Eastern coal to burning PRB coal exclusively without derating units and incurring excessive modification costs. Why? PRB coal has a lower heating value than Eastern coals, and its higher friability results in the formation of more slag on boiler and furnace surfaces. As a result, many plants are choosing to blend Eastern coal with PRB or another low-sulfur coal at the maximum reasonable percentage.
For a new power plant, keeping the blending option open requires considerable attention during the design of its coal-handling facility. It may be economically beneficial to blend a small percentage of a second fuel with the design coal. The second fuel may be an out-of-specification coal either received in error or purchased at a substantial price advantage, a coal with a higher heating value, or even a non-coal fuel such as recycled tires or wood chips.
Coal-handling systems have three major subsystems that must be considered both individually and collectively for their compatibility with blending: the unloading system, the stockout system, and the reclaim system.
Unloading upgrades
Traditionally, more than half of the total coal tonnage received by U.S. power plants has arrived by rail. That percentage continues to rise. With the switch to PRB coal, plants along major rivers such as the Mississippi and Ohio that used to have their fuel barged in from local mines now get it delivered by railroad.
Many PRB coal-burning plants either have had to add new facilities for unloading coal from rail cars or upgrade their existing rail-unloading facilities to handle higher-capacity railcars and more railcars per train. An example of the unloading system modifications required is switching from a rotary car dumper or a conventional bottom dump hopper to a high-capacity, rapid-discharge bottom dump system.
Staying stocked
Some existing coal stockout systems can immediately accommodate stockout and storage of a second coal type, but others must be modified to do so. Generally, those in the former category can stock out two or more coals in segregated piles. Such systems normally form piles whose relative size matches the blending ratio. It's usually good practice to set aside enough space for each pile to accommodate an entire shipment of its coal type.
Following are the most common configurations of blending-ready stockout systems and recommendations for modifying existing systems for handling multiple fuels:
- Two stacking tubes (lowering wells) or fixed stockout conveyors. This configuration produces two storage piles of equal height, ideal for a blending ratio of about 50/50. If your plant has only one stacking tube or fixed stockout conveyor, add a second one of the same type. The addition can be a separate system deployed at a transfer point or a modification of the discharge of the existing system, so it can feed the second system.
- One radial stacker can produce two piles of any size for any blending ratio.
- One traveling stacker can produce two or more storage piles of any size, suitable for any blending ratio.
- Two stacker/reclaimers, which normally produce two equal-size storage piles (based on travel length), are also suitable for a near-equal blending ratio. To widen the range of possible blending ratios, use your existing stacker/reclaimer to create two piles of different size and a secondary method for reclaiming from the smaller pile. Add a second stacker/reclaimer only if a near-equal blending ratio is the objective.
- Two coal yard silos or domes normally produce two equal-size storage capacities (based on height) for a near-equal blending ratio. Add a second silo or dome to increase the blending flexibility.
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