Coal-Fired Power Generation Handbook. James G. Speight

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СКАЧАТЬ easier (and more economical) to transport synthetic crude oil to market than to transport low-value oil sand. Similarly, it is much cheaper to transmit electricity over long distances by means of high-voltage wires than to move the equivalent tonnage of coal. However, the capital costs (and inconvenience) of constructing a plant in a remote area with a hostile environment near to a mine may dictate that this concept he impractical. Thus, the suggestion that coal be transported from the mine site completely or, in part, as a coal/water or coal/oil slurry in pipeline systems may have some merit and could afford a ready means of moving coal to markets using already existing pipeline system(s).

      Generally speaking, the majority of mined coal is transported to market by railroad, the remainder being shipped or trucked to its destination or used at the mine. Shipping coal by rail has become a major industry in many parts of the world.

      3.8.1 Unit Train

      The unit train is the most common form of long-distance coal transportation. A unit train is a group of railcars that operate in a dedicated shuttle service between a coal mine and a power plant.

      A typical unit train consists of 100 to 120 railcars and 3 to 5 locomotives, with each railcar holding approximately 100 to 110 tons of coal. Carefully coordinated loading and unloading terminals are necessary to minimize costs. A unit train making a round trip from mine to plant has a typical turnaround time of 72 hours, including a 4-hour loading and 10-hour unloading and servicing time per train.

      The system is designed so that the trains can be loaded and unloaded without stopping the train, thereby providing a continuous means of shipping the coal as well as an increase in the rate at which the coal can be moved from the mine site to the consumer (Lindberg and Provorse, 1977). As a typical example, it is difficult to drive on many roads in eastern Wyoming, on any given day, without passing several such trains carrying coal to market. In fact, rail service is the lifeline of the large majority (95%) of the western coal mines.

      Modern unloaders use rotary dump devices, which eliminate problems with coal freezing in bottom dump cars. The unloader includes a train positioner arm that pulls the entire train to position each car sequentially over a coal hopper. The dumper clamps an individual car against a platform that swivels the car upside down to dump the coal. Swiveling couplers enable the entire operation to occur while the cars are still coupled together. Unloading a unit train typically takes approximately three hours.

      Shorter trains may use railcars with an air-dump, which relies on air pressure from the engine plus a hot shoe on each car. When the hot shoe comes into contact with a hot rail at the unloading trestle, it shoots an electric charge through the air dump apparatus and causes the doors on the bottom of the car to open, dumping the coal through the opening in the trestle. Unloading one of these trains takes anywhere from an hour to an hour and a half. Older unloaders may still use manually operated bottom-dump rail cars and a “shaker” attached to the cars to dump the coal.

      The cost of shipping coal by train is often more than the mining costs. Using a barge or ship to move coal is a lot less expensive. In the United States there are 25,000 miles of waterways, but not enough to reach all destinations in the country. To reduce transportation costs, power plants are sometimes constructed near coal mines.

      3.8.2 Barge

      Barges on rivers and lakes play an important role in coal transport in the United States and Europe. Coal-carrying barges move in tows of fifteen to forty barges, pulled by a single towboat of 2,000 to 10,000 hp. A “jumbo”-size barge carries 1,800 tons of coal, so a large tow can move 72,000 tons of coal, as much as five unit trains. These large volumes result in significant economies of scale and lower costs. Barge rates can run (on a cost-per-mile or cost-per-kilometer basis) a quarter or less of rail rates. However, waterways often follow circuitous routes, resulting in slow delivery times.

      3.8.3 Pipeline

      Another method to transport coal is through a slurry pipeline. This connects a mine with a power plant where the coal is used to generate electricity. Coal slurry pipelines use a slurry of water and pulverized coal. For pipeline shipment, the coal is ground to approximately the size of coffee grounds and mixed with water to form the slurry. At the power plant the coal is either fed directly to the fuel preparation system or to a pond where the coal settles out and, at a later date, is re-slurried and then pumped to the fuel preparation system. The ratio of coal to water is approximately 1 to 1.

      The coal removed from the mine is crushed to a diameter of around one millimeter, and is mixed with water in holding tanks with agitators, which keep the coal in suspension in the water. The pipeline consumes around a billion gallons of water annually. After three days the slurry reaches the end of the pipeline, at the Mohave power plant, where it is held in agitated tanks, for immediate use, and in drying ponds, for later use. Heated centrifuges are used to get the water out. As of 2006, the plant was shut down because the coal and water supply terms are being renegotiated.

      Coal slurry pipelines are potentially the least costly available means for transporting coal to any location, measured in economic terms. Whether this is true with reference to any particular pipeline can only be determined by detailed evaluation of the conditions of the route. The current coal transportation scenario does not offer any choices between slurry pipelines and rail-road, which undoubtedly will necessarily minimize the cost of transporting coal. In this context the present times warrant assessment of the potential economic, environmental, and social implications of coal slurry pipeline development and transportation of coal through it.

      However, there is need for caution. The large water and energy requirements for coal slurry pose a significant barrier to further deployment, especially in arid regions of Australia and the western United States.

The coal log pipeline is another technology for transporting coal in which coal at the mine site is treated and compacted into cylindrical shapes (coal logs) (Liu et al., 1993). Then the coal logs are injected into an underground pipeline filled with water for transportation to destination which may be one or more than one power plants, or to a train station, a barge terminal, or a seaport, for intermodal transportation.

      The coal must have been cleaned and crushed, with a binding agent comprised of coal pitch, bitumen, or wax. The coal mixture is then tightly compressed and compacted as coal logs that are 5% to 10% thinner than the transportation pipeline. The logs are injected into a pipeline and pumped along using water. The pipeline can deliver the coal to coal-fired electric power stations or coal storage areas. The coal logs must then be crushed for use in fluidized bed, cyclone, or chain-grate stoker coal-burning boilers or pulverized for use in pulverized-coal combustors.

      Proponents of the coal log technology claim that in addition to being more cost effective than coal slurry, the capsule pipeline is also more environmentally sound because the coal logs eliminate coal dust erosion of the pipe interior and erosion of coal fines by rain at the power plant storage site.

      Since coal must be relatively dry before it can be burned efficiently, so the coal must be dried after it arrives at the power plant. Coal transported as slurry requires drying and electricity generation will be substantially less if it is not dried effectively.

      Coal logs do not require as much drying because they are packed so tightly that they do not absorb much water, and any water originally in the coal is squeezed out during compression.

      3.8.4 Truck

      Coal-carrying vehicles are typically end-dump trucks with a carrying capacity of roughly 25 to 50 tons. Truck СКАЧАТЬ