Coal-Fired Power Generation Handbook. James G. Speight
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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 delivery is used extensively for small power plants in the eastern United States.
Coal can be moved by truck over regular highways in vehicles with 15 to 30 tons capacity. Coal can also be transported by large off-road trucks with capacities ranging from 100 to 200 net tons. These trucks are almost always diesel-powered with back or bottom dump.
Specially constructed roads for coal hauling are extensively used for mine-mouth power plants in the west, south, and east, while the hauling of coal by trucks on highways is more concentrated at surface mines. Truck hauls on public highways in the United States typically range from approximately 50 to 75 miles while off-road hauls are approximately 5 to 20 miles.
Trucks are the most versatile of all transportation modes for coal hauling because they can operate over the widest areas where roads are available.
However, adverse environmental impacts resulting from truck coal hauling are coal dust particle releases during coal loading or unloading, and coal dust entrainment during transport. Some coal will escape from the trucks during transport because the loads are normally uncovered. The coal dust tends to wash off roadways during rainstorms, causing aesthetic unsightliness and contamination of runoff waters. The air pollutant emissions from diesel fuel combustion add to the emissions.
3.8.5 Ocean
Ocean transport of coal requires a system of (i) transportation from the mine to the port, (ii) coal-handling facilities at the export port, (iii) ocean carrier networks with adequate number and size of ships, contractual obligations, management of the fleet, and route decisions, (iv) coal-handling facilities at the importing port, and (v) transportation from the port to the customer.
Ships are commonly used for international transportation, in sizes ranging from (i) handy size vessel, which is approximately 40,000 to 45,000 dead weight tons, DWT, a term normally taken to mean a vessel of approximately 10,000 to 40,000 DWT, (ii) a panamax-size vessel, which is approximately 60,000 to 80,000 DWT; technically, the maximum size vessel that can pass through the Panama Canal is restricted to a 105-foot beam, and (iii) a cape-size vessel which is capable of carrying >80,000 DWT; this is a vessel that is too large to transit the Panama Canal and thus has to sail via Cape of Good Hope from Pacific to Atlantic, and vice versa.
However, the ability of coal to variously self-heat (spontaneous ignition), emit flammable gases, corrode, and deplete oxygen levels has made the ocean transport of this commodity a particularly hazardous exercise. This is particularly the case in situations where loading is staggered or delayed and the potentially disastrous consequences of a shipboard coal fire can be realized.
3.8.6 Conveyer Belt
Conveyor belts are normally used in mine-mouth power plants to bring coal from the mining area to the storage or usage area. Conveyor belts can be used for coal transport in hilly terrain where roads are relatively inaccessible, typically being used to move coal over distances of 5 miles to 15 miles.
Conveyors have the advantage of being relatively maintenance free but have the disadvantage of location inflexibility, making a truck haul still necessary. Movable conveyor belts have been developed and used. The only adverse environmental impacts of conveyor belts for coal transport are coal dust losses during loading, unloading, or transport.
Conveyor belts do not use water, except for belt cleaning; they can use plant electricity and do not require crude oil as the energy source.