Many operations associated with fossil fuel electric power generation facilities are not directly involved in the production of electricity but serve in a supporting role. This section discusses two of the major supporting processes.
Coal Processing. Fifty-seven percent of coal used in power plants is transported from mines by rail. Coal is also transported by truck and barge. Once coal arrives at the plant, it is unloaded to live storage, dead storage, or directly to the stoker or hopper. Live storage is an enclosed silo or bunker next to conveyors leading to the pulverizer. Dead storage is exposed outdoors and is the backup supply. Coal unloading devices depend on the size and type of plant. Coal arriving by rail may be unloaded directly into the storage area or to conveyors leading directly to generation units. Coal arriving by barge is unloaded by buckets, which are part of coal towers or unloading bridges. Coal shipped by truck generally needs little equipment for unloading. Precautions must be taken in the transportation and storage of coal. In transporting coal during warmer months and in dry climates, dust suppression may be necessary. Dust suppression is typically accomplished through the use of water, oil, or calcium chloride (CaCl2 ). In winter months, antifreeze chemicals are applied to the coal. Because coal oxidizes easily in open air, it should be stored in layered piles to minimize airflow. Hot areas should be removed from the pile to prevent fire; water should not be added to reduce the heat, since the water increases the airflow and, therefore, would increase the oxidation of the coal. Coal may be cleaned and prepared before being either crushed or pulverized. Impurities in coal, such as ash, metals, silica, and sulfur, can cause boiler fouling and slagging. Coal cleaning can be used to reduce sulfur in the coal to meet sulfur dioxide (SO2) emissions regulations. Cleaning the coal is a costly process that increases its fuel efficiency, yet reduces the size of the particles. Coal cleaning is typically performed at the mine by using gravity concentration, flotation, or dewatering methods. Some smaller stoker plants purchase pre-cleaned, pre-crushed coal.
Coal is transported from the coal bunker or silo to be crushed, ground, and dried further before it is fired in the burner or combustion system. Many mechanisms can be used to grind the coal and prepare it for firing. Pulverizers, cyclones, and stokers are all used to grind and dry the coal. Increasing the coal’s particle surface area and decreasing its moisture content greatly increases its heating capacity. Once prepared, the coal is transported within the system to the combustion system, or boiler. Devices at the bottom of the boilers catch ash and/or slag.
Air Pollution Control Processes. Air pollution control devices found in fossil fuel-fired systems (particularly steam electric power facilities) include particulate removal equipment, sulfur oxide (SOX) removal equipment, and nitrogen oxide (NOX) removal equipment. Particulate removal equipment includes electrostatic precipitators, fabric filters, or mechanical particulate collectors, such as cyclones. SOX removal equipment includes sorbent injection technologies and wet and dry scrubbers. Both types of scrubbers result in the formation of calcium sulfate and sulfite as waste products. NOX emission control systems include low NOX burners and selective catalytic or non-catalytic reduction technologies. The selective catalytic and non-catalytic reduction technologies convert oxides of nitrogen into nitrogen gas and water.
Control technologies are used at many utility electric power generation facilities to mitigate the environmental impacts of cooling water intake structures. These technologies may include intake screening systems, passive intake system (physical exclusion devices), or fish diversion and avoidance systems. Technologies used to mitigate thermal pollution include cooling towers, cooling ponds or lakes, and sprinklers. Other control technologies may include recycling and reuse equipment for metals recovery; evaporators; and physical, chemical, and biological wastewater treatment.