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WEEK 02: GENERATION: FOSSIL-FUEL / HYDRO PLANTS Sections: Introduction | Boiler Design | Environment A. Fossil-Fuel Plants. The most common power plant since the Industrial Age. This involved the combustion of fuel produces high-pressure (typically 2400 to 3500 psig) and high temperature (most commonly 1000°F) steam, which used to drive a turbine at 3600 rpm. The turbine drives an electrical generator. Fossil Fuels used for steam generation in utility and industrial power plants maybe classified into solid, liquid, and gaseous fuels. Each fuel maybe further classified as natural, manufactured, or by-product fuel. Obvious examples of natural fuels are coal, crude oil and natural gas.
Sections: Introduction | Boiler Design | Environment The boiler designer must proportion heat-absorbing and heat-recovery surfaces to make best use of the heat released by the fuel. In addition to the basics of unit size, steam pressure, and steam temperature, other factors influence the overall design of the steam generator, such as: (1) Fuel The furnace size, the equipment to prepare and burn the fuel, the amount of heating surface and its placement, the type and size of heat-recovery equipment and the flue-gas-treatment devices are all fuel-dependent. Firing oil results in relatively small amounts of ash; there is no ash from natural gas. The ash in coal, however, consists a number of objectionable chemical elements and compounds which has serious effect on furnace performance. At high temperatures, fractions of ash become partially fused and sticky. Depending on the quantity and fusion temperature, the partially fused ash may adhere to surfaces contacted by the ash-containing combustion gases, causing slag buildup, Chemicals in the ash may attack materials such as alloy steel used in superheaters and reheaters. The air heater is also subject to corrosion and plugging of gas passages from sulfur compounds in the fuel acting on combination with moisture present in the flue gas. The fuel-ash properties that are particularly important in coal-fired furnaces include a) ash fusibility temperature, b) the ratio of basic to acidic ash constituents, c) the iron/calcium ratio, d) the fuel-ash content (pounds of ash per million Btu's), and e) ash friability. (2) Furnace The size of the furnace will dictate the size of the structural-steel framing, the boiler building and its foundations, as well as on the sootblowers, platforms, stairways, steam piping, and duct work. The furnace plan area, volume, and the fuel burnout zone, h, (the distance from the top fuel nozzle to the furnace arch) increases as lower grade coals with poorer ash characteristics are fired. The following is the effect of coal rank on furnace sizing:
Heat generated in the combustion process appears as furnace radiation and sensible heat in the products of combustion. Water circulating through tubes that form the furnace wall lining absorbs as much as 50% of this heat which, in turn, generates steam by the evaporation of part of the circulated water. Furnace design must consider water heating and steam generation in the wall tubes as well as the process of combustion. Tube diameter and thickness are of concern from the standpoints of circulation and metal temperatures. (3) Superheaters and Reheaters. The function of a superheater is to raise the boiler steam temperature above the saturated temperature level. As steam enters the superheater in an essentially dry condition, further absorption of heat sensibly increases the steam temperature. The reheater receives the superheated steam and re-superheat this steam to desired temperature. Their design depends on the specific duty to be performed. For relatively low final outlet temperatures, superheaters solely of the convection type are generally used, otherwise radiant types, where superheater elements are located at very-high-gas-temperature zones, are employed. (4) Economizers help improve boiler efficiency by extracting heat from flue gases discharged from the final superheater section of a radiant/reheat unit. In the economizer, heat is transferred to the feedwater, which enters at a temperature lower than that of saturated steam. They are usually arranged for downward flow of gas and upward flow of a water. (5) Air Heaters cool the gases before they pass to the atmosphere, thereby increasing fuel0firing efficiency and raising the temperature of the incoming air of combustion. The latter increases the rate of burning and helps raise adiabatic temperature. Sections: Introduction | Boiler Design | Environment Three classes of emissions are of major concern: (1) Nitrogen Oxides can be controlled by in-furnace and postcombustion techniques. The common characteristic of design concepts for reducing this emissions include a careful regulation of the fuel/air ratio in the firing zone where the fraction of the fuel nitrogen compounds are liberated and control of the heat liberation pattern in the furnace. In situ combustion process, 90 to 95% of SO2is captured during combustion by a sorbet, usually limestone. In this process the NOxproduction is low, approximately below 0.25 lb/MBtu fired, because of the low temperature at which the combustion reaction takes place. In situ control of NOxby modifications to firing technology and over-fire air can reduce NOxas much as 50%. Postcombustion reduction methods utilizes reagents, such as ammonia or urea, with or without catalysts, which also capture sulfur and particulate. (2) Sulfur Oxides are removed through the use of Flue-Gas Desulfurization (FGD) systems. The most common is a lime/limestone wet scrubber. After the flue gas has been treated in the precipitation or baghouse, it passes through the induced fans and enters the SO2scrubber. Precombustion reduction methods also clean the coal of sulfur-bearing compounds by wet separation, coal gassification, and coal liquefication techniques. Coal gassification involves the partial oxidation of coal to produce a clean gas or by production of a clean fuel through coal liquefication. Sulfur and ash are removed in these process. (3) Particulate Control, traditionally uses the electrostatic precipitator, but recently shifted to fabric filters, also called baghouses. In electrostatic precipitation, suspended particles in the gas are electrically charged, then driven to collecting electrodes by an electrical field; the electrodes are rapped to cause the particles to drop into collecting hoppers. In fabric filtration recover chemicals or control stack emissions through dry and wet means. When dirty gas flows through a fabric, the particulate matter in the gas forms a cake on the fiber. This deposit increases both the filtration efficiency of the cloth and its resistance to gas flow. Selection of filters depends on gas temperature, particulate characteristics, and the type of cleaning mechanism for removing the collected dust (cake) from the cloth. Particulate removal from the fabric involves a) vigorous shaking which causes the cake to fracture and fall into the collection and disposal hopper. In the b) reverse-air system, clean flue gas taken from the outlet duct is blown in the direction counter to normal flow through the fabric of one or two compartments not in service. The cleaning frequency depends upon the porosity of the cake being formed, the inlet grain loading, and the predetermined allowable system pressure drop. In c) pulse-jet outside collector filters the gases through the bag from the outside to inside. Cake removal uses a short-duration pulse of compressed air injected into the open end of the filter bag. The action fractures the cake, which falls into the collection and disposal hopper. 1. Author. TitleBook Title Publisher, Place, Year, page. |
