Steam generation by coal-fired boiler

Steam generation by coal-fired boilers can be one of the cheapest options (if WHRB is not possible) as compared to use of other fuels. 

Captive power generators may be therefore installed in many chemical plants which are to be run by steam turbines. These can get the steam supply from coal-fired boilers or from waste heat recovery boilers operating on hot process gases. They can reduce dependence on supply of power from external grid. They... 

Coal bottom ash (BA) and boiler slag (BS) are the coarse, granular, incombustible by-products that are collected from the bottom of furnaces that burn coal for the generation of steam, the production of electric power, or both. The majority of these coal by-products are produced at coal-foed electric utiUty generating stations, although considerable BA and/or BS are also produced from many smaller industrial or institutional coal-fired boilers and from coal-burning independent power production facilities [50-57]. The type of by-product (BA or BS) produced depends on the type of finnace used to burn the coal. The main differences between coal bottom ash and boiler slag are summarized in Table 3. 

Flue Ga.s Desulfuriza.tion. Citric acid can be used to buffer systems that can scmb sulfur dioxide from flue gas produced by large coal and gas-fired boilers generating steam for electrical power (134—143). The optimum pH for sulfur dioxide absorption is pH 4.5, which is where citrate has buffer capacity. Sulfur dioxide is the primary contributor to acid rain, which can cause environmental damage. 

The failure took place in a large water-tube boiler used for generating steam in a chemical plant. The layout of the boiler is shown in Fig. 13.1. At the bottom of the boiler is a cylindrical pressure vessel - the mud drum - which contains water and sediments. At the top of the boiler is the steam drum, which contains water and steam. The two drums are connected by 200 tubes through which the water circulates. The tubes are heated from the outside by the flue gases from a coal-fired furnace. The water in the "hot" tubes moves upwards from the mud drum to the steam drum, and the water in the "cool" tubes moves downwards from the steam drum to the mud drum. A convection circuit is therefore set up where water circulates around the boiler and picks up heat in the process. The water tubes are 10 m long, have an outside diameter of 100 mm and are 5 mm thick in the wall. They are made from a steel of composition Fe-0.18% C, 0.45% Mn, 0.20% Si. The boiler operates with a working pressure of 50 bar and a water temperature of 264°C. 

Steam engines and steam turbines require steam boilers, which, until the advent of the nuclear reactor, were fired by vegetable or fossil fuels. During most of the nineteenth century, coal was the principal fuel, although some oil was used for steam generation late in the century. 

Densified fuel pellets may be used as boiler fuel in place of coal in stoker-fired furnaces. Direct substitution of fuel pellets was demonstrated in two informally reported instances with no furnace adjustments. The particulate emission from each of these installations were monitored with the emission rate results shown in Figure 2. As a comparison, the North Carolina Administrative Code allowable particulate emission rate schedule (a function of heating rate) is also shown. The environmentally acceptable particulate emission contributes to the economic justification of the conversion of small-to-medium size steam generators to the use of pelleted biomass fuels. Two such installations are now using pelleted fuels on a routine basis, and a third has concluded the economic benefit by entering into a contract agreement to purchase pelleted fuel. 

The first demand was met by exclusion of reference to quantities measured in the laboratory (heating microscope temperatures, viscosity measurements). These values were replaced by evaluation of the performance in operating boilers and results from pilot scale combustors. The tendencies used in this paper have been determined predominantly from the different behavior of coals from various mines fired in the same steam generators based on the opinions of experts. 

Power Production. Steam cycles for generation of electric power use various types of boilers, steam generators, and nuclear reactors operate at subcritical or supercritical pressures and use makeup and often also condensate water purification systems as well as chemical additives for feedwater and boiler-water treatment. These cycles are designed to maximize cycle efficiency and reliability. The fuel distribution of sources installed in the United States from 1990—1995 are as follow coal, 45% combined cycle, 27% miscellaneous, 14% nuclear, 11% solar, oil, and geothermal, 1% each and natural gas, 0.3%. The 1995 summer peak generation in the United States was 620 GW (26). The combined cycle plants are predominantly fired by natural gas. The miscellaneous sources include bagasse, black liquor from paper mills, landfill gas, and refuse (see Fuels frombiomass Fuels fromwaste). 

Complete WAO produces a mixture of steam and gas at pressures of hundreds of atmospheres which can be expanded through turbines to give power whereas by heat interchange, pure steam is generated at a somewhat lower pressure. The cost of a WAO plant for power production is competitive with that of a modem coal-fired steam boiler unit, as is also the cost of its operation (24). 

In the SRC-11 process, the process steam is generated by direct gas-fired boilers and the process heating by direct gas firing. The fuels utilized are hydrocarbon-rich gas, or CO-rich gas, and purified syngas (i.e., no feed coal is used for fuel). It was shown that a 2 X 600 MW(t) PS/C-MHR can supply these thermal requirements principally by substituting for the fuel gases previously employed (Shenoy 1995). The displaced gases, which are treated already, may then be marketed. 

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

W.A. Parish Generating Station boiler 8 is a 555 MWe (net) tangentially-fired boiler supplied by Combustion Engineering (now Alstom) and commissioned in 1982. It is fired wifii Powder River Basin coal, and equipped with roller type pulverizers from CE [35]. The unit has the capacity to generate 530 kg/s (4.2x10 Ib/hr) of 165 bar/540°C/540°C (2400 psig/1000 F/1000°F) steam [36]. In 1995 tests were conducted at this generating station in collaboration with Q RL... 

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