Fossil fired boilers

FIGURE 3.4 As-received tube and view of ID. (French D.N. Metallurgical Failures in Fossil Fired Boilers, 2nd ed. 1993. Copyright Wiley-VCH Verlag GmhH Co. KGaA. Reproduced with permission.)... 

French DN, Metallurgical Failures in Fossil Fired Boilers, 2nd ed. John Wiley Sons Inc., NY, USA, 1993. Gleeson B, Cheung WH, Da Costa W and Young DJ, The hot-corrosion behavior of novel co-deposited chromium-modified aluminide coatings. Oxidation of Metals, 1992, 38(3-5) 407-424. 

In several industrial environments, the deposition of ashes and salts on the surface of the components may occur at high temperatures. In this case, corrosion is determined by both the deposit and the gaseous environment. Underneath the deposits, the formation of a protective oxide scale may be impeded or, as is often observed, the formerly protective oxide scale is destroyed by a chemical reaction or by the formation of a low melting eutectic of the deposit and the oxide scale. This type of corrosion is often observed in fossil-fired boilers (fuel ash corrosion) and gas turbine components (hot corrosion). In recent years, it was also discovered that, in particular, low melting de-... 

The plant system of the constant pressure supercritical fossil-fired boiler is shown in Fig. 5.1. The startup bypass system includes a flash tank, pressure-reducing valves, and bypass valves. First, a minimum feedwater flow is established in the furnace prior to the firing of the boiler to prevent overheating of the tube walls. During the cold cleanup mode of operation, the flow is bypassed from the inlet of the primary superheater to the flash tank, until the water chemistry is brought to a predetermined level and the boiler firing starts. 

The plant system of the sliding pressure supercritical fossil-fired boiler is shown in Fig. 5.2. It requires a steam-water separator, a separator drain tank, drain valves, and recirculation pumps. A minimum flow rate is maintained through the furnace walls by using a recirculation pump to add a recirculating flow to that provided by a boiler feedwater pump. The water leaving the furnace is passed to the steam-water separators. The water from the separators is collected in the drain tank and routed back to the economizer inlet via the boiler recirculation pump. 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Superheater/reheater corrosion in fossil-fuel-fired boilers is caused by the deposition of alkali sulphates on to the tube surfaceThe corrosion rates increase rapidly at temperatures above 600°C as the sulphates become molten. These molten sulphates contain free SO3 which dissolves the protective oxide to form Fe-based sulphates. The corrosivity of the molten sulphates depends strongly upon their melting points, which are themselves strongly dependent upon the ratio of Na and K in the deposits. 

Fluidised-bed systems produce higher combustion intensities at lower temperatures than combustion of pulverised fuel in conventional fossil-fuel-fired boilers. The mineral matter for corrosion does not form fused salts and is not expected to release corrosive species. Fluidised bed combustors can, therefore, burn lower grade, cheaper fuel in smaller plant with better pollution control than traditional boilers... 

Solid waste treatment steps include dewatering of sludge and combustion in an incinerator, bark boiler, or fossil-fuel-fired boiler. Sludges from a clarifier are dewatered and may be incinerated otherwise, they are landfilled. 

Fossil-fuel-fired boilers, 23 216 Fossil fuel plants, combined cycle,... 

Another study by the California Energy Commission found that wood-fired boilers can be installed for about 20% less than a coal unit. Biomass conversion plants are often smaller than fossil fuel units and can be built more quickly and with less capital investment. 

FIGURE 1 Tangentially fired boiler. [Reproduced with permission from Singer, J. G. (ed.), (1981). Combustion Fossil Fuel Systems, 3rd Ed., Combustion Engineering, Windsor, CT. 

FR 51397) Mixed-fossil-fuel-fired boilers. Particulate matter 43 g/10 kJ No requirement... 

C.J. Davis, P.J. James, L.W. Pinder, and A.K. Mehta, Effects of fuel composition and combustion parameters on furnace wall fireside corrosion in pulverized coal-fired boilers, Materials Science Forum 369-372 857-864, 2001 B.S. Covino, Jr., S.J. Bullard, G.R. Holcomb, M. Ziomek-Moroz, and D.A. Eden, Fireside corrosion probes for fossil fuel combustion. Corrosion 2006 Paper 6472, NACE International, Houston, Tex., 2006. 

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