Furnace boiler, water-tube wall

At this time, 1977-1978, it appears that the current best boiler-furnace design in use for large, high pressure units is the completely water-tube-walled furnace and radiant section, studded and coated with thin plastic refractory in the intense burning zone, followed by one or more long, open, vertical radiation passes preceding a convention-type superheater and boiler-convection passes and economizer. 

The development of the modern water-tube wall furnace/boiler was in part due to the need for proper air pollution control of incinerators. Flue gases can be cooled with massive air dilution, water spray or boiler. Energy recovery in boilers is the preferred cooling method if a reasonable market can be assured or anticipated. The almost universally accepted method for particulate removal is the electrostatic precipitator. Scrubbers alone have usually failed to meet the particulate standards. 

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. 

The development of integral furnace boilers during the 1930s allowed walls constructed of banks of vertical tubes welded together to form a continuous membrane (membrane walls or water walls), which provided for simultaneous heat transfer from the furnace and a furnace water cooling surface. (Earlier WT boilers had completely separate cooling systems.)... 

Furnaces of tbis type, such as the steam locomotive furnace—boiler design, had the obvious disadvantage that pressure was limited to ca 1 MPa (150 psi). The development of seamless, thick-wall tubing for stationary power plants (ie, water-tube furnaces) and other engines for motive power, such as diesel—electric, has in many cases eclipsed the fire-tube boiler. For applications calling for moderate amounts of lower pressure steam, however, the modem fire-tube boiler continues to be the indicated choice (5). 

Fig. 4.2. Waste heat boiler for a copper smelting flash furnace (Peippo et al, 1999). Note, left to right (i) flash furnace gas offtake (ii) boiler radiation section with water tubes in walls (iii) suspended water tube baffles in radiation section to evenly distribute gas flow (iv) convection section with hanging water tubes. Steam from the boiler is used to generate electricity, to power the acid plant s main blower and for general heating and drying.

Fire 1.4. Steam boiler and furnace arrangements. [Steam, Babcock and Wilcox, Barberton, OH, 1972, pp. 3.14, 12.2 (Fig. 2), and 25.7 (Fig. 5)]. (a) Natural circulation of water in a two-drum boiler. Upper drum is for steam disengagement the lower one for accumulation and eventual blowdown of sediment, (b) A two-drum boiler. Preheat tubes along the Roor and walls are cormected to heaters that feed into the upper drum, (c) Cross section of a Stirling-type steam boiler with provisions for superheating, air preheating, and flue gas economizing for maximum production of 550,000 Ib/hr of steam at 1575 psia and 900°F. 

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. 

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. 

In water-wall incinerators. The internal walls of the combustion chamber are lined with boiler tubes that are arranged vertically and welded together in continuous sections. When water walls are employed in place of refrac toiy materials, they are not only useful for the recovery of steam but also extremely effective in controlling furnace temperature without introducing excess air however, they are subject to corrosion by the hydrochloric acid produced from the burning of some plastic compounds and the molten ash containing salts (chlorides and sulfates) that attach to the tubes. 

When a boiler design employs a radiant superheater, it is located within the furnace section. Also within the furnace section are the water-cooled, membrane walls (water-wall tubes), risers, headers, and... 

The furnace areas of WT boilers require the periodic deployment of retractable soot blowers to remove the buildup of soot and combustion products from water-wall tubes to maintain heat transfer and furnace cooling efficiency, as well as to minimize the interference of flue gas pathways. 

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