Hot flue

The Megalopohs station (Fig. 4d) uses hot flue gas to dry the lignite. A cyclone separator and electrostatic precipitator permit rejection of some of the water vapor to the atmosphere rather than to the boiler. Another drying method uses a vertical shaft, heated by combustion gases, for partial drying prior to grinding. 

After the waterwaH tubes deHver the saturated steam back into the top of the boHer dmm, moisture is separated out by a series ofbaffl.es, steam separators, and cormgated screens. The water removed drops down into the hot water contained in the steam dmm. The steam travels out through either a dry pipe, which leads to a superheater header, or a series of superheater tubes that connect directiy into the top of the steam dmm. The superheater tubes wind back into the top of the furnace and/or a hot flue-gas backpass section, next to the economizer, where heat from the combustion gases exiting the furnace superheats the steam traveling through the tubes. 

In ECS s 1986 repowefing project Babcock and Wilcox (B W) constmcted a bubbling-bed section to ECS s existing 125 MWe pulverized-coal furnace to produce 31.3 t/h of lime, usiag cmshed coal as the source of heat to calciae limestone ia the fluidized bed. A portion of the lime is drawn from the bed as bottom ash and a portion is collected as fly ash. Both portions are transferred to a cement (qv) plant adjacent to the boiler. The hot flue gas from the EBC flows iato the existing main pulverized-coal furnace, ia which a B W LIMB system was also iastaHed to absorb sulfur dioxide dufing those times when the EBC is not operating. 

A steam buffered shaft seal shall be provided to ensure that hot flue gas cannot escape from the casing at any specified operating condition. The steam supply shall be controlled by... 

A train that generates only eleetrieity from the hot flue gas is ealled a Total Power Generation (TPG) train. A TPG train eonsists of a power reeovery expander, gear, generator, and sometimes a steam turbine (Figure 6-39). The valve arrangement for a TPG train requires speeial attention beeause the train operation and the proeess operation are independent. 

In die original system eonfiguration, die hot flue gas leaving die regenerator was expanded in die double slide valve and orifiee ehamber to atmospherie pressure, and dien passed via die waste heat boiler to die main staek. This mode of operation remains possible following die expander retrofit. 

After die eonversion, die hot flue gas is dueled through the expander and the power extraeted from the flue gas is eonverted into eleetrie power. The exhaust gas from the expander is dueled to the existing waste heat boiler and the downstream eleetrostatie preeipitator, then diseharged into the atmosphere through the main staek. 

As previously mentioned, FCC units produee hot flue gas as a byproduet. Hot gas is expanded in the expander turbine and then supplies meehanieal energy for proeessing. The exeess power is eonverted into eleetrieal energy by the motor-generator. Therefore, the power reeovery system is likely to raise the energy effieieney and profitability of the plant by substantial margins. 

For PM control from combustion sources, the tlue gas enters a coagulation area (e.g., ductwork, a chamber, or a cyclone) to reduce the number of ultrafine particles, and then a gas conditioner to cool the gas to a suitable temperature and saturation state. This is generally accomplished by means of a waste heat recovery heat exchanger to reduce the temperature of the flue gas or by spraying water directly into the hot flue gas stream. 

As another example of calculation and dimensioning of pneumatic conveying systems we consider an ejector shown in Fig. 14.20. In fluidized bed combus tion systems a part of the ash is circulated with the hot flue gas. The task of the ejector, is to increase the pressure of the circulating gas to compensate the pressure losses of the circulation flow. The motivation for using an ejector, rather than a compressor, is the high temperature of the flue gas. The energy... 

Gas-fired water heaters use the same general method of construction, except that the elements are replaced with a burner beneath the tank. The combustion products from the burner are vented through a flue made out of the same thickness steel as the tank, that goes up through the center of the tank. To increase heat transfer from the hot flue gases to the inner wall of the flue, a baffle is inserted down the flue. This baffle is a twisted strip of sheet metal with folds and tabs on it. The folds and tabs are designed to... 

For example, a hot flue gas flows outside a tube and shell exchanger at 900°F (C) while a hot liquid is flowing into the tubes at 325°F (k). The film coefficients have been estimated to be hj = 225°F and h = 16 Btu/(hr) (fti) (°F). Estimate the tube wall temperature using hj as hj corrected to the outside surface for the inside coefficient ... 

Smittle, D., Maximize Heat Recovery from Hot Flue Gases with Finned Tubing, Power, V. 124, No. 8 (1980) p.76. 

Several common arrangements have been developed to improve the efficiency and/or to accomplish specific purposes, such as furnishing hot flue gas to boilers or to process gas exchangers in addition to the simultaneous generation of power or driving equipment. Figures 14-37A-D illustrate several common arrangements or cycles. 

The flue gas exits the cyclones to a plenum chamber in the top of the regenerator. The hot flue gas holds an appreciable amount of energy. Various heat recovery schemes are used to recover this energy. In some units, the flue gas is sent to a CO boiler where both the sensible and combustible heat are used to generate high-pressure steam. In other units, the flue gas is exchanged with boiler feed water to produce steam via the use of a shell/tube or box heat exchanger. 

The slide or plug valve regulates the flow of catalyst between the reactor and regenerator. The slide valve also provides a positive seal against reversal of the hydrocarbons into the regenerator or hot flue gas into the reactor. Table 7-6 summarizes typical process and mechanical parameters for designing slide valves. 

To ensure proper control and to protect from reversal of hydrocarbons to the regenerator or the back-flow of hot flue gas to the... 

Similarly, a negative pressure differential across the spent catalyst slide valve can allow hot flue gas to backflow to the reactor and the main fractionator, severely damaging the mechanical integrity of these vessels. 

Lees and Whitehead" have shown that differences in boiler design lead to differences in furnace atmospheres, which are subsequently reflected in differences in scale morphology and corrosion performance. Hence they report that there is no unique scale morphology which is characteristic of furnace wall corrosion. They also warn that the scale that is examined during an investigation may not be an exact reflection of the scale on the tube surfaces during operation due to the possible hydrolysis of the scale on cooling (when hot flue gas is replaced by moist air) and the redistribution of phases in the scale due to the loss of the incident heat flux. 

After the flue gas leaves the combustion chamber, most furnace designs extract further heat from the flue gas in horizontal banks of tubes in a convection section, before the flue gas is vented to the atmosphere. The temperature of the flue gases at the exit of the radiant section is usually in the range 700 to 900°C. The first few rows of tubes at the exit of the radiant section are plain tubes, known as shock tubes or shield tubes. These tubes need to be robust enough to be able to withstand high temperatures and receive significant radiant heat from the radiant section. Heat transfer to the shock tubes is both by radiation and by convection. After the shock tubes, the hot flue gases flow across banks of tubes that usually have extended surfaces to increase the rate of heat transfer to the flue gas. The heat transferred in the radiant section will usually be between 50 and 70% of the total heat transferred. 

All the CO resulting from the pseudo solid-solid reaction is conducted, together with entrained char, from the top fluidized section through a constriction, in which the high-velocity gas flow prevents backflow, to a transport combustor, where the CO is burned to C02 with preheated air, along with as much of the char as is called for by heat balance to maintain the endothermic FeO-C reaction. The heated recycled char is separated from the off gas at the top of this transport combustor in a hot cyclone and is returned as a thermal carrier to the lower part of the lowest j igged section, while the hot flue gas from the transport combustor is used to preheat the incoming air in a recuperator. 

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