Flue gas fan

Some concepts for regulating the air ratio in an atmospheric appliance involve local intervention at specific parts of the air intake. In a wider approach, the total flow can be adjusted. Fig. 3.22 shows the airflow in a sealed atmospheric appliance with a flue gas fan [9]. 

In fuel-lean premixed burners, the primary air ratio determines the quality of combustion changing the rotational speed of the flue gas fan has also some influence. An ionization probe is used to determine the quality of combustion. A dedicated system, developed at GWI, provides accurate detection and analysis of the ionization signal. This includes a metering device, which is provided with a rectangular supply voltage, thus warranting very accurate ionization signals. 

Fig. 3.22 Schematic view of an atmospheric appliance with a flue gas fan.

Capacity of the unit con onents, such as the flue gas fans, bottom ash discharge system and air heaters. Very often the biomass has a high moisture content, as a result of which the flue gas flows significantly increase when a large proportion of coal is replaced by (wet) biomass. 

Combustion air supply for inductive burners is similar to combustion air supply for natural suction burners. However, the draft in a combustion chamber is created by flue gas fan. This method is used whenever there is insufficient natural suction or if a pressure drop disables suction of a sufficient amount of combustion air (e.g., fhrough convective heat exchangers, filters, etc.). Installation of burners with an inductive supply of combustion air as opposed to natural draft burners is less common. 

The second digit indicates if present and the position of fhe combustion air/flue gas fan ... 

Cx4 There is a flue gas fan positioned inside the chimney after the flue gas damper. 

Flue gas recirculation (FGR) is the rerouting of some of the flue gases back to the furnace. By using the flue gas from the economizer outlet, both the furnace air temperature and the furnace oxygen concentration can be reduced. However, in retrofits FGR can be very expensive. Flue gas recirculation is typically applied to oil- and gas-fired boilers and reduces NO, emissions by 20 to 50%. Modifications to the boiler in the form of ducting and an energy efficiency loss due to the power requirements of the recirculation fans can make the cost of this option higher. 

Figure 23.8 is a schematic illustration of such a unit. It is desirable for each boiler to have its own economizer. Where one economizer is installed to take the exhaust gases from more than one boiler special considerations must be taken into account. These will include gas-tight isolation dampers. Consideration must be made of flue-gas pressures at varying loads and maximum and minimum combined heat load to match economizer and a pumped feedwater ringmain. Economizers may be used for both forced-draft and induced-draft boilers, and in both cases, the pressure drop through the economizer must be taken into account when sizing the fans. 

The air and flue gas management system includes the various baffles, dampers, fans, dust collectors, scrubbers, and other FGD and emission control equipment, and the boiler flues and stack (chimney). 

Induced draft fans (ID fans) are located near the end of the flue-gas path and provide a negative static pressure below that of atmospheric pressure (creating a partial vacuum). This induces outside air to infiltrate any openings in the boiler. 

Large WT boilers generally have a balanced draft system (i.e., a combination of both FD and ID fans) and usually operate at negative furnace pressure (-0.1 to -0.5 inch of water) to provide for better control of both the furnace flue-gas pressure and the fuel-air combustion process. 

Checks to the air and flue gas system include visually inspecting the furnace and periodically monitoring all fans, levels of draft, furnace pressure, excess air demands, and combustion efficiency. 

Cleaning procedures include an internal washdown with a 0.5 to 2% soda ash solution, using a high-pressure jet. All wash water must be drained and deposits removed from the boiler. Usually, all external surfaces of the boiler are cleaned as well, in addition to the flue gas side of the economizer, air heater, and ID fans. 

Electrostatic precipitators These are cold precipitators, located after the ID fans. Particularly efficient precipitators (> 99%) often remove the alkaline fly ash before it can react with acids in the flue gas. Where scrubbers are not employed, the acid is ultimately liberated to the atmosphere. 

Gupta, H., M. Iyer, B. Sakadjian, and L.S. Fan, Separation of COz from flue gas by high reactivity calcium-based sorbents, Pittsburgh Coal Conference, Pittsburgh, PA, September 2002. 

The burners at the reformer s top are in an enclosure called a penthouse. The flue gas is collected at the bottom in horizontal fire-brick ducts called tunnels. Flue gas exits horizontally into a waste heat recovery (WHR) unit. Combustion gas is drawn through the WHR unit by an induced-draft fan and then discharged to the atmosphere through a stack. 

Flue gas recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO,. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal recirculation can be accomplished by jet entrainment using either combustion air or fuel jet energy external recirculation requires a fan or a jet pump (driven by the combustion air). When combined with staged-air or staged-fuel methods, NO emissions from gas-fired burners can be reduced by 50 to 90 percent. In some applications, external flue-gas recirculation can decrease thermal efficiency. Condensation in the recirculation loop can cause operating problems and increase maintenance requirements. 

FIG. 24-58 Standard equipment arrangement, flue gas condensing economizer waste heat recovery system (flow left to right). The ID fan draws hot flue gas from the boiler, propelling it into the top of the condensing economizer. Courtesy CHX Condensing Heat Exchanger Co. www.chxheat.com.)... 

Anhydrous Ammonia Anhydrous ammonia is injected into the flue gas as a vapor. Anhydrous Ammonia is stored at approximately 265 psig with an electric heater that cycles to maintain pressure in the tank. The anhydrous ammonia is mixed with dilution air supplied by fans. The ammonia concentration is typically 3-8% by volume, which is half of the explosion range of 16-25%. The NHj/air mixture is then injected into the flue gas through an AIG. Anhydrous ammonia is the most economical reagent to use, however, it is more difficult to transport, handle, and store than other reagents. 

Consider a forced-draft boiler producing 600-psig steam as shown in Fig. 20.2. The fuel rate on this boiler is fixed and we are going to optimize the oxygen (02) content of the flue gas by adjusting the speed of the forced-draft fan. Do we simply adjust the forced-draft (FD) fan to give 2 percent 02 in the stack because someone once said that 2 percent 02 in the stack was a good number ... 

Gupta, H., Thomas, T.J., Park, A.-H.A., Iyer, M.V., Gupta, P Agnihotri, R Jadhav, R.A., Walker, H.W., Weavers, L.K., Butalia, T., Fan, L.-S. et al. (2007) Pilot-scale demonstration of the OSCAR process for high-temperature multipollutant control of coal combustion flue gas, using carbonated fly ash and mesoporous calcium carbonate. Industrial and Engineering Chemistry Research, 46(14), 5051-60. 

In most developed countries, coal combustion and smelting facilities are required to treat gaseous and particulate arsenic in flue gases before the gases are released into the atmosphere (Chapters 5 and 7). Potentially effective sorbents for flue gas arsenic include hydrated lime (Ca(OH)2, portlandite), lime (CaO), calcium carbonate, limestone, fly ash, and sometimes activated carbon ((Jadhav and Fan, 2001 Lopez-Anton et al., 2007 Helsen and Van den Bulck, 2004), 287, 289 (Taerakul et al., 2006 Gupta et al., 2007) Chapters 5 and 7). The injection of hydrated lime is especially effective and probably... 

Corrosion penetrations 1n the flue gas ductwork downstream of the air preheater necessitated the replacement of some ductwork as the fourth modification. Although air Inleakage on the suction side of the ID fan was not operationally troublesome, any dilution of the flue gas upstream of the proposed emission extraction grid would bias the analytical results 1n measuring combustion emissions. When this ductwork was replaced, an access platform for the sampling crews was added around the flue gas extraction ports. 

Compute the quantity of flue gas handled. The quantity of gas reaching the induced draft fan is the sum of the actual air required for combustion from step 1, air leakage in the boiler and furnace, and the weight of fuel burned. With an air leakage of 10 percent in the boiler and furnace (this is a typical leakage factor applied in practice), the gas flow is as follows ... 

Induced-draft fans handle hot, dusty combustion products. For this reason, extreme care must be used to choose units specifically designed for induced-draft service. The usual choice for large boilers is a centrifugal-type unit with forward- or backward-curved, or flat blades, depending on the type of gas handled. Flat blades are popular when the flue gas contains large quantities of dust. Fan bearings... 

Compute the correction factors for the induced-draft fan. The flue-gas density is 0.045 lb/ft3 at 385°F. Interpolate in the temperature correction-factor table because a value of 385°F is not tabulated. Find the correction factor for 385°F thus (Actual temperature — lower temperature)/(higher temperature — lower temperature) x (higher temperature-correction factor — lower temperature-correction factor) + lower-temperature-correction factor. Or, [(385 — 375)/(400 — 375)](1.273 — 1.255)+ 1.255 = 1.262. 

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