Recirculation furnaces

For medium or low temperature furnaces/ovens/dryers operating below about 1400 F (760 C), a forced recirculation furnace or recirculating oven delivers better temperature uniformity and better fuel economy. The recirculation can be by a fan and duct arrangement, by ceiling plug fans, or by the jet momentum of burners (especially type H high-velocity burners—fig. 6.2). 

The furnace charge consists of 2iac—lead siater, metallurgical coke, and recirculating metallic drosses and flux. The charge cycle is fully automatic. Hoist... 

Assessments of control, operabiHty and part load performance of MHD—steam plants are discussed elsewhere (rl44 and rl45). Analyses have shown that relatively high plant efficiency can be maintained at part load, by reduction of fuel input, mass flow, and MHD combustor pressure. In order to achieve efficient part load operation the steam temperature to the turbine must be maintained. This is accompHshed by the use of flue gas recirculation in the heat recovery furnace at load conditions less than about 75% of fiiU load. 

FIG. 27-35 Low-NOj burner with air-staging and flue-gas recirculation for use in bigh-temperature furnaces. (Hauck Manufactuiing Company. Developed and patented by the Gas Research Institute.)... 

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. 

If acceptable working conditions must be maintained in the enclosure during furnace operation, attention must be given to internal airflow patterns, i.e., minimization of fume recirculation in the enclosure. 

By recirculating a part of the flue gas to the furnace, the combustion zone turbulence is increased, the temperature is lowered and the oxygen concentration is reduced. All of these factors lead to a reduction of NO, fonnation. 

With regard to operational considerations, the recirculation pump pressure gauges (which measure pressure differentials and consequently indicate possible blockage in the coiled tube) require frequent calibration. Regular cleaning and good housekeeping are also required, especially within the furnace area. 

Circulating Beds These fluidized beds operate at higher velocities, and virtually all the solids are elutriated from the furnace. The majority of the elutriated sohds, still at combustion temperature, are captured by reverse-flow cyclone(s) and recirculated to the foot of the combustor. The foot of the combustor is a potentially very erosive region, as it contains large particles not elutriated from the bed, and they are being fluidized at high velocity. Consequently the lower reaches of the combustor do not contain heat-transfer tubes and the water walls are protected with refractory. Some combustors have... 

Air-staged burners Low-NO air-staged burners for firing gas (or oil) are shown in Fig. 24-28. A high-performance, low-NO, burner for high-temperature furnaces is shown in Fig. 24-32. In this design, both air-staging and external flue-gas recirculation are used to achieve extremely low levels of NO emissions (approximately 90 percent lower than conventional burners). The flue gas is recirculated by a jet-pump driven by the primary combustion air. 

Fuel-staged burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain httle or no fixed nitrogen. Figure 24-33 illustrates a fuel-staged natural draft refinery process heater burner. The fuel is spht into primary (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirculated by the primary gas jets for additional NO control. NO reductions of 80 to 90 percent nave been achieved by staging fuel combustion. 

In this process, ethylene and water are combined with a recycle stream in the ratio ethylene/water 1/0.6 (mole ratio), a furnace heats the mixture to 300°C, and the gases react over the catalyst of phosphoric acid absorbed on diatomaceous earth. Unreacted reagents are separated and recirculated. By-product acetaldehyde (CH3CHO) is hydrogenated over a catalyst to form more ethyl alcohol. 

An electrical resistance heater with more turns at the tube ends (to compensate for heat losses) surrounds each tube. There is a vertical laminar flow hood over the loading area to minimize particle contamination of the wafers being loaded. As we can see, there are temperature controls for the furnace tubes, and a power module to provide the electrical power. When operated as a LPCVD system, a unit including both the gas flow and vacuum systems is positioned on the right side. Such a unit is shown in Figure 8. Here we can see the vacuum pumps on the left, and the mass flow controllers on the right. The vacuum pump oil recirculation systems are shown in the slide out drawers. As can be seen in Figure 9, this system, as well as most current similar systems, operate under computer control. 

In most practical systems such as a furnace or a rocket engine, the combustion process is much more complicated due to two important factors. First, to a large extent the mixing of the fuel and oxidant takes place in the combustion chamber and thus the mechanics of the mixing process plays an important role. Second, the flow patterns are complicated by turbulence or recirculation and frequently cannot be represented by simplified theories. 

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