Air preheaters

Figure 6.28 Increasing the theoretical flame temperature by reducing excess air or combustion air preheat reduces the stack loss.

Heat/Solvent Recovery. The primary appHcation of heat pipes in the chemical industry is for combustion air preheat on various types of process furnaces which simultaneously increases furnace efficiency and throughput and conserves fuel. Advantages include modular design, isothermal tube temperature eliminating cold corner corrosion, high thermal effectiveness, high reHabiHty and options for removable tubes, alternative materials and arrangements, and replacement or add-on sections for increased performance (see Furnaces, fuel-FIREd). 

Fig. 4. (a) Air preheater usiag heat pipes (b) typical heat pipe used ia air preheater. 

The basic fluid-bed unit consists of a refractory-lined vessel, a perforated plate that supports a bed of granular material and distributes air, a section above the fluid bed referred to as freeboard, an air blower to move air through the unit, a cyclone to remove all but the smallest particulates and return them to the fluid bed, an air preheater for thermal economy, an auxiUary heater for start-up, and a system to move and distribute the feed in the bed. Air is distributed across the cross section of the bed by a distributor to fluidize the granular soflds. Over a proper range of airflow velocities, usually 0.8-3.0 m/s, the sohds become suspended in the air and move freely through the bed. 

Typical heat-recuperation devices are finned gas exchangers, ceramic heat wheels, and Ljungstrom air preheaters. 

Blast air, preheated to 650°C, is deflvered by centrifugal blowers through a refractory-lined busde main to the furnace. Zinc vapor from the reduced sinter is carried out with the furnace gases to a condenser fitted with mechanical rotors that are partly immersed in a shallow pool of molten lead. The lead flows countercurrenfly to the gas and is vigorously agitated by the rotors to create an intense shower of lead droplets throughout the condenser. 

Air preheat temperature requirements of 2250—2300 K are anticipated for natural gas-fired systems, and about 2000 K for oil or coal-fired systems (11). Use of 32—40% oxygen enrichment lowers the preheat temperature requirement to a moderate 900—1000 K, which can be attained with conventional metal-type tubular heat exchangers. Depending on the cost of oxygen, this is a viable alternative to the use of separately fired high temperature preheaters. 

Heat Recovery and Feed Preheating. The objective is to bring the reactants to and from reaction temperature at the least utihty cost, and to recover maximum waste heat at maximum temperature. The impact of feed preheating merits a more careful look. In an exothermic reaction, preheated feed permits the reactor to act as a heat pump, ie, to buy low and sell high. The most common example is combustion-air preheating for a furnace. 

Fired Hester a.s a. Reactor. When viewed as a reactor, the fired heater adds a unique set of energy considerations, such as. Can the heater be designed to operate with less air by O2 and CO analy2ers How does air preheating affect fuel use and efficiency How can a lower cost fuel (coal) be used Can the high energy potential of the fuel be used upstream in a gas turbine ... 

Air Preheating. Use of unpreheated air in the combustion step is probably the biggest waste of thermodynamic potential in industry (Table 2). 

Air preheating has the unique benefit of giving a direct cut in fuel consumed. It also can increase the heat-input capabiUty of the firebox because of the hotter flame temperature. The drawback is that it tends to increase NO formation. 

The most common type of air preheater on new units is the rotating wheel. On retrofits, heat pipes or hot-water loops are often more cost-effective because of ductwork costs or space limits. 

Fig. 5. (a) By-product coke oven of the Koppers-Becker type and (b) a section therein, where A represents the coke oven chamber containing coal B, oven wall heating flue C, sole heating flue D, air preheating E, standpipe F, collecting main and G, relationship between the two sections. 

Fig. 4. Use of gas turbine air preheat for ethylene cracking furnace. The gas turbine exhaust duct contains 17% oxygen at 400°C.

A more obvious energy loss is the heat to the stack flue gases. The sensible heat losses can be minimized by reduced total air flow, ie, low excess air operation. Flue gas losses are also minimized by lowering the discharge temperature via increased heat recovery in economizers, air preheaters, etc. When fuels containing sulfur are burned, the final exit flue gas temperature is usually not permitted to go below about 100°C because of severe problems relating to sulfuric acid corrosion. Special economizers having Teflon-coated tubes permit lower temperatures but are not commonly used. 

Some of the other energy conservation approaches appHcable to dryers are interchange between the stack vapor and the incoming dryer air recovering sensible heat from the product use of waste heat from another operation for air preheat and using less, but hotter drying air. This last is limited to nonheat-sensitive materials. 

W. F. Kenney, "Combustion Air Preheat on Steam Cracker Furnaces," Proceedings, 1983 Industrial Energy Conservation Technology Conference, Texas Industdal Commission, p. 595. 

Instead of gas turbine exhaust, air preheat has been used in some plants to reduce fuel consumption. Flue gas leaving the furnace stack passes through an air preheater, and the preheated air is suppHed to the burners. By using mostly hearth burners, the duct work and the investment cost can be minimised with air preheat and gas turbine exhaust. It is also possible with 100% waH-fired furnaces, and has been proven in commercial operation (34). 

These various reactions should be minimized to avoid plugging the catalyst and to prevent fouling of the downstream air preheaters, when these components condense from the gas at the lower temperatures. 

Other problems that can be associated with the high dust plant can include alkaH deterioration from sodium or potassium in the stack gas deposition on the bed, calcium deposition, when calcium in the flue gas reacts with sulfur trioxide, or formation and deposition of ammonium bisulfate. In addition, plugging of the air preheater as weU as contamination of flyash and EGD wastewater discharges by ammonia are avoided if the SCR system is located after the FGD (23). 

FIG. 17-32 Hot windbox incinerator/reactor with air preheating. (Dotr-Oliver, Inc.)... 

Excess air is usually 30 to 40 percent for stationary and dumping grates, while traveling grates are operated with from 22 to 30 percent excess air. Preheated air can be supplied for all types of grates but the temperature is usually limited to 395 to 422 K (250 to 300°F) to prevent excessive slagging of the fuel bed. 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

Ammonia vapor is mixed with air and converted into nitrogen oxide at an elevated temperature in the presence of a catalyst, which generally contains noble metals such as platinum and rhodium. The optimal gauge temperature is maintained by controlled ammonia and combustion air preheating. The reaction is highly exothermic ... 

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