Air Preheating Systems

The overall thermal efficiency of a furnace is improved if the air that enters the raditmt section around the burners is preheated. Economic studies are performed early in a project to determine die feasibility of such a system. The two most common preheating sys- 

A conventional recuperative (closed-loop) system s illustrated in Exhibit 7-13. Oil is circulated through fie convection section tubes, heated, and sent through Sie inlet air duct, where it releases its heat. The hot air len enters the burner for combustion, and the hot oil 5 recirculated to a storage tank for recycling. From a lyout standpoint, the hot oil system with a storage 

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Boilers are higher NO , producers than fired heaters because the firebox temperatures are higher and they typically have air preheat systems. Firebox temperatures typically run in the 2100" F range, at full loads. 

Boilers frequently have air preheat systems. The induced draft (ID) fans used for air preheat can be used to recirculate the flue gas. 

Another very important quality characteristic is the combustion efficiency (fuel efficiency) of radiant tubes. The efficiency strongly depends on the flue gas outlet temperature of radiant tube systems. In order to achieve maximum efficiency, air preheating systems use the enthalpy of the flue gas for preheating the combustion air simultaneously reducing the flue gas outlet temperature of such systems. High efficiencies with less flue gas losses result in lower fuel consumptions and lead to a reduction of the CO2 emission per ton of product. 

Draw a box furnace with an air preheater system and label its parts. Label and show flows with a red pen. 

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. 

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. 

Acid cleaners based on sulfamic acid are used in a large variety of appHcations, eg, air-conditioning systems marine equipment, including salt water stills wells (water, oil, and gas) household equipment, eg, copper-ware, steam irons, humidifiers, dishwashers, toilet bowls, and brick and other masonry tartar removal of false teeth (50) dairy equipment, eg, pasteurizers, evaporators, preheaters, and storage tanks industrial boilers, condensers, heat exchangers, and preheaters food-processing equipment brewery equipment (see Beer) sugar evaporators and paper-mill equipment (see also Evaporation Metal surface treati nts Pulp). 

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. 30-1. Integrated environmental control for electric generating plants (5). APH, air preheater ESP, electrostatic precipitator TREAT, water treatment system.

Heat recovery water heater and the saturated air preheater. Balance-of-plant equipment and systems, including interconnecting piping, valves, controls, etc. 

Ensuring the pressure in the air blower discharge piping system, particularly across the check valve and air preheater, is not excessive... 

Air pollution dispersion modeling, for odor impacts, 26 725 Air preheater, 19 512 Air products and chem systems, methanol process, 26 310-311 Air quality... 

Natural gas systems with endothermic steam reformers often make use of the residual fuel from the anode in a reformer burner. Alternatively, the residual fuel could be combusted prior to a gas expander to boost performance. In an MCFC system, the residual fuel often is combusted to maximize the supply of CO2 to the cathode while at the same time providing air preheating. In an SOFC system, the residual fuel often is combusted to provide high-temperature air preheating. 

Harada and Mori, 1988 Yamamoto, Kaneko and Takahashi, 1988). The system consisted of a SOFC generator module containing 144 cells, electrical air preheaters for generator temperature control and an air and fuel handling system. The system operated continuously and successfully for over six months. Now, 25 kW scale SOFC systems from Westinghouse are being tested. 

Preheater vibration. Air preheaters or any type of waste-heat recovery device designed for horizontal flow across vertical tubes, may be subject to vibration produced by the velocity of gas across the tube banks. The velocity produces a vortex-shedding wave pattern that could correspond to the natural harmonic frequency of the tube bank. If the natural harmonic frequency is reached, excessive vibration of the tubes will occur. Redesign of the internal baffle system by inserting dummy baffles can stop the vibration. 

In a real SOFC system, heat is exchanged within the SOFC in several ways including fuel processing, air preheating, flue gas cooling, etc. The excess air is commonly required to prevent overheating, while the conversion of hydrocarbons into hydrogen and carbon monoxide often absorbs heat. 

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