Combustion air preheating

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). 

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. 

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

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 ... 

The burner should be designed for handling preheated combustion air. Preheated combustion air is obtained by diverting part of the exhaust from the gas turbine. The air from the turbine is clean, hot air. To recover additional heat energy from the exhaust flue gases, a steam coil is placed... 

When combustion air preheat is used, the air preheat unit may replace the boiler feed water coil. Flue gas exits this unit at about 300 degrees F. This provides a typical heat loss of 3% of the overall reformer efficiency. Steam is also made in a process steam generator which extracts heat from the reformer outlet process gas. The heat recovery unit and process steam generator normally have a common steam drum. 

The cooler part of the combustion air preheater - the tail end of the flue-gas-heat-recovery train - is more likely to corrode due to sulfur dioxide (S02) condensing from the flue gas. In this area, cast iron or glass will resist the acid attack. Carbon steel preheater tubes, joined with 1.5 to 2 meters of SS 304 tubes at the cold end of the tube sheet, can ensure reasonable service life. Typically, the flue gas temperature to the stack is maintained above the dew point of S02 to prevent condensation. During startup and shutdown, condensation of S02 will occur88. 

High Efficiency Reformer Design. Maximum feedstock and combustion air preheat is used to balance steam generation with steam requirements. 

Other measures, involving more additional hardware and engineering work, are introduction of combustion air preheating and reducing the primary reformer load. 

Fired duty can be reduced by either reduction of dilution steam or combustion air preheating or to a lesser degree by reduction of excess air to the burners. The selection of the optimum steam dilution was discussed previously. 

The second part of the test rig consists of a combustion air preheater, a specially designed LCV gas burner, a flue gas cooler and a pressure control valve. The LCV gas combustor is situated in a water cooled pressure vessel. This swirl-diffusion combustor has a centric position in the ceramic combustion chamber. From the central symmetry axis of the burner to the outer wall the gas flows are in annular spaces whereby in the inner annulus primary air is added with swirl to the LCV gas and in the outer annulus secondary air is added. 

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