Convective section heat-recovery

Use duct firing of heat recovery steam generators. Increase steam generation through WHR from fired heater s convection section, heat recovery steam generators and process heat exchangers/steam generators... 

Efficiency is the ratio of total heat absorbed in radiant, convection, and heat recovery sections of the heater to the heat released by combustion. The released heat is based on the lower heating... 

The steam generator is a balanced draft, controlled circulation, multichamber unit which incorporates NO control and final burnout of the fuel-rich MHD combustion gases. The MHD generator exhaust is cooled in a primary radiant chamber from about 2310 to 1860 K in two seconds, and secondary air for afterburning and final oxidation of the gas is introduced in the secondary chamber where seed also condenses. Subsequent to afterburning and after the gas has been cooled down sufftciendy to soHdify condensed seed in the gas, the gas passes through the remaining convective sections of the heat recovery system. 

Selection of the high pressure steam conditions is an economic optimisation based on energy savings and equipment costs. Heat recovery iato the high pressure system is usually available from the process ia the secondary reformer and ammonia converter effluents, and the flue gas ia the reformer convection section. Recovery is ia the form of latent, superheat, or high pressure boiler feedwater sensible heat. Low level heat recovery is limited by the operating conditions of the deaerator. 

Since the mid 1970 s, changes in the design of reformers have produced improvements in overall thermal efficiency, accomplished primarily by improved heat recovery in the convection section, ultimately reducing the temperature of the flue gas to the limit imposed by the condensation point of water, or in the case of some fuels, sulfur trioxide. Extensive use of air preheat in newer designs lowers fired fuel requirements. 

Figure 46. Example of the heat recovery in leh convection section (top-fired furnace, plant with auxiliary boiler)...

Modify convection section for better heat recovery. This may include ... 

One manufacturer uses single-drum, watertube type waste heat boilers on incineration systems. Watertube bodets are also used by other manufacturers in installations where high steam pressures and flow rates are required. Another manufacturer offers heat recovery systems with water wall or radiant sections in the primary chamber. These water wall sections, which are usually installed in series with a convective type waste heat boiler, can increase overall heat recovery efficiencies by as much as 10 to 15%. 

Combustion flue gases travel downward parallel to the tubes and pass through tunnels at the radiant section floor into the convection section where a high level of heat recovery is obtained assuring maximum fuel efficiency and optimum operating economics. 

Cold air can be sucked into the furnace convective section through holes in the furnace skin. This reduces the efficiency of heat recovery from the hot flue gas. At lower crude rates, flue gas flow drops, but cold air in-leakage remains constant. Thus, at lower crude rates, holes in the furnace exterior will hurt efficiency more than at higher throughputs. A roll of aluminum tape can go a long way toward correcting this problem. 

Reformer In a reformer furnace, shown in Exhibit 7-5, preheated process fluid flows through catalyst-filled tubes, which are usually located in the center of the radiant section. This type of furnace may have single or multiple compartments burners may be mounted in the roof, wall, or floor. Heat recovery systems may also be employed through the use of waste heat boilers or the convection section s steam generation coils. 

The flue gas at 1900°F must then be cooled to about 300°F to achieve efficient heat recovery. This heat is recovered in the convection section of the SMR. The process gas at 1600°F must then be cooled to about 100°F before final product purification. 

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