STEAM ATTEMPERATION CONCEPTS

• The attemperation water, which is 2-5% of the total steam flow, may come from the boiler feed water pump outlet, or further downstream, just before the economizer inlet.
• From an energy efficiency standpoint, attemperation would be more effective and require lower flows if sprayed further downstream. The risk of water damage to the turbine, however, requires that water have time to mix and evaporate thoroughly before entering the turbine.
• Bypassing feed water heating has a negative impact on cycle efficiency.
• Other means to control steam temperatures are; tilting burners away from the superheater tubes, using bypass dampers in the back-pass or convection portion of the unit, or using flue gas recirculation (FGR) to moderate combustion.
• These methods are typically used for longer-term adjustment compared to the quick action of spray attemperation to provide control of transients.
• Attemperation flows can be eliminated on supercritical boilers by controlling the firing rate to match the feed water flow.
• Steam temperature excursions are a result of an imbalance between the heat added to the steam by the firing rate and the heat withdrawn, by the steam flow rate.
•  A control strategy will control steam temperature for a drum boiler operating in either constant or sliding pressure mode.

Benefits of Steam Temperature Control

• Improved temperature control on process transients caused by changing load, soot blowing, starting and stopping of mills in service.
• Improved steady state temperature control when operating under conditions of reduced pressure, slagging.
• Faster ramp rate.
• Reduce the need for attemperation, which improves plant heat rate.
• Flexibility to control main steam temperature on drum boilers in either a fixed or variable pressure operation.
• Avoiding unscheduled outages for superheater tube failure.
• The benefits of the higher temperature increased generation of backpressure power, better condensing power efficiency, and reduced steam use by the drive turbines, all due to the higher steam expansion work available in the turbines.
• Operating with higher energy content steam allows specific steam consumption to be reduced, with consequent reduction in auxiliary boiler feed pump power.

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