Economizers (as well as air heaters) perform a key function in providing high overall boiler thermal efficiency by recovering low level (i.e., low temperature) energy from the flue gas. For each 40F (22C) that the flue gas is cooled by an economizer or air heater in a conventional boiler with air as the oxidant and normal excess air, the overall boiler efficiency typically increases by approximately 1%.

Economizers also reduce the potential of thermal shock and large water temperature fluctuations as the feedwater enters the steam drum or waterwalls.

B&W’s economizer designs are proven to provide effective performance for optimal thermal efficiency. We consider a wide range of factors based on current or expected operating conditions such as low load operation and on/off cycling. These operating conditions require special consideration to reduce the potential damage of thermal shock on an economizer.

Design properties and operating factors include:

  • Surface type and tube geometry
  • Velocity limits
  • Bank size
  • Support systems
  • Cleaning requirements
  • Access requirements

SCR considerations

B&W has also developed a unique solution for units installed with selective catalytic reduction (SCR) for NOx control. SCR systems, and more specifically the catalyst, operate optimally within a narrow flue gas temperature range. At reduced boiler loads, it is often difficult to maintain the minimum required flue gas temperature entering the SCR.

Primary superheater sections and economizer

Our V-Temp™ system is a unique design solution which allows the boiler to operate at reduced loads with the SCR in service by maintaining the required minimum flue gas temperature at the SCR inlet. With this system, SCR performance is optimized, regardless of boiler load. The result is a system that maintains unit efficiency at full load while optimizing SCR operation at reduced loads.

Compared to other methods of controlling SCR inlet temperatures, the V-Temp system requires

  • no extensive convection pass casing or waterwall modifications
  • no large, high temperature flues to route through limited areas
  • no large, high maintenance dampers
  • no issues with ash accumulation
  • no flue gas mixing points upstream of the SCR

The V-Temp system is an economical, low maintenance solution which provides improved load control and optimized SCR performance.


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