Air Preheaters on Boilers
Air preheaters transfer heat from stack gases to incoming combustion air. In comparing an air preheater to an economizer, the following factors should be considered:
1. Is there an alternative source of heat for the BFW? If there is, the air preheater would become the better choice.
2. Is there a limitation on NOx? An air preheater, with its hot combustion air produces more NOx at the burner than if it were firing air at ambient temperature.
3. How much sulfur is in the fuel? What is the acid dewpoint? Heating ambient air in a preheater (70°F typically) or makeup water (60°F typically) in an economizer, will cause the exchanger metal temperatures to be lower than heating BFW from a deareator (225°F typically). See Section 3421 for a discussion of a dewpoint.
4. At the pressure selected, will an economizer with an arbitrary approach, i.e., a difference in the flue gas exit temperature and the economizer inlet temperature of say 50°F, recover more heat than an air preheater that may have an outlet-temperature restriction due to the burner or NOx control?
5. Investment and operating costs for each. This would have to include any operating horsepower for fans to overcome incremental draft losses. Economizers have a significant edge with lower investments. They also have lower draft losses; flue gases normally have to be ducted to an air preheater at grade and then back up to the stack.
6. All boiler manufacturers have arrangements that can satisfy either an economizer or an air preheater on new boilers. They can also add them to existing boilers.