Purpose of an air preheater in a boiler system An air preheater in the flue-gas path primarily serves to:

Introduction / Context:
An air preheater is a common boiler accessory that transfers heat from hot flue gases to the incoming combustion air. This device improves overall thermal performance and combustion quality, especially in coal- and oil-fired units, and is a staple in modern steam generators.

Given Data / Assumptions:

• Exhaust (stack) gases are hotter than the ambient combustion air.
• Counterflow or regenerative heat exchange increases air temperature before the burners or grate.
• Boiler firing rate and draft are properly controlled.

Concept / Approach:
Preheating combustion air increases the adiabatic flame temperature and enhances fuel–air mixing and ignition stability. This yields more complete combustion, reduced unburnt losses, and better heat transfer to the working fluid, thus increasing boiler efficiency. With hotter air, the same heat input can evaporate more water per hour (higher evaporative capacity) or allow satisfactory burning of lower-grade fuels with higher moisture or ash.

Step-by-Step Solution:
Recover sensible heat from flue gas into incoming air.Hotter air improves combustion quality and flame stability.Improved combustion reduces losses and increases efficiency.Higher furnace exit gas temperature margin can be traded for greater steaming capacity.

Verification / Alternative check:
Heat balance sheets from boiler trials show reduced stack losses after installing an air preheater; efficiency rises by a few percentage points depending on fuel and design.

Why Other Options Are Wrong:
Option (e) misstates the function; preheaters do not directly reduce steam pressure.

Common Pitfalls:
Operating with excessive air preheat can increase NOx; control must be coordinated with excess air and burner settings.

Final Answer:
all of the above