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Coming Up For Air: Improving Combustion Efficiency

Key Points
  • An overabundance of excess air results in heat loss and reduced boiler efficiency.
  • Operating with the right amount of excess air minimizes heat loss and improves combustion efficiency.
  • There are two types of burner control systems: mechanical and electronic.

Combustion efficiency is a measure of how effectively the heat content of a fuel is transferred into usable heat. The stack temperatures and flue gas oxygen (or carbon dioxide) concentrations are good indicators of combustion efficiency. Operating your boiler with the right amount of air will reduce heat loss through the stack and improve combustion efficiency, which can reduce fuel costs by up to 10 percent.

How much excess air is too much?

Combustion efficiency decreases when excess air or flue gas temperature increases. Excess air levels will vary for different firing rates and boiler types and applications. For natural gas boilers, high firing rates may require less than 2 percent excess oxygen (10 percent excess air), while low-firing rates may need more than 6 percent excess oxygen (80 percent excess air) to ensure complete combustion.

Oil-fired boilers typically require more excess air than gas-fired systems. Because colder air is denser than warmer air, colder supply air will result in an increased level of excess air.

You can determine the correct amount of excess air by analyzing flue-gas oxygen concentrations. Low-cost test kits measure the percentage of oxygen in the flue gas. Higher-priced analysis equipment displays oxygen, temperature, and boiler efficiency, but it's recommended for larger boiler systems.

Online oxygen analyzers should be considered when fuel composition or steam flows are highly variable. They provide immediate feedback to burner controls to reduce excess air and optimize fuel-to-air ratios.

Controlling excess air levels

Burner controls can be used to optimize excess levels and combustion efficiency. There are two types of burner control systems:

  • Mechanical systems can only be tuned to a limited number of firing rates. Mechanical controls should be calibrated frequently, at least on a monthly basis.
  • Electronic or linkageless digital control systems can be optimized across the entire range of firing rates. Electronic control systems require calibration three to four times per year and allow the fuel valve and combustion air damper to operate independently of one another.

At a pulp and paper company, optimum tuning of main oxygen trim loops, as well as master and drum level control loops, were completed on two boilers. Once the digital controllers were configured, the oxygen loops were tuned to meet the responses of the combustion loops.

These changes resulted in a more stable and robust boiler operation; variability in oxygen was reduced by 50 to 60 percent for one boiler. Overall, the control system reduced boiler energy use by 2 percent.