Improve Combustion Efficiency with Excess Air Control

Boilers, fired furnaces, and calciners are among some of the most common pieces of equipment in any processing plant, and often consume the majority of the site’s fuel. The increase in their fuel efficiency will often result in significant economic benefits and the simplest way to do this is through excess air control.

Combustion is used in an extremely wide range of practices - boiling water into steam as a heat transfer media, calcining product such as limestone or alumina, transforming chemical energy into mechanical energy or electricity. Combustion requires a certain amount of oxygen for the reaction to fully occur, which is provided through air. Unfortunately air only contains approximately 21% oxygen, with the majority of the remainder being nitrogen. Due to the exothermic nature of the combustion reaction the nitrogen is also heated, which is then vented to the atmosphere through a stack. This is a direct loss of energy as heat to the atmosphere.

Most furnaces are run with too much excess air to ensure complete combustion, as well as to reduce the risk of an explosion if there is a potential for secondary ignition - i.e. the presence of an electrostatic precipitator to clean the waste gas on fired calciners. Insufficient air will also result in incomplete combustion, leading to high levels of Carbon monoxide, soot, ash, and unburnt fuel in the waste gas emissions.

Depending on the accuracy and confidence in the air flow meters, an air to fuel ratio control can be used to stabilize the excess air regardless of the furnace load. From this an online O2 or CO monitor can be used as a trim to ensure the optimal amount of air is being used at all times. The optimal excess O2 can be determined by measuring the CO concentration in the flue gas and reducing the oxygen content until the CO concentration begins to increase.

This control system requires strict control over both the fuel flow rate and the air flow rate through blower output/speed.

With the ever decreasing cost of instrumentation control systems such as this can be installed on smaller and smaller pieces of equipment and still prove to be economically viable.

The potential energy savings can be easily estimated:

  • Calculate the stoichiometric oxygen requirement for a set fuel flow
  • Determine an appropriate level of excess oxygen - usually 2-3%
  • Convert the oxygen requirements into total mass flow of air
  • Calculate the total enthalpy of the stack gases
  • Compare the difference between current stack energy losses and with excess oxygen control
  • Determine the cost of energy through fuel cost and calorific value

Introducing an excess air control system will also result in the following additional benefits:

  • Increased combustion efficiency
  • Reduced fuel costs
  • Reduced NOx emissions

Once excess air control is in place further fuel efficiency upgrades can be made, such as economizers to reduce stack temperature, or extra insulation to reduce shell losses.

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