In part two of this series on combustion control of industrial heat sources, we consider the air-fuel ratio and the balance of using excess air to consume combustibles while minimizing energy from going up the stack in industrial heating sources. In Part I, Stoichiometric Combustion and its Impact on Boiler Efficiency, we discussed stoichiometric combustion, the theoretical position of the optimal amount of oxygen and fuel mixture to produce the most heat possible while achieving maximum combustion efficiency.

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Air Fuel Ratio and Excess Air

Combustion efficiency depends on using the right amount of air to consume the fuel.

In fuel-fired process heating, the largest energy loss source is the exhaust stack, so managing airflow is essential to combustion efficiency. When fuel burns in the presence of oxygen, it converts to carbon dioxide, water, and heat. Consider the combustion of methane (CH4).

CH4 + 2O2 → CO2 + 2H2O + Heat (1,013 Btu/ft.3)

Air contains approximately 21% oxygen and 79% nitrogen. In this case, the reaction for complete combustion becomes:

CH4 + 2O2 + 7.53N2→ CO2 + 2H2O + 7.53 N2 + Heat (1,013 Btu/ft.3)

The amount of air required will vary depending on the type of fuel. Ideally, you would like to add enough oxygen to consume all the fuel so that little or no combustibles are exhausted while minimizing the excess air to prevent energy loss out of the stack.

Air Fuel Ratio Effect on Combustion Efficiency
The air-to-fuel ratio defines the amount of air needed to burn a specific fuel.

The air-to-fuel ratio defines the amount of air needed to burn a specific fuel. The conventional fuels used in the combustion process are oil (#2, 4, and 6), diesel oil, gasoline, natural gas, propane, and wood—ratios for common gases, liquid, and solid fuels noted in Tables 1.1 and 1.2.

Optimizing Air-to-Fuel Ratio

There is a balance between losing energy from using too much air and wasting energy from running too richly in any combustion process. The best combustion efficiency occurs at the optimum air-to-fuel ratio, and controlling this provides the highest efficiency. A liquid and gas fuel burner achieve this desired balance in most scenarios by operating at 105% to 120% of the optimal theoretical air. For natural gas-fired burners,  the stoichiometric air required is 9.4-11 ft.3 / 1.0 ft.3 of natural gas or approximately an air-to-gas ratio of approximately 10:1. In this case, there is an excess oxygen level of 2%.

In the combustion zone, it is challenging to measure excess air. However, in the stack, it can be easily measured using Oxygen analyzers. Operating with 5%-20% excess air would correspond to a 1% to 3% oxygen measurement in the stack.

The ideal air-to-fuel relationship will vary at different operating loads. Tuning is the act of establishing the desired air-to-fuel relationship under various operating conditions. It can be accomplished when evaluating specifics in the stack: temperature, oxygen concentration, carbon monoxide, and NOx emissions.

In part three of this five-part series, we consider analyzing flue gas oxygen and combustibles, along with varying air and fuel pre-combustion flows, to improve the combustion efficiency of industrial boilers, steam generators, furnaces, ovens, smelters, and process heaters.

If you are interested in reading the Sage Metering white paper on this topic, see Combustion Efficiency and Thermal Mass Flow Meters.

Boiler Combustion Efficiency Explained

Stoichiometric Combustion Impact on Boiler Efficiency

Flue Gas Analysis and Air/Fuel Flow – Combustion Efficiency

Mass Flow, Differential Pressure and Air Flow Meters

Air Flow Meter for Combustion Efficiency | Industrial Boilers

11 thoughts on “Air Fuel Ratio Effect Combustion Efficiency and Air Fuel Ratio

  1. Avatar
    A.Geetha says:

    Dear sir

    We are global service providers & suppliers of Industrial Automation & Electrical Equipment.

    Our client is having the following requirement. Description is given below. Kindly suggest suitable Air fuel controller & necessary field instrument with price & availability.

    CO2 Plant built in 1982.

    Single burner fire tube boiler combustion control

    Air fuel ratio controller : local controller(pneumatic type) used to maintain ratio between air & fuel for a good combustion.

    As the item got obsolete, we are facing problem with combustion control. We are willing to replace this complete system even including primary sensing elements & final control element.

    Moreover we are also interested to upgrade complete CO2 instrument & control system including field instrument.

  2. Avatar
    Tamas Polya says:

    Dear Sir,
    In small appliances, like in gas fired local room heaters there is no air/fuel ratio control. The air/fuel rate is factory-adjusted to be the best possible at the maximum power of these appliances by the manufacturers. These appliances have natural draught combustion chambers, without fan to assist in the flow in the combustion chamber. So the resistance of the combustion chamber against the flow of the air and the combustion gases is set to get the optimal air/fuel ratio at the maximum power.
    However when these appliances turn to reduced rate, only the fuel flow is reduced. The result is 4 – 5 times more air in the combustion chamber than necessary. It goes to an efficiency reduction about 15% at the reduced rate mode. In case of installing an air/fuel ratio control device into these appliances the efficiency is higher by 5 – 8% than at the nominal power. The 2/3 of the yearly gas consumption of these appliances burns on the reduced rate if there is a built in room thermostat. The remaining 1/3 of the yearly gas consumption is burnt on pilot light mode, where the efficiency is almost equal to the efficiency measured at the reduced rate.
    So the seasonal efficiency difference is about 20% when there is an air/fuel ratio device in the heating appliance compared to the situation when there is no air/fuel ratio deice built into the appliance.
    It would be easy to apply factory-built air/fuel devices even in the simply gas stove: We only need a butterfly valve disk in the flue gas way, which turns to right angle position, which rising the resistance of the combustion chamber at the reduced rate of these appliances in such a degree, which result the best air/fuel ratio at the reduced rate as well. The disk of the butterfly valve is on the extended spindle of the gas tap. So the gas flow and the air flow are mechanically synchronised to each other at the maximum and at the reduced power of the appliance as well. The additional cost of such an air/fuel ratio device return to the user in 0,5 – 1 year.
    I believe it is important to let the readers know, what happens if there is no air/fuel ratio device in an equipment.

    Best regards,

    • Avatar
      Sam Short says:

      I hope you’ll share this or find it informative. Combustion = air, fuel, and an ignition source. (Spark) change a value of one and it has the potential change the rest.

      Changing any of those considering basic laws of physics.
      Owners can control ( to a limiting degree,) Air/Fuel aka AF or AFr just by….
      Filter changing .. proper installation location and have the proper area and volume of a space for a system designed and engineers as such.
      Manually adjusting air vents, shutters, louvers, or even adding a dam to restrict air.
      You can control AFr by changing the ignition source such as direct ignition spark electrode gapping. Or making sure the ignition transformer doesn’t have unnecessary resistance in the coil winding by test via Megohmeter.

      Sensors can require calibration and often the smallest particulate matter (see hot film wire Mass air sensors) can adversely effect readings.

      -Certified HVAC-R Tech with an emphasis of
      Commercial and industrial applications
      -Automotive enthusiast focused on forced induction tuning
      Hobbiest of pc building/gamer and networking including image design.
      Studying BAC, building automation controls currently while learning about advanced controls and anything in the IoT area 🙂

  3. Avatar
    Luc Colman says:

    I like to comment on everything that is said about burning efficiency. What the normally forget is the coolling of the furnace bij the air that is needed. e.g. for burning 1 kg gas you need 5 kg air. Heating up the air consumes a lot of the generated heat. If you add this to the balance you com out with maximum effeciences of 20 or 30 % and not 80 or 90 %

  4. Avatar
    sanjay kumar says:

    Dear sir I job in bladt furnace and I operating the kalugin regenrative stove I want to increase hot blast temperature but I can’t setting air and bf gas ratio. Please help me air gas ratio setting. Which ratio put so hot blast temperature and dome temperature increased.

  5. Avatar
    Jun says:

    How is the proper way of adjustment of oil/air ratio example if i want to increase the excess oxygen from example 2.5 to 3.5 mmH2O what should i do? increase or decrease the ratio?

  6. Avatar
    ABED says:

    Performance of Insulated LPG Burner with Ball Bearings as Porous Medium.

    Part 1 – To determine the values and determine the ratios for the fuel-air ratio, and then each fuel-air ratio is one we set and determine the maximum temperature, whether without preheating and without pre-mixing or with preheating and pre-mixing.
    Part 2- To know the components of the combustion products, and whether the presence of pre-mixing or preheating will improve the combustion products or not?

  7. Avatar
    Ella says:

    I have no idea whether I am on the correct site for the answer to my question. I have a small Palazzetti Ecofire 54T pellet burner installed in my apartment which is supposed to be thermostatically controlled, but since I have had it the control has never worked. An example being – having set the room temperature at 21C the control panel of the stove shows a temperature of 31.2 and rising. Would this be due to the air/fuel mixture being incorrect? If so, how could that be overcome? It has now become a safety issue and I am in dispute with the installers. Your comments would be appreciated.

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