What is a flame fired process?

Every flame fired process is dictated by the reaction of a fuel in the presence of oxygen or an oxidized environment; this reaction is more commonly known as combustion or more simply burning. Fuels used may be solid, liquid or gaseous and common examples include wood, coal, natural gas and other hydrocarbon variations, biodegradable waste and used tires.

The most common example of a combustion process occurs in the engine of your car. When gasoline (fuel) is ignited by a flame within a very small area of a car’s engine it releases a great deal of gas which then pushes the car’s pistons, which then rotates the car’s crankshaft, and ultimately this will then turn the car’s wheels. Another common example is a coal power boiler. The stored energy in coal is released by combustion and converted into electricity. When coal is burned in air the carbon in the coal and the oxygen in the air react to produce CO2 and heat. The heat is then used to convert the water within the boiler into steam. The high pressure steam is used to turn a turbine and a generator converts this mechanical energy into electrical energy. About half of the world’s electricity is created this way. Waste treatment is another example of a flame fired process. Hazardous waste is fed into a rotating combustion chamber of an inclined rotary kiln. The combustion of these waste materials creates heat, ash, and flue gas which is then treated before being released into the atmosphere.

As you can see, the presence of a burning flame is the catalyst in these kinds of processes. Therefore close temperature control of the flame is absolutely essential to ensure that these processes are occurring efficiently.

FlameOther Flame fired processes include:

  • Power Boilers
  • Incinerators
  • Industrial Furnaces & Kilns
  • Thermal Reactors

The Nature of Flames

Temperature control of a flame fired process is tricky because of the elevated temperatures along with the presence of a burning voluminous flame. A non-contact infrared pyrometer is recommended because of the harsh conditions found in these areas. Thermocouples have proven to be inaccurate and constant replacement of these contact devices become costly. To better understand the challenges of making temperature measurements of flame fired processes we must understand the nature of flames.

Flames may have solid, liquid, gas and plasma components. Infrared emission bands depend upon fuel type. Most flames share a few common emission bands and transmission bands. For example, let’s take a look at hydrocarbon fuels where the products of combustion = CO2 – carbon dioxide and H2O – water vapor. Looking at the transmission curves for these fuels it is clear that there are wavelength bands where these products are very strong emitters and bands where they are not as strong (indicated in the regions shaded in green). Selecting a wavelength where the transmittance is very weak will allow the sensor to view through the flame as if it were transparent, and vice-versa, selecting a wavelength where the transmittance is very strong will render the flame opaque in the sensor’s view and it will make a temperature reading of the flame.

Further, flames can be characterized as non-luminous (clean) or luminous (dirty) and this is determined by the amount of available oxygen. A clean flame or a non-luminous flame will burn blue and these flames will have access to as much oxygen as can be consumed allowing them to burn very efficiently. This means that all of the available carbon is reacting with oxygen to form carbon dioxide leaving behind zero carbon or soot, the black stuff found in smoke. Conversely, a dirty or luminous flame is going to burn yellow/white/orange in color. This kind of flame produces soot because there isn’t enough oxygen available to react with all of the carbon present in the process.

For non-luminous flames, a single-wavelength sensor will be appropriate because the flame temperature will be uniform. Dual-wavelength technology measures the hottest temperature viewed in a given area and will be required for luminous flames because of the presence of particulate. The dual-wavelength will report the particulate temperature which will represent the temperature of the hottest portion of the flame.

Why Does Wavelength Matter?

Careful attention to wavelength is critical when choosing a pyrometer to monitor any flame fired process. As we have discussed, an infrared pyrometer will be sensitive to certain emitters depending upon wavelength. Depending on our area of interest we can choose a wavelength where the flame is opaque (looking at the flame) or a wavelength where the flame is transparent (looking through the flame). This will effectively allow us to eliminate errors associated with making these kind of measurements. Illustrated below is a rotary kiln application where the flame temperature and the product temperature are both being monitored. Because the flame here is luminous (orange) and contains particulate a dual-wavelength pyrometer is preferred. For a more detailed overview of our different pyrometer technologies please check out our comprehensive blog post here.

Rotary Kiln

Measurement Points


In some specific flame fired processes such as a coal power boiler, fine particles known as fly ash are suspended in the air within the boiler or furnace. Monitoring the temperature of these particles can be achieved by monitoring the temperature of the flame as these particles will be the same temperature and is actually a representation of the bulk gas temperature at the center of the process.


Monitoring the temperature of the gas contained within the heating zone also allows for careful control of the combustion process. For applications without particulate present, a wavelength that views the hot gas and avoids the flame temperature must be selected. For applications where particulate is present the particulate temperature will often represent the true bulk temperature of the gas stream.


Many of these flame fired processes occur within a refractory lined furnace, kiln or incinerator. The refractory material is designed to withstand high temperatures but ultimately if temperatures get too high the refractory will fail. Although the refractory is meant to be disposable, monitoring the refractory temperature helps to prevent costly repairs and rapid replacements. This is achieved by selecting a wavelength that views clearly through combustion gas and the flame and looks at the refractory wall.


The flame temperature is usually tightly controlled for process optimization. As we have discussed, flames can be luminous or non-luminous. Depending on the nature of the flame, single-wavelength or dual-wavelength technology may be selected. Careful wavelength selection will allow us to view through hot gasses but not the flame.

Non-Luminous Highly Transparent Clean Flames – Single-Wavelength

  • Natural gas
  • Propane
  • Coke oven gas
  • Oxygen
  • Hydrogen
  • Ammonia

Luminous Less Transparent Flames – Dual-Wavelength

  • Coal
  • Oil
  • Biomass
  • Municipal Waste
  • Pulverized Tires
  • Wood Chips

If you are wondering if an infrared pyrometer would be suitable for your application, please click below for a free Williamson consultation with one of our temperature measurement experts.

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