There are many terms used in the combustion industry. One is the concept of “combustibles” for detecting the onset of incomplete combustion for safety, and for optimizing the combustion process for fuel efficiency. This article provides a basis for understanding combustibles measurements and related terms, such as “COe.”
What are combustibles?
Three elements are needed to ignite a fire and generate combustion: oxygen, fuel, and energy (such as a spark). Under perfect conditions, hydrocarbon fuels, such as methane, react to form carbon dioxide (CO2) and water (H2O). However, in practice, there is always a small amount of incomplete combustion present, often taking the form of ppm-levels of carbon monoxide (CO) and hydrogen (H2). These byproducts of incomplete combustion are known as “combustibles” and primarily consist of both CO and H2.
The term “combustibles” originated in the mid-1900s from the handheld gas detection industry. During the 1970s and 1980s, the technology used for combustible gas detection was adapted to make industrial measurements to detect process upset conditions for safety, and later for fuel efficiency.
How are combustibles measured?
Combustibles are measured using specialized catalytic detectors which have active and reference elements that respond differently to sample gas. Given the catalytic nature of these sensors, the measurement itself is not speciated for a specific compound. Instead, it is grouped in an umbrella measurement tuned for compounds within a specific reactivity range. Thermox combustibles detectors are tuned specifically for accurate CO and H2 readings. It is possible for a combustibles measurement to detect certain hydrocarbons with similar reactivity to CO and H2, but this is with varying levels of accuracy and is not recommended for primary measurement. Combustibles detectors are specifically designed to detect the onset of incomplete combustion in the form of CO and H2.
Can a combustibles detector measure methane?
Not exactly – the catalytic detector needs to be tuned for methane (CH4) specifically, and this requires a specific methane detector. Methane is the hardest hydrocarbon to crack, requiring very high temperatures. A methane detector operates at a much higher temperature than a combustibles detector in order to crack methane and provide more sensitive CH4 readings. Note that methane detectors are also able to detect other hydrocarbons (such as ethane, propane, butane), according to their specific reactivities, in an umbrella-like measurement.
What is COe and how is this different?
In the combustion industry, the abbreviation “COe” stands for “Carbon Monoxide equivalent.” As the name suggests, COe measures all combustibles relative to (or equivalent to) the reactivity of CO. COe and combustibles are similar in that they both detect the onset of incomplete combustion, but with one key difference: calibration.
To improve sensitivity to both CO and H2, combustibles detectors are calibrated with a ppm mixture of CO and H2. For Thermox analyzers, the factory uses a calibration gas mixture of 800 ppm CO and 800 ppm H2, 3-4% O2 and balance nitrogen. For COe measurements, the calibration gas consists of ppm-levels of CO only in air. COe detectors are tuned to measure CO specifically, and all other compounds respond to the detector based on their specific reactivities.
By itself, H2 is much more reactive compared to CO. Given the difference in calibration, a COe detector might overestimate the levels of H2 in the sample gas due to its much higher reactivity compared to CO. This is important to consider when burning fuels with high percent levels of H2, such as refinery fuel gas.
When to use either detector?
Either measurement type is suitable for combustion safety and fuel efficiency. Both are similar approaches to reporting the presence of incomplete combustion. In an instance of using refinery fuel gas (containing high H2) an operator might be more inclined to use a combustibles measurement to more accurately detect unburnt H2 levels.
In summary, both combustibles and COe represent the byproducts of incomplete combustion in the form of an umbrella measurement. Combustibles measurements are calibrated to a mixture of ppm-levels of CO and H2 to more accurately respond to both byproducts of incomplete combustion. COe measurements are calibrated to only ppm-levels of CO and all readings are relative to CO (hence equivalent of CO). For methane, a separate detector is needed with the tuning to crack methane and provide accurate CH4 measurements (among other hydrocarbons as well). Finally, despite some differences, combustibles and COe measurements are equally informative and either can be used for monitoring process safety and for optimizing combustion efficiency.
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