What?
In combustion, oxygen (O2) measurement is critical to ensure the proper balance (or ratio) of air to fully combust the fuel and generate heat. With existing technologies, two measurement types are available to industry: gross oxygen and net oxygen.
Gross oxygen measurement represents the exact amount of oxygen in the flue gas, regardless of any other constituents. Typically, in combustion applications, there are minor amounts of combustibles (sub-500 ppm), and gross oxygen measurements assume that any free oxygen will not with combine with the combustibles. Thus, a gross value is reported, representing the exact in-process oxygen concentration in the flue gas. Gross oxygen technologies include paramagnetic oxygen analyzers, wet electrochemical cell oxygen analyzers, and tunable diode lasers (TDL).
Net oxygen measurement represents the residual or excess oxygen in the flue gas, after all combustible compounds are consumed. As a result of the technology itself, the oxygen measurement is made AFTER burning whatever free combustibles are present. Thus, an excess or net oxygen measurement is made, representing the remaining oxygen in the flue stream. Zirconium oxide is among the most common of net oxygen technologies, and is used throughout Thermox products.
So what?
For combustion efficiency and normal operation, the difference between gross and net measurements are small since combustibles are generally in the ppm range, while oxygen is usually in the percent range. However, gross oxygen and net oxygen measurements respond very differently during safety-related incidents, such as substoichiometric combustion, also known as fuel-rich combustion or flooding/bogging. For example, loss of flame in a fired heater may prompt a gradual decline in the gross oxygen measurement as the percent of fuel in the heater increases; whereas a net oxygen measurement would drop significantly – sounding an immediate low or low low O2 alarm – as the combustibles are reflected in the excess oxygen measurement. Thus, gross oxygen and net oxygen measurements respond very differently in an upset scenario, having different ramifications on personnel and process safety.
From a safety standpoint, technologies, such as zirconium oxide, provide the ability to monitor net oxygen for normal operation and also provide rapid response to alert on process upsets. Technologies, such as paramagnetic and TDL, will provide the ability to monitor gross oxygen for normal operation, but secondary measurements are required to detect process upsets, which include using a combustibles or CO/CH4 detector.
Now what?
Historically, industry has standardized on the concept of excess oxygen measurement in combustion processes to ensure all combustibles are considered and only residual oxygen is reported. As gross oxygen measurements are adopted into combustion analysis, it is critical to understand these differences in measurement and how it impacts your safety system.
For more information about AMETEK’s zirconium oxide analyzers, click here.