Understanding Oxygen Measurement in Flue Gas Streams

Looking at the total amount of O2 in the gas stream can be misleading because the stoichiometric excess O2 amount is the more important measurement.

Automated combustion processes are all around us, from residential appliances and HVAC systems to large industrial boilers, fired heaters, and power plants. For all the differences in scale and purpose, the common element that all try to achieve is high efficiency through effective combustion control. Fuel cost is a major factor, and hence the emphasis on efficiency, but safety and emission considerations are also top of mind, particularly the latter for industrial applications. The type of combustion discussed in this article is a chemical reaction between the fuel and oxygen (O2) and is therefore subject to basic stoichiometric factors. The correct number of O2 molecules must be available to react with the corresponding number of fuel molecules. As a practical matter, most combustors use atmospheric air, with air flow measured to control the O2 supply. Air flow imbalance in either direction is problematic. If there is insufficient air (below the stoichiometric requirement, or fuel-rich combustion), unburned fuel goes out the stack. This wastes fuel, creates emissions and hazardous air pollutants. It also creates a potential safety issue should enough fuel subsequently mix with O2 and ignite. To further complicate matters, in the real world, combustion is rarely one hundred percent complete. There is typically some amount of unburned fuel in the flue gas, although trace amounts won't have a material effect on total versus excess O2 levels. However, significant levels of unburned fueled are likely at some point in a facility's operating life and is more common than realized. This is unavoidable even in the most efficient burners. More on what this means in a moment. If there is too much air (above the stoichiometric requirement, resulting in fuel-lean combustion), efficiency is reduced due to energy wasted heating the unnecessary volume of air. This is inescapable to some extent since approximately 80% of air is nitrogen, but excess air is less problematic for efficiency and safer for operation, although nitrogen oxides (NOx) emissions can increase with increasing excess air. For most combustors there is an ideal excess air to achieve good combustion, low emissions and high efficiency. Excess air and excess fuel both reduce efficiency, but excess air doesn't reduce efficiency as much as the same volume of excess fuel.

Anyone who has worked with an old-fashioned gas stove or heater can see the mixing process working by adjusting the burner air intake to achieve a perfectly blue flame. But the question emerges, what is the most practical way to optimize large-scale combustion for safety, efficiency and emissions? The most common answer is to measure and control the amount of O2 remaining in the flue gas exhaust, but what is ideal? As just mentioned, combustion is often not perfectly complete, so some unburned fuel and O2 go out the stack, even if the air and fuel mixture going into the burner is just right. The area of concern is the amount of O2 in excess of what is required to burn the amount of fuel, but looking at total O2 content in the flue-gas stream can be deceiving if operators don't fully understand what the measurement represents. The challenge is to determine how much of the O2 in the flue gas is in excess of the stoichiometric amount. Operators normally want some amount of excess O2 because it is undesirable to reduce air flow below the stoichiometric amount (Figure 1), but the exact amount depends on the fuel and combustion system. Erring on the fuel-lean side in most situations is more desirable than running fuel-rich.

For industrial installations, there is a wide range of control strategies. At minimum, there will be an instrument monitoring fuel flow. Air flow will be metered, or at least controlled, to correspond to fuel flow. This type of scheme can be implemented using some formula (air volume per unit of fuel) for a rough calculation, but variability in oxygen demand from different fuel sources, and precision of fuel and air flow measurements, results in the necessity to monitor the actual O2 content of the flue gas as well. There are two techniques commonly applied for flue gas O2 measurement: a tunable-diode laser (TDL) analyzer and a zirconia sensor analyzer. A TDL analyzer uses two sensing components (Figure 2), a laser source and detector.

Controls for boilers, fired heaters, and other combustion processes are built to regulate combustion air to ensure operation with adequate, but not too much, excess air. Zirconium sensors measure excess O2, not total O2, which is directly related to excess air. Other technologies, such as TDL, measure total O2, which does not provide a direct measure of excess air when unburned fuel is present. Total O2 is only equal to excess O2 when combustion is 100% complete and no unburned fuel exists. While an operation is never intended to operate with unburned fuel, the risk remains and happens to some extent in virtually all applications. When using a TDL analyzer, its total O2 measurement can therefore cause operators or automated controllers to drive to unsafe fuel-rich conditions. A simple and durable zirconium sensor addresses this issue by measuring only excess O2, allowing operators to control combustion processes while maintaining safe and optimum conditions.

Neil Widmer is the combustion business development manager at Emerson. He has 30 years of experience in the combustion and thermal energy industry. Neil has 13 US patents, 2 peer-reviewed publications, and has composed numerous papers and presentations. He holds a BSME degree from the University of California-Davis.Jesse Sumstad is a global product manager for Emerson's measurement solutions business. He manages the analytical instruments combustion portfolio, which consists of in-situ oxygen analyzers, extractive oxygen and combustible transmitters, and accessories. Jesse graduated with a Bachelor of Science in Industrial Engineering from the University of Iowa and an MBA from the University of St. Thomas.

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