The four critical measurements of combustion analyzers


Combustion measurements are essential to ensure the safe and efficient operation of the powered equipment. These measurements provide the information needed to make key decisions, such as adjusting the air / fuel ratios at the burner, lowering excess air set points, detecting the presence of a carbon monoxide breakthrough. carbon and warning of the build-up of a fuel-rich mixture. Each of the following critical measurements provides operators with the critical information necessary to fully monitor and safely operate fired equipment.

Fundamentals of combustion

Before we dive into the measurements, we must first consider some of the fundamentals of combustion. To begin with, the Fire Triangle summarizes the three elements necessary to start and maintain a fire: oxygen, fuel, and heat (or energy in the form of a spark). If one of the elements is removed, the flame goes out. Likewise, oxygen and fuel can mix together at the tip of the burner, but without an initial spark they do not react to form a flame.

Fire triangle.

Additionally, combustion reactions provide additional context for monitoring combustion. Under perfect conditions, hydrocarbon fuels, such as methane, react to form carbon dioxide (CO2) and water (H2O). However, in practice combustion is never perfect and there are always small amounts of fuel present in the flue gases, often in the form of ppm carbon monoxide (CO) and hydrogen (H2). Only in cases where the burner is starved of oxygen or in low air (fuel rich) operation can the fuel level rise enough to signal a potentially hazardous condition.


Simplified sub-reactions of the combustion of oxygen (O2) and methane (CH4).

The Fire Triangle and the combustion sub-reactions provide valuable insight and context for each of the following four critical combustion measures.

1. Oxygen measurement (O2)

The first and most critical measure in combustion is oxygen. Oxygen is one of the three key elements of the Fire Triangle. It provides a critical operating set point for combustion air fans.

In practice, there are two types of oxygen measurement: raw oxygen and net oxygen. Raw oxygen represents the exact amount of oxygen in the flue gas, regardless of any other constituent. The net oxygen measurement represents residual or excess oxygen in the flue gases, after all combustible compounds have been consumed.

During normal operation, the difference between raw and net oxygen readings is negligible. However, during a disturbance event, they react differently as a net oxygen reading would decrease as it consumes the incoming combustible material while the raw oxygen reading would remain rather flat and unchanged. This distinction made net oxygen measurements more readily achievable for adjusting the burner’s air / fuel ratios, as its readings take into account any unburned content in the flue gases and are directly correlated with excess air levels at the burner. burner level.

Historically, furnace and boiler manufacturers have used net oxygen measurements to define their air / fuel ratios and ensure safe excess air levels in the combustion chamber. Some technologies, such as zirconium oxide, provide this measurement of net oxygen very reliably.

2. Measurement of fuels (CO + H2)

The second most critical measure is that of fuels. These measures play two very important roles in the industry: safety oversight and process efficiency.

For safety monitoring, fuel measurements detect and signal the start of incomplete combustion. Catalytic detectors can provide ppm level indication of combined carbon monoxide (CO) and hydrogen (H).2) in the process flow – in a single umbrella reading – again, known as “measuring fuels”. Other approaches only monitor CO. In either case, the fuel measurement provides a method to monitor both the CO breakthrough and insufficient air levels at the burner.

From an efficiency perspective, the measurement of fuels provides a secondary benchmark to allow operators to lower their levels of combustion air to the burner. Oxygen measurement alone provides an operating set point, but fuel measurement gives operators additional information to reduce excess air levels safely – before reaching the CO breakthrough. . Lower excess air levels mean less additional air to heat and therefore less fuel consumed at the burner.

3. Measurement of methane and hydrocarbons (CH4+)

While you would expect methane (CH4) and hydrocarbon fuels to burn completely in a hot fireplace, they may gradually build up from extinguishing during normal operation, or fuel leakage during start-up. A methane and hydrocarbon measurement provides an additional layer of visibility to safely detect the build-up of a fuel-rich mixture.

Similar to fuel detectors, methane / hydrocarbon detectors are also catalytic in nature and provide all-in-one overall measurement. However, unlike the fuel detector, methane detectors operate much hotter. Methane is the most difficult hydrocarbon to crack, and the methane detector operates at a temperature high enough to crack the methane molecule and provide accurate measurements of the percentage level. Note that methane detectors are also capable of detecting other hydrocarbons (such as ethane, propane, butane), depending on their specific reactivities.

Importantly, methane / hydrocarbon measurement provides additional monitoring to detect unburned fuels, process tube leaks and flame losses during start-up and normal operation.

4. Indication of the process representation

Beyond the measurement of oxygen, fuels and methane / hydrocarbon, it is important to ensure that these measurements are representative of the process itself. The fourth critical measure is exactly that: the indication of a process representation. This fourth measurement can be very variable depending on the type of combustion analyzer, the measurement technologies and the sampling system used, but its importance is underlined because this indication justifies the relevance of the other three measurements.

In an extractive analyzer, it is important to ensure that the sample system is not clogged or clogged with particles. Clogging would reduce the volume of gas sample sent to the detectors and be less representative of the process itself. In this case, a flow sensor would provide this fourth measurement to ensure that a sample large enough is taken to be representative of the process. A flow sensor would also trigger an alarm in the event of a potential clogging problem.

Other technologies and arrangements may depend on a temperature or pressure measurement to ensure that the measurement is representative of the process. In these cases, it would be essential to have this information available at all times to ensure the validity of the oxygen, fuel and methane / hydrocarbon readings.

Summary

In summary, there are four critical measurements for any combustion analyzer. First, an oxygen measurement provides a set point to operate the powered equipment and adjust the air / fuel ratio at the burner. Second, a fuel measurement provides a safety mechanism to monitor the CO breakout and also a point of optimization to reduce excess air levels and thereby reduce fuel consumption. Third, methane / hydrocarbon measurement provides additional protection to monitor and detect flames and gas leaks during start-up and normal operation. Fourth, an indication of the process representation is important to ensure the relevance of the other three measurements, and this can take the form of a sample flow measurement or even a temperature or pressure measurement, depending on the technologies. and the arrangement of the combustion analyzer used. These four metrics provide operators with a bigger picture to fully monitor and safely operate their heating equipment.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/03092021/the-four-critical-measurements-of-combustion-analysers/


Kevin A. Perras

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