In any combustion process, it is necessary to monitor the inlet flow rates of fuel and air to the burner. While these flow rates provide baseline parameters to set a flame, they do not provide feedback to reveal or alert any potential concerns with the combustion reaction, such as incomplete combustion from imperfect mixing in the burner or safety risks such as fuel leaks or loss of flame. Flue gas analysis offers one approach to monitor the process and provide feedback. It is especially important to consider when firing both hydrocarbon-based and high hydrogen fuels.
Flue gas analysis plays three critical roles in the combustion process:
- Excess oxygen setpoint
- Combustion optimization
- Process safety
Each role will be highlighted along with its corresponding measurement. Together, the measurements ensure combustion safety, proper operation, and fuel efficiency.
The role of the excess oxygen setpoint
As the first, most critical role, flue gas analysis provides the operational air-fuel ratio set point at the burner, using the excess oxygen measurement. To sustain a flame, a sufficient amount of air must be diverted to the burner to consume ALL the fuel. This balance of air and fuel is also known as the “air-to-fuel” or the “air-fuel” ratio. In practice, the burner must have enough excess oxygen in the flue gas to ensure complete consumption of the fuel – along with an additional margin for safety.
Unlike the total oxygen concentration in the flue gas, the excess oxygen measurement is unique in that it correlates directly to the air-fuel ratio. This excess oxygen level refers to the amount of oxygen present AFTER all the combustible content in the stream is consumed – hence it monitors the excess of the remaining oxygen. This measurement is also referred to as the “residual oxygen” or “net oxygen” reading. It is also important to note that a zero percent (0%) excess oxygen reading means that there is no safety margin of extra or excess air at the burner to consume all the fuel at the burner. Insufficient excess oxygen levels present an unsafe condition as they allow no room for fluctuations in fuel composition, changes in humidity, sudden changes in fuel flow, etc.
When it comes to setting the air-fuel ratio, the excess oxygen measurement is the operational setpoint to ensure that the burner operates with sufficient excess oxygen at all times. In most cases, operators set the excess oxygen setpoint anywhere between 1% and 5%, depending on the fuel type and the variability of the fuel composition over time. If the reading is too low, more combustion air may be needed at the burner, although the addition of a combustibles reading would give a fuller picture of the combustion process (described in the next section). That said, insufficient oxygen at the burner causes incomplete combustion, an obvious waste of fuel, and a potential safety hazard. Through flue gas analysis, operators can ensure that the burner has enough air to maintain a safe and stable flame. This is especially true when using both hydrocarbon-based and high hydrogen fuels.
The role of combustion optimization
As its second role, flue gas analysis offers the ability to optimize the excess oxygen setpoint using a combustibles measurement. For context, combustibles are generated as a byproduct of incomplete combustion at the burner. Under perfect conditions, hydrocarbon fuels react to form carbon dioxide (CO2) and water (H2O). However, in practice combustion is never perfect because of poor air/fuel mixing at the burner, changing load conditions, malfunctioning burners, and variable fuels. As a result, a small amount of unburned combustibles is always generated, usually in the form of ppm-levels of carbon monoxide (CO) and hydrogen (H2). A combustibles detector uses catalytic elements to measure both CO and H2 together in a single measurement. As shown in Figure 1, the combustibles detector uses a catalytic active element and a reference element to provide this single, combined, non-speciated, umbrella measurement. In the case of high hydrogen fuels, these catalytic combustibles detectors have higher sensitivity to monitor for unburnt hydrogen, but they can also monitor for CO if hydrocarbons are present in the fuel source as well.
Figure 1. An example combustibles detector which uses catalytic elements to monitor for ppm-levels of incomplete combustion, including H2 and CO, in a single, combined combustibles measurement.
The combustibles measurement can be used to monitor the health of the combustion process. Combustibles inform the operator of how much incomplete combustion is present. As the excess oxygen increases, less combustibles are formed. However, if not enough excess oxygen is present, the combustibles increase dramatically. In extreme cases where the burner has too low of an excess oxygen level, the combustibles concentration can hit a point of breakthrough and increase exponentially - as can be seen in Figure 2 - creating an unsafe condition.
Figure 2. Measurement of excess oxygen and combustibles enables optimized combustion and lower fuel consumption and stack emissions
Operators can use the combustibles measurement (in conjunction with the excess oxygen measurement) to reduce their carbon emissions and fuel consumption – ultimately ensuring safety while also optimizing their combustion process. As noted earlier, operators use the excess oxygen measurement as a set point for their burners, but excess oxygen alone does not tell the full story in the flue gas. Too much excess oxygen reduces fuel efficiency. Lower excess air levels mean there is less air and flue gas to heat, and thus, less heat is lost through the stack. By using a combustibles measurement, operators can reduce their excess oxygen reading to a safe range well before reaching the combustibles breakthrough point. The combustibles measurement provides the secondary reference point to allow operators to thoughtfully lower their combustion air levels at the burner. Flue gas analysis provides the excess oxygen reading and the combustibles measurement necessary to optimize the combustion process safely.
The role of process safety
Finally, as its third critical role, flue gas analysis plays an important part in detecting unsafe conditions, using a combustibles detector as well as a methane / hydrocarbons detector. As noted earlier, a combustibles detector measures the CO and H2 from normal incomplete combustion caused by poor mixing or changing dynamics at the burner. However, the combustibles measurement can also provide a safety measurement to detect and alert the onset of combustibles breakthrough. Using the combustibles measurement, operators have the ppm-level visibility to monitor for CO breakthrough and insufficient levels of air at the burner.
Operators can also use a methane / hydrocarbon catalytic detector to provide percent-level measurements of methane, hydrocarbons, and any unburnt hydrogen in the flue gas. By itself, a combustibles detector does not run hot enough to crack and measure methane and other small hydrocarbons. However, with a catalytic design similar to the combustibles detector, a methane / hydrocarbons detector can be used instead to detect and signal high percent levels of unburnt hydrocarbons in the flue gas, often as a result of fuel leaks at the burner, process tube leaks, and loss of flame during start-up and normal operation. Even when using high hydrogen fuels, the hydrocarbons detector will respond to any unburnt hydrogen present, and it can be used to detect these percent-level unsafe conditions. It is the catalytic nature of the detector which enables these measurements to include methane, hydrocarbons, and hydrogen in a single, combined, percent level value.
As shown in Figure 3, the WDG-V analyzer is an example of a combustion flue gas analyzer which provides all three of these critical measurements in one sensor to ensure safe and efficient combustion control.
Figure 3. The THERMOX WDG-V is a combustion flue gas analyzer which is capable of measuring excess oxygen, combustibles, and methane/hydrocarbons in one unit.
Flue gas analysis for safe, efficient hydrogen combustion
Flue gas analysis plays the three critical roles of (1) operational setpoint for “excess oxygen” to ensure adequate air (and air-fuel) ratio at the burner, (2) optimization mechanism to reduce fuel consumption and emissions caused by operating too high of an “excess air” level at the burner, and (3) safety monitoring to detect high levels of incomplete combustion and unburnt fuels. Together, these measurements ensure safe operation of the combustion process, regardless of the fuel source.