T. Tallon, AMETEK Process Instruments, Pittsburgh, Pennsylvania
Combustion processes are critical for producing heat and power in nearly every industry. As such, these processes rely on several measurements and controls for proper operation, including main fuel control valves, air flow valves, thermocouples and exhaust gas (flue gas) measurements. A single flue gas measurement can provide a useful operational setpoint for the overall system. However, multiple flue gas measurements can be combined to drive greater process safety and combustion optimization, reducing carbon dioxide (CO2) and nitrogen oxide (NOx) emissions and fuel consumption simultaneously.
Balancing safety margins at the burner. An excess oxygen measurement provides the first glimpse of post-combustion effectiveness in operating any combustion process. Excess oxygen represents the amount of oxygen in the process after all flammable compounds are consumed. As such, excess oxygen is also referred to as net oxygen and residual oxygen. As a unique flue gas measurement, excess oxygen is typically measured using a zirconium oxide ceramic sensor, which often carries a platinum coating that oxides all flammable compounds when in contact with the sensor before the excess oxygen measurement is made.
In any combustion process, measuring excess oxygen in the exhaust gas provides an important operation feedback mechanism. By measuring the amount of oxygen only after all other combustible compounds are consumed, the excess oxygen in the flue gas correlates directly with the amount of excess air supplied to the burner, revealing the air-to-fuel ratio of the burner. For example, in a typical boiler, fired heater or thermal oxidizer application, normal operation should have a slight excess of combustion air added to the burner at all times.
Excess air is beyond what is required for full fuel consumption, but it provides an important safety margin for combustion. If a burner fires natural gas and operates with an excess of 20% combustion air as a safety margin, the process will measure approximately 3% excess oxygen in the flue gas. In the same scenario, a burner operating with about a 12% safety margin of additional combustion air will see approximately 2% excess oxygen in the exhaust flue gas. As shown in FIG. 1, this correlation between excess oxygen (in the flue gas) and excess air (at the burner) enables the excess oxygen measurement to be used as an operational setpoint to monitor this safety margin of excess air at the burner. Some combustion systems even leverage the excess oxygen measurement to drive secondary (downstream) combustion air dampers to maintain adequate excess oxygen levels during normal operation.
Note: One important distinction to highlight is the difference between excess and total oxygen. Excess oxygen provides a useful correlation that reveals the air-fuel ratio at the burner. However, the measurement of total oxygen does not have this same relationship. As the name would suggest, total oxygen measures the total amount of oxygen in the process, regardless of any combustible compounds present. As such, there is no trend nor correlation with the excess air of the burner when measuring total oxygen, and any increases in partially burned fuel or incomplete combustion would be undetected. Only through the excess oxygen measurement can operators monitor the safety margin of additional air at the burner.
Recognizing the safety-efficiency challenge. While the excess oxygen reading provides an operational setpoint, no specific recommended value exists for every combustion application. Coal-fired burners may operate at 4%–6% excess oxygen during normal operation, while oil-fired burners may operate at 2%–4% excess oxygen in the flue gas. Natural gas-fired burners may target even lower levels at 1%–3% excess oxygen, depending on the expected fluctuations of the fuel composition and/or load variability conditions from the process. Bluntly stated, there is no fixed excess oxygen setpoint for all cases.
One challenge in combustion is selecting the correct excess oxygen level for safe and efficient operation. Operating at high excess oxygen levels is safer but less efficient for the system overall. As more excess air is added to the burner, the excess air passes through the system untouched, as it does not participate in the combustion reaction. However, this additional pass-through excess air is effectively inert, and this larger inert load causes the burner to increase its fuel consumption to maintain the same operating process temperature. As a result, the increased fuel consumption also increases the CO2 emissions of the process, and the increased availability of oxygen increases the risk of heightened NOx emissions.
Some amount of excess air will always be required at the burner to prevent a dangerous, unplanned, fuel-rich condition. However, a careful balance between safety and efficiency must be maintained when setting the optimal control point.
Identifying the optimum efficiency point: Combustion optimization. Beyond the excess oxygen measurement, another flue gas measurement to consider for operation is the combustibles measurement. The term combustibles refers to those compounds derived from partially burned fuel and incomplete combustion, typically carbon monoxide (CO) and hydrogen (H2). Combustible measurement is often done through catalytic detectors, which provide an unspecified, all-in-one measurement within a specific reactivity zone. Most combustible detectors are designed to measure parts per million (ppm) levels of the reactive zone between CO and H2; as a result, the combustibles typically do not operate hot enough to crack methane or other hydrocarbons in the fuel. However, measuring combustibles alone can provide a mechanism to monitor the onset of incomplete combustion in the flue gas.
In the combustion process, as excess oxygen levels increase, the combustibles levels decrease, as more oxygen is available at the burner to fully react with the fuel. However, in the reverse case, as excess oxygen levels are lowered, combustible levels increase, and at some point, near stoichiometric conditions, they increase exponentially—this is also referred to as the combustibles breakthrough point. At excess oxygen levels below this inflection point, the burners are nearly starved of oxygen and do not have sufficient air to consume the fuel entirely, posing a safety risk and a major inefficiency to the process. As such, the combustibles measurement provides a mechanism to monitor this breakthrough point, alerting operators to the possibility of an unsafe condition.
The excess oxygen and combustibles measurements monitor the process for safe post-combustion operation. However, these two safety measurements offer a path for optimizing combustion. Using the combustible measurement as a cross-reference point, operators can lower the excess oxygen level below the typical setpoint while also monitoring for sufficient safety margin above the combustibles breakthrough point (FIG. 2). While it is important to consider operating modes where the fuel composition varies significantly, this same process can be repeated to determine an ideal excess oxygen setpoint for the process. Thus, combustion optimization is ultimately achieved when measuring excess oxygen and combustibles simultaneously to balance safety and efficiency.
Increasing safety during startup and process upset. In addition to measuring excess oxygen and combustibles, one other flue gas measurement can be used to increase overall combustion safety: the measurement of methane, hydrocarbons and other fuels. In many cases, a single detector can measure percent levels of methane and hydrocarbons in a manner similar to the combustible detector. Methane and hydrocarbon detectors also typically use catalytic elements, providing an unspecified, all-in-one measurement. However, methane and hydrocarbon detectors operate at higher temperatures, which allows them to crack and measure methane and various other hydrocarbons in the fuel. Furthermore, detecting methane and hydrocarbons provides one other mechanism for detecting fuel leaks and loss of flame during startup and normal operation.
During startup, the combustion process must be adequately purged with multiple volumes of air before lighting off. If the process undergoes too many failed light-off attempts within a specified period, the standard protocol is to repeat the purging cycle with multiple volumes of air until successful. A methane/hydrocarbon detector becomes useful in monitoring for any large accumulation of an unburned fuel-rich mixture during the light-off phase or, worse, a fuel leak. Although downstream of the burner, the catalytic methane/hydrocarbon detector gives operators one more reference point to ensure safety during startup conditions.
In addition, unburned fuel should not exist during normal operation. However, a fuel leak or loss of flame in combination with poor mixing in the firebox can create a scenario where cold spots of unburned fuel rise and escape the process. In these cases, a methane/hydrocarbon detector has also been used to detect and alert a fuel-rich condition.
Monitoring for combustion safety and efficiency. Operators rely on several measurements and controls for proper operation in any combustion process. Through flue gas measurements, operators can monitor downstream of the burner for feedback on the effectiveness of their combustion process. An operational setpoint can be set and monitored through excess oxygen measurement to determine the operating air-fuel ratio and highlight the amount of excess air at the burner as its own safety margin.
With the combustibles measurement, operators can detect the combustibles breakthrough (when excess oxygen is too low) and identify the optimum combustion control setpoint in combination with the excess oxygen measurement. Notably, combustion optimization reduces the amount of excess air in the burner, which lowers CO2 emissions, NOx emissions and fuel consumption. Finally, using a methane/hydrocarbon measurement in the flue gas provides a safety mechanism to monitor for fuel leaks and loss of flame during startup and normal operation. Above all, these flue gas measurements provide a path for optimizing combustion safely and efficiently. HP