As the economy tightens, operators of industrial combustion systems such as steam boilers, thermal oxidizers, fired heaters, waste-heat boilers, dryers, furnaces, incinerators, and kilns are called upon to save fuel costs and minimize downtime. Each of these applications burns fuel in the presence of air (oxygen) to create process heat.
Saving fuel costs and minimizing downtime requires maximizing the combustion efficiency by tightly controlling the flow of both air and fuel introduced to the burner. The trick is knowing how much of each to provide—and that goes back to understanding the stoichiometric air-fuel ratio.
Efficient Air-to-Fuel Ratio
In chemistry, stoichiometry is the method for balancing chemical equations to calculate the exact amount of individual reactants needed to ensure that all the reactants are used up with no excess left over from the reaction. For example, a simple combustion equation for burning the methane (CH4) in natural gas is:
CH4 + 2O2 → CO2 + 2H2O + Heat
According to this stoichiometric equation, a perfectly balanced combustion process would require exactly two oxygen (O2) molecules to burn each methane molecule. The reaction would produce one molecule of carbon dioxide (CO2) and two molecules of water (H2O) along with excess heat to be used for the process.
If the equation is not perfectly balanced, for example, if there is not enough O2 to completely combust or burn each CH4 molecule, the reaction will produce some carbon monoxide (CO) instead of CO2.
Real-World vs. Ideal Air-to-Fuel Ratio
In theory, a perfectly efficient combustion system would operate with a perfect stoichiometric ratio of fuel to oxygen. But in practice, real-world industrial combustion systems need to operate with a slight excess of oxygen in the ratio of air to fuel.
This ensures complete combustion and avoids potential safety issues associated with burning fuel-rich mixtures, which can lead to safety issues.
The result, as you can see from the illustration, is that operators try to maintain a slightly lean mixture of fuel with only a small amount of excess oxygen because too much excess oxygen degrades the combustion efficiency and increases cost. This happens for a few reasons.
First, by requiring the system to heat a larger volume of gas than is needed decreases the ability for the system to transfer heat from the flame to the steam (in the case of a boiler) or to the process fluid.
Second, too much excess oxygen will force the fan to operate at elevated speeds, which wastes energy over time and increases emissions. A common margin of excess air is about 2-4% more air than fuel.
This target maximizes efficiency and minimizes formation of NOx—a pollutant generated from nitrogen in the air reacting with oxygen—while still ensuring the complete combustion of fuel.
3 Methods for Maintaining an Efficient Air-to-Fuel Ratio
To accurately maintain this small excess margin in industrial combustion systems, combustion control systems need accurate airflow measurement and control in addition to fuel flow monitoring. In general, operators use one of three different methods to monitor and maintain the proper amount of air and fuel in an industrial plant:
- Parallel positioning (PP)
- PP with O2 trim
- Fully metered airflow control systems
Parallel positioning systems are relatively straightforward technically and are generally the least expensive option for controlling airflow. These provide some degree of control for stable loads and consistent burner function and offer the lowest capital cost of the three options.
However, PP is not effective for variable loads or rapid swings and is the least able to maintain optimal efficiency.
PP with O2 trim systems continuously monitor the amount of oxygen in the exhaust gas and provide feedback to an automatic positioner for the air damper. The objective is to “trim” the airflow in order to drive excess oxygen lower (closer to 2-4% excess O2).
PP with O2 trim is the most common type of control that is applied to combustion systems across a number of industries. This type of system, however, is best applied to boilers at steady-state operation, while real-world boilers run at constantly varying steam loads. The degree of actual control enabled with a PP/O2 trim system is not very large and this system is not the best choice for boilers with broad load ranges.
Fully metered airflow control systems
Minimizing the amount of excess air without creating a fuel-rich environment can best be achieved with the help of fully metered systems that effectively monitor and control airflow. These systems have the best ability to optimize the efficiency of burners and can address multiple challenges at once including efficiency, maintenance, safety, and emissions.
They allow tighter control of air-to-fuel ratio throughout the full operating range of the boiler and ensure that emissions are within specifications.
Fully metered systems allow a much higher degree of control for operations over tuning the system using PP with O2 trim. With a fully metered system, operators can closely match the optimal air-fuel ratio even throughout process temperature swings, barometric pressure, and static pressure changes.
The greater degree of control means there are fewer boiler trips and therefore less downtime across full operating ranges of the combustion burner when the air-to-fuel ratio gets out of balance. Fully metered systems also can easily adapt to fuel type or composition changes such as switching between natural gas and fuel oil or changing natural gas suppliers.
Because a poor ratio of air to fuel can contribute to failures of burner internals as well as soot buildup, there is a connection to unplanned shutdowns. Better airflow control enabled by fully metered systems results in fewer chronic maintenance issues, such as burner replacements.
For large-sized boilers, good air monitoring allows the possibility of using a predictive emissions control system (PEMS) versus continuous (CEMS) to report emissions to air districts or the EPA.
A PEMS system is one-third the cost of a CEMS but requires that plant operators can maintain high levels of confidence in the control system for airflow to ensure the measurement is accurate.
A Fully Metered Airflow Control System Can Benefit Your Industrial Application
The benefits of a fully metered airflow control system come with costs and added complexity— however, the benefits far outweigh the costs. The cost of this type of system varies but is slightly more than that of a PP with O2 trim.
The challenge of maximizing fuel economy using the stoichiometric air-to-fuel ratio is felt by plant managers everywhere. With a fully metered combustion airflow measurement system, operators can expect to see measurable efficiency improvements and emissions reductions that will increase their bottom line.