Of all the improvements available for fuel-based process heating systems, controlling the air-to-fuel ratio may be the most accessible. It doesn’t require new equipment or significant capital. It requires understanding how combustion works and making sure your system is operating within the right parameters.
The core principle is straightforward: burning too much fuel relative to air wastes fuel. Burning too much air relative to fuel wastes energy by carrying it out of the system as hot exhaust.
How Combustion Efficiency Works
When a hydrocarbon fuel burns in air, the ideal condition — called stoichiometric combustion — consumes all the fuel with just enough oxygen to complete the reaction. In practice, stoichiometric operation isn’t achievable in most industrial burners because perfect mixing of fuel and oxidant would be required. Unburned hydrocarbons in the exhaust stream are both wasteful and potentially hazardous.
As a result, systems are designed to run with some excess air — typically around 3% according to the sourcebook — to ensure complete combustion. This small margin provides safety without significant energy penalty.
The Problem with Too Much Excess Air
Excess air beyond what’s needed for complete combustion must be heated along with the combustion products, and all of that heat ends up in the exhaust gases rather than in the product.
For systems that have drifted out of calibration, reducing excess air to the appropriate level can produce immediate, measurable fuel savings without any other changes.
What to Monitor
The sourcebook identifies practical indicators that suggest the air-to-fuel ratio needs attention: excess oxygen in furnace exhaust gases (indicating too much excess air), unstable flame behavior (indicating improper fuel/air control), and linkage conditions that cause poor control of the fuel/air mixture across the range of operating conditions.
Monitoring exhaust gas oxygen content — using an oxygen analyzer or portable combustion analyzer — is the standard method for verifying that the system is operating at the correct ratio.
While air-to-fuel optimization is a fuel-based system improvement, it works in combination with other measures including heat containment. A well-insulated system transfers more heat to the product per unit of fuel burned, which means the gains from proper burner calibration and proper insulation compound each other. UniTherm’s insulation solutions address the containment side of that equation.
For more on combustion optimization, see the DOE’s Process Heating Tip Sheet series, available at energy.gov/eere/amo.
For our standard removable insulation solutions, visit shop.unitherm.com.
To request a quote for a custom insulation solutions click here.
Source: U.S. DOE, Improving Process Heating System Performance: A Sourcebook for Industry (Third Edition), 2015.