Is it possible to achieve perfect combustion?
The term “Stoichiometric Combustion” refers to the theoretical ideal mixture of oxygen and fuel that produces the most complete combustion, with the highest efficiency and fewer harmful greenhouse gases.
Can We Achieve It?
Although we strive for this condition, the variances in fuel composition, temperatures, pressures, and humidity make it impossible to predict and achieve, we can come very close by maximizing burner efficiency and minimizing energy losses and greenhouse gas emissions.
For example, for a particular burner operating at a certain firing rate, we can predict the amount of air that will be required to balance the air-gas ratio to achieve complete combustion without running the burner either too rich or too lean.
A Burner Running Rich or Lean
Excess air in a burner system is the air flow above the point of stoichiometric combustion that provides a sufficient volume for complete combustion, plus a little extra to make sure all the fuel is burned safely and efficiently. Carbon Dioxide (CO2) and water vapor (H2O) are natural results of complete natural gas combustion, and the excess air is manipulated to minimize Carbon Monoxide (CO), a dangerous greenhouse gas emission.
This also maintains flame stability and reliability with fluctuating conditions, such as changes in air temperature, humidity, and loss of precision due to wear and hysteresis on mechanical linkages.
Beyond this point, additional excess air reduces efficiency as additional unburned air cools the exhaust stream and limits heat transfer through the heat exchanger.
This difference between a burner running rich or lean can be illustrated in the chart to the right.
As the fuel-air mixture becomes richer, to the left of the Stoichiometric Combustion line, unburned fuel is exhausted through the heat exchanger and can be measured in CO content. The efficiency curve moves downward as less fuel is burned and CO emissions are increased.
Gas flow is regulated along a curve from the minimum firing rate to the maximum firing rate.
As the fuel-air mixture becomes leaner, to the right of the Stoichiometric line, additional excess air is produced, efficiency rises slightly (the “Highest Efficiency” range), after which the CO2 levels and efficiency both start to drop as higher excess air, as measured in the flue gas exhaust, rises.
A typical goal for tuning most burners is 15% excess air, or a 3% O2 content in the exhaust stream as measured by a combustion analyzer. By selecting the right burner system, and having it properly maintained, we can achieve these levels consistently through the firing range. This helps to maximize efficiency and safety, while minimizing fuel costs.