If a burner is set "rich", which element would NOT be found in the exhaust analysis?

When a burner is set "rich," this typically means that the fuel-air mixture has an excess of fuel relative to air. In such conditions, the combustion process does not have enough oxygen (O2) to completely oxidize all the fuel, resulting in the production of various combustion byproducts.

In a rich combustion scenario, hydrogen (H2) may be present in the exhaust because not all of the fuel fully combusts, allowing for unburned hydrogen to escape. Similarly, nitrogen (N2) is usually present in combustion exhaust since it is a major component of the air used in the combustion process, and remains largely unchanged throughout.

While carbon dioxide (CO2) can be created during combustion, its levels in the exhaust will be lower in a rich mixture compared to a stoichiometric (balanced) mixture due to incomplete combustion, which produces carbon monoxide (CO), unburned hydrocarbons, and other compounds instead. However, CO2 is still often found in some quantities because some degree of combustion does take place.

On the other hand, oxygen (O2) would be expected in higher quantities in the exhaust for a rich burner setup. The excess fuel means that there is not enough O2 to react with all the fuel