What happens to stack O2 levels when steam temperature and pressure decrease?

When steam temperature and pressure decrease, the stack oxygen levels typically rise due to incomplete combustion of the fuel being used. This happens because lower temperatures and pressures can lead to less efficient combustion, resulting in a higher concentration of unburned fuel and, consequently, more oxygen in the exhaust gases.

The process here involves the relationship between the efficiency of fuel combustion and the heat produced. As you decrease steam temperature and pressure, the combustion process may not generate enough energy to fully utilize the available oxygen, leading to an increase in stack O2 levels. This situation is often described as one where there is "stopped fuel," which suggests that the fuel being fed into the system cannot burn completely due to insufficient conditions created by the lowered temperature and pressure.

In contrast, other options suggest actions that do not necessarily correlate with the decreased steam conditions affecting stack O2 levels. Fuel supply and airflow adjustments are typically responses to maintain or enhance combustion efficiency rather than consequences of decreased steam conditions. Excessive heat application would not align with decreased temperature, as it implies the opposite effect. Thus, the rise in stack O2 levels indicates an issue related to fuel efficiency, which aligns with the notion of stopped fuel.