Final answer:
A decrease in primary air would likely lead to incomplete combustion, which in turn would decrease the efficiency of the process and reduce the percentage of CO₂ produced.
Step-by-step explanation:
If a decrease in primary air occurs, it would affect the combustion process. Primary air refers to the air that is mixed with the fuel before or at the point of combustion. This air is essential in providing the necessary oxygen for the fuel to burn efficiently. In combustion, stoichiometric conditions, where the fuel and oxygen are in perfect ratio, result in complete combustion and maximum production of CO₂.
If the primary air is decreased, there will be less oxygen available for combustion, which can lead to incomplete burning of the fuel. Consequently, this tends to increase the production of carbon monoxide (CO) and other unburned hydrocarbons due to incomplete combustion, while reducing the percentage of CO₂. Therefore, a decrease in primary air would most likely result in a decrease in combustion efficiency and a decrease in the percentage of CO₂.