Question 24 QMED03 - Oiler

The Correct Answer is C ### 2. Explanation for Option C (Correct Answer) Combustion is a chemical process where a fuel reacts with an oxidant (usually oxygen in the air) to produce heat and combustion products. * **Complete Combustion** primarily yields carbon dioxide ($\text{CO}_2$) and water ($\text{H}_2\text{O}$). In an ideal complete scenario, the carbon in the fuel is fully oxidized to $\text{CO}_2$, and there is minimal to no carbon monoxide ($\text{CO}$) produced. * **Incomplete Combustion** occurs when there is insufficient oxygen, poor mixing, or low temperatures, leading to the formation of undesirable products, most notably carbon monoxide ($\text{CO}$) and unburned hydrocarbons (soot). The **LEAST complete combustion** means the process has failed significantly to fully oxidize the carbon. Therefore, the stack gas analysis will show: 1. **Relatively high CO percentage:** A large amount of the carbon was only partially oxidized (to $\text{CO}$), indicating poor combustion efficiency and high inefficiency. $\text{CO}$ is the primary measurable indicator of incomplete combustion. 2. **Relatively low $\text{CO}_2$ percentage:** Since much of the carbon was converted to $\text{CO}$ instead of $\text{CO}_2$, the final concentration of the fully oxidized product ($\text{CO}_2$) will be comparatively lower than in an efficiently operating boiler. ### 3. Explanation of Incorrect Options **A) Relatively low $\text{CO}_2$ percentage and relatively low $\text{CO}$ percentage:** This combination usually indicates high levels of excess air (dilution). If both $\text{CO}_2$ and $\text{CO}$ are low, it means the sample is heavily diluted by unused air (nitrogen and excess oxygen). While low $\text{CO}_2$ suggests inefficiency, the very low $\text{CO}$ level does not indicate poor oxidation (incomplete combustion); rather, it indicates effective oxidation or significant dilution. **B) Relatively high $\text{CO}_2$ percentage and relatively low $\text{CO}$ percentage:** This is the indication of the **MOST complete and efficient combustion** possible for a given fuel/air ratio. High $\text{CO}_2$ means the fuel was effectively oxidized, and low $\text{CO}$ confirms that minimal partial oxidation occurred. **D) Relatively high $\text{CO}_2$ percentage and relatively high $\text{CO}$ percentage:** It is physically difficult to have both products simultaneously high in an operating boiler, as the formation of one generally sacrifices the other (as the total available carbon is finite). However, if this scenario were somehow observed, the high $\text{CO}_2$ would contradict the notion of **LEAST** complete combustion. High $\text{CO}$ is a strong indicator of incomplete combustion, but the high $\text{CO}_2$ suggests a high degree of successful complete oxidation, making this combination contradictory to the worst possible efficiency.