A fuel with composition by mass is given as $78 \%$ carbon $(\mathrm{C}), 16 \%$ hydrogen $\left(\mathrm{H}_{2}\right)$, $3.2 \%$ sulphur $(\mathrm{S}), 1.6 \%$ oxygen $\left(\mathrm{O}_{2}\right)$, and ash. The fuel is fired in a boiler with excess air for the complete combustion and no carbon monoxide $\text{(CO)}$ is detected in the flue gases. The dry flue gas contains $0.2 \%$ sulphur dioxide $\left(\mathrm{SO}_{2}\right)$ by volume. Which one of the following options is closest to the amount of excess air provided (in $\%$ by mass)?
Assume molar masses $\left(\mathrm{g} \mathrm{~mol}^{-1}\right)$ of $\mathrm{C}, \mathrm{H}_{2}, \mathrm{~S}$, and $\mathrm{O}_{2}$ are $12,2,32$, and 32 , respectively, and the volumetric ratio of nitrogen $\left(\mathrm{N}_{2}\right)$ and oxygen $\left(\mathrm{O}_{2}\right)$ in the air as $3.76:1$.
- $14.8$
- $7.4$
- $22.2$
- $3.1$
