Question 1.1.3: A sample of coal was found to have the following composition......

Engineering Chemistry: Fundamentals and Applications [EXP-169979]

A sample of coal was found to have the following composition by weight

C = 75%; H = 5.2%; O = 12.1%; N = 3.2% and ash = 4.5%

Calculate (i) minimum amount of $O_2$ and air necessary for complete combustion of 1 kg of coal; (ii) weight of air required if 40% excess air is supplied.

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\begin{aligned}O_2  \text{required from air} &=  \text{Total}  O_2  \text{required}  –  O_2  \text{in fuel}\\ & =  2.416 ~kg  –  0.121 ~kg =  2.295 ~kg\end{aligned}

Minimum weight of air required = $2.295 × \frac{100}{ 23} = 9.978 ~kg$ .

Weight of excess air = $9.978 × \frac{40}{100} = 3.9912 ~kg$

Total air supplied = 9.978 + 3.9912 = 13.969 kg. $\quad \quad \quad [Ans = 13.969 ~kg]$

Constituent	Amount per kg of the coal sample	Combustion reaction	Weight of $\bf O_2$ required
C	0.75 kg	$C +O_2 \rightarrow CO_2$	$0.75 × \frac{32}{12} = 2 ~kg$
H	0.052 kg	$H_2 +\frac{1}{2} O_2 \rightarrow H_2O$	$0.052 × \frac{16}{2} = 0.416 ~kg$
O	0.121 kg	–	–
N	0.032 kg	–	–
Ash	0.045 kg	–	–
		–	$\text{Total} O_2 \text{required} = 2.416 ~kg$

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