Ultimate analysis
Ultimate analysis is carried out in order to determine the percentage of carbon, hydrogen, nitrogen, sulphur and oxygen in the coal. This is useful in combustion calculation.
(i) Carbon and hydrogen
- Carbon and hydrogen in coal can be determined by heating 0.2g of accurately weighed coal sample in combustion apparatus in a current of air.
- As a result, carbon is converted into CO2 and hydrogen is converted into H2O as shown below
- CO2 is absorbed in the KOH tube, whose weight is noted accurately.
2KOH + CO2 → K2CO3 + H2O
- H2O is absorbed in the CaCl2 tube, whose weight is also calculated accurately.
CaCl2 + 7H2O → CaCl2 + 7H2O
- The increase in weights of these two tubes gives the amounts CO2 and H2O formed as a result of combustion.
\left.\begin{array}{r} \% \text { of Carbon } \\ \text { in coal } \end{array}\right\}=\frac{\text { Increase in weight of } \mathrm{KOH} \text { tube }}{\text { Weight of coal sample taken }} \times \frac{12}{44} \times 100
\% \text { of Hydrogen }=\frac{\text { Increase in weight of } \mathrm{CaCl}_2 \text { tube }}{\text { Weight of coal sample taken }} \times \frac{2}{18} \times 100
Significance
Greater the percentage of carbon and hydrogen better is the coal in terms of quality and calorific value.
- The percentage of nitrogen can be calculated by using Kjeldahl method.
- One gram of pure dry powdered coal is heated with conc entrated H2SO4 in the presence of K2 SO4 as catalyst in Kjeldahl’s flask.
- The clear solution obtained is treated with excess of KOH solution.
- The NH3 liberated is absorbed in a known volume of standard acid solution.
- The unreacted acid is determined by the back titration with standard solution of NaOH .
- The percentage of nitrogen in the coal can be determined by knowing the volume of acid used and ammonia liberated.
Significance
- Nitrogen has no calorfic value and hence its presence in coal is undesirable.
- Hence, a high-quality coal should have little nitrogen content.
(iii) Sulphur
- Take 1g of pure dry powdered coal in a platinum crusible and cover it by adding of 10g of Eschka mixture.
- Eschka mixture is a mixture of porous calcined magnesium oxide and anhydrous sodium carbonate in 2:1 ratio.
- The crucible is heated gradually in an asbestos sheet in the presence of air.
- After about half-an hour, the temperature is increased so that the whole carbon is burnt out.
- The heating source is removed, when the content becomes yellow or light brown in colour.
- The insoluble matter is filtered and then washed with boiled distilled water for 5 6 times. 2 3ml of bromine and then 4ml of 5% HCl are also added.
- This sulphate is treated with excess of BaCl2 solution. BaSO4 thus formed is precipitated by gravimetric analysis.
\left.\begin{array}{c} \% \text { of carbon } \\ \text { in coal } \end{array}\right\}=\frac{32 \times \text { Weight of } \mathrm{BaSO}_4 \text { obtained }}{233 \times \text { Weight of coal sample }} \times 100
Significance
(i) Sulphur is present to the extent of 0.5 3.0%
(ii) Presence of sulphur is highly undesirable in coal to be used for making coke.
(iv) Oxygen
The percentage of oxygen is calculated as follows :
| Percentage of oxygen in coal = 100- Percentage of [ C+ H + N+ S+ Ash] |
Significance
(i) Oxygen is combined with hydrogen in coal and thus hydrogen availability for combustion is decreased.
(ii) Thus, the presence of oxygen decreases the calorific value of coal and its presence its undesirable.
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