Analysis of Coal

ANALYSIS OF COAL

In order to assess the quality of coal the following two types of analysis are made.

1. Proximate Analysis

It is the analysis involving the determination of physical constituents like percentage of

(i) Moisture content.

(ii) Volatile matter.

(iii) Ash content.

(iv) Fixed carbon in coal.

1. Moisture content

About 1 gm of powdered air-dried coal sample is taken in a crusible, and is heated at 100 – 105°C in an electric hot-air oven for 1 hour. The loss in weight of the sample is found out and the % of moisture is calculated as

% of moisture in coal = loss in weight of the coal / weight of air-dried coal × 100

2. Volatile matter

After the analysis of moisture content the crusible with residual coal sample is covered with a lid, and is heated at 950 ± 20°C for 7 minutes in a muffle furnace. The loss in weight of the sample is found out and the % of volatile matter is calculated as

% of volatile matter in coal = loss in weight of the coal / weight of air-dried coal × 100

3. Ash content

After the analysis of volatile matter, the crusible with residual coal sample is heated without lid at 700 ± 50°C for 1/2 an hour in a muffle furnace. The loss in weight of the sample is found out and the % of ash content is calculated as

% of ash content in coal = weight of ash formed / weight of air-dried coal × 100

4. Fixed carbon

It is determined by subtracting the sum total of moisture, volatile and ash contents from 100.

% of fixed carbon in coal

= 100 – % of (moisture content + volatile matter + ash content)

Significance (or) Importance of Proximate Analysis

1. Moisture content

High percentage of moisture is undesirable because

(i) it reduces the calorific value of coal,

(ii) moisture in coal consumes more heat in the form of latent heat of evaporation and hence more heat is to be supplied to the coal,

(iii) it increases the transport cost.

2. Volatile matter

High percentage of volatile matter is undesirable because

(i) it reduces the calorific value of coal,

(ii) large proportion of fuel on heating will distill over as vapour, which escapes out unburnt,

(iii) coal with high percentage of volatile matter burns with a long flame with high smoke,

(iv) the coal containing high percentage of volatile matter do not coke well.

3. Ash content

High percentage of ash content is undesirable because

(i) it reduces the calorific value of coal,

(ii) ash causes hindrance to heat flow as well as produces clinkers, which blocks the air supply through the fuel,

 (iii) it increases the transporting, handling and storage costs,

(iv) it involves additional cost in ash disposal.

4. Fixed carbon

(i) High percentage of fixed carbon is desirable because higher the percentage of fixed carbon in a coal, greater is its calorific value,

(ii) the percentage of fixed carbon helps in designing the furnace and the shape of the fire-box.

 

2. Ultimate Analysis

It is the analysis involving the determination of chemical constituents like percentage of

(i) carbon and hydrogen contents

(ii) nitrogen content

(iii) sulphur content

(iv) ash content

(v) oxygen content

1. Carbon and Hydrogen contents

A known amount of the coal sample is burnt in a current of O₂ in a combustion apparatus. The carbon and hydrogen, present in the coal sample, are converted into CO₂ and H₂O respectively according to the following equations.

C + O₂  → CO₂ ↑

H₂ + 1/2O₂ → H₂O ↑

The liberated CO2 and H20 vapours are absorbed respectively in KOH and anhydrous CaCl₂ tubes of known weights. The increase in weight of KOH tube is due to the formation of CO₂ while increase in weight of CaCl₂ tube is due to the formation of H₂O. From the weights of CO₂ and H₂O formed, the % of carbon and hydrogen present n the coal can be calculated as follows.

Calculations

2KOH + CO₂ → K₂CO₃ + H₂O

CaCl₂ + 7H₂O → CaCl₂ . 7H₂O

Let

m = weight of the coal sample taken.

x = increase in weight of KOH tube.

y = increase in weight of CaCl₂ tube.

(a) % of carbon

(b) % of hydrogen

2. Nitrogen content

The determination of nitrogen content is carried out by Kjeldahl’s method. A known amount of powdered coal sample is heated with con. H₂SO₄ in presence of K₂SO₄ (catalyst) in a long necked flask (called Kjeldahl's flask). Nitrogen in the coal is converted into ammonium sulphate and a clear solution is obtained

2N + 3H₂ + H₂SO₄ → (NH₄)₂SO₄

The clear solution is then heated with excess of NaOH and the liberated ammonia is distilled over and is absorbed in a known volume of standard N/10 HCl.

(NH₄)₂SO₄ + 2NaOH → 2NH₃ + Na₂SO₄ + 2H₂O

NH₃ + HCI → NH₄Cl

The volume of unused N/10 HCl is then determined by titrating it against standard N/10 NaOH. Thus the amount of acid neutralised by liberated ammonia from coal is determined. From this the percentage of nitrogen is calculated as follows.

Calculation

Let, the weight of the coal sample taken = m gms

3. Sulphur content

A known amount of coal sample is burnt completely in a bomb calorimeter. During this process sulphur is converted into sulphate, which is extracted with water. The extract is then treated with BaCl₂ solution so that sulphates are precipitated as BaSO₄. The precipitate is filtered, dried and weighed. From the weight of BaSO₄ obtained, the sulphur present in the coal is calculated as follows.

Calculation

Let, the weight of coal sample = m gms

weight of BaSO₄ obtained = x gms

4. Ash content

Determination of ash content is carried out as in proximate analysis

5. Oxygen content

The percentage of oxygen is calculated as follows.

 % of oxygen in coal = 100 – % of (C+H+N+S+ash)

Significance (or) Importance of Ultimate Analysis

1. Carbon and hydrogen contents

(i) Higher the % of carbon and hydrogen, better is the quality of coal and higher is its calorific value.

(ii) The % of carbon is helpful in the classification of coal.

(iii) Higher % of carbon in coal reduces the size of combustion chamber required.

2. Nitrogen content

(i) Nitrogen does not have any calorific value, and its presence in coal is undesirable.

(ii) Good quality coal should have very little nitrogen content.

3. Sulphur content

Though sulphur increases the calorific value, its presence in coal is undesirable because

(i) The combustion products of sulphur, i.e., SO₂ and SO₃ are harmful and have corrosion effects on equipments.

(ii) The coal containing sulphur is not suitable for the preparation of metallurgical coke as it affects the properties of the metal.

4. Oxygen content

(i) Lower the % of oxygen higher is its calorific value.

(ii) As the oxygen content increases its moisture holding capacity increases, and the calorific value of the fuel is reduced.

Table 5.1 Differences between proximate analysis and ultimate analysis

Proximate analysis

1. It involves the determinations of physical constituents like moisture, volatile, ash and fixed carbon contents in coal.

2. It gives the approximate composition of the main constituents of coal.

Ultimate analysis

1. It involves the determination of chemical constituents like carbon, hydrogen, nitrogen and sulphur and oxygen contents in coal.

2. It gives the exact composition of the elementary constituents of coal.