Flue Gas Analysis (Orsat Method)

FLUE GAS ANALYSIS (ORSAT METHOD)

The mixture of gases (like CO₂, O₂, CO, etc) coming out from the combustion chamber is called flue gases. The analysis of a flue gas would give an idea about the complete or incomplete combustion process. The analysis of flue gases is carried out by using orsat's apparatus.

Description of orsat's apparatus

It consists of a horizontal tube. At one end of this tube, U-tube containing fused CaCl2 is connected through 3-way stop cock. The other end of this tube is connected with a graduated burette. The burette is surrounded by a water-jacket to keep the temperature of gas constant. The lower end of the burette is connected to a water reservoir by means of a rubber tube. The level of water in the burette can be raised or lowered by raising or lowering the reservoir (fig 6.1).

The horizontal tube is also connected with three different absorption bulbs 1, 2, and 3 for absorbing CO₂, O₂ and CO.

Working

The 3-way stop-cock is opened to the atmosphere and the reservoir is raised, till the burette is completely filled with water and air is excluded from the burette. The 3-way stop-cock is now connected to the flue gas supply and the flue gas is sucked into the burette and the volume of flue gas is adjused to 100 cc by raising and lowering the reservoir. Then the 3-way stop cock is closed.

(a) Absorption of CO₂

The stopper of the absorption bulb-1, containing KOH solution, is opened and all the gas is passed into the bulb-1 by raising the level of water in the burette. The gas enters into the bulb-1, where CO₂ present in the flue gas is absorbed by KOH

The gas is again sent to the burette. This process is repeated several times to ensure complete absorption of CO₂. The decrease in volume of the flue gas in the burette indicates the volume of CO₂ in 100 cc of the flue gas.

 (b) Absorption of O₂

Stop-cock of bulb-1 is closed and stop cock of bulb-2 is opened. The gas is again sent into the absorption bulb-2, where O₂ present in the flue gas is absorbed by alkaline pyrogallol. The decrease in volume of the flue gas in the burette indicates the volume of O₂.

(c) Absorption of CO

Now stop-cock of bulb-2 is closed and stop-cock of bulb-3 is opened. The remaining gas is sent into the absorption bulb-3, where CO present in the flue gas is absorbed by ammoniacal cuprous chloride. The decrease in volume of the flue gas in the burette indicates the volume of CO. The remaining gas in the burette after the absorption of CO₂, O₂ & CO is taken as nitrogen.

Significance (or) uses of flue gas analysis

1. Flue gas analysis gives an idea about the complete or incomplete combustion process.

2. If the flue gases contain considerable amount of CO, it indicates that incomplete combustion is occuring and it also indicates that the short supply of O₂.

3. If the flue gases contain considerable amount of O₂, it indicates that complete combustion is occuring and also it indicates that the excess of O₂ is supplied.

Precautions

1. Care must be taken in such a way that, the reagents in the absorption bulb 1, 2 and 3 should be brought to the etched marked level one by one by raising and lowering reservoir bottle.

2. All the air from the reservoir bottle is expelled to atmosphere by lifting the reservoir bottle.

3. It is essential that CO₂, O₂ and CO are absorbed in that order only.

4. As the CO content in flue gas is very small, it should be measured quite carefully.