Flue Gas Analysis by Orsat Apparatus | Combustion | Thermodynamics

When the combustion of the fuel takes place, the products of combustion will be carbon dioxide, (sometimes carbon monoxide may also be present) sulphur dioxide, vapour and nitrogen all the while, oxygen also will be present in the flue gases.

Generally, the composition of solid or liquid fuels is expressed by weight, whereas that of the gaseous fuels is expressed by volume. Products of combustion are in the gaseous state and the analysis of the products of combustion will be in volumes or volumetric analysis will be obtained.

In order to get the analysis of the exhaust gases by volume, first the analysis of the exhaust gases is found by weight, i.e., the weights of the individual products are first determined and then the percentage analysis of the gas is found. When the weight analysis is known, it can be converted into volumetric analysis by the help of Avogadro’s hypothesis.

Analysis by Weight or Gravimetric Analysis:

(a) Theoretical Air Supplied:

As before, let C, H, S and O be the weight of carbon, Hydrogen sulphur and oxygen per kg of fuel. Then the chemical equations can be written as-

When H₂O vapours are included in the exhaust or flue gases, then the products of combustion are wet. When H₂O vapours are excluded from the analysis, then the products of combustion are known as Dry Products of Combustion and in practice we consider the dry products of combustion. With the exclusion of the H₂O vapours, the table will be slightly modified accordingly. Note here that hydrogen has to be considered for finding the total oxygen required. N₂ will depend on this total oxygen required.

(b) Excess Air Supplied:

Let x be the excess air supplied per kg of fuel. Then as before, consider 1 kg fuel to contain carbon (C), hydrogen (H), sulphur (S) and oxygen (O) and the products of combustion will be-

Analysis by Volume or Volumetric Analysis:

Once we have the analysis of the products of combustion by weight, we can convert this into volumetric analysis of the products of combustion. Remember, for this purpose we make use of Avagadro’s law or hypothesis (molecu­lar weights of all gases at NTP occupy equal volumes). Therefore, weight or gravimetric analysis can be converted into volumetric analysis by dividing the weight of the gas by its molecule weight to get the volume of the gas. Now consider the percentage analysis of the wet products.

Now, when the volumetric analysis of the products of combustion is known, we can convert the volumetric analysis into gravimetric analysis in converting volume into weight of the gas by multiplying the volume with the molecular weight of the gas. Thereby, weights of all constituents of the flue or exhaust gases are determined and therefore total weight of the gas and then the percentage composition of the gas can be determined. Table given below will show this procedure of converting volumetric analysis into gravimetric analysis.

The illustrative examples will give the reader idea about the conversion of gravimetric into volumetric analysis and vice-versa.

Conversion between Gravimetric and Volumetric Analysis:

For converting the gravimetric analysis of the products of combustion into volumetric analysis and vice-versa, two tables are given below:

Flue Gas Analysis (Using Orsat Apparatus):

A simple and convenient apparatus used for analysis of dry flur gases by volume is called an Orsat Appartus. It consists of a graduated measuring bottle also called as eudiometer, an aspirating bottle, three double reagant pipettes to absorb CO₂, O₂ and CO. Eudiometer is connected to aspirating bottle by means of rubber tube.

The first reagent pipette next to eudiometer contains the KOH solution (33% KOH + 67% water by weight) to absorb CO₂. The second pipette contains alkaline solution of pyrogallic acid (5 gm of pyrogallic acid in 15 cc of water + 33 gm KOH in 67 gm H₂O) to absorb O, and the third pipette contains acidic solution of cuprous chloride (5 cc of cuo + 1000 cc of HCl) to absorb CO.

Each pipette is made of two pipettes joined together by a glass tube at the bottom as shown. Front row of pipette is connected to the front flue gas tube and rear row of pipette are connected to another tube which runs parallel to flue gas sample tube. Each front pipette is provided with number of glass tubes inside to increase the wetted surface area hence it accelerates action of the gas absorption.

Firstly, the existing air or the gas is expelled from the eudiometer by raising the aspirating bottle and keeping three way valve open to atmosphere. The three cocks a, b, c and a three way cock are closed. The aspirating bottle is lowered so that the eudiometer reads zero on graduated scale. The three way cock is open and the sample of flue gas is drawn in the measuring bottle. The flue gas is expelled by raising the aspirating bottle. Now the three way cock is kept in the closed position.

Now the cock a is opened and the sample of the gas is forced into the first reagent pipette containing KOH by raising the aspirating bottle. Here the CO₂ component of the flue gas is absorbed. Aspirating bottle is moved up and down several times to ensure the complete absorption of CO₂ by KOH solution in reagent pipette. The sample of gas is returned to eudiometer and cock a is closed by keeping the reagent to its original level of solution. The eudiometer reading is taken and the difference of two readings gives the percentage of CO₂ by volume in the gas sample.

The same procedure is repeated with the pipettes b and c to find the percentage analysis of O₂ and CO respec­tively in the flue gas. The remainder is N₂ gas.

Thus, Orsat apparatus gives analysis of the dry gas.

Determination of the Quantity of Air Supplied per kg of Fuel from the Analysis of Flue Gas given in Weight (Gravimetric Analysis Given):

Let C be the percentage of carbon in one kg of fuel and the percentage analysis of the dry flue gas by weight is as given below:

Determination of Actual Air Supplied per kg Fuel when Volumetric Analysis of the Dry Flue Gases is Given:

In the power plants the amount of fuel used is very large and consequently the amount of air supplied to a boiler furnace is also great and so it cannot be measured directly. Its indirect measurement can be done if we know the volumetric analysis of the dry flue gas. The main constituents of the flue gases are carbon dioxide, carbon monoxide, oxygen and nitrogen. Quantity of sulphur dioxide is negligibly small and the water vapours are condensed and do not enter the flue gas.

Let C be the percentage by weight of carbon in the fuel. And the percentage volumetric analysis of the dry products of combustion is given as-

Carbon Dioxide = C₁

Carbon Monoxide = C₂

Oxygen = O

Nitrogen = N

We will convert this volumetric analysis into gravimetric analysis and then the procedure is same as the above article.

∴ To convert volumetric analysis into gravimetric analysis, multiply volume by its molecular weights to get relative weights.

∴ Relative weights of these constituents are-

Carbon dioxide = 44 C₁

Carbon monoxide = 28 C₂

Carbon content of carbon dioxide

Note: All values are in percentage

To find excess air supplied, first we have to consider the oxygen required for combustion of carbon monoxide to carbon dioxide and after deducting this oxygen from the oxygen present in the flue gases, the excess oxygen is obtained. Once this excess oxygen is obtained, then we can find the weight of excess air supplied.

The illustrative example given below will give you an procedure for accounting oxygen for the combustion of carbon monoxide to carbon dioxide.

Approximate equation for excess air supplied is-

This equation does take into consideration the complete combustion of carbon to carbon dioxide and hence approximation.

Note: All values are in percentages.

Determination of Air Fuel Ratio with the Help of Dry Flue Gas Analysis:

When the volumetric analysis of dry gas is given, then the amount of air supplied per kg of fuel can be obtained by any one of the following methods (Carbon content of the fuel is not given directly):

1. Carbon balance

2. Hydrogen balance

3. Carbon-hydrogen balance

(i) With known composition of fuel

(ii) With unknown composition of fuel

4. Oxidized product method

Of this method (2) and (3) required that three should be sufficient proportion of hydrogen in the fuel and hence they are not applicable for boiler fuel as coal.

We can write the chemical equation by considering a certain moles of fuel and corresponding moles of air such that the analysis of dry products given will be obtained.

For balancing purpose, we have to consider the moles of H₂O produced:

(i) Carbon Balance:

In this method, carbons in the reactants (fuel) and the products are balanced and thereby getting the moles of fuel taken. The condition required for this method is that the formation of free carbon does not take place.

(ii) Hydrogen Balance:

After writing the combustion equation, we can proceed to balance the hydrogen also and the unknown.

(iii) Carbon-Hydrogen Balance:

This method is very much useful when the composition of the fuel is now known.

These methods will be better understood with the help of illustrative examples.