How do you calculate the mass of flue gas generated in Boilers???

 How do you calculate the mass of flue gas generated in Boilers???

 

The mass of flue gas generated in boilers can be calculated using the principles of stoichiometry and the knowledge of the fuel composition and combustion process. Here are the steps to calculate the mass of flue gas:

1-Determine the Fuel Composition:

You need to know the composition of the fuel being used in the boiler. Typically, this includes information about the types and proportions of elements in the fuel, such as carbon (C), hydrogen (H), sulfur (S), oxygen (O), and other impurities. This information is usually provided in the fuel's specifications.

2-Write the Combustion Equation:

Write the balanced chemical equation for the combustion of the fuel. For example, if you're burning natural gas (CH4) in air (which contains oxygen), the combustion equation would be:

CH4 + 2O2 → CO2 + 2H2O

3-Calculate the Stoichiometric Air-Fuel Ratio:

Calculate the stoichiometric air-fuel ratio, which is the theoretical amount of air required for complete combustion. This ratio depends on the chemical composition of the fuel and the combustion equation. For the example above, one mole of methane requires two moles of oxygen for complete combustion.

4-Determine the Actual Air-Fuel Ratio:

In real-world situations, the actual air-fuel ratio is usually not exactly stoichiometric due to factors like incomplete combustion, excess air, and variations in combustion efficiency. You may need to measure or estimate the actual air-fuel ratio in your specific boiler operation.

5-Calculate the Mass of Fuel:

Determine the mass of fuel being burned in the boiler. This is typically measured or known based on the flow rate and properties of the fuel being supplied to the boiler.

6-Calculate the Mass of Air:

Using the actual air-fuel ratio and the mass of fuel burned, calculate the mass of air required for combustion. You can do this by multiplying the mass of fuel by the actual air-fuel ratio.

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7-Calculate the Mass of Flue Gas:

The mass of flue gas is equal to the mass of the combustion products, which includes the mass of the carbon dioxide (CO2), water vapor (H2O), and any other combustion products produced in the combustion process. Use the balanced combustion equation to calculate the masses of these products.

For example, in the combustion of methane (CH4) from step 2, you can calculate the mass of CO2 and H2O produced based on the moles of CH4 burned and their molar masses.

8-Sum Up the Masses:

Add up the masses of all the combustion products to find the total mass of flue gas generated in the boiler.

Keep in mind that this is a simplified calculation, and real-world combustion processes can be more complex due to factors like incomplete combustion, impurities in the fuel, and variations in combustion efficiency.

Therefore, it's important to consider these factors for a more accurate estimation of flue gas mass in a specific boiler system. Additionally, measuring instruments and gas analyzers can provide real-time data on flue gas composition and mass flow rates in practical applications.

Calculations:

A Boiler uses imported coal to generate 150 TPH of steam, the O2 & CO2 in flue gases are 6% & 15% respectively. Calculate the mass of flue gas generated if following is the ultimate analysis of fuel.

Carbon C = 54%

Hydrogen H2 = 3.4%

Oxygen O2 = 9.1%

Sulphur S = 0.6%

Nitrogen N2 = 1.3%

We have Theoretical air, Th = (11.6 X %C + 34.8 X (H2-O2/8) + 4.35 X S) / 100

                     Th = (11.6 X 54 + 34.8 X (3.4-9.1/100) + 4.35 X 0.6) / 100

                     Th = 7.44 kg/kg of fuel

We have excess air EA = O2 X 100 / (21-O2)

                                      = 6 X 100 /(21-6) = 40%

Total air = (1 + EA/100) X Theoretical air

Total air = (1 + 40/100) X 7.44 = 10.42 kg of air per kg of fuel burnt

Mass of flue gas generated Mfg = Mass of CO2 in flue gas + Mass of N2 in fuel + Mass of N2 in air + Mass of O2 in the flue gas + Mass of SO2 in the flue gas

Mass of flue gas generated Mfg = (Carbon percentage in fuel X Mol.weight of CO2) / Mol.weight of Carbon + 0.013 + (10.42 X 77 / 100) + ((10.42-7.44) X 23 / 100) + (0.006 X Mol.weight of SO2) / Molecular weight of sulphur

Mass of flue gas generated Mfg = (0.54 X 44 / 12) + 0.013 + 8.02 + 0.68 + (0.006 X 64) / 32 =10.7 kg of flue gas per kg of fuel burnt.

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