Understanding Dry Flue Gas Composition

Flue gas is the gas that exits a combustion process. Its composition provides valuable information about how efficiently the fuel is burning and whether the right amount of air is being supplied.

Analyzing the Given Flue Gas Composition

The dry flue gas composition provided is:

• CO₂ = 10.4%
• O₂ = 9.6%
• N₂ = 80%

Let's look at what these components tell us about the combustion:

• CO₂ (Carbon Dioxide): This is a product of complete combustion of carbon in the fuel. Its percentage relates to the amount of carbon burned.
• N₂ (Nitrogen): Air, which is typically used for combustion, is about 79% nitrogen. Nitrogen is mostly inert and passes through the combustion process unchanged, ending up in the flue gas. The high percentage of nitrogen (80%) is expected as it comes from the air supplied.
• O₂ (Oxygen): Oxygen is supplied by the air for the combustion reaction. Ideally, in theoretically perfect combustion, all the oxygen supplied would react with the fuel. However, in real-world systems, some excess air is often supplied to ensure complete combustion. Any oxygen that does not react with the fuel ends up in the flue gas.

Interpreting the Oxygen Level in Dry Flue Gas

The presence of a significant percentage of oxygen (9.6%) in the dry flue gas composition is a key indicator. This oxygen is unreacted oxygen that was supplied as part of the combustion air.

• If insufficient air were supplied, there would be very little or no free oxygen in the flue gas. Instead, you would likely find products of incomplete combustion, such as carbon monoxide (CO), along with CO₂ and N₂.
• If just sufficient air (stoichiometric air) were supplied for complete combustion, the amount of oxygen in the flue gas would theoretically be zero or very close to zero.
• If excess air is supplied, more air than is theoretically needed for complete combustion is introduced into the furnace. This extra air contains oxygen and nitrogen. While the oxygen required for combustion reacts with the fuel, the excess oxygen passes through unreacted and appears in the flue gas along with the nitrogen from all the supplied air (both the theoretically required amount and the excess amount).

Since the flue gas contains 9.6% O₂, it clearly indicates that more oxygen (and therefore more air) was supplied than was needed to burn the fuel completely. This condition is known as using excess air.

The percentage of N₂ (80%) is also consistent with the presence of excess air. The nitrogen in the flue gas comes from all the air supplied. If excess air is used, more nitrogen is also introduced into the system, leading to a higher percentage of N₂ in the dry flue gas compared to scenarios with insufficient or just sufficient air for the same fuel.

Conclusion

The presence of 9.6% oxygen in the dry flue gas, along with carbon dioxide and a high percentage of nitrogen, is a direct indication that excess air was used during the combustion process.