Combustion Air Requirements

As mentioned before, combustion is a chemical reaction. Therefore a specific amount of air is needed for complete combustion of fuel. In the industry, in this context, we use a certain phrase known as the “theoretical air.” The theoretical air is the amount of air necessary for complete combustion of fuel – no more and no less.

However, perfect conditions do not exist in a furnace. That means if we introduce an amount of air equal to the "theoretical air" into the furnace, we are likely to end up with some problems.

No matter how hard we try, we cannot mix air and fuel in a perfect manner. That means, in an operating furnace, there will be pockets unmixed substances, some of which will be oxygen-rich while others will be deficient in oxygen. The oxygen-rich regions will have complete combustion (while some oxygen will be left over). On the other hand, in the oxygen-deficient regions, due to insufficiency of oxygen, some of the fuel will be left unburned. This is potentially a hazardous condition. Here’s why:

As the mass of flue gas flows through the furnace toward the stack, it will carry pockets of unburned fuel along with it. It is possible that this unburned fuel will ignite, at some point along its path and cause an explosion.