Excess Air Control
As previously stated the amount of excess air is an important aspect of boiler efficiency. The idea behind excess air control is to identify the point where losses from unburnt fuel and heat losses in the flue are minimized. This point is commonly referred to as the smoke point, and since the rate of energy loss due to incomplete combustion is six times greater than that for excess air, it is preferable to operate the boiler as close to this point as possible without falling below it. In many boilers the level of excess air is set to a fixed value that is found through calibration. In such a system the amount of excess air is often set to a rather high value in order to make sure that the boiler operates within safety limits. In more sophisticated control systems the ex- cess air level is continuously monitored and adjusted. This allows for a more conservative control and the concentration of excess oxygen in the flue gases can be reduced. This is done by measuring oxygen with sensors placed at strategic locations in the flue, and adjusting the air flow rate to achieve a certain set-point in oxygen (Turner & Doty, 2007).