Excess Oxygen Control in Boilers. Excess oxygen (or excess air) is also a key
factor in the efficiency of all types of furnaces and boilers. Ideally, only the stoichiometric
amount of oxygen in air should be supplied to a boiler or furnace, but in order to ensure
essentially complete combustion, it is always necessary to provide some excess. This
increases the flue gas flow rate, and as the flue gas is hotter than ambient temperature, it also
increases the amount of heat lost up the stack. Flue gas oxygen should, therefore, be
controlled to minimize excess oxygen subject to the requirement that the fuel supplied to
the furnace or boiler is, for practical purposes, fully combusted [8].
In order to control excess oxygen, it is necessary to measure it. This is accomplished
using an analyzer—either a portable device used periodically, or a permanent one
installed within the boiler or furnace. As a minimum, the analyzer measures the excess
oxygen content. Some analyzers also measure combustibles or carbon monoxide (to
ensure substantially complete combustion), or NOx and other chemical species required
for environmental compliance.
A control mechanism is also required. Control is generally achieved in boilers by
using a combustion airflow control device such as a fan outlet damper, inlet guide vane
(IGV) control, or variable speed control (see Chapter 15).
Table 9.1 shows typical excess oxygen control limits for different types of boilers,
based on full gas sample (wet basis) measurements2 leaving the combustion zone.
Table 9.1 also shows two different types of control. Positioning control is typically
used in boilers without continuous flue gas oxygen measurement. Combustion airflow is