In order to reduce NOx, which is particularly a prob-lem in the exhaust gas composition, it is basically desir-able to set the O2 concentration at the outlet of the fur-nace to a low value. However, if the O2 concentration is reduced excessively, the CO concentration will increase due to increases in unburned gas and unburned fixed car-bon. Furthermore, changes in the O2 concentration also influence the SOx concentration and the temperature in various parts of the furnace.

Therefore, a rule-based type multipurpose control system was developed, taking into account the inverse relationship between NOx and CO, and a total consid-eration of all operational indexes, such as constraints of SOx and furnace temperature.

In this control, a total judgment of the combustion condition is made by rule-based control, and the O2 con-centration target is changed corresponding to the furnace operational indexes/constraints.

The concrete method of revising the target value is outlined below (Fig. 1).

(1)The O2 concentration set point (SP) is set based on the present boiler load.

(2)The set point is corrected based on the temperature in the middle of furnace.

(3)The set point is corrected based on the cyclone out-let temperature.

(4)Corrected values are determined by rule-based con-trol based on the CO, NOx, and SOx concentrations.

The present optimum value is determined so as to satisfy all of these conditions, and this is used as the corrected O2 concentration value.

Fig. 1 Exhaust gas stabilization control flow diagram