Air running amount is usually expressed in excess air coefficient. Within a certain range, to increase the excess air coefficient can improve combustion efficiency, because of the increasing of regional oxygen concentration can improve the combustion efficiency and the degree of burnout, but continuing to increasing after excess air coefficient over 1.15 will have little impact on the combustion efficiency; High excess air coefficient will lower the bed temperature, increase CO concentration, decrease the total combustion efficiency and increase fan power consumption, therefore, the oxygen at outlet of hearth of the boiler typically shall be kept at 3% 4% advisable.   (lsh)