With the increasing share of renewable energy in power system

more and more coal-fired power units participate flexibility generation. For clarifying the air emission characteristics of flexibility generation

the operation history data of two subcritical coal-fired power units (600 MW and 300 MW) are used for regression analysis. In normal cyclic operation

power ramping-down will lead to a little increase in coal consumption rate and CO

2

emissions factor. Meanwhile

the NO

x

emissions factor will decrease firstly and then increase when ramping down and the amplitude of variation is little in clean flue gas. The SO

2

emissions factor will decrease a little while dust factor will increase a little when ramping down. In deep cycling operation

the increase in coal consumption rate and CO

2

emissions factor will be much larger

while the NO

x

and dust emissions factors will increase substantially when SO

2

e

missions factor continues to decrease in clean flue gas. In start-up process

the NO

x

and dust emissions are much larger while SO

2

emissions are slightly smaller than regular operating conditions. In addition

larger ramping rate has remarkable impact on NO

x

and SO

2

in untreated flue gas and on dust in clean flue gas in deep cycling conditions.