The CO₂emission from fossil fuel plant is a function of the power generated overall efficiency, fuel carbon content and calorific value. For typical solid fuels at an efficiency of 40% the emission will be approximately 950kg of CO₂per MW of total generation.
Increase air preheater pressure arising from fouling will inevitably increase CO₂emissions because of the increase in total generation required to maintain net generation. It is not unusual for air preheaters to be operating 50% above the designed clean value because of fouling e.g. 15 mbar cf 10 mbar. The limit is governed by the fan power margin and if fan power margins are great enough, air preheater’s pressure losses of twice the design value are not uncommon and considered acceptable because the perceived limit for plant operators is the point where fans become incapable of maintaining generated output.
The emission will be influenced by the works power required to generate the stated output. Part of this works power is expended by the fans required to force air into the boiler (Forced Draft) and to remove combustion gas (Induced Draft) and exhaust to the chimney. Typically fan power is approximately 1% of generation e.g. 5MW per 500MW. Any increase in fan power induced by the air preheater will increase CO₂ emission. The increase will depend on the proportion of total fan power required by the air preheaters to overcome pressure loss and to supply air to gas cross leakage within the air preheater. Depending on the boiler design, air preheaters consume between 30% and 60% of the total fan power. The implications of this are that clean air preheaters can account for 0.6% of the total CO₂ emissions from coal fired power stations.