power plant engineering

here's something you must ALWAYS remember Complete Combustion always produces the products of Carbion dioxide and Water.  INComplete Combustion tends to produce the products of Carbon Monoxide and water...  Carbon monoxide is produced from the lack of oxygen in the atmosphere during combustion, hence smaller carbon compounds are made...  Ok...you HAVEN'T done the reaction yet, as seen in your question, so you CAN'T tell whether it's complete or not...  go to the reaction and balance it out if there are CO2, then it was COMPLETE, and ...  if there was CO, then it was INCOMPLETE Complete combustion:  CH4(g) + 2 O2(g) →  CO2(g) + 2 H2O(l) Incomplete combustion: CH4(g) + 1.5 O2(g) → CO(g) + 2 H2O(l) Incomplete combustion: CH4(g) + 1 O2(g) → C(s) + 2 H2O(l) Complete combustions needs the most oxygen. Insufficient oxygen produces incomplete combustion.

primary air density (cold air) more than flue gas (hot air)

Depending are two types of boiler depending on pressure:- Positive pressure :- When the pressure in combustion area of the boiler is more than atmospheric pressure, normally forced draft boilers, HRSG are positive pressure. Negative pressure :-When the pressure in combustion area of the boiler is less than atmospheric pressure. This is achieved in balanced draft boilers by using Forced draft fans to force air into boiler (pressuring it)and Induced draft fans to remove air from the boilers to create negative pressure. In general negative pressure boilers are preferred as any flames or exhaust from boiler don't have a tendency to spread outside as the pressure inside is lower. The air ingresses inside incase of small leaks between boiler tube fins( large boiler are not perfectly sealed and have small gaps between fins) and contains the flames in the boiler. It also reduces the risk of explosions in boilers w.r.t. positive pressure boilers.

Deaerator at height not only provides Available suction head at boiler feed pump at higher side than required NPSH but also makes recirculation valve discharge to remain in liquid state and not the gaseous state. because feed water temperature to the BFP (and also the temperature after the throttling in ARC valve) is lower than the saturation temperature of the return line of re-circulation to deaerator.