To safeguard boilers against fuel explosion in the furnace, operators must ensure that especially during startup and shutdown a fuel rich environment is not created in the furnace.
One of the most dangerous situations in the operation of a boiler is that of a fuel explosion in the furnace. A fuel rich mixture is one of the major reasons for a fuel explosion to take place in the boiler. A fuel rich mixture is formed when the quantity of fuel in an air fuel mixture is higher than that required by the stoiciometric ratio.
Theoretically if the right amount of air is supplied, fuel will consume the entire oxygen in air and this ratio of air to fuel is called the stoiciometric ratio for combustion. When the air supplied is less than this ratio, incomplete combustion of fuel takes place which creates high quantity of unburnts.
Herein lays the danger of a fuel-rich mixture. When the high concentration of accumulated unburned fuel ignites it can do so in a very rapid or explosive manner. Thus it is imperative that always sufficient air be provided for combustion.
Fuel-rich mixtures can occur any time that insufficient air is supplied for the amount of fuel being burned. Thus to avoid insufficient air whenever a boiler is shutdown or started from shutdown, it should be purged with sufficient amount of air. While most modern burner control system will have this as an inherent logic, the operators should be cautious to ensure air purge during startup and shutdown.