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Principles of Combustion in Boilers

Combustion may be defined as the rapid oxidation of a fuel resulting in the release of usable heat and the production of a visible flame. Only two combustible, chemical elements are of any significance - carbon and hydrogen. In a boiler, fuel burns in the combustion chamber where the reaction is isolated and confined so that it can be controlled. The convection surfaces are the areas that the heat travels to outside the combustion furnace. In the convection areas, additional heat is removed and transferred to the water.

The burner is the principal device for firing the fuel oil and/or gas. Burners are normally located in the vertical walls of the furnace. Burners along with the furnaces in which they are installed are designed to mix and burn the fuel and air properly.

The fuel system includes all equipment (regulators, valves, piping) necessary to provide fuel to the burner. The equipment required in the fuel system depends on the type of fuel used in the system. All fuels are combustible and dangerous if necessary safety standards are not followed. Fuels commonly used are fossil fuels, electricity, and certain process wastes. The approximate heat values of typical fossil fuels used in boiler applications are:

  • Natural Gas 1,000 Btu/cubic foot
  • #2 Oil 142,000 Btu/gallon
  • #4 Oil 148,000 Btu/gallon
  • #5 Oil 149,000 Btu/gallon
  • #6 Oil 152,000 Btu/gallon
  • Coal 12,500 Btu/ton
  • Wood (Dry) 8,000 Btu/ton
  • Wood (Wet) 4,000 Btu/ton

In a natural gas-fired boiler, gas is supplied at a set pressure that varies depending on the gas source. Gas is drawn into the burner and mixed with air supplied by a blower. This mixture is directed to the burner where it is ignited with a pilot light. Some boilers have combination burners that can burn gas or fuel oil or a combination of both gas and fuel oil.

In a fuel oil-fired boiler, fuel oil leaves the tank through a suction line and duplex strainer traveling to the fuel oil pump. The fuel oil is then forced through the pump and then through the discharge line. From the discharge line some fuel oil is burned and some returned to the tank through a regulating valve.
Coal-fired boilers use mechanical feeders or stokers to feed fuel to the burner at a consistent rate. In a chain grate stoker, coal is fed through the hopper and regulated before passing under the ignition arch. The coal continues on a conveyor that carries the ignited coal slowly under the heating surface. Ash, slag and unburned parts or clinkers are discharged at the other side of the conveyor.

The draft system regulates the flow of air to and from the burner. For fuel to burn efficiently the right amount of oxygen must be provided. The proper flow of air must also be provided to direct the gases of combustion out of the furnace to the breaching. A forced draft system uses a fan to force (or push) air through the furnace. An induced draft system uses a fan to draw (or pull) air through the furnace. A combination or balanced draft system uses forced and induced draft fans. Gases of combustion enter the stack from the breaching and are released to the atmosphere.