Gases to be flared must have sufficient calerofic fuel value for complete combustion. Generally calerofic fuel value of 200 to 250 Btu/ft3 is sufficient for complete combustion otherwise fuel must be added. Flares provided with external fuel to waste gas are known as endothermic or fuel assisted flares.
In some cases, even flaring waste gases having the necessary heat content will also require supplemental heat. If fuel-bound nitrogen is present, flaring ammonia with a heating value of 13,600 kJ/m3 (365 Btu/ft3) will require higher heat to minimize nitrogen oxides (NOx) formation.