A flare combustion control system comprising: a sound speed measurement device for measuring sound speed in a flare vent gas; a supplemental fuel gas conduit including a supplemental fuel gas control valve coupled in line for controlling an amount of supplemental fuel gas flow into the flare vent gas to facilitate increasing a heating value of the flare vent gas; an assist gas conduit including an assist gas control valve coupled in line for controlling an amount of assist gas flow to a flare stack to facilitate reducing a heating value of the flare vent gas; characterized by a pressure sensor and a temperature sensor to measure respectively a pressure and a temperature of the flare vent gas; and a flare combustion controller comprising a memory and a processor, wherein said processor is configured to: receive the measured sound speed from the sound speed measurement device and pressure and temperature information of the flare vent gas from the pressure and temperature sensors respectively; determine, based on the measured sound speed, pressure and temperature a molecular weight of the flare vent gas; and determine, based on the determined molecular weight, a net heating value of the flare vent gas; adjust the net heating value of the flare vent gas by regulating an amount of a supplemental fuel gas in the flare vent gas via the supplemental fuel gas control valve; calculate, based on the determined molecular weight, an amount of an assist gas to achieve smokeless combustion of the flare vent gas; and regulate a flow of the assist gas via the assist gas control valve for mixing with the flare vent gas to achieve an assist gas ratio to produce smokeless combustion of the flare vent gas. a sound speed measurement device for measuring sound speed in a flare vent gas; a supplemental fuel gas conduit including a supplemental fuel gas control valve coupled in line for controlling an amount of supplemental fuel gas flow into the flare vent gas to facilitate increasing a heating value of the flare vent gas; an assist gas conduit including an assist gas control valve coupled in line for controlling an amount of assist gas flow to a flare stack to facilitate reducing a heating value of the flare vent gas; characterized by a pressure sensor and a temperature sensor to measure respectively a pressure and a temperature of the flare vent gas; and a flare combustion controller comprising a memory and a processor, wherein said processor is configured to: receive the measured sound speed from the sound speed measurement device and pressure and temperature information of the flare vent gas from the pressure and temperature sensors respectively; determine, based on the measured sound speed, pressure and temperature a molecular weight of the flare vent gas; and determine, based on the determined molecular weight, a net heating value of the flare vent gas; adjust the net heating value of the flare vent gas by regulating an amount of a supplemental fuel gas in the flare vent gas via the supplemental fuel gas control valve; calculate, based on the determined molecular weight, an amount of an assist gas to achieve smokeless combustion of the flare vent gas; and regulate a flow of the assist gas via the assist gas control valve for mixing with the flare vent gas to achieve an assist gas ratio to produce smokeless combustion of the flare vent gas. receive the measured sound speed from the sound speed measurement device and pressure and temperature information of the flare vent gas from the pressure and temperature sensors respectively; determine, based on the measured sound speed, pressure and temperature a molecular weight of the flare vent gas; and determine, based on the determined molecular weight, a net heating value of the flare vent gas; adjust the net heating value of the flare vent gas by regulating an amount of a supplemental fuel gas in the flare vent gas via the supplemental fuel gas control valve; calculate, based on the determined molecular weight, an amount of an assist gas to achieve smokeless combustion of the flare vent gas; and regulate a flow of the assist gas via the assist gas control valve for mixing with the flare vent gas to achieve an assist gas ratio to produce smokeless combustion of the flare vent gas.