A system for flare combustion control includes a sound speed measurement device for measuring sound speed in a flare vent gas, and a flare combustion controller including a memory and a processor. The processor is configured to receive the measured sound speed and determine, based on the measured sound speed, a molecular weight of the flare vent gas. The processor is further configured to determine, based on the determined molecular weight, a net heating value of the flare vent gas, and adjust the net heating value of the flare vent gas by regulating an amount of a supplemental fuel gas in the flare vent gas.