Integrated flare combustion control

1. A flare combustion control system comprising:

a sound speed measurement device for measuring sound speed in a flare vent gas flowing within a flare gas conduit, wherein the flare vent gas includes a flare gas flow or a mixture of the flare gas flow and a supplemental fuel gas flow;

a pressure sensor and a temperature sensor, each configured to respectively measure a temperature and pressure of the flare vent gas;

a fuel gas control valve for controlling a flow rate of the supplemental fuel gas flow delivered from a supplemental fuel gas conduit to the flare gas conduit upstream from the sound speed measurement device, wherein the supplemental fuel gas increases a heating value of the flare gas flow;

an assist gas control valve for controlling a flow rate of an assist gas flow delivered by an assist gas conduit to a flare stack proximate a flare tip that receives the flare vent gas from the flare gas conduit, wherein the assist gas decreases a heating value of the flare vent gas; and

a flare combustion controller comprising a memory and a processor, wherein said processor is in communication with the sound speed measurement device, the fuel gas control valve, the assist gas control valve, the temperature sensor, and the pressure sensor and is configured to:

receive the measured sound speed from the sound speed measurement device, a measured temperature of the flare vent gas from the temperature sensor, and a measured pressure of the flare vent gas from the pressure sensor;

determine, based on the measured sound speed, the measured temperature of the flare vent gas, and the measured pressure of the flare vent gas, a molecular weight of the flare vent gas;

determine, based on the determined molecular weight, a measured net heating value of the flare vent gas;

adjust the measured net heating value of the flare vent gas when the measured net heating value is outside of a net heating value operating range between a predetermined net heating value lower limit and a predetermined net heating value upper limit by opening or closing the fuel gas control valve to regulate an amount of the supplemental fuel gas in the flare vent gas;

determine, based upon the flow rate of the assist gas, the flow rate of the fuel gas, and the flow rate of the flare vent gas, a measured assist gas ratio for the flare vent gas; and

adjust the measured assist gas ratio when the measured assist gas ratio is outside of a predetermined assist gas ratio operating range between a predetermined assist gas ratio lower limit and a predetermined assist gas ratio upper limit by opening or closing the assist gas control valve to regulate an amount of the assist gas in the flare vent gas, wherein the assist gas ratio operating range is selected to achieve smokeless combustion of the flare vent gas.

2. The flare combustion control system of claim 1 , wherein said processor is further configured to determine the molecular weight of the flare vent gas by iteratively estimating the molecular weight, predicting a sound speed based on the estimated molecular weight, and comparing the predicted sound speed against the measured sound speed.

3. The flare combustion control system of claim 2 , wherein said processor is further configured to predict the sound speed based on the estimated molecular weight using virial equations.

4. The flare combustion control system of claim 1 , wherein the net heating value determined based on the determined molecular weight is a coarse net heating value determined at coarse tuning intervals, said combustion control system further comprising a gas chromatograph configured to determine, from a sample of the flare vent gas, a fine net heating value at fine tuning intervals, wherein the fine tuning intervals are longer than the coarse tuning intervals.

5. The flare combustion control system of claim 4 , wherein the coarse tuning intervals include intervals of at least one of 2 minutes, 1 minute, 30 seconds, 15 seconds, and continuous.