Integrated flare combustion control

1. A flare combustion control system comprising:

an ultrasonic flowmeter configured to measure a sound speed of a flare vent gas flowing within a flare gas conduit;

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

a fuel gas control valve configured to control a flow rate of a supplemental fuel gas flow delivered from a supplemental fuel gas conduit to the flare gas conduit upstream from the ultrasonic flowmeter; and

a flare combustion controller communicatively coupled to the ultrasonic flowmeter, the pressure sensor, the temperature sensor and the fuel gas control valve, and comprising at least one processor programmed to perform operations including:

receive the measured sound speed, the pressure, and the temperature from the ultrasonic flowmeter, the pressure sensor, and the temperature sensor, respectively;

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

determine, based on the measured sound speed, the pressure, the temperature, and the estimated molecular weight, a predicted sound speed;

compare the measured sound speed to the predicted sound speed;

update the estimated molecular weight based on the comparing;

repeat the steps of determining the estimated sound speed, comparing the measured sound speed to the predicted sound speed, and updating the estimated molecular weight until the predicted sound speed is approximately equal to the measured sound speed;

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

adjust a valve position of the fuel gas control valve based on the net heating value of the flare vent gas.

2. The flare combustion control system of claim 1 , wherein the flare combustion controller includes a memory storing information regarding hydrocarbon gases including a plurality of critical pressures, compressibility factors, acentric factors, and heat capacities, tabulated as functions of molecular weight, wherein the at least one processor is further configured to predict the sound speed based on the estimated molecular weight using virial equations;

determine, based on the estimated molecular weight, a plurality of coefficients for the virial equations; and

determine, using the virial equations, the predicted sound speed.

3. The flare combustion control system of claim 1 , wherein the net heating value determined based on the estimated molecular weight is a coarse net heating value determined at coarse tuning intervals, said flare combustion control system further comprising:

a gas chromatograph configured to determine a fine net heating value at fine tuning intervals, wherein the fine tuning intervals are longer than the coarse tuning intervals.

4. The flare combustion control system of claim 3 , wherein the at least one processor is further programmed to determine the net heating value of the flare vent gas at coarse tuning intervals of at least one of 2 minutes, 1 minute, 30 seconds, 15 seconds, or continuously.

5. The flare combustion control system of claim 1 , further comprising an assist gas control valve communicatively coupled to the flare combustion controller and configured to control 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 at least one processor is further configured to:

determine, based on the estimated molecular weight, a necessary flow rate of assist gas to achieve smokeless combustion of the flare vent gas; and

adjust, via the assist gas control valve, a flow of the assist gas based on the necessary flow rate.

6. The flare combustion control system of claim 5 , wherein the at least one processor is further configured to adjust the flow of the assist gas by adjusting a flow of steam from a steam source.