Method and ultrasound flow measurement unit for the determination of the CO2 emission factor in flare gas plants

A method for the determination of the CO2 emission factor in flare gas plants comprising the steps of:

• a) using an ultrasound flow measurement unit having an ultrasonic conversion capability for converting transmission and reception of ultrasonic signals and having an evaluation unit configured to determine a speed of sound from the ultrasonic signals to measure the speed of sound (SoSm) in the flare gas and to determine a gas flow via the measured flow speed from the time of flight difference of two ultrasonic signals which have respective components with and against the gas flow, wherein the ultrasound flow measurement unit comprises first and second ultrasonic transducers for respectively generating and receiving the two ultrasonic signals, the first and second ultrasonic transducers being arranged at an angle in a wall of a pipeline through which the flare gas flows, and wherein the two ultrasonic signals are accelerated by the flare gas in a direction of the gas flow and are decelerated by the flare gas in a direction opposite the direction of the gas flow;
• b) detecting known proportions (fN2, fH2O, fCO2) of N2, CO2 and H2O;
• c1) calculating a maximum speed of sound (SoS(fhigh=0)) of the flare gas as measured by the ultrasound flow measurement unit under the assumption that the hydrocarbon proportion (fCH) of the flare gas only consists of alkanes having a chain length i;
• c2) calculating a minimal speed of sound (SoS(flow=0)) of the flare gas under the assumption that the hydrocarbon proportion (fCH) of the flare gas only consists of alkanes having a chain length i+1
• c3) varying the chain length i for so long until the measured speed of sound (SoSm) lies between the calculated minimal speed of sound and the calculated maximum speed of sound;
• d) varying the proportions (flow and fhigh) of the alkanes having the chain length i and the chain length i+1 determined in steps c1) to c3) for so long until the speed of sound (SoS) calculated with these proportions (flow and fhigh) lies within a predefined difference from the measured speed of sound (SoSm);
• e) calculating an equivalent chain length;
• and
• f) calculating an emission factor.