Method and ultrasound flow measurement unit for the determination of the CO

1. A method for the determination of the CO 2 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 (SoS m ) 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 (f N2 , f H2O , f CO2 ) of N 2 , CO 2 and H 2 O;

c1) calculating a maximum speed of sound (SoS(f high =0)) of the flare gas as measured by the ultrasound flow measurement unit under the assumption that the hydrocarbon proportion (f CH ) of the flare gas only consists of alkanes having a chain length i;

c2) calculating a minimal speed of sound (SoS(f low =0)) of the flare gas under the assumption that the hydrocarbon proportion (f CH ) 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 (SoS m ) lies between the calculated minimal speed of sound and the calculated maximum speed of sound;

d) varying the proportions (f low and f high ) 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 (f low and f high ) lies within a predefined difference from the measured speed of sound (SoS m );

e) calculating an equivalent chain length; and

f) calculating an emission factor.

2. The method in accordance with claim 1 , wherein the chain length is determined in iterative steps starting at i=1 in step e).

3. The method in accordance with claim 1 , wherein the proportions are determined in iterative steps in step d).

4. The method in accordance with claim 1 , wherein the step of calculating the speed of sound takes place in accordance with the GERG-2004 XT08 algorithm.

5. The method in accordance with claim 1 , further comprising the steps of: determining a gas flow using an ultrasound flow measurement unit and calculating the CO 2 emission using the flow and the emission factor.

6. The method in accordance with claim 5 , further comprising the step of using the measured speed of sound for the calculation of the density of the flare gas.

7. An ultrasound flow measurement unit for the carrying out of a method in accordance with claim 1 , having an input unit for inputting the proportions (f N2 , f H2O , f CO2 ) of N 2 , CO 2 and H 2 O, having ultrasonic transducers for transmitting and receiving ultrasonic signals, having an evaluation unit which is configured to measure a speed of sound (SoS m ) and to carry out the steps a) to h) in accordance with claim 1 , and to determine a gas flow via the measured flow speed, wherein the flow speed can be determined from the time of flight difference of two ultrasonic signals which have one component with the flow, on the one hand, and one component against the flow, on the other hand.