Nitrogen subtraction on reported CO2 emission using ultrasonic flare gas meter

The CO2 emission from flaring is typically measured by ultrasonic flare gas meters. In order to reduce the CO2 emissions, nitrogen purging is often utilized in situations of low flow in the flare. At such purging conditions, a significant amount of the gas flow in the flare is nitrogen. The CO2 emission data are to be reported to authorities. In order to get a realistic report of the CO2 emissions, the nitrogen purging should be subtracted from the total combustible gas flow.

Ultrasonic flare gas meters measure primarily the flow velocity through the flowmeter. From this, the volumetric flow rate at line conditions can be calculated using dimensions of the pipe, and by using measured pressure and temperature, the volumetric flow rate at a reference condition (for example 15 °C and 1.01325 bar) can be calculated. Such flow meters also measure the velocity of sound. From this measured velocity of sound, in combination with pressure and temperature, the density of the flare gas is estimated, and also the mass flow rate can thus be found. In the models relating the velocity of sound to the density, there are underlying assumptions regarding the gas composition. Typically, the assumption is that the gas contains hydrocarbons, in addition to up to some few percents of inert gases like nitrogen and carbon dioxide. Through the measured velocity of sound there is also a potential for estimation of nitrogen molar fraction in cases where nitrogen purging is a significant part of the flow. In the present paper, tests of such an algorithm in real flow tests at StatoilHydro’s process plant at Kollsnes is reported.