The emission of nitrous oxides (e.g. NO2) into our atmosphere is a global issue these days. Everywhere researchers and developers are working on better and more accurate simulation and measurement methods, as well as on the development of more efficient catalysts. This applies both to stationary combustion processes (e.g. power plants, steel production and chemical base materials) and to mobile applications in the automotive sector to reduce the NO2 with selective catalytic reduction (SCR). Ammonia or ammonia forming compounds (urea) are added to form pure nitrogen and water.
NOx, a generic term for nitrogen oxides that are most relevant for air pollutions, is a mixture of different nitrogen oxides; nitric oxide (NO) and nitrogen dioxide (NO2). The focus here is on NO2 radicals and its dimer dinitrogen tetroxide N2O4. NO2 is toxic and emissions to the environment should be kept as low as possible. However, NO2 occurs as a by-product in a large number of combustion processes, so that both technical developers in the industry and developers of occupational and preventive medicine have to deal with this substance.
However, this equilibrium also poses the problem of measuring and controlling gas flows containing NO2 in higher concentrations. Especially when using pure NO2, which is in balance with its dimeric form N2O4, which is temperature and pressure dependent and additionally influenced by light and surface conditions (at 27°C only 20% is present as NO2, the remaining 80% as dimer N2O4). The mixture is very sensitive to moisture and can react with humidity to nitric acid (HNO3) and nitrous acid (HNO2), which in turn are highly corrosive.
Gas mixtures with NO2
For investigations of combustion processes with NO2 emission or the testing/ new development of catalysts, a precisely known flow rate of gas mixtures with NO2 must be realised. This applies not only to catalysis but also to the effect of NO2 on the organism and the environment, because NO2 is highly toxic due to its reactivity.
In one of our projects a system consisting of a gas cylinder, needle valve, backwash unit, transfer lines and mass flow controller should be constructed, which can dose nitrogen dioxide (NO2) in the range between 0- 6 g/h against room pressure.
Challenges with thermal mass flow
Common mass flow meters and mass flow controllers work with thermal measuring principles (with bypass sensor or according the CTA principle (Constant Temperature Anemometry)). Thermal sensors operate on the principle of heat transport in the sensor element. This method depends on the type of gas, since the heat transport depends directly on the heat capacity and the thermal conductivity of the gas to be metered.
Since NO2 has a temperature and pressure-dependent equilibrium with N2O4, the parameters in the sensor element can change constantly. Consideration of the equilibrium using a single conversion factor to a reference gas is not sufficient, especially for pure NO2 or N2O4. Through gravimetric tests, we have determined that massive under-dosing can occur at a dosage of pure NO2 (approx. 10 % of the target value).
A further challenge with a thermal mass flow controller in the closed state, corresponding to a flow rate of 0 ml/min, is that it can produce pseudo signals of up to 10% of the maximum dosing range. The reason for this is that the sensor element contains a mixture of NO2 and N2O4, which is constantly influenced by the active heating of the sensor element. Thus, a heat transport in the device is faked and a flow rate is indicated.
The solution: Use of a Coriolis mass flow controller
The remedy here is a Coriolis mass flow controller instead of a thermal mass flow controller due to its different working principle. It does not matter to what extent the balance of NO2 and N2O4 is on one side or the other, since it is all about the transported mass. When using a Coriolis mass flow controller, however, it must be ensured that the medium to be metered is in a defined physical state, i.e. either in a completely liquid or gaseous state.
The boiling point of NO2 at atmospheric pressure is 21 °C, so the complete dosing system, consisting of gas cylinder, needle valve, backwash unit, transfer lines and mass flow controller, can be heated here. Since evaporative cooling occurs inside the mass flow controller when dosing NO2 at the pressure relief point, the temperature there must be set significantly higher than 21 °C. Only at a temperature of at least 45 °C is it ensured that the dosing functions in the range between 0-6 g/h without fluctuations due to condensing and re-evaporating NO2. In this setup a Bronkhorst mini CORI-FLOW ML120 was used, which is the Coriolis instrument with the lowest flow control range in the world. So it is possible to dose even these small gas amounts of NO2.
Check the nitrous oxides (NO2) dosage
The dosed NO2 quantity is checked with the aid of gravimetric measurements. NO2 is transferred via a heated transfer line to a glass U-tube with stopcocks where it is frozen out at -50 °C. The shut-off valves are then closed, the condensate thawed to room temperature and weighed. A total of five different mass flows were tested. The figure shows the result of the check and confirms the very small deviations between the desired and actual dosing quantities. In addition, it can be seen that the mass flow controller operates linearly in the tested range between 0.1 and 4.0 g/h (single points: 0.1; 1.0; 2.5 and 4.0 g/h with error bars drawn in).
This proves that precise control for small quantities of NO2 can be achieved even at low inlet pressures. As mentioned, nitrogen dioxide (NO2) is a substance of the mixture nitrogen oxides (NOx). Reducing the level of NOx can also be done with Selective Catalytic Reduction (SCR). In case of Selective Catalytic Reduction (SCR) Ammonia or ammonia forming compounds are added to form pure nitrogen and water.
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