Study on NOx emissions from ternary molten nitrate salts contacting several kind of metals in thermal energy storage process

Abstract

To research the different emissions of NOx when HITEC (an eutectic mixture of ternary molten nitrates with the component of 7wt.%NaNO3-53wt.%KNO3-40wt.%NaNO2) contacts with metals during the thermal energy storage (TES) process, this paper studies the accumulative absorption concentrations of NOx in tail gases generated from the molten salts exposure to 45# carbon steel (1045, ASTM), stainless steels(321, 310S), and nickel-based alloys(In625, Has C276) respectively under different temperatures, according to the national environmental protection standard of PR China, HJ479-2009. The results indicated that HITEC could release NOx when it contacted with those metals within the range from 300°C to 600°C. The emissions of NOx slowly increased with the increasing temperature under 450°C, but they were all less than 200mg/m3. The NOx emissions had a sharp rise as HITEC contacting with 321 and 310S above 450°C, and their quantities were 1300 mg/m3 and 770 mg/m3 at 550°C respectively. When HITEC contacted with Has C276 and In625 above 500°C, the NOx emissions had a greatly increase, and their quantities were 340 mg/m3 and 265 mg/m3 at 550°C respectively. As for 45# carbon steel, the emissions were higher than 200 mg/m3 at 585°C.The chemical thermodynamic computation revealed that NOx were generated by various redox reactions between nitrate or nitrite and Fe, Ni, Cr, Mo, W below 450°C, but the emissions were not large for oxide films covered on metal surface could restrain the reactions. The formed Cr2O3, on surface of stainless steel and nickel based alloy, were further oxidized with NaNO3 and KNO3 above 500°C, leading to the oxide films to be damaged, and then NO2 and NO to be released. There was little Cr element in 45# carbon steel, so there was no sharp increase of NOx.