Research Of NOx Emissions In Thermal Energy Storage Process Of Molten Nitrate Salts

Solar thermal power generation technology becomes the sweet pastry that scrambled byall the countries in the world to develop for its advantages that it can provide high qualityelectricity and relief energy crisis. High temperature thermal energy storage technology is thekey to the solar thermal power generation technology; high temperature thermal energystorage material is the core of high temperature thermal energy storage technology. Moltennitrate salts with advantages of low melting point and price, wider using temperature rangeand low viscosity become one of the perfect materials of for solar thermal power generation,NaNO3-KNO3binary molten nitrate salts has been applied in commercial solar thermal plants.But molten nitrate salts at high temperature are not stable; its thermal decompositionmay produce NOx. Hence, this paper referred to national standard of NOx monitored in the airto study whether NOx could be emitted or not in heat storage process of common moltennitrate salts systematically, and explored the effect of temperature and the container material(carbon steel and stainless steel) on NOx emissions of molten nitrate salts. And the cause ofNOx emissions of molten nitrate salts also was explained theoretically by thermodynamiccalculation.Gases emitted in heat storage process of molten nitrate salts contained in quartz boatwere monitored in the experiment, temperature was changed and different steel materials wereadded to repeat the experiment. Results showed that molten nitrate salts would release NOxunder this experimental condition and the1h average concentrations of NOx emissionsincreased with the rise of temperature gradually. The1h average concentrations of NOxemissions of ternary and quaternary molten nitrate salts were larger than that of binary moltennitrate salts at the same condition. Much more NOx was released after molten nitrate saltscontact with45#carbon steel or stainless steel, after the process of heat storage, theconcentrations of NO2–in binary molten salts samples increased with the rise of temperature,and the contents of NO3–and NO2–in ternary and quaternary molten salts samples changedwith the rising temperature irregularly.Thermodynamic calculation showed that K/NaNO3decomposed to K/NaNO2and O2were reactions that can occur thermodynamically in binary molten salts; ternary and quaternary molten salts contained less NaNO3but more KNO3and NaNO2, the reactions thatKNO3decomposed to KNO2and O2, NaNO2was oxidized by O2in the air to NaNO3andNaNO2decomposed to Na2O, N2and NaNO3could both occur thermodynamically, butcouldn’t produce NOx gas, considering the container material, found that nitrates could reactwith SiO2to generate NO and NO2thermodynamically; it was the real reason that NOx can bedetected in the experiment. Metals (Fe, Cr, Ni, Mn) could react with K/NaNO3to form NO;and reactions that Fe/Cr/Ni respectively or any two of them could react with K/NaNO3toform NO2and with NaNO2to form NO could occur thermodynamically, these reactions madeNOx emissions of molten nitrate salts increased after contacting with carbon steel or stainlesssteel. In addition to the thermal decomposition of K/NaNO3to produce NO2–, it could alsoreact with metals to produce K/NaNO2, making NO2–in binary molten nitrate salts samplesincrease after contacting with steel materials; And except for these two kinds of reactions,NaNO2itself in ternary or quaternary molten salts could be oxidized or participated in otherreaction, the result of several kinds of reactions made the content of NO3–and NO2–in moltensalts change irregularly.The study showed that molten nitrate salts could emit NOx inevitably when looped inthermal energy storage system made of metal materials, NOx contact with water vapor willcause corrosion to the pipeline; the tail gas absorption device should be added if used in opensystem, otherwise it will damage the environment.