Non-Thermal Plasma Induced Oxidation Of Concentrated Ammonium Sulfite

In recent years, with the rapid economic growth, coal consumption rises sharply and SO2 and NOx emission continuously increase. Both the area-size and frequency of heavy acid rain tend to larger and higher, respectively. Their caused air pollution is still a serious problem. Coal-fired boilers are the main sources for SO2, NOx and smoke dust in atmospheric environment. Coal used by thermal power plants accounts over 50% of its total consumption in China.Flue gas desulfurization (FGD) is one of the most effective methods to control SO2 emission. The wet ammonia based FGD not only has efficient desulfurization efficieny, it also has the advantage of recovery and utilization of ammonium sulfite. The valuable by-products not only lead to reduce its costs, but also solve the problem of sulphur fertilizer shortage.Traditional ammonium sulfite oxidation techniques are based on air-forced oxidation processes. Its main drawback is its slow oxidation rate for concentrated ammonium sulfite (>lmol/L), which is much smaller than its value for an industrial FGD. If the solution is diluted for oxidation, the energy consumption for late-stage crystallization would greatly rise.Since 1970s, investigations on plasma assisted FGD have been carried out over the world. It has been anticipated that plasma assisted FGD will enhance the efficiency and elimiate NH3, SO3 and fine particle or aerosol emission. Plasma induced radicals, such as OH, H, and O, play the key role for sulfite oxidation. The AC/DC power source is used in this work to generate the uniform stream corona plasma inside a saw-plate type reactor. Pulse repetition rate (pulses per second, pps) and added capacitator are optimized to improve the energy conversition efficiency of AC/DC source. It is observed that for the present system, the energy consumption for sulfite oxidation is 33 Wh/mol when without using the additional capacitor and under the repetition rate of 70 pps (or Hz).Within this study, effects of the pH value (6-8), the initial concentration (0.5-2.5 mol/L), the reactor temperature (10-25℃) and the plasma energy density are experimentally studied. When sulfite concentration ranges from 1 to 2.5mol/l, the optimum pH value is around 7 under the temperature of 20℃. The oxidation rate tends to approach a constant value with an energy consumption of 33 Wh/mol.Moverever, effects of metal ions Fe2+ and Co2+ and gas pressure are also experimentally investigated. It is obeased that both of them do not play significantly role for the plasma-induced oxidation. The oxidation efficiency shows a linear correlation with the square root of the plasma energy density, which indicates that radical-radical termination reactions dominate the oxidation process.