Numerical Study of NOx Abatement Using Ozone Injection Integrated with Wet Absorption

Nitrogen oxides emitted from power plants and the chemical industry are poisonous to humans and animals, contribute to ozone depletion, and cause acid rain. More than 90% of nitrogen oxides (NO_x) consist of nitric oxide (NO), which is insoluble in water. Among the various available techniques of NO_x abatement, ozone injection is a promising method in which NO is oxidized to higher-order nitrogen oxides (NO₃, N₂O₃, N₂O₄, and N₂O₅), which can easily be absorbed in a wet scrubber. In this article, the ozone injection process integrated with an absorber column is numerically modeled and simulated at various operating conditions. The predicted results of NO_x oxidation with ozone injection and absorption in water agree with the published experimental results. The ozone injection process is modeled using a plug flow reactor, while the wet absorption is based on a rigorous rate-based RateFrac model. Detailed kinetic mechanisms of O₃-NO_x oxidation and absorption of nitrogen oxides in water are incorporated in the model to simultaneously predict the performance efficiency of the ozone reactor and absorber column. Thermodynamic properties of the components are estimated using an Electrolyte NRTL model. The influence of performance parameters (such as feed gas flow rate, inlet gas temperature, reactor configurations, ozone concentration, and NO/NO₂ molar ratio) on the oxidation efficiency of NO_x in the reactor and absorber column is investigated to predict the optimal operating conditions.     

Nitrogen Oxides Ozonation as a Method for NOx Emission Abatement

Attempts to develop new technologies of reduction of NO_x emission are still carried out all around the world. However, most of them as literature survey suggests is focused on NO_x emission control from power plants and mobile vehicles. Fewer investigations are conducted on the NO_x emission abatement from the chemical industry. One of the relatively new approaches is the application of ozone injection into exhaust gas stream followed by an absorption process. Ozone is used to transform NO_x to higher nitrogen oxides that are more soluble in water, and therefore the higher yield of nitric acid is expected. The main objective of this article is to present results of our studies in which the effectiveness of the ozonation process, as well as the dependence of the conversion rate and the selectivities of NO ozonation into NO₂, N₂O₅ and HNO₃ on the residence time of reagents in the reactor space were studied. Results of laboratory investigations were confirmed during ozonation experiments with real exhaust gases from a nitric acid pilot plant in Fertilizers Research Institute in Pulawy, Poland.     

Removal of sulfur dioxide and nitrogen oxides by using ozone injection and absorption–reduction technique

A two-step process capable of removing NO_x and SO₂ simultaneously was proposed, which was made up of an ozonizing chamber and an absorber containing a reducing agent solution. Nitrogen oxides (NO plus NO₂) in most practical exhaust gases consist chiefly of NO. The injection of ozone into the exhaust gas gives rise to a rapid oxidation of NO to NO₂. Compared to NO, NO₂ has relatively high solubility in water, and it can readily be reduced to N₂ when the NO₂-rich exhaust gas is brought into contact with the reducing agent solution. Sodium sulfide (Na₂S) used as the reducing agent in this study can also remove SO₂, effectively. As the exhaust gas passed through the ozonizing chamber and the absorber sequentially, NO_x removal efficiency of about 95% and SO₂ removal efficiency of 100% were obtained. The formation of H₂S from sodium sulfide could be suppressed by using a basic reagent, together with the reducing agent. The rate of depletion of the reducing agent during the treatment of the exhaust gas was much faster than expected by reaction stoichiometry, obviously due to the oxygen in the exhaust gas. The amount of sodium sulfide required was found to be about four times the amount of NO_x and SO₂ removed.     

Improvement in selective catalytic reduction of nitrogen oxides by using dielectric barrier discharge

The behavior of the selective catalytic reduction of nitrogen oxides (NO_x) assisted by a dielectric barrier discharge was investigated. The principal function of the dielectric barrier discharge in the present system is to generate ozone, which is continuously fed to a chamber where the ozone and NO-rich exhaust gas (NO forms the large majority of NO_x) are mixed. In the ozonization chamber, a part of NO contained in the exhaust gas is oxidized to NO₂, and then the mixture of NO and NO₂ enters the catalytic reactor. The ozonization method proposed in this study was found to be more energy-efficient for the oxidation of NO to NO₂ than the typical nonthermal plasma process. The degree of NO oxidation was approximately equal to the amount of ozone added to the exhaust gas, implying that the decomposition of ozone into molecular oxygen was relatively slow, compared to its reaction with NO. When the exhaust gas was first treated by ozone to produce a mixture of NO and NO₂, a remarkable enhancement in the catalytic reduction of nitrogen oxides was observed. Neither NO₃ nor N₂O₅ was formed in the present system, but small amounts of ozone and N₂O (less than 5 ppm) were detected in the outlet gas.     

Kinetic model of NOx ozonation and its experimental verification

The most of new technologies of reduction of NO_x emission, as literature survey (Skalska et al., 2010b) suggests is focused on NO_x emission control from power plants and mobile vehicles. Fewer investigations are conducted on the NO_x emission abatement from chemical industry. Recently, Chacuk et al. (2007) proposed the model for the nitrous acid oxidation with the use of ozone in gas–liquid contactor. It is well known that not all of NO_x can be totally absorbed in water or nitrous/nitric acid solution, as well as ozone is not totally consumed in the acidic liquid. The reaction between ozone and NO_x can take place also in the gas phase. The ozone injection into exhaust gas stream followed by absorption was proposed as the NO_x emission abatement. The objective of these studies was to propose kinetic model of the process and to determine the rate constants of NO_x ozonation in the laboratory scale batch reactor. The process was carried out in the 0.5 dm³ volume batch reactor for different concentrations of NO, and NO₂ and varying molar ratios of O₃/NO at temperature 25 °C. Gaseous reagents were analyzed using a Fourier Transform Infrared Spectrometer Jasco FTIR-4200. The kinetic model of NO_x ozonation process was proposed and rate constants were estimated based on experimental data.     

Reduction of nitrogen oxides by ozonization-catalysis hybrid process

Treatment of nitrogen oxides (NO_x) by using a hybrid process consisting of ozonization and catalysis was investigated. The ozonization method may be an alternative for the oxidation of NO to NO₂. It was found that nitric oxide (NO) was easily oxidized to nitrogen dioxide (NO₂) in the ozonization chamber without using any hydrocarbon additive. In a temperature range of 443 to 503 K, the decomposition of ozone into molecular oxygen was not significant, and one mole of ozone approximately reacted with one mole of NO. A kinetic study revealed that the oxidation of NO to NO₂ by ozone was very fast, almost completed in a few tens of milliseconds. When the amount of ozone added was less than stoichiometric ratio with respect to the initial concentration of NO, negligible NO₃ and N₂O₅ were formed. The oxidation of a part of NO to NO₂ in the ozonization chamber enhanced the selective reduction of NO_x to N₂ by a catalyst (V₂O₅/TiO₂), indicating that the mixture of NO and NO₂ reacts faster than NO.     

Experimental Study on Ozone Synthesis via Dielectric Barrier Discharges

The characteristics of ozone generation using a dielectric barrier discharge reactor were investigated experimentally. Results indicate that ozone concentration increases with increasing applied voltage and gas residence time. In addition to applied voltage, ozone generation rate varies with reactor configuration as well. Optimum ozone generation rates can be reached at the specific gas residence time for a given applied voltage and gas composition. At the same applied voltage, the reactor with a single dielectric barrier results in a higher ozone generation rate in comparison with the reactor having double dielectric barriers. Given a constant N₂/O₂ ratio in the feed gas, NO_x concentration increases as applied voltage and gas residence time increase. Results indicate that maximum NO_x concentration is reached when the N₂/O₂ ratio of feed gas is 4.     

Simultaneous removal of NOx, SO2 and Hg in nitrogen flow in a narrow reactor by ozone injection: Experimental results

A process capable of removing NO_x, SO₂ and mercury simultaneously was proposed, which utilizes the injection of ozone and assist with a glass made alkaline washing tower. Experiments were conducted in a quartz flow reactor within an electrical heated furnace. Oxidation properties of NO and Hg, removal efficiency of NO and SO₂ behind the washing tower were investigated. Results show that the oxidation efficiency of NO and Hg greatly depends on the amounts of ozone injected. With the increasing amounts of ozone added to the main flow, NO and Hg oxidation efficiency all improved individually. About 85% of NO can be oxidized with 200 ppm of ozone added and 89% of elemental Hg can be oxidized with 250 ppm of ozone added. The optimal temperature for NO oxidation should be lower than 473 K, and the optimal temperature range for mercury was 473 K to 523 K. The appearance of SO₂ has little effect on the NO oxidation process. NO has priority compared to mercury when react with ozone. With the assistance of washing tower, about 97% of NO and nearly 100% of SO₂ can be removed simultaneously with 360 ppm of ozone added.     

Efficient Decomposition of Trichloroethylene (TCE) in Groundwater by Ozone-Hydrogen Peroxide Treatment

It was verified that the ozone-hydrogen peroxide treatment (O₃/H₂O₂) was very effective for decomposing trichloroethylene (TCE) in groundwater. The reaction efficiency of O₃/H₂O₂ was 40% better than that of ozone treatment by using a diffuser type reactor and the optimum operating conditions were also revealed by laboratory-scale experiments and computer simulations. Taking these results into consideration, the 2-port ozone injection method using an ejector-type reactor was investigated to decompose TCE more rapidly and efficiently, and it was revealed that the ozone dose necessary for satisfying the Japanese water quality standard for drinking water (0.03mg/L) by the 2-port ozone injection method was 13% less than that by the conventional 1-port ozone injection method under the condition of the same amount of ozone injected. Also, the effectiveness of the 2-port injection method was proved by a sequential plant-scale experiment for about 40 days.     

Reduction of NO in the exhaust gas by reaction with N radicals

N atoms can serve as a reducing agent for NO in the exhaust gases from fossil fuel power plants. In order to investigate the reduction of NO, synthetic exhaust gas was plasma treated in a dielectric barrier discharge (DBD) (direct treatment) as well as treated by adding nitrogen atoms generated in a pure nitrogen DBD (remote treatment). A DBD with a coaxial electrode geometry was used. Oxygen could be added to the synthetic exhaust. A complete NO reduction was achieved in dry, oxygen free exhaust gas with direct treatment, but oxygen and humidity in the exhaust gas promotes formation of N_xO_y and reduces the efficiency of NO reduction. A 90% NO reduction was achieved with remote treatment. Remote treatment is insensitive to oxygen in the exhaust, but requires large amounts of nitrogen and energy. Fourier transform infrared spectroscopy and ultraviolet absorption spectroscopy were employed in order to detect NO, NO₂, and N₂O in the treated exhaust gas stream.     

Reduction of nitrogen oxides from simulated exhaust gas by using plasma–catalytic process

Removal of nitrogen oxides (NO_x) using a nonthermal plasma reactor (dielectric-packed bed reactor) combined with monolith V₂O₅/TiO₂ catalyst was investigated. The effect of initial NO_x concentration, feed gas flow rate (space velocity), humidity, and reaction temperature on the removal of NO_x was examined. The plasma reactor used can be energized by either ac or pulse voltage. An attempt was made to utilize the electrical ignition system of an internal combustion engine as a high-voltage pulse generator for the plasma reactor. When the plasma reactor was energized by the electrical ignition system, NO was readily oxidized to NO₂. Performance was as good as with ac energization. Increasing the fraction of NO₂ in NO_x, which is the main role of the plasma reactor, largely enhanced the NO_x removal efficiency. In the plasma–catalytic reactor, the increases in initial NO_x concentration, space velocity (feed gas flow rate) and humidity lowered the NO_x removal efficiency. However, the reaction temperature in the range up to 473 K did not significantly affect the NO_x removal efficiency in the presence of plasma discharge.     

Removal of Nitric Oxide in a Pulsed Corona Discharge Reactor

Conversion of nitric oxide (NO) in a pulsed corona discharge reactor was investigated. Relative importance of each of the active species such as O, OH, HO₂ and O₃ produced by corona discharge was evaluated with respect to the oxidation of NO to NO₂. Of those species, O₃ was found to be the most important one in the oxidation of NO. In order to reduce the energy required to convert NO, olefins (ethylene, propylene) were used as additive, and the scheme for the oxidation of NO promoted by the hydrocarbon was discussed. In the presence of hydrocarbon, ozone also played very important role in the reaction mechanism. The concentration of ozone was measured at the reactor outlet to verify the importance of ozone in the oxidation chemistry. Compared to ethylene, propylene gave much better performance in the conversion of NO at the same specific energy. When propylene was added to the gas stream at identical amount to initial NO_x (300 ppm), 60 % of NO could be converted with a specific energy of only 2.6 Wh m^–3.     

Experimental Study of Ozone Generation by Negative Corona Discharge in Mixtures of N2 and O2

Ozone generation by negative DC corona discharge in N₂-O₂ mixtures has been experimentally investigated using a coaxial wire-cylinder corona reactor operating at room temperature and atmospheric pressure. The experiments have been carried out under different gas flows (15 cm³ min^?1 to 200 cm³ min^?1) and gas compositions (5% to 90% of O₂), and the effect of these parameters on the corona current, the ozone density and the efficiency of the ozone generator have been analyzed. The global rate coefficients for ozone formation and destruction have also been evaluated, and their values compared with those reported by other authors. The maximum efficiency for ozone production was found in gas mixtures with oxygen content about 70–80%.     

NOx generation from the combustion of Australian brown and subbituminous coals

The formation of NO_x during the combustion of pulverized brown and subbituminous coals from Victoria and Queensland respectively was investigated in an entrainment reactor. As no NO₂ was detected, all the NO_x was present in the form of NO. The brown coals exhibited a significantly greater potential for NO emission under fuel-lean conditions than did the subbituminous coal, even though the latter coal had a higher nitrogen content. However, under fuel-rich conditions the conversion of coal nitrogen to NO for the subbituminous coal was higher than for the brown coals. The differences in conversion efficiency may have been related in part to the nature and reactivity of the volatile nitrogen species. Reactivity differences between the chars produced from the brown and subbituminous coals may also have accounted for different extents of removal of NO. There was a significant reduction in the amount of NO emitted when brown coal was added to a combustion gas stream containing an appreciable quantity of NO before coal injection.     

Catalytic process for decolorizing yellow plume

Yellow-colored exhaust gas streams from internal engines or gas turbines, frequently referred to as “yellow plume,” contain nitrogen dioxide (NO₂) at concentrations as low as 15 ppm. The process developed in this work for decolorizing the yellow plume is based on reduction of NO₂ to NO utilizing a combination of a Pt catalyst and a reducing agent. A stoichiometric excess of carbon monoxide, diesel oil, methanol or ethanol were used as reducing agents. Depending on the type of the reductant, the active temperature window of NO₂ reduction was varied with methanol and CO being active at lower temperatures and ethanol and diesel oil at higher temperatures. By changing the Pt loading of the catalysts the active temperature window of NO₂ reduction was also changed, higher loading Pt catalysts being active at lower temperatures. This scheme of NO₂ reduction process was verified in a pilot-scale test with the real exhaust gas from the gas turbine power plant, showing 96% of NO₂ reduction at the stack temperatures of 102–123 °C and at space velocities of 28,000–95,000 h⁻¹ with inherent CO in the exhaust gas as the reducing agent.     

Simultaneous oxidation of nitrogen oxides and sulfur dioxide with ozone and hydrogen peroxide

The use of ozone and hydrogen peroxide for the simultaneous oxidation of nitrogen and sulfur oxides was studied in experiments carried out in a stirred cell. It was found that in a gas mixture, containing both nitrogen and sulfur oxides, only the nitrogen oxides are oxidized by ozone. Contrary to earlier results, sulfur dioxide does not disturb the oxidation of nitrogen oxides under dry conditions. The consumption of ozone in the oxidation of nitric oxide was slightly below the stoichiometric level because the ozone was introduced into the reactor in the oxygen flow. When the molar ratio between ozone and nitric oxide was more than 0.4, some of the nitric oxide was oxidized to higher oxides of nitrogen, the final product being a solid mixture of N₂O₅ and (NO)₂S₂O₇. Some nitrosyl sulfuric acid was formed in the aqueous solution of hydrogen peroxide in addition to sulfuric acid under wet conditions. Some white solid was found on the walls of the reactor. This solid is said it the literature to consist of H₂SO₄, HNOSO₄ and (NO)₂S₂O₇.     

Investigation of flue-gas treatment with O3 injection using NO and NO2 planar laser-induced fluorescence

Direct ozone (O₃) injection is a promising flue-gas treatment technology based on oxidation of NO and Hg into soluble species like NO₂, NO₃, N₂O₅, oxidized mercury, etc. These product gases are then effectively removed from the flue gases with the wet flue gas desulfurization system for SO₂. The kinetics and mixing behaviors of the oxidation process are important phenomena in development of practical applications. In this work, planar laser-induced fluorescence (PLIF) of NO and NO₂ was utilized to investigate the reaction structures between a turbulent O₃ jet (dry air with 2000 ppm O₃) and a laminar co-flow of simulated flue gas (containing 200 ppm NO), prepared in co-axial tubes. The shape of the reaction zone and the NO conversion rate along with the downstream length were determined from the NO-PLIF measurements. About 62% of NO was oxidized at 15d (d, jet orifice diameter) by a 30 m/s O₃ jet with an influence width of about 6d in radius. The NO₂ PLIF results support the conclusions deduced from the NO-PLIF measurements.     

Modelling of NOx adsorption over NOx adsorbers

Modelling of the phenomena involved during the adsorption of NO_x on NO_x trap catalysts was developed. The aim of the model is the prediction of the quantity of stocked barium nitrate as well as the emissions of NO and NO₂, as a function of time and temperature. The mechanism of the process is sounded on the adsorption of gas species (NO, NO₂, O₂) on platinum sites, equilibrium reaction between adsorbed species followed by the formation of Ba(NO₃)₂. This formation of barium nitrate is limited by the thermal decomposition reaction which liberates NO in the gas phase. The kinetic constant of decomposition of barium nitrate was determined by temperature programmed thermogravimetry on pure Ba(NO₃)₂, using the method of Freeman and Carroll. Other kinetic constants bound to the mechanism were estimated by fitting the results of the model to experimental results.The mechanism was validated for various values of the molar fraction of O₂, the molar fraction of NO and various values of the NO/NO₂ ratio in the gas entering the reactor. It was also tested with different catalyst compositions (variation of the platinum and BaO concentrations). The importance of oxygen in the process was clearly demonstrated as well as the promoting role of NO₂.     

Mechanism and Kinetic Study on Elemental Mercury Oxidation in Flue Gas by Ozone Injection

The mechanism and kinetics of the elemental Hg oxidation in flue gas by ozone injection are investigated in detail by using quantum chemistry, kinetic simulation and experimental research. The reaction processes, activation energies and kinetic parameters are calculated and analyzed by quantum chemistry. From the comparison of activation energies, the Hg⁰ oxidation ability of oxidizing radicals is that: NO₃>O₃>NO₂. The calculated results are in good agreement with literature experimental results. The calculated kinetic parameters are employed for kinetic simulation. The results of kinetic simulation are in good agreement with the experimental results. Results show that, the Hg⁰ oxidization increases linearly when the mole ratio of O₃/NO becomes larger or the reaction temperature becomes higher. The reaction Hg+NO₃ = NO₂+HgO is the key elemental reaction and the concentration of NO₃ is the most important factor for affecting Hg⁰ oxidation.     

Katalysatoren für den Umweltschutz

Catalysts for environmental protection. The main emitters of anthropogenic air pollution are internal combustion engines, power plants, and production processes. Components of exhaust gases which are regarded as pollutants are hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO_x), sulfur dioxide (SO₂), and dust. Three main types of catalyst are understood to improve the environment; namely automotive emission control, NO_x abatement and oxidation. To reduce the pollutants HC, CO, and NO_x in automobile exhaust gas, three-way catalysts are currently applied. The reduction of particle emissions in diesel exhaust gas is achieved by diesel filters and oxidation catalysts. Pollutants from power plants are mainly the inorganic components NO_x and SO₂. In the SCR process, NO_x is catalytically reduced to nitrogen and water. The DESONOX process is suited for the simultaneous catalytic abatement of NO_x and SO_x. Exhaust gases from production processes in many areas require after-treatment. Therefore catalyst formulations depend on process parameters and exhaust gas components. This overview presents and explains catalyst types, design, mode of operation, and processes.