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Simultaneous purification of NOX and SO2 in flue gas using an alkaline phosphorus yellow emulsion Author: Shen Dixin, Research Center for Eco-Environment, Chinese Academy of Sciences Abstract: This paper presents an overview of the experiments and demonstration projects related to the simultaneous purification of NOX and SO2 in flue gas using an alkaline phosphorus yellow emulsion. Keywords: yellow phosphorus emulsion ； Denitration and desulfurization ； Flue gas purification: The flue gas emitted from the combustion of fossil fuels contains NOX and SO2. Some industrial production processes generate NOX emissions; such emissions occur in the production of nitric acid, sulfuric acid in tower processes, nitrogen fertilizers, fuels, as well as in various nitration processes. Methods for purifying NOX and SO2 from exhaust gases can be classified into three categories based on their principles: absorption, adsorption, and catalytic reduction. Based on the working medium, they can be divided into wet and dry methods. Wet flue gas desulfurization (FGD) systems make extensive use of CaCO3 as a desulfurizing agent. However, the effect on NOX purification is not significant. Relatively speaking, it is more difficult to purify NOX than to purify SO2. The most widely used, technologically mature, and effective method for removing NOX is selective catalytic reduction. However, whether it is to remove SO2 or NOX, there are high economic costs involved. Since the late 1980s, efforts have been made both domestically and internationally to develop methods for simultaneously purifying NOX and SO2. An alkaline-containing yellow phosphorus emulsion can remove both NOX and SO2 simultaneously; this method was developed by the Lawrence Berkeley Laboratory in the United States and is known as the PhoSNOX method. An alkaline-containing yellow phosphorus emulsion is sprayed into flue gas containing NOX and SO2 and brought into countercurrent contact with it; there, the yellow phosphorus reacts with the oxygen in the flue gas to produce ozone (O3) and oxygen atoms (O), which rapidly oxidize NO to NO2. NO2 dissolves in the solution and is converted into NO2- and NO3-, while SO2 is transformed into HSO3- / SO32-, which react with NO2 to produce HSO3·/SO3· free radicals. These radicals react with O2 in the flue gas to produce SO42-, and some of the HSO3-/SO32- ions react with NO2- to form N–S intermediates; the hydrolysis of these intermediates ultimately results in the formation of (NH4)2SO4 and gypsum, thereby purifying the flue gas of NOX and SO2. In 1990, the British journal “Nature” reported on this new technology. I. Experiment on simultaneously purifying NOX and SO2 in flue gas using an alkaline yellow phosphorus emulsion. The experimental setup is shown in Figure 1. Figure 1: The 0.9 L wash solution in the P4/CaCO3 simultaneous NOX and SO2 removal device consists of 0.3% P4 and 5.0% CaCO3. Flue gas containing 560 ppm of NO, 2900 ppm of SO2, 10% O2, and the remainder as N2 was sparged into the reactor, with the reaction temperature maintained at 55°C. The initial pH of the washing solution was 7.5, and it dropped to pH 4.2 after 3 hours ； Filter to separate solids from liquids in the wash solution and absorption solution. Solids are analyzed using a Raman laser spectrometer ； Liquid samples are analyzed using an ion chromatograph to determine the oxoacid ions of nitrogen, sulfur, and phosphorus. Tests found that the NO removal rate is related to the O2 content in the flue gas, the amount of P4 used, and the pH value of the emulsion. The removal of NO using yellow phosphorus emulsion requires the presence of O2; the removal efficiency of NO increases as the O2 concentration rises (from 0% to 20%) ； If O2 is not present, the NO removal rate is zero. It is clear that in the process of removing NO using P4, P4 must be oxidized, and the remaining O2 concentration in the flue gas has been shown to be sufficient to meet the requirements. Figure 2 shows the effect of P4 concentration on NO removal. Figure 2 shows the effect of P4 content on the NO removal rate. Figure 3 indicates that both the NO and SO2 removal rates reach 100% within 3 hours. Figure 3: Experiments on the simultaneous removal of NO and SO2 using P4/CaCO3 show that the pH value of the yellow phosphorus emulsion gradually decreases as the reaction proceeds, and accordingly the removal efficiency of NO also decreases. Therefore, by adding an alkaline substance during the reaction and maintaining the concentration of yellow phosphorus, it is possible to achieve a high removal efficiency for both NO and SO2. The reaction mechanism for the absorption of NO involves reactions with P4 and O2, which take place both in the liquid phase and in the gas phase. The liquid-phase reaction is the gradual oxidation of the surface of P4 nanoparticles by O2. The NO removal rate depends on the degree of dispersion of P4 in water, as well as reactor design, temperature, and additives that alter the dielectric constant of the liquid phase, among other factors. In the gas phase, the reaction of P4 in removing NO is as follows: The oxidation of P4 to various phosphorus oxyacids is a complex process, producing a series of oxidation products such as PO, PO2, P2O, and P4O. II. Amplification experiment on the simultaneous purification of NOX and SO2 in flue gas using a yellow phosphorus emulsion – Figure 4 shows the experimental setup (Environ. Prog. 11(1) 66, 1992). Figure 4 Enlarged view of the test setup: LN – liquid nitrogen cylinder; EV – liquid nitrogen evaporator; CA – compressed air; H – heater; SC – spray absorber; B – P4 dosing unit; P – pump; FM – flow meter. Simulated flue gas components: NO at 60–600 ppm, SO2 at 1500–2000 ppm, CO2 at 10%, with the remainder being N2. The simulated flue gas is preheated to about 177°C by an electric heater (H) and then enters the spray absorber (SC), a glass column with a diameter of 0.1 m and a height of 1.2 m, which contains two sets of 10 nozzles each, to allow the alkaline yellow phosphorus emulsion to be sprayed into the flue gas; the flue gas and the emulsion then come into counterflow contact to absorb NOX and SO2. The flue gas treatment capacity is 34 m3/h, with NO concentrations of 58 ppm, 300 ppm, and 610 ppm respectively, and SO2 at 1500 ppm. The liquid-to-gas ratio L/P is 7.48 (L/m3); the P4 emulsion is reused, the gas-liquid contact time is within 2 seconds, and the removal rates of both NO and SO2 are above 95%. In summary, yellow phosphorus is oxidized by O2 to form P4O10 and P4O6, and reacts with water in the scrubber to produce H3PO4 and H3PO3 fumes. The analysis results show that only 10%–15% of the phosphoric acid fumes are absorbed in the washing solution. 90%–85% of the white phosphoric acid smoke enters another scrubber along with the flue gas, where 40%–60% of phosphoric acid is produced, with a ratio of H3PO4 to H3PO3 of 9:1. In addition, trace amounts of H3PO2 are present. NO is quickly oxidized to NO2 by O3, and is directly absorbed into the washing solution (6) ； After dissolution, NO2 forms N2O3 (7) or reacts with SO32- and HSO3- (8). In the wash solution, the HSO3-/SO32- concentration is greater than 1 mmol, which is similar to typical limestone/limestone systems. Most NO, through a series of reactions, forms nitrogen-sulfur compounds, with the overall reaction equation given as (9). The final by-products of NOX and SO2 in flue gas are (NH4)2SO4 and gypsum. III. Demonstration Project for Purifying NOX and SO2 Using Alkali-Containing Yellow Phosphorus Emulsion Figure 5: Equipment for the demonstration project of purifying NOX and SO2 using alkali-containing yellow phosphorus emulsion 1. Flue gas 2. NO converter 3. N2 4. P4 emulsion 5. Water 6. P4 storage tank 7. Connection to the existing scrubber tower 8. Purified flue gas discharged into the chimney International Thermal Energy Corporation developed a method called Thermalonox, which was first used for removing NOX from the flue gas of a 375MW coal-fired power plant owned by an American electric utility; the installation was completed in June 2001. The flue gas treatment capacity is 1.3×106 m3/h, with a NOX removal rate of 75%–90%, achieving a NOX emission concentration of 50 ppm. Conventional selective catalytic reduction (SCR) requires the flue gas to be heated to 500°F in order for the Pt catalyst to effectively convert NOX using NH3. The Thermalonox French yellow phosphorus aqueous emulsion is sprayed into flue gases at 280°F–320°F, and within 0.3–8 seconds, NO is converted to NO2. The demonstration project setup is shown in Figure 5. In the second step, NO2 is absorbed and hydrolyzed into ammonium molybdate sulfate within the existing wet flue gas desulfurization (WFGD) system, eventually converting to ammonium sulfate, while water-soluble P2O5 is removed. The cost is 30% lower than that of SCR treatment since no catalyst is required. For power plants already equipped with WFGD systems, treatment costs can be reduced by 70%. The investment cost for this method is $35/KW, while to achieve the same removal rate, the SCR method requires an investment cost of up to $100/KW. The cost of removing 1 ton of NOx using this method is $700–$900, whereas it is $2500–$3500 for the SCR method. The maintenance time for this method is only a few days, whereas it takes several months for SCR (see Chem, Eng107(8)212000, Chem. Eng.108(4)212001).