Experiment And Mechanism Research Of A Novel Process Of Removing SO₂ And NO₂ From Flue Gas By Sodium Humate

With the development of economy and society, the atmospheric pollution caused by SO₂ and NO₂ from the combustion of fossil fuels has been gradually more serious. Although the wet flue gas desulfurization (FGD) processes based mainly on limestone scrubbing are frequently used, they have many disadvantages such as high capital operating costs, a lager water requirement, poor quality of byproducts, and even producing second pollution, Which is difficult to accept for the developing countries. Therefore, it is important to investigate and develop the cost effective technologies of removing SO₂ and NO_x for resolving the atmospheric pollution in china.In this thesis, a novel process of simultaneous removing SO₂ and NO₂ by sodium humate (HA-Na) was proposed. This process is a resourceful type of environmental protection technology for flue gas desulfurization and denitrifcation (FGDD) and has many advantages including: (a) lower costs and energy requirements, (b) almost no waste sludge, (c) the recovery of sulfur and nitrogen as a useful sulfur-containing nitrogen fertilizer, and (d) the simultaneous removal of SO₂ and NO₂. Therefore, it is hopeful to be used in a large scale. For developing the novel process, investigations were conducted in this thesis as following:(1) Preparation of HA-Na/?-Aluminum (?-Al₂O₃) and its desulfurization properties.A new composite adsorbent of HA-Na/?-Al₂O₃ for FGD was prepared using the impregnation method. The desulfurization property of the adsorbent was studied in a fixed-bed. The experimental results show that the HA-Na-coating on the?-Al₂O₃ fibers improved the property of?-Al₂O₃ support for FGD. On the other hand, the HA-Na-coating on the adsorbent of HA-Na/?-Al₂O₃ impregnated with ammonia (NH4OH) played an important role in enhancing the desulfurization property of a-Al₂O₃. Due to the strong adsorption capability of HA-Na, more NH4OH was adsorbed in the adsorbent of HA-Na/?-Al₂O₃ the longer a high SO₂ conversation rate was maintained. In addition, the desulfurization products can be flushed from?-Al₂O₃ fibres and make into the compound fertilizer consisting of ammonium sulfate [(NH4)2SO4], ammonium humate (HA-NH4), and HA-Na. the recycling use of?-Al₂O₃ fibres was also easily achieved. (2) Experiment and mechanism research of FGD by HA-Na solution.Experiments were carried out to examine the effect of various operating parameters, such as the HA-Na concentration, pH, temperature, gas flow rate, O₂ concentration, SO₂ inlet concentration, and NO₂ coexiting with SO₂, on the SO₂ absorption efficiency and breakthrough time in a lab-scale bubbling reactor. The experimental results indicate that the HA-Na concentration significantly influences the breakthrough time but has little effect on the SO₂ absorption efficiency. The breakthrough time increases with the HA-Na concentration reaching 0.06 g/mL, and after this value, it begins to decrease. The SO₂ absorption efficiency maintains 99% when pH is above 4.5. The low temperature is favorable to SO₂ absorption. The increase of the SO₂ inlet concentration improves the mass transfer of SO₂ and accelerates the SO₂ consumption rate. NO₂ coexisting with SO₂ can promote SO₂ absorption because it may speed up oxidation of sulfite to sulfate. HA-Na solution shows great performance in SO₂ absorption, and the SO₂ absorption efficiency can be above 98% in the optimal condition. Moreover, the desulfurization products can be made into the humic acid (HA) compound fertilizer, and recycling water can be obtained in this desulfurization process.(3) Experiment and mechanism research of removing NO₂ from flue gas by HA-Na solution.The effects of various factors like the HA-Na concentration, NO₂ inlet concentration, temperature, O₂ concentration, the oxidation extent (the NO₂ volume fraction in NO_x) and SO₂ coexiting with NO₂, have been investigated in a bubbling rector The experimental results show that the NO₂ absorption efficiency is increased with HA-Na concentration, O₂ concentration, and pH. The low absorption temperature is favorable to the absorption of NO₂. The inlet NO₂ concentration has not significant effect on the NO₂ absorption efficiency. It is concluded that SO₂ coexisting with NO₂ may go against removing NO₂ until SO₂ concentration is saturation in HA-Na solution. The NO_x absorption efficiency is proportional to the oxidation extent when the NO₂ concentration is lower than 0.1% in flue gas. HA-Na solution has strong NO₂ absorption capability, and the NO₂ absorption efficiency can be above 95%. Moreover, the denitrification products can be made into the HA compound fertilizer.(4) Simultaneous removal of SO₂ and NO₂ from flue gas by HA-Na solution.This study relates to the SO₂/NO₂ absorption efficiency and products of simultaneous removing SO₂ and NO₂ in a bubbling reactor, especially the effect of recycled water on the SO₂/NO₂ absorption. Under alkaline conditions, the sulfate content in S-containing compound decreases with the increase of NO₂ concentration, whereas there is a contrary result under acidic conditions. Whether the absorption liquid is alkaline or acidic, the presence of NO₂ improves the SO₂ absorption into HA-Na solution. The NO₂ absorption efficiency in the presence of SO₂ is more than that in the absence of SO₂.The NO₂ absorption efficiency decreases slightly with the increase of NO₂ concentration, and NO 3- is the main byproduct of absorption of NO₂.The NO₂ absorption efficiency can be improved with the cycle number rising due to the increasing amount of sulfite. Although all the ion concentrations of Na⁺, SO₄^2-, SO₃^2-, and NO 3- have a gradual increase as the cycle number rises, the ion concentrations of SO₄^2- and Na+ are far more than that of the other ions, which results in a slight decrease of the SO₂ absorption efficiency. However, the initial pH of HA-Na solution prepared by recycled water decreases from 10 to 8.1 with the cycle number increasing from 1 to 10, whereas the final pH remains almost constant (3.3). The SO₂ absorption efficiency is above 98% and the NO₂ absorption efficiency may reach above 95% in the optimal condition in this process. The chief byproduct is a compound fertilizer consisting of HA, sulfate, and nitrate. The oxygen content in the byproducts increases with the cycle number rising, which indicates that the O-containing functional groups of HA is increased and the activity of HA is improved by oxygenolysis.(5) Modeling and numerical simulation of mass transferring for FGD by HA-Na in bubbling reactor.According to the double-film theory, the theoretical analysis of gas-liquid mass transfer process for absorption of SO₂ by HA-Na solution was carried out, and the mass-transfer model was established. The parameters in this model were determined on the basis of experimental results, such as pseudo-first-order rate constant, average bubble diameter, gas holdup, and specific gas-liquid interfacial area. The nonlinear regression model of SO₂ equilibrium pressure was constructed by the losopt-software. According to the SO₂ mass-transfer model, the SO₂ mass-transfer during the FGD by HA-Na was numerical simulated, and the simulated result is in agreement well with the experimental result.(6) Design and make the spray absorber and process units for the simultaneous removal of SO₂ and NO₂ process by HA-Na solution.According the working principle, process characteristics and design principle of spray absorber, the spray absorber and process units for removing SO₂ and NO₂ by HA-Na solution were designed and made. The results show the simultaneous removal of SO₂ and NO₂ process by HA-Na has a better performance on the whole, and the SO₂ absorption efficiency is above 98% and the NO₂ absorption efficiency is above 95% in the optimal condition. During this process, not only the high SO₂/NO₂ absorption efficiency, but also the precipitation separation of by-products, the neutralization and recycling of acidic water solution can be achieved. The economic evaluation of FGD process by HA-Na was also carried out, and this process has good economy.