Research On Recycling Of Sodium Alkaline Flue Gas Desulfurization Wastewater

Sodium alkali FGD is the method that NaOH(or Na2CO3) solution was joined directly into the absorption tower to absorb the SO2 in the flue gas. Compared with the traditional limestone/gypsum method, sodium alkaline FGD has the advantages of higher efficiency, less fouling phenomenon, less by-products and no secondary pollution, etc. The desulfurization wastewater produced during the sodium alkaline FGD technology which contains more than 1% of the inorganic material belongs to the high salinity wastewater. Direct emissions of such wastewater will not only make the surrounding soil and water salinity increased and the environment damaged, but also waste the resources of minerals. Thus, it is of great importance for the protection of the environment to realize the minimizing emission or zero emissions of the high salinity wastewater.The object of this study was the sodium alkali FGD wastewater of a thermal power plant. The results of quality analysis of this wastewater showed that the main components were inorganic salts, in which the content of Na2SO4 was the highest between 40 ~ 60 g/L. Therefore, physical methods should be adopted to enrich the wastewater to recycle the inorganic salt and water, respectively. Combined with the characteristic of high salt content of the desulfurization wastewater and the comparative analysis of several high salinity wastewater treatment technologies in such aspects as economy, technology and environmental protection, the mechanical vapor recompression(MVR) evaporation technology has more advantages in the enrichment of the high salinity wastewater. As the application examples of MVR are less and the technology is not mature yet, most of the researches on the MVR are still in the laboratory stage and more concentrated on their performance. Therefore, the composition of MVR system was introduced simply in this article, and the performances of the system were simply analyzed taking the desulfurization wastewater as the research object.Through the calculation of the material balance, energy balance and heat transfer equations of the MVR system, a mathematical model of the MVR system was set up. And the impact of such factors as the evaporation pressure in the evaporator, the compressor compression ratio and the raw material concentration ratio on the heat transfer area, the power consumption of the compressor and the heating coefficient were analyzed according to the mathematical model. By analyzing the relative curve between the factors and the performances and taking the investment costs and operating expenses of the system into account, the optimum operating conditions of the system were obtained. The optimum operating conditions were as following: the evaporation pressure was at 60 ~ 80 kPa, the compressor compression ratio from 1.6 to 2.2 was appropriate. And the concentrate can be returned to the evaporator repeatedly to achieve the desired concentration in the case of maintaining the enrichment ratio constant. In this paper, the energy saving of MVR system was compared with the triple effect evaporation system under the specific process operating conditions. The energy consumption and cooling water treatment costs were compared between the two systems, which showed that the MVR system could save nearly 0.663 million of operating expenses annually compared with the triple effect evaporation system under the condition that the amount of the wastewater treatment was 1800 kg/h. Accordingly, the energy saving effect of the MVR system relative to the triple effect evaporation system was remarkable, and the MVR system significantly reduced the operation cost of the enterprise, had a good application prospect.Finally, according to the characteristic that the solubility of sodium sulfate decreased rapidly with the decrease of temperature below 32.4 ℃, the freezing crystallization technology was adopted to dispose the concentrated solution discharged from the MVR system, then the sodium sulfate was precipitated in the form of glauber’s salt which could meet the requirements of recycling.