| As the super-critical, ultra-supercritical generating unit gradually become the main force in electricity production, the quality of boiler feedwater is higher and higher, its impurity ions contents control at several?.g/L or less, its conductivity is less than 0.1?.S/cm which is more strict than national standard for first-grade reagent water. But the dissolved orangic matter such as humic acid (HA) exists in the make-up water still, because the pretreatment and ion exchange technology at present can not effectively remove it, the organic matter in water-steam system was the root causes for decreasing in boiler water pH value, scaling in water-wall tubes and corroding severely in turbine low-pressure cylinder. In order to research the damage degree for organic matter in boiler make-up water, and know it how to affect pH value in boiler water, corrode metal in boiler-front system and turbine nascent condensed section, the mathematical models was derived to characterize the specific influence degree at first time, the specific numerical results were obtained according to actual control standard or experimental data, which can be used as the practice operation guide or to predict the possible influence by organic matter in field application, which being uesd to judge whether qualified for residual organic matter in following study.The conventional pretreatment methods such as enhanced coagulation, activated carbon adsorption were carried to remove humic acid (HA) in experiment, the HA removal rate under the best conditions were 60%,59% respectively, which can not meet the field requirements. So the multi-walled carbon nanotubes (MWNT) with stable structure and excellent absorption performance was choiced as a basic material, its kinetics, thermodynamics and mechanism for MWNT adsorbing HA were investigated, a new and recycling adsorbent was prepared with MWNT and polyurethane foaming agent, the MWNT/TiO2 nano-particles and nanotubes composite photocatalysts were synthesized, the mechanism of removal HA with above materials was studied systematicly, the results of this study were hopeful to provide a new method and technique for make-up treatment in ultra-supercritical boiler.The experimental results of MWNT adsorbing HA showed that adsorbing efficiency decreased with pH value increased, increased with MWNT dosage enhanced and reduced with initial concentration of HA added. The adsorbing efficiency and adsorption capacity with pH at 5.5, MWNT dosage at lg/L and HA concentration at 30mg/L were 85%,25.5 mg/g respectively, the HA removal rate was higher than that in enhanced coagulation and activated carbon adsorption process. The adsorption kinetics data was fitted with pseudo second-order model well, its quation was qt-1=0.037+0.955/t, the equilibrium adsorption capacity obtained from fitting curve was 27mg/g, consistent with the experimental results, indicating that it correctly described kinetics adsorption process for MWNT adsorbing HA. The adsorption isotherm test was carried at 25?-50?, its thermodynamic data was fitted with Langmuir and Freundlich model well, their linear correlation was more than 0.9, and equilibrium adsorption capacity was 29.7mg/g by Langmuir model, which being similar with experiment result. The free energy (?G), enthalpy (?H) and entropy (?S) were obtanined with Clapeyron-Clausius and Gibbs-Helmholtz equation in the adsorption isotherm. Experiment results showed that MWNT were a hollow tubular molecule with large specific surface area, its outer surfaces, layer interspace, inner cavity and aggregated pores became efficient adsorption space for HA, aggregated pores belonged to mesopore or macropore accoding to measurement, HA molecules was in the 3-10nm in diameter, so HA adsorbed by MWNT favorablely, which meet requirement for adsorbent pore size matching the adsorbate particle size.The MWNT dispersed solution was prepared with physical dispersion method at room temperature, the MWNT polyurethane foam was synthesized with MWNT dispersed solution and polyurethane foaming agent at first time, which was used to adsorb HA. The adsorbing efficiency and adsorption capacity were 64%?19.2 mg/g respectively with pH at 5.5, dosage at 1g/L and HA concentration at 30mg/L, the HA can be removed basically in the two series adsorption process. The adsorption kinetics data was fitted with pseudo second-order model well, its quation was qt-1=0.0495+1.424/t, the adsorption isotherm test and fitting results of Langmuir and Freundlich model showed that HA absorbing being exothermic process of entropy reduction, the experimental results of desorption and recycling experiments showed that MWNT polyurethane foam holded its adsorption capacity after recycling three times. The polyurethane foam was porous, the diffusion process between MWNT and HA was not affected, so the single MWNT in polyurethane foam can adsorb HA at outer surfaces, layer interspace and inner cavity more effectively.The MWNT/TiO2 nanotubes and nano-particles composite photocatalyst were synthesized with sol-gel and hydrothermal method. Their properties were characterized by TEM (transmission electron microscopy) and FL (Fluorescence spectrum). Their photocatalysis activity was evaluated with HA photodegradation under ultraviolet radiation, the HA maximum degradation rate at 5h for the three kinds of samples which prepared by sol-gel, hydrothermal and pure TiO2 were 83.5%?72.4%?56.7% respectively, the first-order reaction parameters of samples prepared by sol-gel were 2 times than pure TiO2. The MWNT/TiO2 nanotubes composite photocatalyst were produced by hydrothermal method, their properties were characterized by SEM, XRD (X-ray diffraction) and UV-Vis spectroscopy. Their photocatalysis activity was evaluated with HA photodegradation under ultraviolet radiation.20%MWNT/TiO2 nanotubes had synergetic effect, and had the highest removal rate for HA at 77%, its first-order reaction parameters was great 1.6 times than pure TiO2. MWNT played the role of electronic transmission in photocatalyst, which improved the life time for optical electron and hole. Meanwhile, MWNT were an excellent adsorbent, the HA and oxygen can be absorbed efficiently to its surface, accelerated the photocatalytic process, reduced recombination rate of electron and hole, improved photocatalytic efficiency. The MWNT/TiO2 composite photocatalyst which obtained in this study can significantly reduce the residual organic matter in boiler make-up water, has greatly improved the HA removal rate Comparing with enhanced coagulation and activated carbon adsorption method.The above mentioned results provided a theoretical basis and technical support for MWNT used in practical boiler make-up treatment and TiO2 applied in high efficiency. |
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