Studies On Descaling Of Geothermal Water By Adsorption And Performances Of Antifouling And Anticorrosion Of Coatings For Heat Exchanger Utilized In HDR Geothermal Energy System

Fouling and corrosion of heat exchanger and pipeline caused by geothermal water is one of the bottlenecks that restrict the efficient utilization of geothermal energy. The research progress on antifouling and anticorrosion of heat exchanger in geothermal energy utilization system was summarized in the dissertation. The modified zeolite A was prepared and employed to calcium ion removal from water and geothermal water by ion-exchange adsorption. Besides, the SiO2, SiO2-FPS and TiO2 coatings on AISI304 stainless steel(SS304) substrate were prepared by sol-gel and liquid phase deposition(LPD) methods, respectively. The pressure device for anticorrosion and antifouling evaluation was established, by which antifouling and anticorrosion performances of the coatings in 150 °C simulated hot dry rock(HDR) geothermal water were systematically studied. The main contents are organized as follows.Modified zeolite A was prepared by a two-step crystallization method. The calcium ion adsorption capacity of the modified zeolite A prepared under optimum conditions was 129.3 mg/g at 298 K. The kinetics, mechanism and thermodynamics of calcium adsorption with the zeolite in aqueous solution were studied. The adsorption rate was fitted well with pseudo-second-order rate model. The adsorption process was controlled by film diffusion at calcium ion concentration less than 250 mg/L, and it was controlled by intraparticle diffusion at calcium ion concentration larger than 250 mg/L. The calculated mass-transfer coefficient ranged from 2.23×10-5 cm/s to 2.80×10-4 cm/s. Dubinin–Astakhov isotherm model could appropriately describe the adsorption thermodynamic properties when combined with Langmuir model. The adsorption process included not only ion exchange but also complexation between calcium and hydroxyl ions. The relatively higher calcium adsorption capacity indicated that the calcium removal method had a potential application for scale removal from geothermal water.The HDR geothermal water in Yingshen, Daqing region China was studied by means of Shoka Lev’s classification. The results showed that hydrochemical types mainly belonged to HCO3(-Cl/SO4)-Na type. The classification method could preliminarily predict the scaling and corrosion tendencies of geothermal water. However, Langelier index model could more accurately predict the scaling tendency for serious scaling type of geothermal water. Corrosion electrochemistry analysis method could predict and verify the corrosion tendencies of different types of geothermal water.Based on the characteristic of two-dimension roughness profile curve, the surfaces with different roughness degree were classified into roughness surface and microscopic roughness surface. For microscopic roughness surface, the roughness coefficient(I) was defined, which had definite physical meaning and preferably describe the characteristic of microscopic roughness degree. Besides, the ideal antifouling surface was brought up, the surface should be the hydrophobic surface having no defects but possessing molecular level flatness(For crystallization fouling or particulate fouling).Fouling and corrosion behaviors of different coatings and SS304 in 150 °C simulated HDR geothermal water were investigated systematically under flow condition. The results showed that compared to SS304 tube, for calcium bicarbonate-type simulated HDR geothermal water, the heat exchange tubes coated with LPD TiO2 and Sol-gel TiO2 could reduce fouling resistance by over 48% in experimental periods. Moreover, compared to SS304 tube, for the simulated HDR geothermal water containing corrosive components(total mineralization at about 7000 mg/L), the heat exchange tubes coated with Sol-gel SiO2 and SiO2-FPS could reduce fouling resistance by over 30% in experimental periods. In addition, the existence of chloride ions and increase of total mineralization in the geothermal water system over 150 °C would aggravate corrosion apparently. To solve the corrosion problem at higher temperature is the key of anticorrosion and antifouling in the HDR utilization system. Corrosion electrochemistry tests were performed after the SS304, SiO2 and SiO2-FPS coating specimens corroded in the 150 °C simulated HDR geothermal water for 14 d. The testing results showed that compared with SS304, both of the coatings could reduce the corrosion rate by over 60%. The Sol-gel SiO2 and SiO2-FPS coatings have a good application prospect for anticorrosion and antifouling in moderately corrosive HDR geothermal water.