Flue gas injection control of silica in cooling towers.

Brady, Patrick Vane ; Anderson, Howard L., Jr. ; Altman, Susan Jeanne

关键词: ACID NEUTRALIZING CAPACITY;    BLOWDOWN;    CALCITE;    CALCIUM;    COOLING TOWERS;    FLUE GAS;    KINETICS;    POLYMERIZATION;    POWER PLANTS;    SILICA;    SOLUBILITY;   

DOI  :  10.2172/1020504
RP-ID  :  SAND2011-2859
PID  :  OSTI ID: 1020504
Others  :  TRN: US201116%%772

美国|英语

来源: SciTech Connect

【 摘 要 】

Injection of CO{sub 2}-laden flue gas can decrease the potential for silica and calcite scale formation in cooling tower blowdown by lowering solution pH to decrease equilibrium calcite solubility and kinetic rates of silica polymerization. Flue gas injection might best inhibit scale formation in power plant cooling towers that use impaired makeup waters - for example, groundwaters that contain relatively high levels of calcium, alkalinity, and silica. Groundwaters brought to the surface for cooling will degas CO{sub 2} and increase their pH by 1-2 units, possibly precipitating calcite in the process. Recarbonation with flue gas can lower the pHs of these fluids back to roughly their initial pH. Flue gas carbonation probably cannot lower pHs to much below pH 6 because the pHs of impaired waters, once outgassed at the surface, are likely to be relatively alkaline. Silica polymerization to form scale occurs most rapidly at pH {approx} 8.3 at 25 C; polymerization is slower at higher and lower pH. pH 7 fluids containing {approx}220 ppm SiO{sub 2} require > 180 hours equilibration to begin forming scale whereas at pH 8.3 scale formation is complete within 36 hours. Flue gas injection that lowers pHs to {approx} 7 should allow substantially higher concentration factors. Periodic cycling to lower recoveries - hence lower silica concentrations - might be required though. Higher concentration factors enabled by flue gas injection should decrease concentrate volumes and disposal costs by roughly half.