Bleed stream oxidation, limestone

Limestone Tests with Bleed Stream Oxidation. A major advantage of the bleed stream oxidation is its simple flow configuration. In operation without forced oxidation, the scrubber bleed stream would be sent directly to the solids dewatering system. 

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

Table 7 gives the results of a typical bleed stream oxidation test, Run 915-1C, which was conducted with adipic acid-enhanced limestone on the venturi/spray tower system. The effluent slurries from the venturi and the spray tower were discharged into a common effluent hold tank. The scrubber bleed stream was pumped from the effluent hold tank to an oxidation tank into which air was injected through a 3-inch diameter pipe. The final system bleed was withdrawn from the oxidation tank and sent to the solids dewatering system. 

Adipic Acid-Enhanced Limestone Test on the Venturi/Spray Tower System with Bleed Stream Oxidation... 

Forced oxidation is achieved by air sparging of the slurry in an oxidation tank, either on the bleed stream to the solids dewatering system or on the recirculated slurry within the scrubber slurry loop. For a one-scrubber-loop forced oxidation system, the slurry effluent from all scrubbers in the system (e.g., the venturi scrubber and spray tower at Shawnee constitute a two-scrubber system, and the spray tower alone or TCA, a one-scrubber system) are sent to a single effluent hold tank, which is the oxidation tank. For a two-loop forced oxidation system, there are two scrubbers in series (e.g., venturi and spray tower at Shawnee) with effluent from each scrubber going to a separate tank the effluent hold tank for the upstream scrubber (with respect to gas flow) is the oxidation tank. For either one-loop or two-loop forced oxidation systems, the oxidation tank may be followed by a second tank, in series, to provide further limestone dissolution and gypsum desupersaturation time prior to recycle to the scrubber. 

Case 2 — A limestone case with MgO addition. Oxidation of the scrubber bleed stream was chosen because in-loop oxidation is incompatible with magnesium-enhanced scrubbing. As in Case 1, long-term reliability has not been demonstrated at Shawnee for this mode of operation. 

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