Challenge

This Midwestern power utility produces 200 megawatts of electricity from one 2400 psig coal-fired boiler. The plant can sell all of the power it generates. It is critical that the heat exchangers are kept free of any heat-transfer limiting biofilm. The process is cooled using a once-through system utilizing 170,000 gallons per minute. Microbiological control of this cooling system had historically been a problem due to wastewater discharges from a food processing plant upstream of the plant, discharging wastewater with a high microbiological nutrient loading. Chlorine gas fed continuously was required to maintain control of the system. Up to 10, one ton cylinders were kept on site at any one time. De-chlorination was necessary to meet NPDES discharge limits. Normal operating backpressure on the turbine was 2 mm Hg. If this increased to 6-8 mm Hg, the plant was unable to produce power at capacity and was derated.

Solution

A full system survey was completed. A recommendation to treat the process water with chlorine dioxide intermittently at 0.5 mg/L was made. Optimization of the system was attained by varying the frequency and duration of chlorine dioxide. During the summer months, the feed rate of chlorine dioxide was for 4, 30-minute periods. This was reduced during the winter to meet lower system demand. Microbiological control was successfully achieved

Results

This program resulted in operating cost savings of over $700,000 during a three-year period. The heat exchangers operated clear of biofilm and the plant was able to run at capacity.