Coal, O&M

Controls cutchemistry costs

Issue 8 and Volume 100.

Controls cutchemistry costs

A coal-fired utility plant, operating three units rated at more than 2,000 MW, recently selected Nalco Trasar technology to assist its chemistry department with goals of reducing dependence on inaccurate wet chemistry tests used to control product residuals, gaining confidence that the actual operating chemical concentration is within the targeted control range and minimizing program costs by maximizing program control. The recirculation rate of each unit is 300,000 gpm and the primary chemical treatment program consists of an organic phosphonate for scale inhibition and a general dispersant for control of silt and suspended solids. Sulfuric acid is fed for pH/alkalinity adjustment and microbio control is achieved by intermittent feed of chlorine.

Mineral solubility calculations showed that the plant could assure system cleanliness by controlling the scale inhibitor/dispersant blend at 10 to 15 ppm as product. However, product residuals in the system were maintained at approximately 15 to 25 ppm as product. At this higher targeted feed rate, the plant was assured of system protection in all periods of operation.

To further define the performance of the treatment control strategy, mass balances were done on makeup flow rates, chemical inventory and feed rates. Results were compared with wet chemistry tests to verify the inaccurate control of treatment chemicals. Higher target feed rates assured unfouled heat exchangers, but program costs were higher than needed. The goal was to control chemical treatment in dosages within a safe, verifiable control range to optimize program performance at lowest cost.

Control strategy

In addition to improving chemical feed strategy, this utility was interested in upgrading controls for the entire feed system. This was accomplished by installing a Trasar control to continuously monitor and control dosage of cooling water chemicals to within 1.5 ppm as product. The plant also chose to replace existing ac pump motors with dc motors capable of accepting a 4-20 mA signal from the Trasar control. The upgraded control strategy employs the use of a proportional controller to fine tune chemical additions.

To incorporate the new controls with existing equipment, outputs were run to the station`s existing controller in a remote building, allowing plant operators to adjust chemical dosage setpoint from the same location as other plant process. After five months of running chemical treatment under Trasar, the plant evaluated the program performance. Chemical usage decreased by more than 40 percent on Units 1 and 2, and by more than 20 percent on Unit 3.

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