OPTIMISATION OF CHEMICAL DOSING FOR TE MIHI COOLING TOWERS

Abstract

Te Mihi Uenukukopako (eew-new-koo-kor-par-kor) (Te Mihi) Power Station was commissioned in May 2014 but started experiencing cooling water pH control issues after 12 months operation. The alternating dual biocide programme of dosing a quaternary ammonium biodispersant/biocide (quat) one week then glutaraldehyde biocide (glut) the following week was no longer able to maintain control.
Alternative biocides were trialed to initially regain pH control, but it was then determined that the most cost and performance effective solution was to implement continuous caustic dosing, to compensate for the low concentration of ammonia in the steam and hence poor buffering ability to changes in pH, and to move to a less frequent, performance-based biocide dosing regime triggered by on-line conductivity monitoring and caustic consumption rate.
Te Mihi Power Station is a 2 x 83 MW double flash plant fed by the Wairakei geothermal field. Each unit has a c.4,400 m3 main cooling water system designed to withstand the corrosive environment associated with aerated geothermal fluid for a pH range of 4.5 to 8. Exposed materials are fiberglass, PVC, 316L stainless steel and epoxy-coated concrete.