Cooling Water

Crucial Aspects Of Bacterial Control In Geothermal Power Station Cooling Towers

By Ian Richardson, Simon Addison and David Addison

BACTERIA BIOFILMS BOILERS CARBAMATES COOLING TOWERS COOLING WATER CORROSION CYCLES OF CONCENTRATION DEPOSITION ECONOMICS GEOTHERMAL GLUTARALDEHYDE LEGIONELLA NON-OXIDIZING BIOCIDES OXIDIZING BIOCIDES pH POWER SCALING SODIUM HYDROXIDE TROUBLESHOOTING

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Abstract

Mighty River Power (MRP) operates a diverse fleet of geothermal power plants, including two geothermal power plants with condensing steam turbines. The cooling water used to condense the exhausted steam is condensed geothermal steam, recirculated through a mechanical draft evaporative cooling tower. The use of condensed geothermal steam as cooling water presents both opportunities and challenges when compared to the use of surface water or groundwater for cooling. The condensed steam has low levels of dissolved solids, and very low levels of suspended solids, reducing the likelihood of mineral scale formation and erosion within the cooling water system. 

The presence of hydrogen sulfide in the geothermal steam (and the condensed steam) presents several challenges to the management of the cooling water system, including the build-up of sulfur deposits, the management of sulfur metabolizing bacteria and the limited choice of sulfide compatible biocides. This article discusses the implications of these challenges to the management of cooling water systems using condensed geothermal steam, including discussion of significant cooling water events resulting from these challenges.

Introduction

Mighty River Power (MRP) operates a diverse fleet of geothermal power plants, including flash plants, binary plants, and combined flash-binary plants. MRP currently operate two flash plants that use condensing steam turbines in the power generation process, the 138-megawatt electrical (MWe) triple-flash Nga Awa Purua (NAP) Power Plant and the 100-MWe double-flash Kawerau Power Plant.

The use of condensing steam turbines increases the efficiency of the power generation process (compared to a back-pressure steam turbine) by extracting more energy from the working fluid, which results in a lower working fluid exhaust temperature. Condensing the exhausted steam requires the removal of the latent heat of vaporization of the steam; this is achieved at MRP’s two flash plants through the use of evaporative, re-circulating cooling water systems. The use of a re-circulating cooling water system minimizes the requirement for an external source of cooling water while also removing the need to discharge large volumes of wastewater into the environment.

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