| Title | Microbially Induced Degradation of Fibre Reinforced Plastic (FRP) in the Cooling Tower of a Geothermal Power Station |
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| Authors | T. Clark, K. Wade, N. Gibson |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Cooling towers, FRP, sulfuric acid, corrosion, microbial degradation, sulfur oxidizing bacteria, geothermal |
| Abstract | Evaporative cooling water systems in geothermal power plants can pose several challenges for geothermal operators. The challenges of corrosion, deposition and biological control can be magnified in systems utilising direct contact condensers, due to the presence of hydrogen sulfide in the cooling water. Hydrogen sulfide provides an energy source for the growth of sulfur-oxidising bacteria, with the production of sulfuric acid as the main end-product. The sulfur oxidising bacteria can readily colonise such cooling towers, forming a biofilm on the large surface area provided by the tower’s structure and fill material. This paper describes a technical investigation, undertaken following discovery of wide-spread degradation of the Fibre Reinforced Plastic (FRP) structure of a geothermal cooling tower, after only 7 years’ of service. The degraded FRP was found only in the top section of the tower, above the Drift Eliminator level. The subsequent investigation identified the cause of degradation as microbially-induced corrosion of the FRP material, due to the presence of sulfuric acid at about 1% concentration (pH 1). Several studies were conducted to understand the nature of the FRP degradation, its effect on the strength of the structural elements, and the extent of degradation over the tower’s structure. Subsequent work considered a few possible ways to control bacterial growth and prevent the acid from accumulating. However, no suitable method could be found, with the decision taken to replace the affected FRP with new, acid-resistant FRP material. Because this type of degradation had not been previously reported, selection of new material required the development of an accelerated aging test. This test was extensively applied to FRP samples, covering a range of material formulations, allowing selection of suitable product for use in the tower’s repair. This paper concludes with some observations about FRP formulations most suited to this type of cooling tower service. |