Scale build up on heat transfer equipment in the geothermal industry has been a problem since its first ever utilisation as a smart and renewable form of energy nearly 60 years ago. Over the last 20 years with the growth and success of geothermal power generation, the knowledge, experience and data illustrating the increased performance through regular chemical cleaning to ensure maximum heat transfer has been gathered. Chemical cleaning as the method to remove scale has the added benefit of increased plant availability through minimising down time for cleaning.
Chemical cleaning to remove silica build-up.
At the lowest point of generation, circa 4.2mW of power would be produced. After execution of a chemical cleaning program power generation is returned to 8.0mW.
Other data analysed include the types of scale encountered, types of equipment cleaned, the various solvent chemistry employed, chemical cleaning equipment, and techniques for treatment and/or disposal of spent solvent.
The graph below illustrates the combined output of binary units. A gradual degrease in generation can be seen over time as the pre-heaters and vaporisers being to scale up with silica. The arrows indicate unit shutdown for chemical cleaning, followed by return to service post cleaning and back to full generation.
Fig 1. Power generation showing increased output at regular chemical cleaning intervals.
An added benefit is the removal of silica and calcite deposits from the condensate well upon reinjection of the depleted Hydrofluoric Acid post chemical cleaning. This step is soon to be expanded to include fresh blended inhibited Hydrochloric Acid to further improve re-injection scale removal.
The total cleaning programme can be executed in as little as 12 hours from isolations on to isolations off.
Fig 2. Silica Scale build up prior to chemical cleaning on Pentane vaporiser.
Fig 3. Clear tubes post chemical cleaning on Pentane vaporiser tubes.