Coatings to Regulate Scaling in Geothermal Applications

This paper explores the possibility of using thermal spray coatings to engineer the substrate surfaces and modify the scaling behaviour in simulated geothermal environments. The coatings were pre-selected based on the performance data in literature, ease of application, availability and cost. The philosophy was to select coating types that facilitate and/or retard the formation of surface scales, by investigating modifying the coating properties through a matrix of process parameters. Gas pressure was selected as a variable to modify the topography and surface roughness of the deposits. Carbon steel substrates were coated with different coating materials and water contact angle measurements were carried out. These measurements provided an indication of possible wettability of the coatings, and allowed further down-selection in conjunction with optical profilometry. The down-selected coatings were tested in simulated scale-forming conditions for 96 h followed by detailed post-test characterisation (photography, light and electron microscopy).

Geothermal Energy is currently engineered as an "always on" baseload supply, due to the limited flexibility to throttle the well without scaling and fatigue issues, and it is engineered for maximal efficiency at this output level.^1,2 Scaling is a major problem in geothermal plants, particularly in cases where the geothermal fluid composition and plant operation make it difficult to control scaling.^3-5 In such areas, particularly where scale inhibitors cannot be employed,⁶ the formation of scales can make the process less efficient and in extreme cases can lead to unexpected shutdown.

Scaling is not only dependent on the geothermal fluid chemistry and operational conditions,³ it also depends on the surface properties of the materials⁷ in contact with the geothermal fluid. Geothermal fluids containing high silicate⁸ and carbonate⁹ content have shown propensity to scale due to changes in pressure and temperature during unit operations and processes associated with geothermal plants. Carbonate scaling, particularly in the form of calcium carbonate is well known in geothermal plants working with limestone or dolomite aquifers.¹⁰ These are more common in low to medium enthalpy geothermal facilities.¹¹ In contrast, silicates (or silica) is widespread in high enthalpy geothermal fields.¹² Sulfates, sulfides, oxides and hydrated oxides can also form scales.^13,14 Some chlorides can also form scales (silver chloride, lead chloride etc), but nitrates do not as metal nitrates are soluble in water.