Surface roughness affects early stages of silica scale formation more strongly than chemical and structural properties of the substrate
Introduction
Silica (SiO2) is the most common chemical compound in the Earth’s crust and dissolved silica a major component in most high-enthalpy geothermal fluids. When such fluids are flashed and cooled during power generation, they become supersaturated with respect to amorphous silica, which leads to rapid precipitation. This unwanted precipitation (scaling) inside pipelines and onto fluid-handling equipment is a major issue in geothermal power plants around the world, decreasing the efficiency of geothermal energy production (Gudmundsson and Bott, 1979; Rothbaum et al., 1979; Harrar et al., 1982; Yokoyama et al., 1993; Gunnarsson and Arnórsson, 2005; Padilla et al., 2005; Meier et al., 2014; Dixit et al., 2016; Mroczek et al., 2017).
Amorphous silica scales exhibit two different morphologies: (1) “fluffy” and soft silica precipitates consisting of individually deposited and (partly) cemented particles and (2) dense, hard silica layers, often with a botryoidal surface and no internal structure (Thórhallsson et al., 1975; Gudmundsson and Bott, 1979; Rothbaum et al., 1979; Brown and McDowell, 1983; Carroll et al., 1998). Our recent study (van den Heuvel et al., 2018) of silica scales inside the pipelines of the Hellisheiði power plant (SW-Iceland) found that these two morphologies can be attributed to two different silica precipitation pathways: The homogeneous pathway starts with homogeneous nucleation of nanoparticles in the liquid, which then grow by addition of dissolved silica or are aggregated to μm-sized spheres. Subsequently these μm-sized particles are deposited onto available surfaces where they can form complex 3D structures. For the heterogeneous pathway, silica precipitates by heterogeneous nucleation directly onto available surfaces. The nuclei subsequently grow to individual half-spheres by addition of dissolved silica from the liquid. Over time, this leads to the formation of a continuous botryoidal silica layer.
From a plethora of laboratory studies we know that favourable physicochemical conditions such as high total silica concentrations, high percentage of monomers, elevated temperature and high pH enhance silica polymerisation and thus silica scaling (Alexander et al., 1954; Goto, 1956; Kitahara, 1960; Iler, 1979; Crerar et al., 1981; Weres et al., 1981; Fleming and Crerar, 1982; Gallup, 1997; Gunnarsson and Arnórsson, 2005; Icopini et al., 2005; Tobler et al., 2009; Tobler and Benning, 2013). This in turn favours both types of nucleation as well as growth of silica particles and half-spheres. Besides the physicochemical conditions of the liquid, the heterogeneous pathway also depends on surface properties such as surface roughness and composition and structure of the substrate. Rough surfaces enhance heterogeneous nucleation as they reduce the contact angle of the nuclei with the surface which in turn reduces the interfacial energy and the energetic barrier for nucleation to occur (De Yoreo and Vekilov, 2003; Benning and Waychunas, 2007). This has been shown experimentally and by numerical simulations for a range of different materials onto different substrates (e.g. Qi et al., 2004; Järn et al., 2006; Page and Sear, 2009; Campbell et al., 2013). The effect of surface chemistry on heterogeneous nucleation has been evaluated by investigating mineral precipitation onto different mineral substrates. One factor enhancing mineral precipitation is the presence of ions in the mineral substrate which are needed for the nucleation of the secondary phase (Putnis, 2009 and references therein). Another aspect enhancing mineral nucleation is a lattice match between the substrate and the secondary phase, i.e. surfaces with a similar structure can act as a template for the nucleation of new materials. (e.g. De Yoreo and Vekilov, 2003; Fernandez-Martinez et al., 2012; Murray et al., 2012; Stockmann et al., 2014; Zolles et al., 2015). Based on all these studies, we would expect that rough and/or silica(te)-based surfaces (e.g. the reservoir rocks) are more prone to silica deposition than smooth and/or non-silica(te) surfaces (e.g. the inside of geothermal pipelines). In order to test this hypothesis, we investigated three surfaces with different chemical compositions and roughness: non-precious opal, volcanic glass and carbon steel. In addition, we compared the results presented here to our previous study investigating silica scaling inside the pipelines of the Hellisheiði power plant onto stainless steel (van den Heuvel et al., 2018) as the scaling plates for both studies were deployed at the same time and the silica thus precipitated under identical physicochemical conditions.
Section snippets
Materials and methods
Three different substrates (non-precious opal, volcanic glass and S275 carbon steel) were chosen to investigate the effect of surface properties on silica deposition. Coupons (2 × 1.3 cm) were prepared from each material and then glued onto S316 stainless steel plates (5.4 × 2 cm) using a Loctite Hysol 9455 epoxy adhesive. Equivalent coupons of each material were imaged pre-deployment using a field emission gun scanning electron microscope (FEG-SEM, FEI Quanta 650 at 15 keV). In addition,
Characterisation of the separated water
The separated water at Locations A and B was identical with respect to chemical composition (Table 2). The differences between the two locations were temperature and flow rate, which were both higher at Location A (∼120 °C vs. ∼60 °C and ∼420 L/s vs. 280 L/s, respectively). The total concentration of silica was 800 ppm at both locations. At Location A, 85 % of this silica occurs as molybdate-reactive silica (primarily H4SiO4 but including an unknown proportion of silica dimers and trimers)
Interactions between the separated water and the coupon materials
As the different surfaces were in contact with the separated water for up to 10 weeks, dissolution and/or alteration of the coupon materials were possible. In order to assess the likelihood of dissolution for the opal coupons, we compared the material to the silica gel used by Gunnarsson and Arnórsson (2000), which was used to determine the thermodynamic data, which in turn was used to calculate the SIs reported in Table 2. Non-precious opal consists of irregularly packed submicrometre-sized
Summary
The results of our scaling plate study at Hellisheiði showed that silica can precipitate via a homogeneous and a heterogeneous pathway inside geothermal pipelines. While the homogeneous pathway is independent of the characteristics of available surfaces, the heterogeneous pathway is controlled by their properties. In this study, we investigated materials which are present in a geothermal system (opal = previously formed silica scales, volcanic glass = reservoir rocks, carbon steel = pipelines)
CRediT authorship contribution statement
Daniela B. van den Heuvel: Conceptualization, Investigation, Writing - original draft, Visualization, Funding acquisition. Einar Gunnlaugsson: Investigation, Writing - review & editing. Liane G. Benning: Writing - review & editing, Supervision, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This research was made possible by a Marie Curie grant from the European Commission in the framework of the MINSC ITN (Initial Training Research network), Project number 290040, the 2014 PhD Student Grant by the International Geothermal Association (IGA) awarded to DBH and a UK Natural Environment Research Council grant (NE/J008745/1) awarded to LGB. We thank GT Opals in Coober Peedy for providing the non-precious opal samples and Tony Windross, Stephen Burgess and Hari Williams for preparation
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