Silicate Scale Prevention During Low Temperature Geothermal Brine Production Using a Polymer-Based Scale Inhibitor/Dispersant

Silicate scaling is recognized as a potential operational problem for geothermal power plants. In this work, a static bottle test methodology was developed to assist in identifying efficient silicate inhibitors/dispersants with 80-90% performance, applicable for low enthalpy geothermal heat recovery systems. To investigate products to control amorphous silicate and magnesium silicate scale, the inhibition efficiency (IE) and potential mechanism of a sulphonated polymer-based scale inhibitor/dispersant, denoted A5, were studied. The most common metal ion to combine with silicate in power plants is magnesium. In this work, magnesium at an initial concentration of 120ppm, was allowed to react with silicate ions (1880ppm) for 3 days, during silicate scaling static bottle tests (at 60ºC and 95ºC; pH 8.5). The scale inhibitor A5 IE performance was evaluated over a concentration range of 20-500ppm. To investigate the inhibition mechanism of A5, the concentration of sulphur, contained within its structure, was monitored alongside the scaling ion consumption of magnesium and silicate by Inductively Coupled Plasma – Optical Emissions Spectrometer (ICP-OES) throughout the test duration. The 60ºC results are similar whether the scale inhibitor A5 is deployed in magnesium brine or silicate brine. For 20ppm A5, there is less than 60% silicate and magnesium IE at 2hrs and no performance after 22hrs and 3 days. A5 shows 60-85% silicate and magnesium IE performance at 50ppm and above. The most consistent results are measured for A5 in magnesium brine before brine mixing and hence this method is used for subsequent testing. Due to low A5 consumption (< 20%) at 50-500ppm, despite displaying a moderate 60-85% IE performance, it is believed A5 is performing as a dispersant rather than a scale inhibitor. At higher temperature, 95°C, ≥100ppm A5 is required to control silicate scale effectively (cf. 50ppm A5 at 60oC). It appears A5 is less effective at preventing magnesium silicate scale (60-70% IE) as opposed to amorphous silicate scale (80-90% IE). The highest A5 consumption is found at 50-60% for 20ppm and 50ppm, whilst at 100ppm and 200ppm, less than 10% is consumed. These 95ºC results also confirm that A5 is acting as a dispersant, since it is not being consumed during its effective IE performance. A successful investigation of the initial formulation of scale inhibitor in magnesium brine prior to mixing was performed for the first time, due to observed inconsistencies during SI/silicate brine test regimes. This has provided greater confidence in the A5 inhibition performance results. The novel approach of monitoring the scale inhibitor, based on its sulphur content, by ICP-OES, means further insights relating to the inhibition mechanisms can be determined, thus advancing our knowledge of the mechanism of how silicate inhibitors/dispersants work.