| Title | ON-LINE SILICA INHIBITION AND DISPERSION BY ORGANIC COPOLYMER DOSING AT WAIRAKEI BINARY PLANT, NEW ZEALAND |
|---|---|
| Authors | J. Seastres Jr., K. Wigram, C. Siega, A. Dean, K. Barrie and D. Rodman |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | silica inhibitor, GEO981, aluminium, colloids, binary plant, GEO962, heat exchanger, reinjection |
| Abstract | A pilot test was conducted at the Wairakei Binary Plant (2 units x 7.5 MWe net rated capacity) to assess the effectiveness of controlling silica deposition by chemical dosing. An acrylic copolymer with non-ionic monomer, NALCO GEO981, was tested at one unit (G16) of the binary plant from July 2014 to January 2015. The test results indicated a reduction in output decline rate for this unit from 70% to 15% per annum. Following the successful trial results of GEO981, a chemical dosing system was installed at the main reinjection line (X-Line) to dose both units (G15 and G16) of Wairakei Binary Plant in July 2015. Chemical dosing has reduced the total power decline from around 3.2-3.5 MWe to 1.5-3.0 MWe per 200 days production cycle. The variable extent of plant output improvement suggests that silica scaling was not fully controlled and it was still observed within the heat exchanger tubes. Since chemical dosing commenced, there was no decline in reinjection capacity of Otupu wells in which cooled brine from the binary plant was injected. XRF (X-Ray Fluorescence) analysis of scale samples recovered during testing consist mainly of silica (77%) with significant Al content (≈7%) and minor cation components (Na, K and Ca). Monomeric silica particles can be absorbed on these metal ions reducing the effectiveness of the chemical inhibitor in controlling silica scaling. A co-dosing trial of GEO981 with another chemical GEO962, anionic acrylic copolymer, is being considered to further reduce the scaling rate at the binary plant by keeping these metal ions including aluminium in soluble state so it will not induce silica deposition. |