| Title | CONTROL OF SILICA SCALES AT MOMOTOMBO NICARAGUA |
|---|---|
| Authors | N. Delgado, O. Weres, J. Guido and M. Perez |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Silica Scale Control, Binary Plant, Bottoming Cycle, Brine Line Scale, Momotombo Power Plant |
| Abstract | The brine produced by a two phase steam separator in a water dominated geothermal reservoir can often form tenacious metal silicate and amorphous silica scales which will with time completely obstruct the pipelines taking the brine from the separator pad to a binary plant, a bottoming cycle plant or simply a reinjection well or discharge point. The geothermal fluid from Separators was initially discharged into local lakes, streams, or rivers. However, for both reasons of reservoir management and concern for the environment these fluids are now almost universally reinjected back into the reservoir. Furthermore, with the improvement and experience in production and operation of binary plants, it is becoming more and more common to extract the last kilojoule of energy and latent heat from the brine via these types of facilities. The need therefore to keep the pipelines, heat exchangers and pumps as scale free and deposit free as possible has increased significantly compared to the past. Another advance in operations and technology has been the development of equipment that can use the latent energy in the concentrated brine to provide direct heat via heat exchangers for facilities such as paper mills, green houses, and the production sites for the drying of meats and fruits. The scaling tendency of the concentrated brine can then become an even more problematic issue as the negatives associated with the scaling expand beyond periodic replacement of the liquid brine pipe to one of a constant loss of electrical production capacity in a binary unit or the loss of heat in a building. Removal of scale from the heat exchangers of the binary power plant in order to restore electrical generation capacity can lead over time to damage of the equipment, permanent lost capacity, and a reduction in reliability of the unit. In plants with high saturation levels of metal silicates and amorphous silica, the geothermal industry has developed solutions for control of the problem which are based on pH control, scheduled and periodic maintenance programs, the use of scale inhibitors, and a combination of all three methodologies. The use of strong acids for pH control has been used successfully in the industry for many decades. However, the use of strong acids has also led to major equipment failures, damaged heat exchangers, negative impacts on the reservoir and harmful environmental consequences. US trade patented ideas have also been developed and implemented to utilize the naturally occurring carbon dioxide present in the geothermal steam to produce a weaker carbonic acid to assist in the prevention of silica and metal silicate depositions (Weres, 2012). Periodic and Preventative maintenance programs vary from bi-annual to annual shutdowns in which heat exchangers and piping can be cleaned with high pressure water cleaning systems to the use of hydrofluoric (HF) and hydrochloric (HCL) acid descale recirculation programs. To reduce the impact of the use of the acid options and to extend out the periods of maintenance and the level of complexity involved with each maintenance outage, the development and use of silicate and silica inhibitors has been maturing substantially over the past 15 years. The development of the silica inhibitor has now been proven, in various geothermal fields over the past 10 years, to allow for the elimination for the need of strong acids, either in the role of pH modification or in the application as a descale chemistry. This paper will examine the silica scaling issues that have occurred over the decades at one geothermal field in Momotombo, Nicaragua. The Momotombo Power Corporation (MPC) currently operates a geothermal plant in which silica deposition problems presented themselves on a) the brine line coming off a single flash Separator at the Momotombo Power Station and b) in the binary power plant which was constructed in 2002 to utilize the latent energy remaining in the flashed brine. After a historical review of the scaling problems at Momotombo, this paper will then document the progress made in reducing silica scale formation by the use of scale inhibitors. |