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It is generally recognized that corrosion and scaling problems could seriously affect the operation and electric power production from a geothermal power plant. The Electric Power Research Institute (EPRI) has sponsored a study at Battelle-Northwest (BNW) to develop a brine chemistry data base and analytical tools to analyze how corrosion and scaling affect the degradation of the power output of a geothermal plant. The GEOSCALE computer model is a steady-state thermal hydraulics code that describes the process parameters of the power plant. At present the multistage flash and binary cycle plants are being analyzed. Initially the code computes the power output from a given geothermal brine flow and provides flow rates, temperature, velocities at points from the bottom of the production wells through the plant to the waste injection system. Based on the starting brine chemistry and these process parameters, corrosion and scaling rates will be estimated at points throughout the system. The amount of scale formation in a time interval will be calculated and the impact on brine flows and heat transfer calculated, resulting in a new set of plant process parameters for the next iteration. The iterations continue until some portion of the plant is degraded to a process limit or a plant life of 20 to 30 years is reached. Obviously the most difficult part of this analysis is the lack of valid analytical expressions and supporting rate data to calculate scaling. The general approach to the scaling rate equations is that the rate of buildup is proportional to the degree of insolubility of a mineral minus the rate of mechanical removal. They are very interested in all current scaling work that could help in providing scaling kinetics data related to process parameters so they can test these analytical expressions. The program includes a computer subroutine for calculating mineral insolubilities as brines cool, a chemical and structural analysis of several actual scale samples and a laboratory experimental program to examine scaling kinetics.
This paper summarizes results of recent analyses performed by Battelle-Northwest in EPRI project RP 653-3: Computer Simulation of Scaling in Geothermal Systems. The results reported here are drawn primarily from case evaluations performed over the 12 months since the preceding EPRI Geothermal Symposium held in Monterey in June 1979. The present project is a continuation of a previous project designated RP 653-1. The ultimate objective of research performed in this project is to develop analytical tools (computer codes) and the supporting thermophysical and chemical data base that can be used to predict scaling and corrosion in geothermal power generating systems.