Performance analysis of geothermal turbine set for well-head power plants: case study of Olkaria, Kenya

Abstract

Globally, geothermal energy utilization for power generation has been ongoing for decades through conventional technologies. In 2012, Well-head technology was first utilized in Kenya’s Olkaria geothermal field. However, severe cases of erosion/corrosion and deposition/scaling were observed within the turbine set after hardly five years of operation. These defects greatly impacted on the plant’s performance by reducing its energy conversion efficiency and output. The main objective of this research was to investigate the factors affecting the performance of the turbines at Olkaria’s well-head power plants. Its specific objectives were to determine the composition of geothermal fluid, characterize the solid deposits in comparison with the turbine blade material, determine the causes of observed defects, determine the methods of eliminating the defects and to enhance performance of the turbines by implementing selected methods of eliminating the defects at Olkaria’s well-head power plants. Analyses of geothermal fluids at various sections of the plant were done using the potentiometric, AAS and spectrophotometric techniques to determine its composition. Analyses of solid deposits were done using XRD and XRF techniques. Blade sample analysis was done using Metal scan spectroscopy and XRF techniques. Literature review guided an inferential approach used to determine the causes of the defects. Manufacturing & maintenance standards were used to determine methods of eliminating the defects. The characteristics of geothermal fluid entering the turbine were found to be; pH (4.55), TDS (12.40 ppt), Conductivity (24.78 μs/cm), chloride ions (6.59 ppm), Sulphate ions (6.27 ppm), silica (2.71 ppm), Iron (1.56 mg/L) and Sodium (1.03 mg/L). The solid deposits on the turbine consisted of silica in form of SiO 2 (66.20%) iron, Fe (13.78%), K 2 O (9.08%), Sulphur (3.40%), chlorides (2.48%), P 2 O 5 (1.69%) , Calcium (1.23%), Barium (0.7%), Manganese (0.62%), titanium (0.39%), and Chromium (0.19%).The turbine blade material was characterized as an alloy steel with the highest composition being iron (Fe 82.64%), and an average chromium content of 12.50%. The pH value, chloride and sulphate ions in the fluid signify acidity and its highly corrosive nature. Significant amounts of oxides in deposits indicate oxidation reactions as the fluid interacts with the metals at elevated temperatures leading to deposition/scaling. As observed, the turbine blade was highly affected by corrosion. This was attributed to the parent material, 12.5%Cr steel alloy, having low resistance to corrosion under the operating conditions of the turbine. In conclusion, the results attributed the root cause of the defects to steam quality and blade material resistance to corrosion. Based on this conclusion, hard facing and machining techniques were determined as suitable methods of eliminating the defects. These methods were implemented to refurbish an affected turbine and in the process, a material composed of 23.9%Cr, 13.0% Ni, 1.8%Mn and 0.15%Mo (AWS A5.9:ER309L) was selected as a suitable overlay material for the repair in view of its fusion characteristics, toughness, tensile and creep strengths. The refurbishment resulted in improving the turbine’s output from 2.75 MW to its design rated output of 3.20 MW. Considering the same quantity of steam being consumed, the energy conversion efficiency was increased by 16.4%; hence, the turbine performance was enhanced. However, further research was recommended to investigate the impacts of hard facing and machining techniques on the life span of the turbine.