Gas purification process in a geothermal power plant with total reinjection designed for the Larderello area

摘要

Many geothermal power plants require systems for steam washing and treatment of non-condensable gases (NGCs). Steam washing is an early-stage unit and the NCG treatment is performed before the reinjection. The paper describes the potential methods for those parts of the process and evaluates the approach of scrubbers and the associated models. The first model is developed for the steam washing scrubber, which eliminates the dissolved solids from the extracted steam. The energy analysis, steam loss, and the efficiency of removal are investigated and potential approaches to the H2S removal process are discussed. The second model is dedicated to the packed-bed acid-gas scrubber, which efficiently reduces the H2S content of NCGs before the reinjection into the reservoir. This model is a rate-based approach that considers all the reactions in parallel. Also, a comprehensive model for the sizing of the scrubbers is proposed. The case study is a binary geothermal power plant, designed for the area of Larderello, in which the extracted steam consists of 25 ppm dissolved solids. The power plant design includes the NCG reinjection system, having about 0.3% of H2S content. The design parameters are found to achieve solid removal efficiency higher than 90%, thus abating the dissolved solids in the steam to less than 3 ppm by the first scrubber. The performance of the selective H2S removal treatment is optimized by identifying the best operating parameters for the effective removal of the H2S and keep the CO2. The selectivity is calculated in various conditions considering recycle-ratio, caustic solution temperature, and caustic solution concentration. The configuration of the second scrubber is optimized with an H2S removal efficiency of about 99.96 percent, with a selectivity index around 10.