Ciclo higroscópicoestudio experimental de la tecnología y análisis de sus ventajas respecto a un ciclo rankie

Abstract

ABSTRACT The Hygroscopic cycle is a thermodynamic cycle characterized by working with a rich in hygroscopic compounds cooling reflux current, absorbing and condensing the exhaust steam, coming from the steam turbine of a Rankine cycle or from any other process in a steam absorber. The condensing temperature, at the outlet of the latter, is always higher than the temperature of the pure steam entering it for the same pressure. As the mass concentration in hygroscopic compounds is increased, condensing temperature increases as well, the mixture being able to reach differences higher than 15 ºC. It is the only existing technology capable of condensing pure steam at a cooling temperature higher than its saturation temperature of mentioned steam. In this technology there is a thermal and chemical recovery of the boiler blowdowns, which are rich in hygroscopic compounds. In this case, a solution of lithium bromide (LiBr) in water has been used. The Hygroscopic cycle allows to reduce the condensing pressures of the steam turbine output for the same cooling temperature, or to increase the cooling temperatures for the same condensing pressure. Production gains are, for example, an increase in gross electrical efficiency of 6.50% and a net electrical efficiency of 5.30% compared to a Rankine cycle. Optimum mass concentrations of LiBr in water have been found to be of 45% at the cooling reflux current, and 50% at the boiler blowdowns. The Hygroscopic cycle is designed with dry coolers in order to dissipate the condensation energy of the steam in dry mode, thus reducing the electrical consumption of this equipment according to ambient temperatures, and eliminating the consumption of cooling water. The incorporation of LiBr makes it possible to reduce the average annual electric self-consumption of the dry coolers by almost 4 times, thus increasing the net electrical efficiency of the Hygroscopic cycle. Another interesting point that will be discussed in this doctoral thesis is the behavior of the filming and neutralizing amines used to protect the metallurgy of the system from corrosion, including the tubular copper bundle of the dry coolers. It has been concluded that the protection with aforementioned compounds depends on the electrical conductivity of the cooling reflux current, as a stability band was measured comprising a range of pH values where the corrosion rate is zero or reaches a minimum value. Outside this range, the corrosion rate increases as the pH value moves away from these values, with higher corrosion at higher pH of this band. The basic equations are identified to optimally define the mass and energy balances of a Hygroscopic cycle, as well as the two most important parameters in the design of the steam absorber, which are the diameter and length of the contact zone. These parameters have been reduced to the steam mass flow rate and cooling reflux mass flow rate. In addition, the recommended speed of the live steam at the droplet separator was studied to be less than 1.5 m/s in order to guarantee the maximum tolerable electrical conductivities at steam turbines. The results have been corroborated experimentally by means of a test plant with the capacity to generate 110 kg/h of live steam. The plant was operated with water qualities ranging from demineralized water (with electrical conductivities lower than 2 μS/cm), to a solution of LiBr in water with mass concentration ranging from 45 to 65% at the cooling reflux current. Also, are described the advantages found once the Hygroscopic cycle technology is implemented to the biomass power plant of 12.5 MWe of Vetejar (Córdoba), which suffers lack of water and poor quality of it. The improvements that this technology provides are justified by the 13 ºC increase of the cooling temperature at this site, while maintaining the same condensing pressures at turbine outlet. This allows to increase net electric performance of the facility, reducing electric self-consumption and eliminate the annual consumption of cooling water. This last point has allowed to increase the availability of the installation, and with it the electricity output per year. This technology can be incorporated to any industrial process that condenses steam from different processes, mainly in power plants that use a Rankine cycle, obtaining the maximum gross and net electrical performance, and without the need to consume cooling water.