On the Usage of Saline Water in Spray Assisted Natural Draft Dry Cooling Towers

Abstract

Cooling towers are used to reject heat to the surrounding air in power plants. In thermal power plants the hot temperature of the cycle reaches up to 600-700 oC. As power plant thermal efficiency is limited by the Carnot efficiency (1-Tc/Th), it is important to minimize the heat rejection temperature (Tc) to increase total efficiency. When water is available, wet cooling towers are a good solution. However due to water scarcity and reduced energy consumption during their lifetime, dry cooling towers are becoming the preferred alternative. In dry cooling towers, air is either pushed through the heat exchangers using fans or using the chimney effect. Dry cooling relies on air-dry bulb temperature, which can affect power generation during hot summer days. The efficiency losses during hot days (high Tc) can be mitigated by spray cooling the inlet air. This reduces the air temperature to the wet-bulb temperature, thereby increasing the dry cooling tower efficiency significantly.Industry currently uses fresh water for hybrid cooling, which is expensive and can lead to environmental issues. An alternative is to use saline water or water containing dissolved solids that is created as an industrial by-product. This work focuses on coal seam gas (CSG) water that is produced during the production of natural gas from coal-bed methane and is readily available in arid regions of Queensland.To investigate saline water spray this project is divided into three parts: analysis of single droplet evaporation, investigation of saline water spraying, and performance evaluation between saline water and pure water sprays.To investigate the evaporation from single droplets, an improved four-stage model is presented. This captures the size, temperature, and evaporation rate of solid containing droplets more accurately than those in literature. This new model includes an additional stage that models the transient formation of the crust which starts from the bottom until the entire droplet is enclosed. Based on the wide-ranged experimental tests in this study, a comparison of evaporation potential (ability to absorb energy) of pure and saline droplets was performed. This showed that saline droplet consume less energy per unit mass (7.3% for 3% salinity) due to a portion of the weight being occupied by dissolved solids.After understanding the evaporation process of a single saline water droplet, a lower order numerical model to simulate saline droplets in a spray was developed. This model uses a standard multicomponent droplet model, which is fine tuned using data obtained from observing NaCl crystals using scanning electron microscope, to set correct evaporationproperties and to create the correct solid particles. The resulting modelling framework was used to analyse the effect of saline water on spray cooling. The sensitivity study showed that initial droplets size and water mass flow rate have the largest impact on the minimum distance between nozzle and heat exchangers (wet length) and cooling efficiency. The wet length ranged between 4.3 m and 5.25 m. By performing a non-dimensional analysis, two dimensionless correlations for wet length and cooling efficiency were created. A laboratory test was performed to partially validate the new numerical model for saline water spraying.Finally to explore differences between pure and saline water in cooling towers, sprays in a cooling tower representative geometry are explored. It is shown that in an upwards flow with a nozzle spraying in the co-flow direction, close to the nozzle more cooling is achieved by saline water (8% improve in cooling efficiency compared to pure water). The principle behind this is explained. In addition, wet length for saline water spray is up to 30% shorter than that of pure water spray. Moreover, it was shown that an efficient nozzle arrangement achieves 2.91% higher cooling efficiency.This thesis has developed an improved understanding of the fundamentals of saline water droplet evaporation and new insight in to saline water spray cooling for spray assisted natural draft cooling towers. The work shows that in addition to pure water preservation and cost savings (water purification costs), the use of saline water can also lead to improved cooling efficiency and size reduction of cooling towers.