A thermodynamic and economic modelling study of recovering heat from MSF desalination cogeneration plant

Abstract

This study focuses on an actual cogeneration power and MSF desalination plant and models it, analyses it, and proposes enhancements to MSF desalination at different, real operating scenarios. Based on actual data gathered from the plant for a full operating year, the study has identified the major operating scenarios of this cogeneration plant due to seasonal change to provide a real basis for assessing thermal, economic and environmental performance. It is difficult to standardize thermal evaluation of such systems because the net products, electrical power and water, are different in quality. Exergy analysis has achieved worldwide acceptance for thermal system assessment but no study was found in the literature that addressed the evaluation of power and MSF desalination together using exergy analysis. This thesis, therefore, makes an original contribution to this issue in three areas. Firstly, as simulation is the only practical approach to investigate enhancements to complex plants, the simulation models developed for the power and water desalination plant have been validated against actual operating data to substantiate the credibility of this approach. For the power plant model, validation against actual plant data at the three operating scenarios gave differences between the model and actual data varying from 1.0% to 3.7%. The MSF desalination system was modelled and validated against vendor testing data with the highest difference of 3.9%. Secondly, while previously both power production and desalination have been evaluated separately using the exergy approach, this study has applied it in a standardized approach to a specific cogeneration power and water desalination plant, including exergy analysis of the MSF desalination in detail that has not been found in the literature. It has been shown that the specific coupling of MSF desalination with a combined power plant is not a preferable option for thermal performance, which is contrary to the previous studies using Heat Utilization Factor as a performance indicator. The simulation was used to carry out a pioneer attempt of detailed energy and exergy analysis using the latest published thermodynamics properties, assuming that seawater solution is not an ideal solution (assumed in previous studies). Extraction of the hot distillate water from MSF up to stage 8 could enhance exergy efficiency to 14%. Extraction of hot distillate water from MSF stages was found to increase the unit water production up to 2%. Further, utilizing the hot water to heat up the make-up seawater flow through an Internal Heating (IH) caused an increase of brine recirculation temperature and reduced the powering steam by 5% and therefore reduces natural gas consumption and CO2 emissions by 57000 tonnes. Implementation of this modification has a one-year payback period. Thirdly, this study has, for the first time, studied the recovery of low-grade heat from MSF hot distillate water to enhance power or water production through the Absorption Chiller (AC), the Organic Rankine Cycle (ORC), and the Single Effect Desalination (SED). There appears to be no literature exploring MSF hot distillates to power AC to cool the gas turbine inlet by AC or dedicated SED (though previous studies have investigated steam powered MED). The temperature of these hot distillate stages was between 65ºC and 100ºC, suitable for low-grade heat recovery technologies and it was confirmed that utilizing part of the heat up to 10ºC temperature difference in the AC, ORC, and SED and reconnected back to IH had no adverse impact on the original MSF performance.Utilizing the heat to produce cooling from a single effect H2O/LiBr AC, the produced cooling load could be used to cool down the gas turbine inlet temperature to augment the electrical power generation. The AC was modelled and validated against manufacturer data. Reducing the GT inlet temperature by AC cooling increased the cogeneration plant electrical power production by 3.8% for every 5ºC reduction, with CO2 emissions reduced by 29000 tonnes and a 2.4 year payback period to implement such a modification. An ORC unit was modelled and validated against an existing plant. From both energy and exergy aspects, it was found that R245fa performs better as a working fluid than R134a in this application. Annually this option could increase plant power generation by 9000 MWh and reduce CO2 emissions by 13000 tonne. The economic assessment of this option showed the payback period was the highest at 5.2 years. Powering of hot water SED from hot MSF distillate water was the fourth heat recovery option studied (for the first time). The SED was modelled and validated against manufacturer published data with a 3.2% difference. The SED was able to produce 240000 tonne/year of water. This hybridization saved 11000 tonnes/year in CO2 emissions. The implementation of the modification has a 1.8 years payback period.