이산화탄소 포집 공정을 포함한 발전 및 담수화 공정의 성능 예측에 대한 수치적 연구

Abstract

Climate change means to a significant change in the observations of climate distribution lasting over an extended period of time (from decades to millions of years). According to the Physical Science Basis report of the IPCC, the climate change proceeds more quickly than expected and the observations, such as the increase in the global average surface temperature, the global average sea-level rise and the decrease in the northern hemisphere snow cover, are reported as the evidence of climate change. The increase in greenhouse gases, such as CO2, CH4 and NOx, to the atmosphere is regarded as the main cause of climate change. Among these greenhouse gases, the increase in CO2 plays the largest role in the global warming. Climate change affects not only the global warming but also the global water trends. According to the report published by IPCC, the main cause of the water shortage is the population growth and it is expected that the world population is increased to about 8 billion by 2030 and over 9 billion by 2050.

To solve these problems such as the global warming and the water shortage, the many studies on the carbon capture and storage technology and the desalination technology have been performed by many researchers, while most of these studies have focused on each technology separately. Therefore, in this work, the power and water cogeneration process with CO2 capture was selected as the analysis target to suggest the basic design data for the multi-objective plant. Especially, NGCC, VMD and CO2 absorption processes were used for generating the electricity, producing the fresh water and removing CO2, and then, were integrated. As a result, the performance of power and water cogeneration process with CO2 capture was improved.

The performances of NGCC and CO2 absorption processes were conducted by using Aspen+ 7.3, and VMD process was simulated by using in-house code developed in this work. Then, analysis models of three processes were validated by comparing the present simulation results with experimental or simulation data in the previous literature. As a result, these analysis models were in good agreement with the previous literatures.

For the performance evaluation of power and water cogeneration process with CO2 capture, each unit process was integrated and simulated as following procedures. (1) Integration of NGCC and VMD processes (2) Integration of NGCC and CO2 absorption processes (3) Integration of NGCC, VMD and CO2 absorption processes (4) Integration of gas turbine, VMD and CO2 absorption processes

In each integrated process, the steam extracted from LP turbine was used as the heat source for CO2 absorption processes. In VMD process, not only the steam discharged from LP turbine but also the waste heat in the exhaust gas was used as the heat source for heating the feed seawater. As a result, the loss of thermal energy in the integrated process was minimized. In addition, in order to generate the steams supplied to VMD and CO2 absorption process, the process flow diagram and the operating conditions of NGCC process were modified. In NGCC process with VMD process, four types of VMD processes with respect to the heat recovery, the brine recycle and the vacuum pressure profile were considered. In addition, for maximizing GOR of VMD process, the vacuum pressure profile was optimized by using micro-GA algorithm. Then, the performance of integrated process was analyzed with respect to the split fraction of steam supplied to VMD process. As a result, the maximum total water production of 236.4 kg/s and the minimum net output power of 380.7 MW were achieved in this integrated process.

In NGCC process with CO2 absorption process, the size of absorption and stripping columns was determined to satisfy the design specifications of CO2 absorption process. Then, the performance of integrated process was calculated with respect to the CO2 removal efficiency. As a result, the net output power of 399.4 MW was achieved at the CO2 removal efficiency of about 90%.

Next, the performance prediction of NGCC process with VMD and CO2 absorption processes was conducted based on the simulation results for the above integrated processes. In this integrated process, the net output power of 378.9~387.7 MW was achieved at the CO2 removal efficiency of about 70~97%. In contrast, the total water production was decreased to about 99.0~138.2 kg/s because about 66% of the steam generated in NGCC process was supplied to CO2 absorption process.

In addition, the gas turbine process was integrated with VMD and CO2 absorption process to increase the total water production of VMD process. In this integrated process, most of the steam generated in HRSG was supplied to VMD process and CO2 absorption. As a result, the total water production of 183.1~220.3 kg/s was achieved at the CO2 removal efficiency of about 70~97 %. In contrast, the net output power was decreased to 285.7 MW.

Finally, the costs of electricity and water in the integrated processes were estimated based on the simulation results in this work and the results in the previous literature. In case of without CO2 absorption process, the integrated process of NGCC and VMD processes was economical at LCOW over 4.94 $/tonne. In case of with CO2 absorption process, LCOW of the integrated process of 3.34 $/tonne was achieved. Nevertheless, LCOW obtained in this work was still higher than that in the conventional desalination processes. Therefore, for commercializing VMD process, the development of hydrophobic membrane to enhance the water flux and the optimization of VMD are required.