Carbon dioxide is considered one of the main factors leading to global warming. Considering the significant impacts of CO2 on climate change, various technologies have been developed in recent decades to control carbon emission, such as CO2 capture and storage. The developed cycle of a negative CO2 emission power plant includes some devices, out of which, separator plays an indispensable role. To this end, T-junction separator is widely used as a phase separation component to separate two-phase flow because of its simple structure and low cost. Previous studies suggest that an increase in the number of T-junction branches is conducive to raise phase separation efficiency. In this paper, the numerical simulation in a single T-junction separator is compared with the predicted values generated by experimental models based on air-water. Then, CO2 has been replaced with air and CO2-water in double T-junction separator have been scrutinized. In addition, the pressure distribution and phase separation performance of two-phases flow of CO2-Water in horizontal double T-junctions are studied. Keywords: Numerical simulation, double T-junction separator, Power plant, Two phases flow, CCS