Investigation of Water Requirement Reduction in Natural Gas Combined Cycle Power Plants Equipped with Carbon Capture System

Abstract AbstractReducing water requirements in thermoelectric power plant is a major concern specifically with increasing global warming negative impact on the environment. Furthermore, adding post-combusting-carbon capture unit (PCCC) doubles the amount of water usage through cooling system as has been reported in the literature. Natural gas combined cycle (NGCC) is one of the solutions to reduce water requirements and CO2 emission to the half of its amount in the coal-fired power plant. In the present Ph.D. study, two novel investigations have been performed to decrease water curtailment in the cooling system and its impact on plant performance. The first investigation is involved studying the effect of using four different hybrid cooling system configurations on water requirements and power penalty of a NGCC power plant equipped with (PCCC). The based 630 MWe power plant has been validated with NETL report 2015 and the validation has been performed properly for both cases with and without (PCCC). The configurations include an ACC (Air Contact Cooler)-CT (Cooling Tower) in parallel hybrid cooling system (ACTD), a ACC-CT in series hybrid cooling system (ACTS), direct ACC-wet cooling in parallel hybrid system (DACW), and an indirect air-CT parallel hybrid cooling system (IDACT). It has been shown that IDACT design has the better performance in term of water saving and power penalty reducing when PCCC is not equipped with the plant while with integrating PCCC, IDACT is the best in term of penalty reducing while water usage amount is not the lowest between all the proposed hybrid systems. IDACT design performance has been compared with all the other conventional cooling systems; once-through, closed loop, dry direct and indirect dry cooling systems for both cases with and without PCCC. One interesting finding is that integrating PCCC adds small amount of water to the water system by condensing in the direct contact cooler (DCC) of the gas fuel cooling system before the absorber and in the CO2 condenser after the stripper by which the net water usage in the plant could be reduced.The second investigation of this Ph.D. work is related to develop an optimization model to reduce water requirements in once-through and hybrid indirect dry and wet cooling systems of a NGCC power plant equipped with PCCC. For the once-through cooling system, condition data of Catamaran Brook river, Canada, 1992 have been implemented in the model to calculate the initial and the reduced optimized mass flow rate while for the hybrid cooling system, sensitivity analysis and parametric study have been performed as a bases for the optimization model. Air to Water ratio, humidity content, air ambient wet bulb temperature, and cycles of concentrations at three different wet system cooling load split factors have been studied regarding their effect on water withdrawal and consumption. The three wet cooling system split factors are 40%, 60%, and 80%. When PCCC is being integrated, capture rate and reboiler duty effect on plant net condenser duty and power penalty have been investigated. Results show that increasing air to water ratio increases water withdrawal and consumption in the cooling system exponentially while increasing wet bulb temperature and humidity content reduces water withdrawal and consumption in the cooling system almost linearly. Water usage is changing adversely with cooling tower outlet temperature meaning that the change in water withdrawal and consumption leads to an opposite change in tower outlet temperature. An interesting finding is that after wet system cooling load split factor be equated to 60%, there would be no significant change in the water usage amounts. In addition, as number of cycles of concentrations change adversely and exponentially with water requirements, after number of cycles of concentration be equated to (5-6) cycles, the reduction in water withdrawal and consumption is inconsiderable. When PCCC is being integrated, it has been found that reboiler duty and carbon capture rate affect directly the plant net cooling duty and power penalty as being expected.