Analyses and dynamic modelling of the compressor section in a PCC-process for coal-fired power plants with offshore storage

Abstract

Carbon dioxide (CO2) is one of the main greenhouse gases that contributes to the current climate change. Its increasing share in the Earth atmosphere leads to an increase in global temperature. To prevent the sea level from rising and increased severity of extreme weather, many governments are now trying to reduce their carbon footprint. One third of the global CO2 emissions are produced by fossil-fuelled power plants. Carbon Capture and Storage (CCS) is considered one of the leading technologies for reducing these emissions. This thesis is part of investigating the feasibility of the ROAD 2020 project. This project will be one of the worlds first large-scale Monoethanolamine (MEA)-based Post Combustion Capture (PCC) demonstration project with offshore storage. The power plant in this project is designed so that it can rapidly change between full and part load operation. The combination of the resulting fluctuating flow and offshore storage makes compressing the CO2 a challenging problem. An integrally geared centrifugal compressor train seems the most suited compressor system for a PCC process. However, their operational limitation is their short operating range for compressing CO2 at high pressures. If not properly controlled, this compressor train can significantly reduce the capture unit efficiency. Moreover, possible flow instabilities like surge, two phase flow and hydrate formation could inflict unrepairable damage to the systems in the PCC process. This thesis investigates the dynamic performance of the integrally geared centrifugal compressor train for a PCC process retrofitted to a coal-fired power plant. The dynamic performance of the compressor train is evaluated for different process configurations which include different compressor control strategies, addition of a well control valve and a single and double compressor train configuration. The figures of merit used to assess the dynamic performance are: compressor work, impact on stripper pressure, occurrence of surge and choke, occurrence of two phase flow. To evaluate these figures of merit a dynamic model of the integrally geared centrifugal compressor train has been successfully developed. The model also includes the transportation pipeline and offshore storage well. The open source Modelica language is used for the development of the dynamic model. The simulations performed with this model give more insight in the performance of the compressor train and its impact on the CO2 capture process, transportation pipeline and storage well. Recycle valves seem the most suited solution for the compressor train to extend its operating range during minimum load of the power plant. Simulations show that all evaluated process configurations meet the required performance and guarantee a stable filling of the offshore storage well. However, each configuration has a different power consumption, possible process complications and impact on the process capital and operational costs. To propose an optimal process configuration for the ROAD project a further techno-economic investigation is required. Two future research directions can be distinguished to obtain this goal. The first direction should focus on extending the developed dynamic model with a capture unit model. The second direction should focus on the technology gaps that make the integration of MEA-based PCC processes with offshore storage challenging.