Flexible Operation of Thermal Power Plants with Moving Bed Temperature Swing Adsorption Post-Combustion CO2 Capture

Abstract

To meet our climate targets, global CO2 emissions must be significantly reduced in the coming decades. The power sector is still dominated by fossil fuel-based thermal power plants and stands for around 42% of anthropogenic CO2 emissions. This sector will therefore play an important role in the upcoming transition. One solution for decarbonizing the power sector is carbon capture and storage, which in the form of post-combustion CO2 capture can be retrofitted to existing plants without influencing the core process. Due to increasing shares of intermittent renewable energy sources in the electricity mix, the flexible operation of thermal power plants is expected to be necessary at least in the short to medium term to help balance supply and demand. As a result, the variable operation of thermal power plants with post-combustion carbon capture might play a role in the future energy system. The moving bed temperature-swing adsorption (MBTSA) technology is currently under development and has shown promise for several post-combustion CO2 capture applications. However, the dynamic operation of the entire MBTSA process had not been investigated in the literature prior to this Ph.D project. To address this knowledge gap, the main goal of this work was to study the flexible operation of coal-fired power plants with MBTSA post-combustion CO2 capture. Two main flexibility methods were studied in this project. Firstly, control strategies with the goal of maintaining the performance of the MBTSA process during changes in the power plant operation were developed and eval uated. Secondly, the integration of a steam accumulator thermal energy storage unit into the power plant – carbon capture process was considered. Increasing the maximum load of the power plant and enabling it to rapidly change its load were the primary flexibility modes studied in this work. Two adsorbent materials, activated carbon and Zeolite 13X, and two different power plant scales were investigated. The first main contribution of this thesis was the development and implementation of a control framework for the MBTSA process. The framework consisted of five controlled and manipulated variable pairs in a decentralized structure that was divided into a regulatory and higher-level layer. The regulatory layer included a controller keeping the fraction of heating and cooling delivered by the internal heat recovery loop stable, a controller for the sorbent temperature leaving the cooling section and a controller for the gas velocity at the top of the precooling section. In the higher-level layer, the sorbent flow rate was used to control the CO2 recovery and the heating fluid flow to the desorption section was used to control either the CO2 product purity or the regenerated sorbent temperature. The control framework was added to a mathematical model of the MBTSA process and both the open and closed-loop dynamic responses were studied. A range of scenarios were simulated, including step changes in the incoming flue gas flow rate, ramps in power plant load, setpoint changes for higher-level control variables, variations in flue gas feed CO2 concentration and variations in the external heat source temperature. The simulations showed that the developed control framework was able to maintain the performance of the MBTSA pro cess for power plant-driven flexibility scenarios and changes in the operation of the PCC process. The second main contribution of this thesis was the comparison of several different al ternatives for higher-level control of the MBTSA process. Initially, four different control strategies were investigated: a baseline structure with proportional-integral control of both the CO2 recovery and purity, an option with proportional-integral control of CO2 purity and feedforward control of the sorbent flow rate, a structure with feedforward control of both the sorbent flow rate and heating fluid velocity to the desorption section, and a case with regular proportional-integral control of the CO2 recovery and a cascade con troller for the CO2 purity. Due to imperfect ratio adjustment in the feedforward controllers with system load, steady-state offsets from the control variable setpoints were observed. Furthermore, aggressive tuning of the feedback controllers caused oscillations when the power plant load was reduced. To address these limitations and improve the controller performance, two enhanced single-loop control structures were implemented. By adaptively adjusting the controller tuning parameters (gain and integral time) with the system load, no oscillations were observed and tighter control of the CO2 recovery compared to the standard control structure was achieved. A proportional-integral controller was combined with the feedforward control structure to adjust the ratio based on the control error instead of a parametric relation. This eliminated the steady-state offsets and lead to closer control of the CO2 recovery rate. When testing the enhanced single-loop control structures, the effect of measurement delays were included. Such delays were found to have a large effect on the relative performance of the investigated control strategies. The third main contribution of this Ph.D project was a study of how the integration of a steam accumulator thermal energy storage unit could increase the flexibility of the power plant – carbon capture system. A dynamic process model of the steam accumulator was implemented and validated with both experimental and simulation data from the literature. Combined with a steady-state power plant model, simulations were carried out to quantify how charging and discharging the thermal energy storage affected the net electrical power output of the power plant. Charging the accumulator with reheat steam from the power plant could reduce the net power output by up to 1.4 % for around 200 minutes. Two al ternatives for discharging of the thermal energy storage were considered, namely covering the regeneration duty of the MBTSA process and meeting the demand of two feedwater heaters. Discharging was found to give relative power plant load increases between 1.7 and 11.2% for up to 37.5 minutes, which exceeds the requirement for primary reserve. Sending steam from the accumulator directly to the MBTSA process could increase the net electrical power output by almost 67 MW for a period of 3.2 minutes. An advantage of using a thermal energy storage system to provide flexibility is that the resulting load changes take place without modifying the boiler load or reducing the CO2 recovery rate.