Optimization and control for CO₂ compression and purification unit in oxy-combustion power plants

High CO₂ purity products can be obtained from oxy-combustion power plants through a CO₂ CPU (compression and purification unit) based on phase separation method. To ensure that CPU (with double flash separators) can be operated under optimal conditions, this paper focuses on single variable analysis, multi-variable optimization, dynamic simulation and control system design for CPU in oxy-combustion power plants. It is found that optimal operating conditions are 30 bar, 30.42 °C, −24.64 °C, and −55 °C for multi-stage CO₂ compressor discharge pressure, flue gas temperature after compression, first flash separator temperature, and second flash separator temperature, respectively. The designed double temperature control structure based on a systematic top-down analysis and bottom-up design method is more suitable than the single temperature control structure under different operating scenarios (load change and flue gas composition ramp change). To bear operating disturbances, operating strategies like elevating temperature, manipulating valves and adjusting setpoints are proposed. Influence of SOx would be more obvious than that of NOx, whilst the k_ij mixing parameters in Peng–Robinson property method affects little on process optimization and control system design. Comprehensive dynamic model with specified control system provides possibility to integrate CPU with full-train oxy-combustion power plants. - Highlights: • Dynamic model of CO₂ compression and purification unit is established. • Double temperature control system for compression and purification unit is designed. • The designed control system is tested successfully under load change and disturbance. • Comprehensive dynamic behavior for key operating parameters is obtained. • Alternative control structure is also investigated and compared