It is important for power plants to be able to operate flexibly to respond to changes in consumer demand for electricity. Flexibility is also becoming increasingly important due to the greater use of other low carbon generation technologies, particularly variable renewable generators. The issue of operating flexibility of power plants with carbon capture and storage (CCS) has been the subject of a previous technical study by IEAGHG ("Operating flexibility of power plants with CCS, IEAGHG report 2012/6, June 2012", see here: http://www.ieaghg.org/docs/General_Docs/Reports/2012-06%20Reduced.pdf). The new report contributes to the knowledge base on flexible operation of power plants with CO2 capture by focusing on process control issues.
A team from Imperial College London and Process Systems Enterprise has undertaken this work for IEAGHG.
The study focuses on performing an evaluation of process control strategies for normal, flexible and upset operation conditions of CO2 post-combustion capture (PCC) processes based on solvent scrubbing. PCC is currently the leading near-term technology for large-scale deployment of CO2 capture in the power generation sector. The aim of this study was to develop process control strategies PCC, to select appropriate control variables, and to design efficient control structures for operation with minimum energy requirements and costs for both pulverised coal (PCPP) and combined cycle gas turbine (CCGT) power plants.
The key messages from the report are:
- Electricity market models suggest power plants with CCS will need to adopt flexible operation in the future. Appropriate control strategies will be necessary to ensure their ability to operate in such a market and their profitability.
- An evaluation of process control strategies for normal, flexible and upset conditions of PCC processes (considered the leading technology for deployment in the power sector) based on amine scrubbing has been undertaken.
- This work used a high-fidelity modelling tool that can describe the dynamic operation of the CCS chain to investigate 3 different process control strategies for both PCPP and CCGT, each with PCC.
- The power plant modelling showed the performance of the CO2 capture unit can be maintained even during periods of significant load fluctuation, using industry standard control techniques, thus avoiding other more expensive solutions.
- Manipulating the solvent flow rate generally provided better control of the CO2 capture rate than varying the solvent lean loading, as it results in less oscillation, i.e. more constant hydraulic conditions in the CO2 capture plant.
- For the PCPP, a control strategy that manipulates the CO2 capture rate by varying the solvent flowrate is the more profitable option. For the CCGT, all strategies provided the same benefit, due to the dilute nature of the CCGT flue gas.
- The CO2 capture plant was able to continue operation for a limited amount of time, i.e. 3.5-5 hours, in case of hazardous events, such as injection shutdown or loss of compression.
- In conclusion, this study has shown that simple and well-tuned control strategies can maintain critical operational parameters of a CO2 capture plant.