Integration and synthesis of heat and mass exchanger networks for CO2 capture in power plants

Abstract

CO2 capture and storage (CCS) have been identified as a promising technology that could allow for the continual use of coal in power generation, and yet maintain a near-zero emission of CO2 into the atmosphere. However, the major challenge facing the retrofitting of CO2 capture systems to existing power plants is that most CCS techniques are material and energy-intensive. This subsequently reduces the operational efficiency of the power plants and also increases the energy penalty, as well as operational cost. To date, the blending of sorbents with inhibitors such as piperazine (PZ) solvents, amine-2-amino-2-methyl-1-propanol (AMP), the use of phase change materials, amine scrubbing and the use of external utilities such as steam and cooling water have been the common strategy to minimize energy consumption during CO2 capture in power plants. In this thesis, a technique to minimize the high energy and material requirement during absorptive and adsorptive CO2 capture in coal-fired power plants was proposed, developed and tested. The principles of heat and mass integration were employed in this study to minimize energy consumption and resource usage during CO2 capture in power plants. Heat exchanger networks (HENs) were synthesized in this study to address energy minimization while the application of mass exchanger networks (MENs) were introduced to address the excessive consumption of resources and utilities such as cooling water, steam, sorbents etc during CO2 capture. Furthermore, a systematic method for the synthesis of combined heat and mass exchanger networks (CHAMENs) with regeneration was developed and tested in this thesis to simultaneously minimize the use of energy and mass during CO2 capture considering sorbent regeneration. Also, to ensure the cost-effectiveness of the MENs, a technique was presented to target the minimum mass separating agents (sorbent) as well as the capital costs for the heat and mass exchanger networks. The method used was simple and based on insight, rather than relying on a mathematical 'black-box' approach. Graphical tools new to CCS studies, such as the y-x and the y-y* composite curve plots were introduced to allow the minimum exchanger sizes to be determined before network design. Capital cost targets were traded off against the established operating cost targets to optimize the total cost first with no design being necessary. The total annualized cost (TAC) for the network which includes the cost of mass separating agents, hot v and cold utilities, was minimized using an objective function to reduce process cost. To account for fluctuations in operating parameters during absorptive CO2 capture, a simultaneous (mathematical programming) approach was used to synthesize optimal heat exchanger networks (HEN) with non-isothermal mixing and fluctuating parameters, while minimum utility cost for the CO2 absorption process was determined at selected parameter points. The HENs synthesis procedure presented in this thesis took into consideration quantified uncertainties in inlet temperatures and flow rates to address the major shortcomings observed in previously reported methodologies. Area targeting of heat exchangers was investigated in this study to determine the capital cost of the synthesized multi-period network. Where a simultaneous synthesis technique was used, a multi-period MINLP model was developed to generate a HEN with optimized heat exchanger areas and total annualized costs while the concept of pinch analysis was used for the sequential approach. A linear programming technique was then used to synthesize an effective transport network for captured CO2 from power plants in different locations using a hypothetical case study. The synthesized CHAMENs network confirmed $ 0.1998 million/yr as the estimated total annualized cost accruable if combined network with regeneration is integrated for CCS as suggested in this research. The outcome of this study further revealed that about 19.1 % of sorbent used for adsorptive CO2 capture could be saved by integrating mass exchanger networks in CCS studies. Besides, this study also discovered that about 25-30 % of the energy used during CO2 separation could be saved by integrating the heat exchanger networks developed and described in this thesis. Conclusively, this study established that the application of heat exchanger network (HEN) and mass exchanger network (MEN) to minimize energy and material consumption during CO2 capture is unique and effective in CO2 capture systems where solid and liquid sorbents are used, compared to existing methods. Interestingly, the techniques proposed in this study which are relatively new to CCS can be fully extended to other CO2 capture techniques after minor modifications. The major shortcoming identified in this study is the generation of some process data, hence scientific assumptions had to be made in such cases.