Post-combustion carbon capture goals:
Post-combustion carbon capture refers to capturing carbon dioxide from flue gas after the fossil fuel (e.g., coal, natural gas, or oil) is burned with air in a boiler producing steam to rotate a turbine generator and produce electricity. National Energy Technology Laboratory (NETL), a national laboratory under Department of Energy (DOE), states that coal will continue to play a critical role in powering the Nation’s electricity generation for the foreseeable future, especially for base-load power plants . The biggest challenge in post-combustion capture is separating CO2 generated during combustion from the large amounts of nitrogen (from air) found in the flue gas. The research and development (R&D) effort in post-combustion carbon capture is mainly focused on advanced solvents, solid sorbents, and membrane systems. Solvent-based CO2 capture technology is the most mature technology that involves absorption of CO2 from flue gas into a solvent. Even though high levels of CO2 capture are possible with commercially available chemical solvent-based systems, these systems require significant amounts of energy for regeneration.
In the United States, NETL has been leading many research projects along with industry partners that focus on the development of durable, low-cost, resistant to flue gas impurities, non-corrosive solvents that can effectively capture CO2 using as little energy as possible (e.g., lower regeneration energy requirement than existing amine systems). The focus is also on computational simulation, fundamental research, technology development, large-scale testing, and seamless integration of these new capture processes, equipment, and designs with balance-of-plant operating systems. DOE’s carbon capture program set the goal to develop a transformational CO2 capture technology for a coal-fired power plant with 95 percent CO2 purity at approximately $30 per tonne of CO2 captured . Carbon capture R&D can increase new coal generation capacity by 45 GW, create an additional 1.9 million jobs and add over $200 billion to GDP between now and 2040.
Solvent-based CO2 capture involves chemical or physical absorption of CO2 from flue gas into a liquid carrier. After the absorption, the used or CO2-rich solvent is regenerated by increasing its temperature or reducing its pressure to break the absorbent-CO2 bond. To model such process, a comprehensive thermodynamic model that represents phase and chemical equilibria as well as thermal and volumetric properties of the mixed system is required.
30 wt.% or less monoethanolamine (MEA) solution is generally considered a benchmark solvent for post-combustion solvent-based carbon capture process. MEA has a high rate of reaction with CO2 but requires high regeneration energy (3.2-4.2 GJ/tonne CO2 captured). The host power plant after the MEA-based carbon capture plant is integrated could be de-rated by more than one-third of its capacity . MEA solution entails high solvent circulation rate which leads to large equipment sizes and high energy consumption. CO2-loaded MEA solution is also very corrosive and degrades rapidly. These drawbacks of MEA have been addressed through development of new and transformational solvents which include mixed amine solvents such as mixtures of MEA and MDEA (methyldiethanolamine), AMP (aminomethyl propanol) and PZ (piperazine) among others, ammonia-based solvents, amino acid solvent, biphasic solvents and ionic liquid-based solvents.
The computational simulation and process design starts with defining the underlying chemistry of the entire capture process from flue gas intake to product CO2 transport over the operating ranges and beyond. The chemistry between the flue gas constituents (e.g., CO2, N2, O2, nitrogen oxides, sulfur oxides, carbon monoxide, etc.) and transformational/traditional solvents could be quite complex. A comprehensive thermodynamic model and complete database of species for the system is prerequisite for process design and simulation. Otherwise, poor quality input will always produce faulty output.
Process optimization challenges:
The capture process must be optimized; the energy and utilities required to operate a capture process come from the host power plant, and a suboptimal capture process will overburden the host power plant. At the same time, the per kilowatt-hour cost for electricity generation would go higher. Like any other processes, the optimization requirements for a capture process is multifold. The regeneration process to break the absorbent-CO2 bond is the most energy extensive process. The traditional MEA process uses a substantial amount of thermal energy at the stripper reboiler when CO2 concentration increases. In particular, the thermal energy required by the stripper reboiler overshadows the annualized capital costs of the capture plant. Therefore, minimizing the reboiler heat duty is crucial in this process. A series of heat exchangers to exchange heat between the hot and cold streams are essential for the capture process. The heat exchange network must be operated very efficiently and efforts to recover waste heat is the key.
MEA-based process requires a substantial amount of cooling water that is expected to be provided by the host power plant cooling water systems. However, ammonia-based solvents and some of the transformational solvents may require less cooling water. The additional cooling load imposed by the capture unit will be reflected in the significantly larger cooling water pumps and cooling tower capacity. Thus, it is essential for the capture process to have the raw water withdrawal (consumption + cooling/recirculation) from cooling tower as low as possible. Similarly, the auxiliary power load to operate the capture unit must be kept low to be viable.
With every capture technology, alternative process configurations can be developed by adding extra equipment (e.g. heat exchangers, compressors, flash drum etc.) to the conventional process. Typical examples include configurations involving multi-absorbers, absorber inter-cooling, multi-pressure stripping, and split-flow among others. Each configuration may have an edge over the others, and the choosing the right configuration is crucial in the design. Finding the optimal rich/lean solution feed/withdrawal location is also an important consideration in the design.
It is customary in the design to maintain emission standards as set by the regulators. A high-pressure stripping, if required by the process, may reduce final CO2 product compression energy requirement. A power plant produces different pressure steam (high, intermediate, low pressure) at different cycle, and power plant may prefer capture unit using low pressure steam, however, low pressure steam may not be enough to raise reboiler temperature high, if high temperature is required in the reboiler. Since the capture unit requires significant amount of steam extraction, which significantly reduces the steam turbine output for the host power plant, the net output in the host power plant will be also reduced. Last but not the least, corrosion and solvent degradation need to be considered.
OLI Systems in carbon capture technology:
OLI System’s Mixed-Solvent Electrolyte (MSE) framework is a comprehensive thermodynamic framework and is well-positioned for the carbon capture industry simulation needs. MSE framework has been developed for the calculation of speciation, phase equilibria, enthalpies, heat capacities and densities in mixed solvent electrolyte systems. The model incorporates chemical equilibria to account for chemical speciation in multiphase, multicomponent systems. For this purpose, the model combines standard-state thermochemical properties of solution species with an expression for the excess Gibbs energy. The model is validated using experimental data on vapor-liquid equilibria, solubility, activities and activity coefﬁcients, acid dissociation constants, Gibbs energies of transfer, heats of dilution and mixing, heat capacities, and densities .
OLI systems teamed up with SRI international to develop their mixed-salt process (MSP) based on ammonium and potassium salts through a DOE funding. OLI Systems developed the thermodynamic model for the MSP system K2CO3-CO2-NH3-H2O and a complete database of species in the concentration range and beyond using OLI’s proprietary mixed-solvent electrolyte (MSE) model. OLI Systems also developed the full-scale rate-based process model of the integrated capture process using OLI’s Flowsheet Simulator (OLI Flowsheet: ESP) and optimized the entire capture process . OLI’s Corrosion Analyzer program (OLI Studio: Corrosion Analyzer) was used for predicting corrosion tendency and selecting material of construction. The project is currently in integrated testing phase (e.g., carbon capture unit integrated to fossil fuel-powered power plant) in Norway and OLI Systems has been playing important roles as partner. OLI Systems teamed up with SRI international on another DOE funded project where MDEA will be added to the MSP formulation, resulting in an advanced mixed-salt process (A-MSP). OLI has joined forces with the Norwegian Institute for Energy Technology (IFE) to develop a methodology for predicting when the impurities will pose a danger to the integrity of CO2 transportation facilities.
OLI Systems offers a unique combination of material properties, models and software that uses first-principles based thermodynamic and electrochemical modeling. The OLI property database of over 6,000 species covers over 80 elements of the periodic table. OLI’s physical property methods, and property parameters and data are the ideal choice to suit your simulation needs. OLI Systems has expert scientists, thermodynamicists, engineers and consultants ready to help your thermodynamic data development, defining complex chemistry, conducting sponsored-research and Joint Industry Programs (JIPs) for carbon capture and storage technology.
Contact OLI at https:/www.olisystems.com/contact for more information or to schedule a meeting with an OLI expert.
 National Energy Technology Laboratory (NETL), department of energy (DOE), accessed April 15, 2020, https://netl.doe.gov/coal/carbon-capture
 Mixed-Salt-Based Transformational Solvent Technology for CO2 Capture, NETL, DOE, accessed April 15, 2020, https://netl.doe.gov/project-information?p=FE0031597
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 I. Jayaweera et al., Energy Procedia 114 (2017), 771-780.