Carbon dioxide capture: Technical note

I've?written?this?note. pointing?out?the?CO2?capture's?kinetic?and?thermodynamic?constraints?and?how?they?affect?the?process'?cost. To?overcome?the?thermodynamic?constraints?and?power?the?separation?process,?energy?must?be?added?in?order?to?capture?CO2. The?price?of?removing?CO2?from?industrial?sources?is?impacted?by?this?energy?input. In?this?sense,?the?high?cost?is?a?trade-off?for?making?CO2?capture?practical?despite?the?inherent?thermodynamic?difficulties.

Following are the two equations illustrating the thermodynamic limitations of negative entropy and the hydrolysis of bicarbonate to revert back to CO2 for which CO2 capture has to pay a price.

Thermodynamic Limitations of Negative Entropy

CO2 + H2O ? HCO3- + H+

The conversion of CO2 to bicarbonate ions decreases the degree of randomness (entropy) in the system, it is accompanied by an increase in negative entropy.

Hydrolysis of Bicarbonate to revert to CO2

HCO3- + H2O ? [CO3]2- + H3O+

[CO3]2- + H3O+ ? HCO3- + H2O

Today, CO2 capture is a hot topic. It is imperative that each of us have a foundational understanding of this subject because CO2 capture figures in a variety of formats in most technical discussions.

CO2 capture is a fascinating emerging technology that has gained significant attention in recent years. With the increasing global concern about climate change and the need to reduce greenhouse gas emissions, CO2 capture has become a prominent approach to mitigate CO2 emissions from various industrial processes and power generation.

Broadly speaking, CO2 capture technologies can be classified into three main categories

Post-combustion capture: This technology captures CO2 after the fuel or energy source has been burned. It involves processes such as chemical solvent absorption, adsorption, or membrane separation. The challenges associated with post-combustion capture include the high energy requirements for separation, the large volume of flue gas to be processed, and the cost of implementing and maintaining the capture units.

Pre-combustion capture: This technology involves the gasification or reforming of fossil fuels prior to combustion to produce a syngas, which is then converted into hydrogen and CO2. The CO2 is separated from the hydrogen before combustion, resulting in relatively pure CO2 for capture. The challenges for pre-combustion capture include the higher complexity and cost of gasification or reforming processes and the potential impact on energy efficiency.

Direct air capture (DAC): This technology directly captures CO2 from ambient air and can be used to remove CO2 emissions that are difficult to capture at the source. DAC technologies typically involve chemical processes such as adsorption or absorption. The challenges in DAC include the lower concentration of CO2 in ambient air, making the capture process energy-intensive, and the large infrastructure required for large-scale CO2 removal.

Cost

The average cost of CO2 capture can vary depending on various factors such as the type of technology used, scale of the project, location, and specific requirements. However, on average, the cost of CO2 capture ranges from $30 to $200 per metric ton of CO2 captured. It is important to note that these costs are highly dependent on the specific circumstances of each project and can vary significantly. The costs associated with CO2 capture primarily consist of three categories:

Capital costs: This includes the expenses for building and installing the necessary equipment and infrastructure for CO2 capture, including the capture units, storage infrastructure, and related facilities. Operating costs: These costs encompass ongoing operational expenses such as energy consumption, maintenance, chemicals, and labor required for the operation of the CO2 capture system.

Storage or utilization costs: After capturing CO2, it needs to be stored or utilized. Storage costs involve the transportation and storage of CO2 in geological formations (such as underground storage). Utilization costs involve finding commercial uses for CO2, such as enhanced oil recovery or conversion into valuable products, which can offset the overall capture and storage expenses. Overall, these costs can vary greatly based on the specific technology employed, the scale of the project, the location, the CO2 concentration, and other project-specific factors.

The high cost of CO2 capture is a result of making the process feasible against thermodynamic laws. In order to capture CO2, energy must be supplied to overcome the thermodynamic limitations and drive the separation process. This energy input contributes to the cost of capturing CO2 from industrial sources. So, in a way, the high cost is a trade-off for making CO2 capture feasible despite the inherent thermodynamic challenges.

Following are the two equations illustrating the thermodynamic limitations of negative entropy and the hydrolysis of bicarbonate to revert back to CO2 for which CO2 capture has to pay a price.

Thermodynamic Limitations of Negative Entropy

CO2 + H2O ? HCO3- + H+

The conversion of CO2 to bicarbonate ions decreases the degree of randomness (entropy) in the system, it is accompanied by an increase in negative entropy.

Hydrolysis of Bicarbonate to revert to CO2

HCO3- + H2O ? [CO3]2- + H3O+

[CO3]2- + H3O+ ? HCO3- + H2O

Fundamental technical challenges

?Here are three fundamental principles related to thermodynamics and kinetics that govern CO2 capture:

Thermodynamic Equilibrium: The process of CO2 capture is influenced by the principles of thermodynamics, particularly the concept of equilibrium. During CO2 capture, the system aims to achieve equilibrium between the absorbent material (solvent, adsorbent, etc.) and the CO2 molecules. According to Le Chatelier's principle, the system will adjust its conditions to attain equilibrium, which influences factors such as temperature, pressure, and concentration. Achieving thermodynamic equilibrium is crucial for maximum CO2 capture efficiency and requires careful optimization of operating conditions.

Impact on cost

Capturing CO2 is an endothermic process, it requires supplying energy to drive the reaction. This energy input contributes to the high cost of CO2 capture. Diffusion Kinetics: The kinetics of diffusion play a significant role in CO2 capture. When using processes like absorption or adsorption, CO2 molecules must physically move from the gas phase into the liquid or solid phase of the absorbent material. The rate at which CO2 molecules diffuse into the absorbent material determines the efficiency of the capture process. Factors such as pore size, surface area, and mass transfer coefficients affect the diffusion kinetics and can be optimized to enhance CO2 capture efficiency.

Chemical Reaction Kinetics: Some CO2 capture techniques involve chemical reactions between CO2 and the absorbent material. These reactions, such as chemisorption or carbonation, have their kinetic rates influenced by factors such as temperature, pressure, concentration, and catalysts. Understanding the reaction kinetics is crucial for designing efficient capture systems. By optimizing these factors and selecting suitable catalysts, the rate and extent of CO2 capture can be enhanced.

Impact on cost

Kinetic limitations: The kinetics of CO2 capture reactions can influence the efficiency and cost of the capture process. If the reaction kinetics are slow, it would require longer contact times between the CO2 and the capturing medium, leading to large equipment sizes and high capital costs. Enhancing the reaction kinetics often requires the use of catalysts or novel materials, which can contribute to additional expenses. In summary, the fundamental principles in CO2 capture involving thermodynamics and kinetics include achieving thermodynamic equilibrium between the absorbent material and CO2, optimizing diffusion kinetics for efficient movement of CO2, and understanding the chemical reaction kinetics for enhanced capture rates.

Additional cost elements

Scale and integration challenges: Scaling up CO2 capture technologies to industrial levels presents several challenges. Implementing large-scale capture facilities requires significant investments in infrastructure, such as pipelines and storage sites. Integration with existing industrial processes also adds complexities and costs. These scale and integration challenges further contribute to the high cost of CO2 capture.

Other complexities: Addressing these fundamental issues of thermodynamics and kinetics requires research and development to develop more efficient and cost-effective CO2 capture technologies. Improving the thermodynamic efficiency, developing catalysts to enhance reaction kinetics, and optimizing system integration are all areas of research aimed at reducing the cost of CO2 capture. Additionally, breakthroughs in materials science, process engineering, and innovation in energy sources can also help overcome these challenges