Carbon Capture

Practice flashcards covering core concepts from the lecture notes on carbon capture, including PCC, DAC, 2nd-law efficiency, Henry’s law, mass-transfer design, and related topics.

What is the main goal of carbon capture?

To separate CO2 from gas mixtures (such as power-plant exhaust) in order to mitigate climate change by reducing CO2 emissions.

Define 2nd-law efficiency in CO2 separation.

The ratio of the minimum reversible work to the actual work required, η2nd = Wmin/Wreal, indicating how close a process is to the thermodynamic limit.

What does Wmin represent in CO2 separation?

The minimum work required to separate CO2 from a gas mixture under isothermal, isobaric conditions (the ideal reversible minimum).

Name the three core CO2 capture technologies discussed in the book.

Absorption, Adsorption, and Membrane technology (including related hybrid approaches).

What is Post-Combustion CO2 Capture (PCC)?

Capture of CO2 from fossil-fuel exhaust gases (flue gas) using solvent-based systems, such as amine scrubbing (e.g., MEA).

What is Direct Air Capture (DAC)?

Capture of CO2 directly from ambient air, where CO2 is very dilute (about 390 ppm), making the process energy-intensive and costly.

Which factors strongly influence the choice and cost of CO2 capture technologies?

Concentration of CO2, chemical environment (presence of H2O, SOx, NOx, etc.), temperature, pressure, and integration with energy, transport, and storage.

According to Example 1.1, what is the thermodynamic minimum work per kg CO2 for 12% CO2 in coal flue gas?

172 kJ/kg CO2.

How does the 2nd-law efficiency trend with CO2 concentration, as discussed in the text?

The 2nd-law efficiency decreases as CO2 concentration decreases, meaning more energy is needed above the thermodynamic minimum for dilute mixtures.

What are the main distinctions between PCC (post-combustion) and DAC (direct air capture) in terms of feed CO2 concentration and economics?

PCC deals with relatively high CO2 concentrations in flue gas and is generally more economical; DAC handles very dilute ambient air CO2 (≈390 ppm) and can be orders of magnitude more expensive.

What is Henry’s law form used to relate CO2 solubility to CO2 partial pressure in aqueous solutions?

pCO2 = H xCO2, where H is the Henry’s law constant (temperature- and ionic-strength-dependent).

What is the pinch point in packed-column design for CO2 absorption?

The operating point where the gas–liquid operating line nearly touches the gas–liquid equilibrium curve, implying an infinite number of stages and determining the minimum liquid flow rate.

What is NTU, and why is it important in mass-transfer design?

Number of Transfer Units; a dimensionless measure used to estimate the height of a packed column and overall mass transfer performance.

What is HTU in packed-column design?

Height of a Transfer Unit; HTU × NTU gives the total column height (ZT ≈ HTU × NTU) and reflects separation effectiveness.

What is the difference between ‘captured CO2’ and ‘avoided CO2’ costs?

Captured CO2 is the gross amount removed; avoided CO2 accounts for the energy use and emissions from operating the capture system; avoided CO2 is always less than captured CO2.

What does the Sherwood number (Sh) represent in packed beds, and what is a common correlation for Sh in packed columns?

Sh is a dimensionless mass-transfer coefficient; for packed beds, Sh ≈ 2.0 + 1.1 Re^0.6 Sc^1/3 (valid for typical Re and Sc ranges).

Name three major options for advanced coal-conversion processes with respect to CO2 capture.

Gasification, oxy-combustion, and chemical looping combustion (all aiming to concentrate CO2 in exit streams for easier capture).

What is Henry’s law constant (H) and how does it influence CO2 absorption in aqueous solutions?

H relates dissolved CO2 to gas phase CO2 via pCO2 = H xCO2; H depends on temperature and ionic strength and governs CO2 solubility driving force.

What is the role of water and acidic gases (SO2, NOx) in CO2 capture from flue gas?

Water and acid gases compete for binding in solvents or sorbents, influence mass-transfer resistance, and can affect corrosion and selectivity in absorption/adsorption systems.

What is the basic idea behind the minimum thermodynamic work for CO2 separation shown in Fig. 1.10?

It shows the theoretical minimum energy required to separate CO2 from a gas mixture based on Gibbs free energy changes, setting a lower bound for real processes.