What is Carbon Capture and Sequestration? How does it work? How well does it work? What are the advantages? What are the challenges?

The clean coal technology field is moving in the direction of coal gasification with a second stage so as to produce a concentrated and pressurized carbon dioxide stream followed by its separation and geological storage. This technology has the potential to provide what may be called “zero emissions” – in reality, extremely low emissions of the conventional coal pollutants, and as low-as-engineered carbon dioxide emissions.

Carbon Capture involves the separation of CO₂from coal-based flue gas or syngas. While there are commercially available 1st-Generation CO2 capture technologies that are currently being used in various industrial applications, at their current state of development these technologies are not ready for widespread deployment on fossil fuel based power plants. At the same time, successful development of advanced CO₂ capture technologies is critical to maintaining the cost-effectiveness of fossil fuel based power generation.

The Current State Of Carbon Capture

Carbon Capture technology consists of two core research areas: Post-Combustion Capture and Pre-Combustion Capture. In both post and pre-combustion, R&D is underway to develop Carbon Capture technologies based on advanced solvents, sorbents, and membranes.

Post-Combustion Capture

Primarily applicable to fossil fuel based systems such as conventional pulverized coal (PC)-fired power plants, where the fuel is burned with air in a boiler to produce steam that drives a turbine/generator to produce electricity
The carbon dioxide (CO₂) is captured from the flue gas after fuel combustion
The applicability of post-combustion capture technologies to existing plants differentiates it from other technologies designed to reduce carbon emissions
Given the importance of the existing fleet of coal-based power production, the largest market for post-combustion capture technology is in power plant retrofits
Capture of carbon dioxide from flue gas streams following combustion in air is much more difficult and expensive than from natural gas streams, as the carbon dioxide concentration is only about 14% at best, with nitrogen most of the rest, and the flue gas is hot

Advanced Solvents & Post-Combustion Capture

Solvent-based CO2 capture involves chemical or physical absorption of CO2 from flue gas into a liquid carrier
- The absorption liquid is regenerated by increasing its temperature or reducing its pressure
Research projects in this key technology focus on the development of low-cost, non-corrosive solvents that have
- High CO2 loading capacity
- Improved reaction kinetics
- Low regeneration energy
- Resistance to degradation
Advanced technologies that may be pursued include
- Both switchable and non-switchable ionic liquids
  - Since ionic liquids are physical solvents, less energy is required for regeneration compared to today’s conventional chemical solvents
  - However, the ionic liquid working capacity still needs to be significantly improved to meet cost targets
  - One possible drawback is that the viscosities of many ionic liquids are relatively high upon CO2 absorption compared to those associated with conventional solvents, perhaps adversely affecting the energy requirement to pump ionic liquids in a conventional adsorption/stripping process
  - Although the production cost for newly synthesized ionic liquids is high, the cost could be significantly lower when produced on a commercial scale
- Catalyzed absorption that accelerates CO2 uptake in solvents with lower regeneration energies
- Solvents that change phase in the presence of CO2
- Hybrid systems
Advantages
- Chemical solvents provide a high chemical potential (or driving force) necessary for selective capture from streams with low CO2 partial pressure
- Wet-scrubbing allows good heat integration and ease of heat management
Challenges
- Trade-off between heat of reaction and kinetics
- Current solvents require a significant amount of steam to reverse chemical reactions and regenerate the solvent, which derates power plant
- Energy required to heat, cool, and pump non-reactive carrier liquid (usually water) is often significant
- Vacuum stripping can reduce regeneration steam requirements, but is expensive

Sorbents & Post-Combustion Capture

Solid sorbents are being explored for post-combustion CO2 capture, including
- Sodium and potassium oxides
- Zeolites
- Carbonates
- Amine-enriched sorbents
- Metal organic frameworks
Possible configurations for contacting the flue gas with the sorbents include fixed, moving, and fluidized beds
Research projects in this key technology focus on developing sorbents with the following characteristics
- Low-cost raw materials
- Thermal and chemical stability
- Low attrition rates
- Low heat capacity
- High CO2 adsorption capacity
- High CO2 selectivity
Another important focus of this research is to develop cost-effective process equipment designs that are tailored to the sorbent characteristics
Advanced technologies being considered include
- Structured solid adsorbents (e.g., metal organic frameworks)
- Enhanced pressure swing adsorption (PSA)
- Temperature swing adsorption (TSA) processes
- Hybrid systems
- Electrochemical technologies
Advantages
- Chemical sites provide large capacities and fast kinetics, enabling capture from streams with low CO2 partial pressure
- Higher capacities on a per mass or volume basis than similar wet-scrubbing chemicals
- Lower heating requirements than wet-scrubbing in many cases (CO2 and heat capacity dependent)
- Dry process—less sensible heating requirement than wetscrubbing process
Challenges
- Heat required to reverse chemical reaction (although generally less than in wet-scrubbing cases)
- Heat management in solid systems is difficult, which can limit capacity and/or create operational issues when absorption reaction is exothermic
- Pressure drop can be large in flue gas applications
- Sorbent attrition

A key attribute of CO2 sorbents (compared with solvent-based systems) is that no water is present, thereby reducing sensible heating and stripping energy requirements.

Membranes & Post-Combustion Capture

Membrane-based CO2 capture uses permeable or semi-permeable materials that allow for the selective transport and separation of CO2 from flue gas
- Gas separation is accomplished by some physical or chemical interaction between the membrane and the gas being separated, causing one component in the gas to permeate through the membrane faster than another
- Usually the selectivity of the membrane is insufficient to achieve the desired purities and recoveries
- Therefore multiple stages and recycle streams may be required in an actual operation, leading to increased complexity, energy consumption, and capital costs
Also under development are gas absorption membrane technologies where the separation is caused by the presence of an absorption liquid on one side of the membrane that selectively removes CO2 from a gas stream on the other side of the membrane
Research projects in this key technology address technical challenges to the use of membrane-based systems, such as
- Large flue gas volume
- Relatively low CO2 concentration
- Low flue gas pressure
- Flue gas contaminants
- The need for high membrane surface area
The Department’s research focus for post-combustion membranes includes development of low-cost, durable membranes that have improved
- Permeability and selectivity
- Thermal and physical stability
- Tolerance to contaminants in combustion flue gas
Advanced technologies under investigation include
- Hybrid systems
- Novel process conditions (e.g., systems that operate at subambient temperatures)
- Nanomaterials
Advantages
- No steam load
- No chemicals
- Simple and modular designs
- “Unit operation” versus complex “process”
Challenges
- Membranes tend to be more suitable for high-pressure processes such as IGCC (Integrated Gasification Combined Cycle)
- Trade-off between recovery rate and product purity (difficult to attain both high recovery rate and high purity)
- Requires high selectivity (due to CO2 concentration and low pressure ratio)
- Poor economy of scale
- Multiple stages and recycle streams may be required

Advanced Post-Combustion Capture Technology Timeline

The Carbon Capture program has adopted an aggressive timeline for developing 2nd-Generation and Advanced post-combustion capture technologies
The 2nd-Generation technology timelines are shown in shades of purple and the Advanced technology timelines are shown in shades of orange
Each timeline in the figure consists of three major RD&D phases
- Research testing
- Pilot-scale testing
- Demonstration-scale testing
It is anticipated that the demonstration-scale testing will be conducted through private industry funding
As a result of these efforts, 2nd-Generation and Advanced post-combustion capture technologies will be ready for demonstration-scale testing after 2020 and 2030, respectively.

Pre-Combustion Capture

By carefully controlling the amount of O2, only a portion of the fuel burns to provide the heat necessary to decompose the remaining fuel and produce syngas, a mixture of H2 and carbon monoxide, along with minor amounts of other gaseous constituents (e.g., sulfur)
To enable pre-combustion capture, the syngas is further processed in a WGS reactor, which converts carbon monoxide into CO2 while producing additional H2, thus increasing the CO2 and H2 concentrations
An acid-gas removal system can then be used to separate CO2 from the H2
- After WGS, the CO2 in syngas is present at relatively higher concentrations than in flue gas
- Also, the syngas is at higher pressure relative to flue gas
- These characteristics make pre-combustion carbon capture relatively simpler and less expensive compared to post-combustion carbon capture
After CO2 removal, the H2 is used as a fuel in a combustion turbine combined cycle to generate electricity or other useful, high-value products
The current state-of-the-art pre-combustion CO2 capture technologies that could be applied to IGCC systems—the glycol-based Selexol™ process and the methanol-based Rectisol® process—employ physical solvents that preferentially absorb CO2 from the syngas mixture
Today, these technologies are not considered cost-effective for application to IGCC power plants.

Advanced Solvents & Pre-Combustion Capture

Solvent-based CO2 capture involves chemical or physical absorption of CO2 from flue gas into a liquid carrier
As the name implies, a chemical solvent relies on a chemical reaction for absorption, whereas a physical solvent selectively absorbs CO2 without a chemical reaction
The main benefit of a physical solvent, as compared to a chemical solvent, is that it requires less energy for regeneration
However, chemical solvents offer the advantages of
- Increased mass transfer driving force into solution
- Increased acid gas selectivity, and
- The potential to generate the CO2 at elevated pressure
Advanced technologies include
- Combining temperature-swing and pressure-swing regeneration to lower cost and energy penalty
- Development of hybrid systems
- Pre-combustion solvent R&D activities focus on a number of research objectives that address solvent technology challenges, including
- Increasing CO2 loading capacity and reaction kinetics
- Decreasing regeneration energy
- Ionic liquids can absorb CO2 at elevated temperature, providing a potential option to combine CO2 capture with warm syngas cleanup
Advantages
- CO2 recovery does not require heat to reverse a chemical reaction
- Common for same solvent to have high H2S solubility, allowing for combined CO2/H2S removal
- System concepts in which CO2 is recovered with some steam stripping, rather than flashed, and delivered at a higher pressure may optimize processes for power systems
Challenges
- Modifying regeneration conditions to recover the CO2 at a higher pressure
- Improving selectivity to reduce H2 losses, and
- Developing a solvent that has a high CO2 loading at a higher temperature to improve IGCC efficiency
- CO2 pressure is lost during flash recovery
- Must cool down syngas for CO2 capture, then heat it back up again and rehumidify for firing to turbine
- Low solubilities can require circulating large volumes of solvent, resulting in large pump loads
- Some H2 may be lost with the CO2

Sorbents & Pre-Combustion Capture

The materials, regeneration characteristics, and process configurations for pre-combustion sorbents are similar to those described for post-combustion sorbents but applied to the unique conditions of IGCC systems
Research projects in pre-combustion sorbent technology focus on the development of sorbents with the following characteristics
- High adsorption capacity
- Resistance to attrition over multiple regeneration cycles, and
- Good CO2 separation and selectivity performance at the high temperatures to avoid the need for syngas cooling
- Another important focus of the research is to develop cost-effective process equipment designs that are tailored to the sorbent characteristics
Some of the current pre-combustion sorbents under development include
- Activated carbon
- Alumina
- Calcium carbonate, and
- Magnesium oxide
Advanced technologies include
- Integrating capture directly with the WGS reaction to help drive equilibrium toward CO2 and H2 production while eliminating the need for syngas cooling and development of hybrid systems
- The advantage of an adsorption process is that some solid sorbents can be used at a high temperature
- In a pre-combustion application, this is important since high-temperature (above 500 °F) CO2 capture combined with warm/humid gas sulfur cleanup would eliminate syngas reheating and thus improve the overall thermal efficiency of the IGCC power plant
Advantages
- CO2 recovery does not require heat to reverse a reaction
- Common for H2S to also have high solubility in the same sorbent, meaning CO2 and H2S capture can be combined
- System concepts in which CO2 is recovered with some steam stripping, rather than flashed, and delivered at a higher pressure may optimize processes for power systems
Challenges
- CO2 pressure is lost during flash recovery
- Must cool syngas for CO2 capture, then heat it back up again and rehumidify for firing to turbine
- Some H2 may be lost with the CO2

Membranes & Pre-Combustion Capture

As with sorbents, the general characteristics of precombustion membranes are similar to those for post-combustion
Research is being conducted on CO2 selectivity and permeability in
- Pre-combustion systems
- Thermal and hydrothermal stabilities of the membrane, as well as
- Other physical and chemical properties
Scaleup studies must determine the potential for lower cost and efficient operation in integrated systems
Large-scale manufacturing methods for defect-free membranes and modules must be developed
Better methods are needed to make high-temperature, high-pressure seals
Membrane designs include
- Metallic
- Polymeric, or
- Ceramic materials
- Operating at elevated temperatures, with a variety of chemical and/or physical mechanisms that provide separation
Advanced technologies include
- Integration of a membrane-based system with WGS
- High density and pressure nanoscale membranes
- High-temperature/high-pressure seals
- Process intensification
- Hybrid systems
H2 or CO2 Permeable Membrane
- Advantages
  - No steam load or chemical attrition H2 Permeable Membrane Only
  - Can deliver CO2 at high-pressure, greatly reducing compression costs
  - H2 permeation can drive the carbon monoxide shift reaction toward completion—potentially achieving the shift at lower cost/higher temperatures
- Challenges
  - Membrane separation of H2 and CO2 is more challenging than the difference in molecular weights implies
  - Due to decreasing partial pressure differentials, some H2 will be lost with the CO2
  - In H2 selective membranes, H2 compression is required and offsets the gains of delivering CO2 at pressure
  - In CO2 selective membranes, CO2 is generated at low pressure, requiring compression
Water-Gas-Shift Membranes – Advantages
- Promote higher conversion of carbon monoxide and H2O to CO2 and H2 than is achieved in a conventional WGS reactor
- Reduce CO2 capture costs
- Reduce H2 production costs
- Increase net plant efficiency
- Single-stage WGS with membrane integration
- Improved selectivity of H2 or CO2
- Optimize membranes for WGS reactor conditions

Advanced Pre-Combustion Capture Technology Timeline

The Carbon Capture program has adopted an aggressive timeline for developing 2nd-Generation and Advanced pre-combustion capture technologies
The 2nd-Generation technology timelines are shown in shades of purple and the Advanced technology timelines are shown in shades of orange
Each timeline in the figure consists of three major RD&D phases
- Research testing
- Pilot-scale testing
- Demonstration-scale testing
It is anticipated that the demonstration-scale testing will be conducted through private industry funding
As a result of these efforts, 2nd-Generation and Advanced post-combustion capture technologies will be ready for demonstration-scale testing after 2020 and 2030, respectively.

The Future State Of Carbon Capture

New generation of technologies will essentially be able to overcome potential environmental barriers and meet any projected environmental emission standards. Four primary technology areas include gasification, advanced combustion, advanced turbines, and solid oxide fuel cells.

Gasification Systems research to convert coal into clean high-hydrogen synthesis gas (syngas) that can in-turn be converted into electricity with over 90 percent CCS
Advanced Combustion Systems research that is focused on new high-temperature materials and the continued development of oxy-combustion technologies
Advanced Turbines research, focused on developing advanced technology for the integral electricity-generating component for both gasification and advanced combustion-based clean energy plants fueled with coal by providing advanced hydrogen-fueled turbines, supercritical CO2-based power cycles and advanced steam turbines
Solid Oxide Fuel Cells research is focused on developing low-cost, highly efficient solid oxide fuel cell power systems that are capable of simultaneously producing electric power from coal with carbon capture when integrated with coal gasification

However, Amine Scrubbing is the most developed technique for capturing carbon from emissions.

It involves bubbling the exhaust from burning coal through a solution of
- Water and
- Monoethanolamine
  - MEA is unpleasant: toxic, flammable, and caustic, with an acrid, ammoniacal smell
  - But it bonds to carbon dioxide, separating it from the other gases in the exhaust
The process creates a new chemical compound called, uneuphoniously, MEA carbamate
The MEA carbamate and water are pumped into a “stripper,” where the solution is boiled or the pressure is lowered
Heat or expansion reverses the earlier reaction, breaking up the MEA carbamate into carbon dioxide and MEA
Carbon dioxide and water vapor gush out, ready to be buried
MEA returns to combine with the next batch of coal exhaust

“Amine scrubbing typically recovers 90 percent of a plant’s greenhouse gas emissions.”

Carbon Sequestration

Captured carbon dioxide gas can be put to good use, even on a commercial basis, for enhanced oil recovery (EOR), and a majority of CCS projects are oriented thus. This is well demonstrated in West Texas, and today over 5800 km of pipelines connect oilfields to a number of carbon dioxide sources in the USA. The CO₂ acts to reduce the viscosity of the oil, enhancing its flow to recovery wells. It is then separated and re-injected.

In geologic carbon sequestration, industrial CO2 is compressed into a semi-liquid state, and then injected deep underground into porous rock where it becomes trapped along with existing fluids
Porous rock structures, such as depleted oil fields or brine formations that have held fluid for millions of years, are excellent candidate sites for carbon sequestration
A layer of nonporous stone, called caprock, helps ensure the CO2 is contained
Over time, injected CO2 becomes trapped inside the rock – most immediately through the capillary pressure in the pores, then by dissolving into a liquid, and in the case of brine, by eventually hardening into solid calcium carbonate
The Intergovernmental Panel on Climate Change (IPCC) has estimated there is sufficient global capacity to sequester 10,000 billion tons of CO2.

Categories Of Geologic Carbon Sequestration

Enhanced Oil Recovery (EOR)
Coal Seam Storage
Deep Saline Aquifers

Enhanced Oil Recovery (EOR)

…and exhausted oil fields

“Exhausted” does not mean the field has been pumped dry
- Rather, the remaining petroleum—as much as two-thirds of the total in the ground—is too thick and tarry to extract at a reasonable price
- Injecting carbon dioxide changes the equation
- Flowing into the pores of the rock, the gas mixes with the remaining crude oil, lowering its viscosity and squeezing it toward the wellhead
As a rule, an oil or gas field consists of two layers of stone
- The bottom layer is porous and spongelike, its holes filled with petroleum
- Atop it is the second layer: a cap of nonporous stone… Oil or gas companies drill through the cap, releasing the liquids and gases below

Coal Seams

Coal Bed Methane. Coal beds typically contain large amounts of methane-rich gas that is adsorbed onto the surface of the coal
The current practice for recovering coal bed methane is to depressurize the bed, usually by pumping water out of the reservoir
An alternative approach is to inject CO₂ into the bed. Tests have shown that the adsorption rate for CO₂ to be approximately twice that of methane, giving it the potential to efficiently displace methane and remain stored in the bed
CO₂ recovery of coal bed methane has been demonstrated in limited field tests, but much more work is necessary to understand and optimize the process
The recovered methane provides a value-added revenue stream to the carbon sequestration process. Similar to the by-product value gained from enhanced oil recovery, thus improving economics of carbon storage.

Saline Aquifers

Obvious targets include saline beds—underground reservoirs of salty water… Deep brine or saline formations have great capacity for sequestration. Storage of CO₂ in deep saline formations does not produce value-added by-products, but it has other advantages:

First, the estimated carbon storage capacity of saline formations in the United States is large, making them a viable long-term solution. It has been estimated that deep saline formations in the United States could potentially store more than 12,000 billion tonnes of CO₂
Second, most existing large CO₂ point sources are within easy access to a saline formation injection point, and therefore storage in saline formations is compatible with a strategy of transforming large portions of the existing U.S. energy and industrial assets to near-zero carbon emissions via low-cost carbon storage retrofits
Underground formations of deep porous sedimentary rock such as sandstone, that are saturated with salty water which is unfit for human consumption or agricultural use, and covered by a layer of impermeable cap rock (such as shale or clay), which acts as a seal. Once injected into the formation, the CO2 dissolves into the saline water in the reservoir rock. CO2 storage in deep saline formations usually takes place at depths below 800 metres. At this depth, the CO2 will be at high enough pressures to remain in a liquid-like state
Some CO2 injection to saline aquifers involves acid gas, disposing of hydrogen sulphide and CO2 separated for a natural gas stream. Chevron’s Acheson Field in Canada was one of the first to use this acid gas injection
Saline formations have the largest storage potential globally and a number of CO2 storage demonstration projects are proving their effectiveness to maximise storage capacity and containment