Pre-combustion Carbon Capture and Storage Technology Creating a Buzz in the Power Industry

 What is Carbon Capture?

Carbon capture technology is intended to remarkably minimize the emissions originating from carbon dioxide (CO₂). This technology is specifically essential for the transition from a fossil fuel-oriented economy to an advanced and sustainable energy age. Carbon capture is a subset of the CCUS technology i.e., carbon capture, utilization, and storage, which comprises:

• Capturing CO₂ at enormous stationary sources (including coal-fired plants for power generation)
• Deploying the captured CO₂ in various applications (including gas injection in improved recovery of oil)
• Petrochemicals’ feedstock
• Captured CO₂ transportation to the storage facilities
• Permanently storing the CO₂ in the storage site by injection (sequestration)

The sources of carbon capture include the following:

• Power generation sources: Separation of CO₂ emissions from syngas stream or the plant’s exhaust gas in fossil fuel-based power generation.
• Industrial sources: Separation of CO₂ emissions in industrial facilities such as mineral production plants, hydrogen production plants, iron & steel manufacturing plants, and ethanol plants.
• Emissions control: Separation of CO₂ at power plants from the output of by-products that can be recycled.

A number of technologies for carbon capture and storage are crucial to mitigate fossil fuel-oriented carbon emissions, particularly in the power sector. These technologies are categorized into pre-consumption capture, post-combustion capture, and oxy-combustion capture. Here, we will be elaborating on the pre-combustion carbon capture technology.

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What Says the Process?

Pre-combustion capture involves the removal of carbon dioxide from fossil fuels prior to the completion of combustion. For instance, during the process of gasification, a feedstock, such as coal, is oxidized partially in steam and air/oxygen under high pressure and temperature to create syngas or synthesis gas. Syngas is a combination of various gases, including carbon monoxide, CO₂, hydrogen, and lesser amounts of other gaseous elements, including methane. The syngas then undergoes a shift reaction for water-gas to convert H₂O and CO to H₂ and CO₂, thereby forming an H₂ and CO₂ (with a concentration of 15-50%) gaseous mixture. The CO₂ is captured and divided from the mixture, transported, and ultimately isolated, and the H₂-rich fuel is combusted.

In natural gas reforming, natural gas processing, gasification, and IGCC the eventual CO₂ capture is presently achieved by an acid gas removal (AGR) process, under pressure, followed by the regenerative stripping of solvent for CO₂ release which aids in sending the with subsequent compression to sequestration or it can be supplied for enhanced oil recovery (EOR).

There are two key generic acid gas (i.e., CO₂, COS, and H2S) removal (AGR) solvents – mainly physical and chemical:

Chemical Absorbents:

Chemical absorbents [including Methyl diethanolamine (MDEA) and other amines] react with acid gases and necessitate heat for reversing the reaction and releasing the gases. This process often involves lower capital for AGR in comparison with physical solvents; however, it utilizes larger amount of steam heat required for solvent regeneration.

Physical Absorbents:

Physical absorbents (such as Rectisol and Selexol) help in dissolving the acid gases, specifically with rising pressure. When the temperature is increased and pressure is decreased, the absorbed gases are released from the solvent. Relatively lesser steam-heat is needed for solvent regeneration, compared with the chemical solvents. In the Rectisol process that deploys chilled methanol, the capital costs involved are high, but it offers the most complete form of removal.

Applications of pre-combustion carbon capture and storage:

• Capture of CO₂ from natural gas
• Capture of CO₂ from partial oxidation and natural gas reforming
• Removal of CO₂ from coal gasification plants

Pre-Combustion Capture of CO₂ for Gasification Application

The pre-combustion capture of CO2 in a gasification plant is illustrated above.

In gasification processes, the amount of oxygen or air in the gasifier is controlled cautiously so that only some of the fuel completely burns. This process of partial oxidation offers the required heat for chemically decomposing fuel and generating syngas. The carbon dioxide has a high level of partial pressure, which prominently enhances the driving force for different separation & capture technologies. After the removal of CO₂, the H₂-rich syngas is transformed into power. It can also be deployed as a fuel in combustion turbines for generating electricity. Furthermore, another application of this gas is presently being innovated under the Fuel Cell Program by National Energy Technology Laboratory (NETL); it is to deploy H₂ for fuel cell-oriented power generation, with the motive of remarkably increasing the overall efficiency of a plant. Since the process of gasification functions at high pressure and CO₂ is available in much higher concentration in syngas (compared to flue gas in post-combustion), pre-combustion CO₂ capture technology is considered less expensive in comparison with the post-combustion CO₂ capture technology. For the equivalent amount of the captured CO₂, a much lesser volume of gas is required to be treated, this leads to a requirement of a much smaller size of equipment and lesser capital costs.

The Battle of the Technologies (Pre-combustion vs Post-combustion)

In comparison to the post-combustion technology that extracts diluted CO₂ (~5-15% concentration) at low pressure from the streams of flue gas, the shifted syngas stream is at a higher pressure and CO₂-rich. Due to the higher concentration of CO₂, the pre-combustion capture process is more efficient; however, the costs involved in the base gasification process are higher than in conventional power plants of pulverized coal.

Pre-combustion Carbon Capture Technology

Advantages	Limitations
Higher efficiency	Higher overall capital cost of the base gasification process
Comparatively easier carbon removal	Limited IGCC plants
Produced fuel is less harmful to the environment	Heat transfer challenges
Easy retrofitting in existing plants, lowering the adoption cost	Decay issues with hydrogen-rich fuel utilization

Post-combustion Carbon Capture Technology

Advantages	Limitations
More mature technology	Lower efficiency
Standardized techniques	Low CO2 concentration in flue gas
Ease of installation in new and existing plants	High cost of electrical energy generation
	Large parasitic loads

Some Thoughts on the Costs

The latest commercially existing technology of pre-combustion carbon capture typically deploys chemical or physical adsorption processes and approximately costs USD 60/tonne for capturing CO₂ formed by an integrated gasification combined cycle (IGCC) power plant. Further, it requires over USD 1500 million to build a typical plant of 500MWe.

Present Status and Key Technology Providers

Present Status:

There are numerous commercially existing IGCC projects. Nonetheless, the IGCC plant designs incorporating capture, which deploys the existing capture technology sustain high losses of energy of over 7-8% points by the inclusion of capture. These losses arise in several parts of the flow scheme of IGCC. In an IGCC plant that includes capture based on the existing technology, the technology readiness level (TRL) of the key components of the gas cooling, air separation unit (ASU), gasification, sulfur removal, shift, and CO₂ capture are all at a level 9.

Key Technology Providers:

The key providers of gas turbine-based power slabs in IGCC with capture are Siemens, General Electric, and MHI. All of the aforementioned companies also offer gasification so that they can efficiently provide overall packages of IGCC technology when integrated with shift, ASU, and AGR technology providers. Furthermore, the key providers of acid gas removal process include BASF and Dow for MDEA, Linde and Lurgi/Air Liquide for Rectisol, Honeywell for Selexol, and Shell for ADIP and Sulfinol.