Studying carbon capture technologies is very important because it is one of the most important solutions for mitigating the emission of carbon dioxide from commercial-scale power generation plants. Studies report that carbon capture technologies can lower carbon dioxide emissions by 50% by 2050. This article discusses and compares the three main carbon capture technologies: oxyfuel combustion, post-combustion, and pre-combustion.

Pre-combustion carbon capture

The pre-combustion carbon capture technology is mature and has been used to capture carbon dioxide for many decades. The pre-combustion technology comprises a unit for air separation to separate oxygen. The fuel reacts with air and produces synthesis gas, which is moved to the shift reactor for the production of carbon dioxide and hydrogen. The carbon dioxide is compressed and liquefied for storage and transport purposes.

The separation of carbon dioxide in pre-combustion is easy and has a lower energy requirement than in post-combustion. However, pre-combustion still requires air separation, energy for reforming, and enhancements in the energy recovery efficiency within the process. In addition, further stages of purification are needed when coal or oil is used to remove compounds containing sulfur, ash, and impurities.

The carbon dioxide produced via the pre-combustion carbon capture process has a high-pressure characteristic. Moreover, pre-combustion facilitates hydrogen production as a fuel for use as a building block for producing value-added chemicals for transportation, or fuel cells. The flexibility of the outputs for either power generation or hydrogen production makes pre-combustion highly beneficial.

Separating the carbon dioxide and hydrogen mixture in pre-combustion can be carried out through chemical or physical absorption techniques. This is done through syngas scrubbing using a hydrogen sulfide and carbon dioxide selective liquid solvent. A physical solvent requires higher partial pressure while a chemical solvent requires lower partial pressure.

In pre-combustion carbon capture, the purity of hydrogen is not a priority, whereas the separation of carbon dioxide is. Membrane reactors and adsorptive reactors have the potential for integrating separation and reaction in a single unit at lower energy requirements.

The adsorptive reactor technology uses selective solid carbon dioxide adsorbents to facilitate carbon dioxide removal. The features of these adsorbents include stability during carbon dioxide adsorption and regeneration cycles, selectivity, fast sorption, mechanical robustness, and high capacity for carbon dioxide adsorption.

The commonly used membrane in membrane reactors is the palladium membrane or its alloy. Studies report that palladium has a high tendency for deactivation and sulfur poisoning even at lower reaction temperatures. Silica-based membrane, on the other hand, does not have this tendency.

Oxyfuel combustion carbon capture

Oxyfuel combustion carbon capture follows the combustion of a carbon-based fuel in a pure oxygen and re-circulated fuel gas stream. The commercialization of oxyfuel combustion is limited due to the high cost of separating and producing oxygen. However, the capture and separation of carbon dioxide are easy for this route.

The oxyfuel combustion route is more promising with respect to energy efficiency as compared to pre-combustion and post-combustion. Oxyfuel combustion has a low-efficiency penalty of about 4% in comparison to about 8-12% for post-combustion.

The oxyfuel combustion process can be attached to a power plant, which can decrease the electricity cost and power plant capital cost by 7-12% and 10-18%, respectively. Moreover, an advantage of utilizing the oxyfuel combustion process is that it can be used in existing and new power plants in addition to the use of different fuel types like lignocellulosic biomass or municipal solid wastes.

Integrating bioenergy and carbon capture and storage (BECCS) for climate change mitigation leads to a negative carbon approach. A recent study reported that in oxyfuel combustion of lignocellulosic biomass, the emitted carbon dioxide of net electricity production is -0.27 kgCO2 MJel-1.

Another study reported an emission of -0.70 kgCO2, eq k-1 of wet waste feedstock when carbon capture is integrated with the incineration of municipal solid wastes. This presents BECCS as a potentially effective way to achieve decarbonization and as a negative carbon technology for the abatement of climate change along with oxyfuel combustion carbon capture.

A more recent study reported a reduction of -3.7 megatons of carbon dioxide per year by using biomass in oxyfuel combustion using a supercritical carbon dioxide cycle. Additionally, if the carbon tax goes above $28.3 per ton of carbon dioxide, the BECCS process might be more economical than fossil-based fuels.

However, the BECCS technology still faces several challenges compared to fossil-based fuels. These include lower efficiency, levelized electricity cost, and higher cost of biomass. Also, if carbon dioxide purification and compression units and air separation units are included in the oxyfuel combustion system, the efficiency falls by 9-13%. This is due to the energy-intensive nature of air separation and purification and compression of carbon dioxide.

Linde Engineering is one of the companies working on oxyfuel combustion carbon capture. The company provides air separation units for oxygen production, carbon dioxide compression, drying, and purification systems, and liquefaction systems.

Another company working on oxyfuel combustion is OxyBrightTM, which developed an advanced carbon capture technology for generating steam. OxyBrightTM was created by the Babcock & Wilcox (B&W) company.

Post-combustion carbon capture

Post-combustion carbon capture involves the capturing and separation of dilute carbon dioxide from the flue gas of a combustion system in an oxidant environment. Prior to capturing carbon dioxide, the emitted exhaust flue gas passes through desulfurization and denitrification processes along with the prevention of solvent degradation by removing dust and cooling.

The captured carbon dioxide is compressed into a supercritical fluid and sent for storage in saline aquifers or geological reservoirs. Due to the low concentration of carbon dioxide in the flue gas stream and its high flow rate, in addition to its underlying stable nature, solvent regeneration requires an energy-intensive process.

The commonly used and commercialized post-combustion method is monoethanolamine absorption. Other absorbents include N-methyldiethanolamine and 2-amino-2-methyl-1-propanol. Monoethanolamine chemisorption has a high operating cost.

In post-combustion, the absorption route can be used via the pressure swing or temperature swing adsorption processes in addition to calcium looping. The commonly used solvents are amine solutions due to their good acid gas selectivity and high capacity for carbon dioxide absorption. The disadvantages of using amine solutions include degradation, high energy footprint during regeneration, and corrosivity.

The use of membrane separation can reduce the capital costs of post-combustion technology. The membrane separation process requires low energy, easy retrofitting, low operating cost, and low carbon footprint. It can also be easily scaled up with existing power plants.

Several companies are using post-combustion technology to capture carbon. Some of these companies make use of solvent technology while others use sorbent technology. Several others use membrane technology.

Membrane Technology and Research, Inc. (MTR) is one of the companies using membrane technology. MTR uses low-pressure membrane contactors, which reduces the system’s footprint, energy use, and costs. In a detailed economic and performance analysis, MTR reported the capture of 90% of carbon dioxide from flue gas using Econamine.

GE Global Research developed a large pilot-scale (10 MWe) using a new aminosilicone-based solvent to capture carbon dioxide. This solvent was used to quantify and minimize the risks associated with schedule, costs, and technical success. The company reported in a techno-economic analysis that the cost of carbon dioxide removal utilizing the steam stripper for desorption was $42 per metric ton.

SRI International developed and tested its post-combustion carbon capture process using a novel carbon sorbent composed of carbon microbeads. SRI reported that the carbon microbeads show good resistance to agglomeration, rapid adsorption/desorption kinetics, and excellent selectivity and carbon dioxide capacity.

Future research

Post-combustion carbon capture is a more mature technology compared to pre-combustion and oxyfuel combustion. While the pre-combustion carbon capture may offer higher efficiency than post-combustion carbon capture, pre-combustion is more costly. Research is needed to find a solvent with superior absorbent characteristics to reduce the cost associated with pre-combustion carbon capture.

In oxyfuel combustion carbon capture, further research is needed to develop new routes of air separation. Possible research areas may include oxygen-transport membranes and ion-transport in addition to chemical looping methods.

In post-combustion carbon capture, research can be directed towards novel solvents with suitable properties like low degradation, low corrosiveness, low cost of production, and high cyclic capacities.