If you have been working within carbon capture, utilisation and storage (CCUS) for more than a few weeks, you have probably already come across terms such as “pre-combustion”, “post-combustion” or “oxy-fuel”. But what do they really mean, and how to make sense of the different carbon capture technologies out there? This article will give you an overview of how the main types of carbon capture technologies are classified, and what the main differences are between them.
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Why do we need to categorise carbon capture technologies?
Carbon capture, utilisation and storage (CCUS) is a broad field, with a wide range of technologies and applications. To make sense of the different carbon capture technologies out there, it is useful to categorise them into different types. This can help us to understand the main differences between the technologies, and to identify the most promising options for different applications.
In CaptureMap, we have organised our overview of large CO2 emitters into 70 different activity types, from coal power plants to upstream oil and gas facilities, to waste-to-energy plants, cement plants and many more. Each of these activity types have different characteristics when it comes to fuels, equipment and processes that lead to CO2 emissions. These emissions, in turn, have different characteristics when it comes to CO2 concentration, contaminants, temperature, pressure, and other factors that can influence the choice of carbon capture technology.
By categorising carbon capture technologies into different main types, we can identify the most promising options for each of these facility activities, based on their specific characteristics and requirements. This can help us to develop a more targeted and effective approach to deploying carbon capture technologies, and to accelerate the transition to a low-carbon economy.
Another reason for organising capture technologies into main categories is that it makes it easier to compare technologies and to understand how they relate to each other. For example, if we know that a certain technology is a type of “pre-combustion” capture, we can infer that it is likely to have certain characteristics in common with other pre-combustion technologies, or at least compete with them.
What are the main types of carbon capture technologies?
Some of the main basic types of carbon capture technologies have been around for decades. We can trace the use of the term “post-combustion” to the 1980s or even earlier. Many documents out there also talk about “pre-combustion”, “oxy-fuel”, “inherent capture” or “capture from industrial process streams”, and “direct air capture”.
These terms are widely used in the industry, and we decided to adapt them for the main categories in our overview of carbon capture technologies in CaptureMap. However when we looked into the details we started running into issues linked to different definitions and criteria for categorising capture projects. Therefore we landed on our own definitions and criteria for categorising the technologies, which we will present in the following sections.
You will quickly notice that most categories are centered around combustion, although not all large CO2 emission points include that process. Our take on it is that those capture technology categories were mostly defined at a time where power plants were the main targets for carbon capture, and therefore combustion was the main process to be considered.
Post-combustion
CaptureMap’s definition for post-combustion capture is:
Post-combustion carbon capture separates CO2 from exhaust gases after fuel has been combusted with air. Examples of sources include conventional turbines, engines and boilers, but also kilns and ovens. This technology extracts CO2 from air-diluted flue gases where carbon dioxide is mixed with nitrogen and other combustion byproducts.
This mixture of CO2 with nitrogen and other combustion byproducts is for us the defining element of post-combustion capture. The presence of a large share of nitrogen in the air input to the combustion (typically 78%vol) means that the CO2 concentration at the output can only be so high. In addition, the other combustion byproducts and potential contaminants are a factor that can influence the actual capture technologies applicable to post-combustion.
As mentioned in the definition, typical examples of sources for post-combustion capture include conventional turbines, engines and boilers. Additionally, industrial equipment like kilns in cement plants and incinerators at waste-to-energy facilities are also common sources for post-combustion capture. When it comes to cement, this can be counter-intuitive, since in a typical cement plant 60% of the emissions come from calcination of limestone (CaCO3 -> CaO + CO2). However in conventional cement plants, this process CO2 is mixed with the combustion CO2 from burning fuels and waste in the kiln, and therefore all the emissions are considered as post-combustion.
Next on our overview of carbon capture technologies we will talk about oxy-fuel, since it is, in our view, the category most related to post-combustion.
Oxy-fuel
In CaptureMap, the definition for oxy-fuel is:
Oxy-fuel carbon capture uses pure oxygen instead of air for fuel combustion, creating exhaust gas that consists mainly of CO2 and water vapor. Examples include modified power plants or kilns, and industrial furnaces where oxygen is separated from air before the combustion process. This technology avoids nitrogen in flue gases, producing a high-concentration CO2 stream.
The use of pure oxygen as input to the combustion process is the defining factor for oxy-fuel capture. By removing the nitrogen from air before combustion, we avoid excessively diluting the CO2 in the flue gas, and obtain a much higher CO2 concentration than with post-combustion. This has major consequences when it comes to the downstream capture and conditioning for the CO2.
Most oxy-fuel implementations include significant modifications to existing facilities or the construction of new ones. There are limitations to the applicability of oxy-fuel to certain types of industrial processes or equipment due to the need for pure oxygen in the process, and to the consequences it has on the combustion characteristics.
Typical candidates for oxy-fuel carbon capture include power plants or modified cement kilns.
Now let’s talk about the next topic in our list of carbon capture technologies: pre-combustion.
Pre-combustion
There are variations in definitions for pre-combustion capture across the industry, and this category name can be misleading. After synthesizing available information, we’ve established the following definition for CaptureMap:
Pre-combustion carbon capture converts fuel into a mixture of hydrogen, CO2 and other gases, through gasification or reforming processes. Examples include processing coal, biomass, or natural gas with steam and/or oxygen to produce syngas. This technology separates CO2 from high-pressure, concentrated gas streams.
The term “pre-combustion” can be misinterpreted to mean capturing CO2 before any combustion occurs. However, all technical definitions focus on the same defining characteristic: the generation of syngas (a mixture of hydrogen, CO2, and other gases) and the separation of CO2 from this specific type of gas stream. Whether the remaining gases will be combusted later is not actually relevant to the capture process itself, despite what the name suggests.
So why calling this technology pre-combustion then? The terminology originated in power plant applications, where hydrogen separated from syngas would later be combusted for electricity generation—hence “pre-combustion.” However, modern applications extend beyond power generation to include hydrogen or ammonia production for chemical processes, fertilizers, and plastics—many of which involve no combustion step at all.
While a more technically accurate term might be “syngas separation” or “gasification-based capture,” we’ve chosen to retain the widely established term “pre-combustion” in CaptureMap for consistency with industry convention, while emphasising that users should focus on the technical process rather than the potentially misleading name.
Last on our list of carbon capture technologies: inherent process capture.
Inherent process capture
We gave this category a new name. In other documents it is often not clearly defined, and often referred to as “inherent capture” or “capture from industrial process streams”. CaptureMap’s definition is:
Inherent process capture integrates CO2 separation directly into the fundamental design of industrial processes. Examples include modified kilns that physically separate process emissions from combustion, CO2 separation in natural gas processing, and fermentation-based biofuels production. This technology produces high-purity CO2 streams without requiring additional separation.
At first, inherent process capture might seem like a miscellaneous type of category, where all the capture technologies not fitting in above mentioned categories are grouped. However the main differentiating factor for “inherent process capture” here is that the CO2 separation is the result and an integrated part of the industrial process itself. According to our statistics, inherent process capture is actually the most common type of CO2 capture already in operation. We will come back to that.
The most obvious examples of inherent process capture are CO2 emissions from fermentation within ethanol or biogas production. That CO2 is the direct result of the fermentation process, and is often vented at high concentrations if not captured. Very few modifications are therefore required for capturing and separating that type of CO2.
Natural gas processing (e.g. for sour gas treatment) is another type of inherent process capture, although this categorisation can be unexpected. The reasoning is that the CO2 separation is carried out for the purpose of gas treatment, to make it reach the necessary specs for further export, rather than for the purpose of reducing emissions. The separation is therefore an integrated part of the industrial process itself.
Lastly, a new type of cement and lime kilns physically separate heat generation from the actual calcination process, leading to process emissions from calcination being emitted through a separate stack, which can then be considered as “inherent process capture” rather than post-combustion capture as mentioned earlier.
Things can get complex: example with hydrogen production in SMR
Now that we have reviewed the different types of carbon capture technologies, let’s have a deeper dive in one example: hydrogen production with SMR.
A large share of the world’s hydrogen production is done through Steam Methane Reforming, where natural gas is converted into syngas, from which the hydrogen is separated. The figure below illustrates a typical SMR process, with the different GHG emissions and potential CO2 capture points highlighted.
As illustrated above, CO2 on an SMR can be captured from the pre-reformer boiler (point #1, ca. 40% of the CO2 emissions) or from the purification unit tail gases (point #2, ca. 60%). The CO2 emitted from the pre-reformer boiler only originates from combustion of natural gas with ambient air, it is therefore post-combustion capture. On the other hand, the CO2 emitted from the purification unit tail gases comes from reforming and water shift, which involve syngas, it is therefore pre-combustion CO2.
Carbon capture on SMRs can therefore involve both post-combustion and pre-combustion capture, depending on the source of the CO2 at the unit. Due to the higher CO2 concentration in the purification unit tail gases, carbon capture is more likely to be implemented there, but some sites also capture CO2 from the pre-reformer boiler flue gases, where the it is less concentrated.
What can we learn from CaptureMap’s categorisation of carbon capture technologies?
By categorising each carbon capture project in CaptureMap according to its main capture technology type, we can draw valuable insights and trends. Below is a first take, and we will follow-up this article with other pieces about these insights and trends.
For the figure below we categorised more than 1200 carbon capture projects according to their main capture technology type and to their last known engineering status. Here feasibility refers to all stages before a Final Investment Decision (FID) is taken, while Engineering Procurement and Construction (EPC) is the next phase until start of operations.
As mentioned earlier, most of the capacity for carbon capture projects already in operations is concentrated within inherent process capture and pre-combustion. The main reason for that is the efficiency and effectiveness of these technologies in capturing CO2 emissions. With inherent process capture, the CO2 is almost readily available for further treatment and conditioning. With pre-combustion, the CO2 is also easier to separate from syngas already being produced, compared with other capture technologies.
There are already some projects in operation within post-combustion carbon capture, although the capture capacities there are more limited. That said, it’s also the capture technology with the largest pipeline of projects under development. Oxy-fuel is the category with the least advanced and least amount of capture capacity among the listed technologies.
Finally, for many capture projects the technology type is not explicitly provided and we categorised those as “Unknown” until we have more information available. A detailed look at those projects shows that many are implemented within refineries, ammonia production, upstream oil and gas, hydrogen and ethanol production. This indicates that the actual capture technology is likely to be inherent process capture or pre-combustion, increasing further the share of capture projects capacities within those categories.