CO₂ glossary

CO₂ topics at a glance

Difference between C, CO and CO2

Carbon, C, is also known as the atom of life. It is in DNA and in half of our food – chemistry knows of 200,000 compounds in the world without carbon and 20 million with it. It is found in many compounds (including frequently as CO2) and performs multiple functions: It is used to generate heat and power, as an aid in process technology (e.g. in steel production), and as a basic material in the manufacturing of products such as pharmaceuticals or plastics.

Carbon dioxide, CO2, occurs naturally in the air at a rate of 0.038%. CO2 absorbs part of the heat given off by the earth and radiates it back to the earth, which is why it is one of the greenhouse gases. It is a non-toxic, neither flammable nor explosive gas, and is a natural by-product of cellular respiration in many living organisms. CO2 is also released as dead organisms decay or through natural CO2 sources, such as volcanic gases. It is also produced, however, through combustion of wood, coal, oil or gas due to the carbon they contain (C), which combines with oxygen (O2). 

Carbon monoxide, CO, is a flammable, toxic gas. Unlike CO2, CO does not occur naturally in the atmosphere. It is formed by the incomplete combustion of coal, natural gas or oil. A low oxygen content and low temperatures lead to the formation of carbon monoxide during combustion, but it can be further burned to create CO2 where there is a sufficient oxygen supply.

Unavoidable CO2

Process-related CO2 quantities result from the reaction itself and not through the use of the energy required for the process. In industry, these are considered unavoidable if their formation cannot be prevented despite optimisation of the production process or the product. This is the case when no alternative processes and no alternative products or resources are available to a sufficient extent for the same use case.

These currently include, for example, process-related CO2 quantities from glass production, the chemical industry, lime and cement production, and electric arc furnaces in the steel industry.

Green or biogenic CO2

Biogenic CO2 is obtained from the combustion or fermen­tation of biomass (biological material consisting of carbon, hydrogen and oxygen), such as wood or food waste. The combustion of biomass leads to emissions that are considered CO2-neutral because the carbon is produced from a neutral CO2 cycle. Trees, for example, absorb CO2 from the air as they grow; when the wood is burnt, it is released back into the air. In this case, the total amount of CO2 in the atmosphere remains the same. The use of biogenic CO2 is climate-neutral under the same circum­stances as the utilisation of biomass. 

Grey or fossil CO2

Fossil CO2 is produced when fossil carbons (e.g. coal or lime) are utilized. The use of fossil CO2 to manufacture products in which the carbon is not permanently chemically bound (so-called long-life products) is therefore not climate-neutral. This is because the CO2 escapes into the atmosphere and contributes to climate change. This makes a sensible circular economy all the more important in order to reduce the further use of conventional fossil carbons.

CO2 capture

CO2 capture refers to the process of “capturing” CO2 from the air, from biomass, or from industrial emissions. Here, for example, a technical process makes it possible to capture up to 90% of CO2 emissions produced by industry.

CO2 can be captured directly from the air by means of a special process known as Direct Air Capture.

Carbon Dioxide Removal

Carbon Dioxide Removal (CDR) refers to techniques aimed at extracting CO2 directly from the atmosphere or from renewable bio-based sources. Unlike processes involving CCS or CCU facilities, which mostly work with CO2 from fossil fuels, CDR methods result in net-negative emissions. This is achieved by actively binding more CO2 than is emitted.

CO2 can be captured directly from the air by means of a special process known as Direct Air Capture (DAC).

Negative Emissions

Negative emissions are achieved using Carbon Dioxide Removal (CDR) technologies that actively remove CO2 from the atmosphere or compensate any remaining emissions. These technologies, such as Direct Air Capture (DAC) and the enhancement of natural carbon reservoirs, help to reduce global CO2 concentrations. Their development and application are decisive steps on the way to reducing greenhouse gas emissions.


The abbreviation CCS stands for Carbon Capture and Storage. Following the capture of CO2 from the air, from industrial emissions, or from biomass, there are various processes for storing this CO2, also known as sequestration. Here, a distinction is made between geological storage, where the CO2 is stored underground in rock formations, in natural sinks such as peat, bogs, or forests that naturally sequester carbon, and permanent sequestration in long-lived products.


CCU, Carbon Capture and Utilization, refers to the subsequent use of CO2 after CO2 capture. In the long term, even in a climate-neutral system carbon or hydrocarbons will still be needed, for example for air and sea transport or for the chemical industry. Recycling of carbon-containing products as a raw material source is gaining immense importance in this context. Since not all products can be recycled, on the one hand, and on the other this would most likely cover only part of the demand, CCU measures will be necessary to meet the demand using CO2 from the atmosphere as well as from unavoidable CO2 sources.  


If CO2 is first used after capture before being stored, this process is known as Carbon Capture, Utilization and Storage. 

Types of CO2 use

After the capture of CO2, it can be used in a wide variety of ways or bound long-term in products (CCU), e.g.:

As an energy source: liquid fuels, energy storage, gaseous fuels

As a material: solvents, fertilizers, urea, flexible foams, plastics, rigid foams, concrete and aggregates, cement and mortar

Physically: carbonated drinks, dry ice, fertiliser, greenhouses, fire extinguishers, refrigeration systems