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

Biogenes CO2 wird aus der Verbrennung oder Vergärung von Bio­masse (biologischem Material, welches aus Kohlen­stoff, Wasser­stoff und Sauer­stoff besteht), wie z.B. Holz, gewonnen. Die Ver­brennung von Bio­masse führt zu Emissionen, die als CO2-neutral angesehen werden, weil der Kohlen­stoff aus einem neutralen CO2-Kreislauf entsteht. Bäume beispiels­weise binden beim Wachsen CO2 aus der Luft, wird das Holz verbrannt, gelangt es wieder in die Luft. Die Gesamt­menge an CO2 in der Atmo­sphäre bleibt in diesem Fall die gleiche. Die Nutzung von biogenem CO2 ist unter den gleichen Umständen klima­neutral wie die Ver­wertung von Biomasse. 

Graues bzw. fossiles CO2

Fossiles CO2 entsteht bei der Nutzung fossiler Kohlen­stoffe (z.B. Kohle oder Kalk). Die Verwendung von fossilem CO2 zur Herstellung von Produkten, in welchen der Kohlen­stoff nicht permanent chemisch gebunden ist (sog. langlebigen Produkten), ist somit nicht klima­neutral. Denn das CO2 entweicht in die Atmo­sphäre und trägt zum Klima­wandel bei. Hier ist eine sinn­volle Kreis­lauf­wirt­schaft umso wichtiger, um den weiteren Einsatz konven­tioneller fossiler Kohlen­stoffe zu reduzieren. 

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.


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