HeidelbergCement is using concrete to decarbonise

Multiple pathways are mandatory to mitigate cement CO₂ emissions, yet the potential of concrete to absorb CO₂ is underestimated, Jan Theulen, HeidelbergCement, explains.

Concrete is the second most used commodity in the world after water, and the demand is expected to grow further, due to population growth and urbanisation. Therefore, the potential of concrete to capture a significant amount of the CO₂ emitted during the production of cement – one of main constituents of concrete – needs to be fully exploited. This potential, called re-carbonation, is complementary to other measures that the cement-industry is currently researching. It follows the conviction that none of the options to mitigate CO₂ can be neglected by the cement industry to make their products carbon neutral during its lifecycle.

Carbonation of concrete: a nightmare or blessing?

Within the built environment, it is well known that re-carbonation can lead to deterioration of reinforced concrete structures. Let’s have a look how this works: the steel bars in reinforced concrete are not being corroded, due to the high pH-value of the concrete around it. This prevents the water and chlorides present in concrete from attacking the steel. However, concrete surfaces will react with the CO₂ from ambient air. As a result, the pH value drops, taking away the protection of steel against corrosion.

So, why should we promote re-carbonation as a means of mitigating climate change? When this re-carbonation process is well controlled, the negative impacts, as described above, can be avoided. Moreover, a well-controlled carbonation process increases the strength of concrete, potentially enabling the reduction of the amount of cement in concrete. The climate change benefits will be delivered in two forms:

1. CO₂ absorbed by concrete through the process of carbonation; and
2. Less cement means less CO₂ released during production.

How can this carbonation be applied?

There are three phases in the life cycle of concrete where the re-carbonation effect can be utilised to sequester CO₂:

• During the pouring and hardening time of concrete;
• During the lifetime of the concrete structure, when exposed to ambient air; and
• At the end of the lifetime of concrete – after the demolition of the building or structure it served.

Carbonation at the start of lifetime

The use of CO₂ to harden concrete pre-cast elements is applied in so-called autoclaves. In these temperature-controlled vessels, the concrete elements are put under gas pressure from CO₂. There are different possible applications for the use of CO₂ in hardening concrete elements, but those referred to here are produced by using standard cement types.

HeidelbergCement is engaged in developing and optimising the use of CO₂ in autoclaves for the hardening of concrete structures, and in order to maximise the use of CO₂ for this purpose.

Carbonation during lifetime

During the lifetime of the concrete structure, it is exposed to CO₂ from the ambient air. For example, any slim, smaller or hollow structures, like prefabricated floor or wall elements, have a quite beneficial surface/thickness ratio, and therefore offer greater surface to the air, compared to its total mass. Through this, more CO₂ can be absorbed.

This is particularly efficient for structures without reinforcement steel, or reinforced concrete, which are not exposed to outside climate conditions. For any structures exposed to outside weather attacks, the steel reinforcement can be replaced by carbon or silica fibers, or other non-corroding reinforcement materials.

Carbonation at end-of-lifetime

Finally, when a concrete structure has been demolished, the concrete is usually crushed so that it can serve a second life in replacing virgin aggregates in various applications. Thereby, the cement particles within this concrete grit can sequester CO₂ just by exposing these fines to a CO₂-rich atmosphere.

Also in this domain, HeidelbergCement is exploring the underlying mechanisms and is preparing field tests in order to collect quantitative results for various concrete conditions.

Lowering the costs of capturing CO₂

For the re-carbonation processes described above, a CO₂ concentration of 20-30%, as contained in the exhaust gas of standard cement kilns, is potentially sufficient. However, in many cases, a direct conflation of these two streams – exhaust gas and concrete grit – is not possible for logistical reasons, and it is by far more economic, or simply mandatory, to liquefy CO₂ and store it in standard tanks or transport it by trucks and ships.

HeidelbergCement is therefore engaged in several carbon capture technologies, each with another maturity level.

Carbon capture by amine scrubbing

The end-of-pipe technology of amine scrubbing is state of the art in the power sector, and tests in Brevik, Norway, have demonstrated that it can similarly be applied to cement kilns. Therefore, an industrial scale test project – 400,000 tonnes of CO₂, per year – is scheduled to be realized, with support of the Norwegian government.

Carbon capture in oxyfuel mode

An integrated technology for cement kilns is in using so-called oxyfuel technology, where ambient air is replaced by kiln flue gas, recirculated within the system to increase CO₂ content, while pure oxygen is added to safeguard proper burning conditions. Thereby, CO₂ concentration can be increased to a range of 80-85%.

The European Cement and Research Academy (ECRA) is scheduling two demos at an industrial scale, one at HeidelbergCement’s Colleferro plant in Italy, and the other at Lafarge Holcim’s Retznei plant in Austria. The final results of these tests are expected to be available around 2022.

Carbon separation in the process

A 3rd carbon capture technology applicable to the cement industry, Low Emission Intensity Lime and Cement (LEILAC), is being developed under the EU research programme Horizon 2020, funded with €12m. In this process-integrated technology, the CO₂ released from limestone is separated by indirectly heating the material, and so avoiding mixture with flue gasses from kiln fuels.

The construction of the direct separator reactor (DSR) is in full swing and will be erected at the Belgian Lixhe plant, owned by HeidelbergCement, within the next months. Ten partners from industry and universities synergise their know how in order to bring this breakthrough technology to the next level of maturity. In 2019, an intensive test programme will be executed; if successful, the road map to further commercialisation of the technology will be another key part of the work package, which will be executed within the two to three years.

Using CO₂ for purposes other than concrete

Although the use of CO₂ to recarbonise concrete offers the best opportunity by volume, the cement industry will further push for the development of other applications that can substantially contribute to the decarbonisation of the sector.

As such, various programmes have been launched:

• The mineralisation of natural minerals with CO₂, such as olivine or basalt, as well as industrial byproducts, such as steel slags and fly ashes;
• The use of CO₂ and sunlight to grow micro algae for the purpose of replacing soybean in cattle or fishfeed; this is a particularly environmentally-friendly solution as it uses non-arable land, instead of agricultural or natural land; and
• The use of CO₂ with renewably-produced hydrogen to produce synthetic fuels or base chemicals. This method can contribute in mitigating climate change, where case-sufficient surplus renewable energy is available.

Maximising the use of green energy

For 30 years the cement industry has been using alternative fuels to replace fossil fuels, in turn, covering their demand for thermal energy. It is widely applied in Europe and, also, worldwide. Innovative measures, for example, gasifying waste, are being developed further under the lead of the industry. For certain countries, where abundant renewable, electrical energy is available, the electrification of the thermal intensive kiln process is also being investigated.

All of these different measures only serve one target: to ensure the industry meets the goals of the Paris Agreement.

Jan Theulen
HeidelbergCement Group

https://www.heidelbergcement.com/en