Decarbonisation Technology - May 2023 Issue

Quarrying

Pyroprocessing

Grinding

Transportation

85%

Calcination

* 50%

Raw mill

Clinker cooling

CO% eq

1500˚C

Figure 3 Geogenic emissions upon limestone calcination account for about 50% of the total process emissions, as indicated by *

carbonated products (see Figure 1). The reaction is typically exothermic; hence it is not particularly energy-consuming, and the heat generated can be easily recovered and reused. Most importantly, these processes combine both storage and utilisation, with the resulting materials being fit for use by the large construction and building materials sector. These industries are currently characterised by a considerable carbon footprint, as the cement industry alone is responsible for at least 8% of global anthropogenic emissions (Ellis & et a l., 2020). The production of cement involves heating calcium carbonate, commonly known as limestone, to approximately 1,450°C (2,700°F) to separate calcium and carbon and make clinker. As a result, CO 2 emissions are generated both by the fuel burned to heat up the limestone and by the decomposition of the latter (see Figure 3 ). Hence cement accounts for over 90% of the carbon intensity of concrete, even though it only represents 10-17% of the mass of concrete. The adoption of mineralisation-based CCUS solutions for point source emitters can therefore act as a gateway to an immense market and unlock the sequestration of high volumes of CO2 . Moreover, as buildings and their components are characterised by long service lives, these can be considered carbon storage facilities. When looking at the specific technologies and opportunities available, the mineralisation of captured CO 2 can support the concrete industry through two alternatives. Specifically, carbon

emissions can be used as feedstock to cure cement, also known as carbonate hardening, or to produce synthetic aggregates. Carbon storage via aggregate production The first is initiated by the diffusion of gaseous CO 2 to the external surface of concrete materials, from where it enters the pores. The gas subsequently reacts with calcium sources, such as calcium silicate in Portland clinker, to form calcium carbonate and silica-rich components that reduce the material’s porosity. Therefore, the technique acts as an enforced (or active) carbonation treatment to increase the hydration of cement. The superficial layer of calcium carbonate (CaCO 3 ) generated ultimately improves key properties, such as compressive strength. To date, curing processes can only be performed in batch conditions and require an accurate tuning of temperature and pressure to facilitate the reaction, contributing to higher energy requirements. Also, carbon storage only occurs on the surface of cementitious materials, sequestering limited volumes of CO2 , especially when components are made of large particles and have compact surfaces. Usually, the natural carbonation of concrete is limited to a few millimetres from the exterior surface exposed to the atmosphere (Kaliyavaradhan & Ling, 2017). Conversely, carbon storage through aggregate production can be 10-40 times higher and continuous processing is possible, limiting energy usage and increasing throughput. This method relies on a capture system that