Tue, 30 November, 2021
Carbon capture, utilisation and storage (CCUS) is widely acknowledged as a viable option for the decarbonisation of energy-intensive industries, such as steelmaking and cement production, both tackled by the FORGE project. The cement industry in particular, has been estimated to contribute to approximately 8% of the global CO2 emissions1, thus making it an ideal target for the application of CCUS technologies in the project.
CCUS involves capturing CO2 from the industrial process, its transportation in compressed form, and finally either its use as input or feedstock to create products or permanent storage in underground geological formations. Several capture technologies are available:
- Chemical absorption
- Physical separation
- Oxy-fuel separation
- Membrane separation
- Calcium looping
- Chemical looping
- Direct separation
- Supercritical CO2 power cycles
each with its own advantages/disadvantages depending on the industrial process they are applied to.
Depending on the specific process, CO2 extracted from flue gases inevitably contains a certain amount of impurities, such as SOx, NOx, H2S, O2, Ar, N2, H2, CO and H2O, depending on the specific CCUS technology. These impurities are associated with corrosive degradation mechanisms of the CCUS component material, generally a carbon steel when possible (i.e. if the environment allows). For instance, acid gas impurities (SOx, NOx and H2S) can dissolve in water condensed on the component’s surface, leading to formation of acids (including H2SO4, H2SO3, HNO3, H2CO3, etc.) and increased corrosion rates. The most corrosive environments are generally found in the components at the beginning of the CCUS system, immediately after flue gas extraction. In the worst cases, corrosion resistant alloy (CRA) coatings or even non-metallic materials such as glass-reinforced plastics are used. As the safety of CCUS is one of the most important issues dictating its large scale application, developing resistant materials is of fundamental importance for the future of the technology2.
Accelerated material development in the FORGE project - from thermodynamic calculations to real coatings
In the FORGE project, TWI is involved in developing compositionally complex alloys (CCA) resistant to the most corrosive acidic environments found in CCUS systems. As for the other three material development targets of the project - wear-resistant CCA, H2 embrittlement resistant CCA and corrosion resistant Compositionally Complex Ceramics (CCC) - the strategy involves a combination of high-throughput thermodynamic calculations via the CALPHAD method, high-throughput experiments and their processing by means of machine learning algorithms.
In a first stage of the project, an in-depth literature review was performed to characterise the harsh environments in CCUS systems, as well as identify elements potentially able to produce high-performance alloys for the environments. In this way, it was decided to focus on the de-sulphurisation unit of an oxy-fuel separation system applied to flue gas from clinker production in the cement industry. The actual composition of the flue gases in such an environment will be provided by the cement manufacturer CIMSA, partner in the FORGE project. Moreover, elements Fe, Ni, Cr, Ti, Al, Mo, Nb and Ta were identified as possible constituents of the CCAs developed for this target application. These elements were the constituents of a first set of 10 CCAs predicted by a combination of ML and CALPHAD calculations and prepared by induction/arc melting.
This set of 10 CCAs, together with another 20 CCAs obtained from the development of the other targets in the project (i.e. wear resistance and H2 embrittlement), were then tested by cyclic potentiodynamic polarisation (CPP) in an aerated solution containing a mixture of H2SO4 and HNO3 (corresponding to pH~2), at 25°C (Figure 2). Testing all of the CCAs in the project (i.e. not only the 10 envisaged for the CCUS target), was performed in order to have more data points available to evaluate the correlation between CCA metrics and corrosion performance responses. These CPP tests are currently being analysed to extract corrosion performance metrics relevant for the correlation analysis, such as corrosion potential (Ecorr) and current (icorr), corrosion rate (CR), passivation current (ipass), pitting potential (Epit), etc.
The results from the correlation analysis, together with high-throughput thermodynamic calculations will then be employed to select compositions for the subsequent phase of coating production.
The compositions selected after the induction/arc melting phase will be placed at the centre of 4in. wafers, coated by PVD, therefore creating a compositional gradient. This gradient will allow approximately 60 compositions to be tested for each wafer, for an estimated total of ~300-400 compositions tested for each performance target by the end of the project. Performance data from the test will be employed to train machine learning models for corrosion resistance in CCUS environments which will be used to predict, by also including cost considerations, compositions which will be eventually used to produce the final coatings in the project (i.e. deposited by HVAF, HVOF, Cold Spray and LMD). In order to verify the viability of the alloys predicted at different stages during this process of testing and refinement, TWI will be also involved in the production of alloys by arc melting, thus produced at a much lower solidification rate than that experienced in PVD coatings deposition.
Testing will also increase in complexity as the project progresses, with tests to be performed in environments as close as possible to the actual conditions experienced in a real CCUS system. Temperature, pressure and presence of further gaseous species will be included in the tests performed on the final coatings in the project, by testing in a dedicated glass vessel at the TWI Cambridge site. In the glass vessel, both static exposure and electrochemical tests will be performed, thus allowing an in-depth understanding of the fundamental corrosion mechanisms for the newly developed coatings.
Exciting times ahead!
- Francesco Fanicchia, PhD, TWI Ltd.
 Earth Syst. Sci. Data, 11, 1675–1710, 2019 https://doi.org/10.5194/essd-11-1675-2019
The FORGE project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 958457