Splitting steelmaking pre-combustion gases into hydrogen rich outputs and high purity CO2 streams (CASOH technology)

This innovative technique enables the upgrade and decarbonisation of steelmaking process gases using a calcium / copper assisted looping process which converts a fraction of blast furnace gas into a H2-rich output while a CO2-rich gas is obtained which may undergo further purification for geological storage or reuse. This has been tested at a pilot scale facility built by ArcelorMittal in Asturias (Spain), under the EU Horizon 2020 project C4U over the period 2020 – 2025.

TECHNICAL DESCRIPTION

This innovative technique, designated as the CASOH technology (‘’Calcium Assisted Steel-mill Off-gas Hydrogen’’) is designed to enhance the valorisation of internally generated steelmaking gases while at the same time reducing their associated CO₂ emissions. This dual-purpose approach not only contributes to emissions reduction but also supports the transition toward cleaner hydrogen production. Although initially developed and tested using steelmaking gases, the technology also exhibits promising adaptability to other industrial processes involving CO-containing gases broadening its potential impact across sectors.

In the CASOH process, BFG is transformed into a H2-rich gas free of CO2 with a high energy content (e.g. Lower Heating Value - LHV). The process is carried out in dynamically operated packed bed reactors following a sequence of three reaction stages:
• BFG upgrading;
• Cu oxidation
• CO2 sorbent regeneration.

Temperature and pressure are alternated to accommodate the conditions of the solids bed in every stage.

 CASOH stage: It involves the production of a H2/N2 fuel gas through a water-gas-shift (WGS) reaction of the CO contained in the BFG (around 23 %vol.). The Cu-based particles act as WGS catalyst, and the CaO reacts with CO2 forming CaCO3. The continuous removal of CO2 from the gas phase as soon as it is produced, shifts the WGS equilibrium towards a higher production of H2, so that a very high conversion of the inlet CO is feasible at temperatures ranging between 500-700 C.
In the absence of CaO carbonation (i.e. only WGS reaction), a maximum concentration of 40 %vol. H2 (on a N2-free and dry basis) can be achieved at 550°C, at atmospheric pressure and with a steam-to-CO molar ratio of 1. In contrast, when CO2 is removed from the gas phase by the carbonation of CaO, almost pure H2 (above 98 %vol.) with only 1 %vol. of CO (on a N2-free and dry basis) can be produced at 550°C. A temperature of 650°C and a S/C molar ratio in the feed slightly above 1 must be reasonably close to an economic optimum because they allow reaching CO2 capture efficiencies in this reactor of about 95%, with moderate steam consumption and with both WGS and CaO carbonation reactions sufficiently fast. However, metal dusting can occur during BFG preheating at temperatures between 300°C and 700°C. For this reason, an excess of steam mixed with BFG is desirable.

 Cu oxidation stage: The main purpose of the Cu oxidation stage is to obtain the necessary CuO to carry out the subsequent regeneration of CaO by means of the reduction of CuO with a fuel gas. In this stage, a flow of air reacts with the metal-based particles and the heat released rapidly increases the temperature of the bed until a maximum value is achieved in the oxidation front. A sufficient quantity of inert solids is required in the bed in order to limit the maximum oxidation temperature, and therefore, the partial calcination of the CaCO3 formed in the CASOH stage.
The calcination of CaCO3 is favoured at temperatures above 800°C. In these conditions, the oxidation of Cu proceeds very fast, giving rise to a complete conversion of O2 in the oxidation front and a product gas mainly composed of N2. In these mild oxidation conditions, the chemical stability of the metal oxide has been reported after successive redox cycles. The raise in temperature required to achieve values of around 900°C in the next sorbent regeneration stage is moderate, which allows reasonable Cu/Ca ratios in the composition of the solids bed.
The oxidation of Cu must be carried out at relatively high pressure in the large scale CASOH process, to minimise the partial calcination of CaCO3 in this stage. The oxidation of Cu at 800ºC and at atmospheric pressure would generate a product gas at the reactor exit containing around 21 %vol of CO2, which means that almost 95% of the CaCO3 present in the reactor would be calcined. As the operating pressure increases, the fraction of calcined CaCO3 during oxidation rapidly decreases. If the oxidation is carried out at 10 bars, the maximum concentration of CO2 in the product gas given by the equilibrium is 2.2 vol%, which would lead to only 8% of calcined CaCO3.

 Sorbent regeneration stage: In the sorbent regeneration stage, a balanced CuO/CaCO3 ratio will ensure a suitable reactor performance allowing the reduction and calcination to proceed simultaneously, reach moderate maximum temperatures of around 900ºC and leave totally converted solids in the bed. Different fuel gases can be used, which determines the Cu/Ca ratio required in the bed to sustain the reduction/calcination stage. The use of nitrogen-free fuel gases (such as natural gas or COG in the regeneration stage would allow a higher CO2 purity to be obtained directly from the reactor (i.e. about 95% vol. on a dry basis) at the expense of using a higher Cu/Ca molar ratio of 2.1. Other alternatives are considered also in the scheme of the CASOH process for regeneration such as the addition of steam or the application of vacuum to decompose the calcium carbonate and generate high purity CO2 from the bed of solids.

DEGREE OF MATURITY
The CASOH technology has reached a TRL of 7 since it has been demonstrated in an operational environment using real industrial conditions. In the C4U project, over 2,000 hours of continuous operation were successfully completed at ArcelorMittal Global R&D’s pilot facility, located within a functioning steelmaking plant.
Following the successful pilot demonstration, the next logical step for CASOH is scaling up to a pre-commercial scale. Plans are being considered for deployment in a larger demonstration facility, potentially as part of a follow-up EU-funded project or through private investment.
Besides, this technology could be applicable to a wide range of industrial sectors with CO-containing off-gases.

CROSS-MEDIA EFFECTS
While CASOH offers significant environmental benefits through CO₂ capture and hydrogen production, it is an energy-based process as it occurs at elevated temperatures. Initial gas preheating is required to carry out the process. This implies the necessity to rely on preheating systems that can heat up gases up to 500ºC. Solutions with lower environmental impact are linked with heat recovery from the same exhausted process gases, dedicated H2 product gas from the process to preheat or electrical gas heating. Other alternatives, however with higher environmental impact, include the use of natural gas.
The CASOH process also require steam for proper operation. The steam used is required not for energy but for chemical purposes. Indeed, the WGS reaction needs water to combine with CO for H2 generation. Besides, the steam calcination method would be another process step that require steam. These requirements depend highly on the chemical composition of the inlet gas.
The implementation of CASOH at industrial scale introduces potential safety and operational risks. The handling of high-temperature gases, pressurised systems and reactive materials increases the likelihood of accidents if not properly mitigated through design and operational protocols. Even though the functional materials do not present a high risk, precautions have to be taken into consideration during handling as new material can generate dust emissions

BARRIERS TO IMPLEMENTATION:
Typical obstacles encountered for implementation of carbon capture in general also applies in the case of the CASOH technology, including:

 The availability and consistency of suitable process gases, such as blast furnace gas, may limit deployment in facilities with variable or insufficient gas streams.
 The successful implementation of CASOH requires robust infrastructure for CO₂ transport, utilisation or storage, which is not yet fully developed in many regions. This includes limited access to geological storage sites or CO₂ pipeline.
 Concerns related to industrial emissions, hydrogen safety, and long-term CO₂ storage may influence public perception and policy support (social acceptance).
 Permitting, and regulatory approval processes for new decarbonisation technologies can be complex and time-consuming, particularly when integrated into existing industrial operations.

Basic information about the technique

Participant Companies