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# Solar-aided direct reduction of iron ore with hydrogen targeting carbon-free steel metallurgy

Stéphane Abanades, Sylvain Rodat

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Stéphane Abanades, Sylvain Rodat. Solar-aided direct reduction of iron ore with hydrogen targeting carbon-free steel metallurgy. Renewable Energy, 2024, 235, pp.121297. 10.1016/j.renene.2024.121297. hal-04696411

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Submitted on 13 Sep 2024

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# Solar-aided direct reduction of iron ore with hydrogen targeting carbon-free steel metallurgy

Stéphane Abanades*, Sylvain Rodat

CNRS, Processes, Materials and Solar Energy Laboratory (PROMES), 7 rue du Four Solaire, 66120, Odeillo Font-Romeu, France

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## ARTICLE INFO

### Keywords:

- Direct reduced iron

- Solar metallurgy

- Iron/steel making

- Packed-bed solar reactor

- Hydrogen

- Thermochemical iron ore reduction

## ABSTRACT

Steel metallurgy accounts for about 7 % of global greenhouse gas emissions and its decarbonation is a major issue. The direct reduction of iron ore with $H_2$ using concentrated solar heat offers a carbon-free route for the iron metallurgical process. Such a solar process for clean ironmaking has never been implemented before. This study aims to investigate the reaction characteristics of direct iron ore reduction and further proposes a first experimental demonstration of direct reduced iron production in a packed-bed solar reactor under real solar irradiation. $Fe_2O_3$ reduction with $H_2$ was first studied by thermogravimetric analysis to unravel the effect of temperature, $H_2$ mole fraction, and particle size. The reaction rate increased with temperature (400–800 °C) and $H_2$ mole fraction (25–75 %), and the reaction reached completion with an initial conversion starting from 370 °C. An activation energy of 38.5 and 30.4 kJ/mol was determined for commercial $Fe_2O_3$ and iron ore, respectively. The size of iron ore particles after pellets crushing (in the range 0.25–2 mm) did not significantly affect the reaction rate. However, fine micron size powder with low bulk density showed improved conversion rates. Finally, the reduction process was demonstrated in a solar-heated packed-bed reactor up to 1000 °C, confirming the feasibility of renewable iron production from complete ore reduction at moderate temperatures (starting above 400 °C), and thus paving the way toward decarbonation of the iron and steel metallurgical industry.

## 1. Introduction

In the context of a sustainable energy transition, hydrogen has a strong potential for decarbonation of the hard-to-abate industrial and transportation sectors. Reducing iron ore with non-carbon reducers and with solar energy represents a major breakthrough for decarbonization of the steel industry. In the conventional process, the energy-demanding iron metallurgy involves the reduction of iron oxide into metallic iron, steel and cast iron using coke and coal in blast furnace, leading to large $CO_2$ emissions (approximately 2 tons of $CO_2$ generated per ton of steel) and making the coal-based steel industry one of the most polluting industrial sectors (7 % of world emissions). This old process, originally giving rise to the industrial revolution, benefits from the high energy density and cheapness of coal at the expense of considerable environmental impacts. Blast furnace-basic oxygen furnace (BF-BOF) dominates production (71 %), while secondary steel production using mainly steel scrap in electric arc furnace (EAF-scrap) represents 24 % of global production but it is limited by recycled steel capacity [1]. Alternative iron-making processes include smelting reduction process [2,3] and

direct reduced iron (DRI, also named sponge iron) [4–8]. DRI (in shaft furnaces), usually followed by electric arc steelmaking, accounts for 5 % of the production and is growing [1]. The development of such low-$CO_2$ alternatives to steel or cast iron production at large scale is necessary to reduce the global carbon footprint of this industrial sector [9–12]. To achieve this goal, different options are considered for the replacement of coke and charcoal with carbon-free reductants (such as hydrogen, $H_2$ [13–15]) or bio-sourced reductants (such as biogas, biochar or biomass [16–21]). The possible use of syngas (a mixture of CO and $H_2$) produced from natural gas reforming or biomass/coal gasification is also considered [22,23], and it is applied in the industrial MIDREX and HYL processes. Such a similar reduction pathway of iron oxide is also involved in the cyclic “steam-iron” process for hydrogen production [24]. Among the different possible reductants, the use of hydrogen for ore reduction ($H_2$-DRI) results in a decarbonized reaction with low or no emission provided that hydrogen is produced from renewables (green $H_2$) [25]. In contrast, the reduction process is carbon neutral when bio-based resources are used. Techno-economic analyses and $CO_2$ emission reduction potentials have been reported for $H_2$-DRI [26].

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: stephane.abanades@promes.cnrs.fr (S. Abanades).</footnote>
Steel production by using the direct reduction processes together with the scrap recycling is a suitable and reasonable solution for a sustainable iron and steel making [27]. New ironmaking processes based on fluidized bed technology have been developed [28]. The main advantage of these technologies is that fine ore can be directly used whereas preliminary treatments such as sintering or pelletizing (required in the conventional processes) can be avoided.

In addition, the process energy source based on coal combustion (a non-renewable resource) also needs to be replaced by a renewable energy resource. To this end, the use of electrical power in place of fossil fuels now represents a widespread option in the industrial sector for process decarbonation. The DRI process using a mixture of CO and H₂ (from reforming/gasification) coupled with melting in electric arc furnace has been developed for carbon-free steel production [6,29–33]. The steelmaking route based on hydrogen and electricity is the subject of a great deal of attention and numerous R&D projects. The first step is to chemically reduce iron ore with H₂ (produced by electrolysis of water with low-carbon electricity), and then to transform the direct reduced iron into steel in an electric arc furnace (similar to that used for scrap recycling) [34,35]. However, using renewable electricity for its conversion to industrial process heat is subjected to cost fluctuations due to the electricity market volatility, and moreover suffers from low global energy efficiency and electricity wasting for heat production. The direct use of high-temperature solar heat (T > 400 °C) for industrial processes (SHIP) delivered by solar concentrating systems thus represents an attractive option for decarbonation of energy-intensive industries. Therefore, the development of a solar metallurgical process based on carbon-free reducers offers the following advantages: (i) the greenhouse gas emissions due to combustion are eliminated, (ii) the use of fossil fuels as both reducer and energy source is avoided, (iii) pure solid iron metal can be directly produced from iron ore reduction, (iv) the process temperature is greatly lowered (below 800 °C), thus highlighting the energy-saving potential of this reduction route (which operates at almost half the temperature required for the classical reduction with coke or charcoal).

The reduction of iron oxide pellets using hydrogen or carbon monoxide follows a mixed rate-controlled mechanism, where chemical reaction and internal gas diffusion are competing processes during the first stage of the reduction, while internal gas diffusion becomes the controlling step at the last stage of the process [36]. Iron oxide reduction kinetics by hydrogen rely on several elementary reactions progressing within multiple reduction scheme [37,38]. Single and multi-step models such as nucleation and growth were used to describe the experimental reduction data [39]. Kinetics of iron ore pellets reduced by H₂ under non-isothermal condition were also reported [40,41]. Effect of gas composition on reduction behavior of red mud and iron ore pellets was studied [42]. Regarding the kinetics, reduction with H₂ is definitely faster, compared to CO due to the favorable physical properties (high diffusivity) of hydrogen [35]. The reduction steps from hematite towards iron strongly depend on temperature [43–45]. H₂-DRI is a multi-step solid-gas reaction and the solid undergoes several phase transformations. Three successive and rather separate reduction steps, through magnetite and wüstite to iron were evidenced [46]. When the reduction temperature is above 570 °C, the reactions evolve in the sequence hematite (Fe₂O₃) → magnetite (Fe₃O₄) → wüstite (Fe(1-x)O, where x indicates Fe deficiency) → iron (α-Fe or γ-Fe). For reduction below 570 °C, wüstite is thermodynamically unstable and the reduction proceeds via hematite → magnetite → α-iron [43]. The underlying mechanisms governing this process are characterized by a complex interaction of several chemical (phase transformations), physical (transport), and mechanical (stresses) phenomena [47].

In the field of solar thermochemical processing of metal oxides, various solar reactors have been developed mainly devoted to the reduction step as part of two-step redox cycles for fuel synthesis from H₂O and/or CO₂ splitting [48–51]. In particular, the thermal reduction of iron(III) oxide into wüstite was experimentally studied and cycles

based on Fe₃O₄/FeO redox reactions were developed [52]. Moreover, different reductants other than H₂ were used to favor the reduction step to metallic species (Mg, Zn, Sn, W, Cu, etc.) at lower temperatures and mostly involved carbonaceous species such as solid carbon, biochar or methane [53–56]. The solar reduction of CuO with H₂ was also studied [57]. However, there is currently no mature solar metallurgical process suitable for large-scale applications [58].

In particular, solar process development devoted to the reduction of iron ore for renewable steelmaking has never been done before. This study thus aims to develop a new metallurgical process based on the direct use of concentrated solar energy for process heat and on carbon-free hydrogen as reducer to decrease the reaction temperature below 1000 °C. These benefits allow to propose a sustainable process for producing metallic iron from iron oxide ore with minimized environmental impacts. Using solar energy for process heat in place of fossil fuels for the production of solid metallic iron is a means to reduce the dependence of metallurgical processes on conventional energy resources and to avoid emissions of CO₂ and other pollutants.

A preliminary thermodynamic equilibrium analysis was performed and experimental investigations were then carried out. First, the direct reduction of iron ore with H₂ was studied by thermogravimetry to determine the effect of the main operating parameters on the conversion rate. The impact of temperature, H₂ mole fraction, and particle size was emphasized. The main reaction characteristics were thus determined and kinetic data were derived. The iron production process was then demonstrated in a solar packed-bed reactor to show the feasibility of pure solid iron production at temperatures from 400 °C up to 1000 °C using direct solar heat, as an attractive and sustainable pathway to decarbonize the steel-making industry.

## 2. Materials and methods

Iron(III) oxide powder (Fe₂O₃ ≥ 97 %, -325 mesh, bulk density: 0.34 g/cm³, purchased from VWR Chemicals, CAS N° 1309-37-1) was used as the reference material. In addition, iron ore pellets (supplied by ArcelorMittal) were used as a raw high-grade material containing the following main impurities (H₂O: 1.6 %, SiO₂: 1.68 %, Al₂O₃: 0.36 %, CaO: 0.58 %, MgO: 0.30 %, TiO₂: 0.15 %). The raw pellets (spherical granules of 10–15 mm size) were crushed and sieved in distinct size fractions to obtain different well-defined ranges of particle sizes (1–2 mm, 0.5–1 mm, 0.25–0.5 mm, and <0.25 mm with bulk densities of 1.57, 1.59, 1.59, and 2.07 g/cm³, respectively).

The iron oxide reduction reaction with H₂ was studied by combined TGA/DSC using a Netzsch STA 449 F3 System (with 0.1 µg resolution). A given mass of iron oxide was loaded in an alumina crucible (~23 mg for commercial Fe₂O₃ and 53 mg for iron ore) and then subjected to simultaneous thermal analysis (STA) consisting of TGA (thermogravimetric analysis) coupled with DSC (differential scanning calorimetry). Blank run was used to correct the baseline drift due to the effects of buoyancy and thermal gas expansion. Isothermal TGA was mainly conducted, which means that H₂ was injected once the sample temperature was stable. Typically, the sample was heated up to the targeted temperature set-point in argon (≥99.999 % purity, flow-rate of 40 mL/min) at a heating rate of 20 °C/min. Then, a temperature dwell of 5 min was applied to stabilize the reaction conditions (temperature and mass baseline). Finally, H₂ (≥99.9999 % purity) was injected at the desired concentration in argon (for example 40 mL/min H₂ for 50 % concentration). In addition, non-isothermal TGA was also performed with continuous H₂ injection (50 % H₂/Ar) under constant heating rate (5, 10, and 20 °C/min) to determine the mass loss and rate evolution as a function of the temperature and to identify the start of the reaction.

During H₂ injection, a prompt mass decrease was recorded due to the oxygen loss in the form of steam. As a result, the mass loss was monitored continuously as a function of time and the Fe₂O₃ conversion was calculated.

The maximum sample weight loss corresponding to a complete
release of oxygen from Fe₂O₃ can be calculated as:

$$m_{O,max} / m_{Fe_2O_3} = 3 \cdot M_O / M_{Fe_2O_3} = 30\% \qquad (1)$$

The Fe₂O₃ conversion as a function of time was obtained from the sample mass loss (i.e., oxygen release), Δm(t), continuously measured in TGA:

$$\alpha_{Fe_2O_3} = \Delta m(t) / m_{O,max} = (\Delta m(t) \cdot M_{Fe_2O_3}) / (3 \cdot M_O \cdot m_{Fe_2O_3}) \qquad (2)$$

Where M_O and M_Fe₂O₃ are the molecular weight (g/mol) of oxygen and Fe₂O₃, respectively, m_O,max is the maximum oxygen mass released from iron oxide, and m_Fe₂O₃ is the initial sample mass.

Powder X-Ray diffraction (XRD) analysis was carried out on the fresh and reacted samples using a Malvern-Panalytical Empyrean diffractometer (X-ray tube, tension = 40 kV, intensity = 30 mA, anode material = Co, radiation wavelengths: K-alpha1 = 1.78901 Å, K-alpha2 = 1.79290 Å, K-beta = 1.62083 Å). X-ray diffraction measurements of 2θ symmetrical scans were made over an angular range from 10 to 85°, and

the step size and time per step were fixed at 0.039° and 450 s, respectively. Besides, for raw powdered samples, the particle size distribution was measured by laser diffraction analysis with dry dispersion method using a Malvern Mastersizer 3000 instrument.

# 3. Results and discussion

## 3.1. Thermodynamic equilibrium analysis

Thermodynamic equilibrium calculations were performed using HSC Chemistry code to predict the species distribution as a function of the temperature for the DRI reaction (Eq. (3)).

$$Fe_2O_3 + 3H_2 \rightarrow 2Fe + 3H_2O(g) \quad \Delta H^\circ = 97.5 \text{ kJ/mol} \qquad (3)$$

These calculations are based on the Gibbs free energy minimization of the system assuming a closed system approach without considering any mass transfer and kinetic limitations. This approach aims to

<figure><img src="image_5.png" /><figcaption>Fig. 1. Species distribution versus temperature at thermodynamic equilibrium: (a) amount of iron species for an over-stoichiometric H₂:Fe₂O₃ ratio of 18, (b) amount of solid Fe product for the Fe₂O₃ + λH₂ system at different excess ratios.</figcaption></figure>
determine the theoretical limit of the conversion process and to identify expected products with global trends about the effect of the main operating conditions. The Gibbs free enthalpy of Eq. (3) equals zero at 510 °C, which means that the reaction is thermodynamically favorable above this temperature. However, an excess of H₂ is necessary to shift the reaction toward Fe production. The main iron species formed from Fe₂O₃ reduction with a H₂:Fe₂O₃ stoichiometric ratio of 3 are FeO and non-stoichiometric wüstite (Fe₁,yO). In contrast, Fe is the main species formed above 500 °C with a large excess of H₂ (Fig. 1a). When increasing the H₂:Fe₂O₃ ratio (λ), the amount of Fe increases steadily while the temperature required for Fe formation decreases (Fig. 1b). These results were obtained by taking into account all the iron species including non-stoichiometric oxide compounds. In another work focused on thermodynamic equilibrium [59], introducing excess H₂(g) with respect to Fe₂O₃(s) was also shown to be beneficial for reduction to Fe(s). However, this previous study only included stoichiometric oxides (Fe₂O₃, Fe₃O₄, FeO, and Fe) and the equilibrium results thus differed, highlighting a partial Fe formation predicted between 300 and 1000 °C, but in minor amount compared to FeO.

Besides, in a continuous flow reactor featuring continuous H₂ reactant feeding and H₂O steam product removal in an open system, the input of fresh reactant is expected to displace the thermodynamic equilibrium toward the products formation, since the condition of excess H₂ will be inherently fulfilled. A series of experiments on direct iron ore reduction was then carried out. First, a thermogravimetric analysis of the reaction was performed to assess the influence of the main operating parameters on the kinetic rates. Then, proof-of-concept experiments in a packed-bed reactor heated by high-flux concentrated solar energy were carried out for demonstration of process feasibility under real-case conditions.

## 3.2. Thermogravimetric analysis

The influence of temperature on Fe₂O₃ reduction with H₂ was first investigated. Fig. 2 shows the evolution of Fe₂O₃ conversion with time in the temperature range 400–800 °C. Regardless of the temperature, a complete conversion (100 %) is achieved but the time to reach complete conversion varies from less than 5 min at 800 °C to over 50 min at 400 °C. The reaction rate increases drastically with the temperature.

A similar test with two different samples was reproduced at 500 °C to confirm results repeatability, as ascertained by the perfectly overlapping TGA curves. Above this temperature of 500 °C, a different mechanism appears to occur. Indeed at 550 °C, the initial reaction rate tends to slow

<figure><img src="image_6.png" /><figcaption>Fig. 2. Evolution of Fe₂O₃ conversion during reduction with H₂ in TGA at different temperatures ranging from 400 to 800 °C (50 % H₂/Ar, commercial Fe₂O₃ powder).</figcaption></figure>

down in comparison to that observed at 500 °C, but it then fastens rapidly and the peak rate (occurring when the 2nd order derivative of the TG curve is equal zero) is finally higher at 550 °C compared to 500 °C. To better visualize these temperature effects, a logarithm plot of the first derivative of the TG curve as a function of 1/RT is represented in Fig. 3 for the different temperatures considered according to Eq. (4) (where α is the fractional conversion), which results in an apparent activation energy Eₐ = 38.5 ± 1.5 kJ/mol for commercial Fe₂O₃ reduction with H₂. This value is in agreement with previously reported data [39,60,61].

$$ \ln(d\alpha/dt) = \ln(A) - E_a/RT \quad (4) $$

Non isothermal TG runs at constant heating rates reveal that the reaction starts at about 370 °C (with an onset temperature at 426 °C and 458 °C for 5 and 10 °C/min, respectively), as shown in Fig. 4. However, the temperature of the start of reaction tends to increase when increasing the heating rate (~400 °C at 20 °C/min with an onset of 491 °C) due to a short delay or induction time. Regardless of the heating rate, the final conversion is complete after about 10 min (for 20 °C/min) to 20 min (for 5 °C/min) of constant heating above 400 °C. However, the full conversion is shifted to higher temperatures due to the effect of heat and mass transfer limitations when increasing the heating rate.

The reduction of iron ore particles with H₂ was then considered. The effect of particle size on iron ore conversion was investigated at 800 °C with 50 % H₂ in Ar (Fig. 5a). The raw iron ore pellets were first crushed manually and sieved to obtain four different classes of particle size. The classes 0.25–0.5 mm, 0.5–1 mm, and 1–2 mm formed iron ore gravel with well identified individual particles, whereas the smallest class with a particle size below 0.25 mm contained a mix of the finest crushed particles and dust (with the following characteristics obtained by laser diffraction granulometry: D [3,2] = 17.1 µm, D [4,3] = 94.2 µm, Dv(50) = 66.0 µm and Dv(90) = 223.7 µm). As a result, the rate of iron ore conversion was not drastically affected by the particle size in the considered ranges. The lowest particle size (ore powder) showed the highest reaction rate but the difference with the other classes was not significant, especially during the initial stage of the reaction (in the first 5 min).

The difference in the reaction rate for the other ranges of particle size (from 0.25 to 2 mm) was even less observable. It can thus be concluded that the particle grinding to obtain very fine powder is not required to obtain a complete conversion in the case of the iron ore reduction with H₂. This reduces energy losses associated to the material grinding and further provides flexibility in the operation of solar chemical reactors with continuous particle feeding.

Increasing the temperature from 600 °C to 1000 °C resulted in a much higher initial reaction rate, as shown in Fig. 5a for 0.5–1 mm iron ore particles. The influence of rising temperature in the range of

<figure><img src="image_7.png" /><figcaption>Fig. 3. Logarithm plot of the conversion rates as a function of 1/RT for Fe₂O₃ reduction with H₂ (50 % H₂/Ar, commercial Fe₂O₃ powder). The slope equals the activation energy Eₐ = 38.5 ± 1.5 kJ/mol (R² = 0.9938).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Fig. 4. Evolution of Fe₂O₃ conversion during non-isothermal reduction with H₂ at different heating rates of 5, 10, and 20 °C/min (50 % H₂/Ar, commercial Fe₂O₃ powder).</figcaption></figure>

600–1000 °C thus led to accelerate the reaction while no slowing down was observed at the highest temperature (where the transition from Fe (α) to a denser Fe(γ) phase may occur), in agreement with [45]. A linear fit of the logarithm of the reaction rate versus inverse temperature resulted in an activation energy Eₐ = 30.4 ± 4.0 kJ/mol for iron ore reduction with H₂. During the initial period of the reaction, the conversion increases almost linearly with time, which suggests that the reaction rate is constant and a global zero order reaction can be assumed for iron ore reduction with H₂. By integration of the DSC endothermal peak measured right after H₂ injection, a reaction enthalpy of 258.9 J/g (41.3 kJ/mol) at 1000 °C was determined, which is lower than the theoretical value of 60.6 kJ/mol due to heat losses above the sample surface (no lid was used).

As a comparison, the reaction rate for commercial Fe₂O₃ powder (granulometry analysis: Sauter mean diameter D [3,2] = 17.5 µm, D [4,3] = 99.9 µm, Dv(50) = 83.3 µm and Dv(90) = 215.6 µm) at 800 °C was much higher, compared with its iron ore counterpart (with similar particle size distribution, as measured by laser granulometry), due to a much lower bulk density of the commercial powder bed (0.34 g/cm³ compared with 2.07 g/cm³ for ore), which translates into a higher bed void fraction (93.5 % vs. 60.4 % based on the true density of 5.24 g/cm³ for Fe₂O₃), and denotes the beneficial effect of decreasing drastically the bulk density for improving the conversion rate. Using fine powder with low bulk density thus improves the kinetic rate in comparison with denser particles. It should be noted that the final conversion of iron ore represented in Fig. 5a was below 100 %, although the actual iron oxide conversion was complete, as confirmed by post-reaction material characterization (XRD analysis shown in Section 3.4). The conversion was actually calculated based on the initial iron ore sample mass (mₐre), assuming it was composed of pure Fe₂O₃. However, given that iron ore is not pure (with wFe₂O₃ being the initial Fe₂O₃ mass fraction), the real Fe₂O₃ mass in the sample was actually lower (mFe₂O₃ = mₐre·wFe₂O₃). This final conversion thus reflects the real iron ore purity (i.e., the Fe₂O₃ mass fraction in iron ore wFe₂O₃ was in the range 95–96 %), which was consistent with the chemical composition of the raw material.

The influence of the H₂ mole fraction (γH₂ in the range: 20–75 %) on the iron ore conversion was also examined during isothermal TG runs (Fig. 5b). A complete conversion was achieved regardless of the H₂ mole fraction, assuming that the initial mass fraction of Fe₂O₃ in iron ore was 96 ± 0.5 %. The reaction rate was improved when the H₂ mole fraction increased from 20 % to 75 %, suggesting a kinetic rate dependence on γH₂.

The values of the reaction rate were used to estimate the reaction

order with respect to H₂, which was obtained from the slope of the plot of ln[dα/dt] versus ln[yH₂]. A kinetic order relative to the H₂ mole fraction of m = 1.213 ± 0.016 (R² = 0.9998) was determined (Fig. 6). Based on the reaction rate dependency on both the temperature and H₂ mole fraction, the reaction rate during the initial stage of the reaction (corresponding to the linear increase period) can be represented by the following expression:

$$ \frac{d\alpha}{dt} = k \cdot [y_{H2}]^m \qquad (5) $$

Where k is the kinetic rate constant (s⁻¹) based on the Arrhenius rate expression (k = A·exp(-Eₐ/RT)). After this initial period, diffusion limitations prevail and the reaction rate slows down progressively toward reaction completion.

Fig. 7 shows the characteristic times to reach 50 % and 90 % of full conversion. This information provides insights into the residence time required in a reactor for ensuring a given conversion. For ore particles (in the 0.25–2 mm size range), τ90 increases slightly with the particle size and it is in the range 10–15 min at 800 °C and 50 % H₂. Increasing the temperature or the H₂ mole fraction results in a shortened value of τ90 to about 5 min. The use of powders decreases significantly the characteristic times, but the handling of powders in continuously-fed reactors is less convenient than particles due to cohesive issues and sticking to the walls. This issue shall be further intensified in the case of iron oxide due to the well-known sticking behavior during reduction, attributed to the formation of fibrous whiskers [62,63]. Thus, continuous solar processing with powders remains challenging and larger particles should be preferred for better flowability and enhanced process stability during continuous operation. Therefore, the minimum residence time in a solar reactor should be at least a few minutes to approach complete particle conversion at 1000 °C. In order to confirm these results, additional experiments were carried out in a solar reactor heated under a stream of H₂ reducer.

### 3.3. Iron ore reduction in a packed-bed solar reactor

Solar experiments were carried out to investigate the ore reduction process and assess the time required for complete conversion under both non-isothermal and isothermal heating, with continuous analysis of the outlet H₂ concentration for reaction progress monitoring. The solar reactor, shown in Fig. 8, was composed of a vertical mullite tube (30 × 25 mm) closed at the bottom, in which the bed composed of iron oxide and sand particles (1.25–1.6 mm) was placed (total height of ~40 mm). The flowing gas (Ar + H₂, H₂ mole fraction = 50 %) was injected through a central inlet tube (6 × 3 mm) plunging at the bottom of the bed. The flow rates were regulated by two mass-flow controllers (Brooks Instruments). The bed temperature (Tₐₖₐ) was directly measured by a type K thermocouple along with two type S thermocouples in contact with the external rear wall of the reactor tube (T₁ and T₂). The reactor was placed inside a well-insulated cavity receiver made of high-temperature fibrous insulating materials (polycrystalline mullite/alumina wool). The height of the reactor tube located in the cavity (100 mm height) was heated by concentrated solar radiation entering via an 18 mm diameter aperture in front of the cavity. The reactor was positioned at the focal point of a solar furnace composed of a 2 m diameter parabolic concentrator and a sun-tracking heliostat (solar flux density with a Gaussian distribution and peak flux at 16 MW/m²). The incident solar flux was regulated thanks to a shutter positioned between the heliostat and the parabola. At the reactor outlet, the gas flowed first through a bubbler to remove the steam product (formed by the reduction reaction) and then through a desiccant cartridge to remove residual moisture. The H₂ concentration was measured continuously by a gas analyzer (Emerson NGA2000, thermal conductivity detector, precision ±1 %).

The time-dependent outlet H₂ flow rate (FH₂) was determined from
<figure><img src="image_9.png" /><figcaption>Fig. 5. Iron ore conversion during reduction with H₂ in TGA: (a) effect of particle size (at 50 % H₂/Ar, T = 800 °C), a comparison with ore (0.5–1 mm) at 600 °C and 1000 °C and with commercial Fe₂O₃ is also provided, and (b) effect of H₂ mole fraction (at T = 800 °C, particle size: 0.5–1 mm).</figcaption></figure>

the outlet mole fraction (yH₂) and the Ar flow rate (FAr):

$$F_{H_2,out} = F_{Ar} \cdot y_{H_2} / (1 - y_{H_2}) \quad (6)$$

The total amount of H₂ consumed by the reaction (integrated over the reaction duration) and the resulting Fe₂O₃ conversion were then calculated:

$$n_{H_2} = \int_{0}^{t} (F_{H_2,in} - F_{H_2,out}) / V_m \cdot dt \quad (7)$$

$$\alpha_{Fe_2O_3} = n_{H_2} / 3n_{Fe_2O_3} \quad (8)$$

The pressure drop through the bed of particles (sand + iron oxide) was about 10 mbar for the considered total gas flow rate of 0.3 NL/min (50 % H₂ in Ar). The inlet pressure in the bed was about 0.9 bar. The cavity temperature was homogeneous (close values for T₁ and T₂ measured at the rear tube wall) and the bed temperature (Tbed) was about 50 °C higher at thermal equilibrium (due to the thermal gradient in the radial direction between the rear of the tube and the central zone of the bed because the tube was irradiated at the front face).

During non-isothermal runs (Fig. 9), the reactor temperature was
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Logarithm plot of the conversion rates as a function of ln[y<sub>H2</sub>] for iron ore reduction with H<sub>2</sub> at a constant temperature of 800 °C (particle size: 0.5–1 mm). The slope equals the kinetic order relative to the H<sub>2</sub> mole fraction: m = 1.213 ± 0.016 (R<sup>2</sup> = 0.9998).</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Evolution of the characteristic times required to reach 50 % and 90 % of maximal conversion under the different tested conditions.</figcaption></figure>

increased steadily by opening progressively the shutter while injecting H₂ through the bed (at a H₂ flow-rate of 0.15 NL/min, set point value represented by the dash line). At about 400 °C, the outlet H₂ concentration started to decrease due to the H₂ consumption by the reaction, in agreement with TGA results. Once the reaction was complete, the H₂ concentration returned to the initial inlet value, which denoted the end of Fe₂O₃ reduction. The effect of the amount of Fe₂O₃ was first investigated through non-isothermal reduction runs. The reaction duration was over 40 min when using a packed bed of 5 g Fe₂O₃ placed just above a sand bed heated up to 900–1000 °C (Fig. 9a). Moreover, the final conversion was not totally complete (~93 %) because of iron bed sintering and packed particles agglomeration, which likely hindered the access of the reactive gas to the whole material. In contrast, the reaction was shortened (~25 min) and total conversion was achieved when lowering the amount of loaded Fe₂O₃ (1 g) and mixing the powder with sand bed to favor dispersion and avoid particle agglomeration and bulk diffusion limitations (Fig. 9b). These results suggest a strong effect of mass transfer limitation within the bed. Therefore, a better dispersion of particles favored the solid-gas reaction.

Then, isothermal runs were carried out at 600 °C and 1000 °C with a bed of 2 g Fe₂O₃ mixed with sand particles (Fig. 10). The reactor temperature was first increased to the set-point value and H₂ was then injected through the bed (50 % H₂/Ar). A blank run (dash line) was performed to monitor the dynamic response of the H₂ signal at the reactor outlet after step H₂ injection (step response to the H₂ input flow rate). The amount of H₂ consumed was then determined from the area between the curves of H₂ without and with reaction. At 600 °C, the conversion increased slowly and was not totally completed after 1 h reaction (Fig. 10a). In contrast, the reaction rate was much faster at 1000 °C and the conversion was complete after 15 min reaction (Fig. 10b), thus confirming that high temperatures strongly favor the reduction rate.

Finally, the reduction of iron ore was considered during isothermal runs at 1000 °C by focusing on the effect of the particle size (Fig. 11). For large particles (in the 1–2 mm range obtained by pellets grinding and sieving), the reaction was complete after ~15 min and a two-step process was clearly evidenced from the H₂ evolution profile (Fig. 11a). After a fast initial step, the increase of H₂ tended to level off, suggesting a diffusion-controlled regime in the particle bulk. The global reaction duration in packed-bed was somewhat higher than that measured in TGA for ore particles (about 5 min at 1000 °C and 50 % H₂ in Fig. 5a). Regarding iron ore powder (represented in Fig. 11b), the rate evolution profile was very similar to the case of commercial Fe₂O₃ powder (Fig. 10b), although the reaction duration was longer (~25 min) because of the higher bulk density of iron ore powder. The final

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Scheme and photograph of the packed-bed solar reactor for direct iron ore reduction.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 9.** Evolution of reactor temperatures, H₂ flow rate, and Fe₂O₃ conversion during non-isothermal direct reduction in solar packed-bed reactor: (a) 5 g commercial Fe₂O₃ above sand bed, (b) 1 g commercial Fe₂O₃ mixed with sand bed.</figcaption></figure>

calculated conversion of ~96 % was obtained for iron ore for the same reason as pointed out in section 3.2 (linked to the iron ore purity). These results reveal that the size of iron ore particles does not hinder the reaction rate and large particles should even be preferred to shorten the reaction in a particle bed owed to more favorable mass transfer through the flowing gas around the individually dispersed particles.

Complete conversion was further confirmed with a global mass balance by weighting the material before and after reaction, highlighting a sample mass decrease corresponding to about 30 % (oxygen loss, Eq. (1)) of the initial Fe₂O₃ sample weight. This study performed in a solar packed-bed reactor confirmed the feasibility of direct iron ore reduction with hydrogen. The most favorable conditions were identified: the particle dispersion within the bed promoted solid-gas mass transfer, while a high temperature of 1000 °C enhanced the kinetics and large particles showed faster conversion rates due to their widened dispersion. Pure iron production under real concentrated solar

irradiation was successfully demonstrated.

### 3.4. Phase characterization of the iron products

The different materials after the reduction reaction were collected and analyzed by powder XRD for identification of the main phases produced to confirm metallic iron production (Fig. 12). The raw materials were also characterized for comparison. Regarding the materials reduced in TGA at different temperatures (from 400 to 800 °C), the main phase was Fe (PDF 01-089-7194) regardless of the temperature (Fig. 12a). However, the materials reduced at both 400 and 450 °C still contained residual unconverted Fe₂O₃ (PDF 01-079-1741) and Fe₃O₄ intermediate product (PDF 01-089-0950), without any wüstite species detected. This supports that the reduction process occurs via the formation of magnetite at low temperatures because of wüstite instability [43]. Between 500 and 800 °C, a pure Fe phase with cubic crystal
<figure><img src="image_14.png" /><figcaption>**Fig. 10.** Evolution of reactor temperatures, H₂ flow rate, and Fe₂O₃ conversion during isothermal direct reduction in solar packed-bed reactor (2 g Fe₂O₃ mixed with sand bed): (a) commercial Fe₂O₃ at 600 °C, (b) commercial Fe₂O₃ at 1000 °C.</figcaption></figure>

structure was obtained. A comparison with ore powder reduced at 800 °C showed no difference in the diffractogram compared with the other reduced Fe₂O₃ materials.

As for the materials reduced in packed-bed solar reactor (Fig. 12b), the main phase identified was still Fe. Some minor phases of magnetite Fe₃O₄ (PDF 01-089-0950) and wüstite FeO (PDF 01-074-1886) were identified in the case of the material reduced at 600 °C, which confirms that the reduction was not fully completed and that wüstite was an intermediate product at this temperature. At 1000 °C, a pure Fe phase was produced with minor traces of wüstite Fe₀.₉₆₃O (PDF 01-074-1882). It can be noticed that traces of quartz SiO₂ (PDF 01-079-1906) were also detected in some cases due to the weak presence of either sand from the bed in the collected sample or quartz impurities in raw iron ore. Solar production of iron was thus demonstrated via identification of the

product composition after direct reduction reaction with H₂, and achieving a fully decarbonized solar-driven DRI process was shown to be feasible.

## 4. Conclusion

The synthesis of metallic iron from its ore by reduction in H₂ atmosphere can be achieved by direct supply of concentrated solar energy as the process heat source. Its application to the conventional steelmaking process, highly energy consumer, would entail a drastic lessening of the energy cost and environmental impact. This attractive pathway indeed results in decreasing the carbon footprint of the process due to the direct use of both hydrogen as a reducing agent and solar heat for process decarbonization. Such a solar process for clean ironmaking has never
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Evolution of reactor temperatures, H₂ flow rate, and Fe₂O₃ conversion during isothermal direct reduction at 1000 °C in solar packed-bed reactor (2 g iron ore mixed with sand bed): (a) iron ore (1–2 mm), (b) iron ore powder.</figcaption></figure>

been implemented to date. In this study, the reduction of iron ore with H₂ was completed at temperatures above 400 °C with a reaction rate highly dependent on both the temperature and the H₂ mole fraction, according to thermogravimetric analysis. Indeed, increasing the temperature (from 400 to 800 °C) and the H₂ mole fraction (from 20 to 75 %) hastened the kinetics and thereby shortened the time required for reaction completion. In contrast, the particle size in the considered range of 0.25–2 mm did not show a significant effect on the ore conversion. Noticeably, micron size particles (ore powder) showed higher reaction rates at the initial stage of the reaction, while a fine commercial powder with low bulk density significantly improved the kinetic rate in comparison with a denser powder. A complete ore conversion into solid iron was also demonstrated in a packed-bed solar reactor under real solar irradiation during heating up to 1000 °C. The reaction progress

was directly monitored by continuous analysis of the outlet H₂ concentration, which revealed that particle-gas reaction times should exceed about 15 min to warrant a complete conversion of the raw material into pure solid iron. The formation of a pure Fe phase after reduction was confirmed by XRD analysis of the recovered materials after reaction.

Future work will be devoted to the design and demonstration of solar continuous-flow reactors allowing the continuous injection of iron ore particles and the continuous extraction of reaction products. The process will be tested at the focus of a solar concentrator with direct irradiation of iron ore particles to determine performance such as yields and product quality under different operating parameters. Such a solar process will pave the way toward sustainable and carbon-free iron/steel metallurgy.
<figure><img src="image_16.png" /><figcaption>Fig. 12. XRD analysis of powders before and after direct reduction with H₂: (a) samples collected after TGA at different temperatures (Fe_400 to Fe_800 data shown in Fig. 2), and (b) samples collected from packed-bed solar reactor after non-isothermal (Fe_5g & Fe_1g in Fig. 9) and isothermal reduction (Fe_2g 600 & Fe_2g 1000 in Fig. 10), and Ore 1–2 mm 1000 & Ore powder 1000 in Fig. 11). Diffractograms of raw commercial Fe₂O₃ and iron ore are also given.</figcaption></figure>

**Data availability**

Data will be made available on request.

**CRediT authorship contribution statement**

**Stéphane Abanades:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.
**Sylvain Rodat:** Writing – review & editing, Investigation, Conceptualization.

**Declaration of competing interest**

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

**Acknowledgments**

This work was funded by the French National Agency for Research (ANR, METASOL project, contract N° ANR-20-CE05-0008-02). The authors thank José Barros Lorenzo (ArcellorMittal) for supplying the raw iron ore material. The support of B. Fraisse (ICGM) for XRD materials characterization and of S. Arufe Vilas and R. Domingo (Cirad) for laser
granulometry is gratefully acknowledged.

Nomenclature

A pre-exponential factor ($s^{-1}$)

E_a apparent activation energy (J/mol)

F_Ar Ar flow rate (NL/min at normal conditions 0 °C, 1 atm)

F_H2 H₂ flow rate (NL/min at normal conditions 0 °C, 1 atm)

k kinetic rate constant ($s^{-1}$)

m reaction order with respect to H₂

M_i molecular weight of species i (g/mol)

m_{O,max} maximum oxygen mass released from iron oxide (g)

m_{Fe_2O_3} initial Fe₂O₃ sample mass (g)

m_{ore} initial iron ore sample mass (g)

n_{H_2} total amount of H₂ consumed by the reaction (mol)

n_{Fe_2O_3} initial mole amount of Fe₂O₃ (mol)

T temperature (K)

V_m molar volume (22.4 L/mol at 0 °C, 1 atm)

w_{Fe_2O_3} Fe₂O₃ mass fraction in iron ore

y_{H_2} outlet H₂ mole fraction

α_{Fe_2O_3} Fe₂O₃ conversion

τ90 characteristic time to reach 90 % of full conversion (s)

Δm(t) sample mass loss due to oxygen release (g)

ΔH° reaction enthalpy (J/mol)

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