# Carbon Impact Mitigation of the Iron Ore Direct Reduction Process through Computer-Aided Optimization and Design Changes

^{*}

## Abstract

**:**

_{2}emissions. The so-called direct reduction route, which makes use of reformed natural gas along with top gas recycling to reduce iron oxide pellets with H

_{2}and CO, is responsible for lower CO

_{2}emissions than the classic blast furnace route and is currently under development. The present article focuses on the direct reduction process and discusses means to further decrease the CO

_{2}emission rate. A set of 10 operating parameters were simultaneously changed according to computer-aided optimization. The results provide about 15% improvement over original emissions for comparable output values.

## 1. Introduction

_{2}emissions [1,2]. This can be related to the widely adopted use of the blast furnace for the chemical reduction of iron ore [3]. Efforts have thus been put forth to tackle these emissions as indicated in [4] and [5], which stressed the importance of alternative technologies. One promising and developing process is the so-called Direct Reduction (DR) process, and its primary technique MIDREX (Midland-Ross Direct Iron Reduction) [6]. This technology has seen increased interest, namely in cases with high natural gas reserves [7,8].

_{2}, CO

_{2}, H

_{2}O, CH

_{4}and N

_{2}, with the first components having the highest proportion. Top gas on the other hand has CO

_{2}and H

_{2}O in a higher proportion with the presence nonetheless of other components.

_{2}is emitted along with other gases in the flue gas (7), which is the only gas exiting the system. Carbon emissions ${m}_{C,fluegas}\left({\mathrm{kg}\mathrm{h}}^{-1}\right)$ are usually normalized to the amount of DRI leaving the system ${m}_{DRI}\left({\mathrm{kg}\mathrm{h}}^{-1}\right)$ as indicated in Equation (1).

_{2}) flow rate (thus reducing H

_{2}flow rate correspondingly). This was attributed to CO being a better reducer than H

_{2}. Moreover, the authors stressed the importance of studying the coupling between the shaft reactor and the reformer for the identification of feasible optimal results. Alamsari et al. studied the impact of higher input gas temperature. It was concluded that this condition led to higher cooling gas requirements and ultimately to higher carbon contents in exiting DRI [13]. Nouri et al. [14] found that a higher gas flow rate led to higher solid conversion, and linked it to higher reducing gas concentration near the solid inlet. Moreover, these authors underlined the need for a smaller H

_{2}/CO ratio for higher iron production rates, as well as the importance of having a higher reducing power $\left({\mathrm{CO}+\mathrm{H}}_{2}\right)/\left({\mathrm{CO}}_{2}{+\mathrm{H}}_{2}\mathrm{O}\right)$ ratio for higher solid conversion. A lower limit of 1 and an upper limit of 20 were identified for the two ratios, namely with the concern of reformer operation. These authors further studied the effect of solid flow rate, reactor length and pellet size. It was found that a lower solid flow rate, a higher reactor length and a smaller pellet size all positively affected solid conversion rate, with the inverse being true. Shams et al. modelled the shaft reactor in its entirety (reduction, transition and cooling zones) [15]. They investigated the effect of increasing the cooling gas flow rate on the output solid temperature and carbon deposition. The authors found an optimum and economical value beyond which only a little more solid cooling was observed. They related the increase in solid carbon formation to the increase in the cooling gas input. None of these works modeled or studied the interaction between the shaft reactor and the reformer.

_{2}consumption for a fixed reduction degree [17]. Hamadeh explored the impact of furnace radius, area of gas injection port, cooling and reducing gas flow rates, reducing gas composition, and pellet diameter [18]. It was deduced that the metallization decreased with greater reactor radius, greater solid gas flow rate and temperature. This metallization increased with reduced cooling gas temperature as well as a smaller H

_{2}/CO ratio. He also attempted to model the reformer and couple it with the shaft furnace; no coupled optimization work was, however, performed. Although those works provide ample information on the influence of process parameters, they do not address the important aspect of carbon emission reduction.

_{2}issue was addressed by Duarte and Becerra in [19], where carbon emissions were reduced by 10% via the use of an acid gas absorption system for CO

_{2}removal from the recycled top gas, added to the inclusion of in-situ CH

_{4}reforming. However, no modification to process operating parameters was realized. Tanaka also proposed the use of in-process heat integration as well as the foregoing of the cooling zone for hot DRI production [20]. Knop and Ångström patented the use of a water gas shift reactor in place of the scrubber and natural gas reformer, with a subsequent CO

_{2}removal system, leading to a pure H

_{2}reducing gas [21]. This, however, represents a drastic change over the initial MIDREX process. Using a systems model of the process, which couples the DR shaft with the process gas loop, Bechara et al. investigated the modification of operating parameters, namely H

_{2}, CO, CO

_{2}and H

_{2}O flow rates [9]. By trial-and-error, optimal values were found that respected the trade-off between minimal normalized carbon emissions and feasible process designs.

_{2}emissions.

## 2. Modeling Scheme

- The reducing gas characteristics (composition, temperature), pellet properties (diameter, flow rate) and the reactor dimensions are considered as model inputs, with the cooling gas specified.
- The split of input pellets between Zone 1 and Zone 2 $sp{l}_{Fe}$ is calculated based on the given reducing and cooling gas flow rates.
- The shaft outputs are calculated and the DRI as well as top gas properties are determined.
- The gas loop is calculated with top gas as input with the goal of having a converged recycling. The convergence is obtained when the calculated heated outlet reformed gas characteristics are sufficiently close to the inlet reducing gas characteristics.

## 3. Process Computer-Aided Optimization

#### 3.1. Definition of the Optimization Problem

- The first term is the normalized carbon emissions that need to be minimized. This term was multiplied by a power (a = 0.5) of the relative wustite reduction height $\frac{{h}_{\mathrm{FeO}}}{{h}_{reac}}$. h
_{reac}is the reactor height and h_{FeO}is the height where wustite rate falls below a low value (10^{−6}kg h^{−1}). This was done in order to keep a good column performance. - Two other terms were added to ensure the feasibility of the proposed modifications. The middle term drives the convergence of the gas loop (feasibility), ensured when the H
_{2}flowrate at the reformer outlet is equal to that at the shaft inlet: ${n}_{{\mathrm{H}}_{2},ref}-{n}_{{\mathrm{H}}_{2},reducinggas}\to 0$. The last term was added to get similar metallization rates, via the input pellet split between original and optimized cases: $sp{l}_{pellets,0}-sp{l}_{pellets}\to 0$.$$obj={C}_{normal}\times {\left(\frac{{h}_{\mathrm{FeO}}}{{h}_{reac}}\right)}^{a}+10\times \left|{n}_{{\mathrm{H}}_{2},ref}-{n}_{{\mathrm{H}}_{2},reducinggas}\right|+10\times \left|sp{l}_{pellets,0}-sp{l}_{pellets}\right|$$

#### 3.2. Launching of Optimization Runs

#### 3.3. Results Analysis

_{2}released in the flue gas. As our optimization process minimizes the normalized carbon ratio, it was expected that lower operating temperatures are found. It is important to note that a temperature of 860 °C is indeed suitable for the shaft to run. Laboratory reduction experiments have shown satisfying kinetics of reduction of standard pellets at temperatures even lower [18].

_{2}.

#### 3.4. Comparison of Profiles in the Reduction Zone (Zone 1)

_{2}(b), the temperature of gas and solid phases (c) and the evolution of different gaseous components (d), for reference (1) and optimized (2) cases (run 2).

_{2}for reduction, which is highlighted on the second line (b) graphs where more FeO is reduced by H

_{2}than by CO. Considering that H

_{2}has faster kinetics, the windows of presence of magnetite and wustite are drastically reduced in the shaft. The FeO reaction was the only shown because it is the slowest of the three reduction reactions.

_{2}input rates.

_{2}flow rate does not see a steep increase from the reducing gas inlet (height 0) in the optimal case. This is mainly related to the absence of methane reactions. Also, H

_{2}O, CO and CO

_{2}rates are higher in the optimal case than in the original one. In summary, the output has rather evenly partitioned components.

#### 3.5. Comparison with Previous Literature Works

#### 3.6. Techno-Economic Remarks

_{2}emissions and lower operation costs.

## 4. Conclusions

_{2}emissions. Those operating conditions were obtained by means of computer-aided optimization using Aspen Plus.

_{2}/DRI) could be significantly reduced (about 15%) in comparison to the reference case, while keeping similar productivity, carbon content and metallization of the DRI.

_{2}fractions and lower CH

_{4}fraction. As a result, the optimal shaft reactor configuration showed smaller in-situ methane reforming rates, smaller in-reactor temperature drop, and finally a greater H

_{2}reduction rate. At the process level, this was accompanied by a greater process gas recycling ratio for an equivalent reformer heat consumption. Somewhat paradoxically, lower CO

_{2}emissions could be achieved by the use of higher CO and CO

_{2}rate albeit with a greater gas recycling fraction.

## Author Contributions

## Funding

_{2}in industry’, 2014-18, VALORCO, No. 1382C0245; the authors thank Nathalie Thybaud and Aïcha El Khamlichi, and the coordinator, Eric de Coninck; and (ii) one operated by the National Research Agency (ANR) and referenced by ANR-11-LABX-0008-01 (LabEx DAMAS).

## Acknowledgments

## Conflicts of Interest

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**Figure 3.**Dividing the shaft furnace into four zones (right hand side, as retained in the present Aspen Plus model) on the basis of the CFD simulation results (shown on the left-hand side with the example of the metallic iron mass fraction) given by [11].

**Figure 5.**Evolution of the solid mass flow rates (

**a**), FeO conversion from H

_{2}and CO (

**b**), temperatures (

**c**) and gas molar flow rates (

**d**) along Zone 1 height for the reference (left column .1) and optimized (right column .2) plant operation.

Hematite reduction | $3{\mathrm{Fe}}_{2}{\mathrm{O}}_{3}+{\mathrm{H}}_{2}\left(\mathrm{CO}\right)\to 2{\mathrm{Fe}}_{3}{\mathrm{O}}_{4}+{\mathrm{H}}_{2}\mathrm{O}\left({\mathrm{CO}}_{2}\right)$ |

Magnetite reduction | ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}+\frac{16}{19}{\mathrm{H}}_{2}\left(\mathrm{CO}\right)\to \frac{60}{19}{\mathrm{Fe}}_{0.95}\mathrm{O}+\frac{16}{19}{\mathrm{H}}_{2}\mathrm{O}\left({\mathrm{CO}}_{2}\right)$ |

Wustite reduction | ${\mathrm{Fe}}_{0.95}\mathrm{O}+{\mathrm{H}}_{2}\left(\mathrm{CO}\right)\to 0.95\mathrm{Fe}+{\mathrm{H}}_{2}\mathrm{O}\left({\mathrm{CO}}_{2}\right)$ |

Water gas shift | $\mathrm{CO}+{\mathrm{H}}_{2}\mathrm{O}\rightleftharpoons {\mathrm{CO}}_{2}+{\mathrm{H}}_{2}$ |

Steam methane reforming | ${\mathrm{CH}}_{4}+{\mathrm{H}}_{2}\mathrm{O}\to 3{\mathrm{H}}_{2}+\mathrm{CO}$ |

Methane cracking | ${\mathrm{CH}}_{4}\to \mathrm{C}+2{\mathrm{H}}_{2}$ |

Boudouard reaction | $\mathrm{C}+{\mathrm{CO}}_{2}\rightleftharpoons 2\mathrm{CO}$ |

Variable Number | Variable Name | Description | Original Value/Range |
---|---|---|---|

1 | n_{CO,red} | Component molar flow rates in the reducing gas inlet (kmol/s) | 0.7/[0.65–1.2] |

2 | n_{H2,red} | 1.08/[0.8–1.2] | |

3 | n_{H2O,red} | 0.09/[0.02–0.3] | |

4 | n_{CO2,red} | 0.05/[0.02–0.3] | |

5 | n_{CH4,red} | 0.19/[0.02–0.25] | |

6 | T_{reducing gas} | Reducing gas temperature (°C) | 957/[850–970] |

7 | m_{pellet} | Pellet mass flow rate (kg/s) | 45.54/[42.5–54] |

8 | r_{reac} | Radius of the shaft furnace (m) | 2.75/[2.25–3.25] |

9 | h_{reac} | Height of shaft furnace * | 10/[6–14] |

10 | d_{p} | Pellet diameter (m) | 0.015/[0.01–0.016] |

**Table 3.**Values for key design parameters and results. The variables with a grey background are optimization variables.

Variables | Reference | Run 1 | Run 2 | |
---|---|---|---|---|

1 | ${n}_{\mathrm{CO},red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 0.71 | 0.98 | 0.82 |

2 | ${n}_{{\mathrm{H}}_{2},red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 1.08 | 1.07 | 1.146 |

3 | ${n}_{{\mathrm{H}}_{2}\mathrm{O},red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 0.0916 | 0.096 | 0.075 |

4 | ${n}_{{\mathrm{CO}}_{2},red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 0.05 | 0.12 | 0.15 |

5 | ${n}_{{\mathrm{CH}}_{4},red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 0.1981 | 0.167 | 0.064 |

6 | ${T}_{reducinggas}\left(\xb0\mathrm{C}\right)$ | 957 | 882 | 860 |

7 | ${m}_{pellet}\left(\mathrm{kg}/\mathrm{s}\right)$ | 45.54 | 45.54 | 44.9 |

8 | ${r}_{reac}$ (m) | 2.75 | 2.75 | 2.94 |

9 | ${h}_{reac}$ (m) | 10 | 10 | 9 |

10 | d_{p} (m) | 0.015 | 0.015 | 0.0145 |

Objective | ||||

${\mathit{C}}_{\mathit{n}\mathit{o}\mathit{r}\mathit{m}\mathit{a}\mathit{l}}$(kg/kg) | 0.123 | 0.105 | 0.105 | |

Results | ||||

${m}_{DRI}\left(\mathrm{kg}/\mathrm{s}\right)$ | 33.527 | 33.53 | 32.90 | |

Metallization (%) | 94.2 | 94.1 | 94.2 | |

Carbon mass fraction in DRI | 0.0235 | 0.0237 | 0.02 | |

${\left(\frac{{\mathrm{H}}_{2}}{\mathrm{CO}}\right)}_{inlet}$ | 1.51 | 1.09 | 1.39 | |

${\left(\frac{\mathrm{CO}+{\mathrm{H}}_{2}}{{\mathrm{CO}}_{2}+{\mathrm{H}}_{2}\mathrm{O}}\right)}_{inlet}$ | 12 | 9.4 | 8.7 | |

Recycling ratio * | 0.63 | 0.77 | 0.73 | |

${n}_{tot,red}\left(\mathrm{kmol}/\mathrm{s}\right)$ | 2.178 | 2.479 | 2.29 | |

${m}_{tot,red}\left(\mathrm{kg}/\mathrm{s}\right)$ | 30.35 | 40.44 | 35.34 | |

${n}_{{\mathrm{CH}}_{4},ref}$ (kmol/s) | 0.339 | 0.285 | 0.267 | |

${n}_{{\mathrm{CH}}_{4},bur}$ (kmol/s) | 0 | 0.008 | 0.01 |

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**MDPI and ACS Style**

Béchara, R.; Hamadeh, H.; Mirgaux, O.; Patisson, F. Carbon Impact Mitigation of the Iron Ore Direct Reduction Process through Computer-Aided Optimization and Design Changes. *Metals* **2020**, *10*, 367.
https://doi.org/10.3390/met10030367

**AMA Style**

Béchara R, Hamadeh H, Mirgaux O, Patisson F. Carbon Impact Mitigation of the Iron Ore Direct Reduction Process through Computer-Aided Optimization and Design Changes. *Metals*. 2020; 10(3):367.
https://doi.org/10.3390/met10030367

**Chicago/Turabian Style**

Béchara, Rami, Hamzeh Hamadeh, Olivier Mirgaux, and Fabrice Patisson. 2020. "Carbon Impact Mitigation of the Iron Ore Direct Reduction Process through Computer-Aided Optimization and Design Changes" *Metals* 10, no. 3: 367.
https://doi.org/10.3390/met10030367