Towards Zero CO₂ Continuous Steelmaking Directly from Ore

Abstract

In-line continuous processing of high-grade hematite ore (crushed ore or fines) with a pure hydrogen reductant is assessed. An appraisal is made of the rate controlling mechanisms involved in the reduction of a pure layer of molten wustite being transported by floating on a molten carrier iron carbon-free medium at temperatures just in excess of the iron melting point. Published research clearly indicates that under these conditions the kinetics are principally controlled by molecular gaseous diffusion. Thus, the rate is essentially not influenced by total gas pressure above 1 atmosphere. Accordingly, on safety grounds it is recommended that high pressure should not be used for hydrogen steelmaking in the future, but the operation should be conducted close to atmospheric pressure with low pressure steam encapsulation of the plant items involved. Using hydrogen as the reductant means that sub-surface nucleation of CO bubbles cannot disrupt continuous processing. The operation is then no different to processing a normal liquid phase. The off-gases from the reduction zone of a melt circulation loop are super-clean and only contaminated with iron vapor. Accordingly, the best available technology becomes available for energy conservation without risk of non-fusible solids deposition. The net result is that the energy requirements are expected to be superior to other potential processes.

Appendix

Enhanced Safety Option for Hydrogen Steelmaking

In the event of HAZOP studies concluding back-mixed post combustion of hydrogen with oxygen at essentially atmospheric pressure, all encased in low pressure steam jacketing, is unacceptably risky, there has to be a viable alternative for energy input to support hydrogen reduction of the iron ore charge. The proposed solution is to eliminate hydrogen combustion entirely from the steelmaking process. In its place electrical conductive heating may be employed with generic melt circulation technology to provide the way forward for the global steel industry in safely securing effectively zero carbon dioxide emission at the steel plant.

In an experimental research program at Birmingham University[11] over a six-hour period, some 500 tonnes of conductively heated molten copper matte at about 1573 K (1300 °C) were re-circulated past a given point in a melt circulation loop. Clearly, this must have immediate implications regarding proof of concept for melt circulation employing a comparable approach for full-sized commercial hydrogen steelmaking with enhanced safety employing energy input via conductive heating into a melt circulation loop. The research was funded by the Mineral Industry Research Organization (MIRO) backed-up by substantial contributions from the UK government EPSRC and the European Union.

The experimental melt circulation loop was comprised of two rectangular hearths side by side. The temperature of the matte was raised by means of direct resistive or so-called conductive heating. Electric currents as high as 7,000 A were passed through the copper matte between graphite electrodes. Power to each hearth was supplied by two 180 kVA welding transformers with parallel primary windings, and with their secondary windings in series. The two hearths each 0.35 m wide × 4 m in length were designed so that the matte depth for the lower hearth at about 280 mm was greater than the upper hearth at 150 mm. A RH-type vacuum system connected the hearths via two inclined snorkel legs, which were lowered into the molten matte immediately prior to circulation. Inert gas (nitrogen) was injected into the up-leg (in the deep; lower hearth). The scale of the operation can be gauged from the plant photograph shown in Figure 16. Accounts of this Birmingham University project have been published elsewhere.[3,11]

Fig. 16

Sacrificial heating electrical elements being checked on one of the RH snorkels

Power generation employing an enhanced geothermal system (EGS) or hydroelectric power, available 24 h per day, is the long-term preferred way forward. If EGS or hydroelectricity are not available or are more expensive options, then other renewable energy resources, including concentrated solar power (CSP), photovoltaic solar panels (PV), wind farms and tidal power are alternative options demanding serious consideration.

In the long term, hydrogen gas generated from water is likely to be used as the reductant to produce steel in a future low carbon economy. Very importantly, a melt circulation arrangement with hydrogen is to be seen as the ultimate solution to zero gas emission continuous steelmaking in response to global concerns about climate change.

In the present case, this effectively means that if PV is chosen and thus not available as on a cloudy day or at nighttime, the plant has to have a complementary power supply of AC mains frequency electricity to satisfy the minor power demands to maintain the plant at operating temperature. With large-sized reactors this facility is mandatory. Accordingly, a decision has to be taken at the design stage whether it is financially desirable to effectively close down major production at night to minor levels using mains supply electricity and then start-up again in the morning using renewable energy. This strategy avoids the capital cost involved in providing energy storage, as well as potential safety issues associated with hydrogen storage.

With AC mains current the “skin effect” is used to advantage. Also if the melt circulation loop did freeze or partially freeze, mains frequency AC would be required to slowly re-establish ore reduction operations In view of these considerations, the present proposal, provides the means for using either DC or AC for conductive heating of a shallow bath of molten iron flowing in an open channel configuration. The maximum depth of molten iron in the open channel primary melt circulation loop is accordingly about twice the so-called “skin depth” so that both 50 to 60 cycle per second AC and DC are of comparable effectiveness for conductive heating of an iron melt close to its melting point.

A conservative estimate of the Process Fuel Equivalent (PFE) for the enhanced safety hydrogen steelmaking process employing electrical conductive heating rather than post combustion of hydrogen with oxygen to produce 50 mm thick refined steel slab is 14.3 GJ per tonne of solid product.