(35a) Hydrogen based processing of mineral iron carbonate, process and economics

Hydrogen based reduction of mineral iron carbonate ore is expected to contribute to carbon dioxide emission reduction. At present the annual global iron and steel production capacity is about two billion metric tons, with specific carbon dioxide emissions of about 1.9 to 2.2 metric tons per ton of pig iron. Hydrogenation of carbon dioxide (CO₂) may contribute to the reduction of CO₂ emissions. High carbon dioxide concentration of process gas from direct iron ore reduction with hydrogen recommends conversion of carbon dioxide to either methane or methanol.

Catalytic conversion of carbon dioxide to methane with a MgO/Ni catalyst was successfully investigated in lab scale. The product gas may directly be fed into a gas grid. The spent catalyst may easily be recycled in the steel industry.

The bulk chemical methanol is a suitable alternative product. It is used for paints, plastics, and adhesives, as feedstock for the production of chemicals, and as fuel additive or alternative fuel. It provides promising properties for hydrogen storage, as it can raise the energy density of hydrogen-based energy carriers by one order of magnitude. Low temperature level in reforming also suggests methanol for fuel cell applications. State-of-the-art methanol production is based on syngas, a mixture of carbon monoxide, CO₂, and hydrogen, and preferably Cu/ZnO/Al₂O₃ catalysts. State of the art reactor- and process design suggest looping systems with sophisticated internal heat energy management. Huge efforts have meanwhile been made to directly process carbon dioxide instead of syngas. To our knowledge neither of carbon dioxide based processes has established on the market, mainly because of catalyst life time limitations. To sum up the present status, the establishment of direct methanol synthesis from carbon dioxide suffers from limitations in feed gas preparation and/or appropriate catalysts. In process gas from direct reduction of mineral iron carbonate carbon dioxide at high concentration is available in huge quantities. When processing carbonate based iron ores (siderite) in hydrogen atmosphere our group registered that at ambient pressure within a temperature window of 600 to 800 °C it is possible to alter the carbon monoxide/carbon dioxide ratio of the exiting gas. Methanol synthesis with off-gas from mineral iron ore reduction with hydogen was successfully tested and validated in a lab size CSTR loop reactor system with external cooling loop for condensing the gaseous products. The methanol synthesis process was validated with a Cu/MgO catalyst prepared by wet impregnation and stoichiometric CO₂/H₂ feed at 300 °C and 5 MPa.

The technology of pig iron production from mineral iron carbonate with hydrogen and carbon dioxide utilization has been successfully validated, but it has to be considered at what price we can establish pig iron production without carbon dioxide emissions.

According to the state of the art blast furnace process the specific energy consumption is 17.8 GJ/t pig iron, and the specific carbon dioxide emissions is about 1.9 t/t of pig iron.

Based on hydrogen production by electrolysis (10,000 A/m² at 1.84 V for zero gap electrolysis setup) the iron ore reduction step (FeCO₃ + H₂ = Fe + CO₂+ H₂O) will consume as much as 7.26 GJ/t of pig iron (plus 0.39 GJ/t pig iron for the smelter). Conversion of carbon dioxide to methane (CO₂ + 4 H₂ = CH₄ + 2H₂O) will consume 29 GJ/t pig iron. Alternatively, conversion of carbon dioxide to methanol (CO₂ + 3 H₂ = CH₃OH + H₂O) would consume 21.8 GJ/t of pig iron. Even when considering the LCV of methane and methanol the specific energy consumption for hydrogen based iron ore processing would consume at least 21.3 GJ/t of pig iron in case of methane synthesis and 22.9 GJ/t of pig iron in case of methanol synthesis. In conclusion, the technology of direct reduction of mineral iron carbonate with hydrogen is feasible. The gas quality of process gas from direct reduction of mineral iron carbonate with hydrogen permits utilization of carbon dioxide, but at the expense of the specific energy consumption.