Decarbonisation Technology - November 2022

Composition economically necessary. This is because hydrogen prices are expected to drop to around 2 $/kg, which is equivalent to 15 $/ GJ, i.e., hydrogen will always be a relatively The economics of oxyfuel combustion are typically driven by fuel price, but as steel mills adopt green hydrogen fuel to decarbonise their footprint, oxyfuel combustion will become for capture. Another way to increase the CO₂ concentration is to reduce/eliminate nitrogen in the flue gas with oxyfuel combustion. Most flue gases contain significant amounts of nitrogen from the combustion with air, and this nitrogen is reduced or eliminated by using oxygen enrichment or 100% oxyfuel combustion. Oxyfuel combustion Oxyfuel combustion technology is a solution for immediate decarbonisation, which also allows smooth adaptation to hydrogen fuels whenever viable. Many unit operations in a steel mill use air for the combustion of fuel, which carries 79% ballast (almost all of it nitrogen). This nitrogen is heated up in the furnace and emitted in the flue gas, resulting in wasted energy, higher fuel consumption, and CO₂ emissions. Moreover, it hampers the radiative heat transfer from the products of combustion, which is the dominant mechanism at elevated temperatures. The use of oxygen instead of air, called oxyfuel combustion, eliminates this nitrogen ballast and results in: • Up to 60% fuel and CO₂ savings • 75% reduction in flue-gas volume • Up to 90% NOx reduction • On-demand production increase • Ability to use low calorific gases in heating and reheating operations expensive fuel; accordingly, oxyfuel combustion is required to minimise its use. Therefore, the recommendation to steel mills is to convert to oxyfuel combustion now to achieve 20-50% CO₂ reduction and be prepared to blend green H₂ when it becomes available to achieve full decarbonisation in the future. Given the high fuel prices and the need to decarbonise, oxyfuel combustion offers an immediate first step for steel mills to adopt across their flow sheet. Oxyfuel combustion has been successfully applied to several steel mill operations, including blast furnace stoves, pelletising/sintering furnaces, electric

using green hydrogen as the reductant instead of natural gas, along with green power to the EAF and balance of plant. While up to 70% hydrogen has already been demonstrated in a DRI plant, pushing this limit to 100% hydrogen is the focus of several pilot plants that recently have been initiated. An important difference between use of natural gas and pure hydrogen, however, is the endothermic nature of reduction with hydrogen, which requires a preheating of the hydrogen to temperatures above 1,000°C (this is not needed when using natural gas as reductant). Additionally, a large obstacle to expanding the production of DRI is the availability of suitable iron ore pellets. It is obvious from the above considerations that near complete decarbonisation is possible, but at severe costs which need to be offset with higher costs of CO₂ emission. This decarbonisation also requires significant investments in new facilities and assets – a daunting challenge for this industry to overcome. For example, the Capex for an H 2 -DRI-EAF plant could be 800-1,000 $/t of annual capacity, but after factoring in the cost of the required green power and hydrogen infrastructure, these costs can escalate to 5,000 $/t of annual capacity. Clearly, this is a long-term solution for the industry, beyond the year 2030. exothermic reaction, reduction with H₂, as already mentioned, is endothermic. If the established DR shaft furnaces process uses natural gas as an input for the reduction, a reasonable balance between the heat required and the heat generated is achieved. However, if increasing levels of H₂ are used to carry out the reduction, there will be a need to preheat it beyond 1,000ºC. This can be achieved either by electric heating or combustion of H₂, or a combination of both; at higher temperatures, electric heating becomes less favourable. Accordingly, use of natural gas but combined with CCUS could also be advantageous from that perspective. Sometimes the DRI process flow sheet includes a CO₂ removal step to recycle the top gas. In this case, a high-purity CO₂ stream is readily available Hydrogen and CCUS for DRI While reducing iron ore with CO is an


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