Experimental and techno-economic investigations into the implementation of H₂-DRI in New Zealand

Abstract

Currently, the iron and steel industry produces approximately 7% of the total global CO₂ emissions, and 2% of the total CO₂ emissions in New Zealand. However, steel is an essential material for construction around the world with a growing demand so eliminating this industry is not a possibility. Therefore, fundamental process changes must be implemented which reduce the emissions from iron and steel manufacturing.

Traditional ironmaking involves the carbothermic reduction of iron oxides at high temperatures using coal as both the reducing agent and heat source. Presently, the most viable alternative ironmaking process replaces the coal with hydrogen gas as a reducing agent, resulting in a product known as hydrogen direct reduced iron. Additionally, when the process heat is generated from electricity produced through renewable energy, no CO₂ emissions are produced through the entire process.

There is a sufficient supply of indigenous iron ore, titanomagnetite, available for ironmaking in New Zealand and its use in the production of hydrogen direct reduced iron was the focus of this study. The kinetics of the hydrogen reduction process was investigated through a series of thermogravimetric experiments. The results of the kinetic analysis were used for the development of a process model for the industrial production of hydrogen direct reduced iron. This process model was then analysed with regard to the techno-economics to ultimately estimate the conditions in which the process could viably operate in New Zealand. Kinetic analysis of the reduction of New Zealand titanomagnetite based on the shrinking core model found that the rate limiting step of the reduction reaction was the interfacial chemical reaction between the solid ore and the H2 gas. Additionally, the overall activation energy for this system, 97 kJ/mol, was used to estimate the size of the reactor required to produce the direct reduced iron at an industrial scale. Hence, the reactor should be approximately 6.5 m3 to meet the requirements of this process.

An investigation of the process model’s operating conditions found that the operating costs are dominated by electricity requirements. Overall, the process requires 3.24 MWh of energy per tonne of iron produced which becomes economically viable at an electricity cost of NZ$60 per MWh. The electrolyser efficiency and the design of a gas-gas process heat exchanger were important considerations for minimising the energy requirements of the overall process. An estimated capital cost of NZ$957 per tonne iron was also calculated, however, the process model considered in this study did not take into consideration unit operations upstream or downstream of the direct reduced iron plant, so, this should be considered in future research.

Therefore, based on the results of this experimental and economic analysis, the manufacture of hydrogen direct reduced iron from New Zealand ironsand is technically and economically feasible and has the potential to reduce overall emissions from the iron and steel industry in New Zealand.