Energy saving technologies and optimisation of energy use for decarbonised iron and steel industry

Abstract

The iron and steel industry relies significantly on fossil energy use and is one of the largest energy consumers and carbon emitters in the manufacturing sector. Simultaneously, a huge amount of waste heat is directly discharged into the environment during steel production processes. Conservation of energy and energy-efficient improvement should be a holistic target for iron and steel industry. There is a need to investigate and analyse potential effects of application i.e., a number of primary and secondary energy saving and decarbonisation technologies to the basic energy performance and CO2 emissions profile of iron and steel industry. A 4.7Mt annual steel capacity iron and steel plant in the UK is selected as a case study.
By carrying out a comprehensive literature review of current primary and secondary energy saving and decarbonisation technologies, suitable technologies are categorised based on their purpose of utilisation and installation positions. It is found that fuel substitution technologies and waste heat recovery technologies have wide application prospects in iron and steel industry. To further investigate effects of these technologies on the UK integrated steelwork, a comprehensive model of iron and steel production processes is built by using the software Aspen Plus. The model is fully validated and is used to examine the specific energy consumption and direct CO2 emissions. Energy consumption and CO2 emissions of whole production chain to produce a ton of crude steel are 17.5 GJ and 1.06 t. Waste heat from hot coke and gas cooling could cover 40% of electricity consumed in the plant if coking process has the maximum coke capacity.
To implement primary energy saving and decarbonisation technologies, the performance of blast furnace is optimised first by substituting coke with bio-reducers based on the proposed model. Three biomass substitutions are considered to reduce coke rate and CO2 emissions of ironmaking process. Results show that coke demand of per ton of hot metal and CO2 emissions of the ironmaking process are improved by replacing partial coke with biomass. An optimal coke replacement is operated with 200 kg bio-oil and 222 kg coke when producing one ton of product. The reaction involving bio-syngas has the most potential to reduce CO2 emissions.
To find a sustainable way to capture CO2 and recover waste heat onsite, a model of adopting organic Rankine cycle with amine-based CO2 capture in ironmaking process is introduced. In comparison with different reducing agents injected into BF, bio-oil has the most advantage to improve energy consumption of CO2 capture system. CO2 emissions from total sites can be maximumly reduced by 69% through the method of CO2 capture with waste heat recovery technologies. The combination of various decarbonised technologies creates great opportunity to reduce CO2 emissions.
A mass-thermal network of iron and steel industry is finally built up, where primary and secondary energy saving technologies are implemented to optimise energy use and reduce CO2 emissions. The general guideline i.e., 5-step method is summarised to optimise the mass-thermal network. Exergy analysis is used to evaluate overall network after applications of energy saving and decarbonisation technologies. Injection of biomass-based syngas can maximumly increase the exergy efficiency of ironmaking process. Sinter and BOF steelmaking processes are related with mass ratio of hot metal. Optimisation insights of energy use and decarbonisation for steelwork are revealed based on exergy efficiency and destruction results.