Energy recovery in blast furnace steel production
To improve energy efficiency, energy flows are recovered in the production process (see illustration). Gases, steam and hot water produced in the processes are recovered and utilized in other parts of the process to generate electricity and heat.
Production processes that produces energy
In conjunction with the process where iron ore is reduced to hot metal, energy rich process gases are released. These gases are utilized in SSAB’s steel production. In the blast furnace, the oxygen bound in the iron ore is converted to CO and CO2 through a reaction with the carbon in the coke and coal powder. As its name implies, blast furnace gas (BFG) originates in the blast furnace. It is a gas mixture containing mostly carbon monoxide and carbon dioxide from the reduction process. Although the gas has a low heat value, given the large volume it still contains a considerable amount of energy that can be utilized through either combustion through own processes, where the energy content is used for coke or pig iron production, or in a combined heat and power plant, where the energy content is converted to electricity and heat. The electricity is used in SSAB’s production processes, while the majority of the heat is supplied to the local district heat network.
SSAB produces coke through dry distillation of coal in coking plants. Heating the coal in the coking plant, releases energy rich gases that mainly consist of hydrogen and carbon monoxide. The coke oven gases released are used in the production process instead of sourced fuels such as natural gas, LPG or oil. The BOF process also gives rise to energy rich gases that consist mainly of carbon monoxide. BOF gases (BOFG) are mixed with the blast furnace gas before combustion in the power plant. The combined heat and power plants at SSAB’s production sites in Sweden and Finland produce 50% of SSAB’s electricity requirement in the Nordics, as well as supplying the district heat networks in Luleå, Oxelösund and Raahe with heat.
Energy usage at SSAB
Apart from the energy-rich process gases that are recovered as electricity and heat in SSAB’s steel production, purchased fuels such as natural gas, LPG and oil are also used in the steel production process. Purchased fuels are used to heat up the steel in the hot-rolling process and after treatment, as well as being a supporting fuel in the power plant. In order to maximize energy efficiency, priority is always given to utilizing process gases rather than purchased fuels. To maximize the energy efficiency, heat is recovered from waste gases and machinery; the heat is used internally to reduce the need for purchased energy, or used for district heating.
In 2018, SSAB used 4.69 TWh of purchased fuels in its global operations. Of these purchased fuels, natural gas accounted for 68%, LPG for 25% and oil for 7%.
In 2018, SSAB used 4.73 TWh of electricity in its global operations of which 30% was produced internally. In the Nordics, 2.75 TWh was used, of which 52% was produced internally.
SSAB’s target to reduce the amount of purchased energy
SSAB has a target to reduce the amount of purchased energy, both electricity and fuels. The target is to achieve a lasting reduction of 400 GWh in purchased energy by the end of 2020, compared to the 2014 baseline.
Read more about SSAB's sustainability targets
Use of process gases in combined heat and power plants
The chart illustrates the use of process gases to make electricity and heat in the combined heat and power plants.
Energy efficiency in practice
Energy efficiency applies to the whole production chain and the need for purchased fuels is reduced by optimizing which process gases can be utilized for which processes. Additionally, the production of electricity and heat is maximized in terms of heat value and volume by optimizing the supply of process gases to the power plant. The most fundamental part of improving energy efficiency is to constantly question if energy usage is optimized, as well as analyzing the actual need in different processes (shutdowns, different production levels, etc.). Below are some examples of activities to improve efficiency, based on experiences from SSAB’s operations and split into optimization, productivity, use and maintenance.
- Reduced output pressure in media system (compressed air, etc.)
- Cooling water, adjust output pressure according to need due to inlet temperature
- Optimize pump groups (3 running instead of 4, etc.) to run on optimum operation point (cooling water, hydraulic system, etc.)
- Furnaces, short stops (10 min to 8 h) manual ramp-down and control
- Furnaces, long stops (>8 h), manual set-point for lower temperature
- Furnaces, analyze oxygen level in flue gasses, optimize burners
- Routines to stop equipment not needed when production is shut down
- Increase yield
- Hot charging of reheating furnaces
- Routines to shut down equipment during maintenance, disruption, etc.
- Switch to optimized lighting source
- Customize lighting after need, presence-controlled, etc.
- Leakage control in media system (compressed air, water, air, gases, etc.)
- Analyze efficiency on pumps, fans, compressors, etc. Change equipment if efficiency is poor due to high wear or wrong sizing
- Analyze efficiency on control valves, change if performance is poor due to high wear or wrong sizing