Project nominated for the Excellence in low-carbon steel production Steelie 2022
The Gijón (Asturias, Spain) Coke Oven Batteries had been idled in 2013. After a complete revamping, they were put back in operation in 2020 battery 1 and 2021 battery 2. The coke gas is used in mills’ battery and reheating furnaces, but there was an important excess to be flared.
In the past, this excess gas was sold to an external power plant where it was burnt to produce electricity, but the pipe was obsolete and it would be more profitable in terms of energy efficiency and CO2 reduction to use it inside the factory as a substitute for fossil fuels.
To take advantage of this energy, it was decided to use it as an alternative fuel in the blast furnace (BF) to reduce coke, coal and, hence, CO2.
For this purpose, coke gas is cleaned at the Coke Battery Gas Treatment Plant to reduce naphthalene, ammonia and sulfur as much as possible. After this process, the gas is sent to the factory network, where it can be stored in a gasholder, where it is then compressed in a 2-stage screw compressor at 8 bar.
Keeping the gas in a certain temperature range is important to prevent harmful compounds from transforming and generating clogging or corrosion.
The gas flows through a pipe until a distributor close to the BF, which divides the flow into 29 smaller pipes to bring the gas to the 29 tuyeres. To control the flow, two parallel valves modify it according to the BF pressure behaviour or other pressure losses along the circuit.
The gas is injected through a dedicated lance at the same time that coal is injected in a second lance in the same tuyere, acting as fuel and reducing agent. After combustion simulations were done in R&D, it was concluded that there is not a harmful effect on coal combustion due to the competition for the oxygen.
Each kilogram of coke gas substitutes around 0.65kg of coke or 0.78kg of coal, as well as a small reduction of coke due to process stabilisation.
The calorific power and H2 content of the gas lead to an important reduction of coal and coke (Scope 1 CO2 reduction), not only by the direct reduction but also by a stabilisation of the process created by the presence of hydrogen being used to remove oxide from the iron.
As a secondary effect, the waste gas of the BF (BFG), is richer in H2 and its calorific power and total energy are higher. This extra energy is used to substitute natural gas (Scope 1 CO2 reduction) and electrical energy production (Scope 2 CO2 reduction).