Combined system for producing steel and method for operating the combined system

A plant complex for producing steel, comprising:

• a blast furnace for producing pig iron;
• a converter steel mill for producing crude steel;
• a gas-conducting system for transporting one or more streams of gases that occur in the production of pig iron and crude steel;
• and
• a power-generating plant for electricity generation, the power-generating plant being designed as a gas-turbine power-generating plant or gas-turbine and steam-turbine power generating plant and being operated with a gas supply that comprises at least a partial amount of one of a blast-furnace top gas that occurs in the production of pig iron in the blast furnace, and a converter gas that occurs in the converter steel mill;
• wherein:
• a chemical plant and a biotechnological plant are connected to the gas-conducting system;
• the power-generating plant, the chemical plant, and the biotechnological plant are arranged in a parallel setup, wherein each of the power-generating plant, the chemical plant, and the biotechnological plant receive a partial stream of gas diverted from the gas supply such that the power-generating plant, the chemical plant, and the biotechnological plant are operated simultaneously in parallel;
• and
• the gas-conducting system comprises an operationally adjustable gas-distributing device for dividing and controlling the gas supply that is fed to the power-generating plant, the chemical plant, and the biotechnological plant, wherein the gas-distributing device is adjusted when a price for externally obtained electricity reaches a predetermined threshold;
• the biotechnological plant produces a hydrocarbon compound by a fermentation of a syngas comprising CO and H2 as main constituents;
• and
• the gas supply to the chemical plant remains substantially constant during operation of the plant.