Method for preparing clean steel by coal gas direct reduction one-step method

The invention discloses a method for preparing clean steel by a coal gas direct reduction one-step method. Low-coal-rank coal is pyrolyzed to tar, pyrolyzed coal gas and sponge-shaped fixed carbon at a medium temperature; the sponge-shaped fixed carbon and water vapor are reacted to prepare reducing gas; the reducing gas is introduced in iron pellets or lump ores; iron therein is selectively reduced to obtain pure sponge iron; the pure sponge iron is filled in an electric furnace for smelting to obtain pure liquid-state steel; and meanwhile, the pyrolyzed coal gas is used as heating energy for pyrolysis of the low-coal-rank coal, gasifying reaction of the sponge-shaped fixed carbon and selective gas base direct reduction of the iron pellets or lump ores. The method extracts the tar and the pyrolyzed coal gas in the low-coal-rank coal to a greater extent, and prepares the reducing gas by the sponge-shaped fixed carbon to serve as by-products in the whole process; the energy cost in the whole smelting process is almost zero; and in the selective reducing process, only iron is reduced, and other elements are not reduced, so that the foundation is laid for preparation of clean steel, the oxidation period and the reducing period in the smelting procedure of a traditional electric furnace are synchronously disappeared, and the clean steel is obtained by a simpler flow.