Multi-stage coking for increased tar yield and formed coke of high strength. 1. Pressurized heat treatment of blended coals

Two stage coking of a blend of fresh coking coals was studied to achieve high extracted tar yield and high mechanical strength of the formed coke from the residue. The preheat treatment (at 450 °C for 10 min in nitrogen under 1 MPa) produced 5% gas, 20% pyridine soluble (PS) tar and 75% pyridine insoluble (PI) residue, the last of which exhibited high mechanical strength (over 10 MPa) by carbonization at 3 °C min^-1 heating rate and successive calcination at 800 °C. The carbonization and calcination provided another 5-10% tar. Thus, the pressurized preheat treatment depolymerizes coal macromolecules enough to allow a large tar yield and satisfactory fusibility to the extracted residue. Sufficient strength of the formed coke for blast furnace application developed even at a calcination temperature as low as 700-800 °C. Such high strength may originate from the dense packing of the PI residual grains and their sufficient adhesion. Sufficient fusibility of the grains allowed the development of fine mosaic anisotropy, which is favourable for mechanical strength and gasification reactivity.