(p521) Improvement of coke strength by control of particle size of semi-soft coking coal

摘要

The particle size of the charged coal for carbonization is very important to control the coke strength which is strongly affected by homogeneity and bulk density of blended coal. In Dangjin work of Hyundai Steel, 10-11 coal brands are used in a coal blend where blending ratio of semi-soft coking coal (SSCC) currently reaches up to 30%. As the ratio of SSCC with poorgrindability is increasing, normal crushing makes a big difference in the fraction of below 3 mm particles between single coals, resulting in fluctuation of coke strength. It is known that the particle size of charged coal can be kept fine to improve coke strength. Fine crushing, however, leads to decrease in bulk density and some troubles in a gas refinery process.In this study, group crushing was applied to preparation of charged coal to improve the coke strength without decrease in bulk density. According to grinability (HGI), coal brands were classified into two groups for hard (HGK80) and mild crushing (HGI>80). Most of SSCCs were regarded as the hard crushing group. In pilot oven tests, group crushing showed tendency of increase of 0.9-1.0% in Dl(150/15). When selective fine crushing of only SSCCs was applied, Dl increased by 0.9% at same level of-3 mm% (about 78%). As -3 mm% was varied from 80% to 77% under conventional crushing, Dl decreased from 82.6% to 81.5%. But selective fine crushing of SSCCs could keep Dl unchanged, even with drop of 3% in -3mm%. Based on results of laboratory tests, a commercial plant trial is conducting to optimize operation of two impact hammer crushers with a capacity of 500 t-coal/h for group crushing by adjusting hammer speed, coal loading rate, and gap between hammer-head and grinding-wall.