Coal quality characteristics and its coal-forming environment of No.3 coal seams in Zhangwan Minefield, Southwest Shandong Province

To understand the basic characteristics of coals from the No.3 coal seams in the Zhangwan minefield and discuss its coal-forming environment, coal petrology analysis, industrial analysis and element analysis were carried out on 114 coal samples by means of optical microscopy, microscopic photometer and coal quality analysis. The results of coal petrographic composition show that the coal rock type is mainly semi-bright briquette, and the coal seams are dominated by organic components (total content is more than 85.40%). The vitrinite is the main component, followed by inertinite, and a small amount of semi-vitrinite and crust. The inorganic components are clay minerals, followed by oxides, a small amount of sulfides and carbonate minerals. The maximum reflectivity of coal ranges from 0.77% to 1.21%. Industrial analysis results show that the upper coal seam of the No.3 coal seams in the Zhangwan minefield has extra-low total moisture contents (SLM) (0.45%-4.91%), low ash contents (LA) (5.35%-16.52%), medium to medium-high volatile contents (MV-MHV) (27.62%-36.96%), low to medium fixed carbon contents (LFC-MFC) (44.96%-63.31%), low sulfur contents (LS) (0.19%-0.92%), low to medium phosphorus contents (P-2 to P-3)(0.006 6%-0.007 6%), moderate to strong bonding (ZQNM-TQNM) (48-100) and characteristics of high calorific value (HQ) (22.25-32.49 MJ/kg). The lower coal seam of the No.3 coal seam in the Zhangwan minefield has extremely low total moisture contents (SLM) (0.55%-2.32%), low ash contents (LA) (5.21%-12.76%), medium-medium-high volatile contents (MV-MHV) (27.45%-36.95%), medium-medium fixed carbon contents (LFC-MFC) (52.74%-63.68%), low sulfur contents (LS) (0.25%-0.84%), extra-low to low phosphorus (P-1 to P-2) (0.004%-0.024%), strong to super-strong bonding (QNM-TQNM) (76-98) and high calorific value (HQ) (25.21-32.05 MJ/kg) Characteristics. Coal of the No.3 coal seams in the Zhangwan minefield mainly consist of 1/3 coking coal and coking coal, and a small amount of fat coal. The industrial utilization direction of coal are coking coal and power coal. The mirror inertness ratio (V/I) of Zhangwan No.3 coal seam is between 1.6 and 2.7, which indicates that the coal forming environment is dominated by wet overlying water environment; the ash composition index of upper coal seam 3 is 0.12-0.54, and that of lower coal seam 3 is 0.09-0.41, which indicates that coal seam 3 belongs to swampy environment affected by seawater; The sulfur content of No.3 coal seam is less than 1%, indicating that the coal forming environment is delta coal forming environment with weak influence of seawater. The proportion of organic sulfur in total sulfur of coal seam 3 is 40.22%-66.20%, which indicates that coal seam 3 should be low sulfur coal formed in continental Piedmont plain; all samples of coal seam 3 have V/(V+Ni) values of 0.215-1.0, V/Cr values of 1.06-3.2, and Ni/Co values of 0.57-2.77, all of which show the sedimentary environment with strong reduction. The sedimentary environment of No.3 Coal Seam in Zhangwan coalfield is a freshwater reduction environment, occasionally affected by seawater, which is delta plain subfacies.