POTENTIAL USE OF LOCAL LIGNITE FOR SEMI-COKE PRODUCTION THROUGH LABORATORY-SCALE CARBONIZATION AND PROCESS PARAMETER OPTIMIZATION

Abstract

Pakistan holds an estimated 185 billion tonnes of coal, of which more than 99 % is Sindh lignite characterised by high moisture, ash, sulphur, and volatile-matter (VM) contents. These properties limit its direct use in cement plants and steel mills, which instead rely on costly imported coal. This study evaluates the upgrading of Lakhra lignite into a smokeless, carbon-dense semi-coke through low-temperature carbonization. A custom air-tight iron carbonization chamber coupled with a steel cyclone for tar and gas separation was designed and fabricated for use inside a temperature-controlled crucible furnace. Eighteen carbonization runs were carried out on three size fractions (−25/+10, −10/+5 and −5/+2 mm) at three temperatures (400, 600 and 750 °C) for two carbonization times (2 and 4 h). Raw feed and carbonized products (36 samples) were characterised by proximate, sulphur and gross calorific value (GCV) analysis. Carbonization reduced VM from an average of 57.6 % to as low as 0.4 to 6.7 %, raised fixed carbon (FC) from 32.1 % to 84 to 95 %, and increased GCV from 10,771 to about 13,000 Btu/lb. Treatment at 750 °C lowered sulphur from 8.3 % to 6.6 %. Balancing product quality against energy consumption, the optimum conditions were a −10/+5 mm fraction carbonized at 600 °C for 2 h, producing a semi-coke comparable to commercially reported products. A preliminary cost analysis indicates a favourable margin against imported coal.