Gasification of chars from tetralin liquefaction of < 1.5 g cm−3 carbon-rich residues derived from waste coal fines in South Africa

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The float–sink density separation method was used to produce a < 1.5 g cm−3 coal density fraction. The sample was subjected to tetralin liquefaction experiments and pyrolysed to produce char samples. The < 1.5 g cm−3 coal density fraction and its liquefaction carbon-rich residue chars formed from the pyrolysis experiments were evaluated using the CO2 gasification tests. The chars were characterized using a thermogravimetric analyzer by heating the samples at isothermal temperatures between 880 and 940 °C under a CO2 atmosphere. The results obtained from the gasification experiments revealed that the gasification reactivity values (Ri, Rs, Rtf, Rt/0.5) of the liquefaction carbon-rich residue chars were higher than those of the < 1.5 g cm−3 coal density fraction chars. Some inherent mineral matter in the residues may play a catalytic role during gasification of South African lighter density waste coal density fractions and their liquefaction residue chars. The initial reactivity of the liquefaction residue chars was observed to be approximately double than those of the < 1.5 g cm−3 coal density fraction chars. Significant increases in a number of pores were associated with the liquefaction experiments of these carbon-rich particles, which aid in the acceleration of the gasification reactions. These increases in the number of pores assisted the reduction of the gasification activation energy values. The RPM was shown to fit the experimental data the best and used to determine kinetic parameters. The gasification activation energy of the < 1.5 g cm−3 coal density fraction chars was shown to be 190.5 kJ mol−1 and 236.7 kJ mol−1 for Highveld float (HF) and Waterberg float (WF) chars, respectively. The values of the apparent activation energy for the liquefaction carbon-rich residues from the same two < 1.5 g cm−3 coal density fraction residue samples were 145.3 kJ mol−1 for the Highveld coal sample and 196.0 kJ mol−1 for the Waterberg coal sample. The gasification results obtained in this study indicate that the possibility of utilizing the waste coal floats and their liquefaction carbon-rich residues in the thermochemical processes (pyrolysis and gasification) is high.