Influence of metal additives on pyrolysis behavior of bituminous coal by TG-FTIR analysis

To study the catalytic effects of alkali, alkaline earth and transition metal additives on coal pyrolysis behavior, bituminous coal loaded NaCl, KCl, CaCl2, MgCl2, FeCl3 and NiCl2 was respectively investigated using Thermogravimetry and Fourier Transform Infrared Spectroscopy (TG-FTIR). Results indicated that the maximum mass loss rate decreased under the metal additives in the primary pyrolysis stage. The total mass loss of pyrolysis was reduced in metals catalyzed pyrolysis except for Na loaded sample. Kinetic analysis was taken for all samples adopting the method of Coats-Redfern. Activation energy of raw coal in the primary pyrolysis stage was 92.15 kJ.mol(-1), which was lowered to 44.59-73.42 kJ.mol(-1) under metal additives. The orders of catalytic effect for this bituminous coal were Mg>Fe>Ca>Ni>K>Na according to their activation energies. Several investigated volatiles including CH4, CO2, CO, toluene, phenol and formic acid were identified from FTIR spectra. The yields of CH4, CO2, toluene, phenol and formic acid were decreased, but the evolution of CO was increased. The presence of metals in the coal samples have been involved in a repeated bond-forming and bond-breaking process, which greatly hindered the release of tars during pyrolysis as the tar precursors were connected to coal/char matrix and were thermally cracked, becoming a part of char.