Dependence of pyrite concentration on kinetics and thermodynamics of coal pyrolysis in non-isothermal systems

Thermogravimetric investigation has been undertaken to evaluate the kinetic and thermodynamic parameters and the influence of pyrite concentration on coal pyrolysis in inert atmosphere. Weight loss up to 150 °C are 4.3 wt.%, 5.5 wt.% and 7.0 wt.% in 1%, 5% and 10% pyrite blended coals, respectively, and in the raw coal (8.2 wt.%) suggests that the presence of pyrite can influence loss of moisture and volatile matter characteristics. The essential weight loss is restricted in the temperature range of 220–650 °C for all the samples. DTA shows two peaks in that region; the first one is related with the primary carbonisation of coal and the second, which appears at a relatively higher temperature, is attributed to the decomposition of pyrite, both shifting to higher temperature with increasing pyrite concentration. Further presence of pyrite in substantial amount decreases the reactivity of coal for pyrolysis; however, its negligible amount has no significant impact on carbonisation. Kinetic study reveals that, with increase in temperature, the rate constants continuously increase; however, the trend is irregular for activation energy (range: 20.42–36.17 kJ mol−1 from Arrhenius equation and 28.72–38.29 kJ mol−1 from Coats and Redfern equation) and frequency factor (range: 1.35–2.11×10−4 s−1) for all the samples. It has been inferred that decomposed products of pyrite rather than pyrite itself were the real catalyst for enhancing coal pyrolysis; non-stoichiometric pyrrhotite accelerates carbonisation and/or elemental sulphur stabilises free radicals through cross linking. The pyrolytic reaction in all the systems proceeds with the absorption of heat, decrease in randomness and non-spontaneous in nature. It is suggested that the carbonisation process is a diffusion controlled reaction rather than thermodynamically controlled.