A comprehensive study on the transition metal–catalysed pyrolysis kinetics, thermodynamics and mechanisms of bamboo powder

Abstract

Effects of Co(II), Ni(II) and Fe(III) impregnations on the pyrolysis behaviours of bamboo were investigated by using thermogravimetry and fixed-bed pyrolyser. Results reveal that the impregnated metals have significant effect on biomass devolatilization. Nickel impregnation increases the organic phase yield and relative amounts of tar in liquid phase. Cobalt impregnation drops cellulose degradation temperature and increases aqueous phase and non-condensable gas yields. Similarly, iron-impregnated biomass produces the highest amounts of biochar. Impregnated metals act as catalysts during pyrolysis and change degradation mechanism by lowering apparent activation energies and by changing thermodynamic parameters. Mean activation energies determined by three different isoconversional methods for normal and Co(II)-, Ni(II)- and F(III)-impregnated biomass are around 240, 197, 183 and 180 kJ.mol−1 respectively and similar to the values calculated by combined kinetic model (CK model). However, Kissinger method–based activation energy of Co(II)-impregnated biomass (295.9 kJ.mol−1) is quite high compared to the values predicted by isoconversional methods (~197 kJ.mol−1) and the CK model (197.7 kJ.mol−1). The observed results can be related to the complex degradation behaviour of Co(II)-impregnated biomass and the limitations on the uses of the Kissinger method in such a complex process. The CK model–based single-step pyrolysis mechanisms of normal and Co(II)-, Ni(II)- and Fe(III)-impregnated biomass are f(α) = (1-α)1.118.α−1.828, f(α) = (1-α)1.286.α−1.033, f(α) = (1-α)0.8895.α−1.898.[−ln(1-α)0.360] and f(α) = (1-α)0.8898.α−1.670.[−ln(1-α)0.060] respectively. The single-step CK model poorly explains biomass devolatilization behaviour. However, the same model can explain metal-impregnated biomass pyrolysis quite satisfactorily. Additionally, thermo-kinetic results obtained from various methods adequately explain the results obtained from thermogravimetric and fixed-bed pyrolysis experiments.