The thermal degradation of lignocellulose biomass with an acid leaching pre-treatment using a H-ZSM-5/Al-MCM-41 catalyst mixture

Abstract

Improvements on the pyrolysis process of biomass fuels are needed to obtain a high-quality of bio-oil. Pre-treatment by acid leaching prior to the pyrolysis process is considered to remove Alkali and Alkaline Earth Metal (AAEM) from the biomass, since AAEM adversely affect the catalytic pyrolysis process. Therefore, the main objective of the present work was to determine and compare the effect of mixed-catalysts consisting of H-ZSM-5 and Al-MCM-41, which are used at different ratios in lignocellulose biomass pyrolysis via Thermal Gravimetric Analysis (TGA) for both, un-leached and leached biomass. In addition, the activation energies have been determined based on the solid-state reaction mechanism. The results from the acid leaching treatment showed that the optimum leaching process was set to 30 min, 30 °C and 5 wt% acetic acid in the leaching liquid. This resulted in 59%, 95%, 99%, and 96% reduction degree of Calcium (Ca), Magnesium (Mg), Potassium or Kalium (K), and Natrium (Na), respectively. The Higher Heating Values (HHVs) for un-leached and leached biomass were 19.42 and 19.14 MJ/kg, respectively, calculated by using the Milne formula. The HHV value is not significantly influenced by the acid-leaching pre-treatment. Thermogravimetric analysis showed similar trends for the mass loss as a function of temperature and four stages could be determined from the thermal degradation of biomass. There was no significant shift in the temperature profiles between un-leached and leached biomass degradations. However, the removal of AAEM significantly affected the degradation of hemicellulose, cellulose, and lignin. A 12.4–18.2% increase of mass losses could be found in Phase 2 for leached biomass, compared to un-leached biomass. The mass losses in Phase 2 also increased with increased heating rates. From the un-leached biomass experiments using a catalyst mixture, the percentage of mass loss increased from 64.95 wt% at 10 K min−1 to 68.33 wt% at 50 K min−1. Moreover, for the leached biomass, it rose from 65.71 wt% at 10 K min−1 to 66.73 wt% at 30 K min−1, before decreasing to 62.44 wt% at 50 K min−1. A lower mass loss in Phase 4 for leached biomass compared to un-leached biomass showed the influence of an AAEM removal. The second order (F2) mechanism was able to illustrate the catalytic pyrolysis process, proven by the result that the coefficient of determination (R2) was higher than 0.99, which was high compared to other mechanisms. An acid leaching pre-treatment led to a reduction in the activation energies. The activation energies for the un-leached biomass were 26.94 and 25.56 kJ mol−1 at a heating rate of 10 K min−1 for a process without and with a catalyst mixture, respectively. The activation energies for leached biomass were 24.05 and 21.80 kJ mol−1. The results also showed that the use of the acid leaching process as a treatment prior to catalytic pyrolysis is positive, since it resulted in high devolatilization and reaction rate.