Insights into catalytic co-pyrolysis of spent coffee grounds and high density polyethylene (HDPE) using acid mine drainage (AMD) treated sludge based catalyst: Analysis of kinetics, mechanism and thermodynamic properties

• Synergistic effect was observed for co-pyrolysis of spent coffee grounds and HDPE in terms of activation energy. • Thermogravimetric kinetics, thermodynamics and reaction mechanism was assessed. • Acid mine sludge based catalyst significantly impacted the co-pyrolysis kinetics. • The present investigation will help in optimization, design and scalability of the process. The present study aims to investigate the catalytic effect of acid mine drainage (AMD) treated sludge based catalyst on the co-pyrolysis of spent coffee grounds and HDPE. The sludge was generated during the treatment of AMD using eggshell and hydrogen peroxide. Theromogravimetric analysis of pyrolysis of spent coffee grounds (SC), HDPE (High density polyethylene), blend of spent coffee grounds and HDPE (SC+HDPE) in the ratio of 1:1, and blended feedstock with sludge derived catalyst (SC+HDPE_AMDC) (1:1 wt. ratio) was conducted at various heating rates of 10 °C/min, 20 °C/min, 30 °C/min and 40 °C/min respectively. Iso-conversional models such as Ozawa Flynn Wall (OFW), Kissinger Akahira Sunose (KAS), Friedman, and Starink were utilized for the determination of activation energy (E a ) of the process. The results showed that using AMD treated sludge based catalyst to the pyrolysis process, enhances its overall efficacy by lowering the activation energy (E a ) (OFW- E a : 209.11 to 177.14 KJ/mol, KAS-E a : 208.30 to 173.06 KJ/mol, Friedman- E a :210.54 to 176.28 KJ/mol, and Starink- E a :208.60 to 173.44 KJ/mol). Criado's z-master plot (CZMP) method was utilized to analyze the mechanism of the reaction. The pre-exponential factor and thermodynamic parameters were also evaluated. It is concluded that the incorporation of sludge based catalyst (AMDC) lowered enthalpy and randomness of system. Catalytic co-pyrolysis requires less energy, making it more environmental friendly choice for the sustainable processing of biomass and plastics. The present investigation will aid in the design, optimization and scalability.