Pyrolysis of Spent Coffee Ground Using Landfill Leachate Concentrate as a Pyrolytic Additive

Resource depletion and climate change have fostered sustainable initiatives in the waste management sector. Pyrolysis (Py) has emerged as an option for valorizing spent coffee grounds (SCG). In addition, inorganic compounds can play catalytic effects in the pyrolytic reaction of organic materials, increasing the char yield and material porosity. This study investigates the slow pyrolysis of SCG using concentrated landfill leachate residue (CLLR) (1:1 %wt) as a pyrolitic additive due to its high salinity. Biochars were characterized in terms of their thermal behaviour to discuss environmental benefits and potential application. Slow-py experiments were conducted using a lab-scale pyrolizer at heating rate of 45ºC min^-1. The lab-pyrolizer was operated at atmospheric pressure. Py conditions were as follows: temperature of 600ºC, inert gas flow of 100 cm³ N₂ min^-1, and residence time of 1 h. Thermal characterization was performed using TA Instruments, SDTQ600 model. Biochar samples were heated from 20 to 1000ºC at a rate of 20 °C min^-1 under air-flow rate of 100 mL min^-1. Biochars were characterized by higher water content and heating rate than their feedstocks. Values were 9.18 %wt and 18.11 MJ kg^-1 and 23.25 %wt and 22.05 MJ kg^-1 for biochars produced from SCG and SCG+CLLR (1:1 %wt), respectively. High water content is associated with higher porosity. In the case of the more porous biochar, water loss occurred from 20 to 150°C, followed by the combustion of organics until 550°C. Data indicated that alkali metals of the CLLR catalyzed the carbonization of organic materials making thermal decomposition faster. It is suggested that the high metal content of CLLR could change the biochar's thermal stability, decreasing the decomposition temperature. From elemental composition analysis, the produced biochar owned essential soil minerals (e.g., Na, K, and Ca); therefore, it could be used as a soil amendment for C-sink slow-release inorganic elements or energy storage devices such as batteries and supercapacitors. Future studies will include morphological characterization of biochar and carbon balance.