Effects of wet grinding of coffee wastewater on co-pyrolytic gasification of composite carbon materials: Reaction properties, thermodynamics and gasification kinetics, integral master diagram method and carbon material structure

Abstract

Coffee wastewater (CWW), a byproduct of the coffee-making process, can serve as a grinding medium in the co-pyrolysis gasification of coal and biomass. In this study, the effects of three wet grinding media—water, alcohol, and CWW—were compared with dry grinding for the first time, and their influences on the response characteristics of mixed carbon materials were investigated. Changes in carbon structure, graphitization, morphology, surface area, and pore size were analyzed through XRD, Raman, SEM, and BET technique. Reaction activation energy was evaluated through the Friedman, Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, and Starink methods, while the reaction model was investigated through the integral master curve method. The results showed that CWW-based grinding exhibited the highest reactivity, with an average weightlessness end point of 98.03 %. CWW-based grinding exhibited a specific surface area of 2.0228 m²/g, d₀₀₂ of 0.3777 nm, La of 2.204 nm, and Lc of 0.9119 nm, representing increases of 20.10, 2.18, 16.61 and 11.33 %, respectively, compared with dry grinding. In addition, the average pore diameter of CWW-based grinding was 23.90 % lower. Moreover, the average activation energy decreased from 147.03 kJ/mol for dry grinding to 110.14 kJ/mol for CWW-based grinding. The average activation energy and enthalpy change for the reactions based on the four grinding media followed this order: CWW-based grinding < alcohol-based grinding < H₂O-based grinding < dry grinding. Wet grinding of mixed carbon materials using CWW can modify the carbon skeleton structure, refine particle size, and shift the reaction mechanism from exponential nucleation to diffusion. Overall, this study proposes a new method to enhance the pyrolysis gasification of carbon materials, and can provide new ideas for the industrial silicon industry.