Energy recovery and waste treatment using the co-pyrolysis of biomass waste and polymer

The pyrolysis of spent coffee grounds (SCG) and polymers was examined as a waste treatment option for energy recovery and carbon sequestration. Rice straw-derived biochar was used as control biochar to evaluate the sorption capacity and energy production capability of SCG-derived biochar. SCG are characterised by high levels of volatile matter, rendering them suitable as an energy source. SCG were converted to biochar, bio-oil, and syngas via pyrolysis, with yields of 22%, 33%, and 45%, respectively. The high heating value (HHV) of the biochar and bio-oil was 20.6 and 22.9 MJ kg^-1, respectively, indicating that they could be used as supplementary fuels. Co-pyrolysis with polymers (20 v v%^-1) increased the HHV of biochar. Accordingly, the maximum production of CH₄ and H₂ increased from 0.3 and 0.04 mmol g^-1 to 3.4-6.3 and 0.8-1.3 mmol g^-1, respectively. Polystyrene most strongly enhanced the yields of CH₄ and H₂, followed by polypropylene and polyethylene; this order was likely to be in accordance with the number of carbon and hydrogen atoms present in the monomers. Similar to rice straw-derived biochar, the biochar produced from SCG demonstrated a high sorption capacity for 2,4-dinitrotoluene and chromate due to its high carbon content and anion exchange capacity, respectively. Laboratory pot tests revealed that the coffee grounds-derived biochar was able to increase the growth of young radish. Our results suggest that the pyrolysis of SCG and polymer may be a promising option for waste treatment, energy production, and carbon sequestration.