Low-Temperature Co-pyrolysis of Polypropylene and Coffee Wastes to Fuels

The co-pyrolysis of coffee grounds and polypropylene was studied, at different volumetric fractions of each material, in a fixed bed, under inert flow. With a constant heating rate of 5 degrees C/min, a preset pyrolysis temperature, between 360 and 420 degrees C, was maintained for 3 h. Polymer degradation is not completed at a low temperature (360 degrees C), while coffee is easier to degrade, as confirmed by differential scanning calorimetry (DSC). Liquid product analysis confirms that a two-stage pyrolysis mechanism occurs, with biomass degrading first. When the pyrolysis temperature is increased, the degradation grows noticeably, doubling from 360 to 380 degrees C, becoming >90 wt % at 420 degrees C. When the fraction of polypropylene is increased, the quantity of light liquid products reduces, while the fraction of heavy condensate products rises. The presence of pyridine, phenol, C-12, and C-13, sometimes with hydroxyl groups, is relevant, while among the high-molecular-weight products, large quantities of hexadecanoic acid and caffeine, arising from coffee, and hydrocarbons up to C-30 were detected. Increasing the temperature shows a drop in the amount of light liquids, while medium- and high-molecular-weight products stabilize. Above 400 degrees C, co-pyrolysis allows for the procurement of a liquid product mixture, which is comparable to fossil fuel oil.