Virtuous utilization of carbon dioxide in pyrolysis of polylactic acid

Abstract

Polylactic acid has been adopted as a strategic alternative to petroplastics because of its biodegradability. The waste generation rate could be proportional to its use, considering the short lifespan of polylactic acid. However, a practical disposal or recycling protocol for polylactic acid waste has not yet been developed. Thus, this study suggests a promising thermochemical platform for valorizing polylactic acid waste into energy resources (syngas). Specifically, carbon dioxide-assisted pyrolysis has been suggested to impart environmental features to polylactic acid disposal. Before the pyrolysis tests, the polylactic acid waste sample was characterized by Fourier transform-infrared spectrometer and thermogravimetric analyses, which showed that polylactic acid contained a substantial amount of additives and impurities (∼13 wt%). The impurity containing polylactic acid was converted into pyrogenic gases and biocrudes through pyrolysis process. The pyrolysis was performed under carbon dioxide condition and led to enhanced carbon monoxide formation from simultaneous homogeneous reactions between CO2 and volatile organic compounds evolved from thermal degradation of polylactic acid. CO2 was reduced and the volatile compounds were oxidized. The evolution of carbon monoxide from pyrolysis under carbon dioxide condition was 2 times higher than that from nitrogen condition. The concentration of carbon monoxide from the pyrolysis of polylactic acid waste with respect to plastics and biomass was considerably higher. This observation indicates that the susceptibility of carbon dioxide to the homogeneous reaction is highly sensitive. To seek a way to hasten the homogeneous reaction, silica supported nickel catalysts were applied. The evolution of carbon monoxide from catalytic pyrolysis under carbon dioxide condition was 4.5 times higher than inert atmosphere.