Solar-assisted sorption-enhanced gasification of polyethylene terephthalate (PET) plastic waste - Experimentation and process modelling

Polyethylene terephthalate (PET) is a common plastic that has been widely used in packaging and textiles. In European Union countries, only 10% of PET plastic was recycled in a closed loop, and the remained was discarded in nature. Therefore, we proposed the solar-assisted sorptionenhanced gasification (SEG) to produce H2 and other materials from PET waste. This research aims to study the solar-assisted SEG of PET waste by experiments to obtain the optimized operating conditions, and then use them in Aspen Plus modelling to investigate the techno-economic performance of the system. This research was divided into four tasks: three experiments of PET steam gasification, PET steam gasification with CaO, and solar-assisted Ca looping process in bubbling fluidized beds, and the Aspen Plus modelling of SEG of PET integrated with a steam power plant and a LiBr-H2O absorption chiller to recover the excess heat in the process. Four journal papers were published accordingly to research tasks. Publication 1 revealed that gas yields increased significantly with the increasing temperatures, and the total tar yields dropped. The effect of temperature on gas products was more remarkable than steam-to-PET ratio and residence time. Carbon balance analysis showed that half of the carbon in PET was converted into tars, and CO2 was the dominant gas product. Publication 2 reported that adding CaO promoted H2 and CO2 yields and reduced tars, meaning that CaO mainly acted as a catalyst at the examined conditions at examined temperatures. Correlations between gas products and three operating parameters: temperature, CaO-to-PET ratio, and steam-to-PET ratio were fitted with the help of response surface methodology, and the gas yields were predicted well. Publication 3 studied the solarassisted Ca looping process and obtained the optimized combined temperatures of carbonation 650 °C and calcination 850 °C to minimize CaO deactivation and maximize the energy-carrying capacity, considering a reasonable mass flow rate between the solar calciner and the gasifier. Publication 4 discovered that the energy and exergy efficiencies of the integrated system were 60%–70% for both day and night modes. The annual production of H2 and benzene were 684 t and 6286 t, respectively, with an annual 19 kt of PET feedstock and capturing 21 kt of CO2. The project is feasible when the benzene price is greater than 1092 €/t and the CO2 prices are higher than 80 - 120 €/t. The results show that there is a potential to implement solar-assisted SEG of PET plastic waste to produce H2 and other valuable chemicals. This research provides a novel method for PET plastic waste upcycling sustainably with zero CO2 emission and a renewable heat source without burning additional fuels to achieve the zero plastic waste goal in the future.