(438l) A Two-Stage Process for Converting Waste Plastics into Energy

In this work, a laboratory-scale two-stage reactor for converting waste plastics into energy was constructed. Our process converts waste plastics into synthesis gas (hydrogen and carbon monoxide) and methane that can be used for energy production. This innovative reactor technology employs a pyrolysis reactor as the first step, followed by a chemical looping gasification reactor as the second step. This two-stage process is more suitable than the traditional one-step gasification process, because solid residues associated with the waste, such as ash and tar, are contained in the pyrolysis unit. In addition, our reactor system utilizes metal oxides (Me_xO_y) in the gasification (second) step instead of traditional oxidizers such as oxygen or steam. This further saves the cost for the expensive air separation step to separate desired products from nitrogen. 

Our experimental results using polyethylene as a surrogate plastic waste showed that the two-step pyrolysis-looping process significantly increased the yields of light gaseous products (hydrogen, water, carbon monoxide, carbon dioxide, and methane) compared to only pyrolysis. Polyethylene can be converted into light gaseous products in the presence of iron (III) oxide (Fe₂O₃) oxygen carriers. The total carbon-based mass yields of these products reached as high as 53%, and the total hydrogen-based mass yields reached as high as 86%. The product distributions of light gaseous products can be varied by certain controllable parameters (e.g., metal oxide loading, carrier gas flow rates, and gasification temperature), and key parameters affecting product yields were identified as gasification temperature and metal oxide loading and deactivation.