Leveraging CO2 to directionally control the H2/CO ratio in continuous microwave pyrolysis/gasification of waste plastics: Quantitative analysis of CO2 and density functional theory calculations of regulation mechanism

Abstract

To recover syngas from waste plastics (Low-density polyethylene, LDPE) and CO2 simultaneously, the advanced continuous microwave pyrolysis and in situ reforming of CO2 were used. The ability of CO2 to tune LDPE carbon reallocation was evaluated, as was that of CO2 to directionally control the H2/CO ratio. The CO2 utilization efficiency and energy balance were quantitatively analysed, and the mechanism through which CO2 controlled H2/CO ratio was explored via experimental results and density functional theory calculations. The results showed that increasing pyrolysis temperature from 450 °C to 650 °C and introducing CO2 changed the carbon reallocation ability, increased syngas content, and controlled H2/CO ratio. The pyrolysis gas yield and syngas content reached the maximum values of 96.44% and 66.61 vol%, respectively. However, introducing excessive CO2 led to the double waste of resources and energy due to side reactions and high energy consumption. The maximum value of CO2 utilization efficiency and energy recovery efficiency was 78.95% (75 vol% CO2) and 75.09% (50 vol% CO2), respectively. CO2 directionally controlled the production of syngas and H2/CO ratio (from 0.65 to 2.88) by stimulating the LDPE pyrolysis reaction and the reforming reactions of its pyrolysis intermediates to form oxidized carbon radicals (such as Rn-O-C-O*, Rn-CO*, and Rn-O*) to generate CO. Therefore, different pyrolysis systems could regulate the degree of CO2 reforming reaction by controlling CO2 concentration based on the goal of optimizing the efficiency of resource or energy. This work provides basic theory and technical support for realizing the efficient recycling of waste plastics and CO2.