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

• Continuous microwave pyrolysis realized efficient recycling of waste plastics and CO 2. • H 2 /CO was directionally controlled via instantaneous heating rate and reaction degree. • Utilization and conversion efficiencies of CO 2 were quantitatively analysed. • High-quality syngas from pyrolysis enhanced by CO 2 made high energy efficiencies. • CO 2 control mechanism of H 2 /CO ratio was explored via DFT theoretical calculations. To recover syngas from waste plastics (Low-density polyethylene, LDPE) and CO 2 simultaneously, the advanced continuous microwave pyrolysis and in situ reforming of CO 2 were used. The ability of CO 2 to tune LDPE carbon reallocation was evaluated, as was that of CO 2 to directionally control the H 2 /CO ratio. The CO 2 utilization efficiency and energy balance were quantitatively analysed, and the mechanism through which CO 2 controlled H 2 /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 CO 2 changed the carbon reallocation ability, increased syngas content, and controlled H 2 /CO ratio. The pyrolysis gas yield and syngas content reached the maximum values of 96.44% and 66.61 vol%, respectively. However, introducing excessive CO 2 led to the double waste of resources and energy due to side reactions and high energy consumption. The maximum value of CO 2 utilization efficiency and energy recovery efficiency was 78.95% (75 vol% CO 2 ) and 75.09% (50 vol% CO 2 ), respectively. CO 2 directionally controlled the production of syngas and H 2 /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 R n -O-C-O*, R n -CO*, and R n -O*) to generate CO. Therefore, different pyrolysis systems could regulate the degree of CO 2 reforming reaction by controlling CO 2 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 CO 2 .