Study on gasification characteristics and kinetics of polyformaldehyde plastics in supercritical water

Supercritical water (SCW) treatment technology has been applied in the field of waste plastics recycling due to its advantages of clean, high efficiency and low carbon. Previous research has focused on liquefaction of plastics to produce oil, mainly for some general-purpose plastics. Different plastics have different structures, which will affect their gasification behavior in SCW. As one of the five engineering plastics, polyoxymethylene (POM) plastics will also become one of the main sources of waste plastics recycling industry in the future. In this paper, the supercritical water gasification (SCWG) experiment of POM plastics was carried out. The effects of temperature, residence time, feedstocks concentration, pressure on the gasification reaction were discussed. The carbon balance of the products under different working conditions was investigated. The optimum condition for SCWG of POM plastics was determined. On this basis, the gasification kinetics of POM plastics were studied. It was found that the gasification efficiency of plastics was significantly improved by temperature increase. The total carbon content of gas and liquid phase products increased gradually. At 700 °C, the total conversion rate reached 99.79%. With the prolongation of the reaction time, the organic carbon in the liquid phase was gradually converted into gas and the gasification reaction proceeds more completely. The increase in concentration reduced the contact area between the raw material and the gasification agent. At the same time, the diffusion of organic matter was hindered, resulting in a lower gasification rate. The effect of pressure on the plastics gasification reaction was not significant. Under the optimum gasification reaction conditions, the carbon gasification efficiency reached 97.15%. Finally, a gasification kinetic model was established, and the activation energy of paraformaldehyde was 160.38 kJ/mol.