Manipulating the H₂/CO ratio from dry reforming of simulated mixed waste plastics by the addition of steam

Two-stage pyrolysis-catalytic reforming of plastics was investigated with the aim of producing usable quality synthesis gases (syngas) comprised of H2 and CO. The process consisted of pyrolysis of the plastics in the first stage and catalytic reforming with CO2 and steam as the reforming agents in the second stage. The plastics used were a mixture of waste plastics prepared to represent those found in municipal solid waste and the catalysts studied were Ni-Co/Al2O3 and Ni-Mg/Al2O3 prepared by the rising pH technique. A range of different CO2/steam ratios were considered; 4:0, 4:0.5, 4:1, 4:1.5 and 4:2 for the Ni-Co/Al2O3 catalyst and 4:0, 4:0.5, 4:1, 4:2 and 4:3 for the Ni-Mg/Al2O3 catalyst. The results obtained demonstrated that the catalysts and the CO2/steam ratio influence the syngas quality, as represented by the H2/CO molar ratio value. With the Ni-Co/Al2O3 catalyst, the H2/CO molar ratio was increased from 0.74 (no steam) to 0.94 (CO2:steam ratio; 4:1) however the H2/CO molar ratio decreased with further steam addition. Results using the Ni-Mg/Al2O3 catalyst showed a different trend, wherein the H2/CO molar ratio increased with the increase of steam addition into the system. From the evaluation of the gas composition, the steam addition with the Ni-Mg/Al2O3 catalyst promoted hydrogen production while the Ni-Co/Al2O3 catalyst promoted carbon monoxide production. The addition of steam to the dry reforming of waste plastics has the potential to manipulate the H2/CO molar ratio hence, the quality of syngas produced can be matched to the desired end-use industrial application