Thermochemical conversion through pyrolysis and pyrolysis-catalysis in addition to gasification methods of processing plastic wastes and biomass have attracted research interest. Likewise, co-pyrolysis catalytic steam reforming of lignocellulosic biomass with plastic wastes for converting these wastes into high-value fuel and chemicals remains a novel option as it offers several advantages such as cost reduction for waste management, improvement in product quality and quantity, prevention of wastes into the landfill, etc.
In this research, pyrolysis-catalysis of high-density polyethylene (HPDE) was carried out in a fixed-bed two-stage reactor for the production of up-graded aromatic pyrolysis oils. The catalysts investigated were Y-zeolite impregnated with transition metal promoters with 1 wt.% and 5 wt.% metal loading of Ni, Fe, Mo, Ga, Ru and Co to determine the influence on aromatic fuel composition. Loading of metals on the Y zeolite catalyst led to a higher production of aromatic hydrocarbons (about 98%) in the product oil with greater concentration of single ring aromatic hydrocarbons (up to 99%) produced. The single ring aromatic compounds consisted mainly of toluene, ethylbenzene and xylenes, while the 2-ring hydrocarbons were mainly naphthalene and their alkylated derivatives. There was a reduction in the production of multiple ring aromatic compounds such as phenanthrene and pyrene. However, there was significant carbon deposition on the catalysts in the range 14-22 wt.% for the 1% metal-Y-zeolite catalysts and increased to 18-26 wt.% for the 5 wt.% metal-Y-zeolite catalysts which were mainly filamentous type carbon.
In addition, steam reforming of six agricultural biomass waste samples and the three main components of biomass were investigated in a two-stage fixed bed pyrolysis-catalytic reactor. The waste biomass samples consisted of; rice husk, coconut shell, sugarcane bagasse, palm kernel shell, cotton stalk and wheat straw while the biomass components included: cellulose, hemicellulose (xylan) and lignin. The TGA results showed distinct peaks for the individual biomass components, which were also evident in the biomass waste samples reflecting the existence of the main biomass components in the biomass wastes. The results for the two-stage pyrolysis-catalytic steam reforming showed that introduction of steam and catalyst into the pyrolysis-catalytic steam reforming process significantly increased gas yield and syngas production notably hydrogen. For instance, hydrogen composition increased from 6.62 to 25.35 mmol/g by introducing steam and catalyst into the pyrolysis-catalytic steam reforming of palm kernel shell. Lignin produced the most hydrogen compared to cellulose and hemicellulose at 25.25 mmol/g. The highest residual char production was observed with lignin which produced about 45 wt.% char, more than twice that of cellulose and hemicellulose.
Co-pyrolysis gasification of biomass components (lignin and cellulose) with plastic wastes (HDPE and PS) using novel metal catalysts in a fixed bed two-stage reactor was equally investigated. The introduction of steam and temperature increase, as well as the presence of metal catalysts, markedly increased the yield of hydrogen and syngas. Likewise, in the analyses of the different metals (Ni, Co, Mo and the bimetal NiCe) supported on MCM-41 catalysts presented the 10%Ni/MCM-41 (up to ~52 mmol/g and syngas ~ 79 mmol/g) to have a slightly higher yield of hydrogen compared with the other metals. The temperature increase from 750 to 8500C showed an increase in hydrogen yield from ~52 to 61 mmol/g and increased syngas yield from ~ 74 to 87 mmol/g. Likewise, as the steam flow rate was increased from 0 to 9.7 ml/h, the hydrogen yield increased from ~ 32 to 68 mmol/g and the syngas yield increased from ~ 45 to 96 mmol/g but decreases slightly from 7.7 to 9.7 ml/h steam rate. The blend of lignocellulosic biomass and plastic wastes in co-pyrolysis appears to be a novel option for tackling the problem of waste disposal in landfill, coupled with the production of petrochemical products, and in addition the generation of carbon neutral fuels.
