Thermochemical conversion of algal biomass and agricultural plastic waste to fuels and value-added materials

Abstract

Energy recovery from biomass and plastic wastes has gotten great interest due to high energy efficiency and hindering the wastage of resources. Thermochemical conversion methods are considered promising technologies for handling biomass and plastic-based feedstocks. Algal biomass has become the center of attraction among biomass-based resources due to its perceived advantages. Gasification of algal biomass is seen as a promising clean, and environmentally friendly way for biohydrogen production. This study applied a statistical analysis approach, central composite design, to maximize biohydrogen production from C. vulgaris sp. grew and cultivated in lab conditions. The effects of the two important gasification parameters, temperature (600°C to 900°C) and catalyst loading (CaO) (0 wt.%, 20 wt.%, and 100 wt.%) on biohydrogen production, were investigated, and the optimum conditions were evaluated. According to the results, catalyst loading was the most influential parameter affecting biohydrogen yield. The CaO (lime) catalyst also improved the total gas conversion and decreased tar composition. The highest gas yield (~96 wt.) was achieved with 100 wt.% at 750°C. This study also focused on the relationship between microalgae biomass biochemical composition (carbohydrate, lipid, protein, and ash) fraction and syngas (H2, CO, CO2, and CH4) yields. First, linear fitting graphs showed that since carbohydrates progressed carbon conversion, it enhanced total gas yield. Ash contents raised the percentage of char formation. Additionally, principal component analysis (PCA) was driven to unroll whether they have significant correlations. According to the research, while increased carbohydrate contents reduced biohydrogen yield, proteins had no significant effects on gaseous product fraction. More importantly, lipid and hydrogen production correlated positively.Additionally, a tandem process that combines catalytic pyrolysis with catalytic chemical vapor deposition (CVD) was performed to investigate the conversion of waste plastics to carbon nanomaterials, oil, and hydrogen production. This study primarily explored the effects of different plastic types and the plastic feeding rate on the pyrolysis products and the quality and morphology of carbon nanomaterials. High-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), polystyrene (PS), and polyethylene terephthalate (PET) were used as carbon precursors for the growth of carbon nanomaterials on nickel foams. Results showed that PS produced the highest liquid oil with just 2% of predominantly amorphous structured carbon nanomaterials. PET had the lowest hydrogen yield and approximately 42% amorphous carbon due to the formation of more CO2 and CO. Most importantly since HDPE, LDPE, and PP produced more graphitic carbon nanomaterial with fewer structural defects compared to PET and PS, they are more favorable for the production of hydrogen and carbon nanomaterials. Additionally, HDPE loading significantly affected the quality of the produced carbon nanomaterial, in particular, higher or lower feedstock loading resulting in more defects. This proposed tandem process shows great potential for upcycling waste plastics for secondary use.