Bio-oil production from water hyacinth by hydrothermal liquefaction process using iron oxide-based nanocomposites as catalysts

Abstract

The iron oxide nanocatalyst and iron oxide/nickel oxide nanocomposite were synthesized by the co-precipitation method and used in the hydrothermal liquefaction (HTL) of water hyacinth. The composition and structural morphology of the synthesized catalysts were determined using atomic absorption spectroscopy (AAS), energy dispersive X-ray fluorescence (EDXRF) spectrometry, and scanning electron microscopy (SEM). The particle size distribution of the catalyst nanoparticles was determined by the Image J software and plotted using the Originlab software. Three reaction parameters were optimized through the central composite design of response surface methodology (RSM). These reaction parameters were: temperature, residence time, and catalyst dosage. A maximum bio-oil yield of 58 wt. % and 59.4 wt. % was obtained using iron oxide nanocatalyst and iron oxide/nickel oxide nanocomposite, respectively compared to 50.7 wt. % obtained in absence of the catalyst. The maximum bio-oil yield was obtained at a temperature of 320 °C, 1.5 g of catalyst dosage, and 60 min of residence time. The composition of bio-oil was analyzed using gas chromatography-mass spectroscopy (GC-MS) and elemental analysis. The GC-MS showed an increase of hydrocarbons from 58.3 % for uncatalyzed hydrothermal liquefaction to 83.09 % and 88.66 % using iron oxide nanocatalyst and iron oxide/nickel oxide nanocomposites respectively. Elemental analysis revealed an increase in the hydrogen and carbon content and a reduction in the nitrogen, oxygen and sulphur content of the bio-oil when HTL was done in presence of catalyst nanoparticles compared to HTL in absence of catalyst nanoparticles. The high heating value increased from 33.5 MJ/Kg for uncatalyzed hydrothermal liquefaction to 35.5 MJ/Kg and 38.6 MJ/Kg using iron oxide nanocatalyst and iron oxide/nickel oxide nanocomposite, respectively. The catalyst nanoparticles were recovered from the solid residue by sonication and magnetic separation and recycled. The recycled catalyst nanoparticles were still efficient as hydrothermal liquefaction (HTL) catalysts and were recycled four times. The application of iron oxide-based nanocomposites in the HTL of water hyacinth increases the yield of bio-oil and improves its quality by reducing hetero atoms thus increasing its energy performance as fuel. Iron oxide-based nanocomposites used in this study are cheaper, widely available and can be easily recovered magnetically and recycled. This will potentially lead to an economical, environmentally friendly and sustainable way of producing bio-oil from biomass.