In-situ biodiesel production using liquefaction and supercritical extraction

Abstract

Biodiesel is an alternative source of energy with the potential to replace diesel fossil fuel. Full scale production however, has been hampered by high production costs since the extraction of oil causes excessive losses of energy, time and money when producing biodiesel. Currently, there is a lack of innovative technologies that combine known techniques to collectively overcome individual shortfalls. The aim of this project is to investigate the feasibility of combining liquefaction, in -situ trans-esterification and supercritical carbon dioxide extraction, to develop a biodiesel production process that requires a minimum amount of energy, is fast and efficient in the extraction of oil from biomass, while simultaneously converting extracted oil into biodiesel. Sunflower seeds were used as feedstock for liquefaction in a methanol/-catalyst solution to produce biodiesel under high temperatures and pressures. Experiments were conducted with three different catalysts, potassium hydroxide (KOH), sulphuric acid {H₂SO₄) and calcium carbonate {CaCO₃), at five different temperatures, varying with 10°C intervals from 320°C to 360°C; and with three different biomass loadings of 15wt%, 20wt% and 25wt% of the reaction mixture in the presence of supercritical CO₂. The final product was analysed with gas chromatography, gas chromatography-mass spectrometry, elemental analysis and FT-IR to determine the product quality. The results from the various temperatures peaked at 340 C; this temperature delivered yields of 120 g.kg⁻¹ bio-oil, 84.7 g.kg⁻¹ FAME and 147.9 g.kg⁻¹ biochar. The bio-oil had a HHV of 41.12 MJ.kg⁻¹ and the biochar 28.12 MJ.kg⁻¹. It was evident that temperature had little influence on the elemental composition, but it was a major determinant of bio-oil, FAME, and biochar production. The results obtained for the different types of catalysts varied. Potassium hydroxide catalysed reactions peaked at 320°C, delivering yields of 110.55 g.kg⁻¹ bio-oil, 71.72 g.kg⁻¹ FAME and 108.95 g.kg⁻¹ biochar. The bio-oil had a HHV of 40.12 MJ.kg⁻¹ and the biochar 25.20 MJ.kg⁻¹ The catalyst favoured gas production. Sulphuric acid catalysed reactions peaked at 320°C; this temperature delivered yields of 61.15 g.kg-1 bio-oil, 36.61 g.kg-1 FAME and 367.25 g.kg-1 biochar. The bio-oil had a HHV of 42.28 MJ.kg⁻¹ and the biochar 28.73 MJ.kg⁻¹. The catalyst favoured biochar production. Calcium carbonate catalysed reactions peaked at 340°C, delivering yields of 120 g.kg⁻¹ bio-oil, 84.7 g.kg⁻¹ FAME and 147.9 g.kg⁻¹ biochar. The bio-oil had a HHV of 41.12 MJ.kg⁻¹ and the biochar 28.12 MJ.kg⁻¹. The catalyst was superior to the other two and favoured bio-oil production. The three catalysts had varying influences on bio-oil, FAME, biochar production and subsequently, on calorific values. The results for the biomass loading peaked at 20wt% loading using 20 g.kg⁻¹ CaCO₃ catalysts at 330°C; this load delivered yields of 120 g.kg⁻¹ bio-oil, 84.7 g.kg⁻¹ FAME and 147.9 g.kg⁻¹ biochar. The bio-oil had a HHV of 41.12 MJ.kg⁻¹ and the biochar 28.12 MJ.kg⁻¹. Biomass loading illustrated adverse effects on product distribution between biochar, bio-oil and biogas production. There was an abundance of aromatic compounds, stemming from unsaturated, branched, uneven hydrocarbon chains present in the bio-oil. Analysis of the C/H molar ratios against the C/O content showed the change in oil quality relative to specific fossil fuels. The HHV increased with higher carbon-hydrogen ratios and lower oxygen ratios. The results motioned an apparent threshold limit to de-oxygenation. The bio-oil can therefore be classified as paraffinlike, but a high level of methyl esters means that the bio-oil should be classified as low quality diesel oil. The products of in -situ trans-esterification by liquefaction and supercritical carbon dioxide can compete with fossil fuels regarding the HHV. However, a low quality biodiesel was produced; this requires secondary processing to reduce the cyclic compounds and oxygen content. These findings contribute towards the current growing bed of knowledge to overcome the blatant lack of innovative usages of the existing technology.