Hydrothermal liquefaction of spent coffee grounds for crude bio-oil production

Abstract

The extensive use of fossil fuels has intensified the energy crisis over the recent years and caused series of environmental issues. Biofuels are renewable and carbon-neutral, thus have gained increasing attention and are becoming an irreplaceable proportion in the section of renewable energy. Hydrothermal liquefaction (HTL) is a promising technology that can not only convert wet biomasses to biofuels (crude bio-oil) but also provide a potential pathway to deal with the bio-waste streams.

The first portion of this research is to investigate the HTL of spent coffee grounds (SCG) in subcritical water for crude bio-oil production. The batch-scale experiments were conducted in a 100 cm3 stainless-steel autoclave reactor in N2 atmosphere. The effects of operating parameters, e.g., reaction times (varied from 5 min to 25 min), reaction temperatures (varied from 200 C to 300C), water/feedstock mass ratios (5:1, 10:1, 15:1 and 20:1) and initial pressure of process gas (2.0 MPa and 0.5 MPa) on the yield and properties of the resulting crude bio-oil, were investigated. The highest yield of the crude bio-oil (47.3 % mass fraction) was obtained at conditions of 275 C, reaction time of 10 min and water/feedstock mass ratio of 20:1 with an initial pressure of 2.0 MPa. The elemental analysis of the produced crude bio-oil revealed that the oil product had a higher heating value (HHV) of 31.0 MJ·kg-1, much higher than that of the raw material (20.2 MJ·kg-1). GC-MS and FT-IR measurements showed that the main volatile compounds in the crude bio-oil were long chain aliphatic acids and esters.

In the second part of this research, spent K-Cups were liquefied into crude bio-oil in a water-ethanol co-solvent mixture and reaction conditions were optimized using response surface methodology (RSM) with a central composite design (CCD). The effects of three independent variables on the yield of crude bio-oil were examined, including the reaction temperature (varied from 255 °C to 350 °C), reaction time (varied from 0 min to 25 min) and solvent/feedstock mass ratio (varied from 2:1 to 12:1). The optimum reaction conditions identified were 276 °C, 3 min, and solvent/feedstock mass ratio of 11:1, giving a mass fraction yield of crude bio-oil of 60.0 %. The overall carbon recovery at the optimum conditions was 93 % in mass fraction. The effects of catalyst addition (sodium hydroxide NaOH and sulfuric acids H2SO4) on the yield of crude bio-oil were also investigated under the optimized reaction conditions.

The last portion of this research is to study co-liquefaction of SCG with other lignocellulose biomasses, including paper filter (PF), corn stalk (CS) and white pine bark (WPB), in subcritical water for crude bio-oil production. The effects of reaction temperature (varied from 225 °C to 325 °C) on the product distributions were investigated, aiming to maximize the crude bio-oil yield. The highest crude bio-oil yield was obtained at the reaction temperature of 250 °C when the feedstock combination mass ratio was fixed at 50 % SCG with 50 % the others. The addition of catalyst (5 % of sodium hydroxide NaOH) and various feedstock combination mass ratios were also tested at 250 °C. The results showed that synergistic effects occurred in the co-liquefaction process of these feedstocks with the addition of 5 % NaOH in terms of the crude bio-oil yield. The measurements of elemental analysis, GC-MS, GPC and viscometer revealed that positive synergistic effects appeared in their co-liquefaction process by improving the oil quality.