Supercritical Water Gasification Of Algae

Diversification of our energy supplies – especially in the transport and electricity generation sectors – is required to meet decarbonisation targets. Algae have been identified as suitable alternative feedstocks for third generation biofuels due to their fast growth rates and non-competitiveness with land for food crops. Hydrothermal processing of algae is an appropriate conversion route as it allows the processing of wet feedstock thus removing the energy penalty of drying. In this study, supercritical water gasification was used for (i) the hydrothermal processing of macroalgae for the production of gaseous fuel – mainly hydrogen and methane – and (ii) the upgrading of the process water from hydrothermal liquefaction of microalgae for hydrogen production for biocrude hydrotreating. The supercritical water gasification (SCWG) of the four macroalgae species investigated (Saccharina latissima, Laminaria digitata, Laminaria hyperborea, and Alaria esculenta) produced a gas that mainly consisted of hydrogen, methane and carbon dioxide. Non-catalytic SCWG resulted in hydrogen yields of 3.3-4.2 mol/kg macroalgae and methane yields of 1.6-3.3 mol/kg macroalgae. Catalytic SCWG (using ruthenium) resulted in hydrogen yields of 7.8-10.2 mol/kg macroalgae and methane yields of 4.7-6.4 mol/kg macroalgae. The yield of hydrogen was approximately three times higher when using sodium hydroxide as catalyst (16.3 mol H2 / kg macroalgae) compared to non-catalysed SCWG of L. hyperborea (5.18 mol H2 / kg macroalgae). The energy recovery (an expression of how much chemical energy of the feedstock is recovered in the desired product following hydrothermal processing) was 83% when sodium hydroxide was used as a catalyst, compared to 52% for the non-catalytic SCWG of L. hyperborea. The yield of methane was approximately 2.5 times higher (9.0 mol CH4 kg 1macroalgae) when using ruthenium catalyst compared to the non-catalysed experiment (3.36 mol CH4 / kg macroalgae) and the energy recovery increased by 22% to 74%. The selectivity of methane or hydrogen production during the SCWG of macroalgae can be controlled using ruthenium or sodium hydroxide respectively. Longer hold times and increased reaction temperature favoured methane production when using ruthenium. An increase in catalyst loading had no significant effect on the methane yield. Higher hydrogen yields were obtained through using higher concentrations of sodium hydroxide, lower algal feed concentration and shorter hold times (30 min). Increasing reaction times (>30 min) with a base catalyst (sodium hydroxide) decreased the hydrogen yield. Overall energy recovery was highest at the lowest feed concentrations; 90.5% using ruthenium and 111% using sodium hydroxide. The process waters from the hydrothermal liquefaction (HTL) of microalgae (Chlorella, Pseudochoricystis, and Spirulina) were gasified under supercritical water conditions to maximise hydrogen production. Hydrogen yields ranged from 0.18-0.29 g H2 / g biocrude from SCWG of the process water of HTL along with near complete gasification of the organics (~98%). Compared to the hydrogen requirements for hydrotreating algal biocrude (~0.05 g H2 / g biocrude), excess hydrogen can be produced from upgrading the process water through SCWG. The results indicate that process waters following SCWG are still rich in nutrients that can be recycled for algal cultivation

Hydrogen production from the catalytic supercritical water gasification of process water generated from hydrothermal liquefaction of microalgae

The integration of hydrothermal liquefaction (HTL) and hydrothermal gasification (HTG) is an option for enhanced energy recovery and potential biocrude upgrading. The yields and product distribution obtained from the HTL of Chlorella vulgaris have been investigated. High conversion of algae to biocrude as well as near complete gasification of the remaining organic components in the aqueous phase was achieved. The aqueous phase from HTL was upgraded through catalytic HTG under supercritical water conditions to maximise hydrogen production for biocrude hydrotreating. High yields of hydrogen were produced (∼30 mol H2/kg algae) with near complete gasification of the organics (∼98%). The amount of hydrogen produced was compared to the amounts needed for complete hydrotreating of the biocrude. A maximum of 0.29 g H2 was produced through HTG per gram of biocrude produced by HTL. The nutrient content of the aqueous phase was analysed to determine suitability of nutrient recovery for algal growth. The results indicate the successful integration of HTL and HTG to produce excess hydrogen and maintain nutrient recovery for algal growth

A parametric study on supercritical water gasification of Laminaria hyperborea: a carbohydrate-rich macroalga.

The potential of supercritical water gasification (SCWG) of macroalgae for hydrogen and methane production has been investigated in view of the growing interest in a future macroalgae biorefinery concept. The compositions of syngas from the catalytic SCWG of Laminaria hyperborea under varying parameters including catalyst loading, feed concentration, hold time and temperature have been investigated. Their effects on gas yields, gasification efficiency and energy recovery are presented. Results show that the carbon gasification efficiencies increased with reaction temperature, reaction hold time and catalyst loading but decreased with increasing feed concentrations. In addition, the selectivity towards hydrogen and/or methane production from the SCWG tests could be controlled by the combination of catalysts and varying reaction conditions. For instance, Ru/Al2O3 gave highest carbon conversion and highest methane yield of up to 11 mol/kg, whilst NaOH produced highest hydrogen yield of nearly 30 mol/kg under certain gasification conditions