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

Abstract

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/Al 2 O 3 gave highest carbon conversion and highest methane yield of up to 11 mol/kg, while NaOH produced highest hydrogen yield of nearly 30 mol/kg under certain gasification conditions. These arise from increased land use for growing biomass for fuel which leads to competition with arable land for food crops. As such, biofuels production is shifting to non-food sources -lignocellulosic biomass -but these still require large arable areas as well as sufficient quantities of water and fertilisers to grow.The utilisation of macroalgae as a raw material for energy production compared to terrestrial biomass is appealing due to a number of factors. Macroalgae has a faster growing rate due to no water limitations (Gellenbeck and Chapman, 1983) and a lesser effect on temperature variation. It also has a higher photosynthetic efficiency of 6-8% (FAO, 1997) compared to 1.8-2.2% for terrestrial biomass and a higher productivity than that of terrestrial crops. Cultivated macroalgae (e.g. brown seaweed) demonstrate a productivity 6.5 times the maximum projected yield for sugarcane on an aerial basis (Gao and Mckinley, 1994). However, the feasibility of production of macroalgae for energy production, in a scale similar to terrestrial biomass has thrown up some uncertainties relating to where and how it will be produced and the economics of its production and subsequent conversion to fuels (Elliott, 2008).Despite macroalgae being extensively grown and used as food in Asiatic countries, as well as a source of chemicals, the fuel from algae concept does not face the same challenges compared to first and second generation biofuels in terms of food production and requirement of large areas of land, water and fertilisers.Page 3 of 28The carbohydrates in macroalgae have potential for producing biofuels and while conversion has focused on biogas production by anaerobic digestion (Matsui and Koike, 2010), recent work has focused on utilising the laminarin and mannitol for bioethanol production by fermentation (Borines et al., 2013;Yeon et al., 2011). Thermochemical conversion routes like direct combustion, pyrolysis, gasification and liquefaction have received less attention due to ...

Catalytic conversion of bio-oil in supercritical water: Influence of RuO2/γ-Al2O3 catalysts on gasification efficiencies and bio-methane production

“…The gas products obtained from the reactions were sampled via gas-tight plastic syringes and immediately analysed using gas chromatography [20,22]. Briefly, permanent gases including hydrogen, nitrogen and carbon monoxide were analysed using a column fitted on a GC with a thermal conductivity detector (GC-TCD).…”

“…They also showed that attempts to increase feed concentrations resulted in poor conversion difficulties due to tar formation, reactor pulling and clogging of the pump outlets during continuous operation. In addition, high biomass feedstock concentrations in batch reactors have resulted in low carbon gasification efficiencies and reduction in hydrogen yields [18][19][20].…”

Macroalgae in biofuel production

“…Especially, hydrothermal gasification has been seen as a good processing method for carbohydrate‐rich biomass, like macroalgae (Cherad et al . ).…”

“…The use of macroalgae in the biofuel industry is still under development and even recent results on this topic are widely based on laboratory or meso-scale operations (Bastianoni et al 2008;Bruhn et al 2011;Maceiras et al 2011;Jang et al 2012;Wargacki et al 2012;Cherad et al 2014). This article provides information on the chemical composition of macroalgae in relation to the feasibility of using the different compounds for biofuel production.…”

“…This should be taken into account in anaerobic digestion methods, too (Rupérez 2002;Nkemka & Murto 2012). On the other hand, alkali metals and their salts can favor the conversion processes of pyrolysis and gasification when functioning as catalysts (Ross et al 2008;Schumacher et al 2011;Cherad et al 2014). In biofuel production the selection of right pretreatment can diminish the ash content remarkably, which is shown in the decrease of the ash content from 25.8 w.t.…”