There has been growing concern about the way cultivating biomass for the production of agro-biofuels competes with food production. To avoid this competition biomass production for biofuels will, in the long term, have to be completely decoupled from food production. This is where microalgae have enormous potential. Here we propose a novel process based on microalgae cultivation using dilute fossil CO 2 emissions and the conversion of the algal biomass through a catalytic hydrothermal process. The resulting products are methane as a clean fuel and concentrated CO 2 for sequestration. The proposed gasification process mineralizes nutrient-bearing organics completely. Here we show that complete gasification of microalgae (Spirulina platensis) to a methane-rich gas is now possible in supercritical water using ruthenium catalysts. 60-70% of the heating value contained in the algal biomass would be recovered as methane. Such an efficient algae-to-methane process opens up an elegant way to tackle both climate change and dependence on fossil natural gas without competing with food production.

We describe a potential novel process (SunCHem) for the production of bio-methane via hydrothermal gasification of microalgae, envisioned as a closed-loop system, where the nutrients, water, and CO 2 produced are recycled. The influence on the growth of microalgae of nickel, a trace contaminant that might accumulate upon effluent recycling, was investigated. For all microalgae tested, the growth was adversely affected by the nickel present (1, 5, and 10 ppm). At 25 ppm Ni, complete inhibition of cell division occurred. Successful hydrothermal gasification of the microalgae Phaeodactylum tricornutum to a methane-rich gas with high carbon gasification efficiency (68-74%) and C1-C3 hydrocarbon yields of 0.2 g C1-C3 /g DM (DM, dry matter) was demonstrated. The biomass-released sulfur was shown to adversely affect Ru/C catalyst performance. Liquefaction of P. tricornutum at short residence times around 360°C was possible without coke formation.

Synthetic natural gas (SNG)Supercritical water gasification Sensitivity analysis Bioenergy Process economics a b s t r a c t A techno-economic sensitivity analysis of the production of synthetic natural gas (SNG) via catalytic supercritical water gasification (SCWG) of microalgae produced in raceway ponds (RP), tubular-, or flat-panel-airlift photobioreactors (FPA-PBR) has been perfomed. The aim of combining microalgae production with SCWG is to close material flows with respect to water and nutrients, the so called SunCHem process. The sensitivity analysis is based on an annual production of 86,500 t of microalgae biomass yielding 1.14 PJ of methane per year. The sensitivity analysis showed that with an annual algae productivity of 38.5 t per hectare of RP an energy return on energy invested (EROEI) of 1.84 can be achieved for the self-sufficient base case scenario. An SNG production cost of 194 V GJ À1 was obtained forRP. An EROEI of 0.08 was calculated for tubular PBR with a productivity of 75.1 t ha À1 a À1 in the base case scenario and thus was found to be inappropriate for SNG production. EROEI for FPA-PBR with an assumed microalgae productivity of 79 t ha À1 a À1 was found to be 1.01in the base case scenario and an SNG production cost of 266 V GJ À1. With significantly more optimistic assumptions concerning microalgae productivity, energy input and capital requirement with respect to microalgae cultivation, an EROEI of 3.6e5.8 and SNG production costs of 53e90 V GJ À1 were found for RP, whereas for FPA-PBR an EROEI of 2e3.7 and SNG production costs of 30e103 V GJ À1 were obtained.ª 2013 Elsevier Ltd. All rights reserved. IntroductionThe potential of microalgae as a renewable energy source, as a provider of renewable bulk and high value chemicals for the chemical and pharmaceutical industry, as a source of proteins for animal feedstock and fertilizer has made them the subject of considerable research effort in the past [1,2]. However, the economic viability and energy efficiency of biofuels made from microalgae are presently intensively discussed. The main obstacles to large scale introduction of biofuels from microalgae are the high investment costs and energy input required for microalgae cultivation and harvesting [3,4]. b i o m a s s a n d b i o e n e r g y x x x ( 2 0 1 3 ) 1 e9Please cite this article in press as: Brandenberger M, et al., Producing synthetic natural gas from microalgae via supercritical water gasification: A techno-economic sensitivity analysis, Biomass and Bioenergy (2013), http://dx

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