A Detailed Quali-Quantitative Analysis of Products from Pyrolysis-Zeolite Cracking of the Microalgae Spirulina

The recent EU directive 2015/1513 encourages research efforts for the developments of advanced biofuels, such as those made from algae, that could provide greenhouse gas emission savings with low risks of indirect land-use change and food/feed competition. Harvesting algal biomass is a major obstacle for economical sustainability, but there are species profitably cultivated at industrial scale for non-energetic purposes, such as the microalgae A.platensis (Spirulina). Spirulina is not an oleaginous or carbohydrate-rich microalgae, therefore not a suitablespecies for biodiesel or bioethanol production. An organic liquid (here named as bio-oil) with potential fuel properties (high C and low O/N content) was recently obtained by catalytic pyrolysisof Spirulinawith zeolite HZSM-5 [1]. However,further investigations are needed to fully evaluated the prospective of zeolite cracking in the development of liquid biofuels from protein-rich microalgae.In this study, A.platensiswas cultivated in a 70 L indoor vertical photobioreactor and harvested when the dried biomass reached a concentration of 1 g L-1.In triplicate runs, about 4 g of lyophilised algal biomass were pyrolysed at 500 °C under nitrogen flow that swept vapours through 40 g of pelletized HZSM-5 (SiO2/Al2O3 38) leaving behind a char. Downstreamthe zeolite bed, two liquids, a bio-oil floating over an aqueous phase,werecondensed inside a cold trap (0 °C);thereafter, uncondensed vapours (light fraction) were adsorbed onto a poly(styrene-divinylbenzene) resin (XAD-2), and permanent gas was collected in a tedlar bag. The yields, carbon C and nitrogen N contents of all the fractions excluding gas were determined directly by weight difference and elemental analysis; molecular analysis was performed on the liquid(SPME/GC-MS with internal calibration, 1H-NMR), gas (GCTCD) and char (Py-GC-MS) fractions. C was distributed as inorganic C into the aqueousand gas (CO2/CO)phases, all representing 20% of the original algal C. Most of the feedstockC ended up in char (30%); about 15% of initial Spirulina mass deposited onto the catalyst as coke. Bio-oil with over 80% C content was obtained with a molecular composition dominated by mono and polyaromatic hydrocarbons; a similar pattern was observed in the light fraction and at trace levels in the aqueous phase.Overall, bio-oil and light fraction were obtained with a 10% mass yield accounting for 20%C and energy of algal biomass. Large part of original N was dissolvedinto the aqueous phase (38% of initial N) and incorporated intochar/coke (40%). A minor fraction of N ended up in bio-oil (about 6%); part of it was attributed by GC-MS to N-containing organic compounds derived from the thermal degradation of proteins or their reactions with lipids and carbohydrates (alkyl indoles, pyrroles, nitriles, pyrazines, carbazoles).Decoupling pyrolysis and zeolite cracking enabled the collection of char that could be employed as a fuelto increase the utilization of C and energy from the original biomass. The detailed mass, C and N balance and molecular analysis confirmed previous studies on a similar zeolite/algae system [1] and provided additional information for the quantitative evaluation of the potential of the process towards third generation biofuels. Acknowledgments: Italian Ministry of Economic Development of the program Agreement MISE-CNR "Ricerca di SistemaElettrico". [1] C.Lorenzetti, R.Conti, D.Fabbri, J.Yanik, Fuel 166, pp. 446-452, 2016