(659e) In situ Catalytic Fast Pyrolysis of Nannochloropsis sp. using Co-Mo Catalysts for Value Added Chemicals

Growing global energy need demands an urge for exploitation of renewable and environmentally friendly energy resources. In order to reduce our dependency on fossil fuels, carbon neutral sources such as biomass are seen as potential source of energy. Besides lignocellulosic biomass, fast growing plants such as microalgae are looked upon as a prospective source of energy. Microalgae do not require arable land for its cultivation, and so they do not compete with food crops. Thermochemical conversion of microalgal biomass is a promising strategy to convert them into valuable chemicals and fuel molecules. Fast pyrolysis of microalgae is recognized as a promising technique to produce bio-oil. However, high oxygen and nitrogen content of bio-oil limit its direct application in engines or transportation fuels. The various oxygenates like aldehyde, ketone, carboxylic acid and nitrogen containing nitriles, pyrroles, indoles result in low heating value, poor storage stability and low thermal stability. The problem of complex composition of bio-oil can be addressed by deploying catalysts for enhanced selectivity with an aim of promoting deoxygenation, denitrogenation and dehydration resulting in a stable product.

In this study Nannochloropsis oculata (N. oculata), a high lipid and protein containing microalgae, was fast pyrolyzed in the absence and presence of catalysts. The effect of temperature on catalytic and non-catalytic fast pyrolysis and the effect of catalyst to algae ratio on product distribution was studied. The algae was characterized by proximate, elemental, heating value and thermal stability analyses using various analytical techniques such as thermogravimetric analyzer (TGA), CHNS analyzer and bomb calorimeter. Fast pyrolysis experiments were performed in an analytical micropyrolyzer and the products were analyzed in an online gas chromatograph-mass spectrometer (Py-GC/MS) at different temperatures from 400-600°C. Bifunctional Co-Mo supported on γ-Al₂O₃ was prepared and characterized before using it as a catalyst for Py-GC/MS experiments. Thermal stability, pore size distribution and catalyst composition were studied using various characterization techniques like TGA, porosimetery and ICP-OES (Infrared coupled plasma – optical emission spectroscopy), respectively. This is the first study to investigate the effect of Co-Mo catalysts on N. oculata under the fast pyrolysis conditions.

Aromatic and aliphatic hydrocarbons were the major products (c.a. 50%) produced during the non-catalytic fast pyrolysis at 500°C. The major products were benzene, styrene and alkenes (C₁₄ – C₁₉), nitriles (C₄ – C₆). The yields of oxygenated and nitrogen containing compounds were low in the pyrolysates. Catalyst: algae ratios of 1:9, 1:3, 1:1 and 2:1 were tested for catalytic fast pyrolysis. The use of catalyst resulted in the formation of 1-isocyanobutane and dimethylketene (combined yield of 25-40%) as the major compounds. Long chain nitriles, long chain alkanes and alkenes, polyaromatic and monoaromatic hydrocarbons were the major product groups. Pyrolysates obtained from catalytic fast pyrolysis had lower amount of oxygen containing organic compounds (< 3%) as compared with non-catalytic fast pyrolysis (c.a. 21%). This was evident from the increase in the approximate HHV (33-40 MJ kg^-1) of organics in the pyrolysates from 18 MJ kg^-1 (raw dried N. oculata). Highest deoxygenation was observed with CoMo catalyst to algae ratio 1:3. Mo and support (γ-Al₂O₃) as catalysts resulted in long chain nitriles. The plausible reaction mechanism for the conversion of microalgae to important chemicals was proposed in this study. More interesting results on (i) effect of metal loading, (ii) effect of temperature and (iii) effect of catalyst: algae ratio will be discussed during the presentation.