Algae pyrolysis in alkaline molten salt: Products transformation

Abstract

Algae pyrolysis in alkaline molten salt is a promising technology for hydrogen production, but the pyrolysis products transformation has not clearly been described up-to-date. Moreover, the complex bio-oil especially heavy bio-oil fraction directly influences the products transformation pathways, while limiting the hydrogen production yield and application. Both the light and heavy bio-oil fractions are investigated in detail and the effect of alkaline molten salt on products transformation is obtained. Molten salt promotes the cyclization and aromatization of linear alkenes, thereby increasing aromatic compound content and hydrogen production. The decarbonation of Na2CO3 and the deoxygenation of NaOH lead to the methane release, in turn reducing hydrogen production. The heavy compounds tend to polymerize at higher temperatures into compounds with higher molecular weights and unsaturation, while generating hydrogen. As the temperature rises, char reacts with molten salt to form hydrogen and disappears at 650 °C, while aromatics in light compounds condense into heavy compounds, which then grow to form soot-like at 600 °C, accompanying hydrogen release. Based on the relationship between products transformation, hydrogen release and alkaline molten salt effect, strategies to improve the hydrogen yield and purity of AMSP process are proposed.