Comparison of the effects of Na2CO3, Ca3(PO4)2, and NiO catalysts on the thermochemical liquefaction of microalga Spirulina platensis

This study investigated the effect of three types of catalysts on the yield of biocrude oil from thermochemical liquefaction (TCL) of the microalga, Spirulina platensis. TCL experiments were performed in a 1.8 L batch reactor using an alkali metal catalyst (Na2CO3), an alkaline earth metal (Ca3(PO4)2), and a transition metal oxide (NiO) and compared with non-catalytic TCL results. Na2CO3 was found to increase biocrude oil yield resulting in 51.6% biocrude oil, which was ∼29.2% higher than under non-catalytic conditions and ∼71% and ∼50% higher than when using NiO and Ca3(PO4)2 catalysts, respectively. Presence of NiO and Ca3(PO4)2 increased yields of gaseous products. GC–MS analysis indicated critical differences in chemical composition of the biocrude oil obtained under different catalyst conditions. Biocrude oil from the catalyzed runs had greater abundance of monoaromatic compounds and lesser polyaromatic and aliphatic compounds than that of non-catalyzed reactions. TCL using Na2CO3 reported the lowest energy consumption ratio and recovered highest energy in the form of biocrude oil among all treatments. Algal biocrude oil had an energy density of 34–39 MJ kg−1 compared to 43 MJ kg−1 for petroleum crude, but had higher oxygen and nitrogen levels. In all cases, the solids conversion was more than 94%. Analysis of solids revealed that 40–60% of the initial catalysts were retained in the solid char.