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Catalytic tar removal from producer gas is critical for the economic feasibility of Municipal Solid Waste (MSW) gasification in the waste-to-energy(WtE) approach. Nickel- and noble-metal catalysts have the highest tar cracking activities, but they increase costs, use scarce materials, and generate dangerous byproducts. To overcome these drawbacks, naturally occurring materials should be used for tar cracking. In this paper, two nanomaterials, synthesized from oyster and mussel waste shells respectively, are used to clean syngas from MSW in a secondary tar cracking unit. We observed that they reform class 1 tar (heavy tars that condense at high temperatures at very low concentrations) into class 3 tar (light hydrocarbons that are not important in condensation) and benzene. Although both catalysts' composition and textural properties were identical, crystallite size and especially specific surface area variation was enough to generate a change in product selectivity. A larger crystallite size and SSA shows a soot yield reduction of 95% with respect to the non-catalytic case, simultaneously increasing the H 2 /CO at 1000 °C. • Two CaO rich catalysts doped with Sr were synthesized from oyster and mussels waste shells. • Both catalysts improve syngas H2:CO ratio, reduce class 1 tar and soot yield at high temperatures. • Soot reduction is accompanied by a class 5 tar reduction and by an increase in class 3 tar. • The crystallite size and specific surface area of the two catalysts directly affect the tar cracking product selectivity. • The syngas quality obtained using the oyster derived catalyst makes it ideal to use in tandem with Ni and noble-metal catalysts. [ABSTRACT FROM AUTHOR]