For thermal storage in solar tubular reformers, high-temperature composite materials with alkali carbonate and magnesia have been examined. The new concept of “double-walled reactor tube” was previously proposed to use in solar tubular reformers, which involves storing the molten-salt/ceramic composite material in the annular regions between an internal catalyst tube and the exterior solar absorber wall. The shape and structure of the reactor tube can be modified to match with design of a solar tubular reformer. In this paper, the interior structure of the reactor tube has been modified for use in cavity-type reformers using straight reactor tubes. The Na2CO3, K2CO3 and Li2CO3 composite materials with magnesia were prepared, and tested on heat-discharge mode performances for CO2 reforming of methane in the modified reactor tube of a laboratory scale.