A mixing apparatus for producing a feedstock for a reformer, the mixing apparatus including at least one mixing vessel comprising a cylindrical vessel with a conical bottom; a steam inlet configured for introducing steam into the conical bottom; a carbonaceous material inlet configured for introducing a carbonaceous feed into the cylindrical vessel; and an outlet for a reformer feedstock comprising at least 0.3 pounds of steam per pound of carbonaceous material, with the at least one mixing vessel configured for operation at a pressure of greater than about 10 psig.