To provide a catalyst oxidation system for purifying the discharge air for drying printed paper which extends the duration wherein the temperature of the oxidization catalyst stays in the self-combustion temperature and the safe temperature.

A supply flow volume of the discharge air is selected for every printing paper (a) based on the data of the printing ink consumption in a printer. The supply flow volume of the discharge air is set at the selected value at the start of printing, and the purifying speed is controlled by increasing or decreasing the supply flow volume of the discharge air based on the speed of the printing ink consumption. Furthermore while the temperature of the oxidization catalyst 15 stays in the range of the self-combustion temperature wherein the oxidization reaction arises without preheating by a preheating apparatus 14 and the safe temperature wherein the oxidization catalyst is not overheated, the supply volume of the discharge air increases as the temperature of the oxidization catalyst goes up, and the supply volume of the discharge air decreases as the temperature of the oxidization catalyst goes down. Thus the temperature of the oxidization catalyst is controlled by increasing or decreasing the supply volume of the discharge air in accordance with the temperature of the oxidization catalyst.

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