Exhaust gas treatment apparatus, engine system, and exhaust gas treatment method

An exhaust gas treatment method for oxidizing unburned methane in exhaust gas of an engine which uses gas containing methane as a fuel, comprising:

• supplying a catalyst part with exhaust gas having a temperature lower than a predetermined normal operating temperature;
• making a tuning state of an engine to be a first tuning state in which a concentration of carbon monoxide contained in said exhaust gas is higher than that during a steady operation when the temperature of said catalyst part is lower than a first switching temperature which is lower than said predetermined normal operating temperature, and oxidizing said carbon monoxide contained in said exhaust gas in said catalyst part and using oxidation reaction heat of carbon monoxide, to thereby raise a temperature of said catalyst part up to said predetermined normal operating temperature or higher;
• and
• oxidizing methane in said exhaust gas by said catalyst part having a temperature not lower than said predetermined normal operating temperature after said operation of making the tuning state to be the first tuning state, and
• when the temperature of said catalyst part is raised up to said first switching temperature, making said tuning state of said engine to be a second tuning state in which the concentration of carbon monoxide contained in said exhaust gas is higher than that during said steady operation and is lower than that in said first tuning state.