To provide a denitrification apparatus with an adjusting function for reducing agent injection distribution, the function enabling an air flow for diluting a reducing agent at an injection nozzle part of the reducing agent like NH3to be secured within a safety region.

When automatically controlling NH3distribution, depending on whether a dilution air flow rate is secured more than a specified amount for controlling a distribution adjusting valve, (1) when dilution air flow is more than the specified amount, a distribution adjusting valve corresponding to the lowest NOx concentration section in a NOx concentration measuring section in a denitrification apparatus outlet side duct 7 is selected and put into a control mode, (2) when the dilution air flow is less than the specified amount, a distribution adjusting valve corresponding to the highest NOx concentration section in the NOx concentration measuring section in the duct 7 is selected and put into the control mode, (3) the distribution adjusting valves are controlled for opening and closing, control is performed until the deviation between the outlet NOx concentration of the control section and an average value of the whole sections of the outlet NOx concentration becomes less than an allowable value, and (4) when the deviation of the (3) becomes less than the allowable value, control is returned to controls of (1) and (2) for continuous controlling.

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