Feed back control method in V-shaped characteristic system, and NH3 injection rate control method for NOx removal apparatus using the same

An NH3 injection rate control unit for use in an NOx removal apparatus in which a correlation between a molar ratio of an NH3 injection rate into said NOx removal apparatus with respect to an NOx flow rate at an inlet of said NOx removal apparatus and an NOx concentration at an outlet of said NOx removal apparatus shows a downwardly protrusive V-shaped characteristic curve on a plane where said molar ratio and said NOx concentration are expressed in the form of X-Y coordinates, said control unit comprising:

• NOx concentration detecting means for detecting said NOx concentration at said outlet of said NOx removal apparatus;
• desired NOx concentration setting means for setting a desired NOx concentration at said outlet of said NOx removal apparatus;
• NOx flow rate detecting means for detecting said NOx flow rate at said inlet of said NOx removal apparatus;
• NH3 injection rate adjusting means for adjusting said NH3 injection rate into said NOx removal apparatus;
• virtual NOx concentration setting means for, when an operating area of said NOx removal apparatus is divided into a first area in which said molar ratio is smaller than a molar ratio at a minimum point where said NOx concentration assumes a minimum value and a second area in which said molar ratio is equal to or larger than said minimum point molar ratio, setting a virtual NOx concentration with respect to said molar ratio according to a virtual characteristic line which varies monotonically to stride across a desired operating point without rising with an increase of said molar ratio from said first area to said second area;
• and
• feedback control means for adjusting, through the use of said NH3 injection rate adjusting means, said NH3 injection rate on the basis of said NOx flow rate, detected by said NOx flow rate detecting means, in accordance with a deviation between said desired NOx concentration and said virtual NOx concentration, set by the virtual NOx concentration setting means, in a direction that said virtual NOx concentration is brought close to said desired NOx concentration.

A control unit for use in a V-shaped characteristic system in which a correlation between input and output values shows a V-shaped characteristic curve having one minimum point on a plane where said input and output values are expressed in the form of X-Y coordinates, said control unit comprising:

• detecting means for detecting said output value from said system;
• desired out put value setting means for setting a desired output value;
• inputting means for inputting said input value to said system;
• virtual output value setting means for, when an operating area of said system is divided into a first area in which a deviation between said input value and an input value at said minimum point has a specific sign and a second area having a sign different from said specific sign, setting a virtual output value with respect to said input value according to a virtual characteristic line which varies monotonically to stride across a desired operating point so that an inclination of said output value to said input value in said first area and an inclination of said output value to said input value in said second area do not have different signs;
• and
• feedback control means for changing said input value on the basis of a deviation between said desired output value and said virtual output value in a direction that said virtual output value is brought close to said desired output value.