Patent ID: 6868294

Claim:
An NH 3 injection rate control method for use in an NO x removal apparatus in which a correlation between a molar ratio of an NH 3 injection rate into said NO x removal apparatus with respect to an NO x flow rate at an inlet of said NO x removal apparatus and an NO x concentration at an outlet of said NO x removal apparatus shows a downwardly protrusive V-shaped characteristic curve on a plane where said molar ratio and said NO x concentration are expressed in the form of X-Y coordinates, said NH 3 injection rate control method for suppressing said NO x concentration to below a predetermined desired NO x concentration through the use of said NH 3 injection rate reaching a minimum comprising: a step of, when an operating area of said NO x 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 NO x 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 NO x 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 a step of implementing feedback control for adjusting said NH 3 injection rate with respect to said NO x flow rate on the basis of a deviation between said desired NO x concentration and said virtual NO x concentration in a direction that said virtual NO x concentration is brought close to said desired NO x concentration.