Ammonia control system for NO.sub.x emission control for gas turbine exhaust

An ammonia control system for controlling a flow of ammonia into an exhaust of a fuel combustion apparatus to enable reaction of NO.sub.x in said exhaust with said ammonia in a catalyst for reduction of NO.sub.x effluent into the atmosphere, including a controller effective to produce a predicted NO.sub.x signal related to parameters in said combustion apparatus and at least one NO.sub.x setpoint signal representing a desired NO.sub.x condition downstream of said catalyst, a NO.sub.x analyzer producing a measured NO.sub.x signal related to a relative volume concentration of NO.sub.x downstream of said catalyst, comprising:

• means for converting said measured NO.sub.x signal in terms of relative volume concentration into a NO.sub.x flow rate in terms of moles of NO.sub.x per unit time;
• said at least one NO.sub.x setpoint signal being in terms of moles of NO.sub.x per unit time;
• means for differencing the converted measured NO.sub.x flow rate and the NO.sub.x setpoint signal to produce a NO.sub.x error signal;
• means for producing a mole ratio signal of ammonia to NO.sub.x in response to said NO.sub.x error signal;
• means for multiplying said mole ratio signal by said predicted NO.sub.x signal to produce an ammonia command signal;
• and
• an ammonia supply responsive to said ammonia command signal for injecting ammonia into said exhaust.