Apparatus for treating flue gas

An apparatus for removing nitrogen oxides from a low temperature flue gas flowing through said apparatus, which flue gas contains NO.sub.x and residual O.sub.2 which comprises:

• a source of low temperature flue gas;
• NO.sub.x removal means for removing nitrogen oxides from flue gas;
• a main flue gas passage operably connecting said source of low temperature flue gas with said NO.sub.x removal means, said main flue gas passage allowing at least a first portion of flue gas from said source of said low temperature flue gas to pass to said NO.sub.x removal means;
• heating means for forming a heated flue gas;
• said main flue gas passage being provided with, in series, withdrawal means connected to said main flue gas passage for withdrawing a second portion of flue gas in said main flue gas passage, gas mixing means for mixing the heated flue gas with the first portion of flue gas in said main flue gas passage to form a mixed flue gas, and injection means for injecting an NO.sub.x removal agent into said mixed flue gas in said main flue gas passage, said withdrawal means being provided between said source of low temperature flue gas and said gas mixing means, and said injection means being provided between said gas mixing means and said NO.sub.x removal means;
• an auxiliary flue gas passage for removing said second portion of flue gas from said main flue gas passage, heating the second portion of flue gas removed in the auxiliary flue gas passage and then returning the flue gas removed in the auxiliary flue gas passage to said main flue gas passage, said auxiliary flue gas passage removing the second portion of flue gas from said main flue gas passage at said withdrawal means, passing the removed second portion of flue gas to said heating means and then passing the second portion of flue gas from the heating means to said gas mixing means so as to recombine the second portion of flue gas with the flue gas in the main flue gas passage;
• said heating means being operably connected between said withdrawal means and said gas mixing means, said heating means comprising an auxiliary burner means for burning heating fuel and air and a front burner provided with inlet mixing means for mixing heating fuel with the second portion of flue gas to thereby form a fuel gas mixture, and catalytic combustion means for combusting the fuel gas mixture;
• auxiliary heating fuel flow rate control means for controlling a flow rate of heating fuel to said auxiliary burner means in accordance with a temperature of combustion gas between said auxiliary burner means and said catalytic combustion means, and an air flow rate control means for controlling a flow rate of air to said auxiliary burner means in accordance with a detected flow rate of heating fuel to said auxiliary burner means;
• and
• control circuit means for setting a flue gas temperature of flue gas passing to said NO.sub.x removal means for maintaining a desired percent NO.sub.x removal in accordance with a load and for controlling a flow rate to the heating fuel to said front burner to thereby keep the desired flue gas temperature;
• wherein said control circuit means comprises a flow rate control means for controlling a flow rate of the second portion of flue gas in accordance with a temperature of the heated flue gas between said heating means and said gas mixing means.