Method and system for the removal of noxious compounds from flue-gas using fabric filter bags with an SCR catalyst

Abstract

Method and system for the removal of nitrogen oxides, from flue gas at low temperatures.

Claims

1. A method for the removal of nitrogen oxides from flue gas from combustion facilities, comprising the steps of passing the flue gas through one or more fabric filter bags catalysed with a catalyst for selective reduction of nitrogen oxides in presence of ammonia added to the flue gas either as such or in form of a precursor thereof; injecting an effluent gas containing nitrogen dioxide into the flue gas upstream the one or more fabric filter bags; providing the effluent gas containing nitrogen dioxide by steps of catalytically oxidizing ammonia or a precursor thereof with an oxygen containing atmosphere to an effluent gas containing nitrogen monoxide and oxygen in presence of an oxidation catalyst; cooling the effluent gas to ambient temperature and oxidizing the nitrogen monoxide in the cooled effluent gas to the nitrogen dioxide containing effluent gas.

2. The method of claim 1, wherein the oxygen containing atmosphere comprises flue gas.

3. The method of claim 1, wherein the oxygen atmosphere is ambient air.

4. The method according to claim 1, wherein the nitrogen dioxide containing effluent gas is injected into the flue gas in an amount resulting in 45 to 55% by volume of the nitrogen oxides is nitrogen dioxide at inlet to the catalyst for selective reduction of nitrogen oxides.

5. The method according to claim 1, wherein the oxidation of the nitrogen monoxide in the cooled effluent gas to the nitrogen dioxide containing effluent gas is performed in presence of an oxidation catalyst.

6. System for use in the method according to claim 1, comprising within a flue gas duct a filter bag house with one or more fabric filter bags catalysed with a catalyst for selective reduction of nitrogen oxides; upstream the one or more fabric filter bags or the filter bag house injection means for injection of ammonia or a urea solution into the flue gas duct; upstream the one or more fabric filter bags or the filter bag house, injection means for injection of nitrogen dioxide containing effluent gas; and outside the flue gas duct, an ammonia oxidation catalyst unit; and means for cooling and oxidizing nitrogen monoxide containing effluent gas withdrawn from the ammonia oxidation catalyst to the nitrogen dioxide containing effluent gas connected at its outlet end to the injection means for injection of the nitrogen dioxide containing effluent gas.

7. The system of claim 6, wherein the means for cooling and oxidizing the nitrogen monoxide containing effluent gas is in form of a heat exchanger.

8. The system of claim 6, wherein the means for cooling and oxidizing the nitrogen monoxide containing effluent gas is in form of a spirally wound tube.

9. The system of claim 6, wherein the means for the cooling and oxidizing nitrogen monoxide containing effluent gas is provided with an oxidation catalyst.

Description

(1) In an embodiment, the oxygen containing atmosphere includes hot recirculated gas which provides then additionally part of the oxidation reactor heating duty.

(2) NO formed from NH.sub.3 in a first step by oxidation of the NH.sub.3 in contact with a precious metal containing catalyst is subsequently oxidized to NO.sub.2 in the NO containing effluent gas from the first step by cooling the gas to ambient temperature to push the equilibrium reaction 2NO+O.sub.22NO.sub.2 towards formation of NO.sub.2.

(3) The term ambient temperature as used herein, shall mean any temperature prevailing in the surroundings of a combustion facility employing the method and system of the invention. Typically, the ambient temperature will be between 20 C. and 40 C.

(4) Cooling and oxidation of the NO containing effluent gas can be performed in an aging reactor sized so that the residence time of the gas is about 1 minute or longer.

(5) In an embodiment the oxidation reaction is performed in presence of a catalyst promoting the oxidation of NO to NO.sub.2. Those catalysts are known in the art and include Pt on TiO.sub.2, Pt on SiO.sub.2 and activated carbon or Pt and/or Pd on alumina.

(6) As mentioned hereinbefore the desired fast SCR reaction requires equal amounts of NO and NO.sub.2. Consequently, the amount of NO.sub.2 injected into the flue gas at a temperature below 250 C. is controlled to result in 45 to 55% by volume of the nitrogen oxides content in the flue gas is NO.sub.2 at inlet to the SCR catalyst unit.

(7) In another aspect, the invention provides a system for use in the method according to the invention.

(8) The system comprises within a flue gas duct a filter bag house with one or more fabric filter bags catalysed with a catalyst for selective reduction of nitrogen oxides;

(9) upstream the one or more fabric filter bags or the filter bag house injection means for injection of ammonia or a urea solution into the flue gas duct;

(10) upstream the one or more fabric filter bags or the filter bag house, injection means for injection of nitrogen dioxide containing effluent gas; and

(11) outside the flue gas duct,

(12) an ammonia oxidation catalyst unit; and

(13) means for cooling and oxidizing nitrogen monoxide containing effluent gas withdrawn from the ammonia oxidation catalyst to the nitrogen dioxide containing effluent gas connected at its outlet end to the injection means for injection of the nitrogen dioxide containing effluent gas.

(14) As mentioned above, the oxidation reaction of NO to NO.sub.2 needs a residence time of the NO containing gas of at least 1 minute. Typically, 1-2 minutes.

(15) This can be achieved in a heat exchanger either gas cooled or water cooled or alternatively when shaping the cooling and oxidizing means as a spirally wound tube with a length resulting in the desired residence time of the gas passing through the tube.

(16) In another embodiment, the means for cooling and oxidizing nitrogen monoxide containing effluent gas is provided with an oxidation catalyst promoting the oxidation of NO to NO.sub.2.