Methods and systems for flue gas denitrification

Abstract

A process for nitrification, in the heterogeneous phase, of the flue gases produced by a combustion furnace (1), in particular a furnace for incinerating waste or sludge from a municipal water or industrial water purification plant, according to which the fuel is introduced into a fluidized bed (B) or onto a grate, and combustion air (2) is injected into the furnace; a reducing agent (6) is injected into the fuel and/or into the combustion air, upstream of the combustion chamber (H), and is mixed homogenously with the fuel and/or the combustion air, in order to carry out a reducing treatment promoted by the bed (3) of ash or of solids present in the furnace.

Claims

1. A method of removing NOx from flue gases in a furnace for incinerating waste or sludge from a municipal water or industrial water purification plant, the furnace comprising a combustion chamber with a bed of sand or ash, the method comprising the following steps: introducing the waste or sludge as a fuel into a devolatilization zone of the furnace in which the sand or ash is located and in which the fuel loses volatile components during combustion such that the devolatilization zone is constituted by the bed comprising sand or ash along with the fuel; introducing a reducing agent in a vaporized form into at least one of the combustion air upstream of a wind box of the furnace and the fuel; introducing the combustion air into the wind box of the furnace positioned below the devolatilization zone, the wind box providing distribution of the combustion air into the bed and mixing of the bed with the combustion air, wherein the combustion air is introduced solely through the wind box; wherein the reducing agent is introduced solely upstream of said devolatilization zone; measuring nitrogen oxide and/or ammonia levels of treated flue gases downstream of a post-combustion zone of the furnace located downstream of the devolatilization zone; and regulating the introduction of the reducing agent in order to maintain pre-established nitrogen oxide content and/or ammonia levels; wherein the reducing agent is an ammonia (NH.sub.3) compound causing a reduction of nitrogen oxides to nitrogen, wherein the reduction of nitrogen oxides to nitrogen is carried out in the bed, according to reactions of the following type: 4NH.sub.3+4NO+O.sub.2+C.fwdarw.N.sub.2+H.sub.2O(catalysis of the waste or sludge by carbon), 4NH.sub.3+4NO+O.sub.2+CaO.fwdarw.N.sub.2+H.sub.2O(catalysis of the waste or sludge by ash).

2. The method of claim 1, wherein the reducing agent is diluted with water.

3. The method of claim 1, wherein said introducing of the reducing agent comprises injecting the reducing agent into the fuel prior to the step of introducing the fuel into the devolatilization zone of the furnace.

4. The method of claim 1, wherein the introduction of the reducing agent and the fuel comprises mixing the reducing agent and the fuel in a separate vessel before introduction to the furnace.

5. The method of claim 1, wherein the furnace comprises a fluidized bed furnace in which the bed comprises sand and wherein the distribution of the combustion air provides fluidization of the bed.

6. The method of claim 1, wherein the furnace comprises a fluidized bed furnace in which the bed comprises sand and the distribution of the combustion air provides fluidization of the bed, and wherein temperature of the sand bed is maintained by controlling one or more of: temperature of the fluidizing combustion air; stoichiometry of the combustion air; depth of the sand bed; and/or use of auxiliary fuel.

7. A method of removing NOx from flue gases in a furnace for incinerating solid waste from a municipal water or industrial water purification plant, the furnace comprising a combustion chamber with a bed of sand or ash, the method comprising the following steps: introducing solid waste and sand or ash into a devolatilization zone of the furnace to form the bed, the bed of sand or ash with solid waste constituting the devolatilization zone; introducing combustion air via a wind box into the devolatilization zone of the furnace such that the solid waste and sand or ash are mixed within the devolatilization zone of the furnace by the combustion air, the wind box being the sole entry of the combustion air; vaporizing a reducing agent and adding the reducing agent to at least one of the combustion air in the wind box before the combustion air is introduced into the furnace and the solid waste upstream of the devolatilization zone, such that the reducing agent is combined with the solid waste and sand or ash while being mixed; wherein the reducing agent is introduced solely upstream of said devolatilization zone; measuring nitrogen oxide and/or ammonia levels of treated flue gases downstream of a post-combustion zone of the furnace located downstream of the devolatilization zone; and regulating the introduction of the reducing agent in order to maintain pre-established nitrogen oxide content and/or ammonia levels; wherein the reducing agent is an ammonia (NH.sub.3) compound causing a reduction of nitrogen oxides to nitrogen present in the furnace, wherein the reduction of nitrogen oxides to nitrogen is carried out into the bed, according to reactions of the following type: 4NH.sub.3+4NO+O.sub.2+C.fwdarw.N.sub.2+H.sub.2O(catalysis of the waste by carbon), 4NH.sub.3+4NO+O.sub.2+CaO.fwdarw.N.sub.2+H.sub.2O(catalysis of the waste by ash), wherein the temperature of the combustion chamber lies between 850 C. and 870 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention consists, apart from the arrangements set out above, of a certain number of other arrangements, which will be mentioned more explicitly below regarding exemplary embodiments described with reference to the appended drawings, but which are not limiting. In these drawings:

(2) FIG. 1 is a diagram of a fluidized bed incineration furnace for purification plant waste or sludge, implementing the process of the invention;

(3) FIG. 2 schematically shows, similarly to FIG. 1, a waste incineration furnace containing a grate;

(4) FIG. 3 is a block diagram of the process with injection of the reducing agent into the fuel; and

(5) FIG. 4 is a block diagram of the steps of the process when the reducing agent is introduced into the combustion air.

DESCRIPTION OF THE INVENTION

(6) With reference to FIG. 1 of the drawings, it is possible to see a combustion furnace 1 having a fluidized sand bed B, according to which the combustion air and fluidization air 2 is introduced partly below in a wind box A surmounted by an arch a1 supporting the bed B. Nozzles a2 that ensure the distribution of the air blown into the bed B pass through the arch a1. A furnace of this type is known under the name Thermylis from the company Degrmont. In general, the temperature of the gases in the combustion chamber of the furnace is below 900 C.

(7) The bed B constitutes a devolatilization zone 3 which contains the waste in the solid phase and in which the volatile components devolatilize and partly burn. It is recalled that the devolatilization of a fuel denotes the process via which, during a heat treatment, the fuel loses its volatile components (water, hydrocarbon-based matter, carbon monoxide, hydrogen, etc.).

(8) The fuel is introduced at the bottom part of the bed B via at least one side nozzle 4. A post-combustion zone 5 is located in the chamber of the furnace above the bed B.

(9) The injection 4 of the fuel thus takes place in the devolatilization zone 3. The fuel may consist of the purification plant sludge, household waste, fuel oil, or gas, or any organic waste that is introduced into a furnace in order to burn it.

(10) According to a first aspect of the invention, a reducing agent 6 is injected directly into the fuel, in particular sludge or waste, before introduction into the combustion chamber H level with the devolatilization zone 3. This injection may take place by means of a control member 7 for controlling the flow rate of the reducing agent. The injection of the reducing agent 6 is carried out by a pipe 8 connected to the fuel inlet line 4a. The control member 7 may be of the valve, variable flow pump or rotary screw feeder type or any other device that makes it possible to regulate the flow rate of the reducing agent.

(11) This reducing agent 6 may consist of a solution of aqueous ammonia, of gaseous ammonia, of urea in solution, of cyanhydric acid, or any other reactant which ensures the chemical reduction of the nitrogen oxides NOx. The reducing agent 6 may also be in the form of a pulverulent material, especially in the case where it consists of urea.

(12) The flow rate control member 7 may be controlled by a measurement of the concentration of nitrogen oxides NOx or of ammonia NH.sub.3 downstream of the furnace 1, especially using a sensor 9 located on an outlet line 10 of the flue gases from the furnace 5. Preferably, the sensor 9 is installed at the outlet of the post-combustion zone 5.

(13) A mixing device or member 11 is installed on the fuel inlet line 4a, downstream of the injection of the reducing agent, in order to produce a homogeneous mixture and intimate contact between the fuel and the reducing agent 6. This mixing device may consist of a screw, a kneader, a turbulence zone or simply a long tube or any means that make it possible to have at the inlet of the devolatilization zone 3 the finest possible mixture of the fuel and of the reducing agent 6.

(14) It should be noted that, according to the invention, it is also possible to inject the reducing agent 6 into an auxiliary fuel, fuel oil or gas, which may be introduced into the furnace by a nozzle 4b other than the nozzle 4, in order to stabilize the combustion. It is also possible to inject the reducing agent 6 into any solid product such as ash or sand recirculated in the devolatilization zone 3.

(15) Preferably, a probe 12 for measuring the concentration of nitrogen oxides NOx is provided on the line 10 just downstream of the post-combustion zone 5. A probe 13 for measuring the concentration of ammonia NH.sub.3 in the flue gases is also provided downstream of the combustion on the line 10 in order to monitor the over-stoichiometry of the reaction. One of the two probes 12, 13 may be merged with the sensor 9.

(16) Temperature sensors of the combustion zone are provided in the furnace, in particular a sensor 14 of the temperature of the heterogeneous zone constituted by the bed B with the presence of solids, sand or waste or of gas, These temperature sensors are implanted in order to monitor and adapt the temperature in the zone in question, in particular by varying the preheating, via a preheater E1, of the combustion air that arrives via the line 2, and by adjusting the stoichiometry or over-stoichiometry of the combustion via action on a blower S giving the flow of fuel and/or of combustion air.

(17) According to a second aspect of the invention, which may be combined with the first, the reducing agent 6 may be injected into the combustion air and fluidization air via a line 8a connected to the air inlet line 2a linked to the outlet of a blower S. A mixing device 11a is provided on the line 2a downstream of the injection of the reducing agent in order to ensure a homogeneous and intimate mixture before arrival in the wind box A, also referred to as distribution box of the combustion air.

(18) The flue gases exiting via the line 10 pass through the preheater E1, consisting in particular of a unit that forms a heat exchanger with the combustion air 2. Before release at the chimney stack C, the flue gases may pass through a unit E2 providing a treatment for eliminating the remaining nitrogen oxides by a selective catalytic reaction (SCR).

(19) The invention is preferably used in a fluidized sand bed furnace according to the example in FIG. 1, in order to exploit the highly catalytic nature of this bed.

(20) However, the invention may also be used in a grate furnace 1b (FIG. 2) comprising a grate 15 inclined from the inlet 15e of the materials to be burned, in particular municipal waste, to the outlet 15s of the residues and ash. A bed 3b of combustion materials and of ash is formed on the grate 15. The combustion air is introduced via a line 2b on which a blower Sb is installed. The combustion air 16 is blown into a distribution chamber Ab below the grate 15 while being spread over the entire area of this grate by a spray boom or a device that is not represented.

(21) The reducing agent is injected, into the combustion air, in gaseous form, or liquid form, in particular as a dispersion of droplets, or in solid form, in particular as a powder. The injection takes place in the line 2b. A mixing device 11b, located downstream of the injection, ensures a homogeneous mixture and an intimate contact between the air and the reducing agent. This mixture is blown under the grate 15 and passes through the grate 15 and the bed 3b, which plays a role similar to that of the bed B from FIG. 1.

(22) According to the embodiment from FIG. 2, the reducing agent could also be introduced into the materials to be burned or into the fuel injected at the inlet 15e.

(23) When the denitrifying reducing agent 6 is injected directly into the fuel before the introduction thereof into the furnace, the reducing agent is vaporized at the same time as the water and the volatile component contained in the fuel. The distribution of the reducing agent is ensured, in the same way as the fuel feed, by the fluidization of the bed B or, in the case of a grate furnace (as shown in FIG. 2), by the mixing of the ash and of the combustion material of the bed 3b.

(24) The intense mixing, in addition to ensuring that the homogeneity and the catalytic action of the bed B or of the ash bed 3b are obtained, makes it possible to carry out denitrification reactions that are catalyzed at relatively low temperature, of the order of 800 C., which would only be possible in the homogeneous gas/gas phase at higher temperatures.

(25) The catalytic heterogeneous reactions are of the type:
4NH.sub.3+4NO+O.sub.2+C.fwdarw.N.sub.2+H.sub.2O (catalysis of the waste by carbon),
4NH.sub.3+4NO+O.sub.2+CaO.fwdarw.N.sub.2+H.sub.2O (catalysis of the waste by ash).

(26) If the reactions are not complete, the higher temperature and the residence time of greater than 2 s (2 seconds) in the post-combustion zone 5, 5b, enable denitrification reactions at 850 C.-870 C. in the homogeneous phase, without having the drawbacks of a conventional SNCR process.

(27) In the case where the denitrifying reducing agent is injected into the combustion/fluidization air, the mixture passes through the waste to be burned.

(28) The wind box A located under the combustion chamber is the place that enables the homogeneous and controlled distribution of the combustion air in the chamber. It is an empty zone, for which the connection with the combustion chamber consists of an arch a1 with orifices, or of a grate 15 which can be formed by a simple pierced orifice, or by distribution nozzles, or of a mobile grate or any other means enabling the homogeneous distribution of the air in the combustion chamber.

(29) The combustion air arrives in the wind box after having been preheated, or not, in a preheater E1 that may be an air-flue gases, air-vapor, air-oil or water exchanger or any other system that makes it possible to raise the temperature of the combustion air.

(30) The reducing agent 6 is mixed by means of a distribution rod (not represented) implanted in the re-heated combustion air line or duct 2a,2b between the preheater and the wind box. If the reducing agent is in aqueous form, it is first vaporized by injection of vapor, compressed air or any other means enabling a good distribution in the duct between the preheater and the wind box.

(31) The metering of the amount of reducing agent is ensured by a control valve 7a,7b placed on the pipework of the reducing agent before, or preferably after, the vaporization. This valve may be controlled as a function of the content of NOx and/or of NH.sub.3 measured downstream.

(32) The reducing agent is then conveyed to the furnace via the wind box and the grate.

(33) In particular, in a fluidized bed, such as the bed B from FIG. 1, the pressure loss of the nozzles a2 enables a homogeneous distribution of the stream of combustion air loaded with reducing agent. A similar phenomenon occurs in the case of a grate furnace such as that from FIG. 2. Next, the passage of the combustion air loaded with the reducing agent into the bed B or into the bed 3b ensures an intense mixing between the nitrogen oxides NOx and the reducing agent in order to promote the reduction reaction. In addition to obtaining the homogeneity, the coexistence of the nitrogen oxides NOx/reducing agent/ catalytic bed (sand+ash) enables denitrification reactions to take place that are catalyzed at relatively low temperature (800 C.).

(34) FIG. 3 is a block diagram summarizing the operation of the invention in the case where the reducing agent is injected into the fuel.

(35) The fuel arrives via the line 4a into the mixing member 11, which receives, by means of the control member 7, the reducing agent 6.

(36) The mixture obtained at the outlet of the member 11 is homogeneous and a good contact is ensured between the fuel and the reducing agent. This mixture is introduced into the devolatilization zone 3 of the furnace 1. The combustion air and fluidization air is injected into this zone 3. The combustion is continued in the post-combustion zone 5. The sensors 9, 12 and 13 ensure the measurement of the concentrations of nitrogen oxides NOx and NH.sub.3 in the flue gases which leave via the line 10.

(37) FIG. 4 is a block diagram illustrating the operation of the invention when the reducing agent is injected into the combustion air.

(38) The reducing agent 6 is conducted via the line 8a,8b to the control valve 7a,7b before being injected into the combustion air line 2a,2b, which, generally, has been preheated by a preheater E1 with the aid of the flue gases. It is possible to provide, upstream of the control valve 7a, 7b, a dilution or an injection of air for spraying 17 into the reducing agent 6.

(39) The mixture of combustion air and of reducing agent is introduced into the wind box A or under the grate 15, in the case of a grate furnace, then into the combustion chamber from which the flue gases leave via the line 10.

(40) According to the invention, the nitrogen oxides NOx are destroyed in the sand bed B or in the ash bed 3b, the mineral portion of which acts as a catalyst.

(41) At the start of combustion, the speed of introduction of the sludge or material to be burned is increased in order to form a layer, in particular the bed 3b in the case of a grate furnace. This start-up phase may last around 10 to 15 minutes.

(42) The invention makes it possible to ensure an effective denitrification of the flue gases produced by a combustion furnace, in particular a furnace for incinerating waste or sludge from municipal water or industrial water purification plants.