Patent Grant
10458650
The present invention claims priority from French patent application FR 12 53597, filed Apr. 19, 2012, incorporated by reference in its entirety.
The invention relates to a process for the denitrification of the flue gases produced by a combustion furnace, in which a fuel is introduced into a fluidized bed or onto a grate, and combustion air is injected into the furnace.
The invention relates more particularly, but not exclusively, to such a process for the denitrification of the flue gases produced by a furnace for incinerating waste or sludge of municipal water or industrial water purification plants.
During a combustion, unlike sulfur oxides, acids and heavy metals, the emissions of which are intrinsically linked to the content of sulfur, of Cl (chlorine), Br (bromine), F (fluorine), I (iodine), and of heavy metals of the fuel used, the amount of nitrogen oxides generated is a function, to a certain extent, of the fuel used, but also of the conditions under which the combustion takes place. Therefore there is no univocal relationship between the emissions of nitrogen oxides and the fuel. At the very most, when one has a good knowledge of a given process (coal, heavy fuel oil, natural gas, etc. thermal power plant), it is possible to formulate an emission factor which will be used inter alia as a base reference for the advances and reductions in the emissions of nitrogen oxides which could be obtained by subsequent research and development.
The combustion of a hydrocarbon-based compound is therefore always accompanied, in addition to carbon dioxide CO2, water H2O, and nitrogen N2, by a production of nitrogen oxides. These oxides are represented by nitrogen monoxide (NO), nitrogen protoxide (N2O), and by a very small proportion of nitrogen dioxide (NO2).
From an environmental and health viewpoint, it is important to reduce their emissions therein since each of these nitrogen oxides has a significant impact:
In order to reduce the emissions of NOx, processes have been developed, in particular the following two processes:
The SCR process makes it possible to abate large amounts of NOx, but at the expense of major economic and environmental drawbacks. The more economical SNCR process does not make it possible to achieve a nitrogen oxides removal efficiency as high as the SCR process.
The objective of the invention is, above all, to provide a process for the denitrification of flue gases, of the SNCR process type, the nitrogen oxides removal efficiency of which is higher so as to tend toward the performances of an SCR process, and to have an effective, relatively economic, denitrification process with as low as possible an environmental impact.
Current Problem with the SNCR Process
The SNCR process for reducing nitrogen oxides consists in directly injecting the reducing agent into the combustion chamber in a zone where the temperature of the gaseous effluents is preferably between 850° C. and 1000° C.
By using ammonia as a reducing agent, the chemical reactions given below occur more or less simultaneously. At a temperature below 850° C., the reactions are too slow; at a temperature above 1000° C., the secondary reaction dominates with an increase in nitrogen oxides.
Main reaction:
4NO+4NH3+O2→4N2+6H2O (reduction)
Undesired secondary reaction:
4NH3+5O2→4NO+6H2O (oxidation)
The temperature window takes on considerable importance since, if the temperature is too high, the ammonia is oxidized which leads to a production of nitrogen oxides; if it is too low, the degree of conversion will be too low and the ammonia will be observed downstream (ammonia leak).
The reaction of the nitrogen oxides and of the ammonia, or of the urea, in water and nitrogen depends strongly on the temperature and on the residence time in the required temperature range. The temperature window for an aqueous ammoniacal solution (aqueous ammonia) lies between 800° C. and 1100° C., the optimum temperature being 930° C./960° C.
By way of comparison, the temperature window when urea is used is narrower (between 850 and 1050° C.), the optimum temperature being 960° C./980° C.
A first difficulty with the SNCR process is therefore the narrowness of the optimum temperature window.
A second difficulty consists in thoroughly mixing the gases to be treated with the reducing agent, also referred to as reactant.
In order to achieve a high degree of reduction and a low release of NH3, the reducing agent and the nitrogen oxides NOx of the flue gases must be sufficiently mixed otherwise the reaction does not take place and a portion of the nitrogen oxides NOx is not treated, or else the ammonia NH3 does not react. Most of the problems faced by selective non-catalytic reduction applications relate to the non-uniform distribution of the reducing agent in the combustion chamber.
Besides the distribution and the mixing, another important parameter is the size of the droplets of a liquid reducing agent. Small droplets would evaporate too rapidly and would react at temperatures that are too high, which would lead to a drop in the degree of reduction of the nitrogen oxides, whereas larger droplets would evaporate too slowly and would react at temperatures that are too low, which would lead to a larger release of ammonia NH3.
The choice of the reducing agent also influences the formation of by-products such as nitrogen protoxide (N2O). The use of ammonia and of caustic ammonia produces negligible amounts of N2O whereas relatively large amounts can be measured during the direct injection of urea into the combustion chamber.
The residence time in the required temperature range varies from 0.2 to 0.5 s. This contact time range is rather unstable; this is the reason why the ammonia/nitrogen oxides ratio must be rich in ammonia instead of being stoichiometric. Here too, it is necessary to optimize the NH3/NOx molar ratio. The degree of removal of the NOx is favored by a higher ratio, but the release of ammonia also increases, which leads to an increased pollution of the equipment placed downstream (heat exchangers, flue gas treatment). In order to neutralize these two contrary effects, the NH3/NOx ratio is limited to a range between 1.5 and 2.5.
In the case of a fluidized bed furnace for combustion of purification plant sludge, the temperature in the combustion chamber lies between 850° C. and 870° C. This temperature level is not favorable to the use of the SNCR process under the best conditions, which limits the degree of abatement of the nitrogen oxides to 40-50% and generates reaction by-products such as ammonia NH3 or nitrogen protoxide N2O. This distinctive feature adds to the imperfections of mixing operations linked to the injection of a reducing agent into a large-volume chamber.
Furthermore, during the evaporation of the aqueous ammonia solution, the temperature of the combustion chamber will drop as a function of the necessary amount of reducing agent. This drop is detrimental for obtaining the European regulatory temperature of 850° C. for 2 s (T2 s).
The invention aims, above all, to provide an improved process for the denitrification of the flue gases produced by a combustion furnace, which process, while being of the SNCR process type, makes it possible to address, at least partly, the major drawbacks of the treatment of nitrogen oxides NOx by the SNCR process:
According to the invention, the process for denitrification, in the heterogeneous phase, of the flue gases produced by a combustion furnace, in particular a furnace for incinerating waste or sludge from a municipal water or industrial water purification plant, according to which process a fuel is introduced into a fluidized bed or onto a grate, and combustion air is injected into the furnace, is characterized in that a reducing agent is injected into the fuel and/or into the combustion air, upstream of the combustion chamber, and is mixed homogenously with the fuel and/or the combustion air, the reducing agent being chosen to ensure the reduction of the nitrogen oxides to nitrogen, so that the reducing agent carries out a reducing treatment promoted by the layer of solids or ash present in the furnace.
The temperature of the gases in the combustion chamber of the furnace is, in general, below 900° C.
Preferably, the reducing agent is chosen from ammonia-based chemical compounds.
The application of the denitrification reaction in the heterogeneous phase by using the combustion zone (sand bed or waste grate) as a preferential denitrification reaction zone and by profiting from the heterogeneous catalytic aspect of this zone, makes it possible to address, at least partly, the drawbacks listed above.
This arrangement is a very simple implementation and guarantees the lowest possible consumption of reducing agent. If there is a place where the homogeneity is guaranteed, it is indeed the combustion zone of the solid waste and in particular the sand bed in a fluidized bed furnace.
The invention thus consists in injecting the reducing agent, in particular a solution of denitrifying reactant, directly into the fuel and/or into the combustion air before it is introduced into the furnace, as homogeneously as possible. Thus, the reducing agent is vaporized at the same time as the water and the volatile material contained in the fuel, and its distribution, is provided in the same way as the fuel charge, by the fluidization of the sand bed.
This intense mixing, in addition to obtaining the homogeneity, ensures the coexistence of the NOx/reducing agent/catalytic bed (sand+ash) and enables denitrification reactions to take place that are catalyzed at relatively low temperatures, of the order of 800° C., which are not possible in the homogeneous gas/gas phase at higher temperature according to the conventional SNCR process.
The heterogeneous (catalytic) reactions are of the type:
4NH3+4NO+O2+C→N2+H2O (catalysis of the waste by carbon),
4NH3+4NO+O2+CaO→N2+H2O (catalysis of the waste by ash).
If the reactions are not complete, the higher temperature and the time greater than 2 s (2 seconds) in the post-combustion enable denitrification reactions at 850° C.-870° C. in the homogeneous phase, without the drawbacks of a conventional SNCR process.
The reducing agent may be injected into the solid fuel and mixed therewith before being introduced into the combustion chamber. The reducing agent is injected with a solid, liquid or gaseous co-reactant (lime, limestone, etc.).
It is possible to inject reducing agent into a liquid or gaseous fuel feeding a zone where solid fuels are present in the furnace.
It is possible to inject reducing agent into a solid waste, sand or ash, which is recirculating.
The reducing agent may be injected into the combustion air of the furnace upstream of the combustion chamber.
The reducing agent and the fuel are preferably mixed in a mixing member.
In the case of a fluidized sand bed furnace comprising a device for the homogeneous distribution of air through the sand bed, the reducing agent may be injected into the box for distributing the combustion air, also referred to as a wind box.
The reducing agent may be chosen from ammonia or urea.
Preferably, the reducing agent is vaporized prior to its injection into the combustion air.
The reducing agent may be injected directly in the solid, liquid or gaseous phase with optional dilution with water.
When the reducing agent is injected directly in the liquid phase in sprayed form, the spraying of the reducing agent may be assisted by addition of air using spray nozzles.
Advantageously, in the case of a fluidized sand bed furnace, the temperature of the sand bed where the heterogeneous reduction takes place is controlled by adjusting:
Generally, the content of nitrogen oxides NOx and/or of ammonia NH3 is monitored directly at the outlet of the combustion chamber, and the injection of the reducing agent is regulated as a function of the measured contents of NOx and/or of ammonia NH3.
The reducing agent may be injected, at least partly, into a flue gas recycle stream.
Advantageously, the temperature of the devolatilization zone where the heterogeneous reduction takes place is controlled by adjusting the preheating of the fluidization air, and/or the stoichiometry of the combustion air, and/or the height of the sand layer, and/or the use of auxiliary fuel.
The invention also relates to a plant for the implementation of a process as defined previously, this plant comprising a combustion furnace, in particular a furnace for incinerating waste or sludge from a municipal water or industrial water purification plant, the fuel being introduced into a fluidized bed or onto a grate, and combustion air being injected into the furnace, and being characterized in that it comprises a line for injecting a reducing agent into the fuel line and/or into the combustion air line, upstream of the combustion chamber, and a mixing member in order to carry out a homogeneous mixing of the reducing agent with the fuel and/or the combustion air.
Advantageously, the plant comprises a control member for the flow of reducing agent, and at least one sensor of the NOx content or ammonia NH3 content of the flue gases, at the outlet of the furnace, which sensor controls the control member.
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:
With reference to
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.).
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.
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.
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.
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.
The flow rate control member 7 may be controlled by a measurement of the concentration of nitrogen oxides NOx or of ammonia NH3 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.
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.
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.
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 NH3 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.
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.
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.
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).
The invention is preferably used in a fluidized sand bed furnace according to the example in
However, the invention may also be used in a grate furnace 1b (
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
According to the embodiment from
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
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.
The catalytic heterogeneous reactions are of the type:
4NH3+4NO+O2+C→N2+H2O (catalysis of the waste by carbon),
4NH3+4NO+O2+CaO→N2+H2O (catalysis of the waste by ash).
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.
In the case where the denitrifying reducing agent is injected into the combustion/fluidization air, the mixture passes through the waste to be burned.
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.
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.
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.
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 NH3 measured downstream.
The reducing agent is then conveyed to the furnace via the wind box and the grate.
In particular, in a fluidized bed, such as the bed B from
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.
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 NH3 in the flue gases which leave via the line 10.
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.
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.
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.
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.
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.
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