FR2682459A1 - Method and devices for decreasing the nitrogen monoxide content of the gases from an oxidising combustion of a fluidised-bed reactor - Google Patents

Abstract

Method for decreasing the nitrogen monoxide content in the smoke (fumes) produced by an oxidising combustion in a fluidised-bed reactor (1), characterised in that water or water vapour (steam) is injected during this combustion. Device with fluidised-bed combustion reactor (1) with injection of water or steam. Decrease in the nitrogen monoxide by injection of water or water vapour whilst minimising the effect on the thermal efficiency of the plant.

Description

Method and devices for reducing the nitrous oxide content of the gases of an oxidative combustion of a fluidized bed reactor
STATE OF THE ART
Fluidized beds are now used in thermal power plants whose power reaches 150 MW electric and will therefore take on an increasing importance in the production of electricity. Circulating fluidized beds in particular have the definite advantage of allowing significant desulphurization greater than 90 a and of being the source of a low production of nitrogen oxides (NOx) of the order of 200 mg / nm3 or less (on a reference basis of dry smoke containing 6% 02).

 However, recent measurements have shown that the fluidized bed is also at the origin of the production of nitrous oxide (N20) also called laughing gas, harmful for the environment taking into account in particular its influence on the terrestrial ozone layer. .

 This nitrous oxide, which is practically nonexistent in conventional thermal power plants burning fossil fuels in pulverized form, is produced in a fluidized bed due to the combustion temperature around 850 C, a low temperature if compared to that existing in homes burning pulverized fuel.

 In European patent application No. 0406185 A2, it is proposed to raise, by conventional complementary combustion, the temperature of the gases leaving the cyclone of a circulating fluidized bed to a value between 900 and 1 1000 C, and to maintain it at this value for a time up to 5 seconds to remove this nitrous oxide. This process is however at the origin, in this complementary combustion, of an additional production of nitrogen oxides and sulfur oxides.

 A similar process is described in German patent application No. DE 39 33 286 Ai.

INVENTION
The invention is characterized by the injection of water or steam as a means of reducing the content of nitrous oxide in the gases produced by oxidative combustion in a fluidized bed corresponding to a speed of the fluidizing gases going from the bed dense fluidized in the circulating fluidized bed. The amount of water or vapor required for each fuel will depend on its chemical characteristics, in particular its nitrogen, oxygen and water contents. This amount will also vary depending on the load of the installation.

Water injection takes place
either in the lower part of the reactor under the fuel introductions or also distributed along the periphery of a straight section of the reactor,
either in the return duct of the siphon,
either under the injection nozzles of the pneumatic fuel supply to the reactor,
either around the injection pipes for liquid or gaseous fuels.

Steam injection takes place
either in the lower part of the reactor under the fuel introductions or also distributed along the periphery of a straight section of the reactor,
. either in the gravity descent of fuel,
either in the return duct of the cyclone siphon
either in pneumatic fuel injection,
. either around the injection pipes for liquid or gaseous fuels,
. either in the secondary air injection nozzles,
either in the primary air supply.

 The application of the invention will be described by way of example more particularly in the case of a circulating fluidized bed boiler.

 Figure 1 shows a circulating fluidized bed system.

 FIG. 2 represents a fluidized bed reactor with gravity feeds of fuels in return ducts of the cyclone traps.

 FIG. 3 represents a fluidized bed reactor with gravity feeds of fuel directly into the reactor.

 FIG. 4 represents a fluidized bed reactor with pneumatic fuel inlets.

 FIG. 5 represents a fluidized bed reactor with inlets of liquid or gaseous fuels on the periphery of a section of the reactor.

 FIG. 6 represents a fluidized bed reactor with injection of water or steam on the periphery of a section of the reactor.

 FIG. 7 represents a fluidized bed reactor with injection of steam into the secondary air.

 FIG. 8 represents a fluidized bed reactor with injection of steam into the primary air.

 FIG. 9 represents a circulating fluidized bed system equipped with a device for heating injection water.

 FIG. 10 represents a device for injecting steam drawn from a steam turbine.

 Figure 11 shows a second steam injection device.

 The boiler, in FIG. 1, comprises a reactor 1 cooled by water and steam tubes (not shown) a cyclone 2, a siphon 3 for reintroducing into the reactor materials collected by the cyclone, a set of surfaces d convection exchange (superheater 4, superheater 5, economiser 6), a primary and secondary air heater 7, a dust collector 8, a draft fan 9, a chimney 10, a fuel introduction 11 or 11 'or 11 " , a primary air introduction 12, a fluidization grid 13, a secondary air introduction 14, an introduction of inert bed material 15 and an introduction of sulfurous gas reducing agent 16.

 Note that all of the combustion takes place in reactor 1 and that there is no more combustion of the gases from reactor 1 after their course (4, 5, 6, 7, 8, 9, 10) .

The introduction of a crushed or crushed fuel can be done in several ways
- either pneumatically (11),
- either by gravity fall (11 ') in the reactor,
- or by gravity fall (11 ") in the return of the siphon to the reactor.

 The diagram of a dense fluidized bed boiler would be similar to that of FIG. 1. However, cyclone 2 could be omitted.

To be effective, the introduction of water or steam must be localized so that water vapor is present from the start of combustion or the distillation of volatile matter from the fuel. In FIGS. 2 to 8, examples of fuel introduction and preferred solutions for injecting water or corresponding steam are given.
FIG. 2 corresponds to a gravity supply of fuel 11 "in the return of the siphon 3 of the cyclone 2 of a circulating fluidized bed: the steam is introduced into the gravity descent (V 1) or into the return sheath of the materials of the siphon 3 (V 2); water can be sprayed into the return duct of siphon 3 (E 2) or under the arrival of this in reactor 1 (E 1).

 FIG. 3 corresponding to a gravity supply of fuel via a line 11 ′ directly into the reactor 1 of a fluidized bed, steam is introduced into this gravity descent 11 ′ (V 3); water can be sprayed under the arrival of this gravity descent 11 'in the reactor 1 (E 3).

 FIG. 4 corresponds to one or more pneumatic fuel inlets 11, in the reactor 1 of a fluidized bed: steam is introduced directly into these inlets 11 (V 4) or into the reactor in the lower part below these inlets (V 5 ); water will be introduced into the reactor under these arrivals (E 4).

 FIG. 5 corresponds to the combustion of liquid or gaseous fuels which are introduced into the reactor of a fluidized bed by canes 11 distributed over the perimeter of the reactor: water or steam will be introduced around each cane 11 (V 6 or E 5).

 FIG. 6 corresponds to the case where several fuels are introduced simultaneously into the reactor of a fluidized bed by different methods, without it being possible to be certain of their distribution: in this case, water or steam is introduced by injection pipes 17 distributed over the periphery of the reactor (v 7 or E 6).

 Figure 7 corresponds to the injection of steam into the secondary air (V 8) of a reactor of a fluidized bed.

 FIG. 8 corresponds to the introduction of steam by nozzles 18 into the primary air (V 9) of the fluidized bed reactor, this can also be done by special nozzles mixed with those of the primary air and distributed across the grid 13.

 Even if common coals require much less water vapor in their combustion gases than lignites because of their nitrogen and oxygen content, it is obvious that the injection of cold water into the reactor introduces a loss of significant yield.

 A reduction in this loss can be obtained by reheating this water before its introduction into the reactor by the fumes downstream of the air heater 7, as shown in FIG. 9. An exchanger E is provided arranged between the heater 7 and the dust collector 8 or downstream of the dust collector 8. Water recycling R is also provided so that the temperature of this water on entering the exchanger E is above the condensation temperature of the vapor in the combustion gases . The water arrives at 19 and after reheating is sent to 20 into the reactor 1 as described in the examples above.

But the loss still being significant, the direct use of steam injected into the reactor proves to be interesting
1. Sometimes there are surplus low pressure steam networks in factories where steam has a very low cost.

 2. In thermal power plants, a sample of steam from a drawing off of the turbine at the lowest possible pressure but sufficient for it to be injected into the fluidized bed reactor makes it possible to significantly reduce the corresponding loss of efficiency to an injection of cold water into the reactor.

This solution corresponds to FIG. 10 where the boiler K is represented, equipped with a circulating fluidized bed reactor 1, an air heater 7, a dust collector 8, a draft fan 9 and d 'a chimney 10. The steam turbine consists of an HP body, an MP body and a BP body, followed by a condenser C, a PE extraction pump, water heaters low pressure R1, R2, R3, a degasser R4, a feed pump PA, a high pressure heater R5, as well as the withdrawals S1, S2, S3, S4 and S5 supplying the various heaters. The vapor necessary for the removal of nitrous oxide is taken from the rack S3 (the rackings S1 and
S2 having insufficient pressure) and sent via line 20 for injection into reactor 1 as described in FIGS. 2 to 8.

 This withdrawal of steam from the turbine, however, involves a drawback; it corresponds to a significant consumption of demineralized water. This disadvantage can be overcome by using the diagram shown in Figure 11: the water required for injecting steam into the reactor is previously heated in an exchanger E, arranged on the combustion gas circuit downstream of the heater. air 7, and passes through a still G where it is vaporized by the heat of condensation of the vapor taken from the draw-off S3 of the turbine and this vapor is injected into the reactor 1 via line 21; the condensed water is recovered and reintroduced for example into the low pressure heater R3.

 This device thus has the advantage that the make-up water injected into the reactor in the form of low-pressure steam does not require costly demineralization but only possible simplified treatment. The still needs to be deconcentrated by a purge P which makes it possible to heat the make-up water injected at 22 which, itself, is brought - thanks to a recirculation R - to the temperature sufficient to be above the condensation temperature of the water vapor in the boiler combustion gases.

Claims (17)

CLAIMS: 1. Method for reducing the content of nitrous oxide in the fumes produced by oxidative combustion in a fluidized bed reactor (1), characterized in that water or steam is injected during this combustion 2. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening below the grid (13) means secondary air introduction (14) opening above the grid (13), means for introducing inert bed material (15), a cyclone (2) connected to the reactor (1) followed by a siphon (3) provided with a sheath for reintroducing into the reactor (1) the materials collected by the siphon (3), fuel introduction means (11 ") opening out, including the sheath of the siphon (3), characterized in that that it includes means for injecting steam (V1, V2) into the fuel introduction means (11 ") or into the siphon sheath (3). 3. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), secondary air introduction means (14) opening above the grid (13) means for introducing inert bed material (15), a cyclone (2) connected to the reactor (1) followed by a siphon (3) provided with a sheath for reintroducing into the reactor (1) the materials collected by the siphon (3), fuel introduction means (11 ") opening into the sheath of the siphon (3), characterized in that that it comprises means for injecting water (E1, E2) into the siphon sheath (3) or into the lower part of the reactor (1) below the level of the siphon sheath (3). 4. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), fuel gravitation introduction means comprising a descent (11 ') opening into the reactor (1), characterized in that it comprises means for injecting steam (V3) into said descent (11'). 5. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), fuel gravitation introduction means comprising a descent (11 ') into the reactor, characterized in that it comprises means for injecting water (E3) directly into the reactor (1) below said descent ( 11 '). 6. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), pneumatic fuel injection means comprising a pipe (11) opening into the reactor (1), characterized in that it comprises steam injection means (V4, V5) in said pipe (11) or directly in the reactor (1) below said pipe (11). 7. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), pneumatic fuel injection means comprising a line (11) opening into the reactor, characterized in that it comprises water injection means (E4) below said line (11). 8. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), means for injecting liquid or gaseous fuels comprising a cane (11) opening into the reactor (1), characterized in that it includes means injecting steam (V 6) into the reactor around said canes (11). 9. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), means for injecting liquid or gaseous fuels comprising rods (11) opening into the reactor (1), characterized in that it comprises means injecting water (E5) into the reactor around said rods (11). 10. Fluidized bed device comprising a combustion reactor (1) provided with a fluidizing grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), means for introducing fuel into the reactor, characterized in that it comprises means for injecting steam (V7) over the entire periphery of the reactor at a level situated above the fluidization grid (13) and in below the fuel introduction. 11. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), means for introducing the fuel into the reactor, characterized in that it comprises means for injecting water (E6) over the entire periphery of the reactor at a level situated above the fluidization grid (13) and below the fuel introduction. 12. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), means for introducing secondary air into the reactor, characterized in that it comprises means for injecting steam (V8) into the secondary air. 13. Fluidized bed device comprising a combustion reactor (1) provided with a fluidization grid (13) of the bed, means for introducing primary air (12) opening out below the grid (13), characterized in that it comprises steam injection means (V9) below or at the level of the fluidization grid (13). 14. Device according to one of claims 3, 5, 7, 9, 11 comprising an air heater (7) for heating the primary air (AP) and the secondary air (AS) thanks to the combustion gases before their introduction into the reactor (1), characterized in that it includes an exchanger (E) located in the combustion gas circuit of the boiler downstream of the air heater (7) said exchanger (E) serving to preheat the water injected into the reactor (1). 15. Device according to one of claims 2, 4, 6, 8, 10, 12, 13 used to supply steam to a steam turbine (HP, MP, BP) with several steam withdrawals (S1, S2, S3, S4, S5), characterized in that the steam injected into the reactor (1) is taken from a withdrawal (S3) of the steam turbine. 16. Device according to one of claims 2, 4, 6, 8, 10, 12, 13 used to supply steam to a steam turbine (HP, MP, BP) associated with water heaters (king, R2, R3, R4, R5) supplied by steam withdrawals (S1, S2, S3, S4, S5) from the turbine, characterized in that the steam for injection into the fluidized bed reactor (1) is supplied by a distiller (G) supplied with heat from a racking (S3) of the steam turbine, the water condensed in the still (G) being returned to one of the water heaters (R3). 17. Device according to one of claims 2, 4, 6, 8, 10, 12, 13, 16 in which the injection vapor in the fluidized bed reactor (1) is produced from water which is preheated in an exchanger (E) by the combustion gases at the outlet of the air heater (7) of the reactor.