EP1078203A1

EP1078203A1 - Method for the heat treatment of solids

Info

Publication number: EP1078203A1
Application number: EP99917726A
Authority: EP
Inventors: Hans Rueegg; Beat Stoffel
Original assignee: Alstom Power Schweiz AG
Current assignee: General Electric Switzerland GmbH
Priority date: 1998-05-11
Filing date: 1999-05-10
Publication date: 2001-02-28
Also published as: WO1999058902A1; JP2002514732A; KR20010025004A; CA2332011A1; KR100549654B1; HUP0102798A2; HUP0102798A3; CN1300359A; CN1218141C; US6336415B1

Abstract

The invention relates to a method for the heat treatment of solids (3), especially waste products, during which the solids (3) in a first stage (5) are burnt/gasified or pyrolysed in the presence of an oxygen deficiency, after which the exhaust gases (6) resulting from the first stage (5) are mixed in a secondary combustion chamber (14) with an oxygenated gaseous medium (15) and are fully burnt. Before being mixed with the oxygenated medium the exhaust gases (6) resulting from the first stage (5) are actively homogenized in a mixing stage (7) with addition of a gaseous medium (8) containing little or no oxygen. The homogenized exhaust-gas stream then passes through a steady-state zone (13) in which it remains for at least 0.5 seconds before the medium (15) is added in a secondary combustion stage (14) to ensure that the exhaust gas is fully burnt. The method provided for in the invention is characterized by simple process steps and a reduced content of pollutants, notably NOx, in relation to prior art.

Description

Process for the thermal treatment of solids

Technical field

The invention relates to a method for the thermal treatment of solids, in particular wastes such as domestic and urban waste, in which the solids are burned / gasified or pyrolyzed in a first stage with a lack of oxygen, and the exhaust gases of the first stage then in a post-combustion chamber with an oxygen-containing gaseous Mixed medium and burned with complete burnout.

State of the art

It is known prior art, lumpy solids, such as. B. waste, in a combustion chamber in which primary air is added, and a downstream post-combustion chamber in which secondary air is added to burn. The solid is usually reacted on a combustion grate. The primary air is supplied under the grate and flows through openings in the grate to the solid bed above.

The exhaust gases generated during combustion in and above the bed have a strongly fluctuating composition and temperature in terms of location and time. These exhaust gases are therefore subsequently used in conventional systems of secondary air or secondary air and recirculated flue gas mixed. The secondary air fulfills the following functions:

- Mixing of the gases emerging from the combustion chamber

- Oxygen supply to ensure the combustion of the gases

- cooling of the escaping gases.

The primary air added in the first stage is usually sufficient to burn the fuel completely, the secondary air is used to achieve the cross-mixing of the exhaust gas (mixing of CO-containing gas strands with 0 ₂ -containing gas strands). In order to ensure sufficient mixing, the amount of secondary air blown in must be selected to be correspondingly high. However, this excess of air adversely increases the amount of exhaust gas.

In order to eliminate this disadvantage, EP 0 607 210 B1 describes a method for the combustion of solids, in which, in addition to the primary air, no further combustion air is fed into the combustion boiler. In order to improve the poor burnout of the gases, which is caused by the insufficient mixing in the afterburning chamber and which leads to high pollutant contents in the exhaust gas, EP 0 607 210 B1 proposes, on the one hand, to add as much primary air in the first stage that a Excess oxygen results and, on the other hand, water vapor is injected into the combustion boiler above the combustion chamber and in the lower region of the afterburning chamber at a supersonic speed generated by excess pressure. This method has the disadvantage that the nitrogen contained in the fuel is oxidized to NO to an increased extent when there is excess air in the first combustion stage, and as a result low NOx emissions cannot be achieved.

Furthermore, a process for the thermal treatment of waste is known (Beckmann, M. and R. Scholz: "Gasification of Waste" in "Gasification Process for the Disposal of Waste", Springer-VDI-Verlag GmbH, Düsseldorf, 1998, Pp. 80-109), in which the amount of primary air under the grate is reduced to such an extent that gasification of the fuel occurs and a CO-rich exhaust gas is produced. This exhaust gas is afterburned with air in a subsequent completely separated afterburning chamber. As a result of the significant reduction in air addition in the first stage compared to conventional rust combustion systems, a significant reduction in NOx emissions is reported. So far, however, this procedure has only been carried out on a trial scale. The afterburning chamber was completely separated from the combustion chamber and connected by a pipe. The exhaust gas flow was homogenized by turbulence when flowing through this pipe. As a result of the small system size and the routing of the exhaust gas flow from the primary combustion chamber through a connecting pipe, it was possible to dispense with a mixing device for the exhaust gas flow from the primary combustion chamber without increased pollutant concentrations in the exhaust gas occurring after the afterburning chamber. However, the use of a pipe as a connection between the primary combustion chamber and the afterburning chamber is disadvantageous in the case of a large-scale design (wear and tear).

Presentation of the invention

The invention tries to avoid these disadvantages. It is based on the object of a process for the thermal treatment of solids, in particular waste, in which the solids are burned / gasified or pyrolyzed in a first stage with a lack of oxygen, and subsequently the escaping gases are mixed with the oxygen-containing medium required for a complete burnout and are burned to develop, eliminating local concentration and temperature fluctuations in the exhaust gas of the first stage and thereby minimizing the pollutant concentrations, in particular the NOx emissions. This is achieved according to the invention in that, for the purpose of NOx reduction, the exhaust gases emerging from the first stage are actively homogenized in a mixing stage with the addition of a gaseous oxygen-free or low-oxygen medium before they are mixed with the oxygen-containing medium and the homogenized oxygen-poor exhaust gas stream emerging from the mixing stage passes through a steady-state zone before the addition of the oxygen-containing medium required for complete burnout, the dwell time in the steady-state zone being at least 0.5 seconds.

The advantages of the invention are that the gases emerging from the first stage no longer have any concentration and temperature fluctuations due to their subsequent homogenization when they are mixed with the burnout air. Due to the additional residence of the homogenized gas flow in the persistence zone under lack of air (substoichiometric air ratio), the NO already formed can be reduced to N ₂ by the NH _X , HCN and CO present. As a result, only minimal pollutant emissions occur in the thermal treatment of the solids according to the invention.

It is particularly expedient if recirculated exhaust gas, water vapor, oxygen-depleted air or inert gases such as nitrogen are used as gaseous oxygen-free or low-oxygen media for homogenization. These gases are advantageously injected into the mixing zone perpendicular to the direction of flow of the exhaust gases or, in order to improve the homogenization and mixing effect, at a certain angle opposite or equal to the direction of flow of the exhaust gases from the first stage.

It is also advantageous if the active homogenization of the exhaust gases emerging from the first stage takes place with the aid of internals (static mixers) installed in the mixing zone. These internals cause a flow diversion exhaust gases and thus their efficient mixing. It is expedient if these internals have cavities with a cooling medium, for. B. water, steam or air.

Finally, the active homogenization of the exhaust gases emerging from the first stage is advantageously carried out by narrowing or widening the cross section of the flow channel.

In addition, it is expedient to regulate the temperature of the exhaust gases in the area of the injection of the oxygen-containing medium via the amount of the oxygen-free or low-oxygen gaseous medium fed into the mixing zone. This is a very simple way to keep the temperature constant.

It is advantageous if a grate system with medium-current firing or with counter-current firing is used as the first stage.

Furthermore, it is advantageous if a fluidized bed is used as the first stage, since a very good mass and heat exchange is thereby achieved. Local temperature peaks and locally increased wear on the lining can be prevented. In addition, the ferrous and non-ferrous metals contained in the waste can be recovered from the ashes in very good quality.

It is also expedient if the afterburning stage is a fluidized bed and the oxygen-containing gaseous medium is supplied at the entry into the fluidized bed or directly into the fluidized bed. As an advantage, local hot zones with high thermal NOx formation can then be avoided due to the increased heat transfer due to the presence of particles. In addition, caking on the heat exchanger walls is prevented, which leads to a reduction in corrosion on the heat exchanger surfaces. It can be higher Vapor pressures and temperatures can be set which enable a higher thermal efficiency of the incineration plant.

Finally, it is expedient if the steady-state zone is a fluidized bed and the gaseous oxygen-free or low-oxygen medium is supplied at the entry into the fluidized bed or directly into the fluidized bed.

Brief description of the drawing

Several exemplary embodiments of the invention are shown schematically in the drawing.

Show it:

1: a partial longitudinal section of a plant for the thermal treatment of waste in a first embodiment of the invention, in which a combustion grate is used as the first stage;

2 shows a partial longitudinal section of a plant for the thermal treatment of waste in a second embodiment of the invention, in which a fluidized bed is used as the first stage;

3 shows a partial longitudinal section of a plant for the thermal treatment of waste in a third embodiment variant of the invention, in which a combustion grate is used as the first stage and a fluidized bed is used as the afterburning zone;

4: a partial longitudinal section of a plant for the thermal treatment of waste in a fourth embodiment of the invention, in which the first stage 7

a combustion grate and a fluidized bed are used as a steady zone;

5 shows a partial longitudinal section of a plant analogous to FIG. 3, in which the afterburning zone is a circulating fluidized bed.

Only the elements essential for understanding the invention are shown. The direction of flow of the media is indicated by arrows.

Way of carrying out the invention

The invention is explained in more detail below with reference to several exemplary embodiments and FIGS. 1 to 5.

Figure 1 shows schematically part of a plant for the thermal treatment of solids, for. B. garbage or coal in a first embodiment of the invention. In the present exemplary embodiment, waste is to be used.

A grate 2 is arranged in the lower part of a boiler 1, of which only the first train is shown and whose further radiation trains and its convective part are not shown in FIG. 1. In the waste incineration plant shown, a medium-current grate combustion is implemented, i. H. the afterburning chamber 14 is arranged in the middle above the grate 2.

The solids 3, in this case waste, are charged into the boiler 1 and come to rest on the grate 2. Primary air 4 is blown through the grate 2 from below. Since only a small proportion of primary air 4 is supplied, only partial combustion or gasification of the waste takes place in this first process stage 5 because of the lack of air or oxygen. It arise in this first 8th

Stage 5 CO-containing and 0 ₂ -low exhaust gases 6, which subsequently flow into a mixing zone 7. The exhaust gas 6 emerging from the first stage 5 is actively homogenized in this mixing zone 7.

For the purpose of homogenization, at least one almost oxygen-free or low-oxygen gaseous medium 8 is added to the mixing zone 7. In the present exemplary embodiment, water vapor 9 and, on the other hand, recirculated flue gas 10 are added as medium 8. Nitrogen or other inert gases and air with a reduced oxygen content are also suitable for homogenizing the exhaust gas 6 of the first stage 5. It is sufficient if one of these media 8 is introduced into the mixing zone 7, but of course mixtures between these different media 8 are also suitable. 1, the gaseous medium 8 is injected into the mixing zone 7 approximately perpendicular to the flow direction of the exhaust gases 6 in this exemplary embodiment.

An even more intensive mixing and homogenization is achieved if the medium 8 is added at an angle opposite to the flow direction of the exhaust gases 6 from the first process stage 5. It is also possible to add the medium 8 at an angle equal to the flow direction of the exhaust gases 6 from the first process stage 5. An increased overpressure of the medium 8 also improves the homogenization effect.

In the present example, the mixing zone 7 is characterized by changes in the cross section of the walls of the boiler 1, ie changes in the cross section 11 of the flow channel. These cross-sectional changes can be both narrowing and widening of the flow channel. The cross-sectional changes 11 support the exhaust gas homogenization. 9

Furthermore, in the present exemplary embodiment, according to FIG. 1, additional internals 12 (static mixers) are arranged in the mixing zone 7, which ensure a flow deflection of the exhaust gases 6 and thus further mixing and active homogenization of the exhaust gases 6. The static mixer 12 have cavities (not shown in the figure), which with coolant, for. B. air, water or water vapor.

Of course, in other embodiments, the above. Various technical means (addition of a gaseous, almost oxygen-free medium, built-in elements in the gas flow, changes in cross-section of the flow channel) can also be used alternatively for the homogenization of the exhaust gases 6 from the first stage 5.

The homogenized CO-rich exhaust gas emerging from the mixing zone 7 then passes into a steady-state zone 13 in which there is also a lack of oxygen, that is to say a substoichiometric air ratio is present. In the steady-state zone 13, part of the NO already formed from the furnace is reduced to N ₂ in the presence of CO, NH, and HCN. It is of central importance for the invention that the residence time of the homogenized exhaust gases in the steady-state zone 13 is at least 0.5 seconds. At a normal exhaust gas velocity of approximately 4 m / s, this means that the steady-state zone must be at least approximately 2 m long.

The exhaust gas then flows from the steady-state zone into the post-combustion stage 14. There, an oxygen-containing medium 15, for example air (secondary air), is mixed in, so that a complete burnout of the exhaust gas is ensured.

The process according to the invention for the graded thermal treatment of solids is characterized by simple process steps and by a 10

compared to the known prior art reduced content of NOx emissions. In contrast to the known state of the art, the mixing and homogenization of the gas 6 emerging from the first stage 5 does not take place in the post-combustion zone by means of secondary air, but in an additional mixing stage 7 before the actual post-combustion, with mixing of the exhaust gases 6 and the feed a steady-state zone 13 for the exhaust gas in the absence of oxygen is inserted into the burnout air 15, in which the gases must remain for at least 0.5 seconds. In this way, both pollutant emission values are reduced and complete burnout is achieved.

Furthermore, with the method according to the invention it is very easy to regulate the temperature of the exhaust gases in the area of the injection of the oxygen-containing medium 15 by simply varying the amount of the medium 8 fed into the mixing zone 7 and adapting it to the prevailing operating conditions.

2 shows a further exemplary embodiment of the invention, which differs from the first exemplary embodiment only in that a fluidized bed 16 is used instead of the combustion grate in the first process stage 5. The waste 3 is burned sub-stoichiometrically in the fluidized bed 16, a very good material and heat exchange advantageously taking place and local temperature peaks being prevented. The mixing and homogenization of the gas 6 emerging from the fluidized bed 16 (first stage 5) also takes place, as in the first exemplary embodiment, in the subsequent mixing zone 7, into which a gaseous, virtually oxygen-free or low-oxygen medium 8, for. B. water vapor 9 or rezirkIERT.es exhaust gas 10 is introduced and also static internals 12 are arranged which cause a deflection of the exhaust gases 6 and thus an active mixing and homogenization. The homogenized CO-rich exhaust gas emerging from the mixing zone 7 arrives 1 1

then in a steady-state zone 13 in which there is also a lack of oxygen. In the steady-state zone 13, part of the NO already formed is removed from the furnace in the presence of CO, NH; and HCN reduced to N ₂ . The exhaust gas then flows from the steady-state zone 13 into the post-combustion stage 14. There, an oxygen-containing medium 15, for example air, is mixed in, so that a complete burnout of the exhaust gas is ensured.

FIG. 3 shows an exemplary embodiment in which, in contrast to the example shown in FIG. 1, the afterburning zone 14 is designed as a fluidized bed 16. The oxygen-containing gaseous medium 15 is introduced either directly into the fluidized bed 16 or at the inlet into the fluidized bed 16. These two alternatives are shown in FIG. 3. By designing the afterburning zone 14 as a fluidized bed 16, local hot zones with high thermal NOx formation are avoided due to the increased heat transfer due to the presence of particles. In addition, caking on heat exchanger walls can be prevented and corrosion on heat exchanger surfaces can be considerably reduced. Higher steam pressures and temperatures can also be set, which enable a higher thermal efficiency of the incineration plant.

4 shows a partial longitudinal section of a plant for the thermal treatment of waste in a fourth embodiment variant of the invention, in which a combustion grate 2 is used as the first stage and a fluidized bed 16 is used as the persistence zone 13. In contrast to FIG. 1, the mixing zone 7 in this exemplary embodiment is characterized by a cross-sectional expansion. In the case of the homogenized exhaust gas emerging from the mixing zone 7, an intensive material and heat exchange then advantageously takes place in the fluidized bed 16 (steady-state zone 13). 12

Finally, FIG. 5 shows a further embodiment variant, which differs from FIG. 3 only in that the fluidized bed 16 in the afterburning stage 14 is a circulating fluidized bed in which the empty pipe speed in the riser pipe is increased. The fluidized material is discharged into a cyclone and then returned to the fluidized bed. In the circulating fluidized bed, the average vertical gas velocity in the riser pipe is higher than in the classic fluidized bed, and the average relative velocity between gas and particles also increases. This leads to an increased heat and mass exchange between gas and particles and thus to a reduced temperature and concentration distribution. In addition, with the use of an external fluid bed cooler, the amount of heat removed from the fluidized bed can be varied and the fluidized bed temperature and the temperature at the end of the afterburning zone can be set well.

Of course, the invention is not limited to the exemplary embodiments described. In another exemplary embodiment, for example, the steady-state zone 13 can also be designed as a circulating fluidized bed or a grate system with countercurrent firing is used.

13

Reference list

Boiler rust solids, e.g. B. Waste primary air first process stage exhaust gas from item 5 mixing zone oxygen-free or low-oxygen gaseous medium water vapor recirculated exhaust gas cross-sectional changes of the flow channel internals / static mixer steady-state post-combustion stage oxygen-containing gaseous medium fluid bed

Claims

14 Patent claims

1. Process for the thermal treatment of solids (3), in particular waste, in which the solids (3) are burned / gasified or pyrolyzed in a first stage (5) in the absence of oxygen and the exhaust gases (6) of the first stage (5) subsequently in a post-combustion stage (14) are mixed with an oxygen-containing gaseous medium (15) and burned with complete burnout, characterized in that, for the purpose of NOx reduction, the exhaust gases (6) emerging from the first stage (5) are mixed before being mixed with the oxygen-containing medium (15) are actively homogenized in a mixing stage (7) with the addition of a gaseous oxygen-free or low-oxygen medium (8) into the mixing stage (7) and the homogenized oxygen-poor exhaust gas stream emerging from the mixing stage (7) before the addition of the complete Burnout required oxygen-containing medium (15) passes through a persistence zone (13), the residence time in the persistence zone ( 13) is at least 0.5 seconds.

2. The method according to claim 1, characterized in that recirculated exhaust gas (10) is used as the gaseous medium (8). 15

3. The method according to claim 1, characterized in that water vapor (9) is used as the gaseous medium (8).

4. The method according to claim 1, characterized in that oxygen-depleted air is used as the gaseous medium (8).

5. The method according to claim 1, characterized in that inert gas, preferably nitrogen, is used as the gaseous medium (8).

6. The method according to claim 1, characterized in that the active homogenization of the exhaust gases (6) emerging from the first stage (5) takes place with the aid of internals (12) installed in the mixing zone (7).

7. The method according to claim 6, characterized in that the internals (12) are flowed through by a cooling medium, preferably water, steam or air.

8. The method according to claim 1, characterized in that the active homogenization of the exhaust gases (6) emerging from the first stage (5) is carried out by narrowing or widening the cross section (11) of the flow channel in the mixing zone (7).

9. The method according to any one of claims 1 to 8, characterized in that the temperature of the exhaust gases in the region of the injection of the oxygen-containing medium (15) is regulated via the amount of the medium (8) fed into the mixing zone (7).

10. The method according to any one of claims 1 to 9, characterized in that in the persistence zone (13) the exhaust gases are substoichiometric 16

Have air ratio.

11. The method according to any one of claims 1 to 10, characterized in that a grate system (2) with medium-current grate firing is used as the first stage (5).

12. The method according to any one of claims 1 to 10, characterized in that a grate system (2) with counter-current grate firing is used as the first stage (5).

13. The method according to any one of claims 1 to 10, characterized in that a fluidized bed (16) is used as the first stage (5).

14. The method according to any one of claims 1 to 13, characterized in that the post-combustion stage (14) is a fluidized bed (16) and that the oxygen-containing gaseous medium (15) either the flue gas (6) at the entry into the fluidized bed (16) or is fed directly into the fluidized bed (16).

15. The method according to any one of claims 1 to 13, characterized in that the steady-state zone (13) is a fluidized bed (16) and that the oxygen-free or low-oxygen gaseous medium (8) either the flue gas (6) at the entry into the fluidized bed (16 ) or directly into the fluidized bed (16).

16. The method according to claim 15 or 15, characterized in that a circulating fluidized bed is used as the fluidized bed (16).