EP0745807A1 - Steam boiler - Google Patents

Abstract

The steam generator (1) is inserted into a circuit for the direct, selective catalytic reduction of nitrogen oxide in flue gas (19). The economiser is in two parts between which (2a,2b) is the nitrogen oxide catalytic converter (5). The economiser (2a) positioned in front of the catalytic converter in the flow direction of the gas is divided into at least two sections (10a,10b) through which flow in parallel flue gas and heatable working substances. At least one section (10a) is constantly connected to a drum (13) by a pipe (11a,12). The other sections are sealed off from the water circulation by pipes (11b) which can be blocked by shut-off devices (14).

Description

Technical field

The invention relates to a steam generator with a radiation part downstream of the combustion chamber and an adjoining convective part, the latter consisting essentially of a contact heat exchanger, superheater and economizer connected in series on the flue gas side, which is used for direct selective catalytic reduction (SCR process) of nitrogen oxides (NOx) in the exhaust gas of waste incineration plants (KVA) is used, whereby the NOx catalytic converter is fed directly with the hot flue gases, i.e. it is arranged in the circuit in front of the scrubber.

State of the art

The NOx emissions from thermal waste incineration plants must not exceed legally prescribed values. To reduce NOx emissions, which are generally between 300 and 450 mg / m ³ , it is known that primary firing measures and / or more effective exhaust-side secondary measures are used, whereby the secondary measures are the SNCR process (selective non-catalytic reduction) and the SCR -Procedures (selective catalytic reduction) are available.

In the SNCR process, the NOx reduction takes place thermally by injecting the reducing agent (ammonia or urea) into the furnace or boiler part in a temperature range of around 900 to 1100 ° C.

With the SCR process, however, the nitrogen oxides are converted to nitrogen and water vapor at a much lower temperature with the addition of ammonia water on a catalyst. According to the current state of the art, it is only possible with a catalytic process to reduce the NOx emissions to values <100 mg / m ³ .

According to the known prior art, there are various possibilities for switching the catalyst stage. For example, Boiler for waste incineration plants equipped with NOx catalysts, which are usually used after the scrubber. On the one hand, this has the advantage that the risk of catalyst poisoning or blocking by dust and sulfur dioxide is reduced, but on the other hand the disadvantage that the flue gases have to be reheated before entering the catalyst.

Therefore, the NOx catalyst is provided in front of the scrubber in newer circuits. It is then fed directly with the hot flue gases, so that the exhaust gas does not have to be reheated after washing. With prior dedusting ("direct low dust" switching) to residual dust contents below 10 mg / Nm ³ , the catalytic converters achieve a service life similar to that in the switching systems after exhaust gas scrubbing. The electric filter (e-filter) for dedusting can also be arranged after the NOx catalyst ("direct high dust" circuit).

In order for the catalytic converter to work optimally and as long as possible, it is necessary to keep the gas temperature upstream of the NOx catalytic converter as constant as possible at a predetermined value, for example 350 ° C. The optimal operating temperature the catalyst is at 320 to 350 ° C (KJ Thomé-Kozmiensky: Thermal waste treatment. EF-Verlag für Energie- und Umwelttechnik GmbH, 2nd edition, 1994, pp. 555-557). This range can be even larger depending on the catalyst used, for example a catalyst operating at an operating temperature of 280 ° C. is known in a waste incineration plant.

With the prior art known to date, however, an approximately constant gas temperature is not possible in the various operating states. For example, the gas temperature in a conventional KVA boiler has the following values in two different operating cases: Operating case Gas temperature in ° C after Superheater Evaporator Economizer Dirty at full load 461 343 237 Part load clean 370 290 190

There are therefore considerable differences in the level of the flue gas temperature (here approx. 50 ° C after the economizer), which has an unfavorable effect when the NOx catalyst is charged directly with the hot flue gases.

Presentation of the invention

The invention tries to avoid all these disadvantages. It is based on the task of developing a steam generator which can be used for SCR process circuits in which the NOx catalyst is fed directly with the hot flue gases upstream of the scrubber, with relatively little effort the gas temperature upstream of the catalytic converter can be kept at an approximately constant, predetermined value.

According to the invention, this is achieved in a steam generator according to the preamble of claim 1 in that the economizer is constructed in two parts and a NOx catalyst is arranged between the two parts, the economizer arranged in the flow direction of the gas upstream of the catalyst being divided into at least two sections, which are flowed through one after the other on the flue gas side and on the other hand through the working medium to be heated up, at least one section being permanently connected to the drum via a line and the other section (s) optionally being lockable from the water circuit via lines which can be shut off.

According to the invention, this is achieved in a method for operating the steam generator in that the temperature of the flue gases is measured immediately before they enter the NOx catalytic converter and a number of shut-off elements in the lines, which is dependent on the level of this temperature, is closed. As a result, one or more sections of the economizer in front of the catalytic converter are closed off from the water circuit and part of the heating surface becomes inactive.

The advantages of the invention can be seen, inter alia, in the fact that the regulation of the gas temperature upstream of the NOx catalytic converter is relatively easy to handle and that the NOx catalytic converter works optimally and has a long service life by ensuring an approximately constant admission temperature. The invention can be applied to both "direct-low dust" and "direct-high dust" circuits, ie the e-filter can be arranged either before or after the economizer.

It is particularly expedient if the economizer in front of the catalytic converter is designed so large that in the "full load dirty" mode of operation the inlet temperature of the flue gas into the catalytic converter is less than / equal to the catalytic converter's operating temperature.

Furthermore, it is advantageous if the components upstream of the economizer in front of the catalytic converter, such as superheaters, protective bundles, empty trains, are designed so that, in the "partial load clean" operating mode, the inlet temperature of the flue gas into the economizer in front of the catalytic converter is greater than / equal to the operating temperature of the catalyst.

Brief description of the drawing

In the drawing, exemplary embodiments of the invention are shown using a boiler for a waste incineration plant.

Fig. 1

drei Schaltungssschemata von Kehrrichtverbrennungsanlagen mit SRC-Verfahren nach dem Stand der Technik

• I: konventionell
• II: direkt-low dust
• III: direkt-high dust;

Fig. 2

das erfindungsgemässe Schaltungsschema einer Kehrrichtverbrennungsanlage mit SCR-Verfahren (direkt-high dust);

Fig. 3

eine detailliertere Darstellung eines Teils von Fig. 2 im Bereich des Kessels, des NOx-Katalysators und des Economizers;

Fig. 4

eine schematische Darstellung der Erfindung im Bereich des Kessels, des E-Filters, des NOx-Katalysators und des Economizers (direkt-low dust-SCR-Verfahren).

Show it:

Fig. 1

three circuit diagrams of waste incineration plants with SRC processes according to the prior art

• I: conventional
• II: direct-low dust
• III: direct-high dust;

Fig. 2

the circuit diagram according to the invention of a waste incineration plant with SCR (direct high dust) method;

Fig. 3

a detailed representation of a part of Figure 2 in the area of the boiler, the NOx catalyst and the economizer.

Fig. 4

is a schematic representation of the invention in the area of the boiler, the E-filter, the NOx catalyst and the economizer (direct low dust SCR process).

Only the elements essential for understanding the invention are shown. The system does not show, for example, the loading of the boiler, the combustion system and the wet washing system. The direction of flow of the work equipment is indicated by arrows.

Way of carrying out the invention

The invention is explained in more detail below on the basis of exemplary embodiments and FIGS. 1 to 4.

For a better understanding of the invention, three SCR circuits known from the prior art are initially shown in FIG. 1 with the temperature level of the gas which can be achieved in each case after the individual treatment steps. Part I shows a circuit in which the apparatus boiler 1 / economizer 2, e-filter 3, scrubber 4, NOx catalytic converter 5 and cooler 6 are arranged in the order in which they flow through, due to the low temperature of the flue gas after the scrubber 4 (eg 70 ° C) the gas must be heated again before entering the NOx catalyst 5 (eg to 350 ° C). This heating does not apply to the "low dust" circuit shown in Part II, in which the units are arranged in the order boiler 1, electric filter 3, NOx catalytic converter 5, economizer 2 and scrubber 4, as well as that in part III shown "high dust" circuit with an arrangement in the order boiler 1, NOx catalyst 5, economizer 2, e-filter 3 and scrubber 4th

Since, in the last two cases, no constant gas temperature can be guaranteed upstream of the NOx catalytic converter 5 for different operating states, the method according to the invention becomes Solution, of which an embodiment variant is shown in FIGS. 2 and 3, applied. The basis is that the steam generator 1 has a two-part economizer 2. This consists of a part 2a, which is arranged on the gas side in front of the NOx catalytic converter 5 and a part 2b, which is arranged after the NOx catalytic converter 5. According to the circuit arrangement according to FIG. 2, a filter 3 and then a scrubber 4 are then arranged in the order in which the gases flow. The temperature upstream of the NOx catalytic converter is approximately constant for different operating states (350 ° C. in the exemplary embodiment shown), it can differ by +/- 10 ° C.

FIG. 3 shows a more detailed schematic illustration of the steam generator according to the invention as used in the high dust circuit according to FIG. 2. Above a combustion chamber 7, two vertical empty trains 8 are arranged, which form the radiation part of the steam generator. In the subsequent horizontal part of the steam generator, a superheater 9 and an economizer 2, which is divided into two main parts 2a and 2b, are arranged in the order in which they flow through, the NOx catalyst 5, which is used for the selective catalytic reduction, between the two parts 2a, 2b the nitrogen oxides is needed.

The economizer in front of the NOx catalytic converter 2a is divided into a plurality of separate sections 10 (here 4 sections), through which the gas flows in succession while the working medium flows through them, ie in parallel from bottom to top. These parallel connecting lines 11 finally open into a line 12 which is connected to the drum 13. With one exception, in all lines 11 connected in parallel, a shut-off device 14, for example a valve, is arranged after the individual sections 10 of the economizer in front of the NOx catalytic converter 2a, so that these sections are optionally closed off from the water cycle can be, while a section 10 of the economizer in front of the catalytic converter is connected to the drum 13 in any case, ie also when all other sections 10 are shut off.

The economizer in front of the catalytic converter 2a is designed so that partial evaporation can occur. It is designed so large that in the "full load dirty" operating case the inlet temperature of the flue gas into the catalytic converter 5 is less than / equal to the operating temperature of the catalytic converter 5.

The components upstream of the economizer in front of the catalytic converter 2a in the steam generator 1, such as superheaters 9, contact heat generators 18 (“protective bundles” which are first acted upon by the flue gas), idle trains 8, are designed such that the operating temperature of the flue gas is “partial load clean” in the economizer before the catalytic converter is greater than / equal to the operating temperature of the catalytic converter.

A temperature measuring element 15 is arranged after the last section 10 in the gas flow direction. The second part of the economizer 2b, which is located on the gas side after the catalytic converter 5, is essentially realized in a counterflow circuit. A pump 16 pumps water via line 17 into part 2b of the economizer, which is arranged after the NOx catalytic converter 5. The water cools the denitrified flue gases that emerge from the catalytic converter 5 before they are dedusted in the filter 3 (not shown here) and fed to the scrubber 4. The water is then conducted past the catalyst 5 in parallel into the sections 10 which are flowed through from bottom to top, with a further heat exchange taking place with flue gas which is even hotter here. In order to ensure an approximately constant inlet temperature of the flue gases into the NOx catalytic converter, the flue gas temperature is measured by means of the temperature measuring element 15. Depending on the level of this temperature, the flue gas temperature can be influenced by closing or opening the respective one Shut-off devices 14 individual sections 10 of the economizer in front of the catalytic converter 2a are closed off or reconnected from the water cycle. This changes the active heating surface. The SCR process itself then runs according to the known prior art.

Of course, the invention is not limited to the embodiment just described. It can also be implemented, for example, in a steam generator with a vertical convective train.

4 schematically shows a steam generator according to the invention for the SCR low dust process. In contrast to FIG. 3, an e-filter 3 is arranged here between the economizer in front of the catalytic converter 2a and the NOx catalytic converter 5. In addition, this exemplary embodiment shows that the economizer after the catalyst 2b can also be designed with a vertical gas flow. In a further variant, not shown, the economizer is also arranged after the catalyst 2b at a greater spatial distance from the NOx catalyst.

Reference list

1

boiler

2nd

Economizer

2a

Part of the economizer in front of the NOx catalytic converter

2 B

Part of the economizer after the NOx catalyst

3rd

E filter

4th

washer

5

NOx catalyst

6

Cooler

7

Firebox

8th

Empty train

9

Superheater

10th

Sections from item 2a

10a

Section not lockable from the water cycle

10b

Section lockable from the water cycle

11

parallel connection lines

11a

Connection line without shut-off device

11b

Connection line with shut-off device

12th

management

13

drum

14

Shut-off device

15

Temperature measuring element

16

pump

17th

management

18th

Touch heat generator

Claims (9)

Steam generator (1) with a radiation part downstream of the combustion chamber (7) and an adjoining convective part, the latter consisting essentially of a contact heat exchanger (18) connected in series on the flue gas side, superheater (9) and economizer (2), the steam generator (1) in a circuit for the direct selective catalytic reduction (SCR process) of the nitrogen oxides in the flue gas (19) is used, characterized in that the economizer (2) is constructed in two parts and between the two parts (2a, 2b) there is a NOx catalyst ( 5), the economizer (2a) arranged upstream of the catalytic converter (5) in the flow direction of the gas (19) being subdivided into at least two sections (10a, 10b), which are flowed through one after the other on the flue gas side and flowed through by the working medium to be heated in parallel, wherein at least one section (10a) is continuously connected to the drum (13) via a line (11a, 12) and di e other section (s) (10b) can be optionally locked from the water circuit via lockable lines (11b). Steam generator according to claim 1, characterized in that in the lines (11b), which of the sections (10b) of the economizer upstream of the catalytic converter (2a), which can be closed by the water circuit, into the line (12) lead to the drum, a shut-off device (14) is arranged. Steam generator according to claim 1 or 2, characterized in that the economizer in front of the catalytic converter (2b) is designed so large that in the "full load dirty" mode of operation the inlet temperature of the flue gas (19) into the catalytic converter (5) is less than / equal to the operating temperature of the Catalyst (5). Steam generator according to claim 1 or 2, characterized in that the components upstream of the economizer upstream of the catalytic converter (2a) in the steam generator (1), such as superheater (9), contact heat generator (18), empty trains (8), are designed such that in "Part load clean" operating case, the inlet temperature of the flue gas (19) into the economizer upstream of the catalytic converter (2a) is greater than / equal to the operating temperature of the catalytic converter (5). Steam generator according to claim 1, characterized in that an e-filter (3) is arranged between the economizer in front of the catalytic converter (2a) and the NOx catalytic converter (5). Steam generator according to claim 1, characterized in that the economizer after the catalyst (2b) is flowed through essentially in a counterflow circuit. Steam generator according to claim 1, characterized in that the economizer after the catalyst (2b) is flown vertically by the flue gas (19). Steam generator according to claim 1, 6 or 7, characterized in that the economizer is arranged after the NOx catalyst (2b) spatially distant from the NOx catalyst (5). Method for operating a steam generator according to one of claims 1 to 8, characterized in that the temperature of the flue gases (19) is measured immediately before they enter the NOx catalyst (5) and a number of shut-off elements (depending on the level of this temperature) 14) is closed in the lines (11b).

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