WO2000032987A1

WO2000032987A1 - Method for operating a steam generator and steam generator for carrying out this method

Info

Publication number: WO2000032987A1
Application number: PCT/DE1999/003654
Authority: WO
Inventors: Hans-Gerd Brummel; Siegfried BÄHR; Sergej Reissig; Kurt Kirstein
Original assignee: Siemens Aktiengesellschaft
Priority date: 1998-12-02
Filing date: 1999-11-17
Publication date: 2000-06-08
Also published as: ID28804A; BR9915876A; DE19855670A1; EE200100300A; EP1135650A1

Abstract

The invention relates to a steam generator (1) having high steam efficiency, comprising a combustion chamber (8), a chimney (30) and an overheater (12), which has several heating surfaces (18). A flue gas flows through the combustion chamber (8), the overheater (12) and the chimney (30). The aim of the invention is to achievearticularly high efficiency and to avoid pollution and slagging during the operation of said steam generator (1). To this end, the flue gas temperature (T1) which has an effect on the heating surfaces (18), which are positioned first as seen in the flow direction of the flue gas, is adjusted in such a way that the softening temperature for ash particles contained in the flue gas is not reached and that fresh air (F) having a fresh air temperature (T2) reduced of at least T2 = 15K is supplied to the combustion chamber (8).

Description

description

Method for operating a steam generator and steam generator for carrying out the method

The invention relates to a method for operating a steam generator with a combustion chamber and a superheater, which has a number of heating surfaces, wherein a medium flows through the combustion chamber and the superheater. It also relates to a steam generator for performing the method.

In a steam power plant, the energy content of a fuel is converted into a technologically usable form of energy. By burning the fuel, a hot gas is generated as the working medium, the heat content of which is used to completely or partially evaporate and / or overheat water in an evaporator. The steam generated in this way is fed to a steam turbine, where it relaxes and in the process transfers energy to a generator system.

The fossil fuels oil shale or coal dust or garbage are used as fuel. Common to these fuels is that they release ash particles when they are burned. In addition, the flame generated during combustion emits a considerable heat flow to the combustion chamber wall due to radiation due to particularly high combustion temperatures. The flame radiation is largely determined by the proportion of the triatomic gases C0 ₂ and H ₂ 0 and by the proportion of

Soot, coal dust or ash particles. The flame radiation enables the heat released by the combustion to be transferred to the combustion chamber wall. Further downstream, the remaining flue gas heat is transferred by convection to heat exchangers arranged in the boiler, eg superheater, reheater or economizer. The effectiveness of the combustion process, ie the size of the heat flow emitted during combustion, is largely determined by the combustion temperature. High combustion temperatures lead to particularly high heat transfer, which improves thermal efficiency. The disadvantage here is that these high combustion temperatures result in sticky ash particles from the minerals contained in the fuel. Such softening of the ash particles, particularly in the area of the combustion chamber wall and in the entrance area of the heating surfaces of the superheater, however results in a steadily growing glass or sinter-like coating or coating on the combustion chamber wall or on the heating surfaces, which is reinforced by the fact that by obstruction of the Heat transfer in the combustion chamber continues to increase the smoke gases.

From DE 37 12 801 a method for operating a steam generator is known, in which, when an upper limit of the flue gas temperature is exceeded, the flue gas temperature acting on the first heating surfaces seen in the flow direction of the flue gas stream is set such that the softening temperature for ash particles contained in the flue gas stream is set remains below.

For this reason, cleaning systems, for example so-called soot blowers or water lances, are usually provided to reduce the contamination and / or slagging of the combustion chamber wall and the heating surfaces. The steam generator is cleaned in so-called cleaning intervals, which can be traced back to the experience of plant operation. The effectiveness of these cleaning systems is limited with regard to the removal of glass or sinter-like coatings or deposits. However, these coatings generally result in a reduction in the thermal output of the steam generator and additionally lead to an increase in the flue gas temperature and thus to consequential damage to the heating surface due to temperature shifts in the steam generator which inevitably occur due to the coatings. chen. This in turn can lead to undesirably short cleaning intervals with correspondingly high losses in efficiency or even to the replacement of heavily soiled or corroded heating surfaces and thus to a shutdown and longer shutdown of the steam generator.

The invention is therefore based on the object of specifying a method for operating a steam generator in which the contamination and / or slagging of the combustion chamber wall and the heating surfaces is reliably avoided while achieving a particularly high degree of efficiency.

With regard to the method, the object is achieved according to the invention by the features of claim 1. To this end, when an upper limit value for the flue gas temperature in the chimney and / or a lower limit value for the steam output is undershot, the flue gas temperature acting on the first heating surfaces as seen in the flow direction of the flue gas stream is determined set such that the softening temperature for ash particles contained in the flue gas stream remains below.

The invention is based on the consideration that when the steam generator is in operation, safe and reliable cleaning of the combustion chamber walls and the heating surfaces should be ensured with a particularly high thermal efficiency by suitably fulfilling two conditions which contradict each other. On the one hand, the temperature of the flue gas and consequently the combustion temperature should be chosen as low as possible. It can thereby be achieved that the solidified ash particles are applied to the combustion chamber wall and the heating surfaces instead of softened ash particles. This leads to a particularly simple removal of the contamination occurring on the heating surfaces and the combustion chamber wall, as a result of which the heat transfer is sustainably improved, which in turn leads to an increase in the steam output. On the other hand, the temperature of the flue gas and the combustion temperature are generally chosen to be as high as possible in boiler technology in such a way that a particularly high thermal efficiency of the steam generator is achieved. An essential influencing variable for achieving the high thermal efficiency is a high temperature of fresh air to be supplied from the combustion process in order to achieve the required high combustion temperature. There is a connection between the efficiency, which is influenced by the combustion temperature, which largely determines the flue gas temperature, and the degree of contamination of the combustion chamber and the heating surfaces. Contrary to the general trend towards increasing the combustion temperature, the invention thus proposes to reduce and, in particular, to make it more uniform for existing, particularly contaminated steam generators.

Expediently, fresh air with a fresh air temperature reduced by at least 15K is supplied to the combustion chamber. For example, the fresh air temperature is based on a value less than T2 = 280 ° C. The fresh air temperature, which is usually well above 300 ° C. in the prior art, can be reduced to a lower value of approximately T2 = 50 ° C. depending on the ignition parameters of the fuel used. Another criterion for determining a particularly favorable fresh air temperature is that the most uniform possible combustion is achieved with a given fuel. Furthermore, the lowering of the fresh air temperature primarily affects the thermal conditions in the combustion chamber and in the steam generator. This means that the lowering of the fresh air temperature leads to a lowering of the combustion temperature, which in turn leads to a lowering of the flue gas temperature. The fresh air temperature is reduced to such an extent that in particular the flue gas temperature in the area or in the vicinity of the combustion chamber wall and / or the first heating surfaces has a value which is below the softening point of the ash particles. For this purpose, the air preheater is modified, for example, or the fresh air warmed to the fresh air temperature in the air preheater is mixed with colder fresh air. As an alternative or in addition, the preheating of the fresh air can be reduced by further system-technical measures.

The flue gas temperature at the combustion chamber outlet is expediently reduced by at least 15K. The amount of the reduction depends in particular on the deviation of the steam output and / or the flue gas temperature in the chimney from the specified lower or upper limit. That the higher the deviation the higher the reduction in the flue gas temperature at the combustion chamber outlet. The deviations in the steam output and / or the flue gas temperature in the chimney - largely determined by the degree of contamination of the steam generator - can be reliably detected. This enables the flue gas temperature at the combustion chamber outlet to be adapted in a particularly simple manner by means of appropriate system-technical measures.

For example, for coal dust or waste as fuel for the flue gas temperature at the outlet of the combustion chamber, a value of approximately Tl = 1200 ° C is used. For oil shale as fuel, on the other hand, the flue gas temperature at the outlet of the combustion chamber is based on a value of around Tl = 1000 ° C. Taking these boundary conditions into account, the setting mentioned results in a particularly favorable condition of the ash particles, which enable particularly easy cleaning of the combustion chamber wall and the heating surfaces and thus avoid contamination and / or slagging of the heating surfaces and combustion chamber walls. This also leads to an increase in cleaning intervals. These measures extend the service life of the heating surfaces, reduce the high-temperature corrosion of the heating surfaces and reduce the internal requirements that are required for cleaning the steam generator. The ash softening temperature, which is the level of the maximum permissible flue gas temperature at the outlet of the Combustion chamber determined, depends primarily on the ash composition.

In an advantageous further development, the fresh air is divided into at least two partial flows which are fed to the combustion chamber, for example at different heights. As a result, the fresh air is supplied to the combustion chamber in several stages, which on the one hand leads to an increase in the flame, whereby hot flame cores are reliably avoided. On the other hand, the staged supply of fresh air also leads to an equalization of the flames, which in particular significantly reduces the thermal nitrogen oxide formation during the combustion process.

In an advantageous embodiment, as seen in the flow direction of the flue gas, a portion of the flue gas flow is branched off after the superheater and fed to the combustion chamber on the inlet side. Such a recirculation of the flue gas flow also lowers the combustion temperature.

In a further advantageous embodiment, feed water with a feed water inlet temperature reduced by at least 10K is fed to a preheater (hereinafter referred to as economizer) which is connected downstream of the superheater. The setting of the feed water inlet temperature is significantly influenced by the heat content available in the flue gas. This measure is particularly necessary for steam generators that have a particularly long operating time. Compared to the original design of the steam generator with clean heating surfaces and a clean combustion chamber wall, older steam generators cause less heat to be transferred to the heating surfaces and combustion chamber wall comprising the evaporator tubes. Therefore, the exhaust gas temperature in the chimney increases, which reduces the thermal output of the steam generator. To avoid this, the feedwater inlet temperature of the economizer is adjusted to the desired value lowered. For example, the number of preheaters upstream of the economizer is reduced. The shutdown of preheaters, which are fed from a steam turbine downstream of the steam generator, leads to an increase in the power of the steam turbine compared to the actual state, which increases the overall efficiency of a steam turbine system, which comprises a steam generator with the design described above. As an alternative or in addition, the heating surface size of the economizer can be adapted to further reduce the feed water inlet temperature as a function of the remaining heat content of the flue gas.

The second-mentioned object is achieved according to the invention by a steam generator with a steam output, comprising a combustion chamber and a superheater, which has a number of heating surfaces, and a chimney, the combustion chamber and the superheater being traversed by a flue gas stream and the chimney by an exhaust gas stream, wherein if an upper limit value of the flue gas temperature in the chimney and / or a lower limit value for the

Steam output, the smoke gas temperature acting on the first heating surfaces as seen in the flow direction of the flue gas stream is set such that the softening temperature for the ash particles contained in the flue gas stream remains below. The flue gas temperature at the combustion chamber outlet is preferably reduced by at least 15K. A bypass line is connected in parallel to the air preheater. This results in a mixing of the fresh air warmed up in the air preheater and the colder fresh air bypassing the air preheater, so that the temperature of the mixed fresh air is reduced overall.

An air preheater for preheating the fresh air is advantageously connected upstream of the combustion chamber. For this purpose, the air preheater has a corresponding number of heating surfaces depending on the fresh air temperature to be set. In the case of steam generators to be refurbished, this can be done at one order at least 15K reduced fresh air temperature lead to a reduction in the number of heating surfaces of the air preheater that is already in operation, which is usually designed for a significantly higher fresh air temperature, more than 300 ° C.

In an advantageous embodiment, a number of feed lines connected in parallel in terms of flow are led into the combustion chamber. As a result, the fresh air can be supplied to the combustion chamber in a number of partial flows corresponding to the number of supply lines. This makes it possible in a particularly simple manner to enlarge the surface of the flame in the combustion chamber and thus to even out the combustion temperature.

In a further advantageous embodiment, a return line is provided downstream of the superheater, as seen in the flow direction of the flue gas, and is fed to the combustion chamber on the inlet side. In this way, already cooled flue gas is preferably fed to the combustion chamber at the lower end of the combustion chamber, so that the combustion temperature in the combustion chamber is reduced.

The advantages achieved by the invention consist in particular in that, by setting the flue gas temperature of the steam generator to a predetermined value in the area of the combustion chamber wall and in the entrance area of the heating surfaces while avoiding contamination, in particular slagging, heating surfaces and combustion chamber wall have a high effectiveness. degree of the steam generator is ensured. A steam generator designed in this way can be adapted particularly flexibly to varying requirements when using different types of fuel. This is the only way to ensure that when combusting fuels with the formation of ash particles, the special requirements for avoiding contamination and / or slagging by ash on the combustion chamber wall and the heating surfaces of the steam generator, in particular the Superheater is taken into account, but a uniform temperature distribution at the highest possible temperature level in the steam generator, which is required for high efficiency, can nevertheless be achieved.

Exemplary embodiments of the invention are explained in more detail with reference to a drawing. The figure shows a steam generator with a combustion chamber and a superheater in a schematic representation.

The steam generator 1, according to the figure, is connected via a feed water supply 2 and via a steam-side outlet 4 into the water-steam circuit of a steam turbine, not shown. The feed water supply 2 is arranged on a preheater or economizer 6 for heating the feed water S supplied to it. The steam generator 1 comprises a combustion chamber 8 for generating a flue gas R by burning a fuel B. The fuel B is preferably oil shale, coal dust or waste. The fuel B is fed to the combustion chamber 8 via a fuel line 10.

Furthermore, the steam generator 1 comprises a superheater 12 downstream of the combustion chamber 8 in the flow direction of the flue gas R, which superheater 12 is also connected to the water-steam circuit of the steam turbine (not shown) via its inlet 14 and its steam-side outlet 16. The superheater 12 comprises a number of heating surfaces 18 which are located on the outlet side of the combustion chamber 8, i.e. are arranged in the flue gas area of the steam generator 1. For cleaning the heating surfaces

18 and the wall of the combustion chamber 8, a number of cleaning systems 19 are provided, which are arranged at different heights in the combustion chamber wall.

Two feed lines 20, which are combined to form a common fresh air line 22, open into the combustion chamber 8. The fresh air line 22 is on the secondary side in one Switched heat exchanger or air preheater 24, which is connected on the primary side in the flue gas stream. In this case, a flap 21 is connected in the upper supply line 20, which closes this upper supply line 20 depending on the amount of fresh air to be supplied. The air preheater 24 thus serves to preheat fresh air F. A blower 26 is also connected to the fresh air line 22. The flue gas R is fed into a chimney 30 after the air preheater 24 with a flue gas temperature T0. A bypass line 32, in which a flap 33 is arranged, is also connected in parallel to the air preheater 24.

Seen in the flow direction of the flue gas R, a return line 34 is provided after the economizer 6, which is the

Combustion chamber 8 is fed on the input side. A flap 35 and a blower 36 are connected in the return line 34. Alternatively, the return line 34 can be branched off after the air preheater 24 or before the economizer 6. The determining factor for determining the branch position of the return line 34 is the flue gas temperature T1 present in this area and the combustion temperature to be achieved in the combustion chamber 8.

When the steam generator 1 is operating, the combustion chamber 8 is supplied with coal B, for example, as fuel B. In order to avoid ash softening and the resulting contamination of the heating surfaces 18 and the combustion chamber wall, the combustion of the fuel B is carried out in such a way that the flue gas temperature Tl of that generated during the combustion

Flue gas R in the entrance area of superheater 12 falls below the softening temperature of ash particles contained in the flue gas stream. If an upper limit value of the exhaust gas temperature T0 in the chimney 30 is exceeded and / or if the lower limit value for the steam output is not reached, the smoke gas temperature Tl is reduced by at least 15K. For example, the flue gas temperature Tl for a coal dust is approximately Tl = 1200 ° CDh. The first heating surfaces 18 of the superheater 12 seen in the flow direction of the flue gas stream are subjected to a flue gas temperature Tl of less than 1200 ° C. If the exhaust gas temperature TO deviates from the upper limit value, the flue gas temperature Tl can have, for example, a substantially higher value of approximately Tl = 1250 ° C. depending on the degree of contamination of the steam generator. The smoke temperature Tl must therefore be reduced by at least 50K.

For this purpose, the fuel B in the combustion chamber 8 is operated by admixing fresh air F with a fresh air temperature T2 reduced by at least 15K, for example with a value of less than T2 = 280 ° C. As a result, the combustion temperature T3 is reduced by more than 20K (usually significantly higher: between 50K and 200K). This in turn causes the flue gas temperature Tl to drop to the predetermined value in the area of the combustion chamber wall and / or in the area of the first heating surfaces 18

1200 ° C cooled flue gas R causes the softening temperature for the ash particles contained in the flue gas stream to remain below. As a result, excessive contamination or even slagging of the combustion chamber wall and the first heating surfaces 18 by sticky or melted ash particles as a result of the ash softening is reliably avoided.

Analogous to coal dust as fuel B, when using oil shale as fuel B, the flue gas temperature Tl is based on a value of approximately 1000 ° C. Depending on the type of fuel B supplied, the flue gas temperature Tl can assume intermediate values or can also be below the values mentioned.

A heat content of the flue gas R is transferred via the superheater 12 and the economizer 6 to feed water S fed therein or steam D conducted therein. This cools the flue gas R seen in the direction of flow. In addition, depending on the type of steam generator 1, further heat exchangers, for example high-pressure superheaters or intermediate reheaters, can be provided. A portion of the already cooled flue gas stream can be used in addition to the lowering of the fresh air temperature T2 and the lowering of the combustion temperature T3. For this purpose, the partial quantity of the cooled flue gas stream is fed to the combustion chamber 8 on the inlet side via the return line 34. The amount of the cooled flue gas R and the amount of fresh air F can be adjusted by means of the blowers 36 and 26 connected in the return line 34 and in the fresh air line 22, respectively.

A remaining heat content of the flue gas R is transferred to the combustion or fresh air F supplied to the combustion chamber 8 via the air preheater 24 for air preheating. In order to reduce the fresh air temperature by at least 15 K at the entrance to the combustion chamber 8, the fresh air F warmed up in the air preheater 24 is mixed with the cold fresh air F conducted via the bypass line 32 in the fresh air line 22. Alternatively, the number of heating surfaces arranged in the air preheater can be reduced instead of the bypass line 32.

The fresh air F is supplied to the combustion chamber 8 via two supply lines 20 connected in parallel in terms of flow technology. As a result, the fresh air F is divided into two partial flows which are led into the combustion chamber 8 at different locations. This causes the flame to grow and become more uniform, thereby avoiding hot flame cores.

In particular in the case of a steam generator 1 which has a particularly long operating time, there is less heat transfer to the heating surfaces 18 as a result of soiling of the heating surfaces 18, which results in an increase in the flue gas temperature T1 in the chimney 30. To avoid this is fed to the economizer 6 feed water S with a feed water inlet temperature T4 reduced by at least T4 = 10K compared to the original design. This is done, for example, by means of system-technical measures by reducing the number of preheaters required for preheating the feed water S. As a result, the heat content of the flue gas R is used much better. In addition, the output of the steam turbine is increased, for example, by switching off some preheaters which are tapped for feeding on the steam turbine, as a result of which the overall efficiency of the steam turbine system comprising the steam generator 1 is improved.

In a steam generator 1 designed in this way, the contamination and / or slagging of the combustion chamber wall and the heating surfaces 18 is thus very low, so that the maintenance intervals are lengthened and damage is also avoided. In addition, this steam generator 1 has a particularly long service life.

Claims

claims

1. A method for operating a steam generator (1) with a steam output, comprising a combustion chamber (8) and a superheater (12), which has a number of heating surfaces (18), and a chimney (30), in which the combustion chamber (8), the superheater (12) and the chimney (30) are flowed through by a flue gas stream, wherein when an upper limit value of the flue gas temperature (TO) in the chimney (30) and / or a lower limit value for the

Steam output the smoke gas temperature (Tl) acting on the first heating surfaces (18) seen in the flow direction of the flue gas stream is set such that the softening temperature for ash particles contained in the flue gas stream remains below and the combustion chamber (8) fresh air (F) with at least T2 = 15K reduced fresh air temperature (T2) is supplied.

2. The method according to claim 1, wherein the flue gas temperature (Tl) at the outlet of the combustion chamber (8) is reduced by at least Tl = 15K.

3. The method according to claim 1 or 2, wherein the fresh air (F) is divided into at least two partial streams, which the

Combustion chamber (8) are supplied.

4. The method of claim 3, in the flow direction of the flue gas (R) seen after the superheater (12)

Part of the flue gas flow is branched off and fed to the combustion chamber (8) on the inlet side.

5. The method according to any one of claims 1 to 4, in which one of the superheater (12) downstream economizer (6) feed water (S) with a T4 = 10K reduced feed water inlet temperature (T4) is supplied.

6. Steam generator (1) with a steam output, comprising a combustion chamber (8) and a superheater (12), which has a number of heating surfaces (18), and a chimney (30), the combustion chamber (8), the superheater ( 12) and the chimney (30) are flowed through by a flue gas stream, wherein when an upper limit value of the flue gas temperature (TO) in the chimney (30) and / or a lower limit value for the steam output is undershot, the first heating surfaces seen in the flow direction of the flue gas stream (18) acting flue gas temperature (Tl) is set such that the softening temperature for ash particles contained in the flue gas stream remains below and in which a bypass line (32) is connected in parallel to the air preheater (24).

7. A steam generator (1) according to claim 6, in which a number of supply lines (20) connected in parallel in terms of flow technology are guided into the combustion chamber (8).

8. Steam generator (1) according to claim 6 or 7, wherein the flue gas temperature (Tl) is reduced by at least Tl = 15K.

9. Steam generator (1) according to one of claims 6 to 8, in which the combustion chamber (8) is preceded by an air preheater (24) for preheating the fresh air (F).

10. Steam generator (1) according to one of claims 6 to 9, in which, seen in the flow direction of the flue gas (R) after the superheater (12), a return line (34) is provided, which is fed to the combustion chamber (8) on the input side.