AU684526B2 - Steam plant for production of electrical energy - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Saarbergwerke Aktiengesellschaft ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Steam plant for production of electrical energy The following statement is a full description of this invention, including the best method of performing it known to me/us;- The present invention relates to a steam plant for production of electrical energy having a fossil-fuel fired boiler, a water/steam cycle for production of high-pressure superheated steam for a steam turbine, an economizer for transferring exhaust gas heat to feed water, an air pre-heater for transferring exhaust gas heat to fresh air and equipment for dust removal, desulfurization and denitrogenating of the exhaust gases, if required.

In the case of such steam plants operated on fossil fuels, that is, gaseous, liquid or solid fuels, the heat flow capacities (mass flow x specific heat capacity) of the cooling exhaust gas and of the warming fresh air (combustion air) are various so that with a normal temperature difference between exhaust gas and fresh air of ca. 30 k at the warm end of the air pre-heater a temperature difference of up to 90 k arises at the cold end of this heat exchanger. These high temperatures result in corresponding energy losses and *4$ accordingly work to disadvantage on the overall efficiency of the plant.

An additional disadvantage is that in power plants where there is no possibility for turther use of the residual heat still present at a relatively high temperature in the exhaust gas leaving the air pre-heater, whether by reheating of the purified exhaust gases prior to entering the chimney or by balancing out of heat for remote heating purposes, this residual heat is destroyed in the exhaust gas desulfurizing plant, resulting in yet further reduction in the overall efficiency of the power plant.

Furthermore, the known steam plants of the type initially described are not optimum in their start-up procedure. In the case of a coal-operated plant considerable amounts of expensive auxiliary fuels such as oil or gas have to be burnt in the boiler prior to starting of the coal firing until those parts of the plant to be heated up by means of exhaust gas heat, such as the mills for the dry grinding, the catalytic denitrogenating reactor and the air pre-warmer with its large regenerative heat storage masses. have reached the required minimum operating temperature.

Furthermore, the steam generated during the start-up and also the starting phase is generally condensed in the condenser of the steam plant without the use of heat.

-iEM II)PMI.KAMI 86829 ACt 1219M, -2- The object of the present invention is to reduce energy losses in a process of the type initially described, to improve on the heat utilization of the exhaust gas and to more economically structure the overall start-up procedure by using less oil or gas and by more meaningful use of the steam generated in the start-up phase.

Accordingly, this invention provides a steam plant for production of electrical energy having a fossil-fuel fired boiler, a water/steam cycle for production of high-pressure, superheated steam for a steam turbine, an economizer for transferring exhaust gas heat to feed water of the water/steam cycle, an air preheater for transferring exhaust gas to the total mass of the required fresh air, a first heat exchange system having a first cross section through which flows recirculating air and a second cross-section through which said feed water flows, an inlet to the first cross-section being connected with a fresh air outlet of the air pre-heater and an outlet to said first cross-section being connected with a fresh air inlet of the air preheater, and a second heat exchange system for transferring to the fresh air at least a portion of the residual heat still contained in the exhaust gas prior to desulfurisation wherein in the first heat exchange system the predominant portion of the heat yielded is transferable exclusively to high-pressure feed water and the remainder to low-pressure feed water of the water/steam cycle and that within the second heat exchange system, the residual heat of the S exhaust gas is transferable exclusively to fresh air.

S"By way of comparison with the prior art the measures provided by the present invention result in a clear reduction in energy losses in the air pre-heater, an essentially *improved usage of the heat contained in the exhaust gas and thus in a clear increase in the overall efficiency of such a steam plant.

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With the recirculating air flow of the first heat exchange system, which is superimposed on the fresh air in the vicinity of the air pre-heater, there is an extensive S 25 adjustment of the heat flow capacities in both heat exchanger cross-sections of the air preheater, with the consequence of small differences in temperature both at the warm and at the cold end, as well as correspondingly diminished energy losses. A portion of the exhaust heat released in the air pre-heater, which would have been transferred to relatively cold air in a conventional circuit, is now input directly into the water/steam cycle of the power plant by 0 way of the recirculating air flow and the first heat exchanger system at a higher temperature PIO'!RILKA\ 688259 ACL 12/9 -2Alevel.

Since according to the present invention a counterbalance is available with the recirculating air flow to adjust the equilibrium of the heat flow capacities between both crosssections in the air pre-heater, the fresh air can also be pre-heated and conveyed to the air preheater, that is, the residual heat still contained in the exhaust gas prior to desulfurization can be reused in the overall process by being transferred according to the present invention to the fresh air, resulting in further improvement of the overall efficiency.

The present invention proves to be particularly advantageous in power plants where the desulfurized cold exhaust gases are supp .ed directly to the cooling tower of the power plant and are discharged to the atmosphere along with the cool air and where there is no balancing out of the exhaust gas residual heat for remote heating purposes or similar purposes. In this case, on account of the circuit according to the present invention the residual heat still contained in the e

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C C C t e 3 exhaust gas can be completely recycled to the power plant cyclic process under thermodynamically favourable conditions.

For power plants where the exhaust gas residual heat is required either for remott heating purposes or for heating of the desulfurized exhaust gases to be discharged to the atmosphere by s way of a chimney, improvement can be made in the overall efficiency in accordance with another characteristic of the present invention in that the fresh air is again pre-heated prior to entering the air pre-heater in a steam air pre-heater by transfer of low-temperature heat from the water steam cycle.

Fundamentally. carrying into effect the process according to the present invention makes it possible to raise io a higher temperature level the low-temperature heat accumulating in the power plant by transfer to the fresh air and input into the air pre-heater, and to return it to the watersteam cycle.

Essential advantages relative to the starting of a steam plant emerge when-as provided for according to the present invention the first heat exchanger system is used as a start-up heat 15 exchanger. Prior to the beginning of the start-up procedure of the steam plant. that is. already 0* betore ignition of the burner in the boiler, the regenerative heat storage masses of the air pre- :warmer can be pre-heated.

In the start-up heat exchanger heat from any heat-transfer agent is transferred to the recirculation air. which is guided in the cycle between the still cold air pre-heater and the start-up heat exchanger, and thus heats up the regenerative heat storage masses of the air pre-heater.

Due to the pre-heating of the air pre-heater, during the next start-up of the steam plant the fresh combustion air flowing to the boiler by way of the air pre-heater is accordingly heated to favourable effect both during a cold start with already cooled boiler heating surfaces and during a warm start with still hot boiler heating surfaces. During a cold start the plant is heated more "25 quickly, that is. either oil or gas can be economized on in the boiler, whereas during a warm start less undercooling occurs by means of the inflowing cold combustion air.

With coal-fired steam plants and here the essential application of the invention is evident there is the additional advantage that through pre-heating of the air pre-heater the minimum temperatures behind the air pre-heater required for starting of the coal firing/dry grinding can be achieved more quickly, with the consequence that there can be a faster changeover from the standard oil or gas firing during start-up to normal coal firing. A further advantage is that the operating temperatures required for the denitrogenating reactor are faster to achieve, with corresponding effects on the efficiency of the denitrogenation and thus the environment.

A further advantage because of the pre-heating of the air pre-heater arises from the fact that with the first contact of the air pre-heater with the yet to be desulfurized exhaust gas. excessive cooling with associated falling below the dew point is avoided, with the consequence of correspondingly reduced corrosion in the air pre-heater and in other connected plant components such as the electrofilter.

.As already mentioned, the start-up heat supplied to the air pre-heater by way of the start-up heat exchanger may originate from any source. By way of example this could be heat from another steam plant available at the same site, or waste heat of another industrial plant.

In accordance with a further feature of the invention feed water from the feed water container of the water/steam cycle is used as heat-transfer agent, which has already been brought to temperature by means of start-up steam from the generation of energy. from adjoining plants or separate boilers.

5. If necessary. the temperature of the removed feed water can be raised further prior to being cooled in the start-up heat exchanger in a further heat exchanger in heat exchange with condensing start-up steam.

Further explanation of the present invention will emerge from the embodiments represented diagrammatically in the accompanying figures, in which a0 Figure 1 shows an exemplary circuit arrangement for reducing heat loss during operation of a steam plant, Figure 2 shows an exemplary circuit arrangement for reducing heat loss during start-up of a steam plant.

According to the figure hot exhaust gas from a st-am generator of a mineral coal power plant is supplied by way of piping 1 first to a denitrogenating plant 2 and then to an air pre-heater 3 at a temperature of ca. 3SO8C. The exhaust gas is cooled down to ca. 13 0 °C in air pre-heater 3 in a heat exchange with air. After it has passed through an electrofilter 4 and induced draft bellows 5 it is further cooled from 130°C to ca. SO-90°C in a second heat exchanger system

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I~ comprising heat exchangers 6 and 11 as well as a recycled water system 12. Next, the cooled exhaust gas is conveyed to an exhaust gas desulfurization plant 8 by way of piping 7 and is finally discharged to the atmosphere along with the cool air by way of the cooling tower of the power plant. not illustrated here.

The combustion air required in the steam generator is supplied to the power plant by way of piping 9 and bellows 10 and pre-heated in a heat exchanger to a temperature of ca. 70-80C.

The heat required for pre-heating is transferred by means of a closed circuit water system 12 trom heat exchanger 6 to heat exchanger 11.

T,.e fresh air pre-heated in heat exchanger 11 is admixed at mixing point 13 with recycled air. whose temperature and mass flow are determined such that a heat flow equilibrium is adjusted in air pre-heater 3, that is, both at the cold and the warm end of the air pre-heater there are now the desired slight differences in temperature between the exhaust gas and the air.

After passing through the air pre-heater the recycled air flow is again separated from the fresh air flow at a branching-off point 14. While the fresh air is supplied at a temperature of s 350°C by way of piping 15 to the furnace of the steam generator. the recycled air is again cooled in a heat exchanger 16 in heat exchange with high-pressure feed water and, if necessary, in another heat exchanger 17 in heat exchange with low-pressure feed water and then conveyed to mixing point 13 by way of adjustable bellows 18.

Figure 2 diagrammatically illustrates sections from the circuit of a coal-fired steam plant. In detail, hot exhaust gas from an economizer 21 of the steam generation plant is supplied by way of piping 22 to a catalytic denitrification reactor 23 and finally to an air pre-heater 24. In economizer 22 the exhaust gas is cooled in heat exchange with feed water to the optimum operating temperature of denitrification reactor 23 from ca. 350 380°C. It is further cooled to ca. 130'C in heat exchange with fresh combustion air in following air pre-heater 24. After being cooled the exhaust gas is dedusted or desulfurized in apparatus, not illustrated here, and then discharged to the atmosphere along with the cool air by way of a cooling tower, also not illustrated here.

The combustion air required for the boiler is conveyed to the plant by way of piping heated to ca. 350°C in air pre-heater 24 and then fed on by way of piping 26 for firing or dry grinding.

'1 -'I Il~aa~slaarr The illustrated section of the water/steam cycle of the plant shows a feed water container 27 in which the condensate, flowing in by way of piping 28, is heated with steam from piping 29 The heated water (feed water) is drawn off from feed water container 27 by way of piping 30. pumped in a high-pressure pump 31 at ca. 250 300 bar. then pre-heated to a temperature of ca. 250 300 0 C in a standard high-pressure pre-heater 32. The pre-heated feed water flows through piping 33 into economizer 21, where it is further heated in heat exchange with hot exhaust gas.

The feed water is then fed through piping 34 into the further heat exchange system of the boiler where it is vaporized or superheated to the starting temperature of the steam turbine of ca. 530 After pressure decrease in the turbine the vapour is condensed and rerouted to feed water container 27 by way of piping 28.

The foregoing system description of a steam plant is based essentially on normal operation at full load or also at partial load.

The aim of the present invention is to create a more economical design of the start-up .1s procedure of such a plant. Provided for this purpose is a start-up heat exchanger 35 with recirculating air flowing through a cross-section which is connected on the inlet side to the fresh air outlet by way of piping 36. and is connected on the outlet side to the fresh air inlet of air preheater 24 by way of piping 37 and bellows 38.

Before or during start-up of the plant the recirculating air conveyed in the cycle between air r.Sf pre-heater 24 and start-up heat exchanger 35 is heated in start-up heat exchanger 35 and then cooled down in air pre-heater 24. whereby the regenerative heat storage masses of the air preheater are warmed. One consequence of this pre-heating is that the exhaust gas accumulating at the beginning of the start-up procedure is cooled down less in the air pre-heater, effectively pre. enting falling below the dew point and also avoiding associated corrosion damage in the air pre-heater and in subsequently connected plant components. Moreover. during the start-up procedure supplementary heat can be transferred by means of the recirculating air to the combustion air. whereby the temperatures required for activating the dry grinding and thus the coal burner of the boiler can be reached more quickly, with the result that the auxiliary burners operating on expensive oil or gas can now be shut down earlier and the operating temperatures on the exhaust gas side are reached more quickly (corrosion).

In accordance with the embodiment illustrated in the figure heating of the recirculating air of heat exchanger 35 takes place in heat exchange with hot feed water which is drawn off in feed ~~~a~ll water container 27 by injection of start-up steam supplied by way of piping 29 through piping, 39, 41 with the then open valves 42 and 43, cooled in heat exchanger 35 and then recycled through piping 44 and 45 as well as open valve 46 into feed water container '7.

If necessary, the feed water can be reheated before it passes into heat exchange 35 by at least one partial flow of the feed heated water flowing in piping 39 being conveyed through a now open valve 47 and piping 48 into another heat exchanger 49. Heat exchanger 49 is heated by condensing start-up steam from a source 50, which is then conveyed to feed water container 27 through piping 51.

The feed water flow further heated up in hea, exchanger 49 is fed through piping 52 and an open valve 53 first to piping 41 and then to heat exchanger In accordance with another design of the present invention additional heat is supplied to not only air pre-heater 24, but also to economizer 31 when the steam plant starts up. According to the embodiment illustrated in the figure at least a partial flow of the feed water heated in heat exchanger is fed through an open valve 54 and piping 44 and 55 into piping 33 where it is a admixed with feed water from high-pressure pre-heater path 32. Start-up heat can be used for this purpose. On the basis of the temperature increase of the feed water the exhaust gas is cooled less in the economizer during the start-up procedure. so that the minimum temperature for start-up of connected denitrogenating reactor 23 is reached more quickly.

The heat transfer to the storage mass of air pre-heater 24 or the additional heat transfer to the combustionl air or to the feed water enables use of the heat from the steam generated in the start-up phase: accumulation in air pre-heater heat surfaces 4 also enables use of the steam generated during the start-up phase.

The proposed injection of additional heat into the economizer proves effective not only during the start-up procedure of the plant, but also can be used during off-peak ope.-tion of the plant for supporting the exhaust gas temperature and thus for maintaining the optimum opt rating temperature of denitrogenating reactor 23.

Furthermore, by variation of the recirculating air mass through bellows 38 and of the transferred heat mass in heat exchanger 35 the exhaust gas temperature can be kept constant after air pre-heater 24 over the entire load zone; an optimum lower exhaust gas temperature is also possible at full load, without unduly low temperatures, for example at partial load, and thus corrosion having to be considered.

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Claims (9)

1. A steam plant for production of electrical energy having a fossil-fuel fired boiler, a water/steam cycle for production of high-pressure, superheated steam for a steam tirbine, an economizer for transferring exhaust gas heat to feed water of the water/steam cycle, an air preheater for transferring exhaust gas to the total mass of the required fresh air, a first heat exchange system ;lving a first cross section through which flows recirculating air and a second cross-section through which said feed water flows, an inlet to the first cross-section being connected with a fresh air outlet of the air pre-heater and an outlet to said first cross- section being connected with a fresh air inlet of the air preheater and a second heat exchange system for transferring to the fresh air at least a portion of the residual heat still con;tained in the exhaust gas prior to desulfurisation wherein in the first heat exchange system the predominant portion of the heat yielded is transferable exclusively to high-pressure feed water and the remainder to low-pressure feed water of the water/steam cycle and that within the second heat exchange system, the residual heat of the exhaust gas is transferable exclusively to fresh air.

3. 2. A steam plant as claimed in Claim 1, wherein air flow of the first heat exchanger system can be regulated. 3. A steam plant as claimed in Claim 1 or Claim 2, wherein the second heat exchanger system is designed as a closed circuit system with water as heat transfer medium. a S o a* I' \)i'll!tI.KAul259 ACL. 19A -9-

4. A steam plant as claimed in any one of claims 1 to 3, wherein prior to entering the air pre-heater the fresh air is heated in heat exchange with draw-off steam from the water/steam cycle of the plant. A steam plant as claimed in any one of claims 1 to 4, wherein the first heat exchange system is designed as the start-up heat exchanger.

6. A steam plant as claimed in Claim 5, wherein the heat transfer medium in the start-up heat exchanger is hot water from the feed water container of the water/steam cycle of the steam plant.

7. A steam plant as claimed in Claim 6, wherein prior to being cooled in the start-up heat exchanppr the water feed can be heated further with the condensing steam-heated heat exchanger.

8. A steam plant as claimed in Claim 7, wherein the cooled water feeder and the condensed steam are conveyed to the feAd water container.

9. A steam plant as claimed in any one of claims 5 to 8, wherein at least a portion of the heat transfer medium for the start- .p heat exchanger can be used for increasing the feed water S temperature before the economizer.

10. A steam plant as claimed in Claim 9, wherein the heat transfer medium is hot feed water which, after further heating in heat exchange with condensed steam, can be admixed with the feed water flowing to the economizer. 9 0 ~LW~ i m"I 1p!VOPr-1kXAIG259~A(t. 1219MV

11. A steam plant substantially as hereinbefore described with reference to the drawings. DATED this TWELFTH day of SEPTEMBER 1997. Saarbergwerke Aktiengesellschaft by DAVIES COLLISON CAVE Patent Attorneys for the Applicant(s) 'I L Abstract In a steam plant for production of electrical energy having a fossil-fired boiler, a water'steam cycle for production of high-pressure superheated steam for a steam turbine, an economizer for transferring exhaust gas heat to feed water, an air pre-heater for transferring exhaust gas heat to fresh air and equipment for dust removal, desulfurization and denitrogenating of the exhaust gases, if required, it is proposed, for optimizing the heat exchange in the air pre-heater during operation and for reducing heat loss during start-up of the steam plant. to provide a heat exchanger system having a cross-section through which flows recirculating air and a heat transfer medium, whereby the air-guided cross-section is connected with the fresh air outlet of the air pre- heater on the inlet side and with the fresh air inlet of the air pre-heater on the outlet side. S. S SoS osolJ o