GB2138555A

GB2138555A - Process for Utilising Heat Removed on Cooling a Flue Gas Stream

Info

Publication number: GB2138555A
Application number: GB08409025A
Authority: GB
Inventors: Ullrich Neumann; Manfred Weigel; Karl Wintrup
Original assignee: Davy McKee AG
Current assignee: Davy McKee AG
Priority date: 1983-04-19
Filing date: 1984-04-06
Publication date: 1984-10-24
Also published as: NL8401282A; GB8409025D0; ATA129784A; AT389824B; GB2138555B

Abstract

The process serves for utilising the heat removed on further cooling of a pre-cooled SO2-containing flue gas stream before it enters a desulphurising stage at low temperature, by transferring the heat to an air stream, which is thereby preheated, further raising of the temperature of the preheated air stream with hot flue gas and separating the hot-air stream into two partial streams, of which one is used as air for combustion. <IMAGE>

Description

SPECIFICATION Process for Utilising Heat Removed on Cooling a Flue Gas Stream The invention relates to a process for utilising the heat removed on further cooling of a precooled SO2-containing flue gas stream before it enters a desulphurising stage at low temperature (cold process).

Numerous processes are known for the purpose of removing SO, from flue gases by absorption with aqueous solutions or suspensions. According to the Wellman-Lord process, the flue gas containing SO, is brought into contact with an aqueous sodium sulphite solution, which thereby takes up SO, from the gas, with partial formation of sodium hydrosulphite. The absorption occurs at temperatures of, for example, approximately 50 to 550C, so that the hot flue gas must initially be cooled approximately to this temperature.After leaving the absorption stage, the purified or cleansed flue gas has a temperature of about 50 to 55cm. In order that a sufficient upward thrust is imposed on this gas, both in the chimney and on discharge into the atmosphere, it has to be heated to approximately 80 to 1 000C before it enters the chimney. This requirement is applicable to all flue gas desulphurising processes, which operate with aqueous media for the removal of the 502.

In addition to sulphur dioxide, the uncleansed flue gas also contains smaller amounts of sulphur trioxide and frequently other corrosive impurities, so that, on cooling of the gas to the temperature level of the desulphurising stage, the sulphuric acid dew point is passed through and, as a consequence, considerable corrosion can occur on the heat exchanger being used for the cooling of the flue gas. Corrosion and/or caking effects can also occur on the heat exchanger serving for the re-heating of the cleansed gas, the said caking being formed by absorption agents entrained by the clean gas and deposited in the exchanger. As regards the Wellman-Lord process, these depositions are water soluble and consequently cause no problem.The high danger of corrosion of the exchangers, both with the cooling of the contaminated flue gas and with the reheating of the desulphurised flue gas, make necessary a high-quality corrosion-resistant material in both stages, with the consequence that there is a considerable expense factor. One proposal as regards achieving the heat exchange is the use of a Ljungström regenerator which, as is known, is frequently used with steam boiler installations for the pre-heating of the air for combustion by flue gas to a substantially higher temperature level (Ullman, Enz. Techn. Chem., Vol. 1, pages 2789).The use of Ljungström regenerator on the inflow side of the flue gas desulphurising plant and in the low temperature range also makes it necessary for its entire transfer surface to be made of corrosion-resistant material, for example, enamelled sheet material. Such appartuses are costly and do not function in a leakage-free manner, that is to say, depending on the pressure conditions, one medium or the other becomes contaminated by the opposing medium. As a consequence, it is necessary to operate on the side of the purified or cleansed gas at a higher pressure than on the side of the S02-containing gas in order to avoid the S02-containing untreated gas flowing over to the cleansed gas and having a detrimental effect on the degree of desulphurisation.This disadvantage also occurs in those cases where it is not pure gas, but another gas, as for example air, which is heated with the hot, S02-containing flue gas via a Ljungström regenerator.

Two variants in procedure concerning the regenerative reheating of the cold flue gases after wet desulphurisation are described in VGB Kraftwerkstechnik 63 (1983) page 332 et seq.

Apart from the defects which are connected with the use of the regenerator (caking effects, leakages), the heat is not utilised to the best possible extent with these processes. In the one case, the heat to be removed from the gas prior to the desulphurisation is transferred in its entirety and regeneratively to the cold desulphurized gas.

However, for the reheating of the cold pure gas, generally only a part of the heat thus available is required, so that heat is lost through the chimney with this arrangement. As regards the other variant, all the available heat is transferred regeneratively to an air stream, which is then further heated in the boilder air preheater. Some of this further heated air stream is admixed with the cold desulphurised flue gas, in order to give it the necessary upward thrust. This constructional form is unfavourable from the point of view of heat technology, because the heat at low temperature extracted from the flue gas is transformed to high temperature in the boiler air preheater, but then, by being admixed, only partially serves the purpose of heating the cold flue gas to 90 to 1000C.

The invention seeks to give the best possible utilisation of the heat to be removed on further cooling of a pre-cooled S02-containing flue gas stream before it enters a desulphurising stage at low temperature. More especially, by using this heat, the temperature of the purified flue gas is to be raised to the delivery temperature necessary for the upward thrust and the excess heat is to be made available in a useful form. In addition, the heat to be removed on flue gas cooling to about the working temperature of the wet flue gas desulphurisation in the low temperature range is to be used for producing additional boiler steam, when it is not used for the reheating of the desulphurised gas.

Using the process as initially referred to, according to the invention the heat is transferred with a liquid or vaporisable heat carrier to an air stream, which is thereby preheated, the temperature of the preheated air stream being further raised with hot flue gas and the hot air stream being separated into two partial streams, of which one is used as air for combustion. As a result of the uncoupling of the crude gas cooling and pure gas reheating, not only is it possible to optimise the utilisation of the flue gas heat, but, in addition, substantial savings as regards capital expenditure are produced.

Since no problems as regards corrosion are caused by the transfer or heat from the hot heat carrier to the cold air, this heat exchange stage can be carried out in ordinary steel, i.e.

economically. It is only the exchange stage, in which the S02-containing, untreated gas gives off its heat to the heat carrier, which has to be manufactured from a high-grade, corrosionresistant material. Such materials are known from sulphuric acid technology. As compared with the reheating of the pure gas, the preheating of the air thus has the advantage that no longer are there present heating surfaces contacted by pure or clean gas and consequently there is produced a considerable saving of high-grade, corrosionresistant material. In addition, the possibility is provided of minimising the cooling surface contacted by the sulphur-containing untreated gas in relation to the air-contacted heating surface, by the temperature of the heating medium being kept low.Corrosion-resistant material in the untreated gas cooling stage is likewise saved, as opposed to an increased use of ordinary carbon steel in the air preheater.

Furthermore, an improvement in energy consumption is produced. By the heat removed with the cooling of the untreated gas to the low temperature level of the latter being transferred to the air for combustion, the boiler efficiency is improved, i.e. the heat removed in a low temperature range is utilised for producing high pressure steam, whereas the heat directly available from the untreated gas in the temperature range from about 150 to about 600C would otherwise only be suitable for producing low-pressure steam. By the return of heat produced according to the invention from the low temperature range into the boiler installation, there is produced an improvement in the boiler efficiency which is of the order of 1% (with a ?00 MW power station).Provided as an additional advantage of the process according to the invention are substantial savings as regards the necessary flue gas pipes.

An additional advantage which is provided is that the heat removed from the precooled flue gas stream is only partly-transferred to the cold air stream, while the other part is transferred to the desulphurised flue gas and produces steam with that partial stream of the hot air which is used as air for combustion. The heat extracted from the flue gas in the low temperature range (e.g. 150 to 500C) is sufficient, temperature-wise, for the reheating of the clean gas. The necessary partial quantity of the heat can be transferred in suitable manner to the clean gas.The remaining part of tbe e heat extracted from the S02-containing flue gas is transferred to fresh air, which is thereby preheated from, for example, 30 to 8000. The air stream is thereafter further heated in the conventional boiler air preheater to, for example, 300 to 38000 and then divided into two partial streams. The main stream serves as air for combustion in the boiler installation, while the other partial stream flows through a steam generator, in which a large part of the sensible heat of the air stream is used for generating steam.By varying the proportions of the heat transferred to the cleansed gas and to the air stream it is readily possible to raise the delivery temperature of the cleansed gas in preference to the steam generation or to increase the generation of steam with a lowering of the delivery temperature.

According to a preferred embodiment of the invention, the heat carrier is brought into heat exchange in a first stage with the hot flue gas and in a second stage with cold air and the heat carrier is circulated between the two heat exchange stages. Since the second heat exchange stage has air admitted thereto at about ambient temperature or at a slightly higher temperature, the heat carrier is able to circulate at relatively low temperature, so that also the temperature of the flue gas to be desulphurised can be lowered on entering the flue gas desulphurising stage. As a result, there is also a fall in the operating temperature of the SO, absorption, the absorption is improved and the regeneration accordingly requires a smaller heat consumption (heating steam).When the SO, is absorbed with an aqueous solution containing Na2SO3 in accordance with the Wellman-Lord process, the lowering of the gas temperature which can thus be produced leads to a saving of steam in the regeneration of about 10%.

According to a preferred form of the process according to the invention, the hot flue gas is cooled with the heat carrier from a temperature in the range from 1 10 to 19000 to a temperature in the range from 40 to 8000 and the air is heated with the heat carrier from approximately ambient temperature or slightly above to a temperature in the range from 70 to 11000. More especially, the hot flue gas is cooled from a temperature in the range from 140 to 16000 to a temperature in the range from 50 to 7000 and the air is pre-heated from a temperature in the range from 20 to 3500 to a temperature in the range from 80 to 10000.

Provision is also made for the temperature of the cold desulphurised gas to be raised by the admixture of hot air from 40 to 7000 to a temperature in the range from 70 to 11000. After the further heating, the hot air has a temperature in the range from about 250 to 40000. The further heating is effected in conventional manner in a boiler air preheater, which may more especially be a Ljungström regenerator.

In accordance with a preferred form of the invention, provision is made for 10 to 30% of the heated air to be admixed with the cold flue gas discharging from the flue gas desulphurising stage. It is true that this proportion of air has to be drawn by suction, in addition to the necessary air for combustion, by the air fan. However, since the pressure drop in the air preheater and in the boiler air preheater is comparatively low, the additional expense for the increased fan or blower efficiency is small in comparison with the previously indicated advantages of the process according to the invention.

The first heat exchange stage is advantageously operated with a larger temperature difference and a smaller heat exchange surface than the second heat exchange stage. In this way, the capital expense for the two heat exchange stages is minimised.

The pre-cooled flue gas stream is preferably cooled in two stages and the heat extracted in the first stage is transferred to the desulphurised flue gas and the heat extracted in the second stage is transferred to the air stream. Thus, the low temperature heat available in the upper temperature range is made effective for the further heating of the clean gas, while the heat available in the lower temperature range (generally below 10000) still serves for the heating of the fresh air.

According to one form of the process according to the invention, the partial air stream utilised for the production of steam is again combined with the main air stream before it is further heated. Due to this cyclic procedure, the heat which is still contained in the partial air stream after the production of steam is not lost, but is once again transformed to high temperature in the boiler air preheater, together with the air for combustion, so that it can once again serve for producing steam. In this connection, according to a first embodiment, the partial air stream used for the production of steam can be combined with the stream of cold fresh air.

The combining of the streams preferably takes place before the fresh air blower or fan, since it is then possible to dispense with a separate fan or blower for the partial air stream. In another form of the invention, the partial air stream used for the production of steam is combined with the already pre-heated air stream. In this case, it is more expedient for the partial air stream used for the production of steam to be further cooled by heat exchange before being combined with the preheated air stream and for the heat thereby extracted to be also transferred to the desulphurised flue gas.

In accordance with another embodiment of the process according to the invention, the partial air stream utilised for the production of steam is at least partially mixed with desulphurised flue gas.

Since the partial air stream, after the generation of steam, still has a temperature which is above the desired discharge temperature of the desulphurised gas, a further raising of the clean gas temperature is obtained by the admixing procedure. With this embodiment, generally first of all the cold desulphurised clean gas is heated by heat transfer directly by the pre-cooled flue gas and its temperature is then raised by the admixture of air to the required delivery temperature. In this case, the major part of the heat necessary for the further heating of the clean gas is transferred directly from the pre-cooled flue gas stream and only a relatively small proportion is supplied by the admixing of air.

With one particular arrangement, heat can be extracted, likewise by production of steam, from the other partial stream before being used as air for combustion.

With the preferred form of the process according to the invention, the heat is transferred from the pre-cooled flue gas and possibly the partial air stream used for the generation of steam to the desulphurised flue gas or respectively the air stream with circulating liquid or vaporisable heat carriers.

As compared with the previously proposed and practised regenerative heat exchange, the participation of circulating heat carriers offers the advantage that the heat-delivering side is completely separated from the heat-absorbing side and thus the contamination of the clean gas side, occurring with regenerative heat exchange, and any impairment of the degree of desulphurisation, are avoided. Furthermore, the transfer of heat by circulating heat carriers is less liable to disruption that that with circulating heating surfaces, more especially when the danger of incrustation of the heating surfaces is taken into account.

In accordance with a preferred form of the process according to the invention, the partial air stream is cooled by the generation of steam from a temperature in the range from 300 to 38000 to a temperature in the range f.om 120 to 24500.

After the heat content of the partial air stream has been fully utilised in this manner, the said stream may, possibly after further heat extraction, be returned in the circuit to the boiler air preheater or even be admixed with the desulphurised gas.

Combinations of both variants are possible, that is to say, the partial air stream used for the generation of steam is partially circulated and partially admixed with the clean gas. The admixing can already take place on the inflow side of the heat transfer to the clean gas. In this way, there may possibly be achieved the effect that the dew point of the clean gas stream is already exceeded before entry into the heat exchanger and thus any corrosion effects in the heat exchanger are reduced or avoided. The cold desulphurised exhaust gas of the flue gas desulphurising plant is expediently heated from a temperature in the range from 40 to 7000 to a temperature in the range from 70 to 11000. The cooling of the pre-cooled SO2-containing flue gas on the inflow side of the flue gas desulphurisation unit is preferably carried out in two stages from a temperature in the range from 120 to 25000 to a temperature in the range from 40 to 10000.

The invention is hereinafter more fully described by way of example with reference to the drawings, wherein: Figure 1 is a flow diagram of an installation for carrying into effect a first embodiment of the process according to the invention; Figure 2 is a diagrammatic flow diagram of an installation for carrying into effect a second embodiment of the process according to the invention, in which the partial air stream utilised for the generation of steam is combined with the desulphurised gas; Figure 3 is a diagrammatic flow diagram of an installation for carryng into effect a third embodiment of the process, in which the partial air stream utilised for the generation of steam is circulated after being combined with the fresh air; and Figure 4 is a diagrammatic flow diagram of an installation for carrying into effect a fourth embodiment of the process according to the invention, in which the partial air stream utilised for the generation of steam, after further heat extraction, is combined and circulated with the pre-heated fresh air.

Accordig to Figure 1, the flue gas leaves the boiler installation 1 at a temperature of, for example, 4500C and is first of all cooled in a boiler air preheater 2 to about 1 500C. After passing through the electrofilter 3 and the suction duct fan 4, the flue gas is cooled in the gas cooler 5 to about 600C. It then passes through the absorption section of the flue gas desulphurising plant 6, in which the SO, and also other acid gas constituents, such as S03, are removed from the gas by means of an aqueous absorption solution, more especially a sodium sulphite solution. The flue gas as thus purified then continues its flow to the base of the chimney 7.

The cooling of the S2-containing flue gas in the gas cooler 5 is effected by indirect heat exchange with a liquid heat carrier, such as water, which, for example, is supplied to the cooler through a pipe 8a. The heat carrier, moderately heated in the cooler 5, passes by way of pipe 8b to a primary preheater 9, in which it comes into heat exchange with air, which is drawn in from the atmosphere by the air fan 10 and is forced into the pre-heater 9. In this way, the air is heated to about 90 C, while the heat carrier is cooled to 40 to 600C. The pre-heated air then passes to the conventional boiler air pre-heater 2, which is constructed as a Ljungström regenerator. The air is thereby heated to about 3000C by the hot flue gas flowing from the boiler installation 1.

80% of this heated air is supplied as air for combustion to the boiler installation 1, while the remaining 20% of the air heated to approximately 3000C is combined with the cleansed flue gas at a temperature of 500C which is discharging from the flue gas desulphurising unit 6. On entering the chimney 7, the combined gases have a temperature of about 900C and thus an upward thrust sufficient for the ascent in the chimney and the dispersion in the atmosphere.

The process according to the invention makes it possible for the use of the high-grade corrosion resistant material to be restricted to the gas cooler 5 and this also, at the expense of the primary preheater, which can be constructed as a conventional air preheater of carbon steel, to minimise the surface area thereof. An additional advantage is to be seen in the fact that the heat dissipated at low temperature in the gas cooler 5 is returned to the boiler installation 1 and, as a consequence, serves for the generation of the high-pressure steam, unless it is supplied to the cleansed gas by way of the warm air stream for re-heating purposes.

According to Figure 2, the hot flue gas coming from a boiler installation 1 is drawn in by the suction fan or blower 4, first of all through an air pre-heater 2 and then through an electrofilter 3.

The air preheater 2 is more particularly a conventional Ljungström regenerator, in which the flue gas, as opposed to air for combustion, is cooled to, for example, 1 300C. The flue gas then flows through a two-stage heat exchanger 5, in which it is further cooled to the entry temperature of the flue gas desulphurising unit 6.

In this case, in the two stages, the heat of the flue gas is delivered to separate heat carrier circuits 7 and 8, respectively. The heat absorbed by the circuit 7 is transferred in the heat exchanger 9 to the desulphurised gas coming from the flue gas desulphurising unit 6, more especially a Wellman Lord installation. The heat extracted from the flue gas in the second stage of the exchanger 5 passes by way of the heat carrier circuit 8 into the exchanger 10, in which the fresh air drawn in by the fresh air fan 1 1 is pre-heated. This pre-heated air then flows through the boiler air pre-heater 2, in which its temperature is raised, for example, to 3400C. The major part of this heated air is used as air for combustion in the boiler unit 1.The minor part of the heated air flows, according to the invention, through a steam generator 12 and, as a consequence, is cooled to approximately 1 500 C. This air is then mixed with the desulphurised gas coming from the heat exchanger 9 and delivered to the chimney 13.

In Figures 3 and 4, similar parts of the installation also bear the same reference numerals as those used in Figure 2. The embodiment according to Figure 3 differs from that according to Figure 2 by the fact that the air stream coming from the steam generator 12, combined with fresh air, is once again drawn in by the fresh air fan 1 1 and is once again caused to circulate through the air p#-heaters 10 and 2.

In the constructional form according to Figure 4, the air stream coming from the steam generator 12 flows through an additional heat exchanger 14, in which it is further cooled, for example, to the temperature of the air pre-heated in the exchanger 10. In this case, the air likewise gives of its heat to the heat carrier circuit 7, which transfers its heat in the exchanger 9 to the desulphurised flue gas. The purpose of the fan or blower 15 is to circulate the partial air stream through the exchangers 12 and 14.

The process according to the invention makes it possible to optimise the heat efficiency with the desulphurisation of flue gases by low-temperature processes. With this procedure, not only is the consumption of steam for the pre-heating of the air for combustion dispensed with (cf. VGB Kraftwerkstechnik 63 (1983), page 333, Figures 1 and 2), but the excess low temperature heat is even utilised for the generation of steam.

Constructional Example In the installation according to Figure 2, 218.2 Nm3/s of flue gas at 1 340C enter the twostage heat exchanger 5. In the first stage, having an exchange surface or area of 2900 m2, the flue gas is heated to 105.1 0C, while in the second stage, with an exchange area of 5800 m2, the flue gas is cooled to approximately 650C. 221.2 Nm2/s of gas leaves the flue gas desulphurising unit 6 (Wellman-Lord installation) at 470C. The temperature of the gas is raised to 760C in the exchanger 9, having an exchange area of 2900 m2, the temperature of the heat carrier falling from 1050 to 760C. By means of the fresh air fan 11, 1 80 Nm2/s of air at 300C are introduced and are pre-heated in the exchanger 10 to 800C. The temperature thereof is further raised to 331 0C in the boiler air pre-heater 2. 140 Nm3/s of air are supplied as air for combustion to the boiler installation 1. With the remaining 40 Nm3/s of air, 12 14 t/h of steam are generated in the steam generator. As a consequence, the temperature of the air stream falls to 1 660C. The air stream is then combined with the desulphurised gas coming from the exchanger 9. 261.2 Nm3/s of exhaust gas at a temperature of 880C are delivered to the chimney.

Claims

1. Process for utilising the heat removed on further cooling of a pre-cooled S02-containing flue gas stream before it enters a desulphurising stage at low temperature, wherein the heat is transferred with a liquid or vaporisable heat carrier to an air stream, which is thereby preheated, the temperature of the preheated air stream is further raised with hot flue gas and the hot air stream is separated into two partial streams, of which one is used as air for combustion.

2. Process according to claim 1, wherein the other, smaller, partial stream of the hot air is admixed with the desulphurised cold gas.

3. Process according to claim 1 or 2, wherein the heat removed from the precooled flue gas stream is only partly transferred to the cold air stream and the remainder is transferred to the desulphurised flue gas and produces steam with that partial stream of the hot air which is not usea as air for combustion.

4. Process according to any one of claims 1 to 3, wherein the heat carrier is circulated between a first stage and a second stage, in which it is brought into heat exchange with the hot 502 containing flue gas and cold air, respectively.

5. Process according to any one of claims 1 to 4, wherein the pre-cooled flue gas stream is cooled with the heat carrier from 1 to 1900C to 40 to 80 C 'and the air stream is pre-heated with the heat carrier from about ambient temperature up to 70 to 1 10cm.

6. Process according to any one of claims 2 to 5, wherein the temperature of the desuiphurised cold gas is raised by the mixing with hot air from 40 to 700C up to 70 to 1 1 OOC.

7. Process according to any one of claims 1 to 6, wherein the temperature of the preheated air stream is raised to a temperature in the range from 250 to 4000C.

8. Process according to any one of claims 2 to 7, wherein 10 to 30% of the hot air is admixed with the cold gas.

9. Process according to any one of claims 4 to 8, wherein the first heat exchange stage is operated with a larger temperature difference between the heat-oxchanging#media and a smaller heat exchange surface than the second heat exchange stage.

10. Process according to claim 3, wherein the precooled flue gas stream is cooled in two stages and the heat removed in the first stage is transferred to the desulphurised flue gas and the heat removed in the second stage is transferred to the air stream.

11. Process according to any one of claims 3 to 10, wherein the component air stream used for generating steam is again combined with the air stream prior to its further heating.

12. Process according to any one of claims 3 to 11, wherein the component air stream used for the generation of steam is combined with the cold stream of-fresh air.

13. Process according to any one of claims 3 to 11, wherein the component air stream used for the generation of steam is combined with the preheated fresh air stream.

14. Process according to claim 12 or 13, wherein the component air stream used for generating steam is further cooled by heat exchange and the heat thereby removed is likewise transferred to the desulphurised flue gas.

15. Process according to any one of claims 3 to 14, wherein the component air stream used for the generation of steam is at least partially admixed with the desulphurised flue gas.

16. Process according to claim 15, wherein the cold desulphurised flue gas is initially preheated with heat from the pre-cooled flue gas and its temperature is then raised by the admixture of air to the delivery temperature.

17. Process according to any one of claims 1 to 16, wherein heat is extracted from the other component air stream by production of steam before being used as air for combustion.

18. Process according to any one of claims 1 to 17, wherein the heat from the pre-cooled flue gas and possibly the component air stream used for the production of steam is transferred to the desulphurised flue gas and the air stream, respectively, with circulating liquid or vaporisable heat carriers.

19. Process according to any one of claims 3 to 18, wherein the component air stream is cooled, by the production of steam, from a temperature in the range from 300 to 3800C to a temperature in the range from 1 50 to 2450C.

20. Process according to any one of claims 3 to 19, wherein the pre-cooled flue gas is cooled in two stages from a temperature in the range from 120 to 2500C to a temperature in the range from 40 to 1000C.

21. Process for utilising the heat removed on further cooling of a pre-cooled S02-containing flue gas stream substantially as described herein with reference to any one of Figures 1 to 4.