GB2299768A

GB2299768A - Cooling hot crude gas charged with pollutants prior to scrubbing

Info

Publication number: GB2299768A
Application number: GB9507750A
Authority: GB
Inventors: Winfried Voges; Dieter Wurz
Original assignee: GEA Luftkuehler GmbH
Current assignee: GEA Luftkuehler GmbH
Priority date: 1994-03-02
Filing date: 1995-04-13
Publication date: 1996-10-16
Also published as: IT1276534B1; DE4406772A1; GB9507750D0; FR2733304A3; FR2733304B3; DE4406772C2; ITRM950241A0; ITRM950241A1; BE1009340A3; ATA138594A; NL1000280C2; AT405376B

Description

DESCRIPTION OF INVENTION

2299768 Title: "A method of cooling hot crude gas charged with pollutants, and an apparatus for carrying out the method" THE INVENTION relates to a method of cooling hot crude gas charged with pollutants, whereby hot crude gas charged with pollutants is cooled upstream of a gas scrubber for removing sulphur from the crude gas, which is brought to a lower temperature level by a heat-withdrawing cooling fluid while flowing through a cooling zone.

The invention also relates to an apparatus for cooling such gas, comprising a crude-gas cooler in front, in the direction of the crude-gas flow, of a gas scrubber, withdrawing sulphur from the crude gas, the cooler being subj ected to the crude gas on the one hand and a heatwithdrawing cooling fluid on the other hand.

Burning of sulphur-containing fuels results in sulphur dioxide (S02) and a smaller quantity of sulphur trioxide (S03). In a medium containing water-vapour, at a temperature below a value depending mainly on the S03 concentration and the content of water vapour, sulphur trioxide condenses as sulphuric acid (H2S04) (the dew-point temperature).

The general process of condensation of sulphuric acid is complex. If the gas containing sulphuric acid has access to a surface at a temperature below the dew point, sulphuric acid will condense on this surface. It has also unexpectedly been discovered that supersaturation of S03 is avoided not only by condensation of H2S04 on the cold surface but also by condensation of sulphuric acid on 2 condensation nuclei entrained in the crude gas. Sulphuricacid droplets grow on and surround the condensation nuclei, which in some cases are very small and so are the sulphuric acid droplets, which usually have a mass equal to a multiple of the mass of the condensation nuclei.

A high percentage of S02 is separated in flue-gas desulphurising plants (gas scrubbers), but there are serious practical problems in separating gaseous S03 or H2S04. particularly small H2S04 aerosols. The reason is that these aerosols are much too large for diffusion-controlled separation processes ("gas scrubbing"). On the other hand, in spite of their increase in size through absorption of water vapour in a medium saturated with water in a scrubber, they are too small for significant inertia separation. In any case they are below the boundary conditions prevailing in a scrubber. As a result, when a crude gas is highly charged with S02. most of the sulphur is emitted in the form of sulphuric acid aerosols.

Emission of sulphuric acid aerosols, however, should not only be considered with respect to the total sulphur emission. On the contrary, the optical effect of a plume of waste gas charged with sulphuric acid aerosols (clouds round chimneys) may be so striking that it is experienced as unpleasant, to say the least, by the general public and will therefore inevitably give rise to complaints. In addition, a waste-gas plume charged with sulphuric acid aerosols will undoubtedly inject small droplets of sulphuric acid into the atmosphere, which does not contain sufficient neutralising substances. It is consequently impossible to avoid immission of larger drops containing sulphuric acid in the neighbourhood of chimneys which convey a stream of crude gas charged with sulphuricacid aerosol.

1 3 Among the objects of the invention are to provide a method of cooling hot crude gas charged with pollutants and to provide an apparatus f or carrying out the method such that sulphur trioxide or sulphuric acid can he largely separated from the crude gas and the formation of sulphuricacid aerosols can be avoided without a substantial additional energy requirement.

According to one aspect of the invention, there is provided a method whereby hot crude gas charged with pollutants is cooled upstream of a gas scrubber for removing sulphur from the crude gas, which is brought to a lower temperature level by a heat-withdrawing cooling fluid while f lowing through a cooling zone characterised in that, depending on the degree of saturation of the crude gas with water vapour and sulphur trioxide (S03). the crude gas in each longitudinal compartment of the cooling zone is kept at a cross-sectionally averaged temperature at or above the saturation curve defined by the specific dew-point temperature in the S031C temperature graph.

Basically according to the invention, the crude gas is cooled "gently" while " flowing through a cooling zone before entering the scrubber. In the cooling zone the crude gas is cooled in controlled manner so that in each longitudinal compartment of the cooling zone it is always at a crosssectionally averaged temperature allowing for the specific dew-point temperature determined by the water vapour and sulphur trioxide, in the case of a cooling zone with realistic practical dimensions. At no place, therefore, does the temperature of the crude gas fall below the temperature for permanently triggering aerosol formation. As a result of this deliberate temperature control of the crude gas when flowing through the cooling zone, S03 or H2S04 is reliably separated from the crude gas 4 and also the f orination of H2S04 aerosols is avoided. Owing to the low pressure losses, little or no additional energy is needed and additional useful heat is also obtained.

The crude gas can be cooled in accordance with a temperature curve which permanently decreases above the saturation curve. Another alternative is a method in which, when the temperature reaches or briefly falls below the saturation curve, the crude gas is re-heated by a certain amount (sawtooth curve), optionally a number of times during the cooling process. Some of the previouslyextracted heat and/or external energy can be used for intermediate heating.

Preferably, in the cooling zone, the crude gas is conveyed in countercurrent to a cooling fluid such as water, which absorbs heat from the crude gas and delivers it either to a stream of pure gas substantially freed from sulphur dioxide or to an external consumer of useful heat, such as a district-heating line.

According to another aspect of the invention there is provided apparatus for carrying out the method comprising a crude-gas cooler in front, in the direction of the crude-gas flow, of a gas scrubber withdrawing sulphur from the crude gas, the cooler being subjected to the crude gas on the one hand and a heat-withdrawing cooling fluid on the other hand, characterised in that the heat-transmitting surfaces in the crude-gas cooler are so dimensioned relatively to the quantity and speed of the crude gas that in every region of the crude-gas cooler the crude gas taken in indirect counter-current to the cooling fluid flowing in a closed circuit has a temperature at or above the saturation curve defined by the specific dew-point temperature in the S03/OC temperature graph.

The crude-gas cooler, accordingly, is designed so that the cooling fluid, which serves as a storage material and flows in a closed circuit, is at temperatures such that the surface temperatures in the crude-gas cooler are at no place below the expected S03/H2S04 dew point. To this end, only a slight temperature difference is provided at every contact between the crude gas and the cooling fluid (indirect contact). This ensures the necessary and desired heat transfer from the crude gas to the cooling fluid, but prevents excessive cooling at the surfaces of the crude-gas cooler resulting in boundary temperatures capable of forming condensate nuclei which are not deposited in the parts of the power station downstream of the crude-gas cooler (e.g. in the flue-gas desulphurisation or re-heating plant) and consequently remain in the gas and may ultimately reach the atmosphere.

A maximum length of contact between the crude gas and the cooling fluid at low relative temperatures in the various regions of the crude-gas cooler is achieved by means of the features in claim 4. The result is a typical counter-current crude-gas cooler, which can be equipped with pipes, flexible tubes or plates through which the cooling fluid flows and which are exposed to the flow of crude gas.

One important embodiment in this connection consists in the features in claim 5. The pipe coils are exposed to the cooling f luid in the exit region of the crude gas from the crude-gas cooler. The cooling f luid f lows through the pipe coils in counter-current to the crude gas and leaves the crude-gas cooler at the place where the crude gas enters it. Consequently, gentle cooling of the crude gas is assisted by the fact that the 6 crude-gas cooler is exposed to the already-heated cooling fluid in the region through which the crude gas has first flowed.

A preferred embodiment of the crude-gas cooler is characterised by the features in claim 6, according to which the crude-gas cooler comprises a number of sections disposed one behind the other in the direction of flow of the crude gas and each with U-shaped bunches of pipes or flexible tubes. The flexible tubes in particular are made entirely of a full plastic such as perfluralkoxide (PFA). Depending on the temperature of the crude gas for cooling or on the portions of water vapour and sulphur trioxide, a number of sections can be operated so as to cool the crude gas gently, distributed over a number of stages, without the temperature of the crude gas during the cooling process sinking into the aerosol region below the saturation curve in the S0314C temperature graph.

The invention basically increases the size of the crude-gas cooler. In that case, larger oncoming-flow cross-sections are also desirable, to reduce the pressure drop. Advantageously in this connection, according to claim 7, the crude-gas cooler is incorporated in the exit region of an electrostatic filter disposed in front of the scrubber in the stream of crude gas. The successive compartments of the electrostatic f i lter for separating flue dust can be combined directly, e.g. without an intermediate duct, with at least one additional compartment for cooling the crude gas and for removing sulphuric acid. This avoids the need for a flue-gas duct between the electrostatic filter and the crude-gas cooler, or hoods at the inflow and outflow ends of the crude-gas cooler. The loss of pressure by the crude gas (the external energy requirement) is substantially reduced by simplifying the 7 ducting and by reducing the flow speed through the crudegas cooler. The conditions of flow into the crude-gas cooler are significantly improved.

In order also to take account of variations in the load on a plant, e.g. a power station, for burning sulphurcontaining fuel and the resulting temperature fluctuations in the crude gas, advantageously according to the features of claim 8 the outward-flow and return-flow lines for cooling fluid connected to the crude-gas cooler are coupled by a bypass and by a mixing valve incorporated in the return-flow line. The outward and return flow lines can be connected both to a pure-gas heater behind the scrubber, and at least indirectly to an external consumer of useful heat, e.g. a heating line.

An embodiment of the invention according to the features of claim 9 is advantageous in this connection. Irrespectively of whether the crude-gas cooler is disposed between an electrostatic filter and a scrubber or directly incorporated in the electrostatic filter, heat is displaced from the crude-gas cooler via the scrubber to the pure-gas heater downstream of the scrubber.

If a pure-gas heater is incorporated directly in the scrubber or in the exit region thereof (claim 10) as when the crude-gas cooler is incorporated in an electrostatic filter, the installation costs can be reduced, the conditions of flow to the pure-gas heater can be improved and the pressure loss can be reduced. optimum separation of droplets can be achieved by disposing a drop separator in front of the pure-gas heater.

Alternatively according to claim 11, as already mentioned, the crude-gas cooler can be coupled to an 8 external consumer of useful heat, such as a heating line, via the cooling fluid conveyed in a closed circuit. Another alternative according to the invention is to couple the crude-gas cooler in parallel with a pure-gas heater and an external consumer of useful heat, via separate cooling circuits.

According to the features of the invention in claim 12, the bunches of pipes or tubes in the sections of the crude-gas cooler are connected in series in the direction of flow of the crude gas. The cooling fluid successively flows through the individual sections in counter-current to the crude gas, from the exit region of the crude-gas cooler to the entry region thereof.

The features in claim 13 are preferably applied when the crude gas for cooling contains extremely high proportions of S03. In this manner, some of the energy obtained after cooling the crude gas can be used to reheat the crude gas, while still in the crude-gas cooler, sufficiently above the sulphuric acid dew-point to prevent the temperature falling below the dew point, even in the neighbourhood of any cold bridges formed in the connection to the flue-gas desulphurising plant (gas scrubber). The heating stages (sections) can form a part of a pure-gas heater incorporated in the pure-gas flow downstream of a scrubber. The heatabsorbing fluid will then be the pure gas. Alternatively the sections can be incorporated in a district heating line.

In this connection, to ensure that all sulphuric acid aerosols are evaporated, according to claim 14 at least the last section of the crudegas cooler in the direction of flow of the crude gas can be connected to that cooling-fluid circuit which couples the second heating 9 stage to the first section in the direction of flow of the crude gas.

The embodiment according to the features of claim 15 likewise enables the temperature of the crude gas in the crude-gas cooler to be kept always above the saturation curve, even at extremely high proportions Of S03 Another advantageous embodiment consists in the features in claim 16. This feature also brings about gentle cooling of the crude gas, with improved separation Of S03 and a reduction in aerosol formation.

Finally, a possible embodiment consists in the features in claim 17. In this case, 503 is separated without aerosol formation, by a combination of heat recovery and use of heat.

The invention will now be explained in detail with reference to embodiments shown in the drawings, in which:

Figs. 1 to 8 are diagrams of apparatuses for cooling hot crude gas charged with pollutants; Fig. 9 is a vertical longitudinal section through another embodiment of a crude-gas cooler, and Figs. 10 and 11 are two S03/OC temperature graphs.

Fig. 1 shows a stream 1 of crude gas originating from a plant (not shown) for burning sulphur-containing fuel.

Dust is removed from the stream 1 of crude gas by conveying it through an electrostatic filter 2, from which the stream 1 at a temperature of about 17011C enters a crude-gas cooler 3. The cooler 3 comprises two sections A, B disposed one behind the other in the direction of flow and comprising U-shaped bunches 4, 5 of flexible tubes made of perfluralkoxide.

In the cooler 3, the crude gas is cooled by a fluid consisting of water, so that the crude gas leaves the cooler 3 at a temperature of about 1350C, at which it is transferred to a gas scrubber 6 (flue-gas desulphurising plant) for the purpose of removing sulphur.

A stream of pure gas 7 leaves the scrubber 6 at a temperature of about 55, 1C. The pure gas stream 7 is conveyed to a pure-gas heater 8, in which it is heated by the water heated in the crude-gas cooler 3 to a temperature of about 850C, at which the pure gas enters a chimney 9, through which it is discharged to atmosphere.

In the embodiment in Fig. 1, the pure-gas heater 8 comprises U-shaped bunches 10 of perfluralkoxide flexible tubes.

The water for cooling the crude gas is conveyed in a closed circuit 11 between the crude-gas cooler 3 and the pure-gas heater 8. To this end, a pump 13 driven by an electric motor 12 is incorporated in the circuit 11. Also, a buffer reservoir 14 is connected to the circuit 11.

In the exit region 15 of the crude gas from the cooler 3, at 78, the water is introduced into the bunch 5 of flexible tubes in the exit section B and flows through the tubes 4, 5 in counter-current to the crude gas in the sections A and B, which are connected in series. The water flows out at 17 in the region 16 where the crude gas enters 11 the cooler 3. The water is then returned through the circuit 11 to the pure-gas heater 8.

The embodiment in Fig. 2 dif f ers f rom Fig. 1 in that a mixing valve 19 is incorporated in the return-flow line 18 of the circuit 11 between the crude-gas cooler 3 and the pure-gas heater 8 and is connected by a bypass 20 to the outward-f low line 21. The mixing valve 19 can allow f or variations in the load on the plant generating the crude gas.

In the embodiment in Fig. 3, in contrast to the embodiment in Figs. 1 and 2, the crude-gas cooler 3 is incorporated in the exit region 22 of the electrostatic f ilter 2. This arrangement can also be provided with a mixing valve 19 and a bypass 20 as per Fig. 2.

In the embodiment in Fig. 4, the pure-gas heater 8 is incorporated in the exit region 23 of the scrubber 6. In this case also, the circuit 11 can be constructed as in Fig. 1 or Fig. 2.

In the embodiment in Fig. 5, a crude-gas cooler 3a consisting of three sections C, D, E connected in series in the direction of f low of the crude gas is coupled to the pure-gas heater 8 in the stream 7 of pure gas behind the scrubber 6; the outward-flow line 24 of the circuit 11a coupled to the outlet 36 of the tubes 10 of the pure-gas heater 8 is connected to the entry 25 of the tubes 26 in the middle section D. The exit 27 of the pipes 26 is connected in series with the front entry 28 of the pipes 29 in the direction of flow of the crude gas. The rear exit 30 of the pipes 29 in the first section C in the direction of flow of the crude gas is connected to the front entry 31 of the pipes 33 of the last section E in the direction of 12 flow of the crude gas. The rear exit 32 of the pipes 33 of the last section C in the direction of f low of the crude gas is coupled via the return-f low line 34 to the front entry 35 of the corresponding tubes 10 in the direction of the stream 7 of pure gas.

The circuit 11a can he designed like the circuit 11 in Fig. 1 or as per Fig. 2.

Fig. 6 shows an embodiment in which a crude-gas cooler 3b comprising four sections F, G, H, I containing Ushaped bunches of flexible tubes 37 is coupled via a circuit 11b to a pure-gas heater 8 in the stream 7 of pure gas behind the scrubber 6 and via an additional independent circuit llc to a heat exchanger 41, which is also exposed to a f luid for supplying heat to a useful-heat consumer (not shown). The connections for the f luid to the heat exchanger 41 are marked 42 and 43.

As can be seen, the two first sections F, G of the crude-gas cooler 3b in the direction of flow of the crude gas are coupled to the pure-gas heater 8, and the two last sections H, I are coupled to the heat exchanger 41. The two sections F, G and H, I respectively of each circuit 11b, 11c are connected in series, and in each case the front entry 46, 47 of the tubes 38, 40 in the direction of flow of the crude gas is connected to the outward-flow line 44, 45 of the circuit Ilb, 11c and the rear exit 48, 49 in the direction of flow of the crude gas is connected to the return-flow line 50, 51 of the circuit 11b, llc.

The circuits 11b, llc can be designed as per Fig. 1 or Fig. 2.

13 Fig. 7 shows an arrangement comprising a crude-gas cooler 3c made up of three sections K. LO M with bunches of flexible tubes 52, 53, 54 and a pure-gas heater 8a comprising two section N, 0 with bunches of tubes 55, 56 in the stream of pure gas 7 behind the scrubber 6. In this circuit, the second section 0 of the pure-gas heater 8a in the direction of f low of the pure gas is coupled via a closed circuit lid to the first section K of the crude-gas cooler 3c in the direction of flow of the crude gas. The circuit lid is also coupled to the last section M.

By contrast, the f irst section N of the pure-gas heater 8a in the direction of f low of the pure gas is connected via a closed circuit lie to the middle section L of the crude-gas cooler 3c.

The embodiment in Fig. 8 contains a crude-gas cooler 3d comprising three sections P, Q, R with bunches of tubes 57, 58 and 59. The crude-gas cooler 3d is coupled via a circuit llf to the tubes 10 in the pure-gas heater 8 behind the scrubber 6. The outward-f low line 60 of the circuit llf is connected to the front entry 61 of the tubes 59 in the direction of flow of the crude gas. The exit 62 of these tubes 59 is connected to the entry 63 of the tubes 57 of the first section P in the direction of flow of the crude gas. The exit 64 of the tubes 7 is connected to the rear entry 65 of the tubes 58 of section Q in the direction of flow of the crude gas. The exit 66 of the tubes 58 is connected to the return- flow line 67 of the circuit llf.

This circuit likewise, as in Fig. 11, can be used to keep the temperature of the crude gas above the saturation curve 76 in the S03P5C temperature graph, by stepwise re-heating.

14 The circuit lif can be designed like the circuits 11 in Figs. 1 and 2.

In the embodiments in Figs. 1 to 8, both the crudegas coolers 3 - 3d and the pure-gas heaters 8, 8a are always provided with U-shaped bunches of flexible perfluralkoxide (PFA) tubes 4, 5, 10, 26, 29, 33, 37 - 40, 52 59.

Fig. 9 shows an embodiment in which a crude-gas cooler 3e is equipped with a number of side-by-side pipe coils 68 which convey a cooling fluid and are surrounded by a flow of crude gas. The pipe coils 68 lie side by side in the plane of the drawing. This embodiment likewise ensures that the entry 69 of the cooling fluid into the crude-gas cooler 3e lies in the exit region 70 of the crude-gas stream 1 from the cooler 3e and the exit 71 of the cooling fluid from the crude-gas cooler 3e is provided in the entry region 72 of the crude-gas stream 1 into the cooler 3e. The pipe coils 68 can be made of plastic.

Alternatively of course a pure-gas heater 8, Sa can be constructed in similar manner. Another alternative is a combination of heat exchangers comprising U-shaped bunches of flexible tubes on the one hand and pipe coils on the other hand.

Fig. 10 is a graph in which the S03 content of the crude gas in mg/NM3 is plotted on the abscissa 73 and the temperature of the crude gas in C is plotted along the ordinate 74. By way of example, the small rectangular compartment 75 shows the temperature curve at which the heat energy required for re-heating pure gas from 560C to 8511C is taken from a stream of crude gas which at the beginning of the cooling process contains about 12% by volume of H20.

Accordingly, the crude gas is gently cooled from about 1700C to about 1350C, ensuring that the temperature of the crude gas always remains above the saturation curve 76 (chain line) defined by the specific dew- point temperature and does not f all into the aerosol region 77 underneath. During this temperature exchange, the water temperature rises from about 1150C to about 1500C and the temperature of the pure gas in the pure-gas stream 7 downstream of the scrubber 6 increases from about 560C to about 850C. These temperatures are shown by circles marked I to VI in Fig. 1.

Corresponding remarks apply to the graph in Fig.

11.

16

Claims

1. A method whereby hot crude gas charged with pollutants is cooled upstream of a gas scrubber (6) f or removing sulphur from the crude gas, which is brought to a lower temperature level by a heat-withdrawing cooling fluid while flowing through a cooling zone (3, 3a - 3e), characterised in that, depending on the degree of saturation of the crude gas with water vapour and sulphur trioxide (S03) 9 the crude gas in each longitudinal compartment of the cooling zone (3, 3a - 3e) is kept at a cross-sectionally averaged temperature at or above the saturation curve (76) defined by the specific dew-point temperature in the S03/OC temperature graph.

2. A method according to claim 1, characterised in that the crude gas is cooled in indirect counter-current to the cooling fluid flowing in the closed circuit (11, 11a llf).

3. Apparatus for carrying out the method according to claim 1 or 2, comprising a crude-gas cooler (3, 3a - 3e) in front, in the direction of the crude-gas flow (1), of a gas scrubber (6) withdrawing sulphur from the crude gas, the cooler being subjected to the crude gas on the one hand and a heat-withdrawing cooling fluid on the other hand, characterised in that the heat-transmitting surfaces in the crude-gas cooler (3, 3a - 3e) are so dimensioned relatively to the quantity and speed of the crude gas that in every region of the crude-gas cooler (3, 3a - 3e) the crude gas taken in indirect counter-current to the cooling fluid flowing in a closed circuit (11, 11a - 11f) has a temperature at or above the saturation curve (76) defined 17 by the specific dew-point temperature in the S0310C temperature graph.

4. Apparatus according to claim 3, characterised in that the entry (78, 47, 69) of the cooling fluid into the crude-gas cooler (3, 3b, 3e) is disposed in the region of the exit (15, 70) of the crude gas from the crude-gas cooler (3, 3b, 3e) and the exit (17, 48, 71) of the cooling fluid from the crude-gas cooler (3, 3b, 3e) is disposed in the region of the entry (16, 72) of the crude gas into the crude-gas cooler (3, 3b, 3e).

5. Apparatus according to claim 3 or 4, characterised in that the crudegas cooler (3e) is equipped with a number of pipe coils (68) disposed side by side and conveying the cooling fluid and having the stream of crude gas flowing therearound.

6. Apparatus according to claim 3 or 4, characterised in that the crudegas cooler (3, 3a - 3d) is made up of a number of sections (A, B; C, D, E; F, G, H, I; K, L, M; P, Q, R) disposed one behind the other in the direction of flow of the crude gas and each comprising U-shaped bunches of pipes or flexible tubes (4, 5; 29, 26, 33; 37, 38, 39, 40; 52, 53, 54; 57, 58, 59).

7. Apparatus according to any of claims 3 to 6, characterised in that the crude-gas cooler (3, 3a - 3c) is incorporated in the exit region (22) of an electrostatic filter (2) disposed in the crude-gas stream (1) in front of the scrubber (6).

8. Apparatus according to any of claims 3 to 7, characterised in that the outward-flow and return-flow lines (21, 18; 24, 34; 44, 50; 45, 51; 60, 67) for the 18 cooling f luid connected to the crude-gas cooler (3, 3a, 3b, 3d) are interconnected by a bypass (20) and by a mixing valve (19) incorporated in the return-f low line (18, 34, 50, 51, 57).

9. Apparatus according to any of claims 3 to 8, characterised in that the crude-gas cooler (3, 3a - 3d) is coupled, via the cooling fluid conveyed in a closed circuit (11, 11a, 11b, 11d - 11f), to a pure-gas heater (8, 8a) incorporated in the pure-gas stream (7) downstream of the gas scrubber (6).

10. Apparatus according to any of claims 3 to 8, characterised in that the crude-gas cooler (3) is coupled, via the cooling fluid conveyed in the closed circuit (11), to a pure-gas heater (8) incorporated directly in the scrubber (6) or in the exit region (23) thereof.

11. Apparatus according to any of claims 3 to 8, characterised in that the crude-gas cooler (3b) is coupled, via the cooling fluid conveyed in a closed circuit (11c), to an external consumer (41) of useful heat.

12. Apparatus according to any of claims 6 to 11, characterised in that the bunches (4, 5) of tubes or flexible pipes in the sections (A, B) of the crude-gas cooler (3) are connected in series in the direction of flow of the crude gas.

13. Apparatus according to any of claims 6 to 11, characterised in that the fluid absorbing heat from a cooling fluid is conveyed through two successive heating stages (H, 0), the first heating stage (N) in the direction of flow of the fluid being coupled to at least one section (L) of the crude-gas cooler (3c) via a cooling fluid 19 conveyed in the closed circuit (11c) and subjected to the already-cooled crude gas, whereas the second heating stage (0) is coupled, via a cooling fluid conveyed in the closed circuit (lid), to at least that section (K) which is subjected to the as yet uncooled crude gas (Fig. 7).

14. Apparatus according to claim 13, characterised in that at least the last section (M) of the crude-gas cooler (3c) in the direction of flow of the crude gas is connected to that cooling-fluid circuit (lid) whereby the first section (K) in the direction of f low of the crude gas is coupled to the second heating stage (0) (Fig. 7).

15. Apparatus according to any of claims 6 to 11, characterised in that the fluid absorbing heat from a cooling f luid is conveyed through a heat exchanger (8) connected to two sections (D, E) of the crude-gas cooler (3a) disposed downstream of the f irst section (C) in the direction of f low of the crude gas, the exit (30) of the f irst section (C) being coupled to the entry (31) of the last section (E) (Fig. 5).

16. Apparatus according to any of claims 6 to 11, characterised in that the f irst sections (F, G) in the direction of flow of the crude gas, when connected in series, are coupled via a cooling f luid conveyed in the closed circuit (11b) to a pure-gas heater (8) incorporated in the pure-gas stream (7) behind the scrubber (6), and the last sections (H, I) in the direction of flow of the crude gas, when connected in series, are coupled via a cooling fluid conveyed in the closed circuit (11c) to an external consumer (41) of useful heat (Fig. 6).

17. Apparatus according to any of claims 6 to 11, characterised in that, in the case of a crude-gas cooler (3d) comprising three successive sections (P, Q, R) in the direction of flow of the crude gas, the outward-flow line (60) of the cooling-fluid circuit (I1f) is connected to the front entry (61) of the last section (R) in the direction of f low of the crude gas, the rear exit (62) of the last section (R) is connected to the f ront entry (63) of the first section (P), the rear exit (64) of the first section (P) is connected to the rear entry (65) of the middle section (Q) and the front exit (66) of the middle section (Q) is connected to the return-flow line (67) of the circuit (11f) (Fig. 8).

18. A method of treating hot crude gas charged with pollutants, substantially as hereinbefore described.

19. Apparatus for treating hot crude gas charged with pollutants, substantially as hereinbefore described with reference to and as shown in any of Figures 1 to 9 of the accompanying drawings.

20. Any novel feature or combination of features described herein.

List of reference numbers 1 2 3 4 5 6 8 9 11 12 13 14 is 16 17 18 19 20 21 21 Crude-gas stream Electrostatic filter Crude-gas cooler 3a Crude-gas cooler 3b Crude-gas cooler 3c Crude-gas cooler 3d Crude-gas cooler 3e Crude-gas cooler Bunch of flexible tubes Bunch of flexible tubes Gas scrubber Pure-gas stream Pure-gas heater Sa Pure-gas heater Chimney Bunch of flexible tubes in 8, 8a circuit lla circuit 11b Circuit Ilc Circuit lid Circuit lie Circuit lif Circuit Electric motor Pump Buffer reservoir Exit region of 3, 3a - 3d Entry region of 3, 3a 3d Exit of 4 Return-flow line of 11 Mixing valve Bypass outward-flow line of 11 22 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 46 47 48 49 50 51 52 53 54 55 56 Exit region of 2 Exit region of 6 Outward-flow line of 11a Entry of 26 Bunch of flexible tubes in D Exit of 26 Entry of 29 Bunch of flexible tubes in C Exit of 29 Entry of 33 Exit of 33 Bunch of flexible tubes in E Return-flow line of 11a Entry of 10 Exit of 10 Bunch of flexible tubes in F Bunch of flexible tubes in G Bunch of flexible tubes in H Bunch of flexible tubes in I Heat exchanger Connection to 41 Connection to 41 Outward-flow line of 11b Outward-flow line of lic Entry of 38 Entry of 40 Exit of 37 Exit of 39 Return-flow line of 11b Return-flow line of 11c Bunch of flexible pipes in K Bunch of flexible pipes in L Bunch of flexible pipes in M Bunch of flexible pipes in N Bunch of flexible pipes in 0 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 76 77 78 23 Bunch of flexible pipes in P Bunch of flexible pipes in Q Bunch of flexible pipes in R Outward-flow line of Ilf Entry of 59 Exit of 59 Entry of 57 Exit of 57 Entry of 58 Exit of 58 Return-flow line of Ilf Pipe coils Entry of 68 Exit region of 3e Exit of 68 Entry region of 3e Abscissa Ordinate Rectangular compartment Saturation curve Aerosol region Entry of 5 A Section of 3 B Section of 3 c Section of 3a D Section of 3a E Section of 3a F Section of 3b G Section of 3b H Section of 3b I Section of 3b K Section of 3c L Section of 3c m Section of 3c 24 N 0 p Q R I II III IV vi Section of 8a Section of 8a Section section Section Temperature between 2 and 3 Temperature between 3 and 6 Temperature between 6 and 8 Temperature between 8 and 9 Temperature behind 3 Temperature in front of 3