WO1994008697A1

WO1994008697A1 - Method for flue-gas cleaning

Info

Publication number: WO1994008697A1
Application number: PCT/SE1993/000861
Authority: WO
Inventors: Björn LÜNING
Original assignee: Senea Miljöteknik Ab
Priority date: 1992-10-22
Filing date: 1993-10-20
Publication date: 1994-04-28
Also published as: CZ103695A3; FI951911A0; SE500037C2; SE9203105L; PL308479A1; CZ285424B6; SE9203105D0; FI951911A; AU5347194A

Abstract

In a method for cleaning SOx-polluted flue gases from a combustion process, the flue gases are, in a gas-cleaning zone (20), passed through a gas-cleaning bed (22) of an SOx-absorbing substance which, once polluted and heated by the flue gases, is circulated out of and back to the gas-cleaning zone (20) in a circulation path which passes through a cooling zone (30), where the substance is directly contacted with a cooling medium in order to be cooled, and a separating zone (40), where sulphur pollutants are removed. The gas-cleaning bed (22) is gradually displaced into the cooling zone (30) where it instead forms a cooling bed (32) through which the cooling medium is passed. The bed is displaced through a channel (21, 50, 31) of annular cross section, such that the substance is fed as a tubular body (22, 53, 32) through the channel running through the gas-cleaning zone (20) as well as the cooling zone (30). The flue gases and the cooling medium are passed substantially radially through the bed in the gas-cleaning zone (20) and the cooling zone (30), respectively.

Description

METHOD FOR FLUE-GAS CLEANING

This invention relates to a method for cleaning SO - polluted flue gases from a combustion process, specifi- cally a method comprising the steps of passing the flue gases through a substantially dry gas-cleaning bed of an SO -absorbing substance in a gas-cleaning zone; and cir- culating said substance, once polluted and heated by the flue gases, out of and back to the gas-cleaning zone in a circulation path which passes through a cooling zone, where the substance is directly contacted with a cool¬ ing medium in order to be cooled, and a separating zone, where sulphur pollutants are removed from the substance. The preamble to appended claim 1 is based on a method, described in EP-Al-0,205,866, including the above steps. In a prior-art method for flue-gas cleaning, use is made of a fluidised combustion bed of an SOx-absorb- ing substance (e.g. lime, limestone or dolomite) absorbing oxidised sulphur which is released during combustion. How- ever, this prior-art method requires comparatively compli¬ cated and expensive equipment and involves problems of taking care of the waste from the combustion bed.

In another prior-art method for flue-gas cleaning, use is made of a gas-cleaning apparatus (scrubber) with liquid as separating medium. This method particularly involves problems of taking care of the wet waste from the gas-cleaning apparatus.

SE 422,777 teaches a method using two or more maga¬ zines, each having an SO -absorbing bed through which flue gases and air flow alternately, the flue gases being cleaned and the air absorbing the heat which has been emitted to the bed by the flue gases in a preceding state of flow. The SE specification teaches the possibility of conducting the heated air as preheated combustion air to a combustion process. This technique requires a large number of movable components for alternating between the flue-gas flow and the air flow, which leads not only to high con- struction costs, but also to problems in respect of reliability and repairing. The alternation between the flows also results in undesirable air and flue-gas blasts which, inter alia, entrain fine particles from the bed material into the chimney where they are discharged into the atmosphere. Further, there are problems related to the renewal or cleaning of the substance in the bed. After some time of operation, granules are discharged from the beds to a separating device, where sulphur pollutants and dust particles are removed, whereupon the granules are returned to the bed. Because also bed material that has not been completely cooled is discharged from the maga¬ zines to the separating device, thermal energy is lost. In addition, the bed is not continuously cleaned, and thus shows an uneven gas-cleaning effect.

The cleaning method described in the above-mentioned EP-A1-0 205 866 and comprising the steps enumerated in the introductory paragraph of this specification, has the fol¬ lowing advantages over the method described in SE 442,777: there is no need for using a number of separate magazines and valve mechanisms for alternately controlling the flue- gas flow and the air flow; both the flue-gas flow and the cooling-medium flow can be continuous; the SO -absorbing substance can be continuously circulated; and the energy content of the heated substance is utilised more effi¬ ciently, because cooling takes place in a separate cooling zone.

Despite its advantages, the method according to EP- Al-0 205 866 is deficient in some ways. Thus, EP-A1- 0 205 866 teaches a cleaning plant having an upper sepa¬ rate container for a gas-cleaning bed of limestone or the like, and a lower separate container for a cooling bed of the same material. The lower part of the upper container is in the form of a downwardly-directed funnel extending through a horizontal partition between the containers and opening into the upper part of the lower container. Clean¬ ed bed material is returned to an inlet at the top of the upper container, and polluted and cooled bed material is removed via a valve means at the bottom of the lower con¬ tainer. The geometry and the inlets and outlets for the flue gases and the cooling air are such that the flue gases as well as the cooling air move countercurrent (upwards) to the direction of movement (downwards) of, respectively, the gas-cleaning bed and the cooling bed. The plant according to EP-A1-0 205 866, described in the preceding paragraph, suffers from the following drawbacks. First, the overall size of the plant is con¬ siderable, the bed material being utilised comparatively ineffectively. Second, there is a relatively high pressure drop over the respective beds. As a result, this known plant is expensive to build as well as to run. Third, there is a risk of clogging in the funnel-shaped passage between the containers, which of course is undesirable.

The invention has for its object a substantially dry method for flue-gas cleaning which provides efficient, expedient and inexpensive removal of SO from the flue gases from a combustion process and which also enables efficient utilisation of the energy content of the fuel.

Especially, the invention has for its object a method for flue-gas cleaning in which an SO -absorbing substance is continuously circulated, but which does not suffer from the above-mentioned drawbacks of the method according to EP-A1-0 205 866.

Thus, a particular object of the invention is to pro¬ vide such a method which reduces the overall size of the plant by utilising the SO -absorbing substance more effi- ciently, reduces the pressure drops over, respectively, the gas-cleaning bed and the cooling bed, and reduces the risk of clogging.

These objects are attained by a method having the features recited in appended claim 1, preferred embodi- ments of the invention being defined in the appended sub- claims. Thus, the invention provides a method for flue-gas cleaning comprising the steps enumerated in the introduc¬ tory paragraph and being characterised by gradually displacing the gas-cleaning bed into the cooling zone where it instead forms a cooling bed through which the cooling medium is passed, causing the gradual displacement of the gas-cleaning bed from the gas-cleaning zone to the cooling zone to take place through a channel of annular cross-section which extends in the direction of displacement and runs through the gas-cleaning zone as well as the cooling zone, the substance being fed as a tubular body through said channel from an inlet end located at the gas-cleaning zone to an outlet end located at the cooling zone, and passing the flue gases and the cooling medium sub¬ stantially radially through said tubular body in the gas- cleaning zone and the cooling zone, respectively.

The inventive method enables efficient utilisation, and consequently low consumption, of the SO -absorbing substance, as well as a reduced pressure drop over the respective beds, which renders the plant less expensive to build (reduced overall size, smaller conveying means, smaller fans, and so forth) and to run (reduced power con¬ sumption of the fans and the conveying means). Neither is there any risk of clogging at the transition between the gas-cleaning zone and the cooling zone. The invention yields a high cleaning effect of the SO -absorbing sub- stance owing to this being cooled, the possibility of recovering heat from the flue gases, as well as a, gene- rally speaking, rational cleaning process.

In tests using a plant operating according to the inventive method, it has proved sufficient, in terms of cleaning and heat transfer, for the flue gases to be passed through a bed having a thickness of about 0.5 m. Since (i) the bed is in the form of a hollow, tubular body and (ii) the flue-gas flow is passed radially through the wall of this tubular body, the flue-gas flow will have a large impingement surface for a comparatively small floor surface of the reactor. Also the height of the reactor is reduced, which affects not only the space required by the plant, but also the height to which the absorbing sub- stance has to be conveyed, which is more expensive in a higher plant, both in installation and in operation.

For a given particle size of the absorbing substance, the pressure drop of the gas across the bed is essentially the same per unit length in the plant according to the invention as in the prior-art plant according to EP-A1- 0 205 866. With the aid of comparative computations and on the assumption that the wall size is 0.5 as above, it can be proved that the pressure drop may be reduced by at least a factor 16 with the inventive method, resulting in considerable savings on fan capacity and power consump¬ tion.

The design of the bed as a tubular body has the addi¬ tional advantage of making it possible, when dimensioning the impingement surface of the flue gases, to take more into account the fact that the volume of the flue gases decreases as the gases are cooled during their passage through the bed. If the bed were flat, the velocity of the flue-gas flow in the bed would be lower at the end than at the beginning. If, in accordance with a preferred embodi- ment of the invention where the flue gases pass radially inwards through the bed, the impingement surface of the gases however decreases as the volume of the flue gases decreases when they are passed through the bed, the flue gases can have an optimal, linear and substantially con- stant velocity through the whole bed.

The SO -absorbing substance can be transferred between the gas-cleaning zone and the cooling zone through a connecting channel having a suitable length and the same, or essentially the same, cross-section as the tubu- lar bed and connecting to an outlet opening of correspond¬ ing cross-section at the gas-cleaning zone as well as an inlet opening of corresponding cross-section at the cool- ing zone. An effective "material lock" counteracting leak¬ age between the zones is thus obtained. For enhanced safe¬ ty, the pressure of the gases can be checked at the respective inlets and outlets, so that the gases will not flow in the wrong direction. Tests have shown that minimum leakage of air to the flue gases and, which is more impor¬ tant, of flue gases to the air, is attained if the respec¬ tive fans are arranged downstream from (after) the asso¬ ciated sections of the bed. As known from SE 442,777 and EP-A1-0 205 866, the cooling medium may be air, which is passed through the cooling zone directly contacting the substance and which, after being thus heated in the cooling zone, is used as combustion air in the combustion process. The heat absorbed may also be used in other ways, e.g. as combustion air in another combustion process. It is further conceivable to use a liquid cooling medium.

The SO -absorbing substance may be limestone, dolo- mite or some other mineral of similar properties. EP-A1- 0 205 866 indicates other conceivable materials. For the sake of simplicity, the term "limestone" will be used in the following description of embodiments as a non- restricting instance of a suitable substance, but it should be emphasised that every mention of limestone also encompasses all other SO .ft.-absorbing substances conceivable for use in the invention.

The expressions "channel of annular cross-section" and "tubular body" used in this specification are intended to cover also the alternative of a broken annular cross- section, enabling the flue gases and the cooling medium to be conducted substantially radially into and/or out of the central compartment inside the bed through a gap or open¬ ing at the side of the bed.

Embodiments of the invention will now be described in more detail with reference to the accompanying drawings schematically illustrating the inventive method. In the drawings, Fig. 1 is a flow chart of a cleaning plant implement¬ ing the inventive method;

Fig. 2 is a schematic and enlarged vertical section of a gas-cleaning zone and a cooling zone of the plant in Fig. 1; and

Fig. 3 is a schematic and enlarged vertical section of an alternative embodiment comprising two gas-cleaning zones and two cooling zones.

Like elements have like reference numerals in the drawings. The flue-gas flows and the cooling-medium flows are throughout indicated by, respectively, filled short arrows and unfilled short arrows, the limestone flows being indicated by more slender arrows.

In the flow chart in Fig. 1, flue gases are fed from a boiler (not shown) through a channel 1 to a register 2 so set as to conduct the flue gases to a cleaning plant 10 via a channel 3. After cleaning, the flue gases are discharged through a chimney at 6 via a channel 4 and a fan 5. The cleaning plant 10 comprises three zones: a gas- cleaning zone 20, a cooling zone 30 and a separating zone 40. The three zones are disposed in this order in a cir¬ culation path for limestone.

The gas-cleaning zone 20 has a container 21 of annu- lar cross-section. The container 21 holds a gas-cleaning bed 22 of limestone or equivalent material, received via inlet openings 23 at the upper part of the container. A central compartment 26 inside the container 21 communi¬ cates with the outlet channel 4. The container 21 is sur- rounded by an inlet box 24 connected to the inlet channel 3.

The cooling zone 30 has a container 31 having the same annular cross-section as the container 21 and being disposed concentrically with respect to the container 21 and at a distance below it. The container 31 holds a cool¬ ing bed 32 of limestone which is received from the con¬ tainer 21 of the gas-cleaning zone 20 via an intermediate connection 50. The limestone is discharged from the cool¬ ing bed 32 through a conical outlet unit 51 at the lower part of the container 31 under the control of a bottom valve 52 preventing air from leaking out of the container 31. A central compartment 36 inside the container 31 com¬ municates with an inlet channel 7 for incoming cooling medium, here being air. The container 31 is surrounded by an outlet box 34 for outgoing heated cooling air which is conducted as preheated combustion air to the boiler via a fan 8.

How these components of the gas-cleaning zone 20 and the cooling zone 30 operate and are designed will be described in more detail below with reference to Fig. 2. Fig. 1 shows a separating zone 40, receiving lime- stone from the cooling zone 30 via a channel 41, which comprises in succession a drum 42, a sieve unit 43 and a buffer container 44. The limestone is transferred from the separating zone 40 to the gas-cleaning zone 10 via a chan¬ nel 45, to be returned to the gas-cleaning bed 22 via the inlet openings 23.

The plant illustrated in Fig. 1 essentially operates as follows. Hot SO -polluted flue gases from the combus- tion process in the boiler are fed, via the channel 3, into the inlet box 24 and through the gas-cleaning bed 22 in the gas-cleaning zone 20. The hot and polluted flue gases are then directly contacted with the limestone, which absorbs the sulphur oxides, resulting in that "scales" of pollutants deposit on the limestone grains. The cleaning effect of the limestone decreases as the pol- lutant scales accumulate and the limestone is heated by the flue gases. The cleaned and cooled flue gases are con¬ ducted to the chimney 6 via the outlet channel 4 and the fan 5.

The flue gases are continuously conducted into the cleaning plant 10 during combustion. In order that the gas-cleaning bed 22 should operate effectively all the time, the limestone is continuously replaced by polluted and heated limestone being discharged from the gas-clean¬ ing bed 22 and by cleaned and cooled limestone being sup¬ plied through the inlets 23.

The polluted and heated limestone discharged from the gas-cleaning zone 20 is, under the action of gravity and through the connection 50, conducted into the container 31 of the cooling zone 30 to replenish the cooling bed 32. At the same time, cooled limestone is removed from the cool¬ ing bed 32 through the bottom valve 52 controlling the rate of displacement of the limestone. In order to cool the limestone, air is conducted from the air channel 7 through the cooling bed 32 in direct contact with the limestone, and the heated air is then used as preheated combustion air in the boiler. Limestone cooled in the cooling zone 30 is fed to the separating zone 40, where the pollutant scales are sepa¬ rated from the limestone by passing this first through the drum 42, where the pollutant scales are decomposed, and then through the sieve unit 43, where the decomposed pol- lutant scales are removed by sieving, to be deposited as indicated at 46. If need be, fresh limestone is supplied to the circulation path, as indicated at 47.

The limestone is primarily conveyed under the action of gravity from the gas-cleaning zone 20 to the drum 42, via the cooling zone 30, but is mechanically conveyed from the drum 42 to the buffer container 44, via the sieve unit 43. Cleaned and cooled limestone is returned through the channel 45 by mechanical or pneumatic means.

The limestone cleaned in stages 42 and 43 is then conducted to the buffer container 44 to be supplied on to the gas-cleaning bed 22.

Fig. 2 is an enlarged view showing the gas-cleaning zone 20 and the cooling zone 30, as well as the associated pipe connections and fans. The container 21, having an annular cross-section, of the gas-cleaning zone 20 is delimited by a cylindrical outer wall 21a and a cylindrical inner wall 21b, which both are perforated to allow the flue gases to pass. Similarly, the container 31 of the cooling zone 30 has a cylindrical outer wall 31a and a cylindrical inner wall 31b, which both are perforated to permit the cooling air to pass.

The connection 50 between the "gas-cleaning zone 20 and the cooling zone 30 have the same annular cross-sec¬ tion as the containers 21 and 31 and is delimited by an outer wall 50a and an inner wall 50b, which interconnect the outer walls 21a and 31a and the inner walls 21b and 31b, respectively. Thus, the container 21, the connection 50 and the container 31 together form a straight channel extending in the direction of displacement of the lime¬ stone and running through the gas-cleaning zone 20 as well as the cooling zone 30. Through this channel, the lime¬ stone is gradually fed in the form of a tubular, conti¬ nuous or coherent body from an inlet end at 23 to an out¬ let end at 37. The portion of the coherent limestone bed located in the connection 50 is designated 53. The cross-section of the annular channel is prefer¬ ably, but not necessarily, circular, and is preferably, but not necessarily, constant.

The containers 21 and 31 are each surrounded by an annular compartment 25 and 35, respectively. These com- partments are outwardly delimited by the inlet box 24 and the outlet box 34, respectively, and are radially inwardly delimited by the perforated outer walls 21a and 31a, respectively.

Thus, the flue gases from the boiler are conducted to the annular compartment 25 of the inlet box 24 via the channel 3, and then pass substantially radially inwards through the gas-cleaning bed 22 to the central compart¬ ment 26. The cleaned flue gases are thereafter conducted upwards and away via channel 4 and the fan 5. Cooling air from the channel 7 is conducted upwards in the central compartment 36 and thereafter passes sub- stantially radially outwards through the cooling bed 32 to the annular compartment 35 of the outlet box 34, to be subsequently removed via the fan 8.

In the portion 53 between the gas-cleaning zone 20 and the cooling zone 30, the limestone acts as a material lock 53 preventing the cooling air from reaching the gas- cleaning zone 20 and, which is more important, preventing the flue gases from reaching the cooling bed 32. To enhance this effect, the cooling-air fan 8 is arranged downstream from (after) the cooling bed 32 and the flue- gas fan 5 is arranged downstream from (after) the gas- cleaning bed 22. As a result, the pressure of the flue gases in the gas-cleaning zone 20 is slightly lower than the pressure of the cooling air in the cooling zone 30. Any gas conveyed through the material lock 53 will then consist of air that has reached the gas-cleaning zone 20 by overcoming the pressure drop caused by the material lock 53.

When the limestone in the embodiment illustrated in Fig. 2 leaves the gas-cleaning zone 20, it is polluted as well as heated. After passing through the cooling zone 30, the limestone is still polluted but may now be enough cooled to be used for gas cleaning before the cleaning stage in the separating zone 40. A further development of the embodiment in Fig. 2 is illustrated in Fig. 3. Here, the limestone is thus repeatedly used in each circulation.

The embodiment illustrated in Fig. 3 comprises an upper gas-cleaning zone 20. and a lower gas-cleaning zone 20_r essentially similar to the gas-cleaning zone 20 in Fig. 2, as well as an upper cooling zone 30_π and a lower cooling zone 30-. essentially similar to the cooling zone 30 in Fig. 2. As seen from above in Fig. 3, these zones are vertically spaced apart in the following order: 20.., 30-., 20 30_L. Each of these zones has a perforated con- tainer 21y, 31-_j, 21 and 31., respectively, as well as a surrounding inlet or outlet box 24 34 24_τ and

U U L

34 respectively, having annular compartments 25_π, 35 , 25_τ la, 35_τL and central compartments 26_τU_τ, 36_IU_T, 26_τ la and 36_τ , respectively. The four containers are interconnected by connections 50 to form an elongate annular channel of con¬ stant, circular and annular cross-section. The limestone is gradually fed as a coherent tubular body through this channel.

The flue gases from the boiler are conducted in parallel manner into the two inlet boxes 24_I U_T and 24_τLi so as to pass, via the annular compartments 25_τ U_τ and 25_τ la, sub- stantially radially inwards through an upper and a lower gas-cleaning bed 22 and 22 respectively. The central compartments 26_ττ and 26_τ are interconnected by a connect- ing tube 28 which, with suitable seals, passes through the outlet box 34B and which removes cleaned flue gases from the lower gas-cleaning zone 20^.

The cooling air is, via the channel 27, conducted to the lower central compartment 36.. and, via a connecting tube 38 corresponding to the tube 28, conducted to the upper central compartment 36... Thereafter, the cooling air passes substantially radially outwards through a lower and an upper cooling bed 32 and 32.., respectively.

It goes without saying that many modifications of the embodiments described above are conceivable within the scope of the appended claims. For instance, the perforated walls of the containers 21 and 31 may be replaced with walls composed of lamellae that are vertically spaced apart and directed obliquely outwards and upwards. Fur¬ thermore, the direction of flow of the flue gases and the cooling air can be reversed, although the illustrated embodiment is the most preferred.

Claims

1. A method for cleaning SO -polluted flue gases from a combustion process, said method comprising the steps of: passing the flue gases through a substantially dry gas-cleaning bed (22) of an SO -absorbing substance in a gas-cleaning zone (20), and circulating said substance, once polluted and heat¬ ed by the flue gases, out of and back to the gas-cleaning zone (20) in a circulation path which passes through a cooling zone (30) where the substance is directly con¬ tacted with a cooling medium in order to be cooled, and a separating zone (40) where sulphur pollutants are remov¬ ed from the substance, c h a r a c t e r i s e d by gradually displacing the gas-cleaning bed (22) into the cooling zone (30) where it instead forms a cooling bed (32) through which the cooling medium is passed, causing the gradual displacement of the gas-cleaning bed from the gas-cleaning zone (20) to the cooling zone (30) to take place through a channel (21, 50, 31) of annu¬ lar cross-section which extends in the direction of dis¬ placement and runs through the gas-cleaning zone (20) as well as the cooling zone (30), the substance being fed as a tubular body (22, 53, 32) through said channel from an inlet end (23) located at the gas-cleaning zone (20) to an outlet end (37) located at the cooling zone (30), and passing the flue gases and the cooling medium sub- stantially radially through the bed in the gas-cleaning zone (20) and the cooling zone (30), respectively.

2. A method as set forth in claim 1, c h a r a c ¬ t e r i s e d in that the flue gases are passed radial¬ ly inwards through the bed (22) in the gas-cleaning zone (20).

3. A method as set forth in claim 1 or 2, c h a r ¬ a c t e r i s e d in that the cooling medium is passed radially outwards through the bed (32) in the cooling zone (30).

4. A method as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the annular cross-section of the channel (21, 50, 31) is substantially constant in the direction of displacement of the tubular body.

5. A method as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the annular cross-section of the channel (21, 50, 31) is circular.

6. A method as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the channel comprises an intermediate portion (50) located between the gas-cleaning zone (20) and the cooling zone (30), and that the portion (53) of the tubular body situated in said intermediate portion (50) is not subjected to any radial flow-through of flue gas or cooling medium, whereby to serve as a material lock.

7. A method as set forth in any one of the preceding claims, c h a r a c t e r i s e d in that the circula¬ tion path includes, in addition to the gas-cleaning zone (20-_j) and the cooling zone (30 ), one or more additional gas-cleaning zones (20 ) and one or more additional cool¬ ing zones (30-. ).

8. A method as set forth in claim 7, c h a r a c ¬ t e r i s e d in that the gas-cleaning zones (20_T U_I, 20_τL) and the cooling zones (30 , 30 ) forming part of the cir- culation path are arranged alternately, one after the other.

9. A method as set forth in claim 7 or 8, c h a r ¬ a c t e r i s e d in that the channel (21, 50, 31) passes through all the gas-cleaning zones (20 U 20L_τi) and all the cooling zones (30 30 ) forming part of the circulation path.

10. A method as set forth in any one of the preced¬ ing claims, c h a r a c t e r i s e d in that the SO X- absorbing substance is continuously, or at least substan¬ tially continuously, circulated during the combustion pro- cess.