WO1992004122A1 - Process and device for cleaning waste gases - Google Patents

Abstract

Dust-free hot gas (10) is taken to a gas turbine for the rational production of electric power. Hot gas (1) containing dust is generated by fluidized bed pressure firing or coal gasification at a temperature of 850 C and a pressure of 16 bar and then cleaned in a dust-removing plant (4, 8, 9). The dust removal plant consists of a coagulator (4) for bringing the dust particles together and two downstream cyclones (8, 9) to separate the dust (13, 14). The coagulator has a nest of coagulator pipes (5) with vibrators (6) attached thereto. A first cyclone is fitted with a cyclone electrode (11) to separate the dust more efficiently. It is possible to fit packed bed filters or chemical injection devices before, in or after the coagulator (4) to reduce greenhouse gases and/or corrosive gases.

Description

 Method and device for cleaning exhaust gases

TECHNICAL AREA

The invention is based on a method for cleaning a gas stream and a cleaning system for carrying out the method according to the preamble of claims 1 and 7.

STATE OF THE ART

With the preambles of claims 1 and 7, the

 Invention related to a prior art, as known from G3-A-2 055 623. There one becomes one

 Contaminated hot gas resulting from coal dust combustion is fed to a gas turbine via an electrostatic separator and 2 cyclones connected in series. The contaminated hot gas contains sodium and potassium particles with a diameter in the range of 0.1 μm - 1 μm and ash particles in the range of 2 μm - 50 μm. Smaller ones

Particles are preferably in the electrostatic

Separator that has multiple channels with wires at negative high voltage potential, separated.

Aerosol particles tend to attach themselves to large ash particles rather than to the plates of the separator, from which they are torn off or carried away by vibration and the gas flow. Larger ash particles are mainly separated in the cyclones.

From DE-PS 844 593 a method and a

Electrostatic filter for flocculation of soot and by weight similar, conductive floating particles with a downstream settling space for electrically flocculated particles are known. Raw gas is passed from top to bottom through a tube bundle with outflow electrodes. That from the lower ones

Gas escaping from the pipe ends is deflected upwards within the jacket and, with an enlarged cross-section, fed to a large-scale cyclone, in which the remaining flocculation occurs at a low flow rate

attack.

From US-A-4,478,613 it is known to remove the combustion gases of a diesel engine for removing particles and

Aerosols successively through a coagulator and an

Send cyclone. In the grounded coagulator chamber, several stacks with disk-shaped spray electrodes are arranged at a negative potential. The coagulator can also be built into the cyclone. To avoid deposits on the

To avoid the coagulator chamber wall, the exhaust gas flows over it at high speed or it is exposed to mechanical shocks or vibrations.

From the conference publication: AIAA-81-0393 by R. R. Boericke et al., Electrocyclone for High Temperature, High Pressure Dust Removal, AIAA 19th AEROSPACE SCIENCES

MEETING, January 12-15, 1931, / St. Louis, Missouri, is known to connect several electrocyclones in series to separate dust from the hot gas stream of a power plant with pressure fluidized bed combustion. Rod-shaped high-voltage electrodes protrude into the upper part of the electrocyclones, which generate a maximum electric field strength of 5 kV / cm. One advantage of this device is that the electrical forces that act on the particles can be selected independently of the cyclone size and the volume flow in the cycles. In contrast, the inertial forces that are otherwise effective during deposition decrease with decreasing volume flow and with increasing cyclone size. If the degree of charge of the particles is low, the advantages of the electrocycles become ineffective. Due to the very short dwell time of the particles in the part of the first electrocyclone intended for particle charging, this is the case.

Fluidized bed combustion is a promising new combustion technology for the efficient, environmentally friendly generation of electricity from coal. SO ₂ produced during combustion can be bound directly to coal by adding lime compounds. The NO _x formation is low due to the low combustion temperature of 850 ° C, but not yet so low that on

 Secondary measures can be dispensed with. Electricity is usually generated using a steam process. A

 but can significantly increase efficiency

 can be achieved if the exhaust gases, which have a temperature of 850 ° C and a pressure of 16 bar at full load, are directed to a gas turbine. In order to protect the gas turbine blades from erosion, the exhaust gases are made of particles by means of 2- or multi-stage cyclones

 cleaned. With the known method, especially large particles (> 5 μm), which can damage a turbine blade on impact, are removed from the exhaust gas. There is still no satisfactory solution for removing smaller particles.

The separation of unstable coagulates is difficult. Coagulates of suspended particles are stable when cohesive forces between the particles are high compared to inertial and aerodynamic forces. However, there are many applications with unstable coagulates, which are characterized only by the fact that the suspended

Particles only appear in much higher density than in

adjacent flow field. The advantage of coagulated

Particles easier to separate, can be destroyed if the coagulates z. B. break apart in one cycle.

For the relevant prior art, reference is also made to CH-A-673 411, from which an electrostatic Filter device for a continuous separation of solid and / or liquid particles from a gas stream is known.

A packed bed filter for the separation of dust and gaseous pollutants is from the German magazine: Dust cleanliness of the air 48 (1988), pp. 379 - 386,

known.

PRESENTATION OF THE INVENTION

The invention, as defined in claims 1 and 7, achieves the object of specifying a method for cleaning a gas stream and a cleaning system for carrying out the method, which are more complete

Enable cleaning of pollutants from a gas stream.

An advantage of the invention is that it is less

Pollution from dust. An improved degree of dust separation is achieved with relatively simple means.

Another distribution lies in the low pressure and

Temperature loss of the agglomerator or coagulator.

When used in power plants to generate electricity, the service life of gas turbine blades can be increased and the economy can be increased.

Another advantage is the extremely low level

Space requirement for a given exhaust gas volume flow. The small amount of space required for the application in a pressure fluidized bed power plant enables the accommodation of the

Cleaning system within the pressure vessel for the

Combustion chamber.

According to an advantageous embodiment of the invention, the removal of small particles in an electrostatic filter modified as a coagulator and particle charger in combination with a subsequent one

 Electrocyclone. The modified electrostatic precipitator is operated on the one hand so that the

 Fly ash particles coagulate into large particles in an interplay of separation and re-entry, so that in particular small particles can be separated in a subsequent cyclone. On the other hand, the long dwell time of the particles in the electrostatic filter due to the interplay helps them to a very high degree

 electrostatic charging, so that the subsequent use of an electrocyclone is very effective. This increases the separation effect overall and especially with small particles of <5 μm. The size spectrum is shifted to large particles in the electrostatic precipitator, which simplifies a subsequent inertial separation. In this way, the advantages of the electrostatic filter are used, namely the simple and robust construction with low maintenance requirements, the very low pressure drop, the high rate of precipitation of the particles at high

 Temperature and high pressure as well as the small construction at high pressure. At the same time, the im

 Given the high dwell time of the particles, the degree of charge of the particles is maximized, as a result of which the advantages of an electrocyclone can subsequently be used.

According to a further advantageous embodiment of the

Invention can corona discharge intrinsically exploited in the electrostatic filter by suitable electrodes and

Field strength selection are used so that NO _x or greenhouse gases are broken down into harmless substances and thus removed.

The cleaning system according to the invention can be

advantageous with an additional chemical

Combine exhaust gas treatment. It is particularly suitable for coal gasification, pressure fluidization and waste incineration plants. To do this various chemicals (additives, such as NH ₃ ) in the

Exhaust gas stream or injected directly into the combustion boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below using exemplary embodiments. Show it:

1 shows a cleaning system with a coagulator, which has a plurality of coagulator chambers separated by plates and which are followed by 2 cyclones connected in series,

Fig. 2 shows a cleaning system according to FIG. 1, but with a coagulator, which is constructed from several coagulator tubes with vibrators and in which the

1. downstream cyclone is an electrocyclone, Fig. 3 is a diagram showing the displacement of the

 Particle size distribution by the coagulator under atmospheric conditions,

Fig. 4 is an electrostatic filter with dust separator and

 Exhaust gas deflection as a coagulator,

Fig. 5 2 series connected electrostatic precipitators with

 Dust separators, exhaust gas deflection and one

 Bulk bed filter for chemical

 Exhaust gas treatment as well

Fig. 6 shows a coagulator, the one

 Packed bed filter is connected to a cyclone.

WAYS OF CARRYING OUT THE INVENTION

In Fig. 1, (1) denotes a particle-laden or dust-laden gas or hot gas, which comes from a fluidized bed combustion system of a power plant and is not shown via a dedusting system (4, 8, 9)

Gas turbine blades is fed to a gas turbine. The hot gas (1) has a temperature of 850 ° C and at Vollatet a pressure of 16 bar. The dust particles are electrically charged in a grounded plate electrostatic filter or a coagulator or coagulator particle charger (4) with earthed plates or coagulator chambers or coagulator particle charging chambers (5) on high-voltage electrodes or coagulator electrodes or coagulator particle charging electrodes (3) and on the inner wall of the coagulator particle charger (4). The wire-shaped coagulator particle charging electrodes (3) are arranged in the middle between the plates (5) or on the edge between the respective edge plate and the inner wall of the coagulator particle charger (4). The on the separation plates or coagulator particle loading chambers (5) and on the

 The agglomerates formed on the inside wall of the coagulator particle charger (4) are influenced by the gas flow, the average speed of which is between 3 m / s and 10 m / s.

 sporadically carried away again. If necessary, tearing off can be supported by sporadically tapping the plates (5). In downstream cyclones (8, 9), the agglomerates are separated as ash or dust (13) or (14), while dedusted cyclone exhaust gas (10) is directed to a gas turbine (not shown).

The coagulator particle charging electrodes (3) are normally negatively charged; but they can also be positively charged or alternately put on negative and positive potential. In addition to coagulation on the inner wall of the coagulator particle charger, bipolar coagulation in the gas space can also be used to enlarge the particles.

Fig. 2 shows the separation of dust from a hot gas (1) in a process sketch. With (2) is one

High-voltage source referred to, which is connected to wire or rod-shaped coagulator particle charging electrodes (3) in the middle of a plurality of tubular coagulator particle loading chambers or coagulator particle charger tubes (5) of the coagulator particle charger (4). At every coauulator Particle charger tube is fitted with a vibrator (6) in order to re-enter agglomerates deposited on the walls of the coagulator particle charger tubes (5) into the hot air stream or to assist detachment. The coagulator particle loading chambers (5) are connected to a ground potential (12); they have a diameter in the range from 4 cm to 50 cm, in particular in the range from 5 cm to 25 cm, preferably from 25 cm, and a length in the range from 3 m to 7 m, in particular in the range from 4 m to 6 m, preferably of 5 m. The flow velocity of the gas in the coagulator particle charger (4) is high, it is in the range between 1 m / s - 20 m / s, preferably in the range between

3 m / s - 10 m / s. With an exhaust gas flow of 200 kg / s in a pressure fluidized bed power plant under full load, the tube bundle consists of about 25 coagulator particle charger tubes (5) with a diameter of 25 cm, the flow velocity in the coagula particle charger (4) being 8 m / s.

Dust particles coagulate on the tube walls of the coagulator particle charger tubes (5) or in their immediate vicinity. There is an interplay between electrostatic separation and re-entry of the particles by turbulent shear stress due to the flow or by mechanical vibrations or shocks. The resulting long residence time of the particles in the coagulator particle charger (4) causes a very high particle charge, which is used in the subsequent electrocyclone (8).

With (7) a coagulator exhaust gas or a coagulator particle charger exhaust gas is designated, the particles in

Are larger on average than those of the dust-laden

Hot gas (1) and in which the degree of particle charging is higher than in a normal electrocyclone. The

Coagulator particle charger exhaust gas is a 1st cyclone (8) with a central, rod-shaped high voltage or

Cyclone electrode (11) and then fed to a second cyclone (9) without a high-voltage electrode. The 2nd cyclone delivers Cyclone exhaust gas (10) well cleaned of small and large particles on the clean gas side, which is directed to a gas turbine. The coagulates in the clean gas or coagulator-particle charger exhaust gas (7) of the coagulator-particle charger (4) are separated in the downstream cyclones (8, 9) with as little pressure loss as ash (13, 14).

Fig. 3 shows the effect of the coagulator particle charger (4) under atmospheric temperature and pressure conditions at a flow rate of 8 m / s, one

Field strength of 4 kV / cm and a particle concentration of 17 g / m ³ . The cumulative size distribution (V) is plotted in% on the ordinate and the particle diameter (d) in μm on the abscissa. A curve labeled (15) represents the size distribution (V) when switched off

 Coagulator particle loader (4). (16) denotes measured value ranges which are achieved when the coagulator particle loader (4) is switched on. The proportion of particles with a diameter of less than 5.8 μm is included

 switched off coagulator particle charger (4) at about 15% of the total mass of all particles. With the on

 Coagulator particle loader (4) this proportion can be reduced to about 7%.

It goes without saying that in a large power plant, several dedusting systems (4, 8, 9) can be provided in parallel. The gas temperature can be in the range from 250 ° C. to 1400 ° C., preferably in the range from 500 ° C. to 1200 ° C., and the pressure in the range from 5 bar to 100 bar, preferably in the range from 5 bar to 20 bar.

4 shows a vertical, electrostatic or

Electrofilter (17) as a coagulator with a baffle or dust separator (18) substantially below the

vertical, cylindrical filter tube and with a

horizontal deflection outlet (20) for a gas flow (22) deflected by 90 °. In the cylindrical filter tube, mutually parallel filter chambers or Filter tubes are provided, corresponding to the coagulator chambers (5) according to FIGS. 1 and 2. Coarse dust particles of the dust-laden hot gas (1) are in a

Electrofilter (17) upstream cyclone (8) separated as ash (13).

Hot gas (1) containing fly ash occurs in

unipolar electrostatic filter (17) under certain

Operating conditions that u. a. of the

 Flow rate of the hot gas (1) and depend on the corona tension, coagulate (30). As a rule, coagulates (30) arise when particles on a deposition electrode, not shown, correspond to the

grounded plates or coagulator chambers (5) according to FIGS. 1 and 2 come into contact with each other, so that cohesive surface forces, such as. B. liquid bridges, electrostatic forces, etc. are effective. If the

Particles again from the deposition electrode z. B. are replaced by shear stresses, mechanical vibrations or the like, the coherent coagulates (30) are present in the gas stream. These are often loose

coherent flakes of low inherent stability. Through suitable flow deflections, in which

Inertia are exploited, these can be

Coagulates (30) nevertheless from the gas stream z. B. continuously in the baffle (18) from the gas stream without destroying it. The coagulates (30) deposited in the impact pot (18) can be continuously removed by means of a screw conveyor or a moving conveyor belt (19). Instead of the conveyor belt (19), pneumatic cleaning can also be provided (not

shown).

This arrangement can be supplemented by a

electrostatic filter device according to the aforementioned CH-A-673 411 or through a band filter (21) in the deflected flow (22) at the end of the deflection outlet (20). In this way, the separation effect of this electrostatic filter (17) is optimally used for smaller particles.

For chemical exhaust gas treatment, chemical injections (31, 32) can be provided at the inlet and / or outlet of the electrostatic filter (17) in order to reduce undesirable corrosive and / or greenhouse gases such as CO ₂ , N ₂ O, NO _X from combustion plants.

With the corona discharge occurring in the electrostatic filter (17), NO _x molecules are reduced and greenhouse gases are broken down, so that harmless substances are produced. In discharges under high pressure of> 3 bar and high temperature of> 500 ° C, such as occur with hot gases (1) from power plants, electrons from the coagulator electrodes (3) are also emitted by thermal and thermal field emissions in addition to the known field emission triggered. This effect is intensified if the coagulator electrodes (3) have a greater thickness than normal corona electrodes. Such electrodes can either be correspondingly thick due to dust loading or can be produced using suitable materials. The ratio of the surface of one

 Coagulator electrode (3) to the surface of the second electrode (5) surrounding it, cf. 1, 2 and 6, is in

 Range from 1: 400 to 2: 1, preferably in the range from 1:20 to 1: 1. Those materials are preferred in which the work function of the electrons is small. This is e.g. B. in the case of metallic electrodes above 750 ° C if they with fly ash

are coated. The work function is about 0.6 eV. Another advantage of this discharge is that it is possible to denitrate directly with relatively homogeneously distributed electrons, in contrast to pure corona discharges, where electrons and ions are distributed inhomogeneously. In addition, the energy of the charged particles is too great to be used optimally. Electrons out

Thermal field emissions have less energy and are therefore more economical. Chemical additives such as NH ₃ ,

Methanol and other alcohols are injected into, before or between coagulators (4). Due to the existing temperature, the purely thermal excitation of the additives is sufficient for a DeNO _X reaction. Such methods are known. However, the effect is significantly enhanced if they are activated by electrons in discharges. The charges from the corona of the electrostatic filter (field and thermal field emissions) are ideally suited for this. Unstable intermediate substances, such as NH ₂ , then form optimally

denitrify. Catalytic lining or doping of the walls of the electrodes of the coagulator (4) can also improve activation. A chemical injection (32) behind the coagulator (4) or in front of the cyclone (8) is particularly advantageous, cf. Fig. 6. Another

Chemical injection can take place before or on the belt filter (21) and on a packed bed filter (28) according to FIGS. 5 and 6. This chemical injection (32) can take place depending on the load. Power plants with

 Due to different temperatures, pressure fluidized bed firing requires more denitrification at part load than at full load.

5 shows a different version of the optimized dust separation. The flow conditions can be better used if the exhaust gas is deflected by an exhaust gas deflection angle α from 80 °, preferably in the range from 140 ° to <180 °. One marked with (ß)

Deposition deflection angle can be> 90 °, so that in an impact pot (25) of an electrostatic filter (23)

The coagulates are less likely to be torn back.

A tapered area (24) at the bottom of the

Coagulating part of the electrostatic filter (23) serves to accelerate the flow of hot gas (1). So that the outside walls are angled upwards

Deflection outlets (26) from a deflected gas flow (29) not build up deposits, can in the

 Deflection areas liquid films (27) are provided, which rinse these areas.

As indicated in Fig. 5, 2 such

 Electrofilter (23) can be operated in series. This has the advantage that it significantly increases efficiency. The transition from the 1st to the 2nd stage takes place in tubes. Measures can be taken in the pipework between the stages, e.g. B. unwanted

 Reduce the chemical effects of the exhaust gas. So z. E. near the entrance to the 2nd stage

 Bed layer filter (28) with silicon and / or

 Aluminum oxides can be provided, which acts as an alkali getter by gas sublimation.

In gasification processes, instead of

 Fill layer filter (23) a chemical injection, e.g. B. of oxygen, can be provided, which improves the calorific value of the synthesis gas or brings about other properties.

To reduce greenhouse gases such as CO ₂ , N ₂ O, other additives can be added. Especially for

Gasification plants can convert or reform CO ₂ into CH ₄ , methanol or other usable fuels. This is for applications in hot gases

Coal dust firing is important.

The speed of the hot gas in the electrostatic precipitators is in the range of 3 m / s - 50 m / s, preferably in

Range of 5 m / s - 10 m / s, so that deposits on the

Separation electrodes can be prevented.

Fig. 6 shows a cleaning system in which a

Coagulator (4) with vertical from top to bottom

extending coagulator chambers (5) is connected on the output side to a cyclone (8) via a packed bed filter (28). The length of the coagulator tubes (5) is in the range of 2.5 m - 28 m. Additionally or alternatively, at least one cyclone can be provided in front of the coagulator (4).

With these coagulators and electrostatic filters is one

Unstable coagulates can be separated. This separation takes place continuously and essentially uses

Inertial forces that do not require additional energy. The dedusting system is not only suitable for separating particles from atmospheric or

Combustion chamber flue gases, but e.g. B. also for the

Separation of coagulates that are to be manufactured as a product.

(Only intended for the examination center; not part of the registration)

 LIST OF DESIGNATIONS

 1 dust-laden hot gas

 2 high voltage source

 3 coagulator electrodes, coagulator particle charging electrodes

 4 coagulator, coagulator particle charger,

 Agglomerator, plate electrostatic precipitator

 5 coagulator chambers, coagulator particle loading chambers, tubes, plates

6 vibrators

 7 Coagulator exhaust gas, coagulator particle charger exhaust gas 8, 9 1st cyclone or 2nd cyclone

 10 dedusted cyclone exhaust gas

 11 cyclone electrode

 12 Earth potential

 13, 14 ashes, dust

 15 Coagulator off, coagulator particle loader off

16 coagulator on, coagulator particle charger on 17, 23 electrostatic precipitator

 13 baffle, dust collector

 19 conveyor belt

 2C deflection outlet from 17

 21 band filter

 22 deflected flow

 24 taper area

 25 baffle from 21, dust collector

25 deflection outlet of 23

 27 liquid film

 28 packed bed filters

 29 deflected flow in 23

 30 coagulates

 31, 32 chemical injection

d diameter

 V cumulative size distribution

α exhaust gas deflection angle

ß Deposition deflection angle

Claims

PATENT CLAIMS 1. Process for cleaning a gas stream,  a) wherein a to be cleaned, dust-laden gas stream (1) in parallel through several coagulator chambers (5) of an electrostatic coagulator (4, 17, 23) for the mutual accumulation of dust particles  is promoted  characterized,  b) that the dust-laden gas stream (1) with a  Speed in the range of 3 m / s - 50 m / s through the at least one coagulator (4, 17, 23)  is promoted. 2. The method according to claim 1, characterized in that  a) that the dust-laden gas stream (1) is a hot gas stream with a temperature in the range from 250 ° C. to 1400 ° C.,  b) is in particular in the range from 500 ° C to 1200 ° C. 3. The method according to claim 1 or 2, characterized  featured,  a) that the dust-laden gas stream (1) is a hot gas stream with a pressure in the range of 5 bar - 100 bar,  b) is in particular in the range from 5 bar to 20 bar. 4. The method according to any one of claims 1 to 3, characterized  characterized in that the dust-laden gas stream (1) after electrostatic coagulation of the  Dust particles  a) by a predeterminable deflection angle (α) in the range from 80 ° - <180 °,  b) is deflected in particular by a deflection angle (α) in the range from 140 ° to 170 °,  c) that was not redirected due to inertia Coagulates (30) of the gas stream (1) are continuously separated from the coagulator (17, 23) and d) the deflected part of the gas stream (1) is subjected to continuous electrostatic filtering. 5. The method according to any one of claims 1 to 4, characterized in that the gas stream (1) to be cleaned before and / or after the coagulation in the electrostatic coagulator (4, 17, 23) by at least one 1st cyclone (8, 9 ) for the separation of dust (13, 14) from the  Gas flow is transported. 6. The method according to any one of claims 1 to 5, characterized  characterized in that the gas stream (1) to be cleaned is subjected to a chemical treatment (31, 32) or cleaning (23) before and / or after the coagulation in the electrostatic coagulator (4, 17, 23). 7. Cleaning system (4, 8, 9) for cleaning a  Gas flow (1)  a) with at least one coagulator (4, 17, 23), the  has at least 2 coagulator chambers (5), each with a coagulator electrode (3),  characterized,  b) that the length of the coagulator chamber (4) is in the range between 2.50 m and 23 m. 8. Cleaning system according to claim 7, characterized  featured,  a) that the ratio of the surface of the at least one coagulator electrode (3) to the surface of a surrounding second electrode (5) in the range of 1: 400 to 2: 1,  b) is in particular in the range from 1:20 to 1: 1. 9. Cleaning system according to claim 7 or 8, characterized  featured,  a) that the coagulator (4, 17, 23) at least one Cyclone (8, 9) is connected upstream and / or downstream, b) in particular that a downstream 1st cyclone (8) has at least one cyclone electrode (11). 10. Cleaning system according to one of claims 7 to 9, characterized in that  a) that the at least one coagulator  Has electrostatic filter (17, 23), with an essentially straight, vertical part for electrostatic coagulation of the particles of the dust-laden gas stream (1),  b) an essentially straight line  subsequent dust separator or baffle (18, 25),  c) in particular with a continuous  Separating device (19) for coagulates (30) separated in the baffle pot (18, 25), and  d) with at least one deflection outlet (20, 26) for the gas flow (1) in the area between the coagulation part and the baffle pot (13, 25),  e) its direction with respect to that of the  Coagulation part an angle (a) in the range of 30 ° - <180 °,  f) forms in particular in the range of 140 ° - 170 °. 11. Cleaning system according to one of claims 7 to 10, characterized in  a) that at least one outlet (20, 26) at least  a coagulator (4, 17, 23) with a  electrostatic filter device (21) for separating solid and / or liquid particles,  b) in particular that this filter device  Band filter (21). 12. Cleaning system according to one of claims 7 to 11, characterized in that a) that before and / or in and / or after the coagulator (4, 17, 23) a device for Chemical injection (31, 32) and / or a  chemical filter (28) is provided for the separation and / or conversion of gaseous pollutants and compounds, b) in particular that the chemical filter  Bed layer filter (28).