The present invention relates to a method for sepa¬ rating dust from hot process gases, such as flue gases, in which method the process gases are conducted through a gas inlet duct to a dust separator, in which dust is separated from the process gases and from which the cleaned process gases are discharged through an outlet duct, part of the dust separated in the dust separator being passed to a device for agglomeration of the dust and then recirculated by being introduced into the pro- cess gases in the gas inlet duct.

A method of this kind is disclosed in US Patent 3,834,123. This document thus describes how gases con¬ taining fine particles are passed through a dust sepa¬ rator, in which particles are separated from the gases, and how agglomerated particles are drawn off from the dust separator and recirculated by being introduced into the gases upstream of the dust separator.

By thus recirculating agglomerated dust, the effi¬ ciency of the dust separator is increased in relation to the efficiency obtained without such recirculation.

The object of the present invention is to provide a method for separating dust from hot process gases, said method additionally improving the efficiency of the dust separator. This object is achieved by a method which is of the type described by way of introduction and characterised in that liquid is supplied to the dust in the agglomerat¬ ing device in such an amount that the relative humidity in the cleaned process gases in the outlet duct is higher than 30%, preferably 40-60%.

The invention will now be described in more detail with reference to the accompanying drawings, in which

Fig. 1 is a schematic view of a plant for cleaning flue gases from a coal-fired central heating plant, the

cleaning plant being provided with equipment for carrying out the method according to the invention,

Fig. 2 is a side view in which certain parts have been broken away and which shows a mixer used for carry- ing out the method according to the invention,

Fig. 3 shows a mixer according to Fig. 2 from above, and

Fig. 4 is a cross-sectional view along line IV-IV in Fig. 3. Fig. 1 schematically shows a plant for cleaning flue gases from a coal-fired central heating plant 1, said flue gases containing dust, such as fly ash. A preheater 2 is arranged to transmit heat from the hot flue gases to combustion air, which is supplied through a duct 2a to the central heating plant 1 by means of a fan 3.

The hot flue gases are passed through a duct 4 to a dust separator 5, which in the shown embodiment is a fabric filter, which in known manner comprises a plura¬ lity of rows of filter bags and through which the flue gases are passed to be cleaned. The thus cleaned flue gases are passed through a duct 6 to a flue gas blower 7, which through a duct 8 feeds them to a chimney 9 to be discharged into the atmosphere. The dust separator can also be, for instance, an electrostatic precipitator. The duct 4 comprises a vertical portion 10. A mixer 11 communicates with this portion 10 in the lower part thereof. The mixer 11 introduces in the manner that will be described in more detail below moistened and agglome¬ rated dust particles into the flue gases in the lower part of the duct portion 10.

The dust particles separated in the dust separator 5 are collected in the hoppers 12 of the dust separator 5. Part of the collected dust particles is recirculated in the system by being passed to the mixer 11 (arrow Pl ) . The remainder of the collected dust particles is trans¬ ported away in a manner not described in detail, for instance, by means of a screw conveyor.

The mixer 11 is a mixer of the type described in WO 96/16727 and shown in more detail in Figs 2-4.

The mixer 11 shown in Figs 2-4 comprises a contai¬ ner 13, which essentially is in the shape of an elongate, parallelepipedal box. The container 13 has two vertical side walls 14 and 15, a vertical rear end wall 16, a ver¬ tical front end wall 17, a horizontal upper bottom 18, a horizontal lower bottom 19 and a horizontal top or lid 20. At the rear end, the container 13 has an inlet 21, through which dust particles from the hoppers 12 are supplied from above into the container 13 (arrow Pl in Figs 1 and 2), and at its front end, an outlet 22, through which a homogeneous mixture of agglomerated dust particles and water is discharged (arrows P2 in Figs 3 and 4 ) .

In the example shown in the drawings, the front end of the container 13 is inserted in the vertical duct por¬ tion 10, through which the flue gases are conducted up- wards (arrows P3 in Figs 1, 2 and 4). In this applica¬ tion, the outlet 22 is an overflow formed as a result of the side walls 14 and 15 being lower in the part of the container 13 inserted in the duct portion 10 than in the container part located outside the duct portion 10. As appears from Figs 2 and 3, the top 20 extends from the inlet 21 to the outlet 22, i.e. up to the duct portion 10.

Between them, the two bottoms 18 and 19 define a chamber 23 which, in the lateral direction, is delimited by the two side walls 14 and 15 and, in the longitudinal direction, is delimited by the two end walls 16 and 17. The ceiling of the chamber 23, i.e. the upper bottom 18, consists of an air-permeable fluidisation cloth of poly¬ ester mounted in stretched state in the container 13. An air-supply means, which in the embodiment shown consists of two air inlets 24 and 25, is arranged to supply air

to the chamber 23 ( arrows P4 in Figs 2 and 3 ) , so as to fluidise the dust particles in the container 13.

A water-supply line 26, which disposed above the container 13 is connected to a plurality of nozzles 27 arranged in the upper part of the container 13 to spray water in finely-divided form over the dust particles in the container. The nozzles 27, of which but a few are shown in the drawings, are arranged in two parallel rows extending along the container 13. Two juxtaposed horizontal shafts 28, 28' extend along the entire container 13 and are rotatably mounted in the two end walls 16 and 17 with the aid of bearings 29, 29' and 30, 30', respectively. A motor 31 is arranged to rotate the shafts 28, 28' via a transmission unit 32. Each shaft 28, 28' supports a plurality of elliptic discs 33, 33', which, about their minor axes, are mounted in inclined state on the shafts 28, 28' at an axial dis¬ tance from one another. The shafts 28, 28' extend through the centres of the respective discs 33, 33' . In the Exam- pie illustrated, each disc 33, 33' is so inclined in re¬ lation to the shaft 28, 28' that the angle α between the major axis of the disc and the shaft 28, 28' is about 60° (see Fig. 2). This angle α may vary between 45° and 80°. The discs 33, 33' are so inclined in relation to the respective shafts 28, 28' and have such an elliptic shape as to have a circular axial projection, as illustrated in Fig. 4. The discs 33, 33' are so positioned on the respective shafts 28, 28' that the discs of the one shaft project into the spaces between the discs of the other shaft.

Each of the discs 33, 33' arranged and designed in the manner indicated above performs, during the rotation of the shafts 28, 28', a throwing movement conducive to thorough mixing of the dust particles. By a partition 34 in the front part of the container 13, the chamber 23 is divided into a front part chamber 23a, which is situated in the duct portion 10, and a rear

part chamber 23b. As appears from Fig. 2, the air inlet 24 opens into the rear part chamber 23b, while the air inlet 25 opens into the front part chamber 23a. With this division of the chamber 23, it is possible to achieve different fluidisation conditions in the two part cham¬ bers 23a and 23b, especially with a view to so adapting the air supply to the front part chamber 23a that one there obtains a suitable fluidisation state for the mate¬ rial discharge. That part of the dust particles collected in the hoppers 12 of the dust separator 5 which is to be recir¬ culated in the system, is supplied to the mixer 11 via the inlet 21. The dust particles are moistened with water which is supplied through the nozzles 27. By the design of the mechanical mixing mechanism and by fluidisation of the dust particles supplied to the mixer, the mixer 11 produces a homogeneously moistened, homogeneous mixture of dust particles which is, via the overflow 22 of the mixer 11, continuously introduced into the duct portion 10 as agglomerated dust particles.

The function of the plant will now be described in more detail by means of five different experiments that have been carried out. Experiment 1 The flow rate of the flue gases was 45,000 Nm^/h. At A (Fig. 1) the flue gases had a dust content of 22.5 g/Nm3 and a temperature of 130°C. The average par¬ ticle size of the dust was 20 μm. The differential pres¬ sure over the fabric filter 5, which comprised 20 rows of filter bags, was constantly kept at 2,000 Pa by cleaning every row of filter bags by means of a strong compressed- air pulse, an entire cycle for cleaning all the rows of filter bags of the filter being passed in about 10 min. The interval between the pulses thus was about 30 s. The recirculation circuit comprising the mixer 11 was not used. The dust content of the flue gases at B in the duct 6 was measured to be 21.2 mg/Nm-3.

Experiment 2

Like in Experiment 1, the flow rate of the flue gases was 45,000 Nm-3/h. At A, the flue gases had a dust content of 19.3 g/Nrn^ and a temperature of 130°C. The differential pressure over the fabric filter 5, which comprised 20 rows of filter bags, was constantly kept at 2,000 Pa by cleaning each row of filter bags by means of a strong compressed-air pulse, an entire cycle for clean¬ ing all the rows of filter bags of the filter being pass- ed in about 30 s. The interval between the pulses thus was about 1.5 s. Via the mixer 11, separated dust par¬ ticles were recirculated in an amount of about 60 t/h. The recirculated dust particles had an average particle size of 26 μm. The dust content of the flue gases at B in the duct 6 was measured to be 35 mg/Nm^. Experiment 3

Like in Experiments 1 and 2, the flow rate of the flue gases was 45,000 Nm^/h. At A, the flue gases had a dust content of 20.5 g/Nm-3 and a temperature of 130°C. The differential pressure over the fabric filter 5, which comprised 20 rows of filter bags, was constantly kept at 2,000 Pa by cleaning each row of filter bags by means of a strong compressed-air pulse, an entire cycle for clean¬ ing all rows of filter bags of the filter being passed in about 16 min. The interval between the pulses thus was about 48 s. Via the mixer 11, separated dust particles were recirculated in an amount of about 60 t/h. The recirculated dust particles had an average particle size of 33 μm. Water was supplied to the mixer 11 in an amount of about 1.3 m^/h, thereby cooling the flue gases to about 75°C. The dust content of the flue gases at B in the duct 6 was measured to be 15.1 mg/N ^. Experiment 4

Like in Experiments 1, 2 and 3, the flow rate of the flue gases was 45,000 Nm^/h. At A, the flue gases had a dust content of 20.2 g/Nm^ and a temperature of 130°C. The differential pressure over the fabric filter 5, which

comprised 20 rows of filter bags, was constantly kept at 2,000 Pa by cleaning each row of filter bags by means of a strong compressed-air pulse, an entire cycle for clean¬ ing all rows of filter bags of the filter being passed in about 28 min. Thus, the interval between the pulses was about 84 s. Via the mixer 11, separated dust particles were recirculated in an amount of about 60 t/h. The re¬ circulated dust particles had an average particle size of 40 μm. Water was supplied to the mixer 11 in an amount of about 1.4 m-^/h, thereby cooling the flue gases to about

70°C. The dust content of the flue gases at B in the duct 6 was measured to be 8.8 mg/Nm^. Experiment 5 Like in Experiments 1, 2, 3 and 4, the flow rate of the flue gases was 45,000 Nm^/h. At A, the flue gases had a dust content of 18.5 g/Nm^ and a temperature of 130°C. The differential pressure over the fabric filter 5, which comprised 20 rows of filter bags, was constantly kept at 2,000 Pa by cleaning each row of filter bags by means of a strong compressed-air pulse, an entire cycle for clean¬ ing all rows of filter bags of the filter being passed in about 62 min. Thus, the interval between the pulses was about 186 s. Via the mixer 11, separated dust particles were recirculated in an amount of about 60 t/h. The re- circulated dust particles had an average particle size of 36 μm. Water was supplied to the mixer 11 in an amount of about 1.5 τβ/h, thereby cooling the flue gases to about 65°C. The dust content of the flue gases at B in the duct 6 was measured to be 1.7 mg/Nm^. As is evident, even without supplying water to the mixer 11, a certain agglomeration of the dust particles takes place in the mixer 11 (Experiment 2). When supply¬ ing water, an additional agglomeration of the dust par¬ ticles takes place (Experiments 3, 4 and 5). The great reduction of the dust content at B in the duct 6 and, thus, of the discharge of dust into the atmosphere that was obtained in Experiments 3, 4 and 5 indicates that the

increased water content of the flue gases, which in Expe¬ riments 2, 3, 4 and 5 was 4%, 30%, 38% and 51%, respec¬ tively, and the decreased temperature thereof, which in Experiments 2, 3, 4 and 5, as indicated above, was 130°C, 75°C, 70°C and 65°C, respectively, had a great effect on the result. A high humidity in coaction with partly agglomerated dust particles has been found to give sur¬ prisingly good cleaning results.