GB2038468A

GB2038468A - Cooling and moistening dust- containing gases

Info

Publication number: GB2038468A
Application number: GB7931863A
Authority: GB
Original assignee: DOTTERNHAUS PORTLAND ZEMENT
Current assignee: DOTTERNHAUS PORTLAND ZEMENT
Priority date: 1978-10-19
Filing date: 1979-09-13
Publication date: 1980-07-23
Also published as: GB2038468B; CH641970A5; DE2845593A1; FR2439037A1; JPS5556818A

Abstract

To cool and moisten dust- containing gases, such as waste gases from furnaces and boilers, water is sprayed into the gas from a spray nozzle opening (2), an enveloping jet of compressed dust-free gas being formed around the water droplets issuing from the nozzle opening (2) by the supply of compressed air or gas to outer duct (6'), whereby the droplets can evaporate before coming into contact with the dust-containing gas, and whereby the tendency for sludge formation is reduced. A spraying agent such as steam or compressed gas may be supplied through an intermediate duct (3'), both gas supplies being caused to swirl by means (4, 8). The water spray nozzle may be of the return-flow type. <IMAGE>

Description

SPECIFICATION Method and apparatus for cooling and moistening dust-containing gases This invention relates to the cooling and moistening of dust-containing hot gases and waste gases, for example the gases obtained from furnaces, boiler plants, and plants in the stone and earth industries and in the iron and steel industries, by spraying water into the gases through one-component or two component pressure spray nozzles or through back-flow spray nozzles.

The waste gases and other gases from these industries are frequently obtained in a state in which purification in a dust extraction plant e.g. in an electricaí separator, is not possible, or is possible only to a very poor degree of dust extraction or is possible in very largescale plants.

The degree of separation of an electrical separator is, apart from other variables, known to be greatly dependent upon the temperature and the water dew point of the gas to be purified (Zement-Kalk Gips 29 (1976),251).

Compared with pure gases, dust-containing gases, as are produced in the industries referred to above, present great problems with respect to the introduction into them of the quantity of water required to increase their dew point anc! to reduce their temperature. Direct spraying of water into the gas is almost always unsatisfactory, due to the formation of sediment and/or sludge.

It is true that a substantial improvement is obtained by spraying water into the gas in evaporation coolers or injection towers expressly designed for the respective throughput of gas (Zement-Kalk-Gips 23 (1970), 106 and 30 (1977), 438).

However, in the case of the gas throughputs of the industrial processing plants used in the stone and earth industries and in the iron and steel industries, the quantities of water to be sprayed into the gases necessitates the use of very large injection coolers. The residence time of the gas in the cooler must be great enough such that even the largest droplets of liquid are safely evaporated before the gas leaves the tower. Thus, for example, in the case of a cement clinker combustion plant of a capacity of 3000 t/day two injection coolers connected in parallel, each 6 m in diameter and 24 m in height, are used (ZKG 29 (1976), 93).

Moreover, in some cases, the use of such iargescale cooling towers does not prevent the -formation of sediment of sludge. Furthermore, such evaporation coolers are intended to be used for cooling dust-containing waste gases with a temperature in excess of approximately 2500C.

For gases of a temperature in the range of 100 to 2000C the prevention of the formation of sediment or sludge in virtually impossible.

Even when using back-flow nozzles (i.e. nozzles in which the throughput of liquid is regulated by the amount of back-flow and in which the droplet size is largely dependent upon the throughput) it is not always possible, particularly in the case of varying plant conditions, to keep the injection cooler free from baked-on matter or sludge.

Furthermore, even when employing twocomponent nozzles (i.e. nozzles in which the liquid is sprayed by means of steam or compressed gas) it is not possible to attain shorter residence times than are usual in evaporation coolers.

It is not possible to spray, into the gas, quantities of water sufficient for reducing gas temperature by more than, for example, 200K in existing plants, without the use of evaporation coolers and without the consequently greater residence times.

According to the present invention, there is provided a method of cooling and a moistening dust-containing gas, comprising (a) spraying water into the dust-containing gas by means of a onecomponent or two-component pressure spray nozzle or by means of a back-flow spray nozzle, and (b) forming an enveloping jet of dust-free compressed gas or compressed air around the cone of water droplets issuing from the spray nozzle.

The invention thus provides a method which allows a liquid, preferably water, to be sprayed into a hot dust-containing gas, without the need for expensive installations, for example evaporation coolers, and with residence times adapted to the droplet size of the liquid sprayed into the gas.

In the method of the invention, the sprayed liquid and the dust-containing gas are prevented from being mixed prematurely.

The enveloping jet of air or gas results, surprisingly, in the reduction or elimination of baked-on matter in the vicinity of the nozzle through re-mixing, and evaporation, even of the largest droplets of liquid, takes place in a relatively short time, with the result that baked-on matter and sludge formation can also be reduced or prevented in gas lines or gas chambers and with the result that higher gas velocities and shorter-residence times can be attained. An explanation for this is that the enveloping jet allows even the largest droplets to be substantially evaporated without coming into contact with dust particles of the gas to be cooled.

According to a preferred embodiment of the invention, the exit velocity of the enveloping jet is from 50 to 300 m/s, more preferably from 100 to 200 m/s. The enveloping jet can be given a moment of rotation of the same or opposite direction as the moment of rotation of the jet droplets.

An advahtageous apparatus for implementing the method according to the invention comprises a one-component or two-component spray nozzle or a back-flow-spray nozzle enclosed by and spaced apart from an additional outer casing provided with a supply line for the compressed gas or the compressed air, which outer casing terminates at the level of the nozzle opening of the spray nozzle and forms therewith an annular opening. It is beneficial if, according to a further embodiment of the invention, a turbulenceproducing device is arranged in the annular opening between the outer casing and the casing of the spray nozzle, in order to give the enveloping jet of compressed gas or compressed air a moment of rotation.

Embodiments of the invention are described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic longitudinal section through a two-component spray nozzle; Figure 2 is a schematic longitudinal section through a one-component spray nozzle; and Figure 3 is a schematic representation of the two-component spray nozzle of Figure 1 in a conduit for dust-containing gas.

Referring to Figure 1, the liquid to be sprayed is supplied via line 1 to a nozzle opening 2 of a spray nozzle 7. Steam or compressed gas as a spraying agent is supplied via line 3 to the casing 3', this spraying agent being given a moment of rotation by means of a turbulence-producing device 4, before issuing from the nozzle opening. The spray nozzle 7 is a two-component nozzle.

Compressed gas or air is additionally supplied to the spray nozzle through a line 6 and a coaxial outer casing 6', and the compressed gas or air flows out through annular nozzle opening 9 at a velocity of 50-300 m/s, depending upon the quantity of gas or air. By means of a turbulent producing device; a moment rotation can be given to the enveloping gas issuing from opening 9, this moment of rotation being in the same or opposite direction relative to the driving jet issuing at 5.

Figure 2 shows a one-component spray nozzle 11 in which the liquid is sprayed by means of pressure, without the use of a spraying agent. The liquid to be sprayed is supplied via supply line 1 to the nozzle 11, which nozzle is provided with a turbulence-producing device 10 and which nozzle, if desired, can be constructed as a back-flow nozzle with a controllable back-flow 12. The issuing jet 13 of droplets is surrounded by a dustfree jet of enveloping gas 14, the dust-free gas being fed under pressure to the spray nozzle via line 15.

Figure 3 shows by way of example the use of the two-component-spray nozzle shown in Figure 1. Dust-containing gas flows in the direction 16 to an electrical separator 19 via a conduit 17. A spray nozzle 7 of the type shown in Figure 1 is located in the ascending portion of the conduit 17, the direction 1 8 of the spray from the nozzle preferably being the same as the.direction of flow of the gas. The liquid to be sprayed, the spraying agent (for example water vapour or compressed air) and the gas or air for producing the enveloping jet are supplied via lines 1,3 and 6, respectively.

The invention will now be illustrated by the following examples.

EXAMPLE 1 A waste gas from a waste-heat boiler and having a flow rate of 39,000 m3/h, a temperature of 1 600C and a dust content of 64 g/m3 was passed to an electrical separator. Without conditioning the gas, the residual dust content thereof downstream of the separator was 625 mg/m3.

By employing two spray nozzles of the type described above, upstream of the electrical separator, with which approximately 400 kg/h of water (namely decanted water from the boiler) were sprayed into the gas with the aid of 125 kg/h of steam, a reduction in the waste gas temperature by approximately 200K to 1400C and an increase in the water dew point by 30K were obtained.

The quantity of air for the enveloping jet was approximately 250 m3/h per nozzle.

The residual dust content downstream of the electrical separator suprisingly fell to below 60 mg/m3, i.e. to less than 10% of the original value.

Even with continuous operation over several months, no sludge formation and no formation of baked-on matter in the locality of the nozzle was encountered, even though a free path of only approximately 3.5 m in length was available for evaporation of the water sprayed into the gas.

EXAMPLE 2 A waste gas issuing from a grinding and drying device and having a flow rate of 230,000 m3/h, a temperature of 1 350C and a dust content of approximately 30 g/m3 was passed to an electrical separator. Without water being sprayed in, the residual dust content downstream of the electrical separator was in excess of 190 mg/m3. The use high-pressure spray nozzles in an evaporation cooler 6 m in diameter and 15 m in cylindrical height resulted in baked-on matter and sludge formation, due to the low gas temperature. Surprisingly, however, by employing a device of the invention as described above, it was possible to introduce up to 2,500 kg/h of water into the gas in a continuous operation, without disorder. Thus, for example, when 1,500 kg/h of water were sprayed into the gas, a reduction in the gas temperature by 1 80K and an increase in the dew point by approximately 20K were obtained, and the residual dust content of the gas downstream of the electrical separator fell to below 30 mg/m3 since the filter functions at a higher level of efficiency at a lower gas temperature and at a higher dew point.

Claims

1. Method of cooling and a moistening dustcontaining gas, comprising (a) spraying water into the dust-containing gas by means of a onecomponent or two-component pressure spray nozzle or by means of a back-flow spray nozzle, and (b) forming an enveloping jet of dust-free compressed gas or compressed air around the cone of water droplets issuing from the spray nozzle.

2. Method according to claim 1, wherein the exit velocity of the enveloping jet is from 50 to 300 m/s.

3. Method according to claim 2, wherein the exit velocity of the enveloping jet is from 100 to 200 m/s.

4. Method according to any of claims 1 to 3, wherein the enveloping jet is given a moment of rotation of the same or the opposite direction as the moment of rotation of the jet of droplets.

5. Method according to any of claims 1 to 4, wherein the dust-containing gas is a hot gas or waste gas emanating from a furnace, a boiler, a plant used in the stone or earth industries, or a plant used in the iron or steel industries.

6. Method according to any of claims 1 to 5, wherein the spray nozzle is a spray nozzle substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.

7. Method according to claim 1, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

8. Method according to claim 1, substantially as described in either of the foregoing examples.

9. Apparatus for implementing the method according to claim 1, comprising a onecomponent or two-component spray nozzle or a back-flow spray nozzle; an additional outer casing which encloses said spray nozzle and is spaced apart therefrom and forms therewith an annular opening; and a supply line for the supply of compressed gas or compressed air to said annular opening.

10. Apparatus according to claim 6, wherein a turbulence-producing device is disposed in said annular space.

11. Apparatus according to claim 9, substantially as hereinbefore described with reference to, and as shown in, Figure 1 or Figure 2 of the accompanying drawings.