GB2059799A - Method for scrubbing material from flue gases - Google Patents

Abstract

Flue gases are scrubbed by injecting a liquid, such as water into the gas, e.g. from a circular array 20 of sprays on the inner wall of a chimney 10 above a position 3 where the flue is turbulent, the liquid being heated by the gas to produce a mist which can trap dust and noxious materials from the flue gas. A liquid-sludge results which falls to the base 60 of the chimney 10 and can be extracted via a grate 40 into a precipitating vessel 50. The feed pipe 21 to the sprays may contain a solenoid control valve 27 which is interlocked with a boiler so as to adjust the temperature of the chimney depending on the combustion within the boiler. <IMAGE>

Description

SPECIFICATION Method for removing material from flue gases This invention relates to a method for removing material, for example fly-ash and noxious chemicals, from flue gases, especially within tall chimneys. The invention also relates to an apparatus suitable for carrying out this method. In this method, mist layers, partially saturated with scrubbing liquid, are formed, preferably at a vertical portion of the stack, so as to catch readily fly-ash and soluble noxious compounds contained in the flue gases. A liquid sludge falls to the base of the stack and can be discharged to a separate treating vessel without interferring with normal ventilation of the stack or with proper combustion in the boiler producing the flue.

In conventional steam power plants or factories which require a large quantity of steam, a boiler heated by coal or by heavy oil is usually a main component. In general, commercial methods of separating material such as dust and noxious chemicals from flue gases caused by combustion of fuels can be classed as mechanical methods or electrostatic methods. Usually, mechanical methods use, by means of a multi-cyclone or wet scrubbers, a ventilator to discharge burnt gas from a flue through a by-pass pipe to a cyclone, and a pump is used to spray water into the gas. This type of method is not sufficiently economical, since a relatively large quantity of water and electricity are consumed.Electrostatic methods, however, use an electrostatic precipitator or a Cottrell electrical dust collector positioned at opposite inner sides of a flue, and employ a DC potential of from 25,000 to 100,000 volts to charge and to collect any fine particles. Particles deposited on the charging plates are then scrubbed or rapped to a waste treating vessel. Conventional electrostatic precipitators are capable of efficiencies of from 90 to 95%, but their initial and operating costs are higher than those of mechanical separators.

There is, therefore, a demand for a method by which the initial and operating costs can be significantly reduced. I have now discovered a method wherein water, preferably pre-heated water, is sprayed under pressure (preferably about 0.5 Kg/cm2) into flue gas in a chimney under normal ventilation conditions, which can save a considerable amount of electrical energy without increasing the operating cost of the boiler that produces the flue. Also, the temperature of the chimney can be reduced to a certain value without increasing fuel consumption. I have also discovered an apparatus that can be mounted on an inner wall of a chimney at a much lower initial cost than that of conventional dust collectors. This apparatus can also save considerable space.

Thus, the present invention consists in a method of scrubbing flue gas, in which scrubbing liquid, preferably water, injected into the gas is heated by latent heat of the gas to form a mist which can trap dust and noxious materials from the flue gases. The injection is preferably carried out up-stream of a region of turbulent flow of the gas.

I prefer that the liquid is pre-heated by the flue gas before injection by passing the liquid along a pipe within the flue. It is preferred that at least part of the mist is saturated with liquid.

The invention also consists in an apparatus which comprises a set of spray jets mounted on an inner wall of a stack, such as a tall chimney, preferably immediately after a spent portion of the stack where a turbulent flow may take place. Heated water is sprayed from the jet to form mist comprising regions of saturated vapour. The mist layers contact the flue gases and trap fine particles and soluble noxious chemicals, such as sulphur dioxide. This forms a liquid sludge which can be discharged from the stack under normal ventilation conditions and at a reasonable temperature drop. The layers of mist can be increased to enlarge the area of contact with the flue gas.

The present invention will be further illustrated with reference to the accompanying drawings in which: Figure 1 is a longitudinal section illustrating a chimney containing the apparatus for scrubbing; Figure 2 is a top view of the apparatus of Fig. 1; and Figure 3 is a top view of a spraying apparatus; Figure 4 is a sectional view along line 4-4 of Fig. 3; and Figure 5 is an enlarged section of a nozzle suitable for use in the illustrated apparatus.

Figs. 1 and 2 show a spraying device 20 positioned around the inner wall of a chimney 10 above a connecting portion 3 between the vertical chimney 10 and a horizontal flue 11. The direction of arrow A approximates the flowing direction in flue 11 of exhaust gases, whose temperature is generally above 1 80 C. The flow of the exhaust gases may be turbulent at position 3 due to the sharpened sudden change of flow direction. Water, which is preferably pre-heted to a certain temperature within a main supply pipe 21 by latent heat of the gases around it, is sprayed from nozzles 30 of the spraying device 20 to form layers of mist 4. Parts of the layers of mist are preferably heated to form saturated vapour by the latent heat of the exhaust gases, so that the volume of mist layers will expand by a factor of least 1,500, which reduces the amount of water required.The mist with partially saturated vapour layers 4 can positively catch fine particles, and react with soluble noxious materials such as sulphur dioxide entrained in the gases which pass through the mist. The result is a liquid sludge which falls to an inclined bottom 60 of chimney 10. Provided on the lower portion of the chimney 10 at the lower end of the bottom 60 is an outlet 12 through which liquid sludge can be discharged to a precipitating vessel 50. After the fine particles of liquid sludge precipitate the dirty liquid can flow into a suitable water-treating vessel 51, and then can be discharged into a drainage system 52.

Figs. 3 and 4 show a suitable construction of spraying device 20, which consists of a main pipe 21, a circular pipe 28 communicating with one end of the pipe 21, and a plurality of nozzles 30 mounted on the circular pipe 28. The orifices of the nozzle 30 face the centre of the circular pipe 28 to enhance spraying. As is best shown in Fig. 5, a preferred nozzle includes the following features: an outer supporting member 24 having an external screw thread at its upper end; a nozzle body 34 containing a spraying chamber 31 and provided with female screw thread at its lower portion to engage with the outer supporting member 24, and with screw thread perpendicular to this; an atomizing nozzle 32 having external screw thread to engage with the second-mentioned screw thread of the spraying chamber.A branch pripe 22 is provided on the circular pipe 28 extending through the outer supporting member 24 and terminating at the lower portion of the spraying chamber 31. In order to avoid corrosion or wear, all parts of the spraying device 20 are preferably made of an anti-acid stainless steel, except for nozzles 30 which are preferably made of a certain type of brass.

A checkered plate 40 is provided at the out!et 12 to restrict the entry of cold air into the chimney 10 which would reduce the temperature of the flue gases. The outlet 12 is therefore designed, not only to allow discharge of liquid sludge, but also to restrict entry of cold air.

The lower portion of the inner wall of the chimney, the bottom 60 of the chimney and the inner wall of the outlet 12 can be provided with a stainless steel plate 62 to prevent the refractory inner walls of the chimney 10 being etched by the layers of mist.

Since the mist resulting from the liquid sprayed from the device 20 absorbs latent heat from the flue gases, the temperature of the flue gases will be reduced, and this may result in bad ventilation of the chimney or in incomplete combustion. In order to avoid this, a solenoid valve 27, a pressure regulating valve 23 and a by-pass valve 26 may be provided on the main pipe 21 to control the water supply. Pressure gauges 25 may be provided upstream and downstream of the pressure regulating value 23.

Before the boiler pressure reaches a certain value, for instance 8 Kg/cm2, or when the boiler is under a lower load of operation, the solenoid valve 27 will automatically close the main pipe 21 until the boiler pressure reaches a pre-determined value or until operation of the boiler has returned to normal. A conventional electric control can be used, and it will not therefore be described in detail.

Some detailed testing data obtained carrying out the process of the invention will now be given.

Testing period: March 1976 to August 1979 Testing Place: Chung-Ho Textile Co.

Wu-Tz Factory, Taichung, Taiwan, R.O.C.

Capacity of the boiler 10 tons/hour Type of boiler double drum water tube boiler Fuel: slack coal Calorific capacity of the fuel 6428 Kcal/Kg.

Size of chain grate stoker 5m X 2.6m Grate area 13m2 Rate of combustion 120 Kg/m2hr Coal combusted per hour 1560 Kg Analysis of Elements in the coal C H2 O2 N2 S Water Ash 62% 6.3% 13% 1.3% 1.2% 1.2% 13% Height of the chimney 45m Diameter of the chimney 2 m Spraying pressure of the nozzles 0.25 Kg/m2 Diameter of the Orifice 4.7 mm Number of the nozzles 20 each Jet velocity of the nozzle 2.26 m/sec Total quantity of the spraying water per minute 46.6 Kg Heat loss 1210.21 Kcal/min Temperature drop 32,41 C Temperature of the flue 186 C Results of dust collection: : (A) Coal combusted per day 25 tons Spraying pressure 0.15 Kg/cm2 Spraying volume per day 20.12m3 Recovered fine particles 2.45 tons Percentage of the recovered fine particles in total combusted coal 9.8% (B) Coal combusted per day 37.5 tons Spraying pressure 0.25 Kg/cm2 Spraying volume per day 40 m3 Recovered fine particles 4.234 tons/day Percentage of the recovered fine particles in total combusted coal 11.28% From various tests, it appears that the spraying pressure to the nozzles is preferably 0.25 Kg/cm2. It also appears that the fine particles recovered can be burnt to release a considerable amount of heat, about 2,044 Kcal/Kg, and these fine particles may therefore be mixed with fresh fuel and conveyed to the combustion chamber of the boiler.

Claims (12)

1. A method of scrubbing flue gas, in which scrubbing liquid injected into the gas is heated by latent heat of the gas to form a mist which can trap dust and noxious material from the flue gas.

2. A method according to Claim 1, carried out upstream of a region of turbulent flow of the gas.

3. A method according to Claim 1 or Claim 2, in which the liquid is water.

4. A method according to any one of Claims 1, 2 and 3, in which the liquid is pre-heated by the flue gas before injection, by its passage along a pipe in the flue.

5. A method according to any one of the preceding Claims, in which at least part of the mist is saturated with the liquid.

6. A method for removing dust and noxious materials from flue gases within a tall chimney, comprising the steps of forming mist layers above a portion, where said chimney is connected with a flue and may occur a turbulent flow therein, by spraying water with at least a set of spraying device assembly provided around the inner wall of said chimney, heating a part of said mist layers to form saturated vapour layers by the latent heat of said flue gases; catching fine particles and noxious materials entrained in said flue gases which pass through the mist layers; forming condensed droplets down to the bottom of said chimney to form liquid-sludge; and discharging said liquid-sludge through an outlet provided at the lower portion of said chimney into a precipitating vessel.

7. A method as claimed in Claim 6, wherein the spraying pressure of said spraying device assembly is preferably set at a range from 0.05 Kg/cm2 to 0.50 Kg/cm2.

8. A method as claimed in either one of Claims 6 and 7, wherein the spraying pressure of said spraying device assembly is controlled by a solenoid valve which is interlocked with a boiler so as to adjust the temperature of said chimney depending on the combustion within said boiler.

9. A method as claimed in any one of Claims 6 to 8, wherein said bottom of said chimney is provided with an inclined surface, and said outlet is located at the lower end of the inclined bottom so as to permit said liquid-sludge deposit on said inclined bottom to be discharged naturally.

10. A method as claimed in any one of Claims 6 to 9, wherein said outlet is provided with a checkered plate extending into said precipitating vessel for preventing overmuch cold air from entering said chimney.

11. A method as claimed in any one of Claims 6 to 9, wherein on the surface of the inner, lower wall of said chimney, inner wall of said outlet and said inclined bottom are provided with a stainless steel plate so as to prolong the life of said chimney.

12. A spraying device assembly provided around the inner wall of a tall chimney comprising a main pipe for supplying water and positioned such that the water can receive latent heat from flue gases in the chimney, a circular pipe communicated with one end of said main pipe, and a plurality of nozzles mounting on and communicated with said circular pipe wherein orifice of each said nozzle faces the centre point of said circular pipe so as to enhance the spraying effect.

12. A spraying device assembly provided around the inner wall of a tall chimney comprising a main pipe for supplying water, a circular pipe communicated with one end of said main pipe, and a plurality of nozzles mounting on and communicated with said circular pipe wherein orifice of each said nozzle faces the centre point of said circular pipe so as to enhance the spraying effect.

13. A spraying device assembly as claimed in Claim 12, where in each said nozzle is surrounded by a hollow outer supporting member.

14. A method of scrubbing flue gas, substantially as herein described with reference to any one of the accompanying drawings.

15. An apparatus for scrubbing flue gas, substantially as herein described with reference to any one of the accompanying drawings.

CLAIMS (9 April 1980)