GB2082086A - Treatment of waste gases - Google Patents

Abstract

An electric-arc steelmaking vessel 2 has electrodes 4 and oxy-fuel burners. During operation of the vessel 2, a hot fume-laden gas mixture containing combustible gas is evolved. The combustible gas is burned in a combustion chamber 14. The resultant gas mixture is then cooled in a conventional conditioning unit 20. Requirements for cooling beyond those that can readily be met by the conditioning unit are met by spraying liquid nitrogen through sprayer 39 into the hot fume-laden gas upstream of a filtration house 24 in which fume is removed from the gas; alternatively liquid noble gas or liquid CO2 may be used. Injection of liquid gas may be controlled in response to sensors 30,32 sensing temperature and/or humidity. <IMAGE>

Description

SPECIFICATION Treatment of gases This invention relates to the treatment of gases, particularly fume-laden gases emitted from a furnace, for example, an electric-arc steel making furnace.

Electric arc furnaces have been used for making steel for more than 50 years. In a typical procedure a charge comprising steel scrap is introduced into the furnace (or steel-making vessel) and an electric arc struck between the charge and the furnace electrodes (or between the electrodes themselves) so as to heat and thus melt the charge. When the charge is substantially molten it is refined by oxygen and its carbon content is thus reduced. During this melting phase relatively large quantities of iron oxide fume and combustible gas are formed. The chief gaseous constituents of this "off-gas", as it is sometimes called, are carbon monoxide, carbon dioxide, nitrogen and oxygen with the possibility of there being present in the gas significant amounts of hydrogen if the initial charge is wet and of hydrocarbon if the initial charge is wet or oily.In addition, there may be a considerable evolution of gas at the start of the melting of the charge. In addition, the operation of oxy-fuel burners to assist in the melting of the charge may create large volumes of "off-gas". Such oxy-fuels burners are typically employed to counteract any tendency for some parts of the charge, away from the vicinity of the arc, to remain at a temperature significantly lower than that of other parts of the charge in the vicinity of the arc. During the refining phase the carbon monoxide and iron oxide fume content of the off gases rise sharply.

It is therefore necessary to employ plant for burning or oxidising the combustible gases and removing the bulk of the fume.

Typically, such plant includes a combustion chamber in which the combustible gases are burned, a conditioning apparatus in which the gases leaving the combustion chamber are cooled, and apparatus for removing fume from the gases. In one arrangement an elbow-shaped pipe is fitted to the furnace and communicates with an orifice in the furnace roof through which the off-gases are taken. The other end of the elbow almost registers with a duct conveying the off-gas to the combustion chamber; there being means for varying the size of the gap between the two pipes. A flow of air is thus induced into the fume-laden gases, the magnitude of the flowing depending on the size ofthe slice gap.

A separate stream of air is caused to flow into the combustion chamber to support the combustion of the combustible gases. Substantially all of the combustible gases are burnt in the combustion chamber. Carbon monoxide is therefore oxidised to carbon dioxide, hydrogen to water vapour, and hydrocarbons to carbon dioxide and water vapour.

The resultant gas mixture is then passed into the conditioning unit. This unit may be one in which the gas is cooled by being diluted with air, by being contacted with water, or by being passed through tubes which radiate heat to air that is passed over the tubes.

The apparatus for removing the fume from the gases may be a scrubber or an electrostatic precipitator. However, in order to meet modern requirements regarding pollution of the atmosphere, the means for removing the fume is typically a filtration apparatus comprising an array of bags of a suitable fabric or other filter material. It is necessary, particularly with filters of the kind described, to ensure that the temperature and humidity of the gas are below chosen limits. If the humidity is too high there is a tendency for a cement-like substance to form in the filter-bags and clog them. If the temperature is too high the material from the bags are formed is damaged.Typically, this limiting temperature is in the order of 135"C. In practice, it is often found that the conditioning unit does not reduce the tempera tureto below this limiting value. It is therefore found necessary on occasions to dilute the fume-laden gas with air. However, sometimes the temperature of the fume-laden gas is so excessive that the requirement for air dilution would, if met, mean that the total volume of gas entering the filtration apparatus would be more than the plant could cope with. In such circumstances, it is necessary to shut down the furnace rather than exceed the limit on the total volume of gas.Moreover, since there are limits on the maximum gas temperature and volume with which the plant can cope, it may not be posible to supply the amount of additional thermal energy from oxy-fuel burners that it would otherwise be desirable to provide.

It is to be appreciated that analogous problems can arise with types of fume removal apparatus other than that which uses filter-bags and with other metallurgical processes than steel making in an electric arc furnace.

According to the present invention there is provided a method of removing fume from hot fumeladen gas including the steps of introducing the gas into means for removing fume therefrom; and introducing fluid having a temperature of -50 C or below into the hot fume-laden gas upstream of the fume-removal means so as to keep the temperature of the gas below a value at which it would cause damage to the fume removal means.

The invention also provides apparatus for removing fume from hot fume-laden gas, including means for removing fume from the gas; means for sensing the temperature upstream of the fume removal means; and means for introducing fluid having a temperature of -50 C or below into the hot fumeladen gas upstream of the fume-removal means if the sensed temperature is above a chosen value.

The method and apparatus according to the present invention are particularly suited for use in protecting a filtration apparatus comprising bags formed of a suitable fabric or other material from the effects of having to filter gas at an excessive temperature.

The aforesaid fluid is preferably liquid nitrogen.

Alternatively, a liquid noble gas or liquid carbon dioxide may be employed. It is possible to evaporate the liquefied gas before it encounters the hot fume-laden gas. This practice is, however, not to be recommended as it is wasteful of the cooling effect of the liquid nitrogen, in particular the latent heat of evaporation. Instead, it is preferred to introduce liquefied gas, be it liquid nitrogen or another of the liquefied gases mentioned herein, directly into the fume-laden gas upstream ofthefume-removal means.

If liquid nitrogen is employed as the aforementioned fluid it will be appreciated that owing to its low temperature of -196"C, the volu me of it (measured as gas) required to produce a given temperature reduction in a given volume of fume-laden gas is far less than the volume of air (at ambient temperature) required to give the same temperature reduction in the same volume of fume-laden gas.

Thus, the use of liquid nitrogen makes it possible to operate the filtration orotherfume removal means in circumstances in which, were merely air dilution to be relied upon to reduce the temperature of the fume-laden gas, it would not be possible to operate the filtration means. In the example of an electric arc furnace in which steel is made, by adopting the method according to the invention with liquid nitrogen as the aforesaid fluid the length of time for which the furnace needs to be shut down owing to problems concerning the treatment of the off-gases may be reduced and/or a greater amount of supplementary heat can be supplied by operating oxy-fuel burners without making it necessary for the furnace to be shut down for longer periods. These advantages can be obtained using liquid carbon dioxide, instead of liquid nitrogen, butto a lesser extent.In general, we believe that using a liquefied noble gas will not be as economically attractive as using liquid nitrogen or liquid carbon dioxide.

There are other advantages associated with the use of liquid nitrogen (or other "inert" liquefied gas).

For example, substituting such liquefied gas for dilution airwouldtend inevitably to reduce the volume of gas that is handled by equipment for removing fume from the fume-laden gas and may therefore make a marginal reduction in the power consumption. Futhermore, having a facility to dilute the fume-laden gas with liquid nitrogen is a useful safety provision which makes it possible to purge the fume removal equipment in the event of failure of a fan or fans used to draw the gas through the filters.

Since liquid nitrogen (or a liquefied noble gas) is dry, its addition to the fume-laden gas will reduce the humidity of that gas. Accordingly, it desired, there may be means for sensing the humidity of the gas upstream of the fume removal means, and liquid nitrogen or other fluid having a temperature of -500C or below may be introduced into the fumeladen gas should its sensed humidity reach a chosen value.

Liquefied gas may be sprayed or otherwise introduced into a pipe through which the fume-laden gas flows into the fume removal means.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawing which is a schematic diagram illustrating plant for treating off-gas from an electric-arc furnace.

Referring to the drawing, an electric arc furnace 2 for making steel has electrodes 4 and oxy-fuel burners 6 for melting the furnace charge. In thereof of the furnace vessel is an orifice 7. In operation, fume-laden gases produced as a result or the steel making process leave the furnace through the orifice 7.

An elbow-shaped pipe 8 is attached to the furnace and is aligned with the orifice 7 and is out of register with the duct such that a gap 9 is left therebetween.

The size of this gap may be varied by means (not shown) so as to vary the amount of air that flows into the gases leaving the furnace 2 through the orifice 7.

The duct 10 which leads to a combustion chamber 14. The gases flowing through the pipe 10 are diluted with air supplied from pipe 12 communicating with the pipe 10.

The gases from the pipe 10 are supplied to burners in the combustion chamber 14. These burners oxidise any carbon monoxide in the off-gas to carbon dioxide, hydrogen to water vapour, and hydrocarbons to carbon dioxide and water vapour.

Secondary air for the combustion is supplied to the chamber 14 via a pipe 16.

The combustion gases (which will be laden with fume) pass out of the combustion chamber 14 into a pipe 18 and enter a conditioning unit 20 in which they are cooled. The conditioning unit may be of a kind conventionally used in treating fume-laden gases evolved from an electric arc steel making furnace and is preferaly of a kind which does not involve the addition of moisture to the fume-laden gases (although a kind of conditioning unit which does involve such addition of moisture may alternatively be employed).

These gases leave the conditioning unit 20 and pass into a pipe 22 which conveys them to a filtration house containing an array of bags of fabric filter material. Substantially all the iron oxide fume is removed from the gas in the filtration house 24. A fan or fans 26 are operated in order to induce a flow of gas through the filtration apparatus 24. The fan or fans 26 supplying the clean gas to a stack 28.

In operation of the electric arc furnace or vessel 2, the temperature and composition of the off-gases will vary according to the stage of the steel making cycle. In particular, the proportion and quantity of combustible gases may vary. Accordingly, the heat generated by the combustion of these gases in the combustion chamber 14 will vary. Thus, the temperature of the fume-laden gas leaving the conditioning unit 20 will vary. Typically, the conditioning unit may be designed so as to reduce the temperature of the gas entering it to below the maximum temperature at which the filters may be safely operated. Typically, this maximum temperature is in the order of 1350C.

In accordance with the present invention, the plant shown in Figure 1 is provided with means for' reducing the temperature of the gas in the pipe 22 should it approach a temperature of 135"C. Thus, a spray header 39 is located in the pipe 22 and communicates with a pipe 38 which is connected to a source 42 of liquid nitrogen. A solenoid valve 40 is located in the pipe 38. Downstream of the spray header 39 but upstream of the filter house 24 are located a temperature sensor 30, a humidity sensor 32 and a flow meter 34. These are all adapted to generate electrical signals representing respectively the temperature, humidity, and the flow rate of the rias about to enter the filter house.The electrical signals generated by the sensors 30 and 32, and the flow meter 34 are converted by means of a controller 36 into signals which control the opening and closing of the valve 40. The controller is programmed to open the valve should the temperature sensed by the sensor 30 reach, say, 1300C. It is programmed to close the valve 40 again when the temperature falls to, say, 125"C. The controller 36 is also programmed so as to open the valve 40 (unless it is already open) if the sensed humidity ever rises to a chosen value, ad to close the valve 40 when the humidity falls to a chosen lower value. (Instead of the arrangement shown there may be one valve controlled by the sensor 30 and another valve controlled by the sensor 32).The controller 36 is also programmed such that the temperature sensor 30 and the humidity sensor 32 are "overriden" should the flow rate reach a chosen maximum. In such circumstances, the valve 40 is closed and operation of the electric arc furnace is discontinued. It is to be appreciated, however, that using liquid nitrogen to cool the gas downstream of the conditioning unit 20 instead of air, reduces the total volume of gas flowing into the filter house 24 and thus tends to reduce the likelihood of the maximum flow rate being reached.

Generally, the bulk of the cooling will be provided by the conditioning unit 20, and liquid nitrogen cooling will come into operation only during periods in which there is a particularly large cooling load.

It will, we believe, frequently be possible to operate the method and apparatus according to the invention without means for controlling the introduction of liquefied gas into the hot fume-laden gas responsive to changes in sensed humidity of the gas.

Claims (16)

1. A method of removing fume from hot fumeladen gas including the steps of introducing the gas into means for removing fume therefrom; and introducing fluid having a temperature of -50 C or below into the hot fume-laden gas upstream of the fume removal means so as to keep the temperature of the gas below a value at which it would cause damage to the fume removal means.

2. A method as claimed 1, in which the said fluid is liquid nitrogen.

3. A method as claimed in claim 1, in which the said fluid is liquid carbon dioxide or a liquid noble 9as.

4. A method as claimed in any one of the preceding claims in which the fume removal means includes filter bags of a suitable material.

5. A method as claimed in any one of the preceding claims, in which the said fluid provides only part of the cooling for the fume-laden gas.

6. A method as claimed in claim 5, in which tge fume-laden gas is additionally cooled by being diluted with air, contacted with water or by being passed through tubes which radiate heat to air that is passed over the tubes.

7. A method as claimed in any one of the preceding claims in which introduction of the said fluid into the fume-laden gas is initiated only when the temperature of such gas sensed immediately upstream of the fume removal is at or above a chosen value.

8. A method as claimes in claim 7, in which the said introduction is stopped once the sensed temperature falls to a chosen value.

9. A method as claimed in any one of the preceding claims, in which the fume-laden gas is evolved from an electric arc steel making vessel.

10. A method as claimed in claim 9, in which oxy-fuel burners are employed to assit melting of the charge in the steel making vessel.

11. A method of removing fume from hot fumeladen gas, as claimed in any of the preceding claims, comprising the steps of cooling the gas by means of air and/or water, passing the gas through means for removing fume therefrom, sensing the temperature of the cooled gas and introducing liquefied gas having a temperature of -50 C or below into the cooled gas upstream of the fume removal means when the sensed temperature rises above a chosen value so as to restore the sensed temperature to below the chosen value.

12. A method of removing fume from hot fumeladen gas substantially as herein described with reference to the accompanying drawings.

13. Apparatus for removing fume from hot fumeladen including means for removing fume from the gas; means for sensing the temperature upstream of the fume removal means; and means for introducing fluid having a temperature -50OC or below into the hot fume-laden gas upstream ofthefume removal means if the sensed temperature is above a chosen value.

14. Apparatus as claimed in claim 13, in which the fluid-introduction means is a sprayer for spraying liquefied gas into the fume-ladeb gas.

15. Apparatus as claimed in claim 13 or claim 14, additionally including means for sensing the humidity of the fume-laden gas upstream of the fume removal means, the arrangement being such that the said fluid is introduced into the fume-laden gas should, in operation of the apparatus, its sensed humidity reach a chosen value.

16. Apparatus for removing fume from hot fumeladen gas substantially as herein described with reference to, and as shown in, the accompanying drawing.