GB2225002A

GB2225002A - Control of oxides of nitrogen in post-combustion waste gases

Info

Publication number: GB2225002A
Application number: GB8826797A
Authority: GB
Inventors: Keith Russell Mcneill
Original assignee: Individual
Current assignee: Individual
Priority date: 1988-11-16
Filing date: 1988-11-16
Publication date: 1990-05-23
Also published as: WO1990005578A1; GB8826797D0

Abstract

The NOx content of post-combustion waste gases is reduced by adding atomised ammoniacal water having an ammonia concentration of 10 % to 30 % by weight, and preferably 20 to 22 % by weight, to the waste gases at temperatures in the range 870 DEG C to 1090 DEG C. Preferably the reaction of the NH3, with the NOx in the waste gases is performed in a chamber containing a moving pebble bed, and the waste gases are passed from this chamber to a second pebble bed chamber in which removal of SOx is effected, particulates also being removed by the filter effect of the pebble beds.

Description

CONTROL OF OXIDES OF NITROGEN IN POST-COMBUSTION WASTE GASES This invention relates to the control of the content of the oxides of nitrogen (NOx) in post-combustion waste gases by a treatment to reduce the content of oxides of nitrogen.

Any combustion process where nitrogen and oxygen gases are present at temperatures in excess of 800 C produces oxides of nitrogen (NOx) in the waste gas.

Oxides of nitrogen are undesirable pollutants which, when mixed with water, are commonly referred to as acid rain.

It is inevitable that induStrial processes in which furnaces, incinerators or ovens are heated by burning a fossil fuel or organic material with air to provide some or all of the heat requirement will give rise to oxides of nitrogen as constituents of the waste gases. The higher the flame temperature employed in the process, the higher is the concentration of oxides of nitrogen in the waste gases and, since it is the higher flame temperatures which give higher heating and melting efficiency in fossil fuel firing, the more efficient fossil fired processes produce the higher levels of the pollutant oxides of nitrogen.

In consequence of increased environmental conker over the production of acid rain as a result of the emission of oxides of nitrogen into the atmosphere, considerable attention has been directed to methods of reducing the content of oxides of nitrogen in postcombustion waste gases.

It is known that the content of oxides of nitrogen may be reduced by limiting the flame temperature, but this is not a satisfactory method because it inevitably reduces the efficiency and output of the industrial process.

It is also known to add ammonia gas to the waste gases in order to reduce the content of oxides of nitrogen by the following reaction: 4NH3 + 4NO + 02 = 4N2 + 6H20 However this reaction only proceeds in a temperature range of 8700C to 10900C. At temperatures above 1090go the ammonia reacts to form more pollutant oxide of nitrogen by the process: 4NH3 + 502 = 4N0 + 6H20 while below 8700C neither reaction proceeds and the result is the emission of unreacted ammonia. In practice it is difficult to achieve significant reductions in the NO, content of waste gases by the addition of ammonia gas.

It is further known to use water to reduce the NOX content of post-combustion waste gases. This is done by adding water to the combustion air before this is fed to the combustion zone. In consequence the water vapour becomes super-heated and the water dissociates in the combustion air. As a result the flame temperature in the combustion zone is reduced, thereby reducing the quantity of oxides of nitrogen formed, although the emissivity of the flame is increased because of the water content, particularly when natural gas is the fuel, so that the decrease in the efficiency of the process is not as great as the reduction in formation of oxides of nitrogen.

In accordance with the present invention the content of oxides of nitrogen (NOx) in post-combustion gases is reduced and/or controlled by adding atomised ammoniacal water to the post-combustion waste gases, the ammoniacal water having a concentration of from 10% to 30% by weight of ammonia dissolved in water.

Surprisingly, it is found that the addition of atomised ammoniacal water to waste gases containing NOX pollutants has a substantially greater effect in reducing the NO, content of the post-combustion waste gases than would be expected from experience in using water and ammonia separately for the same purpose. It is thought that this surprising effect may be due to the rapid vapourization of the water and then its dissociation which causes thorough mixing of the ammonia with the waste gases. At the same time the temperature of the waste gases is reduced.

Also the dissociation of the water inhibits the oxidation of the ammonia at high temperatures, i.e. above 1090 C, thus allowing the reduction of NO, to take place at these higher temperatures.

In consequence the atomised ammoniacal water may be added to the waste gases very soon after combustion, for example at the top of a conventional regenerator, thus allowing a longer period within which the reduction of the NO, can take place. The temperature of the waste gases to which the atomised ammoniacal water is added may be in the range from 16000C to 20000C, the higher temperatures being present when the fuel is natural gas, the waste gases from oil combustion being in the range of 16000C to 1800 C.

Alternatively, however, the addition of the ammoniacal water to the waste gases may be effected by supplying the atomised ammoniacal water to a chamber in a regenerator packing through which the waste gases are passed. In such a case the temperature of the waste gases will be lower, probably of the order of 1600 C, the refractory of the regenerator being at a temperature of the order of 1500 C.

A result of the addition of the ammoniacal water to the waste gases is an increase in the volume of the waste gases due to the water vapour now present, thereby increasing the emmissivity of the waste gases, again due to the water vapour, so that the waste gases give up their heat to the regenerator packing as the waste gases pass down the regenerator. In consequence, when a regenerator is used, the process of the present invention will not significantly alter the pre-heat temperature of combustion air passed through the regenerator and the fuel consumption of the process will remain substantially unchanged.

The present invention also has application in a recuperator, in which case the atomised ammoniacal water may be added to the waste gases as they leave the furnace.

Preferably the ammoniacal water has a concentration of from 20coo to 22% by weight of ammonia dissolved in water. Ammoniacal water having such an ammonia concentration is available as a by-product from a coking plant. Such a by-product contains other impurities including hydrogen sulphide, and organic compounds such as pyridine and hydrogen cyanide.

Provided that the hydrogen sulphide content is below 1 by weight, such ammoniacal water is acceptable for treatment of post-combustion waste gases in accordance with the present invention as the organic materials, which in any case are only present in low quantities, are destroyed at temperatures above 10000C.

The addition of the atomised ammoniacal water to the waste gases is monitored and controlled to compensate for variations in the rate of fuel input and in the temperature of the combustion air. The lower the temperature of the combustion air the less of the ammoniacal water is necessary.

The addition of ammoniacal water to post-combustion waste gases in accordance with the present invention is particularly effective in enabling natural gases to be used as an industrial fuel without discharging large quantities of pollutant oxides of nitrogen into the atmosphere. This is particularly valuable because, as already indicated, natural gas flames are hotter than oil flames and are therefore preferred in many industrial furnaces, for example in glass-melting furnaces.

In accordance with a preferred feature of the present invention the introduction of the atomized ammoniacal water to reduce the NOX content of the waste gases is combined with a treatment to remove oxides of sulphur (SOx) and particulates from the post-combustion waste gases.

Conveniently the combined treatment of the waste gases to remove NOx, SOx and particulates is effected by including a secondary regenerator to which the waste gases are fed from a conventional regenerator.

Advantageously the secondary regenerator comprises a chamber including a bed of heat tolerant metal or ceramic bodies of a size to provide a filter effect within the secondary regenerator, and means for moving the metal or ceramic bodies through the secondary regenerator.

The present invention will be better understood from the following detailed description which is made, by way of example, with reference to the accompanying drawings in which: Figure 1 shows diagrammatically a regenerator including means for adding ammoniacal water to postcombustion waste gases in accordance with the present invention, Figure 2 shows diagrammatically an alternative regenerator including a special chamber for adding ammoniacal water in accordance with the present invention, Figure 3 shows diagrammatically the position in a recuperator at which ammoniacal water is added to post-combustion waste gases in accordance with the present invention, Figure 4 is a flow diagram of a preferred process in accordance with the present invention in which a secondary regenerator is provided in addition to a conventional regenerator, and Figures 5 and 6 are respectively a diagram=,atic cross-sectional representation and a diaracatic plan view of a secondary regenerator used in the process of Figure 4.

Referring to Figure 1 of the drawings there is shown diagrammatically a furnace regenerator system having a first regenerator stack 1 through which combustion air is fed and in which the combustion air is heated by residual heat retained in the bricks of the first regenerator stack 1 from the previous cycle during which the first regenerator stack 1 was passing post-combustion waste gases to flue. The warmed combustion air is passed through a port 2 to be mixed with fuel, for example natural gas or oil, provided from a fuel injector 3 where the fuel is burnt and heat provided for a furnace chamber 4 over a melt 5, for example, of glassforming materials.

As the waste gases from the combustion leave the furnace chamber 4 through a port 6, cooling air is added through a feed 7. The waste gases proceed to the top of a second regenerator stack 8 and, immediately before entering the second regenerator stack 8, atomized ammoniacal water is added to the waste gases by a jet 9.

In practice the jet 9 for introducing the ammoniacal water in accordance with the present invention will be duplicated at a position between the port 2 and the first regenerator stack 1 for use in the alternative cycle of the regenerator system.

Figure 2 shows diagrammatically an alternative regenerator stack 10 in which a compartment 11 in the body of the regenerator stack 10 is provided for the purpose of injecting ammoniacal water vapour to be mixed with the waste gases as they pass through the compartment 11 in the regenerator stack 10.

Figure 3 shows diagrammatically a recuperator 14, in which the path of the post-combustion waste gases from a furnace chamber 15 over a melt 16 is indicated by arrows 17 and the path of pre-combustion air to be heated in the recuperator 14 is indicated by arrows 18. In accordance with the present invention ammoniacal water is added to the waste gases through a jet 19 in the recuperator 14.

A preferred method in accordance with the present invention will now be described with reference to the flow diagram which is Figure 4. In this preferred method a secondary regenerator is provided in addition to the conventional regenerator. The secondary regenerator is a separate and additional regenerator which enables a number of advantages to be provided.

In the first place it enables the combustion air fed to the furnace to be heated to higher temperatures for more efficient combustion. Secondly, it enables the conventional regenerator to be maintained at a higher temperature so that the conventional regenerator is preferably partly or wholly at a temperature within the range from 8700C to 10900C at which ammonia reacts to reduce the NO, content of the waste gases. Thirdly, the secondary regenerator provides a medium for removing oxides of sulphur and/or particulate matter from the waste gases.

Referring to Figure 4, combustion air, that is air for use in the combustion process, is passed in the direction of the arrow 21 into a secondary regenerator 22. During passage of the combustion air through secondary regenerator 22 an aqueous desulphurizing agent is added to the secondary regenerator 22 as represented at 23, such that the water content of the aqueous desulphurizing agent is vapourized and mixed with the combustion air so that combustion air with added water vapour passes, as represented by arrow 24, to a conventional regenerator stack 25. The desulphurizing agent is meanwhile deposited within the secondary regenerator 22 for reaction with SO, in the waste gases during the alternative cycle when a secondary regenerator 22 will be receiving partially cooled post-combustion waste gases from the conventional regenerator stack 25.

The combustion air with added water vapour is further heated in the conventional regenerator stack 25 and is then fed to a furnace 25 where it is burnt with the fossil fuel, such as natural gas, conventionally provided. The presence of super-heated water vapour in the combustion air depresss the flame temperature and thereby aids control of NO, production.

Post-combustion waste gases from the furnace 26 pass, as represented by arrow 27, to a second conventional regenerator stack 28 to the top of which ammoniacal water is added in accordance with the present invention as indicated at 29. A heater 30 is further provided near the top of the second conventional regenerator stack 28 for the purpose of selectively adding heat in order to maintain the top of the second conventional regenerator stack 28 near to 10900C so that reaction of ammonia with NOX in regenerator stack 28 is promoted.

Maintaining the second conventional regenerator stack 28 at a higher temperature range than conventional processes prevents much of the sulphonate deposit normally experienced in a conventional regenerator stack. Waste gases with a sharply reduced NO, content are passed from the second conventional regenerator stack 28 to a second secondary regenerator 31 in which a desulphurizing agent has been deposited to react with the SO, content of the waste gases. The second secondary regenerator 31 preferably also includes an appropriate filter for removing particulate matter from the waste gas so that the waste gas passed from the second secondary regnerator 31 to flue has NOx, SO, and particulate contents within acceptable limits for discharge into the atmosphere.

A preferred construction of secondary regenerator used for both the secondary regenerators 22 and 31 will be described with reference to Figures 5 and 6 of the accompanying drawings.

As shown in the diagrammatic cross-sectional side view of a secondary regenerator which is Figure 5 and the diagrammatic plan view which is Figure 6, there are two similar vertical chambers 32 and 33 which are arranged to contain respective beds of suitable sized bodies of metal or ceramic heat tolerant material, for example spherical balls 34. These balls 34 fill a substantial portion of each of the vertical chambers 32 and 33 and effectively provide a filter medium on which particulates will be deposited as the waste gases pass from a flue 35, past an open damper 36, upwardly through the chamber 32 along a linking pipe 37 and downwardly through the chamber 33 before being exhausted through a passage 38 past another open damper 39 and being fed to a chimney (not shown).

The spherical balls 34 of metal or ceramic heattolerant material are continuously cycled through the chambers 32 and 33 by a suitable mechanism indicated diagrammatically at 40, so that the deposits of sulphur and particulates may be removed from the balls 34 in a cleaning bath 44 and clean balls may be re-introduced at the top of each of chambers 32 and. 33.

An injector jet 41 is provided to the first chamber 32 enabling a spray of an alkali slurry to be applied to the balls as they move through chamber 32 when the secondary regenerator is included in the combustion air feed side of the apparatus as secondary regenerator 22 of Figure 4.

When the secondary regenerator is in use a damper 42 in the flue 43 from the conventional regenerator is closed. If the secondary regenerator is not to be used in a particular process, the damper 42 is opened and the dampers 36 and 39 are closed.

The processes in accordance with the present invention, and particularly the preferred process of Figure 4, have particular application in the glass industry. The use of the preferred process in accordance with the present invention enables furnaces fuelled by fossil fuel to be used for forming glass and strict requirements for the levels of NOX, SO, and particulate matter in the gases exhausted to atmosphere to be met.

Claims

CLAIMS:

1. A method of reducing the content of oxides of nitrogen (N0x) in post-combustion waste gases which comprises the step of adding atomised ammoniacal water to the waste gases, the ammoniacal water having a concentration of from 10% to 30% by weight of ammonia dissolved in water.

2. A method according to Claim 1 in which the atomised ammoniacal water is added to the waste gases immediately after combustion.

3. A method according to Claim 1 in which the waste gases are passed through a chamber containing ammoniacal water.

4. A method of controlling the content of oxides of nitrogen (N0x) in the waste gas vented to air from a furnace in which natural gas is burnt in air comprising the step of reacting oxides of nitrogen in the waste gases with atomised ammoniacal water having a concentration of from 10% to 30% by weight of ammonia.

5. A method according to any one of the preceding claims in which the ammoniacal water has a concentration of from 20% to 22% by weight of ammonia.

6. A method according to any one of the preceding claims wherein the post-combustion waste gases are passed through a conventional regenerator and a secondary regenerator.

7. A method according to Claim 6 wherein the secondary regenerator comprises a filter for removal of particulate material from the waste gases.

8. A method according to Claim 6 or Claim 7 wherein the secondary regenerator comprises a filter having a desulphurizing agent deposited therein for removal of SO, from the waste gases.

9. A method according to Claim 7 or Claim 8 wherein the filter in the secondary regenerator comprises a moving bed of heat-tolerant metal or ceramic bodies.

10. A method of treating post-combustion waste gases containing oxides of nitrogen (NOx) with ammoniacal water substantially as hereinbefore described with reference to the accompanying drawings.