WO1990005578A1

WO1990005578A1 - Control of oxides of nitrogen in post-combustion waste gases

Info

Publication number: WO1990005578A1
Application number: PCT/GB1989/001361
Authority: WO
Inventors: Keith Russell Mcneill
Original assignee: Keith Russell Mcneill
Priority date: 1988-11-16
Filing date: 1989-11-16
Publication date: 1990-05-31
Also published as: GB2225002A; 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 870C to 1090C. 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 (N0_X) 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 (N0_X) 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 f rnaces, 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 concern 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 post- combustion 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 + 4N0 + O2 = N2 + 6H2O However this reaction only proceeds in a temperature range of 870°C to 1090°C. At temperatures above 1090°C the ammonia reacts to form more pollutant oxide of nitrogen by the process:

4NH3 + 502 = 4N0 + 6H2O while below 870°C neither reaction proceeds and the result is the emission of unreacted ammonia. In practice it is difficult to achieve significant reductions in the N0_X content of waste gases by the addition of ammonia gas.

It is further known to use water to reduce the N0_X 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 (N0_X) 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 107o to 307o by weight of ammonia dissolved in water.

Surprisingly, it is found that the addition of atomised ammoniacal water to waste gases containing N0_X pollutants has a substantially greater effect in reducing the N0_X 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 N0_X 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 N0_X can take place. The temperature of the waste gases to which the atomised ammoniacal water is added may be in the range from 1600°C to 2000°C, the higher temperatures being present when the fuel is natural gas, the waste gases from oil combustion being in the range of 1600°C 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 1300°C to 1500°C, the refractory of the regenerator being at a temperature of the order of 1200°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 e issivity 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 20% to 227o 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 17o 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 1000°C.

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 gas 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 f rnaces.

In accordance with a preferred feature of the present invention the introduction of the atomized ammoniacal water to reduce the N0_X content of the waste gases is combined .with a treatment to remove oxides of sulphur (S0_X) and particulates from the post-combustion waste gases.

Conveniently the combined treatment of the waste gases to remove N0_X, S0_X 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.

Such a secondary regenerator may be constructed with a moving pebble bed acting as a heat exchanger in an essentially similar manner to the pebble bed heater described by C. L. Norton Jr. in the Journal of The American Ceramic Society, Volume 29, (1946) No. 7, pages 187 to 193.

In the application of the present invention to the cleaning of gaseous emissions resulting from the burning of domestic or industrial waste, a pair of moving pebble beds may be used, the gaseous emissions being passed first through a pebble bed to which ammoniacal water is added and then through a pebble bed to which a desulphurising agent is added.

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 post- combustion 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,

Figures 4 and 4a_ are flow diagrams of preferred processes in accordance with the present invention in which a secondary regenerator is provided in addition to a conventional regenerator,

Figures 5 and 6 are respectively a diagrammatic cross-sectional representation and a diagrammatic plan view of a secondary regenerator used in the processes of Figures 4 and 4a_,

Figure 7 is a diagrammatic representation of a system incorporating the present invention for disposing of industrial or domestic refuse by incineration.

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. 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. The fuel injector 3 is similarly duplicated for use in the alternative cycle.

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 870°C to 1090°C at which ammonia reacts to reduce the N0_X 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 S0_X 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 26 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 depresses the flame temperature and thereby aids control of N0_X 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 so that reaction of ammonia with N0_X in regenerator stack 28 takes place.

Waste gases with a sharply reduced N0_X 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 S0_X 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 secondar regenerator 31 to the exit flue has N0_X, S0_X and particulate contents within acceptable limits for discharge into the atmosphere.

In an alternative arrangement illustrated in Figure 4_a_, the ammoniacal water is added at a point in the connecting flue between the second conventional regenerator stack 28 and the second secondary regenerator 31. In order to ensure that the waste gases are in the temperature q^ange 870°C to 1090°C when the ammoniacal water is added, additional heat is supplied by a heater 30 to the waste gases leaving the second conventional regenerator stack 28. The reaction to remove N0_X then proceeds partly in the flue connecting the second conventional regenerator stack 28 and second secondary regenerator 31 and partly within the second secondary regenerator 31.

In addition to maintaining a correct temperature range for effecting reduction of N0_X in the waste gases in the forward cycle illustrated in Figure 4a_, the addition of heat by heater 30 has the further beneficial effect that the temperature of the second conventional regenerator stack 28 is raised when the combustion gases flow into regenerator stack 28 during the alternative or reverse cycle. This has the effect of maintaining the second conventional regenerator stack 28 at a higher temperature range than conventional processes so that, in the next forward cycle, much of the sulphate deposit normally experienced in a conventional regenerator stack is prevented.

A preferred construction of secondary regenerator used for both the secondary regenerators 22 and 31 of Figures 4 and a_ will now 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 heat- tolerant 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 close. 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 secondary regenerator illustrated in Figures 5 and 6 may be modified by the provision of means for applying infra-sound across the beds of metal or ceramic heat tolerant material at 90° to the movement of the waste gases or the combustion air. The infrasound has the effect of imparting to the gas passing through the bed a shuttling movement to and fro transversely to the gas flow, thereby evening out any preferential gas flow through the bed and ensuring a more uniform temperature throughout the gases. The processes in accordance with the present invention as so far described, and particularly the preferred processes of Figures 4 and 4_a, have particular application in the glass industry. The use of this 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 N0_X, S0_X and particulate matter in the gases exhausted to atmosphere to be met.

Referring now to Figure 7 there is shown diagrammatically apparatus for removing oxides of nitrogen, N0_X, and oxides of sulphur, S0_X, from gaseous emissions resulting from the burning of domestic or industrial waste. The gaseous emissions emerging from a chimney 51 of an incinerator 52 are fed at a temperature of the order of 1000°C to an inlet 53 of a pebble bed chamber 54 together with ammoniacal water, NH OH. The gaseous oxides of nitrogen in the gas emissions from the incinerator 52 react with the ammonia in the ammoniacal water in pebble bed chamber 54 at temperatures in the range of 1090°C to 870°C and the oxides of nitrogen are removed from the gaseous stream together with particulates which are filtered out by passage of the gaseous stream through the pebble bed in chamber 54.

The gaseous stream is then passed from exit 55 of pebble bed chamber 54 to inlet 56 of pebble bed chamber 57 to which an alkali such as NaOH or Ca(0H)₂ is added in order to remove oxides of sulphur from the gaseous stream. The temperature of the gaseous stream emerging from exit 55 of pebble bed chamber 54 is preferably of the order of 600°C because the reaction to reduce S0_X and deposit sulphur on the pebbles in the pebble bed chamber 57 is best conducted at a temperature of 600°C. The gaseous stream emerging from exit 58 of pebble bed chamber 57 is then passed to atmosphere either directly or after passage through a bag filter system.

The pebble beds in the chambers 54 and 57 effect a physical - filtration of particulates in addition to the chemical filtrations to remove sufficient N0_X and S0_X for the levels of these oxides to be acceptable in accordance with environmental regulations for discharge of the gaseous stream into the atmosphere.

The pebble bed chambers 54 and 57 are essentially similar to chambers 32 and 33 and include moving beds of spherical pellets or pebbles of heat resistant steel which are cleaned and recycled through the respective chambers 54 and 57. Pebble -bed chambers 54 and 57 may also be provided with means for applying infra-sound in order to vibrate the gases passing through the pebble beds transversely to the gas flow to avoid any preferential gas flows through the pebble beds. The pebble bed chambers 54 and 57 are illustrated in Figure 7 as consisting of a single cylindrical chamber but each of them could be constructed to have upper and lower cylindrical chambers in the manner illustrated for a pebble bed heater as described by C. L. Norton Jr. in the reference cited above.

Claims

CLAIMS :

1. A method of reducing the content of oxides of nitrogen (N0_X) 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 107_o to 307o 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 (N0_X) 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 107o to 307. 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 207o to 227o 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_x 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 ceramics bodies.

10. A method according to Claim 3 wherein the chamber comprises a moving bed of heat-tolerant metal or ceramic bodies.

11. A method according to Claim 10 wherein the gases exiting from the chamber are passed through a further chamber containing a desulphurising agent and a moving bed of heat-tolerant metal or ceramic bodies.