METHOD OF CARRYING OUT A COMBUSTION PROCESS AND APPARATUS FOR USE IN THE METHOD

Abstract: A regenerative furnace is provided with means (25) for introducing ammonia to products of combustion leaving a combustion space (12). The rate of addition of ammonia is controlled in accordance with the difference between the oxygen concentration in the gaseous mixture exhausted from the combustion space and the oxygen concentration downstream of the heat store (20) receiving heat from the gaseous mixture.

Description: Title: "Method of carrying out a combustion process and apparatus for use in the method" According to a first aspect of the present invention, there is provided a method of treating a gaseous mixture resulting from a combustion process and containing free oxygen and oxides of nitrogen. In a method in accordance with the first aspect of the invention, a stream of said gaseous mixture is directed along a flowpath, there are obtained first and second signals which are respectively dependant on the concentration of oxygen in the stream at first and second positions along said path, a reducing agent is fed to said stream at a controlled rate to react between said first and second positions with said oxides and with oxygen present in the mixture and wherein said first and second signals are used together in controlling the rate at which the reducing agent is fed to said stream. We have found that a large reduction in the amount of oxides of nitrogen present in products of combustion can be achieved without excessive use of reducing agent, by means of the present invention. It will be understood that use of excessive reducing agent is undesirable because the presence of a substantial amount of reducing agent in gases discharged to the atmosphere may be objectionable, as is the presence of substantial amounts of oxides of nitrogen in gases discharged to the atmosphere. The method of the invention may be applied to the combustion of fuel using regenerative burners. A regenerative burner has a heat store used to cool products of combustion and, subsequently, to impart heat to air which is to be used for combustion. Regenerative burners are normally used in pairs, the burners of the pair being operated alternately. In a first part of a cycle of operation, air flows through the heat store of one burner and is discharged from the burner with fuel into a combustion space which is common to the pair of burners. Hot products of combustion pass from the combustion space through the heat store associated with the other burner and transfer heat to that heat store. In the second part of the cycle, the direction of airflow is reversed and air flows through the hot heat store to the combustion space, being heated by heat exchange with the heat store, so that heat recovered from the products of combustion during the first part of the cycle is returned to the combustion space. This results in efficient operation, by which we mean that the products of combustion carry to the atmosphere a relatively small proportion of the heat released by combustion of the fuel. A disadvantageous consequence is that the temperature attained in the combustion space is relatively high. This promotes formation of oxides of nitrogen. During the operation of a regenerative burner, the temperature in the heat storage means varies. The variation may be with respect to time, with respect to position or with respect to both of these parameters. For example, in a case where hot products of combustion are directed through a heat store during a first part of a cycle and combustion air is passed through that heat store along substantially the same path during a second part of the cycle, the temperature at a selected position along the path will increase during the first part of the cycle and will decrease during the second part of the cycle. There will be a temperature gradient along the path and this gradient will differ from time to time, particularly from the first part of the cycle to the second part of the cycle. In the combustion of fuel using regenerative burners and treatment of the products of combustion by a method in accordance with the first aspect of the present Invention, the first position is preferably in or immediately adjacent to one of the burners and the second position is downstream of or is at the downstream end of the heat store associated with that burner, considering the direction of gas flow which applies when products of combustion are discharged from the combustion space through that burner and heat store. The reducing agent may be added downstream of the combustion space and is preferably introduced Into the stream of gaseous mixture upstream of the heat store which receives heat from that mixture, preferably within the associated burner. According to a second aspect of the invention, there is provided apparatus for the combustion of a fuel in air, the apparatus comprising means defining a combustion space, means defining a flowpath for conducting products of combustion from the combustion space to the atmosphere, first and second signalling means arranged to provide respectively first and second signals which are dependant on the concentration of oxygen in the contents of said flowpath at first and second sensing positions respectively, said sensing positions being spaced apart along the flowpath, means for feeding a reducing agent into said flowpath at a position between said first and second sensing positions and control means for controlling the rate at which reducing agent is fed into the flowpath, the control means being arranged to receive said first and second signals and to vary the rate of feed of reducing agent in a manner dependant on the relation between said first and second signals. An example of a furnace embodying the second aspect of the invention and which is used in a method according to the first aspect will now be described, with reference to the accompanying drawing, which shows a diagrammatic representation of the furnace. The apparatus illustrated in the drawing comprises a pair of burners 10 and 1 1 , each of which is arranged for discharging fuel and air into a common combustion space 12. The combustion space may be the interior of a furnace or the interior of a radiant heating element. The radiant heating element may be disposed in a heating chamber of a furnace. The furnace is of known construction having thermally insulated walls with a refractory lining and will not be further described. The furnace may be arranged for heating workpieces which are fed continuously through the furnace chamber, for heating workpieces which are placed in the chamber for a predetermined period or for heating a fluid contained in a heat exchanger disposed in the furnace chamber or incorporated in the walls thereof. Fuel supply means is provided for supplying fluid fuel to the burners. The fuel supply means includes ducts 13 extending from a supply of fuel, for example a natural gas main, to the burners and appropriate valves 14 and 15 for controlling flow of fuel to the burners. By way of example, two only valves are shown in the fuel supply means but a larger number of valves would normally be provided, some being intended to exercise control over the fuel flow rate and others intended to prevent the supply of fuel to a burner unless it is appropriate and safe for fuel to be supplied to that burner. There is associated with the burner 10 a heat store 16 comprising a vessel which contains a bed of refractory elements 17. At one of its ends, the vessel communicates with the burner 10 and at its opposite end the vessel communicates through a duct 18 with a change-over valve 19. A similar vessel 20 containing a bed 21 of refractory elements is associated with the burner 1 1 and connected with the changeover valve by a duct 22. Air supply means of the apparatus comprises a fan 23 arranged for blowing air along one or other of the ducts 18 and 22 through one or other of the heat stores to the corresponding burner. There is also provided an exhaust fan 24 for drawing products of combustion through one of the burners and the associated heat store and discharging such products of combustion to the atmosphere. The vessels 16 and 20 may be steel vessels and may have thermally Insulated walls. The refractory elements contained in these vessels, or at least some of these refractory elements, are preferably ceramic bodies. The size of each ceramic body is small, as compared with the volume of the vessel in which it is disposed. The parts thus far described may be constructed and arranged in a known manner. Furthermore, there would be provided for each burner a pilot burner which will Ignite fuel discharged from that burner into the combustion space 12. The apparatus includes a control system comprising appropriate sensors, timing devices and valves for operating the burners alternately and checking that the conditions of operation are safe. The control means may comprise a micro-processor. When the apparatus is brought into operation, the refractory elements In both of the heat stores may be cold. The valve 1 Is set to direct air from the fan 23 along the duct 18 to the burner 10 and fuel is discharged with air from this burner into the combustion space 12 and is ignited by the associated pilot burner. The valve 19 also directs products of combustion from the combustion space 12, through the burner 1 1 and the heat store 20 to the exhaust fan 24. After a period of operation, which may be a predetermined period (for example a period within the range 1 to 4 minutes) or which may be such period as Is required to attain a predetermined temperature at a predetermined position in the apparatus, the valve 19 Is operated to bring to an end a first part of the cycle. During the first part of the cycle, hot products of combustion give up heat to the ceramic elements 21 in the heat store 20 and are thereby cooled, so that the exhaust fan 24 is not subjected to excessively high temperatures. Throughout the first part of the cycle, fuel and air are delivered continuously and, preferably, at substantially uniform rates to the burner 10. The duration of the first part of the cycle is generally at least one minute. During the second part of the cycle, the valve 19 directs air from the fan 23 through the duct 22 and the heat store 20 to the burner 1 1. Fuel Is discharged with the air from this burner into the combustion space and is ignited by the associated pilot burner. Air which flows through the heat store 20 is heated by contact with the hot, ceramic elements 21 and these elements are cooled. The cycle is brought to an end by operation of the valve 1 when either the predetermined period has elapsed or a predetermined temperature has been attained at a further predetermined position in the apparatus. During the second part of the cycle, hot products of combustion pass through the burner 10 to the heat store 16 and impart heat to the refractory elements 17 before passing to the exhaust fan 24. The second part of the cycle also has a duration of at least one minute. During the second part of the cycle, air and fuel are fed continuously to the burner 1 1, preferably at rates which are the same as those at which the fuel and air were fed to the burner 10 during the first part of the cycle. Heating in the manner described hereinbefore of air which flows to a burner results in a relatively high flame temperature being attained. This results. in significant formation of oxides of nitrogen. The apparatus includes means for reacting at least lower oxides of nitrogen to reduce the concentration thereof in gases discharged to the atmosphere. The apparatus includes feed means 25 for feeding a reducing agent into the hot products of combustion. The feed means includes a bulk supply 26 of the reducing agent and a number of ducts, examples of which are represented at 27, 28, 29 and 30, for discharging the reducing agent into the paths along which products of combustion flow rrom the combustion space 12 to the valve 19 at alternative positions along those paths. Valves are provided for controlling flow along the ducts 27 to 30. These valves are incorporated in the control means and subjected to control by the microprocessor. When the burner 10 is firing and hot products of combustion leave the combustion space 12 through the burner I I , reducing agents may be discharged into those products of combustion through the duct 28 only at a position upstream of the heat store 20, prefarably into the burner I I , as shown. Flow of the gaseous mixture through the bed of refractory elements 2 ! promotes thorough mixing of the gases so that the reducing agent is distributed throughout the products of combustion. It will be understood that the reducing agent is a gas, at least at the temperatures prevailing in the passage connecting the burner 1 1 with the heat store 20. Oxides of nitrogen are reduced by the reducing agent in the heat store 20 so that the concentration of oxides of nitrogen in the gases passing through the exhaust fan 24 are small, as compared with the concentration present in the combustion space 12. Alternatively, if the temperature conditions in the burner and/or in the passage leading from the burner 1 1 to the heat store 20 are unfavourable, for example are so high that unwanted dissociation of the reducing agent occurs, the reducing agent may be admitted through the duct 27 into the bed of refractory elements 21 at a position where the temperature is substantially below that in the combustion space 12. Whilst a single duct for discharging reducing agent Into the bed of elements 21 has been illustrated, a number of alternative ducts for introducing the reducing agent at a selected one of several alternative positions in the bed may be provided. Information received by the microprocessor from temperature sensing devices at appropriate positions may be used to determine the position at which the reducing agent should be introduced into the products of combustion. When flow of products of combustion from the combustion space 12 I o through the burner 1 1 to the heat store 20 is terminated, feeding of reducing agent into the bed of refractory elements 21 or via the duct 28 Is also terminated. During the next half cycle, reducing agent is fed through either the duct 29 or the duct 30 to mix with products of combustion flowing from the combustion space 12 into the heat store 16. The reducing agent is 15 preferably introduced into the burner 10 to mix with the stream of gases flowing from the combustion space 12 to the heat store 16. A suitable reducing agent is ammonia. The bulk supply 26 of ammonia may be liquified ammonia or an aqueous solution of ammonia. The control means Includes first sensing means 31 and second sensing 0 means 32 which are spaced apart along the flowpath defined collectively by the burner 1 1 , the heat store 20 and the duct 22. In the particular example illustrated, the sensing means 31 is associated with the burner I I and is arranged to provide an electrical signal representative of the concentration of oxygen in gases flowing through the burner 1 1 , when products of 5 combustion are transferring heat to the heat store 20. The sensing means 32 is arranged adjacent to the downstream end of the heat store 20 to provide an electrical signal representative of the concentration of oxygen in the gaseous mixture leaving the heat store and flowing towards the valve 19. The sensing means 31 and 32 are commercially available oxygen-concentration sensors. 0 During that part of the cycle in which heat is transferred to the contents of the heat store 20, signals are fed from the sensing means 31 and 32 to the control means and are compared by the control means. A difference signal Is provided, representing the change in the concentration of oxygen which occurs between the burner 1 1 and the downstream end of the 5 heat store 20. This difference signal Is used to control the rate of flow of reducing agent into the burner 1 1 through the line 28 or into the heat store through the line 27. In this way, the concentration of oxides of nitrogen in the products of combustion can be reduced by a factor in excess of 80%, without the concentration of reducing agent in the gaseous mixture leaving the heat store 20 being as great as the concentration of oxides of nitrogen in that mixture. Typically, the products of combustion leaving the combustion chamber contain up to 4,000 parts per million of oxides of nitrogen and up to 2% oxygen. Ammonia is fed into the gaseous mixture at a rate such that reaction of approximately one tenth of the ammonia with the oxides of nitrogen (mainly nitric oxide) will reduce the concentration of oxides of nitrogen by more than 90%, typically leaving 400 parts per million of oxides of nitrogen in the gases which are discharged to atmosphere, almost all of the remainder of the ammonia reacting with oxygen present in the products of combustion to leave the concentration of oxygen downstream of the heat store 20 in the region of 1.5%. We have found it advantageous to introduce ammonia to the products of combustion at a position within the burner 1 1 , where the temperature is relatively high, for example, around 1000 C and preferably in excess of 800 C. Furthermore, it is found advantageous to direct the ammonia onto surfaces defined by the burner, for example surfaces of ceramic components of the burner. During the next half cycle of operation, sensing means 33 and 34 associated respectively with the burner 10 and duct 18 and corresponding to the sensing means 31 and 32 are used to control the rate of feed of reducing agent into the flowpath defined by the burner 10, heat store 16 and duct 18 in dependence on the change in the concentration of oxygen between the products of combustion leaving the combustion space and the products of combustion flowing to the valve 19. With the arrangement illustrated In the drawing, the reducing agent is introduced to the stream of gases being exhausted from the combustion chamber at a position in the burner through which the gases are exhausted. The delay between introduction of the reducing agent and cooling of the gas stream is short, owing to the proximity of the introduction position to the heat exchanger 20. In order to provide a greater opportunity for mixing of the reducing agent with the other gases and/or a longer period for reaction, prior to cooling, the reducing agent may be introduced at a position upstream of that illustrated in the drawing, for example at a position in the burner near to the combustion chamber, in the combustion chamber rather than in the burner or even in the burner through which fuel and air are supplied. The furnace illustrated in the accompanying drawing may be modified by omission of the burner 1 1 and substitution of a single heat storage means for the separate heat stores 16 and 20 shown in the drawing. The single heat storage means may comprise a rotating body of refractory elements arranged in a known manner so that the combustion air flowing along the duct 18 to the burner 10 is directed through the heat storage means along a first path whilst the hot products of combustion flow from the combustion space 12 to the duct 22 via a second path through the heat storage means. The first and second paths through the heat storage means are separate from each other and each scan the entire heat storage means as the latter rotates. This arrangement is generally called a heat wheel. In the modified arrangement, where a single heat storage means is used, there may be provided a single pair of oxygen-concentration sensors, these being disposed one upstream of and one downstream of the heat storage means In the path along which products of combustion are exhausted from the combustion space 12. The reducing agent would be admitted to this path via the duct 28 at a position between the sensors. The duct 29 shown in the drawing for admitting reducing agent to the burner 10 would be omitted from the modified furnace. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately or any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims: CLAIMS:- I . A method of treating a gaseous mixture resulting from a combustion process and containing free oxygen and oxides of nitrogen, wherein a stream of said gaseous mixture is directed along a flowpath, there are obtained first and second signals which are respectively dependant on the concentration of oxygen in the stream at first and second positions along said path, a reducing agent is fed to said streams at a controlled rate to react between said first and second positions with said oxides and with oxygen present in the mixture and wherein said first and second signals are used together in controlling the rate at which the reducing agent is fed to said stream. 2. A process wherein respective streams of fuel and air flow separately to a burner, said streams are mixed by the burner, the fuel and air burn in a combustion space at the downstream side of the burner to produce a gaseous mixture which includes free oxygen, oxides of nitrogen and products of combustion of the fuel and air, said mixture flows from the combustion space along a path which extends through heat storage means, heat is extracted from the products of combustion and is stored in the heat storage means, that heat is subsequently imparted to combustion air which Is passed through the heat storage means, there are obtained first and second signals which are respectively dependant on the concentration of oxygen in the gaseous mixture at first and second positions along said flowpath, a reducing agent is fed to said flowpath at a controlled rate to react between said first and second positions with the oxides and with oxygen present in the gaseous mixture and wherein said first and second signals are used together in controlling the rate at which the reducing agent is fed to said path. 3. A method according to Claim I or Claim 2 wherein a difference signal representing the difference between said first and second signals is provided and wherein the rate at which the reducing agent is fed is varied in accordance with the difference signal. 4. A method according to any preceding Claim wherein the reducing agent is fed into said gaseous mixture at a position between the first and second iTioP . 5. A method according to Claim 4 wherein the reducing agent is fed into the gaseous mixture at a position where the temperature of that mixture is in excess of 800 ° C. 6. A method according to Claim I wherein the gaseous mixture results from combustion carried out using a pair of regenerative burners, wherein the first position is In or is immediately adjacent to one of said burners and wherein a heat store associated with that burner defines a part of the flowpath which lies between the first and second positions. 7. Apparatus for the combustion of a fuel in air, the apparatus comprising means defining a combustion space, means defining a flowpath for conducting products of combustion from the combustion space to the atmosphere, first and second signalling means arranged to provide respectively first and second signals which are dependant on the concentration of oxygen in the contents of said flowpath at first and second sensing positions respectively, said sensing positions being spaced apart along the flowpath, means for feeding a reducing agent into said flowpath at a position between said first and second sensing positions and control means for controlling the rate at which reducing agent is fed into the flowpath, the control means being arranged to receive said first and second signals and to vary the rate of feed of reducing agent in a manner dependant on the relation between said first and second signals. 8. A furnace for supplying heat released by combustion of a fuel and comprising means defining a combustion space, a burner arranged for discharging a burning mixture of fuel and air into the combustion space, heat storage means, feed means for feeding a fuef and combustion air separately and at substantially steady rates to the burner, the combustion air being fed through the heat storage means, means defining a flowpath leading from the combustion space through the heat storage means for conducting products of combustion of the fuel and air away from the combustion space Into heat exchange relation with the heat storage means, first and second signalling means arranged to provide respectively first and second signals which are dependant upon the concentration of oxygen in the contents of said flowpath at first and second sensing positions respectively, said sensing positions being spaced apart along the flowpath, means for feeding a reducing agent into said flowpath at a position between said first and second sensing positions and I I control means for controlling the rate at which reducing agent is fed into th flowpath, the control means being arranged to receive said first and secon signals and to vary the rate of feed of reducing agent in a manner dependan upon the relation between said first and second signals. 9. Apparatus according to Claim 7 wherein said means defining th flowpath includes a regenerative burner. 10. Apparatus according to Claim 9 wherein said first position is in sai regenerative burner or between the burner and an associated heat store an wherein said associated heat store defines a part of the flowpath which lie between the first and second sensing positions. 1 1. Apparatus substantially as herein described with reference to th accompanying drawing. 12. Any novel feature or novel combination of features disclosed herein o in the accompanying drawing.

Inventors: JOHNS GARY, WILLS BRIAN, JOHNS, GARY, WILLS, BRIAN
Applicants: STORDY COMBUSTION ENG, STORDY COMBUSTION ENGINEERING LIMITED
Application year: 1987
Main classification: F23J15
IPC codes: B01D53, C03B5, F23J15, F23L15