Claims
- 1. A method for limiting the amount of nitrogen oxides in exhaust gas from a regenerator by causing water vapor to enter a combustion chamber of the regenerator during a firing phase in a form in which the water vapor is entrained in preheated combustion air, the method comprising injecting water or water vapor into the combustion air at a point upstream of the preheated storage bed with respect to the direction of movement of the combustion air so that the water or water vapor reaches the combustion chamber by way of the heat storage bed.
- 2. The method as claimed in claim 1, wherein the heat storage bed is arranged so that a surface farthest from the combustion chamber is uppermost.
- 3. The method as claimed in claim 1, whereby the temperature of the combustion air is sensed before the air enters the combustion chamber and the air leaves the heat storage bed, and when the temperature of combustion air is below a preset level the injection of water or water vapor into the regenerator is ceased.
Priority Claims (2)
| Number |
Date |
Country |
Kind |
| 8531277 |
Dec 1985 |
GBX |
|
| 8604168 |
Feb 1986 |
GBX |
|
Parent Case Info
This application is a continuation of application Ser. No. 227,517, filed on Aug. 2, 1988, now abandoned, which is a continuation of application Ser. No. 096,284, filed on Sept. 1987, now Pat. No. 4,768,949, issued Sept. 6, 1988.
The present invention relates to the limiting of the presence of oxides of nitrogen in the exhaust gas emitted by a regenerative heating system and is also concerned with a regenerator adapted for this purpose and also adapted for the removal from the heat storage bed of the regenerator susceptible material as hereinafter defined tending to block the pores of the bed.
Regenerative heating systems usually comprise two or more regenerators which are connected to a furnace containing a charge to be heated by the combustion products supplied through the wall of the furnace by the regenerators. The system is designed so that the combustion products supplied by one regenerator during its so called firing phase provide waste heat in the form of waste gas leaving the furnace, the waste heat being recovered by another regenerator operating in a heat collecting or recovery phase.
Each regenerator usually comprises a shaft housing a heat storage bed disposed between two openings.
One opening communicates directly with the furnace to serve during a firing phase of the regenerator as an outlet to discharge combustion products to the furnace and during a heat collecting phase of the regenerator as an inlet to receive waste gas from the furnace. The other opening serves during a firing phase of the regenerator as an inlet to receive combustion air to provide combustion of the fuel in a combustion chamber and during a heat collecting phase of the regenerator as an outlet to discharge the waste gas as exhaust gas to the atmosphere.
The heat storage bed recovers heat from the waste gas during a heat collecting phase before allowing it to be discharged as exhaust gas from the system. The bed then releases its stored heat during a firing phase to preheat the combustion air as it passes through the bed before it reaches the combustion chamber. The bed itself is permeable to fluids such as waste gas and air of course and comprises a porous structure frequently of discrete particles of a heat collecting refractory material.
The combustion chamber, which lies between the heat storage bed and the opening communicating directly with the furnace serves to provide combustion of fuel with the preheated combustion air during a firing phase whereby the combustion products are produced. The fuel is injected into the chamber during the firing phase, means being provided to ignite the fuel so that combustion can take place.
The combustion chamber may form part of a burner incorporated within the regenerator forming part of a regenerative heating system. Typical such regenerative heating systems are fully described in U.S. Pat. No. 4522588 corresponding to published GB Patent Application No. 2128724A and in our copending UK Patent Application No. 8527894.
The preheating of combustion air in the manner described above has now become a well established technique for improving the efficiency of high temperature furnaces by recovering heat from the waste gas.
One effect of preheating combustion air is generally to raise the temperature of the flame ensuing from the fuel combustion process. This effects the combustion reaction and tends to increase the formation of the oxides of nitrogen mainly NO and NO.sub.2. In fact where the fuel is natural gas, the production of these oxides increases markedly with flame temperature and is overwhelmingly caused by the thermal fixation of atmospheric nitrogen present in the combustion air. The quantity of oxides produced is also determined by the time (residence time) for which the flame is held at these high (peak) flame temperatures.
The production of the oxides of nitrogen (known as NO.sub.x for short) as a consequence of the combustion of fuel with air is undesirable in its effect upon the environment as the NO.sub.x in the exhaust gas leaving the regenerative heating system is now known to produce air pollution contributing to acidification of rain, the production of photochemical oxidants in the interaction of the NO.sub.x with the hydrocarbons in the atmosphere and the degradation of visibility where high levels of NO.sub.2 are allowed to be emitted.
Legislation already exists in some countries e.g. Japan, to limit the presence of NO.sub.x in emitted exhaust gas and it is quite likely that similar legislation will be introduced in Europe and possibly North America and it will be essential to limit NO.sub.x emission if the more efficient heat recovery systems of the regenerative type are to be widely adopted.
A known method of reducing the level of NO.sub.x produced by a burner is the provision of water vapour at the point of combustion. The water may be provided in the form of liquid water injected into the flame, or water vapour in the form of steam injected into the flame. Where the water is liquid it forms steam on contact with the flame.
The actual presence of water vapour in the combustion chamber increases the volume and thermal mass of the gases in the combustion zone. This results in a reduction in the level of NO.sub.x in the combustion gases. However, the injection of water or water vapour at the point of combustion causes heat to be extracted from the flame and unfortunately it is not possible to recover this heat from the subsequent waste gas. Consequently the overall efficiency of the process is reduced.
It is an object of the present invention to provide a technique whereby the presence of the oxides of nitrogen in the exhaust gas leaving a regenerator are limited with an accompanying smaller reduction in overall process efficiency than is obtained with the methods described above.
It is also an object of the present invention to provide a regenerator adapted for this purpose and as a consequence also adapted for the removal from the heat storage bed of the regenerator of susceptible material tending to block the pores of the bed.
According to one aspect of the present invention we provide a method for limiting the presence of the oxides of nitrogen in the exhaust gas leaving a regenerator, the method comprising injecting water or water vapour into the regenerator during its firing phase and causing the water or water vapour to reach the combustion chamber of the regenerator by way of its heat storage bed which has been preheated during a previous heat collecting phase.
According to another aspect of the present invention we provide a regenerator adapted to limit the presence of the oxides of nitrogen in the exhaust gas leaving the regenerator, the regenerator comprising a shaft housing a heat storage bed, and means for injecting water or water vapour into the regenerator during firing in such a manner that the water or water vapour enters the combustion chamber of the regenerator by way of the heat storage bed which, in use, has been preheated during a previous heat collecting phase.
At the normal temperatures to which the heat storage bed of a regenerator is preheated, e.g. 1200.degree. C., water (if injected rather than water vapour) will vapourise on passing through the bed and the resulting vapour will also be preheated before it enters the combustion chamber. In the case where vapour e.g. steam, is injected rather than water the vapour will simply be preheated on passing through the bed and will then enter the combustion chamber.
Since by either method water vapour is present in the combustion chamber of the regenerator during combustion, the volume and thermal mass of the combustion gases will be increased and this will result in the level of NO.sub.x in the gases being limited. However, since the water vapour has already been preheated by the heat storage bed and a large proportion of this heat will be collected as the water vapour leaves the system by way of another regenerator bed, little additional heat input is necessary to raise the temperature of this water vapour to process temperature. Consequently the overall efficiency of the process is not unduly reduced.
While the heat storage bed is cooled as the water especially or the water vapour passes through it we have surprisingly found that this has only a marginal effect in reducing the overall process efficiency.
We recommend that water rather than steam be injected if the steam has to be generated in a special plant since we have found that it is more efficient to generate the steam within the regenerator as a result of heat released from the bed rather than as a separate operation in say a steam-raising boiler. However, where the steam is available free, so to speak, as a waste product of a parallel process we would recommend injecting this steam rather than water in order to save the heat released from the bed in vapourising the water.
Preferably the water or water vapour is injected into the combustion air at a point upstream of the heat storage bed with respect to the direction of movement of the combustion air and the heat storage bed is arranged so that its surface farthest from the combustion chamber is uppermost. This will be the coldest part of the bed at all times and will thus ensure that heat removal from the bed will not reduce the peak air preheat temperature below the level which is a natural consequence of heat removal in the colder zones of the bed.
In a preferred embodiment of the invention during a firing phase of the regenerator the temperature of the preheated combustion air is sensed by temperature sensing means before the preheated air reaches the combustion chamber of the regenerator and if the temperature of the combustion air is below a preset level the injection of water or water vapour into the regenerator is ceased.
The value of this threshold temperature will be determined by the level of NO.sub.x in the exhaust gas permitted by the country in which the regenerative system is operating it being appreciated that the higher the temperature of the combustion air, the higher the flame temperature and therefore the higher the levels of NO.sub.x in the exhaust gas. As will also be appreciated, from the user's point of view it is generally desirable to operate with a flame temperature as high as possible to increase the heating efficiency of the process.
By monitoring the temperature of the preheated combustion air it is possible to maintain the flame temperature at no lower a level than is necessary to satisfy the legislation on permitted NO.sub.x levels.
According to a futher aspect of the present invention we provide a regenerator comprising a shaft housing a heat storage bed which, in use, is generally upwardly directed, means for injecting water or water vapour onto that surface of the heat storage bed which is uppermost when the shaft is in its generally upwardly directed position, an inlet for supplying water or water vapour to the injecting means, and an outlet for discharging from the regenerator water or water vapour which has percolated through the bed, the outlet being disposed beneath that surface of the bed which is lowermost when the shaft is in its generally upwardly directed position.
This regenerator, apart from enabling the level of NO.sub.x in the exhaust gas to be reduced, is also adapted for the removal from the heat storage bed of the regenerator "susceptible" material tending to block the pores of the bed. By "susceptible" material we mean material which is either water soluble or if not soluble is capable of being carried along by water, eg small or at any rate light particulate material. Such material is carried over by the waste gas from the furnace and deposits in the pores in the regenerator bed. Such material may be fluxes, and dust from the charge being heated, eg metal or glass dust. In use, water or vapour sprayed onto the uppermost surface of the bed when the regenerator is not in operation would percolate down through the bed dissolving any water soluble matter or washing away any light particulate matter, this matter and the transporting water being discharged through the outlet. This avoids the necessity to remove the bed to clean the particles and in fact promotes the useful life of the bed.
Preferably the inlet has a valve controlling the supply of water or vapour from the inlet to the injecting means and means are provided to control the valve such that the valve is open during a firing phase of the regenerator if the temperature of the combustion air sensed after preheat is above a preset level.
Suitably the means for controlling the inlet valve is also adapted to open the valve when the regenerator is neither firing nor collecting heat.
Conveniently the outlet has a valve controlling the discharge of water or vapour from the outlet and means are provided to control the valve such that the valve is closed during a firing or heat collecting phase and can be opened if the regenerator is neither firing nor collecting heat.
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| Number |
Date |
Country |
| 1054556 |
Feb 1954 |
FRX |
| 51-148569 |
Nov 1976 |
JPX |
| 51-47131 |
Dec 1976 |
JPX |
| 52-14230 |
Feb 1977 |
JPX |
| 53-105727 |
Sep 1978 |
JPX |
| 244906 |
Dec 1925 |
GBX |
| 717782 |
Nov 1954 |
GBX |
| 895463 |
May 1962 |
GBX |
Continuations (2)
|
Number |
Date |
Country |
| Parent |
227517 |
Aug 1988 |
|
| Parent |
96284 |
Sep 1987 |
|
Patent Grant
4957430
