1. Field of the Invention
This invention lies in the field of liquid and gaseous fuel burning. More particularly, this invention concerns fuel burning apparatus in which the design of the burner and control of the fuel and air supply is such as to maintain a minimum value of NOx in the effluent gases.
2. Description of the Prior Art
The burning of fuels, however it is accomplished in burners, as they are known in the art of fuel burning, is productive of oxides of nitrogen (NOx) in normal operations. Such oxides of nitrogen as are produced in combination with olefinic hydrocarbons which may be present in the atmosphere constitute a source of smog.
Smog, while not necessarily lethal, is recognized universally as potentially damaging to animal tissue. Consequently, severe limitations on the NOx content of stack gases vented to the atmosphere as a result of fuels burning, have been imposed by various governmental authorities and agencies. Emission of olefinic hydrocarbons is also subject to limitations, but is a matter separate from the invention of this application.
The prior art is best represented by U.S. Pat. No. 4,004,875. This patent has been the basis of a wide application of low NOx burners in the natural gas field. Scores of burners which are based on this patent are in commercial service, where they have suppressed NOx as intended. However, the optimum operation of this prior patent has been for fixed rates of burning, where a good balance can be provided between the primary and secondary air supplies to a first combustion chamber and the supply of additional tertiary air downstream of the first combustion chamber.
The weakness of the prior design is that, for one condition of furnace draft or firing rate, the operation is ideal. However, when the firing rate changes significantly, such as from 100% to 80%, as is typical of daily process heater firing, there is difficulty in maintaining NOx suppression. The reason for this is that at reduced firing rate the furnace draft remains constant or approximately so, and increased air-to-fuel ratios destory the less-than-stoichiometric burning zone prior to tertiary air delivery/entry, which results in less than optimum NOx reduction plus higher than desirable excess air.
What is required is a burner which provides means for correction for any condition of firing, such as might be required when the furnace draft remains substantially constant as changes in firing rate are made. If such corrections can be made, the result is continuation of NOx suppression and maintenance of optimum excess air for high thermal efficiency. In the prior art burner there is no control of the tertiary air, which is caused to flow by furnace draft (less than atmospheric pressure within the furnace), while the primary and secondary air also flow for the same reason. The total air flow will vary as the square root of the furnace draft. Thus, only one rate of fuel burning or firing rate, at a condition of furnace draft will provide required excess air and NOx suppression. This would seem to indicate that control of air flow would provide some benefit.
What is not immediately evident is, that the air entry control must be proportionately controlled for maintenance of a less-than-stoichiometric burning zone prior to entry of tertiary air to the less-than-stoichiometric gases, for completion of fuel burning plus preferred excess air when firing rate is caused to vary. If the conditions as outlined are maintained, there is suitable NOx suppression in any condition of draft and firing rate, and furnace excess air remains best for high thermal efficiency. This is to say that control must be proportional and simultaneous for primary, secondary and tertiary air for best and most assured operation in all firing conditions.