Patent Application
20090061369
1. Field of the Invention
This invention relates to a combustion system and method for reducing or eliminating combustion driven acoustic pulsations that occur in industrial combustion systems employing premix or partial premix industrial burners. More particularly, this invention relates to premix or partial premix burners or burner systems for use in boilers, process heaters and the like which reduce or eliminate combustion driven acoustic pulsations normally generated by conventional premix or partial premix burners or burner systems.
2. Description of Related Art
Conventional premix or partial premix burners utilized, for example, in boilers, are inherently subject to combustion instability in the combustion chamber as represented by dynamic pressure oscillations in the combustion chamber. These pressure oscillations, which may occur at various fundamental or predominant resonant frequencies and other higher order harmonic, are capable of generating unacceptable amounts of acoustic noise.
For most industrial burners, fuel and oxidant, the latter typically being air, are typically available at different pressures. Fuel pressures are frequently greater than 5 pounds per square inch (psi) whereas air pressures are usually less than about 1 psi and often less than about 20 inches of water column. As a result, whenever a pressure disturbance occurs in the combustion chamber of a combustion system, the air flow to a given burner of a multi-burner combustion system or to a given nozzle of a multi-nozzle burner can be significantly affected, whereas the corresponding effect to the fuel flow may be minimal. The result may be a dramatic change in the fuel/air ratio which, in turn, will affect the heat release pattern in the combustion chamber and, in some instances, may even cause flame blowout when the fuel-rich limit is reached.
Most multi-firing nozzle premix burners and multi-burner combustion systems are designed with a single fuel/air delivery response time which provides a substantially uniform response time to a pressure disturbance in the combustion chamber for all of the burner nozzles or burners. Heat release peaks in these conventional burner systems from all firing burners or firing nozzles occur at substantially the same time inside the combustion chamber whenever there is a pressure disturbance affecting the fuel/air delivery rate or ratio. When this heat release frequency matches one of the natural frequencies of the combustion chamber, according to Rayleigh criteria, acoustic resonance will occur due to the heat release oscillation being in phase with one of the natural frequencies of the combustion system. This is referred to as combustion pulsation. Combustion pulsation, in addition to the unacceptable acoustic noise generated that is harmful to human health, is undesirable because it can dramatically affect the burner performance and cause combustion system structural damage.
It is known that such combustion pulsations may be reduced by the application of asymmetry in the heat release or by axially distributing or spreading out the heat release. One known method for producing the asymmetry to reduce the combustion pulsations is to bias fuel to one or more burners, generating more local heat release. An alternative method involves distributing the flame axially by physically offsetting one or more fuel injectors within the combustion chamber. U.S. Pat. No. 6,269,646 B1 teaches a combustion system employing pre-mixers for reducing combustion pulsations in which a portion of the pre-mixers comprise an altered flameholding capability so as to distribute the resulting combustion flames from the respective portion of the pre-mixers axially downstream with respect to the non-altered pre-mixers, thereby reducing the dynamic pressure amplitude of the combustion flames.
It is, thus, one object of this invention to provide a method and combustion system for substantially reducing combustion driven pulsations in a combustion chamber.
It is another object of this invention to provide a method and combustion system for reducing noise generated by combustion systems employing premix or partial premix burners and burner nozzles.
These and other objects of this invention are addressed by a combustion system comprising at least one wall enclosing a combustion chamber and forming a plurality of burner nozzle openings, and a premix burner nozzle, i.e. a burner nozzle suitable for premixing or partial premixing of a fuel and an oxidant, disposed within each of the burner nozzle openings, where each premix burner nozzle has a premixed fuel/oxidant outlet proximate the combustion chamber and a fuel/oxidant inlet distal from the combustion chamber, and at least a portion of the premix burner nozzles are sized to produce different fuel/oxidant delivery response times. We have discovered that if the premix burner nozzles are designed such that at least one or more burner nozzles has significantly different internal dimensions than the internal dimensions of other premix burner nozzles of the combustion system, a longer amount of time will be required for the unaffected fuel/oxidant mixtures to be exhausted from the burner nozzles having larger internal dimensions, e.g. volumes, than from the burner nozzles having smaller internal dimensions. As a result, the burner nozzles having larger internal dimensions respond to a pressure disturbance within the combustion chamber affecting the fuel/oxidant ratio slower than the burner nozzles having smaller internal dimensions, resulting in time lags between the heat release peaks arriving at the combustion chamber. That is, by dividing a single large group of burner nozzles having the same fuel/oxidant delivery response time into several nozzles having different fuel/oxidant delivery response times, the single large heat release peak or pulsation normally produced by the single large group of burner nozzles is divided into several smaller heat release peaks or pulsations with phase shifts. Thus, the heat release pulsation represented by the combination of the smaller heat release pulsations has a much smaller amplitude than the single large heat release pulsation and the smaller heat release pulsations may cancel each other out as a result of the lags in response times between each group of burners or burner nozzles, i.e. phase differences.
In accordance with the method of this invention, a burner nozzle is positioned in each of a plurality of burner nozzle openings formed by at least one wall of a combustion chamber with at least a portion of the burner nozzles having internal dimensions different from others of the burner nozzles. A fuel and oxidant are premixed by introduction into each of the burner nozzles, resulting in a fuel/oxidant mixture within the burner nozzles. The fuel/oxidant mixtures are ignited, resulting in combustion of the fuel/oxidant mixtures and producing a plurality of heat release patterns with phase shift within the combustion chamber. Phase shift refers to the relative axial locations of the heat release pulsations within the combustion chamber. Thus, while one of the burner nozzles may produce a heat release pulsation which matches one of the natural frequencies of the combustion chamber, the other nozzles will have heat release pulsations which most probably do not match the natural frequencies of the combustion chamber, thereby reducing the potential for generation of unacceptable acoustic pulsations.
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
As used herein, the term “delivery response time” refers to the period of time extending from the occurrence of a pressure disturbance within a combustion chamber to the point in time at which the altered fuel/oxidant mixture, having been altered by the pressure disturbance, is ignited in the combustion chamber. As used herein, the term “disturbance response time” is interchangeable with the delivery response time.
The disturbance response time T of a burner nozzle is calculated as
T=T
1
+T
2
+T
3
where T1 is the time for a pressure disturbance in the combustion system to travel at the speed of sound to the nozzle fuel/air inlet to affect the fuel and air flow; T2 is the time for the fuel/air mixture to travel from the nozzle inlet to its outlet, i.e.
T
2
=Vol/Q
where Vol is the interior volume of the burner nozzle and Q is the volume flow rate through the corresponding burner nozzle. T3 is the time period from the fuel/air mixture exiting from the nozzle tip to the onset of the flame. Typically, T2 is much longer than T1 and T3, and T1 and T3 are nearly constant for a certain combustion system and not easy to change. Thus, changing T2 is the easiest and most effective way to change the disturbance response time. That is, burner nozzle disturbance response time can be modified by changing Vol and/or Q.
The fundamental concept of this invention is the use of multi-response time premix or partial premix burners or burner nozzles to reduce or eliminate combustion driven acoustic pulsations. The basic idea is to provide a combustion system comprising a plurality of burners or burner nozzles having significantly different fuel/oxidant delivery response times. This may be achieved with burner nozzles having different internal dimensions, preferably by at least about 25%, e.g. different mixing chamber volumes which create different volumes of unaffected buffer mixtures, different fuel/oxidant mixture flow rates through the burner nozzles, or both. It will be understood that, as used herein, the term “oxidant” refers to air, oxygen, and oxygen-enriched air. By arranging the burner nozzles in this manner whenever there is a pressure disturbance occurring within the combustion chamber of the combustion system, the different burner nozzles having different internal dimensions will respond to the disturbance at different times, which, in turn, will create response time lags between the burner nozzles so that the final affected fuel/oxidant mixtures arrive at the combustion chamber with delay times relative to each other. Accordingly, the conventional single high heat release peak or pulse produced by conventional systems is split into multiple, non-reinforcing peaks or pulses, which may also cancel each other out if the time lags are properly timed. The reduced amplitude of the heat release pulsations will input less energy in phase with the acoustic oscillation so that sustained combustion pulsation may be eliminated.
As previously stated, conventional premix and partial premix industrial and commercial burner systems having a plurality of burners or burner nozzles are designed so as to produce a single fuel/air delivery response time, which design provides a substantially uniform response time to a disturbance, such as a pressure disturbance, within the combustion system.
In contrast thereto,
The method of this invention comprises positioning a burner nozzle in each of a plurality of burner nozzle openings formed by at least one wall of a combustion chamber where at least a portion of the burner nozzles have different internal dimensions from others of the burner nozzles. A fuel and oxidant are introduced into each of the burner nozzles resulting in a fuel/oxidant mixture within each of the burner nozzles, which mixture is ignited, resulting in combustion of the fuel/oxidant mixtures and production of a plurality of heat release patterns within the combustion chamber. In accordance with one embodiment of this invention, differences in fuel/oxidant delivery response times among the burner nozzles may further be enhanced by modifying the fuel/oxidant ratios of the mixtures.
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.
