Abstract:
An oscillating firing system for targeted adjustment/readjustment of amplitudes of oscillations of static pressure and/or hot gas velocity in an oscillating firing system for thermal treatment of materials or materials synthesis includes at least one burner, with which an oscillating (pulsating) flame is generated, and at least one combustion chamber (resonator), into which the flame is directed. Upstream of the burner outlet an oscillation volume is inserted into the supply lines leading to the burner through which oscillation volume air, fuel or fuel-air mixture can flow. Preferably, the size of the oscillation volume may be adjustable. Thus, it is possible to change the amplitude of oscillation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority under 35 USC 119 to German Patent Application Serial No. 10 2015 005 224.1 filed Apr. 23, 2015, the entire disclosure of which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and apparatus for setting the oscillation amplitudes of oscillating firing systems for treatment or synthesis of materials. More specifically, the invention relates to a process for selective adjustment of oscillation amplitude (oscillation level) of an oscillating combustion process in an oscillating firing system (e.g., a pulse dryer, pulse combustor, pulsation reactor) in a plant for the thermal treatment or synthesis of materials, having at least one burner, which provides a burner outlet to which a fuel and/or air is fed via a pipe and in which a pulsating flame is present forming at the burner outlet, which burns in a combustion chamber, and having at least one gas column capable of resonating (e.g., in the combustion chamber or in a resonance tube), into which material to be treated is fed, and a corresponding device for implementing the method and a corresponding method of application. 
       BACKGROUND INFORMATION 
       [0003]    By far the maximum number of all technical or industrial furnaces and combustion systems are designed and operated so that the combustion process except for slight turbulent fluctuations, whose size is at least one size smaller than the average size of the combustion process (such as mean flow velocity, mean temperature of flame or the exhaust gas flow, average static pressure in the combustion chamber, etc.) runs on an in average time-constant manner. 
         [0004]    This means that the conversion of fuel used is carried out continuously over time and, as a consequence, the release of heat from the combustion process as well as the mass flow of resulting exhaust gas (combustion products) for a fixed burner setting have time constant values. 
         [0005]    Deviating from this occasionally occur phenomena or “abnormalities” which are referred to in the literature as “combustion chamber oscillations,” “self-excited combustion instabilities” or “thermo-acoustic oscillations.” These phenomena are effective in that the at first steady-state (i.e. time-constant) combustion process when reaching a stability limit suddenly turns into a periodic, oscillating combustion process, whose time function may be referred to a good approximation as sinusoidal. 
         [0006]    Along with this change, also the heat release rate(s) of flame(s), and thus the thermal firing capacity of the firing system as well as the exhaust gas flow in and out of the combustion chamber and the static pressure in the combustion chamber itself are periodically-transient, i.e., oscillating. 
         [0007]    As against steady-state operation of the furnace, the occurrence of these combustion instabilities often brings about a modified pollutant emission behaviour and causes, in addition to increased noise pollution of the plant environment, a significantly increased mechanical and/or thermal load of the plant structure (e.g., combustion liners, combustion chamber liner, etc.), which can lead to destruction of the furnace or of individual components. 
         [0008]    It is, therefore, easy to see that the undesirable occurrence of the phenomena as described are preferably avoided in firing systems, which are designed for a time-constant combustion process, in which even the static pressure in combustion chambers or in plant components located upstream or downstream should have constant values, the so-called equal-pressure combustion. 
         [0009]    However, quite different is the situation in a small number of very special combustion-technical systems, in which the above illustrated phenomenon of self-excited, periodic combustion instability is caused deliberately. These systems are to be used in treatment or synthesis of materials for generating a periodic combustion process with periodic heat release of the flame and thus periodically oscillating or pulsating exhaust hot gas flow in the combustion chamber and in upstream components such as heat exchangers, chemical reactors, etc. 
         [0010]    One can classify these special plants into oscillating firing systems, in which heat is transferred from the oscillating combustion process to generate heat or hot water or steam generation (purely thermal equipment use), and also in oscillating reactors, as they stand in the foreground here, in which physical and/or chemical treatment of the material is foreseen such as drying, calcination, a thermally controlled synthesis, etc. Such reactors are often called “pulse dryer”, “pulse combustor” or referred to as a “pulsation reactor.” 
         [0011]    The advantage of these special systems over conventional systems with stationary operating combustion systems is averaged over time, periodic-unsteady and turbulent exhaust gas flow in the combustion chamber and/or in downstream components, such as heat exchangers, reaction chambers or resonance tubes, etc. 
         [0012]    Both against fixed walls, such as combustion chamber walls, the wall of a heat exchanger or steam generator, etc., and in particular, also with respect to material which is introduced for treatment in such an oscillating flow of hot gas with a defined treatment temperature, the heat transfer from hot gas rises to the walls or to the material significantly by 2 to 5 times as compared with a steady-state turbulent flow having the same mean flow velocity and the same temperature. 
         [0013]    Therefore, the materials to be handled, as per the invention, in pulsating streams of hot gas flows, experience high heating gradients, which are also referred to as “thermal shock treatment”. 
         [0014]    Analogously, a mass transfer of materials to be treated is improved in such special equipment: In the case of periodic-unsteady, oscillating flow the transition rate of gas and or vaporous substances from the hot gas into the material to be treated or from the material to be treated into the hot gas flow increases by similar values as mentioned above. 
         [0015]    For this, the almost complete absence of boundary layers is considered to be the cause, which boundary layers resulting in the known manner from steady flows and represent diffusion or transfer resistances. 
         [0016]    On the other hand, the advantages of unsteady process as explained have to be seen vis-à-vis numerous disadvantages. 
         [0017]    Firstly, the pressure or velocity amplitudes occurring during combustion must be restricted upwards to avoid certain mechanical and/or thermal overload of the plant structure. 
         [0018]    Secondly, even flashbacks or rising of the flame(s) from the burner outlet or a flameout must be reliably avoided, in order to ensure a stable-wielding permanent operation. 
         [0019]    Further, increased noise emission from the combustion process to the plant environment poses a significant problem for occupational safety and reasonable working conditions. 
         [0020]    As regards the above-mentioned basic requirements to guarantee safe operation of the oscillating firing systems or pulsation reactors, it is equally important that the two main parameters of combustion oscillations, namely, the oscillation frequency and the oscillation amplitude, are kept constant within the possibilities of an industrial process. If they change significantly during a process then it can be reckoned with that the benefits of heat transfer or in the materials treatment vis-à-vis conventional, steady-state processes and/or the homogeneity of the resulting product are lost. 
         [0021]    With respect to the oscillation frequency, which occurs during formation of self-excited combustion instabilities due to in-phase feedback in the sphere of burner-flame-combustion chamber-resonator, there is consensus in the literature that this essentially depends on the geometry of acoustically-active or resonance-capable volumes such as the combustion chamber and/or the resonance tube, as well as on the gas temperature. 
         [0022]    If these two main variables, which mainly determine the oscillation frequency, namely the aforementioned geometry and the gas temperature, during a process, for example, a heat transfer or a treatment or synthesis of material, are not changed, then the frequency of the combustion/pressure/flow oscillations remains constant in good approximation. 
         [0023]    But in addition to the oscillation frequency, the amplitude of oscillation is of importance. 
         [0024]    It must be realized that a decrease in the amplitude of oscillation, characterized as amplitude of oscillation of static pressure in the combustion chamber or the oscillation velocity of hot gas flow in the combustion chamber or in the resonance tube, leads to a decrease of convective heat and/or mass transfer between hot gas and walls or between hot gas and material to be treated. Thus, the benefits of a non-steady-state process can disappear, e.g., with regard to the desired specific material properties of the treated material. 
         [0025]    In contrast to the oscillation frequency, which remains constant while maintaining the geometry and gas temperature, the oscillation amplitude of the periodically-transient combustion process can change significantly in an oscillating firing system or a pulsating reactor during a process. 
         [0026]    This is evident from the following considerations: If in a pulsating reactor with a fixed predetermined frequency and amplitude of the combustion oscillation, raw material is added for thermal treatment in different mass flows (e.g., 50 kg/h or 100 kg/h), then these different mass flows of raw material would bring about a differently strong damping of oscillation amplitude. The reason is that the (oscillational) energy of the original oscillation of only the hot gas after the addition of the raw material must divide between different levels of particle or drop loads of the hot gas. 
         [0027]    In the limit case of a sufficiently large addition of material to be treated per unit time, it is even possible to bring the oscillation in the reactor to a standstill, whereby the advantages of the periodic-unsteady process, e.g. for generating specific material characteristics (for example, particle sizes, specific surface, reactivity, etc.) disappear in the product. 
         [0028]    This can be explained by the fact that any change either in the mass flow of the added raw material or the specific material characteristics of different reactants (such as raw material density, moisture content, particle size distribution, solids contents in suspensions, etc.) changes the oscillation amplitude of the oscillating stream of hot gas and thus the result of materials treatment. 
         [0029]    In the relevant patent literature pertaining to the problems, e.g. setting the oscillation amplitudes in oscillating firing systems or pulsation reactors and readjusting during a treatment process or matching different reactor throughputs, so as to ensure consistent product quality; there are almost no statements: either no values are indicated in oscillation amplitudes to be observed in the reactor, or it is explained that the amplitudes result “automatically.” 
         [0030]    It is assumed that in the case where there is no possibility of independent adjustment of oscillation amplitude, but the oscillation amplitude “automatically” adjusts with given reactor geometry, hot gas temperature and burner settings, physically there is a link between oscillation frequency and oscillation amplitude. It should be considered that the higher the (set) oscillation frequency at which a pulsed reactor operates, the higher is the oscillation damping of the system at this frequency and the lower are the “automatically” resulting oscillation amplitudes which are established as a result of increased oscillation damping. 
         [0031]    Thus, if the oscillation frequency is increased to achieve a desired increase in heat transfer in an oscillating firing system or at a targeted adjustment of particle/material properties during thermal treatment of the material in a pulsed reactor, then the resulting benefit to the process and/or the product is reckoned partially nullified again, and that simultaneously with the increase of oscillation frequency, the oscillation amplitude “automatically” decreases because of increased oscillation damping. 
         [0032]    As indicated in U.S. Pat. No. 5,015,171, the frequency and amplitude of pulsation is said to be controlled by “different combinations of flame holder in various positions within the hot gas generator”. But a concrete procedure for the selection and positioning of corresponding flame holder or a physical, traceable mechanism of action that could be the base here, has not been specified. 
         [0033]    In EP 2,092,976 other statements are made with regard to the adjustment/regulation of oscillation amplitude by changing the burner settings. It is proposed to change the absolute fuel or combustion-air mass flows, the fuel/air ratio or the air ratio. 
         [0034]    In fact, these measures are not appropriate as the change in these parameters and other important process parameters are changed, which significantly influence the material properties of the products treated. In case of other process parameters it relates to materials treating temperature or the residence time of the material to be treated in the hot gas flow etc. 
         [0035]    According to the teaching of this document, it is maybe possible to produce comparable amplitudes of materials treatment in the pulsation reactor at two different raw material feed rates, yet the material properties achieved in each case would not, however, be comparable. 
       SUMMARY 
       [0036]    An object of an embodiment is to further develop the controllability of the oscillation amplitude with an oscillating firing system for treatment or synthesis of materials, as described above, having at least a burner, to which fuel and/or air is fed to a burner outlet via at least one pipe, and in which a pulsating flame is present at the burner outlet, which burns in a combustion chamber, and that also provides a resonance-capable gas column. It is important that the oscillation frequency should remain essentially unchanged. 
         [0037]    The afore-mentioned object can be achieved, in that the at least one pipe that carries the fuel and/or air to the burner outlet, is to be fluidically connected upstream of the burner outlet with at least one oscillation volume. 
         [0038]    The embodiment is based on the finding that the amplitude of an oscillating combustion process is mainly determined by the pulsation of the flame. This can be particularly affected by a pulsation of the flame feeding currents. It is assumed that in a normal burner operation, the supply of air and/or fuel takes place at a constant admission pressure and thus the pressure at the burner outlet is constant in a first approximation. 
         [0039]    As the fuel and/or air feeding pipe is now to be connected upstream of the burner outlet with an oscillation volume, there is the possibility that pulsating pressure acting at the burner outlet in the combustion chamber can impact the amount of inflowing gas. The gas flowing with constant inlet pressure, such as air, fuel or fuel-air mixture can flow into and out of the oscillation volume periodically, which results in a periodically varying flow at the burner outlet. 
         [0040]    Since the amount of air, fuel or fuel-air mixture averaged over time does not change, the parameters essentially influencing the oscillation frequency remain unchanged and only the amplitude of the oscillation changes. 
         [0041]    By the magnitude of the selected oscillation volume, the arising periodic change and thus the size of the amplitude can be influenced. 
         [0042]    It has been found that the amplitude is modified such that, with increasingly larger oscillation volume that is positioned on the cold side of the burner, the amplitude of oscillation of the static pressure rises in the combustion chamber and also in the system components carrying hot gas downstream without having to modify other parameters of the furnace or burner adjustment. 
         [0043]    Thus, with the embodiment described herein only the strength of the pressure of oscillation is changed that is characterized by the amplitude of oscillation of the static pressure in the combustion chamber. 
         [0044]    The amplitude can thus be altered purposefully by choosing the right size of the oscillation volume being arranged on the cold side of the burner, e.g. so as to adapt this amplitude to a changed feed rate of a raw material, without changing the frequency. 
         [0045]    In a particularly preferred embodiment of the invention, the oscillation volume, which is connected to the at least one pipe for fuel and/or air, can, therefore, be varied in its magnitude. The oscillation volume can thus be adjusted in its volume as a resonator. 
         [0046]    This offers the possibility to influence the flow that periodically attunes on the burner outlet accordingly and thus to adjust the oscillation amplitude as per requirement. 
         [0047]    It is indeed possible, for this purpose, to provide several oscillation volumes, possibly even of different sizes and even combinable. In a particularly preferred embodiment, instead, a cylinder-piston unit adjustable in size of its volume is proposed as oscillation volume for different oscillation amplitudes. The size of the volume is to be changed by an adjustment of the piston within the cylinder-piston unit. 
         [0048]    With a corresponding design, there is the possibility to adjust the amplitude of oscillation frequency that otherwise would set plant-specifically, without changing the oscillation frequency. 
         [0049]    The method according to this embodiment is thus characterized in that during operation of an oscillating firing system, as described above, an oscillation volume is first connected fluidically with at least one pipe. In a further development of the method, the oscillation volume is then changed in its size. 
         [0050]    As described, the change in the magnitude of volume can be carried out, for example, continuously by displacement of the piston of a cylinder-piston unit, the piston is sealed towards the container wall and the surrounding area. 
         [0051]    Alternatively, it would be possible to adjust its size steplessly with an oscillation volume by partially filling with a non-compressible material (for example, water, sand, etc.). 
         [0052]    A corresponding oscillation volume may be of any shape (cylindrical, box, sphere, etc.). These should be designed respectively in a pressure-resistant manner. Preferably each can have a defined setting of hermetically separated partial volumes. 
         [0053]    The function of the oscillation volume can also be met with a structural shape integrated in the burner housing. It has, for example, a burner housing with an oscillation volume arranged on the cold side of the burner, particularly with infinitely variable size. 
         [0054]    It should be noted that the oscillation volume can flow through both by air as well as a fully-premixed combustion control (premixed combustion) of a fuel gas/air mixture. 
         [0055]    Like this always the goal is reached, to be able to adjust the oscillation frequency and the oscillation amplitude of the pulsating stream of hot gas flow independent of one another and without mutual influence. 
         [0056]    Such a method can be used, in particular to produce fine particles having an average particle size in the range of 5 nm to 3 mm. 
         [0057]    Further advantages and features of the invention become apparent from the following description of embodiments. 
         [0058]    In accordance with one embodiment, an oscillating firing system for treatment or synthesis of materials includes at least one burner having a burner outlet to which fuel and/or air is fed via at least one pipe, and in which a pulsating flame is disposed at the burner outlet, which burns in a combustion chamber, and a selectively resonating gas column, wherein the at least one pipe is fluidly coupled to at least one oscillation volume. 
         [0059]    In accordance with another embodiment, a method for operating an oscillating firing system for treatment or synthesis of materials having at least one oscillation volume defining a volume having a size, at least one burner having a burner outlet to which fuel and/or air is fed via at least one pipe, and in which a pulsating flame is disposed at the burner outlet, which burns in a combustion chamber, and a selectively resonating gas column, includes the step of fluidly coupling the oscillation volume to the at least one pipe. 
         [0060]    In accordance with a further embodiment, a method for operating an oscillating firing system for treatment or synthesis of materials having at least one oscillation volume defining a volume having a size, at least one burner having a burner outlet to which fuel and/or air is fed via at least one pipe, and in which a pulsating flame is disposed at the burner outlet, which burns in a combustion chamber, and a selectively resonating gas column, the oscillation volume being fluidly coupled to the at least one pipe, includes the step of employing the oscillating firing system to produce particles having an average particle size in the range from 5 nm to 3 mm. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0061]      FIG. 1  is a schematic diagram of a device according to the present invention; and 
           [0062]      FIG. 2  is a schematic diagram of an alternative embodiment of the device according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0063]      FIG. 1  shows a schematic diagram of a device according to an embodiment. 
         [0064]    A combustion chamber  1  can be seen here, into which a flame  2  is directed. 
         [0065]    The flame  2  forms at a burner outlet  3  of a burner  4 . The flow-technical or fluidical sections of the device lying upstream of the burner outlet of the device are considered a cold side of the burner. 
         [0066]    The flame  2  pulsates and causes a pulsation of an existing plant-specific resonating gas column being in the combustion chamber and/or in a reactor chamber downstream of this reactor chamber. Into this combustion chamber or the reactor chamber, a reactant is fed, that then is treated in the oscillating gas column of hot gas, and finally the product thus produced is removed from the hot gas stream, for example, by a hot gas filter, a cyclone or the like. 
         [0067]    The pulsation of the flame  2  is a self-excited combustion instability, wherein the frequency of their self-excited oscillation is determined in large part by the geometrical dimensions of the combustion chamber, the reaction chamber and the appropriate plant engineering attachments and installations, which can be found first of all on the hot side of the burner, which are located downstream of the burner outlet  3 . 
         [0068]    In the burner housing  5  of the burner  4  there is a swirl generator  6 , with which airflows  8  flowing through pipes  7  to the swirl generator receive a swirl, to then mingle at the burner outlet  3  with fuel gas  9  which is guided through a fuel gas line  10  to the burner outlet  3 . 
         [0069]    Upstream of the pipes  7  flows gas  8  passing through these pipes  7  through an oscillation volume  11 , to which gas  14  is supplied through a supply line  12 . 
         [0070]    Gas  8  flowing through the oscillation volume  11  is guided, in this example, through a laminar flow rectifier  13 , so that all pipes  7  in burner  4  are essentially exposed to flow in equal measures. 
         [0071]    Gas  14  flowing in the supply line  12  is either pure air  15 , which received in a compressor or compactor or a blower  16  an admission pressure. 
         [0072]    Alternatively gas  14 , is a fuel-air mixture when fuel gas  18  is supplied to pure air  15  through a fuel line  17 . In this case, the fuel gas line  10  can be omitted in the burner  4 , through which pure combustible gas  9  is fed to the burner outlet  3 . 
         [0073]    It is essential in the device shown here that the oscillation volume  11  has a variable size, in that a piston  19  is displaced in a cylinder  20  and, if necessary, can be locked. 
         [0074]    The piston  19  can be moved according to the double arrow  21  between a minimum and a maximum volume position, wherein volume, which is present in the minimum case between the piston  19  and the lamella flow rectifier  13 , lies at about 5 to 10 litres, while the maximum attainable volume lies between 200 and 2000 litre. 
         [0075]    For the sake of completeness it should be noted that the oscillation volume is adjusted one-off in its size for achieving a certain amplitude of pressure oscillation and is then fixed at a suitable size. 
         [0076]    The above-mentioned self-excited oscillation is thus not impacted by an externally induced back-and-forth movement of the piston  19  according to the double arrow  21 , as it concerns a combustion instability that results from an in-phase feedback in the sphere of activity burner-flame-combustion chamber-reaction chamber/resonator. 
         [0077]    The described fixed size of oscillation volume  11  influences the compression strength of the burner supply via the compressibility of the gas present in the oscillation volume  11 . 
         [0078]    Between piston  19  and the wall of the cylinder  20  a conventional seal can be present, which ensures gas-tightness, like for example, an O-ring seal, or a special piston seal. 
         [0079]    The gas  14  flowing through the supply line  12  with essentially constant inlet pressure from compressor  16  first enters the oscillation volume  11 . In this oscillation volume, there is a pulsating pressure, since the pressure in this oscillation volume is influenced by the pressure in the combustion chamber  1 , which also pulsates due to the pulsed combustion of the flame  2 . 
         [0080]    Due to pulsating pressure, gas  14  flows indeed with approximately constant volume flow into the oscillation volume  11  but with a pulsating flow from it outwards. Then, it flows as air  8  (optionally mixed with fuel gas  18  from the fuel line  17  as a fuel-air mixture) and passes via the pipes  7  through the swirl generator  6  at the burner outlet opening  3  into flame  2 . 
         [0081]    The pulsating flow of gas (i.e., air  8  or here air-fuel mixture) intensifies the pulsation of flame  2 , but changes only the amplitude of oscillation generated by this pulsation of the flame but not its frequency. 
         [0082]    Via the adjustment of magnitude of oscillation volume  11  the mass in the oscillation volume  11  can influence the continuously inflowing but pulsating output gas  14  or  7  and hence the amplitude of oscillation of a gas column (which is fed by hot gas from combustion) present in the combustion chamber  1  or in a downstream (not shown) resonance tube as the reaction chamber. 
         [0083]    The amplitude of the oscillation of static pressure in the combustion chamber  1  and the system components leading hot gas downstream so changes that it increases with magnifying oscillation volume  11  and decreases with a reduction of the oscillation volume  11 , without having to change other parameters of the furnace or burner adjustment. 
         [0084]    In principle, it is therefore assumed that with a device as described, the compressive strength or stiffness of the burner supply with air and/or fuel gas can be adjusted—preferably continuously—and thus, adapted to the oscillation characteristics of hot gas carrying plant components, which include a combustion chamber, resonance tube and reactor. 
         [0085]      FIG. 2  shows an alternative embodiment. The same parts are provided with the same reference numerals. 
         [0086]    A difference between  FIG. 1  and  FIG. 2  is that in the embodiment of  FIG. 2 , the oscillation volume  11  is connected through a connecting pipe  22 , in fluid dynamic or fluidical way, with the pipe that leads fuel and/or air to the burner outlet  3 . 
         [0087]    While in the embodiment of  FIG. 1 , as explained above, the gas  14  flows through the oscillation volume  11  to reach burner  4 , in the embodiment of  FIG. 2 , it is foreseen that this gas  14  partially flows as pulsation flow  23  into the oscillation volume  11  and immediately flows out. 
         [0088]    The diameter of the connecting pipe  22  is preferably selected such that the gas that flows into and flows out of the oscillation volume  11  is not hindered and thus the oscillation volume  11 , being adjustable in size, may affect the compressive strength or stiffness of the burner  4  as described above. 
         [0089]    Alternative to the oscillation volume  11  being connected to the line that leads the fuel and/or gas to the burner outlet  3 , it can be directly connected within the burner  4  so that it influences the compressive strength or stiffness of the burner supply. 
         [0090]    Thus, the present invention offers a working and reliable solution to a specific set of oscillation amplitudes in oscillating combustion processes. Thus, for example, the attenuation of amplitude can be specifically counteracted, which is caused by adding of the raw material to be treated into the vibrating hot gas column. 
         [0091]    Although illustrative embodiments of the present invention have been described above, it is foreseen that changes can be made without departing from the scope and spirit of the invention. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           1  combustion chamber 
           2  flame 
           3  burner outlet 
           4  burner 
           5  burner housing 
           6  swirl producer 
           7  pipelines 
           8  airflows 
           9  fuel gas 
           10  fuel gas line 
           11  oscillation volume 
           12  supply line 
           13  fin flow rectifier 
           14  gas 
           15  clean air 
           16  compressor 
           17  fuel line 
           18  fuel gas 
           19  piston 
           20  cylinders 
           21  arrow 
           22  connecting pipe 
           23  pulsation flow