ITRM20120037A1 - BURNER - Google Patents

Description

BURNER

Field of invention

The present invention refers to a burner, used for example for combustion in propulsive (aircraft, space, naval engines, etc.), thermal (ovens, cement factories, boilers, etc.) and for the production of electrical energy (power plants gas, fuel oil, coal, waste or solid biomass).

State of the art

The energy necessary to operate all the apparatuses that characterize an industrialized society is mostly produced by the combustion of substances of various origins and nature (fossil fuels or renewable sources, such as biomass) in the solid, liquid or gaseous state. Combustion takes place by means of suitable devices, called burners, which facilitate the mixing of the fuel, which can be solid, liquid or gaseous, with the comburent (air, oxygen, etc.). This mixing must be as rapid as possible and the flow of reagents obtained as uniform as possible.

It is well known that to obtain a good combustion, energy efficient and clean from the point of view of polluting emissions, it is first of all necessary that the initially separated reagents mix intimately inside the burner as quickly as possible. A slow and inefficient mixing, in fact, leads to a strong uneven temperature and concentration of the various chemical species, with a consequent decrease in combustion efficiency, production of unburnt products and nitrogen oxides (the latter if the comburent is 'air) and non-uniform temperature distribution of the components affected by the hot flow potentially harmful due to temperature peaks dangerous for the materials used, thermal stresses, etc. The presence of certain pollutants, such as nitrogen oxides, also causes corrosive phenomena, especially in the hottest areas. Hence the need to design combustors and burners that optimize mixing, making it as fast as possible, and consequently making combustion more efficient.

As an accessory result of efficient mixing, we obtain the reduction of the path that the reagents must travel before combustion is complete, and consequently the reduction of the overall dimensions and weight of the entire combustion chamber. This aspect can be crucial in the aeronautical field and even more so in the space field.

Current burners and combustion chambers try to achieve good mixing by assuming different configurations depending on the fuel used.

When the fuel is in the fluid state (liquid or gaseous), the two reactants (oxidizing and combustible) are divided into a plurality of small diameter jets that move in the same direction at different speeds, which causes the formation of a layer of mixing which gradually thickens. As an alternative to such parallel jets, some configurations provide for the injection of the reagents by means of non-parallel jets, with a modest relative angle, since in such conditions an accelerated mixing was noted. The flames resulting from these jets can be as diffusive (mixing occurs during combustion, which means that there is no temporal separation between mixing and combustion) as premixed (the two reagents mix before starting to react). In any case, the fine subdivision of the reagents into a large number of jets involves a large number of injectors, with a consequent considerable complexity of the combustor and high pressure drops. This last problem is particularly felt in the space sector, as the additional power that the pumps must supply in a liquid propellant rocket reactor is considerable, and involves a far from negligible increase in the consumption of propellant to power the turbines that move the pumps themselves. This leads to an inevitable reduction in the overall performance of the launcher.

If, on the other hand, the fuel is solid (coal, solid urban waste, woody biomass), there is no possibility in the devices currently used to accurately dose the rate of fuel that reacts (unless it has been finely chopped, such as coal dust), so it is not possible to have an optimal control of the thermal power that is instantly developed. Furthermore, the contact between the reactants is very limited compared to the case of a gaseous fuel unless, as already mentioned, a solid fuel with a very small particle size is used. This situation can also occur in the fluidized bed combustion chambers which allow a high temperature constancy thanks to the thermal inertia of the refractory bed. As a consequence, when solid fuels are used, there is a significant reduction in the combustion efficiency (it is around 85%) and a greater production of polluting substances. These serious drawbacks make the use of solid fuels anti-ecological and consequently unpopular. Consider, for example, the hostility with which the construction of coal-fired thermoelectric plants, waste-to-energy plants or even just woody biomass plants is often greeted). However, these fuels are not in themselves more polluting than gaseous or liquid ones. In this regard, it must be taken into account that today considerable efforts of technological research are aimed at improving the combustion of solid substances, as it is now aware that there are not so many intrinsically â € œecologicalâ € or â € “pollutants”, but rather correct or wrong ways of burning them.

There are currently innovative solutions such as the aforementioned fluidized bed, or the use of chopped fuels (powder), but the gap between the performance (in terms of polluting emissions and combustion efficiency) of combustors that use solid fuels and those that burning fluid fuels remains remarkable. It should also be noted that a plant equipped with a solid fuel shredder is more expensive, and the cost increase connected to the use of the aforementioned device is not always justified by the increase in combustion efficiency and the reduction of polluting emissions.

Ultimately, current configurations fail to achieve good energy performance, reduced emissions and high fluid dynamic efficiency at the same time.

The need is therefore felt to provide a burner which allows the above drawbacks to be overcome.

Summary of the invention

The primary purpose of the present invention is to produce a burner which allows at the same time to obtain good energy performance, reduced emissions and high fluid dynamic efficiency.

A further object of the invention is to provide for a related combustion process.

The present invention therefore proposes to achieve the objects discussed above by realizing a burner for carrying out a combustion between a fuel and a comburent, which, according to claim 1, comprises a combustion chamber and at least a first injection nozzle of at least one jet of pressurized comburent in said combustion chamber, said first nozzle being configured to direct the jet of comburent towards a zone of the combustion chamber in which the fuel is provided in such a way as to produce a mixing by frontal impact between comburent and fuel.

A second aspect of the present invention provides a combustion process which, according to claim 12, comprises an injection of at least one jet of pressurized comburent into a combustion chamber, said comburent jet being directed towards a zone of the combustion chamber in which à The fuel is provided in such a way as to produce a mixing by frontal impact between the comburent and the fuel.

Advantageously, the burner of the invention has an operation based on the mixing by frontal impact (ie from opposite directions) of a fluid reagent (gaseous or liquid) and another reagent (gaseous, liquid or even solid). The flame that occurs in these conditions is called triple flame: it is a particular diffusive flame in which the composition varies continuously with a gradient from poor to rich mixture in a direction parallel to that of the injectors. The frontal impact of the fuel and the oxidizer (comburent) has the effect of crushing the opposing jets, even if they are of considerable diameter, with consequent very rapid mixing of the same.

The high level of turbulence that is generated tends to very quickly uniform the fields of all fluid dynamics (species concentrations, temperature, velocity, etc.), cf. Figs. 1, 2 and 3.

Furthermore, a natural anchorage zone for the flame is obtained around the zone or stopping point of the jets and a system of vortices, which are small in number and large in size. Furthermore, said eddies or secondary gaseous flows, being at the edge of the main gaseous flow, are fixed, and constitute low-speed regions where combustion stabilizes. The vortices themselves contribute to the mixing of the reactants. Finally, precisely because of the greater stability of the flame, the gaseous flow moving towards the exhaust can be faster without the danger of stopping the combustion.

The advantages that this burner configuration entails are:

- a very efficient, fast and clean combustion,

- a limited number of large injectors and, therefore, reduced fluid-dynamic losses and lower construction cost of the device,

- a more stable flame,

- a higher combustion intensity,

- greater power and, at the same time, greater compactness and lightness.

The dependent claims describe preferred embodiments of the invention, forming an integral part of the description.

Brief description of the figures

Further characteristics and advantages of the invention will become more evident in the light of the detailed description of preferred, but not exclusive, embodiments of a burner illustrated by way of non-limiting example, with the aid of the accompanying drawing tables in which:

Figure 1 represents the results of a numerical simulation relating to the pressure field with current lines on a section of a plane burner according to the invention;

Figure 2 represents the results of a numerical simulation relating to the mass fraction of methane on a section of a plane burner according to the invention; Figure 3 represents the results of a numerical simulation relating to the turbulent kinetic energy on a section of a plane burner according to the invention; Figure 4 represents a schematic sectional view of a first embodiment of a burner according to the invention;

Figure 5 represents a schematic sectional view of a second embodiment of a burner according to the invention;

Figure 6 represents a three-dimensional schematic view of a third embodiment of a burner according to the invention.

The same reference numbers in the figures identify the same elements or components.

Detailed description of preferred embodiments of the invention

With reference to Figures 4 to 6 there are represented embodiments of burner according to the invention which can use fluid or solid fuels.

A first embodiment of the burner, globally indicated with the numerical reference 1, is illustrated in Figure 4.

This burner comprises a combustion chamber 2, at least a first injection nozzle or injector 3 of at least one jet of pressurized comburent in chamber 2, and at least a second injection nozzle or injector 4 of at least one jet of fuel in said chamber 2.

The first nozzle 3 and the second nozzle 4 are advantageously configured so that the respective combustion and fuel jets are opposite each other so that the combustion agent jet impacts frontally with the corresponding fuel jet causing a crushing of the jets with instant mixing of fuel and comburent and defining a natural anchoring of the flame generated by said mixing in correspondence with the front impact zone. First nozzle 3 and second nozzle 4 preferably have a common longitudinal axis and the outflow section of the first nozzle 3 is suitably spaced from the outflow section of the second nozzle 4. Said outflow sections are axially symmetrical, flat or of other shape.

The first nozzle 3 and the second nozzle 4 are preferably but not necessarily contained within a casing 5 which separates them from the external environment and at least partially delimits the combustion chamber 2. The casing 5 can constitute a liner of the combustion chamber which allows the flame that develops and that touches one of the two nozzles 3, 4 to be directed towards an exhaust 6. In the example of Figure 4, the flame touches the nozzle 3 and the combustion system producing a preheating of the comburent, thus making the chemical kinetics faster and reducing the risk of extinguishing the flame itself.

The nozzles 3, 4 can preferably have a diameter of at least 10 cm. The combustion agent feeding system to the at least one injection nozzle 3 preferably comprises two delivery pipes 3â € ™, 3â € ™ â € ™ having the same longitudinal axis and defining, together with the injection nozzle 3, a T-shaped sectional configuration. The comburent is fed into the first delivery pipe 3â € ™ in the opposite direction to the direction of the comburent supply into the second delivery pipe 3â € ™ â € ™. The combustion agent coming from these delivery pipes 3â € ™, 3â € ™ â € ™ then flows into the nozzle 3 which injects it at a predetermined pressure into the combustion chamber 2.

This first embodiment of the burner is suitable for using reagents in the fluid state, for example methane and air. Nozzles 3 and 4 inject fuel and oxidizer in opposite directions (Figure 4). The jets are perfectly centered between them, ie they define the same longitudinal axis, in order to avoid slipping phenomena of one on the other without the immediate mixing of the reactants. The frontal impact that derives from this injection mode immediately shatters the jets, which can also be large in diameter, for example 10 cm or even greater than 10 cm. A very high level of turbulence is produced at the stopping point of the jets, which makes mixing almost instantaneous. Inside the flame, the temperature and concentration gradients of the species are reduced compared to those found in current combustors, so the polluting emissions are lower and the temperature distribution in the objects placed downstream of the flame, for example the vanes of a turbine is more uniform, resulting in less thermal stress and less possibility of local settlements. The stopping point of the jets is also a natural anchorage area for the flame, so the risk of flame extinction is reduced. In addition to this anchoring area, recirculation areas are created on the sides of the nozzles which further facilitate the anchoring of the flame. Furthermore, by creating a diffusive flame in this way, it happens that even if a globally poor combination of reactants is used, i.e. a combination more prone to causing a combustion arrest than a richer combination of reactants, the flame can still be ignited in the â € œrichâ € area of the combustion chamber (excess fuel) and then spread to the more â € œpoorestâ € area (fuel deficiency). The speed of mixing also allows to obtain, as mentioned above, a remarkable uniformity of the flue gas flow. In other words, the advantages of a diffusive flame are combined with those of a premixed flame, which is obtained when the reagents have been premixed before the flame develops.

A second embodiment of the burner, globally indicated with the numerical reference 1, is illustrated in Figure 5. The operation is substantially identical to that described above for the first embodiment.

With respect to the first embodiment, the burner of Figure 5 is also provided with a first combustion agent delivery manifold 7 and a second fuel delivery manifold 8.

The first manifold 7 is fed at its two lateral ends, opposite each other, by the delivery pipes 3â € ™, 3â € ™ â € ™ and has a flat bottom 9 provided with a plurality of injection nozzles 3 arranged on at least one row. The injection nozzles 3 can be arranged along rows and columns so as to define a matrix. In Fig. 5, for example, the nozzles 3 are provided in three rows and four columns.

The second manifold 8, on the other hand, is fed from below by a fuel delivery pipe 4 and has a flat upper surface 11 provided with a plurality of injection nozzles 4 arranged on at least one row. The injection nozzles 4 can be arranged along rows and columns so as to define a matrix. In Fig. 5, for example, the nozzles 4 are provided on three rows and four columns in a manner corresponding to the nozzles 3.

The first nozzles 3 and the second nozzles 4 are advantageously configured in such a way that the respective combustion and fuel jets are mutually opposed so that the combustion agent jets impact frontally with the corresponding fuel jets, causing the jets to crush with instant mixing of fuel and combustion agent. and defining a natural anchoring of the flame generated by said mixing in correspondence with the front impact zone. First nozzles 3 and second nozzles 4 preferably have a common longitudinal axis and the outflow section of the first nozzles 3 is suitably spaced from the outflow section of the second nozzles 4. Said outflow sections are axially symmetrical, flat or of other shape.

The first nozzles 3 and the second nozzles 4 are preferably but not necessarily contained within a casing 5 which separates them from the external environment and at least partially delimits the combustion chamber 2. The casing 5 can form a liner of the combustion chamber which allows the flame that develops and that licks the nozzles 3 and the comburent supply system, including the pipes 3â € ™, 3â € ™ â € ™ and the first manifold 7, to be directed towards a discharge 6, producing a preheating of the comburent.

The nozzles 3, 4 can have a variable diameter from a few centimeters up to exceeding 10 cm in applications with greater power and flow rate (for example, thermoelectric power plants).

A third embodiment of the burner, globally indicated with the numerical reference 1, is illustrated in Figure 6.

This burner comprises a combustion chamber 2 and at least a first injection nozzle 3 of at least one jet of pressurized comburent in said combustion chamber 2. In the diagram of Fig. 6 three nozzles 3 are visible, arranged along a row, configured to direct the combustion agent jet towards a zone 12 of the combustion chamber 2 in which a solid fuel is positioned in such a way as to produce a mixing by frontal impact between fluid, gaseous or liquid comburent and solid fuel. Alternatively, a plurality of injection nozzles 3 can be provided, arranged in several rows, along rows and columns in order to define a matrix.

The system for feeding the comburent to the plurality of injection nozzles 3 preferably comprises two delivery pipes 3â € ™, 3â € ™ â € ™ having the same longitudinal axis and which together with the injection nozzles 3 define respective cross-sectional configurations in the shape of T (Fig. 6). The comburent is fed into the first delivery pipe 3â € ™ in the opposite direction to the direction of supply of the comburent into the second delivery pipe 3â € ™ â € ™. The combustion agent coming from said delivery pipes 3â € ™, 3â € ™ â € ™ then flows into the nozzles 3 which inject it at a predetermined pressure orthogonally towards the solid fuel positioned in zone 12 of the combustion chamber.

This third embodiment of the burner is suitable for using one of the two reactants, fuel, in a solid state, for example coal, waste, wood. The solid fuel is contained in a container or storage tank 13 on the bottom of which there is a loading device 14, for example a worm screw, which pushes it, as it is consumed, towards the proximal part of the tank. to said zone 12 of chamber 2.

The jets of fluid comburent impact frontally with the solid fuel of not too large size, for example coal, wood chips, waste.

The mixing is rapid and the flame that is generated preheats the combustion agent feeding system which therefore has a high reactivity towards the fuel.

The ash produced is mechanically removed from the combustion agent jets, thus making the solid fuel always exposed to the action of the other reagent. This greatly reduces the production of unburnt pollutants, such as carbon monoxide, and makes combustion much more efficient, without the need to chop up the solid reagent, as is done for example in thermoelectric plants that burn coal or wood dust.

The nozzles 3 are preferably but not necessarily contained inside a casing 5 which separates them from the external environment and at least partially delimits the combustion chamber 2. The casing 5 can form a liner for the combustion chamber which allows hot fumes and ashes to be directed towards an exhaust 6. The nozzles 3 can preferably have a diameter of at least 10 cm, as already explained above (second embodiment).

Considering a burner according to the invention and planning to use methane as fuel and air as comburent, Figures 1, 2 and 3 respectively show the results of the numerical simulation relating to the pressure field with current lines, to the mass fraction of methane and turbulent kinetic energy on a section of a flat burner. By flat burner we mean a burner that can be considered perfectly two-dimensional, since each section orthogonal to the third axis has the same geometric and thermo-fluid dynamic characteristics.

The resulting gaseous flow was calculated cold, that is due only to mixing and not to combustion. Note the practically perfect mixing, the reduced pressure losses, the swirling system and the high level of turbulence in the areas where mixing takes place. Experimentally it has been found that the crushing of the jets is immediate and the complete combustion (blue flame).

Claims (13)

CLAIMS 1. Burner (1) to carry out a combustion between a fuel and a comburent comprising a combustion chamber (2) and at least a first injection nozzle (3) of at least one jet of pressurized comburent in said combustion chamber, said first nozzle (3) being configured to direct the jet of combustion agent towards a zone of the combustion chamber where the fuel is provided in such a way as to produce a mixing by frontal impact between the combustion agent and the fuel. 2. Burner according to claim 1, wherein at least one second injection nozzle (4) of at least one pressurized fuel jet is provided in said zone of the combustion chamber, first nozzle and second nozzle being configured so that the respective jets of comburent and fuel are opposed to each other so that the jet of comburent impacts frontally with the corresponding jet of fuel causing a crushing of the jets with instant mixing of fuel and comburent and defining a natural anchorage of the flame generated by said mixing in correspondence with the area of frontal impact. 3. Burner according to claim 2, wherein said at least one first nozzle (3) and said at least one second nozzle (4) have a common longitudinal axis and the outflow section of the first nozzle is suitably spaced from the outflow section of the second nozzle. 4. Burner according to claim 2 or 3, in which a plurality of first nozzles (3) and a plurality of second nozzles (4) are provided, in an equal number. 5. Burner according to claim 4, wherein first nozzles (3) and second nozzles (4) have an axially symmetrical, flat or other shaped outlet section. 6. Burner according to any one of claims 2 to 5, wherein said at least one first nozzle (3) and said at least one second nozzle (4) are contained inside a casing (5) which separates them from the external environment and at least partially delimits the combustion chamber (2). 7. Burner any one of the preceding claims, in which there is provided a system for feeding the comburent to the at least one injection nozzle (3) comprising two delivery pipes (3â € ™, 3â € ™ â € ™) having one same longitudinal axis and which define, together with the injection nozzle (3), a sectional configuration in the shape of a T. 8. Burner according to claims 4 and 7, in which a first manifold (7) is provided, fed at its two lateral ends, opposite each other, by the delivery pipes (3â € ™, 3â € ™ â € ™) and having a bottom (9) provided with a plurality of first injection nozzles (3) arranged on at least one row. 9. Burner according to claim 8, wherein a second manifold (8) is provided, fed from below by a delivery pipe (4â € ™) of the fuel and having an upper surface (11) provided with a plurality of second nozzles injection (4) arranged in at least one row. 10. Burner according to claim 9, wherein both the first nozzles (3) and the second nozzles (4) are arranged along rows and columns so as to define a matrix. 11. Burner according to claim 1, wherein an accumulation tank (13) is provided for containing a solid fuel, a loading device (14) being arranged on the bottom of said tank for advancing said solid fuel towards a part of the tub proximal to said zone of the combustion chamber (2). 12. Combustion process, achievable by means of a burner according to any one of the preceding claims, comprising an injection of at least one jet of pressurized comburent into a combustion chamber, said comburent jet being directed towards a zone of the combustion chamber in which it is the fuel is provided in such a way as to produce a mixing by frontal impact between the comburent and the fuel. 13. Process according to claim 12, wherein an injection of at least one jet of fuel is provided in said zone of the combustion chamber, simultaneously with the injection of the at least one jet of comburent, the respective comburent and fuel jets being between opposite them so that the jet of comburent impacts frontally with the corresponding jet of fuel causing a crushing of the jets with instant mixing of fuel and comburent and producing a natural anchoring of the flame generated by said mixing in correspondence with the frontal impact zone.