Patent Description:
The present invention relates to a burner assembly for a boiler unit for steam generation and to a boiler unit comprising said burner assembly.

The growing attention to air pollution and climate change on a global scale, confirmed by the COP26 protocols, requires, along with an acceleration of the energy transition towards renewable energy sources, a reduction in the use of fossil fuels.

Global emissions of nitrogen oxides are expected to fall by <NUM>% by <NUM>, so both a significant reduction of the emissions and high efficiency are crucial to meet market demands in terms of cost and sustainability.

The need to reduce greenhouse gas emissions in a context of increasing steam demand for process/industrial applications is therefore leading to the development of a new generation of burners capable of accompanying and promoting the energy transition from fossil to renewable and waste fuels.

Next-generation burners will need to be able to burn several fuels with high efficiency and low emissions.

The fuels available may vary depending on availability and range from natural gas to gases with different compositions and/or low calorific value (e.g. flare gas, biogas, etc.), up to pure hydrogen or mixed in a variable percentage with other gases. Each of <CIT> and <CIT> discloses a burner assembly according to the preamble of claim <NUM>.

It is therefore an aim of the present invention to provide a burner assembly which is capable of burning a wide range of gases at high efficiency and which is capable, at the same time, of keeping emission levels below legal limits.

In other words, it is an aim of the present invention to provide a burner assembly characterized by maximum flexibility in terms of fuel with which the burner assembly can be supplied, high thermal efficiency and low pollutant emissions.

In accordance with these aims, the present invention relates to a burner assembly for a boiler for steam generation; the burner assembly extending along a longitudinal axis and comprising:.

In the burner assembly according to the present invention
the injection of fuel takes place through the injection nozzles surrounding the air duct and are axially rearward with respect to the outlet of the air duct. In this way, the fuel injection zone and the air discharge zone are substantially separated. This allows the generation of an oxygen-lean combustion zone in the annular region surrounding the air duct.

The possibility of rotating the end portions of the fuel conduits allows a further adjustment of the injection direction, for example depending on the fuel injected.

For some types of fuel, it may be useful to adjust the injection direction by trying to move the injection zone further away from the air discharge zone. For other types of fuel, on the other hand, the exact opposite could be useful (i.e. to bring the injection zone closer to the air discharge zone).

A further aim of the invention is to provide a boiler unit for steam generation with high efficiency and able to comply, at the same time, with the legal limits in terms of emission of pollutants.

In accordance with these aims, the present invention relates to a boiler unit for steam generation as claimed in claim <NUM>.

Finally, it is a further aim of the invention to provide a method for adjusting a burner assembly that is capable of increasing its efficiency while respecting at the same time the legal limits in terms of emission of pollutants.

In accordance with these aims, the present invention relates to a method for adjusting a burner assembly as claimed in claim <NUM>.

Further characteristics and advantages of the present invention will become clear from the following description of a non-limiting example of an embodiment thereof, with reference to the figures of the attached drawings, in which:.

In <FIG>, reference number <NUM> denotes a boiler unit for steam generation.

The boiler unit <NUM> comprises a boiler <NUM> configured to generate steam and provided with a combustion chamber <NUM>, at least one burner assembly <NUM>, a plurality of evaporation ducts <NUM> (only some of which are schematically represented in the accompanying figures), and at least one cylindrical body <NUM>.

The evaporation ducts <NUM> extend into the combustion chamber <NUM>. Each evaporation duct <NUM> is provided with a water inlet <NUM> and with a steam outlet <NUM>.

The cylindrical body <NUM> is connected to the water inlet <NUM> and to the steam outlet <NUM> of each evaporation duct <NUM> and is further provided with an inlet <NUM> for receiving water to be supplied to the evaporation ducts <NUM> through the water inlets <NUM>, and with an outlet <NUM> through which the steam coming from the evaporation ducts <NUM> flows through the steam outlets <NUM>.

In use, the burner assembly <NUM> heats the combustion chamber <NUM>; the water in the cylindrical body <NUM> is supplied to the evaporation ducts <NUM> and transformed into steam by exploiting the heat present in the combustion chamber <NUM>.

The steam generated by each evaporation duct <NUM> is discharged into the cylindrical body <NUM>, and then escapes through the outlet <NUM>. The combustion fumes in the combustion chamber <NUM> are discharged into the atmosphere through a flue <NUM> after passing through the zone of the combustion chamber <NUM> in which the evaporation ducts <NUM> are arranged and, if provided, also an economizer (not shown).

In the non-limiting example described and shown herein, the boiler <NUM> comprises only one burner assembly <NUM>.

The burner assembly <NUM> is provided with a burner body <NUM>, an air supply system <NUM>, a fuel supply system <NUM> and a control device <NUM>.

Preferably, the air supply system <NUM> comprises an air supply duct <NUM> which connects the burner body <NUM> to an air supply line <NUM> and is configured to supply the burner assembly <NUM> with a certain air flow rate.

The air supply line <NUM> preferably comprises a fan <NUM> arranged at the inlet of the line and a heater <NUM> configured to heat the air before it reaches the burner body <NUM>, if necessary.

The heater <NUM> and the fan <NUM> are adjusted by the control device <NUM>.

With reference to <FIG>, the fuel supply system <NUM> is configured to supply more than one fuel to the burner body <NUM>. In the non-limiting example described and shown herein, the fuel supply system <NUM> comprises a stabilization line <NUM> (which supplies a stabilizing gas, like natural gas or gas with known characteristics that are stable over time), a liquid fuel supply line <NUM> (if provided), and three fuel supply lines 30a 30b 30c.

In the non-limiting example described and shown herein, the three fuels supplied to the fuel supply lines 30a, 30b, 30c are preferably different from each other.

For example the three fuels supplied to the fuel supply lines 30a 30b 30c may be different in terms of provenance (e.g. they may come from different sources depending on current availability) or in terms of composition (e.g. they may comprise different percentages of hydrogen) or in terms of fuel type (e.g. flare gas, biogas, natural gas, hydrogen, etc.).

A variant not shown provides that at least two of the fuel supply lines 30a 30b 30c are supplied with the same fuel.

A variant not shown provides that the number of fuel supply lines is different from three, for example two or more than three, depending on the types of fuel available or depending on the flexibility required of the burner assembly <NUM>.

With reference to <FIG>, the stabilization line <NUM> is provided with a stabilizing fuel regulating valve <NUM>, the liquid fuel supply line <NUM> is provided with a liquid fuel regulating valve <NUM>, while each fuel supply line 30a 30b 30c is provided with a respective fuel regulating valve 33a 33b 33c.

The fuel supply system <NUM> further comprises a first manifold 34a connected to the fuel supply line 30a, a second manifold 34b connected to the fuel supply line 30b, a third manifold 34c connected to the fuel supply line 30c.

Preferably, the manifolds 34a, 34b and 34c are connected to each other.

In the non-limiting example described and shown herein, the fuel supply system <NUM> comprises a first connection line 36a which connects the first manifold 34a to the second manifold 34b, a second connection line 36b which connects the second manifold 34b to the third manifold 34c, and a third connection line 36c which connects the third manifold 34c to the first manifold 34a.

Each connection line 36a 36b 36c is provided with a respective connection valve 37a 37b 37c.

The stabilizing fuel regulating valves <NUM>, the liquid fuel regulating valve <NUM>, the fuel regulating valves 33a 33b 33c and the connection valves 37a 37b 37c are controlled by the control device <NUM>. The control mode will be detailed below.

With reference to <FIG> and to <FIG>, the burner body <NUM> of the burner assembly <NUM> extends along a longitudinal axis A and comprises an air duct <NUM> centred on the axis A, a main burner <NUM>, which extends around the air duct <NUM> and a stabilization burner <NUM>, which is at least in part housed in the air passage duct <NUM>. In the non-limiting example described and shown herein, the burner body <NUM> further comprises a liquid fuel lance <NUM>, which is also arranged along the air duct <NUM>.

As highlighted by the arrow in <FIG>, air and fuel flow along the burner body <NUM> in a forward direction D.

The air duct <NUM> has an inlet <NUM> and an outlet <NUM>. The air in the air duct <NUM>, therefore, flows from the inlet <NUM> to the outlet <NUM>.

The air duct <NUM> comprises an inlet element <NUM>, which comprises an inlet edge <NUM> defining the inlet <NUM> of the air duct <NUM>, an outlet element <NUM>, which comprises an outlet edge <NUM> defining the outlet <NUM> of the air duct <NUM>, and an outlet swirler <NUM>.

In the non-limiting example described and shown herein, the inlet element <NUM> and the outlet element <NUM> are coupled together. In particular, in the example of <FIG> and <FIG>, the outlet element <NUM> is keyed onto the inlet element <NUM> and wraps it for a stretch.

According to a variant not shown, one or more connection elements may be present between the inlet element <NUM> and the outlet element <NUM>.

The inlet edge <NUM> has a preferably frusto-conical shape converging along the forward direction D.

In the example shown in <FIG> and <FIG>, the inlet element <NUM> comprises an inner annular face <NUM>, which defines an inlet portion <NUM> of the air duct having a constant passage section along the forward direction D.

The outlet element <NUM> is annular and made of refractory material and is preferably defined by sectors that are assembled together.

The outlet element <NUM> comprises an inner annular face <NUM> provided with a first inner portion <NUM> proximal to the outlet edge <NUM>, which defines an outlet portion <NUM> of the air duct <NUM> having an increasing passage section along the forward direction D.

Preferably, the first inner portion <NUM> of the inner annular face <NUM> has a frusto-conical shape diverging along the forward direction D.

In particular, the first frusto-conical inner portion <NUM> is inclined with respect to the axial direction by an angle α comprised between <NUM>° and <NUM>°.

The inner annular face <NUM> of the outlet element <NUM> is further provided with a second inner portion <NUM> which is configured so as to define a constant passage section along the forward direction D. In detail, the second inner portion <NUM> is arranged upstream of the first portion along the forward direction D.

Preferably, the second inner portion <NUM> is cylindrical and contiguous to the first inner portion <NUM>.

The outlet element <NUM> is further provided with a plurality of axial and through seats <NUM>, which, as we will see in detail later, house portions of the main burner <NUM>.

Preferably, the axial seats <NUM> are aligned along a circular path that surrounds the air duct <NUM>.

More preferably, the axial seats <NUM> are arranged equidistant from each other.

According to a variant not shown, the seats <NUM> are arranged along non-axial directions and suitably inclined with respect to the longitudinal axis A (for example converging along the forward direction D or diverging along the forward direction D).

In detail, the outlet element <NUM> is provided with an outer face <NUM> comprising a first cylindrical outer portion <NUM>, which is proximal to the outlet edge <NUM>, and a second annular outer portion <NUM>, which extends transversely to the first cylindrical outer portion <NUM> and is provided with a plurality of holes <NUM> that define the outlet of the axial seats <NUM>.

Preferably, the outer face <NUM> of the outlet element <NUM> also comprises a third outer portion <NUM>, preferably cylindrical, which extends from the second annular outer portion <NUM> in a direction opposite to the forward direction D.

The third outer portion <NUM> is coupled to a wall <NUM> of the boiler <NUM> (dashed and only partially visible).

The wall <NUM> of the boiler <NUM> and the second annular outer portion <NUM> are therefore arranged substantially flush with each other.

The inlet <NUM> faces a casing <NUM>, which defines a plenum <NUM> supplied with air from the air supply duct <NUM> of the air supply system <NUM>.

Inside the plenum <NUM> and at the inlet <NUM>, the burner body <NUM> comprises a perforated pipe <NUM>, which extends mainly externally to the air passage duct <NUM> and coaxially to the axis A.

Preferably, the perforated pipe <NUM> extends axially between the inlet <NUM> and an end wall <NUM> of the casing <NUM>.

Preferably, a portion of the perforated pipe <NUM> is arranged within the air duct <NUM>.

In use, the perforated pipe <NUM> helps to generate vorticity in the air flow passing through the inlet <NUM> by limiting the amount of air at the centre of the air duct <NUM>.

Preferably, the burner body <NUM> also comprises an air flow distributing element <NUM> arranged within the casing <NUM>.

The air flow distributing element <NUM> is substantially cylindrical, centred on the axis A and arranged about at least a portion of the perforated pipe <NUM>.

In the non-limiting example described and shown herein, the air flow distributing element <NUM> is fixed to the end wall <NUM> of the casing <NUM> and extends around the perforated pipe <NUM> until it reaches approximately half the axial length of the perforated pipe <NUM>.

Preferably, the air flow distributing element <NUM> is dimensioned such that the distance between the perforated pipe <NUM> and the inner wall of the air flow distributing element <NUM> is such as to cause a homogeneous air distribution effect around the perforated pipe <NUM>.

The outlet swirler <NUM> is arranged in the outlet portion <NUM> of the air duct <NUM> and is supported by an axial conduit <NUM> that is concentric to the axis A.

The outlet swirler <NUM> preferably faces the first inner portion <NUM> of the inner face of the outlet element <NUM> and is provided with a plurality of blades <NUM>, which extend along respective radial directions and have one end 87a fixed to the axial conduit <NUM> and one end 87b fixed to an outer support ring <NUM>.

The outer support ring <NUM> is provided with a substantially cylindrical outer surface 89a, which faces the first inner portion <NUM> and is preferably provided, along the edge proximal to the outlet <NUM>, with a plurality of teeth 89b which preferably project orthogonal to the outer surface 89a. The teeth 89b are preferably trapezoidal in shape.

In use, the outlet swirler <NUM> generates, downstream, a low axial velocity zone to stabilize the diffusion flame created by the stabilization burner <NUM>, as we will see later.

In the non-limiting example described and shown herein, a plurality of races <NUM> project radially from the axial conduit <NUM>, which have a structural function and are fixed to the inlet element <NUM> of the air duct <NUM>.

The main burner <NUM> extends around the air duct <NUM> and comprises a plurality of fuel conduits <NUM> and a plurality of fuel injection nozzles <NUM> arranged at an end <NUM> of the respective fuel conduits <NUM>.

The fuel conduits <NUM> comprise an end portion <NUM>, which comprises the end <NUM>, and is housed in the seats <NUM>.

Preferably, the fuel conduits <NUM> are cylindrical.

Each fuel conduit <NUM> is provided with an end <NUM> opposite the end <NUM>, which is connected to a respective manifold of the manifolds 34a, 34b and 34c. The end portion <NUM> is rotatably coupled to the remaining part of the fuel conduit <NUM> (detail not clearly visible in the accompanying figures). Preferably, the end portion <NUM> is configured so as to rotate between -<NUM>° and <NUM>° around its own axis B.

Preferably, a first group of fuel conduits 90a is connected to the first manifold 34a, a second group of fuel conduits 90b is connected to the second manifold 34b, and a third group of fuel conduits 90b is connected to the third manifold 34b.

The fuel conduits 90a of the first group are preferably interposed to the fuel conduits 90b of the second group and to the fuel conduits 90c of the third group.

In this way, the fuel conduits 90a, 90b and 90c alternate along the circumference along which they are arranged. This allows an equal distribution of the fuels supplied by means of the main burner <NUM>.

In the non-limiting example described and shown herein, the manifolds 34a, 34b and 34c are preferably defined by annular channels 98a, 98b, 98c, which extend externally to the casing <NUM>. Preferably, a wall of the manifolds 34a, 34b and 34c is defined by the wall of the casing <NUM>.

It is understood that, in accordance with a variant not shown, the manifolds 34a, 34b and 34c can be housed in the plenum <NUM> defined by the casing <NUM>.

In the non-limiting example described and shown herein, the manifolds 34a, 34b and 34c are substantially identical. In accordance with a variant not shown, the manifolds can have different structure and different passage sections.

Each injection nozzle <NUM> is provided with at least one injection hole <NUM> (better visible in the enlargement of <FIG>).

In the non-limiting example described and shown herein, the at least one injection hole <NUM> is arranged offset from the axis B of the respective end portion <NUM> of the fuel conduit <NUM>.

In this way, the rotation of the end portion <NUM> of the respective fuel conduit <NUM> causes a displacement of the injection hole <NUM> and a consequent variation in the injection direction through the injection hole <NUM>.

In use, for example depending on the type of fuel supplied, the orientation of the end portion <NUM> of each fuel conduit <NUM> may be suitably adjusted so as to obtain an adjustment in the direction of injection of the fuel into the combustion chamber <NUM> through the injection holes <NUM>.

Preferably, the end portion <NUM> of each fuel conduit <NUM> is adjusted during installation of the burner assembly or during ordinary and/or extraordinary maintenance operations.

The stabilization burner <NUM> is, at least in part, housed in the air duct <NUM>.

The stabilization burner <NUM> is preferably supplied with natural gas or with gases of known characteristics that are stable over time and is connected to the stabilization line <NUM>.

Preferably, the stabilization burner <NUM> extends substantially axially within the air duct <NUM>.

In the non-limiting example described and shown herein, the stabilization burner <NUM> comprises two conduits <NUM> connected to the stabilization line <NUM>, which extend in the air duct and are provided with a discharge portion <NUM> provided with a plurality of discharge nozzles <NUM>.

The conduits <NUM> are preferably arranged on opposite sides with respect to the axial conduit <NUM>.

Preferably, the discharge portion <NUM> is arranged transverse to the axis A and the discharge nozzles <NUM> are arranged so as to inject fuel into the air duct <NUM> in a direction other than the forward direction D.

Preferably, the discharge portion <NUM> of each conduit <NUM> is arranged upstream of the outlet swirler <NUM>.

In use, a limited amount of natural gas is injected into the air duct <NUM> through the stabilization burner <NUM> to produce, in diffusion mode, a stabilization flame particularly useful during the ignition, heating and full ignition phases.

The injection of a very small portion of fuel into the central part of the burner assembly <NUM> can be useful for stabilizing the flame when the main burner <NUM> is supplied with fuels with low calorific value and low reactivity (e.g. waste gas) helping to keep the combustion stable.

Injecting a very small portion of fuel into the central part of the burner assembly <NUM> may also be useful when the main burner <NUM> is supplied with standard fuels (e.g. natural gas) because it creates a conventional diffusive combustion zone. Advantageously, in this way the presence of a flame is detectable even when the injection of the fuel into the outer portion can generate a "MILD" type combustion as a result of the demixing with more difficult flame detection.

In the non-limiting example described and shown herein, the burner body <NUM> further comprises a liquid fuel lance <NUM>, which is also arranged along the air duct <NUM>.

In particular, the liquid fuel lance <NUM> is preferably arranged in the axial conduit <NUM> which supports the outlet swirler <NUM> and is connected to the liquid fuel supply line <NUM>.

In use, the control device <NUM> regulates the fuel regulating valves 33a, 33b, 33c, the stabilizing fuel regulating valve <NUM>, the liquid fuel regulating valve <NUM> depending on the availability of fuel and the energy demand to the boiler <NUM>.

In particular, the control device <NUM> is configured to regulate the opening of the fuel regulating valves 33a, 33b, 33c on the basis of the energy demand of the boiler <NUM> and on the basis of characteristic parameters of the type of fuel that is supplied through the respective valve (e.g. Wobbe number, fuel pressure, fuel density, fuel calorific value).

In essence, the control device <NUM> defines the flow rate of each fuel to be supplied to the burner body <NUM> on the basis of the energy demand of the boiler <NUM> and on the basis of characteristic parameters of the fuel available.

In particular, the control device <NUM> regulates the flow rate supplied to the main burner <NUM>, and therefore the opening of the fuel regulating valves 33a, 33b, 33c, on the basis of the following relationship: <MAT> Where:.

The thermal power required for the fuel supplied to the n-th manifold is preferably defined a priori. Preferably, the thermal power demand is distributed among the fuels so that only one of the fuels is supplied to the respective manifold being affected by the variations in thermal power demand, while the other manifolds (n-<NUM>) are preferably supplied with an amount of fuel that is substantially constant or subject to variations only in cases of actual need.

In other words, if the n manifolds are all active, one manifold supplies fuel in modulating mode and is affected by variations in thermal power demand, while the other n-<NUM> manifolds supply fuel in substantially constant mode or regulated according to criteria not related to thermal power demand.

The calorific value of the fuel supplied to the n-th manifold is detected upstream of the respective fuel regulating valve 33a 33b 33c.

Alternatively, instead of the calorific value of the fuel supplied to the n-th manifold, it is also possible to consider the Wobbe index, which combines calorific value and density.

Preferably, the control device <NUM> is also configured to take into account the availability of current fuel and to optionally regulate the thermal power required for the fuel supplied to the n-th manifold PT(n-th) on the basis of actual availabilities. Preferably, the control device <NUM> detects the availability of fuel based on pressure values detected on the fuel supply lines 30a 30b 30c upstream of the respective fuel regulating valves 33a 33b 33c. In this way, the control device is able to establish the actual availability of the fuels in the network and to regulate the thermal power required for the fuel supplied to the n-th manifold PT(n-th) if this exceeds the actual availability.

The same philosophy is basically adopted in the regulation of the connection valves 37a 37b 37c controlled by the control device <NUM>.

The control device <NUM>, in fact, regulates the opening of the connection valves 37a 37b 37c to selectively put the manifolds 34a, 34b and 34c in connection on the basis of the energy demand of the boiler <NUM> and on the basis of characteristic data of the fuel present in the respective manifold 34a, 34b and 34c (for example the pressure of the fuel detected by means of pressure sensors 35a 35b 35c arranged downstream of the fuel regulating valves 33a 33b 33c).

The opening of the connection valves 37a, 37b, 37c is carried out in order to obtain a distribution of the fuel on more than one manifold of the manifolds 34a, 34b and 34c and the closing is of course carried out in order to reduce said distribution.

Preferably, the control device <NUM> is configured to open at least one connection valve of the connection valves 37a 37b 37c on the basis of the pressure values detected by the pressure sensors 35a 35b 35c downstream of the fuel regulating valves 33a 33b 33c.

In particular, if the pressure value detected on a supply line exceeds a threshold value, the opening of at least the connection valve connecting the manifold supplied by said line with a further manifold is carried out.

Similarly, once the opening of the at least one connection valve has been carried out, the closing of said valve will take place when the value of the data detected on the line (in the example considered herein the fuel pressure detected by means of pressure sensors 35a 35b 35c arranged downstream of the fuel regulating valves 33a 33b 33c) is again acceptable. In other words, if the pressure value drops to a second minimum threshold, then the connection valve that had been opened is closed again.

Making a practical example to better understand the mechanism, if the pressure value of the pressure sensor 35a exceeds a certain threshold value, at least one of the connection valves 37a 37c is opened to connect the manifold 34a supplied by the line 30a to a further manifold (34b or 34c).

The opening of a connection valve due to an excess of pressure on one of the connection valve lines will preferably coincide with the closing of the line regulating valve that supplies the manifold shared with the fuel supply line where an excess of pressure has occurred.

Returning to the example described above, then, if the open connection valve is 37a, the valve 33b supplying the manifold 34b is closed.

In this way the manifold 34b is used for the distribution of the first fuel.

In other words, the connection valve 37a is opened to put two fuel manifolds in communication in order to increase the outflow section of the injection nozzles and keep the pressure at the injection nozzles within acceptable limits.

In essence, the connection valves 37a 37b 37c allow any type of fuel to be managed even if it is not stable. Similarly, the opening of the liquid fuel regulating valve <NUM> is regulated by the control device <NUM> on the basis of the energy demand of the boiler <NUM> and on the basis of the availability of liquid fuel and, if required, also on the basis of the indications of the need for fuel disposal (decided by the plant operator). Sometimes, in fact, it is required to dispose of liquid fuel with priority, especially when this represents, for example, a waste produced by the plant.

The stabilizing fuel regulating valve <NUM>, on the other hand, is regulated based on the operating conditions of the burner assembly <NUM> (i.e. on the basis of the energy demand of the boiler <NUM>) and on the basis of the characteristic parameters of the fuel that is supplied to the main burner <NUM>.

Preferably, the control device <NUM> is configured to carry out a regulation of the flow rate of the stabilizing fuel based on, for example, the pressure of the fuels supplied to the fuel supply lines 30a 30b 30c. Preferably, the pressure on which the flow rate control of the stabilizing fuel is based is carried out on the basis of the pressure detected upstream of the fuel regulating valves 33a, 33b, 33c.

Preferably, the stabilization burner <NUM>, in fact, is activated in the ignition phases and optionally kept active or reactivated in the event that the characteristics of the fuel (or of the fuels) supplied to the main burner <NUM> detect particularly lean fuels. The presence of the stabilization burner <NUM>, in these cases, ensures flame stability.

In case the main burner <NUM> is supplied with particularly reactive fuels (e.g. hydrogen-based) the stabilization burner <NUM> can be switched off (because it is not necessary) further reducing the overall NOx emissions.

A burner assembly <NUM> in accordance with a second embodiment is shown in <FIG> and <FIG>.

Hereinafter, the same reference numerals adopted in relation to <FIG> will be maintained to indicate identical or similar parts.

The burner assembly <NUM> has a structure very similar to the burner assembly <NUM> and differs in some details which we specify hereinbelow.

The burner assembly <NUM> is devoid of a liquid fuel burner and comprises a stabilization burner <NUM>, which engages the axial conduit <NUM>.

In other words, the stabilization burner <NUM> injects fuel just downstream of the outlet swirler <NUM> and in the central position.

The burner assembly <NUM> is further characterized by a different structure of the casing <NUM> (schematically represented in <FIG> only), and by a different structure of the air flow distributing element <NUM> arranged within the casing <NUM>.

The casing <NUM> has a plenum <NUM> supplied with air from the air supply duct <NUM> of the air supply system <NUM>.

According to a variant not shown, wherein the boiler is provided with a plurality of burner assemblies, the casing <NUM> may be dimensioned so as to supply several air ducts of different burner assemblies.

The air flow distributing element <NUM> has a substantially cylindrical structure and is centred on the axis A. Preferably, the air flow distributing element <NUM> is fixed to the end wall <NUM> of the casing <NUM> and extends axially until it surrounds the inlet <NUM> of the air duct <NUM>.

The air flow distributing element <NUM> is defined by a plurality of axial fins <NUM>, among which a plurality of air passages <NUM> are defined.

Further, the burner assembly <NUM> differs from the burner assembly <NUM> by a different aerodynamic conformation of the inlet element <NUM> of the air duct <NUM>.

The inlet element <NUM> comprises an inner annular face <NUM>, which defines an inlet portion <NUM> of the air duct <NUM> having a variable passage section along the forward direction D.

In particular, the inner annular face <NUM> of the inlet element <NUM> is shaped so as to define, along the forward direction D, a decreasing portion <NUM> having decreasing passage section, a constant portion <NUM> having constant passage section and an increasing portion <NUM> having increasing passage section.

Advantageously, the conformation of the inner annular face <NUM> results in a better air distribution on the outer parts of the air duct <NUM> thanks to a slight flow acceleration effect.

Although schematically shown in <FIG>, the burner assembly <NUM> comprises the fuel supply system <NUM> provided with the manifolds 34a, 34b and 34c as described and shown with reference to <FIG>.

Preferably, the manifolds 34a, 34b, and 34c are arranged externally to the casing <NUM>.

Advantageously, the structure of the burner assembly <NUM> allows the use of fuel conduits <NUM> that are substantially longitudinal and arranged parallel to the axis A. This allows the adjustment of the position of the nozzles <NUM> also from the outside of the burner assembly <NUM> by rotating the fuel conduits <NUM>. The end portions <NUM> of each fuel conduit <NUM> are, in this case, integral with the respective fuel conduit <NUM>.

It is understood that the features described above and highlighted as peculiar features of the burner assembly <NUM> can also be implemented individually in the burner assembly <NUM>.

The burner assembly <NUM>, <NUM> according to the present invention therefore allows the generation of a demixed type combustion (with MILD characteristics) with low NOx emissions and allows to manage a plurality of fuels with time-varying characteristics.

The main burner <NUM>, in fact, injects fuel into a low-oxygen combustion zone, which surrounds an oxygen-rich combustion zone (i.e. the zone that faces the outlet <NUM> of the air duct <NUM> and is supplied by the stabilization burner <NUM> and optionally by the liquid fuel lance <NUM>).

In this configuration, in essence, the comburent (air) coming from the air duct <NUM> and the fuel injected from the main burner <NUM> are substantially decoupled. This triggers an internal discharge gas recirculation (Flue Gas Recirculation) that dilutes the oxygen concentration. Due to the diluted conditions, the injection of fuel by the main burner <NUM> produces a combustion with significantly reduced temperature levels.

As a result, there is a drastic reduction in thermal NOx emissions.

Advantageously, the burner assembly <NUM>, <NUM> according to the present invention is able to operate with several different gaseous fuels at the same time, exclusively with high reactivity fuel flows (e.g. <NUM>% hydrogen) or even exclusively with low calorific value fuel flows.

Thanks to the structure of the burner assembly <NUM>, <NUM> in fact, the combustion of lean gases is supported by the presence of the outlet element <NUM> in refractory material, which acts as a thermal flywheel to release energy in the case of lean fuels and to absorb energy in the case of fuels with high energy content.

Claim 1:
Burner assembly for a boiler (<NUM>) for steam generation; the burner assembly (<NUM>; <NUM>) extending along a longitudinal axis (A) and comprising:
• an air duct (<NUM>) centred along the axis (A), into which, in use, air flows in a forward direction (D); the air duct (<NUM>) being provided with an outlet (<NUM>) discharging into a combustion chamber (<NUM>) of the boiler unit (<NUM>);
• a plurality of fuel conduits (<NUM>; 90a, 90b, 90c) into which, in use, at least one fuel flows in the forward direction (D);
• a plurality of fuel injection nozzles (<NUM>) arranged around the air duct (<NUM>); each fuel injection nozzle (<NUM>) being connected to an end portion (<NUM>) of a respective fuel conduit (<NUM>; 90a, 90b, 90c) for discharging, in use, fuel into the combustion chamber (<NUM>) ;
• the outlet (<NUM>) of the air duct (<NUM>) being arranged downstream of the fuel injection nozzles (<NUM>) along the forward direction (D); characterised in that
• the end portion (<NUM>) of at least one fuel conduit (<NUM>; 90a, 90b, 90c) is rotatable about its extension axis (B) to allow an adjustment of the injection direction.