The hyper-stoichiometric mixture makes possible to reduce the temperature of the flames thus reducing the quantity of NOx formed during combustion.

The burners fed with an hyper-stoichiometric mixture involve the problem of flame stability because the flame tends to be extinguished intermittently and, as a consequence, to get detached from the surface of the burner, which causes an unsatisfactory operation of the burner, emission of unburned substances and high quantity of CO in the fumes produced by the combustion.

The higher the ratio between air and fuel in the air-fuel mixture burnt in the burner is, the higher the instability of the flame is.

This problem may be made worse by the presence of air or nitrogen in the gas fuel feeding the burner and by pressure and moisture conditions of the environment wherefrom air, which is mixed with the gas fuel, is taken.

According to the state of art, one has tried to solve this problem by distributing through parallel rows on the burner diffuser discharge openings, shaped as slots, for the air-fuel mixture, said rows, continuous or non-continuous, being separated from each other by a diffuser portion devoid of slots. A vacuum and a temperature higher than the temperature of the other parts of the diffuser are generated during combustion in said diffuser portion, which oppose flame extinction and consequent detachment of the flame from the surface of the burner.

However, the secondary air which laps the flames tends to cool them and to dilute further the mixture of air and gas-fuel flowing through the slots, with the consequence that the instability of the flame takes nevertheless place when the aeration rate is high and a reduction of the content of NOx in the combustion products over a certain limit can not be achieved.

It is an object of the present invention to obtain a burner having a very low content of NOx in the combustion products, whilst the flame is kept stable.

The invention solves the above mentioned technical problem by adopting a burner fed with a hyper-stoichiometric mixture comprising a diffuser on which parallel continuous, or non-continuous, rows of openings shaped as slots are made, said rows being separated from each other by an intermediate portion of diffuser having a substantially constant width, characterised in that said intermediate portion is provided with a plurality of further openings for the flow of the mixture of air and gas-fuel. Said further openings may be arranged through one or more continuous or non-continuous rows parallel to the median line of said intermediate portion of diffuser. Said further openings may be shaped as further slots perpendicular to, or anyhow inclined with respect to, said slots. Said further openings may be arranged parallel to, or even inclined with respect to, the median line of said intermediate portion of diffuser, and may be rectilinear or curvilinear.

Said embodiment has the advantage of generating, in said intermediate portion of diffuser, auxiliary flames between the main flames generated by two adjacent rows of slots, said auxiliary flames heating those portions of the main flames facing towards said intermediate portion, thus preventing the extinction of the main flames and their instability.

In an advantageous embodiment of the present invention, said further openings are so shaped that the speed of the mixture of air and gas-fuel flowing through them is lower than the speed of the mixture flowing through said slots.

Said embodiment has the advantage that very stable auxiliary flames are generated, which act as anchor means for the main flames.

In a further advantageous embodiment of the present invention, the rate of flow of the mixture of air and gas-fuei flowing through said further openings of said intermediate portion of diffuser is lower than the rate of flow of the mixture flowing through said slots: this may be obtained by means of a whole surface of said further openings substantially lower than the whole surface of said slots, or by so shaping the further openings that the mixture is subject to heavy energy losses flowing through them.

Said embodiment has the advantage that the mixture of air and gas-fuel flowing through said further openings of said intermediate portion of diffuser does not affect significantly the structure of the flame formed on said rows of slots.

In a further advantageous embodiment of the present invention said intermediate portion of diffuser comprises a heat-resistant material which may reach, without being damaged, a temperature higher than the remaining portions of the diffuser.

Said embodiment has the advantage that the mixture of air and gas-fuel flowing through said apertures is heated by the heat stored in said intermediate portion of diffuser, which increases the combustion speed of the mixture allowing extremely stable auxiliary flames to be obtained, said auxiliary flames acting as anchor means for the main flames.

In a further advantageous embodiment of the present invention, said heat- resistant material has a porous or spongy structure, which allows said mixture of air and gas-fuel to pass through it; said material having a porous or spongy structure may be a ceramic material.

Said embodiment has the advantage that the mixture of air and gas-fuel flowing through said further apertures is heated and its speed is the same time reduced.

In a further advantageous embodiment, said material having a porous or spongy structure is accommodated in a housing provided in said intermediate portion of diffuser. said embodiment has the advantage to make possible to replace said material quickly and easy, if necessary.

In a further advantageous embodiment, said heat-resistant material has a reticular structure.

Said embodiment has the advantage the the thermal deformations to which said material is subjected do not cause transmission of considerable mechanical stresses to the other parts of the burner.

The invention will be now described in detail referring to the attached drawings, illustrating some embodiments of the invention, in which: - figure 1 is a top view of a burner according to the state of art, fed with a hyper-stoichiometric mixture of air and gas-fuel and provided with two continuous parallel rows of slots; - figure 2 is a section through a line ll-ll of figure 1; - figure 3 is a view as in figure 1, but showing a burner according to the invention provided with a row of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 4 is a section through a line IV-IV of figure 3; - figure 5 is a view as in figure 4, but with two rows of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 6 is a view as in figure 4, but with three rows of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 7 is a view of a burner according to the invention, provided with four rows of slots, separated from each other by respective intermediate portions of diffuser each provided with a row of further openings; - figure 8 is a view of a burner according to the invention, provided with two non-continuous rows of slots, with a non-continuous row of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 9 is an interrupted view of a burner according to the invention provided with two rows of inclined slots and a row of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 10 is a view of a burner as in figure 9, provided with two rows of slots and a row of further openings in the intermediate region between the two rows of slots, said further openings being shaped as slots inclined in the same direction - figure 11 is a view as in figure 10, but with the further openings shaped as slots alternately inclined in opposite directions; - figure 12 is a view of a burner according to the invention provided with two non-continuous and staggered rows of slots and two continuous rows of further openings in the intermediate portion of diffuser between the two rows of slots; - figure 13 is an interrupted view of a burner according to the invention provided with two parallel rows of slots separated by an intermediate portion of diffuser wherein a housing is made for an insert of a porous or spongy heat- resistant material; - figure 14 is a section though a line XIV-XIV of figure 13; - figure 15 is a section as in figure 14, but showing a different shape of the housing for the insert of porous or spongy material, made in said intermediate portion of diffuser.

With reference to figures 1 to 14, 1 shows the diffuser of a burner provided on its surface with two continuous parallel rows 2 and 2a of openings 3 shaped as slots, through which a mixture of air and gas-fuel flows, said mixture generating a pair of flames, for instance bladed flames. The rows 2 and 2a are separated from each other by an intermediate portion 10 of diffuser provided with further openings 5, shaped as slots (figure 2), said further openings 5 being arranged in a continuous row 4. The further openings are arranged parallel to a median line of said intermediate region and are substantially perpendicular to the slots 3 of said parallel rows 2, 2a. The mixture of air and gas-fuel exiting from said further openings 5 generates an auxiliary flame 22 which anchors the flames 21 on the surface of the diffuser, thus ensuring the stability of said flames even when the aeration rate of the mixture is very high.

Figure 5 shows a diffuser 1 of a burner having two rows 6 of further openings 5 shaped as slots provided in the intermediate portion 10 of diffuser, said openings being parallel to a median line of said intermediate portion 10 of diffuser and substantially perpendicular to the slots 3.

Figure 6 shows a diffuser 1 of a burner having three rows 7 of further openings 5 shaped as slots provided in the intermediate portion 10 of diffuser, said further openings 5 being parallel to a median line of said intermediate portion 10 of diffuser and substantially perpendicular to the slots 3.

Figures 7 shows a diffuser 1 of a burner provided with four continuous rows 2, 2a, 2b, 2c, 2d of slots 3, parallel to each other. Adjacent rows are separated from each other by respective intermediate portions 10, 10a, 1 0b of diffuser, each of which is provided with a respective continuous row 4, 4a, 4b of said further openings 5.

Figure 8 shows a diffuser 1 of a burner provided with two non-continuous rows 8, 8a of slots 3, separated by said intermediate portion 10 of diffuser, which is provided with a non-continuous row 9 of further openings 5, shaped as slots.

The slots 3 are grouped in groups 24 each of which comprises two parallel rows of slots. An end slot 11 of each group may have dimensions greater than the dimensions of the slots 3 and extend through the whole combined width of the two rows of slots and intermediate portion 10 of diffuser.

Figure 9 shows a diffuser 1 of a burner similar to that of figure 3, wherein the rows 12, 12a of slots consist of slots 13 which are non-perpendicular to the further slots 5.

Figure 10 shows a diffuser 1 of a burner similar to that of figure 3, wherein said intermediate portion 10 of diffuser is provided with a row 15 of further openings 14 shaped as slots, all of them being inclined in the same direction with respect to a median line of the intermediate portion 10 of diffuser.

Figure 11 shows a diffuser 1 of a burner similar to that of figure 3, wherein said intermediate portion 10 of diffuser is provided with a row 16 of further openings 17 shaped as slots and alternately inclined in opposite directions with respect to a median line of said intermediate portion 10 of diffuser.

Figure 12 shows a diffuser 1 of a burner provided with two non-continuous, staggered and parallel rows 25, 25a of slots 3, separated by said intermediate portion 10 of diffuser, in which two rows 6 of further openings 5 shaped as slots and parallel to a median line of the intermediate portion 10 of diffuser are provided.

Figures 13 to 15 show a diffuser 1 of a burner provided with two parallel, continuous rows 2, 2a of slots 3, separated by said intermediate portion 10 of diffuser, in which a housing 19, 23 is provided inside which an insert 20 of porous or spongy heat-resistant material, for instance a ceramic material, may be inserted. The bottom of the housing 19, 23 is provided with passage openings 18 for the mixture of air and gas-fuel, which flows through said openings 18 and passes through the porous or spongy material of the insert 20. In addition. the insert 20 may be made of a material having a reticular structure.

The intermediate portion 10 itself may be made of a heat-resistant material, for instance a ceramic material, which may have a reticular structure.

The use of said heat-resistant material makes possible to heat the mixture of air and gas fuel mixture flowing through the further openings made in said intermediate portion 10 of diffuser, which increases the combustion speed of the mixture allowing an auxiliary flame 22 extremely stable to be obtained, which anchor the main flames 21.

The use of a porous or spongy material makes possible to reduce the speed of the mixture of air and gas-fuei, which passes through said material flowing through the passage openings 18 and generates the auxiliary flames 22, thus preventing the structure of the main flames 21 from being significantly affected.