"Industrial burner and related combustion process for heat treatment furnaces" Field of the invention The present invention relates to a high speed burner with lean and compact flame capable of obtaining very low NOx emissions while keeping the flame stable at any chamber temperature. The burner implements combustion techniques such as dividing the main combustion into three stages (air and fuel staging), recirculating flue gases and further diluting some reagents. State of the art Over the last years, the market demand concerning a reduction in gas consumption has forced industrial fumace manufacturers to develop their products by taking advantage of the heat of the combustion flue gases for heating the combustion air by means of exchangers. Therefore, the need has been created to develop burners which operate with comburent air pre-heated at a temperature of about 400-550''C. At the same time, the laws on nitrogen monoxide and dioxide emissions have established increasingly restrictive limitations and have forced manufacturers to make burners capable of falling within the limitations set. However, it is known that the formation of the NOx also increases according to the temperature of the combustion air. The higher the temperature of the air, the more the temperature of the flame increases thus generating the formation of themrial NOx. To keep the emissions low, low NOx burners use techniques for decreasing the temperature of the flame such as flame staging, flue gas dilution and flameless combustion. The burners which use the air staging technique operate by Injecting the air into two different zones at various distances from the burner and create two combustions, one with sub-stoichiometric ratio and the other with super-stoichiometric ratio and which completes the main combustion. Instead, the fuel staging operates by injecting the gas into two different zones at various distance from the burner thus generating two combustions with super- and sub-stoichiometric ratios. The object of both the techniques is the one of decreasing the flame temperature when the percentage of oxygen is high in combustion. However, the drawback of the burners which use these techniques Is the cold instability, i.e. below the spontaneous ignition temperature as the two combustions are fed by unbalanced reagents, one with air in excess and the other with gas in excess. The way to cold stabilize the flame Is to increase the excess air of the main combustion so as the primary combustion does not have gas in excess with the consequent increase of the NOx (there is no more sub-stoichiometric combustion) and the consumption of gas. Once the temperature of spontaneous ignition of the fuel In the chamber is exceeded, the bumer no longer requires the excess air and operates in stoichiometric ratio with low NOx emissions. The burners which use flameless combustion have significantly low NOx emissions but have the significant limitation of not being able to operate in flameless mode below the temperature of spontaneous Ignition of the fuel. Therefore, even to heat the chamber with the same bumer, these burners should be able to operate in traditional mode (non-flameless), and consequently are to be equipped with some valves for modulating the gas or the air inlet according to the type of operation. The burners which operate v\^th the mentioned combustion techniques have significant advantages when the chamber temperature is high but have significant limitations when the temperature is below or about the temperature of spontaneous ignition of the fuel. In specific types of process, such as in furnaces for heat treatments of the load, the zone temperatures are relatively low and vary significantly according to the type of treatment. Under these conditions, the burners with staged combustion and the burners with flameless combustion are not capable of operating constantly in low NOx mode but should vary their excess air or the operating mode (flameless -non flameless) according to the chamber temperature, respectively. The result is that the zone adjustment valves and the valves which control the injection of the fuel (for flameless bumers) operate continuously during the operation of the furnace. Heat treatment furnaces heat the load in free atmosphere by convection by means of the high speed of the flue gases which sweep it and not by means of radiation as occurs for reheating furnaces. Thus, there is a need in this type of appiication for burners which are capabie of deveioping a high speed, lean and compact fiame and not a diffused flame or with voluminous combustion. Thus, the need is felt to malte a burner and a related combustion process which allows the aforesaid drawbaci(s to be overcome. Summarv of the invention It is the primary object of the present invention to make a burner which allows very low NOx emissions to be obtained both in the ignition phase, that is cold, and in the operation at speed thereof and which is also capable of obtaining a combustion compatible with the process of the heat treatment fumaces of the load in free atmosphere. It is another object of the invention to obtain a related combustion process for heat treatment fumaces in free atmosphere which always allows very low NOx emissions to be obtained. Thus, the present invention proposes to achieve the above-discussed objects by malting a low NOx emission industrial burner which, according to claim 1, comprises: - a hollow body defining a longitudinal axis X; - a tubular element arranged within said hollow body at one open end thereof; - a diffuser arranged within said tubular element at a first end thereof inside the hollow body, so as to define a volume within said hollow body; - a pipe for feeding comburent air into said volume; - a combustible gas Injection lance, an-anged within said hollow body and connected to said diffuser; in which said diffuser is provided with - first holes for injecting a first portion of combustible gas from said injection lance, into a first combustion zone provided within said tubular element and adjacent to said diffuser; - second holes for injecting a first portion of comburent air into said first combustion zone; - third holes for injecting a second portion of comburent air into a second combustion zone, provided inside said tubular element, downstream of the first combustion zone and communicating witli the latter, said second portion of comburent air crossing said first combustion zone; - a tube for injecting a second portion of combustible gas, from said injection lance, into a third combustion zone outside said tubular element, provided downstream of the second combustion zone and communicating with the latter, said second portion of combustible gas crossing said second combustion zone; the burner bdng also provided with a longitudinal passage, defined between the outer profile of said tubular element and the inner profile of said open end of the hollow body, for injecting a third portion of comburent air into said third combustion zone. A second aspect of the present invention provides a low NOx emission combustion process, obtainable by means of the aforesaid burner and a furnace with which said burner cooperates by defining a combustion chamber, the process sequentially comprising the following combustion steps, according to claim 11: - a primary combustion which occurs by mixing, in the first combustion zone, a first portion of comburent air injected through the second holes with a first portion of combustible gas injected through the first holes in a first, in essence stoichiometric air/fuel ratio Ai, thus obtaining primary combustion products and primary unburnt gas; - a secondary combustion which occurs by mixing, in the second combustion zone, a second portion of comburent air injected through the third holes with said primary combustion products and said primary unburnt gas with air in excess, so as to obtain a second super-stoichiometric air/fuel ratio A2, thus obtaining secondary combustion products and secondary unbumt air; - a tertiary combustion which occurs by mixing, in the third combustion zone, a second portion of combustible gas injected through the tube with a third portion of comburent air injected through the longitudinal passage, with said primary and secondary combustion products and said secondary unburnt air with gas in excess, so as to obtain a third sub-stoichiometric air/fuel ratio A3, such as to complete the total combustion within the combustion chamber of the furnace. The low NOx emission burner object of the present invention advantageously generates a high speed and compact flame and bases its principles on the staged combustion of air and gas. The combustion technique used is called "fuel staging" combined with recirculating flue gases and diluting the flame. The combustion air or comburent air and the combustible gas are injected into different zones of the combustion chamber through a combustion head or diffuser so as to obtain various combustions in which the combination is controlled and restricted of the high percentage of oxygen in the combustion and the high flame temperature, caused by the generation of the NOx of thermal origin. Advantageously, the burner provides three main combustions In series, called primary combustion, secondary combustion and tertiary combustion, respectively. These three main combustions are carried out at respective different distances from the combustion head and with respective different combustion ratios: the primary combustion provides a combustion ratio (air/fuel) near the stoichiometric; the secondary combustion is achieved by means of a combustion ratio with significant air In excess so as to achieve a super-stoichiometric air/fuel riatio; the tertiary combustion has a combustion ratio with gas in excess so as to achieve a sub-stoichiometric alr/fuel ratio such as to complete the total combustion. Furthermore, the burner of the invention provides that the primary combustion is broken down into primary sub-combustions in which the combustion ratio is kept constant but for which the primary combustible gas and the primary comburent air are injected and mixed at different distances from the combustion head. Thereby, although the combustion ratio is kept unaltered, the flame is further diluted with particularly advantageous effects for reducing the formation of the NOx. Due to the geometry of the combustion head or diffuser and to the configuration of the holes for injecting the secondary air (partially comburent in the secondary combustion and partially in the tertiary combustion) and the tertiary air (comburent in the tertiary combustion) the flame generated by the burner is particularly lean and compact. The holes of the secondary air are advantageously configured so as to create a "swirled" and compact flame. A particular advantage for the type of process to be achieved is that the burner does not require differentiating the combustion technique according to the temperature of the combustion chamber to reduce the NOx emissions. Indeed, the burner is made to operate at temperatures about the temperature of'spontaneous ignition of the fuel, hence it is impossible to use combustion techniques which have as their main prerogative a chamber temperature always higher than the one of spontaneous ignition of the combustible (flameless combustion). Advantageously, for the s£ime reason the burner of the invention does not require a double inlet for the comburent air or for the combustible gas. Moreover, as Is known, most flameless burners on the market require a higher pressure of the combusttWe gas wrth v^ped to the owe of tvadWonat buTvvers who the burner of the invention succeeds in obtaining excellent perfonnances with the same air and gas pressures of a traditional burner. The fuel and the combustion air are injected into the combustion chamber at such a speed whereby a high recirculation of the bumt gases is obtained. In particular, the speed of the tertiary air and of the secondary gas (fuel in the tertiary combustion) are crucial for obtaining a strong mixing with the bumt gases and consequently decreasing the percentage of oxygen in the combustion. The main innovation of the burner of the invention concerns the fact that by simultaneously using the "fuel staging", heavily diluting the flame and recirculating the flue gases, the NOx emission are always very low at any chamber temperature and the burner succeeds in entering an operating range in which the flameless combustion Is automatically generated. Advantageously, the burner of the Invention, conceived for heat treatment furnaces of load in free atmosphere may operate both with significant air in excess and in stoichiometric ratio regardless of the chamber temperature. In particular, when the chamber temperature is above SOCC, by decreasing the excess air up to A<1,1, the burner automatically enters an operating range in which the recirculation of the flue gases (Kv=4) and the low percentage of oxygen in the combustion is such as to make the combustion with invisible flame (Fig. 6). It is preferable to control the burner in on/off mode to constantly have a high recirculating factor of the flue gases Kv, and accordingly to keep the air and gas injection speeds high; i.e. operate the burner according to the power requirement of the heating of the furnace, at maximum power or by keeping it off. In any event, the burner is also capable of operating correctly when it is proportionately controlled by increasing or decreasing the power according to the process requirement. Considering that the burner of the invention operates in a range of temperatures spanning the emperature of spontaneous ignition of the fuel and with a control in on/off mode, it Is provided with ignition and flame detection instruments to pemiit the execution of the Ignition and the shutdown cycles in complete safety. In particular, the combustion head is prepared with two holes for accommodating an \9n\\\on device, such as an ignition eiectrode or piiol burner, and a flame detection device, such as a UV cell or a flame detection electrode, respectively. Considering the importance of the stability of the burner upon igniUon and when the flame Is triggered, by means of the ignition device, the primary combustion has a combustion ratio such that the flame has a very anchored root and has a particularly strong emission spectrum, to be detected by the flame detection device at any chamber temperature, and not very sensitive to the variation of the combustion ratio. Advantageously, the burner of the invention does not require significant excess air (with combustion ratio A>1,5), at chamber temperatures below the one of spontaneous Ignition of the fuel, as occurs for most burners which use staged combustion, but right from the cold ignition is capable of operating with a combustion ratio dose to the stoichiometric ratio, thus l