The environmental impact of oxides of nitrogen formed with high temperature combustion has limited the exploitation of such energy efficient combustion technology.

It is an object of the invention to provide a burner which causes little or no emission of NOx, even with compact regenerative combustion.

The invention is based on the realisation that by drawing waste gases into the supply of fuel before staged mixing there will be a reduction in NOx formation.

According to the invention in one aspect, there is provided a method of burning combustible fuel so as to cause low emissions of NOx, the method comprising: creating a high velocity stream of combustible fuel and passing it from a small diameter nozzle facing the inlet of a passageway leading to a furnace; * combining the emergent stream with waste gas supplied from the furnace and passing the combined stream into the passageway which is of increased diameter whereby the combined stream expands in volume; * directing the expanded stream through the main port of a hot baffle plate in an inlet of the furnace, the baffle plate having subsidiary ports adjacent the perimeter of the baffle plate; * intersecting the expanded stream in the region of the baffle plate with a tangential flow of combustion air in order to stage the air supply and create a rotary disturbance in the mixture prior to passing through the main port to join the remainder of combustion air and waste gases in the furnace to complete the combustion.

In another aspect of the invention there is provided a burner accessory for a furnace having an inlet containing a baffle plate having a main port and subsidiary ports which are located near the perimeter of the baffle plate, the accessory comprising a housing containing a small diameter nozzle arranged to direct a high velocity air, fuel and waste gas stream towards the inlet of a passageway having a wider diameter portion leading to the baffle plate; means for supplying fuel to the nozzle to provide the high velocity stream by utilising the delivery pressure of the fuel; means for internally directing waste gases from the furnace to combine the high velocity stream; means defining a physical barrier between the combustion air and the combined and expanded stream; means for passing combustion air to the baffle plate via a liner containing the main passageway.

Preferably the dilution of the combustion air takes place by the induced principle within the furnace chamber, to maintain the minimum burner dimensions.

It is preferred to have metered air injection points. By incorporating metered air injection points any low inherent cold instability is overcome without jeopardising the NOx reduction at working temperature.

Preferably other parameters are controlled to reduce NOx and these include peak flame temperature, residence time at peak temperature and oxygen concentration within particular reaction zones.

Preferably the burner incorporates a quarl shaped to retain sufficient flame for UV scanning whilst also creating a high velocity discharge of reacting mixtures. The optimisation of both load penetration for melting and chamber circulation for product uniformity are maintained with this controlled discharge. The flameless reaction is preferably controlled to provide a uniform heat flux which, when complemented with the desired velocity contours around the load, allows for uniform temperature control.

It is an advantage that the initial generation of the fuel rich mixture ensures stable combustion over the entire burner operating range. Preferably, the air flow is less than 10% of the flow required for stoichiometric combustion of the fuel at maximum flow.

The waste gas flow is preferably facilitated by either a venturi or jet pump effect via the fuel delivery velocity, or by an external delivery system, and is assisted by a low volume, high velocity air jet stream.

The tangential staged supply of combustion air allows for more stable combustion at low operating temperatures, i. e. below 800°C, in addition to swirling the outer layers of the air/fuel/waste gas mixture at higher temperatures. (The volume flow rate of this tangential air flow may be less than 20% of the air required for stoichiometric combustion.) The cold air inlet allows for up to 20% of the air required for combustion to be supplied when below 800°C, at the position normally taken up by the induced waste gas flow when at higher temperatures. The combustion air may be supplied pressurised by a fan system and can be either cold or preheated to temperatures in excess of 1000°C.

In order that the invention may be well understood it will now be described with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a sectional view showing an accessory mounted on a side of a furnace; Figure 2 is a view of area ringed A in Figure 1, drawn to an enlarged scale showing the flow of waste gas by short arrows and the flow of fuel by large arrows; Figure 3 is a view showing the flow of fuel, from the nozzle into the passage and waste gases being drawn into the fuel stream; Figure 4A is a view showing the flow of the fuel and waste gas mixture to the baffle plate and Figure 4B is a sectional view taken on lines AA-AA on Figure 4A detailing the rotary disturbance created by the staging of the combustion air supply; Figure 5 is an elevation of the baffle plate hot face and fuel lance, illustrating localised visible combustion; Figure 6 shows the inside of the furnace downstream of the baffle plate showing the flows of combustion air, waste gas, and the combustible mixture with illustration of visible and flameless combustion.

The burner shown in Figure 1 comprises a burner body 1 connected to an inlet 2 in the quarl 3 of a furnace wall 4 in sealed manner. The inlet 2 contains a circular baffle plate 5. As shown in Figure 5, the plate has a central relatively large port 6, and a plurality of radially spaced apart smaller ports 7 which are near the perimeter.

The body 1 comprises an outer casing 10 having an end wall 11. The casing houses a cylindrical refractory liner 12 the front end 13 of which sits on the baffle plate 5 in the region between the main centre port 6 and the row of outer subsidiary ports 7. The rear end of the liner is a wider diameter wall 14 which is spaced from the inner face of the end wall 11 to form a chamber 15.

The liner has an inlet port 16 which flares at 17 into a wider chamber 18. A port 19 in the side wall of the casing 10 is arranged to supply combustion air to the small ports 7 in the baffle plate. Two inlets are present in the end wall, one 21 being a cold start air supply inlet. A fuel lance 23 is mounted in an opening 22, and has a nose 24 which is frustoconical and projects in to the inlet port 16 of the liner 12 with a clearance 25 in between. The diameter of the nozzle is selected to discharge the fuel at a high, preferably the maximum, firing rate. A fuel supply inlet 26 is present in a side of the lance 23, and an air jet device 27 extends through the nozzle to stop short of the outlet end 20 of the fuel lance 23.

Passageway 28 is present in the wall 4 of the furnace and/or burner quarl 3 adjacent to the inlet 2, and leads to one or more passageways 29 which enter the chamber 15 formed by the end wall 11 of the casing and the facing rear wall 14 of the liner 12.

In use, fuel is supplied to the burner via the lance 23 and is forced through the single small diameter nozzle 20 to create a high velocity fuel stream, say about 80 to about 110 m/s, to induce a suction around the nozzle in the region 15. Air introduced via the air jet 27 at about 80 to about 120 m/s and maintains suction and initiates a combustible mixture at low flow rates, say about 10 to 13% of the maximum rating. The suction draws waste gases via passageways 29 into the fuel stream at the junction between the nozzle and burner liner 12, i. e. in the area 16. As a result high velocity fuel rich mixture is created at the entry to the burner by an air jet flowing a low volume, high velocity stream concentric. If a liquid fuel is used the air jet is mixed with the fuel flow. The flow of waste gas is taken from the furnace and introduced into the high velocity fuel rich stream to form a mixture of fuel, waste gas and air.

The gas stream flows forward axially of the burner and now comprising of fuel, air and waste gases is now allowed to expand within the area 18 of the liner 12 and intersect with a tangential flow of combustion air 9 (from the inlet 19) close to the exit into the furnace chamber near the baffle plate 5. The combustible mixture now further mixes on the perimeter of this stream with staged combustion air 8, on the upstream side of baffle plate 5, rotating the outer section of the stream 9. The baffle air will flow at up to about 40 m/s for cold air and up to about 300 m/s for heated air. In other words the partially reacting air/fuel/waste gas mixture is directed through an aperture in an air distribution baffle where the balance of the air required for complete combustion is delivered at high velocity. The remaining combustible mixture enters the furnace chamber at the burner baffle plate hot face and there expands and further proceeds to visibly burn local to the baffle air combustion streams 7. The resulting mixing of the fuel mixture with the balance of combustion air enables combustion to be completed within the confines of the retention quarl and furnace volume in an essentially flameless manner.

The velocity of the air streams exiting the baffle plate 5 induce waste gases into and so diluting the combustion air. Complete combustion does not occur until an equilibrium of velocities and mixture is achieved some way into the furnace chamber when flameless combustion 30 (Figure 6) takes place.

It can be seen that the invention provides an industrial burner and quarl assembly which generates reduced levels of oxides of nitrogen (NOx) emissions by premixing fuel and internally recirculated waste gases drawn directly from the furnace chamber to within the confines of the burner body prior to staged mixing with the air required for combustion. Dilution of fuel by waste gases is achieved within the confines of the burner body by venturi induction using a fuel lance. Both flame stability and the induction of waste gas diluent into the fuel stream are assisted at low fire rates by an air jet or jets. The burner quarl is designed to maintain a stable flame pattern and to develop a high velocity discharge of the reacting mixture.

The fuel may be natural gas, liquid petroleum gas or a combustible gaseous industrial by-product. The fuel may also be a liquid or atomised liquid such as fuel oils.

The combustion air may be cold or pre-heated. Where appropriate, the combustion air is pre-heated in a compact regenerator directly attached to the burner body.