A boiler arrangement and a method of burning oil Field of invention This invention relates to a method of burning oil by atomising it and burning it with air and recirculated flue gas. It relates also to a boiler arrangement Background of the invention and prior art The flue gas from conventional oil burners is normally very clean today as to the amount of carbon monoxide, hydrocarbons, and flue dust. The content of nitrogen oxides (NOX or NOX), however, is usually still high. GB-B 1 464 169 describes a method for reducing air pollution agents in the flue gas. Particularly, a method is described that reduces the amount of the nitrogen oxides by recirculating flue gas. A small amount of the air is mixed with recirculated flue gas but most of the combustion air is added at various points without being mixed with flue gas. This method makes the apparatus very complicated and probably also difficult to adjust.

GB-A 2 259 567 describes a method for reducing the amount of nitrogen oxides in a natural gas burner by recirculating flue gas and mixing it with the combustion air before ejecting it in the burner. The flue gas is additionally heated before the mixing.

Object of invention It is an object of the invention to reduce the nitrogen oxides in a simple and safe way that can be applied to a simple conventional oil burner.

Brief description of the invention The recirculated flue gas and the air are mixed to a combustion air that comprises at least 80% of the total mass flow of combustion air. The flue gas is cooled before being mixed with the air. The cooling doubles the effect of the flue gas recirculation. Preferably, the flue gas is sucked into a mixing chamber for flue gas and air. This mixing chamber can be an inlet chamber in a fan housing so that the air and flue gas mix in the fan.

Brief description of the drawings Figure 1 is a side view of a boiler with an oil burner.

Figure 2 is a fragmentary front view seen as indicated by the arrows 2-2 in figure 1 and partly in section.

Figure 3 is a section through a somewhat modified burner.

Figure 4 shows enlarged a detail from fig 3.

Figure 5 is a section through another modified burner.

Detailed description of preferred embodiments Figure 1 shows a boiler 11 with a conventional oil burner ! 2. A conduit 13 lead from the stack 14 to the burner 12 and there is a cooler 16 in the conduit 13 for cooling the flue gas that is recirculated from the stack to the burner 12. There is also a valve 17 in the conduit 13, by which the flow of recirculated flue gas can be adjusted.

Figure 2 shows the burner 12 on a larger scale. It comprises an electric motor 20 which has a shaft 21 that rotates a rotor 22 of a fan and the rotor of an oil pump 23. The fan housing has been denoted by 31. The conduit 13 leads to the interior of the fan rotor 22 and ends with a funnel 24. The inlet for atmospheric air is through a housing 25 with a non-illustrated valve by which the inlet area for air can be adjusted. Since the air inlet is restricted, there will be a reduced pressure inside the fan rotor which sucks flue gas through the conduit 13 so that air and a minor amount of flue gas will mix in the fan and form the combustion air. The amount of flue gas can be adjusted by the valve 17. Normally, there should be 10-20 % flue gas by weight in the combustion gas. A secondary burning may be arranged, but at least 80% of the total of the atmospheric air should be supplied as this primary combustion air.

Figure 3 shows the burner in a section and it shows the atomising nozzle 27 and the high- pressure tube 28 that lead from the oil pump 23 to the nozzle 27. At the very front end of the burner, there is also a turbulator 29 that gives the combustion air a swirling motion. In this figure, the flue gas inlet has been modified from the one in figures 1 and 2, and it is indicated by numeral 30 in the figure. The modified inlet 30 is shown on a larger scale in figure 4. The conduit 13 leads to holes 32, 33 in the fan housing 31 and vanes 34,35 make an ejector that draws flue gas into the fan housing.

Fig 4 shows still another way of drawing the flue gas through the conduit 13 and into the burner. On the pressure side of the fan, there is an annular ejector 36. The ejector 36 has an annular inlet chamber 37 with an inlet nipple 38 to which the conduit 13 is to be coupled. In this modified design, the tube 39 form a mixing chamber for the atmospheric air and the flue gas. In the designs according to figure 2 and 3, the fan housing 31 forms the mixing chamber.

Normally, about 15 % of the flue gas is recirculated and the recirculated gas should be cooled in the cooler 16 to as low a temperature as possible since the emission of nitrogen oxides will be lower the lower the temperature is. The temperature should preferably not exceed 50 °C.

A normal temperature of 30-40 °C can be reached when a non-expensive and simple cooler is used.