Liquid fuel burner and atomizer having low emissions of NOx...

Details Liquid fuel burner and atomizer having low emissions of NOx... Liquid fuel burner and atomizer having low emissions of NOx...

2000-05-31

2002-07-30

Bennett, Henry (Department: 3743)

Combustion

Fuel disperser installed in furnace

C431S187000, C431S182000, C431S154000, C239S554000, C239S596000, C239S600000

Reexamination Certificate

active

06425755

ABSTRACT:

The present invention relates to liquid fuel burners having low emission of NO
x
and of dust, and also to liquid atomizers (or spray nozzles) for such burners.
FIELD OF THE INVENTION
The technical field of the invention is that of manufacturing industrial burners of liquid fuel (in particular fuel-oil).
BACKGROUND OF THE INVENTION
Recent developments in liquid fuel burners have sought to reduce polluting emissions (un-burned solids containing carbon, CO, NO
x
, O
2
involved in forming SO
3
), while maintaining the operating performance of boilers, hot gas generators, and incinerators, and in particular while maintaining the radiation from flames in the hearth, the dimensions of hearths (and thus flame dimensions), the ability to accommodate possible variations in load, and the absence of deposits of coke or scale on the tubes of the hearth, on the stabilizer of the burner, or on its refractory door.
A technique known as flue gas recirculation (FGR) consists in recirculating (or “recycling”) flue gases having a low O
2
content in the combustion air of burners, with the effects firstly of reducing the concentration of O
2
in the oxygen carrier so as to decrease “combustible” NO
x
, and secondly of reducing the combustion temperature so as to reduce “thermal” NO
x
that is formed by oxidizing the nitrogen in the oxygen carrier (where such formation is highly temperature sensitive). That technique has its limits: in particular it slows down the combustion process and consequently often leads to an unacceptable increase in flame length, and also to a large increase in the emission of unburned carbon-containing solids, and also of CO and of unburned hydrocarbons.
A technique sometimes known as over firing air (OFA) ports consists in spreading out combustion as a whole by causing the burners to operate with too little air, and then adding air via ports further downstream in the flue gas circuit. That technique also has its limits: the quality of the mixing obtained between air and the combustion gas with such a method is poor, given that the air injected through the walls of the hearth cannot mix homogenously throughout the entire combustion gas stream, and as a result performance is unsatisfactory in terms of unburned solids and gases since mixing is poorly organized. It would be preferable to organize such mixing at each injection of fuel, i.e. at each burner.
Finally, a technique known as “reburning” consists in injecting fuel, often a gaseous fuel, at the end of the hearth to reduce NO
x
to N, and then to add combustion air for that fuel at a subsequent stage. That technique is expensive and complicated and in any event gives a better final result when the NO
x
has already been kept to a minimum at each burner.
Consequently, it is still necessary, even when using the FGR technique, OFA port spreading, or reburning, to keep down emissions of NO
x
while maintaining emissions of unburned solids or gases and of O
2
that are as low as possible at each burner. It is advantageous to keep down pollutant emission from the beginning of combustion, i.e. at the nozzle-carrying heads even if additional methods are also used for reducing emissions overall.
Studies performed on reducing NO
x
at burners (without significantly increasing unburned content) have shown that it is advantageous to cause flames to split up into a plurality of individual flames so as to cause NO
x
emission to drop by various effects as described in EP 0 435 735 (IFP-Pillard), which corresponds to U.S. Pat. No. 5,169,304.
The advantage of increasing flame separation while shortening each individual flame by staged injection of air directed exactly on each individual flame has been demonstrated, since it makes it possible to maintain flame diameters and lengths unchanged and to further improve performance in terms of unburned content and in reduction of NO
x
, as described in patent U.S. Pat. No. 5,562,437 (Pillard).
Those improvements do not eliminate the need to improve atomizing (or spraying) of liquid fuels by using an auxiliary atomizing fluid (saturated or superheated steam, compressed air, compressed gas).
To this end, patent FR 2 641 365 proposes a burner having a primary spray piece which defines an axial mixing chamber into which the liquid fuel is introduced axially and into which an auxiliary fluid under pressure is introduced radially or tangentially. The chamber has primary spray nozzles each having an outlet orifice from which a jet of fuel spray issues. The burner also has a secondary spray piece which surrounds the primary spray piece and co-operates therewith to define an auxiliary chamber. The secondary spray piece carries a plurality of secondary nozzles, each of which is coaxial with a primary nozzle and has a diameter greater than the outlet orifice of the primary nozzle. The burner has means for injecting an auxiliary fluid radially into the auxiliary chamber at a pressure which is sufficient to enable it to expand in the secondary nozzle around the jets issuing from the primary nozzles, and to do so at an axial speed that is greater than the speed of the peripheral layers of the sprayed fuel jets.
In addition to its primary and secondary nozzle-carrying pieces, the burner head (or end) described in that document also includes a plurality of removable pieces disposed between the nozzle-carrying pieces and the burner body. An emulsifier defines a mixing chamber for the fuel and the auxiliary fluid and has passages (for the auxiliary fluid) fitted with fins, and also has a diaphragm (or pellet) fitted with a calibrated central orifice for introducing the fuel into the mixing chamber, and a fuel guide piece (sometimes referred to as an “atomizer”) upstream from the diaphragm. Those various pieces are generally in the form of thick disks and they are “stacked” face to face and secured to the burner head by a cap which surrounds them, which is screwed onto the burner head, and which has a shoulder bearing against the periphery of the nozzle-carrying spray piece.
OBJECTS AND BRIEF SUMMARY OF THE INVENTION
An object of the present invention is to propose improved burners, in particular burners of the kind described in patent FR 2 766 557 (Pillard).
The invention seeks to simplify the implementation of liquid fuel atomizers, and in particular those described in patents FR 1 305 391 and FR 2 641 365, while leaving their functions unchanged and providing performance that is at least as good.
The invention seeks; to optimize the atomizer/burner “pair” by improvements relating specifically to the design of atomizers and dismountable liquid fuel atomizing heads, and also concerning the burner bodies proper, so as to make it easy to control optimization of this “pair”.
To this end, in a first aspect, the invention consists in providing a nozzle-carrier piece suitable for being removably mounted on an atomizer head of a liquid fuel burner, the piece including a rotary link member for constraining it to turn with said head, which member is preferably integrated in the structure of the nozzle-carrier piece to increase the strength of the link and avoid any faulty relative positioning, in particular during repeated disassembly and reassembly operations for maintenance purposes.
By means of this rotary constraint, the angle of orientation about the axis of the burner of the liquid fuel jets produced by the nozzle-carrier piece can be adjusted to correspond exactly to the angular position of the secondary air injection nozzles. Specifically with the burners shown in
FIG. 2
of patents EP 0 774 620 and U.S. Pat. No. 5,562,437, this makes it possible to adjust and control the angular offset that exists between the liquid fuel injection orifice and the position of the secondary nozzle, which depends on,the direction of the “rotation” imparted by the burner to the combustion air. The invention thus makes it possible to control the angular position of the piece in which the liquid fuel outlet orifices are made in such a manner that the angle of orientation of the liquid fuel injection head can be adjusted accura

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