EP0690263A2 - Method for operating a combustion plant - Google Patents

Abstract

The method involves feeding a mixture (6) of air (3) and returned exhaust gas (4) into a burner (100) for the first combustion phase (1) . The hot gases from this first phase are calorie-moderated before entering the second combustion phase (2). At the top of the second combustion phase (2) a mixture (14) of fuel (15) and returned exhaust gas (4) is introduced into the hot gases. Combustion in this second phase is triggered by self-ignition. A ring chamber (12) is mounted downstream of the first combustion phase on the top side of the second phase. The wall of the ring chamber has openings (13) for feeding in the mixture (14) of exhaust gas and fuel. The burner (100) operates with a compressed combustion air (115). <IMAGE>

Description

Technical field

The present invention relates to a method according to the preamble of claim 1. It also relates to a furnace for carrying out the method.

State of the art

In conventional combustion plants, the fuel is injected into a combustion chamber via a nozzle and burned there with the supply of combustion air. In principle, such firing systems can be operated with a gaseous and / or liquid fuel. When using a liquid fuel, the weak point with regard to clean combustion with regard to NOx, CO, UHC emissions (UHC = unsaturated coal-water substances) is primarily that the atomization of the fuel causes a high degree of mixing (gasification) with the combustion air must achieve. When using a gaseous fuel, the combustion therefore takes place with a substantial reduction in pollutant emissions. However, in the case of firing systems for boilers, gas-powered burners, despite the many advantages, have not really been able to establish themselves. The reason for this may be that the logistics for gaseous fuels require a complex infrastructure. Therefore, if the operation of combustion plants with liquid fuel is created, the quality of the combustion with regard to low pollutant emissions depends largely on whether it is possible to achieve an optimal degree of mixing between the fuel and the combustion air, ie whether complete gasification of the liquid fuel is guaranteed is. The route via a premixing section, which acts upstream of the actual burner head, did not lead to the goal, because with such a configuration there must always be fear that the flame may re-ignite inside the premixing zone. It is true that premix burners, which work with 100% excess air, have become known so that the flame can be operated shortly before the point of extinguishing. However, it should be borne in mind here that, because of the boiler efficiency, a maximum of 15% excess air is permitted, which is why the use of such burners in atmospheric combustion systems does not guarantee optimal operation. Furthermore, even if the necessary degree of gasification of the liquid fuel could be approximately achieved, the high flame temperatures, which are known to be responsible for the formation of NOx emissions, would not have been acted on. The desired combustion at low flame temperatures and with a homogeneous fuel / air mixture cannot be achieved with the possibilities known from the prior art.

Presentation of the invention

The invention seeks to remedy this. The invention, as characterized in the claims, is based on the object of minimizing the pollutant emissions, in particular as far as the NOx emissions are concerned, in a method and a furnace of the type mentioned both when using a liquid fuel, a gaseous fuel, and in a mixed operation with the fuels mentioned.

The underlying idea of the invention differs from the classic principles in that the grading is carried out exclusively in the excess air area by double addition of fuel and with recirculated flue gas. In the first stage, the combustion air is fed to an aerodynamically stabilized premix burner via a heat exchanger. Depending on the design of the heat exchanger, the combustion air can be preheated to approx. 400 ° C, which leads to very good pre-evaporation when burning oil. The combustion air ratio in this so-called lean stage is approx. 2.1, corresponding to approx. 11% residual oxygen, which means that at flame temperatures of approx. 1300 ° C the NOx emissions, in the atmospheric case, are below 1 vppm. On the way to the second stage, heat is extracted from the medium so that when entering the second stage the temperature is still approx. 1000 ° C. A further fuel / flue gas mixture is preferably injected axially offset there via an annular chamber until a residual oxygen content of approximately 3% is reached in the exhaust gas. The injected mixture is ignited by the hot flue gases from the first stage. The complete burnout then takes place in the combustion chamber at a temperature of approx. 1400 ° C.

The main advantage of the invention can be seen in the fact that the arrangement of the injection openings of the fuel / flue gas mixture control a time offset of the ignition in the combustion chamber and thus influence the oxygen content during the burnout, in such a way that, with optimal trimming of the system, the expected NOx -Emissions, when burned out completely, are between 5-8 vppm. According to current knowledge, this value marks the theoretical one lower limit for the near-stoichiometric combustion of fossil fuels.

Another advantage of the invention is that the combustion air of the first stage can be supplied with calorically conditioned flue gas, on the one hand to influence the preheating temperature and on the other hand to be able to further reduce the residual oxygen content after the second stage if necessary.

Advantageous and expedient developments of the task solution according to the invention are characterized in the further claims.

An exemplary embodiment of the invention is explained in more detail below with reference to the drawings. All elements not necessary for the immediate understanding of the invention have been omitted. The direction of flow of the different media is indicated by arrows. Identical elements are provided with the same reference symbols in the various figures.

Brief description of the drawings

Fig. 1

eine Kesselanlage für eine gestufte Verbrennung,

Fig. 2

einen Vormischbrenner in der Ausführung als "Doppelkegelbrenner" in perspektivischer Darstellung, entsprechend aufgeschnitten und

Fig. 3-5

entsprechende Schnitte durch verschiedene Ebenen des Vormischbrenners gemäss Fig. 2.

It shows:

Fig. 1

a boiler system for a staged combustion,

Fig. 2

a premix burner in the version as a "double cone burner" in perspective, cut open accordingly and

Fig. 3-5

corresponding cuts through different levels of the premix burner according to FIG. 2.

Ways of carrying out the invention, commercial usability

Fig. 1 shows a boiler system, which is divided into a lean stage 1 and a near-stoichiometric stage 2. The lean stage 1 essentially consists of a premix burner 100 with a downstream combustion chamber 122, in which a flame temperature of approximately 1300 ° C. prevails. The premix burner 100 is operated with a liquid 112 and / or gaseous fuel 113. The combustion air 115 for the premix burner 100 is a mixture 6 which is composed of fresh air 3 and of recirculated, calorically conditioned flue gas 4. The degree of mixing is maintained on the air side by a controllable throttle valve 7, this air 3 being unconditioned, that is to say at ambient temperature. The flue gas 4 comes from a flue gas distributor 8, which comes from the flue gases 9 from the near-stoichiometric stage 2. These flue gases 9 occur at a temperature of approximately 300 ° C. and are cooled to approximately 260 ° C. in the flue gas distributor 8 mentioned by a heat exchange system 10. These cooled flue gases 4 and the fresh air 3 are mixed upstream of the premix burner 100 and compressed in a compressor 11 acting there, the temperature of this compressed air / flue gas mixture being approximately 260 ° C. This mixture 6 is then further calorically processed by a further heat exchange induced by the wall of the combustion chamber 122, which is symbolized by the arrow 16, in such a way that the combustion air 115 for the premix burner 100 flows in there at approximately 400 ° C. On the outflow side of the combustion chamber 122 there is an annular chamber 12, which already belongs to the near-stoichiometric stage 2. The slightly cooled hot gases from the lean stage 1, which is operated with combustion air 115 at approx. 11% O2, flow into this annular chamber 12, so that at a flame temperature of approx. 1300 ° C. the NOx emissions in the atmospheric case are below 1 vppm. Furthermore is perforated this annular chamber 12 with a number of injection holes 13 through which a fuel / flue gas mixture 14 flows. This mixture 14 is composed of a portion of flue gas 4 from the flue gas distributor 8 and a further portion of fuel 15, which is preferably a gaseous fuel. On the way to the near-stoichiometric stage 2, heat is extracted from the hot gases provided in the lean stage 1 by the already mentioned heat exchange 16, so that a temperature of approximately 1000 ° C. still prevails when entering the annular chamber 12. The fuel / flue gas mixture 14 injected into the annular chamber 12 by axial displacement reduces the residual oxygen content of the conditioned hot gases from the lean stage 1 to approximately 3%. Furthermore, the mixture 14 injected into the annular chamber 12 experiences self-ignition due to the hot gases of approximately 1000 ° C., the complete burnout then taking place in the boiler furnace 17 at a temperature of approximately 1400 ° C. After leaving the boiler furnace 17, the flue gases 9 still have a temperature of approximately 300 ° C., a part of which, as already explained above, is introduced into the flue gas distributor 8. The non-branched flue gases 18 are blown into the open at a low temperature via a chimney 19. With optimal control of the various media that induce a complete burnout within the near-stoichiometric level 2, the expected NOx emissions are between 5-8 vppm, which, according to current knowledge, represents a lower limit for the near-stoichiometric combustion of fossil fuels.

In order to better understand the structure of the premix burner 100, it is advantageous if the individual cuts according to FIGS. 3-5 are used simultaneously with FIG. 2. Furthermore, in order not to make FIG. 2 unnecessarily confusing, the guide plates 121a, 121b shown schematically according to FIGS. 3-5 have only been hinted at.

In the description of FIG. 2, reference is made below to the remaining FIGS. 3-5 as required.

The premix burner 100 according to FIG. 2 consists of two hollow, conical partial bodies 101, 102 which are nested in one another in a staggered manner. The offset of the respective central axis or longitudinal axis of symmetry 201b, 202b of the conical partial bodies 101, 102 to one another creates a tangential air inlet slot 119, 120 on both sides, in a mirror-image arrangement (FIGS. 3-5), through which the combustion air 115 enters the interior of the Premix burner 100, ie flows into the cone cavity 114. The conical shape of the partial bodies 101, 102 shown in the flow direction has a specific fixed angle. Of course, depending on the operational use, the partial bodies 101, 102 can have an increasing or decreasing cone inclination in the direction of flow, similar to a trumpet or. Tulip. The last two forms are not included in the drawing, since they can be easily understood by a person skilled in the art. The two tapered partial bodies 101, 102 each have a cylindrical starting part 101a, 102a, which, similarly to the tapered partial bodies 101, 102, also run offset from one another, so that the tangential air inlet slots 119, 120 are present over the entire length of the premix burner 100. In the area of the cylindrical initial part, a nozzle 103 is accommodated, the injection 104 of which coincides approximately with the narrowest cross section of the conical cavity 114 formed by the conical partial bodies 101, 102. The injection capacity and the type of this nozzle 103 depend on the specified parameters of the respective premix burner 100. Of course, the premix burner can be of a purely conical design, that is to say without cylindrical starting parts 101a, 102a. The tapered partial bodies 101, 102 further each have a fuel line 108, 109, which are arranged along the tangential inlet slots 119, 120 and are provided with injection openings 117, through which preferably a gaseous fuel 113 is injected into the combustion air 115 flowing through there, as is shown by the arrows 116. These fuel lines 108, 109 are preferably placed at the latest at the end of the tangential inflow, before entering the cone cavity 114, in order to obtain an optimal air / fuel mixture. On the combustion chamber side 122, the outlet opening of the premix burner 100 merges into a front wall 110, in which a number of bores 110a are provided. The latter come into operation when necessary and ensure that dilution air or cooling air 110b is supplied to the front part of the combustion chamber 122. In addition, this air supply ensures flame stabilization at the outlet of the premix burner 100. This flame stabilization becomes important when it comes to supporting the compactness of the flame due to a radial flattening. The fuel brought in through the nozzle 103 is a liquid fuel 112, which can be enriched with a recirculated exhaust gas at most. This fuel 112 is injected into the cone cavity 114 at an acute angle. A conical fuel profile 105 is thus formed from the nozzle 103 and is enclosed by the rotating combustion air 115 flowing in tangentially. In the axial direction, the concentration of the fuel 112 is continuously reduced to an optimal mixing by the incoming combustion air 115. If the premix burner 100 is operated with a gaseous fuel 113, this is preferably done via opening nozzles 117, the formation of this fuel / air mixture taking place directly at the end of the air inlet slots 119, 120. When the fuel 112 is injected via the nozzle 103, the optimal, homogeneous fuel concentration over the cross section is achieved in the region of the vortex run, that is to say in the region of the backflow zone 106 at the end of the premix burner 100. The ignition takes place at the tip of the backflow zone 106. Only at this point can a stable flame front 107 arise. A Flashback of the flame into the interior of the premix burner 100, as is latently the case with known premixing sections, while remedial measures are sought there with complicated flame holders is not to be feared here. If the combustion air 115 is additionally preheated or enriched with a recirculated exhaust gas, this supports the evaporation of the liquid fuel 112 before the combustion zone is reached. The same considerations also apply if, instead of gaseous, liquid fuels are supplied via the lines 108, 109. When designing the conical partial bodies 101, 102 with regard to the cone angle and width of the tangential air inlet slots 119, 120, narrow limits must be observed so that the desired flow field of the combustion air 115 with the flow zone 106 at the outlet of the premix burner 100 can be set. In general, it can be said that a reduction in the cross section of the tangential air inlet slots 119, 120 shifts the backflow zone 106 further upstream, which, however, then causes the mixture to ignite earlier. At least it must be determined that the backflow zone 106, once fixed, is positionally stable, because the swirl number increases in the direction of flow in the region of the cone shape of the premix burner 100. The axial speed within the premix burner 100 can be changed by a corresponding supply, not shown, of an axial combustion air flow. The design of the premix burner 100 is furthermore excellently suitable for changing the size of the tangential air inlet slots 119, 120, with which a relatively large operating range can be recorded without changing the overall length of the premix burner 100.

3-5 now shows the geometrical configuration of the guide plates 121a, 121b. They have a flow introduction function, which, depending on their length, extend the respective end of the tapered partial bodies 101, 102 in the direction of flow relative to the combustion air 115. The Channeling the combustion air 115 into the cone cavity 114 can be optimized by opening or closing the guide plates 121a, 121b about a pivot point 123 located in the region of the entry of this channel into the cone cavity 114, in particular this is necessary if the original gap size of the tangential air inlet slots 119 , 120 is changed. Of course, these dynamic arrangements can also be provided statically, in that guide baffles as required form a fixed component with the tapered partial bodies 101, 102. The premix burner 100 can also be operated without baffles, or other aids can be provided for this.

Reference list

1

First combustion stage, lean stage

2nd

Second combustion stage, near-stoichiometric stage

3rd

air

4th

Flue gas conditioned

6

Air / flue gas mixture

7

throttle

8th

Flue gas distributor

9

Smoke gases from level 2

10th

Heat exchanger

11

compressor

12th

Ring chamber

13

Injection holes

14

Fuel / flue gas mixture

15

fuel

16

Heat exchanger

17th

Boiler firebox

18th

Flue gas fireplace

19th

stack

100

burner

101, 102

Partial body

101a, 102a

Cylindrical starting parts

101b, 102b

Longitudinal symmetry axes

103

Fuel nozzle

104

Fuel injection

105

Fuel injection profile

106

Reverse flow zone (vortex breakdown)

107

Flame front

108, 109

Fuel lines

110

Front wall

110a

Air holes

110b

Cooling air

112

Liquid fuel

113

Gaseous fuel

114

Cone cavity

115

Combustion air

116

Fuel injection

117

Fuel nozzles

119, 120

Tangential air inlet slots

121a, 121b

Baffles

122

Combustion chamber

123

Pivot point of the guide plates

Claims (9)

Method for operating a furnace, which essentially consists of a first combustion stage that can be operated with a burner and a second combustion stage connected downstream thereof, characterized in that a mixture (6) of air (3) is used as combustion air (115) for the first combustion stage (1) ) and returned flue gas (4) flows into the burner (100), that the hot gases from this first combustion stage (1) are calorically moderated before entry into the second combustion stage (2), that the top side of the second combustion stage (2) enters the hot gases Mixture (14) of fuel (15) and recirculated flue gas (4) is entered, and that the combustion in this second combustion stage (2) is triggered by self-ignition. Method according to claim 1, characterized in that the recirculated flue gases (4) for the first and second combustion stages (1, 2) are calorically moderated before they are mixed with another medium (3, 15). A method according to claim 1, characterized in that the first combustion stage (1) is operated as a lean stage with an oxygen content of 9-13%, and that the second combustion stage (2) is operated as a near-stoichiometric stage with an oxygen content of 2-4%. Furnace system for carrying out the method according to claim 1, wherein the furnace system essentially consists of a first combustion stage operable with a burner and a downstream second combustion stage, characterized in that an annular chamber (2) downstream of the first combustion stage (1) on the head side of the second combustion stage (2) 12) that the wall of the annular chamber (12) has openings (13) for the injection of a mixture (14) of recirculated flue gas (4) and fuel (15), and that the burner (100) contains compressed combustion air ( 115) can be operated. Furnace system according to claim 4, characterized in that the burner (100) consists of at least two hollow, conical partial bodies (101, 102) nested one inside the other in the flow direction, the respective longitudinal axes of symmetry (101b, 102b) of which are offset with respect to one another, such that the adjacent walls of the partial bodies (101, 102) in their longitudinal extent form tangential channels (119, 120) for a combustion air flow (115) that at least one fuel nozzle (103) is present in the cone cavity (114) formed by the partial bodies (101, 102). Apparatus according to claim 5, characterized in that further fuel nozzles (117) are arranged in the region of the tangential channels (119, 120) in their longitudinal extent. Apparatus according to claim 5, characterized in that the partial bodies (101, 102) expand conically in the flow direction at a fixed angle. Apparatus according to claim 5, characterized in that the partial bodies (101, 102) have an increasing taper in the direction of flow. Apparatus according to claim 5, characterized in that the partial bodies (101, 102) have a decreasing taper in the direction of flow.

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