EP0639742A2 - Method and device for low emission combustion of fluid and or gaseous fuels with internal recirculation of fluegas - Google Patents

Abstract

Method and device for low emission combustion of fluid and/or gaseous fuels with internal recirculation of flue gas, particularly in water tube boilers, a first part of the combustion air being guided in a swirled manner as primary air 1 and primary air 2, coaxially with the fuel and essentially in rotational symmetry with respect to the longitudinal axis of the burner, to a primary exit position. A second part of the combustion air is guided in two stages as secondary air to a secondary exit position. The secondary air is fed in in the form of a number of free jets essentially in the direction of the primary air flow, the jets of the first stage each beginning at a first radial distance from the longitudinal axis and being set so as to converge towards the longitudinal axis, and the jets of the second stage each beginning at a second radial distance from the longitudinal axis and being guided parallel to the latter, and the radial distance of the jets of the first stage from the longitudinal axis is preferably smaller than the radial distance of the jets of the second stage. <IMAGE>

Description

The invention relates to a method for low-emission combustion of flowable and / or gaseous fuels with internal flue gas recirculation, in which a first part of the combustion air swirls as primary air coaxially with the fuel is guided essentially rotationally symmetrically to a longitudinal axis to a primary exit position and a second Part of the combustion air is supplied radially outside the primary air as secondary air at a secondary exit position in two stages.

The invention further relates to a combustion device for low-emission combustion of flowable and / or gaseous fuels with internal flue gas recirculation, in particular for carrying out the invention Method, with a device for essentially rotationally symmetrical supply and swirl of a first part of the combustion air as primary air and for coaxial supply of the fuel to a primary exit position, and with a device for two-stage supply of a second part of the combustion air radially outside the primary air as secondary air a secondary exit position.

On the one hand, technical combustion plants should, of course, convert the chemically bound primary energy contained in the fuel as completely as possible into thermal energy. Another goal is to reduce the emission of pollutants (SO _x , NO _x , soot ...) as much as possible. While most pollutants can only be dealt with through complex secondary measures (filters, desulfurization systems ...), there is a considerable reduction potential with regard to NO _x solely through firing technology or primary measures, ie by optimizing the combustion conditions in the combustion chamber. The reason for this can be seen in the fact that NO _x in technical firing systems for fuels with a low organic nitrogen content (e.g. heating oil EL, natural gas) usually arises predominantly as so-called thermal NO _x , ie by oxidation of the nitrogen contained in the ambient air used for combustion at high temperatures. From the formation mechanism for thermal NO _{x it} can be deduced that in a combustion with low NO _x formation the spatial and / or temporal coincidence of high temperatures (noticeable NO _x formation above approx. 1400 ° C.) with (relatively) high oxygen partial pressures is avoided should be. The minimum requirement from this is that (spatial or temporal) peak temperatures should be avoided as far as possible.

The maximum permissible NO _x emission values for combustion plants are determined in the 1st, 4th and 13th BImSchV depending on fuel and thermal output. Within the scope of the 4th BImSchV, a limit value of 200 mg / m³ NO _x applies for gaseous fuels and 250 mg / m³ for heating oil EL, which according to the current state of the art can be maintained by means of firing measures. According to the dynamics clause of the 4th BImSchV, limit values of 100 mg / m³ gas or 150 mg / m³ (heating oil EL) are often required.

• 1. Luft- und/oder Brennstoffstufung,
• 2. interne oder externe Rauchgasrezirkulation.

There are generally two tried and tested solutions to avoid high peak temperatures:

• 1. air and / or fuel grading,
• 2. internal or external flue gas recirculation.

When fuel and / or air are graded, the heat is released over a longer distance at several points, so that temperature peaks are avoided which arise from the rapid reaction of the entire fuel with the air. Especially in the substoichiometric (fuel-rich) zones, little NO _x is produced due to the reaction kinetics. The reaction of the substances in the burnout zone must also not take place too quickly, but must be complete. A disadvantage of the grading of fuel / air is the high need for the combustion chamber volume. The flame is very long due to the formation of at least two combustion zones and the defined, gradual mixing of the reactants. This places demands on the furnace, which are often not feasible.

The external recirculation of flue gas has been going on for years State of the art. In this process, relatively "cold" flue gas is drawn off at the end of the boiler by a fan and fed to the combustion air. The inert flue gas lowers the temperature in the flame and the O₂ partial pressure. The disadvantage of this process is high investment and operating costs.

The internal flue gas recirculation offers numerous advantages in this situation, although guide devices for guiding the flame and flue gases are required in numerous known designs. Such guidance devices have, in addition to constructional effort and inevitable space requirements, the additional disadvantage that solid particles from combustion can be deposited on them, which makes regular cleaning necessary.

The object of the invention is to provide a method and a device for combustion with internal flue gas recirculation, in which the mentioned and other disadvantages are avoided and in which a considerable reduction in NO _x is achieved with a simple structural design.

The solution to this problem with regard to the method is that the secondary air is supplied in the form of a number of (free) jets essentially in the direction of the primary air flow, the jets of the first stage each beginning and converging at a first radial distance from the longitudinal axis are set to the longitudinal axis and the beams of the second stage each begin at a second radial distance from the longitudinal axis and are directed parallel to the latter.

The secondary air free jets suck from their surroundings Flue gas on, the arrangement of the jets according to the invention drawing in from areas of the combustion chamber in which the flue gases have already (partially) cooled.

The swirled primary air induces a highly turbulent backflow zone and thus brings about an excellent mixing of the reactants with the recirculated flue gas, as a result of which temperature peaks are effectively avoided.

The (first) radial distance of the rays of the first stage from the longitudinal axis is preferably chosen to be smaller than the corresponding (second) radial distance of the rays of the second stage.

The angle of incidence of the beams of the first stage is preferably 15 °.

It is further proposed that the primary air be divided into a first (primary air 1) and a second (primary air 2) air stream, which are guided coaxially, the gaseous fuel preferably being supplied between these two streams.

In a preferred embodiment, the number of jets in the first stage of the secondary air is equal to the number of jets in the second stage.

In the first and second stages 4 to 12, but preferably six beams are provided.

The initial cross-sectional areas of the beams of the first and second stages can be either symmetrical or asymmetrical with respect to the longitudinal axis (7) be arranged in order to obtain an adaptation of the flame shape to the geometry of the combustion chamber.

The jets of the two stages (in the case of a symmetrical arrangement of the secondary air) are expediently offset symmetrically in relation to one another in the circumferential direction.

The initial cross-sectional area of the rays of the first stage may or may not be equal to the corresponding area of the rays of the second stage.

The primary air or at least a partial flow thereof is advantageously swirled (with a swirl angle of approximately 70 °).

The proportion of primary air is preferably 10% -30% of the total combustion air supplied.

It is also provided that the flowable fuel is guided centrally to the primary exit position and is atomized there in the form of jets, symmetrically or asymmetrically with respect to the longitudinal axis (7). These are preferably aligned in the circumferential direction with the rays of the secondary air of the first stage.

It is preferably provided that the secondary exit position is in the axial direction downstream of the primary exit position, the flow consisting of primary air and fuel being guided between the primary and secondary exit positions in a parallel or diffuser-like manner.

The solution to the part of the task related to the device is that the device for feeding the secondary air has a number of nozzles, the nozzles for the first stage each having a first radial distance from the longitudinal axis and being converged to the longitudinal axis, and the nozzles for the second stage each having a second radial distance from the longitudinal axis and essentially are directed in parallel.

The (first) radial distance of the nozzles of the first stage from the longitudinal axis is preferably smaller than the corresponding (second) radial distance of the nozzles of the second stage.

It has proven to be useful if the angle of attack of the nozzles of the first stage is 15 °.

It is advantageous if the first stage of the secondary air supply has as many nozzles as the second stage. The number of nozzles in each of the two stages can be between 4 and 12, with a number of six nozzles in both stages being found to be favorable.

It is also advantageous if the nozzles of both stages are symmetrically offset from one another in the circumferential direction.

The area proportions of the secondary air nozzles can be the same or different in both stages, and they can be arranged symmetrically or asymmetrically with respect to the longitudinal axis, which can influence the shape of the flame.

In a preferred embodiment, the device for supplying primary air and fuel has an outer burner tube and two concentric therein and in each other arranged, first and second leads. The annular space between the burner tube and the first feed line serves to supply the primary air or a part thereof, it being possible for a swirl-generating device to be arranged between the burner tube and the (first) feed line lying next to it. The annular space between the first and the second supply line located within it can serve to supply part of the primary air (primary air 1), in which case a (further) swirl-generating device can then be arranged between the first and second supply lines.

The swirl-generating device (s) for the primary air can be adjustable, but preferably have a swirl angle of 70 °, whereby the two portions of the primary air (primary air 1 and primary air 2) can be swirled in the same or in opposite directions.

It has proven to be expedient to arrange a number of feed lines for fuel gas in the annular space between the first and second feed lines, which feed lines can be rotatable, so that the gas outlet bores can be aligned tangentially or radially to the burner axis in the region of the primary outlet position, thereby creating a flame shape and Stability can be influenced.

In a preferred embodiment, a third supply line for flowable fuel is arranged concentrically within the second supply line. An atomizer nozzle for liquid or flowable fuel is arranged in the area of the primary outlet position, which can be designed in a known manner as a two-substance vapor pressure atomizer nozzle.

The atomizer nozzle preferably has as many individual bores as there are secondary air nozzles in the first stage, the individual bores being aligned with the nozzles. The individual bores, like the nozzles, can be aligned symmetrically or asymmetrically with respect to the longitudinal axis.

With regard to flame stability, it has proven advantageous to arrange the secondary exit position in the axial direction downstream from the primary exit position, the outer burner tube extending between the primary and secondary exit positions with a constant diameter or expanded like a diffuser.

• Fig. 1 einen schematischen Längsschnitt durch eine erfindungsgemäße Verbrennungsvorrichtung zeigt,
• Fig. 2 eine Draufsicht auf die Sekundärluftbohrungen zeigt, und
• Fig. 3 und 4 Meßergebnisse von NO_x für Erdgas bzw. Heizöl El zeigen, die an einer erfindungsgemäßen Verbrennungsvorrichtung erhalten wurden.

The invention is explained below using an exemplary embodiment with reference to a drawing, in which

• 1 shows a schematic longitudinal section through a combustion device according to the invention,
• Fig. 2 shows a plan view of the secondary air holes, and
• 3 and 4 show measurement results of NO _x for natural gas or heating oil El, which were obtained on a combustion device according to the invention.

• 1. Primärluft 1 (bei 1),
• 2. Primärluft 2 (bei 2),
• 3. Sekundärluft 1 ( bei 3), und
• 4. Sekundärluft 2 (bei 4).

The burner shown in FIG. 1 is inserted into the boiler wall of a water tube boiler, designated by 21, and works, indicated by the arrows 10c, in the combustion chamber 20. The combustion air supplied radially from the outside by fans is divided into four partial flows:

• 1. Primary air 1 (at 1),
• 2. Primary air 2 (at 2),
• 3. Secondary air 1 (at 3), and
• 4. Secondary air 2 (at 4).

The first partial flow of the primary air, the primary air 1, passes through a feed 1a into the annular space between a first feed line 5 and a second feed line 6, in which it is guided in the direction of the longitudinal axis 7 of the burner to the primary outlet position 8. At this point, a (possibly adjustable) device 9 for swirling the primary air 1 is arranged between the first and second feed lines.

The second part of the primary air, the primary air 2, is fed in the direction of the first feed line 5 and then deflected in the axial direction, being in the annular space between the (outer) burner tube 11 (or a brick lining or the like) and the first Lead 5 is guided in the direction of the primary exit position 8. A (adjustable) swirling device 12 is also arranged in a first, tapering section 11a of the burner tube 11, which swirls the primary air. The primary air has the task of stabilizing the flame by strong swirling with as little as possible of the total air, forming back vortices in the flame core and eliminating temperature peaks by good mixing of the reactants.

Fuel gas is fed via a pipeline 13a to an annular distributor space 13b, to which a number of gas lances 13 are connected. At the primary outlet position 8, the fuel gas emerges from each gas lance 13 through a number of gas outlet bores 14 essentially radially from, the bores 14 can be pivoted about their longitudinal axis in a partially tangential direction by rotating the gas lances 13. Flame shape and stability can be influenced. A conically widened section 11b of the burner tube 11 follows downstream from the primary exit position 8, which further increases the tendency of the swirled primary air flow to form a central recirculation flow in the region of the flame core 10, as is illustrated by the arrows 10a which indicate the direction of flow .

Liquid or flowable fuel passes through the annular space between the second feed line 6 and a third feed line 16 arranged within it to the primary outlet position 8, where the liquid fuel is fed into a two-component fuel with the aid of steam which is fed through the third feed line. Atomizer nozzle 17 is atomized finely, the two-phase mixture of two substances emerging at the speed of sound from the outlet openings.

The secondary air supply takes place in two partial streams 3 and 4, each of which reaches the nozzles 18 for the first stage and 19 for the second stage via annular distribution spaces and feed lines 3a and 4a. It should be pointed out that it is not absolutely necessary to provide nozzles in order to achieve the desired purpose, rather the secondary air supply could also be carried out through smooth pipes of appropriate diameter. The area ratio of the air outlet areas of the first and second stages is selected in this area so that the area of the bores or nozzles 18 of the first stage is approximately 30% of the total air outlet area of the secondary air, while the cross sections of the bores or nozzles 19 of the second stage 4 about 70% of it. In the shown here Embodiment there are six nozzles 18 and 19 each, which are each offset symmetrically in the circumferential direction, so that the arrangement shown in Fig. 2 results.

The method of supplying the secondary air according to the invention results in the formation of free jets 18a, 19a, which suck in flue gas as it travels. Due to the special arrangement of the nozzles, suction takes place from areas of the combustion chamber 20 in which the flue gases have already partially cooled. The good mixing of the flue gases in the flame core by primary and secondary air ensures the required temperature peak reduction.

The volume flows of primary air 1, primary air 2, secondary air 1 and secondary air 2 are individually controlled depending on the load and from other points of view by means of an electronic fuel-air ratio controller, so that on the one hand minimal NO _x values with a free-burning flame in the combustion chamber are ensured and on the other hand the flame geometry can be optimally designed for different combustion chambers.

The division between primary and secondary air is usually done so that a primary air ratio of 0.15 ... 0.35 is set at full load, which is increased accordingly at part load.

3 and 4 show measurement results of NO _x , which were obtained on a burner according to the invention in an 8 MW test facility when natural gas (FIG. 3) and heating oil EL (FIG. 4) were burned. With the method according to the invention the values can obviously be 100 mg / m³ (natural gas) or 150 mg / m³ (Heating oil EL) are safely undercut. The control range is 1: 8 for the combustion of natural gas and 1: 7 for heating oil EL.

Reference list

1

Primary air 1

1a

Feed

2nd

Primary air 2

3rd

Secondary air 1

3a

Supply

4th

Secondary air 2

4a

Supply

5

first supply line

6

second supply line

7

Longitudinal axis

8th

Primary exit position

9

Swirl device (primary air 1)

10th

Flame core

10a

arrow

10b

arrow

10c

arrow

11

Burner tube

12th

Swirl device (primary air 2)

13

Gas lance

13a

Pipeline (gas)

13b

Distribution room (gas)

13

Gas outlet bore

15

Secondary exit position

16

third supply line

17th

Two-component atomizer nozzle

18th

(Secondary air) nozzle (first stage)

18a

Free jet

19th

(Secondary air) nozzle (second stage)

19a

Free jet

20th

Firebox

21

Boiler wall

Claims (37)

Method for low-emission combustion of flowable and / or gaseous fuels with internal flue gas recirculation, in which a first part of the combustion air swirled as primary air is guided coaxially with the fuel essentially rotationally symmetrically to a longitudinal axis up to a primary exit position and a second part of the combustion air as Secondary air is supplied radially outside the primary air at a secondary outlet position in two stages, characterized in that the secondary air (3, 4) in the form of a number of (free) jets (18a, 19a) essentially in the direction of the primary air flow (2nd ) is supplied, the beams (18a) of the first stage each having a first radial distance from the longitudinal axis (7) begin and are convergent to the longitudinal axis, and the rays (19a) of the second stage each begin at a second radial distance from the longitudinal axis (7) and are directed parallel to the latter. Method according to claim 1, characterized in that the (first) radial distance of the rays (18a) of the first stage from the longitudinal axis (7) is smaller than the corresponding (second) radial distance of the rays (19a) of the second stage. Method according to claim 1 or 2, characterized in that the angle of attack of the beams (19a) of the first stage is 15 °. Method according to one of the preceding claims, characterized in that the primary air (1, 2) is divided into a first (primary air 1) (1) and a second (primary air 2) (2) air flow which are guided coaxially. Method according to claim 4, characterized in that the gaseous fuel is supplied between the two streams (1, 2). Method according to one of the preceding claims, characterized in that the number of jets (18a) in the first stage of the secondary air is equal to the number of jets (19a) in the second stage. Method according to Claim 6, characterized in that 4 to 12 beams (18a, 19a) are present in the first and second stages. A method according to claim 7, characterized in that there are six beams (18a, 19a) in each of the first and second stages. Method according to one of the preceding claims, characterized in that the initial cross-sectional areas of the beams (18a, 19a) of the first and second stages are arranged symmetrically or asymmetrically with respect to the longitudinal axis (7). Method according to Claim 6, 7 or 8, characterized in that the jets (18a, 19a) of both stages of the secondary air are offset symmetrically with respect to one another in the circumferential direction. Method according to one of the preceding claims, characterized in that the initial cross-sectional area of the rays (18a) of the first stage is equal to or not equal to the corresponding area of the rays (19a) of the second stage. Method according to one of the preceding claims, characterized in that the primary air 1 (1) and / or the primary air 2 (2) are swirled. A method according to claim 12, characterized by a swirl angle of 70 °. Method according to one of the preceding claims, characterized in that the proportion of primary air (1, 2) is 10% - 30% of the total combustion air supplied (1, 2, 3, 4). Method according to one of the preceding claims, characterized characterized in that flowable fuel is guided centrally to the primary exit position (8) and is atomized there, symmetrically or asymmetrically with respect to the longitudinal axis (7), in the form of individual jets. A method according to claim 15, characterized in that the atomizing jets of the flowable fuel are aligned in the circumferential direction with the jets (18a) of the secondary air of the first stage. Method according to one of the preceding claims, characterized in that the secondary exit position (15) is downstream of the primary exit position (8), the flow consisting of primary air (1, 2) and fuel between the primary (8) and secondary - Exit position (15) is guided parallel to the longitudinal axis (7) or expanded like a diffuser. Device for low-emission combustion of flowable and / or gaseous fuels with internal flue gas recirculation, in particular for carrying out the method according to one of claims 1 to 17, with a device for the essentially rotationally symmetrical supply and swirl of a first part of the combustion air as primary air and for the coaxial supply of the Fuel to a primary exit position, and with a device for the two-stage supply of a second part of the combustion air radially outside the primary air as secondary air at a secondary exit position, characterized in that the device for supplying the secondary air (3, 4) has a number of Has nozzles (18, 19), wherein the nozzles (18) for the first stage each have a first radial distance from the longitudinal axis (7) and are converged to the longitudinal axis (7) and the nozzles (19) for the second stage each have a second radial distance from the longitudinal axis (7) and are directed essentially parallel to it. Apparatus according to claim 18, characterized in that the (first) radial distance of the nozzles (18) of the first stage from the longitudinal axis (7) is smaller than the corresponding (second) radial distance of the nozzles (19) of the second stage. Apparatus according to claim 18 or 19, characterized in that the angle of attack of the nozzles of the first stage is 15 °. Apparatus according to claim 18, 19 or 20, characterized in that the first stage of the secondary air supply has as many nozzles as the second stage. Apparatus according to claim 21, characterized in that 4 to 12 nozzles (18, 19) are provided in each of the two stages. Apparatus according to claim 21 or 22, characterized in that there are six nozzles (18, 19) in each of the two stages. Apparatus according to claim 21, 22 or 23, characterized in that the nozzles (18, 19) of both stages are symmetrically offset from one another in the circumferential direction. Device according to one of the preceding claims, characterized in that the area portions of the secondary air nozzles (18, 19) are the same or different in both stages are. Device according to one of the preceding claims, characterized in that the surface portions of the secondary air nozzles (18, 19) are arranged symmetrically or asymmetrically with respect to the longitudinal axis (7). Device according to one of claims 18 to 26, characterized in that the device for supplying primary air (1, 2) and fuel has an outer burner tube (11) and two first (5) and second (6) feed lines arranged concentrically therein and one inside the other having. Apparatus according to claim 27, characterized in that an adjustable, swirl-generating device (12) is arranged between the burner tube (11) and the (first) feed line (5) lying next to it. Apparatus according to claim 27 or 28, characterized in that a further adjustable, swirl-generating device (9) is arranged in the annular space between the first (5) and the second supply line (6) located therein. Apparatus according to claim 28 or 29, characterized in that the swirl-generating devices (9, 12) have a swirl angle of 70 °. Device according to one of claims 27 to 30, characterized in that a number of fuel gas lines (13) are arranged in the annular space between the first (5) and second (6) supply lines. Apparatus according to claim 31, characterized in that the fuel gas lines (13) are rotatable about their longitudinal axis. Device according to one of claims 27 to 32, characterized in that a third feed line (16) is arranged within the second feed line (6). Device according to one of claims 18 to 33, characterized in that an atomizing nozzle (17) for flowable fuel is arranged in the primary exit position (8). Apparatus according to claim 34, characterized in that the atomizing nozzle (17) has as many individual bores as there are secondary air nozzles (18) in the first stage, the individual bores being aligned with the nozzles (18) in the circumferential direction. Apparatus according to claim 35, characterized in that the individual bores are arranged symmetrically or asymmetrically with respect to the longitudinal axis (7). Device according to one of claims 18 to 36, characterized in that the secondary exit position (15) is arranged downstream of the primary exit position (8), the burner tube (11) between the primary and secondary exit positions having a constant diameter or being diffuser-like extends.

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