DE3332565A1 - Underfeed stoker - Google Patents

Abstract

The invention relates to an underfeed stoker which can be very well arranged, with two or three combustion troughs arranged one behind the other, in long flame tubes with comparatively small diameters. This underfeed stoker is thus well suited for the conversion of oil/gas-fired flame tube boilers to coal firing, the full nominal steam outputs of the boilers being achieved again even after the conversion. In a variant embodiment of the proposed underfeed stoker, this has four, six or eight combustion troughs, which are arranged in successive pairs. The embodiment with four combustion troughs, the air chest of which has a vertical cross-section in the form of a sector of a circle, can be very well accommodated, on flame tube boilers and the like, in a separate, double-walled, water-cooled precombustion chamber of a circular cylindrical basic shape. The proposed new underfeed stoker can be used as a multiple underfeed stoker, even for the firing conversion of small water tube boilers (with nominal steam productions of up to 50 t/h). It is of particular advantage in this connection that the provision of a precombustion chamber can be dispensed with, since the cross-section and volume of the existing boiler combustion chambers are also sufficient for this kind of coal firing. <IMAGE>

Description

Underfeed

An improved underfeed firing was already proposed in the parent application, which increases the boiler steam output of flame tube boilers from around 4 to 12 t / h.

This additional registration is now based on the task of the techn. To develop the solution of the parent application so that their Fire output is sufficient for even larger steam boilers.

It should especially be with these further developed underfeed firings - as multiple underfeed firing - be possible, also smaller industrial water tube boilers (up to nominal steam output of around 50 t / h), preferably converting these boilers from oil / gas to coal as fuel.

Another aim of the further development of the parent application is to design this underfeed firing so that it can be arranged in the - relatively narrow and long - flame tubes previously oil or gas-fired three-pass boilers, whereby - while avoiding the subsequent arrangement of a pre-combustion chamber on the boiler - after Installation of the underfeed firing system, the full nominal steam output of the boiler can also be achieved with coal as fuel. This is a particularly economical way of converting the furnace from oil / gas to coal. There are also no problems with the arrangement of the pre-combustion chamber, for which there is often not enough space.

In all cases, the ash must be removed mechanically.

Underfeed firing is used to achieve these goals proposed a one-part or multi-part air box and facilities that are delimited on all sides by metallic walls for the fuel supply, and which is characterized according to the invention that - from the operator side Front wall as seen - in the air box several fire pits one behind the other or one behind the other and next to each other are recessed.

With this type of arrangement, up to four - or more - fire troughs can be combined in an underfeed furnace, each of which has a maximum furnace output with which about 6 t / h of steam can be generated. (A maximum of approx. 750 kg / h of gas coal or gas flame coal can be burned in each fire trough.)

According to a special feature of the invention, the underfeed firing in the one-part or multi-part air box on two or three or four fire troughs, which are arranged one behind the other and in the vertical longitudinal plane of symmetry of the Air box. This gives particularly long and slim devices that are easily housed in smaller flame tubes can be.

A construction variant of the subject of the invention has four or six or eight fire pits, which in relation to the vertical longitudinal plane of symmetry of the air box too Pairs side by side, and these pairs of fire pits are arranged one behind the other. This version is suitable i.a. for arrangement in large flame tubes or in separate, double-walled, water-cooled pre-combustion chambers of flame tube boilers, which have a circular cylindrical basic shape.

In these designs, each fire trough is or are one or two sub-feed screws in a manner known per se assigned.

All underfeed screws of an underfeed firing system are arranged inside the air box and through the operator's side Front wall of the same passed. This enables a central supply of the coal.

If the underfeed firing has several fire troughs arranged one behind the other, then the underfeed screw (s) the rear fire trough (s) under or to the side under the foremost Fire pit passed through. This gives the device a particularly slim design.

If the underfeed firing has several pairs of adjacent fire pits, which are arranged one behind the other, so are the feed screws of the rear fire troughs below or to the side carried out under the two foremost fire pits (= the first pair of them).

If the underfeed firing has two fire pits arranged one behind the other and the normal section of your air box is the Has the shape of a circular segment, the rear fire trough is appropriately assigned two - smaller - feed screws, which are passed inside the air box on both sides of the front fire trough. This solution enables above all the accommodation of a sufficiently powerful furnace within flame tubes with smaller diameters.

When the underfeed firing has three fire pits arranged one behind the other and the normal section of your air box has the shape of a segment of a circle, so are the second

and third fire trough each assigned a feed screw, which are led past the foremost (first) fire trough inside the air box on both sides of the latter.

In order to increase the fire performance of a fire pit without increasing the width of the underfeed firing, is the maximum length (L) of the trough at least twice its maximum width (B-).

In this case - to achieve a good burnout and to prevent the coal from "rising" prematurely - the Sub-feed screw adjacent ignition area of each fire pit or its grate at the top of at least one plate or a plate-like one Covered tubular structure. These plates or plate-like tube structures are one-part or multi-part Air box releasably connected. The plates or plate-like tubular structures are also provided with air nozzles and during of the operation for the purpose of cooling some of the combustion air flows through. The plates have through-holes for the carbonization gas from the coal.

Furthermore, there is at least one in each fire trough under the covering plate or under the plate-like tube construction Ignition firing or the pipe mouth of such arranged for the supply of the ignition gas.

This facility is also used during operation permanent supply of gas for a supporting fire possible, whereby In addition to gas and gas flame coal, other types of coal can also be burned in the underfeed furnace.

The heating output of the auxiliary firing is below 10% the total heating output (nominal heating output) of the underfeed furnace.

In the type of underfeed firing in which several pairs of fire pits are arranged one behind the other, Air supply webs are arranged between the two associated fire troughs of the trough pairs, which are parallel or approximately parallel to the vertical longitudinal plane of symmetry of the underfeed furnace get lost. In the side walls of the air supply webs there are air nozzles for some of the combustion air to pass through arranged. This ensures an even and complete burnout of the coal.

In order to be able to switch the fire troughs on and off individually on the firing side during operation, the air box has several separate combustion air chambers or it consists of several parts screwed together with separate Combustion air spaces. Each fire trough is assigned a separate combustion air space. In addition, in is on As is known, an underwind fan is assigned to each fire trough.

To increase its dimensional stability, the air box can have one or more transverse ribs with air passage openings.

That underfeed firing, in which several fire troughs in the vertical longitudinal plane of symmetry of the furnace one behind the other are arranged, carries two ash discharge screws (or scraper belts), which are above the air box at its two peripheral Long sides are arranged in gutters.

On the other hand, the underfeed firing system, in which pairs of fire pits are arranged one behind the other, has ash from it wear the screw (or a scraper) over the air supply bars and two further ash discharge screws (or scraper belts) over the longitudinal, peripheral areas of the or multi-part air box. These ash discharge screws (or scraper belts) are also arranged in channels.

Sometimes just arranging an ash discharge screw is enough (or a scratch tape) on the top of the air box (e.g. with multiple underfeed firing of water tube boilers) .

The ash discharge screws (or scraper belts) are made of refractory material in a manner known per se.

to improve the removal of the ash, is on the ash exit side (Front side) of each fire trough on the upper, horizontal boundary wall of the air box an ash hinge shield attached, which has a ploughshare-like horizontal section and its cutting edge to the associated fire pit is directed towards.

To cool the ash hinge shields, these are expediently designed as box structures and provided with air nozzles. They are part of the combustion air during operation flows through.

Instead of the ash discharge screws, the underfeed firing systems can be used also be equipped with a pneumatic suction device with suction openings at the edges the fire troughs or on the edges of the grates are arranged. The suction pipes are also to be arranged inside the air box and to guide it through the front wall on the operator's side.

The advanced underfeed firing systems described above can also - in whole or in part - in a separate, double-walled, water-cooled pre-combustion chamber Flame tube boiler with one or more technical features of German patent application P 30 04 217 be arranged.

Due to the significantly increased fire performance of the underfeed firing systems described here, it is possible to use them as Multiple underfeed firing - also on smaller industrial water tube boilers (with nominal steam capacities up to approx. 50 t / h) to be used. Such a water tube boiler has in his Combustion chamber several underfeed firings with the features described, whose longitudinal axes to the vertical longitudinal plane of symmetry of the boiler run in parallel or approximately in parallel, the supply of coal in a manner known per se takes place from the front of the combustion chamber.

The underfeed firings are arranged side by side and avoiding gaps so that the upper, horizontal boundary walls of the air boxes form the combustion chamber floor - or at least part of it.

The main advantage of such a multiple underfeed firing for smaller water tube boilers is that they can be accommodated in the existing combustion chamber of the boiler and that an additional pre-combustion chamber is not required whose arrangement is often not possible due to limited space.

Since the combustion of the coal takes place mainly in the fire troughs of the multiple underfeed firing, this is also the case The volume of the existing combustion chamber is sufficient to achieve extensive burnout of the flame gases.

For the implementation of the above suggestions it is necessary to operate the underfeed firing in such a way that no ' Melting of the coal ash occurs, which then leads to the well-known formation of "slag clods".

With the "previous operating mode of underfeed firing the burning of the coal in the fire pits takes place at higher levels

COPY

Loads at 1500 - 1400 ° C. These temperatures are mostly Well above the softening point of the coal ash and from the coal slag, clods form, which contain large proportions of unburned Contain coal and which gradually fill the entire fire pit.

The firing must then be temporarily shut down and the fire pits emptied of the slag clods by hand, since the latter cannot be removed mechanically.

To avoid these malfunctions and to enable mechanical ash removal as well as to reduce the losses of unburned coal and the formation of nitrogen oxides To reduce (NO gases), a method for operating an underfeed furnace with a or several fire troughs proposed, which is characterized in that the primary combustion air is mixed with cold flue gas is, and that the primary air-cold gas mixture thus formed via the air box and its air nozzles as well as the grate or the Grates is blown into the fire trough (s).

By adding cold flue gas to the primary combustion air, a primary air / cold gas mixture is formed, the oxygen concentration of which is lower than the 0 _? -Concentration of the primary air. The combustion processes on the pieces of coal are therefore slower and the release of heat is reduced over a longer period of time.

In addition, the secondary throughput of gas is increased by the furnace, which lowers its overall temperature level. (The combustion then takes place at temperatures of 1000-1100 ^° C.).

The weight of the cold flue gas returned to the furnace can - depending on the type of coal and its ash softening point - 10 to 50 $> the weight of the primary combustion air

- \ Jrb--

". This proportion can decrease with partial load of the furnace. No cold gas is added at very low partial loads.

In order to achieve a primary air / cold gas mixture that is as homogeneous as possible reach, the cold flue gas is expediently in the pressure line of the primary air blower or in the pressure lines of the Primary air blower introduced. If necessary, however, admixing can also take place in the air box.

The drawings show three exemplary embodiments of the subject matter of the invention and a water tube boiler shown with a multiple underfeed firing.

It shows:

Fig. 1 is a vertical longitudinal section along the line A-B in Fig. 3 of a first embodiment in which two fire troughs are arranged one behind the other in the vertical longitudinal plane of symmetry of the furnace.

Fig. 2 is a plan view of this first embodiment

3 is a side elevation of this first embodiment according to FIG Line C-D in Figure 1

4 shows a vertical longitudinal section along the line E-F in FIG. 6 of a second embodiment in which three fire troughs are arranged one behind the other in the vertical longitudinal plane of symmetry of the furnace.

Fig. 5 is a plan view of this second embodiment

Figure 6 is a side elevation of this second embodiment the line G-H in FIG. 4

COPY

7 shows a vertical longitudinal section along the line J-K in FIG. 9 of a third embodiment in which two pairs of Fire pits are arranged one behind the other and the two fire pits of a pair are next to each other.

Fig. 8 is a plan view of this third embodiment

Fig. 9 is a side elevation of this third embodiment the line L-M in FIG. 7.

FIG. 10 shows a vertical longitudinal section of a water tube boiler with multiple underfeed firing along the line N-O in FIG. 11.

11 shows a vertical cross section through this water tube boiler along the line P-Q in FIG. 10.

Pig. 12 a vertical longitudinal section through a flame tube boiler with pre-combustion chamber, in its underfeed firing during operation cold flue gas is recycled.

In the first embodiment shown in FIGS. 1, 2 and 3 the new underfeed firing are within the air box 1, the two fire troughs 2a and 2b one behind the other recessed, which lie in the vertical longitudinal plane of symmetry of the furnace and in the peripheral areas of its interior each carry a stepped, closed grate 3a and 3b.

The maximum length L of a fire pit 2a, 2b is more than twice as large as its maximum width B-. this makes possible with high heat output, a relatively small overall width of the underfeed firing.

COPY

The ignition areas 2a ¹ and 2b ^{1 of} the fire troughs 2a and 2b, which are adjacent to the feed screws 4 or 5a and 5b, are each covered by a plate 6a, 6b that guides the two coal flows longitudinally to the feed screws 4 or 5a and 5b take over and - as a result of the supply of combustion air via the plates 6a, 6b and the screw-side subregions of the grids 3a and 3b - enable the coal to be ignited.

Each plate 6a, 6b is connected to the air box 1 via a double flange connector 7 and is in operation during operation a part of the combustion air flows through it, which exits from the plates 6a, 6b via air nozzles 8. In the plates 6a, 6b through bores 9 are attached for the carbonization gases of the coal.

Under each plate 6a, 6b is the muzzle of a gas igniter attached (ignition not shown).

The front fire trough 2a is the central - larger sub-feed screw 4 assigned.

The rear fire trough 2b are the two - smaller sub-feed screws 5a and 5b assigned, which - located inside the air box 1 - on both sides of the front fire trough 2a are passed by this.

The three feed screws 4, 5a and 5b penetrate the operator-side end wall 1 a of the air box 1.

For the mechanical discharge of the ashes are over the In both longitudinal peripheral areas of the air box 1, the two ash discharge screws 10a and 10b in channels 11

arranged. These screws 10a and 10b are made of refractory material (cast). Instead of the ash discharge screw 10a and 10b can also be arranged in the grooves 11 scraper belts.

The two halves of the air box 1 lying one behind the other are separated from one another by the transverse rib 12, has the passage openings for the combustion air.

The air box 1 has in its vertical normal section the shape of a segment of a circle, whereby it fits well into the surrounding flame tube.

At the end of each fire trough on which the ash is pushed out of it, there is a hinged ash plate 13, which has a ploughshare-like horizontal section and its cutting edge is directed towards the associated fire trough 2a or 2b.

These hinged ash shields 13 favor the outflow of the ash emerging from the fire troughs 2a and 2b to the two Ash discharge screws 10a and 10b.

The ash deflector shields 13 are designed as box structures and during operation of part of the combustion air flows through (cooled), which blow through nozzles 8 'into the flame gases and swirl them through, causing an afterburning is achieved.

An underwind fan is assigned to each of the two halves of the air box 1 lying one behind the other. Through this it is possible to switch off half of the furnace - except for the required amount of cooling air - when the boiler is at low partial load.

The combustion air of the rear air box half is fed to this by means of pipes that are passed through the front air box half (pipes not shown).

In the second embodiment of the underfeed firing system according to the invention shown in FIGS. 4, 5 and 6, ^{three combustion troughs 2a, 2b and 2c are arranged one} behind the other in the air box I 1, which in turn lie in the vertical longitudinal plane of symmetry of the firing system.

The first (foremost) fire trough 2a is the feed screw 4, the second fire trough 2b is the feed screw 5a and the third (last) fire trough 2c the feed screw 5b is assigned.

The feed screws 5a and 5b are again guided past the first fire trough 2a on both sides of the latter. the Sub-feed screw 5b is also guided past the second fire trough 2b to the side of the latter.

The air box 1 'is in three one behind the other Subdivided sub-areas which are separated from one another by transverse ribs 12.

In this embodiment, the ignition areas 2a ¹ , 2b ¹ and 2c 'of the fire troughs 2a, 2b and 2c are each covered by a plate-like tubular construction 14.

Each of these plate-like pipe structures 14 consists of a transverse distributor pipe 14a and five

- 3-He ~

Longitudinal tubes 14b (with air nozzles 8) and is reconnected to the air box I ¹ on the combustion air side via the double flange connector 7. The carbonization gases from the coal can be drawn off upwards through the gaps between the longitudinal tubes 14b.

The other positions of the second embodiment are correct with those of the first embodiment when the item numbers are the same.

In the third illustrated in FIGS. 7, 8 and 9 The new underfeed firing is implemented in the air box 1 "four fire troughs 2a, 2a *, 2b and 2b * left out.

The two adjacent fire pits 2a and 2a * are combined into a pair of fire pits, which is through a Air supply web 15a is separated. So are the two adjacent fire troughs 2b and 2b * combined to form a further pair of fire troughs, which through the air supply web 15b is separated.

The pair of fire pits 2a / 2a * and the pair of fire pits 2b / 2b * are arranged one behind the other.

The two sub-feed screws 4a and 4b are assigned to the pair of fire troughs 2a / 2a *. On the other hand are the pair of fire pits 2b / 2b * the two sub-feed screws 5a and 5b assigned to each side under the fire trough 2a and 2a * - and mirror images of the vertical longitudinal plane of symmetry of the furnace - are arranged.

In the side walls of the two air supply bars 15a and 15b air nozzles are also arranged 8 ⁿ through which a part of the combustion air over the located in front portions of the shelves 3a, 3a *, 3b and * enters the fire troughs 3b (these air nozzles are directed obliquely against the movement direction of the coal in order to improve the ignition of the same). An all-round supply of combustion air into the four fire troughs 2a, 2a *, 2b, 2b * is guaranteed, which causes the coal to burn evenly.

For the mechanical discharge of the ash, over the two air supply webs 15a, 15b and over the two longitudinal, In the peripheral areas of the air box 1 ″, the three ash discharge screws 10, 10a and 10b are arranged in channels 11.

The air box 1 ″ has in its vertical normal section approximately the shape of a segment of a circle. This allows this third embodiment of underfeed firing - if required - very well in a separate double-walled, water-cooled pre-combustion chamber with a circular cylindrical basic shape (according to the German patent application P 30 04 217).

The other positions of the third embodiment are the same as those of the first embodiment when the position numbers are the same.

In Fig. 10 and Fig. 11, the lower part of a smaller earlier oil / gas-fired water tube boiler is shown, which with a multiple underfeed firing was retrofitted.

COPY

The contact heating surfaces 17 are located above the combustion chamber 16 of the water tube boiler arranged.

In the combustion chamber 12 there are six underfeed firings close to one another 18 arranged in such a way that their upper, horizontal boundary surfaces Ib form part of the combustion chamber bottom form.

The longitudinal axes of the six underfeed firings are parallel to the vertical longitudinal plane of symmetry of the water tube boiler.

The coal is fed to the six underfeed firing systems in a manner known per se from the front of the combustion chamber 16 ago.

Each of the six underfeed firings 18 has four in a row arranged fire troughs, each of which is assigned a feed screw. The feed screws of the second, third and fourth fire troughs are passed under or laterally under the first (foremost) fire trough.

Of the six underfeed firings, the one on the left has two ash discharge screws. The other five underfeed firings each have only one ash discharge screw.

In Pig. 12 is a flame tube boiler in a vertical longitudinal section shown with pre-combustion chamber.

In the cylindrical boiler body 19 is the flame tube 20, the internal turning chamber 21 and the Heizrohrbundel 22 arranged. The double-walled, cylindrical pre-combustion chamber 23 (according to the German patent application P 30 04 217) is available via connecting pipes 24, 25 and the collector 26 in connection with the boiler body 19 and contains the underfeed firing 18 '.

COPY

This is the primary air fan 27 with pressure line 28 assigned.

For the return of cold flue gas from the outlet nozzle 29 of the flame tube boiler, a return fan 30 is provided, the pressure line of which leads to a flue gas cooler 31.

The cold gas line 32 contains a quantity measuring nozzle 33 and a throttle element 34 and connects to the pressure line 28 of the primary air fan 27, where the primary air / cold gas mixture is formed during operation. This flows through the air box and the air nozzles and the grate in the firing trough of the stoker 18 'where a delayed combustion at a low temperature level - expires (1000 110o ⁰ C). This prevents the formation of slag clods and enables mechanical ash removal. The proportion of unburned coal and NO formation are also significantly reduced.

From the coal bunker 35, the feed screw 4 leads to Firing 18 '.

, -27-

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Claims (1)

claims 33θ2θθ5 Underfeed firing with a one-part or multi-part air box bounded on all sides by metallic walls, as well as devices for the fuel supply, characterized in that several fire troughs (2a, 2b, 2c, 2a ¹ , 2b ¹ ) in the air box (1, I ^{1, 1 ")} are recessed one behind the other or one behind the other and next to one another. Underfeed firing according to claim 1, of elongated extent, characterized in that it has two or three or four fire troughs (2a, 2b, 2c) in the air box (1, I ¹ ), which are arranged one behind the other and in the vertical longitudinal plane of symmetry (AB, EF ) of the air box (1, I ¹ ). Underfeed firing according to claim 1, of elongated extent, with four or six or eight fire pits, characterized in that two fire pits (2a, 2a *) in pairs next to one another and these pairs (2a / 2a *, 2b / 2b *) of the fire pits are arranged one behind the other (Fig. 7, Fig. 8, Fig. 9). Underfeed firing system according to claim 1 as well as according to claim 2 or 3, characterized in that each fire trough (2a, 2b, 2c, 2a *, 2b *) is assigned one or two underfeed screws (4, 5a, 5b) in a manner known per se. are. GOPY Underfeed firing according to claims 1 and 4, characterized in that all underfeed screws (4, 4a, 4b, 5a, 5b) of the fire troughs (2a, 2b, 2c) are arranged inside the air box (1, I ¹ , 1 ") and through the operator-side Front wall (la) of the same are passed. 6. underfeed firing according to claim 1, 2 and 4, characterized in that the underfeed screw (s) (5a, 5b) of the rear fire trough (s) (2b, 2c) is passed under or laterally under the foremost fire trough (2a) or are. 7. underfeed firing according to claim 1, 3 and 4, characterized in that the underfeed screws (5a, 5b) of the rear fire troughs (2b, 2b *) under or laterally under the two foremost fire troughs (2a, 2a *) are passed. 8. underfeed firing according to claim 1 and 2, with two fire troughs arranged one behind the other, characterized in that the rear fire trough (2b) is assigned two underfeed screws (5a, 5b) which are inside the air box (1) on both sides of the front fire trough (2a) these are passed, and that the normal section of the air box (1) preferably has the shape of a segment of a circle. 9. underfeed firing according to claim 1 and 2, with three fire troughs arranged one behind the other, characterized in that the second and third fire trough (2b, 2c) is assigned a respective underfeed screw (5a, 5b) which is within of the air box (I ¹ ) on both sides of the foremost fire trough (2a) are guided past this, and that the normal section of the air box (I ¹ ) preferably has the shape of a segment of a circle. 10. underfeed firing according to claim 1 or 2 or 3 _f, characterized in that the maximum length (L) of a fire trough (2a, 2b, 2c) is at least twice its maximum width (B.). 11. underfeed firing according to claim 1 or 2 or 3, characterized in that the underfeed screws (4, 5a, 5b) adjacent ignition areas (2a ¹ , 2b ¹ , 2c ') of the fire troughs (2a, 2b, 2c) or their Grids (3a, 3b, 3c) are each covered at the top by at least one plate (6a, 6b, 6c) or a plate-like tubular structure (14). 12. Underfeed firing according to claim 1 and 11, characterized in that the plates (6a, 6b) or the plate-like tubular structures (14) are releasably connected to the one-part or multi-part air box (1, 1 ', 1 "). 13. Underfeed firing according to claim 1 and 11, characterized in that the plates (6a, 6b) or the plate-like tubular structures (14) are provided with air nozzles (8) and part of the combustion air flows through them during operation. 14. Underfeed firing according to claim 1 and 11, characterized in that the plates (6a, 6b) have through bores (9) for the carbonization gas of the fuel. 15. Underfeed firing according to claim 1 or 2 or 3, characterized in that in each fire trough (2a, 2b, 2c) under the plate (6a, 6b) or under the plate-like tubular structure (14) at least one ignition or the muzzle such is arranged. 16. Underfeed firing according to claim 1 and 3, characterized in that the air supply webs (15a, 15b) run parallel or approximately parallel to the vertical longitudinal plane of symmetry of the underfeed firing. 17. Underfeed firing according to claim 1 and 3, characterized in that air nozzles (8 ") are arranged for the passage of part of the combustion air in the side walls of the air supply webs (15a, 15b). 18. Underfeed firing according to claim 1 or 2 or 3, characterized in that the air box (1) has several separate combustion air chambers, or consists of several parts screwed together with separate combustion air chambers. 19. Underfeed firing according to claim 1 or 2 or 3, characterized in that several underwind fans - each fire trough (2a, 2b, 2c) one - are assigned to it in a manner known per se. 20. Underfeed firing according to claim 1 or 2 or 3, characterized in that the normal cross-section of its one-part or multi-part air box (1, l ' _f 1 ") in a manner known per se - at least approximately - has the shape of a segment of a circle. 21. Underfeed firing according to claim 1 or 2 or 3, characterized in that the one-part or multi-part air box (1, I ¹ , 1 ") has one or more transverse ribs (12) with air passage openings to increase its dimensional stability. 22. Underfeed firing according to claim 1 and 2, characterized in that it has two ash discharge screws (10a, 10b) or scraper belts which are arranged above the air box (1, I ¹ ) on its two peripheral long sides in grooves (11). 23. Underfeed firing according to claim 1 and 3, characterized in that an ash discharge screw (10) or a scraper belt over the air supply webs (15a, 15b) and two more ash discharge screws over the longitudinal, peripheral areas of the one-piece or multi-part air box (1 ") (10a, 10b) or scratch belts are arranged in grooves (11). 24. Underfeed firing according to claim 1, characterized in that at least one ash discharge screw (10a) or a scraper belt in a channel (11) is arranged on the upper side of the air box (1, I ^{1, 1 ").} 25. Underfeed firing according to claim 1 and 22 or 23 or 24, characterized in that the ash discharge screws (10, 10a, 10b) or the scraper belts are made of refractory material in a manner known per se. 26, underfeeding according to claim 1 or 2 or 3, characterized in that attached to the ash outlet side of each fire trough (2a, 2b, 2c) on the upper, horizontal boundary wall of the air box (1, I ¹⁾ an ash-steering plate (13), which has a ploughshare-like horizontal section and the cutting edge of which is directed towards the associated fire trough (2a or 2b or 2c). 27. Underfeed firing according to claim 1 or 2 or 3 as well as according to claim 26, characterized in that the ash hinge shields (13) are designed as box structures and are provided with air nozzles (8 ¹ ) and some of the combustion air flows through them during operation. 28. Underfeed firing according to claim 1 or 2 or 3, characterized in that it has at least one pneumatic suction device, the suction openings of which on the edges of the fire troughs (2a, 2b, 2c) or on the edges of the grates (3a, 3b, 3c ) are arranged. 29. Underfeed firing according to claim 1 or 2 or 3 and one or more of claims 4 to 28, characterized in that it is on a flame tube boiler - entirely or partially - in a separate, double-walled, water-cooled Pre-combustion chamber with one or more technical Features of the German patent application P 30 04 is arranged. 30. Water tube boiler, characterized in that in its combustion chamber (16) several underfeed firings (18) are arranged according to claim 1, their longitudinal axes running parallel or approximately parallel to the vertical longitudinal plane of symmetry of the boiler, and that the supply of coal in a manner known per se takes place from the end face of the combustion chamber (16). 31. Water tube boiler according to claim 30 with several underfeed firings, characterized in that the underfeed firings (18) are arranged close together and that the upper horizontal boundary walls (Ib) of the air boxes (1) form the combustion chamber floor - or at least part of it - form. 32. A method for operating an underfeed firing with a one-part or multi-part air box, bounded on all sides by metallic walls and containing one or more fire troughs, as well as devices for the fuel supply, characterized in that cold flue gas is mixed with the primary combustion air, and that the primary air thus formed -Cold gas mixture via the air box (1) and its air nozzles as well as the grate or grids (3a, 3b, 3c, 3a ^x , 3b ^x ) into the fire trough (s) (2a, 2b, 2c, 2a ^x , 2b ^x ) is blown in. 33 · Method for operating an underfeed furnace according to claim 32, characterized in that at the nominal load of the furnace the weight of the cold flue gas returned to the furnace (18 ·) is 10 to 50 # of the weight of the primary combustion air. 34. A method for operating an underfeed furnace according to claim 32, characterized in that the cold flue gas is introduced into the pressure line (28) of the primary air blower (27) or into the pressure lines of the primary air blower.