METHOD AND APPARATUS FOR CASCADED BIOMASS OXIDATION WITH THERMAL FEEDBACK

Abstract

The invention relates to a method for cascaded biomass oxidation in a dish burner with ejection firing. The fuel (18), together with oxygen-containing primary air, is fed to a gasifier dish (8) having high thermal conductivity, in which the fuel is gasified by pyrolysis in a first combustion step, the resulting gas is conducted through guiding devices (6, 9) over the dish edge (11) of the gasifier dish, or over recesses on the upper dish edge, to the outside wall of the dish (8), and is enriched with oxygen-containing secondary air in the intermediate chamber (10) and converted into a cyclone flow around the outer shell dish during a second combustion step, through the convection of which strong thermal feedback is created, together with the high reflection of the thermal radiation on the guiding devices.

Description

The present invention concerns a method for cascaded biomass oxidation in a dish burner with ejection firing and an apparatus for carrying out the method. It concerns the field of heating technology, in particular furnace technology, for solid fuels, with a focus on biomass combustion systems, preferably those for pellets and wood chips.

This method and the apparatus are preferably used for water heating, for heating, and to a small extent for supplying power through cogeneration, or combined heat and power, in single-family and multi-family homes. The method serves the purpose of optimum combustion of fuels in a dish burner with ejection firing, preferably in burner systems for pellets and wood chips. The apparatus represents the central device of a heating boiler, and permits compact, efficient, and low-residue combustion in order to obtain heat.

PRIOR ART

Modern burner systems are divided primarily into top-fed systems with and without grates, stoker feed systems, and retort burners.

In the first type, the fuel (pellets, wood chips, shavings, grain and the like), after being delivered through a feed auger, falls through a chute onto a grate or a firebox with the fire bed. Initial ignition is accomplished by a hot air blower or electric heating devices. The flames reach upward and the hot flue gases are discharged upward to the heat exchanger through flues. These systems are not suitable for straw-like materials. Fill level monitoring is difficult (visually or by lambda probe). A horizontal arrangement of the combustion chamber is referred to as a tunnel burner, which likewise functions without a grate. The pellets are gasified in the combustion zone with the aid of primary air. The secondary air is introduced into an attached combustion cylinder or through laterally arranged nozzle bores. As a general rule, a relatively small intake air flow is additionally introduced through the chute in order to reduce the risk of burn-back. In tilting and vibrating grate systems, the quantity of ash that is produced is automatically dropped periodically into the ash collector located underneath.

In general, however, a pellet stove or furnace assumes a leading position with regard to low emissions and high efficiency, not least on account of the high homogeneity of the fuel; the carbon monoxide emissions are far lower than for other individual heat-producing appliances, and the efficiency reaches values of over 90%.

Stoker feed systems are subdivided into lateral feed (transverse feed) and underfeed systems, where stoking takes place from the side or from below to a grate (fixed, possibly with a tilt function, or traveling as a moving grate) or a steel plate (as a moving floor with or without water-cooling). In the case of transverse feed burners, a portion of the combustion air is blown in through the grate when present, through air nozzles in the side region of the burner trough, or—in the case of moving grate burners—through end-face air channels on the grate elements. The primary air also fulfils the function of grate cooling here. The secondary air is delivered above the grate or fire bed or ahead of the entrance to the secondary combustion chamber.

In retort burners, the fuel is supplied from below the retort (fuel trough) by means of a stoking auger. The drying, pyrolytic decomposition, and gasification of the fuel take place in the retort, in addition to burn-off of the charcoal by means of primary air that is blown in. The secondary air is mixed with the combustible gases ahead of the entrance to the hot secondary combustion zone. Advantages here are the low inertia and the residual heat, although disadvantages include high wear of steel parts, inhomogeneous fire bed and fuel compaction, high residual fuel content in the ash, and high pollutant production during the shutdown process.

For the combustion principles, a distinction is drawn between burn-through, top firing, and bottom firing. In burn-through, combustion air is passed through a grate and the fuel layer. A disadvantage here is the difficult adaptation of the quantity of combustion air to the differences in combustion gas release.

In top firing, the combustion air arrives laterally at the fire bed zone.

In bottom firing, only the bottommost layer of the fuel takes part in the combustion. The combustion gases released in the region of the primary air feed are diverted by a forced draft to the combustion chamber where they undergo secondary combustion under the supply of secondary air. If the combustion chamber is at the bottom, it is called downdraft combustion, if the combustion chamber is to the side, it is referred to as lateral bottom combustion. In general, the control variable for regulating the different combustion phases (start-up phase, stationary phase with constant output, partial load phase and burnout phase) is the air. When there is a separation between primary air flow and secondary air flow, there are two control variables. The burner output can be adjusted from approximately 50% to 100% with the primary air. The complete burn-off of the combustible gases is controlled with the secondary air. For output-regulated boilers, an induced-draft or forced-draft blower serves to control output. The fuel feed is regulated at the same time. Lambda probes are used to measure the excess air in the exhaust gas flow.

DE 10 2005 033 320 dated Feb. 15, 2007 discloses a method for burning solid fuels and a device according to that method. Shown here is a tray-like grate with at least one air chamber, wherein primary air is fed into this air chamber and secondary air is conducted around the air chamber. WO 99/15833 dated Apr. 1, 1999 shows a “Self-Stoking Wood Pellet Stove” with the features: worm conveyor, discharge chute, and regulator valves for supply of primary and secondary air, as well as a flue leading upward.

Because of the asymmetrical arrangement of the elements, the above systems are beset by the following disadvantages: the difficulty of achieving a state of optimal combustion even under partial load, the complex design or cleaning of the grate and the great space requirement for the combustion device.

Each of the cited systems has advantages and disadvantages, with no one of these systems combining the majority of advantages.

OBJECT OF THE INVENTION

Based on this prior art, a method and an apparatus were sought for optimal combustion of biomass in which the advantages of all existing systems are combined to the greatest possible degree and, moreover, which provide good results even in partial load conditions. The design should be compact, the material requirements low, and the construction should be minimal with respect to the achievable performance yield. In addition, the number of wearing components should be reduced to an acceptable minimum, or else the service life of the wearing parts should be as long as possible. Moreover, a very low-noise combustion system is desired with the most homogeneous burn-off possible and low turbulence in the heating. Uncontrolled convection of the heat should be avoided to the greatest extent possible. In like manner, sooting and burn-back should be precluded. In addition, for maintenance, the easiest possible accessibility to the areas to be cleaned should be provided and simple means for replacement of parts should be provided.

Application for Protection

In order to attain this object, provision is made for fuel to be supplied together with primary air containing oxygen to a gasifier box with high thermal conductivity, wherein the fuel is gasified through pyrolysis in the first combustion step, the resultant gas is directed to the outside wall of the box by guide devices above the box edge of the gasifier box or through recesses at the top edge of the box to the outside wall of the box, and is enriched in the intermediate space with secondary air containing oxygen and, during a second combustion step, is transformed into a cyclonic flow about the outside box wall, the high convection of which, in conjunction with the high reflection of the heat radiation at the guide devices, results in strong thermal feedback.

Here, fuel is supplied together with oxygen-containing primary air to a gasifier box in a dish burner with ejection firing with high thermal conductivity, wherein the fuel is gasified through pyrolysis in the first combustion step. The resultant gas is directed by special guide devices above the box and around the box. For example, due to the suction caused by a suction fan or the pressure caused by a forced-draft blower, the gases travel across the box edge of the gasifier box or through recesses at the top edge of the box directly to the outside wall of the box. In the narrow-walled intermediate space, enrichment with oxygen-containing secondary air takes place, while a cyclonic flow about the box outside wall is simultaneously imposed by suitable devices. During this process, intensive mixing and uniform distribution of the exothermically reacting gases takes place in the second combustion step. Reflections of the heat radiation at the guide devices and high convection of the gas flow serve the purpose of strong thermal feedback of the materials to one another, and hence to very good burnout of the flue gases. The pyrolysis is improved or can take place in a smaller space. The dense and efficient gas conduction brings about a flue gas with an extremely low concentration of pollutants, even under partial load conditions. Particles of ash are forced against the burner wall by the cyclonic flow, and fall out of the main gas flow. Soot particles and fine particulates burn up at the hot guide devices because of their catalytic effect.

It is especially advantageous when the predominant flow direction of the cyclonic flow points downward and the gas flow is conveyed directly into the diffuser chamber at the lowest point directly beneath the box. As a result, the fuel gases are guided downward in a targeted and homogeneous manner, opposite to the customary upward chimney direction.

As a refinement, the flow can be forced into additional cascades with the aid of alternating dish-like and funnel-like guide devices. Then an upwardly directed flow follows the downward flow again, and vice versa. The cascades thus produced represent flow layers with respect to the diffuser chamber, which can deliver heat to the environment or heat exchanger in an optimized manner.

It is advantageous if the flow speed of the primary air in the gasifier box and the flow speed of the secondary air in the zones outside the gasifier box are different or can be regulated separately from one another.

In an apparatus according to the invention, guide devices are provided above and below a gasifier box to guide the gas flow after pyrolysis into close contact with the outside of the gasifier box, wherein at least one secondary air nozzle is provided between the gasifier box and the guide devices whose axial flow outlet direction has a horizontal component and runs at least approximately tangentially to the gasifier box wall.

Essentially, this consists of a two-dimensional upper boundary with the exception of the feed pipe, and a two-dimensional boundary arranged laterally around the gasifier box. The guide devices serve to guide the gas flow after pyrolysis into close contact with the outside of the gasifier box and to reflect the heat of combustion in alternation depending on the operating state. A cyclonic flow is imposed with the aid of at least one secondary air nozzle between the gasifier box and the guide devices. To this end, the axial flow outlet direction of the nozzle is advantageously such that it has a horizontal component. The nozzle axis here runs at least approximately tangentially to the gasifier box wall.

A simple embodiment of the top guide device is in the form of a cover constructed in two layers with a hollow space. This hollow space should have at least one air intake opening. The bottom boundary of the cover can be connected to at least one nozzle whose air intake opening is connected to the hollow space of the cover. A fill opening with feed pipe for the fuel is left free here. This feed pipe is also nearly gas-tight toward the top, with the result that the flue gas can only escape downward.

Guidance takes place by means of the bottom guide device, which, like a funnel, is designed as a second box around the outer wall of the gasifier box with the exception of a bottom recess, with parallel spacing or with spacing which changes toward the bottom. In this design, the bottom guide device is attached to the top guide device.

The bottom guide device, like the gasifier box, can have pyramidal, spherical, conical, paraboloidal or hyperboloidal surface segments.

The gasifier box and its surrounding guide device can advantageously be arranged coaxially and concentrically about a burner axis. They may be made to be rotationally symmetrical.

It is advantageous for at least one air outlet opening of a primary air nozzle to be positioned inside the box just above the lowest point of the box, near the burn-off position of the biomass, by which means the optimal quantity of air can be supplied to the pyrolysis. The position of at least one air outlet opening, specifically of at least one secondary air nozzle, should usefully be provided on the outside of the box.

At least one of the air supply nozzles can be designed to good advantage as an annular gap nozzle about a feed pipe. Alternatively or in addition, multiple primary air nozzles or multiple secondary air nozzles may be arranged about the burner axis at uniform angular spacing.

It is useful for the flow speed through the air supply nozzles to be regulated and controlled by at least one suction blower as exhaust gas fan. In addition, it can be regulated by means of a rotary valve around the air intake openings.

Bimetallic regulators can advantageously be provided in the air supply nozzles for temperature-dependent regulation of the flow speed.

The air supply nozzles can be made by deep drawing, but can also be made from welded-on metal tube sections or ceramic parts.

It is useful for the top guide device to be spherical in shape to avoid stress deformations or cracks, or else to have individual, separate convexities as an expansion reserve.

The radial spacing between the gasifier box and the bottom guide device can be made mutually adjustable. This spacing is constant in the axial direction, or tapers in the flow direction.

To avoid heat losses at the top, a thermal insulating layer, preferably made of mineral wool or ceramic fiber, is provided above the top guide device.

The gasifier box can be provided with a spring-mounted impact cone or grate-like components for removal of residual ash.

The invention is explained in detail below with reference to the attached drawings. They show:

FIG. 1 a vertical section through the invention.

FIG. 2 the horizontal section A-A′ from FIG. 1.

In FIG. 1, the method and the apparatus are illustrated using the vertical section. Shown is the burner 1 with its double-box construction that is clearly visible here. Above the central feed pipe 2, the fuel 18, preferably pellets, wood chips, or wood shavings, is fed into the gasifier box 8 with the aid of an auger (not shown here). The fuel falls through the feed pipe because of gravity at intervals in time because of the auger advance. In doing so, it passes through the fill opening 20 in the burner cover. The feed pipe has a flow resistance towards the top that is accomplished through suitable check valves, through seals, or through a slight primary air flow on account of an overpressure on account of a forced-draft blower or a suction blower in the exhaust duct, not shown here. The fuel falls during the course of combustion, and the burned fuel is replaced by fresh fuel. Since the optimum operating condition is achieved at a constant box level, even at different outputs, and the fuel from above tends to be cold, the level for precise fuel metering can be achieved through detection by a sensor 23, even with imprecise conveyor systems. In the case of a cold start, the fuel 18 consisting of biomass is heated by electric heating elements or by hot air. This initial heat can be supplied from above through the primary air nozzles 3 with integrated glow wires or from below through a possible grate in order to start the pyrolysis process. In FIG. 1, four primary air nozzles 3 are shown (three visible), which are arranged about the burner axis 19 at an angle of 90° and extend diagonally into the gasifier box 8. The biomass fuel 18 may also cover the primary air nozzles 3. The primary air can also be supplied through a possible grate mounted on the floor or in the vicinity of the floor of the gasifier box. Because of the initial heat, the fuel is gasified by the pyrolysis, with additional heat being released in the process, with the result that the hot air or electric heat is no longer required. The gasification produces combustible gases consisting of carbon compounds, thereby bringing about temperatures of up to 1000° C. at the floor of the gasifier box. The gases are directed by the blower up the inside wall of the box to the box edge 11, where transfer into the intermediate space 10 occurs. If the box is suspended at multiple points or separated from the bottom guide device by spacers, then the flow slips directly over the edge of the box. However, a direct connection of the box edge to the top guide device 6, in particular its lower boundary 22, is also possible. In this case recesses, such as holes in the top edge of the box, must be provided in order to allow flow into the intermediate space 10 between the gasifier box and the bottom guide device 9. Here, the gasifier box represents the surface of a truncated cone that is made of sheet steel approximately 2 mm thick, with a level floor. The angle of opening is about 90° and the bottom guide device 9 forms a conical funnel at the radial spacing 12, and is displaced axially and concentrically thereto. This produces a double wall with constant spacing in the axial direction. Between these walls, the primary combustion of the pyrolysis gases takes place with secondary air supplied through secondary air nozzles 13. The shape of the box does not have to be conical, any box shape that forms a trough is possible, and only minor restrictions are placed on the shape of the guide device 9 as well. Here, the elements critical to the shape are the production of thermal stresses, the desired flow profiles for the combustion gas flow, and the aspects of aging-induced wear and maintenance. Thus, spherical, conical, paraboloidal, hyperboloidal or even multi-sided pyramidal shapes or composite shapes are possible. When a spacing that narrows towards the bottom is chosen, the rotational speed of the gas flow increases towards the bottom. Also, a spiral-shaped guide groove may be provided on the outside wall of the box and the inside wall of the guide device 9 in order to achieve dimensional stability coupled with laminar cyclonic flow. This cyclonic flow is also achieved by the shape of the secondary air nozzles 13. Here, six nozzles are distributed evenly about the box circumference in the region where the combustion gases from the gasifier box enter the intermediate flow space 10. The angular spacing is 60° here. The nozzle opening 15 in this design is not aimed perpendicularly downward, but instead is tilted at an angle to the burner axis 19. It is useful for this angle to be chosen such that the dwell time of the flow about the gasifier box brings about good feedback or interaction with the first stage of combustion. The load is determined by the feed rate of the fuel 18 by the auger in conjunction with the strength of the blower draft and the removal of heat. Regardless of this, the gasifier box 8 should be kept at an optimum temperature level as activation energy for the pyrolysis. The tight guidance of the secondary combustion air along the gasifier box 8, coupled with the high reflection of the heat radiation by the guide device 9 on the one hand and by the gasifier box 8 on the other hand, has a self-regulating effect, causing the gases to be fully oxidized independently of load before they can strike the cold diffuser surfaces and incompletely combusted pollutant gases such as carbon monoxide can exit the chimney.

The number and shape of the secondary air nozzles are chosen as desired here. The imagination is unfettered by restrictions. Thus, an annular gap nozzle can also open onto the intermediate space 10 formed by the bottom boundary 22 of the top guide device 6. Additional guide panels can then bring about the defined cyclonic flow.

Here, the primary air and secondary air originate directly from the hollow space 6 of the two-layer top guide device (burner cover). The air is supplied from outside through the air intake opening (shown here as a circumferential gap). The conduction of heat by the panels employed and the heat radiation of the combustion gases accomplish the result that the burner cover reaches high temperatures and thus also bring about high preheating for the primary and secondary air. The primary air nozzles 3 and the secondary air nozzles 13 are connected directly to the hollow space 5 of the burner cover 6 through the air inlet openings 4 and 14. Thermal losses are prevented by the insulating layer 17 above the burner cover 6.

FIG. 2 shows a view from above of the section along line A-A′ from FIG. 1, and makes clear the symmetrical design of the embodiment. The air inlet openings of the nozzles 3, 13 are visible in cross-section, and the gasifier box is visible.

LIST OF REFERENCE CHARACTERS

1 dish burner

2 feed pipe

3 primary air nozzle

4 air inlet opening in the primary air nozzle

5 hollow space of a two-layer cover

6 top guide device (cover)

7 air outlet opening from the primary air nozzle

8 gasifier box

9 bottom guide device (wall, cyclone nozzle, funnel)

10 intermediate space (2^nd combustion stage)

11 box edge of the gasifier box

12 spacing between box and bottom guide device

13 secondary air nozzle

14 air inlet opening in the secondary air nozzle

15 air outlet opening from the secondary air nozzle

16 air intake opening

17 insulating layer

18 fuel with ash from biomass

19 burner axis

20 fill opening for the fuel

21 opening of the bottom guide device (nozzle, funnel) into the diffuser chamber

22 bottom boundary of the top guide device (cover)

23 sensor for determining fuel quantity

Claims

1-20. (canceled)

21. A method for cascaded biomass oxidation in a dish burner with ejection firing, the method comprising: supplying fuel together with primary air containing oxygen to a gasifier box with high thermal conductivity;gasifying the fuel through pyrolysis in a first combustion step;directing the resultant gas towards the outside wall of the box via guide devices arranged above a box edge of the gasifier box or through recesses arranged at the top edge of the gasifier box towards the outside wall of the box; andenriching the resultant gas in an intermediate space with secondary air containing oxygen and, during a second combustion step, transforming the resultant gas into a cyclonic flow about the outside box wall, a high convection of which, in conjunction with a high reflection of heat radiation at the guide devices, resulting in strong thermal feedback,wherein the flow can be forced into additional cascades with the aid of additional alternating dish-like and funnel-like guide devices, andwherein a downward flow is followed again by an upward flow, and conversely an upward flow is followed again by a downward flow, and the cascades thus produced represent flow layers with respect to the subsequent diffuser chamber.

22. The method according to claim 21, wherein a predominant flow direction of the cyclonic flow points downward and the gas flow is conveyed directly into a diffuser chamber at the lowest point directly beneath the box.

23. The method according to claim 21, wherein the flow speed of the primary air in the gasifier box and that of the secondary air in the zones outside the gasifier box are different or can be regulated separately from one another.

24. An apparatus for cascaded biomass oxidation in a dish burner with ejection firing, the apparatus comprising: guide devices arranged above and below a gasifier box to guide the gas flow after pyrolysis into close contact with an outside of the gasifier box; andat least one secondary air nozzle provided between the gasifier box and the guide devices whose axial flow outlet direction has a horizontal component and runs at least approximately tangentially to the gasifier box wall.

25. The apparatus according to claim 24, wherein the top guide device is designed as a cover constructed in two layers with a hollow space and has at least one air intake opening, has at least one nozzle with air inlet opening that is connectable to the a bottom boundary of the cover and to a hollow space, and has a fill opening with feed pipe for the fuel.

26. The apparatus according to claim 24, wherein the bottom guide device, with the exception of a bottom recess, is designed as a second box around the outer wall of the gasifier box, with parallel spacing or with spacing that narrows towards the bottom, and wherein the bottom guide device is attached to the top guide device.

27. The apparatus according to claim 26, wherein the bottom guide device and the gasifier box have pyramidal, spherical, conical, paraboloidal or hyperboloidal surface segments.

28. The apparatus according to claim 24, wherein the gasifier box and its surrounding guide device are arranged coaxially and concentrically about a burner axis and are configured to be rotationally symmetrical.

29. The apparatus according to claim 24, wherein at least one air outlet opening has at least one primary air nozzle just above the lowest point of the box inside the box, near the burn-off position of the biomass, and/or has at least one air outlet opening of at least one secondary air nozzle on the outside of the box.

30. The apparatus according to claim 24, wherein at least one of the air supply nozzles is configured as an annular gap nozzle about the feed pipe.

31. The apparatus according to claim 24, wherein multiple primary air nozzles and/or multiple secondary air nozzles are arranged about the burner axis at a uniform angular spacing.

32. The apparatus according to claim 24, wherein the flow speed through the air supply nozzles is regulatable by at least one suction blower as exhaust gas fan and/or a rotary valve around at least one air intake opening.

33. The apparatus according to claim 24, wherein the regulating devices are provided in the air supply nozzles.

34. The apparatus according to claim 24, wherein the air supply nozzles are made by deep drawing, or from welded-on metal or ceramic tube sections.

35. The apparatus according to claims 24, wherein the top guide device is spherical in shape, or has individual, separate convexities.

36. The apparatus according to claim 24, wherein the gasifier box and the bottom guide device are arranged with a small, constant radial spacing, or a radial spacing that tapers in the flow direction, and wherein the spacing is adjustable via lifting spindles.

37. The apparatus according to claim 24, wherein a thermal insulating layer made of mineral wool or ceramic fiber is arranged above the top guide device.

38. The apparatus according to claim 24, wherein the gasifier box has a grate or a spring-mounted impact cone for removal of residual ash.

39. The apparatus according to claim 24, wherein the mass flow rate of the fuel is regulatable by the level of the fuel at its input-side surface in the gasifier box via a sensor.