Step grate and method for burning solid fuel on a step grate

Abstract

Method for burning solid fuel on a stepped grate (20) of a mobile solid fuel combustion plant (2), which has several steps arranged between side walls (42), each consisting of an intermediate plate (38) and a pusher plate (36) movable relative to the side walls (42) and the intermediate plate (38), in which the pusher plates (36) push burning solid fuel in the direction of the steps (56) from one step to the next lower step by a relative movement to the respective adjacent intermediate plates (38), characterized in that the solid fuel is burned on the pusher plates (36) and, after the end of a burning period, is conveyed during a pusher period to the next lower pusher plate (36) and burns there during the following burning period.

Description

[0001] The invention relates to a stepped grate and a method for burning solid fuel on a stepped grate.

[0002] Mobile solid fuel combustion plants are used to generate large quantities of heat for a limited period at a single location. Possible applications include generating heat for hay drying, heating a large tent, or providing short-term heat for industrial processes. For these purposes, the solid fuel combustion plant is moved to the site, set up, and started. A solid fuel combustion plant—hereinafter referred to simply as a combustion plant—must not be too heavy, as the entire unit should be liftable with simple means, such as a forklift.However, a solid fuel combustion plant must be built very robustly, since the combustion of solid fuel is not as controllable as the combustion of liquids or gases and therefore places very high demands on the combustion chamber, the heat exchanger and other components of the combustion plant exposed to combustion heat.

[0003] AT 505 162 A4 relates to a boiler for lump fuel with a stepped grate consisting of grate elements that closes off a combustion chamber at the bottom. With the exception of the lowest grate element, the grate elements are fixedly arranged in a frame. This frame, with the exception of the lowest of the fixed grate elements, has slides below the fixed grate elements. These slides can be moved by means of an actuator through openings in the steps between the successive grate stages, over the subsequent grate elements.

[0004] DE 10 2008 063 709 A1 relates to a flame-tube boiler with a boiler shell and an internal flame tube. A mechanical stepped grate for the combustion of solid fuels is installed in the flame tube.

[0005] DE 339 751 A relates to a combustion process in which a burning mass is passed over a stepped grate and broken up from stage to stage with ample air supply. The process utilizes a stepped grate equipped with moving brush grates, the air supply, step design, and brush grate arrangement of which enable the mass to be continuously broken up from stage to stage and the complete combustion of even fuels of very different types and sizes.

[0006] WO 1995 / 018 333 A1 concerns a push-type incineration grate module for incinerating waste in large-scale plants. One grate plate of the module consists of two welded stainless steel sheet shells. On the underside of the grate plate are two connection ports for connecting a supply and discharge line for a medium flowing through the grate plate.

[0007] It is therefore an object of the present invention to provide a mobile solid fuel combustion plant and a method for burning solid fuel on a stepped grate of a mobile solid fuel combustion plant, with which a high combustion performance combined with a relatively low weight of the combustion chamber can be achieved.

[0008] The problem addressed in the method is solved by a method according to claim 1. By moving the push plates relative to the intermediate plates, the burning solid can be distributed in the combustion chamber using lightweight means in such a way that it is well ventilated, thus resulting in hot, efficient, and high-performance combustion. A very good power-to-weight ratio can be achieved.

[0009] The combustion system is expediently a wood-fired combustion system, which is advantageously prepared for operation with wood chips. The stepped grate according to the invention is particularly suitable for use in a mobile solid fuel combustion system; however, its use is not limited to mobile combustion systems, as its advantages can also be utilized in stationary combustion systems.

[0010] Furthermore, it is advantageous if the combustion system is air-cooled, in which the heat generated in the combustion chamber is transferred directly to the air via the combustion chamber wall. An intermediate cooling medium, such as water, is not required. This also allows the combustion system to be designed with a lighter weight.

[0011] The direction of travel, or stair direction, refers to the direction of the stairs from top to bottom. This corresponds to the conveying direction, i.e., the direction in which the fuel is conveyed from step to step. The push plates are advantageously connected to one another by a connecting element, the movement of which allows the push plates to be moved back and forth together, thus eliminating the need to move each individual push plate separately.

[0012] When burning solid fuel on a stepped grate, the fuel can be combusted on stationary intermediate plates, which then serve as grate plates. After a while, the pusher plates are activated, and the burning solid fuel is pushed from the intermediate plates to the next lower intermediate plate. In such a process, the intermediate plates are advantageously made particularly robust, as the thermal stress acting on them is very high and limits their service life.

[0013] According to the invention, this type of stepped grate combustion is dispensed with, and the fuel is burned on the pusher plates. After a combustion period, the burning solid is conveyed in the step direction to the next lower pusher plate during a pusher cycle, in order to continue burning there during the following combustion period. The combustion periods and the shorter pusher cycles alternate, with the combustion period advantageously being at least five times longer than the pusher cycle. Furthermore, it is advantageous if more than 80% of the combustion takes place during the combustion period and thus on the pusher plates.

[0014] This method has the advantage that the intermediate plates can be made thinner and therefore lighter. The thermal load is shifted to the movable sliding plates, which are consequently subjected to greater stress and thus wear out faster. However, the sliding plates can be replaced more easily because they are movably mounted within the stepped grid. If the sliding plates are conveniently mounted on the stepped grid in such a way that they can be pulled out, they can be easily replaced and renewed. The requirement for a particularly long service life of the sliding plates can therefore be reduced, as they can be replaced easily and cost-effectively.

[0015] Advantageously, the push plates for conveying the burning solid are first pulled backwards one step down and then pushed forwards again. "Forward" here refers to the direction of the steps or stairs, excluding the vertical component of the step direction. This movement allows for the easy conveyance of the burning solid along with the main combustion on the push plates.

[0016] With the same advantage, the push plates are advanced forward during a combustion period, are retracted to the rear after the combustion period to expel the burning solid, and are then pushed forward again into the combustion position.

[0017] A good supply of combustion air is essential for hot and efficient combustion of the solid fuel. For this purpose, the combustion plates could be perforated so that the combustion air can be supplied evenly and across the entire surface from below. However, this design weakens the plates and leads to faster wear, which must be compensated for with thicker plates and thus increased weight. The plates can be thin, and the weight of the combustion system kept low, if, during operation, combustion air—preferably primary combustion air—is supplied to the burning solid fuel at least predominantly, and ideally exclusively, between the intermediate plates and the pusher plates. The plates can then be smooth and solid, thus keeping the overall weight down.

[0018] A combustion air supply system as described is particularly advantageous when the stages are relatively short, allowing unburned combustion air to reach the front of the stage to enable combustion there as well. Accordingly, it is advantageous if the number of stages in the stepped grate and the solid fuel combustion system are coordinated so that the combustion per stage, calculated uniformly across all stages, has a maximum firing capacity of 70 kW per stage at nominal load. Power outputs between 20 kW and 70 kW are particularly advantageous, especially between 30 kW and 50 kW.

[0019] With a simple feed system, the solid fuel is conveyed to the top step at only one point, essentially at a single location. An excessively wide staircase – the width of which should be considered perpendicular to the direction of travel – therefore has the disadvantage that no fuel reaches the edges of the steps, as the width increases only slightly from step to step. A step width between 30 cm and 70 cm is therefore advantageous. However, if multiple feed points are planned, the step width can be greater. Consequently, the combustion output per step can also be higher.

[0020] Generally speaking, it is advantageous if the number of steps in the staircase and the solid fuel combustion system are matched so that the combustion per step, calculated evenly across all steps, results in a maximum firing rate of 150 kW per meter of staircase width at nominal load operation. A firing rate between 40 kW and 150 kW per meter of staircase width is particularly suitable, especially between 60 kW and 100 kW. Therefore, with a nominal output of 250 kW, a staircase width of at least 2.5 m is required. With a step width of 50 cm, at least 5 steps should be present. With a nominal output of 750 kW, a staircase width of at least 7.5 m is required. With a step width of 70 cm, at least 11 steps should be present.

[0021] In this way, the combustion output per stage area is limited to ensure efficient combustion even with the weight-saving combustion air supply described above. An advantageous stage dimensioning can be achieved if the combustion output at nominal load reaches a maximum of 50 kW per stage – again, distributed evenly across all stages.

[0022] To minimize the load on the ignition device for igniting the solid fuel, it is advantageous if the combustion air, directed between the intermediate plates and the pusher plates, heats up as it passes through the plates to such an extent that it ignites the unburned solid fuel. After an initial ignition, automatic re-ignition of the newly introduced unburned solid can thus be achieved, eliminating the need for continuous ignition of the solid fuel on the top stage. The ignition device can therefore be simple and lightweight. The high temperature of the primary air at the solid fuel has the further advantage of promoting low-emission combustion, as it contributes to more complete combustion.

[0023] A stepped grate of a mobile solid fuel combustion plant has two side walls and several intermediate plates anchored in the side walls and arranged in the manner of steps, and several push plates arranged between two intermediate plates, which are movable in the step direction to push burning solid fuel from one step to the next lower step relative to the respective adjacent intermediate plates.

[0024] A particularly lightweight design for the stepped grating can be achieved if both the intermediate and sliding plates are made of sheet metal. Heavy, cast-iron plates are unnecessary. Ideally, the sheets should be flat. This allows for excellent airflow between the plates.

[0025] A good compromise between good wear resistance and a lightweight design of the sheets can be achieved if their thickness is between 3 mm and 10 mm. Thicknesses between 4 mm and 7 mm, and especially 6 mm, are particularly advantageous.

[0026] Particularly high wear resistance can be achieved if the sheets are made of stainless steel. However, sheets made of structural steel are especially advantageous because they have low thermal expansion. This is important for the very simple anchoring of the sheets in the side walls, as it allows for a particularly simple and therefore weight-saving construction of the entire stepped grating. Furthermore, structural steel is sufficiently temperature-resistant and inexpensive, so that replacing the sliding plates involves only minimal costs. Structural steel can be defined as steels that bear the prefix "S" for "structural steel" in the European standard EN 10027. Steels from the St37 group, which originates from the old German standard, are particularly advantageous.

[0027] Furthermore, a weight-saving design for the step grating is facilitated if the intermediate plates and the sliding plates are essentially the same thickness. A ratio of intermediate plate thickness to sliding plate thickness between 0.7 cm and 1.7 cm for all steps is advantageous, particularly between 0.9 cm and 1.1 cm.

[0028] A good balance between ventilation and low weight can be achieved if all steps, consisting of an intermediate plate and a sliding plate, are no more than 3 cm thick. The height difference between one step and the next is therefore a maximum of 3 cm.

[0029] A weight-saving design for the step grate is also possible if the intermediate plates are movable and the side panels are inserted. Welding and a rigid structure of the step grate are unnecessary. Furthermore, the step grate remains low-stress even under large temperature fluctuations.

[0030] Another advantageous embodiment of the invention provides that the stepped grate comprises a front wall and a base with an ash discharge opening, which together with the side walls form a tray. This also allows for a compact and weight-saving design.

[0031] In combustion plants, the ash from the burned solid fuel is typically discharged through an opening in the bottom of the combustion chamber. The ash can then be conveyed, for example by a screw conveyor, into an ash container. However, the ash container is usually located in an environment with normal ambient pressure, whereas a negative pressure should be maintained in the combustion chamber to prevent unwanted flames from escaping. Therefore, it is common practice to install a pressure lock, such as a rotary valve, between the combustion chamber and the ash container.

[0032] A pressure lock can be dispensed with and its weight saved if the bottom sliding plate of the staircase at least largely closes the ash discharge opening. In this way, the bottom sliding plate can largely take over the function of the pressure lock. The sliding plate, in conjunction with the floor, is designed to counteract or at least largely prevent air from entering through the ash discharge opening, thus reducing it by more than 90%. Advantageously, the bottom sliding plate closes at least 90% of the opening cross-section of the ash discharge opening.

[0033] Furthermore, it is proposed that the sliding plates are each supported at several points by sliding supports, so that the sliding plates and sliding supports form a self-supporting unit that can be pulled out as a whole from a frame containing the side walls. Replacing the sliding plates can be done particularly easily, and therefore quickly and cost-effectively, thus ensuring a long service life for the stepped grating.

[0034] Another advantageous embodiment of the invention provides that the push plates and intermediate plates increase in length from step to step, at least over the top three steps in the direction of the staircase. The top step is thus shorter in the direction of the staircase than the second-to-top step, which in turn is shorter in the direction of the staircase than the third-to-top step. This increasing length of the steps is advantageously provided at least up to the middle of the staircase, i.e., in the case of seven steps, at least up to the fourth step. An increase in step length beyond the middle is also advantageous. The advantage of such a step geometry lies in the fact that the solid fuel on the top step is not yet fully combusted, and the heat of combustion there is therefore relatively low. This heat increases continuously from step to step in the direction of the staircase, so that the heat load on the steps increases from top to bottom in the direction of the staircase, at least up to the middle.As the steps lengthen in the direction of travel, the cooling air, or combustion air, can cool the longer steps more effectively, thus reducing their thermal load. The longer the steps are in the direction of travel, the greater the cooling effect of the combustion air.

[0035] On the final step, the fuel should ideally be completely burned, so that – after the last combustion cycle – only fully combusted ash remains. The heat generated on the lowest step is therefore lower than on the steps above it. It is thus advantageous for the length of the steps to decrease towards the final step, ideally extending over at least three steps. The lowest step can therefore be shorter than the step above it, and this in turn shorter than the step above that. If the lowest sliding plate is used as a pressure lock to close the ash discharge opening, the decrease in length can apply to the penultimate step, which is ideally shorter than the third-to-last step.

[0036] The invention further advantageously relates to a mobile solid fuel combustion plant with a stepped grate as previously described and shown in the Fig. description.

[0037] The preceding description of advantageous embodiments of the invention contains numerous features, some of which are summarized in the individual dependent claims. However, it is advantageous for those skilled in the art to consider these features individually and combine them into meaningful further combinations. In particular, these features can each be combined individually and in any suitable combination with the inventive method for, or the inventive step grate. The same applies to features of each embodiment described in the following figures, which can be explicitly considered in isolation and combined with the inventive method or the inventive step grate.

[0038] They show: Fig. 1 a schematic representation of a mobile and air-cooled solid fuel combustion plant with a stepped grate, Fig. 2 of the step grate made of Fig. 1 in a sectional view, Fig. 3 of the step grate made of Fig. 2 in the same sectional view but with retracted sliding plates, Fig. 4 of the step grate in a side view, Fig. 5 of the step grate in a front view, Fig. 6 of the step grate in a top view and Fig. 7 another stepped platform with shorter steps at the bottom.

[0039] Fig. Figure 1 shows a schematic representation of a mobile solid fuel combustion plant 2 and a solid fuel storage facility 4. The solid fuel storage facility 4 is also mobile, i.e., transportable. For this purpose, both the solid fuel storage facility 4 and the combustion plant 2 are each equipped with a transport device 6, 8 (shown only schematically), by means of which the two elements 2, 4 can be lifted, for example, by a forklift. The transport devices 6, 8 comprise a stable frame that holds the other components of the respective element 2, 4.

[0040] The solid fuel storage unit 4 is connected to the combustion plant 2 via a connection 10, which includes a joint or other angle adjustment device to compensate for any unevenness in the installation of the two elements 2 and 4. For this purpose, the connection 10 is also equipped with a height adjustment device for adapting a discharge unit 12. The discharge unit 12 is, for example, a screw conveyor and serves to transport solid fuel, such as wood chips, located in the solid fuel storage unit 4 to the combustion plant 2.

[0041] To operate the combustion plant 2, it is transported to its operating location, for example on a truck, and placed on the ground. The solid fuel storage unit 4 is also transported to the operating location and placed next to the combustion plant 2. The two elements 2 and 4 are then connected via connection 10. Position adjustment of the two elements 2 and 4 relative to each other is generally not necessary, as connection 10 sufficiently compensates for unevenness in the ground. Solid fuel, such as wood chips, pellets, or other suitable solid fuel, can now be filled into the solid fuel storage unit 4, for example, using a wheel loader. During operation of the combustion plant 2, the solid fuel is conveyed to the combustion plant 2 via the discharge unit 12 and connection 10.

[0042] In an alternative embodiment, the combustion unit 2 and the solid fuel storage unit 4 are housed in a single, integrated frame and can be transported together. This solution is particularly advantageous for units up to 500 kW, as it simplifies transport and eliminates the need to connect the two elements 2 and 4 to each other.

[0043] After passing through a backfire protection device 14, the solid fuel is conveyed via a further conveying element 16, designed as a screw conveyor, into the combustion chamber 18 of the combustion system 2. It is transported by the conveying element 16 to the upper end of a stepped grate 20, i.e., to its uppermost step. There, the solid fuel is ignited and thus burns over time on the stepped grate 20. During the combustion period, the solid fuel is conveyed further downwards, step by step, and finally reaches an ash discharge opening 22 into an ash container 24. From there, the ash is conveyed via an ash screw conveyor 26 into a further ash container 28.

[0044] The hot flue gases produced during combustion are fed through a combustion chamber 30 to a heat exchanger 32, which is cooled with ambient air. This air is heated in the heat exchanger 32 and is then available for further use. The exhaust gases, cooled in the heat exchanger 32, are fed to a spark arrestor 34, for example, a cyclone separator. Larger systems can incorporate several cyclone separators operating in parallel. Separated ash is also collected in the ash container 28, and the cleaned exhaust gases are discharged upwards from the combustion unit 2.

[0045] Fig. Figure 2 shows the stepped grating 20 from the side in a sectioned view. Seven steps are visible, each with a sliding panel 36 and an intermediate panel 38 above it. The uppermost intermediate panel 38 is not actually an intermediate panel, as it is the uppermost panel. For clarity, the sliding panels 36 are shown with narrow longitudinal hatching, and the intermediate panels 38 are shown with wide diagonal hatching.

[0046] The stepped grate 20 is in the Fig. 3 to 6 shown in four different views. Fig. Figure 3 shows the stepped grid 20 analogous to Fig. 2, however in a different position of the push plates 36. Fig. Figure 4 shows the stepped grating 20 from the side, i.e., in the same perspective as in the Fig. 2 and Fig. 3, but not cut. Fig. Figure 5 shows the stepped grid 20 from the front and Fig. Figure 6 shows the stepped grating 20 in a top view. The stepped grating 20 will be described below with reference to all figures.

[0047] The stepped grate 20 comprises a frame 40 made of two opposing side walls 42, which are folded upwards for reinforcement. The frame 40 is closed at the front by a front wall 44. At the bottom, the frame 40 has a base 46, at the front end of which the ash removal opening 22 is incorporated, so that the base 46 is open at the bottom there. The frame 40 is open at the rear, allowing combustion air 48 to flow into the frame 40 from behind. The entire stepped grate 20 is assembled solely from sheet metal.

[0048] The intermediate plates 38 are mounted in the side walls 42 and are held rigidly to the side walls 42 and to each other, except for a small amount of play. This play in the mounting compensates for thermal expansion and contraction, allowing the intermediate plates 38 to move slightly within the side walls 42 and relative to each other. For this purpose, the side walls 42 are provided with longitudinal slots into which the intermediate plates 38 are simply inserted. An example of such a mounting is shown in Fig. 6 can be seen from the uppermost intermediate plate 38, which is inserted into the side cheeks 42 with two tapered sections, the uppermost intermediate plate 38 being removable upwards as an exception.

[0049] The sliding plates 36 are each rigidly connected to four sliding units 50 in the form of supports, which are arranged perpendicular to the sliding plates 36. Such a connection can be made, for example, by welding. The four sliding units 50 are essentially identical to one another and each has a fork 52 for each sliding plate 36, between the two prongs of which the sliding plate 36 is inserted. In this way, each sliding plate 36 is held by four forks 52: one fork 52 holds the sliding plate 36 on the outside and two forks 52 hold the sliding plate in the middle.

[0050] Each sliding unit 50 is provided with a recess 54 into which a linkage (not shown) engages, thus engaging all four sliding units 50. Through this linkage, all four sliding units 50, and with them all sliding plates 36, move synchronously forward, i.e., in the direction of the stairs 56.

[0051] Direction of conveyance, or movable backwards. The movement is parallel to the ground 46.

[0052] During the operation of combustion plant 2, solid fuel is fed through the in Fig. The two conveying means 16, shown only schematically, are transported onto the uppermost push plate 36. For this purpose, the push units 50 and with them the push plates 36 are pushed all the way forward, as shown in Fig. Figure 2 shows the solid fuel being placed on the uppermost pusher plate 36 and ignited there by means of an ignition device (not shown). Combustion on this stage lasts for a combustion period of five minutes.

[0053] Following this firing period, a shearing period takes place. During the shearing period, the load-bearing assembly consisting of shear units 50 and shear plates 36 is pulled backwards, as shown in Fig. Figure 3 illustrates this process. During this backward pull, the burning fuel is pushed down from the push plate 36 by the intermediate plate 38 above it and falls onto the intermediate plate 38 below. The push plates 36 are then pushed forward again. The uppermost push plate 36 pushes the burning fuel from the intermediate plate onto the next push plate 36 below, where the fuel continues to burn for another combustion period of approximately five minutes. The uppermost push plate 36 is supplied with fresh fuel by the conveying device 16, which is then ignited.

[0054] In the further course of the process, combustion periods and push-off periods alternate, so that the burning fuel is pushed one stage lower with each push-off period until it finally reaches the lowest stage and thus the lowest push-off plate 36. The fuel is essentially burned, so that ash accumulates on the lowest push-off plate 36. This is conveyed to the floor 46 or through the ash discharge opening 22 and enters the ash container 24.

[0055] As in Fig. As shown in Figure 2, combustion takes place at all stages. However, the combustion energy released, or the combustion power, is not the same at all stages. While the combustion power is still low at the first stage because not all the fuel has ignited yet, the combustion power increases from stage to stage, as shown in Figure 2. Fig. 2 is illustrated by the increasing size of the flames. The highest combustion power is achieved on the middle three stages, especially on the fourth stage, with the combustion power then decreasing downwards until it reaches its lowest power on the lowest stage.

[0056] The combustion process causes the push plates 36 and the intermediate plates 38 to heat up considerably. To cool the plates 36 and 38, the primary combustion air 48 is passed between them, as indicated by the arrows in the figure. Fig. As indicated in Figure 2, the combustion air 48 flows between the plates 36 and 38, cooling them. The cooling effect is greatest the longer the airflow between the plates 36 and 38. This cooling path of the combustion air 48 is shortest in the first stage, i.e., between the uppermost intermediate plate 38 and the sliding plate 36 below it. Therefore, the lowest cooling capacity occurs there. As the depth of the stages increases, each stage is longer, so that the second stage in the direction of the staircase 56 is longer than the first, the third stage is longer than the second, the fourth stage is longer than the third, and so on. This also increases the cooling capacity of the combustion air 48 from stage to stage, so that the cooling capacity increases with increasing combustion capacity.

[0057] This allows an increased heat input into plates 36, 38 due to the increased combustion power to be fully or partially compensated for by increased cooling capacity, so that the middle plates 36, 38 are not subjected to a greater load, or only a slightly greater load, than the upper plates 36, 38. The geometry of the individual stages ensures that the thermal load on plates 36, 38 remains uniform, preventing them from being oversized and thus saving weight.

[0058] At the in Fig. However, in the embodiment shown in 2, the lowest steps are very long and therefore heavy in relation to the combustion capacity. This deficiency can be remedied by a different step grate 58 from the embodiment shown in 2. Fig. 7. The length of the lower plates 36, 38 decreases with the combustion power, so that the in Fig. The step grate shown in section 7, 58, is lighter in weight than the one shown in [reference missing]. Fig. 2 shown. The push unit 60 was adapted accordingly to the length of the plates 36, 38. This design of the step grate 58 does not result in an increased load on the plates 36, 38, so it is just as durable as the step grate 20.

[0059] In both embodiments, both the push plates 36 and the intermediate plates 38 are formed from flat sheets of structural steel, specifically ST37. The plates 36, 38 have a thickness of 6 mm, and the width of the staircase, or of the individual steps and plates 36, 38, perpendicular to the direction of travel 56, is 450 mm. Accordingly, the combustion surface of the uppermost six steps, on which combustion takes place, is as shown in Fig. The plate 2, shown in Figure 2, measures 450 mm × 50 mm. The length of each stage is very short at 50 mm. This results in each stage being subjected to only a low combustion output. In the illustrated embodiment, the combustion system 2 is designed for a nominal output of 250 kW. Distributed across seven stages, this equates to a combustion output of 35 kW per stage, assuming an even distribution of combustion output. While this is only a theoretical value, as the combustion output is higher in the middle stages, it provides an indication that the plates 36 and 38 are subjected to relatively low thermal stress. This reduces scaling and thus thickness shrinkage of the plates 36 and 38 to an acceptable level, allowing them to withstand several thousand combustion hours without damage and remain fully functional.

[0060] By distributing the total combustion power of a maximum of 250 kW across the many stages, high cooling capacities can also be achieved through the combustion air 48 per combustion power, since the combustion air 48 passes through fourteen flat spaces between the plates 36, 38.

[0061] Once the service life of the sliding plates 36 is reached after several thousand operating hours, they can be replaced very easily. For this purpose, the entire load-bearing assembly consisting of the sliding units 50 and the sliding plates 36 is pulled out of the stepped grating 20 or the frame 40 from the rear. Finally, a new and identical assembly can be slid back in from rear to front and connected with the linkage (not shown), so that the stepped grating 20 is ready for operation again. This replacement process is very quick and therefore cost-effective. Since the materials of this load-bearing assembly are very simple, this assembly of sliding units 50 and sliding plates 36 is very inexpensive.

[0062] A further advantage is achieved by the large-area flow of combustion air 48 between the plates 36 and 38. This heats the combustion air to approximately 200 degrees Celsius or even more during operation, exceeding the ignition temperature of around 150 degrees Celsius for dry wood. The air flowing between the uppermost intermediate plate 38 and the uppermost push plate 36 thus reaches the fuel at a temperature of at least 200 degrees Celsius, so that the combustion air 48 alone ignites the fuel. A continuous ignition process using the ignition mechanism (not shown) is therefore unnecessary.

[0063] The height of the stepped grate 20 is approximately 180 mm, so that, after deducting a clearance above the top step, a height of approximately 170 mm is available for all seven steps. This height is divided equally among the seven steps, so that each step has a height of approximately 25 mm. After deducting the thickness of the plates 36, 38, each 6 mm thick, a height of approximately 6 mm remains for the spaces between the plates 36, 38, through which the combustion air 48 flows evenly between the plates 36, 38.

[0064] The combustion surface of the lowest step plate is longer than that of the steps above it. This means the fuel remains on the lowest step longer than on the steps above. As a result, the fuel is more thoroughly combusted, and a large amount of unburned material doesn't fall through the ash discharge opening.

[0065] As in Fig.As shown in Figure 2, the ash discharge opening 22 is largely closed by the lowest push plate 36 during combustion cycles, as this extends forward to the front plate 44, except for a small clearance provided for thermal expansion. A combustion chamber 62 located within the frame 40 is thus aerodynamically shielded by the lowest push plate 36 from a space below the ash discharge opening 22, so that the inflow of air from this space into the combustion chamber 62 is at least largely prevented. During a push cycle, the push plates 36 are indeed pulled backward and the ash discharge opening 22 is opened, allowing air to flow into the combustion chamber 62 from below. However, during the push cycles, the auger in the ash screw 26 remains stationary, so that airflow through the ash screw 26 is largely inhibited.

[0066] In another variation of the stepped grate 20, the intermediate plates 38 can be made thinner than the push plates 36. In principle, a thickness of 2 mm is sufficient for a sheet of the intermediate plates 38, as these are subjected to only minimal stress from combustion. This allows for considerable weight savings. However, the intermediate plates 38 should then be thickened at the front, e.g., folded downwards, to reliably ensure that the fuel is pushed away from the push plates 36. This would, however, slightly disrupt the airflow under the intermediate plates 38, thus somewhat impairing combustion on the push plates. A continuous increase in the thickness of the intermediate plates 38 towards the front over a predetermined distance would solve this problem, but would involve greater manufacturing effort.

[0067] The invention of the stepped grate 20 and the combustion process is not limited to the described embodiments. In particular, the stepped grate according to the invention is also suitable and advantageous for stationary combustion plants. Reference symbol list 2 Solid fuel combustion plant 4 solid fuel storage 6 means of transport 8 means of transport 10 connection 12 discharge units 14 Backfire protection 16 funding opportunities 18 Combustion chamber 20-step grate 22 Ash discharge opening 24 ash containers 26 ash snail 28 ash containers 30 Burnout room 32 heat exchangers 34 spark arresters 36 drawer plate 38 Intermediate plate 40 frames 42 Side cheek 44 Front panel 46 Floor 48 Combustion air 50 shear element 52 Fork 54 Exclusion 56 Stair direction 58-step grate 60 Push support 62 Combustion chamber

Claims

[1] Method for burning solid fuel on a stepped grate (20) of a mobile solid fuel combustion plant (2), which has several steps arranged between side walls (42), each consisting of an intermediate plate (38) and a pusher plate (36) movable to the side walls (42) and the intermediate plate (38), wherein the pusher plates (36) push burning solid fuel in the direction of the steps (56) from one step to the next lower step by a relative movement to the respective adjacent intermediate plates (38), characterized by , that the solid fuel is burned on the push plates (36) and, after the end of a burning period, is conveyed during a push period to the next lower push plate (36) and burns there during the following burning period. [2] Method according to claim 1, characterized by, that the push plates (36) are advanced in the direction of the steps (56) during a combustion period, are withdrawn to the rear after the combustion period to expel the burning solid fuel and are then pushed forward again into the combustion position. [3] Method according to any one of the preceding claims, characterized by , that during operation combustion air (48) is supplied exclusively between the intermediate plates (38) and the push plates (36) to the burning solid fuel. [4] Method according to any one of the preceding claims, characterized by , that the number of steps of the stepped grate (20) and the solid fuel combustion plant (2) are coordinated in such a way that the combustion per step, calculated evenly across all steps, has a firing capacity of a maximum of 100 kW per meter of stair width during nominal load operation. [5] Method according to any one of the preceding claims, characterized by, that the combustion air (48) directed between the intermediate plates (38) and the push plates (36) towards the burning solid fuel is heated as it passes through the plates (36, 38) so that it ignites the unburned solid fuel. [6] Mobile solid fuel combustion plant (2) - a stepped grate (20) with two side stringers (42), several intermediate plates (38) anchored in the side stringers (42) and arranged in the manner of steps, and several push plates (36) arranged between two intermediate plates (38), which are movable in the direction of the stairs (56) for pushing burning solid fuel from one step to the next lower step relative to the respective adjacent intermediate plates (38) and - a conveying means (16) for conveying solid fuel onto the uppermost, fully forward-pushed push plate (36), so that the solid fuel comes to rest on the uppermost push plate (36). [7] Mobile solid fuel combustion plant (2) according to claim 6, characterized by , that both the intermediate plates (38) and the push plates (36) are sheet metal. [8] Mobile solid fuel combustion plant (2) according to claim 7, characterized by that the metal sheets are flat. [9] Mobile solid fuel combustion plant (2) according to claim 7 or 8, characterized by that the sheets are made of structural steel. [10] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 9, characterized by , that the ratio of the thicknesses of the intermediate plates (38) to the thicknesses of the shear plates (36) is between 0.7 and 1.4 at all stages, in particular between 0.9 and 1.

1. [11] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 10, characterized by , that all steps, each consisting of an intermediate plate (38) and a sliding plate (36), are a maximum of 3 cm thick. [12] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 11, characterized by , that the intermediate plates (38) are movably inserted into the side walls (42). [13] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 12, characterized by an arrangement of the side walls (42), intermediate plates (38) and push plates (36) such that the air supply to the burning solid fuel is provided at least predominantly through longitudinal slots between the intermediate plates (38) and push plates (36). [14] Mobile solid fuel combustion plant (2) according to claim 13, characterized by that the air supply within all stages is completely through the longitudinal slots between the intermediate plates (38) and push plates (36). [15] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 14, characterized bya front wall (44) and a floor (46) with an ash removal opening (22) which together with the side walls (42) form a trough, wherein the lowest sliding plate (36) of the staircase at least largely closes the ash removal opening (22). [16] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 15, characterized by , that the shear plates (36) are each supported at several points in shear supports (50), so that shear plates (36) and shear supports (50) form a self-supporting composite which as a whole can be pulled out of a frame (40) containing the side walls (42). [17] Mobile solid fuel combustion plant (2) according to any one of claims 6 to 16, characterized by , that the push plates (36) and the intermediate plates (38) become longer from step to step, at least over the top three steps in the direction of the stairs (56).

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