NO326110B1 - grate furnace - Google Patents

Abstract

There is mentioned a solid fuel grate furnace with supply of primary air through holes / slots (4) in a grate plate (3) and a feed mechanism for fuel over the grate (3), where at least parts of the feed mechanism are arranged in a duplex arrangement and includes bar feeders (1, 2) with intermediate scrapers (5) and counter scrapers (6). The grate surface (3) is substantially a flat surface without any kind of construction, and said scrapers (5) and counter-scrapers (6) are at least by rearward movement arranged to undercut the fuel over the grate (3). A method for using the rice oven is also discussed.

Description

The present invention relates to a grate stove for solid fuel with the supply of primary air through holes/slots in a grate plate and a feeding mechanism for fuel over the grate, where at least parts of the feeding mechanism are arranged in a duplex arrangement and include rod feeders with intermediate scrapers and counter scrapers. The invention also relates to a method for using the rice cooker.

Grate stoves for solid fuel consist of a grate and a subsequent gas combustion zone. On the grate, combustion air is added to the fuel bed and the combustible gases that are formed from the fuel through various devices. In addition, a transport mechanism is needed which ensures that the solid fuel is transported forward on the grate from the furnace inlet to its ash outlet. A distinction is made between inclined gratings, stepped gratings and flat gratings. For inclined grates, the transport mechanism can be as simple as natural gravity - the angle of the inclined grate is adapted to the fuel's angle of descent so that the fuel naturally slides forward on the grate as it is fed in and burned, or an explicit physical transport mechanism can be used, for example in the form of a scraper device or a screw device. For stair grates, the transport mechanism is often integrated into the grate itself - for example, a number of movable grate rods can be given a coordinated movement pattern reminiscent of the leg movement of a millipede, so that the fuel can "walk" down the steps in the stair grate. Both series of rollers laid one after the other, conveyor belts and various scraping and screwing devices are known for planar grates. A common feature for the vast majority of grate stoves, however, is that a large part of the combustion air needed to dry, gasify and burn out the solid fuel is supplied to the fuel bed through holes, slits or other supply devices in the grate itself. This addition of air through the grate is often called the primary air. Additional air that is added above the grate in one or more stages so that flammable gases formed on the grate are fully burned out is often called secondary air, tertiary air, etc.

Rod feeders are a well-known feeding mechanism in furnaces for solid fuel, described in the patents SE 501 226 and in NO 304 450. The rod feeder can be considered a "ladder" that is carried back and forth over the grate in the furnace where the "steps" in the ladder, called carriers or scrapers, are shaped like triangles. Between the scrapers there are often barriers called counter-holding irons or counter-scrapers, which prevent that too large a proportion of the fuel that is carried forward and over the barriers by the vertical front of the scrapers is also transported back by the inclined back of the scrapers. The angle of the scrapers, which is given by the length ratio between their horizontal height and their horizontal underside, is one of the decisive parameters for what proportion of the fuel that is brought forward is also brought back. In order for the rod feeder to be operated with low energy consumption, it is important that its scraper undercuts the fuel bed. This can be illustrated by the following analogy: If a shovel is to be used to move a pile of gravel lying on a flat surface (e.g. an asphalt surface), much less force is required to push the shovel into the pile of gravel if the shovel is pushed along the asphalt surface than if it is pressed into the gravel pile a little further up.

In Patent SE 501 226, the counter-scrapers are permanently mounted on the grate, and primary air is introduced into the fuel bed through holes in the counter-scrapers. Several facilities have been realized based on this principle, and facilities of this type are today marketed by Swedish TPS under the brand name "Stepfire".

Two bar feeders can also be combined so that together they form a Duplex bar feeder. This transport mechanism is, among other things, known from deliveries from the company Saxlund, which supplies Duplex bar feeders designed for the discharge from silos of materials such as powder, grain and the like. The counter-holding irons or counter-scrapers found in feed mechanisms based on a single bar feeder are replaced in the Duplex arrangement by the scrapers on an intermediate bar feeder. One bar feeder's scraper operates m.a.o. in the space between the second rod feeder's counter-rail.

NO 304 450, hereinafter called the NO patent, refers to Patent SE 501 226, hereinafter called the SE patent. The NO patent document argues that the SE patent has problems with fuel accumulations around the SE patent's fixed counter-scraper. Several combustion plants have been built with Duplex rod feeders. In all these plants, however, the Duplex mechanism is realized in a suboptimal way: The drive for each of the two bar feeders in the duplex arrangement moves the bar feeders between fixed outer points, and both bar feeders are operated at the same speed in both forward and backward movement.

Bar feeder one is moved forward at the same time as bar feeder two is moved backwards until the scrapers on one bar feeder meet the scrapers of the other bar feeder. The movement is reversed and repeated. This means that the meeting points between scraper and counter-scraper - the positions on the grate where one bar feeder's scraper meets the other bar feeder's corresponding counter-scraper, are always fixed - in the middle between two scrapers on the same bar feeder. Both practically and in principle, this leads to the fact that the problem that the NO patent document points out in relation to the SE patent's accumulation of fuel cannot be considered satisfactorily resolved - the accumulation is only moved: While the SE patent describes a solution that entails an accumulation of fuel around the fixed the counter-scrapers, the NO patent will be able to provide a corresponding accumulation of fuel around the meeting points.

A further practical and principled problem linked to the NO patent is that it prescribes primary air supply through the grate using raised rows of nozzles on the grate, also called grate bars. The primary air is led out through slits in these rows of nozzles in a direction parallel to the grate surface. The Duplex arrangement is moved over these rows of nozzles in a longitudinal direction, but since the rows of nozzles represent elevations on the grate where the fuel is located, the NO patent's prescribed Duplex arrangement will not be able to undercut the fuel. Instead, scrapers and counter-scrapers move back and forth some distance up in the fuel bed. This means that the force that must be used to drive each of the rod feeders in the Duplex arrangement increases significantly compared to a more ideal arrangement where the rod feeders' scraper and counter scraper undercut the fuel. Increased power consumption here leads to increased strain on both the bar feeders and the grate, which causes increased wear and tear and the risk of breakage.

For compressive fuels, a transport mechanism based on scrapers and counter-scrapers that move towards each other some distance up in the fuel bed will cause the fuel to be compressed between scraper and counter-scraper, instead of the opposite movement causing the fuel to be pushed by the scraper over the counter-scraper. When the scraper and counterscraper then move apart again after the meeting point between scraper and counterscraper is reached, the fuel expands more or less back to its original volume. This compression and expansion of compressible fuel results in reduced feed efficiency, which means that a Duplex arrangement that moves some distance up the fuel bed must be pushed back and forth more times than a similar arrangement that undercuts the fuel to achieve the same fuel feed. This reduction in feed efficiency with the associated need for an increased degree of movement back and forth causes unnecessary wear and tear on bar feeders and grates.

Combustible gases formed in the fuel bed will ignite when they come into contact with supplied primary air. Since the primary air when it is introduced onto the grate is colder than the combustible gases, this ignition will not be instantaneous - the primary air will be transported a short distance into the beds before it is sufficiently heated for ignition to occur. This means that a rod feeder mechanism such as the one described in the NO patent will be more exposed to thermal stresses from ignited flammable gases than a rod feeder mechanism that undercuts the fuel on the grate. Furthermore, the individual scrapers and counter-scrapers will be carried out in accordance with the instructions in the NO patent due to the design of the nozzle arrays is exposed to uneven thermal load across the direction of movement of the scrapers: the nozzle arrays described in the NO patent must, so that wedging of fuel does not occur, oriented parallel to the direction of movement of the scrapers. Water ignited gases will thus hit the underside of the scraper / counter-scraper at the same given places throughout the scraper / counter-scraper's movement interval between its extreme points. In these places, the scraper / counter-scraper is exposed to a significantly increased thermal load - a "welding flame effect" which over time could lead to the scraper and counter-scraper burning to pieces.

In the SE patent, the scrapers are not guided over the holes where air is supplied through the grate - the counter-scrapers are permanently mounted on the grate and the air supply through the grate goes through holes in the permanently mounted counter-scrapers. However, this means that only a smaller part of the grate is used for air supply, so more air must pass through each individual supply point than if the entire grate had been used for air supply. This gives increased combustion intensity around the limited parts of the grate that are used for air supply, compared to a more ideal design where the air supply is distributed over the entire grate. This increased combustion intensity around the counter-scrapers will result in increased thermal load around the counter-scrapers with associated wear, and there is also good reason to expect that the increased combustion intensity will result in locally increased production of NOx beyond what would have been possible with a more distributed air addition.

Solid fuel that is combusted or gasified in grate furnaces undergoes three fundamentally different processes as the fuel is transported forward on the grate:

Drying, pyrolysis / gasification and finally combustion of residual carbon.

1) Drying: The moisture in the fuel must first be driven off before the fuel can be brought up to temperatures that cause the pyrolysis / gasification to occur to a significant extent. In the drying phase, the fuel retains a large part of its original volume, but it is significantly reduced in weight (density) as the water is driven off. 2) Pyrolysis / gasification: Combustible volatile components (gas / liquid) are formed by thermal decomposition of parts of the organic content of the fuel when it reaches temperatures of 250 - 550 degrees. In the gasification phase, the fuel loses a significant part of its original volume, and a significant part of its weight; it "collapses". 3) After the fuel has collapsed, it still contains significant amounts of residual carbon. This is gasified or slowly burnt out in the burn-out phase in the temperature range from 550 degrees and up to 900 - 1000 degrees.

Grate stoves for burning solid fuel where the internal feed mechanism in the stove is based on rod feeders generally face the following challenges that must be dealt with in order for them to function satisfactorily.

1) The transport properties of the fuel change dramatically from the start to the end of the grate. The rod feeders should therefore be designed so that the feed speed in each of the three phases is appropriate and adapted to the characteristics of the fuel. If the feed rate in the drying zone is too low, this will cause the height of the fuel bed here to be too great, and the drying efficiency will decrease. If the feed rate in the burnout zone is too high, you will both be at risk

a. that the fuel is not sufficiently burned out, and

b. that the ash layer that protects the grate surface and bar feeders becomes too low to be able to provide sufficient protection against thermal stress. 2) To avoid unnecessary loads on the rod feeders, these should be able to undercut the fuel, and they should be able to be assigned an operating mode that provides a high feeding efficiency. 3) The grate should be designed so as to sufficiently avoid a. overgrowth of the air supply holes,

b. penetration of unburnt or burnt fuel through the air supply holes,

c. burning to pieces of thermally exposed components, and

d. build-up of sintering on the grid surface.

It is consequently an object of the invention to produce a grate oven in which the above-mentioned points are fulfilled.

Said purpose is achieved with a grate stove as stated in the characteristic in the independent claim 1, in that the grate surface is mainly a flat surface without any kind of structures, and that said scrapers and counter-scrapers, at least when moving backwards, are designed to undercut the fuel above the grate .

Alternative designs are characterized by the independent claims 2-9.

In the furnace's burnout zone, the feed forward mechanism can comprise a simple bar feeder, one of said bar feeders in a duplex arrangement being truncated in relation to the other. In the furnace's burn-out zone, scrapers and/or counter-scrapers can be arranged at a greater distance on said bar feeders than further forward on the grate.

In the furnace's burnout zone, scrapers and/or counter-scrapers can be designed with a geometry that deviates from scrapers and/or counter-scrapers further forward on the grate. Furthermore, scrapers and/or counter-scrapers in the oven's burn-out zone can be designed with a larger angle at the rear edge than scrapers and/or counter-scrapers further forward on the grate.

Alternatively, scrapers and/or counter-scrapers in the oven's burn-out zone can be designed with a smaller angle at the front than scrapers and/or counter-scrapers further forward on the grate. Scrapers and/or counter-scrapers in the furnace's burnout zone can be designed with a trapezoidal shape.

Preferably, said holes/slits in the grid plate are arranged in an inclined pattern in relation to the direction of movement of the scrapers. At least through the oven's drying zone and gasification zone, said bar feeders can be arranged to move cyclically in relation to each other, at least through a meeting zone between scrapers and counter-scrapers of respective bar feeders.

The above-mentioned purpose is also achieved with a method as stated in independent claim 10, in that at least through the oven's drying zone and gasification zone, respective bar feeders are moved in relation to each other, in that a movement pattern is produced so that the scrapers and counter-scrapers of the bar feeders are cyclically moved through stationary zones that occur between fixed extremes.

In an alternative embodiment of the method, the steps are included:

a) to place a first bar feeder and a second bar feeder in relation to each other so that the scrapers of the first bar feeder are positioned at a distance to,

and preferably in between, the other rod feeds his scrapers,

b) to move both bar feeders forward simultaneously in a long step without the mutual distance between the scrapers on the two bar feeders changing, c) to move the second bar feeder backwards in a short step, at most so far that its scraper meets the scrapers of the first bar feeder , and then to move it

first bar feeder backwards in a short step, maximum as far as its scraper meets the scrapers of the second bar feeder,

The invention will now be described in more detail with the help of the attached figures, in which: Figures 1a and 1b show a rod feeder arrangement, seen respectively from above and from the side, according to the invention.

Figure 2 shows part of a bar feeder over a flat grate, according to the invention.

Figure 3 shows various designs of scrapers for a bar feeder in the burnout zone, according to the invention.

The rod feeders according to the invention are assembled in a mainly standard way. That is a bar feeder 1 comprises two parallel stays 1a, 1b, where a number of scrapers 5 are arranged between the stays. Correspondingly, a bar feeder 2 is composed of two parallel stays 2a, 2b, where a number of counter scrapers 6 are arranged between the stays. As shown in figures 1a and 1b, the second rod feeder 2 is arranged lying above the first rod feeder 1, where the rod feeders 1, 2 can be moved relative to each other, as previously explained.

In the drying phase, the fuel bed above the grate is high and heavy, and the flow resistance in the fuel bed itself will be more decisive than the grate design for air distribution in and over the beds. The use of rod feeders 1, 2 in a duplex arrangement in this phase will provide a movement characteristic of breaking up and partial stirring of the wet bed, which increases the heat and mass transfer efficiency between the bed and hot combustion gases above the beds. The drying phase is therefore optimally designed with rod feeders in a duplex arrangement.

Efficient fuel feeding inside the furnace requires that the scrapers undercut the fuel. The scrapers should therefore move back and forth over the grate with a small distance to the grate, and the grate 3 should be realized as a flat surface without grate bars / rows of nozzles or other forms of structures. Said scrapers 5 and counter-scrapers 6 are therefore arranged to undercut the fuel above the grate, and for example at least through the oven's drying zone and gasification zone, said bar feeders 1, 2 are arranged to move cyclically in relation to each other, at least through a meeting zone between scrapers 5 and counter scrapers 6 to respective rod feeders 1, 2.

If scraper 5 and counter-scraper 6 move between fixed extreme points in such a way that the meeting zone between scraper and counter-scraper never moves, this meeting zone will represent a stationary zone where no direct forces are exerted by scraper / counter-scraper on fuel located in the zone . This is therefore a zone where fuel can easily remain without being transported either forwards or backwards, and there is a risk of an accumulation of fuel in such zones. However, this problem can be avoided by designing the bar feeders 1, 2 movement pattern so that the scraper / counter-scraper is regularly moved through the stationary zones. This can be realized, for example, by giving the bar feeders a movement pattern analogous to the cyclical movement pattern in a walking floor: a. Step 1: Bar feeder 1 and bar feeder 2 are placed in relation to each other so that bar feeder 1's scrapers are in the middle between bar feeder 2's scrapers. b. Step 2: Both bar feeders 1, 2 are moved forward simultaneously in a long step without the mutual distance between the scrapers on the two bar feeders changing. c.. Step 3: Bar feeder 2 is moved backwards in a short step, at most until its scraper meets the scrapers of bar feeder 1. Bar feeder 1 is then moved backwards in a short step, at most until its scraper meets the scrapers of bar feeder 2.

d. Step 4: Step 3 is repeated one or more times.

e. Repeat all steps above.

In the burn-out phase after the beds have collapsed, the air addition must be designed so that entrainment of ash is avoided and so that the air distribution over the beds is as even as possible. As an alternative to the use of grating rods described in the NO patent, this can be achieved by using a flat grating without grating rods if an appropriate number of holes per square is used on this part of the grating. surface unit where each hole is given an appropriate diameter. Experiments carried out with a prototype grate element have shown that it is possible to find hole diameters and hole geometries that satisfy both the requirements to avoid overgrowth and to avoid fuel / ash falling through. These experiments are described in more detail later in this patent document. The temperature above the ash layer in the burnout zone is high, and here it is important to avoid a feed rate that is so high that you get a direct exposure of bare grate without a protective ash layer.

This can be achieved in several alternative ways:

a. On this last part of the grate, one can change from a duplex configuration of bar feeders to a single bar feeder (not duplex), which will give a lower feed rate than a duplex arrangement. This is realized e.g. simply by shortening the length of one of the two bar feeders in the duplex arrangement relative to the other, so that the last scrapers on the grate are realized without a counter scraper. This will work without a counter-holding iron / counter-scraper because the counter-pressure from ash and fuel that is further forward on the grate will provide good enough counter-resistance for the scrapers in this area.

b. Alternatively, every second counterscratch in the burnout zone can be removed.

c. Alternatively, scrapers and counter-scrapers in a duplex arrangement that run all the way to the end of the grate can be realized with scrapers and counter-scrapers with changed geometry in relation to the scrapers / counter-scrapers further forward on the grate, either by i. increasing the angle at the rear edge of the scraper, as illustrated by the scratch cross-section in Figure 3b relative to Figure 3a ii. the angle at the leading edge of the scraper is reduced, as illustrated in figure 3c ii. one replaces the triangular shape with a lower trapezoid, as illustrated in figure 3d.

The number of holes 4 in the grid 3 for primary air supply and the respective diameter of the holes must be determined through trials, as described later in this document. However, the holes should be arranged in an inclined pattern in relation to the direction of movement of the scrapers, as illustrated in Figure 2. The inclined pattern means that when the scrapers are moving, the thermal load from the air supply holes is evenly distributed over the entire width of the scraper, in contrast to the concentrated load on single positions on the scraper which will occur if the air supply holes are in rows parallel to the direction of movement of the scrapers.

Description of grating test:

Experiments have been carried out over a period of time on a grate stove with Duplex fuel feed. A segment of the grating was replaced during the experiments with a test segment with a flat grating surface with holes of different geometry and size. The purpose of the experiment was to identify the necessary number of holes per surface unit, as well as suitable hole geometries and hole sizes. The experiments showed that holes of certain geometries had an easier time growing back than other geometries, and that holes with a diameter below a given limit would grow back in the drying zone. The tests also showed that the passage of fuel and ash through the holes was modest. The experiments have thus documented that it is possible to design a grid without rows of nozzles or other devices that represent structures on the grid. The experiments have also demonstrated that the wear on a flat grate without structures will be marginal.

Claims (11)

1. Solid fuel grate stove with supply of primary air through holes/slits (4) in a grate plate (3) and a feed mechanism for fuel over the grate (3), where at least parts of the feed mechanism are arranged in a duplex arrangement and include rod feeders (1, 2) with intermediate scrapers (5) and counter-scrapers (6), characterized in that the grate surface (3) is mainly a flat surface without any kind of structures, and that said scrapers (5) and counter-scrapers (6) at least at backward movement is designed to undercut the fuel above the grate (3). 2. Grate oven in accordance with claim 1, characterized in that in the oven's burnout zone, the feeding mechanism comprises a single bar feeder (1), one of said bar feeders in the duplex arrangement being truncated in relation to the other. 3. Grate oven in accordance with claim 1, characterized in that in the oven's burn-out zone, scrapers (5) and/or counter-scrapers (6) are arranged at a greater distance on said bar feeders (1, 2) than further forward on the grate (3). 4. Grate oven in accordance with claim 1, characterized in that in the oven's burnout zone, scrapers (5) and/or counter-scrapers (6) are designed with a geometry that deviates from scrapers and/or counter-scrapers further forward on the grate (3). 5. Grate oven in accordance with claim 4, characterized in that scrapers (5) and/or counter-scrapers (6) in the oven's burn-out zone are designed with a greater angle at the rear edge than scrapers and/or counter-scrapers further forward on the grate (3). 6. Grate oven in accordance with claim 4, characterized in that scrapers (5) and/or counter-scrapers (6) in the oven's burn-out zone are designed with a smaller angle at the front than scrapers and/or counter-scrapers further forward on the grate (3). 7. Grate oven in accordance with claim 4, characterized in that scrapers (5) and/or counter scrapers (6) in the oven's burnout zone are designed with a trapezoidal shape. 8. Grate oven in accordance with claim 1, characterized in that said holes/slits (4) in the grate plate (3) are arranged in an inclined pattern in relation to the direction of movement of the scrapers (5, 6). 9. Grill oven in accordance with claim 1, characterized in that at least through the drying zone and gasification zone of the oven, said bar feeders (1, 2) are arranged to move cyclically in relation to each other, at least through a meeting zone between scrapers (5) and counter scrapers (6) to respective bar feeders (1, 2). 10. Method for feeding solid fuel into a grate furnace with supply of primary air through holes/slits (4) in a grate plate (3) and a feeding mechanism for fuel over the grate (3), where at least parts of the feeding mechanism are arranged in a duplex arrangement and comprises rod feeders (1, 2) with intermediate scrapers (5) and counter-scrapers (6), characterized in that at least through the oven's drying zone and gasification zone, respective rod feeders (1, 2) are moved in relation to each other, by producing a movement pattern so that the bar feeders' scrapers (5) and counter-scrapers (6) move cyclically through stationary zones that occur between fixed extreme points. 11. Method in accordance with claim 9, characterized by) placing a first bar feeder (1) and a second bar feeder (2) in relation to each other so that the scrapers (5) of the first bar feeder (1) are positioned at a distance to, and preferably in the middle, the other bar feeder (2) has its scrapers (6), b) to move both bar feeders (1, 2) forward simultaneously in a long step without the mutual distance between the scrapers (5, 6) of the two bar feeders (1, 2) is changed, c) to move the second bar feeder (2) backwards in a short step, at most until its scraper (6) meets the scrapers (5) of the first bar feeder (1), and then to move it first bar feeder (1) backwards in a short step, maximum as far as its scraper (5) meets the scrapers (6) of the second bar feeder (2), d) to repeat step c) one or more times, e) to repeat all the steps above as long as necessary.