WO2008156368A1

WO2008156368A1 - Grate furnace for solid fuel. method for forward feeding of solid fuel

Info

Publication number: WO2008156368A1
Application number: PCT/NO2008/000216
Authority: WO
Inventors: Kristian M. Lien; Torborn Pettersen
Original assignee: Thermos As
Priority date: 2007-06-18
Filing date: 2008-06-16
Publication date: 2008-12-24
Also published as: EP2171355A1; NO20073090A; NO326110B1

Abstract

A grate furnace for solid fuel with a supply of primary air through holes/slits (4) in a grate plate (3) and a forward feeding mechanism for fuel across the grate (3) are described where at least parts of the forward feeding mechanism are arranged in a duplex arrangement and comprise bar feeders (1,2) with intermediate scrapers (5) and counter scrapers (6). The grate surface (3) is, in the main, a plane surface without any form of elevations, and that said scrapers (5) and counter scrapers (6), at least in the backward directed movement, are arranged to undercut the fuel across the grate (3). A method for use of the grate furnaces is also described.

Description

Grate furnace for solid fuel . Method for forward feeding of solid fuel

The present invention relates to a grate furnace for solid fuel with supply of primary air through holes/slits in a grate plate and a forward feeding mechanism for fuel across the grate, where, at least parts of the forward feeding mechanism are arranged in a duplex arrangement and comprise bar feeders with intermediate scrapers and counter scrapers. The invention also relates to a method for use of the grate furnace.

Grate furnaces for solid fuel comprise a grate and a subsequent gas burning-out zone. On the grate, combustion air, for the fuel bed and the combustible gases that are formed from the fuel, is added through different appliances. In addition, a transportation mechanism is needed that ensures that the solid fuel is brought forward on the grate from the inlet of the furnace to the outlet point for the ash. Normally, one separates between inclined grates, stair grates and plane grates. For inclined grates, the transport mechanism can simply be natural gravitation - the angle of the inclined grate is adjusted to the fall angle of the fuel so that the fuel naturally falls forward on the grate as it is fed in and burnt, or an explicit physical transport mechanism can be used, for example, in the form of a scraping device or a screw device. For stair grates, the transport mechanism is typically integrated into the grate itself - for example, a series of movable grate struts are given a coordinated movement pattern akin to that of the leg movement of a caterpillar so that one gets the fuel to "wander" down the steps in the stair grate. For plane grates, a series of rollers placed after each other, conveyor belts and different scraping and screwing devices, are known.

However, a common feature for most of the grate furnaces is that a large part of the combustion air which is required to dry, gasify and burn the solid fuel is supplied to the fuel bed via holes, slits or other supply devices in the grate itself. This supply of air through the grate is normally called the primary air. Further air, which is added over the grate in one or more steps so that the combustible gases that are formed on the grid are completely burnt, is normally called secondary air, tertiary air, etc.

Bar feeders are a known forward feeding mechanism in furnaces for solid fuel, described in the patents SE 501 226 and in NO 304 450. The bar feeder can be considered as a ladder that is brought forwards and backwards across the grate in the furnace where the steps in the ladder, called carriers or scrapers, are formed as triangles. Barriers, which are called restraining irons or restraining scrapers, can often be found between the scrapers, which prevent that too large a part of the fuel which is led forward and across the barriers by the vertical front side of the scrapers is also transported back by the inclined rear side of the scraper. The angle of the scrapers, which is given by the length ratio between their horizontal height and their horizontal underside, is one of the parameters that decides how large a part of the fuel that is brought forward is also brought back. So that the bar feeder shall be able to operate with a low energy consumption, it is important that its scraper cuts under the fuel bed. This can be illustrated by the following analogy: If one shall use a spade to move a pile of gravel that lies on a plane surface (for example, a base of asphalt) , much less force is required to push the spade into the pile of gravel if the spade is pushed along the asphalt surface than if it is forced higher up into the gravel pile.

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

Two bar feeders can also be placed together so that they together make up a Duplex bar feeder. This transport mechanism is known, among others, from deliveries from the firm Saxlund, which supplies Duplex bar feeders arranged to remove materials such as powders, cereals and the like from silos. The restraining irons or the counter scrapers, which one finds in forward feeding mechanisms based on a single bar feeder, are replaced in the Duplex arrangement by the scrapers on an intermediate bar feeder. The scraper of the one bar feeder operates, in other words, in the intermediary space between the restraining iron of the second bar feeder.

NO 304 450, hereinafter called the NO patent, refers to the patent SE 501 226, hereinafter called the SE patent. The NO patent argues that the SE patent has problems with fuel collections around the permanently fixed counter scraper of the SE patent. Several combustion installations are built with Duplex bar feeders. However, in all these installations, the Duplex mechanism is realised in a suboptimal manner. The drive system for each of the two bar feeders in the duplex arrangement moves the bar feeders between permanent extreme positions, and both the bar feeders are operated at the same speed of movement in the forward direction and backward direction.

The first bar feeder is moved forward at the same time as the second bar feeder is moved backward until the scrapers on the first bar feeder meet the scraper of the second bar feeder. The movement is reversed and repeated. This leads to the meeting points between scraper and counter scraper - the positions on the grate where the scraper of the first bar feeder meets the corresponding counter scraper of the second bar feeder always lie in the same place - midway between the two scrapers on the same bar feeder. Both practically and fundamentally, this leads to the problem which the NO patent points out in relation to the SE patent, that the piling up of fuel can not be seen to be satisfactorily resolved - the piles are only moved. While the SE patent describes a solution which leads to piling up of fuel around the permanently fixed counter scrapers, the NO patent will be able to give a corresponding piling up of fuel around the meeting points.

A further practical and fundamental problem associated with the NO patent is that it describes supply of primary air through the grate with the use of elevated rows of nozzles on the grate, also called grate struts. The primary air is led out through slits in these rows of nozzles in a direction parallel to the surface of the grate. The Duplex arrangement is moved across these rows of nozzles in the longitudinal direction, but since the rows of nozzles represent elevations on the grate where the fuel lies, the described Duplex arrangement of the NO patent will not be able to undercut the fuel. Instead, the scraper and counter scraper move back and forth a distance up into the fuel bed. This leads to the power required to drive each of the bar feeders in the Duplex arrangement increasing significantly in relation to a more ideal arrangement where the scraper and counter scraper of the bar feeders undercut the fuel. Increased power consumption leads to increased strain on both bar feeders and grate, something which in turn leads to increased wear and risk of breakdowns.

For compressed fuels, a transport mechanism which is based on a scraper and a counter scraper that move towards each other some distance up in the fuel bed will lead to the fuel becoming compressed between the scraper and the counter scraper instead of the coming together movement leading to the fuel being pushed by the scraper over the counter scraper. When the scraper and the counter scraper thereafter move apart after the meeting point between the scraper and the counter scraper is reached, the fuel expands more or less back to its original volume. This compression and expansion of the compressible fuel results in a reduced forward feeding effectiveness which in turn means that a Duplex arrangement that moves some distance up in the fuel bed must be pushed back and forth more times than a corresponding arrangement that cuts under the fuel to obtain the same forward feeding of the fuel. This reduction in forward feeding effectiveness with the corresponding need for an increased degree of movement forward and backward gives an unnecessary large wear on bar feeders and grate.

Combustible gases that are formed in the fuel bed will be ignited when they come into contact with supplied primary- air. As the primary air when it is led into the grate is colder than the combustible gases, this ignition will not be spontaneous - the primary air will be transported some distance into the bed before it becomes sufficiently heated up for ignition to occur. This means that a bar mechanism such as the one described in the NO patent will be more exposed to thermal strains from ignited combustible gases than a bar mechanism that cuts under the fuel on the grate. Furthermore, the individual scrapers and counter scrapers constructed according to the descriptions in the NO patent will, because of the shape of the row of nozzles, be exposed to an uneven thermal load across the direction of movement of the scrapers. The row of nozzles described in the NO patent must be oriented in parallel with the direction of movement of the scrapers so that the fuel does not get stuck. Hot ignited gases will thereby hit the underside of the scraper / counter scraper at the same given places in the whole of the movement interval of the scraper / counter scraper between its extreme points. At these places the scraper / counter scraper is subjected to a considerably elevated thermal strain - an "acetylene torch" effect which over time will possibly lead to the scraper and counter scraper being burnt to pieces.

In the SE patent, the scrapers are not led across the holes where the air is supplied through the grate - the counter scrapers are permanently mounted to the grate and the supply of air through the grate goes through holes in the permanently mounted counter scrapers. However, this leads to only a smaller part of the grate being used for air supply, so that more air must go through each supply point than if the whole of the grate was used for supply of air. This gives an elevated combustion intensity around the limited parts of the grate that are used for air supply, compared with a more ideal design where the supply of air is distributed over the whole of the grate. This elevated combustion intensity around the counter scrapers will give an elevated thermal load around the counter scrapers with an associated wear, and there is also a good reason to expect that the elevated combustion intensity will lead to a local elevated production of NOx beyond what would have been possible with a more evenly distributed supply of air.

Solid fuel that is burnt or gasified in grate furnaces goes through three fundamentally different processes as the fuel is transported forward on the grate: Drying, pyrolysis / gasification and finally burning out of the residual carbon.

1) Drying: Firstly, moisture in the fuel must be driven off before the fuel can reach the temperatures required for significant pyrolysis / gasification to take place. In the drying phase, the fuel largely retains its original volume, but it is considerably 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 temperatures of 250-550 degrees are reached. In the gasification phase, the fuel loses a considerable part of its original volume, and a considerable part of its weight; it "collapses".

3) After the fuel has collapsed it still contains considerable amounts of residual carbon. This is gasified or burnt slowly in the burning-out phase in the temperature region from 550 degrees and up to 900-1000 degrees.

Grate furnaces for burning solid fuel where the internal forward feeding mechanism in the furnace is based on bar feeders normally meet the following challenges which must be handled so that they function satisfactorily:

1) The transport characteristics of the fuel change dramatically from the start to the end of the grate. Bar feeders should therefore be formed such that the forward feeding velocity in each of the three phases is appropriate and adapted to the characteristic of the fuel. If the forward feeding velocity in the drying zone is too low, this will lead to the height of the fuel bed being too great and the drying effectiveness decreasing. If the forward feeding velocity in the burning-out zone is too high, one will risk a. that the fuel is not burnt out sufficiently, and b. that the layer of ash that protects the grate surface and the bar feeders is too thin to be able to give sufficient protection against thermal stress.

2) To avoid unnecessary loads on the bar feeders, these ought to be able to undercut the fuel and they ought to be arranged in an operating modus which gives a high forward feeding effectiveness.

3) The grate ought to be formed so that one avoids, to a sufficient extent, a. blockage of the air supply holes b. non-burnt fuel or burnt fuel falling through the air supply holes c. components exposed thermally being burnt to destruction and d. build-up of sintering on the grate surface.

Consequently, it is an object of the invention to provide a grate furnace where the above mentioned points are met.

Said objects are achieved with a grate furnace as described in the characteristic in the independent claim 1, in that the grate surface is mainly a plane surface without any form of elevations and that said scrapers and counter scrapers are, in at least in backward directed movement, arranged to undercut the fuel across the grate.

Alternative embodiments are characterised by the dependent claims 2-9.

In the burning-out zone of the furnace, the forward feeding mechanism can comprise a single bar feeder, as one of the mentioned bar feeders in the duplex arrangement is shortened in relation to the other. In the burning-out zone of the furnace, the scrapers and/or counter scrapers can be arranged with a greater distance on said bar feeders than further forward on the grate.

In the burning-out zone, the scrapers and/or counter scrapers can be formed with a geometry that deviates from scrapers and/or counter scrapers further forward on the grate. Furthermore, scrapers and/or counter scrapers in the burning-out zone of the furnace can be formed 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 burning-out zone of the furnace can be formed with a smaller angle at the front edge than scrapers and/or counter scrapers further forward on the grate. Scrapers and/or counter scrapers in the burning-out zone of the furnace can be formed in the shape of a trapezium. Said holes/slits in the grate plate are preferably arranged in a diagonal pattern in relation to the direction of movement of the scrapers. At least through the drying zone and the gasification zone of the furnace, said bar feeders can be arranged to move cyclically in relation to each other, at least through a meeting zone between the scrapers and the counter scrapers of the respective bar feeders.

The above mentioned objects are also reached with a method as described in the independent claim 10, in that, at least through the drying zone and the gasification zone of the furnace, the respective bar feeders are moved mutually in relation to each other, in that a movement pattern is provided such that the scrapers and the counter scrapers of the bar feeders are moved through stationary zones that arise between fixed extreme points.

In an alternative embodiment of the method, the phases include: 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 a distance to, and preferably in the middle of, the scrapers of the second bar feeder. b) to move both bar feeders forward a long step at the same time without the mutual distance between the scrapers on the two bar feeders being altered. c) to move the second bar feeder a short step back, at most so far that its scraper meets the scrapers of the first bar scraper, and thereafter to move the first bar feeder back a short step, at most so far that its scraper meet the scrapers on the second bar feeder.

The invention will now be described in more detail with the help of the enclosed figures, in which: Figures Ia and Ib show a bar feeder arrangement viewed from above and from the side, respectively, according to the invention.

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

Figure 3 shows different embodiments of scrapers of a bar feeder in the burning-out zone according to the invention.

According to the invention, the bar feeders are put together in a mainly standard way, i.e. a bar feeder 1 comprises two parallel struts Ia, Ib, where a number of scrapers 5 are placed between the struts. Correspondingly, a bar feeder 2 is put together of two parallel struts 2a, 2b, where a number of counter scrapers 6 are located between the struts. As shown in figures Ia and Ib the second bar feeder 2 is arranged lying above the first bar feeder 1 where the bar feeders 1, 2 can be moved mutually in relation to each other as explained previously.

In the drying phase, the fuel bed above the grate is high and heavy, and the resistance to flow in the fuel bed itself will be more decisive than the form of the grate for the distribution of air in and above the bed. Use of bar feeders 1, 2 in the duplex arrangement in this phase will give a movement characterised by the breaking up and partial mixing of the wet bed, something which increases the heat and mass transfer effectiveness between the bed and the hot combustion gases above the bed. The drying phase is therefore shaped in an optimal way with bar feeders in a duplex arrangement.

Effective forward feeding of fuel inside the furnace presupposes that the scrapers undercut the fuel. Therefore, the scrapers ought to move back and forth across the grate with a short distance to the grate, and the grate 3 ought to be realised as a plane surface without grate struts / rows of nozzles or other forms of elevations. Said scrapers 5 and counter scrapers 6 are therefore arranged to undercut the fuel across the grate and, for example, said bar feeders 1, 2 are, at least through the drying zone and the gasification zone of the furnace, arranged to move cyclically in relation to each other, at least through a meeting zone between the scrapers 5 and the counter scrapers 6 of the respective bar 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 the scraper / counter scraper on the fuel that is in the zone. Therefore, this is a zone where the fuel can easily be left lying without being transported back and forth, and one risks a piling up of fuel in such zones. However, this problem can be avoided in that the movement pattern of the bar feeders 1, 2 is designed such that the scraper /counter scraper is regularly moved through the stationary zones. This can, for example, be realised in that the bar feeders are given a movement pattern analogous to the cyclical movement pattern of a travelator : a. Phase 1: Bar feeder 1 and bar feeder 2 are placed in relation to each other such that the scraper of bar feeder 1 stands in the middle of the scraper of bar feeder 2. b. Phase 2: Both bar feeders 1, 2 are moved simultaneously forward in a long step without the mutual distance between the scrapers of the two bar feeders being altered. c. Phase 3: Bar feeder 2 is moved back a small step, at most so far that its scraper meets the scrapers of the bar feeder 1. The bar feeder 1 is then moved back a short step, at most so far that its scraper meets the scrapers of bar feeder 2. d. Phase 4: Phase 3 is repeated one or more times. e. Repeat all the phases above. In the burning-out phase after the bed has collapsed, the air supply must be formed so that any dragging along of ash is avoided and such that the distribution of air across the bed is as even as possible. As an alternative to the use of the grate strut described in the NO patent, this can be achieved by using a plane grate without grate struts if one, at this part of the grate, uses an appropriate number of holes per unit area where each hole is given an appropriate diameter. Tests which have been carried out with a prototype grate element have shown that it is possible to find hole diameters and hole geometries which satisfy both the requirement of avoiding blockage and avoiding fuel / ash falling through. These tests are described in detail later in this document. The temperature above the layer of ash in the burning-out zone is high and here it is important to avoid a forward feeding velocity which is so high that one gets a direct exposure of a naked grate without a protective layer of ash.

This can be achieved in several alternative ways: a. In the last part of the grate one can go over from a duplex configuration of the bar feeders to a single bar feeder (non duplex) which will give a lower forward feeding velocity than a duplex arrangement. This is simply realised, for example, 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 do not have counter scrapers. This will function without having a restraining iron /counter scraper because the counter pressure from the ash and fuel, that lie furthest forward on the grate will provide enough resistance for the scrapers in this area. b. Alternatively, every other counter scraper in the burning-out zone can be removed. c. Alternatively, scraper and counter scraper in a duplex arrangement that runs all the way to the end of the grate can be realised with scraper and counter scraper with altered geometry in relation to the scrapers / counter scrapers further forward on the grate, either in that i. one increases the angle of the rear edge of the scraper as illustrated by the scraper cross-section in figure 3 (b relative to figure 3 (a ii. one reduces the angle of the front edge of the scraper, as illustrated in figure 3 (c iii. one replaces the triangular shape with a lower trapezium, as illustrated in figure 3 (d.

The number of holes 4 in the grate 3 for supply of primary- air and the respective diameter of the holes must be decided through tests as described later in this document. However, the holes ought to be arranged in a diagonal pattern in relation to the movement direction of the scraper as shown in figure 2. When they move, the diagonal pattern leads to the scrapers getting the thermal load from the air supply holes evenly distributed over the whole width of the scraper, in contrast to the concentrated load in individual positions on the scraper which will arise if the air supply holes lie in rows parallel to the direction of movement of the scraper.

Description of grate tests:

Tests have been carried out over a period on a grate furnace with Duplex forward feeding of the fuel. During the tests, a segment of the grate was replaced by a test segment with a plane grate surface with holes of different geometries and sizes. The aim of the tests was to identify the necessary number of holes per unit area, and also suitable hole geometries and hole sizes. The tests showed that holes of different geometries got more easily blocked than other geometries, and also that holes of a diameter below a given limit would become blocked in the drying zone. Furthermore, the tests showed that the amount of fuel and ash falling through the holes was small. Thus, the tests have documented that it is possible to design a grate without rows of nozzles or other devices which represent elevations on the grate. The tests have also shown that wear on a plane grate without elevations will be marginal.

Claims

C L A I M S

1. Grate furnace for solid fuel with supply of primary air through holes/slits (4) in a grate plate (3) and a forward feeding mechanism for fuel across the grate (3) , where at least parts of the forward feeding mechanism are arranged in a duplex arrangement and comprise bar feeders (1, 2) with intermediate scrapers (5) and counter scrapers (6), c h a r a c t e r i s e d i n that the grate surface (3) is, in the main, a plane surface without any form for elevations and that said scrapers (5) and counter scrapers (6), at least in backward directed movement, are arranged to undercut the fuel across the grate (3) .

2. Grate furnace according to claim 1, c h a r a c t e r i s e d i n that the burning-out zone of the furnace comprises a forward feeding mechanism of a single bar feeder (1) , as the one of said bar feeders in the duplex arrangement is shortened with respect to the other.

3. Grate furnace according to claim 1, c h a r a c t e r i s e d i n that in the burning-out zone of the furnace the scrapers (5) and/or counter scrapers (6) are arranged with a greater distance on said bar feeders (1, 2) than further forward on the grate (3) .

4. Grate furnace according to claim 1, c h a r a c t e r i s e d i n that the scrapers (5) and/or counter scrapers (6) in the burning-out zone of the furnace are shaped with a geometry that deviates from the scrapers and/or counter scrapers further forward on the grate (3) .

5. Grate furnace according to claim 4, c h a r a c t e r i s e d i n that the scrapers (5) and/or counter scrapers (6) in the burning-out zone of the furnace are shaped with a larger angle at the rear edge than the scrapers and/or counter scrapers further forward on the grate (3) .

6. Grate furnace according to claim 4, c h a r a c t e r i s e d i n that the scrapers (5) and/or counter scrapers (6) in the burning-out zone of the furnace are shaped with an smaller angle at the front edge than the scrapers and/or counter scrapers further forward on the grate (3) .

7. Grate furnace according to claim 4, c h a r a c t e r i s e d i n that the scrapers (5) and/or the counter scrapers (6) in the burning-out zone of the furnace are formed in the shape of a trapezium.

8. Grate furnace according to claim 1, c h a r a c t e r i s e d i n that said holes/slits (4) in the grate plate (3) are arranged in a diagonal pattern in relation to the direction of movement of the scrapers (5, 6) .

9. Grate furnace according to claim 1, c h a r a c t e r i s e d i n that said bar feeders (1, 2) are arranged to move, at least through the drying zone and the gasification zone of the furnace, cyclically in relation to each other, at least through a meeting zone between the scrapers (5) and the counter scrapers (6) of the respective bar feeders (1, 2) .

10. Method for forward feeding of solid fuel in a grate furnace with supply of primary air through holes/slits (4) in a grate plate (3) , and a forward feeding mechanism for fuel across the grate (3) where at least part of the forward feeding mechanism is arranged in a duplex arrangement and comprises bar feeders (1, 2) with intermediate scrapers (5) and counter scrapers (6), c h a r a c t e r i s e d i n that at least through the drying zone and the gasification zone of the furnace, the respective bar feeders (1,2 ) are moved mutually in relation to each other in that a movement pattern is provided such that the scrapers (5) and the counter scrapers (6) of the bar feeders are moved cyclically through stationary zones that arise between fixed extreme points .

11. Method according to claim 9, c h a r a c t e r i s e d i n a) to place 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) is positioned with a distance to, and preferably in the middle of, the scrapers

(6) of the second bar feeder (2), b) to move both bar feeders (1, 2) forward simultaneously a long step without the mutual distance between the scrapers (5,6) on the two bar feeders (1,2) being altered, c) to move the second bar feeder (2) a short step backwards, at most so far that its scraper (6) meets the scrapers (5) of the first bar feeder (1) , and thereafter to move the first bar feeder (1) backwards a small step, at most so far that its scraper (5) meets the scrapers (6) of the second bar feeder (2) , d) to repeat phase c) one or more times, e) to repeat all of the above phases as often as necessary.