SE1951417A1 - An incinerator with a firing grate and a method of operating such an incinerator - Google Patents

Abstract

An incinerator (10) with a firing grate (12) comprising an inclining grate track extending in a first direction and comprising a plurality of grate steps, wherein a first set of grate steps comprises a plurality of grate bars (14) arranged in parallel, and a second set of grate steps comprises sheets (16), said first set and said second set being arranged alternately, said grate bars (14) are arranged to move reciprocally in parallel with said first direction while engaging an upper surface of said sheets (16); said sheets (16) are pivotally arranged to rotate around a first axis extending perpendicularly to the first direction; and said sheets (16) are provided with a cooling liquid inlet (24) and a cooling liquid outlet (26), wherein the sheets (16) are provided with an interior space where cooling liquid is circulated.A method for operating a firing grate of the incinerator comprising directing a flow of a cooling liquid into an interior of the sheets; intermittently moving said grate bars (14) reciprocally in parallel with said first direction; and mounting said sheets (16) to be rotatable around a first axis extending perpendicularly to said first direction.

Description

1 AN INCINERATOR WITH A FIRING GRATE AND A METHOD OF OPERATINGSUCH AN INCINERATOR TECHNICAL FIELD The present invention relates to an incinerator with a firing grate comprisingan inclining grate track extending in a first direction and comprising a plurality ofgrate steps, wherein a first set of grate steps comprises a plurality of grate bars ar-ranged in parallel, and a second set of grate steps comprises sheets, said first set and said second set being arranged alternately.

BACKGROUND ln the field of combustion technology different technologies are used forcombustion of solid fuels biomass fuels. These solid fuels can, for example, com-prise waste, substitute fuels such as, for example, sorted and/or prepared waste,secondary fuels, biomass, wood-chips or similar. A technology commonly used forthermal outputs in the range of 1-8 MW is moving grate.

An incinerator with a firing grate may consist of a grate construction havinga plurality of grate steps arranged one above the other in the manner of roofing tiles,each comprising a plurality of grate bars mounted parallel adjacent to one another.The roofing- tile-like arrangement of the grate bars thereby forms the grate layer onwhich the fuel bed is transported through the combustion chamber and on which thecombustion takes place. The combustion is maintained by primary air, which is usu-ally passed in, among other things, through gaps between the grate bars from belowinto the combustion chamber.

For movement of the fuel bed, the firing grate is frequently configured sothat the grate bars of every other grate step can be moved back and forth in the lon-gitudinal direction of the grate bars whilst the grate bars of the remaining grate stepsare held fixed. Due to the cyclic back and forth movement of every other grate step,during combustion the fuel bed is transported obliquely downwards on the roofing-tile-like grate construction from a preheating zone into a main combustion zone andfurther to a post-combustion zone. ln such an incineration plant temperature of con-tinuously 800 to 1300° C. or briefly even higher are produced.

The entire front part of the grate bars which project below the grate bars ofthe grate step located there above inside the roofing- tile-like layering is conse-quently exposed to high thermal stresses. Added to this are appreciable mechanical stresses due to transport of the fuel bed and the back and forth movement of every 2 other grate step. ln addition, particularly in solid fuel incineration plants the gratebars are exposed to chemical attack resulting from the special composition of thefuel in this area of application.

From the above it is understood that there is room for improvements and the invention aims to solve or at least mitigate the above and other problems.

SUMMARY The invention is defined by the appended independent claims. Additionalfeatures and advantages of the concepts disclosed herein are set forth in the de-scription which follows, and in part will be obvious from the description, or may belearned by practice of the described technologies. The features and advantages ofthe concepts may be realized and obtained by means of the instruments and combi-nations particularly pointed out in the appended claims. These and other features ofthe described technologies will become more fully apparent from the following de-scription and appended claims, or may be learned by the practice of the disclosedconcepts as set forth herein.

The disclosed incinerator comprises a firing grate with a plurality of differentsets of grate steps. A first set of grate steps comprises a plurality of grate barsmounted parallel adjacent to one another. A second set of grate steps comprisessheets that are cooled. The sheets cover a substantial part of the grate steps withgrate bars in a normal operation position. The sheets and the grate bars therebyform the grate layer extending in a first direction and on which a fuel bed is trans-ported through a combustion chamber and on which the combustion takes place.The combustion is maintained by primary air, which is usually passed in, amongother things, through gaps betvveen the grate bars from below into the combustionchamber. Grate steps with grate bars can be moved back and forth in a reciprocalmotion. The sheets can be rotated around an axis extending perpendicularly to thefirst direction. Due to a cyclic back and forth movement of grate steps with gratebars during combustion, the fuel bed is transported obliquely downwards. ln a first aspect, there is disclosed an incinerator with a firing grate compris-ing an inclining grate track extending in a first direction and comprising a plurality ofgrate steps, wherein a first set of grate steps comprises a plurality of grate bars ar-ranged in parallel, and a second set of grate steps comprises sheets, said first setand said second set being arranged alternately, said grate bars (14) are arranged tomove reciprocally in parallel with said first direction while engaging an upper surface of said sheets (16); said sheets (16) are pivotally arranged to rotate around a first 3 axis extending perpendicularly to the first direction; and said sheets are providedwith a cooling liquid inlet and a cooling liquid outlet, wherein the sheets are providedwith an interior space where cooling liquid is circulated.ln a second aspect, there is disclosed a method for operating a firing grateof an incinerator comprising an inclining grate track extending in a first direction andcomprising a plurality of grate steps, wherein a first set of grate steps comprises aplurality of grate bars arranged in parallel, and a second set of grate steps com-prises sheets, said first set and said second set being arranged alternately, saidmethod comprising directing a flow of a cooling liquid into an interior of the sheets;intermittently moving said grate bars reciprocally in parallel with said first directionwhile engaging an upper surface of an adjacent sheet; and mounting said sheets tobe rotatable around a first axis extending perpendicularly to said first direction.ln a third aspect, there is disclosed a sheet for a firing grate of an incinera- tor, wherein said sheets are rectangular boxes with four long side walls and twoshort side walls, and wherein said sheets are provided with means for rotatingaround a first axis extending in parallel with the long side walls; a cooling liquid inletand a cooling liquid outlet, wherein the sheets are provided with an interior space where cooling liquid is circulated.

BRIEF DESCRIPTION OF THE DRAWINGS ln order to best describe the manner in which the above-described embodi-ments are implemented, as well as define other advantages and features of the dis-closure, a more particular description is provided below and is illustrated in the ap-pended drawings. Understanding that these drawings depict only exemplary embod-iments of the invention and are not therefore to be considered to be limiting inscope, the examples will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: Fig. 1 is a schematic side elevation view partly in section showing an embodi-ment of the disclosed incinerator, Fig. 2 is a schematic side elevation view a firing gate mechanism of the incinera-tor in Fig. 1, Fig. 3 is a schematic perspective view of a disclosed firing grate with grate barsin a first position, Fig. 4 is a schematic perspective view of the firing grate in Fig. 3 with grate bars in a second, extended position, Fig. 5 is a schematic top view showing the disclosed firing grate with inlets and outlets for cooling media, Fig. 6 is a schematic partly cut off perspective view of a disclosed sheet showingflow of a cooling liquid,Fig. 7 shows schematically a basic function of a reciprocating mechanism, Fig. 8a-c show schematically the operation of reciprocating grate bars and rotatingsheets, and Fig. 9 shows schematically in a perspective view an embodiment of a grate bar.

Further, in the figures like reference characters designate like or corre- sponding parts throughout the several figures.

DETAILED DESCRIPTION Various embodiments of the disclosed methods and arrangements are dis-cussed in detail below. While specific implementations are discussed, it should beunderstood that this is done for illustration purposes only. A person skilled in the rel-evant art will recognize that other components, configurations, and steps may beused without parting from the spirit and scope of the disclosure. ln the description and claims the word “comprise” and variations of theword, such as “comprising” and “comprises”, does not exclude other elements orsteps.

Hereinafter, certain embodiments will be described more fully with refer-ence to the accompanying drawings. lt will be apparent to those skilled in the art thatvarious modifications and variations can be made without departing from the in-ventive concept. Other embodiments will be apparent to those skilled in the art fromconsideration of the specification and practice disclosed herein. The embodimentsherein are provided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the inventive concept, and that theclaims be construed as encompassing all equivalents of the present inventive con-cept which are apparent to those skilled in the art to which the inventive conceptpertains. lf nothing else is stated, different embodiments may be combined witheach other. ln the embodiment shown in Fig. 1 and Fig. 2, the disclosed incinerator 10is provided with a firing grate 12 comprising a plurality of grate steps arranged oneabove the other in the manner of roofing tiles. Every second step is formed by a plu- rality of grate bars 14 mounted parallel adjacent to one another leaving some gaps there between for supply of air. Between the steps with grate bars there are pro-vided grate steps formed by sheets 16. All grate bars 14 in one step are connectedto a reciprocating mechanism 18 shown only schematically in Fig. 1. During use ofthe disclosed incinerator 10, the reciprocating mechanism 18 is in operation and willmove each grate step with grate bars 14 back and forth, c.f. Fig. 7 and Fig. 8a Fig.8c, in a synchronized way. The reciprocating mechanism 18 can be provided in sep-arate sections. Each section can be driven by a hydraulic device 22.

The incinerator 10 is provided with an inlet 17 for fuel and is dimensionedfor thermal outputs at least in the range 1-8 MW. ln the embodiment shown in Fig. 1,a conveyor 19 is arranged at the inlet 17 for moving the fuel into the interior of theincinerator 10. An electrically powered burner 21 is arranged to produce hot air thatis directed into a first section of the incinerator 10 to ignite the fuel. The hot air canhave a temperature around 700° C. The heat that is produced can be used to heatwater in a heating system in a conventional manner. A boiler (not shown) operateswith a water pressure of about 16 bar and water is circulated by a pump (notshown). The incinerator 10 shown in the drawings is made of steel and an interior ofit is covered with firesafe sandwich elements 23. The firing grate 12 is arranged in-clining down from the inlet 17 for fuel at a first end to ash collectors and a slag col-lecting and conveying device 25 at an opposing second end section of the incinera-tor 10. A fan 27 is arranged for providing primary air to the incinerator10.

During operation of the incinerator 10, the fuel such as wood-chips ismoved into the first section by the conveyor 19 and ignited by the burner 21. Tem-perature transducers and other transducers in different positions within the incinera-tor 10 are used to provide inputs to a control device for control the conveyor, burner,pump and other elements. After an initial startup process, fuel is fed intermittently tothe incinerator 10 through the inlet 17 for fuel.

The disclosed incinerator 10 with firing grate 12 comprises a grate con-struction having a plurality of grate steps arranged one above the other in the man-ner of roofing tiles, every second step comprising a plurality of grate bars 14mounted parallel adjacent to one another. Every second grate step comprises thesheets 16. The roofing- tile-like arrangement of the grate bars and sheets therebyforms the grate layer on which the fuel bed is transported downwards through thecombustion chamber and on which the combustion takes place. The combustion ismaintained by primary air, which is usually passed in, among other things, throughgaps between the grate bars from below into the combustion chamber. ln various embodiments, the number of sheets 16 is higher than the number of steps with grate 6 bars 14. The grate construction could have a top and bottom step of sheets. ltshould also be noted that sheets 16 could be arranged in less than every secondstep, such as for instance in every third step.

For movement of the fuel bed, the grate bars 14 are moved back and forthin the longitudinal direction of the grate bars, as indicated by arrow A. Due to the cy-clic back and forth movement of every other grate step during combustion, the fuelbed is transported obliquely downwards on the roofing-tile-like grate constructionfrom a preheating zone into a main combustion zone and further to a post-combus-tion zone. ln the incinerator a temperature of continuously 800 to 1300° C. or brieflyeven higher can be produced.

Located below the firing grate 12 are ash collectors which collect the ashproduced during combustion, which drops down through the air slots between thegrate bars, and supply it to subsequent conveying devices. The slag conveying de-vice 25 is located at the lower end of the firing grate. The ash and slag are furtherremoved by suitable devices not shown here in detail.

During most of the production time of the incinerator 10, the grate bars 14of each step are in a withdrawn position as shown in Fig. 3. ln this position, most ofan upper surface of each grate bar 14 is protected by an adjacent sheet 16 abovethe grate bar 14. At regular intervals, the grape steps of grate bars 14 are pushedfon/vard in the longitudinal direction of the grate bars as indicated by arrow A to a po-sition shown in Fig. 4. During the movement fonNard, the grate bars 14 will push un-burnt fuel and residuals 15 fon/vard and down to the next grate step. After reachingan end position, the grate bars 14 will return to the position shown in Fig. 3. Thus,the grate bars 14 will move in a reciprocating movement. The sheets 16 are pivotallyarranged around an axis extending perpendicularly to the direction A and will beable to move in a pivotal movement as indicated by arrow B. The pivotal movementcan be caused by residuals 15 and/or fuel that do not come easily off the grate bars14 during the reciprocating movement.

The sheets 16 are provided with a cooling media inlet 24 and a cooling liq-uid outlet 26 at opposite end sections as shown also in Fig. 5. By feeding cooling liq-uid at an appropriate temperature through the sheets 16 the temperature of themcan be kept sufficiently low to prevent damages and harmful coating of slag. The di-rection of flow of the cooling liquid is of less importance, since the temperature dif-ference of the cooling liquid between the inlet and the outlet into and respectivelyfrom the sheets 16 is relatively small. ln various embodiments, the cooling liquid is circulated through the sheets 16 and also through other parts of the incinerator 10. 7 ln various embodiments, the sheets are rectangular boxes with four longside walls and two short side walls. The sheet 16 shown in Fig. 6 is provided with afirst side wall 40 and a second side wall 42 extending in parallel with the first sidewall 40, both side walls being long side walls. The sheet 16 is provided also with aflat bottom side 44 and a top cover (not shown) to enclose an interior space 45. Thetop cover of the sheet 16 has been removed to show the interior of the sheet 16.Cooling liquid enters through the cooling liquid inlet 24 provided at a first end section46 and will flow out of the sheet 16 through the cooling liquid outlet 26 at a secondend section 48. The cooling liquid will flow through the interior guided by a first sepa-rating wall 28 and a second separating wall 29. The first separating wall 28 is L-shaped with a short leg 31 extending from the first side wall 40 at the first end sec-tion 46 and a long leg 33 extending along the second side wall 42 towards the sec-ond end section 48. The second separating wall 29 extends from the second endsection 48 and along the first side wall 40 towards but not the full distance to theshort leg 31, so as to allow a flow of cooling liquid. The second separating wall 29extends between the long leg 33 and the second side wall. The cooling fluid will becirculated through the interior space 45 to transfer heat efficiently from the walls, thebottom side and the cover. lt should be noted, that the flow direction of the coolingliquid can be opposite to what is shown in Fig. 6.

The sheets 16 are rotatably connected to a support arrangement 47 or sidewalls (not shown) of the incinerator 10, so as to allow some rotational movementduring use. ln the embodiment shown in Fig. 5 and Fig. 6, the rotation will be arounda first axis X extending through the cooling liquid inlet 24 and the cooling liquid outlet26. ln various embodiments, the rotational axis X will be offset from a center line ofthe sheet 16.

The basic function of the reciprocating mechanism 18 is shown in Fig. 7and Fig. 8a - Fig. 8c. ln the embodiment shown, the grate bars 14 of each step areinterconnected by a rod 20 and connected to the reciprocating mechanism 18. Asindicated in Fig. 2 the reciprocating mechanism 18 can be provided in separate sec-tions. Each section can be driven by a hydraulic device 22. Preferably, all sectionsare operated synchronously to move the fuel bed appropriately. When the incinera-tor 10 is in operation, all grate bars 14 are positioned as shown in the figures andmainly covered and protected by an adjacent sheet 16 above the grate bars 14 of agrate step. The position of the grate bars 14 as shown in Fig. 8A is the basic posi- tion that will be maintained during most of the heating process. Most of an upper 8 surface of the grate bars 14 is covered and protected by an adjacent sheet 16above.

Fig. 8b and Fig. 8c show momentary positions of the grate bars 14 duringthe reciprocal motion. The grate bars 14 are arranged rotatable around a secondaxis P extending along and in the center of the rod 20. The position of the grate bars14 shown in Fig. 8c is an end position where the grate bars 14 starts a return move-ment back to the position shown in Fig. 8a via the position shown in Fig. 8b.

At regular intervals, the reciprocating mechanism 18 is activated and eachgrape step of grate bars 14 is pushed fon/vard in the longitudinal direction of thegrate bars as indicated by arrow A. ln various embodiments, the intervals are 2-3movements per minute. Normally, there is at least one movement per minute andnot more than 6 movements per minute. The grate bars are pushed fonNard a dis-tance Y sufficiently large to provide a desired transportation of the fuel bed obliquelydownwards. ln various embodiments, the distance Y is 60-100 mm or around 85mm. All sheets 16 remain fixed in the direction A, but can rotate around axis X as in-dicated by arrow B. By allowing rotation of the sheets 16, forces on and tensions inthe sheets 16 during movements of the grate bars 14 due to residuals or fuel can beeliminated or at least lowered. As a result, the sheets16 will endure for a longertime.

An embodiment of a grate bar 14 is shown in Fig. 9 The grate bar 14 canbe made by cast steel and, at least partly, covered by ceramic material. The gratebar 14 is elongated with a retaining section 50 at one end and a front section 52 atan opposite end. The retaining section 50 has an open space dimensioned to re-ceive the rod 20.

The various embodiments described above are provided by way of illustra-tion only and should not be construed to limit the invention. For example, the princi-ples herein may be applied to any remotely controlled device. Those skilled in theart will readily recognize various modifications and changes that may be made to thepresent invention without following the example embodiments and applications illus-trated and described herein, and without departing from the scope of the present disclosure.

Claims (14)

1. An incinerator (10) with a firing grate (12) comprising an inclining grate track ex-tending in a first direction and comprising a plurality of grate steps, wherein a firstset of grate steps comprises a plurality of grate bars (14) arranged in parallel, and asecond set of grate steps comprises sheets (16), said first set and said second set being arranged alternately, said grate bars (14) are arranged to move reciprocally in parallel with said first direc- tion while engaging an upper surface of said sheets (16); said sheets (16) are pivotally arranged to rotate around a first axis extending per- pendicularly to the first direction; and said sheets (16) are provided with a cooling liquid inlet (24) and a cooling liquid out-let (26), wherein the sheets (16) are provided with an interior space where cooling liquid is circulated.

2. The incinerator (10) of claim 1, wherein a flow path is formed in the interior space to direct a flow of a cooling liquid in two opposite directions through the sheets (16).

3. The incinerator (10) of claim 2, wherein the flow path is formed by a first separat- ing wall (28) and a second separating wall (29) in the interior space.

4. The incinerator (10) of claim 1, wherein the sheets (16) are rectangular boxes with four long side walls and two short side walls.

5. The incinerator (10) of claims 2-4, wherein the first separating wall (28) is L-shaped with a short leg (31) extending from a first long side wall (40) at a first endsection (46) of the sheet (16), and a long leg (33) extending along a second longside wall (42); and the second separating wall (29) extends from a short side wall ata second end section (48) of the sheet (16) opposite to the first end section (46) and along said first long side wall (40).

6. The incinerator (10) of claim 5, wherein the lengths of the long leg (33) and thesecond separating wall (29) are shorter than the length of the interior of the sheet(16).

7. The incinerator (10) of claim 1, wherein said first axis is offset from a center line extending through the sheets (16) in parallel with said first axis.

8. The incinerator (10) of claim 1, wherein the cooling liquid inlet (24) and the cool- ing liquid outlet (26) are arranged in line with said first axis.

9. A method for Operating a firing grate of an incinerator comprising an inclininggrate track extending in a first direction and comprising a plurality of grate steps,wherein a first set of grate steps comprises a plurality of grate bars (14) arranged inparallel, and a second set of grate steps comprises sheets (16), said first set and said second set being arranged alternately, said method comprising:directing a flow of a cooling liquid into an interior of the sheets; intermittently moving said grate bars (14) reciprocally in parallel with said first direction while engaging an upper surface of an adjacent sheet (16); and mounting said sheets (16) to be rotatable around a first axis extending perpendicularly to said first direction.

10. The method of c|aim 9, further comprising directing the flow of the cooling liquid along in opposite directions in parallel with said first axis in the interior of the sheets.

11. A sheet (16) for a firing grate of an incinerator, wherein said sheets (16) are rec-tangular boxes with four long side walls and two short side walls, and wherein saidsheets (16) are provided with means for rotating around a first axis extending in par-a||e| with the long side walls; and a cooling liquid inlet (24) and a cooling liquid outlet(26), wherein the sheets (16) are provided with an interior space where cooling liq- uid is circulated.

12. The sheet (16) of c|aim 11, comprising a first separating wall (28) and a second separating wall (29) in the interior space.

13. The sheet (16) of c|aim 12, wherein the first separating wall (28) is L-shapedwith a short leg (31) extending from a first long side wall (40) at a first end section(46) of the sheet (16), and a long leg (33) extending along a second long side wall(42); and the second separating wall (29) extends from a short side wall at a secondend section (48) of the sheet (16) opposite to the first end section (46) and alongsaid first long side wall (40).

14. The sheet (16) of c|aim 13, wherein a length of the long leg (33) and a length ofthe second separating wall (29) are shorter than the length of the interior of thesheet (16).