WO2019174714A1

WO2019174714A1 - Process and plant for energy efficient thermal treatment of bulk material

Info

Publication number: WO2019174714A1
Application number: PCT/EP2018/056105
Authority: WO
Inventors: Matthias Bergmann
Original assignee: Outotec (Finland) Oy
Priority date: 2018-03-12
Filing date: 2018-03-12
Publication date: 2019-09-19
Also published as: CN213624299U; BR212020018583U2; RU206530U1

Abstract

A method for thermal treatment of bulk material, whereby the bulk material passes in at least one drying zone, one preheating zone, one firing zone and a cooling zones via a travelling grate chain. The travelling grate chain is capable of revolving in direction of movement comprising an endless travelling grate with moveable links. It features a plurality of grate carriages each consisting of a frame with wheels and grate rods being arranged on crossbars. Gas flows through the grate carriages and their grate rods from or into wind boxes. From a wind box in the cooling zone, which is installed downwards of a first wind box of the cooling zone and which sucks the cooling gas medium in, the cooling gas is recirculated via a recycling conduit featuring at least one fan into one wind box of the drying zone. Downwards of the fan, parts of the cooling gas are branched off via a bypass conduit and led back into the wind box of the first cooling zone blowing the gas medium. The cooling gas in the recycling and the bypass conduit is controlled with at least one pressure control valve, which enables draining of cooling gas as an exhaust.

Description

Process and plant for energy efficient thermal treatment of bulk material

The invention relates to a method for thermal treatment of bulk material, where- by the bulk material passes in at least one drying zone, one preheating zone, one firing zone and a cooling zones via a travelling grate chain being capable of revolving in direction of movement comprising an endless travelling grate with moveable links, a plurality of grate carriages each consisting of a frame with wheels and grate rods being arranged on crossbars and whereby gas flows through the grate carriages and their grate rods from or into wind boxes, and whereby from a wind box in the cooling zone which is installed downwards of a first wind box of the cooling zone and which sucks the cooling gas medium in, the cooling gas is recirculated via a recycling conduit featuring at least one fan into one wind box of the drying zone.

In pellet burning or sintering machines the bulk material to be treated, such as for example iron ore, iron oxides or also zinc ore, is loaded onto grate carriages. These grate carriages consist of a frame being provided with wheels and con- sisting of end pieces and crossbars, and grate rods being arranged between the crossbars. A plurality of such grate carriages forms an endless grate carriage chain, which is also referred to as travelling grate.

In figure 1 , for example, a pellet burning machine 1 for burning pellets of iron ore is shown, wherein here the present invention has been employed. At a feeder station before a hood 2 the bulk material is loaded onto grate carriages 3 form- ing an endless grate carriage chain which is referred to as travelling grate 4. Under the hood 2 the bulk material being transported on the grate carriages 3 passes through a number of thermal treatment stations. In detail, these stations are 1. the charging zone,

2. the first drying zone,

3. the second drying zone,

4. the preburning zone,

5. the burning zone,

6. the afterburning zone,

7. the cooling zone and

8. the exit zone.

In these zones the grate carriages are charged with the material, the bulk mate- rial is dried, preheated, fired and subsequently cooled again. At the treatment stations under the hood 2 the travelling grate is guided on a carrying run 5 of a continuous conveyer 6, wherein the rollers 7 of the grate carriages 3 are guided between an inner rail guide 8 and an outer rail guide 9. The drive of the travel- ling grate 4 is effected via a driving or elevating wheel 10 being designed as a gear wheel and engaging with its tooth spaces (recesses 11 ) at the rollers 7 of the grate carriages 3.

After passing through the hood 2 the grate carriages 3 of the travelling grate 4 reach a discharge station which is assigned to a descending or output wheel 13 of the continuous conveyer 6. Like in the case of the elevating wheel 10 at the descending wheel 13 tooth spaces 14 of the output gear wheel engage with the rollers 7 of the grate carriages 3. The grate carriages 3 are tilted so that their load is discharged by gravitational force. Since the grate carriages 3 are guided by the outer rail guide 9, they itself do not fall down, but will be returned back to the elevating wheel 10 upside down in a lower run 15 of the continuous convey- er 6. Below the hood 2 wind boxes 16 are arranged which allow a controlled gas flow. In the region of the carrying run 5 the grate carriages 3 travel along be- tween the above lying hood 2 and the below lying wind boxes 16 without coll id- ing with the components of the hood 2 or the wind boxes 16. During normal operation, the travelling grate 4 revolves on the continuous con- veyer 6 in an endless manner and transports the bulk material to be treated through the treatment stations under the hood 2, before it is discharged at the discharge station and is processed further in a manner which is not described in detail here.

As already mentioned, in the two drying zones, the preburning zone, the burning zone, the afterburning zone and the cooling zone the bulk material contained in the grate carriages is thermally treated by the fact that hot or cold gas flows either from the wind box through the bulk bed in the grate carriage into the hood or from the hood through the bulk bed in the grate carriage into the wind box. In the charging zone and the exit zone no flow prevails.

To improve energy efficiency of such grate carriages, often heated gas streams are recycled, e.g. off-gas of the firing is recycled to the drying and/or the pre- heating stage. Other heat recycling concepts are found in documents EP 1 974 066 B1 and DE 195 13 549 B4.

Document DE 42 34 085 A1 shows a two-sections cooling zone with a recycling from the second section into the first cooling section and the drying zone via lines.

Document DE 1 433 339 describes a cooling zone divided in two sectors, whereby air from the second sector is partly recycled to the first cooling sector and partly to the drying stage. Moreover, DE 1 433 339 discloses a recycling line from gas of the first cooling sector to the pre-heating stage. However, this energy concept does not involve electrical energy and investment costs since two fans are needed to control pressure and gas mass flow in the wind boxes the gas is recycled into. So the overall energy consumption directed to thermal end electrical energy is still high.

In this context it is the object of the current invention to reduce the overall ener- gy requirements of pelletizing plants.

This object is solved with a method according to the features of claim 1.

The method is directed to an improved cooling of pellet machines, namely a cooling step with two independent hood sections and a bypass, passing off- gases from the last cooling stage not only in the drying stage but also into the first cooling stage whereby a common fan is used.

In details, the method for thermal treatment of bulk material comprises treat- ments in at least one drying zone, one preheating zone, one firing zone and two cooling zone. These zones are passed by the bulk material by the help of a travelling grate chain being capable of revolving in direction of movement corn- prising an endless travelling grate with moveable links, a plurality of grate car- riages each consisting of a frame with wheels and grate rods being arranged on crossbars. The bulk material is loaded on the grate carriages. The thermal treatment is achieved by a gas flow through the grate carriages and their grate rods from or into wind boxes, whereby the direction of the gas flow from the blowing wind box installed above or below the grate into the sucking wind box belongs to the different zones. With regard to the cooling zone it is to say that it features at least two pairs of wind boxes (two upper and two lower). From a wind box in the cooling zone which is installed downwards of a first wind box of the cooling zone and which sucks the cooling gas medium in, the cooling gas is recirculated via a recycling conduit featuring at least one fan into one wind box of the drying zone. It is the essential part of the invention that downwards of the fan parts of the cooling hood 2 off-gas gas are branched off via a bypass conduit and led back into the wind box of the first cooling zone blowing the gas medium. Moreover, the cooling gas in the recycling and the bypass conduit is controlled with at least one pressure control valve in each conduit which enable draining of cooling gas as an exhaust. So, only one fan can be used but the different process require- ments of the wind boxes the gas is recycled in can be met. As a result, capex and opex costs are reduced which leads also to an economical as well as an ecological benefit.

In a preferred embodiment of the invention, the cooling off-gas is recirculated from the last wind box of the cooling zone, which sucks the cooling gas medium in. Thereby, the temperature difference between the first cooling zone wherein parts of the gas are recycled and, as a result, the thermal recycling effect, is maximized.

It is also preferred that the recycling gas has a temperature between 280 and 420 °C which fit best to the requirements of the drying and the first cooling zone. In this context, it is also preferred to feed gas with a temperature between 160 and 200 °C into wind box, which is opponent from the wind box the gas stream is recycled from.

It is most efficient to use ambient air as the cooling gas since it is available without any additional costs.

Moreover, it is preferred that the recycling conduit and/or the bypass conduit has a negative pressure for safety reasons. For a further improvement of the method's energy efficiency, gas is recirculated as a heat transfer medium from the first cooling zone into the pre-heating zone and/or from the first cooling zone into the firing zone and/or from the pre-heating zone into the drying zone and/or from the firing zone into the pre-heating zone. Each recycling enables an at least partly heat recovery.

In addition, the invention covers also a plant with the features of claim XX, pref- erably to operate a process with the features of any of claims 1 to YY.

Such a plant for thermal treatment of bulk material comprises at least one drying zone, one preheating zone, one firing zone and at least two cooling zones. Moreover, it features a travelling grate chain being capable of revolving in direc- tion of movement comprising an endless travelling grate with moveable links, a plurality of grate carriages each consisting of a frame with wheels and grate rods being arranged on crossbars. To establish a gas flow through the grate carriages and their grate rods, wind boxes are arranged such that gas flows from or into the wind boxes. In the context of the invention, the already men- tioned wind box features at least two cooling zones, whereby from a wind box in the cooling zone which is installed downwards of the first wind box of the cooling zone and which sucks a cooling gas medium in, the cooling gas is recirculated via a recycling conduit featuring at least one fan into one wind box of the drying zone.

The invention is characterized by a bypass conduit, which is branched off downwards of the fan and leads back into the wind box of the first cooling zone blowing the gas medium. Furthermore the bypass conduit and the recycling conduit features at least one pressure control valve each which enables draining of cooling gas as an exhaust. So, the overall energy efficiency covering thermal and electrical energy is improved. In a preferred embodiment the fan is a UDD fan. The UDD fan provides the needed gas volume for the first drying stage and controls the pressure in the second cooling zone hood. It is also preferred that the cooling zone features two cooling zones with two upper and two lower wind boxes. Thereby, with the current concept a maximized energy recycling rate is achieved.

In another preferred embodiment, the pressure control valves in the bypass conduit and the recycling conduit lead to a common exhaust conduit. So, only one hood exhaust fan and less conduits are necessary, which further reduces capex and opex costs.

Further features, advantages and possible applications of the invention follow from the description of the figures below. Here, all described and/or depicted features form on its own or in arbitrary combination the subject matter of the invention, independently from their summary in the patent claims or their back references. Shown are: in fig. 1 the construction of a travelling grate, in fig. 2 a first process design according to the state of the art in fig. 3 a second process design according to the state of the art in fig. 4 a process design according to the present invention. Fig. 1 has already been explained in detail and shows in principle the arrange- ment of a travelling grate as is also the basis of the present invention.

In fig. 2, the different zone of the pellet burning machine 1 and the relating wind boxes and their conduits are shown in more detail. The grate carriage 3 passes the first and the second drying zones with the relating upper wind boxes 10 and 11 as well as their lower wind boxes 10' and 1 T. From there, it comes to the pre-heating zone with upper wind 20 and lower wind box 20'. Downwards of the described pre-treatment, the firing zone with upper wind box and lower wind box is found. As a distinctive feature, the firing zone shows an second upper wind box 31 coupled to the lower wind box 30' for an after- burning. Finally, the bulk material loaded on the grate carriage passes the different cool- ing zones, which e.g. show three upper wind boxed 40, 41 and 42 and one lower wind box 40'. Fresh air is fed via conduits 60, 62 and fan 61 into the lower wind box 40' and blown though the bulk material into the upper sucking wind boxed 40, 41 and 42.

Off-Gas blown out from the first upper wind box 41 of the cooling zone is fed into the upper firing wind box 31 for the after-burning via conduit 32. From the sec- ond upper wind box 40, off-gas is recirculated via conduit 80, 81 and 82 in the upper wind box 30 of the firing and the upper wind box 22 of the pre-heating. Off-gas from the third wind box of the cooling zone 42 is recycled via conduit 50, fan 51 and conduits 52,53 into the first lower wind box 20' of the drying zone. Parts of said recycled gas are branched of via conduit 54 and passed via valve 55 and conduit 56 into conduit 12. The collected off-gas from the first upper wind box 10 of the first drying zone and said conduit 56 is fed into conduit 90 and blown out via fan 01 and conduit 92. Off-gas from the firing is recirculated via conduit 70, fan 71 and conduit 72 into lower wind box 22' of the pre-heating zone.

Off-gas from the lower wind boxes 21 ' of the second drying is drawn off via conduits 73, fan 74 and conduit 75, while off-gas from the lower wind box 22' of the pre-heating zone is drawn off via conduits 76 and 78 and relating fan 77. Both off-gases are collected in conduit 79.

Fig. 3 shows another possibility of increasing energy efficiency already found in the state of the arte. Therein, the cooling zone shows four upper wind boxes 40, 41 , 42, and 43 and three lower wind boxes 44, 45 and 46, whereby lower wind box 46 corresponds to the upper wind boxed 42 and 43.

From the upper wind box 46 of the cooling zone passed by the bulk material last, the heated off-gas is withdrawn and recirculated via conduit 56, fan 57 and conduit 58 as cooling medium in the lower first wind box 44 of the cooling zone.

Moreover, the other two lower wind boxes of the cooling zone 45 and 46 show a separate introduction of cooling gas via conduit 60, fan 61 and conduit 62 for wind box 45 and via conduit 63, fan 64 and conduit 65 for wind box 46.

Fig 4 describes the current invention. Thereby, only three upper wind boxes 40, 41 , and 42 as well as two lower wind boxes 47 and 48 are foreseen without a direct correlation between the upper and the lower wind box. Via conduit 50, partly heated off-gas is drawn of from the wind box of the cooling zone 42 which is passed last by the bulk material. The heat transfer gas is pres- surized in fan 94. Via by pass conduit 96 the gas is partly injected in the first lower wind box of the cooling zone 47. Bypass conduit 96 shows a control valve 97.

The remaining gas stream is recirculated via conduits 95, 51 and conduit 52 into the first lower wind box 20' of the pre-heating. The pressure and flow in conduit 52 is controlled by valve 55 via conduits 53, 54 into conduit 90.

List of reference signs:

1 pellet burning machine

2 hood

3 grate carriage

4 travelling grate, grate carriage chain

5 carrying run

6 continuous conveyer

7 roller of the grate carriage

8 inner rail guide

9 outer rain guide

10 elevating or driving wheel

1 1 tooth space

13 descending or output wheel

14 tooth space

15 lower run

20 first upper wind box of the drying zone 20 first lower wind box of the drying zone 21 second upper wind box of the drying zone 21 second lower wind box of the drying zone 22 upper wind box of the pre-heating zone 22 lower wind box of the pre-heating zone

30 first upper wind box of the firing zone 30 lower wind box of the firing zone

31 second upper wind box of the firing zone

40 second upper wind box of the cooling zone

41 first upper wind box of the cooling zone

42 third upper wind box of the cooling zone

43 forth upper wind box of the cooling zone 40 lower wind box of the cooling zone 44 first lower wind box of the cooling zone

45 second lower wind box of the cooling zone

46 third lower wind box of the cooling zone

50 conduit

51 fan

52-54 conduit

55 control unit

60 conduit

61 fan

62 conduit

70 conduit

71 fan

72, 73 conduit

74 fan

75, 76 conduit

77 fan

78, 79 conduit

80-84 conduit

75, 76 conduit

77 fan

78, 79 conduit

80 - 84 conduit

90, 91 conduit

92 fan

93 conduit

94 fan

95, 96 conduit

97 control unit

98 conduit

Claims

Claims:

1 . A method for thermal treatment of bulk material, whereby the bulk materi- al passes in at least one drying zone, one preheating zone, one firing zone and a cooling zones via a travelling grate chain being capable of re- volving in direction of movement comprising an endless travelling grate with moveable links, a plurality of grate carriages each consisting of a frame with wheels and grate rods being arranged on crossbars and whereby gas flows through the grate carriages and their grate rods from or into wind boxes, and whereby from a wind box in the cooling zone, which is installed downwards of a first wind box of the cooling zone and which sucks the cooling gas medium in, the cooling gas is recirculated via a recycling conduit featuring at least one fan into one wind box of the dry- ing zone characterized in that downwards of the fan, parts of the cooling gas are branched off via a bypass conduit and led back into the wind box of the first cooling zone blowing the gas medium and that the cooling gas in the recycling and the bypass conduit is controlled with at least one pressure control valve which enables draining of cooling gas as an ex- haust.

2. Method according to claim 1 , characterized in that the cooling gas is recirculated from the last first wind box of the cooling zone which sucks the cooling gas medium in.

3. Method according to claim 1 or 2, characterized in that the recycling gas has a temperature between 280 and 420 °C.

4. Method according to any of the previous claims, characterized in that ambient air is used as the cooling gas.

5. Method according to any of the previous claims, characterized in that ambient air is used as the cooling gas.

6. Method according to any of the previous claims, characterized in that the recycling conduit and/or the bypass conduit has a negative pressure.

7. Method according to any of the previous claims, characterized in that gas is recirculated as a heat transfer medium from the first cooling zone into the pre-heating zone and/or from the first cooling zone into the firing zone and/or from the pre-heating zone into the drying zone and/or from the fir ing zone into the pre-heating zone.

8. A plant (1 ) for thermal treatment of bulk material in at least one drying zone, one preheating zone, one firing zone and at least two cooling zones, comprising an endless travelling grate (4) with moveable links, a plurality of grate carriages (3) each consisting of a frame with wheels and grate rods being arranged on crossbars and wind boxes (20, 20', 21 , 21 ', 22, 22', 30, 30', 31 , 40, 40', 41 , 42, 43, 44, 45) which are arranged such that gas flows through the grate carriages (3) and their grate rods from or into the wind boxes (20, 20', 21 , 21 ', 22, 22', 30, 30', 31 , 40, 40', 41 , 42, 43, 44, 45), whereby from a wind box in the cooling zone (43) which is in- stalled downwards of the first wind box (41 ) of the cooling zone and which sucks a cooling gas medium in, the cooling gas is recirculated via a recy- cling conduit (50, 51 , 52) featuring at least one fan (94) into one wind box (20') of the drying zone characterized in that downwards of the fan (94) a bypass conduit (96) is branched of and leads back into the wind box (47) of the first cooling zone blowing the gas medium and whereby the bypass conduit and the recycling conduit features at least one pressure control valve (55, 97) which enables draining of cooling gas as an exhaust

9. A plant according to claim 8, characterized in that the fan (94) is a UDD fan.

10. A plant according to claim 8 or 9, characterized in that the cooling zone features two upper wind boxes (40, 41 , 42, 43).

11 . A plant according to any of claims 8 to 10, characterized in that the by- pass conduit (96) and the recycling conduit (50, 51 , 52) features one pressure control valve (55, 97) each.

12. A plant according to claim 11 , characterized in that the pressure control valves (55, 97) in the bypass conduit (96) and the recycling conduit (50, 51 , 52) leads to a common exhaust conduit (91 , 93).