GB2190099A - Process for the manufacture of briquettes by hot briquetting - Google Patents

Abstract

In the manufacture of briquettes by hot briquetting, the mixer gas (that is, the gas (27) produced when the pre-heated components (11, 12) of the briquetting mixture are mixed together in a mixer (13)) is freed of dust-containing tar in a tar extraction process (41). The tar is then freed of dust particles in a centrifuge (44) and the dust-free tar is burnt in a combustion chamber (4) to produce flue gases for the pre-heating of the components of the briquetting mixture. <IMAGE>

Description

SPECIFICATION Processforthe manufacture of briquettes by hot briquetting The invention relates to a processforthe manufacture of briquettes by hot briquetting. In particular, the invention relates to a process in which briquettes reformed by the hot briquetting of a mixture of two components, one of which softens at the pressing temperature and the other of which does not soften (or hardly softens).

A process of this generic type, known as the Ancit process ("Ancit" is a Trade Mark) is described, for example, in "Stahl und Eisen" 100,1980, page 805.

The two components, having been preheated, are mixed in a mixer before the hot briquetting process.

During the mixing, a dust-containing gas is produced and is drawn off, from the mixer, for use as fuel in a combustion chamber.

According to DE-OS 22 36 338, which also relates two a process of this type, the dust-containing gas from the mixer is used as fuel to produce flue gases forpreheating the coal componentsforthe briquetting process. Since, in the plants used nowadays, flue gases must be heated to a temperature in the range 1200 and 1 600"C, use ofthe dust-containing gas from the mixer would prove two bee problem because the ash portion in the dust contained in the gas would soften and make it virtuallyimpossibletooperatetheplant. Itis therefore possible to use only a purified gas or a dust-free ta r as fuel for preheating the coal components.

The problem with which the present invention is concerned isto increase the proportion offuel available in the plant and thus reduce the amount of external energy which must be supplied to produce the flue gases for heating the briquetting components.

The present invention provides a processforthe manufacture of briquettes by the hot briquetting of a mixture which contains a component that softens at the briquette pressing temperature and a component that does not soften, or hardly softens, at that temperature, the process comprising the steps of: pre-heating the briquetting components, before mixing, with flue gases; separating the pre-heated components from the flue gases and using some at least oftheflue gases in a waste-gas combustion chamberforthe production of steam; mixing the pre-heated components in a mixer and drawing-off the gas produced in the mixer; separating tar from the mixer gas and removing solids from the tar, and using some at least of the tar in the production offlue gases for heating purposes.Preferably, some at least of the tar is used in the production of the flue gases for pre-heating the briquetting components.

Alternatively, or in addition, some at least of the tar is used in the waste-gas combustion chamber, for the production of steam.

Advantageously, some at least of the mixer gas obtained afterthe tar separation is used in the production of the flue gases, for pre-heating the briquetting components. Alternatively, or in addition, some at least ofthe mixer gas obtained afterthetarseparation is used in the waste-gas combustion chamber for the production of steam.

In orderthatthe mixer gas obtained afterthetar extraction process should contain as little moisture as possible, the mixer gas is cooled, for example to approximately 30"C, following the tar separation.

Advantageously, the aqueous condensate containing oily constituents, which is obtained during the cooling operation is used in the waste-gas combustion chamberforthe production of steam.

In a process in accordance with the invention, gases from the waste-gas combustion chamber are used for heating purposes in a waste-heat boiler for the production of steam. Some of the gases from the waste-gas combustion chamber (typically 2 to 10%) may, selectively, by-pass the waste-heat boiler.

The steam produced by a process in accordance with the invention may be used for driving a steam turbine, for use either in electricity production or as a drive unit.

An embodimentofthe invention is described in greater detail below, by way of example, with reference to the accompanying diagrammatic Figures 1 to 3, of which: Figure 1 illustrates a known method ofcarrying out a briquetting process; Figure2 illustrates apparatus for carrying out a process in accordance with the invention, and Figure 3 illustrates apparatus for use with that shown in Figure 2.

In the process for hot briquetting illustrated in Figure 1 (known as the Ancit process), fuel (6) and air (5) are burnt in a combustion chamber (4) and the resulting flue gases are used to heat the briquetting components which comprise non-softening substances (2,3) (for example, inert coals) and a softening substance (1) (for example, caking coals).

Initially, the flue gases (having a temperature of approximately 1400"C) are used for heating the substances (2), (3) that do not soften to approximately 620"C in a reactortube (7). The non-softening substances are separated-off in a cyclone (8), andthesubstance (1)thatsoftens isthen heated to approximately 320"C i n a reactortube (10) by the hot gases which are obtained downstream of the cyclone (8) and which now have a temperature of approximately 800 C. The softening substance is then separated-offin a cyclone (9).

The non-softening and the softening substance (1, 2,3) areconveyed in a ratio of 70:30 to a mixer(13) via the lines (11) and (12), respectively. A mixing temperature of approximately from 480to500"C resu Its in the mixer (13) and the non-softening inert components (2,3) become bound-in intensively by the softening caking coal (1).Thefinished briquetting mixture is hot briquetted atthis temperature (approximately from 480 to 500"C) on a double-roller press (14). The so-called "green" briquettes are then conveyed via the outlet (15) to a hardening installation, where the strength of the moulded bodies is increased considerably. When the briquettes have cooled, they are available as smokeless fuel.

This hot briquetting process produces the following waste-gas streams: (a)-As a result of heating the caking coal to approximately 500"C in the mixer (13) a mixer gas is released into the line (17), in the form of a rich gas which has a net calorificvalue of approximately 6000 kcal/m3 under standard conditions, and which, in addition, contains a high proportion ofvaporoustar and entrained dust.

(b)- Downstream ofthe second cyclone (9), in which the heated caking coal is separated off, a large amount of lean gas having a net calorific value of approximately from 500 to 800 kcal/m3 under standard conditions is obtained atthe outlet (16).

This gas is composed oftheflue gases produced in the combustion chamber (4), including the volatile constituents freed from the inert components and the vaporised water from these products, including dust particles not separated off in the cyclones (8,9).

Thetwowaste-gas streams (16) and (17) are brought together and burnt in a waste-gas combustion chamber (20). The flue gases from the waste-gas combustion chamber, which are ata temperature of approximately 1 000"C, are then cooled to approximately 250"C in a waste-heat boiler (21), the heat removed being used for the production of steam. The problems associated with this mode of operation areasfollows: - a high rate of steam production is achieved and this cannot always be accommodated, and - external energy must be supplied to the initial combustion chamber (4) at a rate of approximately 0.5 x 10s kcal/t product.

This supply of external energy has an increasingly adverse effect on the efficiency ofthe process. For example, approximately 800 m3/h of natural gas are required in the combustion chamber (4) for the production of approximately 13t/h of saleable product (smokeless fuel) and, at the same time, approximately 16 t/h of steam (15 bar, 2800C) are produced in the waste-heat boiler (21).

The process according to the invention shown diagrammatically in Figure 2 provides for the mixer gas drawn offfrom the mixer (13), via the line (27), at a temperature of approximately 500"C to be treated as follows. The gas is cooled to approximately 120"C in a tar extractor (41) and the higher boiling tar fraction condenses, a large part of the dust also being separated off. The dust-containing tar fraction is drawn off as excess via a line (43). The mixer gas is then fed to a cooler (59) via a line (50) and is there cooled indirectly, by means of a cooling water circuit (57), (58), to approximately 30"C. Afterthe last dust particles and tar aerosols have been separated off in an electrostaticfilter (63), a cooled and purified rich gas is available, downstream of a fan (68), forfurther use.

The dust-containing tar drawn off via the line (43) in Figure 2 is freed of solids in a centrifuge (44), and some or all ofthe resulting dust-free tar, drawn off via the devices (46) to (49), is returned as fuel to the initial combustion chamber (4) where it can be burnt without difficulty.

Some or all ofthe purified rich gas available downstream ofthefan (68) can also be returned to the initial combustion chamber (4) and, when both the dust-free tar and the purified mixer gas are recycled in this way, it is possible (if, for example, the caking coal (1) contains approximately 25%waf (ie water-and-ash-free) volatile constituents) to supply 70% of the energy required for the combustion chamber (4). The proportion can be increased to from 80 to 90% if a caking coal containing a high proportion of volatile constituents (for example, from 28 to 32% waf) is used as the softening component (1 ) in the hot briquetting process.Thus, for example, the proportion oftarfreed from the mixer gas is, in the case of a caking coal containing 32% volatile matter (waf), increased by approximately 50% byweight compared with a caking coal containing 25% volatile matter (waf).

As an alternative to the illustrated arrangement of the centrifuge (44), it is possible to incorporate a centrifuge and/or a candle filter (54) into the circuit of the tar extractor (41), as shown by broken lines in Figure 2, the solids being separated off and the dust-freetarbeing drawn off accordingly.

As in the system illustrated in Figure 1, the lean gas obtained at the outlet (26) downstream of the cyclone (9) in the briquetting apparatus is supplied to, and burnt in, a waste-gas combustion chamber (30, Figure 3).

The waste-gas combustion chamber (30) should preferably be operated at a waste-gas temperature of from 950 to 1000 C, in order to avoid softening of the ash content of the dust introduced with the lean gas.

With the mode of operation illustrated in Figure 2, in which the mixer gas obtained downstream of fan (68) is recycled to the initial combustion chamber (4), an amount of energy of from 1 O to 15% remains to be supplied to the waste gas combustion chamber (30) in order to raisethetemperature ofthis combustion chamber to the desired value of from 950 to 1 000 C.

Thisamountcan be supplied in full bythe dust-containing tar scurry obtained (at 45) downstream ofthe centrifuge (44) and by the aqueous condensate (containing oily constituents) which is obtained at the outlets (60, 64) of the rich gas cooler (59) and the filter (63) and which is drawn-off via the line 65 from a receiver (61) by the pump (66).

Approximately 10 to 15% of this aqueous condensate is composed of oily constituents (such as benzene, toluene, etc) and these could only be separated from the remainder of the aqueous phase with very great difficulty, in a separation tank. In addition, phenol, ammonia, H2S, etc are dissolved in the aqueous phase and make it impossible to drain off the waste water into the drainage system withoutfurther treatment. From the point of view of reducing emissions, the described method of burning the small amount of condensate in the waste-gas combustion chamber (30) together with the remaining dust-and-tar-containing waste gas (lean gas), is especially efficient. The burning ofthetar slurry from centrifuge (44) in the waste-gas combustion chamber (30) is also particularly efficient and disposes of the residue from the centrifuge (44) in a simple manner. In the example given above (with 13 t/h of saleable product), the amount of steam produced in the waste-heat boiler (31, Figure 3) would be reduced (using a process in accordance with the invention) to approximately 8 t/h (at 15 bar and 280"C) compared with the 1 6t/h of steam produced in the process practiced hitherto.

Figure 3 shows how flue gases produced inthe waste-gas combustion chamber (30) can by-pass the waste-heat boiler (31), when appropriate.

Depending on the local conditions, it has been found that, for example at weekends and on public holidays, the quantity of steam produced in the waste-heat boiler (31) cannot be accommodated by certain consumers, although the briquetting plant must still be operated. In order to avoid the working-up ofthe entire amount of boilerfeed water at this time, the flue gases, having a temperature of approximately 1000doc, are drawn off downstream of the combustion chamber (30) and cooled to approximately 25000 in a cooler (40) by the injection of water (at 37) and are passed into the atmosphere via an electrostaticfilter (34) (in which the gases are freed of dust) and chimney (35).In order to keep the waste-heat boiler ready for operation at all times and to avoid condensation, a small portion oftheflue gases (from 2to 10%) continues to be passed through the waste-heat boiler (31) at the same time, to keep the boilerwarm.

As a modification ofthe process described above with reference to Figures 2 and 3, some or all of the dust-containing tar obtained from the tar extractor (41) can be burnt in the waste-gas combustion chamber (30) for the production of steam, instead of being used as fuel in the initial combustion chamber (4). In this way, the amount of steam produced inthe waste-heat boiler (31) can be regulated in accordance with requirements.

As a further modification, mixer gas obtained downstream of the fan (68) can also be burnt in the waste-gas combustion chamber (30).

The steam produced in the waste-heat boiler (31) can be used in any appropriate way but is advantageously used to drive steam turbines in the plant, the turbines being used either to generate electricity to meet the electricity requirements ofthe briquetting plant or as drive units.

Claims (19)

1. Process for the manufacture of briquettes by thehotbriquettingofa mixture which contains a component that softens at the briquette pressing temperature and a component that does not soften, or hardly softens, atthattemperature,the process comprising the steps of: pre-heating the briquetting components, before mixing, with flue gases; separating the pre-heated components from the flue gases and using some at least of the flue gases in a waste-gas combustion chamberforthe production of steam; mixing the pre-heated components in a mixer and drawing-offthe gas produced in the mixer; separating tarfrom the mixer gas and removing solids from the tar, and using some at least of the tar in the production of flue gases for heating purposes.

2. A process according to claim 1, in which some at least ofthe tar is used in the production ofthe flue gases for pre-heating the briquetting components.

3. A process according to claim 1 or claim 2, in which some at least ofthetar is used in the waste-gas combustion chamber, for the production of steam.

4. Aprocessaccordingto anyoneofthe preceding claims, in which some at least of the mixer gas obtained after the tar separation is used in the production oftheflue gases, for pre-heating the briquetting components.

5. A process according to anyone ofthe preceding claims, in which some at least of the mixer gas obtained after the tar separation is used in the waste-gas combustion chamberforthe production of steam.

6. Aprocessaccordingto anyoneofthe preceding claims, in which the mixer gas is cooled following the tar separation.

7. A process according to claim 6, in which the mixer gas is cooled to 30 C.

8. A process according to claim 6 or claim 7, in which the aqueous condensate containing oily constituents, which is obtained during the cooling operation is used in the waste-gas combustion chamberforthe production of steam.

9. A process according to any one of the preceding claims, in which gases from the waste-gas combustion chamber are used for heating purposes in a waste-heat boiler for the production of steam.

10. A process according to claim 9, in which some of the gases from the waste-gas combustion chamber selectively by-pass the waste-heat boiler.

11. A process according to claim 10, in which from 2 to 10%ofthegasesfromthewaste-gas combustion chamber pass through the waste-heat boiler.

12. A process according to claim 10 orciaim 11, in which the gases that by-pass the boiler are cooled and freed of dust and then discharged to atmosphere.

13. A process according to any one of the preceding claims, in which the steam produced is usedfordriving a steam turbine which is used either in electricity production or as a drive unit.

14. A process according to any one of the preceding claims, in which the briquetting component that does not soften or hardly softens is heated to from 550 to 7000C by flue gases having a temperature of from 1200 to 160000.

15. A process according to claim 14, in which after the component that does not soften or hardly softens has been separated off, the flue gases are used for drying the component that softens and for preheating it to a temperature of from 220 to 380"C.

16. A process according to any one of the preceding claims, in which the gas produced in the mixer is drawn-off at a temperature of from 400 to 550"C.

17. A process according to any one of the preceding claims, in which solids are removed from the tar in a centrifuge and/or a candle filter.

18. A process according to any one of the preceding claims, substantially as described herein.

19. Apparatus for carrying out a process according to any one ofthe preceding claims, substantially as described herein with reference to, and as shown in, Figures 2 and 3 ofthe accompanying drawings.