GB2146747A - Method and apparatus for precalcining pulverous raw material - Google Patents

Abstract

A kiln plant for treating material such as cement raw material comprises a kiln (1), a cooler (2), a precalciner (9) and a suspension preheater (15, 17, 19). A suspension of preheated material is introduced tangentially via a duct (11) into the top or bottom of a vertical tubular chamber of the precalciner (9) near to fuel/oxygen- containing gas inlets (7, 8) to generate a swirling movement of the suspension in the chamber in such a way that the major part of the suspension remains as a protective layer near to the wall of the tubular chamber without intimate mixing with the fuel/gas mixture, thus allowing the fuel to burn out in the oxygen-containing gas near to the axis of the chamber at a temperature above the calcination temperature of the raw material, whereupon precalcination of the material is carried out in the heated atmosphere in the remainder of the chamber. <IMAGE>

Description

SPECIFICATION Method and apparatus for precalcining pulverous raw material The invention relates to a method and an apparatus for precalcining pulverous raw material containing calcium carbonate, for instance cement raw material, in a separate precalciner of a plant for burning and sintering such raw material, the plant having a suspension preheater for preheating the raw material by means of warm smoke gases, a suspension precalciner for precalcining the material, i.e. expelling CO2 from the latter, a kiln for burning and sintering the material and an adjoining air cooler for cooling the material.The precalciner may be a vertical, tubular chamber with an inwardly sloping frustoconical bottom, the chamber being fed with oxygen-containing combustion gas in the form of spent cooler air which is introduced axially into the chamber, and with fuel which is introduced immediately adjacent to the combustion air inlet. Such precalciners may be an integrated part of the preheater string in onestring preheater plants, in which case kiln smoke gases are passed through the precalciner to a preheater proper. Instead of mixing the kiln smoke gases with hot air from the cooler in the precalciner before feeding the mixture to the preheater the gas stream and the air stream may also be fed separately from the kiln and the precalciner, respectively, to the preheater.In constructions of the muitistring type the precalciner may be an integrated part of a separate "calciner" string in which the preheating is effected solely by the smoke gases from the precalciner. Plants of the above mentioned types are known for instance from GB 1489416, GB 1441733, FR 2442595 and GB 1434091, while the precalciners as such are known for instance from FR 234111 3 and Japanese UM Application No. 45855/81.

When using oil as fuel the hitherto precalciner types have generally functioned satisfactorily in creating inside the precalcination chamber the optimal conditions for suspending and precalcining the preheated pulverous raw material in the fuel-gas mixture irrespectively of the suspension passing upwards or downwards through the chamber. The need for changing to solid fuel, such as crushed coal, as precalciner fuel for reasons of energy cost saving has, however, caused problems in that the complete burning out of the solid fuel in the precalcination chamber has not been accomplished, causing partly an unfavourable afterburning of coal particles in the ducts or the precipitator unit after the precalciner, and partly an undesired occurrence of unburnt coal in the precalcined product which is fed to the kiln.The same problem has occured, although to a lesser extent, when natural gas has been used as the fuel.

It is therefore the object of the invention to provide a method of precalcining pulverous raw materials, especially in using solid fuel or natural gas, where the above drawbacks are reduced or overcome.

According to the invention, this is achieved by suspending the preheated raw material in kiln smoke gas or in spent cooler air and then feeding the suspension tangentially into a substantially vertical tubular precalcination chamber near to fuel/oxygen-containing gas inlets of the chamber to generate a swirling movement of the suspension in the chamber in such a way that the major part of the suspension remains as a protective layer near to the wall of the tubular chamber without intimate mixing with the fuel/gas mixture, thus allowing the fuel to burn out in the oxygen-containing gas near to the axis of the chamber at a temperature above the calcination temperature of the raw material, whereupon precalcination of the material is carried out in the heated atmosphere in the remainder of the chamber.

The burning of the fuel near to the chamber axis takes place at a temperature of between 900 and 1400"C, preferably between 950 and 1200"C, while the temperature of the pre-heated, tangentially introduced suspension which forms a layer adjacent to the chamber wall near to the fuel/gas and suspension inlets and protects the chamber construction against the heating action of the fuel-burning, may lie between 650 and 880"C.

A precalciner for carrying out the new method consists of a precalcination chamber with means for introducing the raw materialgas or -hot air suspension formed outside the chamber in kiln gas or hot air ducts tangentially into the chamber through an inlet near the bottom or the top of the chamber. In case the suspension inlet is at the top of the precalcination chamber, forcing the suspension to pass downwards through the chamber, the latter may have at its top an adjoining separate cyclone burning chamber for mixing fuel with "clean" hot air, i.e. hot air or tertiary air not containing any raw material particles, and may thereby create especially favourable burning out conditions for the solid fuel before its mixing with the raw material suspension further down in the precalcination chamber.When the calciner is of the in-line type, i.e. the mixing of the raw material-kiln gas suspension with the fuel-hot air mixture taking place in the precalcination chamber proper, the riser pipe between the upper kiln end and the chamber may have such a construction and be connected in such a way to the chamber and to the smoke gas outlet of the chamber that at least one movable damper mounted in the riser pipe enables the kiln smoke gas to by-pass the precalcination chamber particularly during starting-up of the plant.

Although the new method and precalciner may be used with any conventional fuel, such as oil, their use with natural gas and particularly with solid fuel, is of greatest benefit and importance.

The invention wiil now be described in more details with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a one-string preheater plant with an in-line precalciner where the raw material-kiln gas suspension is fed to the bottom of the precalcination chamber and the suspension, during its precalcination, passes upwards through the chamber; Figure 2 shows part of a one-string plant similar to that of Fig. 1 but in which the suspension is fed to the top of the precalcination chamber, the suspension passing downwards through the chamber and where the chamber is at its top provided with a separate cyclone burning chamber;; Figure 3 shows a two-string preheater plant with a separate "calciner" string in which the spent cooler air provides one substream for the suspension of raw material, the suspension being introduced at the bottom of the precalcination chamber and passing upwards through the chamber, and another substream providing the combustion air introduced axially in the precalciner; Figure 4 shows part of a two-string plant similar to that shown in Fig. 3, but where the suspension is fed to the top of the precalcination chamber, the suspension passing downwards through the chamber, and where the chamber at its top is further provided with a separate cyclone burning chamber; and, Figure 5 shows a detail of a one-string preheater plant with an in-line precalciner, and where the kiln riser pipe has a by-pass arrangement.

The same reference numerals are used in all five Figures for corresponding parts. As shown, the plants have a rotary kiln 1 with a burner 3, an air cooler 2 with an air inlet 4 and a material outlet 5, a kiln smoke gas chamber 6 with a riser pipe 10 and a precalciner, i.e. a precalcination chamber 9, having a hot air inlet duct 7 for feeding spent cooler air as combustion air to the chamber and a burner 8. There is a precipitator 1 3 for separating precalcined raw material from the suspension withdrawn from the precalciner 9, the precipitated precalcined raw material being fed to the kiln material inlet through a duct 14 and the smoke gases being fed to the remainder of the preheater string through a duct 18.

In the one string preheater plant of Fig. 1, 25 is a raw material inlet, 15, 1 7 and 19 cyclone stages with appertaining riser pipes 20, 22 and 24 and material ducts 12, 21 and 23. Raw material preheated in these stages is fed through the duct 1 2 to the kiln riser pipe 10 to be suspended and partly precalcined therein, the suspension being introduced tangentially at the duct inlet 11 near the bottom of the precalcination chamber 9.

The tangential eddy effect keeps the suspension near the chamber wall at the lower part of chamber 9, allowing solid fuel from the burner 8 to burn out in the hot combustion air from the duct 7 without any significant intimate mixing with the raw material suspension. The latter moves upwards through the chamber 9 due to the combined eddy effect from the tangential movement and the axial movement from the fuel burning and is mixed with the fuel-air mixture in the remaining part of the chamber 9 for further precalcination.

Through a duct 1 6 the suspension withdrawn from the chamber 9 is introduced intothe cyclone separator 1 3 from which the separated precalcined material is fed to the kiln through the duct 1 4 while the smoke gas is fed through the riser pipe 1 8 to the remainder of the preheater. to be withdrawn therefrom and fed to a filter (not shown) through a pipe 24.

In Fig. 2 the precalciner 9 has at its top an adjoining, separate cyclone burning chamber 28 to which fuel is fed through burner 8 and combustion air through duct 7. The raw material suspension created in the kiln riser pipe 10 is introduced tangentially through a duct inlet 26 into the chamber 9. In the upper part of the chamber the tangential centrifugal effect keeps the suspension near the chamber wall allowing for a burning out of the solid fuel without any intimate mixing with the suspension while the precalcination takes place in the remainder of the chamber during the movement of the suspension downwards through the chamber. The suspension with the precalcined product is withdrawn from the bottom of the chamber through a duct 27.

In Fig. 3 is shown a two string suspension preheater, the left string of which is mainly identical to the preheater-string described in connection with Figs. 1 and 2, while the right string, the kiln string, has a kiln riser pipe 10, cyclone stages 31, 33, 35 and 37 with appertaining riser pipes 32, 34 and 36 and material ducts 39, 40 and 41. Raw material is fed to the preheater through a duct 42 and discharged as preheated material through a duct 30 to the precalciner 9. Kiln smoke gas is withdrawn from the preheater through a pipe 38 leading to a filter (not shown). The hot air duct 7 allows, in known ways, for division of the air stream into two substreams, one of which, through a branch 44, provides the precalciner 9 with axially introduced "clean" combustion air while the other substream in branch 43 is used for creating the suspension of preheated and partly precal cined raw material from material ducts 1 2 and 30, the suspension being introduced tangenti ally near the bottom of the precalcination chamber. The function of the latter and the treatment of the suspension are in principle as described above in connection with Fig. 1.

In Fig. 4 the precalciner 9 has like the precalciner shown in Fig. 2 at its top an adjoining, separate cyclone burning chamber 28 into which "clean" hot combustion air from duct 7 is introduced via a duct branch 48 and fuel via burner 8. The raw material suspension is created in a duct branch 47 with raw material from ducts 1 2 (calciner string) and 30 (kiln string). The suspension is tangentially introduced into the chamber 9, passes downwards through the chamber and is withdrawn through a pipe 45. The function proper of the precalcination chamber and the treatment of the suspension therein is as described above in connection with Fig. 2.

Fig. 5 shows a modification of the plant described in Fig. 1. For starting-up purposes the kiln riser pipe 10 has a movable damper 49 allowing for branching-off the precalciner 9 during the starting phase of the plant while under normal running conditions the upper part of the riser pipe 10 is branched-off.

Claims (11)

1. A method of precalcining pulverous raw material containing calcium carbonate, in a separate precalciner of a plant for burning and sintering such raw material, the plant having a suspension preheater for preheating the raw material by means of warm smoke gases, a suspension precalciner for precalcining the material, a kiln for burning and sintering the material and an adjoining air cooler for cooling the material, characterised by suspending the preheated raw material in kiln smoke gas or in spent cooler air and then feeding the suspension tangentially into a substantially vertical tubular precalcination chamber near to fuel/oxygen-containing gas inlets of the chamber to generate a swirling movement of the suspension in the chamber in such a way that the major part of the suspension remains as a protective layer near to the wall of the tubular chamber without intimate mixing with the fuel/gas mixture, thus allowing the fuel to burn out in the oxygen-containing gas near to the axis of the chamber at a temperature above the calcination temperature of the raw material, whereupon precalcination of the material is carried out in the heated atmosphere in the remainder of the chamber.

2. A method according to claim 1, wherein the fuel is solid fuel or natural gas.

3. A method according to claim 1 or claim 2, wherein the burning of the fuel near to the chamber axis takes place at a temperature of between 900 and 1400'C.

4. A method according to claim 3, wherein the temperature is between 950 and 1200"C.

5. A method according to any one of the preceding claims, wherein the temperature of the protective layer of the suspension near to the chamber wall is between 650 and 880"C.

6. A method of precalcining pulverous raw material, substantially as described with reference to the accompanying drawings.

7. A precalciner for carrying out the method according to any one of the preceding claims, the calciner comprising a substantially vertical tubular precalcination chamber with means for introducing the raw material-gas or -hot air suspension formed outside the chamber in kiln gas or hot air ducts into the chamber, characterised in that the means is arranged to introduce the suspension tangentially into the chamber through an inlet near to the top or bottom of the chamber.

8. A precalciner according to claim 7, wherein the suspension inlet is at the top of the precalcination chamber, and the chamber has at its top an adjoining separate cyclone burning chamber for mixing fuel with clean hot air.

9. A precalciner according to claim 8, wherein there are means for introducing the suspension as a hot air substream suspension tangentially into the top of the chamber, and means for leading a separate clean hot air substream to the cyclone burning chamber.

10. A precalciner according to claim 7, which is arranged to be used in-line with a kiln, wherein a riser pipe which is arranged to connect the upper kiln end and the chamber has such a construction and is connected in such a way to the chamber and to a smoke gas outlet of the chamber that at least one movable damper mounted in the riser pipe enables the kiln smoke gas alternatively to bypass the calcination chamber.

11. A kiln plant comprising a suspension preheater for preheating raw material by means of warm smoke gases, a suspension precalciner according to any one of claims 7 to 10 for precalcining the preheated raw material, a kiln for burning and sintering the precalcined raw material, and an adjoining air cooler for cooling the burned and sintered material.

1 2. A kiln plant, substantially as described with reference to the accompanying drawings.