GB2142126A

GB2142126A - Heat treatment of a pulverulent solid material

Info

Publication number: GB2142126A
Application number: GB08413019A
Authority: GB
Inventors: Pierre Javelle
Original assignee: Creusot Loire SA
Current assignee: Creusot Loire SA
Priority date: 1983-06-03
Filing date: 1984-05-22
Publication date: 1985-01-09
Also published as: FR2547033B1; GB2142126B; IT1205407B; GB8413019D0; YU91984A; FR2547033A1; ES532889A0; ES8506396A1; BR8402660A; IT8467571A0

Abstract

Apparatus for heat treatment of pulverulent solid material, comprises a tubular mixing chamber (2) with a vertical axis, provided in its lower part with a lower orifice (21) for introducing a rising stream (C1) of hot gas, a side orifice (22) for introducing a transverse stream (C2) of combustive gas, and means for introducing the material and a fuel into the chamber (2). The transverse stream (C2) of combustive gas is directed towards the centre of the column (2) and the lower orifice (21) for the entry of the rising stream (C1) of gas is offset relative to the axis (20) of the column (2) towards the side orifice (22), the sidewall (26) of the column (2) facing the side orifice being connected to the lower orifice (21) by a sloping wall (27) for deflecting the rising stream (C1) of gas away from the side orifice (22) to provide in the region of the side orifice (22) a zone (A) for introducing at least the major part of the fuel. <IMAGE>

Description

SPECIFICATION Improvements in and relating to the heat treatment of a pulverulent solid material The invention relates to an apparatus for heat treatment of a pulverulent solid material, which may for example, be used as a precalciner in a plant for the production of cement clinker.

In plantsforthe production of cement, which are known as dry process plants, the pulverulent raw material, before arriving at the clinkering furnace, passes through a device for preheating it as a suspension, incorporating a series of superposed mixing chambers, of the cyclone type, which are connected byflues and through which the gases leaving the furnace are drawn by a fan. The gases pass upwards through the various cyclones in succession, while the material undergoes preheating as it moves downwards from one chamber to another. Each cyclone includes an upper gas outlet orifice, opening into the flue connected to the cyclone placed on the higher stage and a lower orifice for the outlet of the recovered material opening into the flue feeding the cyclone placed on the lower stage.As a result, the material goes into suspension upstream of each cyclone in the rising stream of gas and descends in this way as far as the last cyclone, which is generally placed on the fourth level, and whose lower orifice is connected to the furnace entry by a material feed duct which in the majority of cases, opens on the sloping bottom of a hood placed at the end of the furnace to collect the gases leaving the furnace and to direct them towards a vertical connecting flue opening into the last cyclone and into which is introduced the preheated material coming from the upper stages of the preheater.

The heat needed for the clinkering reaction is produced by introducing a fuel into the furnace, which is equipped with a burner located at the end of the furnace opposite the exit hood, and an entry for air which is preheated in a device for cooling the clinker leaving the furnace. The heat of the gases leaving the furnace is thus employed in the set of cyclones for preheating the material. However, it is known that the preparation of cement clinker takes place in two stages, one corresponding to an endothermic reaction of decarbonation of the raw material and the other to the exothermic reaction of clinkering. These two reactions, which take place at different temperatures, are advantageously carried out in different enclosures.

In this case, only the clinkering reaction is carried out in the furnace and the decarbonation reaction, also called calcination, is carried out in a separate apparatus for heat treatment, which is called a precalciner and which comprises a mixing chamber interposed in the path of the material, between the penultimate and the last cyclone of the preheating device.

The purpose of the precalciner is to carry out a substantial part of the decarbonation of the material, previously preheated in the first three stages of the preheater. It is therefore equipped with means for introducing a fuel, such as burners, and is crossed by a stream of combustive gas consisting of the air heated in the clinker cooler and, in some cases, of the gases leaving the furnace and containing a quantity of oxygen.

Numerous precalciners are known, all of which aim to ensure an excellent heat transfer which is obtained by ensuring a good combustion of the fuel and, by producing inside the chamber a vigorous mixing of the materials and the various gas streams to homogenise the temperatures as much as possible and to facilitate heat transfer. In some cases, these two functions are carried out in separate chambers, a combustion chamber into which the fuel, the combustive air and, if appropriate, the material, are introduced, and a mixing chamber which is crossed by the gases coming from the furnace and thus contains a quantity of oxygen which depends on the combustion conditions in the furnace.

In other cases, the gases coming from the furnace, the combustive air, the fuel and the preheated material are introduced into the same chamber. It is then usually preferred to divide this single chamber into several zones, a precombustion zone in which the fuel is introduced in the stream of air, a material dispersion zone and a zone for intensive mixing with the gases coming from the furnace. In an arrangement which is frequently adopted, in order to separate the various zones, the mixing chamber is provided in its lower part with a side orifice, usually forming a volute, through which emerges a transverse stream of gas forming a vortex inside the mixing chamber and the latter is provided axially with a lower orifice, usually of a reduced crosssection, through which enters a rising stream of a second gas which rises along the axis of the vortex before mixing with the latter.Usually, the transverse stream consists of hot air and forms the combustive gas needed for the combustion of the fuel introduced through the burners which are placed on the side wall of the chambers. The pulverulent material is dispersed in this swirling stream above the burners and the whole is then mixed with the axial stream in the upper part of the mixing chamber which is connected to the last separator of the preheating device.

All the known precalcination chambers are usually costly and bulky because of the performance which is required. When installed in existing plants, they require very considerable aiterations of the latter and particularly of the preheating device. Of course, these alterations are usually amortised by the increase in production resulting from installing the precalciner but such an installation is less justified in the case of smaller plants in which only a moderate increase in the production is sought after, which does not justify costly conversions of the preheating device.

According to the invention, there is provided apparatus for heat treating a pulverulent solid material, comprising a tubular mixing chamber in the form of a column with a vertical axis, provided in its lower part with a lower orifice, with a crosssection which is reduced relative to that of the column, for connection to a duct for a rising stream of hot gas, a side orifice into which opens a duct for introducing a transverse stream of combustive gas, and means for introducing the material and a fuel into the chamber, the material and gases being in operation entrained towards the top part of the chamber for discharge to a device for separating the solids and the gases, wherein the side orifice is arranged so that the transverse stream of combustive gas is directed towards the centre of the column and the lower orifice is offset relative to the axis of the column towards the side orifice, the side wall of the chamber facing the side orifice being connected to the lower orifice by a sloping wall for deflecting the rising stream of gas away from the side orifice to provide in the region of the side orifice a first zone for introducing the transverse stream of combustive gas, the means for introducing at least the major part of the fuel being placed in the first zone.

Preferably, the mixing chamber has a rectangular cross-section.

The solid material is preferably introduced into the transverse stream of gas by a descending duct opening into the feed duct of the transverse stream of gas in a substantially vertical direction, immediately upstream of the outlet orifice of the gas feed duct. The feed duct of the transverse stream of gas may be provided with a plurality of horizontal bars for dividing the stream of gas arranged across the duct in a substantially vertical plane and tangential to the material feed duct on the side facing the transverse stream.

According to another important characteristic, the apparatus incorporates additional means which may be provided for introduction of fuel into the zone of the rising stream. Preferably a plurality of orifices for fuel introduction means are distributed over the two zones of the bottom part of the mixing chamber and the means for introducing the fuel consist of burners which are fixed in a removable manner in some of the orifices, the other orifices being closed in a removable manner. The orifices for the burners and the respective fuel feed flows are then determined as a function of the desired operating conditions, taking account in particular of the temperatures observed in the various zones of the apparatus.

The accompanying drawings show, by way of example only, an embodiment of apparatus according to the invention, suitable for a plant for the production of cement.

Figure 1 shows diagrammatically a plant for the production of cement equipped with an embodiment of apparatus according to the invention; Figure 2 is a larger-scale view, in axial section, of the lower part of the apparatus of Figure 1; and Figure 3 is a transverse section along the line Ill-Ill of Figure 2.

Figure 1 shows diagrammatically the lower part of a plant for the production of cement comprising a rotary clinkering furnace 1, an embodiment of a precalciner comprising a mixing chamber 2 according to the invention, a device 3 for preheating in suspension, only the lower part of which is shown, and a device 4 for cooling the clinker.

The inclined rotary furnace 1 is equipped, at its lowest end, with a burner 11 supplied with fuel and at its upper end with a hood 12 which directs the gases, which have crossed the furnace, towards the mixing chamber 2 of the precalcinator. The latter comprises a tubular column, with a vertical axis, which is connected at its lower part to a lower entry orifice 21 of reduced cross-section relative to the section of the chamber through which enters a rising stream C1 consisting of the gases coming from the hood 12. The clinker formed in the rotary furnace 1 descends into the cooling device 4 which is traversed by air introduced through inlet 41 and removed along a duct 42, after being heated in contact with the clinker.The duct 42 feeds a collector 43 connected by a cranked duct 44 to a side orifice 22 in the side wall of the chamber 2, in the lower part of the latter, and through which enters a transverse stream C2 of gas, in a substantially horizontal direction.

In its upper part, the chamber 2 is connected by an exit flue 23 to the last cyclone 5 of the preheating device 3. The latter incorporates a lower outlet 51 through which the solid materials separated from the gases in the cyclone 5 are removed through a duct 52 opening onto the sloping bottom 13 of the hood 12 which thus conveys them toward the furnace, and an upper outlet 53 which opens into a duct 31 connecting with the penultimate cyclone 6 of the preheating device.

The material introduced into the upper part (not shown) of the preheater 3 moves down from one floor to the next, is dispersed at 32 in the gases coming from the cyclone 5 and is carried away through the duct 31 to the cyclone 6 from which it is removed through outlet 61 and descending duct 62 which opens at its lower end into the duct 44 for the entry of the transverse stream C2, just upstream of the side orifice 22 of the chamber 2.

Figures 2 and 3 show in greater detail the operation of the precalcinator chamber 2. The latter consists of a tubular column with a vertical axis 20, preferably having, as shown in Figure 3, a rectangular cross-section defined by two long side walls 24 and two short walls, being a front wall 25 in which is placed the side orifice 22 for the transverse stream, and a rear wall 26 opposite the wall 25.

The axis 10 of the entry orifice 21 for the rising stream C1 is offset relative to the axis 20 of the column 2 towards the side orifice 22 for the transverse stream C2. The lower orifice 21 preferably has a square section and is connected to the rear wall 26 by a sloping wall 27. This arrangement permits the rising stream C1 to be deflected towards the rear wall 26 as it enters the chamber 2 under the action of the stream C2 which emerges in a transverse direction through the orifice 22. Thus, the lower part of the mixing chamber 2 is divided into two zones iocated substantially on either side of the median plane P parallel to the front and rear walls 25,26 of the chamber 2, being a front zone A for introducing the transverse stream C2 and a rear zone B for passage of the rising stream C1 after deflection.

In the branching method of Figure 1, in which the rising stream C1 consists of the gases coming from the furnace and the transverse stream C2 consists of reheated air, the rear zone B contains less oxygen than the front zone A and it is therefore in the latter that means for introducing the fuel formed by burners 7 are placed.

As a result, the preheated material entering by the duct 62 is dispersed in the transverse stream C2 which then entrains it towards the front zone A of the mixing chamber where the fuel is introduced. The material is thus dispersed in the stream of hot air in a uniform manner before being subjected to the heat produced by the combustion ofthefuel and the latter takes place under good conditions, given the high proportion of oxygen in the front zone A. It is only then, in the middle part of the mixing chamber 2 and in the top part, that the combustion gases and the material mix with the furnace gases of stream C2 which make it possible to raise the temperature further and to complete the combustion, the whole being discharged through the outlet flue 23 towards the cyclone 5 which feeds the furnace.

Naturally, the calcination yield of the precalcinator chamber 2 can be less good than in the more highly improved chambers but, as can be seen in Figure 1, the precalcinator chamber 2 offers the advantage of being capable of being installed in place of the connecting flue between the hood of the furnace and the last cyclone without a significant modification of the circuits for gas and materials or of the positioning of the various units.

Furthermore, other arrangements make it possible to improve the operation of the mixing chamber.

Thus, it is advantageous to place horizontal bars 45 across the duct 44 for the introduction of the transverse stream C2. As shown in Figures 2 and 3, the bars 45 are preferably arranged in a plane which is substantially vertical and tangential to the material feed duct 62, on the side facing the transverse stream C2. As a result, the transverse bars 45 ensure the division of the stream C2 into several parallel streams which pick up the material M descending along the duct 62 and any clumps of material which may be present are broken by falling on the bars 45 which thereby ensure a better dispersion of the material.

The lower wall of the duct 44 is connected to the wall of the entry orifice 21 by a sloping wall 46 placed below the outlet of the feed duct 62 so as to pick up the heaviest particles which may not have been entrained by the stream C2 and to direct them towards the stream C1 which, thereupon, causes them to rise into the mixing chamber 2.

Although the major part of the fuel supplied to chamber 2 is introduced into zone A, it is advantageous to place additional burners 71 in the rear part of the side walls 24 of the chamber 2, in the bottom part of the latter, that is to say in the zone B of the rising stream C1 after deflection. The flow rate of the fuel introduced depends on the proportion of oxygen present in the gases leaving the furnace and it is possible in this way to employ all the oxygen drawn in through the irilet 41 of the cooling device 4.

In fact, the primary air 15 serving for the burner combustion 11 and the secondary air 16 coming from the clinker cooler 4 and rising in the furnace are combined to form the rising stream C1 which thus contains all the oxygen required relative to the quantity of fuel introduced into the furnace.

The auxiliary burners 71 thus make it possible to introduce an additional quantity of fuel and to employ as well as possible the quantity of oxygen present in the gases drawn in by the fan through the preheater 3. By thus placing in series the precalciner and the furnace, which form the two combustion units of the plant, a difficult adjustment of the proportionality of the airflows, and of fuel in each of these is avoided.

Naturally, without departing from the scope of protection defined by the claims, modifications may be introduced in the devices which have just been described in detail by way of example only.

Thus, in order to take account of the distribution of the gas streams between the two zones A and B of the mixing chamber underthe best conditions, it is possible to provide several orifices for removably fixing the burners 7 in the side walls 24.

In this way, by noting the conditions of combus tion and/or measuring the temperatures at various points in the chamber, it is possible to place the burners 7 in the orifices which are the most aptly placed depending on the operating conditions (com position and the flow rate of the materials and gases, for example, the unused orifices 72 being closed by removable plugs.

A precalcination chamber is thus provided which can be incorporated easily and economically into an existing plant or a plant of moderate capacity, and which, despite its simplicity, can yield an appreciable result.

Claims

1. Apparatus for heat treating a pulverulent solid material, comprising a tubular mixing chamber in the form of a column with a vertical axis, provided in its lower part with a lower orifice, with a crosssection which is reduced relative to that of the column, for connection to a duct for a rising stream of hot gas, a side orifice into which opens a duct for introducing a transverse stream of combustive gase, and means for introducing the material and a fuel into the chamber, the material and gases being in operation entrained towards the top part of the chamber for discharge to a device for separating the solids and the gases, wherein the side orifice is arranged so that the transverse stream of combustive gas is directed towards the centre of the column and the lower orifice is offset relative to the axis of the column towards the side orifice, the side, wall of the chamber facing the side orifice being connected to the lower orifice by a sloping wall for deflecting the rising stream of gas away from the side orifice to provide in the region of the side orifice a first zone for introducing the transverse stream of combustive gas, the means for introducing at least the major part of the fuel being placed in the first zone.

2. Apparatus according to claim 1, wherein the mixing chamber has a rectangular cross-section.

3. Apparatus according to either claim 1 or claim 2, wherein the solid material is introduced into the transverse stream of gas through a descending duct opening into the duct for introducing the transverse stream in a substantially vertical direction and immediately upstream of the side orifice.

4. Apparatus according to claim 3, wherein the duct for introducing the transverse stream of gas is provided with a plurality of horizontal bars for dividing the stream of gas, which bars are arranged across the duct in a plane which is substantially vertical and tangential to the material feed duct on the side facing the transverse stream.

5. Apparatus according to either claim 2 or claim 3, including additional means for introducing fuel in a second zone for the rising stream of gas.

6. Apparatus for heat treatment according to claim 5, including a plurality of orifices for receiving fuel introducing means which are arranged over the two zones of the bottom part of the mixing chamber, and the means for introducing fuel are provided by burners which are fixed in a removable manner in some of the orifices, the other orifices being closed in a removable manner, the positioning of the burners and the respective feed flow rates of fuel being determined as a function of the desired operating conditions.

7. Apparatus according to one of the preceding claims, forming a device for precalcining the raw material, interposed in a plant for the manufacture of cement, in the path of the gases flowing from a clinkering furnace to a unit for preheating the material as a suspension in the gas, incorporating a series of separation stages for the material, the rising stream being formed by the gases coming from the furnace and the transverse stream by hot air coming from a clinker cooling apparatus, and the mixing chamber opening in its upper part into the last separation stage of the preheating unit

8. Apparatus for heat treating pulverulent solid material subtantially as herein described with reference to the accompanying drawings.