GB2098305A

GB2098305A - Utilising heat of discharge gases in cement plant

Info

Publication number: GB2098305A
Application number: GB8135245A
Authority: GB
Original assignee: Italcementi SpA; Italcementi Fabriche Riunite Cemento SpA
Current assignee: Italcementi SpA
Priority date: 1981-05-11
Filing date: 1981-11-23
Publication date: 1982-11-17
Also published as: IT1137384B; BE891354A; NL8105329A; FR2505473A1; GR76377B; DE3151823A1; IT8121626A0; DK522381A; LU83829A1

Abstract

The invention relates to a plant for the dry production of portland clinker in a kiln with a multi-stage suspension preheater, arranged to provide advantageous recovery of at least part of the enthalpy of the exit gases from the preheater, characterised by comprising on the outlet side of said preheater a steam production boiler (31) divided into at least one pair of sections (33, 34) of different pressure, and a superheater (19) for the steam produced in said boiler situated in a zone of said preheater where the temperature of said gases is at least 500 DEG C. The steam thus produced can be used for example for the production of electrical energy. <IMAGE>

Description

SPECIFICATION Portland clinker production plant In the dry production of portland cement clinker, rotary kilns with a suspension preheater for the raw pulverised feed are known. In these plants, the gases leaving the preheater have a temperature of about 3500C, and part of their heat content can be used for drying the raw materials when their moisture content requires it.

Dust removal from these gases is carried out in electrostatic filters after conditioning with regard to humidity and temperature in a suitable column.

The most recent method using cement kilns with a suspension preheater -- in the case of large plants -- utilises the so-called precalcining of the pulverised feed (after its preheating but before being fed into the rotary kiln) in a suitable reactor supplied with 3060% of the total fuel required by the process. Precalcining enables higher production capacities of the rotary kiln to be obtained for the same size, and removes volatile components from the internal cycle.

However, these advantages are normally accompanied by an increase of about 30"C in the temperature of the exit gases from the suspension preheater (about 3800 C), which results in a greater specific heat consumption. When the moisture content of the raw materials is very low, only a small part of this heat can be reused for drying them, whereas most is dissipated in the electrostatic filter conditioning column.

The object of the present invention is to provide an advantageous method for recovering the heat contained in the exit gases from the suspension preheater of a cement kiln, particularly if provided with a precalciner, where the raw materials have such a low moisture content as to be dried by another heat source which would otherwise be unused, such as the hot air expelled to atmosphere by the clinker cooler. In the aforesaid cases of gases leaving the preheater at a temperature of 3500C and beyond, it is possible to recover part of their enthalpy in order to produce electrical energy by means of a boiler, steam turbine and alternator. It must however be considered that the extent of possible recovery is strictly linked to the characteristics of the steam produced in relation to the various useful gas temperature levels.With a gas temperature for example of 3500 C, a known waste heat boiler would return, in terms of electrical energy, about.

2.8% of the heat consumed for calcining the clinker, i.e. would allow a production of about 10 kWh per ton of clinker produced. With a boiler operating with gases at 3500C, there is also the risk that the superheated steam entering the turbine at about 2500C can contain traces of liquid, thus inevitably prejudicing the turbine life.

It therefore does not appear to be particularly advantageous to recover heat by means of waste heat boilers located in the exit gases from the preheater if these gases are at a temperature of around 3500C.

According to the present invention, the aforesaid object can be attained by means of a plant for the dry production of portland clinker in a kiln, comprising a multi-stage suspension preheater, arranged to provide advantageous recovery of at least part of the enthalpy of the exit gases from the preheater, characterised by comprising on the outlet side of said preheater a steam production boiler divided into at least one pair of sections of different pressure, and a superheater for the steam produced in said boiler located in a zone of said preheater in which the gas temperature is at least 5000 C.

The characteristics and advantages of the invention will be more apparent from the description of non-limiting embodiments given hereinafter with reference to the figures of the accompanying drawings.

Figure 1 is a flow diagram of a plant according to the invention.

Figure 2 is a flow diagram of a further embodiment of a plant according to the invention.

With reference to Figure 1, a plant according to the invention comprises a rotary kiln 21 for producing portland clinker, provided with a grid cooler 22 for the clinker. A fan 23 feeds the excess hot air 24 from the cooler outlet to a raw pulverised feed mill 25.

The kiln 21 is connected to a precalciner 14, in which a proportion ranging from about 30 to 60% of the total fuel is burnt. The precalciner 14 is connected to a pulverised feed preheater 9, comprising a series of stages in succession, indicated by 1 5 (fourth stage), 1 6 (third), 1 7 (second) and 1 8 (first stage).

A steam preheater 1 9 is provided between said second stage 17 and said first stage 1 8. The preheater stage 18 is connected to a boiler 31 by a duct 30 provided with fans 32. Said boiler 31 is divided into a pair of sections 33 and 34 of low pressure and high pressure respectively. The boiler is connected to a bag filter 35, on the outlet side of which a fan 36 acts. Finally, a turbine 10 driving an alternator 11, and a steam condenser 12 are connected to said preheater 1 9.

In the following description of operation of the plant shown in Figure 1, no reference is made to those aspects considered known to an expert. The gases 8 leaving the precalciner 14 pass in succession through the stages 1 5, 1 6 and 1 7 of the preheater, to reach the superheater 1 9 at a temperature of about 5500 C. They are then drawn by the fans 32 into the first stage of the preheater, and fed to the boiler 31, to pass in succession through the two sections 34 and 33.

The gases 8 produce steam in the boiler. They then pass through the filter 35 and are drawn in at 36 and expelled to atmosphere. The steam produced in the boiler flows through a circuit indicated by 7. On the outlet side of the boiler section 34, the steam reaches the superheater 19, and is superheated therein to about 4500C by heat transfer against the gases 8 which are at about 5500 C. The steam flows from the superheater to the turbine 10, which drives the alternator 11.

The turbine steam is condensed in 12, and the water produced is fed back to the boiler along a circuit 6.

The alternator 11 produces electrical energy, to recover about 8.3% of the total heat utilised in calcining the clinker (30 kWh pert of clinker produced). The heat consumption necessary for producing 1 kg of clinker +0.03 kWh of electrical energy is of the same magnitude as the classical heat consumption of a kiln with a preheater and pulverised feed precalciner (about 3500 kJ/kg of clinker).

The advantage obtained from the described plant is therefore apparent. Such an advantageous energy recovery is possible essentially because of the fact that the superheater is located in a zone of the plant in which the gases have a temperature which is sufficiently high to allow production of superheated steam at about 4500 C.

The plant shown in Figure 2 represents another embodiment of the invention, which differs from the example described in Figure 1 firstly in that a steam production boiler 37 is divided into a pair of sections, namely a low pressure section 38 and a high pressure section 39, which are disposed in quite different zones of the gas outlet duct 30 of the preheater 9. More specifically, the high pressure zone 39 is located between the preheater 9 and the fans 32. This arrangement prevents the high pressure boiler section from operating with excessive quantities of false air. Secondly, in the case of the plant of Figure 2, a hot water quantity is produced in the section 38 of the low pressure boiler which is greater than that which can be evaporated in the high pressure section. This excess is fed wholly or partly, directly through 40, to a suitable point of the superheater 1 9.This latter, besides superheating the steam fed to it, then also performs the further function of an evaporator. By varying the flow to the steam preheater of this hot water excess produced in the boiler, it is also possible to keep the temperature of the superheated steam constant as the temperature of the exit gases from the second stage of the pulverised feed preheater varies. With this modified embodiment of the invention, the electrical energy production is raised from 30 to 33 kWh pert of clinker (again with a total heat consumption of about 3500 kJ/kg of clinker).A further recovery increase can be attained if in addition to the hot water quantity which can be evaporated in the high pressure section and in the superheater, saturated steam 41 is produced in the boiler low pressure section for injecting into a suitable point of the turbine or for use in directly driving turbo-motors. The technical limit to the degree of enthalpy recovery from the gases in the coldest part of the low pressure boiler section is fixed by the gas SO2 content.

A still more efficient embodiment of the invention is attained if the clinker calcining process is carried out with low-intensity internal circulation of the so-called volatile minor components, i.e. when the raw materials used for the raw pulverised feed and the fuel used in the process are particularly low in easily vaporised chlorides, sulphates and alkalies. Under such conditions, the steam preheater can be located between the 3rd and 2nd stage of the pulverised feed preheater instead of between the 2nd and 1 st stage as heretofore described, without damaging sulphate and alkaline and alkaline-earth chloride incrustations forming on the outer surface of the superheater tubes. The superheater then operates with gases at about 7500C, and can produce superheated steam at about 500"C.

Whether the steam preheater is located between the 2nd and 1 st stage or between the 3rd and 2nd stage, the invention offers the further advantage of providing gases at the boiler outlet which are at a sufficiently low temperature and moisture content for dust removal in bag filters instead of in electrostatic filters. This treatment avoids the need for installing a conditioning column, while at the same time attains a dust concentration level in the filtered gases which is less than 15 mg per Nm3.

The most interesting use for the energy recovered from the hot exit gases from the preheater in a cement plant would appear to be for producing electrical energy as heretofore stated.

However, any other use must be considered to be included within the scope of the invention as herein described.

Claims

1. A plant for the dry production of portland clinker in a kiln with a multi-stage suspension preheater, arranged to provide advantageous recovery of at least part of the enthalpy of the exit gases from the preheater, characterised by comprising on the outlet side of said preheater a steam production boiler divided into at least one pair of sections of different pressure, and a superheater for the steam produced in said boiler, situated in a zone of said preheater where the temperature of said gases is at least 5000 C.

2. A plant as claimed in claim 1, characterised in that said steam superheater is situated between the second and first stage of said suspension preheater.

3. A plant as claimed in claim 1, characterised in that said steam superheater is situated between the third and second stage of said preheater.

4. A plant as claimed in claim 1, characterised in that superheated water from the lower pressure section of said boiler is fed into said superheater in order to be evaporated in this latter.

5. A plant as claimed in claim 1, characterised in that saturated steam for feeding to a user item is produced in the lower pressure section of said boiler.

6. A plant as claimed in claim 1, characterised in that the sections of said boiler are adjacent.

7. A plant as claimed in claim 1, characterised in that the sections of said boiler are situated in separate zones.

8. A plant as claimed in claim 1, characterised in that the steam thus produced is fed to a turbine coupled to an alternator for producing electrical energy.

9. A plant as claimed in claim 1, characterised in that at least part of the superheated steam thus produced is fed to one or more turbomotors.

10. A plant substantially as heretofore described and illustrated in the figures of the accompanying drawings.