GB2198508A

GB2198508A - Improvements relating to coal-fired kiln plants

Info

Publication number: GB2198508A
Application number: GB08724825A
Authority: GB
Inventors: Peter Bechtoft Nielsen
Original assignee: FLSmidth and Co AS
Current assignee: FLSmidth and Co AS
Priority date: 1986-12-09
Filing date: 1987-10-23
Publication date: 1988-06-15
Anticipated expiration: 2007-10-23
Also published as: GB8629372D0; FR2607802B1; BE1003542A4; CN1021482C; FR2607802A1; ES2005726A6; AU588718B2; GB8724825D0; BR8706611A; AU8167887A; GB2198508B; CN87107329A; IN170574B

Abstract

A coal-fired kiln plant e.g. for cement production comprising means (12) for continually taking, in use, a measurement of the ash content of coal meal fed to the plant, and means (23) controlled by the first means (12) for metering, in use, correction material into the plant whereby variations in the composition of the clinker, which would otherwise result from variation in the ash content, are substantially compensated. Ash content may be derived from an analyser in the feed path to the burner (5) or from a temperature responsive coal meal feed control system (17) for a precalciner (2). The control system (17) additionally controls input of coal meal to precalciner to maintain a substantially constant temperature at (18), and the means (12) also controls the input of coal meal to the burner (5) to maintain a substantially constant energy supply at the burner. <IMAGE>

Description

IMPROVEMENTS RELATING TO COAL-FIRED KILN PLANTS In many plants, e.g. for manufacturing cement, rotary or other kilns are fired by coal having a continually varying ash content. This causes undesirable variation in the extent of burning of the clinker manufactured in the kiln and, as the ash is incoporated in the clinker, also results in an undesirable variation in the chemical composition of the clinker and hence of the final product. The problem is particularly acute when the coal has a high ash content, such as the case with low-grade coal which, for present purposes, may be defined as coal of which the mineral content in a dried sample exceeds 30% by weight and/or the net calorific value of such dried coal sample is below 5000 kcal/kg.

With an ash content at this level, even small proportional changes in the ash content in the coal correspond to a large absolute variation in the clinker composition. As a result, there have been problems in using low-grade coal, particularly when a clinker product with a precisely controlled chemical composition is needed.

In accordance with the present invention, in the burning of clinker in a coal-fired kiln plant, a measurement is continually taken of the ash content of the coal meal fed to a burner installation of the plant, and correction material is metered into the plant whereby variations in the composition of the clinker, which would otherwise result from variation in the ash content, are substantially compensated.

The correction material may be added either to the raw material supply to the kiln plant, or to the coal meal fed to the plant, or at some other location in the plant where intimate mixing with the raw material being treated in the plant is ensured. This is important because the correction material should be added before nodulisation of the clinker commences so that the correction material is homogeneously dispersed in the clinker. A most appropriate point to introduce the correction material is the raw material inlet to a preheater of the plant.

The invention is particularly applicable to the production of cement clinker as the minerals forming the ash content of coal correspond quite closely to the clay component of the cement raw material.

Consequently, in the case of cement clinker, the correction material could be for instance fly ash or ground limestone, and the metering of the correction material would be adjusted so that the lime saturation factor (LSF) in the clinker is kept substantially constant. For instance using limestone as correction material the system could be set up so that no correction material is added when there is negliable ash formation, for example if the kiln is optionally coal- or oil-fired, and oil firing occurs. During coal-firing, the correction material would then be added in an amount corresponding to the fluctuating ash content of the coal.

The ash content of the coal may be measured directly by bulk feed on-line analysis equipment, which is set up to monitor the ash content of the coal meal fed to the plant. An output signal from the analyser, corresponding to the ash content, would then be used automatically to meter the correction material into the plant.

Alternatively, the ash content of the coal meal may be measured indirectly. For example, in plants of the kind in which the coal meal fed to a precalciner is controlled to maintain constant a temperature in a separating cyclone associated with the precalcincer outlet, the instantaneous specific coal meal supply to the precalciner may be used as a measure of the calorific value of the coal meal, and consequently of the ash content in the coal meal. A relatively high value of this parameter will indicate a high ash content in the coal meal, and a relatively low value a low ash content.

In addition to controlling the metering of the correction material, the measurement of the ash content in the coal meal fed to the plant may also be used to control the coal meal supply to a burner in the kilns in such a way that the energy supply at the burner is maintained substantially constant irrespective of fluctuations in the calorific value of the coal meal caused by the varying ash content.

An example of a rotary kiln plant for burning cement clinker is illustrated diagrammatically in the accompanying drawing.

In conventional manner the plant comprises a rotary kiln 1, a precalciner 2, a cyclone preheater 3, a clinker cooler 4, a kiln burner installation 5, an exhaust gas duct 6, a raw material inlet 7, and a tertiary air duct 8, leading from the cooler to the precalciner. The -cooler 4 has an outlet 9 and contains a pipe 10 for preheating combustion air supplied to the burner 5 by a fan 11. A controller 12 acquires and responds to a signal corresponding to the ash content in the coal meal fed to the burner 5 from an inlet 13 via a conveyor 14 and also to the precalciner 2 from an inlet 15 via a conveyor 16.

The controller 12 may acquire the ash content signal from an analyser through which at least a proportion of the coal meal feed passes, or from a control system 17 associated with the precalciner. This comprises a temperature sensor 18 in a gas outlet from a cyclone 19 associated with the outlet of the precalciner 2. Any variation in the gas temperature sensed by the sensor 18 is detected by a unit 20 which controls a motor 21 for the conveyor 16 so as to adjust the coal meal feed rate, and hence the energy input to the precalciner 2, to restore the temperature at the sensor 18 to its required value.

A further unit 22 senses the speed of the motor 21 and the signal from a conveyor load cell 30, and hence the instantaneous coal meal supply rate, and feeds it to the controller 12, which thereby deduces the ash content of the coal from its calorific value.

The primary purpose of the controller 12 is to adjust, via a unit 23, which responds to the signal from a load cell 24 of a conveyor 25, the speed of a motor 26 which drives the conveyor 25 feeding correction material from an inlet 27 to the primary raw material inlet 7, where it is mixed with the primary raw material feed 28.

Optionally in addition, the controller 12 controls, via a unit 29 which responds to the signal from a load cell 31 of the conveyor 14, a motor 32 of the conveyor 14 so as to maintain substantially constant the energy supplied to the burner 5.

Claims

1. A method of burning clinker in a coal-fired kiln plant, wherein a measurement is continually taken of the ash content of the coal meal fed-to a burner installation of the plant, and correction material is metered into the plant whereby variations in the composition of the clinker, which would otherwise result from variation in the ash content, are substantially compensated.

2. A method according to claim 1, wherein the correction material is added to the raw material supply to the kiln plant.

3. A method according to claim 1, wherein the correction material is added to the coal meal fed to the plant.

4. A method according to any one of the preceding claims, of burning cement clinker, wherein the correction material comprises fly ash or ground limestone and the metering of the correction material is adjusted so that the lime saturation factor in the clinker is kept substantially constant.

5. A method according to any one of the preceding claims, wherein the ash content of the coal is measured directly by bulk feed on-line analysis equipment, which is set up to monitor the ash content of the coal meal fed to the plant, an output signal from the analyser, corresponding to the ash content, being used automatically to meter the correction material into the plant.

6. A method according to any one of claims 1 to 4, wherein the ash content of the coal meal is measured indirectly by measuring the instantaneous specific coal meal supply to a precalciner of the plant so as to maintain constant a temperature in a separating cyclone associated with an outlet of the precalciner.

7. A method according to any one of the preceding claims, wherein the measurement of the ash content in the coal meal fed to the plant is also used to control the coal meal supply to a burner in the kiln in such a way that the energy supply at the burner is maintained substantially constant irrespective of fluctuations in the calorific value of the coal meal caused by the varying ash content.

8. A method of burning clinker in a coal-fired kiln plant, substantially as described with reference to the accompanying drawing.

9. A coal-fired kiln plant for carrying out the method according to any one of the preceding claims, the plant comprising means for continually taking, in use, a measurement of the ash content of coal meal fed to a burner installation of the plant, and means for metering, in use, correction material into the plant whereby variations in the composition of the clinker, which would otherwise result from variation in the ash content, are substantially compensated.

10. A coal fired kiln plant, substantially as described with reference to the accompanying drawing.