GB2050584A - A combination of a coal- crushing/drying process and a coal- firing process with increased heat recovery - Google Patents

Abstract

A combination coal- crushing/drying and coal-firing process in which the volume of primary air is minimised, wherein the proportion of mill exhaust air laden with coal dust and not required as primary air is used for heating the secondary air and/or cooling the hot material. Preferably the process is used for the burning or roasting of cement clinker, lime or expanded clay using a rotary-kiln system having connected at the outlet side a planetary cooler, tubular cooler, shaft cooler or fluidised-bed cooler.

Description

SPECIFICATION A combination of a coal-crushing/drying process and a coal-firing process with increased heat recovery The invention relates to an environmentally acceptable and energy-saving coalcrushing/drying and coal-firing process in which the volume of primary air used is minimised, even in the case of coal having a high moisture content.

The flame configuration has an important effect on the energy consumption of process furnaces.

For a low heat consumption a rapid final combustion is required with favourable heat transfer through a shot and hot flame. In addition to other factors the following are important variables for this: the proportion of primary air, the secondary-air temperature and the primary-air temperature, as well as injection feed conditions.

The proportion of primary air should be as small as possible and the temperatures for primary and secondary air should be as high as possible. The injection rate and degree of turbulence of the furnace combustion should be freely selectable within the optimum range.

For coal firing the following processes have been known hitherto: 1. Indirect firing 2. Direct firing 3. Semi-direct firing with or without additional filter.

In the case of indirect firing, the coalcrushing/drying plant is operated completely independently of the firing of the furnace. In this process it is in fact possible to optimise the critical influencing factors for the flame configuration independently of the crushing/drying plant.

However, it is necessary to have a separate filter for cleaning.the dust-laden exhaust air from the mill and an intermediate storage container for the pulverised or crushed, dried coal. Accordingly, this process is exposed to the danger of coal-dust explosions and fires. Moreover, the costs for the necessary investment, service and maintenance are high.

In the case of direct firing, all the mill exhaust air containing coal dust is blown into the process furnace. Considerably lower investment costs arise, because it is possible to dispense with, for example, a separate filter for cleaning the dust laden mill exhaust air and with an intermediate storage container for the crushed, dried coal. The danger of coal explosions and fires is considerably reduced as a result of the omission of these intermediate units and necessary intermediate stages. However, the occurrence of high proportions of primary air is disadvantageous.

Consequently, optimum injection feed conditions cannot be attained.

In the case of semi-direct firing, the mill exhaust air laden with coal dust undergoes preliminary dedusting in a cyclone. A portion of this air is directed back into the mill system as recirculated air and is used as additional carrier air in the mill, whereas the remainder together with the coal dust separated in the cyclone is blown into the furnace. In the case of a high coalmoisture content and with low-temperature mill heating, there arises from the drying a greater volume of exhaust air than corresponds to the necessary volume of carrier air. In these cases therefore, it is necessary either to accept a high volume of primary air or to install an additional filter. Accordingly, disadvantages arise as in the case of direct and indirect firing.A high volume of primary air thus prevents optimisation of the injection feed conditions at the furnace burner and an additional filter means additional investment costs and an increased danger of coal explosions and fires.

The invention is thus based on the object of devising a process in which, independently of the coal-moisture content and mill heating, it is possible for the primary air proportion to be kept as low as desired and the injection feed conditions at the burner nozzle can be adjusted independently of the crushing/drying process, without it being necessary to take into consideration a separate coal-dust filter or an intermediate storage container and the associated safety risks and increased costs, as in the case of an indirect installation, and in which the heat of the mill exhaust air can be recovered.

This object is achieved according to the invention in that the proportion of mill exhaust air laden with coal dust and not required as primary air is used for heating the secondary air and/or cooling the hot material.

In one preferred embodiment, that portion of the mill exhaust air which is not required as primary air is subjected to preliminary dedusting through a precleaning stage, in particular a cyclone, and is used for cooling the hot material and/or for heating the secondary air, so that the heat from the hot material is also recovered for heating the mill exhaust air.

In another preferred embodiment, the warm mill exhaust air laden with coal dust is blown into the front hot part of the clinker cooler, so that here too the heat is recovered and, moreover, the residual coal dust ignites in this hot zone so that it is substantially eliminated thereby and also usefully heats the secondary air.

In another preferred embodiment, the mill exhaust air is injected below the grating of the clinker cooler, after it has undergone preliminary dedusting through a precleaning stage.

In another preferred embodiment, the mill exhaust air is injected above the grating of the clinker cooler, after it has undergone preliminary dedusting through a precleaning stage.

In another embodiment of the process of the invention, hot air from the clinker cooler or from the kiln head is used for coal drying.

In another embodiment, hot exhaust gas from the kiln installation is used for coal drying.

In another, special embodiment, however, an individual firing unit is used for coal drying.

The process of the invention is particularly suitable for the burning or roasting process of cement clinker, lime or expanded clay using a rotary-kiln system having connected at the outlet side a planetary cooler, tubular cooler, shaft cooler or fluidised-bed cooler.

It is also suitable for the burning or roasting process of cement clinker, lime or expanded clay when using a shaft furnace or reciprocating-plate furnace.

Furthermore, the process according to the invention is also suitable if a roller-dish mill or ball mill is used for the crushing/drying.

Finally, the process of the invention is also suitable if pure coal firing or mixed firing with coal and other fuels is carried out.

These aforementioned application possibilities of the process of the invention merely represent examples and should not imply any restriction.

In this way it is possible to limit the proportion of primary-air volume for the kiln burner, irrespective of the crushing/drying system and coal-moisture content, to the minimum required for optimum combustion and at the same time to utilise in an energy-saving and environmentally acceptable manner the remaining residual volume of mill exhaust air, without a separate filter installation. Therefore, the process of the invention operates in an energy-saving manner because the heat from the hot mill exhaust air is used to heat the secondary air. Moreover, the residual coal dust from the mill exhaust air is fed to the secondary air so that it does not have to be filtered out. In particular, this residual coal dust can be burnt in the secondary-air stream, according to a special embodiment of the process of the invention.This means a more direct and complete utilisation of all the coal dust in the mill exhaust air, so that the process of the invention operates not only in a manner of the invention operates not only in a manner which is economical in respect of energy and raw materials, but also in an inexpensive and environmentally acceptable manner, because the residual coal dust is eliminated and thus a separate filter installation is not required.

In addition, as a result of omitting an intermediate container for coal dust and also the filter installation, and as a result of the possibility of operating and rendering inert at low hot-air temperatures the associated crushing/drying plant even in the case of high coal-moisture contents, without heat economy losses, there is ensured in the process of the invention a high degree of safety from coal-dust explosions and fires and it is readily possible to overcome even high coalmoisture contents by the extensive use of the waste heat from the kiln and/or from the clinker cooler.

The process according to the invention is illustrated on the basis of the accompanying drawing which represents diagrammatically a system operating according to the process of the invention. The system illustrated therein comprises in particular a cement kiln with clinker/slide grating cooler.

The coal, dried and crushed in a coal mill 1, is separated in a high-power cyclone 2 and is charged via a shut-off member 3 into the primary air stream or into a partial air stream of the primary air 4.

The gas volume and air volume required for drying and as carrier air respectively is sucked by a process blower 5 through the coal mill 1 and highpower cyclone 2. As the carrying medium and drying medium upstream of the coal mill 1 there are used either hot cooler air through the coolerair duct 6 or hot kiln-head air via the kiln-head air duct 7, kiln exhaust gas via the kiln exhaust-gas duct 8, hot gas from a combustion chamber 9 or combinations of cooler air, kiln-head air, kiln exhaust gas and hot gas. Here for controlling the cooler-air flap there are used the kiln air flap 11, the kiln exhaust flap 12, the combustion-chamber flap 1 3 or the cold-air flap 30, and the dust-laden hot air is precleaned in a cyclone 14.The mill exhaust air laden with coal dust and subjected to preliminary dedusting in the high-power cyclone 2 is split up downstream of the process blower 5 and may be fed to the primary-air blower 15, to the mill inlet via the connecting-duct flap 19 through the connecting duct 18, or to a cooling-air blower 1 6 of the clinker cooler 17. The proportions of the different volumes may be optimised according to the requirements of the mill system, the coal-moisture content and the flame configuration, and may be adjusted respectively within a range of 0-100%. For this purpose there are used the connecting-duct flap 19, the coolingair feed flap 20 and the auxiliary flap 21, as well as the primary-air flap 22 and the cold-air mixing flap 23.

As a result of these adjustment possibilities it is possible for the proportion of primary air to be reduced as desired and an extremely wide variety of burner nozzles 24 and combination firings may be used, for example with additional fuel 36, without it being necessary to take into consideration the coal-moisture content and the conditions at the crushing/drying plant.

Therefore, particularly in the case of high coalmoisture contents, a higher proportion of mill exhaust air is blown into the clinker cooler. The injection is effected, for example, with an existing cooling-air blower 16, as shown in the Figure, or directly into one of the cooling chambers 31 (not shown). The injection may also be split up among several cooling-air blowers 16, or may even be fed with a separate cooling-air feed blower 34 into one or more cooling-air chambers 31 or directly into the hot-air part of the clinker cooler 37 via the grating 25. Preferably, however, blowing takes place into the cooling-air chambers 31 situated nearest the hot-air part of the clinker cooler 37. In this way the thermal heat of the mill exhaust air may be fully utilised for heating secondary air.

Moreover, the non-thermal heat portion still contained in the mill exhaust air in the form of the residual coal dust content is ignited upon the passing through the hot clinker layer and is thus utilised. It further heats the secondary air. The excess cooler exhaust air is fed through the exhaust-air duct 32 to the exhaust-air dust separator 27 in which dust is removed therefrom.

By means of the exhaust-air blower 28 and the control flap 29 the exhaust air is discharged into the atmosphere, depending on kiln-head pressure.

In the process of the invention, in contrast to the indirect process, all the exhaust-air heat is recovered, i.e. both the thermal heat and the heat present in the residual coal dust of the exhaust air. Furthermore, as a result of the burning of the residual coal dust content, no coal filter is required. This constitutes a considerable saving and increase in operating safety. Accordingly, coal dust is no longer discharged into the free atmosphere so that the process of the invention is substantially more favourable to the environment than the hitherto known processes. It is also unnecessary to effect any intermediate bunkering of coal dust, so that the danger of coal-dust fires and explosions is considerably reduced.

Altogether, of course, the investment, maintenance and operating costs are lower.

In contrast to the direct and semi-direct processes, it is possible to have free choice of the proportion of primary air. Optimisation of the flame is thus possible and the proportion of primary air can be kept considerably lower. This results in fuel economies which, in the case of high coal-moisture contents, may be as high as about 100 kcal/kg of clinker (0.42 GJ/t clinker).

Claims (13)

1. A combination coal-crushing/drying and coal-firing process in which the volume of primary air is minimised, wherein the proportion of mill exhaust air laden with coal dust and not required as primary air is used for heating the secondary air and/or cooling the hot material.

2. A process as claimed in claim 1 wherein the proportion of mill exhaust air not required as primary air and subjected to preliminary dedusting through a precleaning stage, in particular a cyclone, is used for cooling the hot material and/or for heating the secondary air.

3. A process as claimed in claim 1 or claim 2 wherein the warm mill exhaust air laden with coal dust is blown into the hot part of the clinker cooler, so that not only the thermal heat of the air but also the combustion heat of the residual coal dust in the mill exhaust air, which dust ignites in the hot part of the clinker cooler, is recovered for heating the secondary air.

4. A process as claimed in any one of claims 1 to 3 wherein the mill exhaust air subjected to preliminary dedusting through a precleaning stage is injected below the grating.

5. A process as claimed in any one of claims 1 to 3 wherein the mill exhaust air subjected to preliminary dedusting through a precleaning stage is injected above the grating.

6. A process as claimed in any one of claims 1 to 5 wherein hot air from the clinker cooler or from the kiln head is used for coal drying.

7. A process as claimed in any one of claims 1 to 6 wherein hot exhaust gas from the kiln installation is used for coal drying.

8. A process as claimed in any one of claims 1 to 7 wherein an individual firing unit is used for coal drying.

9. A process as claimed in any one of claims 1 to 8 when used for the burning or roasting of cement clinker, lime or expanded clay using a rotary-kiln system having connected at the outlet side a planetary cooler, tubular cooler, shaft cooler or fluidised-bed cooler.

10. A process as claimed in any one of claims 1 to 8 when used for the burning or roasting of cement clinker, lime or expanded clay using a shaft furnace or reciprocating-plate furnace.

11. A process as claimed in any one of the preceding claims wherein a roller-dish mill or a ball mill is used for the crushing/drying.

12. A process as claimed in any one of the preceding claims wherein pure coal firing or mixed firing with coal and other fuels is carried out.

13. A combination coal-crushing/drying and coal-firing process substantially as hereinbefore described with reference to the accompanying drawing.