WO2001023318A1

WO2001023318A1 - Method for reducing nox emission from a plant for manufacturing cement clinker

Info

Publication number: WO2001023318A1
Application number: PCT/EP2000/007760
Authority: WO
Inventors: Lars Skaarup Jensen; Peter Glarborg; Kim Allan Dam-Johansen
Original assignee: F L Smidth & Co. A/S
Priority date: 1999-09-24
Filing date: 2000-08-10
Publication date: 2001-04-05
Also published as: AU6994400A

Abstract

Described is a method for reducing the NOx emission from a plant for manufacturing cement clinker in which cement raw meal is preheated in a preheater (1). The raw meal is then calcined whilst being subject to simultaneous input of fuel, in a calciner (3) which has at least a primary burning zone (17) and a secondary burning zone (19), and burned into clinker in a kiln (5). Thereafter, the clinker is subsequently cooled in a clinker cooler (7). The method of the invention involves ensuring that the temperature in the primary burning zone (17) of the calciner is maintained at a level above 900 °C while simultaneously ensuring, during the burn-out of the fuel in this zone (17) of the calciner, a gradual mixing of combustion air and fuel. Thus there can be obtained a greater reduction of NOx emissions than by previously known methods.

Description

METHOD FOR REDUCING NO_x EMISSION FROM A PLANT FOR MANUFACTURING CEMENT CLINKER

The present invention relates to a method for reducing the NO_x emission from a plant for manufacturing cement clinker in which cement raw meal is preheated in a preheater, calcined, whilst being subject to simultaneous input of fuel, in a calciner comprising at least a primary and a secondary burning zone, burned into clinker in a kiln, and cooled in a subsequent clinker cooler.

Plants of this kind for manufacturing cement are known from the literature. During the combustion process in the calciner, the nitrogen oxides NO and NO₂ (NO_x) are formed by oxidation of nitrogen in the fuel or by oxidation of nitrogen in the combustion air. If, during the combustion process the temperature is lower than 1500°C, which is nearly always the case in a calciner, NO_x is substantially completely formed on the basis of nitrogen contained in the fuel. If the temperature is higher than 1500°C, a significant formation of NO_x on the basis of nitrogen in the combustion air will take place through so- called "thermal mechanisms". Approximately 95 % of NO_x formed during the combustion process consists of nitrogen oxide.

In connection with the known methods for reducing the NO_x emission from the aforementioned type of kiln plant, reductions or attempts at reductions, of the NO_x emission levels are made by means of various measures. Cement-making plants are known where the NO_x emission is restricted by direct firing of fuel in the exhaust gases from the rotary kiln prior to input of combustion air. Following a sufficient retention time in a zone with a surplus of fuel relative to the existing amount of oxygen, the combustion air and raw materials for the cement-making process are introduced, subsequently causing the fuel to burn out. This procedure for reducing the NO_x emission is often referred to as rebuming. In other types of plants, attempts at reducing NO_x emission is made by ensuring a slow mixture of air and fuel streams or by gradually introducing airflow and fuel into the kiln plant. Also, it is a recognized fact that a rise in temperature will nearly always produce a reduction of NO_x emission from some of the mentioned types of kiln plants. However, a temperature rise during the combustion process will not always result in a reduction of the NO_x emission. If high temperatures are attained on the basis of a homogeneous mixture of fuel and air being generated into a premixed flame without raw meal addition, the hot spot zone thus formed will prevent a reduction of the NO_x emission on the basis of a rise in temperature. In the calciner, combustion of the fuel usually takes place at a temperature ranging between 830 and 900°C, since a temperature within this range is set as equilibrium by means of the lime being calcined. This causes it to act as a thermal buffer if the energy content in the applied amount of fuel is insufficient to ensure complete calcination of the raw materials involved. In certain specific circumstances, the combustion temperature may be higher than 900° C. This will be the case if the energy content of the input fuel is sufficient to ensure complete calcination of the raw materials in the kiln. This may also be the case if the mixture between fuel and the calcined raw materials exhibits substantial deviations in terms of homogeneity. Finally, the scenario indicated above may occur if a very reactive fuel is burned in the kiln plant.

It is an object of the present invention to provide a method by means of which it will be possible to reduce NO_x emission from a kiln plant for manufacturing cement while simultaneously allowing utilization of solid fuels, such as coal and petrocoke.

According to the invention this is achieved by a method of the kind mentioned in the introduction, characterised in that the temperature in the primary burning zone of the calciner is maintained at a level above 900° C while simultaneously ensuring, during the burn-out of the fuel in this zone of the calciner, a gradual mixing of combustion air and fuel.

Thus a greater reduction of the NO_x emission is obtained than by previously known methods. This is attributable to the new appreciation that an elevated combustion temperature in calciners reduces the net formation of NO_x when burning nitrogen-laden fuel if an elevated combustion temperature causes the local O₂ concentration to decrease during the combustion process. The local O₂ concentration will only decrease during the combustion process if a controlled mixing of the combustion air and fuel is ensured. The reason for this is that the rate at which 0₂ is consumed by combustion reactions increases rapidly subject to a temperature rise, whereas the mixing velocity of the airflow does not increase at a correspondingly high rate. A lower 0₂ concentration locally during the combustion process will lead to a reduction in the net formation of NO_x because the relationship between the rate of formation and the rate of reduction for NO_x will be more advantageous for NO_x reduction. In case of a rise in the combustion temperature in a situation where the rate of mixing is not sufficiently slow, i.e. the time-scale for the combustion reactions is slower than the time scale for the mixing process, any rise in the combustion temperature will increase the level of NO_x emission. The reason for this is that the activating energies for NO_x formation are greater than the activating energies for NO_x reduction in case of a homogeneous gas phase reaction and that NO_x formation from heterogeneous combustion of coke increases slightly in case of a temperature rise. The temperature in the primary burning zone of the calciner can be regulated by regulating the feed rate of raw meal to the zone, and it is primarily regulated within the range 900 - 1300°C.

Any residual fuel is burned out in the secondary burning zone of the calciner. In a preferred embodiment of the invention, the raw meal may be fed gradually to the calciner after fuel and, as a minimum, a stoichiomethc airflow has been introduced into the calciner.

To obtain slow mixing of the combustion air and fuel, the combustion air may be introduced substantially tangentially into the calciner. By introducing the combustion air tangentially it will follow a helical path at the wall of the calciner, thereby encircling the upward-flowing gas/fuel stream so that the air is gradually mixed into the gas/fuel stream when the inner layers of air lose their rotation, causing them to be entrained in and carried along by the gas/fuel stream.

The rate at which the combustion air is thus mixed into the gas/fuel stream depends on the velocity of the airflow as it flows into the calciner through the tangential inlet, thereby imparting the rotational energy to the airflow when it is forcibly swirled by the cylindrical wall of the calciner. According to the invention it will therefore be possible to control the inlet velocity of the airflow. For example, this may be done by regulating the cross-sectional area of the tangential inlet. This may, for example, be done by means of dampers fitted in the inlet duct.

An example of the invention will now be described in further detail with reference to the accompanying diagrammatic drawings, in which:

Fig. 1 shows a plant according to the invention, and Fig. 2 shows a detail of a preferred embodiment of the invention. In Fig. 1 is shown a plant for manufacturing cement clinker. The plant comprises a cyclone preheater 1 , a calciner 3, a rotary kiln 5 and a clinker cooler 7. During operation, cement raw meal is directed from a raw meal store (not shown) to the raw meal inlet F of preheater 1. The raw meal subsequently flows towards the rotary kiln 5 through the cyclones of preheater 1 and the calciner 3 in counterf low to hot exhaust gases coming from the rotary kiln 5, causing the raw meal to be heated and calcined. In the rotary kiln 5 the calcined raw meal is burned into cement clinker which, in the subsequent cooler 7, is cooled by means of atmospheric air. Some of the air thus heated is directed from the clinker cooler 7 via a duct 11 to the calciner 3. In the calciner 3 fuel is fired via a number of burners 6 which are preferably fitted in the lowermost part of the calciner. The calciner 3 shown in Fig. 2 comprises a so-called rebuming zone 15, a primary burning zone 17 and a secondary burning zone 19. Hot exhaust gases from the kiln 5 are introduced axially into the reburning zone 15, whereas combustion air, primarily from the cooler 7, is fed to the primary burning zone 17.

According to the invention it is aimed at that the combustion air is mixed slowly with the fuel. This may be donein different ways. For example, the combustion air may be introdued tangentially into the calciner via the duct 11 as shown in Fig. 2. Hereby, subject to swirling action in a helical path at the wall of the calciner and while encircling the upward-flowing gas/fuel mixture, the combustion air will be gradually mixed into the gas/fuel stream as the inner layers of air lose their rotation, being entrained in and carried along by the gas/fuel stream. In order to regulate the rate at which the combustion air is mixed into the gas/fuel flow, the velocity of the air, as it flows into the calciner through the tangential inlet, may be controlled. As illustrated, this can be done by adjusting the cross- sectional area of the tangential inlet by means of dampers 12 fitted in the inlet duct 11. Alternatively, the combustion air may be introduced gradually at several points of entry distributed in the axial direction of the calciner, or it may be mixed with the fuel by intermingling the airstream and the fuel stream over a predetermined distance in the axial direction of the calciner.

In order to maintain a temperature in the primary burning zone 17 of the calciner which is higher than 900°C which is also a requirement according to the invention, the mixing of the raw meal into the fuel-rich stream in this zone 17 of the calciner may occur at a rate which is slower than that at which combustion air is mixed into the same stream. In other words, the mixing of the raw meal into the fuel-rich stream must be done in such a way that the necessary heat consumed for calcination of the quantity of raw meal which, up to a specific point in time, is mixed into the fuel-rich stream does not exceed the amount of heat released by fuel burned in this stream during the same time period. This can be done by introducing the raw meal gradually into the fuel-rich stream in the calciner 3 via two or more inlets. Alternatively or supplementarily, raw meal may be introduced in such a way that it will flow along the wall inside the calciner, causing it to be gradually mixed into the fuel-rich stream. The temperature in the primary burning zone 17 of the calciner may thus be controlled within the desirable range, partly by controlling the quantity of raw meal being fed to the zone and partly by gradually introducing the raw meal into the calciner. The temperature in the primary burning zone 17 of the calciner is controlled, as previously noted, within the range 900 - 1300°C.

Residual fuel, if any, is burned in the secondary burning zone 19 of the calciner.

Claims

1. A method for reducing the NO_x emission from, a plant for manufacturing cement clinker in which cement raw meal is preheated in a preheater (1), calcined whilst being subject to simultaneous input of fuel, in a calciner (3) comprising at least primary and a secondary burning zones (17, 19), burned into clinker in a kiln (5), and cooled in a subsequent clinker cooler (7), characterised in that the temperature in the primary burning zone (17) of the calciner is maintained at a level above 900° C while simultaneously ensuring, during the burn-out of the fuel in this zone (17) of the calciner, a gradual mixing of combustion air and fuel.

2. A method according to claim 1 , characterised in that the temperature in the primary burning zone (17) of the calciner is regulated by regulating the feed rate of raw meal to the zone.

3. A method according to claim 2, characterised in that the temperature in the primary burning zone (17) of the calciner is regulated to be within the range 900 - 1300°C.

4. A method according to any of claims 1 to 3, characterised in that residual fuel is burned out in the secondary burning zone (19) of the calciner (3).

5. A method according to claim 1 , characterised in that the raw meal is fed gradually to the calciner (3) after fuel and, as a minimum, a stoichiometric airflow has been introduced into the calciner.

6. A method according to any of claims 1 to 5, characterised in that the combustion air is introduced substantially tangentially into the calciner through a duct (11).

7. A method according to any of claims 1 to 6, characterised in that the inlet velocity of the combustion air into the calciner (3) is regulated.

8. A method according to claim 7, characterised in that the cross-sectional area of the inlet (11) into the calciner is regulated.

9. A method according to claim 8, characterised in that the cross-sectional area of the inlet (1 1 ) into the calciner is regulated by means of dampers disposed in the inlet duct (11).