Method for reducing the combustion temperature and thermal radiation in a lime kiln

Abstract

A method for reducing the combustion temperature and / or heat radiation in a lime kiln of a slurry production plant, the lime kiln being a rotary kiln having a kiln tube (1) and having a burner (2) which is supplied with fuel for heating the rotary kiln by means of a flame (3), the inside of the kiln tube being covered with refractory bricks (13). These effects are achieved by supplying calcium carbonate-containing particles to the flame (3) and / or to the surrounding area around the flame (3). Preferably, the particles are supplied into the rotary kiln by at least one lance (9) to the upper part of the flame (3). Calcium oxide-containing particles can be supplied to the area around the flame (3) in the rotary kiln for reducing the heat radiation, preferably to the area above the flame (3) and / or to the area at the side of the flame (3) where the refractory bricks (13) of the kiln rotate downwards.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for reducing the combustion temperature and heat radiation in a lime kiln of a pulp production plant. BACKGROUND

[0002] Lime reburning is performed at high temperatures well above 900 degrees Celsius. Therefore, the flame temperature is always high so that the formation of nitrogen oxides (NOx) can occur. A large number of NOx reduction catalysts and chemical treatments have been developed to reduce the NOx content in the flue gas emitted by different types of combustion processes. Generally, flue gas treatment is more expensive than NOx reduction in the combustion zone.

[0003] The formation of NOx in coal, gas and oil flames has been extensively studied. One of the key factors for the formation of NOx is the combustion temperature. At temperatures below 760 °C, the formation of nitric oxide is an irrelevant phenomenon. The molar concentration of nitrogen and oxygen and the combustion temperature are the main factors related to thermal NOx formation. Thermal formation of NO starts already at 650 °C, but is significant only at 1300 °C. For example, the publication “Reduction of nitrogen oxide emissions in limekiln” by Timo Hakkarainen considers a number of ways to solve the problem of lime kiln. Different fuels produce different flame temperature profiles. The most commonly used fuel in pulp plants is natural gas, which has a higher peak temperature than, for example, heavy oil fuel. The combustion temperature can be reduced by using a mixture rich in fuel to limit the amount of available oxygen, using a lean fuel mixture by reducing the energy input to limit the temperature, injecting cooled low-oxygen flue gas into the combustion air, injecting cooled flue gas with added fuel or injecting water or steam. The use of water injection to reduce the peak temperature increases the heat consumption. Steam / water injection is only recommended for cement kilns, not for lime kilns, because the raw materials are usually wet, requiring a lot of drying and preheating. Injection of cooled flue gas is considered a technically unfeasible control technique for lime kilns, as it excessively reduces the flame temperature, thus weakening the lime quality.

[0004] US2019093950 and US5746144 describe a two-stage combustion process. The secondary combustion allows the primary combustion zone to have a lower combustion temperature, but the combined heat input can be higher. US5746144 also suggests injecting lime or other substances into the secondary combustion stage to reduce the emission of sulfur oxides. SUMMARY

[0005] As NOx emission requirements will become more stringent and emission costs will increase, there is an urgent need for an efficient method to reduce NOx emissions from lime kilns. When other bottlenecks of the slurry plant are opened, the emission requirements will prevent an increase in lime kiln capacity. The lime kiln will therefore hinder the increase in capacity of the entire plant. Known methods of NOx reduction will result in a reduction of fuel economy, thereby increasing carbon dioxide emissions, or they have other disadvantages.

[0006] The high temperature flame causes a high level of thermal energy radiation, which is absorbed by the refractory brick lining of the rotary kiln. Especially the bricks around the flame will be under constant changing high thermal stress and can deteriorate and fall off due to the high cyclic thermal radiation. This radiation also causes the brick surface to rise to a level where the calcium oxide particles on the brick surface will melt. The melted particles are not a chemical component of the white liquor and should therefore be avoided from melting.

[0007] The new invention aims to reduce the combustion temperature and thermal radiation in the lime kiln of a slurry production plant. The rotary kiln has a rotary kiln tube, which is internally covered with refractory bricks, and the rotary kiln has a burner at the lower end of the kiln tube. The burner is supplied with fuel for heating the interior of the tube by a flame. The reduction of the combustion temperature and / or the thermal radiation around the flame is achieved by supplying calcium carbonate containing particles to the flame and / or the surrounding area around the flame. These particles will form the end product, calcium oxide. This reaction is endothermic and will cool the flame. The heated supplied dust particles also take energy from the flame. The cooler flame will also radiate less energy around it. The dust particles in the surrounding area around the flame will filter the thermal radiation and limit the temperature level on the bricks directly affected by the radiation.

[0008] These and other objects will become apparent from the following summary and description. The invention enables a higher heat input, thereby increasing the production rate, without unwanted side effects.

[0009] The fuel economy is not compromised, as the reaction and the heating of the particles are preferred effects and they will normally occur on the material being treated anyway. The heated dust particles will efficiently transfer their thermal energy to the surrounding gases, which will transfer their energy to the treated calcium carbonate bed. The particles will also radiate thermal energy to the treated material bed as they flow upwards in the kiln. All the energy taken away that reduces the temperature of the flame is therefore efficiently conserved for the production of the product, without the losses as when spraying steam or water. Any residual water of the supplied particles will vaporize to steam and cool the temperature, but the evaporation of water from the supplied particles in the previous stage will require the same energy. However, it is more energy efficient to use a preheater to dry the supplied material than to supply wet particles to reduce the flame temperature.

[0010] According to the invention, a part of the processed raw material is supplied to the hot end of the kiln as small particles. The particles have enough surface for reaction and thermal convection. The supply rate of the particles is preferably controlled by measuring the NOx emission level, the product quality and / or observing the temperature and / or form of the flame.

[0011] Lime kilns normally do not burn fuels with high sulphur content. The fuel should be free from contaminants that would be enriched in the chemical cycle of the slurry plant. This is why gaseous fuels that produce hot flames, such as methane and hydrogen, are preferred. The invention can also lead to a reduction of sulphur dioxide in the flue gas. The invention can be used in combination with other known NOx reduction practices, but the temperature profile of the flame and the total energy input must remain sufficient to complete the reaction of the calcium carbonate being treated. In practice, two-stage combustion, which is commonly used in cement kilns to suppress the formation of NOx, is not an available option for lime kilns of the slurry plant.

[0012] The supplied particles will achieve the best efficiency when the particle stream is directed to the upper part of the flame where the temperature is the highest. When particles containing calcium oxide and / or calcium carbonate are supplied to the area around the flame in the rotary kiln, the supplied particle stream should be directed at least to the area above the flame and / or to the area at the sides of the flame where the refractory bricks of the kiln rotate downwards. There, the bricks are not covered by the processed material and the reduction of heat radiation is most beneficial.

[0013] The particles are preferably supplied to the kiln from a flue gas dust separation device, such as a cyclone and / or an electrostatic precipitator. The transport of the particles is preferably carried out by a pneumatic conveying device. The carrier gas can be flue gas or combustion air. The particles can be directed into the kiln tube with primary combustion air or preferably with at least one lance, which can be designed and adjusted for the best guiding and dispersing properties. Preferably, the nozzle of the lance has an increasing internal cross-section towards the outside, at least when the intention is to disperse the particles widely around the flame. For the area around the flame, secondary combustion air can also be supplied to the particles.

[0014] The source of the supplied particles can also be added raw material, which can be ground into particles of suitable size. Then they do not need to be transported from the other end of the rotary kiln a long distance. The supplied calcium carbonate should be very dry so that it will act as a dry powder. It can also be in the form of granules, whereupon the water content will evaporate into dust inside the kiln and / or in the flame. The granules should be very small in order to be dispersed effectively in the form of dust. If the particles are supplied to the kiln in the form of sludge, the reduction of the combustion temperature will be mainly based on the evaporation of water. Lime sludge will be difficult to disperse effectively and evenly around. The water content of the supplied calcium carbonate should be less than 25% by weight and preferably less than 15% by weight in order to obtain particles that create dust that are small enough and their even dispersal. BRIEF DESCRIPTION OF DRAWINGS

[0015] Examples of embodiments of the present invention will now be described in more detail with reference to the drawings, in which:

[0016] Figure 1 A side view of an embodiment of the present invention is shown, and

[0017] Figure 2 A cross-sectional view of the interior of the kiln tube is shown. DETAILED DESCRIPTION

[0018] US5667582 describes a prior art solution for blowing calcium oxide and / or calcium hydroxide dust as an additive into a cement kiln to react with sulphur oxides in the exhaust gas. The intention is to blow the dust at high speed from the lower end of the kiln through the flame to an area after the flame, where the entire upper part of the kiln can act as a reaction zone for the additive. The intention of the present invention is the opposite. It is intended to supply calcium carbonate particles to the flame area and / or the area around the flame to achieve the benefits of the present invention. The elements of the rotary kiln are still very similar, but the effect is very different when the particles are directed and supplied close to the burner end of the kiln tube. Our invention enables protection of the refractory bricks of the kiln tube and reduction of the maximum temperature of the flame.

[0019] Figure 1 A preferred embodiment for reducing NOx production and heat radiation in a lime kiln with a rotary kiln tube 1 is shown. A longitudinal flame 3 is created in the centre of the kiln tube 1 by supplying compressed primary air 8 and fuel to a burner 2, which has concentric tubes for supplying fluids. The burner is attached to the stationary end of the kiln lower part. Calcium carbonate particles are blown to the flame 3 and / or the area around the flame 3 by the combustion air 6, 8 or other fluids. The supply parameters, like the speed of the carrier gas and the circulation movement of the primary combustion air 8, are set to achieve a cloud of dust particles in the target area within and / or around the flame. The endothermic decomposition reaction of calcium carbonate to calcium oxide and carbon dioxide will take place within and around the flame 3, so that the high temperature profile peak is smoothed to a temperature level that reduces the formation of NOx to an acceptable level.

[0020] The particles are preferably supplied to the flame by at least one lance 9. The particles can also be supplied via the primary combustion air 8 channel of the burner 2. Secondary combustion air 6 is supplied around the burner 2 to complete the combustion. The secondary combustion air 6 is preheated by a preheater 5, which is located around the burner end of the kiln tube 1. The preheater 5 also cools the outgoing product 1, which is calcium oxide particles. The mixing and supply parameters of the primary combustion air 8 and the secondary combustion air 6 also affect the formation of NOx.

[0021] The supplied particles are preferably taken from the dust separator of the exhaust gas. The particles from a cyclone are larger than those from an electrostatic precipitator. These particles are very dry and the source can be chosen to optimize the process. The internally circulating and externally added raw materials can also be ground or otherwise prepared to supply the particles.

[0022] The particles are preferably supplied to the flame 3 by at least one separate lance 9 above the burner 2. The lance 9 directs the flow of conveying air or other fluid and particles, preferably mostly to the upper half of the flame 3 where the temperature peak is highest.

[0023] Figure 2 A cross-sectional view of the inside of the kiln tube 1 is shown to illustrate the preferred locations 14 and 15 where the particles should be supplied and dispersed. In order to filter the radiated energy that is absorbed by the refractory bricks 13 next to the flame 3, these particles can be supplied in addition to or instead of the flame 3 to the area between the flame 3 and the refractory bricks 13 around the flame 3 area. Preferably, the nozzle 10 of the lance 9 has an increasing internal cross-sectional area outwards so that the velocity of the carrying fluid and the supplied particles is reduced to a level that creates a cloud of dust particles at the burner 2 end of the kiln tube 1. The particles do not need to be supplied below the flame 3 nor to the ascending side of the rotary kiln tube 1 where the refractory bricks 13 are covered by particles of the bed 12 of treated material. In the area covered by the bed 12, radiation is useful because it will end the reaction of the treated material. For the purpose of radiation filtering, it is not necessary to reduce the temperature of the area around the flame 3 by the decomposition reaction of calcium carbonate, so the particles supplied for this purpose can be, for example, discharged calcium oxide. The particles supplied only for the purpose of reducing radiation should be supplied by at least one separate lance 9.

[0024] The amount of blown particles can be more than necessary to reduce the temperature of the flame 3, especially when it is not supplied to the flame 3 area but is widely dispersed within the burner 2 end of the kiln. The heat output of the flame 3 can still be kept within a sufficient level to achieve complete calcination of the feed material, because the supplied particles will absorb the radiation and disperse the absorbed energy to the treated material within the kiln tube 1.

Claims

1. A method for reducing the combustion temperature and / or thermal radiation within a lime kiln of a slurry production equipment, said lime kiln being a rotary kiln having a rotary kiln tube (1) and a burner (2), the rotary kiln tube (1) being internally covered with refractory bricks (13), the burner (2) being attached to a burner end of said rotary kiln, said burner (2) being supplied with fuel for heating said rotary kiln by a flame (3), wherein calcium carbonate-containing particles are supplied to said flame (3) and / or the surrounding area around said flame (3), and wherein the supplied particle stream is directed to the upper part of said flame (3).

2. The method according to claim 1, wherein at least a portion of the supplied particles is supplied to the rotary kiln by at least one spray gun (9).

3. The method according to claim 2, wherein the nozzle (10) of the spray gun (9) has an outwardly increasing internal cross-sectional area.

4. The method according to any one of claims 1 to 3, wherein the supply particle stream is directed to a region above the flame (3) and / or a region on the side of the flame (3), in which the refractory bricks (13) of the kiln rotate downward.

5. The method according to any one of claims 1 to 3, wherein the fuel of the burner (2) is gaseous.

6. The method according to any one of claims 1 to 3, wherein the fuel comprises methane and / or hydrogen.

7. The method according to any one of claims 1 to 3, wherein at least a portion of the particles supplied to the kiln is taken from a dust separation device.

8. The method according to any one of claims 1 to 3, wherein at least a portion of the particles supplied to the kiln has been ground into particles from the supplied raw materials.

9. The method according to any one of claims 1 to 3, wherein the particles are supplied to the kiln together with primary combustion air (8) and / or secondary combustion air (6).

10. The method according to any one of claims 1 to 3, wherein the combustion is carried out in only one stage.

11. The method according to any one of claims 1 to 3, wherein the particle supply rate is controlled by measuring NOx emission levels, product quality and / or observing the temperature and / or form of the flame (3).

12. The method according to any one of claims 1 to 3, wherein the particles supplied to the flame (3) and / or around the flame (3) have a water content of less than 25% per weight.

13. The method according to any one of claims 1 to 3, wherein the particles supplied to the flame (3) and / or around the flame (3) have a water content of less than 15% per weight.

14. The method according to claim 7, wherein the dust separation device is a cyclone dust collector and / or an electrostatic precipitator.

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