A METHOD FOR REDUCING COMBUSTION TEMPERATURE AND THERMAL RADIATION WITHIN A LIME KILN

Abstract

A method for reducing combustion temperature and/or thermal radiation within a lime kiln of a pulp production plant, which kiln is a rotary kiln having a kiln tube (1) internally covered with refractory tiles (13) and having a burner (2) supplied by fuel for heating of the rotary kiln by a flame (3). The effects are achieved by supplying calcium carbonate containing particles to the flame (3) and/or to surrounding area around the flame (3). 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 may be supplied to the rotary kiln to areas surrounding the flame (3) for reducing the thermal radiation to an area over the flame (3) and/or to the area at the side of the flame (3), where the refractory tiles (13) of the kiln are rotating downwards.

Claims

1. A method for reducing combustion temperature and/or thermal radiation within a lime kiln of a pulp production plant, wherein the lime kiln is a rotary kiln having a rotary kiln tube internally covered with refractory tiles and a burner attached to a burner end of the rotary kiln, the method comprising: supplying the burner with fuel; heating the rotary kiln by a flame produced by the burner burning the fuel; supplying particles containing calcium carbonate to the flame and/or to an area surrounding the flame.

2. The method of claim 1, wherein the step of supplying the particles includes supplying at least some of the particles into the rotary kiln by at least one lance.

3. The method of claim 1, wherein the step of supplying the particles includes supplying a flow the particles and directly the flow to an upper part of the flame.

4. The method of claim 3, wherein the lance includes a nozzle having an outwards increasing internal cross sectional area, and the flow of the particles flows through the nozzle.

5. The method of claim 1, wherein the flow of the particles is directed to area over the flame and/or to an area at a side of the flame facing a side of the rotary kiln tube where the refractory tiles are rotating downwards.

6. The method of claim 1, wherein the fuel of the burner is gaseous.

7. The method of claim 1, wherein the fuel includes methane and/or hydrogen.

8. The method of claim 1, wherein at least some of the particles supplied to the rotary kiln are obtain from a dust separating device.

9. The method of claim 1, wherein at least some of the particles supplied to the rotary kiln are ground to particles from supplied raw material.

10. The method of claim 1, wherein the step of supplying the particles includes supplying the particles with primary combustion air and/or secondary combustion air supplied to the burner of the rotary kiln.

11. The method of claim 1, wherein the combustion is performed only in one phase.

12. The method of claim 1, wherein a rate of supply of the particles is controlled by at least one of: measuring a level of NOx emissions, measuring a quality of product, measuring a temperature of the flame and an observed form of the flame.

13. The method of claim 1, wherein the particles supplied to and/or around the flame have no more than 25% per weight water content.

14. The method of claim 1, wherein the particles supplied to and/or around the flame have no more than 15% per weight water content.

Description

LIST OF DRAWINGS

[0016] Examples of embodiments of the invention will now be described in more detail with reference to the appended drawings in which:

[0017] FIG. 1. illustrates a side view of an embodiment of the invention and

[0018] FIG. 2 illustrates a cut view of the interior of the kiln tube.

DETAILED DESCRIPTION OF THE INVENTION

[0019] U.S. Pat. No. 5,667,582 describes a prior at solution for blowing calcium oxide and/or calcium hydroxide dust as an additive to a cement kiln for reacting with sulfur oxides of exhausted gases. The aim is to blow the dust with high speed from the lower end of the kiln over the flame to the area after the flame, where the whole upper part of the kiln can act as a reaction zone for the additive. The aim of the present invention is opposite. It aims to supply calcium carbonate particles to flame area and/or to area surrounding the flame for realizing the benefits of the invention. The elements of the rotary kiln are still quite similar, but as the particles are directed and supplied close to the burner end of the kiln tube, the effects are very different. Our invention can achieve protection of refractory tiles of the kiln tube and reduction of the highest temperatures of the flame.

[0020] FIG. 1. illustrates a preferred embodiment for reducing generation of NOx and thermal radiation within a lime kiln having a rotary kiln tube 1. A longitudinal flame 3 is generated within the center of the kiln tube 1 by supplying compressed primary air 8 and fuel to a burner 2 having concentric tubes for supplied fluids. The burner is attached to lower, stationary end of the kiln. Calcium carbonate particles are blown with combustion air 6, 8 or other fluids to the flame 3 and/or area surrounding the flame 3. The feeding parameters like velocity of carrying gas and circulating motion of primary combustion air 8 are set to achieve a cloud of dust particles within the targeted area within and/or around the flame. The endothermic dissolution reaction of calcium carbonate to calcium oxide and carbon dioxide will happen within and around the flame 3 so that high temperature profile peak is smoothed to temperature levels which lowers NOx formation to acceptable levels.

[0021] The particles are preferably supplied to the flame by at least one lance 9. The particles can also be supplied via primary combustion air 8 channels of the burner 2. Secondary combustion air 6 is supplied around the burner 2 for completing the combustion. The secondary combustion air 6 is preheated by a preheater 5, which is around the burner end of the kiln tube 1. The preheater 5 also cools discharged product 1 which is granules of calcium oxide. The mixture and feeding parameters of the primary 8 and secondary combustion air 6 will also affect to formation of NOx.

[0022] The supplied particles are preferably taken from dust separators of exhaust gas. The particles are bigger from a cyclone than taken from an electrostatic precipitator. Those particles are very dry and the source can be chosen for optimizing the process. Internally circulating and externally added raw material may also be ground or otherwise prepared for the supply of the particles.

[0023] The particles are preferably supplied to flame 3 by at least one separate lance 9 over 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 wherein the temperature peak is highest.

[0024] FIG. 2. shows a transversally cut view of the interior of the kiln tube 1 for illustrating the preferred positions 14 and 15 where the particles should be supplied and spread. For filtering radiation energy absorbed by the refractory tiles 13 next to the flame 3, the particles can be supplied to areas between the flame 3 and the refractory tiles 13 surrounding the flame 3 area also or instead of supplying them to the flame 3. Preferably the nozzle 10 of the lance 9 has an outwards increasing internal cross sectional area so that the velocity of carrying fluid and supplied particles are reduced to a level which creates a cloud of dust particles at the burner 2 end of the kiln tube 1. The particles do not need to be supplied under the flame 3 nor to the rising side of the rotary kiln tube 1 where the refractory tiles 13 are covered by the particles of the bed 12 of treated material. At areas covered by the bed 12, the radiation is useful as it will finalize the reactions of the treated material. For the radiation filtering purpose, there is no need to lower the temperature of the areas surrounding the flame 3 by the dissolution reaction of calcium carbonate, so the supplied particles for the purpose can be for example discharged calcium oxide. The particles which are supplied only for reducing radiation, should be supplied by at least one separate lance 9.

[0025] The amount of blown particles can be more than needed for the reduction of the temperature of the flame 3, especially when it is not supplied to the flame 3 area but spread widely within the burner 2 end of the kiln. Still heat output of the flame 3 can be kept within sufficient level for achieving full calcination of fed raw material as the supplied particles will absorb the radiation and will spread the absorbed energy to treated material within the kiln tube 1.