Method and arrangement for burning lime mud

Abstract

The present invention relates to a method and an arrangement for burning lime mud into lime in a lime kiln. The lime mud flows counter-currently to flue gases from a feed end to a firing end and the fuel used is flue gas that is produced by gasifying a fuel in the presence of combustion air in a gasifier. The combustion air for gasification is preheated with heat generated in lime mud combustion. The arrangement is provided with a conduit between the lime kiln and the gasifier for leading air from the lime kiln into the gasifier as combustion air. At least a portion of the combustion air for gasification is preheated with heat generated in the lime mud combustion so that air is led into cooling of lime obtained in the combustion and further into the kiln, from or through the firing end of which air is taken into the gasification.

Claims

1. A method for burning lime mud in a rotary drum kiln comprising: moving lime mud through the rotary drum kiln while flue gas flows through the kiln counter-currently to the lime mud; producing lime from the lime mud in the rotary drum kiln and discharging the lime from the rotary drum kiln; forming the flue gas with a burner positioned proximate to an end of the rotary drum kiln; fueling the burner with a fuel gas; producing the fuel gas by gasifying fuel in the presence of combustion air in a gasifier including a solid bed or fluidized bed, wherein the gasifier is physically separate from the rotary drum kiln and burner; heating air for use as the combustion air with heat from the lime discharged from the rotary drum kiln.

2. The method according to claim 1, wherein the air is heated with heat recovered by cooling the lime or a housing of the rotary drum kiln.

3. The method according to claim 1, further comprising obtaining additional combustion air for gasification from an interior of the rotary drum kiln upstream in the direction of lime mud flow from a dam in the rotary drum kiln.

4. The method according to claim 2, further comprising cooling the lime mud or the housing in a lime cooler section of the rotary drum kiln by ducting the air through the lime cooler before the air is used as the combustion air.

5. The method according to claim 1, wherein the heating of the air for the combustion air includes cooling a housing of a cooler section of the rotary drum kiln.

6. The method according to claim 5, wherein the heating of the air includes passing the air between a radiation shield surrounding the cooler section and the cooler section.

7. The method according to claim 1, wherein the fuel to be gasified includes at least one of a bio-based material, a wood-based material or a biomass material.

8. A method to convert lime mud to lime in a rotary drum kiln comprising: moving lime mud through the rotary drum kiln while flue gas flows through the kiln counter-currently to the movement of the lime mud; producing lime from the lime mud in the rotary drum kiln and discharging the lime from the rotary drum kiln; combusting a fuel gas in a burner to produce the flue gas, wherein the flue gas flows through the rotary drum kiln counter-currently to the movement of the lime mud; producing the fuel gas by gasifying fuel in a gasifier including a solid bed or fluidized bed, wherein the gasifier is physically separate from the rotary drum kiln and burner; heating air for use as the combustion air in the gasifier with heat from the lime discharged from the rotary drum kiln.

9. The method of claim 8 further comprising cooling the lime with the air.

Description

SUMMARY OF DRAWINGS

(1) The present invention is described in more detail by means of embodiments according to the invention and with reference to the appended figures, in which

(2) FIG. 1 illustrates schematically a prior art and an inventive arrangement of a gasifier and a lime kiln;

(3) FIG. 2 illustrates an arrangement for implementing some embodiments according to the invention; and

(4) FIG. 3 illustrates an arrangement for implementing some of the embodiments of the invention.

DETAILED DESCRIPTION OF INVENTION

(5) FIG. 1 illustrates components of a lime mud combustion plant and a gasifier that are needed for describing the invention. The gasification unit is in this case a circulating fluidized bed gasifier 1, i.e. a CFB-gasifier. It comprises a gasification reactor 2, a grate 3, a cyclone 4 and a cyclone return pipe 5. A fluidized bed is arranged at the lower part of the gasifier, above which bed a fuel is introduced. The fuel is typically solid bio-based fuel 6, such as bark, wood chips etc. Ash generated in the gasification is removed via conduit 6.

(6) The hot product gas exits the gasifier via duct 7 and is partially cooled in an air preheater 8. Then the product gas is led into a burner 9 of the lime kiln 10 via the gas duct 7.

(7) In this known arrangement, the combustion air for the gasifier is introduced by means of a fan 11 from ambient conditions. Prior to feeding into the gasifier, the air is, heated typically to 300-400 C. by cooling hot product gas from the gasifier in the heat exchanger 8. The heat exchanger is typically a twin-housing heat exchanger located at the initial end of the gas duct downstream of the cyclone. Hot product gas flows in the inner tube of the heat exchanger and air flows to the same direction in a dedicated channel around the inner tube. This kind of exchanger is space-consuming.

(8) According to the embodiments of the present invention, air is introduced by means of a fan 12 from the firing end of the lime kiln 10 into the gasifier via air duct 13.

(9) An advantage of the present invention is that it is simple as an apparatus solution, since it utilizes a lime cooler required as such in any case. Only the air duct from the firing end of the lime kiln into the gasifier is to be constructed, and no separate air preheater is needed, which would be considerably more space-consuming and more expensive. Further, a heat exchanger is susceptible to erosion caused by ash entrained in gas.

(10) FIG. 2 illustrates in more detail the leading of the gasifier combustion air out of the lime kiln 10, which is a rotary drum kiln. The cooler 14 of the lime kiln comprises two cylindrical housings 15 and 16, disposed one inside the other, surrounding the kiln and rotating together with the kiln, which housings are mounted at the discharge end of the kiln concentrically with the kiln. An annular space 17 is formed between them. The inner cylindrical housing 15 is attached at its initial end to the kiln via drop chutes 18. Via the drop chutes, an inlet 19 of the annular space 17 of the cooler communicates with the discharge opening 20 of the kiln for leading the hot material from the kiln into the cooler. A plurality of drop chutes 18 are provided around the circumference of the kiln. Transportation of the material being cooled inside the cooler is accomplished by means of feed devices known per se, such as feed vanes (not shown) that can be located on any wall of the cooler space. The transportation can also be accomplished by means of separate members constructed inside the cooler space. The cooled material is discharged from the cooler 14 into a discharge hopper 21. The flow of lime is shown with black arrows 31.

(11) The cooler 14 of the lime kiln is surrounded by a stationary cylindrical radiation shield 22 that is insulated from the outside and tightly connected to the discharge hopper 21. The function of the radiation shield is to act as thermal insulation outwards and to prevent dust leakages. Its end facing the discharge hopper 21 is partly open. The cooling air is withdrawn into the cooler through a gap 23 between this partly open end and the firing end of the kiln. Most of the cooling air is directed from a canal 24 between the kiln housing and the inner housing 15 of the cooler via the discharge hopper 21 into the cooler to a sector part, where cooling of the lime takes place according to the counter-current principle. The flow of the air is shown with white arrows 25. A smaller portion of the airs is directed through the slot between the radiation shield and the cooler duct 26 between them.

(12) The flow of air in the canal 24 prevents excess heating of the kiln housing. From the cooler, preheated air flows further via drop chutes 18 into the kiln 10 as secondary combustion air. In accordance with an embodiment of the invention, a duct 13 is mounted in the firing end of the lime kiln, which duct is provided with a fan 12 for leading secondary air as combustion air into the gasifier. The air is led into a wind box 27 of the gasifier below the grate 3. The air duct can extend into the kiln to a desired depth; preferably it extends to a so-called dam 28 of the kiln. The choice of the depth is dependent on optimizing between the dust-content of the air, the temperature and strength of the materials and air temperature.

(13) Product gas is led from the gasifier along channel 7 into the gasifier 9 of the gasifier for combustion fuel.

(14) According to an embodiment, air can be obtained from the kiln also from canal 26 20 between the radiation shield and the cooler. Then a duct 30 provided with a fan 29 is arranged in an opening 32 in the outer wall of the canal, which duct leads the air heated. by lime into the gasifier 1 as combustion air. In FIG. 2, the air duct 30 is connected to the lower part of the kiln, but it is more advantageous to withdraw air through the upper part of the kiln cooler.

(15) According to an embodiment, the combustion air for gasification is taken through the housing of the lime kiln at a point located at a distance from the firing end and downstream of the lime cooler in the direction of the longitudinal axis, P, of the kiln. For accomplishing this, an additional part of the housing, a hood 33, is mounted around the kiln. The hood can be formed of a cylindrical piece concentric with the kiln, which piece in the circumferential direction surrounds the whole kiln or a portion thereof and which in the longitudinal direction of the kiln has a desired length L. Air is taken between the cylinder and the kiln housing, whereby the air has been heated by heat radiating from the kiln. Air is withdrawn from between the cylindrical piece 33 and the kiln by means of a fan 34 and led via duct 35 to be used as combustion air for the gasifier.

(16) The air ducts 13, 30 and 35 are provided with valves 36, such as flap valves, for regulating the air amount. This allows also regulating the use of different air streams in a desired proportion, if other air sources 30 and 35 are used in addition to air 13 taken from the firing end of the kiln.

(17) FIG. 3 illustrates another embodiment for arranging the air ducts. In this case other air streams 30 and 35 (air from canal 26 between the radiation shield and the cooler and/or air from between the hood 33 and the kiln housing) are led into duct 13 for the air stream taken from the firing end of the lime kiln upstream of the fan 12. Additionally, for further regulation of the gasification air temperature, air of lower temperature can be introduced from the surroundings of the kiln via conduit 37. In this embodiment, each air duct is provided with a regulation valve 36, but the fan 12 is common.

(18) As presented in the above, a lime kiln has several locations where air heated by the heat of the lime mud combustion process can be taken and led into the gasifier as combustion air. In the simplest case, only a pipeline provided with a fan or corresponding needs to be arranged between the lime kiln and the gasifier.

(19) Though the above description relates to an embodiment of the invention that is at the light of present knowledge the most preferred, it is obvious for one skilled in the art that the invention can be modified in several different ways within the broadest possible scope defined by the appended claims only.