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What are shaft kilns for lime

manufacture?

A Combustion File downloaded from the IFRF Online Combustion HandbookISSN 1607-9116

Combustion File No: 248

Version No: 1

Date: 13-10-2003

Author(s): Barrie Jenkins

Source(s): Author

Sub-editor: Barrie Jenkins

Referee(s): Marc Vansnick

Status: Published

Sponsor: Barrie G Jenkins, Consulting Engineers

1. BackgroundIn a shaft kilnthere are three zones in which distinct operations involving heat transfer

take place. They are:

The preheating zone, in which the limestoneis dried and heated to

calcinationtemperature.

The calcination zone, in which the stone dissociates.

The cooling zone, in which the quicklimeis cooled.

The quality of the limeis most influenced by the conditions in the calcination zone.

Here, heat must be supplied at a temperature above the calcination temperature; which

consequently results in a sufficiency of heat in the combustion gases and dissociated

carbon dioxide to adequately heat the stone in the preheating zone. Unless the

required product is a hard burnt lime, or dead burnt lime, the rate of heating in the

calcination zone must be carefully controlled to ensure that there are no 'hot spots' or

cool channels. Most of the design variations of shaft kilns are attempts to address this

aspect of the kiln performance.

2. Types of shaft kilnsThe available shaft kiln technologies fall into four types.

1. Single shaft

2. Double shaft (recuperative)

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3. Annular

4. Inclined

The following pages give brief descriptions of typical kilns of each type, and a summary

table of their operating characteristics is given at the end of this file.

3. Single Shaft KilnsThese are the most common forms of kiln in use. The vintage and number of

manufacturers are large, but included in this list would be, in no particular order of

preference; -

Mixed feed kilns

The mixed feed kiln is the most basic and oldest shaft kiln design, in which alternatelayers of stone and fuel are laid on to the top of the preheating zone, and are then

drawn down through the kiln as material is discharged at the bottom. Mixed feed kilns

are still widely used in the ammonia soda process, and in regions of the world where the

infrastructure and technical support are limited. The designs range from very basic

small open top, hand loaded kilns relying on natural chimney draught to induce air

through the bed, to large automatic feed and discharge, forced draft units. The latter

are highly efficient, whereas the former are not.

A significant advantage of this type of kiln is that it can be operated to produce a

consistently lowlime reactivity. Higher reactivity is usually only obtained at the

expense of a higher level of residual CaCO3, which is not the case with more modern kilndesigns. They are typically fired on a low volatile coal (anthracite) or metallurgical coke,

both of which have high ignition temperatures (~800oC). Higher volatile solid fuels, such

as wood, tend to release the volatile components of the fuel in the preheating zone,

which results in the excessive emission of smoke and a loss of some of the calorific

valueof the fuel.

West kilns

These kilns are designed for two basic outputs,

50 and 100 tons/day lime, although more recent

experience with these units has shown that they

are capable of considerably increased output

with suitable modification. They were originally

designed for oil firing, using four, five or six

'carburetors' equally spaced around the kiln at

the base of the calcining zone, the idea being to

flash vaporizethe oil on the hot walls of the

carburetor chamber. This was achieved by

injecting a solid rotating jet of oil which

partially combusted with about 15%stoichiometricair, before entering the burning

Waste Gas

170C

SubmergedTake-off 370C

Secondary Air

Primary Air

Oil Vapour

20% ofWaste Gas

1200-1300C

5m

4-5m

8-9m

5 burners

Fig 1 Schematic of West Kiln

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zone, where it meets the cooling zone air and some recycled flue gas. An original

schematic is shown in Fig 1. The original oil-firing concept was later abandoned for

simple rotating oil injection onto the stone bed, and they have also been successfully

operated using natural gas as the fuel.

Esjornsson

This kiln is of Swedish design in which the preheating zone is hexagonal, tapering into a

rectangular burning zone of approximately 3.0m by 1.2m, and then opening out into a

square cooling/discharge zone. A separate gasification unit is used to provide the heat

to the calcining zone. Water sprays cool the gasified fuel before being injected into the

calcining zone at two levels.

Fercalx (Union Carbide)

These kilns were originally designed by Union Carbide as mixed-feed, coke-fired unitsThey were first converted in 1952 to gas or oil fired using water cooled burner beams

at two levels and are now marketed by Fercalx. The ability to inject fuel at a large

number of discrete points within the bed in the calcining zone enables a more even heat

distribution to be achieved, although mechanical failure, distortion and the higher heat

usage due to the beam cooling tend to outweigh this potential advantage.

Azbe

There are a large number of Azbe kilns in the USA, and other parts of the world, and

there are a number of variations as the technology has developed. Typically, these kilns

are of curved rectangular cross section, with a multiple level refractory burner beam

for fuel, air and flue gases at the base of the burning zone on the longer kiln axis.

WestofenWaste Gas

Secondary Air

Primary Air @ 3bar

Oil

Throttle

Throttle150C

2:1 air:waste

gas ratio

80-100C

1300C

waste Gas 270-300

4m

13m

6 burners

Fig 2 Schematic of Westofen Kiln

This kiln design is similar to the West

kiln using flue gas recycle and side firing

at the base of the burning zone. Figure

2 shows a schematic of the design.

SIC (Societa Impiante Calce srl)

SIC have two single shaft kiln designs,

the CBK (central burner kiln) and the

HPK (high performance kiln), the latter

being designed for smaller stone.

Chisaki Koma

This kiln design of Japanese origin is

based on fixed bed preheater

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technology as developed in the Davis preheater, or the Kraus-Maffai preheater.

4. Double Shaft KilnsDouble shaft kilns have been in use since the 1960's, and were developed to overcome

the problems of increased bed resistance to gas

flow with smaller sized stone. They employ the

technique of regenerative flow, whereby two, or

more, shafts are fired singly in a cyclic

sequence, the flue gases exhausting through the

non-fired shaft(s), thereby effecting heat

recovery. They generally have very good

specific fuel consumption, but are more costly in

electrical requirements due to the need for airblowers, rather than fans. There follows a

resume of the currently available double shaft

kilns.

Maerz

The Maerz kiln design is probably accepted as

the most successful double shaft kiln. Fuel is

burnt in the upper end of the burning zone in

one cylindrical shaft, and the hot gases flow co-

currently with the charge in that shaft, and

then via a connecting duct to the other shaft, where they flow counter-currently to the

charge. The second shaft thus acts as a recuperator. After a given time, the roles of

the shafts are reversed. It is claimed that since the flow of gases and stone are in the

same direction in the burning zone, the risk of overheating the lime is reduced, and a

softer burnt, more reactive lime results. The kiln designs come in two forms, a standard

and a finelimekiln. The finelime kiln, which is a later development of the standard

design, has a smaller rated capacity for any given geometric size relative to the

standard kiln, and contains more refractory. Fig 3 shows a schematic of the kiln system.

Waste gas 100CCombustion air

Fuel

Connecting

air duct

Cooling air

Combustion

and cooling

air compressor

Lime 100C

Fig 3 Schematic of Maerz Kiln

Cimprogetti

The Cim-Reversy kiln operates in a similar manner to the Maerz kiln, but is formed from

two D shaped chambers, flat sides adjacent, which gives a very short gas transfer duct,

thus reducing the propensity for dust settling and deposition. These kilns have also

been designed for small stone operation.

Voest-Alpine

The Valec kiln is a double shaft design using cylindrical shafts in the same manner as the

Maerz design.

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5. Annular Kilns

Annular kilns have been developed in

Germany as an alternative method of

ensuring even heat distribution. Thecalcining and burning zones have an

annular cross-section, and the

preheating zone is circular. Figure 4

shows a schematic of this type of kiln

system. Rheinische Kalksteinwerke

(RKW) developed the original design,

but they are generally available from

Beckenbach and FLS.

There are two common versions of this

type of kiln, in which the upper heat

exchanger is optional. In the case

where the kiln is equipped with two

inner cylinders, see figure 5, the upper

cylinder extracts ~30% of the flue

gases to a recuperator place in a

vitiated air environment. The

compressed air used for cooling in the

annular sections of the cylinders is

subsequently used as primary air to the

burners (lower cylinder) or for other purposes

(upper cylinder). The upper cylinder isnormally omitted from the design if the stone

is wet as all the flue gases are then used for

drying.to preheat the air for the lower level

burners. The lower cylinder extracts gases at

the bottom of the calcining zone for

recirculation to the lower burners where

combustion takes

Fuel

Compressed cooling

air to annulus

Combustion

and primary

air

waste

gas

Upper

burners (5)

Lower

Burners (5)

Hot air

Primary air

HeatExchanger

Recycled

Waste Gas

Lime

Fig 4 Schematic of Annular Kiln

Beckenbach

Most of the kiln shaft has an annular cross-section with five gasified fuel entry ports at

each of two levels in the kiln, and staggered so

as to deflect the charge as it falls in the

outer annular space. Some of the waste gases

go up the central cylinder space together with

air from the cooling zone, and this can be

deflected back to the lower burners and thus

regulate the temperature.

Fig 5 Double cylinder annular kiln

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6. Inclined KilnsThere are two types of inclined kilns, which have been developed to cater for small

stone.

Oil or Gas

Oil or Gas

Combustion air

Stone and

Coke

2 or more burners

1200 - 1300C

7m

9m

3m

Fig 6 Schematic of Double Incline Kiln

Double Incline

The double incline kiln was developed by

Warmestelle, Steine und Erde Gmbh, and is

now made by Beckenbach. The principal is

to lengthen the burning zone, producing a

milder flame, with more even heatdistribution and transfer without the need

for flue gas recycle. The kiln cross-section

is rectangular, widening for two firing

chambers. Considerable care is needed in

selection of the refractory lining, due to

the complex geometry. Waste gas and dust

losses tend to be high, but the material

residence time is typically about half of a

conventional shaft kiln. Fig 6 shows a

schematic of the kiln system.

IAF Multi-Chamber

The IAF multi-chamber kiln was first built in 1972, and an improved design was

introduced in 1987. The kiln is basically rectangular in cross-section, but is stepped into

a number (4 to 6) of combustion chambers through the burning zone, which allows a

degree of control of combustion conditions to suit the burning requirements of the

stone.

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7. Summary of typical shaft kiln characteristics

Kiln Type Maker /Design Rate of

OutputTPD

Feed

Sizemm

Fuels Lime

QualityCaCO3%

Reactivity Capital

CostRelative

Basis

Running

CostRelative

Basis

Product

ControlRelative

basis

Energy

UsageMJ/kg

lime (fuel)

Energy

UsagekWh/t

(power)

Shaft Mixed feed 10 - 300 30 - 150 C.W 1 - 5 Low Low Low Medium 4.0 6.0 5-15

Shaft West 30-170 60-130 G.O.C 0.5 - 3 Variable Low High Medium 4.2 5.0 25

Shaft Esjornsson 50-100 100-150 G.GF 1.35 n/d Low High n/d 5.4 30

Shaft Fercalx (UC) 40-800 80-350 G.O.C 2 High Medium Medium n/d 5.5 30

Shaft Azbe 50-150 60-200 G.GF 1 - 3 High Low High Poor 4.2 5.0 10 - 15

Shaft Westofen 100 30-120 O n/d n/d n/d n/d n/d 4.2 5.0 25

Shaft SIC CBK 40-80 40-150 G n/d n/d Low High Medium 4.4 20

Shaft SIC HPK 15-125 25-120 G.O.C 1 - 2 High Medium High Medium 4.4 20

Top

Shaped

Shaft

Chisaki 30-100 5-40 G.O.C 1 - 3 Variable Medium Medium n/d 4.6 5.2 40

Double

Shaft

Maerz

Standard

100-800 25-200 G.O.C 1 - 2 High High Low Good 3.6 4.2 25 - 40

Double

Shaft

Maerz Finelime 100-300 10-30 G 1 - 2 High High Low Medium 3.6 4.2 35 - 45

Double

Shaft

Cimprogetti 100-400 25-125 G.O.C 1.8 High High Low Good 3.8 4.2 22

Double

Shaft

Voest Alpine up to 300 10-150 n/d 1 - 2 High High Low Good 3.8 4.2 25 - 35

Double

Shaft

SIC 150-400 20-120 G.O.C n/d n/d High Low Good 3.6 4.2 25 - 35

Annular Beckenbach 80-850 10-250 G.O.GF 0.5 - 2 High High Low Good 4.0 4.6 18 - 35

Double

Incline

Beckenbach 120 10-60 G.O 0.6 n/d Medium Medium n/d 4,000 20 - 30

Multi-

chamber

IAF 40-225 20-150 G.O.C 0.3 - 1 Medium Medium High n/d 4,200 20 - 45

G gas; O oil; C - coal/coke; W, - wood; GF - gasified fuel

Glossary termsAnthracite - The highest rank coal characterised by low volatile matter - always less

than 10% - and high carbon content it has a semi-metallic lustre and is capable of

burning, relatively easily, without smoke see also Semi-anthracite.

Burner beam A water cooled or refractory conduit passing horizontally across a shaft

kiln through which air and/or fuel is injected into the bed of material. There are usually

multiple injection ports in each beam

Calcination -The heating of a substance so that a physical, or chemical change occurs. In

the case of limestone this refers to the dissociation of calcium and magnesium

carbonates

Calorific value- The quantity of energy released as heat when a unit of fuel is

completely combusted

Carburetor A chamber in which liquid fuel and air are premixed prior to ignition

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Dead burnt lime Sintered quicklime, which does not slake readily under normal

conditions

Finelime Manufactures trade name for shaft kiln designed to process small (10-30mm)

limestone lumps

Flash vaporize To evaporate fuel oil by spraying on to a hot surface

Hardburnt lime Lime that has been sintered as a result of over-burning at a high

temperature

Lime A general term for the various forms of calcium oxide and/or hydroxide with

lesser amount of magnesium oxide and/or hydroxide

Lime Reactivity A measure of the rate at which quicklime reacts with water. The

terms very high, high, moderate, medium and low are used as broad classifications.

These can be related to a variety of standard tests

Limestone- Sedimentary rock composed mainly of calcium carbonate derived from the

shells and skeletons of marine micro-organisms.

Metallurgical coke - A dark porous solid fuel, mainly carbon, formed as a pyrolysis or

carbonisation product of coal, produced either as by-product of Town Gas production or

as the main product of Coke Ovens metallurgical or hard coke. Optimum Coke

properties depend upon the end-use which include presently, the main energy supply and

ore reducing agent in blast furnaces, and in former times a domestic and industrial

smokeless fuel and a basic fuel for gas producers

Quicklime Consists mainly of calcium oxide and magnesium oxide, which, when

incorporated into a mortar mix slowly hardens in air by reaction with atmospheric carbon

dioxide

Shaft kiln Generic name for a vertical, refractory lined furnace in which a gravity

driven packed bed of material is processed

Stoichiometric- In a combustion system, the fuel and comburent supply ratio necessary

to burn completely, all the hydrocarbons and other combustible species present in a fuel

KeywordsMinerals; processing; lime; shaft; kiln; calcination; limestone;

Related Combustion FilesCF255 What are the minerals processing industries

Sources[1] Oates J A H, Lime and Limestone, Chemistry and Technology, Production and Uses,

Wiley-VCH, 1998, ISNB 3-527-29527-5

[2] Boynton R S, Chemistry and Technology of Lime and Limestone, John Wiley & Sons,

1980, ISBN 0-471-02771-5

[3] Wingate M, Small Scale Lime Burning, Intermediate Technology Publications, 1985,

ISBN 0-946688-01-X

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AcknowledgementsNone

File Placing[Minerals processing]; [Lime]; [Processes and equipment]

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