The MAERZParallel Flow Regenerative Lime Kiln

31. Limestone, Lime and Dolomite

Lime is one of the key elements in life. This natural raw mate-rial is involved in the production of the majority of modernproducts. The production of iron and steel, gold, silver, copperand plastics as well as many chemical products and foodstuffs,just to mention a few, requires lime and, to a lesser extent,dolomite. The most important fields of application for lime anddolomitic lime are: Iron and steel Non-ferrous metals Building industry Pulp and paper Chemical industry PCC - Precipitated calcium carbonate Sugar Glass Flue gas desulphurisation Agriculture Soil stabilisation Water treatment Sewage treatment.

World wide more than 120 million tons per year of lime anddolomitic lime are produced. The iron and steel industry is theprimary consumer with an annual demand of approx. 40 mil-lion tons.

High quality limestone contains 97 to 99% CaCO3. It requiresapproximately 1.75 tons of limestone to produce one ton oflime. High quality dolomite contains 40 to 43% MgCO3 and 57to 60% CaCO3. It requires approximately 2 tons of dolomiticstone to produce one ton of dolomitic lime.

The calcination or burning of limestone and dolomite is a sim-ple chemical process. When heated the carbonate decomposesaccording to its respective equation.

CaCO3 + approx. 3180 kJ (760 kcal) = CaO + CO2CaMg(CO3)2 + approx. 3050 kJ (725 kcal) = CaO.MgO + 2 CO2

The decomposition temperature depends on the partial pressu-re of the carbon dioxide present in the process atmosphere. Ina combustion gas atmosphere of normal pressure and 25%CO2, the dissociation of limestone commences at 810 C. In anatmosphere of 100% CO2, the initial dissociation temperaturewould be 900 C. Dolomite decomposes in two stages startingat approx. 550 C for the MgCO3 portion and approx. 810 Cfor CaCO3.

In order to fully calcine the stone and to have no residual core,heat supplied to the stone surface must penetrate via conduc-

tive heat transfer to the core. A temperature of 900 C has tobe reached in the core at least for a short period of time sincethe atmosphere inside the material is pure CO2. The stone sur-face must be heated to greater than 900 C to maintain therequired temperature gradient and overcome the insulatingeffect of the calcined material on the stone surface. When pro-ducing soft-burnt lime the surface temperature must notexceed 1100 to 1150 C as otherwise re-crystallisation of theCaO will occur and result in lower reactivity and thus reducedslaking properties of the burnt product.

A certain retention or residence time is required to transfer heatfrom the combustion gases to the surface of the stone andthen from the surface to the core of the stone. Larger stonesrequire longer time to calcinate than smaller ones. In principal,calcining at higher temperatures reduces the retention timeneeded. However, too high temperatures will adversely affectthe reactivity of the product. The relation between burningtemperature and retention time required for different stonesizes is shown in the following table.

Stone size Calcining temperature Approx. residence time [mm] [C] [hours]50 1200 0.7

1000 2.1100 1200 2.9

1000 8.3

Throughout this paper, the word lime is used interchangeab-ly to mean high calcium lime or dolomitic lime.

2. Lime Production Equipment

Two types of kilns are primarily used to calcine limestone anddolomite in todays lime industry: Rotary kilns, and Vertical shaft kilns.

Rotary kilns, with or without preheater, usually process grainsizes between 6 and 50 mm. The heat balance of this type ofkilns is characterised by rather high losses with the off-gasesand through the kiln shell. Typical figures for off-gas losses arein the range of 20 to 25% and for kiln shell losses 15 to 20%of the total heat requirement. Only approx. 60% of the fuelenergy introduced into preheater type kilns is used for the cal-cining process itself.

For all types of vertical single shaft kilns there is an imbalancebetween the heat available from the burning zone and the heatrequired in the preheating zone. Even with an ideal calcinationprocess (having an excess air factor of 1.0) a waste gas tempe-

The MAERZ Parallel Flow Regenerative Lime Kiln

rature of 100C may only be achieved with limestone contai-ning less than 88% CaCO3. However, lime produced from suchlow quality limestone has only a restricted field of application.In practice limestones with much higher carbonate content areprocessed resulting in higher waste gas temperature which isthe consequence of excess available heat in the preheatingzone.

The question now is: How can the surplus heat available in thecalcining zone of the kiln be utilised to minimise heat con-sumption and how do the modern kiln types match this aspect.An almost perfect solution for this problem is offered by theMaerz Parallel Flow Regenerative Lime Kiln (PFR-Kiln).

3. The PFR-Kiln

Two main types of vertical shaft kilns exist. The single shaftcounter flow heating kiln and the multiple shaft parallel flowheating kiln. The standard PFR-Kiln is a two-shaft kiln definedby alternating burning and non-burning shaft operation. Thereare two key characteristics of the PFR-Kiln: 1) the parallel flowof hot gases and stone in the burning zone, and 2) the rege-nerative preheating of all combustion air in the process. Thekiln is ideally suited to produce soft-burnt, high reactive limeand dolomitic lime because of the conditions created by theparallel flow of the stone and the combustion gases in the bur-ning shaft. Additionally, the regenerative process provides thelowest heat consumption of all modern kilns available today.The difference in the temperature profile of conventional sin-gle shaft kilns and PFR-Kilns is depicted in Fig. 1. The curvesshow the temperatures of the material, of the air and of thecombustion gases flowing through the kiln.

Fig. 1 compares parallel flow heating with counter flow hea-ting. In single shaft kilns usually counter flow heating is app-lied, a typical temperature profile is shown in Fig. 1a. Thegreen line shows the temperature of the material. The blue lineshows the temperature of the cooling air and the red line thetemperature of the combustion gas and kiln off-gas. As theamount of cooling air is not sufficient for complete combus-tion of the fuel additional air has to be introduced via the late-ral burners. As in this type of kiln the fuel is introduced at thelower end of the burning zone (where the material is alreadycalcined) the temperature in this area is significantly higherthan required for production of high reactive lime.

In parallel flow kilns the fuel is introduced at the upper end ofthe burning zone and the combustion gases travel parallel tothe material. Fig. 1b shows a typical temperature profile wherethe green line represents the material, the blue line in the pre-heating zone the combustion air, the blue line in the coolingzone the cooling air and the red line the combustion gas andkiln off-gas. As the fuel is injected at the upper end of the bur-ning zone where the material can absorb most of the heat rele-

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Fig. 1a: Temperature Profile in a Counter Flow Kiln

Fig. 1b: Temperature Profile in a Maerz PFR-Kiln

ased by the fuel the temperature in the burning zone is typi-cally 950C in average. Because of this, parallel flow heating isthe best solution for the production of soft-burnt, reactive limeand dolomitic lime.

The second important characteristic of the PFR-Kiln is theregenerative preheating a part of the combustion air. In kilnswith counter flow heating, the combustion air is preheated inthe cooling zone by the sensible heat contained in the calcinedlime. The amount of preheating is limited, however, by the ent-halpy of the lime. In the counter flow heating process there isa surplus of usable sensible heat contained in the off-gas thatis not recovered prior to being exhausted. Some single shaftkiln designs therefore have incorporated recuperators in an eff-ort to recover this waste heat, but such heat exchangers aresusceptible to disruptions caused by dust contained in the hotoff-gases.

In the parallel flow regenerative kiln the combustionair is preheated in an ideal manner. The regenerati-ve process requires two connected shafts. Each shaftis subject to two distinct modes of operation, bur-ning and non-burning. One shaft operates in theburning mode and simultaneously, the second shaftoperates in the non-burning or exhaust mode. Eachshaft spends an equal amount of time in both theburning and non-burning modes of operation.

In burning mode, a shaft is characterised by theparallel flow of combustion gases and raw stone,whereas, in non-burning mode a shaft is character-ised by the counter-current flow of off-gases andraw stone. The combustion gases exit the burningshaft through a crossover channel into the non-bur-ning shaft. The alternating burning / non-burningshaft sequence serves as a regenerative prehea-ting process. Heat is transferred to the rawstone from the off-gases during the non-burning mode and then reclaimed by thecombustion air from the raw stone duringthe burning mode. The stone preheatingzone acts as a regenerator with the stonecharge as chequers. This kind of regeneratoris completely insensitive to dust-laden orcorroding gases and, at the same time, showsexcellent heat transfer characteristics.

The regenerative preheating of the combustionair makes the thermal efficiency of the kilnpractically independent from the excess com-bustion air factor. This considerably simplifiesthe setting of the correct length of the flame toproduce the desired quality of soft-burnt lime. Alarger quantity of excess air produces a shorterflame and less excess air produces a longer flame. Thelength of the flame is one of the key factors to control the

reactivity of burnt lime. Generally shorter and hotter flamesreduce the reactivity of the burnt product.

4. The Operating Principle of the Maerz PFR-Kiln

Fig. 2 shows the basic operating principle of the PFR-Kiln andillustrates the two phases of gas flow. Two shafts, designated1 and 2, contain the material to be calcined. The stone char-ging system, the reversal traps for fuel, combustion air, andoff-gas, and the lime discharge system have been omitted fromthis diagram. The shafts are either alternately or simultaneous-ly charged with stone depending on kiln capacity. Lime isdischarged continuously at the bottom of both shafts.

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Fig. 2: Operating Principle of the Maerz PFR-Kiln

6Fuel is supplied to only one of the two shafts. In Fig. 2 it issupplied to shaft 1 thus shaft 1 is designated the burning shaftand shaft 2 is designated the non-burning shaft. The fuel isintroduced through multiple lance tubes that vertically extendto the bottom of the preheating zone. The lower end of thelance tubes marks the changeover to burning zone from thepreheating zone. Fuel is injected through these lances andevenly distributed over the cross sectional area of the shaft.

Combustion air is introduced under pressure at the top of thepreheating zone above the stone bed. The complete system ispressurised. The combustion air is preheated by the stone inthe regenerator (preheating zone) prior to mixing with the fuel.The air/fuel flame is in direct contact with the calcining mate-rial as it passes through the burning zone from top to bottom(parallel flow heating).

The off-gases leave the burning shaft and enter the non-bur-ning shaft through the crossover channel, travelling up incounter flow to the stone. The off-gases transfer heat to thestone bed in the non-burning shaft and even calcine it to asmall degree. The off-gases then regenerate the stone bed inthe preheating zone in preparation for the next burning cycleon that particular shaft.

Each shaft cycles through the burning and non-burning modeat intervals of approximately 12 minutes. The changeover fromburning to non-burning is called reversal period. During eachreversal period a measured amount of stone is charged to thekiln. Calcined product is discharged from both shafts continu-ously throughout the burning cycle by discharge tables into apressurised hopper. Cooling air is continuously introduced atthe bottom of both shafts to reduce the temperature of theproduct prior to being discharged into the lime storage hopper.During reversal periods, when the kiln is depressurised, the pro-duct is discharged from the storage hopper onto vibrating fee-ders and conveyor belts.

The excellent thermal conception of the PFR-Kiln can be satis-factorily proven by means of the heat balance. The sum ofeffective heat, i.e. heat required for dissociation, and of theheat losses provides the thermal requirement of the kiln.

The heat losses consist of: the loss through the kiln wall equal to approximately 170 kJ

(40 kcal)/kg of lime, the sensible heat of the discharged burnt lime equal to

approximately 80 kJ (20 kcal)/kg of lime at a discharge temperature of 100 C, and

the sensible heat contained in the off-gases equal to approximately 290 kJ (70 kcal)/kg of lime at a dischargetemperature of 100 C.

Because the kiln has no moving shell as a rotary kiln it can bewell insulated and the loss through the walls can be kept to aminimum by using the appropriate insulating refractory lining.The refractory lining installed has been determined to providethe lowest heat loss for the money invested. Additional insula-tion to further reduce the wall losses would be too costly forthe corresponding savings.

A sufficient amount of cooling air is used to reduce the tem-perature of the calcined lime in the cooling zone. The heatedair is subsequently used in the process thereby improving thekiln efficiency.

Although it is theoretically possible to reduce the off-gas tem-perature below 100 C, operating below this value is not advi-sable because of condensation and corrosion problems whenoperating in the range of the gases dew point.

Considering these design criteria for heat losses of the kilnwhen producing lime with 96% CaO the total thermal require-ment is approx. 3500 kJ (840 kcal)/kg or 3.02 million Btu perton of burnt lime.

PFR-Kilns are typically designed with two shafts of either rect-angular or circular cross sectional shape. The shafts are con-nected by a crossover channel at the bottom of the burningzone. The crossover channel serves as the transport duct toallow the hot gases to exit the burning shaft and enter thenon-burning shaft.

Fig. 3a: Gas Flow in the Rectangular Maerz PFR-Kiln Fig. 3b: Gas Flow in the Circular Maerz PFR-Kiln

The simplest design is to lengthwise place two shafts with rect-angular cross section side by side in such a manner that thekiln gases can flow directly from one shaft to the other (Fig.3a). A disadvantage of this design occurs at larger kiln capaci-ties (and consequently larger shaft cross sections) where thehot gases have the tendency to concentrate on the crossover-channel side of the shafts and the gas distribution is not uni-form. Therefore at larger capacities, kilns of circular cross sec-tion are proposed. These kilns have circular connecting chan-nels, as illustrated in Fig. 3b. The off-gases exit the burningshaft and enter the non-burning shaft radially around the com-plete shaft perimeter thereby guaranteeing an absolute evenheat distribution which is a key factor for a high quality of theburnt lime.

5. Kiln Components and Equipment

Fig. 4 shows the general arrangement of the PFR-Kiln andillustrates various components and equipment related to thiskiln type.

5.1 Kiln ShaftsIn the early days of PFR-Kiln design, two-shaft PFR-Kilns usedstone sized between 40 mm and 120 mm. When the require-ments were for high output using stone less than 40 mm insize, three shafts were used. Small stone size creates a greaterpressure drop in the shaft and increases the pressure inside thekiln. When three shafts were used, the off-gases of the burningshaft were distributed into two exhaust shafts thereby reducingthe gas speed by one half and the pressure drop by approxi-mately three fourths. Technical development and experiencehas allowed the use of two-shaft kilns for almost all applica-tions and as such has eliminated the need for three-shaft kilns.

The PFR-Kiln operates under pressure therefore the steel shellmust be sealed air tight. All openings at the top of the kiln forcharging stone and the bottom of the shafts for discharginglime are sealed by hydraulically operated traps.

5.2 Refractory LiningThe preheating and cooling zones of the kiln are lined with anabrasion resistant wear lining backed by insulating firebricks.The wear lining in the burning zone is made of high qualitymagnesite bricks with an insulating secondary lining. The wor-king lining has a thickness of 250 mm and is backed by aninsulating lining made from light fireclay bricks and calciumsilicate boards.

The arrangement of the brickwork is simple as shown in Fig. 5.There are no burner bridges or other devices in the shafts thatwould hinder the free flow of stone and calcined product as itpasses through the kiln. In the case of rectangular shafts, stan-dard brick shapes can be used to a large extent with a minimalnumber of special shapes required. This provides low cost

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Fig. 4: The Maerz PFR-Kiln

8lining and a simple inventory of spare bricks for repairs. Cir-cular shafts are not as simple and require somewhat more spe-cial shapes. However, due to the static nature of the kiln andthe constantly improving refractory materials, a long life withminimal maintenance can be expected.

5.3 Air BlowersThe kiln system is pressurised up to typically 40 kPa (400mbar). Rotary piston (Roots type) blowers produce combustionair as well as lime and lance cooling air. Rotary piston blowerssupply practically constant volumes of air, independent fromthe pressure drop created by the resistance of the stonecolumn. Depending upon the kiln capacity, a varying numberof blowers is installed to supply combustion air and cooling air.One combustion air blower and one lime cooling air blower aredriven with variable speed with the remaining blowers driven atfixed speed. The variable speed blowers provide the accurate

control of the required volumes of combustion and cooling air. The variable control can be either manual or automatic and itis used to provide the correct air flows at any kiln output andto meet all product quality requirements.

From the combustion air blowers an air duct leads to the topof the kiln, from the cooling air blowers a similar duct leads tothe discharge device. The combustion air is fed to the kilnabove the stone charge, and the cooling air enters the limecharge through the discharge devices.

The blowers are installed in a room designed to minimise thesound emissions (Fig. 6). All blowers are provided with inletand outlet silencers. During the reversal period all air flow tothe kiln must be stopped and the kiln de-pressurised. Bypassvalves are provided so that the blowers can remain in operationduring this time.

5.4 Firing EquipmentThe main requirement to produce a high, uniform quality pro-duct, i.e. quicklime or burnt dolomite, is to achieve a uniformdistribution of the fuel over the entire shaft cross sectionalarea. This is accomplished by installing vertically suspendedburner lances inside the stone charge as shown in Figure 7.

Fig. 5: Refractory Lining

Fig. 6: Rotary Piston Blowers

Fig. 7: Burner Lance System in Maerz Lime Kilns

9The PFR-Kiln can be fired with virtually all kinds of gaseous,liquid and pulverised solid fuels as described in the followingchapters.

5.4.1 Natural GasEven distribution of gaseous fuels is accomplished by feedinggas into a main ring duct (Figure 8). From the main ring ductit is distributed to the individual burner lances. Each burnerlance contains a limiting nozzle to control the actual gas flowand the distribution between the lances.

The burner lances are in direct contact with hot gas and hotstone therefore cooling air is required to cool the lances and toprevent dust from entering into the lances in the non-burningshaft during the exhaust gas cycle. Positive pressure, Roots-type blowers supply air to cool and purge the lances therebymaximising lance life.

5.4.2 Fuel OilVertical burner lances similar to those used for gas firing,however, consisting of two concentric pipes are used for oilfiring (Figure 9).

Fig. 8: Gas Firing System

Fig. 9: Fuel Oil Firing System

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Cooling air passes through the space between the inner andouter pipes. Steam or compressed air is used to atomise thefuel oil and to purge the lances to avoid clogging and/orcoking at the lance tip. Individual dosing pumps or control val-ves assure constant and even oil flow to each burner lance.

5.4.3 CoalIn many areas, coal, lignite or petcoke are less expensive andmore readily available than natural gas and fuel oil. A systemsimilar to the gaseous fuel design was developed to allow pul-verised lignite, coal and petcoke be injected through the bur-ner lances. Air is used as the carrier gas and lance coolingmedium. Figure 10 shows the basic scheme of the MaerzPulverised Solid Fuel firing system (PSF). Since its introductionin 1980 the PSF system has been proven in more than 40installations.

Pulverised fuel is stored in a bin to allow batch discharge of thefuel into the weigh hopper beneath. The outlet cone of the sto-rage bin is equipped with fluidising devices operated with com-pressed air or inert gas such as nitrogen or carbon dioxide. Therequired weight of coal for one burning cycle is fed from thebin into the weigh hopper during reversal periods when theflow of coal and combustion air to the kiln is stopped. The bot-tom of the weigh hopper is designed with evenly spaced out-lets around the circumference of the hopper leading to rotaryvalve feeders that discharge into conveying pipes.

Contrary to the firing systems for gaseous and liquid fuels thesystem for solid fuels operates in two steps. In the first step therequired amount of coal for one burning cycle is fed into theweigh hopper. In the second step the coal is conveyed from theweigh hopper via rotary dosing valves to the burner lances.

5.4.4 LPG (Propane, Butane)Propane and butane (LPG), as well as LVN (Light VirginNaphtha), are excellent fuels for lime kilns. They are used inspecific cases where available at favourable prices and wherehigh purity lime is required. In some cases LPG is used in com-bination with other fuels, such as sulphur containing coal, tokeep the overall sulphur content of the product at acceptablelevels.

The firing system used for LPG on Maerz lime kilns is very simi-lar to that used for natural gas. In most cases LPG is vaporisedbefore being fed to the burner lances via the main ring ductand nozzles. Occasionally, liquid gas is fed directly to the bur-ner lances by dosing pumps.

5.4.5 Coke Oven Gas and Low Calorific Value GasSteadily rising fuel prices coupled with the requirement toreduce the use of non-renewable fossil fuels has created thedesire to use low calorific value gases known as lean gases.These LCV gases are typically off-gases from pyrolytic proces-ses and the manufacture of iron and steel. Converter gas, Corexgas, blast furnace gas or a mixture of all are successfully usedon Maerz PFR-Kilns. These waste gases are produced at lowpressure so their pressure must be boosted by means of rotarycompressors or Roots type blowers before being used. Thesame procedure applies to coke oven gas.

5.4.6 WoodThe earliest lime kilns used wood as the standard fuel. Woodwas substituted first by coal and coke and later by fuel oil andnatural gas. In recent years the use of wood waste, readily avai-lable in certain areas, has gained importance. Maerz has deve-loped a firing system, similar to the one for coal, where wood

Fig. 10: Maerz PSF Pulverised Solid Fuel Firing System

waste (specifically sawdust and grinding dust from the furnitu-re industry) is injected through the burner lances. The particlesize should be less than 3 mm before being fed to the kiln.

5.4.7 Waste OilThe recycling of waste oil, primarily used lubricants, has beco-me an important and necessary routine in our industrialisedworld. There are several Maerz lime kilns using converted oilas fuel. Many waste fuels contain elevated levels of impuritiesthat can result in the production of toxic gases or the dischar-ge of heavy metals. Special attention must be paid to environ-mental issues and the use of waste fuels may be restricted byenvironmental regulations.

5.4.8 Simultaneous Use of FuelsBesides the use of single fuels as described above the MaerzPFR-Kiln may also be operated using two fuels simultaneouslyin order to optimise overall fuel costs. Typical combinations offuels are coal dust and natural gas, fuel oil and natural gas aswell as coal dust and fuel oil.

It is also common practice to design the kiln for a single fuelin the first stage and only later upgrade the firing system forthe use of two types of fuel.

5.5 Hydraulic EquipmentThe kiln operation requires the alternating burning and non-burning shaft procedure. The kiln must be opened, closed, sea-led, pressurised, fired, and de-pressurised. These actions requi-re the use of hydraulically operated, movable parts. Thesemoveable parts include: reversal traps for combustion air and off-gas shaft closing traps discharge tables discharge traps traps at the weigh hopper relief valves in the air ducts reversal valves for fuel and purging media stone level indicators.

The non-compressible feature of hydraulic oils has the advan-tage of producing strong moving force with small constructionelements. Operation is safe, reliable and requires minimum ser-vice. The hydraulic system consists of a power unit (as shownin Fig. 11) comprising an oil reservoir, pumps and filters, aswell as cylinders and control valve stands.

5.6 Electric, Measuring and Control Equipment

5.6.1 Motor Control CentreThe Motor Control Centre (MCC) generally is of conventionalrelay technology design. The main switches, current, voltageand protection elements as well as the transformer for controlvoltage are all installed in the entry section. The other sectionshouse the control, switch and protection elements for the indi-vidual drives as well as the frequency converters for the blowerdrives.

The last section of the cabinet comprises all electric apparatusthat must be emergency operated in case of power failure. Theprovision and scope of the emergency section depends on thetype of fuel used and customer requirements.

For each drive the operation modes Local - Off - Auto maybe selected with a key-operated switch on local control panelson the kiln or at the control panel depending on customerrequest. Furthermore the individual drives may also be equip-ped with local isolators.

5.6.2 Control Panel and Remote I/O StationsThe control panel houses the following instruments: PLC Input/Output module cards

(if not located in remote I/O stations) Control power supply Interface relays Transmitters (if not located in remote I/O stations) Uninterruptible Power Supply (UPS) for the PLC Data bus interfaces for the visualisation station and

remote I/Os.

All digital and analogue signals from the field or the powercabinet are transmitted to the system via PLC input/outputmodule cards installed in the control panel or the remote I/Ostations. Measuring signals for temperatures and pressures aretransmitted as analogue signals. Signals from limit switchesand other position indicator devices are digitally transmitted.

5.6.3 Operator Control StationThe control station consists of a visualisation industrial PC withmonitor, keyboard and printers. All kiln operation commandssuch as kiln start, conveyor start, etc. can be given via the PC.In Fig. 12 a graphic display of the kiln and its operating para-meters is shown.

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Fig. 11: Hydraulic Power Unit

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Process data and limit value inputs are handled via the visua-lisation PC. Also historical data may be charted from the sta-tions database.

The operator interface system is programmed to provide thefollowing information: Indication of system operating conditions

in a process flow chart Input/output of process data and measured values Output of alarm messages Output of short term trends Storage of data on hard disk for long term trends Output of production reports Print function of all charts/graphs/pictures and reports.

5.6.4 Process Parameters Calculation ModuleAll process parameters are calculated in a program moduleaccording to the input data. The kiln operator can for examplemake the following selections: Production rate (t/d) Amount of stone per cycle (kg) Heat consumption (kJ/kg lime) Excess combustion air factor (-) Cooling air volume (m3n/kg lime)

From these data the following process parameters are calcula-ted among others: Number of cycles per day (cycles/d) Combustion or burning time (sec) Fuel quantity per cycle (kg or m3n) Fuel flow (kg or m3n/hr) Combustion air flow (m3n/hr) Cooling air flow (m3n/hr).

5.6.5 Production Report ModuleThe following reports may be displayed: Cycle report Day report.

The cycle report provides specific data on date and time suchas stone charge weight in each shaft, fuel per cycle, heat con-sumption, crossover channel temperature and actual combus-tion air factor.

The day report is a summary of the cycle reports giving anoverview over the most important operating parameters.

5.6.6 Operation without Human InterventionIn a number of cases Maerz kilns are operated without conti-nuous personnel interaction during night shifts and weekends.

Fig. 12: Graphic Display of the Maerz PFR-Kiln

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Should any serious problem occur, the kiln is automaticallystopped and the appropriate person is electronically notified.The operator could be elsewhere in the plant or even off site.The person notified can analyse the problem via modem and ifcorrected can attempt to restart the kiln. If the problem cannotbe solved through the control system then other appropriateactions can be taken. Upon request Maerz may design suchsystems adapted to customer needs.

5.7 Charging Equipment of the KilnA kiln with such a high thermal efficiency demands constantmaterial throughput. During each reversal a weighed amountof stone is fed to the kiln. The number of charges per hour andthe duration of the heating cycle are regulated to control thekiln output. Typical operation demands a reversal cycle appro-ximately every 12 minutes.

Local site conditions determine the method of transporting thestone to the top of the kiln. The various charging methodscomprise: Standard conveyor belt Skip hoist Vertical conveyor belt.

A stone hopper on top of the kiln receives the limestone.Depending on local conditions this hopper may also serve asweigh hopper.

In its simplest design the stone hopper distributes the stonecharge to the two kiln shafts by having two traps on its bot-tom that open alternatively. The stone slides through the opentrap via a chute into the corresponding shaft. The shafts areopened for charging and sealed by hydraulic traps for opera-tion.

The stone charging system described above is a simple systemused in many applications. Some operations, such as thoserequiring a wide range of stone gradation, demand a moresophisticated charging system. The purpose of the sophistica-tion is to ensure a uniformly distributed stone size across thekiln cross section. This is critical when a wide range of top tobottom size is desired or when small stone is used. Fig. 13shows a system that uses rotating buckets in place of the char-ge chutes. The purpose is to improve the distribution of thestone prior to charging into the shaft as the buckets are loca-ted directly above the shafts. Additional designs include remo-vable distribution cones located in the top of the shafts. Thesecones serve to control the distribution of the varying stone sizeinside the shaft and are successfully used in Finelime Kilns.

5.8 Reversal DeviceKiln reversal is the periodic transfer or swapping of the burningand non-burning shafts. This requires devices to control theflow of fuel, combustion air and off-gas. The fuel flow is swap-ped between shafts by on/off valves. Double-acting hydrauliccylinders insure the correct trap position for the flow of com-bustion air and off-gas in each shaft.

Fig. 14 shows the position of thetraps, the burning shaft on the leftand the non-burning or off-gasshaft on the right. In the diagram onthe left side the combustion air trapis open allowing the flow of com-bustion air to the top of the shaft. Atthe same time, the right side of thediagram shows the off-gas trap openallowing the flow of off-gases fromthe non-burning shaft to the stack.These traps are in the opposite posi-tion after a reversal. The reversal iscontrolled automatically by the kilncontrol system.

Cooling air flows continuously intothe bottom of both shafts.Distribution of the cooling air bet-ween the burning and the non-bur-ning shaft is obtained by butterflycontrol valves.

Fig. 13: Stone Charging Device

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5.9 Discharge DeviceThe calcined lime or dolomitic lime material is continuouslydischarged from both shafts. Kilns with rectangular shafts usereciprocating tables while kilns with circular shafts use twocrosswise operating tables. All types of discharge tables arehydraulically operated. The rate of discharge is automaticallyregulated by the stone level control system located in the pre-heating zone.

Fig. 15 shows the arrangement of the discharge device of adouble-shaft kiln with circular cross section. A small hopper issituated underneath each discharge table to collect the limedischarged from the tables during the 12-minute burning peri-od. The hoppers are sealed by airtight, hydraulically operatedtraps. During each reversal period the traps open, lime dropsinto the pressure-free receiving hopper and is then dischargedby vibrating feeders. A roof-like saddle or a steel cone is con-structed above the discharge table to maintain lime flow anddischarge.

5.10 Environmental Control EquipmentTwo main issues must be considered when looking at environ-mental protection and operation of PFR-Kilns. Noise emissions Emissions into the atmosphere.

5.10.1 Noise ProtectionNoise emissions are mainly generated in three places: charging of stone into the kiln discharging of product from the kiln air blowers.

Fig. 14: Reversal Device

Fig. 15: Discharge Device

Stones falling from buckets, conveyors, and chutes into metalhoppers create excessive noise. Therefore, the upper part of thekiln where the stone is dumped and charged must be comple-tely enclosed to control the noise that escapes into the sur-rounding area. In addition chutes, buckets and hoppers arerubber lined.

Most of the lime discharge device is located within the concre-te foundation structure at the bottom of the kiln and thus isless critical for noise emissions.

The Roots-type blowers that supply combustion and cooling airoperate at high noise levels. To reduce the noise emissions toacceptable levels each blower is equipped with inlet suctionand pressure discharge silencers. Additionally, all blowers arelocated in a special building made from concrete or concretemasonry block. The building is equipped with a silencer loca-ted in the inlet suction channel that supplies all air to the insi-de of the building. All doors to the building are sound proof.

5.10.2 Off-gas EmissionsOff-gas containing particles of carbonate and calcined materi-al exit the non-burning shaft. The particulate matter is usual-ly controlled by bag house filters, an example being shown inFig. 16. The dust content in the raw gases exiting the kiln isusually around 5 g/m3n. The bag house filter reduces the finalemission level to less than 20 mg/m3n with the final leveldepending on local regulations.

Fig. 16: Bag House Filter Plant

Emissions of carbon monoxide, sulphur dioxide, heavy metals,etc. depend to a large extent on the type of stone and fuelused. NOx formation is inherently low in the PFR-Kilns becau-se there is no free flame generated. The flame is produced wit-hin the stone bed, completely surrounded by material of lowertemperature. As the heat is released it is immediately transfer-red thereby minimising the peak flame temperature and conse-quently the formation of nitrous oxides.

6. Kiln Operation

6.1 Principles and Control PhilosophyThe PFR-Kiln operates in cycles: during each 10 to 15 minutecycle, fuel and combustion air are fed into one shaft - the bur-ning shaft - while the other shaft serves as the preheating andoff-gas shaft - the non-burning shaft. During kiln reversal thefuel flow is stopped and combustion and cooling air are ven-ted to the open. In this phase the kiln is de-pressurised, stoneis charged into the kiln and calcined product is discharged fromthe collecting hoppers underneath the discharge tables. Whenthe reversal period ends the shafts switch roles, the burningshaft is now the non-burning shaft and the non-burning shaftbecomes the burning shaft.

The high thermal efficiency of the Maerz PFR-Kiln requires theaccurate control of multiple operating parameters such as: Weight of charged stone Stone level in the kiln shafts Combustion and cooling air flow rate Fuel flow rate, i.e. heat input Temperature and pressure inside the kiln Discharge speed of calcined product.

The batch process of charging stone allows accurate weighingof each charge added to the preheating zone of the shafts.Level control is usually done by either a mechanical level probeconnected to an electronic instrument or by gamma ray instru-mentation.

Continuous measuring of ambient air temperature and pressu-re allows the setting of a constant amount of air under localconditions thereby eliminating the influence of outside airtemperature and barometric pressure fluctuations. Constant airflow is maintained with variable speed blower motors.

Calorimetric equipment may be used to measure the heat valueof gaseous fuel. Although the heat value of natural gas is rat-her constant it can vary significantly with coke oven gas, con-verter gas, electric furnace gas, or a mixture thereof. In thiscase continuous measuring of the heat value and subsequentcontrol of the heat input is critical. Liquid fuels typically do notvary in heat value. The calorific value of solid fuels may varybut continuous measuring is a problem. Regular heat valuechecks help to maintain constant heat input.

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An important objective for kiln operation is to control the tem-perature in the calcining zone in a consistent, uniform manner.Temperature within this zone can vary due the stone grain size,the chemical analysis of the stone, variations in the amountand distribution of air flow, and variations of the heat value ofthe fuel. Fuel input has to be controlled as a function of thecalcining temperature. As the temperature in the crossoverchannel is an excellent indicator for the calcining temperaturean accurate measurement of this temperature by optical pyro-meter is required.

6.2 Grain Size of the StoneA narrow range of grain size is ideal for any kiln, but, due tothe crushing properties of stone, a widely varying grain size isthe typical situation in the quarry. The PFR-Kiln is able to cal-cine a wide range of top to bottom stone size because of itssophisticated charging system. The ideal range is 2:1, but ope-ration using 4:1 is still permissible. The top to bottom sizerange is not the only criteria though as the shape of the grainalso plays a role. The minimum stone size for the standard typePFR-Kiln is approximately 25 mm with a typical maximumstone size of 125 mm. Upon customer request the maximumsize may be as high as 180 mm provided the burning zone aswell as the feeding and discharge equipment have been ade-quately designed for it.

6.3 Quality of the StoneAs for all types of vertical shaft kilns the use of hard, non-decrepitating, high purity limestone is an ideal condition fortrouble-free operation of the PFR-Kiln. Nevertheless, due tothe fact that the shafts of the PFR-Kiln are virtually a pipewithout any devices which could obstruct the free flow oflimestone and lime the movement of the material column isslow and uniform minimising abrasion and formation of fines.This means that also soft limestone can be calcined in the PFR-Kiln.

In case the limestone has a tendency to decrepitate during thecalcining process an increased percentage of fines will be gene-rated. The installation of so called air cannons in the crossoverchannel area where dust particles could stick to the refractorylining facilitates the calcination of soft and decrepitatingstone.

High quality limestone and dolomite with consistent chemicalproperties is often not available or is scarce. Varying contentsof carbonates and impurities can result in the production ofoverburnt or underburnt product with inconsistent values forresidual CO2 and loss on ignition. For such cases a fully auto-matic temperature control system of the Maerz PFR-Kiln maybe implemented to adjust the heat input to maintain uniformquality of the calcined product.

6.4 Excess Combustion AirExcess air has a considerable influence on fuel consumption inthe typical counter flow shaft kiln. But this is not the case inthe parallel flow regenerative kiln where the excess air factorhas hardly any effect. The same amount of heat is recovered inthe stone of the non-burning shaft regardless of the introdu-ced excess combustion air. Therefore the air volume can beadjusted to produce a short or long flame and adapt the bur-ning zone temperature to produce the desired product. Limecooling air does not take part in the combustion and dilutesthe combustion gases thereby making the CO2 content in theoff-gas of PFR-Kilns lower than in a conventional single shaftkiln.

7. Performance, Product Quality, EnergyConsumption, Maintenance

7.1 Kiln CapacityThe trend in todays market is to focus on large capacity kilns.PFR-Kilns with a daily output of 600 tons have been in opera-tion for years with up to 1000 tpd available. Small capacitykilns are restricted by economic factors. The relation betweenthe cost to install a large kiln and a small kiln is not linear. Itis generally recognised and accepted that the investment costsper ton of burnt lime are higher on small kilns than they are ona larger kiln. Even so, under certain conditions, PFR-Kilns witha daily output of 50-75 tons have been proven economical.

The output of a PFR-Kiln can be varied within a wide range: itis quite possible to operate the kiln at only one half of thenominal capacity without considerable influence on the speci-fic fuel and power consumption.

7.2 Product Quality

7.2.1 Residual CO2The PFR-Kiln allows the production of lime and dolomitic limewith residual CO2 figures as low as 0.5%, in certain cases evenlower. The steel industry, the biggest consumer of lime anddolomitic lime, generally asks for residual CO2 contents of lessthan 2%.

7.2.2 ReactivityThe parallel flow of material and combustion gases during thecalcining process is the ideal condition to produce high reacti-ve lime and dolomitic lime as required for most applications.For special applications such as the production of porous con-crete, lime with medium or low reactivity is required. By adap-ting operating parameters, such as excess air ratio and heatinput, medium burnt lime can be produced in the PFR-Kilnwith adequate quality of the raw stone. The production of hardburnt lime, however, is in general not possible in this type ofkiln.

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7.3 Fuel and Electric Energy RequirementThe PFR-Kiln has the highest heat efficiency of all modern limekilns. Energy efficiencies of 85% and higher have been achie-ved. Typical heat consumption (based on the net calorific valueof the fuel) is in the range of 3350 to 3600 kJ (800 to 860kcal)/kg or 2.88 to 3.09 million Btu per ton of lime dependingon chemical analysis and grain size of the stone and the typeof fuel.

Electric energy consumption depends on the stone size, thefuel used and the kilns elevation above sea level. Consumptionfigures range between 25 and 35 kWh/ton of product.

7.4 Maintenance WorkLong term operating experience suggests the following refrac-tory repair intervals for normal kiln operation:

Crossover channel area: 3 to 4 years Burning zone: 6 to 8 years Preheating and cooling zones: 9 to 12 years

Using this expected life and normal repairs and maintenance,the average figure for refractory consumption is less than 0.3kg per ton of lime produced.

7.5 Supervision and Maintenance PersonnelAs a standard recommendation one operator per shift is requi-red to supervise operation of one or more kilns. His main workplace is the control room. He can, however, perform duties out-side the control room as the kiln will be shut down automati-cally in case of any serious trouble.

For maintenance and repair work a mechanic as well as an elec-trician should be available.

8. Modernisation and Revamping

8.1 Technical ProgressThe first Maerz PFR-Kilns were built more than 35 years agoand are still operating. Despite the tremendous technical deve-lopment occurred since then the basic and unique principle ofthe PFR-Kiln has remained unchanged. In fact studies publis-hed in the literature have come to the conclusion that the ther-mal efficiency of this kiln type cannot sensibly be improved.

On the other hand extensive tests in the Maerz laboratory, the-oretical investigations and, most important of all, the fruitfulco-operation with kiln operators for so many years have resul-ted in vast experience and subsequently in essential improve-ments in kiln design and operation.

8.2 Revamping of KilnsThe most important factors which make modernising of aMaerz Kiln desirable and interesting are:

Environmental issuesThe installation of modern bag house filters reduces dust emissions to meet the environmental regulations set by local authorities. Better control of the combustion process results in lower CO and NOx emissions.

Improvement of kiln operationKiln operation can be improved by an increased degree of automation and centralised control.

Increased availability and safetyImproved and automatic control of kiln operation increaseavailability and operational safety.

Improved quality of productStricter operational control improves product quality and consistency. An increase in the number of burner lancesleads to more uniform distribution of the fuel and thus further improved product quality.

Flexibility in fuel applicationFlexibility in the use of different fuels, separately or combined, depending on availability, market price and product requirement increase competitiveness (Fig. 17).

Capacity increaseThrough adaptation of the reversal sequence it is possible tocharge the kiln during the burning period. The shortening of the reversal period results in an increase of kiln output.

Enhanced range of stone grain sizeThe use of broader grain size range through improved charging systems will increase the quarry yield and thus reduce raw material costs.

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Fig. 17: Modernised Fuel Oil Firing System

A narrow grain size range of the stone is desirable in shaft kilnoperation. On the other side a high quarry yield and thus theuse of a max. allowable grain size range is imperative to redu-ce production costs. To achieve this goal Maerz has designedthe so-called Sandwich Charging System for its PFR-Kilns.The successive charging of stone in layers of different sizereduces the pressure drop in the stone column compared tocharging a mixture of the two stone fractions. At the same timequality of the calcined product is improved.

8.3 Conversion of Single Shaft KilnsUnder specific conditions it is desirable and feasible to convertexisting single shaft kilns into PFR-Kilns. The reasons for con-version include: There is insufficient area to construct a completely new

installation on site. Existing facilities such as the stone charging equipment, the

discharge arrangement, the dust collecting equipment andthe kiln foundation structure must be re-used. On certainoccasions even the kiln shells have been re-used. Sufficientheight between charging and discharging equipment as well as an appropriate distance between the shaft kilns to be

converted into PFR-Kilns are essential pre-conditions for any conversion. Furthermore the existing foundation structure must have enough bearing capacity to support thenew kiln structure.

Capital expenditure for conversion is lower than for installation of a new kiln.

The above factors may make the conversion of existing kilnsinto PFR-Kilns an attractive solution. To adapt the new kilnsystem to the existing installation requires substantially moreengineering and design work from the kiln constructor.

Conversions carried out by Maerz to date comprise the follo-wing alternatives: Two existing coke fired, single shaft kilns have been

connected by a new crossover channel and thus converted to a PFR-Kiln with gas firing.

A newly built shaft has been added to an existing singleshaft kiln to form a PFR-Kiln.

The two shafts required by a PFR-Kiln have been installedwithin the shell of one existing shaft kiln.

9. Specially Designed Maerz PFR-Kilns

9.1 Maerz Finelime KilnTypical stone sizes processed in conventional vertical shaftkilns are larger than 30 mm. Stone sizes of 6 to 30 mm can beused in rotary kilns, however with lower thermal efficiency.Maerz has developed a special type PFR-Kiln to burn stonesized between approx. 10 and 40 mm at thermal efficienciesequal to or greater than conventional PFR-Kilns. The primaryfeatures of the Maerz Finelime Kiln (Fig. 18) are: New stone charging system the system allows each shaft

to be charged simultaneously and provides control of thesmaller fraction to be charged to the outside of the shaftcross section.

Adapted fuel injection system uses a greater number ofburner lances for increased uniformity of heat input into theburning zone.

Inner shape of kiln the Maerz Finelime Kiln has strictly acircular design. A kiln with rectangular or semi-circular crosssection would neither allow sufficiently uniform charging ofstone nor uniform flow of air and combustion gases.

9.2 Maerz MG-PFR-KilnOne of the key factors for lime producers today is to increasethe yield of their quarry operations. An important factor in thisis the ability to use the maximum grain size range in the calci-ning kilns. In many cases only one kiln type is available and itnormally accepts only a limited size range. Maerz has develo-ped a special type of PFR-Kiln that is able to process top tobottom grain size of 6:1, e.g. 20 to 120mm.

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Fig. 18: 300 tpd Maerz Finelime Kiln

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10. References

Since 1966 more than 350 Maerz PFR-Kilns have been builtaround the world (Fig. 19). Of these, approx. 150 kilns are gasfired, approx. 130 kilns fuel oil fired, approx. 20 kilns solid fuelfired and approx. 50 kilns are fired with a combination of gase-ous, liquid and solid fuels.

Gaseous fuels used include: Natural gas LPG (propane, butane) Coke oven gas Electric furnace gas

Liquid fuels used are: Heavy fuel oil Light fuel oil (Diesel oil) Waste oil

Solid fuels used are: Bituminous coal Lignite Petroleum coke Wood.

Maerz PFR-Kilns have so far been built with daily capacitiesranging between 100 and 600 metric tons of calcined product.The kilns can be operated between 50% and 100% of theirnominal capacity. The following number of kilns have beenbuilt according to daily capacity:

100 to 150 metric tons per day: approx. 80160 to 250 metric tons per day: approx. 90260 to 350 metric tons per day: approx. 110360 to 450 metric tons per day: approx. 40460 to 600 metric tons per day: approx. 30

11. Summary

Since its introduction into lime industry in the late fifties theMaerz PFR-Kiln has become a piece of standard equipment inthe lime and dolomite industry. Its unbeatable thermal effi-ciency together with the flexibility in operation with virtuallyall types of fuel and raw material have made him an excellentchoice whenever the installation of new calcining equipment isan issue.

BibliographyR.S. Boynton, Chemistry and Technology of Lime and Limestone, John Wiley & Sons, 1980J.A.H. Oates, Lime and Limestone, Wiley VCH, 1998E. Schiele / L.W. Berens, Kalk, Verlag Stahleisen, 1972

Fig. 17: Maerz Lime and Dolomite Kilns World Wide

Maerz Ofenbau AGRichard Wagner-Strasse 28CH-8027 ZurichSwitzerland

Phone +41-1-287 27 27Fax +41-1-201 36 34e-mail:info@maerz.comhttp://www.maerz.com

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