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The M&RZ@ Padlel FlOw Regenerative Lime Kiln

1. 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 modem products. The production of iron and steel, gold, silver, copper and 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 and dolomitic lime are:

lron 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 and dolomitic lime are produced. The iron and steel industry is the primary consumer with an annual demand of approx. 40 mil- lion tons.

High quality limestone contains 97 to 99% CaCO,. It requires approximately 1.75 tons of limestone to produce one ton of lime. High quality dolomite contains 40 to 43% MgCO, and 57 to 60% CaCO,. It requires approximately 2 tons of dolomitic stone to produce one ton of dolomitic lime.

The calcination or burning of limestone and dolomite is a sim- Two types of kilns are primarily used to calcine limestone and ple chemical process. When heated the carbonate decomposes dolomite in today’s lime industry: according to its respective equation. l Rotary kilns, and

l Vertical shaft kilns. CaCO, t approx. 3 180 kJ [760 kcal) = CaO t CO, CaMg(CO,), t approx. 3050 kJ (725 kcal) = CaO.MgO t 2 CO,

The decomposition temperature depends on the partial pressu- re of the carbon dioxide present in the process atmosphere. In a combustion gas atmosphere of normal pressure and 25% CO,, the dissociation of limestone commences at 810 "C. In an atmosphere of 100% CO,, the initial dissociation temperature would be 900 “C. Dolomite decomposes in two stages starting at approx. 550 “C for the MgCO, portion and approx. 810 “C for CaCO,.

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 to be reached in the core at least for a short period of time since the atmosphere inside the material is pure CO,. The stone sur- face must be heated to greater than 900 “C to maintain the required temperature gradient and overcome the insulating effect of the calcined material on the stone surface. When pro- ducing soft-burnt lime the surface temperature must not exceed 1100 to 1150 “C as otherwise re-crystallisation of the CaO will occur and result in lower reactivity and thus reduced slaking properties of the burnt product.

A certain retention or residence time is required to transfer heat from the combustion gases to the surface of the stone and then from the surface to the core of the stone. Larger stones require longer time to calcinate than smaller ones. In principal, calcining at higher temperatures reduces the retention time needed. However, too high temperatures will adversely affect the reactivity of the product. The relation between burning temperature and retention time required for different stone sizes is shown in the following table.

Stone size Calcining temperature Approx. residence time b-4 [“Cl (hours] 50 1200 0.7

100 1000 1200

2.1 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 EauiDment

Rotary kilns, with or without preheater, usually process grain sizes between 6 and 50 mm. The heat balance of this type of kilns is characterised by rather high losses with the off-gases and through the kiln shell. Typical figures for off-gas losses are in the range of 20 to 25% and for kiln shell losses 15 to 200/o of the total heat requirement. Only approx. 60% of the fuel energy introduced into preheater type kilns is used for the cal- cining process itself.

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

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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 amount and distribution of air flow, and variations of the heat value of the fuel. Fuel input has to be controlled as a function of the calcining temperature. As the temperature in the crossover channel is an excellent indicator for the calcining temperature an accurate measurement of this temperature by optical pyro- meter is required.

6.2 Grain Size of the Stone A narrow range of grain size is ideal for any kiln, but, due to the crushing properties of stone, a widely varying grain size is the typical situation in the quarry. The PFR-Kiln is able to cal- tine a wide range of top to bottom stone size because of its sophisticated charging system. The ideal range is 2:1, but ope-

*

ration using 4:l is still permissible. The top to bottom size range is not the only criteria though as the shape of the grain also plays a role. The minimum stone size for the standard type PFR-Kiln is approximately 25 mm with a typical maximum stone size of 125 mm. Upon customer request the maximum size may be as high as 180 mm provided the burning zone as well as the feeding and discharge equipment have been ade- quately designed for it.

6.3 Quality of the Stone As for all types of vertical shaft kilns the use of hard, non- decrepitating, high purity limestone is an ideal condition for trouble-free operation of the PFR-Kiln. Nevertheless, due to the fact that the shafts of the PFR-Kiln are virtually a pipe without any devices which could obstruct the free flow of limestone and lime the movement of the material column is slow and uniform minimising abrasion and formation of fines. This means that also soft limestone can be calcined in the PFR- Kiln.

l In case the limestone has a tendency to decrepitate during the calcining process an increased percentage of fines will be gene- rated. The installation of so called air cannons in the crossover channel area where dust particles could stick to the refractory lining facilitates the calcination of soft and decrepitating stone.

High quality limestone and dolomite with consistent chemical properties is often not available or is scarce. Varying contents of carbonates and impurities can result in the production of overbumt or underbumt product with inconsistent values for residual CO, and loss on ignition. For such cases a folly auto- matic temperature control system of the Maerz PFR-Kiln may be implemented to adjust the heat input to maintain uniform quality of the calcined product.

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

7. Performance, Product Quality, Energy Consumption, Maintenance

7.1 Kiln Capacity The trend in today’s 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 capacity kilns are restricted by economic factors. The relation between the cost to install a large kiln and a small kiln is not linear. It is generally recognised and accepted that the investment costs per ton of burnt lime are higher on small kilns than they are on a larger kiln. Even so, under certain conditions, PFR-Kilns with a daily output of 50-75 tons have been proven economical.

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

7.2 Product Quality

7.2.1 Residual CO, The PFR-Kiln allows the production of lime and dolomitic lime with residual CO, figures as low as 0.5%, in certain cases even lower. The steel industry, the biggest consumer of lime and dolomitic lime, generally asks for residual CO, contents of less than 2%.

7.2.2 Reactivity The parallel flow of material and combustion gases during the calcining 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 heat input, medium burnt lime can be produced in the PFR-Kiln with adequate quality of the raw stone. The production of hard burnt lime, however, is in general not possible in this type of kiln.

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