Everything you need to know about calcination and calciners by Juan Ortega

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Calcination Efficiency

How to get an efficient calcination? How to calculate What factors are considered?

The burning of fuel, as well as the residence time of solids depends on the gas flow rate. The calculated gas time varies according to the type of project, from 1.4 to 1.7 seconds in a calciner system with tertiary air flow up to 4 to 5 seconds in total or hybrid flow systems.

Some calciner projects induce a cyclonic or rotational movement in the gas flow inside the calciner, giving the solids a significantly longer residence time.

That is extremely favorable so that a high degree of calcination is obtained, since most of the larger particles will be calcined. If higher calcination rates are reached during the operation, guaranteed kiln production can be exceeded.

The level of calcination will depend mainly:

1. temperature inside the calciner

2. residence time of the raw meal in the system

3. solid gas separation

4. dust circulation effect

5. kinetic behavior of raw materials

The calciner system is normally projected for a minimum cancellation rate of 85% being defined by the following equation:

• C = calcination rate %
• WLsample = weight loss in fire (calcined meal)
• WLmeal = weight loss in fire (raw meal)

It is extremely important to understand the concepts of the degree of true calcination and the degree of apparent calcination. Degree of True Calcination is obtained when the calcination process was complete, that is, all calcium carbonate (CaCO3) was dissociated into free lime (CaO) and carbon dioxide (CO2).

According to this definition, we can consider two extreme cases:

Raw meal – degree of calcination = 0% (weight loss in fire = 35%)

Clinker – degree of calcination = 100% (weight loss in fire = 0%)

In practice, we never managed to determine the true degree of calcination due to the method of collecting a sample of calcined meal extracted from the cyclone feed chute of the last stage.

As there are dust cycles in the area between the kiln, kiln inlet, gas rise duct and lower cyclone, this sample contains a certain amount of dust that was already present inside the kiln, therefore being “ contaminated. ” This means that the collected sample contains both flour calcined by the calciner as well as extremely calcined and recycled powder from the kiln. Therefore, the degree of calcination determined with the collected sample was always to have a higher degree of calcination than with the freshly decarbonated hot meal from the calciner.

In conclusion, the degree of calcination determined according to the sample of hot meal collected in the cyclone chute feed the last stage is not a true degree of calcination plus something we call the Degree of Apparent Calcination. This means that the higher the concentration of dust near the kiln inlet area, depending on the number of dust cycles, the greater the degree of apparent calcination.

Whatever the type of calcination used, that is, a separate calciner or a calciner in-line with the kiln, it is mandatory to use a fan to process induction of the system (ID Fan), that is, the combustion gases together with carbon dioxide released in the calcination.

On the other hand, in order to obtain an effective control over the secondary and tertiary air flows a control device must be used in at least one of the suction branches, for example, in the tertiary air duct.

For efficient heating of the preheater, a damper installed in the tertiary air duct prevents fresh, cold air from being diverted to the main kiln flame. However, the most important task of this damper is to obtain effective control of the oxygen rate necessary for complete combustion of fuels fed to the calciner.

Another form of secondary and tertiary air flow control is to install a restriction damper, normally installed in the gas riser duct. This equipment developed by some manufacturers has been used in several factories but costs more than a gate in the tertiary air duct.

Calcinator Combustion Process I 

What are the differences between a conventional combustion system and the calciner system? What factors influence combustion with calciner? How does the calciner affect the temperature profile of the pre-heater?

The combustion process in a precalciner is carried out under very different conditions compared to those of combustion in the burning zone of the rotary kiln:

• the ambient combustion temperature in the calciner is of the order of 900 ° C, while the flame temperature is of the order of 2000 ° C

• some precalcination systems (in-line calciner) use an air-gas mixture for combustion, while others use pure air (separate calciner)

• in all precalcination systems the preheated flour is suspended in the combustion air or in a mixture of air-gases respectively, to absorb the heat released, thus maintaining the temperature at a relatively low level; In any case, sintering of the material is avoided, which could lead to the formation of coating in the precalcination stage.

Given these less favorable conditions, complete combustion is not always achieved in such a way that some experience is required to achieve optimum performance. Of the various parameters that influence combustion, the following elements can be considered the most important:

• a good mixture of fuel with available oxygen (this is particularly difficult to achieve with in-line calcines); The optimum dispersion of the fuel in the gas stream is essential (in the case of a liquid fuel, this can be obtained by spraying).

• the retention time for combustion must be sufficient; the combustion must be completed in the precalcination stage, otherwise it will continue to the next stage (post-combustion) where the temperature level is lower and, therefore, less favorable for calcination: this will result in the underutilization of heat and possibly an increase in thermal consumption,

• The flow pattern of the gas-air mixture (tertiary air) must be favorable for combustion.

• The distribution of raw meal in the combustion zone should be optimal, that is, it should cause minimal distortion in combustion (CaCO3 and CO2; they can also burn with the carbon of the fuel (producing CO).

From practical experience it is well known that a very high concentration of raw food can seriously prevent complete combustion. Therefore, the introduction of a separate air duct for the combustion air of the calciner has become important to improve the efficiency of the entire combustion process in the calciner.

On the other hand, the introduction of fuel between the furnace inlet and the last cyclone, with secondary cooking or precalcining, necessarily increased the temperature level of the process as a whole.

Normally, the output temperature of the last stage of a typical common preheater is between 790 ° C and 820 ° C, much lower compared to the temperature of the last stage of a preheater with precalciner between 840 and 870 C. Therefore, The preheater outlet temperature is also somewhat higher, which implies a greater thermal loss, which is more pronounced in four-stage preheaters.

Calcinator Combustion Process II 

How does the burning of fuels in the calciner occur? What is the difference of burning solid, liquid and gaseous fuels? What is the correct fineness of solid fuel? How to optimize the burning of the calciner?

 

The performance of precalciner systems can be primarily assessed considering two characteristic values as the basis:

• The differences in temperatures between gas and material, in the stage that receives the flow of the precalciner, should be as small as possible, so as to optimize the thermal losses in the exhaust gases of the system; The reaction temperature of the precalciner depends on the raw meal and the degree of calcination required as well as the tolerated NOX level.
• A complete combustion has to be achieved because this directly influences the overall thermal consumption of the system; It should be emphasized that this is strongly influenced by excess air.

Any solid, liquid or gaseous fuel can be burned successfully in a calciner system, but the location and position of the burner in the precalciner must be adapted to the particularities of the fuel. This is particularly important for gaseous fuels, which seem more difficult to be burned in precalcination chambers than other fuels.

On the other hand, it should be recognized that there is also a uniform temperature distribution in the transverse direction of the calciner, with an almost constant level around 875 ° C during the calcination process. Thus, as long as a reasonable amount of carbonate is present, during the time that the raw meal remains suspended in the hot combustion gases, the system temperature is typically between 850°C and 900°C.

The slight rise in temperature after calcination indicates that the fuel suppression rate is well adjusted for this purpose and that the carbon monoxide (CO) content should be below 0.1% at the outlet of the cyclone preheater.

The purpose of a calciner should be to obtain a complete calcination of the carbonates as soon as the flour remains in suspension in the hot gases produced by the combustion process. As soon as a quantity of carbonate was present, the temperature of the system will be temporarily around a balance, which is typically between 850ºC and 900ºC.

At this level the temperature, the combustion rate of fuels with lower volatile content (petcoke, for example) is also low, becoming problematic because a longer retention time is necessary to complete combustion of residual coke.

This is the main reason why burning a coal such as anthracite or petcoke in a traditional calciner, designed for the burning of fuel oil or high volatile coal, is considerably more difficult than burning inside the kiln .

An obvious means of solving such a burning problem with low-content fuels is to increase the grinding of the material by creating a larger contact surface for the reaction between the fuel and the oxidant.

In cases where problems were experienced with burning petcoke in calcines operated between 850ºC and 900ºC, these were solved with a finer grinding.

As a general rule, it is adopted that the residue in the 90 micron mesh (#170) must be between 50% and 75% volatile content for the main kiln burner. For the calciner, the 45 micron mesh (#325) is adopted for control, with an objective of about 6% + 45 microns, which is much finer than what is required for the kiln.

If the process of increasing the fineness of the ground fuel does not result in considerable improvement in the combustion process, it may be necessary to use a higher temperature for the burning of the fuel. This strategy is extremely effective because the combustion rate is doubled if the temperature is increased by 70ºC

A higher temperature zone in the calciner can be reached through:

• Optimization of the temperature of the grate cooler to obtain a tertiary air with a higher temperature.
• The division of raw fed to the calciner into two flows, with a lower rate in the cone bottom of the calciner

In relation to the combustion process carried out inside a calciner, it is customary to classify such reactors into three main types:

• Total Flow Calciner
• Separated Tertiary Air Flow Calciner
• Hybrid Calciner

Calcinator Combustion Process III

What are the main types of calcinators? What are the main projects? What are the advantages and disadvantages?

In our previous article we said that calcinators are classified into three main types. In this third part of this series of articles, we will detail each of them.

Total Flow Calciner

In Total Flow Calciner, combustion occurs in a mixture of kiln suction gases and pure air (tertiary air) from the Clinker cooler. In this way, combustion begins in gases with about 10% to 14% oxygen and is terminated with 1% to 3% oxygen. The raw meal from the preheater is fed to the bottom of the calciner and transported through it to the last stage along with the hot gases. During this time, fuel burning and heat transfer for raw meal occurs.

Tertiary Air Flow Calcinators

In this type of calciner, combustion is processed in pure hot air flow. It starts with 21% oxygen and the process ends with 1% to 3% oxygen. The raw meal of the penultimate stage of the preheater cyclone is fed to the bottom of the calciner and transported through it to the last stage along with the hot combustion and calcination gases.

Hybrid Calciner

This calciner is a combination of the total flow system with the tertiary air flow system. In the hybrid calciner, combustion begins with hot tertiary air, as in calcined air tertiary calcinators, but it is completely with hot kiln gases, similar to total flow calcinators.

Types of Calcinators According to Combustion Process (Projects)

We will list the main projects:

ILC-E (FLS) In-line Calciner – Excess Air.

ILC calciner (FLS) In-line Calciner

SLC calciner (FLS) Separate Line Calciner

SLC-D (FLS) Separate Line Calciner – Downdraft

Calciner Prepol AT (Polysius) Air Through Kiln

Prepol AS (Polysius) Separated Tertiary Air Calciner

Prepol Calciner AS-CC (Polysius) Separate Combustion Chamber

Prepol Calciner – M.S.C (Polysius) Multi-Stage Combustion

RSP Calciner (Taiheyho) Reinforced Suspension Preheater.

*In a next article we will give more details of each project.

Advantages and Disadvantages

The main advantages of a system with precalcination are:

• More stable kiln operation due to better control by two separate fuel injection points.

• More stable kiln operation due to controlled conditions of raw meal fed into the kiln inlet.

• Reduced thermal load in the burning zone.

• Long service life of the burning area refractory.

• Increased kiln availability.

• Higher production capacities with the same dimensions (10,000 tpd with kiln 6.0m x 95m).

• Possibility of building smaller kilns with two bases (C / D – 12).

• Possibility of increasing production on existing lines.

• Reduction of the circulation of volatile elements (S, Cl, K2O, Na2O, etc.)

• Decrease in NOX emission

However, there are also disadvantages in a precalcination system because not all types can be used in all cases:

• Additional installations (dosing, fuel, tertiary air duct, calciner) as well as the relatively smaller kiln size, set a lower economic limit for calciner systems in new plants with P ≤ 1200 tpd

• Alternative fuels containing hazardous components can only be used in the main burner due to the high temperature level in that room; thus the potential for burning these fuels is less than in common kilns.

• Higher temperatures in the sucked gases and higher pressure losses in the preheater can be a disadvantage in some specific cases.

• Separate line burners for new installations are only possible, a two-branch tower is required for a certain capacity, for example, more than 3500 tpd

Types of Calciners I – according to project.

What are the most common types of calciners, according to your projects? What are their characteristics? How do they work? What are your advantages and disadvantages?

In our previous article we list some types of calcines by projects.

Today we will talk about two of these projects:

Calciner ILC-E (In-line Calciner with Excess Air) – FLSmidth

The ILC-E calciner has a simple project based on the principle of gas suction from the furnace together with the excess air necessary for combustion of the calciner through the kiln itself.

In this way the calciner can be considered as an extension, with larger dimensions, of the ascending duct of the kiln gases. The hot gases from the kiln with combustion air penetrate axially through the conical bottom of the calciner and, after passing through it, leave it on the opposite end that has a flat roof.

The penultimate stage raw meal and fuel are fed into the upper part of the conical bottom, creating a stream of fluidized material in the flow of ascending gases. The speed of the mixture of combustion air and kiln gases at the bottom of the calciner is around 25m/s, preventing a fall of raw meal or fuel for the bedroom.

This mixture has a decreased speed through the expansion of the calciner area in its cylindrical part to increase the retention time along the calciner. This ensures an effective mixture between the fuel and the combustion air as well as the complete burning thereof.

Although the combustion with combustion air through the kiln can only be carried out within certain limits, maintaining a stable temperature at the outlet of the calciner allows obtaining a degree of calcination between 50% and 70%. An additional advantage of this process is that a large amount of excess air decreases the risks of coating, so that it is possible to work with higher contents of components such as sulfur.

Prepol AT Calciner – Polysius

In the Prepol AT calcining system (through the kiln) the combustion air for the burners of the calciner is drawn through the rotary kiln to the calciner. This system allows the use of low-calorie fuels such as granulated bituminous, tires, etc. due to the long cross section in the transition from the kiln to the calciner.

Kiln suction gases flow through the calciner to the lower stage of the pre-heater. The calciner is sized in such a way that all the fuel fed to the system is burned during the time interval until the gases reach the lower cyclone. The penultimate stage raw meal is fed to the calciner so that the heat resulting from combustion is used immediately to increase its decarbonation, avoiding overheating.

Its main characteristics lie in low investment costs, extremely simple operation and low load loss due to the optimized calciner flow project.

In our next article, we will talk about two other calciner projects.