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We were discussing “Cements and its types” and “Material preparation technology in cement manufacturing” in our previous posts. We have also see the concept of cement manufacturing process and its selection in our recent post. 

Today we are going to start here one very important topic i.e. pre-calciner in cement manufacturing process. We will understand the various terminologies and engineering concepts used in cement technology with the help of this category. 

Before reading this post, please find link “Raw meal homogenization and storage in cement industries” and “grinding and drying ofcoal in cement industry” and “Cement manufacturing process: burning technology”

Further we will go for discussion of firing technology, clinker cooling and grinding, cement packing and dispatch, pollution control and much more facts about cement technology in our next post in this category of cement technology.

Precalciner (with cyclone preheater)

Precalciner systems have been derived from the preheater kilns in that the decarbonation of the raw materials is further increased before introduction into the kiln.Thus, the degree of calcination at kiln inlet is much higher compared to that in preheater kiln. This is done by burning part of the fuel in this region, either in a special precalcining chamber or in preheater itself.

The air required for this combustion can either be supplied from the cooler through kiln itself or through a special duct often referred to as tertiary air duct. 

The various precalciner kiln systems may be divided into two categories, those with tertiary duct and those without tertiary duct.

In kiln systems without tertiary duct, almost all the air for combustion has to pass the kiln to obtain a sufficiently high temperature in the burning zone; the excess air must be limited to 20 - 30 %.
This means that a maximum of 20 - 25 5 of the fuel can be burned in the calciner. 

Usually, the calciner is operated with approximately 15 % firing, since more fuel in the calciner will give higher fuel consumption and also a higher exit temperature. So, 80 % or more of the fuel has to be burned in the kiln itself.

 In systems with tertiary air duct, the fuel combustion in the kiln can be reduced to around 40 % with normal excess air. This kiln system will, thus, have a much larger output since the kiln tube is dimensioned on the basis of the amount of fuel to be burned in the kiln.

 Heat consumption during clinkerization process increases with the increase in percentage of kiln exit gas to be bypassed. While the raw material drying capacity of the grinding plant (using kiln exit gas) decreases with the increase in kiln gas bypass volume.

Advantages of Precalcinator Technology

1.      Less thermal loading in kiln as only 40 - 45% of the total fuel is burned in the kiln and the rest in precalciner. With decrease in thermal loading, the life of the brick lining in the burning zone increases and this leads to higher availability of the kiln section and lower inventory cost on refractories.
2.      With preheater and precalciner, the amount of kiln space available for burning increases from 30 % to 65 %. These results in
-         Shorter rotary kiln, resulting in 25 % less construction space for same capacity
-         Lower specific heat losses
3.      Kiln feed while entering the kiln is almost 90 - 92 % calcined as compared to 35 - 40 % for a conventional kiln with preheater. this high degree of calcination has the following advantages
-        Stable coating in the burning zone, so higher refractory life which leads to higher availability of the kiln itself.
-         Due to stable kiln operation, the quality of clinker as well as the throughput from the kiln shall be consistent over a longer period of operation.
4.      Lower ash absorption by the clinker as a result of its lower fuel consumption makes it possible to utilize limestone of relatively lower grade.
5.      Low grade fuel with low calorific value (high ash content and low volatile matter) can be effectively used without affecting the clinker quality.
6.      NOX emission is lower than conventional kiln and can be further reduced by a suitably designed precalciner vessel in a simple and inexpensive way.

Heat exchange in the burning zone of the kiln is affected by direct radiation from the flame. Heat exchange in other zones can be affected more efficiently by suspending the raw material particles in hot gases. The basic concept of a precalciner is, therefore, to effect as much heat transfer as possible outside the kiln.

Higher rate of calcination is accomplished by introducing additional heat into the precalciner Thus, about 40 - 60 % of the total heat required is introduced into the precalciner reducing the heat requirement in the burning zone by about 50 %. As a result, about double the normal capacity can be obtained from the kiln by introducing the precalciner without increasing the thermal loading, which is a limiting factor on the kiln capacity.

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We will see other topic i.e. “Process technology of cement manufacturing: Firing technology” in our next post in the category of Steel and cement technology. 
I am very thankful to Mr. Subrata Bhaumik, Independent cement consultant, for providing such beautiful information and contents about cement technology. 

Mr. Subrata Bhaumik has more than 50 (Fifty) Years (1965 - 2016) of Experience in Cement and other related Industry covering more than 100 assignments in cement plants with capacities ranging from 100 tpd to 10,000 tpd in India and abroad involving visit to 25 countries overseas in connection with work.

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