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Sunday, 18 December 2016

CEMENT KILN FIRING SYSTEM

We were discussing “Cements and its types” and “Safety requirement in cement industries” in our previous posts. We have also seen the concept of “Coal grinding and drying in cement industry" in our recent post. 

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

Further we will go for discussion of 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.

Firing system

Fuels  

The fuels commonly used for the burning of cement are:
1. Medium volatile coal
2. Heavy fuel oil
3. Low sulphur fuel oil
4. Natural gas

Coal

Coal with a volatile content between 18 - 22 % is used. If necessary, a suitable mixture of high volatile coal (coal gas, fat coal) and low volatile coal (lean coal, anthracite) can be fired. Lignite is another fuel fired in pulverized form. This substance generally has a volatile content in excess of 50%., necessitating extra care and precautions against fire and explosion hazards during grinding, storage and handling. 

The ash from coal becomes incorporated in the clinker in the course of sintering process, a fact that must be duly taken into account which determines the chemical composition of raw meal. Coal with more than 20 % ash content, will in some cases; necessitate the addition of pure (high grade) limestone to the raw mix.

Oil

Rotary kilns may be fired with heavy fuel oil. This oil should have as low a sulphur content as possible, or otherwise, a low sulphur oil should be used which would be more expensive.
Oil is viscous at low temperature and has to be heated to approximately 50 C for discharging it from tankers, pumping it and generally handling it. To achieve optimum firing conditions, the pressure (40 - 60 bar) and the temperature (120 C - for good atomization) of oil, should be as nearly constant as possible.

Natural Gas

Gaseous fuels for cement kilns are predominantly, natural gas. It is supplied by pipeline at pressures ranging from 10 - 70 bars, which are reduced to between 3 - 10 bars at pressure regulating station.
The Indian Cement Industry uses coal as fuel for firing. Though coal has the disadvantage of leaving behind residue (ash) on burning and also requires additional power for drying and grinding, storage, dust collection etc. yet it is most widely used because of its availability.

Firing Systems (Pulverized coal firing)

A distinction can be made between direct and indirect firing systems for rotary kilns, with semi - direct firing as an intermediate solution. 

In direct firing, the coal is ground and dried and then supplied direct i.e. without any intermediate storage, from the mill to the burner. All the exhaust air from the mill is fed as primary air to the kiln.
With indirect system, the coal is likewise, simultaneously ground and dried, but is then stored in a bunker or bin. 

Semi - direct firing denotes a system whereby the primary air flow can be reduced to such an extent as is compatible with adequate removal of moisture from the coal drying / grinding mill, while the rest of the mill exhaust air is returned to the mill.

To be sure of maintaining a constant flame in the kiln, fuel must be fed at a constant rate - in terms of weight of pulverized coal supplied per unit time. The combustion air should be preferably preheated and the volumetric flow rate be kept as low as possible in order to achieve maximum utilization of the very hot secondary air. 

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We will see other topic i.e. “Cement kiln refractory lining” 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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