The essential characteristic of a grate cooler is a layer of clinker spread on a more-or-less horizontal perforated grate, through which cold air is blown. This article discusses the importance of grate coolers in cement industry.

A s the demand for cement in India is not increasing at a considerable rate, new cement plants are hardly coming up. Capable owners are going for plant acquisition or upgradation of existing plants. Major capacity upgradation is involved in converting pyro section from suspension preheater to a pre-calciner system. In the last two decades, the Indian cement industry has observed many such upgradation. However, still many, especially lower capacity units with capacity of 1,000 tpd or less are yet to be upgraded at the same line. Owners of such cement plants have now realised the importance of adding external pre-calcinator at least for the reduction of operating cost by saving specific heat consumption (from 1,050 to 1,100 kCal/kG clinker to 800 to 850 kCal/kG clinker – about 15 per cent saving in fuel). Due to small capacity, the sales problem is not as critical as that of large capacity plants, the additional production makes sense for this sector of industry.

Capacity upgradation is carried out on the basis of rotary kiln volumetric loading. Specific volumetric loading increases normally from approximately 2.5 tpd/m3 in case of preheater system to approximately 4.5 tpd/m3 (for Indian plants the figure goes as high as close to 7) for precalciner system. Converting preheater system to precalciner system capacity can be increased by a minimum of 80 per cent. In capacity upgradation, the major challenge comes from increase in capacity of clinker cooler due to layout constriction. In a modern cement plant, whether it is a new plant or upgradation of existing plant, clinker cooler plays a crucial role.

With the leap in cement plant capacity, especially from early 80s, grate coolers have gained enough popularity over other types of coolers like rotary and planetary for better heat recuperation advantages. Other advantages with grate coolers are:

• Improved clinker quality
• Clinkers can be cooled to much lower temperature by using excess cold air
• Excess hot air can be tapped off from cooler to be used in other areas like calciner and for drying fuel if necessary

Cross current heat transfer
In a grate cooler, clinker forms a bed on perforated steel plates and moves horizontally with reciprocating motion. Cold atmospheric air is pumped to undergrate chamber, which passes vertically upward through perforated plates and then hot clinker bed. The cooler acts as a cross current heat exchanger.

As cooling is a heat transmission process, cooling efficiency is greatly dependent on temperature difference between two media – air and clinker. Higher the temperature difference between the two media, higher is the cooler efficiency. In coolers like rotary and planetary type operating with counter current flows, the difference in temperature is practically uniform during cooling so the rate of cooling. In cross current coolers, the difference in temperature pronounced at the start of the process and cooling is therefore slightly faster and more abrupt, a quenching effect is possible improving clinker quality by preserving reactive high temperature silicate polymorphs. Mechanically as well as operationally, a clinker grate cooler may be considered as most complex unit in the clinker burning process. The clinker cooler has a great influence on heat consumption of entire pyro installation.

Specific heat consumption
Specific heat consumption for making clinker comprises of:

• Heat of reaction
• Heat loss from preheater exhaust gas
• Surface loss from preheater
• Surface loss from kiln
• Cooler loss and
• Free heat in material

Heat from most of the preheater exhaust gas is normally used for drying of raw materials and fuel, and therefore this heat is not really lost but utilised in other departments of clinker making. The surface losses of kiln and preheater remain more or less constant and specific heat loss gets reduced with increased capacity. Again, especially preheater surface loss can be reduced by extensive use of insulating materials. Free heat in material does not contribute much and can be reduced by increasing preheater cyclone efficiency.

Total cooler loss comprises of:

• Heat loss with exhaust air;
• surface loss; and
• free heat in clinker.

Normally, a clinker is cooled to a temperature of approximately 1000C or may be even higher; acceptable mainly to a cement mill and also downstream equipment. Hence free heat in clinker in a particular plant is fixed by the requirement. Since the specific heat of air is little higher than that of clinker, heat loss with exhaust air can be theoretically brought to zero, if the amount of combustion air from the cooler is over 1 kG air/kG, clinker if ideal heat exchange conditions were provided.

A cooler offers a great potential for improving the overall heat economy of a kiln installation. An efficient kiln consumes about 1.2 kG air per kG clinker inclusive of excess air required to ensure complete combustion. Use of this air quantity alone in the cooler will result in a clinker temperature of approximately 4000C. So an additional air is needed to obtain an acceptable clinker temperature. The excess air used for only cooling of clinker, goes out of system to atmosphere as waste after getting de-dusted. The heat in the waste air contributes to overall heat consumption. In many cases, a part of this hot waste gas is used for drying fuel. This heat can also be utilised in waste heat recovery system.

Right from the inception of grate coolers, the development work was continuous to reduce the cooling air and thereby reducing cooler loss. The developments in proper cooling air distribution system resulted in reduction of cooling air quantity. This led to a total cooler loss reduction from about 130 kCal/kG clinker for earlier generation of grate cooler to about 100 kCal/kG clinker in modern cooler.

Cooling air distribution
From rotary kiln, a clinker comes out to cooler with varying sizes. Depending on the kiln rotation direction, a clinker gets distributed across the width of cooler from the biggest size in one side to the smallest size in other side. Due to this, in earlier generation of grate coolers where cooling air are supplied from undergrate, a larger portion of air passes through the portion with coarser clinker as the resistance is less.

As the contact time between air and clinker is less, heat transfer becomes very inefficient. On the contrary, due to higher resistance, less air passes through the portion with finer clinker, and the clinker cooling becomes inefficient contributing to operational problems like red river formation and damage and less life of grate plates. The first improvement in cooler came by taking care of distribution of proper air supply across the width of cooler by passing controlled air through hollow beam on which grate plates are fixed. Right side and left side beams/plates are supplied air from two different fans designed with two different total pressures. FLSmidth, the world’s renowned cement machinery supplier, came out with’Self Adjusting Mechanical Flow Regulators’.

Modern grate plates are designed with high flow resistance slots, which result in an even and well-controlled air distribution. The grate plate design and arrangements are made in such a way that it allows cooling air pass through but counteract and prevent the clinker from falling through grate plate slots. These developments resulted in reduction in cooling air quantity from about 2.6 to 2.0 kG air/kG clinker. The quantity of extra air required for cooling has come down by about 60 per cent resulting to lower cooler loss and less quantity of air to be de-dusted.

Mechanical improvements
Mechanical aspects of grate coolers are being taken care of with great importance. The front area where the hot clinker is falling from rotary kiln is most vulnerable to thermal and mechanical action. The first few rows (six to seven) of plates are now converted to fixed rows with a slope.

High capacity cooling air fans are provided to keep the plates cool and to prevent clinker particle cluster formation.

The latest from FLSmidth is a cross-bar cooler, which separates the clinker conveying and air distribution system. Reciprocating bars fitted above stationary air distribution system effectively convey, mix and shear the clinker beneficial for efficient exposure to cooling air. A further benefit to this separation is that gradual wear of the bars has no effect on cooler performance.

The internal flow pattern in the modern grate plate ensures an excellent heat resistance that results in improved life leading to lower maintenance cost.

Cooler efficiency
The efficiency of a cooler is defined as the relationship between the recuperated heats to the kiln/calciner and total heat transferred in cooler.

Efficiency = (Heat Input – Heat Losses)/Heat Input

A grate cooler may be characterised in terms of cooler loss, which is the amount of clinker heat not utilised in pyro process. Cooler loss (VDZ basis) has been remarkably improved from about 130 kCal/kG clinker to approximately 100 kCal/kG clinker from old generation to modern grate cooler. At the same time, due to improvement of heat recuperation, secondary and tertiary air temperatures have increased considerably, resulting to less fuel requirement, stability in kiln operation is a spin-off benefit.

Discussion

• Today, state-of-the-art grate coolers have the following advantages over earlier generation grate coolers.
• Fuel savings of 30-40 kCal/kG clinker
• About 30 per cent less cooling air and about 60 per cent less excess air to be de-dusted
• Low overall power consumption
• Effective and consistent cooling of clinker
• More stable high capacity kiln and cooler performance due to less dust circulation and higher secondary and tertiary air temperature
• Low maintenance costs due to minimum wear on grate plates and movable parts
• Less Red River and Snowman formation tendency
• Less clinker dust fall through grate plate
• Small overall dimensions due to high specific grate load – up to 50 tpd/m2 against 37 tpd/m2 for first generation grate cooler.

Conversion of first generation grate coolers to state-of-the-art grate coolers allow clinkerisation capacity increment by only about 35 per cent as against 80 per cent increase is possible in kiln tube by converting preheater system to pre-calciner system, while cooling clinker to 650C above ambient. Practically, a clinker temperature can go even up to 1,000C above ambient at least in India. That can be achieved by cooler grate load of 60 tpd/m2 with expected cooling air flow of 2.35 kG air/kG clinker resulting to achieving about 65 per cent increase in cooler capacity by conversion to latest generation of cooler. About 15 per cent increment of system upgradation is to be sacrificed if cooler area cannot be increased.

About the author
The article is authored by Jayanta Saha, Cement Process Consultant