Once raw materials have been selected and blended, and ground and homogenized into a fine and uniform kiln feed, they must then be subjected to enough heat to allow the clinkering reactions to proceed. This is the pyroprocessing stage of cement manufacture, beginning with the kiln feed material extracted from storage and weighed and transported to the kiln, and finishing with the clinker from the cooler going to clinker storage.

The main chemical reactions to produce the calcium silicates that later give cement its bonding strength occur in the kiln system. There is a combination of endothermic and exothermic reactions occurring in an extremely complicated chemical reaction sequence. The raw material composition, mineralogical composition and the time and temperature profile of these materials in the kiln determine the ultimate composition and mineralogy of the clinker, which in turn determines the performance of the cement produced.

The pyroprocessing stage is generally regarded as the heart of the cement-making process. It is the stage in which most of the operating costs of cement manufacture appear, and is also therefore the stage where most of the opportunities for process improvement exist. There are many different kiln system designs and enhancements, but they are all in essence performing the following material transformation, in order from the feed end:

1. Evaporating free water, at temperatures up to 100°C.

2. Removal of adsorbed water in clay materials 100° to 300°C.

3. Removal of chemically bound water 450° to 900°C.

4. Calcination of carbonate materials 700° to 850°C.

5. Formation of C2S, aluminates and ferrites 800° to 1,250°C.

6. Formation of liquid phase melt >1,250°C.

7. Formation of C3S 1,330° to 1,450°C.

8. Cooling of clinker to solidify liquid phase 1,300° to 1,240°C.

9. Final clinker microstructure frozen in clinker <1,200°C.

10. Clinker cooled in cooler 1,250° - 100°C.

On the gas flow side, the sequence from the firing end is:

1. Ambient air preheated by hot clinker from kiln 20°C up to 600° to 1,100°C.

2. Fuel burns in preheated combustion air in kiln 2,000° to 2,400°C.

3. Combustion gases and excess air travel along kiln, transferring heat to kiln charge and kiln refractories 2,400° down to 1,000°C.

4. Preheating system for further recovery of heat from kiln gases into the material charge in the kiln system 1,000°C down to 350° to 100°C.

5. Further heat recovery from gases for drying of raw materials or coal.

All kiln systems aspire to optimize heat exchange between the gas streams and material streams at various stages to minimize waste heat and maximize thermal efficiency.

KILN SYSTEMS

Wet Process Kilns — The long wet process kiln, with a length to diameter ratio (L/D) of up to 40, was the main clinker producing plant for most of the 20th century. It is a relatively simple process, with the main advantage of slurry preparation being the eases of milling, handling, blending, storage, pumping, and metering. It is also less prone to low level dust emission.

In wet process systems, the material preheat system is metal chains hanging in the cold end of the kiln, which absorb heat from gases and heat the material which flows over them. The chain actually provides a greater surface area for contact between hot gases and the material clinging to the chains.

The main problem with long wet kilns is their poor fuel efficiency, because of the water to be evaporated from the slurry. This became a severe problem only when the cost of fuel escalated during the 1970s, and only a few wet kilns have been built since that time. However, there are some rare situations where raw material moistures, cheap (waste) fuels, low technology workforce, or other factors may still favor wet process production.

Another disadvantage of a wet process kiln is that it is limited in production rate because of mechanical limitations on kiln size. A 1,500-tpd wet kiln is a large kiln, with 2,000 tpd being an upper economic limit without encountering severe maintenance problems. Apart from the sheer weight and stresses on mechanical drives and supports, shell deflection makes it increasingly difficult to achieve acceptable refractory life.

Long Dry Kilns — Dimensionally, long dry kilns are similar to long wet kilns. These kilns were developed and became popular particularly in North America. Their advantage over wet kilns is potentially improved fuel consumption because the kiln feed is dry. However, without any enhanced heat transfer fittings in the preheating zone, kiln exit temperatures of 700°C or more meant that water spray cooling was required, and very little advantage was realized over wet process. However, kiln internals fitted at a later stage of development included kiln chains (similar to wet kilns), kiln metallic crosses, and ceramic heat exchangers. The crosses and ceramic heat exchangers basically split the kiln into three or four cross-sectional areas over a distance of about 15 to 20 m, splitting both the feed and gas flow, and providing improved heat transfer.

With these enhancements, the kiln gas exit temperatures were reduced to 350C to 400°C, specific fuel consumption improved some 30% and output increased by 35% to 40% compared to wet kilns. Kiln production rates for long dry kilns are marginally higher than long wet kilns.

Travelling Grate Preheater Kilns (Lepol) — The Lepol kiln was invented in 1928 by Otto Lellep and marketed by Polysius, the combination of names leading to “Lepol.” This was a major improvement in kiln thermal efficiency, some 50% over the popular wet kiln process at the time, and led to ready market acceptance of the technology. The technology reached the stage of 3,000-tpd kilns with specific fuel consumption of 3.3 MJ/kg (800 kcal/kg).

These kilns have a short rotary kiln section, L/D of 12 to 15, preceded by a travelling grate covered by a 150 mm to 200 mm layer of nodulized raw meal. The kiln exit gases at about 1,000°C pass through this nodule layer providing preheat of material before it enters the rotary kiln at about 800°C. The gases exit the grate section at around 100°C, implying very efficient recovery of heat. For some grate preheater kiln systems, the kiln feed nodules experience two separate passes of the hot gases, the first for drying and the second for preheating and partial calcination.

The nodules or pellets formed as kiln feed can either be produced from dry raw meal mixed with about 13% moisture in a pan granulator, or produced from raw meal slurry after it has passed through a filter press and been extruded and sliced into cylindrical pellets.

The raw material properties are critical to the performance of the Lepol kiln system. The nodules or pellets formed have to be strong and plastic enough to withstand the mechanical handling and thermal shock on the grate without breaking down. Nodule breakdown causes blinding of the holes on the travelling preheater grate, increasing pressure drop, and reducing airflow and hence capacity of the kiln system.

The output of a particular kiln can vary by almost double depending on the suitability of raw feed for this process and nodule formation in particular.

Cyclone Preheater Kilns — The cyclone preheater was first patented in 1934 in Czechoslovakia by an employee of F.L.Smidth. However, the first preheater kiln was built and commissioned in 1951 by KHD. This system utilizes cyclone separators as the means for promoting heat exchange between the hot kiln exit gases at 1,000°C and the incoming dry raw meal feed.

Cyclone preheater kilns can have any number of stages between 1 and 6, with increasing fuel efficiency with more cyclone preheater stages. The most common is the 4-stage suspension preheater, where gases typically leave the preheater system at around 350°C.

The rotary kiln is relatively short, with L/D typically 15. The material entering the rotary kiln section is already at around 800°C and partly calcined (20% to 30%) with some of the clinkering reactions already started.

Material residence time in the preheater is in the order of 30 seconds and in the kiln about 30 minutes. Kiln speeds are typically 2 rpm. Preheater pressure drops range from 300 mm to 600 mm water, with gas duct velocities typically 20 m/s in the preheater and cyclones.

Kiln capacities up to 3,500 tpd exist, with specific fuel consumption usually around 750 to 800 kcal/kg (3.2 to 3.5 MJ/kg). The larger capacity kilns are built with two preheater tower systems to keep cyclone sizes to economic proportions and required efficiency.

Cyclone Preheater Kilns with Riser Duct Firing — The operation of the cyclone preheater kilns can be improved by firing some fuel in the riser duct to increase the degree of calcination in the preheater. The production rate can also be increased marginally, depending on the limitation to output for a particular plant.

Preheater kilns can be subjected to flushing of material due to the fluidization of raw meal that occurs during calcination in the kiln. Increasing the degree of calcination in the riser (to 50%) before the material enters the kiln reduces this tendency. Furthermore, burning some fuel in the riser duct reduces the fuel requirement and thermal loading in the kiln, thus improving the kiln refractory life.

In this kiln system, the excess air in the burning zone is increased, and the additional oxygen in the riser duct allows additional fuel to be burnt there. This can be an ideal place to burn some waste fuel such as waste oils or tires. The limitation as to how much fuel can be burnt in the riser can be limited by the geometry of the duct, combustion system design, or fuel type. However, even under ideal conditions, the fuel quantity is limited to about 25% of total fuel because of the limitation to the amount of excess air that can be passed through the burning zone. Too much excess air will reduce burning zone temperature to below the levels needed for clinkering. This reduction in burning zone temperature can be countered by oxygen enrichment, as discussed later in the paper.

Precalciner Kilns — In precalciner kilns, the combustion air for burning fuel in the preheater no longer passes through the kiln, but is taken from the cooler region by a special tertiary air duct to a specially designed combustion vessel in the preheater tower. Typically, 60% of the total fuel is burnt in the calciner, and the raw meal is over 90% calcined before it reaches the rotary kiln section. Since the calciner operates at temperatures around the calcination temperature of raw meal (800°C to 900°C), there may not be a flame as such.

The calciner efficiency is dependent on uniform air flow and uniform dispersion of fuel and raw meal in the air. Typically, average residence times calculated on gas flow for early units were about 1 to 2 seconds for coal and oil, and 2 to 3 seconds for natural gas. In recent years, though, there has been a trend toward larger calciner vessels to reduce some of the combustion problems of the earlier designs and provide greater flexibility for using lower grade fuels.

Precalciner kilns can have very large outputs in excess of 10,000 tpd, with specific fuel consumption below 3 MJ/kg (700 kcal/kg). There are many different configurations, with one, two, or three preheater towers operating with one or two calciner vessels in either an in-line configuration or separate line configuration. Some (mostly recent) designs include a separate precombustion chamber. An in-line calciner has kiln exhaust gases and tertiary air making up the combustion air (reduced oxygen levels) for the calciner, while the separate line system has tertiary air only with 21% oxygen forming the combustion air.

A separate line system therefore has a better combustion environment and may be preferred for difficult fuels. It has a further advantage when converting preheater kilns to precalciners in that there is minimal interference with the operating preheater kiln during the construction phase for the new separate line preheater tower and tertiary air duct.

Precalciner kiln systems can operate only in conjunction with grate coolers, as there is no provision for tertiary air off-take with planetary coolers. L/D ratios are typically low at 10 to 14, and kiln speeds are in the order of 3.5 rpm. Kiln residence time is typically 20 to 25 minutes.

Other Systems — The rotary kiln has been the standard clinkering unit for cement production over the past century. During that time, there have been various attempts to produce clinker in other reactors including a flash calcining/clinkering and fluid bed calcining/clinkering vessels.

The motivation has been to reduce capital and operating costs or to allow smaller economic clinkering plants to be built in remote locations.

The author was involved with one such project during the 1980s, where clinker was produced in a fluid bed fed with raw meal. Although this did not proceed to commercialization, it was noted that the clinker produced from this pilot plant was much more reactive than any conventionally produced clinker, and gave cement strengths 20% higher than those produced from good quality rotary kiln clinker. This was thought to be due to the absence of overburning in the fluid bed clinker because of the more uniform temperature that can be achieved. The mineral structure certainly showed much finer alite and belite crystals in the 10- to 20-µm range. This gives some indication of the potential quality improvement possible through better control of rotary kiln temperature.

Critical data on the kiln systems discussed above are summarized in Table 1 (see page 8). The data reflects mechanical, operational, thermal, fuel, production, and efficiency parameters of each kiln system.

VITAL KILN OPERATIONAL PARAMETERS

The following parameters are typical for any kiln operation and considered critical in optimizing the performance of a kiln.

Material Residence Time — The residence time of material in the kiln is governed by the kiln slope, the speed of rotation, and any internal restrictions either by design (dam rings) or through kiln ring formation. The residence time, t, can be calculated from this equation:

1.77 × L × -O × F
p × D × n

Where

t = residence time, min

L = kiln length, meters

p = kiln slope, degrees

D = kiln diameter, meters

t = -----

n = kiln speed, rpm

O = angle of repose of material, (40°)

F = constriction factor (usually1 if no dams, lifters, etc.)

Kiln Degree of Fill — This is the percentage of the kiln cross-sectional area filled by the kiln charge, and is usually in the range of 5% to 17% for most rotary kilns. It should be noted, though, that a fill degree of more than 13% could impair heat transfer in that some of the material in the center of the charge will not be exposed to enough heat. It is sometimes seen that a kiln ring could coincide with high or erratic free lime in the clinker, possibly because the fill degree has exceeded limits for ensuring that all kiln charge material is uniformly heated.

Kiln Slope — Rotary kilns slope from the feed end to the discharge end for material to travel in that direction utilizing gravitational force. The slope is typically 2% to 4%, or 1° to 2°, and is decided in conjunction with the kiln rotational speed. A lesser slope with a higher rotational speed may improve heat transfer because of the greater tumbling of kiln charge.

Kiln Capacity (see accompanying “Design Parameters ” article at right) — There are design limits for all of the above that may vary between different processes, but any of the above could be the limitation to a kiln's output. These limitations will typically manifest themselves as kiln instability and ring or coating buildup, excessive dust loss, poor refractory life, poor clinker quality, or high fuel consumption. Usually, however, the limitation is found to be more a question of a fan capacity, a burner capacity, or milling of raw materials or coal.

CLINKER COOLERS

Cooling of clinker takes place at two locations: 1) in the kiln after the material passes the burning zone region, and 2) in the specially designed clinker coolers after the material falls out of the kiln.

The rate of cooling can be critical to the clinker quality and performance of cement. The rate of cooling in the kiln is determined by the flame and resulting heat flux, flame temperature, and speed of material flow through the kiln. As the clinker temperature exiting the kiln is normally 1,200°C to 1,250°C, the clinker characteristics have been already largely established before the, clinker enters the cooler. A long flame gives slow heat-up and slow cooling of the kiln charge before it falls from the kiln. This will tend to produce clinker with large alite and belite crystals, resulting in a coarse-grained clinker matrix with poor reactivity and poor grindability. Slow cooling can also result in reversion of C₂S from the α' phase to the less reactive β form, or in extreme cases even to the unreactive γ form, and can even allow C₃S to revert to C₂S and CaO. All of these have a negative impact on cement strength.

A further quality problem can arise if there are high levels of MgO in the clinker, because slow cooling allows large periclase crystals to form such that when these hydrate slowly in concrete, the expansion can cause the concrete to rupture.

There are two main types of coolers used in cement clinker production. These are the satellite (or planetary) type and the oscillating grate type. The 1990s saw tremendous advances in clinker cooler technology that greatly improved heat efficiency and potential output from a given kiln system.

Clinker coolers perform the function of:

• Transporting clinker from the kiln to the clinker delivery system;

• Cooling the clinker to a safe temperature for subsequent transport;

• Finalizing the clinker mineralogy through rapid cooling; and

• Preheating combustion air by heat exchange with hot clinker.

Future installments of this overview of “Kiln Burning Systems” will feature information on kiln aerodynamics, considerations in design, kiln burner types, excess air levels, gyro-therm burners, calciner burners, oxygen injection, and thermal considerations.

Con G. Manias is managing director, FCT Group of Companies, Thebarton, South Australia; (+61) 8-8352-9999, www.fctinternational.com

TABLE 1

Summary of Critical Data Information on Different Kiln Systems

Kiln Systems	rpm	tpd/m3	L/D	SFC (kcal/kg)	kWh/t	Residence time, min	Kiln System Exit T, °C	3P, mm H2O	Exit Gas, Nm3/kg clinker
Long Wet	1	0.45-0.8	30-35	1300-1650	17-25	180-240	150-230	150-180	3.4
Long Dry	1	0.5-0.8	30-35	1100-1300	20-30	180-240	380-400	150-200	1.8
Lepol	1.5	1.5-2.2	12-15	950-1200	20-25	30	100-120	250-400	2.0
Cyclone Preheater	2.0	1.5-2.2	14-16	750-900	25	30-40	350	500-700	1.5
Precalciner	3.6	3.5-5.0	10-14	720-850	25	20-30	300-360	500-700	1.4

Design Parameters

When designing a kiln for a certain capacity, or when evaluating an existing kiln for potential output, there are a number of key parameters that must be evaluated. These include:

• Burning zone heat loading
• Secondary air velocity
• Burning zone gas velocity
• Kiln exit gas velocity
• Kiln exit gas temperature
• Preheater tower gas velocities
• Preheater tower pressure drops
• Preheater tower exit gas temperature
• Volatile concentrations
• Material residence time
• Cooler grate loading
• Cooler air supply
• Kiln dust cycles