Cement Clinker Processing Plant

CEMENT PRODUCTION PROCESSES

The two basic methods to produce cement are the wet and dry manufacturing processes. The main difference between wet and dry process is the mix preparation method prior to burning clinker in the kiln. In the wet process water is added to the raw materials to form a raw thick slurry whereas the dry process is based on the preparation of a fine powdered raw meal by raw materials grinding and drying [1, 5]. The choice of the process is mainly based on the nature of the available raw materials [1]. When the moisture content in raw materials is more than 20% (and up to 45%), the wet [12] method is preferred to the dry method [4]. In the past, the wet process was mostly preferred because the homogenization of wet raw materials was easier than that of dry powders. The wet process also enables an easier control of the chemical composition of the raw mix. However, the wet process is more energy intensive and expensive than the dry process as it requires the wet slurry to be evaporated before calcination [1, 4, 6, 8]. The total heat requirement with new dry precalciner kiln systems ranges from 850 to 900 kcal/kg which is approximately 56 to 66% of the energy requirement of old wet process kilns (1300-1600 kcal/kg) [9]. For saving energy and reducing costs, if dry raw materials are available as a basic input, traditional wet process plants are usually converted into dry process plants at the occasions of equipment renewal (plant lifetime in the order of 15-20 years). Semi-wet and semi-dry processes are often used as intermediate steps in the conversion to dry processes cement industry is moving towards more energy-efficient dry process technologies. Over the period 1990 to 2006 the share of dry process kilns in total production capacity increased from some 63% to 80%, while the share of wet process technologies decreased from 16% to 5.6%1 . The quick shift from wet to dry processes is a consequence of the rapid production growth in Asia over the last decade to meet the local demand. To a lesser extent the move is due to the replacement of old equipment in mature countries [6]. In Europe, in 2004 dry process kilns accounted for about 90%, followed by semi-dry/semi-wet technologies (7.5%) and wet process kilns (2.5%). In Europe, new wet cement plants are no longer built.

However, it should be noted that cement production is usually based on locally available raw materials because material transportation costs have a significant impact on final production costs [1, 7], and that wet or semi-wet processes are more suitable for raw materials with high moisture content. When wet raw materials are available locally, an assessment may help find out whether a wet process is economically competitive over a dry process based on materials imported from far regions. On the other hand, wet raw materials are not necessarily usable in dry processes as a significant amount of energy may be needed to reduce moisture content and make the materials ready for a dry kiln system.

Clinker Kiln Systems

The main types of rotary kilns for clinker are discussed here: a) kiln without pre-heater; b) kiln with pre-heater (PH) ; c) kiln with both PH and pre calciner (PC). Kilns with PH are preferred to kilns without PH as they have a lower energy consumption. For this reason, long rotary kilns without PH (long dry kilns) are being replaced over time. Thermal energy requirement is further reduced if a PH kiln is also equipped with a PC.

Cement Clinker Processing Plant

New facilities usually include both PH and PC. A preheater (PH) is a series of vertical cyclones in which the material is passed in counter-flow with exhaust gases counter-flow with exhaust gases from rotary kiln so that heat is transferred from the hot gas to the raw meal, which is therefore preheated and even partially calcinated (30%) before entering the rotary kiln. In the 1970s, a 4-stage cyclone preheater kiln (so-called suspension preheater, SP) was considered the technology of choice for dry and semi-wet processes. However, a number of different SP kilns is available. Most common SP kilns have between 4 and 6 cyclone stages. The number of stages is determined by the moisture content of the raw materials. Where moisture is less than 8.5%, a PH kiln with 4 to 6 stages may be used. The higher the number of cyclone stages, the more the heat recovered. The energy demand of a 6- stage cyclone PH is about 60 MJ/t less than demand of a 5-stage PH, and a 5-stage PH would save approximately 90 MJ/t over a 4-stage PH. The addition of a 4th cyclone stage to a 3-stage PH may decrease the energy needs by 250 MJ/t, but moisture in the raw materials should not exceed 8.5%. If this is the case, a 3-stage cyclone is preferred as the thermal efficiency will not improve when an extra stage is added. The SP unit has a typical unit capacity between 300 and 4000 t/d [1, 5]. In general, a PH tower consists of 1 to 6 cyclone stages, which are disposed one above the other in a tower. The PH kiln performance can be extended using a pre-calcination technology. For the time being, kiln systems with multistage (4 to 6) cyclone preheaters and a precalciner are considered to be the state-of-the-art technology for new dry process plants. Precalciner kilns first appeared in the 1970s. The calciner is a secondary combustion device between the PH and the rotary kiln, where typically some 65% of the total fuel is burnt. In this chamber about 60%-65% of the total kiln emissions are released while limestone (CaCO3) is decomposed into lime (CaO) and carbon dioxides (CO2). The remainder of the emissions is generated from fuel combustion. As calcination is at least 90% completed when the raw meal is fed into the rotary kiln, the PC technique allows a considerable increase in clinker capacity. The average capacity of new European plants ranges from 3000 to 5000 tonnes of clinker per day. From a technical point of view however, capacities of up to 15,000 t per day are feasible. Three PH/PC kilns with a capacity of 10,000t/d are currently in operation in Asia. The addition of a PC also reduces the energy requirements. The PH/PC kiln is the most energy efficient kiln technology. Thermal energy demand for different kilns are listed in Tables 3 and 4. Other kind of kilns includes equipments for semidry and semi-wet processes. For semi-dry processes, the lepol kiln (300 to 2000 t/d) - where a travelling grate preheater is installed outside the rotary kiln - requires less thermal energy than a long dry kiln (300-2800 t/d).

In semi-wet processes, a filter cake is produced from raw materials handling. This cake is either extruded to pellets prior to being fed to the lepol kiln or loaded into a cyclone SP/PC kiln after being dried to a raw meal in an external drier. This latter system offers both the lowest heat consumption and the highest clinker capacity (2000-5000 t/d compared to 300-3000 t/d). If wet raw materials preparation is required, a 2 stage PC with dryer (2000-5000 t cli/d) can provide the lowest thermal energy consumption. The wet slurry is first dried in an integral dryer crusher, after which it is fed to the PH-PC kiln. This modern process is replacing the conventional method which comprises the long wet rotary kiln (300- 3600 t/d) with an internal drying/preheating system [1, 5]. An emerging technique is the fluidised bed cement kiln (FB). In Japan two pilot plants with a capacity of 20 t/d and 200 t/d has been in operation since 1989 and 1996 respectively. In China a pilot kiln with a capacity of 1000 t/d is now under construction.

Clinker Cooling

After sintering the hot clinker is cooled by air in the clinker cooler. There are three main types of cooling technologies, namely the rotary (tube), planetary (satellite) and the grate cooler. The latter is preferred because of key advantages. The grate cooler is suited to large clinker capacities (up to 12,000 t of clinker per day). Secondly, the amount of heated cooling air, which is re-circulated back to the PH kiln ("secondary air") or to the PC kiln ("tertiary air"), is higher and the grate cooler is an efficient heat recovery system that reach heat recovery efficiency of 70%-75%. Third, grate coolers allows for lower clinker temperatures (60-80°C compared to 120-200°C) because they use an excess of cooling air, as the amount of air for cooling is larger than that needed for secondary combustion. In the grate coolers the clinker moves slowly on a travelling or on a reciprocating grate through the cooling zone which is divided into a recuperation and an aftercooling zone. Exhaust air from the aftercooling zone is either used for drying purposes, e.g. raw materials, mineral additives or coal, or leaves the system as waste air after de-dusting. In the 1980s, travelling (or 2nd generation) grate coolers were abandoned in favour of the reciprocating grate coolers because of superior heat recovery. Modern reciprocating (3rd generation) grate coolers were introduced in 1983 to use less cooling air than the conventional devices (800-1700 Nm3 /t of clinker instead of 2000 Nm3 /t). Electricity consumption of the modern grate cooler ranges from 4 to 8 kWh/t of clinker. The economical lifetime is estimated at more than 10 years [1, 2, 5]. Rotary coolers are seldom used and planetary coolers cannot be used with PC kilns as they make it difficult to extract tertiary air for combustion.„ Cement grinding - The grinding of clinker with additives to produce cement requires only electricity (no heat) and accounts for about 38% of total electricity use [2]. The choice of grinding system is mainly determined by the cement type to be produced. Currently, vertical roller mills (for high mineral additions) and high pressure grinding rolls (for limited mineral additions) are state-ofthe-art technologies, as they have the highest electrical efficiency (50.5 and 445 kWh/t of cement) .

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