Limestone (primary source of calcium carbonate CaCO3) and clay (primary source of silica SiO2, alumina Al2O3 and iron oxide Fe2O3) are typically mined in company-owned quarries and pre-blended to a target chemical material composition. Corrective materials like sand, iron ore, bauxite or industrial waste materials (alternative raw materials) are then used to fine-tune and correct the chemical composition of this pre-blend material in the raw mill. Fine ground raw meal is then stored and further homogenized in the raw meal silo.
A kiln system consists of a static preheating system with typically five cyclone stages and a pre-calciner. The raw meal (so-called kiln feed) is heated up to 1,000C and the calcium carbonate portion is calcined: CaCO3-> CaO + CO2 (g). In the attached rotary kiln, the material is then heated up to 1,450C and Portland cement clinker is formed.
Fast cooling and freezing of these artificial minerals then happens in the clinker cooler, the third element of a cement kiln system. A crucial ingredient to make this mineral transformation happen is the fuel. Historically oil and gas were used, then coal and petcoke, but since the 1980s more and more alternative fuels are employed for heating. Here it must be highlighted that the fuel ash is incorporated fully in the clinker so the final clinker composition is always the sum of the calcined raw meal and the remaining fuel ash after burn-out of the fuel. Clinker is the important intermediate product stage in cement manufacturing and is also traded worldwide as a commodity between cement producers.
Gypsum is added to control cement setting, i.e. the hardening process. Mineral components are added to reduce the clinker portion in cements and introduce special product properties. Pure Ordinary Portland Cements (OPC) contain only clinker and gypsum; blended cements are the ones with mineral components.
Cement is a highly consumed material over the world for constructional purposes. Different types and brands of cement products are available in the market. Sand and water are also used, where cement is used as a construction material.
A material similar to what is now called cement was first discovered by Joseph Aspidine in 1756. This was called 'Portland cement' because its similarity to the stones found in the village of Portland.
The calcareous and argillaceous raw materials are dried, mixed and heated to a high temperature (around 10000C) in a furnace. The produced granular form is called clinker. Then it is ground with 4% gypsum to obtain Portland cement. There are two main processes in manufacturing Portland cement.
Then, mixture is heated in an inclined rotary furnace ( inclined angle is 15). This rotary furnace is rotated slowly at about 0.5 turn per minute. Furnace is made of large cylindrical steel tube.It is lined with refractory bricks of either high alumina or high magnesia.
The raw cement ingredients needed for cement production are limestone (calcium), sand and clay (silicon, aluminum, iron), shale, fly ash, mill scale and bauxite. The ore rocks are quarried and crushed to smaller pieces of about 6 inches. Secondary crushers or hammer mills then reduce them to even smaller size of 3 inches. After that, the ingredients are prepared for pyroprocessing.
The crushed raw ingredients are made ready for the cement making process in the kiln by combining them with additives and grinding them to ensure a fine homogenous mixture. The composition of cement is proportioned here depending on the desired properties of the cement. Generally, limestone is 80% and remaining 20% is the clay. In the cement plant, the raw mix is dried (moisture content reduced to less than 1%); heavy wheel type rollers and rotating tables blend the raw mix and then the roller crushes it to a fine powder to be stored in silos and fed to the kiln.
A pre-heating chamber consists of a series of cyclones that utilizes the hot gases produced from the kiln in order to reduce energy consumption and make the cement making process more environment-friendly. The raw materials are passed through here and turned into oxides to be burned in the kiln.
The kiln phase is the principal stage of the cement production process. Here, clinker is produced from the raw mix through a series of chemical reactions between calcium and silicon dioxide compounds. Though the process is complex, the events of the clinker production can be written in the following sequence:
The kiln is angled by 3 degrees to the horizontal to allow the material to pass through it, over a period of 20 to 30 minutes. By the time the raw-mix reaches the lower part of the kiln, clinker forms and comes out of the kiln in marble-sized nodules.
After exiting the kiln, the clinker is rapidly cooled down from 2000C to 100C-200C by passing air over it. At this stage, different additives are combined with the clinker to be ground in order to produce the final product, cement. Gypsum, added to and ground with clinker, regulates the setting time and gives the most important property of cement, compressive strength. It also prevents agglomeration and coating of the powder at the surface of balls and mill wall. Some organic substances, such as Triethanolamine (used at 0.1 wt.%), are added as grinding aids to avoid powder agglomeration. Other additives sometimes used are ethylene glycol, oleic acid and dodecyl-benzene sulphonate.
The heat produced by the clinker is circulated back to the kiln to save energy. The last stage of making cement is the final grinding process. In the cement plant, there are rotating drums fitted with steel balls. Clinker, after being cooled, is transferred to these rotating drums and ground into such a fine powder that each pound of it contains 150 billion grains. This powder is the final product, cement.
Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains or ships, and only a small amount is packed for customers who need small quantities.
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Cement production has undergone tremendous developments since its beginnings some 2,000years ago. While the use of cement in concrete has a very long history (Malinowsky, 1991), the industrial production of cements started in the middle of the 19 th century, first with shaft kilns, which were later replaced by rotary kilns as standard equipment worldwide. Today s annual global cement production has reached 2.8 billion tonnes, and is expected to increase to some 4 billion tonnes per year in 2050 (Schneider et al., 2011). Major growth is foreseen in countries such as China and India as well as in regions like the Middle East and Northern Africa. At the same time, the cement industry is facing challenges such as cost increases in energy supply (Lund, 2007), requirements to reduce CO2 emissions, and the supply of raw materials in sufficient qualities and amounts (WBCSD, 2008).
In this chapter, the environmental impact of cement production will be described. The chapter will first focus on the most common cementitious product: ordinary Portland cement (OPC) and will then evaluate the main perspective in terms of reduction of cement production s environmental impacts. To do so, we will describe the improvement perspective in the cement sector as well as the alternative products that could, at least partially, replace OPC.
Cement production is a thermal energy intensive process, which requires heating solid particles up to 1450C and cooling it down. The process generates hot and CO2 rich exhaust streams. Fin order to study energy efficiency of the process, authors like Mujumdar et al. (2006, 2007) developed detail models of process units, while others highlighted energy and exergy performances of plants (Kolip et al 2010). Some authors have studied the integration of cogeneration units in cement industry as an option of waste heat recovery (Wang et al 2008). The previously mentioned studies are considering, in detail, thermodynamic performances, but they are not investigating the heat integration inside the process. Some other authors, such as Murray et al (2008), Kaantee (2004), Mokrzycki (2003) have focused their analysis in the use of alternative/waste fuels, but they did not show the impact and the potential of these fuels on the heat integration between streams.
This paper explores the use of process integration techniques to improve the energy efficiency of cement plants, focusing on the dry route cement production and the integration of alternative fuels. Flow sheeting modeling, Pinch Analysis and mixed integer linear optimization techniques are applied to study an existing cement production facility.
Cement production is one of the largest industries in the world. Annual world production in 2013 was approximately 4 GT (of which, about half was in China). It is produced in kilns at around 1400oC (2500oF), and approximately 750 kg (1650 lb) of CO2 are released for each tonne (2205 lb.) that is made. This is 400 m3 (524 yd3) of gas; and CO2 normally forms only 0.04% of air. Cement production is responsible for around 7% of worldwide greenhouse gas emissions. The only larger contributors are electricity generation and transport.
Most of the CO2 released when cement is made comes from the chemical reaction. The limestone from which it is made is calcium carbonate, and when this is heated it gives off carbon dioxide, and forms calcium oxide. Thus, while some benefit has been obtained by making the process more energy efficient, there is little scope for any further reductions.
Numerous different solutions to this problem have been proposed, including carbon capture and inventing completely new cements; but the only effective method currently used to achieve significant reductions in greenhouse gas emissions is the use of cement replacements.
One process that has not been extensively investigated is carbon sequestration. In this process, the hydrated cement reacts with CO2 in the air, slowly reversing some of the processes that took place in the kiln when the cement was made (this is the carbonation process; it also causes reinforcement corrosion, see Section 25.3.2). It is estimated that this may reduce the carbon footprint of the cement industry by 35%.
Cement production is an energy-intensive process. The cost of energy constitutes more than 60% of the cost of the cement; hence cement plants have to consider minimizing the cost of energy when planning production. However, there are several challenging issues regarding the production plan. First, there are some operational constraints for the production process itself, such as keeping a proper level of inventory. Second, electricity prices are not fixed; they change on an hourly basis. Hence the cost of the electricity consumed may change substantially depending on the time of production. In this study, we developed a mixed integer programming model to minimize the cost of production (including energy, labor, and storage) by shifting working times to hours with low energy costs and trying to maintain activities during hours with high energy costs.
Cement production processes can be categorized as dry, semidry, semiwet, and wet processes depending on the handling of raw material before being fed to the rotary kiln. Nowadays, almost all new plants are based on the dry process and many old wet plants are also remodeled to dry or semidry processes. Dry cement manufacture has three fundamental stages: preparation of feedstocks, production of clinker, and preparation of cement [15,16].
Preparation of feedstock. This stage includes the process of siege, crushing, and prehomogenization. Typical raw materials used for cement production have 85% cayenne, 13% clay or blackboard, and under 1% each of materials such as silica, alumina, and iron ore. These feedstocks are crushed into particles with a diameter of less than 20mm and mixed with a prehomogenization pile .
Clinker production. As shown in Fig.6.6, the feedstock first enters the raw material to make a fine powder (raw feed), where 85% of the material is slightly smaller than 88m. Then, the feed is transferred to the homogenizing silos to impair the material difference. After that,the meal is thrown into the precalciner tower to start the chemical change to cement. Precyclone towers intermix with raw feed and almost 1000C of exhaust gas to recover energy, preheat feed, and initiate chemical reactions that result in cement. Gases straight from the kiln, but in precalciner facilities, gasoline, and air are provided by a combustion vessel inside the tower and kiln. Typically, 60% of burned calciner and more than 90% calcination have been reached before the material enters the rotary kiln. Inside the kiln, temperatures reach approximately 1400C to complete the process of chemical reactions and produce calcium silicates, called clinkers, with a diameter of 1025mm. The gas from the preheater tower is usually blended in a rawmill, which will help stabilize the future feedstock. After flowing through the rawmill, the gases are eventually released by a dust collector, which also obtains good particles when feedstuffs are milled. The dust will then be recycled into homogenizing silos and served as part of the kiln feed.
Preparation of cement. This stage completes the manufacturing process where clinker nodules are milled into cement. Following clinker milling, the cement is ready for use as a binder in various concrete mixes.
The cement production process, for example, starts with mining of limestone, which is then crushed and ground to powder. It is then preheated to save energy before being transferred to the kiln, the heart of the process. The kiln is then heated to a high temperature of up to 1480 degrees to convert the material to a molten form called clinker. The clinker is then cooled and ground to a fine powder with other additives and transferred to storage silos for bagging or bulk transportation (Portland Cement Association, 2014). The production of cement is either through the wet or dry process with the dry process as the preferred option because of the lower energy intensity. Cement production accounts for about 5% of total anthropogenic emissions (IFC, 2017). Cement-based structures constitute the largest surface area of all man-made structures (Odigure, 2009). World cement demand was about 2.283 billion tons in 2005, 2035 million tons in 2007, and 2836 million tons in 2010 with an annual estimated increase of about 130 million tons (Madlool, Saidur, Hossaina & Rahim, 2011; Odigure, 2009). World total cement production for 2016 was about 4.2 billion tons with emerging markets playing a dominant role (IFC, 2017). The energy intensity of cement production ranges from 3.6 to 6.5 GJ/ton depending on production process and location of the production (Hammond and Jones, 2011; Ohunakin et al., 2013; Worrell et al., 2000).
Cement production is one of the largest contributors to CO2 emissions. SCMs have been partially or completely used as replacement of cement or fine aggregates in construction to reduce the demand of cement and corresponding CO2 emissions (Al-Harthy etal., 2003; Babu and Kumar, 2000; Bondar and Coakley, 2014; Cheng etal., 2005; Jia, 2012; Khan and Siddique, 2011; Kunal etal., 2012; Limbachiya and Roberts, 2004; Lothenbach etal., 2011; Maslehuddin etal., 2009; Najim etal., 2014; Nochaiya etal., 2010; Siddique, 2011; Siddique and Bennacer, 2012; Toutanji etal., 2004). Some of the established SCMs are fly ash, silica fume, blast furnace slag, steel slag, etc. Pozzolanic materials, such as fly ash, steel slag, and cement kiln dust (CKD) when used as replacement to cement, improve the long-term performance of concrete as the pozzolanic reaction takes time. But, the early age strength of SCMs is a concern, as the reduction in cement content causes lesser hydration and, consequently, lesser formation of CSH gel (Lothenbach etal., 2011). The problem of low early strength of SCMs can be solved by using carbonation curing at early ages.
Apart from CO2 sequestration, carbonation curing has also been found to act as an activation mechanism for SCMs (Monkman etal., 2018). Many studies have tried to assess the effect of ACC on use of SCMs (Monkman and Shao, 2006; Sharma and Goyal, 2018; Zhan etal., 2016; Zhang etal., 2016; Zhang and Shao, 2018). ACC not only enhances the hydration degree of alternative cementitious materials but also improves the early age performance of concrete. Monkman and Shao (2006) assessed the carbonation behavior of blast furnace slag, fly ash, electric arc furnace (EAF) slag, and lime. All four materials reacted differently when subjected to carbonation curing of 2h. Fly ash and lime showed highest degrees of carbonation, followed by EAF slag, whereas ground granulated blast slag (GGBS) showed least reactivity towards CO2. Calcite was the major reaction product from fly ash, lime, and EAF slag, whereas aragonite was produced by carbonation of GGBS. Sharma and Goyal (2018) studied the effect of ACC on cement mortars made with CKD as cement replacement. ACC was found to improve the early age strength of cement mortars by 20%, even for mortars with higher CKD content. Several studies tried to assess the CO2 sequestration ability of steel slag binders (Bonenfant etal., 2008; He etal., 2013; Huijgen etal., 2005; Huijgen and Comans, 2006; Ukwattage etal., 2017). Presence of C2S component in steel slag makes it a potential cementitious material that could act as a carbon sink for CO2 sequestration (Johnson etal., 2003).
Zhang etal. (2016) in their study found that fly ash concrete was more reactive to CO2 as compared with OPC concrete. With the reduction in OPC content, a porous microstructure was generated due to insufficient hydration reaction. The enlarged distance between cement grains facilitated higher possibility of reaction with CO2, and hence, a higher degree of CO2 sequestration. The performance of SCMs subjected to carbonation curing is majorly dependent upon fineness of material and water content postcarbonation. Finer particle size of SCMs provides a higher specific area for effective carbonation reaction. Due to this, it was observed in many studies that concrete made with SCMs had better reactivity toward CO2 than OPC (Monkman and Shao, 2006). Water content postcarbonation also plays a dominant role in determining the performance of SCMs. Sufficient water content postcarbonation is necessary for complete hydration and pozzolanic reaction of SCMs (Monkman and Shao, 2006).
In Europe, cement production decreased by 26.9% from 1990 to 2012, whereas CO2 emissions decreased by 38.6%, showing an improvement in the cement production (CEMBUREAU, 2014). However, to reach the objectives of various sustainability programs, further efforts must be made in order to improve every step in the concrete production line. For concrete, the main solutions for reducing the environmental impact of modern construction are (Flatt et al., 2012):
Some other technical solutions to improve cement production are also of interest, including CO2 capture and storage. If this solution were to present a major impact on the final objective, it would need technological breakthroughs that are still in the research phase and are not planned to be ready for users before 2030, but they could potentially capture up to 45% of the CO2 produced by cement (IEA and WBCSD, 2010). By 2050 the baseline emissions will be 2.34 gigatonne (Gt) but will be reduced to blue emissions of 1.55Gt by the contribution of energy efficiency (10%), use of alternative fuels and other fuel switching (24%), clinker substitution (10%) and carbon capture and storage (56%) (IEA and WBCSD, 2010).
The worldwide use of blended cement in the production of cement is already a commonly used improvement, with significant investments for research made by cement producers (Table 15.7). Many blended cement types exist because of the great variability of usable SCMs. These cement types are produced by replacing part of clinker with SCMs, which leads to cement with unchanged or improved properties for both general and special applications. The same properties must be maintained; otherwise, for a functional unit (concrete beam) and on a product scale (cement), more cement may be necessary to obtain the same durability. Therefore the beneficial impact of replacing a part of clinker can be useless (Li et al., 2004). In the European context, binary cement incorporates up to 35% of each type of SCM even in the case of inert mineral fillers, and ternary cement uses up to 80% in mass replacement of clinker (EN 197-1, 2001). A final trend is the development of quaternary binder, but a lot of improvements remain necessary in order to ensure a better understanding of the interactions between clinker and SCMs.
As the CO2 resulting from the decarbonation of limestone during the calcination process is a fixed amount by clinker volume, the two major solutions are optimisation of the heating process to reduce the energy needed to reach 1450C and the use of blended cements.
The improvement of a specific burning process leads to no or very little change in the reduction of CO2 emissions, but a change from a wet process to a dry process with preheating and precalciner kilns can lead to significant improvements (WBCSD, 2013). The energy used by a wet kiln is estimated to be between 5.9 and 6.7GJ/tonne of clinker, whereas a kiln with preheating and precalciner kilns uses only 2.9 to 3.3GJ/tonne of clinker (reduction by 50%). Shifting the cement production from wet to dry with preheating and precalciner kilns can lead to a reduction of 20% (International Energy Agency, 2010) of the energy needed and 17% in the amount of CO2 emitted per tonne of clinker (Damtoft et al., 2008). It can be seen that the increase in part of the clinker produced by this technology could partially overcome the increase in cement production (WBCSD, 2013). These savings could be increased by the use of more alternative fuels (Nielsen and Glavind, 2007) (to coal, which actually accounts for 60% of the fuel used in cement production).
The use of SCMs in concrete is different from using them to produce blended cements. There is no guarantee on the potential strength achieved by a mixture of cement and SCM because of the great variability in the physicochemical properties of SCM. In order to control the amount used in concrete, use of SCM has been standardised through the k-value concept (Smith, 1967). This concept is based on the potential reactivity of each SCM, which helps in fixing a maximal replacement rate of cement to achieve the same mechanical and durability properties. The k-value for each addition differs depending on the type, on the concrete exposure conditions (frost, salt, sulphate, etc.)and on the local national standards. Depending on the type of SCM, the volume used and the targeted concrete strength, the savings in terms of CO2 emissions can be more than 20% (Table 15.9).
The previous solutions for improving the environmental aspect of concrete are widely used and remain of interest for further development, and some new, challenging solutions are also being studied but will need either breakthrough technology or huge investments in development. So far emissions of CO2 are inherent to cement production, so finding ways to prevent this gas from getting into the atmosphere need to be explored, as illustrated next.
Carbon capture and sequestration (CCS): This process consists basically of capturing the CO2 before it is released into the atmosphere and then compress it and store it underground (in mines, caves, oceans). Unfortunately, currently this process remains quite expensive and energy-consuming. Nevertheless, it is a new technology which can be improved in the future. On the other hand, it can also be argued that far from dealing with the problem, CCS is just a way to avoid the problem and leave it for later. Therefore it is totally in contradiction with sustainable development, which is defined by the Brundtland report (United Nations, 1987) as a development that meets the needs of the present without compromising the ability of future generations to meet their own needs.
Biofuel production: Although this approach is not yet totally explored, some researches have shown that captured carbon can be used in the production of algae that can then be transformed to biofuel, agricultural fertilizer or even animal protein (Potgieter, 2012).
Electrochemical carbon reduction (ECR): This process involves creating a reaction which will transform carbon into formic acid. This product is often used in the pharmaceutical industry. It is usually manufactured with high energy consumption but ERC requires less energy (Potgieter, 2012).
The first approach will be to reduce the volume of CO2 in the production of cement which is emitted into the atmosphere. But the most interesting solution in the short-term will be the development of alternative binders with a lower environmental impact, and some proposed solutions already provide excellent results on a laboratory scale or event in pilot projects (Aldred and Day, 2012; Duxson and Provis, 2008; Owens et al., 2010). Alkali-activated cements are aluminosilicatepozzolana-based materials (glass furnace slag, natural volcanic glass, manmade glass, fly ash, metakaolin). The activation occurs between the alumina-rich pozzolana and a strong alkali base, which dissolves the silicate and aluminate groups to form a cementitious gel to form the structure of the matrix. The so-called geopolymer is proven to have similar mechanical properties as cement-based concrete with a lower environmental impact (Duxson et al., 2007). This solution is already implemented in Australia by using fly ash and slag (Duxson and Provis, 2008). This solution is, however, hindered by some practical aspects, including the robustness of the design of the mix, the heat needed for a proper reaction to occur in a reasonable amount of time, the limited amount of available aluminosilicate compounds, the handling of the alkaline activator on the job site, the cost, and the environmental impact of the activator (Flatt et al., 2012).
Belite-rich cement, with a combination of calcium sulphoaluminate and calcium sulphoferrite, have been tested with success. Compared to ordinary Portland cement, belite-rich cement contains relatively higher belite (C2S) and lower alite (C3S) contents (Chatterjee, 1996). Belite-rich cement produces performances comparable to cement with the production of 2030% less CO2 (Li et al., 2007).
Some specially designed concretes have been developed with the eco-efficiency of products at a structural level in mind. This is particularly the case of HPC, including SCC (Hossain et al., 2013; Sonebi and Bartos, 1999, 2002). First, SCC will not act directly on the environmental aspect of sustainability, but rather on economical and societal aspects. When properly designed from the beginning, a construction made of SCC brings economic benefits by increasing the productivity because of the higher casting rate and the reduction in manpower which results in the elimination of vibration. This latter aspect also has a major societal aspect because it reduces the noise at construction sites and concrete factories and removes the risk of injury to workers related to crowded construction sites and vibration (Nielsen, 2007). The use of SCC in concrete factories (Yahia et al., 2011) can also be a solution for increasing mould service life and saving energy. In the case of concrete pipes, this reduction can be about 1.0GWh (De Schutter et al., 2010). Note also the reduced need for post-treatment of a surface by plaster, which accounts for 0.57kg of equivalent CO2 per m2, or by paint because of the better finish of concrete surfaces (Witkowski, 2015). This latter aspect can also increase the CO2 uptake by concrete carbonation because the concrete surface has no applied post-treatment. The major issue of SCC is that proportioning of a mix involves a significant amount of cementitious material in order to improve the workability of the concrete. This problem can be overcome with the development of high-performance chemical admixtures, use of high SCM content (Diederich et al., 2013), and an adequate selection of the granular skeleton of solid particles to achieve proper rheological properties of concrete. The latter approach has allowed the production of Eco-SCC with 40% fewer CO2 emissions than standard vibrated concrete, along with a 150-year service life (Mansour et al., 2013).
The development of high-strength concrete and high-strength, fibre-reinforced concrete is also a new approach to sustainability in construction materials. These concrete types are designed with a relatively greater amount of binder (cement and SCM) and chemical admixture, which achieves better performance than conventional concrete, including 316 times more compressive strength, 10 times more flexural strength and 10100 times more durability than conventional concrete does (Wang et al., 2015). These high performances allow a reduction in the size of elements for similar structural performance, therefore leading to a lower volume of concrete for the same structure. This reduction in volume can lead to a reduction of 65% in raw materials consumed, 51% in the primary energy used, and 47% in CO2 emissions (Batoz and Rivallain, 2009).
Currently in the United Kingdom, concrete debris is not sent to landfills but is treated so it can have a second life in the construction sector. Indeed, concrete is a material that can be fully recycled. In 2011 recycled and secondary aggregates represented about 5.3% of all aggregates used in concrete (Concrete Centre, 2011). This material is used mainly in road construction, but recycled concrete aggregates (RCA) can count for up to 30% of the aggregates in a concrete mix. In the United Kingdom recycled and secondary materials represent 28% of the total aggregates used in the marketplace, the highest in the Europe. The precast concrete industry provides greater opportunities for using recycled aggregates over to 20% (Concrete Centre, 2011). However, the following issues need to be considered when using RCA:
CO2 emissions from cement production are incurred through the consumption of fossil fuels, the use of electricity, and the chemical decomposition of limestone during clinkerization, which can take place at around 1400C. The decarbonation of limestone to give the calcium required to form silicates and aluminates in clinker releases roughly 0.53t CO2 per ton of clinker . In 2005, cement production (total cementitious sales including ordinary Portland cement (OPC) and OPC blends) had an average emission intensity of 0.89 with a range of 0.650.92t CO2 per ton of cement binder . Therefore, the decarbonation of limestone contributes about 60% of the carbon emissions of Portland cement, with the remaining 40% attributed to energy consumption, most of which is related to clinker kiln operations; the WWF-Lafarge Conservation Partnership  estimated that the production of clinker is responsible for over 90% of total cement production emissions.
In view of the fact that the requirement for decarbonation of limestone presents a lower limit on CO2 emissions in clinker production, and that there exist technical issues associated with the addition of supplementary cementitious materials (SCMs, including fly ash and ground granulated blast furnace slag), which restrict the viability of direct Portland cement supplementation by SCM above certain limits, the possibility to reduce CO2 emissions using Portland chemistry is limited. The WWF-Lafarge Conservation Partnership  expects that the emissions intensity of cement, including SCM, could be reduced to 0.70t CO2 per ton of cement by 2030, which still amounts to around 2 billion tons of CO2 per annum worldwide, even if cement production does not increase from its current level.
Fig. 10.6 shows the CO2 emissions of various binder designs as a function of Portland cement content. There have been a limited number of life-cycle analyses (LCA) of geopolymer technology. One reasonably extensive research program carried out in Germany  has provided information regarding the selection of precursors and mix designs for a range of geopolymer-based materials. However, geographic specificity plays a significant role in a full LCA, so there is the need for further studies considering different locations in addition to a wider range of mix designs spanning the broader spectrum of geopolymers. The main carbon-intensive and also the most expensive ingredient in geopolymer cement is the alkali activator, which should be minimized in mix design. McLellan et al.  provided further detail, while Habert et al.  concluded that geopolymer cement does not offer any reduction in carbon emissions; such a conclusion needs to be drawn with caution.
Sodium carbonate is the usual Na source for the production of sodium silicate. The different processes for conversion of Na2CO3 (or NaOH) and SiO2 to sodium silicate, via either furnace or hydrothermal routes, differ by a factor of 23 in CO2 emissions, and up to a factor of 800 in other emissions categories . It is therefore essential to state which of these processes is used as the basis of any LCA. Moreover, the best available data for emissions due to sodium silicate production were published in the mid-1990s , so improvements in emissions since that time have not been considered. Sodium carbonate itself can be produced via two main routes, which vary greatly in terms of CO2 emissions. The Solvay process, which converts CaCO3 and NaCl to Na2CO3 and CaCl2, has emissions between 2 and 4t CO2 per ton of Na2CO3, depending on the energy source used. Conversely, the mining and thermal treatment of trona for conversion to Na2CO3 has emissions of around 0.14t CO2 per ton of Na2CO3 produced plus a similar level of emissions attributed to the electricity used. This indicates an overall factor of 510 difference in emissions between the two sources of Na2CO3 .
A commercial LCA was conducted by the NetBalance Foundation, Australia, on Zeobonds E-Crete geopolymer cement, as reported in the Factor Five report published by the Club of Rome . This LCA compared the geopolymer binder to the standard Portland blended cement available in Australia in 2007 on the basis of both binder-to-binder comparison and concrete-to-concrete comparison. The binder-to-binder comparison showed an 80% reduction in CO2 emissions, whereas the comparison on a concrete-to-concrete basis showed slightly greater than 60% savings, as the energy cost of aggregate production and transport was identical for the two materials. However, this study was again specific to a single location and a specific product, and it will be necessary to conduct further analyses of new products as they reach development and marketing stages internationally. Fig. 10.6 shows a comparison of the CO2 emissions of four different E-Crete products against the Business as Usual, Best Practice 2011, and a Stretch/Aspirational target for OPC blends. It is noted that in some parts of the world (particularly Europe), some of the blends shown here in the Stretch/Aspirational category are in relatively common use for specific applications, particularly CEM III-type Portland cement/slag blends, but this is neither achievable on a routine scale worldwide at present, nor across the full range of applications in which Portland cement is used in large volumes.
Environmental impact of cement production is calculated based on the data provided by Ecoinvent (2012). Detailed information on the production process and on all inputs can be taken from Knniger et al. (2001). The functional unit is the production of 1kg of Portland cement strength class 42.5 (CEM I 42.5 R). Average fuel for clinker production is composed of 6.81103MJ natural gas (high pressure), 3.74104kg light fuel oil, 2.55102kg heavy fuel oil, 3.54102kg hard coal and 3.91103kg petroleum coke. In addition 5.80102 kWh electricity (medium voltage) is considered. Besides 0.912kg of Portland cement clinker, an input of 0.063kg gypsum (not balanced, origin from flue gas desulphurization), 0.025kg additional milling substances (not balanced as it is taken as waste without environmental burdens, e.g. dust from the cement rotary kiln, fly ash, silica dust, limestone) and 3.50104kg ethylene glycol (process material for grinding) is taken into account. A total energy for grinding and packing of 4.85102 kWh (electricity, medium voltage) as well as transport processes corresponding to 4.40103 tkm (lorry, 16t) is necessary. The production of 1kg Portland cement CEM I 42.5R yields an environmental impact of 0.833kg CO2-eq. (GWP100), 2.241108kg CFC-11-eq (ODP), 4.211105kg C2H4-eq. (POCP), 1.138103kg SO2-eq (AP) and 1.702104kg PO43-eq. (NP).
The cement industry is one of the most intensive energy consumers in the industrial sectors. The energy consumption represents 40% to 60% of production cost. Additionally, the cement industry contributes around 5% to 8% of all man-made CO2 emissions. Physiochemical and thermochemical reactions involved in cement kilns are still not well understood because of their complexity. The reactions have a decisive influence on energy consumption, environmental degradation, and the cost of cement production. There are technical difficulties in achieving direct measurements of critical process variables in kiln systems. Furthermore, process simulation is used for design, development, analysis, and optimization of processes, when experimental tests are difficult to conduct. Moreover, there are several models for the purpose of studying the use of alternative fuels, cement clinker burning process, phase chemistry, and physical parameters. Nonetheless, most of them do not address real inefficiency taking place in the processes, equipment, and the overall system. This paper presents parametric study results of the four-stage preheater dry Rotary Kiln System (RKS) with a planetary cooler. The RKS at the Mbeya Cement Company (MCC) in Tanzania is used as a case study. The study investigated the effects of varying the RKS parameters against system behaviour, process operation, environment, and energy consumptions. Necessary data for the modelling of the RKS at the MCC plant were obtained either by daily operational measurements or laboratory analyses. The steady-state simulation model of the RKS was carried out through the Aspen Plus software. The simulation results were successfully validated using real operating data. Predictions from parametric studies suggest that monitoring and regulating exhaust gases could improve combustion efficiency, which, in turn, leads to conserving fuels and lowering production costs. Composition of exhaust gases also depends both on the type of fuel used and the amount of combustion air. The volume of exit flue gases depends on the amount of combustion air and infiltrating air in the RKS. The results obtained from the study suggest a potential of coal saving at a minimum of about , which approximates to 76,126 tons per year at the current kiln feed of 58,000kgh-1. Thus, this translates to a specific energy saving of about 1849.12kJkgcl-1, with relatively higher clinker throughput. In this vein, process modelling provides effective, safe, and economical ways for assessing the performance of the RKS.
There are several process parameters in a cement rotary kiln system, which should be studied in order to observe trends that may indicate problems and provide necessary mean data for process analysis. The most important kiln controlling parameters are clinker production rate, fuel flow rate, specific heat consumption, secondary air temperature, kiln feed-end temperature, preheater exhaust gas temperature, ID fan pressure drop, kiln feed-end percentage oxygen, percentage downcomer oxygen, primary air flow rate, specific kiln volume loading, specific heat loading of burning zone cross-section area, and cooler air flow rate including temperature, pressure, and oxygen profile of the preheater . However, the principal control variables are burning zone solid material temperature typically aimed at ; feed-end gas temperature typical at ; and feed-end oxygen typical at 2% . Control is managed by adjustments of kiln feed, fuel flow rate, and ID fan speed .
A process simulation software is used for the description of different processes in flow diagrams. The objectives of simulation models are to deliver a comprehensive report of material and energy streams, determine the correlation between the reaction and separation systems, study how to eliminate wastes and prevent environmental pollution, evaluate plant flexibility to changes in feedstock or product policy, investigate the formation and separation of by-products and impurities, optimize the economic performance of the plant, validate the process instrumentation, and enhance process safety and control.
Cement production processes involve complex chemical and physical reactions during the conversion of raw materials to the final product. Moreover, the clinker burning process, which has a decisive influence on energy consumption and the cost of cement production, involves the combustion reaction of fossil fuel and a complex heat exchange between solids from raw materials and hot combustion gases [2, 3]. It also involves mixing, as well as separation of solid and fluids at various compositions, temperatures, and pressures. Therefore, following these complex issues which contribute to the inefficient energy use and emissions in cement kiln systems, there is a strong need to use computer-aided modelling to simplify the work of analyses. Other studies have tried to vary fuel properties, primary- and secondary air settings, and fuel feed location to study the effect of the operational setting on refuse-derived fuel, where the results show a good applicability of the presented modelling procedure .
Cement manufacturing is a high volume and energy intensive process, and according to the authors in [6, 7], the price of consuming large amounts of nonrenewable resources and energy (principally thermal fuels and electrical power) in those plants contributes to about 40% to 60% of the total manufacturing cost. In addition, the cement plants are also intensive in terms of CO2 and other effluent emissions. For that reason, sustainability can be viewed as a broad and complex concept in the cement industry sector, as it includes a variety of key issues, such as (i) efficiency of resource and energy use, (ii) reduced emissions, (iii) health and safety protection, and (iv) competitiveness and profitability, which are essential for its economic survival and social acceptance .
The term cement includes a range of substances utilized as binders or adhesives, even though the cement produced in the greatest volume and most widely used in concrete for construction is Portland cement. Cement plants basically consist of three manufacturing parts: (i) raw material and fuel supply preparation, (ii) clinker production (commonly named as the pyroprocessing part), and (iii) intergrinding and blending of cement clinker with other active ingredients to produce the required types of cement.
The cement manufacturing process starts by handling a mix of raw materials: (i) naturally occurring limestone, which is the source of calcium, (ii) clay minerals and (iii) sand, which are the sources of silicon and aluminium, and (iv) iron-containing components. The raw materials are ground and mixed together in controlled proportions to form a homogeneous blend, termed as a raw meal or raw-mix, with the required chemical composition.
Raw meal is then subjected to the continuous, high temperature operations in the pyroprocessing part of the plant, namely the rotary kiln system (RKS). The progressive increase of temperature along RKS initiates a series of consecutive reactions of raw meal, ranging from the evaporation of free water to the decomposition of raw materials and the combination of lime and clay oxides. This means that raw meal passes through a series of functional zones where it is dried, preheated, calcined, and sintered to produce clinker minerals, which, in turn, form the semifused pellets of cement clinker. Regarding the type of pyroprocessing employed in RKS, the overall technology for cement production can be roughly divided into (i) the dry process, (ii) the wet process and its modification, (iii) the semidry process, and (iv) the semiwet process. Each of the enumerated processes are characterized by different raw material preparations and different configurations of RKS, and in practice, they have to be selected according to consideration given to properties of raw materials and costs of fuel and electricity, as well as conditions of location, etc. The major technologies in use today, including their configurations, respective temperature, and functional zones inside the RKS, are illustrated in Figure 1 .
Generally, although the wet processes are more energy intensive due to the evaporation of high moisture contained in raw materials, the investment cost of those plants is rather low and high-quality products are manufactured easily . On the other hand, the plants based on the dry processes consume less energy, which results in much lower operational costs of manufacturing. However, since the progress of technology almost eliminates the differences in final quality products between technologies and as the need for energy conservation is getting increasingly stronger, in the future, the wet process will not necessarily be required. Currently, all cement plants in Tanzania use the technology based on the dry process.
In the final stage of production, Portland cement is produced by intergrinding cement clinker with sulphates such as gypsum and anhydrite in order to obtain a fine homogeneous powder. In blended (composite) cements, there are other constituents such as artificial pozzolana, sand, limestone, granulated blast furnace slag, fly ash, and natural or inert fillers. These are interground with the cement clinker or may need to be dried and ground separately. The kind of cement intergrinding and blending process and the corresponding plant concept, chosen at a specific site, depend on the cement type to be produced, with special importance of the grindability, humidity, and the abrasive behaviour of its compounds. Sometimes, those processes may be performed at the plants that are in separate locations from the clinker production plants. About 70% of the total energy required for cement productions is thermal energy, and 30% is used as electrical energy , in which the pyroprocessing part of the plant (RKS) takes around 90% of the total energy consumption. Most of the thermal heat losses occur in the same part of the plant, due to temperature variations of the feed solid streams caused by chemical reactions, as well as the heat exchange with hot flue gases (in the heating section of RKS) and ambient air streams (in the cooling section of RKS) . Some authors pointed out that those heat losses can lead to up to 20% of initial energy wastage .
Cement production also has a significant contribution to environmental degradation originating both from anthropogenic pollutant emissions and mining activities of raw materials and coal, which is the most usual source of energy in the cement plant. In this way, it contributes to about 5 to 8% of anthropogenic GHG emissions [13, 14, 21]. These emissions have two main sources, which are both located in the RKS: (i) process CO2 released by the calcination of carbonate minerals (about 62% of the total direct CO2 emissions) and (ii) energy-derived CO2 released by the combustion of fuels used in the clinker production (about 38% of the total direct CO2) .
Many studies have evaluated energy and environmental performance of the cement manufacturing plants worldwide. Farag and Taghian  investigated the energy performance of five Egyptian cement plants, where according to them, the energy efficiency varied between 41.6% and 55.5%. Grtrk and Oztop  investigated the thermal performance of a plaster production plant, whereby the energy efficiency of RKS was 69%. A similar investigation was conducted by Parmar et al. , where the energy efficiency of RKS was 51.90%. Kolip and Savas  analysed the RKS with a four-stage cyclone preheater and with a precalciner and reported first and second law efficiencies of 51% and 28%, respectively. Koroneos et al. in  examined cement production in Greece using energy and exergy analysis, and their results uncovered that the energy efficiency of a typical RKS was 68.8%. Furthermore, their results indicated that the biggest thermal energy losses in the plant were due to irreversibility, which occurred during the preheating of feed, the cooling of clinker, and the combustion of pet coke. The energy efficiency of a raw material preparation unit of 84.30% in a cement plant in Turkey was calculated by Utlu et al. in , while Atmaca and Yumrutas in  conducted exergoeconomic analysis of a 4-stage dry rotary cement plant and found that the overall energy efficiency of the plant was 59.37%. Evaluation results of the thermal performance of a clinker grate cooler system was undertaken by Madlool et al. , who found that the energy efficiency varied between 46.18% and 45.19%. The influence of calcium oxide formation, CO2 emissions, and environmental effects of pyroprocessing in an RKS was studied by Boyaghchi in . The energy performance of processes in lime vertical shaft kilns was conducted by Gutirrez et al. in  and in a cement trass mill by Sogut et al. in . Rasul et al. in  investigated the use of energy recovery systems in Indonesias cement plants and reported that energy efficiency could be significantly improved. In their calculations, clinker burning efficiency was 52.07% and cooler efficiency was 47.75%.
In their recently published paper, Rahman et al.  pointed out four commercial software packages, namely, Aspen Plus, Aspen HYSYS, ANSYS Fluent, and CHEMCAD, as the commonly used computer-aided modelling and simulation tools in cement manufacturing processes. The authors observed that most of the studies concerning modelling and simulation of cement manufacturing processes that can be found in the literature are based on computational fluid dynamics (CFD) and use the ANSYS Fluent package. This software allows the modelling of the effect of surface condition and phase changes of the material, as well as the optimization of fluid flow, material feed, and containing structure . Furthermore, by considering the nature of cement production, specific needs and purpose of conceptual process design, and research experience from the other authors , they identified the Aspen Plus software as the most suitable tool for flow sheeting modelling and simulation of cement plants. Aspen Plus uses a flow sheet simulator to graphically represent each stage of the process and enable quick and easy alterations to a process, without requiring a new model for each change. In addition, the Aspen Plus software has a rich database and has the ability to simulate chemical reactions within solid, liquid, and vapour phases. For that reason, the Aspen Plus software is used in this study for modelling and simulating the MCC plant, focusing on the clinker chemistry and thermodynamics in RKS.
In cement production processes, there are several models for the purpose of studying the use of alternative fuels [4, 36, 39]: phase chemistry , oxidation process of coal tar pitch , cement raw material blending process , reduction of CO2 , sensitivity analysis of a model used for the design of rotary kiln processes , and a nonlinear model predictive control . However, all these models do not address issues of real inefficiency taking place in cement production processes, equipment, and the overall system. Most models found in the literature are based on the first law of thermodynamics alone, thereby giving no insights into minimizations of irreversibility due to chemical reactions. Furthermore, it has been very difficult to simulate processes which include thermodynamic properties of fluids and solids in the same simulating environment software. Therefore, the current study sheds light on providing a model which combines both solids and fluids in the same simulating environment for the purpose of improving the performance of cement production processes in the kiln system. The study provides a model that deals specifically with the optimization of energy use in cement production processes using both first and second laws of thermodynamics. With due regard, the developed model was advanced by making use of exergy analysis, so as to identify inefficient processes and components within the system . The thermodynamic model of RKS developed is not only used to calculate the energy and environmental indicators of RKS, but to also provide a useful clue for reducing energy consumption, as well as predict the system behaviour under alternative configurations and different production parameters. In this work, parametric analysis studies were conducted with a view to investigate the effect of varying the kiln system parameters to the system behaviour, process operation, environment, and energy use while maintaining the quality of clinker produced within acceptable values. The aim was also to arrive at a kiln system which performs better in terms of energy use and environmental conservation. Parametric analyses were carried out by varying coal flow rate, as well as cooling and primary air flow rates. Other comparative parametric analyses carried out were temperature versus fuel flow rate, fuel flow rate versus composition of combustion gases, fuel flow rate versus exhaust gas composition, coal moisture content versus combustion efficiency, and air flow rate versus exhaust gas composition.
The rest of the paper is organized as follows: the whole production process in the MCC is described in Section 2. The process is illustrated by a simplified block flow diagram, where the most important technological operations are presented as interrelated subsystems. In Subsection 2.2, key issues related to the modelling of RKS in Aspen Plus software are presented and discussed. Subsection 2.3 reports the simulation results and validation. Section 3 presents parametric study results of the kiln system using the model with respect to the key material and thermodynamic parameters in RKS. Section 4 is devoted to the general conclusion of research presented in this paper and directions for future works.
This paper presents the parametric study results of the MCC four-stage preheater dry rotary kiln system with a planetary cooler  built in the Mbeya region of Tanzania in 1978. The plant current production capacity is about 770 tons of cement per day, and it has been growing dramatically during recent years as a result of the growing demand for cement in the country. The cement production in the MCC plant is based on the dry process technology, and according to the preliminary energy audit, the thermal energy consumption in MCC is about 3.5GJ per ton of produced clinker. The main source of energy in the plant is the coal that is obtained from Tanzania (Tancoal) and from Malawi (both, Mchenga and Erland). The major product of MCC is composite, pozzolana cement, while the ordinary Portland cement is a minor product.
The energy performance of most existing cement manufacturing plants in Tanzania is similar to the other plants in sub-Saharan Africa (SSA), and it is low when compared to the average global available best practice. Studies indicate that the specific electrical energy consumption in some cement plants in East and Central Africa varied between 105kWh and 140kWh per ton of produced cement, where the specific thermal energy consumption is between 3.35GJ and 4.19GJ per ton of produced clinker . Obviously, this is very far when compared to the typical plants of India, for instance, where the specific electrical energy consumption is about 85kWh per ton of produced cement, where the specific thermal energy consumption is usually less than 3.18GJ per ton of produced clinker.
During the preliminary energy audit in the MCC plant, it was observed that the energy performance of the plant is more than 20% lower compared to similar plants in other parts of the world. For this reason, the goal of the study presented in this paper is to identify process improvement opportunities which could increase sustainability indicators of the cement production process in the plant. The research was performed by modern computer-aided modelling and simulation tools which were also used for comprehensive energy analysis of RKS. As it was mentioned before, this part of the plant consumes more than 90% of the total energy input to the production process .
The necessary data for modelling and simulating RKS in the MCC plant were obtained either from daily operational measurements and laboratory analyses or from the plant automatic control system database, and they have been roughly classified into two types: (i) system data that included types and performance of process equipment in the plant and (ii) operation data that included the various parameters of every day operation, such as the rotation of the rotary kiln, rated power, temperature and pressure profiles along RKS, electrical power consumption, chemical analysis of raw meals, coal, ash, dust, and produced clinker.
A simplified block flow diagram (BFD) of the MCC plant with focus on the rotary kiln system is presented in Figure 2. It consists of the few most important subsystems (illustrated as a single block in the figure), which performs the specific technological operations in cement manufacturing. A short explanation of BFD and a description of the subsystems and main material streams that are shown in Figure 2 follows.
Raw material preparation (RMa single block in Figure 2) is a subsystem, where the raw material feeds (Rmfthe material stream in Figure 2) are converted into the raw meal or raw-mix (Rmx). The proportioned raw material is dried, homogenized, and fine-grounded to the required size by the raw mill. The drying process is supported by the hot flue gasses (Hfg2) from the next subsystem of the preheater tower (PH). In the PH subsystem, the raw meal (Rmx) is heated by direct contact with the hot flue gases (Hfg1) from the rotary kiln (RK) subsystem. The preheater tower in MCC consists of four suspension cyclone stages which are arranged one above the other. The number of the stages depends on the heat demand for raw material drying. The uppermost stage comprises two parallel cyclones for better dust separation. The hot flue gases move through the cyclone stages in countercurrent flow to the Rmx stream feed. The dry Rmx is added to the exhaust Hfg in the riser duct before the uppermost cyclone stage. It is separated from the Hfg in the cyclones and remixed with the Hfg from the next cyclone stage. This procedure is repeated until the Rmx that is preheated to about 950C is fed to the RK. The temperature of the hot flue gasses that exit the PH varies depending on the number of stages. Generally, for a 4-stage PH, the exit temperature of the hot flue gasses is in the range of 300C to 380C and in the case of 5- and 6-stage PH, the exit temperature is in the range of 260C to 300C.
After leaving PH, the hot flue gasses are divided in two streams: the first one leads to RM (Hfg2) and to the exhaust gas conditioning unit (EGCn) (Hfg3), and the second leads to the fuel preparation subsystem (FP) (Hfg4). The cyclones in PH are not only used for the purpose of Rmx preheating but also as gas-solid precleaners of gases containing solid dusts. In addition, they are connected in series with other high-efficiency gas-solid cleaners such as bag filters and electrostatic precipitators, which are presented as exhaust gas cleaning units (EGCl unit and EGCn unit). In EGCL, the hot flue gases are cleaned from the dust particles originating from raw materials or coal, while in EGCn they are cooled to a suitable temperature before they are released to the environment.
The raw meal from PH is then transferred to the next subsystem, the rotary kiln (RK), which is a highly refractory-lined cylindrical steel shell (3.95m dia, 58m long), inclined at an angle of 3% and equipped with an electrical drive to rotate at 1.5rpm. It is a countercurrent heating device, whereby its inclination facilitates a continuous transport, so that preheated Rmx, fed into the upper end, travels slowly by gravity, to be discharged as a clinker into the clinker cooler (CC) at the lower discharge end. During the process, the rotary movement enables a continuous rolling motion of the solid material and helps to create the clinker pellets (Clp). The combustion of coal in burners, at the firing end of RK, produces a current of hot flue gases that initiate the clinker burning process. The temperature of the Hfg in RK depends on the volume flow rate and temperature of cooling air (Ca), primary air (Pa), and secondary air (Sa) and is usually regulated by the temperature of secondary air (Sa). Before the use in RK, the raw coal feed (Flf) passes the fuel preparation subsystem (FP), where it is pulverized and preheated. The pulverization of coal is performed in the coal mill, while the hot flue gasses from PH (Hfg4) are used for preheating and drying of the raw coal. The stream of preheated coal (Flph) is then transferred into the rotary kiln burner.
A planetary clinker cooler (CC) is used to cool down the clinker from approximately 1400C to 100C. The planetary cooler is a set of tubes (9 to 11) fixed to the kiln and without a separate drive. Cooling of the clinker starts in the RK cooling zone which is created 1.5m to 2.5m behind a flame. The heat exchange between the hot clinker (Clp) and cooling air (Ca) in a CC takes place countercurrently and maintains a minimum cooling velocity in order to avoid unfavourable mineralogical clinker phases and crystal sizes. During the energy audit, it is observed that a considerable amount of thermal energy is transferred to the environment since approximately three quarters of the cooler shell is not insulated. The cooled clinker material stream (Clco) which passes the CC is then transferred to the subsystem named the cement mill (CM), where it is ground and blended with additives such as gypsum (Gy) and pozzolana (Pz) to form the cement (Cm). From Figure 2, it can be noted that the boundary of RKS considered in this study involved three major subsystems: the preheater tower (PH), the rotary kiln (RK), and the clinker cooler (CC). Other subsystems, as well as facilities such as storing, packaging, logistics, offices, laboratory, and transport are not considered, due to the fact that they are not intrinsic or specific for the cement manufacturing processes .
In this section, a description of the model that is used for the simulation of RKS is presented . The discussion is supported by a flow sheet of a research object created by Aspen Plus software and presented in Figure 3 and by a generalized summary of all Aspen unit operation models used in the modelling process presented in Table 1. The simulation model was considered as a steady state control volume system with steady flow processes that operate in a direct mode and under the following assumptions: (i)Variations of potential and kinetic energies were neglected(ii)Pressure drops were considered only for the preheater tower cyclone simulation(iii)All gas streams were assumed to be ideal gases(iv)The raw material feed and coal particles were considered as homogeneous(v)Reactors operated in adiabatic and uniform conditions, and all outlet streams left the reactors at the same temperature(vi)Chemical equilibrium was assumed for all chemical reactions within the system(vii)Thirty percent of calcination was considered to take place at the last stage of the cyclone preheater tower
The calculations were performed under the following assumptions: (i) cyclone efficiency was selected from the range of 95% to 75%, and a corresponding pressure drop in cyclones were in the range of 1.51mbar to 0.83mbar, respectively; (ii) PH-stage mixed pressure drops were set to 0.8195bar, 0.8390bar, 0.8514bar, and 0.8170bar; (iii) isentropic and mechanical efficiency of ID-FAN was assumed to be 85%, and in the case of a cooling air fan it was 75%; (iv) pressure drops in EGCl and EGCn were set to the values of 21mbar and 50mbar, respectively; and (v) the minimal available temperature difference in CC was set to 15C.
The IDEAL property method in Aspen Plus software was considered as a good choice for the RKS simulation, since the process involves conventional components (such as H2O (g), N2 (g), and O2(g)) at low pressure and high temperature. For nonconventional and heterogeneous components (like coal), the HCOALGEN and DCOALIGT models were employed. For hot flue gas streams, a combination of mixed conventional inert nonconventional particle size distribution (MCINCPSD) was used. The choice was suitable since the simulation includes CISOLID and NCPSD solid with particle size distributions, as well as conventional components. Defining substream NCPSD allowed the inclusion of component attributes such as proximate analysis (PROXANA), ultimate analysis (ULTANAL), and sulfur analysis (SULFANAL) for coal combustion.
Combustion processes in the rotary kiln burner at MCC was modelled by using the Aspen unit operation model RGibbs. The same unit operation model was also chosen for chemical reactions in pyroprocessing, because it was the only model that can calculate phase and chemical equilibrium between solid solutions, liquids, and gases. It was assumed that 30% of the calcination process takes place at the final stage of the preheater cyclone, and the remaining 70% was carried out in a Gibbs reactor. The planetary cooler was simulated using a countercurrent two-stream heat exchanger model (HeatX), and heat losses through the cooler shell were modelled using the Heater model. Cooling air was drawn into the COOLER by a cooling air fan (model Compr) in a countercurrent direction with the incoming hot clinker.
Each of the preheater stage cyclones were modelled using the combination of cyclones and RGibbs model blocks, except for the 4th stage, whereby combinations of the Cyclone and RStoic models were used. The 4-stage preheater models were connected in a series with Cyclone models (removal of large particles) and FabFl models (removal of smaller particles). The cyclones were specified by using a design mode, where the efficiency correlation used was the Shepherd and Lapple model and the type was set to medium efficiency. The simulation of the 4-stage PH starts with the injection of raw meal to the mixer (the RGibbs model), which represents a 1st-stage riser duct. Furthermore, hot exhaust gases from RK entered the PH through the rotary kiln riser duct. The purpose of splitting stream 43 was to control the pressure drop in the 1st-stage preheater cyclones by reducing the mass flow rates inside the cyclones. When the plant is in compound operation, precleaned gases (stream 44) are directed to the raw mill for raw meal drying. However, when the raw mill was off, precleaned exhaust gases were fed to the cooling tower (the Heater model), where they were cooled down. The booster fan was used to draw out cooled precleaned gas from the cooling tower to the Bag Filter (the FabFl model), where it was further cleaned. The clean gas stream 6 was finally drawn out by fan to the environment. The residue solid (stream 7) could be directed to the raw mill depending on its chemical composition.
The fuel specific heat energy consumption was calculated as follows: where (kJkg-1) is specific heat energy consumption, HHV (kJkg-1) encompasses fuel higher heating values, (kgs-1) is fuel flow rate, and (kgs-1) is clinker flow rate.
Validation of the results started by comparing the weight percent composition of clinker, one obtained by simulation and the other obtained from chemical analysis of clinker that is produced in the MCC plant. The values of the weight percent composition that are presented in Table 2 could be generally rated to be in a good agreement, except in the case of some minor elements like sulphur trioxide (SO3), potassium oxide (K2O), and sodium oxide (Na2O). The main reason for the observed disagreement between the calculated and real plant data could be the fact that some substances such as sodium sulphate (Na2SO4), potassium sulphate (K2SO4), calcium sulphate (CaSO4), and tetracalcium aluminium ferrite (C4AF) were not considered in the simulation model. Additionally, the simulation model assumed that dicalcium silicate (C2S) had completely reacted with free lime, calcium oxide (CaO), to form the tricalcium aluminate (C3S).
Deeper analysis presented in  also indicates a relatively significant disagreement of the O2 mass fraction in preheater exhaust gas between the calculated value and real plant data. The difference can be contributed to the unavoidable flow rate of in-leakages of air in the RKS, which was not accounted for in the simulation model. However, since the energy audit indicates 3.77% of the O2 mass fraction in preheater exhaust gas, it became obvious that the useful thermal energy consumption in the plant was far from the optimum level. The high value of percentage oxygen also implied the increase of specific fuel consumption, which, in turn, would lead to more chemical irreversibility in the RKS and more CO2 emission to the environment.
A second validation of the results, concerning the mass flow rate and temperature of some process streams, is illustrated in Figure 4. Like in the case above, most of the results obtained by simulation are in good agreement with the real operating data of the plant. However, there is a slight disagreement between the simulation results and real plant data in the case of coal, i.e., 5000 vs. 4158 in (kg/s) and clinker, i.e., 35,000 vs. 35,528 in (kg/s) mass flow rate. The reason for this could be the too high assumed values of cyclones and clinker burning efficiency in the simulation model. However, the simulation results indicated that the improvement of processes in cyclones, the rotary kiln, and the dust cleaning system can lead to the increase of material-use efficiency in RKS. Also, from the same figure, it can be observed that the temperature of secondary air, i.e., 800 vs. 772 in (C), obtained by simulation, deviated slightly from the real plant data, but was within the allowable range of secondary air temperature in similar plants. The deviation could be due to the type of process unit selected in the simulation model, as well as due to the lower value of minimal available temperature differences used for the simulation of a heat exchanger. Another important parameter calculated by simulation, which deviates from the real plant data, is the temperature of combustion gases, i.e., 2000 vs. 2128 in (C). This deviation could be due to the fact that the model did not consider imperfections of the real plant.
The parametric studies generate vital information for evaluation of cement kiln system production processes. Thus, the parametric analysis could be used to improve kiln system performance and evaluate environmental performance of the plant due to emissions arising from physiochemical reactions.
From the thermodynamics of combustion of rotary kiln systems point of view, increasing coal flow rate raises burning temperature, as well as clinker production rate. However, increasing the fuel flow rate at stoichiometric conditions reduces the flame temperature . In order to maintain the flame temperature, fuel and air flow rates are varied proportionately . The results from Figure 5 suggest that the maximum temperature of is achieved at . By increasing beyond 5580Ckgh-1, flame temperature decreases to 1660C at stoichiometric air.
Figure 5 also indicates that clinker production increases linearly with an increasing coal flow rate. This could be due to the temperature increase which accelerates calcination, as well as the contribution of ash content to the amount of clinker formed. It should be noted that the current operating coal flow rate at MCC is , which gives a burning zone flame temperature of combustion gases of . The result indicates that there is an opportunity for fuel saving. Suitable combustion gas temperatures for clinker burning found in the literature are from up to . Figure 5 indicates that these temperatures can be achieved at () and (), respectively. This could provide a minimum potential coal saving of about , which approximates to 76,126 tons per year at the current kiln feed of 58,000kgh-1. It should be pointed out that the current specific energy consumption of the kiln system is 4200kJkgcl-1 at a clinker throughput of . Therefore, the coal input of to the kiln is equivalent to the specific useful energy consumption of 2350.88kJkgcl-1, which gives a clinker output of . Thus, this translates to a specific energy saving of about 1849.12kJkgcl-1, with a relatively higher clinker throughput. Note that the specific useful energy consumption was calculated using equation (2). However, such fuel saving is only possible if the plant runs without any heat losses.
Moreover, Figure 5 indicates that increasing the coal flow rate above an optimum value will lower combustion efficiency, which is indicated by an increase in carbon monoxide (CO). This is explained by the fact that increasing the coal flow rate at a constant supply of combustion air results into a dramatic decrease in O2 as indicated in the same figure. The decreases in O2, in turn, result into the incomplete combustion of coal.
Traditionally, rotary kiln burning operation control is done by the adjustment of the raw feed, fuel flow rate, and ID fan speed. However, oxygen target level is very important not only for the complete combustion of fuel, but also for better clinker burning conditions.
The right amount of oxygen for complete combustion is very important for the thermal performance of a rotary kiln. However, an exceeding oxygen level indicates more combustion air into the system than expected. This will result into a significant amount of useful energy from the combustion of fuel being used for heating up of excess air, thereby cooling down the burning zone temperature as indicated in Figure 6. The latter will result into kiln system heat losses. In other words, it can be stated that the higher the percentage of excess air, the greater the exergy destroyed by the thermal exergy of combustion gases. Figure 6 indicates that varying primary air beyond 15,000kgh-1clinker production is unstable or varies irregularly. It is also indicated that NO emission increases, but when primary air is beyond , emission starts to decrease. The increase in NO emission below primary air flow rate is due to the increased amount of N2 contained in primary air at elevated temperature, but above the combustion temperature is cooled down by excess air volume at lower temperature. The NOX level in the outgoing gases gives information about combustion processes. It should be pointed out that a high peak temperature in the combustion zone leads to a higher NOX level, among other things. Thus, for any given type of kiln, the amount of NOX formed is directly related to the amount of useful energy consumed in the clinker burning process. Therefore, measures that improve the energy efficiency of this process should reduce NOX emissions as well.
It should also be noted from Figure 6 that CO decreases with an increase in the primary air flow rate, indicating that complete combustion is approached with excess air. It should be noted further that increasing excess air at a certain point may improve combustion efficiency, but when it exceeds the acceptable value between 1% and 2% O2 (10%-15% excess air), combustion efficiency is lowered followed by unstable kiln operation indicated by irregular clinker production in Figure 6. Furthermore, it can be noted that O2 increases with primary air increase. Exceeding the primary air above brings errors to the simulation, and the simulation fails to converge, probably due to an excessive air flow rate, which contributes to an excessive mass flow rate above optimal values allowed to some components such as cyclones, thereby, causing excessive pressure drops and blocking cyclone outlets, due to overloading. An excessive air mass flow rate may also cause problems to reactors due to excessive cooling.
In a rotary kiln system, it is very important to keep the secondary combustion air temperature at a constant acceptable level of between and 1000C . This is very important for the stable and smooth operation of the kiln. Furthermore, the efficiency of the clinker cooler and the kiln system, at large, is mainly constrained by heat recovered from a hot clinker by secondary air. Therefore, results from Figure 7 suggest that an excessive increase in cooling air flow rate lowers the combustion temperature. Increasing the cooling air flow rate above the optimal value causes an unstable kiln operation. That is, varying the cooling air flow rate causes a fluctuating secondary air temperature, thereby causing the cycling of the kiln operation. The latter results into irregular clinker production as indicated in Figure 7. This phenomenon is also supported by the findings of [39, 53]. In Reference , it was observed that too large an amount of secondary air fed to the burning zone interrupted clinker formation, where the flame becomes unstable, the burning zone is cooled, and a lot of dust is in circulation in the kiln and precalciner system. Furthermore, the findings of  indicate that a variation of secondary combustion air changes the formation of clinker minerals, whereby alite content goes down and the belite content grows rapidly. These changes are usually caused by the increase of oxygen. Arad et al.  pointed out that a nonuniform clinker product output suggests that large temperature gradients exist near and within the rotary kiln bed. Generally, the demand of combustion air in the burning zone appears to have a large influence on the results.
Furthermore, increasing the cooling air volume above an optimal value lowers combustion gas temperatures, thereby lowering kiln thermal efficiency (Figure 7). It can also be noted from Figure 7 that increasing the cooling air flow rate will lower CO emission, while increasing NO emission and O2 flow rate.
Coal flow rate was varied in order to study the contribution of coal burning to environmental pollutions. It can be noted from Figure 8 that CO increases exponentially with the coal flow rate. It is also noted that CO2 increases with a coal flow rate up to when it starts to decline with an increasing coal flow rate. Such decrease in CO2 can be explained by the fact that a further increase in the coal flow rate will increase flame temperature, which, in turn, results into CO2 dissociation to form CO.
Furthermore, Figure 8 indicates that NO2 increases with a decreasing coal flow rate, while NO increases up to a coal flow rate of , from where it starts to decline. The decrease in NO with an increasing coal flow rate beyond could be due to a decrease in O2 with an increasing coal flow rate as predicted in the same figure. Normally, it is expected that thermal NOX emission should increase at elevated temperatures, although that depends on the availability of oxygen and nitrogen from excess air.
The findings in Figure 9 reveal that CO decreases with an increasing primary air flow rate. The decrease in CO could be a result of more oxygen supply, which, in turn, facilitates complete combustion. It should be pointed out that the presence of CO near the main flame has a negative influence on clinker quality. NO increases with the air flow rate up to the primary air flow rate of and starts to decrease with an air flow rate increase. It is important to mention that NO in cement kilns has correlation to free-lime content in clinker and, hence, is used to determine clinker quality. The figure further predicts that NO2 increases linearly with the primary air flow rate, while O2 increases with the increasing primary air flow rate. It is also observed that CO2 increases exponentially with the primary air flow rate.
The raw coal at MCC has a moisture content of 8%. Thus, it can be observed from Figure 10 that at the coal moisture content of 8% the combustion flame temperature of combustion gases is lowered to . Therefore, it can be concluded that burning coal with a higher moisture content in a rotary kiln will affect the thermal efficiency of the kiln.
In general, results obtained from the parametric studies of the model give important insights into the possibilities available for the improvements of specific energy use and emissions in the cement dry rotary kiln system. Additionally, parametric studies generate vital information for the evaluation of cement kiln system production processes. Thus, the parametric analysis could be used to improve kiln system performance and evaluate the environmental performance of the plant caused by emissions. Parametric analysis studies uncovered the following: (i)Monitoring of flue gases for O2 and combustibles allows the process to be operated more safely and efficiently(ii)Maintaining the air/fuel ratio within a specific range is very important for the performance of the kiln system, including the quality of clinker produced(iii)Monitoring CO and combustibles could prevent build-ups to levels that can cause an explosion in the ESP, bag house, as well as ID fans(iv)Significant reduction in emissions of NOX, together with other pollutants, could be achieved by maintaining good combustion control(v)Reliable flue gas analysis could provide information for the effective control of the kiln system
Predictions from parametric studies suggest that monitoring and regulating exhaust gases could improve combustion efficiency, which, in turn, could lead to conserving fuels and lowering production costs. Complete combustion will occur when proper amounts of fuel and air (fuel/air ratios) are mixed for correct amounts of time under appropriate conditions of turbulence, as well as temperature. The amount of fuel supplied to the system depends on its calorific value, which means, the higher the calorific value, the lesser the amount of fuel required, and vice versa. The composition of exhaust gases also depends on the type of fuel used and amount of combustion air. The volume and temperature of exit flue gases from a preheater cyclone could also affect fuel consumption in the kiln system. The volume of exit flue gases depends on the amount of combustion air and infiltrating air in the kiln system.
Results obtained from parametric analysis suggest that the maximum fuel flow rate possible at the MCC kiln burner is (current ). Increasing the coal flow rate above could lower the thermal efficiency of the kiln system. The current coal flow rate of gives a simulated burning zone gas temperature at , which is above the typical plant temperature between and , suggesting for an opportunity of fuel saving or more clinker production.
It was observed from the parametric analysis that the higher the excess air for combustion above optimal values, the greater the exergy destroyed due to the thermal cooling of the exergy of combustion gases. Generally, the demand of combustion air in the burning zone appears to have a large influence on the results.
In the future, further parametric studies to explore physical parameters such as specific kiln volume loading, flame height, ID fan speed, and particle size within the rotary kiln bed using Aspen Plus, together with CFD software could be considered. Furthermore, a heat recovery feasibility study could be performed based on the exhaust gas condition predictions made under this study.
I, the author, would like to acknowledge the contributions of my PhD supervisors, Prof. Dr.-Ing. George Tsatsaronis and Prof. Dr. Tetyana Morozyuk. This paper is part of the authors PhD thesis which was funded by the then Ministry of Education and Vocation Training- (MoEVT-) Tanzania, DAAD, and Mbeya University of Science and Technology.
Copyright 2020 John P. John. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.