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Cement A reference book for the Industry
Promotion of Benchmarking Tools for Energy Conservation in Energy Intensive Industries in China
Cement Production – A Reference Book for the Industry
Imprint Contract:
Promotion of Benchmarking Tools for Energy Conservation in Energy Intensive Industries in China
Contract No.: EuropeAid/123870/D/SER/CN Contractor:
The Administrative Centre for China’s Agenda 21 (ACCA21) Room 609, No. 8 Yuyuantan South Road, Haidian District, Beijing, P.R. China, Postal Code: 100038
Partners:
CENTRIC AUSTRIA INTERNATIONAL (CAI) Beijing Energy Conservation & Environment Protection Center (BEEC)
Disclaimer This publication has been produced within the frame of the EU-China Energy and Environment Programme project “Promotion of Benchmarking Tools for Energy Conservation in Energy Intensive Industries in China”. The EU-China Energy and Environment Programme (EEP) was established to correspond to the policies of the Chinese Government and the European Commission to strengthen the EU-China cooperation in the area of energy. The project was formally started on the 1. September 2008. The total duration is 12 months. The contents of this publication are the sole responsibility of the project team and can in no way be taken to reflect the views of the European Union. Beijing, 2009
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Cement Production – A Reference Book for the Industry
Table of Contents Summary and Acknowledgments.............................................................................. 5 1
Cement Production Worldwide with Particular Reference to China ........... 6
2
Cement Manufacturing - Overview ................................................................... 9
3
Processes and Techniques ................................................................................. 12 3.1
Main Technologies and Process Routes.................................................. 13
3.2
Technology Choice .................................................................................... 15
3.3
Winning of Raw Materials .......................................................................... 16
3.4
Kiln Feed Preparation ................................................................................. 16
3.4.1
Raw Material Storage ............................................................................ 17
3.4.2
Grinding of Raw Materials ..................................................................... 17
3.5 3.5.1
Storage of Fuels ....................................................................................... 19
3.5.2
Preparation of Fuels................................................................................ 20
3.5.3
Use of Waste as Fuel............................................................................... 20
3.6
4
Fuel, storage and preparation ................................................................. 18
Clinker burning (pyro-processing) ............................................................ 21
3.6.1
Rotary Kilns Equipped with Preheaters and Precalciner .................. 22
3.6.2
Kiln Exhaust Gases................................................................................... 24
3.6.3
Clinker Cooling ........................................................................................ 24
3.7
Cement grinding......................................................................................... 25
3.8
Packing and storage.................................................................................. 26
Consumption/Emission Levels and Benchmarks ............................................ 27 4.1
Consumption of raw materials ................................................................. 27
4.2
Use of energy............................................................................................... 28
4.2.1
Energy Consumption – Benchmarks (Average Ranges).................. 28
4.2.2
Energy Consumption – Benchmark (World Best)............................... 29
4.2.3
Consumption data overview................................................................ 29 2
Cement Production – A Reference Book for the Industry
4.2.4
Specific Characteristics of Kiln Processes ........................................... 30
4.2.5
Energy Consumption for Different Types of Cement........................ 30
4.3 5
Emissions........................................................................................................ 31
Energy Efficiency Technologies and Measures .............................................. 33 5.1
Overview of Measures................................................................................ 34
5.2
Raw materials preparation........................................................................ 36
5.2.1
Efficient Transport Systems (Dry Process)............................................. 36
5.2.2
Raw Meal Blending (Homogenizing) Systems (Dry Process) ........... 36
5.2.3
Slurry Blending and Homogenizing (Wet Process) ............................ 37
5.2.4
Wash Mills with Closed Circuit Classifier (Wet Process) .................... 37
5.2.5
Use of Roller Mills (Dry Process) ............................................................. 37
5.2.6
High-Efficiency Classifiers/Separators.................................................. 38
5.2.7
Fuel Preparation ...................................................................................... 38
5.2.8
Roller Press for Coal Grinding................................................................ 39
5.3
Clinker Production – All Kilns...................................................................... 39
5.3.1
Process Control & Management Systems for Kilns ............................ 39
5.3.2
Kiln Combustion System Improvements .............................................. 40
5.3.3
Indirect Firing............................................................................................ 41
5.3.4
Oxygen Enrichment................................................................................ 41
5.3.5
Seals .......................................................................................................... 42
5.3.6
Kiln Shell Heat Loss Reduction and Refractories................................ 42
5.3.7
Kiln Drives.................................................................................................. 42
5.3.8
Use of Waste-Derived Fuels ................................................................... 43
5.3.9
Conversion to Reciprocating Grate Cooler....................................... 44
5.3.10 5.4 5.4.1
Optimization of Heat Recovery/Upgrade Clinker Cooler ........... 44
Clinker Production - Wet Process Kilns..................................................... 45 Wet Process Conversion to Semi-Dry Process (Slurry Drier) .............. 45
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Cement Production – A Reference Book for the Industry
5.4.2
Wet Process Conversion to Semi-Wet Process (Filter Press System) 45
5.4.3
Wet Process Conversion to Pre-Heater/Pre-calciner Kiln................. 46
5.5 5.5.1
Low Pressure Drop Cyclones for Suspension Preheaters .................. 46
5.5.2
Heat Recovery for Cogeneration ........................................................ 46
5.5.3
Dry Process Conversion to Multi-Stage Preheater Kiln...................... 47
5.5.4
Upgrading to a Preheater/Precalciner Kiln........................................ 47
5.5.5
Conversion of Long Dry Kilns to Preheater/Precalciner Kiln ............ 48
5.6
Finish Grinding.............................................................................................. 48
5.6.1
Process Control and Management – Grinding Mills ......................... 48
5.6.2
Advanced Grinding Concepts ............................................................ 48
5.6.3
High-Efficiency Classifiers....................................................................... 49
5.6.4
Improved Grinding Media..................................................................... 50
5.7
Plant-Wide Measures.................................................................................. 50
5.7.1
Energy Management............................................................................. 50
5.7.2
Preventative Maintenance ................................................................... 51
5.7.3
Motor Systems.......................................................................................... 51
5.7.4
Compressed Air Systems........................................................................ 52
5.7.5
Lighting ..................................................................................................... 52
5.8
Product Changes........................................................................................ 53
5.8.1
Alkali Content .......................................................................................... 53
5.8.2
Blended Cements ................................................................................... 53
5.8.3
Limestone Portland Cement ................................................................. 54
5.8.4
Reducing the Fineness for Particular Applications............................ 54
5.9 6
Clinker production - dry process preheater kilns................................... 46
Conclusion ................................................................................................... 55
References and Links .......................................................................................... 56
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Cement Production – A Reference Book for the Industry
Summary and Acknowledgments This reference book for the cement production industry is a compilation of best available technologies methodologies and future development in the cement industry based on research carried out by CAI. A very useful source for this work was the “Reference Document on Best Available Techniques in the Cement and Lime Manufacturing Industries (BREF)” published by the Integrated Pollution Prevention and Control (IPPC) Board of the European Commission (2001). Particular reference is also made to the excellent study “Energy Efficiency Improvement and Cost Saving Opportunities for Cement Making” by Ernst Worrell and Christina Galitsky (2008). As part of the BMT-Tool set (BMT = Benchmarking – Monitoring – Targeting) this reference book provides sector specific information regarding the cement industry in general, frequently used technologies, energy consumption of key processes and other relevant aspects connected with the energy and environment performance of cement manufacturing. Energy benchmarks are discussed as typical ranges of energy consumption (MJ or kg standard coal) per production unit (ton of cement or clinker). References are made to world best performances. A larger part of this reference book is dedicated to describing the ample options for energy improvements which exist for this industry, even within relatively modern and advanced plants. The report is divided into a number of Chapters as follows: •
The first chapter provides an overview of cement production worldwide and the predominant role of China in this context.
•
The second chapter summarizes the main features of cement manufacture.
•
The third chapter describes the most relevant processes and technologies in detail.
•
The fourth chapter reviews typical emission and energy consumption data with reference to related benchmarks.
•
The fifth chapter describes the options for energy efficiency improvements through the whole cement manufacturing processes.
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Cement Production – A Reference Book for the Industry
1 Cement Production Worldwide with Particular Reference to China Cement is a basic material for the building and civil engineering construction industries. Output from the cement industry is directly related to the state of the construction business in general and therefore closely tracks the overall economic situation in a region or country. Cement is mainly used to make concrete, and is sort of the "active ingredient" in concrete - it is combined with sand and gravel in roughly fixed proportions. So cement production can be considered a rough indicator for the total amount of construction going on in a region or country. New cement production statistics show China to be by far the largest cement producer in the world. This is a reflection of the size and population of China, but also the massive reconstruction programme that are being undertaken at the present time, particularly in the Eastern Provinces. Also interesting is the percentage of the world's production of cement that China took up in 2007 (50%) compared to 2004 (42.5%); some of this can no doubt be due to preparation for the Beijjing Olympics of 2008 and the Shanghai World Expo of 2010, but these are only the most publicly aware projects the enormous infrastructure development and redevelopment that is currently occurring in China While the cement production in China increased by only 30% between 1960 and the late 90’s (a period of almost 40 years), It has increased from 0.573 to 0,813 billion metric tons between 1999 and 2003 , an increase of 42% over a period of 5 years only; This has further increased between 2005 to 2008 by another 37 %. China is the world's largest producer of cement, producing over 1.2 billion metric tons annually. The next highest producers are India and the US. China’s cement production has grown about 10 percent per year over the past two decades and is expected to reach a saturation point of 1.3 million tons around 2010.
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Cement Production – A Reference Book for the Industry
Annual production of cement by country in billions of metric tons.
Percentage of yearly worldwide cement usage.
Source: USGS 2006 report and the USGS 2008 report.
Percentage growth in cement consumption 2005-2008. Source: USGS 2006 report and the USGS 2008 report.
The Chinese cement industry is characterized by the use of many different technology types. There are basically two types of cement kilns used for the production of clinker (the first production stage of cement manufacture). These are 7
Cement Production – A Reference Book for the Industry
vertical (or shaft) kilns and rotary kilns, but many variations of each type exist in China. Over half of China's cement production is still by vertical shaft kilns. The remainder is produced by both wet and dry rotary kilns; wet kilns comprised only 5% of production in 2003. About 30% of China's cement production in 2004 was from advanced dry rotary kilns that have new suspension preheaters which include precalciners (NSP or precalciner kilns) or suspension preheaters (SP kilns). Energy savings and GHG emissions reductions in the Chinese cement industry can be realized through energy-efficiency retrofits, increased use of blended cements, and substitution of coal with waste fuels, use of waste heat for power generation, and structural shifts (closing older shaft kilns and building modern rotary kilns).
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Cement Production – A Reference Book for the Industry
2 Cement Manufacturing - Overview Cement is a finely ground, non-metallic, inorganic powder, which when mixed with water forms a paste that sets and hardens. This hydraulic hardening is primarily due to the formation of calcium silicate hydrates as a result of the reaction between mixing water and the constituents of the cement. In the case of aluminous cements hydraulic hardening involves the formation of calcium aluminate hydrates... In Europe the use of cement and concrete (a mixture of cement, aggregates, sand and water) in large civic works can be traced back to antiquity. Portland cement, the most widely used cement in concrete construction, was patented in 1824. After quarrying, crushing, grinding and homogenization of raw materials; the first step in cement manufacture is calcination of calcium carbonate followed by burning the resulting calcium oxide together with silica, alumina, and ferrous oxide at high temperatures to form clinker. The clinker is then ground or milled together with gypsum and other constituents to produce cement. The following figure shows a simplified flow chart of cement making.
Naturally occurring calcareous deposits such as limestone, marl or chalk provide the source for calcium carbonate. Silica, iron oxide and alumina are found in various ores and minerals, such as sand, shale, clay and iron ore. Power station ash, blast furnace slag, and other process residues can also be used as partial replacements for the natural raw materials. 9
Cement Production – A Reference Book for the Industry
To produce 1 ton of clinker the typical average consumption of raw materials in the EU is 1.57 tons. Most of the balance is lost from the process as carbon dioxide (CO2) emission to air in the calcination reaction (CaCO3 → CaO + CO2). Clinker burning usually takes place in a rotary kiln which can be part of a wet or dry long kiln system, a semi-wet or semi-dry grate preheater (Lepol) kiln system, a dry suspension preheater kiln system or a preheater/precalciner kiln system. According to state of the art modern cement production uses dry process kilns. Vertical (or shaft) kilns are almost never used now in developed countries largely because of energy inefficiencies. Where semi-dry, semi-wet, and wet process kilns are still in use then they are generally expected to be converted to dry process kiln systems when renewed for similar reasons. In addition there are further grinding plants (mills) without kilns, which are not considered further in this reference book, because the major part of energy consumption in cement production concerns the clinker production with kilns. The best available technique for the production of cement clinker is considered to be a dry process kiln with multi-stage suspension preheating and precalcination. In recent years typical kiln size has come to be around 3000 tons clinker/day. The associated average benchmark heat balance value is 3000 MJ/ton clinker. The clinker burning is the most important part of the cement manufacturing process in terms of the key environmental issues of energy use and emissions to air. Clinker making accounts for up to 90% of the total energy consumption of cement manufacturing. Further key environmental emissions are nitrogen oxides (NOx), sulphur dioxide (SO2) and dust. Whilst dust abatement has been widely applied for more than 50 years and SO2 abatement is a plant specific issue, the abatement of NOx is a relatively new issue for the cement industry. Many modern cement plants have adopted general primary measures, such as process control optimization, use of gravimetric solid fuel feed systems, optimized cooler connections and use of power management systems. These measures usually improve clinker quality and lower production costs but they also reduce the energy use and air emissions. A number of environmental issues, especially its large carbon dioxide emissions, potentially affect the cement industry. As a rule of the thumb, one ton of cement produced releases one ton of CO2. Carbon dioxide reduction strategies by the cement industry aim at lowering emissions per ton of cement product rather than by
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Cement Production – A Reference Book for the Industry
plant. These strategies include installation of more fuel-efficient kiln technologies, partial substitution of noncarbonated sources of calcium oxide in the kiln raw materials, and partial substitution of supplementary cementitious materials (SCM) additives, such as pozzolans, for Portland Cement in the finished cement products and in concrete. Because SCM do not require the energy-intensive clinker manufacturing (kiln) phase of cement production, their use therefore reduce the monetary and environmental costs of the cement component of concrete. Fossil fuel cost are a main concern of the cement industry; even in times of cement shortages, the industry has historically found it difficult to fully pass on the cost increases of fuels to the customers. Some cement plants burn waste materials in their kilns as a low-cost substitute for fossil fuels. Cement kilns can be an effective way of destroying such wastes, but can result in other emission related problems. The viability of the practice and the type of waste burned hinge on current and future environmental regulations and their associated costs. The trend appears to be toward increased use of waste fuels, while separate emission related issues are being addressed more directly through the waste incineration industry.
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Cement Production – A Reference Book for the Industry
3 Processes and Techniques The basic chemistry of the cement manufacturing process begins with the decomposition of calcium carbonate (CaCO3) at about 900°C to leave calcium oxide (CaO, lime) and liberate gaseous carbon dioxide (CO2). This process is known as calcination. This is followed by the clinkering process in which the calcium oxide reacts at high temperature (typically 1400-1500°C) with silica, alumina, and ferrous oxide to form the silicates, aluminates, and ferrites of calcium which comprise the clinker. The clinker is then ground or milled together with gypsum and other additives to produce cement.
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Cement Production – A Reference Book for the Industry
3.1 Main Technologies and Process Routes There are four main process routes for the manufacturing of cement; the dry, semidry, semi-wet and wet processes (see figures below: •
In the dry process, the raw materials are ground and dried to raw meal in the form of a flowable powder. The dry raw meal is fed to the preheater or precalciner kiln or, more rarely, to a long dry kiln.
•
In the semi-dry process, dry raw meal is palletized with water and fed into a grate preheater before the kiln or to a long kiln equipped with crosses.
•
In the semi-wet process the slurry is first dewatered in filter presses. The filter cake is extruded into pellets and fed either to a grate preheater or directly to a filter cake drier for raw meal production.
•
In the wet process, the raw materials (often with high moisture content) are ground in water to form pumpable slurry. The slurry is either fed directly into the kiln or first to slurry drier.
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Cement Production – A Reference Book for the Industry
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Cement Production – A Reference Book for the Industry
3.2 Technology Choice The choice of process is to a large extent determined by the state of the raw materials (dry or wet). A large part of world clinker production is still based on wet processes. However, in developed countries, more than 75% of production has converted to the dry processes thanks to the availability of dry raw materials. Wet processes are more energy consuming, and thus more expensive. Plants using semidry processes are likely to change to dry technologies whenever expansion or major improvement is required. Plants using wet or semi-wet processes normally only have access to moist raw materials, as is the situation in Denmark and Belgium, and to some extent in the UK and North America. A similar tendency is observed in China, too. Plants using wet processes are going to be closed or changed to dry processes.
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Cement Production – A Reference Book for the Industry
The following sub-processes are discussed in more detail in the next chapters: •
Winning of raw materials
•
Kiln feed preparation
•
Fuels storage and preparation
•
Clinker burning
•
Cement grinding and storage
•
Packing and dispatch
3.3 Winning of Raw Materials The most common raw materials for cement production are limestone, chalk and clay. The major component of the raw materials, the limestone or chalk, is usually extracted from a quarry adjacent to or very close to the plant. Limestone provides the required calcium oxide and some of the other oxides, while clay, shale and other materials provide most of the silicon, aluminum and iron oxides required for the manufacturing of Portland cement. The limestone is most often extracted from openface quarries, sometimes from underground mining.
3.4 Kiln Feed Preparation Preparation of the raw material is of great importance to the subsequent kiln system both in getting the chemistry of the raw feed right and in ensuring that the feed is sufficiently fine. The raw materials are selected, crushed, ground, and proportioned so that the resulting mixture has the desired fineness and chemical composition for the pyro-processing (clinker production) systems. It is often necessary to raise the content of silicon oxides or iron oxides by adding quartz sand and iron ore. Power station ash, blast furnace slag, and other process residues can also be used as partial replacements for the natural raw materials,
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Cement Production – A Reference Book for the Industry
depending on their chemical suitability. More than 1.5 tons of raw materials are required to produce one ton of Portland cement. After primary crushing the raw materials are transported to the cement plant for storage and further preparation. Other raw materials, such as bauxite, iron ore, blast furnace slag or foundry sand, are brought in from elsewhere.
3.4.1 Raw Material Storage The mined and crushed or purchased and delivered raw materials may need to be stored in covered storages depending on climatic conditions and the amount of fines in the raw material. In the case of a 3000 tons/day plant these buildings may hold between 20000 and 40000 tons of material. The raw material fed to a kiln system needs to be as chemically homogeneous as practicable. This is achieved by controlling the feed into the raw grinding plant. When the material from the quarry varies in quality, initial pre-blending can be achieved by stacking the material in rows or layers along the length (or around the circumference) of the store and extracting it by taking cross-sections across the pile. When the material from the quarry is fairly homogeneous, simpler stacking and reclaiming systems can be used. Raw materials used in relatively small quantities, mineral additives for example, are usually stored in smaller silos or bunkers. Any raw materials with potentially harmful properties, such as fly ash and phosphogypsum, must be stored and prepared according to individual controlled conditions.
3.4.2 Grinding of Raw Materials After primary size reduction (crushing), the raw materials are further reduced in size by grinding. The grinding differs with the pyro-processing process used. In dry processing, the materials are ground into a flowable powder in mills. Typical dry grinding systems used are: tube mills (centre discharge), tube mills (air-swept), vertical roller mills and horizontal roller mills (only a few installations in operation so far). In a ball (or tube) mill, steel-alloy balls (or tubes) are responsible for decreasing the size of the raw material pieces in a rotating cylinder, referred to as a rotary mill. Rollers on a round table fulfill this task of comminution in a roller mill. 17
Cement Production – A Reference Book for the Industry
The fineness and particle size distribution of the product leaving a raw grinding system is of great importance in the subsequent burning process. The target given for these parameters is achieved by adjusting the separator used for classifying the product leaving the grinding mill. For dry classification, air separators are used. The newest generation, rotor cage type separators, have several advantages including: •
lower specific energy consumption of the grinding system (less over-grinding),
•
increased system throughput (efficiency of particle separation),
•
A more favorable particle size distribution and product uniformity.
Accurate metering and proportioning of the mill feed components by weight is important for achieving a consistent chemical composition. This is essential for steady kiln operation and a high-quality product. Metering and proportioning is also an important factor in the energy efficiency of the grinding system. Raw material preparation is an electricity-intensive production step requiring generally about 25-35 kWh/ton raw material, although it could require as little as 11 kWh/ton. Further energy is required to dry the raw material. The moisture content in the kiln feed of the dry kiln is typically around 0.5 % (0 – 0.7 %); waste heat from the kiln exhaust clinker cooler is commonly used for this purpose. For raw materials with relatively high moisture content, and for start up procedures, an auxiliary furnace may be needed to provide additional heat.
3.5 Fuel, storage and preparation Various fuels can be used to provide the heat required for the pyro-process (clinker burning). Three different types of fuels are mainly used in cement kiln firing; in decreasing order of importance these are: pulverized coal and petcoke, (heavy) fuel oil, and natural gas. In order to keep heat losses at minimum, cement kilns are operated at lowest reasonable excess oxygen levels. This requires highly uniform and reliable fuel metering and fuel presentation in a form allowing rapid and complete combustion. These conditions are fulfilled by all liquid and gaseous fuels. For pulverized solid fuels, good design of hoppers, conveyors and feeders is essential to meet these conditions. The main fuel input (65-85%) has to be of this easily combustible type, 18
Cement Production – A Reference Book for the Industry
whereas the remaining 15-35% may be fed in coarse crushed or lump form. The main fuels used in the cement industry are petcoke and coal (black coal and lignite), which has also process specific benefits: the main ash constituents of these fuels are silica and alumina compounds. These combine with the raw materials to become part of the clinker. This has to be considered in calculating the raw material composition. Thus it is desirable to use fuel with a consistent, though not necessarily low, ash content. Low rank lignitic and sub-bituminous coals are therefore sometimes favored for cement manufacture. Cost normally precludes the use of natural gas or oil, but the selection of fuels depends on the local situation (such as availability of domestic coal). However, the high temperatures and long residence times in the kiln system implies considerable potential for destruction of organic substances. This makes a wide variety of less expensive fuel options possible, in particular different types of wastes.
3.5.1 Storage of Fuels Raw coal and petcoke are stored similarly to other raw materials, in many cases, in covered stores. Outside storage in large, compacted stockpiles is used for long-term stocks. Such stockpiles may be seeded with grass to prevent rainwater and wind erosion. Pulverized coal and petcoke are potentially explosive and are stored exclusively in silos. For safety reasons . These can be triggered by smouldering fires and static electricity spark-overs) these silos have to be of the mass flow extraction type and have to be equipped with standard safety devices. Fuel oil is stored in vertical steel tanks. These are sometimes insulated to help keep the oil at pumpable temperature (50 to 60 °C). They may also be equipped with heatable suction points to maintain the oil at the correct temperature locally. Natural gas is not stored at the cement plant. National high pressure gas distribution network acts as the primary gas storage facility. Specific logistics are employed in case of the use of waste as alternative fuel.
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Cement Production – A Reference Book for the Industry
3.5.2 Preparation of Fuels Solid fuel preparation (crushing, grinding and drying) is usually carried out on site. Coal and petcoke are pulverized to about raw meal fineness in grinding plants using equipment similar to the raw-material grinding plants. The fineness of the pulverized fuel is important: too fine fuel can cause excessively high flame temperatures; too coarse fuel can cause poor combustion. Low volatility solid fuel will need finer grinding. If sufficient hot air for drying is not available from the kiln or from the cooler, an auxiliary furnace may be needed. Ground solid fuel may be fired directly into the kiln, but in most modern installations it is usually stored in silos to allow the use of more thermally efficient burners (indirect firing) using low primary air. Fuel oil and natural gas preparation has to be carried out in accordance with National safety standards for and allow for required metering and combustion.
3.5.3 Use of Waste as Fuel The following types of waste are most frequently used as fuels today (e.g. in Europe): used tyres, waste oils, sewage sludge, rubber, waste woods, plastics, paper waste, paper sludge, spent solvents. Preparation of different types of waste for use as fuel is usually performed outside the cement plant by the supplier or by waste-treatment specialists. This means they only need to be stored at the cement plant and then proportioned for feeding to the cement kiln. Since supplies of suitable waste for use as fuel tend to be variable whilst waste material markets are rapidly developing, it is advisable to design storage/preparation plants to be multi-purpose.
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Cement Production – A Reference Book for the Industry
3.6 Clinker burning (pyro-processing) This part of the process is the most critical in terms of energy efficiency, atmospheric emission, product quality, and cost. In clinker burning, the raw meal is fed to the kiln system where it is dried, pre-heated, calcined and sintered to produce cement clinker. The clinker is cooled with air and then stored. In the clinker burning process it is essential to maintain kiln charge temperatures of between 1400 to 1500°C and gas temperatures of about 1800 to 2000°C under oxidizing conditions. Therefore an excess of air is required in the sintering zone of a cement clinker kiln. Clinker production is the most energy-intensive stage in cement production, accounting for over 90% of total industry energy use, and virtually all of the fuel use. Clinker is produced by pyro-processing in large kilns. The first rotary kilns were long wet kilns, where the raw meal typically contains approximately 36% moisture. These kilns were developed as an upgrade of the original long dry kiln to improve the chemical uniformity in the raw meal. The water (due to the high moisture content of the raw meal) is first evaporated in the kiln in the low temperature zone. The evaporation step makes a long kiln necessary. The length to diameter ratio may be up to 38, with lengths up to 230 meters. The main pyro-processing kiln type used today in developed countries is the rotary kiln. In these rotary kilns a tube with a diameter up to 8 meters is installed at a 3-4o angle that rotates 1-3 times per minute. The ground raw material, fed into the top of the kiln, moves down the tube countercurrent to the flow of gases and toward the flame-end of the rotary kiln, where the raw meal is dried, calcined, and enters into the sintering zone. In the sintering (or clinkering) zone, the combustion gas reaches a temperature of 1800-2000°C. Coal is still the fuel mainly used worldwide and of course in China. In a dry rotary kiln, feed material with much lower moisture content (0.5%) is used, thereby reducing the need for evaporation and reducing kiln length. The first development of the dry process took place in the U.S. and was a long dry kiln without preheating. Later developments have added multi-stage suspension preheaters (i.e. a cyclone) or shaft preheater. Pre-calciner technology was developed more recently A second combustion chamber has been added between the kiln and a conventional pre-heater that allows for further reduction of kiln fuel requirements. The typical fuel 21
Cement Production – A Reference Book for the Industry
consumption of a dry kiln with 4 or 5-stage preheating can vary between 3.2 and 3.5 GJ/ton clinker (109-120kgce/t); electricity use increases slightly due to the increased pressure drop across the system. A six stage preheater kiln can theoretically use as low as 2.9-3.0 GJ/ton clinker (98-102kgce/t). The most efficient pre-heater, precalciner kilns use approximately 2.9 GJ/ton clinker (97kgce/t). Alkali or kiln dust (KD) bypass systems may be required in kilns to remove alkalis, sulfates, and/or chlorides. But such systems lead to additional energy losses since sensible heat is removed with the bypass gas and dust.
3.6.1 Rotary Kilns Equipped with Preheaters and Precalciner The four-stage cyclone preheater kiln system (see left figure below) was standard technology in the 1970s when many plants were built in the 1000 to 3000 tons/day range.
Left: Four stage cyclone preheater. Right: Four stage cyclone preheater plus precalciner
The exhaust gas, which has a temperature of around 330 °C, is normally used for raw material drying. When the meal enters the rotary kiln, calcination is already about 30% completed. Severe problems have been encountered in the past with four stage preheaters in cases where inputs of circulating elements (chlorides, sulphur, and alkalis) from the feed and/or fuel were excessive. Highly enriched cycles of these elements lead to build-ups in cyclone and duct walls, which frequently cause
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Cement Production – A Reference Book for the Industry
blockages and kiln stops lasting several days. Kiln gas bypass, i.e. extraction of part of the particulate laden gas stream leaving the kiln so that it bypasses the cyclone system, is a frequently used solution to the problem. This bypass gas is cooled to condense the alkalis and then passed through a dust collector before discharge. Whilst in some regions it is necessary, for the control of clinker alkali levels, to send the bypass dust and part of the kiln dust to landfills, in all other cases it is fed back into the production process. Almost all four-stage suspension preheaters operate with rotary kilns with three supports. This has been the standard design since around 1970. Kilns with diameters from 3.5 to 6 m have been built with length to diameter ratios in the range 13:1 to 16:1. Mechanically simpler than the long wet and dry kilns, it is probably the most widely used kiln type today. The precalcination technique (see right figure above) has been available to the cement industry since about 1970. In this procedure the heat input is divided between two points. Primary fuel combustion occurs in the kiln burning zone. Secondary burning takes place in a special combustion chamber between the rotary kiln and the preheater. In this chamber up to 60% of the total fuel can be burned in a typical precalciner kiln. This energy is basically used to calcine the raw meal, which is almost completely calcined when it enters the kiln. Hot air for combustion in the calciner is ducted from the cooler. Material leaves the calciner at about 870 ºC. For a given rotary kiln size precalcining increases the clinker capacity. Earlier precalciner systems had only four preheater stages with accordingly higher exhaust gas temperature and fuel consumption. Kiln systems with five cyclone preheater stages and precalciner are considered standard technology for new dry process plants. Where natural raw material moisture is low, six-stage preheaters can be the preferred choice, particularly in combination with bag-filter de-dusting. The size of a new plant is primarily determined by predicted market developments, but also by economy of scale. Typical unit capacity for new plants in Europe today is from 3000 to 5000 tons/day. Technically, larger units with up to 15000 tons/day are possible, and three 10000 tons/day kilns are currently in operation in Asian markets. Where excessive inputs of circulating elements are present, a kiln gas bypass is required to maintain continuous kiln operation. However, due to the different gas flow characteristics, a bypass in a precalciner kiln is much more efficient than in a straight preheater kiln. In spite of the fact that the meal enters the kiln 75 to 95% calcined, most precalciner kilns are still equipped with a rotary kiln with a calcining zone, i.e. 23
Cement Production – A Reference Book for the Industry
with an L/D ratio of 13:1 to 16:1 as in the case of the straight preheater kilns.
3.6.2 Kiln Exhaust Gases In all kiln systems the exhaust gases are finally passed through an air pollution control device (electrostatic precipitator or bag filter) for separation of the dust before going to the main stack. In the dry processes the exhaust gases can be at a relatively high temperature and may provide heat for the raw mill when it is running (compound operation).
3.6.3 Clinker Cooling Once the clinker is formed in the rotary kiln, it is cooled rapidly to minimize the formation of a glass phase and ensure the maximum yield of alite (tricalcium silicate) formation, an important component for the hardening properties of cement. The clinker cooler is an integral part of the kiln system and has a decisive influence on performance and economy of the pyro-processing plant. The cooler has two tasks: to recover as much heat as possible from the hot (1450°C) clinker, which is returned to the process; and to reduce the clinker temperature to a level suitable for the downstream equipment. The main cooling technologies are either the grate cooler or the tube or planetary cooler. In the grate cooler, the clinker is transported over a reciprocating grate through which air flows across the passage of clinker. In the planetary cooler (a series of tubes surrounding the discharge end of the rotary kiln), the clinker is cooled in a counter-current air stream. The cooling air is used as secondary combustion air for the kiln.
24
Cement Production – A Reference Book for the Industry
3.7 Cement grinding After cooling, the clinker can be stored either in the clinker dome, silos, and bins or in open air. Larger stocks can be stored in the open if the necessary precautions against dust formation are taken. The material handling equipment used to transport clinker from the clinker coolers to storage and then to the finish mill is similar to that used to transport raw materials (e.g. belt conveyors, deep bucket conveyors, and bucket elevators). To produce powdered cement, the nodules of cement clinker are ground to the consistency of face powder. Grinding of cement clinker, together with additions (3-5% gypsum to control the setting properties of the cement) can be done in ball mills, ball mills in combination with roller presses, roller mills, or roller presses. While vertical roller mills are feasible, they have not found wide acceptance. Coarse material is separated in a classifier that is re-circulated and returned to the mill for additional grinding to ensure a uniform specific surface area of the final product. Portland Cement is produced by inter-grinding cement clinker and sulphates such as gypsum and anhydrite. In blended cements (composite cements) there are other constituents, such as granulated blast furnace slag, natural or artificial pozzolanas, limestone, or inert fillers. These will be inter-ground with the clinker or may need to be dried and ground separately. (Grinding plants may be at separate locations from clinker production plants.) The kind of cement grinding process and the plant concept chosen at a specific site depend on the cement type to be produced. Of special importance are the grindability, the humidity and the abrasive behavior of the compounds of the cement type produced. Most mills work in a closed circuit, that is, they can separate cement with the required fineness from the material being ground and return coarse material to the mill. The accuracy and reliability of metering and proportioning of the mill feed components by weight is of great importance for maintaining high energy efficiency of the grinding system. The predominant metering and proportioning equipment for the material feed to mills is the belt weigh feeder. Due to the variety of cement types required by the market, latest-generation grinding systems equipped with a dynamic air separator predominate. Power consumption for grinding depends on the specific surface area required for the final product and the additives used. Electricity use for raw meal and finish grinding 25
Cement Production – A Reference Book for the Industry
depends strongly on the hardness of the material and the desired fineness of the cement as well as the amount of additives. Blast furnace slags are harder to grind and hence use more grinding power, between 50 and 70 kWh/ton for a 3,500 Blaine1 (expressed in cm2/g). Traditionally, ball mills are used in finish grinding, while many plants use vertical roller mills. In ball or tube mills, the clinker and gypsum are fed into one end of a horizontal cylinder and partially ground cement exits from the other end. Modern ball mills may use between 32 and 37 kWh/ton for 3,500 Blaine cement. Modern state-of-the-art concepts utilize a high-pressure roller mill and the horizontal roller mill that are claimed to use 20-50% less energy than a ball mill. The roller press is a relatively new technology, and is more common in Europe than in North America. Various new grinding mill concepts are under development or have been demonstrated
3.8 Packing and storage Finished cement is stored in silos, tested and filled into bags, or shipped in bulk on bulk cement trucks, railcars, barges or ships. Additional power is consumed for conveyor belts and packing of cement. The total consumption for these purposes is generally low and not more than 5% of total power use. Both pneumatic and mechanical conveying systems can be used for cement transport to storage silos. Mechanical systems normally have a higher investment cost but a much lower operating cost than pneumatic transport. A combination of airslide or screw/chain conveyors with a chain bucket elevator is presently the most commonly used system.
1
Blaine is a measure of the total surface of the particles in a given quantity of cement, or an indicator of the fineness
of cement. It is defined in terms of square centimeters per gram. The higher the Blaine, the more energy required to grind the clinker and additive.
26
Cement Production – A Reference Book for the Industry
4 Consumption/Emission Levels and Benchmarks The main environmental issues associated with cement production are emissions to air and energy use. Waste water discharge is usually limited to surface run off and cooling water only and causes no substantial contribution to water pollution. The storage and handling of fuels is a potential source of contamination of soil and groundwater. The following figure (based on data from Austria) shows a mass balance for the production of 1 kg of cement with the dry process, using heavy fuel oil as fuel.
4.1 Consumption of raw materials Cement manufacture is a high volume process. The figures in the following table indicate typical average consumptions of raw materials for the production of cement in the European Union. The figures in the final column are for a plant with a clinker production of 3000 tons/day or 1 million tons/year, corresponding to 1.23 million tons cement per year based on the average clinker content in European cement. 27
Cement Production – A Reference Book for the Industry
4.2 Use of energy The dominant use of energy in cement manufacture is fuel for the kiln. The main users of electricity are the mills (finish grinding and raw grinding) and the exhaust fans (kiln/raw mill and cement mill) which together account for more than 80 % of electrical energy usage. On average, energy costs - in the form of fuel and electricity – represent about 50% of the total production cost involved in producing a ton of cement. Electrical energy represents approximately 20 % of this overall energy requirement. The theoretical energy use for the burning process (chemical reactions) is about 1700 to 1800 MJ/ton clinker.
4.2.1 Energy Consumption – Benchmarks (Average Ranges) The actual fuel energy use (primary energy = final energy) for different kiln systems is in the following ranges (MJ/ton clinker): •
about 3000
for dry process, multi-stage cyclone preheater and precalciner kilns
•
3100-4200
for dry process rotary kilns equipped with cyclone preheaters
•
3300-4500
for semi-dry/semi-wet processes (Lepol-kiln)
•
up to 5000
for dry process long kilns
•
5000-6000
for wet process long kilns, and
•
3100-4200
for shaft kilns (max. capacity 200t/day)
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Cement Production – A Reference Book for the Industry
The final energy electricity demand - for raw material, solid fuel and additives preparation and grinding - is about 90-130 kWh/ton (~320-430 MJ/ton) cement. This number is equivalent to 270-400 kWh/ton (~990-1420 MJ/ton) cement of primary energy electricity, which includes electricity generation, transmission and distribution losses.
4.2.2 Energy Consumption – Benchmark (World Best) The world best practice value reported in the literature is a fuel energy use (primary energy = final energy) of 2850 MJ/ton clinker. The final energy electricity demand - for raw material, solid fuel and additives preparation and grinding - is 70 kWh/ton (~250 MJ/ton) cement; this number is equivalent to 210 kWh/ton (~760 MJ/ton) cement of primary energy electricity, which includes electricity generation, transmission and distribution losses.
4.2.3 Consumption data overview
Energy consumption clinker burning International benchmarks
Primary energy intensity
Final energy intensity kgce/t clinker
MJ/t clinker
kgce/t clinker
MJ/t clinker
from
up to
from
up to
from
up to
from
up to
dry process, multi-stage cyclone preheater and precalciner kilns
97.2
102.4
2,850
3,000
97.2
102.4
2,850
3,000
dry process rotary kilns equipped with cyclone preheaters
105.8
143.3
3,100
4,200
105.8
143.3
3,100
4,200
semi-dry/semi-wet processes (Lepolkiln)
112.6
153.5
3,300
4,500
112.6
153.5
3,300
4,500
dry process long kilns
170.6
5,000
170.6
5,000
wet process long kilns
170.6
204.7
5,000
6,000
170.6
204.7
5,000
6,000
shaft kilns (100 t/day)
105.8
143.3
3,100
4,200
105.8
143.3
3,100
4,200
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Cement Production – A Reference Book for the Industry
Energy consumption electricity International benchmarks
Final energy intensity kgce/t cement
Primary energy intensity
kWh/t cement
kgce/t cement
kWh/t cement
from
up to
from
up to
from
up to
from
up to
Raw material preparation
2.9
4.3
24
35
8.9
13.0
73
106
Kiln frives, blasters, fans
2.7
3.1
22
25
8.2
9.3
67
76
Finish grinding
3.1
8.6
25
70
9.3
26.1
76
212
Total electricity consumtion
8.7
16.0
71
130
26.4
48.4
215
394
4.2.4 Specific Characteristics of Kiln Processes
4.2.5 Energy Consumption for Different Types of Cement Because clinker making accounts for the most energy consumed in the cement making process (about 90%), reducing the ratio of clinker to final cement produced by mixing clinker with additives can greatly reduce the energy used for manufacturing of cement. Best practice values for additive use are based on the European ENV 197-2 standards: •
Typical Portland Cements (CEM I) have only up to 5 % additives and 95 % 30
Cement Production – A Reference Book for the Industry
clinker. The above reported energy consumption figures relate to these types of cement. •
For composite Portland Cements (CEM II), up to 35% can be fly ash and 65% clinker. Total energy consumption for the production of these types of cements can up to about 20% less.
•
For blast furnace slag cements (CEM III/A), up to 65% can be blast furnace slag and only 35% clinker. The total energy consumption for the production of these types of cement can be up to about 45 % less.
4.3 Emissions This topic was included because the Chinese cement industry. like its international counterparts, is under increasingly pressure to adopt stricter environmental protection standards. The main polutants from the production of cement are releases to air from the kiln system. These derive from the physical and chemical reactions involving the raw materials and the combustion of fuels. The main constituents of the exit gases from a cement kiln are nitrogen from the combustion air; CO2 from calcination of CaCO3 and combustion of fuel; water vapor from the combustion process and from the raw materials; and excess oxygen. There are also releases of particulates from all milling operations including raw materials, solid fuel and product. There is potential for the release of particulates from any outside storage of raw materials and solid fuels as well as from any materials transport systems, including cement product loading. The magnitude of these releases can be significant if they are not well engineered or maintained. Even at low levels, such releases can lead to local nuisance problems. Cement plant operation and literature on air pollution and abatement techniques generally focus on three pollutants, which are also used for fixing emission limit values: •
oxides of nitrogen (NOx) and other nitrogen compounds
•
sulphur dioxide (SO2) and other sulphur compounds 31
Cement Production – A Reference Book for the Industry
•
dust
The following pollutants are also considered to be of concern for the production of cement: •
carbon monoxide (CO)
•
volatile organic compounds (VOC)
Other pollutants to be considered in relation to the production of cement in case of waste fuels are: •
polychlorinated dibenzodioxins and dibenzofurans (PCDDs and PCDFs)
•
metals and their compounds
•
hydrogen fluoride (HF) – or hyroflouric acid
•
hydrogen chloride (HCl) – or hydrochloric acid
Not mentioned in the list, but considered to be relevant for cement production is carbon dioxide (CO2). Other emissions the effect of which is normally slight and/or local, are waste, noise and odor.
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Cement Production – A Reference Book for the Industry
5 Energy Efficiency Technologies and Measures In this chapter techniques are discussed that may have a positive effect on (i.e. reduce) the consumption of raw materials and the use of energy. many options exist for cement plants to improve energy efficiency while maintaining or enhancing productivity. Improving energy efficiency at a cement plant could be approached from several directions. Several technologies and measures exist that can reduce the energy intensity (i.e. the electricity or fuel consumption per unit of output) of the various process stages of cement production. This section provides more detailed estimates on the technologies and measures, their costs, and potential for implementation. For new plants and major upgrades a dry process kiln with multi-stage preheating and precalcination is considered to be the most energy efficient. The wet process kilns operating in Europe and other developed countries are generally expected to convert to the dry process when renewed, and so are semi-dry and semi-wet processes. The same is expected to happen in China, too. Kiln systems with 5 cyclone preheater stages and precalciner are considered standard technology for ordinary new plants; such a configuration will use between 2900-3200 MJ/ton clinker (98-109kgce/t). To optimize the input of energy in other kiln systems it is possible to change the configuration of the kiln to a short dry process kiln with multi stage preheating and precalcination. However, this is usually not feasible unless done as part of a major upgrade with an increase in production. Given these constraints, short term options for energy efficiency improvements in Chinese cement plants are discussed mainly for dry process plants, with some reference to wet processes.
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Cement Production – A Reference Book for the Industry
5.1 Overview of Measures
Raw materials preparation
Dry
Wet
process
process
•
Efficient transport systems
•
Slurry blending and homogenization
•
Raw meal blending systems
•
Conversion to closed circuit wash mill
•
High-efficiency roller mills
X
•
High-efficiency classifiers
X
•
Fuel Preparation: Roller mills
X
X
Dry
Wet
process
process
X X X
Clinker production
X
•
Energy management and process control
X
X
•
Seal replacement
X
X
•
Kiln combustion system improvements
X
X
•
Kiln shell heat loss reduction
X
X
•
Use of waste fuels
X
X
•
Conversion to modern grate cooler
X
X
•
Refractories
X
X
•
Optimize grate coolers
X
X
•
Addition of pre-calciner to pre-heater kiln
X
•
Low pressure drop cyclones suspension pre-heaters
X
•
Heat recovery for power generation
X
•
Long dry kiln conversion to multi-stage pre-heater kiln
X
•
Conversion to pre-heater, pre-calciner kilns
X
•
Conversion to semi-dry kiln (slurry drier)
X
•
Conversion to semi-wet kiln
X
•
Efficient kiln drives
X
•
Oxygen enrichment
X
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Cement Production – A Reference Book for the Industry
Finish grinding
Dry
Wet
process
process
•
Energy management and process control
X
X
•
Improved grinding media (ball mills)
X
X
•
Kiln combustion system improvements
X
X
•
High-pressure roller press
X
X
•
High efficiency classifiers
X
X
Dry
Wet
process
process
General measures
•
Preventative maintenance
X
X
•
High efficiency motors
X
X
•
Efficient fans with variable speed drives
X
X
•
High-pressure roller press
X
X
•
Efficient lighting
X
X
Dry
Wet
process
process
Product & Feedstock Changes
•
Blended Cements
X
X
•
Limestone cement
X
X
•
Low Alkali cement
X
X
•
Use of steel slag in kiln
X
X
•
Reducing fineness of cement for selected uses
X
X
Not all measures mentioned above will apply to all plants. Application will depend on the current and future situation in individual plants. For example, expansion and large capital projects are likely to be implemented only if the company has about 50 years of remaining limestone reserves onsite. Plants that have a shorter remaining supply are unlikely to implement large capital projects, and would rather focus on minor upgrades and energy management measures.
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Cement Production – A Reference Book for the Industry
5.2 Raw materials preparation
5.2.1 Efficient Transport Systems (Dry Process) Transport systems are required to convey powdered materials such as kiln feed, kiln dust, and finished cement through the plant. These materials are usually transported by means of either pneumatic or mechanical conveyors. Mechanical conveyors use less power than pneumatic systems. The average energy savings are estimated at 2.0 kWh/ton by switching to mechanical conveyor systems. Conversion to mechanical conveyors is cost-effective when replacement of conveyor systems is needed to increase reliability and reduce downtime.
5.2.2 Raw Meal Blending (Homogenizing) Systems (Dry Process) To produce a good quality product and to maintain optimal and efficient combustion conditions in the kiln, it is crucial that the raw meal is completely homogenized. Quality control starts in the quarry and continues to the blending silo. On-line analyzers for raw mix control are an integral part of the quality control system. Most plants use compressed air to agitate the powdered meal in so-called airfluidized homogenizing silos using 1-1.4 kWh/ton raw meal. Older dry process plants use mechanical systems, which simultaneously withdraws material from 6-8 different silos at variable rates, using 2-2.4 kWh/ton raw meal. Modern plants use gravity-type homogenizing silos (or continuous blending and storage silos) reducing power consumption. In these silos, material funnels down one of many discharge points, where it is mixed in an inverted cone. Gravity-type silos may not give the same blending efficiency as air-fluidized systems. Although older plants use mechanical or air-fluidized bed systems, more and more new plants seem to have gravity-type silos, because of the significant reduction in power consumption. Silo retrofit options are cost-effective when the silo can be partitioned with air slides and divided into compartments which are sequentially agitated, as opposed to the construction of a whole new silo system. The energy savings are estimated at 0.9-2.3 kWh/ton raw meal. Careful selection and control of substances entering the kiln can reduce emissions. For example, limiting the sulphur content of both raw materials and fuels can reduce releases of SO2. The same is valid for raw materials and fuels containing other 36
Cement Production – A Reference Book for the Industry
substances, for example nitrogen, metals and organic compounds. There are, however, some differences between different kiln systems and feeding points. For example, fuel sulphur is not a problem for dry preheater and precalciner kiln systems, and all organic compounds in fuels fed through the main burner will be completely destroyed.
5.2.3 Slurry Blending and Homogenizing (Wet Process) In the wet process the slurry is blended and homogenized in a batch process. The mixing is done using compressed air and rotating stirrers. The use of compressed air may lead to relatively high energy losses because of its poor efficiency. An efficiently run mixing system may use 0.3 – 0.5 kWh/ton raw material. The main energy efficiency improvement measures for slurry blending systems are to be found in the finding of improvements to the compressed air system.
5.2.4 Wash Mills with Closed Circuit Classifier (Wet Process) In most wet process kilns, tube mills are used in combination with closed or open circuit classifiers. An efficient tube mill system consumes about 13 kWh/ton of raw material. Replacing the tube mill by a wash mill would reduce electricity consumption to between 5-7 kWh/ton of raw material at comparable investment and operation costs as a tube mill system. When replacing a tube mill a wash mill should be considered as an alternative, reducing electricity consumption for raw grinding by 5-7 kWh/ton of raw material, or 40-60%.
5.2.5 Use of Roller Mills (Dry Process) Traditional ball mills used for grinding certain raw materials (mainly hard limestone) can be replaced by high-efficiency roller mills combined with high-pressure roller presses, or by horizontal roller mills. The use of these advanced mills saves energy without compromising product quality. Energy savings of 6-7 kWh/t of raw material are assumed through the installation of a vertical or horizontal roller mill. A further advantage of the inline vertical roller mills is that they can combine raw material drying with the grinding process by using the large quantities of low grade waste heat available from the kilns or clinker coolers. Various roller mill designs are marketed. 37
Cement Production – A Reference Book for the Industry
5.2.6 High-Efficiency Classifiers/Separators A recent development in efficient grinding technologies is the use of high-efficiency classifiers or separators. Classifiers separate the finely ground particles from the coarse particles. The large particles are then recycled back to the mill. High efficiency classifiers can be used in both the raw materials mill and in the finish grinding mill. Standard classifiers may have a low separation efficiency, which leads to the recycling of fine particles, which results in extra power use in the grinding mill. Various concepts of high-efficiency classifiers have been developed. In highefficiency classifiers, the material stays longer in the separator, leading to sharper separation, thus reducing over-grinding. Electricity savings through implementing high-efficiency classifiers are estimated at 8 % of the specific electricity use. Replacing a conventional classifier by a high-efficiency classifier can lead to 15 % increases in the grinding mill capacity and improved product quality due to a more uniform particle size, both in raw meal and cement. The better size distribution of the raw meal may lead to fuel savings in the kiln and improved clinker quality.
5.2.7 Fuel Preparation Coal is the most widely used fuel in the cement industry. Fuels preparation is most often performed on-site and may include crushing, grinding and drying of coal. Coal is normally shipped “wet” to prevent dust formation and fire during transport. Most commonly a ball mill or a roller mill is used for coal grinding. An impact mill would consume around 45-60 kWh/ton and a tube mill around 25-26 kWh/ton (total system requirements). Waste heat of the kiln system (e.g. the clinker cooler) can be used to dry the coal if needed. Advantages of a roller mill are that it is able to handle larger sizes of coal (no precrushing needed) and coal types with a higher moisture content, and can manage larger variations in throughput. However, tube mills are preferred for more abrasive coal types. Currently, roller mills are the most common coal mills in the cement industry. Coal roller mills are available for throughputs of 5 to 200 tons/hour. Coal grinding roller mills can be found in many countries around the world, including: US, Brazil, Canada, China, Denmark, Germany, Japan and Thailand. All major suppliers of cement technology offer roller mills for coal grinding.
38
Cement Production – A Reference Book for the Industry
Electricity consumption for a vertical roller mill is estimated at 16-18 kWh/ton coal. The investment costs for a roller mill are typically higher than that of a tube mill or an impact mill, but the operating costs are also lower; roughly 20% compared to a tube mill and over 50% compared to an impact mill.
Energy savings are estimated
between 7-10 kWh/ton of coal.
5.2.8 Roller Press for Coal Grinding Roller presses, like those used for cement and raw material grinding, are generally more efficient than conventional grinding mills. Roller presses can be used to grind raw materials and coal interchangeably, although coal-grinding equipment needs special protection against explosions. But penetration of roller presses into the cement industry is still at a relatively low level.
5.3 Clinker Production – All Kilns A smooth and stable kiln process, operating close to the process parameter set points, is beneficial for all kiln emissions as well as the energy use. This can be achieved by applying Process control optimization, including computer-based automatic control systems and the use of modern, gravimetric solid fuel feed systems. Kiln optimization is applicable to all kilns and can include many elements ranging from instruction/training of the kiln operators up to installation of new equipment such as dosing systems, homogenization silos, pre-blending beds and new clinker coolers.
5.3.1 Process Control & Management Systems for Kilns Optimization of the clinker burning process is usually done to reduce the heat consumption, to improve the clinker quality and to increase the lifetime of the equipment (the refractory lining, for example) by stabilizing process parameters. Reduction of emissions, such as NOx, SO2 and dust, are secondary effects of this optimization. Smooth and stable kiln operation close to design values for process parameters is beneficial for all kiln emissions. Optimization includes measures like 39
Cement Production – A Reference Book for the Industry
homogenizing the raw material, ensuring uniform coal dosing and improving the cooler’s operation. the maintenance of a steady fuel feed rate, with few peaks, is of great importance and requires good designs of hopper, transport conveyor and feeder systems and gravimetric solid fuel feed systems to achieve this objective. Heat from the kiln may be lost through non-optimal process conditions or process management. Automated computer control systems may help to optimize the combustion process and conditions. Improved process control will also help to improve the product quality and grindability, such as reactivity and hardness of the produced clinker, which may lead to more efficient clinker grinding. A number of management systems are marketed through the cement industry manufacturers and available and in use throughout the world. Most modern systems use so-called ‘expert control’ (also known as 'fuzzy logic' or rule-based control strategies). Expert control systems do not use a modeled process to control process conditions, but try to simulate the best human operator, using information from various stages in the process. An alternative to expert systems or fuzzy logic is model-predictive control using dynamic models of the processes in the kiln. Additional process control systems include the use of on-line analyzers that permit operators to keep track of the chemical composition of raw materials being processed in the plant. This enables rapid changes to be made to the blend of raw materials. A uniform feed allows for steadier kiln operation, thereby saving fuel. Energy savings from process control systems may vary between 2.5% and 10%, and the typical savings are estimated at 2.5-5%. The economics of advanced process control systems are very good and payback periods can be as short as 3 months.
5.3.2 Kiln Combustion System Improvements Fuel combustion systems in kilns can be contributors to kiln inefficiencies with such problems such as poorly adjusted firing, incomplete fuel burn-out, high CO formation, and combustion with excess air. Improved combustion systems aim to optimize the shape of the flame, the mixing of combustion air and fuel and the reduction of the use of excess air.
40
Cement Production – A Reference Book for the Industry
5.3.3 Indirect Firing Historically the most common firing system is known as the direct-fired system. Coal is dried, pulverized and classified in a continuous stream, and fed directly to the kiln. This can lead to high levels of primary air (up to 40 % of stoichiometric). These high levels of primary air limit the amount of secondary air that can be introduced to the kiln from the clinker cooler. Primary air percentages vary widely, and non-optimized matching can cause severe operational problems including the creation of reducing products on the kiln wall and clinker, refractory wear and reduced efficiency due to having to run at high excess air levels to ensure effective burnout of the fuel within the kiln. In modern cement plants, indirect fired systems are becoming increasingly common. In these systems, neither primary air nor coal is fed directly to the kiln. All moisture from coal drying is vented to the atmosphere and the pulverized coal is transported to storage via cyclone or bag filters. Pulverized coal is then densely conveyed to the burner with a small amount of primary transport air. As the primary air supply is decoupled from the coal mill in multi-channel designs, lower primary air percentages are used, normally between 5 and 10%. The multi-channel arrangement also allows for a degree of flame optimization. This is an important feature if a range of fuels is fired. Input conditions to the multi-channel burner must be optimized to secondary air and kiln aerodynamics for optimum operation. The optimization of the combustion conditions will lead to reduced NOx emissions, better operation with varying fuel mixtures, and reduced energy losses. This technology is standard for modern plants. The majority of modern plants in Europe and US have indirect firing systems. The advantages of improved combustion conditions will lead to a longer lifetime of the kiln refractories and reduced NOx emissions. These co-benefits may result in larger cost savings than the energy savings alone. The disadvantage of an indirect firing system is the additional capital cost.
5.3.4 Oxygen Enrichment Several plants use oxygen enrichment to increase production if the local market demand for cement can justify the additional costs for oxygen purchase or production. Experiences exist with wet and dry process kilns. Production my increase by around 3-7% on the basis of annual production; but oxygen enrichment is unlikely to result in net energy savings, because additional electricity is consumed for oxygen 41
Cement Production – A Reference Book for the Industry
generation.
5.3.5 Seals Seals are used at the kiln inlet and outlet to reduce false air penetration, as well as heat losses. Seals may start leaking, hence increasing the heat requirement of the kiln. Most often pneumatic and lamella-type seals are used, although other designs are available (e.g. spring-type). Although seals can last up to 10,000 to 20,000 hours, regular inspection may be needed to reduce leaks. Energy losses resulting from leaking seals may vary, but are generally relatively small. The payback period for improved maintenance of kiln seals is estimated at 6 months or less.
5.3.6 Kiln Shell Heat Loss Reduction and Refractories There can be considerable heat losses through the shell of a cement kiln, especially in the burning zone. The use of better insulating refractories can reduce heat losses. Refractory choice is the function of insulating qualities of the brick and the ability to develop and maintain a coating. The coating helps to reduce heat losses and to protect the burning zone refractory bricks. Structural considerations may limit the use of new insulation materials. The use of improved kiln-refractories may also lead to improved reliability of the kiln and reduced downtime, reducing production costs considerably, and reducing high energy demands during start-ups. Refractories protect the steel kiln shell against heat, chemical and mechanical stress. The choice of refractory material depends on the combination of raw materials, fuels and operating conditions. Extended lifetime of the refractories will lead to longer operating periods and reduced lost production time between relining of the kiln. These savings may offset the higher costs of better quality refractories. It may also lead to additional energy savings due to the reduction in start-up time energy costs. These energy savings are difficult to quantify, as they will depend on the existing lining choice and management.
5.3.7 Kiln Drives A substantial amount of power is used to rotate the kiln. Mostly synchronous motors are used up to 1,000 hp. The highest efficiencies are achieved using a single pinion
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drive with an air clutch and a synchronous motor. The system would reduce power use for kiln drives by a few percent, or roughly 0.5 kWh/ton of clinker at slightly higher capital costs (+6%). More recently, the use of AC motors is advocated to replace the traditionally used DC drive. The AC motor system may result in slightly higher efficiencies (0.5 – 1% reduction in electricity use of the kiln drive) and has lower investment costs. Using high-efficiency motors to replace older motors or instead of re-winding old motors may reduce power costs by 2 to 8%. Adjustable or variable speed drives (ASDs) for the kiln fan result in reduced power use and reduced maintenance costs.
5.3.8 Use of Waste-Derived Fuels Waste fuels can be substituted for traditional commercial fuels in the kiln. The cement industry is increasingly using waste fuels. New waste streams include carpet and plastic wastes, filter cake, paint residue and (dewatered) sewage sludge. Cement kilns also use hazardous wastes. The revenues from waste intake have helped to reduce the production costs of all waste-burning cement kilns, and especially of wet process kilns. Waste-derived fuels may replace the use of commercial fuels, and may result in net energy savings and reduced CO2 emissions, depending on the alternative use of the wastes (e.g. incineration with or without energy recovery). A cement kiln is an efficient way to recover energy from waste. The carbon dioxide emission reduction depends on the carbon content of the waste-derived fuel, as well as the alternative use of the waste and efficiency of use (e.g. incineration with or without heat recovery). The high temperatures and long residence times in the kiln destroy virtually all organic compounds, while efficient dust filters may reduce any potential emissions to safe levels. The use of waste containing volatile metals (mercury, thallium) or volatile organic compounds can result in an increase of the emissions of mercury, thallium or VOCs when improperly used. Wastes that are fed through the main burner will be decomposed in the primary burning zone, at temperatures up to 2000°C. However, waste fed to a secondary burner, preheater or precalciner will be burnt at lower temperatures, which are not always is enough to decompose halogenated organic substances. Volatile components in materials that are fed at the upper end of the kiln or as lump fuel can evaporate. These components do not pass the primary burning zone and may not be decomposed or bound in the cement clinker. 43
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5.3.9 Conversion to Reciprocating Grate Cooler Four main types of coolers are used in the cooling of clinker: shaft, rotary, planetary and travelling and reciprocating grate coolers. There are no longer any rotary or shaft coolers in operation in Europe and North America. Some travelling grate coolers may still be in operation, but planetary and grate coolers are the coolers of choice. The grate cooler is the modern variant and is used in almost all modern kilns. The advantages of the grate cooler are its large capacity (allowing large kiln capacities) and efficient heat recovery (the temperature of the clinker leaving the cooler can be as low as 83°C, instead of 120-200°C, which is the norm from planetary coolers). Tertiary heat recovery (needed for pre-calciners) is impossible with planetary coolers, limiting heat recovery efficiency. Grate coolers recover more heat than the other types of coolers. For large capacity plants, grate coolers are the preferred equipment. For plants producing less than 500 ton of clinker per day, the cost of a grate cooler may be too great. Replacement of planetary coolers by grate coolers is not uncommon. Grate coolers are standard technology for modern large-scale kilns. Modern reciprocating coolers have a higher capacity of heat recovery than older variants, increasing heat recovery efficiency to 65% or higher, while reducing fluctuations in recuperation efficiency (i.e. increasing productivity of the kiln). When compared to a planetary cooler, additional heat recovery is possible with grate coolers at an extra power consumption of approximately 2.7 kWh/ton clinker. The savings are estimated to be up to 8% of the fuel consumption in the kiln. Cooler conversion is generally economically attractive only when installing a precalciner, which is necessary to produce the tertiary air (see above), or when expanding production capacity.
5.3.10 Optimization of Heat Recovery/Upgrade Clinker Cooler The aim of the clinker cooler is to drop the clinker temperature from around 1200°C down to below 100°C. The most common cooler designs are of the planetary (or satellite), traveling and reciprocating grate type. the most popular coolers in use in developed countries are grate coolers. All coolers heat the secondary air for the kiln combustion process and sometimes also tertiary air for the precalciner. Reciprocating grate coolers are the modern variant and are suitable for large-scale kilns. Grate coolers use electric fans and excess air. The highest temperature portion of the 44
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remaining air can be used as tertiary air for the precalciner. Rotary coolers (used for approximately 5% of the world clinker capacity for plants up to 2200-5000 tons/day of clinker) and planetary coolers (used for 10% of the world capacity for plants up to 3300-4400 tons/day of clinker do not need combustion air fans and use little excess air, resulting in lower heat losses. Improving heat recovery efficiency in the cooler results in fuel savings, but may also influence product quality and emission levels. Heat recovery can be improved through reduction of excess air volume; control of clinker bed depth and new grates such as ring grates. Control of cooling air distribution over the grate may result in lower clinker temperatures and high air temperatures. Additional heat recovery results in reduced energy use in the kiln and precalciner, due to higher combustion air temperatures. A recent innovation in clinker coolers is the installation of a static grate section at the hot end of the clinker cooler. This has resulted in improved heat recovery and reduced maintenance of the cooler. Modification of the cooler would result in improved heat recovery rates of 2-5% over a conventional grate cooler.
5.4 Clinker Production - Wet Process Kilns 5.4.1 Wet Process Conversion to Semi-Dry Process (Slurry Drier) In modernized wet kilns, compounds can be added to dry the slurry before entering the kiln using waste heat from the kiln. This reduces energy consumption considerably and increases productivity.
5.4.2 Wet Process Conversion to Semi-Wet Process (Filter Press System) In the wet process the slurry typically contains 36% water (range of 24-48%). A filter press can be installed in a wet process kiln in order to reduce the moisture content of the slurry to about 20% and obtain a paste ready for extrusion into pellets. Additional electricity consumption is 3-5 kWh/ton of clinker.
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5.4.3 Wet Process Conversion to Pre-Heater/Pre-calciner Kiln A wet process kiln can be converted to a state-of-the art dry process production facility that includes either a multi-stage preheater, or a pre-heater/pre-calciner. The cost of converting a wet plant to a dry process plant may be high, as it involves the full reconstruction of an existing facility, and economic feasibility is an issue.
5.5 Clinker production - dry process preheater kilns
5.5.1 Low Pressure Drop Cyclones for Suspension Preheaters Cyclones are a basic component of plants with pre-heating systems. The installation of newer cyclones in a plant with lower pressure losses will reduce the power consumption of the kiln exhaust gas fan system. Depending on the efficiency of the fan, 0.6-0.7 kWh/ton of clinker can be saved for each 50 mm water column (W.C) of pressure reduction. For older kilns this amounts to savings of 0.6-1.0 kWh/ton of clinker. Installation of the cyclones can be expensive, however, since it may often entail the rebuilding or the substantial modification of the preheater tower. The costs are very site specific. New cyclone systems may also increase overall dust loading and increase dust carryover from the preheater tower. However, if followed by an inline raw mill, the dust carryover problem becomes less of an issue.
5.5.2 Heat Recovery for Cogeneration Waste gas discharged from the kiln exit gases, the clinker cooler system, and the kiln pre-heater system all contain useful energy that can be utilised. Only in long-dry kilns is the temperature of the exhaust gas sufficiently high to cost-effectively recover the heat through power generation. Cogeneration systems can either be direct gas turbines that utilize the waste heat (top cycle), or the installation of a waste heat boiler system that runs a steam turbine (bottom cycle). Steam turbine systems have been installed in many plants worldwide and have proven to be economic. Heat recovery has limited application for plants with in-line raw mills, as the heat in the kiln exhaust is used for raw material drying. While electrical efficiencies are still relatively low (18%), based on several case studies power generation may vary between 10 46
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and 23 kWh/ton of clinker. Electricity savings of 20 kWh/ton of clinker are assumed.
5.5.3 Dry Process Conversion to Multi-Stage Preheater Kiln Older dry kilns may only preheat in the chain section of the long kiln, or may have single- or two-stage preheater vessels. Especially, long dry kilns may not have any preheater vessels installed at all. This leads to a low efficiency in heat transfer and higher energy consumption. Installing multi-stage suspension preheating (i.e. four- or five-stage) may reduce the heat losses and thus increase efficiency. Modern cyclone or suspension preheaters also have a reduced pressure drop, leading to increased heat recovery efficiency and reduced power use in fans (see low pressure drop cyclones above). By installing new preheaters, the productivity of the kiln will increase, due to a higher degree of pre-calcination (up to 30-40%) as the feed enters the kiln. Also, the kiln length may be shortened by 20-30% thereby reducing radiation losses. As the capacity increases, the clinker cooler may have to be adapted to be able to cool the larger amounts of clinker. The conversion of older kilns is attractive economically when the old kiln needs replacement and a new kiln would be too expensive, assuming that limestone reserves are adequate. Energy savings depend strongly on the specific energy consumption of the dry process kiln to be converted as well as the number of preheaters to be installed.
5.5.4 Upgrading to a Preheater/Precalciner Kiln An existing preheater kiln may be converted to a multi-stage preheater precalciner kiln by adding a precalciner and, when possible an extra preheater. The addition of a precalciner will generally increase the capacity of the plant, while lowering the specific fuel consumption and reducing thermal NOx emissions (due to lower combustion temperatures in the pre-calciner). Using as many features of the existing plant and infrastructure as possible special precalciners have been developed by various manufacturers to up grade existing plants. Generally, the kiln, foundation and towers are used in the new plant, while cooler and preheaters are replaced. Cooler replacement may be necessary in order to increase the cooling capacity for larger production volumes. The conversion of a plant in Italy, using the existing rotary kiln, led to a capacity increase of 80-100%, while reducing specific fuel consumption with resulting savings of 11-14%. Generally, fuel savings will depend strongly on the
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efficiency of the existing kiln and on the new process parameters (e.g. degree of precalcination and cooler efficiency). Older calciners can also be retrofitted for energy efficiency improvement and NOx emission reduction.
5.5.5 Conversion of Long Dry Kilns to Preheater/Precalciner Kiln A long dry kiln can be upgraded to the current state of the art multi-stage preheater/precalciner kiln. Energy savings reflect the difference between the average dry kiln specific fuel consumption and that of a modern preheater, pre-calciner kiln. However, economic feasibility may be an issue.
5.6 Finish Grinding
5.6.1 Process Control and Management – Grinding Mills Control systems for grinding operations are developed using the same approaches as for kilns (see above). The systems control the flow in the mill and classifiers, producing a stable and high quality product. Several systems are marketed by a number of manufacturers. Expert systems have been commercially available since the early 1990’s. The payback is estimated to be between 1 and 2 years. Energy savings range between 2.5 and 10 %, with increased product quality (lower deviation) and production increases of 2.5 –10 %, after installing control systems in finishing mills
5.6.2 Advanced Grinding Concepts The energy efficiency of ball mills for use in finish grinding is relatively low, consuming up to 30-42 kWh/ton of clinker depending on the fineness of the cement. Several new mill concepts exist that can significantly reduce power consumption in the finish mill to 20-30 kWh/ton of clinker, including roller presses, roller mills, and roller presses used for pre-grinding in combination with ball mills. Roller mills employ a mix of compression and shearing, using 2-4 grinding rollers carried on hinged arms 48
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riding on a horizontal grinding table. In a high-pressure roller press, two rollers pressurize the material up to 3,500 bar, improving the grinding efficiency dramatically. A variation of the roller mill is the air swept ring roller mill, which has been shown to achieve an electricity consumption of 23 kWh/ton of cement with a Blaine of 3000. Another mill concept is the Horomill; it is a horizontal roller within a cylinder. The centrifugal forces resulting from the movement of the cylinder cause a uniformly distributed layer to be carried on the inside of the cylinder. The layer passes the roller (with a pressure of 700-1000 bar. The finished product is collected in a dust filter. The Horomill is a compact mill that can produce a finished product in one step and hence has relatively low capital costs. Grinding Portland Cement with a Blaine of 3200 cm2/g consumes approximately 21 kWh/ton and even for pozzolanic cement with a Blaine of 4000, power use may be as low as 25 kWh/ton of cement. Today, high-pressure roller presses are most often used to expand the capacity of existing grinding mills, and are found especially in countries with high electricity costs or with poor power supply. New designs of the roller mills allow for longer operation times (> 20,000 hours). The electricity savings of a new finish grinding mill when replacing a ball mill are estimated at 25 kWh/ton cement. The addition of a pregrinding system to a ball mill will result in savings of 6-22 kWh/ton cement. Some new mill concepts may lead to a reduction in operation costs of as much as 30-40%.
5.6.3 High-Efficiency Classifiers A recent development in efficient grinding technologies is the use of high-efficiency classifiers or separators. Classifiers separate the finely ground particles from the coarse particles. The large particles are then recycled back to the mill. Standard classifiers may have a low separation efficiency, which leads to the recycling of fine particles, resulting in extra power use in the grinding mill. In high-efficiency classifiers, the material is more cleanly separated, thus reducing over-grinding. High efficiency classifiers or separators have had the greatest impact on improved product quality and reducing electricity consumption. Newer designs of high-efficiency separators aim to improve the separation efficiency further and reduce the required volume of air (hence reducing power use), while optimizing the design.
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5.6.4 Improved Grinding Media Improved wear resistant materials can be installed for grinding media, especially in ball mills. Grinding media are usually selected according to their wear characteristics. Increases in the ball charge distribution and surface hardness of grinding media and wear resistant mill linings have shown a potential for reducing wear as well as energy consumption. Improved balls and liners made of high chromium steel is one such material but other materials are also being developed. Other improvements include the use of improved liner designs, such as grooved classifying liners. These have the potential to reduce grinding energy use by 5-10% in some mills, which is equivalent to an estimated savings of 1.8 kWh/ton cement.
5.7 Plant-Wide Measures
5.7.1 Energy Management Prior to all technological changes and modifications an energy management program is the most powerful approach and the most successful and cost-effective way to bring about energy efficiency improvements. Energy efficiency does not happen on its own. A strong energy management program is required to create a foundation for positive change and to provide guidance for managing energy throughout an organization. Energy management programs also help to ensure that energy efficiency improvements do not just happen on a one-time basis, but rather are continuously identified and implemented in an ongoing process of continuous improvement. Furthermore, without the backing of a sound energy management program, energy efficiency improvements might not reach their full potential due to lack of a systems perspective and/or proper maintenance and follow-up. Such a strong energy management program, which has the management commitment from top to down, is also the ideal framework to establish a benchmarking system, as it is described in the BMT instruction manual. Vice versa a pilot benchmarking project should lead to the establishment of such an energy management program. Right from the beginning the plant management must be aware that although technological changes in equipment can help to reduce energy use, changes in staff 50
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behavior and attitude may have a greater impact. Staff should be trained in both skills and the company’s general approach to energy efficiency in their day-to-day practices. Personnel at all levels should be aware of energy use and objectives for energy efficiency improvement. Often this information is acquired by lower level managers but not passed to upper management or down to staff. Programs with regular feedback on staff behavior, such as reward schemes, have had the good results. Though changes in staff behavior, such as switching off lights or closing windows and doors, often save only small amounts of energy at one time, taken continuously over longer periods they may have a much greater effect than more costly technological improvements. Most importantly, companies need to institute strong energy management programs that oversee energy efficiency improvement across the corporation. An energy management program will see to it that all employees actively contribute to energy efficiency improvements.
5.7.2 Preventative Maintenance Preventative maintenance includes training personnel to be attentive to energy consumption and efficiency. Successful programs have been launched in a variety of industries. While many processes in cement production are primarily automated, there are still opportunities, requiring minimal training of employees, to increase energy savings. Preventative maintenance (e.g. for the kiln refractory) can also increase a plant’s utilization ratio through reduced downtime over the long term. For example the reduction of false air input into the kiln at the kiln hood has the potential to save energy.
5.7.3 Motor Systems When considering energy efficiency improvements to a facility’s motor systems, it is important to take a “systems approach.” A systems approach strives to optimize the energy efficiency of entire motor systems (i.e., motors, drives, driven equipment such as pumps, fans, and compressors, and controls), not just the energy efficiency of motors as individual components. A systems approach analyzes both the energy supply and energy demand sides of motor systems as well as how these sides interact to optimize total system performance These include not only energy use but also system uptime and productivity.
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A systems approach typically involves the following steps: •
First, all applications of motors in a facility should be located and identified.
•
Second, the conditions and specifications of each motor should be documented to provide a current systems inventory.
•
Third, the needs and the actual use of the motor systems should be assessed to determine whether or not motors are properly sized and also how well each motor meets the needs of its driven equipment.
•
Fourth, information on potential repairs and upgrades to the motor systems should be collected, including the economic costs and benefits of implementing repairs and upgrades to enable the energy efficiency improvement decision-making process.
•
Finally, if upgrades are pursued, the performance of the upgraded motor systems should be monitored to determine the actual costs savings.
5.7.4 Compressed Air Systems Compressed air systems are used in different parts of the plants. Total energy consumption by compressed air systems is relatively small in cement plants. However, it can amount to a considerable expense if the systems run continuously and end-uses are offline. Compressed air is probably the most expensive form of energy available in a plant because of its poor efficiency, typically around 10%. Because of this inefficiency, if compressed air is used, it should be of minimum quantity for the shortest possible time, constantly monitored and weighed against alternatives.
5.7.5 Lighting Energy use for lighting in the cement industry is very small. Still, energy efficiency opportunities may be found that can reduce energy use cost-effectively. Lighting is used either to provide overall ambient lighting throughout the manufacturing, storage and office spaces or to provide low-bay and task lighting to specific areas. Highintensity discharge (HID) sources are used for the former, including metal halide, high-pressure sodium and mercury vapor lamps. Fluorescent, compact fluorescent (CFL) and incandescent lights are typically used for task lighting in offices. 52
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5.8 Product Changes
5.8.1 Alkali Content Sometimes low-alkali cements are required by the cement company’s customers. Low alkali cement production leads to higher energy consumption. Reducing the alkali content is achieved by venting (called the by-pass) hot gases and particulates from the plant, loaded with alkali metals. This becomes cement kiln dust (CKD) which has to be disposed. Some customers demand a lower alkali content, as it allows greater freedom in the choice of aggregates. The use of fly-ash or blast-furnace slags as aggregates (or in the production of blended cement, see below) may reduce the need for low-alkali cement. There are no investments involved in this product change, although cement users (e.g. ready-mix producers) may need to change the type of aggregates used (which may result in costs). Hence, this measure, as well as the following (see below), is most successfully implemented in coordination with ready-mix producers and other large cement users.
5.8.2 Blended Cements The production of blended cements involves the inter-grinding of clinker with one or more additives (fly ash, pozzolans, granulated blast furnace slag, silica fume, and volcanic ash) in various proportions. The use of blended cements is a particularly attractive efficiency option since the inter-grinding of clinker with other additives not only allows for a reduction in the energy used (and carbon emissions) in clinker production, but also corresponds to a reduction in carbon dioxide emissions in calcination. Blended cement has been used for many decades and longer around the world. Blended cements are very common in Europe, and blast furnace and pozzolanic cements account for about 12% of total cement production with Portland composite cement accounting for an additional 44%. Blended cement was introduced to reduce production costs for cement (especially energy costs), expand capacity without extensive capital costs, and to reduce emissions from the kiln. In Europe a common 53
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standard has been developed for 25 types of cement (using different compositions for different applications). The European standard allows wider applications of additives. Many other countries around the world use blended cement. Blended cements demonstrate a higher long-term strength, as well as improved resistance to acids and sulfates, while using waste materials for high-value applications. Shortterm strength (measured after less than 7 days) may be lower, although cement containing less than 30% additives will generally have setting times comparable to concrete based on Portland cement. The costs of applying additives in cement production may vary. Capital costs are limited to extra storage capacity for the additives. However, blast furnace slag may need to be dried before use in cement production. This can be done in the grinding mill, using exhaust from the kiln, or supplemental firing, either from a gas turbine used to generate power or a supplemental air heater. The operational cost savings will depend on the purchase (including transport) costs of the additives, the increased electricity costs for (finer) grinding, the reduced fuel costs for clinker production and electricity costs for raw material grinding and kiln drives, as well as the reduced handling and mining costs. These costs will vary by location, and would need to be assessed on the basis of individual plants.
5.8.3 Limestone Portland Cement Similar to blended cement, limestone is inter-ground with clinker to produce cement, reducing the needs for clinker-making and calcination. This reduces energy use in the kiln and clinker grinding and CO2 emissions from calcination and energy use. Addition of up to 5% limestone has shown to have no negative impacts on the performance of Portland cement, while optimized limestone cement would improve the workability slightly. Adding 5% limestone would reduce fuel consumption by 5%, power consumption for grinding by 3.0 kWh/ton of cement, and CO2 emissions by almost 5%. Additional costs would be minimal, limited to material storage and distribution, while reducing kiln operation costs by 5%.
5.8.4 Reducing the Fineness for Particular Applications Cement is normally ground to a uniform fineness. However, the applications of cement vary widely, and so does the optimal fineness. The grinding of the cement to
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the desired fineness could reduce the energy demand for grinding. The exact savings will depend on the grindability of the clinker. As a rule of thumb, for each 100 additional Blaine points, grinding power requirements increase by 5 %. Note that finer cement may reduce the amount of concrete needed for a structure, due to the higher strength. It is hard to estimate the total savings due to the many factors affecting strength of concrete and grinding energy requirements. Also, without a detailed assessment of the market and applications of cement, it is difficult to estimate the total potential contribution of this measure to potential energy savings in the cement industry.
5.9 Conclusion The cost of energy as part of the total production costs in the cement industry is significant. Attention to improve energy efficiency is high thus. Historically, energy intensity has declined; mainly between 1970 and 1999, primary physical energy intensity for cement production dropped 1%/year, although more recently energy intensity seems to have stabilized with the gains. Coal and coke are currently the primary fuels for the sector; natural gas is an option to mitigate CO2 emissions, but expensive compared to coal as fuel. More recently, there is a slight increase in the use of waste fuels, addressing both the cost factor and CO2 mitigation by partly replacing fossil fuels with waste as fuel. E.g. The British Cement Association has formulated a target to reduce fossil fuels consumption until 2010 by up to 30% compared to the baseline of 1998. Despite the historic progress, there is ample room for energy efficiency improvement. The relatively high share of other than state of the art dry process preheater and precalciner plants suggests the existence of a considerable potential. Substantial potential for energy efficiency improvement exists in the cement industry and in individual plants. A portion of this potential will be achieved as part of (natural) modernization and expansion of existing facilities, as well as construction of new plants in particular regions. Still, a relatively large potential for improved energy management practices exists.
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6 References and Links BEST AVAILABLE TECHNIQUES FOR THE CEMENT INDUSTRY, CEMBUREAU 2000 http://www.cembureau.be/Documents/Publications/CEMBUREAU_BAT_Reference_ Document_2000-03.pdf BREF in the Cement and Lime Manufacturing Industries – EU Environmental Protection Agency 2001 - http://www.epa.ie/downloads/advice/brefs/cement.pdf Energy Efficiency Improvement Opportunities for the Cement Industry - Ernst Worrell, Christina Galitsky and Lynn Price - Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 2008 http://ies.lbl.gov/iespubs/Cement_EEMs_English.pdf Energy Consumption Benchmark Guide: Cement Clinker Production - CIPEC, Canada 2001 - http://oee.nrcan.gc.ca/publications/industrial/BenchmCement_e.pdf Energy Efficiency and CO2 Emission Reduction Potentials and Policies in the Cement Industry: Towards a plan of action - British Cement Association 2006 http://www.iea.org/Textbase/work/2006/cement/proceedings.pdf Sustainable cement production: Co-processing of Alternative Fuels and Raw Materials in the Cement Industry, CEMBUREAU 2009 http://www.cembureau.be/Documents/Press%20Release/Sustainable%20cement%2 0production%20Brochure.pdf Alternative Fuels in Cement Manufacture - Technical and Environmental Review, CEMBUREAU 1997 http://www.cembureau.be/Documents/Publications/Alternative_Fuels_in_Cement_Ma nufacture_CEMBUREAU_Brochure_EN.pdf World Best Practice Energy Intensity Values for Selected Industrial Sectors - Ernst Worrell, Maarten Neelis - Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, 2007 - http://ies.lbl.gov/iespubs/62806.pdf U.S. Geological Survey - http://minerals.usgs.gov/minerals/pubs/commodity/cement/ Final report: IMPEL Workshop on Licensing and Enforcement Practices in a Cement Plant using Alternative Fuel - CENTRIC AUSTRIA INTERNATIONAL 1998 http://kundencenter.linea7.com/files/users/centric.at/download_area/impel1998ceme nt.pdf Portland Cement Association (PCA) - http://www.cement.org/ British Cement Association (BCA) - http://www.cementindustry.co.uk Canadian Cement Association - http://www.cement.ca/
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The European Cement Association (CEMBUREAU) - http://www.cembureau.be/ Indian Cement Manufacturers Association - http://www.cmaindia.org/ Cement Concrete & Aggregates Australia - http://www.concrete.net.au/ Japan Cement Association - http://www.jcassoc.or.jp/cement/2eng/ea.html German Cement Works' Association - http://www.vdz-online.de/314.html?&lang=en Austrian Cement Industry Association - http://www.zement.at/
More information can be found by searching the cited links.
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