Cement is a global commodity, manufactured at thousands of plants. The industry is consolidating globally, but large international firms account for only 30% of the worldwide market. The principal and most visible market for cement is the construction industry in a multitude of applications where it is combined with water to make concrete. Most modern civil engineering projects, office buildings, apartments and domestic housing projects use concrete, often in association with steel reinforcement systems. In many developed countries, market growth is very slow, with cement used in bulk primarily for infrastructure construction, based on UNEPTIE. In developing country markets (e.g. China), growth rates are more rapid. Because it is both global and local, the cement industry faces a unique set of issues, which attract attention from both local and international level.
Cement accounts for 83% of total energy use in the production of non-metallic minerals and 94% of CO2 emissions. Energy represents 20% to 40% of the total cost of cement production. The production of cement clinker from limestone and chalk by heating limestone to temperatures above 950°C is the main energy consuming process. Portland cement, the most widely used cement type, contains 95% cement clinker. Large amounts of electricity are used grinding the raw materials and finished cement.
The clinker-making process also emits CO2 as a by-product during the calcination of limestone. These process emissions are unrelated to energy use and account for about 3.5% of CO2 emissions worldwide and for 57% of the total CO2 emissions from cement production. Emissions from limestone calcination cannot be reduced through energy-efficiency measures or fuel substitution, but can be diminished through production of blended cement and raw material selection.
Cement is a global commodity, manufactured at thousands of plants. The industry is consolidating globally, but large international firms account for only 30% of the worldwide market (European Commission, 1997). The principal and most visible market for cement is the construction industry in a multitude of applications where it is combined with water to make concrete. Manufacturing industries in general account for one-third of global energy use. Direct industrial energy and process CO2 emissions amount to 6.7 gigatonnes (Gt), about 25% of total worldwide emissions, of which 30% comes from the iron and steel industry, 27% from non-metallic minerals (mainly cement) and 16% from chemicals and petrochemicals production (IEA, 2008). Cement production involves the heating, calcining and sintering of blended and ground materials to form clicker. As a result, cement manufacturing is the third largest cause of man-made CO2 emissions due to the production of lime, the key ingredient in cement. Therefore, energy savings during cement production could lead to lower environmental impact. In the cement/concrete industry improvement of energy efficiency and reduction of CO2 emissions could be mainly achieved through two procedures: (i) by changes in the manufacturing and production processes, and (ii) by adjusting the chemical composition of cement. Manufacturing and production processes can be improved by changing energy management and by investing in new equipment and/or upgrades. Changes in the chemical formulation of cement have been demonstrated to save energy and reduce CO2 emissions, but their widespread adoption has thus far been hampered by the fact that developing a new industrial standard is complex and requires time. This holds in particular for the cement industry which is a highly capital intensive and competitive sector with long economic lifetimes of existing facilities so that changes in the existing capital stock cannot easily be made.
The largest opportunities for improving energy efficiency and reducing CO2 emissions can be achieved by improving the cement manufacturing process. In the cement industry pyroprocessing (processing the raw material into cement under a high temperature, e.g., above 8000C) is a very common technological procedure, which accounts for 74% of the energy consumption in global cement/concrete industries. Since the thermal efficiency through the use of this conventional technology of pyroprocessing is slightly higher than 30% on average (Mersmann, 2007), there could be considerable scope for improvements. Grinding and milling account for 5.8% of cement/concrete energy consumption (Choate, 2003). These operations have an energy efficiency ranging from 6 to 25% and also offer a large opportunity for energy saving. The following figure presents the cement production process.
The potential opportunities for improving energy efficiency and lower CO2 emissions in raw material generation and production of concrete are smaller than in cement manufacturing. For instance, CO2 emissions during transport could be reduced by replacing diesel fuel with biodiesel. Normally, energy efficiency improvements proportionally reduce the emissions of CO2 generated from fossil fuel combustion and electricity generation. However, it should be noted that reducing CO2 emissions from cement manufacturing by a percentage proportional to energy efficiency improvements is not possible. More than half of the CO2 emissions associated with cement/concrete are a result of the chemical reactions necessary for converting raw materials and not a result of the energy required to produce these reactions. For example, if a near-zero CO2 emitting fuel (e.g. nuclear energy, biomass) were utilised for all pyroprocessing energy needs, then CO2 emissions could be reduced by 54%.
Another way to reduce emissions is to substitute fossil fuels with waste or biomass. Cement kilns are well suited for waste-combustion because of their high process temperature and because the clinker product and limestone feedstock act as gascleaning agents. Used tyres, wood, plastics, chemicals and other types of waste are co-combusted in cement kilns in large quantities. Plants in Belgium, France, Germany, the Netherlands and Switzerland have reached average substitution rates of from 35% to more than 70%. Some individual plants have even achieved 100% substitution using appropriate waste materials. However, very high substitution rates can only be accomplished if a tailored pre-treatment and surveillance system is in place. Municipal solid waste, for example, needs to be pre-treated to obtain homogeneous calorific values and feed characteristics. The cement industry in the United States burns 53 million used tyres per year, which is 41% of all tyres that are burnt and is equivalent to 0.39 Mt or 15 PJ. About 50 million tyres, or 20% of the total, are still used as landfill. Another potential source of energy is carpets: the equivalent of about 100 PJ per year are dumped in landfills – these could instead be burnt in cement kilns. Although these alternative materials are widely used, their use is still controversial, as cement kilns are not subject to the same tight emission controls as waste-incineration installations. According to IEA statistics, the cement industry in OECD countries used 1.6 Mtoe of combustible renewables and waste in 2005, half of it industrial waste and half wood waste (Taylor, 2006). Worldwide, the sector consumed 2.7 Mtoe of biomass and 0.8 Mtoe of waste. This equals less than 2% of total fuel use in this sector. From a technical perspective, the use of alternative fuels could be raised to 24 Mtoe to 48 Mtoe, although there would be differences among regions due to the varying availability of such fuels. This would yield CO2 reductions in the range of 100 Mt to 200 Mt a year.
Yet another way to reduce energy and process emissions in cement production is to blend cements with increased proportions of alternative (non-clinker) feedstocks, such as volcanic ash, granulated blast furnace slag from iron production, or fly ash from coal-fired power generation. The use of such blended cements varies widely from country to country. It is high in continental Europe, but low in the United States and the United Kingdom. In the United States and in China, other clinker substitutes are added directly at the concrete-making stage. For the long run, cement lacks a viable carbon-free alternative, and the IEA scenarios imply a heavy reliance on Carbon Capture and Storage (CCS) cement kilns with oxy-fuelling (IEA, 2008).
In the cement pyroprocessing process it is important to keep in mind that waste materials combust and burn at different temperatures under different conditions. Therefore, solid waste fuels need to be introduced into the kiln in such a manner that they do not significantly change the temperature profile and chemical reactions in the overall pyroprocessing. Sometimes it is necessary to add solid waste through a hatch or valve structure in the kiln shell, which implies a technical challenge and which partly offsets the efficiency gains and CO2 emission reductions. Finally, receiving and handling of alternate or waste fuels can raise technical liability and political concerns. Cement manufacturing companies do not desire to be labeled as handlers of hazardous wastes and surrounding communities may have concerns about hazardous waste transport and handling in a nearby cement plant.
Furthermore, blended cements offer a major opportunity for energy conservation and emission reductions, but their use would in many cases require revisions to construction standards, codes and practices.
Of the cement production chain steps, grinding and milling operations are rather energy inefficient. As mentioned before, typical systems routinely run at 6 to 25% on-site energy efficiency (US Department of Energy, 2003). Energy improvement of grinding and milling could be increased by using modern mill systems which comprise several units of process equipment with high-pressure, twin-roll presses, tube mills, ball mills, and conventional or high-efficiency separators (IEA, 2009).
The main potential in reducing energy consumption and CO2 emissions from cement/concrete production is in improvement of cement pyroprocessing. Pyroprocessing transforms the raw mix into clinkers. At present, about 78% of Europe's cement production is from dry process kilns, a further 16% of production is accounted for by semi-dry and semi-wet process kilns, with the remainder of European production, about 6%, coming from wet process kilns. The wet process kilns operating in Europe are generally expected to be converted to dry process kiln systems when renewed, as are semi-dry and semi-wet processes kiln systems. On average, pyroprocessing systems in the EU and US operate at below 35% thermal efficiency, which is rather low. The percentage is even lower for developing countries (Karstensen, no date). These process improvements will come from better energy management, upgrading existing equipment (e.g. replacing wet kilns, upgrading to preheater and precalciners), adopting new pyroprocessing technologies (e.g. fluidised bed systems) and, in the longer term, performing the R&D necessary to develop new concepts for the cement manufacturing processes.
Japan is the leading country when it comes to energy efficiency in the cement sector. Europe (4.1 GJ/t cement on average) could not compete with Japan (3.1 GJ/t), but many other parts of the world show much higher energy consumption patterns, e.g. the average US (5.3 GJ/t) or Chinese plant are well above the European average plant, regarding energy consumption (Worrell et al., 2004).
The typical energy balances for the major pyroprocessing systems are shown below. These balances show where energy losses occur and which thus represent an opportunity for improving energy efficiency and lowering fuel-based CO2 emissions. In particular, the table shows that significant improvements can be made by switching from wet to dry cement processes. The individual energy use areas (e.g. clinker discharge, kiln shell, etc.) in the table show the area and the magnitude of the opportunities available from managing energy losses by improving specific equipment or practices.
Through energy audits, including kiln system performance testing and calculation of mass and heat balances, specific opportunities for improving energy efficiency and lowering CO2 emissions can be identified. A cement manufacturing energy audit should at a minimum address the energy use and recommend potential actions, such as:
- Lower kiln exit gas losses
- install devices to provide better conductive heat transfer from the gases to the materials, e.g., kiln chains
- operate at optimal oxygen levels (control combustion air input)
- optimise burner flame shape and temperature
- improve or add additional preheater capacity
- Lower moisture absorption opportunities for raw meal and fuels: avoiding the need to evaporate adsorbed water
- Lower dust in exhaust gases by minimizing gas turbulence: dust carries energy away from the kiln where it is captured in dust collectors; the dust is recycled into the raw meal and fed into the kiln where it is reheated
- Lower clinker discharge temperature, retaining more heat within the pyroprocessing system
- Lower clinker cooler stack temperature
- recycle excess cooler air
- reclaim cooler air by using it for drying raw materials and fuels or preheating fuels or air
- Lower kiln radiation losses by using the correct mix and more energy efficient refractories to control kiln temperature zones
- Lower cold air leakage
- close unnecessary openings
- provide more energy efficient seals
- operate with as high a primary air temperature as possible
- Optimise kiln operations to avoid upsets.
Wet cement production involves mixing raw materials (limestone and clay or loam) with water in order to produce slurry. Further in the process, water is evaporated from the homogenized mixture and this step in the production requires significant amounts of energy. The raw meal (dried slurry) is subjected to high temperatures in a rotary kiln, where the reaction of calcination takes place (its final products are lime and CO2). The lime is further influenced by the temperatures of 1,400 to 1,450 oC. This reaction, called sintering, results in clinker. The final stage of cement production is fine crushing of clinker and mixing the substance with mineral components, such as slag, fly ash or gypsum.
In the case of dry cement production, the raw materials are mixed without water and therefore the evaporation process can be omitted. The latter technology could reduction the energy consumption from the ‘wet’ to the ‘dry’ process by over 50%.
Existing technology in the cement industry can be upgraded in several ways. Table 26‑3 shows, based on data from US cement plants, the impact of possible upgrading measures such as from wet to dry processes and within the latter category, the impact of using preheater and precalciner technologies. The Table shows that if all US plants would upgrade their pyroprocessing to the level of the best US plant (i.e. a dry process system with preheater with precalciner technology), the industry would lower its energy consumption by 30% to approximately 3,407,650 Jouls/tonne of cement and reduce CO2 emissions by 13% to 75.3 Mt/year.
In terms of new technologies in the cement sector, several technologies are being tested and demonstrated, such as fluidised-bed kilns. Several large-scale fluidised-bed kiln pilots (200 tonnes/day) have been developed since the mid-1990s and have demonstrated significant energy savings. For instance, it is estimated that a full-scale fluidised-bed (3,000 tonnes/day) system would be as efficient as the most advanced US kiln utilising a preheater and precalciner, and 37% more efficient than an average US plant. For fluidised-bed systems the required capital costs are about 12% lower than those of a modern cement facility and their operating costs are about 75% of a modern cement facility’s operating costs (US Department of Energy, 2003). However, in comparison with older, fully capitalised kiln-based plants, the fluidised bed systems are relatively expensive so that they are likely to be considered only for future capacity expansion. Another barrier to adoption of fluidised-bed systems is the reluctance to invest in such large capital expenditures, as the systems have been demonstrated only at small-scale facilities.
Cement plants, given their large-scale industrial thermal energy demand, offer opportunities for co-generation of electricity and/or steam production, particularly if the co-generation system is part of the initial plant design. This could significantly improve the overall energy efficiency of some manufacturing operations. Presently, five cement manufacturing plants produce electricity on-site through co-generation (US Department of Energy, 2003). Moreover, utilisation of waste heat in preheater heat exchange systems is usually more energy efficient than the co-generation of electricity with its inherently low conversion efficiency of thermal to electrical energy (typically about 10,481 Jouls are required to produce 1 kWh). Although co-generation of steam at a cement plant is possible, cement plants typically require little steam and are located in isolated areas where markets for excess steam generation are often not available.
An important benefit of enhancing energy efficiency in the cement industry would be the reduction in energy costs. Broadly speaking, in the EU cement industry the energy bill represents about 40% of total production costs, while European cement production techniques are amongst the most energy efficient in the world. Since the 1970s, in Europe the energy required for producing cement has fallen by about 30% and the scope for further improvements has became rather small. However, larger energy cost savings are still possible in other parts of the world.
In cement manufacturing, cost-effective efficiency gains in the order of 10% to 20% are already possible using commercially available technologies. The energy intensity of most industrial processes is at least 50% higher than the theoretical minimum determined by the basic laws of thermodynamics. Energy efficiency tends to be lower in regions with low energy prices. Cross-cutting technologies for motor and steam systems would yield efficiency improvements in all industries, with typical energy savings in the range of 15% to 30%. The payback period can be as short as two years, and in the best cases, the financial savings over the operating life of improved systems can run as high as 30% to 50%. In those processes where efficiency is close to the practical maximum, innovations in materials and processes would enable even further gains (IEA, 2008).
Cement manufacturing produces CO2 as it requires very high temperatures to burn raw materials and give the clinker its unique properties. CO2 is generated from three independent sources: de-carbonation of limestone in the kiln (about 525 kg CO2 per tonne of clinker), combustion of fuel in the kiln (about 335 kg CO2 per tonne of cement) and use of electricity (about 50 kg CO2 per tonne of cement). There are three central measures by which the cement industry may save direct CO2 emissions in the immediate future:
- Improvement of energy efficiency (a maximum of 2% is still feasible),
- Reduction of clinker/cement ratio (introduction of useful industrial by-products), and
- Increase in the use of waste as alternative fuel (national initiatives, adequate national implementation of certain directives regarding specific waste).
Based on the IEA (2008) analysis for blended cements, in total, the savings potential in this case amounts to 300 Mt CO2 to 450 Mt CO2 by 2050. The main approaches to this are to use:
- Blast-furnace slag that has been cooled with water, rather than air. About half of all blast-furnace slag is already used for cement-making where the slag is water-cooled and where transport distances and costs are acceptable. If all blast-furnace slag were used, this would yield a CO2 reduction of approximately 100 Mt CO2.
- Fly ash from coal-fired power plants. But the carbon content of fly ash can affect the concrete setting time, which determines the quality of the cement. To be used as clinker substitute, high-carbon fly ash must be upgraded. Technologies for this are just emerging. Special grinding methods are also being studied as a way to increase the reaction rate of fly ash, allowing the fly ash content of cement to increase to 70% compared with a maximum of 30% today (Justnes et al., 2005). China and India have the potential to significantly increase the use of fly ash. If the 50% of all fly ash that currently goes to landfill could be used, this would yield a CO2 reduction of approximately 75 Mt.
- Steel slag. The CemStar process, which uses a 15% charge of air-cooled steel slag pebbles in the rotary kiln feedstock mix, has been developed and successfully applied in the United States, resulting in a CO2 reduction of approximately 0.47 t/t steel slag added (Yates et al., 2004). In China, there are about 30 steel slag cement plants with a combined annual output of 4.8 Mt.However, steel slag quality varies and it is difficult to process, which limits its use. If the total worldwide BOF and EAF steel slag resource of 100 Mt to 200 Mt per year was used this way, the CO2 reduction potential would be 50 Mt to 100 Mt per year. Further analysis is needed to validate the viability of this option. Other materials that could be used to a greater extent as clinker substitutes include volcanic ash, ground limestone and broken glass. Such approaches could alleviate clinker substitute availability problems, and possibly pave the way to a 50% reduction of energy use and CO2 emissions. In the long term, new cement types may be developed that do not use limestone as a primary resource. These new types are called synthetic pozzolans. The technological feasibility, economics and energy effects of such alternative cements remain speculative.
- Blended cements offer a major opportunity for energy conservation and emission reductions, but their use would in many cases require revisions to construction standards, codes and practices. In total, the savings potential for blended cements amounts to 300 Mt CO2 to 450 Mt CO2 by 2050. Learning rate for CCS cement kilns under a current cost of 200 USD/ tCO2, is around 5%, while the cost target to reach commercialization in USD is 75.
For calculation of these GHG emission reductions, it is recommended to apply the approved methodologies for consolidated methodology for increasing the blend in cement production, methodology for greenhouse gas reductions through waste heat recovery and utilization for power generation at cement plans, switching fossil fuels, energy efficiency and fuel switching measures for industrial facilities, Emissions reduction through partial substitution of fossil fuels with alternative fuels or less carbon intensive fuels in cement manufacture project (large scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html.
Global demand for cement is forecast to grow by 4.7% annually to 2.8 billion metric tons in 2010. China, which is already by far the largest market for cement in the world, will show the largest increase in total amount of cement sold. Other developing parts of the Asia/Pacific region and Eastern Europe, as well as a number of nations in the Africa/Middle-East and Latin American regions will also record above-average cement market gains, fueled by a robust construction outlook. Vietnam, Thailand, Ukraine, Turkey and Indonesia are also expected to record strong increases in percentage terms. Market advances will be less robust in the developed areas of the USA, Japan and Western Europe, with maintenance and repair construction accounting for most of the growth in cement demand through 2010. However, a pickup a construction spending in Germany and Japan following an extended period of decline will help bolster overall developed world market growth.
Cement industry has devoted substantial effort to introducing innovative procedures in cement production. Considerable resources have been spent in recent years to investigate emerging and hopefully non-controversial and non-polluting technologies. Unfortunately, many such technologies have low capacities (some are still under development), are technically sophisticated, and currently not affordable by many developing countries. When comparing the state of the art technologies in terms of sustainability, suitability, performance, robustness, cost-efficiency, patent restrictions (availability), and competence requirements it can be concluding that at least in the short term cement industries are going to be based on pyroprocessing and grinding mills.
As described above, the most conventional way of producing cement is in kilns. Although in the developed world this is a standardised procedure, in the developing world we may face financial requirements which can not be easily met. High temperature cement kilns are common and available in most developing countries and can constitute an affordable, environmentally sound and sustainable treatment alternative. The choice of grinding mill will vary at different facilities due to a number of factors. While power consumption (and hence energy costs) at tube mills is higher, they have lower operating and maintenance costs than the other types of mills. Investment costs are difficult to compare in a general way, because site-specific constraints play an important role. Non-cost factors that affect investment decisions include the moisture content of the raw materials, vertical roller mills can both dry and grind materials, and so are the most suitable for raw materials with higher moisture content, while roller presses and horizontal roller mills may require a separate dryer.
In 1999, ten leading cement companies – representing approximately the one-third of the world’s cement production – voluntarily embarked on what became the Cement Sustainability Initiative (CSI), a member-led programme of the World Business Council for Sustainable Development (WBCSD). Its purpose is to find new ways for the industry to reduce its ecological footprint, understand its social contribution potential, and increase stakeholder engagement.
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