cement is a global commodity

8/10/2019 Cement is a Global Commodity

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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 CO2emissions. 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 950C 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.

Theclinker-making process also emits CO2as a by-product during the calcination of

limestone. These process emissions are unrelated to energy use and account for about

3.5% of CO2emissions worldwide and for 57% of the total CO2emissions 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.

Introductiontop:

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 CO2emissions

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
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cement) and 16% from chemicals and petrochemicals production (IEA,

2008). Cement production involves the heating, calcining and sintering of blended

and ground materials to formclicker.As a result, cement manufacturing is the third

largest cause of man-made CO2emissions due to the production oflime,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 CO2emissions 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 CO2emissions, but their widespreadadoption 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 thecement industrypyroprocessing (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.
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Figure 1: Cement production process (Source: Lootahgroup)

The potential opportunities for improving energy efficiency and lower CO2emissions

in raw material generation and production of concrete are smaller than in cement

manufacturing. For instance, CO2emissions during transport could be reduced byreplacing 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 CO2emissions from

cement manufacturing by a percentage proportional to energy efficiency

improvements is not possible. More than half of the CO2emissions 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 CO2emitting fuel (e.g.nuclear energy, biomass) were utilised

for all pyroprocessing energy needs, then CO2emissions could be reduced by 54%.
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Figure 2: Energy use and CO2

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 landfillsthese 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


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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).

Feasibility of technology and operational necessitiestop:

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 CO2emission 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.
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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).

Status of the technology and its future market potentialtop:

The main potential in reducing energy consumption and CO2emissions 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). Theseprocess 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 CO2emissions.

In particular, the table shows that significant improvements can be made by switching

fromwet 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
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opportunities available from managing energy losses by improving specific

equipment or practices.

Figure 3: Thermal energy balances (Source: The Rotary Cement Kiln, Kurt E. Perry)

Through energy audits, including kiln system performance testing and calculation of

mass and heat balances, specific opportunities for improving energy efficiency and

lowering CO2emissions 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


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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 amountsof 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 1,450oC. 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.


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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 CO2emissions by 13% to 75.3 Mt/year.

Figure 4: Energy use in US kilns

In terms of new technologies in the cement sector, several technologies are being

tested and demonstrated, such asfluidised-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
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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 facilitys 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-generationof 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.

Contribution of the technology to economic development (including energy market

support)top:

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


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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).

Climatetop:

Cement manufacturing produces CO2as it requires very high temperatures to burn

raw materials and give the clinker its unique properties. CO2is generated from three

independent sources: de-carbonation of limestone in the kiln (about 525 kg CO2per

tonne of clinker), combustion of fuel in the kiln (about 335 kg CO2per tonne of

cement) and use of electricity (about 50 kg CO2per tonne of cement). There are three

central measures by which the cement industry may save direct CO2emissions in the

immediate future:

Improvement of energy efficiency (a maximum of 2% is still feasible),

o Reduction of clinker/cement ratio (introduction of useful industrial by-

products), and

o 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 CO2to 450 Mt CO2by 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 CO2reduction of

approximately 100 Mt CO2.


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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 CO2reduction of approximately 75

Mt.

Steel slag. TheCemStar process,which uses a 15% charge of air-cooled steel

slag pebbles in the rotary kiln feedstock mix, has been developed andsuccessfully applied in the United States, resulting in a CO2reduction 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 CO2reduction 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 CO2emissions. 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 CO2to 450 Mt CO2by 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.
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Figure 8: CO2 emission reduction potentials based on best available technology(Source: IEA, 2008)

For calculation of these GHG emission reductions, it is recommended to apply the

approved methodologies forconsolidated 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 lesscarbon intensive fuels in cement manufactureproject (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.

Financial requirements and coststop:

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-averagecement market gains, fueled by a robust construction outlook. Vietnam, Thailand,
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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 decisionsinclude 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 companiesrepresenting approximately the one-third of

the worlds cement production voluntarily embarked on what became the Cement

Sustainability Initiative (CSI), a member-led programme of the World BusinessCouncil for Sustainable Development (WBCSD). Its purpose is to find new ways for
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the industry to reduce its ecological footprint, understand its social contribution

potential, and increase stakeholder engagement.

Referencestop:

American Coal Ash Association (ACAA), 2001. Coal Combustion Product Survey.

Choate, W., 2003. Energy and Emission reduction opportunities for the cement

industry. US Department of Energy.

European Commission, 1997. 4th Framework Programme for Research and

Technological Development (RTD), ATLAS Project. Available at:

http://ec.europa.eu/energy/atlas/html/hydotech.html

IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050.

International Energy Agency.

IEA, WBSCD, 2009. Cement Technology Roadmap 2009Carbon Emissions

Reductions up to 2050. International Energy Agency.

Justnes, H., Elfgren, L. and Ronin, V., 2005. Mechanism for Performance of

Energetically Modified Cement Versus Corresponding Blended Cement, Cement and

Concrete Research, 35 (2), pp. 315-323.

Karstensen, K.H., (no date). Sound Destruction of Obsolete Pesticides in Cement

Kilns in Developing Countries, The Foundation for Scientific and Industrial Research

(SINTEF).

Mersmann, M., 2007. Pyro-process Technology.Cement Industry Technical

Conference Record,IEEE, pp. 90-102.

Perry, Kurt E., 1986. Energy and Emission Reduction Opportunities for the Cement

Industry - The Rotary Cement Kiln, Chemical Publishing Co., Inc., New York, page

107

Scalon, J., 1992. Mineral Admixturer, ACI Compilation 22.
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Taylor, M. 2006. Energy Efficiency and CO2Emissions from the Global Cement

Industry. Paper prepeared for the IEA-WBCSD workshop, International Energy

Agency.

U.S. Department of Energy, 2003. Energy and Emission Reduction Opportunities for

the Cement Industry, Washington, D.C., USA.

Worrell, E., Price, L. and Galitsky, C., 2004. Emerging Energy-Efficient

Technologies in Industry: Case Studies of Selected Technologies, Nr. LBNL-54828:

Energy Analysis Department, Environmental Energy Technologies Division, Ernest

Orlando Lawrence Berkeley National Laboratory, University of California, Berkeley,

CA 94720.

Yates, J.R., Perkins, D. and Sankaranarayanan, R., 2004. Cemstar Process and

Technology for Lowering Greenhouse Gases and Other Emissions While Increasing

Cement Production, Hatch, Canada. Available at:

http://hatch.ca/Environment_Community/Sustainable_Development/Projects/Copy%2

0of%20CemStar-Process-final4-30-03.pdf
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Manufacturing - the cement kiln

Most Portland cement is made in a rotary kiln. Basically, this is a long cylinder

rotating about its axis once every minute or two. The axis is inclined at a slight angle,

the end with the burner being lower.

The rotation causes the raw meal to gradually pass along from where it enters at the

cool end, to the hot end where it eventually drops out and cools. They were

introduced in the 1890s and became widespread in the early part of the 20th century

and were a great improvement on the earlier shaft kilns, giving continuous production

and a more uniform product in larger quantities.

For information onreactions in the kiln see the clinker pages.

Principle of a basic wet-process kiln.

Wet process kilns

The original rotary cement kilns were called 'wet process' kilns. In their basic form

they were relatively simple compared with modern developments. The raw meal was

supplied at ambient temperature in the form of a slurry.
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A wet process kiln may be up to 200m long and 6m in diameter. It has to be long

because a lot of water has to be evaporated and the process of heat transfer is not very

efficient.

The slurry may contain about 40% water. This takes a lot of energy to evaporate and

various developments of the wet process were aimed at reducing the water content ofthe raw meal. An example of this is the 'filter press' (imagine a musical accordion 10-

20 metres long and several metres across) - such adaptions were described as 'semi-

wet' processes.

The wet process has survived for over a century because many raw materials are

suited to blending as a slurry. Also, for many years, it was technically difficult to get

dry powders to blend adequately.

Quite a few wet process kilns are still in operation, usually now with higher-tech bits

bolted on. However, new cement kilns are of the 'dry process' type.

Dry process kilns

In a modern works, the blended raw material enters the kiln via the pre-heater tower.

Here, hot gases from the kiln, and probably the cooled clinker at the far end of thekiln, are used to heat the raw meal. As a result, the raw meal is already hot before it

enters the kiln.

The dry process is much more thermally efficient than the wet process.

Firstly, and most obviously, this is because the meal is a dry powder and there is little

or no water that has to be evaporated.

Secondly, and less obviously, the process of transferring heat is much more efficient

in a dry process kiln.

An integral part of the process is a heat exchanger called a 'suspension preheater'. This

is a tower with a series of cyclones in which fast-moving hot gases keep the meal

powder suspended in air. All the time, the meal gets hotter and the gas gets cooler

until the meal is at almost the same temperature as the gas.

The basic dry process system consists of the kiln and a suspension preheater. The raw

materials, limestone and shale for example, are ground finely and blended to produce

the raw meal. The raw meal is fed in at the top of the preheater tower and passes

through the series of cyclones in the tower. Hot gas from the kiln and, often, hot air

from the clinker cooler are blown through the cyclones. Heat is transferred efficiently

from the hot gases to the raw meal.

The heating process is efficient because the meal particles have a very high surface

area in relation to their size and because of the large difference in temperature

between the hot gas and the cooler meal. Typically, 30%-40% of the meal is

decarbonated before entering the kiln.


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A development of this process is the 'precalciner' kiln. Most new cement plant is of

this type. The principle is similar to that of the dry process preheater system but with

the major addition of another burner, or precalciner. With the additional heat, about

85%-95% of the meal is decarbonated before it enters the kiln.

Basic principle of precalciner kiln.

Since meal enters the kiln at about 900 C, (compared with about 20 C in the wet

process), the kiln can be shorter and of smaller diameter for the same output. This

reduces the capital costs of a new cement plant. A dry process kiln might be only 70m

long and 6m wide but produce a similar quantity of clinker (usually measured in

tonnes per day) as a wet process kiln of the same diameter but 200m in length. For the

same output, a dry process kiln without a precalciner would be shorter than a wetprocess kiln but longer than a dry process kiln with a precalciner.


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Kiln and preheater tower: raw meal passes down the tower while hot gases rise up,

heating the raw meal. At 'A,' the raw meal largely decarbonates; at 'B,' the

temperature is 1000 C - 1200 C and intermediate compounds are forming and at 'C,'

the burning zone, clinker nodules and the final clinker minerals form. A preheater

tower is likely to have 4-6 stages, not the three shown here. Many designs are morecomplex but this diagram illustrates the principle. See the 'Clinker' pages for more

information on reactions in the kiln.

The kiln is made of a steel casing lined with refractory bricks. There are many

different types of refractory brick and they have to withstand not only the high

temperatures in the kiln but reactions with the meal and gases in the kiln, abrasion and

mechanical stresses induced by deformation of the kiln shell as it rotates.

Bricks in the burning zone are in a more aggressive environment compared with those

at the cooler end of the kiln (the 'back end'), so different parts of the kiln are linedwith different types of brick.

Periodically, the brick lining, or part of it, has to be replaced. Refractory life is

reduced by severe changes in temperature, such as occur if the kiln has to be stopped.

As the cost of refractories is a major expense in operating a cement plant, kiln

stoppages are avoided as far as possible.

As the meal passes through the burning zone, it reaches clinkering temperatures of

about 1400 C - 1500 C. Nodules form as the burning zone is approached. When the

clinker has passed the burning zone, it starts to cool, slowly at first, then much more

quickly as it passes over the 'nose ring' at the end of the kiln and drops out into thecooler.


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cement is a global commodity

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