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CLOSE THIS BOOKGlobal Overview of Construction Technology Trends: Energy Efficiency in Construction (HABITAT, 1995, 210 p.)
2. Energy efficiency in the production of high-energy content building materials
VIEW THE DOCUMENT2.1. Cement
VIEW THE DOCUMENT2.2. Lime
VIEW THE DOCUMENT2.3. Clay bricks
VIEW THE DOCUMENT2.4. Ceramic wall and floor tiles (2)

Global Overview of Construction Technology Trends: Energy Efficiency in Construction (HABITAT, 1995, 210 p.)

2. Energy efficiency in the production of high-energy content building materials

2.1. Cement

Cement production is an energy-intensive industry and the cost of required energy constitutes approximately 25 per cent of the price of the finished product (1). The total cement production in the world was about 1200 million tons in 1993 which used about 8 × 109 GJ of primary energy which is more than 1 per cent of world’s total primary energy consumption. Thus, the potential impact of energy savings in cement industry is considerable. One of the characteristics of cement industry is that different production technologies consume different amounts of energy for the same amount of output.

The important stages of cement production include:

(i) Raw material winning;
(ii) Raw mix preparation (grinding);
(iii) Firing the raw mix (producing clinker); and
(iv) Clinker grinding and mixing.

All these stages require energy with various intensities, however, the most vital energy consuming stages are mechanical processing of raw material and clinker (grinding) and firing the raw mix (kiln process).

Cement production is basically a choice between rotary kiln technology and vertical shaft kiln technology. The rotary kiln is more popular, due to several technical advantages. However, on account of energy consumption alone, the shaft kiln is more efficient - as illustrated in table 1.

Table 1. Typical energy consumption patterns of cement manufacturing processes in Europe (fossil fuel only)

Type of kiln

Energy consumption in kcal/kg of clinker

Shaft kiln

750

Rotary kiln types


Dry (long kiln)

860

Wet (long kiln)

1300

Dry (suspension pre-heater)

790

Source: Spence, R. J. S., Reference No. 6

There are two basic types of cement production technology in the rotary kiln process: dry and wet processes. But intermediate processes are also used: semi-dry and semi-wet. In the dry process, all raw material grinding and blending operations are done with dry materials and the resulting powder is fed into the kiln system. In the wet process, the raw mix is wet ground and then in the form of slurry containing 30 to 40 per cent moisture is pumped to homogenization tanks where the final mixing is accomplished. In the past, wet process in cement manufacture was very common, but today the global trend is towards the dry process.

Comparison of the two basic cement manufacturing processes from the viewpoint of energy requirements shows the following:

Wet process

Dry process

Electric power

85-105 kwh/ton

105-125 kwh/ton

Fuels

5,000-6,000 kJ/kg

3,000-3,600 kJ/kg

Energy consumption depends not only on specific conditions prevailing in each factory but also on the choice of suitable equipment and proper technology, on the condition of the equipment and on its professional maintenance and operation (2).

Table 2 shows some representative figures for the energy requirements of the different kiln processes, and typical values for power consumption in the form of electrical energy. These are process-energy rather than gross-energy requirements. However, they are a reasonable approximation to gross-energy requirements and they indicate the relative-energy requirements of the different processes (3).

Table 2. Process-energy requirements in various processes of cement manufacture

Process

Process energy requirement
(MJ/ton)

Source of data

Dry process (kiln energy) suspension preheater

3300

NATO (Europe)

3300

ETSU (United Kingdom)

3600-4000

Rai (India)

Semi dry

5074

Ming-yu (China)

Wet process (kiln energy)

5400

NATO

6100

ETSU

5700-6500

Rai

Vertical-shaft (kiln energy)

Europe

3150

NATO (Europe)

India (mini-cement)

4180-4600

Sinha

China

4850

(ave) Ming-yu

Source: UNCHS (Habitat), Energy for Building, reference No. 1

Energy requirements for the preparation of raw mix to be fed into the kiln are rather substantial. Electric power is primarily used for crushers, grinding mills and for transporting materials. In some cement plants using dry process production, the moisture of raw materials should be removed before grinding. Removing the moisture in such cases is done by hot air from special furnaces or using heat from the waste gas of the cement kiln. The amount of heat required for drying raw materials is low as compared to the heat requirements of the cement kiln which is the major consumer of energy in cement plants. Clinker burning, in general, accounts for about 90 per cent of heat energy used in cement making (4).

The thermal energy consumption of the rotary kiln ranges widely depending on the type of process used, the quality and type of clinker required, kiln insulation, the effectiveness of the operation control system, etc. As a result, the actual specific heat consumption varies from about 3 to 7.5 GJ/t of clinker (4).

Wet and semi-wet processes

The advantages of the wet process, such as a simpler technological scheme for raw mix preparations and its control, lower-labour input and less pollution of the environment, led to its early extensive use in many countries. The basic disadvantage of this process as compared to the dry process has been a higher consumption of heat for clinker burning, as the water added to the raw mix has to be driven off in the kiln. Various efforts have been made to increase energy efficiency in the wet process kiln through design improvements, particularly by installing different types of heat exchangers in the kiln to improve the heat transfer (4).

In the semi-wet process, the water content of the slurry is reduced before feeding it into the kiln. Different types of filters can be used to reduce the water content to about 20 per cent. The resulting mass is fed into the kiln, which can be equipped with a pre-heater. The nodules obtained by filter-pressing of slurry and a shaping process are fed into the kiln. The preheater-kiln system consumes 5 to 5.4 GJ/t of clinker (4).


Figure 1. Energy consumption in cement grinding (kwh/t of cement). Courtesy Holtec Engineering Private Ltd., India


Figure 2. Specific power consumption (kwh/t of cement). Courtesy Holtec Engineering Private Ltd., India

Dry and semi-dry processes

Dry process kilns came into existence along with wet process kilns. The major development in dry process kilns was the introduction of external pre-heaters installed before the kiln inlet. This has led to better heat economy, increase in kiln capacity and, particularly in connection with the application of pre-calciners, making it possible to use smaller kilns for a given capacity. The main advantage of dry process kilns with pre-heaters is that the kiln gases pass over the raw mix thereby transferring their heat to the raw mix before it enters the kiln. Thus, the raw mix undergoes the final drying and partial calcination by using waste heat (4).

The four-stage cyclone suspension pre-heater-kiln system reduced specific heat consumption to 3.3 to 3.6 GJ/t of clinker and allowed the creation of large-scale units of upto 5,500 tonnes per day of clinker. The relatively high temperature of the raw mix entering the kiln permits a significant reduction of the size of the rotary kiln. Both the operation and maintenance of the cyclone suspension pre-heater are relatively simple, because the pre-heater does not have any moving parts. The four-stage cyclone suspension pre-heater-kiln system is today the most widely used conventional dry process kiln in the cement industry. Further developments in clinker production have been achieved by the introduction of the cyclone suspension pre-heaters with pre-calciners (4).

Although the reduction in specific heat consumption by adding pre-calciners to the suspension pre-heater-kiln system is modest, namely, about 0.084 GJ/t of clinker, this development has some advantages, which have led to its increasing use in developed and developing countries. The main advantage is increased output per unit of kiln volume. For a given capacity, the volume of the kiln can be reduced by approximately 60 per cent as compared to the suspension pre-heater-kiln system, because the raw meal is decarbonized up to 90 per cent in the pre-calciner. Another advantage, which is particularly important from the energy point of view, is that low-grade fuels can be burned in the pre-calciner thus saving high-grade fuels such as fuel oil, gas and high-grade coal. A third positive factor relates to the environment: NOx emissions are significantly decreased (4).

Energy-saving opportunities

Bearing in mind that about 85 per cent of the gross energy requirements in the production of cement, is consumed in the kiln process where temperatures of about 1450°C are reached (1), the potential for energy saving in the kiln process is remarkable. Considerable improvements in energy efficiency are possible through the replacement of wet-process with dry-process plants, through the installation of suspension heaters and through improved kiln insulation. Studies have shown that the variations for energy consumption between wet and dry process in rotary kilns could reach up to 86 per cent (a wet process could consume 1400 kcal/kg of cement compared to 750 kcal/kg of cement in the dry process) (5).

Small-scale plants using vertical-shaft kiln process are also better in terms of energy-efficiency compared to wet-process, but less efficient than the best and modern dry process plants.

Vertical shaft kilns have the lowest energy consumption, although they are smaller (in general 20-200 t/d) than the large-scale rotary kilns (1000 to 4000 t/d). In India, for example, it was established that while a vertical shaft kiln consumed around 750 kcal/kg of clinker, a rotary kiln consumed up to 2000 kcal/kg of clinker. In addition to the advantage of lower fuel consumption, the vertical shaft kilns are known to have operated efficiently on a variety of solid fuels, sometimes with an ash content as high as 50 per cent (6). Small-scale cement plants with vertical shaft kilns have inherent advantages of being located in the rural areas of developing countries meeting local demand resulting in reduced energy consumption for transportation. For a detailed treatment of the small-scale production of cement see UNCHS (Habitat) “small-scale production of Portland cement”, reference No. 7.

As mentioned earlier, the improvement of heat-use efficiency and the reduction of heat losses are of great importance in reducing the energy consumption in cement plants. To that effect some important measures include (4):

(a) Energy savings in heating processes

A significant reduction of specific heat consumption in rotary kilns is achieved by using modern dry process kilns with suspension pre-heaters or more advanced kilns with both suspension pre-heaters and pre-calciners, where the heat of the exhaust gas of the kiln is used for pre-heating the kiln feed. This can be realized when building new plants and/or when expanding existing ones.

Another useful method to economize on fuel is changing the mineralogy in the cement and utilizing mineralizers. Low-melting slags, fluoride and calcium sulphate are a few of the numerous substances that render a mineralizing effect on clinker formation. A reduction of clinker formation temperature, when utilizing the latter, leads eventually to saving of energy.

The specific fuel consumption for calcining clinker could be reduced by approximately 10 per cent by changing the clinker mineralogy. Since the clinker produced by this process features a high grinding ability, the specific consumption of electric power decreases considerably and the capacity of the cement mills increases.

Significant attention is given to the development of industrial production of cement by low-temperature technology with the use of calcium chloride, wherein the reaction in mineral formation is completed at temperatures lower than in currently utilize technologies.

Among the measures widely applied to improve heat use in rotary cement kilns are the use of internal heat exchangers, e.g., chain systems used in wet and dry process kilns and different thinners permitting reduction of the slurry moisture in the wet process that was described above.

Substantial reductions in heat losses from the kiln can be achieved by proper maintenance of the kiln seals, control of combustion and improved refractories and cooling of the kiln shell.

Losses from the wall of the rotary kiln make up a substantial share of total energy lost. The heat transfer is reduced when the surface is insulated, but at the same time the temperature of both the metal jacket and the lining rises. Thus, external insulation may be detrimental to operating safely. One way of overcoming this is to utilize heat losses rather than minimizing heat transfer. To this end, a recuperator is fitted at the top or on the side of the rotary kiln. It is heated by some of the radiation emitted by the kiln.

Another opportunity for decreasing heat utilization in a cement plant is to use waste heat from clinker coolers. This waste heat can be used in a number of ways, particularly to dry raw materials, to dry coal when it is used for firing the kiln, or to generate steam and power if necessary.

(b) Electric power savings

Cement mills are the major consumers of electric power at cement plants, accounting for 40 per cent of total electric power consumption. Therefore, primary attention is given to improve efficiency in electric power utilization in these units as well as in the raw material grinding mills, which are in second place in terms of consumption of electric power in cement plants. Currently, clinker is ground mainly in ball mills whose energy efficiency is low - only 5 to 9 per cent.

The basic method for reducing energy consumption in ball mills is their adjustment to establish an optimum operating regime and to ensure maximum output with minimum energy consumption while preserving the predetermined fineness of grinding. Another possibility of reducing the consumption of electric power is to improve the technology of grinding in ball mills by pre-engagement of the crusher-drier. Preliminary crushing may save energy in grinding up to 6 kwh/t raw material or 9 kwh/t of clinker.

To further increase the efficiency of the grinding process, it is necessary to employ grinding units of other designs such as roller mills. But, when developing new methods of grinding, it is necessary to ensure that economy in energy use does not cause offsetting increases in capital and operating costs. Evidently, cement will be ground in the future mostly in vertical mills.

The utilization of grinding intensifiers also ensures a certain energy saving. During recent years many cement plants have started using grinding aids for clinker grinding as a means of reducing electric power consumption and achieving increased output in the cement mill.

At present, roller mills appear to have some basic advantages in raw material grinding. The use of a roller mill in raw material grinding can reduce electric power consumption by about 20 to 25 per cent compared with that of a tube mill. However, a roller mill is in general not suitable for processing abrasive raw materials.

(c) Substitution of fuels

Energy conservation means not only reduction of specific energy consumption per unit of finished product but also preservation of scarce fuels such as fuel oil.

Many countries started reversing to coal after sharp increase in prices for oil-based fuels in the 1970s. Even countries possessing significant oil deposits like Indonesia aim at switching their cement plants from oil to coal and other low-grade fuels.

Widespread opportunities for using low-grade fuels for clinker burning were opened up by the development of systems with cyclone pre-heaters and pre-calciners. Pre-calcining or secondary firing, apart from the possibility of achieving higher outputs, has also opened the way for the saving of high-grade fuels and combustible industrial wastes such as wood chips and bark, waste tyres, urban wastes, etc.

This is possible because the pre-calciner operates at a temperature of about 900°C which is needed for decarbonation of the raw mix.

Only about a quarter of the total amount of heat used for clinker burning is consumed in the kiln itself. Therefore, it is possible to feed up to 75 per cent of the fuel into the secondary furnace in kilns with pre-calciners (in practice normally up to 60 per cent is burnt in the pre-calciner). This allows the differentiation of fuel for calcining the clinker: higher calorific fuel for ensuring high temperatures in the sintering zone of the rotary kiln and less calorific fuel for ensuring decomposition of the carbonate component in the pre-calciner.

In addition, many of the substances harmful to the environment are introduced into the process together with the fuel and are bound almost completely in the cement clinker without impairing its quality. Moreover, low-grade fuel can normally be used in the process directly, i.e., without pre-treatment.

The potentialities for saving energy in the production of cement by utilizing energy-saving technologies and measures are given in table 3.

Table 3. Potentialities for saving energy in the production of cement

Energy-saving technologies, equipment and measures

Potential savings

Calcination of clinker

Fuel (percentage)

Utilization of predehumified slurry, including the use of diluentsa

1-1.2 per one per cent of moisture content reduction

Production of cement by dry method in kiln with decarbonizer:b

as compared with kilns with cyclone pre-heaters

5-7

as compared with the wet method

40-50

Intensification of the calcining process, including reduced suction of ambient air, utilization of mineralizers, automatic control systems, efficient heat exchangers, fuel combustion systems etc.

10-15

Utilization of ashes, slags and other materials, containing CaO, calcium silicates or aluminatesc

10 or more (depending on additive)

Utilization of combustible industrial waste and domestic garbage

equivalent to amount of waste used as fuel

Utilization of fluidized kilns for calcining clicker (as compared with rotary kilns)d

25-35

Utilization of secondary energy for electric power generation, raw material drying

8-20

Clinker grinding

Electric power

Milling cement in roller mills (as compared with ball mills)e

15-25

Utilization of milling intensifiers, optimization of granulometric composition, etc.

10-15

Source: UNIDO/former CSSR Joint Programme, reference No. 2

a Increase of kiln capacity by 1.5-2 per cent of moisture content decrease.

b Increase of specific capacity by 2.5 and 7.5 times, respectively and decrease of specific consumption of refractories by 4 and 5 times.

c Savings in raw materials components.

d Reduction of capital investments by 20-30 per cent, potentiality for burning shale and low-grade coal.

e Reduction in capital investments for construction, potentiality for additional cooling of clinker in milling processes.

In addition to the above opportunities, other studies provide more information on measures and possibilities for optimizing the use of energy in cement production. Table 4 is one of such compilation of information which provides examples from some countries.

Table 4. Selected examples of improvements in energy conservation and in specific energy consumption in cement production

Plant type/location

Energy savings

Measure taken

A. Energy conservation

Wet Process

Savings of 150 kcal/kg

Adding a vent air recirculation system to clinker cooler thereby reducing dust wastage and increasing heat recuperation.

Dry Process

Savings of 14 kcal/kg

Addition of new kiln seal at end discharge end to cut out air infiltration.

B. Lowering specific energy consumption

Wet Process (Canada)

10 per cent (from 1,416 kcal/kg to 1,280 kcal/kg)

Recirculating clinker cooler air.

Wet Process (Canada)

9 per cent (from 1,441 kcal/kg to 1,280 kcal/kg)

Thinner to lower slurry moisture from 35.8 per cent to 31.2 per cent with increase in clinker production by 9 per cent.

Wet Process (USA)

17 per cent (from 1,876 kcal/kg to 1,560 kcal/kg)

Reduction in slurry moisture, new seals and closing holes, new cooler grates, and fans, new chain systems.

Wet Process (Brazil)

11 per cent (from 1,841 kcal/kg to 1,637 kcal/kg)

Changing clay component, modifying chain system.

Wet Process (USA)

15 per cent (from 1,617 kcal/kg to 1,381 kcal/kg)

Slurry water reduction, adding lifters, insulating bricks, raw feed chemistry control, chain maintenance, and cooler modification.

Source: Fog, M. H. and Nadkarni, K.L., reference No. 5

Another innovation regarding energy-savings in cement production is the technology of blended cements. The blending of certain carbonaceous materials such as granulated slag, fly-ash and other pozzolanas with cement makes it possible to produce more cement from the same amount of clinker and, thus, reduce the final consumption of energy per ton of cement produced. Experience has shown that up to 25 per cent by blast furnace slag without changing the performance of blended cements in comparison to Portland cement for general application in construction. In some countries, this mode of production has led to an estimated 20 to 40 per cent savings in fuel consumption. It has been estimated that at least one half of all Portland cement used in developing countries is used in applications for which a material of much lower strength would be adequate (8).

Note 1.

For a detailed treatment of small scale cement production refer to UNCHS (Habitat), “Small scale production of Portland Cement”, reference No. 7



Note 2.

For a detailed treatment of blended cements and other types of binding materials see UNCHS (Habitat), Endogenous Capacity-Building for the Production of Binding Materials in the Construction Industry-Selected Case Studies, reference No. 9.

2.2. Lime

The production process of lime, like cement production, is highly energy-intensive, but energy requirements in lime production are lower than cement and the types of fuels required could be a variety of low-grade fuels.

Quicklime is manufactured by calcining limestone at temperatures around 900°C which is almost 35 per cent lower than the heat required for cement clinker production. A high proportion of the total energy requirement in lime production is used in kiln for calcining the limestone. Thus, as in the case of cement, the principal means of achieving energy-efficiency lies in improving the performance of the kilns. In industrialized countries, production of quicklime is done by burning the limestone in large and fully automated rotary kilns. However, in most developing countries, a vertical shaft kiln, using simple masonry for the wall of the kiln is more common.

Rotary kilns are similar to those used in the cement industry. Rotary kilns have evolved into short rotary kilns with separate pre-heaters, resulting in lower waste gas temperatures. The original design had a specific energy consumption of 6.6 to 8.4 GJ/t and capacities up to 560 tonnes per day. The short kiln can be made more energy efficient with specific energy consumption as low as 5.0 GJ/t, though there are instances of such kilns which do not capitalize on this energy-saving potential. Capacities of the short kiln range from 200 to 1,000 tonnes per day (2).

Vertical-shaft lime kilns, however, have, proved to be one of the most suitable and economical methods of small-scale lime production in many developing countries (3 to 10 t/d capacity). For example, a vertical-shaft lime kiln can burn lime on a continuous basis resulting in considerable savings in heat losses. The kiln, having three distinct zones of operations (preheating, calcining and cooling), is attractive and efficient in terms of use of fuel. Further efficiency can be achieved by incorporating insulation in the wall of the kiln. These are only a few characteristics that make the use of the vertical-shaft lime-kiln technology more advantageous than the other traditional methods. For a detailed treatment of the vertical-shaft lime-kiln technology see UNCHS (Habitat), Vertical-shaft Lime-kiln Technology, reference No. 10.

Energy use in quicklime production

As mentioned earlier, the main energy requirements for the production of quicklime is in burning the lime stone in the kiln. The other energy inputs are required for limestone quarrying, grinding, and hydrating/slaking the quicklime. Table 1 shows the energy requirements for quicklime production in various types of kilns and countries.

Table 1. Energy requirements of quicklime production from various sources

Process and scale

Scale of production
(ton/day)

Energy requirement
(GJ/ton)

Efficiency
(percentage)

Source of data

Traditional intermittent kiln, India

Very small

12.6

25

Rai

Conventional shaft kiln, India

10-20

9.03

35

Rai

Improved shaft kiln, India

10-20

6.24

51

Rai

Improved shaft kiln, Malawi

3

6.92

46

Spiropoulos

Mechanized vertical

20-100

4.76

67

Rai

Rotary kiln

<100

6.71

48

Rai

National studies


Argentina

3.8


Germany

8.8


India

6.34

Source: UNCHS (Habitat), Energy for Building, reference No. 1

Table 2. Fuel consumption in lime production using different technologies

Country

Type of kiln

Proportion of production cost

Primary energy consumption MJ/kg quicklime

Federal Republic of Germany

Traditional vertical kiln

35

4.760


Ring annular kiln

22

4.865

Rotary kiln

20

6.715

Traditional kiln

-

12.60

Conventional

-

9.03

Improved shaft kiln (CBRI)

-

6.24

Source: UNCHS (Habitat), Technical Note No. 12

As can be seen, traditional intermittent kilns can be very wasteful of fuel, with only 25 per cent of efficiency. Whereas the improved kilns can have an efficiency of up to 50 per cent even if in small-scale.

Fuel consumption in alternative technologies for production of hydrated lime tends to show the importance of choice of technology in achieving energy efficiency. Using the case study of the Federal Republic of Germany in table 2, an energy saving of about 40 per cent is achieved when rotary kiln technology is adopted in place of a traditional vertical kiln.

Energy-saving opportunities

In vertical-shaft small-scale lime production, almost 60 per cent of the production cost is constituted by the cost of fuel alone.

The main fuel sources for lime-burning are coal, coke, cinder, wood, oil, gas etc. With the depleting resources of fossil fuel, it has become necessary to reduce the consumption of energy for the production of quicklime. Therefore, any saving in the consumption of energy will result in lowering the cost of production (10).

Energy savings could be achieved by changing the design parameters of the kiln. Studies and experiments carried out indicate that heat-energy savings could be achieved to the tune of 50 per cent by changing the design only. This indicates a potential for further savings by continuous research and development in limekilns (10).

An example of the energy savings achieved by improved limekiln design is illustrated in table 3 (10).

Table 3. Average energy inputs for 1 kg quicklime (CBRI limekiln)

Constituent of energy consumption

Energy consumption (kcal/kg)


Country type kiln

Conventional shaft kiln

CBRI improved kiln

Theoretical heat requirement for calcination for pure limestone

750.0

750.0

750.0

Sensible heat loss through exhaust

1050.0

850.0

850.0

Heat loss due to underburnt limestone

500.0

200.0

100.0

Wall heat loss

75.0

75.0

5.0

Sensible heat loss from quicklime

250.0

25.0

25.0

Radiation losses from top surfaces

150.0

125.0

75.0

Heat loss due to incomplete burning of fuel

100.0

75.0

10.0

Excess air loss

125.0

50.0

25.0

Gross energy input (kcal/kg CaO)

3000.0

2150.0

1660.0

Source: Central Building Research Institute (CBRI), Roorkee, India


Figure 1. Flow chart of vertical-shaft lime kiln production in Balaka, Malawi. Courtesy Brian Jones

The CBRI improved shaft kiln has been achieved through the principles of uniformity of heat distribution over the cross section of the kiln plus the provision of a good draught system. In detail, the CBRI kiln is a tall cylindro-conical structure constructed of masonry material with an internal lining of fired-clay bricks. The effective height of this kiln for a 10 tonne per day capacity is 11m and the calcining zone maintains a temperature of 950°C to 1100°C (11).


Figure 2. An improved shaft kiln developed by KVIC, India

In rotary kilns, the highest losses are waste gases leaving the plant at a temperature of 250° to 350°C. These gases are always mixed with secondary air getting in at the interface between rotary kiln and pre-heater. If the plant were completely sealed, the waste gas temperature would rise to 430°C. This would not affect the balance sum of waste gas losses, because the over-all amount of waste gas would be reduced accordingly (4).

All energy-saving opportunities seem to be already realized in the most efficient shaft furnaces, particularly in the parallel-flow-counterflow-regenerative furnace. Waste gases emitted from this furnace range between only 60° and 140°C; further reduction would risk condensation and corrosion of equipment (4).

When a new kiln is constructed, the preferred kiln types are of the regenerative shaft kiln type. Specific heat consumption as low as 3.5 kj/kg is possible in this type of kiln. However, an exception to this may occur if, for example, the disintegrating property of chalks and some coarsely crystalline limestones results in the shaft kiln being choked off by dust formed from the complete disintegration of these materials during burning. In this situation, the rotary kiln, fitted with a heat exchanger to recover waste gas heat, would be the most energy efficient solution (4).

For further information on lime production and some case studies see UNCHS (Habitat), “Endogenous Capacity-Building for the Production of Binding Materials in the Construction Industry-Selected Case Studies”, reference No. 9 as well as UNCHS (Habitat), Vertical-shaft Limekiln Technology, reference No. 10.

2.3. Clay bricks

As in the cement and lime industry, most of the energy used in brick manufacture is required to fire the bricks - typically more than 95 per cent of all energy use. There are considerable differences between the energy requirements for different types of brick kilns, depending on whether the firing is continuous or intermittent, on the size and heat-transfer efficiency of the kiln and on whether the brick-earth used contains combustible materials (12).

Experience has shown that, beyond a daily output of 10,000 bricks, economies of scale are small and that low-capital-cost kilns, such as the Indian Bull’s trench kiln, can have a fuel efficiency comparable with the expensive covered Hoffman kilns used in European countries. A recent study of the brick industry in Delhi (13) showed that detailed differences in the design of the trench kiln contributed to significant variations in energy efficiency. Overall, the bricks produced by the process had an average fuel consumption of 1.8 MJ per kg of bricks produced, but with individual kilns’ fuel consumption varying from 100 to over 300 kilograms of coal per 1000 bricks. Research on the kiln process at the Central Building Research Institute (CBRI), Roorkee, India, has led to the development of an improved but more expensive high-draught kiln design which is reported to reduce energy consumption by 25 per cent, compared with a typical trench kiln (13).

The scarcity and rising prices of coal, the predominant fuel used for brick-firing, are forcing brick producers to look for cheap fuel substitutes as well as energy economies. The study of the Delhi industry showed that increasing numbers of producers were using a proportion of unconventional fuels, such as fuelwood, rice husks, sawdust and agricultural waste, and that these producers were able to lower costs without it being detrimental to the quality of the bricks, though at the cost of a small lowering of energy efficiency. A small number of producers had eliminated coal entirely, with a resulting increase in energy consumption of only 14 per cent compared with producers using coal alone (14).

The theoretical energy requirement for firing clay bricks in small-scale kilns is about 20 to 35 per cent of the actual energy consumption in production practice. Thus, most existing brick production technologies are by definition energy-inefficient. Table 1 shows some examples for energy requirements related to the size of plants.

Since transportation of bricks from the point of production to the point of use accounts for a significant amount of energy consumption, it could be argued that the scale of production is a critical determinant of efficiency in energy utilization. Large-scale production technologies predetermine high-energy consumption for distribution because a single plant often has a wide catchment zone, sometimes an entire country. Large scale brick industries are examples of prevailing error in choice of scale of technology as far as energy-efficiency in transportation is concerned.

Table 1. Energy consumption in brick-making technologies

Technology

Scale of production
(No. of bricks)

Labour required
(man-hr for 1000 solid bricks)

Over-all energy consumption
(MJ/1000 solid bricks)

Small-scale production, all manual methods, clamps, stoves, scotch kilns

2,000

20 to 30

7,000 to 10,000

Small-scale production, all manual methods, up draught and down draught kilns

2,000

30 to 40

10,000 to 15,000

Medium-scale production, all manual methods, Bull’s kilns

20,000

30 to 40

4,000

Medium-scale production, semi-mechanized method, Hoftmann on zig-zag kiln

30,000

30 to 35

3,000 to 3,500

Large-scale production, full mechanized tunnel kiln

150,000

10 to 15

3000 to 4000

Source: UNCHS (Habitat), Technical Note No. 12

In addition to the size of kilns, the actual amount of fuel required to burn a given amount of bricks would depend, among others, on the following factors:

(a) characteristics of raw material (clay);
(b) porosity of bricks;
(c) volume of water (moisture) to be evaporated;
(d) quality of the carbonaceous matters in the bricks;
(e) temperature to be attained;
(f) reuse of hot air generated in the kiln;
(g) quality of fuel.

Table 2 shows fuel requirements of intermittent and continuous kilns which is based on a study carried out in Pakistan (15).

Table 2. Typical fuel requirements of kilns

Type of Kiln

Heat Requirement
(MJ/1000 Bricks)

Quantity of Fuel Requirement
(Tons/1000 Bricks)

Wood

Coal

Oil

Intermittent

7,000 to 15.000

0.50-1.0

0.25-0.6

0.15-0.35

Continuous

2,000 to 5,000

0.15-0.3

0.10-0.2

0.05-0.1

Source: Mohammed Khalid Farooq, Pakistan, reference No. 15

Due to scarcity of coal in many developing countries (which are not covered in high priority segment for allotment of coal or railway wagons for its transportation) and rising prices of electricity or petroleum products, the introduction of modern technology in the industries manufacturing bricks, lime, ceramics, etc is a difficult task. Summary of a study carried out by UNIDO and ILO on the average fuel requirements for kilns is given in table 3.

Table 3. Fuel requirements for brick-making using different kiln types

Type of kiln

Heat requirement
(MJ/1,000 bricks)

Quality of fuel required
(tons/1,000 bricks)



Wood

Coal

Oil

Intermittent





Clamp

7,000

(0.44)

0.26

(0.16)


Stove

16,000

1.00

0.59

0.36


Scotch

16,000

1.00

0.59

0.36


Downdraught

15,500

0.97

0.57

(0.35)

Continuous


Original Hoffmann

2,000

0.13

0.07

0.05


Modern Hoffmann

5,000

0.31

0.19

0.11


Bulls’s Trench

4,500

0.28

0.17

(0.10)


Habla

3,000

0.19

0.11

(0.07)


Tunnel

4,000

(0.25)

(0.15)

0.09

Source: UNIDO/ILO, Technical Memorandum No. 5

Similarly the continuing rise in the cost and shortage of wood as a source of energy for brick-making has forced many producers to switch to low-grade fuels such as agricultural wastes and cow dung. The change-over to these residues has been taking place despite their several disadvantages in comparison to fuelwood of those having lower calorific values and lower heat intensity and more difficult to feed in furnaces. Table 4 shows the price range of different fuels in India together with their respective heating values.

Table 4. Comparison of heating value and prices of different fuels

Fuel type

Unit

Price in Rs./unit

Heat value
(air dry)

MJ/Rs. total heat

Conversion efficiency
(percentage)

MJ/Rs. useful heat

Fuelwood

kg.

0.50-0.70

19.7 MJ/kg

28.1-39.4

10-30

2.8-3.9
7.9-11.0

Agri-residue

kg.

0.04-0.09

14.7 MJ/kg

163-268

8-12

13.0-29.4
19.6-44.2

Dung Cake

kg.

0.30-0.40

8.8 MJ/kg

22.0-29.3

8-12

1.8-2.3
2.6-3.5

Charcoal

kg.

2.50-3.00

29.0 MJ/kg

9.7-11.6

23-30

2.2-2.7
2.9-3.5

Soft Coal

kg.

0.40-0.50

24.2 MJ/kg

48.4-60.5

20-30

9.7-12.1
14.5-18.2

Kerosene

ltr

1.80-2.55

38.2 MJ/ltr

17.0-21.2

37-52

6.3-7.8
8.8-11.0

Electricity

kwh.

1.50-2.00

3.6 MJ/kwh

1.8-2.4

45-50

0.8-1.1
0.9-1.2

LPG

kg.

4.05

24.5 MJ/kg

11.2

60-65

6.7-7.3

Source: Adapted from Energy Conservation Book, Utility Publication, 1988

Due to large variety of processes and the wide range of fuel types and products, it is almost impossible to fix a standard value for the total energy input in brick-making. For example, intermittent kilns consume twice as much energy, in general, as continuous kilns.


Figure 1. Working method of the Bull’s Trench continuous kiln. Courtesy BRE, U.K.

It is, therefore, advisable to establish necessary measures which should be based on trial firing in each case and/or replicate identical processes as far as it is possible. Obviously, maintaining uniformity of raw materials and fuel types and continuous maintenance of kiln operation would lead to reduced deviations in the amount of fuel required in a given plant. Hence, reasonably accurate forecast of fuel consumption for a foreseeable future of plant operation.

Energy-saving opportunities

As mentioned earlier, one of the major energy-saving opportunities lies in converting, where possible and feasible, the intermittent kilns such as clays or simple updraught kilns, to continuous kilns: the latter include the Bull’s trench kiln, the Hoffman kiln and the tunnel kiln. Experience has shown that a coal-fired Bull’s trench kiln, of the type common in India and Pakistan, is several times more efficient as a sophisticated oil-fired clamp one and almost as efficient as a sophisticated oil-fired tunnel kiln. In China, Fuyin, Y., and others have developed a small-scale continuous updraught kiln, fired on coal, wood or agricultural waste, which has excellent efficiency.

Where it is considered necessary to use intermittent kilns, improvement may be made to enhance fuel efficiency by:

- ensuring that even temperatures are obtained throughout the kiln;

- reducing heat losses through the sides and top surface of the kiln; and

- recovering heat from the combustion gases. This can be achieved through use of scotch or updraught kilns with permanent sidewalls rather than open clamps, and improvement of insulation of clamp kilns.

where continuous kilns are used, strategies for energy savings can be achieved by:

- increasing chimney heights;

- adopting a fixed rather than the traditional moving chimney design; and

- careful control of the levelling of the kiln floor. Process control such as sealing the kiln, adequate drying of the green bricks, and uniform feeding of the fuel also contribute to energy saving.

The Central Building Research Institute in Roorkee, India, has carried out extensive research on kiln processes and has found out that a reduction of 25 per cent on fuel consumption can be achieved by improvements in the high-draught kiln designs compared with the typical trench kilns. But the capital requirements of such a kiln is almost 10 times that of the traditional kiln which is a major bottleneck for small-scale brick producers to afford such an initial investment (3).

Changes in fuel type and addition of solid-fuel particles such as coal dust, sawdust, coffee or rice hulls or, even, plastics such as polystyrene, to the clay can increase energy efficiency but affect the characteristics of the brick, e.g., porosity, strength. This is not necessarily a disadvantage, but it is necessary to take these different properties into consideration during design and construction stages. Similarly, hollow bricks will save energy (5 to 6 per cent per 10 per cent of hollowness) but will have different properties from solid bricks. Several of the improvements noted above have been implemented in brick-making in Indonesia, as part of a Technical Assistance Programme, and include:

(a) introduction of perforations in hand-made and extruded bricks;

(b) use of combustible filler (e.g., rice husk) in the clay body;

(c) improved body-clay preparation; and

(d) control of air supply and waste gas in kilns. The results of this programme have not yet been assessed (16).

Waste engine oil can be an alternative fuel in some areas. Waste engine oil is often disposed of in such a way that it causes serious water pollution, and using it as a fuel would greatly reduce this problem. Waste engine oil can be utilized in simple gravity drip-feed flat-plate burners which vaporize the oil: such a method has been successfully used to fire a 35 cubic-foot pottery kiln at Arusha, the United Republic of Tanzania. Alternatively, sophisticated forced-air burners can be employed which use blowers to atomize the fuels. To use waste oil most effectively, a combination of fuels is required. Moreover, to start the firing using flat-plate burners, wood or other biomass can be used (17).

Another opportunity for saving energy in the overall process of brick-making is the use of solar energy for drying the green brick in countries with dry and warm climate.


Figure 2. A rectangular down-draught kiln. Courtesy UNIDO/ILO, Small-scale brick making


Figure 3. Sun drying of green bricks is common in dry and warm climates. Courtesy BRE, U.K. (a)


Figure 3. Sun drying of green bricks is common in dry and warm climates. Courtesy BRE, U.K. (b)


Figure 4. Design of brick kiln using rice-husk as fuel. Courtesy Satya Prakash and F. U. Ahmed, CBRI, Roorkee, India

2.4. Ceramic wall and floor tiles (2)

Ceramic products for interior and exterior protection and decoration of all types of commercial and residential buildings are of great importance. Earthenware, tiles and mosaics are included in this group. Energy consumption of both glazed and unglazed products is high and covers a broad range depending on the individual product.

Energy consumption in ceramics production is concentrated in the following processes and activities:

- Dressing and shaping;
- Handling and internal transport;
- Drying and firing, which together account for about 85 per cent of energy consumption.

Wall tile

The traditional manufacturing process consists, with the exception of body preparation and pressing, of three phases: drying, bisque and glost firing after glazing.

The traditional manufacturing process, based on kaolinitic-clay, semi-silicious or feldspar body composition, requires bisque firing temperatures in the range of 1,230°C to 1,280°C. Glazed bisque is fired at 1,080°C to 1,120°C. Decreased firing temperatures, of 1,050-1,080°C for bisque firing and 960-1,040°C for glazing, can be achieved by replacing those bodies with lime-silicious, dolomite-silicious and certain other raw materials. Significant energy savings are possible through the lower-temperature firings, which newer materials allow.

Ceramic glazes

Properties of ceramic glazes have to correspond to the ceramic body properties and to quality and appearance requirements.

The last 50 years of technological progress in the production of ceramic glazes has paralleled that of ceramic bodies. Opaque zircon glazes based on potash feldspar are fired at temperatures of about 1,120°C. These glazes are for glazing kaolinitic-clay bodies. The application of calcium-silicious body in the manufacture has allowed the development of new types of glazes with sodium-lime feldspar and lead content, which are melted at firing temperatures of only 960° to 1,040°C.

Further development of glazes should result in single ceramic processes at temperatures as low as 900°C.

Opportunities for energy conservation

There exist several possibilities for energy conservation in the ceramic floor and wall tile industry. The most important among these are the following:

- Further reduction of firing temperatures through the application of fluxes and corresponding glazes;

- Replacement of the double firing by the single firing process for energy savings of 40 to 50 per cent;

- Proper adjustment of existing kilns to reduce energy consumption usually by 10 to 20 per cent;

- Use of waste heat from the cooling zones of kilns, for example, to pre-heat combustion air;

- Use of waste heat from flue gases in technological processes;

- Use of modern equipment for drying and firing where about 85 per cent of total energy in the overall production process is consumed.

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