Kilns represent one of the main sources of "throwaway" energy in the production cycle, and offer a vast range of possibilities for the recovery and use of energy dispersed in the form of heat.

Over the last decade, many tile producing countries have been strongly affected by the increase in the cost of energy and legislative restrictions regarding the environment. The impact of these factors has become a larger percentage of the final product cost, especially for small- and medium-sized producers. Due to a strange irony of the market, even in countries like Russia where there is a surplus of available energy and raw materials, tile producers have seen themselves hedged into a "quota" system in which the possibility of obtaining low-price natural gas depends on the amount reserved for the internal market compared with that reserved for the much more profitable export market.

Manufacturers of heat treatment machinery for ceramic products have been pressed to improve existing machinery and urged to design and construct more energy-efficient equipment. Reduction in consumption, which formerly focused on individual machines, has become a strategy that aims to optimize the entire ceramic production cycle and recover quantities of energy considered, until recently, to be unworthy of interest.

The kiln is the chief piece of heat treatment machinery around which this philosophy has developed. It represents one of the main sources of "throwaway" energy in the production cycle and offers a vast range of possibilities for the recovery and use of energy dispersed in the form of heat. The roller kiln will be our focus here, as it is the most common machine in tile production. However, some of the ideas that will be discussed are also applicable to other types of products and processes.

Figure 1. Heat balance in a roller kiln.

Loss Potential

A general example of the heat balance in a roller kiln producing porcelain tile is illustrated in Figure 1, which shows the percentages of heat used and dispersed when firing with a specific consumption of 490 kcal per kg of cotto. It is clear how a large quantity of energy is released in the form of heat from the chimney flue (stack) and the chimney in the cooling area. (For the purposes of this article, we will ignore the structure's heat loss, the reduction of which would require expenditures with an almost exponential effect on the cost of the machine, as well as the energy required for the endothermic transformations that could be adjusted by changing the formulation of the tile body.)

A kiln that produces 8500 kg/h (approximately 9000 m2 per day) of porcelain tile requires a quantity of heat roughly equal to 4.2 million kcal/h for the heat treatment, of which 2.3 million  is exhausted through the hot air chimney and about 800,000 kcal/h through the chimney flue. Considering that the calorific power of methane is about 8200 kcal/Nm3, and knowing its unit cost (which varies from country to country), it is easy to realize how much money is lost through chimneys each year.

Figure 2. Various solutions exist for improving the energy efficiency of the heat treatment machinery that operates within the tile production process.

If, in absolute terms, the figures given are substantial, the effect in percentage terms on the cost per square meter of product drastically reduces the impact. This also explains why small and small/medium size producers were the first to concern themselves with reducing consumption when fuel costs reached their current levels.

Various solutions exist for improving the energy efficiency of the heat treatment machinery that operates within the tile production process. Diverse technical solutions are also available for ensuring considerable savings on operating costs in view of the current and future costs of fuel (see Figure 2).

Figure 3. Modified cooling zone. Hot air can be recovered from the cooling area through a technical modification that consists of splitting the final cooling suction in two and adding a second expulsion chimney.

Combustion Processes

Preheating the combustion air offers many possibilities in terms of savings and optimization of the thermal efficiency of a kiln. Hot air can be recovered from the cooling area through a technical modification that consists of splitting the final cooling suction in two and adding a second expulsion chimney (see Figure 3). At the main suction, hot air reaches temperatures of 170-190°C (338-374°F). Part of the hot air is recovered, sent through an exchanger in the rapid cooling section, and then directed to the burners at a temperature of 200-220°C (392-428°F), achieving a fuel (methane) savings of 10-12%.

The quantity of air taken off at the main suction in the cooling zone and used as combustion air is rather small compared with the actual amount available. This opens up other possibilities of use for the air, including supplying the spray dryer burner or using it as recirculation air in the dryer(s) after the press section. Also available is the air at the second cooling suction, where the hot air temperature can reach more than 90°C (194°F). By means of simple filtering, this air can also be used in additional ways, such as sanitary water production, depending on the individual needs of the customer (see Figure 2).

To get an idea of the quantity of hot air available from a roller kiln, let us go back to the previously mentioned example of a kiln that produces 9000 m2 of porcelain tile a day at 22 kg/m2 and with fumes at the chimney reaching 250°C (482°F). For each kg of fired product, there is about 2 kg of hot air (fumes) available at the chimney, 4 kg at the main cooling suction and 5 kg at the second one. The quantity of combustion air required by the burners is about 1.1 kg/kg of fired product, so we have approximately 8 kg of air per kg of product available free of charge.

Spray Dryer Operations

The idea of recycling lost heat through the chimney flue to feed the spray dryer burner is certainly not new. However, it has only recently become a cost-effective option because of the exponential increase in the cost of energy. The type of filters generally used in the ceramic industry (aramidic fiber, Nomex®) require that the fumes that reach the bag filter at 250°C (482°F) must be cooled to 130-140°C (266-284°F) before being filtered and sent to the spray dryer.

The temperature reduction is generally achieved by placing a heat exchanger upstream from the filtering system, but this is costly due to the required maintenance and the loss of energy it causes. Replacing Nomex with PTFE (Teflon®) bags that can withstand higher temperatures (~230°C, 446°F) allows the heat exchanger to be eliminated and the air to reach the spray dryer at temperatures that are about 80°C (176°F) higher, achieving worthwhile savings and a payoff of the required plant modifications in only about a year.

Another possibility involves recovering heat for the spray dryer from the chimney flue (generally belonging to one or more kilns, depending on the evaporative capacity of the spray dryer), as well as from the main cooling suction. This option must be studied in relation to the needs of the user and the current legislation in the country in which the plant is located. For countries in which filtering the fumes is compulsory, it may be worthwhile to recover the heat. In other countries, it may be necessary to heat the working or production environments for several months a year using the exchangers. In still other countries, there is almost no interest in this method of energy recovery.

Reject and Storage Reduction

One factor that is rarely given consideration when it comes to reduced energy consumption is improved production efficiencies through reduced rejects. For those designing machinery that will form part of the production cycle, the aim should be to improve the relationship between operator and machine in order to reduce production rejects and the resultant wasted energy. Examples include the use of multifunctional computers and the in-depth study of machine ergonomics. Another possibility involves managing unstable situations, such as gaps in production, through the use of appropriate software.

Storage of the finished product is also an important consideration. The amount of space that has become necessary for storage, whether covered or not, has reached untenable proportions. A survey carried out by the Sacmi Group on a large sample of ceramic companies found that 4-7% per year of stored product simply disappears. Whether the product has been destroyed, stolen or sold off at cost, the energy required to create these lost products is completely wasted.

New "slab" production techniques are being developed that aim to drastically reduce storage. Slabs of suitable dimensions can be cut into the required sizes when the order arrives in the production department, eliminating the need to house large numbers and varieties of "sized" product. This "on demand" philosophy will certainly find its application in the tile sector (just think of technical porcelain) and result in greater production efficiencies.

For additional information regarding energy conservation in tile production, contact Sacmi Forni SpA, Via Dell'Artigianato,10, 42010 Salvaterra di Casalgrande RE, Italy; (39) 0522-997011; fax (39) 0522-840875; e-mail sacmi_forni@sacmi.it; or visit www.sacmiforni.com.com.


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