Firing and Drying

SPECIAL SECTION/FIRING & DRYING: Firing Evolution

February 1, 2011
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A kiln has been developed to meet multiple manufacturing needs and act as a starting point for a new approach to firing.

New challenges are forcing manufacturers to rethink past achievements and cast a critical eye on processes that previously seemed mature and consolidated. The latest generation of production plants must meet a variety of important needs, including increased productivity and control, and reduced energy consumption and environmental impact. A new kiln has been developed to provide a single machine that will satisfy all these requirements and act as a starting point for a new approach to firing.*

Figure 1. Temperature distribution over the kiln cross section in the standard kiln (top) and the new kiln (bottom).

Kiln Architecture

The new kiln architecture has changed in substance, not form: a glance at the exterior shows no great difference compared to the standard model. However, the adoption of self-recovery burners and a new control/supervision system has made it possible to sub-divide the kiln into thermal cells that are independent of each other.

The kiln consists of a sequence of these thermal cells, which constitute the basic building blocks of the machine's heat and pressure control system. Each thermal cell consists of three physical modules. The burners are installed in the first physical module of the kiln, thus aiding in the accurate control of the firing curve.

Sub-division into thermal cells that are thermally independent allows manufacturers to fire the product in a significantly different way. A compromise between the needs of the material and the thermodynamic requirements of machine operation is no longer necessary.

The material's firing curve is first established on the basis of lab tests as a function of chemical-physical reactions. It can then be reproduced exactly as desired in the new kiln by controlling both temperature and atmosphere inside the individual cells.

Figure 2. The new kiln maintains pressure throughout.

Technology Updates

The substantial difference between the standard operational concept and the new kiln lies in fume movements inside the tunnel. In a traditional kiln, the fumes travel parallel to the longitudinal kiln axis and against the flow of material; in the new kiln, they travel crossways with respect to product flow. This substantial change in the direction of fume flow provides many advantages.

The energy exchange between fumes and the product is improved due to the more turbulent convective motion and the fact that the fumes remain in the firing chamber longer. The volume of fumes needed to transfer thermal energy from air to the product is reduced, which consequently reduces fuel consumption. In addition, the reduced fume volume also results in a reduction in the amount of fumes that are sent to the filter, with less polluting vapors and gases needing to be purified. Table 1 shows the reduced quantity of CO2 introduced into the atmosphere per unit of product.

The new kiln also results in more uniform temperatures (see Figure 1). Since the fumes move transversely, they are mixed together more efficiently. Turbulence prevents their stratification and ensures that they remain in the kiln longer. This results in greater temperature homogeneity of materials that are in contact with fume flows and eliminates the need for radial-flame burners to heat the walls. In addition, control over size and color defects is enhanced.

Pressure varies along the longitudinal axis in a traditional roller kiln. In the new kiln, however, pressure is maintained throughout, thus providing greater stability and preventing the influx of exhaust air (see Figure 2). In this way, no uncontrolled flows occur above or below the roller plane, thus reducing the risk of planarity defects on the tile.

Since the manufacturer has complete control, it is possible to maintain thermal conditions (positive gradient, no gradient, negative gradient) and atmospheres (oxidizing, neutral, reducing) in individual cells. Each thermal condition can be associated with an atmosphere optimal for aiding chemical reactions, when necessary. The management of these two key combustion characteristics (gradient and atmosphere) improves the material's heat treatment, especially when not vitrified.

Absolute and specific fuel consumption are reduced in the new kiln. Consumption is calculated as follows:

Total consumption = Fixed consumption + Variable consumption

Total consumption is calculated via hourly fuel consumption and expressed in Nm3/h. Fixed consumption is dependent on kiln characteristics and independent of product presence. Fixed consumption is that which is necessary to create the thermal conditions that allow the material to be transformed into finished product.

Figure 3. The self-recovery burner is designed so that fumes and air meet (without mixing) in a counter-flow configuration.

Variable consumption is dependent on the specific heat of the material. It is given by the product of the mass of material to be brought up to temperature and the volume of fuel needed to heat a kilogram of the same material.

During times of reduced hourly output (i.e., fewer sales), it is evident that specific consumption per unit of product increases in traditional kilns. In the new kiln, however, it is possible to keep specific consumption constant by shutting down some of the thermal cells (i.e., by "shortening" the kiln), thus limiting the fixed consumption component.

The self-recovery burner, which has been on the market for many years and is used in steelmaking and other industries, has previously never been applied in ceramics. The self-recovery burner is designed so that fumes and air meet (without mixing) in a counter-flow configuration, allowing the air to draw heat from the fumes before their expulsion (see Figure 3). Combustion air that is pre-heated before being mixed with the fuel can reach temperatures of up to 700°C, with evident energy-saving benefits.

In addition, executing heat exchange inside the kiln wall, and therefore at high temperature, provides the following benefits:
  • Improved overall heat exchange efficiency. A radiating component is also used in the firing zone, as opposed to using only a convective component in the low-temperature exchanger outside the kiln.
  • No technical solutions are needed to thermally insulate the air-fume exchange zones.
  • Thermal dispersion is limited.
  • It is not necessary to dilute fumes with external air (in a way that dissipates energy) before arriving at the fume fan, as is the case with traditional kilns.
  • The fume intake and expulsion fan operates at lower temperatures with respect to traditional kilns (from 180-220°C to about 150°C).


Kiln Control

The kiln is managed by an advanced control system with a double touch-screen interface. Traditional temperature regulators are no longer present. Process control occurs through an innovative system of temperature and pressure curves.

All kiln phases (ignition, shutdown, curve change, etc.) are managed automatically to ensure optimum kiln operation control. In addition, the system features diagnostic software that continually keeps operational status under control.

Future Firing

This new kiln provides a technological step-change and may unlock as-yet undiscovered firing potential for the ceramic industry. For example, manufacturers may choose to interpose glaze application in the pre-heat stage to obtain new aesthetic effects.

For additional information, visit the company's website at www.sacmi.com.

*The EKO kiln, developed by Sacmi.

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