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Kiln Energy AnalysisIn the ceramic industry, shuttle (periodic) and tunnel (continuous) kilns are used in the vast majority of firing applications. The overall energy balance for a shuttle kiln is shown schematically in Figure 1. As indicated, the energy input and gas flows generally vary during the course of the firing cycle (except for the soak time at peak temperature). The fundamental objective is to subject the kiln load to the required firing curve, but only a fraction of the total energy input goes to this use. The bulk of the energy input leaves a shuttle kiln in the exhaust gas flow, which consists of the products of combustion of the fuel, the excess combustion air, and the air infiltration (if any). Unless some form of energy recovery (recuperation) is employed, this energy is totally lost. Additional energy is lost in re-heating the kiln lining during each firing and by convection and radiation from the external surface of the kiln.
Kiln Thermal EfficiencyKiln thermal efficiency is defined as the ratio of the energy delivered to the load (product plus kiln furniture) to the total energy input (x 100). Using the sanitaryware industry as an example, some typical values of kiln thermal efficiency are shown in Table 1. For a tunnel kiln, the longer the firing cycle, the lower the kiln thermal efficiency. This is due primarily to the fact that the kiln surface heat losses are essentially the same for a fixed firing curve, regardless of the firing cycle time. As the cycle time increases (and the kiln throughput decreases), those losses represent a greater portion of the total energy input. For sanitaryware applications, tunnel kiln thermal efficiencies are generally in the range of 35 to 45%.
As indicated above, shuttle kiln thermal efficiency is lower because all of the energy in the exhaust gas is lost and because the kiln lining must be re-heated during each cycle. For sanitaryware applications, shuttle kiln thermal efficiencies are in the range of 20 to 25%. Shuttle/periodic kiln thermal efficiency falls rapidly as peak firing temperature increases. For example, a periodic kiln firing to 3000?F could have a thermal efficiency as low as 13 to 15%.
Low-Mass Kiln FurnitureLow-mass kiln furniture is produced from several types of silicon carbide (SiC) refractory materials. The important characteristics of those SiC refractories are summarized in Table 2, with the properties of the traditional refractory, cordierite, shown for reference.
The conventional nitride-bonded silicon carbide is a cost-effective SiC alternative for plates and lavatory setters in the sanitaryware industry. Plates and setters produced from the advanced nitride-bonded SiC will have minimum thickness and mass, while beams extruded from the reaction-sintered, silicon-infiltrated SiC have exceptional load-carrying capability up to the use limit of 1350?.
The SiC refractories are 10 to 100 times stronger than cordierite and thus allow major reductions in the thickness and/or cross section of kiln furniture components such as plates and beams. The corresponding mass reduction translates directly into energy savings. In addition, the SiC refractories have significantly better oxidation resistance and thermal shock resistance than cordierite. These advantages result in longer life and reduced life-cycle cost. Although low-mass SiC kiln furniture has a higher initial cost than cordierite, a thorough economical analysis typically indicates a rapid return of that additional investment.
Low-Mass Construction EconomicsA typical traditional kiln furniture arrangement for a sanitaryware shuttle kiln, consisting of heavy cordierite posts, is shown in the photo at right, while the low-mass version is shown in the opening photo. The mass of the cordierite kiln furniture is approximately 7000 pounds per car, while the mass of the alternative SiC furniture is approximately 3000 pounds per car. Since there are four cars in the kiln during a firing, the reduction in “dead weight” is almost 16,000 pounds per firing.
As shown in Table 3, based on a natural gas price of $5 per MMBTU, the monthly fuel savings for low-mass construction is approximately $10,000. With respect to the return on investment, two cases are considered. The first case considers the time to return the difference in cost between the cordierite and low-mass designs and would be applicable for a new kiln, when one or the other of the kiln furniture systems is to be purchased. The second case considers the time to return the total investment in the low-mass kiln furniture and would be applicable for a complete replacement of the cordierite kiln furniture in an existing kiln.
For this example, the time to return the additional investment in low-mass kiln furniture was determined to be about six months, and the time to return the total investment was found to be about 10 months. In the current economic climate, projects generally must show payback times of 12 months or less to be given serious consideration, and the low-mass conversion in this example certainly clears that hurdle.
Recent economic analyses of other shuttle kiln and tunnel kiln applications indicate similar economics, with the investment in low-mass kiln furniture returned in less than one year. All of these analyses have assumed fuel costs in the range of $4 to $6 per MMBTU. Clearly any increase in fuel cost only makes the economics more attractive.
Other AdvantagesLow-mass kiln furniture can be justified based on energy savings alone, but it also has the following additional advantages relative to traditional cordierite kiln furniture:
- Longer life and lower life-cycle cost
- Plates remain flat longer (better product quality)
- More open construction (better circulation and temperature uniformity in the kiln)
- Less air required for combustion and cooling (electrical energy savings in blowers)
- Possibility of increased kiln capacity due to reduction in “dead load” (shorter firing cycle)