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Kiln Connection: Where Does The Energy Go?

January 3, 2001
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Table 1. Characteristics of two periodic kilns firing at different temperatures.
High fuel prices of late have many manufacturers asking this question for the first time in several years—“Where does the energy go?” Understanding what happens to the fuel that you pay for when you fire your kilns will enable you to make informed decisions when you are asked—or told—to reduce the energy consumption of your firing systems. Since periodic kilns are generally the SUVs of fuel consumption, using twice the equivalent fuel of their continuous counterparts, this column will start with them.

Figure 1. Gross fuel input for both kilns.

An Example

Let’s look at two periodic kilns, one firing whiteware at 2200°F, and the other firing technical ceramics at 3000°F. These are two basic kilns, each with 400 cubic feet of setting. Table 1 shows the characteristics of the two kilns, and Figure 1 illustrates their gross fuel output.

Each kiln has excess air input, gradually decreasing from lightup to 1700°F. The load shown is the gross load, which equals ware plus furniture. Wall storage is the input necessary to heat the wall refractories to equilibrium. Heat losses are a combination of wall, floor and roof heat losses, plus radiation losses through the flue. Water vapor losses are the heat of vaporization losses in BTUs that occur when methane is oxidized. Finally, the losses are reported as gross input values, and as such, have the flue gas losses built into them.

As far as efficiency goes, the facts are startling—and pitiful. The BTU/pound of ware values are about 11,600 for the 3000°F kiln, and 3800 BTU/pound for the 2200°F kiln. Absolute thermal efficiencies are 6.0% and 13.3%, respectively! Without a doubt, the periodic kilns have the lowest overall thermal efficiency in the entire factory.

Improving the Situation

So, how do you make improvements? Since a large part of the inefficiency arises because all gases exit each of these kilns at kiln temperature, heat recovery would be a good place to start. Normally it is most effective to use this wasted heat in the kiln as preheated air. In the case of the high temperature kiln, preheated combustion air through recuperation could readily reduce the fuel consumption by 20% or more. Using additional waste heat from the stack after the recuperator for plant water heating can be an additional energy saver.

Reducing the furniture-to-ware ratio through better furniture design will also pay dividends. The use of high strength SiC beams could be most helpful. Fiber (lightweight) linings are also a possibility for the high temperature kiln, and they would reduce the energy necessary to heat the walls while possibly reducing heat losses, too. Reduction of excess air could also have major benefits; pulse firing might be a firing technique to consider. Faster firing (and therefore lower losses) can also make a difference.

Energy costs have nearly tripled in some cases, and significant curtailments in supply could be ahead. Since it will take a few years for supply to meet demand given the historically low fuel prices, begin now to make these improvements. Plan ahead—contact experts in combustion, kilns and refractory insulation systems to develop a logical and secure energy reduction program for the days ahead.

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