A kiln I am familiar with is 300 ft long, with a setting width of 10 ft, firing sanitaryware on a 10-hour cycle. The gross fuel consumption of this kiln is 2000 Btu/pound of green ware. The kiln has insulating brick and fiber refractories, with lightweight fiber kiln cars and SiC product support beams. Pieces are set one high with the appropriate setting furniture. In short, it is a modern kiln with many good features relative to energy efficiency. With these parameters in mind, let’s look at the energy performance of this kiln.
Second, we are heating our kiln cars and furniture, as well as processing ware. Even with lightweight kiln furniture and cars, we still consume roughly 25% of the total input of the kiln with our kiln furniture, and another 13% of the fuel is consumed heating the kiln cars themselves.
Wall losses are another factor. While the kiln in question is quite well insulated, the losses due to conduction through the walls, crown and kiln cars add another 9% to our losses. Last, the losses in our kiln exhaust, plus air infiltration and radiation losses, consume the final 18% of the fuel input.
A reduction of the “dead” load to be fired—i.e. kiln furniture and car mass—will pay handsome dividends in reducing the quantity of fuel that is used. Lighter weight kiln furniture has good potential. For example, with the use of thin profile setting plates (normally SiC, nitride bonded), fuel savings would be measurable. Furthermore, careful placement of products to be fired could potentially increase the loading per car, and thereby reduce the level of “dead load” compared to the ware being fired. This will reduce the fuel consumption because the pushing rate could be reduced, ultimately lowering the fuel required to heat the kiln car refractories.
“Other” losses are predominately associated with the heat content of exhaust gases. Quite often, these exhaust products are contaminated by a variety of hydrocarbon byproducts (e.g., Styrofoam setting pads) as well as fluorides and sulfates from the product firing. Consequently, applying recuperation to these gases can be tricky. It can be successful, however, by careful design.
A better route is to use the cooling zone exhaust air—which is normally clean and hot—directly as combustion air to the kiln burners. Almost 3⁄4 of the input to this kiln is in the hot zone, and with careful design, 500°F combustion air can be applied to the hot zone burners. This can prove successful in saving about 10% of the total fuel usage—in this case, around $70K per year at current prices. Applying warm air to the air jets in the preheating zone can save some energy as well.
Remember that this is a modern kiln. Some companies with older kilns firing similar products have fuel consumption levels per pound that are double or triple these values. They won’t survive the energy crunch if it continues too long because they can’t be competitive. In addition to modifications to the equipment, however, there are often modifications to the operation that can reduce fuel consumption dramatically. For example, reducing air infiltration, using cooling air as preheated combustion air through drafting techniques, minimizing the use of unnecessary burners, applying baffles to the kiln cars, monitoring and adjusting oxygen levels, evaluating and improving cycle and car loading all provide some areas to look at.
The dramatic change in energy costs means that we all have to rethink our energy conservation strategy.