Kiln Connection: Tunnel Kiln Energy Considerations

Since my January Kiln Connection column, high natural gas costs and curtailed fuel availability have really made the news. As of this writing, natural gas prices have been as high as $10/Mcf, and storage inventories are at their lowest levels since they began being monitored. By the time you read this, the news may well be worse. Some of my clients have already put together an energy analysis and started the work toward improved energy efficiency. How are you doing?

Case in Point

Tunnel kiln energy calculations are a little more complex than their periodic brethren are, because a variety of assumptions must be made regarding modes of operation. To simplify matters, we will work backwards from a known tunnel kiln and develop the energy evaluation starting with the existing fuel consumption.

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.

Energy Performance

The theoretical energy required to heat our product is 504 Btu per pound (specific heat x weight x T), yielding a thermal efficiency of about 25%. What happened to the other 75%? First, when we burn methane, we have an automatic loss of about 10% of our fuel input due to the conversion of water that is produced in the reaction to water vapor. The formula is CH₄ + 2O₂ + 8N₂ CO₂ + 2H₂O + 8N₂; the heat of vaporization of the H2O accounts for the first loss.

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.

Improving Performance

At a given cycle time, with a constant firing temperature, the losses associated with heating the kiln cars and furniture are constant, along with the wall losses. This means that these losses are more or less fixed as long as the kiln is firing at a constant rate. It is difficult to reduce the wall losses significantly, because additional insulation can often lead to refractory failure by overheating the existing refractories (if the additional insulation is applied to the cold face).

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.