The firing process is arguably the most important stage in ceramic and brick manufacturing. Naturally, continuous kilns are the principal means of firing due to their higher efficiency compared to intermittent kilns, but there is always room for improvement.

Each of the three functional zones (preheating, firing and cooling) in a tunnel kiln can be considered a separate heat exchanger. In the preheating and cooling zones, heat transfer occurs predominately by convention, while heat is transferred by convention and radiation in the firing zone. The calculation of the heat required for firing in each segment of the kiln and the comparison to the heat absorption capability of the treated material is the basis for discovering optimal process parameters.

When the ware being fired cannot absorb the essential amount of heat, manufacturers must adjust the gas/air flows in the kiln. The fine-tuning of the kiln’s firing cycles can improve productivity while decreasing overall costs.

Tunnel Kiln Example

Enlarge this picture Figure 1. Tunnel kiln parameters.

One brick manufacturer was faced with unacceptable rejects (up to 15%) and high energy consumption (1590 Btu/lb) with its tunnel kiln firing process. The kiln parameters are shown in Figure 1. The kiln measured 336 ft long x 11 ft wide x 5.8 ft high, and fired 3696 brick/hr (13,948 lbs/hr). The brick being fired measured 75/8 x 35/8 x 21/4 in., and weighed 4 lbs each.

Each of the kiln’s 28 cars carried 12 packs of 616 brick each, and the decks were 12 ft 6 in. x 111/2 ft. The total insulation mass was 800 lbs. Brick entered the kiln at 70ºF. It moved into the firing zone at 1200ºF, and the cooling zone at 2000ºF, before exiting the kiln at 120ºF.

Enlarge this picture Figure 2. Theoretical gas/air flows in the kiln.

An independent calculation of the heat balance for each of the functional zones of the tunnel kiln defined the theoretical required gas/air flows in the kiln channel (see Figure 2).

Enlarge this picture Figure 3. Comparison of heat required vs. absorption capability in the kiln’s functional zones for the original firing cycle.

However, as shown in Figure 3, the heat transfer/absorption capabilities of the existing kiln zones did not provide the total heatwork needed for preheating the brick to 1200ºF (heat deficiency = 2.86 MBtu/hr). Similarly, the cooling zone lacked 3.36 MBtu/hr of capacity when cooling the fired brick to 120ºF.

Simple Adjustments

Enlarge this picture Figure 4. The mathematical model recommended a new breakdown of gas/air flows in the kiln.

The mathematical model recommended a new breakdown of gas/air flows without expensive kiln reconstruction (see Figure 4). The new firing cycle solved problems by increasing the input hot gas in the preheating zone from 18,007 lbs/hr to 29,328 lbs/hr and the number of carts from 8 to 10. In the cooling zone, the input of cold air was changed from 9124 lbs/hr to 13,245 lbs/hr.

Enlarge this picture Figure 5. Comparison of heat required vs. absorption capability in the kiln’s functional zones following the updated firing cycle.

This approach allowed for the recovery of heat (Figure 5) in the preheating and cooling zones. On the other hand, increasing the flow of cooling air, which helped the preheating energy requirement, increased the energy input into the firing zone. Despite the increased hot zone input, overall input based on the load was reduced to 1011 Btu/Lb.

The implementation of these recommendations reduced production rejects and decreased heat consumption from 1590 Btu/hr to 1011 Btu/hr. Total cost savings was $450,000 per year without any additional investment for kiln modernization.