With the ongoing debates regarding climate change, global warming, carbon footprints and greenhouse gas generation, it’s difficult to come to any firm conclusion. Regardless of anyone’s opinion, we are being forced into making changes in our lives to reduce the use of fossil fuels and reduce CO₂ generation. Whether these new rules will be directed to the ceramic industry or not, let’s look at steps we’ve taken to “do our share”—and what we can do in the future.

History

Early kilns using solid fuels required long firing times to achieve the necessary properties. The CO₂ and CO generated were substantial and uncontrolled (along with the generation of soot, which also was a byproduct of many of these kilns). As fluid fuels became available, the combustion processes became more efficient, even though many were fired with raw fuel burners. Less fuel and shorter firing times were required to achieve the same production rates. With these changes, our “contribution” to the greenhouse gases was decreased.

Inspirator burners were the next improvement in the process of making our kilns more energy efficient and further reducing emissions, even though excess air was required for stability of the flame. As premix and excess air burners were developed, it was possible to reduce fuel requirements further by being able to better direct the products of combustion into the setting, again reducing what was sent out the stack.

The advent of the high-velocity burner enabled many kilns to reduce the numbers of burners needed for the same production levels, or enabled more production through fewer kilns. In combination with improved control of fuel-to-air ratios and pulsed-fire combustion, this continued to provide improvements to our processes while reducing our dependence on high fuel consumption and reducing total gas exhaust and the related emissions. Roof-fired tunnel kilns with air/fuel injectors and recuperated air from the cooling sections were another approach to reducing fuel dependence and total emissions, bringing us up to the present time.

With all of these accomplishments discussed, it would be too easy to just sit back and say, “I’ve done my share, now let someone else carry the burden.” We need to keep forging ahead, not only to reduce our operating costs, but also to continue protecting our environment. What can we do?

Proper Burner Air/Fuel Ratios

Maintaining proper air/fuel ratios is necessary not only for the efficient operation of the kiln, but to provide the proper atmosphere for the product as well. If an oxidizing atmosphere is required, care should be taken to not use too much excess air.

Why use excess air at all? Most products require an oxidizing atmosphere to fire properly. Changes in ambient conditions (temperature and humidity) can shift the amount of oxygen in the air substantially, shifting the ratio to more excess air—or worse, excess fuel. For example:

•  Ambient temperature of 60°F and relative humidity of 0% gives an oxygen concentration of 20.99% (by volume)

•  Ambient temperature of 90°F and relative humidity of 100% gives an oxygen concentration of 19.99% (by volume), a reduction of 5% of the required oxygen

•  Higher ambient temperatures reduce the oxygen concentration further

While many burners provide data charts showing the flow rates of air and gas for measured pressures at the burner or across drilled orifices in the burner castings to be used as a quick setting, they are only guides. The only true way to properly set air/fuel rations is to use properly designed orifice plates external to the burner, where changing burner backpressures will not affect the readings.

Regardless of the method used, pressure drops are also dependent on the air and gas temperatures, so it is important to use the gas laws to get the actual flows converted to standard conditions:

T1 x S1 = T2 x S2

where T1 = absolute temperature for 60°F (520), T2 = ambient absolute temperature (460°F + ambient temperature), S1 = known (published) flow at 60°F and S2 = actual flow at standard conditions (60°F).

Once the burners are under control, we’re home free, right? Not quite; there are other areas in which the amount of total gas exhaust affects the process, including infiltration of ambient air from openings in the refractory construction (e.g., cracks in the refractory, gaps around burners and observation ports) and gaps between the kiln cars (e.g., seals, sand plates, low sand in the troughs). Air from these sources cools the hot mix temperature and the product as the infiltrated air passes across the product before mixing with the products of combustion, forcing the kiln to use more fuel in the mix with the burner gases.

So what’s the solution? An obvious place to start is to repair the areas where the infiltration enters the chamber. With rotary calcine kilns, target the firing hood and its seal. Too much air from the cooler is another area of infiltrated air, as are any observation ports that are left open.

What are a few leak areas going to do? A 1 sq in. opening with a negative kiln pressure of 0.1 in. wc will allow 450 cfh of outside air into the process. A 0.4 in. wc differential increases the leak rate to 900 cfh/sq in. (see Figure 1). One of the best investments for controlling this issue is a good pressure control system connected to a variable frequency drive (VFD) for the exhaust fan. Another (less ideal) option is to connect the control system to a damper at the exhaust fan, preferably on the upstream side of the fan. This will help to keep the cold air infiltration to a minimum. In addition, the VFD on the fan can save considerable money by reducing the power required for operation and less products of combustion to handle.

Finally, consider the use of atmosphere analysis and control of the kiln. Portable devices for periodic sampling are available, and manual adjustments can be made accordingly. A better option is a continuous monitoring system with automatic control for making the proper adjustments.

Periodic Kilns

Many periodic kilns are operated with a fixed-air variable-fuel control, which is good for low-temperature control. However, once the organic burnout phase is complete, the kilns should be switched to on-ratio control. At higher temperatures, oxygen control becomes important as the amount of available heat to the load decreases and excess air becomes costly (see Figures 2, 2a and 2b, p. 29). Exhaust gases are also increased. A typical control schematic is shown in Figure 3.

For processes that must have a minimum amount of excess air (e.g., carbon baking), this arrangement can be used throughout the process, with the entrained gases bearing combustibles being incinerated within the furnace and becoming a part of the fuel supply. This also allows the soot that normally builds up on the furnace walls to be eliminated.

Tunnel Kilns

Regardless of the temperatures used to fire the products, controlling the excess air is crucial in three areas:

•  Preheat section, to assure that the organics are completely oxidized before shrinkage and vitrification begin

•  The main firing sections, especially if operating the burners close to ratio or somewhat fuel-rich, to take advantage of preheated combustion air from the cooling section

•  Prevent backdrafting of the kiln, sending products of combustion to the dryers and adversely affecting the main firing sections

In the preheat (oxidation) section of the kiln, the control arrangement is similar to that for the periodic kiln (see Figure 3). The probe would be located in the upper sidewall to get a representative sample of the oxygen level in that area where the carbonaceous material is being oxidized. These gases (mainly CO) are then entrained through the high-velocity burner flame envelope and become a part of the kiln fuel supply, rather than being an exhausted pollutant.

In the main burner section (high-fire zones), a probe would be located in the upper sidewall of the kiln, about a quarter of the distance from the discharge end of the main burner sections. The measured oxygen level would control the amount of air being taken to the dryer (or exhausted) through the ware cooling exhaust fan. This parameter, along with kiln pressure control, has been effective in reducing fuel and electrical costs due to less products of combustion that have to be exhausted.

Maintaining proper flow of the cooling air is important for good kiln control. If the kiln is allowed to be backdrafted, the fuel from the main firing section burners are moved backward and sent to the dryers. This reduces the efficiencies of the burners if they are dependent on cooling air for part of the total combustion air, and it effectively reduces the length of the main firing section. This is particularly a problem when the load and cross-section of the product varies on the cars.

In this arrangement, the probe would be located closely after the burners but before the air offtake to the dryers (see Figure 4). Final control would be the air input through the exit end cooling fan. The rapid cool fan should be controlled by a thermocouple in the area where the air is being injected.

Rotary Calcine Kilns

Rotary calcine kilns can be challenging to control. First, good hood pressure control is a necessity in order to prevent uncontrolled air from leaking into the kiln and/or hot products of combustion gases being forced into the work area. The oxygen probe would be mounted at the gas discharge (feed end) of the kiln and properly inserted to prevent any ambient air from the seal area from distorting the sample.

The control arrangement will vary depending on the type of cooler used. Pressure control works with the exhaust fan damper or VFD, while the oxygen sensor works with the amount of cooling air into the kiln.  

 Author’s acknowledgement: Some of the information in this article is based on and used with the permission of Fives North American, and can be found in the company’s Combustion Handbook, Third Ed., Vol. I. Thanks to Fives North American for supplying charts and graphs for this article.