Taming the Energy Eaters

February 28, 2006
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"I've replaced all my old burners with high-velocity burners. What else can I do to reduce energy use on my kilns?"

Sound familiar? Other areas pertaining to kilns can be energy eaters, but unless you know where to look, it can be difficult to formulate a successful energy saving strategy. While this discussion focuses on tunnel kiln applications, many of the concepts are directly applicable to kilns of all types.



"I've replaced all my old burners with high-velocity burners. What else can I do to reduce energy use on my kilns?"

Sound familiar? Other areas pertaining to kilns can be energy eaters, but unless you know where to look, it can be difficult to formulate a successful energy saving strategy. While this discussion focuses on tunnel kiln applications, many of the concepts are directly applicable to kilns of all types.



Figure 1. Since ideal conditions are difficult to achieve and maintain in production facilities, some of the cooling air must be carried into the firing zones to prevent backdrafting.

Burner Capacity

Before leaving the topic of burners, there are a few things to consider. Specifically, are the burners properly sized and in the best locations? With regard to the former, are they operating at or above 70% of their rated capacity (not the output of control devices)? If so, you are in relatively good shape regardless of the type of control system you have (modulating control, thermal turndown, StepFire or pulse fire). If not, you should look at a burner of a different size or consider turning off some of the burners. While a StepFire or pulse fire system can be used to overcome some of the problems associated with oversized burners, too much time spent at the low-fire setting could cause premature failure of the burner tile.

Are the burners operating at more than 95% of their rated capacity? If so, you might not have enough capacity to react to major upsets to the system and should consider installing some additional or larger burners.

If you have a roof-fired tunnel kiln, you should ask the same questions about the injector burners being used, especially if the air stays on when the fuel is in the "off" mode.1



Burner Settings and Metering Devices

Metering devices in the individual burner air and fuel lines provide the means to set the proper air-to-fuel ratios for each burner. Too much excess air or excess fuel can be costly--a system using 100% excess air requires 25-30% more fuel just to heat the excess air, compared to a system using only 10% excess air. In addition, the load on the products of combustion (POC) exhaust fan is considerably greater. Excess fuel through the burner can be used to properly combine with hot combustion air in the kiln as long as the fuel gets sufficient oxygen for complete combustion. If it doesn't, product quality can suffer, and fuel consumption will increase.

Energy is rapidly becoming the most costly portion of the manufacturing process.

Preheated Combustion Air

Running heated combustion air through the burners has long been considered as a method to reduce fuel use. Based on available heat charts, this method seemed unnecessary for air temperatures below 600 degrees F because of the low tunnel kiln POC exhaust temperatures. However, many people have seen fuel reductions when using preheated combustion air of only 300 degrees F.

Why does this concept seem to contradict all the engineering tables? When the preheated combustion air is passed through the burners and the burner ratios are not recalculated for the change in air temperature, the air-to-fuel ratio gets closer to stoic and can become excess fuel. Operating closer to a stoic ratio increases the available heat. The excess fuel uses the (hot) excess air that is in the tunnel kiln, thereby lowering the overall excess air and increasing the available heat.

So why not just set the burners for excess fuel and accomplish the same results? This can be done if a sufficient amount of distance exists between the burners and the product, and if sufficient mixing occurs with the kiln gases to prevent flame impingement and "striping" of the product. The heated air through the burner helps to shorten the flame (reducing the reaction time for combustion) and prevent flame impingement.

Before adding hot air through your blower, make certain that the device is compatible with the air temperature you intend to run. This normally means a steel impeller and housing instead of the more common aluminum. Also, be aware that more heat will be conducted back to the motor, which could cause premature motor failure. The blower manufacturer can help you take all of these factors into consideration. Often it is more cost effective to replace the existing blower with one that is designed for use with higher temperatures.



Smart Air Management

How the gases are handled through a tunnel kiln can have a major effect on fuel consumption. Improper control of the gas flow through the tunnel can outweigh any potential fuel savings achieved from burner settings or controls.

Under ideal conditions for a typical side-fired tunnel kiln, all of the air used for cooling the product and kiln cars would be removed before the firing zones and used in other processes, such as drying. Additionally, all of the POC would be used to preheat the load before being exhausted at the charge end of the tunnel. Since ideal conditions are difficult to achieve and maintain in production facilities, some of the cooling air must be carried into the firing zones to prevent backdrafting (see Figure 1).

Some modern top-fired kilns are designed to bring most of the cooling air through the firing zones, where it is used as combustion air. In this case, the burners are really fuel injectors with very little combustion air supplied through them.



Backdrafting

Backdrafting is a condition in which the products of combustion flow from the latter portion of the burner section to the ware cooling exhaust fan. Plants sometimes backdraft their kilns on purpose to obtain more heat for their dryers, but this can be extremely costly for several reasons:
  • The cooling zone is shortened. Less cooling means less waste heat is sent to the dryers.
  • The cooling curve can no longer be controlled.
  • Fuel is wasted since the POC are being exhausted at higher temperatures than at the charge end of the kiln. In some cases, backdrafting has caused as much as 35% excess fuel to be used.


Air Infiltration

Air infiltration increases fuel use because the air must be heated to the temperature of the kiln in the area where it comes in. Common sources of air infiltration include:
  • Defective or missing end seals between the kiln cars
  • Low sand levels, the wrong sized sand or warped troughs in the kiln sand seal area
  • Improperly sized or attached, warped, or missing car sand plates

The amount of air infiltration will vary with the area of the gaps and the negative differential pressure between the undercar area and the top of the kiln car. (See the "Air Infiltration Study" sidebar.) Air infiltration is usually greatest in the front portion of tunnel kilns and is increased if forced undercar cooling is used.



Other Process Considerations

Keep in mind that making changes to your kiln equipment and/or operation to save fuel can affect other areas of your production process. For example, changes in firing schedules and variations in setting areas and weights can create product quality problems such as non-uniformity, inconsistent colors and inconsistent physical and chemical properties. The kiln must have adequate controls and firepower to respond appropriately to changes and maintain the proper firing curve and temperature uniformity.

Improper burner settings can also cause quality problems, especially when the burners are set with a ratio too close to stoichiometric or are burning excess fuel. Likewise, burner groupings that swing from excess air to excess fuel and back can cause cracking and direct flame impingement on the product.

If the waste heat from the kiln is being used in the drying process, the POC can cause scumming or efflorescence in brick manufacturing. These problems might not be apparent as the brick is packaged but can show up after they are installed.

Steps taken to reduce fuel consumption will also typically reduce electricity consumption since the amount of air and POC that need to be handled by the various fans decreases. However, if the changes made to conserve fuel decrease the uniformity in the recirculating sections of the kiln, fans might need to handle more volume to compensate, and this will increase electricity requirements. Using burners to accomplish the required uniformity might be more cost effective.

In some cases, electricity consumption can be further reduced by using variable frequency drives (VFDs) instead of dampers to control the waste heat and POC fans. Since the volume handled by a fan is directly proportional to the speed of rotation, VFDs also provide more linear control than conventional motorized dampers.



A Collaborative Approach

Energy is rapidly becoming the most costly portion of the manufacturing process. To remain competitive, all manufacturers must take a serious look at how to reduce these costs. Make sure you are using efficient burners that are properly sized and located, with the correct air-to-fuel ratio for each burner. Consider using preheated combustion air, and make sure the gas flow through the kiln is controlled. Also check for backdrafting and air infiltration, which can be significant sources of wasted fuel consumption.

To fully optimize your energy savings, enlist your combustion equipment and controls supplier(s) in your endeavors. These companies are continuously working on advances to improve heat transfer and product uniformity and help reduce energy consumption in a variety of industries, and they can often suggest approaches that are not inherent in your particular process. Sharing your specific problems and goals with your suppliers will help them to direct their efforts on your behalf.



Click on image to see larger version.

SIDEBAR: Air Infiltration Study

Air infiltration was studied in a tunnel kiln with 40 cars, in which 15 cars were in the negative front end. The cars were 8 ft wide and 10 ft long, and the average pressure in the front end of the tunnel kiln was 0.2 in. water column.



Click on image to see larger version.

The sources of the leaks are shown in Table 1, and the data are summarized in Table 2.



For more information about combustion control and energy savings, contact North American Mfg. Co. Ltd., 4455 East 71st St., Cleveland, OH 44105; (800) 626-3477; fax (216) 641-7852; e-mail sales@namfg.com; or visit www.namfg.com.

J.J. Lukacs is industry coordinator-ceramics and Fred McMann is southern regional manager for North American Mfg. Co., Cleveland, Ohio.

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