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Much of my kiln work these days is associated with fuel consumption and energy conservation. Companies are struggling with high fuel costs as natural gas prices continue to rise. Extra energy costs impose additional profit erosion on many businesses, particularly those firing commodity products that compete with offshore companies employing cheap labor and raw materials, and enjoying government-subsidized energy and minimal pollution requirements.
Compounding the problem is the fact that many companies no longer employ skilled staff engineers who can analyze how to conserve energy effectively without eroding product quality. As a result, many American companies are nearly paralyzed as fuel prices continue to increase.
The Good Old DaysIn the '70s, the application of high-velocity excess air (XSA) burners was the norm. These burners increased circulation inside kilns to a great degree, resulting in faster firing cycles with higher product yields. The burners were designed with extraordinary XSA capabilities-in excess of 2000%-and were able to operate with jet temperatures as low as 200°F when used with high levels of XSA.
Periodic kilns could be started with all burners lighted simultaneously and were able to adhere to the time/temperature curves desired, even when starting at low temperatures. The downside was that large quantities of XSA increased the kilns' fuel consumption significantly. The extra fuel consumption imposed by these systems was not a major cost issue at the time since fuel costs were low, however, and the improved yield and higher productivity resulted in lower costs overall.
To produce favorable circulation within a kiln, the burner jet velocity must be high. Not only does the high-velocity (400+ mph) jet create circulation, but the entrainment of air into the burner stream decreases the ultimate jet temperature, eliminating hot spots. Designers tended to use fixed air input to all of the burners because high air inputs assured high velocity and circulation; control of temperature was achieved by varying the fuel input. The process maximized circulation but fostered high fuel usage.
Many XSA design schemes were used well into the '90s to enhance temperature capability, but the end result is universally high fuel consumption. XSA also increases the level of NOx produced, as the oxygen content of the kiln atmosphere is maximized in most of these systems.
What Can You Do?I've studied combustion design and the fuel consumption of hundreds of kilns, and have participated actively in adjustments and modifications of systems to conserve fuel. Many older kilns are well designed with excellent mechanical systems and lightweight refractory linings. If you have such systems, you may find relief from the burden of high fuel costs. If fuel consumption is a concern, however, modifying the kiln's combustion system can provide relief.
In the past four years, eight older kilns (produced between the 1970s and the late 1990s) have been modified with a system that was first produced in 1985.* All of the kilns that were upgraded had originally employed an "oscillation" system, in which burners firing beneath and above the load varied in air and fuel output to effect a sweeping action of the burner gases. Generally, as the burners' output on one side of the kiln increased, the burner output on the other side of the kiln decreased. This output oscillation continued in cycles of 60 to 120 seconds (see Figure 1).
The sweeping action of the burners gradually moved the impact point of the opposing burners across the width of the kiln setting, and assisted in obtaining temperature uniformity across the bottom of the load. Temperature was controlled by increasing or decreasing the fuel input of the burners, which normally resulted in high levels of excess air.
* Integrated Multi-Zone Pulsing SystemTM, developed by GFC Kilns International Ltd., Melbourne, Australia.
Making the UpgradeThe design of the modified combustion system involves considerably more than simply adding pulsing components to the existing burners. Generally, the modifications include a rearrangement of the kiln burners to provide better circulation around the load, and, typically, the elimination of 30% of the existing burners.
Kiln zoning is also modified. Normally, these large kilns only use four control zones before modification, and the upgrade system converts the kiln to as many as 21 control zones after the retrofit. Further, the zones communicate with each other via PLC logic algorithms to avoid zone interactions.
New PLC-based kiln controls are added, along with an interface program. The system plots time and temperature, along with fuel and kiln oxygen levels, pressure, etc., on a real-time basis. The software was written by kiln engineers and developed jointly with customer input. The result is both user friendly and informative.
Safety equipment (i.e., fuel and air controls) is brought into current NFPA or FM compliance during the upgrade, and additional air injection ports are included to provide increased air input for cooling. The burners are controlled in different modes depending on the cycle and product. Typical modes include:
- Excess Air-very high levels of excess air are used at the start of the firing cycle to create a gentle start. All zones control at a starting set point of 200°F, thereby avoiding any initial temperature "bumps" that might create product cracking.
- Partial Excess Air-some excess air is used and the amount is continuously controlled. The objective of this mode is to provide good circulation while eliminating the excessive air that consumes high amounts of energy.
- On Ratio-the burners fire with the slightest amount of excess air possible to provide approximately 2.5% O2 content in the kiln atmosphere.
- Cooling-the burners are extinguished, and air is injected into the kiln burners and extra cooling ports as needed to maintain the desired cooling curve.
Normally, system installation requires a couple of weeks and is followed by burner calibration and test firings. During the test firings, fuel use in each of the segments of the firing curve is analyzed, and the use of each mode is fine tuned to provide the best production quality at the lowest amount of fuel consumption. As shown in Table 1, modifications have achieved documented fuel savings ranging from 30 to 45%.
Reaping the RewardsThe payback on a retrofitted system is very attractive, ranging from as little as six months to two years. The return is particularly impressive because, in addition to fuel savings, the upgrade provides a renewed control system with PLC, software and updated safety compliance.
Fuel costs have tripled-or more-in the past decade, and are projected to continue their rise. If you have an older kiln in good condition, modification of the combustion design can offer a very handsome fuel benefit, while at the same time replacing obsolete controls and upgrading the safety system.
For more information regarding kiln combustion design modifications, contact GFC Kilns International Ltd., 227 Princes Hwy., Dandenong, Melbourne, Victoria, 3175, Australia; (613) 9792-5211; fax (613) 9792-5605; e-mail email@example.com; or visit www.gfckilns.com.au.