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Combustion and Kiln DesignTo begin the seminar, Jim Feese, Hauck’s project engineer, presented an overview of the combustion process. Air is the most important factor in a combustion system, said Feese, and almost all combustion systems are designed for their air handling capabilities. A good design will help ensure complete combustion.
The three basic types of burners are: nozzle mix, in which the gas and combustion air do not mix until they leave the ports; delayed mixing, in which the gas and air travel a considerable distance from the burner before complete mixing and burning; and pre-mix, in which the primary air and gas are mixed at some point upstream from the burner ports by an inspirator mixer, an aspirator mixer or a mechanical mixer. High velocity or super velocity burners, which are part of the nozzle mix group, are becoming a popular choice among many ceramic manufacturers. The burners are very stable and are capable of high excess air limits, Feese said. About 70% of the combustion takes place outside the tile, making this a “cool” running burner with less wear and tear on the tile and lower maintenance requirements. It also offers high turndown ratios and produces relatively low NOx. Numerous styles of high velocity burners have been developed to suit virtually any application.
Ralph Ruark from Ruark Engineering, Inc., and Ceramic Industry’s senior technical editor, discussed kiln firing strategies for tunnel kilns. The modern tunnel kiln comprises four zones: preheat, main firing, rapid cooling, and slow and final cooling. The rapid cooling zone is especially important when flashing, Ruark said, because it “freezes” the process and keeps it from continuing and causing unwanted effects. To avoid damaging the product, the kiln should have the capability to remove air as well as inject air. Hot air moving toward the entrance can be used to lower the fuel consumption of the kiln, said Ruark. Other important factors to consider in kiln design include the kiln scale, pressure control requirements and body/glaze development.
Ruark also discussed some basic design requirements of an accurate, fast firing kiln. The doorway should prevent the suction of cold air into the ware space, and there should be a sufficient number of exhaust offtakes extending far enough up the tunnel to draft the kiln. In the preheat zone, there should be enough burners and zones to accommodate the body firing requirements, and enough circulation to promote top-to-bottom temperature uniformity. The main firing zone should be designed for firing accuracy, and the rapid cooling zone should be engineered to provide uniform cooling throughout the entire settling area. In the slow cooling zone, good circulation and multiple exhaust locations are important.
Emissions and Pulse FiringEmissions were the topic of a presentation by Bruce Irwin, director of sales engineering for Hauck. Combustion takes time, Irwin said. To avoid incomplete combustion from “quenching,” adequate temperatures must be achieved in a given period of time. Carbon monoxide (CO), volatile organic compounds (VOCs) and total hydrocarbon (THC) are some of the byproducts of incomplete combustion. To some extent, NOx emissions can be controlled with the right burner design—for example, super velocity burners are staggered and provide a layer of air around the flame, which helps lower the NOx emissions. Pulse firing is another way to reduce NOx emissions by maintaining peak design velocities, Irwin said, with side benefits of saving fuel and improving temperature uniformity.
Fred Fuhrman, controls engineer for Hauck, discussed pulse firing in more detail. The technology was developed about 20 years ago in Europe to make the best use of high velocity burners, Fuhrman said. During pulse firing, low fire gas input is set by a bypass in or around the pulse regulator. The burner is ignited at low fire. Each time the air solenoid is energized, the burner switches from low fire to high fire. This uses the entrainment and stirring action benefits of high velocity burners to their best advantage. Additionally, since the air and fuel piping to each burner is identical, changing the pulse sequence or even moving a burner from one zone to another is all done electronically.
Other Topics and EventsReed Slevin from Harrop Industries, Inc., presented a discussion on achieving faster, safer drying by understanding what happens during the drying stage. A laboratory instrument called the TGDA dryer (a combination of a thermal gravimetric analyzer [TGA] and a thermal dilatometric analyzer [TDA]) can be used to quantify the drying characteristics of a ceramic body. This information can be used to understand the effects of temperature, humidity and air circulation on the body, so that the right adjustments can be made to the drying schedule. (See “Speed Up Your Drying Cycle,” by Reed Slevin, Ed Whalen and Alex Blum, Ceramic Industry, May 1999, pp. 51-57.)
Another guest speaker, Nathan Fields manager of technical services for Southern Color & Chemical Co., offered a solution to the troublesome process of flashing bricks—a new coating that simulates the flashing effect. (See “No More Flashing!” on pp. 3-4 in this issue.)
The seminar closed with burner demonstrations and a tour of the Glen Gery brick plant in nearby Shoemakersville, Pa.