OCR Text |
Show from 10 - 30% of the materials theoretical porosity). Mean pore sizes range from about depending on the grade selected. Figure typical reticulated ceramic structure. density (70-90% 300 um to 1700 um 1 illustrates a These materials have been used for over 30 years to filter molten metals. The purpose of filtration is to reduce the size and frequency of included defects such as oxides, borides and carbides. Aluminum, copper, steel, and superalloys are all being filtered for many applications. In more recent years other applications have developed for reticulated ceramics including kiln furniture, furnace linings, catalyst supports, and diesel particulate traps. Considerable interest has also been shown in low density ceramics with properties tailored for electronic applications. In this paper the use of reticulated ceramics in low NOx gas fired burners and pollution abatement devices will be discussed. GAS FIRED BURNERS Gas fired infrared burners are being used in a variety of industrial applications primarily because of low operating costs resulting from both a high concentration of radiation in the infrared region and the relatively low cost of natural gas. Typical applications include paper and coating drying, paint curing, plastic forming and space heating. As the trend towards cleaner air continues with the implementation of legislation to reduce NOx and CO emissions, it is anticipated that new applications will evolve for this type of burner. Water heating for residential and commercial use, supplementary heat for metal processing and process heaters for a variety of fluids in the chemical and petrochemical industries are potential users of this technology. There are several types of IR surface combustion burners commercially available, the most common based on ported ceramic tiles, ceramic or metal fiber and reticulated ceramics. The general operating principle of these types of burners is similar. Combustion iniates at or near the surface of the material. The material begins to heat up as it adsorbs energy from the flame and subsequently emits radiation in proportion to its temperataure. The flame becomes stable as it is essentially quenched on the surface. The Infracel Dual reticulated ceramic burner is shown schematically in Fig. 2. The gas/air mixture passes through a relatively fine reticulate structure (65 ppi). This tortutous path provides excellent mixing with low pressure drops. In addition to acting to uniformily deliver the |