OCR Text |
Show main advantages of catalytic combustion include low NOx emissions (due to the surface temperature remaining below about l200 0C), complete combustion, low pressure drop and an extremely high volumetric heat release ratg -- up to j.l x 103 w/m3- Pa (20 x 10 Btu/hr-ft -atm) for a honeycomb ceramic monolith combustor. The catalytic radiant tube concept employs a combustion catalyst, such as platinum, that is coated on the inside surface of a ceramic tube. Figure 1 illustrates the concept. A ::::RATlO~CONDUCTION ON SURFACE , . RADIATION AIR/FUEL IN-CATALYST ' ON SURFACE ' 0 .0 . t" 10 2" END VIEW jTUBE ................... _....................................... COMBUSTION PRODUCTS ._._._._.-._._. _ . _ . _ . _ . -. OUT ............................................................. f.-- LI D-tO 1020 -------l SIDE VIEW Fig. 1 - Catalytic radiant tube concept preheated, combustible mixture of natural gas and air passing through the tube burns catalytically on the tube surface. Some of the heat generated conducts through the wall and is transmitted by radiation from the outside surface to a heat sink. Since the only source of heat is the catalytic surface, the bulk gases remain at a temperature close to the original preheat level. As a result, the gas density stays relatively high and the velocity and pressure drop through the tube remain relatively low. Thus, the radiant tube concept should have significant advantages over existing gas-fired radiant tubes by offering greater tube heating uniformity, smaller physical size and potentially higher flux density. When commercialized, this radiant tube concept could be used in high temperature heat treating furnaces in place of the currently popular electrical resistance elements. Therefore, the focus of this project was on meeting or exceeding the heat flux capability of existing electric resistance elements. Currently, electric elements in an industrial furnace operating at 22000F produce a flux d2nsity ranging from 69.8 to 116.3 kW/m (45 to 75 W/in2) • Figure 2 shows how the tubes mi~ht be installed in a 120 kW (400 x 10 Btu/hr) standard box-type furnace with a 1.52 m (5 foot) long working space. The tubes are positioned vertically against the side walls of the 30 REACTANT MANIFOLD EXHAUST OUT _ EXHAUST MANIFOLD WORKING VOLUME 2'x2'xS' RADIANT TUBE Fig. 2 - Concept for using catalytic radiant tubes in box furnace working volume, and are connected between an upper reactant manifold and a lower exhaust manifold. The tubes (61 cm (2 feet) long and 3.8 cm (1.5 inches) in diameter are spaced evenly apart on 15.2 cm (6 inch) centers, for a total of 20 tubes. The heat released from each tube would be 5.9 kW (20 MBtu/hr) at a 2 heat flu~ density of about 77.5 kW/m (50 W/in ), which would generate the required furnace power of about 120 kW (400 MBtu/hr). TUBE AND CATALYST SELECTION Due to their high service temperature (compared to metals) and potentially greater heat flux density, it was decided to concentrate only on ceramic materials. From Alzetats background in catalytic combustion, three ceramic materials were initially considered: mullite (3A1 20 3 ·2Si02 ) cordierite (2MgO·2A1 20 3 ·5Si02 ) and silicon carbide (SiC). All three are manufac-tured in tubular form, and have service temperatures in the targeted range of 11000C to l2000C (2000 F to 22000F). This is very close to the maximum use temperature of cordierite, and the major limitation of the mullite material is its susceptibility to thermal shock during on/off cycling. Silicon carbide was finally selected due to its reported resistance to thermal shock and extremely hiqh maximum use temperature of l650 0C (3000 0F). Silicon carbide (Hexolloy SA) tubes from Sohio Engineered Materials Company mea sur in g 7 6 • 2 cm ( 2 • 5 fee t ) in length and 4.4 cm (1.75 inches) in outside diameter with a wall thickness of approximately 0.64 cm (0.25 inch) were procured. |