OCR Text |
Show combustion of the supp lied gas. This variable t urned out to be the critical factor in initi at i ng and sustaining surface combustion. The dependent v a riables we r e a s follows: • Emissi ons -- NO , 02' CO 2 , UHC and CO leve!s were measured; these data enabled the combustion efficiency to be calculated. • Temperature -- measured at various locations within the tube and flow system at the given firing rates and stoichiometries. To initiate the combustion reaction on the inside surface of the tube, it was necessary to heat the outside of the tube with a propane torch flame. After ignition was achieved, a layer of insulation was installed around the tube to reduce heat loss and maintain the tube temperature at a level sufficient to sustain internal surface combustion. Testing showed that a radiant condition on the tube surface could be achieved. The salient results of testing were the following: • Internal surface catalytic combustion sustained the SiC tube wall temperatures above 11000C (20000F) for a period of at least 5 hours. • Emissions of NOx were very low (less than 5 ppmv). • Combustion efficiency was calculated by means of a carbon-carbon balance to be approximately 50 percent. • Under the selected conditions of reactant flow rate, stoichiometry and preheat, Cr203 failed to burn enough fuel to result in tube radiance. • At a given stoichiometry and sufficiently low flow rate, the end of the tube ceased radiating and became dark due to lack of fuel. A high flow rate caused the tube inlet to become dark due to an excessively high mass transfer rate away from the tube wall. • Continued operation of the tube generally resulted in the dimming of the end o f the tube and a spread i ng darkening of the tube fr om the inlet end. 32 • If the system was operated fuel-rich (less than 100 percent theoretical air), a sharp increase in tempera t ure waS d initially experienc ed, foll o~e by a rapid cooling of the tu e inlet. COMBUSTION AND HEAT TRANSFER ANALYSIS The results of the t e sts revealed the following information with respect to the combustion and heat t r ansfer performance of the tube. TUBE INCANDESCENCE ACHIEVED - The key achievement of this project was the attainment of self-sustaining, high temperature (llOOoC) radiant operation of the silicon carbide tube. This radiant condition was caused solely by the conduction of heat from the catalytic combustion of natural gas and air on the inside surface of the tube. HEAT FLUX DENSITY COMPARABLE TO ELECTRIC RESISTANCE ELEMENTS AND EXISTING GAS-FIRED RADIANT TUBES - A heat balance calculation showed that the catalytic radiant tube's heat flu~ density ranged f 20m 47.3 to 94.6 kW/m (30.5 to 61 W/in ). This is comparable to the flux density of conventional (silicon carbide) electric resistance elements for furnaces operating at 12000C (2200 0F); t~eir flux ran ges 2 from 69.8 to 116.3 kW/m (45 to 75 W/in ). Also, the flux density of existing gasfire~ radiant tubes -- 1~.9 to 25.3 kW/m (12.2 to 16.3 W/in ) -- was surpassed by the catalytic tube. This should be considered a preliminary result, however, as the maximum time period of maintaining this flux density from the tube in a practical application has yet to be determined. However, it is a significant finding in that these heat fluxes were achieved without heat recuperation, at a low combustion efficiency, and with a material (SiC) with thermal response properties that will be of significant benefit in a future heat treating furnace application. DOMINANCE OF HETEROGENEOUS (SURFACE) COMBUSTION - It was noted that the tube outlet temperature was only about 1500c (300 0F) above the i nl e t temperature. This is an indication of the absence of homogeneous c ombustion, because the outlet temperature would have been much higher (probabl y c loser to the 11000C wall tempe r atu r e) i f the bulk gases had been r eact i ng. Instead, the fuel t hat was c o nsumed was burned on the ca talyti c surface. |