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Show - 2- Greek Letters (3 extinction coefficient, '" + u f emissivity v kinematic viscosity of the gas p reflectivity or density 11 parameter defined by equation (11) l1t thermal efficiency defined by equation (16) '" absorption coefficient To optical dimension (opacity), {3H ¢ porosity w single scattering albedo, u / {3 u Stefan-Boltzmann constant or scattering coefficient Subscripts a refers to ambient g reCers to gas i reCers to inlet conditions o reCers to outlet conditions s reCers to porous solid w reCers to tube wall INTRODUCTION The surCace combustor-heater is a device that is based on the concept in which the relatively cold heat exchange surCaces are embedded in a stationary bed oC reCractory material where gaseous Cuel is burned. The heat liberated is extracted by the embedded heat exchanger. Removing heat simultaneously with the combustion process has the advantage of reducing the combustion temperature and suppressing Cormation oC nitrogen oxides (Jasionowski et aI., 1QS7). In a conventional combustor, the presence oC large areas of cold heat exchange surCaces within the combustion zone would quench the flame, thereby producing increased carbon monoxide and total hydrocarbon emissions. In an advanced surCace combustor-heater concept fired by natural gas, heat exchange surfaces (tubes) are embedded in a packed stationary bed oC alumina chips where natural gas is burned (Jasionowski et aI., 1QS7; Simpson, 1QS7). As the bed (porous material) is heated by the combustion products, the heat is extracted by a coolant, such as water circulating in the tubes, and simultaneously the combustion products are cooled. The combustor-heater has the benefit oC reducing NOx Cormation by decreasing the combustion temperature. The additional incentives Cor the development oC an advanced natural gas-fired surCace combustor-heater with high combustion efficiency and firing density are the high heat transCer rate to the load, high thermal efficiency, ultra-low emissions, low pressure drop, and adequate turndown ratio (Jasionowski et aI., 1QS7). Besides the very large market Cor natural gas Cor process heaters, the surCace combustor-heater concept can provide the basis for development oC advanced and more efficient ultra-low pollutant emissions boilers, water heaters, thermal fluid heaters, and air heaters Cor industrial |
