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Show INTRODUCTION The surface combustor-heater is a combustion-heat transfer device that is based on the concept in which the relatively cold heat-exchange surfaces are embedded in a stationary bed (porous matrix) where gaseous fuel is burned. As the bed is heated by the combustion products, the heat is extracted from the bed by the embedded heat exchanger and is transferred to a working fluid circulating in the tubes. The overall heat-transfer rate from the porous matrix to the heat-exchange surfaces will be higher than that in a conventional convective heat exchanger because gas flow across the tubes is intensively mixed and turbulized by the solid particles of the porous matrix - providing a high convective heat-transfer rate - and the radiant heat transfer from the solid particles to the tubes provides a significant contribution to the total rate. Also, by removing heat simultaneously with the combustion process, the combustor-heater reduces NOx formation by suppressing the combustion temperature.1 There are many additional incentives for the development of the advanced natural gasfired surface combustor-heater with high combustion efficiency and intensity, high heat-transfer rate to the load, high thermal efficiency, ultra-low emissions, low pressure drop, and adequate turndown ratio. The idea of burning a fuel/air mixture in a porous matrix of refractory material is probably more than 50 years old. High combustion intensity, low excess air, and high combustion efficiency can be achieved in this bed. However, several drawbacks, such as short refractory service life, the high combustion air pressure required, and relatively high NOx emissions are apparent in using this approach. About 15 years ago, another design was tested in the U.S.S.R.2,3 Water-cooled tubes were inserted in a stationary bed supported by a ceramic distribution grate. High combustion efficiencies, low emissions (NOx ~20 vppm; CO ~20 vppm), and much longer refractory-chip ervice life were achieved. Nevertheless, several major drawbacks remained: high combustion air pressure (more than 40 in. wc) is necessary for operating without flashback at a practical jrndown ratio (4:1), difficulties with scale-up of the distribution grate for industrial applications, relatively short grate service life, and high capital and operating costs. Based on the technical principle of the surface combustor-heater concept described bove, a 24-inch by 24-inch cold model and a 6-inch by 6-inch bench-scale surface combustorheater (Figure 1) were built and preliminarily tested at the Institute of Gas Technology (IGT).1 Although only a limited number of tests were performed during the evaluation stage, the results 2 INSTITUTE o F GAS TEe H N 0 LOG Y |