OCR Text |
Show opacity. There are many techniques that have been applied to increase convective heattransfer rates from the combustion products to the heat exchanger, such as surface modifications (for example, treated surfaces, rough surfaces, extended surfaces, etc.) and flow enhancement (for example, swirl flow, stirring flow, additives for liquid and gas, flow oscillation, etc.). Regardless of the technique, a basic approach for improving convection heat transfer of the gas flow across the heat-exchanger surfaces is to increase either velocity (or turbulence) or temperature, or both. However, an increase in gas velocity substantially increases the pressure drop that results in the reduction of the performance of the entire system, and an increase in the gas temperature requires higher flame temperatures that normally result in higher thermal NOx emissions. Therefore, further improvement of heat transfer in conventional heat exchangers are restricted. To reduce NOx formation in a combustor, one of the most effective approaches is to remove heat from the combustion zone. This could be achieved by placing a heat sink or injecting a diluent in the flame zone. However, in a conventional combustor, the presence of cold heat-exchange surfaces within the combustion zone tends to quench the flame, thereby producing CO and total hydrocarbon (THC) emissions. In the surface combustor-heater, however, the relatively cold heat-exchange surfaces are embedded in a stationary bed (porous matrix) where the fuel is burned, as shown in Figure 1. 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 to the heat-exchange surfaces is higher than that in a conventional heat exchanger because gas flow across the tubes is intensively mixed and turbulized by the solid particles in the bed, and the radiant heat transfer from the solid particles to the tubes accounts for the 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. The problem with flame quenching can be avoided because combustion reaction takes place in a great number of the small pores between the particles as well as at the hot surfaces of the particles. Therefore, combustion intensity can be very high, which allows perfect completion of combustion. CXJCL.C) DIITIUIUTIOH CIlIATE Figure 1. SCHEMATIC OF SURFACE COMBUSTOR-HEATER CONCEPT 2 |