OCR Text |
Show 3 A shell and tube recuperator has been widely used to preheat air. In this case, air is preheated up to 920K, while the maximum gas temperature exhausted from a reheating furnace is over 1600K. This is due to the temperature endurance limit of the tube material. Regenerative Ceramic Burner (RCB) heating system [1], developed jointly by Hotwork Development Ltd. and British Gas in the early 1980s, as shown in Fig.1, in which ceramic ball/nuggets is used as heat storage medium in regenerators, can raise preheated air temperature much higher than that of recuperator system. The RCB system has at least a pair of burners ( A and B, each of them equipped with a regenerator), a pair of fuel valves and switching valves. When burner A is firing, the fuel valve for burner A is opened and fuel is supplied to burner A. Combustion air at ambient temperature is supplied thorough regenerator A and its temperature is raised. Furnace gas is directly drawn by burner B and passes through regenerator B to store heat and then exhausted to the atmosphere at a low temperature. After a short time (30 seconds ~ 2 minutes), the system is switched and burner B is ignited. Now, the fuel valve for burner B is opened and the fuel valve for burner A is closed. Combustion air is supplied through regenerator B. Combusti on ai r for burner B i s preheated by the regenerator whi ch has already stored heat before switching. This time, furnace gas is drawn by burner A and passes through regenerator A to store heat. By repeating this process, the system can transfer apparent heat of furnace gas to the combustion air, and can heat up the air. The RCB system shown in Fig.1 had several aspects to be improved before being widely used in Japan. a) Highly preheated air combustion increases the potential of NOx emission, therefore, a method for reducing NOx emission is strongly required. b) Time averaged preheated air temperature is 150K to 200K below furnace gas temperature [2], therefore, some measures to raise air temperature are necessary. c) The regenerator is considerably heavy and large [2], and application of the RCB system to a large-scale reheating furnace was limited to partial applications[3],[4]. |