OCR Text |
Show Typical operation ot a periodic kiln involves a heat-up stage, a soaking stage, and a cooldown stage as shown in Fig. 2. During the first two stages, hot flue gases can be routed to a TES unit; during the cooldown stage air heated while passing over the hot product can also be routed to the TES device. Therefore, the TES unit is charged by air normally vented to the ambient. Three possible uses for the stored heat are: 1. Preheating combustion air to 540°C und above. For SIC-32 industrial operating conditions, air preheating can mean a 25% fuel reduction. 2. Preheating kiln ware up to 540°C prior to firing of kiln burner. 3. Greenware drying utilizing heat from the TES unit in the 25 to 85°C range. All of the energy normally input to the dryers can be obtained from the TES device. A centrally located TES unit divided into several modules can be designed to accept heat from several sources and reject heat to other heat-requirin_ operations in the plant. Figure 3 shows a 2 kiln-1 dryer brick plant schematic incorporating a TES unit consisting of one TES module per heat-using device. A sensible storage unit using rocks, taconite, or scrap iron in dire contact with the gas streams could be used in this application. However, an advanced TES concept using carbonate salt/ceramic composite materials appears to offer economic and performance benefits. This concept is being developed by the Institute of Gas Technology under contract to the Oak Ridge National Laboratory as part of the DOE Industrial Thermal Storage Program. This advanced composite medium consists of phase-change carbonate salt supported and immobilized within the submicron-size capillary structure of a particulate ceramic matrix or porous sintered ceramic. Immobilization of the molten salt within the porous ceramic structure permits operation of the composite pellets, bricks, or Other suitable shapes in direct contact with compatible fluids, thus eliminating expensive heat-exchanger tubes required in shell-and-tube HX designs. Such a direct-contact HX design could consist of composite media shapes fabricated as cylindrical pellets, briquetts spheres, etc. and utilized in a packed-bed arrangement. 8-8 |