OCR Text |
Show 5 detached. Two techniques were used to reduce the production of nitric oxide. In Case 4 the coal was carried with nitrogen and methane. The overall heat release increased slightly but because there was no oxygen in the core of the jet, NO production is reduced. In Case 5 the burner was modified to incorporate an annular gas pilot between the axial coal jet and the secondary air. This represents a very simple permanent pilot. The predicted NO is less than 150 ppm dry at 0% oxygen. Ve do not claim that this represents the actual emission for the real system if it were built. As stated earlier it is probably within twenty percent. However, using the" model in this way helps plan the development program by guiding the choice of different design and operational parameters in this complex system. Figure 5 compares the integrated NO for the five cases as a function of axial location. As expected the majority of the NO is produced in the near burner region. But the model indicates two distinct regions of NO formation, one in the first two meters which is not associated with NO formation in the bulk of the jet. Increasing wall temperature, reducing excess air level or using a permanent pilot all reduce the amount of NO formed on the boundary of the coal jet in the first two meters. The success of our HIPPS design will ultimately depend on the corrosion resistance of the ceramic air heater and the tailoring of the combustion process. Our HITAF is being designed to minimize ash buildup on the ceramic walls which requires holding the combustion gas temperature below that for sticking ash, a temperature that varies greatly from coal to coal. The cycle analysis depicted in Figure 1 established the coupled relationships that constrain the operating conditions. To achieve the operating efficiency of ~ 47% a high efficiency Brayton cycle is required with a turbine inlet temperature of ~ 2400 F. Using the recommended maximum amount of natural gas in the duct heater, then the ceramic air heater must raise the temperature of the working fluid from 730 F to about 1800 F. For the optimized size and effectiveness of the heat exchanger, the coal combustion products are cooled from 2490 to 1550 F resulting in a maximum ceramic surface temperature of about 2000 F in contact with corrosive ash deposits. Many coals produce a sticky ash at 2000 F and our designs are concentrating on minimizing deposition and ash buildup. Temperatures around 2000 F are tolerable for state-of-the-art SiC in non-corrosive atmospheres. The extent of corrosion and its long terms effect on thermomechanical properties of the ceramic are a major concern of our study. |