OCR Text |
Show (Fig.4) for natural gas has been kept at %"/10mmi for propane ~n/6.Smmi for butane Ya"/3.Smm. Figures 11 and 12 show the temperature distribution along the tube and temperature uniformity vs. furnace temperature. As shown, propane operation provides a smoother tube temperature distribution. However, Butane combustion issues the best uniformity when relative tube temperature difference T' achieves 4.2% at 98S0 C furnace temperature. Comparison with SER burner temperature uniformity demonstrates that U-Type burners appear to have SO% less uniformity due to the bigger length. Comparison of different fuels for each furnace temperature level determines the excess air rate reduction when switching over from natural gas to butane (Fig.13). Smokey combustion, high CO level (>100ppm), or long flames through the exhaust are restrictive parameters for minimum excess air curves. Tube vibration caused by an unstable flame is the top limit for the operational zone envelope. The butane operational zone envelope is much smaller than the natural gas or propane envelope. This is due to the earlier vibration at high excess air combustion. Emissions data during testing do not exceed 30ppm for CO, and lS0ppm for NOx inside the operational envelope. There was no significant soot build-up on the inner surface of the nozzle cone during propane and butane combustion tests. "pilot" input of 1.S Kwt. (S,OOO Btu/hr) is characterized by a high CO >100ppm level in the flue gases. Fig.14 illustrates NOx emission vs. excess air inside of the operational zone envelope. NOx level is considerably lower than for SER operation. At lS% excess air I the highest point is obtained from Propane - 142ppmi the lowest for natural gas - 90ppm. Butane combustion forms approximately 130ppm NOx. SER burner NOx emission is higher due to the higher combustion air temperature and higher temperature levels inside the firing tube, with the same radiant tube temperature. -lS- |