OCR Text |
Show constant. C and P are calculated from the chosen C/P ratio. The two-gas pulsation dephasing is mechanically set. Since the two valves are moved by the same electric motor, the dephasing does not vary versus time. 4. Experimental conditions In order to test the parameters which characterize the pulsating combustion process, the basic combustion parameters were established : burner firing rate, flue gas temperature, furnace pressure, oxygen excess in flue gas and nitrogen content of natural gas. 4.1. Firing rate, flue gas temperature and furnace pressure. On furnace n01, power was set to 20 kY, yielding flue gases at 1300 oC, and on furnace n02 at 60 kY, with flue gases leaving the furnace at 1500 oC. In order to mInImIze air entrainment into the furnace, experiments were carried out under a slight positive pressure of pressure about 2 mm water column. 4.2. Nitrogen content in natural gas A relatively high N2 content in natural gas (7 to 15%) was selected for the evaluation of pulsating combustion. One reason for this was to supply a source of N2 for NOx formation which ~ould be easily measured (non-pulsated NOx emission levels between 1500 and 2000 ppm). The other was to illustrate relative unfavourable conditions which are however representati ve of european operations (Groningue natural gas contains 14.7% N2 before is diluted to about a 10% range at point of use). A clear relation between the NOx emission level and the N2 contained in natural gas is evidenced in figure 4 at non-pulsating combustion conditions. These results have been obtained at 3% excess 02 in the dry flue gas. Consequently we worked in relatively "polluting" conditions. 4.3. Excess oxygen in dry flue gas. NOx formation is closed related to the excess O2% in the dry flue gas . This is shown on figure 5. In the range between 1% and 4% oxygen in dry flue gas, NOx increase drasti cally. Beyond 4% 02 the slope is smoother, bu t still NOx emissions increase. Above 8%, the impact of lower flame temperature due to diluti on by the ex ra oxygen , becomes more important than the increased oxygen availabil ity. As flame emperature decreases, so do NOx emissions. To operate near th "flat" NOx generating excess 02 range, it was decided to work at 6% O2 fo all experi ments . 5. Experimental results 10 |