| OCR Text |
Show 4. Fuel and air The fuel was Western Canadian natural gas, as received from the pipeline. The gas composition was very stable. A typical chemical analysis and the resultant combustion properties are shown in Table 3. In analyzing furnace performance, the excess air level is best estimated from the exhaust-gas analysis rather than the air and fuel feed rates, provided, of course, that infiltration is negligible (when it is not, interpretation of burner and furnace behaviour can be very difficult). The significant species in this calculation are CO2, CO, H2O, O2 and N2. For our natural gas, the excess air ratio, £, is related to the exhaust gas composition by 4.289X02 - 0.25335XCO /(XCOi + Xco) € = l . (1) 1-4.774 JT02 When the oxygen level is substantial (X0 >0.005), the CO level is normally very low, giving effectively 4.289^ s = ^-, e>0. (2) 1- 4.774 X0i When the oxygen is very low (X0 < 0.001), the excess air level becomes negative and effectively 2.1444Xco £ = -, s <0. (3) 1 - 0.5XCO V ; Comparison of (2) and (3) shows, as should be expected, that at modest CO contents, say Xco < 0.03, C O acts like negative oxygen in relation to excess air, with (- X0 ) = 0.5 Xco . 5. Traversable probes and combustion gas analyses The only traversable probe used in the present work was the CAGCT gas sampling probe. The probe tip consists of a ceramic tube about 6 m m long and of 0.40 m m i.d.. This is followed by a small-diameter line of about 3 m m i.d. maintained at a wall temperature of 60 °C or over and at a pressure of about 3 kPa (98 kPa, or 740 m m Hg, vacuum). The gas sample then passes through a sample conditioning unit in which water vapor and any particulates are removed, and thence to the gas analyzers. A paramagnetic analyzer was used for oxygen, a chemiluminesce analyzer for NOx , and infrared instruments for CH4, C O and CO2. The sampling rate was maintained at about 1/2 L/min. In the successor project currently underway, several other probes are being employed as well: a microthermocouple probe for local gas temperature, a two-component L D V probe (part fiber-optic) for two components of gas velocity, and a radiometric probe for local directional irradiance. The method of gas analysis, removing the water vapour first, as is commonly done, determines the gas composition in terms of mole ratios X\ = x,/(l - xH 0 ) , moles of species i per mole dry gas, or "mole fractions, dry basis", where the x, are the mole fractions in the full gas 5 |
