OCR Text |
Show 4 The effect of rumdown on exit NOx emission was investigated by increasing the thermal input from 1.0 MW to 1.8 MW for the same burner air distribution. The overall and local stoichiometries were changed by adding air through the burner or the over fire section of the combustion tunnel. The transport air to coal mass ratio was increased from 1.38 to 1.82, for a constant thermal input. Different flame characteristics were also investigated by varying the primary, secondary and tertiary air fractions and swirl numbers, respectively. External staging was achieved by introducing overfire air into the combustion chamber at two axial positions, 10.5 and 13.5 burner diameters downstream of the flame. (The burner diameter is 0.30 m). The measured exit NOx concentrations are plotted in Figure 2 as a function of the first stage stoichiometric ratio for the two modes of operation: IVM and PPM. The PPM non-staged flame produced about 700 to 900 ppm exit NOx concentration. This was reduced to 217 ppm in the IVM flame. The minimum exit NOx concentration, 55 ppm NOx at 3% 02, was obtained by a PPM flame with a stoichiometric ratio in the primary stage of 0.6. A mildly staged, PPM flame was chosen as the baseline for investigating the effect of transport air to coal mass ratio, thermal input, exit O2 concentration, air preheat and the location of the over fire air injection (first stage residence time) on NOx emission. The details of the baseline flame are given in the second data column of Table 1. The heat input was 1.5 MW and the overall and burner stoichiometric ratio was measured as 1.13 and 0.92, respectively. The total air flow rate was 1460 Nm3/h. The transport air to coal mass ratio was set to 1.5, resulting in a transport air initial velocity of 23.8 m/s. The transport air fraction of the total air was 14.7 %. The coal gun was recessed from the flush position by 5 inches. High primary air flow rate and swirl number were used to promote early and uniform mixing between coal particles and primary air. The secondary air flow rate was set at the minimum necessary to cool the hot primary barrel and to avoid fly ash deposition on the primary and secondary barrels. The tertiary air fraction was set to 18.5 % of the total air and was physically separated from the initial flame front by the secondary air annulus. The over fire air was introduced at XID= 13.5 burner diameters downstream of the flame through an injection bank consisting of two, triple air nozzles. The inner diameter of each of the six nozzles was 1 inch, producing a high momentum air jet which enhanced mixing between the over fire air and the combustion products of the first stage. Measuremems of exit concentrations of the main gas components, ~ C~, CO and NOx were carried out in steady state conditions. Ash samples were also taken in isokinetic conditions at the exit of the furnace. For the base case, the CO concentration was low, about 33 ppm. The high CO2 concentration indicates good carbon conversion. The carbon conversion was determined to be 99.52% (from the carbon content of the fly ash) by using a Perkin-Elmer analyzer. The exit NO" concentration was 107 ppm at 3% 02. |