OCR Text |
Show The CFB system was warmed-up to around 600 0C by a gas burner before coal was fed to the riser. Temperature and pressure profiles were measured by thermocouples and pressure transducers, respectively. Concentrations of combustion gases were measured by on-line analysers. Here, 02 concentration was measured by a paramagnetic analyser, CO concentration was measured by an NDIR. analyser and the concentration of NOx was measured by a chemiluminescent analyser. Dry flue gas was also sampled with Teflon bags which enabled the concentration of N20 to be measured using gas chromatography. The CFB was operated at a solids circulation rate and a superficial gas velocity (above the secondary air jets) of 20 kglm2s and 5 mis, respectively. Table 1 Ultimate and Proximate Analyses of the Coal Used Ultimate Proximate (wt.%) (wt.%) C 74.4 Moisture 6.3 H 4.7 Ash 4.5 0 7.5 Volatile 36.6 N 1.1 Fixed carbon 52.6 S 1.4 RESUL TS AND DISCUSSION An example of the concentration profiles of NO x and N20 is shown in Fig. 2. Here, the CFB was operated at 830 0C and at an excess air factor of 20%. Secondary air was injected at 1. 83 m above the primary air inlet, and the secondary air/total air ratio was 0.20. The detected high level of NOx concentration in the bottom of the riser is thought to be due to volatile NOx formation during devolatilization of coal by oxidation of coal-No The NOx formed from coal devolatilization then diffuses into the boundary layer around the burning char particles with CO, where it is either reduced by direct reaction with CO (catalysed by the char surface) or by the reaction between CO and chemisorbed oxygen deposited by NO on the char surface (Suzuki et al, 1990). The N20 concentration profile in Fig. 2 suggests that N20 is continuously generated along the riser height, and that the N20 generation could occur preferentially in the secondary combustion zone where the char combustion is predominant. This result agrees with that of Moritomi et al (1990). It is believed that the N20 could be generated from HCN during volatiles combustion and from char-N during .char combustion. |