OCR Text |
Show guide for comparing the effectiveness of air staging technique under investigation, hence enabling selection of most effective staging level. The S02 emissions were reduced by about 69% when limestone at a Ca/s ratio 3:1, was added at bed temperature of 850°C. The SO retention as a function of bed temperature is shown in fig.4. (Note that t~e values of S02 shown in this figure are corrected for the corresponding oxygen level in the fl ue) . Th is fi gure shows that SO retent ion is very sens it i ve to bed temperature and falls from 73% at 830 C to 43% at 880 C and 40% excess air. This 30% decrease in SO retention at 880 C may be due to more rapid pore plugging of CaO by the layer of CaS04, formed by a faster surface reaction of S02 and CaO, thereby decreasing the overall utilisation of limestone. It is also apparent from this figure that S02 retention is slightly better at the high excess air level of 40%, which is consistent with the findings of Tatebayashi (4). S02 Reduction by Limestone During Staged Combustion. Three levels of air staging were used to investigate the effect of PAF on S02 reduct i on by 1 i meston-e. The percentage 1 eve 1 s of primary/secondary air used were 85:15, 70:30, and 60:40 at 40% excess air. Figure 5 presents graphically the effect of the PAF on sulphur retention. This figure shows that as the primary stage, the bed, becomes progressively more substoichiometric, and as the PAF is decreased the sulphur retention deteriorates. This trend could be due to a combination of lower oxygen concentration in the bed available for the sulphation react i on and increased sulphur generat ion in the reduc i n9 reg i on of the bed, as sulphur retention is known to be proportional to gas residence time in the oxidizing region of the bed. At lower PAF a large amount of carryover of unburnt fuel sulphur species into the freeboard occurs where it subsequently oxidizes to SO)' effectively by-passing the limestone which would also increase S02 emiSSlon. Similar findings were obtained by Nack (3) and Tatabayashi (4). Data of Tatebayashi (4) show the onset of a marked decrease in SO~ retention occurred at a PAF of 0.9. whereas M Valk (5) found a two fold lncrease in S02 emissions at a 0.6 PAF and 10% overall excess air level (equivalent to 55:45 staging). The plot of Figure 5 shows that the general trend is for lower sulphur retention as the primary stage becomes progressively more substoichiometric (ie PAF is reduced). In-bed oxygen partial pressures in the range of 10-12-10-3 atm (6) at 1.0 m/s and 40% excess air level during unstaged combustion show that approximately 4% of the particulate phase of the bed operates under reducing conditions. Similar measurements at a PAF of 0.84 indicate that about 20% of the particulate phase can be considered to under reducing conditions. The effect of bed temperature on sulphur retention, with PAF in the range 0.84 to 1.4 is shown ;n Figures 5 to 7. Bed temperature variation has a dramatic effect on sulphur retention. A maximum retention of 68% (similar to that obtained in conventional combustion) was obtained for 70:30 staging at 830 C bed temperature (Fig 7). When the bed temperature was increased to 880 C, sulphur retention went down to 9%. The rapid fall in S02 retention might be caused by two possible mechanisms. Firstly by lower inbed oxygen partial pressure during staged combustion, limiting the sulphation reaction and secondly by decomposition of calcium sulphate under (5) |