| OCR Text |
Show The proximate and ultimate analyses and particle size distribution of the coals are given in Table 1. The sorbent used in the study was a local calcitic limestone from Ontario. RESULTS AND DISCUSSIONS S03 CONDITIONING OF LUSCAR COAL ASH Luscar coal is a low sulphur (0.3%) Western Canadian coal produceslblue gases wii~ 225 ppm S02 content. Ash resistivities range between 2.7 x 10 to 1.4 x 10 ohm.cm depending on the carbon content of the ash. The pilot precipitator performed only with a 79% efficiency. Chemical composition and particle size distribution of typical Luscar ash samples collected isokinetically are shown in Table 2. Such high ash resistivity is known to limit the performance of the ESP. Even a thin layer of high resistivity ash reduces the current flow between the discharge electrode and the collecting plates. With reduced current levels, the time required to charge the particles is lengthened and consequently particle migration veloci ty is decreased resulting in rapid deterioration of ESP performance. S03 conditioning, the injection of S03 vapour into the flue gas stream was studied by injecting various amount of S03' ranging from 30 to 80 ppm into the flue gas at approximately 140?c. The residence time from the injection point to the precipitator was 1.5 s. The results are summarized in Table 3. The effect of increasing S03 concentration in the flue gas on ash resistivity is plotted in Figure 5. The two curves correspond to a high (3.3%) and a low (0.8-2.8%) ash carbon level. Ash resistivities decrease with increasing S03 concentration in the flue gas, up to about 60 ppm. Further increase in S03 level has only a very small effect on resistivity. S03 conditioning reduces ash resiftivity by 0.7 to 1 order of magnitude: in the low carbon ash Srom 1.3 x 1~ 1 to 1.2 x 1010 ohm.cm and in the high carbon ash from 2.7 x 101 to 4 x 10 ohm. cm. Lower ash resistivities are reflected in improv~d ESP performance: outlet dust loading was reduced from 0.16 to 0.12 g/m , and collection efficiency increased by about 6%, from 79 to 85%. MOISTURE AND S03 CONDITIONING OF LIMESTONE INJECTION WASTE Limestone injection waste results from the injection of limestone into the furnace and consists of a mixture of ash and calcium compounds (mainly CaO, CaS04 and CaC03). The waste contains high levels of CaO ranging between 206-337 g/kg and CaS04 ranging between 77-256 g/kg/5,6/. Because of the high level of CaO, the S03 in the flue gas is neutralized and hence the conductive sulphuric acid layer on the fly ash particles is no longer present. This decreases the electrical conductivity of the ash. The problem is further compounded by the two to three-fold increased dust loading to the ESP. An increase in inlet loading results in a corresponding increase in outlet emissions if the particle size distribution and ash resistivity remain the same. Since these increased emissions are not acceptable, methods for restoring ESP performance have been evaluated. The upgrade options include: increased collection area by enlarging the ESP, pulse energization and methods to decrease ash resistivity such as S03 and water conditioning. S03 and water |
