OCR Text |
Show conditioning reduces the ash resistivity and helps to restore the ESP electrical operating conditions. Water conditioning also lowers gas volume resulting in a higher effective precipitator SCA, increases the gas density which results in higher field strength and increased migration velocity. It also promotes a low temperature reaction between S02 and unreacted CaO resulting in additional S02 capture in the humidifier. Conditioning of the limestone injection waste from the coal blend was carried out using both water and S03. The results are summarized in Table 4. Injecting 100 to 140 ml/min of water correspondirf to a relative humhdity of 51 to 58% reduced waste resistivity from 2.8 x 10 ohm.cm to 1.8 x 10 ohm.cm and improved the efficiency of the ESP by 7% from 89 to 96%. S02 capture in the humidifier was up to 14% bringing the S02 capture from 45 to 59%/7/. The effect of 60 ppm S03 injection on waste resistivity, as a function of residence time between the injection point and to ESP is shown in Figure 6. The data show that witjbn about o. 5 ~, resistivity drops two orders of magnitude from 1.3 x 10 to 2.0 x 10 ohm.cm. The corresponding ESP performance improvement was 5-6% from 87 to 92-93% and dust emission was reduced by a factor of 1.2 to 1.6. CHEMICAL CONDITIONING OF LOW SULPHUR US COAL ASH Chemical conditioning of fly ash either lowers the resistivity of the ash or improves the ash cohesivity or impacts on both resistivity and cohesivity, so that particulate emissions and opacity compliance could be met. Improving ash cohesivity is particularly useful for ashes with high carbon content. Carbon enrichment contributes significantly to plume opacity and a chemical conditioner which can improve ash cohesivity and collectability will reduce particulate emissions. A proprietary chemical supplied by Calgon Ltd was also evaluated and used to condition the ash from a low sulphur eastern US coal, burned at Ontario Hydro's Lambton Thermal Generating Station (TGS). The chemical was dissolved in water and injected concurrently into the flue gas stream via a twin fluid atomizer, while burning a 1.1% sulphur eastern US coal from Lambton TGS. The results are summarized in Table 5. Baseline ash resistivity was measured to be 1.3 x 106 ohm.cm at carbon in ash level of about 8%. The pilot precipitator performed with an efficiency of 8S%. In run 2, water alone was injected into the gas stream to establish the effect of the water on the ash resistivity. No effect on the resistivity was observed. Injecting 1.5 g/kg coal of the chemical at the rate of 200 mlfh also had no effect on the ash resistivity. This is expected at high carbon, low ash resistivity levels. Bench scale experiments were also conducted to correlate ash resistivities with carbon levels. These results, plotted in Figure 7 indicate that resistivities of ash s~Oles froTlLambton TGS drop significantly at about 2-3% carbon in ash from 10 to 10 ohm.cm to about 10 ohm.cm, Beyond about 4% carbon in ash, no further change in resistivity was measured. In run 5 (Table 5) enriched oxygen combustion br~ught the carbon in ash down to 2.5% and resistivity was measured to be 10 ohm.cm. The proprietary chemical was injected at 200 mlfh and again no change in resistivity was |