Resistivity is also determined isothermally as a function of electric field intensity using an air environment containing concentrations of moisture and sulfuric acid vapor or other specified agents. This procedure is described in EPA Report 600/7-78-035.
Instead of the guarded, parallel plate test cell geometry described in IEEE Standard 548, a radial test cell is used. An ash layer 5 mm wide and 1 mm thick is subjected to the applied potential. The ash is held in the test environment for 48 hours prior to determining resistivity as a function of electric field intensity from 2 kV/cm to 12 kV/cm or break down, whichever comes first. Typically the test is performed at two temperatures to bracket the process temperature. One or more sulfuric acid concentrations are employed depending on the circumstances.
Figure 2 shows the results for this type of test. Electric field can have a much greater effect on resistivity when conduction is dependent of adsorbed acid vapor. In some cases, the increase in field strength from 4 kV/cm to 12 kV/cm causes a decrease in resistivity of more than an order of magnitude. In Figure , the ash resistivity determined in accordance with IEEE Standard 548 at 4 kV/cm and a similar temperature (311°F) and environment without acid vapor was 5 x 1011 ohm-cm. Figure 2 shows the great effect acid vapor can have on ash resistivity.
As stated above, the IEEE Standard 548 test determines a maximum expected resistivity. If sulfuric acid vapor is present in the flue gas, the laboratory resistivity test should also be conducted with a commensurate acid concentration to determine a true picture of the resistivity level that the precipitator will experience.
Using the ultimate coal analysis or a given flue gas analysis and the ash chemical composition, resistivity can be predicted as a function of temperature and environmental conditions. Resistivity is modeled in accordance with EPA Report 600/7-86-010. An example of the model's output using the data supplied in Table 1, is shown in Figure 3. If one wishes to check the validity of the modeled data, a minimum amount of laboratory data can be acquired for comparison. A reasonable agreement between the modeled data and laboratory data allows one to utilize the modeled information to anticipate resistivity at all temperatures and environmental conditions.