Particle Size Distribution
- 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.
- The image below shows resistivity as a function of applied electric field potential
- Not really a compound-specific analysis. This algorithm determines the electrical properties of a dust. These properties can be used to determine the effectiveness of:
- Coal-switching studies that evaluate the resistivity of select coals and blends to predict precipitator collection performance;
- Flue gas conditioning used to improve precipitator collection performance; and
- Precipitator design and optimization studies.
- A bulk sample (about ten grams) is required for this analysis. Optimally, samples should be pulled from every precipitator hopper and blended into a representative inlet sample. This is simple across the rows, a little more involved from inlet to outlet. The Deutsch-Anderson equation is used along with hopper mapping to blend samples in a row from inlet to outlet fields.
- Isokinetic samples can also be used for this analysis. One major inconvenience is the bulk quantity required for the measurements.
- Flyash samples are generally non-hazardous for shipping purposes.
- Samples should be shipped in airtight containers.
- Modeled resistivity is well-suited for predictions of how well a candidate coal will work. It can also predict:
- How well sulfuric acid conditioning might help a given situation;
- How well temperature changes might affect a given situation;
- The resistivity of fuel blends.
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