Cement, Energy and Environment

sulfuric acid (in case of Power plant Boi ler ESP) could cause corona quenching. Either type of non-uniformity can cause excessive sparking and a severe reduction in the useful current density in the ESP having an impact oN overall collection efficiency. Step 4.3. Determine i f performance is limited by non-ideal effects If poor performance cannot be traced to the electrical operating conditions, then the effects of non-uniform gas velocity distribution, gas sneakage past electrified fields, and particle re– entrainment should be investigated. Certain measurements are required for this investigation. The inlet gas velocity distribution should be measured by making velocity measurement traverses over the face of the precipitator. The parameter is the mathematical standard deviation of the matrix of velocity measurements, divided by the average value. If the measured value of standard deviation is greater than 0.25, then improving the gas velocity distribution will likely improve the ESP performance. The gas flow above and below the electrified fields should also be measured during the gas flow measurement. If the sneakage is greater than 10% then the baffling to control the gas sneakage through the non-electrified regions should be upgraded. If the measured values of standard deviation and sneakage are not large enough to cause poor performance, or if improvements in standard deviation and sneakage do not cause a match between measured and designed efficiencies, then particle re-entrainment should be investigated. Collection efficiency measurements should be made with and without the rappers in operation to determine the fraction of the total emissions from rapping re-entrainment. If rapping re-entrainment accounts for more than 40% of the total emissions, then efforts should be made to reduce the rapping re-entrainment by changing the rapping frequency, rapping logic or rapping intensity. If rapping re-entrainment is not a significant problem, then particle re-entrainment without rapping may be limiting the performance. Non– rapping re-entrainment can result from high average gas velocity, a very non-uniform gas velocity distribution, a low value of dust resistivity, excessive sparking, low operating current densities, hopper dust buildup, air in-leakage through the hoppers, or malfunction of the hopper emptying mechanism. The measured gas velocity distribution and the measured gas flow below the electrified fields (above the hoppers) should be analyzed to see if re-entrainment is resulting from poor gas flow. The ash collection hoppers should be checked to see if they are functioning properly. Erratic re-entrainment without rapping can be detected by a continuous opacity monitor at the precipitator outlet. Summary In summary, the precipitator evaluation method described in this paper can be a valuable aid in diagnosing the causes of poor precipitator performance. Costly modifications to the precipitator that will not result in a significant improvement in performance can be avoided. Again, a precipitator should be placed in good mechanical condition and its performance should be properly diagnosed before initiating any type of up-gradation. ****** 20 ·4.