Cement, Energy and Environment

I distributions in the inlet and outlet ducts, inlet particle size distribution, dust resistivity, T-R secondary voltage-current readings, and electrical operating points. These data, in addition to careful inspection of precipitator components, can be used to evaluate a precipitator performance. Comparing measured precipitator data with the ESP guaranteed performance provides the opportunity to diagnose the possible reasons for a degraded level of precipitator performance. There are various ways to conduct such an analysis; The main objective of this article is to provide plant operators and engineers with a systematic method for 1. Determining whether emissions limits can be achieved wi th an existing ESP or whether an upgrade will be necessary; 2. Diagnosing the cause(s) of ESP under– performance; and 3. Identifying the best corrective actions. A simplified summary of the structured approach is presented in table 1. Precipitator Performance Evaluation If the actual performance is less than the original designed performance under these conditions, the following steps are taken to examine the potential causes for the degraded performance. Step 1. Determine the operating collection efficiency It should be emphasized that the measured operating collection efficiency may not necessarily represent the best possible performance for an ESP, even though it has good gas velocity distribution, low gas sneakage, and low re– entrainment losses without rapping . Excessive accumulation of dust on the discharge and collection electrodes, broken discharge electrodes, electrode misalignment, and warped collection plates will result in degraded electrical operating points and degraded collection efficiency. It is important to take steps to optimize the electrical conditions in the precipitator before obtaining data for use in this evaluation procedure. These steps involve correcting any existing electrical problems and perhaps cleaning the electrodes. Step 2. Compare the measured collection efficiency performance with the guaranteed original designed performance If the measured collection efficiency is slightly lower than the designed performance, the discrepancy may be caused by operating conditions that are different from the basis of design; refer to Step 3. If the measured collection efficiency is significantly less than the designed value, the precipitator is performing poorly. Refer to Step 4. Step 3. If measured collection efficiency is only slightly less than the designed value, investigate operating parameters. A measured collection efficiency that is only slightly less than the designed value may be caused by operating conditions that are different from the original designed data. Check to see if small differences in the designed and measured efficiencies is caused by differences in factors such as the inlet particle size distribution or in the electrical data. A greater mass loading of sub– micron particles or a lower average electric field will result in a lower value of measured collection efficiency. First, evaluate rapping re-entrainment for the unit. Vary the rapping frequencies and intensities (if possible) to minimize losses in collecting efficiency from rapping re-entrainment. Increasing the time between raps can sometimes reduce emissions. If this is not successful in bringing the measured performance to match the designed performance, then the gas velocity distribution over the face of the ESP and the fraction of gas passing below and above the electrodes should be measured under air load conditions. If the measured data are such that velocity standard deviation is greater than 0.25, then the gas velocity distribution and gas flow baffles should be improved. If these steps are not helpful, there is a possibility that non-rapping re-entrainment is degrading the precipitator performance. A high average gas velocity, excessive sparking, dust resistivity, low electrical operating points, or re– entrainment this from the hoppers could be the cause. These potential contributing factors should be examined. If non-rapping re-entrainment is periodic, a continuous opacity monitor at the precipitator outlet can detect it. 18 (