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Many energy companies need to reduce sulfur trioxide (SO3) emissions and/or sulfur dioxide (SO2) emissions. Injection of an alkaline sorbent into the boiler or into the duct after the air heater is a relatively simple and inexpensive means of meeting this need. These processes can, however, have a negative effect on the performance of the electrostatic precipitator (ESP) most plants are equipped with for particulate control. Hence, there is a need to understand and be able to predict the impact of these sorbents on ESP performance. While some of these sorbents are currently used to control SO3 emissions or reduce SO2 emissions, data available from these applications are insufficient to predict the impact of sorbent addition on ESP performance. This issue is of particular concern because the negative effect is limiting use of some of the least expensive sorbents. This project was initiated to develop the needed data, and this report summarizes the results of the first two project field tests.
ObjectiveThis project's ultimate goal is to characterize the affect of sorbent injection on ash/ sorbent mixtures with sufficient accuracy and in sufficient detail to allow useful predictions of the impact of sorbent injection processes on ESP operation. The key to achieving this goal is to predict the properties of these mixtures. In particular, two properties are of great importance: resistivity and particle size. Both properties are difficult to measure in the field; hence, both extensive field and laboratory measurements are needed to ensure reliable results. Extensive laboratory tests of ash samples gathered during the field tests did, in fact, make it possible to develop correlations needed for the processes that use low injection rates needed for SO3 control.
ApproachThe project team began with a literature search from an earlier report (Report 1013348, The Effect of SO3 Sorbents on Electrostatic Precipitator (ESP) Performance, A State-of-the-Art Review). The team determined there were insufficient sorbent data to produce reliable correlations for the critical particulate properties needed to use in a model such as EPRI's ESPM to predict ESP performance. Two major tests to gather the needed data have been completed, and the results are in this report. It was necessary to supplement the field data with extensive laboratory tests, and further field tests are planned to gather additional data. In particular, tests at sites that inject sorbents at much higher levels for SO2 control are needed to extend the correlations. When these tests are completed, the results will be applied to generate new correlations and/or new rules-of-thumb that can be used in EPRI's ESPM to predict ESP performance. Results of these predictions will be compared with full-scale field measurements, and refinements will be made to the correlations and rules-of-thumb, if needed.
ResultsTwo sorbents were identified as principal candidates for further research during the literature search for this project: hydrated lime and trona. Two host site utilities that use these alkaline reagents for SO3 control were identified, and extensive tests were conducted at these sites. Hydrated lime was used at one site, and it was determined that SO3 concentrations at the stack could be reduced to acceptable levels without a significant impact on ESP power levels. Particulate mass loading exiting the electrostatic precipitator did increase slightly at the higher hydrated lime injection rates with the highest emissions measured at the maximum hydrated lime injection rate. However, the plant does not normally operate at these high injection rate which were only tested to advance the state-of-the-science. The plant reports that stack particulate emission rates have not increased at normal, lower injection rates. At the plant using trona for SO3 control, sulfuric acid emissions were typically reduced by 60 to 70%. A demonstrable decrease in particulate loading at the precipitator outlet occurred as the amount of trona injected at the air heater outlet increased. These results demonstrated that both sorbents can reduce SO3 concentrations to acceptable levels with little or no increase in outlet particulate emissions.
Application, Value and UseMany power producers are looking for inexpensive ways to reduce SO3 and/or SO2 emissions from their coal-fired boilers. Sorbent injection processes that are the subject of this study are two of the leading candidates in the low-cost technology category. Any operator considering one of these processes will benefit from this study’s results because they make possible reasonable predictions of sorbent impact on particulate emissions without an expensive field test. Although the study is not complete, there are enough data in this report to allow a preliminary evaluation of the impact of both hydrated lime and trona on ESP particulate emissions when the sorbents are injected at low rates. At the end of the project, enough data should be available to update the correlations in EPRI's ESPM computer model. These updated correlations will enable the model to make accurate and reliable predictions of the effects of these processes and of processes that use the higher rates of injection needed for SO2 control on ESP performance and emissions.
EPRI PerspectiveThe addition of selective catalytic reduction (SCR) nitrogen oxide (NOX) reduction systems has produced the undesirable side effect of increasing sulfuric acid concentration in treated flue gas. The level of increase is great enough to introduce both particulate emission and opacity problems at many plants where the systems are installed. In addition, current and future regulations will require utilities to reduce SO2 emissions from coal-fired units. Sorbent addition is currently being used at a number of plants to reduce SO3 emissions, and many utilities are considering sorbent addition to achieve moderate rates of SO2 removal. In either case, the impact of sorbent addition on ESP performance is a critical consideration. This research is necessary to help energy companies accurately predict such impacts.
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