Impacts of Hydrated Lime Injection on Electrostatic Precipitator Performance
ASTM Symposium on Lime Utilization
June 28, 2012
Robert A. Mastropietro
Vice President of Technology
2319 Timberlock Place – Suite E
The Woodlands, Texas 77380 USA
Electrostatic precipitators (ESPs), which are particulate collectors, are now used as part of the flue gas scrubbing strategy. In these combined systems, hydrated lime is injected into the flue gas ahead of the ESP, to either neutralize or absorb a gaseous pollutant. Then the ESP must remove the combined fly ash from fuel combustion, plus the unreacted reagent and reaction products.
The primary parameter in ESP performance is the particulate resistivity. Particulate resistivity is a measure of how well the particulate, when deposited on the ESP collecting electrodes, conducts electricity to ground. Variations in resistivity from optimum to extremely high can change ESP particulate emissions by significant amounts. When injecting hydrated lime into the flue gas, we must be very concerned with the impacts of injected lime and reaction products on combined particulate resistivity in the ESP. Resistivity will have a greater impact on ESP performance than all other parameters combined.
This paper is a study of the impacts on resistivity from injecting hydrated lime to treat flue gas. Lime and some of its reaction products are known to be somewhat high in resistivity, and its addition to the fly ash could be a concern in performance modeling. The impact of typical lime injection rated will be analyzed for impacts on fly ash resistivity. In addition, the paper will also discuss the impacts of these resistivity changes on ESP particulate emissions.
Laboratory resistivity (OHM-CM) of a dust is the ratio of the applied electric potential across the dust layer to the induced current density. The value of the resistivity for a dust sample depends upon a number of variables, including dust chemistry, dust porosity, dust temperature, composition of gaseous environment (i.e. gas moisture), magnitude of applied electric field strength, and test procedure.
In working with electrostatic precipitators (ESP), resistivities are encountered in the range from about 1E4 to 1E14 OHM-CM. The optimum value for resistivity is generally considered to be in the range of 1E8 to 1E11 OHM-CM. In this range the dust is conductive enough that charge does not build-up in the collected dust layer and insulate the collecting plates. Additionally the dust does not hold too much charge and is adequately cleaned from the collecting plates by normal rapping. If the resistivity is in the range 1E12 to 1E14 OHM-CM, it is considered to be high resistivity dust. This dust is tightly held to the collecting plates, because the dust particles do not easily conduct their charge to ground. This insulates the collecting plates and high ESP sparking levels result (also poor ESP collection efficiencies). Conversely if the dust is low resistivity, 1E4 to 1E7 OHM-CM, the dust easily conducts its charge to the grounded collecting plates. Then there is not residual charge on the low residual charge on the low resistivity dust particles to hold them on the plates. Thus these particles are easily dislodged and re-entrain back into the gas stream. ESP gas velocities are generally designed in the 2.5-3.5 FT/S range, if the high carbon particles are to be collected. There are a number of publications that provide a depth of discussion on the resistivity impacts on the operation of ESPs ¹²³⁴.
In looking at resistivity data, the resistivity of particulates is temperature dependent and “curves” generally peak out in the range of 280-360 F. On the high side of the peak, thermal conduction effects cause the resistivity to decrease as temperature increases. On the cold side of the resistivity peak, condensation of moisture on the surface of the particulate causes the resistivity to decrease as well.
The laboratory resistivity testing in this paper was done strictly with humidity for surface conditioning. So these laboratory measurements in this report are for fly ash, hydrated lime, and potential reaction products only. In the actual flue gas (especially with high sulfur content fuels) there will be surface conditioning from sulfuric acid, which could reduce the particulate resistivity down to even lower values than shown in this report. However, in most cases the fly ash from high sulfur coal contains relatively low levels of dielectric (i.e. silica+alumina+CaO). So there is never a situation where we have anything but a good resistivity predicted for any of the high sulfur fuel cases. Therefore no matter what sorbent we inject with high sulfur coal, we have good resistivity before and good resistivity after injection.
Calcium Hydroxide Injection
The chemical formula of calcium hydroxide is Ca(OH)2. F primary importance to resistivity measurements, is that this material contains calcium. In the ESP industry, calcium compounds (CaO, CaSO₄, CaCO₃) have been observed for many years to be highly resistive. In these ESP uses involving high amounts of calcium compounds, such as cement plants, the resistivity of the calcium compounds has been controlled by injecting moisture and operating on the cold side on the resistivity peak.
In recent years, the Ca(OH)₂ is being injected as a reagent for gaseous scrubbing purposes. But at the same time this hydrated lime and its reaction products must be collected by the ESP. Note that at the ESP, some of the calcium may exist as reagent, Ca(OH)₂, and some as a reactant, such as CaSO₄. To better understand the impacts of this injection, resistivity studies were undertaken with both Powder River Basin sub-bituminous fly ash and Eastern high sulfur bituminous fly ash. The results of resistivity tests for PRB coal are shown on Figures 1.
There are several things to note on Figure 1. First the resistivity of the 100% PRB fly ash was in the high range (i.e. >1E12 OHM-CM), which on its own would cause difficulty for ESP performance. Then tests of 100% CaSO₄ showed the resistivity to “peak out” even higher at 4E12 OHM-CM. This is a high/bad value for electrostatic precipitation. So as expected, the pure calcium reaction products can be very high in resistivity. With any cases of very high injection rates vs. fly ash rate, there could be a negative impact on resistivity/ESP performance.
However, the typical injection rate for Ca(OH)₂ injection is in the 10% reagent to 90% fly ash range. In this more dilute case, the combined flyash/reagent resistivity is hardly impacted by the injection. This means that really the only impact on the ESP would be from a 10% higher inlet loading coming to the ESP. Inlet loading is a much less powerful impactor on ESP performance than resistivity. This is especially true in this case, where the particle size of the injected reagent is created from milling. It is typical for the particle size from pulverized-coal firing to be much finer. This is because the particle size of fly ash is created by milling and then burning off of the carbon in the coal. Therefore the Ca(OH)₂ impact in this case would depend on ESP design and sizing. If the ESP is conservative (i.e. properly designed for high resistivity), the prediction would be very little increase in particulate emissions in this case.
Fly ash from high sulfur Eastern bituminous coal is quite different in resistivity from PRB fly ash. Figure 2 shows tests for the Eastern coal fly ash.
On Figure 2, we can note that the bulk resistivity of 100% high sulfur Eastern coal fly ash has a good resistivity on its own. The addition of the typical injection quantity of 10% Ca(OH)₂ does increase resistivity, by up to ½ order of magnitude. This does not increase resistivity to a severe condition, but it is a small move in the poorer direction. At the same time the 10% increase in inlet dust loading is also a small move in a poorer direction. So there is some possibility of an increase in particulate emission. In this case, the ESP must be studied specifically to see if the increase in inlet dust loading would cause a “bogging down” of the inlet fields of the ESP. This will be dependent on ESP size, inlet field electrode geometry, and ESP rapping density. There is potential that injection could cause higher particulate emissions, if the ESP is marginal in size or design. In addition to unreacted Ca(OH)2 , there will be products of the reaction mixed with the fly ash. Other species might be CaSO4, CaCL2, CaCO3, and Ca(SO3) . Resistivity tests on all these species are shown on Figure 3.
As can be seen from this data, calcium carbonate (limestone or CaCO3) does indeed have a high resistivity. Calcium carbonate is probably the source of most of the concern in cement plants with ESPs. Cement plants are a well-known difficult application for ESPs. However, this is with a particulate chemistry that has a high level of calcium species, while in contrast utility plants the injection rates will typically only result in low levels (i.e. 5-15%) of lime in the fly ash. Of the reaction products, calcium sulfate had the highest resistivity. The calcium sulfite and calcium chloride actually have low/good resistivities in the temperature range of typical utility flue gas (i.e. 250-330 F). These species would actually serve to reduce resistivity if admixed with fly ash. In the Figure 1 and Figure 2 studies, the hydrated lime was added in varying concentrations in the laboratory. However, resistivity studies were also conducted on actual flyash samples, with and without hydrated lime injection in the ductwork of a utility coal fired boiler. These results are shown in figure 4.
Within the experimental accuracy of the resistivity equipment, these results show almost no change in resistivity between; 100% fly ash (i.e. 20.1% CaO, 4.3% MgO, and 39.4% SiO2) 86% flyash:14% injected CaO (i.e. 34.2% CaO, 3.3% MgO, 27.3% SiO2) At high temperatures, the flyash with injected lime was very slightly higher in resistivity. But at typical utility cold-side operating temperatures, approximately 300 F, the 100% fly ash was actually slightly higher in resistivity than the flyash with injected lime. The measurements were so close that they even crossed over each other, depending on temperature. Thus the low injection rates of hydrated lime were judged to have no significant effect on the fly ash resistivity for this application.
Sufficient data now exists on the subject of resistivity impacts of hydrated lime injection to draw educated conclusions about the performance of electrostatic precipitators. In general, the utility boiler impacts were not as dire as once expected based upon cement plant experience. For low sulfur coals, low injection rates appear to have very little impact on fly ash resistivity. For high sulfur coal, low fly ash bulk resistivity and high levels of sulfuric acid surface conditioning result in good resistivities. In both cases, ESP performance can be predicted using ESP sizing models and resistivity predictions based upon laboratory measurements. In operation, resistivity can be modified by changing temperature and flue gas moisture content. So there are process options available if the products of sorbent injection do cause a detrimental impact on resistivity.
Acknowledgement to Michael Tate and Graymont Corporation for providing samples and chemical analyses for many of the samples tested for this paper.
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