By Rod Truce and John Wilkins, Indigo Technologies Group Pty Ltd. and Robert Crynack, Indigo Technologies LLC
Activated carbon injection (ACI) at the air-heater is an effective technique for controlling mercury emissions from coal-fired boilers. Therefore, many utilities are planning to use this technology to obtain the mercury reductions required by 2010. A key concern with applying this technology before an electrostatic precipitator (ESP) is increased particulate emissions. Because of its low resisitivity, carbon is not collected efficiently in an ESP and can increase particulate matter emissions. It takes an increase in particulate emissions of only 25 tons a year to trigger a New Source Review (NSR), therefore, even though it is a preferred technology for capturing mercury, activated carbon injection’s tendency to increase particulate emission is a major drawback.
ESPs are the preferred particulate collection equipment on coal-fired utility boilers in the United States, with more than 70 percent of boilers using the technology. Having almost 100 years of commercial use, ESPs are well understood and both their capital and operating costs are low. Two physical characteristics of particulate matter that impact an ESP’s performance are resistivity and particle size. ESPs are efficient collectors of particulate with resisitivity between 107 ohm-cm (units of resistivity) and 1011 ohm-cm. However, as Figure 1 illustrates, the collection efficiency reduces rapidly for particulate with a resisitivity below 106 ohm-cm. The migration velocity, which can be thought of simply as the rate that the particles are removed from the gas stream, is in fact a variable in the following Deutsch Equation for ESP performance:
Efficiency = 1-e-wk
Where: W = migration velocity (m/sec)
k = specific collection area (sec/m)
= A/Q area of collector plate per cubic meter of gas treated
The ESP collection efficiency drops off rapidly with reducing migration velocity. Carbon has a low resitivity, less than 10 ohm-cm with specifications as low as 0.3 ohm-cm, and therefore is not collected efficiently in an ESP. Low resistivity particles, such as activated carbon, if collected in the ESP discharge rapidly and tend to re-entrain back into the gas stream. Also, buildup on electrical insulators, which may result in electrical tracking, can have a detrimental effect on ESP operation, reducing the emitter voltage and hence the electrical power that is directed to particle collection. This will reduce the ESP collection efficiency for all particulates.
Figure 2 illustrates a typical ESP particulate concentration emission for particle sizes from 0.05 micrometers (μm) to 10μm. Figure 3 illustrates the percentage not collected, termed “slip.” It should be noted that slip is 1 – ESP efficiency, so 90 percent collection efficiency is the same as 10 percent slip. The ESP is efficient (greater than 99.9 percent) at collecting large particles, those greater than 10μm. But as the particle size falls below 2μm, the ESP efficiency falls off dramatically. In extreme cases, the collection efficiency can drop below 50 percent but will generally be less than 90 percent for particles between 0.5μm and 2μm (Figure 3). This is greater than two orders of magnitude (more than 100 times) increase in the emission of this particle size range.
Any injection of particulate in the 0.5μm to 10μm size range will result in a significant percentage, generally from 10 percent to more than 50 percent, being emitted to the atmosphere, thus increasing particulate emissions. Activated carbon size specifications give D50 size as low as 10μm, which would place a significant portion below the 2μm point where ESP collection efficiency drops off significantly. In practice, activated carbon with a larger component of fine particles will have better mercury adsorption efficiencies, so there is some degree of balance between mercury removal efficiency and ESP collection efficiency. As the activated carbon particle size decreases, the mercury adsorption efficiency increases and the ESP particle collection efficiency gets lower, hence the particulate matter emissions increase.
Because SO3 inhibits the adsorption of mercury by activated carbon, it may be necessary to reduce the SO3 concentration prior to injecting the activated carbon. This will most likely be required when selective catalytic reduction is present because the catalyst will oxidize some SO2 to SO3, resulting in increased SO3 generation. Hydrated lime (Ca(OH)2) or similar compound injections will remove the SO3 at low temperatures but will increase the ESP’s inlet particle load and may have a high resistivity, thus reducing ESP collection efficiency for the injected particulate.
A typical injection rate for activated carbon to significantly reduce mercury emissions is about 5 pounds per actual million cubic feet (lbs/MMacf). This results in an annual injection of 5 lbs/minute X 60 minutes X 8,760 hours/year, which equates to 1,314 tons/year/MMacf. Based on the 25 ton/year particulate emissions increase that is required to trigger an NSR, the following ESP collection efficiency for the injected activated carbon is required to avoid NSR:
- 100 MW uses 525 tons/year of activated carbon, which requires greater than 96 percent ESP collection efficiency of the injected activated carbon
- 250 MW uses 1,314 tons/year of activated carbon, which requires greater than 98 percent ESP collection efficiency of the injected activated carbon
- 500 MW uses 2,628 tons/year of activated carbon, which requires greater than 99 percent ESP collection efficiency of the injected activated carbon to avoid a potential NSR trigger
Given that activated carbon has low resisitivity and a component of fine particulate may exist, which may be increased due to abrasion in handling, obtaining the required ESP collection efficiency may be difficult. If additional particulate must be added to reduce SO3, the problem would be further aggravated. Based on this analysis, there is a significant risk of exceeding, possibly by a significant amount, the 25 ton/year allowed under NSR.
To eliminate the risk of triggering an NSR, the collection efficiency of the ESP must be increased, particularly in relation to fine particles and particles with low resistivity. One option is to increase the size of the ESP or the specific collection area (SCA) by adding an extra section. This can be expensive, requiring additional foundations and structural support as well as auxiliary services, such as ash handling. An alternative and potentially lower-cost solution is to install a technology that pretreats the particulate before it enters the ESP.
Australian-based Indigo Technologies developed such a technology, the Indigo Agglomerator, about seven years ago. It has been tested on a range of Australian, U.S. and South American coals with success in reducing fine particles emissions. The Indigo Agglomerator is installed in the inlet duct immediately prior to the ESP. It uses a combination of electrostatic and fluidic processes to reduce the inlet fine particle concentration by attaching the fine particles to the large particles. These large agglomerates are then collected in the ESP, increasing the ESP’s fine particle collection efficiency and hence reducing the particle slip as shown in Figure 4. The reduction in the ESP inlet fine particle loading also improves the overall ESP collection efficiency by allowing increased power, which is the result of reduced space charge and emitter build-up. By attaching fine ash particulate to the surface of larger activated carbon particles and fine activated carbon particles to larger ash particles, these agglomerates containing activated carbon particles will be retained within the collected ash in the ESP.
Three sets of tests to assess the impact of the Indigo Agglomerator on mercury removal were carried out at Mississippi Power’s Watson Plant Unit 4 (see October 2003 Power Engineering magazine article “Coal-fired Opacity Technology Being Demonstrated in the U.S.”). Unit 4 is a 250 MW opposed wall-fired boiler with two separate exhaust gas treatment and monitoring systems. It burns a variety of foreign and domestic coals.
Two of the tests involved activated carbon injection. Without adding activated carbon, these tests showed significant improvement in native mercury removal: 60 percent to 78 percent with the Indigo Agglomerator compared to 19 percent to 34 percent without it. When activated carbon injection was used to adsorb the mercury, the mercury removal improvement provided by the Indigo Agglomerator reduced with increasing activated carbon injection rates. Opacity monitors showed no measurable increase with increasing activated carbon injections with the Indigo Agglomerator in service, even when the activated carbon injection was increased to above 20lb/MMacf. Method 17 (Determination of Particulate Matter Emissions from Stationary Sources) test results were also conducted on the A-side and B-side (Figure 4) while injecting 12 lb/MMacf (360 lb/hr) of activated carbon. Test results revealed that the particulate emissions on the A-side were 0.057 lb/MMBtu and 0.045 lb/MMBtu on the B-side with the Indigo Agglomerator. The particulate emissions levels are similar to those taken during other testing with the Indigo Agglomerator operating without activated carbon injection.
As shown in Figure 5, the Indigo Agglomerator has two sections, the bipolar charger followed by the mixing section. The gas flow is split into a number of streams, each of which enters a passage in the charger section of the Indigo Agglomerator. Alternate passages are positive or negative charging. That is, the even passages may be positive and the odd passages negative, or visa versa. Following the charger, a mixing process takes place such that the fine particles leaving the positive passages are diverted into the stream of large particles leaving the negative passage and the fine particles from the negative passage are diverted into the stream of large particles leaving the positive passage. Thus, the oppositely charged particles are brought within close proximity of each other causing them to electrostatically attach to each other. These agglomerates then enter the precipitator where they are easily collected.
The percentage of dust emitting to the atmosphere is reduced by 60 percent at 10μm, 75 percent at 1μm and 90 percent at 0.1μm, as shown in Figure 4. Testing has shown that this will provide a significant total mass reduction of between 30 percent and 60 percent depending on the ESP performance and fine particle concentration.
In general, the better the ESP is performing, the higher the fine particle component of the particulate emissions. Therefore, the higher the mass reduction with the installation of an Indigo Agglomerator. Of course, the higher the fine particle load at the inlet to the ESP, the higher the outlet emission reduction, because the Indigo Agglomerator will improve ESP performance. Because fine particles limit ESP performance, due to the space charge effect and emitter build-up, reducing the fine particle inlet load will improve the ESP collection efficiency.
Owing to the low particulate resistivity and potential increase in fine particle loading generated when an activated carbon injection system is installed to reduce mercury emissions, there is a significant risk that the 25 ton/year particulate matter emission needed to trigger a NSR will be exceeded. To eliminate this risk, plant operators must ensure the PM emissions do not increase when an activated carbon injection system is installed. The Indigo Agglomerator could be a viable way to do that.
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