Figure 1. Carbon Injection Skid Installed at Plant Gaston
Figure 2. Distribution Manifold for Injection Lances at Plant Gaston
Full-Scale Evaluation of Mercury Control with Sorbent Injection
and COHPAC at Alabama Power E.C. GastonDurham, M. et al 20aug01
ABSTRACT
|
C. Jean Bustard,
Michael Durham, Ph.D., Charles Lindsey, Travis Starns, Ken
Baldrey, Cameron Martin Sharon Sjostrom,
Rick Slye Scott Renninger Larry Monroe,
Ph.D. Presented at A&WMA Specialty Conference on Mercury Emissions: Fate, Effects, and Control and The US EPA/DOE/EPRI Combined Power Plant Air Pollutant Control Symposium: The Mega Symposium Chicago, IL August 20 -23, 2001 ADA-ES Publication No. 01002 |
The overall objective of this project is to determine the cost and impacts of mercury control using sorbent injection into a COHPAC baghouse at Alabama Power's Gaston Unit 3. This test is part of a program funded by the Department of Energy's National Energy Technology Laboratory (NETL) to obtain the necessary information to assess the costs of controlling mercury from coal-fired utility plants that do not have scrubbers for SO2 control. The economics will be developed based on various levels of mercury control.
Gaston Unit 3 was chosen for this evaluation because COHPAC represents a cost-effective retrofit option for utilities with electrostatic precipitators (ESPs). COHPAC is an EPRI patented concept that places a high air-to-cloth ratio baghouse downstream of an existing ESP to improve overall particulate collection efficiency. Dry sorbents were injected upstream of COHPAC, downstream of the ESP to obtain performance and operational data and residue hopper ash and carbon samples was collected to evaluate the impact on disposal byproduct reuse. A series of parametric tests were conducted to determine the optimum operating conditions for several levels of mercury control up to 90% mercury removal. Based on results from these tests, a two-week test with one sorbent and optimized conditions was conducted to assess longer term impacts to COHPAC and auxiliary equipment.
This paper presents preliminary results from the testing in March and April 2001.
INTRODUCTION
In December 2000 EPA announced their intent to regulate mercury emissions from the nation's coal-fired power plants. In anticipation of these regulations, a great deal of research has been conducted during the past decade to characterize the emission and control of mercury compounds from the combustion of coal. Much of this research was funded by the Department of Energy, EPA, and EPRI. The results are summarized in the comprehensive AWMA Critical Review Article'. As a result of these efforts, the following was determined:
1. Trace concentrations of mercury in flue gas can be measured relatively accurately;
2. Mercury is emitted in a variety of forms;
3. Mercury species vary with fuel source and combustion conditions; and
4. Control of mercury from utility boilers will be both difficult and expensive.
This latter point is one of the most important and dramatic findings from the research conducted to date. Because of the large volumes of gas to be treated, low concentrations of mercury, and presence of difficult to capture species such as elemental mercury, some estimates show that 90% mercury reduction for utilities could cost the industry as much as $5 billion per year. Most of these costs will be borne by power plants that burn low-sulfur coal and do not have wet scrubbers as part of the air pollution equipment.
With regulations rapidly approaching, it is important to concentrate efforts on the most mature retrofit control technologies. Injection of dry sorbents such as powdered activated carbon (PAC) into the flue gas and further collection of the sorbent by ESPs and fabric filters represents the most mature and potentially most cost-effective control technology for power plants. However, all of the work to date has been conducted using bench-scale and pilot experiments. Although these reduced-scale programs provide valuable insight into many important issues, they cannot fully account for impacts of additional control technology on plant-wide equipment.
Therefore, it is necessary to scale-up the technology and perform full-scale field tests to document actual performance levels and determine accurate cost information. Under a DOE/NETL cooperative agreement, ADA-ES is working in partnership with PG&E National Energy Group (NEG), Wisconsin Electric, a subsidiary of Wisconsin Energy Corp., Alabama Power Company, a subsidiary of Southern Company, EPRI, and Ontario Power Generation on a field evaluation program of sorbent injection upstream of existing particulate control devices for mercury control2. Other organizations participating in this program as team members include EPRI, Apogee Scientific, URS Radian, Energy & Environmental Strategies, Physical Sciences, hie, Southern Research Institute, Hamon Research Cottrell, Environmental Elements Corporation, Norit Americas, and EnviroCare International.
DESCRIPTION OF OVERALL PROGRAM
The Department of Energy's National Energy Technology Laboratory (NETL) is the primary funding agency on an industry cost-shared test program to obtain the necessary information to assess the costs of controlling mercury from coal-fired utility plants that do not have scrubbers for SO, control. The method for mercury control evaluated in this program is the injection of dry sorbents, such as activated carbon, upstream of the existing particulate control device. The economics will be developed based on various levels of mercury control at four different host sites. The four sites, shown below, burn coal and have particulate control equipment that are representative of 75% of the coal-fired generation.
| Test Site | Coal | Particulate Control |
| PG&E
NEG Salem Harbor |
Low S. Bituminous | Cold-Side ESP |
| PG&E
NEG Bra ton Point |
Low S. Bituminous | Cold-Side ESP |
| Wisconsin
Electric Pleasant Prairie |
PRB | Cold-Side ESP |
| Alabama
Power Gaston |
Low S. Bituminous | Hot-Side
ESP COHPAC FF |
Gaston Unit 3 was chosen as one of the test sites because COHPAC represents a cost-effective retrofit option for utilities with electrostatic precipitators (ESPs). COHPAC is an EPRI patented concept that places a high air-to-cloth ratio baghouse downstream of an existing ESP to improve overall particulate collection efficiency. The advantages of this configuration are:
1. Sorbents are mixed with a small fraction of the ash (nominally 1 %) which reduces the impact on ash reuse and waste disposal.
2. Pilot plant studies and theory indicate that compared to ESPs, baghouses require one-tenth the sorbent to achieve similar removal efficiencies.
3. Capital costs for COHPAC are less than other options such as replacing the ESP with a baghouse or larger ESP.
In this test, carbon-based dry sorbents were injected upstream of COHPAC, downstream of the ESP over an eight week period.
SITE DESCRIPTION
Alabama Power Company, a subsidiary of Southern Company, owns and operates the E.C. Gaston Electric Generating Plant located in Wilsonville, Alabama. The plant has four 270 MW balanced draft and one 880 MW forced draft coal fired boilers. All units fire a variety of low-sulfur, washed, Eastern bituminous coals.
The primary particulate control equipment on all units are hot-side ESPs. Units #1 and #2 and Units #3 and #4 share common stacks. In 1996 Alabama Power contracted with Hamon Research-Cottrell to install COHPAC downstream of the hot-side ESP on Unit 3. This COHPAC system was designed to maintain Unit #3 and #4's stack opacity levels below 5% on a 6 minute average'.
The COHPAC system is a pulse jet cleaned baghouse designed to treat flue gas volumes of 1,070,000 acfm at 290°E (gross air-to-cloth ratio of 8.5 ft/min with on-line cleaning). The COHPAC baghouse consists of four (4) isolatable compartments, two compartments per air-preheater identified as either A or B-Side. Each compartment consists of two bag bundles, each having a total of 544, 23-foot long, Ryton™ felt filter bags, 18 oz/yd2 2 nominal weight. This results in a total of 1,088 bags per compartment, or 2,176 bags per casing'.
The hot-side ESP is a Research-Cottrell weighted wire design. The specific collection area (SCA) is 274 ft2/1000 acfm. Depending on the operating condition of the hot-side ESP, nominally 97 to 99+% of the flyash is collected in the ESP. The remaining flyash is collected in the COHPAC system. The average inlet particulate mass concentration into COHPAC between 1/97 and 4/99 was 0.0413 gr/acf4.
Hopper ash is sent to a wet ash pond for disposal. A hydrovactor system delivers the flyash to the pond.
Design parameters for Gaston Unit 3 are presented in Table 1.
PROJECT OBJECTIVE AND TECHNICAL APPROACH
The overall objective of testing at Alabama Power's Gaston Unit 3 was to determine the cost and impacts of sorbent injection into the COHPAC baghouse for mercury control. The evaluation was conducted on one-half of the gas stream, nominally 135 MW. The side chosen for testing was B-side. A-side was monitored as the control unit.
To achieve the overall objective, the program was designed with an extensive field evaluation and an equally extensive laboratory testing and analysis effort. Details of the overall test plan are presented elsewhere2. A brief description of the test plan specific to Gaston is presented herein.
Table 1. Site Description Summary, Gaston Unit 3.
| Parameter Identification | Description |
| Boiler Manufacturer | B&W wall-fired |
| Burner Type | B&W XCL |
| Low NOx Burners | Yes |
| NOx Control (Post Combustion) | None |
| Temperature APH Outlet | 290°F |
| Coal (Typical - this unit fires a variety of coals | |
| Type | Eastern Bituminous |
| Heating Value Btu/Ib | 13,744 |
| Moisture % | 6.9 |
| Sulfur % | 0.9 |
| Ash (%) | 13.1 |
| H | 0.06 |
| Cl (%) | 0.03 |
| Control Device | |
| Type | Hot-Site ESP with COHPAC |
| ESP Manufacturer | Research Cottrell |
| Design | Weighted Wire |
| Specific Collection Area (ft2/1000afcm) | 274 |
| Flue Gas Condifionin | None |
| Baghouse Manufacturer | Hamon Research-Cottrell |
| Design | Pulse-Jet, Low Pressure-High Volume |
| Air-to-Cloth Ratio (acfm/ft2) | 8.5:1 (gross) |
FIELD EVALUATION
The critical elements of the program were the actual field tests and measurements, which relied upon accurate, rapid measurements of mercury concentration and an injection system that realistically represented commercially available technology.
Near real-time vapor phase mercury measurements were made using a Semi-Continuous Emissions Monitor (S-CEM) designed and operated by Apogee Scientific. This instrument was developed with EPRI funding to facilitate EPRI research and development efforts'. The S-CEMs operated continuously for over seven weeks providing speciated, vapor phase mercury concentrations at the inlet and outlet of COHPAC.
Norit America's supplied a portable dilute phase pneumatic injection system that is typical of those used at Municipal Solid Waste (MSW) facilities for mercury control with activated carbon. ADA-ES designed the distribution and injection components of the system.
Site Specific Equipment
Sorbent requirements for various levels of mercury control were predicted based on empirical models developed through EPRI funding 3. Rates used to design equipment for the Gaston test are presented in Table 2. The system was sized for a maximum injection rate of 100 Ibs/h.
Table 2. Predicted Injection Rates for FGD Carbon on B-Side of COMPAC3
| Target
Hg Removal Efficiency (%) |
Predicted
Injection Concentration (lbs/MMacf) |
Predicted
Injection Ratea (lbs/h) |
| 50 | 0.5 | <30 |
| 75 | 1.5 | 45 |
| 90 | 3.0 | 90 |
a. Injection rate based on nominal flow at full load of 500, 000 acfm.
Figure 1 is a picture of the portable injection skid supplied by Norit Americas and installed for injection into Plant Gaston Unit 3B. Activated carbon delivered to the plant in 900 lb supersacks was loaded onto the skid by a hoist. The sorbent was metered by a variable speed screw feeder into an eductor that provided the motive force to carry the sorbent -100 ft to the injection point.
Sorbent was carried via flexible hose from the feeder to a distribution manifold at the injection level and injected into the flue gas through six injection probes (three/duct). Figure 3 is a photograph of the distribution manifold The injection system operated without plugging while injecting carbon based products with D50 particle size of 15 micron. The distribution system plugged once while feeding a finer material with a D50 of 6-7 microns.
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Figure 1. Carbon Injection Skid Installed at Plant Gaston |
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Figure 2. Distribution Manifold for Injection Lances at Plant Gaston |
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Sorbent Selection
The test plan was designed to allow for the evaluation of two sorbents during the parametric tests. The test protocol required that one of the sorbents be a lignite-derived powdered activated carbon (PAC). The second sorbent was selected based on results from fixed-bed screening tests.
Darco FGD powdered activated carbon from Norit was considered the benchmark for these tests because of its wide use in DOE/EPRI/EPA sponsored testing. Alternate sorbents were considered based on their capacity to adsorb mercury, cost and availability.
Because of the economic impact of sorbent cost on the overall cost of mercury control, it is desirable to find less expensive sorbents. The test plan included time to evaluate an alternative sorbent to Darco FGD activated carbon. This alternative sorbent could have been flyash derived, or a less expensive form of activated carbon, or a novel sorbent.
The Scope of Work supplied by DOE/NETL for this program specifically defined that only sorbents that were well along in the development and commercialization stages could be considered for testing. However, in some cases it is of interest to consider using carbon found in ash from plants within the host sites system. A sorbent selection criteria was developed so that sorbent vendors/developers could clearly understand the needs and requirements of this program. In summary an alternative sorbent must:
1. be at least 25% less expensive than Darco FGD carbon;
2. be available in quantities of at least 15,000 lbs and 250,000 lbs for site tests;
3. show that the sorbent will be available in sufficient quantities to supply at least 100,000 tons per year by 2007; and
4. have a mercury adsorption capacity of at least 100 µg/g as measured in the laboratory by URS Corporation.
Field Tests
The field tests were separated into four different test phases:
1. Pre-baseline and Sorbent Screening;
2. Baseline;
3. Parametric; and
4. Long-Term.
1. Pre-baseline Measurements and Screening
The first field measurements were made prior to installing the injection equipment. The objectives for the prebaseline tests were to:
1. Measure vapor phase mercury concentrations at several locations using the S-CEM to compare results with Ontario Hydro measurements (draft wet method used by EPA) made in 1999 (these measurements were made across the hot-side ESP on Unitl);
2. Document mercury emissions across COHPAC; and
3. Perform screening tests for mercury adsorption characteristics of several activated carbons that were candidate sorbents for the full-scale tests.
Vapor phase mercury measurements were made with the S-CEMs upstream of the hot-side ESP, upstream of the COHPAC baghouse (downstream of the hot-side ESP) and downstream of COHPAC, as shown in Figure 3. Measurements across the hot-side ESP were compared to measurements made as part of Phase III of the Information Collection Request (ICR) on Gaston Unit 1. Since no previous measurements of mercury removal across the COHPAC baghouse had been made, these tests provided important insight for planning of the actual injection tests.
Five carbon-based sorbents, three variations of ash from Gaston, and one non-carbon based sorbent were screened by URS Radian in a laboratory mercury adsorption test fixture. Six of these sorbents were then evaluated in a similar test device on a slip stream of flue gas at Gaston. A description of this screening device is presented elsewhere'.
Figure 3. Flow Schematic of Gaston Unit 3, Showing Injection and Measurement Locations
 |
2. Baseline Testing
After equipment installation and checkout, a set of baseline tests were conducted immediately prior to the first parametric test series to document current operating conditions. During this test boiler load was held steady at "full-load" conditions during testing hours, nominally 7:00 am to 7:00 pm. Mercury across B-Side of COHPAC was measured using two separate methods:
1. S-CEMs; and
2. Modified Ontario Hydro Method.
In addition to monitoring mercury removal, it was also important to document the performance of COHPAC during sorbent injection which is critical to the success of sorbent injection for mercury control. The primary performance indicators are:
Pressure drop and drag are both used to monitor the permeability of the filter and dustcake. Pressure drop is a direct measurement of pressure loss across the fabric filters. Drag is a calculated number that normalizes pressure drop to flow by dividing pressure drop by the air-to-cloth ratio. These values are a function of inlet grain loading, filtering characteristics of the particulate matter, and flow and time between cleaning. Of particular interest is the change in rate of pressure drop increase with sorbent injection and whether pressure drop/drag returns to baseline levels when injection is stopped.Pressure Drop/Drag:
Cleaning Frequency: Pressure drop/drag is controlled in a baghouse by the cleaning frequency. It is expected that cleaning frequency will increase with the increased particulate loading from sorbent injection. Cleaning frequency was monitored before, during and after sorbent injection.
Opacity/Emissions: Cleaning frequency and particulate matter characteristics can affect collection efficiency across the baghouse. Most emissions occur immediately following a clean, so increasing the cleaning frequency can increase outlet emissions. The emissions could also increase if the particulate does not form a high efficiency filter calve, but tends to work through the fabrics.
Bag Strength: The filter bags in COHPAC are made from Ryton™ felt. The Ryton bags at Gaston have experienced very little loss in fabric strength, as measured by Mullen Burst tests, in the four years of operation. To assure that carbon injection will not adversely affect fabric strength, samples of both old and new bags were pulled periodically throughout the test. Prior to the baseline tests, several new bags were installed in both the A- and B- side to monitor short term strength loss.
During the baseline tests, COHPAC and plant operating data were collected.
3. Parametric Testing
A series of parametric tests was conducted to determine the optimum operating conditions for several levels of mercury control up to 90% mercury removal, for several activated carbon products. To minimize permitting issues, only coal-based sorbents were considered at this site. Norit Americas lignite-based PAC, Darco FGD, was chosen as the benchmark sorbent. Darco FGD is Norit's standard product for mercury removal at MSW and incineration sites. Sorbent and injection concentration for the long term tests were chosen based on results from these tests.
In all, 15 different parametric conditions were tested. The primary variables were carbon type and target mercury removal level. Other variables included COHPAC cleaning settings and flow through the baghouse. Although lower flue gas temperatures have been correlated with increased mercury removal, temperature was not a variable during these tests because normal operating temperatures at this plant were between 250°F and 270°F, which is cool enough for acceptable removal. A summary of the parametric tests is presented in Table 3. Unless noted, all tests were conducted with the boiler at full load conditions and COHPAC cleaning at a drag initiate setpoint of 0.6 inches w.c./ft/min. A description of the different carbons used in these tests is presented in Table 4.
Table 3. Summary of Parametric Test Conditions.
| Test Series | Carbon Name | Target Hg Removal Efficiency (%) | Non
Standard Conditions |
| 1-5 | Darco FGD | 50, 75 and 90 | Standard |
| 6-9 | Norit PAC2B | 50, 75, 90 | Standard |
| 10 | None | Baseline | Standard |
| 11 | Darco Insul | 90 | Standard |
| 12 | HydroDarco-C | 90 | Standard |
| 13 a-c | Darco FGD | 75 | Change to
pressure drop initiate clean |
| 14 | Darco FGD | 50 | Lower A/C to 4 ft/min |
| 15 | Darco FGD | 50 | Compare to
test 14 with A/C = 7 ft/min |
Table 4. Description of Norit Carbons Used in Parametric Tests.
| Name | Description | Particle Size Distribution' | ||
| D95 | D50 | D5 | ||
| Darco FGD | Lignite AC | 52 | 15-20 | <3 |
| Norit PAC2B | Subbit/Bit Blend AC | 52 | 15-20 | <3 |
| Darco Insul | Fine chemically washed specialty product | 25 | 6-7 | <2 |
| I HydroDarco-C | Coarser FGD | 100 | 30 | 3 |
a. Percent of particles less than size in microns
4. Long-Term Performance Tests
Long-term testing at "optimum" plant operating conditions (lowest cost/highest mercury removal), as determined from the parametric tests, were planned to gather data on:
1. Mercury removal efficiency over time;
2. The effects on COHPAC and balance of plant equipment of sorbent injection; and
3. Operation of the injection equipment to determine the viability and economics of the process.
During these tests, carbon was injected continuously 24 hours per day, for 10 days. Darco FGD activated carbon was chosen as the sorbent for these tests. Injection rate was determined taking into consideration both mercury removal and the projected increase in COHPAC cleaning frequency.
Similar to the baseline test series, mercury was measured by both the S-CEMs and manual methods (Ontario Hydro). COHPAC performance, coal and ash samples, plant CEM data were collected. During these tests an EPA audit of the manual measurements was performed
Preliminary Test Results
Testing was concluded on April 29, 2001. At the time that this paper was written, only limited data were available, including results from S-CEMs measurements, COHPAC performance, preliminary leaching tests on COHPAC hopper ash, and coal analyses from the baseline tests.
1. Pre-Baseline Tests
Table 5 presents vapor phase mercury measurements during the pre-baseline tests in January. Two analyzers were used for these tests. The analyzers were set-up to measure simultaneously either across the hot-side ESP or COHPAC.
The results show that vapor phase mercury varied between 7 and 10 µg/dNm3 at all three locations. There was no measurable removal of vapor phase mercury across either the hot-side ESP or COHPAC.
These results are comparable to those made during ICR measurements on Unit 1 for total mercury concentrations and removal efficiencies. ICR measurements showed total mercury concentrations between 6.0 and 7.5 µg/dNm3 and no mercury removal across the hot-side ESP'.
It was somewhat surprising that there was no measured mercury removal across COHPAC, especially at operating temperatures below 270°F. Review of data collected through the ICR at other plants shows that there was significant natural mercury capture on units with baghouses when firing bituminous coals'. This natural collection is assumed to occur because of exposure of the flue gas to ash on the bag dustcake. The ash at Gaston was tested for mercury adsorption capacity by URS Radian. Analysis of ADA-ES Publication No. 01002 13
the ash showed high carbon content throughout the total size distribution and an adsorption capacity that was reasonable when compared to other ashes. However, since COHPAC is downstream of the hotside ESP and the ESP was in excellent condition at the time of the tests, the inlet loading was very low (0.04 gr/acf4. on average' and less than 0.01 during the tests) and there was a relatively small amount of ash present to react with mercury.
Table 5. Pre-Baseline Mercury Measurement Results (S-CEM).
| Location | Total
Mercury µg/dNm3 @ 3% Oz |
Oxidized Mercury |
| ESP Inlet | 7 - 10 | 5 - 33 |
| ESP Outlet/COHPAC Inlet | 7 - 10 | 29 - 51 |
| COHPAC Outlet | 7 - 10 | 52 - 76 |
| Mercury Removal Across ESP | 0% | |
| Mercury Removal Across COHPAC | 0% | |
The portion of vapor phase mercury in the oxidized state increased in the direction of flow. There was a greater percentage of elemental mercury at the hot-side inlet (economizer outlet) than there was at either the COHPAC inlet or outlet. The most significant oxidation occurred across the COHPAC baghouse. Similar phenomena have been documented across baghouses with fiberglass fabric bags'.
Results from fixed bed screening tests on a slip stream of flue gas were similar to the laboratory results showing that the activated carbons had adsorption capacities 100 times greater than ash or a noncarbonbased sorbent. Because of the accelerated schedule at this site, it was not possible to consider either ash, or non-carbon based alternate sorbents.
2. Baseline Tests
Baseline testing was conducted the week of March 5`°. Results from the Ontario Hydro tests were not yet available at the time this paper was written. S-CEM measurements showed vapor phase mercury varied between 8 to 12.5 µg/Nm3. Coal analyses showed mercury levels in the three coal samples varied between 0.06 and 0.17 µg/g. Since Gaston burns several coals per day it is difficult to correlate a mercury level in the coal to a specific flue gas measurement.
3. Parametric Tests
Parametric testing showed mercury removal as a function of injection concentration and sorbent type, and the impact of sorbent injection on COHPAC performance. The parametric test conditions are presented in Table 3. Feedback from the S-CEMs were invaluable in malting timely, real-time decisions on test conditions. Examples of the data provided from the S-CEMs are presented in Figure 4. These data are from the first week of parametric tests, test numbers 1 - 5, with Darco FGD.
Reduction in outlet mercury concentration can be seen to correlate with relative injection rates. Actual mercury concentrations are not shown in this figure as final results have not yet been approved for release to the public.
Results with Darco FGD showed mercury removal efficiencies as high as 90% at injection concentrations about 2.0 lbs/Mmacf. This is less that the empirically predicted rate of 3.0 lbs/Mmacf3. Other carbon based products tested and described in Table 4, showed similar performance.
Carbon injection significantly increased the cleaning frequency of the COHPAC baghouse. At an injection concentration of 2.0 Ibs/Mmacf the cleaning frequency increased from 0.5 to 2 pulses/bag/hour, or a factor of 4. An acceptable cleaning frequency at this site is 1.5 pulses/bag/hour, to maintain good bag life.
Figure 4. S-CEM Mercury Measurements During the First Week of Parametric Tests with Norit Darco FGD PAC
4. Long-Term Tests
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Based on results from the parametric tests, Darco FGD was chosen as the sorbent and a target injection rate of 1.5 Ibs/Mmacf was chosen to maintain COHPAC cleaning frequency below 1.5 pulses/bag/hour.
Results from the S-CEMs showed 80 - 90% mercury removal at an injection rate of nominally 1.5 Ibs/Mmacf. COHPAC cleaning frequency varied between 1.0 and 1.3 pulses/bag/hour
5. Coal and Ash Characterization
Coal and ash samples were collected daily during the baseline, parametric and long-term tests. Gaston fires a variety of washed, low sulfur eastern bituminous coals. Because several coals can be fired in a day, the daily coal samples will provide relative mercury concentrations, but may not be representative of specific test periods. Standard ultimate and proximate analyses will be performed, plus measurements for mercury, chlorine, and sulfur.
Ash samples were collected from the hot-side ESP, control side (A-side) COHPAC, and test side (Bside) COHPAC hoppers.
Ash generated from the E.C. Gaston Plant is impounded using a wet ash handling system. The ash is not currently beneficially reused, therefore the waste characterization testing was aimed at assessing the stability of the mercury contained on the COHPAC collected materials.
The standard testing technique used for assessing hazardous waste characteristics is the Toxicity Characteristic Leaching Procedure (TCLP, SW846-1311). The test protocol involves exposing a 100gram sample of ash to 1-liter of acidic solution (acetic acid-or acetate based) for 24 hours. The solution is then analyzed for several metals (including mercury) to determine how much of each target metal was leached from the solid sample. Results are compared against limits established by regulation. In the case of mercury, a maximum leachable level of 0.2 mg/liter has been established.
A second series of tests will be performed to answer the question of the stability of the mercury. The potential long-term environmental impact of the mercury-laden ash will be determined using two techniques, leaching and thermal desorption. Leaching tests are done using a method known as the synthetic groundwater leaching procedure (SGLP)9. This test is modeled after the TCLP, but modified to allow for disposal scenarios.
Thermal desorption tests will be performed using a special test fixture that is heated using a programmable temperature controller. The temperature of the ash sample is ramped to 500 °C at a rate of 20°C per minute. Mercury that is released by the sample is swept to a spectrophotometer for mercury measurement as a function of time and temperature.
6. Economic Analysis
After completion of testing and analysis of the data, the requirements and costs for full-scale, permanent commercial implementation of the necessary equipment for mercury control using sorbent injection technology will be determined. Process equipment will be sized and designed based on test results and the plant specific requirements (reagent storage capacity, plant arrangement, retrofit issues, winterization, controls interface, etc.). A conceptual design document will be developed with drawings and equipment lists. Modifications to existing plant equipment will be determined and a work scope document developed based on input from the plant which may include modifications to the particulate collector, ash handling system, compressed air supply, electric power capacity, other plant auxiliary equipment, utilities and other balance of plant engineering requirements. Reagent type and sources will be evaluated to determine the most cost-effective reagent(s) for the site.
A cost estimate to implement the control technology will be developed. This will include capital cost estimates for mercury control process equipment as well as projected annual operating costs. Where possible, order-of-magnitude estimates will be included for plant modifications and balance of plant items.
7. Data Analysis and Reporting
Data collected during the field evaluation will be used to prepare a summary report on the effect of sorbent injection on mercury control and the impact on existing pollution control equipment. Different plant signals will be monitored to determine if any correlation exists between changes in mercury concentration and measured plant operating conditions. This analysis will include a characterization of mercury levels and plant operation for baseline conditions, various injection rates, and different sorbents. This analysis will also identify effects of sorbent injection on operation and predict long term impacts.
This report shall be a stand alone document providing a comprehensive review of the testing and data analysis.
CONCLUSIONS
A comprehensive evaluation of mercury control using activated carbon injection upstream of a COHPAC baghouse was conducted at Alabama Power Company's Plant Gaston Unit 3. Final results were not available at the time that this paper was due; however, preliminary results and trends were encouraging.
• Effective mercury removal, up to 90% efficiency, was obtained with Darco FGD powdered activated carbon.
• A significant increase in the cleaning frequency of the COHPAC baghouse occurred with the injection of activated carbons. At this site, the maximum acceptable cleaning frequency and pressure drop limited the amount of sorbent that could be injected.
• Actual and theoretical removals were in reasonably close agreement for 80 to 90% removal (1.5 to 2 vs 3 lbs/Mmacf) considering that the model is based on a uniform particle size of 15 microns while the actual FGD carbon used has a wide size distribution with significant numbers below 15 microns.
• Using data obtained from these tests, future COHPAC (TOXECON) baghouses can be designed to operate acceptably with carbon injection.
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9. Bassett, D.J., D.F. Pflughoeft-Hassett, D.L. Laudal and J.H. Pavlish. "Mercury Release from Coal-Combustion By-Products to the Environment," Mercury in the Environment Specialty Conference, Minneapolis, MN, September 15-17, 1999.
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