Recovering Lost Generation Capacity and Reducing Air Toxics Through Improved ESP Performance by Flue Gas Conditioning
Benetech, Inc.
Montgomery, IL
Existing electrostatic precipitators (ESP) were designed to provide a specific efficiency of collection for the coals in use or proposed at the time of design. In the past, many power companies decided to switch to low sulfur coal as a means of meeting environmental restrictions on sulfur dioxide emissions. Uncertain fuel supplies have often forced the burning of lower quality fuels or less expensive western fuels. These and the more recent trend to Powder River Basin coal has in many cases, resulted in the deterioration of precipitator efficiency and subsequent plant derating to meet opacity emission limits. Stack emissions are being further complicated by new regulations for ultrafine air borne particulate matter less than 2.5 µm in diameter and potential regulations for air toxics.
Considerable field experience indicates that fly ash conditioning can materially improve the performance of these precipitators at relatively low cost. Two of the major factors affecting the performance or efficiency of an electrostatic precipitator are the electrical resistivity of the fly ash and fly ash particle size distribution. Various chemical treatments have been used to bring the resistivity of the fly ash to the level required for high efficiency operation of the precipitator. Sulfur trioxide has been applied to restore the resistivity to the proper range for efficient precipitator operation. However, not all fly ashes respond consistently to this treatment and under certain circumstances can produce sulfur trioxide emissions leading to acid rain fallout. The capitals cost to install these systems are often prohibitive for many plants.
Benetech developed a flue gas conditioning (FGC) system that has shown consistent performance for more than 15 years on a wide range of coals. This system can be quickly installed, rapidly proven, and is economic compared to mechanical alternatives. A number of units were brought into particulate emission and opacity compliance through the use of this chemical treatment. Work during the past several years was oriented toward expanding the range of coals tested and correlating coal ash composition and precipitator performance with various chemicals, in order to predict the most effective formulations. The objective of this paper is to review case histories of flue gas conditioning demonstrations and long-term deployment programs based on Benetech's technology. The paper also discusses the relationship that improved ESP performance has on PM2.5emissions and control of air toxics.
Benetech's chemical blends are based on aqueous combinations of various nitrogen and sulfur containing materials that pose no on-site safety storage issues. The product is atomized into a flue gas duct well ahead of the cold side precipitator, to provide thorough mixing with the fly ash and time for conditioning. One formulation has also proven effective in hot side ESP operation. The entire system has been automated so that essentially no station personnel involvement is necessary. The product is delivered ready to feed, so no on-site chemical operations are required.
Despite the costs associated with meeting the Clean Air Act Amendments (CAAA), increases in electrical demand have resulted in an increase of coal use in the United States. Projections published in the Department of Energy's (DOE) Energy Information Administration (EIA) Annual Energy Outlook predict a nearly 30% growth in coal generated electricity between 1994 and 2015. Currently, approximately 56% of all energy in the US is generated by burning coal. Although the CAAA has not reduced the consumption of coal, it has forced changes in the quality of coal used and the mine source. To meet the CAAA sulfur dioxide (SO2) emission regulations and avoid the cost of SO2 scrubber systems, utilities have reduced the use of higher sulfur midwestern and northeastern coals in favor of low-sulfur subbituminous western coals found in Colorado, UT, and especially the Powder River Basin (PRB). PRB coal production alone has nearly doubled in the past decade.1
Fuel switching to low-sulfur coals, although a necessity for SO2 compliance, has adversely impacted ESP performance. Many utility generating units have also experienced deratings due to the typically lower heating value of subbituminous western coals relative to bituminous eastern coals. Unit deratings are most noticed during periods of high demand, such as this past summer, when spot market prices exceeded $4,000 per MWh. Unit mechanical changes and/or fuel blending can minimize derating, but any resulting increase in flue gas volume can adversely affect ESP performance. Fuel switching, from the original unit design coals, has also meant a change in ash composition and consequently the electrical resistivity and particle size distribution of the fly ash. High fly ash electrical resistivity will lower the performance of the ESP by limiting the amount of corona current from the discharge electrodes through the collected dust layer to the grounded collecting plate surface. Corona current is limited by electrical breakdown in the high resistivity dust layer, which limits operating voltage and reduces ESP efficiency. When properly sized and operated, ESPs offer an effective means of collecting fine PM2.5 particles and air toxics present as aerosols.2 PM2.5 regulations also impact SOx, NOx, and volatile organic compound (VOC) emissions since, according to the EPA, they are fine particle precursors.3
Flue gas conditioning (FGC) to improve ESP performance is a well-proven technology used to reduce fly ash resistivity and increase the cohesivity of fly ash particles. Worldwide, more than 120,000 MWe of installed capacity use this technology to enhance ESP performance. Combinations of chemicals have been used to change fly ash resistivity and/or the cohesivity of the fly ash particles. The cohesivity of the fly ash particles is a critical parameter for the collection of PMS2.5s and forms of air toxics. Cohesivity affects reentrainment of particles caused by ESP rapping and dustbin discharge. Specialty chemicals are available to control resistivity and fine particle collection and reentrainment by agglomeration. Agglomeration of the fine sized particles also has the potential of reducing the air toxic levels if these substances are not present as vapors at the ESP operating temperatures.
The use of FGC to improve ESP performance is potentially relevant in the context of the recent EPA Utility Air Toxics report to Congress.4 Although uncertainties exist in the analysis, mercury emissions from coal-fired plants is the hazardous air pollutant posing the greatest public health concern. The report estimated that one-third of U.S. manmade mercury emissions are generated from coal-burning utility power plant. Mercury air toxics are eventually deposited onto water and land resources, contaminating fish, which are the primary source of exposure to humans. Methylmercury, the most toxic form of mercury, found in fish has been linked to air-borne mercury emissions. Mercury is the most frequent basis for EPA contaminated fish advisories, more than any other pollutant. Nine states have issued statewide advisories and 39 states have issued advisories for at least one body of water. Recently, the EPA has required the Tennessee Valley Authority (TVA) to monitor mercury air toxic emissions from its 11 plants. EPA is also requiring weekly coal sample monitoring for mercury at more than 400 coal-fired power plants. Quarterly stack emission testing is being conducted at 30 randomly selected coal-fired power plants for elemental and oxidized species of mercury.5
The objective of this paper is to review Benetech's flue gas conditioning programs to improve ESP performance which have recovered lost generating capacity. The paper will also explore the potential impact that FGC may have on PM2.5 and air toxics emissions.
The earliest application of FGC dates back to 1912. Dust from copper converter furnaces at Garfield, UT became difficult to collect when processing low sulfur ores. Dust collection was improved by evaporating sulfuric acid in the high temperature stack gases. Other examples include water injection into gases from cement kilns and steel refining vessels. Other FGC chemicals reported in the literature include SO3 and SO3 based compounds, ammonia and ammonia based compounds, dual SO3 and NH3 or SO3 and NH3 compounds, sodium based electrolytes, and phosphorus based compounds. Ammonia was reportedly used as early as 1942 to treat high resistivity catalyst dust in the petroleum industry. Ammonia FGC produced intermittent success at an Australian coal fired power plant in 1966. Although ammonia only conditioning has had a mixed history of success, several successful applications have occurred in the U.S. since 1968.
The collection efficiency of an ESP is a function of the chemical and physical properties of the fly ash, constituents of the flue gas such as moisture, SO3 generated from coal borne sulfur, excess air, and flue gas temperature. One of the major factors affecting the performance of an ESP is the electrical resistivity of the fly ash. The chemical and physical properties of the fly ash are determined by the coal type (rank), coal ash composition (acidic/basic), quantity of ash, boiler unit design and operation. Flue gas conditioning must consider the different resistivity mechanisms, which are a function of temperature. In a cold side precipitator operating at an air preheater (APH) outlet temperature of 120°C to 180 °C, resistivity is generally attributed to surface conductivity produced by the movement of ions in the adsorbed sulfuric acid originating from the naturally present sulfur, which was converted to sulfur trioxide. For hot side ESPs operating at APH inlet temperatures of 300°C to 425°C, resistivity is a function of conduction through the bodies of the particles, known as bulk or volume conductivity. Fly ash resistivity is generally classified as low (104 - 108 ohm-cm), normal (108 - 1010 ohm-cm), moderate (1010 - 1011 ohm-cm), high (1011 - 1013 ohm-cm), and severe (> 1013 ohm-cm). The electrical resistivity of fly ash typically increases as the ratio of sulfur to ash content decreases, assuming other variables remain constant. If the resistivity is too high, which often occurs when low sulfur coal is burned, the collection efficiency is poor because the amount of useful power that can be supplied to the ESP is limited, for the most part, by the electrical resistance of the fly ash layer on the collecting plates. Corona current from the discharge electrodes must pass through the collected dust layer on the plates to reach the grounded collecting plate surface. The passage of current builds up a voltage across the dust layer per Ohm's law. When the dust resistivity exceeds about the mid-1010 ohm-cm, corona current is limited by electrical breakdown in the dust layer. This limits operating voltage and reduces ESP efficiency.
Modern ESPs have been designed with efficiencies exceeding 99.9% and measured emissions below 10 mg/Nm3. In doing so, according to the right operating conditions, a well-designed ESP can remove a high portion of particles below 2.5 µm. For a pulverized coal-fired utility plant, fine particle content is typically about 5% less than 2 µm.6 At cold side ESP conditions mercury, with a boiling point of 356.66°C, will typically be present as a condensed vapor aerosol, an oxidized form of mercury, or adsorbed on a fly ash particle. Therefore, the potential exists to capture mercury in an ESP. However, typical installed ESPs may have a designed capture efficiency of 99.75% to achieve an emission level of less than 50 mg/Nm3 when operating on design coal. FGC offers the opportunity to enhance the performance of these ESPs to achieve a higher capture efficiency of PM 2.5, certain air toxics such as mercury, as well as particulates contributing to plume opacity. This is especially significant in light of EPA rules which, will monitor PM 2.5 through the year 2003, designate areas in 2005, seek state implementation plans by 2008, and require complete compliance by 2017. The new PM2.5 rule sets an annual limit of the particulate mass in the air of 65 µg per cubic meter, with a daily limit of 15 µg/cu.m.7 Figure 1 presents a map of potential PM2.5 non-compliance areas in the U.S.
High resistivity fly ash has become more of a problem since many electric utilities have chosen to switch to low sulfur coals to reduce sulfur dioxide emissions.8 Reduced sulfur dioxide generation results in less sulfur trioxide production, which adversely affects the performance of the precipitator in the normal range of operating temperatures. If less sulfur trioxide (sulfuric acid) is naturally available for adsorption on the ash, the electrical resistivity will be raised.
Chemical Conditioning Mechanism
Sulfur trioxide (SO3) is the most widely used form of FGC. The SO3 concentration is determined by the kinetics of sulfur oxidation and is typically limited to a small fraction (1 to 2%) of the total sulfur in the coal. The SO3 generated during the combustion process begins to react with water vapor in the flue gas at around 300°C to form H2SO4 vapor. This reaction is essentially complete at 150°C. The H2SO4 vapor is adsorbed/condenses on the otherwise poorly conducting fly ash surface and directly contributes to the electrical conduction process. For very low concentrations of H2SO4 vapor on the fly ash surface, alkali metal (Na & K) ions react with the water vapor to form the conducting layer on the fly ash surface. The H2SO4 vapor may however react with basic constituents like calcium to form poorly conducting compounds on the fly ash surface. This is the reason why high calcium ash and low sulfur PRB coals or lignite give rise to high resistivity ash that is difficult to collect, producing high stack opacity readings. Therefore, two coals of similar sulfur content with different alkaline constituents can have substantially different fly ash resistivity and collectability under identical operating conditions. It should be noted that SO3 conditioning can be effective at flue gas temperatures of less than 200°C where the conduction process is on the surface of the fly ash. At temperatures greater than 200°C, the higher vapor pressure and equilibrium characteristics reduce the effective acid layer and resistivity becomes more a function of bulk volume conduction.
The mechanisms for ammonia based FGC include electrical resistivity modification, fly ash cohesivity improvement, and space charge effect. Ammonia and ammonia based compounds are absorbed in the H2SO4/H2O of the flue gas to form ammonium bi-sulfate and ammonium sulfate. The chemistry is as follows:
Chemisorption of the ammonium bi-sulfate and ammonium sulfate on the fly ash forms a conductive film. This reduces resistivity and permits higher ESP operating voltages to be maintained without sparking or back corona. Ammonium bi-sulfate and ammonium sulfate are also hygroscopic and semi-liquid at typical flue gas temperatures. This semi-liquid state promotes cohesion that minimizes reentrainment caused by rapping and dust bin discharge, thereby improving overall ESP performance. Ammonium bi-sulfate and ammonium sulfate fumes and sub-micron size particles also increase the charge level of the fly ash particles and the field near the collecting plate. The higher field increases collection efficiency.
Dual FGC conditioning is possible using NH3 and SO3 based compounds to reduce resistivity and improve cohesivity. This approach is typically used when SO3 or NH3 alone are not effective or when flue gas temperatures are > 200°C. Dual conditioning supplies needed SO3 while controlling the thickness of the acid film on the fly ash. The ammonium bi-sulfate and ammonium sulfate formed during dual conditioning reduce resistivity and act as a binding agent that increases cohesivity to promote particle collection. This has been discernable by opacity monitoring during rapping and non-rapping conditions. Forms of dual conditioning are currently deployed in about 15,000 MWe of generating installations.
The chemical blend and concentration selected for a specific site is a function of a number of parameters, but is typically an aqueous combination of various nitrogen and sulfur containing compounds. The product is atomized ahead of the precipitator (in a cold ESP application) to provide thorough mixing with the fly ash and time for conditioning. The entire system can be automated so that essentially no station personnel involvement is necessary. Product is delivered ready to feed with no on-site preparation.
A typical treatment program includes complete baseline analysis of the unit followed by treatment application and optimization. A testing program involves:
- Review of boiler operating data including fuel use rate
- Review of ESP design operating parameters and current operating conditions
- Coal analysis for ash, sulfur and heat content
- Fly ash in-situ resistivity measurements
- Review of opacity data, both visual and instrumental
- Velocity traverses before the ESP.
Benetech has demonstrated the effectiveness of FGC in a wide range of operating conditions and coal types including low sulfur eastern fuels, Texas lignite, and low sulfur western fuels. In a hot side precipitator application, a Benetech formulation has been successfully used intermittently for more than 20 years as a trim whenever opacity levels exceeded 20%. Other recent demonstrations have shown Benetech formulations to be effective in enhancing cold side ESP performance. In one demonstration the plant target opacity was exceeded when run at base load. Benetech BGC-620 controlled opacity at 12% during base load operation while burning Black Thunder western coal with a low sulfur level of 0.3% and an ash content of 5.4%. In a more recent demonstration, nearly 50 MWe of opacity limited capacity was recovered by FGC. A more detailed discussion of this program follows.
Recently, Benetech conducted a FGC trial at a Midwestern coal-fired electric generating station that had adopted a low-sulfur coal strategy to comply with SO2 emission regulations. To minimize coal costs, a number of eastern coals were being purchased with varying sulfur content and coal properties. This resulted in a decrease in unit performance and/or reduced output depending on the coal used and resulting opacity levels/restrictions. Variations in coal sulfur content and properties affected the maximum plant output by as much as 80 MWe. In addition, stack-out procedures produce a coal of mixed blend, which, when fired, further complicates plant operation.
To help control particulate emissions for increased plant output, Wahlco SO3 injection was deployed to modify the ash resistivity and enhance precipitator performance when combusting lower sulfur coals. This opacity reduction strategy was not fully effective on fly ash from low sulfur coals or during the numerous fuel switching periods due to the fact that ash agglomeration issues had been largely ignored.
The restriction on plant output and the failure of in-place technology to sufficiently recover lost capacity prompted plant management to seek alternative solutions. Benetech's FGC system represented a unique solution to recovering lost plant capacity caused by opacity limitations. Factors influencing their decision included the cost-effective approach and the fact that the system can work over a broad range of coal types and operating conditions. Another advantage is that the system can respond rapidly to changes in coal type and need only be applied when combusting low-sulfur coal during peaking periods.
Based on interactions with administrative personnel, a test program was proposed utilizing Benetech to review existing equipment performance and conduct a FGC trial based on three aqueous chemical formulations. The objective of the program would allow the unit to regain megawatt capacity, which had not been achievable to that date.
The program was conducted in a month period to accommodate different fuels and plant operating conditions. Three chemical formulations, dictated by the selection of three different coal types being utilized were targeted to modify the fly ash resistivity and/or address fly ash particle agglomeration. Benetech supplied a team of FGC professionals consisting of combustion engineers, chemical application engineers and FGC field service installation specialists. The team conducted an extensive review of the existing operations including performance reports, maintenance records, fuel specifications, and precipitator operation. Parameters of the program such as injection points and application rates were formulated based on the conclusions of our analysis and coordination with Benetech's team of chemists.
When burning low sulfur eastern coal the results showed that chemical injection improved indicated precipitator power levels and dramatically increased generating capability by nearly 50 MWe while operating within the regulated opacity limit. Select test results presented in Figure 2 dramatically shows the difference in performance between FGC and no FGC. During this period the plant load reached 611 MW at 7:00 a.m. and remained stable until 3:00 p.m. when BGC-610 chemical injection was terminated. At this point, load was reduced from 595 MW down to 544 MW to meet opacity limitations. The stack opacity CEM quickly climbed to 20.69% when injection was stopped and stabilized at 18.77% at 5:00 p.m. corresponding to a load of 544 MW.
Additional increases in maximum generating capacity were also achieved by mechanical changes. Many of the mechanical changes implemented were the result of interaction between Benetech and plant personnel during the test program. Normal Benetech practice is to review mechanical system status. Benetech professionals deployed for this effort have proven combustion and emission related experience. As with most units, plant configuration is often unique, which required us to assess the impact of these unique elements such as the duct configuration between the air pre-heater and precipitator. The initial review of the system included:
Fire Side
- LOI
- Burner condition
- Firing rate/angle
- Heat transfer surfaces
- Existing coal ash problems
- Soot blower performance
- Air/fuel ratios
- Mill performance
- Supperheater performance
- Gas velocities in boiler
- Furnace heat loss
- Economizer performance
- Burner balance
- Slag / ash removal
- Pneumatic ash conveyor system performance
- ID/FD fan performance
Flue Gas Side
- ESP configuration
- Plate surfaces
- Power setting
- Rapper sequence
- Flue gas velocities
- Flue gas distribution
- Opacity reading
- Pressure differential
- Spark rate
- Maintenance / performance issues
- ESP Ash hopper sequencing / ash blowback
As part of this process, Benetech personnel interviewed plant personnel about operating problems and general equipment performance. Through the course of these interviews we gathered information concerning previous attempts to correct known deficiencies. This included operating restrictions and the suspected causes of those restrictions. Next, Benetech personnel reviewed available equipment maintenance records and reports to determine the general state of repair for all equipment related to the FGC test program. Any equipment modifications were noted along with reported performance benefits.
The FGC equipment deployed by Benetech consists of liquid injection spray probes that are strategically placed within the ductwork prior to the cold side precipitator. An example of the spray pattern produced by a probe is presented in Figure 3. These probes are constructed of stainless steel to provide long life with minimal maintenance. The specially designed nozzles and placement ensure even dispersion of chemical throughout the duct and complete evaporation of the liquid prior to entering the ESP. The test chemical injection unit used in the above demonstration program is shown in Figure 4 (still in wooden packing enclosure). As can be seen, this low cost unit consists of a number of chemical injection pumps, flow distribution control elements, and system controls. The system controls can be interfaced with plant controls or operated as a stand-alone unit. FGC liquid phase chemicals can be stored in 1 kiloliter totes or large 22 kiloliter heat traced and insulated containers installed outside and near the chemical injection system. Unlike ammonia or SO3, these chemicals do not pose a severe storage hazard.
The economic benefits from the above demonstration program were significant. Recovery of 50 megawatts per hour can quickly translate to 150,000 MWH per year, if the recovered capacity is utilized only 35% of the time. The value of the recovered megawatts could exceed $100 per MWH on those "capacity short" summer peak days. A very conservative average value of $10 / MWH equates to a revenue of $1,500,000 per year. Based on preliminary estimates, the equipment cost and the first year's chemical cost could be paid for in less than 6 months.
The Use of FGC to Control PM2.5 and Air Toxics
A number of technologies have been proposed to increase the particle collection efficiency of ESPs. These include advanced controls, expert systems, new compact hybrid particle collectors, advanced separator technologies, reverse gas and pulse jet baghouse additions, wet precipitator additions, precharging, and pulsed generation, to name a few. In an environment of regulatory emissions uncertainty, deregulation, and limited plant budgets, low capital cost approaches such as chemical FGC provide a clear avenue to address today's operating problems and recover critically needed lost generating capacity related to opacity restrictions. Coal switching has also proven a valuable tool to manage plant emission requirements. The use of low-sulfur PRB coals has helped to control SOx emissions. Studies have also shown that coals vary in elements related to air toxics such as mercury, lead, arsenic, and nickel. Fuels such as Powder River, Hanna and Carbon Basins coals are lower in trace elements related the most onerous air toxics. As with coal sulfur levels, coals with lower bad trace elements can be selected to control air toxics.9 Furthermore, additional research is required on the forms of air toxics generated from these trace elements. Research has indicated that the coal chlorine content can also play a significant role in compounds formed and their fate.10 Capture of these trace elements in the bottom ash or fly ash, as opposed to release as air toxics, is not yet fully understood.
Through the optimization of fly ash resistivity and cohesivity, FGC can play an important role in minimizing PM 2.5 and air toxics emissions. Numerous scientific papers indicate that ESPs are capable of collecting fine particles and aerosols, such as sulfuric acid droplets with a mean particle diameter of 0.4 microns, formed by condensing vapors. Furthermore, the addition of cohesive agents will minimize reentrainment of these particles and air toxics during rapping, dustbin discharge, and other upset conditions. The dynamics of FGC allows for quick adjustments to operating conditions that can hamper ESP operations such as higher exit temperatures and velocities. Again, FGC resistivity and cohesivity adjustments can be made quickly to help control particulate emissions.
One alternative to emission compliance is to reduce the load to a point where particulate emissions are below the compliance level. This, however, is a costly solution compared to chemical treatment, which will often allow full load operation below compliance levels. Another alternative is to retrofit the unit by increasing precipitator capacity at significant capital costs and downtime for installation. Benetech field experience with FGC conditioning on existing units has shown a significant capital cost advantage and no downtime requirements to install the system. Benetech's use of aqueous chemical solutions also mitigates costly hazard control measures for other chemical solutions such as SO3 and ammonia. This is an important consideration given recent Toxic R Release Inventory reporting requirements. The use of chemicals for enhancing the collectability of coal fly ash and improving precipitator efficiency has been well documented in more than 120,000 MWe of applications and in the literature. Benetech has developed a system that is effective on a wide variety of coals and has shown consistent performance in a period of nearly 20 years. This system can be quickly installed and rapidly proven and is economic compared to mechanical alternatives. A number of units have been brought into particulate emission and opacity compliance through the use of this chemical treatment. Work in the past several years has been oriented toward expanding the range of coals tested and correlating coal ash composition and precipitator performance with various chemicals, in order to predict the most effective formulation. This knowledge is valuable for proper formulation selection, treatment minimization, and the prediction of ESP performance relative to coal, unit design, operation, and compliance requirements.
References