Power Engineering

Developing a Low-Cost MATS Rule Compliance Strategy

The Mercury Air and Toxics Standard (MATS),
The Mercury Air and Toxics Standard (MATS), which was finalized by the Environmental Protection Agency last year, established emission limits for mercury and hazardous air pollutants, including lead, arsenic, hydrogen chloride, hydrogen fluoride and dioxins/furans. Power generators have until April 2015 to comply.

By Will Boward and Danielle Flagg, Sargent & Lundy, and Kurt Sangster and William Cain, NIPSCO

On March 16, 2011, U.S. Environmental Protection Agency (EPA) issued a proposed rule that included standards for the Utility Maximum Achievable Control Technology (MACT) Rule standards for coal- and oil-fired electric generating units (EGUs). NIPSCO engaged Sargent & Lundy, L.L.C. (S&L) to assist in assessing the implications of the proposed MACT Rule regulations on the coal-fired EGUs within NIPSCO's fleet.

On February 16, 2012, the EPA published in the Federal Register the final Mercury and Air Toxics Standards (MATS Rule), which effectively replaced the proposed MACT Rule regulations. The MATS Rule for the existing coal-fired EGUs includes emission limits for: mercury (Hg), hydrochloric acid (HCl), and particulate matter (PM).

The MATS Rule also establishes alternative standards for HCl (via SO2 control) and an alternative standard for non-mercury metallic hazardous air pollutants (HAPs) via a filterable particulate matter (FPM) limit.

Summary of MATS Rule requirements

The main emissions criteria established by EPA as part of the MATS Rule applicable to NIPSCO's stations are summarized in Table 1.

table 1

In addition to the emission limits summarized above, the MATS Rule contains a work practice standard for organic HAP emissions, including emissions of dioxins and furans (D/F), non-D/F organic compounds, and hazardous volatile organic compounds (VOCs). The work practice standard requires maintaining and inspecting the burners and associated combustion controls, tuning the specific burner type to optimize combustion, and obtaining and recording carbon monoxide (CO) and nitrogen oxides (NOX) values before and after burner adjustment.

This paper summarizes operating history of the NIPSCO coal-fired EGUs affected by the MATS Rule, and presents the evolution of the strategy developed for the units to assess the implications of the proposed MACT Rule regulations through the continuing assessment of the finalized MATS Rule regulations. This strategy has been an evolutionary process reflecting events through the proposed and finalized versions of the legislation and the assessment and development of initial unit performance baseline data, followed by updated data resulting from additional testing. The process has been iterative. What may be believed at the beginning of the process may change throughout the development of the strategy. The strategy is an attempt to utilize and leverage NIPSCO's existing environmental assets to support the development of a cost-effective, yet flexible, response to the MATS Rule legislation.

NIPSCO Coal-Fired EGUs

NIPSCO operates seven EGUs that are affected by the MATS Rule regulations. A summary-level description of the coal-fired EGUs included in the strategy development and the environmental equipment that is currently installed or that is planned to be installed prior to the MATS Rule implementation date follows.

Bailly Generating Station (BGS) - Units 7 and 8

BGS is located on a 100-acre site on the shores of Lake Michigan in Porter County, Indiana. The BGS two base load units and one peaking unit came on line over a six-year period, ending in 1968.

Unit 7 is a 1962 B&W cyclone boiler with a net full-load output of 160 MW. BGS Unit 8 is a B&W cyclone boiler with a net full-load output of 320 MW installed in 1968. Both of the boilers at BGS currently burn Illinois bituminous (Illinois Basin) fuels. The station has also burned Indiana (Illinois Basin)-sourced fuels that are lower in chloride content.

Unit 7 is currently equipped with the following air pollution control equipment:

  • OFA and selective catalytic reduction (SCR) technology for NOX control
  • Electrostatic precipitator (ESP) for particulate control
  • Limestone wet FGD for SO2 control
  • Sodium Bi-Carbonate (SBC) injection for SO3 control

Unit 8 is currently equipped with the following air pollution control equipment:

  • OFA and selective catalytic reduction (SCR) technology for NOX control
  • ESP for particulate control
  • Limestone wet FGD for SO2 control
  • SBC injection for SO3 control

BGS Units 7 and 8 share a common FGD system and have a common stack, and their emissions will be evaluated together.

Michigan City Generating Station (MCGS) - Unit 12

MCGS is located on a 134-acre site on the shores of Lake Michigan in Michigan City, Indiana, and comprises one base load coal-fired steam unit.

Unit 12 is a 1974 B&W cyclone boiler with a net full-load output of 469 MW. Unit 12 currently burns a blend of 85% low-sulfur Powder River Basin (PRB) and 15% bituminous fuels. Unit 12 is currently equipped with the following control technologies:

  • OFA and SCR technology for NOX control
  • ESP with a SO3 flue gas conditioning system for particulate control

There is currently no FGD technology installed on Unit 12; however, NIPSCO had plans to install a dry FGD in 2018. These plans were developed prior to the promulgation of the MATS Rule regulations.

R.M. Schahfer Generating Station (RMSGS) - Units 14, 15, 17, and 18

RMSGS is located on approximately a 2,900-acre site two miles south of the Kankakee River in Jasper County, near Wheatfield, Indiana, and is the largest of NIPSCO's generating stations. The SGS four base load and two peaking units came on line over an 11-year period, ending in 1986.

Unit 14 is a 1976 B&W cyclone boiler with a net full-load output of 431 MW. The unit currently burns a blend of Illinois bituminous and low-sulfur PRB fuels; in the future, Unit 14 may burn up to 100% bituminous fuel. The unit is equipped with the following air pollution control equipment:

  • OFA and SCR technology for NOX control
  • ESP for particulate control
  • Currently being retrofit with wet FGD technology for SO2 control and sorbent injection for SO3 control (will be in operation by the end of 2013)

Unit 15 is a 1979 Foster Wheeler opposed-firing pulverized coal (PC) boiler with a net full-load output of 472 MW. Unit 15 was designed for PRB fuel and currently burns a blend comprising of 90% PRB and 10% Illinois bituminous; in the future, the unit may burn up to 25% bituminous fuel. Unit 15 is equipped with the following air pollution control equipment:

  • Low-NOX burner (LNB) and OFA technology for NOX control
  • ESP for particulate control
  • Currently being retrofit with wet FGD technology for SO2 control and sorbent injection for SO3 control (in conjunction with Unit 14 installation; will be in operation by the end of 2015)

Unit 17 is a 1983 CE tangential firing PC boiler with a net full-load output of 361 MW, and currently burns Illinois bituminous fuel. Unit 17 is currently equipped with the following air pollution control technologies:

  • LNB/OFA technology for NOX control
  • ESP for particulate control
  • Limestone wet FGD for SO2 control

Unit 18 is an identical, sister to Unit 17, installed in 1986. Unit 18 has the same control technologies installed as Unit 17.

MACT Rule Considerations

All of NIPSCO's units are in compliance with their permitted emission requirements. Due to the various control technologies equipped on each unit and the overall age of the units and equipment, the units have a wide range of emissions. The first step in the process was to consider the requirements of the MACT and MATS Rules with respect to current unit performance. Once the units' baseline emissions were established, with respect to the proposed and final regulations, the potential control technologies for each pollutant were identified and evaluated. This step was revisited several times throughout the project when the final MATS Rule was issued and when additional emission test data became available. The following summarizes the initial strategy in relation to the major requirements of the MATS Rule.

Acid Gas Removal Requirement

Both the proposed MACT Rule and the final MATS Rule regulations allowed the use of a surrogate SO2 removal limit of 0.20 lbs SO2/MMBtu to show compliance with the acid gas provisions. For NIPSCO's system, this was the simplest emission limit with which to comply. All of NIPSCO's coal-fired EGUs either have existing wet FGD technology installed or will have wet FGD technology installed, capable of reducing the SO2 levels below the 0.20 lbs SO2/MMBtu limit. The exception is MCGS Unit 12. For a number of reasons that are explained under the other regulated pollutants portion of this paper, it was decided that implementation of the planned MCGS Unit 12 dry FGD system, originally scheduled in 2018, would be accelerated in order for the system to be operational in time for compliance with the MATS Rule acid gas requirement.

Other options were considered, including installation of a baghouse along with dry sorbent for HCl removal to operate between the MATS Rule compliance date and the dry FGD installation in 2018. The economics favored early installation of the dry FGD. With that decision, NIPSCO will have operational FGD systems on all units capable of complying with the 0.20 lbs SO2/MMBtu requirement prior to the MATS Rule deadline. The acid gas removal requirement will be met by using existing or planned FGD systems.

Mercury Removal Requirement

The original MACT Rule requirements for Hg included an outlet emission rate below 1.0 lbs Hg/TBtu. The finalized MATS Rule has a requirement of 1.2 lbs Hg/Btu. Of the seven applicable NIPSCO units, six either already have wet FGD installed or are in the process of having it installed. NIPSCO instituted a testing program to characterize the units' Hg emissions. As was expected, based on the fuel analyses, those units burning Illinois basin bituminous fuels with relatively high chloride content were shown to have stack and sorbent trap emissions for many of the point tests lower than the MATS Rule Hg limit.

Under these conditions it appeared that the existing FGD units were capable of controlling Hg emissions below the MATS Rule limit. As the majority of emissions data to date are from individual stack tests, some uncertainty regarding whether the units can comply with the MATS Rule limits on a continuous 30-day rolling average basis exists. NIPSCO has recently installed Hg continuous emission monitoring systems (CEMS) on all units and particulate (CEMS) on selected units. Some of NIPSCO units burn a blend of PRB and bituminous fuels; these units typically have higher Hg emissions than the MATS Rule emission limit.

The preliminary compliance strategy was to use fuel additives, which are designed to oxidize Hg to a form that is more readily captured in the FGD systems. This would be the most cost-effective manner of reducing the Hg emissions; however, the use of fuel additives was contingent upon a successful demonstration test. An alternative strategy, especially for units that burn primarily PRB fuels, includes activated carbon injection (ACI) and halogenated powdered activated carbon (PAC).

Non-Mercury Metal Hazardous Air Pollutants Removal Requirements

The final MATS Rule allows utilities to comply with requirements for non-mercury metal (NMM) HAPS by controlling filterable particulate matter (FPM). Judging compliance performance for this requirement was not as straightforward as for the acid gas requirements.

The proposed MACT Rule required compliance with a total particulate matter (TPM) limit, which included both FPM and condensable particulate matter (CPM), rather than just FPM.

The TPM emission limit under the proposed MACT Rule was 0.030 lb/MMBtu. Very little data existed to evaluate the condensable contribution to the TPM.

Although NIPSCO conducted stack testing historically at least every two years on all their units, the particulate regulations they were required to meet prior to the proposed MACT Rule did not encompass CPM; therefore, it was not measured.

NIPSCO instituted a testing program to develop TPM data on their units. The final MATS Rule eliminated the CPM component of the limit and identifies a FPM limit for NMM compliance. The final MATS Rule FPM limit is 0.030 lb/MMBtu.

The inclusion of CPM in the limit when the MACT Rule regulations were first proposed put the units firing primarily bituminous fuels at risk. There could potentially be sufficient SO3 in the flue gas that would appear as CPM, resulting in a TPM emission above the 0.03 lb/MMBtu MACT Rule limit. The changes in the final MATS Rule made the NMM removal requirements more straightforward to obtain.

Upon the initial assessment, it became clear that the greatest risk involved with the NMM removal requirements was the condition of the existing ESPs and their ability to control FPM below the MATS Rule limits. In addition, future increases in particulate loading to the ESPs due to the potential addition of halogenated activated carbon for Hg control could pose further complications.

Options Considered for MATS Rule Compliance

FPM Particulate Control

The assessment of the particulate removal capability was begun by reviewing the biennial stack testing data that NIPSCO had accumulated. A number of the NIPSCO existing ESPs showed removals within the compliance levels of MATS Rule, even though none of the original performance guarantees were for a continuous 0.030 lb/MMBtu outlet. The particulate results varied widely from test to test, with test results above and below the MATS Rule limit. The only major exception to this was MCGS Unit 12, which was consistently above the limit. However, since the decision has been made to install dry FGD to comply with the acid gas provisions of MATS Rule, the existing ESP at MCGS Unit 12 will be replaced with a baghouse, bringing this unit into compliance; although, the baghouse would need to be installed prior to the MATS Rule compliance date.

As discussed above, the main issue for the NIPSCO system once the decision was made to install dry FGD at MCGS Unit 12 for MATS Rule compliance was how to ensure compliance with the particulate requirements. This is complicated by the likely need for ACI to control Hg emissions below the MATS Rule limit. The decision to accelerate the installation of the dry FGD system in time for the MATS Rule compliance date was finalized, as this would provide compliance with both the acid gas and NMM requirements for MCGS Unit 12.

In reviewing the design specifications for NIPSCO's existing ESPs, it was noticed that the designs included boxes that appeared to be conservatively sized. One parameter used to evaluate the potential for ESPs to readily collect FPM is specific collecting area (SCA). The ESPs have substantial SCAs, with the exception of MCGS Unit 12, which has an SCA of ~ 177 ft2/1,000 acfm. The other ESPs range from a low SCA of 312, up to 495 for RMSGS Units 17 and 18. These generous SCAs served as the impetus to investigate whether there might be benefit in upgrading the installed ESPs to achieve the MATS Rule level particulate removal.

ESP Upgrade Options

There are several available ESP upgrade options that may be capable of reducing the FPM emissions from the existing ESPs. The potential ESP upgrades include:

  • Install high-frequency transformer-rectifier (TR) sets
  • Upgrade collector and emitting electrode rapping systems
  • Replace the ESP internals
  • Add an additional field or additional plate area to the ESP
  • Convert part of the ESP to a baghouse (COHPAC II)1
  • Convert existing ESP into baghouse

After reviewing the basic size of NIPSCO's ESPs, it was decided, based on S&L judgment, that upgrades under consideration would include wider plate spacing, improved rapping systems, rigid electrodes, and high-frequency TR sets. As it is S&L's opinion that each of the ESPs (with the exception of MCGS Unit 12) can be upgraded to provide reduced FPM emissions, more capital-intensive options, such as the COHPAC II or conversion of the ESP into a baghouse, were not evaluated further.

Implementation of Baghouse for Particulate Compliance

The underlying reason to upgrade the existing ESPs was that the sizes of the precipitators (normally described by SCA) are quite large. With upgraded capability, we would expect sufficiently low particulate emissions that should allow NIPSCO to continue to produce outlet FPM emissions in compliance with MATS Rule limitations in the future.

A baghouse addition to the units would also represent a means to comply with the particulate requirements of the MATS Rule. A number of utilities that do not have large ESPs may utilize this option.

The following is a high-level analysis of the cost difference to install baghouse technology as compared to ESP upgrades. S&L did not perform walkdowns for the implementation of baghouse technology at the NIPSCO units other than MCGS Unit 12. Such walkdowns are necessary to more accurately predict baghouse arrangements and installation costs. However, S&L has been involved in a number of baghouse retrofit projects over the recent years and believes an analogous cost based on the units' size may prove illustrative. The vast majority of the costs for baghouses scale with the volume of flue gas being treated. The flue gas volume is proportional to the heat input of the unit, which is directly proportional to the megawatts produced by the unit.

Without taking into account the site-specific constraints of the individual units, such as potentially longer ductwork on some units due to where the baghouse would have to be built, whether ID fans would have to be replaced to provide the additional pressure drop, whether foundations would be more costly due to unusual soil conditions, and so forth, the costs for units similar in size to NIPSCO's range from $150/kw to $200/kw. These costs do not include AFUDC, taxes, owner's cost, or escalation, and are expressed as 2011 dollars.

Table 2 shows the analogous capital cost differences between the ESP upgrades (which were scaled from budgetary quotes on similarly sized units in June 2011 costs) and the baghouse installation.

table 2

These costs do not include demolition costs of the existing ESPs once the baghouse is operational. In addition to capital cost, the baghouse option will require an additional flue gas pressure drop of roughly 4 to 6 inches w.g. This will increase parasitic auxiliary power and may require new induced draft (ID) fans and re-enforcement of portions of the flue gas path.

It is important to note that since detailed assessments have not been made on each unit, the costs for some NIPSCO units may be less than the analogous costs. Some ESP boxes may be candidates for gutting and stuffing with baghouse technology. This may provide some cost savings over the values in Table 2. However, this may require longer outages to install this option and would be highly dependent on detailed condition assessment of the current condition of ESP and ductwork internals, and their ability to cope with the increased pressure requirements of the retrofit baghouses.

For these reasons, S&L has recommended compliance through a program of upgrading the ESPs throughout NIPSCO's fleet (with the exception of MCGS Unit 12, which is too small to be effectively upgraded).

ESP Condition Assessment

Most of the subject ESPs had been in service for 20 to 30 years. In order to determine the current state of repair of the ESPs, a condition assessment evaluation on each ESP was undertaken. Precipitators were inspected based on short-outage opportunity. Inspections included:

  • Assessing corrosion or material thinning in key areas
  • Mapping misaligned and corroded plates and emitting electrodes
  • Inspecting internal support structures and electrical insulators
  • Inspecting rapper systems
  • Inspecting TRs and electrical components
  • Inspecting hoppers where possible

Through the condition assessments, we were able to ascertain a reasonable determination as to the state of the ESPs, as well as the number and type of repairs that would be necessary to establish an operating base moving forward. In general, the inspections indicated that the ESPs were in better condition than might otherwise be expected based strictly on their age and operating history on largely bituminous fuels and blends.

More Particulate Testing Results

NIPSCO's testing program encompassed all of their coal-fired units in 2011, according to the FPM protocol called for by the MATS Rule legislation. While most of the NIPSCO unit stack test results are currently below the FPM limit for the MATS Rule, it is important to note that none of these units have demonstrated compliance with the MATS Rule limit on a continuous 30-day rolling-average basis using particulate CEMS, which is the method of compliance monitoring NIPSCO has chosen in response to the MATS Rule. Stack testing results from RMSGS Unit 14 for 2011, were only marginally below the MATS Rule limit for FPM. Emissions from MCGS Unit 12 are significantly higher than the MATS Rule limit for FPM.

Combining the results from the condition assessments, the latest emission testing, and the outage schedule before the MATS Rule compliance date, a series of upgrades and rebuilds of the ESPs will be developed on a unit-by-unit basis.

Mercury Control

When coal is combusted in a boiler, the Hg contained in the coal is released predominantly in three forms: particulate-bound mercury (Hgp), ionic mercury (Hg+2), and elemental mercury (Hg0). The amount of each form of mercury that develops during combustion depends on a number of factors, including other constituents of the coal itself, such as the halogen content. The various types of mercury formed are referred to as speciation.

Particulate-bound mercury exists in solid form, is typically a small fraction of the mercury present, and is removed to a significant degree by conventional particulate control equipment.

Elemental mercury is insoluble in water and is not removed to a great extent in normal particulate control devices or in an FGD system. In contrast to elemental mercury, oxidized mercury is highly water-soluble. Downstream FGD systems readily capture oxidized mercury. The formation of oxidized mercury is directly related to the halogen (chlorides, bromides, fluorides) level in the coal. NIPSCO's FGD units collect oxidized mercury present in the flue gas.

Some mercury removal technologies involve converting elemental mercury to water-soluble, ionic mercury, for capture in a downstream FGD. Others involve adsorption of mercury on activated carbon by injection of carbon in the flue gas. Units with wet FGD may also require additives to retain captured mercury in the FGD byproduct if re-emission is evident. NIPSCO's units have not shown evidence of re-emission under testing performed to date.

The speciation of the mercury plays a significant role in the ease of its capture. The conversion of elemental mercury to oxidized (ionic) mercury depends upon several factors:

  • Cooling rate of the gas
  • Presence of halogens or SO3 in the flue gas
  • Amount and composition of fly ash
  • Presence of unburned carbon

The strategy first developed to address Hg emissions used a layered approach. As stated previously, all the subject units will have FGD installed prior to the MATS Rule compliance date. Early data for the units firing bituminous fuel showed that the 30-day rolling average for Hg emissions was below the MATS Rule compliance limit. It was believed that with slight fuel variations, fuel additives represented a cost-effective option to increase the amount of oxidized mercury that would be collected by the FGD system and, therefore, lower the outlet Hg emissions. This was especially true for RMSGS Units 14 and 15, which are in the process of installing wet FGD systems and fire a high percentage of western PRB fuels. The PRB fuels do not have sufficient halogen content to ensure that a large majority of the mercury ends up in the oxidized form, which is readily collected in a wet FGD system. Because of the fuel mix of the subject units, it is also planned to install ACI systems to trim the Hg emissions, if needed.

Testing of Fuel Additives

Fuel additive testing was undertaken by NIPSCO to confirm that the fuel additives adequately oxidized the Hg and that the FGD system collected sufficient mercury to maintain the Hg emissions below the 1.2 lbs/TBtu MATS Rule compliance limit. The results of the testing were mixed.

Section 45 Tax Credit Additives

Section 45 of the U.S. tax code was designed to stimulate early NOX and Hg reductions utilizing coal treatment. RMSGS Unit 14 was the only NIPSCO unit to run a demonstration test with tax credit additives. The main focus of this demonstration was to evaluate whether additives could be used to meet the requirements of the Section 45 tax credit. In order to qualify for the Section 45 tax credit, a fuel additive has to concurrently reduce NOX and mercury emissions from the unit. The preliminary results of this test show that the demonstration was successful, which suggests that mercury-oxidizing fuel additives would effectively oxidize the mercury present in the flue gas.

Commercial Fuel Additives

NIPSCO also began commercial fuel additive testing in 2012. Fuel additives were tested on BGS Units 7 and 8 in June of 2012; the results of this testing showed that while additional mercury was converted to the oxidized form, additional mercury removal may be required to meet the MATS Rule requirement. Recent fuel additive testing on RMSGS Units 17 and 18 has shown higher mercury emissions (particularly at higher loads) than the historical CEMS data would indicate. Analyses are pending and no root cause for the differences has as of yet been determined. At the time of this paper, it is believed that additional mercury control technologies will be required for NIPSCO's units to comply with the MATS Rule.

Meeting Mercury Emissions Limit

The mercury MATS Rule emissions limit will be achieved through a multi-faceted strategy. Existing ESPs and the planned future upgrades to the ESPs will remove a portion of the particulate-bound mercury. (In addition, depending on the unburned carbon level of the fly ash, some of the oxidized mercury may also be collected in the ESP.) A significant component of oxidized mercury removal will occur in the existing pollution controls and planned FGD systems. The availability of oxidized mercury will be improved with the utilization of fuel additives, which are designed to increase the amount of oxidized mercury in the flue gas. Finally, an ACI storage and injection system will be installed to complete the mercury emission reduction strategy. The mercury reduction system will be designed to achieve continuous compliance of the mercury emission standard via ACI, along with mercury oxidation and removal in existing and new pollution controls, while managing operations and maintenance costs by selecting the mix of options that meets the mercury limits and minimizes cost. S&L recommends that ACI testing on selected units be incorporated into the schedule in an appropriate time frame to allow for confirmation of ACI effectiveness in mercury removal.

The specific mercury compliance strategy selected for each unit will include a combination of these options, and will account for unit-specific operating conditions (e.g., fuel composition, temperatures, etc.). The strategy will attempt to first utilize any naturally occurring mercury oxidation due to the halogen content in the fuel and the ability of the ESP and FGD collection to reduce mercury emissions. Secondarily, fuel additives, where shown to be effective, will be utilized to achieve incremental oxidation, which should lead to additional mercury removal in the existing systems. Finally, if further mercury removal is required to meet the MATS Rule requirements, it will be accomplished by the addition of ACI upstream of the ESPs. The goal of the current strategy is to minimize the amount of ACI needed (optimize ACI usage). For units that burn either high-sulfur bituminous fuels or a significant bituminous portion of their coal mix, SO3 control to acceptable levels through the use of reagent injection, may be needed, to ensure the ACI usage is effective. (SO3 competes with mercury for active sites on the ACI.)

The design of the ACI injection systems will be based on equipment sized to remove all the required mercury to meet the MATS Rule requirements without any removal by the FGD system. Due to the cost of compliance via activated carbon alone, the system will be designed to optimize (minimize) the amount of ACI added, while removing as much Hg as possible in the FGD and other emission equipment. To this end, S&L has reviewed the impact that ACI and dry sorbent injection (DSI) will have on particulate emission levels. Based on the size of the existing ESPs and their performance testing completed to date, the additional particulate loading should be accommodated without problems. Specific loadings will be incorporated into the ESP design upgrades and will be factored into project-specific guarantees and warranties.


By utilizing existing emissions control systems to the greatest extent practical, NIPSCO can leverage its previous environmental investment for future emission reductions. This strategy is being achieved through an iterative process of utilizing existing emissions data to develop a flexible compliance strategy, while a parallel path is forged for obtaining additional emissions data and testing portions of the strategy in a timely manner to allow the maximum amount of schedule flexibility.

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