# Trace Mercury Removal from Flue Gas

D esulfurization Wastewater

Mark Owens, P.E. Degremont Technologies, Richmond, VA

H. Robert Goltz, Ph.D. The Dow Chemical Company, Midland, MI

And

D ean M ingee, R obert Kelly , Ph.D. Degremont North American

Research & Development

, Richmond, VA

KEYWORDS: Mercury, Flue Gas Desulfurization (FGD) Wastewater, Trace Contaminant Removal, Selective Removal

, Ion Exchange

.

ABSTRACT: Discharges from Wet FGD Systems are facing new, very stringent, effluent

limitations for mercury. To comply with these new limitations, an end

- of

- pipe system to remove trace mercury contamination to <12 parts per trillion was developed . This report will present the results of a successful pilot test performed in 2008.

INTRODUCTION

Many coal

- fired power plants have installed or plan to install wet flue gas desulfurization (FGD) scrubbers in
response to increasing pressure to control

air emissions. The blowdown from these units contains elevated levels of solids, chlorides, sulfates, and heavy metals as well as nutrients (nitrates and/or ammonia)

This wastewater is generally treated in a physical/chemical system including

desaturation of sulfates and chemical precipitation. These systems may or may not include filtration depending on ef fluent requirements. Physical/chemical treatment with sand filtration can remove mercury

from FGD wastewater to concentrations as

low as 2

- 10 parts per billion (ppb). In general, physical/chemical treatment of

FGD blowdown wastewater has been

sufficien t to achieve discharge standards for many years. Many installations in the

US have been operating from the early

1990s. However, new and more stringent water quality based standards a re being introduced, which requir e additional removal of specific contam inants such as mercury, selenium, boron, or nutrients.

These standards are set based on the ability of the receiving water to accept the discharge without significant impact. They are not set based on any available technology.M ercury is of particular interest and

concern because of its toxicity as well as

the fact that it bio

- accumulates. Due to water quality based standards such as the Great Lakes Initiative (GLI), some states are setting effluent mercury standards as

low as 1

- 10 parts per trillion (ppt).

The information discussed in this paper is

the result of collaborative research studies conducted by the Dow Chemical Company and Degremont Technologies in an effort to achieve trace mercury removal down to these new regulatory standards. The pilo t test was performed at a power station in

the eastern United States.

Prior to pilot testing, a bench

- scale treatability test was performed by Degremont’s North American R&D Center (DENARD) in Richmond, Virginia using the mercury selective resins

. With preliminary treatability data in hand, the treatment train was developed and the necessary pilot equipment was assembled.

The pilot treatment train included:

**•** Addition of organosulfide

**•** Sand Filtration

**•** Cartridge Filtration (5 micron)

**•** Ultrafiltration ( 0.03 micron) **•** Selective Ion Exchange

It is important to note that all of these technologies are w ell established and can be implemented commercially. The novel part of this system is the selective resin

itself.

The goals of the pilot study included:

**•** T h e determin ation of the lowest

achievable effluent mercury

concentration

**•** T he determin ation of the most

effective mercury

- selective resin

**•** T he determin ation of the effectiveness of differing levels of

filtration

**•** T he identif ication of potential operations and maintenance issues

The pilot operated from July through

December 2008 and achieved all of the above goals

.

A number of interesting observations have been made during the course of these investigations. A patent disclosure has

been filed on these observ ations.

DEVELOPMENT OF MERCURY SELECTIVE RESINS

Modern ion exchange resins are made from styrene divinylbenzene plastic. This material is a tough and proven base for attachment of a variety of functional chemistries. Liquid styrene and divinyl

b enzene are combined with diluents, solvents, catalysts and stabilizers and formed into organic phase droplets by suspension polymerization processes. The suspension is heated to polymerize the styrene and DVB into a solid plastic sphere. The functional groups are added by first activating the benzyl rings of the plastic by chloromethylation with chloromethyl, methyl ether that adds a

- CH 2 - Cl functional group. It is through this group that a wide variety of nucleophiles can be reacted to

create the wide variety of selective media that are enjoyed today.

For selective mercury removal, the chemical form of the mercury is a primary concern. Mercury is a very reactive

molecule , often referred to as “sticky” inthat it associates with a wide range of othe r molecules, forming a wide variety of complexes. In addition, mercury can exist

in various form s such as Hg 0 the zero valence or metallic form, Hg +2 the ionic form and organo

- mercury such as methylmercury. Each of these forms of mercury has different p hysical and chemical properties that must be taken into account in the design of a selective media.

A speciation study of the form of mercury that is present is recommended so that the right media is used for the right form of the molecule. Speciation studies are difficult to perform, as relatively few laboratories are capable of this kind of study at these very low levels. Speciation studies are subject to interpretation as the error in the analysis can be significant. In this case, the ionic

form of the mercury was identified as the primary target for treatment.

Since ionic mercury is a cation, removal

with a cation exchange resin is logical. In the case of FGD wastewater, this treatment has not been successful, presumably due to the high salt

ba ckground of the wastewater matrix.

The literature describes Iminodiacetic acid (IDA) functionalized resins as having very high affinities for mercury.

Figure 1A IDA Functional Group

While this may be true for neat solut i ons, the high salt background of FGD wastewater likely causes interference with the IDA group causing lower affinity in testing

. Another chemistry of interest is the Thiol functional group. The interactions of sulfur and mercury are used for mercury re moval from vapor phase treatment of natural gas.

Figure 1B Thiol Functional Group

Other Thiol chemistries attach the S

- H directly to the benzyl ring changing the chemical properties of the functional group.

Yet another fu nctional group of interest is the Thiouronium chemistry. Here a thiourea moiety is the active group that forms an attractive charge transfer

interaction.

Figure 1C Thiouronium Functional Group

A final note on selective medi a is that for

removal of con taminants like mercury at
parts per trillion levels, the total loading capacity of the media may not be the limiting factor . These media

, when challenged with mercury at high levels (mg/L range ) will load metal to wt/wt%

levels . To even come close to approaching th is level of loading, the media would need to be exposed to billions

of bed volumes of solution. Thus the media is less likely to become fully loaded with the trace contaminant and more likely to fail due to fouling. Thus the issues of concern are fouling resistance of the 

### S

C

H 2 

### N NH 2 

### +

CH

2 

### COOH N HOOCCH

2 

### S

Hmedia, an open pore structure of the media creating easy access and allow ing fast flow rates and short contact times for cost effective system design.

BENCH

- SCALE TREATABILITY STUDY

Prior to performing on

- site pilot testing, a

bench

- scale study was performed to screen various ion exchange resins for performance evaluation.

Bench

- scale treatability for mercury with selective ion exchange resin was determined using a ‘shake flask’ technique.

T his method includes adding a measured mass of resin to a flask containing the

wastewater to be treated. The flasks are

left on a flask shaker for 24 hours. After

the 24

- hour shake period, the resin is allowed to settle and the supernatant is analyzed.

The purpose of this test was not to determine the achievable mercury concentration, but to determine which resin

achieved the best relative result.

Table 1

Shake Flask Results

Resin Type Ratio (g/mL) Hg (ppt) Percent

Removal

Blank

--- --- Raw

--- 1 26

Thiol Resin 0.01 7.98 94%

0.0 0 5 10.7 92%

Thiouronium 0.01 5.45 96%

Resin 0.0 0 5 8.61 93% Silver Impregnated 0.01 73.8 41%

Alumina Resin A 0.0 0 5 101 20% Silver Impregnated 0.01 87.9 30%

Alumina Resin B 0.005 89.4 29%

The above results clear ly indicated the superior performance delivered by the

Thiol and Thiouronium resins.

PILOT EQUIPMENT AND PROCESS FLOW DIAGRAMS

For the purposes of this study, the mercury
present was loosely classified as ‘colloidal’

or ‘soluble.’ On this basis, the pilot equipment was divided into a chemical precipitation and filtration portion designed to remove the colloidal mercury followed by the selective ion exchange step to remove the remaining soluble mercury.

Chemical Precipitation Prior to filtration, t he wastewater is dosed

wi th an organosulfide precipitant. The purpose of this chemical is to coagulate mercury as a sulfide complex prior to

filtration.

Filtration Removal of colloidal mercury by

filtration is dependent on particle size. Accordingly, th e pilot system was designed

with serial filt ration using decreasing pore size. In this way, the particle distribution could be loosely determined.

Initially, the filtration sequence included a sand/anthracite filter followed by a 5

- micron cartridge fil ter as represented

below.

Figure 2A

Initial Treatment Train

Organo

sulfide dosing

tank S and

anthracite

filter 5 μm cartridge

filter PumpHowever, initial analytical data suggested that colloidal mercury was passing through

the 5

- micron filter and contaminating the

resin downstream. To eliminate the

r emaining colloidal mercury, a second cartridge filter rated at 0.35 microns was

added as represented below. At this time, the sand filtration step was bypassed as

redundant with the downstream filtration.

Figure 2B

Revised Treatment Train

Even with this change, ongoing analytical data indicated that colloidal mercury was still passing through filtration to

contaminate the resins. At this early stage of the pilot study it was decided to add an ultrafiltration step to repla ce the 0.35

- micron filter in order to completely remove the colloidal mercury.

The UF membrane module was a hollow fiber, PVDF design, employ ing an outside

in flow configuration, with an effective pore

size of 0.03 microns. The new pilot configuration is represented below: Figure 2C

F inal Treatment Train

Selective Ion Exchange The selective ion exchange skid was comprised of six columns housing three

different resins. The resin columns employed a packed

- bed, d o w n

- flow design and were pump fed from a tank through a common distribution header. Each pair of

columns could be connected in series allowing for evaluation of an effective resin depth of either 48” or 96”.

Figure 3 Ion Exchange Beds

Resin

Column Flow Meter

Quick

Connect

Effluent To drain Sample

TapThioUr AThioUr BL07 AL07 BThiol AThiol B Organo

sulfide dosing 5 μm cartridge

filter

U ltra

filtration
system S and

filter Pump

Break

t ank Pump

Organo

sulfide dosing

tank 5 μm cartridge

filter 0.35 μm cartridge

filter PumpBased on this setup, the effluent from any

individual column could be collected for analysis.

PILOT PLANT OPERATION

The operation of the pilot proceeded without any major operational issues. The sand filter required periodic backwashi ng as expected. The cartridge filter s also required periodic changeout. A clean in place system was included with the ultrafiltration skid and weekly chemical cleanings were performed.

T he resins were kept in place for the

remainder of the test. Some compaction of

the resin did occur with in the columns. The one operational issue that was

observed was a significant fouling of the

ultrafiltration membranes. Even after physical/chemical treatment, the FGD

wastewater remains very high in dissolved

soli ds and makes a difficult matrix for ultrafiltration. Several different cleaning regimes were used before the appropriate cleaning regime was determined.

SAMPLING AND ANALYTICAL METHODS

To obtain reliable data in the single digit part per trillion r ange, sampling and analytical analysis are extremely important. All sampling collection followed EPA’s “Sampling Guidance” (EPA Method 1669)

to reduce or eliminate contamination. Using this m ethod, s amples were collected using the “clean hands/dirty hand s” technique in fluoropolymer (FLPE) and glass sample collection bottles. Samples were shipped unpreserved via overnight delivery and then acid

- preserved upon receipt by the analytical lab.

EPA developed Method 1631 specifically

to address the need for reliable measurement of mercury to concentrations in the range of 0.5 to 100 ng/L. Method 1631 supports technology

- based and water quality

- based monitoring programs

authorized under the Clean Water Act for mercury. Following EPA Method 1631,

measurement of mercury is accomplished via the oxidation of mercury with bromine monochloride (BrCl), sequential reduction with ammonium hydroxide and stannous chloride to convert Hg(II) to volatile Hg(0)

for ultimate detection via cold

- vapor atomic

fluo rescence spec trometry (CVAFS ).

PILOT TEST RESULTS

The initial results revealed limited removal (approximately 40%) of the total mercury through the treatment train. Subsequently, this hydraulic loading rate was considered too high for the influent pollutant load

, which during that time ranged from 800 to 1,700 ppt as highlighted in the table below. Lowering the hydraulic loading rate produced an immediate and significant improvement in treatment performance, yield ing 65 to 80% removal of influent mercury for a similar influent concentration range. However, the effluent mercuryconcentration remained well above the ultimate objective of 12 ppt.

It was suspected that the resin performance was being hindered by non

specific fouling with particulate or colloida l mercury. A serial filtration study was

conducted t o characterize the form of mercury present. These results illustrated that a significant fraction of mercury thought to be soluble was in fact removed via filtration through a 0.2

- micron filter,

demonst rating the presence of a significant colloidal mercury fraction smaller than 0.2

microns. Accordingly, the ultrafiltration stage was added to the pilot treatment train. The new configuration showed immediate improvement.

Using this configuration, th e results

continued to show significant variability, presumabl y due to colloidal m ercury remaining on the resin. However, t he system performance improved over the next few weeks. Results are presented

below:

Table 2 Microfiltration & Ion Exchange Resul ts

Sample

Date Results

Influent Influent

Flow Effluent Resin/ Depth Column

Flow

ppt gpm ppt gpm

07/23/08 825 2.33 530 Thiol/96" 0.5

08/06/08 647 1.11 221 Thiol/96" 0.2

08/13/08 1640 2.59 337 Thiol/96" 0.2

08/19/08 375 2.50 56.8 ThioUr/48" 0.1

0 8/20/08 337 2.38 47.9 ThioUr/48" 0.2

08/20/08 385 2.38 42.3 ThioUr/48" 0.2

08/27/08 816 2.55 333 ThioUr/48" 0.2

09/03/08 1670 2.31 112 ThioUr/48" 0.2

09/10/08 2030 2.42 30.4 ThioUr/48" 0.2

09/17/08 1460 1.92 494* ThioUr/48" 0.2

09/24/08 1080 2.42 88. 7 ThioUr/48" 0.2

10/01/08 553 2.51 45.6 ThioUr/48" 0.2

Min 337 1.11 30.4 0.1

Max 2030 2.59 530 0.5

Average 985 2 168 0.2Table 3 Ultrafiltration & Ion Exchange

Sample

Date Results

Influent Influent

Flow Effluent Resin/ Depth Flow

ppt gpm ppt gpm

10/22/08 421 2.48 < 5 ThioUr/48" 0.2

10/29/08 167 2.6 < 5 ThioUr/48" 0.2

11/05/08 168 2.47 < 5 ThioUr/48" 0.2

11/12/08 588 2.22 < 5 ThioUr/48" 0.2

11/19/08 372 2.24 < 5 ThioUr/48" 0.2

12/09/08 130 2.09 < 5 ThioUr/48" 0.2

12/10/08 189 1.69 < 5 ThioUr/48" 0.2

Min 130 1.7 < 5 0.2

Max 588 2.6 < 5 0.2

Average 291 2.3 < 5 0.2

RESIN LOADING AND DISPOSITION

As mentioned earlier in the paper, bulk loading of the mercury was not the primary challenge for the media tested here. Both

the Thi ol and the Thiouronium functionalized resins were capable of removing mercury to the desired limits. Post mortem analysis of the resin showed that loading of other metals via the selective functional groups or non

selectively via inorganic precipitation o r fouling may be a more important consideration for the long term application

of these media. Since variation in coal sources, variations in station operations and variations in pretreatment processes all affect the type and level of potentially

interferi ng ions, pilot evaluation of the various media options is likely to be needed to sort through these issues.

PILOT STUDY CONCLUSIONS

Achievable Effluent Mercury The pilot was successful in removing trace mercury from FGD wastewater to

concentrations below 5 ppt . Thus the most important goal of the study was

achieved.

Most Effective Resin

The most effective resin was the

thiouronium functional group

. While t he

T hiol resin was also very effective, the performance of the T hiouronium resin was sligh tly more consistent. O nly minimal/ incidental removal w as achieved by the silver impregnated resin.

Effect of Filtration The majority of the mercury removal (on a mass basis) was via filtration. While sand and cartridge filters removed the mercury

to so me degree, they did not remove all of the colloidal mercury. Complete removal of colloidal mercury was only achieved upon incorporation of the u ltrafiltration
treatment stage

. It must be noted that the

observed result s are based up on the

distribution and form of mercury present 3with in the FGD wastewater at the specific facility tested.

In any application where selective ion

exchange is to be used downstream, it is necessary to remove ALL of the colloidal

mercury to avoid its deposit ion on th e resin.

The form and concentration of mercury is known to vary widely in FGD wastewater

based on a number of variables such as coal composition and wet scrubber operational practices. Limited d ata from other pilot testing show removal of mercury to single

- digit ppt concentrations without the use of ultrafiltration. Because of this, a bench scale analysis of particle size

distribution is recommended prior to pilot or

full

- scale testing of any trace mercury

removal application. Operational Difficulties As previou sly mentioned, the only significant operational difficulty

encountered during the pilot study involved scaling of the ultrafiltration membrane.

T his issue was re solved by optimizing the chemical cleaning regime including reagent chemistry and dosage, leng th of cleaning cycle and cleaning frequency

.

Technology Status All of the technologies included in the pilot

test are well established and can be implemented commercially. With the proper application of the selective ion

resin

, it is possible to achiev e mercury concentrations below 5 ppt in FGD

Wastewater . With proper design work (including treatability testing), this technology is ready for commercial application.

REFERENCES

1. Goltz, H. Robert, “Trace

Contaminant Removal from Ground

and Waste Waters with Selective

and Non

- Selective Media” IWC

- 08

- 76 (2008).

2. Pudvay, Michael & Goltz, H. Robert,

“ Mercury Removal in Flue Gas

Desulfurization Wastewater ” IWC

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- XX (200 7 )