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Chemical Engineering Science 63 (2008) 782–790www.elsevier.com/locate/ces

Evaluation of mercury sorbents in a lab-scale multiphase flow reactor, apilot-scale slipstream reactor and full-scale power plant

Jiang Wua,b, Yan Caoa, Weiguo Panb, Minqiang Shenb, Jianxing Renb, Yuying Dub, Ping Heb,Du Wangb, Jingjing Xub, Andy Wua, Songgeng Lia, Ping Luc, Wei-Ping Pana,∗

aInstitute for Combustion Science and Environmental Technology, Western Kentucky University, KY 42101, USAbSchool of Energy and Environmental Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China

cSchool of Power Engineering, Nanjing Normal University, Nanjing 210042, PR China

Received 19 September 2006; received in revised form 22 August 2007; accepted 13 September 2007Available online 6 October 2007

Abstract

Due to its adverse effects on human health and ecosystem, mercury emission from the coal-fired utility boiler has been generating moreand more concern. Sorbent injection upstream of the electrostatic precipitator (ESP) or bag-house has been deemed one of the recommendedmature technologies to reduce mercury emission. Before a sorbent is used in practice, its mercury capture ability needs to be evaluated, but hasuntil recently only been demonstrated in bench-, pilot- or full-scale experiments separately. In this paper, a lab-scale multiphase flow reactorand a pilot-scale slipstream reactor were set up and conducted such evaluation on the two scales. After that, some kinds of sorbents wereinjected at a full-scale power station. The experimental results show that the lab- and pilot-scale reactor systems in this paper can provideaccurate information of sorbent evaluation under flue gas atmosphere. There was significant difference between the mercury removal efficiencyof tested sorbents, varying from 98.3% down to 23%. SO2 in the flue gas was shown to inhibit mercury oxidization and capture. The sorbentshave higher mercury capturing efficiency with higher injection rate and longer residence time when other conditions were held constant. Inthe pilot-scale, four injection ports vertical to the flue gas flow direction could help improve mixture of sorbent and flue gas so that themercury removal efficiency became higher. The pilot-scale data can be used to predict the full-scale results. Some of the chemical and physicalmechanisms responsible for the mercury removal of the sorbents were identified.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Mercury; Removal efficiency; Evaluation; Mercury sorbent; Lab-, pilot- and full-scale; Prediction

1. Introduction

As reported by the U.S. Environmental Protection Agency(EPA), the Canadian Council of Minister of Environment(CCME) and the European Commission, the major anthro-pogenic source of mercury emissions, which are among themost toxic pollutants to human health and the ecosystem, isfrom coal-fired power plants (Brown, 1999; US EPA, 1998;Pavlish et al., 2004; Keating et al., 1997). Coal contains natu-rally occurring mercury that varies in concentration with boththe type of coal and its place of origin. The U.S. EPA hasdetermined that mercury emitted from utility power plants

∗ Corresponding author. Tel.: +1 270 745 2272; fax: +1 270 745 2221.E-mail address: wei-ping.pan@wku.edu (W.-P. Pan).

0009-2509/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2007.09.041

should be controlled and it has set the final regulation on mer-cury emission from coal-fired power generation on March 15,2005 (US Environmental Protection Agency, 2005). This deci-sion will affect both economic and environmental aspects of theU.S. U.S. EPA announced the Clean Air Interstate Rule (CAIR)that will cap emissions of sulfur dioxide (SOx) and nitrogenoxides (NOx) and also mercury (Hg) from coal-fired powerplants. This is a market-based cap-and-trade program, whichwill reduce electric utility mercury emissions by nearly 70%from 1999 levels when fully implemented. Sorbent injectionupstream of the ESP or bag-house is one of the recommendedmethods for mercury emission control. Sorbent injected intothe flue gas ducts absorbs both of the elemental and oxidizedmercury in flue gas, then the ESP captures the sorbent and flyash simultaneously. However, the mercury capturing efficiency

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J. Wu et al. / Chemical Engineering Science 63 (2008) 782 – 790 783

of sorbents is extremely important to avoid increasing the ESPload and to control the cost of adsorbent. Before a sorbentis used in practice, its mercury capturing ability needs to beevaluated. However, such evaluation has until recently onlybeen demonstrated in bench-, pilot- or full-scale experimentsseparately (Cao et al., 2004; Wu et al., 2006a,b; Yan et al.,2004). In this paper, a lab-scale multiphase flow reactor and apilot-scale slipstream reactor were set up and evaluated mercurycapture by the injection of some commercial sorbents. Afterthat, some kinds of sorbents were chosen to inject at a full-scalepower station to test their mercury capturing efficiency.

2. Experimental

Two phases of testing were conducted on the lab-scale multi-phase flow reactor to evaluate the mercury capture efficiency ofthe commercial sorbents. In the first phase, elemental mercurygenerated by a special Cav-Kit�, which provides a stable mer-cury concentration, was introduced to the lab-scale multiphaseflow reactor. An on-line Hg analyzer, semi-continuous emissionmonitor (Hg-SCEM) (Kellie et al., 2004; Wu et al., 2005), wasadopted to get the quasi-real-time mercury concentration in theflue gas. The change of the mercury concentration with sorbentinjection was monitored. The adopted Hg analyzer uses goldamalgamation cold-vapor atomic fluorescence to measure Hg(0) concentrations. A proprietary flue gas-conditioning systemwas used to remove acid gases and reduce any Hg (2+) presentto Hg (0) for subsequently measuring total mercury. The on-line instrument measures Hg (0) and Hg (T) continuously byswitching from channel to channel.

The selected bituminous coal ash was injected together withthe sorbents into the lab-scale reactor so that the real workingconditions of the sorbents were simulated. The sorbent feed-ing and ash separation are very important for the experiments.The feeding system was optimized and suitable transfer gaswas added so that the sorbent could be injected smoothly, and acyclone and inertial filter were added to remove the ash. Com-pressed air and simulated flue gas consisted of 9.8% of O2,9.5% of CO2 and 1106 ppm of SO2 were used as the carriergases, respectively, for the lab-scale multiphase flow reactor.The dimensions of this reactor are listed in Table 1.

The tubular reactor is a 0.05 m I.D. stainless steel pipe. Itslength is 1.22 m. To protect the stainless steel pipe and elimi-nate its effect on mercury due to possible oxidization and ab-sorption of mercury, a 0.04 m I.D. ceramic pipe is inserted intothis stainless steel pipe. The multiphase flow reactor is heatedup by two electric furnaces. A thermocouple is inserted intothe reactor to monitor the inside temperature. A two-channeltemperature controller is utilized to control the furnaces to the

Table 1Dimensions of lab-scale multiphase flow reactor

Stainless steel Ceramic pipe Total Length of heatingpipe I.D. (m) I.D. (m) length (m) section (m)

0.05 0.04 1.016 0.914

desired temperatures. The gaseous products flowed out the mul-tiphase flow reactor from the outlet in the bottom end of thestainless steel pipe, where they entered the gaseous samplingsystem. The schematic of lab-scale multiphase flow reactor isshown as Fig. 1.

In the pilot-scale slipstream testing facility, the flue gas wasdirectly introduced from the air pre-heater duct of utility boilerto simulate the real flue gas atmosphere for sorbent evaluation.The pilot-scale slipstream reactor was set up in a selected powerstation in Kentucky that burns medium-sulfur bituminous coal.The pilot-scale slipstream reactor is shown as Fig. 2. The fluegas was taken out from the duct into the slipstream reactor andthen went back to the duct so that real flue gas was attained.Some insulation tape was put on the surface of the duct con-necting the duct and slipstream reactor. The position and thick-ness of the insulation tape are adjustable so that the temperatureinside the slipstream reactor can be adjusted to expected value.The temperature inside the multiphase reactor and the residencetime of the flue gas in the reactor were controlled by adjustingthe insulation tape and use of the cooling fan. The temperatureinside the slipstream reactor kept very well, and the tempera-ture difference between the two ends of the slipstream reactorwas around 1 ◦C. A special feeder was designed to make thesorbent enter the reactor easily and distribute evenly.

In the second phase of the experiments on the lab-scalemultiphase flow reactor, the reactor was taken to the powerstation where the pilot-scale slipstream reactor was set up. Theflue gas was taken out directly from the air pre-heater duct ofthe utility boiler and introduced into the multiphase reactor toinvestigate the real flue gas atmosphere for sorbent evaluation.The flue gas was taken out from the duct into the multiphase

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1. Simulated flue gas4. Cavkit Box7. Flue gas port

10. Feeder Controller13. Temperature controller16. Mercury injection port19. Computer

17. CEM analyzer14. Mini cyclone11.Tube flow reactor8. Thermocouple5. Carrier gas2. Regulator

18. Conversion Unit15. Solid collection vessel12. Electric furnace9. Mini screw feeder6. Flowmeter3. MFC

Fig. 1. The schematic of lab-scale multiphase flow reactor.

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Lab-scale Multi-phase

Reactor

Pilot-scale Slipstream

Reactor

Flue Gas from Air

Preheater

A A

A-A

Fig. 2. The schematic diagram of pilot-scale slipstream reactor.

reactor and returned back to the boiler duct through the pilot-scale slipstream reactor. The process that introduces real fluegas into the lab-scale multiphase reactor is shown as Fig. 2.

Several kinds of sorbents were tested at both of the twophases on the lab-scale multiphase reactor and the pilot-scaleslipstream reactor under different residence time, sorbent in-jection rate and temperature. The injection system was alsooptimized to improve the mixture and capturing process. Theresidence time was 1–2.5 s and temperature inside the reac-tors was 150–170 ◦C. Sorbent injection changed from 3.85 ×10−5.7.7×10−5 kg/m3 in the lab-scale, and 5.13×10−5.2.57×10−4 kg/m3 in the pilot-scale experiments.

3. Results and discussion

3.1. Definition of mercury adsorption efficiency

During the first phase of the lab-scale experiments on themultiphase flow reactor, elemental mercury was introducedwith the flue gas into the multiphase flow reactor and resid-ual Hg concentration changed during the sorbent injection. Atypical curve of mercury concentration at the lab-scale multi-phase flow reactor during sorbent injection is shown as Fig. 3.It demonstrates that the mercury concentration begins to dropas soon as sorbent injection starts and gradually returns to theoriginal concentration level when sorbent injection ends; how-ever, it is difficult to recover completely possibly because offine sorbent build-up on the wall of the reactor and inertial filter.

0

4000

8000

12000

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ury

Concentr

ation,

ng / N

m3

Hg (T)Start injection

Stop injecting

Time (date)

14:24 16:48 19:12

Fig. 3. The history of Hg concentration changing during sorbent injection onlab-scale multi-phase flow reactor.

To describe the phenomena, maximum and minimum mercuryadsorption efficiency can be defined as

�max = (Ci − Cmin)/Ci × 100% (1)

and

�min = (C0 − Cmin)/Ci × 100%, (2)

where Ci is concentration of the injected mercury, Cmin is thelowest concentration of the mercury in the flue gas after sorbentinjection, and C0 is the recovered concentration of the mercuryafter ending sorbent injection. They are shown as Fig. 4.

According to the experimental data, �max is a function ofboth of the characteristics of a sorbent and the experimental

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Cmin

CiCi

C0

Fig. 4. The schematic of Hg concentration changing during sorbent injection.

conditions. For a given experimental run, a sorbent may havedifferent �max and �min. This is possibly a result of the sorbentsticking to the wall of the reactor or sorbent accumulating inthe horizontal duct of the reactor so that the contact time of thesorbent with the flue gas is longer than the calculated residencetime.

�min can be adopted to study the mechanism of the sorbentadsorption. When the sorbent is injected into the reactor, it willbe distributed by the aerodynamic effect and start to fly in thereactor. The mercury inside the reactor will possibly collidewith the sorbent and adhere to the surface of the sorbent, and themercury will be oxidized not depending on the characteristicsof the sorbent. The mercury on the sorbent surface will enterthe sorbent inside or just adhere to the surface. This process isphysical and/or chemical sorption, and it can be called flyingadsorption. On the other hand, some sorbent may collide withthe horizontal tube or the wall of the reactor and stick on itssurface and accumulate. The accumulated sorbent will go onadsorbing mercury as soon as there is a chance since it has moremercury capturing capability after flying adsorption. This is themain reason why the mercury concentration could not recoverits original level even after ending sorbent injection for sometime. It will take a very long time for this recovering process.The yielding mechanism of the difference between �max and�min will be described in a separated paper. This paper focuseson �max, and it will be called mercury removal efficiency � inthis paper.

During the second phase of experiments on the lab-scalereactor and experiments on the pilot-scale slipstream reactor,elemental and oxidized mercury in the flue gas are both intro-duced into the reactors, and their concentration dropped dur-ing the sorbent injection and recovered gradually when sorbentinjection was ended. The specific process is shown in Fig. 5.The relevant definition of mercury adsorption efficiency is alsoapplied to the second phase of testing.

3.2. Mass balance calculation

During sorbent injection, mercury was introduced into themultiphase flow reactor and it would be adsorbed by the injectedsorbent and would stay in bottom container, horizontal tube,cyclone or went out together with the flue gas. This process isso complex that mercury mass balance calculation needs to beconducted to help understand mercury transportation.

The sorbent was injected together with the selected repre-sentative bituminous coal fly ash from a power station at a ra-tio of 4.4:1000, so the mercury in the ash is also a mercurysource in the material balance calculation. The compressed air

or simulated flue gas was believed to be mercury-free. The ashin the horizontal tube was found to be so little that it couldbe ignored. Although there was a bottom container and mini-cyclone to collect the ash, a part of ash still escaped with theflue gas, so the ratio of injected ash to the sum of bottom ashand cyclone ash was adopted as a reference.

Based on the above analysis, the mercury mass balance wascalculated as

MHg, input = CHg, gas phase, introduced × Q × t + YHg, in ash

× Minjected ash, (3)

MHg, output = CHg, gas phase, introduced × (1.0 − �) × Q × t

+ (YHg, in bottom ash × Mbottom ash

+ YHg, in cyclone ash × Mcyclone ash)/(Mbottom ash

+ Mcyclone ash) × Minjected ash, (4)

R = MHg, output/MHg, input, (5)

where the flow rate Q was 8.33 m3/s and injected ash was0.05 kg.

The calculated mercury mass balance results for 18 exper-imental runs of sorbent injection tests show that the recoveryof mercury ranged from 90% to 110%; so the experimentaldata are acceptable. On the other hand, most of them are under100%. This is possibly because the little ash together with theinjected sorbent escaping with the flue gas was fine and con-tained more mercury; however, such ash entered the exhaustand was lost to the atmosphere.

During the second phase of experiments on the lab-scalereactor and experiments on the pilot-scale slipstream reactor,the ash collected at the bottom and cyclone was insufficient toanalyze the mercury concentration in it; so the mercury massbalance was not calculated for these experiments.

3.3. Sorbent adsorption efficiencies under different injectionconditions

Different sorbents were injected under varying conditions onthe lab-scale multiphase flow reactor. The residence time was1–2.5 s, temperature inside the reactors was 150–170 ◦C, andsorbent injection rate was 3.85 × 10−5.7.7 × 10−5 kg/m3. Inthe first phase, compressed air and simulated flue gas were usedas carrier gases in the multiphase flow reactor and real flue gaswas introduced into the reactor during the second phase.

The experimental results in the first phase show that differentsorbents have different mercury removal ability, ranging from98.3% to 23.0%. A part of the experimental results is shown asTable 2. It demonstrates that, compared with compressed air,simulated flue gas inhibited the mercury removal efficiency. Itwas possibly because SO2 in the simulated reduced mercuryoxidization and capture. On the other hand, longer residencetime and higher injection rate help improve the mercury re-moval. The temperature inside the multiphase flow reactor alsohas effect on the mercury adsorption efficiency, and sorbent hashigher mercury adsorption efficiency at lower temperature.

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0.00

5000.00

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19:12

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ury

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ation,

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Nm

3

HgT Hg (0)start injection

stop injectionstart injection

stop injection

Time (date)

21:36 0:00 2:24

Fig. 5. The changing history of Hg concentration during sorbent injection in flue gas.

Table 2Experimental results on the lab-scale multiphase flow rate

Injected sorbent Carrier gas Temperature inside Residence time (s) Sorbent injection rate Hg (0) adsorptionthe reactor (◦C ) (∗10−5 kb m3) efficiency (%)

A1 Compressed air 150 1 3.85 84.1A1 Simulated flue gas 150 1 5.78 77.6A1 Simulated flue gas 150 2.5 7.70 82.1

B1 Compressed air 150 1 3.85 32.9B1 Simulated flue gas 150 2.5 7.70 50.1

C1 Compressed air 150 1 3.85 98.3C1 Compressed air 170 1 5.13 97.0

Fig. 6. Hg adsorption at difference SO2 in the flue gas.

According to the data in Table 2, we can get Hg adsorp-tion efficiencies at different SO2 concentration in the flue gasand at different residence time. They are shown in Figs. 6and 7. The mercury adsorption efficiency of the sorbent C1changed much when the SO2 concentration changed from2 ppm in the air to 1000 ppm. For sorbent A1, its mercuryadsorption efficiency reduced from 84.1% to 77.6% when

the SO2 concentration changed from 2 ppm in the air to1106 ppm in the simulated flue gas even the injection rate wasincreased from 3.85 × 10−5 to 5.78 × 10−5 kg/m3. When res-idence time increased to 2.5 s and injection rate increased to7.7×10−5 kg/m3, the mercury adsorption of sorbent A1 in thesimulated flue gas was still lower than that in the compressedair. It shows that SO2 has big inhibition on mercury adsorption

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Fig. 7. Hg adsorption at difference residence time.

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injection condition

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(%)

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Fig. 8. Hg adsorption efficiencies at different injection conditions.

ability. The mercury adsorption efficiency of sorbent B1 in-creased much when residence extended from 1.0 to 2.5 s;however, there was little mercury adsorption efficiency changeof sorbent A1 when residence time changed. This is possiblydue to different physical and chemical characteristics of thesorbents.

In the second phase, real flue gas from the utility duct wasintroduced into the multiphase flow reactor. The experimentaldata were compared with that in the first phase, and the resultsare shown in Fig. 8. The temperature inside the reactor was170 ◦C, the residence time was 1.0 s, and the sorbent injec-tion rate was 5.13 × 10−5 kg/m3. It was shown that differenttypes of sorbents have different mercury removal efficiency.This was possibly due to different physical and chemical char-acteristics of the sorbents. The typical SEM (scanning elec-tron microscopy) results for sorbent C1 are shown as Fig. 9.The SEM analysis results show that there is bromine as well

as carbon in the sorbent C1. The main content of sorbentB was carbon and no halogen was in the sorbent B. Thecarbon mainly adsorbs the oxidized mercury. It is easy to yieldchemical bond between bromine and elemental mercury, sobromine can help sorbent to improve elemental adsorptionefficiency. Fig. 8 demonstrates that sorbent C1 mainly adsorbelemental mercury and sorbent B adsorb more oxidized mer-cury than elemental mercury, and that the sorbent C1 has muchhigher mercury adsorption efficiency than that of sorbent B.The bromine helps to improve the mercury adsorption effi-ciency. The sorbent E is a non-carbon based sorbent, and therewere aluminum, silicon, sulfur, chlorine, calcium, manganese,iron, copper, and other inorganic contents instead of carbon init. The main mercury adsorption mechanism of the sorbent Emay be its porosity and specific surface area. At the same time,chlorine in sorbent E helps capture and oxidize elemental mer-cury through chemical bond to improve the mercury adsorption

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Fig. 9. SEM results for sorbent C1.

efficiency, so that sorbent E has higher mercury removal abilitythan sorbent B. There was also chlorine in sorbent D, which canhelp explain why it has high adsorption efficiency on elementalmercury.

The sorbent has lower mercury adsorption efficiency in realflue gas than that in the compressed air. This was possiblybecause SO2, NOx and other contents in the flue gas inhibitedthe mercury capture and/or oxidization by the sorbent.

During experiments on the pilot-scale slipstream reactor, thesorbent injection feeder was modified. Before modification, thesorbent entered the reactor with one port vertical to the reactorand its injection direction was same as the flue gas flow. Af-ter modification, the sorbent entered the reactor through fourports uniformly distributed at each side of the reactor and theirinjection directions were vertical to the reactor and vertical tothe flue gas flow. When the residence time, temperature insidethe reactor and injection rate were kept at 1.0 s, 170 ◦C and2.57 × 10−4 kg/m3, respectively, the experimental results ofdifferent sorbent injection before and after the sorbent injectionfeeder modification were shown in Table 3. It demonstrated

that the capture efficiencies of the total mercury and elemen-tal mercury of all the sorbents were improved at differentlevels after the sorbent injection system modification. It waspossibly because the modified sorbent injection systemenhanced the mixture between the sorbent and mercury andlengthened their contact and reaction time, which helped theprocess of mercury capturing of the sorbents. On the otherhand, the mercury removal efficiency with 2.57 × 10−4 kg/m3

sorbent injection at the pilot slipstream reactor was similarto that with 5.13 × 10−5.7.7 × 10−5 kg/m3 at full-scale. Itmay be because the aerodynamic field inside the pilot-scaleslipstream reactor needs to be further optimized to improvethe distribution of the injected sorbent and mercury capturingprocess, and it is in consideration.

The changing of mercury removal efficiency with the in-jection rate on the pilot-scale slipstream reactor is shown inFig. 10. It shows that the mercury removal efficiency willbe higher when the injection rate increased; however, the in-creasing extent became less and less. Different type of sor-bent has different mercury adsorption efficiency relative to itsown physical and chemical characteristics. For the purposeof comparison, the Hg adsorption efficiency of standard PAC,as derived from the literature (Withum et al., 2005) is plot-ted in Fig. 10 along with our experimental results. The PACwas injected at Pleasant Prairie Power Plant (PPPP) combust-ing sub-bituminous coal. The shape of the PAC curve followsthe same trend as that of the data in the present paper. Thedifference in magnitude of Hg removal between PAC and ad-sorbents C1, D and E is reflective of experimental conditions,particularly adsorbent injection efficiency. The sorbent C1 iscarbon based and its removal efficiency on mercury total at-tained on the pilot-scale slipstream reactor with injection rateof 2.57 × 10−4 kg/m3 is 49.6%. The removal efficiency ofPAC on mercury total at the full-scale power station with in-jection rate of 7.7 × 10−5 kg/m3 is 56.0%. The absolute dif-ference between them is 6.4% and the relative difference is11.4%. The injection rate is one of the reasons. The sorbent in-jection rate of 2.57 × 10−4 kg/m3 at the pilot-scale slipstreamreactor was around equal to 5.13 × 10−5.7.7 × 10−5 kg/m3 atthe full scale. In fact, the removal efficiency of PAC on mer-cury total at the full-scale power station with injection rate of5.13 × 10−5 kg/m3 is 50.0%, which is close to 49.6% at thepilot scale slipstream reactor. At the same time, the coal typeand boiler operation conditions may make part contribution tothe difference.

Sorbent E was chosen to conduct injection test at a powerstation burning bituminous coal. The sorbent injection portwas as before ESP. The mercury concentration changing withthe sorbent injection was monitored by Hg SCEM and On-tario hydro (OH) method. The injection rate was around 7.7 ×10−5 kg/m3. The mercury removal efficiency was 41.4% on Hg(0) and 35.4% on Hg (T). The result attained on the pilot-scaleslipstream reactor with injection rate of 2.57×10−4 kg/m3 was36.1% and 46.2%. The comparison between them was shownas Fig. 11. It demonstrates that the data of mercury removalefficiency attained from pilot-scale slipstream reactor can beused to predict the results at the full-scale power station. For

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Table 3Experimental data for the pilot-scale tests

Injected sorbent Hg(VT) capture efficiency (%) Hg(VT) capture efficiency (%) Hg(0) capture efficiency (%) Hg(0) capture efficiency (%)after feeder modification before feeder modification after feeder modification before feeder modification

C1 49.6 40.1 63.5 50.1D 14.5 10.5 80.1 67.0E 36.1 28.8 46.2 36.4

Fig. 10. The mercury removal efficiencies at different injection rates (pilot-scale).

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aptu

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Fig. 11. The mercury removal efficiencies of sorbent E at different scales.

the removal efficiency on the mercury total, the absolute dif-ference between them is 5.3% and the relative difference is12.8%. Considering relationship between the sorbent injectionrate (2.57 × 10−4 kg/m3) at the pilot scale slipstream reactor

and that at the full scale (5.13 × 10−5.7.7 × 10−5 kg/m3), thedifference may be less.

Together with the comparison result between sorbent C1 in-jected on the pilot-scale slipstream reactor and PAC injected

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on the full-scale power station, the predicted results are witharound 6% of absolute difference and 12% or less of relativedifference.

4. Conclusions

1. The sorbent injection tests on the lab-scale multiphaseflow reactor and pilot-scale slipstream reactor systems couldsimulate the working conditions of the full-scale power plantand provide accurate information of sorbent evaluation in theflue gas atmosphere. The data of mercury removal efficiencyattained from pilot-scale system can be used to predict theresults at the full-scale power plant, and the relative differenceis around 12%.

2. There was significant difference between the mercuryremoval efficiencies of tested sorbents, varying from 98.3%down to 23%. It is due to different physical and chemicalcharacteristics of the sorbents and different reaction con-ditions. The halogen in the sorbent can help improve themercury capture efficiency. The coal type and boiler opera-tion parameters may impact the mercury removal efficiencies.SO2 in the flue gas may inhibit the mercury oxidation andcapturing.

3. The mercury capturing efficiency of the sorbents is af-fected by the injection rate, residence time and mixture betweenthe sorbent and flue gas. The tested sorbents had higher mer-cury capture efficiency with higher injection rate and longerresidence time when other conditions were held constant. Atthe pilot-scale slipstream reactor, four injection ports verticalto the flue gas flow direction could help improve mixture ofsorbent and flue gas so that the mercury removal efficiencybecame higher.

Notation

C concentration, kg/m3

Hg (0) elemental mercuryHg (2+) oxidized mercuryHg (T) mercury totalM mass, kgppm Parts per millionQ flow rate, m3/sR ratio, %t injection duration time, sY concentration, kg/kg

Greek letter

� mercury adsorption efficiency

Acknowledgments

This work was partially supported by the U.S. Department ofEnergy (Cooperative Agreement No. DE-FC26-03NT41840),Key Fund of Shanghai Science Technology Committee (GrantNo. 062312059), Shanghai Pujiang Program (07PJ14045) andShanghai Leading Academic Discipline Project (No. P1302).The authors would like to thank Mr. John Smith, Martin Cohron,and Stan Herren for their help during the setup of the reactorsystems.

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