Experimental studies on gas-phase mercury oxidation removal and denitration of coal combustion with NH_4 Br addition

In order to remove gas-phase mercury and NOx from flue gas, experimental studies on flue gas mercury oxidation removal and denitration of Guizhou anthracite combustion with NH4Br addition were carried out. The influence of NH4Br addition on the ignition temperature and combustion characteristics was studied using a thermogravimetric analyzer. The effects of the NHaBr addition amount on gas-phase mercury oxidation and removal were investigated in a bench scale of 6 kW fluidized bed combustor （FBC）. Mercury concentrations in flue gas were determined by the Ontario hydro method （OHM） and the mercury mass balance was obtained. Results show that the NH4Br addition has little influence on the ignition temperature of Guizhou anthracite. With the mercury mass balance of 95.47%, the proportion of particulate mercury Hg^p, gaseous mercury Hg^0 and Hg^2＋ are 75.28%, 11.60% and 13. 12%, respectively, as raw coal combustion. The high particulate mercury Hg^p in flue gas is caused by the high unburned carbon content in fly ash. When the NH4Br addition amount increases from 0 to 0. 3%, the concentration of gaseous Hg^0 and Hg^2＋ in flue gas decreases continuously, leading to the Hg^p increase accordingly. The oxidation rate of Hg^0 is positively correlated to the Br addition amount. It demonstrates that coal combustion with NH4Br addition can promote Hg^0 oxidation and removal. NOx concentration in flue gas exhibits a descending trend with the NHaBr addition and the removal rate reaches 17.31% with the addition amount of 0.3%. Adding NH4Br to coal also plays a synergistic role in denitration.

Evaluation of mercury speciation and removal through air pollution control devices of a 190 MW boiler

Air pollution control devices （APCDs） are installed at coal-fired power plants for air pollutant regulation. Selective catalytic reduction （SCR） and wet flue gas desulftLrization （FGD） systems have the co-benefits of air pollutant and mercury removal. Configuration and operational conditions of APCDs and mercury speciation affect mercury removal efficiently at coal-fired utilities. The Ontario Hydro Method （OHM） recommended by the U.S. Environmental Protection Agency （EPA） was used to determine mercury speciation simultaneously at five sampling locations through SCR-ESP-FGD at a 190 MW unit. Chlorine in coal had been suggested as a factor affecting the mercury speciation in flue gas; and low-chlorine coal was purported to produce less oxidized mercury （Hg^2＋） and more elemental mercury （Hg^0） at the SCR inlet compared to higher chlorine coal. SCR could oxidize elemental mercury into oxidized mercury when SCR was in service, and oxidation efficiency reached 71.0%. Therefore, oxidized mercury removal efficiency was enhanced through a wet FGD system. In the non-ozone season, about 89.5%-96.8% of oxidized mercury was controlled, but only 54.9%-68.8% of the total mercury was captured through wet FGD. Oxidized mercury removal efficiency was 95.9%-98.0%, and there was a big difference in the total mercury removal efficiencies from 78.0% to 90.2% in the ozone season. Mercury mass balance was evaluated to validate reliability of OHM testing data, and the ratio of mercury input in the coal to mercury output at the stack was from 0.84 to 1.08.

Mercury speciation and removal across full-scale wet FGD systems at coal-fired power plants

The Ontario Hydro Method （OHM） recommended by the United States Environmental Protection Agency （EPA） was used to determine mercury speciation in the combustion flue gas across wet FGD systems. Four coal-fired units with wet FGD systems were chosen to evaluate mercury speciation and mercury removal efficiencies through these wet FGD systems. Chlorine content in coal had been suggested as a main factor that affects mercury speciation in flue gas. It is shown that the higher the chlorine concentration in coal is, the higher the percentage of oxidized mercury （Hg2＋） is removed in wet FGD systems, which can increase overall mercury removal efficiencies through wet FGD systems. The selective catalyst reduction （SCR） system has a function of oxidizing ele- mental mercury （Hg0） to oxidized mercury. A higher percentage of oxidized mercury in the total vapor mercury at the FGD inlet is observed when SCR is in service. Therefore, higher overall mercury removal efficiencies through wet FGD are attained. Because of different wet FGD operating conditions, there are different mercury removal efficiencies in different units. Elemental mercury reemission took place when a fraction of oxidized mercury absorbed in the slurry is reduced to elemental mercury, and Hg2＋ is reemitted from stack, which results in decreases in mercury removal efficiencies through wet FGD systems.

Role of mangrove in mercury cycling and removal in the Jiulong Estuary

Baseline of mercury in mangrove swamp The mercury content in sediments in the Jiulong Estuary was determined in 1984 and 1985(Table 1). The highest content of mercury in sediments (300×10) was found in Ditou, contrast-ing strongly with the low content of 36×10in Port Dongzhai in the Hainan Island. Tab1e 1 showsthat the average content of mercury in sediments in the Jiulong Estuary is 140×10and the averageof four areas is 72×10. This value means that these areas have not been heavily contaminated by

Removal of Mercury(Hg(Ⅱ)) from Seaweed Extracts by Electrodialysis and Process Optimization Using Response Surface Methodology

In this work,response surface methodology(RSM)was employed to model and optimize electrodialysis process for mercury(Hg(II))removal from seaweed extracts.Box-Behnken design(BBD)was utilized to evaluate the effects and the interaction of influential variables such as operating voltage,influent flow rate,initial concentration of Hg(II)on the removal rate of Hg(II).The developed regression model for removal rate response was validated by analysis of variance,and presented a good agreement of the experimental data with the quadratic equation with high value coefficient of determination value(R2=0.9913,RAdj 2=0.9678).The optimum operating parameters were determined as 7.17V operating voltage,72.54L h−1 influent flow rate and 5.04mgL−1 initial concentration of mercury.Hg(II)removal rate of 76.45%was acquired under the optimum conditions,which showed good agreement with model-predicted(75.81%)result.The results revealed that electrodialysis can be considered as a promising strategy for removal of Hg(II)from seaweed extracts.

Analysis of solid sorbents for control and removal processes for elemental mercury from gas streams:a review

Due to the restriction such as the Minamata Convention as well as the IED of the European Commission,mercury removal from flue gases of coal-fired power plants(CPP)is an increasingly important environmental issue.This makes this topic very crucial for both the energy industry and scientists.This paper shows how mercury arises from natural resources,i.e.,coals,through their combustion processes in CPP and considers the issue of mercury content in flue gases and solid-state coal combustion by-products.The main part of this paper presents a review of the solid sorbents available for elemental mercury control and removal processes,tested on a laboratory scale.The described solutions have a potential for wider usage in exhaust gas treatment processes in the energy production sector.These solutions represent the latest developments in the field of elemental mercury removal from gases.The authors present an overview of the wide range of solid sorbents and their modifications intended to increase affinity for Hg^(0).Among the presented sorbents are the wellknown activated carbon solutions but also novel modifications to these and other innovative sorbent proposals based on,e.g.,zeolites,biochars,other carbon-based materials,metal-organic frameworks.The paper presents a wide range of characteristics of the described sorbents,as well as the conditions for the Hg^(0) removal experiments summarizing the compendium of novel solid sorbent solutions dedicated to the removal of elemental mercury from gases.

REMOVAL OF MERCURY FROM WATER USING POLYPYRROLE AND ITS COMPOSITES

One of the suitable methods for removing heavy metals from water is by using surface adsorption process. In this paper, the preparation of polypyrrole and its composites as adsorbents are discussed and the capability of separating mercury from water is investigated. The results indicated that the polypyrrole and its composites are able to remove mercury from aqueous media. Furthermore the adsorption percentage is related to the surface morphology, type of additives and its concentration.

Pre-combustion mercury removal with co-production of hydrogen via coal electrolysis

Pre-combustion mercury removal via coal electrolysis was performed and investigated on a bench-scale coal electrolytic cell(CEC)systemically,and factorial design was used to determine the effect of different operating conditions(coal particle size,operating temperature,operating cell voltage,and flow rate of slurry)on the percentage of mercury removal,percentage of ash removal,and dry heating value change.The results showed that the operating cell voltage,as well as the interaction between operating cell voltage and coal particle size,are significant factors in the percentage of mercury removal.There is no significant factor in the percentage of ash removal and the dry heating value change,but the coal could be purified while keeping the dry heating value almost constant after electrolysis.A co-product of hydrogen could be produced during coal electrolysis with 50%lower energy consumption compared with water electrolysis.Meanwhile,a mechanism for mercury removal in coal was proposed.The facts indicate that coal electrolysis is a promising method for precombustion mercury removal.

Graphene oxide based carbon composite as adsorbent for Hg removal:Preparation, characterization, kinetics and isotherm studies

The presence of Hg in the aqueous media is known to cause severe health issues in both humans and animals.Many technologies and especially adsorbents have been applied for its removal. In this study, a graphene oxide–carbon composite(GO–CC) as a new adsorbent was prepared by sol gel procedure and characterized using field emission scanning electron microscopy, BET and EDX. The effects of different variables including solution p H, contact time, adsorbent dose and GO ratio in adsorbent matrix on the removal capacity of Hg were studied. The isotherm data correlated well with the Langmuir isotherm model. Further analysis recommended that the Hg^(2+) adsorption process is governed by the intra-particle and external mass transfer, in which the film diffusion was the rate restrictive step. The presented composite has maximum absorption capacity, q_(max) of 68.8 mg·g^(-1), which is comparable with carbon based adsorbent reported in the previous publications.

Development of new technology for coal gasification purification and research on the formation mechanism of pollutants

Coal-fired power generation is the main source of CO_(2)emission in China.To solve the problems of declined efficiency and increased costs caused by CO_(2)capture in coal-fired power systems,an integrated gasification fuel cell(IGFC)power generation technology was developed.The interaction mechanisms among coal gasification and purification,fuel cell and other components were further studied for IGFCs.Towards the direction of coal gasification and purification,we studied gasification reaction characteristics of ultrafine coal particles,ash melting characteristics and their effects on coal gasification reactions,the formation mechanism of pollutants.We further develop an elevated temperature/pressure swing adsorption rig for simultaneous H_(2)S and CO_(2)removals.The results show the validity of the Miura-Maki model to describe the gasification of Shenhua bituminous coal with a good fit between the predicted DTG curves and experimental data.The designed 8–6–1 cycle procedure can effectively remove CO_(2)and H_(2)S simultaneously with removal rate over 99.9%.In addition,transition metal oxides used as mercury removal adsorbents in coal gasified syngas were shown with great potential.The techniques presented in this paper can improve the gasification efficiency and reduce the formation of pollutants in IGFCs.

Elemental mercury (Hg^(0)) removal from coal syngas using magnetic tea-biochar: Experimental and theoretical insights

Mercury is ranked 3^(rd)as a global pollutant because of its long persistence in the environment. Approximately 65% of its anthropogenic emission (Hg^(0)) to the atmosphere is from coal-thermal power plants. Thus, the Hg^(0)emission control from coal-thermal power plants is inevitable. Therefore, multiple sorbent materials were synthesized using a one-step pyrolysis method to capture the Hg^(0)from simulated coal syngas. Results showed, the Hg^(0)removal performance of the sorbents increased by the citric acid/ultrasonic application.T5CUF_(0.3)demonstrated the highest Hg^(0)capturing performance with an adsorption capacity of 106.81 μg/g within 60 min at 200 °C under complex simulated syngas mixture (20% CO,20% H_(2), 10 ppm V HCl, 6% H_(2)O, and 400 ppm V H_(2)S). The Hg^(0)removal mechanism was proposed, revealing that the chemisorption governs the Hg^(0)removal process. Besides, the active Hg^(0)removal performance is attributed to the high dispersion of valence Fe_(3)O_(4)and lattice oxygen (α) contents over the T5CUF_(0.3)surface. In addition, the temperature programmed desorption (TPD) and XPS analysis confirmed that H_(2)S/HCl gases generate active sites over the sorbent surface, facilitating high Hg^(0)adsorption from syngas. This work represented a facile and practical pathway for utilizing cheap and eco-friendly tea waste to control the Hg^(0)emission.

Mercury removal and recovery by immobilized Bacillus megaterium MB1

From several mercury removing microorgan- isms, we selected Bacillus megaterium MB 1, which is non- pathogenic, broad-spectrum mercury resistant, mercuric ion reducing, heat tolerant, and spore-forming, as a useful bacterium for bioremediation of mercury pollution. In this study, mercury removal performance of the immobilized B. megaterium MB1 was investigated to develop safe, efficient and stable catalytic bio-agent for mercury bioremediation. The results showed that the alginate gel immobilized B. megaterium MB 1 cells efficiently removed 80% of mercury from the solution containing 10mg/L mercuric chloride within 24 h. These cells still had high activity of mercury removal even after mercuric ion loading was repeated for nine times. The analysis of mercury contents of the alginate beads with and without immobilized B. megaterium MB1 suggested that a large portion of reduced metallic mercury was trapped in the gel beads. It was concluded that the alginate gel immobilized B. megaterium MB 1 cells have potential to remove and recover mercury from mercury-containing water.

New insights into mercury removal mechanism on CeO2- based catalysts： A first-principles study

First-principles calculanons were performed to investigate the mechamsm of Hg0 adsorpnon and oxidation on CeO2（111）. Surface oxygen acnvated by the reducnon of Ce to Ce a vlta to Hg~ adsorption and oxidation processes. Hg0 was fully oxidized by the surface lattice oxygen on CeO2（111）, without using any other oxidizing agents. HCI could dissociate and react with the Hg adatom on CeO2（111） to form adsorbed Hg CI or CI-Hg-Cl groups, which promoted the desorption of oxidized Hg and prevented CeO2 catalyst deactivation. In contrast, O-H and H-O-H groups formed during HC1 adsorption consumed the active surface oxygen and prohibited Hg oxidation. The consumed surface oxygen was replenished by adding O2 into the flue gas. We proposed that oxidized Hg desorption and maintenance of sufficient active surface oxygen were the rate-determining steps of Hg0 removal on CeO2-based catalysts. We believe that our thorough understanding and new insights into the mechanism of the Hg0 removal process will help provide guidelines for developing novel CeO2-based catalysts and enhance the Hg removal efficiency.

Mercury removal from flue gas using nitrate as an electron acceptor in a membrane biofilm reactor

Mercury(Hg^(0))is a hazardous air pollutant for its toxicity,and bioaccumulation.This study reported that membrane biofilm reactor achieved mercury removal from flue gas using nitrate as the electron acceptor.Hg^(0) removal efficiency was up to 88.7%in 280 days of operation.Oxygen content in flue gas affected mercury redox reactions,mercury biooxidation and microbial methylation.The biological mercury oxidation increased with the increase of oxygen concentration(2%‒17%),methylation of mercury reduced with the increase of oxygen concentration.The dominant bacteria at oxygen concentration of 2%,6%,17%,21%were Halomonas,Anaerobacillus,Halomonas and Pseudomonas,respectively.The addition of ferrous sulfide could immobilize Hg^(2+) effectively,and make both Hg^(2+) and MeHg transform into HgS-like substances,which could achieve the inhibition effect of methylation,and promote conversion of mercury.The dominant bacteria changed from Halomonas to Planctopirus after FeS addition.Nitrate drives mercury oxidation through katE,katG,nar,nir,nor,and nos for Hg^(0) removal in flue gas.

Mercury removal from aqueous solution using petal-like MoS_(2) nanosheets

Recently,different nanomaterial-based adsorbents have received greater attention for the removal of environmental pollutants,specifically heavy metals from aqueous media.In this work,we synthesized few-layered MoS2 nanosheets via a surfactant-assisted hydrothermal method and utilized them as an efficient adsorbent for the removal of mercury from aqueous media.The synthesized MoS2 nanosheets showed petal-like morphology as confirmed by scanning electron microscope and high-resolution transmission electron microscopic analysis.The average thickness of the nanosheets is found to be about 57 nm.Possessing high stability and negative zeta potential makes this material suitable for efficient adsorption of mercury from aqueous media.The adsorption efficiency of the adsorbent was investigated as a function of pH,contact time and adsorbent dose.The kinetics of adsorption and reusability potential of the adsorbent were also performed.A pseudo-second-order kinetics for mercury adsorption was observed.As prepared MoS_(2) nanosheets showed 93%mercury removal efficiency,whereas regenerated adsorbent showed 91%and 79%removal efficiency in the respective 2^(nd) and 3^(rd) cycles.The adsorption capacity of the adsorbent was found to be 289 mg/g at room temperature.

Mercury removal performance over a Ce-doped V-W/TiO_(2) catalyst in an internally illuminated honeycomb photoreactor

A new type of internally illuminated honeycomb photoreactor was designed. The honeycomb catalyst prepared by using Cedoped TiO2 with 1%–2% vanadium and tungsten was employed for mercury removal from simulated industrial flue gas. The adsorption kinetics in the reaction process were studied. The results showed that the internally illuminated honeycomb photoreactor had good mercury removal performance. When the temperature was 25℃ and the ultraviolet(UV) light intensity reached 80 μW/cm2, the mercury removal efficiency reached 92.5%. The mercury removal efficiency increased significantly with the doping ratio of Ce. XPS analysis showed that the oxidation state of Ce changed from 4 to 3 in the mercury removal reaction and produced lattice oxygen, which acts as an oxidant. O2 can promote mercury removal by honeycomb catalysts;SO2 and HCl also had positive effects, while NO had an inhibitory effect on mercury removal. Kinetic research in the reaction process showed that the quasi-first-order dynamic model had good fitting results, and the correlation coefficients of the fitting results for multiple sets of experimental data were more than 0.999.

Elemental mercury removal from coal gas by CeMnTi sorbents and their regeneration performance

Ce and Mn modified TiO_(2) sorbents(CeMnTi) were prepared by a co-precipitation method,and their ability to remove elemental mercury from coal gas in a fixed bed reactor was studied.Based on results of Brunauer-Emmett-Teller(BET),X-ray diffraction(XRD),scanning electron microscope(SEM),and X-ray photoelectron spectroscopy(XPS) studies,the modification mechanisms of the CeMnTi sorbents are discussed.Mn doping improved the specific surface area and dispersion of cerium oxides on the sorbent surface,while Ce doping increased the proportion of Mn^(4+)in manganese oxides by a synergetic effect between manganese oxides and cerium oxides.The effects of the active component,temperature,and coal gas components on the mercury removal performance of the sorbents were investigated.The results showed that the CeMnTi sorbents exhibited high mercury removal efficiency.Ce_(0.2)Mn_(0.1)Ti adsorbed 91.55% elemental mercury from coal gas at 160℃.H2 S and O2 significantly improved the ability of sorbents to remove mercury.Part of the H2_(S) formed stable sulfates or sulfites through a series of oxidation reaction chains on the sorbent surface.HCl also improved the mercury removal performance,but reduced the promotion effect of H2_(S) for mercury removal when coexisting with H2_(S).CO and H2 had a minor inhibitory effect on mercury adsorption.The recycling performance of the sorbents was investigated by thermal regeneration.The thermal decomposition of the used sorbents indicated that mercury compounds were present mainly in the form of HgO and HgS,and higher temperature was beneficial for regeneration.The formation of sulfates and sulfites in the presence of H2_(S) led to a decrease in mercury removal efficiency.

MoS_(2) quantum dots based MoS_(2)/HKUST-1 composites for the highly efficient catalytic oxidation of elementary mercury

Due to the ever-tightening regulation on mercury emission in recent decades,there is an urgent need to develop novel materials for the removal of elemental mercury at coal-fired power plants.In this study,a series of MoS_(2) quantum dots(QDs)-based MoS_(2)/HKUST-1 composite materials were prepared.It is found that MoS_(2)QDs were encapsulated by HKUST-1and enhanced the crystallinity and specific surface area of HKUST-1.The MoS_(2)/HKUST-1 showed excellent performance in catalytic oxidation of Hg~0as compared with pristine HKUST-1.It is found that surface layer of lattice oxygens is active and participates in Hg^(0) oxidation,while the consumption of surface oxygens then leads to the formation of oxygen vacancies on the surface.These vacancies are effective in the adsorption and dissociation of O_(2),which subsequently participates in the oxidation of Hg^(0).Moreover,the study on the influence of commonly seen gas components,such as SO_(2),NO,NH_(3) and H_(2)O,etc.,on Hg^(0) oxidation demonstrated that synergistic effects exist among these gas species.It is found that the presence of NO promotes the oxidation of Hg^(0) using oxygen as the oxidant.