Review of Mercury Formation and Capture from CO2-Enriched Oxy

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Review of Mercury Formation and Capture from CO2‑Enriched OxyFuel Combustion Flue Gas Fumei Wang,† Boxiong Shen,*,† Jiancheng Yang,† and Surjit Singh‡ †

School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin, China, 300401 Doosan Babcock Limited, Technology and Engineering, Porterfield Road, Renfrew, PA4 8DJ, U.K.

‡

ABSTRACT: Oxy-fuel combustion is the ideal option among low carbon combustion technologies for post CO2 capture. Mercury in CO2-enriched flue gas must be removed efficiently because mercury can cause metal embrittlement and aluminum corrosion during post CO2 processing. To date, there is little research that has reported mercury speciation, emissions, and removal from the oxy-fuel combustion systems. Therefore, a comprehensive review of mercury chemistry and mercury removal from the oxy-fuel combustion system is necessary. This paper reviews the formation, emission, and removal of mercury from oxyfuel combustion systems. The aim is to evaluate and summarize the fate of mercury and the status of mercury-control technologies for the oxy-fuel combustion process. It is recommended that the modification of existing ESP, SCR, or FGD systems to enhance mercury control should be developed. The investigation of mercury capture by activated carbon or other lowcost sorbents requires further research to evaluate its suitability for oxy-fuel combustion plants. The installation of additional mercury oxidation catalysts downstream of an SCR unit, where mercury would be oxidized along the economizer exit exhaust, is a promising solution. It is also recognized that the injection of a chemical oxidizing agent to control mercury emissions requires further investigation to realize the full potential of this application.

1. INTRODUCTION Coal is a major fossil-fuel energy resource for the immediate future. Carbon dioxide (CO2), which is emitted from coal combustion, is an important global warming gas. Oxy-fuel combustion technology is considered to be a promising technology that meets target limits for reducing greenhouse gas emissions from stationary sources. This technology was initially proposed by Horn and Steinberg1 and Abraham et al.2 Compared with conventional air combustion, the fuel in an oxy-combustion system is burned in either pure oxygen (O2) or an oxygen-enriched mixture, with recycled flue gas (∼60%). Oxy-combusted flue gas consists primarily of CO2 and H2O, and impurities such as SOx, NOx, and fly ash. The concentrated CO2 rich flue gas is further compressed, transported, and sequestered. Oxy-fuel technology is currently undergoing development toward commercialization in a number of manufacturing industries, such as glass, ceramics, cement, steel, and coal-fired power generation.3−6 The world’s first successful demonstration of oxy-fuel combustion and carbon capture technology took place in Australia as part of the Callide Oxyfuel Project. Oxy-fuel technology was applied to this existing power station, which generates electricity, to lower CO2 emissions.7 A possible configuration of an oxy-fuel power plant is presented in Figure 1. The solid lines represent standard process flow paths and/or control equipment for the operation of the oxy-fuel power plant. The dashed lines denote optional flow paths for pollutant removal equipment, such as secondary recycle and selective catalytic reduction (SCR). High concentrations of O2 from the air separation unit (ASU) are provided for coal combustion. Conventional pollutants, such as NOx, fly ash, and SOx, can be removed by the SCR, the electrostatic precipitator (ESP), and the flue gas desulfurization © XXXX American Chemical Society

Figure 1. Possible configuration of an oxy-fuel power plant13

(FGD) systems, respectively. A portion of the flue gas exiting the SCR, ESP, and FGD is recycled back to the combustion boiler (flue gas recycle). The difference in the composition of the postcombustion flue gas for air and oxygen is presented in Table 1. It is clearly evident that the concentration of CO2 and H2O for the oxy-combusted flue gas is significantly higher when compared to air-combusted flue gas. The SOx and NOx species, along with trace amounts of elemental mercury (Hg), are also observed to be higher for oxy-combusted flue gas. There are several comprehensive reviews of oxy-fuel combustion that address such issues, including cost, combustion performance, heat transfer, emissivity, coal reactivity, ignition, and ash deposition.3,8,9 These reviews also address the formation and control of the acidic pollutant gases (SO2 and NOx), as well as the capture and sequestration of CO2. Recent studies have Received: September 20, 2016 Revised: December 23, 2016 Published: January 2, 2017 A

DOI: 10.1021/acs.energyfuels.6b02420 Energy Fuels XXXX, XXX, XXX−XXX

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CO2 processing unit. Corrosion is regarded as the main challenge of unit operation for oxy-fuel combustion systems. Consequently, mercury must be removed prior to CO2 purification and compression. The formation and destructive mechanisms for mercury speciation that are associated with air-fired power plants have been comprehensively examined and reviewed.18−22 Previous studies have reviewed the oxy-combustion technology involving its characteristics, including recent developments in pilot-scale and commercial-scale demonstration plants.3−6,8,9 In recent years, increasing attention has been paid to the fate of Hg0 in oxy-fuel combustion technology due to the corrosion of unit operation for oxy-fuel combustion systems. To date, the fate of mercury under oxy-fuel combustion conditions had been investigated, while these research studies were mainly limited to experimental study and modeling studies under certain conditions or special technologies. This article provides a comprehensive review of the progress and recent developments of Hg0 under oxy-fuel combustion conditions, with emphasis on the formation, emissions, and capture as well as optimal removal techniques involved. The crucial factors that affect the Hg speciation formation and emissions during the oxycombustion process are summarized. The Hg capture in the operating unit and control of Hg from oxy-combustion of flue gas are also comprehensively reviewed, with insights into the challenges for large-scale applications. This review is intended to advance our understanding of the fate of Hg speciation under oxy-fuel combustion conditions and outline directions for future developments of this research field.

Table 1. Gas Composition between Combustion with Air and Oxygen14,15 Air Combustion composition (wet basis)

Oxy-Combustion composition (wet basis)

O2

21 vol %

N2 CO2

79 vol % 0 vol %

H2O

small

21 vol % 30 vol % (0)-10 vol % 40 vol % 50 vol % 10 vol % 20 vol % NOx, SOx, ... 3 vol % - 4 vol % (0) vol % 10 vol % 60 vol % 70 vol % 20 vol % 25 vol % 600−1800 ppmv for black coal 300−900 ppmv for brown coal 300−700 ppmv for black coal 100−200 ppmv for brown coal 0.3 μg/Nm3 1.0 μg/Nm3

Item Inlet Wind box

Outlet Flue Gas

Gaseous species

Others O2 N2 CO2 H2O Total sulfur concentration (SO2, H2S, COS)

Total nitrogen concentration (NO, NO2, NH3, HCN, etc.) Hg

3 vol % - 4 vol % 70 vol % 75 vol % 12 vol % 14 vol % 10 vol % 15 vol % 100−2000 ppmv

100−1000 ppmv

reported the release behavior of trace elements (Hg, Se, Cr, and As) from oxy-fuel combustion systems.10,11 Stam et al.12 reported that the concentration of trace elements in oxy-fuel combustion is higher because of the recycled flue gases. However, only limited data on trace element speciation within oxy-fuel combustion systems has been published. It has been reported that a potential risk associated with oxyfuel combustion is the inherent mercuric species present in the flue gas. The fate of mercury during oxy-fuel combustion is important because of environmental and downstream corrosion problems. Mercury has a strong affinity toward aluminum. They form a corrosive amalgam, which causes liquid metal embrittlement and material failure.16 Santos et al.17 notes that any mercuric species, if not adequately removed, are liable to cause failure of the aluminum-based equipment used in the

2. MERCURY FORMATION AND EMISSION IN OXY-FUEL COMBUSTION Mercury is present within the flue gas in the following forms: elemental mercury vapor (Hg0), oxidized mercury (Hg2+), and particulate-bound mercury (Hgp). Among these forms, Hg0 is the most difficult to capture due to its high volatility and low solubility in water. However, the oxidized and particulate forms of mercury can be more efficiently captured by postcombustion emission reduction units, such as a wet flue gas desulfurization scrubber (WFGD) and an electrostatic precipitator system (ESP). Mercury speciation, behavior, and fate under oxy-fuel conditions have been experimentally investigated at the 20

Figure 2. Hg-transformation path in oxy-coal combustion.26 B


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C

A higher sulfur in ash content reported by several studies could lead to a lower mercury adsorption capacity.

Carbon in ash It has been reported that slightly higher carbon in ash could provide a natural sink for sequestration of mercury. Sulfur in ash It has been reported that higher sulphation in ash will compete with adsorption of mercury in ash.

ESP/FF performance

Oxy-combustion firing mode for a coal with a lower inherent sulfur content may not require an FGD due to flue gas recycling. Thus, the cobenefit of mercury removal via SCR and FGD would not be realized.

In a coal fired power plant equipped with a fabric filter (FF) or ESP and SCR, a 90% reduction in mercury emissions can be achieved. Under normal operation, FF or an ESP can be operated for higher particulate capture efficiency. FF or ESP FGD/SCR

Oxy-combustion Air-combustion

Table 2. Comparison of the Potential Impact on Mercury Emissions (Capture) between Air- and Oxy-Combustion17

MWth oxy-pulverized Coal Combustion (oxy-PCC) demonstration plant at CIUDEN (Fundacion Ciudad de la Energia).23 This study has shown that, under oxy-fuel conditions, the CO2rich recycled flue gas influences Hg partitioning significantly. It is further noted that the complex homogeneous and heterogeneous reaction pathways are governed by the interactions between Hg, unburned carbon, halogen species, and SO3.24 In a 50 kWth oxy-fuel circulating fluidized bed combustor (with heated recycled flue gas), the oxy atmosphere has a higher NO, SO2, and H2O concentration, and mercury oxidation is enhanced.25 The fate of mercury in oxy-coal combustion (air-enriched: O2/N2 = 30/70 vol %, O2/CO2 = 21/79 vol %, and O2/CO2 = 30/70 vol %) was assessed at the University of Leeds.26 The study identified the important parameters for the homogeneous transformation of mercury with changes in temperature, chlorine and sulfur content of the coal, and residence time. The Hg-transformation path in oxycoal combustion is shown in Figure 2.26 In addition, oxygen enrichment promotes Hg oxidation in both air and oxycombustion systems. Chlorine is a dominant species for the oxidation of mercury. However, this process tends to be inhibited by the presence of sulfur. Pavlish et al.27 summarized the factors that govern mercury emissions as follows: (1) fuel inherent mercury and other input streams, such as recycled flue gas; (2) other fuel constituents or system inputs, such as halogens, ash components, and quantity of sulfur/acid gases and alkali components; (3) operational factors, such as temperature zones, O2, recycle loops, and flue gas composition; (4) installation and operation of existing control equipment for NOx, SO2, particulate matter (PM), and CO2 control. The potential impact of air and oxy-combustion conditions on mercury emissions is summarized in Table 2. A higher amount of carbon in ash would provide the natural capture of mercury, while a higher sulfur content in ash will lower mercury adsorption. 2.1. Effect of Coal Types and Operational Factors on Mercury Emission. The mechanism of mercury formation consists of a series of complex functions that incorporate many factors, such as coal type and properties, combustion conditions, temperature, excess air ratios, residence time, and the flue gas purification process. The complexity of these factors is reflected in the speciation of mercury. It has been reported that the characteristics of mercury oxidation seem to differ with the concentration of iron, sulfur, and calcium in the coal.18 The release of mercury from four different coals was studied in a drop tube reactor. The results showed that mercury speciation is greatly affected by coal composition (S, Cl, and moisture).28 A high conversion ratio of elemental mercury to oxidized mercury (∼80%) was found for bituminous coal. In contrast, the conversion ratio was approximately 40% for petroleum coke. These were investigated in a 0.8 MWth circulating fluidized bed (CFB) boiler.29 When compared to air combustion, the oxy-fuel combustion of bituminous and sub-bituminous coals resulted in lower emissions of mercury within the flue gas. However, Suriyawong et al.30 evaluated the combustion of pulverized sub-bituminous coal under oxygen/carbon dioxide mixtures (O2/CO2 = 20/80 Vol.%). The goal was to estimate the effects of the O2/CO2 mixtures on submicron particle formation and mercury speciation. They found no significant differences in the speciation of mercury (mainly in Hg0 form) in O2/CO2 coal combustion compared with coal combustion in air. Sethi et al.31

Oxy-fuel combustion produces a flue gas with a higher dew point temperature to ensure acidic flue gas species (NOx, SO2, SO3) do not condense preSCR-FGD treatment if the flue gas is maintained at a higher temperature. This may lower Hg capture efficiency by FF or ESP, as Hg remains in an aerosol phase, leading to poor adsorption via carbon in ash. In an oxy-combustion boiler, the carbon in ash is reported to be lower than air-combustion boilers. This may result in lower mercury capture efficiencies by the ash.

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(HCl/Cl2) in the flue gas is a strong oxidizer of mercury. Mercuric chloride (HgCl2) is considered to be the main oxidation product. Roy et al.11 investigated the effect of chlorine on mercury distribution within an oxy-fuel combusting environment. The results indicated that higher chlorine content, particularly at low temperatures, produced a greater excess of oxidized mercury (Hg2+) under oxy-fuel combustion conditions. The homogeneous oxidation of Hg0(g) by Cl2 and HCl can vary significantly and is influenced by HCl/Cl2 concentrations, flue gas temperature, and O2 concentration.42 The results showed that Cl2, compared to HCl, is a stronger oxidizer of Hg0(g) at higher temperatures (>873 K). The oxidation of Hg0(g) by the same concentrations of Cl2 and HCl in a CO2 environment was lower when compared to an N2 environment. One probable reason for this may be due to CO2 inhibiting the oxidation activity of Cl2. The suppression of Hg0(g) oxidation by other chlorine species, such as atomic Cl, may also be occurring. However, the addition of O2 into the CO2 matrix may enhance the formation of atomic Cl or HOCl, which are important radicals for the homogeneous oxidation of Hg0(g). The reactions between Hg0(g) and gaseous Cl2 are summarized as follows:

described the relative performance of a 250,000 Btu/h (∼73 kWh) combustion test facility for three types of coals (a lignite, a sub-bituminous, and a bituminous coal) in air, in a O2/CO2 = 21/79 vol % mixture, and in a O2/CO2 = 27/73 Vol.% mixture. Compared to the air combustion results, they found that, in oxy-fuel combustion, the total mercury content was higher for the bituminous and sub-bituminous coals, but not as large for the lignite. Roy et al.11 modeled thermodynamic equilibrium calculations for firing Victorian brown coal at different temperatures (800−1400 °C). The mercury emissions were found to be marginally higher for oxy-fuel firing (O2/CO2 = 10/74 vol %) when compared to air firing. Temperature was found to be an important parameter for the release of mercury; a higher furnace temperature resulted in a higher release rate of total mercury. Moreover, elemental mercury is the dominant species over the entire temperature range, with a minor contribution of zerovalent Hg0 (g) at lower temperatures. The release rate of total mercury under CO2 conditions was found to be strongly dependent on the volatile yield and temperature for the different coals.32,33 According to temperature dependent thermodynamic calculations (350− 1200 °C), the percent of Hg0 reached its peak. The percent of HgO in the flue gas decreased in the following order: O2/CO2 = 40/60 Vol.% > O2/CO2 = 30/70 Vol.% > O2/CO2 = 21/79 Vol.% > air.34 Gharebagi et al.26,35 modeled the mechanism of mercury chlorination for flue gas under oxy-fuel conditions. They considered mercury adsorption on fly ash in a carbon burnout kinetic model. It was found that temperature determined the retention of mercury within the fly ash. Other parameters that affect the transformation of mercury species during oxy-coal combustion include the boiler design, the combustion environment, the heat transfer rate, the residence time at lower temperatures during convective cooling, the use of air pollution control devices, and operating practices. Rohan et al.36 suggested that operational conditions have a primary impact on the capture of Hg and SOx. This occurs during the secondary impact of the firing mode (air or oxy) and is determined by measuring mercury and SO3 on the fabric filter. This work was performed at the Callide Oxy-fuel Project (COP) during air and oxy-fuel firing transitions. In general, oxy-fuel combustion is associated with higher temperatures compared with conventional air combustion. However, a moderation of the temperatures in the combustion zone and in the heat-transfer sections is required due to the metal temperatures of the various heat exchanger sections of the boiler. This moderation in temperature is accomplished by recycled flue gas. Oxy-fuel combustion conditions are likely to result in a lower adiabatic flame temperature when the high recycle ratio is adopted.4,37−40 As a result, the vaporization of the volatile metals and metal suboxides, and the particle formation rates, are reduced under oxy-fuel combustion conditions. These metals include sodium (Na), lead (Pb), cadmium (Cd), and mercury (Hg). Some of them may undergo gas phase chemical reactions and, subsequently, nucleate to form new particles or condense on the surface of existing particles.4 This is especially the case for mercury because all forms of mercury in coal decompose at the high temperature combustion zone to form elemental Hg0.24 Schofield et al.41 studied the heterogeneous mercury processes in coal derived flue gases and suggested that they were highly dependent on residence time and the availability of reaction surfaces. 2.2. Effect of Gas Compositions on Mercury Emission. 2.2.1. Halogens. It is generally accepted that the presence of Cl

Hg 0(g) + Cl 2(g) → HgCl2(g)

(1)

2Hg 0(g) + Cl 2(g) → Hg 2Cl 2(g)

(2)

The reactions between Hg0(g) and gaseous HC1 are Hg 0(g) + 2HCl(g) → HgCl2(g) + H 2(g)

(3)

Hg 0(g) + 2HCl(g) → HgCl2(g) + H 2(g)

(4)

4Hg 0(g) + 4HCl(g) + O2 (g) → 2Hg 2Cl 2(g) + 2H 2O(g) (5)

Hg 2Cl 2(s) → Hg 0(g) + HgC12 (g)

(6)

An increase in the concentration of chlorine in oxy-fuel combustion flue gas was observed, which is suggested to be beneficial for mercury oxidation.43−46 Conversely, the increased volume of steam within the oxy-fuel combustion process can also inhibit mercury oxidation even in the presence of Cl. Bromine is another strong oxidizer of mercury. Preciado et al.47 determined that mercury oxidation by bromine is approximately 2−4 times greater than chlorine. This has been further supported by mercury oxidation experiments conducted at the University of Utah.48 In the presence of 400 ppmv chlorine (HCl), the oxidation of mercury in oxy-firing versus air firing was approximately 80% and 5%, respectively. In the presence of 25 and 50 ppmv bromine (HBr), the oxidation levels of mercury ranged from 80 to 95%, respectively. 2.2.2. NOx. NOx is always formed during combustion processes. At high temperatures, NO is the dominant species, although NO2 may also be present to some extent. The results have shown that the mass of NOx released per unit of energy under oxy-fuel firing conditions is significantly lower than that generated under air firing conditions. Approximately one-third of the NOx is produced by air combustion.3 NO is recognized as an accelerating agent for mercury oxidation in air fired coal combustion flue gas. Preliminary experimental data has indicated that NO has a minor effect on Hg oxidation in a simulated O2/CO2 flue gas environment.20 The Hg0 (g) oxidation by NO was approximately 5% to 32%, depending on D


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Energy & Fuels the NO concentration in flue gas. This amount decreased as the temperature increased,42 as can be observed in reaction 7.49 2Hg 0(g) + 2NO(g) → 2HgO(g) + N2(g)

both atmospheres. It is suggested that mercury oxidation may have occurred as a consequence of reaction 14. 2Hg 0(g) + 2SO2 (g) + 2O2 (g) → 2HgO(g) + 2SO3(g)

(7)

(14)

Some studies have revealed that a partial amount of NO was converted into NO2 in an atmosphere with 70% CO2, whereas only 5 ppm of NO2 was formed in an atmosphere free of CO2. Therefore, a higher oxidation of mercury was observed in the presence of NO, CO2, and O2 due to the formation of a higher proportion of NO2.49 There are several possible reaction products in the Hg0 (g) + NO2 reaction system, including Hg nitrites and nitrates. However, most of them are unstable at temperatures above 200 °C. The decomposition of HgO will occur at temperatures above 400 °C. This could be the reason why the Hg0(g) oxidation by NO2 decreases as temperature increases. A probable reaction for mercury oxidation by NO2 is the formation of HgO(g) or Hg nitrites and nitrates, which are as follows. NO(g) + O2 → NO2 (g)

(8)

NO2 + Hg 0(g) → HgO + NO

(9)

NO2 + Hg 0(g) → HgNO2

2.2.4. Fly Ash. Oxy-fuel combustion produces a lower gas volume compared to air combustion. As a result, the dust concentration is 1.5 times greater, with no significant differences in the dust particle size distribution. Fly ash has been determined to be a deposition carrier for the mercury species. Mercury was found to be more enriched in ultrafine fly ash (9058

NA

good capture of Hgp or sorbent-bound mercury; SCR enhanced capture for low capture for low-rank coals58 bituminous coals by oxidizing Hg0 to Hg2+.58 by oxidizing Hg0 to Hg2+ poor capture of Hgp in general and total mercury for low rank coals58 58 high mercury capture for all coals high energy consumption

75−9663

high mercury capture

SNCR SDA+FF SCR+ESP-hs FF+WFGD SDA+FF+SCR ESP-cs +WFGD ESP-hs +WFGD SCR+ESP-cs+ WFGD SCR+ESP-hs+ WFGD SCR+FF +WFGD SCR+FF+Wet scrubber

NA

efficiency efficiency efficiency efficiency

for for for for

Hgp capture57 Hg2+57 Hg2+60 oxidizing of Hg0 to Hg2+62

high efficiency for mercury capture;58 benefit for NOx and PM controls high efficiency for Hgp and Hg2+ capture high efficiency for mercury capture

low capture for low-rank coals low efficiency at high temperature;57 probably requires specially sorbents complex operation, periodical regeneration57

a NA: not applicable. bFor bituminous; ESP-cs: cold-side ESP; ESP-hs: hot-side ESP; SNCR: selective noncatalytic reduction; SDA: spray dryer adsorbent.

mercury control technologies, such as sorbents or additives, oxidizing chemicals, and catalysts. The main control options involve fuel blending, controlling combustion processes, sorbent or chemical addition and injection, and the addition of oxidizing chemicals (shown in Figure 3).56,57 A portion of the Hg0 tends to be oxidized to Hg2+ by the SCR catalyst. As the flue gas passes through an air preheater, the flue gas temperature decreases, thereby favoring the formation of Hg2+ and Hgp in the spray dryer adsorber (SDA). Most of the Hgp is removed by the ESP/FF system, and a portion of the remaining mercury, including Hg2+ and Hgp, is removed by the FGD. 3.1. Mercury Removal in Oxy-Fuel Combustion by APCDs. In a conventional air-firing power plant, the cocapture of mercury by APCDs is the optimal option. The APCDs, such

as FF, ESP, and WFGD, can remove some of the mercury. A portion of the mercury tends to be oxidized while passing through the SCR and the ESP systems. Another portion of the mercury, which includes oxidized and particulate-bound mercury, is removed by the ESP and WFGD systems. As a result, the measured mercury concentration decreases across the APCDs. Thus, the mercury transformation in the APCDs is a dynamic process, which continues to change across the APCDs, and ultimately affects the removal efficiency. The APCDs for mercury removal in conventional air-firing power plants are summarized in Table 3.22,57,58,60−65 High mercury capture is observed by the combination of APCDs for air-firing. The APCDs for mercury removal in oxy-firing will be discussed in greater detail. F


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The process has been tested on a small scale slip-stream at the Doosan Babcock oxy-fuel furnace in Renfrew, Scotland.72,75,76 The oxidation of NO promoted the conversion of SO2 to H2SO4 in the presence of water. Mercury was expected to dissolve in the formed nitric acid.77 In addition, research on NOx, SOx and Hg0 removal, during the compression of oxy-fuel flue gas, has become a hot topic.78 Timothy et al.79 studied the absorption of gaseous Hg0into nitric acid and the gas phase reaction between mercury and nitrogen dioxide in the compression system. Stanger et al.80 investigated the behavior of gaseous Hg0 in pressurized oxy-fuel systems. This was performed in terms of the potential capture in acidic condensates, the interaction with NOx gases, and liquid stability on depressurization. The results showed that at 30 barg, capture rates of 100% Hg and 75−83% NOx were measured at the compressor exit. 3.2. Mercury Removal in Oxy-Fuel Combustion System by Sorbents. To date, there are many papers that have investigated sorbents for mercury capture in aircombustion flue gas. The main sorbents include activated carbon,81−84 modified carbon-based sorbents85−89 (such as with sulfur,90 halogen impregnated activated carbon,91 and metal oxides loaded onto carbon-based sorbents), fly ash,92,93 noncarbon sorbents,94−98 and novel sorbents99−104 (such as chemical sorbents105 and binary metal oxides106−108). These papers have comprehensively studied the mercury removal mechanism and other influencing factors of the air combustion flue gas. However, they did not discuss mercury removal under oxy-combustion conditions. Activated carbon was evaluated to be the most effective sorbent for mercury capture. Activated carbon and modified carbon-based sorbents (such as sulfur, bromine, iodine, and chlorine impregnated activated carbon) were used in flue gas. High mercury removal in the oxy-fuel systems was obtained in an activated carbon bed, which was placed ahead of the CO2 compression and sequestration system.77,109,110 Fry et al.111 proved that a higher native capture of mercury, as absorbed onto activated carbon, was obtained under oxy-fuel combustion conditions when compared to air combustion conditions. Lopez-Anton et al.112 evaluated the capacity of a series of activated carbons to retain mercury. These activated carbons were obtained from leather industry waste, and high mercury oxidation was produced in an oxy-combustion atmosphere (64%CO2). Mercury removal tends to increase with the addition of activated carbon or halogen compounds to the oxy-fuel combustion system when coal with low chlorine levels is burnt. This method has been proved by Babcock and Wilcox (B&W)113 to be effective in achieving essentially the complete removal of mercury. Zhuang et al.114 reported that mercury capture with activated carbon under CO2-enriched conditions exhibited a similar performance to typical high-acid coal combustion flue gas (Figure 4). During oxy-coal combustion, the elevated concentrations of SOx (SO2 and SO3) are believed to inhibit mercury capture, which may pose significant challenges for mercury control. This is because SOx competes with mercury for adsorption sites on the sorbent surface.115 Activated carbon injection is considered to be effective for mercury control at coal-fired power plants. However, the presence of SO3 can dramatically decrease mercury capture on the carbon surface by competing for the binding sites.116 In addition, the moisture level in oxy-fuel combustion flue gas tends to be higher because of flue gas recycling, which also affects the performance of mercury sorbents.115 Dimantopou-

Mercury may homogeneously oxidize within the gas phase, heterogeneously on metal surfaces and heterogeneously on unburned carbon particles in the fly ash, or it may simply be adsorbed onto the fly ash particles. Mercury can be removed via the ESP or FF in both oxy-fuel and air combustion systems. Coal combustion experiments conducted in a 1.5 MWth facility showed that mercury removal across an ESP under oxy-firing versus air-firing conditions were 75% and 63%, respectively.66 The sulfur content of the coal was a key factor that affected mercury removal. Lower temperatures and lower sulfur content led to a higher rate of mercury removal. The EERC (Energy and Environmental Research Center),67 US EPA (Environmental Protection Agency) Method 29 and EPA Method 30B were used to collect samples from the inlet and outlet of the ESP and scrubber units. The measurements indicated that when high-sulfur coals were fired, 97% of the mercury existed in the gas phase and almost no capture occurred across the ESP. In contrast, 40% to 50% mercury capture occurred at the scrubber. Moreover, it was concluded that a lower ESP collection efficiency for particulate matter was obtained for O2/CO2 mixtures. This is attributed to the electrical mobility of the CO2, since it is only approximately 0.6 times of an air-fired atmosphere.68,69 The behavior of mercury and SO3 was studied in a Babcock-Hitachi 1.5 MWth test facility. The results revealed that the installation of a clean energy recuperator and reducing the temperature at the ESP outlet could enhance mercury capture within the plant.16 The results of this study are as follows: (a) mercury removal across the ESP increased with decreasing sulfur content in coal; (b) the adsorption of mercury on ash particles was inhibited under high SOx conditions; (c) mercury removal at the stack increased from 94% to 97% (i.e., the outlet concentration decreased to 1/3) by decreasing the precipitator temperature from 160 to 90 °C. A comprehensive study on mercury oxidation and capture behavior of three Australian coals under air and oxy-fuel conditions, with variable flue gas cleaning devices, was performed at the University of Stuttgart.53 Lower mercury capture efficiency (∼11%) was obtained under oxy-fuel conditions when the flue gas passed through the bag house filter. It is possible that the higher SO2 and SO3 concentrations in the oxy-fuel experiments were responsible for the reduced mercury capture efficiencies. Raquel et al.70 observed that the higher concentration of CO2 in the oxy-fuel-combusted flue gas may have decreased the reformation of Hg0 due to the solubility of CO2 in the suspension and lower pH. Furthermore, O2 affects the stabilization of the Hg2+ species within gypsum slurries in the WFGD. Wu et al.71 examined the migration of Hg across the WFGD at high CO2 concentrations. They used the typical simulated desulfurization slurry, and considered the influences of solids (CaSO4, CaSO3) and anions (Cl−, NO3−, SO32−) in the slurry. The results showed that the CaSO3 solids were more favored for Hg retention than CaSO4 regardless of atmosphere or the existence of other anions. The lower pH of the slurry hindered Hg retention. In the case of anions, both NO3− and SO32− transformed larger amounts of Hg by the CaSO4 solids and liquid into the gas phase. In contrast, the existence of Cl− retained more Hg in the liquid, especially when NO3− and SO32− coexisted. Air Products has developed an alternative approach termed the “Sour Gas Compression” technology.72−74 Two scrubbers and one adsorption bed at 15 and 30 bar are used to increase the surface area and residence time to facilitate gas dissolution. G


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atmosphere.118 Spörl et al.48 studied mercury emissions and the removal behavior of ash in an oxy-fuel combustion system. The study showed that mercury oxidation was enhanced in an oxy-combustion atmosphere. Another researcher observed that the presence of Ca-based sorbents in the combustor favored mercury fixation as particle-bound mercury in the oxy-fuel combustion process.119 Activated carbon is expensive, ranging in price from $500 to $3000 per ton. It is estimated that the annual cost of using activated carbon for mercury removal at a typical 500-MWe coal-burning power plant is approximately five million dollars.120 It is suggested that further development of more cost-effective mercury sorbents/additives is required for oxy-fuel combustion systems. 3.3. Mercury Removal in Oxy-Fuel Flue Gas by Catalysts. It is well-known that a catalyst can oxidize elemental mercury to oxidized mercury, which can be easily removed. Yang et al.121 investigated the Hg0 removal behavior and reaction mechanisms under oxy-fuel combustion conditions using cobalt oxide-loaded magnetosphere catalysts from fly ash (Co−MF catalyst). In addition, SCR units have been widely applied to the removal of NOx. The SCR catalyst has been effective in oxidizing elemental mercury. This additional beneficial use of the SCR renders the cost of mercury control more economical than with the sorbents or the additives injection method. To date, great efforts have been deployed, and various types of catalysts for mercury oxidation have been developed for air-combustion flue gas, as summarized in detail by Gao et al.57 Although there have been some difficulties, some studies on mercury oxidation across the SCR were performed in an oxycombustion power plant. A high oxidation performance was achieved by the TRAC (Triple Action catalyst) catalyst under oxy-fuel combustion conditions.16 Mercury oxidation across the SCR increased with HCl concentration at the SCR inlet. However, the effects of the oxy-fuel combustion conditions were relatively small because the original chlorine content of the coal was high enough to oxidize mercury efficiently. Our

Figure 4. Comparison of mercury breakthrough with activated carbon sorbent in different flue gases.109

lou et al.117 evaluated the ability of activated carbon to remove mercury in the presence of other flue gas components in conventional combustion. The results indicated that mercury adsorption was prevented by CO2 because it partially fills the microporous structure of the activated carbon. A possible explanation is the competitive occupation of similar chemisorption sites by elemental mercury and CO2. These active sites are represented mainly by lactones and carbonyls and are located at the edges of the activated carbon graphitic layers. Thus, the utilization of activated carbon for both elemental mercury and carbon dioxide retention seems to be difficult. Figure 5 shows the removal of mercury over activated carbon sorbents in various oxy-coal combustion flue gases.26 Currently, studies on the mechanism of mercury removal by activated carbons in oxy-fuel flue gas are limited, and further studies are required to confirm the influencing factors. However, other sorbents for mercury capture in oxy-fuel combustion flue gas have not been significantly investigated. Research has shown that high mercury retention efficiency was achieved by an Au/C regenerable sorbent under CO2

Figure 5. Factors in mercury capture by activated carbon in oxy-coal combustion.26 H


Review

Energy & Fuels team’s studies have shown that mercury oxidation by V2O5− WO3/TiO2 in the absence of HCl would be easier in the oxyfuel combustion system (80 vol % CO2) than in the air combustion system (12 vol % CO2) (shown in Figure 6).122

Table 4. Using Oxidant for Mercury Oxidation from Conventional Air-Firing Flues Gas Injection location Coal Flue gas SCR FGD Sorbents Flue gas

Oxidation efficiency/%

Additive types CaCl263 HCl, HBr, Br2 HCl126 Ca(OH)2 BrCl,124 ICl,127 I2128 Ozone or secondary hydroxyl radicals129

50 98.5 90

removed because mercury can cause metal embrittlement, and aluminum corrosion in CO2 postprocessing units. To date, there is little research that has reported mercury speciation, emissions, and removal from the oxy-fuel combustion systems. Therefore, a comprehensive review on mercury chemistry and mercury removal from oxy-coal combustion has been conducted. This study focused on the formation and emissions of Hg speciation and its control techniques under oxy-fuel combustion conditions. Despite the limited studies of Hg behavior under oxy-combustion, it is clearly evident that the flue gas composition has a significant impact on the fate of Hg speciation mechanisms. It is recommended that more detailed thermodynamic predictive modeling tools are required in order to further understand mercury speciation within air- and oxy-fired combustion systems. The predictive models require further evaluation based on the impact of coal type, composition, and plant operating conditions for a more detailed understanding as to the fate of mercury during oxy-fuel firing. A practical approach to controlling mercury emissions at existing utility plants is to minimize capital costs by adapting existing equipment for mercury capture. Due to the high capital costs associated with ESP, SCR, and FGD, these technologies are expected to be installed not only to control the primary pollutants but also to remove mercury. Modification to existing ESP, SCR, or FGD systems to enhance mercury control could be developed, such as injection of a mercury sorbent upstream of the PM control devices. Another potential application could be the installation of additional oxidation catalysts downstream of existing SCR units for enhanced oxidation and removal of Hg from the flue gas stream pre-CO2 compression. Despite the feasibility of these techniques in lab-scale study, the technology for testing, validating, and developing oxidation catalysts warrants further investigation, in order to develop multifunction low cost catalysts. Moreover, Hg removal via activated carbon injection has demonstrated its potential within air firing combustion. However, further investigation is needed to assess if this technology can be applied to oxy-fuel combustion plants, where there was the higher CO2, H2O, SO2, and SO3 concentrations under oxy-fuel combustion conditions with flue gas recycling. Therefore, research is also required to identify highly effective sorbents or regenerable sorbents for mercury capture at a low cost.

Figure 6. Effects of CO2 on Hg0 removal over V2O5−WO3/TiO2. Reaction conditions: 12−80 vol % CO2, 5 vol % O2, N2 as balance gas, GHSV = 75,000 h−1.

Fernández-Miranda et al.123 evaluated the oxidation and capture of Hg0 by SCR catalysts (V/W/TiO2 and Fe/Zeolite with/without Mn as a doping agent) under a simulated oxycombustion flue gas. The results showed that higher mercury oxidation was observed in the CO2-enriched atmosphere because a higher concentration of NO and NO2 was free to homogeneously oxidize mercury, while NOx conversion was lower. The results were almost identical to our previous study.122 However, high amounts of CO2 and H2O may block the active sites for mercury adsorption. Moreover, the performance of titanium dioxide with UV (ultraviolet) irradiation for mercury capture was evaluated in three systems: a bench-scale coal combustor, a slip stream from a pilot scale system, and a pilot scale system for oxy-fuel combustion.124 The tests indicated that the performance of TiO2 is favorable for mercury capture under realistic operating conditions. Further considerations are required to maximize the effectiveness of TiO2 for mercury capture. 3.4. Mercury Removal in Oxy-Combustion by Additional Oxidizing Agents. The conversion of Hg0 to Hg2+ can also be implemented by the addition of an oxidant. The addition of oxidants (e.g., halogens or halide salts, such as CaBr2 or NaBr) into the SCR catalyst systems or flue gas upstream of the WFGD has been demonstrated to be effective in oxidizing elemental mercury.125 The major contributions to the oxidant reagents that promoted oxidation of mercury are reviewed in Table 4. However, this Hg0 oxidation approach for mercury removal has been rarely researched and has not matured yet. In addition, the technology has not yet been reported for oxy-coal combustion flue gas. Furthermore, there is still concern that adding too much oxidant directly into the system may possibly result in secondary pollution.

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4. CONCLUSIONS AND OUTLOOK Oxy-fuel combustion is the ideal option among low carbon combustion technologies for post CO2 capture. The mercury present in the CO2-enriched flue gas should be efficiently

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Boxiong Shen: 0000-0003-1445-5332 I


Review

Energy & Fuels Notes

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The authors declare no competing financial interest. Fumei Wang and Boxiong Shen are the cofirst authors.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Hebei Province (E2016202361), Young Scientists of Hebei University of Technology (2015009), and National key research and development program of China (2016YFC0209202).

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ABBREVIATIONS COP = Callide Oxyfuel Project ASU = Air Separation Unit SCR = Selective Catalytic Reduction ESP = Slectrostatic Precipitator FF = Fabric Filter FGD = Flue-Gas Desulfurization WFGD = Wet Flue-Gas Desulfurization CCS = Carbon Capture and Storage APCD = Air Pollution Control Device PM = Particulate Matter ESP-cs = Cold-side ESP ESP-hs = Hot-side ESP SDA = Spray Dryer Adsorber EERC = Energy and Environmental Research Center CANMET = Canada’s Center for Mineral and Energy Technology CFB = Circulating Fluidized Bed EPA = Environmental Protection Agency TRAC = TRiple Action catalyst UV = Ultraviolet

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Review of Mercury Formation and Capture from CO2-Enriched Oxy

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