Application of Activated Carbon for Mercury Removal in Coal-fired Power Plants

Coal-fired power generation still occupies an important position in the global energy structure. However, the associated mercury (Hg) emissions pose a serious threat to the ecological environment and human health. With the implementation of the Minamata Convention in many countries, especially in China, the United States, the European Union and other regions, mercury emission control in coal-fired power plants has shifted from an "option" to a mandatory constraint. Among various mercury removal technologies, flue gas mercury adsorption technology based on Activated Carbon Injection (ACI) is widely used in in-service units due to its mature process, relatively convenient retrofitting and strong adaptability.

 

Ⅰ. Mercury Emission Issues in Coal-fired Power Plants

In the current global primary energy consumption pattern, coal still acts as a "ballast stone". Although the proportion of renewable energy is increasing, coal-fired units are still difficult to be completely replaced for a long time in terms of power system dispatchability and power supply reliability. Meanwhile, with the expansion of coal combustion, the release of trace heavy metals in coal has gradually shifted from academic discussion to regulatory focus.

 

Mercury has the following characteristics:

1. High toxicity and neurotoxicity: Organic mercury such as methylmercury can accumulate through the food chain and cause irreversible damage to the neurodevelopment of fetuses and children;

2. Long-distance migration ability: After being emitted as gaseous elemental mercury (Hg⁰), it can migrate thousands of kilometers in the atmosphere, forming global pollution;

3. Refractory and environmental persistence: After settling into soil and water bodies, it may re-enter the biosphere cycle through microbial methylation.

 

 

Therefore, mercury emissions have typical trans-regional and intergenerational externalities. Governance by a single country is often insufficient, and overall control requires international coordination. Since its adoption in 2013, the Minamata Convention has been ratified by more than 100 contracting parties including China. One of its core objectives is to limit and reduce mercury emissions from industrial sources such as coal-fired power plants.

 

In engineering practice, the flue gas purification systems of coal-fired power plants are relatively mature, generally equipped with the following devices:

• Selective Catalytic Reduction (SCR) or Selective Non-Catalytic Reduction (SNCR) for denitrification;
• Electrostatic Precipitators (ESP) or Fabric Filters (FF) for particulate matter control;
• Wet or semi-dry Flue Gas Desulfurization (FGD) systems for SO₂ removal.

 

These devices have a "synergistic removal" effect on some forms of mercury. However, for the most difficult-to-control gaseous elemental mercury (Hg⁰), the conventional flue gas treatment chain cannot guarantee stable and efficient capture. In this context, mercury removal technology centered on activated carbon injection is regarded as an important supplement to traditional flue gas treatment paths, and even becomes a key link to achieve mercury emission compliance under some working conditions.

 

Ⅱ. Mercury Speciation and Transformation Pathways in Flue Gas

 

Understanding mercury speciation distribution from the perspective of thermodynamics and kinetics is a prerequisite for discussing the mechanism of mercury removal by activated carbon.

 

2.1 Release and Initial Speciation of Mercury in Coal

Mercury in coal usually exists in trace amounts, with a content generally ranging from 0.01–0.5 mg/kg, hosted in organic matter, pyrite and clay minerals. Under high-temperature furnace combustion conditions (about 1300–1600°C), these mercury species are almost completely volatilized and converted into gaseous mercury. Most of the initially released mercury exists as elemental mercury (Hg⁰), with only a small amount oxidized in the high-temperature zone.

Gaseous Hg⁰ has the following characteristics:Strong chemical inertness; insoluble in water; limited interaction with conventional capture devices in flue gas. Therefore, without special measures, Hg⁰ can easily be discharged into the atmosphere with purified flue gas.

 

2.2 Speciation Transformation of Mercury in Flue Gas Systems

As the flue gas temperature decreases from the furnace outlet and passes through the economizer, air preheater, SCR, dust collector and desulfurization device in sequence, mercury undergoes a series of homogeneous and heterogeneous reactions in different equipment and temperature zones, forming three key forms:

1. Elemental Mercury Hg⁰: Gaseous, electrically neutral, highly volatile, low chemical reactivity; not easily absorbed by wet desulfurization or physically adsorbed on fly ash surfaces; the most difficult to control and the main target species of ACI technology.

2. Oxidized Mercury Hg²⁺: Usually exists as highly soluble salts such as HgCl₂ and HgSO₄; Hg⁰ can be oxidized to Hg²⁺ in the presence of HCl, O₂, NO₂ and catalytic surfaces; easily soluble in FGD slurry or captured on fly ash surfaces, thus partially removed by traditional desulfurization and dust removal devices.

3. Particulate-bound Mercury: Mercury combined with minerals on the surface of fly ash particles or unburned carbon to form solid-phase components; can be captured in high proportion by high-efficiency dust collectors, the easiest mercury form to control.

 

In actual working conditions, the relative proportions of the three forms depend on coal characteristics (especially chlorine, sulfur and alkali metal content), temperature distribution in the furnace and tail flue, combustion and reducing atmosphere, SCR operation status and physical and chemical properties of fly ash. Generally, bituminous coal with high chlorine content in flue gas is more conducive to promoting Hg⁰ oxidation, thereby increasing the Hg²⁺ ratio and enabling the FGD device to undertake more mercury removal functions; when burning low-chlorine coal (such as some lignite and sub-bituminous coal), the Hg⁰ ratio is significantly higher, and such working conditions rely more on special mercury removal technologies (such as ACI).

 

Ⅲ. Basic Principles and Material Properties of Activated Carbon for Mercury Removal

 

Activated carbon is widely used for flue gas mercury removal in coal-fired power plants due to its unique pore structure and easily adjustable surface chemical properties.

 

3.1 Pore Structure and Physical Adsorption

Activated carbon is generally prepared from carbon sources such as coal, biomass, coconut shell, wood or petroleum coke through high-temperature carbonization and activation. The activation process (physical or chemical activation) produces a large number of micropores (<2 nm)**, **mesopores (2–50 nm)** and **macropores (>50 nm) inside the material, forming a highly connected hierarchical pore network, making its specific surface area reach thousands of m²/g.

Physical adsorption mainly relies on van der Waals forces and weak polarization interactions, characterized by:

• The adsorption process is usually reversible;
• Low adsorption heat, close to condensation heat;
• Adsorption capacity is sensitive to temperature, and temperature increase significantly reduces adsorption capacity.

Within the flue gas temperature range of coal-fired power plants (generally 100–200°C), the efficiency of capturing Hg⁰ solely by physical adsorption is often insufficient, especially when the mercury concentration in flue gas is low (usually at the μg/m³ level). At this time, elemental mercury has no advantage in competitive adsorption compared with other gas molecules, and the adsorption equilibrium tends to the desorption side under high temperature conditions. Therefore, although the pore structure is the basis of adsorption, a stronger chemical adsorption mechanism must be introduced to achieve efficient mercury removal.

 

3.2 Surface Functional Groups and Chemical Adsorption

There are a certain number of oxygen-containing functional groups on the surface of activated carbon, such as hydroxyl groups (-OH), carboxyl groups (-COOH), carbonyl groups (C=O), lactone structures, etc. These functional groups can act as weak Lewis acid or base centers to a certain extent and react with some components in flue gas. For mercury, these functional groups on virgin activated carbon can provide limited chemical adsorption sites, causing partial Hg⁰ oxidation or surface complex formation, but this ability is far from meeting strict emission control requirements.

Therefore, surface modification of activated carbon is usually required in engineering applications. By introducing specific elements or chemical species, active centers with higher mercury affinity are created, transforming mercury capture from physical adsorption-dominated to chemical adsorption and surface reaction-dominated.

 

Ⅳ. Modified Activated Carbon: From Physical Adsorption to Reaction Control

 

Activated carbon modification technology can be understood as "constructing a specific functional surface on a carbon skeleton". Among many modification methods, halogen modification, especially bromine modification, has been the most widely studied and applied in the field of coal-fired flue gas mercury removal.

 

4.1 Core Mechanism of Brominated Activated Carbon

Brominated Activated Carbon is usually prepared by introducing bromine in molecular or ionic form into the pores and surface of activated carbon through impregnation, spraying or gas-phase adsorption. Bromine can form C-Br bonds on the carbon surface or exist as bromide salts and adsorbed bromine molecules, forming new surface active sites.

 

Existing studies generally believe that mercury capture on brominated activated carbon generally undergoes the following basic steps:

1. Mass transfer and physical entry of Hg⁰ into poresHg⁰ first diffuses to the surface of activated carbon particles by convection, and then further approaches the active sites through molecular diffusion in the pore structure. This process is mainly controlled by pore structure and flue gas flow state.

2. Heterogeneous oxidation of Hg⁰ by bromine active sitesWhen Hg⁰ reaches the vicinity of bromine-containing sites, heterogeneous oxidation reaction occurs with the participation of bromine species. A simplified expression can be written as:Hg⁰(g) + 2Br(s) → HgBr₂(ads)Where Br represents the active bromine species on the activated carbon surface. Some studies show that this oxidation process may go through intermediate valence states or form HgBr(ads) before further conversion to HgBr₂.

3. Stabilization and strong binding of HgBr₂The generated HgBr₂ can be adsorbed by van der Waals forces inside and on the surface of the pores, and can further form a more stable complex structure with surface functional groups, making it difficult to desorb at a flue gas temperature of 120–180°C, thus achieving efficient mercury fixation.

 

Brominated activated carbon has the following significant advantages over chlorinated activated carbon:

• Bromine is more favorable for the oxidation kinetics of Hg⁰, and the reaction rate is usually higher;
• The generated HgBr₂ has good thermal stability at high temperatures, and is less likely to decompose or reduce than HgCl₂;
• In the presence of competitive gases such as SO₂ and H₂O, the poisoning degree of active bromine sites is relatively light, and the ability to resist working condition fluctuations is stronger.

From engineering practice, for low-chlorine coal combustion conditions, injecting an appropriate dose of brominated activated carbon can often achieve a total mercury removal rate of more than 80–90% before the downstream dust collector, meeting the current emission limit requirements in most regions.

 

4.2 Other Modification Paths: Sulfur, Chlorine and Composite Modification

In addition to bromine modification, several other modification ideas have certain application potential:

1. Sulfur-modified activated carbonBased on the strong affinity between Hg and S (finally forming extremely stable mercury sulfide HgS), researchers strengthen the chemical adsorption capacity by loading sulfur-containing species (such as elemental sulfur, sulfides) on the activated carbon surface. Sulfur-modified activated carbon has a strong capture ability for oxidized mercury and part of Hg⁰, especially at low temperatures (<150°C), but it is highly sensitive to working condition changes, and regeneration and stability problems remain to be solved.

2. Chlorine-modified activated carbonChlorine modification is similar to bromine modification in mechanism, mainly improving mercury removal efficiency by promoting the oxidation of Hg⁰ to HgCl₂. Since flue gas itself often contains a certain concentration of HCl, supplemented by surface reactions such as SCR catalysts, some power plants do not additionally use chlorine-modified activated carbon, but utilize the synergistic effect of existing systems. Compared with bromine, chlorine modification is usually slightly inferior in efficiency and temperature adaptability, but has certain advantages in cost and corrosivity.

3. Multi-component composite modificationIn recent years, some studies have attempted to introduce multiple functional components (such as bromine + sulfur, chlorine + transition metals, etc.) on activated carbon at the same time to achieve synergistic capture of multi-form mercury in a wider temperature range. Such composite modified materials show good application prospects at the laboratory level, but their industrial scale-up, long-term stability and potential impact on downstream equipment still need more engineering verification.

 

Ⅴ. Key Factors Affecting Mercury Removal Efficiency of Activated Carbon

 

Under the same activated carbon material conditions, the mercury removal rate in actual power plant working conditions may still show significant differences, which is rooted in the coupling effect of many factors such as flue gas composition, temperature, injection position and downstream equipment.

 

5.1 Coupling Effect of Flue Gas Components

1. SO₂ and SO₃The concentration of SO₂ is usually in the order of thousands to tens of thousands of mg/m³, much higher than the mercury concentration (μg/m³ level), and has a natural competitive advantage in the adsorption process. Part of SO₂ can be further oxidized to SO₃, which easily forms sulfuric acid mist with H₂O in the low-temperature section or condenses in the activated carbon pores, thereby blocking pores and covering active sites, leading to activated carbon deactivation. Under high SO₃ working conditions, it is often necessary to increase the activated carbon injection rate or adopt sulfur poisoning-resistant modification schemes.

2. NOₓ and its derivativesNO itself has weak oxidizing properties and limited impact on mercury adsorption; NO₂ may participate in the Hg⁰ oxidation process and act as an auxiliary oxidant. At the same time, NO₂ can also interact with surface oxidation sites to change the surface chemical environment. Overall, the impact of NOₓ on ACI mercury removal is less significant than that of SOₓ, but the interaction mechanism under SCR+ACI combined working conditions deserves further in-depth research.

3. HCl and other halidesHCl is one of the key gases promoting the conversion of Hg⁰ to HgCl₂. In flue gas with high chlorine content, even without ACI, partial mercury removal can be achieved only through SCR catalysis and FGD synergy. For low-chlorine coal, it is often necessary to compensate for this deficiency by injecting brominated activated carbon. Some power plants add chlorine-containing combustion improvers before the boiler or add an appropriate amount of HCl to the flue gas as auxiliary measures, but the corrosion risk caused by this needs to be vigilant.

4. Water vapor and oxygenWater vapor competes with adsorption sites to a certain extent, especially weak adsorption sites; but an appropriate amount of H₂O can also participate in some surface oxidation reactions. Oxygen is an important background gas for maintaining mercury oxidation reactions, especially affecting the re-oxidation process involved in multi-step oxidation mechanisms.

 

5.2 Temperature and Residence Time

Temperature is one of the most critical operating parameters in ACI system design and optimization. Practice shows:

When the flue gas temperature is too high (>200°C), the physical adsorption capacity decreases significantly, and some chemically adsorbed mercury may begin to destabilize, leading to desorption;

When the temperature is too low (<100°C), although physical adsorption is enhanced, the risk of SO₃/H₂O condensation increases, and flue gas condensation and corrosion problems worsen, which is not conducive to long-term stable operation.

Therefore, the typical ACI design usually arranges the injection point after the air preheater or economizer, controlling it within a window of about 100–180°C to balance adsorption thermodynamics and equipment corrosion risks.

In addition, the residence time of activated carbon particles in the flue is also one of the key factors determining mercury removal efficiency. Too short residence time will lead to capture by the dust collector before full contact with mercury; appropriately extending the residence time is conducive to the completion of mass transfer and reaction processes between mercury and active sites. By adjusting nozzle arrangement, optimizing flue structure and flow field distribution, the effective residence time can be improved without significantly increasing resistance loss.

 

5.3 Injection Rate, Particle Size and Mixing Uniformity

The activated carbon injection rate (expressed as mgAC/m³ flue gas or C/Hg mass ratio) directly affects the total number of available adsorption sites, and is the most intuitive variable for controlling mercury removal rate. In actual engineering, when burning low-chlorine coal and pursuing a mercury removal rate of about 90%, unmodified ordinary powdered activated carbon often requires thousands to tens of thousands of times the C/Hg mass ratio, resulting in high cost pressure; after using brominated activated carbon, the injection rate can be greatly reduced under the same mercury removal rate.

Activated carbon particle size affects its dispersion and mass transfer characteristics in flue gas. Smaller particle size is conducive to increasing specific surface area and enhancing contact frequency with mercury molecules, but too small particle size may increase flying loss and affect dust collector performance. Generally, the particle size of powdered activated carbon used for ACI is mostly in the range of 10–30 μm.

Mixing uniformity is also an easily overlooked but very critical factor. Even if the activated carbon injection rate is sufficient, if the cross-sectional mixing with flue gas is uneven, there will be local excessive adsorption and local insufficient mercury removal. Optimizing the injection position and nozzle layout through CFD simulation, combined with online concentration monitoring to adjust injection conditions, is an effective means to improve the overall system performance.

 

Ⅵ. Engineering Application, Economy and Market Development of ACI Technology

 

6.1 Current Status of Engineering Application

Since the United States gradually implemented Mercury and Air Toxics Standards (MATS) in the 2000s, ACI technology has taken the lead in large-scale application in the North American power industry, gradually covering various units burning different coal types. Some European countries have also deployed ACI systems on large coal-fired units to meet stricter comprehensive emission requirements. In recent years, with China's ultra-low emission retrofitting of coal-fired units and increased attention to mercury emissions, ACI and other mercury-related technologies have gradually been included in the technical selection vision, especially on units with low-chlorine, low-ash activity coal types, ACI is regarded as one of the feasible technical paths.

ACI systems usually have the following engineering characteristics:

• Relatively simple device structure, generally including storage bins, metering and feeding systems, pneumatic conveying pipelines, injection lances and control systems;
• Good compatibility with the original flue gas treatment system, without large-scale retrofitting of the main engine and flue structure;
• The injection rate can be flexibly adjusted according to mercury emission concentration and regulatory requirements, with certain scalability.

 

6.2 Economic Cost and By-product Disposal Issues

Although ACI technology has obvious advantages in technical maturity, its economy and environmental externality have always been important factors restricting its wide promotion.

1. Activated carbon costAlthough the price of powdered activated carbon itself is low (domestic market), high-performance modified products such as brominated activated carbon have a high unit price. Taking medium and large coal-fired units as an example, to achieve a high mercury removal rate, the annual activated carbon consumption may reach hundreds to thousands of tons, with an annual operating cost ranging from millions of RMB to a higher level. With the compression of profit margins in the energy industry, this continuous operating cost has become a factor that must be carefully weighed in power plant decision-making.

2. Disposal of mercury-containing fly ashThe activated carbon injected into the flue gas will eventually be captured by the dust removal system together with the fly ash, thereby changing the physical and chemical properties of the fly ash. On the one hand, the increase in unburned carbon content and color change may affect its utilization value in the cement and concrete industries; on the other hand, fly ash adsorbed with toxic metals such as mercury is often managed as hazardous waste in accordance with legal and environmental standards, increasing the cost of transportation, disposal and long-term environmental risk control. At present, safe landfill and solidification/stabilization treatment are the main options, but how to improve its resource utilization rate on the premise of ensuring environmental safety is still an unsolved problem.

3. Corrosion and system reliabilityWhile brominated activated carbon exerts high-efficiency mercury removal capacity, there is also a risk that bromides may cause corrosion to downstream FGD systems, flue and chimney materials. Especially in wet desulfurization, the increase of halogen concentration in the liquid phase may adversely affect rubber linings and alloy components, increasing maintenance frequency and spare parts replacement costs. When designing an ACI system, corrosion mechanism, material selection and anti-corrosion strategies must be comprehensively considered.

 

6.3 Market Scale and Development Trends

According to global market research reports in recent years, the activated carbon market for industrial flue gas treatment (including coal-fired power plants, waste incineration, metal smelting, etc.) has reached a scale of billions of US dollars, among which special powdered activated carbon for mercury removal is one of the fast-growing segments. Forecasts show that the segment will maintain positive growth in the next few years, with main driving forces including:

• Tightening mercury emission standards for new and retrofitted coal-fired units in developing countries;
• Stricter regulation of heavy metal control in the waste incineration and hazardous waste incineration industries;
• Application expansion brought by the commercial promotion of new high-performance modified activated carbon products.

 

Ⅶ. Future Technical Directions and Comprehensive Governance Ideas

 

With the further refinement of environmental supervision and the continuous advancement of energy transformation, the "symptomatic treatment" governance model of a single technology is gradually being replaced by a systematic, multi-pollutant synergistic control comprehensive plan. In this context, ACI technology itself also faces the needs of iterative upgrading and functional integration.

 

7.1 New Low-cost and Renewable Adsorbents

From the perspective of resources and costs, adsorption materials prepared from biomass or industrial solid waste (such as crop straw, fruit shells, sludge, fly ash, etc.) are receiving increasing attention. Through appropriate activation and chemical modification, these materials can achieve mercury removal performance close to or even exceeding traditional coal-based activated carbon under some working conditions, while greatly reducing raw material costs and carbon footprint. Furthermore, developing adsorbents that can be recycled and reused on-site in power plants or centralized facilities will help reduce the generation of hazardous solid waste and improve overall environmental sustainability.

 

7.2 Multi-pollutant Synergistic Control Integration

One of the important development directions of future flue gas treatment technology is to deeply couple mercury removal functions with existing denitrification, desulfurization or dust removal systems to build an "all-in-one" synergistic control platform. For example:

• Develop SCR catalysts with dual functions of NOₓ reduction and Hg⁰ oxidation to reduce dependence on independent ACI systems;
• Introduce multi-functional powders with mercury adsorption function in semi-dry desulfurization or dry injection processes to achieve synergistic control of SO₂, HCl and Hg;
• Combine with fly ash classification and collection technology to separate mercury-loaded activated carbon from utilizable fly ash, reducing the impact on building material utilization.

This integrated solution is expected to reduce overall investment and operating costs while reducing system complexity and maintenance burden.

 

7.3 Digitalization and Intelligent Control

With the development of online mercury monitoring technology, more and more power plants have begun to deploy Mercury Continuous Emission Monitoring Systems (HgCEMS) to obtain real-time data on flue gas mercury concentration and speciation changes. Combined with CFD simulation, data-driven modeling and optimization algorithms, dynamic optimal control of activated carbon injection rate, injection position and even modifier formula can be realized, forming a closed-loop regulation system to minimize activated carbon consumption on the premise of ensuring emission compliance.

With the advancement of digital and intelligent transformation of power plants, ACI systems can also be linked with unit load adjustment, fuel switching, desulfurization and denitrification working condition adjustment to achieve more accurate and economical synergistic control.

 

7.4 From Mechanism Simulation to Material Targeted Design

Quantum chemical calculation methods represented by Density Functional Theory (DFT) have gradually become important tools for studying the adsorption and reaction mechanisms of mercury on the surface of different adsorption materials. By systematically investigating the electronic structure interactions between mercury and different elements and surface structures at the theoretical level, it can provide a scientific basis for the "targeted design" of new adsorption materials, reducing the time and resources required for traditional trial-and-error experimental research and development. On the basis of predecessors, more extensive combination of such simulations with high-throughput experiments and machine learning algorithms is expected to promote the research and development of activated carbon and alternative materials into a new "design-driven" stage.

 

7.5 Granular Activated Carbon (GAC) Fixed Bed Mercury Removal Technology

Granular Activated Carbon (GAC) fixed bed technology represents another idea for mercury pollution control in coal-fired power plants. Different from the mainstream ACI technology, the GAC fixed bed is placed downstream of the wet desulfurization (FGD) device, and the flue gas temperature drops to about 50°C, serving as the final purification means to achieve deep mercury removal. This layout strategy has significant advantages: the low-temperature environment is conducive to the synergistic effect of physical adsorption and chemical adsorption, breaking through the bottleneck of limited adsorption capacity of ACI in the high-temperature section (100-180°C); at the same time, the granular form of GAC avoids the problem of mixing with fly ash affecting its resource utilization.

 

 

Based on in-depth research on the mercury adsorption mechanism, the design of GAC fixed bed can fully learn from the modification strategy of brominated activated carbon. Studies show that mercury follows the Langmuir-Hinshelwood oxidation-adsorption mechanism on the activated carbon surface, and finally forms stable compounds with surface halogens in the form of Hg²⁺. Loading pre-brominated GAC in a fixed bed can achieve efficient oxidation and fixation of Hg⁰ at low temperatures, and avoid the competitive adsorption interference of SO₂ – because sulfates have been removed in the FGD system. In addition, the modular structure of the fixed bed facilitates adsorbent replacement and regeneration, providing a feasible path for the engineering application of high-performance materials such as precious metal-modified activated carbon, and promoting mercury control in coal-fired power plants towards the goal of near-zero emission.

Against the realistic background that the global clean energy transformation still takes time, implementing high-level pollution control on existing coal-fired units is still a necessary transitional task. The continuous improvement and innovation of activated carbon mercury removal technology are not only related to the technical choice of a single industry, but also to environmental quality improvement and public health protection, and it will still play an important role in engineering practice in the next few decades.