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Review

Combustion Utilization of High-Chlorine Coal: Current Status and Future Prospects

by
Kang Hong
1,2,
Tuo Zhou
2,
Man Zhang
2,
Yuyang Zeng
3,
Weicheng Li
4,* and
Hairui Yang
1,*
1
School of Electrical Engineering, Xinjiang University, Urumqi 830017, China
2
Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China
3
School of Energy and Power Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
4
Dongfang Boiler Co., Ltd., Dongfang Electric Group, Zigong 643099, China
*
Authors to whom correspondence should be addressed.
Energies 2025, 18(12), 3011; https://doi.org/10.3390/en18123011
Submission received: 11 April 2025 / Revised: 30 May 2025 / Accepted: 4 June 2025 / Published: 6 June 2025
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Abstract

Under China’s “dual carbon” goals (carbon peaking and carbon neutrality), the utilization of high-chlorine coal faces significant challenges due to its abundant reserves in regions such as Xinjiang and its notable environmental impacts. This study systematically investigates the combustion characteristics, environmental risks, and control strategies for high-chlorine coal. Key findings reveal that chlorine release occurs in three distinct stages, namely low-temperature desorption, medium-temperature organic bond cleavage, and high-temperature inorganic decomposition, with release kinetics governed by coal metamorphism and the reaction atmosphere. Chlorine synergistically enhances mercury oxidation through low-activation-energy pathways but exacerbates boiler corrosion via chloride–sulfate interactions. Advanced control technologies—such as water washing, calcium-based sorbents, and integrated pyrolysis–gasification systems—demonstrate substantial emission reductions. However, challenges remain in addressing high-temperature corrosion and optimizing multi-pollutant synergistic control. This study provides critical insights into the clean utilization of high-chlorine coal, supporting sustainable energy transitions.

1. Introduction

Against the backdrop of the global energy transition, coal, as a traditional energy source, continues to hold significant importance. High-chlorine coal, a specific type of coal, has seen its combustion and utilization issues garner increasing attention. With growing international emphasis on environmental protection and sustainable development, the clean and efficient utilization of high-chlorine coal has become a focal research area. This paper systematically examines the combustion characteristics, environmental risks, and control strategies associated with high-chlorine coal, providing valuable insights for its cleaner utilization.
Concurrently, an analysis of publication trends and citation counts for high-chlorine coal-related research from 1999 to 2025 (Figure 1) reveals a continuous upward trajectory in research activity within this field. Notably, since 2010, both the number of publications and citation counts have shown a sharp increase, further underscoring the dynamism and significance of research on high-chlorine coal combustion and utilization.
However, significant challenges remain in the combustion utilization of high-chlorine coal. These include chlorine release-induced environmental pollution, equipment corrosion during combustion, and the need for effective multi-pollutant control. Consequently, conducting in-depth studies on the combustion behavior of high-chlorine coal, developing effective pollution control technologies, and formulating robust anti-corrosion strategies are imperative. Such advancements are crucial for achieving the clean and efficient utilization of high-chlorine coal and promoting sustainable energy development.

2. Global Perspectives on Geochemical Characteristics of Chlorine in Coal

Research on chlorine in coal has emerged as a pivotal topic in global energy science, with international scholars unveiling the characteristics of this “stealth corrodent” from diverse perspectives. UK researchers discovered that chlorine in high-Cl coals (up to 1% content) predominantly exists in ionic form within coal pore water, adsorbed onto organic surfaces through sponge-like water absorption, with this “semi-organic chlorine” constituting over 60% [1]. Polish teams identified a “moisture resurgence” phenomenon during coke production, where chlorine re-adsorbs via wet quenching processes, spiking coke chlorine content by 50%, prompting novel dechlorination technologies like ultrasonic cleaning [2]. Russian scholars proposed the “geological archive” theory, suggesting deep coalbed chlorine originates from ancient brine infiltration—a stratigraphic “pickling” process—often co-occurring with arsenic, lead, and other heavy metals as composite pollutants [3]. U.S. DOE laboratories revealed through real-time molecular beam mass spectrometry that chlorine migrates as HCl and NaCl vapors during coal gasification, an “invisible chemical dagger” accelerating equipment corrosion [4]. Spanish scientists uncovered a geological paradox at the Puertollano coalfield: vitrain exhibits 8× higher Cl content than fusain. This “molecular sieve effect” arises from vitrain’s microporous structure acting as nanoscale traps for brine-derived Cl− [5]. Canadian researchers achieved breakthroughs in fluidized bed combustion, finding that sulfur addition reduces chlorine deposition by 40%, demonstrating sulfur–chlorine antagonistic interactions that inform pollution control strategies [6]. The IEA synthesis report highlights global coal-Cl distribution patterns: coastal > inland coalfields; deep > shallow seams. These “geological fingerprints” enable pollution source tracing [7]. Cutting-edge methodologies like synchrotron X-ray absorption spectroscopy now decode chlorine’s molecular-scale occurrence mechanisms [8], laying theoretical foundations for targeted dechlorination technologies. Table 1 presents a comparative analysis of the main controlling geological factors, dominant occurrence forms of chlorine, and release mechanism characteristics for coal from global representative basins, including regions like Germany/Norway, US Illinois, etc.

3. Combustion Characteristics and Mechanisms of High-Chlorine Coal

3.1. Occurrence Modes of Chlorine in Coal

The occurrence modes of chlorine in coal are primarily categorized into inorganic and organic chlorine, with their specific forms closely correlated with coal-forming environments, coal rank, and geological conditions [9]. Inorganic chlorine mainly comprises chloride ions dissolved in coal pore water (e.g., ionic forms like NaCl and KCl), chlorine-bearing halide minerals (e.g., halite and sylvite), and adsorbed chlorine, typically associated with mineral matter in coal [10]. For instance, NaCl crystals dominate inorganic chlorine in the Zhundong Hongshaquan Coalfield, where water-soluble chlorine accounts for 74.3–82.2%, while inorganic chlorine in the Jining Coalfield exhibits a negative correlation with ash content in low-chlorine coals [11]. Organic chlorine includes Cl− bound to coal macromolecules through ion exchange (e.g., H+ Cl− associated with carboxyl groups), covalently bonded organochlorides (e.g., C-Cl bonds), and metal–organic complexes [12]. Figure 2 presents a schematic diagram of organochlorine transport mechanisms. DFT studies reveal methyl chloride as the most stable organic chlorine form in coal, formed through interactions between groundwater-derived Cl− and organic matter during coalification [13].

3.2. Release Behavior of Chlorine

The release characteristics of chlorine in coal are regulated by multiple factors including temperature, atmosphere, and coal properties [14]. During pyrolysis, inorganic chlorine is primarily released at 300–600 °C, while organic chlorine exhibits multi-stage release behavior: ion-exchangeable Cl preferentially desorbs at 200–400 °C, and covalently bonded Cl decomposes at 500–800 °C [15]. For instance, Australian YRB coal demonstrates dual chlorine release peaks at 210–580 °C, whereas Vietnamese HGI coal shows a single release peak at 580 °C, which is directly correlated with structural differences in organic chlorine compounds [16]. During combustion, inorganic chlorides (e.g., NaCl) generate HCl via gas–solid reactions above 800 °C, achieving release efficiencies up to 89% [17], while organic chlorine conversion to HCl exhibits lower energy barriers (<100 kJ/mol) [18]. Atmospheric conditions significantly influence chlorine migration pathways. Under CO2 atmosphere, organic chlorine release rates increase by 20–30% compared to N2 atmosphere, with enhanced formation of S-Cl complexes like COS [19]. Elevated O2 concentrations accelerate C-Cl bond cleavage, shifting HCl emission peak temperatures forward by 50–100 °C [20]. At elevated pressures (3 bar), chlorine release in circulating fluidized bed (CFB) combustion becomes suppressed, accompanied by 56% and 35% reductions in NOx and SO2 emissions, respectively, likely attributable to modified gas-phase reaction kinetics [21]. Regarding coal properties, high-volatile coals (Vdaf > 30%) exhibit lower chlorine release activation energies (~65 kJ/mol), contrasting with low-volatile coals (Vdaf < 20%) showing elevated energy barriers (85 kJ/mol) [22]. In Xinjiang high-sodium coals, ionic association between Na+ and Cl− results in 80% water-soluble chlorine content, with 60–75% gaseous chlorine release during combustion [23]. Ash composition (e.g., Al2O3/SiO2 ratio) modulates high-temperature (>800 °C) chlorine retention by altering NaCl-mineral reactivity [24].

3.3. Formation Mechanisms of Corrosive Gases

HCl, as the predominant gaseous product of chlorine release from coal, is generated through three principal pathways: organic chlorine pyrolysis involving C-Cl bond cleavage at 200–400 °C to form Cl radicals that combine with hydrogen, a process catalyzed by O/N-containing functional groups in coal [25]; inorganic chlorine conversion where NaCl reacts with H2O/H2 at temperatures exceeding 700 °C to produce HCl with activation energies of 120–150 kJ/mol [26]; and ion exchange mechanisms wherein thermal decomposition of coal carboxylates (R-COO−Na+) releases Na+ ions that combine with Cl− to form gaseous NaCl, which subsequently reacts with SO2 to generate HCl [27]. As shown in Figure 3, the schematic diagram illustrates the formation, bonding, and volatilization mechanisms of hydrogen chloride (HCl) during coal combustion.
Cl2 formation predominantly occurs under oxy-fuel combustion conditions through the Deacon reaction, which reaches equilibrium at 400–600 °C [28]. Experimental studies demonstrate that increasing O2 concentration from 5% to 21% elevates the Cl2/HCl ratio from 0.05 to 0.18 [29]. Additionally, gas-phase decomposition of metal chlorides such as FeCl3 at 300–500 °C contributes to Cl2 release [30].

4. Migration and Transformation Mechanisms and Pollution Control of Chlorine During Coal Combustion

4.1. Transport and Deposition Characteristics of Chloride Salts in Fly Ash

During high-chlorine coal combustion, chlorine is predominantly released as alkali metal chlorides such as NaCl and KCl [31]. Experimental studies demonstrate that the release rate of chlorine accelerates significantly within the temperature range of 550–815 °C, while sodium rapidly volatilizes at 550–700 °C [32]. This volatilization behavior results in a non-monotonic trend in flue gas Cl− concentration, which initially increases and subsequently decreases with rising combustion temperatures [33]. At 1050 °C, calcium silicates become the dominant ash component, facilitating chlorine fixation in solid phases [34]. The formation of ultrafine particulate matter (PM0.4) is closely linked to Cl-Na synergistic effects, with Cl content in PM0.4 from Sha’erhu coal combustion accounting for 28.9% of the total coal chlorine [35].
Chloride migration is strongly influenced by mineral composition. Silicon-based additives reduce ash fusion temperatures, promoting the volatilization of low-melting-point NaCl, whereas aluminum-based additives suppress Cl release by forming high-melting-point aluminosilicates (e.g., NaAlSiO4) [36]. Elevated calcium content in coal facilitates Cl retention via CaCl2 formation in ash residues, a process particularly pronounced under circulating fluidized bed (CFB) combustion conditions [37]. Microstructural analysis of fly ash reveals Cl-Na complexes enriched on fine particle surfaces (<10 μm), exhibiting dense spherical morphologies, while coarse particles (>10 μm) primarily consist of aluminosilicates [38]. Figure 4 presents a micrograph revealing the morphological characteristics of fly ash particles.

4.2. Speciation and Retention Mechanisms of Chlorine in Ash Residues

Chlorine retention in ash residues comprises both inorganic and organic forms. Pyrolysis experiments reveal that inorganic chlorine accounts for 68.9–69.4% in coke treated at 350–450 °C, predominantly as water-soluble NaCl, while organic chlorine increases to 58.4% at 550 °C, likely associated with C-Cl bond formation [39]. Industrial boiler ash exhibits distinct stratified structures: an inner NaCl deposition layer, an intermediate Ca2Al2SiO7-dominated zone, and an outer aluminosilicate layer [40]. This stratification is particularly pronounced in the embedded tubes of circulating fluidized bed (CFB) boilers, reflecting thermal gradients’ regulatory effects on chlorine migration.
Water washing pretreatment removes over 80% of inorganic chlorine but exhibits limited efficacy (<20%) for organic chlorine removal [41]. Co-firing with high-silica-alumina coal (e.g., Yuanbaoshan coal) triggers mineralization reactions between Si/Al oxides and chlorine, forming stable minerals such as CaAl2Si2O8, which reduces ash residue chlorine retention by 40–60% [38]. Under oxy-fuel combustion (O2/CO2), elevated SO3 concentrations promote sulfation of NaCl, yielding Na2SO4 and releasing HCl, thereby significantly decreasing ash chlorine content [42].

4.3. Formation Pathways and Control Strategies of Secondary Pollution

Gaseous HCl constitutes the predominant primary pollutant, with emission concentrations reaching 200–400 mg/m3 during Sha’erhu coal combustion [43]. PM2.5-bound Cl− concentrations exhibit a positive correlation with biomass co-firing ratios, showing a 30% increase when co-firing 40% industrial organic waste [44]. Chlorinated organics (e.g., polychlorinated biphenyls) form through high-temperature (>800 °C) cracking and accumulate on ultrafine particle surfaces via vapor condensation mechanisms [45]. Heavy metal migration studies reveal that chlorine significantly enhances the volatilization of Pb, Cu, and Zn, with Pb volatility increasing from 23.7% to 82.3% when co-firing high-chlorine coal [46].
Control strategies include (1) application of aluminum-based additives to immobilize chlorine through mineral phases like CaAl2Si2O8; (2) integrated pyrolysis (350–450 °C) and water washing pretreatment, achieving >80% soluble chlorine removal; (3) optimized co-firing ratios, where >30% high-Si/Al coal blending reduces chlorine partitioning in fly ash by 50% [47]; and (4) flue gas calcium injection technology, attaining >60% chlorine fixation efficiency within 600–800 °C [48].

5. Advances in Chemical Synergistic Effects Between Chlorine and Mercury in Coal with Mercury Removal Technologies

During coal combustion, mercury is released in three primary forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate-bound mercury (HgP). Hg0, characterized by its high volatility and low water solubility, evades capture by conventional pollution control devices, constituting the dominant atmospheric mercury emission form [49]. Figure 5 illustrates a schematic diagram depicting the speciation transformation of mercury during coal combustion. The neurotoxicity and bioaccumulative nature of mercury pose severe threats to ecosystems, prompting the international Minamata Convention to mandate strict mercury emission controls [50]. As the largest anthropogenic mercury emission source, coal-fired power plants necessitate prioritized research on mercury control technologies with critical practical implications [51].

5.1. Synergistic Mechanism of Chlorine in Mercury Speciation Transformation

5.1.1. Homogeneous Oxidation Reaction

Chlorine species (Cl2, HCl) play pivotal roles in gas-phase oxidation processes. Cl2 dominates Hg0 oxidation at lower temperatures (<450 °C) through the reaction pathway Cl2 → 2Cl·, where Cl· radicals combine with Hg0 to form HgCl2, while HCl promotes Hg0 oxidation at elevated temperatures (>600 °C) via radical chain reactions [53]. Studies demonstrate that a 10 ppm increase in flue gas Cl concentration enhances Hg0 oxidation efficiency by 15–20% [54]. Under oxy-fuel combustion conditions, high CO2 concentrations suppress Cl reactivity, yet HCl remains effective in facilitating Hg0 oxidation [55].

5.1.2. Heterogeneous Catalysis

Chlorine species on fly ash surfaces participate in mercury oxidation via the Eley–Rideal mechanism. Inorganic chlorine (e.g., NaCl) decomposes into active Cl* species on fly ash surfaces, directly reacting with gaseous Hg0 to form HgCl2, while organic C-Cl groups serve as chemisorption sites for Hg0 immobilization [56]. In char derived from the pyrolysis of Turpan–Hami high-chlorine coal (Cl content > 1%), chlorine exists as C-Cl bonds and NaCl, exhibiting a mercury adsorption capacity of 400 μg/g [57]. The synergistic effects between chlorine and transition metals (e.g., Fe3+, Cu2+) enhance surface oxidation activity, with CuCl2-modified activated carbon demonstrating a threefold enhancement in mercury adsorption efficiency compared to unmodified materials [58].

5.2. Synergistic Chlorine-Mediated Mercury Removal Technologies

5.2.1. Adsorbent Modification Techniques

Chlorination modification significantly enhances the performance of carbon-based materials. Impregnation of biomass char with NH4Cl solution introduces C-Cl and C=O functional groups, elevating Hg0 adsorption capacity from 0.3 mg/g to 2.1 mg/g [59]. Non-thermal plasma treatment of multi-walled carbon nanotubes in HCl/N2 atmosphere generates Cl· radicals, increasing surface C–Cl content fivefold and improving Hg0 removal efficiency from 20% to 98% [60]. FeCl3- and CuCl2-modified activated carbons achieve chemisorption via Hg-Cl coordination bonds, with CuCl2 exhibiting superior Hg adsorption energy (−1.78 eV) compared to FeCl3 (−1.12 eV) [61].

5.2.2. Synergistic Abatement via Existing Pollution Control Devices

Selective catalytic reduction (SCR) systems enhance Hg0 oxidation, with V2O5-WO3/TiO2 catalysts increasing Hg0 oxidation efficiency from 15% to 79% in the presence of chlorine species [62]. Electrostatic precipitators (ESPs) capture particulate-bound mercury (HgP) through fly ash adsorption, demonstrating a 65% average removal efficiency positively correlated with unburned carbon content in fly ash [62]. Wet Flue Gas Desulfurization (WFGD) achieves 72% capture efficiency for Hg2+, though pH control (>5) is critical to preventing Hg0 re-emission [63]. Actual measurements from an ultra-low emission power plant reveal an 89% total mercury removal efficiency using a combined SCR-ESP-WFGD-Wet Electrostatic Precipitator (WESP) process [64].

5.2.3. Advanced Combustion Modulation Technologies

Biomass–coal co-firing enhances mercury control mechanisms. When corn stover (Cl content 0.8%) is blended at a 20% co-firing ratio, flue gas Cl concentration increases by 50%, elevating Hg0 oxidation efficiency by 35% [65]. Under oxy-fuel combustion (O2/CO2), elevated CO2 concentrations suppress homogeneous Hg0 oxidation, but enhance heterogeneous Cl-mediated oxidation on fly ash surfaces, increasing HgP generation by 20% [66]. During circulating fluidized bed (CFB) combustion of high-chlorine coal (Cl > 1%), low-temperature operation (500–600 °C) facilitates Cl conversion to HCl, achieving a 99% HgCl2 formation rate [67]. Figure 6 summarizes various methods for high-chlorine coal utilization across different stages (pre-combustion, combustion optimization, post-combustion, and corrosion mitigation), detailing their efficiencies and limitations.

6. Formation Mechanisms and Emission Control of Dioxins During Combustion Processes

6.1. Mechanistic Role of Chlorine in Dioxin Formation

Chlorine in coal and solid waste serves as a critical factor in polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/Fs) formation [68]. Chlorine speciation (inorganic chlorides like NaCl or organochlorines like PVC) significantly influences dioxin generation pathways: inorganic chlorides are converted to more reactive Cl2 via the Deacon reaction, promoting chlorination of aromatic hydrocarbons. Experimental studies demonstrate that when fuel chlorine content exceeds the 0.8–1.1% threshold, polychlorinated dibenzofurans (PCDFs) form at rates significantly higher than polychlorinated dibenzo-p-dioxins (PCDDs), likely due to enhanced chlorination of incomplete combustion products (e.g., polycyclic aromatic hydrocarbons, PAHs) [69]. Organochlorines (e.g., PVC) directly decompose into HCl and Cl2, exhibiting 2–3 times higher chlorination efficiency compared to inorganic chlorides [70]. Notably, sulfur presence suppresses the Deacon reaction through competitive inhibition, reducing Cl2 generation and consequently lowering dioxin emissions [71].
A nonlinear relationship exists between coal chlorine content and dioxin emissions. Increasing coal chlorine content from 0.07% to 0.31% elevates flue gas dioxin toxicity equivalents (I-TEQ) by three orders of magnitude [9]. Laboratory studies reveal that adding NaCl to low-chlorine coal (0.074% Cl) to reach 0.38% Cl increases dioxin emissions by two orders of magnitude [72]. This nonlinear behavior may relate to chlorine release kinetics and intermediate product formation pathways during combustion [73].

6.2. Impact of Combustion Conditions on Dioxin Formation

Combustion atmospheres critically regulate dioxin formation mechanisms. Oxygen-enriched environments in conventional air incineration promote the Deacon reaction and carbon matrix oxidation [74]. In contrast, chemical looping combustion (CLC) creates anoxic conditions, reducing Cl2 generation by 94% and total dioxin emissions by 89% [75]. Under reducing atmospheres in gasification processes, H2 inhibits chlorination by attacking C-Cl bonds, lowering dioxin formation by 80% compared to incineration [76]. Temperature gradient experiments reveal 500 °C as the peak dioxin formation temperature, with generation rates exhibiting exponential growth below 700 °C [77].
Combustion technology types significantly influence dioxin congener distributions. Fluidized bed combustion (800–830 °C) yields higher PCDF/PCDD ratios (1.76) than grate-fired systems (0.73), likely due to divergent carbon matrix oxidation pathways [78]. Pulverized coal boilers (>1000 °C) effectively suppress high-temperature homogeneous reactions via the “3T + E” principle (Temperature, Time, Turbulence, and Excess air), with dioxins primarily originating from low-temperature (200–400 °C) heterogeneous catalytic processes [79]. In circulating fluidized bed (CFB) boilers, PCDFs dominate in fly ash (76.5–93.1%), while PCDDs prevail in flue gas (86.6–94.2%), reflecting temperature gradient-driven phase partitioning [80].

6.3. Characterization and Congener-Specific Analysis of Dioxin Emissions

Emission concentrations exhibit significant process dependency. Hazardous waste incinerators (HWIs) demonstrate higher flue gas I-TEQ concentrations (0.04–0.18 ng/Nm3) compared to municipal solid waste incinerators (MSWIs, 0.01–0.11 ng/Nm3), likely due to enhanced chlorination from complex waste components [81]. During solid waste co-processing in coal-fired boilers, dioxin emission factors (0.015–0.129ng I-TEQ/kg) show no significant increase relative to pure coal combustion (0.009–0.327ng I-TEQ/kg) [82].
Congener profiles serve as source identifiers. Medical waste incineration sources are dominated by highly chlorinated OCDD (24.5–38.2%) and 1,2,3,4,6,7,8-HpCDF (18.7–29.1%) [83]. Coal combustion sources exhibit higher PCDD contributions (50.1–60.4%) than PCDFs, with O8CDD and H7CDD being prominent [84]. In fluidized bed systems, 2,3,7,8-T4CDF and 1,2,3,7,8-P5CDF account for 46.6–92.9% of electrostatic precipitator ash, highlighting low-temperature heterogeneous synthesis pathways [85].
Phase partitioning is governed by chlorination degree. Low-chlorinated congeners (e.g., 2,3,7,8-T4CDF) predominantly reside in the gas phase (72.5%) due to higher vapor pressure, while highly chlorinated congeners (e.g., O8CDD) adsorb onto fly ash particles [86]. Activated carbon injection (ACI) achieves higher gas-phase dioxin removal efficiency (97.2%) than particulate-phase removal (94.6%), but shows weaker adsorption capacity for highly chlorinated congeners [87].

6.4. Advances in Dioxin Abatement Technologies: Current Research Progress

In combustion optimization, chemical looping combustion (CLC) achieves enhanced dechlorination efficiency and suppression of the Deacon reaction through synergistic effects between Fe2O3/Al2O3 oxygen carriers and CaO, resulting in a 94% reduction in total dioxin emissions [88]. Selective catalytic reduction (SCR) degrades 30–50% of gas-phase dioxins within the 180–230 °C temperature range, though catalyst aging may induce polychlorinated dibenzofuran (PCDF) regeneration [89].
Among flue gas purification technologies, fabric filters coupled with activated carbon injection (FF + ACI) demonstrate 97.2% total I-TEQ removal efficiency, yet exhibit lower efficacy for low-chlorinated congeners such as 2,3,7,8-TCDD [90]. Wet scrubbing may elevate emission concentrations by a factor of 1.95 due to the “memory effect,” primarily promoting re-emission of low-chlorinated congeners [91]. Novel control strategies like thiourea-based inhibitors effectively block copper-catalyzed pathways, reducing dioxin formation by 65–86% [92]. Figure 7 presents a schematic diagram of dioxin accumulation pathways in the human body.

7. Mechanisms of Chlorine-Induced Corrosion in Coal Combustion Systems

7.1. Incipient Stage of Chlorine-Induced Corrosion: Degradation Mechanisms of Oxide Layers

Chlorine directly attacks metal oxide layers through gaseous penetration or liquid-phase deposition. Under oxidizing atmospheres, HCl reacts with metal oxides (e.g., Fe2O3, Cr2O3) to generate metal chlorides (e.g., FeCl2, CrCl2) and Cl2 gas. The resulting metal chlorides exhibit high vapor pressures, volatilizing at elevated temperatures and diffusing to the oxide/metal interface, where they re-oxidize into Cl2, establishing a cyclic corrosion process [93]. This mechanism, termed” active oxidation”, represents the central pathway of chlorine-induced corrosion [94].

7.2. Synergistic Interactions Between Chlorine and Sulfur

Chlorine–sulfur compound interactions (e.g., SO2, SO3) exhibit synergistic effects in corrosive processes. SO2 oxidizes to SO3 at elevated temperatures, reacting with alkali metal chlorides to form sulfates (e.g., Na2SO4, K2SO4). These sulfates possess lower melting points than their parent chlorides (e.g., Na2SO4: 884 °C, K2SO4: 1069 °C), generating low-melting-point eutectic mixtures (e.g., NaCl-K2SO4 with a eutectic point of 690 °C). This phase behavior significantly reduces deposit melting temperatures and accelerates molten salt corrosion [95]. For instance, eutectic reactions between KCl and FeCl2 enhance oxide layer permeability and enable aggressive melt penetration [96].

7.3. Direct Chloride Attack on Metallic Materials

The formation and diffusion of metal chlorides serve as key governing factors in corrosion kinetics. For instance, FeCl2 exhibits high vapor pressure (1576 Pa at 760 °C), enabling volatilization and pore formation within oxide layers, ultimately leading to oxide layer spallation [97]. The Cr2O3 protective layer preferentially oxidizes into volatile CrO3 under Cl− attack, resulting in chromium depletion [98]. Cl− diffusion along grain boundaries induces stress corrosion cracking, particularly accelerating Cr-depleted zone formation in sulfur-containing environments [99].

7.4. Impact of Physicochemical Properties of Deposits

The composition and microstructure of deposits directly dictate corrosion behavior. For instance, NaCl-K2SO4 (melting point: 690 °C) and KCl-FeCl2 (melting point: 355 °C) form continuous liquid films that facilitate Cl− migration toward the metal substrate [100]. Porous deposits (e.g., biomass ash) enhance Cl− diffusion pathways, whereas dense deposits (e.g., high-silicate compositions) inhibit corrosive attack [6,10]. Ion-exchange processes involving K+ and Na+ compromise oxide layer integrity by disrupting their crystalline structure [101]. Figure 8 presents a schematic diagram of the microstructure.

7.5. Regulatory Effects of Temperature and Atmosphere

At low temperatures (<500 °C), Cl− exists as adsorbed species that primarily compromise passive films through chemisorption [103]. During medium-temperature stages (500–700 °C), volatilization of KCl and NaCl dominates, inducing gaseous corrosion [104]. Under high-temperature conditions (>700 °C), direct reactions between Cl2 and metals accelerate oxidation. Oxygen-deficient environments (e.g., reducing atmospheres) suppress Cr2O3 formation, exacerbating Cl− penetration [102]

7.6. Protection Strategies and Material Innovations

Thermal-sprayed NiCr and FeCr alloy coatings demonstrate enhanced resistance to Cl− permeation in substrates, though porosity and bonding strength require further optimization [105]. Incorporating additives such as Al2O3 and SiO2 effectively immobilizes Cl−, mitigating oxidative layer degradation [106]. Pretreatment processes to reduce fuel Cl/S content and co-firing with low-chlorine fuels (e.g., biomass–coal blending) significantly mitigate corrosion risks [107].

8. Ecological Impacts of High-Chlorine Coal Combustion

8.1. Exacerbation of Atmospheric Fine Particulate Matter (PM2.5) Pollution

Coal combustion releases hydrogen chloride (HCl) and particulate-bound chlorine (pCl), which significantly increase PM2.5 concentrations. Studies have shown that approximately 5.5–8.3% of chlorine in PM2.5 during winter in China originates from inorganic chlorine emitted by coal combustion. These particles reduce air visibility and pose respiratory health hazards [108].

8.2. Enhancement of Ground-Level Ozone Formation

Gaseous chlorine species such as Cl2 and ClNO2 emitted from coal combustion decompose under solar irradiation to generate chlorine radicals. These highly reactive species oxidize volatile organic compounds (VOCs) and methane, thereby accelerating ozone formation. Modeling studies indicate that wintertime ozone concentrations in certain regions of China have thus increased by 55% due to this mechanism [109].

8.3. Emission of Ozone-Depleting Substances (ODSs)

Recent research has revealed that household coal combustion emits controlled substances, including chlorofluorocarbons (CFC-11) and hydrochlorofluorocarbons (HCFC-22). Although their emission levels are lower than industrial sources, their ubiquitous distribution may impede ozone layer recovery. Experimental measurements detected ozone-depleting substance (ODS) concentrations as high as 270 ppb in coal combustion flue gas [110].

8.4. Enhancement of Mercury Pollution Toxicity

Chlorine radicals in coal combustion flue gas oxidize gaseous elemental mercury (Hg0) into depositable divalent mercury (Hg2+), elevating mercury bioaccumulation risks. This mechanism has enhanced atmospheric mercury conversion efficiency by 48% during winter in North China [111].

8.5. Modulation of Atmospheric Oxidizing Capacity

Synergistic interactions between chlorine radicals (Cl·) and hydroxyl radicals (OH·) significantly alter atmospheric oxidation pathways. Observational studies reveal that halogen chemistry enhances total oxidant concentrations by 26–73% in wintertime atmospheres over North China, accelerating the formation of secondary pollutants [112].

9. Conclusions

The clean utilization of high-chlorine coal is a critical link in achieving the “dual carbon” goals (carbon peaking and carbon neutrality). This paper systematically reviews the research progress on chlorine occurrence forms, release kinetics, pollutant formation mechanisms, and control technologies. Despite significant achievements in current technologies, the following core problems and challenges persist.

9.1. Existing Problems and Challenges

1.
Bottlenecks in Corrosion Control
Chlorine–sulfur–alkali metal interactions induce molten eutectic corrosion (e.g., KCl-FeCl2 eutectic point at 355 °C). Existing corrosion-resistant materials (e.g., NiCr coatings) exhibit insufficient service life under complex atmospheres. High-temperature corrosion prediction models lack industrial-scale validation, particularly due to an inadequate understanding of the coupled chlorine migration–deposition mechanism in O2/CO2 atmospheres.
2.
Limitations of Pollution Control Technologies
Dechlorination adsorbents face challenges of high-temperature sintering (energy consumption ~4.2 kWh/kg at 1100 °C) and chlorine poisoning (Cl− reduces the half-life of V2O5-WO3/TiO2 catalysts by 40%). End-product environmental risks are prominent: mercury leaching concentrations from desulfurization gypsum (0.0035–0.0711 mg/L) exceed standards, and the transformation mechanism of persistent organic chlorides in fly ash remains unclear.
3.
Difficulties in Multi-Pollutant Synergistic Control
SO2 inhibits chlorine activity, reducing mercury oxidation efficiency. Existing technologies struggle to meet the ultra-low emission requirements of BECCS (bioenergy with carbon capture and storage) systems (Hg < 0.01 μg/m3, dioxins < 0.1 ng-TEQ/Nm3).
4.
Economic and Scaling Barriers
Nanoadsorbents are costly (>150 USD/kg), while calcium-based modifiers are prone to agglomeration and deactivation. Water washing pretreatment leaves residual chlorine >0.06%, and sintering achieves 99.14% dechlorination efficiency but entails unsustainable energy consumption.

9.2. Future Perspectives

1.
Mechanism Research and Model Development
Deepen the understanding of the multi-field (Cl-S-Al) coupling mechanism. Employ DFT calculations and molecular dynamics simulations to reveal heterogeneous corrosion interfacial reaction pathways. Establish dynamic prediction models linking chlorine speciation, release, and pollution risk. Integrate machine learning to optimize multi-objective control parameters.
2.
Development of Novel Materials and Technologies
Design multifunctional core–shell materials (e.g., CaO\CeO2, Fe3O4\CuCl2) to enhance chlorine poisoning resistance via surface oxygen vacancies and enable magnetic recovery of adsorbents (>90% capacity retention after five cycles). Develop low-temperature organic chloride degradation catalysts (target temperature < 400 °C) and porous calcium aluminate-based chlorine fixation agents.
3.
System Integration and Intelligent Control
Construct an integrated “staged pyrolysis–combustion–purification” system: pretreatment (washing/low-temperature pyrolysis) + in-combustion adsorption (modified calcium-based) + end-pipe synergistic control (SCR-ACI-FF). Couple Hg-CEMS online monitoring with an intelligent regulation platform to achieve synergistic optimization of dechlorination, desulfurization, denitrification, and dedusting.
4.
Expansion of Green and Low-Carbon Pathways
Explore green technologies like microbial dechlorination and promote the resource utilization of dechlorination products. Leverage the Cl–alkali metal synergistic effect in BECCS systems via biomass co-combustion to achieve co-reduction of carbon and mercury (targets: Hg capture rate > 95%, carbon capture rate > 90%).
Efficient and clean utilization of high-chlorine coal requires overcoming the triple challenges of “corrosion control–synergistic pollution reduction–economic sustainability”. Future research should focus on multi-scale mechanistic innovation (from molecular simulation to industrial validation), development of disruptive materials, and intelligent system integration. This will provide the foundation for establishing a technological system characterized by “low-chlorine combustion–near-zero emissions–product resource utilization,” thereby supporting the green transition of the energy system.

Author Contributions

K.H.: conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, visualization. H.Y.: conceptualization, methodology, validation, formal analysis, investigation, resources, data curation, writing—review and editing, supervision, project administration, funding acquisition. Y.Z.: methodology, formal analysis, visualization. M.Z.: methodology, validation, resources, funding acquisition. T.Z.: validation, formal analysis, resources, funding acquisition. W.L.: investigation, project administration. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Key Research Plan (2023YFB4104301), Huaneng Group science and technology research project (HNKJ23-H71), and Zhongyuan Electric Laboratory (No. zn20250401).

Conflicts of Interest

Weicheng Li was employed by the Dongfang Boiler Co., Ltd., Dongfang Electric Group. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. Trends in publications and citations of high-chlorine coal-related papers from 1999 to 2025.
Figure 1. Trends in publications and citations of high-chlorine coal-related papers from 1999 to 2025.
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Figure 2. The migration routine of organic-Cl in coal [12].
Figure 2. The migration routine of organic-Cl in coal [12].
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Figure 3. Schematic diagram of the bonding and volatilization mechanism [12].
Figure 3. Schematic diagram of the bonding and volatilization mechanism [12].
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Figure 4. Surface micromorphology of coal ash with different SEH contents: (a) 800 °C, (b) 1000 °C, (c) 1200 °C.
Figure 4. Surface micromorphology of coal ash with different SEH contents: (a) 800 °C, (b) 1000 °C, (c) 1200 °C.
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Figure 5. Schematic of mercury speciation transformation [52].
Figure 5. Schematic of mercury speciation transformation [52].
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Figure 6. Integrated strategies for mercury chloride control and corrosion mitigation in combustion processes.
Figure 6. Integrated strategies for mercury chloride control and corrosion mitigation in combustion processes.
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Figure 7. Schematic of dioxin fate pathways.
Figure 7. Schematic of dioxin fate pathways.
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Figure 8. A schematic diagram of the microstructure [102].
Figure 8. A schematic diagram of the microstructure [102].
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Table 1. Comparative analysis of main controlling geological factors and release mechanism interrelationships for chlorine occurrence in coal from global representative basins.
Table 1. Comparative analysis of main controlling geological factors and release mechanism interrelationships for chlorine occurrence in coal from global representative basins.
Country/RegionMain Controlling
Geological Factors		Dominant Occurrence FormsRelease Mechanism
Characteristics
Germany/NorwayMarine sedimentation, volcanic ash intrusionOrganic-bound state + volcanic glass adsorptionHigh-temperature volatilization-dominated
US, IllinoisTransgression–regression cycles, saline lacustrine environmentInorganic chlorides + organic complexationSignificant water-driven desorption
Spain, PuertollanoVolcanic ash (tonstein) inputClay mineral adsorption + microporous retentionIon exchange-mediated release
Australia, SuratResidual basin brine, microbial activityFree water phase + organic matter-boundPressure-driven seepage release
Poland Hard CoalTerrigenous clastic input, weathering–leachingClay mineral adsorption + pore water occurrenceSignificant particle size effect
UK CoalfieldMarine sedimentation, deep hydrothermal activityPore water occurrence + humic acid complexationMetamorphic degree-controlled release modulation
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Hong, K.; Zhou, T.; Zhang, M.; Zeng, Y.; Li, W.; Yang, H. Combustion Utilization of High-Chlorine Coal: Current Status and Future Prospects. Energies 2025, 18, 3011. https://doi.org/10.3390/en18123011

AMA Style

Hong K, Zhou T, Zhang M, Zeng Y, Li W, Yang H. Combustion Utilization of High-Chlorine Coal: Current Status and Future Prospects. Energies. 2025; 18(12):3011. https://doi.org/10.3390/en18123011

Chicago/Turabian Style

Hong, Kang, Tuo Zhou, Man Zhang, Yuyang Zeng, Weicheng Li, and Hairui Yang. 2025. "Combustion Utilization of High-Chlorine Coal: Current Status and Future Prospects" Energies 18, no. 12: 3011. https://doi.org/10.3390/en18123011

APA Style

Hong, K., Zhou, T., Zhang, M., Zeng, Y., Li, W., & Yang, H. (2025). Combustion Utilization of High-Chlorine Coal: Current Status and Future Prospects. Energies, 18(12), 3011. https://doi.org/10.3390/en18123011

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