Temporal measurements and kinetics of selenium release during coal combustion and gasification in a fluidized bed

Retention of trace elements in coal-fired flue gas by a novel heterogeneous agglomeration technology

Heterogeneous agglomeration (HA) is a very potential technology for coal-fired flue gas treatment. In this paper, the distribution and migration mechanisms of trace elements (TEs) such as Se, As and Pb in CFPPs were studied on a 30,000 m³/hr pilot-scale experimental platform. The influences of HA on the removal efficiency of gaseous and particulate TEs were well analyzed. The results showed that Se, As and Pb were enriched in fly ash, and their sensitivity to particle size is quite different. The content of Se was the highest in PM₁, reaching 193.04 mg/kg at the electrostatic precipitator (ESP) outlet. The average particle size of the total dust before ESP increased significantly from 21.686 to 62.612 µm after injecting the heterogeneous agglomeration adsorbent, conducive to its further removal by ESP. In addition, the concentrations of gaseous Se, As and Pb in the flue gas decreased after adsorbent spray, and accordingly, their contents in the hierarchical particles increased, indicating that the adsorbent could effectively promote the adsorption of gaseous trace elements in fly ash and reduce the possibility of their escape to the atmosphere. Total concentrations of Se, As and Pb emitted by wet flue gas desulfurization (WFGD) are 0.223, 0.668 and 0.076 µg/m³, which decreased by 59.98%, 47.69% and 90.71%, respectively. Finally, a possible HA mechanism model was proposed, where chemical adsorption, physical condensation and collision agglomeration of gaseous TEs and fine particles with adsorbent droplets occurred to form larger agglomerates.

Chemical association of copper and selenium in coals of Sindh by time saving single step strategy and their impact on groundwater

The aim of the present study is to estimate the different chemical fractionations of copper (Cu) and selenium (Se) in coal samples of different coal mining areas. The Cu and Se bound to various chemical fractions of coal collected from two mining fields of Sindh, Pakistan, have been determined by BCR sequential extraction scheme (BCR-SES). The long duration of the BCR sequential scheme (51 h) was reduced by a time-saving shaking device (ultrasonic bath) termed as ultrasonic-assisted extraction (USE) depending on the same operating conditions and extracting solutions used for BCR sequential extraction scheme. The both trace elements were determined in aquifer water, sampled from different depth of both coal mining fields. In addition, the groundwater of dug well in the vicinity of coal mining areas were also analyzed for Cu and Se using reported extraction methodologies. The partitioning of Cu and Se bound with different chemical fractions of coal was successfully made by proposed USE, within 2 h as compared to long duration of BCR-SES (51 h). The Cu and Se concentrations in acid-soluble fractions of coal samples were > 10%, enhanced by USE extraction procedure than those values gained via BCR-SES (p < 0.01). About 67 to 69% of Cu were found in the first three fractions, whereas their remaining amount corresponding to 31 to 33%, respectively bound with crystalline/residual fraction, while up to 66.1 to 71.1% of total Se contents extracted in three extractable phases, followed up to 28.9 to 33.8% of it was bound with residual phase. The concentrations of Cu and Se in groundwater of different aquifers were found in decreasing order as AQ1 > AQII > AQIII; the same trend was observed for two aquifers of Lakhra coal mining, whereas the groundwater samples have two to three folds higher levels of Se than WHO limit. The Cu levels in water samples were significantly lower than the recommended limit of WHO for drinking water (p < 0.01).

Fate and emission behavior of heavy metals during hazardous chemical waste incineration

The fate and emission behavior of heavy metals (As, Cd, Co, Cr, Cu, Ni, Pb, Se, and Zn) from a hazardous chemical waste incinerator were systematically explored. The results show that the main components of incineration fly ashes and slags contain minerals such as salt, plagioclase, pyroxene, gypsum, calcite, and slaked lime. The elements As, Cd, Pb, and Se are enriched in the fly ash particles during flue gas condensation. Co and Ni are more likely to be deposited in the rotary kiln slag and cooling tower slag owing to their lower volatility. Zn, Cr, and Cu are usually volatilized into the flue gas as oxides or chlorides are condensed and enriched in the slag of the cooling tower during the flue gas cooling process. The content of As, Cd, Pb, Ni, Cr, and Se increase with decreasing fly ash particle size. After the flue gas purification equipment was employed, the concentration of particulate metals significantly reduced. In the exhaust flue gas, the concentrations of Cu and Zn are 29.85 and 28.47 μg/m³, those of As, Cr, Ni, Pb, and Se range from 2.54 to 9.25 μg/m³, and those of Co and Cd are 0.42 and 0.13 μg/m³, respectively.

Fine particulate-bound arsenic and selenium from coal-fired power plants: Formation, removal and bioaccessibility

The arsenic (As) and selenium (Se) in fine particulate matter (PM₁₀) have attracted increasing attentions due to their health effects. However, the emission control of fine particulate-bound arsenic and selenium (fine particulate-bound As/Se) from coal-fired power plants still faces various challenges. Understanding the formation and characteristics of fine particulate-bound As/Se is crucial for developing specific control technologies. This study clarifies the formation mechanism, removal characteristics, and inhalation bioaccessibility of fine particulate-bound As/Se from industrial coal-fired power plants through methods including aerosol generation, As/Se speciation determination, and in vitro bioaccessibility testing. The findings demonstrated that PM₁ from pulverized coal-fired (PC) boilers was enriched with As/Se in terms of concentration and mass distribution. Instead, As/Se was mainly distributed in PM_2.5-10 from circulating fluidized bed (CFB) boilers. Limestone injection in CFB boilers promoted As/Se enrichment in coarse PM. Fine particulate-bound As was mainly formed by chemical adsorption of As vapors by Ca-minerals, while the formation of fine particulate-bound Se was closely related to active Ca-minerals and Fe-minerals. Furthermore, Ca-bound As was easy to remove by electrostatic precipitator (ESP) and the removal of physically adsorbed SeO₂(s) was difficult, which was caused by the specific resistivity of different mineral components. Importantly, finer particulate-bound As/Se posed higher inhalation bioaccessibility, following the order of PM₁ ≥ PM_1-2.5 > PM_2.5-10. In particular, Ca-bound Se in fine PM owned high bioaccessibility. Based on these findings, measures were proposed to suppress the formation of fine particulate-bound As/Se in the furnace and/or strengthen its removal in the post-combustion stage.

Behavior of Selenium during Chemical-Looping Gasification of Coal Using Copper-Based Oxygen Carrier

The migration and transformation behavior of selenium during coal chemical looping gasification (CLG) under the impact of a CuO/Bentonite (Ben) oxygen carrier (OC) were studied in a batch fluidized bed reactor. In the CLG process, the total percentage of selenium released in gaseous phase was 73.06%. In the conventional gasification process, 91.71% of the total selenium was released in a gaseous state. The addition of CuO/Ben OC apparently promoted the transformation from gaseous selenium to particulate selenium. The oxygen–carbon ratio (O/C) played an important role in affecting the fraction of gaseous selenium released in the gasification process, with results showing that the amount of selenium adsorbed by CuO/Ben OC was added along with the increase in OC. By means of X-ray photoelectron spectroscopy (XPS) characterization, we found that the reduced CuO/Ben OC contained a small amount of Cu2Se due to the oxidation and adsorption of selenium onto their porous surface. The regeneration performance of the CuO/Ben OC was favorable after 10 regeneration cycles of the CLG process. The increase in the pore volumes and specific surface areas contributed to the enhanced capacity of retaining selenium for CuO/Ben OC.

Condensation and adsorption characteristics of gaseous selenium on coal-fired fly ash at low temperatures

Gaseous selenium is of high saturated vapor pressure, making its retention in solid phases quite difficult during coal combustion. The selenium transformation from gaseous form into solid phases at low temperatures can be essential to control selenium emission. To understand the migration of SeO₂ (g) on ash particles in the low-temperature zone, this study investigated the speciation of selenium in fly ash and simulated the physical retention of SeO₂ (g) on fly ash. The results demonstrated that there was a large proportion of physically-bound Se in the fly ash of pulverized-coal-fired boiler (22.62 %-58.03%), while the content of physically-bound Se in fly ash of circulated fluidized-bed boiler was lower (∼6%). The physically-bound Se was formed through selenium condensation and physical adsorption. The decrease of temperature or the increase of cooling rate could promote the transformation of gaseous selenium to solid phase and the presence of HCl might suppress SeO₂ transformation into Se in the condensation process. Meanwhile the compositions of fly ash had a great influence on the selenium adsorption process. Among typical coal-fired ash components, mullite showed the best performance in the selenium capture in the temperature range of 90-200 °C, contributing to the high content of physically-adsorbed selenium in PC fly ash. These findings provided new ideas for improving the removal rate of volatile selenium.

On-line detection and kinetic study of selenium release during combustion, gasification and pyrolysis of sawdust

An on-line analysis system was firstly developed to quantitatively measure the temporal concentrations of selenium in the flue gas directly. Then the selenium release during air combustion, CO₂/argon gasification, and argon pyrolysis of sawdust was systematically studied using the on-line analysis system, based on the inductively coupled plasma optical emission spectroscopy. The peak of selenium concentration in the flue gas ranges from 0.38 to 1.76 mg∙Nm^-3 with change of reaction temperature and atmosphere. The overall activation energy for selenium release is 75.3 kJ∙mol^-1 in air combustion, 102.4 kJ∙mol^-1 in CO₂/argon gasification, and 81.9 kJ∙mol^-1 in argon pyrolysis, respectively. The results show that the combustion atmosphere contributes to the selenium release more than that in gasification and pyrolysis. The promotion effect of chlorine on selenium release under combustion environment was one to three times higher than that under gasification and pyrolysis atmosphere. Thermodynamic equilibrium calculation showed that selenium oxides were the main gaseous selenium species in combustion, while the dominant gaseous selenium species were H₂Se (g) and Se (g) under gasification/pyrolysis condition. The selenium release was increased with different degrees by additive chlorine species, mainly because of the formation of SeCl₂ (g). The role of chlorine in selenium transformation has been provided in the proposed reaction pathways of selenium release, based on the new findings using on-line analysis system. The selenium species retained in sawdust can be transformed into selenium oxide (SeO₂, SeO, corresponding to the combustion condition) and selenium hydride (H₂Se, corresponding to the gasification/pyrolysis conditions).

Study on arsenic, selenium, and lead produced in coal combustion: bibliometric method

The literature on trace element pollutants (arsenic, selenium, lead) produced during coal burning from 2007 to 2020 was summarized by the bibliometric method, and the characteristics of published articles and research trends were analyzed. Taking 2007 as the starting point for statistics on articles in this research direction, there was a process of rapid growth in the total number of published articles by 2015, and it was increased over time. In the last 5 years of statistics, it is found that the number of articles published in China is the largest, accounting for almost half of the total. Most of the articles are published in the fields of energy, environmental protection, etc. Among them, the research on arsenic, selenium, and lead is mainly related to the use of adsorbents. At the same time, the effects of temperature, catalyst, material, and other conditions on the removal efficiency of arsenic, selenium, and lead in coal were considered. Application of photocatalysis, preparation of new adsorption materials, and mining of the properties of existing materials under different experimental conditions are a good development prospect.

Migration and emission behavior of arsenic and selenium in a circulating fluidized bed power plant burning arsenic/selenium-enriched coal

Arsenic (As) and selenium (Se) pollution caused by coal combustion is receiving increasing concerns. The environmental impacts of As/Se are determined not only by stack emission but also by leaching process from combustion byproducts. For a better control of As/Se emission from As/Se-enriched coal combustion, this study investigated the migration and emission behavior of As/Se in a circulating fluidized bed (CFB) power plant equipped with fabric filter (FF) and wet flue gas desulfurization (WFGD) system. The results demonstrated that arsenic was both enriched in bottom ash (41.4-47.6%) and fly ash (52.4-58.6%), while selenium was mainly captured by fly ash (73.9-83.4%). Limestone injection into furnace promoted As/Se retention in ash residues. Arsenic was mainly converted into arsenate in high-temperature regions and partly trapped in bottom ash as arsenite. In contrast, selenium capture mainly occurred in low-temperature flue gas by the formation of selenite, because of the poor thermal stability of most selenite. Triplet-tank method can totally remove arsenic in WFGD wastewater. And 18.4-58.7% of selenium was removed, resulting from the precipitation of Se⁴⁺ anions with highly soluble Se⁶⁺ anions remaining in wastewater. The concentrations of As and Se in the stack emission were 0.25-1.02 and 0.96-2.24 μg/m³, receptively. The CFB boiler equipped with FF + WFGD was shown to provide good control of the As/Se emission into the atmosphere. Leaching tests suggested that more attention should be paid to As leachability from fly ash/gypsum, and Se leachability from gypsum/sludge.

Volatilization characteristics of selenium during conventional and microwave drying of coal slime: an emerging contaminant in mining industry

The priority feature of coal drying which accounts for its industrial use is moisture removal. Though much improvement is made in thermal-drying process, volatilization of harmful elements during coal slime drying is mostly ignored which has manifold environmental implications. In view of this, the present study attempted to investigate the moisture and selenium (Se) volatilization proportion during coal slime drying by using electric blast oven (conventional) and microwave oven (microwave) drying procedures. X-ray photoelectron spectroscopy (XPS) technique was employed to investigate the chemical changes of raw and dried coal. The results indicated that the microwave-drying process eliminates the moisture in line format, whereas, moisture content was removed slowly (initial stage) during conventional drying and then rapidly (final stage) with increasing drying period. It was confirmed that the microwave drying took less drying time to remove water and make the coal dried compared with the conventional drying. Volatilization proportion of Se to the atmosphere was 38.81 and 61.13% during conventional and microwave drying, respectively. Findings of XPS analysis demonstrated an increase in C-C and C-H content, while a decrease in C-O fraction on the coal surface, after execution of drying procedures. Microwave drying appeared not only energy efficient but also proved an optimized method for removing oxygen-functional groups than conventional drying methods. Peak fittings of XPS spectra gave indication of several oxidation forms including selenide, selenate, and oxide species in coal slime after drying. It was concluded that Se volatilization including speciation characteristics should be considered an indicator of efficiency for coal drying process from environmental safety perspective.

Temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust

The temporal influence of reaction atmosphere and chlorine on arsenic release in combustion, gasification and pyrolysis of sawdust was studied using an on-line analysis system. The arsenic release amount in combustion atmosphere was higher than that in CO₂ gasification and argon pyrolysis. The derived values of activation energy followed the order: combustion < gasification < pyrolysis. Furthermore, the enhancement effect of chlorine species on arsenic release percentage in air combustion was also higher than that in gasification and pyrolysis conditions. The total proportion of arsenic release in combustion with additive chlorine is bigger than the case in gasification and pyrolysis, especially when 20% chlorine is added. According to equilibrium analysis, arsenic oxides were identified as the main gaseous arsenic species and their formation were decreased in the oxygen-deficient environment, mainly accounting for lesser arsenic release proportion in gasification and pyrolysis than combustion. The release of arsenic was promoted to a different extent with additive chlorine, mainly caused by the AsCl₃ (g) formation. By the findings of the experiments and theoretical analyses, the possible reaction pathways and release mechanisms of arsenic species were proposed.

Design of O2/SO2 dual-doped porous carbon as superior sorbent for elemental mercury removal from flue gas

A porous carbon was synthesized via hydrothermal carbonization and CO₂ activation. O₂ and SO₂ were successfully co-doped onto carbon surface by applying non-thermal plasma technique. Porous carbon possessing excellent textural properties is effective to adsorb the radicals generated by plasma. Plasma promotes the adsorption of O₂ and SO₂ on carbon surface with the formation of abundant CO, C-S and C-SO_x (x = 1-3) groups. The O₂/SO₂ dual-doped porous carbon was utilized to adsorb elemental mercury (Hg⁰) from the flue gas of coal combustion. The Hg⁰ adsorption ability of the O₂/SO₂ dual-doped porous carbon is closely related with the concentrations of O₂ and SO₂ for plasma treatment and the treatment time. The optimal O₂/SO₂ dual-doped porous carbon exhibited far greater Hg⁰ adsorption capacity than a commercial brominated activated carbon. Density functional theory was employed to understand the Hg⁰ adsorption mechanism at the molecular level. CO, C-S and C-SO_x (x = 1-3) groups enhanced the interaction of Hg⁰ with surface carbon atom. The activity of them for enhancing Hg⁰ adsorption is in the order of C-SO₂ >  CO > C-S > C-SO > C-SO₃. Porous carbon can be activated by plasma in flue gas containing O₂ and SO₂, and used as superior sorbent for Hg⁰ removal.

Development of O2 and NO Co-Doped Porous Carbon as a High-Capacity Mercury Sorbent

In this study, a novel Hg⁰ adsorption strategy based on nonthermal plasma and porous carbon was proposed and tested. The O₂ and NO in flue gas were used to activate porous carbon with auxiliary plasma. The plasma significantly increased the functionalities on the carbon surface, and it has a negligible effect on the textural properties of porous carbon. The O₂/NO co-doped porous carbon was used to remove elemental mercury (Hg⁰). The sample functionalized by plasma in 4% O₂ and 200 ppm NO (balanced with N₂) for 3 min exhibited superior Hg⁰ adsorption ability, which could be assigned to the formation of a large amount of C═O, C-NO, and C-NO₂. O₂, NO, and HCl have a positive effect on Hg⁰ adsorption, whereas SO₂ and H₂O have an inhibitory effect on Hg⁰ removal. The equilibrium Hg⁰ adsorption capacity of optimal O₂/NO co-doped porous carbon was found to be 12315 μg/g, which was far greater than that of brominated activated carbon (1061 μg/g). Density functional theory was used to investigate the mechanism responsible for Hg⁰ adsorption. C═O and C-NO improved the interaction of Hg⁰ with neighboring carbon sites. C-NO₂ could react with Hg⁰ by forming HgO.

Oxygen-Rich Porous Carbon Derived from Biomass for Mercury Removal: An Experimental and Theoretical Study

A porous carbon was synthesized by the combination of freeze-drying and CO₂ activation from starch. Nonthermal plasma was employed to quickly produce oxygen functional groups on a porous carbon surface. The plasma treatment has a negligible effect on the textural properties of the porous carbon. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analyses suggested that the plasma treatment significantly increased the amount and promoted the evolution of oxygen groups on surface. The unique pore structure of porous carbon was proven favorable to effective oxygen loading. The elemental mercury (Hg⁰) adsorption ability of the oxygen enriched porous carbon was tested. The results indicated that the oxygen-rich porous carbon constitutes an effective sorbent for Hg⁰ removal. The excellent textural properties, surface atomic oxygen concentration, and the type of oxygen group are the three key factors for realizing high Hg⁰ removal performance. Density functional calculations were performed to understand the effect of oxygen groups on Hg⁰ adsorption. Carbonyl and ester groups are beneficial for Hg⁰ adsorption, whereas epoxy, carboxyl, and hydroxyl groups inhibit Hg⁰ adsorption. Plasma treatment enhances Hg⁰ adsorption by increasing the amount of ester and carbonyl groups on surface.

Insights into the effect of chlorine on arsenic release during MSW incineration: An on-line analysis and kinetic study

The effect of chlorine on arsenic (As) release dynamics during municipal solid waste (MSW) incineration in a fluidized bed was studied on the basis of an on-line analysis system. This system can continuously and quantitatively measure the concentrations of trace elements in flue gas. Chlorine addition increases obviously the concentration of arsenic in flue gas, indicating a promoting effect of chlorine on arsenic release during MSW incineration. Based on the temporal concentration of arsenic in flue gas, the overall kinetic parameters of arsenic release during MSW incineration were calculated. A second-order kinetic law r(x) = 81.6e^-66.9/RT (-1.05x² - 0.01x + 1.03) was ascertained for arsenic release during MSW incineration without chlorine addition, and r(x) = 177.3e^-65.3/RT (-0.68x² - 0.43x + 1.08) for arsenic release with chlorine addition. Thermodynamic calculations were performed to predict the partitioning behavior of arsenic during MSW incineration. The addition of chlorine can not only compete with gaseous arsenic to react with mineral, but is also able to increase the volatilization of arsenic by forming volatile arsenic chlorides, thereby affecting the release kinetics of arsenic during MSW incineration.

Thermal effects from the release of selenium from a coal combustion during high-temperature processing: a review

The release of selenium (Se) during coal combustion can have serious impacts on the ecological environment and human health. Therefore, it is very important to study the factors that concern the release of Se from coal combustion. In this paper, the characteristics of the release of Se from coal combustion, pyrolysis, and gasification of different coal species under different conditions are studied. The results show that the amount of released Se increases at higher combustion temperatures. There are obvious increases in the amount of released Se especially in the temperature range of 300 to 800 °C. In addition, more Se is released from the coal gasification than coal combustion process, but more Se is released from coal combustion than pyrolysis. The type of coal, rate of heating, type of mineral ions, and combustion atmosphere have different effects on the released percentage of Se. Therefore, having a good understanding of the factors that surround the release of Se during coal combustion, and then establishing the combustion conditions can reduce the impacts of this toxic element to humans and the environment.