Coal Combustion Environments Research Articles

Effect of NPAH Exposure on Lung Function of Children in Indoor Coal Combustion Environment

Nitropolycyclic aromatic hydrocarbon (NPAH) emissions from the combustion of household solid fuel may cause great harm to public health. Children are one of the most susceptible population groups at risk of indoor air pollutants due to their immature respiratory and immune systems. In this study, a primary school using household coal combustion for heating in winter was selected and forty participants were randomly recruited among schoolchildren. Fine particulate matter samples were collected by both individual portable samplers and fixed middle-flow samplers during the heating and nonheating seasons. The NPAH concentrations in PM2.5 samples were analyzed by a gas chromatograph coupled to a mass spectrometer. Potential sources of NPAHs were identified by NPAH ratios as well as principal component analysis. Lung function of children was tested by an electronic spirometer. The relationship between NPAH exposure level and children’s lung function was studied. Finally, the cancer risk caused by NPAH inhalation was assessed. The results showed significantly higher individual NPAH exposure level in heating season (0.901±0.396 ng·m-3) than that in nonheating season (0.094±0.107 ng·m-3). Coal/biomass combustion and secondary formation were the potential NPAH sources in heating season. Significantly lower lung function of children was also found in heating season compared with that in nonheating season. As a result of the Monte Carlo simulation, the averaged incremental lifetime cancer risk (ILCR) values from the inhalation of NPAHs in the heating and nonheating seasons were 3.50×10−8 and 2.13×10−8, respectively. Our research revealed the association between NPAH exposure and children’s lung function and confirmed the adverse effect of indoor coal combustion. The results also indicated that further control strategies on indoor coal combustion are needed to reduce the risk of NPAH exposure and protect children’s health.

Different Prevention Effects of Ventilation Dilution on Methane Accumulation at High Temperature Zone in Coal Mine Goafs

In coal mine goafs, spontaneous combustion of coal can result in methane accumulation, which raises the danger of methane explosion disasters. As an atmospheric control tool, ventilation is applied to ensure air quality for avoiding disasters in underground mines. However, during the process of the spontaneous combustion of coal in coal mine goafs, the impact of ventilation dilution on the possible methane explosions induced by coal combustion has not been well investigated. In this study, a validated gas flow model for the spontaneous coal combustion environment in goafs of coal mines is adopted to investigate the influence of ventilation dilution at the three stages of the spontaneous combustion of coal. The research conclusions suggest that (1) ventilation dilution is a quick measure to dilute methane concentration and intensify heat transfer in the vertical direction in coal mine goafs; (2) ventilation dilution can lessen the danger of methane explosions by diluting methane concentration to the lower explosive limit for methane when coal combustion takes place on the air-inlet side; (3) however, ventilation dilution increases the methane explosion risk by decreasing methane concentration, resulting in explosive methane limits, if coal combustion occurs on the air-return side. This provides a reference for the management of ventilation during a spontaneous coal combustion disaster in the goafs of coal mines with methane.

An improved model of fine particulate matter formation coupling the mechanism of mineral coalescence and char fragmentation during pulverized coal combustion

An improved model of fine particulate matter formation coupling the mechanism of mineral coalescence and char fragmentation under different pulverized coal combustion environments has been constructed. Firstly, based on the theoretical model of char fragmentation and percolation, the included minerals with different types and particle sizes are constructed in the model, and a three-dimensional char particle sub-model is established. And the type, content and particle size distribution of included minerals are introduced as input parameters by using computer controlled scanning electron microscopy (CCSEM) technology. All of the above makes it more in line with the actual distribution of the included minerals. Then a sub-model of char fragmentation is built based on the sub-model of the char particle. And considering the influence of char combustion reaction on the particle formation process and melting characteristics of included minerals, a sub-model of mineral melting coalescence under different combustion environments is established. Finally, based on this improved model, we compared the calculation results with the experimental data and the calculation results of the traditional model. Fully considering the process of mineral coalescence and char fragmentation, which contains the characteristics of different included minerals, the results show that the newly established model has a good fitting effect for the experiment and is closer to the actual process of char particle combustion to generate particles. By the new model, the influence of the factors (mineral content, particle size distribution and porosity) on the formation of particulate matter is preliminarily analyzed.

Experimental Study of the Influence of H2/CO on the CH4 Explosion Pressure and Thermal Behaviors.

In a spontaneous coal combustion environment and in the coal chemical process, multiple gases, such as CH4, H2, and CO, coexist, and explosion accidents are prone to occur. The causes of these disasters and the explosion characteristics are key to formulating preventive measures. To explore the effect of H2/CO on the explosion pressure and thermal behavior of methane–air, CH4 with initial volume fractions of 7, 9.5, and 12%, which correspond to three states of oxygen enrichment, equivalence ratio, and oxygen depletion, was selected. Moreover, a mixed fuel system is composed of H2/CO with different volume ratios. A 20 L spherical gas explosion experimental system was used to test the peak explosion overpressure Pmax, the maximum explosion overpressure rise rate (dP/dt)max, and the corresponding time parameters of the H2/CO–CH4 mixed system. Combined with the thermodynamic calculation model, laminar burning velocity SL, explosion heat loss qtra, and other parameters were obtained. The results show that due to the existence of the damping effect, CO has the dual characteristic of promoting or weakening methane explosions. Compared with CO, the effect of H2 on the methane explosion is more significant, and the improvement or weakening of the laminar combustion rate of the reaction system by CO “lags” behind that of H2. The heat loss in the process of a gas explosion is affected by factors such as the heat release rate, the propagation speed of the combustion wave, and the heat dissipation effect of the container wall. When H2/CO increases the laminar burning velocity of the mixed system, the heat loss decreases accordingly. This study also found that the laminar burning velocity model of the mixed gas based on the ideal spherical flame propagation theory is not fully applicable to the H2/CO/CH4 mixed system in a spherical closed space, and the calculation results have large errors when the mixed system is close to the upper limit of the explosion.

Polycycl. Aromatic Hydrocarbon Exposure of Children in Typical Household Coal Combustion Environments: Seasonal Variations, Sources, and Carcinogenic Risks.

Polycyclic aromatic hydrocarbon (PAH) emissions from the combustion of household solid coal for cooking and heating cause great harm to public health in China, especially in less developed areas. Children are one of the most susceptible population groups at risk of indoor air pollutants due to their immature respiratory and immune systems. However, information on PAH exposure of children is limited due to limited monitoring data. In this study, we aimed to assess the seasonal differences of PAHs in classrooms, analyze the pollutant sources, and calculate the incremental lifetime cancer risk attributable to PAHs in Shanxi Provence. A typical school using household coal combustion in Shanxi Province was selected. Fine particulate matter (PM2.5)samples were collected by both individual samplers and fixed middle-flow samplers during the heating and non-heating seasons in December 2018 and April 2019. The PAH concentrations in PM2.5 samples were analyzed by a gas chromatograph coupled to a mass spectrometer. The results showed that PAH concentrations in PM2.5 varied between 89.1 ng/m3 in the heating season and 1.75 ng/m3 in the non-heating season. The mean concentrations of benzo[a]pyrene (BaP), a carcinogenic marker of PAHs, were 10.3 and 0.05 ng/m3 in the heating and non-heating seasons, respectively. Source allocation analysis of individual portable and passive samplers revealed that the main contributors during heating and non-heating seasons were coal combustion and gasoline sources, respectively. According to the results of a Monte Carlo simulation, the incremental lifetime cancer risk values from the inhalation of PAHs in the heating and non-heating seasons were 3.1 × 10−6 and 5.7 × 10−8, respectively. The significant increase in PAHs and the incremental lifetime cancer risk in the heating season indicates that children are more exposed to health threats in winter. Further PAH exposure control strategies, including reducing coal usage and promoting clean fuel applications, need to be developed to reduce the risk of PAH-induced cancer.

Oxidation‐related life‐time assessment using the new ISO standards ISO 21608:2012 and ISO 26146:2012

Life‐time extrapolation has often been performed based on a parabolic oxidation law. The new standards, ISO 21608 and ISO 26146, aim at determining the actual oxidation rate constant and exponent in order to describe the oxidation behavior as closely as possible to the observed situation. The present paper extends this approach by the kinetics of subsurface zone depletion of the protective scale‐forming alloying element (for example, chromium). Four different types of characteristic depletion kinetics are combined with considerations on the minimum amount of chromium necessary to maintain the protective situation in order to define the extrapolation basis for oxidation‐related life‐time assessment. This approach is applied to experimental data from exposure in simulated coal combustion environments at temperatures of 550–750 °C and exposure times up to 15 000 h. The materials ranged from low‐alloy steel to high‐alloy austenitic steels and nickel‐base alloys. As a result, it is proposed to use data from metallographic investigations rather than mass change data for life‐time assessment together with the respective kinetics approaches. Furthermore, it is recommended to base life‐time extrapolation on experimental data from tests of at least 10 000 h duration. The investigated materials were categorized with regard to their potential of oxidation‐related life‐time assessment.

Erosion–corrosion of iron and nickel alloys at elevated temperature in a combustion gas environment

This paper reports on the results of a study that compares the erosion–corrosion behavior of a variety of alloys (Fe–2¼Cr–1Mo, 304 SS, 310 SS, Incoloy 800, Haynes 230 and a Fe3Al) in a combustion environment. Advanced coal combustion environments, with higher temperatures, are driving re-examination of traditional and examination of new alloys in these hostile environments. In order to simulate conditions in advanced coal combustion boilers, a special erosion apparatus was used to allow for impingement of particles under a low abrasive flux in a gaseous environment comprised of 20% CO2, 0.05% HCl, 77% N2, 3% O2, and 0.1% SO2. Tests were conducted at room temperature and 700°C with∼270μm silica, using an impact velocity of 20m/s in both air and the simulated combustion gas environment. The erosion–corrosion behavior was characterized by gravimetric measurements and by examination of the degraded surfaces optically and by scanning electron microscopy (SEM). At room temperature most of the alloys had similar loss rates. Not surprisingly, at 700°C the lower chrome-iron alloy had a very high loss rate. The nickel alloys tended to have higher loss rates than the high chrome austenitic alloys.

Effects of Inherent Metals on NO Reduction by Coal Char

Five coal chars were prepared in a flat flame flow reactor (FFR) which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Reactivity of these chars with NO in the temperature range of 1273-1573 K has been characterized by experiments in a high-temperature drop tube furnace (DTF). Two normalized parameters ((X) over bar and m(c)) are proposed to analyze the effect of inherent metal content on char reactivity. The inherent metal catalysts in chars, such as magnesium, potassium, sodium, calcium, and iron, can significantly increase reactivity of char by reducing the activation energy. The reactivity of NO-char reaction increases with m value monotonously and linearly in a certain range. Experimental results showed that the catalytic activity of magnesium, potassium, sodium, calcium, and iron at the temperatures of 1273-1573 K were in decreasing sequence. In this study, the magnesium, which was rarely studied in other literature, appears to have a great catalytic effect on the reduction of NO by chars.

Evaluation of the Corrosion Resistance of Fe–Al–Cr Alloys in Simulated Low NO x Environments

Due to their excellent corrosion resistance, iron aluminum alloys are currently being considered for use as weld claddings in fossil fuel fired power plants. The susceptibility to hydrogen cracking of these alloys at higher aluminum concentrations has led researchers to examine the effect of chromium additions on the corrosion resistance of lower aluminum alloys. In this work, three iron aluminum alloys were exposed to simulated coal combustion environments at 500 and 700 °C for short (100 h) and long (5000 h) isothermal durations. Scanning electron microscopy was used to analyze the corrosion products. All alloys exhibited excellent corrosion resistance during short term exposures. For longer test times, increasing the aluminum concentration improved alloy corrosion resistance. The addition of chromium to the binary iron aluminum alloy prevented the formation iron sulfide and resulted in slower corrosion kinetics. A general classification of the scales developed on these alloys is presented.

Kinetic analysis of NO-Char reaction

Two Chinese coals were used to prepare chars in a flat flame flow reactor which can simulate the temperature and gas composition of a real pulverized coal combustion environment. Acid treatment on the YB and SH chars was applied to obtain demineralized chars. Kinetic characterization of NO-char reaction was performed by isothermal thermogravimetry in the temperature range of 973–1,573 K. Presence of catalytic metal matter can increase the reactivity of chars with NO, which indicates that the catalytic effects of inherent mineral matter play a significant role in the NO-char reaction. The discrete random pore model was applied to describe the NO-char reactions and obtain the intrinsic kinetics. The model can predict the data for all the chars at various temperatures well, but underestimate the reaction rates at high carbon conversions for the raw YB and SH chars, which can be attributed to the accumulation of metal catalyst on char surface.

Studies of NO−Char Reaction Kinetics Obtained from Drop-Tube Furnace and Thermogravimetric Experiments

Four coal chars were prepared in a flat flame flow reactor (FFR), which can simulate the temperature and gas composition of a real pulverized coal combustion environment. The pore structure of chars was measured by mercury porosimetry and nitrogen adsorption, and the Hg and Brunauer−Emmett−Teller (BET) surface areas were obtained. The kinetics of NO−char was studied in a drop-tube furnace (DTF) and thermogravimetric analyzer (TGA). In the TGA experiments, the random pore model (RPM) was applied to describe the NO−char reactions and obtain the intrinsic kinetics. By presenting the data of DTF and TGA experiments on the same Arrhenius plot, it can be concluded that TGA is an available tool to study the kinetics of a high-temperature NO−char reaction. With respect to the DTF experiments, in comparison to the BET surface area, the Hg surface area is a better basis for normalizing the reactivity of different coal chars because of less scatter in the measured values, better agreement with TGA experimental data, and more stable values during the process of reaction. Moreover, by comparing the Hg surface area of chars before and after reactions, it is believed that the Hg surface area basis is more appropriate for high-rank coal chars. The determined kinetic rate constants are in good agreement with other data in the literature, and a new rate constant expression is proposed.

Advantage of Illinois Coal for FGD Removal of Mercury

An investigation was conducted to characterize and modify mercury speciation in Illinois coal combustion flue gas so that a mercury control strategy can be implemented in conventional flue gas desulfurization (FGD) units. Mercury is readily volatilized during coal combustion and leaves the high-temperature zone as gas-phase elemental mercury (Hg°). As the flue gas is cooled, a portion of the Hg° is converted, primarily to mercuric chloride (HgCl 2 ), in the presence of catalytically active surfaces. Unlike Hg 0 , HgCl 2 is highly soluble in water and has a high affinity for alkaline sorbents; it can, therefore, be easily removed in wet scrubbers and spray dryers (FGD units). This study specifically examined the effect of injection of Illinois coal combustion residues (ICCRs) on the conversion of Hg° to HgCl 2 in coal combustion flue gases. Tests were conducted in two tasks. Task I studied Hg° oxidation using a fixed-bed, bench-scale reactor in a simulated Illinois coal combustion environment. Various types of ICCRs were examined to determine active residues in the oxidation of Hg°. A scrubber sludge and two bottom ash samples were determined to be active Hg° catalysts. The most active ICCR (a bottom ash sample) was chosen for Task II. Task II was performed in two subtasks. In Subtask I, the active bottom ash sample was injected in a pilot-scale combustor while burning natural gas doped with appropriate amounts of Hg°, hydrogen chloride (HCl), nitrogen oxides (NO X ), and sulfur dioxide (SO 2 ). Injection of this bottom ash sample did not contribute to the background gas-phase oxidation of Hg°, indicating that in-flight oxidation of Hg° in the pilot-scale combustor may be mass transfer limited. In Subtask II, a representative Illinois coal was combusted in the pilot-scale combustor. As with most bituminous coals, the combustion flue gas of this coal was dominated by oxidized forms of mercury. Additional Hg° was doped into this flue gas, and significant oxidation of Hg° was observed. It was determined that the fly ash generated from combustion of this coal is very active in oxidation of Hg°. Additional pilot-scale tests were performed to investigate the activity of the Illinois coal fly ash. It was determined that packed-bed flow modes, such as those observed in baghouses, are required to catalytically oxidize Hg° using this Illinois coal fly ash.

Waterwall corrosion mechanisms in coal combustion environments

When coal-fired boilers are operating with substoichiometric firing to limit NOx formation in order to comply with emission regulations, the reducing environments formed locally in the firing zone or under deposits can result in sulfidation attack and premature failure of components. Carbon and low-alloy steels are the preferred materials for waterwall applications due to their good thermal conductivity, creep strength (up to 550°C) and relatively low cost. However, these materials, with at best low contents of Cr, are also the most susceptible to sulfidation attack. This paper presents the results of an extensive investigation to evaluate the effect of coal combustion characteristics, gas flow in the boiler, sulfidation potential of the environment, and the formation of deposits, on the kinetics of sulfidation in typical boiler materials. An interdisciplinary approach was adopted involving combustion analysis, corrosion studies and computational fluid dynamics along with information on operational boilers. The objective was to provide support for the low NOx retrofit program and to generate authoritative information for utility customers and also to develop a capability to predict potential combinations of coal and combustion conditions which could cause enhanced rates of attack.

Waterwall corrosion mechanisms in coal combustion environments

Waterwall corrosion mechanisms in coal combustion environments

Degradation of gas turbine blade materials in integrated coal-gasification combined cycle plant

Interest in the integrated coal-gasification combined cycle plant (IGCC) has increased because of its high efficiency. However, degradation of gas turbine component materials has not been studied so far. Therefore, the purpose of this study is to investigate the corrosion behaviors of these materials in an actual coal gas combustion environment. Our test facility was constructed near the coal-gasification test facility (CGT) at the Aioi works of Ishikawajima-Harima Heavy Industries Co., Ltd. Several superalloy (René80H and FSX414) and coating systems (CoCrAlY, CoNiCrAlY and TBC) were exposed in the coal gas combustion environment at 1123, 1223 and 1323 K up to 350 h. After these tests, the weight change of each specimen was measured. Morphologies of products and element distributions at their surfaces were observed with SEM and EPMA. It is concluded that not only high- temperature oxidation and sulfidation but also ash deposition has a remarkable influence on the degradation of gas turbine blade materials.

Mercury transformations in coal combustion flue gas

Mercury chlorination [i.e., formation of HgCl 2(g)] is generally assumed to be the dominant mercury-transformation mechanism in coal combustion flue gas. Other potential mechanisms involve mercury interactions with ash particle surfaces where reactive chemical species, oxidation catalysts, and active sorption sites are available to transform Hg 0(g) to Hg 2+X(g) (e.g., where X is Cl 2 or O) as well as Hg 0(g) and HgCl 2(g) to particulate mercury, Hg(p). Results from an investigation of Hg 0(g)–O 2(g)–HCl(g) and Hg 0,2+(g)–HCl(g)–CaO(s)-fly ash interactions in a 42-MJ/h combustion system are consistent with the following mechanisms: mercury chlorination, catalysis of mercury oxidation by Al 2O 3(s) and/or TiO 2(s), and mercury sorption on a calcium-rich (25.0 wt.% CaO) subbituminous coal fly ash. Additions of 50 and 100 ppmv of HCl(g) and ≈12.6 wt.% of CaO(s) to the subbituminous coal combustion environment inhibited Hg(p) formation, primarily via a change in ash surface chemistry and a decrease in particle surface area, respectively.

98/02372 Strength and corrosion behavior of SiC-based ceramics in hot coal combustion environments : Breder, K. and Parten, R. J. Oak Ridge Natl. Lab. [Rep.] ORNL/FMP, 1996, (ORNL/FMP-96/1, Proceedings of the Tenth Annual Conference on Fossil Energy Materials, 1996), 53–62

Fly ash produced during the combustion of powdered coal could be converted up to 45% into zeolite. By varying the experimental conditions different types of zeolite were produced, e.g. zeolite Na-P1, zeolite K-G and zeolite ZK19. By this zeolitization process the cation exchange capacity (CEC) was raised from 0.02 to circa 2.4 meq/g. Anionic heavy metals were largely extracted by the process water. Sorption experiments indicated that the selectivity series for zeolite Na-P1 is Ba>Cu>Cd≈Zn>Co>Ni. Besides cation exchange reactions, precipitation of hydroxides also played a role in the immobilization of heavy metals in the column experiments carried out. Column leaching experiments showed that relative to the original fly ash, the zeolitized fly ash has a better environmental quality. The results of the sorption experiments suggest that the zeolite product can be applied in environmental technology as an immobilizer of pollutants.

97/02274 A review of the efficacy of silicon carbide hot-gas filters in coal gasification and pressurized fluidized bed combustion environments

97/02274 A review of the efficacy of silicon carbide hot-gas filters in coal gasification and pressurized fluidized bed combustion environments

Sodium partitioning in a pulverzed coal combustion environment

Sodium transformation mechanisms during pulverized coal combustion were investigated as functions of combustion temperature and of the modes in which sodium occurred in the parent coal. The combustion experiments took place in a 17-kW laboratory downflow combustor which was designed to form a link between bench-scale reactor studies and commercial-scale combustors. Three different coals were burned under excess air conditions, and size-segregated fly-ash samples were extracted far from the combustion zone. Atomic absorption on each ash sample yielded bulk composition data. Computer-controlled scanning electron spectroscopy (CCSEM) yielded composition data of individual ash particles and allowed inference of mechanisms governing the formation of these individual particles. It was demonstrated that partitioning of sodium in the vapor, and the subsequent fraction of sodium in the submicron fume, is greatly reduced by the presence of silicates and high temperatures. This was attributed to reaction, of sodium in the vapor with solid silicates. The CCSEM analysed showed that reaction to form sodium aluminum silicates was preferred over reactions forming sodium silicates, alone, although both are formed. Some physical condensation also occurs. Both included and excluded alumino-silicates are effective in reactively scavenging sodium. The maximum amount of sodium scavenged in any one particle was consistent with formation of Na 2 O·Al 2 O 3 ·2SiO 2 . It was demonstrated that sodium from coal reacted with kaolinite additives mixed with the coal, suggesting that this is a strategy to reactively scavenge vaporized sodium. Additional bench-scale experiments and equilibrium calculations suggest that the mechanism of sodium reaction with alumino-silicates involves sodium as sodium hydroxide.

Corrosion of silicon carbides by ash and vapor in a coal combustion environment

Corrosion of silicon carbides by ash and vapor in a coal combustion environment