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Abstract This paper aims to reveal the mechanisms governing the impaction and sticking dynamics of fly ash particles in pulverized coal combustion. The modeling work is of relevance to experiments in a 25 kW self-sustained down-fired furnace, which provides a sequence of real deposit shapes as varied boundary conditions for CFD simulations. Although the formed ash deposit has a comparable length scale with the probe, it has little effect on the global impaction efficiency of newly-coming particles. However, as the deposit builds up, incident particles impact the deposit and probe at generally larger impact angles and smaller normal velocities despite the almost invariant global impaction efficiency. It results in an enhanced local sticking probability in the center region of the probe, but a decreased one in the lateral regions. The incident kinetic energy of newly sticking particles to the deposit exhibits a converse correlation with their impact angle. The relationship of the averaged local sticking probability as a function of the azimuthal angle of probe is illustrated. Finally, the effect of Reynolds number on global particle impaction efficiency is examined. A universal formula is proposed, which is of importance to bridge lab-scale experiments and practical applications. [ABSTRACT FROM AUTHOR]

A reducing-to-oxidizing (R O) environment is characteristic of what a coal particle experiences in the near-burner region of pulverized coal (pc) furnaces. The R O environment can influence early-stage coal combustion processes such as ignition, aerosol formation, and char burnout. However, fundamental studies have focused on either oxidizing conditions (mimicking the post-flame region) or reducing conditions (mimicking the devolatilization region). The effect of this R O environment on early-stage coal combustion has, until now, not been considered. Here, the role of this reducing-to-oxidizing environment on single-particle ignition is evaluated. Powder River Basin (PRB) sub-bituminous coal was used, with a particle size of 125–149 μm and two nominal gas temperatures of 1300 K and 1800 K. The experimental findings for purely-oxidizing conditions with 20 vol% oxygen are compared with those of reducing-to-oxidizing environment. Single particles were tracked using high speed, high resolution videography. Emission intensities of the particles were used to evaluate the prevailing ignition modes, and to determine the characteristic ignition and induction times in both oxidizing and reducing-to-oxidizing environments. Experimental findings show that homogeneous-to-heterogeneous mode of ignition is prevalent for purely oxidizing conditions for both nominal gas temperatures of 1300 K and 1800 K. However, hetero-homogeneous ignition is favored in reducing-to-oxidizing environment at 1800 K and heterogeneous ignition at 1300 K gas flame temperature. The reducing-to-oxidizing environment leads to longer ignition delay times of about 20% and 40% on average for 1300 K and 1800 K nominal gas temperatures respectively but shorter induction times than those of oxidizing condition. The results show that ignition behavior in a reducing-to-oxidizing post-flame environments can be quite different from those in oxidizing environments. • Effects of reducing-to-oxidizing environments on ignition behavior of isolated coal particles are identified and compared to oxidizing environments. • Hetero-homogeneous ignition is favored in reducing-to-oxidizing environments at 1800 K and heterogeneous ignition at 1300 K gas temperature. • Homogeneous-to-heterogeneous ignition is prevalent for oxidizing environments. • The reducing-to-oxidizing environment leads to longer ignition delays but shorter induction times than those of oxidizing environments. [ABSTRACT FROM AUTHOR]

Based on 57 literatures, the quantitative relations among the apparent activation energy, E, apparent pre-exponential factor, k0 and volatile content on dry ash free basis, Vdaf, fuel ratio (FR) data from TGA experiments were obtained for 131 Chinese power coal samples. Regression was made for lg k0, E with Vdaf, FR, the regression results show that ① E, lg k0 decrease linearly with Vdaf, E = 109. 6-0. 365Vdaf lg k0 = 5. 406-0. 04 Vdaf, and lg k0 increases linearly with E, lg k0 = -0. 193+0. 043E. According to Vdaf of Chinese power coals, E and k0 can be calculated. In addition, according to Arrhenius Law, reaction speed constant number during power coal burning processes can be calculated. ② with the increase of FR, E and lg k0 increase with FR generally, E = 90. 891 +3. 097 37FR-0. 138FR², lg k0 = 3.413 2 - 0. 001FR+0. 022 8FR² The two mathematic relationships mentioned above correspond to the relationship of lg k0 increasing linearly with E. [ABSTRACT FROM AUTHOR]

In this paper, we intensively investigate the influence of steam on the hetero-homogeneous joint ignition of dispersed coal particle streams in both O 2 /N 2 and O 2 /CO 2 conditions of an optically accessible flat-flame burner. Two differently ranked coals and one bituminous char were tested for a comparison. The effects of moisture content in the raw coal, steam content or diluents (N 2 /CO 2 ) in the ambience, and fuel types were systematically examined under isothermal conditions. First, it is seen that the moisture content in raw coal delays its ignition because of endothermic vaporization, which becomes weakened at the high ambient temperature. Secondly, three main mechanisms are categorized to explore the profound effects of ambient steam content on ignition of coal particles, i.e., ( i ) the promotion effect of OH radical on the oxidation of volatile hydrocarbons, ( ii ) the inhibition effect of gaseous H 2 O molecule on the equilibrium of hydrocarbon oxidation, ( iii ) the negative effect of endothermic surface gasification reaction (C-CO 2 /C-H 2 O). In O 2 /N 2 ambience (1500 or 1800 K), as the steam content increases, the ignition delay of coal particles is rapidly reduced in an early stage of 0%–5% due to the positive effect of OH radicals, followed by an inappreciable reduction in 5%–13% stage because of both the steam inhibition effect and the endothermic surface gasification. However, in O 2 /CO 2 ambience with sufficiently high steam content (5%–18%), the ignition delay even becomes longer with the further increment of steam. It is because of the synergistic effect between C-H 2 O and C-CO 2 reaction at high temperatures (1500 K and 1800 K). Finally, for fuel types, the OH radical promotion effect of two lignite samples is obviously higher than that of low-volatile char samples, whereas with steam addition the sodium-rich Zhundong coal exhibits lower ignition delay than Hulunbel lignite due to catalytic effect. [ABSTRACT FROM AUTHOR]

The gas-phase and coal ash-related fireside corrosion behavior of a new type of Ni-Fe-Cr based superalloy GH984G and two other contrast alloys were investigated for its application under advanced ultra-supercritical (A-USC) conditions. Results showed that the oxidation activation energy of the three alloys at 600–780 °C in the flue gas environment can be ranked from large to small: C-HRA-1>GH984G>Alloy 625. In the coal ash environment, GH984G and C-HRA-1 exhibited satisfactory corrosion resistance, while Alloy 625 had weaker corrosion resistance due to the presence of molybdenum. The corrosion mechanism of GH984G and the influence of constituent elements were elucidated. • The oxidation kinetics of the tested alloys followed the Wagner law in the flue gas. • The oxidation activation energy could be ranked: C-HRA-1 >GH984G>Alloy 625. • Alloy 625 showed less corrosion resistance than GH984G and C-HRA-1 due to Mo. • Introduction of Fe might decrease diffusion activation energy of Cr in Ni lattice. • The corrosion mechanism of GH984G was elucidated. [ABSTRACT FROM AUTHOR]

Mineral transformation and soot formation are the two governing mechanisms for fine particle formation in the early stage of coal combustion. The interaction between the evaporative alkali species and soot, which can have significant effect on fine particle formation, however, is not fully understood. To investigate the interaction between Na and soot in the early stage of coal combustion, we characterized the particles derived from four coals samples, including acid-washed and Na-loaded samples, under both the pyrolytic and oxidizing atmospheres. It was found that Na can reduce the soot yield and particle size under the pyrolysis condition. The Na-doped soot was formed by flame synthesis of vaporized minerals and soot precursors, and the Na content had a strong effect on soot oxidation activity. The Na-soot interaction was found to reduce the fine particle yield in the early combustion stage, as the high Na concentration and strong sooting tendency of a sodium doped bituminous (YK-Na) resulted in a lower fine particle yield. A schematic of the Na-soot interaction pathways is proposed based on the experimental results. Catalyzing soot oxidation and eliminating ultrafine mineral particles were found to be the two principal ways by which Na-soot interaction affects the formation and evolution of fine particles. [ABSTRACT FROM AUTHOR]

An experimental and theoretical investigation is conducted on the combustion of pulverized coal particle streams in either conventional or oxy-fuel conditions. The laboratory setup consisted of a Hencken flat-flame burner operated at the same temperatures of 1200, 1500 and 1800 K in N 2 and CO 2 environments. The visible light detection technique and the calibrated three-color pyrometry are separately used to characterize the ignition delay time and the luminous char surface temperature. First, experimental results indicate that the ignition delay time is relatively longer in CO 2 environment in heterogeneous mode or even heterogeneous-controlled joint mode. The ignition delay is obviously enhanced in O 2 /CO 2 ambient when the volatile-flame controlled joint mode is prevalent. The model predictions of particle temperature history further reveal that the extra gas flame radiation of CO 2 and the declined difference between the thermal conductivity are two main reasons. Then, during char combustion, the measurement shows that the char surface temperature is lower in an O 2 /CO 2 environment than in an O 2 /N 2 environment. It is noted that the temperature difference between N 2 and CO 2 environments enlarges with the increasing ambient temperature. In contrast to conventional O 2 /N 2 conditions, the lower O 2 diffusivity in CO 2 , the endothermic carbon reaction with CO 2 and the higher heat capacity of CO 2 , which are all temperature dependent, synergistically contribute to the enlargement of char surface temperature differences at high ambient temperature. [ABSTRACT FROM AUTHOR]

Particulate matter (PM) generated from pulverized coal combustion is influenced by a process of vaporization, nucleation and condensation/reaction of semi-volatile minerals. The objective of the work is to identify the controlling mechanism of the enrichment of volatile sodium (Na). Three representative kinds of coal with different rank and mineral content, Zhundong lignite, Hami lignite and high-ash-fusion (HAF) bituminous, were burned in a 25 kW self-sustained pulverized coal combustor. Zhundong lignite, possessing medium rank among the three coal samples, exhibits the highest formation ability of ultrafine PM 0.2 , indicating a prominent effect of mineral content over that of coal rank on PM formation. The size partitioning behavior of sodium is affected by the molar ratio Na 2 O/(SiO 2 +Al 2 O 3 ) instead of the absolute Na content. With this ratio increasing from 0.27 (HAF bituminous) to 20 (Zhundong lignite), the gas-to-particle conversion of Na transits from a surface-reaction controlled process to a nucleation-condensation/coagulation dominated pathway. The Na partitioning behavior of Zhundong lignite is then quantitatively interpreted by a population-balance-based theoretical approach. The model reveals the details on the competing processes of homogeneous nucleation and vapor condensation, resulting in that the simulated final particle mass size distribution exhibits reasonable agreement with the experimental result. The model has also successfully reproduced the d p 0 dependence of Na fraction in the ultrafine size regime from a process of homogeneous nucleation under the decreasing gas temperature from 1420 to 1270 K. [ABSTRACT FROM AUTHOR]

The plasma-assisted ignition enhancement of pulverized lignite particles is intensively studied in a laminar, upward Hencken flat-flame burner. By applying a pair of pin-to-pin electrodes with a high-voltage DC power supply, the apparent reductions of ignition delay time, about 0.8–3.5 ms, are observed for all three ambient temperatures, 1200 K, 1500 K and 1800 K, across which the ignition mode transits from heterogeneous ignition (HI) to hetero-homogeneous combined one. Concentrated on 1200 K ambience where HI mode prevails, the plasmas-induced reduction of ignition delay time as a function of the applied voltage (0–20 kV) is explored, in which the chemical effect is found to play a key role in cases with voltage less than 3 kV. Then, under the conditions of different oxygen mole fractions, the discharge mechanisms and the reductions of ignition delay time are further examined to distinguish the chemical and thermal effects on the ignition enhancement. Finally, the gas temperature rise, both measured and predicted at a point located 6 mm above the electrodes, verifies the thermal effect. This paper provides a preliminary understanding of plasma-assisted ignition mechanisms on dispersed coal particles that may be applied in practical combustion system. [ABSTRACT FROM AUTHOR]

Formation of ultrafine particles during coal combustion is believed to be governed by the solid–vapor–particle process of evaporative inorganic species. In this work, a down-fired flat-flame burner with a two-stage dilution sampling system were used to characterize the particulate matter formed in the early stage of high-sodium coal combustion. It was found that 33.9% of Na and 33.1% of Mg in the coal were enriched in the ultrafine particles through the solid–vapor–particle process at 1500 K, 20% O 2 . The control mechanisms of homogeneous condensation and surface reaction were identified with elemental particle size distributions. Below 1500 K, either increasing the oxygen concentration or the ambience temperature lead to a higher ultrafine particle yield, which was caused by the intensification of gas-phase release. At 1700 K, however, the gas-to-particle conversion process became the control step, resulting in reduced particle yield with lower molar fraction of Na, Mg and S. This change in particle elemental composition is correlated to the thermodynamic behavior of the Na–Mg–Ca–Si–S–Cl system, which is of great importance during the gas-to-particle conversion process at high temperature. [ABSTRACT FROM AUTHOR]

In this work, the pulverized coal combustion of Zhundong lignite was conducted in an optically-accessible, downward Hencken flat-flame burner to investigate the incipient formation of ultrafine particulate matter (PM). The ultrafine PM were collected in - flame from distinct positions under 1800 K, 1500 K and 1200 K using a self-designed thermophoretic sampler, together with extensive conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. A novel in situ low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) was further introduced to diagnose the dynamic behavior of particle-phase sodium during the pulverized coal combustion. The primary particles in the collected ultrafine PM have mean diameter of 8–15 nm, with Si and Na as main mineral components based on TEM/HRTEM–EDS results. The formation of ultrafine PM is regarded as a process of multi-component flame syntheses of mineral precursors, in which the competition between the devolatilization and the coalescence of particles are of most significance. Finally, based on an approach of time scale analysis, three characteristic times of minimum mean diameter of primary particles, the initial increment point of particle-phase sodium and the maximum devolatilization are found to be highly correlated. [ABSTRACT FROM AUTHOR]

• The addition of ion-exchangeable calcium reduces the soot mass yield during coal pyrolysis. • Ion-exchangeable calcium decreases the sizes of soot primary particles and soot aggregates. • Calcium loading shrinks the disordered cores of soot, promotes the soot graphitization, and leads to more compact aggregates. • The size distributions of inorganic species show that calcium and sodium play different roles in soot formation. This work investigated the effect of ion-exchangeable calcium on the mass yield, size distribution and structural properties of carbonaceous particulate matter from coal pyrolysis. The raw, acid washed and calcium-exchanged coal samples were pyrolyzed in a flat-flame burner at 1600 K. The particulate matter was collected with a three-stage dilution sampler. Then, the particle size distributions were measured by a low pressure impactor (DLPI+) and a scan mobility particle sizer (SMPS). The results show that, with the addition of ion-exchangeable calcium, the soot yield reduces, and the size growth of soot particles is inhibited. Transmission electron microscopy (TEM) analysis shows that ion-exchangeable calcium diminishes the sizes of disordered cores in soot primary particles, accelerates the graphitization of soot and makes the soot aggregates more compact. The chemical structures derived from Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) indicate that the graphitization degree of the soot is first enhanced and then inhibited with increasing calcium loading amount. Additionally, the inorganic species in the size-segregated soot particles were characterized. The fractions of calcium and sodium in soot show opposite trend with particle size. Unlike Na, which nucleates before soot inception, Ca tends to participate in both the soot nucleation and surface growth processes, and is enriched at larger particle size. [ABSTRACT FROM AUTHOR]

Abstract: The characteristics of both particulate formation and ash deposition play an important role in retrofitting the conventional air-fired coal power plant into the recycled oxy-fuel plant. In this paper, an intensively comparative study was performed with a 25kW quasi one-dimensional down-fired pulverized coal combustor for clarifying the differences between air combustion and recycled oxy-fuel combustion of bituminous coal. In oxy-fuel mode, the volume recycle ratio of flue gas (dry basis) was kept at 77.8%, and then the oxygen concentration at 30% to provide a similar heat flux output to the air mode. A nitrogen-aspirated, isokinetic particulate sampling probe, followed by either electric low-pressure impactor (ELPI) or scanning mobility particle spectroscopy (SMPS), was introduced to measure fine particulates, while an air-cooled deposition probe was designed to collect ash deposit samples. The results indicated that, under similar furnace temperature profiles, the oxy-fuel combustion leads to the higher fine particulate formation, finer bulk ash particle formation and lower ash deposition. The aerodynamic factor, instead of the chemical composition related to fine particulates, determines the difference in the ash deposition behavior in two combustion modes. [Copyright &y& Elsevier]

Transformation of As in the post-combustion zone has a significant effect on its emission characteristics. In this study, redistribution of As that occurs during the interaction between flue gas and ash has been elucidated using a double-layer fixed-bed reactor at 800–1000 °C. Herein, flue gas and ash samples with different As contents and inorganic components were produced by two varieties of coals. The As content in the ash with high Ca and Al contents increased after interacting with the flue gas with a high As content, indicating gaseous As adsorption. However, the As content in the ash decreased by up to 67% for other cases. The blank experiments showed that the As occurrences in ash were thermally stable, suggesting that the decrease was caused by the action of flue gas. Accordingly, the effect of flue gas components on the release of As from the ash was investigated. It was found that a significant amount of As release was primarily caused by the reaction of gaseous Na in the flue gas with the residual As in ash. A bi-directional interaction was observed between the high-temperature flue gas and ash, which led to the redistribution of As. Unlabelled Image • Interaction between high-temperature flue gas and ash caused As transformation. • Ash with high Ca and Al contents obviously adsorbed gaseous As in the flue gas. • Gaseous Na promoted the residual As release via reaction with aluminosilicate. [ABSTRACT FROM AUTHOR]