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The steady burning and stabilization of the boundary layer diffusion flame over a gasifying condensed fuel surface, commonly called the Emmons flame, is an important problem in the study of boundary layer combustion. We investigate herein, theoretically and numerically, the liftoff distance and blowout limit of the Emmons flame, through the corresponding response of the controlling triple flame in the leading edge of the bulk flame. An explicit solution of the flame liftoff distance and the critical blowout limit is derived, with the theoretical results agreeing well with the numerical simulation for an extensive range of the system parameters. In particular, it is shown that the transversal velocity gradient (TVG) ahead of the triple flame renders the flame harder to liftoff and blowout, with this effect increasing for increasing TVG and decreasing triple flame curvature, which is related to the mixture fraction gradient. Furthermore, the Spalding mass transfer number, B v , for the surface segment ahead of the flame front affects the flame stabilization and blowout limit by modifying the similarity structure of the flow and the location of stoichiometry. Thermal expansion of the flow around the triple flame together with the surface viscous drag also significantly promotes flame stabilization. [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]

With the increasing use of low-grade lignite for energy production, ash related problems have received renewed attention due to their detrimental effects on power plant equipment. This study investigates the fuel effect on the ash deposition in a 25 kW self-sustained down-fired furnace, with practical implications to the efficient utilization of a sodium-rich Zhundong (ZD) lignite. The active fouling control methods include co-firing with Si/Al-dominated HAF-2 bituminous coal and pretreatment using water leaching. Both manipulation methods reduce the propensity to form ultrafine PM 0.2 particles and ash deposition for simulated furnace exit conditions compared with combustion of only ZD lignite. While the deposits formed by HAF-2 bituminous are prone to shed, blending with ZD lignite increases the ash collection efficiency and prevents massive deposit collapse. The morphology of the ZD ash deposits is characterized by a smooth shape with wide spreading angle, whereas those formed by the blended samples with ≥33 wt% bituminous coal have sharp and pointed shapes with smaller spreading angles. Deposits by samples with ≥67 wt% ZD lignite have an inner layer of micro-size particles. The local basic to acid ratio of the deposits gradually decreases from the inner to the outer layers. The apparent particle sticking probability due to the fuel effect derived from the experimental and theoretical results positively correlates with the yield of ultrafine PM 0.2 particles for the various coal samples. [ABSTRACT FROM AUTHOR]

The ash fouling during coal combustion is initialized by a deposition of ‘sticky’ inner layer composed of fine particulate matter (PM). In this work, we present a quantitative investigation on the built-up of the initial deposit layer and its roles in the capture of coarse fly ash particles. The fly ash and ash deposit are sampled in a 25 kW self-sustained pulverized coal combustor for various lignite samples including Zhundong and Hami lignites. The ash collection efficiency onto the probe is positively correlated to the ultrafine particle formation fraction. The deposition rate of fine particle PM 10 is modeled in a combined Eulerian-Lagrangian scheme, with an input particle concentration measured at the entrance of boundary layer. The predicted results divulge that the thermophoresis enhances the impaction efficiencies of submicron and micron sized particles by 1–2 orders. The deposited particles are more concentrated on the central top region of the probe, and the trend is even more remarkable for larger particles, consistent with experimental observations. For both Zhundong and Hami lignites, the deposits are largely composed of fine PM 10 and condensed matter during the initial stage of one minute, but the weight percentages of fine particle deposits over the total deposit mass decrease significantly in the subsequent 5–10 min. Consequently, the average sticking efficiency of the coarse mode PM 10 + increases asymptotically as the deposited PM 10 grows to ~ 1.5 g/m 2 . Zhundong lignite shows higher saturated sticking efficiency of PM 10 + than Hami lignite, which is possibly attributed to the different surface properties of the bulk ash particles. [ABSTRACT FROM AUTHOR]

Agglomeration of aluminum nanoparticles (nano-Al) is inevitable due to the strong van der Waals forces. It is supposed to affect the combustion characteristics of nano-Al. However, the agglomeration effect is not fully understood yet due to the worse description of the morphological evolution of agglomerates during reaction. In this paper the morphological changes of agglomerated nano-Al, which mainly results from the oxide shell thickening when the particle temperature is below the melting point of alumina, are confirmed in the thermogravimetric analyzer (TGA) and Hencken experiments. Three factors ( f A , f ht , and f sp ) are defined to describe the morphological changes quantitatively through proposed formulas. Thus the agglomerate could be equivalent to an effective spherical particle to help studying the effect of agglomeration on combustion. The results indicate two combustion modes for different agglomerates. The small Al agglomerate is hard to combust fully due to the strong heat transfer with ambience, corresponding to the high-temperature oxidation mode. However, at the same environment, as the agglomerate becomes larger, the maximum particle temperature ( T p,max ) increases until that the agglomerate burns fully, corresponding to the fully-fledged combustion mode. At that mode, the burning particle temperature is mainly controlled by the viscous sintering of agglomerates. [ABSTRACT FROM AUTHOR]

Boron is an attractive high-energy fuel additive. But it could not burn efficiently in practical systems due to its high ignition temperature and slow burning velocity. Finding methods to enhance the combustion of boron is desired. This work focused on the combustion characteristics of boron submicroparticles with and without plasma discharges in a hot environment supported by CH 4 /N 2 /O 2 flat flame based on the optical diagnostics. The boron submicroparticles were dispersed by the nebulization method to control the agglomeration. The well-dispersed boron flame exhibited two different burning modes, depending on the ambient temperature. As the ambient temperature was above 1520 K, the boron flame showed definitely two-stage characteristics where the upstream of particle flow was yellow, corresponding to the first-stage flame, while the downstream was green and diffusive, corresponding to the second-stage flame. The first-stage and second-stage burn times were respectively in the range of 0.46–1.08 ms and 0.92–1.87 ms, as the ambient temperature decreased from 1752 K to 1520 K. The chemical kinetics-controlled mechanism was confirmed by the nearly linear size dependence of the burn time (d 1 law). Nevertheless, as the ambient temperature was below 1520 K, the boron submicroparticles were partially burned or oxidized, exhibiting a mildly orange stream. This mild boron flame could be enhanced using a plasma discharge. The ignition delay time was shortened from 3.06 ms to 0.77 ms when the discharge was introduced at the ignition delay stage. The two-stage combustion characteristics occurred when the discharge was introduced at the combustion stage. [ABSTRACT FROM AUTHOR]

Pyrolysis and oxidation of heavy oil is the fundamental of in situ combustion process. Heavy oil from Xinjiang, China and its main components, saturates, aromatics and resins (SAR), were used as samples to study their pyrolysis and oxidation characteristics with a thermogravimetric analyzer (TGA) and a fixed-bed reactor. The DTG curves and the detected gas products differed between pyrolysis and oxidation of the heavy oil. Elemental analysis of pyrolysis and oxidation cokes indicates that low temperature oxidation (LTO) primarily accounted for the differences. Compared with independent pyrolysis and oxidation of saturates, aromatics and resins, they showed little interactions during co-pyrolysis, but significant interactions during co-oxidation. When saturates co-oxidized with aromatics or resins, both the beginning temperatures of LTO were increased, the DTG curve at high temperature oxidation (HTO) remained unchanged during co-oxidation with aromatics, while the DTG peaks at HTO changed from two into one during co-oxidation with resins. Co-oxidation of aromatics and resins showed similar effect at HTO. Oxygen-adding reactions of saturates, aromatics, and resins at low temperature promoted the polycondensation between intermediate products and altered the subsequent cracking reactions, which caused special characteristics during their co-oxidation. [ABSTRACT FROM AUTHOR]

Dispersed palladium on the high-surface-area ceria support is synthesized via one-step flame-assisted spray pyrolysis method, which applies a highly-quenched stagnation-point flame controlling catalyst sizes and structures. The X-ray photoelectron spectroscopy (XPS) spectra of Pd show a significantly high binding energy for flame-made Pd/CeO 2 catalysts, possessing a value of 1.7 eV larger than the reference value in literature. It suggests that the partial electron transfer occurs from metal Pd to their supports during the synthesis process, which creates Pd electron-deficient (cationic Pd δ+ ) and Ce electron-rich (anion Ce δ − ), respectively. The catalytic activities of CH 4 oxidation are performed over the temperatures ranging from 200 °C to 600 °C. In comparison with inert support materials, the synergistic effect is found between palladium and support ceria that leads to the enhanced catalytic activity. During the heating and cooling cycles of CH 4 oxidation, Pd/CeO 2 catalysts exhibit an exceptional inhibition effect against the sintering of Pd cluster and its dispersion decrement, which is related to strong electronic interaction of metal-support interfaces induced by the aforementioned partial electron transfer. [ABSTRACT FROM AUTHOR]

The high-reserve Zhundong lignite, rich in Alkali and Alkaline Earth Metal (AAEM) elements, causes severe fouling and slagging problems in stationary combustion systems. In this paper, the ash deposition propensity as well as its relation to AAEM-rich fine particulates was investigated in a 25 kW down-fired furnace possessing similar conditions to practical combustors. The high content AAEM (mainly calcium) species in Zhundong lignite results in the molten slag at the vicinity of the burner inlet, differing from other case burning high-ash-fusion (HAF) bituminous coal. The ash deposits were collected at a position with gas temperature of 800 °C, whereas the fine particulates were sampled at the same position by a two-stage nitrogen-dilution isokinetic probe. The deposition tendency of ash particles from Zhundong lignite is apparently higher than those from contrast fuels and even herbaceous biomass. It is then related to the sticky surfaces of both bare deposition tube and bulk fly ash particles, forming from large amounts of AAEM species, which enhances the deposition propensity. [ABSTRACT FROM AUTHOR]

The temporal release of K from single pinewood particles of four different initial masses (20–50 mg) in a Hencken burner was measured in-situ by laser-induced break down spectroscopy (LIBS). Particle temperature during the combustion was measured by two-color pyrometry. A calculation method was developed to determine the K release flux across the plume based on LIBS measurement and computational fluid dynamics (CFD) model. For all the 4 particle sizes, the temporal release rate of K showed a peak during devolatilization, a monotonous increase during char combustion and a monotonous decrease during the ash stage. It is found that the cumulative fraction of released K was below 4% in the devolatilization stage and ∼20% in the char combustion stage. The particle mass did not have significant influence on the K release. The mechanism of the K release during the char combustion was analyzed, based on the initial portioning of K as inorganic K and organically-bound Char-K. The inorganic K releases mainly by thermal decomposition. The Char-K releases mainly by converting first to inorganic K when the char is burnt and then by releasing to gas phase from the inorganic K. A kinetic model for the release of K during pinewood char combustion was developed on this basis. The kinetic parameters for the release of K was found to be influenced by the initial partitioning between the inorganic K and the Char-K. The correlated initial proportion of inorganic K in pinewood char was found to be 0.37, while the correlated pre-exponential factor and the activation energy for K release were 12.5 1/s and 89.9 kJ/mol, respectively. [ABSTRACT FROM AUTHOR]

The interaction between flue gas and ash causes the redistribution of heavy metals and affect their emission characteristics. In this study, a double-layer fixed-bed reactor was used to investigate the effect of this interaction on Pb distribution at 800–1000 °C. Obvious gaseous Pb adsorption by ash just occurred in the case of high-Pb flue gas and less ash melting. In other cases, significant Pb release from ash occurred under the action of the high-temperature flue gas. The occurrences of Pb in ash was found to be thermally stable. Ash melting is the main reason for the Pb release. Gaseous Na in the combustion flue gas and reducing components in the pyrolysis flue gas promote the Pb release via promoting the ash melting. H 2 S and HCl in the pyrolysis flue gas also promote the Pb release by forming PbS and PbCl 2 , which are more volatile. [Display omitted] • The interaction between high-temperature flue gas and ash redistributes Pb. • Ash melting causes the significant release of Pb from ash at high temperatures. • Gaseous Na and reducing components in flue gas promote ash melting and Pb release. • H 2 S/HCl in flue gas also promotes Pb release by forming volatile PbS/PbCl 2. [ABSTRACT FROM AUTHOR]