Ignition of single coal particle in a hot furnace under normal- and micro-gravity condition

An experimental study on ignition and combustion of single particles was conducted at normal gravity (1-g) and microgravity (μ-g) for three high volatile coals with initial diameter of 1.5 and 2.0 mm, respectively. The non-intrusive twin-color pyrometry method was used to retrieve the surface temperature of the coal particle through processing the images taken by a color CCD camera. At the same time, a mathematical model considering thermal conduction inside the coal particle was developed to simulate the ignition process.Both experiments and modeling found that ignition occurred homogeneously at the beginning and then heterogeneously for the testing coal particles burning at μ-g. Experimental results confirmed that ignition temperature decreased with increasing volatile content and increasing particle size. However, contradicted to previous studies, this study found that for a given coal with certain particle size, ignition temperature was about 50–80 K lower at μ-g than that at 1-g.The model predictions agreed well with the μ-g experimental data on ignition temperature. The criterion that the temperature gradient in the space away from the particle surface equaled to zero was validated to determine the commence of homogeneous ignition. Thermal conduction inside the particle could have a noticeable effect for determining the ignition temperature. With the consideration of thermal conduction, the critical size for the phase transient from homogeneous to heterogeneous is about 700 μm at ambient temperature 1500 K and oxygen concentration 0.23.     

Ignition and combustion of a falling, single sodium droplet

Ignition and combustion of a single sodium droplet has been studied experimentally, by use of a falling droplet. It is found that the ignition delay time increases first gradually and then rapidly, with decreasing initial temperature and/or oxygen concentration, and reaches the limit of ignitability, because of the suppression of surface reaction in the ignition stage. It is also found that with decreasing droplet diameter, the ignition delay time first decreases gradually, because of the decrease in the droplet mass to be heated, and then increases steeply, because of the enhancement of heat loss from the droplet surface. As for the effect of the relative speed, it is found that the ignition delay time increases with increasing relative speed, because of the enhanced heat loss. Experimental comparisons with the analytical results have also been conducted to elucidate dominant parameters, and it is confirmed that a set of comprehensive parameters in the literature can be useful in correlating dominant parameters that influence the ignition delay and/or the limit of ignitability. Furthermore, the analysis has been extended to determine the critical size for the ignition and that for the minimum ignition delay time. Combustion behavior after the ignition has also been examined, and it is found that d²-law can hold for the sodium droplet combustion. In addition, it is found that the burning rate-constant without forced convection has nearly the same value as those for usual hydrocarbon droplets, while the sodium combustion in air is quite similar to that of the usual hydrocarbon fuel in an oxidizer-rich environment.     

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Direct observations of single particle fragmentation in the early stages of combustion under dry and wet conventional and oxy-fuel conditions

This article investigates the single particle fragmentation of three solid fuels in the early stages of combustion under dry and wet conventional and oxy-fuel conditions. The three solid fuels studied were a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal, and a charcoal residue from black acacia. Particles, with size in the range 125–150 µm, were burned in a drop tube furnace with a constant wall temperature of 1475?K, under six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the fragmentation process during the early stages of combustion and the collected images were treated to characterize the fragmentation mode, probability and time. The observed fragmentation modes are characterized by the occurrence of exfoliation, radial fragmentation or a combination of both. The results disclose that the fragmentation mode is strongly affected by the fuel type, but less affected by the atmosphere; the fragmentation probability is strongly affected by both the fuel type and the atmosphere; and, finally, fragmentation in air occurs significantly dispersed after ignition, but it tends to cluster closer to the ignition under simulated oxy-fuel conditions.     

A new technique for the measurement of the product CO/CO2 ratio at the surface of char particles burning in a fluidized bed

A new experimental technique is proposed to measure the product CO/CO₂ ratio at the surface of spherical char particles during fluidized bed combustion. It is based on the measurement of the burning rate of a single char particle under low oxygen concentration conditions and on the use of an accurate prediction of the particle Sherwood number. This technique was applied to spherical char particles obtained from a bituminous coal which is characterized by a low attrition and fragmentation propensity. The product CO/CO₂ ratio was measured at a bed temperature of 850 °C and at a fluidization velocity of 0.3 m/s in a lab-scale apparatus operated with a bed of 0.5–0.6 mm sand. The char particle size was varied between 2 and 7 mm and the inlet oxygen concentration between 0.1% and 2.0%. Results showed that under the experimental conditions investigated carbon was mostly oxidized to CO₂ within the particle boundary layer, with a maximum fraction of carbon escaping as CO of 10–20% at the lowest oxygen concentrations and largest particle sizes.     

In-situ gasification chemical looping combustion of plastic waste in a semi-continuously operated fluidized bed reactor

Conventional air incineration of plastic waste has been considered as one of important sources of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) through de novo synthesis and precursor conversion. Chemical looping combustion (CLC) is an attractive technology for the conversion of plastic wastes to energy with the potential to drastically suppress the formation of PCDD/Fs. In this paper, the iG-CLC (in-situ gasification CLC) experiments of plastic waste were implemented in a semi-continuously operated fluidized bed reactor, which actually simulates the fuel reactor of a continuously-operated interconnected fluidized bed reactor. A kind of low-cost material, natural iron ore without/with 5 wt% CaO adsorbent through the ultrasonic impregnation method, was used as oxygen carrier (OC). Firstly, some key performances of the reactor system, such as the relevance of the bed inventory to the flow rate of fluidizing agent as well as the relationship between the feeding rate and overflow rate of OC, were calibrated. Then, 90 min of single experiment was conducted for each experimental case and an accumulative operation of more than 10 h was attained. Typically, the combustion efficiency can reach at about 98%, and both the carbon conversion and CO₂ yield can approach to 95% at 900 °C and input thermal power of 150 W with a mixture of 5 vol% H₂O and 95 vol% N₂ as the fluidizing agent (U_FR/U_mf = 3). Moreover, the results obtained in the semi-continuously operated fluidized bed reactor demonstrated that CaO decoration to iron ore is conductive to suppressing the formation of chlorobenzene (as a toxic matter and precursor/intermediate of PCDD/Fs) and does not obviously deteriorate the OC performance.     

Experimental and numerical investigations on the interactions of volatile flame and char combustion of a coal particle

The model that takes chemical reactions, heat and mass transfers in the boundary layer of the particle into account simultaneously, is developed for simulating the combustion of a pulverized coal particle. The FTIR in situ temperature-measurements and the comparison between numerical simulations for the pulverized coal and the devolatilized char show that the volatile flame induces the combustion of the primary product of surface oxidation CO. Due to the influence of volatile flame, the char particle can be ignited at temperature lower than its heterogeneous ignition temperature, which elucidates the physical essence of joint hetero-homogeneous ignition mode discovered by Jüntgen.     

Ignition enhancement by addition of NO and NO2 from a N2/O2 plasma torch in a supersonic flow

The effects of NO and NO₂ produced by using a plasma jet (PJ) of a N₂/O₂ mixture on ignition of hydrogen, methane, and ethylene in a supersonic airflow were experimentally and numerically investigated. Numerical analysis of ignition delay time showed that the addition of a small amount of NO or NO₂ drastically reduced ignition delay times of hydrogen and hydrocarbon fuels at a relatively low initial temperature. In particular, NO and NO₂ were more effective than O radicals for ignition of a CH₄/air mixture at 1200 K or lower. These ignition enhancement effects were examined by including the low temperature chemistry. Ignition tests by a N₂/O₂ PJ in a supersonic flow (M = 1.7) for using hydrogen, methane, and ethylene injected downstream of the PJ were conducted. The results showed that the ignitability of the N₂/O₂ PJ is affected by the composition of the feedstock and that pure O₂ is not the optimum condition for downstream fuel injection. This result of ignition tests with downstream fuel injection demonstrated a significant difference in ignition characteristics of the PJ from the ignition tests with upstream fuel injection.     

Combination of genetic algorithm and computational fluid dynamics in combustion process emission minimization

A computational fluid dynamics (CFD) code was connected with a systematic optimization algorithm to find the optimum settings for nitric oxide (NO) emission minimization in the bubbling fluidized bed boiler. This approach is superior to the typical approach in which CFD is used to calculate only a few intuitively selected cases, and the best case found is treated as the ‘optimum’ solution. Ammonia injection (selective non-catalytic reduction) is applied to control NO emission in the modelled fluidized bed boiler. Design variables consisted of ammonia mass flow from distinct injections. Genetic algorithm (GA) was used for the optimization. The predicted NO emission was reduced approximately 15% with respect to the current operating point by the application of GA and CFD. The use of CFD connected with systematic optimization algorithm seems to be a promising design tool for combustion process optimization.     

Nonpremixed ignition of H2/air in a mixing layer with a vortex

The ignition of a laminar non-premixed H₂/air mixing layer with an embedded vortex was computationally studied with detailed chemistry and transport. The initial vortex velocity and pressure fields were specified based on the stream function of an incompressible nonviscous vortex. The fuel side is pure hydrogen at 300 K, and the oxidizer side is air at 2000 K. The vortex evolution process was found to consist of two ignition events. The first ignition occurs in a diffusion mode with chain branching reactions dominating. The second ignition takes place in the premixed mode, with more chemical reactions involved, and is significantly affected by the heat and species generated in the first ignition event. The coupling between the most reactive mixture fraction and scalar dissipation rate was verified to be crucial to the ignition delay. The effects of the vortex strength, characteristic size, and its center location were individually investigated. For all vortex cases, the ignition delay was shorter than that of the 1D case. Furthermore, the ignition delay has a nonmonotonic dependence on all the vortex parameters.     

Ignition in a supersonic flow by a plasma jet of mixed feedstock including CH4

Ignition tests of hydrocarbon fuels in a supersonic airflow by plasma jet (PJ) torches of mixed feedstock, including methane (CH₄), such as N₂/CH₄ and N₂/CH₄/O₂ mixtures, were conducted. The Mach number of the airflow was 2.0, and the total temperature and total pressure of the main flow were those of room conditions. The wall pressure increase due to combustion of hydrocarbon fuels for the N₂/CH₄ PJ exceeded those of pure O₂ and N₂ PJs at high electric power input. Equilibrium calculations showed that the main species in high-temperature PJ, aside from N₂, were H₂, H, and HCN. Considering the slight impact of the HCN species on ignition delay time, the combustion enhancement by the N₂/CH₄ PJ was caused primarily by the existence of a large amount of H and H₂ dissociated from CH₄ molecules in the PJ. Moreover, the addition of O₂ to the N₂/CH₄ feedstock further enhanced the combustion and stability of the N₂/CH₄ PJ. The existence of O₂ increased the temperature and the number of H radicals in the PJ exhaust.     

Ignition and devolatilization of pulverized bituminous coal particles during oxygen/carbon dioxide coal combustion

Oxygen/carbon dioxide recycle coal combustion is actively being investigated because of its potential to facilitate CO₂ sequestration and to achieve emission reductions. In the work reported here, the effect of enhanced oxygen levels and CO₂ bath gas is independently analyzed for their influence on single-particle pulverized coal ignition of a U.S. eastern bituminous coal. The experiments show that the presence of CO₂ and a lower O₂ concentration increase the ignition delay time but have no measurable effect on the time required to complete volatile combustion, once initiated. For the ignition process observed in the experiments, the CO₂ results are explained by its higher molar specific heat and the O₂ results are explained by the effect of O₂ concentration on the local mixture reactivity. Particle ignition and devolatilization properties in a mixture of 30% O₂ in CO₂ are very similar to those in air.     

A novel continuous process for synthesis of carbon nanotubes using iron floating catalyst and MgO particles for CVD of methane in a fluidized bed reactor

A novel continuous process is used for production of carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CVD) of methane on iron floating catalyst in situ deposited on MgO in a fluidized bed reactor. In the hot zone of the reactor, sublimed ferrocene vapors were contacted with MgO powder fluidized by methane feed to produce Fe/MgO catalyst in situ. An annular tube was used to enhance the ferrocene and MgO contacting efficiency. Multi-wall as well as single-wall CNTs was grown on the Fe/MgO catalyst while falling down the reactor. The CNTs were continuously collected at the bottom of the reactor, only when MgO powder was used. The annular tube enhanced the contacting efficiency and improved both the quality and quantity of CNTs.The SEM and TEM micrographs of the products reveal that the CNTs are mostly entangled bundles with diameters of about 10-20 nm. Raman spectra show that the CNTs have low amount of amorphous/defected carbon with I_G/I_D ratios as high as 10.2 for synthesis at 900 °C. The RBM Raman peaks indicate formation of single-walled carbon nanotubes (SWNTs) of 1.0-1.2 nm diameter.     

Clustering and combustion of dilute aluminum particle clouds in a post-detonation flow field

A hybrid two-phase numerical methodology is used to investigate the flow-field subsequent to the detonation of a spherical charge of TNT with an ambient distribution of a dilute cloud of aluminum particles. The interaction of the particle cloud with the contact surface results in Rayleigh–Taylor instability, which grows in time and gives rise to a mixing layer where the detonation products mix with the air and afterburn. At early times, the ambient particles get engulfed into the detonation products and ignite. Subsequently, they catch up with the Rayleigh–Taylor structures, and the vortex rings around the hydrodynamic structures cause transverse dispersion that results in the clustering of particles. Then, the particles leave the mixing layer and quench, yet preserve their hydrodynamic foot print. Preferential heating and combustion of particles occurs due to clustering. A higher initial mass loading in the ambient cloud results in larger clusters due to stronger/larger vortex rings around the hydrodynamic structures. A larger particle size results in the formation of fewer and degenerate clusters when the initial width of the cloud is larger. A theoretical model is used to predict the bubble amplitudes, and are in good accordance with the simulation results. Overall, this study has provided some useful insights on the explosive dispersal of dilute aluminum particle clouds and the gas dynamics of the flow field in the mixing layer.     

Dynamic evolution of impaction and sticking behaviors of fly ash particle in pulverized coal combustion

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.     

Effect of feedstock water leaching on ignition and PM1.0 emission during biomass combustion in a flat-flame burner reactor

In this work, the effects of feedstock water leaching on ignition and PM_1.0 emission during biomass combustion were studied, for the first time, in a Hencken flat-flame burner reactor (HFFBR). A high-speed video camera and high-resolution electrical low-pressure impactor were respectively employed to diagnose ignition and PM_1.0 along the height of the burner. The mineral composition of PM₁₀₊ was measured as a function of height to demonstrate the potassium release during the early stage of biomass combustion. The results show that water leaching does not change the functional group of the biomass (straw), but increases the BET surface area and pore volume. Water leaching removes 90% of the potassium and all the chlorine, reducing the same amount of PM_1.0 emission. The effect of water leaching on ignition delay observed in the flat-flame burner reactor agrees with the delay of biomass-devolatilization in TGA. Profiles of mineral composition in the PM₁₀₊ with height shows that a large amount of the potassium is released before biomass ignition. This indicates that, at realistic heating rates, the catalytic promotion of water-soluble minerals on biomass ignition is primarily through promoting devolatilization. The ignition delay of biomass particles caused by water leaching is more significant at lower temperature, e.g., ignition is delayed from 20 to 24?ms at 1000?°C, and from 9.2 to 10.2?ms at 1300?°C.     

一个灵敏测定过氧化氢的吖啶红共振散射光谱新方法

在pH为5.0的HAc-NaAc缓冲液中,Fe2 催化H2O2产生·OH,·OH氧化I-为I2.过量I-与I2反应生成的I-3,与吖啶红(AR)形成AR-I3缔合物分子.在疏水作用和分子间力作用下,AR-I3缔合物分子自动聚集形成(AR-I3)n缔合物微粒.该缔合物微粒在320,400,595 nm处产生3个共振散射峰.H2O2的浓度在0.50～16.0×10-6 mol·L-1范围内与400 nm波长的共振散射光强度成线性关系,方法的检出限为2.0×10-7 mol·L-1 H2O2,用于废水中H2O2的测定,结果满意,回收率在97.9%～101.2%之间.     

Effect of a single water molecule on the formations of H2O2 + ClO from HO2 + HOCl reaction under tropospheric conditions

ABSTRACT

In this work, four different channels, represented by H₂O???HO₂ + HOCl, HO₂???H₂O + HOCl, H₂O???HOCl + HO₂ and HOCl???H₂O + HO₂ have been analysed for water-catalysed formations of H₂O₂ + ClO to gain insight into the potential impact of the reaction in the atmosphere. The results at the CCSD(T)/aug-cc-pVTZ//MP2/6-311+G(2df,2p) level show that the H₂O???HO₂ + HOCl reaction is kinetically the most favourable channel among the four channels. Compared to the channel of H₂O₂ + ClO formations without water vapour, the effective rate constant of H₂O???HO₂ + HOCl reaction is estimated to be faster than the naked reaction by 2–3 orders of magnitude, indicating that the single water molecule in the H₂O???HO₂ + HOCl channel exhibits a positive catalytic effect on enhancing the rate of H₂O₂ + ClO formations. Meanwhile, it is interesting that the transfer process between H₂O???HOCl + HO₂ and H₂O???HO₂ + HOCl has an activation energy of 0.6 kcal?mol^?1 and can occur easily under tropospheric conditions.     

Characterization and luminescence of Eu/Sm-coactivated CaYAl3O7 phosphor synthesized by using a combustion method

A novel red-emitting phosphor, CaYAl3O7 : Eu 3+ , Sm3+ , is synthesized by a combustion method at a low temperature (850℃), and the single phase of CaYAl3O7 is confirmed by powder X-ray diffraction measurements. The photoluminescence property results reveal that the red emission intensity of Eu3+ is strongly dependent on the Sm3+ concentration. Only the Eu 3+ luminescence is detected in the Eu 3+ -Sm3+ co-doped CaYAl3O7 phosphor with 393 nm excitation. However, under the characteristic excitation (402 nm) of Sm3+ , not only the Sm3+ emission but also the Eu 3+ emission are observed. A possible mechanism of the energy transfer between Sm3+ and Eu 3+ is investigated in detail.     

The effect of fuel inlet turbulence intensity on H2/CH4 flame structure of MILD combustion using the LES method

Large eddy simulations (LES) are employed to investigate the effect of the inlet turbulence intensity on the H₂/CH₄ flame structure in a hot and diluted co-flow stream which emulates the (Moderate or Intense Low-oxygen Dilution) MILD combustion regime. In this regard, three fuel inlet turbulence intensity profiles with the values of 4%, 7% and 10% are superimposed on the annular mixing layer. The effects of these changes on the flame structure under the MILD condition are studied for two oxygen concentrations of 3% and 9% (by mass) in the oxidiser stream and three hot co-flow temperatures 1300, 1500 and 1750 K. The turbulence-chemistry interaction of the numerically unresolved scales is modelled using the (Partially Stirred Reactor) PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influences of the turbulence intensity on the flame structure under the MILD condition are studied by using the profile of temperature, CO and OH mass fractions in both physical and mixture fraction spaces at two downstream locations. Also, the effects of this parameter are investigated by contours of OH, HCO and CH₂O radicals in an area near the nozzle exit zone. Results show that increasing the fuel inlet turbulence intensity has a profound effect on the flame structure particularly at low oxygen mass fraction. This increment weakens the combustion zone and results in a decrease in the peak values of the flame temperature and OH and CO mass fractions. Furthermore, increasing the inlet turbulence intensity decreases the flame thickness, and increases the MILD flame instability and diffusion of un-burnt fuel through the flame front. These effects are reduced by increasing the hot co-flow temperature which reinforces the reaction zone.