煤焦异相还原NO的实验研究

在U形石英管固定床反应器中实验研究了乌达煤焦对NO的异相还原作用。乌达煤焦在程序升温和快速升温条件下还原NO的实验结果表明,反应温度超过700°C时,NO的还原率开始迅速上升,在实验范围内,温度维持在800°C～1000°C之间,NO的还原率较高。反应区的初始氧气浓度越高,NO的还原率越低,表明煤焦与氧气的反应和煤焦还原NO的反应之间存在着较强的竞争作用。制焦温度越高、煤焦粒径越粗,NO的还原率越低,表明制焦条件对煤焦还原NO的反应性有较大影响。     

Combustion kinetics of coal chars in oxygen-enriched environments

Oxygen-enhanced and oxygen-fired pulverized coal combustion is actively being investigated to achieve emission reductions and reductions in flue gas cleanup costs, as well as for coal-bed methane and enhanced oil recovery applications. To fully understand the results of pilot scale tests and to accurately predict scale-up performance through CFD modeling, accurate rate expressions are needed to describe coal char combustion under these unconventional combustion conditions. In the work reported here, the combustion rates of two pulverized coal chars have been measured in both conventional and oxygen-enriched atmospheres. A combustion-driven entrained flow reactor equipped with an optical particle-sizing pyrometry diagnostic and a rapid-quench sampling probe has been used for this investigation. Highvale subbituminous coal and a high-volatile eastern United States bituminous coal have been investigated, over oxygen concentrations ranging from 6 to 36 mol% and gas temperatures of 1320-1800 K. The results from these experiments demonstrate that pulverized coal char particles burn under increasing kinetic control in elevated oxygen environments, despite their higher burning rates in these environments. Empirical fits to the data have been successfully performed over the entire range of oxygen concentrations using a single-film oxidation model. Both a simple nth-order Arrhenius expression and an nth-order Langmuir-Hinshelwood kinetic equation provide good fits to the data. Local fits of the nth-order Arrhenius expression to the oxygen-enriched and oxygen-depleted data produce lower residuals in comparison to fits of the entire dataset. These fits demonstrate that the apparent reaction order varies from 0.1 under near-diffusion-limit oxygen-depleted conditions to 0.5 under oxygen-enriched conditions. Burnout predictions show good agreement with measurements. Predicted char particle temperatures tend to be low for combustion in oxygen-depleted environments.     

Combustion of nanoaluminum at elevated pressure and temperature behind reflected shock waves

This study presents experimental measurements on the combustion of nanoaluminum particles behind reflected shock waves in a shock tube. These experiments were performed at elevated pressures (4-32 atm) and temperatures (1200-2100 K) in the oxidizers oxygen and carbon dioxide, with nitrogen also present. The light emission from the reacting particles was monitored. For all cases, a brief period of intense light emission was observed soon after exposure to the reflected shock conditions. The time scales of this emission event are quantified by the 10-90% integrated emission intensity method to yield a reaction time for this rapid exothermic process. The duration of the emission is found to be 50-500 μs for the conditions tested here. Reaction times in 50% O₂ and 50% N₂ were shown to decrease significantly with ambient temperature, with Arrhenius-type exponentials fitting reasonably well to the observed experimental data. The reaction times were also dependent on pressure, with the timescales decreasing by 1.6-4 times as the pressure was increased from 8 to 32 atm over the range of temperatures in the experiments. In 50% CO₂ and 50% N₂, the reaction occurs in two sequential stages, with more of the emission at earlier times under higher-temperature conditions. Particle temperatures were also measured. During the bright emission event, the temperature rises above the ambient and then cools to near the ambient as the emission event ends. The peak temperature of the particle varied with ambient temperature, pressure, and oxidizer, with high ambient temperatures (2000 K), high pressures (32 atm), and high oxygen mole fractions (50%) giving the highest particle temperatures (∼3500 K). Conversely, 50% CO₂ atmospheres produced particle temperatures just slightly above the ambient. The spectral output of the light emission was shown to be dominated by broadband emission. At high temperatures and pressures in oxygen, weak emission from the AlO B-X transition was observed.     

Multivariable optimization of reaction order and kinetic parameters for high temperature oxidation of 10 bituminous coal chars

In industrial pulverized fuel combustion, char oxidation is generally limited by the combined effects of chemical reactions and pore diffusion. Under such conditions, char oxidation is frequently predicted by power law models, which despite their simplicity, are widely used in the comprehensive CFD modeling of pulverized coal boilers. However, there is no consensus on the apparent reaction order given by such models. This study developed a systematic approach which gives consistent values over a range of conditions. Apparent reaction orders for 10 bituminous coal chars were investigated with three different oxygen concentrations, ranging from 4 to 12 vol.%, and a gas temperature of 1223 K for each char. Experimental burnout profiles of the chars were obtained by means of an Isothermal Plug Flow Reactor operating at industrially realistic heating rates (10⁴ K/s). For various reaction orders between 0.05 and 2.00, kinetic parameters were independently determined, following numerical procedures recently suggested in the literature. The resulting values were incorporated into an empirical power law model and compared to experimental data for the 10 chars, over a burnout range of 0–75%. The best fit to the experiments occurs with apparent reaction orders of around one for all the chars.     

1-Butanol ignition delay times at low temperatures: An application of the constrained-reaction-volume strategy

Ignition delay times behind reflected shock waves are strongly sensitive to variations in temperature and pressure, yet most current models of reaction kinetics do not properly account for the variations that are often present in shock tube experiments. Particularly at low reaction temperatures with relatively long ignition delay times, substantial increases in pressure and temperature can occur behind the reflected shock even before the main ignition event, and these changes in thermodynamic conditions of the ignition process have proved difficult to interpret and model. To obviate such pressure increases, we applied a new driven-gas loading method that constrains the volume of reactive gases, thereby producing near-constant-pressure test conditions for reflected shock measurements. Using both conventional operation and this new constrained-reaction-volume (CRV) method, we have collected ignition delay times for 1-butanol/O₂/N₂ mixtures over temperatures between 716 and 1121 K and nominal pressures of 20 and 40 atm for equivalence ratios of 0.5, 1.0, and 2.0. The equivalence ratio dependence of 1-butanol ignition delay time was found to be negative when the oxygen concentration was fixed, but positive when the fuel concentration was held constant. Ignition delay times with strong pre-ignition pressure increases in conventional-filling experiments were found to be significantly shorter than those where these pressure increases were mitigated using the CRV strategy. The near-constant-pressure ignition delay times provide a new database for low-temperature 1-butanol mechanism development independent of non-idealities caused by either shock attenuation or pre-ignition perturbations. Comparisons of these near-constant-pressure measurements with predictions using several reaction mechanisms available in the literature were performed. To our knowledge this work is first of its kind that systematically provides accurate near-constant-enthalpy and -pressure target data for chemical kinetic modeling of undiluted fuel/air mixtures at engine relevant conditions.     

Remaining uncertainties in the kinetic mechanism of hydrogen combustion

An analysis of the performance of an updated hydrogen combustion mechanism is presented. Particular attention was paid to different channels of reaction between H atoms and HO₂ radicals, to pressure dependence of the recombination of HO₂ radicals, and to the anomalous rate constant of reaction between OH and HO₂ radicals. The contemporary choice of the reaction rate constants is presented with the emphasis on their uncertainties. Then the predictions of ignition, oxidation, flame burning velocities, and flame structure of hydrogen-oxygen-inert mixtures are shown. The modeling range covers ignition experiments from 950 to 2700 K and from subatmospheric pressures up to 87 atm; hydrogen oxidation in a flow reactor at temperatures around 900 K from 0.3 up to 15.7 atm; flame burning velocities in hydrogen-oxygen-inert mixtures from 0.35 up to 4 atm; and hydrogen flame structure at 1 and 10 atm. Comparison of the modeling and experiments is discussed in terms of the range of applicability of the present detailed mechanism. The necessity for analysis of the mechanism to have an exhaustive list of reactions is emphasized.     

Physicochemical structure characteristics and intrinsic reactivity of demineralized coal char rapidly pyrolyzed at elevated pressure

This study aims to investigate the effect of pyrolysis pressure on the physical and chemical structure characteristics and reactivity of subbituminous demineralized coal char. The pyrolysis experiments were studied under different pressures using a pressurized drop tube reactor (PDTR). Structural properties of coal chars were investigated by the application of scanning electron microscopy (SEM), nitrogen adsorption analyzer, automatic mercury porosimeter, and Raman spectroscopy, respectively. The Random Pore Model was used to determine kinetic parameters and intrinsic reactivity of chars. The specific pore volume of chars pyrolyzed at 900–1000 °C increased first and then decreased with pyrolysis pressure increasing, and the maximum value of the specific pore volume of chars occurred at 1.0 MPa. The degree of graphitization of chars deepened with the increase of temperature or pressure. Intrinsic activation energy of char-O₂ was within the range of 126–134 kJ/mol. The intrinsic reactivity of char-O₂ reaction showed strong correlation the Raman parameters with the change of pyrolysis conditions, and it suggested that the intrinsic reactivity of char-O₂ reaction was mainly affected by aromatic ring structures rather than pore structures.     

煤焦燃烧模型的热重实验研究

基于热重分析法对无烟煤和烟煤的两种煤焦在不同氧浓度下(100％、30％、20％和7％，其余为氩气)的燃烧模型进行了研究．比较和分析了几种主要燃烧动力学模型的特点，采用一种新的统一模型和动力学参数对上述煤焦的燃烧过程进行了模拟．分析了煤焦颗粒比表面积在燃烧过程中的变化规律，改进了原有煤焦模型中的表面积系数的选取办法，模拟结果与实验结果吻合程度较好．该模型能够较好地对煤焦的燃烧过程进行模拟，而且采用变氧浓度的实验方法是一种研究煤焦燃烧动力学参数的有效途径。     

Coal conversion submodels for design applications at elevated pressures. Part I. devolatilization and char oxidation

Numerous process concepts are under development worldwide that convert coal at elevated pressure. These developments rely heavily on CFD and other advanced calculation schemes that require submodels for several stages of coal chemistry, including devolatilization, volatiles combustion and reforming, char oxidation and char gasification. This paper surveys the databases of laboratory testing on devolatilization and char oxidation at elevated pressure, first, to identify the tendencies that are essential to rational design of coal utilization technology and, second, to validate two well-known reaction mechanisms for quantitative design calculations.Devolatilization at elevated pressure generates less volatile matter, especially tar. Low-rank coals are no less sensitive to pressure variations than bituminous coals; in fact, coal quality is just as important at elevated pressure as it is at atmospheric pressure. Faster heating rates do not enhance volatiles yields at the highest operating pressures. The FLASHCHAIN^® predictions for the devolatilization database depict the distinctive devolatilization behavior of individual samples, even among samples with the same nominal rank. The only sample-specific input requirements are the proximate and ultimate analyses of the coal. There were no systematic discrepancies in the predicted total and tar yields across the entire pressure range. Char oxidation rates increase for progressively higher O₂ partial pressures and gas temperatures, but are insensitive to total pressure at constant O₂ mole fraction. Char burning rates become faster with coals of progressively lower rank, although the reactivity is somewhat less sensitive to coal quality at elevated pressure than at atmospheric pressure. An expanded version of the carbon burnout kinetics model was able to represent all datasets except one within useful quantitative tolerances, provided that the initial intrinsic pre-exponential factor was adjusted for each coal sample.     

Kinetic and diffusive data from batch combustion of wood chars in fluidized bed

In this work it is studied the combustion of batches of wood char particles in a shallow fluidized bed at laboratory scale. Commercial and recarbonized chars from nut pine and cork oak parent woods were burned for bed temperatures of 600-750 °C and particle sizes range of 1.8-3.6 mm. A combustion model based on the two-phase theory of fluidization is presented to evaluate the global combustion resistance. Sherwood numbers and kinetic constants for the heterogeneous phase reaction are also assessed. Through the comparison among theoretical and experimental results, conclusions are drawn on the combustion mechanism as well as on the combustion controlling resistance. The Arrhenius law is proposed to predict the kinetic constants for the studied chars.     

Self-ignition of a lean mixture of n-pentane and air over a wide range of pressures

The shock-tube technique is used to measure the ignition delay time of a lean (?=0.5) mixture of n-pentane and air in a wide range of temperatures from 867 to 1534 K and pressures from 11 to 530 atm. The previously developed detailed kinetic model of ignition of hydrocarbons [Kinet. Catal. (2005), in press] is used to interpret the experimental data. The kinetic model includes mechanisms of ignition at high and low temperatures and a mechanism of ignition in the range of intermediate (1000-1200 K) temperatures. Each of these mechanisms is analyzed. The effect of the mixture pressure on the ignition at a temperature of 1000-1100 K is demonstrated.     

A detailed experimental and kinetic modeling study of n-decane oxidation at elevated pressures

The oxidation of n-decane/oxygen/nitrogen is studied at stoichiometric conditions of 1000 ppm fuel in the Princeton variable pressure flow reactor at temperatures of 520–830 K and pressures of 8 and 12.5 atm. The overall oxidative reactivity of n-decane is observed in detail to show low temperature, negative temperature coefficient (NTC) and hot ignition regimes. Detailed temporal speciation studies are performed at reactor initial temperatures of 533 K and 740 K at 12.5 atm pressure and 830 K at 8 atm pressure. Significant amounts of large olefins are produced at 830 K, at conditions of transition from NTC to hot ignition behavior. The predictions using available chemical kinetic models for n-decane oxidation are compared against each other and the experiments. Only the kinetic models of Westbrook et al., Ranzi et al., and Biet et al. capture the NTC behavior exhibited by n-decane. However, each of these models yields varying disparities in the mechanistic predictions of major intermediate species, including ethylene and formaldehyde. Analyses of the Westbrook et al. model are compared with the new data. The predicted double-peaked species yield of ethylene, a behavior not found for the other models or in the experimental observations results from deficiencies in the C₂ chemistry. Mechanistic validation information about fuel oxidation chemistry is also provided by the measurement of various larger carbon number alkene isomers at 830 K and 8 atm. The modeling analysis suggests that in addition to n-alkyl beta-scission chemistry, alkyl peroxy radical chemistry contributes significantly to the formation of these alkenes. Specific reaction pathways and rate constants which affect the computation of these observations are discussed.     

催化剂对贫煤焦还原NO动力学参数的影响

通过对中国一种贫煤焦还原N0的实验，确定不同条件下该反应的动力学参数及催化剂对这些参数的影响。结果表明，在贫氧条件下(a＜1)，催化剂对贫煤焦还原N0具有重要的催化作用，可降低贫煤焦与N0还原反应的活化能，增大频率因子，从而提高反应速率和N0的还原率。     

Aerosol-based method for investigating biomass char reactivity at high temperatures

An aerosol-based method was proposed and developed to characterize particles fragmented from biomass chars during oxidation. The chars were prepared from both wood and miscanthus pellets under various pyrolysis conditions. Char fragments with aerodynamic diameters in the range of 0.5–10 μm were suspended and transported in a reactive gas through an aerosol reactor, which was heated by an electric oven. The oxidation of char particles in the reactor was investigated by determining on-line the particle size distributions before and after passage through the reactor using an aerodynamic particle sizer (APS) spectrometer. The interpretation of APS data was evaluated by both experiments and models in which the fine char particles were assumed to keep either constant density or constant diameter during the oxidation process. The results indicate that the aerosol-based method can be used to determine reaction kinetics of char particles in the high-temperature range, where oxidation is normally controlled by diffusion limitation if measuring with the conventional techniques. The application of the aerosol method indicated that high pyrolysis temperature and prolonged retention time will reduce the char reactivity.     

Gas-solids kinetics of CuO/Al2O3 as an oxygen carrier for high-pressure chemical looping processes: The influence of the total pressure

Copper oxide on alumina is often used as oxygen carrier for chemical looping combustion owing to its very high reduction rates at lower temperatures and its very good mechanical and chemical stability at not too high temperatures. In this work, the redox kinetics of CuO/Al₂O₃ have been studied at elevated pressures and temperatures. All the experiments have been started under the same initial conditions to assure the same starting point. While other studies reported a negative effect of the total pressure on the redox kinetics, this study shows that this negative effect of the pressure is most probably caused by external mass transfer limitations in previous studies. Additionally, as long as external mass transfer limitations are prevented, the total pressure at which the reduction is performed does not affect the redox kinetics nor the morphological and chemical structure of the oxygen carrier. The sudden decrease in the reduction rate at higher particle conversions was not influenced by the operating pressure and was attributed to limitations in the spinel reduction kinetics.     

Burning characteristics of high-ash bangladeshi peat

The combustion characteristics of high-ash Bangladeshi peat was investigated in an atmospheric fluidized-bed combustor at bed temperatures of 750, 800 and 850°C. The particle size was varied from 8.05 mm to 22.76 mm. Combustion of this variety of peat occurred at constant size and thus followed a shrinking core model. Volatile evolution is complete before the particles reached bed temperature and during char combustion the particle temperature exceeded the bed temperature. Combustion times can be represented by power law equations. Results indicated that the burning rate of peat char is a combination of diffusion and kinetic control mechanisms.     

Coal conversion submodels for design applications at elevated pressures. Part II. Char gasification

This paper surveys the database on char gasification at elevated pressures, first, to identify the tendencies that are essential to rational design of coal utilization technology, and second, to validate a gasification mechanism for quantitative design calculations. Four hundred and fifty-three independent tests with 28 different coals characterized pressures from 0.02 to 3.0 MPa, CO₂ and steam mole percentages from 0 to 100%, CO and H₂ levels to 50%, gas temperatures from 800 to 1500 °C, and most of coal rank spectrum. Only a handful of cases characterized inhibition by CO and H₂, and only a single dataset represented the complex mixtures of H₂O, CO₂, CO, and H₂ that arise in practical applications. With uniform gas composition, gasification rates increase for progressively higher pressures, especially at lower pressures. Whereas the pressure effect saturates at the higher pressures with bituminous chars, no saturation is evident with low-rank chars. With fixed partial pressures of the gasification agents, the pressure effect is much weaker. Gasification rates increase for progressively higher gas temperatures. In general, gasification rates diminish for coals of progressively higher rank, but the data exhibit this tendency only for ranks of hv bituminous and higher.

These tendencies are interpreted with CBK/G, a comprehensive gasification mechanism based on the Carbon Burnout Kinetics Model. CBK/G incorporates three surface reactions for char oxidation plus four reactions for gasification by CO₂, H₂O, CO and H₂. Based on a one-point calibration of rate parameters for each coal in the database, CBK/G predicted extents of char conversion within ±11.4 daf wt% and gasification rates within ±22.7%. The predicted pressure, temperature, and concentration dependencies and the predicted inhibiting effects of CO and H₂ were generally confirmed in the data evaluations. The combination of the annealing mechanism and the random pore model imparts the correct form to the predicted rate reductions with conversion. CBK/G in conjunction with equilibrated gas compositions accurately described the lone dataset on complex mixtures with all the most important gasification agents, but many more such datasets are needed for stringent model evaluations.

Practical implications are illustrated with single-particle simulations of various coals, and a 1D gasifier simulation for realistic O₂ and steam stoichiometries. The rank dependence of gasification rates is the determining factor for predicted extents of char conversion at the gasifier outlet. But soot gasification kinetics will determine the unburned carbon emissions for all but the highest rank fuels. Only gasification kinetics for gas mixtures with widely variable levels of H₂O, H₂, and CO are directly relevant to gasifier performance evaluations.     

Thermodynamic assessment of an electrically-enhanced thermochemical hydrogen production (EETHP) concept for renewable hydrogen generation

A novel concept for coupling a thermochemical cycle with an electrochemical separation device for the generation of hydrogen from steam is reported and a thermodynamic analysis of the system is presented. In a conventional thermochemical cycle, an oxygen carrier material is thermally reduced, cooled, and then reoxidized in steam thereby generating hydrogen. However, this process often requires high temperatures (>1700 K) and/or low oxygen partial pressures (<0.001 atm) in order to meet thermodynamic requirements. Such extreme conditions can adversely affect the stability of the reactive oxides, reactor materials, and system efficiency. In our proposed technology, we seek to decrease the required reduction temperature by several hundred degrees Kelvin by relaxing the requirement for spontaneous oxidation reaction at atmospheric pressure. This is accomplished by incorporating a proton-conducting membrane (PCM) to separate hydrogen produced at equilibrium concentrations from reactant steam. We also suggest the use of mixed ionic-electronic conducting (MIEC) oxygen carrier materials that reduce through a continuum of oxidation states at lower temperatures (～1200 °C). This concept allows the generation of a high-quality hydrogen stream while avoiding the challenging high temperatures/low partial pressures required in conventional water-splitting reaction schemes.     

A numerical study of high pressure,laminar, sooting,ethane–air coflow diffusion flames

A numerical study is conducted of ethane–air coflow diffusion flames at pressures from 2 to 15 atm. The model employed uses a detailed gas phase chemical kinetic mechanism that includes PAH formation and growth, and is coupled to a detailed sectional soot particle dynamics model. The model is able to accurately predict the trends observed experimentally with increasing pressure without any tuning of the model for different pressures. The model shows good agreement with the experimental data on both the flame wings and centerline regions. Peak wing and centerline soot volume fractions are found to scale with P^2.49 and P^2.02 respectively. This scaling compares well to that observed experimentally for methane–air and ethylene–air flames. As pressure is increased, the flame cross-sectional area decreases according to P^?1.0 due to a constant mass flux and a thinning of the flame, which is consistent with experimental observations. Soot formation along the wings is seen to be surface growth dominated, while PAH condensation dominates centerline soot formation. Surface growth and PAH condensation increase with increasing pressure primarily due to both of these processes being a function of surface area. This causes increases in soot volume fraction to further accelerate surface growth and PAH condensation, acting in a positive feedback manner. This positive feedback mechanism is initiated by increases in reaction rates caused by increases in gas phase density. Additionally, the significance of surface growth decreases with increasing pressure, while the role of PAH condensation increases.     

Experimental and modeling study of the oxidation of n-butylbenzene

New experimental results for the oxidation of n-butylbenzene, a component of diesel fuel, have been obtained using three different devices. A rapid compression machine has been used to measure autoignition delay times after compression at temperatures in the range 640–960 K, at pressures from 13 to 23 bar, and at equivalence ratios from 0.3 to 0.5. Results show low-temperature behavior, with the appearance of cool flames and a negative temperature coefficient (NTC) region for the richest mixtures. To investigate this reaction at higher temperatures, a shock tube has been used. The shock tube study was performed over a wide range of experimental temperatures, pressures, and equivalence ratios, with air used as the fuel diluent. The ignition temperatures were recorded over the range 980–1740 K, at reflected shock pressures of 1, 10, and 30 atm. Mixtures were investigated at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in order to determine the effects of fuel concentration on reactivity over the entire temperature range. Using a jet-stirred reactor, the formation of numerous reaction products has been followed at temperatures from 550 to 1100 K, at atmospheric pressure, and at equivalence ratios of 0.25, 1.0, and 2.0. Slight low-temperature reactivity (below 750 K) with a NTC region has been observed, especially for the leanest mixtures. A detailed chemical kinetic model has been written based on rules similar to those considered for alkanes by the system EXGAS developed at Nancy. Simulations using this model have been compared to the experimental results presented in this study, but also to results in the literature obtained in a jet-stirred reactor at 10 bar, in the same rapid compression machine for stoichiometric mixtures, in a plug flow reactor at 1069 K and atmospheric pressure, and in a low-pressure (0.066 bar) laminar premixed methane flame doped with n-butylbenzene. The observed agreement is globally better than that obtained with models from the literature. Flow rate and sensitivity analyses have revealed a preponderant role played by the addition to molecular oxygen of resonantly stabilized, 4-phenylbut-4-yl radicals.