Kinetics-controlled Regime Research Articles

Flame propagation mechanism of methanol fuel spray explosion in a square closed vessel

Methanol, as an important chemical raw material and clean energy source, easily forms explosive droplet clouds during its production, processing, and usage. Therefore, understanding the evolution and propagation mechanisms of methanol spray explosion flames is crucial for its widespread use and the design of safety measures. This paper studies the flame propagation behavior of methanol spray explosions using a 16.2-L visualized enclosed vessel. The results indicate that the flame structure of methanol spray explosions is similar to that of premixed gas explosions but significantly differs from traditional fossil fuels. It demonstrates homogeneous combustion characteristics primarily governed by the kinetics-controlled regime. As the methanol spray concentration increases, both the maximum flame propagation speed and the maximum explosion pressure initially increase and then decrease, reaching their peak at a concentration of 224.68 g/m3, with values of 9.96 m/s and 0.72 MPa, respectively. The heat losses during combustion exhibit a trend opposite to that of explosion pressure. Numerical simulation results indicate that the concentrations of key radicals such as O, H, and OH vary significantly between lean and rich combustion states. The increase in H radical concentration enhances the elementary reactions that suppress flame temperature and promotes chain-terminating reactions.

Reaction behaviors in pulverized coal MILD-oxy combustion affected by physical and chemical effects of diluents

In pulverized coal MILD-oxy combustion process, the oxidizer diluents are switched from N2 to CO2 and H2O. Hence, the remarkable discrepancies in combustion characteristics directly originate from the differences in physical and chemical properties of diluents, namely, the physical and chemical effects. This work utilizes the fictious materials, i.e., FCO2 and FH2O, which have the same physical properties with the real ones but are completely non-reactive, to isolate these two dilution effects. Their individual and combined impacts on the global reaction behaviors are numerically evaluated. Results reveal that the physical effect of CO2 and H2O plays an important role in enhancing the internal flue gas recirculation. Compared with H2O-dilution, CO2-dilution better inhibits the temperature rise and promotes the in-furnace thermal uniformity, which predominantly originates from its physical effect. Hence, the MILD reaction regime with high temperature and strong dilution levels occur in a wider region. The homogeneous diffusion/kinetics-controlled regime (0.2 < Dat < 2) is more susceptible to the chemical effect of CO2 and H2O. The kinetics-controlled regime of heterogeneous reaction is facilitated under CO2- and H2O-dilutions, where the physical effect is dominant in the former case while the physical and chemical effects act jointly in the latter case. The chemical effect of CO2 and H2O is responsible for greatly strengthening gasification reaction by direct participating in the reaction process, which in turn slows down the burnout of char. Importantly, H2O-dilution shows greater potential to lower NO emission level than CO2-dilution. The physical effect is revealed to play a dominant role in reducing the global NO formation rate, while the chemical effect enhances the reburning of NO in a wide lean-O2 reduction region. The findings of this work are expected to help gain a more comprehensive understanding in MILD-oxy combustion technology.

Study of the Oxidation Behavior of Fine-Grained Graphite ET-10 by Combining X-ray μCT with Mercury Porosimetry.

By combining X-ray micro-computed tomography with mercury porosimetry, the evolution of the oxygen supply, porous structure, mass loss and oxidized compositions were investigated to characterize the oxidation behavior of fine-grained graphite ET-10, regarding the geometry of the specimen and its oxidation temperature. Here, the porous structure and the gas flows out of and into the porous structure were comprehensively compared for two kinds of specimens-large pure graphite (D = H = 25.4 mm), oxidized at a test facility based on ASTM D7542, and small partially SiC-coated graphite (D ≈ 1 mm and H = 1.95 mm), oxidized in the bottom section of a U-type tube. The fine grains and large geometry resulted in small pores and long flow distances, which exhausted the oxygen in the small stream to the interior of the specimen, making its oxidation deviate from the kinetics-controlled regime. In addition, the well-known three-regime theory was reasonably reinterpreted regarding the oxidation of different compositions, binders and fillers. The kinetics-controlled uniform oxidation mainly oxidizing binders is restricted by their limited contents, while the rate of surface-dominated oxidation increases continuously via the consumption of more fillers. Furthermore, we proposed a new design for the test facility used for the oxidation experiment, wherein a partially shielded millimeter specimen can be oxidized in the long straight bottom section of a U-tube, and this will be discussed further in related future studies.

Internal association between combustion behavior and NOx emissions of pulverized coal MILD-oxy combustion affected by adding H2O

In practical pulverized coal Moderate or Intense Low-oxygen Dilution oxy-fuel combustion (MILD-oxy) process, the accumulation of steam results in the recycled flue gas consisting of highly concentrated H2O, particularly under wet flue gas recirculation. This work numerically studies the individual effect of H2O on the turbulence-chemistry interaction, aiming at revealing the internal association between combustion behavior and NOx emissions. Results show that H2O addition reduces the flame temperature by enhancing the radiative heat transfer. The extremum values of turbulent Damkӧhler number (Dat,max) and Karlovitz number (Kamin) are reduced and increased, respectively, denoting the slow-chemistry feature (Dat < 10 and Ka >> 1) in the reaction zone. With the promoted H2O level, the heterogeneous reactions on the char surface tend to be governed by kinetics due to the reduced reaction rate under low flame temperature. The gasification reaction is enhanced to produce more synthesis gas, and the burnout of char particle is prolonged. The in-furnace NO formation is inhibited under low Dat,max, while the NOx emissions are reduced directly due to the enhanced gasification reaction under low maximum surface Damkӧhler number (Das,max). The addition of H2O essentially establishes a moderate diffusion and reaction process under the kinetics-controlled regime to obtain the uniform thermal field and low-NOx emissions.

Bluff-body MILD combustion of large-proportion semicoke-blend under various secondary air velocities

Secondary air jet velocity is strongly related to flue gas entrainment in semicoke-blend MILD combustion, which exerts a critical influence on the turbulence–chemistry interaction and combustion characteristics. To extend the application of moderate or intense low oxygen dilution (MILD) combustion to tangential fired boilers, this paper numerically investigates the effect of secondary air velocity (Vsec) on the turbulent flow and chemical reaction interaction behaviors of large-proportion semicoke and bituminous coal co-firing to establish the bluff-body MILD combustion regime. The results show that increasing Vsec abates the peak temperature by enlarging radiative heat transfer and enhances flue gas recirculation. The MILD regime first occurs at Vsec = 51.8 m/s and then extends quickly from 0.4 m to 1.2 m with further increasing Vsec. Increasing Vsec decreases the maximum of the turbulent Damköhler number Dat and increases the minimum of Karlovitz number Ka, implying that the predominant role of turbulent mixing gradually is transferred into the comparable role. The local-MILD regime (1 < Dat < 10) is enhanced significantly and an ideal MILD combustion regime occurs at a velocity of 158.6 m/s, i.e., Dat≈1, and Ka≫1, denoting a slow-chemistry regime. Meanwhile, the maximum heterogeneous Damköhler number Da,C-O2 decreases from 1.09 to 0.86, while the maximum Da,C-CO2 and Da,C-H2O decrease from 0.013 to 0.007 and from 0.003 to 0.0018, respectively, hinting that the char-O2 reaction is controlled by diffusion/kinetics, char-CO2 and char-H2O reactions are determined by kinetics, and they all proceed toward the kinetics-controlled regime with increasing Vsec. Low NOx emissions can be captured and coupled with small Dat by increasing Vsec, and weakening the transitional MILD regime (10 < Dat < 30) is of great importance when establishing the MILD regime.

Investigation of air-MILD and oxy-MILD combustion characteristics of semicoke and bituminous coal mixtures in a 0.3 MW fuel-rich/lean fired furnace

To study the combination of fuel-rich/lean combustion with oxy-MILD (moderate and intense low-oxygen dilution) combustion for the reduction of NOx emissions by adopting carbon capture and storage, this paper numerically investigates the turbulence−chemistry interactions and NOx emission behaviours of semicoke mixtures under different secondary velocities Vsec (from air-fuel combustion to air-MILD combustion) and under different CO2 dilution levels (oxy-MILD combustion). The bias concentration ratio (BCR) hardly affects the establishment of air-MILD combustion, which is obtained at Vsec = 158.6 m/s. The oxy-MILD combustion, which is composed of small flamelets diffusing in large-scale eddies (turbulent Damköhler number Dat = 0.1–1), is enhanced, and the ignition and char burnout times are prolonged by increasing CO2 from 0 vol% to 79 vol%. The char oxidation reaction in the diffusion/kinetics-controlled regime and the char gasification reactions in the kinetics-controlled regime both shift towards the kinetics-controlled regime with increasing velocities and CO2 dilution, thus leading to lower burnout rates. The heterogeneous MILD regime of the fuel-lean side is more easily established for the air-MILD combustion, while that of the fuel-rich side is more easily established for oxy-MILD combustion. This is because the intensive turbulence increases the O2 dilution level in the fuel-lean jet, while a high concentration of CO2 reduces the oxygen diffusivity in the fuel-rich jet, which also leads to a larger XNO in the fuel-lean jet compared to the fuel-lean jet. The NOx concentration at the furnace outlet decreases significantly from air-fuel combustion to air-MILD combustion and further decreases as the oxy-MILD combustion is enhanced, along with relatively lower homogeneous and heterogeneous Da.

Investigation of macro-kinetics of coal-oxygen reactions under varying oxygen concentrations: Towards the understanding of combustion characteristics in underground coal fires

The typical combustion mode of underground coal fires (UCFs) is smoldering during which the reaction zone is not exposed to a constant oxygen concentration. Understanding the macro-kinetics of coal-oxygen reactions under varying oxygen concentrations, especially the extremely oxygen-depleted condition, is of both theoretical importance and practical relevance to the control and extinguishment of UCFs. Considering the actual conditions of UCFs, thermal analysis tests under four oxygen concentrations (from 21% to 3%) and three heating rates (1, 2 and 5 °C /min) were carried out. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) results were obtained for a bituminous coal sample from Inner Mongolia, China. With the global reaction assumption, the macro-kinetics parameters (the apparent activation energy, the pre-exponential factor and the kinetics model function) were determined. On the profiles of the apparent activation energy (Ea), three peak values were observed, physically interpreted as the depletion of volatiles, the formation of plastic mass and the depletion of char, respectively. This interpretation was verified by characteristic temperatures extracted from the experimental data. With the decrease of the oxygen concentration from 21% to 9%, two peak values diminish gradually. The case with 3% oxygen concentration gives a nearly monotonically declining Ea, indicating that under that particular condition, oxygen diffusion stands as the only limiting factor across all stages of coal-oxygen reactions. The best-fit kinetics model functions suggest that the char oxidation stage falls into the kinetics-controlled regime when the oxygen concentration is as low as 9%. For the volatiles burning stage, the universal ignition index (Fz) is found to be effectively related to the reaction regime for a variety of coal ranks. The quantitative results obtained can be integrated into any CFD multi-physics models as a sub-model for chemical kinetics.

Oxidation Experiments and Kinetics Analysis of Nuclear Graphite ET-10 by Gas Analysis and Microstructure Observation

Nuclear graphite can be used in fission and fusion systems due to its excellent nuclear performance and mechanical properties where the ability of oxidation resistance is usually concerned. Although the excellent performance of new graphite ET-10 was revealed by previous experiments regarding the accident conditions of a fission reactor, further studies are needed to oxidize the graphite under the conditions recommended by the ASTM D7542 standard. A test facility was designed and developed to oxidize the cylindrical specimen with a 10 L/min airflow. According to oxidation rates and microstructures of specimens, the chemical kinetics-controlled regime was determined as 675–750 °C, where the activation energy was obtained as 172.52 kJ/mol. The experiment results revealed the excellent ability of graphite ET-10 for oxidation resistance with lower oxidation rates and longer oxidation times compared with some mainstream graphite. The main reasons are the low contents of some impurities and the binder and the low active surface area due to the non-impregnation baking process undertaken to produce graphite with coal tar pitch coke. It should be noted that the evolution of oxidation behavior at the bottom part of the specimen (facing the airflow) was quicker than that at the upper part of the specimen. We also suggest that the abundance of oxygen supply and the good linearity of the Arrhenius plot are prerequisites of the chemical kinetics-controlled regime rather than sufficient conditions.

Studies into the kinetic compensation effects of Loy Yang Brown coal during gasification in a steam environment – A mechanistic view

This study aims to investigate the kinetic compensation effects (KCE) and gain insights into the mechanisms, during gasification of Loy Yang brown coal char with steam, in an in-situ fluidized bed gasification operation for two-particle size ranges (106–150 and 180–212 µm). The instantaneous rate of char gasification and CO, CO₂, and H₂ formation was measured by continuous monitoring of product gas composition through a quadrupole mass spectrometer. Gasification of the smaller particle size range (106–150 µm) in kinetics-controlled regime shows less importance of char-catalyzed element on the WGS reaction, as revealed by the apparent activation energy and apparent frequency factor for CO and CO₂ formation. However, the study of kinetic parameters of char consumption on gasification using coal char with larger particle sizes (180–212 µm) indicated the limitations of intraparticle diffusion. That potentially affects the CO₂ formation by catalyzed WGS through re-adsorption of CO on catalytic char surface at a higher conversion level (> 0.3) as revealed by the difference in the extent of KCE for CO and CO₂ formation. The difference in the extent of KCE for char consumption and H₂ formation for bigger particles indicates the intraparticle diffusion limitations also appear to affect the route of H₂ formation, i.e., significantly produced through adsorption on catalytical active site with less involving the carbon active sites on char surface.

General rate equation theory for gas–solid reaction kinetics and its application to CaO carbonation

The kinetics of reactions between gases and solids are typically modeled using shrinking-core or shrinking-pore theoretical frameworks, in which the formation of a uniform solid product layer covering the entire solid surface with a sharp interface between the solid reactant and the product is assumed. However, theories involving a uniform solid product layer cannot predict the kinetic transition behavior that occurs in some gas–solid reactions. This work proposes that the growth of solid product islands occurs instead of the progressive formation of a uniform solid product layer in traditional shrinking-core and shrinking-pore models and establishes a general rate equation theory to model the kinetics of gas–solid reactions for solid reactants of various shapes. The growth of the product islands is calculated using the rate equation theory and is integrated into the shrinking-core model and shrinking-pore model, in which the microstructure of the solid reactant is also considered. Elemental steps of chemical reaction, surface diffusion and product layer diffusion are included in the general rate equation theory. The kinetic parameters included in the model are chemical reaction rate constant ks, surface diffusion coefficient Ds, and product layer diffusion coefficient Dp. The resulting model is analyzed at the limits where either the chemical reaction or product layer diffusion is the rate-controlling step. At very small or very high Ds values, the rate equation theory is equal to the kinetics-controlled regime or diffusion-controlled regime in traditional models; therefore, the traditional models represent special cases of this theoretical rate equation model. At intermediate Ds values, the so-called two-stage kinetic behavior occurs. The transition from the fast initial stage to the diffusion-controlled stage depends on the value of Ds, with the conversion at the transition point increasing with increasing Ds. The rate equation theory is demonstrated to successfully predict the CaO carbonation kinetics under a wide range of experimental conditions including temperature, CO2 concentration, sorbent type, and amount of added inert supports as well as for systems that exhibit a temperature rise during the carbonation reaction The rate equation theory is integrated into a particle model to account for the external diffusion of gas around the particle and intraparticle diffusion as well as the effect of particle structural changes on intraparticle diffusion, such as the pore plugging phenomenon. The developed rate equation theory is an analytical model that can be easily used in reactor-scale modeling or computational fluid dynamics simulations.

Reaction kinetics of char-O2/H2O combustion under high-temperature entrained flow conditions

The combustion kinetics of char-O2/H2O reactions under high-temperature entrained flow conditions involving char-H2O and char-O2 reactions were investigated experimentally and numerically. A numerical model was established based on the unsteady convection-diffusion equations, and the random pore model (RPM) was adopted locally to predict the change in intraparticle pore surface area. The Langmuir-Hinshelwood form rate equation was used to consider the possible competitive relationship between char-O2 and char-H2O reactions, and the most probable intrinsic kinetics parameters were found by checking the correlation coefficient between the experimental and numerical results. For H2O gasification, the numerical results show that the enrichment of H2 inside char particles decreases the intrinsic char-H2O gasification rate, especially at the outer layer regions. For O2/H2O combustion, the experimental results show that H2O has a promotion effect on the total carbon conversion at low SRs (stoichiometric ratio of O2 to fuel) but has an inhibition effect on the total carbon conversion at high SRs. Using the common active sites assumption, the numerical results match well with the experimental results. According to the numerical results, H2O penetrates deeper inside the char particle than does O2, contributing more to the local carbon conversion at the inner layer region, while H2O inhibits the char-O2 combustion rate at the outer layer regions due to H2 formation. As a result, H2O pushes the kinetics-diffusion-controlled reaction towards the kinetics-controlled regime. As the SR increases, the char-O2 combustion contributes more, magnifying the inhibition effects of H2O on the char-O2 combustion rate; therefore, the total O2/H2O combustion is gradually inhibited by H2O.

A surface activation function method to determine the intrinsic reactivity of coal char oxyfuel conversion

The kinetics of coal/char oxyfuel combustion by isothermal and non-isothermal thermogravimetric analysis is still controversial. The two methods were compared according to the kinetics of two coal chars (NCP anthracite, DT bituminous) combustion in kinetics-controlled regime I under different O2/CO2 and O2/N2 atmospheres. We have developed a surface activation function (SAF, F(X)) to describe the reactive specific surface area of char conversion, and then the intrinsic reaction rate can be expressed by R(X)=kPO2mF(X), the autocatalysis model (ACM) of FX=Xa1-Xc was found to be suitable under both isothermal and non-isothermal conditions, however, the exponents a and c in ACM as well as the activation energies (E) differ under these two conditions. These controversial results may be caused by the exponential function exp(−E/RT), which changes with the conversion ratio under non-isothermal conditions, which in turn affects the nonlinear modeling results of SAF. According to the isothermal study, temperature has no effect on the form of SAF in Regime I. Thus it is reasonable to use the intrinsic SAF obtained from isothermal experiments (F(X)iso) to fit the non-isothermal data. The modeled results show that ENCP = 207–220 kJ/mol under different O2/CO2 and O2/N2 atmospheres, greater than those under isothermal conditions (178–179 kJ/mol). While EDT = 159–165 kJ/mol, this agrees well with the isothermal result (163–167 kJ/mol). Furthermore, it indicates that the O2 partial pressure (PO2) has no influence on E, even though increasing PO2 or replacing CO2 in the O2/CO2 mixture with N2 increases the intrinsic reaction rate.

Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under High-Temperature Entrained Flow Combustion Conditions

The char-O2 reaction under high-temperature entrained flow conditions was investigated experimentally and numerically to study the effect of the stoichiometric ratio (SR) on char-nitrogen conversion. A numerical model was established based on the unsteady convection-diffusion and reaction equations, in which Stefan flow was considered. The change of specific surface area during char combustion was considered by locally using the Random Pore Model (RPM). The intrinsic reactivity parameters of char-O2 and char-NO reactions were measured using a thermal gravimetric analyzer (TGA) and drop tube furnace (DTF), respectively. The numerical results indicated that the predictions of carbon conversion and NO release agreed well with experimental data. Based on numerical results, for a given carbon conversion, with the decrease of SR, the kinetics-diffusion controlled char-O2 reaction is pushed toward the kinetics-controlled regime, leading to more significant local carbon conversion and thus more developed pore structure inside char particles, including higher porosity and larger specific surface area. Therefore, the oxidation of char-nitrogen occurs more deeply inside char particles and NO release prefers to diffuse toward the particle center, rather than outward. The larger specific surface area deep inside the char particle also gives rise to the total char-NO reaction rate at a lower SR. As a result, the total fractional conversion of char-N to NO decreases as SR decreases under high-temperature entrained flow combustion conditions.

Ethylene/polyethylene hybrid explosions: Part 1. Effects of ethylene concentrations on flame propagations

To reveal the explosion regimes in ethylene/polyethylene dust hybrid explosions, flame propagations in ethylene/polyethylene hybrid explosions with different ethylene concentrations were experimentally studied. The results showed that minimum explosible concentration of polyethylene particles significantly decreased with the increase of ethylene concentration due to the synergistic effect of ethylene and polyethylene particles. The MEC was reduced from 200 g/m3 to 42 g/m3 with the addition of 2.3% ethylene gas. Flame propagation velocities, maximum flame temperatures, and temperature rising rates obviously increased as ethylene concentration increased. The maximum average flame propagation velocities of 0.5%, 1.2%, and 2.3% mixtures were 6.22 m/s, 8.31 m/s, and 12.91 m/s respectively. With the increase of ethylene concentration, flame propagation mechanism was transited from the devolatilization-controlled regime to the kinetics-controlled regime. Hybrid explosions of ethylene/polyethylene dust mixtures can be identified into four different regimes, which are no-explosion, synergistic explosion, gas-driven explosion, and dust-driven explosion. In the synergistic explosion region, maximum flame temperatures were obviously sensitive to the ethylene concentration and dust concentration. Hybrid explosion regime was transited from the dust-driven explosion to the gas-driven explosion with the increase of ethylene concentration (below LFL).

Nano-Iron Carbide-Catalyzed Fischer-Tropsch Synthesis of Green Fuel: Surface Reaction Kinetics-controlled Regimes in a 3-φ Slurry-Continuous Stirred Tank Reactor

Liquid fuel derived from renewable resources is one of the technologies under development as part of biorefining platforms. Fisher-Tropsch synthesis (FTS) is a commercial technology producing alternative fuels from coal (CTL- Coal-To-Liquid) and natural gas (GTL; Gas-To-Liquid). FTS Biomass-To-Liquid (BTL), although not yet at the market level, is a continuously-growing field, and its successful commercialization depends on improving techno-economic sustainability. In a previous study by the authors' research team, a plasma-synthesized nano-iron carbide catalyst (PS-Nano- FeC) demonstrated direct relationship to the presence of iron carbide high-FTS and water-gas-shift (WGS) activity, with high-catalyst stability and regenerability in a 3-phase slurry, continuously-stirred tank reactor (S-CSTR). Although these results, along with a recently-published phenomenological model, are indicators of the industrial potential of this catalyst, transport phenomena, chemical mechanisms and their intrinsic kinetics are needed as reactor scale-up tools. In the work reported here, the PS-Nano-FeC catalyst was tested in a S-CSTR with hexadecane as liquid carrier. We evaluated the optimal operating conditions for a surface-reaction, kinetics-controlled regime. The results include: (1) Reactant conversion and product yields; (2) Fresh and used catalyst instrumental analyses; (3) A model considering all transfer and surface kinetics, accompanied by proof of the rate-controlling step.

Flame propagation mechanisms in dust explosions

To reveal the effects of particle characteristics, including particle thermal characteristics and size distributions, on flame propagation mechanisms during dust explosions clearly, the flame structures of dust clouds formed by different materials and particle size distributions were recorded using an approach combining high-speed photography and a band-pass filter. Two obviously different flame propagation mechanisms were observed in the experiments: kinetics-controlled regime and devolatilization-controlled regime. Kinetics-controlled regime was characterized by a regular shape and spatially continuous combustion zone structure, which was similar to the premixed gas explosions. On the contrary, devolatilization-controlled regime was characterized by a complicated structure that exhibited heterogeneous combustion characteristics, discrete blue luminous spots appeared surrounding the yellow luminous zone. It was also demonstrated experimentally that the flame propagation mechanisms transited from kinetics-controlled to devolatilization-controlled while decreasing the volatility of the materials or increasing the size of the particles. Damköhler number was defined as the ratio of the heating and devolatilization characteristic time to the combustion reaction characteristic time, to reflect the transition of flame propagation mechanisms in dust explosions. It was found that the kinetics-controlled regime and devolatilization-controlled regime can be categorized by whether Damköhler number was less than 1 or larger than 1.

Effects of entrainment and agglomeration of particles on combustion of nano-aluminum and water mixtures

The combustion of nano-aluminum and water mixtures is studied theoretically for a particle size of 80nm and over a pressure range of 1–10MPa. Emphasis is placed on the effects of entrainment and agglomeration of particles on the burning rate and its dependence on pressure. The flame thickness increases by a factor of ∼10, when particle entrainment is considered. This lowers the conductive heat flux at the ignition front, thereby reducing the burning rate. The pressure dependence of the burning rate is attributed to the changes in the burning time and velocity of particles with pressure. In the diffusion limit, the pressure exponent increases from 0 to 0.5, when the entrainment index increases from 0 to 1.0. A similar trend is observed in the kinetics-controlled regime, although the corresponding value exceeds the diffusion counterpart by 0.5. The kinetics-controlled model significantly over-predicts the burning rate and its pressure exponent, depending on the entrainment index. The present analysis suggests that nano-particles formed closely-packed agglomerates of diameter 3–5μm, which may burn under diffusion-controlled conditions at high pressures.

The growth of carbon nanotube stacks in the kinetics-controlled regime

A CNT stack formation technique is used for the pseudo in-situ monitoring of CNT growth in the kinetics-controlled regime. CNT stacks are fabricated by water-assisted selective etching and the cyclic introduction of ethylene into the chemical vapor deposition (CVD) reactor. By varying the growth temperature and the ethylene flow rate within growth cycles, the reaction was shown to be first order with an activation energy of 201.2 kJ/mol. This technique can be implemented to monitor the initial stage of CNT growth by deceasing the ethylene flow time in growth cycles. Within one minute, the CNT growth reaches steady state. The formation of CNT stacks could be tailored to investigate the effects of growth parameters, such as pressure, temperature, and carbon source flow rate, on the CNT growth.

Experimental Study on Pore Structure and Apparent Kinetic Parameters of Char Combustion in Kinetics-Controlled Regime

Pore structure has a critical role during the combustion process of char particles. The pore structures of six char samples were tested by both gas adsorption and mercury porosimetry. The pore distribution dimensions were obtained, which were observed to be related to the fractal dimension of pore structures. To study the intrinsic reactivity of char samples, the char samples were also burned in a thermogravimetric analyzer. The ramp heating rates were devised at 5, 10, 15, 20, and 25 K/min, so that the char samples were burned in the kinetic regime. The so-called “isoconversional method” was used to investigate the apparent kinetic parameters. The results show that the apparent activation energies of the six char samples share almost the same value: 150 kJ/mol. Two exponents in the deduced model function are determined, relative to the pore distribution dimensions. The model function reveals the relationship between the initial pore structure and the evolution of pore surface area during the combustion ...

Reaction Pathways in Pentachlorophenol Synthesis. 1. Temperature-Programmed Reaction

Pentachlorophenol, a wood preservative for nonresidential applications, contains parts per million levels of dibenzodioxins and dibenzofurans with six or more chlorine atoms. There is interest in reducing the levels of these microcontaminants in pentachlorophenol. We conducted pentachlorophenol synthesis reactions in the laboratory to demonstrate the ability to mimic the commercially practiced temperature-programmed synthesis and to identify an operating region where mass-transfer limitations were effectively eliminated. The laboratory system was operated in a temperature-programmed fashion and produced pentachlorophenol with a yield and microcontaminant content similar to those of the commercially produced material. The experiments also revealed a stirring rate, number of gas spargers, and chlorine flow rate that enabled the laboratory-scale reaction to proceed in the kinetics-controlled regime. Results indicated that microcontaminant formation was low until near the end of the run where the microcontami...