Gas-solid Reaction Mechanism Research Articles

Enhancing the combustion of nAl with AlF3 coating: gas-solid reaction mechanism for reducing combustion agglomeration of Al powder.

The combustion agglomeration of nano-aluminum (nAl) powder leads to incomplete combustion, which seriously hinders its application as metal fuel. In this work, nAl@AlF3 composites were produced by coating nAl with AlF3via a facile chemical deposition method. TEM and SEM analyses indicated that the AlF3 layer was evenly coated on the surface of nAl with a thickness of 4.6-9.1 nm, thereby varying the quantity of AlF3 applied. Experimental results from combustion indicated that the prepared nAl@AlF3 composites exhibit superior combustion efficiency, a higher combustion rate, and reduced combustion agglomeration as compared to raw nAl. Contrary to the widely accepted explanation that volatilization of AlF3 hinders Al combustion agglomeration, we proved that the gas-solid reaction between nAl and AlF3 plays an important role in inhibiting the sintering of nAl particles produced. The gaseous intermediate (i.e., AlOF and HF) released from the hydrolysis of AlF3 could reduce the diffusion barrier of Al2O3 to facilitate the reaction of Al core, which enhances the combustion reaction kinetics. More importantly, these gaseous products actively participate in the reaction cycle to continuously exert their catalytic effects.

Experimental investigation on sulfur transformation during coal steam gasification with Ca-based absorbent

To investigate the effects of temperature, Ca-based absorbent (CaO) and atmosphere on the sulfur transformation characteristics in coal steam gasification, experiments were conducted in a fixed-bed reactor. As temperature increased from 600 °C to 750 °C, H2S-S (sulfur in H2S) was elevated by 21.86% and absorbent-S decreased by 7.66%. More gaseous sulfur was captured by organic groups to form organic sulfur in char at higher temperature. CaO promoted the sulfur conversion, and played the role of gaseous sulfur absorbent at the same time, and showed an ability of organic sulfur removal. CaO slowed down the H2S release by reacting with it to form CaS, resulting in that the peak concentration of H2S without CaO reached 1026.7 ppm, which decreased to 426.3 ppm with CaO. The increase in Ca/C promoted the sulfur conversion slightly and reduced the sulfate content in char. Compared with inert atmosphere, H2S-S in the steam atmosphere increased by 35.07%, while absorbent-S decreased by 23.42%. The solid–solid reaction mechanism is more suitable for the sulfate transformation in inert atmosphere, while the gas–solid reaction mechanism is more suitable in steam atmosphere. The sulfur conversion in inert atmosphere was lower, but sulfate had a higher decomposition rate.

New Analysis Method for Adsorption in Gas (H2, CO)-Solid (SnO2) Systems Based on Gas Sensing.

The chemisorption phenomenon is widely used in the explanation of catalysis, gas–solid reactions, and gas sensing mechanisms. Generally, some properties of adsorbents, such as adsorption sites and dispersion, can be predicted by traditional methods through the variation of the chemisorption capacity with the temperature, pressure, and gas–solid interaction potential. However, these methods could not capture the information of the interaction between adsorbents, the adsorption rate, and the competitive adsorption relationship between adsorbents. In this paper, metal oxide semiconductors (MOSs) are employed to study the adsorption behavior. The gas sensing responses (GSRs) of MOSs caused by the gas adsorption process are measured as a new method to capture some adsorption behaviors, which are impossible for the traditional methods to obtain. The following adsorption behaviors characterized by this new method are presented for the first time: (1) distinguishing the adsorption type using an example of two reducing gases: the adsorption type of the two gases is single-molecular layer adsorption in this work; (2) detecting the interaction between different gases: this will be a promising method to provide original characterization data in the fields of gas–solid reaction mechanisms and heterogeneous catalysis; and (3) measuring the adsorption rate based on the GSR.

High-Performance Large-Area Perovskite Solar Cells Enabled by Confined Space Sublimation.

Large-area devices with high power-conversion efficiency (PCE) are required for the further development of perovskite solar cells (PSCs). However, the major obstacle restricting the commercialization of large-area PSCs is the lack of reliable deposition technology for scalable perovskite thin films. Herein, a confined space sublimation method compatible with the preparation of a large-area perovskite film is introduced. First, pure PbX2 (X is I, Br, and Cl) films are exposed to CH3NH3I vapor, and the gas-solid reaction mechanisms for different lead halide layers are investigated; the perovskite films fabricated by retarded displacement between multiple halogens show large grains and a controllable band gap. Then, through mixed halogen adjustments of the PbX2 film, a maximum PCE of 20.44% (0.07 cm2) for high-efficiency PSCs with large grains (over 2.0 μm) is obtained, while the fabrication of wide-band gap PSCs with a champion PCE of 17.99% (0.07 cm2) verified the band gap-controlled compatibility of the confined space sublimation approach. Furthermore, the confined space sublimation approach is applied to fabricate large-area (6.75 cm2) devices with uniform photovoltaic performance, demonstrating the scalable potential of this approach.

Thermal and kinetic analysis on the co-combustion behaviors of anthracite and PVC

The co-combustion characteristics of anthracite and PVC were investigated by thermogravimetric analysis to study its application in BF. First, the combustion characteristics were investigated. It was found the initial combustion temperature (Ti) of blends decreased with the increase of PVC ratio, while its decreasing rate reached maximum when 10% PVC. Aiming for further characterizing the combustion kinetics, ten gas-solid reaction mechanism functions were adopted for different groups. Results showed that Dimensional Diffusion Model is the best model to describe the combustion kinetics of anthracite and PVC. With this model, combustion kinetic parameters were calculated at 5 °C/min, and theEadecreases with the addition of PVC. Kinetics compensation effect between theEaandAwas also observed. By Fourier Transform Infrared Spectroscopy (FTIR) analysis, significant differences of elements and functional groups were observed. This study provides theoretical guidance for the utilization of PVC as alternative fuels for BF ironmaking.

Determination of Carbonation/Calcination Reaction Kinetics of a Limestone Sorbent in low CO2 Partial Pressures Using TGA Experiments

The carbonation/calcination reaction cycle of a limestone sorbent was used in a novel greenhouse calcium looping process to provide the heat and CO2 requirement of greenhouses. The unique conditions of this process (low temperature and low CO2 partial pressure) led to the determination of the carbonation/calcination reaction kinetic parameters in a temperature range of 400 to 800 oC and CO2 partial pressures of 0.04 to 0.16% (400 to 1600ppm) which were carried out experimentally via a thermo-gravimetric analyzer (TGA). Various gas-solid reaction mechanisms were considered to determine the best reaction mechanism for the carbonation reaction. The diffusion function or G(x) = x2 had the best least-square linear fit, which resulted in a first order reaction for the carbonation reaction in the greenhouse calcium looping process. Moreover, the activation energy and pre-exponential factor of the carbonation reaction were established to be 19.7kJ mol-1 and 295.8 min-1 kPa-1. The same methodology for reaction mechanism determination was carried out for the calcination reaction and G(x)=1-(1-x)1/3 was found to have the best linear fit. The activation energy and pre-exponential factor determined for the calcination reaction were 103.6kJ mol-1 and 6.9 × 106 min-1, respectively.

Derivation of Kinetics and Design Parameters for a Carbonator Reactor in a Greenhouse Calcium Looping Process

AbstractThe carbonation/calcination reaction cycle of a limestone sorbent was studied experimentally by using a thermogravimetric analyzer for unique conditions, such as low temperature (400 to 500 °C) and low CO2 partial pressure (0.05 to 0.1 %), pertinent to a novel greenhouse calcium looping process. The kinetic parameters were obtained and compared with those reported in the literature. Various gas–solid reaction mechanisms were considered to determine the best reaction mechanism for the carbonation reaction. The diffusion function, or G(x)=x2, had the best least‐squares linear fit, which resulted in a first‐order reaction for the carbonation reaction in the greenhouse calcium looping process. Moreover, the activation energy and pre‐exponential factor of the carbonation reaction were established to be 19.7 kJ mol−1 and 295.8 min−1 kPa−1, respectively. The derived kinetic parameters were used in Aspen Plus to optimize the carbonator reactor size. The required size of the reactor decreased with increasing operating temperature of the reactor. Exergy analysis revealed that the overall exergetic efficiency of greenhouse calcium looping could be more than 80 %.

O2 Release of Mn-Based Oxygen Carrier for Chemical Looping Air Separation (CLAS): An Insight into Kinetic Studies

ABSTRACTChemical looping air separation (CLAS) is an air separation technology with a relatively small energy footprint. In this contribution, the kinetic study of Mn-based oxygen carriers has been carried out in a CLAS process. The O2 release behavior was investigated under Ar in a fixed bed. Results showed O2 release amounts increase gradually with an increasing temperature. Moreover, fitting various gas-solid reaction mechanisms with the experimental data was used to obtain the chemical reaction kinetics for MnO2 and Mn2O3 for O2 release. As for all involved gas–solid reaction mechanisms, the first-order chemical reaction model (C1) and Avrami–Erofe’ev random nucleation and subsequence growth model (A2) fitted well with O2 release experimental data for MnO2 and Mn2O3, respectively. Furthermore, activation energy, pre-exponential factor and reaction order were also determined for these models.

Characterization of the kinetics of NF3-fluorination of NpO2

Solid NpO2 has been contacted by gaseous NF3 under isothermal conditions at 450°C, 475°C, and 500°C; and the resulting reactions have been monitored using thermogravimetric analysis. In each case, at least two sequential reactions are clearly observed. The first reaction is fluorination of NpO2 to NpF4 and the second is oxidation and fluorination of NpF4 to NpF6. Careful observation of the experimental reaction curves reveals evidence of several physical and chemical mechanisms occurring sequentially and at times simultaneously. As such, a mathematical modeling approach utilizing a combination of sequential and parallel fundamental gas-solid reaction mechanisms (chemical reaction, diffusion, and phase boundary) is, in general, found to provide representative reaction curves that are in good agreement with experimental reaction curves. The correspondence of fundamental reaction mechanisms with distinctive characteristics of the experimental reaction curves (maximums and inflection points) provides insight into the physical and chemical nature of each reaction being monitored.

Interfacial Reactions between Polymer Derived SiC Fiber and Ti<sub>3</sub>Si(Al)C<sub>2</sub>

In this paper, interfacial reactions between polymer derived SiC fiber and Ti3Si(Al)C2were analyzed. Strong interfacial reactions occurred in 20 vol.% SiCf/Ti3Si(Al)C2composite during the vacuum spark plasma sintering at 1350 °C. At the begining, the Ti3Si(Al)C2suffered spontaneous decomposition and resulted in the sublimation of gaseous Ti, Si and Al, and de-intercalation of TiC from the crystal structure, when it was combined with SiC fiber during sintering. Then, Ti reacted with the SiC phase and turbostratic graphite in the fiber and TiC was formed, and also Al was oxidized into Al2O3. Meanwhile, most of gaseous Si released from Ti3Si(Al)C2escaped from the outer surfaces of the composite under the compressive stress of 30 MPa and the vacuum environment, the rest Si was cooled into solid state following gas-solid reaction mechanism.

A Gas–Solid Reaction Growth of Dense TiO<SUB>2</SUB> Nanowire Arrays on Ti Foils at Ambient Atmosphere

In this paper, a facile method is demonstrated to directly fabricate dense titania nanowire arrays on titanium foils under the atmosphere without extra moist conditions. The influences of temperature, time, different catalysts, and concentrations of the respective catalysts on the growth of titania nanowires are discussed in detail. The morphology, composition and crystal structure of the titania nanostructures are revealed by scanning electron microscopy (SEM), powder-X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques, by which a gas-solid reaction mechanism is suggested to explain the growth process of TiO2 nanowires on Ti substrate.

Measurement of characteristics of solid flow in the cyclone separators with fiber optical probe

In some applications, e.g. circulating fluidized beds (CFB), cyclones are usually operated at high solid loadings. Under high inlet solid concentration, most of the particles are collected at the wall and form a dense particle spiral band because of high separation efficiency. As a result, gas-solid reactions should occur mostly in the near-wall region. To understand the gas-solid reaction mechanism in the cyclone, an experimental study was conducted in a plexiglass CFB cold apparatus, with a riser of 0.2m I.D. and 5m high, and a standard Lapple cyclone. Fiber optical probe was used to measure the characteristics of solid flow in the cyclone, including particle velocity and volumetric solid concentration, especially in the near-wall region of the cyclone. Based on the experiment results, the combustion of carbon particles in the cyclone of a CFB boiler was estimated with group combustion theory. The calculated results show that combustion effectiveness factor ηeff of near-wall particle cloud is smaller than 1/25, which means the combustion rate of a carbon particle in the near-wall region is greatly restricted by other particles in the cloud.

Synthesis mechanism and properties of Mg–Mg2Ni composite hydrogen storage alloy produced by hydriding combustion synthesis

Hydriding combustion synthesis (HCS) has been regarded as an innovative process to produce magnesium based hydrogen storage alloys. In the present paper, a Mg–Mg2Ni composite hydrogen storage alloy was prepared by the HCS process under moderate conditions. Phase composition, synthesis mechanism and hydrogen storage properties were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and pressure composition temperature (PCT). The results indicated that the HCS product was composed mainly of MgH2, Mg2NiH4 and Mg2NiH0·3. A gas–solid reaction mechanism played an important role in the HCS process. The hydriding activity of the as synthesised product was very high so that it absorbed 4·61 mass% hydrogen in the first hydriding process without any activation treatment and the maximum hydrogen storage capacity achieved was 5·24 mass%. Moreover, the hydriding rate was excellent. For example, it could absorb >3·20 and 4·05 mass% hydrogen within 4 min in the first and forth hydriding/dehydriding cycle respectively. The relationships between the plateau pressure and temperature were: lgP(0·1 MPa)=−4250·5/T+7·9635 (hydriding), lgP(0·1 MPa)=−4125·4/T+7·6502 (dehydriding) for the lower plateau, and lgP(0·1 MPa)=−2894·6/T+5·9656 (hydriding), lgP(0·1 MPa)=−3855·9/T+7·3435 (dehydriding) for the higher plateau.

Carbon Reduction Kinetics of NiO by Microwave Heating of the Separated Electric and Magnetic Fields

A powder mixture consisting of NiO and graphite was heated by microwave (MW) irradiation, and the reduction reaction kinetics was studied. The heating was performed in a single-mode MW applicator operated at 2.45 GHz. The specimen was placed in the waveguide at the maximum positions of either the electric (E-) field or the magnetic (H-) field. The reaction kinetics was investigated mostly at the E-field with variation of the reaction time, temperature, and grain sizes of NiO and graphite. It was shown that the kinetics was enhanced at the higher temperature and using smaller sized powder particles. The difference of the kinetics in the E-field and the H-field was investigated. A tendency was observed that the reduction rate was larger in the H-field than in the E-field at 600 °C. Considering the fact that NiO was heated well in the E-field, while graphite was heated in both fields, graphite particles in the powder mixture might be selectively heated in the H-field. The obtained results were discussed considering the thermodynamics and the gas-solid reaction mechanisms.

Carbon Monoxide Reduction and Accompanying Swelling of Iron Oxide Compacts.

Pure Fe2O3 compacts were fired at 1373K for one hour then reduced with CO at 973-1373K. Reduction was followed up by means of weight-loss technique whereas volume change was measured by displacement method. The structure of the fired and reduced compacts was examined by reflected light microscope while the different phases were identified by X-ray diffraction technique. These measurements together with kinetic data were correlated for better understanding the gas-solid reaction mechanisms and the accompanying swelling phenomenon. A highest swelling value of 176% was obtained for compacts completely reduced at 1173K whereas a maximum swelling of 224% was obtained for compacts 90% reduced at the same temperature. At all reduction temperatures swelling increased with the increase in reduction extent up to 90% where a maximum swelling value was obtained followed by a small decrease in swelling when compacts were completely reduced. Excessive swelling was attributed to the carbon in metallic iron and/or iron carbide formed and their reactions with oxygen forming CO and/or CO2. A mechanism of disintegration of iron grains and swelling of the compacts has been proposed and correlated with reduction conditions.

High Pressure Cell for Infrared and Raman Gas-Solid Reaction Mechanism Studies

Both infrared and Raman techniques, the latter to a lesser extent, have been applied to in situ mechanism studies at the gas-solid interface. Few of these studies, however, have been extended to include structure elucidation of intermediates forming during gross gas-solid reactions. For this purpose a versatile high pressure metal cell suitable for use in both Raman and infrared gas-solid mechanism studies has been constructed and is described in this communication.