Mechanism Of Carbon Deposition Research Articles

The Research on Formation Mechanism of Carbon Deposition in Remanufacturing Engines

The Research on Formation Mechanism of Carbon Deposition in Remanufacturing Engines

Ex-situ characterization of synchrotron radiation induced carbon contamination on LiF window

Synchrotron radiation induced carbon contaminations on optical elements is a major concern for a beamline designer. Growth mechanism of carbon deposition is not well understood. Structural characterization of radiation (3.5–11.8eV) induced contamination layer on LiF window is carried out by soft x-ray reflectivity (SXR) and grazing incidence x-ray diffraction (GIXRD) techniques. Diffraction peaks at 2θ=28.3° (002) shows a graphitic structures and peak at 75.3° (220) show a mixed phase (carbonado) of carbon. Out-of-plane cluster/grain sizes ∼6.05nm and ∼2.45nm are measured respectively for crystalline graphite and carbonado phase of carbon by Debye Scherrer equation. For elemental analysis x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy techniques are used. In Raman spectroscopy measurements the crystalline graphite G band (1556cm−1) and disordered graphite D bands (1350cm−1) are observed. In-plane cluster size∼ 9.62nm was calculated by empirical relation using ID and IG ratio of Raman peaks. XPS peak at 291eV indicates that most of the carbon is in the form of carbonate (ROCOOR’) that is polymerizing in form of rings/chains and converted into polymer like (PLCH) and graphitic like hydrogenated (GLCH) carbon due to continuous exposure by radiation. Contamination layer properties also compared with commercial graphitic structure.

Effect of the addition of Ce and Zr on the structure and performances of Ni–Mo/CeZr–MgAl(O) catalysts for CH4–CO2 reforming

A series of Ni–Mo/CeZr–MgAl(O) catalysts consisting of different mass fraction of CeZrO2 were prepared using a two-step coprecipitation method. The performances of the catalysts for CH4–CO2 reforming were investigated at 800°C under atmospheric pressure and gas hourly space velocity (GHSV) of 60L/(hgcat). Two representative catalysts which contained 0% and 10% CeZrO2 were further reacted at 900°C to preliminarily study the feasibility of establishing a balance between carbon deposition and elimination on the two catalysts. All catalysts were characterized using N2 adsorption–desorption, X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray spectrometry, temperature-programmed reduction, temperature programmed desorption of CO2, thermogravimetry, and temperature-programmed oxidation. Compared with the catalytic performance of CZ0 without the addition of CeZrO2 at 800°C, the results show that the formation of the admixture Ce0.8Zr0.2–MgAl(O) is beneficial to the activation of CH4. In addition, the stability test of CZ10 catalyst for 220h revealed that the circulation of Ce4+/Ce3+ can eliminate the coke on the catalysts and extend stability when the reforming reaction is carried out at above 900°C. The results may be explained on the basis of assumption about the mechanism of carbon deposition and elimination over Ni–Mo/CeZr–MgAl(O) metal oxide catalysts.

Carbon deposition on iron surfaces in CO–CO2 atmosphere

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO2 atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T < Tm) and a high carbon potential (αc>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO2 at 650, 706 and 750°C. Cementite (Fe3C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe3C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe3C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.

Morphology, composition, and structure of carbon deposits from diesel and biomass oil/diesel blends on a pintle-type fuel injector nozzle

A biomass oil/diesel blend was prepared using an emulsion method and combusted in a diesel engine. An injector was then removed and the morphology, composition, and structure of the carbonaceous deposits on the pintle-type nozzle were characterized using a combination of HRTEM, SEM/EDAX, Raman and XRD. Results showed that the carbon deposition of the emulsified fuel with high crystallinity was greater than that of diesel. The agglomerated particulate diameters of the deposited carbon from diesel and emulsified fuel were approximately 10–30μm and 50μm, respectively. The carbon deposition mechanism from the emulsified fuel was attributed to the high oxygen content of the groups leading to increased polymerization and subsequent condensation on the nozzle surfaces that was then carbonised.

Parameter study of transient carbon deposition effect on the performance of a planar solid oxide fuel cell

Carbon deposition has a serious effect on the performance of solid oxide fuel cells (SOFCs). An unsteady-state 2D model based on COMSOL software is used to study the carbon deposition process in a planar SOFC. The carbon deposition, catalyst activity, reaction rate and temperature fields are obtained to analyse the mechanism of carbon deposition in the SOFC at different operating time. The effects of the operating voltage, inlet H-2 molar fraction, operating pressure and operating temperature on the performance of the SOFC are investigated in detail. It is found that the biggest variation of the performances caused by carbon deposition occurs in the inlet domain of the anode support layer. The increase of operating voltage, inlet H-2 molar fraction, operating pressure and temperature accelerates the carbon deposition process. The predicted results could deepen our understanding of carbon deposition and its transient quantitative effects on the catalyst, structure and cell performance. (C) 2014 Elsevier Ltd. All rights reserved. (Less)

A Raman spectroscopic study of the carbon deposition mechanism on Ni/CGO electrodes during CO/CO2 electrolysis

In situ and ex situ Raman analyses of porous Ni/CGO electrodes reveal differences in the amount, location and type of carbon formed during CO/CO2 electrolysis. The results demonstrate the limitations of optical in situ techniques applied to Solid Oxide Cells (SOCs) operated in electrolysis conditions. Increased carbon deposition close to the electrode-electrolyte interface is likely to be the result of high charge-transfer current in that area. The positive effect of a CGO interlayer on reducing carbon formation on the fuel electrode is demonstrated.

A Model for Carbon Deposition During Hydrogen Production by the Steam Reforming of Bio-oil

H2 production by steam reforming of fast pyrolyzed bio-oil over Ni/MgO-La2O3-Al2O3 catalyst was carried out in a fixed-bed reactor by using model compounds (acetic acid, furfural, cyclopentanone, and m-cresol) and real bio-oil as the starting materials. The carbon deposition mechanism was discussed and a model of carbon deposition was built based on the amount of coke formed under different reaction temperatures, reaction times, steam to carbon molar ratios (S/C), and liquid hourly space velocities. The activation energies in the carbon deposition reaction and in the carbon elimination reaction were calculated as 28 and 71 kJ/mol, respectively, in terms of the carbon deposition model employed.

First-principles study of the mechanism of carbon deposition on Fe(111) surface and subsurface

A theoretical study of the carbon atoms adsorption and diffusion on the surface and into the subsurface of Fe (111) is performed using DFT calculations. Before the carbon coverages up to 1 ML, the adsorbed carbons tend to exist in an isolated atomic state and cause a reconstruction of Fe (111) surface. The configurations of “mC2+nC” are energetically favorable on the Fe (111) surface at 1 ML ≤ θC ≤ 2 ML. At a higher coverage, complicated adsorbed patterns such as chains and islands are found, and we predict that these carbon islands can function as the nucleation center of the precipitation of graphite or carbon nanotubes on the Fe(111) surface. In the subsurface region, the carbon atom prefers the octahedral site. The barriers for diffusion on and into the Fe (111) surface and subsurface are 0.45 eV and 0.73 eV, respectively. Actually, C2 formation is thermodynamically favored, whereas C migration into the subsurface region is kinetically feasible.

Raman spectroscopic monitoring of carbon deposition on hydrocarbon-fed solid oxide fuel cell anodes

Solid oxide fuel cells (SOFCs) are potentially the most efficient and cost-effective solution for the utilization of a wide variety of fuels beyond hydrogen. One of the chief obstacles to true fuel flexibility lies in anode deactivation by coking as well as a limited mechanistic understanding of coking and its prevention. Here we report Raman spectroscopic mapping and monitoring of carbon deposition on SOFC anode surfaces under both ex situ and in situ conditions. Carbon mapping was successfully demonstrated with a model Ni–YSZ electrode exposed to a CH4-containing atmosphere at high temperature (625 °C), while carbon deposition over time in a wet C3H8 atmosphere was directly monitored on a similar anode system as well as a BaO-modified system. This spectroscopic technique provides valuable insight into the mechanism of carbon deposition, which is vital in achieving rational design of carbon-tolerant anode materials.

Electrical conductivity and optical properties of thin carbon films grown from ethanol vapor

Transparent, ultrathin carbon films have been grown through the pyrolysis of ethanol vapor at a reduced pressure on copper substrates at temperatures from 600 to 950°C. The electrical conductivity of the films increases with deposition temperature. Depending on deposition temperature, ethanol vapor pyrolysis may follow different mechanisms and the carbon deposition process has different key features, which influence the properties of the films. In the range 600–750°C, ethanol vapor pyrolysis is a catalytic process, which results in selective growth of a thin carbon film with an optical transmittance of ∼95% only on the copper surface. At higher temperatures, carbon deposition is nonselective, and the resultant films are darker. The carbon deposition mechanism is discussed in relation to the ethanol vapor pyrolysis temperature. The present results suggest that carbon deposition from ethanol vapor is a promising approach to producing transparent, conductive carbon films.

Preparation of carbon-coated stainless steels and their properties as bipolar plate materials of polymer electrolyte fuel cells

In order to develop metal bipolar plates for polymer electrolyte fuel cells (PEFC), various grades of stainless steels (SUS304, SUS316, and SUS430) were coated with carbonaceous thin films by plasma-assisted chemical vapor deposition method. The resulting carbon-coated stainless steels were characterized by Raman spectroscopy, interfacial contact resistance (ICR) measurement, and scanning electron microscopy (SEM). Corrosion properties were examined by dynamic polarization measurement in H2SO4 solution. ICR values were decreased by carbon coating due to the deposition of a conductive carbonaceous thin film. The corrosion resistance of carbon-coated SUS304 and SUS430 was superior to that of carboncoated SUS316 at ambient temperature. Based on the SEM images obtained after dynamic polarization measurements, it was found that carbonaceous thin-film layers on SUS304 and SUS430 were dense and did not peel. At 80°C (simulated operating temperature of PEFCs), corrosion resistance of carbon-coated SUS304 was superior to that of carbon-coated SUS430. The influence of the stainless steel grade on the corrosion properties provided by carbon-coating was discussed in relation to the carbon deposition mechanism.

Preparation of carbon-coated stainless steels and their properties as bipolar plate materials of polymer electrolyte fuel cells

In order to develop metal bipolar plates for polymer electrolyte fuel cells (PEFC), various grades of stainless steels (SUS304, SUS316, and SUS430) were coated with carbonaceous thin films by plasma-assisted chemical vapor deposition method. The resulting carbon-coated stainless steels were characterized by Raman spectroscopy, interfacial contact resistance (ICR) measurement, and scanning electron microscopy (SEM). Corrosion properties were examined by dynamic polarization measurement in H2SO4 solution. ICR values were decreased by carbon coating due to the deposition of a conductive carbonaceous thin film. The corrosion resistance of carbon-coated SUS304 and SUS430 was superior to that of carboncoated SUS316 at ambient temperature. Based on the SEM images obtained after dynamic polarization measurements, it was found that carbonaceous thin-film layers on SUS304 and SUS430 were dense and did not peel. At 80°C (simulated operating temperature of PEFCs), corrosion resistance of carbon-coated SUS304 was superior to that of carbon-coated SUS430. The influence of the stainless steel grade on the corrosion properties provided by carbon-coating was discussed in relation to the carbon deposition mechanism.

Reactivity of a CaSO4-oxygen carrier in chemical-looping combustion of methane in a fixed bed reactor

Chemical-looping combustion (CLC) is a promising technology for the combustion of gas or solid fuel with efficient use of energy and inherent separation of CO2. A reactivity study of CaSO4 oxygen carrier in CLC of methane was conducted in a laboratory scale fixed bed reactor. The oxygen carrier particles were exposed in six cycles of alternating reduction methane and oxidation air. A majority of CH4 reacted with CaSO4 to form CO2 and H2O. The oxidation was incomplete, possibly due to the CaSO4 product layer. The reactivity of CaSO4 oxygen carrier increased for the initial cycles but slightly decreased after four cycles. The product gas yields of CO2, CH4, and CO with cycles were analyzed. Carbon deposition during the reduction period was confirmed with the combustible gas (CO+H2) in the product gas and slight CO2 formed during the early stage of oxidation. The mechanism of carbon deposition and effect was also discussed. SO2 release behavior during reduction and oxidation was investigated, and the possible formation mechanism and mitigation method was discussed. The oxygen carrier conversion after the reduction decreased gradually in the cyclic test while it could not restore its oxygen capacity after the oxidation. The mass-based reaction rates during the reduction and oxidation also demonstrated the variation of reactivity of CaSO4 oxygen carrier. XRD analysis illustrated the phase change of CaSO4 oxygen carrier. CaS was the main reduction product, while a slight amount of CaO also formed in the cyclic test. ESEM analysis demonstrated the surface change of particles during the cyclic test. The reacted particles tested in the fixed bed reactor were not uniform in porosity. EDS analysis demonstrated the transfer of oxygen from CaSO4 to fuel gas while leaving CaS as the dominant reduced product. The results show that CaSO4 oxygen carrier may be an interesting candidate for oxygen carrier in CLC.

Catalyst deactivation and engineering control for steam reforming of higher hydrocarbons in a novel membrane reformer

The catalyst deactivation and reformer performance in a novel circulating fluidized bed membrane reformer (CFBMR) for steam reforming of higher hydrocarbons are investigated using mathematical models. A catalyst deactivation model is developed based on a random carbon deposition mechanism over nickel reforming catalyst. The results show that the reformer has a strong tendency for carbon formation and catalyst deactivation at low steam to carbon feed ratios (<1.4 mol/mol) for high reaction temperatures (>700 K) and high pressures (>506.5 kPa) . The trend is similar for the cases without and with hydrogen selective membranes. Based on this preliminary investigation, an engineering control approach, i.e., in-site control with a concept of critical/minimum steam to carbon feed ratio, is proposed and used to determine the carbon deposition free regions for both cases without and with hydrogen membranes. The comparison between the reported data and model simulation shows that the critical steam to carbon feed ratio predicted by the model agrees well with the reported industrial/experimental operating data.

Influence of pressure, temperature and surface area/volume ratio on the texture of pyrolytic carbon deposited from methane

The kinetics of carbon deposition from methane were studied over broad ranges of pressures, temperatures and reciprocal surface area/volume ratios. Based on these results, it was possible to distinguish between a growth and a nucleation mechanism of carbon deposition and to select conditions for the preparation of well-defined samples for texture analysis by transmission electron microscopy and selected area electron diffraction. Maximal texture degrees were obtained at medium or high values of the above parameters, but never at low values, at which carbon formation is based on the growth mechanism and dominated by small linear hydrocarbons. High-textured carbon resulting from the growth mechanism is concluded to be formed from a gas phase with an optimum ratio of aromatic to small linear hydrocarbons, which supports the earlier proposed particle-filler model of carbon formation. High-textured carbon may also be formed from a gas phase dominated by polycyclic aromatic hydrocarbons (nucleation mechanism) provided that the residence time is sufficiently long that fully condensed, planar polycyclic aromatic hydrocarbons can be formed in the gas phase.

Chemical vapor infiltration of carbon fiber felt: optimization of densification and carbon microstructure

A carbon fiber felt with a fiber volume fraction of 7.1% was infiltrated at temperatures of 1070 and 1095 °C and methane pressures from 5 to 30 kPa to confirm the inside–outside densification derived from model studies with capillaries 1 mm in diameter. Bulk densities and residual open porosities were determined as a function of infiltration depth at various heights of the felt. The texture of the infiltrated carbon was studied by polarized-light microscopy and characterized with the aid of the extinction angle. Inside–outside densification was demonstrated up to the maximum pressure of 30 kPa at 1070 °C and up to 13.5 kPa at 1095 °C, leading to bulk densities above 1.9 g/cm 3. A pure, high-textured carbon matrix is formed in the pressure range from 9.5 to 11 kPa at 1095 °C. At lower and higher methane pressures and lower temperature, a less textured carbon is formed. The results are based on the growth mechanism of carbon deposition. They strongly support recent conclusions that high-textured carbon is formed from a gas phase exhibiting an optimum ratio of aromatic hydrocarbons to small linear hydrocarbons, preferentially ethine. This model is called the particle-filler model. Aromatic hydrocarbons are the molecular particles and small linear hydrocarbons are the molecular filler, necessary to generate fully condensed planar structures.

Hydrogen–Deuterium Exchange Studies on the Decomposition of Methane over Ni/SiO2

The reaction mechanism of methane decomposition over silica-supported Ni catalyst (Ni(5 wt%)/SiO2) was investigated by performing kinetic studies of the reactions using isotopic methane (CH4, CD4, and 13CH4) and hydrogen (H2 and D2). An isotope effect between CH4 and CD4 was observed in the decomposition of methane over the Ni/SiO2 catalyst. In the decomposition of a gas mixture of CH4 and CD4 over the Ni/SiO2 catalyst, the H–D exchanged methanes were not formed. These results suggested that the first C–H bond cleavage in CH4 was the rate-determining step in the decomposition of CH4 into carbon and H2. The idea well explained the reverse isotopic effect between H2 and D2 in the hydrogenation of the carbons deposited on the catalyst back into methane. Moreover, the rate-determining step was supported by the result that only CD4 was formed in the early part of the exchange reaction between CH4 and D2 (CH4⪡D2). The mechanism of carbon deposition on the Ni/SiO2 catalyst was examined by performing successive depositions from 12CH4 and 13CH4, followed by hydrogenation of the deposited carbon with H2. The carbon atoms deposited last were found to be hydrogenated to CH4 at first, suggesting that the carbon atoms did not scramble and the amorphous parts in a carbon fiber would be hydrogenated back into CH4 catalyzed by a Ni particle present at the tip of a carbon fiber.

A study of carbon deposition on fuel cell power plants — morphology of deposited carbon and catalytic metal in carbon deposition reactions on stainless steel

Carbon deposited on SUS304 stainless steel (18Cr 8Ni) has been observed by two different methods. One method was Field Emission Transmission Electron Microscopy (FE-TEM), with developed preparation for in situ observation of a cross-section of the deposited carbon from the base (SUS) to the top. The other method was X-ray Photoelectron Spectroscopy (XPS), obtaining composition–depth profiles by argon ion sputtering. Carbon was deposited on SUS304, 550°C, 1 atm, H 2/CO/CO 2=75/15/10, after drained the steam-reformed natural gas composition. One result from FE-TEM identified the major form of deposited carbon was tubular in shape with a variety of diameters, ranging from approximately 7 to 100 nm. Some tubes contained metallic particles which were about 20 nm in size at their tips. Therefore, it can be established that the carbon deposition mechanism is similar to that reported for metals such as Fe, Ni, and that the deposited carbon can grow after the SUS surface is covered with deposits under the above conditions. Observations from EDX attached to FE-TEM also determined that most of the particles consisted of Fe and from XPS, that the content of Fe on the surface of the reaction plate was lower than on the unreacted sample. This indicates that carbon deposition on stainless steel was influenced by Fe rather than Ni and Cr.

Influence of Coal Moisture Control on Carbon Deposition in the Coke Oven Chamber.

The carbon deposition mechanism in the free space of coke oven chambers has been studied under Coal Moisture Control (CMC) operation by the actual oven measurements. It was found that the amount of carbon deposits in CMC operation increases as much as 45 to 75 mass% compared with those in wet coal operation. The carryover fines are not negligible mass for the carbon deposits, namely, the carbon deposits on the base portion of an ascension pipe in CMC operation contained about 20 mass% of materials derived from carryover fines, and the majority of the deposits at the initial stage of the carbonization were derived from the carryover fines. The influence of volatile matter on the carbon deposits in CMC operation was greater than that in wet coal operation.