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This study focuses on analyzing the effect of both the peak temperature and pressure on the properties of biochar produced through slow pyrolysis of corn stover, which is a common agricultural waste that currently has little or no value. The pyrolysis experiments were carried out in a fixed-bed reactor at different peak temperatures (400, 525 and 650 °C) and absolute pressures (0.1, 0.85 and 1.6 MPa). The inert mass flow rate (at NTP conditions) was adjusted in each test to keep the gas residence time constant within the reactor. The as-received corn stover was pyrolyzed into a biochar without any physical pre-treatment as a way to reduce the operating costs. The properties of biochars showed that high peak temperature led to high fixed-carbon contents, high aromaticity and low molar H:C and O:C ratios; whereas a high pressure only resulted in a further decrease in the O:C ratio and a further increase in the fixed-carbon content. Increasing the operating pressure also resulted in a higher production of pyrolysis gas at the expense of water formation.

In this work, we present the application of a phenomenological kinetic model to the analysis of experimental results obtained with a LDH-based Ni–Mg–Al catalyst during the CCVD of methane. This model takes into account all the main stages implicated in CNT growth: methane decomposition, metal surface carburization, carbon diffusion, CNT nucleation and growth. The main causes of growth cessation, catalyst deactivation and steric hindrance have also been considered. The effect of the main operational variables, reduction (activation) and reaction temperatures and feed composition (hydrogen and methane composition), have been studied. The model fits very well the data obtained with the studied catalyst at the different operating conditions. As a consequence of the application of the model to the kinetic results, we can describe the influence of the operating conditions on each of the individual stages involved in the CNT formation.

A Phenomenological Kinetic Model has been developed that includes all the relevant steps involved in CNT growth by CCVD,that is, carbon source decomposition, nanoparticle surface carburization, carbon diffusion, nucleation, CNT growth, and growth termination by catalyst deactivation or by the effect of steric hindrance. Here we emphasize the importance of using a proper kinetic description of all the stages, in particular the initial carburization-nucleation and the growth cessation. We have discussed the different mechanisms proposed to explain the critical step of carburization−nucleation and have used an autocatalytic kinetic model to describe it. The two parameters involved in this autocatalytic equation allow a very good fit of the initial induction period usually observed during the growth of CNTs. In addition, rigorous formulations of the main causes of CNT growth cessation (catalyst deactivation by several causes and steric hindrance) have been proposed. The developed model is a versatile tool of ...

Catalytic decomposition of methane is a potential alternative route for the production of hydrogen and nanocarbonaceous materials from natural gas and other hydrocarbon feedstocks. In the present paper, we report the results of characterization and catalytic behaviour during the methane decomposition reaction of a spinel-like Ni–Mg–Al catalyst prepared by coprecipitation. The influence of reaction temperature and feed composition on carbon content, carbon formation rate and carbon morphology has also been studied. The main consequence of MgO addition to the support is the increase in the activity and stability of the Ni–Al catalysts. The better performance of Ni–Mg–Al catalysts is due to the higher interaction generated between Ni particles and the support in this catalyst, which prevents the formation of large metallic particles. The carbonaceous products are carbon nanofibres (diameters ~10–35 nm) and amorphous carbon, which causes the catalyst deactivation by encapsulation. The amount of each type of carbonaceous material depends on the different operating conditions used. The reduction–reaction–regeneration cycles lead to a remarkable sintering of the Ni crystallites due to weakening of the metal-support interaction.

cited By 57; International audience; Catalytic decomposition of methane is a potential alternative route for the production of hydrogen and carbon nanofilaments from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of Ni-Cu-Mg-Al catalysts. The effect of the Cu addition in the catalyst composition on activity and stability has been investigated. The influence of operating temperature and feed composition on carbon content and carbon formation rate has also been studied. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. A higher reaction temperature increases both the deactivation rate and the growth rate of filaments. An increase in the methane concentration generates a rise of the rate of carbon filament formation. The size of metal particles and the carbon filaments as well as the nanofilament texture depends on the copper content of the catalyst. When the Cu content is 7.6 wt.%, the carbon nanofilaments are nanofibers with a platelet texture, and the particles and CNFs sizes are widely distributed (50-400 nm). © 2006 Elsevier B.V. All rights reserved.

The catalytic decomposition of methane over nickel catalysts is a potential alternative route to steam reforming or partial oxidation for the production of hydrogen from natural gas and other feedstocks. In the present paper, we report the results of characterization and catalytic behaviour of a Ni(30%)/Al2O3 catalyst during the catalytic decomposition of methane. The influence of the operating and reduction temperatures and feed composition on the methane conversion, hydrogen production and coking rate has been studied. The effects of the regeneration cycles with oxygen on activity and carbon formation are also investigated. It has been shown that H2 inhibits both the carbon filament formation and the encapsulation of metallic particles by coke. An increase in the reaction temperature increases both the deactivation rate and the growth rate of filaments. However, at high reduction temperatures, there is a decrease in the number of filaments formed due to sintering of the Ni particles. A kinetic model has been developed for the prediction of H2 production and of carbon, taking into account both stages of carbon formation, nucleation and filament growth.