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Patterns and kinetics of pyrolysis of tobacco under inert and oxidative conditions
No full textThe present work is part of a more extensive research carried out in Naples on cigarette combustion. Theoretical and modelling work highlighted the existence of a misdistribution of the gas flow across the cigarette cross-section, which results into locally variable conditions. As a consequence, oxygen-starving versus oxygen-rich conditions do establish in the pre-heat zone of the cigarette, where pyrolysis of cigarette components takes place. More specifically, oxygen-starving conditions should characterize the process of reaction front propagation in the inner part of the cigarette, close to its axis, while oxygen-rich conditions should establish at the cigarette periphery, close to the paper burn line. The present paper addresses the pattern and the kinetics of pyrolysis under inert and under oxidative conditions of three types of tobacco. The experimental work consists of non-isothermal thermogravimetric analysis at heating rates comprised between 5 and 20 degrees C/min and with different He-oxygen mixtures. Kinetic expressions for the rate of pyrolysis tinder inert and under oxidative conditions have been obtained for each ingredient investigated. Results, albeit obtained at sample heating rates smaller than those relevant to actual cigarette burning, highlight the profound differences arising under purely pyrolytic or oxy-pyrolytic conditions as regards the number of reaction steps, the rate, the temperature ranges and yields in solid versus gaseous products of thermal decomposition. The effect of inert/oxidative conditions on the chemical composition of the gaseous products of pyrolysis is discussed in a companion paper [O. Senneca, S. Ciaravolo, A. Nunziata, J. Anal. Appl. Pyrol. 78 (2007) 452]. (c) 2006 Elsevier B.V. All rights reserved
Loss of gasification reactivity toward O-2 and CO2 upon heat treatment of carbons
No full textThe effect of heat treatment on the reactivity of different carbons toward oxygen and carbon dioxide was studied for heat-treatment temperatures in the range 500-2000 degreesC. Results were fitted to a kinetic model of annealing based on an nth order power law reaction kinetics. The sensitivity of the model to the choice of the ultimate char reactivity upon prolonged heat treatment was assessed. The thermodeactivation model was used to fit other experimental data of annealing available in the literature. As far as the loss of oxyreactivity is concerned, different thermodeactivation patterns (reflected by a change of annealing kinetic parameters) are observed at heat-treatment temperatures below and above about 1200 degreesC, depending on the carbon. Activation energy of thermodeactivation toward oxygen is larger under severe heat-treatment conditions than under moderate ones for the coals; it is relatively insensitive to heat-treatment temperature for the petroleum coke. On the contrary, a single set of annealing kinetic parameters fairly well represents the loss of carbon reactivity toward carbon dioxide throughout the temperature range investigated and for all the carbons. It is speculated that changes of the turbostratic carbon structure and modifications of the ash constituents, affecting the heterogeneous reaction of carbon along different pathways for the two oxidants, may both be relevant to the observed behavior
LOSS OF GASIFICATION REACTIVITY TOWARDS O2 AND CO2 UPON HEAT TREATMENT OF CARBONS, PROCEEDINGS OF COMBUSTION AND SUSTAINABLE DEVELOPMENT
No full textOverlapping of heterogeneous and purely thermally activated solid-state processes in the combustion of a bituminous coal
No full textMechanistic studies of coal cornbustion have long highlighted the variety of reaction pathways along which gasification may take place. These involve chemisorption of reactants, formation of surface oxides, surface mobility of chemisorbed species, and product desorption. At the same time, exposure of the solid fuel to high temperatures is associated with solid-state thermally activated processes. Altogether, the course of gasification may be profoundly affected by the overlapping and interplay of heterogeneous oxidation with purely thermally activated solid-state reactions. In the present work the cornbustion of a South African bituminous coal is analyzed in the framework of a simplified reaction network that embodies heterogeneous oxidative and thermally activated processes (pyrolysis, thermal annealing, coal combustion, char cornbustion, oxygen chernisorption) active both on the raw coal and on its char. The kinetics of each process of the network is assessed by a combination of thermogravimetric and gas analysis on coal and char samples. The analysis is directed to the determination of the prevailing combustion pathway, established from the interplay of oxidative and solid-state thermally activated processes, as a function of combustion conditions (temperature, heating rate, particle size). (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved
Oxidative pyrolysis of solid fuels
No full textThis study addresses the dependence of the rate and pattern of pyrolysis of solid fuels from the oxidizing versus inert nature of the gaseous atmosphere. A selection of four solid fuels is considered in the study, namely two plastics (polyethylene and polyethylene terephthalate), one lignocellulosic material (Robinia Pseudoacacia) and a South African bituminous coal. Fuels are pyrolyzed in a thermogravimetric apparatus at different heating rates, under inert conditions or in the presence of oxygen at different concentration. Results indicate that the action exerted by oxygen during pyrolysis depends on the nature of the fuel and on the process conditions such as heating rate and oxygen concentration. Larger heating rates and larger oxygen concentration may indeed emphasize differences between inert and oxidative pyrolysis. Further analysis is directed to check the adequacy of a power low kinetic expression to describe the dependence of the rate of oxidative pyrolysis from the level of oxygen concentration. (C) 2004 Elsevier B.V. All rights reserved
A thermogravimetric study of nonfossil solid fuels. 2. Oxidative pyrolysis and char combustion
No full textThe study addresses the competition between the course of purely pyrolytic processes and heterogeneous oxidation during oxidative pyrolysis of nonconventional fuels. This feature is one key, together with volatile matter flammability and effectiveness of oxygen transport to the particle, to establish whether flaming or glowing ignition of the fuel takes place. A selection of six nonconventional high volatile solid fuels is considered in the study, namely two plastics (polyethylene and poly(ethylene terephthalate)), two lignocellulosic materials (Robinia Pseudoacacia and waste wood), and two rubber-derived materials (scrap tires and ebonite). The analysis is based on the comparison of the behavior of fuels when heated in a thermogravimetric apparatus under inert and oxidizing atmospheres. Further analysis is directed to assess the heterogeneous combustion kinetics of chars obtained by pre-pyrolyzing in inert conditions samples of the fuels. The study is complementary to a companion paper (Senneca, O.; Chirone, R.; Masi, S.; Salatino, P. A Energy Fuels 2002, 16, 653) where pyrolysis in nitrogen of the same fuels has been addressed. Results indicate that heterogeneous oxidation and pyrolytic processes play different roles depending on the nature of the fuel. A general feature of all fuels tested is that burnoff cannot be simply described as sequential reaction paths corresponding to purely thermal degradation and heterogeneous oxidation. Synergistic effects between these processes are significant and need to be taken into account
The influence of thermal annealing on oxygen uptake and combustion rates of a bituminous coal char
No full textThe effect of thermal annealing on the combustion reactivity of a bituminous coal char has been investigated with a focus on the role of the formation of surface oxides by oxygen chemisorption. The combined use of thermogravimetric analysis and of analysis of the off-gas during isothermal combustion of char samples enabled the determination of the rate and extent of oxygen uptake along burn-off. Combustion was carried out at temperatures between 350 and 510 degrees C. Char samples were prepared by controlled isothermal heat treatment of coal for different times (in the range between 1 s and 30 min) at different temperatures (in the range 900-2000 degrees C). Results indicate that oxygen uptake is extensive along burn off of chars prepared under mild heat treatment conditions. The maximum oxygen uptake is barely affected by the combustion temperature within the range of combustion conditions investigated. The severity of heat treatment has a pronounced effect on char combustion rate as well as on the extent and rate at which surface oxides are built up by oxygen chemisorption. Chars prepared under severe beat treatment conditions show negligible oxygen uptake and strongly reduced combustion rates. Altogether it appears that a close correlation can be established between the extent and the accessibility of active sites on the carbon surface and the combustion rate. Despite the investigation has been carried out at temperatures well below those of practical interest, results provide useful insight into the relationship existing between thermal annealing, formation of surface oxide and combustion reactivity which is relevant to the proper formulation of detailed kinetic models of char combustion. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved
The influence of char surface oxidation on thermal annealing and loss of combustion reactivity
No full textThermal annealing associated with heat treatment of coal chars affects gasification reactivity and levels of unburned carbon in residual ash from coal-fired furnaces. The present study addresses the effect of char surface oxidation, occurring upon exposure to oxygen, on the course of thermal annealing, and related loss of combustion reactivity. This goal is pursued by comparing the extent of thermal annealing suffered by coal char upon heat treatment in a nitrogen atmosphere with that of chars that underwent oxidation prior to or during heat treatment. Oxidation of char was accomplished by supplying single or multiple pulses of air during the heat treatment, which were sufficient to oxidize the char surface but small enough to limit carbon gasification to less than 5%. The extent of thermal annealing was characterized both in terms of the loss of combustion reactivity and of the development of structural anisotropy of char samples, investigated by HRTEM. Results of the present study confirm that heat treatment reduces oxyreactivity of char samples, the effect being more pronounced at temperatures exceeding 1200 degrees C. Oxidation of samples mitigates the effects of heat treatment, as demonstrated by the smaller loss of gasification reactivity and by the more limited development of structural anisotropy of oxidized samples. Correspondingly, elemental analysis of samples indicates the formation of stable surface oxides upon oxidation, that are subsequently desorbed upon heat treatment. At temperatures exceeding 1200 degrees C, the effect of oxidation vanishes. Results are analysed and discussed in the light of the possible hindrance of thermal annealing due to the formation of stable surface oxides and of the parallel modifications occurring to the ash constituents. (c) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved
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