Thermogravimetric and reaction kinetic analysis of biomass samples from an energy plantation

The products of a Hungarian experimental plantation for energy crops were investigated. Young shoots of poplar clones (Populus x euramericana and Populus x interamericana), black locust (Robinia pseudoacacia), willow (Salix alba), and an herbaceous plant (Miscanthus sinensis) revealed unexpectedly similar thermal behavior in inert and oxidative atmospheres, as well. An 8-fold difference in the level of grinding did not result in substantial differences in the thermal decomposition. The effect of oxygen in the ambient gas was studied at low sample masses (0.2-0.4 mg) that excluded the overheating due to the high reaction heat of the combustion process. The presence of oxygen affects the decomposition from ca. 220 degreesC. Nevertheless, the extrapolated onset temperature of the hemicellulose decomposition is practically the same at 0, 5, and 21 V/V% oxygen. A group of 12 experiments, representing two grinding levels, three plant genera and four different heating programs were evaluated simultaneously by the method of least squares employing the model of independent pseudocomponents. All evaluated experiments were well described by the same set of kinetic parameters; only the parameters describing the peak area of the partial processes differed. A technique was recommended for the appropriate handling of the nonrandom errors in the simultaneous evaluation of experiment series

Overlapping of heterogeneous and purely thermally activated solid-state processes in the combustion of a bituminous coal

Mechanistic 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

Loss of gasification reactivity toward O-2 and CO2 upon heat treatment of carbons

The 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

A Semi-Detailed Kinetic Model of Char Combustion with Consideration of Thermal Annealing

The present paper presents a semi-detailed kinetic model of coal char combustion which embodies consideration of thermal annealing as a mechanism leading to the loss of char combustion reactivity along burn off. The distinctive feature of this model is that deactivation induced by thermal annealing is followed along with combustion. Thermodeactivation is modelled according to the power-law equation proposed by Senneca and Salatino [1]. A semi-detailed combustion mechanism was taken after Hurt and Calo [2] and includes three steps: formation of carbon–oxygen complexes (chemisorption), switch-over of surface oxides and desorption of oxygen complexes to yield combustion products. Computation results allow to discuss the impact of thermal annealing on char combustion under conditions of practical interest. 2010 The Combustion Institute

Impianto e processo per combustione di tipo looping di solidi carboniosi

L’invenzione è relativa ad un impianto e processo per la “looping combustion” di combustibili solidi carboniosi con produzione di una corrente di anidride carbonica (CO2). Detto processo realizza la conversione del carbonio senza l’ausilio di vettori solidi di ossigeno del tipo , e comprende gli stadi di: (i) Ossidazione, in cui i solidi carboniosi vengono posti in contatto con una corrente gassosa comprendente ossigeno, per un tempo e ad una temperatura sufficienti per formare un complesso superficiale ossidato; (ii) Desorbimento, in cui i complessi superficiali ossidati generati per adsorbimento di ossigeno al punto i) sono rilasciati per decomposizione in carenza di O2 sotto forma gassosa