Fanor Mondragón

Reactivity of coal gasification with steam and CO2

A description of the main parameters affecting char gasification, as well as an evaluation of the kinetic models used for describing the char gasification reaction, are presented. By reviewing literature on char gasification reactivity it is shown that of all the parameters involved in char gasification, the thermal history of the char, the pore structure and the coal chemical composition are those which present the most different results. The examination of reactive models indicates that their use depends mainly on their purpose in use.

New perspectives for coal ash utilization: synthesis of zeolitic materials☆

Abstract An alternative use for coal ash was investigated, based on the metakaolin-type material present in the ash. In this study, fly ash from Amaga coal (Colombia) was used. The ash was treated with sodium hydroxide at different concentrations and various times and temperatures of crystallization. The zeolite produced contained 50–75% of a faujasite type material, and had an adsorption capacity ≈70–80% of that of the commercial zeolite 13X.

CO2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO2 chemisorption on carbonaceous surfaces. The differential heat of CO2 adsorption at low and high coverages was determined in the temperature range 553–593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO2 chemisorption on carbonaceous surfaces was performed. Several of the carbon–oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO2 adsorption. In the high-coverage region, our theoretical results suggest that CO2 molecules are likely to adsorb on surface oxygen complexes and on graphene planes.

The use of differential scanning calorimetry to identify coals susceptible to spontaneous combustion

Non-isothermal oxidation enthalpies have been measured for three Colombian coals which have been weathered under ambient conditions for period up to 105 days. It was found that although the total oxidation enthalpies decreased with increasing oxidation the decrease was not systematic. It was thought that the random behaviour was due to weight losses during the course of the experiments. It was noted that the onset temperature of oxidation increased with oxidation in a more systematic way and also increased with increasing coal rank. It was therefore proposed that the onset temperatures was a better indicator of the propensity of the coals to oxidation.

Effect of low-temperature oxidation of coal on hydrogen-transfer capability

Abstract Coal oxidation with molecular oxygen at low temperatures is a process that can lead to spontaneous combustion. In this study, four Colombian coals were oxidized in a stove at temperatures between 30 and 150°C. The oxidized samples were heat-treated in an autoclave under nitrogen pressure at 420°C in the presence of anthracene. The main hydrogenated product was 9,10-dihydroanthracene. The amount of hydrogen transferred to the aromatic solvent was drastically reduced by oxidation of the sample and this varies with the coal rank. Fourier transform infra-red analysis, solvent swelling and coal conversion to soluble products in tetrahydrofuran were carried out for all of the samples. The mechanism by which coal reacts with molecular oxygen is strongly dependent on temperature. The initial stage of coal oxidation was apparently the same for all coals studied, i.e. the attack by molecular oxygen on the hydrogen in the α position. However, the path followed thereafter depends on the chemical functionality to which the α CH 2 group is attached. In this way it is possible to differentiate the products from low- and high-rank coals.

Formation of CO precursors during char gasification with O2, CO2 and H2O

The nature of some of the carbon–oxygen complexes formed after chemisorption of O2 ,C O2 and H2O on carbonaceous surfaces was determined. The analysis was done by means of density functional theories. Among the three reactions studied, CO2 chemisorption is the less exothermic. The nature of carbon–oxygen complexes depends on the oxidant agents. However, surface transformations of those complexes produce common surface oxygen groups that can desorb CO. Therefore, new data are presented to get insight into an unified mechanism of uncatalyzed carbon gasification. D 2002 Elsevier Science B.V. All rights reserved.

Heterogeneous CO2 Evolution from Oxidation of Aromatic Carbon-Based Materials

Carbon dioxide is one of the main gaseous products in oxidation of carbonaceous materials via both homogeneous and heterogeneous reactions. However, the mechanisms of heterogeneous CO(2) evolution during oxidation of aromatic carbon-based materials are not known in detail. Using density functional theory, a new oxidation mechanism of aromatic hydrocarbons with atomic oxygen was suggested to consist of four main steps, namely, (1) adsorption of oxygen atom, (2) insertion of O atom into the ring, (3) rearrangement to form a five-membered ring and four-membered ring lactone group, and (4) desorption of CO(2). Using naphthoxy radical as a model system, the proposed reaction pathway can explain how some of the experimentally observed CO(2) is formed.

Nitrogen complexes formation during NO–C reaction at low temperature in presence of O2 and H2O

Oxygen and nitrogen complexes formation in coal char due to its reaction with NO at 100 °C in presence or absence of O2 and/or H2O was studied by XPS. Formation of NO2 and pyridinic complexes and a considerable increment of oxygen complex was observed. The NO2 complexes can be formed in all cases, even when there is no molecular oxygen present in the gas mixture. The presence of O2 and/or H2O promotes their formation. Apparently, this complex can be formed by the presence of C(O) complexes next to the site capable of chemisorbing NO.

Adsorption on carbonaceous surfaces: cost-effective computational strategies for quantum chemistry studies of aromatic systems

We present a systematic analysis of the accuracy and efficiency of several computational quantum chemistry models for studying reactions involving aromatic systems. In particular, we have examined different multi-layer ONIOM models in which the whole system is divided into subsystems that can be treated at different levels of theory. The carbonaceous surface is modeled by a graphene layer that has unsaturated carbon atoms to represent active sites and has different shapes to simulate the local environment of the active sites of a carbonized material. We emphasized the model performance in predicting geometrical parameters, interaction energies and infrared spectra of carbon-oxygen complexes. We found that any attempt to partition the graphene layer into subsystems for employing different levels of theory yields considerable errors. However, it is possible to obtain reasonable accuracy by using the same level of theory for the whole system at different basis sets. This computational strategy can predict accurate geometrical parameters, interaction energies and infrared spectra of common oxygen complexes at lower computational cost.  2002 Elsevier Science Ltd. All rights reserved.

CO2 strong chemisorption as an estimate of coal char gasification reactivity

Abstract In this article, coal char gasification reactivity was correlated with the strong chemisorption of CO 2 at 300°C. Chars of as-received, demineralized, K and Fe loaded coals were prepared at 800°C, under high purity nitrogen. The CO 2 chemisorption method described in this article allows differentiation between two types of chemisorption that takes place at low temperatures: strong CO 2 chemisorption (irreversible) which is related to the presence of the active inorganic components of the char, and weak CO 2 chemisorption (reversible) which is because of the organic matter of the char. The char doped with K showed the highest CO 2 strong chemisorption and at the same time the highest reactivity in the CO 2 gasification, while the char loaded with Fe had the highest amount of weak chemisorption. It was found that total chemisorption (weak+strong) at 300°C depends on the CO 2 pressure of the analysis. The reactivity of the CO 2 gasification of the char was normalized using the value of the amount of CO 2 strongly chemisorbed at 300°C.