I. Fernández-Morales

Study of heat-treated Spanish lignites: characteristics and behaviour in CO2 and O2 gasification reactions

Abstract Six Spanish lignites (raw and demineralized) have been charred to 1113 K in a N 2 atmosphere. The surface area, porosity and mineral matter content of the char coals so obtained have been studied, as well as their reactivity in CO 2 flow in the range 1073–1113 K, and in dry air in the temperature range 733–773 K. The reactivities of the raw chars in CO 2 may be explained according to the different inorganic matter content that may act as catalyst. The demineralization process brings about a lowering in reactivity and an increase, in general, in the apparent activation energy that may be interpreted as being due to a fall in mineral matter content and/or an increase in the amount of feeder pores. With regard to reactivity and apparent activation energy, in the case of dry air three groups of raw chars have been established. The differences between these three groups may be due to the different inorganic impurities present in the raw chars that catalyse the reaction of carbon with O 2 more than the porous texture parameters. Demineralization brings about a lowering in the reactivity values and a levelling off of apparent activation energies. The catalytic effect of iron has also been studied by adding different amounts of this metal to a demineralized char. The burn-off versus time curves of the different char coals have been adjusted by using the τ 0.5 parameter.

Gasification reaction of a lignite char catalysed by Cr, Mn, Fe, Co, Ni, Cu and Zn in dry and wet air

Abstract A Spanish lignite was demineralized and pyrolysed in N 2 atmosphere at 1133 K. On the resulting char, metals from Cr to Zn were deposited from nitrates dissolved in ethanol. The behaviour of these supported catalysts in the reaction with dry and wet air was studied by thermogravimetry in the temperature range 733–773 K. To determine the reactivity per unit catalyst surface site, the dispersion of the metals or metal oxides was measured by X-ray diffraction. The dispersion values found were low; the turnover frequencies (TOF) showed that Cu and Cr (probably in the form of Cr 2 O 3 ) are the best catalysts in this reaction when using either dry or wet air. Apparent activation energies were also obtained in the temperature range used; the Arrhenius plot for the copper-char sample shows that in this case the reaction may be diffusion-limited, although there is considerable increase in reactivity close to Tammanns temperature for copper (678 K). Oxygen chemisorption by Cr 2 O 3 may justify an oxygen transfer mechanism.

Adsorption capacity of Saran carbons at high temperatures and under dynamic conditions

Abstract Several activated carbons were prepared by carbonizing Saran 872 at different temperatures. The capacity of these carbons to adsorb several hydrocarbons was studied using a gas-chromatographic technique at high temperatures. All the carbons had pores of less than 0.62 nm in size. The sample obtained at 1573K in a He atmosphere had slit-shaped pores which acted as a molecular sieve for cyclohexane. These pores were opened by mild gasification of the sample, resulting in the loss of molecular sieve properties. Heats of adsorption of the hydrocarbons were calculated from the specific net retention volumes. The effect of oxygen surface complexes on hydrocarbon specific net retention volumes was also studied.

The dynamic adsorption of several hydrocarbons on active carbons

Abstract The adsorption of several hydrocarbons on different active carbons prepared from agricultural byproducts has been studied at high temperatures using a gas-solid chromatographic technique. The thermodynamic functions of adsorption were determined and the differences in the heats of adsorption have been used to evaluate porosity of the samples. An attempt to determine the London component of surface free energy of the active carbons has been made.

A gas-solid chromatographic study of the adsorption of hydrocarbons and alcohols on graphite

SummaryThe Henrys law region of the adsorption isotherms for several hydrocarbons and alcohols on a graphite was studied by gas-solid chromatography. The thermodynamic functions at zero surface coverage were determined. The low heats of adsorption indicate a highly homogeneous surface. A constant increment of the standard free energy with the number of carbon atoms in the lineal adsorbates was found. Attemps were made to estimate the London contribution to the surface free energy of the adsorbents. A non-specific interaction between the graphite and the adsorbates was shown.

Hydrogenolysis of n-butane and hydrogenation of carbon monoxide on Ni and Co catalysts supported on saran carbons

Abstract Ni and Co catalysts were supported on Saran carbons and characterized by CO and H 2 chemisorption and X-ray diffraction. For Ni catalysts, the mean Hydrogenation of CO is not a demanding reaction either for Ni or Co catalysts in the dispersion range we have studied. Methane was the main product obt The formation of olefins is suppressed in the supported Co catalysts as compared with the bulk one.

Adsorption of hydrocarbons on graphites and graphitized carbon black at zero surface coverage

Abstract The Henrys law region of the adsorption isotherms for a homologous series of n -alkanes on graphites and a graphitized carbon black was studied by gas chromatography. The three thermodynamic functions at zero surface coverage were determined. A linear relationship between the standard free energy, the enthalpy and the entropy and the number of carbon atoms was found. The low heats of adsorption at zero coverage indicated a virtual absence of high-energy sites. Attempts were made to estimate the London contribution to the surface free energy of the adsorbents; it was estimated to be 100 mN m −1 for the graphitized carbon black and 80 mN m −1 for the graphites.

The striking behaviour of copper catalysing the gasification reaction of coal chars in dry air

Abstract When copper acts as catalyst in the reaction of oxygen with coal chars, at a certain temperature there is a very high increase in the reactivity value, reaching a region of very low apparent activation energy. This temperature has been identified with that in which the particles of the catalyst wet the surface of the carbon and become mobile. The influences of copper salt and method of preparation of the catalysts, rank of parent coals, residence temperature, time and percentage of Cu and the temperature at which the reactivity is increased have been studied. In some cases this temperature is up to 190 K below the Tammann temperature of CuO (831 K).

Carbon molecular sieves produced by the fixation of sulphur surface complexes

SummaryA Saran carbon has been treated with carbon disulphide and sulphur in order to introduce sulphur surface complexes, a commercial activated carbon was also used. Samples obtained were characterized by their surface areas and porosities. The adsorption of n-hexane, benzene and cyclohexane has been measured out at high temperatures and under dynamic conditions using a gas chromatographic technique. Results were compared with those obtained when oxygen-surface complexes were introduced and it can be concluded that when CS2 is used with a highly microporous carbon, such as Saran, a molecular sieve for cyclohexane (mean size 0.58 nm) is produced at determined amounts of sulphur fixed.

Dynamic adsorption of methyl iodide on activated carbons

Abstract The adsorption at the Henrys law region of CH3I on several activated carbons was studied by gas-solid chromatography. The thermodynamic functions at zero surface coverage were obtained. The difference between the standard differential enthalpy of adsorption for each carbon, ΔH°A, and the heat of liquefaction of the adsorptive, ΔH°L, indicates that the adsorption takes place on micropores whose sizes are commensurate with the size of CH3I, although adsorbent-adsorbate interactions are nonspecific. Finally, from the ΔH°A and ΔG°A values, the evolution of the microporosity of the samples with the increase of the period of treatment was established.