Ximena García

Steam gasification of tars using a CaO catalyst

Abstract Catalytic gasification of three tars from different sources (oil refinery, Petrox; coking, Huachipato; coal gasification, Gasco) was carried out at a laboratory scale fixed bed reactor using CaO as catalyst, obtained by limestone calcination. Gasification runs were performed keeping constant the tar feed flow rate, as well as the steam concentration. The residence time, τ , was regulated through the carrier gas (argon) flow. For steam to tar ratios, R V o (g/g) equal to, or larger than 3.5 and temperatures over 750°C, total conversion to gaseous products was obtained; solid carbon deposition was observed at lower values of R V o . Gas yields and product distribution can be explained and partially predicted by extensive tar characterization, reported elsewhere. A kinetic model is proposed, which coupled with mass balances in a flowing reactor led to an expression of carbon conversion to gaseous products as a function of the operating variables, R V o and τ . The data for the catalytic gasification of these tars corroborate the mechanisms proposed at earlier studies of catalytic gasification, using naphthalene as a model compound.

Coal blend combustion: link between unburnt carbon in fly ashes and maceral composition

Abstract Coal blends are increasingly utilised at power plants with significant savings and without breaking environment regulations. However, evidence of interaction among the coals requires the study of some parameters that affect combustion efficiency and related opacity of emissions. Actual plant data was available for the combustion of five families of binary blends (single coals and approximately 25%/75%, 50%/50% and 75%/25% blends) with variable contents of ash, volatiles and maceral composition. Size distribution of particles was determined for the coals fed to the plant boilers and the fly ashes, as well as for unburnt carbon in the latter. The almost homogeneously sized feed from different coals generates a size distribution in the fly ash where 250-μm particles vary up to 1.29%, while particles smaller than 38 μm vary between 21.74% and 62.41%. Unburnt carbon increases with size of ash particles from a maximum of 12.2% for fractions smaller than 38 μm up to 73.9% for the fraction bigger than 150 μm. Total content of unburnt carbon in the fly ash from combustion of coal blends show deviations from the expected weighted average of the constituent coals (K, L, T, P, F, S and N). These deviations are related to maceral composition and rank based on reflectance values. The smallest deviation is shown by the blend (T/P) with coals having low values of reflectance and homogeneity of maceral contents. Larger deviations were found for blends K/L, P/F and S/N with higher difference of rank and greater heterogeneity of maceral composition. The K/L, R/N and S/N blends show positive deviations with respect to the expected weighted average, that is, blending was detrimental to the combustion efficiency, while blend P–F showed an enhance of the combustion efficiency as measured by unburnt carbon in the fly ash. A “reactive maceral index” introduced in this work plays a useful role. If a ratio of reactive maceral index is established for a binary blend as the ratio of the respective values of the index for the single coals, then the magnitude of the deviation from the expected average is proportional to the ratio of reactive maceral index of the blend. Thus, the K/L and P/F blends, with values of 4.6 and 65.6 for the ratio of reactive maceral index, differ significantly from the expected average, while blends T/P and R/N, whose binary families have a ratio of reactive maceral index close to 1, show a behaviour (in terms of unburnt carbon) which closely approach the expected weighted average.

Structural characterization of tar from a coal gasification plant : Comparison with a coke oven tar and a crude oil flash-column residue

Residual tar from a coal gasification plant was characterized using a wide array of analytical techniques. Structural comparisons were made with a coke oven tar and a crude oil flash-column residue. The gasification tar residue was found to have a carbon aromaticity >94%, with extensively dealkylated aromatic structures and only small amounts of oxygenates, compared with the 56% carbon aromaticity and significant occurrence of alkyl substitution in the coke oven tar. The proportion of material boiling > 450°C as well as the elemental carbon and ‘fixed carbon’ contents of the gasification tar residue were also higher than those of the other samples. As expected, the petroleum flash-column residue was mainly aliphatic (fa = 0.12), with small aromatic ring systems attached to long alkyl chains. For all three samples, MALDI mass spectra showed a signal at molecular masses > 20 000 u, with the gasification tar residue showing the narrowest range of molecular masses. Taken together, structural differences between the gasification tar and coke oven tar reflect the more severe thermal treatment and chemical history of the gasification tar, leading to the survival of low amounts (∼ 1 wt% of coal feed) of an apparently, chemically, very stable residue.

Gasification reactivity of Chilean coals

Abstract Gasification reactivities of raw and acid-washed coal chars obtained from the three most important coal-bearing regions in Chile have been determined in 0.1 MPa of oxygen using a thermobalance. Oxygen chemisorption capacities of the demineralized chars were also measured gravimetrically at 373 K in 0.1 MPa of oxygen. The subbituminous coals of Catamutun and Peckett are more reactive than the bituminous coals of Lota and Trongol due to the catalytic effect of their inorganic constituents. However, in the absence of catalytically significant mineral matter, coal rank is not an important parameter of char reactivity. The reactivity of chars based on carbon active surface areas, estimated from gravimetric chemisorption measurements, agreed very well with the previously reported value based on active surface areas obtained in a volumetric system. These results supply additional evidence that active surface area is the fundamental parameter that can explain most of the observed differences in the kinetic behaviour of coal chars and carbons in general.

Coal blend combustion: fusibility ranking from mineral matter composition☆ ☆

Although coal blends are increasingly utilized at power plants, ash slagging propensity is a non-additive property of the pure coals and hence difficult to predict. Coal ash tendency to slag is related to its bulk chemistry and ash fusion temperatures, and the present study aims to compare the results obtained from thermodynamic simulation with characterization of samples obtained as outcomes of plant-based coal-blend combustion trials at three utilities located in the Centre and North of Chile. Pulverized coal and plant residues samples from five families of binary blends tested in an experimental program were characterized for chemistry, mineralogy and maceral composition. The slagging was evaluated by determination of fusion curves using the MTDATA software and NPLOX3 database for the main coal ash oxides. The ranking obtained was approximately the same as obtained from carbon in the fly ashes and from plant residues observations. The thermodynamic modeling was a valid option to predict the fusibility during the combustion of blends.

A kinetic approach to catalytic pyrolysis of tars

Abstract A kinetic model to describe the catalytic pyrolysis of tars is proposed and validated through the experimental pyrolysis of three tars of different characteristics and origin; calcined limestone (11 m 2 /g) was used as catalyst. Experiments were carried out in a catalytic fixed bed reactor. The model assumes that tars are composed of two pseudo-components, described as heavy and light tars , which can be experimentally determined by a split temperature. Tar pyrolysis is described by two simultaneous chemical reactions; catalyst deactivation due to carbon deposition is also considered. Expressions for product concentrations as functions of residence time, selectivity and deactivation were obtained and the kinetic parameters evaluated.

CaO-MgO CATALYSTS FOR SOOT COMBUSTION: KNO3 AS SOURCE FOR DOPING WITH POTASSIUM

MgO, CaO, an equal ratio mixture of the oxides (50-CaO) and a sample with 10% potassium added to the previous mixture, showed increasing activity for the catalytic combustion of carbon black, a model compound for soot. These catalysts were characterized by XPS, BET, DRIFTS and AAS. Activity was measured by TGA and by TPO; CO and CO2 concentration in the outlet gas flow were measured by NDIR. Pure MgO was barely active. The 50-CaO catalyst was as active as the pure CaO in spite of a lower content of calcium. This was attributed to greater contact area and better dispersion of calcium, lesser bulk and surface carbonation, cooperative effect of magnesium that diminishes the stability of adsorbed surface compounds, and a greater accessibility of O2(g) to calcium sites. Potassium introduction over the 50-CaO produced high activity, related to a increase of the surface concentration of oxygen on the catalyst and larger CO2/CO ratio in the exit gases. Moreover, potassium addition resulted in a lower content of calcium on the surface, hence less carbonation of the catalyst

EFFECT OF Ag ADDITION ON THE THERMAL STABILITY AND CATALYTIC PROPERTIES OF LaFeO3 PEROVSKITE

SUMMARY The effect of low Ag substitution in LaFeO 3 perovskite on the thermal stability and catalytic oxidation of toluene was studied. Perovskites of the type La 1-x Ag x FeO 3 (x Ag =0.0, 0.05, 0.10) were prepared by the amorphous citrate method; characterized by nitrogen adsorption, XRD, FTIR, SEM-EDAX, TPR and O 2 -TPD techniques; and tested for the catalytic combustion of toluene in a flow reactor under excess oxygen. In comparison with the pure perovskite LaFeO 3 , the partially substituted La 1-x Ag x FeO 3 present a perovskite structure with well-defined crystalline phases and greater reduction ability (attributable to the redox pair Fe +3 /Fe +4 ). Noticeable changes in the α and β-oxygen contents upon silver substitution were detected. Larger content of α-oxygen in the pure LaFeO 3 perovskite is associated with greater catalytic activity. Two important effects upon Ag substitution in a pure LaFeO 3 perovskites were observed: i) a greater reduction ability, ii) higher occurrence of oxygen vacancies due to β-oxygen desorption, which were not active in the studied reaction.

Magnesia-supported potassium oxide catalysts for soot combustion: effect of Fe addition on the catalyst activity and stability

This work shows that loading Fe into the K/MgO catalyst for soot combustion can reduce K volatilization due to the formation of potassium-iron oxide compounds. K/MgO and K–Fe/MgO catalysts were prepared by the impregnation of potassium and iron precursors on MgO, and their activity for soot combustion was assessed by thermogravimetric analysis. Catalysts were characterized by atomic absorption spectroscopy, BET surface area, X-ray diffraction and X-ray photoelectron spectroscopy. Fe addition on K/MgO catalyst enhanced its catalytic activity, which is ascribed to a larger K retention into the catalysts either as potassium oxides or as K–Fe–O compounds. The non-stoichiometric potassium oxide (KXOY) is considered as the active phase of this kind of catalysts; it tends to migrate to the surface and to coat the Fe and K–Fe oxides compounds, increasing the surface concentration of active sites for soot combustion. Thus, molecular oxygen from the gas phase is activated over these potassium oxides to generate highly active oxygen species that finally oxidize the carbonaceous matter through an oxygen-transfer mechanism.