No abstract

This work investigated the kinetics and mechanism of CO 2 gasification of chars from 11 types of Mongolian lignites (ash contents of 7−27 wt % dry) and the corresponding acid-washed (catalyst-free) lignites. The catalytic gasification contributed to 83−99% of the initial rate of char conversion to gas. The non-catalytic gasification obeyed first-order kinetics with respect to the fraction of unconverted char, 1 − X, with little or no effect of the physical property of char, such as specific surface area and porosity. The proposed kinetic model, which considered progress in parallel of non-catalytic and catalytic gasification, assumed the presence of three to four different types of catalysts together with a type of precursor, if necessary. The optimized kinetic parameters enabled quantitative description of the measured changes with time of X over its near entire range, X = 0−0.9995. The kinetic analysis revealed that the most and least active catalysts in each char had initial activities differing by 2−3 orders of magnitude. The initial rate of catalytic gasification, which varied with the lignite type by a factor of 16, was correlated well and linearly with (Ca + Na)/Si or (Ca + Na + Fe)/Si molar ratio. It was suggested that more or less portions of catalytic species had lost their activities prior to the gasification by reaction with Si and Al species. The catalyst also underwent deactivation during the gasification. The rate constant for the deactivation was a function of neither Si nor Si + Al content in the char but depended upon the initial activity of the catalyst.

No abstract

No abstract

No abstract

Ion-exchange reactions of brown and sub-bituminous coals with natural soda ash, composed of >99% Na2CO3, have been studied at 20–40 °C without any pH-adjusting reagents, and the pyrolysis and subsequent steam gasification of the resulting Na+-exchanged coals have been conducted using a fixed-bed quartz reactor at 700 °C. When the Na+ concentration and pH of an aqueous mixture of coal and soda ash are monitored during the ion-exchange process, both values decrease at a greater rate with brown coal with a higher content of COOH groups, indicating that ion exchange of Na+ with H+ of the COOH group is the predominant process. About 65% of COOH can be exchanged with Na+ ions under optimal conditions, irrespective of the coal type. The reactivity of these raw coals in steam at 700 °C is similar, with char conversions of less than 20 mass %, even after 2 h of reaction. Exchanged Na promoted the gasification of both coals at this temperature, but the rate profiles were different: conversion of brown coal increased linearly with time and reached nearly 100% at 1 h, whereas sub-bituminous coal needed approximately 2 h to be gasified completely. The temperature dependence of the conversion with this coal revealed that the use of a Na catalyst can lower the reaction temperature by about 120 °C, and the apparent activation energies were estimated to be 190 and 120 kJ/mol without and with the catalyst, respectively, from Arrhenius plots of the initial specific rate. The scanning electron microscopy–electron probe microanalysis and X-ray diffraction analysis of Na-containing chars recovered after pyrolysis and gasification suggested that the Na catalysts were finely dispersed at the initial stage of the reaction but that they may be deactivated by the formation of sodium silicates at high char conversions at temperatures higher than 90%, even at the low temperature of 700 °C.

No abstract

Properties, chemical compositions and hydrotreatment reactivities of Mongolian crude oils and their distillation fractions were investigated and compared with those of Chinese and Middle East crudes. Mongolian crude oils were highly paraffinic with high pour point ( 17 ) and low contents of sulfur (0.09-0.24 %), vanadium ( 1 ppm) and carbon residue ( 4.6 %), but contained large amounts of atmospheric residue (68-83 %). These features are very similar to Chinese Daqing crude. The middle and heavy distillates included large amounts of saturates ( 85 %) and n-paraffins (35-50 %). The nitrogen contents were comparable with the other crude oils or distillation fractions examined. The distillates and atmospheric residues were hydrotreated, and the reactivities of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) were compared. The HDS rates of low-sulfur feedstocks were much larger than those of high-sulfur feedstocks derived from the Middle East, whereas the HDN rates of the same boiling range fractions were similar. The relative HDS rates of the Mongolian feeds to the corresponding mixed Middle East feed were 6-14 for the middle distillates, 7-12 for the heavy distillates, and 5-6 for the atmospheric residues, and the relative HDN rates were 1-2 , 1-2.3 and 0.6-1, respectively. Polyaromatics were readily hydrogenated to monoaromatics but not to saturates due to the difficulty in further saturation.