Yasuo Ohtsuka

Highly active catalysts from inexpensive raw materials for coal gasification

Abstract The present review article focuses on novel methods of converting inexpensive raw materials to active catalysts for low-temperature coal gasification, which can produce clean fuels and valuable feedstock with high thermal efficiency. Precipitation methods using NH3, urea, and Ca(OH)2 make it possible to prepare active, Cl-free iron catalysts on brown coals from an aqueous solution of FeCl3 as the major component in acid wastes. The use of Ca(OH)2 provides the most active iron, which achieves complete gasification within 60 min in a thermogravimetric run at 973 K. Ca(OH)2 and CaCO3 are other promising catalyst sources. Ca(OH)2 promotes the steam gasification of many coals with different ranks at 973 K when kneaded with them in water. The calcium shows a larger catalytic effect for low-rank coals with higher contents of oxygen-functional groups as ion-exchangeable sites. CaCO3, as a raw material of Ca(OH)2, reacts with COON groups to form ion-exchanged Ca and CO2 when mixed with brown coals in water. Ion-exchange reactions proceed more readily with aragonite naturally present in seashells than with calcite from limestone. The exchanged calcium shows higher catalytic activity than the precipitated iron and provides at the largest 40–60-fold rate enhancement during steam gasification at 973 K. Catalysis of coal gasification by the iron and calcium is discussed in terms of catalyst dispersion, reactive sites, and sulfur poisoning.

Calcium catalysed steam gasification of Yallourn brown coal

Abstract Steam gasification of calcium-loaded Yallourn coal has been carried out with a thermobalance. Calcium catalyst showed a high activity at ≈950K. Calcium hydroxide, carbonate, acetate, nitrate and chloride exhibited similar catalyst effectiveness. The gasification rate increased with increasing the calcium loading, and at a loading of 5 wt%, complete gasification was attained within 25 min at 973 K. Comparison of the uncatalysed and catalysed rates showed that calcium catalyst can lower the reaction temperature by 150K. The impregnation of calcium salt on devolatilized char in place of raw coal resulted in the formation of rather large catalyst particles, and their activity was low. High temperature X-ray diffraction analysis revealed that the interconversion between calcium carbonate and oxide takes place readily in the gasification temperature region. Calcium carbonate was the predominant species during the gasification at 923 K. Catalysis of calcium was discussed in terms of a carbonate-oxide cycle mechanism.

Effect of alkali and alkaline earth metals on nitrogen release during temperature programmed pyrolysis of coal

Abstract Formation of HCN, NH 3 , and N 2 during fixed-bed pyrolysis at 10K min −1 has been studied using coal samples after partial demineralization followed by addition of metal hydroxides from aqueous systems. Without additives, NH 3 is the predominant product at ≤ 700°C, showing the two peaks in the formation rate profile, whereas N 2 is the only product at ≥ 800°C. The presence of NaOH, KOH and Ca(OH) 2 promotes considerable NH 3 formation between 450 and 600°C, but in contrast suppresses HCN formation in this region. The Ca shows the largest effect on both the promotion and suppression. It is likely that the NH 3 increased by Ca addition arises partly from HCN, but mainly from secondary reactions of tar-N. These hydroxides affect N 2 formation in quite different manners: the Na decreases the rate between 700 and 950°C, and the K changes it less significantly than the Na, but the Ca remarkably increases the rate in a low temperature region of 550–700°C. These different features are discussed in terms of solid-phase reactions of alkali metal carbonates with char-N and secondary decomposition reactions of tar-N on CaO particles. As a result, total conversion of coal-N to HCN, NH 3 and N 2 up to 1000°C increases in the sequence of Na

Synthesis of ethane and ethylene from methane and carbon dioxide over praseodymium oxide catalysts

Abstract Catalytic effectiveness for the synthesis of ethane and ethylene from methane and carbon dioxide has been examined mainly at 1123 K over 14 lanthanide oxides. Praseodymium (Pr) and terbium oxides show high yield and high selectivity of C2 hydrocarbons among lanthanide oxides. The catalytic performances of Pr oxides are influenced by the preparation method; a commercial Pr oxide prepared from the oxalate shows the highest C2 selectivity of about 50%, and the stable performance for 20 h. Higher partial pressures of CO2 and CH4 are favorable for C2 formation over the oxide catalyst. Reaction mechanisms over Pr oxides are discussed in terms of a redox mechanism involving the unstable lattice oxygen.

Low temperature gasification of brown coals catalysed by nickel

Abstract Nickel catalyst exhibited an extremely high activity in the gasification of some low rank coals at a temperature as low as 750 K. Approximately 85% of Yallourn coal was converted within 30 min in steam at 773 K. A high nickel loading, > 4 wt% was necessary. It seems essential for coal high in oxygen and low in sulphur to be gasified in this manner. Oxygen-containing functional groups on the coal surface seemed to play an important role in keeping the nickel catalyst in a finely dispersed state. Hydrogen sulphide was strongly adsorbed on the nickel catalyst and retarded this reaction. Hydrogen and carbon dioxide were the main products of low-temperature steam gasification. Similar low-temperature gasification reactions were also observed in hydrogen and in carbon dioxide.

Chemical form of iron catalysts during the CO2-gasification of carbon☆

Abstract The chemical form of iron catalysts during the gasification of carbon with a CO 2 CO mixture was examined in situ by a controlled atmosphere high temperature X-ray diffraction method. Several iron compounds, Fe3O4, Fe1 − xO, Fe3C, α-Fe and γ-Fe, were identified between 700–1000 °C. Both the gasification temperature and the ratio of CO 2 CO affect the chemical form. The gasification reactivity was determined by thermogravimetry under the same experimental conditions as the X-ray study, and considered in relation to the chemical form of the catalyst.

Gasification of brown coal and char with carbon dioxide in the presence of finely dispersed iron catalysts

Abstract Gasification of brown coal and char with CO 2 using iron catalysts precipitated from an aqueous solution of FeCl 3 has been studied. When the pyrolyzed char is gasified in the temperature-programmed mode, the presence of the iron can lower the temperature giving the maximal rate of CO formation by 130–160 K, a larger lowering being observed at a higher loading in the range of ≤ 3 wt.% Fe. The specific rates of the isothermal gasification of iron-bearing chars at 1173 and 1223 K increase with increasing char conversion, resulting in complete gasification within a short reaction time. Comparison of the initial rates of uncatalyzed and catalyzed gasification reveals that iron addition can lower the reaction temperature by 120 K. Mossbauer spectra show that the precipitated iron exists as fine FeOOH particles, which are reduced mainly to Fe 3 C on charring at 1123 K. Most of the Fe 3 C is transformed into α-Fe and γ-Fe at the initial stage of gasification, and subsequently these species are oxidized to FeO and Fe 3 O 4 . The changes during gasification are discussed in terms of solid-gas and solid-solid reactions.

Nickel-catalysed gasification of brown coal in a fluidized bed reactor at atmospheric pressure

Abstract Pyrolysis and steam gasification of nickel-loaded Yallourn coal were carried out in a fluidized bed reactor at ambient pressure. The pyrolysis mode was influenced by the addition of nickel catalyst. The yield of total volatile matter decreased whereas the gas yield markedly increased, when compared with uncatalysed pyrolysis. This is considered to result from tar decomposing on the catalyst and being converted to gases and deposited carbon. For the catalysed steam gasification, ≈ 80 wt% of coal conversion was achieved at 873 K and the gas yield was twelve times as much as that for the uncatalysed reaction. The homogeneous equilibrium in the gas phase controlled the composition of the product gas. The product gas contained little tarry material and a negligible amount of hydrogen sulphide. Nickel was efficiently recovered from the residue by an ammonia-leaching method.

Application of microscopy to the investigation of brown coal pyrolysis

Abstract To examine the influence of calcium on the mechanisms of brown coal pyrolysis and gasification, the morphology of chars from raw and calcium-exchanged Yallourn brown coal was analysed. The chars were obtained by slow pyrolysis in a thermo gravimetric analyser and rapid pyrolysis in fluidized bed reactors operating at atmospheric pressure and at 1.1 MPa. They were examined by optical microscopy to determine reflectance and the percentage of particles that had become plastic during pyrolysis. In addition to confirming calciums inhibiting effect on tar yield, the results from the rapid pyrolysis experiments show that in the presence of calcium, char reflectivity decreases, char H/C ratio increases, and the proportion of particles going through a plastic stage decreases. Calciums inhibition of plasticity development is augmented by high pressure in the fluidized bed reactor. The effects appear to be attributable to the action of carboxylate calcium as a cross-linking agent, leading to the formation of a tighter char structure which traps the organic material that would otherwise be liberated as tar. The presence of Ca also increases the H/C ratio of the chars produced by slow pyrolysis, but the mechanism of pyrolysis differs, since in slow pyrolysis none of the particles showed evidence of plasticity. In slow pyrolysis, calciums influence on char reflectivity depends on the holding temperature, since temperature determines the extents of both coal devolatilization and catalytic transformations. The roles of calcium in these processes and their influence on optical anisotropy and reflectance are discussed.

Conversion of methane with carbon dioxide into C2 hydrocarbons over metal oxides

Abstract Conversion of methane with carbon dioxide has been performed over seventeen metal oxides using a flow reaction system at 1123 K. The conversion ratio of carbon dioxide to methane is approximately two in most cases. Ethane and ethene (C 2 hydrocarbons) are formed over many oxides, and C 2 yield is higher over yttrium and manganese oxides. Rare earth catalysts such as yttrium, lanthanum, and samarium show higher C 2 selectivities of about 30%. C 2 selectivity is almost unchanged over yttrium and zirconium oxides during reaction while it decreases over samarium oxide. X-ray diffraction analysis reveals that some oxides such as manganese and iron oxides are reduced during reaction. The reaction mechanism for C 2 formation is discussed in terms of both adsorbed oxygen species dissociated from carbon dioxide and the lattice oxygen in metal oxides.