René Cypres

Mecanismes de fragmentation pyrolytique du phenol et des cresols

The thermal cracking of phenol and ortho and para-cresol at atmospheric pressure, between 665°C and 865°C has been studied with a contact time of 2·5 seconds. 14C- and 3H-labelled substrates have been used in mechanistic studies of these processes. Elimination of one carbon monoxide produced quasi-exclusively by loss of the carbon of the hydroxyl function of the phenols is the first step and leads to the formation of cyclopentadiene or of a C5H6 compound. A condensation of two phenol molecules with loss of water and hydrogen to form dibenzofuran occurs simultaneously. The formation mechanism of this last compound is different according to the cracking temperatures.

Fixed-bed pyrolysis of coal under hydrogen pressure at low heating rates☆

Abstract Fixed-bed hydropyrolysis has been investigated by treating 100 g coal up to 900°C and 10 MPa. The devolatilization rate of Beringen coal (32.8 wt% volatile matter) treated on a fixed bed approximates to that obtained by flash hydropyrolysis. However, the oil yield is smaller because of the slower heating of the coal and the rather longer residence time of the primary volatile matter in the reaction space. The product gas is mainly methane. The oil composition depends on the temperature of pyrolysis. The benzene content of the oil rises with temperature. At constant temperature, the influence of hydrogen partial pressure is important between 0–1 MPa. At higher pressure, the yields and compositions vary only slightly with pressure. It has also been shown that from 580°C pyrolysis under hydrogen yields an additional quantity of water, when compared with pyrolysis under inert atmospheres or under atmospheric pressure. This additional water comes from the hydrogenation reactions of the hydroxyl functions of heavy phenols and xylenols. This implies a hydrogen consumption (from 0.2–0.3 wt% of the coal), varying with the pyrolysis temperature.

La formation de la plupart des composes aromatiques produits lors de la pyrolyse du phenol, ne fait pas intervenir le carbone porteur de la fonction hydroxyle

Abstract The thermal cracking of phenol 1- 14 C and of phenols tritiated in specific positions has been studied at atmospheric pressure between 665 and 865°C, with a contact time of 2·5 seconds, suggesting the most probable reactions occuring during the thermal cracking. All the aromatic compounds, except benzene and to a less extent toluene, obtained by thermal cracking of phenol, are not produced by conservation of the aromatic ring, but by an intermediate structure C 10 H 12 derived from the condensation of two C 5 fragments. The results have shown that the carbon atom carrying the hydroxyl function is always the one eliminated in forming the C 5 fragment.

Pyrolysis of coal and iron oxides mixtures. 1. Influence of iron oxides on the pyrolysis of coal

Abstract The influence of iron oxide additions on coal pyrolysis has been studied by thermogravimetry and the analysis of the evolved gases. Liquid yields and tar composition were studied by complementary experiments on a larger scale. The presence of iron oxides reduces the primary-devolatilization rate of coal, between 300 and 600 °C. Tar and gaseous hydrocarbon yields decrease, but the tar composition does not change. Addition of magnetite, which is not reduced below 600 °C, results in a slight increase in hydrogen production at low temperature. However, at under 600 °C, Fe 2 O 3 is reduced to Fe 3 O 4 , yielding a little CO 2 and water. Towards the end of the primary devolatilization of coal, in the presence of iron oxide, the production of methane increases. In every case hematite has a greater influence than magnetite. The effects are smaller at low heating rates, and they occur with small additions of oxide, further additions having less influence. Iron oxides do not affect the primary devolatilization of lignite. With caking coals, the influence of rank is small. These observations are attributed to a superposition of three effects: 1. (1) the physical effect of oxide particles among coal particles, impeding the formation of the plastic state, 2. (2) the catalytic effect upon aromatization reactions of the metaplast, to form semi-coke, and upon secondary-formation reactions of methane, and 3. (3) the chemical effect of hematite reduction, preferentially using hydrogen required to form tar and gaseous hydrocarbons. The iron oxide structure, responsible for contact between the coal and oxide particles, is also important.

Hydropyrolysis of a high-sulphur-high-calcite Italian Sulcis coal. 1. Hydropyrolysis yields and catalytic effect of the calcite

Abstract Hydropyrolysis (HyPy) of a high-sulphur (4.3 wt% mf) and high-calcite (7.3 wt% mf) subbituminous coal (Sulcis coal) has been studied in a semi-batch fixed-bed reactor under a pressure of 1 or 3 MPa from 580 to 850 °C. The maximum temperature attained is not necessarily the temperature that the reactor is set but depends on the pressure and nature (reactive or not) of the gas; this phenomenon is due to the heat from the exothermic HyPy reaction. There is a correlation between the amount of heat released during the hydrogenation and the amount of water formed. The maximum conversion obtained is 62.5 wt% maf under H 2 at 3 MPa and 850 °C. The char, oil, water, gas (CH 4 , C 2 H 4 , C 2 H 6 , CO, C0 2 ) yields and the oil analysis are reported. A significant proportion of the C0 2 evolved during the reaction results from the decomposition of the mineral matter rich in carbonates. A proportion of the CO evolved results from the degradation of phenols, a reaction which is catalysed by calcite and/or lime, and as a consequence the oil yield is reduced.

Pyrolysis of coal and iron oxides mixtures. 2. Reduction of iron oxides

Abstract The reduction of iron oxides during the pyrolysis of blends of coal and iron oxides on a laboratory scale, has been studied. The pyrolysis of blends of bituminous coal and 30 wt% of magnetite or hematite has been studied by thermogravimetry and analysis of gases, using a heating rate of 3.2 K min −1 . The state of iron in ferrocoke has been established by X-ray diffraction. A primary reduction by hydrogen and carbon monoxide of the hematite has been observed at between 400 °C and 500 °C, but hidden in thermogravimetric measurements by primary volatilization of the coal. At ≈600 °C magnetite is progressively reduced to wustite and then to iron. This reduction starts a little earlier if the heating rate is slow and the coal rank is low and progresses more rapidly when using hematite. Except for higher heating rates in the coal-magnetite blends, the reduction is complete at 1000 °C. The reductants are H 2 and CO, with production of H 2 O and CO 2 . When the temperature is increased the reduction by CO becomes of increasing importance, being mainly produced from the coke by the Boudouard reaction. The consumption of coke for the reduction of iron oxides is therefore more important at higher temperatures. Lignite is clearly a better reducing agent than the other coals, because of larger quantities of CO produced from the start of its pyrolysis, and the good reactivity of its char towards CO 2 and H 2 O.

Pyrolyse thermique des [14C] et [3H] ortho et para-cresols

Abstract The thermal cracking of ortho and para-cresols labelled in specific position with [ 14 C] and [ 3 H] shows that the formation of water and methane are directly related to the formation of toluene and phenol respectively. On the other hand, the formation of benzene results from a rearrangement to a cycloheptatriene followed by elimination of carbon monoxide, produced quasi exclusively by loss of the carbon supporting the hydroxyl function. The minor products involve either a C 5 intermediate (cyclopentadiene), or a C 6 analogue (methylcyclopentadiene). The carbon supporting the hydroxyl group is not involved in the formation of minor products.

Behaviour of pyrite during hydrogenation of graphite at atmospheric pressure

Abstract The pyrite behaviour during hydrogenation of graphite is investigated. Kinetic experiments were carried out using thermogravimetry. The solid burn up residue was examined by SEM, and by X-Ray and electronic diffraction. The results show how graphite hydrogenation at temperatures lower than 1000 °C can be achieved in the presence of metallic iron obtained from pyrite. Pyrite, or its reduced form pyrrhotite, appears to have no catalytic behaviour during hydrogenation of graphite.

Low-temperature hydropyrolysis of coal under pressure of H2-CH4 mixtures

Abstract Hydropyrolysis of a Beringen bituminous coal (VM, 32.8wt%) has been studied in a fixed bed reactor with different gas flows of H2-CH4 and H2-N2 mixtures. At 580 °C, various hydrogen partial pressures between 0 and 1 MPa were used with a total pressure of 1 and 4 MPa. Oil yield increased significantly with increasing hydrogen partial pressure. However, if the difference between partial and total pressure is too large, the oil yield is affected more by the total than the hydrogen partial pressure. Similar effects are observed for the yields of BTX, PCX and naphthalenes except that for the latter the total pressure does not have a significant effect. In the conditions investigated the methane is chemically inert. Thus it is possible to recycle the gas during coal hydropyrolysis with only a slight decrease of the yields.

Direct post-cracking of volatiles from coal hydropyrolysis: 1. Influence of post-cracking temperature

Abstract Direct post-cracking of volatiles from fixed-bed hydropyrolysis of bituminous coal at 580 °C and 1 MPa hydrogen pressure has been studied between 600 and 900 °C at residence times of 0.1 and 1 s. Results showed that post-cracking promotes the formation of gas, mainly methane, at the expense of oil yield. However, the oil composition was richer in benzene, toluene and xylenes (BTX fraction), in naphthalene and methylnaphthalenes, and poorer in phenol, cresols and xylenols (PCX) content. The optimum temperature for post-cracking under conditions investigated was ≈800 °C, but at this temperature the PCX yield was reduced by 40–60%. The PCX formation rate, from heavier phenols, was lower than the PCX dehydroxylation.