Direct post-cracking of volatiles from coal hydropyrolysis: 2. Influence of pressure

Direct post-cracking of volatile material produced by hydropyrolysis of bituminous coal at 580 °C under hydrogen pressure 1–5 MPa has been investigated at 700 °C under constant hydrogen pressure with 0.1 and 1 s residence times. Results show that pressure promotes the formation of benzene, toluene and xylenes (BTX) and naphthalenes during post-cracking, while phenol, cresols and xylenols (PCX) are not affected. The transformation of heavy Ohenols into PCX is not influenced by the hydrogen pressure. During post-cracking the BTX yield can be more than doyble that reached in simple hydropyrolysis. Post-cracking applied to high oil yield hydropyrolysis processes will be a valuable BTX source.     

Pyrolysis and hydropyrolysis of Louisiana lignite

Pyrolysis and hydropyrolysis kinetics of five samples of Louisiana lignite have been studied in an atmospheric pressure TG system as a function of heating rate and atmosphere. Final pyrolysis temperature was always 800 °C. The total volatile yield (dry basis) was 33.5–43.8wt%. For all lignites the volatile yield in hydrogen was greater than that obtained in nitrogen at similar conditions. However, variation in heating rate produced an opposite result in the two atmospheres with volatile yield increasing with heating rate in nitrogen but decreasing in hydrogen. Results have been analysed using the distributed activation energy pyrolysis model and parameters compared to similar studies using North Dakota and Montana lignites.     

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

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.     

Flash hydropyrolysis of Zalannoer lignite

Responses of Northern Chinese Zalannoer lignite to flash hydropyrolysis (FHP) and flash pyrolysis (FP) in a small entrained reactor were explored in experiments at temperatures between 500 and 900°C at pressure up to 6.0 MPa. Compared to FP, FHP can produce more hydrocarbon liquids (HCL) and CH₄. At the conditions of 800°C and 6.0 MPa (hydrogen pressure) the maximum yield of HCL (6.62, wt% C) was obtained.     

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

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.     

Rapid devolatilization and hydrogasification of bituminous coal

Rapid devolatilization and hydrogasification of a Pittsburgh Seam bituminous coal were studied and an appropriate coal conversion (weight loss) model was developed that accounts for thermal decomposition of the coal, secondary char-forming reactions of volatiles, and homogeneous and heterogeneous reactions involving hydrogen. Approximately monolayer samples of coal particles supported on wire mesh heating elements were electrically heated in hydrogen, helium, and mixtures thereof. Coal weight loss (volatiles yield) was measured as a function of residence time (0–20 s), heating rate (65–10000 °C/s), final temperature (400–1100 °C), total pressure (0.0001–7 MPa), hydrogen partial pressure (0–7 MPa), and particle size (70–1000 μm). Volatiles yield under these conditions increases significantly with decreasing pressure, decreasing particle size, increasing hydrogen partial pressure and increasing final temperature, but only slightly with increasing heating rate. The data support the view that coal conversion under these conditions involves numerous parallel thermal decomposition reactions forming primary volatiles and initiating a sequence of secondary reactions leading to char. Intermediates in this char-forming sequence can escape as tar if residence time in the presence of hot coal surfaces is sufficiently short (e.g. low pressures and small particles well dispersed). Hydrogen at sufficiently high partial pressure can interrupt the char-forming sequence thereby increasing volatile yield. Rate of total product generation is largely controlled by coal pyrolysis while competition between mass transfer, secondary reactions, and rapid hydrogenation affects only the relative proportions of volatile and solid products formed.     

大同煤热解和加氢热解过程中产物生成规律的研究

在压力2MPa,温度350—650℃范围内,对比研究了大同煤分别在氮气和氢气气氛下热解过程中产物的分布和气体生成规律。研究表明,煤的热解和加氢热解转化率和水产率都随温度上升而增加;在热解条件下,焦油产率在500℃出现最大值。氢气对煤热解转化只有超过一定温度才具有促进作用,此时与热解相比具有较高的CO、CH4和C2^＋产率以及较低的CO2产率。     

Thermogravimetric study of the devolatilization of hydropyrolysis chars

The devolatilization of hydropyrolysis (HyPy) chars formed at 485–850 °C under 3 MPa and chars formed at 580 °C under 0–5 MPa of H₂ and 3 MPa He is investigated in a thermobalance coupled to two gas chromatographs. The H₂, CH₄ and CO₂ released are analysed every 4 min and all are analysed at the end of the experiment. The amount of residual volatile matter in the chars decreases rectilinearly with the HyPy temperature, whereas their decrease is asymptotic with the HyPy pressure. The char formed under He contains 45% more residual volatile matter than that formed in the same conditions under H₂. The HyPy temperature must be limited if the char is to be burned in a boiler. The CH₄ formation is strongly dependant of the HyPy temperature.     

Product compositions in rapid hydropyrolysis of coal

The rapid hydropyrolysis of a Montana lignite and a Pittsburgh Seam bituminous coal has been examined at a high heating rate (1000 °C s^?1). The results are interpreted in the light of those presented previously for pyrolysis of the two coals. A complex situation involving simultaneous chemical and mass transfer rate control is revealed, and it is apparent that the pyrolysis phase cannot be treated separately from the hydropyrolysis phase. Three key process variables, hydrogen pressure, temperature, and particle diameter, are seen to have major effects on the total yields of products. There appears to be an important tradeoff between high hydrogen pressures and high temperatures. Under the present conditions of almost zero vapour residence time at elevated temperatures, methane is the principal product of reaction between hydrogen and coal, and the yields of light aromatic liquids are small.     

Nitrogen evolution during rapid hydropyrolysis of coal

The behavior of nitrogen evolution during rapid hydropyrolysis of coal has been investigated at temperatures ranging from 923 to 1123 K and hydrogen pressure up to 5 MPa using a continuous free fall pyrolyzer. Three coals have been tested in this study. The dominant nitrogen gaseous species is ammonia, together with a little amount of HCN because most of HCN is converted to NH₃ through secondary reactions. The results show that the evolution of nitrogen in coal is caused mainly by devolatilization at temperatures below 973 K, while the evolution of volatile nitrogen in char is accelerated with increasing temperature and hydrogen pressure. The mineral matter in coal act as catalysts to promote the evolution of volatile nitrogen in char to N₂ apparently at high temperatures of 1123 K, as found during pyrolysis of coal by Ohtsuka et al. A pseudo-first-order kinetic model was applied to the evolution of nitrogen in coal during rapid hydropyrolysis. The model shows the activation energy for the nitrogen evolution from coal is 36.6–58.6 kJ/mol while the rate of the nitrogen evolution depends on hydrogen pressure in the order of 0.16–0.24.     

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

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 hydrogénation and the amount of water formed. The maximum conversion obtained is 62.5 wt% maf under H₂ at 3 MPa and 850 °C. The char, oil, water, gas (CH₄, C₂H₄, C₂H₆, CO, C0₂) yields and the oil analysis are reported. A significant proportion of the C0₂ 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.     

Effect of pressure and temperature on the reaction of coal with alcohol-alkali

The ethanol-NaOH reaction of Taiheiyo coal (C, 77.5; H, 6.3 wt%) was examined under a pressure of 0.1–8 MPa nitrogen or hydrogen, at 300 °C for 1 h. Almost all of the products are extracted with pyridine for the entire pressure range and the extraction yield with ethanol increases with pressure, even under nitrogen. The yield of the products extracted with ethanol is higher when the coal is reacted under hydrogen than when reacted under nitrogen. The explanation for these results is that, under pressure, the hydrogen produced from the reaction of alcohol and NaOH is enclosed for a longer period in the solvent, thus accelerating the hydrogenation reaction of the coal, also under hydrogen pressure, the reaction is particularly accelerated because the hydrogenation takes place with the hydrogen gas. At 300 °C, the ethanol-extraction yield is much higher than the benzene-extraction yield, but the latter increases rapidly and approaches that of the ethanol-extraction yield as the temperature rises to 400 °C. This is because the polar groups, e.g. as hydroxyl groups which are rich in the low-temperature products, decrease with the temperature rise.     

Hydropyrolysis of a high-sulphur-high-calcite Italian Sulcis coal. 2. Importance of the mineral matter on the sulphur behaviour

The sulphur distribution among the char, oil and gas obtained after hydropyrolysis of a high-sulphur (4.3 wt%) and high-calcite (7.3 wt%) coal has been investigated. The chars were examined by scanning electron microscopy coupled with an energy dispersion analyser and by X-ray diffraction. The proportion of the combustible and non-combustible sulphur in the char has been determined. Hydrogen pressure promotes reaction with sulphur but the sulphur content of the chars increases from 3 to 4.5 wt% when the temperature is increased from 616 to 845 °C. This increase in sulphur is a consequence of the reaction between hydrogen sulphide, produced during hydropyrolysis of coal, with the alkaline-earth mineral matter to produce alkaline-earth sulphide. The SEM and X-ray diffraction images show that in the char formed at 780 °C the sulphur, calcium and magnesium are localized in the same compounds. This is not the case when hydropyrolysis is performed at lower temperature. Combustion of the chars produces only <0.6 S0₂ MJ^?1 compared to 2.2 g S0₂ MJ^?1 for untreated coal. X-ray diffraction has shown that the sulphur in the char is oxidized and fixed in the ashes mainly as CaS0₄ and also some as MgS0₄. Although sulphur remains partly in the chars after hydropyrolysis, 75% of it is non-combustible. The hydropyrolysis of a high-sulphur coal containing calcite, produces a char which may be used as a clean fuel.     

Secondary coking and cracking of shale oil vapours from pyrolysis or hydropyrolysis of a Kentucky Cleveland oil shale in a two-stage reactor

It is widely recognized that secondary reactions which are mainly associated with minerals during oil shale retorting have a marked influence on the product yields and compositions. To understand these phenomena more clearly, the secondary reactions of shale oil vapours from the pyrolysis (or hydropyrolysis) of Kentucky Cleveland oil shale were examined in a two-stage, fixed-bed reactor in flowing nitrogen or hydrogen at pressures of 0.1–15 MPa. The vapours from pyrolysis (first stage) were passed through a second stage containing combusted shale, upgrading catalyst or neither. Carbon conversion to volatile products in the first stage increased from 49% during thermal pyrolysis to 81% at 15 MPa H₂ partial pressure. During thermal pyrolysis, total pressure had only a slight effect on carbon removal from the raw shale and subsequent deposition on to the porous solids in the second stage. Carbon deposition on to the combusted shale in the second stage was reduced to zero at 15 MPa H₂ partial pressure. The n-alkane distributions of the oils as determined by gas chromatography clearly demonstrated that higher hydrogen pressure, contact with combusted shale, or both contributed to lower-molecular-weight products.     

Hydropyrolysis of 2,4-xylenol under pressure

The hydrocracking of 2,4-xylenol, chosen as a model compound for the hydropyrolysis of phenolic fractions was studied as a function of the following parameters: temperature, pressure, residence time and hydrogen partial pressure so as to determine the yield of light aromatic hydrocarbons (B T X) and the formation of light phenols (PC). These results, compared with those obtained by thermal pyrolysis, allow the determination of the experimental conditions under which the presence of hydrogen increases the yields of BTX, phenol and cresols instead of inducing the dehydroxylation of 2,4-xylenol. Under such conditions the amounts of solids and heavy liquid products are decreased. A postcracking temperature of 775 °C, a residence time less than 4 s, a total pressure of 1 MPa and a mole fraction of hydrogen of at least 0.25 lead to the highest yields of phenols and cresols; the yield of BTX increases with temperature, but is independent of pressure.     

Thermodynamic equilibria in the C-H system: Stability and distribution of hydrocarbon compounds

To give theoretical support to research on the hydropyrolysis of coal, thermodynamic equilibrium concentrations of hydrocarbon compounds were computed over a wide range of conditions of temperature (400–1000 °C), pressure (0.1–20 MPa), and total hydrogen to carbon atom ratio (H/C) of the system (0.5–10). Of 409 hydrocarbon compounds studied only 52 were found to be stable (i.e., to account for at least 0.01% of the total carbon in the system in some part of the range of conditions). Methane and aromatic compounds, particularly multi-ring compounds, are very stable. The distributions of stable compounds grouped into classes are presented as functions of T, P, and H/C. Optimum yields of benzene, toluene and xylene (BTX) are obtained at overall H/C in the range 2 to 3. Multi-ring aromatics can be hydrogenated at an overall H/C > 4, with T < 800 °C, and P > 5 MPa. The results give a theoretical basis for optimizing high temperature processes such as hydrogenation of coal and oil, and for maximizing BTX production by non-catalytic flash hydrogenation of coal.     

Flash pyrolysis of coals: influence of cations on the devolatilization behaviour of brown coals

The influence of cations on the pyrolysis behaviour of brown coals under flash heating conditions was investigated by means of a small fluidized-bed pyrolyser. A stream of coal particles in nitrogen was injected at rates of 1–3 g coal/h directly into a heated bed of sand fluidized by nitrogen. Yields of tar, C₁–C₃ hydrocarbons and total volatile matter from four Gelliondale brown coals and a Montana lignite were determined as a function of pyrolysis temperature. With all coals the maximum tar yield was obtained at 600 °C. Removal of cations present in the coals markedly increased the yields of tar and total volatile matter, with little effect on the yields of hydrocarbon gases. The converse was also observed in that the addition of Ca²⁺ to a cation-free coal decreased the yields of tar and total volatile matter. The extent of the reduction in tar yield at 600 °C in the presence of cations was found to be similar for all coals. After acid washing, tar yields appear to correlate with the atomic $\text{H}\text{C}$ ratios of the coals in a manner similar to that observed previously with bituminous coals.     

Optical properties of vitrinite carbonized at different pressures

The effect of pressure on the optical properties of cokes from a medium volatile bituminous coal (carbon = 87.9 wt% daf), some carbonized at atmospheric pressure and others under hydraulic pressure (21–310 MPa), over temperatures ranging from 350 to 600 °C at 50 °C intervals, has been studied. The cokes formed at atmospheric pressure developed fine grained mosaics, while medium-flow type mosaics formed in coke carbonized under hydraulic pressure. The thermal decomposition stage began at lower temperatures with increasing hydraulic pressure, resulting in a prolonged devolatilization phase for coke formed at a pressure of 21 MPa. Hence the fluidity of samples carbonized under pressure decreases with increasing hydraulic pressure. Pressure promotes the optical anisotropy apparent from the level of bireflectance. The reflectance of coke formed at atmospheric pressure is higher than that of cokes carbonized under hydraulic pressure, perhaps due to the inhibitory effect of entrapped volatile matter during carbonization under hydraulic pressure. The morphological features of vitirinite carbonized under pressure resemble those of coals naturally affected by heat.     

Coal liquefaction using iron complexes as catalysts

Hydroliquefaction of Japanese Miike and Taiheiyo coals was carried out using various iron complexes as catalysts in tetralin at 375–445 °C. Iron pentacarbonyl (Fe(CO)₅) showed the highest catalytic activity, increasing coal conversion by about 10% at 425 °C under an initial hydrogen pressure of 5 MPa. Amounts of hydrogen transferred to coal increased from 1.4–2.3 wt% of daf coal in the absence of the catalyst to 2.5–4.2 wt% of daf coal in the presence of Fe(CO)₅ at 425 °C.     

Iron sulfate/sulfur-catalyzed liquefaction of Wandoan coal using syngas–water as a hydrogen source

Water-soluble iron sulfate/sulfur-catalyzed coal liquefaction using three kinds of hydrogen sources including syngas–water has been investigated. The liquefaction of Wandoan coal, an Australian subbituminous, with iron sulfate/sulfur as a catalyst precursor using syngas–water or carbon monoxide–water afforded higher coal conversions and oil yields than those using pressurized hydrogen gas. The pretreatment at relatively low temperature (200°C) was indispensable to achieve the high coal conversion. In the two-staged liquefaction (400°C, 60 min+425°C, 60 min), the use of syngas–water as a hydrogen source afforded higher coal conversion of 90.1% together with a high oil yield of 46.2% than those using pure hydrogen, and almost comparable to those using carbon monoxide–water, indicating the presence of synergistic effects of two hydrogen sources. At the early stage of the reaction, the contribution of carbon monoxide–water was predominant, whereas hydrogen gas significantly took effect at the latter stage. The XRD and XPS study revealed the formation of pyrrhotite, a possible active species, covered with a small amount of sulfate species.