Co-current combustion of oil shale - Part 1: Characterization of the solid and gaseous products

Co-current combustion front propagation in a bed of crushed oil shale (OS) leads to the production of liquid oil, of a flue gas and of a solid residue. The objective of this paper was to provide a detailed chemical characterization of Timahdit oil shale and of its smoldering combustion products. The amount of fixed carbon (FC) formed during devolatilization is measured at 4.7% of the initial mass of oil shale whatever the heating rate in the range 50-900 K min⁻¹. The combustion of oil shale was operated using a mix of 75/25 wt. of OS/sand with an air supply of 1460 l min⁻¹ m⁻². In these conditions, not all the FC is oxidized at the passage of the front, but 88% only, with a partitioning of 56.5% into CO and the rest into CO₂. A calorific gas with a lower calorific value of 54 kJ mol⁻¹ is produced. Approximately 52% of the organic matter from OS is recovered as liquid oil. The front decarbonates 83% of carbonates.     

Co-current combustion of oil shale - Part 2: Structure of the combustion front

Timahdit oil shale was used as a porous medium to characterize the structure of a combustion front propagating with co-current downward air supply. A new 1D experimental device was first calibrated using a model porous medium. With the model porous medium, the front propagates as a plane and horizontal surface while using oil shale the front propagates as an inclined curved surface. The peak temperature was 1100 °C; despite the relatively large diameter of the cell (91 mm) and the good thermal insulation, the heat losses were estimated at 42% of the heat released by the combustion. The thickness of the front was characterized using a new gas micro-sampling system: the char oxidation and the carbonate decarbonation zones are approximately 10 and 15 mm thick, respectively. The oil formed during the pyrolysis is adsorbed in the porous medium in the course of the experiment, and expulsed from the cell by the end.     

Utilization of Estonian oil shale semicoke

In thermal processing of oil shale in vertical retorts huge quantities of a solid waste — semicoke are formed. It has been shown that circulating fluidized bed combustion of semicoke could be a promising technology allowing utilization of its high residual energy potential. The main parameters of combustion process and the additional heat produced were calculated and verified by combustion tests in a fluidized bed device with a thermal capacity of 50 kW_th. The experiments indicated that semicoke with low moisture content can be burnt directly in fluidized bed. For the combustion of semicoke with higher moisture content (over 10%) about 10% of oil shale must be added. In addition, possibilities for utilizing residual carbon present in semicoke by obtaining carbon-rich materials with further production, for example, activated carbon were discussed. A series of experiments accompanied by SEM and EDAX analysis was carried out in order to elucidate the distribution of carbon and mineral part in semicoke and to find possibilities for their separation and subsequent enrichment. Different separation methods — selective grinding and subsequent screening, pneumatic separation and triboelectroseparation method were analyzed. It was shown that due to close integration of mineral and organic part in semicoke, the separation of carbon-rich ingredients by these methods was not enough effective to obtain enriched products suitable for the production of activated carbon.     

大庆油页岩及干馏产物的利用途径分析

大庆油页岩的舍油率大部分都在10%以上,具有很好的经济开发价值.对大庆油页岩及其干馏产物性质的实验研究表明,油页岩的机械强度较低,应选择粉末、颗粒干馏炉进行加工处理;页岩油主要由柴油馏分和重油馏分组成,分别可加工成成品油和直接用作燃料油;热解干馏气热值约为17MJ/m3,可以在除作自身干馏所需的热量燃料外,用作城市煤气或工业锅炉的燃料;半焦着火点低,热值约为23 MJ/kg,可作为清洁燃料用于发电或民用;页岩灰的主要组分是氧化钙争氧化硅,可用于生产建筑材料.     

Characterization of Chinese, American and Estonian oil shale semicokes and their sorptive potential

The primary byproduct of current oil shale oil extraction processes is semicoke. Its landfill deposition presents a potential threat to the environment and represents a waste of a potentially useable byproduct. Here we examine the sorptive characteristics of oil shale semicoke. Oil shale samples from Estonia, China and the United States were pyrolyzed at 500 and 1000 °C and their products analyzed for organic char content, surface area and porosity. Pyrolysis of the oil shales at temperatures of 500-1000 °C yields semicokes with organic char contents from 1.7% to 17.5% and BET surface areas of 4.4-57 m² g⁻¹, corresponding to 100-550 m² g⁻¹ of organic char. For comparison, the BET surface areas of class F coal fly ashes (combustion byproducts of bituminous coals) typically range from 2 to 5 m² g⁻¹, corresponding to 30-60 m² g⁻¹ of carbon while class C fly ash (from low rank coals) have carbon BET surface areas comparable to oil shale semicoke organic char surface areas.     

Effect of demineralization of El-lajjun Jordanian oil shale on oil yield

The effect of demineralization on oil yield and mineral composition of Jordanian oil shale was investigated. A standard digestion procedure using a range of inorganic and organic acids including HCl, HNO₃, HF, and CH₃COOH was used to enhance the oil recovery of oil shale samples collected from the El-lajjun area. The total yield of the digested samples, as determined by Fischer Assay, has shown a maximum value (two folds the untreated sample) obtained when using CH₃COOH. The kaolin in the treated oil shale with a high concentration of CH₃COOH is believed to have transformed to illite as found in the XRD analysis. The treatment of oil shale using HCl has shown an increased ratio of oil to gas as a result of the digestion of calcite in the oil shale. At higher concentrations of HNO₃, the acid is believed to react with the kerogen in the oil shale resulting in high levels of low molecular weight compounds. Therefore, the amount of non-condensable gases produced by Fischer assay after treatment with a high concentration of HNO₃ is relatively high. HF is believed to drive off water from the oil shale by dissolving the clay minerals leading to increased oil to gas ratio.     

Kinetics of pyrolysis and combustion of oil shale sample from thermogravimetric data

Thermogravimetric (TG) data of oil shale obtained at MI (Waste to Energy laboratory) were studied to evaluate the kinetic parameters for El-Lujjun oil shale samples. Different heating rates were employed simulating pyrolysis reaction using Nitrogen as purging gas up to ∼800 °C. The extent of char combustion was found out by relating TG data for pyrolysis and combustion with the ultimate analysis. Due to distinct behavior of oil shale during pyrolysis, TG curves were divided into three separate events: moisture release; devolatization; and evolution of fixed carbon/char, where for each event, kinetic parameters, based on Arrhenius theory, were calculated. Three methods were used and compared: integral method; direct Arrhenius plot method; and temperature integral approximation method. Results showed that integral method is closer to the experiment, while no relationship was observed between activation energy and the heating rate.     

Effect of toluene proportion on the yield and composition of oils obtained by supercritical extraction of Moroccan oil shale

Supercritical extraction of Tarfaya's oil shale by toluene revealed that the solvent proportion has a significant effect on the yield and the composition of the obtained oils. The analyses carried out on the recovered oils allowed to establish the optimal operating conditions giving the highest oil yields. In addition, it was observed that these oils contain a large proportion of aromatics compounds.     

Interactions between oil shale and its semi-coke during co-combustion

In the present work, thermogravimetric analysis was employed to investigate the interactions between oil shale and its semi-coke during co-combustion process. During the test, the blends of semi-coke and oil shale were prepared at different blending ratios of 1:0, 4:1, 3:1, 2:1, 1:1, and 0:1. The results indicated some interactions were detected between oil shale and semi-coke during the tests. The rapid combustion of organic matter in oil shale, which led to the fuel particle temperature’s rapid increase and made semi-coke ignite in advance, improved the co-combustion characteristics in terms of ignition temperature (T_i), the temperature reaching the maximum mass loss rate (T_max), the maximum mass loss rate (R_max), the combustibility index (C) and the specific reactivity of the co-combustion. With the increase in semi-coke mass fraction in the blends, the specific reactivity of the samples was found to decrease gradually due to lower volatile content and high carbon condensation structure in semi-coke. As the temperature increased, the specific reactivity of the samples first gradually increased, then decreased attributed to the oxidation of selectivity. The poor diffusion caused mainly by the ash shells made the decomposition of minerals more complicated. It was also analyzed that the ‘particle groups’ phenomenon that the dispersed particles are agglomerated by some forces in semi-coke ash went against the decomposition of minerals.Activation energies E from distributed activation energy (DAEM) slowly decreased at the initial stage, then increased sharply over a narrow conversion region, which indicted a difficult burnout stage. Meantime, a power law model was employed to investigate quantitatively the interactions. The experimental reaction orders could be predicted accurately from the calculated values. At the third stage, the kinetic parameters E₃ and A₃ were almost as much as the calculated values. At the low temperature region (470-540 °C), there was little difference between kinetic parameters E and A from the experiment data and the calculated data.     

小颗粒油页岩及半焦CFB锅炉燃烧适应性研究

油页岩经低温干馏可以得到页岩油,因生产工艺限制,干馏炉无法使用粒径12 mm以下的油页岩,同时会产生大量副产品(页岩半焦)。为提高副产品的利用能力,实现资源利用最大化。通过在1 MW_(th)CFB燃烧试验台对小颗粒页岩及页岩半焦进行试烧试验,研究小颗粒页岩及页岩半焦的理化特性、着火特性、燃尽特性、结焦特性。试验结果表明,控制床温在720~850℃内,由油页岩小颗粒和半焦掺混而成的设计燃料在试验台采用CFB方式能够稳定燃烧,试验各工况灰渣含碳量均低于1.81%,试验燃料较易燃尽。CFB锅炉适合油页岩小颗粒与半焦掺烧利用,且燃烧效率高,燃烧稳定性较好。     

Synthesis of Na-A and -X zeolites from oil shale ash

The solid by-product of oil shale processing (PETROBRAS-Brazil) was used as a raw material to synthesize Na-A and -X zeolites. Two preparation methods using the same starting material composition were carried out. In Method (1), alkaline fusion was used to prepare a glass, which was then hydrated by refluxing. The largest amount of crystallinity was reached with 2 h 30 min of refluxing. In Method (2), alkaline fusion was followed by hydrothermal treatment. The most crystalline sample was obtained after 12 h of heat treatment, and after 96 h hydroxysodalite zeolite was formed. In both procedures, the synthesis products were mainly composed of Na-X zeolite, whose content was influenced by the crystallization time, and of Na-A zeolite, with a practically constant content.     

Modeling calcium dissolution from oil shale ash: Part 1. Ca dissolution during ash washing in a batch reactor

Batch dissolution experiments were carried out to investigate Ca leachability from oil shale ashes formed in boilers operating with different combustion technologies. The main characteristics of Ca dissolution equilibrium and dynamics, including Ca internal mass transfer through effective diffusion coefficients inside the ash particle were evaluated. Based on the collected data, models allowing simulation of the Ca dissolution process from oil shale ashes during ash washing in a batch reactor were developed. The models are a set of differential equations that describe the changes in Ca content in the solid and liquid phase of the ash-water suspension.     

Isothermal oil shale pyrolysis. 1. Oil generation and composition at various pressures

A rich (250 I (59 gal) per ton) Green River oil shale was retorted in a helium atmosphere. Isothermal retort experiments from 375 to 500 °C were carried out at 78 kPa and 765 kPa. Oil was collected as a function of time and a comprehensive analytical procedure was developed and utilized to determine seven oil fractions: straight-chain pentane de-asphaltened dry whole oil (DDWO); solids; saturates (S); aromatics (A); olefins (0); ‘weak’ polars (WP); and polars (P). The objectives of this work were: (a) to develop data to show oil generation and composition at different temperatures and pressures under isothermal, isobaric conditions; (b) to determine the effect of pressure on total oil yield and product-oil composition. Total oil yield was reduced as the pressure was increased. Much of this reduction can be accounted for in the reduced amounts of polar compounds formed at higher pressure because the polar fraction comprises approximately 35–45 wt% of the DDWO. In general, the amounts of aromatics present increase, the amounts of olefins decrease, and the amounts of saturates and ‘weak’ polar compounds remain relatively constant with increased system pressure.     

Investigation on pyrolysis of Moroccan oil shale/plastic mixtures by thermogravimetric analysis

Thermal degradation processes for a series of mixtures of oil shale/plastic were investigated using thermogravimetric analysis (TGA) at four heating rates of 2, 10, 20 and 50 K min^− 1 from ambient temperature to 1273 K. High density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) were selected as plastic samples. Based on the results obtained, three thermal stages were identified during the thermal degradation. The first is attributed to the drying of absorbed water; the second was dominated by the overlapping of organic matter and plastic pyrolysis, while the third was linked to the mineral matter pyrolysis, which occurred at much higher temperatures. Discrepancies between the experimental and calculated TG/DTG profiles were considered as a measurement of the extent of interactions occurring on co-pyrolysis. The maximum degradation temperatures of each component in the mixture were higher than those of the individual components; thus an increase in thermal stability was expected. In addition, a kinetic analysis was performed to fit thermogravimetric data. A reasonable fit to the experimental data was obtained for all materials and their mixtures.     

The chemistry and mineralogy of the Negev oil shale ashes

The characteristic feature of the oil shale ashes produced by fluidized bed combustion (700-800 °C) in PAMA's demonstration power plant is the large amount of amorphous phases, Ca-Al-silicates and Al-silicates, together with anhydrite and lime. Practically all the S and heavy metals in the oil shales are retained in the ash, which, from an ecological point of view, is important. Two kinds of ashes were examined: industrial ashes produced at PAMA's demonstration plant and ashes produced in laboratory experiments. Three different types of ash are produced at the demonstration plant. Ash Cooler (AC), which is comparable to bottom ash in coal power plants. This ash is produced from oil shale subjected to the lowest temperatures and is the most coarse-grained. It contains relatively larger quantities of unaltered minerals (calcite, clays, apatite, etc.) than the other two. The two other ashes Boiler Bank (BB) and Fly Ash (FAS) are not much different from each other and both may be compared to fly ash in coal power plants. Both BB and FAS ashes contain more authigenic (formed in the boiler) phases than AC. The results of the laboratory experiments show that the main factor in the raw material controlling the mineralogy and chemistry of the oil shale ashes is the Al₂O₃ concentration (clay content), and not the organic matter concentration.     

Characterisation of some Australian oil shale using thermal, X-ray and IR techniques

Thermogravimetric analysis (TGA), Diffuse Reflectance Infrared Fourier Transforms Spectroscopy (DRIFTS) and X-ray diffraction (XRD) were used in conjunction to characterise oil shale samples from an Australian Tertiary oil shale deposit. Results from these techniques were compared with conventional Modified Fisher Assay (MFA) data. DRIFTS and TGA results showed clear correlations with each other as well as with the MFA values. DRIFTS results indicated that most of the kerogen is in aliphatic hydrocarbon form. It was evident from TGA analysis that the weight loss in the 450-550 °C temperature region has a strong and direct correlation with the amount of oil in the samples, as determined by the MFA method. Calibration curves were generated in which oil content can be predicted from TGA and DRIFTS data. The combination of TGA and DRIFTS is mostly useful in examining organic matter in oil shale while DRIFTS and XRD combination is useful in examining the minerals phases. XRD and DRIFTS showed good agreement in identifying the presence of minerals such as quartz, clay and carbonates. Combination of these three techniques can provide an alternative and inexpensive method to the MFA analysis in determining the kerogen content, while overcoming the limitations of each other.     

Aromatic and hetero-aromatic compositional changes during catalytic hydrotreatment of shale oil

Oil shale from the Kimmeridge Clay, of Jurassic age from the UK was pyrolysed in a 5 kg fixed bed reactor at 525°C in a nitrogen atmosphere. The derived shale oil was then hydrotreated at 15.0 Mpa pressure and 400°C in a stirred reactor with a nickel–molybdenum (Ni–Mo) catalyst and residence times from 8 to 56 h. The shale oils were analysed for polycyclic aromatic hydrocarbons (PAH) and for nitrogen-PAH (PANH) and sulphur-PAH (PASH), before and after hydrotreatment. The results showed that generally the higher molecular weight three and four ring PAH decreased with increasing hydrotreatment time, however, single ring aromatic compounds and two ring PAH were increased. Nitrogen and sulphur containing PAH were significantly reduced in concentration in the oils with increasing hydrotreatment time to reach negligible concentrations after 56 h. The reduction in PANH and PASH coincided with a reduction in the overall nitrogen and sulphur contents of the oils.     

Effect of demineralization and heating rate on the pyrolysis kinetics of Jordanian oil shales

The effect of mineral matter content on the activation energy of oil shale pyrolysis has been studied. Kerogen was isolated from raw oil shale by sequential HCl and HCl/HF digestion. Oil shale and kerogen samples were pyrolyzed in a Thermogravimetric Analyzer at different heating rates (1, 3, 5, 10, 30, and 50 °C/min) up to a temperature of 1000 °C. Total mass loss of all oil shale samples remained almost constant irrespective of the heating rate employed, whereas it decreased with the increase of heating rate for kerogen (74.5 to 71.4%). From the pyrolysis profile activation energy (Ea) was found to vary between 70 and 83 kJ/mol for oil shale, while 82-112 kJ/mol has been determined for isolated kerogen. An increase of both Ea and pre-exponential factor was observed with an increasing heating rate. It is concluded that the mineral matter in oil shale enhances catalytic cracking as is evident from the reduced Ea values of oil shale compared with those for kerogen.     

Variation of kerogen content and mineralogy in some Australian tertiary oil shales

Oil shale is a potential alternative source of petroleum products. The processes of retorting oil shale and refining shale oil are both affected by the composition of the parent rock. Mineralogical and geochemical data obtained from two bore holes in Queensland oil shale deposits are presented and discussed here. The data includes the variation with depth of mineralogy, ash content, moisture content and kerogen content. A strong correlation of hydrogen and organic carbon, and higher H/C ratios suggest that organic matter is present mainly as aliphatic compounds. Pyrite was identified as the major source of sulphur. This may provide some possibilities for easier removal of sulphur before the oil shale is processed.