Thermal conversion and green utilisation of Chinese oil shale

Abstract

Oil shales are widely defined as fine-grained sedimentary rocks containing a considerable amount of organic matters, known as kerogen. Through retorting, oil shale could be thermally converted to shale oil, which makes utilisation a possibility. The shale oil production is largely governed by the kerogen content, as well as the interactions between kerogen and mineral matrix. To clarify these phenomena, the AGR experiments were carried out using six Chinese oil shales from different regions distributed in Northeast China, namely Fushun (FS), Xingsheng (XS), Xinghua (XH), Laoheishan (LHS), Jinzhou (JZ) and Wulin (WL) oil shale. Compared with coal or biomass, these samples are predominantly comprised of quartz and clays, which together make up to approximately 50-80 wt%. Economically, it requires more energy to process oil shale during aboveground retorting (AGR), hence less profit. From an environmental viewpoint, a sufficient amount of solid wastes containing organic residues would induce unique challenges in disposal. To maximise the use of oil shales, this project aims to (1) characterise physicochemical properties of oil shale; (2) develop advanced thermal conversion technologies; (3) explore the potential of co-gasification with coal and (4) synthesise value-added nanoparticles from oil shale wastes. Oil shale shows a poor grindability owing to the nature of oil shale being a sedimentary rock. Given its high grinding resistance, reducing the size of oil shale particles requires excessive energy. The conventional heating and microwave-enhanced pre-treatment of oil shale was conducted to enhance oil shale grindability. The samples were milled and sieved into a uniform size fraction of 1-1.18 mm, whilst a separate set of samples were cut into cube-shaped specimens. The prepared samples were pre-treated and crushed to study the failure mechanisms, as well as changes in uniaxial peak strength and specific breakage rate in comparison to that of untreated samples. Results show that the strength of FS oil shale was significantly reduced by 62.5% whilst the specific breakage rate increased by 42% after a short exposure to 1 kW microwave irradiation. The improvement was approximately 50% higher than that observed after conventional heating. Hence, results suggest that microwave processing could lead to a more significant and rapid improvement in milling compared to conventional pre-heating, and without adversely affecting fuel quality. Subsequently, the thermal behaviour and kinetic triplets of oil shales (and their respective blends) were studied using a thermogravimetric analyser via a model-free method, whilst the product distribution and oil composition were analysed by Fischer assay test and gas chromatography-mass spectrometry, respectively. The results have shown that the activation energy generally exhibited a decreasing trend with increasing WL fraction due to the catalytic effects of minerals and positive, non-catalytic effects of hydrogen-rich organic constituents on cracking and ring-open reactions. The coupling effect could also increase the gas yield by restricting the coke formation. Moreover, shale oil was redirected towards lighter oil with limited oxygenated compounds. However, extensive dehydrogenation and aromatisation reactions would inversely enhance the coke formation, in particular at low H/C ratio. Hence, upon optimum blending ratio, co-pyrolysis of different oil shales has the potential to lower energy barrier and improve oil quality without the expense of catalysts. Additionally, oil shale, which has high SiO2 and Al2O3 content, is a potential refractory agent due to its high melting point. Herein, the ash fusion characteristics of different oil shales together with the commonly used Qinghai bituminous coal (QH), and their respective blends were investigated under a CO/CO2 atmosphere. The automated image-based ash fusion test was used to monitor and quantify the complete melting behaviour of ash samples from room temperature to 1520 oC. The transformation of mineral matters was characterized experimentally via X-ray diffraction and verified via thermodynamic modelling in FactSage software. Results showed that the excessive melting point (EMP) of QH coal ash is the lowest but would increase with oil shale addition. This is due to the formation of anorthite which restricts the formation of gehlenite and subsequently low-melting hercynite. However, the sintering point of QH coal ash decreased with the addition of oil shale due to the formation of low-melting aluminosilicate. Hence, blending coal with oil shale can modify the melting behaviour by complementing ash chemistry to meet the requirements of a gasifier, but without adversely affecting thermal performance. The amorphous SiO2 in oil shale ash can be effectively extracted as an inorganic silica precursor. Herein, mesostructured siliceous form materials (SFM) were synthesised from oil shale ash via a green and time-saving microemulsion templating method and modified with polyethyleneimine (PEI) for carbon capture. The effects of preparation conditions on sample morphology and pore structure were studied to understand the characteristics of inorganic silica precursors. Results show that the SFMs possess 3-dimensional porous framework with a large surface area of 901.3 m3/g and a pore volume of 2.0 cm3/g. The CO2 adsorption performance of PEI-impregnated SFMs was investigated in terms of CO2 adsorption capacity and cycling stability under a simulated flue gas condition. The results show that pore-expanded SFMs with 50 wt% PEI loading exhibited higher CO2 uptake (109 mg/g-adsorbent) compared to the baseline materials (50 mg/g-adsorbent) and no appreciable reduction in CO2 uptake was observed in the first 12 cycles. The DRIFTS spectra of CO2-evacuated adsorbents illustrated the presence of alkylammonium carbamates residuals and newly formed urea species, illustrating degradation mechanisms. To summarise, this project has demonstrated that (1) microwave-enhanced pre-treatment could reduce energy consumption of comminution due to a significant improvement on oil shale grindability; (2) co-pyrolysis of oil shales from different regions could promote the conversion owing to the positive synergistic interactions; (3) the high-melting oil shale could act as refractory agent to alleviate ash-related problems during co-gasification; (4) with a simple and green synthesis route, the “dirty” oil shale waste could be converted to value-added material, which is a promising candidate for carbon capture. The findings of this work could provide useful information for the development of AGR technologies, especially whilst using low-grade oil shale. The proposed synthesis method is a potential strategy in managing solid waste and reducing the carbon footprint of oil shale industries.

Date of Award	14 Nov 2020
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Cheng Heng Pang (Supervisor), Tao Wu (Supervisor), Cheng gong Sun (Supervisor) & Collin E. Snape (Supervisor)