This paper proposes a novel energy-efficient oil shale pyrolysis process triggered by a topochemical reaction that can be applied in horizontal oil shale formations. The process starts by feeding preheated air to oil shale to initiate a topochemical reaction and the onset of self-pyrolysis. As the temperature in the virgin oil shale increases (to 250–300°C), the hot air can be replaced by ambient-temperature air, allowing heat to be released by internal topochemical reactions to complete the pyrolysis. The propagation of fronts formed in this process, the temperature evolution, and the reaction mechanism of oil shale pyrolysis in porous media are discussed and compared with those in a traditional oxygen-free process.
You-Hong Sun1, Feng-Tian Bai1, Xiao-Shu Lu1,2, Qiang Li1, Yu-Min Liu1, Ming-Yi Guo1, Wei Guo1 & Bao-Chang Liu1
1College of Construction Engineering, Jilin University, Changchun 130021, PR China, 2Department of Civil and Structural.
Engineering, School of Engineering, Aalto University, PO Box 12100, FIN-02015 Espoo, Finland.
Received 18 July 2014, Accepted 14 January 2015, Published 6 February 2015
The results show that the self-pyrolysis of oil shale can be achieved with the proposed method without any need for external heat. The results also verify that fractured oil shale may be more suitable for underground retorting. Moreover, the gas and liquid products from this method were characterised, and a highly instrumented experimental device designed specifically for this process is described. This study can serve as a reference for new ideas on oil shale in situ pyrolysis processes.
The tremendous growth in global energy demand and the rapid depletion of conventional oil resources have created a demand for alternative energy sources. Oil shale1,2,3,4, an organic-rich petroleum source rock, has a high content of solid insoluble kerogen that can release a petroleum-like liquid, that is, shale oil, and provide secure access to transportation fuels5,6. The huge reserves of oil shale, approximately 35% of the world’s total energy reserves7, make it commercially viable. Accordingly, scientists have been attempting for decades to develop environmentally responsible methods of exploiting oil shale8,9,10,11,12,13,14.
Oil shale can be used in various ways15,16,17,18,19,20, from electrical energy via direct combustion to a wide range of petrochemical products (including shale oil and other liquid fuels) via the pyrolysis of kerogen. Two primary types of processes for oil shale retorting have been developed: ex situ and in situ processes. Ex situ (aboveground) processes16,20, such as retorts of the Kiviter, Petrosix, ATP, Tosco-II and Fushun-type, are the primary extraction methods. However, such methods suffer critical problems8,16,17,21,22, including potentially harmful semicoke waste, large land use, and gas pollution. Additionally, ex situ processes are unsuitable for low-grade oil shale buried in deep formations because of mining difficulties and negative economic impacts.
In situ processes10,11, such as ICP, Electrofrac™, Chevron’s technology, and the Occidental Modified in-situ (MIS) process, have recently attracted great attention, without regard to the heating modes and environmental issues related to these methods. The ICP and Electrofrac™ processes use electricity to heat the surrounding shale deposit as a physical heat treatment, consuming significant energy10. Chevron’s technology uses heated and pressurised carbon dioxide to heat the oil shale, which also requires large quantities of water and damages the environment. The MIS method induces combustion air to permeate and burn oil shale underground after mining 20% of the oil shale and fracturing the rest to create a void space of 20–25%; this method was conducted in situ in 1972 at Logan Wash, Colorado10.
We recently introduced the oil shale topochemical reaction23 as a low-energy shale oil recovery method that is triggered by the topochemical reaction between oil shale and a limited amount of air to drive the spontaneous pyrolysis of oil shale in the absence of an external heat supply (denoted as a self-pyrolysis process). In this method, the pyrolysis represents a chemical-enhanced heating process rather than a physical heating process, like ICP and Electrofrac™, or a complete combustion process. Moreover, the shale oil product still consists primarily of hydrocarbons, like the oils obtained from traditional processes. The proposed technique23 has the potential to consume much less energy, making it more suitable for underground retorting.
However, neither the heat transfer mechanisms of the proposed method nor the importance of process design in realising energy savings has been elucidated. Furthermore, previous experiments have been limited to data obtained from a tube furnace for the analysis of products, with little data on heat transport properties. Results cannot therefore be directly applied to energy-efficient pyrolysis solutions. The purpose of this paper is to address these gaps. We aim to provide new insights into the mechanism of the topochemical-induced thermal process of oil shale conversion to gain a novel understanding of the self-pyrolysis process of oil shale and demonstrate a more feasible energy-efficient approach through a laboratory-scale process.
Oil shale generally exhibits poor thermal conductivity and permeability and is therefore very inefficient at transmitting heat and gas. For ex situ retorting, shale is always crushed into smaller particles (commonly < 100 mm) to increase its specific surface and contact areas. For in situ methods, porosity and permeability are two important factors that enable the transfer of heat to the shale within the geologic formation14. Many methods14,24,25, including the use of explosives, hydraulic fracturing, and horizontal drilling, can be used to rubblize the formation to improve the stratum permeability and increase the reaction area. In these experiments, various sizes (2–100 mm) of oil shale were introduced into a fixed bed to form a heterogeneous system to mimic different permeabilities and porosities of shale in the field.
Figure 1: Schematic of the experimental device. (a), The overall layout of the device. (b), The fixed bed with short gas pipeline. (c), The fixed bed with long gas pipeline. (d), The temperature sensor.
In the present study, a self-pyrolysis retorting process of oil shale with a low energy input using high- and normal-temperature air in sequence as carrier gases is described. A highly instrumented fixed bed (Figure 1a) simulating the horizontal stratum was developed to investigate this novel pyrolysis in a porous medium under controlled conditions. The propagation of fronts formed in this process, the temperature evolution, and the reaction mechanism of oil shale pyrolysis in porous media are discussed and compared with those in a traditional oxygen-free process. Two different circular gas pipelines (Figures 1b, c) were specifically designed to simulate the in situ hydraulic fracturing and horizontal drilling modes. The products from this method were also characterised. We anticipate that this study can serve as a starting point to initiate new approaches to oil shale in situ pyrolysis processes.
Thermogravimetric (TG) analysis, differential thermogravimetric (DTG) analysis and differential scanning calorimetry (DSC).
The results of the TG, DTG, and DSC experiments on Huadian (HD) oil shale under both air and inert (N2) atmospheres are shown in Figure 2. The experiments were repeated several times and exhibited good reproducibility. The stages are as follows.
Figure 2: TG/DTG/DSC results for virgin HD oil shale under air and under nitrogen.
- At temperatures less than 200°C, a small mass loss is observed, due primarily to the evaporation of water, including adsorbed and interlayer water, from clay minerals.
- Under an air atmosphere, a slight rebound is observed at approximately 300°C because of the absorption of ambient gas17; this result differs from the results obtained under an N2 atmosphere.
- In the temperature range from 300°C to 550°C, a major mass loss is observed. This stage is attributed primarily to the decomposition of kerogen into volatiles. Under an air atmosphere, the reaction is more rapid, and two peaks are observed in both the DTG and DSC curves. The first peak, referred to as the low-temperature oxidation peak, is observed at 336°C and is due to the oxidation reaction of light hydrocarbons formed by cracking. The second peak, referred to as the high-temperature oxidation peak, appears at 405°C and originates from the oxidation of heavy hydrocarbons, fixed carbon and possibly other components. The two processes together gave off 10.693 MJ kg−1 of heat, which is consistent with the proximate analysis results. However, these two exothermic peaks did not appear when the experiment was performed under N2, and the total endothermic heat was 1.191 MJ kg−1 under an N2 atmosphere.
These results reveal that the exothermic reaction heat from oil shale oxidation is approximately nine times the heat required for oil shale pyrolysis. Therefore, the introduction of a reasonable amount of oxygen to partially oxidise the oil shale and release heat for the surrounding oil shale pyrolysis would facilitate the production of petrochemical goods from oil shale.
- At temperatures greater than 600°C, a final mass loss is observed because of the thermal decomposition of carbonates and clay minerals; the endothermic peak appears in two experiments. The endothermic quantity under air was 49.4% of that under N2.
Evolution of temperature in oil shale self-pyrolysis
The gas pipeline shown in Figure 1b was used for the oil shale self-pyrolysis experiments, and the rubblized oil shale was introduced into the bed to study its heat transfer process. An air compressor was used in these experiments to provide air. Air was preheated and maintained at 400–500°C with a flow rate of 16 m3 h−1 before being fed into the fixed bed. The temperatures at different locations inside the bed are reported in Figure 3a. For sensor T1, which was positioned close to the gas entrance, the temperature of T1-#1 (sensor location 1; sensor number 1) slowly increased to 220°C in the first 27 min before rapidly increasing to 600°C within 3 min, triggering a topochemical reaction in the oil shale.
Figure 3: The temperature evolution curves and experimental timeline. (a), Temperature evolution of the thermocouples in the self-pyrolysis experiment between 18 and 33 min. (b), Interaction between temperature evolution and gas flow. (c), Timeline of time-interval trials. (d), Temperature evolution of the #1 thermocouples during the time-interval of the self-pyrolysis trials. (e), Temperature evolution of the thermocouples in the pyrolysis process (25 m3 h−1). (f), Temperature evolution of thermocouple T1-#1 in different experimental processes.
The temperature successively increased along the vertical direction of the gas pipeline, indicating that the front propagated as a vertical surface. The temperature propagation was consistent with the gas flow direction, i.e., co-current transmit (the reaction front and feeding gas take place on the same side of the bed), spreading from left to right along the horizontal direction of the gas pipeline.
Figure 3b clearly shows that the temperature is affected by the fluctuation in the gas flow and that it is characterised by a positive relationship. Temperatures increased as the gas flow increased and rapidly decreased as the flow decreased in all stages of the reaction. After the topochemical reaction is triggered, the temperature can reach 800°C or higher if the flow is excessive. As shown in Figure 3b, the temperature of T1-#1 quickly increased to 800°C in 33 min.
This continuous high-temperature condition can consume a large amount of organic matter and is not beneficial to the extraction of shale oil. Conversely, the reactions will stop and the oil shale will cool if the gas source is cut off. A reasonable flow can maintain a steadily propagating front with a slowly increasing temperature, and the oil shale can be pyrolysed smoothly without external heat provision. This lack of need of external heat is due to the topochemical reaction between the oil shale and the oxygen from the air, which spontaneously increases the temperature of the oil shale in the beginning before driving the continuous pyrolysis of kerogen and accomplishing the retorting of the oil shale.
Time-interval trials of oil shale self-pyrolysis
As shown in Figure 3c, three time-interval trials of self-pyrolysis were run to verify the feasibility and explore the mechanism of the proposed method. Hot air was firstly fed into the fixed bed. When the temperature of T1 increased to 250–300°C, the gas preheating was stopped and the hot gas was replaced by ambient-temperature air. The gas flow was adjusted simultaneously. After a given period of time, the gas source was shut off and the fixed bed was allowed to cool naturally. This trial process was repeated twice. The interval of the first discontinuation was 630 min, and the second (2520 min) was longer than the first to allow the fixed bed to completely cool.
As shown in Figure 3c, the pyrolysis was activated by hot air sweeping for 30 min in the first trials, whereas, in the second and third trials, much longer times were required to induce the reaction. The reason for the longer times was that, during the first trial, the oil shale near sensor T1 was pyrolysed and the front propagated and reached T2 (see Figure 3d), which led to the reduction of the thermal parameters of the residues (Table 1); this reduction resulted in more exchange heat being consumed by the residues during the initial stage of the second and third trials. However, the self-pyrolysis reaction of oil shale is easily triggered, and once started, it can spontaneously and smoothly propagate in the absence of any external heat supply.
Table 1: Physical properties of the HD oil shale.
The peak temperature of the #1 thermocouples reached in sequence, as shown in Figure 3d, again reveals that, for the bed equipped with a short pipeline, the pyrolysis fronts caused by the topochemical reaction were co-current with the feeding of air. The temperature fluctuation of the sensors shown in Figure 3d might be explained by the complex flow developed in the bed, which was caused by the conversion of solid kerogen into liquid oil and then into gas26,27, generating overpressure in the bed.
The four thermocouples of all sensors reached their peak temperatures at approximately the same time, indicating that sharp fronts were generated during propagation. For all sensors except T1, the #3 thermocouple was always the first one to register a temperature increase and the #1 thermocouple always registered the highest temperature. Such a phenomenon could be caused by the flow’s upward mobility and the significant heat loss at the wall of the bed.
Evolution of temperature in an oxygen-free pyrolysis process
In a traditional oxygen-free oil shale pyrolysis experiment, the gas was replaced by hot nitrogen at two different speeds of 16 and 25 m3 h−1. Figure 3e shows the results for a gas flow of 25 m3 h−1. During this experiment, ten cylinders (4 m3 cylinder−1) of nitrogen were used; the small zigzags in the curves are associated with the replacement of the nitrogen cylinders. The nitrogen gas had been preheated before being fed into the fixed bed within 105 min; however, the highest temperature in this bed was only 325°C. When the nitrogen gas was no longer preheated, the fixed-bed temperature began to plunge, indicating that nitrogen was just a heat carrier. Moreover, the fixed bed could only reach a temperature of 76°C in 30 min when the gas flow rate was adjusted to 16 m3 h−1, as shown in Figure 3f.
The structure of the reaction fronts for different gas pipelines
Horizontal drilling connecting two boreholes is regarded as a suitable method for oil shale in situ retorting, especially for a thinner shale layer, because it can increase the interaction between the gas and the oil shale layer, which is also beneficial to the transmission of the shale oil. In the present study, the gas channel shown in Figure 1c was designed to mimic the in situ horizontal drilling mode. With a long gas pipeline, the first sensor to register an increase was T7-#1 (located at the outlet of the bed), followed successively by T6-#1, T7-#2, T6-#2, T7-#3, and T6-#3. The highest entrance temperature near T1 was less than 150°C. The front propagation in the long pipeline, from right to left, was completely opposite to that of the short pipeline because it followed a counter-current mode (with the reaction front and feed of gas occurring on opposite sides of the bed). Moreover, the temperature of T4 increased slowly, and that of T3 required a longer time to increase.
Analysis of the produced gas and liquid oil and the solid residue
In this part, the products of the oil shale self-pyrolysis were analysed. During each experiment, a total of 110–130 kg of oil shale was fed into the bed. The shale oil, water and residue collected from three separate and repeated experiments are reported in Figure 4a. Compared with the Fischer analysis of both virgin and spent oil shale results in Table 1, only a small amount of shale oil (an average of 1.9%) was consumed in the oil shale self-pyrolysis process with air. The amounts of water and gas increased and the amount of residue decreased because of the oxidation of fixed carbon and volatile substances. The main components of the products were analysed as follows.
Figure 4: The product analysis. (a), The yield of products from the self-pyrolysis experiments. (b), Shale gas ignition tests. (c), Shale oil collected during the experiments. (d), Chromatogram of gas collected from an experiment. (e), TG/DTG analysis of the shale oil. (f), GC-MS spectrum of shale oil collected from the experiment.
A large quantity of pyrolysed gas was generated with the increase in temperature. The colour of the vapour changed to dense-white as the temperature increased, and a large amount of shale gas discharged even when the air was no longer supplied to the fixed bed. The gas exhibited a pungent odour and was flammable (Figure 4b). The shale gases were determined by gas chromatography (GC) (Figure 4d). The chromatograms revealed that the major hydrocarbon components were methane, ethylene, ethane, propylene, propane, butene, and butane.
Oil was gradually expelled from the bed into a jerrican during the experiment. When the experiment was stopped midway through, no liquid oil was observed in the porous medium. Like petroleum, shale oil (Figure 4c) is a black-brown material with an irritating odour. In addition to its use as a transport fuel5,6, shale oil can also serve as a virgin material for the production of numerous different value-added products28,29, such as rubber softeners, chemical intermediates, and asphalt additives.
TG analysis under N2 (Figure 4e) was carried out primarily to study the characteristics of the boiling point/range of the oil samples. The mass loss between the ambient temperature and 400°C was primarily due to the distillation of the volatile hydrocarbons at 20–280°C and of the low-molecular-weight hydrocarbons at 280–400°C; the mass loss between 400 and 500°C was due to a combination of distillation and thermal cracking of medium-molecular-weight hydrocarbons30,31,32. In this study, the mass loss at 20–280°C was 85%, indicating that the shale oil obtained from our experiments was primarily volatile hydrocarbons.
A small amount of residue, approximately 7%, remained in the pan even after the sample was to above 600°C, similar to the residue observed for crude oil30. The GC results for the liquids (Figure 4f) show that numerous hydrocarbons and derivatives were present in the collected oils, mainly as aliphatic (63.66%), aromatic (22.80%), and heteroatom (13.54%) compounds. Among these hydrocarbons, aromatic hydrocarbons were primarily produced in the first 16 min and aliphatic hydrocarbons were dominated by a bimodal distribution of n-alkane and n-alkene doublets that extended up to C34.