Secondary Cracking and Upgrading of Shale Oil from Pyrolyzing Oil

Mar 16, 2015 - (3) This can be implemented through combining oil shale pyrolysis and ... the lower oil yield, say, by 20–30% for the Condor and Stua...
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Secondary Cracking and Upgrading of Shale Oil from Pyrolyzing Oil Shale over Shale Ash Dengguo Lai,†,‡ Zhaohui Chen,†,‡ Lanxin Lin,†,‡ Yuming Zhang,§ Shiqiu Gao,*,† and Guangwen Xu*,† †

State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, China ABSTRACT: Considering oil shale pyrolysis with a solid heat carrier, this article investigated the effect of shale ash, as the bed material for secondary reactions of pyrolysis volatile, on final product distribution and quality of oil shale pyrolysis in a laboratory dual-stage fixed bed reactor. The examined factors included the residence time of pyrolysis volatile in a shale ash bed and the temperature of shale ash (i.e., cracking temperature). Prolonging the pyrolysis volatile residence time in the shale ash bed from 0 to 10 s decreased the shale oil yield by 31.0% but increased the fraction of gasoline and diesel (boiling point 773 K) was cracked to increase the yields of light oil and pyrolysis gas (especially H2 and CH4). The shale ash bed temperature was the key factor affecting the product distribution. The catalytic effect of shale ash lowered the shale oil cracking temperature for achieving the same degree of oil cracking. The catalytic activity of shale ash for cracking shale oil was shown to be closely dependent on the metal oxides in the ash. While CaO and Na2O tended to inhibit the formation of coke and to promote catalytic reforming, Fe2O3 showed good activity in cracking shale oil and forming coke. shale pyrolysis reactions. Gai et al.11 reported that the inherent pyrite in oil shale improved the oil yield, while the extra pyrite added to the oil shale promoted the formation of volatiles. Thus, it is definite that both the external conditions and internal metal oxide composition can affirmatively affect the oil shale pyrolysis based on SHC. Literature studies12−14 have shown that the shale oil from oil shale pyrolysis can be upgraded by shale ash to result in the higher oil quality or lower heavy oil content which makes the oil easy to process. Comparing with the externally heated pyrolysis (indirect heating), the pyrolysis using shale ash as the heat carrier has usually the lower oil yield, say, by 20−30% for the Condor and Stuart oil shale.8,15 The lost oil fraction was mainly heavy fraction (boiling point >623 K) through its adsorption onto shale ash and in turn deep cracking.7,8 Some researchers3,4,16,17 have investigated the catalytic cracking effect of shale ash in oil shale pyrolysis by blending ash and oil shale. Results indicated that shale ash, as the solid heat carrier, had a certain catalytic function on oil shale retorting to improve the shale oil quality. However, the method blending oil shale and ash cannot clearly define the conditions of shale oil cracking and upgrading by shale ash so that the understanding is still far from systematic and the mechanism is yet unclear. The dual-stage fixed bed reactor could separate the volatile cracking from the oil shale pyrolysis. On the basis of this, it is able to study individually the effects of primary volatile residence time in shale ash bed and cracking temperature over shale ash particles. The purpose of this study is devoted to determining how the individual factor affects the shale oil yield

1. INTRODUCTION Oil shale is a representative nonconventional oil resource, and the potential shale oil production is predicted to be more than 4 times of the proved petroleum reserves of the world.1 Oil shale pyrolysis by solid heat carrier (SHC) is a feasible approach to treat fine oil shale particles and to produce shale oil.2 For the pyrolysis processes with SHC, shale ash, sand, and ceramic ball have been used as the solid heat carrier particles, but the use of shale ash, with also unburnt carbon in it, should be most common.3 This can be implemented through combining oil shale pyrolysis and oil shale char combustion in a circulating fluidized bed (CFB) system. In the pyrolyzer, oil shale is heated up by the circulated hot SHC particles which are mainly shale ash from the combustor. Thus, the physical and chemical interactions between shale ash and oil shale have to impact the oil shale pyrolysis behavior and affect the pyrolysis product through secondary reactions including cracking and coking between pyrolysis volatiles and ash.4,5 It is predicted that shale oil cracking could be significant in the SHC process to either decompose or upgrade the shale oil over shale ash.6,7 The primary volatile released from pyrolyzing oil shale can adhere to the surface of shale ash particles to lead to secondary reactions toward the released volatile.5,8 Both thermal and catalytic effects of shale ash particles are the main factors varying the shale oil composition and properties through the secondary cracking and upgrading reactions.9 Many studies have examined the effects of alkali and alkaline earth metals, pyrite, and other minerals in shale ash on oil shale pyrolysis. Taylor et al.5 found that montmorillonite decreased the oil yield by forming coke while kaolinite caused oil cracking and alkene isomerization. Ballice10 reported that alkali and alkaline earth metals increased the reactivity of oil shale, whereas silicate minerals manifested an inhibitive effect on oil © XXXX American Chemical Society

Received: December 16, 2014 Revised: March 9, 2015

A

DOI: 10.1021/ef502821c Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Proximate and Ultimate Analyses of Yilan Oil Shale proximate analysis (wt %, dry basis)

ultimate analysis (wt %, dry basis)

Fischer assay (wt %, dry basis)

A

V

FC

C

H

O

N

S

52.04

30.04

17.93

32.45

3.26

10.57

0.92

0.76

9.8

Table 2. XRF Analysis and Specific Surface Areas of the Tested Ashes content/wt % sample

a

YL YL-Ca YL-Fe YL-Na a

SiO2

Al2O3

Fe2O3

CaO

Na2O

MgO

K2O

others

surface area (m2/g)

67.95 61.69 62.62 61.97

23.53 19.30 19.23 19.74

5.46 6.28 14.97 6.14

0.40 9.71 0.44 0.45

0.09 0.11 0.11 9.03

0.49 0.51 0.45 0.53

1.02 0.93 0.92 0.98

1.06 1.47 1.26 1.16

15.82 15.13 14.96 15.45

YL, Yilan shale ash; YL-M means the Yilan shale ash loaded with the oxide of metal M, M = Ca, Fe, and Na.

and quality through the secondary upgrading and cracking over shale ash, and this hopes to understand more the pyrolysis process using SHC. The actions of individual metal oxides in shale ash are also tested by adjusting the contents of metal oxides in ash, thus giving an understanding of the working mechanism of shale ash.

2. EXPERIMENTAL SECTION 2.1. Material and Properties. The tested oil shale in this article was from Yilan coal mine (called YL oil shale), Heilongjiang province of China. Before the experiment, the oil shale was crushed and sieved to 0.4−1.0 mm and then dried in an atmospheric oven of 383 K for 24 h. The employed shale ash was obtained by combusting the Yilan oil shale in a muffle furnace at 1073 K for 10 h, and the ash was also sieved to the sizes of 0.4−1.0 mm. Table 1 shows the results of proximate and ultimate analyses for the tested YL oil shale. It contained more than 50 wt % ash (A), and the volatile content was about 30 wt % (V). The oil yield from the Fischer Assay was 9.8 wt % of dry oil shale mass. The metal oxides of Ca, Fe, and Na were impregnated on the YL shale ash to test the catalytic activity of individual metallic matters in shale ash. The impregnation was implemented by dosing the shale ash into an aqueous solution of the specified metal nitrate, say, NaNO3, Ca(NO3)2, or Fe(NO3)3 containing 10 wt % of the targeted metal oxide. The formed slurry was in turn vibrated continuously in an ultrasonator at 313 K for 12 h to ensure the dispersion of metallic species onto ash particles. Subsequently, the mixture was dried in an atmospheric oven of 383 K and calcined at 923 K to produce the shale ash loaded with a metal oxide. The raw YL shale ash and also the oxide-loaded ashes were all characterized in Table 2 with the X-ray fluorescence (XRF) analysis, showing that the major components in ash were SiO2, Al2O3, and Fe2O3, while the ash was also with low contents of CaO, SO3, K2O, MgO, and Na2O. 2.2. Apparatus and Methods. Figure 1 shows a schematic diagram of the adopted experimental apparatus consisting mainly of a dual-stage fixed bed reactor having an upper stage for oil shale pyrolysis and a lower stage for volatile cracking.18,19 The length of the upper tube was 400 mm with an inner diameter of 30 mm and the lower tube was 850 mm in length with an inner diameter of 38 mm. The oil shale and shale ash were loaded on their porous sintering quartz plates, which were 330 and 570 mm from the reactor top, respectively. The pyrolysis and cracking temperatures were measured by K-type thermocouples inside the reactor. From a cylinder high purity N2 flew at 100 mL min−1 from the top to bottom of the reactor as the sweeping gas. The effect of pyrolysis volatile residence time in the ash bed was conducted by adding 20 g of oil shale into the upper stage and different amounts of shale ash into the lower stage of the reactor. A specific amount of shale ash was put into the lower stage before heating the reactor, and 20 g of oil shale was in turn quickly put into the upper stage to start the experiment by opening the top cover of the reactor when the reactor reached its desired temperatures for

Figure 1. Schematic diagram of experimental apparatus: (1) mass flow meter, (2) electric furnace, (3) upper tube, (4) lower tube, (5) thermocouple, (6) oil shale, (7) shale ash, (8) condenser, (9) acetone trap, (10) dry silica gel bottle, (11) valve, (12) gas collection bottle (water replacement), and (13) measuring cylinder. the upper and lower stages. The generated primary volatile in the upper stage passed through the shale ash bed in the lower stage to implement the secondary cracking and upgrading reactions. In a similar way, we also tested the influences of shale ash bed temperature (i.e., cracking temperature) and individual metal oxide impregnated on shale ash. The pyrolysis product from the reactor was quickly cooled down in a pipe condenser and in turn passed through three acetone-washing bottles immersed in an ice−water bath to collect shale oil. Further through a fabric filter, the gas was finally collected using gas bags to measure its composition in a micro GC. The oil-containing liquid was collected from the acetone-washing bottles and also from washing all the pipes between the reactor and the acetone-washing bottles using acetone. Then, MgSO4 was added into the obtained oil-containing liquid to remove water. After filtration to remove solid particles, evaporation of acetone was carried out at about 298 K at slightly reduced pressure to recover the shale oil, which was in turn weighted to calculate the oil yield and analyzed in a simulated distillation GC to characterize its boiling point distribution. Tests were first carried out to determine the reaction (pyrolysis) time maximizing the shale oil yield in the adopted dual-stage fixed bed reactor. The results showed that the shale oil yield for Yilan oil shale tended to become constant after 30 min from loading oil shale into the reactor. There were a few minutes for heating up the oil shale particles, but in this work all tests were under the same conditions of pyrolysis in B

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Energy & Fuels the first stage and the comparison was made from the overall oil yield and quality from each batch-wise test. Consequently, the oil shale pyrolysis was all performed for 30 min to ensure the maximal shale oil yield in the experiments shown herein. 2.3. Analysis and Characterization. The shale oil after removal of moisture and dust was analyzed using a simulated distillation GC (Agilent 7890A) to determine its distillation fractions at different boiling points. In this work, the shale oil components were divided into three groups: gasoline and diesel oils (boiling point 773 K). The noncondensable gas was analyzed using a micro GC (Agilent 3000A) to get the concentrations of H2, CH4, CO, CO2, C2H4, C2H6, C3H6, and C3H8. In the context, the hydrocarbons C2H4, C2H6, C3H6, and C3H8 were overall defined as the C2−C3 hydrocarbons. The shale oil was also analyzed using a gas chromatography-mass spectrometer (Agilent 6890A GC and Agilent 5375 MS), while the crystal patterns of shale ash were obtained from an X-ray diffraction analyzer (XRD, Smartlab) working at 45 kV and 200 mA with a step size of 0.02° in 10° to 90° for 2θ. The scanning electron microscopy (SEM, JSM-7001F JEOL) was used to observe the morphology of raw and spent shale ash. The energy dispersive spectroscopy (EDS) analysis was conducted to test the surface element distribution of spent ash, while the nitrogen adsorption in a BET was adopted to determine the specific surface area of shale ash (Micrometrics Tristar II 3020). The shale oil yield (wt %) was defined against the dry mass of oil shale, as shale oil yield = Mshale oil /M

Figure 2. Variation with volatile residence time in ash bed: (a) shale oil yield and composition, (b) yields of major gas species per gram of oil shale (cracking temperature is 823 K).

(1)

where Mshale oil was the mass of shale oil obtained from pyrolyzing oil shale with a dry mass of M. The volume of pyrolysis gas was measured in a volumetric gas meter at room temperature to estimate the gas yield (mL/g), as

gas yield = Vi /M

time from 0 to 10 s. In shale oil, the content of gasoline and diesel was elevated by 46.4%, while their absolute yield had a slight increase. Both the yields and contents of the VGO and heavy oil fractions significantly decreased. These show that the shale oil was upgraded to lower its fractions of VGO and heavy oil but with the kept yield of gasoline and diesel when passing the primary pyrolysis product through a shale ash bed at 823 K. It is even possible to completely crack the heavy oil when the volatile residence time exceeded 8 s in the ash bed. The decrease in the total shale oil yield was mainly due to the lowered yield of heavy fraction (boiling point >623 K) that was more likely to be adsorbed on the shale ash particles and further converted to coke, pyrolysis gas and also some light oil.6,7,20 This shows that the shale ash has a kind of catalytic cracking or upgrading effect on shale oil, especially on removal or reducing of heavy oil. Nonetheless, this cracking must be at its suitable extent because the excessive catalytic cracking would considerably lower the shale oil yield. Figure 2b shows the gas yield and composition at different pyrolysis volatile residence times in the shale ash bed. In changing the volatile residence time from 0 to 10 s, the yields of H2 and CH4 increased by 44.3% and 28.8%, respectively. Both CO and CO2 remained to have little change in their yields, while there was a slight increase in the yield of C2−C3 hydrocarbons. Accordingly, the increase of hydrogen and hydrocarbon gases indicates that shale oil cracking or coking has taken place in the presence of shale ash. Previous studies21−23 reported that CH4 is related to the breakage and hydrogenation of methyl chains in oil, while H2 is from the breakage of C−H bonds and other hydrogen sources. We suggest the catalytic cracking of aliphatic hydrocarbons containing in shale oil for the increased production of CH4 and H2. Figure 3 compares the gas chromatography-mass spectrometry (GC-MS) analysis results of shale oils at different pyrolysis volatile residence times in the shale ash bed. The relative

(2)

where Vi refers to the gas volume of individual gas species i in the pyrolysis gas under the standard conditions of 273 K and 1.0 atm. After each test, the amount of carbon deposited on spent shale ash was determined by combustion of the ash in a muffle furnace of 1073 K for 2 h. For the volatile from oil shale pyrolysis in the upper stage, its residence time inside the lower-stage shale ash bed was calculated from the flow rate of carrier (sweeping) gas and the volume of volatile generated. By adding 2 g of Yilan shale ash into the lower stage, the residence time of volatile in the ash bed was calculated to be about 1 s under the experimental conditions of this study. Thus, the residence times of volatile through the lower-stage ash bed were about 2 s, 4 s, 6 s, 8 s, and 10 s corresponding to the shale ash amount of 4 g, 8 g, 12 g, 16 g, and 20 g loaded into the lower stage, respectively.

3. RESULTS AND DISCUSSION In order to determine the proper pyrolysis temperature leading to the highest shale oil yield, pyrolysis of 20 g of oil shale loaded into the upper-stage was conducted in the dual-stage fixed bed by maintaining the upper and lower stages at the same temperature in 773−873 K. It was shown that increasing the reaction temperature first increased and then decreased the shale oil yield, and at 823 K it resulted in a peak oil yield of about 8.7 wt % for the pyrolysis. This gave the shale oil recovery percent of 88.7% against the yield of Fischer Assay. Consequently, herein all tests adopt 823 K for the first stage to ensure the possibly highest volatile yield from pyrolyzing the oil shale. 3.1. At Different Volatile Residence Times in Ash. Figure 2a shows the effect from varying the residence time of pyrolysis volatile in the lower-stage shale ash bed on the final shale oil yield and composition. The shale oil yield decreased from 8.7 wt % to 6.0 wt % by 31.0% in varying the residence C

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short residence time of pyrolysis volatile in the ash bed is critical to ensure the high oil yield and also the less deposition of carbon or coke on ash particles. From the sense of selective cracking of heavy species, Figures 2 and 3 show that the residence time should be about 4 s at the cracking temperature of 823 K for the secondary upgrading of primary shale oil over shale ash particles. 3.2. At Different Ash Bed Temperatures. Figures 4−6 show the influences of shale ash bed temperature, which is also

Figure 3. GC-MS analysis of shale oils from different residence times of pyrolysis volatile in shale ash bed: (a) 0 s (without passing ash bed), (b) 4 s, and (c) 8 s. Figure 4. Shale oil and gas yields of oil shale pyrolysis with and without ash in the lower stage at different cracking temperatures.

intensity of the MS spectrum was estimated according to the area ratio of a peak to the largest peak in the spectrum. The composition in shale oil can be divided into groups of C5−C10, C11−C20, and above C20 according to the carbon number. It is obvious that raising the residence time gradually decreases the content of shale oil fraction with high carbon number, especially the fraction above C20. This provides a direct evidence for the secondary cracking and upgrading of heavy species over shale ash. Sonoyama et al.24 have reported that the cracking of shale oil would cause macromolecular components to crack into small molecules and accordingly to reduce the average molecular weight of the shale oil and make the oil distribution simpler and concentrated. Table 3 presents the carbon deposition characteristics on shale ash after the tests at different residence times of volatile in

the secondary cracking temperature, on the distribution of the final pyrolysis product. For the tests the residence time of pyrolysis volatile in the shale ash bed was 6 s (12 g ash in the lower stage). Figure 4 compares the shale oil and gas yields of oil shale pyrolysis with and without ash cracking in the lowerstage of the reactor. The shale oil yield with shale ash in the lower stage was obviously lower than that without shale ash, whereas the pyrolysis gas yield was higher. Although the shale oil yield did not show obvious decrease from 723 to 773 K, there was a detectable increase in gas yield between such two temperatures. It is because the gas yield increase in mL/g for cracking every milliliter of shale oil is actually much more obvious or more detectable. The secondary cracking of shale oil over shale ash was not obvious at temperatures below 773 K. At rather high temperatures, the shale oil cracking in a shale ash bed would become excessive to convert oil into gas. Examining the tendencies of oil yield decrease and gas yield increase for the case with shale ash, we can see that 823 K is critical and above this temperature the variations in both oil and gas yields became obviously quicker. Thus, 823 K should be the suitable temperature for secondary cracking leading to the expected oil upgrading. Figure 5 shows the influence of cracking temperature on the contents of three oil fractions in the produced shale oil. At each temperature the sum of the three contents is 100% as for a given case with or without shale ash in the lower stage. Obviously, raising the cracking temperature evidently decreased the contents of VGO and heavy oil but gradually increased the content of gasoline and diesel in the produced shale oil. Comparing the results with and without shale ash, the contents of VGO and heavy oil fractions were obviously higher when there was not shale ash. From the variations of both VGO and heavy oil contents in Figure 5, one can see that the temperature influence is more pronounced for the case with shale ash in the lower stage. The content changes for the three oil fractions revealed again that the shale ash had a definite catalytic effect,

Table 3. Carbon Deposition Calculation in YL Ash at Different Pyrolysis Volatile Residence Times volatile residence time (s) total carbon deposition (mg) specific C deposition (mg/g)a

2 102.7

4 191.5

6 263.0

8 344.6

10 383.9

25.68

23.94

21.91

21.54

19.19

a

Specific C deposition refers to the deposited carbon in mg on every gram shale ash.

the shale ash bed. Increasing the pyrolysis volatile residence time from 2 to 10 s resulted in an increase in the carbon deposition, but the specific C deposition (refers to the deposited carbon in milligrams on every gram of shale ash) slightly decreased to indicate the gradually lower rate of carbon deposition with the longer volatile residence time. As shown in Figure 2, the shale oil passing through the ash bed tends to have its heavy fractions adhered on the surface of shale ash particles, and the adsorbed oil is in turn cracked on the particles into light oil, gas, and coke. Its formed coke on the ash particles should be the measured carbon in Table 3. This carbon would surely decrease the reactivity of the ash for oil cracking, thus decreasing the carbon deposition rate.12 As a consequence, a D

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catalysis of shale ash. Overall, all the gas yields showed the quicker increase with raising temperature above 873 K. It shows that this temperature is already too high for upgrading shale oil via secondary cracking, agreeing with our preceding demonstration that the optimal cracking temperature is around 823 K. 3.3. Effect of Individual Metal Oxides in Ash. Figure 7 compares the shale oil yields from pyrolysis with the same

Figure 7. Shale oil yield from pyrolyzing oil shale with quartz sand and shale ash in the lower stage at different volatile residence times. Figure 5. Three oil fractions in shale oil produced from pyrolysis with and without shale ash in the lower stage at different cracking temperatures.

amount of quartz sand and shale ash in the lower-stage of the reactor. The oil yield was higher and showed a slower decrease with prolonging the residence time for the case using quartz sand. This reveals a higher activity of shale ash for cracking shale oil than that of quartz sand. The major difference between quartz sand and shale ash is that the latter contains metal oxides including the oxides of alkali, alkaline earth, and transition metals.25 Thus, the catalytic activity of shale ash loaded with a given metal oxide was further tested to identify the different catalytic effects of different metal oxides. Figure 8 shows the shale oil yields and specific C deposition (per gram of ash) for the shale ashes loaded with oxides of Ca, Na, and Fe by impregnation (denoted as YL-M, while YL being for Yilan shale ash only). From the shale oil yield one can see that for the same amount of loaded oxides, the activity to catalyze the shale oil cracking is subject to an order of YL-Fe > YL-Na > YL-Ca > Raw YL at the tested cracking temperature of

and the heavy fraction in shale oil is easier to be cracked because they are more likely to be adhered on the surface of shale ash particles. For realizing the similar cracking effect, shown with the similar contents of VGO and light oil (gasoline and diesel oils) in Figure 5, the presence of shale ash in the lower stage of the reactor reduced the cracking temperature due to the facilitating effects of shale ash, including its enhanced adsorption and catalysis. Furthermore, the temperature influence is more obvious than in Figure 2 for the volatile residence time in the ash bed. Figure 6 reports the variations in yields (per gram oil shale) of major noncondensable gases with the cracking temperature. While elevating temperature increased all the gas yields, especially the H2 and CH4 yields, the case with shale ash had generally the higher yields of gases. It is true that breakage and hydrogenation of methyl chains can easier occur if with the

Figure 6. Yield of major gas species (per gram oil shale) from oil shale pyrolysis with and without ash in the lower stage at different cracking temperatures (volatile residence time in ash bed is 6 s).

Figure 8. Shale oil yield and carbon deposition (per gram ash) varying with volatile residence time for shale ashes impregnated with different metal oxides. E

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Energy & Fuels 823 K. The specific C deposition on the ash particles followed the sequence of YL-Fe > Raw YL > YL-Ca > YL-Na. The change of shale oil composition over different metal oxides impregnated on shale ash further proved the realized different extents of cracking reactions for different components. As shown in Figure 9, the increase of gasoline and diesel content

Figure 10. Gas yields per gram oil shale varying with volatile residence time for shale ashes impregnated with different metal oxides.

11 which showed that some Fe2O3 peaks had decreased intensity or disappeared in the XRD patterns of spent shale ash after its participation in the shale oil cracking. Both YL-Na and YL-Ca facilitated the shale oil cracking through increasing the catalytic activity of the ash to the reactions of, for example, decomposition and reforming.27 Their carbon formations were the lowest but their CO and CH4 yields were relatively high. For them, the identified CO2 yield that was lower than the case of raw YL should be due to the adsorption of CO2 by CaO and Na2O.28,29 Comparing the fresh and spent shale ashes shown in Figure 11, one can see that Fe2O3 was detected in all fresh ashes but CaO was only for YL-Ca ash. There was not a distinctive peak for any of sodium chemicals in the fresh ashes, and the possible reason for this was the high-degree dispersion of Na in impregnated ashes. Element Fe could be found in the spent shale ashes, while CaCO3 and Na2CO3 appeared for the spent YL-Ca and YL-Na ashes. These are indicative of the reduction of Fe2O3 and the adsorption of CO2 during oil cracking.21 As a consequence, we believe that the shale oil cracking over shale ash should be mainly attributed to the catalytic cracking by the oxides of alkali and alkaline earth metals, whereas certain other oxides like Fe2O3 promote likely the adsorption of heavy oil species onto the ash particles. It also found that the specific surface area of the spent YL, YL-Ca, YL-Na, and YL-Fe shale ashes were, respectively, 11.09 m2/g, 12.25 m2/g, 12.16 m2/g, and 5.33 m2/g after the oil cracking tests at 823 K (volatile residence time is 6 s). Comparing with the surface areas of the fresh shale ashes in Table 2 (about 15 m2/g and a little different), the specific surface areas of all such spent ashes decreased more or less after experiencing their respective oil cracking tests. The decrement in surface area followed a sequence of YL-Fe > Raw YL > YLNa > YL-Ca, in the same sequence as for carbon deposition shown in Figure 8b. Thus, the carbon deposition was rather in

Figure 9. Shale oil composition from tests over shale ashes impregnated with different metal oxides at different volatile residence times in ash bed (cracking temperature is 823 K).

followed the order of YL-Fe > YL-Na > YL-Ca > Raw YL at each volatile residence time from 2 to 6 s in ash, which agrees with the decrease in the shale oil yield as well as VGO and heavy oil contents. These results indicated that the loaded metal oxides could upgrade the shale oil quality by converting the heavy fraction in shale oil into light fraction. The variations in gas yields (mL/g-oil shale) shown in Figure 10 for the same ashes demonstrated that all the metal-loaded ashes caused obviously the higher yields of CH4 and C2−C3 hydrocarbon. The H2 yield was particularly high for YL-Fe but was very close for all the other ashes. In comparison with raw YL ash, both YL-Na and YL-Ca resulted in the lower CO2 yield but the higher CO yield. Especially, YL-Fe tended to lower the CO yield. Combining Figures 8−10 shows further that prolonging the volatile residence time in the shale ash bed from 0 to 6 s caused the more obvious cracking of shale oil to have the lower oil yield, higher light oil quality, and higher gas yield at the longer volatile residence time. The above variations in the yields and qualities of shale oil and pyrolysis gas clarified that the impregnated metal oxides all facilitated the cracking of shale oil to lower the oil yield and to raise the gas production and oil quality. However, different oxides had different working mechanisms. The YL-Fe greatly promoted the adsorption of heavy oil species but the adsorbed species were rather thermally cracked than catalytically decomposed so that it led to the highest H2 production and carbon deposition. The iron oxide such as Fe2O3 would be reduced by CO in the process of oil cracking so that this ash caused the lowest CO yield and the highest CO2 yield.26 The occurrence of this reduction reaction was confirmed in Figure F

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Figure 11. XRD patterns of (a) fresh and (b) spent shale ashes of shale oil cracking at 823 K and with a volatile residence time of 6 s.

uniformly distributed over ash particles for all the samples. Generally, all spent ashes had obviously decreased O content and increased carbon content. These indicate the occurrence of oxide reductions and carbon deposition with the oil cracking. It is also interesting to find that the C content determined with EDS in the spent shale ashes (Table 4) followed the same order as the specific carbon deposition determined by oxidizing a spent ash in a muffle oven (Figure 8b), which is YL-Fe > Raw YL > YL-Na ≈ YL-Ca. Table 4 shows also the highly dynamic variations in the other ash components between the fresh and spent ashes. This should be a comprehensive result from the factors including different complicated reactions for different ashes involved in secondary reactions, the measurement uncertainties by EDS, and the originally nonuniform element distributions in ash particles.

the internal channels or pores of shale ash so that the surface area becomes lower. Table 4 summarizes the SEM photographs and elemental composition determined by EDS of all fresh and spent ashes, Table 4. SEM Photographs and EDS Analysis of Fresh and Spent Ashes from Cracking Tests at 823 K

4. CONCLUSIONS This article presented results from testing the influences of major factors on secondary cracking and upgrading of in situ generated shale oil through oil shale pyrolysis over shale ash in a laboratory dual-stage fixed bed reactor. The examined major factors included the pyrolysis volatile residence time passing through a shale ash bed and the shale ash bed temperature (or cracking temperature). Comparing to quartz sand, the shale ash exhibited obvious catalytic effect on secondary cracking and upgrading of shale oil. It facilitated the conversion of heavy oil fraction in shale oil into gas and light oil. The optimal temperatures for shale oil pyrolysis and secondary upgrading over shale ash were found to about 823 K. The secondary cracking was not obvious at temperatures below 773, whereas above 873 K excessive cracking should occur to convert most of shale oil into gas. At 823 K, varying the pyrolysis volatile residence time from 0 to 10 s decreased the shale oil yield by about 30% but it increased the gasoline and diesel content in shale oil by about 45%. The heavy oil fraction in shale oil was completely converted when the volatile residence time was above 8 s in the ash bed. The results of these parametric studies are suggested to be a good guidance to the process design and control of oil shale pyrolysis using solid heat carrier (SHC). By investigating the actions of a few individual metallic elements impregnated onto ash with their oxides on the secondary cracking and upgrading of shale oil, it further clarified that the dominant component SiO2 in shale ash indeed

where EDS analysis was performed for two different local positions a and b to show the uniformity of element distribution in ash particles. In appearance, the pores in the spent ashes were coated with deposited carbon to make the surface flat. The EDS results showed that all elements are not G

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Energy & Fuels

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has not obvious catalytic effect than thermal cracking action. Both CaO and Na2O provide catalytic effects on shale oil cracking to inhibit the carbon or coke formation and to give the reasonably good shale oil yield, but Fe2O3 shows more the effect on promoting thermal cracking to lower the shale oil yield (than over ash-Ca and ash-Na) and to give the higher carbon deposition. With shale oil cracking/upgrading over shale ash particles, it also occurs the reduction of some oxides such as Fe2O3 to form element metal or other low-valence oxides. Meanwhile, carbon deposition definitely occurs in/on the internal pores or surfaces to decrease the surface area of ash particles.

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*Phone: +86 10 82544885. Fax: +86 10 82629912. E-mail: [email protected]. *Phone: +86 10 82544886. Fax: +86 10 82629912. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The study was financed by the National Basic Research Program of China (Grant 2014CB744303).

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REFERENCES

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DOI: 10.1021/ef502821c Energy Fuels XXXX, XXX, XXX−XXX