Conceptual Design of an Oil Shale Comprehensive Refinery Process

Article pubs.acs.org/EF

Conceptual Design of an Oil Shale Comprehensive Refinery Process with High Resource Utilization Huairong Zhou, Yu Qian, Qingchun Yang, and Siyu Yang* School of Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, P.R. China ABSTRACT: With the decrease of conventional energy reserves, countries such as China began to explore efficient ways to use oil shale as an alternative energy source. The refinery technology used for oil shale is retorting. However, in this process, fineparticle shale, semi-coke, and retorting gas are not used efficiently; rather, they are dismissed as solid and gaseous wastes. Resource utilization is extremely low in retorting. In addition, the price of crude retorting oil is very low, making the whole retorting process suffer from a low economic benefit. In this paper, a new oil shale comprehensive refinery process is proposed, aiming to process shale particles from all ranges of sizes. The new process includes a gas circulation shale retorting unit, a Dagong shale retorting unit, an oil−gas separation unit, a retorting gas steam reforming unit for hydrogen production, and a shale oil hydrogenation unit. Fine-particle shale from the gas circulation shale retorting unit is used as raw material in the Dagong shale retorting unit. A part of the retorting gas from the oil−gas separation unit is converted into hydrogen in the retorting gas steam reforming unit. The heat balance of the retorting gas steam reforming unit is provided by burning semi-coke from the gas circulation shale retorting unit. Hydrogen from the retorting gas steam reforming unit is used for shale oil hydrogenation, in order to upgrade crude retorting oil for an increased economic benefit. The new process is modeled using Aspen Plus software. Based on the simulation, a techno-economic analysis is carried out to explore its strengths over the conventional gas circulation shale retorting process. Results show that the oil yield is increased by 2.3%. The capital investment is higher, and the production cost for producing 1 t of oil is 1−1.3 times that of the conventional process, but the return on investment increases by 14%.

1. INTRODUCTION It has been reported that China has surpassed the United States to become the country with the largest energy consumption in the world.1 In 2014, total energy production in China was 3.6 × 109 tons of standard coal equivalent (tce), and total consumption was 4.3 × 109 tce.2 The energy reserves in China are characterized by richness in coal but scarcity in oil. More than half of the oil consumed is imported from abroad. For example, oil production was 2.1 × 108 t, while apparent consumption was 5.2 × 108 t in 2014. It is obvious that 60% of the oil used was imported,3 and this number is projected to increase to 75% by 2030.4 The continuous growth of oil consumption requires development of alternative energy resources. The Chinese government has enforced a set of policies to promote exploration and exploitation of unconventional energy resources,5 and their proportion of the energy supply increased from 4% in 2011 to 11% in 2015.6 Oil shale is one such unconventional resource. Its abundant reserve has the potential to mitigate the shortage of oil supply. According to statistics, the extractable amount of shale oil in the world is 654.7 × 109 t, four times the world’s crude oil reserves.7 The amount of that in China is 47.6 × 109 t, double China’s crude oil reserves.8 Now, exploration and exploitation of oil shale have been commercialized. In China, shale mining is mainly concerned with surface mining of shallow shale seams stored 100−400 m below the ground. There are mainly two types of retorting technologies: in situ and ex situ. In China, only ex situ retorting technologies are applied. Ex situ retorting technologies include gas-heat-carrier technologies and solid-heat-carrier technologies. Gas-heat-carrier technologies have been successfully applied in Estonian Kiviter-type retort, Brazilian Petrosix-type © 2016 American Chemical Society

retort, and Chinese Fushun-type retort and gas-circulation-type retort.9 Fushun-type retorting technology has a better adaptability to lean shale, with the lowest oil content of 6%. In addition, it has the advantages of a simple structure as well as easier equipment maintenance. However, the small handling scale of a single Fushun-type retort (100 t/d) and low oil yield (65%) limited its further development.10 Gas-circulation-type retorting technology is based on Fushun-type retorting technology. Its processing scale for a single retort is 300 t/d, and the oil yield is increased to 90%.11 As a consequence, gascirculation-type retorting technology has been broadly applied in different plants. Liaoning Chengda Co., Ltd. applied it in a retorting plant in Huadian, Jilin province. The processing capacity of this plant is 3 Mt/y, and its oil production is 0.25 Mt/y.12 Another plant, in Jimsar, Xinjiang province, has a processing scale of 11 Mt/y. Its oil production is 0.48 Mt/y in the first stage of the project.13 A schematic diagram of the gas circulation shale retorting (GCSR) process is shown in Figure 1. Shale is first ground and then divided into two types of particles, based on size: fine particles smaller than 10 mm in diameter, and large particles between 10 and 75 mm in diameter. Large particles are fed into the gas-circulation-type retort and converted into semi-coke and an oil−gas mixture. The oil−gas mixture flowing out from the top of the retort is separated into oil and retorting gas by the oil−gas separation (OGS) unit. Most of the retorting gas is recycled back to the retort after being heated by burning a part of the retorting gas Received: May 27, 2016 Revised: July 18, 2016 Published: August 10, 2016 7786

DOI: 10.1021/acs.energyfuels.6b01279 Energy Fuels 2016, 30, 7786−7801

Article

Energy & Fuels

Figure 1. Schematic diagram of the GCSR process.

Figure 2. Schematic diagram of the DGSR process.

Figure 3. Schematic diagram of the OSCR process.

and outsourcing fuel gas. The rest is recycled back to the bottom of the retort to exchange heat with the semi-coke. The GCSR process deals with large-particles shale bigger than 10 mm in diameter. However, fine-particles shale from the grinding and screening unit accounts for 20% of the total.10 As a consequence, the maximum shale utilization of the GCSR process is 80%. In addition, high-valued semi-coke is abandoned and not effectively used. To solve these problems, Jiang et al.14 investigated a comprehensive oil shale utilization process. In this process, fine-particles shale and semi-coke were mixed and fed into a circulating fluidized bed (CFB) furnace to burn to generate high-pressure steam and then convert it to electricity in a steam turbine. It was shown that the resource utilization increased dramatically, although the economic

benefit was not enhanced obviously due to the large capital investment in the power generation unit.9 There are some industrialized solid-heat-carrier technologies. They have the advantage of using fine-particles shale to produce oil, which can effectively improve shale utilization and economic benefit. Representative technologies include Estonian Galoter-type retorting, Canadian ATP-type retorting, and Chinese Dagong-type retorting. Daqing Oilfield Co., Ltd. applied Dagong-type retorting technology and built an industrialized pilot-scale plant with a processing capacity of 0.6 Mt/y shale.15 Its oil production is 0.03 Mt/y. A schematic diagram of the Dagong shale retorting (DGSR) process is shown in Figure 2. Shale particles smaller than 10 mm in diameter are fed into the heat exchanger and preheated to 120 7787


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Energy & Fuels °C by hot flue gas generated from combustion of semi-coke. The hot shale is mixed with recycled hot ash and sent into the Dagong-type retort. The shale is then converted into semi-coke and an oil−gas mixture. The semi-coke is burned by preheated air in the burning furnace. Ash from the semi-coke combustion is divided into two parts: most of the ash is recycled back to the retort to supply heat for the retorting reaction, and the rest is discharged after exchanging heat with air. The oil−gas mixture goes into the OGS unit and is separated into oil and retorting gas. In China, larger-scale shale retorting plants mainly adopt the gas-circulation-type retorting technology.10 To solve the problems caused by fine-particles shale, technical improvement is necessary, rather than reconstructing the plants with the Dagong-type retorting technology. It is clear that the fineparticles shale abandoned from the GCSR process is of a suitable diameter for the DGSR process. Therefore, fineparticles shale can be used as raw material for the DGSR process. In that case, high utilization of oil shale would be achieved, and high oil yield would be obtained. Furthermore, the oil−gas separation mechanism in the GCSR process is the same as that in the DGSR process. These two processes can be coupled and share the same OGS unit. A novel oil shale comprehensive refinery (OSCR) process is therefore proposed in this paper. The new process is modeled and simulated by using Aspen Plus software. Based on the simulation, a technoeconomic analysis is performed to explore the advantages of the new process.

reactions and shale pyrolysis process. The feedstock of Huadian oil shale in Liaoning province is selected for simulation. Its Fischer assay is 9% oil content. In addition, its proximate and elemental analyses are given in Table 1.16 Based on the practical Table 1. Proximate and Elemental Analyses of Huadian Shale Proximate Analysis (wt%, ar)a M

FC

V

A

3.43

1.77

22.83

71.97

Elemental Analysis (wt%, ar) C

H

O

N

S

50.16

6.37

38.12

1.99

3.35

“M”, “FC”, “V”, and “A” denote moisture, fixed carbon, volatile, and ash, respectively. “ar” denotes as-received basis of shale.

a

scale of a retorting plant in Huadian, the capacity of the OSCR process is set to 3 Mt/y shale.12 The shale is ground and then screened into two kinds of particles, based on size. Particles bigger than 10 mm in diameter occupy 80% of the total. They meet the size demand of the gas-circulation-type retort. The remaining 20% of the shale is available to the Dagong-type retort. Thus, we set the capacity of the GCSR unit to 2.4 Mt/y, while that of the DGSR unit to 0.6 Mt/y. The composition of oil shale is very complex. For simulation, shale is defined as a mixture composed of water, minerals, and kerogen. These components can be modeled as conventional, solid, and nonconventional stream classes in Aspen Plus.8 In our model, the stream class in which conventional and nonconventional solids are present but there is no particle size distribution is chosen as the global stream class. Kerogen is a type of nonconventional matter. Thus, ENTHGEN and DCHARIGT models are used to calculate the enthalpy and density of kerogen.17,18 Kerogen decomposition produces gases (CO, H2, H2O, CH4, CO2, and CnHm (n ≤ 4)) and oil components. These components are nonpolar or weakly polar types of matter. Redlich−Kwong (RK)-SOAVE is selected as the physical method.8 Figure 4 shows the Aspen Plus simulation calculation procedure and Aspen Plus simulation model of retort. According to the actual process, four Aspen Plus blocks are used to simulate the retort-drying stage and retorting stage. First, a RYield block is utilized to model the drying stage. Its temperature is 200 °C and pressure is 0.1 MPa. In addition, a Sep block is adopted to separate water from the dried shale by specifying split fractions. Its temperature is 200 °C and pressure is 0.1 MPa. Next, a RCSTR block is used for modeling of the shale retorting stage. The temperature and pressure of the RCSTR reactor are set to 500−550 °C and 0.1 MPa. The details of different block conditions and operations in the Aspen Plus model are given in Table 2. Before simulation, we made some simplifying assumptions, as follows: (1) Shale retorting and semi-coke combustion are in steady state, and operational conditions are not changed during simulation.14 (2) Decomposition of kerogen and thermobitumen occurs at the same time, while minerals, as inert matter, do not react.19,20 (3) Part of the elemental N and S from kerogen is left in the oil, while another part is converted into NH3 and H2S. (4) Fine-particles shale is mixed with hot recycle ash completely and quickly at the retorting module. (5) There is no pressure loss in the whole process.14

2. OIL SHALE COMPREHENSIVE REFINERY PROCESS A schematic diagram of the OSCR process is shown in Figure 3. The OSCR process consists of a GCSR unit, a DGSR unit, an OGS unit, a retorting gas steam reforming for hydrogen production (RGSR) unit, a shale oil hydrogenation (SOH) unit, and a building material (BM) unit. After grinding and screening, shale is separated into two types of particles, based on size. The particles bigger than 10 mm in diameter are sent into the GCSR unit, while the rest are fed into the DGSR unit. The oil−gas mixture retorted from these two units is fed into the OGS unit and separated into oil and retorting gas. Most of the retorting gas is recycled to heat shale for retorting in the GCSR unit, and the rest is used to synthesize hydrogen by steam reforming reaction in the RGSR unit. The heat needed for the RGSR unit is supplied by combustion of the semi-coke from the GCSR unit. The hydrogen is used for shale oil hydrogenation in the SOH unit to improve oil quality. In the DGSR unit, the semi-coke discharged from the retort is fed into the CFB furnace. Part of the ash from combustion of the semicoke is recycled back to the retort to heat shale; the rest is transported into the BM unit to prepare building materials. The OSCR process includes advantages of both the GCSR process and the DGSR process, mainly (1) high utilization of oil shale, (2) high yield of high-grade oil products, and (3) low production cost for unit oil with high corresponding benefit. A detailed quantitative analysis of the new process will be explained in following sections. 3. MODELING AND SIMULATION OF OIL SHALE COMPREHENSIVE REFINERY PROCESS This work uses Aspen Plus (version 7.2) software to model and simulate the OSCR process. Additionally, FORTRAN 11.0 and MATLAB 2014a are used to facilitate the modeling of kinetic 7788


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vaporization of internal moisture occurs and the surface temperature increases to 200 °C. Meanwhile, the internal temperature is about 150 °C. The dehydration reaction of particles is formulated as eq 1: shale (wet) → ΦH 2O + shale (dry)

(1)

According to mass balance, the vaporization moisture content mH2O is calculated by eq 2, m H2O = mshale,inMshale,in − mshale,out Mshale,out

(2)

where mshale,in and mshale,out denote the mass flow rate of shale before and after drying; Mshale,in and Mshale,out denote the moisture content of shale before and after drying. 3.1.2. Retorting Mechanism and Model. After drying, particles are heated continuously by gas/solid-heat-carrier. The heat is transferred to internal part and the dry shale started to be retorted to oil, gas, and semi-coke, as formulated in eq 3. shale (dry) → oil + gas + semi‐coke

In eq 3, “oil” represents the long-chain hydrocarbons, with molecular weights usually bigger than C6; “gas” refers to those components lighter than C4, including CO, H2, H2O, CH4, CO2, and CnHm (n ≤ 4); and “semi-coke” is defined as a mixture consisting of fixed carbon and shale ash. Jiang et al.14 and Lü et al.22 studied the mechanism of shale particles retorting. They reported that retorting also involves two stages. The first stage is decomposition of kerogen when the surface temperature is between 200 and 350 °C. In the second stage, the surface temperature is increased to 500−550 °C, and the main reaction is decomposition of bitumen to oil, gas, and C(S). However, the composition of the kerogen is more complex. Furthermore, there are several pyrolysis reactions for each component. According to the assumptions above, kerogen and bitumen are pyrolyzed simultaneously, as given in eq 4.

Figure 4. (a) Aspen Plus simulation calculation procedure and (b) Aspen Plus models of retort.

Table 2. Description of Block Conditions and Operations in Aspen Plus Model block ID

block type

DRYING

RYield with built-in MATLAB code

(1) RCSTR reactor converts dry shale into oil, gas, and semi-coke (2) temperature = 500/550 °C (3) pressure = 0.1 MPa

SEP

separator

(1) separates water/oil−gas from shale/ semi-coke by specifying split fractions (2) pressure = 0.1 MPa (3) SEP 1/2 temperature =200/500 (550) °C

mixer

+ C3H6 + C3H8 + H 2 + NH3 + H 2S + C(S)

(1) removes moisture content of oil shale (2) temperature = 200 °C (3) pressure = 0.1 MPa

RCSTR with builtin FORTRAN code

MIX

kerogen → oil + CO + CO2 + CH4 + C2H4 + C2H6

description

PYROLYS

(3)

(4)

The kinetic equation of the pyrolysis adopts a single reaction model referred to the work,23 as formulated in eq 5, dx /dt = k(1 − x)n

(5)

where x is the shale conversion rate, t is the reaction time, k is the reaction rate constant, and n is the order of reaction. According to the Arrhenius equation, the reaction constant k is expressed as follows:24 k = A exp( −E /RT )

(6)

where A is the apparent frequency factor, E is the apparent activation energy, R is the gas constant (R = 8.314 J/(mol·K)), and T is temperature. Syed et al.23 and Tiwari and Deo25 carried out pyrolysis experiments on shale particles at different heating rates. It was concluded that the heating rate has an influence on shale conversion and kinetic parameter E. Thus, the heating rate β should be included in the kinetic equation. Its expression is as follows:

(1) pressure = 0.1 MPa (2) heat loss = 0

3.1. Mechanism and Model. 3.1.1. Drying Mechanism and Model. Wang et al.21 investigated the effect of constant heating rate (3 °C/min) on surface and internal temperature of shale particles. It was found that particles drying process comprises of two stages. In the first stage, the surface temperature increases to 120 °C. The main reaction is vaporization of surface moisture. In the second stage, the

β = dT /dt

(7)

Equation 8 can be obtained by combining eqs 6 and 7 into eq 5: dx /dT = k(1 − x)n = (A /β) exp(−E /RT )(1 − x)n 7789

(8)


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Energy & Fuels Equations 9 and 10 can be obtained by using Coats−Redfern integration in eq 8: ln(− ln(1 − x)/T 2) = ln(AR /βE) − E /RT ,

Equation 15 can be obtained by using Coats−Redfern integration in eq 14: δ = δ(T , E) = 1 − exp[(−A /β )(RT 2/E) exp(− E /RT )]

n=1

(15)

(9)

Coats and Redfern thought that the activation energy E is a constant in a pyrolysis temperature range.30 However, each volatile light gas undergoes a number of reactions in the devolatilization process. Each reaction has a corresponding E. Therefore, we think E of the devolatilization model is presented as a continuous distribution. Merrick31 found that E follows a Rosin−Rammler equation, as shown in eq 16,

ln[(1 − (1 − x)1 − n )/(T 2(1 − n))] = ln(AR /βE) − E /RT ,

n≠1

(10)

Based on eqs 9 and 10, kinetic parameters E and A can be estimated by a graphical method. However, the Arrhenius equation is a nonlinear equation, so a graphical method cannot give exact values for E and A.26 Additionally, graphical methods are confronted with poor statistical characteristics.27 Lü et al.22 proposed an approximate Arrhenius temperature integral method. This method is accurate and used to simulate morecomplicated kinetic models. Lü proposed that the pyrolysis equation can be expressed using a first-order kinetic model. Its equation is formulated in eq 11, ln(1 − xi) = ln(1 − xi − 1) − γT + γT , i

i−1

Fk(E) = exp[− ((E − E0)/γ )φ ]

where E0 is the initial activation energy, and γ and φ are related to the volatiles. Equation 17 is the accumulation yield of the kth volatile, Yk(T ) = −Y k′

i = 1, 2, ...

where i is the number of iterations, and γT is a function of heating rate β and temperature T, as expressed in eq 12:

δ(T , E)Fk′(E) dE

(17)

0

(18)

To simplify the calculation, the upper integral limit ∞ in eq 17 can be substituted by E0 + 4σ. σ is the standard deviation of Gaussian activation, set according to ref 32. The simplified equation is expressed as eq 19,

(12)

Yk(T ) = −Y k′ × ⎧ E 0 + 4σ ⎨ ⎡⎣1 − exp[(−A /β )(RT 2/E) exp(−E /RT )]⎤⎦ × ⎩ E0

∫

⎫ ⎡⎣( −φ /γ )((E − E0)/γ )φ − 1 exp((E0 − E)/γ )φ ⎤⎦ dE⎬ ⎭ = Y k′δk′(T , E)

(19)

where γ is related to the volatile content V of oil shale, as shown in eq 20. γ = γ1 − γ2 ln V

(20)

The values of the γ1, γ2, and φ components are shown in Table 3. Table 3. Parameters of Devolatilization Model γ1 γ2 φ

CO

CO2

CH4

C2H4

H2

NH3

H2S

H2O

93 0 4

78 0 4

110 0 2

61 0 4

165 0 4

106 0 4

114 0 4

23.6 17.6 8

3.2. Process Simulation. The OSCR process includes a GCSR unit, a DGSR unit, an OGS unit, a RGSR unit, a SOH unit, and a BM unit. A simplified version is shown in Figure 5. Some key parameters are listed in Table 4.8,9,20,33−35 3.2.1. Gas Circulation Shale Retorting Unit. After being crushed and screened, large-particles shale bigger than 10 mm in diameter is fed into the gas-circulation-type retort and retorted to oil, gas, and semi-coke at 0.1 MPa and 500 °C. The elemental analyses of shale oil and semi-coke are shown in Table 5.10 Oil is assumed to be the complex compound,

(13)

where δ is the release rate of volatiles. Qian et al.10 studied the effect of heating rate on the yields of volatiles. The result was equally same as that observed for oil yield. The maximum yields of CO, H2, and CH4 were reached when the heating rate was 10 °C/min. Equation 14 can be obtained by substituting eq 7 in eq 13: dδ /dT = (A /β) exp(−E /RT )(1 − δ)

∞

Fk′(E) = ( −φ /γ )((E − E0)/γ )φ − 1 exp[(E0 − E)/γ ]φ

Campbell et al.28 studied the effect of heating rate β on oil yield. They found that oil yield increased gradually and then reached the highest value as heating rate increased from 5 to 10 °C/min. After that, oil yield decreased due to secondary decomposition during oil diffusion from the interior of particles. Thus, β is set to 10 °C/min. 3.1.3. Devolatilization Mechanism and Model. Bai et al.29 studied the devolatilization characteristics of the Huadian shale pyrolysis process. Their results showed that the release rates of CO, H2, CH4, CO2, and CnHm (n ≤ 4) are similar and coherent with a Gaussian distribution. As the surface temperature increased to 350 °C, light gases were released. The release rate increased slowly in the first stage and then quickly as the temperature ranged from 350 to 480 °C. After reaching a maximum value, the release rate declined as the temperature increased further from 480 to 550 °C. We consider that the devolatilization comprises several parallel reactions. For example, H2 is mainly produced by polycondensation and dehydrogenation reactions of free radicals and aromatic structure. CO results from from bond-breaking reactions of phenols, ether bonds, heterocyclic oxygen, and a small amount of short-chain fatty acids. In this paper, the diffusion of light gases from the interior to the surface of the particles is not taken into account. The devolatilization equation adopts an infinite number of independent parallel first-order reactions models, as formulated in eq 13, dδ /dt = k(1 − δ) = A exp(−E /RT )(1 − δ)

∫E

where Y′k is the final yield of the kth volatile. F′k is associated with γ, φ, and E, as given in eq 18:

(11)

γT = (A /β)(RT 2/E) exp[(−E /RT )/(1 + 2RT /E)]

(16)

(14) 7790


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Figure 5. Flow diagram of the OSCR process.

C14.62H10.46O0.78N0.02S0.07, based on its elemental analysis. In this paper, we use C14H10O to denote oil. Here the molecular formula C 14 H 10 O is insufficient to define the actual components of oil, but it is adequate to estimate the mass and energy balances of the whole process. In this work, a mathematical model including the retorting kinetic equation and devolatilization kinetic equation is used to calculate the yields of oil, volatile gases, and semi-coke, as described below. (1) The sets of all atoms Atj and pyrolysis products Vpk,pyr are defined as follows:

Table 4. Key Operational Parameters of the OSCR Process key parameters

value

GCSR unit [33] retorting temperature (°C) retorting pressure (MPa) flow rate of oil shale (t/h) flow rate of recycle gas (1) (km3/h) flow rate of recycle gas (2) (km3/h) OGS unit [9] washing tower temperature (°C) cooling tower temperature (°C) electrostatic tower temperature (°C) separation pressure (MPa) oil yield of electrostatic tower (%) SOH unit [35] hydrogenation temperature (°C) hydrogenation pressure (MPa) m (H2)/m (oil) a

key parameters

value

500

DGSR unit [8] retorting temperature (°C)

550

0.1

retorting pressure (MPa)

0.1

300

750

69.9

semi-coke combustion temperature (°C) flow rate of oil shale (t/h)

75

144.1

flow rate of air (t/h)

110

At j = {C, H, O, N, S}

99.95

, C3H8, H 2 , NH3, H 2S, H 2O, C14 H10O}

0.1

1.0

k = 1, 2, ..., 13

6

H2O/Ca (mol/mol)

4.0

390

BM unit [20] silica modulus

2.3

0.1

alumina modulus

1.3

0.004

lime saturation factor

0.9

90 43 43

(21)

650

Vpk,pyr = {semi‐coke, CO, CO2 , CH4 , C2H4 , C2H6, C3H6

83

13

C

H

O

N

S

9.90 5.13

5.90 33.73

0.10 2.58

1.10 3.61

13

∑ ∑ Al ,k wk ,pyr = bl l=1 k=1

l = 1, 2, ..., 13;

k = 1, 2, ..., 13

(23)

where Al,k denotes the model constants, wk,pyr denotes the final yields of pyrolysis products, and bl denotes the vector constants. The first five equations are the constraints for atom balance of C, H, O, N, and S, as formulated in eq 24 (below). Al,k is equal to Aj,k (l = j = 1, 2, ..., 5; k = 1, 2, ..., 13), representing the mass fraction of each atom in the pyrolysis products. In addition, bl is equal to bj, meaning the elemental analysis of shale.

Table 5. Elemental Analyses of Shale Oil and Semi-coke of the GCSR Unit (wt%) 83.00 54.95

(22)

where j and k denote each atom and pyrolysis product in the pyrolysis reaction. (2) Pyrolysis product yields are calculated as shown in eq 23,

C denotes the mixture of CH4, CO, and CO2.

shale oil semi-coke

j = 1, 2, ..., 5

RGSR unit [34] reforming reaction temperature (°C) PSA efficiency for hydrogen (%) hydrogen concentration (%) CaO/Ca (mol/mol)

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Energy & Fuels Chart 1. Matrix for Calculating the Yields of Pyrolysis Products in the GCSR Unit

5

13

the scale of a single gas-circulation-type retort is set to 12.5 t/h shale. The dehydration reaction occurs at the top of the retort. Equation 25 is used to estimate the heat of moisture vaporization, Qvap,

∑ ∑ (MWj ,k/MWk)wk ,pyr = bj j=1 k=1

j = 1, 2, ..., 5;

k = 1, 2, ..., 13

(24)

Q vap = m H2O[Cp,H2O(l)(100 − 50) + rH2O

where MW represents the molar weight. The 6th equation of eq 23 is used to estimate the yield of semi-coke. Referring to the work of Chi et al.,36 the kinetic parameter A for shale pyrolysis at 500 °C is set to 2.03 × 109 S−1 and E to 158 kJ/ mol. The shale conversion rate x is calculated on the basis of eqs 11 and 12. Thus, the final yield of semi-coke is 1 − x. The 7−13th equations of eq 23 are devolatilization kinetic equations for CO, CH4, C2H4, C2H6, C3H6, H2, and H2O, as shown in eq 19. According to ref 32, the kinetic parameter A is set to 2.03 × 109 S−1, E0 to 158 kJ/mol, and E0 + 4σ to 345 kJ/mol. (3) The matrix for calculating the yields of pyrolysis products is shown in Chart 1. Simulation data for the GCSR unit are shown in Table 6. The compositions of the pyrolysis products are close to the industrial compositions.17 In the GCSR unit, the heat for the shale retorting reaction is supplied by the recycle gas. The heat is mainly expended in (1) moisture vaporization, (2) oil shale heating, (3) oil shale pyrolysis, and (4) heat loss. According to the industrial practice,

+ Cp,H2O(g)(280 − 100)]

where mH2O represents the mass flow rate of moisture, Cp,H2O(l) and Cp,H2O(g) denote the specific heat capacities of water as liquid phase and gas phase, and rH2O denotes the latent heat of water vaporization. Referring to ref 37, Cp,H2O(l) and Cp,H2O(g) are set to 4.19 and 1.99 kJ/(kg·°C), respectively, and rH2O = 2261 kJ/kg. The simulated temperature of moisture out from the retort is 280 °C. Based on eq 25, Qvap = 409.2 × 104 kJ/h. Equation 26 is used for calculation of the heat Qhet for heating shale to pyrolysis temperature, Q het = (mshale − m H2O)Cp,shale(Tret − 50)

Q pyr = (mshale − m H2O)Q̇ pyr

industrial data

semi-coke CO CO2 CH4 C2H4 C2H6 C3H6 C3H8 H2 NH3 H2S H2O C14H10O

75.04 0.31 2.42 0.35 0.13 0.26 0.12 0.22 0.06 0.05 0.57 11.62 9.20

76.65 0.29 2.36 0.33 0.12 0.28 0.13 0.25 0.06 0.06 0.16 11.01 8.30

(27)

where Q̇ pyr is the unit pyrolysis heat, equal to 235.7 kJ/kg shale.9 Based on eq 27, Qpyr = 260.5 × 104 kJ/h. It is assumed that the heat loss Qloss accounts for 5% of the entire heat. Thus, the retorting heat Qret is equal to the sum of Qvap, Qhet, Qpyr, and Qloss:

mass fraction (%) simulation data

(26)

where mshale is the mass flow rate of shale, Cp,shale is the specific heat capacity of shale, and Tret is the pyrolysis temperature. According to ref 38, Cp,shale of Huadian shale is set to 1.05 kJ/ (kg·°C). After calculation, Qhet = 522.2 × 104 kJ/h. The calculation of the pyrolysis heat Qpyr is expressed in eq 27,

Table 6. Comparison between Simulation and Industrial Data of the GCSR Unit

component

(25)

Q ret = Q vap + Q het + Q pyr + Q loss

(28)

After calculation, Qret = 1251.5 × 10 kJ/h. The retorting heat is supplied by recycle gas, which consists of two parts: recycle gas (1) heated by hot semi-coke, and recycle gas (2) heated by burning a part of the recycle gas. In this work, the volume flow rates of these two streams of recycle gas are estimated on the basis of heat balance. After retorting, the semicoke is discharged from the bottom of the retort to exchange heat with recycle gas (1). The temperature of the semi-coke 4

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Energy & Fuels decreases from 500 to 180 °C. Equation 29 gives the recovery heat Qrev from semi-coke and can be used to calculate the volume flow rate of the recycle gas (1), Vgas(1):

mol.10 According to eqs 11 and 12 and eqs 19−24, the calculation matrix is made as shown in Chart 2. Simulated data for the DGSR unit are shown in Table 8. The values are verified by comparing with industrial data.

Q rev = msemi‐cokeCp,semi‐coke(Tret − 180) 2 1 = Vgas(1)(Cp,gas(1) × 500 − Cp,gas(1) × 40)

Table 8. Comparison between Simulation and Industrial Data of the DGSR Unit

(29)

where m semi‑coke is the mass flow rate of semi-coke, Cp,semi‑coke is the specific heat capacity of semi-coke, and C1p,gas (1) and C2p,gas (1) denote the specific heat capacities of recycle gas (1) at 40 and 500 °C, which are 1.84 and 2.30 kJ/(m3·°C), respectively.9 Based on eq 29, Vgas(1) = 2913.8 m3/h. The heat of the recycle gas (2), Qtran, is supplied by burning part of the recycle gas in a gas furnace. The temperature of the recycle gas (2) increases from 120 to 700 °C. Equation 30 is used to estimate the volume flow rate of the recycle gas (2), Vgas(2),

mass fraction (%)

2 Q tran = Q ret − Q rev = Vgas(2)(Cp,gas(2) × 700 1 − Cp,gas(2) × 120)

C1p,gas(2)

(30)

C2p,gas(2)

where and denote the specific heat capacities of recycle gas (2) at 40 and 500 °C, which are 1.88 and 2.55 kJ/ (m3·°C), respectively.9 Based on eq 30, Vgas(2) = 6003.9 m3/h. 3.2.2. Dagong Shale Retorting Unit. Fine-particles shale, with particles smaller than 10 mm in diameter, is sent to the Dagong-type retort, where shale is retorted to oil, gas, and semi-coke at 0.1 MPa and 550 °C. The elemental analyses of oil and semi-coke are shown in Table 7.10 Based on elemental analysis of oil, its molecular formula is assumed as C14.62H10.90O1.64N1.64S0.03, which is approximately equal to C14H10O.

shale oil semi-coke

H

O

N

S

82.62 54.38

4.95 6.01

10.00 35.89

1.63 0.50

0.80 3.22

simulation data

industrial data

semi-coke CO CO2 CH4 C2H4 C2H6 C3H6 C3H8 H2 NH3 H2S H2O C14H10O

78.23 0.43 2.90 0.36 0.16 0.27 0.07 0.12 0.14 0.02 0.03 7.57 9.70

79.10 0.41 3.05 0.34 0.15 0.28 0.06 0.11 0.15 0.03 0.05 7.28 8.99

The semi-coke is discharged from the bottom of the retort and combusted in the CFB furnace. Part of the ash from the semi-coke combustion is recycled to supply heat for the retorting reaction. In this paper, we calculate the heat balance of the CFB furnace and estimate the mass flow rate of the recycle ash. First, some assumptions are made: (1) The flue gas includes CO2, SO2, N2, and H2O, because C is converted to CO2, H to H2O, N and N2 of air to N2, and S to SO2 completely. (2) Excess air coefficient is set to 1.2. (3) Semi-coke combustion efficiency is set to 95%. Semi-coke combustion needs air. Equation 31 is used to calculate the theoretical volume flow rate of air, V0air,

Table 7. Elemental Analyses of Shale Oil and Semi-coke of the DGSR Unit (wt%) C

component

The calculation of product yields of the DGSR unit refers to that of the GCSR unit. As the retorting reaction temperature of the Dagong-type retort is 550 °C, kinetic parameter A is set to 5.46 × 1012 S−1, E0 to 183 kJ/mol, and E0 + 4σ to 370.5 kJ/

0 V air = 8.89(ωC + 0.375ωS) + 26.5ωH − 3.33ωO

(31)

where wC, wH, wO, and wS are taken from the elemental analysis of the semi-coke. Considering the excess air coefficient α,

Chart 2. Matrix for Calculating the Product Yields of the DGSR Unit

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Energy & Fuels calculation of the practical volume flow rate of air is formulated as eq 32. 0 Vair = αV air

Q flue + Q ash = Q carr + Q comb

Based on eqs 38−44, mrec = 322.5 t/h. The mass ratio of the recycle ash to the input shale is 4.3. 3.2.3. Oil−Gas Separation Unit. As compositions of the oil−gas mixture from the GCSR unit are similar to that of the DGSR unit, these two units could use the same OGS unit. The flow diagram of the OGS unit is shown in Figure 6. The oil−gas

(32)

Equations 33−36 are used to calculate the mass flow rates of the combustion products: mCO2 = msemi‐cokeωC(MWCO2/MWC)

(33)

mSO2 = msemi‐cokeωS(MWSO2/MWS)

(34)

(44)

m N2 = msemi‐cokeω N + 0.79msemi‐cokeVair(MWN2/22.4) (35)

m H2O = msemi‐cokeωH(MWH2O/2MWH)

(36)

After combustion, the remaining O2 in air is calculated by eq 37. mO2 = 0.21(α − 1)(Vair /22.4)MWO2

(37)

The mass flow rate of the semi-coke, msemi‑coke, is 58.6 t/h. Based on eqs 33−36, the mass flow rates of the combustion products CO2, SO2, N2, and H2O are calculated to be 19.9, 0.9, 62.1, and 3.7 t/h, respectively. In addition, the mass flow rate of the remaining O2 is 4.7 t/h. The heat of flue gas Qflue is equal to the sum of the heat of gaseous water, QH2O, and the heat of other gases, Qother, as shown in eq 38. Q flue = Q H O + Q other 2

Figure 6. Flow diagram of the OGS unit.

mixture flows into the washing tower, where it is countercurrently contacted and washed by recycle water. The oil− water mixture from the washing tower is sent to the oil−water separation tank. The separated water is recycled back to the washing tower. In the washing process, the acid gases, such as CO2, H2S, and NH3, are dissolved in water. The main reactions are as follow:

(38)

The heat carried by gaseous water is calculated by eq 39, Q H O = m H2O[Cp,H2O(l)(100 − 25) + rH2O 2

+ Cp,H2O(g)(Tfurn − 100)]

(39)

where Tfurn denotes the temperature of the CFB furnace. The heat carried by other gases is calculated by eq 40, Q other =

∑ mζ Cp,ζ(Tfurn − 25)

(40)

CO2 + H 2O ↔ H3O+ + HCO3−

(46)

HCO3− + H 2O ↔ H3O+ + CO32 −

(47)

−

H 2S + H 2O ↔ H3O + HS

(48)

HS− + H 2O ↔ H3O+ + S2 −

(49)

NH3 + H 2O ↔ NH4 + + OH−

(50)

HCO3−

(51)

−

+ NH3 ↔ NH 2COO + H 2O

The oil−gas mixture from the washing tower is fed into the indirect cooling tower, in which the oil−gas mixture is condensed by the cooling water to separate oil and gas. After heat exchange, the hot water is sent to the cooling tower, where it is cooled to 30 °C. The oil−gas mixture from the indirect cooling tower is sent to the electrostatic oil separator to obtain more oil. In this work, the washing tower employs a plate column on which mass transfer and heat transfer in the gas phase and liquid phase occur. Equation 52 is used to calculate the diameter D of the washing tower,

(41)

The input heat of the CFB furnace involves the heat of semicoke and recycle ash, Qcarr, and the heat released from the semicoke combustion, Qcomb. Equations 42 and 43 are used for the calculation of Qcarr and Qcomb:

D = (4Vs/πu)0.5

Q carr = (msemi‐cokeCp,semi‐coke + mrecCp ,ash)(Tret − 25)

(52)

where Vs is the volume flow rate of oil−gas mixture and u is the superficial velocity, as expressed in eq 53, u = 0.8umax (53)

(42)

Q comb = msemi‐cokeηHL,semi‐coke

(45)

+

where ζ indicates CO2, SO2, N2, and O2. Their specific heat capacities are 1.11, 0.78, 1.10, and 1.02 kJ/(kg·K), respectively. After calculation based on eqs 38−40, Qflue = 943.4 × 105 kJ/h. The output heat of the CFB furnace includes the heat of flue gas and the heat of ash. The output ash is comprised of two parts. One is the recycle ash, and the other is generated from the semi-coke combustion. The heat of the ash Qash is estimated by eq 41. Q ash = (mash + mrec)Cp,ash(Tfurn − 25)

2H 2O ↔ H3O+ + OH−

(43)

where mrec is the mass flow rate of the recycle ash, Tret is the retention temperature, η is the semi-coke combustion coefficient, and HL,semi‑coke is the low-heating value of semicoke. Referring to the work of Lin et al.,39 HL,semi‑coke = 3485.9 kJ/kg. The heat balance of the CFB furnace is given as follows:

where umax is the maximum superficial velocity, as defined in eq 54, umax = ψ ((ρL − ρV )/ρV )0.5 7794

(54) DOI: 10.1021/acs.energyfuels.6b01279 Energy Fuels 2016, 30, 7786−7801

Article

Energy & Fuels Table 9. Simulation of the OGS Unit stream

1

2

3

4

5

6

7

8

9

H2O (t/h) gas (t/h) oil (t/h) mass flow (t/h) temp (°C) pressure (MPa)

40.91 15.38 37.30 93.59 360 0.1

170.45 15.07 15.67 201.19 90 0.1

310.06 0.31 21.63 332.00 90 0.1

4.13 15.07 6.72 25.92 43 0.1

166.32 0.00 8.95 175.27 43 0.1

439.60 0.00 0.00 439.60 74 0.1

946.76 0.00 0.00 946.76 30 0.1

4.13 15.07 4.48 23.68 43 0.1

0.00 0.00 32.82 32.82 72 0.1

Figure 7. Mass and heat balances of the RGSR unit.

where ρL and ρV are the densities of water and oil−gas mixture, equal to 1000 and 0.31 kg/m3, respectively. In addition, ψ is the load factor, set to 0.08.40 Based on eqs 52−54, D = 4 m. From the diameter, we can calculate the volume flow rate of the washing water VL by following eq 55, VL = ρspr π (D/2)2

of H2. However, it is inevitable to release CO2 in the MSR process, which impacts the environment. To decrease the amount of CO2 emissions, it is important for industries to develop means to capture CO2. In situ capture of CO2 from the MSR process has advantages of sequestrating CO2 and increasing CH4 conversion and H2 yield. Xiu et al.41 proposed a sorption-enhanced methane steam reforming (SEMSR) process. A kind of sorbent is sent to the reforming reactor to capture CO2, and simultaneously the thermodynamic equilibrium is shifted to the product side. Chen et al.42 investigated the absorption performance of four different sorbents: CaO, Li4SiO4, Li2ZrO3, and HTC. Chen pointed out that the CO2 absorption capacity and absorption rate of CaO are better than those of other sorbents. In this work, CaO is chosen as a sorbent. The RGSR unit mainly includes a SEMSR reactor, a CaO regenerator (CaOR), and a pressure swing adsorption device (PSA). The remaining gas is first fed into the SEMSR reactor, where four reactions occur. CH4 is converted to CO/CO2 and H2 by a methane steam reforming reaction, as eqs 56 and 57. CO is converted to CO2 and H2 by a water gas shift reaction, as eq 58. CaO works as a sorbent to capture CO2 by a carbonation reaction, as eq 59.

(55)

where ρspr is the spray density. According to Liu et al.,40 ρspr = 35 m3/(m2·h). After calculation, the mass flow rate of the washing water is 439.6 t/h. In this paper, a RadFrac block is used to model the washing tower. The number of trays is set to 60. The washing water is fed into the washing tower from the first tray. Its mass flow rate is 439.6 t/h, temperature is 74 °C, and pressure is 0.1 MPa. The oil−gas mixture is fed at the bottom of the tower. Its mass flow rate, temperature, and pressure are 52.7 t/h, 360 °C, and 0.1 MPa, respectively. Additionally, the pressure drop of the tower is set to 0. The indirect cooling tower is modeled by “heater” and “flash” modules. The temperature and pressure drop of the heater module are set to 45 °C and 0. There is no heat duty and pressure drop in the flash module. According to the work of Qian et al.,10 the electrostatic oil separator can obtain 6% oil of the total. Thus, a “sep” block is employed to model the electrostatic oil separator by specifying split fractions. The simulation of the OGS unit is shown in Table 9. It is seen that the washing tower can obtain 58% of the total oil, and the indirect cooling tower has 34% of the oil. 3.2.4. Retorting Gas Steam Reforming Unit. The retorting gas from the OGS unit is divided into two parts: recycle gas and remaining gas. The recycle gas is used to supply heat for the retorting reaction of the GCSR unit. The remaining gas is used to synthesize hydrogen in the RGSR unit. Methane steam reforming (MSR) is a major route for the industrial production

CH4 + H 2O(g) ↔ CO + 3H 2

ΔH = + 206 kJ/mol (56)

CH4 + 2H 2O(g) ↔ CO2 + 4H 2 ΔH = + 165 kJ/mol (57) CO + H 2O(g) ↔ CO2 + H 2

ΔH = − 41 kJ/mol

CO2 + CaO ↔ CaCO3

ΔH = − 178 kJ/mol (59)

(58)

The reforming gas generated from the SEMSR reactor is fed into the PSA device to separate H2 and other gas. CaCO3 from the SEMSR reactor is sent to the CaOR for CaO regeneration. The heat of CaCO3 calcination is supplied by combustion of 7795


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two processes have the same feedstock input, 375.0 t/h shale. Because the new OSCR process includes a DGSR unit, a RGSR unit, and a SOH unit, this increases utilities consumption. However, the degree of increase is small: only 1.2 times the conventional GCSR process’s consumption. This is mainly because that the additional units, including the RGSR unit and the SOH unit, can achieve self-heat-balance and do not consume extra utilities. In addition, the GCSR process need to combust 108.9 t/h fuel gas to supply heat for the retorting reaction. The oil production totals of the GCSR process and the OSCR process are 25.8 and 32.8 t/h, respectively; their oil yields (the ratio of oil production to input shale consumption) are 6.5% and 8.8%. The oil yield of the OSCR process increases by 2.3%. This is mainly because that the OSCR process effectively uses 75 t/h fine-particles shale from the GCSR process. In addition, the OSCR process produces high-quality oil, because the process makes full use of the remaining gas for hydrogen production. Hydrogen can be used for oil hydrogenation. The OSCR process can implement the efficient utilization of the semi-coke dismissed by the GCSR process. Thereby, 225.1 t/h semi-coke is combusted to supply heat for the RGSR unit of the OSCR process. Ash from semi-coke combustion is 236.3 t/h. In this work, ash utilization is not considered.

the semi-coke from the GCSR unit. Its calcination reaction is formulated in eq 60. CaCO3 ↔ CO2 + CaO

ΔH = + 178 kJ/mol

(60)

In this work, an RGibbs block is used to model the SEMSR reactor according to minimum Gibbs free energy. It temperature is 650 °C and pressure is 1.5 MPa. In addition, the mole ratio of CaO to carbon (CH4, CO, and CO2) is set to 1, and that of steam to carbon is set to 4.34,43−45 The methane steam reforming reaction is an endothermic reaction, while the carbonation reaction is an exothermic reaction. Thus, the heat of the carbonation reaction can be used for heating the methane steam reforming reaction. Other heat is supplied by hot flue gas generated from the CaOR reactor. Hot flue gas is sent to the waste-heat recovery boiler to generate mediumpressure steam which is used for the methane steam reforming reaction. The ADSIM module is used to model the PSA device. The recovery rate of H2 is set to 83%,46 and the concentration of H2 is 99.95%. The mass and heat balances of the RGSR unit are shown in Figure 7. In the SEMSR reactor, 2.9 t/h remaining gas is reacted at 650 °C and 1.5 MPa with 4.9 t/h steam and 3.8 t/h CaO to produce 0.2 t/h H2. In the CaOR reactor, 4.4 t/h CaCO3 is calcined to regenerate CaO. The heat of calcination is mainly supplied by the combustion of 225 t/h semi-coke and 411 m3/ h air. The steam out of the SEMSR reactor is used to generate electricity by a steam turbine. Electricity is used for remaining gas compression and water compression. The heat for HE1, HE2, HE3, HE6, and HE8 is supplied by the heat recovery of HE4, HE5, and HE7. Thus, the energy of the RGSR unit is selfbalanced. 3.2.5. Shale Oil Hydrogenation Unit. The oil from the OGS unit is mixed with hydrogen generated from the RGSR unit. The mixing stream is heated by the furnace and then sent to the hydrogenation reactor, in which a desulfurization reaction, a denitrogenation reaction, and a hydrogenation reaction occur at the same time. The products from the hydrogenation reactor are sent into the high-pressure separator to separate gaseous and liquid phases. Most of the gas is recycled back, while the low-nitrogen oil is sold as a product. The detailed modeling and simulation of the SOH unit are referred to our previous works.33,47 After simulation, the production of the low-nitrogen oil is 32.8 t/h. 3.3. Simulation Results. The input−output of the GCSR process and the OSCR process is shown in Table 10. These

4. ECONOMIC PERFORMANCE ANALYSIS Economic analysis takes 1 t oil production as the evaluation base. In this section, we use three economic indexes: total capital investment (TCI), production cost (PC), and return on investment (ROI). 4.1. Total Capital Investment Analysis. A large budget is paid for purchase and installation of equipment. Fixed capital investment is used to buy plant facilities, while working capital is used to maintain operation. The sum of fixed capital and working capital is total capital investment. In this paper, the calculation of equipment investments of the GCSR process and the OSCR process follows eq 61,48 I2 = I1(S2/S1)sf

where I1 and S1 are the equipment investment and the production scale of the reference project, I2 and S2 are the equipment investment and the production scale of the practical project, and sf is the scale exponent; sf is a common value of chemical processes and fixed at 0.6. The equipment investment of the OSCR process mainly includes that of the GCSR, the DGSR, the RGSR, and the SOH units. The reference scale and reference equipment investment of the GCSR are estimated from ref 20, and that of the DGSR from ref 50, that of the RGSR from ref 49, and that of the SOH from ref 35. Details of scale and equipment investment are listed in Table 11. Based on Table 11, the practical equipment investments for the GCSR process and the OSCR process, calculated from eq 61, are 320.9 × 106 and 530.1 × 106 CNY, respectively. Others in the fixed investment are equivalent with the equipment investment multiplied by corresponding factors (RF), as shown in Table 12.51,52 Moreover, the working capital is fixed to 20% of the total capital investment.53 Calculation of the TCI can be formulated by eq 62,54

Table 10. Input−Output of the GCSR Process and the OSCR Process component

GCSR

OSCR

Input shale (t/h) electricity (MW) steam (MW) fuel gas (t/h)

oil (t/h) fine shale (t/h) semi-coke (t/h) ash (t/h) flue gas (km3/h)

375.0 5.7 15.3 109.8 Output 25.8 75.0 225.1 183.3

(61)

375.0 7.0 18.6

32.8

236.3 456.8

TCI = EI(1 + 7796

∑ RF)i

(62) DOI: 10.1021/acs.energyfuels.6b01279 Energy Fuels 2016, 30, 7786−7801

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Energy & Fuels

of the simulation. Raw materials and utilities prices are the average prices in 2014, as shown in Table 13.8,20 The

Table 11. Benchmark Case for Equipment Investments unit

benchmark

S1

GCSR DGSR RGSR SOH

shale input shale input hydrogen output oil input

343 t/h 33 t/h 668 kmol/h 10 t/h

I1 (CNY)

ref

× × × ×

20 50 49 35

3.0 1.1 1.1 2.1

108 108 108 107

Table 13. Assumptions for Calculation of Production Cost component (1) Raw materials cost (2) Utilities cost

where EI is the equipment investment, RFi is the ratio of other fixed investment to the equipment investment, and i is the component of the investment.

(3) Operating & maintenance (3.1) Operating labor (3.2) Direct supervisory and clerical labor (3.3) Maintenance and repairs (3.4) Operating supplies (3.5) Laboratory charge (4) Depreciation (5) Plant overhead cost (6) Administrative cost (7) Distribution and selling cost (8) Production cost

Table 12. Ratio Factors of Capital Investment component (1) Direct investment (1.1) Equipment (1.2) Installation (1.3) Piping (1.4) Instrumentation and controls (1.5) Electrical (1.6) Land (1.7) Buildings (including services) (2) Indirect investment (2.1) Engineering and supervision (2.2) Construction (2.3) Contractors’ fees (2.4) Contingency (3) Fixed capital (4) Working capital (5) Total capital

RF (%) 100 10 15 15 10 6 25

economic assumption shale, 120 CNY/t; fuel gas, 75 CNY/t H2O, 2 CNY/t; electricity, 0.5 CNY/kWh; steam, 42 CNY/GJ GCSR, 140 labors; OSCR, 400 labors; 100,000 CNY/labor/year 10% of operating labor 2% of fixed capital investment 0.7% of fixed capital investment 10% of operating labor lifetime, 20 years; salvage value, 4% 5% (3.1 + 3.2 + 3.3) 2% of production cost 2% of production cost (1) + (2) + (3) + (4) + (5) + (6) + (7)

calculation of the OSCR process’s operating labor includes the labors of the GCSR unit, DGSR unit, RGSR unit, and SOH unit. The labors of the GCSR unit and the DGSR unit have been calculated in our previous work,8 where we take 10% of total production cost to calculate operating labor.55 The final results are that the GCSR unit needs 140 labors with the processing capacity of 2.4 Mt/y shale, and the DGSR unit needs 160 labors with the processing capacity of 0.6 Mt/y shale. In addition, the labors of the RGSR unit and the SOH unit are estimated from ref 47, indicating that the RGSR unit requires 70 labors and the SOH unit requires 30 labors. A straight-line method is adopted to calculate the depreciation cost under the assumption of 20 years lifetime and 4% salvage value. Other costs of production cost are calculated depending on some assumptions, as explained in Table 13.55 Production cost is calculated on the basis of eq 63,

15 25 20 15 246 50 296

After calculation based on eq 62, the TCI values of the GCSR process and the OSCR process are 1027.0 × 106 and 1696.3 × 106 CNY, respectively; their investments for producing 1 t of oil are 4946.9 and 6462.1 CNY/t. The new OSCR process includes the DGSR unit, the RGSR unit, and the SOH unit. These additional units need much more equipment investment. Meanwhile, the capacity of the OGS unit is 1.2 times that of the conventional GCSR process. This increases the capital investment of the OSCR process. Therefore, the TCI of the OSCR process increases by 31% compared to the GCSR process. The breakdowns of the TCI are shown in Figure 8. 4.2. Production Cost Analysis. Production cost includes raw materials cost, utilities cost, and others. In this paper, the raw materials cost and utilities cost are calculated on the basis

PC = C R + C U + CO&M + C D + C PO + CA + C DS (63)

where CR is the raw materials cost, CU is the utilities cost, CO&M is the operating and maintenance cost, CD is the depreciation cost, CPO is the plant overhead cost, CA is the administrative cost, and CDS is the distribution and selling cost. The production costs of the two processes are calculated following eq 63 and shown in Figure 9. It is seen that the production costs of the GCSR process and the OSCR process are 3319.1 and 2594.3 CNY/t/y, respectively. The production cost of the OSCR process decreases by 22% compared to that of the conventional GCSR process for several reasons: (1) The OSCR process has a higher utilization of oil shale. It consumes 11 t of shale to produce 1 t of oil, while the GCSR process consumes 15 t of shale. (2) The GCSR process has to burn fuel gas to supply heat for the retorting reaction, while the OSCR process does not require outsourcing fuel gas. (3) The OSCR process increases the oil yield to 1.4 times that of the GCSR process. This big increase is requires only a small utilities cost. (4) For the big increase in oil yield, the depreciation cost and the operating and maintenance cost of the OSCR process will not increase much along, with the increase of the capital investment.

Figure 8. Total capital investments of the GCSR and the OSCR processes. 7797


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Figure 9. Production costs of the GCSR and the OSCR processes.

Figure 10. Return on investment of the GCSR and the OSCR processes.

4.3. Return on Investment Analysis. The fluctuations of crude oil price would inevitably influence the fluctuations of shale oil price. In this work, we consider two scenarios of shale oil price. One adopts an average price of shale oil in 2014, 3600 CNY/t, and the other adopts a low price of shale oil, 2250 CNY/t (according to low price of crude oil, 50 $/bbl). For the OSCR process, the stability of shale oil is increased, because amount of nitrogen of shale oil is removed. Thus, the lownitrogen oil price is 500 CNY/t higher than that of the highnitrogen oil price. The incomes of the GCSR process and the OSCR process are 789.4/509.1 × 106 and 1076.3/723.3 × 106 CNY/y, respectively. The details of the calculation are shown in Table 14. Total profit is equal to the income minus the production cost. Thus, the total profits of the GCSR process and the OSCR process are 100.3/−179.9 × 106 and 395.3/42.3 × 106 CNY/y, respectively. Total net profit is equivalent with the total profit minus the sales tax. The sales tax includes urban construction tax, education surcharge, and resource tax. In this paper, we assume that the urban construction tax accounts for 0.9% of the income, and education surcharge is for 0.5% of the income. The resource tax is fixed to 2 CNY/t. The total net profits of the two processes are 83.6/−192.8 × 106 and 374.6/ 26.5 × 106 CNY/y, respectively. Return on investment is the ratio of total net profit over the total capital investment.56 The ROIs of the GCSR process and the OSCR process are 8.1/− 18.8% and 22.1/1.9%, respectively, as shown in Figure 10. For the average shale oil price scenario, the GCSR and the OSCR processes are both profitable. The ROI of the OSCR process increases by 14% compared to that of the GCSR process. This is because that the OSCR process can produce more than 5.5 × 104 t oil. In addition, the OSCR process produces high-quality oil increasing the income much. Thus, the OSCR process is considered to be a promising option for

further use of oil shale. For the low shale oil price scenario, the GCSR process is unprofitable, while the OSCR process is still profitable although the profit space is small.

5. CONCLUSIONS Based on the current exploitation and utilization of oil shale, a comprehensive new oil shale refinery process was proposed. The new process involves a GCSR unit, where large particles (10−75 mm) are retorted to oil−gas mixture and semi-coke; a DGSR unit, where fine particles (0−10 mm) are retorted to oil−gas mixture; an OGS unit, where the oil−gas mixture is separated to oil and retorting gas, and the gas is mostly recycled to provide heat for the retorting reaction; a RGSR unit, where the remaining gas is used to produce hydrogen, and heat balance of the entire unit is achieved by combustion of the semi-coke; and a SOH unit, where the hydrogen is used for oil hydrogenation to improve the quality of the oil products. Compared with the conventional GCSR process, the new process has a higher utilization of shale, a higher oil yield, and high-quality oil. In this paper, the new process was designed and modeled. Based on the simulation, a techno-economic analysis was estimated. The results show that resource and energy utilization efficiency of the new process have been effectively improved. The capital investment of the new process is 6462.1 CNY/t, which is 1.3 times that of the conventional GCSR process. However, the new process’s ROI is 14% higher than that of the conventional process. This reflects the advantages of the new process in terms of technical and economic performance.

Table 14. Incomes of the GCSR and the OSCR Processes GCSR item high-nitrogen oil (t) low-nitrogen oil (t) fine shale (t) semi-coke (t)

price (CNY) b

c

3600 /2250 4100d/2750d 40 10

OSCR

production

incomea

20.8 × 104

747.4/467.1

60.0 × 10 18.0 × 105 4

total income

production

incomea

26.3 × 104

1076.3/723.3

24.0 18.0 789.4/509.1

1076.3/723.3

a

The unit of income is 106 CNY/y. bAverage price of oil in 2014. cLow price of oil at 50 $/bbl. dThe price of low-nitrogen oil is 500 CNY/t higher than that of high-nitrogen oil. 7798


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Vgas(1) = volume flow rate of recycle gas (1), m3/h Vgas(2) = volume flow rate of recycle gas (2), m3/h Vair = volume flow rate of air, m3/h Vs = volume flow rate of oil−gas mixture, m3/h VL = volume flow rate of washing water, m3/h wk,pyr = yield of the kth pyrolysis product, wt% x = shale conversion rate, wt% Yk = accumulation yield of the kth volatile component, % Yk′ = final yield of the kth volatile component, %

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-20-87112056. Fax: +86-18588887467. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful for the National Basic Research Program (2014CB744306) and financial support from the China NSF projects (21676101).

Abbreviations

BM = building materials CFB = circulating fluidized bed CaOR = CaO regenerator DGSR = Dagong shale retorting OSCR = oil shale comprehensive refinery GCSR = gas-circulation shale retorting MSR = methane steam reforming MIXCINC = both conventional and nonconventional solids are present, but there is no particle size distribution OGS = oil−gas separation PSA = pressure swing adsorption PC = production cost RGSR = retorting gas steam reforming RCSTR = continuous stirred-tank reactor ROI = return on investment SOH = shale oil hydrogenation SEMSR = sorption enhanced methane steam reforming TCI = total capital investment

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NOMENCLATURE A = apparent frequency factor At = all atom set in pyrolysis reaction b = vector constant Cp = specific heat capacity, kJ/(kg·°C) CR = raw material cost, CNY/y CU = utilities cost, CNY/y CO&M = operating and maintenance cost, CNY/y CD = depreciation cost, CNY/y CPO = plant overhead cost, CNY/y CA = administrative cost, CNY/y CDS = distribution and selling cost, CNY/y D = diameter of washing tower, m E = apparent activation energy, kJ/mol EI = equipment investment, CNY HL,semi‑coke = low-heating value of semi-coke, kJ/kg I = project investment, CNY k = reaction rate constant, kmol/(m3·s·Pa) mshale = mass flow rate of shale, kg/h msemi‑coke = mass flow rate of semi-coke, kg/h mH2O = mass flow rate of moisture, kg/h mrec = mass flow rate of recycle ash, kg/h mCO2 = mass flow rate of CO2, kg/h mSO2 = mass flow rate of SO2, kg/h mN2 = mass flow rate of N2, kg/h mO2 = mass flow rate of O2, kg/h mash = mass flow rate of ash, kg/h Mshale = moisture content of shale, wt% MW = molar weight, g/mol Qvap = heat of moisture vaporization, kJ/h Qhet = heat of heating dry shale to pyrolysis temperature, kJ/ h Qpyr = shale pyrolysis heat, kJ/h Q̇ pyr = unit pyrolysis heat, kJ/kg Qloss = heat loss, kJ/h Qrev = heat recovered by recycle gas (1), kJ/h Qtran = heat supplied by recycle gas (2), kJ/h Qflue = heat of flue gas, kJ/h Qash = heat of ash, kJ/h Qcarr = heat carried by semi-coke and recycle ash, kJ/h Qcomb = semi-coke combustion heat, kJ/h rH2O = latent heat of water vaporization, kJ/kg R = gas constant, J/(mol K) RFi = ratio factor of capital investment of component i S = production capacity t = reaction time, s Tret = retorting temperature, °C Tfurn = CFB furnace temperature, °C u = superficial velocity, m/h Vp = pyrolysis products set in pyrolysis process

Subscripts

carr = carry comb = combustion flue = flue gas furn = furnace gas = recycle gas het = heating pyr = pyrolysis ret = retorting rev = recovery rec = recycle tran = transformation spr = spray vap = vaporization Superscripts

n = reaction order sf = scale factor Greek letters

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α = excess air coefficient β = heating rate, °C/min Φ = release rate of moisture, % δ = release rate of volatiles, % σ = standard deviation of Gaussian activation distribution, kJ/mol η = combustion efficiency, % ψ = load factor, m/h ρ = density, kg/m3

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Conceptual Design of an Oil Shale Comprehensive Refinery Process

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