Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis

Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis

Accepted Manuscript Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis Mao Mu, Xiangxi...

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Accepted Manuscript Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis

Mao Mu, Xiangxin Han, Xiumin Jiang PII:

S0360-5442(18)30509-7

DOI:

10.1016/j.energy.2018.03.100

Reference:

EGY 12558

To appear in:

Energy

Received Date:

04 February 2018

Accepted Date:

18 March 2018

Please cite this article as: Mao Mu, Xiangxin Han, Xiumin Jiang, Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis, Energy (2018), doi: 10.1016/j.energy.2018.03.100

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ACCEPTED MANUSCRIPT

Combined fluidized bed retorting and circulating fluidized bed combustion system of oil shale: 3. Exergy analysis Mao Mu, Xiangxin Han, Xiumin Jiang Institute of Thermal Energy Engineering, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Abstract Exergy analysis as well as energy analysis is applied to Chinese comprehensive utilization system, in which oil shale is firstly introduced to fluidized bed (FB) retort for obtaining oil and resulting semicoke is fed to circulating fluidized bed (CFB) reactor for further utilization. During the calculation, linear programming helps optimize the efficiency of FB retort in the system. In the light of the results, the process flow diagram of the whole system is redrawn. Also, this paper discusses how three operating parameters (the retorting temperature, the mass of burned fuel gas and the temperature of circulating ash) influence the system, especially exergy efficiency. With the retorting temperature increasing, the exergy efficiency first increase and then decrease, and the highest exergy efficiency is in the temperature range 460-490 oC. Compared with retorting temperature, the mass of burned fuel gas and circulating ash temperature have less effect on the exergy efficiency of the whole system. But it is a good strategy for stable operation of the system to burn more fuel gas to provide more energy and exergy to FB retorting unit. This work could give further detailed suggestions and more reference data to operate the system stably and efficiently.

*

To whom correspondence should be addressed. E-mail: [email protected]

ACCEPTED MANUSCRIPT Key words: oil shale; retorting; combustion; exergy; energy; linear programming

ACCEPTED MANUSCRIPT 1

1. Introduction

2

For its giant reserve, oil shale, a substitute of crude, has been utilized and also still under

3

research [1, 2]. The utilization systems of oil shale are classified into three kinds of modes:

4

retorting modes of obtaining oil and fuel gas, direct combustion modes of obtaining electricity,

5

and the comprehensive modes of obtaining diverse products [3]. And the first two are

6

conventional modes. Conventional retorting modes, only producing oil and little fuel gas, has poor

7

efficiency [4]. In China, Fushun retorting system (FS) is such an example of conventional

8

retorting system. Energy and exergy analysis of the Fushun retorting system show that the greatest

9

exergy destruction happens to the retort unit, and the optimization only improves the exergy

10

efficiency from 34.17% to 36.58% [5, 6]. Compared with the conventional retorting modes, the

11

comprehensive utilization system usually has higher efficiency, and less pollutant emissions.

12

Several comprehensive utilization systems are recommended, such as British tri-generation system

13

[7], Estonian SPC technology [8] and Chinese comprehensive utilization system[9, 10]. Chinese

14

comprehensive utilization system (CCU) is put forward recently by the authors. In the system, the

15

oil shale is firstly introduced to the fluidized bed (FB) retort to have oil shale decomposed into oil,

16

gas and semicoke; the resulting semicoke is the fuel of circulating fluidized bed (CFB) boiler.

17

Finally, the ash from the CFB would be ingredients of building materials. It should be noted that

18

the required energy for pyrolysis reaction in the FB retort is complemented by hot circulating ash

19

from CFB boiler and hot circulating fuel gas from the fuel gas heater.

20

The hot circulating fuel gas not only provide the energy for pyrolysis, but also fluidize the

21

materials in the retort unit. There has been a research on pyrolysis that use nitrogen and steam as

22

fluidizing gas, and electric heater supply all the energy the pyrolytic reactions need [11]. 3

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According to the research, oil from the fluidized bed retort unit with nitrogen and steam

24

atmosphere is 7% and 15% more respectively than that from the Fischer assay. The beneficial

25

results are mainly attributed to two reasons: First, the gas and solids are mixed well, and the heat

26

transfers more easily and efficiently; Second, the reaction time is short and the decomposition of

27

oil is slight. Besides, one pilot scale experiment in which there are two fluidized bed reactors is

28

conducted in Harbin China [12]. One is the retort unit, and another is the combustor to burn the

29

resulting semicoke that has several toxic compounds to avoid the environment problems [13].

30

Then the hot ash from the combustor is introduced to the retorting unit to heat the oil shale. The

31

experiments indicate that two units could operate stably and consecutively, and the ash from the

32

fluidized bed combustor has only 0.32% residual carbon. What’s most important, the received oil

33

has more light fractions and less density, but it is just 80% of oil from Fischer assay. The yield

34

decline of oil is adverse and ascribed to that ash could not only adsorb the oil but also catalyze the

35

decomposition of oil [14]. That is, the more energy provided by the hot circulating fuel gas, the

36

better for oil yield.

37

For optimizing CCU system further, the authors have applied energy analysis to

38

comprehensive utilization system [15]. And the results indicate the retorting temperature should

39

be about 460~490 oC. As a more powerful tool than energy analysis, exergy analysis is widely

40

used in different system [16, 17], also in the Fushun retorting system (FS) [5, 6, 18]. But the

41

exergy analysis has not been made to the CCU system, and the energy and exergy loss

42

distributions in the system also are not obtained, which is critical for enhancing efficiency of the

43

comprehensive system. So, in this paper, both energy and exergy analysis are applied to CCU

44

system. And then the energy and exergy loss distributions of the system are determined. 4

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Furthermore, the effects of several operating parameters are discussed. These results can give

46

further suggestions and reference data on how to operate the comprehensive system stably and

47

efficiently.

48

2. Calculated basis

49

2.1 Process flow diagram

1. raw oil shale; 2. flue gas; 3. dried oil shale; 4. exhausted gas; 5. hot circulating fuel gas; 6. hot circulating ash; 7. semicoke& circulating ash; 8. vapor& gases(hot); 9. vapor& gases(cool); 10. fuel gas 2; 11. fuel gas 3; 12. fuel gas 1; 13. cool water; 14. shale oil; 15. water; 16. fuel gas 4; 17. fuel gas 5; 18 remaining fuel gas; 19. air 1; 20. burned fuel gas; 21. fuel gas 6; 22. flue gas 1; 23. feed water;24. air 2; 25. steam; 26. ash; 27.flue gas 2; Figure 1 process flow diagram of Chinese comprehensive utilization system of oil shale 50

Based on the description in the previous research [9], the process flow diagram of Chinese

51

comprehensive utilization system is shown in Figure 1. In Drier, the flue gas (No.27& 22) from

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circulating fluidized bed boiler (CFB) boiler and fuel gas heater (FGH) heat and dry the raw oil

53

shale (No.1). Next, the dried oil shale (No.3) is introduced to fluidized bed retorting furnace and

54

transmutes into semicoke, shale oil, water and fuel gas. This reaction requires extra energy which

55

is complemented by the hot circulating ash (No.6) from CFB and hot circulating fuel gas (No.5) 5

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from FGH. Also, the circulating fuel gas fluidizes the materials, heats the materials and takes the

57

gaseous products out from retorting furnace. The hot gaseous stream (No.8) flow through a cooler

58

and separator system in turn, finally is classified into oil (No.14), water and fuel gas (No.12). The

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non-condensable fuel gases are divided into four parts: one part of them (No.10) are heated in two

60

heaters, the cooler and FGH; one part of them (No.21) are only heated in FGH, then mixed up

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with the previous describing part, and the joined two parts finally are fed into retorting furnace as

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hot circulating fuel gases; one part of them (No.20) are directly introduced to fuel gas heater as

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fuel to be burned. The remain part are stored in gas can for further use.

64

The solid product, semicoke along with circulating ash (No.7), is directly fed into CFB boiler

65

as fuel. And energy from the semicoke convert into three shares. One proportion heat the water

66

and generate superheated steam; one proportion heat the circulating ash (No.6) which is then

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introduced to retorting furnace; the last are remaining in flue gas and bottom ash (No.26). The flue

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gas from the CFB boiler are merged with flue gas from fuel gas heater, and then flow through the

69

oil shale drier.

70

Based on the previous discussing process flow diagram, self-developed programs are used to

71

calculate the energy and exergy distribution of the whole system. Especially, linear programing

72

helps optimize the energy and exergy efficiency of FB retort simultaneously; and the self-

73

developed C programs calculate the energy flow of CFB boiler. Such calculations help determine

74

the parameters of every unit and every stream. And detail parameters of every material stream in

75

the system are listed in the Table 1.

76

2.2 Input parameters

77

The oil shale introduced to the system comes from Dachengzi mine, Huadian. A series of 6

ACCEPTED MANUSCRIPT 78

experiments have investigated the pyrolysis of this oil shale [9, 19, 20], and the data from these

79

researches are base for the further calculation. Table 2 summarizes the basic operating conditions

80

of the system. Also, physical parameters of materials are critical for calculation, such as the heat

81

capacity of ash, semicoke, and oil shale, which are discussed in the following parts.

82

Table 1 parameters of every material stream at 490 oC

fuel gas heater input

output retorting unit input

output cooler input out put CFB boiler input

output

separators input output

air burned fuel gas fuel gas 6 fuel gas 3 flue gas hot circulating fuel gas

No 19 20 21 11 22 5

mass(kg/h) 29888 5000 30000 135000 34888 165000

pressure 1 atm 1 atm 1 atm 1 atm 1 atm 1 atm

Temp(oC) 25 65 65 457 130 617

dried oil shale hot circulating fuel gas hot circulating ash semicoke & circulating ash vapor & gases

3 5 6 7 8

91621 165000 280000 344002 192619

1 atm 1 atm 1 atm 1 atm 1 atm

50 617 825 490 490

vapor & gases fuel gas 2 vapor & gases (cool) fuel gas 3

8 10 9 11

192619 135000 192619 135000

1 atm

490 65 250 457

semicoke & circulating ash air 2 feed water steam ash hot circulating ash flue gas

7 24 23 25 26 6 27

344002 164042 91400 91400 47029 280000 180679

vapor & gases (cool) cool water fuel gas 1 shale oil water

9 13 12 14 15

192619 350000 170547 18577 353494

83 7

6.17Mpa 5.39Mpa

490 25 150 450 130 825 130 250 25 65 65 65

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2.3 Specific heat capacity of ash and oil shale semi-coke

85

Specific heat capacity is critical for energy and exergy calculation. The process involves

86

three solid materials, the oil shale, oil shale semicoke and the resulting ash, and they are blends.

87

Specific heat capacity of them could be simulated in the Aspen Plus, and also tested by

88

experiments. A few researches have tested the specific heat capacity of them. And if the two

89

results are narrowly close, the tested results are preferred; otherwise, more discussions should be

90

made.

91

According to Wang’s research [21], the tested specific heat capacity of oil shale is near to the

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simulated test by Aspen Plus. Furthermore, so is the heat capacity of oil shale semicoke tested by

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Liu [22]. But the calculated heat capacity of oil shale ash according to formula proposed by

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research [21] is rather different from that of simulation. The simulated specific heat capacity of

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ash by Aspen Plus is 0.995 kJ/(kg·oC), and this value is preferred because specific heat capacity of

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similar materials, such as soil, and brick, is just around 0.8 kJ/(kg·oC).

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Table 2 base operating conditions for calculation

Process unit

parameters

unit

values

Drier FB retort

Feed rate of raw oil shale temperature pressure Heat loss, q5 pressure Circulating ash ratio Excess air ratio temperature pressure Excess air ratio

t/h oC atm % atm

100 490 1 [2,5] 1 6 1.2 850 1 1.2

CFB

Fuel gas burner

98 99 100

oC

atm

2.4 Assumptions This work focuses on the energy and exergy flow of Chinese comprehensive utilization (CCU) system, and some basic assumptions for the calculation are listed as follows: 8

ACCEPTED MANUSCRIPT 101

1.The temperature (T0) and pressure(p0) of surroundings are respectively 25oC and 1atm; Air

102

mentioned in this paper consists of 21%O2 and 79% N2 in volume.

103

2.The components of shale oil are still assumed same as previous research [15]. Without chemical

104

reaction, physical properties of the assumed oil (specific heating capacity, entropy change) are still

105

applied to calculate the energy and exergy flow; if chemical reaction (pyrolysis and combustion)

106

happens, the heating value of assumed oil is not suitable for the calculation. The lower heating

107

value of actual shale oil is smaller than that of presumed shale oil, there should be a ratio between

108

the heating value of actual oil and that of assumed oil. And the ratio between LHV of actual oil

109

and that of assumed oil is given after several times iterations. Besides, all the heating value

110

involved in this paper is lower heating value, thus, the enthalpy of steam in the flue gas should

111

exclude the latent heat of evaporation.

112

3.The whole system is under steady operation, and every unit in the system is under equilibrium.

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Also, even though the hot circulating ash that declines the yields of oil is introduced to the

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fluidized bed retort, the obtained oil is supposed to equal the oil yield in the previous experiments.

115

4. The pipeline and auxiliary facilities combine the system together. The energy and exergy loss of

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pipeline are negligible; but, the energy that auxiliary facilities (pumps, fans and so on) need is

117

from the electricity and it equals 22.5% oil shale exergy [23].

118

5. The stream from retorting unit consist of two phases: gas phase is a mixture of fuel gas, oil

119

steam and water; solid phase contains semicoke and circulating ash. There is difference between

120

the exergy of gas mixture and the summary exergy of all individual pure gas, but, for the whole

121

system, the differences are counteracted because they are calculated in even times. On the other

122

hand, the exergy of solid blend equals the summary exergy of pure solid. 9

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2.5 Mathematic calculation of exergy Previous assumptions simplify the calculation of exergy, and the equations used are listed in

125

the following.

126

2.5.1 Exergy of streams

127 128

When a substance experience non-adiabatic process, the exergy change could be formulated as following differential equation: 𝑇0

129

𝑑𝐸𝑥𝑠,ℎ = 𝑑𝑞(1 ‒ 𝑇 )

130

In which, q is heat gain or loss in the process, T is the temperature of substance, T0 is the

131

temperature of backgrounds, and in this paper T0 is 298.15K.

(1)

132

When the temperature of heat source is isothermal, the formula (1) could transmute into

133

𝐸𝑥𝑠,ℎ = 𝑞(1 ‒ 𝑇 )

134 135 136

𝑇0

(2)

This equation can calculate the exergy loss of retorting unit. If temperature changes, the exergy could be determined through integrating equation (1), and result is: 𝑑𝑇

137

𝐸𝑥𝑠,ℎ = ∫𝐶𝑝𝑑𝑇 ‒ 𝑇0∫𝐶𝑝 𝑇

(3)

138

Where Cp is specific heat capacity, and the equation (3) could transform into equation (4) if Cp

139

doesn’t vary with temperature. Equation (4) could be applied to calculate exergy of solid

140

substance (ash, semicoke and oil shale), when Cp is the average specific heat capacity of the

141

process.

142

𝑇

𝐸𝑥𝑠,ℎ = 𝐶𝑝(𝑇 ‒ 𝑇0 ‒ 𝑇0ln 𝑇 ) 0

(4)

143

For gas phase, the enthalpy and the entropy of every individual component could be obtained

144

easily by looking into database in Aspen plus. So, exergy could be calculated as following 10

ACCEPTED MANUSCRIPT 145

equation:

146

𝐸𝑥𝑠,ℎ = ℎ ‒ ℎ0 ‒ 𝑇0(𝑠 ‒ 𝑠0)

147

In which, h and h0 respectively represent the enthalpy when temperature is T and T0; similarly, s

148

and s0 are the entropy corresponding to T and T0, respectively.

(5)

149

However, exergy in the previous calculations is just physical exergy (Exs,h), and does not

150

include the chemical exergy. In the retorting unit and fuel gas burner where chemical reactions

151

happen, chemical exergy play significant role. So the chemical exergy of oil shale, oil shale

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semicoke and fuel gas are calculated according to the following equations [24].

153

𝐸𝑥𝑠,𝑐ℎ = 𝐿𝐻𝑉

154

𝐸𝑥𝑠,𝑐ℎ = 𝐿𝐻𝑉 ∗ 1.0038 + 0.1365 𝐶 + 0.0308𝐶 + 0.0104𝐶

155

𝐸𝑥𝑠,𝑐ℎ = 𝐿𝐻𝑉 ∗ 1.0064 + 0.1519 𝐶 + 0.0616𝐶 + 0.0429 𝐶

(6)

(

𝐻

𝑂

𝑆

)

(7)

(

𝐻

𝑂

𝑁

)

(8)

156

Where C, H, O, N, and S are the ultimate analysis results of corresponding fuel, LHV is

157

abbreviation of low heating value. And chemical exergy (Exs,ch ) of gas, liquid and solid fuel are

158

calculated according to equation (6), (7), and (8) respectively. So, for the combustible material

159

stream introduced to the retorting unit and fuel gas burner unit, exergy (Exs) is determined as

160

equation (9), so does the products from the two units:

161 162

𝐸𝑥𝑠 = 𝐸𝑥𝑠,𝑐ℎ + 𝐸𝑥𝑠,ℎ

(9)

2.5.2 Exergy and energy balance model for crucial retort part

163

FB retort is one of the crucial units in the system, and hot fuel gases and circulating ash are

164

introduced to retorting furnace to provide the energy and exergy that pyrolysis reaction of oil shale

165

require. Products out from the retorting furnace consists of two phase, solids and gas. It would

166

make the calculation of the whole system more effective to ascertain the operation conditions of

11

ACCEPTED MANUSCRIPT 167

FB retort firstly. To make the calculation of FB retort more succinct, the schematic of enthalpy

168

flow and exergy flow was shown in the Figure 2. According to the thermodynamics laws, the

169

energy and exergy balance equations are established.

170

𝐻𝑓𝑔 + 𝐻𝑎𝑠ℎ + 𝐻𝑜𝑖𝑙 = 𝐻𝑙𝑜𝑠𝑠 + 𝐻𝑔𝑎𝑠 + 𝐻𝑠𝑜𝑙𝑖𝑑𝑠

(10)

171

𝐸𝑥𝑓𝑔 + 𝐸𝑥𝑎𝑠ℎ + 𝐸𝑥𝑜𝑖𝑙 > 𝐸𝑥𝑙𝑜𝑠𝑠,ℎ + 𝐸𝑥𝑔𝑎𝑠 + 𝐸𝑥𝑠𝑜𝑙𝑖𝑑𝑠

(11)

Figure 2 skematic of energy and exergy balance in the retorting unit Subscripts: solids and gas mean the phase of products;fg and os are short for fuel gas and oil shale respectively;

172 173

Exergy destruction inevitably occur to FB retort unit, and consists of two parts: one part is

174

accompanied with energy loss (Exloss,h), and determined by equation (2); another part is the exergy

175

loss of retorting reaction because the pyrolysis reaction is irreversible. In the equation (11), the

176

latter part is not given. Furthermore, exergy destruction in the retorting unit Exdes is determined as

177

follows:

178 179 180 181

𝐸𝑥𝑑𝑒𝑠 = 𝐸𝑥𝑓𝑔 + 𝐸𝑥𝑔𝑎𝑠 + 𝐸𝑥𝑠𝑜𝑙𝑖𝑑𝑠 ‒ (𝐸𝑥𝑎𝑠ℎ + 𝐸𝑥𝑜𝑖𝑙 + 𝐸𝑥𝑓𝑔)

(12)

2.6 Linear programming of retorting unit Just considering per kilogram dried oil shale introduced to the FB retort, energy and exergy balance equations could be simplified as following:

12

ACCEPTED MANUSCRIPT

{

182

𝑚𝑎𝑠ℎℎ𝑎𝑠ℎ + 𝑚𝑓𝑔ℎ𝑓𝑔 + ℎ𝑜𝑠 = ℎ𝑙𝑜𝑠𝑠 + 𝑚𝑠𝑜𝑙𝑖𝑑𝑠ℎ𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑚𝑔𝑎𝑠ℎ𝑔𝑎𝑠 𝑚𝑎𝑠ℎ𝑒𝑥𝑎𝑠ℎ + 𝑚𝑓𝑔𝑒𝑥𝑓𝑔 + 𝑒𝑥𝑜𝑠 > 𝑒𝑥𝑙𝑜𝑠𝑠,ℎ + 𝑚𝑠𝑜𝑙𝑖𝑑𝑠𝑒𝑥𝑠𝑜𝑙𝑖𝑑𝑠 + 𝑚𝑔𝑎𝑠𝑒𝑥𝑔𝑎𝑠

(14)

183

More specifically, hloss should be constrained. Firstly, the constraints are presumably described as

184

equation (15); and after iterations the final hloss should follow the constraints of q5 listed in Table

185

2.

{

186

ℎ𝑙𝑜𝑠𝑠 < 0.008ℎ𝑜𝑠 ℎ𝑙𝑜𝑠𝑠 > 0.001ℎ𝑜𝑠

(15)

187

When temperature of circulating ash (Tash) and circulating fuel gas (Tfg) introduced to retorting

188

unit are determined, the energy (hash, hfg) and exergy (exash, exfg) per kilogram medium introduced to

189

retorting furnace could be calculated. However, the mass of circulating ash (mash) and circulating fuel

190

gas (mfg) remain uncertain. Fortunately, the ranges of these two parameters, can be narrowed by linear

191

programming.

192 193

Taking temperature of retorting furnace TR=490 oC, Tash=825 oC and Tfg=617oC as an example, the equations (14) and (15) are further simplified as:

{

194

334𝑚𝑎𝑠ℎ + 302.73𝑚𝑓𝑔 > 1471.35 334𝑚𝑎𝑠ℎ + 302.73𝑚𝑓𝑔 < 1555.97 22.3𝑚𝑎𝑠ℎ + 7.40𝑚𝑓𝑔 > 26.1

(16)

195

In fact, the mass of media introduced to retorting furnace is also limited, especially the hot

196

circulating fuel gas. Because the energy heating the fuel gas comes from the combustion of the

197

producing fuel gas. Therefore, it needs several times iteration of the whole system to get proper

198

mash and mfg in Figure 3.

13

ACCEPTED MANUSCRIPT

a) TR=490oC b) TR=520 oC Figure 3 skematic of optimizing the mass of circulating ash and circualting fuel gas 199

3. Results and discussions

200

Aspen Plus provides database to calculate the entropy, enthalpy and exergy of gas and shale

201

oil, as well as the specific heat capacity of ash; these fundamental data help establish the balance

202

equation of energy and exergy for every unit in the comprehensive system. Specifically, the

203

energy balance of CFB is calculated by a self-developed program. After several times iteration,

204

how much the circulating ash and fuel gas should be introduced to the retorting unit is determined.

205

Then, the energy loss and exergy destruction of the whole system are determined. In addition,

206

several operating parameters are also investigated to optimize the performance of whole system.

207

3.1 Effect of retorting temperature

208

Retorting temperature, as one of the key operating parameters, has great influence on mass

209

yields and components of pyrolysis products, and influence the energy and exergy balance of

210

retorting unit, even of whole system.

211

Previous studies have investigated the effects of retorting temperature on yields and pyrolytic

212

products components of Huadian oil shale [19, 20, 25]. Higher retorting temperature results in

213

more yields of shale oil, but the mass yield of oil only increases by 0.81% when temperature 14

ACCEPTED MANUSCRIPT 214

improves from 460 to 520 oC [9]. On the other hand, the higher retorting temperature also makes

215

for secondary cracking and coking reactions of oil [26-28], which indicates the lower heating

216

value (LHV) of oil obtained from escalating retorting temperature should decline gradually.

217

Furthermore, escalating retorting temperature make for the secondary coking and cracking

218

reactions and promotes both the yield of total fuel gas and the individual H2 mass in the fuel

219

gas[29, 30]. That is, the calculated LHV and specific heat capacity of fuel gas escalate because H2

220

has high LHV and specific heat capacity. Finally, the increasing retorting temperature leads to

221

more sufficient decomposition of organics within semicoke, so both the mass yield and LHV of

222

semicoke decline. All the details could be seen in the Figure 4 a). In general, taking the LHV and

223

mass yield of products into consideration, the energy distribution of products in different retorting

224

temperature could be obtained and showed in Figure 4 b). As previous discussions indicate, the

225

energy of semicoke declines with retorting temperature improving. On the contrary, the energy of

226

fuel gas increase. What’s more, the energy of shale oil firstly improves, then declines and should

227

have a local maximum during the range, 460~490 oC.

a) products’ lower heating value b) the energy distribution of products Figure 4 Effect of retorting temperature on products 228

In the Figure 4 b), the energy of oil shale introduced to the retorting unit (Hos) is showed as a

229

straight line, and the energy over the straight line is the energy that should be compensated for the 15

ACCEPTED MANUSCRIPT 230

pyrolysis reaction. And the extra needed energy of pyrolysis reaction could be determined

231

according to Hess’s Law and the first law of thermodynamics [31]. The pyrolysis reaction is

232

supposed to be divided into two steps. First, the pyrolysis reaction happens at room temperature,

233

and reaction heat of this pyrolysis reaction (HI) could be determined by Hess’s Law. The results

234

are showed in Figure 4 b). Second, all the products take in the extra energy (HⅡ) and are heated

235

from room temperature to the retorting temperature. Figure 4 b) also indicates HⅡ. In all, the

236

energy need for the pyrolysis reaction at retorting temperature (H) equals the sum of reaction heat

237

in first step reaction (HI) and the extra energy in the second step process (HⅡ). The exergy for

238

pyrolysis reaction (Ex) in different retorting temperature are also calculated in the same way, and

239

both exergy and energy that pyrolysis reaction requires are shown in Figure 5. It is obvious that

240

the total needed energy and exergy increase with retorting temperature.

241

The total energy needed for pyrolysis reaction is provided by hot circulating ash and hot fuel

242

gas. If the temperature and mass of hot circulating ash are constants, increasing retorting

243

temperature would reduce the temperature drop of hot circulating ash, therefore the energy and

244

exergy provided by hot circulating ash decrease progressively as retorting temperature increase.

245

Complementarily, the hot fuel gas should offer more energy and exergy, which is shown in the

246

Figure 5. But the mass of fuel gas burned to heat the circulating fuel gas shows a declining trend

247

when retorting temperature increase from 490 to 520 oC, because retorting temperature above 500

248

oC

249

produce more H2 and CH4.

would result in secondary decomposition of shale oil and organics within semicoke, which

250

As energy loss and exergy destruction exist during the reaction, the summary energy and

251

exergy provided by the two mediums must be bigger than the total energy and exergy needed for 16

ACCEPTED MANUSCRIPT 252

reaction. The discrepancy between them are used to determine the loss ratio of retorting unit as

253

following equations:

254

𝜂𝐻 =

255 256

𝜂𝐸𝑥 =

𝐻𝑓𝑔 + 𝐻𝑎𝑠ℎ ‒ 𝐻 𝐻𝑓𝑔 + 𝐻𝑎𝑠ℎ

𝐸𝑥𝑓𝑔 + 𝐸𝑥𝑎𝑠ℎ ‒ 𝐸𝑥 𝐸𝑥𝑓𝑔 + 𝐸𝑥𝑎𝑠ℎ

(17) (18)

Where, ηH and ηEx are energy loss ratio and exergy destruction ratio respectively.

a) Energy b) Exergy Figure 5 comparison of energy and exergy needed and provided 257

a) ηH and ηEx of retorting unit

b) exergy destruction distribution in the system

Figure 6 the effect of retorting temperature on exergy of individual unit in CCU system

258

As we can see in Figure 6 a), the energy loss ratio and exergy destruction ratio also vary with

259

different retorting temperature. Take Tash=825 oC as an example. Energy loss ratio escalates with

260

increasing retorting temperature (TR) except 490 oC. because only the operating conditions at 490

261

oC

262

slightly adjusted and optimized on the basis of that at 490 oC. For exergy loss ratio, the exergy

get optimized completely, and operating conditions at other retorting temperature (TR) are

17

ACCEPTED MANUSCRIPT 263

destruction ratio at 430 oC is the biggest, and the exergy destruction ratios at other retorting

264

temperature are close.

265

As Figure 6 b) shows, retorting temperature also has great influence on exergy destruction of

266

other units in CCU system. Specifically, exergy destructions of three units (fuel gas heater,

267

separators and cooler) increase with retorting temperature increase; but exergy destruction of other

268

two units (CFB boiler and retorting unit) show opposite trend. It can be concluded that bigger

269

exergy destruction take place where bigger temperature difference exists and fiercer combustion

270

reaction happens. In addition, it is evident that exergy destruction from the combustion process (of

271

semicoke and fuel gas) predominantly makes up of the whole exergy destruction of system.

272

3.2 Effect of circulating ash temperature

273

The circulating ash introduced to retorting unit is from CFB furnace, and its temperature as

274

well as the retorting temperature determines the temperature drop of circulating ash. That is, the

275

energy and exergy from circulating ash are also influenced by ash temperature. So are the energy

276

loss and exergy destruction ratio. As we can see in the Figure 7, both energy loss and exergy

277

destruction ratio declines with ash temperature decreasing. If the ratios decline under 0, the failure

278

of energy and exergy balance happen to the retorting unit. It is of great interest to note that the

279

energy loss ratio versus ash temperature shows a straight line, that is, the change of ash

280

temperature easily leads to failure of energy balance when the energy loss ratio close to 0. So, the

281

advisable Tash is above 820 oC. However, if the energy balance failed unfortunately, the further

282

needed energy could be complemented by burning more producing fuel gas. Therefore, the mass

283

of burned fuel gas is of great significance.

18

ACCEPTED MANUSCRIPT

Figure 7 energy and exergy loss ratios’ variation with retorting temperature 284

3.3 Effect of burned fuel gas

285

Circulating fuel gas is heated in the fuel gas heater, and the heat supply comes from the

286

combustion of fuel gas. In addition, how much the producing fuel gas is burned influences the

287

energy and exergy introduced to retorting furnace by circulating fuel gas. On the previous

288

discussions, not all the producing fuel gas is burned, and the major energy and exergy pyrolysis

289

reaction need are mainly from circulating ash, shown in the Figure 5. More burned fuel gas

290

indicates the circulating fuel gas could transports more energy to retorting furnace, and therefore

291

circulating ash could provide less energy and exergy. That is, either the temperature of circulating

292

ash or the flow rate of circulating ash can decline in some degree, which is good for both the

293

stable operation and increment of oil yield. Because the less circulating ash introduced to retorting

294

furnace is, the less fine ash that is hard to purify in oil is, and also the less intense the crack and

295

coking reactions of oil accelerated by the minerals in ash is. Besides, the excess fuel gas is also a

296

mixture that contains CO2 in 20~50% volume fraction. This indicates the it is less cost effective to

297

purify the fuel gas than burn the fuel gas directly, especially when the retorting temperature is

298

under 490 oC. For better economic performance, higher shale oil yield and more stable operation,

19

ACCEPTED MANUSCRIPT 299

it is advisable to have more producing fuel gas burned to provide the energy and exergy for

300

pyrolysis reaction. So, the condition that all the fuel gas was burned is investigated.

301

As we can see in the Figure 8, when all the produced fuel gas is burned, the energy

302

introduced to retorting unit from circulating fuel gas increase with retorting temperature rising,

303

and therefore the energy from circulating ash could be diminished. But it is of great importance to

304

note that both the energy and exergy from circulating ash plunge as the retorting temperature

305

increase from 490 to 520 oC, which is ascribed to that the LHV of fuel gas obtained from 520 oC is

306

much greater than that obtained from 490 oC. That is, the flow rate of circulating ash decreases

307

abruptly, which is difficult for regulator system to adjust and would bring about turbulence of heat

308

duty in CFB furnace. So, from this perspective, the retorting temperature range, 460~490 oC, is

309

suggested.

a) energy b) exergy Figure 8 energy and exergy from hot ciculating ash and fuel gas (burned all producing gas ) 310

3.4 Comparison of exergy efficiency with other system

311

Many operation conditions would escalate the exergy efficiency of the comprehensive

312

utilization system, but it’s uncertain that, compared with other retorting system, how many

313

advantages the comprehensive utilization system has. The efficiency of exergy is a reliable

20

ACCEPTED MANUSCRIPT 314

parameter to make a further comparison between the systems, even though different kinds of oil

315

shale are introduced to Fushun retorting system (FS) and Chinese comprehensive utilization

316

system (CCU). Proximate and ultimate analysis of the used oil shale in Fushun retorting system

317

are given in reference [6]. And the exergy efficiency of FS system is defined as follows:

318

𝜀=

𝐸𝑥𝑜𝑖𝑙 + 𝐸𝑥𝑟𝑔 ‒ 𝐸𝑥𝑒𝑙 𝐸𝑥𝑜𝑠

(19)

× 100%

319

where Exos, Exoil, Exrg are respectively exergy of oil shale, and shale oil, remaining fuel gas. They

320

are listed in Table 3 [5]. Besides, as discussed previously, Exel/Exos equals 22.5%. In Fushun

321

retorting system, the producing char that has potential exergy is discorded. So, the exergy within

322

the char is overlooked in the calculation. For CCU system, the exergy of products should contain

323

that of steam. What’s more, the exergy efficiency also could be determined as:

324

𝜀 = (1 ‒ (𝐸𝑥𝐷 + 𝐸𝑥𝑒𝑙)/𝐸𝑥𝑠ℎ𝑎𝑙𝑒) × 100%

325

where ExD/Exos is exergy destruction ratio. And the exergy destruction ratio of Fushun retorting

326

system is listed in Table 3.

(20)

Table 3 Exergy efficiency comparison between the systems FS

stream

exergy(mW)

Exergy destruction ratio

𝜀1

𝜀2

input

oil shale

773.61

0.422

0.222

0.353

output

shale oil

241.81

fuel gas

104.25

char

-0.248

0.517

0.523

CCU

327

input

oil shale

321.52

output

shale oil

201.64

fuel gas

10.77

steam

27.47

𝜀1 ,𝜀2 are respectively exergy efficiency calculated according to equation (19) and (20).

328

The references give only the exergy analysis of Fushun (FS) retorting system at retorting

329

temperature 525 oC, and so only the exergy efficiency at 525 oC is obtained. For the 21

ACCEPTED MANUSCRIPT 330

comprehensive system, the exergy efficiency at retorting temperature varying from 430 to 520 oC

331

are calculated. All the exergy efficiencies are shown in Figure 9.

332

It is obvious that CCU system always has much higher efficiency than FS system.

333

Furthermore, both the two efficiencies calculated according to equation (19) and (20) show

334

discrepancy because the exergy of flue gas, ash and waste water is overlooked while using the

335

equation (20). In addition, for FS system, the exergy efficiency according to equation (20) is

336

13.1% bigger than the exergy efficiency calculated by equation (19). Because the producing char

337

has exergy, but is discarded in FS system. However, for CCU system, the discrepancy between the

338

two calculated exergy efficiencies at RT=520oC is 0.6%, smaller than that of FS system. And

339

further calculation shows the exergy of flue gas, ash, and water is about 0.7% oil shale energy. So,

340

it is reasonable that for the comprehensive system, the exergy efficiency at other retorting

341

temperature are calculated as equation (20).

FGC: fuel gas is completely burned, and the ciculating ash is same as that when part of fuel gas is burned; FGC,m: fuel gas is completely burned, and the mass of ciculating ash is reduced.

Figure 9 exergy comparision 342

Generally speaking, Chinese comprehensive utilization system (CCU), compared with

343

Fushun retorting system (FS), has much higher exergy efficiency. But exergy efficiency of CCU

22

ACCEPTED MANUSCRIPT 344

system is also influenced by many operating factors, especially retorting temperature. With

345

retorting temperature increase, exergy efficiency first escalates but then slightly drops; as Figure 9

346

shows, the comprehensive system would reach highest exergy efficiency at temperature range

347

460~490 oC. Also, the exergy efficiency is influenced by the amount of combustion fuel gas. If all

348

the fuel gas is burned (FGC), the exergy efficiency is above the exergy efficiency of partially

349

combusted fuel gas as retorting temperature under 460 oC, but it’s opposite since retorting

350

temperature above 490 oC. Because, above 490 oC, the producing fuel gas has much higher LHV,

351

directly combustion would lead to huge exergy destruction. Even though further modification

352

(FGC,m) is carried out to decline the exergy destruction of retorting unit, the exergy efficiency of

353

whole system is still lower than that of part of fuel gas burned if TR is above 490 oC.

354

4. Conclusions

355

Both exergy analysis and energy analysis are applied to the comprehensive utilization system

356

of oil shale. According to energy and exergy balance of retorting unit, linear programing helps to

357

optimize the operating conditions, such as the amount of hot circulating ash and hot circulating

358

fuel gas, the temperature of hot circulating ash and so on. The detailed operation suggestions are

359

as follows:

360

1. Chinese comprehensive utilization system is beneficial than Fushun system from the aspect of

361

exergy efficiency. No matter what retorting temperature is, Chinese comprehensive utilization

362

system has higher exergy efficiency than the Fushun system. So, investigating how to operate the

363

comprehensive utilization system of oil shale stably and efficiently make significant sense.

364

2. For the aspect of efficiency, the highest exergy efficiency is in the temperature range 460-490

365

oC.

Besides, it is beneficial to have all the producing fuel gas combusted to heat the circulating 23

ACCEPTED MANUSCRIPT 366

fuel gas. But as retorting temperature over 490 oC, more shale oil decompose into the fuel gas and

367

the low heating value of fuel gas would increase steeply, so combustion of all producing gas

368

would result in turbulence of system. In general, the retorting temperature should be around

369

460~490 oC.

370

3. High temperature of circulating ash helps decline the mass of ash, and improve the oil yields.

371

Besides, temperature of hot circulating ash should not be under 820 oC; otherwise, the retorting

372

unit easily fails in energy and exergy balance even through all the fuel gas could be burned to

373

complement the excess required energy.

374

Acknowledgements

375

This work was sponsored by Shanghai Pujiang Program of China [Grant No. 15PJD018].

24

ACCEPTED MANUSCRIPT Nomenclature TR Tash Tfg mash mfg HI HII H Hos hos Hash hash Hfg hfg Hoil hoil Hloss hloss Exall Exh Exs Exs,ch Exs,h Exash exash Exfg exfg Exos exos Exoil exoil Exloss,h Exdes ηH ηEx ε1 ε2

Retorting temperature Temperature of circulating ash Temperature of circulating fuel gas Mass of circulating ash introduced to retorting furnace Mass of circulating fuel gas introduced to retorting furnace Reaction heat calculated according Hess law at room temperature Energy for heating products from room temperature to retorting temperature Total calculated energy for pyrolysis reaction Energy of oil shale Energy of per kilogram oil shale Energy from circulating ash to retort furnace Energy from per kilogram circulating ash to retort furnace Energy from circulating fuel gas to retort furnace Energy from per kilogram circulating fuel gas to retort furnace Energy of shale oil Energy of per kilogram shale oil Energy loss Energy loss (when just one kilogram oil shale is introduced to retorting unit) Calculated exergy for pyrolysis reaction Exergy for heating products from room temperature to retorting temperature Exergy of individual stream Chemical exergy of individual stream Physical exergy of individual stream Exergy from circulating ash to retort furnace Exergy from per kilogram circulating ash to retort furnace Exergy from circulating fuel gas to retort furnace Exergy from per kilogram circulating fuel gas to retort furnace Exergy of oil shale Exergy of per kilogram oil shale Exergy of shale oil Exergy of per kilogram shale oil Exergy loss accompanied with heat loss Exergy destruction Energy loss ratio Exergy destruction ratio Exergy efficiency calculated according to equation (19) Exergy efficiency calculated according to equation (20)

25

ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT Highlights Exergy analysis is applied to Chinese comprehensive utilization system. Linear programming helps optimize the retorting unit. The preferable retorting temperature range is 460~490 oC. All fuel gas produced at preferred temperature range is advised to be burn. The temperature of hot circulating ash should be no less than 820 oC.