Modified exergoeconomic modeling and analysis of combined cooling heating and power system integrated with biomass-steam gasification

Accepted Manuscript Modified exergoeconomic modeling and analysis of combined cooling heating and power system integrated with biomass-steam gasification

Jiangjiang Wang, Tianzhi Mao, Jing Wu PII:

S0360-5442(17)31406-8

DOI:

10.1016/j.energy.2017.08.030

Reference:

EGY 11399

To appear in:

Energy

Received Date:

13 January 2017

Revised Date:

31 March 2017

Accepted Date:

08 August 2017

Please cite this article as: Jiangjiang Wang, Tianzhi Mao, Jing Wu, Modified exergoeconomic modeling and analysis of combined cooling heating and power system integrated with biomasssteam gasification, Energy (2017), doi: 10.1016/j.energy.2017.08.030

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Modified exergoeconomic modeling and analysis of combined cooling heating

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and power system integrated with biomass-steam gasification

3

Jiangjiang Wang*, Tianzhi Mao, Jing Wu

4

School of Energy, Power and Mechanical Engineering, North China Electric Power

5

University, Baoding, Hebei Province, 071003, China

6



Corresponding author. Jiangjiang Wang, E-mail address: [email protected]

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Abstract Biomass-steam gasification is an efficient unitization technology of biomass to

10

produce gas fuel for a combined cooling, heating and power (CCHP) system. The aim

11

of this paper is to modify the exergoeconomic method and analyze the cost allocations

12

of multi-products from CCHP system. Firstly, two integrated CCHP schemes with

13

biomass-steam gasification are designed. The difference lies in the gasification

14

endothermic process driven by electricity and thermal energy from the product gas,

15

respectively. The thermodynamic models are presented and validated. Then, a

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modified exergoeconomic method based on energy level is proposed to accord with

17

the principle of high quality and high price. Finally, a case study is presented to

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analyze the thermodynamic performances of two CCHP schemes and the production

19

cost allocations including electricity, chilled water for cooling (hot water for heating)

20

and domestic hot water in different operation modes. Compared with the previous

21

exergoeconomic method, the unit exergy cost of electricity with higher energy level

22

increases 0.09 Yuan/kWh while the cost of other products with lower energy level

23

decrease. The results show that the modified exergoeconomic method is more

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reasonable and efficient.

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Keywords: combined cooling heating and power (CCHP) system; biomass-steam

29

gasification; energy level; exergoeconomic analysis

30 31

Contents (only for review)

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1 Introduction .........................................................................................................................................3

33

2 CCHP schemes integrated with biomass-steam gasification ...........................................................7

34

2.1.1 Biomass-steam gasification driven by electricity .........................................................7

35

2.1.2 Biomass-steam gasification driven by thermal energy from product gas .................9

36

2.2 Models..............................................................................................................................................10

37

2.3 Operation mode and design parameters ......................................................................................12

38

3 Modified exergoeconomic method ...................................................................................................13

39

3.1 Exergoeconomic balance equations ...............................................................................13

40

3.2 Auxiliary costing equations.............................................................................................13

41

3.3 Product costs ....................................................................................................................16

42

4 Results and discussions .....................................................................................................................17

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4.1 Thermodynamic performances .....................................................................................................17

44

4.2 Exergoeconomic performances .....................................................................................................21

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4.2.1 Validity check................................................................................................................21

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4.2.2 Cost allocations .............................................................................................................23

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4.2.3 Sensitivity analysis ........................................................................................................25

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5 Conclusions ........................................................................................................................................27

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Acknowledgements ...............................................................................................................................28

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References .............................................................................................................................................28

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Nomenclature

55

CCHP

combined cooling heating and power

56

CHP

combined heat and power

57

COP

coefficient of performance

58

FEL

following the electrical loads

59

HPHE

heat pipe heat exchanger

60

ICE

internal combustion engine

61

LHV

lower heating value

62

PHE

plate heat exchanger

63 64

Symbols

65

a

mole amount of steam

66

A

energy level

67

c

cost per exergy unit (Yuan/kWh)

68

C

cost rate (Yuan/h)

69

E

exergy (kW)

70

f

exergoeconomic factor

71

H

enthalpy of stream (kJ)

72

K

equilibrium constant

73

m

mass flow rate (kg s-1)

74

p

unit price of product or stream (Yuan/kWh or Yuan/kg)

75

r

mole of the ingredient

76

S

entropy of stream (kJ)

77

T

temperature (K) 3

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w

number of atoms of sulfur

79

x

number of atoms of hydrogen

80

y

number of atoms of oxygen

81

z

number of atoms of nitrogen

82

Z

investment cost rate of component (Yuan/h)

83

β

coefficient of exhaust and residual of char

84

δ

coefficient of exhaust and residual of tar

85 86

Subscripts

87

f

fuel

88

l

loss

89

p

product

90 91 92

1 Introduction Currently, the increasing energy demand and limited fossil fuels have promoted

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the exploitation and utilization of renewable energy sources. Biomass can be used as a

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clean, renewable and relatively abundant energy resource for electricity generation

95

and other purposes [1]. Biomass can be converted to product gas for convenient

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utilization through various types of gasification, which can replace the natural gas to

97

some degree. A combined cooling, heating and power (CCHP) system is an ideal

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energetic, economic and environmental system. Integrating a CCHP system with

99

biomass gasification is an excellent way to use renewable energy sources and improve

100 101 102

energy efficiency at the same time [2, 3]. Some researchers have proposed and developed some CCHP systems integrated with biomass gasification. J. Wang et al. [4] proposed a co-fired CCHP (combined 4

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cooling, heating and power) system based on the mixture gas of natural gas and

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biomass gasification gas. The thermodynamic performance analysis for the increasing

105

volumetric mixture ratio of 0–1.0 indicates that the energy and exergy efficiencies are

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improved by 9.5% and 13.7%, respectively. Furthermore, the costs of multi-products,

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including electricity, chilled water and hot water, based on exergoeconomic analysis

108

are analyzed and discussed based on the influences of the mixture ratio of the two gas

109

fuels, investment cost and biomass cost. H. Wang et al. [5] used syngas directly as a

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fuel source for the renewable CCHP system, which can be produced through a

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biomass gasification process. The advantages and limitations of entrained flow

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gasifier were compared, followed by a discussion on the key parameters that are

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critical for the optimum production of syngas. E. Gholamian et al. [6] proposed a

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biomass-fueled combined cooling, heating and power (CCHP) system and assessed its

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thermodynamic properties. Taking into account the environmental considerations,

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energy and exergy analyses are conducted and its performance is compared with the

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corresponding power generation unit and the CHP system. Through a parametric

118

study, it is observed that the current density and fuel utilization factor play key roles

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on the system performance.

120

Exergoeconomic analysis is a powerful method that combines exergy analysis

121

with economic studies [7]. Exergoeconomic theories have been applied to different

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energy systems, such as conventional power plant [8], energy storage system [9, 10],

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diesel engine powered cogeneration [11, 12], integrated solar combined cycle system

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[13], biomass combined cycle power plant energy system [14-19], and biomass

125

combined heating and power (CHP) system [20, 21]. Many researches have been done

126

to study the exergoeconomic performance of the CCHP system. T. Kohl et al.[22]

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assessed the exergoeconomic performance of three biomass upgrading processes, 5

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namely wood pellets, torrefied wood pellets and pyrolysis slurry (a mixture of

129

pyrolysis char and oil), integrated with a municipal combined heat and power plant.

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They concluded that the highest exergy destruction is caused in the combustion

131

equipment, whereas the upgrading processes appear highly efficient. The systems’

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exergetic efficiency can be improved by 22%, 26% and 31% when integrated with

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pyrolysis slurry, torrefied wood pellets and wood pellets, respectively, making wood

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pellets the most efficient integration option. A. Abuadala and I. Dincer [23] developed

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a conceptual hybrid biomass gasification system to produce hydrogen and analyzed

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the exergoeconomic performance. They found that for a gasification temperature

137

ranging from 1023-1423 K and with an electricity cost of 0.1046 $/kWh considered,

138

the unit exergy cost of hydrogen ranges from 0.258 to 0.211 $/kWh. A. Sahin et al.[24]

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carried out an exergoeconomic analysis for a combined cycle power plant using the

140

first law and the second law of thermodynamics, and the economic principles while

141

incorporating GT PRO/PEACE Software Packages. They compared four scenarios

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and found that the optimum size and the configuration of the power plant differ for

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each scenarios considered. The selection and optimization of the size and

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configuration of the power plant are found to be depending on the user priorities and

145

the weight factors assigned to the performance indicators. S. Khanmohammadi et al.

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[25] made a thermodynamic and economic analysis of a combined gas turbine and

147

Organic Rankine Cycle integrated with a biomass gasifier. The result of multi-

148

objective optimization shows that the exergy efficiency of the system is 15.6%, which

149

can be increase to 17.9% in the optimal state, regardless of the total cost rate of

150

system as objective function. A. Abusoglu and M. Kanoglu [11, 12] proposed the

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comprehensive exergoeconomic analysis of diesel engine powered cogeneration based

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on specific cost method, and the specific unit exergetic costs of power and steam

153

produced by the cogeneration plant are 10.31 $/GJ and 33.71 $/GJ respectively.

154

Exergoeconomic analyses focus on the cost allocation. Common cost allocation

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methods include the equivalent method, the extraction method and the by-product

156

method [26]. The above three methods ignore the unit cost of exergy flows changing

157

along with energy level, which are against the principle of high quality and high price.

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To solve this problem, H. Qi et al.[27] analyzed the change of energy level during the

159

energy utilization and built the function relationship between the cost and exergy to

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propose a new cost allocation method. Based on the new method, the influence of the

161

main design parameters on the thermal power cost has been researched, and the

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results show that improving the efficiency of the compressor and turbine can reduce

163

thermal power cost at the same time.

164

The originality of this paper lies in proposing two CCHP schemes integrated

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with biomass-steam gasification and modifying the exergoeconomic method based on

166

energy level to analyze the cost allocation of multi-products in the CCHP system.

167

Section 2 presents the energy flowcharts and thermodynamic models of two CCHP

168

schemes integrated with biomass-steam gasification, Section 3 proposes the modified

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exergoeconomic method based on energy level, Section 4 analyzes the

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thermodynamic performances and illustrates the validity of the new method, and

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finally, the conclusions are obtained in Section 5.

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2 CCHP schemes integrated with biomass-steam gasification

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The CCHP system integrated with biomass-steam gasification can reasonably

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utilize the renewable biomass energy and provide multiple energy sources for users.

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However, biomass-steam gasification is an endothermic process, it is necessary to

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provide heat to the biomass-steam gasification. The common methods are using 7

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electricity to drive the gasification process or burning the product gas to drive the

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gasification process. Based on this, two CCHP schemes integrated with biomass-

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steam gasification have been proposed.

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2.1 Schemes

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2.1.1 Biomass-steam gasification driven by electricity

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The flowchart of the CCHP system integrated with biomass-steam gasification

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driven by electricity energy is shown in Fig. 1 (It is called with electricity-driving

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CCHP system). The electricity-driving CCHP system integrated with biomass steam

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gasification mainly includes gasifier, electric heater unit, heat pipe heat exchanger

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(HPHE), product gas conditioning subsystem (i.e., cyclone, spray scrubber), Roots

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blower, gas storage tank, internal combustion engine (ICE), two-stage Libr-H2O

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absorption chiller/heater, plate heat exchanger (PHE), and hot water tank.

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The biomass with steam is gasified in the gasifier, and then, the high temperature

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product gas is cooled in the HPHE to transform water into steam. The biomass tar in

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the product gas is condensed when the temperature is below 473 K [28], and it easily

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combines with water and carbon granules to pollute the equipment. To prevent

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pollution, the high temperature gas is cooled above 473 K. The cooled product gas is

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then purified in the cyclone and further cooled in the spray scrubber, and the tar is

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separated. The clean product gas is stored in a tank as fuel for the CCHP system. The

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product gas is used as fuel in the ICE to generate electricity and part of the electricity

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is provided to the users and other part of the electricity is used to drive the electric

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heater unit because of the endothermic gasification process. The jacket water from the

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ICE and the exhausted gas is sent to a two-stage Libr-H2O absorption chiller/heater to

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produce cooling in summer and heating in winter, and the domestic hot water,

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respectively. Additionally, the product gas is supplements to drive absorption 8

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chiller/heater when the recovered heat from jacket water and exhausted gas is

203

insufficient.

204

The system has realized the rational and step utilization of energy. The HPHE is

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used to utilize the sensible heat of the product gas for getting steam, which prevents

206

the need for a separate steam generator. The exhausted gas is used by the two-stage

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Libr-H2O absorption chiller/heater and the PHE successively, which realizes the

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cascade utilization of exhausted gas.

209

2.1.2 Biomass-steam gasification driven by thermal energy from product gas

210

The difference of the CCHP system integrated with biomass-steam gasification

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driven by thermal energy from product gas is to use gas high-speed burner to replace

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the electric heater unit, and the flowchart is shown in Fig. 2 (It is called with product-

213

gas-driving CCHP system). Compared to the electricity-driving CCHP system, the

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heat provided to the gasification process in this system comes from burning product

215

gas straightly, and the processes of generating electricity and heat utilization are

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almost the same.

217

2.2 Models

218

The models in the two CCHP systems mainly include biomass-steam gasification,

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ICE, absorption chiller/heater and heat exchanger. The ICE, absorption chiller/heater

220

and heat exchanger have been modeled and presented in our previous work [2]. The

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ICE model was modified at the base of natural gas ICE because of the difference

222

characteristics between natural gas and product gas [2]. Herein, they are not

223

introduced in detail. Due to the difference with the previous works, the biomass-steam

224

gasification model is constructed as follows.

225

The gasification process can be simplified to

9

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226

CH x O y N z Sw +aH 2 O   rchar Char   rtar Tar  rCO CO  rH 2 H 2  rCO2 CO 2

(1)

 rCH 4 CH 4  rH 2O H 2 O  rN2 N 2  rSO2 SO 2

227

where x, y, z and w are the numbers of atoms of hydrogen, oxygen, nitrogen and sulfur

228

per number of atoms of carbon in the feedstock, respectively; a is the mole amount of

229

steam;  rchar ,  rtar , rCO , rH 2 , rCO2 , rCH 4 , rH 2O , rN2 and rSO2 are the numbers of moles

230

of the species in the final product gas; rchar and rtar are the numbers of mole of the

231

char and tar produced in the pyrolysis unit; and  and  are the coefficients of

232

exhaust and residual of the production in the pyrolysis unit after gasification.

233

The aim of modeling biomass-steam gasification is to predict the compositions

234

and the lower heating value (LHV) of product gas. Herein, thermochemical

235

equilibrium modeling [29] is adopted, and the equilibrium constants K1 and K2 for

236

water-gas shift reaction and methane reaction mainly affect the calculation results.

237

They are estimated according to the formulations from Zainal et al. [29] as follows:

238

ln K1 =

239

ln K 2 =

240 241

5870.53 58200  1.86 ln TG  2.7 104 TG   18.007 TG TG 2

(2)

7082.848 7.446 103 2.164 106 2 0.701105 -6.567 ln TG  TG  TG + +32.541 TG 2 6 2TG 2 (3)

Figure 3 shows the simulation procedure of gasification. Firstly, the

242

compositions based on biomass ultimate analysis ( x, y, z , w ) and the coefficients of

243

exhaust and residual based on experiment data (  =40% and  =5% ) are set, and the

244

gasification temperature (TG) to affect the equilibrium constants is initialized. Then,

245

the pyrolysis temperature (TG), equilibrium constants (K1 and K2), and char and tar

246

moles ( rchar and rtar ) are calculated, and the species moles in the product gas ( ri ) are

247

calculated and predicted. Finally, the enthalpy balance between biomass input and gas 10

ACCEPTED MANUSCRIPT 248

output is judged and verified. The simulation procedure ends when the judged

249

condition is satisfied. Otherwise, the gasification temperature is modified and the

250

simulation procedure is repeated again. The detail information can be found in

251

literature [4].

252

The model of biomass-steam gasification is compared with the experiment from

253

literature [30]. The experiment conditions in [30] are inputted into the simulation

254

model as follows: the gasification temperature is 1200 ℃, the mass rate of biomass

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and steam is 0.325, and the steam is 0.2 MPa and 450 ℃. The comparison results are

256

listed to in Table 1. The root-mean square error is 0.62%, and the precision of the

257

simulation model can be satisfied the calculation demand of the integrated CCHP

258

system.

259

2.3 Operation mode and design parameters

260

The proposed two CCHP systems both take the ‘following the electrical loads

261

(FEL)’ mode to work, which means the system should first meet the demand of

262

building electric load and when the recoverable heat is larger than the heat demand,

263

the redundant heat is released to the environment or provided to other users. When the

264

recoverable heat is smaller than the heat demand, the absent heat is provided through

265

burning the product gas. The common design parameters are listed in Table 2 to

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compare the thermodynamic performance of two CCHP schemes.

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3 Modified exergoeconomic method

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3.1 Exergoeconomic balance equations

269

The exergoeconomic analysis is an effective way to calculate the cost per exergy

270

unit of the product streams of the system and find the influence factors. The base is

271

the exergy balance, and the exergy balance of k-th component is expressed to:

272

El ,k  E f ,k  E p ,k 11

(4)

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where E is the exergy of each stream, E f ,k , E p ,k and El ,k are exergy of fuel, product,

274

and loss, respectively.

275

A cost balance applied to the k-th component shows that the sum of product costs

276

equals the sum of fuel costs plus the appropriate charges (cost rate) due to investment

277

capital and operating and maintenance expenses. The sum of the last two terms is

278

defined by Z . Accordingly, for the k-th component: c f ,k E f ,k  Z k  c f ,k E p ,k

279

(5)

280

where C  cE and C is the total cost of per exergy of each stream, c is the cost per

281

exergy unit of each stream. Z k is the cost rate, and it consists of the levelized

282

investment capital and the levelized operation and maintenance cost.

283

Taken product-gas-driving CCHP system in Fig. 2 as an example, there are eight

284

equipments and two separation points, and ten balance equations can be written and

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summarized into Table 3.

286

3.2 Auxiliary costing equations

287

A proper ‘fuel-product-loss’ (F-P-L) definition of the system is the key to

288

indicate the real production purpose of its subsystem. The F and P principles in

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exergoeconomic methodology are the key points of the specific exergy costing

290

approach that is employed in literature [31]. The costing principles for multi-products

291

[7] are:

292

(1) The F principle states the specific cost (cost per exergy unit) associated with

293

this removal of exergy from a fuel stream must be equal to the average specific cost at

294

which the removed exergy was supplied to the same stream in upstream components.

295 296

(2) The P principle states that each exergy unit is supplied to any stream associated with the product at the same average cost.

12

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According to the F-P principles, the following auxiliary equations in the cascade heat utilization of exhausted gas in Fig. 4 are listed as:

299

c16  c20  c28

(6)

300

c22 21  c27

(7)

301

where c16 , c20 and c28 are the unit exergy costs of exhausted gas, respectively, and

302

they are equal because of the same fuel stream. c21 22 and c27 are the unit exergy costs

303

of chilled water and domestic hot water, respectively, and they are equal according to the P

304

principle.

305

Obviously, the input exhausted gas of the absorption chiller/heater has the higher

306

energy level than the input exhausted gas of the PHE because of their different

307

temperature. It is against the principle of the high quality and high price. In fact,

308

during the use of the exhausted gas, its cost should decrease along with its decreasing

309

energy level. Additionally, the output exhausted gas is released to the air and not be

310

used so that its unit exergy cost should be zero. According to the principle of the high

311

energy level and high price, the rate of cost of per exergy unit is directly proportional

312

to energy level. Therefore, Eq.(6) and Eq.(7) can be modified to

313

c16 c20 c28   A16 A20 A28

(8)

314

c22 21 c27  A22 21 A27

(9)

315

where A is the energy level per exergy unit of each stream. Energy level is used to

316

evaluate the ability of energy transforming into useful work, which reflects the quality

317

of the energy and shows the physical and chemical nature at the same time. The

318

definition formula is: 13

ACCEPTED MANUSCRIPT A

319

dE dS  1  T0 ( ) dH dH

(10)

320

where dE is the max power capability, dH and dS is the enthalpy change and entropy

321

change in the process, and T0 is the reference temperature and

322

c21 20 and A21 20 are the unit cost and energy level of chilled water. During heating/cooling

323

supply, the heating/cooling energy amount being product is sold to users. Consequently,

324

c21 20 and A21 20 represent the unit cost and energy level of the heat exchange energy

325

in the process of water from high temperature state to low temperature state. The

326

energy levels of heating energy and cooling energy can be estimated respectively：

A1415  327



328

A22 21 

T0  298.15 K .

In Eq.(9),

ET  ET15 dE1415  14 dH1415 H T14  H T15 TT140 mc p (1 

TT022 mc p (

T0 T )dT  TT150 mc p (1  0 )dT T0 T T T  1 ln 14 mc p (T14  T15 ) T14  T15 T15

T0 T  1)dT  TT021 mc p ( 0  1)dT T0 T T T  ln 22  1 mc p (T22  T21 ) T22  T21 T21

(11)

(12)

329

where m and c p are mass flow rate and specific heat of chilled/hot water respectively, T14

330

and T15 are temperatures of supply and return hot water respectively ( T14  T15 ), and T22 and

331

T21 are temperatures of supply and return hot water respectively ( T22  T21 ).

332

Based on the above costing principle, the auxiliary equations of the product-gas-

333

driving CCHP system are listed into Table 3.

334

3.3 Product costs

335

In above equations, the cost of the income air streams (such as states 6, 13 and

336

18 in Fig. 2 and C16  C13  C18  0 ) and the cost of the exhausted gas stream (state

14

ACCEPTED MANUSCRIPT 337

28 in the Fig. 2 and C28  0 ) is also assumed to be zero. The cost of the biomass

338

stream to the system C1 is calculated by the following formula:

C1  3600 p1m1

339 340

where p1 is the unit price of biomass (Yuan/kg) and

341

biomass stream to the system (kg/s).

342

(13)

m1 is the mass flow rate of the

The domestic hot water cannot be cycle used like the chilled water or the hot

343

water for heating. To calculate the unit exergy cost of domestic hot water, it is

344

necessary to take the additional price of tap water into account. The cost of tap water

345

to the system C2 is calculated as:

C2  3600 p2 m2

346

(14)

347

where p2 is the unit price of tap water (Yuan/kg)， m2 is the mass flow rate of the tap

348

water (kg/s).

349

Finally, the product costs of the CCHP system can be obtained as:

351

C12 E12

(15)

C12  C21 E22  E21

(16)

C29 E29

(17)

ce 

350

cchilled / hot  water 

cd  hot  water 

352

353

where ce , cchilled / hot  water and cd  hot  water are the unit costs of exergy of electricity,

354

chilled/hot water and domestic hot water, respectively.

355

4 Results and discussions

356

4.1 Thermodynamic performances

357 358

To compare the thermodynamic performances of CCHP schemes integrated with different biomass gasification, the following schemes are considered: 15

ACCEPTED MANUSCRIPT 359

S1: Electricity-driving system in Fig. 1

360

S2: Product-gas-driving system in Fig. 2 operating in the cooling mode

361

S3: Product-gas-driving system in Fig. 2 operating in the heating mode

362

S4: CCHP system integrated with biomass-air gasification in literature [2]

363

S1 and S2 are taken consideration into the thermodynamic analysis due to the

364

different cooling/heating loads. The gasification results of the four CCHP schemes are

365

shown in Figure 5. Compared between CCHP schemes integrated with biomass-steam

366

gasification, the LHV of product gas through electricity-driving, 9.7 MJ/Nm3 is larger

367

than the LHV of the product-gas-driving CCHP, 6.3 MJ/Nm3. The reason is that, to

368

the product-gas-driving CCHP system, the exhausted gas from the gas high-speed

369

burner is inlet to the gasifier to provide heat for gasification process, which is mixed

370

with the gasification gas and affects its composition and LHV. Consequently, the

371

nitrogen content accounts for 28.3% approximately in the product-gas-driving CCHP

372

system while only 0.4% in the electricity-driving system. Meanwhile, the hydrogen

373

content reaches approximately 60.7% in the electricity-driving system and it is main

374

constituent in the product gas. Then, compared the gasification of product-gas-driving

375

CCHP system in cooling and heating modes, the compositions are similar, but they

376

are different. It is resulted from the different heat balances in different biomass flow

377

rates under cooling and heating modes. Moreover, the biomass gasification results

378

with steam and air are compared, and the LHV is the lowest in the biomass-air

379

gasification because the nitrogen content accounts for 79% while the gasifying agent

380

is only 21%.

381

Then, the thermodynamic results of the four CCHP schemes are shown in Figure

382

6. It can be found that the three CCHP systems have larger system energy efficiency

383

in cooling condition than heating condition. This phenomenon is caused by the larger 16

ACCEPTED MANUSCRIPT 384

COP of the two-stage Libr-H2O absorption chiller/heater in cooling condition than

385

heating condition. However, the system exergy efficiency shows the opposite results.

386

The system exergy efficiencies of electricity-driving and product-gas-driving CCHP

387

systems in the cooling condition are 13.27% and 16.38%, which are smaller than the

388

14.13% and 21.20% efficiencies in the heating condition. The reason is that the

389

exergy of chilled water with 280/285K is much lower than the exergy of hot water

390

with 338/328 K. Compared between CCHP schemes integrated with biomass-steam

391

gasification, the gasification efficiency of the electricity-driving CCHP system, 87%,

392

is larger than the product-gas-driving CCHP system. However, the system energy

393

efficiency and exergy efficiency of the product-gas-driving CCHP system are larger

394

than the electricity-driving CCHP system both in the cooling and heating conditions.

395

Analyzed the influence mechanism, the main factor is that the biomass self-

396

consumption ratio in the electricity-driving gasification process is larger than the

397

product-gas-driving gasification. The gasification process consumptions are shown in

398

Table 4. The electricity-driving CCHP system consumes 67.61% of power in the

399

cooling condition and 64.29% of power in the heating condition respectively. In

400

contrast, the product-gas-driving CCHP system consumes only 20.65% and 20.50%

401

respectively.

402

Compared between CCHP schemes integrated with biomass-steam and biomass-

403

air gasification, the two biomass-steam gasification CHHP systems both have the

404

higher energy and exergy efficiencies. The key factor is that biomass-steam

405

gasification has the higher gasification efficiency than the biomass-air gasification.

406

Finally, it can be concluded that the CCHP scheme integrated with product-gas-

407

driving gasification is the best scheme from the energy and exergy efficiencies.

408

4.2 Exergoeconomic performances 17

ACCEPTED MANUSCRIPT 409

According to the thermodynamic results, the exergoeconomic performances of

410

the product-gas-driving CCHP system with the highest efficiency are analyzed. The

411

initial investment and parameters of the CCHP system integrated with biomass steam

412

gasification are shown in Table 5. Gasification system includes gasifier, gas

413

conditioning and auxiliary equipment, which the gasifier accounts for 95% of the

414

investment and the gas conditioning and other auxiliary components account for 5%.

415

At the building loads in Table 2, the absorption chiller/heater is only used to produce

416

chilled/hot water for cooling/heating and no domestic hot water is outputted. Thus,

417

C27  0 , and the tank is only used to store hot water so that it can be neglected.

418

4.2.1 Validity check

419

To verify the validity of the modified exergoeconomic method based on the

420

energy level, the multi-products of ICE including electricity, exhausted gas and jacket

421

water are calculated, and the unit costs are shown in Fig. 7 in the cooling mode. If the

422

previous method adopting the F-P principles is applied, the unit exergy costs of three

423

products are same and approximately 0.38 Yuan/kWh, which cannot reflect the

424

principle of high quality with high price. Comparably, the costs adopting the modified

425

exergoeconomic method are 0.48 Yuan/kWh, 0.23 Yuan/kWh and 0.07 Yuan/kWh,

426

which are proportional to their energy levels 1.0, 0.48 and 0.15 respectively. The

427

electricity with the highest energy level has the highest cost while the jacket water has

428

the lowest cost due to its lowest energy level. The comparisons between three

429

products from the same component indicate the principle of high quality with high

430

price. Therefore, the modified method based on energy level is more reasonable and

431

validate.

432 433

Additionally, the unit exergy cost of the output exhausted gas of the PHE is equal to the input exhausted gas in the previous exergoeconomic method. However, in the 18

ACCEPTED MANUSCRIPT 434

modified method, the unit exergy cost of the output exhausted gas is apportioned by

435

the products and is set to zero. There are two different methods of apportionment. One

436

is that since the hot water is the last product which using the exhausted gas and the

437

output exhausted gas is released to the air after the PHE, the cost of the output

438

exhausted gas is shared by the hot water. The result is that the unit exergy cost of the

439

three products is 0.45 Yuan/kWh, 2.63 Yuan/kWh and 6.08 Yuan/kWh (electricity,

440

chilled water and domestic hot water, respectively) in the cooling mode.

441

The other one is that, the ICE produces the exhausted gas, the absorption

442

chiller/heater and the PHE both use the exhausted gas. So, the cost of the output

443

exhausted gas should be shared by all the three products. The result is that the unit

444

exergy cost of the three products is 0.48 Yuan/kWh, 2.69 Yuan/kWh and 5.06

445

Yuan/kWh (electricity, chilled water and domestic hot water). It can be found that the

446

costs of electricity and chilled water increase while the hot water cost decreases due to

447

the cost shares by ICE, absorption chiller and PHE. This method is more reasonable to

448

the principle of high quality with high price.

449

4.2.2 Cost allocations

450

Under this apportionment method, Table 6 shows the unit energy cost and unit

451

exergy cost of the three products of the product gas-driving CCHP system by taking

452

the energy level into consideration.

453

From Table 6, the unit energy cost and unit exergy cost of the hot water is the

454

highest, which is mainly caused by the unit price of tap water because the hot water

455

cannot be cycle used. The unit price of tap water (7.15 Yuan/t) is equivalently

456

converted to 3.31 Yuan/kWh. The cost of tap water accounts for a large proportion

457

over 65.42% of the unit exergy cost of hot water in the cooling mode and 46.36% of

458

the unit exergy cost of hot water in the heating mode. Moreover, the electricity with 19

ACCEPTED MANUSCRIPT 459

high energy level has the low unit exergy cost, which is caused by two reasons: 1) The

460

power unit is in the front of the system process, while making chilled water and hot

461

water is the follow-up process, thus their unit exergy costs increase with the

462

equipment initial investment. 2) The energy levels of the chilled water (heating water)

463

and hot water are low and their exergy values are small, so their unit exergy costs are

464

high.

465

Comparing the unit exergy cost of products in cooling and heating modes, it can

466

be seen that the unit exergy cost of products in cooling mode is larger than these in

467

heating mode. The reason is that the loads in summer is larger than the loads in winter,

468

so the needs of biomass is larger and the unit exergy cost of product gas is smaller,

469

which leads to the smaller unit exergy cost of products.

470

Similarly, the unit exergy cost of products of the product-gas-driving CCHP

471

system in the two operation modes (S2 and S3 schemes) using the previous and

472

modified methods are compared in Fig. 8. The results of the modified method show

473

that the unit cost of electricity is higher and other unit cost of chilled water and

474

domestic hot water are lower than before. This phenomenon is in keeping with the

475

principle of high quality with high price. Moreover, the unit cost differences of the

476

three products are 0.08, 0.24 and 0.75 Yuan/kWh (electricity, chilled water and

477

domestic hot water, respectively) in the cooling mode and 0.09, 1.16 and 0.51

478

Yuan/kWh (electricity, hot water for heating and domestic hot water, respectively) in

479

the heating mode. The cost ratio of the three products of the product-gas-driving

480

CCHP system is 1.0: 5.6: 10.6 (electricity: chilled water: domestic hot water) in the

481

cooling mode and 1.0: 6.8: 13.1 (electricity: hot water for heating: domestic hot water)

482

in the heating mode.

20

ACCEPTED MANUSCRIPT 483

Compared to the unit exergy costs of the biomass-air gasification CCHP system

484

(S3 scheme) in the reference [31] in Fig. 8, the biomass-steam gasification CCHP

485

system has the larger unit exergy costs of products except to the chilled water, which

486

is mainly caused by the larger initial investment. The chilled water cost calculated in

487

the modified method is lower than that in the previous method. It is caused by the

488

different energy flowchart that the higher upstream cost of gasification system is early

489

allotted to chiller (the recovered heat from gasification is utilized to drive absorption

490

chiller) in the biomass-air gasification system.

491

4.2.3 Sensitivity analysis

492

Sensitivity analysis can visually display the variations of the unit cost of products

493

caused by changes in important parameters. Figure 9 shows the variation of the unit

494

cost of products with biomass cost in the cooling mode. The biomass cost is set to the

495

increase or decrease from 5% to 25% from the base design. The unit exergy cost of

496

electricity, chilled water, and domestic hot water increase linearly with increasing

497

biomass cost. The different products have different increasing rates. The chilled water

498

has the biggest increasing rate, while the electricity has the smallest increasing rate.

499

This phenomenon is caused by the energy levels of the products. The smaller the

500

energy level is, the more sensitive the unit cost will be.

501

Figure 10 shows the variation of the unit cost of products with operation

502

coefficient (which is defined to the ratio of annual operation hours to 8760 hours). It

503

can be seen that the unit exergy cost of product decreases with the increasing

504

operation time, which is mainly caused by the decreasing levelized investment capital.

505

The decreasing rate of electricity in the cooling mode is larger than in the heating

506

mode.

507

5 Conclusions 21

ACCEPTED MANUSCRIPT 508

This paper proposed two biomass CCHP systems in which the gasification

509

endothermic process is driven by electricity energy and thermal energy from product

510

gas, and modified the exergoeconomic analysis method based on energy level. A case

511

study was presented to analyze the thermodynamic and exergoeconomic performances

512

and to verify the validity of the modified exergoeconomic method. The following

513

conclusions were obtained:

514

The biomass-steam gasification driven by electricity is helpful to improve the

515

LHV of product gas. The LHV of product gas in electricity-driving, 9.7 MJ/Nm3 is

516

larger 53% than that in product-gas-driving, 6.3 MJ/Nm3. However, the system energy

517

and exergy efficiencies of product-gas-driving CCHP system are higher than that of

518

electricity-driving CCHP system because the electricity-driving gasification process

519

consumes overlarge power driven by product gas. From the aspects of energy and

520

exergy efficiencies, the product-gas-driving CCHP system is better than the

521

electricity-driving CCHP system.

522

The costing principles for multi-products in the CCHP systems dramatically

523

influence the cost collations. The modified exergoeconomic method illustrates the

524

principle of high energy level with high cost. Compared with the previous

525

exergoeconomic method, the results of the modified method show that the electricity

526

with higher energy level increases 0.09 Yuan/kWh while the cost of other products

527

with lower energy level decrease. To the product-gas-driving CCHP system under the

528

base design, the average unit exergy cost of electricity water for cooling and heating,

529

and domestic hot water are respectively 0.52 Yuan/kWh, 2.69 Yuan/kWh, 3.74

530

Yuan/kWh and 6.10 Yuan/kWh when the biomass cost is 0.35 Yuan/kg. The

531

sensitivity analysis indicates that the chilled water cost is more sensitive to biomass

532

cost while the electricity cost is influenced slightly. The long operation time is helpful 22

ACCEPTED MANUSCRIPT 533

to decrease the product cost due to the lower levelized investment capital in the

534

exergoeconomic balance equations.

535

The modified exergoeconomic model combines energy level with energy quality

536

to obtain the cost allocations of multi-products from the CCHP system. The results

537

indicate that the product cost is more reasonable, and the allocations are in agreement

538

with the principle of high energy level with high cost. Especially, the energy levels of

539

different forms of energy sources such as renewable energy, thermal energy, high-

540

temperature exhausted gas, and the gas emitted to ambient are emphasized and

541

obtained during the application analysis.

542

Acknowledgements

543

This research has been supported by National Natural Science Foundation of

544

China (51406054).

545

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638 639

27

ACCEPTED MANUSCRIPT 640

Table captions

641

Table 1 Comparison between simulation and experimental data

642

Table 2 Base design parameters

643

Table 3 Exergoeconomic and auxiliary costing equations of the components

644

Table 4 The gasification process consumption results

645

Table 5 Initial investment and parameters in economic analysis

646

Table 6 The unit energy cost and unit exergy cost of the products

647

28

ACCEPTED MANUSCRIPT 648

Figure captions

649

Fig. 1 Flowchart of the CCHP system integrated with biomass-steam gasification

650

driven by electricity

651

Fig. 2 Flowchart of the CCHP system integrated with biomass steam gasification

652

driven by thermal energy from product gas

653

Fig. 3 Simulation procedure of biomass-steam gasification model

654

Fig. 4 The cascade heat utilization of exhausted gas

655

Fig. 5 Product gas parameters in different gasification methods

656

Fig. 6 thermodynamic performances of different CCHP schemes

657

Fig. 7 Unit exergy costs of multi-products from ICE adopting previous and modified

658

methods

659

Fig. 8 Unit exergy cost of products in the two operation modes

660

Fig. 9 Sensitivity of unit exergy cost of products to biomass cost

661

Fig. 10 Sensitivity of unit exergy cost of electricity to operation coefficient

662 663

29

ACCEPTED MANUSCRIPT Gasifier 1

Biomass

Gas conditioning

4 Steam

HPHE 7

6

5

8 Biomass gas

3 Water

2

10 Air 24

28 Absorption chiller/heater

PHE

16

Gas ICE 9

13

23 26 Exhausted gas

14 15 Jacket water

Tank

25

22

27

Domestic hot water

20 21 Cooling water

Biomass Steam Gasification gas

11 12 5 17 Power

19 18 Chilled water Jacket water Domestic hot water Exhausted gas

Product gas Air Power

Air

Chilled water Water Cooling water

Fig. 1 Flowchart of the CCHP system integrated with biomass-steam gasification driven by electricity

Gasifier 1

Gas high-speed burner

5

Biomass

6

Air

7

4 Steam

Gas conditioning

HPHE

8

9 10 Biomass gas

3 2

Water

12 Air 23

19 Absorption chiller/heater

PHE

18

11

16

20

28 Exhausted gas

Gas ICE

14

24

Tank

Domestic hot water

29

Biomass Steam Gasification gas

13

15 Jacket water

Air

27 25 26 Cooling water Product gas Air Power

22 21 Chilled water Jacket water Domestic hot water Exhausted gas

17 Power Chilled water Water Cooling water

Fig. 2 Flowchart of the CCHP system integrated with biomass steam gasification driven by thermal energy from product gas

ACCEPTED MANUSCRIPT

Begin

Initialization： Gasification temperatureTG

x, y, z, w, a,  , 

Calculate pyrolysis temperature TP , equilibrium constants( K1 , K 2 ), and rchar , rtar

Calculate specie moles ri

Modify TG

Calculate enthalpy balance difference H

No

H  0? Yes End

Fig. 3 Simulation procedure of biomass-steam gasification model

c16 Exhausted gas

c27 Hot water

Fig. 4

c20

Absorption chiller/heater

22

21

Chilled water

c22 21

PHE

c24 Hot water

The cascade heat utilization of exhausted gas

c28

ACCEPTED MANUSCRIPT 21

19.9 19.8 S1:Electricity-driving S1:Electricity-driving system system 60.7

59.7

Ultimate analysis, %

60

18

S2: Product-gas-driving S2: Product-gas-driving system- system15.2 cooling mode cooling mode 13.8 13.3systemS3:Product-gas-driving S3:Product-gas-driving system12.1 38.8 39.0 heating mode heating mode 11.0 S4: Biomass-air S4:11.1 Biomass-air gasification gasification

50 40

15 12 9.7 28.4 28.2

30 20 10 0

1.9 1.8 0.5 0.5 1.9 1.81.1 1.1

CH CH4 4

15.2 13.8 11.0 11.1

9

23.2 19.9 19.8

6.3 6.3

LHV, MJ/Nm3

70

6

13.3

12.1

3.3 9.7 6.3 6.3 3.3

0.4 0.4

3 0

CO

CO2 CO2

H2 H2

N2 N2

LHV

Fig. 5 Product gas parameters in different gasification methods

100

100

Efficiency, % Efficiency, %

10080 80 60 60 40

83.9

87.0

87.0

81.3

83.1

80 83.9 66.1

58.3 58.350.0

81.3

Efficiency, %

87.0

60 66.1 40

64.3 58.3 50.8 50.0

37.8

40

13.316.4 20

20 20

0 0

13.3

6.2

21.2 14.1 12.5 16.4 6.2

0 Gasification Energy Gasification Cooling mode S1:Electricity-driving system

Exergy

Gasification Energy

S2/S3: Product-gas-driving system

Energy

Heating mode

Exergy Exergy

S4: Biomass-air gasification

Fig. 6 thermodynamic performances of different CCHP schemes

ACCEPTED MANUSCRIPT

1.00

0.5

Previous method Modified method Energy level

0.48

0.38

0.4

0.38

0.3

0.38

1.0 0.8 0.6

0.23

0.48

1.2

0.2

0.4

Energy level

Unit exergy cost, Yuan/kWh

0.6

9

0.1

7.65

8

0.15 0.07 7.14

Cost, Yuan/kWh

7

0.0

6

0.0

4.90

5

Electricity

4

Exhuasted 3.74 gas

3

0.2

Jacket water

4.86

2.64

2 Fig. 7 Unit exergy costs of multi-products from ICE adopting previous and modified methods 0.46

1

0.55

0.44

0

8

Cost, Yuan/kWh

7 6 5 4 3

Heating water Domestic hot water 9 S2/S3:Previous method S2/S3:Modified method 7.65 8 7.14 S4:Biomass-air gasification CCHP system 7 5.81 6 5.06 4.90 4.86 4.75 5 3.74 4 2.932.69 3.08 3 2.64

Cost, Yuan/kWh

Electricity

9

2 1

2 1

0.40 0.48 0.41

0

0.46 0.55 0.44

0 Electricity

Chilled water

Cooling mode

Domestic hot water

Electricity

Heating water

Heating mode

Fig. 8 Unit exergy cost of products in the two operation modes

Domestic hot water

ACCEPTED MANUSCRIPT

5.0

y = 0.042x + 4.811

4.0

Electricity Chilled water Hot water

3.0

y = 0.066x + 2.295

2.0 1.0 y = 0.011x + 0.406

0.0 -25% -20% -15% -10%

-5%

0.350

+5% +10% +15% +20% +25%

Biomass cost, Yuan/kg

Fig. 9 Sensitivity of unit exergy cost of products to biomass cost 1.0

Unit cost of electricity , Yuan/kWh

Unit costs of three products, Yuan/kWh

6.0

0.9 Cooling mode

0.8

Heating mode 0.7 0.6 0.5 0.4 0.3

0.4

0.5

0.6

0.7

0.8

0.9

Operation coefficient

Fig. 10 Sensitivity of unit exergy cost of electricity to operation coefficient

1.0

ACCEPTED MANUSCRIPT Highlights: > Propose two CCHP schemes integrated biomass-steam gasification. > Modify the exergoecnomic analysis method based on energy level. > Compare thermodynamic performances of CCHP systems integrated with different gasification methods. >Analyze the cost allocations of multi-products with energy level consideration.

ACCEPTED MANUSCRIPT Table 1 Comparison between simulation and experimental data Compositions（%） H2

CO

CO2

Experiment

54.42

44.22

1.36

Simulation

54.38

43.25

1.80

Root-mean-square error

0.62

Table 2 Base design parameters Building loads Biomass

Air Technical parameters

Parameters Cooling condition, kW Heating condition, kW Material Moisture content, % Ultimate analysis, % Low heating value (LHV), kJ/kg Ultimate analysis, % Pressure, kPa Temperature, K Chilled water temperature, K Hot water temperature for heating, K Domestic hot water temperature, K ICE Absorption chiller HPHE

Values Electricity: 446, cooling: 1804, hot water: 335 Electricity: 446, heating: 595, hot water: 335 Wheat straw 3.28 C: 45.17, H: 5.75, O: 35.66, N: 0.86, S: 0.14 19.054 N2: 79, O2: 21 101.325 298 280/285 338/328 333 Generation efficiency 35% Coefficient of performance (COP): 1.27 (cooling), 0.93 (heating) Thermal efficiency 94%

ACCEPTED MANUSCRIPT Table 3 Exergoeconomic and auxiliary costing equations of the components. Component

Exergoeconomic balance equations

Gasifier

C8  C1  C4  C5  Z G

Gas high-speed burner

C5  C6  C7  Z GH

HPHE

C4  C3  C8  C9  Z HPHE

Gas conditioning

C10  C9  Z GC

c8 A8  c9 A9

(C22  C21 )  (C27  C19 ) 

Absorption chiller/heater

(C12  C18  C16  C20 )  (C14  C15 )  Z AC / H

Gas ICE

C17  C16  (C14  C15 )  C11  C13  Z GI

PHE

C24  C23  C20  C28  Z PHE C29  C24  C27  ZT C10  C7  C11  C12 C2  C24  C28  C3

Tank Separation point

Auxiliary costing equations

c22 21 A22 21 c16 A16  ,  c27 19 A27 19 c20 A20 c17 A17 c1415 A1415  ,  c16 A16 c17 A17

c7  c11 , c11  c12 c2  c24 , c24  c28 , c28  c3

Table 4 The gasification process consumption results CCHP system

Mode

Electricitydriving Product-gasdriving

Cooing Heating Cooing Heating

Total power/product gas (kW) 1377 1249 3306 2265

Power/Product gas for gasification (kW) 931 803 682 464

Self-consumption ratio (%) 67.61 64.29 20.63 20.49

Table 5 Initial investment and parameters in economic analysis Items

Component

Investment

Gasification system Gas high-speed burner Electric heater unit Gas ICE Absorption chiller/heater HPHE PHE

Parameter

Unit cost (Yuan/kW) 2500 1200 220 4800 1200 210 210

Capacity (kW) 3164 695 1377 446 1804 612 182

Investment (103 Yuana) 7910 834 302.9 2140.8 2164.8 128.52 38.22

Service life, year 20 Interest rate, % 6.15 Operation hours, h 6000 Maintenance cost ratiob, % 2.5 Unit price of biomass, Yuan/kg 0.35 Unit price of tap water, 10-3Yuan/kg 7.15 a 1 US$=6.12 Yuan. b The maintenance cost ratio is defined as the ratio of the maintenance cost to the investment cost.

ACCEPTED MANUSCRIPT

Table 6 The unit energy cost and unit exergy cost of the products Cooling mode Energy Exergy (Yuan/kWh) (Yuan/kWh) Electricity 0.48 0.48 Chilled/Heating water 0.14 2.69 Hot water a 0.28 5.06 Hot water b 0.10 1.75 a: the hot water cost includes tap water cost. b: the hot water cost doesn’t include tap water cost. Products

Heating mode Energy Exergy (Yuan/kWh) (Yuan/kWh) 0.55 0.55 0.17 3.74 0.40 7.14 0.21 3.83