Life cycle and economic assessments of biogas production from microalgae biomass with hydrothermal pretreatment via anaerobic digestion

Journal Pre-proof Life cycle and economic assessments of biogas production from microalgae biomass with hydrothermal pretreatment via anaerobic digestion Chao Xiao, Qian Fu, Qiang Liao, Yun Huang, Ao Xia, Hao Chen, Xun Zhu PII:

S0960-1481(19)31642-8

DOI:

https://doi.org/10.1016/j.renene.2019.10.145

Reference:

RENE 12514

To appear in:

Renewable Energy

Received Date: 7 August 2019 Revised Date:

12 October 2019

Accepted Date: 27 October 2019

Please cite this article as: Xiao C, Fu Q, Liao Q, Huang Y, Xia A, Chen H, Zhu X, Life cycle and economic assessments of biogas production from microalgae biomass with hydrothermal pretreatment via anaerobic digestion, Renewable Energy (2019), doi: https://doi.org/10.1016/j.renene.2019.10.145. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

1

Life cycle and economic assessments of biogas production from

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microalgae biomass with hydrothermal pretreatment via

3

anaerobic digestion

4

Chao Xiao a, b, Qian Fu a, b, Qiang Liao * a, b, Yun Huang a, b, Ao Xia a, b, Hao Chen a, b,

5

Xun Zhu a, b

6 7

a

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Chongqing University, Ministry of Education, Chongqing 400044, China;

9

b

Key Laboratory of Low-grade Energy Utilization Technologies and Systems,

Institute of Engineering Thermophysics, School of Energy and Power Engineering,

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Chongqing University, Chongqing 400044, China;

11

*

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Emails: [email protected] (Qiang Liao);

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Tel./fax: +86 23 65102474

Corresponding authors:

1

14

Abstract

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Biogas production from microalgae biomass via anaerobic digestion can be

16

enhanced by hydrothermal pretreatment. The process of hydrothermal pretreatment

17

has a significant impact on the energy gain, greenhouse gas emissions, and levelized

18

cost of energy in biogas production from microalgae biomass, which has not been

19

reported until now. In this study, life cycle and economic assessments of biogas

20

production from microalgae biomass with hydrothermal pretreatment and with

21

solar-driven hydrothermal pretreatment were conducted. The results showed that

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both types of pretreatment methods improved the biogas yield, promoted the energy

23

gain, and reduced the levelized cost of energy. In biogas production through

24

hydrothermal pretreatment, the net energy ratio (Energy input/Energy output),

25

greenhouse gas emissions, and levelized cost of energy were 0.54, -129.4 g

26

CO2-eq/(kWh biogas), and 0.22 $/m3, respectively, whereas in biogas production

27

through solar-driven hydrothermal pretreatment, the corresponding values were 0.69,

28

-166.13 g CO2-eq/(kWh biogas), and 0.17 $/m3, respectively. The biogas yield had

29

the maximum effect on the net energy ratio and economic benefit. The efficiency in

30

nitrogen recovery from the biogas residual had the maximum effect on greenhouse

31

gas emissions. This work provides a theoretical guide to promote the environmental

32

and economic benefits of biogas production from microalgae biomass.

33

Key words: Life cycle assessment; Economic assessment; Microalgae biomass;

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Biogas production; Hydrothermal pretreatment; Anaerobic digestion

2

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1. Introduction

36

The increasing environmental pollution and energy crisis issues urge human

37

beings to develop new renewable energy technologies. The conversion of microalgae

38

biomass to biofuels is considered as a promising technology [1], because microalgae

39

biomass can produce biomass for biofuel production, while absorbing nitrogen (N)

40

and phosphorus (P) from waste water [2] and further purifying waste water. In

41

addition, microalgae have extra advantages over other energy crops [3], for instance,

42

a high photosynthetic efficiency, no competition with food crop in lands, and well

43

tolerance of high CO2 concentration.

44

Microalgae biomass has been used as a feedstock for biofuel production, for

45

example, biodiesel production by oil extraction, hydrothermal liquefaction [4], and

46

pyrolysis [5]; biogas production via anaerobic digestion (AD) [2]. Remarkably, the

47

microalgae slurry has high moisture content after harvesting. Dewatering of the

48

microalgae slurry, which is energy intensive, is required before biodiesel production

49

from microalgae biomass [6]. In oil extraction employed for biodiesel production,

50

only lipids are utilized, resulting in the waste of carbohydrates and proteins of the

51

microalgae biomass [7]. Additionally, hydrothermal liquefaction and pyrolysis,

52

which are generally conducted at high temperatures and pressures, have a negative

53

energy gain [8]. In comparison, the microalgae slurry, having whole cell biomass,

54

can be directly converted into biogas via AD at room temperature without further

55

dewatering [9]. Notably, a previous life cycle assessment showed that biogas

56

production from microalgae biomass via AD achieved more energy, compared with 3

57

biodiesel production [10]. Therefore, AD is regarded as an efficient approach for

58

biofuel production from microalgae biomass.

59

However, the microalgae cell inherently has a compact cell wall, which leads to

60

a low biogas yield from microalgae biomass via AD [9]. For example, Sialve et al.

61

[11] reported that all organic matter (i.e., carbohydrates, proteins, and lipids) in the

62

microalgae biomass can be converted into biogas, and in theory, the methane yields

63

from proteins, lipids, and carbohydrates were 0.851, 1.014, and 0.415 L/g of volatile

64

solids, respectively. However, in practice, the methane yield from microalgae

65

biomass was merely 0.28 L/g of volatile solids [12]. To this end, hydrothermal

66

pretreatment (HTP) was proposed to destroy the compact structure of the microalgae

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cell wall and further enhance the performance of biogas production from microalgae

68

biomass [13]. Lee et al. [14] demonstrated that the methane yield from microalgae

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biomass after HTP increased by 20.5% in comparison to that without pretreatment.

70

Perez-Elvira et al. [15] showed that the methane yield from microalgae biomass with

71

HTP was 39% higher than that without pretreatment in AD. Gonzalez-Fernandez et

72

al. [16] reported that the methane yield from microalgae biomass with HTP was

73

enhanced by 1.93 times against that using raw microalgae biomass as a substrate in

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AD. However, the development of HTP is limited owing to the large amount of

75

energy required. For example, Lee et al. [14] reported that the net energy gain of

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biogas production from microalgae biomass with HTP was 47.1% lower than that

77

from microalgae biomass without any pretreatment in AD. To save energy in HTP,

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Liao et al. [17] constructed a solar-driven HTP system and used solar energy as a 4

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resource to provide heat for the HTP of microalgae biomass. They found that the

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methane yield increased by 1.57 times in microalgae biomass with solar-driven HTP,

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in comparison to that without pretreatment.

82

In biogas production from microalgae slurry with/without pretreatment, the

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environmental benefit is affected by the energy and material exchange with the

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surroundings. Correspondingly, the economic benefit is affected by the equipment

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allocation and capital investment. On one hand, positive environmental and

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economic benefits are required for biogas production from microalgae biomass in the

87

industry. On the other hand, the improvement of environmental and economic

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benefits relies on the removal of the bottleneck in biogas production from

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microalgae biomass. Environmental and economic assessments can provide

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important guidance for the development of biogas production from microalgae

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biomass. Life cycle and economic assessments are, respectively, the primary tools to

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assess the environmental and economic benefits of renewable energy production.

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Grierson et al. [18] conducted a life cycle assessment of microalgae biomass

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cultivation, bio-oil extraction, and pyrolysis processing. Collet et al. [19] performed

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a life cycle assessment of biogas production from microalgae biomass without

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pretreatment. Verstraete et al. [20] investigated the economic benefit of a

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cogeneration system, in which microalgae biomass without pretreatment was

98

converted into biogas and then into power. However, until now, life cycle and

99

economic assessments of biogas production from microalgae biomass with HTP and

100

with solar-driven HTP have not been reported. 5

101

Thus, in this work, life cycle and economic assessments of biogas production

102

from microalgae biomass with HTP, and with solar-driven HTP were carried out.

103

Additionally, life cycle and economic assessments of biogas production from

104

microalgae biomass without pretreatment was also investigated, for comparison. The

105

net energy ratio (NER), greenhouse gas (GHG) emissions, and levelized cost of

106

energy (LCOE) were investigated, and sensitivity analyses were conducted.

107 108

2. Methods

109

2.1 Process description and system boundary

110

Fig. 1 shows the system boundary of biogas production from microalgae

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biomass with different pathways. Microalgae biomass was cultivated in

112

photobioreactors and was harvested through settling and centrifugation. The

113

microalgae slurry was converted into biogas through three different pathways, i.e.,

114

AD without pretreatment (Fig. 1A), AD with HTP (Fig. 1B), and AD with

115

solar-driven HTP (Fig. 1C). Generally, the composition of biogas obtained from

116

microalgae biomass via AD is 70% CH4 and 30% CO2 [10]. Microalgae hydrolysates,

117

which were acquired in our previous study [17], were used as substrates for biogas

118

production via AD. It was assumed that the biogas production from microalgae

119

biomass with HTP is same as that with solar-driven HTP. The digestate after AD was

120

separated into residue and slurry through centrifugation. During biogas production

121

from microalgae biomass, the water from microalgae harvesting and the digestate

122

slurry containing nitrogen and phosphorus were recirculated to the photobioreactors. 6

123

124

7

125 126

Fig. 1. System boundary of biogas production from microalgae biomass without

127

pretreatment (A), with hydrothermal pretreatment (HTP) (B), and with solar-driven

128

HTP (C).

129

2.2

Cultivation in open raceway pond

130

Microalgae cultivation in an open raceway pond has advantages in terms of

131

NER and economic cost over that in flat-plate photobioreactors [19]. Therefore, an

132

open raceway pond was used for microalgae cultivation in this study. A cultivation

133

area of 100 ha was taken into consideration, and an open raceway pond with a length

134

of 690 m and a radius of 60 m was designed [21].

135

Chlorella sp. was considered as ideal feedstock for biofuel because of its high

136

growth rate [22] and high environmental tolerance [23]. The average areal

137

productivity of Chlorella sp. was assumed to be 25 g/(m2·d) [24] in an open raceway

138

pond with a depth of 0.3 m [25], which implies a productivity of 25,000 kg/d for the

139

open raceway pond of 100 ha. The requirements of CO2 and fertilizers were based on 8

140

the molecular formula of Chlorella sp. (C106H181O45N16P) [26] in microalgae

141

cultivation. CO2 was obtained from flue gas from a power plant, and the energy

142

required for CO2 injection was evaluated to be 22.2 Wh/kg [27]. The conversion

143

efficiency of CO2 was assumed to be 75% in microalgae cultivation [21]. The

144

nitrogenous fertilizer and phosphorus fertilizer were CO(NH2)2 and (NH4)2HPO4,

145

respectively. We assumed that there was no nutrient loss in microalgae cultivation.

146

The energy requirement of the fertilizers was evaluated with the openLCA software.

147

The microalgae slurry was moved by paddlewheels at a velocity of 0.25 m/s in the

148

open raceway pond [19]. It was assumed that the composition of volatile solids

149

accounted for 90% of total solid of microalgae biomass.

150

The flow resistance of the microalgae slurry came from two bends of 180°, two

151

straight channels, and two aerators of CO2 in each open raceway pond. The head loss

152

from a bend of 180° and an aerator were estimated by Eq. (1) [21], the head loss

153

from the straight channel was estimated by Eq. (2) [21], and the power used for

154

overcoming the total head loss was estimated by Eq. (3) [21].

K1v2 2g

155

hb =

156

 L  hc = v2n2  4 3  (2)  R1 

157

 Q ρh  P1 = 9.8  1   e 

(1)

(3)

158

where hb is the head loss in a bend of 180° (m) and an aerator of the open raceway

159

pond; K1 is the kinetic loss coefficient for a bend of 180° (theoretically K1 = 2), and

160

v and g are the average velocity of the microalgae slurry (m/s) and the acceleration 9

161

of gravity (m/s2), respectively. hc is the head loss in the straight channel (m) of the

162

open raceway pond, n is the roughness factor for clay channels (n = 0.018), and R1

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and L are the channel hydraulic radius (m) and channel length (m) of the open

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raceway pond, respectively. P1 is the power required (W), Q1 is the channel flow

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(m3/s), 9.8 is the conversion factor (W·s/(kg·m)), ρ is the density of the microalgae

166

slurry (approximately 998 kg/m3), and h and e are the total head loss (m) and

167

efficiency of the paddlewheel and drive system (0.4 (assumed)), respectively.

168

2.3 Harvesting

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Natural settling and centrifugation are used for harvesting the microalgae

170

biomass. After natural settling of an hour, 65% of the microalgae was collected with

171

a concentration 20 times higher than that in the open raceway pond [19]. A volume

172

of 76,923 m3 of microalgae suspension was moved to the settlers via pumps, and the

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electrical consumption was 3,825 kWh/d for the pumps [19]. Afterwards, 2,500 m3

174

of microalgae slurry with a concentration of 10 g/L was further concentrated via

175

centrifugation. Finally, 500 m3 of microalgae slurry with a concentration of 50 g/L

176

was acquired. The energy required to convert microalgae slurry from a concentration

177

of 10 g/L to 50 g/L in centrifugation is 0.42 MJ/m3 [19]. The water from the natural

178

settling and centrifugation was recirculated to the open raceway pond.

179

2.4 Biogas production from microalgae biomass without pretreatment

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The concentrated microalgae slurry was moved into the fermentation reactor

181

directly, under the drive of a pump with an energy requirement of 0.036 kWh/m3 and

182

a price of 10,112 $ [28] (approximately 0.00185 $/m3). In the fermentation reactor, 10

183

the energy required for heating microalgae slurry from 25 to 35°C was obtained from

184

the biogas combustion in an auxiliary burner (60 $/kWh [28]) (Fig. 1A), and the

185

power of the auxiliary burner was estimated by Eq. (4). The biogas requirement for

186

heating the microalgae slurry was estimated by Eq. (5). The AD was conducted at

187

the temperature of 35°C with a hydraulic retention time of 28 days. During AD, the

188

microalgae biomass was mixed (300 kJ/m3) [29], and the heat loss was compensated

189

with electricity. The energy requirement for heat loss was calculated by Eq. (6) [29].

190

From our previous study [17], the methane yield obtained from microalgae biomass

191

via AD was 222 L/kg of volatile solids.

192

P2 = ρQ2cp (Td −Ta )

193

V1 =

194

P3 = h ( 2π R2 H + 2π R22 ) (Td − Ta )

P2

ϕξη1

(4)

(5) (6)

195

where P2 is the energy required (W) for heating the microalgae slurry from 25 to

196

35°C, Q2 is the volume flow of the microalgae slurry (m3/s), cp is the specific heat

197

capacity of the microalgae slurry (4.18 kJ/(kg·°C)), Td is the temperature of AD

198

(35°C), Ta is the ambient temperature (25°C), V1 is the biogas consumption in the

199

auxiliary burner (m3), φ is the volume fraction of CH4 in the biogas (70%), ξ is the

200

lower heating value of methane (35,800 kJ/m3), η1 is the energy conversion

201

efficiency of the auxiliary burner (90%), P3 is the heat loss in the fermentation

202

reactor (W), h is the convective heat transfer coefficient (1 W/(m2·°C)), R2 is the

203

radius of the fermentation reactor (m), and H is the height of the fermentation reactor

204

(H = R2). 11

205 206

2.5 Biogas production from microalgae biomass with HTP and with solar-driven HTP

207

The concentrated microalgae slurry was transported into the HTP reactor under

208

the drive of a pump with a power of 0.68 kWh/m3 and a price of 21,498 $ [28]

209

(approximately 0.00393 $/m3). In biogas production from microalgae biomass with

210

HTP, the microalgae slurry firstly absorbed the waste heat from pretreated

211

microalgae slurry in a heat exchange, and then absorbed the heat from biogas

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combustion in an auxiliary burner (60 $/kWh [28]). The biogas for combustion was

213

derived from the anaerobic digestion of microalgae biomass with HTP (Fig. 1B).

214

Finally, the microalgae slurry was heat up to HTP temperature of 160°C. The power

215

of the auxiliary burner was estimated by Eq. (7), and the biogas requirement for

216

heating the microalgae slurry was estimated by Eq. (5). The pretreated microalgae

217

slurry was cooled down to 35°C through a heat exchanger, after which it was

218

allowed to flow into a fermentation reactor. The AD was conducted at the

219

temperature of 35°C with a hydraulic retention time of 28 days. During AD, the

220

microalgae biomass was mixed (300 kJ/m3) [29], and the heat loss was compensated

221

with electricity. The energy required to compensate for the heat loss was calculated

222

by Eq. (6) [29]. From our previous study [17], the methane yield from the pretreated

223

microalgae biomass in AD was 348 L/kg of volatile solids.

224

P4 = ρ Q2 c p (Tp − Ta ) − ρ Q2 c p (Tp − Td )η 2

(7)

225

where P4 is the energy requirement (W) for heating the microalgae slurry, Tp is the

226

temperature of HTP (°C), and η2 is the energy conversion efficiency of the heat 12

227

exchanger (85%).

228

On the other hand, the biogas production from microalgae biomass with

229

solar-driven HTP mainly differed from that with HTP in heat resource for HTP. In

230

biogas production from microalgae biomass with solar-driven HTP, the microalgae

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slurry firstly absorbed the waste heat from pretreated microalgae slurry in a heat

232

exchange, and then absorbed the solar energy from a solar collector (Fig. 1C). The

233

power of the solar collector was estimated by Eq. (8).

234

P5 = AIη3 (8)

235

where P5 is the energy requirement (W) from the solar collector, A is the area of the

236

collector (m2), I is the direct normal irradiation, and η3 is the energy conversion

237

efficiency of the solar collector (78%) [30]. Generally, the direct normal irradiation

238

is 750 W/m2 [31], and the price of a solar collector is 150 $/m2 [32].

239

2.6 Liquid digestate recycling

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The digestate after AD of the microalgae biomass contains organic and

241

mineralized matter, which can be used as nutrients for microalgae cultivation [33].

242

Thus, recycling of the liquid digestate was considered in biogas production from

243

microalgae biomass with/without pretreatment. The digestate can be divided into

244

slurry and residue via centrifugation. The electricity required for centrifugation of

245

the digestate was 1.26 kWh/m3 [19]. It was assumed that 70% of nitrogen and 50%

246

of phosphorus were recycled [20].

247

2.7 Life cycle and economic assessments

248

Generally, the NER (Eq. (9)) [8] and GHG emissions are utilized to evaluate the 13

249

environmental benefit of the production of a renewable fuel. The GHG emissions

250

from biogas production from microalgae biomass are attributed to fertilizer

251

production and electricity generation and were evaluated with the openLCA software.

252

Additionally, the GHG emissions from biogas combustion were calculated by mass

253

conservation.

Energy input (9) Energy output

254

NER =

255

The net present value (NPV) indicates whether a project can be profitable,

256

given the time value of the monetary flows, i.e., revenue, capital investments, and

257

operational costs [20]. LCOE is widely used for comparing the cost of different

258

energy generation technologies over their economic life (Their economic life was

259

assumed to be 30 years). The NPV and LCOE were calculated by Eq. (10) and Eq.

260

(11) [20], respectively.

261

NPV = ∑t

262

LCOE =

(((C + P ) − ( I + O&M + A + D )) ×(1+ r ) ) −t

t

∑ (( I t

t

t

t

t

t

+ O & M + At − Ct + Dt ) × (1 + r )

∑( t

Et × (1 + r )

−t

)

−t

)

(10)

(11)

263

where Ct stands for the annual revenues from carbon credits in year t (30 $/t [34]), Pt

264

stands for the profit from the produced biogas in year t, It is the investment cost in

265

year t, O&Mt is the operation and maintenance cost in year t, At is the cost of

266

microalgae biomass production in year t, Dt stands for the decommissioning cost at

267

the end of the lifetime of the plant, which in this study is assumed to be zero, r stands

268

for the discount rate for year t, which is assumed to be 3% [34], and Et is the amount

269

of energy produced in year t. 14

270

Additionally, sensitivity analyses were conducted to evaluate the effect of the

271

parameters on the NER, GHG emissions, and LCOE in biogas production from

272

microalgae biomass with/without pretreatment. These values were calculated by

273

increasing and decreasing by 20% the value of the parameters. The change rate of

274

the NER was defined in Eq. (12). The difference in GHG emissions and the

275

difference in LCOE were defined in Eq. (13) and Eq. (14), respectively.

276

Change rate of NER =

NERa − NERi NERi

(12)

277

Difference in GHGs = GHGsa -GHGsi

(13)

278

Difference in LCOE = LCOEa − LCOEi (14)

279

where NERa and NERi are the altered and initial values of the NER, respectively;

280

GHGsa and GHGsi are the altered and initial values of the GHG emissions,

281

respectively; and LCOEa and LCOEi are the altered and initial values of the LCOE,

282

respectively.

283 284

3. Result and discussion

285

3.1 Life cycle assessment

286

Table 1 presents the mass and energy flow based on 1 kg of microalgae biomass

287

in biogas production with/without pretreatment. The NERs of the processes without

288

pretreatment, with HTP, and with solar-driven HTP were 0.74, 0.54, and 0.69,

289

respectively. Consequently, all the methods of biogas production obtained a positive

290

energy gain. The difference in NERs was mainly ascribed to the different net

291

methane yield, energy requirement for AD, and energy requirement for HTP. As 15

292

listed in Table 1, the net methane yields in the processes without pretreatment, with

293

HTP, and with solar-driven HTP were 0.174, 0.239, and 0.313 m3/kg, respectively.

294

The energy requirement for the production without pretreatment, which was needed

295

for heating the microalgae slurry from 25 to 35°C, was 0.258 kWh. Comparatively,

296

the energy requirements for production with HTP and solar-driven HTP were 0.74

297

and 0.854 kWh, respectively. The net methane yield from microalgae biomass with

298

HTP increased in comparison with that of biomass without pretreatment. Thus, HTP

299

was useful for enhancing biogas production from microalgae biomass, a result

300

different from that obtained by Lee et al [14]. The reason for this was that the

301

methane yield was substantially increased, and the recovery of waste heat reduced

302

the energy requirement for HTP. The methane yield from microalgae biomass with

303

solar-driven HTP was higher than that without pretreatment and with HTP. The

304

reason was that the methane yield from microalgae biomass was promoted by the

305

solar-driven HTP, and the internal requirement of energy for the HTP was reduced.

306

Notably, the maximum energy requirements were in the HTP and solar-driven HTP

307

processes, which accounted for 36.5% and 39.8% of the total energy requirement in

308

the production with HTP and with solar-driven HTP, respectively. The energy

309

requirement for the HTP was derived from the biogas combustion, and it caused an

310

internal waste of biofuel from the microalgae biomass. In contrast, the energy

311

requirement for the solar-driven HTP was derived from renewable solar energy.

312

Therefore, the solar-driven HTP is a promising system for enhancing the biogas yield

313

from microalgae biomass via AD. 16

314 315

Table 1. Mass and energy flow based on 1 kg of microalgae biomass in biogas

316

production via anaerobic digestion (AD) from microalgae biomass without

317

pretreatment, with hydrothermal pretreatment (HTP), and with solar-driven HTP. Step Cultivation CO2 consumption Energy requirement for CO2 injection Nitrogenous fertilizer requirement Nitrogenous fertilizer production Phosphorus fertilizer requirement Phosphorus fertilizer production Paddlewheel Harvesting Microalgae slurry transport Centrifugation of microalgae Pretreatment and AD Pump power Energy requirement for HTP Energy requirement for AD (heating microalgae slurry to 35°C) Mixing Heat losses in AD Centrifugation of digestate Net methane yield Net output energy Net energy ratio (NER)

318

a

319

calculation.

Without pretreatment

HTP

Solar-driven HTP

Units

1.92 0.0569 0.0556 0.609 0.0272 0.0298 0.223

1.92 0.0569 0.0556 0.609 0.0272 0.0298 0.223

1.92 0.0569 0.0556 0.609 0.0272 0.0298 0.223

kg kWh kg kWh kg kWh kWh

0.153 0.117

0.153 0.117

0.153 0.117

kWh kWh

0.00072

0.0136 0.740a

0.0136 0.854

kWh kWh

0.258a 0.0467 0.0163 0.0252 0.174 1.73 0.74

kWh 0.0467 0.0163 0.0252 0.239 2.37 0.54

0.0467 0.0163 0.0252 0.313 3.11 0.69

kWh kWh kWh m3 kWh -

Internal requirement which cannot be taken into consideration for NER

320 321

Table 2 presents the GHG emissions from the production of 1 kWh of biogas in

322

biogas production from microalgae biomass with/without pretreatment. The total

323

GHG emissions from the production without pretreatment, with HTP, and with 17

324

solar-driven HTP were -264.87, -129.94, and -166.13 g CO2-eq/(kWh biogas),

325

respectively. The difference in GHG emissions was mainly ascribed to the different

326

net methane yields, energy requirements for AD, and energy requirements for HTP.

327

With the increase in methane yield, the requirement of microalgae biomass for the

328

production of 1 kWh of biogas decreased, leading to decreasing requirements of CO2,

329

nutrients, and electricity. Therefore, the CO2 requirement for microalgae cultivation

330

was higher in the production without pretreatment than in that with HTP. GHG

331

emissions from the operational unit were higher in the production without

332

pretreatment than that in the production with HTP. Correspondingly, the CO2

333

requirement for microalgae cultivation increased in the production with HTP,

334

compared with that in the production with solar-driven HTP. GHG emissions from

335

the operational unit in the production with HTP were higher than those from the

336

operational unit in the production with solar-driven HTP. Meanwhile, GHG

337

emissions due to the energy requirement in AD were 42.02 g CO2-eq/(kWh biogas) in

338

biogas production from the microalgae slurry without pretreatment. GHG emissions

339

due to the energy requirement in the HTP were 87.99 g CO2-eq/(kWh biogas) in

340

biogas production from microalgae slurry with HTP. Comparatively, the GHG

341

emissions due to energy requirement in the solar-driven HTP were zero in biogas

342

production from microalgae slurry with solar-driven HTP, because the energy

343

requirement for the solar-driven HTP was derived from renewable solar energy.

344 345

Table 2. Greenhouse gas (GHG) emissions from the production of 1 kWh of biogas 18

346

via anaerobic digestion (AD) without pretreatment, with hydrothermal pretreatment

347

(HTP), and with solar-driven HTP. Without pretreatment HTP (g CO2-eq) (g CO2-eq)

Step Cultivation CO2 consumption Electricity for CO2 injection Nitrogenous fertilizer production Phosphorus fertilizer production Paddlewheel Harvesting Microalgae slurry transition Centrifugation of microalgae Pretreatment and AD Pump power Energy requirement for HTP Energy requirement for AD (heating microalgae slurry to 35°C) Mixing Heat losses Centrifugation of digestate Total GHG emissions

Solar-driven HTP (g CO2-eq)

−1,110.75 33.90 417.15 6.14 132.80

−809.06 24.69 303.85 4.47 96.73

−616.75 18.82 231.63 3.41 73.74

91.30 69.55

66.51 50.66

50.70 38.62

0.43

5.92 87.99

4.51

20.26 7.09 10.94 −129.94

15.45 5.41 8.34 −166.13

42.02 27.82 9.74 15.02 −264.87

348 349

The life cycle assessment result was compared with previous studies, as show

350

in Table 3. The maximum NER and GHGs were all obtained at biodiesel production

351

from microalgae biomass by pyrolysis. It was because that the microalgae slurry

352

must be dewatered to 80% solids before pyrolysis [8], and the drying of microalgae

353

slurry was an energy intensive process. The NER of biogas production from

354

microalgae biomass without pretreatment in this study (0.74) was higher than the

355

previous research (0.66) reported by Collet et al [19]. The reason was that the

356

methane yield from microalgae biomass without pretreatment in this study (222 L/kg)

357

was less than the previous study (262.8 L/kg). Therefore, the output energy in this 19

358

study was lower than that reported by Collet et al, and further the NER of biogas

359

production from microalgae biomass without pretreatment in this study was higher

360

than the previous study [19].

361 362

Table 3. Comparison of net energy ratios (NER) and greenhouse gas (GHG)

363

emissions from different pathways for biofuel production. Pathway

NER

GHG emissions (g/kWh biofuel)

Reference

Hydrothermal liquefaction

1.23

-41.04

[8]

Pyrolysis

2.27

756

[8]

Anaerobic digestion

0.66 a

-

[19]

Anaerobic digestion

0.74 a

-264.87 a

This study

Anaerobic digestion

0.54 b

-129.94 b

This study

Anaerobic digestion

0.69 c

-166.13 c

This study

364

a

Biogas production from microalgae biomass without pretreatment.

365

b

Biogas production from microalgae biomass with HTP.

366

c

Biogas production from microalgae biomass with solar-driven HTP.

367 368

3.2 Economic assessment

369

Table 4 presents the capital and operational costs of biogas production from

370

microalgae biomass with/without pretreatment. The NPVs of the production without

371

pretreatment, with HTP and with solar-driven HTP were 0.04, 0.07, and 0.11 $/m3,

372

respectively. Obviously, the NPV of the process without pretreatment was less than 20

373

that with HTP. Additionally, the NPV of the production with HTP was less than that

374

with solar-driven HTP. The main reason for this was that the biogas yields in the

375

processes without pretreatment, with HTP, and with solar-driven HTP were different

376

(Table 1), and the sales of biogas were the main economic sources in the biogas

377

production. With the increase in net methane yield, the NPV in the biogas production

378

increased. The LCOEs in the production without pretreatment, with HTP, and with

379

solar-driven HTP were 0.30, 0.22, and 0.17 $/m3, respectively. The market price of

380

biogas was 0.0432 $/kWh (approximately 0.3 $/m3) [35]. Therefore, the biogas

381

production from microalgae biomass with HTP and with solar-driven HTP achieved

382

economic profits. Notably, although the capital investment in the solar-driven HTP

383

(372,059 $) was higher than the required investment for the auxiliary burner in the

384

HTP (48,842 $), the production with the solar-driven HTP resulted in an excellent

385

economic benefit compared to that with the HTP. The reason was that the capital

386

investment for the solar-driven HTP system just accounted for a small proportion

387

(5.2%) of total plant cost. Although the biogas production from microalgae biomass

388

with solar-driven HTP obtained the positive economic benefits, the corresponding

389

payback period was quite long (12.3 years). The investment in cultivation and

390

harvesting of microalgae biomass accounted for the main parts of biogas production,

391

as shown in Table 4. Thus, it was imperative for the reduction of payback period to

392

develop advanced cultivation and harvesting technologies of microalgae biomass.

393 394

Table 4. Capital and operational costs ($/100 ha) for biogas production without 21

395

pretreatment, with hydrothermal pretreatment (HTP), and with solar-driven HTP via

396

anaerobic digestion (AD). Item Cultivation Land a Site preparation, grading and compacting a Pond levees and geotextiles a Paddlewheel a Flue gas sumps diffusers a Flue gas supply, distribution a Water and nutrient supply, distribution a Drainage a Harvesting Settling a Centrifugation a Pretreatment and AD Heating supply system AD reactor a Others Buildings and roads a Electrical supply and distribution a Instrumentation and machinery a Sub-total Engineering and supervision (15% of sub-total) b Contingency (10% of sub-total) b Total plant cost Operation and maintenance costs (6% of sub-total) c Financial indicators Net present value (NPV) at 3% ($/m3) Levelized cost of energy (LCOE) at 3% ($/m3) Payback period (years)

Without pretreatment

HTP

Solar-driven HTP

200,000

200,000

200,000

250,000

250,000

250,000

350,000 500,000 500,000 500,000

350,000 500,000 500,000 500,000

350,000 500,000 500,000 500,000

520,000

520,000

520,000

100,000

100,000

100,000

700,000 1,041,700

700,000 1,041,700

700,000 1,041,700

24,695 270,800

48,842 270,800

372,059 270,800

200,000 200,000 50,000 5,407,195.3

200,000 200,000 50,000 5,431,341.8

200,000 200,000 50,000 5,754,558.9

811,079.3

814,701.3

863,183.8

540,719.5

543,134.2

575,455.9

6,758,994.2

6,789,177.2

7,193,198.6

324,431.7

325,880.5

345,273.5

0.04

0.07

0.11

0.30

0.22

0.17

22.6

16.6

12.3

397

a

Data from the report of Benemann et al. [36]

398

b

Data from the report of Verstraete et al. [20] 22

399

c

Data from the report of Mehrpooya et al. [37]

400 401

The economic assessment results were compared with previous studies, as show

402

in Table 5. The LCOE of biodiesel production from microalgae biomass by

403

hydrothermal liquefaction was lower than that by pyrolysis. The reason was that the

404

drying biomass required amount of capital in biodiesel production from microalgae

405

biomass by pyrolysis. Additionally, the LCOE of biogas production from microalgae

406

biomass in this study was higher than that of biodiesel production by hydrothermal

407

liquefaction, which was ascribed to the higher heat value of biodiesel than that of

408

biogas. However, the LCOE of biodiesel from the reports by Ranganathan et al. [38]

409

and DeRose et al. [39] were 3.85 and 4.3 $/gasoline gallon equivalent, respectively,

410

and both of them were higher than market price (2.88 $/gasoline gallon equivalent).

411 412

Table 5. Comparison of levelized cost of energy (LCOE) from different pathway for

413

biofuel production. Pathway

LCOE ($/kWh biofuel)

Reference

Hydrothermal liquefaction

0.11 a

[38]

Hydrothermal liquefaction

0.13 b

[39]

Pyrolysis

0.18

[40]

Anaerobic digestion

0.3 c

This study

Anaerobic digestion

0.22 d

This study

Anaerobic digestion

0.17 e

This study

23

414

a

Integrated hydrothermal liquefaction and wastewater treatment.

415

a

Integrated hydrothermal liquefaction and fermention.

416

c

Biogas production from microalgae biomass without pretreatment.

417

d

Biogas production from microalgae biomass with HTP.

418

e

Biogas production from microalgae biomass with solar-driven HTP.

419 420

3.3 Sensitivity analysis

421

The effects of the variation in parameters on the NER, GHG emissions, and

422

LCOE of biogas production from microalgae biomass with/without pretreatment

423

were investigated. Among the parameters, the biogas yield, flow velocity of

424

microalgae slurry, efficiency of the paddlewheel, and efficiency in nitrogen recovery

425

affected the NER and GHG emissions significantly. Correspondingly, the LCOE was

426

significantly affected by the biogas yield, price of centrifugation, price of settling,

427

and price of the water and nutrient supply.

428

Fig. 2 shows the effects of the variation in parameters on the NER. The biogas

429

yield had the largest impact on the NER, followed by the efficiency in nitrogen

430

recovery from the digestate. Therefore, for improving the energy efficiency, it was

431

imperative to enhance the biogas yield and improve the efficiency in nitrogen

432

recovery. With the increasing biogas yield, the net output energy increased, and

433

further the NER decreased. In addition, the nitrogen resource for microalgae

434

cultivation stemmed from nitrogenous fertilizer and nitrogen recovery from liquid

435

digestate. With the increasing efficiency in nitrogen recovery, the nitrogenous 24

436

fertilizer requirement decreased, and energy requirement for nitrogenous fertilizer

437

production decreased, leading to the decrease of NER. Additionally, with the

438

increasing efficiency of the paddlewheel, the energy requirement decreased, and

439

further the NER decreased. Inversely, with the increasing flow velocity, the flow

440

resistance of microalgae slurry in the open raceway pond increased, and the energy

441

requirement for paddlewheel increased, causing the increasing NER.

Change rate of NER (%)

50

-20% +20%

25

0

-25

-50

Velocity Biogas yield Efficiency of nitrogen recovery Efficiency of paddlewheel Without Solar-driven HTP pretreatment HTP

442 443

Fig. 2. Effects of the variation in parameters on the net energy ratio (NER) in biogas

444

production without pretreatment, with hydrothermal pretreatment (HTP), and with

445

solar-driven HTP.

446 447

Fig. 3 shows the effect of the variation in parameters on the GHG emissions.

448

The nitrogen recovery from the digestate had the largest impact on the GHG

449

emissions, followed by the flow velocity of microalgae slurry in the open raceway

450

pond. Therefore, for reducing GHG emissions, it was essential to improve the

451

efficiency in nitrogen recovery and reduce the flow velocity of the microalgae slurry.

452

Notably, microalgae biomass with a low flow velocity of the microalgae slurry had a 25

453

lower growth rate than that with a high flow velocity in the open raceway pond [21].

454

Therefore, it was necessary for the reduction of GHG emissions to enhance the

455

growth rate of the microalgae biomass with a low flow velocity of the microalgae

456

slurry in the open raceway pond. With the increasing flow velocity of the microalgae

457

slurry in the open raceway pond, the energy requirement increased, and further the

458

GHG emissions increased. Inversely, with the increasing efficiency of the

459

paddlewheel and in nitrogen recovery, the energy requirement and GHG emissions

460

decreased. With the increase in biogas yield, the GHG emissions increased because

461

the CO2 requirement in microalgae cultivation decreased for the production of 1

462

kWh of biogas from microalgae biomass.

Difference in GHGs (g CO2-eq/kWh)

300

463

+20% -20% 150

0

-150

Biogas yield Velocity Efficiency of paddlewheel Efficiency of N recovery

-300 Without pretreatment

HTP

Solar-driven HTP

464

Fig. 3. Sensitivity analysis of greenhouse gas (GHG) emissions from biogas

465

production without pretreatment, with hydrothermal pretreatment (HTP), and with

466

solar-driven HTP.

467 468

Fig. 4 shows the effect of the variation in parameters on the LCOE in biogas

469

production from microalgae biomass with/without pretreatment. The biogas yield 26

470

had the largest impact on the LCOE. Therefore, further enhancement of the biogas

471

yield in AD is extremely important to promote the economic benefit in these

472

processes of biogas production. The prices of centrifugation, settling, and of the

473

water and nutrient supply in the cultivation had a great impact on the LCOE. Thus, it

474

is imperative for improving the economic benefits to develop a low-cost microalgae

475

harvesting technology, for example, flocculation by fungi mediated [41, 42].

476

Furthermore, the optimization of the water and nutrient supply was beneficial for the

477

reduction of the LCOE. In addition, the LCOE decreased with the increase in biogas

478

yield, which was ascribed to the increasing sales of biogas. In contrast, with the

479

increasing price of centrifugation, settling, and of the water and nutrient supply, the

480

LCOE increased because of the increasing capital investment.

Difference in LCOE ($)

0.150

-20% +20%

0.075

0.000

-0.075

-0.150

Centrifugation Settling Water and nutrient supply, distribution Biogas yield

Without pretreatment

481

HTP

Solar-driven HTP

482

Fig. 4. Effect of variation of parameters on the levelized cost of energy (LCOE) in

483

biogas production without pretreatment, with hydrothermal pretreatment (HTP), and

484

with solar-driven HTP.

485 486

4. Conclusion 27

487

In this study, life cycle and economic assessments of biogas production from

488

microalgae biomass with hydrothermal pretreatment and with solar-driven

489

hydrothermal pretreatment were conducted. The biogas production with these

490

processes obtain good environmental and economic benefits. Biogas produced from

491

microalgae biomass with a solar-driven hydrothermal pretreatment has lower

492

levelized cost of energy than that with hydrothermal pretreatment. The net energy

493

ratio (Energy input/Energy output), greenhouse gas emissions, and levelized cost of

494

energy with the solar-driven hydrothermal pretreatment were 0.69, -166.13 g

495

CO2-eq/(kWh biogas), and 0.17 $/m3, respectively. The increment in biogas yield was

496

beneficial to decreasing the net energy ratio and levelized cost of energy.

497

Additionally, the improvement in efficiency in nitrogen recovery can reduce

498

greenhouse gas emissions. This work provides a theoretical guide to promote the

499

environmental and economic benefits of biogas production from microalgae

500

biomass.

501

28

502 503

Acknowledgment The authors are grateful for the financial support provided by the State Key

504

Program of National Natural Science of China (Grant number 51836001), National

505

Natural Science Foundation of China (No. 51776025), the Venture & Innovation

506

Support Program for Chongqing Overseas and Returnees (No. cx2017017), and the

507

Fundamental Research Funds for the Central Universities (2018CDQYDL0049). No

508

conflicts of interest, informed consent, human or animal rights are applicable to this

509

investigation.

510

29

511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553

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32

Highlights Biogas production from microalgae with solar-driven HTP obtained the lowest LCOE. The biogas yield from microalgae had the maximum impact on net energy ratio. The biogas yield from microalgae had the maximum impact on economic benefit. The efficiency of nitrogen recovery had the maximum impact on GHGs.

Abbreviations: hydrothermal pretreatment (HTP), levelized cost of energy (LCOE), greenhouse gas emissions (GHGs).

Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.