Co-pyrolysis of microalgae and sewage sludge: Biocrude assessment and char yield prediction

Co-pyrolysis of microalgae and sewage sludge: Biocrude assessment and char yield prediction

Energy Conversion and Management 117 (2016) 326–334 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 117 (2016) 326–334

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Co-pyrolysis of microalgae and sewage sludge: Biocrude assessment and char yield prediction Xin Wang, Bingwei Zhao, Xiaoyi Yang ⇑ School of Energy and Power Engineering, Energy and Environment International Center, Beihang University, 37 Xueyuan Road, Haidian District, Beijing, PR China

a r t i c l e

i n f o

Article history: Received 17 December 2015 Accepted 5 March 2016

Keywords: Microalgae Sewage sludge Co-pyrolysis Kinetics Interaction H/C

a b s t r a c t High feedstock price is an important barrier for microalgae pyrolysis to alternative biofuels, while high ash content and low heat value affect the stable operation of sewage sludge pyrolysis reactors. Copyrolysis of microalgae and sewage sludge can avoid the drawbacks in individual sludge pyrolysis and improve pyrolysis performances. For better understanding co-pyrolysis kinetics, biocrude characteristics and interaction of sewage sludge and microalgae, thermogrametric analysis (TGA) and fixed pyrolysis bed experiment have been conducted and carbon distribution and components in biocrude were evaluated based on quality and quantity. For TGA, there was nearly no difference between individual pyrolysis and co-pyrolysis below 550 °C, while obvious interaction was found from 550 °C to 700 °C in copyrolysis. According to analysis of co-pyrolysis kinetics, solid-phase decomposition reaction mechanism in sewage sludge individual pyrolysis turned into random nucleation and subsequent growth mechanism above 550 °C. For co-pyrolysis, there was a yield increase of C4 and C7 and a yield decrease of C9 in carbon distribution. Detailed comparison of biocrude composition and carbon distribution indicated interaction in the product of co-pyrolysis. Excellent linear relationship between H/C of feedstocks and pyrolysis char was observed. Co-pyrolysis products reduced in hydrocarbons and N-containing compounds but increased in ketones and aldehyde. Co-pyrolysis with microalgae biomass was superior to pyrolysis individual with sewage sludge, which could improve stable operation of sewage sludge pyrolysis system due to higher heat value of microalgae addition. Co-pyrolysis of microalgae and sewage sludge is a promising way to decrease feedstock cost and realize alternative fuel production. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction There is an increasing demand for alternative liquid fuels due to shortage of fossil fuels, environmental pollution and large greenhouse gases emission. Many biomass conversion methods have been developed to produce alternative liquid fuels, such as pyrolysis [1], transesterification [2], hydrothermal liquefaction [3], and gasification/FT synthesis [4]. Fast pyrolysis is a promising pathway to produce liquid fuels at medium temperature 400–500 °C under oxygen-free environment. The main pyrolysis products are biocrude, gases and char, which are both valuable fuels and chemicals source. Pyrolysis feedstocks include wood [5], agricultural residue [6], microalgae [7], sewage sludge [8] and solid waste [9]. Microalgae are good feedstock for biofuel production because of higher photosynthetic efficiency, higher biomass yield and faster growth [10]. The pyrolysis temperature was 425–500 °C and the ⇑ Corresponding author at: 37 Xueyuan Road, Haidian District, Beijing 100191, PR China. Tel./fax: +86 01 82317346. E-mail address: [email protected] (X. Yang). http://dx.doi.org/10.1016/j.enconman.2016.03.013 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

maximum biocrude yield was up to 49.36%, which could be simulated based on the content of protein, lipid and carbohydrate [11]. Direct and indirect pyrolysis of microalgae indicated that higher total oil yield (lipid and biocrude) can be observed in indirect pyrolysis (59% than 41%) [12]. Valuable chemicals, aromatics and ammonia, can be produced by catalytic pyrolysis of microalgae [13]. However, the high feedstock cost is still an important challenge for large scale commercial microalgae pyrolysis. The production of microalgae cultivated in wastewater has been a promising way to reduce feedstock cost [14,15]. During wastewater treatment sewage sludge is the main byproduct including organic and inorganic composition, which can be treated by agricultural use, landfill and incineration. More attention has been paid to sewage sludge pyrolysis treatment due to energy recovery, volume reduction and environmental concern [16]. Sewage sludge pyrolysis can also produce biocrude, pyrolysis gas and char. There are some defects for sewage sludge pyrolysis process, such as high ash content [17], low energy efficiency and unstable operation of pyrolysis reactor due to low heat value [18]. There is nearly no

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feedstock cost for sewage sludge pyrolysis, and it can be copyrolyzed with microalgae [19]. Co-pyrolysis feedstocks are biomass, coal, sewage sludge, polymer and municipal solid waste. Co-pyrolysis of biomass and coal can reduce CO2 emission during the downstream utilization of pyrolysis products comparing with coal pyrolysis [20,21]. Copyrolysis of biomass and polymer can improve quality of pyrolysis oil, such as higher heating value [22], increased valuable chemical content (aromatic hydrocarbons) [23] and lower oxygen content [24]. Co-pyrolysis can also reduce the feedstock cost and solve waste management [25]. Co-pyrolysis of microalgae and textile dyeing sludge was conducted by TG-FTIR and the result indicated that the potential optimum blending ratio of microalgae was 80% [19]. Interaction between co-pyrolysis feedstocks is an important research aspect. No chemical interaction had been observed from kinetic data with modified distributed activation energy model, but char-N yields reduced with the increase of volatile-N yields. Interaction was observed during other co-pyrolysis researches [26,27]. Individual proline and leucine pyrolysis formed no char, while they produced a significant amount of nitrogen-containing char when co-pyrolyzed with cellulose, hemicellulose and lignin [28]. Co-pyrolysis process is not the sum of Pingshuo coal and biomass conversion based on the experimental and calculated differential thermogrametry (DTG) curves [29]. Interaction can be observed in gas phase reaction mostly [30,31], while cannot be easily observed in solid phase reaction [21,32]. However, little information is known about co-pyrolysis of microalgae and sludge. In this study, TGA and fixed bed co-pyrolysis experiments of microalgae and sludge were investigated to reveal the kinetics and interaction. Interaction was evaluated based on DTG curves, product yield and biocrude composition. Relationship between H/ C in feedstock and char yield was established. Energy conversion efficiency of co-pyrolysis was also conducted.

2. Material and methods 2.1. Raw materials Microalgae Isochrysis sp. (ISO) were provided by Shenyang Research Institute of Chemical Industry (Shenyang, China) and sewage sludge (SLU) was provided by a wastewater treatment plant with traditional aeration process (Beijing, China). Mixtures of sewage sludge and microalgae were prepared with the ratios of 1:1, 1:2 and 2:1 in weight, named acronym SI11, SI12 and SI21 respectively. Samples were dried at 105 °C for 24 h and grinded into fine powder (less than 150 lm). Proximate analysis (National Standard in China GB/T 212-2001), ultimate analyses and chemical composition of microalgae and sewage sludge were shown in Table 1. Heat value was measured by a bomber calorimeter (PARR1281, USA). Elemental analysis (Elementar Vario EL, Germany) was conducted to measure C, H, N, S and O content in the samples. Protein content was determined according to National Standard in China GB/T 6432-1994 (Kjeldahl method), while lipid content was determined according to solvent extraction method (chloroform/methanol).

2.2. Pyrolysis experiment procedure Thermogravimetric experiments were conducted using a thermal analyzer (Thermal Analysis Q50, American) under a nitrogen flow rate of 60 mL/min. Each sample was heated from ambient temperature to 900 °C under five heating rates of 5, 10, 15, 20 and 25 °C/min respectively. The initial sample weight was con-

Table 1 Proximate, ultimate analysis and chemical composition of samples.

Moisture (%) Volatile matter (%) Fixed carbon (%)a Ash (%) Heat value (kJ/kg) C (%) H (%) O (%) N (%) S (%) Protein (%) Lipid (%) Carbohydrate (%)a

Microalgae

Sewage sludge

The maximum error for test method

1.69 79.79 11.63 6.89 23524 49.26 7.50 31.74 6.24 0.96 35.9 42.9 15.2

2.44 60.22 2.49 34.85 13043 34.77 4.27 24.69 2.59 / / / /

±0.20 ±0.80 / ±0.20 ±120 ±0.50 ±0.15 ±0.30 ±0.08 ±0.05 ±1.00 ±0.05 /

a Carbohydrate and fixed carbon content were calculated by mass balance method.

trolled about 5 mg for each run. The detailed kinetic method can refer to previous researches [11]. Fixed bed experiments were carried out and pyrolysis reactor could refer to previous research [11]. The quartz tube is 10 mm in internal diameter and 120 cm in length. In each experiment about 2.5 g sample was put in the quartz tube. Pyrolysis experiments were performed at 425–500 °C with nitrogen flow rate of 400 mL/min. Nitrogen flow was kept at least 10 min to ensure an oxygen-free condition. Before the reactor reaching the set pyrolysis temperature, the sample was placed at unheated zone and then was moved to heated zone quickly to realize fast pyrolysis. All condensable volatile products were collected as biocrude, which were determined by weighting condenser mass. Char yield was weighted directly at ambient temperature and gas yield was calculated based on mass balance. The biocrude obtained at the optimal pyrolysis temperature was analyzed by Gas Chromatography Mass Spectrometry (GC– MS) (Agilent Technologies, 7890A-5975C), which was diluted to 9% with acetone (v/v) and filtered by 0.45 lm PTFE Filter. The detailed GC–MS parameters were shown in previous researches [11].

3. Results and discussion 3.1. Co-pyrolysis and individual pyrolysis kinetics In this section co-pyrolysis and individual pyrolysis kinetics was researched. For all samples pyrolysis was conducted at the heating rates of 5, 10, 15, 20 and 25 °C/min. The DTG curves of sewage sludge, microalgae and their blends were shown in Fig. 1 at the heating rate of 20 °C/min as an example. Three stages (drying stage, main devolatilisation stage and high temperature weight loss stage) can be distinguished from DTG curves. For individual sewage sludge pyrolysis, there were two overlapping mass loss peaks at 200–550 °C and a higher mass loss peak at 600–750 °C. For individual microalgae pyrolysis, there was an obvious weight loss peak with a shoulder peak below 550 °C, while mass loss was unapparent at high temperature zone. The residence weights of sewage sludge, microalgae and their blends were 39.0% (SLU), 15.1% (ISO), 28.5% (SI11), 24.8% (SI12) and 33.0% (SI21) respectively at the heating rate of 20 °C/min. Residual weights increased with the ratio increase of sewage sludge due to high ash content (34.85 %) of sewage sludge. DTG curves showed that most compounds decomposed below 500 °C for microalgae, while it was below 725 °C for sewage sludge.

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Fig. 1. DTG curves of co-pyrolysis at 20 °C/min.

For co-pyrolysis, the temperature corresponding to the first weight loss peak was at 300–350 °C and nearly no change for different blending ratios, while the peak at high temperature (>650 °C) moved right gradually with the increased ratio of sewage sludge content, while their temperature were 696.6 °C (SLU), 677.4 °C (SI21), 671.3 °C (SI11), 659.4 °C (SI12). The DTG curves in co-pyrolysis have similar trend toward individual pyrolysis curves of sewage sludge and microalgae below 550 °C. There is an obvious difference on DTG curves between individual sewage sludge and microalgae at high temperature zone (600–750 °C). To evaluate the effect of co-pyrolysis and sample blend ratios, initial pyrolysis temperature was calculated and shown in Fig. 2. The detailed mathematical method can refer to previous research [33]. Sewage sludge had the lowest initial pyrolysis temperature and it increased from 208.3 °C to 221.5 °C with the ratio increase of microalgae in blend samples, which was lower than copyrolysis results of microalgae and textile dyeing sludge [19]. It is likely due to strain and composition difference between microalgae used in this study and microalgae Chlorella vulgaris. To illuminate co-pyrolysis interaction, DTG curves of copyrolysis process were compared with individual pyrolysis process and their difference value was used as a criterion of interaction degree. The values in individual pyrolysis corresponding to three co-pyrolysis samples are determined by Eq. (1).

Y cal ¼ wss  Y ss þ wiso  Y iso

ð1Þ

where Ycal means calculated DTG curve value or pyrolysis product yield in individual pyrolysis, Yss means DTG curves value or pyrolysis product yield of sewage sludge, Yiso means DTG curves value or pyrolysis product yield of microalgae, wss means weight ratio of sewage sludge in co-pyrolysis samples and wiso means weight ratio of microalgae in co-pyrolysis samples.

Fig. 2. Initial pyrolysis temperature in individual pyrolysis and co-pyrolysis.

Fig. 3 shows experimental and calculated DTG curves of mixture sample SI11 at 10 °C/min to reveal interaction of co-pyrolysis. For the high-temperature zone (above 550 °C), there was an obvious interaction between sewage sludge and microalgae promoting the decomposition reaction. There were similar results for copyrolysis of lignocellulosic biomass model components (cellulose and lignin) with bituminous coal, which was expressed as negative 4W value (the difference of experimental and calculated results) [34]. At high temperature zone, the temperature respond to the maximum weight loss peak for DTG curve of co-pyrolysis was 650 °C, while it was 677 °C for DTG curve of individual pyrolysis. DTG value of these two temperature point were 0.208%/°C in copyrolysis and 0.181%/°C in individual pyrolysis. Before 550 °C there was nearly no interaction observed and DTG curve of co-pyrolysis can be calculated by summation of the DTG values of individual samples based on their blending weight ratio. Pyrolysis process is an overlapping decomposition process and DTG curves can be divided into several fitting curves with mathematical functions based on biomass composition and mathematical accuracy. Multi-peaks simulation results with Lorentzian function were shown in Fig. 4. The DTG curves of microalgae and their blend with sewage sludge were divided into four fitting peaks below 550 °C and two fitting peaks above 550 °C, while the DTG curves of sewage sludge were divided into three fitting peaks below 550 °C and two fitting peaks above 550 °C. This is because that there is nearly no mass loss peak in sewage sludge pyrolysis compared with microalgae pyrolysis at 200 °C. Multi-peak fitting with Lorentzian function shows perfect fitting results and coefficient of determination R2 is ranging from 0.96854 to 0.99426. It can be concluded that DTG curve is a synthesis of several independent reaction curves and the whole pyrolysis process accompany with several main reaction processes, which are responding to decomposition of lipid, algal cell, protein, carbohydrate, fixed carbon and ash. Multi-heating rate method was used to analyze pyrolysis kinetic triplets of pyrolysis process after Lorentzian multi-peaks fitting. Kinetic parameters included apparent activation energy Ea, optimal pyrolysis mechanism function f(a) and pre-exponential factor A, which were shown in Table 2. Main mechanism functions during individual pyrolysis and co-pyrolysis are Random Nucleation and Subsequent Growth reaction mechanism. At high temperature zones, solid-phase decomposition reaction mechanism in sewage sludge individual pyrolysis turned into Random Nucleation and Subsequent Growth mechanism in co-pyrolysis. Activation energy of fitting peaks ranged from 107.5 kJ/mol to 308.0 kJ/mol and increased gradually with the increase of pyrolysis temperature which was similar to kinetics of sewage sludge and pine sawdust blends [23], while pre-exponential factor ln A ranged from 19.2 to 38.0. Activation energy of fitting peaks at high temperature zone also increased from 202.4–264.8 kJ/mol to 263.8–305.6 kJ/mol with the sewage sludge content increase.

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329

Fig. 3. Comparison of DTG curves in individual pyrolysis and co-pyrolysis.

Fig. 4. Multi-peak fitting with Lorentzian function.

3.2. Biocrude yield Biocrude yield of individual pyrolysis and co-pyrolysis are shown in Fig. 5. Biocrude yield of microalgae ranged from 37.36% to 49.36% and the maximum yield was 49.36% at 450 °C, while that of sewage sludge was from 21.07% to 25.72% and the maximum

yield was 25.72% at 500 °C. The biocrude yields in co-pyrolysis varied with the feedstocks blend ratios. Biocrude yield of co-pyrolysis process is between microalgae and sewage sludge, but not a linear relationship with their blend ratios. For the three ratios of sewage sludge and microalgae, biocrude yields are compared as follow: microalgae > co-pyrolysis (SI12 > SI11 > SI21) > sewage sludge

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Table 2 Pyrolysis kinetic parameters of co-pyrolysis and individual pyrolysis. Sample

a1 , %

Sewage sludge

19.7

308.5

Random nucleation and subsequent growth

10.1

439.1

Random nucleation and subsequent growth

1.7

458.1

Random nucleation and subsequent growth

9.8

674.6

5.8 SI21

SI11

SI12

Microalgae

ln A

f(a)

107.5

19.2

0:3875ð1  aÞ½ lnð1  aÞ

223.5

34.4

0:40ð1  aÞ½ lnð1  aÞ

11=0:40

270.9

40.2

0:07ð1  aÞ½ lnð1  aÞ

11=0:07

Solid-phrase decomposition Reaction

263.8

32.0

698.9

Solid-phrase decomposition Reaction

305.6

36.8

að1  aÞ a2 ð1  aÞ2

11.9

259.7

Random nucleation and subsequent growth

118.4

27.5

1=3ð1  aÞ½ lnð1  aÞ

2

28.9

319.6

Random nucleation and subsequent growth

125.8

26.8

1=3ð1  aÞ½ lnð1  aÞ

2

7.7

414.7

Random nucleation and subsequent growth

133.0

23.7

1=3ð1  aÞ½ lnð1  aÞ

2

7.2

458.4

Random nucleation and subsequent growth

175.7

29.6

1=4ð1  aÞ½ lnð1  aÞ

3

9.8

661.6

Random nucleation and subsequent growth

229.0

31.7

1=4ð1  aÞ½ lnð1  aÞ

3

4.9

680.6

Random nucleation and subsequent growth

296.0

39.3

1=4ð1  aÞ½ lnð1  aÞ

3

14.8

262.9

Random nucleation and subsequent growth

124.8

27.9

1=4ð1  aÞ½ lnð1  aÞ

3

33.4

321.3

Random nucleation and subsequent growth

126.1

26.9

1=3ð1  aÞ½ lnð1  aÞ

2

7.2

413.9

Random nucleation and subsequent growth

163.0

29.2

1=4ð1  aÞ½ lnð1  aÞ

3

6.1

458.3

Chemical reaction

161.0

28.2

ð1  aÞ2

6.4

656.3

Random nucleation and subsequent growth

228.7

32.1

1=4ð1  aÞ½ lnð1  aÞ

3

3.6

673.9

Random nucleation and subsequent growth

282.6

38.2

1=4ð1  aÞ½ lnð1  aÞ

3

19.1

267.1

Random nucleation and subsequent growth

129.3

29.3

1=4ð1  aÞ½ lnð1  aÞ

3

37.7

322.3

Random nucleation and subsequent growth

131.8

27.0

1=4ð1  aÞ½ lnð1  aÞ

3

6.8

412.3

Random nucleation and subsequent growth

129.5

20.4

1=2ð1  aÞ½ lnð1  aÞ

1

4.8

455.3

Random nucleation and subsequent growth

193.9

32.8

1=4ð1  aÞ½ lnð1  aÞ

3

2.8

643.9

Random nucleation and subsequent growth

202.4

28.1

1=4ð1  aÞ½ lnð1  aÞ

3

3.3

662.3

Random nucleation and subsequent growth

264.8

36.3

1=4ð1  aÞ½ lnð1  aÞ

3 3

Peak temp., °C

Pyrolysis mechanism

Ea, kJ/mol

11=0:3875

2

4.6

205.8

Random nucleation and subsequent growth

112.5

26.9

1=4ð1  aÞ½ lnð1  aÞ

21.1

280.2

Three-dimensional diffusion

142.7

31.1

3=2ð1  aÞ

41.8

328.3

Random nucleation and subsequent growth

145.8

32.0

1=2ð1  aÞ½ lnð1  aÞ

1

9.2

417.7

Random nucleation and subsequent growth

209.9

38.0

1=4ð1  aÞ½ lnð1  aÞ

3

4=3

1=3

½ð1  aÞ

1

 1

Table 3 Biocrude yield in individual pyrolysis and co-pyrolysis.

Fig. 5. Biocrude yields in individual pyrolysis and co-pyrolysis.

a

excepting at 500 °C. For co-pyrolysis, the maximum biocrude yield for SI11 was 34.37% at 500 °C, 34.9% at 450 °C for SI12 and 30.76% at 500 °C for SI21. Char yields of microalgae, SI12, SI11, SI21 and sewage sludge were 23.37%, 35.34%, 41.53%, 46.17% and 57.01% respectively at 500 °C, while gas yields were ranged from 17.0% to 37.0%. Pyrolysis biocrude yield difference is shown in Table 3. The difference of biocrude yields in co-pyrolysis and individual pyrolysis were ranged from 11.12% to 3.415%. From Table 3, it can be concluded that interaction of co-pyrolysis decreased biocrude yield of SI11 blend sample up to 7.74% at 450 °C and 7.95% at 475 °C, while increased biocrude yield up to 3.415% at 425 °C and 0.15% at 500 °C. Co-pyrolysis biocrude yields decreased slightly 1.34– 5.65% compared with individual pyrolysis process based on the maximum biocrude yield.

b

500

Max biocrudea

23.84 48.90 32.19 36.37 40.55

25.72 42.72 31.39 34.22 37.05

25.72 49.36 32.19 35.71 40.55

26.35 28.42 29.43

30.76 34.37 27.51

30.76 34.37 34.90

Difference between co-pyrolysis and individual pyrolysis SI21 0.98 8.03 5.84 0.63 SI11 3.42 7.74 7.95 0.15 SI12 1.30 5.36 11.12 9.54

1.43 1.34 5.65

Temperature (°C)

425

450

Individual pyrolysis Sewage sludge Microalgae SI21b SI11b SI12b

21.07 37.36 26.50 29.22 31.93

22.06 49.36 31.16 35.71 40.26

Co-pyrolysis SI21 SI11 SI12

25.52 32.63 33.23

23.13 27.97 34.90

475

Max biocrude means the maximum biocrude yield at 425–500 °C. The value was calculated based on microalgae and sewage sludge data.

3.3. Biocrude assessment 3.3.1. Biocrude assessment qualitatively In this section, the composition and carbon distribution of biocrude was assessed qualitatively. The detailed composition of each biocrude sample was detected by GC–MS and their chromatograms can refer to Supplementary Material. Component type in biocrude include aliphatic hydrocarbons, alicyclic hydrocarbons, aromatic hydrocarons, nitriles, amines, amides, N-heterocyclic compounds, alcohols, furans, phenols, esters and ethers. The percentage content of all components in biocrude is shown in Fig. 6. N-heterocyclic compounds were the maximum compounds in biocrude, and their

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hydrocarbons content up to 7.22%. N-heterocyclic compounds content in three co-pyrolysis biocrude were higher than that in individual sludge and microalgae. To evaluate the potential of biocrude upgrading to liquid transportation fuels, carbon number distribution was studied and shown in Fig. 7. For biocrude from co-pyrolysis, there is a similar carbon distribution trend. The carbon number distribution mainly focused on C5–C9, C14, C16, C18 and C20. For five biocrude samples C4–C10 content (gasoline precursors) was in the range of 69.16–84.32%, C8–C16 content (kerosene precursors) was from 55.60% to 69.73% and C14–C20 content (diesel precursors) is from 12.48% to 30.16%. The maximum contents based on carbon number are C8 or C9 for individual and co-pyrolysis biocrude.

Fig. 6. Biocrude quality evaluation based on their composition.

percentage content were up to 34.72%, 42.05%, 38.76%, 39.53% and 38.40% derived from sewage sludge, SI21, SI11, SI12 and microalgae respectively. The maximum content of hydrocarbon (aliphatic, alicyclic and aromatic hydrocarbons) has been detected in biocrude from sewage sludge up to 26.20%. There were high carboxylic acids content for individual microalgae biocrude up to 14.24% than that of individual sewage sludge biocrude (0.54%). The maximum percentage content of all biocrude was the compound of 2,2,6,6-tetramethyl-4-piperidone. For co-pyrolysis biocrude SI21, there were the maximum ketones and aldehyde yield up to 11.32% than biocrude from sewage sludge (4.82%) and microalgae (5.54%). Biocrude from SI11 has maximum alicyclic

3.3.2. Biocrude assessment quantitatively In this section, the composition and carbon distribution of biocrude was evaluated quantitatively. Biocrude carbon distribution comparison was shown in Table 4. There was a yield decrease for C5, C6, C8, C9–12 and C14–17 and an yield increase for C4 and C7 yield. For C7, the difference was 21.9–29.6 mg/g due to yield increase of 2,2,3,3-tetramethyl-Azetidine (C7H15N) and 3-ethyl-2hydroxy-2-Cyclopenten-1-one (C7H10O2). C9 content of copyrolysis biocrude was lower (27.5–35.5 mg/g) than that of both individual pyrolysis biocrudes. The reason was that the methyl benzenes in sewage sludge and 1-Methyl-2-tert-butylpyrrole content in individual microagle biocrude, both decreased and even

Fig. 7. Carbon distribution of biocrude in invidual pyrolysis and co-pyrolysis.

Table 4 Carbon distribution comparison of biocrude between co-pyrolysis and individual pyrolysis. Carbon number

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 P C4–10 P C8–16 P C14–20 a

Co-pyrolysis (mg/g biomass)

Individual pyrolysis (mg/g biomass)

SI21

Sludge

SI11

SI12

Differences (mg/g biomass)

SI21a

SI11a

SI12a

Algae

SI21

SI11

SI12

7.9 14.6 32.8 65.5 64.0 57.0 7.8 2.7 5.8 10.2 18.8 1.9 2.9 1.5 2.8 0.0 11.1

7.6 12.4 37.9 61.3 64.2 68.3 9.0 2.0 7.4 1.8 33.3 1.1 12.4 1.7 8.6 0.0 14.9

7.3 7.7 44.9 59.6 67.4 71.3 8.7 4.0 8.5 1.6 30.8 1.8 10.7 1.5 8.0 0.0 15.1

3.6 18.2 24.4 33.9 65.0 63.1 8.7 7.4 10.0 5.7 5.2 1.7 2.0 1.9 4.2 0.0 1.4

5.0 16.6 34.8 35.8 71.4 84.9 9.9 6.0 11.1 4.7 25.7 2.4 11.1 1.9 5.0 0.0 9.0

5.6 15.9 40.0 36.8 74.5 95.9 10.5 5.3 11.7 4.2 36.0 2.7 15.7 1.8 5.5 0.0 12.9

6.3 15.1 45.2 37.7 77.7 106.8 11.1 4.7 12.3 3.7 46.3 3.1 20.2 1.8 5.9 0.0 16.7

7.6 13.5 55.7 39.7 84.0 128.6 12.2 3.3 13.4 2.7 66.8 3.8 29.3 1.8 6.8 0.0 24.3

3.0 2.0 2.0 29.6 7.4 27.9 2.1 3.3 5.4 5.4 7.0 0.5 8.2 0.3 2.3 0.0 2.1

2.0 3.4 2.1 24.5 10.3 27.5 1.5 3.4 4.3 2.4 2.7 1.7 3.3 0.2 3.1 0.0 2.1

1.0 7.3 0.3 21.9 10.3 35.5 2.4 0.7 3.8 2.0 15.5 1.3 9.5 0.3 2.1 0.0 1.6

249.5 171.0 39.0

260.8 199.3 71.9

267.0 204.9 67.9

216.9 168.8 16.4

258.4 227.3 55.2

279.1 256.5 74.6

299.9 285.7 94.0

341.4 344.2 132.8

8.8 56.3 16.2

18.4 57.2 2.7

32.9 80.8 26.1

Means that the results were calculated based on the result of sewage sludge and microalgae invidual pyrolysis.

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Table 5 Component distribution comparison of biocrude between co-pyrolysis and individual pyrolysis. Compounds

Aliphatic hydrocarbons Alicyclic hydrocarbons Aromatic hydrocarbons Nitriles Amines and Amides N-heterocyclic compounds Carboxylic acids Ketones and Aldehyde Alcohols Furans Phenols Esters Ethers Others Hydrocarbons N-containing components O-containing compounds a

Co-pyrolysis (mg/g biomass)

Individual pyrolysis (mg/g biomass) a

SI21

SI11

SI12

Sludge

SI21

18.9 19.3 12.9 4.9 11.8 129.3 10.9 34.8 18.5 2.7 29.6 10.4 1.0 2.5 51.1 146.0 97.0

18.0 24.8 9.5 5.4 13.9 133.2 38.5 34.2 20.2 3.8 33.8 3.6 2.0 1.6 52.3 152.5 97.6

18.0 23.8 9.3 6.3 13.8 138.0 33.7 38.4 22.3 3.2 32.5 5.3 3.5 1.0 51.1 158.1 105.2

25.3 13.9 28.1 5.9 20.2 89.3 1.4 12.4 4.6 0.9 31.0 21.7 0.3 2.2 67.3 115.4 70.9

25.6 24.3 22.4 7.4 18.0 122.7 24.4 17.4 15.7 1.8 35.4 16.9 2.0 2.2 72.3 148.1 89.2

SI11

a

25.8 29.5 19.5 8.1 16.9 139.4 35.8 19.9 21.3 2.2 37.6 14.4 2.8 2.2 74.8 164.4 98.2

Difference (mg/g biomass) SI12

a

25.9 34.7 16.6 8.8 15.8 156.1 47.3 22.4 26.8 2.6 39.8 12.0 3.6 2.2 77.2 180.7 107.2

Algae

SI21

SI11

SI12

26.2 45.1 10.9 10.3 13.6 189.5 70.3 27.3 38.0 3.5 44.1 7.2 5.3 2.3 82.2 213.4 125.4

6.7 5.0 9.4 2.5 6.2 6.6 13.5 17.4 2.8 1.0 5.8 6.5 1.0 0.3 21.1 2.1 7.9

7.8 4.7 10.0 2.8 3.0 6.2 2.7 14.3 1.0 1.6 3.8 10.8 0.8 0.6 22.5 12.0 0.5

7.9 10.9 7.3 2.5 2.0 18.2 13.6 16.0 4.6 0.5 7.3 6.7 0.1 1.2 26.1 22.7 2.2

Means that the results were calculated based on the result of sewage sludge and microalgae individual pyrolysis.

increase, it was mainly due to the component of 3-ethyl-2-hydro xy-2-Cyclopenten-1-one. Hydrocarbon and N-containing component yield both decreased in co-pyrolysis biocrude with the range of 21.1–26.1 mg/g and 2.1–22.7 mg/g. In conclusion, biocrude composition and carbon distribution comparison indicates interaction in co-pyrolysis. 3.4. Char yield prediction

Fig. 8. Linear relationship between H/C of feedstocks and pyrolysis char yield.

cann’t be detected in co-pyrolysis biocrude. Moreove, there was a decrease (56.3–80.8 mg/g) of C8–C16 componennts (kerosene precursors). Biocrude composition comparison was shown in Table 5. N-heterocyclic compounds were the maximum compounds in biocrude, and their yield were up to 89.3 mg/g, 129.3 mg/g, 133.2 mg/ g, 138.0 mg/g and 128.6 mg/g in biocrude of sewage sludge, SI21, SI11, SI12 and microalgae respectively. There was an amount of hydrocarbons (aliphatic, alicyclic and aromatic hydrocarbons) which were easy to be upgraded to liquid transportation fuels. Biocrude from microalgae has the maximum hydrocarbon yield up to 82.2 mg/g. There were higher carboxylic acids content for individual microalgae biocrude up to 70.3 mg/g than that of individual sewage sludge biocrude (1.4 mg/g), which was likely due to high lipid content in microalgae. For co-pyrolysis biocrude from SI12 there were the maximum ketones and aldehyde yield up to 38.4 mg/g than biocrude from sewage sludge and microalgae. Biocrude from SI11 has maximum alicyclic hydrocarbons yield up to 24.8 mg/g. N-heterocyclic compounds content in three co-pyrolysis biocrude were higher than that in individual sludge and microalgae. However, there were slightly increase for ketones/aldehyde (14.3–17.4 mg/g) and furans (0.5–1.6 mg/g), which indicated no obvious effect on carbonhydrate pyrolysis in co-pyrolysis. For ketones/aldehyde component

Char is an important pyrolysis product with high carbon content, which can be used as fuels or underground carbon sink [35]. Char formation during pyrolysis is tightly related to hydrogen element content in biomass feedstock. In our previous research [11], biocrude yield was simulated based on microalgae composition (lipid, protein and carbohydrate) and their liquefaction ability index (N  0.3, 0.6 and 0.7). In this section, pyrolysis char yield was simulated based on the H/C ratio of microalgae and sewage sludge blend feedstocks, which was shown in Fig. 8. The H/C of microalgae, sewage sludge and their blends were ranged from 0.1523 to 0.1228. Pyrolysis char yield is ranged from 23.57% to 57.56% at 500 °C. It can be concluded that pyrolysis char yield is related to H/C in biomass feedstock by the linear equation of y = 1147.1x + 198.6. The fitting accuracy R2 was 0.99842 which was similar to the co-pyrolysis of sewage sludge and pine sawdust blends [23] and the same liner relationship was also found at the copyrolysis of energy grass and lignite [36]. This linear relationship can be used to predict pyrolysis char yield in co-pyrolysis of microalgae and sewage sludge for the future research works. 3.5. Energy conversion efficiency assessment of co-pyrolysis The energy ratio ge, energy efficiency g0e and energy consumption/output ratio ECR during co-pyrolysis are shown in Table 6. The energy ratio is defined as:

ge ¼

mbiocrude  HHV biocrude mbiomass  HHV biomass

ð2Þ

where HHVbiocrude and HHVbiomass are the higher heating value of the biocrude and the feedstock. mbiocrude and mbiomass are the mass of pyrolysis biocrude and microalgae/sewage sludge feedstock. The energy efficiency for co-pyrolysis process is calculated as:

g0e ¼

mbiocrude  HHV biocrude mbiomass  HHV biomass þ Q

ð3Þ

333

X. Wang et al. / Energy Conversion and Management 117 (2016) 326–334 Table 6 Energy conversion efficiency assessment during co-pyrolysis of sewage sludge and microalgae. Feedstocks

Biomass (g)

HHVbiomass (kJ/ kg)

Biocrude (g)

HHVbiocrude (kJ/ kg)

Char (g)

Sewage Sludge SI21 SI11 SI12 Microalgae

2.4055

13,040

0.6186

36,536

1.3845

2.402 2.4015 2.4015 2.5064

16,532 18,280 20,028 23,524

0.7389 0.8254 0.8381 1.2371

38,410 37,907 36,908 38,029

1.1091 0.9973 0.9052 0.6991

where the net process energy Q was measured by the pyrolysis DSC curves (TA SDT Q600, USA). The DSC experiment sample was controlled on 5 mg and the heating rate was 5 °C/min to enhance the resolution of DSC curves. The nitrogen gas flow was 100 mL/min and the pyrolysis temperature was ranged from 30 °C to 600 °C. A modified ECR is calculated as following:

ECR ¼ Q=½HHV biocrude Y biocrude þ HHV char Y char 

ð4Þ

where HHVbiocrude and HHVchar are the heating values of the produced biocrude and char; and Ybiocrude and Ychar are the yields of biocrude and char, respectively. If the ECR is greater than one, then the overall energy consumption to produce the biofuel is higher than the energy contained in the produced biofuel. The energy ratios of co-pyrolysis process were ranged from 64.31% to 79.79%, while energy efficiency was ranged from 56.77% to 70.94%. The ECR increased from 0.1188 to 0.2267 with the increase of sewage sludge blend ratio. The microalgae individual pyrolysis process has the highest energy ratio, energy efficiency and the lowest ECR. The sewage sludge individual pyrolysis process has the highest ECR, which is reduced by blending with microalgae. For co-pyrolysis process, the energy ratio and energy efficiency was apparently decreased when the blend ratio was 1:2 (sewage sludge/microalgae, wt/wt). It can be concluded that co-pyrolysis can improve the energy conversion and benefit to stable operation of sewage sludge individual pyrolysis process. 4. Conclusions Co-pyrolysis kinetics, biocrude composition and interaction of microalgae and sewage sludge was researched by TGA and fixed bed reactor. Main pyrolysis mechanism of sewage sludge, microalgae and their mixture are random nucleation and subsequent growth reaction mechanism below 550 °C. Biocrude yields were compared as follow: microalgae > co-pyrolysis (SI12 > SI11 > SI21) > sewage sludge excepting results at 500 °C. N-heterocyclic compounds contained the maximum weight in biocrude for all biocrude samples. Interaction of co-pyrolysis process was evaluated based on DTG curves, biocrude yields and composition. Based on the DTG curves in individual pyrolysis and co-pyrolysis before 550 °C, there was nearly no interaction observed during co-pyrolysis. From 550 °C to 700 °C, there was an obvious interaction between pyrolysis char of sewage sludge and microalgae. Carbon distribution and composition variation indicated interaction in co-pyrolysis. The relationship between biomass feedstock H/C ratio and pyrolysis char yield was fitted linearly with the R2 of 0.99842. Sewage sludge pyrolysis process was improved by blending with microalgae based on energy assessment. Acknowledgments This project was supported by the Program of International Science and Technology Cooperation Program of China (2013DFA61590).

Gas (g)

Q (kJ/ kg)

Biocrude yield (%)

ge

g0e

ECR

3940

0.4024

2644.63

25.72

72.05

59.90

0.2267

6494 8251 11,673 21,274

0.554 0.5788 0.6582 0.5702

2680.68 2261.50 2659.26 2935.18

30.76 34.37 34.90 49.36

71.47 71.27 64.31 79.79

61.50 63.43 56.77 70.94

0.1810 0.1374 0.1539 0.1188

HHVchar (kJ/ kg)

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