Characteristics of biochar produced from slow pyrolysis of Geodae-Uksae 1

Bioresource Technology 130 (2013) 345–350

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Characteristics of biochar produced from slow pyrolysis of Geodae-Uksae 1 Yongwoon Lee a, Pu-Reun-Byul Eum a, Changkook Ryu a,⇑, Young-Kwon Park b, Jin-Ho Jung c, Seunghun Hyun c a

School of Mechanical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Faculty of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea c Division of Environmental Science and Ecological Engineering, Korea University, Seoul 136-713, Republic of Korea b

h i g h l i g h t s " Geodae-Uksae 1 (Giant Miscanthus) is a variety of Miscanthus sacchariflorus for energy crop. " Ideal temperature to produce biochar by slow pyrolysis was 500 °C. " The biochar had a mass yield of 27 wt.% at 500 °C with a carbon content of 79 wt.%. " The surface area and large pores of biochar was well-developed at 500 °C for application to soil.

a r t i c l e

i n f o

Article history: Received 30 March 2012 Received in revised form 2 December 2012 Accepted 5 December 2012 Available online 13 December 2012 Keywords: Biochar Biomass Geodae-Uksae 1 Miscanthus Slow pyrolysis

a b s t r a c t s This study investigated producing biochar from Geodae-Uksae 1 for soil applications to sequestrate carbon from the atmosphere and improve the productivity of crops. Using a lab-scale packed bed reactor, pyrolysis products of Geodae-Uksae 1 were produced over a temperature range of 300–700 °C with a heating rate of 10 °C/min. Pyrolysis at 500 °C was found appropriate for biochar production considering the properties of char and the amount of heat required. It yielded biochar of 27.2 wt.% that contained approximately 48% carbon in the raw biomass. The surface area of the biochar rapidly increased to 181 m2/g. Large cylindrical pores with diameters of 5–40 lm developed within the biochar due to the vascular cell structure of the parent biomass. The byproducts (bio-oil and gases) were also analyzed for use as fuel. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Geodae-Uksae is the Korean term for giant Miscanthus. GeodaeUksae 1 is a variety of Miscanthus sacchariflorus (Amur silvergrass) recently discovered in Korea (Moon and Koo, 2011) that grows approximately 4 m tall with an average stalk diameter of 1 cm, which is approximately twice as tall and thick as common M. sacchariflorus. The mass yield of the dry stalk is as much as 30 ton/ha, which is twice that of common Miscanthus. Due to the superior yield, Geodae-Uksae 1 is being mass-cultivated in Korea as an energy crop for bioenergy. Various methods are being considered for the energy conversion of Geodae-Uksae 1, including hydrolysis and fermentation for bioethanol production, combustion through pelletization and fast pyrolysis for the production of bio-oil. This study investigates a method for producing biochar from Geodae-Uksae 1 for the sequestration of carbon in soil and to in⇑ Corresponding author. Tel.: +82 31 299 4841; fax: +82 31 290 5889. E-mail address: [email protected] (C. Ryu). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.012

crease the productivity of various food crops (Lehmann, 2007). Biochar is the highly carbonaceous solid product of the pyrolysis of biomass, which can be used to improve the yield of various agricultural crops as a soil amendment. Due to its strong resistance to biological decomposition, the carbon in biochar can be removed from the atmosphere to mitigate climate change. Since carbon originates from atmospheric carbon dioxide, the application of biochar to soil may contribute to reductions of CO2 concentration. Biochar has been used in horticulture and agriculture with its appearance in literature as early as 1697 (Lehmann and Joseph, 2009). Biochar has drawn interest from a wider scientific community due to a study by Lehmann et al. (2003) examining the sustained fertility of Amazonian dark soil, also known as Terra Preta. When applied to soil, biochar can effectively retain nutrients and water, and therefore reduce the need for fertilizers. In addition to carbon removal, biochar in soil reduces the emissions of other major greenhouse gases, such as N2O and CH4 (Van Zweiten et al., 2009), which have a global warming potential of 298 and 25, respectively, compared to the greenhouse effect of CO2 over a 100 year period (IPCC, 2007). These

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benefits suggest that the application of biochar from biomass to soil could be as effective as producing energy from the valuable resource. Depending on the type of biomass, the amount of biomass available for energy production widely varies by location and time of year. Due to its typically low bulk density, it is sometimes not economical to collect and transport biomass to a large-scale bioenergy plant. In contrast, biochar that is locally produced by a smallscale pyrolysis unit can be consumed locally, minimizing transport needs. Slow pyrolysis is an ideal technology to produce biochar, which involves thermal decomposition in an inert atmosphere at a slow heating rate (10 °C/min) (Mohan et al., 2006). Pyrolysis converts solid fuels, such as coal and biomass, into char (solid), vapors of condensable hydrocarbons (called ‘oil’ for use after condensation) and non-condensable gases (e.g., CO, CO2, H2 and CH4). Biochar, or char originating from biomass, is typically 20–40 wt.% of dry lignocellulosic biomass. However, the yield and characteristics of the pyrolysis products are strongly influenced by the operating conditions (e.g., temperature, heating rate, pressure, purge gas and particle size) and the properties of the feedstock (Antal and Grønli, 2003; Enders et al., 2012). Therefore, the operating conditions of the pyrolysis process can be adjusted to meet the product requirements, however the actual process needs to be carefully designed and performed. In this study, Geodae-Uksae 1 was pyrolyzed in a lab-scale reactor to investigate the yield and properties of biochar for applications to soil. Biochar was produced by slow pyrolysis at a temperature of 300–700 °C and characterized for elemental composition, morphology, surface area and distribution of pore sizes and volumes. The byproducts of pyrolysis, i.e., bio-oil and gases, were analyzed for further use as energy sources. 2. Experimental 2.1. Geodae-Uksae 1 samples Geodae-Uksae 1 samples were provided by the Korean Rural Development Administration (RDA), which discovered and cultivated the strain. Geodae-Uksae 1 was harvested in the second year of planting, in early spring of 2011. Late winter or early spring is suitable for harvest since the moisture content of the crop naturally drops at that time (Lewandowski et al., 2000; Fernando et al., 2008). The nutrients in the plant are transported in winter and stored in underground rhizomes for the formation of new shoots (Beale et al., 1996). The harvested samples were maintained in dry indoor storage. The sample consisted mostly of stalks with a few leaves, since the leaves of Miscanthus naturally fall in winter (Beale et al., 1996). The stalks were cut into 4 cm long pieces for feeding into a pyrolysis reactor. Each piece was cylindrical and hollow, with a diameter of 4–12 mm. 2.2. Pyrolysis reactor The products of slow pyrolysis were produced at final temperatures ranging from 300 to 700 °C using a lab scale reactor. Fig. 1 shows a schematic diagram of the system. The reactor was made of stainless steel with a diameter of 10 cm and a height of 30 cm. It was placed inside an electrically-heated furnace with a temperature control. In each test, 20 g of sample was heated within the reactor from room temperature to the target temperature at a heating rate of approximately 10 °C/min. Once the temperature inside the reactor reached the target temperature, it was maintained for at least one hour to allow sufficient time for complete pyrolysis. Nitrogen was continuously supplied at a flow rate of 1.2 min 1 to purge the pyrolysis vapors from the reactor. The gas flow forced

the pyrolysis vapors to pass through a series of condensers submerged in coolants at 20 °C (water) and 20 °C (acetone), respectively, for the separation of condensable (bio-oil) and non-condensable gases. Past the particulate filter, the gas flow rate was recorded using a mass flow meter (Tylan, FM-360). The gas compositions of the main gas species (O2, CO, CO2, H2 and CH4) were measured using an on-line gas analyzer (A&D System, A&D 9000). The reactor temperatures, gas flow rates and compositions were logged using a data acquisition system. The gases were also sampled into Tedlar bags for detailed compositional analysis by a gas chromatograph (Perkin-Elmer, Clarus 680 GC). After each test, biochar and bio-oil were collected from the reactor and condensers, respectively, to measure the mass yield and for detailed property analyses. The mass yield of gases was calculated by difference. The pyrolysis tests were repeated at least three times for key target temperatures (400, 500 and 600 °C). Pyrolysis at 450 °C, 550 °C and 700 °C was tested once in order to check the variations around 500 °C. The average mass yields are presented in this study as the deviation of the values in each test was less than 1.5 wt.% from the average, except for 300 °C. 2.3. Characterization of biomass and pyrolysis products The biomass and biochar compositions were analyzed by proximate analysis based on standard methods (moisture content: ASTM E871-82, ash: ASTM D1102-84, volatile matter: ASTM E872-82 and fixed carbon: by difference) and ultimate analysis using an elemental analyzer (CE Instruments, EA 1108/NA 2000). The higher heating value (HHV) of biomass was measured using a bomb calorimeter (Parr-1261, Parr Instrument). Thermogravimetric analysis (TGA) for the biomass was carried out by using a Labsys. EVO TGA analyzer (Setaram) for 7 mg of powdered sample at a heating rate of 10 °C/min under nitrogen atmosphere (30 ml/min). Detailed characteristics of biochar were analyzed using a scanning electron microscope (SEM, JEOL, JSM-7600F) for surface morphology, N2–BET (Micrometrics, Tristar 3020) for surface area, and a porosimeter (Micromeritics, AutoPore 4 9250) for distribution of pore volumes. The hydrocarbon compositions in the bio-oil were analyzed by a GC–MS (Hewlett–Packard, HP5890/HP5972). To determine the elemental composition of bio-oil, it was separated into light and heavy phases in a centrifugal separator (ROTINA, 35R). Then, each phase was analyzed by an elemental analyzer (CE Instruments, EA 1108/NA 2000) to determine the C, H, O and N compositions and Karl-Fisher titration (Metrohm, 870 KF Titrino plus) to assess the water content. Based on the elemental compositions, the HHV of biochar and bio-oil were calculated using an empirical correlation proposed by Channiwala and Parikh (2002). The HHV of the gases was estimated from the net mass yield and HHV of each gas component. The net mass yield of a gas component was acquired by integrating the concentrations history measured by the gas analyzer over the test duration. 3. Results and discussion 3.1. Properties of Geodae-Uksae 1 The air-dried sample of Geodae-Uksae 1 contained 7.3% moisture, 73.2% volatile matter, 15.9% fixed carbon and 3.6% ash. The volatile matter to fixed carbon ratio was 4.6, which is in a typical range for lignocellulosic biomass. The elemental composition on a dry-ash-free basis was 47.6% C, 5.5% H, 46.1% O and 0.8% N, which was equivalent to C1.00H1.39O0.73N0.01. Analyses were also conducted to compare different sections of stalks, but no significant differences were found. The HHV experimentally determined was

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Fig. 1. Schematic diagram of a lab-scale pyrolysis reactor.

17.28 MJ/kg which was similar to the value (17.05 MJ/kg) estimated by the correlation of Channiwala and Parikh (2002). The cellulose and hemicellulose contents were 40.8% and 31.4%, respectively. The lignin content for the sample was 23.0%, which was 5–9% higher than the common M. sacchariflorus in Korea (Kim et al., 2012). Such high lignin content is favorable for biochar production, since lignin contributes more to the formation of char. Overall, Geodae-Uksae 1 has a very good quality as a fuel for thermo-chemical conversion to produce heat, electricity or chemical feedstock. The TGA analysis of Geodae Uksae 1 exhibited a typical decomposition trend of lignocellulosic biomass (Yang et al., 2007; Sanchez-Silva et al., 2012). The moisture evaporated near 100 °C. Hemicellulose began to decompose at approximately 200 °C and reached a peak rate (0.35 wt.%/°C) of mass loss at 290 °C. The hemicellulose peak overlapped with cellulose decomposition which continued to approximately 380 °C with a peak rate of 1.21 wt.%/ °C appearing at 358 °C. The residual weight at 380 °C was 28.5 wt.%. The pyrolysis above 380 °C was due to lignin, which slowly decomposes over a wide temperature range. The final weight of the solid residue was approximately 17 wt.% at 800 °C.

3.2. Product yields of pyrolysis Table 1 lists the product yields from pyrolysis of Geodae-Uksae 1 in the lab-scale reactor. The solid (biochar) yield decreased with increasing temperatures up to 500 °C. The decrease in the yield above 500 °C was very small, since the decomposition of hemicellulose and cellulose was complete. The mass yield of bio-oil including water condensation was about 50% at 400 °C and above. A peak in the bio-oil yield appeared at 550 °C (50.57 wt.%) but was only 2.26 wt.% higher than that at 400 °C. In many studies for pyrolysis of biomass, the maximum oil yield appears at 500–550 °C for various heating conditions, sizes and types (Ertasß and Alam, 2010; Heo et al., 2010; Phan et al., 2008). The slight decrease above 600 °C may be the result of vapor-phase cracking of heavy hydrocarbon compounds inside the hot reactor. However, the decrease in the bio-oil yield was not sig-

nificant since the pyrolysis vapors were continuously purged from the reactor by nitrogen while the reactor was heated. The gas yield was calculated by the difference, which gradually increased from approximately 20%, consisting largely of CO and CO2 at lower temperatures. The detailed composition of the released gases will be presented later. 3.3. Biochar properties Table 2 summarizes the key properties of biochar from GeodaeUksae 1. As the pyrolysis temperature increased, the fixed carbon and elemental carbon content gradually increased. The carbon content of biochar at 500 °C was 79.42 wt.% on a dry basis, representing 48.36 wt.% carbon in the raw biomass. Therefore, biochar is the preferred product of pyrolysis in terms of carbon. Although the difference in its mass yield between 500 °C and 700 °C was less than 2 wt.%, the increase in elemental carbon content was about 6.5 wt.% dry. This implies that the biochar became increasingly carbonaceous at high temperatures, releasing H and O. One of the main purposes of biochar production is to remove carbon in soil. Therefore, the high carbon content of biochar is beneficial in terms of maximizing the amount of carbon storage. The weight of carbon in the biochar from 500 °C corresponds to 21.56 wt.% (=0.7942  27.15%) of the weight of the raw biomass. The equivalent amount for CO2 becomes 79.1 wt.% (=21.56  44/ 12) of the raw biomass, if all of the carbon remains in the soil in the long term. Note that biochar can reduce the amount of fertilizer required and the emission of N2O and CH4 from the soil. Therefore, the amount of carbon emissions prevented by biochar can be significant, although it is influenced by many factors (Gaunt and Lehmann, 2008; Galinato et al., 2011). Fig. 2 shows a van Krevelen diagram of biochar to illustrate the changes in its elemental composition. As pyrolysis progressed, H and O were depleted in the biochar and it became carbon-rich. The decrease in the H/C and O/C ratios was approximately linear, approaching the origin (pure carbon) as the pyrolysis temperature increased. This trend coincided well with other types of biomass, such as pine and straw (Karaosmanog˘lu et al., 2000; Keiluweit et al., 2010), also plotted in the figure.

Table 1 Mass yields (wt.%) of pyrolysis products from Geodae-Uksae 1. Temperature (°C)

300

400

450

500

550

600

700

Biochar Bio-oil Gases

49.54(±1.41) 30.70(±2.52) 19.75(±2.14)

30.95(±0.32) 48.31(±1.05) 20.74(±1.36)

29.42 49.01 21.56

27.15(±0.23) 50.03(±1.11) 22.82(±1.23)

26.21 50.57 23.23

25.92(±0.76) 49.78(±1.13) 24.30(±1.05)

25.10 48.24 26.66

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Table 2 Key properties of biochar from Geodae-Uksae 1. Temperature (°C)

300

400

450

500

600

700

Fixed carbon content (wt.% dry) C content (wt.% dry) C yield (wt.% dry of C in raw biomass) N2–BET surface area (m2/g)

53.34 66.19 73.55 0.49

73.89 74.69 51.86 3.11

78.48 78.29 51.67 21.93

82.59 79.42 48.36 180.96

88.07 83.67 48.65 293.04

91.66 85.93 48.38 368.98

2.0

2

N2-BET surface area (m /g)

1.6

H/C atomic ratio

(a) 500

Geodae-Uksae #1 Brassica napus L Straw-stalka Ponderosa Pineb Tall Fescue Strawc

1.2

Raw biomass

0.8 400o C 500o C

0.4 600o C 700o C

0.0 0.0

0.2

0.4

0.6

400

Goedea-Uksae #1 Ponderosa Pinea Tall Fescue Strawa Alderb Beechb

300

200

100

0

0.8

300

400

O/C atomic ratio

Fig. 3a compares the N2–BET surface area of biochar to that of other lignocellulosic biomass with a pore size range of 2–50 nm. The surface area determines the nutrient adsorption capability of biochar in soil. Biochar with a large surface area can fix nutrients, preventing them from being washed away by water and therefore reducing the need for fertilizers. The biochar produced using Geodae-Uksae 1 exhibited a rapid increase in surface area to approximately 180 m2/g at 500 °C, which was one order of magnitude greater than at 450 °C. The surface area gradually increased at higher temperatures, and became similar to that of a soft wood, ponderosa pine (Keiluweit et al., 2010), also plotted in Fig. 3a. In contrast, two hard woods, alder and beech (Gray et al., 1985), exhibited greater surface areas from low temperatures, while the increases in surface area were smaller at higher temperatures. The surface area of the wood samples and Geodae-Uksae 1 converged at 700 °C. In contrast, the straw sample (tall fescue straw) (Keiluweit et al., 2010) had a very low surface area, even at 700 °C. Fig. 3b shows the distribution of pore volumes in biochar ranging from 10 nm to 100 lm. Compared to the biochar at 300 °C, the pore volume significantly increased at 500 °C over the entire size range. When the surface morphology of biochar was examined using SEM, the char from 300 °C was filled with tissue that was not devolatilized and therefore the pores were not fully developed. Once the cellulose and hemicellulose were decomposed at 500 °C, the char became honeycomb-like with large cylindrical holes of 5– 40 lm in diameter interconnected by thin walls. These holes originated from the pith and vascular system of the parent plant. The pore volumes of mesopores and macropores up to 1 lm, which developed on the cell walls, were also significant. When biochar is applied to the soil, the roles of the pores depend on their sizes. Micro and mesopores up to 50 nm in diameter adsorb nutrients (e.g., NH4+, HPO42 , H2PO4 and dissolved organic carbon) and gases (e.g., O2 and CO2), while large pores (10 lm) provide habi-

600

700

o

Temperature ( C)

(b) Log differential intrusion (mL/g)

Fig. 2. Van Krevelen diagram of biochar at different pyrolysis temperatures (a: data taken from Karaosmanog˘lu et al., 2000; b and c: data taken from Keiluweit et al. (2010)).

500

6 o

5

300 C o 500 C o 600 C

4 3 2 1 0 0.01

0.1

1

10

100

Pore diameter (µm) Fig. 3. N2–BET surface area (a) and pore volumes distribution (b) of biochar (a: data taken from Keiluweit et al. (2010), b: data taken from Gray et al. (1985)).

tats for symbiotic microorganisms, such as bacteria (0.3–3 lm), fungi (2–80 lm) and protozoa (7–30 lm) (Thies and Rillig, 2009). Biochar becomes less hydrophobic and its capacity to hold water also increases at 500 °C (Kinney et al., 2012). Based on the properties of biochar presented in this study, an appropriate temperature for biochar production by slow pyrolysis would be 500 °C. The mass yield of biochar was higher at 300 °C (Table 1), but the high volatile matter content at the temperature inhibits plant growth and reduced N uptake in soil (Deenik et al., 2009). At 500 °C, the macro- and micropores rapidly develop to increase the pore volumes and surface area. Temperatures above 500 °C further increased the surface area of biochar, but required a larger amount of heat supply. Furthermore, the bio-oil, an important byproduct of biochar production, had a decrease in its mass yield at higher temperatures. Evaluating the performance of the

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Y. Lee et al. / Bioresource Technology 130 (2013) 345–350 Table 3 Composition of bio-oil and the estimated heating value.

300 400 500 600

Phase (wt.%) Aqueous

Heavy oil

91.29 67.76 57.16 47.31

8.71 32.24 42.84 52.69

Water content (wt.%)

Elemental composition (wt.%) C

H

O

N

59.77 42.39 36.93 36.18

19.17 29.55 32.24 38.51

9.97 8.47 9.07 9.03

70.34 61.41 57.74 51.54

0.51 0.58 0.96 0.92

biochar for improving crop productivity requires further extensive studies. Enders et al. (2012) suggested that the most effective approach is to define the limiting factors of a particular soilcrop-climate condition and apply biochar tailored to address the growth constraints.

Table 4 Key compounds identified in bio-oil by GC–MS for pyrolysis at 500 °C. Retention time (min)

Compounds

Area (%)

2.44 4.77 10.43 12.39 13.95 14.86 15.53 16.03 16.64 17.80 18.77 19.71 20.77 23.48

Acetic acid 2-Furan carboxaldehyde Phenol 2-Methoxy phenol 2-Ethyl phenol 2,3-Dihydro benzofuran 4-Ethyl-2-methoxy phenol 2-Methoxy-4-vinyl phenol 2,6-Dimethoxy phenol 4-Hydroxy-3-methoxy benzoic acid 2,3,5-Trimethoxy toluene 2,4-Hexadienedioic acid 2,6-Dimethoxy-4 phenol Hexadecanoic acid

10.7 5.53 4.42 5.95 5.67 7.89 4.90 3.45 6.32 4.17 1.62 1.17 1.66 0.71

600

30

Temp.

CO2

500

25

400

20 CO

300

15 200

10 CH4

100

5 H2

0 0

1000

2000

o

Bio-oil is a pyrolysis byproduct of biochar production. The properties of bio-oil have been widely reported in the literature, and therefore only a summary of the key properties is presented here for the bio-oil derived from Geodae-Uksae 1. Table 3 describes the composition of bio-oil and the estimated HHV. The bio-oil has two strata of light aqueous and heavy oil fractions. At lower temperatures, the oil is mostly the light fraction with approximately 60 wt.% overall water content. With increasing temperatures, the proportion of heavy oil fraction and carbon content increases. At temperatures greater than 500 °C, the water content in the bio-oil was approximately 37 wt.% and the carbon content was 32 wt.%. The bio-oils produced at 500 °C and 600 °C were not very different, since the pyrolysis vapors were continuously purged from the reactor into the condenser. The HHV estimated from the elemental composition was 15.94 MJ/kg, while commercial heavy oil is approximately 45 MJ/kg (Turns, 2011). This low HHV is due to the high water content and large concentration of oxygenated hydrocarbon compounds in the bio-oil. Table 4 lists the key compounds identified by GC–MS in the biooil produced at 500 °C. The bio-oil contained numerous hydrocarbons, which were primarily acids and phenols, typical of lignocellulosic biomass bio-oil both from slow and fast pyrolysis (Phan et al., 2008; Heo et al., 2010). Overall, the quality of the bio-oil was not high enough for use as primary fuel due to the high water content, inhomogeneity and low energy content. Bio-oil is also acidic, corrosive, viscous, repolymerizes to form heavier compounds and causes delayed ignition when combusted (Oasmaa and Czernik, 1999; Mohan et al., 2006; Chiaramonti et al., 2007). Such properties of bio-oil require caution during handling, storage and use. Therefore, a simple

11.14 13.92 15.94 18.72

Temperature ( C)

3.4. Bio-oil properties

HHV (MJ/kg)

35

Gas concentration (vol.%)

Temperature (°C)

3000

0 4000

5000

6000

Time (s) Fig. 4. Concentration of non-condensable gases and the reactor temperature for the pyrolysis test up to 500 °C.

Table 5 Composition of non-condensable gases and estimated heating values. Temperature (oC)

300

400

500

600

Mass yield (wt.%)

19.75

20.74

22.82

24.30

Species mass fraction (wt.%) CO 38.27 CO2 60.13 CH4 1.59 H2 0.01

25.14 62.47 12.38 0.01

26.40 57.65 15.82 0.13

26.81 53.44 19.03 0.72

Elemental composition (wt.%) C 33.99 H 0.41 O 65.60

37.10 3.10 59.80

38.90 4.09 57.01

40.34 5.47 54.19

9.42

11.64

14.30

HHV(MJ/kg)

4.76

method of utilizing bio-oil as a renewable fuel may be to co-combust it with conventional fuels in an existing furnace. A solvent, such as methanol and ethanol, could be mixed into the bio-oil to prevent repolymerization during storage and decrease its viscosity for easier handling (Oasmaa and Czernik, 1999; Mohan et al., 2006).

3.5. Properties of non-condensable gases Fig. 4 shows the gas release profiles from a pyrolysis test at a final temperature of 500 °C measured by an on-line gas analyzer. CO2 and CO were the two dominant species in the early stage of pyrolysis, mainly due to the presence of cellulose and hemicellulose. Release of CH4 followed with a peak at approximately 460 °C. H2 appeared above 470 °C at a low concentration mainly from the decomposition

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of lignin (Yang et al., 2007) and partially from vapor phase cracking of heavy hydrocarbons (Morf et al., 2002; Gilbert et al., 2009). Table 5 summarizes the composition of non-condensable gases and the HHV. The estimated HHV was approximately 11.64 MJ/kg at 500 °C, which is not high due to the large proportion of CO2. However, the gases can be burned to provide the heat required for pyrolysis. The heat required to increase the biomass temperature from 25 °C to 500 °C is 0.89 MJ/kgbiomass for a specific heat of 1.4 kJ/kg K (Incropera et al., 2007). The energy content in the pyrolytic gases corresponds to 11.64 MJ/kg  (22.82 wt.%) = 2.66 MJ/ kgbiomass. Therefore, 33.5% (0.89/2.66) of the heat exchange efficiency between the pyrolysis reactor and hot combustion gases of the non-condensable gases is sufficient to maintain the reactor at 500 °C. However, this is not straightforward to achieve for small-scale pyrolysis systems. The process requires indirect heat exchange in the pyrolysis reactor to avoid contamination of the pyrolysis vapors. The pyrolysis vapors must be quenched to condense bio-oil, and such quenching is often accomplished by spraying already-quenched bio-oil. The remaining gases can then be burned to generate hot gas. The biochar should be collected and cooled to prevent contact with the air. Incorporating these features requires complex design and control, which is difficult for smallscale systems. Therefore, a careful design of the pyrolysis system is essential to minimize the requirements for the external heat supply and efficiently operate the pyrolysis process.

4. Conclusions This study presented the pyrolysis characteristics of GeodaeUksae 1 to produce biochar for soil quality improvement and CO2 sequestration. A pyrolysis temperature of 500 °C was found appropriate considering the properties of biochar and the heat supplied. The biochar yield was of 27 wt.% at the temperature with a carbon content of 79 wt.% and surface area of 180 m2/g. Large pores of 5– 40 lm developed in the biochar, which is beneficial for symbiotic micro-organisms in soil. The bio-oil consisted largely of water, acids and phenols. The energy content in the product gases was sufficient for use as process heat. Acknowledgements This work was performed with the support of the Cooperative Research Program for Agriculture Science & Technology Development (Project No. PJ0079662011), Rural Development Administration, Republic of Korea. The authors would like to thank Dr. Y-H. Moon (RDA) for providing Geodae-Uksae 1 samples. References Antal, M.J., Grønli, M., 2003. The art, science, and technology of charcoal production. Ind. Eng. Chem. Res. 42, 1619–1640. Beale, C.V., Bint, D.A., Long, S.P., 1996. Leaf photosynthesis in the C4-grass Miscanthus giganteus, growing in the cool temperate climate of southern England. J. Exp. Bot. 47, 267–273. Channiwala, S.A., Parikh, P.P., 2002. A unified correlation for estimating HHV of solid, liquid and gaseous fuels. Fuel 81, 1051–1063. Chiaramonti, D., Oasmaa, A., Solantausta, Y., 2007. Power generation using fast pyrolysis liquids from biomass. Renew. Sust. Energ. Rev. 11 (6), 1056–1086.

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