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Hydrothermally carbonized plant materials: Patterns of volatile organic compounds detected by gas chromatography
Bioresource Technology 130 (2013) 621–628
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Hydrothermally carbonized plant materials: Patterns of volatile organic compounds detected by gas chromatography Roland Becker a,⇑, Ute Dorgerloh a, Mario Helmis b, Jan Mumme c, Mamadou Diakité c, Irene Nehls a a
Federal Institute for Materials Research and Testing (BAM), Richard-Willststätter-Strasse 11, 12489 Berlin, Germany Beuth Hochschule für Technik Berlin, Luxemburger Straße 10, 13353 Berlin, Germany c Leibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany b
h i g h l i g h t s " Digestate, straw and various wood feedstock are carbonized between 190 and 270 °C. " Volatile organic compounds are determined using head space gas chromatography. " Relative amount and composition of volatiles from different feedstock are compared. " Benzenes, phenols, furans, ketones and aldehydes increase with process temperature. " Monitoring of VOC is an option to optimize hydrochars for environmental application.
a r t i c l e
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Article history: Received 28 June 2012 Received in revised form 11 December 2012 Accepted 14 December 2012 Available online 22 December 2012 Keywords: Hydrochar Mass spectrometry Flame ionization detection Phenols Benzenes
a b s t r a c t The nature and concentrations of volatile organic compounds (VOCs) in chars generated by hydrothermal carbonization (HTC) is of concern considering their application as soil amendment. Therefore, the presence of VOCs in solid HTC products obtained from wheat straw, biogas digestate and four woody materials was investigated using headspace gas chromatography. A variety of potentially harmful benzenic, phenolic and furanic volatiles along with various aldehydes and ketones were identified in feedstock- and temperature-specific patterns. The total amount of VOCs observed after equilibration between headspace and char samples produced at 270 °C ranged between 2000 and 16,000 lg/g (0.2–1.6 wt.%). Depending on feedstock 50–9000 lg/g of benzenes and 300–1800 lg/g of phenols were observed. Substances potentially harmful to soil ecology such as benzofurans (200–800 lg/g) and p-cymene (up to 6000 lg/g in pine wood char) exhibited concentrations that suggest restrained application of fresh hydrochar as soil amendment or for water purification. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Solid carbonaceous products termed hydrochars have been suggested for various applications such as soil amendment and carbon sequestration (Titirici et al., 2007; Sevilla et al., 2011). They are obtained from biomass by means of hydrothermal carbonization (HTC), typically at 180–250 °C, 2–10 MPa and in the presence of liquid water. In comparison with chars obtained from pyrolysis, hydrochars show a lower carbon content, less aromatic structure, and less biological stability, but HTC allows a higher char yield as well as a higher overall energy efficiency for wet feedstock (Libra et al., 2011). The option to use wet organic waste such as biogas digestate (Mumme et al., 2011) makes HTC specifically interesting compared to pyrolysis, which requires dry feedstocks. The use of ⇑ Corresponding author. Tel.: +49 30 8104 1121; fax: +49 30 8104 1127. E-mail address: [email protected] (R. Becker). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.102
hydrochars for soil amendment promises various benefits such as improved water holding capacity and long-term storage of carbon branched off the biomass cycle that may lead to an overall negative greenhouse gas balance. Furthermore, chars are chemically related to black carbon components in soil, which have been discussed as a sink for persistent pollutants (Koelmanns et al., 2006). However, the experimental application of hydrochars from various sources to soils in laboratory and field trials revealed various proven or potentially detrimental effects on seed germination (Busch et al., 2011), microbial activity (Steinbeiss et al., 2009), and mycorrhiza growth (Rillig et al., 2010), and gene repression in soil nematodes (Chakrabarti et al., 2011). Phytotoxic behavior is not an exclusive effect of hydrochars but has also been reported for chars obtained from the pyrolysis of bioenergy residues such as digestates (Gell et al., 2011). The adverse effects of fresh hydrochars may partly be attributable to volatile organic compounds (VOCs) (Busch et al., 2011). As a consequence, the VOC content is
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‘‘Japan Clone NM 105’’) was received as dry chips with a diameter of 4–5 cm from the short rotational plantation run by ATB (Scholz et al., 2010). Garapa (Apuleia leicarpa), massaranduba (Manikara bedento), and pine (Pinus sylvestris) wood were obtained from a local timber retailer and cut into chips of about 1 cm in length. All feedstocks were dried at 60 °C for 48 h and stored airtight at room temperature until being milled to a particle size below 4 mm with a Retsch SM100 cutting mill (Retsch, Haan, Germany). Elemental analyses of feedstock materials and HTC chars were carried out with a Vario EL III elemental analyzer (Elementar Analysen Systeme GmbH, Hanau, Germany) using an intake of 10 mg. For reference purposes, several other types of carbonized materials were included in this study. A commercial pyrolysis char produced from a mixture of 65% wood and 35% compost at 700–850 °C was obtained from Pyreg GmbH (Dörth, Germany). A pyrolysis char from wheat straw treated at 275 °C and two pyrolysis chars from olive pomace (OP) treated at 275 and 400 °C were produced at ATB using a rotary kiln. A commercial hydrochar made from maize silage was received from AVA-CO2 Forschung GmbH, Karlsruhe, Germany. A further reference material was a lignite sample from the German Lausitz region designated for domestic heat generation.
considered an important property of chars, especially if they are designated for soil use (Schimmelpfennig et al., 2012). Furthermore, the olfactory characteristics of hydrochars and literature reports on the presence of a variety of volatiles in various pyrochars and hydrochars (Spokas et al., 2011) suggest attention to the relevance of VOCs for work-place safety in case of large scale hydrochar production. Consequently, the aim of the present study was to investigate the composition and significance of VOC substances in hydrochars. Therefore, a number of different types of biomass were hydrothermally carbonized under identical conditions and the solid products were screened with headspace gas chromatography. The objective was to compare the relative content and composition of VOC in the HTC products of the different feedstocks with increasing degree of carbonization. 2. Methods 2.1. Feedstocks for the hydrothermal carbonization and reference materials Wheat straw was received as 4–5 cm long pieces from the Dittmannsdorfer Milch GmbH, Kitzscher, Germany. Digestate was obtained from an anaerobic test reactor operated at 55 °C at the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) (Mumme et al., 2010), which was fed with the wheat straw as sole substrate. Poplar wood (Populus nigra x P. maximowiczii
2.2. Hydrothermal carbonization Between 3 and 5 g of the respective feedstock, in each case corresponding to 2 g of carbon, were mixed with 75 mL of water
2,000 2.000
feedstocks
H/C (mol%)
190 °C 250 °C
1,500 1.500
230 °C
Wheat straw
270 °C
Digestate
1,000 1.000
Biomass
Pine wood
Turf Hard coal
0,500 0.500
Poplar wood
Lignite
Anthracite 0.500 0,000
0,000 0.500
0,100 0.100
0,200 0.200
0,300 0.300
0,400 0.400
0,500 0.500
0,600 0.600
0,700 0.700
O/C (mol%) 2,000 2.000
feedstocks
H/C (mol%)
190 °C 230 °C
1,500 1.500
Masaranduba Hydrochar Garapa Hydrochar
Lignite 1,000 1.000
AVA Hydrochar Bergius Cellulose Hydrochar Bergius Lignin Hydrochar
0,500 0.500
270 °C
Pyrochar wheat straw 275
250 °C
Pyrochar OP 275 Pyrochar OP 400 Pyreg pyrochar
0.500 0,000
0,000 0.500
0,100 0.100
0,200 0.200
0,300 0.300
0,400 0.400
0,500 0.500
0,600 0.600
0,700 0.700
O/C (mol%) Fig. 1. Van-Krevelen diagram illustrating the development of carbon ratios of the products of hydrothermal carbonization with process temperature.
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1 2 3 4 5 6 7 8 9 10 11 12 13
2-methylpropanal 2-butanone 1-methylcycylopentene benzene 3-methylbutanal 2-methylbutanal 2-pentanone 3-pentanone toluene cyclopentanone ethylbenzene 2-methylcyclopentanone furfural
14 15 16 17 18 19 20 21 22 23 24 25
1,3-dimethylbenzene 2-methylcyclopenten-1-one benzofuran 2,3-dimethylcyclopen-2-en-1-one phenol 2-methoxyphenol 2-methylbenzofuran 4-ethylphenol 4,7-dimethylbenzofuran 2-methoxy-4-ethylphenol 2,6-dimethoxyphenol 2-methoxy-4-ethylphenol
190 °C
19 18
56
1 6.00
13
15.00
10.00
25.00
20.00
16
34.00
30.00
19
250 °C
21 22 23
19
17
18
23
15 20 21
25.00
30.00
22 9
6 1 5.00
5
3
2 10.00
78
4 15.00
270 °C
20.00
12 14 10 11
25 24
25.00
35.00
30.00
19
17 15 17
23
18 16 20 21 25.00
30.00
9
2
1
5.00
10.00
3
4
15.00
5
6
11 12 10 14
22 25
7
24
8
20.00
25.00
30.00
Fig. 2. Chromatograms of total volatiles in selected carbonization products of digestate (headspace gas chromatography/mass selective detection).
to obtain a carbon content of 26.7 g/L. Each mixture was homogenized and submitted to the HTC experiments carried out in 125-mL unstirred Parr pressure vessels 4748 (Parr Instrument Company, Moline, IL, USA) featuring removable polytetrafluoroethylene liners. The vessels were placed in a Nabertherm B 180 oven (Nabertherm GmbH, Lilienthal, Germany) using a heating rate of 5 K/min until the desired set-point (190, 230, 250, and 270 °C) was reached. The process temperatures were maintained for 6 h,
the reactors were removed from the oven, and cooled by ambient air. After 2.5 h, the reactor slurry was filtered through fluted cellulose filter paper, type RotilaboÒ 113P (Carl Roth GmbH, Karlsruhe, Germany) and the solids were dried for 48 h at 60 °C. Samples of 100 mg were filled into 10-mL headspace vials and stored at 20 °C until GC analysis. In order to obtain a sufficient amount of char and to assess the reproducibility of the HTC process, each experiment was run at least in triplicate.
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2000
25000
1800 62000
1600
20000
1400
before HTC
µg/g
190 °C 230 °C 250 °C
190 °C
800
230 °C
600
250 °C
400
270 °C
Garapa
Massaranduba
0
Pine wood
200
Garapa
Massaranduba
Pine wood
Poplar wood
Wheat straw
0
Digestate
270 °C
1000
Poplar wood
5000
before HTC
Wheat straw
10000
1200
Digestate
µg/g
15000
Fig. 3. Total volatile compounds in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
Fig. 5. Content of total volatile phenols in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
2.3. Headspace gas chromatography of volatile compounds
compounds. The quantification of volatiles was achieved by means of an Agilent 6890 GC equipped with a CombiPal autosampler (CTC Analytics, Zwingen, Switzerland) and coupled to a flame ionization detector (FID) operated at 300 °C. Headspace conditions, injection mode, column, and oven program were identical to those described for GC/MS. For quantification, 200 ng of d8-toluene was added as internal standard to each vial as a solution in 10 lL methanol directly before analysis. d8-Toluene (2000 ng/lL in methanol) from UltraScientific (Kingstown, RI, USA).
Volatile compounds were identified using an Agilent HP 6890 gas chromatograph (Agilent, Waldbronn, Germany) equipped with an MPS 2 XL autosampler (Gerstel, Mühlheim, Germany) enabling fully automated headspace analysis and coupled to an Agilent HP 5973N quadrupole mass selective detector (EI, 70 eV). Sample vials were kept at 100 °C for 30 min and the syringe temperature was 110 °C. A split injection mode was used (split ratio 5:1) and the injection volume was 1 mL. A 60 m 0.32 mm VF624MS capillary column (Varian, Darmstadt, Germany) coated with a 1.8 lm thick film of 6% cyanopropylphenyl and 94% dimethylpolysiloxane was used with helium as carrier gas at a constant flow rate of 5 mL/ min. The oven temperature started at 40 °C (held for 15 min) and was raised to 140 °C (10 °C/min) and then to 220 °C (35 °C/min) and held for 15 min. The injector, ion source, transfer line, and MS quadrupole temperatures were maintained at 250 °C, 230 °C, 300 °C, and 150 °C, respectively. Data were acquired in the full scan mode between m/z 10 and 400. Chromatographic peaks were identified by means of the NIST mass spectral data library (version 2.0d) and in some cases from their retention times using standard
1400 1200
2.4. Reproducibility: measures and statistics Measures were taken to ensure that process conditions, including heating and cooling periods, were identical over all HTC runs. The HTC products were processed in an identical manner to avoid analytical bias. The variability of the HS-GC/MS and HS-GC/FID procedures was investigated and seen to be negligible compared to the variability of the HTC runs. At least three experiments were run for each temperature/feedstock combination and error bars in Figs. 3–5 represent the standard deviations of the means of three replicate HTC runs.
12000
a
b 10000
1000
before HTC
600
6000
190 °C 230 °C
400
µg/g
µg/g
8000 800
4000
250 °C 200
Garapa
Massaranduba
Pine wood
Poplar wood
Wheat straw
Digestate
Garapa
Massaranduba
Pine wood
Poplar wood
Wheat straw
Digestate
0
2000
270 °C
0
Fig. 4. Summary content of benzene, toluene, ethylbenzene, xylene (a) and total volatile benzenes (b) in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
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Table 1 Characterization of feedstock and hydrothermal carbonization products. Hydrochar and carbon yields (means and standard deviations of the means, n = 3)a are given as percentages of the original biomass. Before carbonization
After carbonization (6 h)
Biomass
Digestate Wheat straw Pine wood Poplar wood Masanduba wood Garapa wood a
190 °C
230 °C
250 °C
270 °C
Cellulose (mass%)
Hemicellulose (mass%)
Lignin (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
44.6 45.8
18.9 27.8
21.3 8.46
70.0 ± 0.60 60.4 ± 0.24
77.6 ± 0.67 70.2 ± 0.27
48.6 ± 0.30 41.0 ± 1.04
67.7 ± 0.41 58.3 ± 1.48
41.1 ± 1.79 38.1 ± 1.77
59.5 ± 1.80 59.9 ± 2.79
32.7 ± 1.83 33.1 ± 1.15
48.8 ± 2.73 54.1 ± 1.88
44.6 60.1
12.5 12.2
26.0 19.0
64.2 ± 0.18 64.2 ± 0.37
73.7 ± 0.21 68.1 ± 0.40
45.3 ± 0.05 58.3 ± 0.58
63.2 ± 0.20 66.4 ± 0.67
43.2 ± 0.89 47.7 ± 1.28
64.8 ± 0.77 66.3 ± 1.64
38.5 ± 2.05 39.1 ± 1.14
59.2 ± 3.15 58.6 ± 1.71
48.7
10.6
30.3
68.9 ± 0.45
70.6 ± 0.46
60.7 ± 0.76
68.7 ± 0.46
43.9 ± 2.50
62.3 ± 3.54
45.6 ± 0.37
63.3 ± 0.52
23.6
66.7 ± 0.13
69.8 ± 0.14
52.9 ± 4.12
63.7 ± 4.96
44.7 ± 2.75
58.7 ± 3.62
42.4 ± 1.10
59.6 ± 1.55
55.0
8.24
See Section 2.4.
3. Results and discussion 3.1. Selection of feedstocks, process conditions and headspace parameters In order to obtain an overview of the nature and amount of volatile substances in hydrochars with a number of different biomass, feedstock was selected to cover a range with potential practical relevance. Thus, wheat straw and a digestate derived thereof where selected as examples for major biomass waste flows. In addition, Scots pine wood and poplar wood were chosen to represent potential European wood sources along with the two tropical hardwood species massanranduba and garapa widely used as timber in Europe. The process period was set to 6 h because earlier experiments (Mumme et al., 2011) and literature reports (Bergius, 1928; Liu et al., 2010; Berge et al., 2011) suggested a completion of carbonization during this time at temperatures above 250 °C. The process temperatures ranged from 190 to 270 °C in order to monitor the development of volatiles with emphasis on compounds that display a significant vapor pressure at ambient temperatures. Headspace GC was employed since it provides a comprehensive screening of solid HTC products from highly volatile propanal to semi-volatiles such as phenols and naphthalenes with retention times ranging from 5.9 to 36 min. The adjusted headspace parameters covered the range of these compounds completely as no further volatiles could be detected in test runs of up to 42 min. It should be noted that an equilibrium is achieved between headspace and chars sample. Therefore, the determination of the VOCs in the headspace provides an excellent representation of the relative amount of volatiles between char samples but tends to underestimate the total VOC amount in the chars. Mass selective detection was chosen to identify individual substances and the combination of identical chromatographic conditions with FID was chosen for their quantification. d8-Toluene was used as internal standard for GC/FID since it was baseline separated from toluene and occupied a time window free of any compounds in all chromatograms. A similar combination of MS and FID has recently been adopted for the quantification of volatiles in biofuels (Stavova et al., 2012) and qualitative characterization of volatiles in biochars (Spokas et al. 2011). The collection of compounds detectable by this procedure is designated as hydrochar ‘‘volatilome’’ in analogy to what has been suggested for soil (Insam and Seewald, 2010) and plant materials (Maffei, 2011).
out with its relative low lignin content; however, after anaerobic digestion, the organic composition was similar to that of woody materials due to the selective degradation of cellulose and hemicellulose. At 190 °C, the yield of solid products was 60–70% of the initial mass; whereas at 270 °C, only 32–45% remained. The portion of the initial carbon recovered in solid form decreased from 68–78% (190 °C) to 48–64% (270 °C). In some cases, product and carbon yields decreased significantly between 250 and 270 °C. These results fall in the range observed in the literature (Libra et al., 2011). Maize silage yielded 40–43% carbon in the chars after 2–10 h at 270 °C (Mumme et al., 2011) while food (57%) and paper (67%) retained more of the original carbon after 20 h at 250 °C (Berge et al., 2011). Fig. 1 depicts the extent of carbonization of the employed feedstock as molar ratios of carbon to hydrogen and oxygen. The increase in the process temperature from 190 to 270 °C resulted in increasing carbonization. For reference purposes, a number of biochars and a lignite are also shown against the background of carbon ratios typical for the diagenesis of biomass towards hard coal and anthracite. Carbonization at 270 °C produced H/C ratios of 0.86–1.1 and O/C ratios of 0.12–0.22 typically associated with hydrochars (Mumme et al., 2011; Bergius, 1928; Berge et al., 2011; Liu et al., 2010) and even superior to those of the commercial product AVA hydrochar. With the exception of poplar wood, the composition of the chars after 6 h at 250 °C was largely similar to those obtained at 270 °C. The atomic ratios of paper and food hydrochars (H/C: 1.0; O/C: 0.18), (Berge et al., 2011)) were close to those observed in the current work while the maize hydrochars displayed H/C ratios of 1.13–1.16 and comparably low O/C ratios between 0.09 and 0.12. As anticipated (Libra et al., 2011), HTC products tend to display significant lower degrees of carbonization compared to pyrochars as also reported for pine wood feedstock (Liu et al., 2010). However, prolonged HTC runs yielded products with degrees of carbonization close to those of pyrochars. Bergius (1928) obtained chars from cellulose and lignin after HTC for 20 h at 340 °C, which displayed H/C-ratios of 0.8 and O/C ratio of 0.1. Similar results (H/C: 0.70); O/C: 0.11) were reported for wood submitted to an HTC-like treatment at 280 °C for 72 h (Tsukashima, 1966). Seen against this background, the hydrochars investigated in this work cover a range of practical relevance regarding feedstock, process conditions, and degree of carbonization. 3.3. Volatile organic compounds and carbonization progress
3.2. Characterization of the HTC chars The results of the characterization of the feedstocks and the respective chars are presented in Table 1. Fresh wheat straw stands
Fig. 2 depicts chromatograms obtained from selected HTC products of digestate. Among the major components were a wide range of aromatic, phenolic, and oxo-compounds. Comprehensive lists
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with all compounds identified in all HTC products and at all process temperatures are presented as Supplementary data (Tables S1–S19). Fig. 2 shows that furfural dominated the chromatograms after 6 h at lower process temperatures. Furfural is the main intermediate derived from pentoses from the hemicelluloses of the substrates during the HTC process (Titirici et al., 2008; Funke et al., 2010; Libra et al., 2011). The presence of furfural indicates that the process was not completed at this stage corresponding to the carbonization progress as seen in Fig. 1. In contrast, after treatment at 250 and 270 °C, furfural was no longer observed in the carbonization products. Massaranduba wood was an exception since its HTC product obtained at 270 °C still contained significant amounts of furfural and other HTC intermediates indicative of incomplete transformation of cellulose such as 5-methyl-furancarboxaldehyd and hydroxymethylfurfural. These findings suggest that HS-GC could be used to assess the progress of HTC of a specific feedstock. The increase in volatile compounds in the HTC products with process temperature is obvious from Fig. 2 and comprised for all HTC runs in Fig. 3. It should be noted that most feedstock displays a significant increase in total VOCs between 250 and 270 °C. Not surprisingly, wheat straw and its digestate displayed largely similar results. Massaranduba wood stands out in that it solely displayed no significant increase in total volatiles after treatment at different process temperatures. Pine wood displayed the greatest amount of total VOCs among the HTC products, but its substantial amounts of volatile mono- and sesquiterpenes disappeared during HTC. Headspace GC/MS chromatograms of the pine wood feedstock and selected HTC products are displayed in Supplemental Fig. S1. The total amount of VOCs observed after equilibration between headspace and char sample obtained at 270 °C ranged between 2000 and 16,000 lg volatiles per gram of char. Though this amounts to just 0.2–1.6% of the solid product and does not make the VOCs a major factor in the overall mass balance of HTC, it means that volatiles in the lower g/kg range are contained in the chars which may lead to adverse effects in soil or the workplace. 3.4. Major volatilome components of the carbonization products Fig. 4a displays the summation of benzene, toluene, xylenes, and ethyl benzene (BTEX) in the HTC products. All BTEX components were observed in all HTC products obtained at 250 °C and above. Toluene was present in pine wood already before HTC and dominates in the HTC products, followed by xylenes, ethylbenzene, and benzene. The toluene/benzene ratio was 3:1 in the case of garapa and 12:1 in case of pine wood hydrochar. Details are given in Supplemental Table S20. Though benzene and toluene are also found in minor amounts in volatilomes of composts (Kumar et al., 2011), concentrations of BTEX in the ppm range are hazardous to water and soil life as well as human health. Fig. 4b depicts the sum of all detected benzenic hydrocarbons. Besides BTEX, these are C10–C15 benzenes displaying methyl, ethyl and isopropyl substituents. The most abundant compounds were isopropylbenzene and isopropyltoluene (cymene). In case of the poplar and digestate hydrochars, the BTEX amounted to about half of the total benzenes and were accompanied by significant amounts of isopropylbenzene while cymene was nearly absent. In the case of straw hydrochar, cymene and BTEX each amounted to a third of the total benzenes. Garapa and massanranduba hydrochars displayed p-cymene as most abundant benzenic compound. Pine wood is unique in that p-cymene amounted to 80% of the total benzenic compounds in the HTC product obtained at 270 °C. Surprisingly, an isomer of p-cymene was found in small amounts before HTC and the amount of this isomer increased until 250 °C and was replaced with p-cymene in the 270 °C runs, which was itself formed at temperatures of 190 °C or above. The ratios of BTEX and cymene in the different volatilomes are collected in Sup-
plemental Table S21. Cymenes display a monoterpenoid structure but seem to be formed mainly during HTC. A source might be bicyclic and tricyclic terpenes such as pinene, carene, and camphene in the original pine wood. p-Cymene appears to be the end point of their stepwise aromatization. Likely intermediates are a number of non-aromatic monoterpenic compounds, namely methylisopropylcyclohexene and methylisopropylcyclohexdiene isomers. Since these compounds seem to be formed mostly during HTC but do not accumulate with process temperature, they are transformed further, most probably into cymene. This concept of aromatization during HTC is outlined in the supplemental data for monoterpenes and sesquiterpenes such as cadinene isomers. The latter are observed in the original pine wood but are no longer found after HTC for 6 h while naphthalenic structures with similar alkylation patterns emerge at the same time. The increase in the levels of volatile aromatics due to char amendment may impact soil ecology. pCymene has been reported to affect arthropod assemblies in soil (Lenardis et al., 2007) and to inhibit phytopathogenic fungi in soil (Sekine et al., 2007) when present in plant material at a lower ppm range. Furthermore, p-cymene inhibited certain fungi symbiotic to pine beetles (Hofstetter et al., 2005) in concentrations typically found in pine wood. Therefore, chars enriched in substances such as p-cymene may affect soil ecology and fungi when added in significant amounts. Fig. 5 depicts the content of total volatile phenols in the HTC products. This group consists basically of 2-methoxyphenol (guaiacol), well-known as major product of the thermal degradation of lignins (Kleinert and Barth, 2008). It was accompanied by phenol, cresols, xylenoles, ethyl phenols, methyl, ethyl and propyl methoxyphenols, and dimethoxyphenol (syringol). Guaiacol was the most abundant phenolic compounds in the hydrochars with the exception of poplar hydrochar where unsubstituted phenol was most abundant. Ratios of the individual phenolic substances and their ratios are given in Supplemental Table S22. It should be noted that the contents of volatile phenols increased significantly between runs at 250 and 270 °C. It is assumed that the thermal breakdown of lignins from biomass is slower than the formation of hydroxymethylfurfural and furfural via hydrolysis and dehydratization of hexoses and pentoses. Thus, the lignin structure is sufficiently retained to provide increasing amounts of guaiacol and other phenols over the investigated process period of 6 h. Phenolic compounds are suspected to contribute to the inhibitory effect of hydrochar on seed germination, and optimization of the HTC products designed for agricultural purposes may therefore have to include monitoring of phenolic compounds by HS-GC. The VOCs may be further classified as oxygen-containing or oxygen-free compounds. All feedstock displayed absolute increases in oxygen-containing and oxygen-free compounds with process temperature. Details are shown in Supplemental Fig. S2. With the exception of pine wood, oxygen-containing compounds dominated over oxygen-free compounds. The ratio of total oxygen-free to oxy compounds tended to decrease in HTC products obtained at higher temperatures and ranged between 2 and 7 in the hydrochars. The oxygen-free compounds consist mainly of BTEX, other benzenic aromates, and cyclohexaenes. The oxygen-containing compounds included, aside from the phenols, mainly oxo-compounds (aldehydes and ketones) and furans. For details on the increasing trends with process temperature, see Supplemental Figs. S3 and S4. The ratios between oxo-compounds, phenols, and furans in the hydrochars obtained at 270 °C ranged between 1:(0.62–1.22): (0.31– 0.78). While the volatile furans in the HTC products obtained at 190 and 230 °C included mainly the HTC intermediates furfural, hydroxymethylfurfural, and 5-methylfurfural, the HTC products obtained above 230 °C displayed increasing amounts of benzofurans. In products treated at 250 °C and above, they are the dominating furans. The most abundant one is 2-methylbenzofuran
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followed by 2-ethylbenzofuran (except in digestate and pine wood) and 4,7-dimethyl-benzofuran. Poplar, digestate and massaranduba displayed also a significant abundance of non-alkylated benzofuran. Details are summarized in Supplemental Table S23. Amounts and structural patterns of benzofurans in hydrochars may be of concern in the connection with their use as soil amendment, as benzofuran and 2-methylbenzofuran have been reported to display eco-toxicological effects in daphnia immobilization tests at concentrations below 10 mg/L (Eisentraeger et al., 2008). The concentration of benzofurans ranged between 200 and 800 lg/g in chars obtained at 270 °C and may thus pose a toxicologically relevant reservoir. The alkylation pattern of benzofurans may be a tool for the assessment of the HTC progress similar to the methylbenzofuran/benzofuran ratio suggested as indicator for thermal alteration of biomass during pyrolysis (Kaal and Rumpel, 2009). The oxo-compounds included C3–C7 aldehydes and ketones, among them methyl butanals. The most abundant volatile ketone in all hydrochars was 2-methyl-2-cyclopenten-1-one followed by 2,3-dimethyl-2-cyclopenten-1-one in case of massaranduba, straw and digestate and by 2-cyclo-penten-1-one in case of poplar and garapa and by cyclopentanone in case pine wood, respectively. A list of the most abundant oxo-compounds and their ratios is given in Supplemental Table S24. Methylated cyclopentenones were reported in biomass conversion products of ferns in sub- and supercritical water above 300 °C (Carrier et al., 2011) and were suggested as marker substances for kraft mill effluents displaying adverse effects on fish reproduction (Martel et al., 2011; Kovacs et al., 2011). Alkylated cyclopentenones have also been identified as major constituents in the volatile fraction of HTC products derived from Chinese privet seeds (Eberhardt et al., 2010). Methylated cyclopentenones were also found in extracts of HTC products from water lettuce obtained under conditions comparable to those employed in the present study (Luo et al., 2011). Nitrogen-containing compounds were not identified in the volatilomes of the HTC products and it is assumed that volatile amino compounds are protonated due to the acid pH of HTC products and therefore no longer volatile. All feedstock displayed minor amounts of thiophene and methylated thiophenes after treatment at 250 or 270 °C.
3.5. Volatile organic compounds and optimization of the carbonization process Headspace GC provided specific information on the amount of key substances for the assessment of the HTC progress such as furfural and hydroxymethylfurfural as well potentially hazardous components. The facile and automated sample pretreatment and the standard GC equipment required for headspace analysis makes this procedure appealing for extended measurement series in routine laboratories. It covers a wide range of compounds from very volatiles such as ethanol to semi volatile such as alkyloxy phenols and alkylated naphthalenes in a single analytical run and allows monitoring the alteration in content and composition of individual components at the same time. The complete volatilome and specific marker substances which cause concern about their possible release to soil, water, or ambient air are suitable parameters for the assessment of chars. Thus, the effect of a systematic variation of relevant process parameters such as process duration and temperature composition of bulk feedstock and auxiliary additives on VOC content and pattern is an additional tool to arrive at a sound compromise between char yield, beneficial elemental composition (e.g. sufficiently low H/C and O/C ratios) and application-oriented emission characteristics. Further research on the environmental impact of mobile hydrochar constituents should include the water soluble fraction. Volatilomes and solubles may be separated from
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their char matrix and submitted to systematic investigations of their impact on soil ecology. 4. Conclusion Headspace GC of solid HTC products obtained from a variety of biomass feedstock between 190 and 270 °C revealed a significant dependency of volatilomes on feedstock and process temperature. The different chars displayed reproducible total VOC amounts and specific patterns of individual compounds from major substance classes such as benzenes, phenols, furans, and ketones. The knowledge of composition and content of potentially detrimental volatile components and elemental composition provides a basis for the optimization of HTC chars for the application as soil amendments or for waste water purification. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech. 2012.12.102. References Berge, N.D., Ro, K.S., Mao, J., Flora, J.R.V., Chappell, M.A., Bae, S., 2011. Hydrothermal carbonization of municipal waste streams. Environmental Science and Technology 45, 5696–5703. Bergius, F., 1928. Beiträge zur Theorie der Kohleentstehung. Die Naturwissenschaften 16, 1–10. Busch, D., Kammann, C., Grünhage, L., Müller, C., 2011. Simple biotoxicity tests for evaluation of carbonaceous soil additives: establishment and reproducibility of four test procedures. Journal of Environmental Quality 40, 1–10. Carrier, M., Loppinet-Serani, A., Absalon, C., Marias, F., Aymonier, C., Mench, M., 2011. Conversion of fern (Pteris vittata L.) biomass from a phytoremediation trial in sub- and supercritical water conditions. Biomass and Bioenergy 35, 872– 883. Chakrabarti, S., Kern, J., Menzel, R., Steinberg, C.E.W., 2011. Selected natural humic materials induce and char substrates repress a gene in Caenorhabditis elegans homolog to human anticancer P53. Annals of Environmental Science 5, 1–6. Eberhardt, T.J., Catallo, W.J., Shupe, T.F., 2010. Hydrothermal transformation of Chinese privet seed biomass to gas-phase and semi-volatile products. Bioresource Technology 101, 4198–4204. Eisentraeger, A., Brinkmann, C., Hollert, H., Sagner, A., Tiehm, A., Neuwoehner, J., 2008. Heterocyclic compounds: Toxic effects using algae, daphnids, and the salmonella/microsome test taking methodical quantitative aspects into account. Environmental Toxicology and Chemistry 27, 1590–1596. Funke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels Bioproducts and Biorefining 4, 160–177. Gell, K., van Groenigen, J.W., Cayuela, M.L., 2011. Residues of bioenergy production chains as soil amendments: immediate and temporal phytotoxicity. Journal of Hazardous Materials 186, 2017–2025. Hofstetter, R.W., Mahfouz, J.B., Klepzig, K.D., Ayres, M.P., 2005. Effects of tree phytochemistry on the interactions among endophloedic fungi associated with the southern pine beetle. Journal of Chemical Ecology 31, 539–560. Insam, H., Seewald, M.S.A., 2010. Volatile organic compounds (VOCs) in soils. Biology and Fertility of Soils 46, 199–213. Kaal, J., Rumpel, C., 2009. Can pyrolysis-GC/MS be used to estimate the degree of thermal alteration of black carbon? Organic Geochemistry 40, 1179–1187. Kleinert, M., Barth, T., 2008. Phenols from lignin. Chemical Engineering and Technology 31, 736–745. Koelmans, A.A., Jonker, M.T.O., Cornelissen, G., Bucheli, T.D., Van Noort, P.C.M., Gustafsson, O., 2006. Black carbon: the reverse of its dark side. Chemosphere 63, 365–377. Kovacs, T.G., O’Connor, B.I., Parrott, J.L., McMaster, M.E., Van Der Kraak, G.J., MacLatchy, D.L., van den Heuvel, M.R., Hewitt, M.L., 2011. Kraft mill effluent survey: progress toward best management practices for reducing effects on fish reproduction. Environmental Toxicology and Chemistry 30, 1421–1429. Kumar, A., Alaimo, C.P., Horowitz, R., Mitloehner, F.M., Kleeman, M.J., Green, P.G., 2011. Volatile organic compound emissions from green waste composting: characterization and ozone formation. Atmospheric Environment 45, 1841– 1848. Lenardis, A.E., van Baren, C., Di Leo Lira, P., Ghersa, C.M., 2007. Plant–soil interactions in wheat and coriander crops driving arthropod assemblies through volatile compounds. European Journal of Agronomy 26, 410–417. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.D., Neubauer, Y., Titirici, M.M., Fühner, C., Bens, O., Kern, J., Emmerich, K.H., 2011. Hydrothermal carbonization
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of biomass residuals: a comparative review of the chemistry, processes and applications of wet and dry pyrolysis. Biofuels 2, 89–124. Liu, Z., Zhang, F.-S., Wu, J., 2010. Characterization and application of chars produced from pinewood pyrolysis and hydrothermal treatment. Fuel 89, 510–514. Luo, K.G., Shi, W., Chen, X., Ni, W., Strong, P.J., Jia, Y., Wang, H., 2011. Hydrothermal conversion of water lettuce biomass at 473 or 523 K. Biomass and Bioenergy 35, 4855–4861. Maffei, M., 2011. The plant volatilome, Chapter 13, Essay 13.7. In: Taiz, L., Zeiger, E. (Eds.), Plant Physiology Online, fifth edition. http://5e.plantphys.net/index.php (accessed: 5 March 2012). Martel, P.H., Kovacs, T.G., O’Connor, B.I., Semeniuk, S., Hewitt, L.M., MacLatchy, D.L., McMaster, M.E., Parrott, J.L., van den Heuvel, M.R., Van Der Kraak, G.J., 2011. Effluent monitoring at a bleached kraft mill: directions for best management practices for eliminating effects on fish reproduction. Journal of Environmental Science and Health, Part A 46, 833–843. Mumme, J., Linke, B., Tolle, R., 2010. Novel upflow anaerobic solid-state (UASS) reactor. Bioresource Technology 101, 592–599. Mumme, J., Eckervogt, L., Pielert, J., Diakité, M., Rupp, F., Kern, J., 2011. Hydrothermal carbonization of anaerobically digested maize silage. Bioresource Technology 102, 9255–9260. Rillig, M.C., Wagner, M., Salem, M., Antunes, P.M., George, C., Ramke, H.-G., Titirici, M.M., Antonietti, M., 2010. Material derived from hydrothermal carbonization: effects on plant growth and arbuscular mycorrhiza. Applied Soil Ecology 45, 238–242. Schimmelpfennig, S., Glaser, B., 2012. One step forward toward characterization: some important material properties to distinguish biochars. Journal of Environmental Quality 41, 1–13.
Scholz, V., Kern, J., Kaulfuss, P., 2010. Environmental effects of energy crop cultivation – results of a long-term field trial. Agronomy Research 8, 445–452. Sekine, T., Sugano, M., Majid, A., Fujii, Y., 2007. Antifungal effects of volatile compounds from Black Zira (Bunium persicum) and other spices and herbs. Journal of Chemical Ecology 33, 2123–2132. Sevilla, M., Maciá-Agulló, J.A., Fuertes, A.B., 2011. Hydrothermal carbonization of biomass as a route for the sequestration of CO2: chemical and structural properties of the carbonized products. Biomass and Bioenergy 35, 3152–3159. Spokas, K.A., Novak, J.M., Stewart, C.E., Cantrell, K.B., Uchimiya, M., DuSaire, M.G., Ro, K.S., 2011. Qualitative analysis of volatile organic compounds on biochar. Chemosphere 85, 869–882. Stavova, J., Stahl, D.C., Seames, W.S., Kubatova, A., 2012. Method development for the characterization of biofuel intermediate products using gas chromatography with simultaneous mass spectrometric and flame ionization detections. Journal of Chromatography A 1224, 79–88. Steinbeiss, S., Gleixner, G., Antonietti, M., 2009. Effect of biochar amendment on soil carbon balance and soil microbial activity. Soil Biology and Biochemistry 41, 1301–1310. Titirici, M.M., Thomas, A., Antonietti, M., 2007. Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO2 problem? New Journal of Chemistry 31, 787–789. Titirici, M.M., Antonietti, M., Baccile, N., 2008. Hydrothermal carbon from biomass: a comparison of the local structure from poly- to monosaccharides and pentoses/hexoses. Green Chemistry 10, 1204–1212. Tsukashima, H., 1966. The infrared spectra of artificial coal made from submerged wood at Uozu, Toyama prefecture, Japan. Bulletin of the Chemical Society of Japan 39, 460–465.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Hydrothermally carbonized plant materials: Patterns of volatile organic compounds detected by gas chromatography Roland Becker a,⇑, Ute Dorgerloh a, Mario Helmis b, Jan Mumme c, Mamadou Diakité c, Irene Nehls a a
Federal Institute for Materials Research and Testing (BAM), Richard-Willststätter-Strasse 11, 12489 Berlin, Germany Beuth Hochschule für Technik Berlin, Luxemburger Straße 10, 13353 Berlin, Germany c Leibniz Institute for Agricultural Engineering Potsdam-Bornim, Max-Eyth-Allee 100, 14469 Potsdam, Germany b
h i g h l i g h t s " Digestate, straw and various wood feedstock are carbonized between 190 and 270 °C. " Volatile organic compounds are determined using head space gas chromatography. " Relative amount and composition of volatiles from different feedstock are compared. " Benzenes, phenols, furans, ketones and aldehydes increase with process temperature. " Monitoring of VOC is an option to optimize hydrochars for environmental application.
a r t i c l e
i n f o
Article history: Received 28 June 2012 Received in revised form 11 December 2012 Accepted 14 December 2012 Available online 22 December 2012 Keywords: Hydrochar Mass spectrometry Flame ionization detection Phenols Benzenes
a b s t r a c t The nature and concentrations of volatile organic compounds (VOCs) in chars generated by hydrothermal carbonization (HTC) is of concern considering their application as soil amendment. Therefore, the presence of VOCs in solid HTC products obtained from wheat straw, biogas digestate and four woody materials was investigated using headspace gas chromatography. A variety of potentially harmful benzenic, phenolic and furanic volatiles along with various aldehydes and ketones were identified in feedstock- and temperature-specific patterns. The total amount of VOCs observed after equilibration between headspace and char samples produced at 270 °C ranged between 2000 and 16,000 lg/g (0.2–1.6 wt.%). Depending on feedstock 50–9000 lg/g of benzenes and 300–1800 lg/g of phenols were observed. Substances potentially harmful to soil ecology such as benzofurans (200–800 lg/g) and p-cymene (up to 6000 lg/g in pine wood char) exhibited concentrations that suggest restrained application of fresh hydrochar as soil amendment or for water purification. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Solid carbonaceous products termed hydrochars have been suggested for various applications such as soil amendment and carbon sequestration (Titirici et al., 2007; Sevilla et al., 2011). They are obtained from biomass by means of hydrothermal carbonization (HTC), typically at 180–250 °C, 2–10 MPa and in the presence of liquid water. In comparison with chars obtained from pyrolysis, hydrochars show a lower carbon content, less aromatic structure, and less biological stability, but HTC allows a higher char yield as well as a higher overall energy efficiency for wet feedstock (Libra et al., 2011). The option to use wet organic waste such as biogas digestate (Mumme et al., 2011) makes HTC specifically interesting compared to pyrolysis, which requires dry feedstocks. The use of ⇑ Corresponding author. Tel.: +49 30 8104 1121; fax: +49 30 8104 1127. E-mail address: [email protected] (R. Becker). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.12.102
hydrochars for soil amendment promises various benefits such as improved water holding capacity and long-term storage of carbon branched off the biomass cycle that may lead to an overall negative greenhouse gas balance. Furthermore, chars are chemically related to black carbon components in soil, which have been discussed as a sink for persistent pollutants (Koelmanns et al., 2006). However, the experimental application of hydrochars from various sources to soils in laboratory and field trials revealed various proven or potentially detrimental effects on seed germination (Busch et al., 2011), microbial activity (Steinbeiss et al., 2009), and mycorrhiza growth (Rillig et al., 2010), and gene repression in soil nematodes (Chakrabarti et al., 2011). Phytotoxic behavior is not an exclusive effect of hydrochars but has also been reported for chars obtained from the pyrolysis of bioenergy residues such as digestates (Gell et al., 2011). The adverse effects of fresh hydrochars may partly be attributable to volatile organic compounds (VOCs) (Busch et al., 2011). As a consequence, the VOC content is
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‘‘Japan Clone NM 105’’) was received as dry chips with a diameter of 4–5 cm from the short rotational plantation run by ATB (Scholz et al., 2010). Garapa (Apuleia leicarpa), massaranduba (Manikara bedento), and pine (Pinus sylvestris) wood were obtained from a local timber retailer and cut into chips of about 1 cm in length. All feedstocks were dried at 60 °C for 48 h and stored airtight at room temperature until being milled to a particle size below 4 mm with a Retsch SM100 cutting mill (Retsch, Haan, Germany). Elemental analyses of feedstock materials and HTC chars were carried out with a Vario EL III elemental analyzer (Elementar Analysen Systeme GmbH, Hanau, Germany) using an intake of 10 mg. For reference purposes, several other types of carbonized materials were included in this study. A commercial pyrolysis char produced from a mixture of 65% wood and 35% compost at 700–850 °C was obtained from Pyreg GmbH (Dörth, Germany). A pyrolysis char from wheat straw treated at 275 °C and two pyrolysis chars from olive pomace (OP) treated at 275 and 400 °C were produced at ATB using a rotary kiln. A commercial hydrochar made from maize silage was received from AVA-CO2 Forschung GmbH, Karlsruhe, Germany. A further reference material was a lignite sample from the German Lausitz region designated for domestic heat generation.
considered an important property of chars, especially if they are designated for soil use (Schimmelpfennig et al., 2012). Furthermore, the olfactory characteristics of hydrochars and literature reports on the presence of a variety of volatiles in various pyrochars and hydrochars (Spokas et al., 2011) suggest attention to the relevance of VOCs for work-place safety in case of large scale hydrochar production. Consequently, the aim of the present study was to investigate the composition and significance of VOC substances in hydrochars. Therefore, a number of different types of biomass were hydrothermally carbonized under identical conditions and the solid products were screened with headspace gas chromatography. The objective was to compare the relative content and composition of VOC in the HTC products of the different feedstocks with increasing degree of carbonization. 2. Methods 2.1. Feedstocks for the hydrothermal carbonization and reference materials Wheat straw was received as 4–5 cm long pieces from the Dittmannsdorfer Milch GmbH, Kitzscher, Germany. Digestate was obtained from an anaerobic test reactor operated at 55 °C at the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB) (Mumme et al., 2010), which was fed with the wheat straw as sole substrate. Poplar wood (Populus nigra x P. maximowiczii
2.2. Hydrothermal carbonization Between 3 and 5 g of the respective feedstock, in each case corresponding to 2 g of carbon, were mixed with 75 mL of water
2,000 2.000
feedstocks
H/C (mol%)
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1,500 1.500
230 °C
Wheat straw
270 °C
Digestate
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Biomass
Pine wood
Turf Hard coal
0,500 0.500
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Lignite
Anthracite 0.500 0,000
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feedstocks
H/C (mol%)
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Masaranduba Hydrochar Garapa Hydrochar
Lignite 1,000 1.000
AVA Hydrochar Bergius Cellulose Hydrochar Bergius Lignin Hydrochar
0,500 0.500
270 °C
Pyrochar wheat straw 275
250 °C
Pyrochar OP 275 Pyrochar OP 400 Pyreg pyrochar
0.500 0,000
0,000 0.500
0,100 0.100
0,200 0.200
0,300 0.300
0,400 0.400
0,500 0.500
0,600 0.600
0,700 0.700
O/C (mol%) Fig. 1. Van-Krevelen diagram illustrating the development of carbon ratios of the products of hydrothermal carbonization with process temperature.
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1 2 3 4 5 6 7 8 9 10 11 12 13
2-methylpropanal 2-butanone 1-methylcycylopentene benzene 3-methylbutanal 2-methylbutanal 2-pentanone 3-pentanone toluene cyclopentanone ethylbenzene 2-methylcyclopentanone furfural
14 15 16 17 18 19 20 21 22 23 24 25
1,3-dimethylbenzene 2-methylcyclopenten-1-one benzofuran 2,3-dimethylcyclopen-2-en-1-one phenol 2-methoxyphenol 2-methylbenzofuran 4-ethylphenol 4,7-dimethylbenzofuran 2-methoxy-4-ethylphenol 2,6-dimethoxyphenol 2-methoxy-4-ethylphenol
190 °C
19 18
56
1 6.00
13
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21 22 23
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15 20 21
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Fig. 2. Chromatograms of total volatiles in selected carbonization products of digestate (headspace gas chromatography/mass selective detection).
to obtain a carbon content of 26.7 g/L. Each mixture was homogenized and submitted to the HTC experiments carried out in 125-mL unstirred Parr pressure vessels 4748 (Parr Instrument Company, Moline, IL, USA) featuring removable polytetrafluoroethylene liners. The vessels were placed in a Nabertherm B 180 oven (Nabertherm GmbH, Lilienthal, Germany) using a heating rate of 5 K/min until the desired set-point (190, 230, 250, and 270 °C) was reached. The process temperatures were maintained for 6 h,
the reactors were removed from the oven, and cooled by ambient air. After 2.5 h, the reactor slurry was filtered through fluted cellulose filter paper, type RotilaboÒ 113P (Carl Roth GmbH, Karlsruhe, Germany) and the solids were dried for 48 h at 60 °C. Samples of 100 mg were filled into 10-mL headspace vials and stored at 20 °C until GC analysis. In order to obtain a sufficient amount of char and to assess the reproducibility of the HTC process, each experiment was run at least in triplicate.
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2000
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Fig. 3. Total volatile compounds in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
Fig. 5. Content of total volatile phenols in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
2.3. Headspace gas chromatography of volatile compounds
compounds. The quantification of volatiles was achieved by means of an Agilent 6890 GC equipped with a CombiPal autosampler (CTC Analytics, Zwingen, Switzerland) and coupled to a flame ionization detector (FID) operated at 300 °C. Headspace conditions, injection mode, column, and oven program were identical to those described for GC/MS. For quantification, 200 ng of d8-toluene was added as internal standard to each vial as a solution in 10 lL methanol directly before analysis. d8-Toluene (2000 ng/lL in methanol) from UltraScientific (Kingstown, RI, USA).
Volatile compounds were identified using an Agilent HP 6890 gas chromatograph (Agilent, Waldbronn, Germany) equipped with an MPS 2 XL autosampler (Gerstel, Mühlheim, Germany) enabling fully automated headspace analysis and coupled to an Agilent HP 5973N quadrupole mass selective detector (EI, 70 eV). Sample vials were kept at 100 °C for 30 min and the syringe temperature was 110 °C. A split injection mode was used (split ratio 5:1) and the injection volume was 1 mL. A 60 m 0.32 mm VF624MS capillary column (Varian, Darmstadt, Germany) coated with a 1.8 lm thick film of 6% cyanopropylphenyl and 94% dimethylpolysiloxane was used with helium as carrier gas at a constant flow rate of 5 mL/ min. The oven temperature started at 40 °C (held for 15 min) and was raised to 140 °C (10 °C/min) and then to 220 °C (35 °C/min) and held for 15 min. The injector, ion source, transfer line, and MS quadrupole temperatures were maintained at 250 °C, 230 °C, 300 °C, and 150 °C, respectively. Data were acquired in the full scan mode between m/z 10 and 400. Chromatographic peaks were identified by means of the NIST mass spectral data library (version 2.0d) and in some cases from their retention times using standard
1400 1200
2.4. Reproducibility: measures and statistics Measures were taken to ensure that process conditions, including heating and cooling periods, were identical over all HTC runs. The HTC products were processed in an identical manner to avoid analytical bias. The variability of the HS-GC/MS and HS-GC/FID procedures was investigated and seen to be negligible compared to the variability of the HTC runs. At least three experiments were run for each temperature/feedstock combination and error bars in Figs. 3–5 represent the standard deviations of the means of three replicate HTC runs.
12000
a
b 10000
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Fig. 4. Summary content of benzene, toluene, ethylbenzene, xylene (a) and total volatile benzenes (b) in the carbonization products (headspace gas chromatography/flame ionization detection). Means and standard deviations (n = 3).
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Table 1 Characterization of feedstock and hydrothermal carbonization products. Hydrochar and carbon yields (means and standard deviations of the means, n = 3)a are given as percentages of the original biomass. Before carbonization
After carbonization (6 h)
Biomass
Digestate Wheat straw Pine wood Poplar wood Masanduba wood Garapa wood a
190 °C
230 °C
250 °C
270 °C
Cellulose (mass%)
Hemicellulose (mass%)
Lignin (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
Product yield (mass%)
Carbon yield (mass%)
44.6 45.8
18.9 27.8
21.3 8.46
70.0 ± 0.60 60.4 ± 0.24
77.6 ± 0.67 70.2 ± 0.27
48.6 ± 0.30 41.0 ± 1.04
67.7 ± 0.41 58.3 ± 1.48
41.1 ± 1.79 38.1 ± 1.77
59.5 ± 1.80 59.9 ± 2.79
32.7 ± 1.83 33.1 ± 1.15
48.8 ± 2.73 54.1 ± 1.88
44.6 60.1
12.5 12.2
26.0 19.0
64.2 ± 0.18 64.2 ± 0.37
73.7 ± 0.21 68.1 ± 0.40
45.3 ± 0.05 58.3 ± 0.58
63.2 ± 0.20 66.4 ± 0.67
43.2 ± 0.89 47.7 ± 1.28
64.8 ± 0.77 66.3 ± 1.64
38.5 ± 2.05 39.1 ± 1.14
59.2 ± 3.15 58.6 ± 1.71
48.7
10.6
30.3
68.9 ± 0.45
70.6 ± 0.46
60.7 ± 0.76
68.7 ± 0.46
43.9 ± 2.50
62.3 ± 3.54
45.6 ± 0.37
63.3 ± 0.52
23.6
66.7 ± 0.13
69.8 ± 0.14
52.9 ± 4.12
63.7 ± 4.96
44.7 ± 2.75
58.7 ± 3.62
42.4 ± 1.10
59.6 ± 1.55
55.0
8.24
See Section 2.4.
3. Results and discussion 3.1. Selection of feedstocks, process conditions and headspace parameters In order to obtain an overview of the nature and amount of volatile substances in hydrochars with a number of different biomass, feedstock was selected to cover a range with potential practical relevance. Thus, wheat straw and a digestate derived thereof where selected as examples for major biomass waste flows. In addition, Scots pine wood and poplar wood were chosen to represent potential European wood sources along with the two tropical hardwood species massanranduba and garapa widely used as timber in Europe. The process period was set to 6 h because earlier experiments (Mumme et al., 2011) and literature reports (Bergius, 1928; Liu et al., 2010; Berge et al., 2011) suggested a completion of carbonization during this time at temperatures above 250 °C. The process temperatures ranged from 190 to 270 °C in order to monitor the development of volatiles with emphasis on compounds that display a significant vapor pressure at ambient temperatures. Headspace GC was employed since it provides a comprehensive screening of solid HTC products from highly volatile propanal to semi-volatiles such as phenols and naphthalenes with retention times ranging from 5.9 to 36 min. The adjusted headspace parameters covered the range of these compounds completely as no further volatiles could be detected in test runs of up to 42 min. It should be noted that an equilibrium is achieved between headspace and chars sample. Therefore, the determination of the VOCs in the headspace provides an excellent representation of the relative amount of volatiles between char samples but tends to underestimate the total VOC amount in the chars. Mass selective detection was chosen to identify individual substances and the combination of identical chromatographic conditions with FID was chosen for their quantification. d8-Toluene was used as internal standard for GC/FID since it was baseline separated from toluene and occupied a time window free of any compounds in all chromatograms. A similar combination of MS and FID has recently been adopted for the quantification of volatiles in biofuels (Stavova et al., 2012) and qualitative characterization of volatiles in biochars (Spokas et al. 2011). The collection of compounds detectable by this procedure is designated as hydrochar ‘‘volatilome’’ in analogy to what has been suggested for soil (Insam and Seewald, 2010) and plant materials (Maffei, 2011).
out with its relative low lignin content; however, after anaerobic digestion, the organic composition was similar to that of woody materials due to the selective degradation of cellulose and hemicellulose. At 190 °C, the yield of solid products was 60–70% of the initial mass; whereas at 270 °C, only 32–45% remained. The portion of the initial carbon recovered in solid form decreased from 68–78% (190 °C) to 48–64% (270 °C). In some cases, product and carbon yields decreased significantly between 250 and 270 °C. These results fall in the range observed in the literature (Libra et al., 2011). Maize silage yielded 40–43% carbon in the chars after 2–10 h at 270 °C (Mumme et al., 2011) while food (57%) and paper (67%) retained more of the original carbon after 20 h at 250 °C (Berge et al., 2011). Fig. 1 depicts the extent of carbonization of the employed feedstock as molar ratios of carbon to hydrogen and oxygen. The increase in the process temperature from 190 to 270 °C resulted in increasing carbonization. For reference purposes, a number of biochars and a lignite are also shown against the background of carbon ratios typical for the diagenesis of biomass towards hard coal and anthracite. Carbonization at 270 °C produced H/C ratios of 0.86–1.1 and O/C ratios of 0.12–0.22 typically associated with hydrochars (Mumme et al., 2011; Bergius, 1928; Berge et al., 2011; Liu et al., 2010) and even superior to those of the commercial product AVA hydrochar. With the exception of poplar wood, the composition of the chars after 6 h at 250 °C was largely similar to those obtained at 270 °C. The atomic ratios of paper and food hydrochars (H/C: 1.0; O/C: 0.18), (Berge et al., 2011)) were close to those observed in the current work while the maize hydrochars displayed H/C ratios of 1.13–1.16 and comparably low O/C ratios between 0.09 and 0.12. As anticipated (Libra et al., 2011), HTC products tend to display significant lower degrees of carbonization compared to pyrochars as also reported for pine wood feedstock (Liu et al., 2010). However, prolonged HTC runs yielded products with degrees of carbonization close to those of pyrochars. Bergius (1928) obtained chars from cellulose and lignin after HTC for 20 h at 340 °C, which displayed H/C-ratios of 0.8 and O/C ratio of 0.1. Similar results (H/C: 0.70); O/C: 0.11) were reported for wood submitted to an HTC-like treatment at 280 °C for 72 h (Tsukashima, 1966). Seen against this background, the hydrochars investigated in this work cover a range of practical relevance regarding feedstock, process conditions, and degree of carbonization. 3.3. Volatile organic compounds and carbonization progress
3.2. Characterization of the HTC chars The results of the characterization of the feedstocks and the respective chars are presented in Table 1. Fresh wheat straw stands
Fig. 2 depicts chromatograms obtained from selected HTC products of digestate. Among the major components were a wide range of aromatic, phenolic, and oxo-compounds. Comprehensive lists
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with all compounds identified in all HTC products and at all process temperatures are presented as Supplementary data (Tables S1–S19). Fig. 2 shows that furfural dominated the chromatograms after 6 h at lower process temperatures. Furfural is the main intermediate derived from pentoses from the hemicelluloses of the substrates during the HTC process (Titirici et al., 2008; Funke et al., 2010; Libra et al., 2011). The presence of furfural indicates that the process was not completed at this stage corresponding to the carbonization progress as seen in Fig. 1. In contrast, after treatment at 250 and 270 °C, furfural was no longer observed in the carbonization products. Massaranduba wood was an exception since its HTC product obtained at 270 °C still contained significant amounts of furfural and other HTC intermediates indicative of incomplete transformation of cellulose such as 5-methyl-furancarboxaldehyd and hydroxymethylfurfural. These findings suggest that HS-GC could be used to assess the progress of HTC of a specific feedstock. The increase in volatile compounds in the HTC products with process temperature is obvious from Fig. 2 and comprised for all HTC runs in Fig. 3. It should be noted that most feedstock displays a significant increase in total VOCs between 250 and 270 °C. Not surprisingly, wheat straw and its digestate displayed largely similar results. Massaranduba wood stands out in that it solely displayed no significant increase in total volatiles after treatment at different process temperatures. Pine wood displayed the greatest amount of total VOCs among the HTC products, but its substantial amounts of volatile mono- and sesquiterpenes disappeared during HTC. Headspace GC/MS chromatograms of the pine wood feedstock and selected HTC products are displayed in Supplemental Fig. S1. The total amount of VOCs observed after equilibration between headspace and char sample obtained at 270 °C ranged between 2000 and 16,000 lg volatiles per gram of char. Though this amounts to just 0.2–1.6% of the solid product and does not make the VOCs a major factor in the overall mass balance of HTC, it means that volatiles in the lower g/kg range are contained in the chars which may lead to adverse effects in soil or the workplace. 3.4. Major volatilome components of the carbonization products Fig. 4a displays the summation of benzene, toluene, xylenes, and ethyl benzene (BTEX) in the HTC products. All BTEX components were observed in all HTC products obtained at 250 °C and above. Toluene was present in pine wood already before HTC and dominates in the HTC products, followed by xylenes, ethylbenzene, and benzene. The toluene/benzene ratio was 3:1 in the case of garapa and 12:1 in case of pine wood hydrochar. Details are given in Supplemental Table S20. Though benzene and toluene are also found in minor amounts in volatilomes of composts (Kumar et al., 2011), concentrations of BTEX in the ppm range are hazardous to water and soil life as well as human health. Fig. 4b depicts the sum of all detected benzenic hydrocarbons. Besides BTEX, these are C10–C15 benzenes displaying methyl, ethyl and isopropyl substituents. The most abundant compounds were isopropylbenzene and isopropyltoluene (cymene). In case of the poplar and digestate hydrochars, the BTEX amounted to about half of the total benzenes and were accompanied by significant amounts of isopropylbenzene while cymene was nearly absent. In the case of straw hydrochar, cymene and BTEX each amounted to a third of the total benzenes. Garapa and massanranduba hydrochars displayed p-cymene as most abundant benzenic compound. Pine wood is unique in that p-cymene amounted to 80% of the total benzenic compounds in the HTC product obtained at 270 °C. Surprisingly, an isomer of p-cymene was found in small amounts before HTC and the amount of this isomer increased until 250 °C and was replaced with p-cymene in the 270 °C runs, which was itself formed at temperatures of 190 °C or above. The ratios of BTEX and cymene in the different volatilomes are collected in Sup-
plemental Table S21. Cymenes display a monoterpenoid structure but seem to be formed mainly during HTC. A source might be bicyclic and tricyclic terpenes such as pinene, carene, and camphene in the original pine wood. p-Cymene appears to be the end point of their stepwise aromatization. Likely intermediates are a number of non-aromatic monoterpenic compounds, namely methylisopropylcyclohexene and methylisopropylcyclohexdiene isomers. Since these compounds seem to be formed mostly during HTC but do not accumulate with process temperature, they are transformed further, most probably into cymene. This concept of aromatization during HTC is outlined in the supplemental data for monoterpenes and sesquiterpenes such as cadinene isomers. The latter are observed in the original pine wood but are no longer found after HTC for 6 h while naphthalenic structures with similar alkylation patterns emerge at the same time. The increase in the levels of volatile aromatics due to char amendment may impact soil ecology. pCymene has been reported to affect arthropod assemblies in soil (Lenardis et al., 2007) and to inhibit phytopathogenic fungi in soil (Sekine et al., 2007) when present in plant material at a lower ppm range. Furthermore, p-cymene inhibited certain fungi symbiotic to pine beetles (Hofstetter et al., 2005) in concentrations typically found in pine wood. Therefore, chars enriched in substances such as p-cymene may affect soil ecology and fungi when added in significant amounts. Fig. 5 depicts the content of total volatile phenols in the HTC products. This group consists basically of 2-methoxyphenol (guaiacol), well-known as major product of the thermal degradation of lignins (Kleinert and Barth, 2008). It was accompanied by phenol, cresols, xylenoles, ethyl phenols, methyl, ethyl and propyl methoxyphenols, and dimethoxyphenol (syringol). Guaiacol was the most abundant phenolic compounds in the hydrochars with the exception of poplar hydrochar where unsubstituted phenol was most abundant. Ratios of the individual phenolic substances and their ratios are given in Supplemental Table S22. It should be noted that the contents of volatile phenols increased significantly between runs at 250 and 270 °C. It is assumed that the thermal breakdown of lignins from biomass is slower than the formation of hydroxymethylfurfural and furfural via hydrolysis and dehydratization of hexoses and pentoses. Thus, the lignin structure is sufficiently retained to provide increasing amounts of guaiacol and other phenols over the investigated process period of 6 h. Phenolic compounds are suspected to contribute to the inhibitory effect of hydrochar on seed germination, and optimization of the HTC products designed for agricultural purposes may therefore have to include monitoring of phenolic compounds by HS-GC. The VOCs may be further classified as oxygen-containing or oxygen-free compounds. All feedstock displayed absolute increases in oxygen-containing and oxygen-free compounds with process temperature. Details are shown in Supplemental Fig. S2. With the exception of pine wood, oxygen-containing compounds dominated over oxygen-free compounds. The ratio of total oxygen-free to oxy compounds tended to decrease in HTC products obtained at higher temperatures and ranged between 2 and 7 in the hydrochars. The oxygen-free compounds consist mainly of BTEX, other benzenic aromates, and cyclohexaenes. The oxygen-containing compounds included, aside from the phenols, mainly oxo-compounds (aldehydes and ketones) and furans. For details on the increasing trends with process temperature, see Supplemental Figs. S3 and S4. The ratios between oxo-compounds, phenols, and furans in the hydrochars obtained at 270 °C ranged between 1:(0.62–1.22): (0.31– 0.78). While the volatile furans in the HTC products obtained at 190 and 230 °C included mainly the HTC intermediates furfural, hydroxymethylfurfural, and 5-methylfurfural, the HTC products obtained above 230 °C displayed increasing amounts of benzofurans. In products treated at 250 °C and above, they are the dominating furans. The most abundant one is 2-methylbenzofuran
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followed by 2-ethylbenzofuran (except in digestate and pine wood) and 4,7-dimethyl-benzofuran. Poplar, digestate and massaranduba displayed also a significant abundance of non-alkylated benzofuran. Details are summarized in Supplemental Table S23. Amounts and structural patterns of benzofurans in hydrochars may be of concern in the connection with their use as soil amendment, as benzofuran and 2-methylbenzofuran have been reported to display eco-toxicological effects in daphnia immobilization tests at concentrations below 10 mg/L (Eisentraeger et al., 2008). The concentration of benzofurans ranged between 200 and 800 lg/g in chars obtained at 270 °C and may thus pose a toxicologically relevant reservoir. The alkylation pattern of benzofurans may be a tool for the assessment of the HTC progress similar to the methylbenzofuran/benzofuran ratio suggested as indicator for thermal alteration of biomass during pyrolysis (Kaal and Rumpel, 2009). The oxo-compounds included C3–C7 aldehydes and ketones, among them methyl butanals. The most abundant volatile ketone in all hydrochars was 2-methyl-2-cyclopenten-1-one followed by 2,3-dimethyl-2-cyclopenten-1-one in case of massaranduba, straw and digestate and by 2-cyclo-penten-1-one in case of poplar and garapa and by cyclopentanone in case pine wood, respectively. A list of the most abundant oxo-compounds and their ratios is given in Supplemental Table S24. Methylated cyclopentenones were reported in biomass conversion products of ferns in sub- and supercritical water above 300 °C (Carrier et al., 2011) and were suggested as marker substances for kraft mill effluents displaying adverse effects on fish reproduction (Martel et al., 2011; Kovacs et al., 2011). Alkylated cyclopentenones have also been identified as major constituents in the volatile fraction of HTC products derived from Chinese privet seeds (Eberhardt et al., 2010). Methylated cyclopentenones were also found in extracts of HTC products from water lettuce obtained under conditions comparable to those employed in the present study (Luo et al., 2011). Nitrogen-containing compounds were not identified in the volatilomes of the HTC products and it is assumed that volatile amino compounds are protonated due to the acid pH of HTC products and therefore no longer volatile. All feedstock displayed minor amounts of thiophene and methylated thiophenes after treatment at 250 or 270 °C.
3.5. Volatile organic compounds and optimization of the carbonization process Headspace GC provided specific information on the amount of key substances for the assessment of the HTC progress such as furfural and hydroxymethylfurfural as well potentially hazardous components. The facile and automated sample pretreatment and the standard GC equipment required for headspace analysis makes this procedure appealing for extended measurement series in routine laboratories. It covers a wide range of compounds from very volatiles such as ethanol to semi volatile such as alkyloxy phenols and alkylated naphthalenes in a single analytical run and allows monitoring the alteration in content and composition of individual components at the same time. The complete volatilome and specific marker substances which cause concern about their possible release to soil, water, or ambient air are suitable parameters for the assessment of chars. Thus, the effect of a systematic variation of relevant process parameters such as process duration and temperature composition of bulk feedstock and auxiliary additives on VOC content and pattern is an additional tool to arrive at a sound compromise between char yield, beneficial elemental composition (e.g. sufficiently low H/C and O/C ratios) and application-oriented emission characteristics. Further research on the environmental impact of mobile hydrochar constituents should include the water soluble fraction. Volatilomes and solubles may be separated from
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their char matrix and submitted to systematic investigations of their impact on soil ecology. 4. Conclusion Headspace GC of solid HTC products obtained from a variety of biomass feedstock between 190 and 270 °C revealed a significant dependency of volatilomes on feedstock and process temperature. The different chars displayed reproducible total VOC amounts and specific patterns of individual compounds from major substance classes such as benzenes, phenols, furans, and ketones. The knowledge of composition and content of potentially detrimental volatile components and elemental composition provides a basis for the optimization of HTC chars for the application as soil amendments or for waste water purification. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.biortech. 2012.12.102. References Berge, N.D., Ro, K.S., Mao, J., Flora, J.R.V., Chappell, M.A., Bae, S., 2011. Hydrothermal carbonization of municipal waste streams. Environmental Science and Technology 45, 5696–5703. Bergius, F., 1928. Beiträge zur Theorie der Kohleentstehung. Die Naturwissenschaften 16, 1–10. Busch, D., Kammann, C., Grünhage, L., Müller, C., 2011. Simple biotoxicity tests for evaluation of carbonaceous soil additives: establishment and reproducibility of four test procedures. Journal of Environmental Quality 40, 1–10. Carrier, M., Loppinet-Serani, A., Absalon, C., Marias, F., Aymonier, C., Mench, M., 2011. Conversion of fern (Pteris vittata L.) biomass from a phytoremediation trial in sub- and supercritical water conditions. Biomass and Bioenergy 35, 872– 883. Chakrabarti, S., Kern, J., Menzel, R., Steinberg, C.E.W., 2011. Selected natural humic materials induce and char substrates repress a gene in Caenorhabditis elegans homolog to human anticancer P53. Annals of Environmental Science 5, 1–6. Eberhardt, T.J., Catallo, W.J., Shupe, T.F., 2010. Hydrothermal transformation of Chinese privet seed biomass to gas-phase and semi-volatile products. Bioresource Technology 101, 4198–4204. Eisentraeger, A., Brinkmann, C., Hollert, H., Sagner, A., Tiehm, A., Neuwoehner, J., 2008. Heterocyclic compounds: Toxic effects using algae, daphnids, and the salmonella/microsome test taking methodical quantitative aspects into account. Environmental Toxicology and Chemistry 27, 1590–1596. Funke, A., Ziegler, F., 2010. Hydrothermal carbonization of biomass: a summary and discussion of chemical mechanisms for process engineering. Biofuels Bioproducts and Biorefining 4, 160–177. Gell, K., van Groenigen, J.W., Cayuela, M.L., 2011. Residues of bioenergy production chains as soil amendments: immediate and temporal phytotoxicity. Journal of Hazardous Materials 186, 2017–2025. Hofstetter, R.W., Mahfouz, J.B., Klepzig, K.D., Ayres, M.P., 2005. Effects of tree phytochemistry on the interactions among endophloedic fungi associated with the southern pine beetle. Journal of Chemical Ecology 31, 539–560. Insam, H., Seewald, M.S.A., 2010. Volatile organic compounds (VOCs) in soils. Biology and Fertility of Soils 46, 199–213. Kaal, J., Rumpel, C., 2009. Can pyrolysis-GC/MS be used to estimate the degree of thermal alteration of black carbon? Organic Geochemistry 40, 1179–1187. Kleinert, M., Barth, T., 2008. Phenols from lignin. Chemical Engineering and Technology 31, 736–745. Koelmans, A.A., Jonker, M.T.O., Cornelissen, G., Bucheli, T.D., Van Noort, P.C.M., Gustafsson, O., 2006. Black carbon: the reverse of its dark side. Chemosphere 63, 365–377. Kovacs, T.G., O’Connor, B.I., Parrott, J.L., McMaster, M.E., Van Der Kraak, G.J., MacLatchy, D.L., van den Heuvel, M.R., Hewitt, M.L., 2011. Kraft mill effluent survey: progress toward best management practices for reducing effects on fish reproduction. Environmental Toxicology and Chemistry 30, 1421–1429. Kumar, A., Alaimo, C.P., Horowitz, R., Mitloehner, F.M., Kleeman, M.J., Green, P.G., 2011. Volatile organic compound emissions from green waste composting: characterization and ozone formation. Atmospheric Environment 45, 1841– 1848. Lenardis, A.E., van Baren, C., Di Leo Lira, P., Ghersa, C.M., 2007. Plant–soil interactions in wheat and coriander crops driving arthropod assemblies through volatile compounds. European Journal of Agronomy 26, 410–417. Libra, J.A., Ro, K.S., Kammann, C., Funke, A., Berge, N.D., Neubauer, Y., Titirici, M.M., Fühner, C., Bens, O., Kern, J., Emmerich, K.H., 2011. Hydrothermal carbonization
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