energies
Article
Towards Biochar and Hydrochar
Engineering—Influence of Process Conditions on
Surface Physical and Chemical Properties,
Thermal Stability, Nutrient Availability,
Toxicity and Wettability
Alba Dieguez-Alonso 1,∗ID , Axel Funke 2ID , Andrés Anca-Couce 3ID ,
Alessandro Girolamo Rombolà 4, Gerardo Ojeda 5, Jörg Bachmann 6and Frank Behrendt 1
1Institute of Energy Engineering, Technische Universität Berlin,
Chair for Energy Process Engineering and Conversion Technologies for Renewable Energies, Fasanenstr. 89,
2Institute of Catalysis Research and Technology (IKFT), Karlsruhe Institute of Technology,
3Institute of Thermal Engineering, Graz University of Technology, Inffeldgasse 25b, 8010 Graz, Austria;
4Department of Chemistry “Giacomo Ciamician”, C.I.R.I. Energia Ambiente and C.I.R.S.A.,
Università di Bologna, Ravenna Campus, Via S. Alberto 163, 48123 Ravenna, Italy;
alessandro.r[email protected]
5Ecological and Forestry Applications Research Centre (CREAF), 08193 Cendanyola del Vallès, Spain;
6Institute of Soil Science, Leibniz University of Hannover, Herrenhaeuser Str. 2, 30419 Hannover, Germany;
*Correspondence: [email protected]; Tel.: +49-(30)-314-24381
Received: 31 December 2017; Accepted: 11 February 2018; Published: 27 February 2018
Abstract:
The impact of conversion process parameters in pyrolysis (maximum temperature, inert gas
flow rate) and hydrothermal carbonization (maximum temperature, residence time and post-washing)
on biochar and hydrochar properties is investigated. Pine wood (PW) and corn digestate (CD),
with low and high inorganic species content respectively, are used as feedstock. CD biochars
show lower H/C ratios, thermal recalcitrance and total specific surface area than PW biochars,
but higher mesoporosity. CD and PW biochars present higher naphthalene and phenanthrene
contents, respectively, which may indicate different reaction pathways. High temperatures (
>
500
◦
C)
lead to lower PAH (polycyclic aromatic hydrocarbons) content (
<
12 mg/kg) and higher specific
surface area. With increasing process severity the biochars carbon content is also enhanced, as well
as the thermal stability. High inert gas flow rates increase the microporosity and wettability of
biochars. In hydrochars the high inorganic content favor decarboxylation over dehydration reactions.
Hydrochars show mainly mesoporosity, with a higher pore volume but generally lower specific
surface area than biochars. Biochars present negligible availability of NO
−
3
and NH
+
4
, irrespective of
the nitrogen content of the feedstock. For hydrochars, a potential increase in availability of NO
−
3
, NH
+
4
,
PO
3−
4
, and K
+
with respect to the feedstock is possible. The results from this work can be applied to
“engineer” appropriate biochars with respect to soil demands and certification requirements.
Keywords:
pyrolysis; hydrothermal carbonization; biochar engineering; porosity; nutrients;
polycyclic aromatic hydrocarbon (PAH)
Energies 2018,11, 496; doi:10.3390/en11030496 www.mdpi.com/journal/energies
Energies 2018,11, 496 2 of 26
1. Introduction
The definitions of biochar provided by the European Biochar Certificate (EBC) [
1
] and the IBI
(International Biochar Initiative) Biochar Standards [
2
] address biochar properties requirements for its
application, as well as methodological standards to assess them, but they do not describe in detail the
production process and its influence on biochar properties. As an example, the EBC considers mainly
pyrolysis as the process to produce biochar, including gasification as part of the pyrolysis technology
spectrum. However, the IBI Biochar Standards consider all forms of thermochemical conversion with
limited presence of oxygen as appropriate technologies to produce biochar. Therefore, hydrothermal
carbonization (HTC) could be included as well. In recent years, a lot of research has been done in
the field of biochar to understand the impact of process conditions on biochar properties, as well
as biochar dynamics once applied to soil and, consequently, its potential positive and/or negative
impacts on crops, environment and soil. However, as Abiven et al. [
3
] pointed out, it is necessary to
develop tailor-made biochar systems according to individual applications, taking into account soil
type, climate and social environments, instead of considering biochar as an universal soil enhancer.
Besides, biochar can have other applications, such as its use for soil and water remediation through the
adsorption and immobilization of organic and inorganic contaminants [
4
–
7
]. Within its agronomic use,
and according to a review by Mohan et al. [
8
], biochar can potentially increase nutrient availability
(direct and indirect pathways), water retention, soil organic matter, microbial activity and crop yields.
This may contribute to a reduction in nutrient leaching, fertilizer demand or emissions [9–11].
•
Direct biochar contribution to nutrient availability relies on the macro- and/or micronutrients
that biochar contains and their availability [
5
,
9
,
10
]. This is strongly dependent on the composition
of the initial feedstock [
10
] and on the pyrolysis conditions. High retention of alkali species
and P is reported for pyrolysis temperatures below 600–700
◦
C and low heating rates [
12
–
15
],
with increase in total, soluble and exchangeable base cations with increasing process severity [
13
].
However, Uchimiya et al. [
16
] showed that NaOH-EDTA extractable P increased at 300–350
◦
C
pyrolysis temperature and decreased at higher temperatures for plant and manure biochars.
Hossain et al. [
17
] showed that total N content and N availability decreased with increasing
pyrolysis temperature in wastewater sludge biochars.
•
Indirect contribution to nutrient availability relies on the capacity of biochars to retain applied
nutrients, potentially reducing leaching and increasing fertilizer-use efficiency, on their liming
effect or on their impact on other soil properties, such as the potential increase in water-holding
capacity, among others, as reviewed by Chan and Xu in [
10
]. Biochars nutrient retention capacity
is associated to their surface charge and surface physical properties as stated by Chan and Xu
in [10].
•
Biochar surface charge will impact the biochar capacity to adsorb positively or negatively charged
nutrients. The cation exchange capacity (CEC) is a measure of the biochar capacity to retain
positively charged species, mainly through electrostatic interactions [
7
,
18
]. Budai et al. [
19
]
reported that CEC is influenced by different factors, with no clear dominance. These may
include: the presence of negatively charged oxygen-containing functional groups on the biochar
surface, such as carboxylate (from carboxylic acid) and hydroxyl functional groups [
13
,
18
–
20
],
which are reported to decrease with increasing conversion temperature [
13
,
18
,
19
,
21
,
22
]; the pH
of the solution [
18
]; the content and composition of the inorganic fraction [
23
,
24
]. Values for
biochars CEC and its dependence on process conditions and feedstock vary significantly in
literature [
18
,
19
,
24
]. Biochars anion exchange capacity (AEC) is scarcely reported in literature [
20
],
with studies even showing negligible AEC [
18
]. Little is known about the origin of AEC on
the surface of biochar [
20
]. However, due to the relevance of this topic, especially towards the
reduction in negatively charged nutrients leaching, such as phosphates and nitrates, plenty of
recent research activity focuses on the modification of biochar surfaces to enhance their capacity
or adsorb such species.
Energies 2018,11, 496 3 of 26
•
Biochar specific surface area and pore volume increase with pyrolysis temperature [
19
,
21
,
25
] up to
approximately 600–700
◦
C, when further ordering of the biochar structure may lead to a decrease
in surface area [
19
,
25
]. Mainly micropores contribute to the specific surface area in biochars [
21
].
Other process parameters, such as biomass composition, specially inorganic species content,
heating rate and pressure can have also a significant impact on porosity development [
26
,
27
].
Surface area and pore network may impact biochar adsorption capacity due to the influence on
accessibility [18] and availability of adsorption sites.
•
With respect to water retention, this depends on both the physical and chemical surface properties
of biochar. The nature of surface functional groups impacts the hydrophobic/hydrophilic behavior
or biochars [
20
]. As reported by Schimmelpfenning et al. [
21
], polar functional groups on the
biochar surface may increase the uptake of water due to electrostatic interactions and hydrogen
bonds. Their reduction can lead to higher hydrophobic caracter [
28
]. Hagemann et al. [
29
] reported
that biochar-water interactions enhancement, leading to higher nutrient retention, can be achieved
with higher mesoporosity, redox-active sites and hydrophilicity on the surface of biochars.
•
Biochars pH is of high relevance for its application in soil, since it determines not only its potential
liming effect, but it also has an influence on other properties, such as CEC, water holding capacity
or adsorption capacity for different nutrients or heavy metals [18,30].
Consequently, in order to achieve the capacity of “engineering biochar”, not only further
knowledge of the mechanisms behind biochar interactions in soil (including quantification and
durability of the impact and variability in response) [
9
,
31
] is required, but also a deeper mechanistic
understanding of the influence of conversion process and feedstock on biochar properties and its
function in soil. This is a complex issue due to the highly relevant influence of (1) feedstock, (2) type of
process, and (3) process parameters on biochar characteristics, as briefly introduced.
For pyrolysis, temperature and feedstock properties are the main process parameters under
consideration when investigating the influence of conversion process on biochar properties. However,
other process variables such as inert gas flow rate, heating rate, particle size, atmosphere and
pressure can dramatically change biochar properties as well [
26
,
32
,
33
]. Furthermore, with HTC
the chemical pathways involved differ significantly from those of pyrolysis. The different nature of
these chemical reactions open up the potential to design carbon materials with a variety of interesting
functionalities [34].
Comparison of chars produced from pyrolysis (termed biochars in the following) and chars
produced from HTC (termed hydrochars in the following) has been previously done in literature,
as summarized in Table 1. Huff et al. [
35
] reported higher O/C ratios for hydrochars than for biochars,
as well as a significantly higher CEC, which, however, did not correlate well with the capacity of these
chars to adsorb organic compounds (methylene blue). Budai et al. [
19
] showed that volatile matter
and H/C and O/C ratios can be good parameters to determine the carbonization degree, irrespective
of conversion process and feedstock, but poor predictors for agronomic properties (SSA and CEC).
Liu et al. [
28
] reported that HTC creates oxygen-containing functional groups on the hydrochar surface
with respect to the feedstock, while pyrolysis reduces their presence. Both biochar and hydrochar
develop a rough surface and pore structure and are able to remove metal species from wastewater,
such as copper, although different adsorption mechanisms can be responsible [
28
]. Based on a wide
comparison of biochars, produced through different methods, and hydrochars, quality indicators
for biochar production were suggested, such as H/C ratios below 0.6, O/C ratios below 0.4 and
black carbon content higher than 15% (based on C), to ensure stability in soil [
21
]. Moreover, BET
SSA higher than 100 m
2
g
−1
and PAHs content according to the precautionary values defined by the
German federal soil protection ordinance (“Bundesbodenschutzverordnung”) should be achieved [
21
].
Regarding chemical structure, it was shown that while biochar is mostly aromatic, with small traces of
alkyl carbons, hydrochars contain mostly alkyl moieties [
36
]. This was further confirmed in [
37
], where
it was also reported that hydrochars have higher nutrient retention capacity than biochars. At the
same time, both were within the EBC [
1
] recommended limits for PAHs content [
37
]. It was concluded
Energies 2018,11, 496 4 of 26
that conversion conditions strongly affect the char properties and that, while biochar standards are
described in the EBC [
1
], hydrochars need their own directive [
37
]. In [
36
], it was also shown that the
aromatic cluster size of biochar is larger than the one of hydrochar. Furthermore, acid prewashing
and water and acetone washing can change the chemical structure of hydrochars [
36
]. Genotoxic and
phytotoxic risk assessment showed hydrochars to be phytotoxic, inhibiting germination, although this
effect could be eliminated with biological post-treatment [
38
]. Moreover, hydrochar is less suitable for
long-term C sequestration in comparison to biochar, due to a lower stability, but it can deliver essential
nutrients, having therefore potential for soil amelioration [39].
The objective of the present work is to contribute to the comparison between biochars and
hydrochars by investigating the influence of conversion process conditions and feedstock on relevant
properties. For this purpose, biochars and hydrochars were produced from two biomass feedstocks
with varying mineral content (pine wood and corn digestate). Biochar was produced by pyrolysis
in a fixed bed reactor at two maximum conversion temperatures (400 and 600
◦
C) and two inert gas
flow rates (20 and 40 NL min
−1
of N
2
). Hydrochars were produced by HTC in a batch autoclave
at two conversion temperatures (200 and 240
◦
C), two residence times (10 and 360 min) and with
and without water washing after production (3 and 6 times). Together with primary characterization
properties, such as proximate and elemental analyses, other characteristics, which can contribute to
the understanding of biochar and hydrochar behavior once applied to soil, are analyzed: pH, acidic
surface functional groups, pore size distribution with N
2
and CO
2
adsorption, nutrient availability,
thermal stability, PAHs content and wettability.
Comparing the objectives of the present work and the previously reviewed literature, this study
introduces a more detailed analysis on (1) the influence of conversion process parameters and
(2) the characterization of products properties. With respect to process conditions, temperature,
time and water washing have been chosen for HTC because very little data is available in the above
mentioned publications regarding their combined influence on hydrochars. Moreover, the influence of
inert gas flow rate in pyrolysis, and therefore the retention time of volatiles in the bed, is a novelty in
this comparative study. This work represents a fundamental characterization of char produced under
different and relevant process conditions and, therefore, issues regarding the energetic costs associated
with inert gas flow rates, process temperatures or residence times cannot be evaluated based on these
laboratory scale results. Furthermore, beyond the investigated product properties in the reviewed
literature, a deep analysis is performed for the first time on the porous structure of both biochars
and hydrochars combining N
2
and CO
2
adsorption, as well as research and comparison on nutrient
availability and wettability. Finally, the results are analyzed from a conversion process perspective,
in order to achieve an advance in the state-of-the-art of the “engineering biochar” concept.
Energies 2018,11, 496 5 of 26
Table 1.
Summary of literature studies comparing biochars and hydrochars. CEC: cation exchange capacity, FTIR: Fourier-transform infrared spectroscopy, SSA:
specific surface area, SEM: scanning electron microscopy, EDS: energy-dispersive X-ray spectroscopy, CT: CarbonTerra (Augsburg, Germany), BC: Black-Carbon
(Barrit, Denmark), AGT: Advanced Gasification Technology (Arosio, Italy).
Authors Process Conditions Feedstock Methods
Budai et al. [19]
Biochars from slow pyrolysis (235–800
◦
C), flash
carbonization char (600
◦
C) and hydrochars (230
◦
C for
6 h)
Corncob and miscanthus
Proximate and elemental (CHNO) analyses, pH, CEC, SSA
with N2adsorption
Schimmelpfennig
and Glaser [21]
Biochars from slow pyrolysis (Pyreg unit) (550
◦
C),
hydrochars (200
◦
C), rotary kiln biochars (350–550, 750
◦
C),
gasification biochars (800
◦
C), traditional kiln biochars
(350 ◦C), other biochars (350, 500, 650, 800 ◦C)
Wood, animal meal, sugar cane/beet, wheat, bamboo,
maize, rice hulls, peanut shells, sewage sludge, walnut
shells, girasol, coconut shells, lop, bark/needles
Elemental (CHNO) analysis, black carbon content, SSA
with N2adsorption, PAHs content
Liu et al. [28]
Biochars from slow pyrolysis (700
◦
C), hydrochars (300
◦
C
for 20 min) Pine wood
Proximate and elemental (CHNO) analyses, Boehm
titration, pH, FTIR spectroscopy, surface morphology with
SEM, SSA and micro-, meso- and macropores volume with
N
2
adsorption, copper removal capacity (adsorption) from
wastewater
Huff et al. [35]
Biochars from slow pyrolysis (300, 400 and 500
◦
C) and
hydrochars (300 ◦C for 30 min) Pine wood, peanut shell and bamboo
Proximate and elemental (CHN) analyses, methylene blue
adsorption, CEC, FTIR spectroscopy
Cao et al. [36]
Biochars from slow pyrolysis (620
◦
C) and hydrochars
(with acid prewashing and water and acetone washing)
(250 ◦C for 20 h)
Swine-manure
Major chemical structural components with Quantitative
13
C Direct Polarization/Magic-Angle Spinning (DP/MAS)
NMR,
13
C Cross-Polarization/Total Suppression of
Sidebands (CP/TOSS) and
13
C CP/TOSS Plus Dipolar
Dephasing,
13
C Chemical-Shift-Anisotropy (CSA) Filter.
Connectivities of different functional groups in hydrochars
with
1
H–
13
C Two-Dimensional Heteronuclear Correlation
(2D HETCOR) NMR. Aromatic cluster sizes with
1
H–
13
C Long-Range Recoupled H–C Dipolar Dephasing
Experiments
Wiedner et al. [37]
Biochars (pyrolysis-Pyreg up to 850
◦
C, pyrolysis/
gasification-BC up to 760
◦
C, gasification-AGT up to
1200
◦
C, gasification-CT up to 550
◦
C) and hydrochars
(1st stage at 230
◦
C for at least 15 min and 2nd stage at
180 ◦C for at least 75 min; 170 ◦C for 90 min)
Wheat straw-AGT, wood chips-AGT, poplar-AGT,
sorghum-AGT, olive residues-AGT, wood chips-CT, wood
chips-BC, draff-Pyreg, miscanthus-Pyreg. Maize silage,
leftover food, biogas digestate, grass greenery and sewage
sludge for hydorchars
pH, EC, ash content, elemental composition (CHNO),
13
C NMR, SEM-EDS, black carbon content, PAHs
content, polychlorinated dibenzodioxines (PCDDs) and
polychlorinated dibenzofurans (PCDFs) content
Busch et al. [38]
Biochars (gasification-AGT up to 1200
◦
C and pyrolysis at
400–600
◦
C) and hydrochars (230
◦
C for 1 h and 180
◦
C for
at least 4 h; 170 ◦C)
Olive residues-AGT, poplar wood chips-AGT, wheat
straw-AGT, miscanthus-pyrolysis. Maize silage, food
leftovers, biogas digestate grass and sewage eludge for
hydrochars
Tradescantia genotoxicity assay, plant germination and
growth tests and impact on soil pH
Busch and Glasser [
39
]
Biochar (pyrolysis-Pyreg), hydrochar (230
◦
C for 1 h and
180
◦
C for at least 4 h), co-composted biochar (1:1 w/w)
and hydrochar (1:3 w/w) with raw biomass residues from
vegetable waste from horticulture and landscaping, kept
for 4 weeks in temperature range 55–75 ◦C)
Conifer wood bark residues for biochar and maize silage
for hydrochar
Stability of co-composted hydrochar and biochar under
field conditions in a temperate soil. TOC, total N content,
pH, EC and black carbon analysis of biochars, hydrochars,
compost and co-composted biochars and hydrochars
before and after application in soil
Energies 2018,11, 496 6 of 26
2. Materials and Methods
Two different raw materials, pine wood chips (PW) and corn digestate (CD), were used for the
production of biochar (pyrolysis) and hydrochar (HTC). PW was provided by Robeta Holz OHG
(Milmersdorf, Germany) while CD was obtained from an anaerobic digester in Dorf Mecklenburg
(Germany). The substrate used for mesophilic (39
◦
C) anaerobic digestion was 70% corn silage and 30%
cow manure. The digestate was dried and stored on site. Due to storing conditions, the dry digestate
contained a significant amount of calcium carbonate. The properties of both raw materials are shown
in Table 2.
Table 2.
Raw material, conversion process, experimental conditions and solid yields for biochar
and hydrochar production. Ash content and elemental composition of pine wood chips (PW) and
corn digestate (CD). Determination of ash content was done according to the DIN norm 51719.
Elemental analysis was performed using a Vario EL elemental analyzer. Oxygen content was
obtained by difference. The values are given in % mass, dry basis (%, db). * Maximum conversion
temperature measured in the reference position, according to the description provided in the Section 1
of Supplementary Material.
Material Ash (%, db) C (%, db) H (%, db) O (%, db) N (%, db)
CD 19.6 ±0.1 40.7 ±0.7 5.5 ±0.1 31.7 ±0.8 2.4 ±0.5
PW 0.2 ±0.0 49.4 ±0.4 6.7 ±0.1 43.7 ±0.4 0.1 ±0.0
Sample Raw Process HTT * Flow Char yield
material (◦C) (NL min−1) (%, db)
PW400-20 Pine wood Pyrolysis 400 20 30.01 ±0.18
PW400-40 Pine wood Pyrolysis 400 40 25.83 ±2.12
PW600-20 Pine wood Pyrolysis 600 20 24.94
PW600-40 Pine wood Pyrolysis 600 40 22.82 ±0.49
CD400-20 Corn digest. Pyrolysis 400 20 50.89 ±0.36
CD400-40 Corn digest. Pyrolysis 400 40 52.00 ±0.21
CD600-20 Corn digest. Pyrolysis 600 20 49.76 ±0.40
CD600-40 Corn digest. Pyrolysis 600 40 48.54 ±1.08
Sample Raw Process HTT Time Wash. Char yield
material (◦C) (min) times (%, db)
PW200-10 Pine wood HTC 200 10 - 82.30 ±0.35
PW200-360 Pine wood HTC 200 360 - 75.68 ±0.01
PW240-10 Pine wood HTC 240 10 - 76.26 ±0.07
PW240-360 Pine wood HTC 240 360 - 59.09 ±0.19
CD200-10-0 Corn digest. HTC 200 10 0 83.49
CD200-10-6 Corn digest. HTC 200 10 6 73.49
CD200-360-0 Corn digest. HTC 200 360 0 74.38
CD200-360-6 Corn digest. HTC 200 360 6 70.29
CD220-185-3 Corn digest. HTC 220 185 3 64.57 ±2.02
CD240-10-0 Corn digest. HTC 240 10 0 70.82
CD240-10-6 Corn digest. HTC 240 10 6 66.85
CD240-360-0 Corn digest. HTC 240 360 0 62.06
CD and PW biochars were produced at the Technische Universität Berlin (Berlin, Germany),
in a technical-scale fixed-bed reactor, with capacity for several kg of biomass. The setup used in these
experiments has been presented elsewhere [
40
]. In these experiments, the influence of maximum pyrolysis
temperature (400 and 600
◦
C) and residence time of the volatiles in contact with the solid bed (with inert
gas flow rates of 20 and 40 NL min−1) was investigated. Experiments were conducted in duplicates.
Energies 2018,11, 496 7 of 26
The hydrothermal carbonization experiments were carried out at the Leibniz Institute for
Agricultural Engineering and Bioeconomy (Potsdam, Germany). Distilled water was added to the dry
digestate in each experiment to achieve the desired solid content of 20% by mass. The experiments
were conducted in a 1 L autoclave (Series 4520, stainless steel T 316, Parr Instruments, Moline, IL,
USA) equipped with a temperature controller. The influence of maximum conversion temperature
(200, 220 and 240
◦
C), time at this temperature (10, 185 and 360 min) and washing effect (0, 3 and
6 times with distilled water) on hydrochar properties was investigated.
Further details on biochar and hydrochar production conditions are given in Table 2and in the
Suplementary Material.
Analytical Methods
Biochars ash content determination was done following the DIN norm 51719. Hydrochars ash
content was characterized according to the method described in Chapter 8.4 from the VDLUFA
Methodenbuch Band III [
41
]. Validation of both methods for both raw materials showed that the
results according to the DIN norm are within the standard deviation of the results from the VDLUFA
method. For elemental analysis (CHNS), a Vario EL elemental analyzer (Elementar Analysensysteme
GmbH, Lagenselbold, Germany) was used for hydrochars and biochars. Inorganic elemental analysis
for hydrochars was performed in an ICP-OES (iCap 6300 Duo, from Thermo Scientific, Schwerte,
Germany). Prior to the analysis, milled samples were digested according to the method exposed
in Chapter 10.8 of the VDLUFA Methodenbuch Band III [
41
], using a microwave (ultraClave-IV
high pressure microwave, from MWS Vertriebs GmbH, Leutkirch, Germany) with nitric acid 65%
(260
◦
C, 140 bar, 2 h) and stored at room temperature. Biochar inorganic elemental characterization
was also performed with an ICP-OES (Varian 720-ES, now Agilent Technologies, Inc., Santa Clara,
CA, USA) after digestion in a microwave (Multiwave 3000, from Anton Paar, Graz, Austria) with
nitric acid 65% (Rotipuran
R
p.a., ISO, max 0.005 ppm Hg, Carl Roth GmbH + Co. Kg, Karlsruhe,
Germany) and hydrogen peroxide 30% (Rotipuran
R
p.a., ISO, stabilisiert, Carl Roth GmbH + Co. Kg,
Karlsruhe, Germany).
The pH of biochars and hydrochars was measured in deionized water solution. The determination
of oxygen-containing functional groups on the char surface was done applying the Boehm titration
method [
42
]. The experimental procedure is based on the description by Goertzen et al. [
43
],
including the sample pretreament proposed by Tsechansky and Graber [
44
]. Further details on
this characterization method are given in the Section 2 of Supplementary Material.
The pore structure, including specific surface area, pore volume and pore size distribution,
was characterized with gas adsorption, combining the measurement of N
2
and CO
2
adsorption
isotherms at 77 K and 273 K respectively. The pretreatment of the samples, prior to adsorption
measurements, included milling and degassing in vacuum at 150
◦
C during 6 h, with the objective
of cleaning the surfaces from other adsorbed species and reducing transport limitations (milling).
For both degassing and adsorption/desorption measurements the gas sorption system Nova 2200,
provided by Quantachrome Instruments (Boynton Beach, FL, USA), was used. Several methods were
applied to the isotherms in order to derive pore surface areas and volumes, all of them included in the
NovaWin software (Version 11.04, also from Quantachrome Instruments). The methods applied to the
N
2
adsorption/desorption isotherms included: the BET method [
45
] to meaure the total specific surface
area; the BJH method [
46
] to characterize the specific surface area and volume of pores bigger than
≈
3 nm; and the density functional theory (DFT) [
47
] method, in particular, the quenched solid density
functional theory (QSDFT) method, considering slit/cylindrical pores (Quantachrome Instruments).
With the QSDFT method, the specific surface area and volume of micropores and mesopores up to
≈
34 nm were characterized. The total pore volume was determined from the volume of N
2
adsorbed
at a relative pressure P/P
0≈
1, assuming that at this point the pores are filled with liquid N
2
. For CO
2
adsorption, the non local density functional theory (NLDFT) method (Quantachrome Instruments)
was applied to determine the specific surface area and volume due to micropores up to ≈1.5 nm.
Energies 2018,11, 496 8 of 26
Biochar thermal stability was analyzed by thermogravimetric analysis (TGA 1, STARe system,
from Mettler Toledo, Columbus, OH, USA) with a heating rate of 5
◦
C min
−1
, in synthetic air (20.5%
O
2
, 200 mL min
−1
air flow) and up to a temperature of 600
◦
C. Initial mass below 5 mg was used to
guarantee kinetic regime and avoid self-heating due to the exothermicity of the oxidation reactions.
Conversion rate of biochar oxidation is defined here as the change of conversion (
α
) over time (
dα
dt
).
Conversion (
α
) is defined as 1
−
(m
o−
m)/(m
o−
m
f
), where m is the biochar mass at each time step,
mois the initial biochar mass and mfis the mass of the combustion product at 600 ◦C, i.e., ash.
Plant available ammonium (NH
+
4
) and nitrate (NO
−
3
) were determined according to the DIN
19746:2005-06 (extraction of the samples with 0.0125 mol L
−1
of CaCl
2
in distilled water). Plant available
phosphate (PO
3−
4
) and potassium (K
+
) were determined according to VDLUFA Methodenbuch Band
I [
48
], Chapter 6.2.1.1 (extraction with a solution of 0.02 mol L
−1
of calcium lactate and 0.02 mol L
−1
hydrochloric acid in distilled water). The analysis of the extracts was conducted by IC-CD (ICS 1000,
from DIONEX). Anions were analyzed with an IonPac AS9-HC 4
×
250 mm column from DIONEX
(chromatographic conditions: 9 mmol Na
2
CO
3
, 1.2 mL min
−1
, 160 bar) and cations with an IonPac CS
16 5
×
250 mm column from DIONEX (chromatographic conditions: 30 mmol methanesulfonic acid
(MSA), 1.0 mL min−1, 80 bar, 40 ◦C).
The method applied for characterization of the 16 USEPA (United States Environmental Protection
Agency) PAHs has been developed by Fabbri et al. [
49
], with the difference that in the present study
a standard solution of deuterated 16 PAHs (PAH-Mix 9 deuterated, 10 mg L
−1
, Dr. Ehrenstorfer,
Augsburg, Germany) was used, instead of 3 deuterated PAHs as in [
49
]. Briefly, the biochar samples,
spiked with 0.1 mL of a 5 mg L
−1
solution of the above deuterated 16 PAHs solution, were Soxhlet
extracted with an acetone (CHROMASOLV, for HPLC,
≥
99.9%, Sigma–Aldrich, St. Louis, MO,
USA)/cyclohexane (ACS reagent,
≥
99.5%, Sigma–Aldrich, St. Louis, MO, USA) (1:1, v/v) solvent
mixture for 36 h and then the PAHs were analyzed in an Agilent HP 6850 gas chromatograph (GC)
coupled to an Agilent HP 5975 quadrupole mass spectrometer (MS) (Agilent Technologies, Inc.,
Santa Clara, CA, USA); GC-MS conditions were those detailed in [49].
Biochar wettability measurements were performed according to the method described by
Ojeda et al. [
50
]. Briefly, a glass tube 6 cm high with 0.9 cm of internal diameter was filled with
the different biochar samples up to a height of 3 cm. Then the tube was attached to a high precision
scale (DCAT, dynamic contact angle measuring instrument, DataPhysics
c
, Germany). Below the tube,
a glass filled with distilled water or n-hexane (AnalaR NORMAPUR –for synthesis–
≥
95%) was placed
and carefully risen until touching the bottom part of the tube. Distilled water/n-hexane capillary
absorption was measured by registering the weight increase in the biochar column during 200 s.
3. Results and Discussion
3.1. Primary Characterization: Yields, Elemental and Proximate Analysis
In Table 2biochars and hydrochars yields are presented. Hydrochars show significantly higher
char yields than biochars, consistent with the lower conversion temperatures and the higher pressures.
For biochars, the lower the conversion temperature the higher the char yield. CD biochars also present
higher yields than PW biochars, due to the significantly higher inorganic species content, which remain
in the solid product after pyrolysis. Besides, a higher N
2
flow rate during pyrolysis generally reduces
the char yield, due to a reduction in the residence time of the volatiles in the bed, minimizing secondary
reactions and formation of secondary char [
12
]. In the case of hydrochars, the solid yields for both
raw materials are similar. Besides, both conversion temperature and retention time at the conversion
temperature play an important role. Noteworthy to mention in the case of hydrochars is the impact
of washing. With the same conversion temperature and retention time, washing the hydrochar after
production can reduce up to 10% the solid yield. This indicates that a significant part of the solid is
actually condensed volatiles and water soluble inorganic species.
Energies 2018,11, 496 9 of 26
In Figure 1(left), the Van Krevelen diagram, i.e., the representation of H/C molar ratio versus
O/C molar ratio is presented for biochars and hydrochars. This diagram is often used to illustrate
the degree of maturity and aromaticity of chars [
51
]. Results in Figure 1show that hydrochar and
biochar chemical composition depends on the conversion conditions, but also on the initial feedstock
composition, having potentially a significant impact on final char chemical properties. This is illustrated
with the results of CD biochars/hydrochars versus those for PW biochars/hydrochars. For the same
O/C molar ratio, the H/C molar ratio of CD biochars is lower than for PW biochars. Looking at the
relation of these molar ratios with the volatiles content (% mass, dry ash free), it is observed that while
biochar O/C molar ratio follows a linear trend with respect to the volatiles content, the behavior of
H/C ratio seems only linear for PW biochars, while very scattered for CD biochars.
As it has been introduced in Section 2, corn digestate contains calcium carbonates, mainly due to
storing conditions. The inorganic C content of CD biochars, due to carbonates, is approximately 4–5%,
as determined by Rombolà et al. [
52
]. The presence of these carbonates could explain, to some
extent, the higher volatiles content of CD biochars in comparison with PW biochars, since at the
conversion temperatures used in this study (400 and 600
◦
C) carbonates are stable and therefore
remain in the biochar. However, this could not explain the lower content in hydrogen for CD biochars.
Besides, the elemental composition of CD presented in Table 2is similar to the one reported by
Mumme et al. [
53
] for maize silage digestate. This suggests different reaction pathways for low
and high inorganic matter content biochars, with a potential enhancement of aromatization for high
inorganic matter content biochars. Li et al. [
54
] also reported lower H/C ratios for biochar produced
from pyrolysis of rice husk, blended with CaCO
3
prior pyrolysis, than for unmodified biochars,
which they correlated with an increase in the carbon aromaticity degree. The potential formation of
oxides or other metal composites on the biochar surface, associated to the high content of inorganic
matter, could also play a role [55].
With respect to hydrochars, CD hydrochars present in general higher H/C ratios and lower
O/C ratios than PW hydrochars, in some cases even lower than CD biochars. This indicates different
reaction mechanisms for both materials. The higher H/C ratios of CD hydrochars suggests that
decarboxylation reactions are favored over dehydration reactions [
53
]. In the case of PW hydrochars,
dehydration reactions seem to be more significant in comparison to CD hydrochars, while the influence
of decarboxylation reactions is lower, except for the highest conversion temperature and highest
retention time (240
◦
C – 360 min), where both are relevant. This is further supported by the composition
of the producer gas (collected during the HTC experiments in gas bags and analyzed with a SSM6000
analyzer from Pronova, Germany), with a concentration of CO
2
increasing from around 50% vol.
to approximately 80% vol. for the highest conversion degree. It is also important to point out that
retention time at the conversion temperature is as significant as the conversion temperature itself.
With respect to CD hydrochars, CO
2
increases from approximately 58% vol. from the mildest conditions
to around 75% for the most severe conditions. In this case, a significant emission of CH
4
(2.5% vol.) was
also registered. Alkaline conditions during HTC enhance the formation of carboxylic acids, reducing
therefore the O/C ratio of the hydrochar [
56
]. In the present study, although the pH of both solid and
liquid products are acid, they are higher for CD hydrochars than for PW hydrochars, as shown in
Section 3.2. Similar composition for digestate hydrochars has been also shown by Mumme et al. [53],
who highlighted that this could be due to the decarboxylation reactions. Liu et al. [
28
], who compared
a hydrochar produced at 300
◦
C with a biochar produced at 700
◦
C using pine wood, reported similar
H/C and O/C ratios for their biochar as for the PW biochars produced at 600
◦
C in the present study.
The values for the hydrochar were slightly higher in comparison to PW hydrochars produced at 240
◦
C
during 360 min in the present study. Besides, for similar hydrochar H/C ratio, the hydrochar O/C
ratio is lower in the present study than in [
28
]. This could be related to the high amount of ash reported
by Liu et al. [28].
Energies 2018,11, 496 10 of 26
According to the guidelines given by the European Biochar Certificate (EBC) [
1
] and the
International Biochar Initiative (IBI) Standards [
2
] on H/C and O/C ratios, the biochars presented in
this Section could be considered as certified biochars. With respect to hydrochars, it is shown that it is
also possible to produce hydrochars able to fulfill those requirements, with a conversion temperature
of at least 240
◦
C and a retention time of 360 min, but only with starting HTC conditions enhancing
both dehydration and decarboxylation reactions (as it is the case for PW), not only decarboxylation
reactions (as in the case of CD).
Inorganic elemental characterization is shown in Table 4 and in the Supplementary Material
(Table S1). It is observed that the elemental inorganic composition of CD hydrochars and biochars
is much higher than the one for PW hydrochars and biochars, in good agreement with their high
ash content.
0 0.2 0.4 0.6 0.8
O/C molar
0
0.5
1
1.5
2
H/C molar
CD HTC
PW HTC
CD Pyro
PW Pyro
CD
PW
Raw material
CD HTC
PW HTC
CD Pyro
PW Pyro
0 20 40 60 80 100
Volatiles (%daf)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
Molar ratio
O/C CD pyro
H/C CD pyro
O/C PW pyro
H/C PW pyro
O/C CD HTC
H/C CD HTC
O/C PW HTC
H/C PW HTC
O/C PW
H/C PW
O/C CD
H/C CD
H/C
O/C
H/C CD Pyro
O/C CD HTC
Figure 1. Left
: Van Krevelen diagram, i.e., H/C molar ratio versus O/C molar ratio of PW and CD
biochars and hydrochars.
Right
: H/C and O/C molar ratios versus volatiles content (% mass, daf) for
PW and CD biochars and hydrochars.
3.2. pH and Surface Functional Groups
It has been reported in literature that pH may be dependent on both, the surface chemistry
and the ash content [
22
,
23
,
57
,
58
]. These both influences are further confirmed in the present study.
In Figure 2a,b the variation of pH with ash content is presented. Biochars with higher ash content
show also higher pH. However, the strongest correlations for biochars are not between pH and the
ash content, but between pH and H/C and O/C molar ratios. Specially remarkable is the correlation
between pH of CD biochars and H/C molar ratio (r =
−
0.95, shown in Figure 2c, being r the Pearson
correlation coefficient), when considering that the latter shows such scattered behavior in Figure 1.
It must be also taken into account that CD contains calcium carbonate due to storing conditions,
which has a high liming effect. However, the correlation between CD biochars ash content and pH is
significantly worse than between pH and H/C. With respect to PW biochars, pH presents also stronger
relations with H/C and O/C molar ratios (r =
−
0.91 and r =
−
0.89 respectively, shown in Figure 2d,f)
than with ash content, being this correlation similar in both cases.
Therefore, with respect to biochars pH, it can be concluded that despite the fact that ash content
has an influence on the pH value, a better correlation exists between the pH value and H/C and O/C
ratios. In the case of CD biochars, the relation between pH and H/C ratio is strongly linear.
For hydrochars the behavior is slightly different. Figure 2a shows that pH of CD hydrochars has
a stronger correlation with ash content (r = 0.88) than with H/C (Figure 2c) or O/C ratios (Figure 2e).
Despite the fact that the slope of the linear fitting for pH-ash in Figure 2a for CD hydrochars is lower
than those for pH-H/C for CD and PW biochars in Figure 2c,d respectively, the correlation is still
much better for pH-ash than for pH-H/C or pH-O/C. This indicates that the pH in CD hydrochars is
Energies 2018,11, 496 11 of 26
mainly correlated to ash content, although with a weaker dependency than PW and CD biochars pH
with H/C ratio. PW hydrochars show similar relations with H/C and O/C molar ratios.
In Figure 3, information about the acidic functional groups present on the chars surface is given,
in particular, for carboxylic, phenolic and lactonic groups. In Figure 3a the total acidic functional
groups (carboxylic, lactonic and phenolic) and the carboxylic functional goups with respect to the
organic fraction of the chars (mmol g
−1
org
, db) are plotted versus the O/C molar ratio for PW600-20,
PW400-20, PW600-40, PW400-40, CD600-20, CD400-20, PW240-10. For biochars, independently of
the inorganic matter content, a good correlation (r = 0.80) is observed between total acidic groups
and O/C ratio. Hydrochar PW240-10 presents a high O/C molar ratio which does not correlate
well with the total acidic functional groups, according to the biochars behavior. This may indicate
that still a significant amount of oxygen is embedded in the hydrochar skeleton, which is in good
agreement with the structure of hydrochar formed mainly by alkyl moieties reported by Cao et al. [
36
].
In Figure 3b the total acidic groups and the carboxylic goups (with respect to the organic fraction
of the chars) are plotted versus H/C molar ratio. No good correlations are observed. However, in
Figure 3c a quite strong linear correlation (r = 0.84) is present between the total surface acidic groups
with respect to the total char mass (mmol g
−1
, db) and the H/C molar ratio, independently of the
inorganic matter content and the conversion process. Figure 3d shows the comparison of these groups
with pH. A relation between the increase in pH and the decrease in total acidity of the biochar surface
is observed.
20 30 40 50
Ash (%,db)
5
10
15
pH
CD HTC
CD Pyro
0 0.5 1 1.5
Ash (%,db)
0
5
10
pH
PW HTC
PW Pyro
0 0.5 1 1.5
H/C
5
10
15
pH
CD HTC
CD Pyro
0 0.5 1 1.5
H/C
0
5
10
pH
PW HTC
PW Pyro
0.1 0.2 0.3 0.4 0.5
O/C
5
10
15
pH
CD HTC
CD Pyro
0 0.2 0.4 0.6 0.8
O/C
0
5
10
pH
PW HTC
PW Pyro
r = 0.76
r = 0.89
r = -0.89
r = -0.03
r = 0.88
r = 0.63 r = 0.26
r = -0.91
r = -0.95
r = 0.82
f)
d)
b)
a)
c)
e)
Figure 2.
pH versus ash (% mass, db) in (
a
,
b
), H/C molar ratio in (
c
,
d
) and O/C molar ratio in (
e
,
f
) for
CD (left) and PW (right) biochars and hydrochars. The strength of the correlations is indicated with the
Pearson correlation coefficient (r).
Energies 2018,11, 496 12 of 26
0 0.1 0.2 0.3 0.4 0.5 0.6
O/C molar
0
0.5
1
mmol/gorg
Total (PW)
Carboxylic (PW)
Total (CD)
Carboxylic (CD)
0 0.2 0.4 0.6 0.8 1
H/C molar
0
0.5
1
mmol/gorg
Total (PW)
Carboxylic (PW)
Total (CD)
Carboxylic (CD)
0 0.2 0.4 0.6 0.8 1
H/C molar
0
0.5
1
mmol/g
Total (PW)
Carboxylic (PW)
Total (CD)
Carboxylic (CD)
4.18 6.84 7.33 7.58 9.05 9.15 10.21 10.59 11.96 12.32
pH
0
0.5
1
1.5
mmol/g
Total
Carboxylic Corn
digestate
c)
a) b)
r = 0.84 Total acidic
groups
PW hydrochar
r = 0.80 Total acidic
groups
d)
Figure 3.
Surface oxygen-containing functional groups of the biochars determined with the Boehm
titration method. (
a
,
b
) show the amount of functional groups with respect to the mass of the chars
organic fraction versus the O/C and H/C molar ratios; (
c
) shows the amount of functional groups with
respect to the total chars mass versus the H/C molar ratio; (
d
) shows the amount of functional groups
versus the pH.
3.3. Porosity Characterization
In Figure 4, the evolution of the total specific surface area (BET method using N
2
adsorption)
is plotted versus the biochars/hydrochars volatiles content (% mass, daf). In general, the lower the
volatiles content, the higher the specific surface area (SSA). However, for CD biochars different BET
SSA behaviors are observed for similar volatiles content. The highest SSA values correspond to biochar
produced at 600
◦
C with a N
2
sweeping flow rate of 40 NL min
−1
. In Figure 4b the SSA evolution is
plotted versus the H/C molar ratio. The decrease of H/C molar ratio is indicative of the formation
of structures containing unsaturated carbon such as aromatic rings [
59
], and therefore, represents
the increase in aromaticity with charring degree, as reported in [
25
]. It is possible to observe that
the behavior of CD biochars differs significantly from the one of PW biochars. Taking into account
that CD biochars have around 40% ash content, the SSA would be higher as function of the organic
fraction, but it would be still lower than for PW biochars. This may be a consequence of two possible
phenomena: (a) porosity development is different for materials with high inorganic content and
(b) potential filling or access blocking of micropores by the inorganic species [
60
,
61
], which could
explain the lower SSA value despite the lower H/C molar ratio in CD biochars.
In order to further characterize the porous structure, in Figure 5the pore size distribution
according to pore surface area (left) and pore volume (right) is presented, characterized with the
methods introduced in Section “Analytical Methods”. SSA and volume from CO
2
adsorption are also
shown in Figure 5. With CO
2
adsorption, micropores up to 1.5 nm in pore width are characterized.
Despite this, the SSA area measured with CO
2
adsorption is higher than the total SSA (up to 100–200 nm
in pore width, depending on the sample porous structure) measured with N
2
adsorption. This indicates
that the samples have a microporous structure which cannot be completely characterized by N
2
adsorption, mostly due to diffusion limitations, consequence of the low temperature measurements in
N2adsorption.
The CO
2
SSA is around 25% higher than the N
2
SSA for PW biochars produced at 600
◦
C,
with inert gas flow rates of 20 and 40 NL min
−1
. However, the CO
2
SSA for CD biochars produced in
the same conditions is 90% and 80% higher respectively. This indicates that the diffusion limitations
are stronger in CD biochars, in good agreement with the previous explanation for the low specific
surface of high inorganic content biochar. That is to say, the inorganic species may provoke partial
blocking or constriction of the porous structure, limiting the access of N
2
to micropores but not of CO
2
.
Energies 2018,11, 496 13 of 26
0 50 100
Volatiles (%daf)
0
100
200
300
400
500
BET SSA (m2/g)
PW Pyro
CD Pyro
PW HTC
CD HTC
(a)
0 0.5 1 1.5
H/C molar ratio
0
100
200
300
400
500
BET SSA (m2/g)
PW Pyro
CD Pyro
PW HTC
CD HTC
(b)
0 0.5 1 1.5
H/C molar ratio
0
200
400
600
800
SSA (CO2) (m2/g)
PW Pyro
CCD Pyro
PW HTC
CD HTC
(c)
Figure 4.
(
a
): Total specific surface area (SSA) determined with the BET method (N
2
adsorption) versus
the volatiles content (% mass, daf) for PW and CD biochars and hydrochars; (
b
): SSA determined with
the BET method (N
2
adsorption) versus H/C molar ratio for PW and CD biochars and hydrochars;
(
c
): SSA determined with CO
2
adsorption applying the NLDFT method versus H/C molar ratio for PW
and CD biochars and hydrochars.
With respect to the pore volume, higher N
2
pore volume than CO
2
pore volume is measured
for PW biochars produced at 600
◦
C. This is due to the smaller size of pores characterized by CO
2
adsorption. Noteworthy to mention is the fact that higher N
2
sweeping flow during the pyrolysis
process leads to higher SSA and pore volume in the biochar. For PW biochars the effect is clearer when
looking at the pore volume distribution. This is also stronger for biochars produced at 400
◦
C than for
biochars produced at 600
◦
C. For CD biochars the difference for biochars produced at 600
◦
C is already
very significant, as introduced in Figure 4. This shows that higher flow rates lead to a more open
porous structure, due potentially to a combination of higher removal of volatiles (more significant
for biochars produced at 400 ◦C) and inorganic species (present in CD biochars), together with lower
extent of intraparticle heterogeneous secondary reactions, leading to the formation of secondary char.
Interestingly, while the major contribution to pores in PW biochars comes from micropores
(observed in the CO
2
adsorption results), in CD biochars the contribution of mesopores is also
significant. Therefore, inorganics in CD biochars would not only reduce the presence of micropores
through potential blocking, but could also enhance the development of mesopores.
Comparing the porous structure of hydrochars with the one of biochars, the former follows quite
a different behavior. According to Figure 4, hydrochars present low SSA, in good agreement with the
high volatiles content and high H/C molar ratio. This suggests low charring and aromatization degree
in comparison with biochars. However, in Figure 5, results for hydrochars pore size distribution,
according to volume (bottom right), show that the volume of these samples is in many cases higher
than the one of biochars. Contribution of micropore volume to the total pore volume of hydrochars is
low. Therefore, it can be concluded that porosity (pore volume) in hydrochars is mainly due to meso-
and macropores. Comparing PW hydrochars and CD hydrochars produced at 240
◦
C, with a retention
time at maximum conversion temperature of 360 min, it is possible to see that PW hydrochars present
significantly higher SSA and volume. This could be attributed to the high inorganic matter content
Energies 2018,11, 496 14 of 26
of CD hydrochars. However, PW hydrochars produced at 240
◦
C during 10 min or at 200
◦
C during
360 and/or 10 min show lower SSA and volume than CD hydrochars produced at 240
◦
C and 200
◦
C
during 360 min. Comparing the behavior of CD hydrochars produced at 240
◦
C and 200
◦
C during
360 min, the SSA and pore volume of the latter are higher despite the lower conversion temperatures.
This is due to the washing of the hydrochar produced at 200
◦
C during 360 min. This washing removes
inorganics and volatiles partially blocking the pores, which leads to an underestimation of the porous
structure with N
2
adsorption, as previously introduced for biochars. However, CO
2
adsorption results
show similar values of microporosity for both cases, indicating that this pore blocking does not affect
CO2adsorption.
BET SSA DFT <2nm >2nm >3nm SSA CO2
0
100
200
300
400
500
600
700
SSA(m2/g)
PW 600-20
PW 600-40
PW 400-20
PW 400-40
CD 600-20
CD 600-40
CD 400-20
TPV TPV DFT <2nm >2nm >3nm Vol. CO2
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
Volume (cm3/g)
PW 600-20
PW 600-40
PW 400-20
PW 400-40
CD 600-20
CD 600-40
CD 400-20
N2N2
BET SSA DFT <2nm >2nm >3nm SSA CO2
0
10
20
30
40
50
60
70
SSA (m2/g)
PW 240-360
PW 240-10
PW 200-360/PW 200-10
CD 240-360-0
CD 200-360-6
TPV TPV DFT <2nm >2nm >3nm Vol.CO2
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Volume (cm3/g)
PW 240-360
PW 240-10
PW 200-360/PW 200-10
CD 240-360-0
CD 200-360-6
N2N2
Figure 5.
Pore size distribution (PSD) of PW and CD biochars (
top
) and hydrochars (
bottom
).
Top left
:
SSA distribution with pore size for biochars according to the BET method (for total specific surface
area), quenched solid density functional theory (QSDFT, for total specific surface area, surface area
due to pores
<
2 nm and pores 2 nm
<
pore size
<
34 nm) method, and BJH method (surface area
due to pores
>
3 nm). Total specific surface area for pores
<
1.5 nm determined with CO
2
adsorption.
Top right
: Volume distribution with pore size for biochars.
Bottom left
: SSA distribution with pore
size for hydrochars according to the aforementioned methods.
Bottom right
: Volume distribution with
pore size for hydrochars. CO2adsorption was not measured for samples PW400-40 and PW240-360.
3.4. Thermal Stability
Oxidation behavior of biochar in the presence of synthetic air has been performed in
a thermogravimetric analyzer (TGA) to assess biochar stability to oxidation [62,63].
In Figure 6, the conversion rate under oxidation conditions is plotted versus the temperature for
hydrochars (top) and biochars (bottom). The higher the temperature correspondent to the peak of the
conversion rate, the lower the reactivity, and therefore, the higher the stability to oxidation.
For most hydrochars, except the ones produced under the highest severity (240
◦
C, 360 min
retention time), a first peak in the conversion rate is observed at temperatures slightly above 300
◦
C,
consistent with further oxidative pyrolysis of the sample [
64
]. The second peak at higher temperatures
is related to char oxidation. For PW hydrochars, these oxidation peaks take place at significantly
higher temperatures than for CD hydrochars. At the same time, water washing of the hydrochars
after production seems to also affect their oxidation behavior. Those not being washed experience an
Energies 2018,11, 496 15 of 26
earlier second oxidation peak. For biochars, the higher the conversion temperature, the higher the
temperature correspondent to the maximum of the oxidation peak. For hydrochars, residence time
at the maximum conversion temperature and post-washing, as previously introduced, also play a
significant role.
200 300 400 500 600
Temperature (°C)
0
0.5
1
1.5
2
Conversion rate (1/s)
×10-3
PW240-360
PW240-10
PW200-360
200 300 400 500 600
Temperature (°C)
0
0.2
0.4
0.6
0.8
1
1.2
Conversion rate (1/s)
×10-3
CD240-360-0
CD240-10-6
CD240-10-0
CD200-360-6
CD200-360-0
200 300 400 500 600
Temperature (°C)
0
0.5
1
1.5
Conversion rate (1/s)
×10-3
PW 600-20
PW 400-20
CD 600-20
CD 600-40
Figure 6.
Conversion rate of char oxidation for PW hydrochars (
top left
), CD hydrochars (
top right
),
PW and CD biochars (bottom).
The initial feedstock seems to have a stronger influence on the thermal stability (based on the
shape and peak position of the char oxidation curve) than the conversion process conditions. It is
known from literature [
12
] that char reactivity (in oxidation and gasification conditions) depends on
the char structure at molecular level, the inorganic matter in the carbon skeleton, specially alkali and
alkaline earth metals (AAEM), and the concentration and type of functional groups in the carbon
skeleton. Consequently, the high inorganic content of CD biochars, including AAEM, could act as
catalyst in biochar oxidation. This is also consistent with the washing of hydrochars, since with
washing, inorganic species can be removed from the hydrochar surface. Harvey et al. [
65
] showed
a similar behaviour with biochars (produced in [
66
]) from loblolly pine and cord grass, i.e., lower
R
50,biochar
for biochars with high inorganic content (up to 30% ash content for cord grass). However,
regardless of the inorganic content, they attributed the differences in biochar thermal stability to the
extent of carbonization/aromatization. Therefore, samples with similar H/C and O/C ratios can have
different recalcitrance depending on the rearragement of these atoms in the biochar structure [65].
Correlation between the thermal stability and the biogeochemical stability of soil organic
matter has been proposed by Plante et al. [
67
]. Harvey et al. [
65
] reported also a strong
negative exponential-type relationship between biochar degradabitlity (based on the amount of
carbon mineralized after 1 year of incubation [
68
]) and the thermal recalcitrance R
50,biochar
, defined
according to Equation (1). T
50,biochar
is the temperature correspondent to 50% conversion of
biochar (i.e., 50% of the total mass loss due to oxidation/volatilization) and T
50,graphite
is analog for
graphite. In Table 3, the R
50,biochar
values for several biochars and hydrochars are shown, considering
T50,graphite = 844 ◦C [62]
. If a T
50,graphite
= 886
◦
C is used, as measured by Harvey et al. [
65
], slightly
lower R50,biochar values are registered (values between parentheses in Table 3).
Energies 2018,11, 496 16 of 26
R50,biochar =T50,biochar
T50,graphite
(1)
Table 3.
R
50,biochar
, C (% mass, daf), H/C and O/C molar ratios for biochar and hydrochar samples.
The R
50,biochar
values are calculated considering a T
50,graphite
= 844
◦
C [
62
]. The R
50,biochar
values
between parentheses correspond to a T50,graphite = 886 ◦C [65].
R50,biochar C (% mass, daf) H/C (molar) O/C (molar)
PW200-360 0.43 (0.41) 56.94 0.96 0.50
PW240-10 0.41(0.39) 55.77 1.00 0.53
PW240-360 0.51 (0.48) 70.33 0.67 0.27
CD200-360-0 0.41 (0.39) 62.93 1.19 0.33
CD200-360-6 0.41 (0.39) 62.86 1.22 0.33
CD240-10-0 0.43 (0.41) 61.69 1.29 0.35
CD240-10-6 0.42 (0.40) 61.77 1.32 0.35
CD240-360-0 0.45 (0.43) 76.35 1.11 0.12
PW400-20 0.53 (0.50) 82.10 0.54 0.13
PW600-20 0.57 (0.54) 93.49 0.22 0.04
CD600-20 0.52 (0.50) 84.62 0.18 0.09
CD600-40 0.52 (0.50) 81.99 0.19 0.13
According to a biochar classification proposed by Harvey et al. [
65
], all analyzed biochars are
Class B biochars (0.50
≤
R
50,biochar <
0.70), with intermediate C sequestration potential, while the rest
are Class C biochars (R
50,biochar <
0.50), which indicate that those chars have carbon sequestration
potential comparable to uncharred biomass. The PW240-360 hydrochar could belong to both classes,
depending on the considered T50,graphite.
3.5. Nutrients
The total content of macronutrients, such as Ca, Mg, N, P and K, is presented in Table 4for PW
and CD biochars and hydrochars. These values are compared with the available nutrient content in
the form of NH+
4, NO−
3, PO3−
4and K+.
Due to methodical issues, the values of hydrochars nutrient availability, determined from the solid
fraction, cannot be directly compared with those for biochars. A significant fraction of the investigated
species is solved in the liquid phase during HTC, which might still be present upon drying of the
hydrochar. This fact leads to measured nutrient availability in the hydrochar that is not a representative
hydrochar characteristic [
69
]. Instead, the behavior of the nutrients under consideration is investigated
via measurements of the liquid phase for the case of hydrochars. Therefore, for hydrochars, the content
of Ca, Mg, K and P in the solid and in the liquid phase are presented.
It is observed that the main factor affecting nutrient content in the final product is the feedstock,
since both present very different composition, with higher inorganic species and N content in CD. It is
obvious that PW has negligible potential for direct nutrient value from this point of view. Such direct
nutrient value is high for CD due to high amount of available N, P, and K (see Table 4). In the case of
CD it becomes important to investigate how the total nutrient content and their availability changes
with the conversion process. PW results are also presented as comparison.
The ratio C/N can be used as a possible indicator for nitrogen mineralization or immobilization,
taking C/N
<
20 as predictor for nitrogen mineralization and C/N
>
30 as indicator of initial net
nitrogen immobilization [
37
,
70
]. For PW biochars the molar C/N ratio is very high, with values
above 500, which could potentially lead to a high degree of N immobilization once applied to soil,
as previously explained. This is due to the low content of nitrogen in the initial feedstock, which would
Energies 2018,11, 496 17 of 26
still decrease during the conversion process due to nitrogen volatilization, leaving negligible amount
of available N-species in the biochar [
17
]. In the case of CD, the initial nitrogen content in feedstock
is significantly higher, leading to C/N ratios between 20 and 30, which would indicate possibility
of mineralization or higher availability for plants once applied to soil [
37
]. However, the amount of
available N-species for CD biochars is negligible, similar to PW biochars. It must be taken into account
that the C/N ratios for hydrochars are indicative, due to the aforementioned methodical issues.
With respect to P and K, PW biochars present higher nutrient availability (PO
3−
4
and K
+
) in
comparison to the raw feedstock. This may be explained by accumulation of these elements due to
release of volatile organic matter during pyrolysis for the case of K
+
availability, which also shows
a highly significant (p
<
0.001) dependency on pyrolysis temperature. PO
3−
4
availability does not seem
to be dependent on temperature but rather on N
2
gas flow at 600
◦
C (p
<
0.05). In contrast to PW, CD
biochars PO
3−
4
availability is reduced in comparison to the feedstock. Also, there is the same significant
(p
<
0.05) dependency on N
2
gas flow at 600
◦
C pyrolysis temperature. The observed reduction in K
+
availability for the case of CD is not significant due to the high uncertainty associated with the results
(which stems from the high heterogeneity of the feedstock).
A reduction in the availability of PO
3−
4
has been reported elsewhere, but different to the observed
results in this study, a dependency on pyrolysis temperature has been reported [
16
,
17
]. Instead, in this
study an increase in N
2
flow leads to a significant increase in PO
3−
4
availability at high pyrolysis
temperature. This hints to the hypothesis that P is somehow included in carbonaceous solids formed
upon recondensation of primary pyrolysis vapors because it has been shown that biochar extractable
P is primarily orthophosphate at pyrolysis temperatures
>
350
◦
C [
16
]. More detailed studies are
required to understand this phenomenon. Finally it has to be concluded that pyrolysis leads to
a significant loss of plant available nutrient species, specifically NH
+
4
, NO
−
3
, and PO
3−
4
, which has to
be considered when nutrient rich feedstock is being used.
In contrast to the observations from pyrolysis experiments, HTC of PW does lead to a significant
increase of N, P, and K availability (as defined by the analytical method applied here). Twice to
ten times the amount of nutrients are recovered in the process water of HTC as compared to the
leachate after extracting with CaCl
2
or double lactate (see Table 4, note that at least four times as
much process water is present after HTC as dry solid input). These leached nutrients are lost to the
hydrochar, but can be potentially re-used together with the process water. Different to pyrolysis,
nitrogen compounds are not lost during the conversion process [
69
,
71
]. HTC of nutrient rich CD
leads to different observations: the same amount of available nutrients of the feedstock are recovered
in the process water for the case of NH
+
4
and K
+
, but there is a significant reduction of NO
−
3
and
PO
3−
4
. This is likely due to the precipitation of salts, leading to a recovery of these nutrients with
the solids [
72
]. Indeed, the observed high amount of extractable PO
3−
4
on the hydrochar (data not
shown) cannot be explained by solved PO
3−
4
in the hydrochars moisture content, i.e., that a significant
amount of solid PO
3−
4
precipitates during the reaction. It is also noted that the P contained in the
liquid phase is almost exclusively detected as PO
3−
4
whereas only a fraction of the hydrochars P is
extractable by the applied methods (10–30%). The recovery of nutrients in the process water allows for
a decoupling of fertilizing and soil amendment with the potential to flexibly combine these two in
a designed hydrochar product. There is only little influence of the applied HTC process conditions on
these findings. For the case of PW, more severe carbonization leads to a significant but small reduction
of NH
+
4
and PO
3−
4
in the liquid phase (p
<
0.05). This might be due to an increase in adsorption
capacity of the produced hydrochar or increased precipitation of these species. For the case of CD,
this decrease with reaction severity is only observed for NO
−
3
and PO
3−
4
(p
<
0.05). These results are in
contrast to observations from other HTC experiments, which reported an increase of NH
+
4
in the liquid
phase with reaction temperature or no significant change at all [
69
,
72
]. These contradicting results can
be possibly explained with the varying nutrient concentrations used in the different experiments.
Energies 2018,11, 496 18 of 26
Table 4.
Total content of N, Mg, Ca, P and K, as well as available N, P and K (mean
±
standard deviation). The inorganic species content is given for the solid phase
(sol) in biochars and for the solid (sol) and liquid phase (liq) in hydrochars.
a
determined by biochar extraction.
b
measured in the liquid phase of the hydrothermal
carbonization experiments. n.d.: not detected. * The ratio C/N is molar ratio.
C/Nsol * Nsol (db) Mgsol (db) Casol (db) Psol (db) Ksol (db) Mgliq Caliq Pliq NH+
4NO−
3PO3−
4K+
- % mg/kg mg/kg mg/kg mg/kg - - - mg/kg amg/kg amg/kg amg/kg a
PW 912.7 0.1 178.1 ±38.7 894.5 ±305.7 - 322.1 ±81.6 - - - 2.3 0.9 13 230
CD 20.1 2.4 - - 13,900 ±440 9600 ±360 - - - 1200 ±110 1800 ±130 1800 ±160 6100 ±350
PW400-20 550.9 ±57.9 0.2 ±0.0 - - - - - - - n.d. 0.9 27 ±9 300 ±30
PW400-40 729.2 ±54.2 0.1 ±0.0 - - - - - - - n.d. n.d. 37 ±2 370 ±60
PW600-20 634.1 ±106.8 0.2 ±0.0 - - - - - - - 0.9 ±0.2 n.d. 16 ±2 900 ±40
PW600-40 694.3 ±91.3 0.2 ±0.0 - - - - - - - 0.4 ±0.1 0.6 43 ±6 750 ±40
CD400-20 22.6 ±3.5 2.4 ±0.4 20,568.0 ±2233.3 139,051.2 ±23,155.2 26,458.0 ±2163.8 11,882.6 ±1303.1 - - - 1.2 ±0.5 n.d. 1360 ±110 4610 ±1680
CD400-40 22.4 ±0.2 2.3 ±0.1 22,608.3 ±3140.2 154,160.6 ±25,380.1 30,319.7 ±3287.0 13,184.2 ±1594.7 - - - 2.6 n.d. 1650 7460
CD600-20 29.4 ±1.5 1.8 ±0.1 21,796.2 ±1781.4 135,010.5 ±15,103.8 27,975.3 ±2270.6 12,410.9 ±1441.5 - - - 1.4 ±0.4 11 960 ±130 7890 ±1580
CD600-40 29.6 ±1.4 1.7 ±0.1 22,150.4 ±2930.0 156,042.0 ±13,715.9 29,015.1 ±3091.5 12,601.1 ±1442.5 - - - 1.3 ±0.1 n.d. 1510 ±50 8210 ±1410
[-] %mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg bmg/kg bmg/kg bmg/kg b
PW200-10 522.8 ±80.7 0.1 ±0.0 95.8 ±9.8 733.1 ±54.3 61.5 ±4.2 213.1 ±19.5 60.0 ±8.8 126.7 ±38.3 31.7 ±9.4 5.1 ±0.2 5.3 ±0.5 24.2 ±7.5 124 ±20
PW200-360 841.7 ±68.6 0.1 ±0.0 99.3 ±14.8 446.6 ±14.1 78.7 ±31.3 255.4 ±9.6 48.2 ±8.7 145.1 ±10.7 16.0 ±1.1 1.1 ±0.1 5.5 ±1.9 11.4 ±1.1 112 ±2
PW240-10 616.1 ±248.0 0.1 ±0.1 72.0 ±0.1 500.1 ±4.0 48.9 ±3.0 183.4 ±9.3 42.8 ±0.4 135.6 ±1.2 22.3 ±0.1 2.1 ±0.2 5.9 ±0.0 17.4 ±0.6 115 ±3
PW240-360 1802.3 ±1351.3 0.1 ±0.1 118.4 ±4.6 624.8 ±16.7 71.2 ±1.3 336.0 ±31.1 40.9 ±1.0 143.3±1.2 10.4 ±1.1 1.2 ±0.3 5.0 ±3.3 3.5 ±0.6 121 ±6
CD200-10-0/6 28.8 ±2.1 1.9 ±0.1 9565.4 44,518.8 ±5535.0 15,735.0 ±1087.1 5359.6 ±2479.4 3.5 1027.5 ±298.6 341.2 ±131.4 230 ±30 19.2 ±4.2 190 ±48 1490 ±20
CD200-360-0/6 27.3 ±1.8 2.0 ±0.1 11,292.0 54,222.0 ±9737.7 18,429.3 ±602.5 4226.9 ±2928.9 189.0 ±263.3 2356.9 ±439.9 63.9 ±19.1 260 ±65 16.0 ±4.1 60 ±31 2180 ±160
CD220-185-3 25.6 ±0.5 2.2 ±0.0 12,970.2 53,535.0±1943.2 19,555.3 ±671.9 2141.8 ±492.8 3.1 1985.0 ±99.0 60.8 ±6.0 180 ±62 8.3 ±2.3 51 ±8 1890 ±360
CD240-10-0/6 27.3 ±1.9 1.8 ±0.1 10,753.2 ±1330.7 101,258.3 ±50,630.1 15,962.0 ±435.4 3158.0 ±1476.1 473.3 ±618.1 2301.6 ±530.4 112.9 ±84.6 360 ±81 8.1 ±1.9 100 ±80 1870 ±240
CD240-360-0 22.5 2.5 - 110,882.0 20,862.4 4656.2 - 2172 .0 50.5 140 1.6 32 1470
Energies 2018,11, 496 19 of 26
3.6. PAH
The PAHs content in the biochar is dependent on the process conditions, mainly maximum
temperature reached in the process, feedstock and mass transport limitations due to potential
enhancement of heterogeneous secondary reactions. For example, as reported in [
73
], the emission
of PAHs (detected in the vapor phase) for pine wood chips starts when the temperatures inside the
solid bed range between 300–350
◦
C, being at those temperatures 2- and 3-ring aromatic compounds
the main species. This is in good agreement with the results shown in Figure 7, where it is already
observable that in the temperature range from 350–400
◦
C the highest amount of PAHs is present in
the PW biochar, being phenanthrene the most significant species, as shown in Figure 7(bottom left and
right). However, as the temperature increases, the presence of PAHs decreases significantly. At those
temperatures almost no more volatiles, which can further react within the solid matrix, are being
produced and the temperatures are also high enough for a good part of the PAHs still present in the
solid matrix to be removed. Inert gas flow rate may have also a significant impact on the formation of
PAHs, since higher flow rates lead to lower volatiles concentration and retention time in the gas phase.
Hence, the potential intraparticle heterogeneous secondary reactions may be reduced, main mechanism
for PAHs formation at conversion temperatures lower than 500
◦
C [
40
,
64
,
73
]. This is further confirmed
by results recently shown by Buss et al. [74].
350-400 400-575 570-600 600-700 420-470 400-650 560-590 600-750
Temperature interval (°C)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Total PAH (ng/g)
×104
CD
PW
600-700
°
C 570-600
°
C 400-575
°
C 350-400
°
C
0
2000
4000
6000
8000
10000
12000
ng/g
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Chrysene
Benzo[a]anthracene
Benzo[b]fluoranthene
Benzo[k]fluoranthene
Benzo[a]pyrene
Indeno[1,2,3-cd]pyrene
Dibenzo[a,h]anthracene
Benzo[ghi]perylene
PW biochar
600-750
°
C 560-590
°
C 400-650
°
C 420-470
°
C
0
2000
4000
6000
8000
10000
12000
ng/g
Naphthalene
Acenaphthylene
Acenaphthene
Fluorene
Phenanthrene
Anthracene
Fluoranthene
Pyrene
Chrysene
Benzo[a]anthracene
Benzo[b]fluoranthene
Benzo[k]fluoranthene
Benzo[a]pyrene
Indeno[1,2,3-cd]pyrene
Dibenzo[a,h]anthracene
Benzo[ghi]perylene
CD biochar
Figure 7. Top
: Total content in PAHs (16 EPA PAHs) for PW and CD biochars. Horizontal lines
correspond to the limits of PAHs content for basic (12 mg/kg) and premium (4 mg/kg) class biochars
(EBC). Bottom: Content of PAHs species for PW and CD biochars.
For CD, however, the trend is not the same, since the highest contents in PAHs are shifted to
higher temperatures, suggesting that the pathways behind their formation are different from the ones
of PW biochar. This is further supported by their composition. In Figure 7, bottom left and right,
it is shown that PAHs present in CD biochar are mainly naphthalene and acenaphthylene, while for
PW biochars phenanthrene and fluoranthene are the main species at low temperatures. At high
temperatures, naphthalene is the main species.
It must be taken also into account that the ash content in the CD biochar is very high, with values
around 40% in mass, dry basis. Since the PAHs are produced in the organic phase, to compare the total
PAHs in the same basis for CD and PW biochars (% mass, dry ash free basis), the PAHs content in the
Energies 2018,11, 496 20 of 26
case of CD should be multiplied by an approximate factor of 1.7. This leads to higher PAHs content,
as function of the organic fraction, in CD than PW, except PW biochars produced at around 400 ◦C.
Regarding the application of biochar to soil, while in almost all cases a basic grade biochar
is obtained, it can be concluded that for a high quality biochar temperatures close to 600
◦
C must
be reached.
According to Schimmelpfennig and Glaser [
21
], hydrochars have very low total PAH content
and low naphthalene/phenanthrene ratios, similar to PW biochars produced at low temperatures.
Therefore, PAHs content in hydrochars should not be a major issue with respect to their application
and they have not been measured in this study.
3.7. Wettability
In Figure 8, the water absorption behavior of several biochars is shown. On the top, those for
PW biochars and, on the bottom, the behavior for CD biochars. CD biochars show increasing capacity
in water absorption with higher conversion severity, probably due to an increase in porosity and
reduction in surface functional groups. PW biochars present two completely different behaviors. While
chars produced at the highest severity (600
◦
C, 40 NL min
−1
) show similar patterns to those from CD
biochars (although with slightly lower capacity to absorb water), those produced at lower temperatures
do not show almost capacity to absorb water, reaching an asymptotic behavior after the first seconds.
0 50 100
Time (s)
0
0.5
1
Mass (g)
0 50 100
Time (s)
0
0.5
1
Mass (g)
0 50 100
Time (s)
0
0.5
1
Mass (g)
0 50 100
Time (s)
0
0.5
1
Mass (g)
CD 600-40 hexane
CD 600-40 water
PW 600-40 hexane
PW 600-40 water
CD 400-20 hexane
CD 400-20 water
PW 400-20 hexane
PW 400-20 water
Figure 8. Water and hexane absorption curves of PW600-40, PW400-20, CD600-40, CD400-20.
When water is substituted by hexane, CD biochars still show similar absorption capacity in
comparison to water, however the capacity of absorbing hexane by pine wood biochars is much higher
than the one for water. If both absorption curves are compared, the contact angle can be determined, as
explained in [
50
]. The contact angles obtained are shown in the Supplementary Material (Table S2). PW
biochars present contact angles close to 90
◦
, except PW600-40, while CD biochars present slightly lower
contact angles. Although the theoretical definition of hydrophobicity is contact angle
>
90
◦
, the fact
that the values reported here are so close to 90
◦
is an indication of high potential hydrophobicity [
50
].
From these results it can be concluded that biochars present mostly hydrophobic behavior, although
this could be reduced with higher inert flow gas rates during the pyrolysis process, suggesting that a
potential source of hydrophobicity is the condensation of volatiles on the char surface. It has been also
shown in literature [50] that after one year in soil, biochars loose part of their hydrophobicity.
4. Conclusions
The influence of process conditions and feedsctock on several biochars and hydrochars properties
has been studied in a comprehensive way and analyzed from mechanistic perspectives. This work
Energies 2018,11, 496 21 of 26
aims at contributing to biochar engineering, i.e., the capacity of designing and producing biochars able
to perform a specific function for given soil, crop and climate requirements and constraints. Based on
the current results, the following conclusions can be derived:
•
Regarding primary characterization, biochars O/C molar ratio is linearly correlated to volatiles
content, independently of the pyrolysis conditions and feedstock. However, H/C molar ratios of
CD biochars are generally lower than those of PW biochars, which may be attributed to the higher
inorganic matter content of CD, leading to different pyrolysis reaction pathways and enhancing
aromatization. PW hydrochars H/C molar ratio and CD hydrochars O/C molar ratio are also
outliers with respect to the linear behavior. This shows that elemental and proximate analysis are
not enough to determine the chemical properties of biochars [
19
], due to the influence of feedstock
properties on reaction pathways.
•
The pH depends on both conversion process parameters and feedstock, having in general
higher pH biochars and hydrochars produced from CD, with much higher inorganic content,
including carbonates. In both PW and CD biochars pH correlates strongly with H/C molar
ratio. PW biochars and PW hydrochars present also correlation with the O/C ratio. Only CD
hydrochars show good correlation (r = 0.88) with the ash content. This shows that the H/C molar
ratio may be a better indicator of surface chemistry than O/C ratios or volatiles content, which is
also supported by measurements of the total acidic functional groups.
•
The porous structure varies greatly among biochars. PW biochars produced at 600
◦
C present
mainly microporosity with N
2
-SSAs around 400 m
2
g
−1
. In CD biochars this microporosity is
lower (
<
100 m
2
g
−1
). The significantly lower microporosity for all CD biochars is attributed to the
high inorganic content, which could potentially block the micropores and enhance mesoporosity.
PW and CD biochars produced at 400
◦
C present also micropores (as shown by CO
2
adsorption),
but much less developed than at higher temperatures, i.e., microporosity must be still developed
through volatiles release, to open the pores, and further solid aromatization. Furthermore, a higher
N
2
sweeping flow during pyrolysis enhances microporosity (more than double for CD biochars at
600
◦
C). This indicates that micropores blocking is related to volatiles, which may further react
to form secondary char or condense in the pores to a greater extent for lower flow rates, when
the concentration and retention time of volatiles is higher. Hydrochars, on the contrary, present
mainly mesoporosity, independently of the raw material and the process severity, having higher
pore volume but generally lower SSA than biochars. Washing of hydrochars further increases the
SSA and pore volume due to removal of condensed species.
•
Regarding nutrients, NO
−
3
and NH
+
4
availability is negligible after pyrolysis, irrespective of
feedstock nitrogen content. The absolute availability of PO
3−
4
is reduced compared to the feedstock.
For PW biochars the relative availability increases with the pyrolysis conversion. Higher inert
gas flow rate leads also to higher PO
3−
4
availability. For hydrochars, HTC can potentially lead
to an increase in availability of NO
−
3
, NH
+
4
, PO
3−
4
, and K
+
; however, these species are primarily
solved in the process water. One exception is PO
3−
4
which appears to form precipitates at
higher concentration, i.e., for feedstocks with high direct nutrient potential. Similar to pyrolysis,
there is the tendency that the availability of the investigated species is decreased at higher
HTC temperatures.
•
Biochars have a higher stability to oxidation than hydrochars. Besides the influence of process
conditions, PW biochars show higher thermal stability than CD biochars. The same holds true for
hydrochars. This may be related to the catalytic effect of AAEM during oxidation reactions.
•
PAHs are already produced in biochars at temperatures as low as 340
◦
C. The maximum quantity
is detected for temperatures around 400
◦
C, exceeding in the case of PW the maximum level for
certification. At higher temperatures the PAHs content is reduced again due to vaporization of
these species from the solid matrix, achieving a premium quality for temperatures above 550
◦
C.
The composition of these species is highly dependent on the feedstock: naphthalene is the main
Energies 2018,11, 496 22 of 26
species in CD biochars, while phenanthrene and fluoranthene are the main species in PW biochars
at low temperatures.
•
Both PW and CD biochars present mostly a hydrophobic behavior, although it can be reduced with
higher conversion temperatures and higher inert gas flow rates during pyrolysis, which reduces
the extent of secondary reactions and recondesation of volatiles.
Based on the previous conclusions, the following general recommendations for biochar and
hydrochar production can be derived. Pyrolysis temperatures above 550
◦
C guarantee low H/C
and O/C ratios, PAHs content below the limit according to quality criteria (EBC), independently
of the initial feedstock, higher SSAs and lower hydrophobicity. N
2
flow during pyrolysis can also
contribute to further microporosity development and hydrophobicity reduction. High inorganic
content in the feedstock potentially reduces SSA and hydrophobicity, but increases mesoporosity and
pH. With respect to hydrochars, the EBC requirement regarding elemental composition can only be
fulfilled for high temperature and residence times PW hydrochars. Hydrochars have a lower SSA
than biochars, but a higher mesoporosity, total pore volume and nutrient availability. Washing of
hydrochars after production further increases their surface area and pore volume. The results from this
work can be applied to “engineer” appropriate biochars with respect to soil demands and certification
requirements, as well as to improve biochar certification, and they are summarized in Figure 9.
inorganic content (%wt, db)
Volatiles content (%wt, daf)
Tem per atur e
Biochars
400 °C
550 °C
600 °C
P availability*
PAHs
PAHs àoptimum
PAHs àbasic
* Increase with higher inert gas flow rates
inorganic content (%wt, db)
Volatiles content (%wt, daf)
Tem per ature, residence time
Mesoporosity*
pH
* Increase with washing times
Hydrochars
K availability
Figure 9. Conclusions. The direction of the arrows indicates increasing values for all properties.
Supplementary Materials: The following are available online at www.mdpi.com/1996-1073/11/3/496/s1.
Acknowledgments:
The authors thank Susanne Hoffmann, Hernán Almuina Villar, Alba Domínguez Yebra,
Alberto García Rueda and Christina Eichenauer (Technische Universität Berlin) for their support with the analytic
characterization of biochars and hydrochars. The authors also thank the Leibniz Institute for Agricultural
Engineering and Bioeconomy (Potsdam, Germany) for preparation of hydrochar samples. The authors
acknowledge support by the German Research Foundation and the Open Access Publication Funds of Technische
Universität Berlin.
Author Contributions:
Alba Dieguez-Alonso, Axel Funke and Andrés Anca-Couce conceived and designed the
experiments; Alba Dieguez-Alonso and Axel Funke performed the pyrolysis and hydrothermal carbonization
experiments respectively; Alba Dieguez-Alonso, Axel Funke, Alessandro Girolamo Rombolà, Gerardo Ojeda
and Jörg Bachmann analyzed the different biochar and hydrochar properties, with the further support
from those mentioned in the acknowledgements; Alba Dieguez-Alonso, Andrés Anca-Couce, Axel Funke,
Alessandro Girolamo Rombolà, Gerardo Ojeda and Jörg Bachmann and Frank Behrendt analyzed the data.
Alba Dieguez-Alonso wrote the paper with important contributions from Andrés Anca-Couce and Axel Funke;
the rest of the co-authors reviewed the manuscript and contributed to it when necessary according to their data
analysis and interpretation. Frank Behrendt supervised the work.
Conflicts of Interest: The authors declare no conflict of interest.
Energies 2018,11, 496 23 of 26
Abbreviations
The following abbreviations are used in this manuscript:
CD Corn digestate
PW Pine wood
HTC Hydrothermal carbonization
PSD Pore size distribution
PAH Polycyclic aromatic hydrocarbon
USEPA United States Environmental Protection Agency
References
1.
European Biochar Certificate (EBC). European Biochar Certificate—Guidelines for Sustainable Production of
Biochar; Version 6.2E; European Biochar Foundation (EBC): Arbaz, Switzerland, 2012. Available online:
http://www.europeanbiochar.org/en/download (accessed on 4 February 2016).
2.
Standardized Product Definition and Product Testing Guidelines for Biochar That Is Used in Soil.
International Biochar Intiative. Version 2.1. 2015. Available online: http://www.biochar-international.org/
characterizationstandard (accessed on 19 October 2016).
3. Abiven, S.; Schmidt, M.; Lehmann, J. Biochar by design. Nat. Geosci. 2014,7, 326–327.
4.
Lee, M.E.; Park, J.H.; Chung, J.W. Adsorption of Pb(II) and Cu(II) by Ginkgo-Leaf-Derived Biochar Produced
under Various Carbonization Temperatures and Times. Int. J. Environ. Res. Public Health 2017,14, 1528.
5.
Frišták, V.; Pipíška, M.; Soja, G. Pyrolysis treatment of sewage sludge: A promising way to produce
phosphorus fertilizer. J. Clean. Prod. 2018,172, 1772–1778.
6.
Frišták, V.; Pipíška, M.; Lesný, J.; Soja, G.; Friesl-Hanl, W.; Packová, A. Utilization of biochar sorbents for
Cd
2+
, Zn
2+
, and Cu
2+
ions separation from aqueous solutions: Comparative study. Environ. Monit. Assess.
2014,187, 4093, doi:10.1007/s10661-014-4093-y.
7.
Inyang, M.I.; Gao, B.; Yao, Y.; Xue, Y.; Zimmerman, A.; Mosa, A.; Pullammanappallil, P.; Ok, Y.S.; Cao, X.
A review of biochar as a low-cost adsorbent for aqueous heavy metal removal. Crit. Rev. Environ. Sci. Technol.
2016,46, 406–433.
8.
Mohan, D.; Sarswat, A.; Ok, Y.S.; Charles, U.P., Jr. Organic and inorganic contaminants removal from
water with biochar, a renewable, low cost and sustainable adsorbent—A critical review. Bioresour. Technol.
2014,160, 191–202.
9.
Sohi, S.P.; Krull, E.; Lopez-Capel, E.; Bol, R. Chapter 2—A Review of Biochar and Its Use and Function in
Soil. In Advances in Agronomy; Sparks, D.L., Ed.; Academic Press: Cambridge, MA, USA, 2010; Volume 105,
pp. 47–82.
10.
Chan, K.Y.; Xu, Z. Nutrient Properties and Their Enhancement. In Biochar for Environmental Management.
Science and Technology; Earthscan: Abingdon, UK, 2009; Chapter 5, pp. 67–84, ISBN 978-1-84407-658-1.
11.
Thies, J.E.; Rillig, M.C. Characteristics of Biochar: Biological Properties. In Biochar for Environmental
Management. Science and Technolgy; Earthscan: Abingdon, UK, 2009; Chapter 6, pp. 85–105.
12.
Anca-Couce, A.; Dieguez-Alonso, A.; Zobel, N.; Berger, A.; Kienzl, N.; Behrendt, F. Influence of
Heterogeneous Secondary Reactions during Slow Pyrolysis on Char Oxidation Reactivity of Woody Biomass.
Energy Fuels 2017,31, 2335–2344.
13.
Yuan, J.H.; Xu, R.K.; Zhang, H. The forms of alkalis in the biochar produced from crop residues at different
temperatures. Bioresour. Technol. 2011,102, 3488–3497.
14.
Angst, T.E.; Sohi, S.P. Establishing release dynamics for plant nutrients from biochar. GCB Bioenergy
2013,5, 221–226.
15.
Zambon, I.; Colosimo, F.; Monarca, D.; Cecchini, M.; Gallucci, F.; Proto, R.A.; Lord, R.; Colantoni, A.
An Innovative Agro-Forestry Supply Chain for Residual Biomass: Physicochemical Characterisation of
Biochar from Olive and Hazelnut Pellets. Energies 2016,9, 526.
16.
Uchimiya, M.; Hiradate, S.; Antal, M.J. Dissolved Phosphorus Speciation of Flash Carbonization,
Slow Pyrolysis, and Fast Pyrolysis Biochars. ACS Sustain. Chem. Eng. 2015,3, 1642–1649.
17.
Hossain, M.K.; Strezov, V.; Chan, K.Y.; Ziolkowski, A.; Nelson, P.F. Influence of pyrolysis temperature on
production and nutrient properties of wastewater sludge biochar. J. Environ. Manag. 2011,92, 223–228.
Energies 2018,11, 496 24 of 26
18.
Mukherjee, A.; Zimmerman, A.; Harris, W. Surface chemistry variations among a series of
laboratory-produced biochars. Geoderma 2011,163, 247–255.
19.
Budai, A.; Wang, L.; Gronli, M.; Strand, L.T.; Antal, M.J.; Abiven, S.; Dieguez-Alonso, A.; Anca-Couce, A.;
Rasse, D.P. Surface Properties and Chemical Composition of Corncob and Miscanthus Biochars: Effects of
Production Temperature and Method. J. Agric. Food Chem. 2014,62, 3791–3799.
20. Lawrinenko, M.; Laird, D.A. Anion exchange capacity of biochar. Green Chem. 2015,17, 4628–4636.
21.
Schimmelpfennig, S.; Glaser, B. One step forward toward characterization: Some important material
properties to distinguish biochars. J. Environ. Qual. 2011,42, 1001–1013.
22.
Zhao, S.X.; Ta, N.; Wang, X.D. Effect of Temperature on the Structural and Physicochemical Properties of
Biochar with Apple Tree Branches as Feedstock Material. Energies 2017,10, 1293.
23.
Carrier, M.; Hardie, A.G.; Uras, A.; Görgens, J.; Knoetze, J.H. Production of char from vacuum pyrolysis
of South-African sugar cane bagasse and its characterization as activated carbon and biochar. J. Anal.
Appl. Pyrolysis 2012,96, 24–32.
24.
Gaskin, J.W.; Steiner, C.; Harris, K.; Das, K.C.; Bibens, B. Effect of low-temperature pyrolysis conditions on
biochar for agricultural use. Trans. ASABE 2008,51, 2061–2069.
25.
Keiluweit, M.; Nico, P.S.; Johnson, M.G.; Kleber, M. Dynamic Molecular Structure of Plant Biomass-Derived
Black Carbon (Biochar). Environ. Sci. Technol. 2010,44, 1247–1253.
26.
Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. Influence of pyrolysis conditions on the structure and gasification
reactivity of biomass chars. Fuel 2004,83, 2139–2150.
27.
Downie, A.; Crosky, A.; Munroe, P. Physical Properties of Biochar. In Biochar for Environmental Management.
Science and Technology; Earthscan: Abingdon, UK, 2009; Chapter 2, pp. 13–32, ISBN 978-1-84407-658-1.
28.
Liu, Z.; Zhang, F.S.; Wu, J. Characterization and application of chars produced from pinewood pyrolysis
and hydrothermal treatment. Fuel 2010,89, 510–514.
29.
Hagemann, N.; Joseph, S.; Schmidt, H.P.; Kammann, C.I.; Harter, J.; Borch, T.; Young, R.B.; Varga, K.;
Taherymoosavi, S.; Elliott, K.W.; et al. Organic coating on biochar explains its nutrient retention and
stimulation of soil fertility. Nat. Commun. 2017,8, 1089, doi:10.1038/s41467-017-01123-0.
30.
Van Zwieten, L.; Kimber, S.; Morris, S.; Chan, K.Y.; Downie, A.; Rust, J.; Joseph, S.; Cowie, A. Effects of
biochar from slow pyrolysis of papermill waste on agronomic performance and soil fertility. Plant Soil
2010,327, 235–246.
31.
McCormack, S.A.; Ostle, N.; Bardgett, R.D.; Hopkins, D.W.; Vanbergen, A.J. Biochar in bioenergy cropping
systems: Impacts on soil faunal communities and linked ecosystem processes. GCB Bioenergy
2013
,5, 81–95.
32.
Antal, M.J.; Grønli, M. The Art, Science, and Technology of Charcoal Production. Ind. Eng. Chem. Res.
2003,42, 1619–1640.
33.
Brassard, P.; Godbout, S.; Raghavan, V.; Palacios, H.J.; Grenier, M.; Zegan, D. The Production of Engineered
Biochars in a Vertical Auger Pyrolysis Reactor for Carbon Sequestration. Energies 2017,10, 288.
34.
Titirici, M.M.; White, R.J.; Falco, C.; Sevilla, M. Black perspectives for a green future: Hydrothermal carbons
for environment protection and energy storage. Energy Environ. Sci. 2012,5, 6796–6822.
35.
Huff, M.D.; Kumar, S.; Lee, J.W. Comparative analysis of pinewood, peanut shell, and bamboo biomass
derived biochars produced via hydrothermal conversion and pyrolysis. J. Environ. Manag.
2014
,146, 303–308.
36.
Cao, X.; Ro, K.S.; Chappell, M.; Li, Y.; Mao, J. Chemical Structures of Swine-Manure Chars Produced under
Different Carbonization Conditions Investigated by Advanced Solid-State 13C Nuclear Magnetic Resonance
(NMR) Spectroscopy. Energy Fuels 2011,25, 388–397.
37.
Wiedner, K.; Naisse, C.; Rumpel, C.; Pozzi, A.; Wieczorek, P.; Glaser, B. Chemical modification of
biomass residues during hydrothermal carbonization—What makes the difference, temperature or feedstock?
Org. Geochem. 2013,54, 91–100.
38.
Busch, D.; Stark, A.; Kammann, C.I.; Glaser, B. Genotoxic and phytotoxic risk assessment of fresh and treated
hydrochar from hydrothermal carbonization compared to biochar from pyrolysis. Ecotoxicol. Environ. Saf.
2013,97, 59–66.
39.
Busch, D.; Glaser, B. Stability of co-composted hydrochar and biochar under field conditions in a temperate
soil. Soil Use Manag. 2015,31, 251–258.
40.
Dieguez-Alonso, A.; Anca-Couce, A.; Zobel, N. On-line tar characterization from pyrolysis of wood particles
in a technical-scale fixed-bed reactor by applying Laser-Induced Fluorescence (LIF). J. Anal. Appl. Pyrolysis
2013,102, 33–46.
Energies 2018,11, 496 25 of 26
41.
Naumann, C.; Bassler, R.; Seibold, R.; Barth, C. Methodenbuch. Band III, Die Chemische Untersuchung von
Futtermitteln; VDLUFA-Verlag: Darmstadt, Germany, 1997.
42.
Boehm, H.P.; Diehl, E.; Heck, W.; Sappok, R. Surface Oxides of Carbon. Angew. Chem. Int. Ed.
1964
,3, 669–677.
43.
Goertzen, S.L.; Theriault, K.D.; Oickle, A.M.; Tarasuk, A.C.; Andreas, H.A. Standardization of the Boehm
titration. Part I. CO2expulsion and endpoint determination. Carbon 2010,48, 1252–1261.
44.
Tsechansky, L.; Graber, E.R. Methodological limitations to determining acidic groups at biochar surfaces via
the Boehm titration. Carbon 2014,66, 730–733.
45.
Brunauer, S.; Emmett, P.H.; Teller, E. Adsorption of Gases in Multimolecular Layers. J. Am. Chem. Soc.
1938,60, 309–319.
46.
Barrett, E.P.; Joyner, L.G.; Halenda, P.P. The Determination of Pore Volume and Area Distributions in Porous
Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951,73, 373–380.
47.
Seaton, N.; Walton, J.; Quirke, N. A new analysis method for the determination of the pore-size distribution
of porous carbons from nitrogen adsorption measurements. Carbon 1989,27, 853–861.
48.
Hoffmann, G. Methodenbuch. Band I, Die Untersuchung von Böden; VDLUFA-Verlag: Darmstadt, Germany, 1991.
49.
Fabbri, D.; Rombola, A.G.; Torri, C.; Spokas, K.A. Determination of polycyclic aromatic hydrocarbons in
biochar and biochar amended soil. J. Anal. Appl. Pyrolysis 2013,103, 60–67.
50.
Ojeda, G.; Mattana, S.; Avila, A.; Alcaniz, J.M.; Volkmann, M.; Bachmann, J. Are soil-water functions affected
by biochar application? Geoderma 2015,249–250, 1–11.
51.
Krull, E.S.; Baldock, J.A.; Skjemstad, J.O.; Smernik, R.J. Characteristics of Biochar: Organo-Chemical
Properties. In Biochar for Environmental Management. Science and Technology; Earthscan: Abingdon, UK,
2009; Chapter 4, pp. 53–65, ISBN 978-1-84407-658-1.
52.
Rombolà, A.G.; Fabbri, D.; Meredith, W.; Snape, C.E.; Dieguez-Alonso, A. Molecular characterization of
the thermally labile fraction of biochar by hydropyrolysis and pyrolysis-GC/MS. J. Anal. Appl. Pyrolysis
2016,121, 230–239.
53.
Mumme, J.; Eckervogt, L.; Pielert, J.; Diakité, M.; Rupp, F.; Kern, J. Hydrothermal carbonization of
anaerobically digested maize silage. Bioresour. Technol. 2011,102, 9255–9260.
54.
Li, F.; Cao, X.; Zhao, L.; Wang, J.; Ding, Z. Effects of Mineral Additives on Biochar Formation: Carbon
Retention, Stability, and Properties. Environ. Sci. Technol. 2014,48, 11211–11217.
55.
Liu, W.J.; Jiang, H.; Yu, H.Q. Development of Biochar-Based Functional Materials: Toward a Sustainable
Platform Carbon Material. Chem. Rev. 2015,115, 12251–12285.
56.
Yin, S.; Mehrotra, A.K.; Tan, Z. Alkaline hydrothermal conversion of cellulose to bio-oil: Influence of
alkalinity on reaction pathway change. Bioresour. Technol. 2011,102, 6605–6610.
57.
Pulido-Novicio, L.; Hata, T.; Kurimoto, Y.; Doi, S.; Ishihara, S.; Imamura, Y. Adsorption capacities and
related characteristics of wood charcoals carbonized using a one-step or two-step process. J. Wood Sci.
2001,47, 48–57.
58.
Nguyen, B.T.; Lehmann, J. Black carbon decomposition under varying water regimes. Org. Geochem.
2009,40, 846–853.
59.
Baldock, J.A.; Smernik, R.J. Chemical composition and bioavailability of thermally altered Pinus resinosa
(Red pine) wood. Org. Geochem. 2002,33, 1093–1109.
60.
Lee, J.W.; Kidder, M.; Evans, B.R.; Paik, S.; Buchanan, A.C., III; Garten, C.T.; Brown, R.C. Characterization of
Biochars Produced from Cornstovers for Soil Amendment. Environ. Sci. Technol. 2010,44, 7970–7974.
61.
Ronsse, F.; van Hecke, S.; Dickinson, D.; Prins, W. Production and characterization of slow pyrolysis biochar:
Influence of feedstock type and pyrolysis conditions. GCB Bioenergy 2013,5, 104–115.
62.
Yang, F.; Zhao, L.; Gao, B.; Xu, X.; Cao, X. The Interfacial Behavior between Biochar and Soil Minerals and
Its Effect on Biochar Stability. Entviron. Sci. Technol. 2016,50, 2264–2271.
63.
Guizani, C.; Jeguirim, M.; Valin, S.; Limousy, L.; Salvador, S. Biomass Chars: The Effects of Pyrolysis
Conditions on Their Morphology, Structure, Chemical Properties and Reactivity. Energies 2017,10, 796.
64.
Anca-Couce, A. Reaction mechanisms and multi-scale modelling of lignocellulosic biomass pyrolysis.
Prog. Energy Combust. Sci. 2016,53, 41–79.
65.
Harvey, O.R.; Kuo, L.J.; Zimmerman, A.R.; Louchouarn, P.; Amonette, J.E.; Herbert, B.E. An Index-Based
Approach to Assessing Recalcitrance and Soil Carbon Sequestration Potential of Engineered Black Carbons
(Biochars). Environ. Sci. Technol. 2012,46, 1415–1421.
Energies 2018,11, 496 26 of 26
66.
Kuo, L.J.; Herbert, B.E.; Louchouarn, P. Can levoglucosan be used to characterize and quantify char/charcoal
black carbon in environmental media? Org. Geochem. 2008,39, 1466–1478.
67.
Plante, A.F.; Fernández, J.M.; Haddix, M.L.; Steinweg, J.M.; Conant, R.T. Biological, chemical and thermal
indices of soil organic matter stability in four grassland soils. Soil Biol. Biochem. 2011,43, 1051–1058.
68.
Zimmerman, A.R. Abiotic and Microbial Oxidation of Laboratory-Produced Black Carbon (Biochar). Environ.
Sci. Technol. 2010,44, 1295–1301.
69.
Funke, A. Fate of Plant Available Nutrients during Hydrothermal Carbonization of Digestate. Chem. Ing. Tech.
2015,87, 1713–1719.
70.
Singh, B.; Singh, B.P.; Cowie, A.L. Characterisation and evaluation of biochars for their application as a soil
amendment. Aust. J. Soil Res. 2010,48, 516–525.
71.
Funke, A.; Mumme, J.; Koon, M.; Diakité, M. Cascaded production of biogas and hydrochar from wheat
straw: Energetic potential and recovery of carbon and plant nutrients. Biomass Bioenergy 2013,58, 229–237.
72.
Kruse, A.; Koch, F.; Stelzl, K.; Wüst, D.; Zeller, M. Fate of Nitrogen during Hydrothermal Carbonization.
Energy Fuels 2016,30, 8037–8042.
73.
Dieguez-Alonso, A.; Anca-Couce, A.; Zobel, N.; Behrendt, F. Understanding the primary and
secondary slow pyrolysis mechanisms of holocellulose, lignin and wood with laser-induced fluorescence.
Fuel 2015,153, 102–109.
74.
Buss, W.; Graham, M.C.; MacKinnon, G.; Masek, O. Strategies for producing biochars with minimum {PAH}
contamination. J. Anal. Appl. Pyrolysis 2016,119, 24–30.
c
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
