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Hydrothermal treatment of industrial solid wastes
under subcritical conditions
Mikko Mäkelä
Research Fellow
Division of Biomass Technology and Chemistry
Department of Forest Biomaterials and Technology
Swedish University of Agricultural Sciences
Umeå, Sweden
2015-04-08
Summary
Hydrothermal carbonization seems promising for converting sludge biomass to a
solid biocoal product approaching the characteristics of low-rank natural coals, and
thereby increasing potential use in energy production and providing significant ease
in handling, storage and transport. The carbonization of two different pulp and paper
mill sludge residues was hence investigated with a laboratory-scale pressure reactor.
The experiments were performed on fibre reject and mixed sludge from SCA Obbola
AB at the Department of Chemical Engineering, University of Alicante, Spain during
September-December, 2014.
Both sludge types are carbonizable with the HTC technique as indicated by the
results. Although reaction temperature mainly governs carbonization reactions, both
reaction temperature (180-260 °C) and retention time (0.5-6.25 h) had a significant
effect on hydrochar properties. The carbon contents of produced hydrochars were
significantly increased and the respective oxygen contents were comparable
subbimituminous coal at higher reaction temperatures. Increasing reaction
temperature also increased biomass dissolution and thus decreased respective
hydrochar energy yields. Analysed BOD/COD –ratios of the reaction medium
suggested that higher temperatures increased the dissolution of non-biodegradable
biomass components from fibre reject. Sludge ash was found slightly soluble only in
the case of fibre reject, insolubility likely caused by the presence of calcium
carbonate, aluminosilicates and quartz. The phosphorus in mixed sludge remained
insoluble during the experiments.
Further experiments are suggested for investigating the use of acid or base catalysts
during sludge HTC and for measuring the energy efficiency of sludge carbonization.
2015-04-08
List of contents
1. Background ........................................................................................................... 4
2. Project description ................................................................................................. 6
3. Project partners ..................................................................................................... 7
4. Project results ........................................................................................................ 8
4.1 Fibre reject ....................................................................................................... 8
4.2 Mixed sludge ................................................................................................. 11
5. Discussion of results ........................................................................................... 13
6. Future work .......................................................................................................... 15
7. Project costs ........................................................................................................ 16
8. Publications and confidentiality ........................................................................... 17
Literature cited ............................................................................................................ 18
Appendix 1
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1. Background
Hydrothermal treatment, combining elevated temperature and pressure, is generally
performed with water acting as a solvent, a reactant and even a catalyst or catalyst
precursor (Fangming et al., 2014) and hence does not require prior drying of a
feedstock or the use of e.g. organic solvents or enzymes. Biomass can be
hydrothermally treated in a range of conditions, sub- and supercritical conditions
separated by the critical temperature (374 °C) and pressure (22 MPa) of water, see
Fig. 1. The reaction temperature can be further limited to approximately 180-260 °C
to minimize the liquefaction and gasification of biomass components thus maximizing
solid recovery through a process known as hydrothermal carbonization, HTC. Under
these conditions the dielectric constant of water is significantly reduced enabling
properties to organic solvents at room temperature while the increasing ionization
constant favours reactions which are typically catalyzed by acids or bases (Peterson
et al., 2008; Toor et al., 2011). Hydrothermal carbonization of biomass was first
reported already in the early 20th century as a method for simulating natural
coalification (Ruyter, 2011), but has not received significant attention for upgrading
various biomass and waste feedstocks until the recent 5-10 years.
Hydrothermal carbonization seems feasible for upgrading especially wet, low-value
fuels by increasing respective energy density, decreasing oxygen and volatile
contents and enhancing drying properties (Funke & Ziegler, 2010; Libra et al., 2011).
Especially for sludge, a low-cost waste feedstock, HTC could be used to convert
biomass to a solid biocoal product approaching the characteristics of low-rank natural
coals thereby increasing potential use in energy production and providing significant
ease in handling, storage and transport. As HTC is ideally operated under saturated
steam pressure (Fig. 1), the energy required for evaporation can be avoided making
the theoretical energy requirement for heating the reaction medium significantly lower
compared to active drying (Peterson et al., 2008; Yoshikawa & Prawisudha, 2014).
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Energy is still required for post-treatment separation of the solid and liquid phases,
but can be significantly lower compared to conventional sludge drying due to e.g.
potential sludge cell breakage and enhanced drying characteristics (Zhao et al.,
2014).
Fig. 1: Water properties as function of temperature and pressure (Lide, 2005), HTC
conditions marked with red.
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2. Project description
This project concentrated on the hydrothermal treatment of industrial sludge for
potential production of waste-derived solid biofuels. Laboratory-scale HTC
experiments (Fig. 2) were performed on fibre reject and mixed sludge provided by
SCA Obbola AB at the Department of Chemical Engineering, University of Alicante
during September-December 2014. Important HTC process parameters, such as
reaction temperature, retention time, reactor solid load and the use of additional
acid/base catalysts (see Tables A1 and A2, Appendix 1) were included as
controlled variables in the adopted experimental designs. Vital response variables,
such as the fuel properties of the attained solid hydrochar, organic carbon loss to the
aqueous medium and ash content and composition of the solid were statistically
evaluated and modelled through the use of response surface methodology (RSM).
Fig. 2: Images of the laboratory HTC procedure. From left to right: HTC reactor;
carbonized sludge suspension; vacuum filtering and; sludge hydrochar.
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3. Project partners
The following partners contributed to the execution of this project:
Swedish University of Agricultural Sciences (SLU), Department of Forest
Biomaterials and Technology, contact person: Mikko Mäkelä (Tel.: +46
(0)72 223 4035, Email: [email protected])
SCA Obbola AB, (SCA), contact person: Nils Gilenstam (Tel.: +46 (0)90
15 400, Email: [email protected])
Metsä Board Sverige AB, Husum (Husum), contact person: Tobias Rudh
(Tel.: +46 (0)72 247 2711, Email: [email protected])
SP Processum AB (Processum), contact person: Liselotte Uhlir (Tel.: +46
(0)70 513 3872, Tmail: [email protected])
University of Alicante (UA), Department of Chemical Engineering, contact
person: Andrés Fullana (Tel.: +34 600 948 787, Email:
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4. Project results
4.1 Fibre reject
The combined solid and liquid recoveries during the fibre reject experiments were at
or above 93% (wb) during the experiments, the rest of which can be allocated to gas
production (74% CO2, 11% N2 and 10% CO). Dry solids contents of filtered
hydrochar samples were in the range of 39-65% with respective solid yields of 59-
98% (db). Reaction temperature correlated with increasing ash content of dried
hydrochar, ranging from 48 to 67% (db). Resulting ash recoveries varied between 82-
102% (db) with an inverse correlation with increasing reaction temperature (p < 0.01).
XRF results indicated that sludge ash was mainly composed of CaO, SiO2, Al2O3 with
minor amounts of MgO, TiO2, Fe2O3, SO3 and K2O. Assuming CaO was present as
calcium carbonate (CaCO3), Al2O3 as kaolinite (Al2Si2O5(OH)4) and the remaining
SiO2 as quartz (SiO2), the respective mineral phases constituted 46, 15 and 8% of
the original feed ash. Only the carbonate content was found inversely correlated with
reaction temperature (p < 0.05), decreasing to a mean value of 41% at 260 °C. No
correlation between process parameters and the content of kaolinite or quartz was
found
The carbon content, O/C –ratio, higher heating value and calculated energy
densification of attained fibre reject hydrochar, see Table A1 (Appendix 1), showed
no apparent change with reaction temperature, retention time or liquid to solid -ratio
until respective corrections for increasing ash contents were made. The resulting
corrected, dry ash-free carbon contents ranged from 37-87% with respective O/C –
ratios of 0.05-1.2. Heating values and energy densification ratios for the dry-ash free
hydrochars varied respective in ranges of 19.9-30.7 MJ kg-1 and 1.0-1.6. In addition,
the corrected dry ash-free solid yields were in the range of 37-95%. Energy yields,
derived from hydrochar mass yields and energy densification ratios, were in the
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range of 54-98%. Predicted response values of the fibre reject experiments are given
in Figs. 3 and 4.
Fig. 3: Model results on fibre reject: a) hydrochar solid content (%), b) ash content
(%), c) solid yield (%, daf) and d) carbon content (%, daf) on minimum liquid to solid
–ratios.
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Fig. 4: Model results on fibre reject: a) hydrochar O/C –ratio (daf), b) energy
densification (daf), c) energy yield (%) and d) liquid chemical oxygen demand, COD
(mg g-1 sample daf) on minimum liquid to solid –ratios.
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4.2 Mixed sludge
The combined solid and liquid recoveries during the mixed sludge experiments were
at or above 91% (wb) during the experiments, the rest of which can be allocated to
gas production (mainly 75% CO2, 19% N2 and 3% CO). Dry solids contents of filtered
hydrochar samples were in the range of 23-53% with respective solid yields of 64-
96% (db). Reaction temperature correlated with increasing ash content of dried
hydrochar, ranging from 35 to 48% (db). Resulting ash recoveries varied between 81-
99% (db) with an inverse correlation with increasing reaction temperature (p < 0.05).
XRF results indicated that sludge ash was mainly composed of SiO2, Al2O3, Fe2O3
and P2O5 with minor amounts of CaO, SO3, K2O and TiO2. Assuming Al2O3 was
present as kaolinite (Al2Si2O5(OH)4) and the remaining SiO2 as quartz (SiO2), the
respective mineral phases constituted 52 and 19% of original feed ash, respectively.
No correlation between process parameters and the content of kaolinite or quartz
was found. It should be noted that the P2O5 content of mixed sludge was found
insoluble during experiments at 260 °C. The measured values were in the range of
13-15% of hydrochar ash, compared to 12% of the ash in the feed. Only four
hydrochar ash samples were analysed with XRF after the experiments on mixed
sludge.
Similar to fibre reject, the carbon content, O/C –ratio, higher heating value and
calculated energy densification of attained mixed sludge hydrochar, see Table A2
(Appendix 1), showed no apparent change with reaction temperature, retention time
or liquid to solid -ratio until respective corrections for increasing ash contents were
made. The resulting corrected, dry ash-free carbon contents ranged from 43-84%
with respective O/C –ratios of 0.01-0.9. Heating values and energy densification
ratios for the dry-ash free hydrochars varied respective in ranges of 24.2-33.1 MJ kg-
1 and 1.0-1.3. In addition, the corrected dry ash-free solid yields were in the range of
54-95%. Energy yields, derived from hydrochar mass yields and energy densification
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ratios, were in the range of 65-97%. The predicted response values based on the
experimental mixed sludge results are given in Figs. 5 and 6.
Fig. 5: Model results on mixed sludge: a) hydrochar solid content (%), b) ash content
(%), c) solid yield (%, daf) and d) carbon content (%, daf) on 75 mL HCl (0.01 N)
addition.
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Fig. 6: Model results on mixed sludge: a) hydrochar O/C –ratio (daf), b) energy
densification (daf), c) energy yield (%) and d) liquid chemical oxygen demand, COD
(mg g-1 sample daf) on 75 mL HCl (0.01 N) addition.
5. Discussion of results
Reaction temperature (180-260 °C) and retention time (1-6.25 h) were found
statistically significant (p < 0.05) for all response models on HTC of fibre reject, as
reactor solid load was insignificant for all acquired models within the original design
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(13-20% dry solids). In the case of mixed sludge, retention time (0.5-5 h) was
insignificant (p > 0.05) for hydrochar carbon content, O/C –ratio and energy
densification as catalyst type was insignificant for all acquired models. With both
sludge types, reaction temperature seemed to govern carbonization reactions and
the properties of produced hydrochar. It must be noted however, that the reliability of
acquired mixed sludge hydrochar models was slightly decreased due to the inclusion
of catalyst as a qualitative factor in the design.
Based on the results both sludge types are carbonizable with the HTC technique. As
illustrated in Figs. 3-6, the carbon contents of the dry, ash-free hydrochars were
significantly increased and the respective oxygen contents were comparable
subbimituminous coal at higher reaction temperatures. Higher temperatures also
increased biomass dissolution and thus decreased respective hydrochar energy
yields. The dissolution of biomass components also increased the COD
concentrations of the reaction medium. In addition to COD, four biological oxygen
demand (BOD) measurements were performed within both designs and resulting
BOD/COD -ratios were in the range of 0.1-0.4 and 0.4-0.6 for fibre reject and mixed
sludge, respectively. Especially with fibre reject higher reaction temperatures
increased the dissolution of non-biodegradable components. Sludge ash was found
slightly soluble only in the case of fibre reject, insolubility likely caused by the
presence of aluminosilicates and quartz. The phosphorus in mixed sludge also
remained insoluble based on the data.
No models on the energy efficiency of the HTC process were included in this report,
as no widely accepted method currently exists for calculating the energy requirement
of the HTC process. At least three different methods have thus far been presented
(see e.g. Areeprasert et al., 2014; Benavente et al., 2014; Wang et al., 2014), based
on specific heat capacity of water, the enthalpies of saturated liquid (water) or the
enthalpies of saturated liquid (water) and gas (water). It is thus recommended that
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work on sludge HTC is continued and attemps on measuring respective energy
requirement are incorporated especially on larger scale investigations.
6. Future work
Funding to continue work on HTC of pulp and paper mill residues has been applied
through an application to Energimyndigheten submitted in the beginning of March,
2015. In the application SLU, Umeå University, Miljöteknisk Centrum, SP Processum,
Metsä Board Husum and SCA Obbola, are looking for possibilities to investigate the
the use HTC for the production of biocoal for replacement of fossil fuels or use in
novel environmental applications. The applied project would run through 2016-2019
with a total budget of 10,7 MSEK. Results of the application process will be published
during June, 2015.
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7. Project costs
The total costs of the project amounted to 43,386 SEK compared with a budgeted
amount of 49,000 SEK. The costs are detailed in Table 1.
Table 1: Project costs.
Activity Detail Cost Budget Surplus
Köpta tjänster
Analysuppdrag -4 009
Analysuppdrag -3 819
Analysuppdrag -19 072
Analysuppdrag -26 900
Driftkostnader
Kontorsmaterial -64
Telekostnader och TV-avgifter -1 903
Porto -1 681
Resor
Frakter och transporter -1 316
Resa, biljetter mm, utrikes -3 732
Resa, hotell o logi ouppdelat -7 791
Total -43 387
49 000 5 613
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8. Publications and confidentiality
The results of this project will be published in the form of at least two peer-reviewed
journal publications most likely focusing on the effect process conditions on
hydrochar properties, the use acid or base catalysts during sludge HTC and
enhancing the drying characteristics of carbonized sludge:
(i) Mäkelä M, Benavente V, Fullana A, Hydrothermal carbonization of lignocellulosic
biomass: effect of process conditions and prediction of hydrochar properties,
submitted to Applied Energy.
(ii) Mäkelä M, Benavente V, Fraikin L, Fullana A, Hydrothermal carbonization and
drying behaviour of mixed pulp and paper sludge (working title), will be written during
June-August, 2015.
Due to the publication of project results as journal articles, it is requested that
publication elsewhere is postponed until the respective journal articles have been
accepted for publication. No pending patent cases exist based on the results.
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Literature cited
Areeprasert C, Zhao P, Ma D, Shen Y, Yoshikawa K. Alternative solid fuel production
from paper sludge employing hydrothermal treatment. Energ Fuel 2014;28:1198-206.
Benavente V, Calabuig E, Fullana A. Upgrading of moist agro-industrial wastes by
hydrothermal treatment. J Anal Appl Pyrol 2014, in press,
DOI:10.11015/j.jaap.2014.11.004.
Fangming J, Yuanqing W, Xu Z, Zheng S, Guodong Y. Water under high temperature
and pressure conditions and its applications to develop green technologies for
biomass conversion. In: Fangming J (editor) Application of hydrothermal reactions to
biomass conversion, Berlin Heiderberg: Springer-Verlag; 2014, p. 3-28.
Funke A, Ziegler F. Hydrothermal carbonization of biomass: A summary and
discussion of chemical mechanisms for process engineering. Biofuels Bioprod
Biorefin 2010;4:160-77.
Libra JA, Ro KS, Kammann C, Funke A, Berge ND, Neubauer Y, Titirici M-M, Fühner
C, Bens O, Kern J, Emmerich K-H. Hydrothermal carbonization of biomass residuals:
a comparative review of the chemistry, processes and applications of wet and dry
pyrolysis. Biofuels 2011;2:71-106.
“Vapor pressure of water from 0 to 370 °C”. In: Lide R (editor) CRC Handbook of
Chemistry and Physics, Boca Raton: CRC Press; 2005, p. 6-8.
Peterson A, Vogel F, Lachance RP, Fröling M, Antal Jr MJ, Tester JW.
Thermochemical biofuel production in hydrothermal media: A review of sub- and
supercritical water technologies. Energ Environ Sci 2008;1:32-65.
Ruyter HP. Coalification model. Fuel 1982;61:1182-7.
Toor SS, Rosendahl L, Rudolf A. Hydrothermal liquefaction of biomass: A review of
subcritical water technologies. Energy 2011;36:2328-42.
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Wang L, Zhang L, Li A. Hydrothermal treatment coupled with mechanical expression
at increased temperature for excess sludge dewatering: influence of operating
conditions and the process energetics. Water Res 2014;65:85-97.
Yoshikawa K, Prawisudha P. Sewage sludge treatment by hydrothermal process for
producing solid fuel. In: Fangming J (editor) Application of hydrothermal reactions to
biomass conversion, Berlin Heiderberg: Springer-Verlag; 2014, p. 385-409.
Zhao R, Ge S, Ma D, Areeprasert C, Yoshikawa K. Effect of hydrothermal
pretreatment on convective drying characteristics of paper sludge. ACS Sustain
Chem Eng 2014;2:665-71.
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Appendix 1
Table A1: Selected process and response variables during the experiments on fibre
reject.
Parameter Unit/specification Range
Reaction temperature °C 180-260
Retention time h 1-6.25
Liquid to solid -ratio - 1-2
Reactor dry solids %, wb 13-20
Solid + liquid recovery %, wb 93-96
Hydrochar dry solids content % 39-65
Solid yield %, db 59-98
Solid yield %, daf 37-95
Ash content %, db 48-67
Ash yield %, db 82-102
Carbon content %, db 19-30
Carbon content %, daf 37-87
O/C –ratio daf 0.1-1.2
Higher heating value (HHV) MJ kg-1, db 9.6-11
Higher heating value (HHV) MJ kg-1, daf 20-31
Energy densification db 0.9-1.0
Energy densification daf 1.0-1.6
Energy yield % 54-98
wb = wet basis
db = dry basis
daf = dry ash-free
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Table A2: Selected process and response variables during the experiments on
mixed sludge.
Parameter Unit/specification Range
Reaction temperature °C 180-260
Retention time h 0.5-5
Catalyst addition - HCl / H2O
/ NaOH
Reactor dry solids %, wb 20
Solid + liquid recovery %, wb 91-94
Hydrochar dry solids content % 23-53
Solid yield %, db 64-96
Solid yield %, daf 35-95
Ash content %, db 35-48
Ash yield %, db 81-99
Carbon content %, db 27-45
Carbon content %, daf 43-84
O/C –ratio daf 0.01-0.86
Higher heating value (HHV) MJ kg-1, db 15-18
Higher heating value (HHV) MJ kg-1, daf 24-33
Energy densification db 0.9-1.1
Energy densification daf 1.0-1.3
Energy yield % 65-97
wb = wet basis
db = dry basis
daf = dry ash-free
