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Magdeldin , Mohamed; Kohl, Thomas; De Blasio, Cataldo; Järvinen, Mika; Won Park, Song;
Giudici, Reinaldo
The BioSCWG Project: Understanding the Trade-Offs in the Process and Thermal Design of
Hydrogen and Synthetic Natural Gas Production
Published in:
Energies
DOI:
10.3390/en9100838
Published: 01/01/2016
Document Version
Publisher's PDF, also known as Version of record
Published under the following license:
CC BY
Please cite the original version:
Magdeldin , M., Kohl, T., De Blasio, C., Järvinen, M., Won Park, S., & Giudici, R. (2016). The BioSCWG Project:
Understanding the Trade-Offs in the Process and Thermal Design of Hydrogen and Synthetic Natural Gas
Production. Energies, 9(10), 1-27. [838]. https://doi.org/10.3390/en9100838
energies
Article
The BioSCWG Project: Understanding the Trade-Offs
in the Process and Thermal Design of Hydrogen and
Synthetic Natural Gas Production
Mohamed Magdeldin
1,
*, Thomas Kohl
1
, Cataldo De Blasio
1,2
, Mika Järvinen
1
,
Song Won Park
3
and Reinaldo Giudici
3
1
Department of Mechanical Engineering, School of Engineering, Aalto University, Aalto FI-00076, Finland;
thomas.kohl@aalto.fi (T.K.); cataldo.de.blasio@aalto.fi (C.D.B.); mika.jarvinen@aalto.fi (M.J.)
2
Department of Chemical Engineering, Åbo Akademi University, Turku 20500, Finland
3
Department of Chemical Engineering, Universidade de São Paulo, São Paulo 05508-010, Brazil;
sonwpark@usp.br (S.W.P.); rgiudici@usp.br (R.G.)
* Correspondence: mohamed.magd@aalto.fi; Tel.: +358-41-470-7014
Academic Editor: Thomas E. Amidon
Received: 29 August 2016; Accepted: 29 September 2016; Published: 18 October 2016
Abstract:
This article presents a summary of the main findings from a collaborative research project
between Aalto University in Finland and partner universities. A comparative process synthesis,
modelling and thermal assessment was conducted for the production of Bio-synthetic natural gas
(SNG) and hydrogen from supercritical water refining of a lipid extracted algae feedstock integrated
with onsite heat and power generation. The developed reactor models for product gas composition,
yield and thermal demand were validated and showed conformity with reported experimental results,
and the balance of plant units were designed based on established technologies or state-of-the-art
pilot operations. The poly-generative cases illustrated the thermo-chemical constraints and design
trade-offs presented by key process parameters such as plant organic throughput, supercritical water
refining temperature, nature of desirable coproducts, downstream indirect production and heat
recovery scenarios. The evaluated cases favoring hydrogen production at 5 wt. % solid content
and 600
◦
C conversion temperature allowed higher gross syngas and CHP production. However,
mainly due to the higher utility demands the net syngas production remained lower compared to
the cases favoring BioSNG production. The latter case, at 450
◦
C reactor temperature, 18 wt. % solid
content and presence of downstream indirect production recorded 66.5%, 66.2% and 57.2% energetic,
fuel-equivalent and exergetic efficiencies respectively.
Keywords:
supercritical water gasification; lipid extracted algae; polygeneration; synthetic natural
gas (SNG); hydrogen; thermodynamic assessment
1. Introduction
The recent announcement at the conclusion of the Paris 2015 United Nations Climate Change
Conference, signed by more than 190 countries, recognized the irreversible negative impact on the
environment by predating and existing industrialization. The adapted emission mitigation pledges
and green energy policy frameworks signaled the growing global awareness and emphasis on the
need for environmentally-conscious human progress and development [
1
]. Bio-based primary energy
sources are considered a renewable source of carbon suitable to replace fossil fuel consumption, and
have the additional potential, due to photosynthetic activity, to act as an emission sink within a carbon
balanced life-cycle. As of today, biomass only constitutes around 10% of the global primary energy
consumption, mainly in the form of traditional heat generation. However, it is expected to expand
Energies 2016, 9, 838; doi:10.3390/en9100838 www.mdpi.com/journal/energies
Energies 2016, 9, 838 2 of 27
into a more integral role within the envisaged environmentally-sustainable energy and chemicals
production sectors, as part of a future emission-constrained economy [2].
Bioenergy production and utilization, in both its traditional and more developed sense, 2nd and
3rd generation biofuels, includes the thermo- and bio-chemical transformation of solid feedstocks into a
broader range of secondary energy carriers. In addition to electricity and heat as traditional coproducts,
a variety of liquid and gaseous fuels or bio-chemical materials can be synthesized as part of future
poly-generative bio-refinery concepts. Several interlinked socio-economic barriers and environmental
concerns restrict the wider deployment and market penetration of the envisaged systems. However,
some of the main challenges and drawbacks remain at heart of technical nature; such as resource
efficient processing and integration within existing infrastructure, both of which could be addressed
through the advancement of innovative conversion technologies [2,3].
Hydrothermal treatment is one of such technologies that has garnered significant attention within
scientific and industrial circles in more recent years, especially the supercritical water gasification
(SCWG) process. Here, the solid carbonaceous feedstock is subjected to pressurized aqueous processing,
near to- and beyond the critical point of water at 374
◦
C and 221 bar. At such acute conditions, above
the vapor pressure of water, the organic constituents of biomass are decomposed and valorized into a
wider range of value added products. Extensive reviews on technology progress, the advantageous
nature of supercritical water’s chemical and physical properties and the up-to-date research findings
can be found in the publications of Yakaboylo et al. [
4
], Akiya et al. [
5
], Kruse et al. [
6
,
7
], Brunner [
8
],
Loppinet-Serani et al. [9], Knez et al. [10], Peterson et al. [11] and many others.
Yakaboylo et al. [
4
] identified the varying properties of viscosity, density and dielectric constant
of water around the critical point as the main reason behind it’s suitability for biomass refining. Liquid
water (at 25
◦
C and 1 bar), when upgraded into supercritical conditions, loses its characteristically polar
tendency due to the destabilization of hydrogen bonds and, the acquirement of vapor-like density,
density dependent di-electric constant and ionic product k
w
properties [
4
,
5
]. Kruse [
6
] summarized it
as “in biomass conversion processes [
. . .
] water (in supercritical form) fulfills every possible role it is
able to fulfill: It is solvent, catalyst or catalyst precursor, and reactant”. The non-polar nature as well
as the richer medium of [H
3
O]
+
and [OH]
−
ions in supercritical water allows for higher miscibility,
selective reactive medium and homogenous processing for the organic constituents without significant
mass or heat transfer limitations [
5
,
7
]. Inorganic or polar constituents of biomass on the other hand are
relatively easily extracted, which fulfills many of the existing processing requirements. Brunner [
8
]
in his review examined the various industrial applications in which the properties of supercritical
fluids in general and for water specifically would provide a technical opportunity. The solubility of
de-polymerized intermediates, some of which are char and tar precursors that account for conversion
inefficiencies in more traditional processes, lead to higher recovery and utilization of the chemical
energy initially stored within the solid feedstock. This feature was attributed by Loppinet-Serani [
9
] as
the main motivation behind the current interest in SCWG and its already commercially available and
energetically self-sufficient sister application of supercritical water oxidation (SCWO). The latter which
is utilized for the degradation and destruction of organic and toxic sludge in water or agro-industrial
waste treatment facilities.
Knez et al. [
10
] classified hydrothermal treatment on the basis of the selected processing
temperature and as a result the predominant yield nature, a similar approach has been adapted in other
works as well [
4
,
7
,
11
–
13
]. It is worth noting that the organic structure, solid-to-water ratio, residence
time, pressure, catalysis and process units’ mechanical configurations also influence the specific yields
of each product as has been reported [
4
–
6
,
9
,
11
,
13
–
16
]. However, such information available in literature
is diverse and fragmented, and temperature levels remain the governing processing parameter that
identify the product phase distribution.
Under subcritical conditions (pressures from 20 to 200 bar):
(1)
At lower temperatures—up to 250
◦
C: the product is primarily a carbon rich solid commonly
known as hydro or bio-char and is reported to be as energetically dense as lignite [4].
Energies 2016, 9, 838 3 of 27
(2)
At higher temperatures—up to 400
◦
C: a de-oxygenated liquid commonly known as bio-oil
or biocrude is the main product, accompanied with an aqueous stream with organic soluble
compounds, a carbon dioxide rich gas and solid char residue as coproducts [
11
,
17
]. The bio-oil,
consisting mainly of hydrolyzed organics, with a carbon partitioning as high as 40–45 wt. % per
carbon feed, has a heating value that could reach between 24 and 37 MJ/kg and offers a potential
substitute for existing liquid fuels. However, it has been reported that significant upgrading
is required to adjust the liquid viscosity levels for longer storage periods and match the lower
oxygen and nitrogen content normally found in the corresponding petroleum crude products [
15
].
Under supercritical conditions (pressures beyond 221 bar):
(1)
At lower temperatures—from 370 to 550
◦
C: under non-catalytic conditions, water soluble
organics are the primary product. While with the introduction of either metallic or alkali
based catalysts, a carbon rich syngas is released due to further de-polymerization, dehydration,
dehydrogenation and decarboxylation reactions taking place. The product gas consists primarily
of a carbon dioxide and methane mixture [5,7,11].
(2)
At higher temperatures—beyond 550
◦
C: catalytic and non-catalytic conditions yield a hydrogen
rich syngas, as a result of kinetically driven gas reforming reactions [
15
]. Some literature has
reported experimental results that show complete partitioning and conversion of carbon from
model compounds or from catalyzed real biomass feedstocks into syngas at temperatures around
600
◦
C and beyond [16].
The appeal for hydrothermal treatment methods does not stem only from the higher organic
conversion rates and selective product nature, but also from thermal design considerations.
Yoshida et al. [18]
, carried out a comparative assessment between hydrothermal and the more
traditional processes such as pyrolysis and thermal gasification. The group showed that for power
production solely and for a combined heat and power (CHP) configuration, SCWG has a higher
overall heat utilization or thermal recovery for carbon feedstocks with a moisture content higher
than 40% and 30% respectively. This is attributed to the elimination of the pretreatment drying load
which significantly leads to higher overall process efficiency. Although higher quality of heat and
power demand, compared to traditional drying, are required to upgrade the processing medium into
supercritical conditions, a quiet similar enthalpy change is required for both cases. Also, aqueous
processing allows for the recovery of high quality heat more efficiently compared to the release of
water vapor at lower temperatures in the traditional drying process. Another significant consideration
is the potential to reduce component sizing and by proxy, lower associated production costs with the
reduced processing volumes. The increased reactivity leads to significantly reduced reaction time,
from a magnitude of hours to seconds, when compared to biochemical processes such as anaerobic
digestion, commonly applied for high moisture feedstocks [19].
As such, a review of the state-of-the-art and an attempt to construe the thermal design trade-offs
through conceptual process design, synthesis and modelling of the SCWG process has been the focus
of a recent collaboration project between the Department of Mechanical Engineering (previously
Department of Energy Technology) at Aalto University in Finland and the University of São Paulo in
Brazil between 2012 and 2015. Earlier publications [
20
,
21
] have presented the developed reactor models
and conceptual plant designs for envisaged integrated bio-refinery concepts for the production of
chemical fuel-either BioSNG or hydrogen, heat and power from an algal feedstock. This article provides
a comparative assessment between the high temperature—hydrogen production and the lower
temperature—BioSNG favorable production pathways. The third partner of the project, Åbo Akademi
University have investigated the hydrothermal treatment of black liquor and some of its model
compounds, their main findings and contributions are reported in another recent publication [16].
Energies 2016, 9, 838 4 of 27
2. State of the Art
2.1. Supercritical Water Gasification Process Synthesis and Simulation
2.1.1. Supercritical Water Gasification Reactor Model
The industrial realization and commercial introduction of an innovative technology is typically
based on the development of pilot and demonstration projects (assumed here to have a nominal
flow higher than 10 kg solid matter per hour), some of which are already in place for the SCWG
application [
22
,
23
]. From which, technical knowledge in the form of collected operational data
would offer reliable guidance for equipment scale up. However, experimental data remains limited
to conditional settings; adjustment to variables of the original setup would lead to deviations
during implementation, especially for parametric sensitive, and chemically complex and energy
intensive processes such as supercritical water refining (SCWR) [
15
]. Due to the absence of
detailed and generally accepted kinetics for processing different heterogeneous structures within
the hydrolyzed aqueous mixtures, thermodynamic modelling is the alternative approach for process
assessment. Thermodynamic equilibrium modelling is commonly divided in literature into two types;
stoichiometric and non-stoichiometric [
4
]. The first approach, as the name implies, requires a clearly
defined and balanced set of reactions. The computation of the equilibrium constant of each reaction
leads to a multivariable optimization problem to be solved in order to obtain yield and product nature
at defined conditions. The work of Marias et al. [
24
] applied a stoichiometric model for a parametric
assessment of a SCWG reactor and a subsequent pressurized phase separator. A predefined set of
7 independent reactions within a single supercritical phase was assessed to represent the principal
organic conversion step, as well as the thermal quality of dry product syngas from the separator.
On the other hand, the non-stoichiometric models are based on the principle of Gibb’s free energy
minimization. The developed models do not require definite knowledge of the detailed chemical
transformations, and as such are widely applied to SCWR processes [
4
]. The reactions are handled with
a black box approach where the only information required are the elemental input and the expected
chemical composition of products [11].
Several published works attempted to derive mathematical models that showed conformity
with experimental findings in literature to enable extended parametric assessments for product
nature [
25
–
30
]. Others utilized commercial computational software equipped with more extensive
databases of property data banks for a wider list of chemical species [
31
–
40
]. Louw et al. [
31
] used
the process simulation tool Aspen Plus
™
(Aspen Technology, Inc., Bedford, MA, USA) to screen a
comprehensive list of 49 real biomass and 5 model compounds. A simplistic plant model that consisted
of a SCWG reactor and subsequent phase separator, for direct dry syngas production was investigated.
Susanti et al. [
32
] and Tushar et al. [
33
] followed suit for the investigation of some expected organic
monomers or intermediates in the SCWG process. However, the scope of their assessment was not on
system-level thermal analysis but rather on deconstructing the specific organic conversion pathways
and predict some of the driving forces for reaction kinetics only.
The principal challenge to ensure the development of a reliable reactor or process model
remains the computation of the specific thermodynamic properties of the highly asymmetric and
multi-dimensional slurry mixtures. The parametrization of the SCWR phase equilibria interactions
for the various processing components of solid and fluid phases, polar and non-polar in nature,
within super- and sub-critical conditions remains an area under development [
41
]. As such, predictive
empirical equations of states (EOS) have garnered significant attention over recent years for SCWG
assessment studies. The advantage of empirical EOS in general compared to their activity coefficient
counterparts is the ability to predict phase equilibria at elevated pressures where infinite dilution
in a single phase is experienced [
41
]. Some of the EOS adapted in literature are the Peng-Robinson
(PR) [
24
,
26
,
28
,
31
,
32
,
35
], Soave Redlich Kwong (SRK) [
36
,
38
], Duan [
27
,
40
], Statistical Association
Fluids Theory (SAFT) [
25
], Virial EOS [
30
] and the original ideal package [
33
]. Some authors employed