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Energy 112 (2016) 121e132
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Energy
journal homepage: www.elsevier .com/locate/energy
Techno-environmental analysis of the biomass gasification and
Fischer-Tropsch integrated process for the co-production of bio-fuel
and power
Karittha Im-orb, Amornchai Arpornwichanop*
Computational Process Engineering Research Unit, Department of Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330,
Thailand
a r t i c l e i n f o
Article history:
Received 14 March 2016
Received in revised form
28 April 2016
Accepted 5 June 2016
Keywords:
Biomass-to-liquid process
Rice straw
Tar removal
Process analysis
Environmental impact
* Corresponding author.
E-mail address: Amornchai.A@chula.ac.th (A. Arpo
http://dx.doi.org/10.1016/j.energy.2016.06.028
0360-5442/© 2016 Elsevier Ltd. All rights reserved.
a b s t r a c t
The present study focuses on the performance analysis of a biomass-to-liquid (BTL) process for the co-
production of green diesel and electricity. The BTL process consists of biomass gasification and Fischer-
Tropsch (FT) synthesis, and rice straw is investigated as the biomass feedstock. The modeling of the BTL
process is performed using Aspen Custom Modeler (ACM). BTL processes with different configurations,
i.e., with and without a tar removal unit based on steam reforming and autothermal (ATO) reforming, are
compared. The amounts of green diesel and electricity produced and the overall potential environmental
impact (PEI) derived from the Waste Reduction (WAR) algorithm are used as technical and environ-
mental performance indicators and subjected to the Analytical Hierarchy Process (AHP) analysis. The
simulation results demonstrate that the BTL process with ATO reforming is the most practical configu-
ration, and the process offering maximum internal heat recovery and minimum external utility re-
quirements is proposed. Based on the parametric analysis of key operating parameters (i.e., gasifying
temperature, FT operating temperature and pressure), the optimal operating conditions of the BTL
process providing the highest AHP index are identified.
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Presently, the increased emissions of greenhouse gases derived
from fossil fuel combustion processes, which leads to global
warming and public health issues, is a topic of concern. To relieve
this problem, the Paris Climate Change Conference (COP21) was
organized, and several countries agreed to limit the rise in global
temperature to less than 2 �C compared to that at the beginning of
the industrial revolution by the year 2035 [1]. Solutions tomaintain
this target, such as the increase of process energy efficiency, the
improvement of carbon capture storage (CCS) technology and the
reduction of fossil fuel utilization in energy production processes
by replacement with the alternative resources, e.g., wind, solar and
biomass, are of increasing interest.
The use of biomass as an energy source has beenwidely studied
and seems to be a suitable practice for agriculture-based countries.
Among the various types of biomass, rice straw is an important crop
rnwichanop).
residue. At present, a high amount of rice straw is left as an agri-
cultural waste after the harvesting season. The conversion of rice
straw to energy has many advantages, including the reduction of
the agricultural waste generated from the rice industry, the
reduction of the environmental impact and the acquisition of a new
alternative energy resource for in-house energy production.
The transportation sector is one that not only consumes a high
amount of energy (e.g., gasoline and diesel) but is also responsible
for a large part of the CO2 emissions. Therefore, replacing the en-
ergy derived from fossil fuel required in this sector with that
derived from a renewable resource such as biomass is a solution
that can relieve global warming and other environmental prob-
lems. A biomass-to-liquid (BTL) process, an integrated process of
biomass gasification and Fischer-Tropsch (FT) synthesis, is a
promising technology used to produce green liquid fuel that can be
applied to existing infrastructure and automotive technologies
[2,3]. However, the BTL process is in the research and development
phases, and the price of the synthesized liquid fuel is still not
competitive with that derived from crude distillation due to the
higher operating cost. Therefore, the study of this process from
several aspects, including technical, economic and environmental,
Delta:1_given name
Delta:1_surname
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Table 1
Ultimate and proximate analyses of the rice straw.
Proximate analysis Ultimate analysis
Moisture wt% 6.71 Carbon wt% 44.4
Fixed carbon wt% 11.09 Hydrogen wt% 5.0
Volatile matter wt% 58.64 Nitrogen wt% 0.6
Ash wt% 23.55 Oxygen wt% 30.8
Sulfur wt% 0.1
Ash wt% 23.55
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132122
to improve its performance has attracted increasing attention.
Hamelinck et al. [4] developed an Aspen plus dynamic model of the
BTL process that can evaluate the influence of each parameter or
device on the investment costs. They concluded that the BTL pro-
cess could become economically viable when the crude oil price
levels are substantially increased or when the environmental
benefits of green FT diesel are more highly valued. Avella et al. [5]
performed an economic analysis by investigating the influence of
various costs associated with plant configurations (i.e., cost of in-
vestment, operation, maintenance, depreciation, and financing
charges) on the cost of the electric energy and synthesized liquid
fuel. Their results showed that the cost of both products strongly
depended on the plant configuration. In the process performance
evaluation, a highest value of 51%, which corresponded to 40%
gasification and 75% Fischer-Tropsch, was reported by Leibbrandt et
al. [6]. An exergy analysis of the BG-FT process was performed, and
a 36.4% exergetic efficiency was found. The largest exergy losses
occurred in the power generation from FT-offgas and in the
biomass gasifier [7]. The production rates of syngas, FT-diesel and
FT-offgas as well as the electricity from the BTL process could be
maximized via the suitable adjustment of the FT-offgas recycle
fraction and selection of the FT reactor volume [8]. Wang et al. [9]
developed a multi-objective mixed-integer nonlinear program-
ming (MINPL) model in which the net present value (NPV) and
global warming potential (GWP) derived from a life cycle assess-
ment procedure were used as economic and environmental in-
dicators, respectively. The optimal solution revealed that the use of
high-temperature gasification, direct cooling, internal hydrogen
production and cobalt catalysis had the best environmental and
economic performances. Reichling and Kulacki [10] found that the
total energy yield (electricity and liquid fuels) and carbon dioxide
emissions of the two processes i.e., the utilization of biomass
through the Fischer-Tropsch (FT) conversion to liquid fuels and that
via the integrated gasification combined cycle (IGCC) electrical
production, were almost identical.
In the BTL process, the syngas derived from the syngas processor
needs to be cleaned and conditioned to achieve the FT-
specification. The tar contained in the raw syngas may cause the
fouling of downstream equipment and deactivation of the FT-
catalyst, resulting in a decrease in process performance. Attempts
at minimizing tar formation, such as selecting suitable operating
conditions, using a catalyst and the installation of secondary
equipment to remove tar from the produced gas, are still topics of
interest [11]. Theconversion of tar to syngas in a reformer via steam
reforming and autothermal (ATO) reforming reactions has been
widely used because it could increase the amounts of syngas and
liquid fuel [12]. Previously, the reforming unit in the BTL process
was mostly considered as a passageway, although some reactions
and heat transfer occur.
The present study therefore focuses on the performance anal-
ysis of the BTL process with different configurations, i.e., with and
without a tar removal unit, based on two reforming processes, i.e.,
steam reforming and ATO reforming. Although the process without
tar reforming is not presently applied in the BTL process due to the
constraint of the FT-feed gas specification which the tar content
must be lower than 1 ppmv [4], it may be possible if the contam-
inant resistance of the FT-catalysts is improved. The performance
analysis of each process configuration is performed, and the results
are compared in terms of the amounts of electricity and green
diesel produced using the BTL model developed in Aspen Custom
Modeler (ACM). Rice straw is considered the feedstock, and its ul-
timate and proximate analyses are shown in Table 1 [13]. The
environmental impact is investigated using the overall potential
environmental impact (PEI), using the waste reduction (WAR) al-
gorithm as an indicator. The integration of the diesel production
rate and the PEI as technical and environmental indicators into one
index using the analytical hierarchy process (AHP) is also investi-
gated. Moreover, the design of a high energy efficiency process is
achieved by performing pinch analysis, which is a promising
methodology used to maximize the energy efficiency of production
processes by minimizing their energy consumption [14]. In this
step, the demands of the hot and cold utilities of the considered
process are determined, and the heat exchanger network offering
the optimal heat integration is identified.
2. Modeling of the BTL process
The model of the BTL process for rice straw feedstock consisting
of gasification, syngas cleaning (e.g., high-temperature resistant
filtration and tar steam reforming or ATO reforming), syngas con-
ditioning (i.e., water gas shift reaction and compression), and
Fischer-Tropsch synthesis and power generation units is developed
in ACM. The process flow diagram is illustrated in Fig. 1, and the
model development is discussed in the following section.
2.1. Model assumptions
The model assumptions of each unit in the BTL process and the
scope of the present study are shown in Fig. 2. For configuration I,
the raw syngas from gasifier is directly fed through the water gas
shift reactor. The installation of tar removal units based on steam
reforming and ATO reforming is considered in process configura-
tion II and III, respectively, in which the raw syngas is fed through
the tar removal unit before it is fed into the water gas shift reactor
and the other downstream units.
2.2. Methodology
The development of the BTL model in ACMwas explained in our
previous work [8]. The developed model is divided into several
parts, i.e., gasification, steam reforming or ATO reforming, water gas
shift reaction, compression, Fischer-Tropsch synthesis, and power
generation. The main reactions in the BTL process are summarized
in Table 2.
2.2.1. Gasification
The gasification model is separated into two sections, the first
explaining the combined pyrolysis and oxidation reactions, which
are relatively fast and assumed to be at thermodynamic equilib-
rium, and the second involving the low reaction rate char reduction
reactions, whose chemical kinetics have to be considered. The
previous studies reported that benzenewas the highest component
found in tar. In downdraft gasifier, around 2 wt% of tar yield was
typically found when one unit mass of biomass was gasified [15].
The present study, therefore, considers benzene as a tar model.
2.2.2. Steam reforming or autothermal reforming
Ash and unreacted carbon contained in the raw syngas are
removed in a solid separation unit. In the tar steam reforming
Gasifier Fillter Heater1
Water sep1
SG2
Steam reformer/ ATO
Cooler1
WGS reactor 150 C
Heater2
SG3
Cooler2
Water sep2
Compressor
75% efficiency
Cooler3
FT reactor
220 C, 20 bar
ASU
O2 purity 99.5%
Raw syngas
700 C
780 C
Steam2,
150 C, 1 atm
50 C
Water1
150 C
Steam3,
150 C, 1 atm
50 C, 1 bar
Water2
Inlet gas 1 bar
Charge gas 20 bar
220 C,
20 bar
HP Off gas
steam
Liquid fuel
Air O2 21%, N2 79%
Unreacted carbon
Biomass 25 C
expander
LP Offgas
Heater3
%
n
C
U
a
r
S
5
2
H
b
Water, 25 C, 1 atm Water 25 C,
1 atm
.
A
Biomass Gasification
Section Gas cleaning and tar removal Section
Air separation unit
Power generation section Fischer Tropsch section Compressor section
H2/CO adjustment
Power
Fig. 1. Process flow diagram of the BTL process.
Fig. 2. Model assumptions and scope of the present study.
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132 123
Table 2
Main reactions in the BTL process.
Process description Main reactions
Gasification
Section 1: combined pyrolysis and oxidation reactions
Combined pyrolysis and oxidation
reactions:
CxHyOz þwH2OþmO2 þ 3:76mN2 ¼ nCOCOþ nCO2 CO2 þ nH2H2 þ nH2OH2Oþ nCH4CH4 þ nC6H6 C6H6 þ nCharCharþ 3:76mN2
Section 2: reduction reactions
Boudouard reaction: Cþ CO242CO
Water gas reaction: Cþ H2O4COþ H2
Methane reaction: Cþ 2H24CH4
Methane steam reforming reaction: CH4þH2O4COþ 3H2
Steam reforming
Methane steam reforming reaction: CH4þH2O4COþ 3H2
Benzene steam reforming reaction: C6H6þ6H2O46COþ 9H2
Water gas shift reaction: COþ H2O4CO2þH2
Autothermal reforming
Autothermal reaction: nC6H6 C6H6 þ nCH4 CH4 þwH2OþmO2 ¼ nCOCOþ nCO2 CO2 þ nH2H2 þ nH2OH2O
Water gas shift reactor
Water gas shift reaction: COþ H2O4CO2þH2
Fischer-Tropsch reactor
Fischer-Tropsch reaction: nCOþ ð2nþ 1ÞH2/CnH2nþ2 þ nH2O
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132124
process, benzene (a model tar compound) and methane are
completely converted to H2 and CO via the steam reforming reac-
tion over a Ni-based catalyst at 1053 K and 1.01 bar [15,16]. A
chemical equilibrium of the water gas shift reaction and a complete
conversion of the methane and benzene steam reforming reactions
are assumed.
In ATO reforming, oxygen is supplied to produce the heat of
combustion for the steam reforming reaction. The operating con-
dition is set at 1053 K and 1.01 bar, and a thermally self-sufficient
operation is assumed. To calculate the composition of the product
gas leaving this unit, the chemical equilibrium of thewater gas shift
reaction and the complete conversion of oxidation, methane and
benzene steam reforming reactions are assumed, and C-, H-, and O-
element balances are performed.
2.2.3. Water gas shift reactor
The H2/CO ratio of the syngas is adjusted to 2.37, which offers
the highest diesel yield [17]. Steam is supplied as a reactant of the
water gas shift reaction, and a chemical equilibrium of this reaction
is considered.
Fig. 3. Analytical hierarchy structure used for the analysis of the three BTL processes.
2.2.4. Compressor
The clean syngas with the desired H2/CO ratio is compressed to
the FT operating pressure of 20 bar. A compressor with 75% effi-
ciency is selected. In this section, the temperature of the effluent
gas is determined.
2.2.5. FT reactor
The considered FT hydrocarbon products are linear paraffins.
The operating temperature and pressure are 473 K and 20 bar, and a
cobalt-based catalyst is used in the FT reactor. The product distri-
bution is assumed to follow the Anderson-Schulz-Flory (ASF) dis-
tribution (Eq. (1)), and the chain growth probability (a) is
determined from the correlations reported in a previous study [4],
as shown in Eqs. (2) and (3).
Mn ¼ an�1ð1� aÞ (1)
a ¼ 0:75� 0:373
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
�logðSC5þÞ
q
þ 0:25SC5þ (2)
SC5þ ¼ 1:7� 0:0024T � 0:088
½H2�
½CO� þ 0:18ð½H2� þ ½CO�Þ
þ 0:0079PTotal (3)
where SC5þ is the selectivity forhydrocarbons with a chain length
longer than 5, [H2] and [CO] are the molar concentrations of H2 and
CO in the FT-feed gas, and T and PTotal are the FT operating tem-
perature (K) and pressure (bar), respectively. The reaction rate used
to determine the conversion of carbon monoxide during the FT
synthesis is derived from the kinetic study of [18].
2.2.6. Expansion turbine
The pressure of the FT-offgas is reduced to the operating pres-
sure of the gasifier (1.01 bar) through the expansion turbine, which
is connected to the generator; as a result, some electricity is
generated. The efficiency of the expansion turbine is assumed to be
75%.
2.2.7. Energy consumption
The overall energy balance for each unit can be calculated by
Eqs. (4)e(6).
Fig. 4. Total output rates of environmental impacts for the BTL processes with steam
reforming, ATO reforming and without a reforming process.
Fig. 5. Total environmental impact outputs per mass of diesel product for the BTL
processes with steam reforming, ATO reforming and no reforming process.
Table 3
Performance of each BTL process configuration (biomass feed rate ¼ 1 kmol/h).
Steam reforming ATO reforming Process without reforming
Syngas processor
Syngas (kmol/h) 1.065 1.054 0.965
Syngas composition (mol%)
C6H6 0.000 0.000 0.465
CH4 0.204 0.000 0.313
CO 23.355 23.102 21.158
CO2 20.928 21.385 20.226
H2 55.410 54.810 50.198
H2O 0.062 0.063 0.060
N2 0.041 0.045 0.041
Fischer-Tropsch synthesis
Diesel (kmol/h) 0.001537 0.001529 0.001526
Gasoline (kmol/h) 0.000659 0.000655 0.000649
Liquid fuel (kmol/h) 0.002288 0.002275 0.002265
FT-offgas (kmol/h) 0.615590 0.607196 0.483003
Water (kmol/h) 0.243597 0.244634 0.251240
Electricity (kW) 0.956728 0.938008 0.721023
Overall energy consumption
BG-FT process (kW) �23.00 �24.54 �25.19
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132 125
Hreactant þ Qin ¼ Hproduct þ Qout (4)
Hreactant ¼
X
reactants
nih
0
fi (5)
Hproduct ¼
X
products
ni
h
h0fi þ DhTi
i
(6)
where h0fi is the enthalpy of formation (kJ/kmol) at the reference
state (298 K, 1.01 bar) and DhTi is the enthalpy difference between a
given state and the reference state.
3. Waste reduction (WAR) algorithm
The WAR algorithm is used to evaluate the environmental
impact of chemical and biochemical processes and to compare
them by determining the overall potential environmental impact
(PEI), which is a quantity representing the average indirect effect
that mass and energy emissions would have on the environment.
The considered impact is separated into two major categories: (1)
the global atmospheric impact, which consists of the global
warming potential (GWP), ozone depletion potential (ODP), acidi-
fication or acid rain potential (AP) and photochemical oxidation or
smog formation potential (PCOP) and (2) the local toxicological
impact, which consists of human toxicity potential by ingestion
(HTPI), human toxicity potential by either inhalation or dermal
exposure (HTPE), aquatic toxicity potential (ATP) and terrestrial
toxicity potential (TTP). The PEI is represented by the total rate of
the environmental impact output ( _I
ðtÞ
out), which is calculated from
the summation of the rates of impact outputs from chemical pro-
cesses ( _I
ðcpÞ
out ), energy processes ( _I
ðepÞ
out ) and waste energy ð _I
ðcpÞ
we ;
_I
ðepÞ
we Þ,
as shown in Eq. (7). As the impact of the energy emissions is low,
only the impact of mass emissions from the chemical process is
considered. In the present study, the PEI is calculated from the FT-
offgas because the potential environmental impact of the gas
stream is higher than that of the solid stream, and the impact of the
produced liquid fuel is not taken into account because it is the
desired product [19]. The calculation of the PEI is based on the
procedure reported in a previous work [20]. The total output rate of
the environmental impact and the total environmental impact
output per mass of desired product ðbIðtÞoutÞ are calculated from Eqs.
Fig. 7. Effect of diesel and electricity production rate on the NPV.
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132126
(7) and (8),
_I
ðtÞ
out ¼ _I
ðcpÞ
out þ _I
ðepÞ
out þ _I
ðcpÞ
we þ _I
ðepÞ
we
¼
Xcp
j
_M
ðoutÞ
j
X
k
xkljk þ
Xep�g
j
MðoutÞj
X
k
xkljk (7)
bIðtÞout ¼
_I
ðcpÞ
out þ _I
ðepÞ
out þ _I
ðcpÞ
we þ _I
ðepÞ
weP
p
_Pp
¼
Pcp
j
_M
ðoutÞ
j
P
k
xkljk þ
Pep�g
j
MðoutÞj
P
k
xkljk
P
p
_Pp
(8)
where _M
ðoutÞ
j is the mass flow rate of stream j, which may be an
input or an output stream, xkl is the mass fraction of component k
for the impact category l, _Pp is the mass flow rate of product p and
jk is the potential environmental impact for chemical k, which can
be calculated from Eq. (9).
jk ¼
X
l
alj
s
kl (9)
where al is the relative weighting factor of impact category l, which
is assumed to have a value of 1 (al ¼ 1) for all impact categories, and
jskl is the specific potential environmental impact of chemical k for
impact category l, which can be calculated from Eq. (10).
jskl ¼
ðScoreÞkl�ðScoreÞk�l (10)
where ðScoreÞkl is the characteristic quantity of chemical k for
impact category l, which can be derived from the literature [21],
and hðScoreÞkil is the average value of all k chemicals in category l.
4. Analytical hierarchy process (AHP)
The AHP is a multi-criteria decision analysis (MCDA) used to
evaluate the relative importance of each criterion [22,23]. The
diesel production rate and the PEI, which are considered as
Fig. 6. Effect of weighting factor of diesel production rate on the AHP index of the BTL
processes with steam reforming, ATO reforming and without a reforming process.
technical and environmental indicators, are integrated into one
AHP index, as shown in Eq. (11). The hierarchy structure used in this
study is illustrated in Fig. 3.
AHP ¼ PDP �weightDP þ PEnv �weightEnv (11)
where PDP is the normalized diesel production rate calculated from
the ratio between the diesel production rate of the process and that
of the sum of all considered processes. However, for ease of anal-
ysis, the environmental impact is represented in terms of envi-
ronmental friendliness, which is calculated by subtracting the PEI
from one (1-PEI); therefore, the normalized environmental
friendliness (PEnv) is the ratio between (1-PEI) for the process and
that of the sum of all considered processes. weightDP and weightEnv
are the weighting factors of the diesel production rate and envi-
ronmental friendliness, respectively. The process with a higher AHP
index offers a higher process performance at a specified weighting
factor of the diesel production rate.
5. Pinch analysis and heat exchanger network (HEN) design
Pinch analysis is a methodology used in whole plant energy
management by determining the optimal structure of the heat
exchanger that offers the maximum internal heat recovery and
minimum external utility requirements. Composite curves are
constructed by combining the hot and cold composite curves into
one temperature-enthalpy (T-H) graph. The minimum temperature
difference (DTmin) is set at 20 �C based on the typical value applied
in a chemical plant. Theminimumhot and cold utility requirements
of the process for a specified DTmin are determined from the over-
shoot at the end of each composite curve. The heat exchanger
network offering the optimum heat integration of the stream can
be designed based on the reported procedure [24,25].
6. Results and discussion
6.1. Performance analysis of the BTL process
The performance of the BTL process with different configura-
tions, i.e., the processes without reforming, with steam reforming
and with ATO reforming, is summarized in Table 3. It is found that
the process with steam reforming offers the highest amount of
Fig. 8. Composite curves, pinch points and minimum energy requirements of the process:es (a) without reforming, (b) with steam reforming and (C) with ATO reforming.
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132 127
syngas (H2þCO) due to the complete conversionof tar andmethane
into syngas, and thereby the highest amounts of electricity and
green diesel are achieved. In the process with ATO reforming, the
combustion reaction occurs, so the amount of syngas is found to
decrease, whereas that of CO2 increases, resulting in decreased
electricity and green diesel products. However, the derived FT-
products are not significantly different from those derived from
the process with steam reforming. As the additional syngas derived
from the reforming reaction does not appear in the process without
reforming, the lowest amount of FT-products is found in this pro-
cess. Moreover, the lifetime of the FT-catalyst may decrease due to
the tar deposition. The overall energy consumption calculated from
the summation of the energy consumption of each sub-unit in the
BTL process is also investigated. As the steam reforming unit in-
volves the highly endothermic tar steam reforming reactions which
large amount of energy from external heat source is required.
Therefore, the process with steam reforming consumes the highest
amount of energy. The process with ATO reforming is the second
mostly energy consumed process, in which the heat of combustion
is produced and supplied to the steam reforming reactions. The ATO
reforming process involves both exothermic combustion reactions
and endothermic steam reforming reactions that can be balanced
by adjusting the amount of supplied oxygen to achieve the thermal
self-sufficient condition, in which external heat sources are not
required during a steady state operation. The process without
reforming consumes the lowest energy because the energy con-
sumption at the reforming unit does not exist and the temperature
of syngas entering the cooler no.1 is lower than that of the other
processes, resulting in the lower energy consumption at this unit
and the overall process.
6.2. Environmental evaluation
The potential environmental impact (PEI) represented by two
output indices, i.e., the total output rate of environmental impact
and the total impact output per mass of diesel product of each
process configuration, which is calculated from the composition
and flow rate of the FT-offgas, are investigated. The effect of CO2
emission is neglected in this study due to the CO2-neutral charac-
teristic of biomass feedstock. It is found from Fig. 4 that the process
with steam reforming has the highest environmental impact due to
the large amount of emitted CO, which has a strong impact on HTPE
and GWP, followed by the process with ATO reforming and that
without reforming. Although the process with steam reforming
Fig. 9. BTL process with heat integration system.
Fig. 10. Effect of operating parameters on diesel production rate: (a) TGs 973 K, (b) TGs 1073 K, (c) TGs 1173 K and (d) TGs 1273 K.
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132128
Fig. 11. CO conversion, liquid fuel and FT-offgas production rate of FT reactor (TGs
1173 K, FT operating pressure 60 bar).
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132 129
offers the highest amount of diesel product, the total impact output
per mass of diesel product has the same trend as that of the total
output rate of environmental impact (Fig. 5). This implies that the
Fig. 12. Effect of operating parameters on overall potential environmental i
amount of diesel product derived from this configuration is not
significantly greater than that from the others.
6.3. Combined technical and environmental impact evaluation
The effect of changes in the weighting factor of the diesel pro-
duction rate from 0 to 1 on the AHP index is investigated. Fig. 6
shows that the AHP index continuously decreases when the
weighting factor of the diesel production rate increases for all
process configurations. However, the process without reforming
offers the best performance when the weighting factor is less than
0.79, followed by the process with ATO reforming and that with
steam reforming. The opposite effect is found when the weighting
factor increases above this value. Moreover, it is found that all
process configurations offer identical performance at theweighting
factor of 0.79. At this condition, the AHP index of 0.40 is achieved. It
is noted that the economic performance can also be represented by
the diesel production rate due to the NPV significantly increases
when the diesel production rate increases, while decreases when
the amount of generated electricity increases as shown in Fig. 7.
Therefore, the result of economic evaluation should reasonably
offer the same trend as that of the technical evaluation.
mpact: (a) TGs 973 K, (b) TGs 1073 K, (c) TGs 1173 K and (d) TGs 1273 K.
Fig. 13. Effect of operating parameters on AHP index: (a) TGs 973 K, (b) TGs 1073 K, (c) TGs 1173 K and (d) TGs 1273 K.
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132130
6.4. Interpretation of composite curve
The composite curves of the three BTL processes are shown in
Fig. 8(a)e(c). At the specified DTmin of 20 �C, the overshoot of the
hot composite curve over the cold composite curve and that of the
cold composite curve over the hot composite curve are found for
the process with steam reforming (Fig. 8(b)). This implies that
thermal energies of approximately 1.59 and 30.05 kW are required
for the hot and cold utilities, respectively. Nevertheless, this process
is not practical because a working temperature of hot utility higher
than 780 �C is required. In the ATO reforming process, the heat of
combustion is produced and supplied to the steam reforming re-
action. Only the overshoot of the hot composite curve over the cold
composite curve is found at both the low and high-temperature
ends of the composite curve, which indicates that only two cold
utilities are required, i.e., with working temperatures lower than 25
and 150 �C, with thermal energies of 3.89 and 27.13 kW, respec-
tively. The process without reforming shows the same trend as that
with ATO reforming, and the demands of cold utilities withworking
temperatures of 25 and 150 �C are quite similar (approximately
4.03 and 26.42 kW).
6.5. Heat exchanger network (HEN) design
As the syngas leaving the syngas processor of the BTL process
without reforming contains tar (C6H6) with 0.465mol%, which does
not meet the FT feed gas specification (<1 ppmv), and the high-
temperature hot utility is required in the process with steam
reforming, the BTL process with ATO reforming is therefore the
most suitable from a technical point of view. This process is selected
to design the optimal heat integration network. Fig. 9 shows that
the heat of the hot reformer effluent gas is recovered and used to
produce steam at the steam generator and also used to heat the
syngas to the operating temperature of the water gas shift reactor.
An additional cooler has to be installed in this newly designed
process, as it is a highly exothermic process that requires a large
amount of cooling media.
6.6. Parametric analysis of newly designed BTL process
There are several operating parameters, i.e., gasifying temper-
ature, FToperating temperature and pressure, that have an effect on
the amounts of derived products as well as the environmental
impact. Therefore, the effect of changes in the values of these pa-
rameters on the diesel production rate, the overall potential envi-
ronmental impact and the combination thereof, which are
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132 131
represented by the AHP index, is investigated in this section. In this
study, the gasifying temperature is considered in the range of
973e1273 K. The FT operating temperature and pressure are varied
in the ranges of 473e523 K and 20e60 bar, respectively.
6.6.1. Effect of operating parameters on the diesel production rate
The changes in the diesel production rate for each gasifying
temperature are shown in Fig. 10(a)e(d). It is found that at constant
FT operating conditions, the diesel production rate increases with
the gasifying temperature due to the increase in the syngas feedrate. However, a slight increase is observed because the syngas feed
rate does not significantly change at the gasifying temperature
higher than 973 K [26]. The same effect is found when the FT
operating pressure increases at constant gasifying and FT operation
temperatures. As the FT operating temperature has less effect on
the CO conversion than the pressure, the CO conversion is therefore
found to be stable when the FT operating temperature increases at
constant gasifying temperature and FT operating pressure. How-
ever, the production rate of diesel is found to continuously decrease
while that of FT-offgas increases at this condition (Fig.11) due to the
decrease in chain growth probability and consequently decrease in
selectivity towards long chain hydrocarbon.
6.6.2. Effect of operating parameters on the overall potential
environmental impact
It is found in Fig. 12(a)e(d) that the overall potential environ-
mental impact (PEI), which depends on the generated FT-offgas,
slightly increases with the gasifying temperature due to the slight
increase in the production rate of syngas and also FT-offgas which
their composition does not significantly change at gasifying tem-
perature higher than 973 K [26]. The opposite effect is found when
the FT operating pressure increases. Regarding the high selectivity
of the FT-offgas at high temperature, therefore the PEI is also found
to increase with the FT operating temperature at a constant gasi-
fying temperature and FT operating pressure.
6.6.3. Effect of operating parameters on the AHP index
Fig. 13(a)e(d) shows the effect of changes in the values of the
gasifying temperature, FT operating temperature and pressure on
the AHP index, which is the integration of the diesel production
rate and environmental friendliness. Weighting factors of 0.82 and
0.18, which are commonly applied for chemical processes [27] are
applied for the diesel production rate and environmental objec-
tives, respectively. As the diesel production rate is considered the
major contributor, the variation of the AHP index offers similar
trend to it. It is noted that the variation of gasifying temperature
does not significantly affect the AHP index because the diesel
production rate increases as the gasifying temperature increases
whereas the environmental friendliness shows inverse effect,
resulting in stable AHP index. Therefore, the results shown in
Fig. 13(a)e(d) are found to be almost identical. The maximum AHP
index is achieved at the gasifying temperature of 1273 K and the FT
operating temperature and pressure of 473 K and 60 bar,
respectively.
7. Conclusions
The performances of three BTL processes, i.e., with and without
a tar removal unit based on steam reforming or ATO reforming, are
compared. The highest amounts of electricity and green diesel are
achieved in the process with steam reforming followed by that with
ATO reforming and that without any reforming. On the other hand,
the last process consumes the least energy and causes the lowest
environment impact. The combined criteria of the diesel produc-
tion rate and environmental friendliness are also investigated. The
process without reforming shows the best performance when the
weighting factor of the diesel production rate is less than 0.79,
followed by the processes with ATO and steam reforming, and the
opposite effect is found when this factor increases higher than this
value. The pinch analysis implied that the process with steam
reforming requires both hot and cold utilities, while the others
require only the cold utility. The process with ATO reforming is the
most practical and can be designed to achieve the maximum in-
ternal heat recovery and the minimum external utility re-
quirements. The gasifying temperature, FT operating temperature
and pressure strongly influence the diesel production rate, the
overall potential environmental impact, and the combination
thereof. The highest AHP index of 0.21 for a newly designed BTL
process with ATO reforming is achieved at the gasifying tempera-
ture of 1273 K, the FT operating temperature of 473 K and the FT
operating pressure of 60 bar when the weighting factors of the
diesel production rate and environmental friendliness are specified
at 0.82 and 0.18, respectively.
Acknowledgments
Support from the National Research University Project, Office of
Higher Education Commission and Chulalongkorn Academic
Advancement into its 2nd Century Project is gratefully
acknowledged.
Nomenclature
m Amount of oxygen per mole of biomass
w Amount of water per mole of biomass
n Number of carbon atoms of hydrocarbon substance
ni Number of moles of component i per mole of biomass
a Chain growth probability
SC5þ Selectivity for hydrocarbons with a chain length longer
than 5
[H2] Molar concentration of H2 in the FT-feed gas
[CO] Molar concentration of CO in the FT-feed gas
T FT operating temperature (K)
PTotal FT operating pressure (bar)
Mn Mole fraction of hydrocarbon with chain length n
h0fi Enthalpy of formation at the reference state (298 K,
1 atm) (kJ/kmol)
DhTi Enthalpy difference between a given state and the
reference state (kJ/kmol)
Hreactant Enthalpy of reactant (kJ)
Hproduct Enthalpy of product (kJ)
PEI Potential environmental impact
_I
ðtÞ
out Total rate of environmental impact output
bIðtÞout Total environmental impact output per mass of desired
product
_I
ðcpÞ
out Rate of environmental impact output from chemical
process
_I
ðepÞ
out Rate of environmental impact output from energy process
_I
ðcpÞ
we Rate of environmental impact output of waste energy
from chemical process
_I
ðepÞ
we Rate of environmental impact output of waste energy
from energy process
_M
ðoutÞ
j Mass flow rate of stream j, which may be an input or an
output stream (kg/h)
xkl Mass fraction of component k for impact category l
_Pp Mass flow rate of product p (kg/h)
K. Im-orb, A. Arpornwichanop / Energy 112 (2016) 121e132132
jk Potential environmental impact for chemical k
al Relative weighting factor of impact category l
jskl Specific potential environmental impact of chemical k for
impact category l
ðScoreÞkl Characteristic quantity of chemical k for impact category l
hðScoreÞkil Average value of all k chemicals in category l
PDP Normalized value of diesel production rate
PEnv Normalized value of environmental friendliness
AHP Analytical hierarchy process
weightDP Weighting factor of diesel production rate
weightEnvWeighting factor of environmental friendliness
DTmin Minimum temperature difference (�C)
TGs Gasifying temperature (K)
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	Techno-environmental analysis of the biomass gasification and Fischer-Tropsch integrated process for the co-production of b ...
	1. Introduction
	2. Modeling of the BTL process
	2.1. Model assumptions
	2.2. Methodology
	2.2.1. Gasification
	2.2.2. Steam reforming or autothermal reforming
	2.2.3. Water gas shift reactor
	2.2.4. Compressor
	2.2.5. FT reactor
	2.2.6. Expansion turbine
	2.2.7. Energy consumption
	3. Waste reduction (WAR) algorithm
	4. Analytical hierarchy process (AHP)
	5. Pinch analysis and heat exchanger network (HEN) design
	6. Results and discussion
	6.1. Performance analysis of the BTL process
	6.2. Environmental evaluation
	6.3. Combined technical and environmental impact evaluation
	6.4. Interpretation of composite curve
	6.5. Heat exchanger network (HEN) design
	6.6. Parametric analysis of newly designed BTL process
	6.6.1. Effect of operating parameters on the diesel production rate
	6.6.2. Effect of operating parameters on the overall potential environmental impact
	6.6.3. Effect of operating parameters on the AHP index
	7. Conclusions
	Acknowledgments
	Nomenclature
	References