cohce_thermo analysis of h2 production from gasification
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8/3/2019 Cohce_Thermo Analysis of H2 Production From Gasification
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Thermodynamic analysis of hydrogen production from
biomass gasification
M.K. Cohce*, I. Dincer, M.A. Rosen
Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario,
Canada L1H 7K4
a r t i c l e i n f o
Article history:
Received 19 June 2009
Received in revised form
26 August 2009
Accepted 31 August 2009
Available online 7 October 2009
Keywords:
Biomass
Gasification
Hydrogen
Thermodynamics
EnergyExergy
Efficiency
Oil palm shell
SMR
a b s t r a c t
An investigation is reported of the thermodynamic performance of the gasification process
followed by the steam-methane reforming (SMR) and shift reactions for producing
hydrogen from oil palm shell, one of the most common biomass resources. Energy and
exergy efficiencies are determined for each component in this system. A process simula-
tion tool is used for assessing the indirectly heated Battelle Columbus Laboratory (BCL)
gasifier, which is included with the decomposition reactor to produce syngas for producing
hydrogen. A simplified model is presented here for biomass gasification based on chemical
equilibrium considerations, with the Gibbs free energy minimization approach. The
gasifier with the decomposition reactor is observed to be one of the most critical compo-
nents of a biomass gasification system, and is modeled to control the produced syngas
yield. Also various thermodynamic efficiencies, namely energy, exergy and cold gas effi-
ciencies are evaluated which may be useful for the design, optimization and modification
of hydrogen production and other related processes.Crown Copyright ª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All
rights reserved.
1. Introduction
Biomass, a relatively large energy source globally which
includes wood, municipal solid wastes and agricultural resi-dues, is being investigated in various countries as a potentially
significant renewable resource. Biomass is derived from solar
energy. Biomass is relatively clean compared to other sources
of energy, as it releases no net CO2 emissions when carefully
managed since CO2 is fixed by photosynthesis during biomass
growth and is released during utilization. This form of energy
can be converted to gaseous fuel through thermochemical
gasification [1]. Such a fuel can be used for various tasks,
including producing hydrogen, which can be used cleanly and
efficiently as a fuel in combustion engines and fuel cells.
Hydrogen is likely to be an important energy carrier in thefuture. Presently, it can be produced by the steam reforming of
natural gas, coal gasification and water electrolysis among
other processes. However these current processes are not
sustainable because they use fossil fuels or electricity from
non-renewable resources. Hydrogen production can be made
more sustainable if it is produced from sustainable energy
resources. In this regard, alternative thermochemical
* Corresponding author.E-mail addresses: mehmet.cohce@uoit.ca (M.K. Cohce), ibrahim.dincer@uoit.ca (I. Dincer), marc.rosen@uoit.ca (M.A. Rosen).
A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / h e
0360-3199/$ – see front matter Crown Copyrightª 2009 Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. All rights reserved.
doi:10.1016/j.ijhydene.2009.08.066
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(pyrolysis and gasification) and biological (biophotolysis,
water-gas shift reaction and fermentation) processes are
practical and can be more sustainable than present processes
[2,3]. Overviews of technologies for hydrogen production from
biomass have been reported [4–8]. Many researchers are
focusing their research on the gasifier portion of this process,
as gasification appears to be more favorable for hydrogen
production than pyrolysis [9].The gasifier is the most important component in any
biomass gasification system [10]. Gasification, which is char-
acterized by partial oxidation, is utilized in numerous clean
energy processes including hydrogen production via biomass
gasification. Currently, 80–85% of the world’s total hydrogen
production is derived from natural gas via steam-methane
reforming (SMR) [11]. Much research has been reported on the
production of hydrogen by SMR [12–15], with many studies
concentrating on analysing the reforming reactor [16]. Gasifier
modelling and simulation, using programs such as Aspen
Plus, have been ongoing [17–19]. Some researchers predict
biomass gasification in supercritical water to be a promising
technology for hydrogen production for it allows the utiliza-tion of wet biomass [20,21]. Recent research has focused on
determining energy and exergy efficiencies [22–25] and
improving understanding and data availability with experi-
mental studies. In this study, the gasifier, the most significant
part of the system, is analyzed in detail.
Exergy analysis is a tool for understanding and improving
efficiency [4], and is used throughout this investigation in
addition to energy analysis. Extensive exergy analyses have
been reported using devices, technologies and systems, in
a variety of fields. Some examples include exergy assessments
of power plants for transportation and power generation,
chemical and metallurgical processing facilities and building
systems [26,27]. Exergy-based assessments and comparisonshave also been performed of such energy technologies as
hydrogen production [28–33], electricity generation with fuel
cells and other technologies, for instance, coal-fired and
nuclear [34,35], and larger energy systems [36,37].
The aim of the present work is to investigate hydrogen
production by thermochemical biomass gasification using
energy and exergy methods, and to evaluate the potential of
hydrogen production from biomass. A parametric analysis of
factors influencing the thermodynamic efficiency of biomass
gasification is carried out. Energy and exergy assessments are
performed and the effects are determined on both system
efficiency and hydrogen yield of varying parameters of the
fractioned syngas. The system considered has two parts, andboth include syngas production as an input for producing
hydrogen in the hydrogen plant. The manner is assessed by
which hydrogen production is changed by fractioning the
syngas streams and by importing methane from external
sources. The focus of this research is on the gasifier since it
requires the most improvement in terms of energy and exergy
utilization. The significance of the present work is its focus on
green energy sources and increasing the efficiency of systems
for hydrogen production. It is anticipated that the results will
assist efforts to optimize and enhance the environmental
performance of energy systems. The use of computer simu-
lation yields a better understanding of the overall system, and
is a particularly useful tool in support of design.
2. Thermodynamic evaluation
A thermodynamic evaluation of a complex system requires
consideration of its components and their characteristics,
chemical reactions and thermal losses. Recently, biomass
gasification in indirectly heated steam gasifiers has received
much attention for the conversion of biomass to combustiblegas [38,39]. In our simulation, we consider the energy effi-
ciency of the gasification reaction as the total energy of the
desired products divided by the total energy of the process
inputs [28]. For this analysis, the products are taken to be
a mixture of H2O, N2, H2, CO2, CH4, CO, NH3 and H2S. Char is
assumed to consist of solid carbon (C) and tar is not taken into
account in the simulation.
Simulations are preformed with the Aspen Plus simulation
software, which is utilized in a wide range of industrial
applications [18]. A Fortran subroutine is applied to control
process yields. In Aspen Plus, streams represent mass or
energy flows. Energy streams may be defined as either work or
heat streams, of which the latter also contain temperatureinformation to avoid infeasible heat transfer. Mass streams
are divided by Aspen Plus into three categories: mixed, solid,
and non-conventional (for substances like biomass). Mixed
streams contain mixtures of components, which can be in
gaseous, liquid and solid phases. The solid phase component
in this simulation is solid carbon (C). Thermodynamic prop-
erties are defined in the Aspen Plus libraries for chemical
components. Components present in the mixed and solid
stream classes may participate in phase and chemical equi-
librium, andare automaticallyflashedby Aspen Plus at stream
temperature and pressure. Non-conventional components are
defined in Aspen Plus by supplying standard enthalpy of
formation and the elementary composition (ultimate andproximate analyses) of the components may also be defined.
Biomassis characterized in thismannerhere. Although Aspen
Plus calculates enthalpies and entropies for conventional
components, ambient temperature and pressure, which are
required in evaluations of exergy, are not readily available in
the result output. A property termed availability by Aspen Plus
is calculated for conventional components, but this does not
include chemical exergy. Forthese reasons, the EESprogram is
used to calculate total exergy (physical and chemical) for each
stream in this simulation.
The following simplifying assumptions are made in the
simulation:
Char only contains solid carbon and ash, and there is no tar
yield.
The process occurs at steady state and isothermally, and
residence time is not considered. Also catalysis is not
used.
The ZNO-bed and the pressure swing adsorption (PSA)
system are not included in the energy and exergy
calculations.
All gases behave ideally.
Air is considered on a volume basis as 79% nitrogen and 21%
oxygen.
A heat stream is used as a heat carrier in Aspen Plus instead
of sand.
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2.1. Balances
Mass and enthalpy values are evaluated with Aspen Plus. For
a general steady-state process, we can write mass and energy
balances, respectively, as
Xi
_mi ¼
Xo
_mo (1)
Xi
_Ein ¼X
o
_Eout (2)
An overall exergy balance can be written for a steady-state
process as follows:
X_Exi
in
¼X
_Ex j
out
þX
_Ex
dest(3)
where
X_Exi
in
¼ _Exair þ _Exdrybio þ _Exbiomoist þ _Exst þ _Exmeth (4)
X _
Ex j
out¼_
Exprodg þ_
Exunconcarb þ_
Exst þ_
Exexh þ_
W turb (5)
These values are available in Table 1
Both physical and chemical exergy inlet and outlet values
are determined for the Gasification, Combustion, SMR, HTS
and LTS product gases, are used to assess exergy destructions.
Some components possess only physical exergy. The specific
flow exergy associated with a specified state is expressible by
the sum of specific physical and specific chemical exergy:
exprodg ¼ exph þ exch (6)
The physical exergy defined as
exph ¼ ðh À hoÞ À T0ðs À soÞ (7)
and the chemical exergy contribution can be calculated for an
ideal gas mixture as follows:
exch ¼X
i
xi
Àexch
i À RT0 ln xi
Á(8)
Here, xi is the mole fraction and exchi the standard chemical
exergy of component i. Standard chemical exergy values used
here are taken from model 2 in Szargut et al. [26].
The entropy balance for a steady-flow reacting system can
be written asX _Q j
_T j
þX
_misi ÀX
_moso þ _Sgen ¼ 0 (9)
The exergy destroyed due to irreversibility can be expressed
as follows:
_Exdest ¼ T0_Sgen (10)
The physical exergy of biomass is zero when it is entering
the system at temperature T0 and pressure P0. Thermody-
namic properties are needed for the calculation of the chem-
ical exergy of biomass. Since such properties for oil palm shell
biomass are not available, a correlation factor for solid
biomass (for O/C < 2) is used based on statistical correlationsdeveloped by Szargut et al. [29]:
b¼1:044 þ 0:0160H=C À 0:3493O=Cð1 þ 0:0531H=CÞ þ 0:0493N=C
1 À 0:4124O=C
(11)
The specific chemical exergy for biomass can then be
determined as
ExchbioðsolidÞ ¼ bLHVbio (12)
Although the magnitude of the physical exergy of biomass
is small, it is calculated in this simulation after the drying
process. The calculation is performed using the heatcapacity of dry biomass which is described by Gronli and
Melaaen [37]:
C pðbioÞ ¼ 1:5 þ 10À3T (13)
where C p(bio) is the heat capacity and T is the temperature of
the biomass.
The change in specific entropy in Eq. (7) can be written for
biomass as
Ds ¼
Z T
T0
C p;i
TdT (14)
Equation (14) can be used to calculate the physical exergy
with Eq. (7) at the specified temperature.The heat capacity of solid carbon is determined using
Aspen Plus property data and substituted into Eq. (7) to find
entropy values, which are used for the thermal exergy calcu-
lation in Eq. (7).
2.2. Energy, cold gas and exergy efficiencies for BCL
gasification
Tables 2 and 3 show the properties and gas composition of the
main streams and the overall system performance based on
the simulation results. The latter can be expressed with the
cold gas efficiency. This measure is the ratio of the chemical
energy of the produced gas to that of the biomass feed energy
Table 1 – Exergy results for the overall hydrogen plant forCase 1.
Exergy flowrate (kJ/h)
Percentage of totalexergy inlet
Inputs
Wet biomass 2,397,523 81.0Water 8,469 3.0
Air 3,432 1.0
Methane (CH4) 527,500 16.0
Purchased electricity 0 0.0
Total 2,936,924 100.0
Outputs and destructions
Hydrogen 653,052 22.3
Electricity 46,800 1.5
Exhaust 303,573 10.3
Water 49,850 1.7
Total output 1,053,265 35.9
Exergy destruction 1,883,658 64.1
Total output
and destruction
2,936,923 100.0
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content. The cold gas energy efficiency of the BCL hydrogen
plant is determined as
hcg ¼_mprodg LHVprodg
_mdrybioLHVdrybio(15)
Hydrogen is the desired product in this simulation, so theproduction efficiency can be described as the system effi-
ciency in general. Energy efficiency h and exergy efficiency j
values are often evaluated for steady-state processes, and can
be written here as follows:
hsys ¼_Eprodg þ _W turb
_Eair þ _Edrybio þ _Ebiomoist þ _Est þ _Emeth
(16)
jsys ¼_Exprodg þ _W turb
_Exair þ _Exdrybio þ _Exbiomoist þ _Exst þ _Exmeth
(17)
where _Eprodg is the rate of product energy output, _Exprodg is the
rate of product exergy output and_
W turb is the turbine workrate. Also the energy efficiency of component i may be written
as follows:
hi ¼ 1 Àð _EoutÞi
ð _EinÞi
(18)
where ð _EoutÞi and ð _EinÞi are the energy output and input rates
for component i. Similarly, the exergy efficiency for compo-
nent i may be written as
ji ¼ 1 Àð _ExdestÞi
ð _ExinÞi
(19)
where ð _ExdestÞi and ð _ExinÞi respectively are the exergy destruc-
tion rate and the exergy input rate for component i.
The steam–biomass ratio (STBR) can be expressed as
STBR ¼_mst
_
mdrybio
(20)
Also, the ratio of exergy destruction xdest for a component
can be evaluated by dividing its exergy destruction by the total
exergy provided to the system. Here, we can write
xdest ¼ð _ExdestÞi
ðP
_ExiÞin
(21)
where ð _ExdestÞi is the exergy destruction for each component
and ðP
_ExiÞin is the exergy flow of all input material streams.
3. Case study
Heating and drying are endothermic processes that require
a source of heat. Heat can be supplied by an external source
via indirect heating. In gasification, indirect heating allows us
to have indirectly heated gasification. More often, a small
amount of air or oxygen, typically not more than 25% of the
stoichiometric requirement for complete combustion of the
fuel, is input for the purpose of partial oxidation, which
releases sufficient heat for drying and pyrolysis as well as for
the subsequent endothermic chemical reactions. These
include the carbon–oxygen, boudovard, carbon–water and
hydrogenation reactions [35].
The system simulated in this paper includes the gasifica-
tion plant and the hydrogen plant. Related energy efficiencyand economic analyses have been performed [38]. In this
investigation, the system analyzed by those authors is modi-
fied and energy and exergy efficiencies are evaluated for
varied conditions (e.g., water streams and the amount of
entered biomass).
In this study, we focus on the gasifier. The R-GIBBS block
reactor uses single-phase chemical equilibrium, or simulta-
neous phase and chemical equilibrium, by minimizing the
Gibbs free energy, subject to atom balance constraints. This
block reactor is useful when the temperature and pressure are
known and the reaction stoichiometry is unknown. The latter
reactor and the decomposed (RYIELD) reactor combined have
been used to model the BCL low-pressure indirectly heated
Table 2 – Properties and composition of the main streams in the H 2 plant.
Quantity Stream
COMP Outlet SMR outlet HTS Outlet LTS outlet PSA inlet Off-gas
T (C) 43.5 850 518 241 43 40
P (bar) 31 28 27 26.5 25.2 1
Flow rate (kg/h) 52.06 121.23 121.23 120.90 70.695 69.70
Dry gas composition (% vol)
H2O 0.0 49.0 38.0 34.0 0.0 0.0
H2 46.0 28.0 40.0 43.0 65.60 26.58
CO 30.0 15.0 4.00 1.0.0 0.40 0.30
CO2 14.0 6.0 17.0 21.0 32.15 72.32
CH4 10.0 0.01 1.00 1.0 1.85 0.8
Table 3 – Simulation results for cases 1 and 2.
Quantity Case 1 Case 2
Biomass flow rate (wet) (kg/h) 166.67 166.67
Biomass flow rate (dry) (kg/h) 88.40 88.40
Steam input to gasifier (kg/h) 33.17 33.17
Syngas fraction to SMR-COMB 0.196 0.0
CH4 input to SMR-COMB (kmol/h) 0.0 0.475
Hydrogen production rate (kg/h) 5.53 6.91
Steam-biomass ratio (STBR) 0.38 0.38
Cold gas efficiency, hcg 0.30 0.40
System energy efficiency, hsys 0.24 0.27
System exergy efficiency, jsys
0.22 0.25
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gasifier. The feedstock used for this analysis is oil palm shell,
delivered at 50 wt% moisture; the ultimate and the proximate
analyses for the feed used in this study are given in Table 4.The plant capacity is designed to be 2000 drytonne/day(83.3 t/
h), and the lower heating value (LHV) of the dry biomass is
22.14 MJ/kg.
In this paper we divide the system into two parts. The first
is the gasification plant in Fig. 1, which produces syngas for
the second part of the system, which is the hydrogen plant. In
this assessment, we consider two cases that have been
designed for producing hydrogen via gasification. In the first
case, 19.6% of the produced syngas and the total amount of
off-gas are fed to the steam-methane reformer combustion
(SMR-COMB) in order to supply heat for the SMR in the
hydrogen plant in Fig. 1. Also, in the first case energy and
exergy analyses for the entire system are carried out. In thesecond case, instead of feeding the SMR-COMB with frac-
tioned syngas, it is fed by just enough methane gas (CH4) from
external sources to enhance the hydrogen yield at the end of
the simulation.
As a case study, biomass and flue gas are mixed in the dry
reactor in order to evaporate the water, and dry the wood
from 50% to 5.7% moisture content. After the drying process
is completed in the stoichiometric reactors (RSTOIC) in Fig. 1,
the gas passes through the decomposition (RYIELD) reactor.
Normally the BCL device is an indirectly heated gasifier
consisting of two main reactors [38]: the gasifier and the
combustor. However, in this simulation the BCL unit contains
three main reactors which are the decomposer, the gasifierand the combustor. First the biomass is decomposed in the
RYIELD reactor (this reactor is used in the Aspen plus simu-
lation); this reactor simulates the decomposition of the feed
at low temperature (394 K, 1 atm). In this step, biomass is
converted into its constituent components including carbon
(C), hydrogen (H2), oxygen (O2), sulphur (S), nitrogen (N2) and
ash, by specifying the yield distribution according to the
biomass ultimate analysis. These components enter the
Gibbs reactor (at 1 atm and 1162 K) to produce syngas using
steam (at 923 K and 1 atm), as seen in Table 5. The heat of
combustion of the actual indirectly heated gasifier system is
transferred to the gasifier by recirculating hot inert material,
usually sand.
In this simulation, however, it is a designed heat stream
using just enoughheat to supply thegasifier heat demand and
the gas cleaning section (see Fig. 1). At the same time
combustion occurs in a third reactor, which is fed with
methane gas (CH4) from an external supply and char gener-
ated by the gasifier. There are two combustors which operate
at different temperatures; the first combustor runs at
a temperature of 1255 K, while the second runs at 1355 K with15% excess air.
Fig. 1 shows that the syngas enters the scrubber which is
designed for syngas cleaning. During this process some of the
toxic gas is cleaned and water in the syngas is condensed.
After entering the scrubber the syngas passes to the separator
(SP1), from which part goes to the SMR-COMB. Note that in
this study, the first case has the fraction of the product
stream at 19.6% but the fraction of the product stream is 0%
for the second case. After, the syngas passes through the five-
stage compressor system, which has polytropic efficiency of
79% for each compressor stage and a mechanical efficiency of
95%. The syngas is cooled, the preferred method being air
cooling as it avoids excess pressure losses. After thecompression and cooling processes, the syngas pressure
increases from 1 to 31 bar while the temperature increases
by 43 K. Before reaching the ZnO-Bed, the syngas is heated
to 653 K because the ZnO-Bed cannot function at a lower
temperature [40].
After sulphur cleaning in the ZnO-Bed, the syngas
undergoes three main reactions: steam-methane reforming
(for which the main reaction is CH4 þ H2O$CO þ 3H2), high-
temperature shift (for which the main reaction is
CO þ H2O$CO2 þ H2), and low-temperature shift. The water-
gas shift reaction is usually performed in two stages in
commercial processes: a high-temperature shift (HTS) in the
range of 643–693 K and a low-temperature shift (LTS) in therange of 473–523 K [38]. The sulphur-free syngas mixes with
the steam from the superheater in mixer 1 to drive SMR. The
reforming condition is fixed at 1123 K and 28 bar because
methane conversion decreases at high pressure. After the
syngas enters the water heat boiler (WHB3), it cools to 677 K
before entering the high-temperature shift reactor (HTS).
While reducing carbon monoxide in the shift reactor, the
hydrogen yield increases by almost 7.5%. As shown in Table 2,
after the HTS, the syngas passes through a heat exchanger
(HE6) and superheater, where its temperature reduces to
473 K. The final treatment for the syngas before pressure
swing adsorption (PSA) is the low-temperature shift reactor
(LTS), where the carbon monoxide is converted and hydrogencontent increased. The outlet of the LTS has the highest
hydrogen flow rate (6.51 kg/h here).
The PSA unit purifies the syngas by separating the
hydrogen from the other components in the shifted gas
stream, mainly CO2 and unreacted CO, CH4 and other
hydrocarbons. Based on studies and data from industrial
gas producers, the shifted gas stream must contain at least
70 mol% hydrogen before it can be economically purified in
the PSA unit. For the present analysis, the concentration of
hydrogen in the shifted stream prior to the PSA unit is
between 60 and 65 mol%. Therefore, part of the PSA unit
hydrogen product stream is recycled back into the PSA
feed. For a 70 mol% hydrogen PSA feed, a hydrogen
Table 4 – Proximate and ultimate analyses and other datafor oil palm shell [41].
Proximate analysis (wt% dry basis)
Volatile matter 73.74
Fixed carbon 18.37
Ash 2.21
Ultimate analysis (wt% dry basis)
C 53.78
H 7.20
O 36.30
N 0.00
S 0.51
Moisture content (wt %) 5.73
Average particle size (mm) 0.25–0.75
Molecular formula CH1.61O0.51
Lower heating value (MJ/kg) 22.14
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recovery rate of 85% is typical with a product purity of
99.9% by volume [40].
For the SMR-COMB unit, air enters at 298 K and 1 atm and
passes through two heat exchangers (HE2 and HE4). The
temperature of the air rises to 1060 K, while concurrently the
off-gas from the PSA and the fractioned syngas passes
through heat exchanger 5 and heat exchanger 3 at 720 K and
1 atm, and enters the SMR-COMB to supply heat for SMR (Case
1). In addition heat exchanger 3 and WHB3 produce the steam
for the turbine which produces electricity.
The effects of fractioned syngas on the hydrogen produc-
tion yield are shown in Fig. 2. That figure shows that the
steam-methane reformer combustion (SMR-COMB) is
supplied with methane gas at a rate of 0.76 kg/h CH4. The
equivalent thermal contribution of this gas is the same as the
19.6% of the fractioned syngas in Case 1.
4. Results and discussion
We now report the results of the energy and exergy analyses,
including the energy and exergy efficiencies and exergy
destructions for each component. The results are reported for
Case 1 in Table 1. It is demonstrated that the inlet and outlet
exergy flows for the hydrogen plant are mainly attributable to
the energy andexergyinlet with the biomass and the methane
gas. The produced electricity also contributes to the system
products and efficiency, on energy and exergy bases. Further,the exergy losses are observed to be due to emissions and
internal consumptions associated with chemical reactions,
particularly those related to combustion and gasification.
Note that inlet exergy values are evaluated for fuels on an LHV
basis.
Some key results from the simulations for the two cases
are presented and compared in Table 3. In this simulation at
1162 K and 1 atm, for the low-pressure indirectly heated
gasifier (R-GIBBS) with the decomposition reactor (RYIELD),
the maximum hydrogen production efficiency for the first
case is found to be 24% while exergy efficiency is 22%, as
determined using Eqs. (16) and (17). These values are relatively
low because of losses for the gasifier, combustor and SMR. For
the second case, performance improvements are noted, as the
energy efficiency is found to be 27% and the exergy efficiency
25%. The efficiency increase occurs because importing
methane gas from external sources increases the hydrogen
production rate from 5.53 kg/h to 6.91 kg/h after the PSA unit.
The energy and exergy efficiency values for Cases 1 and 2
differ because of the improvement applied to the system in
Case 2. These differences are not attributable to uncertainties
associated with computer simulation. The improvements in
Case 2, as previously noted, include non-fractioned syngaswhich passes sequentially through the SMR, HTS and LTS
units in order to increase the hydrogen yield in the reforming
and shift reactions. In Case 1 which used fractioned syngas,
19.6% of the gaseous product cannot pass the SMR, HTS and
LTS processes so less hydrogen is produced, which affects the
overall system energy and exergy efficiencies. Also, the cold
gas efficiencies differ because the second case has a higher
productivity rate than the first.
The products from this process are hydrogen and elec-
tricity, but other energy streams also exit. There are two
sources of flue gas: the char combustor and the second
combustor (SMR-COMB). Together, their energy contents
account for about 4% of the energy in the dried biomass.The simulated hydrogen production flow rates for Cases 1
and 2 are shown in Fig. 3. It is seen there that the hydrogen
production is affected by fractioned and unfractioned syngas
through thecomponent split 1 (point 13 in Fig. 1). It is observed
for the first case that the hydrogen production rate is lower
than for the second case, in which the SMR-COMB unit is fed
with fractioned syngas. At the end of the simulation, there-
fore, the fractioned syngas is prevented from passing through
the SMR, HTS and LTS processes, which are where the
hydrogen yield is increased. In the second case, the unfrac-
tioned syngas passes through of the SMR, HTS and LTS units.
The energy andexergyefficiencies of the main components
involving chemical reactions are shown in Fig. 4. The energy
Table 5 – Conditions at the gasifier outlet.
Quantity Value
Gasifier outlet temperature (C) 890
Combustor 1 outlet temperature (C) 982
Gasifier outlet composition (kg/h)
H2O 38.23
H2 3.52CO 31.87
CH4 5.93
CO2 22.98
NH3 0.08
H2S 0.08
N2 0.70
C (solid) 18.02
Ash 0.77
SMR-COMB
32
40 54
SMR
Q2
CH4
HE1
21
22
AIR from HE2Split syngas
and off gasfrom HE3
Fig. 2 – Aspen Plus simulation flow diagram of a process for
steam-methane reforming with heat supplied by
combustion of methane gas (CH4
) from external source.
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efficiency of the gasifier is approximately 72% while the cor-
responding exergy efficiency is 66% based on Eqs. (18) and (19).
Normally the energy efficiency at these conditions may be
expected to be around 80%; the value here is lower since the
system assessed has unconverted solid carbon as char and
catalysis is not used to promote the gasifier reactions. In
addition, the fuels are over-oxidized in the gasifier in order to
attain the required gasification temperature [41], and this
process may reduce the gasifier efficiency. The gasifier exergy
efficiency is lower than the energy efficiency, mainly due to
chemical reactions and oxidization. Both combustion reactors
operate with high energy and low exergy efficiencies. Thelatter are associated primarily with internal irreversibilities.
For the steam-methane reformer, the energy efficiency is
found to be 83% and the exergy efficiency 77%. These values
are consistent with those reported in the literature [34,42].
Note that in the HTS and LTS units, the shift reactions occur
but there is no combustion. Therefore internal exergy
destructions are very low, leading to high exergy efficiencies
for these devices. It is observed that significant heat is trans-
ferred to water to produce steam in heat exchangers, boilers
and economizers. The results suggest that the low-pressure
indirectly heated gasifier requires improvements in terms of
energy recovery.
For this system, with its feed rate of 4000 kg per day of wet
biomass to the gasification process, the hydrogen production
energy efficiency is 24% (see Table 3). It is determined with the
simulation that 132.7 kg hydrogen can be produced from
4000 kg biomass with an energy rate of 4.5 MW. For the second
case, also with a feed rate of 4000 kg per day of wet biomass tothe gasification process, the hydrogen production energy
efficiency is improved to 27% (see Table 3). Also, it can be
found that 165 kg hydrogen is produced from 4000 kg biomass
with an equivalent thermal input 5.6 MW.
It can be seen in Fig. 5 that the reactors with the highest
exergy destruction rates based on Eq. (21) are the gasifier
Fig. 3 – Simulated hydrogen production rates for Cases 1 and 2.
Fig. 4 – Energy and exergy efficiencies for main system components for Case 1.
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(R-GIBBS), in which 34% of the total exergy inlet is destroyed.
This observation implies that the gasifier is an important
component for efficiency system improvement, especially
since the biomass can be gas, solid and liquid. The combustion
reactors are also responsible for large exergy destructions,
mainly due to irreversibilities associated with the combustion
reactions. These exergy losses mainly relate to chemical
exergy destructions, andfor both combustion units 1 and 2 are
around 11%. It is also interesting that the dry reactor has
a high exergy destruction rate, which is approximately 4.5%
due to the 50% moisture content of the inlet biomass. Thus,
the heat demand is high for this process, resulting in high
exergy destruction rates.
Exergy destruction ratios (on a percentage basis) for Case 1
are shown in Fig. 5 for the system components and in Fig. 6 for
components with lower exergy losses. The exergy destruction
ratio is the ratio of the exergy destruction for a component to
the overall system exergy destruction. The exergy destruction
ratio in Fig. 5 shows that the HE2 and WHB units are each
responsible for about 1% of the exergy destruction rate, and
the SMR, HTS and LTS reactors, which do not include
combustion, also have very low exergy destruction rates.
There are two cases of note in this assessment. The steam-
methane reforming (SMR) heat demand is supplied by
fractioned syngas in one case, and by external methane gas
(CH4) supply in the other. Assessments are required to
understand better how hydrogen production is affected by
changing this parameter, with the aim of investigating the
feasibility of producing hydrogen from biomass and better
understanding the potential of biomass as a renewable energy
source. As pointed out earlier, detailed energy and exergy
assessments were performed for just one system (Case 1). For
Case 2, the improvement of the system performance was
gauged by altering one parameter and observing the impact
on hydrogen production and system overall energy and exergy
efficiencies. The energy and exergy efficiencies and exergy
destruction ratios for the main system components for Case 2
are similar to those for Case 1, so performance figures are not
presented for Case 2. Also, the exergy destruction ratios for
auxiliary components for Cases 1 and 2 are similar. However,
improvements are observed for Cases 1 and 2 in hydrogen
production rate and overall system energy and exergy
efficiencies.
On a broader scale, the results of this study support the
contention by many that biomass may contribute to a future
hydrogen economy. Although biomass has the advantage of
being renewable if managed properly, it has challenges; large
quantities of biomass need to be grown and transported to
Fig. 5 – Exergy destruction ratios for main components for Case 1.
Fig. 6 – Exergy destruction ratio for auxiliary components for Case 1.
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produce a small amount of hydrogen. Transportation
concerns may be alleviated by using pyrolysis of biomass to
produce bio-oil, as opposed to direct gasification. Based on the
costs and availability of hydrogen production processes, it is
likely that hydrogen will be produced by steam-methane
reforming or coal gasification during a transition to
a hydrogen economy. Future advances in water-splitting
processes may allow them to replace fossil fuel processes ascleaner, long-term energy solutions. Many predictions of how
a hydrogen economy will unfold have been published. For
instance, a roadmap was created that provides an overview of
a possible evolution of hydrogen production technologies in
the future [43]. The timing of each step in this evolution
towards a hydrogen economy depends on how quickly tech-
nology advances and other factors.
5. Conclusions
The energy and exergy analyses performed of biomass-based
hydrogen production have yielded energy and exergy effi-
ciencies and an understanding of the impact on performance
of several parameters. The feasibility of producing hydrogen
from biomass and a better understanding the potential of
biomass as a renewable energy source have been attained by
considering two methods: 1) the heat required for steam-
methane reforming is supplied by fractioned syngas, and 2)
the SMR-COMB reactor is provided with externally supplied
methane gas. Oil palm shellis the biomass considered. For the
direct gasification reaction, a BCL-type low-temperature
indirectly heated steam gasifier is examined. The thermody-
namic assessments for the two cases demonstrate that the
processes have low efficiencies. The simulation confirms for
the system that the second case considered, which indicates
performance improvements, has higher energy and exergy
efficiencies than the first case.
Acknowledgments
The authors acknowledge the support provided by the Ontario
Research Excellence Fund and the Natural Sciences and
Engineering Research Council of Canada.
Nomenclature
C p specific heat, kJ/kg K_E energy flow rate, kJ/h_Ex exergy flow rate, kJ/h
ex specific exergy, kJ/kg
h specific enthalpy, kJ/kg
LHV lower heating value, MJ/kg
mi inlet mass, kg
mo outlet mass, kg
Po reference-environment pressure, kPa
Q heat, kJ
R universal gas constant, kJ/kmol K
S entropy, kJ/K
STBR steam–biomass ratio
T temperature, K
T0 reference-environment temperature, K_W work rate, kJ/h
x exergy ratio
Greek symbols
j exergy efficiency, %h energy efficiency, %
b correlation factor, %
Subscripts
bio biomass
biomoist biomass moisture
cg cold gas
dest destroyed
drybio dry biomass
en energy
gen generated
i, j index for components
in input
meth methane gas (CH4)out output
st steam
sys system
turb turbine
prodg produced gas
unconcarb unconverted carbon
Supercripts
ch chemical
ph physical
Acronyms
COMB combustion
COMP compressor
COOL cooling
EC economizer
HE heat exchanger
HTS high-temperature shift
LTS low-temperature shift
PSA pressure swing adsorption
WHB waste heat boiler
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