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Energy Efficiency of an Integrated Process Based on Gasification for Hydrogen Production from Biomass.

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Dev. Chem. Eng. Mineral Process. I4(1/2), pp. 33-49, 2006.
Energy Efficiency of an Integrated Process
Based on Gasification for Hydrogen
Production from Biomass
G. Weber, Q. Fu and H. Wu*
Dept of Chemical Engineering & Centrefor Fuels and Energy,
Curtin University of Technology, GPO Box U1987,Perth 6845,
Western Australia
Hydrogen production from COrneutral biomass is expected to play an important
role in renewable hydrogen supply. This study investigates the overall energy
eflciency of hydrogen production from biomass via an integrated process based
biomass gasipcation. Key parameters and experimental data of the process were
obtained from the literature, considering six cases of air-blown biomass gaslfiers
due to the availability of experimental data in the literature. The results show that
such an integrated system can generally have an overall energy efficiency of
40-6096, depending on biomass properties and process configurations. Significant
efficiency improvement can be achieved i f the gasifier operates at a similar
pressure to the reformer, which typically operates at elevated pressures. Analysis
further suggests that the gasijkation with steam and other oxidiser fe.g. air) is the
most energy efficient way for hydrogen production and such a strategy also delivers
a high amount of hydrogen. Other strategies for efficiency improvements include
increasing reaction conversion in the reformer, enhancing CO conversion in the
shift reactor and improving process heat recovery. Limitations of this work and
future necessary improvements are also discussed in this paper.
* Author for correspondence (H. Wu@exchange.curtin.edu.au).
33
G.Weber, Q.Fu andH. Wu
Introduction
Extensive research is being conducted worldwide on a hydrogen-based energy
system, including hydrogen production, storage, transportation and utilisation. As
hydrogen is only an energy carrier not an energy resource [I], the ultimate
sustainability of the hydrogen-based energy system depends on the source and
method for hydrogen production. Currently, hydrogen is mainly produced from
nonrenewable energy resources, predominantly fossil fuels [2], including natural
gas, petroleum and coal. As a result, net carbon dioxide is produced during the
hydrogen production process as an inevitable by-product. Therefore., for the
hydrogen-based energy system to be truly sustainable, the hydrogen must be
produced from C02-neutral
renewable sources, either water or biomass.
Development of efficient and cheap hydrogen production technologies is one of the
keys to the success of a sustainable hydrogen-based energy system.
For large-scale applications, there are still significant challenges in sustainable
hydrogen production from water. For example, efficient production of hydrogen
from water (such as electrolysis) remains a major difficulty because splitting water
molecular requires a significant amount of energy, which must ultimately be
provided from other sources such as renewable sources, or fossil Fuels [l].
Hydrogen production from renewable biomass appears to be more promising. A
number of possible technical routes are available to produce hydrogen from
biomass. These routes can be broadly divided into two categories, i.e. biological
routes and thermochemical routes, as summarised in Figure 1 [3]. As the biological
routes [4] suffer from low production rates, thermochemical processing is believed
to be the most feasible method for hydrogen production from biomass. The typical
thermochemical routes include biomass gasification [ 5 ] , biomass pyrolysis followed
by steam reforming of the bio-oil [6] and high pressure aqueous processes [7].
These processes can produce either a storable intermediate product, such as ethanol
or bio-oil that can be hrther converted to hydrogen, or yield a hydrogen rich
product through a direct conversion.
This paper evaluates overall energy efficiency for hydrogen production from
biomass through biomass gasification followed by reforming, shift reaction and gas
34
Energy Eficiency of a Process Based on Gasificationfor H?from Biomass
separation. Process parameters and data are sourced from existing design and
experiments in the literature. Energy and mass balances are then established to
evaluate the overall energy efficiency. Sensitivity analysis of key process
parameters is also performed.
1
BioResource
Biohgica I
ThermoAhemical
I
I
Anaerobic
Digestioti
Frriiwntation
1
Metaoolic
Pwqessing
Gasification
j
High Pressure
Aquaous
Priolvsi5
Reforni.ng
Figure 1. Technical routesfor hydrogen production from biomass (31.
Methodology
I. Overall process of hydrogen production from biomass gasification
Figure 2 presents the detailed process that was analysed in this study, The biomass
gasification process is a direct thermochemical route. Although process details may
vary, typical steps of the gasification routes are given below.
The key process steps include drying, gasification, reforming, shift reaction and
gas separation. First biomass feed will be dried up to the moisture content (< 20%)
required by gasifier. In the gasifier, through gasification reactions [3], biomass
is converted to a gasification product gas, containing H2, CH4, CO, C02, other
light hydrocarbons, tars, plus ash and residue carbon particulates.
The operating
35
G. Weber, Q. Fu and H. Wu
Water
Heater
Heater
Gasifier
with gas
deaning
Watar
1
Otlqaaer
Aah +Char
Figure 2. A typical process diagram of hydrogen production from biomass gasification.
I
Table I . Experimental data on biomass properties, gasification operating
conditions and product gas compositions, obtainedjiom references [ I
B
CASES
C
5
10
H
11
84
16
74
0
E
46
5
49
35
3
62
28
3
69
ER: equivalent ratio, defined as the amount o f oxygen in the oxidiser (weight) divided by the amount
of oxygen needed for stoichiometric combustion of the biomass (weight);
SR: steam ratio, defined as the amount of steam (weight) per amount of biomass (weight, daf);
CCE: carbon conversion efficiency, the amount of carbon (mol) that leaves the reactor as gases or
vapours compared to the total amount of carbon (mol) in the biomass.
' Assumed values as these data are not given in the references.
Assumed to be same as equilibrium results as experimental data are not given in references.
36
76
4
20
Energy Eficiency of a Process Based on Gaslficationfor H2 from Biomass
conditions and the product gas compositions strongly depend on biomass
properties, gasifier configurations and operation conditions [8]. Gas cleaning [9] is
generally required to remove various contaminants (particles, alkali, sulphur,
chlorine, etc.) in syn-gas but not considered in the modelling of this paper.
A reforming step is considered in this process because CH4, hydrocarbons and
tar vapours are present in the gasification products (see Table l), and need to be
hrther steam reformed and converted to H2, CO and C02. The gas mixture is then
fed into a shifter reactor, where extra H2is produced together with the conversion
of CO to CO2 by the water-gas shift reaction. The output of the shift reaction is a
gas mixture containing mainly Hz and C02. High purity hydrogen product will be
produced through a gas separation process, such as membrane separation, pressure
swing adsorption (PSA), adsorption or a combination of these technologies.
For a practical process, heaters and compressors are also needed to bring the
reactants and/or intermediates to the required temperature and pressure. For
example, the gasification agents (air/OZand steam) need to be brought to required
conditions from air/02 and water at ambient temperature and pressures, before
being fed into the gasifer. In addition, heat exchangers and furnace are also
deployed for process heat recovery. A typical example is that the reaction
temperature of the reformer is much higher than the shift reactor. In this case, heat
exchangers need to be introduced to recover the heat, which can be used w i t h the
whole process, such as biomass drying and steam generation.
ZZ, Overall approach of process efficiency analysis
To evaluate overall process efficiency, first a detailed flow sheet was developed.
Based on the flow sheet, a mass balance was then established using process
parameters and experimental data from the literature. A process energy balance was
made based on the mass balance and additional thermodynamic data. This paper
considers energy inputs and outputs including: (a) the heat of reaction of the
process steps; (b) the heat for heating up reactants or intermediates to the required
temperatures; (c) the heat of water evaporation; (d) the work for the compression to
the required pressures; (e) the heat that can be recovered from the streams; and
(0
37
G. Weber, Q.Fu and H. Wu
an overall heat loss of the whole process. Using mass and energy balances, the
overall efficiency of the biomass to hydrogen can be calculated. The calculation is
implemented in an Excel template. Sensitivity analysis can be done by varying the
process parameters (e.g. temperature, pressure).
Table 2. Basic process data usedfor calculation (except gasifier parameters).
Literature Data
] References
Ambient conditions
.,-,....
values
Parameters
1
ld
ld
Raw feed
I Pressure,MPa
I la1
I
.I &! Preheated
m
8
L C
w
Oi
dried
Air, water
feed
Preheated
'
Temperature, "C
Pressure, MPa
Temperature, "C
Pressure, MPa
Temperature, "C
Pressure, MPa
Temperature. "C
I
Assumed
Ambient conditions
)
a
'luesusedI
in this paper
0.1
25
0.1
120
0.1
25
Assumed
0.1
120
I
Pressure, MPa
"C
Temperature,
systemC
HI recovery, %
H2 purity, %
Air feed Pressure, MPa
Temperature, "C
Excess ratio
2
k
Off gas Pressure, MPa
Temperature, "C
Efficiency of gas compression, %
Process heat recovery efficiency, %
Heat-to-work conversion efficiency,%
- Other overall process heat loss, %
G a s separation
I II
38
2-4
I [24,25]
Same as output from shift reactor
Up to 90
I [24,25]
u p to 99.99
1 [24,25]
Ambient conditions
Assumed
Atmospheric operations
Assumed
Assumed
Assumed
35
I [27]
Assumed
2.0
250
85
09.99
0.1
25
0.5
0. I
X 50
80
!20
35
I.o
Energy Efficiency of a Process Based on Gasificationfor H2from Biomass
I I . Source data on process parameters and operating conditions
Evaluation of energy efficiency requires the detailed process mass and energy
balances, based on creditable data on process parameters and operating conditions
of each process step. In this study, an exhaustive literature search [lo-271 was
performed to collect practical data and experimental results that provide all the
necessary parameters of these steps for the calculation. Where literature data is not
available, reasonable assumptions (see Tables 1 and 2) have to be made in order to
carry out and simplify the calculations.
Source data on the gasification step
The gasification step determines the gasification product gas and hence affects all
subsequent steps. Although various gasification technologies are available, fluidised
gasification seems to be the most widely used and has attracted significant research
attentions. As listed in Table 1, only six quality datasets can be found in the
literature [ 10-131. Each dataset includes necessary experimental data on biomass
properties, corresponding gasifier parameters, operating conditions and product gas
composition, which make the establishment of mass and heat balances possible.
The six datasets naturally become six case studies in this paper.
It should be noted that all the six cases used air (rather than oxygen) as an
oxidiser in gasifiers. One would argue that a future hydrogen production technology
based on gasification should consider oxygen-blown gasification technology for
easy process integration in subsequent gas separation. Due to the lack of
experimental data or modelling results for biomass gasification under oxygenblown conditions, this paper limits its discussion on these six cases using air-blown
gasification technology as the starting point. The authors are developing biomass
gasification models to predict overall conversion and syn-gas compositions, and
future work is planned to incorporate these modelling results for evaluating an
integrated process based on oxygen-blown gasification.
The moisture contents of biomass in Table 1 is that of the biomass being fed
into the gasifier so that no further drying takes place prior to the gasification as this
would change the output composition. However, sensitivity analysis was performed
39
G. Weber, Q.Fu and H.Wu
in order to evaluate the effect of the moisture content in the raw biomass before
being dried down to the moisture level in Table 1.
Basic process data on other process steps
While there are significant process variations in the gasification step, the drying,
reforming, shift conversion and gas separation steps are similar in various routes for
hydrogen production from biomass gasification. In this paper, a basic set of data for
these steps is used for several processes with different gasifiers. This set of data,
listed in Table 2, is based on typical parameters and experimental data available in
the literature.
I K Energy efficiency
From mass and energy balances, the overall energy input and output of the process
can be calculated. This paper considers two types of energy efficiency, i.e. the coldgas efficiency and the overall efficiency.
The cold-gas efficiency,
qeold-gus,
is an indication of how much energy of the
biomass finally ends up in the hydrogen output. It is defined as the ratio between
the total energy in hydrogen output and that in biomass input (daf) [26:1, as given
by:
...( 1)
HHV o/ Hydrogen Output
~ < o l d - y u i=
HHV of Biomass Input
The overall efficiency, qoverul/,takes into account all the energy in the system,
including total energy in biomass, the net heat and net work of the overall process,
and is defined by:
HHV of Hydrogen Output
%wrn/1
=
HHV of Biomass Input + Tot01 Net Heat + (Total Net Work I Heat - to - Work Eflcency)
An efficiency of 35% for the heat-to-work conversion is considered [27].
40
. * .(2)
Energy Efficiency of a Process Based on Gasification f o r H2from Biomass
Results and Discussion
I. Effects of gasijler operating conditions
Figure 3 presents the calculation results for the whole process, based on the data in
Tables 1 and 2. For hydrogen production in the six cases, where the gasifiers of
Cases A to E operate at 0.1 MPa while Case F at 0.4 MPa, the overall efficiency
ranges from 26.4 to 41.5%, the cold-gas efficiency from 44.7-74.3% and the
hydrogen productivity is between 43.7 and 98.3 g k g biomass (ar).
Effect of gasifier operating pressure
In Figure 3 it is worth noting that the gasifier operating pressure (0.1 MPa in Cases
A to E and 0.4 MPa in Case F) is much lower than the reformer pressure (2.0 MPa).
One would expect that considerable energy would be required to compress the large
quantity of hot gasification product gas before it was fed into the reformer.
To investigate the effect of gasifier pressure on energy efficiency, another set of
calculations were performed by increasing the gasifier operating pressure to
2.0 MPa (same as reformer pressure) with other conditions in Tables 1 and 2 being
kept constant. The calculation results indicate that the cold-gas efficiency and
hydrogen output remain unchanged. However, substantially higher overall energy
efficiency (44.6-54.4%) compared to (26.341 S%) can be achleved when gasifiers
operates at the same pressure as the reformer, as shown in Figure 4. This is
primarily due to significant less energy required to compress low-temperature
reactant gases. Therefore, to achieve high overall energy efficiency the gasifier
should be operated at a pressure similar to that of the reformer.
Effect of steam addition
In Cases A and B, no steam is used for the gasification. No energy input is required
for steam production. The net heat of the gasifier is positive, indicating a large heat
output in the gasifier. However, without steam addition, the S/C (steadcarbon
ratio) is very low in the product gas of the gasifier. More steam has to be provided
to achieve the specified S/C and S/CO (steadcarbon monoxide) ratios in the
reformer and the shift reactor, respectively. The overall net heat is still a large
G. Weber, Q. Fu and H. Wu
output, while the work needed for the process steps is about the same compared to
other cases. The reason why processes without steam do not have a higher overall
efficiency is because less hydrogen is produced, as indicated by considerably lower
cold-gas efficiency compared to Cases C and A with steam addition,
80
hhl
0 Overall effklency
60
3
f
40
E
Y
20
0
A
B
C
D
E
F
Caw.
Figure 3. Cold-gas eficiency, overall efficiency and hydrogen output of six cases
using the data in Tables 1 and 2 (note, gasifier pressure is 0.1 MPa in Cases A-E
and 0.4 MPa in Case F).
-
-
80
-__
-
.-
-
.
I Gaillier pessure. Case A-E 0 1 IA,Case F 0 4Mw
5
0
i
A
I:i
E
8
D
C
F
Caw
Figure 4. Effects of gasifier operating pressure on overall efficiency; other
conditions are fixed to be the same as those in Tables 1 and 2 (the cold-gas
efficiency and hydrogen productivity remains the same as those in Figure 3).
42
Energy Efficiency of a Process Based on Gasificationfor Hzfi.om Biomass
For Cases C-F, steam is added in the gasification. The difference between Cases
C and D is only the ER (equivalent ratio), which is 0.18 and 0.37, respectively. A
high ER indicates more oxygen is available in the gasifier, leading to an increase in
heat evolved from the gasification reactions but a decrease in hydrogen
productivity. Therefore, the cold-gas efficiency is lower and the overall efficiency
decreases as well.
High efficiencies could be achieved with high amounts of steam, 1.7 in Case C
and 1.56 in Case E, and a medium amount of oxygen, 0.18 for Case C and 0.23 for
Case D. The oxygen provides some of the heat for the process, but does not
consume too much hydrogen. In these cases, the cold-gas efficiencies were also
high, 71.8%for Case C and 74.3% for Case E, which shows that a higher amount of
hydrogen can be produced due to the steam added to the process.
The results of Case F show similar results to Cases A & B due to the smaller
amount of steam added (SR
=
0.43) with an ER of 0.28. The results indicate that
less heat is evolved from the gasifier but no steam is necessary for reforming.
The results indicate that the gasification with steam and another oxidiser (e.g.
air) is the most energy efficient way for H2 production. Such a strategy also delivers
a high amount of hydrogen and hence a high cold-gas efficiency (Cases C and E).
II. Effect of methane conversion in reformer
The purpose of the reformer is to convert various components in the gasification
product gas into CO, C 0 2 and Hz. As shown in Table 1, after cleaning the
hydrocarbons in the gasification product gas are mainly CH4 (5.4-8.6%) and a
small amount of C2H4 (0.6-3.1%).
In a reformer, generally a nearly full conversion of CzH4can be achieved [ 181.
However, the conversion of CH4 in a reformer can only reach 70-80% [19, 201.
Therefore, calculations were performed to investigate the influence of methane
conversion in the reformer on energy efficiency and H2 productivity.
The results are presented in Figure 5 . It can be seen that with an improvement of
the methane conversion, the hydrogen output and cold-gas efficiency increase. This
is due to the increase of the amount of hydrogen produced by the steam reforming
43
G.Weber, Q.Fu and H.Wu
of methane to CO and H2.The CO produced in the steam reformer will then be
converted in the shift reactor as well. The overall efficiency has a lower increase
because at higher CHI conversion efficiencies, a higher amount of heat and work is
required in the whole process. The results imply that low methane conversion (e.g.
due to catalyst deactivation) in a reformer has considerable effect on system
performance.
s
--.
80 T---
6 70
60
Eg 50
P
3
6
6
40
30
80
o cH9 conversion: 85%
70
ci CHd conversion: 100%
60
E
1
50
40
30
f
$
I"
100
80
60
40
20
A
0
C
D
E
F
Cams
Figure 5. Effects of methane conversion in the reformer on energy efficiency and
output (gasifier pressure is 2.0 MPa while other conditions are the same as
those in Tables I and 2).
ff2
44
Energy Effiency of a Process Based on Gasificationfor H2 from Biomass
III. Effect of CO conversion in shijit reactor
The shift reactor converts the CO in the reformer output into COz, producing extra
hydrogen via the water-gas shift reaction. As there are significant amounts of CO in
the reformer output, it is expected that the system performance and efficiency can
be influenced by the CO conversion in a shift reactor. In a shift reaction, typical
values achieved for the CO-conversion are between 80 to 99% [21-231.
The calculation results are presented in Figure 6 by varying CO conversion from
80% to 100%. There is a significant amount of CO in the product gas of the gasifier
(see Table 1). The CO amount is even increased by the steam reforming reactions
of methane, ethylene and the tars. Therefore, the improvement of the COconversion increases the hydrogen output, the cold gas efficiency and the overall
efficiency. As with the CH4 conversion in the reformer, the influence of CO
conversion on the cold-gas efficiency is greater than on the overall efficiency.
IV. Effects of biomass moisture content and process heat recovery
Effect of process heat recovery
There are three key opportunities in the overall system to recover process heat for
integrated use, such as biomass drying or steam generation. The first is heat
recovery from the reformer product gas due to its high temperature so that it must
be cooled before entering the shift reactor. The second is the reject stream from the
gas separation unit. It still contains some hydrogen and other unconverted products
from previous steps. The third is the unburnt char materials from the fluidised bed
gasifier. Heat recovery can be realised for the later two through combustion in a
furnace, as shown in Figure 2.
Sensitivity analysis was performed to evaluate the effect of process heat
recovery at two recovery levels, 20% and 90%. The calculation results presented in
Figure 7 shows that overall energy efficiency depends strongly on process heat
recovery. The net absolute increase in overall efficiency is 8-20% for the six cases
when heat recovery improves from 20% to 90%. The results indicate that there is
substantial amount of process heat available which should be recovered in the
system design for efficiency improvement.
45
G. Weber, Q. Fu and H. Wu
nn
_.
80
W
converson 80%
o W conversum BOH
0 W conversm 100%
'*O
1
L
1- - -
-
-
'0 100
2
g
Q
1
80
60
40
20
A
B
C
O
E
F
Caws
Figure 6. Effects of CO conversion in the shift reactor on system efficiency and H2
output (gasifier pressure is 2.0 MPa while other conditions are the same us those in
Tables I and 2).
90% Recovery
E
8
E 40w
-
8
20-
o i
A
B
D
C
E
F
Case
Figure 7. Effects ofprocess heat recovery on overall efficiency (gasif;er pressure is
2.0 MPa while other conditions are the same as those in Tables I and 2).
46
Energy Efliciency of a Process Based on Gasification for H2from Biomass
V. Further discussion
It is important to note that this paper considers the six cases using air-blown
biomass gasification technology. This is mainly due to the lack of experimental data
and modelling predictions on fuel conversion and syn-gas quality for oxygen-blown
gasification conditions. For a fiture
technology, oxygen-blown gasification
technology should be considered for the subsequent efficient gas separation.
Moreover, as the current calculations are based on experimental data, it is difficult
to predict gasifier performance under other gasification conditions.
It should also be noted that a reforming step is considered in the integrated
process in this paper (see Figure 2). This is due to the presence of considerable
amount of CH4, hydrocarbons and tar vapours in the gasification products obtained
for experiments (see Table 1). A reformer is introduced to convert these products to
H2, CO and COz. It is possible that a gasifier can be optimised to minimise the
hydrocarbon production in syn-gas. Future innovative gasification technologies
may even achieve completion gasification of biomass into light molecules such as
H2, CO and COz. In this case, one would expect that the reforming step in Figure 2
can be eliminated.
Therefore, the calculations in this paper are case studies based on the process
and limited experimental data. Further optimisation of the whole process is needed.
It is also clear that a gasification model is essential to predict biomass conversion
and syn-gas compositions under various oxygen-blown gasification conditions. The
authors are currently developing such a biomass gasification model, which will be
incorporated into the integrated system for efficiency evaluation using process
synthesis tools such as ASPEN Plus. The results will be reported in the near future.
Conclusions
This paper evaluates energy efficiency of hydrogen production from biomass via
gasification followed by reforming, shift conversion and separation. The results
show that generally an overall energy efficiency of 40-60% can be achieved,
depending on biomass properties and process conditions. The gasifier should
47
G. Weber, Q.Fu and H.Wu
operate at a similar pressure to the reformer in order to improve energy efficiency.
The gasification with steam and another oxidiser (e.g. air) is the most energy
efficient way for H2production and such a strategy also delivers a high amount of
hydrogen. Energy efficiency can also be improved by higher reaction conversion in
the reformer, hgher CO conversion in the shift reactor and higher process heat
recovery. Future work is needed to evaluate oxygen-blown gasification technology.
Acknowledgments
The authors acknowledge the support from the Australian Research Council (ARC)
through its ARC Discovery Projects Scheme, and from the Sustainable Energy
Development Office of Western Australia State Government. The authors are also
grateful to one reviewer for the constructive comments which led to improvements
of the paper’s clarity and quality.
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