New solar-biomass power generation system integrated a two-stage gasifier

Applied Energy xxx (2016) xxx–xxx

Contents lists available at ScienceDirect

Applied Energy
journal homepage: www.elsevier.com/locate/apenergy

New solar-biomass power generation system integrated a two-stage
gasifier
Zhang Bai a,b, Qibin Liu a,b,⇑, Jing Lei c, Hui Hong a,b, Hongguang Jin a,b
a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, PR China
University of Chinese Academy of Sciences, Beijing 100049, PR China
c
School of Energy, Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, PR China
b

h i g h l i g h t s
 A new solar-biomass power generation system is proposed.
 Endothermic reactions of the biomass gasification are driven by solar energy.
 The thermodynamic properties of the system are numerically investigated.
 The superiorities of the proposed system are validated.

a r t i c l e

i n f o

Article history:
Received 5 March 2016
Received in revised form 3 June 2016

Accepted 17 June 2016
Available online xxxx
Keywords:
Solar energy
Solar thermochemistry
Two-stage biomass gasification
Power generation
Thermodynamics evaluation

a b s t r a c t
A new solar-biomass power generation system that integrates a two-stage gasifier is proposed in this
paper. In this system, two different types of solar collectors, concentrating solar thermal energy at different temperature levels, are applied to drive solar-biomass thermochemical processes of pyrolysis (at
about 643 K) and gasification (at about 1150 K) for production of solar fuel. The produced solar fuel,
namely gasified syngas, is directly utilized by an advanced combined cycle system for power generation.
Numerical simulations are implemented to evaluate the on-design and off-design thermodynamic performances of the system. Results indicate that the proposed system can achieve an overall energy efficiency
of 27.93% and a net solar-to-electric efficiency of 19.89% under the nominal condition. The proposed twostage solar-biomass gasification routine exhibits improved system thermodynamic performance compared to that in one-stage gasification technical mode, and the provided heat resource is in a good match
with the requirements for the biomass gasification procedure. Under given simulation conditions in this
paper, the energy level upgrade ratio in the proposed two-stage solar-biomass gasification system for the
introduced solar thermal energy is as high as 32.35% compared to 21.62% in one-stage gasification mode.
Meanwhile, the daily average net solar-to-electric efficiency on the representative days reaches to the
range of 8.88–19.04%, while that of 9.97–15.71% in one-stage model. The research findings provide a
promising approach for efficient utilization of the abundant solar and biomass resources in western
China and reduction of CO2 emission.
Ó 2016 Published by Elsevier Ltd.

1. Introduction
Renewable energies, including solar energy and biomass, contribute to the alleviation of current energy and environment concerns due to the features of clean utilization and abundant storage
[1–4].
Various types of solar collectors, including flat plate collector,
parabolic trough collector, solar tower and dish receiver, have been

⇑ Corresponding author at: Institute of Engineering Thermophysics, Chinese
Academy of Sciences, Beijing 100190, PR China.
E-mail address: (Q. Liu).

developed to concentrate solar energy at different temperature
levels [5–8]. Currently concentrating solar power (CSP) technologies have been widely applied to generate power in addition to
photovoltaic (PV) technology [9–12]. Thermal energy concentrated
by solar collectors is used to heat feed-water to superheated steam
directly or through a heat transfer fluid (i.e., synthetic oil or molten
salt) and then the superheated steam drives the steam turbine for
power generation. Due to the uneven temporal and spatial distribution of solar energy, storage of solar energy using molten salt
or other phase change materials are investigated [13–17]. Additionally, an emerging technology in solar thermal utilization use
compressed air as heat transfer medium. The first prototype of a

/>0306-2619/Ó 2016 Published by Elsevier Ltd.

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Nomenclature
A
E
H
HHV
m

Q
R
S
t
T
W

energy level (pre-exponential factor)
exergy (kJ molÀ1 or kJ kgÀ1)
enthalpy (kJ molÀ1 or kJ kgÀ1)
high heat value (kJ kgÀ1)
mass flow rate (kg sÀ1)
heat (kW)
the gas constant (8.314 J/(K mol))
heliostat area (m2)
time
temperature (K)
electric power (kW)

Greek letters
a
reaction conversion rate
b
heating rate (K minÀ1)
g
efficiency (%)

solar powered gas turbine system was tested in 2002 without
major problems, and many investigations on the related issues
have been conducted subsequently [18–20].

The inherent properties of solar energy, such as low energy density and intermittency, provide difficulty in keeping the thermodynamic and economic performances of the solar devices at a high
level. Solar thermochemical utilization is a promising solution to
these limitations. Among current solar thermochemical utilization
technologies, solar driven biomass gasification has also attracted
considerable attention [21,22].
Biomass is another type of renewable energy that can be utilized through chemical reactions such as combustion, pyrolysis
and gasification to produce heat, tar and syngas, respectively. In
particular, gasification is one of the most important technique for
processing biomass. While, in gasification, reaction heat from biomass in-situ combustion is needed to drive a set of endothermic
thermochemical conversion reactions for the production of syngas
(a mixture composed of H2 and CO) [23–25]. Therefore, it is possible to introduce solar thermal energy into the thermochemical
reaction of biomass gasification in order to achieve more efficient
biomass utilization. In the process of solar-biomass gasification,
concentrated solar energy is introduced to provide hightemperature heat resource for driving the biomass gasification
reaction, in which solar thermal energy, with an amount equal to
the enthalpy change of the endothermic reactions, is converted
into the chemical energy of the syngas and low-carbon footprint
transportation fuels [22]. It is worth mentioning that biomass is
composed of carbohydrates with high volatile content and exhibits
favorable reactivity. More importantly, the hybridized solar energy
and biomass are renewable which contribute to CO2 emission
reduction.
Currently, numerous prototype reactors, such as two-zone solar
reactor, fluidized bed reactor, packed-bed reactor, have been developed for solid fuel solar gasification and a favorable solar conversion ratio can be achieved through experimental investigations
[26–31]. Additionally, solar gasification acts as a promising pathway for valuable liquid fuels production such as methanol and Fischer–Tropsch diesel, and in some publication, the polygeneration
concept is employed to enhance system performance [32–35]. In
addition, gasified syngas, as a kind of solar fuel, can be directly utilized for power generation with a favorable efficiency by incorporating with the combined Brayton–Rankine cycle [36].
Biomass gasification process is a set of complex reactions, in
which the biomass feedstock is preheated, and then pyrolyzed to
yield tar and char, then the tar is cracked and char is gasified with

#

reaction heat factor

Subscript
ASU
aux
bio
CC
day
net
opt
parasitic
ref
solar
sol-elec
sys
th

air separation unit
auxiliary devices
biomass
combined cycle
daily accumulated or averaged
net output power
optical
parasitic consumption
reference system
solar energy

solar-to-electric
system
thermal

the gasification agent (e.g. CO2 or steam) to produce noncondensable syngas [37–39]. Generally, the biomass preheat and
pyrolysis steps can be implemented under a mid-temperature condition of lower than 673 K. However, most previous publications
only used point focus collectors to concentrate high-temperature
thermal energy to drive the gasification process, which has a relatively high energy loss and capital investment compared to lowtemperature line focus collectors such as parabolic collectors, and
more exergy loss due to higher temperature difference between
solar energy source and biomass preheat and pyrolysis chemical
reaction.
Therefore, the thermal heat resources should be introduced correspondingly to the individual temperature requirement of each
reaction procedure. A two-stage gasification concept, i.e., using
high-temperature heat resource to drive the biomass gasification,
mid-temperature solar thermal energy for biomass preheating
and pyrolysis procedures, is an effective solution. Naturally, the
main objectives of this work include proposing a two-stage solarbiomass gasification concept, developing a novel solar-biomass
hybrid power generation system, and assessing performances of
the solar thermochemical conversion process and the developed
system. The main contributions are summarized as follows:
(1) A novel hybrid power generation system integrated with a
two-stage solar-biomass gasification process is proposed
for effective utilization of solar energy and biomass. The proposed system reduces fossil fuel consumption and mitigates
CO2 emission.
(2) In the proposed system, two solar collection devices are
employed to provide concentrated solar thermal energy at
different temperatures. In addition, the pyrolysis and gasification of the biomass feedstock are driven by concentrated
solar thermal energy at appropriate temperature. In this
method, exergy destruction in solar collection and thermochemical conversion processes can be reduced.
(3) Solar thermal energy can be converted into chemical energy

stored in syngas through solar-biomass gasification. The
energy level of the introduced solar energy is upgraded. An
effective integrated utilization of the renewable energies
can be achieved. In addition, under both design and offdesign working conditions, more favorable thermodynamics
performances of the proposed system are obtained.
The rest of this study is organized as follows. In Section 2, we
propose a novel solar-biomass power generation system integrates

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a two-stage solar gasifier, and the system performance evaluation
criteria are given. In Section 3, the chemical composition of the biomass sample is experimentally determined, and the pyrolysis
kinetic characteristics for the biomass sample are investigated.
The energy level upgrade for the solar-biomass gasification is analyzed in Section 4. The nominal and off-design performances
within the representative days are presented. Finally, we summarize the main conclusions in Section 5.

2. New system and performance analysis

In the power generation subsystem, the Brayton-Rankine combined cycle system uses a SGT-800 type gas turbine. The steam
cycle block is composed of a dual pressure heat recovery steam
generator (HRSG) with a high pressure of 55 bar and a low pressure
of 6.9 bar. We do not include an energy storage subsystem for the
sake of the system simplification, and thus the power output rate
will vary with solar irradiation intensity.

The two-stage solar-biomass gasification concept is applied in
the proposed solar hybrid power generation system, and it has
the following appealing advantages:
(1) Biomass pyrolysis, comprising the feedstock preheating and
steam generation, is driven by mid-temperature solar thermal energy. Since this mid-temperature of solar energy
matches better than single high-temperature with the aforementioned processes, irreversibility in this process can be
reduced.
(2) The introduced mid-temperature solar thermal energy is
concentrated by the LFC, which can achieve a relative
increased collection efficiency than the PFC that operates
in a higher temperature range.
(3) Solar energy can be readily stored in chemical form by driving biomass pyrolysis and gasification, which helps overcome the limitations of solar energy such as low-energy
density and intermittency.
(4) The gasified syngas as a kind of solar fuel can be effectively
utilized by an advanced gas turbine or used for liquid fuels
production, e.g., methanol, and diesel.

2.1. New system
Concentrated solar energy as a heat resource is introduced to
drive the biomass gasification process for chemical fuel production.
However, more exergy destruction will be generated during a typical solar-biomass gasification process which is only driven by
high-temperature solar thermal energy. To overcome this limitation, a two-stage gasification concept is employed to optimize
the reaction process of solar-biomass gasification. In accordance
with the reaction procedure, biomass gasification process can be
divided into two parts, pyrolysis and gasification, at different reaction temperatures. Therefore, a two-stage solar-biomass gasification is employed in this work, and the required heat resource for
each sub-process is suitably provided. The produced syngas, i.e.,
solar fuel, is directly utilized by an advanced combined cycle for
power generation. The flow diagram of this two-stage solarbiomass gasification system is depicted in Fig. 1.
The proposed solar-biomass power generation system consists
of a solar-assisted biomass gasification subsystem and a power

generation subsystem. During the gasification process, the biomass
pyrolysis initially produces tar and char at temperature lower than
673 K. The required solar thermal energy is concentrated by linefocus solar collectors (LFC), such as parabolic trough solar collector,
besides, the steam as the gasification agent is also generated by the
LFC. Subsequently, a point-focus collector (PFC) with the beamdown concept is applied to provide gasification reaction heat for
the processes of tar cracking and char gasification at temperature
above 1000 K, for syngas production. Solid particles of ash and
other corrosive components, such as H2S, are removed from the
produced syngas via condensation and clean-up. Finally, the qualified syngas as gas fuel is directly fed into combined cycle to generate power.

2.2. System designated operation parameters
The PFC provides high-temperature thermal energy by concentrating solar irradiation from the heliostats. Heliostats, with width
and length of 12 m, revolve on dual axes to track sunrays. Meanwhile, solar energy used for pyrolysis is concentrated by the LPC,
e.g., parabolic trough collectors. The solar field is connected by
numerous solar loops. In this work, the ET-150 type collector is
employed. The fundamental parameters of the collectors are
shown in Table 1. The solar field is installed in the south–north
direction and tracks sunrays automatically.
Solar energy resource is abundant in China, especially in western China. The hybrid solar-biomass power generation system is
located in Yanqi (E 86°340 , N 42°050 ), Xinjiang province in western

Hyperboloid
reflector
GT
Compressor

Heliostats

Combustor

GT
Turbine

Air

Biomass
Syngas
clean-up

H 2O

Solar gasifier

Steam
Turbine
Cooling
tower
Condenser

HRSG

Fig. 1. Schematic diagram of the novel solar-biomass power generation system.

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Table 1
Parameters of ET-150 type solar collector.
Items

Unit

Aperture width
Focal length
Glass envelope outer/inner diameter
Absorber tube outer/inner diameter
Mirror reflectivity
Designate optical efficiency

m
m
m
m
%
%

5.75
1.71
0.115/0.109
0.07/0.066
94
85.32

China. The on-design point of the proposed system is the solar time
of 12 h on June 21 and the nominal operation parameters for designation are listed in Table 2.

The economical collection radius of the biomass resource and
the effect of solar field scale on solar tower collection performances
should be taken into account. For PFC, the solar field layout is
designed and optimized with the assistance of the System Advisor
Model (SAM) software, which is developed by the National Renewable Energy Laboratory in USA [40].
The solar field is chosen based on the required thermal solar
energy, the design point condition, and the power capacity. The
solar multiplier is set to 1.0 since the energy storage subsystem
is not considered. The aperture of the LFC is 20,772 m2, while the
heliostat area of the PFC is about 78,830 m2 and the maximum distance of the heliostat from the tower is about 475 m.
Since the solar irradiation and ambient condition are variable,
the operating parameters of the hybrid system should be adjusted
because energy storage systems are not included. The performance
of the gas and steam turbines is influenced by operating condi-

Table 2
Nominal parameters for the proposed solar thermal power system.
Items
Location
Altitude
Direct nominal irradiation (DNI)
Ambient temperature
Pyrolysis temperature
Gasification pressure
Gasification temperature
Solar collection temperature for LFC/PFC
Mass ratio of steam to biomass
Pressure ratio of GT compressor
Gas turbine inert temperature
Gas turbine exhaust temperature

Parameters of the high-pressure steam
Parameters of the low-pressure steam
Pinch-point temperature difference of HRSG

Yanqi, Xinjiang
1055 m
751 W/m2
298.35 K
643 K
18 bar
1150 K
643–1150 K
0.5
20
1561.15 K
819.15 K
805.15 K/55 bar
533.15 K/6.9 bar
20 K

tions, including power load ratio and ambient temperature. As a
result, the off-design behavior of the proposed systems should be
evaluated. Four representative days (19 March, 21 June, 22
September, and 21 December) were selected for analysis. The
direct nominal irradiation profiles vary during the four selected
days, as depicted in Fig. 2.
2.3. Performance evaluation method
In this section, system thermodynamic performance and solar
energy conversion efficiency are evaluated [33,41]. A typical biomass integrated combined cycle (BIGCC) system is selected and
used as the reference system in this work, so that the electric

power generated from the input biomass energy with typical gasification routines and the contribution of the introduced solar
energy for the power generation can be calculated. Biomass is
directly gasified with purified O2, and the gasified syngas is used
in a combined cycle for generating power. Both on-design and
off-design thermodynamic performance evaluations of the proposed solar hybrid power system are implemented. The reference
system is also simulated under the same operating conditions.
For the proposed hybrid power system with two-stage solarbiomass gasification, the net generated electricity Wnet and the
incremental solar power production Wsol,elec are defined as:

W net ¼ W À W CC;aux À W sol;parasitic

ð1Þ

W ref;net ¼ mref;syngas Á HHVref;syngas Á gCC;net À W ASU

ð2Þ

W sol;elec ¼ W net À W ref;net

ð3Þ

where W and Wnet represent the total generated power of the proposed system and the net output power, respectively; WCC,aux, Wsol,parasitic and WASU indicate the power consumption of the combined
cycle auxiliary devices, parasitic consumption of solar field operation and the power consumption of air separation unit, respectively;
Wref, net is the net power output of the reference system, and gCC,net
is the net efficiency of the combined cycle in the proposed system.
The overall energy efficiency gth,sys and the net solar-to-electric
efficiency gsol-elec are used as basic criteria for performance evaluation of the proposed solar-biomass hybrid systems, which are
defined as:

gth; sys ¼

W net
DNI Á Ssolar þ mbio HHVbio

gsolÀelec ¼

W net À W ref;net
DNI Á Ssolar

ð4Þ

ð5Þ

where Ssolar is the area of the solar field; HHV represents the higher
heat value; and m represents the mass rate.
For the off-design performances of the system on representative
days, the accumulated amount of the net generated electricity
Wnet,day is considered, and can be calculated by:

W net;

Fig. 2. DNI profiles of the representative days.

day

¼

X

W net Á Dt

ð6Þ

If the power load ratio of the gas turbine is lower than 10%, the
power generation system will be shut down, because the system
efficiency declines sharply and the generated electricity cannot
even compensate the basic power consumption of the auxiliary
devices at such low power load.
Meanwhile, the one-stage solar-biomass gasification system
with equivalent biomass gasification feed rate is evaluated, and
used to reveal the potential thermodynamic performance improvement of the two-stage solar-biomass gasification power generation
system. The only difference between the two systems is that the

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one-stage solar-biomass gasification system is driven only by hightemperature solar thermal energy collected by the PFC.
3. Biomass sample determination
3.1. Chemical composition of the biomass sample
Corn straw is the most abundant herbaceous biomass resource
in China, and is thus selected as the gasification feedstock. The biomass sample of corn straw was selected follows.
The pyrolysis experiment of corn straw was first conducted, by
a program-controlled electrical furnace, at temperature below
673 K, the tar yield ratio reached 19.5% as reported in Table 3.
The chemical composition as air-dry basis of the biomass sample

and the char (solid product from pyrolysis) were determined and
summarized in Table 4.

Fig. 3. TG and DTG curves for pyrolysis of corn straw.

3.2. Kinetic analysis of biomass pyrolysis

-8.5

a¼

mo À mt
mo À mf

ð7Þ

where mo and mf are the initial and the final masses of the sample,
respectively; mt stands for the mass of the sample at time (t).
It is assumed that in the nonisothermal and heterogeneous
reaction of biomass pyrolysis, the general reaction rate (da/dT) is
calculated as a function of conversion rate a and rate constant k
(T), as follows:

da 1
¼ k Á f ðaÞ
dT b

ð8Þ

where b is the heating rate; and f (a) refers to the reasonable model

of the reaction mechanism.

Table 3
Product yield of pyrolysis (wt.%).

Corn straw

0.15 0.1 0.05

-9.0

ln(β/Τ 2)

To investigate the two-stage solar-biomass gasification concept,
the thermogravimetric analysis (TGA) of the biomass feedstock
pyrolysis has been applied. The reaction temperature of the start
and end can be measured, and the biomass pyrolysis kinetic characteristics can also be obtained by analyzing the TGA data. The TGA
measurements were performed using a Mettler Toledo TGA/DSC-1
with a gas flow of 50 mL/min of nitrogen. In the TGA experiments,
the sample was pulverized to a particle size of about 0.2 mm, and
heated from room temperature to 1000 K at a heating rate of 10–
50 K/min. The thermogravimetric curve (TG) and differential thermogravimetric (DTG) curves of the samples on TGA are shown in
Fig. 3. The main weight loss procedure occurs within the temperature range of 500–650 K, and the solar collection temperature of the
parabolic trough collector can satisfy such technical requirement.
In this study, the reaction conversion rate of the biomass sample denoted by a can be expressed as follows:

0.95

-9.5

-10.0

-10.5
1.50

10K/min
15K/min
20K/min
50K/min
1.55

1.60

1.65

1.70

1.75

1.80

1000/T K-1
Fig. 4. Arrhenius plot of b/T2 versus 1/T at selected conversion ratios.

The reaction rate constant k is dependent on the temperature
and can be expressed by:



E

k ¼ A exp À
RT

ð9Þ

where A is the pre-exponential factor; E represents the activation
energy and R indicates the universal gas constant [8.314 J/(K mol)].
In kinetics investigation, the distributed activation energy
model (DAEM) is widely used because it fits the DTG curve DTG
by a series of parallel, first-order reactions. The DAEM is based
on the assumption that the system consists of a series of irreversible parallel first order reactions. The details were elucidated
by Miura [42–44]. Finally, the reaction conversion rate can be simplified as:

a¼1À

Z

1

UðE; TÞf ðEÞdE

ð10Þ

0

Tar

Water

Char

Gas

19.50

22.13

38.26

20.11

where f(E) is the distribution function of the activation energy, and

U(E,T) can be approximated as follows:

Table 4
Chemical composition of the biomass sample.

a

HHV (MJ kgÀ1)

Sample

Proximate analysis (wt.%)

Ultimate analysis (wt.%)

Mad

Aad

Vad

FCad

Cad

Had

Nad

Sad

Oad

Corn straw
Chara

3.94
0.36

7.1
18.65

69.56
22.81

19.39
58.18

41.49
59.28

6.05
3.90

2.35
4.60

0.19
0.25

38.88
12.96

16.51
25.67

Produced by pyrolysis.

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UðE; TÞ ffi exp À

ART 2 ÀE=RT
e
bE

!
ð11Þ

In the TGA experiments, pyrolysis characteristics were measured at heating rates of 10, 15, 20 and 50 K/min. The Arrhenius
plot of b/T2 versus 1/T was determined through mathematical analysis as illustrated in Fig. 4. The activation energy E at selected conversion ratio can be determined, and the overall activation energy
E is 223.95 kJ/mol.
4. Results and discussion
4.1. Energy level upgrade from solar thermal energy to chemical
energy
The energy level A and energy-utilization diagram methodology
(EUD) proposed by Ishida and Kawamura [45] are applied to analyze energy level upgrading. Energy level A is a dimensionless criterion and defined as the ratio of exergy change DE to energy
change DH, namely A = DE/DH = 1 À T0DS/DH. For transferred
heat, the energy level AT can be simplified to AT = 1 À T0/T.
For an energy-conversion system, energy is released by the
energy donor (Aed) and accepted by the energy acceptor (Aea).
Correspondingly, the EUD can be illustrated and determined by
the energy level (A) versus the energy-conversion quantity (DH),
which graphically shows the variations of energy quality and
energy quantity of the process. With the assistance of the EUD,
exergy destruction can be obtained easily from the shaded area
between the curves for Aed and Aea. Meanwhile, the energy level
degradation of each process and the driving force as the energy
level difference can be graphically shown.
Concentrated solar energy is used to drive the biomass gasification, and the solar energy is converted into chemical energy. From
the viewpoint of the energy level, the energy level of the solar

energy can be upgraded to that of produced solar fuel, which can
then be used in numerous energy applications with increased
efficiency.
For the given solar-biomass gasification process (see Fig. 5), the
energy and exergy balances can be expressed as follows:

m3 h3 ¼ m1 h1 þ DH2

ð12Þ

m3 e3 ¼ m1 e1 þ DE2 À DEw

ð13Þ

where m, h and e represent the mass, specific enthalpy and specific
exergy, respectively; DEw is the reaction exergy destruction during
the gasification process.
In accordance with the definition of the energy level, we obtain
the following:

A1 ¼ m1 e1 =m1 h1 ¼ e1 =h1

ð14Þ

A2 ¼ DE2 =DH2 ¼ 1 À T 0 =T 2

ð15Þ

A3 ¼

m1 e1 þ DE2 À DEw
m1 h1 þ DH2

ð16Þ

The reaction heat factor # denotes the ratio of the absorbed
solar thermal energy to the feedstock chemical energy, namely
# = DH2/m1h1, thus Eq. (13) is changed to:

m1 e1
DE 2 À DE w
þ
¼ A3
m1 h1 þ #m1 h1 DH2 =# þ DH2
A1
#
þ
ðA2 À DAw Þ
¼
1þ# 1þ#

A3 ¼

where DAw = DEw/DH2 is the energy level reduction of the reaction
heat, and it is caused by the mismatch of energy levels between the
solar thermal energy and biomass gasification.
Through the solar thermochemical process of driving biomass
gasification, a part of solar thermal energy is converted to chemical
fuel of syngas with an upgraded energy level. The relative upgrade
ratio in the energy level of solar thermal energy may be formulated

as:



ðA3 À A2 Þ
A1
#
A2
¼
ðA2 À DAw Þ À A2
þ
A2
1þ# 1þ#
¼

ðA1 À A2 Þ
# DA w
À
ð1 þ #ÞA2 1 þ # A2

ð18Þ

The energy level difference between biomass feedstock A1 and
syngas A3 serves as a ‘‘driving force” to improve the solar thermal
energy to the higher one in chemical energy. Meanwhile, the energy
level upgrade ratio is dependent on DAw produced by the mismatch
of energy levels between the reaction heat resource and biomass
gasification. The reduction of DAw is one way to enhance the performance of solar-biomass gasification. Consequently, the two-stage
solar-biomass gasification concept is proposed in this work, with
a main motivation to decrease exergy destruction during the midtemperature reaction procedure. The EUD for the solar-biomass

gasification process is illustrated in Fig. 6. For the typical solarbiomass gasification process with high-temperature solar energy
introduced (1150 K for the case study), the energy level of solar
energy can be improved from 0.74 to 0.9 by conversion into the produced syngas. While, if the gasification process employs the proposed two-stage solar-biomass gasification technical mode, the
energy level of the required solar energy is reduced to 0.68, and
the increased energy level upgrade ratio of 32.35% for the solar
energy conversion can be achieved. In addition, compared to the
one-stage gasification mode, the proposed system can convert more
heat resource of solar energy into the chemical form, accounting for
9.25% of the required net exergy of the solar thermal energy.

1.25

Abiomass

1.0

Asyngas
ATIT

A

Substituting Eqs. (14) and (15) into Eq. (13), the energy level of
the syngas as produced solar fuel can be expressed as:

A'solar

ΔEextra

0.5

biomass
Gasification

steam

H1, E1, A1

ð17Þ

Asolar

syngas

H3, E3, A3
H2,

E2, A2

Solar energy
(thermal resource)
Fig. 5. The process of solar assistant biomass-steam gasification.

0.0

0

50

100

200

ΔH / MW
Fig. 6. EUD of solar-biomass gasification process.

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4.2. System evaluation under the on-design condition
System performance evaluation on two-stage solar-biomass
gasification concept under the nominal condition was first conducted using the aforementioned evaluation criteria, the net generated power is 42.11 MW, with a the biomass feed rate of 4.6 kg/s
and introduced solar energy of 74.8 MW. In the solar-biomass gasification process, the net solar share is about 49.61%, whereas the
overall system energy efficiency and exergy efficiency reach
27.93% and 31.62%, respectively. By comparison with the reference
system, gsol-elec can be calculated and is about 19.89%. In the system with one-stage biomass gasification, if the biomass processing
rate is equal to the proposed system, then the solar energy requirement is increased to 85.39 MW, resulting in gth,sys and gsol-elec
reduction by 6.56% and 12.4%, respectively.
Furthermore, detailed investigations, including energy and
exergy balance analysis, were implemented, and the results are
summarized in Table 5. Under the given on-design operation conditions, the largest energy and exergy loss in the proposed system
are produced in the solar collection processes, which accounts for
the total energy input of 27.36% and total exergy input of 22.13%,
respectively. compared to the one-stage gasification mode, the
exergy loss in solar collection for the proposed system and solar
thermochemical process are reduced by 23.25% and 20.22%,

respectively. Additionally, the heat loss of the stack gas and the
steam condensation contributes to the second largest energy loss
of 29.83%. While, for the exergy analysis, the second largest energy
loss item is generated in the syngas combustion processes and
accounts for 16.37% of the total input.
In particular, for the power generation subsystem in the combined cycle, the illustrated EUD is shown in Fig. 7, five energy conversion sub-processes, namely gas combustion, air compression,
gas expansion, heat exchange in HRSG and steam expansion, are
included. For the gas combustion process in the GT gas combustor,
the combustion of fuel plays the role of energy donor in heating
fuel and air which are energy acceptors, and Aea1 denotes the heating of the fuel gas and air, and the exergy destruction in the combustor reaches 21.8 MW. The gas turbine (Aed,GT) and compressor
(Aea,comp) serve as the energy donors and acceptor, respectively,

Fig. 7. EUD of the power generation subsystem.

with a total exergy destruction of 7.08 MW. While, the width of
Aed,GT and Aea,comp indicates the gas turbine’s work output and
the compressor’s power consumption, the GT net power output
reaches 32.27 MW. During the heat recovery sub-process within
the HRSG, the energy level of flue gas exiting the gas turbine
(Aed2) acts as the energy donor, and its energy level decreased from
0.64 to 0.19, while the feed water of the steam Rankine cycle represents an energy acceptor with energy level improved from 0.06
to 0.63. The area between the curves Aed2 and Aea2 indicates the
exergy destruction in the HRSG, which is 4.04 MW or 3.03% of
the total exergy input.
4.3. Off-design evaluation in representative days
Owing to the fact that the performance of the solar collection
and combined cycle are affected by operation conditions, we evaluated the off-design behavior in four representative days. Solar collection efficiency, including heliostat optical efficiency and receiver
thermal efficiency, is computed based on the optimized solar field
layout and the local meteorological data. In addition, the integrated Brayton-Rankine cycle is referred to SGT-800 type gas tur-

Table 5
Energy and exergy balances in the proposed systems.
Two-stage solar-biomass gasification

One-stage solar-biomass gasification

Energy balance

Energy balance

Exergy balance

Input

Value (kW)

Ratio (%)

Value (kW)

Biomass
Solar energy 643 K
Solar energy 1150 K

75973.00
15599.77
59200.28

50.39
10.35

39.26

80948.82
8366.38
43851.96

Total

150773.05

100.00

133167.16

Output
Generated power

42107.84

27.93

42107.84

Ratio (%)
60.79
6.28
32.93
100.00
31.62

Energy loss/exergy loss

Exergy balance

Value (kW)

Ratio (%)

Value (kW)

Ratio (%)

75973.00
–
85389.33

47.08
–
52.92

80948.82
–
63251.22

56.14
–
43.86

161362.33

100.00

144200.03

100.00

42107.84

26.10

42107.84

29.20

Energy loss/exergy loss

Solar collection-643 K
Solar collection-1150 K
Solar-steam generation
Gasification unit
Syngas condensation
Gas combustor
Gas turbine
Steam turbine
HRSG
Stack loss
Condenser
Aux power
Others

5311.91
35944.60
–
–
14292.45
–
2155.40
1123.86
–
14131.72
30851.00
2632.50
2221.78

3.52
23.84
–
–
9.48
–
1.43
0.75
–
9.37
20.46
1.75
1.47

2848.85
26625.57

1435.40
8299.88
7507.91
21801.00
7083.70
2444.60
4041.00
90.48
1941.50
2632.50
4306.92

Total

150773.05

100.00

133167.16

2.14
19.99
1.08
6.23
5.64
16.37
5.32
1.84
3.03
0.07

1.46
1.98
3.23
100.00

–
51845.79
–
–
14292.45
–
2155.40
1123.86
–
14131.72
30851.00
2632.50
2221.78

–
32.13
–
–
8.86
–
1.34
0.70
–
8.76
19.12

1.63
1.38

–
38404.20
1435.40
10402.97
7507.91
21801.00
7083.70
2444.60
4041.00
90.48
1941.50
2632.50
4306.92

–
26.63
1.00
7.21
5.21
15.12
4.91
1.70
2.80
0.06
1.35
1.83
2.99

161362.33

100.00

144200.03

100.00

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8

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bine, the power cycle off-design performance is simulated considering the influences of the ambient temperature, power load, gas
fuel composition.
The results are calculated based on a one-hour basis, and Fig. 8
presents the system performance represented by system energy

efficiencies of gth,sys and the net power output of Wnet. Electricity
generated by the combined cycle varies instantaneously with solar
irradiation because neither thermal energy storage system nor
chemical energy storage system is adopted in the proposed solar
hybrid power system. On June 21st, the daily maximum power of

Fig. 8. Hourly net generated electricity and the system energy efficiency in the representative days.

Fig. 9. Hourly solar-to-electric efficiency of the systems in the representative days.

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Z. Bai et al. / Applied Energy xxx (2016) xxx–xxx

Wnet / MW⋅h

450

30

300

20

150

10

0

Wnet

ηsys,th

ηsol-elec

600

0

(b) one-stage solar-biomass gasification
3.19
6.21
9.22
12.21

450

40

30

ηsys,th /ηsol-elec / %

3.19
6.21
9.22
12.21

40

Wnet / MW⋅h

(a) two-stage solar-biomass gasification

ηsys,th /ηsol-elec / %

600

300

20

150

10

0

Wnet

ηsys,th

ηsol-elec

0

Fig. 10. Daily average system performances in the representative days.

44.31 MW h with the highest gth,sys of 28.47% is achieved at 12 h,
and the operational time last for 10 h. For the typical days in March
and September, the maximum power is 26.63% and 26.89%, respectively, and gth,sys for both days is above 25%.
Furthermore, in the sunny hours of the December day, compared to other days, the system performances experience a sharp
performance reduction, with the highest hourly net power of only

19.01 MW h and the gth,sys of 16.29%. The main reasons are that the
operation time is significantly reduced to 5 h, and the solar collection efficiencies are weakened, which resulting in a lower efficiency for power cycle and decreased power ratio.
The comparison on performance represented by gsol-elec
between the proposed system with two-stage and one-stage
solar-biomass gasification mode is conducted, and daily results
are summarized in Fig. 9. Generally, gsol-elec for the two-stage gasification mode is higher than that for the one-stage gasification
mode, although the overall variation trends are similar. In summer
time such as in June day, the gsol-elec for the novel two-stage system
is higher than the three other selected days with a peak value of
20.7%. The daily efficiency is also higher than the one-stage gasification mode which in the range of 12.28–30.61%. However, in the
winter time such as in December, the performance of novel twostage solar-biomass gasification mode is worse than the onestage solar-biomass gasification mode since the reduction ratio of
gsol-elec reaches from 0.64% to 16.11%. The reasons are that the solar
collection efficiency of the LFC deteriorates substantially in winter
as the cosine effect is increased, while the performances of the PFC
still remains at a reasonably high level, especially when dual-axis
tracked heliostats are applied.
The daily average performances of the developed system in the
representative days are summarized in Fig. 10. The daily maximum
net generated electricity output produced in June day is
409.65 MW h with the highest averaged gth,sys of 29.14% and gsolelec of 19.04%. Although the daily operation times in March and
September days are relatively shorter than that in June day, gas
turbines can operate with a favorable performance, with an average gsol-elec up to 17.15% and 16.07%, respectively. This is mainly
because the contributions of higher intercepted solar heat and
the lower ambient temperature, while the system with one-stage
gasification technical mode can only achieve 15.71% and 14.81%.
The above studies indicate the proposed novel solar-biomass
power generation system integrated with a two-stage gasifier
exhibits remarkable performance, which achieves the daily average efficiency of the gsol-elec above 15% except in December day.
Although the solar-biomass gasification can only be realized at
the laboratory scale currently, the conventional biomass gasification and the concentrated solar power technologies have become

mature, they will contribute to make a breakthrough and facilitate
the real applications of solar-biomass gasification technology.
Therefore, the solar-biomass thermal gasification technology will

accelerate the commercial operation in the near future, which will
bridge current fossil-fuel-based technologies and future solar thermochemical technologies. Thus, the proposed system provides a
promising approach for efficient utilization of the abundant solar
and biomass resources in the western China.
5. Conclusions
In this work, we propose a new solar hybrid power generation
system that integrated with a two-stage gasifier. The on-design
and off-design thermodynamic performances are simulated and
analyzed, and the main research findings can be outlined as follows:
(1) A new system with two-stage solar-biomass gasification
route is proposed for efficient utilization of the solar energy.
The proposed system applies mid-temperature solar energy
collected by the LPC for the biomass pyrolysis, which
matches well with the reaction temperature, and the exergy
destruction during the gasification process and the solar
energy collection process can be reduced by 23.25% and
20.22% compared to the one-stage solar-biomass gasification mode, respectively.
(2) By using concentrated solar energy to drive the biomass
gasification, solar energy is converted into the chemical fuel
in the form of gasified syngas. The energy level of the introduced solar thermal energy in the proposed two-stage solarbiomass gasification system was improved from 0.68 to 0.9,
which results in an energy level upgrading ratio of 32.35%
compared to 21.62% in one-stage gasification mode.
(3) Under the nominal condition, the overall energy efficiency
and the net solar-to-electric efficiency for the proposed
novel system reached 27.93% and 19.89%, respectively. Additionally, the proposed system exhibits satisfactory thermodynamics performances except in December days during
system off-design evaluation. In addition, and the daily average net solar-to-electric efficiency achieved the improvement in the range of 8.6–21.33% compared to the onestage gasification thermochemical system.

The proposed hybrid solar power generation integrating twostage gasification routine provides a promising approach for the
efficient utilization of the abundant renewable solar and biomass
energy resources in western China.
Acknowledgements
The authors appreciate financial support provided by the
National Natural Science Foundation of China (No. 51276214, No.
51236008).

Please cite this article in press as: Bai Z et al. New solar-biomass power generation system integrated a two-stage gasifier. Appl Energy (2016), http://dx.
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