Simulation and economic evaluation of biomass gasification with sets for heating, cooling and power production

Renewable Energy 99 (2016) 360e368

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Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Simulation and economic evaluation of biomass gasification with sets
for heating, cooling and power production
Hairong Wang a, b, *, Jianbo Yan a, b, Liang Dong a, b
a
b

School of Engineering, Sun Yat-sen University, 510006 Guangzhou, China
Research Institute of Sun Yat-sen University in Shenzhen, 5108075 Shenzhen, China

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 22 December 2015
Received in revised form
14 June 2016
Accepted 1 July 2016

In this work, syngas was used directly as fuel source for the renewable CCHP system, which can be
producted through biomass gasification process. The advantages and limitation of entrained flow gasifier
are compared, followed by discussion on the key parameters that are critical for the optimum production
of syngas. Gasification agent of 450  C temperature and 30 atm pressure has been proposed as a optical
solution to a entrained flow gasifier using air as gasification agent at 0.27 ER (oxygen equivalence ratio),

in that it provides a syngas of 5.665 MJ/m3 LHV and up to 77% gasification efficiency. Depending on the
key parameters of gasification process, the properties of syngas produced can be varied. It is thus
essential to thoroughly understand the cogeneration system to identify the suitable methods for a
renewable CCHP system. These process was simulated using Aspen Plus to perform the rigorous material
and energy balances. The results obtained from simulation and experiment agreed well. This paper later
focused on economic evaluation of the entire process, as well as the environmental benefits. The
renewable CCHP system could able to attain lower CO2 and SO2 emission with total energy efficiency and
gas yield of 75.43% and 2.476 m3/kg respectively.
© 2016 Elsevier Ltd. All rights reserved.

Keywords:
Biomass
Syngas
CCHP
Aspen plus

1. Introduction
Combined heating, cooling, and power production (CCHP) is a
system in which fuel is combusted to generate electricity. The energy for heating and cooling purposes is also formed by utilizing
recoverable waste heat. During the early 21st century, the operation of CCHP systems under electric demand management (EDM)
and thermal demand management (TDM) strategies have been
investigated [2,3,14,24]. However, interest in CCHP has been
renewed due to the recent adjustment of the industrial structure
and an increase in energy needs. Another major issue of concern is
the recent increase in carbon dioxide concentrations in the atmosphere due to combustion of fossil fuels. Biomass which is believed
to be a “renewable and low-carbon energy”, has been suggested as
an energy resource to reduce the buildup of carbon dioxide in the
atmosphere [7,32]. Therefore, a CCHP system with a gasifier not
only integrates the cooling, heating, and power generation processes for higher energy efficiency, but can also bring primary

* Corresponding author. School of Engineering, Sun Yat-sen University, 510006
Guangzhou, China.
E-mail address: (H. Wang).
/>0960-1481/© 2016 Elsevier Ltd. All rights reserved.

energy savings and therefore decrease CO2 emissions.
The purpose of this study was to simulate the conversion of
biomass to combustible gases, followed by simulation of a CCHP
system that uses these syngas as fuel. These simulation results were
used to perform a complete economic and technology analysis of
the entire process. Several pieces of this process have been studied
individually, but no studies that bring together the entire process
have been performed. Some representative gasification projects
include the down-draft gasification furnace by Imbert Energietechnik GMBH in German, the fluidized bed by Omnifuel Gasification System Limited and the fixed bed by Moore Canada Ltd in
Canada. The gasification efficiency of these systems is up to 60e80%
and the heating value of the produced combustion gas is 17e25 MJ/
m3 [11]. Advanced gasification systems is generally larger, higher
degree of automation, and of complex process, which is mainly to
generate electricity and heat. Recently, an integrated gasification
system combined with internal combustion set has been developed, promoting gasification for syngas generation in large-scale
[28]. Liszka et al. [21] studied several integrated gasification combined cycle (IGCC) processes that use biomass or wastes as a main
feedstock. This IGCC system is similar to Narural gas fired gas turbine combined cycle generation, except for the gasification and the


H. Wang et al. / Renewable Energy 99 (2016) 360e368

synthetic gas cleanup process. In recent years, a 1573 K-class gas
turbine combined cycle power plant has been launched for iron
works with a thermal-efficiency as high as 47% [34]. Maraver
et al.[25] evaluated and compared the environmental performance

of bio-fueled CCHP systems and conventional generation in which
the same energy products would be in average stand-alone plants
(i.e. a reference power plant). Many other studies [5,18] maily deal
with the evaluation of natural gas-fueled CCHP and combined heat
and power (CHP) through life cycle assessment (LCA). Very recent
reviews have focused on technologies for renewable CHP and CCHP,
including a brief system design, operation performance, and thermodynamic and economic indexes evolution [4,6,22].
Because actual achievable benefits of CCHP system may vary
depending on the plant operation, its nominal loads, combined
forms and the technologies involved [25]. It is necessary to apply
optimization criteria to guarantee the benefit of CCHP over conventional technologies. In biomass (alone) gasification using an
entrained-flow gasifier process, two cases are well known in the
world. One is a gasification-DME synthesis by CHEMREC Co. Ltd. in
Sweden. The other is a two-stage gasification of woody biomass by
CHOREN and Shell Co. Ltd. in Germany. Both gasification processes
has been developed by companies, and the final target scales
planed are very large, as large as a few thousand ton/day scale,
almost equal to scale of coal gasification [35]. The entrained bed
gasifier has many advantage, for example the tar yield is low and
various kind of biomass are available as feedstock. In this manuscript, syngas was produced by an entrained bed gasifier, combustion turbine of a 9FA type made by General Electric was
choosed. Steam generation in the HRSG was a three-pressure
reheat-type waste heat boiler. The report presents descriptions of
each process, as well as the main design assumption for the gasification reactor, gas turbine, heat recovery stream generator
(HRSG), and stream cycle. The simulation of the rigorous material
and energy conversion of this renewable CCHP was carried out
using Aspen Plus (Aspen Tech, USA) followed by evaluation of the
economy, thermal properties, and environmental indicators. Topics
of interest include the influence of input and operation parameters
on the total energy efficiency and pollutant emissions performance.
The results of this research can be used to optimize energy efficiency and provide the necessary information for maintenance

management.
2. CCHP system description
The renewable CCHP can be conceptualized as a combination of
local subsystems producing syngas via gasification, cooling, heating, and electricity, as illustrated in the schematic (Fig. 1). The core
of the system is represented by two blocks:
1. The biomass gasification block, which can be composed of
different equipment for biomass gasification, treatment, and
conditioning, as well as hydrogen sulfide (H2S) and ammonia
(NH3) removal. Because the gasification production must be
cooled before being cleaned, the treatment and conditioning
units include a low pressure steam heater, air preheater, and
absorption chiller. Through these units the acid gas production
after the gasifier exchanged heat with the low-pressure superheated steam from the boiler, and produced heat to various final
uses including an air preheater, providing hot water to users.
2. The CHP block, which contains a combined cycle (Fig. 1) and 9FA
type combustion turbine as a prime mover, plus a combustion
heat generator composed of a boiler.
The gasifier is an entrained-bed gasifier that consists of a plugflow system where sawdust react with oxygen [29]. Unlike moving

361

bed or fluidized bed gasifiers, entrained flow gasifiers operate at
high temperature of 700e1500  C for biomass [35,36]. The
composition of the product gas is very close to syngas quality [27].
Syngas contains CO and H2 as the main combustible component, as
well as amounts of CH4. Trace constituent of nitrogenous compounds and sulfur heavily depend upon cleanup process [34].
Table 1 shows the typical composition of sawdust and heating
value.
In this type of entrained bed gasifier, the solid feedstock needs
to be grinded into small particle size (<100) for the feed system in

order to achieve high conversion rate [38]. And it usually operates
at high pressure of 2.94e3.43 MPa [23,27]. Tomoko Ogi et al. [35]
studied gasification of oil palm residues in an entrained bed
gasifier. Oil palm was gasified using H2O or H2O þ O2 as a gasification agent at 900  C. During gasification with H2O alone,
hydrogen rich gas was obtained, and tar yield was very low (<1.0 wt
%). The following is the detail parameter of the gasifier represented
in this paper (see Table 2).
3. Subsystem integration
The Braden-Rankine cycle is a promising and innovative technology that greatly benefits a conventional integrated gasification
combined cycle (IGCC) [33]. Similarly, on a large-scale basis, the cogeneration technology most adopted is the gas-steam combined
cycle, which is environmentally friendly and produces high energy
efficiencies under varying load rates. In the gas-steam combined
cycle, the main sets involved chamber (COMBUST), compressors,
gas turbine, HRSG and multi-stage steam turbine. Gas turbine is
graded by combustion temperature, 1100  C for the E-class, 1200  C
the F-class, 1300  C for the G-class and so on. The Central Research
Institute of Electric Power Industry developed an air-blown pressurized, two-stage entrained-flow gasifier, 150 MW 1573 K-class
and 1773 K-class gas turbine combustor technologies for low-Btu
fuel [12,20]. The exhaust gas temperature of chamber in this paper is between 1226 and 1418  C and gas turbine unit of a 9FA type
made by General Electric was choosed. The oxygen and fixed carbon contents of sawdust are respectively about 39 wt.% and 52 wt%,
whereas those of coal are 2e20 wt.% and 50e85% wt.%. The H/C
ratio for sawdust is 0.15, which is higher than that of coal
(0.5e0.85). therefore, sawdust is more degradable compared to
coal. Takeharu Hasegawa et al. [34] shown the typical compositions
of derived gases from coal-based gaisifiers and furnaces, as well as
no-fossil resources. For one thing, syngas derived form water-coal
slurry and sawdust is similar. It is therefore necessary to adopt
suitable working parameter for each gaseous fuel. The performance
characteristics of the gas turbine are shown in Table 3 [8].
In fact, the exhaust gas temperature of sawdust-based gas turbine is about 590  C. The value obtained by simulation is lower than

water-coal slurry as shown in Table .3. A heat recovery stream
generator can be divided into two main categories, single pressure
and multi-pressure. The large-scale modules (greater than
250 MW) operate with exhaust gas up to 590  C, which enables the
use of three-pressure reheat-type HRSG [37]. The smaller combustion turbines, between 90 MW and 200 MW, usually use dualpressure reheat-type HRSG, which cause 3%e5% lower efficiencies
to limit the recovery of the waste heat. During the design of the
boiler temperature gradient, the exhaust gas temperature of the
boiler should be reduced as much as possible to maximize the
utilization of waste heat. The multi-pressure HRSG exhaust gas
temperature, according to experimental data, can be reduced from
approximately 120 to 80  C. In combustion process, most of the
sulfur in the fuel becomes SO2. Under certain conditions, part of SO2
is further oxided to SO3. The SO3 gas and water vapor can be
combined into sulfuric acid vapors, whose condensation dew point


362

H. Wang et al. / Renewable Energy 99 (2016) 360e368

Fig. 1. Schematic of the entire circle.


H. Wang et al. / Renewable Energy 99 (2016) 360e368

principle and operation parameters, an absorption chiller whose
coefficient of performance (COP) is equal to 1.3 has been selected as
the cooling generator. However, the heat source temperature for
heating is often limited to less than 50e60  C.

Table 1
Fuel composition.
Wood dust component

Concentration

Carbon (C) (wt.%)
Hydrogen (H) (wt.%)
Oxygen (O) (wt.%)
Nitrogen (N) (wt.%)
Sulfur (S) (wt.%)
Moisture (%)
Volatile matter (%)
Fixed carbon (%)
Ash(%)

52.07
39.03
0.33
0.59
5.83
71.07
16.07
7.03

4. Parameter analysis
The gasification process plays a critical role in a CCHP plant. In
general, the syngas yield and composition of gases produced from
gasification dependent on parameters including reaction temperature, gasifying agent, type of biomass, particle size, heating rate,
operating pressure, equivalence ratio, catalyst addition and reactor

configuration [26]. Evaluation of a gasification process is mainly
considering the gas yield, LHV, and gasification efficiency. The gas
yield for gasification block is calculated in Eq. (1) as follows [1]:

Note: Lower Heating Value (LHV) is 16.46 (MJ/kg).

Table 2
Performance of the gasifier.
Operating pressure (MPa)
Operating temperature ( C)
Transport nitrogen/sawdust (kg/kg)
Gasifying agent
The moisture of sawdust into the furnace

363

3.1
~900
0.061
Air
2%

GV ¼

temperature can reach up to 110  C when it contains a high acid
content, and 80  C when it contains a low acid content. The exhaust
gas temperature of the three-pressure reheat-type HRSG should be
10  C higher than the acid dew point temperature, so the exhaust
gas temperature must be between 110  C and 120  C when it
contains a high sulfur content and 80  Ce90  C when it contains a

low sulfur content. In this case, mass fraction of sulfur component is
0.0026% and the CO2, SO2, SO3, and other acidic gases mass fraction
of the gas turbine exhaust is about 8.34%, the acid content in the
exhaust gas is relatively low, and the dew point temperature is
80  C. Therefore, the exhaust temperature of boiler is between
80  C and 90  C. Referring to Fig. 1, the heat recovery stream
generator is simulated as a series of cross exchangers that generate
steam at three different pressures, and the multi-stage stream
turbine is simulated as series of turbines with different outlet
pressures [13,39]. Then, steam-water system of HRSG and corresponding T-Q diagram was discussed. Jiao Shujian et al. [16] has
studied the heat transfer process of steam-water system. As a
result, the temperature zone in the T-Q diagram is divided according to the exhaust gas temperature characteristic. And the heat
balance equation of each temperature zone is also obtained. in
accordance with the heat balance equation of each temperature
zone, the operation parameter for steam cycle was calculated. See
Table 4 for detailed parameters.
The operation parameter is slight different with Simens [16].
Absorption chillers, which were created for drawing thermal power
from a “free” heat source, were widely applied in the existing CCHP
system. The heat source temperature required to drive the double
effect lithium bromide units was at 170  C. Based on the working

Vg
Mh

(1)

where GV is gas yield (m3/kg), Vg is syngas volume under standard
conditions (m3), and Mh is the quality of raw materials (kg). Once
the component volume is obtained, the total calorific value is found

by summing the individual component heat value in Eq. (2):

QLHV ¼

XÀ
Á
xi $QLHV;i

(2)

where QLHV is LHV of syngas (kJ/m3), xi is volume percent of the
component gas (%), and QLHV,i is LHV of component gas (kJ/m3).
Gasification efficiency is calculated from the indicator mentioned
above [31] in Eq. (3):

h¼

QLHV Â Gv
 100%
MQ

(3)

where ƞ is gasification efficiency (%) and MQ is LHV of raw materials
Table 4
Operation parameters for steam cycle.
Steam
temperature ( C)

Unit

Gas turbine
exhaust
temperature ( C)
Inlet

Outlet

Inlet

Outlet

High-pressure superheater
Reheater
High pressure evaporator
Medium-pressure superheater
High-pressure economizer 2
Low pressure superheater
Medium pressure evaporator
High-pressure economizer 1
Medium pressure economizer
Low-pressure evaporator
Low-pressure economizer

572
572
/
340
/
271

/
240
240
/
142

/
/
339
/
271
/
240
/
/
142
81

329
/
324
230
225
132
225
128
126
126
15

547
547
329
327
324
260
230
225
225
132
126

Note: /¼ no value.

Table 3
Performance of gas turbine unit.
Feedstock

Syngas from water-coal slurry

Syngas from sawdust

Air flow rate (kg/s)of air compressor
Chamber pressure (bar)
Pressure ratio
pressure ratio of air compressor 1
pressure ratio of air compressor 2
pressure ratio of air compressor 3
Inlet temperature of gas turbin ( C)
Exhaust gas temperature of gas turbin ( C)

Output power (MW)

645
15.801
15.4

623.1
15.801
15.3
9.71
1.46
1.082
1326
590
230

1327
609
250


364

H. Wang et al. / Renewable Energy 99 (2016) 360e368

(kJ/m3). Gasification efficiency is generally used to develop a primary energy saving analysis. Considering the integration of all the
subsystems, the total energy efficiency has been proposed in order
to compare to reference with the same energy product (cooling,
heating, and power). Ignoring differences between the three energy
products, the total energy efficiency can be expressed in Eq. (4) as

[8]:

E¼

Wþ

P

QC þ
QMQ

P

Qh

(4)

where E is total energy efficiency, W is electric power (kW$h), Qc is
cooling output (kJ), Qh is heating output (kJ), and Q is biomass flow
(m3/s). When considering the energy grade of energy product,
exergy efficiency of the system can be obtained in Eq. (5) as [17]:

hy ¼

Aw W þ

P

AC QC;i þ
QMQ

P

Ah Qh;i

(5)

Where Aw is 1, Ac is 0.073, Ah is 0.083, and hy is exergy efficiency.
However, the economic performance of the system is directly
related to its (cooling, heating, and power)price in Eq. (6):

q¼

Bw W þ

P

BC QC;i þ
QMQ

P

Bh Qh;i

(6)

Where q is economic exergy efficiency, Bw is 1, Bc is the ratio of
cooling energy and electricity energy price, and Bh is the ratio of
heating energy and electricity energy price. Commercial prices of
electric power are 0.623 RMB/kW$h, heating at 0.19 RMB/kW$h,

cooling at 0.29 RMB/kW$h in Beijing in 2005 [9]. Therefore, constant Bc is equal to 0.465, and Bh is 0.305. Relative energy saving rate
represents the energy-saving effect of a cogeneration system
comparing against convention individual system when output the
same amount of energy. That is Eq. (7):

Dh ¼

QI À QCCHP
QI

range 50e1000  C, gasification temperature increased from 810.79
to 1040.60  C in this case because gasification temperature rises
with gasifying agent temperature increasing. For comparison purposes, a range of 50e1000  C is presented. The content of CO varied
between 22.08% and 28.86% over the gasification temperature
investigated. J.J. Hernandez et al. [19] compare the effect of operating temperature on the gas production for air-steam gasifying
agents used in the study. The fact that the CO content increases
from 21% to 27% with temperature increasing from 750 to 1100  C.
It can be seen that the CO content in the study is very close to the
result of numerical simulation. Contrary to that the concencentration of CH4 decreases from 2% to 1.19% vol between temperature of
810e871.02  C and reach zero beyond a temperature of 1040  C.
The concentration of CO2 decreased from 11.67% to 6.91% vol. with
increase in temperature from 810  C to 1040  C. These trends may
be consistent with the following, although there are differences in
feedstock and other parameters. P.K.Senapati and S. Behera [30] has
shown that the concencentration of CH4 decreases from 5.4% to
3.0% vol between temperature of 736e816  C and reach zero
beyond a temperature of 1004  C. And the concentration of CO2
decreases form 28.5%e15% rol. with increase in temperature from
736  C to 1088  C. The N2 in the producer gas maily comes from the
gasifying agent. Its content for syngas from sawdust varied between 44.8% and 47.4% over the gas temperature investigated,

while coconut coir dust in the range of 40.7e59.2%. Therefore, the
result of numerical simulation was validated against experimental
and industrial data.
The LHV, gas yield and gasification efficiency at different gasifying agent temperature are shown in Fig. 2. It can be seen that with
the range of 500e600  C (871e895 Cgasification temperature), the
gas yield and gasification efficiency began to increase steadily. Nor
Afzanizam Samiran et al. [27] indicated that in the range of

(7)

WhereDh is relative energy saving rate, QI is fuel consumption of
convention individual system (kg), and QCCHP is fuel consumption of
CCHP system (kg). Eq. (8):
W

hep þ

QI ¼

P

QC
COPE

P !
Q
þ h h
b

MQ

(8)

wherehep is the average efficiency of thermal power plants
(33.3%),hb is HRSG efficiency (90%), and COPE is energy efficiency
ratio of absorption chillers [8]. The parameters mentioned above
are used to carry out a complete economic evaluation in terms of
the CCHP system. Finally, the environmental impact of the assessed
CCHP configuration was taken into account based on the data
collected in simulations.
5. Results and discussion
The relevant plant was modeled using Aspen Plus (Aspen Plus
11; Aspen Tech, USA), the only commercially available software to
handle the rigorous material and energy balances of complex systems. Figs. 2e4 show the gasification results, whereas Fig. 5 shows
the energy efficiency results of the CCHP configurations as depicted
in Fig. 1.
Fig. 2 presents the influence of gasifying agent temperature on
the gasification process. When gasifying agent temperature in the

Fig. 2. Influence of gasifying agent temperature on (a) gas component and (b) gasification efficiency.


H. Wang et al. / Renewable Energy 99 (2016) 360e368

365

Fig. 3. Influence of gasifying agent pressure on (a) gas component and (b) gasification
efficiency.

Fig. 5. Main results of (a) energy production and (b) energy efficiency for an oxygen

equivalence ratio ¼ 0.27.

Fig. 4. Influence of oxygen equivalence ratio on (a) gas component and (b) gasification
efficiency.

900e1100  C the gas yield increases steadily, whereas the char
yield decreases.
There is a relatively lack on information about the effect of
gasifying agent pressure in entrained flow gaisification of biomass.
According to Zhao Hui. et al. [40], the pressure of entrained flow
biomass gasifier is in the range of 20e50 at m. The production of H2
is more sensitive to change in air pressure when air is present in the
gasifying atmosphere. The H2 content at different pressure indicated that there is a reduction from 20.59% to 15.99% vol. with the
increase of gasifying agent pressure from 3 atm to 60 atm in Fig. 3.
The content of CO obtained was around 25.97e24.87% over the
pressure range from 3 to 60 at m. The attainment of syngas yield in
the range of 2.38 m3/kg to 2.25 m3/kg for sawdust in the investigated range of 3 atme60 atm are quite favorable for decrease in
cost of the compressor. In the present study the pressure has been
usually fixed at 3.1 Mpa [27]. Therefore, the high pressure of 30 atm
is advantageous as gasifying agent pressure.
Gasification process is sustained by heat generated from a
controlled amount of oxidant to conserve the reaction of gasification. The most extensively used gasifying agents are air, oxygen and
steam. The advantage of air þ steam gasification are availability and
economy of air, and the possibility of a higher heating value syngas.
A few studies on the effect of equivalence ratio on the gas temperature, when the powdery biomass feedstock along with
air þ steam was injected into the gasifier, have been reported in
literature [30]. The gas temperature increase from 808 to 963  C in
the investigated equivalence ratio range of 0.21e0.31, in agreement
with the results obtained by P.K. Senapati and S. Behera [30]. The
gas component versus the oxygen equivalence ratio is presented in

366

H. Wang et al. / Renewable Energy 99 (2016) 360e368

Fig. 4. With the oxygen equivalence ratio increasing, the H2 content
in the product gas significantly decreased form 20.86%e5.33%,
whereas N2 suffered a steady increase from 39.6% to 66.5%. The
content of CO increased initially and then decreased, and the
maximum value was 25.48% when oxygen equivalence ratio was
0.27. Compare to that the concentration of CO2 decreased from 18.4
to 8.86% and then increased from 8.86 to 12.07%. The content of CH4
decreases from 5.66% to 0.49% vol between equivalence ratio of
0.13e0.29 and reach zero beyond a equivalence ratio of 0.31.
Feng Yipeng et al. [10] indicated that both H2 and CO suffer at
least 10% vol. decrease within a equivalence ratio of 0.2e0.50
(oxygen þ steam gasification). In this paper, H2 content decreasing
from 19.20% to 6.10% vol, and CO content decreasing from 22.25% to
16.10% vol. throughout the whole range of equivalence ratio
mentioned above. The similar behavior and a slight difference can
be observed. It is necessary to take into account that the different
gasifying agent has two contributions: on the one hand, the gas
diluted in nitrogen coming from the air; on the other hand, the
occurrence and the extent of the different gasification reactions are
expected to be modified. In summary, there seems to be a advantageous gasification condition when equivalence ratio is 0.27,
gasifying agent temperature is 400e500  C and gasifying agent
pressure is 30 atm. Figs. 5e6 displays the results obtained in
different gasification condition for the study of the effect of the
gasification process on the whole co-generation system, as

following:
A further step into the understanding is the study of energy
products and energy efficiency of the co-generation system. This
task has been accomplished by keeping sawdust feed rate constant
at 170 tons per hour and the condensed water flow of heater exchangers of HRSG at 330 tons/h. The results obtained are displayed
in Figs. 5 and 6. As seen, the total energy efficiency is about 75%e
77%, with the gasifying agent temperature in the range of
300e500  C, while keeping oxygen equivalence ratio at 0.27. And
the power output of HRSG was about 123.9 MW. At the same time,
the relative energy saving ratio varied in the range of 74.88e80.85,
and the total energy efficiency and equivalent exergy efficiency
reached to the maximum near 410  C.
Fig. 6 presents the effect of oxygen equivalence ratio on energy
product and energy efficiency. It can be observed that for biomass
gaisification with set for heating, cooling and power production, an
increase of oxygen equivalence ratio leads to a rise of cooling
production. With the increase of air flow, the electric power and
heating energy first increases and then decreases. because along
with the increase in air flow, the LHV of syngas in the gasification
furnace increased firstly, then decreased due to the combustion,
partial combustion reactions associated to air. Whereas, If the air
flow is too large, the waste heat boiler system is in an abnormal
operation due to a shortage of heating supply. In severe cases, the
steam turbine is damaged. It is important to note that the total
energy efficiency is increasing about 8.5% with the oxygen equivalence ratio in the range of 0.25e0.35 in the base case. The increase
in air flow leads to a reduction in the exergy efficiency and the
relative energy savings ratio, mainly due to the energy grade influence on the energy efficiency.
The following cases were investigated in this paper for environment impact of co-generation system. In this case, the following
design constrains were maintained: 420  C gasifying agent temperature, 20 atm gasifying agent pressure, 0.28 oxygen equivalence
ratio, 170 tons/h mass flow of sawdust, 330 tons/h condensed water

flow of heater exchangers of HRSG, 224.4 tons/h condensed water
flow in the high pressure (HP) evaporator, 56.1 tons/h condensed
water flow in medium pressure (MP) evaporator, and 49.5 tons/h
condensed water flow in low pressure (LP) evaporator. Table 5
summarizes the simulation results.

Fig. 6. Main results of (a) energy production and (b) energy efficiency for gasifying
agent temperature ¼ 400  C.

It has also been reported that the entrained flow gasifier could
able to produce high temperature(975e1100  C) syngas when
equivalent ratio between 0.21 and 0.3. And the LHV was found to be
5.01 MJ/Nm3 without preheating and steam [30]). At 1150  C, the
use of an air-steam gasifying mixture leads to a 2 MJ/kg improvement in the product gas heating value as compared to air gasification. At 850  C, the LHV of sysgas is 5.665 MJ/Nm3 is obtained
when gasifying with air and equivalent ratio is 0.28, as seen in
Table .5. This gas heat value is acceptable due to CO content of
syngas is sensitive to gasification temperature. For coal-fired power
plants, CO2 emissions are 1.129 kg/kW$h, NOx production is
0.0033 kg/kW$h, and SO2 is emitted at 0.0096 kg/kW$h [15]. In
comparison to a conventional power plant, CO2 and SO2 emissions
were greatly reduced, with a rise in NOx (NO and NO2). The
increasing of NOX was due to high N content and low gasification
temperature. The case assessed in this paper generated about
352 MW of electrical power output, slightly less than an Integrated
Gasification Combined Cycle (IGCC) system. Considering the cooling and heating energy, the total energy efficiency was higher. The
conclusion is that a CCHP system based on sawdust gasification
seems to be a promising concept that has better energy efficiency
and emission performance than conventional systems.

6. Conclusions

1. Biomass gasification with sets (multiple sets of equipment) for
heating, cooling, and power production has been proposed as a
means to use renewable energy and search for a method to


H. Wang et al. / Renewable Energy 99 (2016) 360e368
Table 5
Overall plant performance indicators.
Main plant data

Units

LHV of syngas
Gas yield
Gasification efficiency
Syngas temperature
Gas turbine output
Stream turbine output
Gross electric power output
Cooling energy
Heating energy
Total energy efficiency
NO emission
NO2 emission
CO emission
CO2 emission
SO2 emission

MJ/m3
m3/kg

%

C
MW
MW
MW
MW
MW
%
kg/kW$h
kg/kW$h
kg/kW$h
kg/kW$h
kg/kW$h

5.665
2.476
77.450
850.798
228.357
123.930
352.285
467.085
246.130
75.43
0.67E-02
8.07E-05
4.87E-07
0.38
0.41E-03

improve energy efficiency. As the main design assumption, the
total energy efficiency of the base case is up to 75% and relative
energy savings ratio is up to 74%, with emissions of pollutants
improved for the co-generation system.
2. Another important aspect investigated in this paper was the
effect of the gasifying agent temperature and oxygen equivalence ratio. An increase in the gasifying agent temperature leads
to a improve of gasification temperature. Whereas gasifying
with air, air gasification maily increases the CO and H2. Content
in the product gas. The oxygen equivalence ratio has a great
influence on the performance of the co-generation system. The
syngas temperature increase from 808 to 96  C in the investigated equivalence ratio range of 0.21e0.31. The influence of the
oxygen equivalence ratio on gas turbine exhaust temperature is
the largest. Results show that a 0.27 oxygen equivalence ratio
and 420  C gasifying agent temperature improved overall energy efficiency, exergy efficiency, and relative energy saving
efficiency.
3. Condensate water flow affects the outlet temperature of the
waste heat boiler. Exhaust temperature fell sharply with the
increase of condensate water flow. If the exhaust temperature of
the HRSE dropped to below 80  C, the gas turbine exhaust
temperature may drop below the dew point to acid, and low
temperature corrosion will occur. The increase of condensate
water flow increases the amount of heating and power output
generated by the stream turbine. In addition, increased
condensate water flow can make the temperature and pressure
of the water system drop and make the turbine working conditions unstable, causing cavitation. The effect of water flow on
this process should be estimated at a later time.
Acknowledgments
This work was supported by the Guangzhou Science and Technology Plan “The Thermal Characteristics and Energy Efficiency
Evaluation of DMG System” (Grant No. 2013J4100114), the Guangdong Science and Technology plan (Grant No. 2014A020218005 and

Grant No. 2014B030301034).
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